acrylonitrile-butadiene-styrene nanocomposites reinforced with multilayer graphene

Accepted Manuscript Flammability and thermal properties of polycarbonate /acrylonitrile-butadiene-styrene nanocomposites reinforced with multilayer graphene Raheleh Heidar Pour, Mohammad Soheilmoghaddam, Azman Hassan, Serge Bourbigot PII:

S0141-3910(15)30024-0

DOI:

10.1016/j.polymdegradstab.2015.06.013

Reference:

PDST 7681

To appear in:

Polymer Degradation and Stability

Received Date: 24 December 2014 Revised Date:

25 May 2015

Accepted Date: 15 June 2015

Please cite this article as: Pour RH, Soheilmoghaddam M, Hassan A, Bourbigot S, Flammability and thermal properties of polycarbonate /acrylonitrile-butadiene-styrene nanocomposites reinforced with multilayer graphene, Polymer Degradation and Stability (2015), doi: 10.1016/ j.polymdegradstab.2015.06.013. 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.

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Flammability and thermal properties of polycarbonate /acrylonitrilebutadiene-styrene nanocomposites reinforced with multilayer graphene Raheleh Heidar Pour a, Mohammad Soheilmoghaddam a Azman Hassan a,*, Serge Bourbigot b a

Enhanced Polymer Research Group, Department of Polymer Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Johor, Malaysia Unité Matériaux et Transformations (UMET), CNRS UMR 8207, Equipe Ingénierie des Systèmes Polymères–Ecole

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b

Nationale Supérieure de Chimie de Lille, BP 90108, F-59652 Villeneuve d'Ascq, France

*Corresponding to: Department of Polymer Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81300 Skudai, Johor, Malaysia. Tel.: +60 7 5537835; fax: +60 7 5581 463.

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E-mail address: [email protected]

Abstract

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A series of polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) (70/30 wt.%) nanocomposites with varying concentrations (0-5 wt.%) of multilayer graphene particles (GNP) were fabricated using melt extrusion process. The flammability, thermal, mechanical and morphological properties of the nanocomposites was investigated. Cone calorimeter

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analysis, limiting oxygen index (LOI) and UL94 flame rating tests revealed that addition of GNP to PC/ABS significantly improved the flame retardancy of PC/ABS/GNP nanocomposites. As much as 30.4 % reduction in peak heat release rate was observed for the

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3 wt.% GNP loading. The maximum LOI value of 26% was observed for the nanocomposites with 3wt.% GNP content. UL-94 V-2 rating and less dripping was observed for the

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nanocomposites compared with the pure PC/ABS sample. TGA analysis showed that incorporation of GNP enhanced the thermal stability and char yield of the nanocomposites. Scanning electron microscopy revealed the GNP nanoplatelets were unidirectionally aligned in the PC/ABS parallel to the surface of the nanocomposites. Keywords: Polycarbonate/acrylonitrile butadiene styrene (PC/ABS), Multilayer graphene, Nanocomposites, Flame retardancy, Thermal properties

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Introduction Polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) blends are well-known

commercial polymer blends with unique properties. Due to an appropriate combination of

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two components this blend has been widely applied in various fields such as the automobile industry, electronic components, construction materials and display screens [1-2]. PC/ABS blends offer a unique equilibrium between significant implications arising in demanding applications. The good impact behavior, UV resistance, intrinsic toughness and other

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environmental variables afforded by PC are well complemented by processability, chemical

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resistance and ductility of ABS. (PC)/(ABS) blends are a target for flame retardancy because of their use as engineering polymer blends for electronic engineering and other applications in which flame resistance is a key property [3]. However, PC exhibits a limiting oxygen index of approximate 25% and V-2 rating in UL-94 vertical burning test. In addition, ABS is very combustible and tends to generate heavy black smoke [4] which has restricted their

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applications. Therefore, PC/ABS blends require modifications to decrease their flammability for certain applications [5]. Commercially available flame retardants for PC mostly are

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halogen-containing flame retardants and phosphorus-containing flame retardants, while commercially available flame retardants for ABS mostly are halogen-containing flame

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retardants and silicon-containing flame retardants [6] Several studies have been conducted to investigate the flammability, thermal and

mechanical properties of PC/ABS blends using flame retardants. TPP, RDP and BDP are effective phosphate ester flame retardants in PC/ABS [4, 7] TPP and RDP were studied in polycarbonate by Jang and Wilkie [8], and Murashko and Levchik focused on condensed phase mechanisms of RDP in PC and PC/ABS [9-10]. More recently Pawlowski et al. investigated TPP, RDP and BDP in PC/ABS blends [11-13]. It was concluded that TPP acts mainly in the gas phase through flame inhibition, due to its high volatility and low

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decomposition temperature. RDP and BDP act mainly through flame inhibition, but also through some charring in the condensed phase. Alternative methods for improving the flammability of polymers without halogenated flame retardants involve the incorporation of nanofillers due to their multifold advantages

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including low toxic, low smoke, low corrosion, no corrosive gas, and so on [14-15]. The formation of polymer nanocomposites has become a recent solution to improving the flammability of polymers via an additive approach. A significant advantage of nanofillers is

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that by employing minimal addition levels (<10 wt.%), nanofillers significantly enhance the thermal stability, flame retardancy and mechanical properties of samples [16-17].

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Over the past few years, boehmite (AlOOH) has widely used as a flame retardant for PC/ABS blend which offers a promising potential for nanocomposites that build up surface layers during burning to work as barrier to mass and heat transfer [18-20]. Wang et al. has studied the influence of montmorillonite (MMT) on thermal stability of PC/ABS blend. Other

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researchers have also investigated the thermal stability and flame retardancy of PC/ABC blend reinforced with multiwall-carbon nanotubes (MWCNT) which in turn significant enhancement in thermal stability and flame retardancy, however Because of their high

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surface energy, high aspect ratio and strong van der Waals force, MWCNT show a tendency

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to form agglomerates [21].

In recent years, polymer/ grapheme nanocomposites have drawn more and more

attention from both scientists and engineers due to their unique behaviours [22-25]. Graphene derived from graphite is a new material used in the development of novel nanomaterials in various applications which consist of a single layer of carbon atoms densely packed in a twodimensional honeycomb lattice with high mechanical properties (1 TPa in Young's modulus and ultimate strength of 130 GPa) [26]. In addition to these unique mechanical properties, its high thermal resistance, chemical stability, gas impermeability, high surface area and low

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cost make it an excellent reinforcing nanofiller for polymeric composites. Graphene have begun to be explored to improve the flammability properties of various polymer systems. For example, GNP has been used as intumescent flame retardant in Polycarbonate nanocomposite [27], polyethylene terephthalate (PET)polypr/opylene (PP) blend [28], polyisocyanurate

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(PIR)/polyurethane (PU) [29] and high-density rigid polyurethane (RPU) [30] with an overall improvement of the fire behavior and no worsening of the mechanical properties. The addition of GNP, even at low concentration level (usually less than 5%), into a polymeric

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matrix can significantly improve polymer properties such as flammability, thermal behavior, mechanical properties, electrical conductivity and gas barrier properties [31-35]. Recently, it

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has been recognized that the use of GNP, like other carbon-based nanofillers (carbon nanotubes and buckminsterfullerene), can help to reduce the flammability of polymeric materials [36-38] .

Despite an extensive search, no systematic studies have been done so far to

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investigate the properties of PC/ABS blend reinforced with GNP nanoplatelets. In this study, the thermal behavior, flammability, mechanical properties and morphological features of PC/ABS /GNP nanocomposites prepared by melt extrusion process are investigated as a

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rational alternative to conventional filled polymers. In addition, the mechanical properties the

2. 2.1.

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PC/ABS/GNP nanocomposites were simulated. Experimental Materials

Polycarbonate (PC) grade 2505 used was from Macrolon Bayer. High impact strength

acrylonitrile-butadiene-styrene (ABS) with grade (Toyolac 100-x01) was supplied by Toray Plastic (Malaysia). Multilayer graphene GNP-M-5 grade (8-20 sheets) was purchased from XG Sciences, USA with an average length of 15 µm, thickness of approximately 5–10 nm.

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The BET surface area of the samples used in this experiment is 158 m2/g measured in laboratory. 2.2. Preparation of the PC/ABS/GNP

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The PC/ABS and PC/ABS/GNP were premixed in sealed containers and shaken manually. The compositions of the compounds are listed in Table 1. PC pellets were dried in hopper at 120 ºC for 8 hours in order to remove the moisture before compounding which

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would cause degradation of the polycarbonate and defects in the product PC/ABS. The materials were blended in a twin-screw extruder at a speed of 200 r.p.m. The temperature

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profile used for extruder was 220/230/240/250 ºC for the barrel zone temperatures. The extrudates were then injection molded into standard tensile, flexural and Izod impact samples using an injection-molding machine (JSW (Muraron, Japan) Model NIOOB II) ranging from 240-300 ºC. All test specimens were prepared based on weight percentage (wt.%). The

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PC/ABS/GNP nanocomposites are separated by different GNP content (1, 3 and 5 wt.%) and coded as PC/ABS/GNP1, PC/ABS/GNP3 and PC/ABS/GNP5. The nanocomposite without GNP loading was identified as PC/ABS.

2.3.1

Characterization

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2.3

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Table 1.

X-ray diffraction (XRD) X-ray diffraction experiments were performed at room temperature on an X-ray

diffractometer (D/MAX-2500, Rigaku). The diffracted intensity of Cu Kα radiation (45 kV and 40 mA) was analyzed in a 2θ range between 3° and 60°.

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Field Emission Scanning Electron Microscopy (FE-SEM) The morphology of PC/ABS and PC/ABS/GNP nanocomposites were investigated by

field emission scanning electron microscopy (JEOL JSM-6701F SEM machine) operated at

cryofractured samples previously coated with gold. 2.3.3

Transmission electron microscopy (TEM)

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an accelerated voltage of 5 kV. The micrographs were taken from the surface of the

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Dispersion of the multilayer graphene was observed using transmission electron microscopy (TEM). Thin sections (thickness of 70 nm) used for transmission imaging were

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microtomed using Reichert Jung Ultracut E microtome. Transmission micrographs were collected using a JEOL JEM-2100 microscope, with an operating voltage of 200 kV. 2.3.4

Tensile Properties

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Tensile tests were carried out according to ASTM D638 using Instron 5567 universal testing equipment (Bucks, UK) under ambient conditions with crosshead speed of 50 mm/min. Five specimens of each formulation were tested and the average values were

Thermogravimetric analysis (TGA)

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2.3.5

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reported.

The thermal properties of the PC/ABS/GNP nanocomposites were investigated by

thermogravimetric analysis (TGA) performed with Perkin Elmer TGA7 Instruments, (Perkin Elmer Instruments, USA) under nitrogen atmosphere at the rate of 10 °C/min. TGA tests were carried out at temperatures ranging from 25 to 800˚C. Approximately 10 mg of samples were placed in open alumina pans covered with gold foil with a gas flow rate of 50 mL min-1. Broido's method [39] has been used in the literature to determine the activation energy from the slope of the graph ln(ln1/y) versus 1/T. The activation energy for the thermal

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decomposition for TGA measurements of the present samples, which depends on the residual mass, can be calculated using the equation (1): (1)

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where T is the sample temperature (K), R is the universal gas constant and C is the independent temperature constant. The y parameter is the remaining material weight (W) at

2.3.6

Limiting Oxygen Index (LOI)

(2)

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any given reaction time obtained using equation (2):

Limiting Oxygen Index (LOI) of PC/ABS and its nanocomposites were measured using a Dereke instrument on sheets with the size of 75 mm × 6.5 mm × 3 mm according to the standard oxygen index test (ASTM D2837/77). The test is based on the lowest oxygen gas

2.3.7

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concentration that still sustains combustion of the sample. UL-94

The UL-94 vertical test was measured on a CZF-2 type instrument (Jiangning

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Analysis Instrument Factory, China) with sample dimensions of 127 mm × 12.7 mm × 3 mm.

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UL-94 test results are classified by burning ratings V-0, V-1 or V-2. V-0 rating represents the best flame retardancy of polymeric materials. 2.3.8

Cone calorimeter

The cone calorimeter tests were carried out following the procedures in ISO 5660 using an FTT cone calorimeter. Square specimens (100 × 100 × 3 mm3) were irradiated at a heat flux of 35 kW/m2, corresponding to a mild fire scenario. The heat release rate (HRR) and

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total heat release (THR) were determined. Digital photographs of whole surfaces of burnt samples were also taken. Results and discussion

3.1

X-ray diffraction (XRD)

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3.

XRD patterns of GNP powder, PC/ABS and its nanocomposites are shown in Fig.1. The sharp peak at 2θ = 26.5° corresponds to a (002) plane of GNP with d-spacing of 3.36 Å.

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This result is consistent with previous work by Kim et al. [32]. The GNP diffraction peak shifted to lower angles with less intensity in the PC/ABS/GNP nanocomposites. This peak

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shifted to 2θ = 26.18˚ in PC/ABS/GNP3 nanocomposites and d-spacing increased to 3.42 Å. This shifted GNP diffraction peak along with the observations by TEM and FESEM suggest a uniform dispersion of GNP sheets in the matrix. It can be concluded that GNPs in the composites may not have been substantially exfoliated but homogenously distributed in the

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matrix thereby improving the properties. Similar results have been reported on the incorporation of GNP into the polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene [40]

Fig. 1

FE-SEM morphology

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3.2

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and LLDPE [41].

Fig. 2 shows the FE-SEM images of the cross sectional views of pure PC/ABS and

PC/ABS/GNP nanocomposites. It can be seen that the PC was the continuous phase and the ABS phase appears as spherical inclusions. The uniform dispersion of GNPs which are embedded in PC/ABS matrix is clearly shown in Fig. 2. The surfaces of GNPs are covered with PC/ABS indicating good adhesion between them as a result of interfacial interactions between GNP and PC/ABS matrix. In particular, two-dimensional (2D) GNP with high

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specific surface area and nanoscale surface roughness may result in an enhanced mechanical interlocking with polymer chains, consequently leading to better adhesion at the interface. The higher nanofiller-matrix adhesion/interlocking of epoxy/GNP nanocomposites compared with epoxy/ nanotubes nanocomposites due to wrinkled (rough) surface and two-dimensional

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geometry of multilayer graphene was explained by Rafiee et al. [42]. Fig. 2

In addition, the FE-SEM micrographs show that the GNPs are unidirectionally

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distributed in the PC/ABS matrix and parallel to the surface of samples (Fig. 2(c)), which is

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in strong agreement with the expected result obtained by the Halpine─Tsai (H─T) model discussed in the following sections. This phenomenon is attributed to the unique structure of GNP which tends to lie down along the flow direction during the extrusion and injection molding. Similar results have been reported by other researchers in the preparation of Poly(vinylidene fluoride) (PVF)/ graphene nanoplatelets (GNP) [43], poly(vinyl alcohol)

3.3

TEM analysis

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(PVA)/graphene nanoplatelets [44] and graphene oxide [45].

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TEM micrographs of PC/ABS-GNP3 were collected to gain better understanding of nanoplatelet dispersion (Fig. 3) as the properties of nanosized composite materials are closely

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related to the extent of dispersion of nanoparticles in the matrix and hence to their effectiveness in enhancing the properties of the nanocomposites, such as mechanical, thermal and flammability. Fig. 3(a) confirms the good dispersion of multilayer graphene particles in the PC/ABS matrix. The overlapping of 2-3 sheets of GNP sheets was observed which confirms that the number of sheets was reduced from 5 to 6 sheets originally supplied by the manufacturer. This is attributed to shear mixing in the extruder equipment. In the TEM images, the gray continuous region corresponds to the PC phase and ABS appears as deep gray islands. The black sheets correspond to GNP. The TEM image of the nocomposites (Fig.

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3a) show that the GNPs were almost dispersed in the PC phase. However, Fig. 3c proves that some GNPs were concentrated in the interphase between PC and ABS. Furthermore, the interconnected network of GNP sheets was also observed which would likely produce an enhanced mechanical interlocking. Overall it is evident that good dispersion of GNP sheets

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has been achieved with some degree of exfoliation. Fig. 3 3.4

Tensile strength

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The effect of GNP nanoplatelets on Young’s modulus of PC/ABS nanocomposites has been examined (Fig. 4). The addition of GNP into PC/ABS composites enhanced their

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Young’s modulus. The Young’s modulus of pure PC/ABS was 2.18 GPa while this value for the nanocomposites with 5 wt.% GNP content was 4.15 GPa which is corresponding to an increase of 91 % compared with pure PC/ABS. (Fig. 4(a)). This enhancement in stiffness of the nanocomposites is due to the homogeneously dispersed GNP throughout the PC/ABS

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matrix and the strong interfacial adhesion between them. Similar results have been reported by incorporation of GNP nanoplatelets into a different polymer matrix [42, 44, 46]. In addition, the GNPs alignment within the matrix is an important factor affecting the

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mechanical properties of the nanocomposites. Thus, the well-established H-T model, which

of

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was widely utilized to predict the elastic moduli of unidirectional composites as the function filler

volume

fraction

and

aspect

ratio.

The

longitudinal

and

transverse

moduli E11 and E22 of a composite material in Halpin–Tsai model are generally expressed as: (3)

(4)

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where Ec, Ef and Em are Young's moduli of composites, fillers and the polymer matrix, respectively.

is the filler volume fraction and

filler geometry and loading direction.

is a shape parameter depending on the

= 2(L/t) for platelets when calculating the

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longitudinal elastic modulus E11; whereas, as an approximation, = 2 for transverse elastic modulus E22 due to its relative insensitivity to filler aspect ratio. L, and t, are the length, diameter and thickness of dispersed fillers, respectively.

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The Young’s modulus of the GNP was calculated as around 700 GPa which is in good agreement with other results which previously measured by other researchers for multilayer

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graphene particles [47]. The Young’s modulus of pure PC/ABS was 2.18 GPa as obtained from the experimental data. The density of the PC/ABS matrix and GNS is 1.16 g/cm3 and 2 g/cm3 respectively. The statistical average L and t of GNS was about 15 mm and thickness of 6 nm respectively as determined by the TEM analysis.

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The drawback of above-mentioned modified Halpin-Tsai model lies in the assumption of well-aligned inclusions in unidirectional composites. In reality, it is the general case that most composites contain a certain level of filler disorientation. In particular, plate like GNP

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in composite mats also tend to be dispersed with a certain level of misalignment and random

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orientation [22]. Thus a mathematical laminate model [48] to account for the completely disordered GNPs in all three orthogonal directions can be expressed in the following form:

Where

and

(5)

are composite modulus in the parallel and perpendicular directions

to the major axes of GNPs, respectively. With the combination of

Eqs. (3) and (5), a

modified Halpin-Tsai laminate hybrid model can be established by using

E11 and

accordingly. Such laminate hybrid model is very useful to predict mechanical

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properties of polymer nanocomposites with 3D random-oriented nanofillers with the closer formation to real morphological structures [49].

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Fig. 5 shows that the experimentally measured Young’s modulus of the nanocomposites is very close to the theoretically calculated values under the assumption that the GNP nanofillers are unidirectionally aligned to the surface of nanocomposite films with a

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GNP content up to 3 wt.%. However, as GNP content increases, the experimental value of the Young’s modulus shift towards the random dispersed curve. This indicates that the

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nanoplatelets are aligned parallel to the surface of the nanocomposites especially at less than 3 wt.% GNP content. The random dispersion of nanoplatelets at 5 wt.% of GNP content is attributed to the high viscosity of the nanocomposites which constrains the motion of GNP nanoplatelets in the matrix during the extrusion or injection. However, the low GNP content

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implies low viscosity of the nanocomposites and resulted in GNPs orientation along the flow direction during the extrusion and injection molding. Similar findings have been reported when GNP was incorporated into Polypropylene (PP) [50], poly(vinyl alcohol) (PVA) [44],

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chitosan [51] and epoxy [42]. On the basis of mechanical properties and morphologies of the nanocomposites, the schematic structure of aligned and random orientation of the GNPs in

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PC/ABS matrix is proposed as shown in Fig. 5.

3.5

Fig.4 Fig. 5

Thermal stability analysis Thermogravimetry analysis (TGA) curves of PC/ABS and its nanocomposites are

shown in Fig. 6 and the values of weight loss at different temperatures are summarized in

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Table 2. The data analysis has clearly shown that the PC/ABS/GNP nanocomposites have higher thermal stability than the pure PC/ABS blend. PC/ABS blend and its nanocomposites are decomposed in two overlapping steps in nitrogen environment. As a major result the first decomposition step was related to the ABS decomposition and the second to PC, which was

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confirmed by the characteristic decomposition temperatures and the mass losses. A significant interaction between ABS and PC occurred influencing both the decomposition of ABS and PC (Fig. 6(a)). Similar findings were reported by Pawlowski et al. [11]. As shown

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in Fig. 6, PC/ABS/GNP nanocomposites exhibited higher thermal stabilities as compared to neat PC/ABS blend. Incorporation of the GNP resulted in a significant improvement in the

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thermal stability of the PC/ABS nanocomposites. T10 is the decomposition temperature at 10 %, of the weight loss. For the pure PC/ABS , T10= 408 while for the PC/ABS nanocomposite with 3 wt.% GNPs content these decomposition temperatures increased to 424 °C. Even when the amount of GNP was only 1 wt.% the T10 temperature was increased up to 20°C.

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The results are in agreement with previous work by Villar-Rodil et al. [52] on PMMA/GNPs nanocomposites which reported that the presence of the GNPs improved the thermal stability of the polymer as the degradation process was shifted to higher temperatures by around 10 °C

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with 1 wt.% GNP loading. The peak temperature (Tp) of the DTG curve represents the

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temperature at which the maximum weight loss rate was reached, as shown in Fig. 6(b). The Tp of the pure PC/ABS appeared at 431°C and 473.5°C for ABS and PC respectively, while for the nanocomposite with 3wt.% GNP content these temperatures were increased by 13.9 and 17.3 °C respectively, as compared to pure PC/ABS blend. It can also be seen that the char yields for the nanocomposite increased with GNPs incorporation into the nanocomposite. The char yield of PC/ABS was 14.5 % at 750 ºC, whereas at the same temperature it increased to 20.4 % for the nanocomposites with 5 wt.% GNP content. This can be attributed to higher thermal stability of the GNPs and formation of

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a network of char layers during combustion retards the out-diffusion of gaseous decomposition products. Previous studies conducted by Liang et al. [33] on the graphene/ Poly(vinyl alcohol) (PVA) has reported the higher thermal stability of the nanocomposites

insulation to the underlying polymer from the heat source. Fig.6 Table 2

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due to the formation of char layer by GNP hinders mass transfer and provides thermal

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In accordance with Fig 6, decomposition of PC/ABS is a single step process. The

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decomposition of the PC/ABS/GNP is a two-step process, the first step is in the range of 400–500 °C (ER1) and the second step is in the range of 500–600 °C (ER2). The two step decomposition in TGA may be attributed to the interaction between GNP and polymer matrix. This interaction can lead to a change in thermal stability of PC/ABS blend and emerge two-step process in degradation process. It is believed that the thermal degradation

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mechanism of PC/ABS blends consists of several complex processes such as hydrolysis and thermal degradation; each becomes predominant during different stages of the overall

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process. The activation energies (Ea) of thermal degradation of the PC/ABS blend and PC/ABS/GNPs were determined using Broido’s method (Table 2). A straight line was

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obtained by plotting –log [- log(1 - α)/T2 ] against 1000/T for each sample. The value of calculated activation energy of the pure PC/ABS was 126.8 kJ mol-1. This amount increased to 169.9 kJ mol-1 (first range (EaR1)) and 211.1 kJ mol-1 (second range (EaR1)) for PC/ABS with 3 wt.% GNP content. Increases in activation energy values with increasing in GNP loading indicate higher thermal stability for PC/ABS/GNP nanocomposites. Therefore, the amount of energy required to thermally degrade the PC/ABS/GNP nanocomposites is higher than the pure PC/ABS which demonstrates a certain level of interaction between PC/ABS and GNP. The similar phenomenon have been reported by other researchers [14, 36, 53-56].

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Fire properties The LOI and UL 94 test results are summarized in Table 3. It can be observed that the

GNP loading affected the flame behaviour of the nanocomposites significantly. Pure PC/ABS

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blend exhibited a LOI value of 23.0% which is the same as previously reported values [57]. When the GNP content was as low as 1 wt.% the LOI value increased to 24% indicating a significant influence of GNP on LOI. The maximum LOI value observed for the

nanocomposite with 5wt.% GNPs was nearly constant.

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nanocomposites with 3 wt.% GNP content at 26%. Thereafter, the LOI value for the

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The literature has reported that PC/ABS blend has poor flame retardancy with severe dripping problem due to the PC structure. [58]. Table 3 shows that the PC/ABS blend failed in vertical UL 94 test and burned completely, resulting in HB classification due to its flaming time longer than 30s and dripping behavior. This was attributed to the unique structure of PC.

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The large repeat units of PC and strong interactions (dipole-dipol as well as π-π) between them which in turn the high Tg for PC, but very high chain flexibility close to free rotating chains is resulted in dripping before substantial decomposition [59-60]. PC/ABS/GNP

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nanocomposite with 1 wt.% GNP content exhibited the same UL-94 classification (HB classification), however longer time was required to burn completely. The multilayer

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graphene plates effect was most pronounced for the well-dispersed plates nanocomposites with 3wt.% of the GNP content which reached the UL-94 V-2 rating and less dripping was observed. This is attributed to connected multilayer graphene particles function in the PC matrix as anti-dripping. In fact, residues of multilayer graphene particles act as barriers against heat transport and thus decrease the heating rates of the underlying nanocomposites. In other word, the multilayer graphene heat-shielding layer slows down the escape of volatile products generated from the degrading polymer. The antidirping effect of mulitilayer

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graphene particles which was incorporated into the polypropylene has been reported by other researchers [61-63]. Over the past few years, effect of several flame retardants such as triphenyl phosphate

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(TPP) [64], resorcinol bis(diphenyl phosphate) (RDP)[11], bisphenol A bis(diphenyl phosphate) (BDP)[11], bis(di-2-methylphenyl phosphate) (HMP)[65] and some fillers such as silica [66], boehmite [12], Talc [3] and graphite oxide [14] as an efficient antidripping agents for PC/ABS blend have been investigated. Nevertheless, additional research is required to

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investigation a synergetic influence of a flame retardant in PC/ABS-GNP nanocomposites to

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achieve a V-0 rating. Table 3

Fig. 7 shows the patterns of pure PC/ABS and PC/ABS/GNP nanocomposites after LOI test. The residues at the end of the test still illustrate well the charring and the

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deformation behaviour during the test. During the test, the pure PC/ABS showed selfextinction by dripping, however the addition of GNP inhibited the dripping of the nanocomposites and decreased the burning rate of the nanocomposite samples during

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combustion. This was attributed to the char formation by GNP which serves as a physical

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barrier for heat flux from the flame to the polymer surface, as well as a diffusion barrier for gas transport to the flame. In addition, the GNPs and PC/ABS interactions have increased the viscosity of melt which in turn limited the flame propagation through the inhibition of dripping and the decrease in the rate of release of combustible gas. The same phenomena has been reported by Dittrich et al. [61] where multi-layer graphene was incorporated in polypropylene.

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Cone calorimetry enables quantitative analysis of the flammability of materials. The combustion properties of the PC/ABS and its nanocomposite samples were characterized by means of cone calorimetry. It can be seen that the PHRR (Fig. 8) and THR values of the

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PC/ABS/GNP nanocomposites are decreased as compared with pure PC/ABS (Table 3). The pure PC/ABS burns faster after ignition and PHRR reached a value of 177.5 kW/m2. However this value reduced to 136.1 kW/m2 for the nanocomposites with 3 wt.% GNP

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consent which exhibited a 30.4 % reduction in the PHRR of the PC/ABS/GNP3 nanocomposite. In addition, the GNP incorporation delayed the samples burning after the

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ignition. Similar trend is observed for the total heat release (THR) when the addition GNPs resulted in reduction of THR from 62.1 MJ/m2 for pure PC/ABS to 49.9 MJ/m2 for the nanocomposite with 3 wt.% GNP content. The changed fire behaviour due to the addition of GNP was also reflected in the cone calorimeter residue (Table 3). The residue of PC/ABS

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was 8.7 %, whereas at the almost same initial weight increased to 18.1 % for the nanocomposites with 5 wt.% GNP content. These results, suggesting incorporation of GNP had changed the degradation pathway of nanocomposites by forming higher amount of

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compact and dense char which in turn enhance flammability. Similar trend was observed on

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incorporation of GNP into the poly(methyl methacrylate) (PMMA) matrix [67]. The increase in flame retardancy of PMMA/GNP nanocomposites was attributed to the increased amount of residual char which led to the decrease of total smoke production. During the combustion of the PC/ABS/GNP nanocomposites, the filled GNPs, in

particular its network structure, enhance the strength of intumescent chars and thus prevent the collapse of charred structure. Therefore, the HRR curves for the nanocomposites especially with 3 wt.% GNP content display a plateau unlike the pure PC/ABS system. This is attributed to the presence of GNP which increased the amount of residual char for the

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nanocomposite sample. This is consistent with our recent study on influence of graphene nanoplatelets (GNP) reinforced polyethylene terephthalate (PET)/polypropylene (PP) blend [68] which reported that the mechanism of fire behavior of GNP is through the promotion of

matrix. Fig. 8

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the formation of compact char layers in condensed phase during combustion in polymer

In order to explain how the formation of char affects the combustion behavior of

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PC/ABS/GNP nanocomposites, the morphology of the chars after combustion are investigated with digital camera (Fig. 9). The digital photos indicate that the addition of GNP

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in the nanocomposites resulted in homogenous and compact residue layer acting as an insulating barrier while reducing the escape of volatile decomposition byproducts to the flame.

4.0

Conclusion

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Fig. 9

The PC/ABS nanocomposites with different GNP contents were prepared by twin-

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screw extruder followed by injection molding. The GNP were uniformly dispersed and

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embedded in the nanocomposites. The thermal stability and mechanical properties of RC are significantly improved upon the addition of GNP. The agreement between the experimental modulus data and the values predicted by the H-T model suggest that the GNPs are unidirectionally aligned in the PC/ABS matrix. LOI and UL94 confirmed the enhancement of flame retardancy of PC/ABS nanocomposites samples due to addition of GNPs through the formation of a uniform compact char layer in the condensed phase during decomposition of the polymer matrix. The addition of GNP into PC/ABS exhibits enhanced flame retardancy through reduced heat release, prolonged burning rate and impeded dripping.

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Acknowledgement

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The authors thank the Ministry of Higher Education of Malaysia (MOHE) and Universiti Teknologi Malaysia (UTM) for Research Universiti Grant vote number 02H26 and sub code

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Q.J130000.7113.02H26. References

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[68] Inuwa IM, Hassan A, Wang D-Y, Samsudin SA, Mohamad Haafiz MK, Wong SL, et al. Influence of exfoliated graphite nanoplatelets on the flammability and thermal properties of polyethylene terephthalate/polypropylene nanocomposites. Polymer Degradation and

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ACCEPTED MANUSCRIPT Figure Captions

Fig. 1. XRD patterns of PC/ABS and PC/ABS nanocomposites with different GNP loadings. Fig. 2. Cross sectional FE-SEM images of (a-b) PC/ABS and (c-d) PC/ABS/GNP3

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nanocomposite. Fig. 3. TEM pictures of PC/ABS-GNP3 nanocomposite at (a) 16 k, (b) 25 k, (c) 30 k.

Fig.4. Experimental Young’s modulus and theoretical modulus calculated using the H─T

dispersed and aligned in PC/ABS matrix.

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model and mathematical laminate model under the hypothesis that GNPs are randomly

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Fig. 5. (a) Scheme for possible microstructure of (a) aligned and (b) random orientation of GNP in PC/ABS matrix.

Fig.6. (a) TGA and (b) DTG curves of PC/ABS and PC/ABS/GNPs nanocomposites. Fig. 7. LOI test specimens of PC/ABS and its nanocomposotes after test.

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Fig. 8. Heat release rate (HRR) vs time for PC/ABS and PC/ABS/GNP nanocomposites. Fig. 9. Macroscopic view of PC/ABS and PC/ABS/GNP nanocomposites before and after the

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Cone calorimetry test.

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Fig. 1. XRD patterns of PC/ABS and PC/ABS nanocomposites with different GNP loadings.

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nanocomposite.

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Fig. 2. Cross sectional FE-SEM images of (a-b) PC/ABS and (c-d) PC/ABS/GNP3

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Fig. 3. TEM pictures of PC/ABS-GNP3 nanocomposite at (a) 16 k, (b) 25 k, (c) 30 k.

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Fig.4. Experimental Young’s modulus and theoretical modulus calculated using the H─T

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model and mathematical laminate model under the hypothesis that GNPs are randomly dispersed and aligned in PC/ABS matrix.

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Fig. 5. (a) Scheme for possible microstructure of (a) aligned and (b) random orientation of

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GNP in PC/ABS matrix.

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Fig.6. (a) TGA and (b) DTG curves of PC/ABS and PC/ABS/GNPs nanocomposites.

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Fig. 7. LOI test specimens of PC/ABS and its nanocomposotes after test.

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Fig. 8 Heat release rate (HRR) vs time for PC/ABS and PC/ABS/GNP nanocomposites.

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Fig. 9. Macroscopic view of PC/ABS and PC/ABS/GNP nanocomposites before and after the Cone calorimetry test.

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PC/ABS

PC/ABS with the ratio of GNP (70/30) (wt/wt.%) (wt.%) 100 -

PC/ABS/ GNP1

99

1

PC/ABS/ GNP3

97

3

PC/ABS/ GNP5

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5

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Designations

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Composition of investigated materials.

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TGA, DTG and activation energy (Ea) results of PC/ABS and PC/ABS/GNP nanocomposites. Samples

T10 (°C)

DTG (°C) ABS PC

Residual (%) Ea(kJ mol -1) at (750°C) ER1 ER2

PC/ABS PC/ABS/GNP1 PC/ABS/GNP3 PC/ABS/GNP5

408 432 424 424

431 436 444 441

14.5 17.8 19.3 20.4

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181.3 211.1 166.7

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126.8 163.1 169.9 149.2

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473 490 490 490

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UL-94 ratings

HB HB V-2 V-2

Dripping Moderate dripping Low dripping Low dripping

PHRR

THR

Residue

(kW/m2)

(MJ/m2)

(wt.%)

177.5±5 169.4±6 136.1±5 150.5±4

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PC/ABS PC/ABS/GNP 1 PC/ABS/GNP 3 PC/ABS/GNP 5

LOI (%)

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Compound

62.1±0.8 59.1±0.6 49.9±0.7 52.4±0.6

8.7 14.3 16.9 18.1