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A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites
Accepted Manuscript A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites Kien-Sin Lim, Soo-Tueen Bee, Lee Tin Sin, Tiam-Ting Tee, C.T. Ratnam, David Hui, A.R. Rahmat PII:
S1359-8368(15)00506-5
DOI:
10.1016/j.compositesb.2015.08.066
Reference:
JCOMB 3746
To appear in:
Composites Part B
Received Date: 2 November 2014 Revised Date:
29 December 2014
Accepted Date: 21 August 2015
Please cite this article as: Lim K-S, Bee S-T, Sin LT, Tee T-T, Ratnam CT, Hui D, Rahmat AR, A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.08.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1 A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites
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Kien-Sin Lim1, Soo-Tueen Bee1*, Lee Tin Sin1*, Tiam-Ting Tee1, C. T. Ratnam2, David Hui3, A. R. Rahmat4
Department of Chemical Engineering, Faculty of Engineering and Science, Universiti
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1
Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia. Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000
Kajang, Selangor, Malaysia. 3
Department of Mechanical Engineering, University of New Orleans, New Orleans, LA
70148, USA
Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti
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4
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2
Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia.
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Abstract
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The application of ammonium polyphosphate (APP) in different types of
commodity thermoplastic composites (polyethylene, polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET)) have been discussed in terms of mechanical properties, morphologies and thermal properties. In addition, engineering thermoplastics such as acrylonitrile-butadiene-styrene (ABS), polyamides and poly(vinyl alcohol) (PVOH) and their composites added with APP and
ACCEPTED MANUSCRIPT 2 other additives were analyzed as well. It was suggested that improvement of mechanical properties and morphologies of the thermoplastic composites could be made possible with appropriate amount of APP and other additives such as montmorillonite (MMT),
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pentaerythritol (PER) and different types of layered double hydroxide (LDH). Furthermore, thermal properties such as limiting oxygen index (LOI) values together with cone calorimetry and thermogravimetric analysis (TGA) performance could be enhanced
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through optimum combination of APP, PER and melamine which functions as
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intumescent flame retardant (IFR).
Keywords: A. Polymer-matrix composites (PMC); E. Thermal analysis; C. Thermal properties: Ammonium polyphosphate
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*Corresponding author. Tel: +60 3 4107 9802; Fax: +60 3 4107 9803.
Email: [email protected] (Soo-Tueen Bee) and [email protected] (Lee Tin
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Sin)
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1. Introduction
Incorporation of flame retardant in polymer system is very important nowadays.
Flame retardants are essential in delaying the production and spreading of flames or fires. According to the conference proceeding held in Stamford, Connecticut on 19 May 2014 to 21 May 2014, the global consumption of flame retardants was about 4 billion pounds
ACCEPTED MANUSCRIPT 3 in year 2013. It was predicted that the market for flame retardant will reach 5.2 billion pounds in 2018, accounted for a compounded annual growth rate of about 5 % [1]. Based
to keep up together with the demand in flame retardant market.
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on such a strong and healthy growth, research and development will definitely continued
There are many different types of flame retardants available in the industry market
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with aluminium trihydrate (ATH) or aluminium hydroxide being the most widely used [2,3]. Basically, flame retardants can be either reactive or additive. They can be further
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categorized into several classes which includes minerals (ATH and magnesium hydroxide), organohalogen compounds (organochlorines and organobromines), and organophosphorus compounds (organophosphates, phosphonates and phosphinates). In addition, flame retardants can be categorized into phosphorus-containing, halogen-
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containing, silicon-containing or any other chemical-containing flame retardants based on the chemical type in their structure instead of grouping them according to classes. For instance, both decabromodiphenyl oxide and tris(tribromoneopentyl) phosphate can be
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classified as halogen-containing flame retardants [4,5]. Among the chemical-containing flame retardants, ammonium polyphosphate (APP) which is synthesized from inorganic
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salt of polyphosphoric acid and ammonia is currently getting the attention among the industries. It is known to be preferred over the other flame retardants due to its smaller loadings at lower cost and excellent processability. Most importantly, APP is a halogenfree flame retardant and thus it does not generate additional amount of smokes, making it to be environmentally useful compared to other halogen-containing flame retardants [6].
ACCEPTED MANUSCRIPT 4 APP is able to function as flame retardant in the condensed or polymer phase through intumescence. During intumescence, a material swells when it is exposed to heat or fire to form a porous carbonaceous foam which acts as a barrier to prevent heat, air and
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pyrolysis product from entering the surface of the material [7,8]. When polymeric materials which contain APP are subjected to fire, APP will first decompose into polyphosphoric acid and ammonia. The polyphosphoric acid would then reacts with
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hydroxyl group or other groups of synergists such as hydroxyethylcyanuarates or cyclic urea-formaldehyde resins to form a non-stable phosphate ester [9,10]. Upon dehydration
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of phosphate ester, charring would occur which involves the formation of a carbon foam above the surface of the polymeric materials to against the heat source. Furthermore, other than forming carbon foam, a viscous molten layer or surface glass can also be obtained on the polymeric surface which protects the polymeric materials from heat and
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oxygen [11]. Theoretically, three main components are needed in an intumescent flame retardant (IFR): 1) an acid source such as inorganic acid, acid salt or other acids that elevates the dehydration of carbonizing agent; 2) a carbonizing agent which is a
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carbohydrate that will be dehydrated by the acid source to become a char; 3) a blowing agent that will be decomposed to release gas resulting in the increase of polymer's
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volume and the formation of a swollen multi-cellular layer [12]. In order to achieve an optimum performance, APP needs to be properly dispersed into the polymer system and compatible with the polymer matrix. Besides that, it is better for APP to be insoluble in water to ensure a good weathering performance [13].
ACCEPTED MANUSCRIPT 5 There have been numerous literature published which focus on the flame retardancy and intumescence of APP on thermoset polymers or thermosetting resins [1417]. Over the decades, many improvements have been done by past researchers such as
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microencapsulation to enhance the performance of APP on thermoset polymers [18]. Hence, this paper aims to discuss the application of APP as IFR in thermoplastics and their composites. Mechanical properties (such as the tensile strength and elongation at
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break) and physical properties (such as surface morphology) are discussed here. Also, the thermal behaviour and intumescence are analyzed in order to understand the mechanism
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and effects of APP on thermoplastics and thermoplastics composites.
2. Commodity Thermoplastics and Their Composites
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2.1 Polyethylene
Basfar and coworker have examined the mechanical and thermal properties of
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low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) blends cross-linked by both dicumyl peroxide and ionizing radiation [19]. Besides that, they have performed
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a study to find out the effect of APP content on the mechanical properties of the chemically cross-linked 60/40 LDPE/EVA blends coupled with grafted polyethylene and maleic anhydride. Based on the results in Table 1, it was inferred that the retention of mechanical properties were independent to the amount of APP as both the retention of tensile strength and elongation at break was still remain in the range of about 100 %. However, addition of 35 phr caused a slight decrement in the tensile strength of the 60/40
ACCEPTED MANUSCRIPT 6 LDPE/EVA blend although its elongation at break experienced a small increment. The result was in agreement with the findings by Stark et al. as APP was assumed to have negative effect on the mechanical properties of wood flour-polyethylene composites [20].
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In fact, as shown by Basfar et al. and other researchers, a minimum amount of 35 phr APP is needed to achieve the required fire resistivity of the IFR compounds [19,21-23]. In order to enhance the tensile strength of the LDPE/EVA without reducing the amount
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of APP, small amounts of inorganic additives such as talc can be incorporated. As shown in Table 1, addition of 1 phr talc showed a remarkable increase in terms of tensile
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strength for the LDPE/EVA blends cross-linked by dicumyl peroxide. Also, the elongation at break was found to be increased compared to its previous blends added with 30 phr APP. Both retention of tensile strength and elongation at break were maintained in
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an acceptable range of 93 to 97 %.
In terms of surface morphologies, Cai et al. [24] have synthesized some shape stabilized phase change nanocomposites materials via the blending of high density
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polyethylene (HDPE), EVA, organophilic montmorillonite (MMT), paraffin and IFR compounds which include APP and pentaerythritol (PER). The samples were then
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observed under scanning electron microscopy (SEM) to obtain the surface morphology images for both the shape stabilized and the char residue of the samples. From SEM micrographs illustrated in Figure 1(a) and 1(b), it can be observed that a compact network structure was formed via the entrapment of paraffin and flame retardant by the HDPEEVA composites. Well dispersion of flame retardant in the composite and paraffin have achieved for the shape stabilized materials. On the other hand, when the sample added
ACCEPTED MANUSCRIPT 7 with APP-PER was burned in a muffle furnace under 800 °C to form a multicellular char residue, loose surface morphology was found to present as shown in Figure 1c. Under high magnification, the char residue displayed a rather non-homogeneous microstructure
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as compared to other samples added with organophilic MMT [24]. Hence, it can be concluded that incorporation of IFR compounds (APP and PER) has little or no effect on
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the network structure of the nanocomposites materials.
The effects of halogen-free flame retardant towards the phase change materials
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from paraffin/HDPE composites have been studied extensively by several authors. Cai et al. have performed SEM on the paraffin/HDPE composites mixed with IFR composed of expandable graphite, APP and zinc borate [25]. It was established that the flame retardants were evenly distributed in a three-dimensional net structure of paraffin and
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HDPE. Graphite sheets in round-like lamellar structure having an average size of 300 µm were noticed to be embedded into the phase change materials. After combustion of the materials to produce char residues, worm-like structure of the expanded graphite was
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found, and the addition of APP seems to result in a tighter and denser morphology than the materials added with only expandable graphite. Cai and coworker have further proved
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that the paraffin/HDPE composites with APP produce a form-stable composites which appeared to be uniform with no agglomeration [26]. In order to enhance the dispersions of the paraffin and flame retardant within HDPE, suitable amount of expandable graphite can be supplemented which would cause partial absorption of paraffin into the porous structure of expandable graphite during the extrusion process [27-29].
ACCEPTED MANUSCRIPT 8 Stark et al. have conducted a research to find out the fire performance and effectiveness of five different flame retardant systems (i.e. decabromodiphenyl oxide, magnesium hydroxide, zinc borate, melamine phosphate and APP) in the wood flour-
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polyethylene composites [20]. The composites without flame retardant were first prepared via the blending of 35 wt% polyethylene, 60 wt% wood flour and 5 wt% lubricant to ensure a good composite surface characteristics. Then, five different flame
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retardants were added individually to the composites by replacing 10 % of the wood flour. For instance, APP added composite was made up of 35 wt% polyethylene, 50 wt% wood
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flour, 10 wt% APP and 5 wt% lubricant. Based on Figure 2 which depicts the limiting oxygen index (LOI) for different composites, it was suggested that each of the flame retardant systems increased the LOI of the wood flour-polyethylene composites with APP possessed the highest increment of about 29 % as compared to its composite without
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flame retardant. Thus, it was concluded by the authors that APP functioned effectively as a flame retardant in the wood flour-polyethylene composites, but causing a decline in mechanical properties [30]. On the other hand, it was showed by several literatures that
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enhancement of the protective effect in the APP associated with PER could be done through the combination of zeolites and conventional heat insulating materials [31-33].
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By applying these components, fire-proofing properties could be imparted into the polymer systems such as polyethylene and ethylenic copolymers. For example, the usage of 4A type zeolite in the flame retardant systems consist of APP and PER led to a synergistic effect for the formulations involving polyethylene [32]. The LOI values have increased by 35 % with respect to the conventional APP-PER retardant system when 1.5
ACCEPTED MANUSCRIPT 9 wt% of 4A type zeolite was added. However, it was noted that high zeolite content of more than 5 wt% would result in a dramatic decline of the fire performance.
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Cone calorimetry is an approach which has been widely used in the fire testing field to study the combustion behaviour of a material or substance [34]. It provides a great amount of information regarding the fire performance and risk such as the heat
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release rate (HRR), total heat release, time to ignition, and some limited insight on fire hazards [35-37]. Over the years, the flammability of polyethylene composites with and
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without retardants has been investigated by few researchers. For instance, suitable amounts of polyethylene, wood flour and lubricant were mixed by Stark et al. to produce the wood flour-polyethylene composites [20]. Different types of flame retardants were chosen to evaluate their performances on the fire retardancy. From the HRR curves
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displayed in Figure 3, it is visible that the composites with flame retardants result in lower HRR as compared to their original composites without flame retardant. As reported by the author, APP decreased the HRR significantly while it led to a smaller value of
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HRR for a longer duration. In addition, Cai et al. and Zhang et al. have focused on the study of APP as IFR compounds in phase change materials containing polyethylene
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[25,26,38]. They have found out that addition of appropriate amount of expandable graphite resulted in a noticeable decrease in the peak of HRR due to the synergistic effect between the expandable graphite and APP [25]. Generally, the mechanism of IFR compounds in the phase change materials involves the decomposition of APP which acts as the acid source to induce polyphosphoric acid with intense dehydration effect. The formation of polyphosphoric acid would then participate in the dehydration of
ACCEPTED MANUSCRIPT 10 expandable graphite which yields some carbonaceous and phosphocarbonaceous residues that functions as a physical protective barrier [26].
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Thermal degradation behaviour of APP has been inspected by Camino et al. back in year 1984. According to their research, APP heated under nitrogen gas flow has experienced quantitative volatilisation in three main steps before it was fully burned off
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in temperature of about 900 °C [39]. This degradation process was found to be similar when APP was heated under vacuum, except that a much lower temperature was needed
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for the process to occur [40]. In both scenarios, it was established that ammonia and water were the gaseous products released during the first two steps. In the last step, phosphate chain fragments were obtained in a lesser amount after the condensation of samples in the degradation apparatus under room temperature condition [41]. In the
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present work, thermal stability and degradation properties of APP in polymer system has been widely studied and characterized by thermogravimetric analysis (TGA). As demonstrated by Cai et al., there were two steps of degradation process for the APP and
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other halogen-free flame retardants in the paraffin/HDPE composites [25]. Based on the TGA curves shown in Figure 4, it is obvious that the first step of degradation happens
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from 250 to 450 °C which indicates the degradation of the paraffin molecular chain and the flame retardant system. The second step of degradation that happens in a shorter temperature range of 450 to 500 °C is due to the deterioration of the HDPE main chains. Besides that, it can be discovered that during the first step of degradation, the weight loss of the paraffin/HDPE composites without flame retardant is relatively lower as compared to the composites added with flame retardant. However, the weight loss of the composites
ACCEPTED MANUSCRIPT 11 without flame retardant was found to increase in the second step of degradation while the flame retarded composites gives a lower weight loss. This can be explained by the thermal degradation of the flame retardant in the first degradation step which forms the
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charred residue during the second step. Meanwhile, it can be predicted from Figure 4 that composites with 10 wt% APP has a slightly higher thermal stability than that of the other composites with different flame retardant system. This was due to the action of APP
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which tends to decompose to yield polyphosphoric acid with intense dehydration effect and release gaseous products of ammonia and water during the heating process.
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Thereafter, the polyphosphoric acid would further take part in the dehydration of expandable graphite to form carbonaceous and phosphocarbonaceous residue which functions as a physical protective layer. The volatilized gaseous products would cause the mixtures of carbonaceous and phosphocarbonaceous residue to swell and lead to the
Polypropylene
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2.2
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formation of the intumescent charred residue [42,43].
The mechanical properties of polypropylene (PP) involving APP have been
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studied and published by past few literatures [44-47]. According to the research done by Schacker and Wanzke, PP compounds added with APP would have Charpy impact strength in between 20 to 80 kJ/m2, Young's modulus of about 1300 to 2200 MPa, elongation at break of 15 to 350 % and melt flow index around 5 to 20 g per 10 minutes [48]. In order to prevent the APP flame retardant from decomposition prior to processing stage, polymer compounding can be performed by twin screw extruder with a soft screw
ACCEPTED MANUSCRIPT 12 configuration which used the lowest possible melt temperature that gives a high output rate at low screw speed. Apart from that, sharper screw was recommended to be used during the compounding of PP and APP to produce an excellent flame retarded polymer
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with slower rate of decomposition. Chiu and Wang have conducted an experiment to study the dynamic flammability properties of an intumescent fire retarded PP filled with APP, PER and melamine [49]. From Table 2, it can be seen that increasing the amount of
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APP results in the decline of tensile strength. The inherent superior tensile strength of PP was found to be severely affected by the incorporation of APP in the composites due to
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the reducing crystallinity in the composites [50]. This was proved later by Li et al. when the graph of PP’s crystallinity in PP/PS blends against the side chain length of PP-g-PS copolymer revealed that additives, PS and PP-g-PS copolymer would restrain the PP chain from folding into a growing crystal lamellae, thus reducing the crystallinity of PP
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in the blends [51]. Throughout the whole experiment, Chiu and Wang have concluded that pure PP has poor flame properties. In order to enhance the flammability of original PP and its composites or blends, APP, PER and melamine can be employed which
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function as intumescent flame retardant. However, it is important to note that excessive amount of APP added into the composites would cause a rather low tensile strength
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which limits their applications. In 2011, Lin et al. have showed that incorporation of silane coupling agent KH-550 improved the mechanical properties of PP and APP to a certain extent. The tensile strength, impact strength and elongation were found to be increased by 24 %, 25 % and 14 % respectively compared to its original blend without coupling agent [52].
ACCEPTED MANUSCRIPT 13 Figure 5 shows the SEM micrograph of PP composites mixed with APP flame retardant. From Figure 5 (a), it can be observed that most of the APP particles are distributed in the PP matrix and are exposed to the external surface due to its poor
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compatibility with the PP polymer matrix [53,55]. Besides that, it was reported by Wu et al. that the relatively higher polarity of APP was responsible for the exposure of APP particles to the fractured surface of PP/APP composites [54]. When 70/30 PP/APP
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composites were treated in hot water medium at 50 °C for 24 hours, almost all APP particles have been eluted, causing some defects or voids on the fracture surface as
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displayed in Figure 5 (b) [55]. It was suggested that the water molecules have been absorbed onto the surface of the composites which lead to the dissolution of APP in the water to leave some defects on the surface [56]. For reduced APP amount in 80/20 PP/APP shown in Figure 5 (c), it was found that aggregation of APP occurred in the PP
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matrix which generated cracks and even holes between the components suggesting a poor dispersion and compatibility [52]. The condition becomes even worse when the mechanical properties of 80/20 PP/APP composite were found to decrease drastically due
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to the poor interfacial adhesion and accumulated microcracks at the PP and APP interface. In order to improve the surface morphology, suitable amount of coupling agent can be
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incorporated in the PP/APP composite. As shown by Lin et al., 80/20 PP/APP modified with silane coupling agent gave a rather uniform dispersion of APP in the PP matrix with little or no agglomeration and cracks [52]. This improvement would reduce the adverse effects of inorganic additives on the mechanical properties of thermoplastic composites.
ACCEPTED MANUSCRIPT 14 The flammability and flame properties of PP composites mixed with APP flame retardant and other additives such as PER and melamine have been investigated by Chiu and Wang [49]. From their study, it was first stated that pure PP without APP flame
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retardant and additives is highly combustible as it has a low LOI of only 17.8 % given in Table 3. Through the addition of APP, PER and melamine in the PP, the LOI of the PP composites increased gradually to a maximum value of 38.4 %. Since the amount of
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additives remained constant in the composites, it can be said that APP exhibits a good fire protection to PP composites as it functions as IFR that interrupts the burning process in
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the condensed phase [49]. In addition, it was confirmed by Lewin and Endo that addition of small amount of heavy metal salts such as zinc acetate, zinc borate, manganese acetate and manganese sulphate to APP-PER-PP composites appeared to elevate the flame retardant performance of APP in the PP matrix [57]. Anion type of heavy metal was
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discovered to impose a lesser impact when compared to the cation heavy metal which increased the LOI of the PP composites by 4 to 5 units [58]. On the other hand, it was established by Lin and co-author that PP/APP composites modified with silane coupling
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agent KH-550 reduced the water solubility effectively and improved the interface compatibility between the APP and PP matrix [52]. When increasing APP content in the
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composites modified with coupling agent, the LOI increased as well. This implied that a much higher oxygen concentration is needed to initiate and sustain the continuous combustion of the PP composites via the incorporation of APP. Besides that, APP has been proven to be effective in eliminating the dripping phenomenon and selfextinguished when a certain compound is subjected to fire [59-61].
ACCEPTED MANUSCRIPT 15 Thermal stability of polymer composites can be examined through the usage of cone calorimetry. As shown by Wu and coworker, the presence of intumescent additives in PP decreased the HRR values dramatically when compared to the pure PP without
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additives [55]. For 70/30 PP/APP composite, it was stated that the peak HRR was 1064 kW/m2 which was considered slightly lower than the pure PP with peak HRR of 1177 kW/m2 when both of samples were exposed to an external horizontal heat flux of 35
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kW/m2. Apart from that, it is important to note that the ignition time of the 70/30 PP/APP was only 24 seconds while pure PP has a longer ignition time of 44 seconds. This can be
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attributed to the earlier decomposition of APP than pure PP after the cone heater has irradiated the composite surface, giving off some volatile substances from the decomposition of APP [54]. Meanwhile, the dynamic flammability properties of PP filled with APP, PER and melamine were characterized as well [49]. It was concluded that pure
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PP has a low resistance to combustion and it is easily susceptible to fire or flame. By adding IFR consist of APP, PER and melamine, the maximum HRR was reduced substantially from 687 kW/m2 to 115 kW/m2 under heat flux of 50 kW/m2. The
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mechanism of the intumescence can be explained by the formation of a foamed multicellular char on the material surface when the material was subjected to fire.
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Consequently, thermal insulation was found to be imposed on the material surface which prevents other combustible gases from entering the flame source and separating oxygen from the burning material. As a result, the extinguishment of the fire can be provoked. In the worst scenario where the material absorbed more heat and caused the carbonaceous char to break, some heat would be liberated to the surrounding [49,62,63]. At that moment, new carbonaceous char can be produced subsequently to replace the broken
ACCEPTED MANUSCRIPT 16 char, and the whole phenomenon will continue repetitively. Hence, it was observed that the composite material undergoes several decomposition steps during burning process.
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Generally, there are five major crystalline forms of APP. Crystalline form I is produced by the heating of equimolar ammonium orthophosphate and urea under the ammonia at 280 °C whereas the crystalline form II with orthorhombic symmetry (a =
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4.256 Å; b = 6.475 Å; c = 12.04 Å) is obtained by heating the crystalline form I under high temperature of 200 to 375 °C [64]. During the transition of crystalline form I to form
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II, it was revealed that APP crystalline form III present as an intermediate state. On the other hand, crystalline form IV shows a monoclinic structure while its form V is orthorhombic [64]. Currently, it is known that only APP crystalline form I and II are commercially available in the market. Samyn et al. have examined the crystallinity and
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crystal structure of APP coated with nitrogenous and carbonaceous species at different heat treatment temperature via the X-ray diffraction (XRD) analysis [64]. It was pinpointed that the powdered APP at 20 °C has the same spectrum with the residue of
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treatment at 260 °C, both displaying the characteristic of APP crystalline form II. After thermal treatment of the APP sample at 415 °C, the residue showed a charred structure
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and a broad distribution of spectrum can be observed at 2θ between 22 to 23 degree. This suggests that an amorphous product has been formed as a result of thermal treatment which contains pregraphitic carbon [66,67]. Lin and coworker have investigated the XRD patterns of PP, APP and PP/APP composites modified by silane coupling agent [52]. For PP, it was reported that diffraction peaks at 2θ of about 14.2, 17.1, 18.6 and 21.2 degree were clearly noticed which corresponds to the crystal planes of (110), (040), (130) and
ACCEPTED MANUSCRIPT 17 (131) that exhibits a α-crystal form. However, the addition of APP into the PP composites has shifted the diffraction curves and a new peak was generated at 2θ of around 16 degree. It was predicted that the peak might represent the crystal plane of (300) which
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corresponds to the β-crystal form [68]. Hence, it can be concluded that both α and βcrystal forms were coexist in the PP/APP composites. Incorporation of APP was expected
to be a relatively better crystal structure [68,69].
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to trigger crystal transformation of PP and gave a β-crystal form of PP which is believed
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Thermal degradation of APP has been studied by Camino et al. back in year 1985 [41]. It was established that APP degraded upon heating to release ammonia and water as gaseous products with the formation of cross-linked -P-O-P- structure. Initially, the process started at low temperature of about 165 °C and APP reached thermal stability
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after only 3 % of weight loss. It was suspected that transition to a more stable crystalline form might probably occurred in the first minor degradation step and excess temperature of around 280 °C was required to initiate an extensive degradation. Subsequently, Zhang
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et al. and Hornsby et al. have reported the thermal and thermo-oxidative degradations of APP in a more comprehensive way [70,71]. It was found that degradation of APP
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occurred in two steps whereby the first step involves the elimination of ammonia gas around 300 °C with the formation of a highly cross-linked polyphosphoric acid. For the second step, the polyphosphoric acid would further been evaporated at temperature of over 550 °C and dehydrated to P4O10 which sublimates. Meanwhile, the effect of the APP-PER mixture as intumescent additive on thermal degradation of PP has been investigated as well [72]. From the TGA curve, it was observed that both the
ACCEPTED MANUSCRIPT 18 experimental and calculated values of PP added with 30 % of IFR (APP:PER = 3:1 wt/wt) are in agreement up to 460 °C. Beyond this temperature, a lower weight loss was found for the experimental curves and a larger amount of charred residue was obtained at
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500 °C. It was anticipated that PP was not so effective to exert significant impact during the degradation of the additive whereby gas evolution and charring took place at temperature lower than 400 °C. Furthermore, the additive appeared to induce partial
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charring of PP instead of forming volatile hydrocarbons when PP was heated alone.
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Wu and coworker have studied the intumescent flame retardancy of the PP composites containing APP and microencapsulated APP [53]. It was shown by the TGA curve that microencapsulated APP decomposed at about 270 °C which is almost similar to the break down temperature of APP. Initially, microencapsulated APP decomposed at
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a faster rate than normal APP due to the fact that melamine-formaldehyde resin used for the microencapsulation has a lower thermal stability [73,74]. During heating, melamineformaldehyde resin produced non-flammable gases such as ammonia and carbon dioxide
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which are helpful in forming a honeycomb char structure [75]. After reaching temperature of about 600 °C, the microencapsulated APP was found to be more thermally
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stable than the normal APP. In addition, decomposition of microencapsulated APP at 800 °C left around 3.8 % of residue while the residue of conventional APP at 800 °C is 0.6 %. Despite the similar thermal decomposition behaviour of microencapsulated APP and normal APP, there were still some differences when they were mixed with PP. Based on the TGA curves, it was inferred that 70/30 PP/microencapsulated APP was more thermally stable in the whole range of decomposition as compared to 70/30 PP/APP [53].
ACCEPTED MANUSCRIPT 19 70/30 PP/APP started to experience initial decomposition at about 250 °C whereas 70/30 70/30 PP/microencapsulated APP started to suffer weight loss only at temperature of 270 °C. As confirmed by the authors, both PP/APP and PP/microencapsulated APP
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composites have three steps of decomposition. On the other hand, Wu and coworker have investigated the flame retardancy of APP microencapsulated with poly(vinyl alcohol)melamine-formaldehyde resin [54]. These microcapsules were synthesized through the
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combination of three main components of a typical IFR system: an acid source (APP), a carbonization agent and blowing agent (PVOH-melamine-formaldehyde). Through the
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use of these microcapsules, the flame retardancy and water resistance of PP composites can be enhanced. From the TGA curves, it was revealed that pure PP without additives started to decompose at about 240 °C and has almost decomposed completely at 360 °C. Basically, thermal decomposition of PP/APP composite happened in three steps. The first
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step was the initial decomposition of PP/APP in which its decomposition temperature (250 °C) was said to be slightly higher than of pure PP due to the deterioration of APP. The second step of weight loss was the main decomposition process of the PP/APP
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composite while the third step that took place at 500 °C involve further decomposition of the composite to char. Lastly, it was demonstrated that APP microencapsulated with
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PVOH-melamine-formaldehyde resin can form a more stable charred layer than a conventional APP in PP which can safeguard the polymer composite from being further combusted in the condensed phase.
Figure 6 shows the TGA curves of PP/APP composites modified with silane coupling agent. From the curves, it is clearly observed that the starting decomposition
ACCEPTED MANUSCRIPT 20 temperature of PP/APP composites were slightly higher than pure PP which decomposed at around 264 °C and ended at about 445 °C. In fact, at 445 °C, PP/APP composites still have higher amount of un-decomposed mass with 80/20 PP/APP having the highest
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percentage of 82 % remaining [52]. Therefore, it can be deduced that increasing the weight percent of APP would increase the decomposition temperature of the composite. Moreover, the amount of residue would increase as well as seen in the case of 17 wt% of
SC
APP and 20 wt% of APP which still have 15 % of mass remaining at 600 °C. Hence, it was concluded that thermal stability of composites could be enhanced by the introduction
2.3
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of APP flame retardant.
Polystyrene
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Nyambo et al. have conducted an experiment to explore the chemical interactions between APP and MgAl layered double hydroxide (LDH) on the polystyrene (PS) composites [76]. During their research, magnesium and aluminium type of LDHs were
EP
selected as they appear to be similar to the commonly used magnesium hydroxide and
AC C
aluminium trihydrate fire retardants [77-79]. Transmission electron microscopy (TEM) was employed as a way to assess the presence of intercalated or exfoliated phases of layered inorganic materials within the PS matrix whereby low magnification gives some details about the degree of dispersion while high magnification provides certain information about the morphology, extoliation and intercalation of the materials. Based on Figure 7 which illustrates the TEM micrograph of 95 wt% PS added with different quantity of MgAl LDH and APP, it can be seen that some improvement in the overall
ACCEPTED MANUSCRIPT 21 additive dispersion happened when 2.5 wt% of APP was imparted to replace some MgAl LDH. There was a small number of tactoids exist in the PS composites with average length of 0.5 µm as given in Figure 7 (d) under high magnification. It was suspected that
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the presence of tactoids in the composites might be due to the agglomeration of APP instead of the poor dispersion of LDH [76]. Furthermore, it was anticipated by the authors that despite the improvement in disperion, flammability and thermal stability of
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cause a severe effect on the mechanical properties.
SC
PS composites, incorporation of APP into the PS and MgAl LDH composites would
Back in 1970s, thermally stable phosphonium salts such as Cyagard RF-1 {ethylene-bis[tris(2-cyanoethyl)]phosphonium bromide} combined with APP have been proven to be useful flame retardants which function in the condensed phase of
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thermoplastics such as PP and high impact polystyrene (HIPS) [80,81]. From the literatures, it was discovered that the LOI of HIPS composites increased proportionately with the content of APP although the flame retardant efficiency of APP in HIPS was
EP
found to be lower than in PP by a factor of around 2.5 [81,82]. For the phosphonium salt,
AC C
the flame retardant efficiency was said to increase non-linearly for both PP and HIPS and they tend to remain constant at high APP content. The difference in linearity of the APP and Cyagard RF-1 was believed to be due to different mechanisms operated in its respective PP and HIPS. APP has been reported to be a halogen-free flame retardant used mainly in the condensed phase of polymers whereas Cyagard RF-1 is mostly likely to perform better in the gas phase. Meanwhile, the synergistic effect between oxide nanoparticles and APP on flame behaviour of PS has been published by Cinausero and
ACCEPTED MANUSCRIPT 22 coworker in year 2011 [83]. It was established that the LOI of pure PS has been elevated from 18.5 % to 20.5 % when 15 wt% of APP was mixed with PS to give 85/15 PS/APP composite. Through the substitution of 5 wt% hydrophilic and hydrophobic type of
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alumina and silica into the APP, the LOI was found to be maintained in the range of 19.5 to 20.5 %. However, with 5 wt% APP replaced with hydrophobic silica, the LOI of the composite was able to achieve a highest value of 23.0 %. This can be attributed to the
SC
specific silicon metaphosphate (SiP2O7) crystalline phase which contributes to the
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improved charring and enhanced protective properties of the PS composite [83].
Table 4 shows the cone calorimeter data adapted from Cinausero et al. regarding the study of PS composites filled with different types of additives [83]. By comparing pure PS and 85/15 PS/APP, it can be seen that addition of APP into the PS polymer
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matrix does not convey significant improvement to the flame behaviour of the composite since the total heat release (THR) and peak of HRR of 85/15 PS/APP is comparable to the pure PS. Although the time to ignition (TTI) has somehow been reduced, inorganic
EP
residue was found to build up under the molten state of polymer during combustion,
AC C
indicating the absence of flame retardant efficiency [83]. In fact, incorporation of APP showed its function in modifying the degradation pathway of PS as the ignition of 85/15 PS/APP was about 20 seconds earlier than the pure PS to release some combustible compounds. Also, synergistic effect was said to be achieved when small amount of hydrophilic alumina and silica were added into PS composites, leading to the formation of a solid protective barrier and increased flame retardancy [84,85]. It was inferred that alumina has a catalytic effect on the emission of combustible products prior to the
ACCEPTED MANUSCRIPT 23 ignition of PS polymer since the TTI of 85/10/5 PS/APP/hydrophilic alumina was only 50 seconds compared to the 85/15 PS/APP with TTI of about 62 seconds. Combustion of 85/10/5 PS/APP/hydrophilic alumina demonstrated an intumescent phenomenon with a
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discontinuous char produced on the surface of the composite. Apart from that, as examined by Nyambo and coauthor, the time required to sustained ignition was drastically reduced when APP and MgAl LDH were added individually or in combination
SC
into the PS matrix [76]. This can be explained by the increase in viscosity of the neat polymer and decline in the heat exchange between the sample surface exposed to the
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radiant source and the bulk of the sample [86,87]. Consequently, a sudden increase in the sample's surface temperature would occur and the time needed for the composite sample to reach pyrolysis temperature was reduced. Furthermore, it was also proven by the researchers that replacing APP with MgAl LDH would results in a lower peak of HRR.
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Synergistic effect obtained from the combination of APP and MgAl LDH was able to reduce the peak of HRR by a whole 42 % when compared to the pure PS without any
EP
additives [76].
AC C
As mentioned earlier, thermal degradation of APP was done in a two-step mechanism. The first step basically involves the weight loss at around 250 °C followed by the elimination of ammonia and water as the gaseous products. Maximum degradation peak was observed at temperature of approximately 322 °C. The elimination of water further caused the formation of phosphoric acid accompanied by the cross-linking process to produce vitreous ultraphosphate [40]. Next, the second degradation step represented the fragmentation of ultraphosphate to P2O5 and phosphate compounds at
ACCEPTED MANUSCRIPT 24 higher temperatures. Thermogravimetric study on the flame retardant system in HIPS has been performed by Ferrillo and Granzow in year 1980 [88]. It was predicted that the effect of flame retardants with 15 wt% APP and 15 wt% Cyagard RF-1 {ethylene-
RI PT
bis[tris(2-cyanoethyl)]phosphonium bromide} on the volatilization of HIPS was much smaller than in the case of PP. Besides that, much higher concentration of flame retardants (roughly 35 %) were needed to ensure fire protection of HIPS. Optimum flame
SC
retardancy was obtained for HIPS composites consist of almost equal weight of APP and Cyagard RF-1 {ethylene-bis[tris(2-cyanoethyl)]phosphonium bromide}. On the other
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hand, it was confirmed by Balabanovich and Prokopovich that combination of APP and 2-methyl-1,2-oxaphospholan-5-one 2-oxide in the ratio of 2:1 was more effective to enhance the flame retardancy of HIPS instead of using them separately [89]. Charring has occurred above the surface of HIPS which was mainly caused by the reaction between the
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additives. In the case of 2-methyl-1,2-oxaphospholan-5-one 2-oxide, it was evaporated upon heating to produce some phosphorus-containing volatiles such as aliphatic and aliphatic-aromatic phosphine oxides and formed a small number of polyaromatic char
EP
consist of P-O-P structures. With the addition of APP into the mixtures, the degradation path of 2-methyl-1,2-oxaphospholan-5-one 2-oxide was changed. Formation of
AC C
polyphosphazene type of structure was most probably expected to take place together with formation of a polyphosphoric acid. Moreover, it was pinpointed that the additives and their decomposition products were helpful in generating less volatile styrene oligomers from the pyrolyzing HIPS and thus hinder the emission of pyrolyzed products.
ACCEPTED MANUSCRIPT 25 The influence of APP on thermal and thermo-oxidative degradation of PS has been investigated by Zhu through the means of thermogravimetry [90]. The rate of temperature rise was set at 10 °C/min with air and nitrogen being used as the carrier gas
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flowing at 100 ml/min. From his study, it was established that APP and its thermal degraded products would stimulate char formation in air and to a lesser extent in the presence of nitrogen. Intermolecular hydrogen transfer was reported to be the most
SC
significant during the thermal and thermo-oxidative degradation of PS as there was a decline in the apparent activation energy of the PS composites [90]. In addition, it was
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revealed that APP together with its thermal decomposition products were responsible for the accelerated thermal degradation of PS in nitrogen due to their acidity. Apart from that, the rate of thermo-oxidative degradation of PS was believed to rely on two main factors: the quantity of acid and the protection of the glassy layer formed from APP in air.
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Recently, Cinausero et al. have studied the thermo-oxidative degradation of PS added with nanoparticles [83]. It was disclosed that 85/15 PS/APP followed the thermal behaviour of pure PS until it reached 380 °C. Thereafter, stabilization took place and char
EP
was formed. This modification of PS degradation pathway can be attributed to the presence of APP and polyphosphoric acid [91]. Secondly, it was postulated that
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interactions between the ionic species of phosphorus additive and radical sites led to cyclisation reactions and further promote the formation of char [92]. Lastly, substitution of APP by 5 wt% oxides (alumina and silica) elevated the thermal stability by around 30 to 40 °C in terms of the temperature at maximum degradation rate. This increment was said to be originated from the stabilization effect of both alumina and silica on thermal
ACCEPTED MANUSCRIPT 26
Poly(methyl methacrylate)
SC
2.4
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degradation of PS and from the synergistic effect between the APP and the oxides system.
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Poly(methyl methacrylate) (PMMA) has been widely applied in several areas owing to its good optical properties, excellent mechanical performance, high thermal stability and superior electrical properties [93-95]. Although PMMA was employed in the form of sheets for making shatter resistant panels in building windows, bullet proof
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security barriers, furniture, skylights, signs and displays, it has a fatal disadvantage of being highly combustible [96]. Thus, flame retardants would need to be added to improve its flame behaviour so that the application of PMMA can be sustained. The morphology
EP
and mechanical properties of PMMA added with APP and other constituents have been reported by several authors. For instance, Friederich et al. have discussed the ternary
AC C
system of APP, melamine polyphosphate and titanium dioxide in PMMA from the views of thermal and fire resistance properties, morphology and other chemical properties [97]. It was implied that if titanium dioxide was added individually into PMMA, it was hard to distinguish the phosphorated additives from titanium oxide particles although it was said that the presence of titanium oxide did not bring any negative effects to the dispersion of titanium dioxide [98]. Friederich et al. have concluded that replacement of a small amount of metal oxide with APP flame retardant containing phosphorus led to the
ACCEPTED MANUSCRIPT 27 enhancement in the flame behaviour and thermal properties of PMMA. In particular, 85 wt% of PMMA added with 7.5 wt% of APP and 7.5 wt% of titanium dioxide was suggested to be the best formulation among the ternary system since it exhibits the
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greatest thermal stability increase. Meanwhile, comparison was done by the same author based on two meal oxides (alumina and boehmite) in the mixture of APP and melamine polyphosphate with PMMA as the polymer matrix [99]. From the SEM micrographs, it
SC
was supposed that residues with boehmite were more compact and gave a rather better morphology with fewer voids compared to the residues with alumina. The appearance of
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voids in the residues was believed to be the result of thermal degradation and it is undesirable for its ability to obstruct the efficiency of barrier effect. Hence, it was deduced that boehmite based ternary system especially nanocomposites of 85 wt% of PMMA, 7.5 wt% APP and 7.5 wt% boehmite, displayed a better flammability than the
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system consist of alumina.
Laachachi et al. have studied the effect of Al2O3 and TiO2 nano-oxides together
EP
with APP on the thermal stability and flame retardancy of PMMA [100]. Also, they have compared the two different types of APP (one being the pure APP with particle size of 15
AC C
µm having higher P/N ratio and another being the modified APP with particle size of around 5 to 80 µm having a lower P/N ratio) on the behaviour of PMMA composites. Based on Table 5, it was demonstrated that addition of both pure and modified APP has reduced the TTI and THR of the composites. It was important to note that the peak of HRR experienced a significant reduction when 15 wt% of pure and modified APP were added into the PMMA. Owing to the charred and vitreous or ceramized structure, the
ACCEPTED MANUSCRIPT 28 decomposition pathway of the PMMA composites was expected to be changed [101,102]. Furthermore, it was proved that incorporation of modified APP has led to an intumescent behaviour of the PMMA composites. Comparative analysis via the XRD disclosed that
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part of the APP has been replaced by melamine phosphate for the case of modified APP. This melamine phosphate is thermally less stable than APP because of the melamine release and by other nitrogen compounds. Consequently, mixture of APP and melamine
SC
phosphate would results in an intumescent phenomenon and the incorporation of the mixture in the PMMA would give an intumescent structure with the polymer acting as
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carbon supplier [103].
Cinausero and coworker have investigated the synergism between alumina and silica with APP on the flame behaviour of PMMA composites [83]. From their research,
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it was found out that during the combustion of 85/15 PMMA/APP, small solid islands were formed and developed progressively without covering the whole composite surface. After being taken out from the heating cone, the residue of the composite becomes highly
EP
viscous as a result of entrapment of water in an ultraphosphate structure [104,105]. However, the peak of HRR and mass lost rate values were decreased by about 27 to 34 %
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compared to pure PMMA. With the replacement of 5 wt% APP by nano-oxides, the island regions were present as well followed by the formation of a solid layer to prevent the effervescence of decomposition products. Intumescent phenomenon was seen for PMMA composites added with alumina while discontinuous char was found in the situation of silica filled composites. Moreover, two distinct macroscopic morphologies of char were revealed based on the chemical nature of the nano-oxides. Residues consist of
ACCEPTED MANUSCRIPT 29 aluminium were believed to be shell structured in which the barrier protection is established on the surface of materials while the material is consumed internally. On the contrary, silica based residues were bulk structured whereby their shape is fixed and
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maintained in spite of cracks and filling of carbonaceous substances [106,107].
Ternary system of APP, melamine polyphosphate and metal oxides has been
SC
widely examined by Friederich et al. in the 2000s. For instance, ternary system of APP, melamine polyphosphate and titanium dioxide has been prepared and its thermal
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resistance behaviour has been assessed in details [97]. From the cone calorimeter data, it was reported that THR decreased in comparison with pure PMMA when additives were added into the polymer matrix. In overall, TTI increased for all the formulations except for the case of 85/7.5/7.5 PMMA/APP/melamine polyphosphate and 85/7.5/7.5
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PMMA/melamine polyphosphate/titanium dioxide. 85/7.5/7.5 PMMA/APP/titanium dioxide displayed some unique characteristics as it has the highest fire performance index (ratio of TTI and peak of HRR), lowest THR, rather long combustion period and exert
EP
some synergistic effects on the peak of HRR with 52 % reduction. Hence, it was concluded by the authors that the replacement of a part of metal oxide with nitrogen and
AC C
phosphorus containing flame retardants led to the enhanced properties of PMMA composites
in
terms
of
fire
resistance
and
thermal
behaviour.
85/7.5/7.5
PMMA/APP/titanium dioxide was considered to be the best among the formulations owing to its highest TTI, best decrease in peak of HRR, lowest THR and greatest thermal stability increase as compared to the virgin PMMA. In year 2012, Friederich et al. have conducted another experiment with ternary system prepared from APP, melamine
ACCEPTED MANUSCRIPT 30 polyphosphate and metal oxides of alumina and boehmite [99]. According to the cone calorimetry, it was shown that all formulations of ternary system experienced an overall decrease when compared to its pure PMMA without any additives. The ignition time was
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observed to be delayed when alumina and boehmite were mixed into the PMMA composites. It was cited from other literatures that PMMA composites supplemented with melamine polyphosphate gives a 51 % reduction in peak of HRR due to the
SC
decomposition of melamine polyphosphate above 350 °C which functions as a heat sink to lower down the temperature of the polymer [108,109]. Upon release of phosphoric
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acid, a char would be formed that protects the polymer substrates from heat, flames, oxygen and other free radical gases from entering into the oxygen phase [110]. At the same time, the ammonia liberated as a result of melamine degradation swelled the char to protect the polymer in a higher extent. Besides that, the ammonia helps in slowing down
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the combustion process by diluting the radicals and flammable gases.
Over the past 60 years, several attempts have been done on studying the thermal
EP
degradation of PMMA [111,112]. According to Holland and Hay, the wide variation in the thermal degradation of PMMA can be examined in terms of the PMMA structure and
AC C
the experiment conditions used for preparing the polymer [111]. Generally, a two step degradation process would be resulted if the polymer was prepared in the presence of air. This is because the copolymerization was said to be done with oxygen instead of the weak links formed by combination termination. In addition, thermal degradation of APP and its blends with PMMA has been investigated by Camino et al. back in year 1978 [40]. It was inferred that for APP alone, it was degraded under vacuum in three stages. The
ACCEPTED MANUSCRIPT 31 first stage involves the condensation of APP into an ultraphosphate under temperature of not exceeding 260 °C before the release of ammonia and water. Next, fragmentation occurred in temperature between 260 and 370 °C which gives some high-boiling
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ammonium salts of phosphate fragments with more ammonia and water products. Lastly, the remaining polyphosphoric acid would then undergo extensive fragmentation under temperature of more than 370 °C. For the composites of PMMA and APP, it was
SC
pinpointed that the normal depolymerization of PMMA to monomer competes with the degradations reactions which form high-boiling chain fragments, methanol, carbon
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monoxide, carbon dioxide, dimethyl ether, hydrocarbons and char [40]. These additional reactions were believed to be triggered by the polyphosphoric acid. Apart from that, it was confirmed by Laachachi and coworker that the thermal stability of PMMA could be drastically enhanced by the incorporation of APP-based flame retardants although the
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modified APP with particle size of around 5 to 80 µm and lower P/N ratio would give a lower thermal stability than the pure APP [100].
Poly(ethylene terephthalate)
EP
2.5
AC C
Poly(ethylene terephthalate) (PET) is a thermoplastic resin from the polyester
family which has been commonly used as synthetic fibres. The excellence of its properties such as superior tensile and impact strength, clarity, good processability, high chemical resistance and reasonable thermal stability make it to be extremely useful in a wide variety of applications [113,114]. Zhu et al. have conducted an experiment to explore the effects of APP and coal on the flame retardancy, thermal degradation, char
ACCEPTED MANUSCRIPT 32 formation and mechanical properties of the PET composites [115]. In particular, the tensile and flexural strength of the composites were measured with a Zwick multipurpose tester following ISO 527 and GB 9341-2000. From Table 6, it was stated that the
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mechanical properties of the PET composites added with APP and coal were deteriorated. This can be attributed to the degradation of polyester and the poor interfacial adhesion between the PET and the additives. During the compounding of the composites, it was
SC
emphasized that APP and coal could promote the chain scission of polyester through hydrothermal and catalytic degradations [116]. The degradations can be traced back to
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the adsorption of water and polyacid by coal and APP respectively. Therefore, selection of suitable loading of APP and coal is vital to produce the PET composites with optimum properties. Besides that, pre-treatments of both APP and coal such as the intensive balling and surface coating of interfacial compatibilizer could also be applied before mixing
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them into the PET.
In terms of morphologies, Zhu and coworker have found out that addition of APP
EP
alone into PET would result in a featureless melting surface since both APP and PET would be in molten state by then, and APP was predicted to promote thermal degradation
AC C
and thus melting of PET [115]. On the other hand, APP and poly(acrylic acid) were chosen by Alongi et al. in the construction of a complex, four-layered (quadlayered) hybrid organic-inorganic structure via the layer by layer assembly method [117]. The structure was said to be crucial in the formation of an aromatic and stable carbonaceous structure or char. Besides that, it was able to thermally protect PET, cotton and PET/cotton blends. From the SEM micrographs, it was supposed that the layer by layer
ACCEPTED MANUSCRIPT 33 film formed on the PET surface depends on the quadlayer number. For 5 quadlayers, the PET fibres were not entirely covered unless the high voltage beam employed was able to penetrate beyond any thin film formation and render any such film invisible [118]. As
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confirmed by the authors, increasing the quadlayer number would cause the coverage of the fibres to become even more homogeneous. Furthermore, SEM micrographs showed that the structure and morphology of the deposited coating has greatly enhanced the
SC
thermal and thermo-oxidative stability of the PET, cotton and their blends. Favourable conditions were believed to be achieved for the formation of a stable char and thus
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increasing the final residues at high temperatures and preserving the polymer textures [117].
Table 7 shows the LOI of PET composites added with APP and coal at different
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loading level [115]. From the table, it is obvious that the LOI values increase gradually with the amount of APP in the composites. This is in agreement with the findings by Balabanovich and Brauman [116,119]. The LOI for coal powder was found to be 34.5 %
EP
while the composites of PET and coal exhibited slightly larger LOI values than the mere coal powder but smaller than the PET/APP composites. It was anticipated that coal is
AC C
able to facilitate the degradation of PET and cause the chain scission of PET chains which were able to be ignited at ease [120,121]. Futhermore, the trends of PET/APP/coal composites with different ratio of APP to coal were quite similar although they displayed different values of LOI. This was the result of accelerative degradation at low concentrations and blanketing at high concentrations of APP [115]. Lastly, it was deduced that substitution of APP by coal did not exert much difference to the flame
ACCEPTED MANUSCRIPT 34 retardancy when the combined additive loading level was beyond 30 wt%. In the cone calorimetry test, Zhu et al. have compared the three melt processed composites which include the original PET as well as PET added with 23 wt% of APP and coal in the ratio
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of 1:1 and 2:1 respectively [115]. From the test, it was noticed that the peaks of HRR for two modified composites were only 60 % with respect to their original PET without any additives. Basically, smaller HRR values indicate that less amount of heat would be
SC
supplied back to the degrading polymeric substrates. The adiabatic temperature of a fire situation in a confined space having the flame retarded polymers would be much lower,
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for example, lower than flashover temperature and thus a fire scenario would not happen [115,122]. In addition, it was discovered that the second peak of HRR periods were absent for the two modified composites which suggest that incandescent combustion was notably inhibited. However, the two modified composites could be easily burned when
adding APP and coal.
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they were subjected to fire due to the shorter ignition time than of virgin PET without
Thermal degradation mechanisms of PET have attracted several attentions of past
EP
researchers back in the 20th century. According to the summary by Buxbaum, thermal degradation of PET involved the transfer of a β-CH hydrogen leading to the formation of
AC C
oligomers with olefin and carboxylic end groups [123]. In addition, Zimmermann et al. and McNeill et al. focused on the degradation of PET at high temperatures [124,125]. It was established that the reactions of thermo-oxidative degradation were additionally complicated through the involvement of oxygen. This process was reported to begin with the formation of a hydroperoxide at the methylene group accompanied by the homolytic chain scission. On the other hand, Montaudo et al. have studied the thermal degradation
ACCEPTED MANUSCRIPT 35 mechanisms of PET in the presence of APP [126]. Based on their research, it was inferred that APP affected the pyrolysis by increasing the rate of decomposition and reducing the polymer decomposition temperature of the PET. Nevertheless, addition of
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APP did not alter the nature of the pyrolysis products. For the case of pure PET, it was verified that pyrolysis of PET continue through the primary formation of cyclic oligomers at temperature of about 300 °C and further decompose at around 400 °C to produce
SC
oligomers that contain acetaldehyde and anhydride [127]. The effect of interaction between APP and PET on thermal degradation of PET has been examined by Zhu and
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coworker in year 2010 [115]. It was implied that APP apparently started to degrade at 300 °C with the release of ammonia and formation of polyphosphoric acid. After that, further degradation took place at 650 °C with volatilization of polyphosphoric acid fragments while PET was being degraded at 420 °C due to main chain scission through
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the formation of carboxylic acids and vinyl chain ends [128]. By comparing the calculated and experimental results of TGA, it was observed that rapid degradation of 90/10 PET/APP happened at approximately 380 °C because of the presence of APP that
EP
increased the PET degradation remarkably [115]. This statement also applied for the composites of 75/25 PET/APP and 70/30 PET/APP. Apart from that, interaction between
AC C
APP and coal still displayed the acceleration of initial PET degradation due to APP, but this time a preferential retardation of char residues volatilization was detected at temperature exceeding 550 °C in comparison with the calculated data. Besides that, it was assumed that high weight ratio of APP to coal could be advantageous in hindering the gasification of char residues [129]. In conclusion, synergistic carbonization on PET between APP and coal might probably been achieved at high temperatures.
ACCEPTED MANUSCRIPT
Acrylonitrile-butadiene-styrene
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3.1
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3. Engineering Thermoplastics and Their Composites
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36
Acrylonitrile-butadiene-styrene (ABS) is considered to be one of the vital and prominent type of engineering thermoplastics owing to its superior mechanical properties, good chemical resistance and rather easy processability [130-132]. It has been widely
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applied in several fields such as the electrical industries, medical areas, automotive production and even for households usage. Although ABS possesses overall excellent properties, it shows unsatisfying flame and thermal behaviour as it has a low LOI of only
EP
about 18 % [133,134]. In particular, numerous amount of dense smoke and toxic gas will be liberated during the burning of ABS and its composites, and thus many literatures
AC C
have now been focusing on how to increase the flame retardancy of ABS and its composites through the use of flame retardants and IFR. It has been proposed by several papers that application of APP in ABS composites would results in positive effect with the compensation of the inherent properties of ABS. For instance, as shown by Zhang et al., the storage modulus of ABS blended with 24 wt% APP and 6 wt% poly(diphenolic phenyl phosphate) was lower than the original ABS when the testing temperature exceeds
ACCEPTED MANUSCRIPT 37 105 °C [135]. This can be attributed to the plasticization effect of APP and poly(diphenolic phenyl phosphate). In order to mitigate the plasticization effect and
appropriate amount of expandable graphite can be added.
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enhance the interfacial adhesion between the flame retardants and the polymer matrix,
Figure 8 shows the SEM micrograph of char residues for ABS composites added
SC
with APP and expandable graphite. For pure ABS alone, there were a number of big holes and cracks present in the residue due to the insufficient char formation or formation
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of a rather less condensed char during the combustion process [136]. For the case of 85/15 ABS/APP, it can be observed that a smooth thin film-like glassy coating was formed on the surface of the residue as a consequence of the decomposition of APP which generated viscous phosphoric or polyphosphoric acid. Nevertheless, this formation
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of coating or char was found to be ineffective in preventing the degradation of ABS. On the other hand, as seen in Figure 8 (c) and (d), unique worm-like structures of carbon layer was developed which is different from the pure ABS and ABS/APP composites. It
EP
was believed to be the expandable graphite in the composites which tend to expand and spread in the ABS polymer matrix. However, the char residue of 85/15 ABS/expandable
AC C
graphite seems to be brittle and susceptible to collapse whereas the char residue of ABS added with 15 wt% of expandable graphite and APP in the ratio of 3:1 was found to be more compact, thick and tight with a larger expansion of volume. This char structure was suggested to be more able to inhibit the heat and mass transfer, thus protecting the ABS matrix from heat for a longer time at higher temperature and retarding the degradation of the underlying material [137,138]. Hence, it can be concluded that combination of both
ACCEPTED MANUSCRIPT 38 APP and expandable graphite in a suitable manner could enhance the ABS polymer with excellent flame retardancy.
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Xia et al. have investigated the synergistic effects of organic MMT and IFR made up of APP and PER on the flame retardancy of ABS [139]. Besides that, they have examined the influence of IFR content (with the ratio of APP to PER being 3:1) towards
SC
the LOI and UL-94 rating of ABS composites. From their findings, it was inferred that the LOI values were increasing continuously with the increasing amount of IFR.
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However, it must be noted that only addition of at least 30 wt% of IFR into the ABS would results in LOI of higher than 27 % with V-0 rating (burning ceased within 10 seconds after two applications of flame with 10 seconds each on the test specimen followed by no flaming drips). Although further addition of IFR into the ABS can further
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increase the LOI, other properties such as the mechanical strength and processability of the ABS composites would need to be sacrificed. Meanwhile, the effect of APP and expandable graphite on the flame retardancy of ABS was studied by Ge and coworker
EP
[136]. From the LOI test, it was confirmed that pure ABS is an easily flammable polymer with LOI of only 19 %. The LOI values of ABS added with 15 wt% APP alone and 15
AC C
wt% expandable graphite alone were shown to be 21.5 % and 26.0 % respectively, indicating that expandable graphite is more efficient in improving the LOI of ABS. Through the combination of APP and expandable graphite into the ABS, the LOI values have been increased remarkably above 26.0 % with a maximum of 31.0 % achieved when the ratio of APP to expandable graphite was 1:3. In spite of that, it was established that further increase of the APP content would worsen the LOI of the ABS composites.
ACCEPTED MANUSCRIPT 39
The thermal degradation behaviour of new IFR system consists of APP and expandable graphite incorporated in ABS polymer has been characterized by Ge et al. via
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the TGA [136]. From the TGA and differential thermogravimetric (DTG) curves shown in Figure 9, several observations can be made. First, ABS composite with APP added alone exhibited only one main pyrolysis peak at about 421 °C with char yield of 14 wt%
SC
at 700 °C. For ABS added with expandable graphite alone, two main pyrolysis peaks were seen at temperature around 250 and 420 °C. This can be explained by the
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intumescent process of expandable graphite which appeared in the range of 200 to 300 °C that contribute to the first pyrolysis peak. The second peak was said to be due to the thermal degradation of ABS which happened at 400 to 500 °C. In the case of ABS filled with both APP and expandable graphite, it was suggested that the char yield of the
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composite was about 15 wt% at 700 °C which is clearly higher than of pure ABS. Other than that, it was emphasized that composites containing APP and expandable graphite could lower the maximum decomposition rates, proving that both the flame retardants
EP
were able to enhance the thermal stability of ABS polymer [140,141]. Hence, in overall, it was concluded that combination of APP and expandable graphite works well in
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increasing the char residues of the ABS composites at high temperature and helps in lowering the mass loss rate as compared to the original pure ABS.
3.2
Polyamide
ACCEPTED MANUSCRIPT 40 Polyamide, or generally known as nylon, is a long chain fiber-forming substance with recurring amide groups. It is often applied in monofilaments and yarns in the form of fibers. Over the years, polyamide was chosen as the material of construction in the
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textile, transportation and automotive industries due to its excellent mechanical properties at high temperatures, good chemical resistance, low permeability to gases and high wear and abrasion resistance [142,143]. Up to date, there have been a number of commercially
SC
available types of polyamide such as nylon-4,6, nylon-6,10, nylon-6,12 and nylon-11 with nylon-6 and nylon-6,6 being the most widely used [144-146]. Li et al. have
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conducted tensile test on the nylon-6,6 fabric treated with IFR system made up of APP, melamine and PER [147]. From their study, it was confirmed that the tensile strength of untreated nylon-6,6 fabric was 190.1 MPa with elongation at break of 26 %. When treated with IFR system containing APP, the tensile strength was found to reduce to only
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149.5 MPa although the elongation at break increased to 28.3 %. This can be attributed to the breaking of fibers in nylon-6,6 when the fabric was dipped into the flame retardant solution and passed through the padder with two dips and nips. Furthermore, the high
EP
temperature of 135 °C employed during the preparation of the treated nylon-6,6 was believed to be another factor contributing to the decrease in tensile strength of the final
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polymer [148]. In fact, as proved by Levchik and coworker back in 1995, the loading of halogen-free flame retardants need to be limited as they would cause unfavourable decrease in the mechanical properties of the polyamide materials [149]. Other than mechanical properties, the microstructures of the untreated and treated nylon-6,6 was also characterized by Li et al. and given in Figure 10 [147]. From the SEM micrograph, untreated nylon-6,6 gives a rather flat and smooth surface compared with other treated
ACCEPTED MANUSCRIPT 41 nylon-6,6 samples. For the case of nylon-6,6 treated with APP, the char residue showed some micro-convexities on its surface because of the formation of char promoted by the APP. On the other hand, nylon-6,6 treated with APP and melamine displayed a frothy
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structure contributed by the decomposition of melamine that released gas, whereas nylon6,6 treated with APP and PER exhibited a continuous and thick structure of char residue due to the char formation effect with PER as the char source. Lastly, when nylon-6,6 was
SC
treated with APP, melamine and PER, a typical intumescent char structure was generated which is fairly frothy and swollen. The char formed on the surface was said to be
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effective in preventing the nylon-6,6 from contacting the flame and controlling the release of fuel gas [150,151]. Besides that, the intumescent char layer was able to support the melted nylon-6,6 so as to avoid dripping phenomenon during combustion process.
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The LOI of polyamide 6 or nylon-6 added with APP and other additives have been studied by different researchers in the past few decades. For instance, Levchik et al. have discovered that an increase in LOI was observed when the APP content was over 30
EP
wt% [152]. Nevertheless, intumescent char layer could still form on the surface of the
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nylon-6 composites even in the presence of APP as little as 10 wt%. However, the char layer with lower APP content was predicted to be not continuous and susceptible to flow and drip. This finding was later confirmed again by the same author when they found out that the intumescent char layer was produced only at the edges of the top specimen surface with higher tendency to flow and exposing the underlying nylon-6 polymer to the flame [153]. By increasing the APP content to the range of 40 and 50 wt%, the LOI of the composites has been elevated tremendously to a maximum of about 50 %. In addition, it
ACCEPTED MANUSCRIPT 42 was suggested by Levchik et al. that progressive replacement of APP with fillers such as talc would results in an initial improvement of the fire retardancy of nylon-6 [149]. The improvement can be related to the formation of a protective viscous char which tends to
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stick to the surface of the polymeric material rather than flowing away as in the case of nylon-6/APP composites without the addition of filler. Furthermore, it was emphasized that substitution of as low as 1.8 wt% of APP with manganese dioxide in the nylon-6
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composite would prevent the flowing of intumescent char and increased the LOI from 25% to 37% [154]. The optimum synergistic effect was achieved when the mixtures of
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APP and manganese dioxide was used in the ratio of 6:1.
The cone calorimetry of nylon-6 added with APP was obtained through linear pyrolysis experiment whereby the heat transfer to the composite specimen took place in
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the vertical direction of combustion [153]. At heat flux of 20 kW/m2, it was noticed that all the three composites (pure nylon-6, 80/20 nylon-6/APP and 60/40 nylon-6/APP) did not undergo auto ignition as they have low and constant values of gas temperature. 80/20
EP
nylon-6/APP gave a strong increase in the initial rate of decomposition due to the
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destabilization effect triggered by APP. However, after 15 minutes of decomposition, an intumescent insulating protective char was formed on the surface of the composite which led to the decrease of temperature from 275 to 250 °C. After duration of 30 minutes, the intumescent char layer was diminished when the temperature was risen again to 280 °C. For the case of 60/40 nylon-6/APP, the initial rate of weight loss was slightly lower than 80/20 nylon-6/APP and the intumescent char appeared after 12 minutes of decomposition, displaying better insulating and protective properties which decreased the temperature of
ACCEPTED MANUSCRIPT 43 the composite specimen from 275 to 225 °C. Furthermore, it was showed that substitution of APP with talc caused a slight increase in the final residue although the rate of combustion for the composites remained the same [149]. On the other hand, replacement
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of APP by manganese dioxide was able to enhance the fire retardant performance in the nylon-6 composites. The intumescent char of nylon-6 composites adopting part of manganese dioxide was formed after about 2.5 minutes with high shielding effect. The
SC
char would lose its insulating and protective properties only when the specimen has been
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consumed during the combustion or decomposition process [154].
Thermal degradation of nylon-6 has been studied by Levchik and coworker back in 1992 [152]. It was concluded that nylon-6 undergone competing mechanisms during thermal degradation to produce caprolactum, chain fragments with nitrile and carbon-
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carbon unsaturated chain ends with a small amount of residue stable to 550 °C. Pure APP, on the other hand, started to decompose at 260 °C followed by losing 18 % of its weight up to 450 °C primarily due to the elimination of ammonia and water [153]. For the nylon-
EP
6 composites comprising of APP, thermal decomposition began at temperature of 50 °C
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lower than the pure nylon-6 polymer. The shape of the TGA curves generated was greatly modified. Moreover, the weight loss occurred in three steps which involved initial decomposition at 270 to 330 °C, second decomposition at 330 to 380 °C and final decomposition at 380 to 420 °C. The relative rates of the first and third decomposition steps were found to be increasing with the amount of APP. In addition, incorporation of talc into nylon-6 together with APP has triggered chemical reaction between APP and talc. This reaction was detected at temperature above 350 °C and it was suspected to
ACCEPTED MANUSCRIPT 44 prevent the volatilization of polyphosphoric acid and increase the amount of thermally stable solid residue. As such, the inorganic phosphates that were produced were able to enhance the insulating behaviour of the intumescent layer on the surface of nylon-6 when
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it was exposed to fire.
For the case of nylon-6,6 and nylon-6,10, it was confirmed that both the
SC
polyamides would decompose upon heating through the chain scissioning of N-
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alkylamide bond followed by the formation of vinyl chain ends and primary amide groups that would be dehydrated to nitriles at the temperature of degradation [155]. In nylon-6,6, chain scission of alkylamide bond was also believed to happen, leading to methyl and isocyanate chain ends which can be further dimerized into carbodiimides or trimerized into isocyanurate structures. By adding APP, chemical interaction between
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APP and the polyamides was found to cause chain scission of the polyamides with the formation of a primary amide group and a phosphate ester that would be thermally degraded to generate a vinyl chain end. Furthermore, as shown by Levchik et al., APP
EP
also influenced the thermal degradation process of aliphatic nylon-11 and nylon-12 by
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lowering their decomposition temperatures and modifying the composition of the subsequent volatile products [156]. For both of the polyamides, α and β types of unsaturated nitriles were the primary volatile products of degradation in the presence of APP. It has been proposed that the interaction between APP and the two polyamides involved the formation of intermediate phosphate ester bonds whose further decomposition accounts for the thermal behaviour of the composites.
ACCEPTED MANUSCRIPT 45 3.3
Poly(vinyl alcohol)
Poly(vinyl alcohol) (PVOH) is a type of unique synthetic polymer as it cannot be
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prepared though the conventional polymerization of the corresponding vinyl alcohol monomers. Instead, it is prepared by first polymerizing vinyl acetate followed by the subsequent conversion of the polyvinyl acetate into PVOH [157,158]. Generally, PVOH
SC
possesses high tensile strength and flexibility that rely on the humidity. In another words, higher humidity is resulted when more water is being absorbed into the PVOH and this
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would cause the reduction in tensile strength [159,160]. With the advantage of being highly resistant to oil, grease and solvents, PVOH is mostly applied in textiles, paper making and coating industries. Not long ago, Zhao et al. have verified the mechanical properties of PVOH mixed with flame retardant system consist of APP and 0.3 wt% of
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LDH [161]. The types of LDH used were mainly composed of different combinations of metals such as zinc, aluminium, nickel and iron. From their research, it was revealed that addition of APP alone resulted in the decline of the mechanical properties in terms of
EP
tensile strength, elongation at break and tensile modulus. In particular, significant decrease was observed in the tensile strength as it decreased from 75.0 MPa to a
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remaining value of 33.0 MPa when 15 wt% of APP was added. In order to enhance the mechanical properties of PVOH without abandoning the APP (since it is crucial in increasing the flame retardancy of PVOH), suitable amount of LDH can be incorporated. As shown by the authors, overall mechanical performance of the PVOH composites has been increased when 0.3 wt% of LDH was added. Combination of nickel and aluminium in LDH was found to be the most effective in enhancing the mechanical properties of
ACCEPTED MANUSCRIPT 46 PVOH/APP composites. It was predicted that there exist a physical cross-linking network composed of layered particles, APP molecules and PVOH polymer chains in the composites. This physical cross-linking effect was believed to improve the mechanical
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properties and delay the thermal decomposition of organic-additive/polymer or neat polymer [162,163].
From the perspective of surface morphologies, Zhao et al. have performed SEM
SC
analysis on the char residue of PVOH/APP composites and PVOH/APP added with different types of LDH [161]. Based on Figure 11, it can be seen that the outer surface of
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PVOH/APP char showed a fluffy and puffy morphology while the inner surface showed a rather restricted expansion. For the case of PVOH/APP added with nickel-aluminium type of LDH, continuous and compact outer surface was seen together with the swollen inner structure. This morphology was demonstrated to be able to hinder the heat and mass
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transfer besides than the ability to obstruct flammable gases from entering the surface of composite during fire. As quoted from Liang et al., the outer surface of the charred layer ought to be compact enough to prevent the penetration of gases and to cut off the oxygen
EP
from the degraded volatiles more efficiently [164]. Thus, it was implied that the char of PVOH/APP could not isolate flame, combustion-supporting and flammable gases from
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the inner polymeric material due to the lack of compact outer surface. On the contrary, LDH functions in catalyzing the polyphosphate or the phosphate to cross-link with PVOH during combustion to form expansion charred layer with close outer surface which is helpful in protecting and insulating the underlying polymeric materials from further pyrolysis and burning.
ACCEPTED MANUSCRIPT 47 Lin and coworker have explored the flame retardancy of PVOH through the addition of APP [165]. Also, they have further inspected the effect of MMT towards the mechanical and thermal properties of PVOH and APP composites. According to their
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study, the LOI for pure PVOH was 19.7 % while the incorporation of 15 wt% of APP increased the LOI to 27.9 % although a V-0 rating was still unable to achieve. To increase the flame retardancy of PVOH/APP to a greater extent, small loading of MMT
SC
can be applied. It has been declared that LOI values increased gradually with the amount of MMT in the composites up to a maximum of 30.8 %. However, the LOI values of
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PVOH/APP/MMT composites were reduced when the weight ratio of APP to MMT was lower than 20:1. Therefore, it was concluded that optimum synergistic effect was achieved at APP:MMT of 20:1. No visible dripping occurred during the UL-94 test and the burning of composites specimen ceased within 10 seconds after two applications of
Wang
et
al.
that
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flame with 10 seconds each on the test specimen. Furthermore, it has been proven by ammonium
salt
of
2-hydroxyl-5,5-dimethyl-2,2-oxo-1,3,2-
dioxaphosphorinane was able to improve the flame retardancy of PVOH in combination
EP
of APP and nickel chelate [166]. The optimum formulation of the IFR used in PVOH composites was found to be: 7 wt% APP, 0.5 wt% nickel chelate and 7.5 wt% 2-
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hydroxyl-5,5-dimethyl-2,2-oxo-1,3,2-dioxaphosphorinane. With the formulation, LOI as high as 34.2 % was obtained.
Cone calorimeter was employed to obtain a better understanding regarding the
effects of APP and MMT on the fire and thermal behaviour of various PVOH composites. Basically, HRR and peak of HRR are two important parameters in determining the fire
ACCEPTED MANUSCRIPT 48 safety features of a certain material [35,36]. As shown by Lin et al., 85/15 PVOH/APP and 85/14.3/0.7 PVOH/APP/MMT gave a rather low peaks of HRR (194 and 157 kW/m2 respectively) under heat flux of 35 kW/m2 compared to the pure PVOH with value of 576
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kW/m2 [165]. This deduced that addition of APP with or without MMT can enhance the flame retardancy of PVOH composites. Besides that, the THR of both the PVOH/APP and PVOH/APP/MMT composites were lower than of pure PVOH. This can be explained
SC
by the incomplete burning of the PVOH polymer in the flame retarded composites. The incomplete burning was deemed to be caused by the formation of char on the surface of
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the polymer matrix which functions as a thermal protective and insulating layer to inhibit the pyrolysis of the polymer. Furthermore, it was reported that the peak of HRR for PVOH/APP/MMT was lower than PVOH/APP although the THR for both the composites were almost the same. This can be related to the char formation in
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PVOH/APP/MMT which is more compact than the char in PVOH/APP, leading to synergistic effect between APP and MMT. Hence, it was concluded that introduction of both APP and MMT into the PVOH can strengthen the flame retardance properties. Apart
EP
from that, application of microencapsulated APP was found to exert remarkable effect on certain PVOH derivatives such as the composites of PVOH and glycerol-plasticized
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thermoplastic starch. As established by Wu et al., addition of microencapsulated APP changed the peak of HRR slightly but drastically reduced the THR value during the test of microscale combustion calorimetry [167].
Thermal decomposition of PVOH has been inspected and published by Holland and Hay [168]. It has been justified that thermal degradation of molten PVOH resulted in
ACCEPTED MANUSCRIPT 49 elimination of water and chain scission. This occurred through a six-membered transition state that led to the formation of volatile products consist of both saturated and unsaturated aldehydes and ketones. In the solid state, PVOH degraded thermally with the
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elimination of water only. Owing to the lack of conjugation, elimination in both molten and solid state was considered to be random and the C=C bond formation did not activate the elimination of adjacent hydroxyl units [169]. Recently, Wang and coworker have
SC
synthesized a novel intumescent flame retardant system containing metal chelates for PVOH [166]. It has been confirmed that the general degradation of PVOH was based on
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the loss of side groups from the main chain and thus the degradation can be evaluated from weight loss of a sample subjected to high temperature [170,171]. From TGA curves, it was inferred that the first part of weight loss that happened up to 100 °C was the evaporation of water. The subsequent weight loss which took place between 150 to
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270 °C was believed to be the first stage of degradation followed by the elimination of volatile products. Thereafter, spontaneous degradation of PVOH began above temperature of 270 °C. On the other hand, Shi et al. have conducted an experiment to
EP
study the synergistic effect between APP and LDH towards the flame retardancy of PVOH [137]. Based on the TGA curves of the experiment, the initial decomposition
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temperature and the amount of residues for PVOH/APP were found to be higher than the pure PVOH at temperature around 200 to 700 °C. With the addition of both APP and LDH into the PVOH, initial decomposition temperature was increased as a result of physical cross-linking effect among the layered particles, APP molecules and the PVOH polymer chains. Hence, it was concluded that composites of PVOH, APP and LDH with
ACCEPTED MANUSCRIPT 50 low flammability can effectively suppress the combustion by the major action of condensed phase protection.
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Lin et al. have examined the effect of MMT on the flame retardancy of PVOH/APP composites [165]. The thermal stability of the composites was characterized by TGA analysis. From the TGA curves, it was seen that the thermal stability of PVOH
SC
added with APP and MMT diminished below 350 °C but enhanced at higher temperatures. It was suggested that thermal decomposition of PVOH occurred mainly in
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the temperature range of 220 to 470 °C with two differentiable weight loss zones except for the case of pure PVOH. Besides that, it was disclosed that the initial decomposition temperature of PVOH/APP was being shifted to lower temperatures compared to pure PVOH. The PVOH/APP exhibited higher amount of residue between 480 and 700 °C.
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When MMT was incorporated into the PVOH/APP composites, lower initial decomposition temperature and even higher char residues were obtained at 500, 600 and 700 °C. This can be explained by the effect of MMT in catalyzing water elimination,
EP
cross-linking and decomposition reactions between PVOH and APP and played a vital role in hindering the diffusion of volatile decomposed products contributed by the char
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layer structure [172]. Hence, it was established that the overall flame retardancy of PVOH can be enhance by adding both APP and MMT into the polymer chains. Meanwhile, Wu and coworker have proved remarkable effect of microencapsulated APP on the flame retardancy and thermal behaviour of PVOH/glycerol-plasticized thermoplastic starch [167]. According to the TGA data, presence of microencapsulated
ACCEPTED MANUSCRIPT 51 APP catalyzed the degradation and more gas products were emitted at the earlier thermal
4. Conclusion and future development
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decomposition stage.
This review focuses on the application of APP in a broad range of commodity and
SC
engineering thermoplastic composites. Generally, progressive research and development on the usage of APP is deemed to be necessary in order to fulfill the demand of flame
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retardants to replace the previous halogenated type of flame retardants which are getting banned and prohibited due to their potential health and environmental issues. Throughout this review, it can be concluded that modification of mechanical properties and morphologies of the thermoplastic composites could be made possible with appropriate
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amount of APP as well as applications of other additives such as coupling agent. Subsequent research works should be more focus to include wider range of polymer compounds such as bio-based polymers and rubbers products. Indeed, the study of
EP
addition of APP in rubber insulation products for heating, ventilation and air conditional (HVAC) and acoustic applications can be crucial to meet the fire resistance building code.
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In terms of thermal degradation, it was important to note that the mechanism of IFR can be varied according to the type of polymer matrix. For instance, the polyethylene phase change materials include the decomposition of APP to produce polyphosphoric acid with intense dehydration effect. The polyphosphoric acid would be needed in the dehydration of expandable graphite that give off some carbonaceous and phosphocarbonaceous residues which functions as protective barrier to the polymer surface. Meanwhile, for
ACCEPTED MANUSCRIPT 52 polypropylene composites, it was confirmed that thermal decomposition mechanism of PP/APP composite took place in decomposition and char formation. Despite the fact that the flammability of polymeric materials was made complicated by the extensive
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diversification of polymer matrices available, emergence of new approaches and continuation of research have resulted in a much better understanding of the flame retardancy in different polymers. Importantly, the coverage of study should also include
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combination of APP with other flame retardant additives such as aluminium trihydrate, magnesium dehydrate, zinc borate and etc. as well as variety of polymeric system in
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order to promote applications of APP flame retardant for better safety and environmental friendly products.
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Acknowledgement
The authors are grateful for the financial support from Universiti Tunku Abdul
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References
EP
Rahman Research Fund 6200/B01.
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[172] Li B, Jia H, Guan L, Bing B, Dai J. A novel intumescent flame-retardant system for flame-retarded LLDPE/EVA composites. Journal of Applied Polymer Science 2009; 114(6): 3626-3635.
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Table 1 Effect of APP and talc on the mechanical properties of 60/40 LDPE/EVA blends cross-linked by 3 phr dicumyl peroxide and coupled with 3 phr grafted
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polyethylene and maleic anhydride [19] 35 phr APP +
Property
30 phr APP
35 phr APP
1 phr talc
13.0 ± 0.9
Elongation at break (%)
453 ± 23
487 ± 18
481 ± 10
Retention of tensile strength (%)
113
104
93
Retention of elongation at break (%)
97
99
97
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11.9 ± 0.3
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Tensile strength (MPa)
15.2 ± 0.8
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APP
PER
Melamine
(phr)
(phr)
(phr)
(phr)
100
-
-
-
100
7
14
13
100
15
14
13
100
23
14
13
100
30
14
Tensile strength (Pa)
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20.91
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13
16.23
14.37 13.98 13.49
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APP
PER
Melamine
(phr)
(phr)
(phr)
(phr)
100
-
-
-
100
7
14
13
100
15
14
13
100
23
14
13
100
30
14
LOI (%)
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17.8
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13
27.0
31.9 34.5 38.4
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Peak of HRR
Residue
(s)
(MJ/m2)
(kW/m2)
(%)
pure PS
83±0
131±0
752±10
-
85/15 PS/APP
62±9
118±1
85/10/5 PS/APP/hydrophilic alumina
50±1
119±2
85/10/5 PS/APP/hydrophilic silica
61±1
118±5
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TTI
13.3±0.4
342±19
14.8±0.8
360±3
13.7±0.0
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690±18
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PMMA with
15 wt%
15 wt%
pure APP
modified APP
Pure
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PMMA
69
60
55
Peak of HRR (kW/m2)
624
419
300
THR (MJ/m2)
112
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TTI (sec)
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99
97
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Pure PET
56.5
96.5
95/5 PET/APP and coal
39.9
90/10 PET/APP and coal
19.0
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Tensile strength (MPa)
74.8
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31.7
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25
30
40
50
APP
49.0
57.0
62.0
76.0
>80
Coal
35.0
40.0
37.0
36.5
37.5
APP : coal (2 : 1)
47.0
52.0
61.0
APP : coal (1 : 1)
50.0
54.5
61.0
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Loading level (wt%)
>80
75.0
>80
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>80
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Figure 1 SEM micrograph of the phase change nanocomposite materials: (a) shape stabilized 60 wt% paraffin + 40 wt% HDPE-EVA, (b) shape stabilized 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER, (c) char residue of 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER, (d) char residue of 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER under high magnification [24]
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Figure 2 LOI for wood flour-polyethylene composites (35 wt% polyethylene, 50 wt% wood flour and 5 wt% lubricant) added with 10 wt% flame retardant [20]
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Figure 3 HRR curves for wood flour-polyethylene composites (35 wt% polyethylene, 50 wt% wood flour and 5 wt% lubricant) added with 10 wt% flame retardant [20]
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Figure 4 TGA curves for different flame retardant systems in the paraffin/HDPE composites [25]
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Figure 5 SEM micrograph of PP composites added with APP: (a) 70/30 PP/APP, (b) 70/30 PP/APP treated with 50 °C water for 24 hours, (c) 80/20 PP/APP, (d) 80/20 PP/APP modified with KH-550 coupling agent [51,52]
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Figure 6 TGA curves of PP/APP composites modified with silane coupling agent [51]
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Figure 7 TEM micrograph of 95 wt% PS added with (a) 5 wt% MgAl LDH under low magnification, (b) 5 wt% MgAl LDH under high magnification, (c) 2.5 wt% MgAl LDH and 2.5 wt% APP under low magnification, (d) 2.5 wt% MgAl LDH and 2.5 wt% APP under high magnification [75]
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Figure 8 SEM micrograph of char residues from ABS composites (a) pure ABS, (b) 85/15 ABS/APP, (c) 85/15 ABS/expandable graphite, (d) 85/11.25/3.75 ABS/expandable graphite/APP [135]
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Figure 9 TGA and DTG curves of ABS composites (a) pure ABS, (b) 85/15 ABS/APP, (c) 85/15 ABS/expandable graphite, (d) 85/11.25/3.75 ABS/expandable graphite/APP [135]
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Figure 10 SEM micrograph of char residues for (a) untreated nylon-6,6, (b) nylon-6,6 treated with APP, (c) nylon-6,6 treated with APP and melamine, (d) nylon-6,6 treated with APP and PER, (e) nylon-6,6 treated with APP, melamine and PER [146]
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Figure 11 SEM micrograph of char residues for (a) outer surface of PVOH/APP (b) inner surface of PVOH/APP, (c) outer surface of PVOH/APP added with nickelaluminium LDH, (d) inner surface of PVOH/APP added with nickel-aluminium LDH [160]
S1359-8368(15)00506-5
DOI:
10.1016/j.compositesb.2015.08.066
Reference:
JCOMB 3746
To appear in:
Composites Part B
Received Date: 2 November 2014 Revised Date:
29 December 2014
Accepted Date: 21 August 2015
Please cite this article as: Lim K-S, Bee S-T, Sin LT, Tee T-T, Ratnam CT, Hui D, Rahmat AR, A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.08.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1 A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites
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Kien-Sin Lim1, Soo-Tueen Bee1*, Lee Tin Sin1*, Tiam-Ting Tee1, C. T. Ratnam2, David Hui3, A. R. Rahmat4
Department of Chemical Engineering, Faculty of Engineering and Science, Universiti
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1
Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia. Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000
Kajang, Selangor, Malaysia. 3
Department of Mechanical Engineering, University of New Orleans, New Orleans, LA
70148, USA
Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti
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4
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2
Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia.
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Abstract
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The application of ammonium polyphosphate (APP) in different types of
commodity thermoplastic composites (polyethylene, polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET)) have been discussed in terms of mechanical properties, morphologies and thermal properties. In addition, engineering thermoplastics such as acrylonitrile-butadiene-styrene (ABS), polyamides and poly(vinyl alcohol) (PVOH) and their composites added with APP and
ACCEPTED MANUSCRIPT 2 other additives were analyzed as well. It was suggested that improvement of mechanical properties and morphologies of the thermoplastic composites could be made possible with appropriate amount of APP and other additives such as montmorillonite (MMT),
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pentaerythritol (PER) and different types of layered double hydroxide (LDH). Furthermore, thermal properties such as limiting oxygen index (LOI) values together with cone calorimetry and thermogravimetric analysis (TGA) performance could be enhanced
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through optimum combination of APP, PER and melamine which functions as
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intumescent flame retardant (IFR).
Keywords: A. Polymer-matrix composites (PMC); E. Thermal analysis; C. Thermal properties: Ammonium polyphosphate
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*Corresponding author. Tel: +60 3 4107 9802; Fax: +60 3 4107 9803.
Email: [email protected] (Soo-Tueen Bee) and [email protected] (Lee Tin
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Sin)
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1. Introduction
Incorporation of flame retardant in polymer system is very important nowadays.
Flame retardants are essential in delaying the production and spreading of flames or fires. According to the conference proceeding held in Stamford, Connecticut on 19 May 2014 to 21 May 2014, the global consumption of flame retardants was about 4 billion pounds
ACCEPTED MANUSCRIPT 3 in year 2013. It was predicted that the market for flame retardant will reach 5.2 billion pounds in 2018, accounted for a compounded annual growth rate of about 5 % [1]. Based
to keep up together with the demand in flame retardant market.
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on such a strong and healthy growth, research and development will definitely continued
There are many different types of flame retardants available in the industry market
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with aluminium trihydrate (ATH) or aluminium hydroxide being the most widely used [2,3]. Basically, flame retardants can be either reactive or additive. They can be further
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categorized into several classes which includes minerals (ATH and magnesium hydroxide), organohalogen compounds (organochlorines and organobromines), and organophosphorus compounds (organophosphates, phosphonates and phosphinates). In addition, flame retardants can be categorized into phosphorus-containing, halogen-
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containing, silicon-containing or any other chemical-containing flame retardants based on the chemical type in their structure instead of grouping them according to classes. For instance, both decabromodiphenyl oxide and tris(tribromoneopentyl) phosphate can be
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classified as halogen-containing flame retardants [4,5]. Among the chemical-containing flame retardants, ammonium polyphosphate (APP) which is synthesized from inorganic
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salt of polyphosphoric acid and ammonia is currently getting the attention among the industries. It is known to be preferred over the other flame retardants due to its smaller loadings at lower cost and excellent processability. Most importantly, APP is a halogenfree flame retardant and thus it does not generate additional amount of smokes, making it to be environmentally useful compared to other halogen-containing flame retardants [6].
ACCEPTED MANUSCRIPT 4 APP is able to function as flame retardant in the condensed or polymer phase through intumescence. During intumescence, a material swells when it is exposed to heat or fire to form a porous carbonaceous foam which acts as a barrier to prevent heat, air and
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pyrolysis product from entering the surface of the material [7,8]. When polymeric materials which contain APP are subjected to fire, APP will first decompose into polyphosphoric acid and ammonia. The polyphosphoric acid would then reacts with
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hydroxyl group or other groups of synergists such as hydroxyethylcyanuarates or cyclic urea-formaldehyde resins to form a non-stable phosphate ester [9,10]. Upon dehydration
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of phosphate ester, charring would occur which involves the formation of a carbon foam above the surface of the polymeric materials to against the heat source. Furthermore, other than forming carbon foam, a viscous molten layer or surface glass can also be obtained on the polymeric surface which protects the polymeric materials from heat and
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oxygen [11]. Theoretically, three main components are needed in an intumescent flame retardant (IFR): 1) an acid source such as inorganic acid, acid salt or other acids that elevates the dehydration of carbonizing agent; 2) a carbonizing agent which is a
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carbohydrate that will be dehydrated by the acid source to become a char; 3) a blowing agent that will be decomposed to release gas resulting in the increase of polymer's
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volume and the formation of a swollen multi-cellular layer [12]. In order to achieve an optimum performance, APP needs to be properly dispersed into the polymer system and compatible with the polymer matrix. Besides that, it is better for APP to be insoluble in water to ensure a good weathering performance [13].
ACCEPTED MANUSCRIPT 5 There have been numerous literature published which focus on the flame retardancy and intumescence of APP on thermoset polymers or thermosetting resins [1417]. Over the decades, many improvements have been done by past researchers such as
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microencapsulation to enhance the performance of APP on thermoset polymers [18]. Hence, this paper aims to discuss the application of APP as IFR in thermoplastics and their composites. Mechanical properties (such as the tensile strength and elongation at
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break) and physical properties (such as surface morphology) are discussed here. Also, the thermal behaviour and intumescence are analyzed in order to understand the mechanism
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and effects of APP on thermoplastics and thermoplastics composites.
2. Commodity Thermoplastics and Their Composites
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2.1 Polyethylene
Basfar and coworker have examined the mechanical and thermal properties of
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low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) blends cross-linked by both dicumyl peroxide and ionizing radiation [19]. Besides that, they have performed
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a study to find out the effect of APP content on the mechanical properties of the chemically cross-linked 60/40 LDPE/EVA blends coupled with grafted polyethylene and maleic anhydride. Based on the results in Table 1, it was inferred that the retention of mechanical properties were independent to the amount of APP as both the retention of tensile strength and elongation at break was still remain in the range of about 100 %. However, addition of 35 phr caused a slight decrement in the tensile strength of the 60/40
ACCEPTED MANUSCRIPT 6 LDPE/EVA blend although its elongation at break experienced a small increment. The result was in agreement with the findings by Stark et al. as APP was assumed to have negative effect on the mechanical properties of wood flour-polyethylene composites [20].
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In fact, as shown by Basfar et al. and other researchers, a minimum amount of 35 phr APP is needed to achieve the required fire resistivity of the IFR compounds [19,21-23]. In order to enhance the tensile strength of the LDPE/EVA without reducing the amount
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of APP, small amounts of inorganic additives such as talc can be incorporated. As shown in Table 1, addition of 1 phr talc showed a remarkable increase in terms of tensile
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strength for the LDPE/EVA blends cross-linked by dicumyl peroxide. Also, the elongation at break was found to be increased compared to its previous blends added with 30 phr APP. Both retention of tensile strength and elongation at break were maintained in
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an acceptable range of 93 to 97 %.
In terms of surface morphologies, Cai et al. [24] have synthesized some shape stabilized phase change nanocomposites materials via the blending of high density
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polyethylene (HDPE), EVA, organophilic montmorillonite (MMT), paraffin and IFR compounds which include APP and pentaerythritol (PER). The samples were then
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observed under scanning electron microscopy (SEM) to obtain the surface morphology images for both the shape stabilized and the char residue of the samples. From SEM micrographs illustrated in Figure 1(a) and 1(b), it can be observed that a compact network structure was formed via the entrapment of paraffin and flame retardant by the HDPEEVA composites. Well dispersion of flame retardant in the composite and paraffin have achieved for the shape stabilized materials. On the other hand, when the sample added
ACCEPTED MANUSCRIPT 7 with APP-PER was burned in a muffle furnace under 800 °C to form a multicellular char residue, loose surface morphology was found to present as shown in Figure 1c. Under high magnification, the char residue displayed a rather non-homogeneous microstructure
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as compared to other samples added with organophilic MMT [24]. Hence, it can be concluded that incorporation of IFR compounds (APP and PER) has little or no effect on
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the network structure of the nanocomposites materials.
The effects of halogen-free flame retardant towards the phase change materials
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from paraffin/HDPE composites have been studied extensively by several authors. Cai et al. have performed SEM on the paraffin/HDPE composites mixed with IFR composed of expandable graphite, APP and zinc borate [25]. It was established that the flame retardants were evenly distributed in a three-dimensional net structure of paraffin and
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HDPE. Graphite sheets in round-like lamellar structure having an average size of 300 µm were noticed to be embedded into the phase change materials. After combustion of the materials to produce char residues, worm-like structure of the expanded graphite was
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found, and the addition of APP seems to result in a tighter and denser morphology than the materials added with only expandable graphite. Cai and coworker have further proved
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that the paraffin/HDPE composites with APP produce a form-stable composites which appeared to be uniform with no agglomeration [26]. In order to enhance the dispersions of the paraffin and flame retardant within HDPE, suitable amount of expandable graphite can be supplemented which would cause partial absorption of paraffin into the porous structure of expandable graphite during the extrusion process [27-29].
ACCEPTED MANUSCRIPT 8 Stark et al. have conducted a research to find out the fire performance and effectiveness of five different flame retardant systems (i.e. decabromodiphenyl oxide, magnesium hydroxide, zinc borate, melamine phosphate and APP) in the wood flour-
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polyethylene composites [20]. The composites without flame retardant were first prepared via the blending of 35 wt% polyethylene, 60 wt% wood flour and 5 wt% lubricant to ensure a good composite surface characteristics. Then, five different flame
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retardants were added individually to the composites by replacing 10 % of the wood flour. For instance, APP added composite was made up of 35 wt% polyethylene, 50 wt% wood
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flour, 10 wt% APP and 5 wt% lubricant. Based on Figure 2 which depicts the limiting oxygen index (LOI) for different composites, it was suggested that each of the flame retardant systems increased the LOI of the wood flour-polyethylene composites with APP possessed the highest increment of about 29 % as compared to its composite without
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flame retardant. Thus, it was concluded by the authors that APP functioned effectively as a flame retardant in the wood flour-polyethylene composites, but causing a decline in mechanical properties [30]. On the other hand, it was showed by several literatures that
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enhancement of the protective effect in the APP associated with PER could be done through the combination of zeolites and conventional heat insulating materials [31-33].
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By applying these components, fire-proofing properties could be imparted into the polymer systems such as polyethylene and ethylenic copolymers. For example, the usage of 4A type zeolite in the flame retardant systems consist of APP and PER led to a synergistic effect for the formulations involving polyethylene [32]. The LOI values have increased by 35 % with respect to the conventional APP-PER retardant system when 1.5
ACCEPTED MANUSCRIPT 9 wt% of 4A type zeolite was added. However, it was noted that high zeolite content of more than 5 wt% would result in a dramatic decline of the fire performance.
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Cone calorimetry is an approach which has been widely used in the fire testing field to study the combustion behaviour of a material or substance [34]. It provides a great amount of information regarding the fire performance and risk such as the heat
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release rate (HRR), total heat release, time to ignition, and some limited insight on fire hazards [35-37]. Over the years, the flammability of polyethylene composites with and
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without retardants has been investigated by few researchers. For instance, suitable amounts of polyethylene, wood flour and lubricant were mixed by Stark et al. to produce the wood flour-polyethylene composites [20]. Different types of flame retardants were chosen to evaluate their performances on the fire retardancy. From the HRR curves
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displayed in Figure 3, it is visible that the composites with flame retardants result in lower HRR as compared to their original composites without flame retardant. As reported by the author, APP decreased the HRR significantly while it led to a smaller value of
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HRR for a longer duration. In addition, Cai et al. and Zhang et al. have focused on the study of APP as IFR compounds in phase change materials containing polyethylene
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[25,26,38]. They have found out that addition of appropriate amount of expandable graphite resulted in a noticeable decrease in the peak of HRR due to the synergistic effect between the expandable graphite and APP [25]. Generally, the mechanism of IFR compounds in the phase change materials involves the decomposition of APP which acts as the acid source to induce polyphosphoric acid with intense dehydration effect. The formation of polyphosphoric acid would then participate in the dehydration of
ACCEPTED MANUSCRIPT 10 expandable graphite which yields some carbonaceous and phosphocarbonaceous residues that functions as a physical protective barrier [26].
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Thermal degradation behaviour of APP has been inspected by Camino et al. back in year 1984. According to their research, APP heated under nitrogen gas flow has experienced quantitative volatilisation in three main steps before it was fully burned off
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in temperature of about 900 °C [39]. This degradation process was found to be similar when APP was heated under vacuum, except that a much lower temperature was needed
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for the process to occur [40]. In both scenarios, it was established that ammonia and water were the gaseous products released during the first two steps. In the last step, phosphate chain fragments were obtained in a lesser amount after the condensation of samples in the degradation apparatus under room temperature condition [41]. In the
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present work, thermal stability and degradation properties of APP in polymer system has been widely studied and characterized by thermogravimetric analysis (TGA). As demonstrated by Cai et al., there were two steps of degradation process for the APP and
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other halogen-free flame retardants in the paraffin/HDPE composites [25]. Based on the TGA curves shown in Figure 4, it is obvious that the first step of degradation happens
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from 250 to 450 °C which indicates the degradation of the paraffin molecular chain and the flame retardant system. The second step of degradation that happens in a shorter temperature range of 450 to 500 °C is due to the deterioration of the HDPE main chains. Besides that, it can be discovered that during the first step of degradation, the weight loss of the paraffin/HDPE composites without flame retardant is relatively lower as compared to the composites added with flame retardant. However, the weight loss of the composites
ACCEPTED MANUSCRIPT 11 without flame retardant was found to increase in the second step of degradation while the flame retarded composites gives a lower weight loss. This can be explained by the thermal degradation of the flame retardant in the first degradation step which forms the
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charred residue during the second step. Meanwhile, it can be predicted from Figure 4 that composites with 10 wt% APP has a slightly higher thermal stability than that of the other composites with different flame retardant system. This was due to the action of APP
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which tends to decompose to yield polyphosphoric acid with intense dehydration effect and release gaseous products of ammonia and water during the heating process.
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Thereafter, the polyphosphoric acid would further take part in the dehydration of expandable graphite to form carbonaceous and phosphocarbonaceous residue which functions as a physical protective layer. The volatilized gaseous products would cause the mixtures of carbonaceous and phosphocarbonaceous residue to swell and lead to the
Polypropylene
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formation of the intumescent charred residue [42,43].
The mechanical properties of polypropylene (PP) involving APP have been
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studied and published by past few literatures [44-47]. According to the research done by Schacker and Wanzke, PP compounds added with APP would have Charpy impact strength in between 20 to 80 kJ/m2, Young's modulus of about 1300 to 2200 MPa, elongation at break of 15 to 350 % and melt flow index around 5 to 20 g per 10 minutes [48]. In order to prevent the APP flame retardant from decomposition prior to processing stage, polymer compounding can be performed by twin screw extruder with a soft screw
ACCEPTED MANUSCRIPT 12 configuration which used the lowest possible melt temperature that gives a high output rate at low screw speed. Apart from that, sharper screw was recommended to be used during the compounding of PP and APP to produce an excellent flame retarded polymer
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with slower rate of decomposition. Chiu and Wang have conducted an experiment to study the dynamic flammability properties of an intumescent fire retarded PP filled with APP, PER and melamine [49]. From Table 2, it can be seen that increasing the amount of
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APP results in the decline of tensile strength. The inherent superior tensile strength of PP was found to be severely affected by the incorporation of APP in the composites due to
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the reducing crystallinity in the composites [50]. This was proved later by Li et al. when the graph of PP’s crystallinity in PP/PS blends against the side chain length of PP-g-PS copolymer revealed that additives, PS and PP-g-PS copolymer would restrain the PP chain from folding into a growing crystal lamellae, thus reducing the crystallinity of PP
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in the blends [51]. Throughout the whole experiment, Chiu and Wang have concluded that pure PP has poor flame properties. In order to enhance the flammability of original PP and its composites or blends, APP, PER and melamine can be employed which
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function as intumescent flame retardant. However, it is important to note that excessive amount of APP added into the composites would cause a rather low tensile strength
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which limits their applications. In 2011, Lin et al. have showed that incorporation of silane coupling agent KH-550 improved the mechanical properties of PP and APP to a certain extent. The tensile strength, impact strength and elongation were found to be increased by 24 %, 25 % and 14 % respectively compared to its original blend without coupling agent [52].
ACCEPTED MANUSCRIPT 13 Figure 5 shows the SEM micrograph of PP composites mixed with APP flame retardant. From Figure 5 (a), it can be observed that most of the APP particles are distributed in the PP matrix and are exposed to the external surface due to its poor
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compatibility with the PP polymer matrix [53,55]. Besides that, it was reported by Wu et al. that the relatively higher polarity of APP was responsible for the exposure of APP particles to the fractured surface of PP/APP composites [54]. When 70/30 PP/APP
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composites were treated in hot water medium at 50 °C for 24 hours, almost all APP particles have been eluted, causing some defects or voids on the fracture surface as
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displayed in Figure 5 (b) [55]. It was suggested that the water molecules have been absorbed onto the surface of the composites which lead to the dissolution of APP in the water to leave some defects on the surface [56]. For reduced APP amount in 80/20 PP/APP shown in Figure 5 (c), it was found that aggregation of APP occurred in the PP
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matrix which generated cracks and even holes between the components suggesting a poor dispersion and compatibility [52]. The condition becomes even worse when the mechanical properties of 80/20 PP/APP composite were found to decrease drastically due
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to the poor interfacial adhesion and accumulated microcracks at the PP and APP interface. In order to improve the surface morphology, suitable amount of coupling agent can be
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incorporated in the PP/APP composite. As shown by Lin et al., 80/20 PP/APP modified with silane coupling agent gave a rather uniform dispersion of APP in the PP matrix with little or no agglomeration and cracks [52]. This improvement would reduce the adverse effects of inorganic additives on the mechanical properties of thermoplastic composites.
ACCEPTED MANUSCRIPT 14 The flammability and flame properties of PP composites mixed with APP flame retardant and other additives such as PER and melamine have been investigated by Chiu and Wang [49]. From their study, it was first stated that pure PP without APP flame
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retardant and additives is highly combustible as it has a low LOI of only 17.8 % given in Table 3. Through the addition of APP, PER and melamine in the PP, the LOI of the PP composites increased gradually to a maximum value of 38.4 %. Since the amount of
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additives remained constant in the composites, it can be said that APP exhibits a good fire protection to PP composites as it functions as IFR that interrupts the burning process in
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the condensed phase [49]. In addition, it was confirmed by Lewin and Endo that addition of small amount of heavy metal salts such as zinc acetate, zinc borate, manganese acetate and manganese sulphate to APP-PER-PP composites appeared to elevate the flame retardant performance of APP in the PP matrix [57]. Anion type of heavy metal was
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discovered to impose a lesser impact when compared to the cation heavy metal which increased the LOI of the PP composites by 4 to 5 units [58]. On the other hand, it was established by Lin and co-author that PP/APP composites modified with silane coupling
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agent KH-550 reduced the water solubility effectively and improved the interface compatibility between the APP and PP matrix [52]. When increasing APP content in the
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composites modified with coupling agent, the LOI increased as well. This implied that a much higher oxygen concentration is needed to initiate and sustain the continuous combustion of the PP composites via the incorporation of APP. Besides that, APP has been proven to be effective in eliminating the dripping phenomenon and selfextinguished when a certain compound is subjected to fire [59-61].
ACCEPTED MANUSCRIPT 15 Thermal stability of polymer composites can be examined through the usage of cone calorimetry. As shown by Wu and coworker, the presence of intumescent additives in PP decreased the HRR values dramatically when compared to the pure PP without
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additives [55]. For 70/30 PP/APP composite, it was stated that the peak HRR was 1064 kW/m2 which was considered slightly lower than the pure PP with peak HRR of 1177 kW/m2 when both of samples were exposed to an external horizontal heat flux of 35
SC
kW/m2. Apart from that, it is important to note that the ignition time of the 70/30 PP/APP was only 24 seconds while pure PP has a longer ignition time of 44 seconds. This can be
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attributed to the earlier decomposition of APP than pure PP after the cone heater has irradiated the composite surface, giving off some volatile substances from the decomposition of APP [54]. Meanwhile, the dynamic flammability properties of PP filled with APP, PER and melamine were characterized as well [49]. It was concluded that pure
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PP has a low resistance to combustion and it is easily susceptible to fire or flame. By adding IFR consist of APP, PER and melamine, the maximum HRR was reduced substantially from 687 kW/m2 to 115 kW/m2 under heat flux of 50 kW/m2. The
EP
mechanism of the intumescence can be explained by the formation of a foamed multicellular char on the material surface when the material was subjected to fire.
AC C
Consequently, thermal insulation was found to be imposed on the material surface which prevents other combustible gases from entering the flame source and separating oxygen from the burning material. As a result, the extinguishment of the fire can be provoked. In the worst scenario where the material absorbed more heat and caused the carbonaceous char to break, some heat would be liberated to the surrounding [49,62,63]. At that moment, new carbonaceous char can be produced subsequently to replace the broken
ACCEPTED MANUSCRIPT 16 char, and the whole phenomenon will continue repetitively. Hence, it was observed that the composite material undergoes several decomposition steps during burning process.
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Generally, there are five major crystalline forms of APP. Crystalline form I is produced by the heating of equimolar ammonium orthophosphate and urea under the ammonia at 280 °C whereas the crystalline form II with orthorhombic symmetry (a =
SC
4.256 Å; b = 6.475 Å; c = 12.04 Å) is obtained by heating the crystalline form I under high temperature of 200 to 375 °C [64]. During the transition of crystalline form I to form
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II, it was revealed that APP crystalline form III present as an intermediate state. On the other hand, crystalline form IV shows a monoclinic structure while its form V is orthorhombic [64]. Currently, it is known that only APP crystalline form I and II are commercially available in the market. Samyn et al. have examined the crystallinity and
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crystal structure of APP coated with nitrogenous and carbonaceous species at different heat treatment temperature via the X-ray diffraction (XRD) analysis [64]. It was pinpointed that the powdered APP at 20 °C has the same spectrum with the residue of
EP
treatment at 260 °C, both displaying the characteristic of APP crystalline form II. After thermal treatment of the APP sample at 415 °C, the residue showed a charred structure
AC C
and a broad distribution of spectrum can be observed at 2θ between 22 to 23 degree. This suggests that an amorphous product has been formed as a result of thermal treatment which contains pregraphitic carbon [66,67]. Lin and coworker have investigated the XRD patterns of PP, APP and PP/APP composites modified by silane coupling agent [52]. For PP, it was reported that diffraction peaks at 2θ of about 14.2, 17.1, 18.6 and 21.2 degree were clearly noticed which corresponds to the crystal planes of (110), (040), (130) and
ACCEPTED MANUSCRIPT 17 (131) that exhibits a α-crystal form. However, the addition of APP into the PP composites has shifted the diffraction curves and a new peak was generated at 2θ of around 16 degree. It was predicted that the peak might represent the crystal plane of (300) which
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corresponds to the β-crystal form [68]. Hence, it can be concluded that both α and βcrystal forms were coexist in the PP/APP composites. Incorporation of APP was expected
to be a relatively better crystal structure [68,69].
SC
to trigger crystal transformation of PP and gave a β-crystal form of PP which is believed
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Thermal degradation of APP has been studied by Camino et al. back in year 1985 [41]. It was established that APP degraded upon heating to release ammonia and water as gaseous products with the formation of cross-linked -P-O-P- structure. Initially, the process started at low temperature of about 165 °C and APP reached thermal stability
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after only 3 % of weight loss. It was suspected that transition to a more stable crystalline form might probably occurred in the first minor degradation step and excess temperature of around 280 °C was required to initiate an extensive degradation. Subsequently, Zhang
EP
et al. and Hornsby et al. have reported the thermal and thermo-oxidative degradations of APP in a more comprehensive way [70,71]. It was found that degradation of APP
AC C
occurred in two steps whereby the first step involves the elimination of ammonia gas around 300 °C with the formation of a highly cross-linked polyphosphoric acid. For the second step, the polyphosphoric acid would further been evaporated at temperature of over 550 °C and dehydrated to P4O10 which sublimates. Meanwhile, the effect of the APP-PER mixture as intumescent additive on thermal degradation of PP has been investigated as well [72]. From the TGA curve, it was observed that both the
ACCEPTED MANUSCRIPT 18 experimental and calculated values of PP added with 30 % of IFR (APP:PER = 3:1 wt/wt) are in agreement up to 460 °C. Beyond this temperature, a lower weight loss was found for the experimental curves and a larger amount of charred residue was obtained at
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500 °C. It was anticipated that PP was not so effective to exert significant impact during the degradation of the additive whereby gas evolution and charring took place at temperature lower than 400 °C. Furthermore, the additive appeared to induce partial
SC
charring of PP instead of forming volatile hydrocarbons when PP was heated alone.
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Wu and coworker have studied the intumescent flame retardancy of the PP composites containing APP and microencapsulated APP [53]. It was shown by the TGA curve that microencapsulated APP decomposed at about 270 °C which is almost similar to the break down temperature of APP. Initially, microencapsulated APP decomposed at
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a faster rate than normal APP due to the fact that melamine-formaldehyde resin used for the microencapsulation has a lower thermal stability [73,74]. During heating, melamineformaldehyde resin produced non-flammable gases such as ammonia and carbon dioxide
EP
which are helpful in forming a honeycomb char structure [75]. After reaching temperature of about 600 °C, the microencapsulated APP was found to be more thermally
AC C
stable than the normal APP. In addition, decomposition of microencapsulated APP at 800 °C left around 3.8 % of residue while the residue of conventional APP at 800 °C is 0.6 %. Despite the similar thermal decomposition behaviour of microencapsulated APP and normal APP, there were still some differences when they were mixed with PP. Based on the TGA curves, it was inferred that 70/30 PP/microencapsulated APP was more thermally stable in the whole range of decomposition as compared to 70/30 PP/APP [53].
ACCEPTED MANUSCRIPT 19 70/30 PP/APP started to experience initial decomposition at about 250 °C whereas 70/30 70/30 PP/microencapsulated APP started to suffer weight loss only at temperature of 270 °C. As confirmed by the authors, both PP/APP and PP/microencapsulated APP
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composites have three steps of decomposition. On the other hand, Wu and coworker have investigated the flame retardancy of APP microencapsulated with poly(vinyl alcohol)melamine-formaldehyde resin [54]. These microcapsules were synthesized through the
SC
combination of three main components of a typical IFR system: an acid source (APP), a carbonization agent and blowing agent (PVOH-melamine-formaldehyde). Through the
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use of these microcapsules, the flame retardancy and water resistance of PP composites can be enhanced. From the TGA curves, it was revealed that pure PP without additives started to decompose at about 240 °C and has almost decomposed completely at 360 °C. Basically, thermal decomposition of PP/APP composite happened in three steps. The first
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step was the initial decomposition of PP/APP in which its decomposition temperature (250 °C) was said to be slightly higher than of pure PP due to the deterioration of APP. The second step of weight loss was the main decomposition process of the PP/APP
EP
composite while the third step that took place at 500 °C involve further decomposition of the composite to char. Lastly, it was demonstrated that APP microencapsulated with
AC C
PVOH-melamine-formaldehyde resin can form a more stable charred layer than a conventional APP in PP which can safeguard the polymer composite from being further combusted in the condensed phase.
Figure 6 shows the TGA curves of PP/APP composites modified with silane coupling agent. From the curves, it is clearly observed that the starting decomposition
ACCEPTED MANUSCRIPT 20 temperature of PP/APP composites were slightly higher than pure PP which decomposed at around 264 °C and ended at about 445 °C. In fact, at 445 °C, PP/APP composites still have higher amount of un-decomposed mass with 80/20 PP/APP having the highest
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percentage of 82 % remaining [52]. Therefore, it can be deduced that increasing the weight percent of APP would increase the decomposition temperature of the composite. Moreover, the amount of residue would increase as well as seen in the case of 17 wt% of
SC
APP and 20 wt% of APP which still have 15 % of mass remaining at 600 °C. Hence, it was concluded that thermal stability of composites could be enhanced by the introduction
2.3
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of APP flame retardant.
Polystyrene
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Nyambo et al. have conducted an experiment to explore the chemical interactions between APP and MgAl layered double hydroxide (LDH) on the polystyrene (PS) composites [76]. During their research, magnesium and aluminium type of LDHs were
EP
selected as they appear to be similar to the commonly used magnesium hydroxide and
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aluminium trihydrate fire retardants [77-79]. Transmission electron microscopy (TEM) was employed as a way to assess the presence of intercalated or exfoliated phases of layered inorganic materials within the PS matrix whereby low magnification gives some details about the degree of dispersion while high magnification provides certain information about the morphology, extoliation and intercalation of the materials. Based on Figure 7 which illustrates the TEM micrograph of 95 wt% PS added with different quantity of MgAl LDH and APP, it can be seen that some improvement in the overall
ACCEPTED MANUSCRIPT 21 additive dispersion happened when 2.5 wt% of APP was imparted to replace some MgAl LDH. There was a small number of tactoids exist in the PS composites with average length of 0.5 µm as given in Figure 7 (d) under high magnification. It was suspected that
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the presence of tactoids in the composites might be due to the agglomeration of APP instead of the poor dispersion of LDH [76]. Furthermore, it was anticipated by the authors that despite the improvement in disperion, flammability and thermal stability of
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cause a severe effect on the mechanical properties.
SC
PS composites, incorporation of APP into the PS and MgAl LDH composites would
Back in 1970s, thermally stable phosphonium salts such as Cyagard RF-1 {ethylene-bis[tris(2-cyanoethyl)]phosphonium bromide} combined with APP have been proven to be useful flame retardants which function in the condensed phase of
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thermoplastics such as PP and high impact polystyrene (HIPS) [80,81]. From the literatures, it was discovered that the LOI of HIPS composites increased proportionately with the content of APP although the flame retardant efficiency of APP in HIPS was
EP
found to be lower than in PP by a factor of around 2.5 [81,82]. For the phosphonium salt,
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the flame retardant efficiency was said to increase non-linearly for both PP and HIPS and they tend to remain constant at high APP content. The difference in linearity of the APP and Cyagard RF-1 was believed to be due to different mechanisms operated in its respective PP and HIPS. APP has been reported to be a halogen-free flame retardant used mainly in the condensed phase of polymers whereas Cyagard RF-1 is mostly likely to perform better in the gas phase. Meanwhile, the synergistic effect between oxide nanoparticles and APP on flame behaviour of PS has been published by Cinausero and
ACCEPTED MANUSCRIPT 22 coworker in year 2011 [83]. It was established that the LOI of pure PS has been elevated from 18.5 % to 20.5 % when 15 wt% of APP was mixed with PS to give 85/15 PS/APP composite. Through the substitution of 5 wt% hydrophilic and hydrophobic type of
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alumina and silica into the APP, the LOI was found to be maintained in the range of 19.5 to 20.5 %. However, with 5 wt% APP replaced with hydrophobic silica, the LOI of the composite was able to achieve a highest value of 23.0 %. This can be attributed to the
SC
specific silicon metaphosphate (SiP2O7) crystalline phase which contributes to the
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improved charring and enhanced protective properties of the PS composite [83].
Table 4 shows the cone calorimeter data adapted from Cinausero et al. regarding the study of PS composites filled with different types of additives [83]. By comparing pure PS and 85/15 PS/APP, it can be seen that addition of APP into the PS polymer
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matrix does not convey significant improvement to the flame behaviour of the composite since the total heat release (THR) and peak of HRR of 85/15 PS/APP is comparable to the pure PS. Although the time to ignition (TTI) has somehow been reduced, inorganic
EP
residue was found to build up under the molten state of polymer during combustion,
AC C
indicating the absence of flame retardant efficiency [83]. In fact, incorporation of APP showed its function in modifying the degradation pathway of PS as the ignition of 85/15 PS/APP was about 20 seconds earlier than the pure PS to release some combustible compounds. Also, synergistic effect was said to be achieved when small amount of hydrophilic alumina and silica were added into PS composites, leading to the formation of a solid protective barrier and increased flame retardancy [84,85]. It was inferred that alumina has a catalytic effect on the emission of combustible products prior to the
ACCEPTED MANUSCRIPT 23 ignition of PS polymer since the TTI of 85/10/5 PS/APP/hydrophilic alumina was only 50 seconds compared to the 85/15 PS/APP with TTI of about 62 seconds. Combustion of 85/10/5 PS/APP/hydrophilic alumina demonstrated an intumescent phenomenon with a
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discontinuous char produced on the surface of the composite. Apart from that, as examined by Nyambo and coauthor, the time required to sustained ignition was drastically reduced when APP and MgAl LDH were added individually or in combination
SC
into the PS matrix [76]. This can be explained by the increase in viscosity of the neat polymer and decline in the heat exchange between the sample surface exposed to the
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radiant source and the bulk of the sample [86,87]. Consequently, a sudden increase in the sample's surface temperature would occur and the time needed for the composite sample to reach pyrolysis temperature was reduced. Furthermore, it was also proven by the researchers that replacing APP with MgAl LDH would results in a lower peak of HRR.
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Synergistic effect obtained from the combination of APP and MgAl LDH was able to reduce the peak of HRR by a whole 42 % when compared to the pure PS without any
EP
additives [76].
AC C
As mentioned earlier, thermal degradation of APP was done in a two-step mechanism. The first step basically involves the weight loss at around 250 °C followed by the elimination of ammonia and water as the gaseous products. Maximum degradation peak was observed at temperature of approximately 322 °C. The elimination of water further caused the formation of phosphoric acid accompanied by the cross-linking process to produce vitreous ultraphosphate [40]. Next, the second degradation step represented the fragmentation of ultraphosphate to P2O5 and phosphate compounds at
ACCEPTED MANUSCRIPT 24 higher temperatures. Thermogravimetric study on the flame retardant system in HIPS has been performed by Ferrillo and Granzow in year 1980 [88]. It was predicted that the effect of flame retardants with 15 wt% APP and 15 wt% Cyagard RF-1 {ethylene-
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bis[tris(2-cyanoethyl)]phosphonium bromide} on the volatilization of HIPS was much smaller than in the case of PP. Besides that, much higher concentration of flame retardants (roughly 35 %) were needed to ensure fire protection of HIPS. Optimum flame
SC
retardancy was obtained for HIPS composites consist of almost equal weight of APP and Cyagard RF-1 {ethylene-bis[tris(2-cyanoethyl)]phosphonium bromide}. On the other
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hand, it was confirmed by Balabanovich and Prokopovich that combination of APP and 2-methyl-1,2-oxaphospholan-5-one 2-oxide in the ratio of 2:1 was more effective to enhance the flame retardancy of HIPS instead of using them separately [89]. Charring has occurred above the surface of HIPS which was mainly caused by the reaction between the
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additives. In the case of 2-methyl-1,2-oxaphospholan-5-one 2-oxide, it was evaporated upon heating to produce some phosphorus-containing volatiles such as aliphatic and aliphatic-aromatic phosphine oxides and formed a small number of polyaromatic char
EP
consist of P-O-P structures. With the addition of APP into the mixtures, the degradation path of 2-methyl-1,2-oxaphospholan-5-one 2-oxide was changed. Formation of
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polyphosphazene type of structure was most probably expected to take place together with formation of a polyphosphoric acid. Moreover, it was pinpointed that the additives and their decomposition products were helpful in generating less volatile styrene oligomers from the pyrolyzing HIPS and thus hinder the emission of pyrolyzed products.
ACCEPTED MANUSCRIPT 25 The influence of APP on thermal and thermo-oxidative degradation of PS has been investigated by Zhu through the means of thermogravimetry [90]. The rate of temperature rise was set at 10 °C/min with air and nitrogen being used as the carrier gas
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flowing at 100 ml/min. From his study, it was established that APP and its thermal degraded products would stimulate char formation in air and to a lesser extent in the presence of nitrogen. Intermolecular hydrogen transfer was reported to be the most
SC
significant during the thermal and thermo-oxidative degradation of PS as there was a decline in the apparent activation energy of the PS composites [90]. In addition, it was
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revealed that APP together with its thermal decomposition products were responsible for the accelerated thermal degradation of PS in nitrogen due to their acidity. Apart from that, the rate of thermo-oxidative degradation of PS was believed to rely on two main factors: the quantity of acid and the protection of the glassy layer formed from APP in air.
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Recently, Cinausero et al. have studied the thermo-oxidative degradation of PS added with nanoparticles [83]. It was disclosed that 85/15 PS/APP followed the thermal behaviour of pure PS until it reached 380 °C. Thereafter, stabilization took place and char
EP
was formed. This modification of PS degradation pathway can be attributed to the presence of APP and polyphosphoric acid [91]. Secondly, it was postulated that
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interactions between the ionic species of phosphorus additive and radical sites led to cyclisation reactions and further promote the formation of char [92]. Lastly, substitution of APP by 5 wt% oxides (alumina and silica) elevated the thermal stability by around 30 to 40 °C in terms of the temperature at maximum degradation rate. This increment was said to be originated from the stabilization effect of both alumina and silica on thermal
ACCEPTED MANUSCRIPT 26
Poly(methyl methacrylate)
SC
2.4
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degradation of PS and from the synergistic effect between the APP and the oxides system.
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Poly(methyl methacrylate) (PMMA) has been widely applied in several areas owing to its good optical properties, excellent mechanical performance, high thermal stability and superior electrical properties [93-95]. Although PMMA was employed in the form of sheets for making shatter resistant panels in building windows, bullet proof
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security barriers, furniture, skylights, signs and displays, it has a fatal disadvantage of being highly combustible [96]. Thus, flame retardants would need to be added to improve its flame behaviour so that the application of PMMA can be sustained. The morphology
EP
and mechanical properties of PMMA added with APP and other constituents have been reported by several authors. For instance, Friederich et al. have discussed the ternary
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system of APP, melamine polyphosphate and titanium dioxide in PMMA from the views of thermal and fire resistance properties, morphology and other chemical properties [97]. It was implied that if titanium dioxide was added individually into PMMA, it was hard to distinguish the phosphorated additives from titanium oxide particles although it was said that the presence of titanium oxide did not bring any negative effects to the dispersion of titanium dioxide [98]. Friederich et al. have concluded that replacement of a small amount of metal oxide with APP flame retardant containing phosphorus led to the
ACCEPTED MANUSCRIPT 27 enhancement in the flame behaviour and thermal properties of PMMA. In particular, 85 wt% of PMMA added with 7.5 wt% of APP and 7.5 wt% of titanium dioxide was suggested to be the best formulation among the ternary system since it exhibits the
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greatest thermal stability increase. Meanwhile, comparison was done by the same author based on two meal oxides (alumina and boehmite) in the mixture of APP and melamine polyphosphate with PMMA as the polymer matrix [99]. From the SEM micrographs, it
SC
was supposed that residues with boehmite were more compact and gave a rather better morphology with fewer voids compared to the residues with alumina. The appearance of
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voids in the residues was believed to be the result of thermal degradation and it is undesirable for its ability to obstruct the efficiency of barrier effect. Hence, it was deduced that boehmite based ternary system especially nanocomposites of 85 wt% of PMMA, 7.5 wt% APP and 7.5 wt% boehmite, displayed a better flammability than the
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system consist of alumina.
Laachachi et al. have studied the effect of Al2O3 and TiO2 nano-oxides together
EP
with APP on the thermal stability and flame retardancy of PMMA [100]. Also, they have compared the two different types of APP (one being the pure APP with particle size of 15
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µm having higher P/N ratio and another being the modified APP with particle size of around 5 to 80 µm having a lower P/N ratio) on the behaviour of PMMA composites. Based on Table 5, it was demonstrated that addition of both pure and modified APP has reduced the TTI and THR of the composites. It was important to note that the peak of HRR experienced a significant reduction when 15 wt% of pure and modified APP were added into the PMMA. Owing to the charred and vitreous or ceramized structure, the
ACCEPTED MANUSCRIPT 28 decomposition pathway of the PMMA composites was expected to be changed [101,102]. Furthermore, it was proved that incorporation of modified APP has led to an intumescent behaviour of the PMMA composites. Comparative analysis via the XRD disclosed that
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part of the APP has been replaced by melamine phosphate for the case of modified APP. This melamine phosphate is thermally less stable than APP because of the melamine release and by other nitrogen compounds. Consequently, mixture of APP and melamine
SC
phosphate would results in an intumescent phenomenon and the incorporation of the mixture in the PMMA would give an intumescent structure with the polymer acting as
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carbon supplier [103].
Cinausero and coworker have investigated the synergism between alumina and silica with APP on the flame behaviour of PMMA composites [83]. From their research,
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it was found out that during the combustion of 85/15 PMMA/APP, small solid islands were formed and developed progressively without covering the whole composite surface. After being taken out from the heating cone, the residue of the composite becomes highly
EP
viscous as a result of entrapment of water in an ultraphosphate structure [104,105]. However, the peak of HRR and mass lost rate values were decreased by about 27 to 34 %
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compared to pure PMMA. With the replacement of 5 wt% APP by nano-oxides, the island regions were present as well followed by the formation of a solid layer to prevent the effervescence of decomposition products. Intumescent phenomenon was seen for PMMA composites added with alumina while discontinuous char was found in the situation of silica filled composites. Moreover, two distinct macroscopic morphologies of char were revealed based on the chemical nature of the nano-oxides. Residues consist of
ACCEPTED MANUSCRIPT 29 aluminium were believed to be shell structured in which the barrier protection is established on the surface of materials while the material is consumed internally. On the contrary, silica based residues were bulk structured whereby their shape is fixed and
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maintained in spite of cracks and filling of carbonaceous substances [106,107].
Ternary system of APP, melamine polyphosphate and metal oxides has been
SC
widely examined by Friederich et al. in the 2000s. For instance, ternary system of APP, melamine polyphosphate and titanium dioxide has been prepared and its thermal
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resistance behaviour has been assessed in details [97]. From the cone calorimeter data, it was reported that THR decreased in comparison with pure PMMA when additives were added into the polymer matrix. In overall, TTI increased for all the formulations except for the case of 85/7.5/7.5 PMMA/APP/melamine polyphosphate and 85/7.5/7.5
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PMMA/melamine polyphosphate/titanium dioxide. 85/7.5/7.5 PMMA/APP/titanium dioxide displayed some unique characteristics as it has the highest fire performance index (ratio of TTI and peak of HRR), lowest THR, rather long combustion period and exert
EP
some synergistic effects on the peak of HRR with 52 % reduction. Hence, it was concluded by the authors that the replacement of a part of metal oxide with nitrogen and
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phosphorus containing flame retardants led to the enhanced properties of PMMA composites
in
terms
of
fire
resistance
and
thermal
behaviour.
85/7.5/7.5
PMMA/APP/titanium dioxide was considered to be the best among the formulations owing to its highest TTI, best decrease in peak of HRR, lowest THR and greatest thermal stability increase as compared to the virgin PMMA. In year 2012, Friederich et al. have conducted another experiment with ternary system prepared from APP, melamine
ACCEPTED MANUSCRIPT 30 polyphosphate and metal oxides of alumina and boehmite [99]. According to the cone calorimetry, it was shown that all formulations of ternary system experienced an overall decrease when compared to its pure PMMA without any additives. The ignition time was
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observed to be delayed when alumina and boehmite were mixed into the PMMA composites. It was cited from other literatures that PMMA composites supplemented with melamine polyphosphate gives a 51 % reduction in peak of HRR due to the
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decomposition of melamine polyphosphate above 350 °C which functions as a heat sink to lower down the temperature of the polymer [108,109]. Upon release of phosphoric
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acid, a char would be formed that protects the polymer substrates from heat, flames, oxygen and other free radical gases from entering into the oxygen phase [110]. At the same time, the ammonia liberated as a result of melamine degradation swelled the char to protect the polymer in a higher extent. Besides that, the ammonia helps in slowing down
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the combustion process by diluting the radicals and flammable gases.
Over the past 60 years, several attempts have been done on studying the thermal
EP
degradation of PMMA [111,112]. According to Holland and Hay, the wide variation in the thermal degradation of PMMA can be examined in terms of the PMMA structure and
AC C
the experiment conditions used for preparing the polymer [111]. Generally, a two step degradation process would be resulted if the polymer was prepared in the presence of air. This is because the copolymerization was said to be done with oxygen instead of the weak links formed by combination termination. In addition, thermal degradation of APP and its blends with PMMA has been investigated by Camino et al. back in year 1978 [40]. It was inferred that for APP alone, it was degraded under vacuum in three stages. The
ACCEPTED MANUSCRIPT 31 first stage involves the condensation of APP into an ultraphosphate under temperature of not exceeding 260 °C before the release of ammonia and water. Next, fragmentation occurred in temperature between 260 and 370 °C which gives some high-boiling
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ammonium salts of phosphate fragments with more ammonia and water products. Lastly, the remaining polyphosphoric acid would then undergo extensive fragmentation under temperature of more than 370 °C. For the composites of PMMA and APP, it was
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pinpointed that the normal depolymerization of PMMA to monomer competes with the degradations reactions which form high-boiling chain fragments, methanol, carbon
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monoxide, carbon dioxide, dimethyl ether, hydrocarbons and char [40]. These additional reactions were believed to be triggered by the polyphosphoric acid. Apart from that, it was confirmed by Laachachi and coworker that the thermal stability of PMMA could be drastically enhanced by the incorporation of APP-based flame retardants although the
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modified APP with particle size of around 5 to 80 µm and lower P/N ratio would give a lower thermal stability than the pure APP [100].
Poly(ethylene terephthalate)
EP
2.5
AC C
Poly(ethylene terephthalate) (PET) is a thermoplastic resin from the polyester
family which has been commonly used as synthetic fibres. The excellence of its properties such as superior tensile and impact strength, clarity, good processability, high chemical resistance and reasonable thermal stability make it to be extremely useful in a wide variety of applications [113,114]. Zhu et al. have conducted an experiment to explore the effects of APP and coal on the flame retardancy, thermal degradation, char
ACCEPTED MANUSCRIPT 32 formation and mechanical properties of the PET composites [115]. In particular, the tensile and flexural strength of the composites were measured with a Zwick multipurpose tester following ISO 527 and GB 9341-2000. From Table 6, it was stated that the
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mechanical properties of the PET composites added with APP and coal were deteriorated. This can be attributed to the degradation of polyester and the poor interfacial adhesion between the PET and the additives. During the compounding of the composites, it was
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emphasized that APP and coal could promote the chain scission of polyester through hydrothermal and catalytic degradations [116]. The degradations can be traced back to
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the adsorption of water and polyacid by coal and APP respectively. Therefore, selection of suitable loading of APP and coal is vital to produce the PET composites with optimum properties. Besides that, pre-treatments of both APP and coal such as the intensive balling and surface coating of interfacial compatibilizer could also be applied before mixing
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them into the PET.
In terms of morphologies, Zhu and coworker have found out that addition of APP
EP
alone into PET would result in a featureless melting surface since both APP and PET would be in molten state by then, and APP was predicted to promote thermal degradation
AC C
and thus melting of PET [115]. On the other hand, APP and poly(acrylic acid) were chosen by Alongi et al. in the construction of a complex, four-layered (quadlayered) hybrid organic-inorganic structure via the layer by layer assembly method [117]. The structure was said to be crucial in the formation of an aromatic and stable carbonaceous structure or char. Besides that, it was able to thermally protect PET, cotton and PET/cotton blends. From the SEM micrographs, it was supposed that the layer by layer
ACCEPTED MANUSCRIPT 33 film formed on the PET surface depends on the quadlayer number. For 5 quadlayers, the PET fibres were not entirely covered unless the high voltage beam employed was able to penetrate beyond any thin film formation and render any such film invisible [118]. As
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confirmed by the authors, increasing the quadlayer number would cause the coverage of the fibres to become even more homogeneous. Furthermore, SEM micrographs showed that the structure and morphology of the deposited coating has greatly enhanced the
SC
thermal and thermo-oxidative stability of the PET, cotton and their blends. Favourable conditions were believed to be achieved for the formation of a stable char and thus
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increasing the final residues at high temperatures and preserving the polymer textures [117].
Table 7 shows the LOI of PET composites added with APP and coal at different
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loading level [115]. From the table, it is obvious that the LOI values increase gradually with the amount of APP in the composites. This is in agreement with the findings by Balabanovich and Brauman [116,119]. The LOI for coal powder was found to be 34.5 %
EP
while the composites of PET and coal exhibited slightly larger LOI values than the mere coal powder but smaller than the PET/APP composites. It was anticipated that coal is
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able to facilitate the degradation of PET and cause the chain scission of PET chains which were able to be ignited at ease [120,121]. Futhermore, the trends of PET/APP/coal composites with different ratio of APP to coal were quite similar although they displayed different values of LOI. This was the result of accelerative degradation at low concentrations and blanketing at high concentrations of APP [115]. Lastly, it was deduced that substitution of APP by coal did not exert much difference to the flame
ACCEPTED MANUSCRIPT 34 retardancy when the combined additive loading level was beyond 30 wt%. In the cone calorimetry test, Zhu et al. have compared the three melt processed composites which include the original PET as well as PET added with 23 wt% of APP and coal in the ratio
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of 1:1 and 2:1 respectively [115]. From the test, it was noticed that the peaks of HRR for two modified composites were only 60 % with respect to their original PET without any additives. Basically, smaller HRR values indicate that less amount of heat would be
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supplied back to the degrading polymeric substrates. The adiabatic temperature of a fire situation in a confined space having the flame retarded polymers would be much lower,
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for example, lower than flashover temperature and thus a fire scenario would not happen [115,122]. In addition, it was discovered that the second peak of HRR periods were absent for the two modified composites which suggest that incandescent combustion was notably inhibited. However, the two modified composites could be easily burned when
adding APP and coal.
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they were subjected to fire due to the shorter ignition time than of virgin PET without
Thermal degradation mechanisms of PET have attracted several attentions of past
EP
researchers back in the 20th century. According to the summary by Buxbaum, thermal degradation of PET involved the transfer of a β-CH hydrogen leading to the formation of
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oligomers with olefin and carboxylic end groups [123]. In addition, Zimmermann et al. and McNeill et al. focused on the degradation of PET at high temperatures [124,125]. It was established that the reactions of thermo-oxidative degradation were additionally complicated through the involvement of oxygen. This process was reported to begin with the formation of a hydroperoxide at the methylene group accompanied by the homolytic chain scission. On the other hand, Montaudo et al. have studied the thermal degradation
ACCEPTED MANUSCRIPT 35 mechanisms of PET in the presence of APP [126]. Based on their research, it was inferred that APP affected the pyrolysis by increasing the rate of decomposition and reducing the polymer decomposition temperature of the PET. Nevertheless, addition of
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APP did not alter the nature of the pyrolysis products. For the case of pure PET, it was verified that pyrolysis of PET continue through the primary formation of cyclic oligomers at temperature of about 300 °C and further decompose at around 400 °C to produce
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oligomers that contain acetaldehyde and anhydride [127]. The effect of interaction between APP and PET on thermal degradation of PET has been examined by Zhu and
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coworker in year 2010 [115]. It was implied that APP apparently started to degrade at 300 °C with the release of ammonia and formation of polyphosphoric acid. After that, further degradation took place at 650 °C with volatilization of polyphosphoric acid fragments while PET was being degraded at 420 °C due to main chain scission through
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the formation of carboxylic acids and vinyl chain ends [128]. By comparing the calculated and experimental results of TGA, it was observed that rapid degradation of 90/10 PET/APP happened at approximately 380 °C because of the presence of APP that
EP
increased the PET degradation remarkably [115]. This statement also applied for the composites of 75/25 PET/APP and 70/30 PET/APP. Apart from that, interaction between
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APP and coal still displayed the acceleration of initial PET degradation due to APP, but this time a preferential retardation of char residues volatilization was detected at temperature exceeding 550 °C in comparison with the calculated data. Besides that, it was assumed that high weight ratio of APP to coal could be advantageous in hindering the gasification of char residues [129]. In conclusion, synergistic carbonization on PET between APP and coal might probably been achieved at high temperatures.
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Acrylonitrile-butadiene-styrene
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3.1
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3. Engineering Thermoplastics and Their Composites
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36
Acrylonitrile-butadiene-styrene (ABS) is considered to be one of the vital and prominent type of engineering thermoplastics owing to its superior mechanical properties, good chemical resistance and rather easy processability [130-132]. It has been widely
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applied in several fields such as the electrical industries, medical areas, automotive production and even for households usage. Although ABS possesses overall excellent properties, it shows unsatisfying flame and thermal behaviour as it has a low LOI of only
EP
about 18 % [133,134]. In particular, numerous amount of dense smoke and toxic gas will be liberated during the burning of ABS and its composites, and thus many literatures
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have now been focusing on how to increase the flame retardancy of ABS and its composites through the use of flame retardants and IFR. It has been proposed by several papers that application of APP in ABS composites would results in positive effect with the compensation of the inherent properties of ABS. For instance, as shown by Zhang et al., the storage modulus of ABS blended with 24 wt% APP and 6 wt% poly(diphenolic phenyl phosphate) was lower than the original ABS when the testing temperature exceeds
ACCEPTED MANUSCRIPT 37 105 °C [135]. This can be attributed to the plasticization effect of APP and poly(diphenolic phenyl phosphate). In order to mitigate the plasticization effect and
appropriate amount of expandable graphite can be added.
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enhance the interfacial adhesion between the flame retardants and the polymer matrix,
Figure 8 shows the SEM micrograph of char residues for ABS composites added
SC
with APP and expandable graphite. For pure ABS alone, there were a number of big holes and cracks present in the residue due to the insufficient char formation or formation
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of a rather less condensed char during the combustion process [136]. For the case of 85/15 ABS/APP, it can be observed that a smooth thin film-like glassy coating was formed on the surface of the residue as a consequence of the decomposition of APP which generated viscous phosphoric or polyphosphoric acid. Nevertheless, this formation
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of coating or char was found to be ineffective in preventing the degradation of ABS. On the other hand, as seen in Figure 8 (c) and (d), unique worm-like structures of carbon layer was developed which is different from the pure ABS and ABS/APP composites. It
EP
was believed to be the expandable graphite in the composites which tend to expand and spread in the ABS polymer matrix. However, the char residue of 85/15 ABS/expandable
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graphite seems to be brittle and susceptible to collapse whereas the char residue of ABS added with 15 wt% of expandable graphite and APP in the ratio of 3:1 was found to be more compact, thick and tight with a larger expansion of volume. This char structure was suggested to be more able to inhibit the heat and mass transfer, thus protecting the ABS matrix from heat for a longer time at higher temperature and retarding the degradation of the underlying material [137,138]. Hence, it can be concluded that combination of both
ACCEPTED MANUSCRIPT 38 APP and expandable graphite in a suitable manner could enhance the ABS polymer with excellent flame retardancy.
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Xia et al. have investigated the synergistic effects of organic MMT and IFR made up of APP and PER on the flame retardancy of ABS [139]. Besides that, they have examined the influence of IFR content (with the ratio of APP to PER being 3:1) towards
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the LOI and UL-94 rating of ABS composites. From their findings, it was inferred that the LOI values were increasing continuously with the increasing amount of IFR.
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However, it must be noted that only addition of at least 30 wt% of IFR into the ABS would results in LOI of higher than 27 % with V-0 rating (burning ceased within 10 seconds after two applications of flame with 10 seconds each on the test specimen followed by no flaming drips). Although further addition of IFR into the ABS can further
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increase the LOI, other properties such as the mechanical strength and processability of the ABS composites would need to be sacrificed. Meanwhile, the effect of APP and expandable graphite on the flame retardancy of ABS was studied by Ge and coworker
EP
[136]. From the LOI test, it was confirmed that pure ABS is an easily flammable polymer with LOI of only 19 %. The LOI values of ABS added with 15 wt% APP alone and 15
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wt% expandable graphite alone were shown to be 21.5 % and 26.0 % respectively, indicating that expandable graphite is more efficient in improving the LOI of ABS. Through the combination of APP and expandable graphite into the ABS, the LOI values have been increased remarkably above 26.0 % with a maximum of 31.0 % achieved when the ratio of APP to expandable graphite was 1:3. In spite of that, it was established that further increase of the APP content would worsen the LOI of the ABS composites.
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The thermal degradation behaviour of new IFR system consists of APP and expandable graphite incorporated in ABS polymer has been characterized by Ge et al. via
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the TGA [136]. From the TGA and differential thermogravimetric (DTG) curves shown in Figure 9, several observations can be made. First, ABS composite with APP added alone exhibited only one main pyrolysis peak at about 421 °C with char yield of 14 wt%
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at 700 °C. For ABS added with expandable graphite alone, two main pyrolysis peaks were seen at temperature around 250 and 420 °C. This can be explained by the
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intumescent process of expandable graphite which appeared in the range of 200 to 300 °C that contribute to the first pyrolysis peak. The second peak was said to be due to the thermal degradation of ABS which happened at 400 to 500 °C. In the case of ABS filled with both APP and expandable graphite, it was suggested that the char yield of the
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composite was about 15 wt% at 700 °C which is clearly higher than of pure ABS. Other than that, it was emphasized that composites containing APP and expandable graphite could lower the maximum decomposition rates, proving that both the flame retardants
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were able to enhance the thermal stability of ABS polymer [140,141]. Hence, in overall, it was concluded that combination of APP and expandable graphite works well in
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increasing the char residues of the ABS composites at high temperature and helps in lowering the mass loss rate as compared to the original pure ABS.
3.2
Polyamide
ACCEPTED MANUSCRIPT 40 Polyamide, or generally known as nylon, is a long chain fiber-forming substance with recurring amide groups. It is often applied in monofilaments and yarns in the form of fibers. Over the years, polyamide was chosen as the material of construction in the
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textile, transportation and automotive industries due to its excellent mechanical properties at high temperatures, good chemical resistance, low permeability to gases and high wear and abrasion resistance [142,143]. Up to date, there have been a number of commercially
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available types of polyamide such as nylon-4,6, nylon-6,10, nylon-6,12 and nylon-11 with nylon-6 and nylon-6,6 being the most widely used [144-146]. Li et al. have
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conducted tensile test on the nylon-6,6 fabric treated with IFR system made up of APP, melamine and PER [147]. From their study, it was confirmed that the tensile strength of untreated nylon-6,6 fabric was 190.1 MPa with elongation at break of 26 %. When treated with IFR system containing APP, the tensile strength was found to reduce to only
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149.5 MPa although the elongation at break increased to 28.3 %. This can be attributed to the breaking of fibers in nylon-6,6 when the fabric was dipped into the flame retardant solution and passed through the padder with two dips and nips. Furthermore, the high
EP
temperature of 135 °C employed during the preparation of the treated nylon-6,6 was believed to be another factor contributing to the decrease in tensile strength of the final
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polymer [148]. In fact, as proved by Levchik and coworker back in 1995, the loading of halogen-free flame retardants need to be limited as they would cause unfavourable decrease in the mechanical properties of the polyamide materials [149]. Other than mechanical properties, the microstructures of the untreated and treated nylon-6,6 was also characterized by Li et al. and given in Figure 10 [147]. From the SEM micrograph, untreated nylon-6,6 gives a rather flat and smooth surface compared with other treated
ACCEPTED MANUSCRIPT 41 nylon-6,6 samples. For the case of nylon-6,6 treated with APP, the char residue showed some micro-convexities on its surface because of the formation of char promoted by the APP. On the other hand, nylon-6,6 treated with APP and melamine displayed a frothy
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structure contributed by the decomposition of melamine that released gas, whereas nylon6,6 treated with APP and PER exhibited a continuous and thick structure of char residue due to the char formation effect with PER as the char source. Lastly, when nylon-6,6 was
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treated with APP, melamine and PER, a typical intumescent char structure was generated which is fairly frothy and swollen. The char formed on the surface was said to be
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effective in preventing the nylon-6,6 from contacting the flame and controlling the release of fuel gas [150,151]. Besides that, the intumescent char layer was able to support the melted nylon-6,6 so as to avoid dripping phenomenon during combustion process.
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The LOI of polyamide 6 or nylon-6 added with APP and other additives have been studied by different researchers in the past few decades. For instance, Levchik et al. have discovered that an increase in LOI was observed when the APP content was over 30
EP
wt% [152]. Nevertheless, intumescent char layer could still form on the surface of the
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nylon-6 composites even in the presence of APP as little as 10 wt%. However, the char layer with lower APP content was predicted to be not continuous and susceptible to flow and drip. This finding was later confirmed again by the same author when they found out that the intumescent char layer was produced only at the edges of the top specimen surface with higher tendency to flow and exposing the underlying nylon-6 polymer to the flame [153]. By increasing the APP content to the range of 40 and 50 wt%, the LOI of the composites has been elevated tremendously to a maximum of about 50 %. In addition, it
ACCEPTED MANUSCRIPT 42 was suggested by Levchik et al. that progressive replacement of APP with fillers such as talc would results in an initial improvement of the fire retardancy of nylon-6 [149]. The improvement can be related to the formation of a protective viscous char which tends to
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stick to the surface of the polymeric material rather than flowing away as in the case of nylon-6/APP composites without the addition of filler. Furthermore, it was emphasized that substitution of as low as 1.8 wt% of APP with manganese dioxide in the nylon-6
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composite would prevent the flowing of intumescent char and increased the LOI from 25% to 37% [154]. The optimum synergistic effect was achieved when the mixtures of
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APP and manganese dioxide was used in the ratio of 6:1.
The cone calorimetry of nylon-6 added with APP was obtained through linear pyrolysis experiment whereby the heat transfer to the composite specimen took place in
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the vertical direction of combustion [153]. At heat flux of 20 kW/m2, it was noticed that all the three composites (pure nylon-6, 80/20 nylon-6/APP and 60/40 nylon-6/APP) did not undergo auto ignition as they have low and constant values of gas temperature. 80/20
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nylon-6/APP gave a strong increase in the initial rate of decomposition due to the
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destabilization effect triggered by APP. However, after 15 minutes of decomposition, an intumescent insulating protective char was formed on the surface of the composite which led to the decrease of temperature from 275 to 250 °C. After duration of 30 minutes, the intumescent char layer was diminished when the temperature was risen again to 280 °C. For the case of 60/40 nylon-6/APP, the initial rate of weight loss was slightly lower than 80/20 nylon-6/APP and the intumescent char appeared after 12 minutes of decomposition, displaying better insulating and protective properties which decreased the temperature of
ACCEPTED MANUSCRIPT 43 the composite specimen from 275 to 225 °C. Furthermore, it was showed that substitution of APP with talc caused a slight increase in the final residue although the rate of combustion for the composites remained the same [149]. On the other hand, replacement
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of APP by manganese dioxide was able to enhance the fire retardant performance in the nylon-6 composites. The intumescent char of nylon-6 composites adopting part of manganese dioxide was formed after about 2.5 minutes with high shielding effect. The
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char would lose its insulating and protective properties only when the specimen has been
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consumed during the combustion or decomposition process [154].
Thermal degradation of nylon-6 has been studied by Levchik and coworker back in 1992 [152]. It was concluded that nylon-6 undergone competing mechanisms during thermal degradation to produce caprolactum, chain fragments with nitrile and carbon-
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carbon unsaturated chain ends with a small amount of residue stable to 550 °C. Pure APP, on the other hand, started to decompose at 260 °C followed by losing 18 % of its weight up to 450 °C primarily due to the elimination of ammonia and water [153]. For the nylon-
EP
6 composites comprising of APP, thermal decomposition began at temperature of 50 °C
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lower than the pure nylon-6 polymer. The shape of the TGA curves generated was greatly modified. Moreover, the weight loss occurred in three steps which involved initial decomposition at 270 to 330 °C, second decomposition at 330 to 380 °C and final decomposition at 380 to 420 °C. The relative rates of the first and third decomposition steps were found to be increasing with the amount of APP. In addition, incorporation of talc into nylon-6 together with APP has triggered chemical reaction between APP and talc. This reaction was detected at temperature above 350 °C and it was suspected to
ACCEPTED MANUSCRIPT 44 prevent the volatilization of polyphosphoric acid and increase the amount of thermally stable solid residue. As such, the inorganic phosphates that were produced were able to enhance the insulating behaviour of the intumescent layer on the surface of nylon-6 when
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it was exposed to fire.
For the case of nylon-6,6 and nylon-6,10, it was confirmed that both the
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polyamides would decompose upon heating through the chain scissioning of N-
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alkylamide bond followed by the formation of vinyl chain ends and primary amide groups that would be dehydrated to nitriles at the temperature of degradation [155]. In nylon-6,6, chain scission of alkylamide bond was also believed to happen, leading to methyl and isocyanate chain ends which can be further dimerized into carbodiimides or trimerized into isocyanurate structures. By adding APP, chemical interaction between
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APP and the polyamides was found to cause chain scission of the polyamides with the formation of a primary amide group and a phosphate ester that would be thermally degraded to generate a vinyl chain end. Furthermore, as shown by Levchik et al., APP
EP
also influenced the thermal degradation process of aliphatic nylon-11 and nylon-12 by
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lowering their decomposition temperatures and modifying the composition of the subsequent volatile products [156]. For both of the polyamides, α and β types of unsaturated nitriles were the primary volatile products of degradation in the presence of APP. It has been proposed that the interaction between APP and the two polyamides involved the formation of intermediate phosphate ester bonds whose further decomposition accounts for the thermal behaviour of the composites.
ACCEPTED MANUSCRIPT 45 3.3
Poly(vinyl alcohol)
Poly(vinyl alcohol) (PVOH) is a type of unique synthetic polymer as it cannot be
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prepared though the conventional polymerization of the corresponding vinyl alcohol monomers. Instead, it is prepared by first polymerizing vinyl acetate followed by the subsequent conversion of the polyvinyl acetate into PVOH [157,158]. Generally, PVOH
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possesses high tensile strength and flexibility that rely on the humidity. In another words, higher humidity is resulted when more water is being absorbed into the PVOH and this
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would cause the reduction in tensile strength [159,160]. With the advantage of being highly resistant to oil, grease and solvents, PVOH is mostly applied in textiles, paper making and coating industries. Not long ago, Zhao et al. have verified the mechanical properties of PVOH mixed with flame retardant system consist of APP and 0.3 wt% of
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LDH [161]. The types of LDH used were mainly composed of different combinations of metals such as zinc, aluminium, nickel and iron. From their research, it was revealed that addition of APP alone resulted in the decline of the mechanical properties in terms of
EP
tensile strength, elongation at break and tensile modulus. In particular, significant decrease was observed in the tensile strength as it decreased from 75.0 MPa to a
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remaining value of 33.0 MPa when 15 wt% of APP was added. In order to enhance the mechanical properties of PVOH without abandoning the APP (since it is crucial in increasing the flame retardancy of PVOH), suitable amount of LDH can be incorporated. As shown by the authors, overall mechanical performance of the PVOH composites has been increased when 0.3 wt% of LDH was added. Combination of nickel and aluminium in LDH was found to be the most effective in enhancing the mechanical properties of
ACCEPTED MANUSCRIPT 46 PVOH/APP composites. It was predicted that there exist a physical cross-linking network composed of layered particles, APP molecules and PVOH polymer chains in the composites. This physical cross-linking effect was believed to improve the mechanical
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properties and delay the thermal decomposition of organic-additive/polymer or neat polymer [162,163].
From the perspective of surface morphologies, Zhao et al. have performed SEM
SC
analysis on the char residue of PVOH/APP composites and PVOH/APP added with different types of LDH [161]. Based on Figure 11, it can be seen that the outer surface of
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PVOH/APP char showed a fluffy and puffy morphology while the inner surface showed a rather restricted expansion. For the case of PVOH/APP added with nickel-aluminium type of LDH, continuous and compact outer surface was seen together with the swollen inner structure. This morphology was demonstrated to be able to hinder the heat and mass
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transfer besides than the ability to obstruct flammable gases from entering the surface of composite during fire. As quoted from Liang et al., the outer surface of the charred layer ought to be compact enough to prevent the penetration of gases and to cut off the oxygen
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from the degraded volatiles more efficiently [164]. Thus, it was implied that the char of PVOH/APP could not isolate flame, combustion-supporting and flammable gases from
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the inner polymeric material due to the lack of compact outer surface. On the contrary, LDH functions in catalyzing the polyphosphate or the phosphate to cross-link with PVOH during combustion to form expansion charred layer with close outer surface which is helpful in protecting and insulating the underlying polymeric materials from further pyrolysis and burning.
ACCEPTED MANUSCRIPT 47 Lin and coworker have explored the flame retardancy of PVOH through the addition of APP [165]. Also, they have further inspected the effect of MMT towards the mechanical and thermal properties of PVOH and APP composites. According to their
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study, the LOI for pure PVOH was 19.7 % while the incorporation of 15 wt% of APP increased the LOI to 27.9 % although a V-0 rating was still unable to achieve. To increase the flame retardancy of PVOH/APP to a greater extent, small loading of MMT
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can be applied. It has been declared that LOI values increased gradually with the amount of MMT in the composites up to a maximum of 30.8 %. However, the LOI values of
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PVOH/APP/MMT composites were reduced when the weight ratio of APP to MMT was lower than 20:1. Therefore, it was concluded that optimum synergistic effect was achieved at APP:MMT of 20:1. No visible dripping occurred during the UL-94 test and the burning of composites specimen ceased within 10 seconds after two applications of
Wang
et
al.
that
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flame with 10 seconds each on the test specimen. Furthermore, it has been proven by ammonium
salt
of
2-hydroxyl-5,5-dimethyl-2,2-oxo-1,3,2-
dioxaphosphorinane was able to improve the flame retardancy of PVOH in combination
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of APP and nickel chelate [166]. The optimum formulation of the IFR used in PVOH composites was found to be: 7 wt% APP, 0.5 wt% nickel chelate and 7.5 wt% 2-
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hydroxyl-5,5-dimethyl-2,2-oxo-1,3,2-dioxaphosphorinane. With the formulation, LOI as high as 34.2 % was obtained.
Cone calorimeter was employed to obtain a better understanding regarding the
effects of APP and MMT on the fire and thermal behaviour of various PVOH composites. Basically, HRR and peak of HRR are two important parameters in determining the fire
ACCEPTED MANUSCRIPT 48 safety features of a certain material [35,36]. As shown by Lin et al., 85/15 PVOH/APP and 85/14.3/0.7 PVOH/APP/MMT gave a rather low peaks of HRR (194 and 157 kW/m2 respectively) under heat flux of 35 kW/m2 compared to the pure PVOH with value of 576
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kW/m2 [165]. This deduced that addition of APP with or without MMT can enhance the flame retardancy of PVOH composites. Besides that, the THR of both the PVOH/APP and PVOH/APP/MMT composites were lower than of pure PVOH. This can be explained
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by the incomplete burning of the PVOH polymer in the flame retarded composites. The incomplete burning was deemed to be caused by the formation of char on the surface of
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the polymer matrix which functions as a thermal protective and insulating layer to inhibit the pyrolysis of the polymer. Furthermore, it was reported that the peak of HRR for PVOH/APP/MMT was lower than PVOH/APP although the THR for both the composites were almost the same. This can be related to the char formation in
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PVOH/APP/MMT which is more compact than the char in PVOH/APP, leading to synergistic effect between APP and MMT. Hence, it was concluded that introduction of both APP and MMT into the PVOH can strengthen the flame retardance properties. Apart
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from that, application of microencapsulated APP was found to exert remarkable effect on certain PVOH derivatives such as the composites of PVOH and glycerol-plasticized
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thermoplastic starch. As established by Wu et al., addition of microencapsulated APP changed the peak of HRR slightly but drastically reduced the THR value during the test of microscale combustion calorimetry [167].
Thermal decomposition of PVOH has been inspected and published by Holland and Hay [168]. It has been justified that thermal degradation of molten PVOH resulted in
ACCEPTED MANUSCRIPT 49 elimination of water and chain scission. This occurred through a six-membered transition state that led to the formation of volatile products consist of both saturated and unsaturated aldehydes and ketones. In the solid state, PVOH degraded thermally with the
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elimination of water only. Owing to the lack of conjugation, elimination in both molten and solid state was considered to be random and the C=C bond formation did not activate the elimination of adjacent hydroxyl units [169]. Recently, Wang and coworker have
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synthesized a novel intumescent flame retardant system containing metal chelates for PVOH [166]. It has been confirmed that the general degradation of PVOH was based on
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the loss of side groups from the main chain and thus the degradation can be evaluated from weight loss of a sample subjected to high temperature [170,171]. From TGA curves, it was inferred that the first part of weight loss that happened up to 100 °C was the evaporation of water. The subsequent weight loss which took place between 150 to
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270 °C was believed to be the first stage of degradation followed by the elimination of volatile products. Thereafter, spontaneous degradation of PVOH began above temperature of 270 °C. On the other hand, Shi et al. have conducted an experiment to
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study the synergistic effect between APP and LDH towards the flame retardancy of PVOH [137]. Based on the TGA curves of the experiment, the initial decomposition
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temperature and the amount of residues for PVOH/APP were found to be higher than the pure PVOH at temperature around 200 to 700 °C. With the addition of both APP and LDH into the PVOH, initial decomposition temperature was increased as a result of physical cross-linking effect among the layered particles, APP molecules and the PVOH polymer chains. Hence, it was concluded that composites of PVOH, APP and LDH with
ACCEPTED MANUSCRIPT 50 low flammability can effectively suppress the combustion by the major action of condensed phase protection.
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Lin et al. have examined the effect of MMT on the flame retardancy of PVOH/APP composites [165]. The thermal stability of the composites was characterized by TGA analysis. From the TGA curves, it was seen that the thermal stability of PVOH
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added with APP and MMT diminished below 350 °C but enhanced at higher temperatures. It was suggested that thermal decomposition of PVOH occurred mainly in
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the temperature range of 220 to 470 °C with two differentiable weight loss zones except for the case of pure PVOH. Besides that, it was disclosed that the initial decomposition temperature of PVOH/APP was being shifted to lower temperatures compared to pure PVOH. The PVOH/APP exhibited higher amount of residue between 480 and 700 °C.
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When MMT was incorporated into the PVOH/APP composites, lower initial decomposition temperature and even higher char residues were obtained at 500, 600 and 700 °C. This can be explained by the effect of MMT in catalyzing water elimination,
EP
cross-linking and decomposition reactions between PVOH and APP and played a vital role in hindering the diffusion of volatile decomposed products contributed by the char
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layer structure [172]. Hence, it was established that the overall flame retardancy of PVOH can be enhance by adding both APP and MMT into the polymer chains. Meanwhile, Wu and coworker have proved remarkable effect of microencapsulated APP on the flame retardancy and thermal behaviour of PVOH/glycerol-plasticized thermoplastic starch [167]. According to the TGA data, presence of microencapsulated
ACCEPTED MANUSCRIPT 51 APP catalyzed the degradation and more gas products were emitted at the earlier thermal
4. Conclusion and future development
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decomposition stage.
This review focuses on the application of APP in a broad range of commodity and
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engineering thermoplastic composites. Generally, progressive research and development on the usage of APP is deemed to be necessary in order to fulfill the demand of flame
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retardants to replace the previous halogenated type of flame retardants which are getting banned and prohibited due to their potential health and environmental issues. Throughout this review, it can be concluded that modification of mechanical properties and morphologies of the thermoplastic composites could be made possible with appropriate
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amount of APP as well as applications of other additives such as coupling agent. Subsequent research works should be more focus to include wider range of polymer compounds such as bio-based polymers and rubbers products. Indeed, the study of
EP
addition of APP in rubber insulation products for heating, ventilation and air conditional (HVAC) and acoustic applications can be crucial to meet the fire resistance building code.
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In terms of thermal degradation, it was important to note that the mechanism of IFR can be varied according to the type of polymer matrix. For instance, the polyethylene phase change materials include the decomposition of APP to produce polyphosphoric acid with intense dehydration effect. The polyphosphoric acid would be needed in the dehydration of expandable graphite that give off some carbonaceous and phosphocarbonaceous residues which functions as protective barrier to the polymer surface. Meanwhile, for
ACCEPTED MANUSCRIPT 52 polypropylene composites, it was confirmed that thermal decomposition mechanism of PP/APP composite took place in decomposition and char formation. Despite the fact that the flammability of polymeric materials was made complicated by the extensive
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diversification of polymer matrices available, emergence of new approaches and continuation of research have resulted in a much better understanding of the flame retardancy in different polymers. Importantly, the coverage of study should also include
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combination of APP with other flame retardant additives such as aluminium trihydrate, magnesium dehydrate, zinc borate and etc. as well as variety of polymeric system in
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order to promote applications of APP flame retardant for better safety and environmental friendly products.
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Acknowledgement
The authors are grateful for the financial support from Universiti Tunku Abdul
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References
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Rahman Research Fund 6200/B01.
[1] Recent Advances in Flame Retardancy of Polymeric Materials. In 25th Annual Conference Proceeding, 2014. [2] Beyer G. Flame retardant properties of EVA-nanocomposites and improvements by combination of nanofillers with aluminium trihydrate. Fire and Materials 2001; 25(5): 193-197.
ACCEPTED MANUSCRIPT 53 [3] Sain M, Park SH, Suhara F, Law S. Flame retardant and mechanical properties of natural fibre–PP composites containing magnesium hydroxide. Polymer Degradation and Stability 2004; 83(2): 363-367.
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[4] Georlette P, Simons J, Costa L. Fire Retardancy of Polymeric Materials. New York: Marcel Dekker Inc., 2000.
[5] Georlette P. Fire retardant materials. Cambridge: Woodhead Publishing, 2001.
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[6] Levchik SV, Levchik GF, Balabanovich AI, Camino G, Costa L. Mechanistic study of combustion performance and thermal decomposition behaviour of nylon 6 with added
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halogen-free fire retardants. Polymer Degradation and Stability 1996; 54(2): 217-222. [7] Camino G, Costa L, Luda MP. Mechanistic aspects of intumescent fire retardant systems. Makromolekulare Chemie, Macromolecular Symposia 1993; 74: 71-83. [8] Le Bras M, Bourbigot S, Revel B. Comprehensive study of the degradation of an
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Mechanical, thermal and morphological properties of glass fiber and carbon fiber reinforced polyamide-6 and polyamide-6/clay nanocomposites. Materials Letters 2001;
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properties of polyamide 12/clay and polyamide 6–polyamide 66/clay nanocomposites. Journal of Applied Polymer Science 2006; 100(6): 4782-4794.
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ACCEPTED MANUSCRIPT 72 [149] Levchik SV, Levchik GF, Camino G, Costa L. Mechanism of action of phosphorus-based flame retardants in nylon 6. II. Ammonium polyphosphate/talc. Journal of Fire Sciences 1995; 13(1): 43-58.
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ACCEPTED MANUSCRIPT 74 properties by montmorillonite. Industrial and Engineering Chemistry Research 2011; 50(17): 9998-10005. [166] Wang DL, Liu Y, Wang DY, Zhao CX, Mou YR, Wang YZ. A novel intumescent
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intumescent flame retardant starch-based biodegradable composites. Industrial and Engineering Chemistry Research 2009; 48(6): 3150-3157.
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Table 1 Effect of APP and talc on the mechanical properties of 60/40 LDPE/EVA blends cross-linked by 3 phr dicumyl peroxide and coupled with 3 phr grafted
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polyethylene and maleic anhydride [19] 35 phr APP +
Property
30 phr APP
35 phr APP
1 phr talc
13.0 ± 0.9
Elongation at break (%)
453 ± 23
487 ± 18
481 ± 10
Retention of tensile strength (%)
113
104
93
Retention of elongation at break (%)
97
99
97
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11.9 ± 0.3
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Tensile strength (MPa)
15.2 ± 0.8
ACCEPTED MANUSCRIPT Table 2 Tensile strength of PP composites added with different amount of additives [49] PP
APP
PER
Melamine
(phr)
(phr)
(phr)
(phr)
100
-
-
-
100
7
14
13
100
15
14
13
100
23
14
13
100
30
14
Tensile strength (Pa)
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20.91
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16.23
14.37 13.98 13.49
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APP
PER
Melamine
(phr)
(phr)
(phr)
(phr)
100
-
-
-
100
7
14
13
100
15
14
13
100
23
14
13
100
30
14
LOI (%)
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17.8
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27.0
31.9 34.5 38.4
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Peak of HRR
Residue
(s)
(MJ/m2)
(kW/m2)
(%)
pure PS
83±0
131±0
752±10
-
85/15 PS/APP
62±9
118±1
85/10/5 PS/APP/hydrophilic alumina
50±1
119±2
85/10/5 PS/APP/hydrophilic silica
61±1
118±5
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TTI
13.3±0.4
342±19
14.8±0.8
360±3
13.7±0.0
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690±18
ACCEPTED MANUSCRIPT Table 5 Cone calorimeter data for PMMA composites added with different types of APP [99] PMMA with
PMMA with
15 wt%
15 wt%
pure APP
modified APP
Pure
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PMMA
69
60
55
Peak of HRR (kW/m2)
624
419
300
THR (MJ/m2)
112
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97
ACCEPTED MANUSCRIPT Table 6 Tensile and flexural strength of PET composites added with APP and coal in the ratio of 2 to 1 [114] Flexural strength (MPa)
Pure PET
56.5
96.5
95/5 PET/APP and coal
39.9
90/10 PET/APP and coal
19.0
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Tensile strength (MPa)
74.8
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ACCEPTED MANUSCRIPT Table 7 LOI of PET composites added with APP and coal at different loading level [114] 10
25
30
40
50
APP
49.0
57.0
62.0
76.0
>80
Coal
35.0
40.0
37.0
36.5
37.5
APP : coal (2 : 1)
47.0
52.0
61.0
APP : coal (1 : 1)
50.0
54.5
61.0
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Loading level (wt%)
>80
75.0
>80
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Figure 1 SEM micrograph of the phase change nanocomposite materials: (a) shape stabilized 60 wt% paraffin + 40 wt% HDPE-EVA, (b) shape stabilized 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER, (c) char residue of 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER, (d) char residue of 60 wt% paraffin + 20 wt% HDPE-EVA + 20 wt% APP-PER under high magnification [24]
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Figure 2 LOI for wood flour-polyethylene composites (35 wt% polyethylene, 50 wt% wood flour and 5 wt% lubricant) added with 10 wt% flame retardant [20]
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Figure 3 HRR curves for wood flour-polyethylene composites (35 wt% polyethylene, 50 wt% wood flour and 5 wt% lubricant) added with 10 wt% flame retardant [20]
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Figure 4 TGA curves for different flame retardant systems in the paraffin/HDPE composites [25]
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Figure 5 SEM micrograph of PP composites added with APP: (a) 70/30 PP/APP, (b) 70/30 PP/APP treated with 50 °C water for 24 hours, (c) 80/20 PP/APP, (d) 80/20 PP/APP modified with KH-550 coupling agent [51,52]
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Figure 6 TGA curves of PP/APP composites modified with silane coupling agent [51]
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Figure 7 TEM micrograph of 95 wt% PS added with (a) 5 wt% MgAl LDH under low magnification, (b) 5 wt% MgAl LDH under high magnification, (c) 2.5 wt% MgAl LDH and 2.5 wt% APP under low magnification, (d) 2.5 wt% MgAl LDH and 2.5 wt% APP under high magnification [75]
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Figure 8 SEM micrograph of char residues from ABS composites (a) pure ABS, (b) 85/15 ABS/APP, (c) 85/15 ABS/expandable graphite, (d) 85/11.25/3.75 ABS/expandable graphite/APP [135]
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Figure 9 TGA and DTG curves of ABS composites (a) pure ABS, (b) 85/15 ABS/APP, (c) 85/15 ABS/expandable graphite, (d) 85/11.25/3.75 ABS/expandable graphite/APP [135]
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Figure 10 SEM micrograph of char residues for (a) untreated nylon-6,6, (b) nylon-6,6 treated with APP, (c) nylon-6,6 treated with APP and melamine, (d) nylon-6,6 treated with APP and PER, (e) nylon-6,6 treated with APP, melamine and PER [146]
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Figure 11 SEM micrograph of char residues for (a) outer surface of PVOH/APP (b) inner surface of PVOH/APP, (c) outer surface of PVOH/APP added with nickelaluminium LDH, (d) inner surface of PVOH/APP added with nickel-aluminium LDH [160]