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Synergistic effects of amine-modified ammonium polyphosphate on curing behaviors and flame retardation properties of epoxy composites
Composites Part B 170 (2019) 19–30
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Synergistic effects of amine-modified ammonium polyphosphate on curing behaviors and flame retardation properties of epoxy composites Myounguk Kim, Hyunseok Ko, Sun-Min Park * Fibrous Ceramics & Aerospace Materials Center, Korea Institute of Ceramic Engineering and Technology, Jinju, 52851, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Thermosetting resin Curing behavior Thermal properties Thermal analysis
A flame retardation property of polymer composites is being considered important to minimize the amount of heat release and the smoke production during the combustion. Therefore, this work aims to improve the flame retardation property of epoxy composites using ammonium polyphosphate (APP), one of the intumescent flame retardants (IFRs). However, APP could be migrated to the surface of composite due to the weak compatibility with epoxy resin. To prevent the migration, the surface modification method has taken place using various amines, acting as charring agent. In this study, we select the 4,40 -diaminodiphenylmethane (DDM) as both curing agent and charring agent for the surface modification on the APP, and their epoxy composites were prepared. Interestingly, we observe both the fast curing effect by analyzing the curing behavior of epoxy composite by Friedman method, and significant improvements of the thermal degradation, and flame retardancy of epoxy with adding the amine-modified APP (mAPP). Moreover, the epoxy/mAPP composites showed higher flame retar dation properties and formed the compact char structures due to the attached DDM, which acted as an efficient charring agent to promote to form the carbonaceous char structures. Finally, we also found the correlation between the improvements of flame retardation properties, and char structures of epoxy composites by calcu lating the crosslink density ðMc Þ using the theory of rubber elasticity.
1. Introduction Nowadays, epoxy resins have drawn a great attention to the broad range of industrial applications such as adhesives, electronic devices and aerospace applications, etc. due to its excellent mechanical properties, chemical corrosion resistance, superior electrical properties, and dimensional stability. The properties of epoxy resins could tailored by alkyl groups of polymer chains depending on the final industrial appli cations [1–3]. However, epoxy resins have a severe disadvantage, which is insufficient flame retardation properties. Epoxy resins easily burn and release the massive amounts of toxic gases and heat, which restricts the usage in the fields of functional applications where flame retardation property is a necessity [4–6]. Therefore, some methods have been developed to obtain the superior flame retardation properties of epoxy composites. The first method is the epoxy resin formulation method. The chemical structures of epoxy and curing agents influence the burning rate of epoxy resin. In the case of epoxy-functionalized novolac resins, the novolac was used as curing agent, which leads to the high contents of aromatic units that make
thermally stable and dense structures. Therefore, the epoxyfunctionalized novolac resins shown higher flame retardation proper ties compared with diglycidyl ether of bisphenol A (DGEBA) based epoxy resin [7–10]. The second method is using the additives as flame retardants. Two types of flame retardants commonly used for enhancing flame retardation properties of composites; nanometric flame retardants and intumescent flame retardants (IFRs). Nanometric flame retardants such as nanoclay (montmorillonite and halloysite nanotubes) and other carbon materials (carbon nanotubes and graphene) based on their geo metric hindering effects for improving flame retardation property [11–15]. IFRs are based on the chemical reactions to make the syner gistic effects for flame extinguishment. IFRs are non-toxic, non-haloge nated materials, and are significantly effective in polyolefins. Ammonium polyphosphate (APP) as an acid agent, melamine (MA) as blowing agent, and pentaerythritol (PER) as carbonization agent are commonly used as IFRs for improving flame retardation properties of the polymer composites [11,16–18]. APP have been used among the IFRs for improving flame retardation properties of the polymer composites. It is one of the low-priced
* Corresponding author. E-mail addresses: [email protected] (M. Kim), [email protected] (H. Ko), [email protected] (S.-M. Park). https://doi.org/10.1016/j.compositesb.2019.04.016 Received 25 February 2019; Received in revised form 9 April 2019; Accepted 15 April 2019 Available online 16 April 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Fig. 1. Surface modification mechanism of APP by cationic ion exchange.
inorganic phosphorus-based flame retardants and is also known as an acid agent of IFR system containing dehydration catalyst for char for mations [7]. However, the weak compatibility caused by the different polarities of an additive and polymer matrix, which cause the phase separation. Therefore, an additive was migrated to the surface, resulting in reduced flame retardation properties of polymeric composites. Many other methods have been reported to control the compatibility between an additive and polymer matrix. Microencapsulation techniques have been developed to prevent the migration induced by weak compatibility and they have been intensely focused. The purpose of microencapsula tion was to improve the water resistance and the compatibility between an additive and polymer matrix [19–21]. Moreover, surface modifica tions using mussel-inspired chemistry have received the great attention to control the compatibility and prepare the functional composites for different applications using polydopamine, polyethyleneimine, and melanin-like materials [22–25]. Surface modifications using ion ex change reaction using the various amines like ethanolamine (EA), eth ylenediamine (EDA), diethylenetriamine (DETA) and piperazine have been also used to enhance the compatibility of an additive with the polymer matrix. The amines were chemically reacted on the surface of an additive without structural changes and acted as the efficient char ring agents, which made the synergistic effects with acid agents for improving flame retardation properties of the polymer composites [26–29]. Among them, in our study, 4,40 -diaminodiphenylmethane (DDM) was chosen as both curing agent and charring agent without any addi tional charring agent to improve flame retardation properties and pro mote the char formation of epoxy composites. Therefore, the surface modification of APP was carried out by cationic ion exchange reaction with DDM and characterization was performed. Furthermore, the curing behavior of epoxy composite was observed by differential scanning calorimetry (DSC) and analyzed using the Friedman method, which is one of the iso-conversional method deciphering the curing kinetics at different resin compositions. Moreover, the mechanical property, the thermal degradation property and flame retardation property of epoxy composite was also evaluated to confirm the effect of DDM as a charring agent. For detailed understanding of improvement of flame retardation properties of composites, the char structure formed after CC test was analyzed and the crosslink density of epoxy composite correlated with the flame retardancy was calculated using the theory of rubber elasticity.
Table 1 The compositions of neat epoxy and epoxy composites. Sample code
Epoxy
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
100
DDM
APP
mAPP
Parts per hundred (phr.) 20
– 10 – – –
– – 10 20 30
is 184–190 g/eq. was supplied from Kukdo Chemical Co., LTD., Republic of Korea. The curing agent (DDM) was supplied from TCI Chemicals Industry Co., Ltd., Japan. The active hydrogen equivalent weight is 49.5 g/eq. Commercial APP (n > 1000, form Ⅱ) was purchased by Guangzhou Xijia Chemical Co., Ltd. (Guangzhou, China) and chosen as an intumescent flame retardant. 2.2. Preparation of amine-modified APP Surface modification of the APP was performed by a cationic ion exchange reaction, as shown schematically in Fig. 1. This reaction was proceeded on the APP, forming the aliphatic multi-amines. The mixed solvent of ethanol and water (v/v ¼ 800/30) was transferred into a three-neck flask and magnetically stirred under nitrogen (N2) atmo sphere for half an hour. DDM (49.5 g) and APP (100 g) were poured into the flask and reacted for 4 h at 90 � C under nitrogen (N2) atmosphere. After the reaction, the mixture was cooled down to room temperature, and vacuum filtrated to remove the solvent and washed with ethanol several times. Then, the residues were dried overnight at 80 � C in a vacuum oven to evaporate the residual solvent. The yellowish powder was obtained and denoted as an amine-modified APP (mAPP). 2.3. Preparation of flame-retardant epoxy composites Both APP and mAPP were dried in a vacuum oven at 80 � C for 12 h. Then, pre-weighed DDM, APP, and mAPP added into the epoxy resin according to Table 1. The mixture was magnetic stirred at 80 � C until the homogenous mixture was obtained. Then, the homogeneous mixture was rapidly poured into the silicone mold and pre-cured at the room temperature for 24 h. Finally, the mixture was post-cured in a vacuum oven as following steps; at 80 � C for 2 h and 120 � C for 3 h.
2. Materials and methods
2.4. Characterization
2.1. Materials
Fourier transform infra-red (FT-IR) spectrometer (Frontier, Perki nElmer, UK) in an attenuated total reflection (ATR) mode and the high-
DGEBA (YD128) having the equivalent weight of the epoxide group 20
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by the cone calorimeter instrument (Festec, Korea) according to the ISO 5660-1 standard. The sample dimension for the CC test was 100 � 100 � 3 mm3. The sample was combusted for 10 min at a radiant cone under the heat flux of 50 kW/m2. The particles morphologies of APP and mAPP and char structures of the composites formed after the CC test were observed by FE-SEM (JSM-7160F, JEOL, Japan) at an accelerating voltage of 10 kV. Raman spectra were recorded between 1000 and 2000 cm 1 by NRS-3100 Raman spectrometer (Jasco, USA) with 532 nm laser and TE-cooled CCD camera to observe the char structures formed after the CC test. DMA was performed by MCR 502 (Anton Paar, Austria) from 50 � C to 200 � C at a heating rate of 3 � C/min in torsion mode to calculate the crosslink density of epoxy composites. The flexural tests of a series of epoxy composites were performed using a universal testing machine (UTM, 5900 Series, Instron, USA) following ASTM D790 standard. The span length, strain rate, and preload force were 30 mm, 1 mm/min, and 0.5 N, respectively. The flexural tests were carried out 5 times for each specimen and the data were averaged. 3. Results and discussion Fig. 2. FT-IR spectra of APP and mAPP.
3.1. Characterization of amine-modified APP
1
resolution H-NMR spectra were recorded by AVANCE-600 spectrom eter (Bruker, Germany) using D2O solvent to investigate the changes of the chemical structures on the APP by a cationic ion exchange reaction. The elemental analysis was performed by Flash EA 1112 (Thermo Electron, USA) to investigate the elemental contents of APP depending on the surface modification process. XRD patterns were measured by Ultima Ⅲ X-ray diffractometer (Rigaku, Japan) with Cu Kα1 radiation (λ ¼ 1.54 Å) over the 2θ range of 10–60� at the scanning rate of 0.02� to confirm the effect of surface modification on the crystalline structure of APP. Differential scanning calorimeter (DSC-Q10, TA Instruments, USA) was used to observe the curing behavior of epoxy composite. The sample was heated from a room temperature to 350 � C at the different heating rate of 5, 10, and 15 � C/min under a nitrogen atmosphere. Thermogra vimetric analysis (TGA) was performed by using a thermogravimetric analyzer (STA449F3, Netzsch, Germany) with a constant heating rate of 10 � C/min, up to 800 � C at a nitrogen atmosphere. The LOI test and CC test are employed to measure the flame retardation properties of epoxy composites. The LOI test was also measured by oxygen index instrument (FTT, UK) according to the ASTM D2863 standard. The sample dimen sion for the LOI test was 130 � 6.5 � 3 mm3. The CC test was performed
FT-IR spectra of APP and mAPP are shown in Fig. 2. The several peaks of APP and mAPP were observed in the range with 3030–3400 cm 1, which were attributed to -NHþ 4 asymmetric stretching vibrations. After the amine modification using DDM on the APP, the peaks appeared at 2928 and 2854 cm 1 became sharp due to the CH2 stretches in the DDM. Moreover, the small peak at 1520 cm 1 appeared due to the vibration of -NHþ 3 . These peaks prove that the DDM was attached well on the surface of APP and the salt (aromatic ring-NHþ 3 O-P) was formed in the case of mAPP [26,27]. To further confirm the surface modification of APP, 1H-NMR spectra of the APP and mAPP are recorded in Fig. 3. As shown in Fig. 3(a) for the APP, two characteristic peaks were observed at 4.68 (D2O solvent) and 3.48 ppm (-NHþ 4 -). For mAPP, several peaks in Fig. 3(b) are additionally observed at 6.99–7.00 (Doublet, hydrogen of benzene ring adjacent to NHþ 3 ), 6.67–6.68 (Doublet, hydrogen of benzene ring adjacent to CH2) and 1.80 ppm (Singlet, hydrogen of CH2) compared with the spectrum of the APP. This result also confirmed that the surface of the APP was successfully modified by a cationic ion exchange reaction. Elemental analysis data of APP and mAPP are shown in Table 2. The carbon con tents of mAPP were highly increased compared with the APP due to the
Fig. 3. 1H-NMR spectra of APP and mAPP. 21
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the heat released at a specific time (ΔHt ) over the total heat of reaction (ΔHtotal ) from the DSC curve (Equation (1)).
Table 2 Elemental analysis data of APP and mAPP. Sample
C (wt%)
N (wt%)
H (wt%)
APP mAPP
0.0811 1.1675
15.4219 15.4821
4.3576 4.3548
α¼
ΔHt ΔHtotal
(1)
Iso-conversional methods interpret the curing kinetics using several single-stage equations. In these methods, the activation energy was given as a function of conversion without any specific form of the re action model. Friedman method is one of the well-known differential iso-conversional methods and is based on equation (2) [32,33]. � � � � dα Ea; α ln β (2) ¼ ln½Aα f ðαÞ� dT α RTα where T is absolute temperature (K), R is universal gas constant, β is a heating rate, A is a pre-exponential factor, Ea is activation energy, and fðαÞ is differential reaction model function. The subscript α represents a given value at a specific conversion. The activation energy at different α h � � i dα versus 1= . was obtained by plotting of ln β dT RT α
attached DDM on the APP by cationic ion exchange. The increase of carbon contents has a good agreement with NMR spectra of the APP and mAPP. XRD patterns of the APP and mAPP are shown in Fig. 4. The crystalline structures of the APP were not changed by incorpo ration of the DDM to the surface of the APP, as no significant difference is observed for XRD spectra. However, the peak shifts are observed due to the attached DDM on the APP. The surface morphologies of the APP and mAPP are shown in Fig. 5. The smooth surface is observed for the APP particle as shown in Fig. 5(a). After the surface modification, the surface of the APP particle became rough as shown in Fig. 5(b), which also confirmed that DDM was attached well on the APP by a cationic ion exchange reaction.
Friedman plot and activation energy within the conversion range of 0:3 < α < 0:9 for epoxy composite was shown in Fig. 7 and Table 3. The activation energy of neat epoxy was found in the range of 10.434–14.825 kJ/mol at different conversion, and the average activa tion energy was found as 12.101 kJ/mol. The activation energy of epoxy/APP10 was found in the range of 11.576–14.600 kJ/mol, and the average activation energy was 12.488 kJ/mol, which represents the curing reaction occurred slower than neat epoxy. Usually, the additions of particles decrease the reaction rate of the autocatalytic curing reac tion of epoxy resin [34]. On the other hand, the activation energy of epoxy/mAPP10 was found in the range of 10.801–13.597 kJ/mol and the average activation energy was found as 11.859 kJ/mol. The reaction rate of curing reaction was faster than that of neat epoxy because the attached DDM take part in the curing reaction. The activation energies of epoxy/mAPP20, and epoxy/mAPP30 were found in the range of 9.750–10.692 kJ/mol, and 8.528–9.984 kJ/mol, respectively. The average activation energies of epoxy/mAPP20, and epoxy/mAPP30 were found as 10.178 kJ/mol, and 9.470 kJ/mol, respectively. The curing reaction occurred faster with increasing mAPP contents. As a result, the attached DDM on the APP can promote the reaction rate of the curing reaction of epoxy resin.
3.2. Curing behavior of epoxy composite
3.3. Mechanical property of epoxy composite
The curing reaction of epoxy resin is an exothermic reaction in which an epoxide end group reacts with a curing agent to form a crosslinked network. These curing behaviors, such as curing mechanism and ki netics, affect the physical and mechanical properties of the cured epoxy resin [30,31]. The DSC curve of epoxy composite is shown in Fig. 6. The peak observed within 300–350 � C attributed to the decomposition of APP and release of phosphoric acid, molecular nitrogen and ammonia except for the neat epoxy. The extent of conversion (α) is calculated by
To investigate the mechanical properties of a series of epoxy com posites, the flexural test was performed, as can be seen in the stressstrain curves of Fig. 8(a). The flexural stress and flexural modulus are summarized in Fig. 8(a) and Table 4. The flexural stress (σ) and the flexural modulus (Eb ) are calculated using following equations [35,36].
Fig. 4. XRD patterns of APP and mAPP.
σ¼
3FL 2wt2
Fig. 5. FE-SEM images of APP and mAPP. 22
(3)
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Fig. 6. DSC curves of neat epoxy and epoxy composites at different heating rates; (a) Neat epoxy, (b) Epoxy/APP10, (c) Epoxy/mAPP10, (d) Epoxy/mAPP20, and (e) Epoxy/mAPP30.
Eb ¼
L3 ΔF 4wt3 Δd
curve, Δd is the corresponding deflection variation. Neat epoxy com posite was found to have the average flexural stress of about 100.50 MPa, and average flexural modulus of about 2.61 GPa. In cases of epoxy composites except the neat epoxy composite, the flexural stresses were significantly decreased to about 40 MPa whereas the flexural moduli were significantly increased to about 3.0 GPa by incorporating
(4)
where F is the applied maximum load of the load-extension curve, L is the span length, w is the width of the sample, t is the thickness of the sample, ΔF is the variation in load at the linear portion of load-deflection 23
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Fig. 7. Friedman plots of neat epoxy and epoxy composites at different extent of conversion; (a) Neat epoxy, (b) Epoxy/APP10, (c) Epoxy/mAPP10, (d) Epoxy/ mAPP20, and (e) Epoxy/mAPP30 (Black line: 5 K/min, Red line: 10 K/min, Green line: 15 K/min). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
APP compared to the neat epoxy composite. Generally, for the particle reinforced composites, the reduction of flexural strength might be attributed to the incompatibility and the poor interfacial bonding be tween epoxy matrix and particulate fillers. However, the improvement of the flexural modulus might be attributed to the reduced intra-particle distances between the particulate fillers, which resisted the polymeric deformation [37]. Interestingly, the flexural stress of epoxy/mAPP10 was slightly higher than that of epoxy/APP10 due to attached DDM on
the APP, which took part in the curing reaction of epoxy resin. As a result, the flexural stresses of epoxy/mAPP20 and epoxy/mAPP30 are kept at a certain level with increasing mAPP contents because attached DDM was consistently involved in the curing reaction of epoxy resin. 3.4. Thermal degradation property of epoxy composite Thermal degradation property of the material is a useful key for 24
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Table 3 Activation energy and Crosslink density of neat epoxy and epoxy composites. Activation Energy (Ea, kJ/mol)
Conversion (α)
Sample
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
10.434 12.261 11.208 9.827 9.541
10.250 11.614 11.096 10.393 9.984
11.332 11.576 10.801 10.161 9.771
11.761 12.136 11.661 10.692 9.764
12.662 12.456 12.005 9.928 8.528
13.442 12.774 12.648 9.750 9.453
14.825 14.600 13.597 10.496 9.252
Average Ea (kJ/mol)
Crosslink density (Mc , g/mol)
12.101 12.488 11.859 10.178 9.470
3250.2 2962.5 2232.2 1806.6 1524.9
Fig. 8. (a) Flexural stress-strain curves, and (b) flexural strength of neat epoxy and epoxy composites.
(Tmax ) was shown in Table 5 measured from the peak value of the DTG thermogram. The thermal decomposition of neat epoxy was observed by a single decomposition step with Tmax around 400 � C. However, the thermal decompositions of other epoxy composites were observed by two decomposition steps with a lower Tmax around 340 � C. The decrease of Tmax was known as a characteristic phenomenon of P-containing epoxy composites due to the less stable C-N (305 kJ/mol), and P-O (335 kJ/mol) than the C-C bond (347 kJ/mol) [38]. Therefore, the APP was first decomposed to the chain within a temperature range of 240–380 � C, which yield phosphoric acid, molecular nitrogen, and ammonia (Equation (5)). The released phosphoric acid was known to promote the formation of char formation, which shields flame propa gation (Equation (6)) [7].
Table 4 Flexural properties of neat epoxy and epoxy composites. Sample
Flexural Stress (MPa)
Flexural’s Modulus (GPa)
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
100.50 (� 4.26) 35.79 (� 2.51) 38.47 (� 5.42) 39.24 (� 0.32) 38.48 (� 2.80)
2.61 (� 3.04 (� 2.74 (� 2.89 (� 3.03 (�
0.47) 0.14) 0.30) 0.19) 0.27)
evaluating and comparing the flame retardation property of the mate rial. The TGA and DTG thermograms of neat epoxy and epoxy com posites are shown in Fig. 9. The maximum weight loss temperature
Fig. 9. (a) TGA, and (b) DTG thermograms of neat epoxy and epoxy composites. 25
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Table 5 TGA results of neat epoxy and epoxy composites. Sample
Tmax (� C)
Neat Epoxy Epoxy/APP10 Epoxy/ mAPP10 Epoxy/ mAPP20 Epoxy/ mAPP30
Temperature at 5 wt% loss (� C)
Table 6 LOI and Cone calorimeter (CC) data of neat epoxy and epoxy composites. Residual Weight (%) 500 � C
800 � C
368.07 337.81 337.88
350.20 318.25 318.92
18.34 29.17 29.41
13.81 25.69 25.96
342.74
321.25
32.84
28.96
342.87
321.35
35.60
30.34
>250� C
(5)
ðHPO3 Þn þ Cx ðH2 OÞm →½C�x þ ðHPO3 Þn � m H2 O
(6)
LOI (%)
Peak HRR (kW/m2)
THR (MJ/ m2)
Peak SPR (m2/s)
TSP (m2)
Mass loss (%)
Neat Epoxy Epoxy/ APP10 Epoxy/ mAPP10 Epoxy/ mAPP20 Epoxy/ mAPP30
26.4
463.73
89.75
0.2887
54.58
91.36
26.9
235.77
54.62
0.2075
35.85
62.32
27.3
206.54
52.64
0.201
31.37
55.12
43
134.48
38.01
0.1349
26.06
35.41
44.9
130.66
23.30
0.126
25.35
31.71
the APP. This better dispersion leads to form the char layers as effective protective layers, which contributes to the flame extinguishment. The CC test is recognized as a useful method for measuring flame retardation property of various materials like building, aerospace ma terials and plastics. The heat release rate (HRR) and total heat release (THR) for released heat, smoke production rate (SPR) and total smoke production (TSP) for released smoke, and mass loss during the com bustion were obtained by CC test. The CC test data for a series of epoxy composites are shown in Fig. 11 and Table 6. The peak HRR and THR data of epoxy/APP10 were significantly reduced compared with those of neat epoxy because of the thermal decomposition of APP aforemen tioned in Section 3.3. Interestingly, the peak HRR and THR data of epoxy/mAPP10 were further reduced compared with those of epoxy/APP10. These results demonstrate the attached DDM could act as a charring agent and form the compact char layers reacting the epoxy with the phosphoric acid from the decomposition of the APP. The peak HRR and THR data were proportionally decreased with increasing mAPP contents. The other CC data like peak SPR, TSP and mass loss data of epoxy composites were following same tendencies with the peak HRR and THR data. Moreover, the fire growth rate (FIGRA) was calculated based on the HRR curves to evaluate the hazard during the combustion of the com posites [40,41]. In general, a lower FIGRA data was obtained for the sample that the flashover was hard to occur. The FIGRA curve versus time was shown in Fig. 12. The FIGRA of epoxy/APP10 was remarkably reduced by about 30% compared with that of neat epoxy. Similarly, the FIGRA of epoxy/mAPP10 was slightly lower than that of epoxy/APP10 due to surface modification. Moreover, the FIGRA of epoxy/mAPP20 and epoxy/mAPP30 were also proportionally reduced with increasing mAPP contents. These results describe that mAPP has dramatically influenced the mass loss and the suppression of the heat and smoke during the combustion.
Fig. 10. LOI values of neat epoxy and epoxy composites.
ðNH4 PO3 Þn → ðHPO3 Þn þ n NH3
Sample
The released phosphoric acid was reacted with the epoxy chain and formed the char, which significantly improved the thermal stability observing residual weight at 800 � C shown in Table 5. Furthermore, the epoxy composites containing mAPP showed higher residual weights due to the attached DDM contributed to form the char structures, which lead to the higher thermal stability with the increase of the mAPP contents. 3.5. Flame retardation property of epoxy composite
3.6. Char morphology of epoxy composite
The LOI and CC test were performed to evaluate the flame retarda tion properties of a series of epoxy composites. The LOI value is the minimum fraction of oxygen in the oxygen-nitrogen mixed atmosphere in which is sufficient to maintain the combustion after the materials ignited. The LOI value relatively measures the combustibility of the materials. Higher LOI value indicates that the material is more difficult to burn and maintain the combustion. In general, the material is eval uated as a self-extinguishing material when the LOI value is over 26 [39]. The LOI values of epoxy composites are shown in Fig. 10 and Table 6. The LOI value of epoxy/APP10 was slightly increased compared with that of neat epoxy by decomposition of the APP. On the contrary, the LOI value of epoxy/mAPP10 was more increased than epoxy/APP10 due to attached DDM on the APP, which acted effectively as charring agents during the combustion. Interestingly, the LOI values of epox y/mAPP20 and epoxy/mAPP30 were drastically increased to 43.0% and 44.9%, respectively. This drastic increase was a result of the improved dispersion state of the APP by the curing reaction of attached DDM on
The char morphology was usually analyzed to obtain important in formation about flame retardation property of polymeric material. In Fig. 13, the char morphologies for neat epoxy, epoxy/APP10, and epoxy/mAPP10 are observed to investigate the changes in char struc tures by the addition of the APP and mAPP compared with neat epoxy. The char morphology of neat epoxy shows both broken carbon layers and numerous pores, which confirms that the flame and heat transfer did not prevent during the combustion. However, the char morphology of epoxy/APP10 shows denser carbonaceous char layers than that of neat epoxy due to the decomposition of the APP, but a partially rough carbonaceous char layer was also observed. Moreover, the char morphology of epoxy/mAPP10 shows compact carbonaceous char layers due to the attached DDM on the APP, which acted as charring agents. This compact char structures block the flame and heat transfer during the combustion, which led to the higher flame retardation properties. 26
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Fig. 11. CC data of neat epoxy and epoxy composites. (a) HRR, (b) THR, (c) SPR, and (d) TSP.
Raman spectra have been importantly used to compare flame retar dation property by observing the microstructure of the carbon layer after the epoxy composite was combusted. In Raman spectrum, two characteristic bands were mainly observed, D and G band appearing at approximately 1338 and 1572 cm 1, respectively. The D band appeared by the disorder of the sp2 structure of graphene sheets. The G band appeared by the in-plane tangential stretches of C-C bonds in the sp2 structure of graphene sheets [42,43]. Raman spectra of char residues of neat epoxy, epoxy/APP10, and epoxy/mAPP10 are shown in Fig. 14. The in-plane microcrystalline size was known to be inversely propor tional to the relative intensity ratio of D band over G band (ID/IG). The higher the ID/IG ratio, the smaller the microstructure size of the carbon layer [44]. The ID/IG ratio of epoxy composites was increased from 0.81 of neat epoxy to 0.97 of epoxy/APP10 by addition of APP. Moreover, the ID/IG ratio of epoxy/mAPP10 is 1.02, which is slightly higher than that of epoxy/APP10 due to attached DDM acting as charring agents. This result was consistent with the aforementioned flame retardation prop erties of epoxy composites and FE-SEM images of the carbonaceous char structures formed after the CC test.
can be calculated from the theory of rubber elasticity using storage modulus in the rubbery plateau region. The shear modulus E’ of the cured epoxy is calculated by equation (7). � � r2 dRT 2Mc E’ ¼ 12 1 (7) rf Mc Mn where d is the density, Mn is the number-average molecular weight of polymer chain backbone, and Mc is the molecular weight between crosslinks. Moreover, r21 =r2f is the ratio of the mean square end-to-end
distance of polymer chain to the same quantity in a randomly coiled chain, which was assumed to be unity. The correction factor for chain ends, ð1 2Mc =Mn Þ, also known to be negligible in case of highly crosslinked thermoset resin system ðMn ≫Mc Þ [45]. Therefore, equation (7) could be summarized in equation (8). E’ ¼
dRT Mc
(8)
where E’ is the storage modulus in the rubbery plateau region at Tg þ 40 C. The Mc calculation was known as an indirect method to compare the crosslink density of thermoset resin [45–47]. The crosslink density was higher when the thermoset resin had a lower Mc value. Generally, high crosslink density is known to be beneficial for improving the flame
�
3.7. Dynamic mechanical analysis of epoxy composite Flame retardation properties of composites correlate with the crosslink density. The crosslink density of the cured epoxy composite 27
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Fig. 12. FIGRA curves of neat epoxy and epoxy composites.
Fig. 13. FE-SEM images of char structures of (a, a’) neat epoxy, (b, b’) epoxy/APP10, and (c, c’) epoxy/mAPP10 formed after the CC test. 28
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Fig. 14. Raman spectra of carbonaceous chars of neat epoxy, epoxy/APP10, and epoxy/mAPP10.
retardation properties of thermoset resins [48]. The storage modulus (E’ ) and loss factor (tan δ) curves of epoxy composites are shown in Fig. 15. The Mc values of epoxy composites calculated by equation (6) were shown in Table 3. The Mc value of epoxy/mAPP10 was further decreased than that of epoxy/APP10 due to the attached DDM on the APP. Therefore, this Mc value has a good agreement with the flame retardation properties and the carbonaceous char structures formed after the CC test. The Mc value was proportionally decreased as increasing mAPP contents, because the charring agent gets involved with the additional curing reaction of epoxy resin. This data was also consistent with the curing behavior of epoxy composites.
mAPP were fabricated by hand lay-up process, then their curing behavior and flame retardation property were investigated, focusing on the influences of attached DDM on the APP. The surface modification of the APP contributes to the accelerated curing reaction of epoxy com posites, which is confirmed by the calculated the activation energy by the Friedman method. Moreover, the mechanical property and the flame retardation properties of epoxy composites were also maintained the certain level and considerably improved simultaneously due to the surface modification of the APP. This is because the attached DDM on the APP acted as charring agent, contributed to form the crosslinked structure and promoted the formation of the compact carbonaceous char structure. Also, the compact carbonaceous char structures had a rela tionship with the crosslink density, which was calculated by the theory of rubber elasticity. The crosslink density was proportionally decreased with increasing mAPP contents because the charring agent gets involved with the additional curing reaction of epoxy resin. These data were in good agreements with curing behavior and flame retardation properties of epoxy composites.
4. Conclusions In this study, the surface of the APP was chemically modified by cationic ion exchange with DDM as an efficient charring agent to obtain both fast curing behavior and improved flame retardation properties of epoxy composites. A series of epoxy composites containing APP and
Fig. 15. DMA curves of neat epoxy and epoxy composites. 29
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Acknowledgements
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Synergistic effects of amine-modified ammonium polyphosphate on curing behaviors and flame retardation properties of epoxy composites Myounguk Kim, Hyunseok Ko, Sun-Min Park * Fibrous Ceramics & Aerospace Materials Center, Korea Institute of Ceramic Engineering and Technology, Jinju, 52851, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Thermosetting resin Curing behavior Thermal properties Thermal analysis
A flame retardation property of polymer composites is being considered important to minimize the amount of heat release and the smoke production during the combustion. Therefore, this work aims to improve the flame retardation property of epoxy composites using ammonium polyphosphate (APP), one of the intumescent flame retardants (IFRs). However, APP could be migrated to the surface of composite due to the weak compatibility with epoxy resin. To prevent the migration, the surface modification method has taken place using various amines, acting as charring agent. In this study, we select the 4,40 -diaminodiphenylmethane (DDM) as both curing agent and charring agent for the surface modification on the APP, and their epoxy composites were prepared. Interestingly, we observe both the fast curing effect by analyzing the curing behavior of epoxy composite by Friedman method, and significant improvements of the thermal degradation, and flame retardancy of epoxy with adding the amine-modified APP (mAPP). Moreover, the epoxy/mAPP composites showed higher flame retar dation properties and formed the compact char structures due to the attached DDM, which acted as an efficient charring agent to promote to form the carbonaceous char structures. Finally, we also found the correlation between the improvements of flame retardation properties, and char structures of epoxy composites by calcu lating the crosslink density ðMc Þ using the theory of rubber elasticity.
1. Introduction Nowadays, epoxy resins have drawn a great attention to the broad range of industrial applications such as adhesives, electronic devices and aerospace applications, etc. due to its excellent mechanical properties, chemical corrosion resistance, superior electrical properties, and dimensional stability. The properties of epoxy resins could tailored by alkyl groups of polymer chains depending on the final industrial appli cations [1–3]. However, epoxy resins have a severe disadvantage, which is insufficient flame retardation properties. Epoxy resins easily burn and release the massive amounts of toxic gases and heat, which restricts the usage in the fields of functional applications where flame retardation property is a necessity [4–6]. Therefore, some methods have been developed to obtain the superior flame retardation properties of epoxy composites. The first method is the epoxy resin formulation method. The chemical structures of epoxy and curing agents influence the burning rate of epoxy resin. In the case of epoxy-functionalized novolac resins, the novolac was used as curing agent, which leads to the high contents of aromatic units that make
thermally stable and dense structures. Therefore, the epoxyfunctionalized novolac resins shown higher flame retardation proper ties compared with diglycidyl ether of bisphenol A (DGEBA) based epoxy resin [7–10]. The second method is using the additives as flame retardants. Two types of flame retardants commonly used for enhancing flame retardation properties of composites; nanometric flame retardants and intumescent flame retardants (IFRs). Nanometric flame retardants such as nanoclay (montmorillonite and halloysite nanotubes) and other carbon materials (carbon nanotubes and graphene) based on their geo metric hindering effects for improving flame retardation property [11–15]. IFRs are based on the chemical reactions to make the syner gistic effects for flame extinguishment. IFRs are non-toxic, non-haloge nated materials, and are significantly effective in polyolefins. Ammonium polyphosphate (APP) as an acid agent, melamine (MA) as blowing agent, and pentaerythritol (PER) as carbonization agent are commonly used as IFRs for improving flame retardation properties of the polymer composites [11,16–18]. APP have been used among the IFRs for improving flame retardation properties of the polymer composites. It is one of the low-priced
* Corresponding author. E-mail addresses: [email protected] (M. Kim), [email protected] (H. Ko), [email protected] (S.-M. Park). https://doi.org/10.1016/j.compositesb.2019.04.016 Received 25 February 2019; Received in revised form 9 April 2019; Accepted 15 April 2019 Available online 16 April 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Fig. 1. Surface modification mechanism of APP by cationic ion exchange.
inorganic phosphorus-based flame retardants and is also known as an acid agent of IFR system containing dehydration catalyst for char for mations [7]. However, the weak compatibility caused by the different polarities of an additive and polymer matrix, which cause the phase separation. Therefore, an additive was migrated to the surface, resulting in reduced flame retardation properties of polymeric composites. Many other methods have been reported to control the compatibility between an additive and polymer matrix. Microencapsulation techniques have been developed to prevent the migration induced by weak compatibility and they have been intensely focused. The purpose of microencapsula tion was to improve the water resistance and the compatibility between an additive and polymer matrix [19–21]. Moreover, surface modifica tions using mussel-inspired chemistry have received the great attention to control the compatibility and prepare the functional composites for different applications using polydopamine, polyethyleneimine, and melanin-like materials [22–25]. Surface modifications using ion ex change reaction using the various amines like ethanolamine (EA), eth ylenediamine (EDA), diethylenetriamine (DETA) and piperazine have been also used to enhance the compatibility of an additive with the polymer matrix. The amines were chemically reacted on the surface of an additive without structural changes and acted as the efficient char ring agents, which made the synergistic effects with acid agents for improving flame retardation properties of the polymer composites [26–29]. Among them, in our study, 4,40 -diaminodiphenylmethane (DDM) was chosen as both curing agent and charring agent without any addi tional charring agent to improve flame retardation properties and pro mote the char formation of epoxy composites. Therefore, the surface modification of APP was carried out by cationic ion exchange reaction with DDM and characterization was performed. Furthermore, the curing behavior of epoxy composite was observed by differential scanning calorimetry (DSC) and analyzed using the Friedman method, which is one of the iso-conversional method deciphering the curing kinetics at different resin compositions. Moreover, the mechanical property, the thermal degradation property and flame retardation property of epoxy composite was also evaluated to confirm the effect of DDM as a charring agent. For detailed understanding of improvement of flame retardation properties of composites, the char structure formed after CC test was analyzed and the crosslink density of epoxy composite correlated with the flame retardancy was calculated using the theory of rubber elasticity.
Table 1 The compositions of neat epoxy and epoxy composites. Sample code
Epoxy
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
100
DDM
APP
mAPP
Parts per hundred (phr.) 20
– 10 – – –
– – 10 20 30
is 184–190 g/eq. was supplied from Kukdo Chemical Co., LTD., Republic of Korea. The curing agent (DDM) was supplied from TCI Chemicals Industry Co., Ltd., Japan. The active hydrogen equivalent weight is 49.5 g/eq. Commercial APP (n > 1000, form Ⅱ) was purchased by Guangzhou Xijia Chemical Co., Ltd. (Guangzhou, China) and chosen as an intumescent flame retardant. 2.2. Preparation of amine-modified APP Surface modification of the APP was performed by a cationic ion exchange reaction, as shown schematically in Fig. 1. This reaction was proceeded on the APP, forming the aliphatic multi-amines. The mixed solvent of ethanol and water (v/v ¼ 800/30) was transferred into a three-neck flask and magnetically stirred under nitrogen (N2) atmo sphere for half an hour. DDM (49.5 g) and APP (100 g) were poured into the flask and reacted for 4 h at 90 � C under nitrogen (N2) atmosphere. After the reaction, the mixture was cooled down to room temperature, and vacuum filtrated to remove the solvent and washed with ethanol several times. Then, the residues were dried overnight at 80 � C in a vacuum oven to evaporate the residual solvent. The yellowish powder was obtained and denoted as an amine-modified APP (mAPP). 2.3. Preparation of flame-retardant epoxy composites Both APP and mAPP were dried in a vacuum oven at 80 � C for 12 h. Then, pre-weighed DDM, APP, and mAPP added into the epoxy resin according to Table 1. The mixture was magnetic stirred at 80 � C until the homogenous mixture was obtained. Then, the homogeneous mixture was rapidly poured into the silicone mold and pre-cured at the room temperature for 24 h. Finally, the mixture was post-cured in a vacuum oven as following steps; at 80 � C for 2 h and 120 � C for 3 h.
2. Materials and methods
2.4. Characterization
2.1. Materials
Fourier transform infra-red (FT-IR) spectrometer (Frontier, Perki nElmer, UK) in an attenuated total reflection (ATR) mode and the high-
DGEBA (YD128) having the equivalent weight of the epoxide group 20
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by the cone calorimeter instrument (Festec, Korea) according to the ISO 5660-1 standard. The sample dimension for the CC test was 100 � 100 � 3 mm3. The sample was combusted for 10 min at a radiant cone under the heat flux of 50 kW/m2. The particles morphologies of APP and mAPP and char structures of the composites formed after the CC test were observed by FE-SEM (JSM-7160F, JEOL, Japan) at an accelerating voltage of 10 kV. Raman spectra were recorded between 1000 and 2000 cm 1 by NRS-3100 Raman spectrometer (Jasco, USA) with 532 nm laser and TE-cooled CCD camera to observe the char structures formed after the CC test. DMA was performed by MCR 502 (Anton Paar, Austria) from 50 � C to 200 � C at a heating rate of 3 � C/min in torsion mode to calculate the crosslink density of epoxy composites. The flexural tests of a series of epoxy composites were performed using a universal testing machine (UTM, 5900 Series, Instron, USA) following ASTM D790 standard. The span length, strain rate, and preload force were 30 mm, 1 mm/min, and 0.5 N, respectively. The flexural tests were carried out 5 times for each specimen and the data were averaged. 3. Results and discussion Fig. 2. FT-IR spectra of APP and mAPP.
3.1. Characterization of amine-modified APP
1
resolution H-NMR spectra were recorded by AVANCE-600 spectrom eter (Bruker, Germany) using D2O solvent to investigate the changes of the chemical structures on the APP by a cationic ion exchange reaction. The elemental analysis was performed by Flash EA 1112 (Thermo Electron, USA) to investigate the elemental contents of APP depending on the surface modification process. XRD patterns were measured by Ultima Ⅲ X-ray diffractometer (Rigaku, Japan) with Cu Kα1 radiation (λ ¼ 1.54 Å) over the 2θ range of 10–60� at the scanning rate of 0.02� to confirm the effect of surface modification on the crystalline structure of APP. Differential scanning calorimeter (DSC-Q10, TA Instruments, USA) was used to observe the curing behavior of epoxy composite. The sample was heated from a room temperature to 350 � C at the different heating rate of 5, 10, and 15 � C/min under a nitrogen atmosphere. Thermogra vimetric analysis (TGA) was performed by using a thermogravimetric analyzer (STA449F3, Netzsch, Germany) with a constant heating rate of 10 � C/min, up to 800 � C at a nitrogen atmosphere. The LOI test and CC test are employed to measure the flame retardation properties of epoxy composites. The LOI test was also measured by oxygen index instrument (FTT, UK) according to the ASTM D2863 standard. The sample dimen sion for the LOI test was 130 � 6.5 � 3 mm3. The CC test was performed
FT-IR spectra of APP and mAPP are shown in Fig. 2. The several peaks of APP and mAPP were observed in the range with 3030–3400 cm 1, which were attributed to -NHþ 4 asymmetric stretching vibrations. After the amine modification using DDM on the APP, the peaks appeared at 2928 and 2854 cm 1 became sharp due to the CH2 stretches in the DDM. Moreover, the small peak at 1520 cm 1 appeared due to the vibration of -NHþ 3 . These peaks prove that the DDM was attached well on the surface of APP and the salt (aromatic ring-NHþ 3 O-P) was formed in the case of mAPP [26,27]. To further confirm the surface modification of APP, 1H-NMR spectra of the APP and mAPP are recorded in Fig. 3. As shown in Fig. 3(a) for the APP, two characteristic peaks were observed at 4.68 (D2O solvent) and 3.48 ppm (-NHþ 4 -). For mAPP, several peaks in Fig. 3(b) are additionally observed at 6.99–7.00 (Doublet, hydrogen of benzene ring adjacent to NHþ 3 ), 6.67–6.68 (Doublet, hydrogen of benzene ring adjacent to CH2) and 1.80 ppm (Singlet, hydrogen of CH2) compared with the spectrum of the APP. This result also confirmed that the surface of the APP was successfully modified by a cationic ion exchange reaction. Elemental analysis data of APP and mAPP are shown in Table 2. The carbon con tents of mAPP were highly increased compared with the APP due to the
Fig. 3. 1H-NMR spectra of APP and mAPP. 21
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the heat released at a specific time (ΔHt ) over the total heat of reaction (ΔHtotal ) from the DSC curve (Equation (1)).
Table 2 Elemental analysis data of APP and mAPP. Sample
C (wt%)
N (wt%)
H (wt%)
APP mAPP
0.0811 1.1675
15.4219 15.4821
4.3576 4.3548
α¼
ΔHt ΔHtotal
(1)
Iso-conversional methods interpret the curing kinetics using several single-stage equations. In these methods, the activation energy was given as a function of conversion without any specific form of the re action model. Friedman method is one of the well-known differential iso-conversional methods and is based on equation (2) [32,33]. � � � � dα Ea; α ln β (2) ¼ ln½Aα f ðαÞ� dT α RTα where T is absolute temperature (K), R is universal gas constant, β is a heating rate, A is a pre-exponential factor, Ea is activation energy, and fðαÞ is differential reaction model function. The subscript α represents a given value at a specific conversion. The activation energy at different α h � � i dα versus 1= . was obtained by plotting of ln β dT RT α
attached DDM on the APP by cationic ion exchange. The increase of carbon contents has a good agreement with NMR spectra of the APP and mAPP. XRD patterns of the APP and mAPP are shown in Fig. 4. The crystalline structures of the APP were not changed by incorpo ration of the DDM to the surface of the APP, as no significant difference is observed for XRD spectra. However, the peak shifts are observed due to the attached DDM on the APP. The surface morphologies of the APP and mAPP are shown in Fig. 5. The smooth surface is observed for the APP particle as shown in Fig. 5(a). After the surface modification, the surface of the APP particle became rough as shown in Fig. 5(b), which also confirmed that DDM was attached well on the APP by a cationic ion exchange reaction.
Friedman plot and activation energy within the conversion range of 0:3 < α < 0:9 for epoxy composite was shown in Fig. 7 and Table 3. The activation energy of neat epoxy was found in the range of 10.434–14.825 kJ/mol at different conversion, and the average activa tion energy was found as 12.101 kJ/mol. The activation energy of epoxy/APP10 was found in the range of 11.576–14.600 kJ/mol, and the average activation energy was 12.488 kJ/mol, which represents the curing reaction occurred slower than neat epoxy. Usually, the additions of particles decrease the reaction rate of the autocatalytic curing reac tion of epoxy resin [34]. On the other hand, the activation energy of epoxy/mAPP10 was found in the range of 10.801–13.597 kJ/mol and the average activation energy was found as 11.859 kJ/mol. The reaction rate of curing reaction was faster than that of neat epoxy because the attached DDM take part in the curing reaction. The activation energies of epoxy/mAPP20, and epoxy/mAPP30 were found in the range of 9.750–10.692 kJ/mol, and 8.528–9.984 kJ/mol, respectively. The average activation energies of epoxy/mAPP20, and epoxy/mAPP30 were found as 10.178 kJ/mol, and 9.470 kJ/mol, respectively. The curing reaction occurred faster with increasing mAPP contents. As a result, the attached DDM on the APP can promote the reaction rate of the curing reaction of epoxy resin.
3.2. Curing behavior of epoxy composite
3.3. Mechanical property of epoxy composite
The curing reaction of epoxy resin is an exothermic reaction in which an epoxide end group reacts with a curing agent to form a crosslinked network. These curing behaviors, such as curing mechanism and ki netics, affect the physical and mechanical properties of the cured epoxy resin [30,31]. The DSC curve of epoxy composite is shown in Fig. 6. The peak observed within 300–350 � C attributed to the decomposition of APP and release of phosphoric acid, molecular nitrogen and ammonia except for the neat epoxy. The extent of conversion (α) is calculated by
To investigate the mechanical properties of a series of epoxy com posites, the flexural test was performed, as can be seen in the stressstrain curves of Fig. 8(a). The flexural stress and flexural modulus are summarized in Fig. 8(a) and Table 4. The flexural stress (σ) and the flexural modulus (Eb ) are calculated using following equations [35,36].
Fig. 4. XRD patterns of APP and mAPP.
σ¼
3FL 2wt2
Fig. 5. FE-SEM images of APP and mAPP. 22
(3)
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Fig. 6. DSC curves of neat epoxy and epoxy composites at different heating rates; (a) Neat epoxy, (b) Epoxy/APP10, (c) Epoxy/mAPP10, (d) Epoxy/mAPP20, and (e) Epoxy/mAPP30.
Eb ¼
L3 ΔF 4wt3 Δd
curve, Δd is the corresponding deflection variation. Neat epoxy com posite was found to have the average flexural stress of about 100.50 MPa, and average flexural modulus of about 2.61 GPa. In cases of epoxy composites except the neat epoxy composite, the flexural stresses were significantly decreased to about 40 MPa whereas the flexural moduli were significantly increased to about 3.0 GPa by incorporating
(4)
where F is the applied maximum load of the load-extension curve, L is the span length, w is the width of the sample, t is the thickness of the sample, ΔF is the variation in load at the linear portion of load-deflection 23
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Fig. 7. Friedman plots of neat epoxy and epoxy composites at different extent of conversion; (a) Neat epoxy, (b) Epoxy/APP10, (c) Epoxy/mAPP10, (d) Epoxy/ mAPP20, and (e) Epoxy/mAPP30 (Black line: 5 K/min, Red line: 10 K/min, Green line: 15 K/min). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
APP compared to the neat epoxy composite. Generally, for the particle reinforced composites, the reduction of flexural strength might be attributed to the incompatibility and the poor interfacial bonding be tween epoxy matrix and particulate fillers. However, the improvement of the flexural modulus might be attributed to the reduced intra-particle distances between the particulate fillers, which resisted the polymeric deformation [37]. Interestingly, the flexural stress of epoxy/mAPP10 was slightly higher than that of epoxy/APP10 due to attached DDM on
the APP, which took part in the curing reaction of epoxy resin. As a result, the flexural stresses of epoxy/mAPP20 and epoxy/mAPP30 are kept at a certain level with increasing mAPP contents because attached DDM was consistently involved in the curing reaction of epoxy resin. 3.4. Thermal degradation property of epoxy composite Thermal degradation property of the material is a useful key for 24
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Table 3 Activation energy and Crosslink density of neat epoxy and epoxy composites. Activation Energy (Ea, kJ/mol)
Conversion (α)
Sample
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
10.434 12.261 11.208 9.827 9.541
10.250 11.614 11.096 10.393 9.984
11.332 11.576 10.801 10.161 9.771
11.761 12.136 11.661 10.692 9.764
12.662 12.456 12.005 9.928 8.528
13.442 12.774 12.648 9.750 9.453
14.825 14.600 13.597 10.496 9.252
Average Ea (kJ/mol)
Crosslink density (Mc , g/mol)
12.101 12.488 11.859 10.178 9.470
3250.2 2962.5 2232.2 1806.6 1524.9
Fig. 8. (a) Flexural stress-strain curves, and (b) flexural strength of neat epoxy and epoxy composites.
(Tmax ) was shown in Table 5 measured from the peak value of the DTG thermogram. The thermal decomposition of neat epoxy was observed by a single decomposition step with Tmax around 400 � C. However, the thermal decompositions of other epoxy composites were observed by two decomposition steps with a lower Tmax around 340 � C. The decrease of Tmax was known as a characteristic phenomenon of P-containing epoxy composites due to the less stable C-N (305 kJ/mol), and P-O (335 kJ/mol) than the C-C bond (347 kJ/mol) [38]. Therefore, the APP was first decomposed to the chain within a temperature range of 240–380 � C, which yield phosphoric acid, molecular nitrogen, and ammonia (Equation (5)). The released phosphoric acid was known to promote the formation of char formation, which shields flame propa gation (Equation (6)) [7].
Table 4 Flexural properties of neat epoxy and epoxy composites. Sample
Flexural Stress (MPa)
Flexural’s Modulus (GPa)
Neat Epoxy Epoxy/APP10 Epoxy/mAPP10 Epoxy/mAPP20 Epoxy/mAPP30
100.50 (� 4.26) 35.79 (� 2.51) 38.47 (� 5.42) 39.24 (� 0.32) 38.48 (� 2.80)
2.61 (� 3.04 (� 2.74 (� 2.89 (� 3.03 (�
0.47) 0.14) 0.30) 0.19) 0.27)
evaluating and comparing the flame retardation property of the mate rial. The TGA and DTG thermograms of neat epoxy and epoxy com posites are shown in Fig. 9. The maximum weight loss temperature
Fig. 9. (a) TGA, and (b) DTG thermograms of neat epoxy and epoxy composites. 25
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Composites Part B 170 (2019) 19–30
Table 5 TGA results of neat epoxy and epoxy composites. Sample
Tmax (� C)
Neat Epoxy Epoxy/APP10 Epoxy/ mAPP10 Epoxy/ mAPP20 Epoxy/ mAPP30
Temperature at 5 wt% loss (� C)
Table 6 LOI and Cone calorimeter (CC) data of neat epoxy and epoxy composites. Residual Weight (%) 500 � C
800 � C
368.07 337.81 337.88
350.20 318.25 318.92
18.34 29.17 29.41
13.81 25.69 25.96
342.74
321.25
32.84
28.96
342.87
321.35
35.60
30.34
>250� C
(5)
ðHPO3 Þn þ Cx ðH2 OÞm →½C�x þ ðHPO3 Þn � m H2 O
(6)
LOI (%)
Peak HRR (kW/m2)
THR (MJ/ m2)
Peak SPR (m2/s)
TSP (m2)
Mass loss (%)
Neat Epoxy Epoxy/ APP10 Epoxy/ mAPP10 Epoxy/ mAPP20 Epoxy/ mAPP30
26.4
463.73
89.75
0.2887
54.58
91.36
26.9
235.77
54.62
0.2075
35.85
62.32
27.3
206.54
52.64
0.201
31.37
55.12
43
134.48
38.01
0.1349
26.06
35.41
44.9
130.66
23.30
0.126
25.35
31.71
the APP. This better dispersion leads to form the char layers as effective protective layers, which contributes to the flame extinguishment. The CC test is recognized as a useful method for measuring flame retardation property of various materials like building, aerospace ma terials and plastics. The heat release rate (HRR) and total heat release (THR) for released heat, smoke production rate (SPR) and total smoke production (TSP) for released smoke, and mass loss during the com bustion were obtained by CC test. The CC test data for a series of epoxy composites are shown in Fig. 11 and Table 6. The peak HRR and THR data of epoxy/APP10 were significantly reduced compared with those of neat epoxy because of the thermal decomposition of APP aforemen tioned in Section 3.3. Interestingly, the peak HRR and THR data of epoxy/mAPP10 were further reduced compared with those of epoxy/APP10. These results demonstrate the attached DDM could act as a charring agent and form the compact char layers reacting the epoxy with the phosphoric acid from the decomposition of the APP. The peak HRR and THR data were proportionally decreased with increasing mAPP contents. The other CC data like peak SPR, TSP and mass loss data of epoxy composites were following same tendencies with the peak HRR and THR data. Moreover, the fire growth rate (FIGRA) was calculated based on the HRR curves to evaluate the hazard during the combustion of the com posites [40,41]. In general, a lower FIGRA data was obtained for the sample that the flashover was hard to occur. The FIGRA curve versus time was shown in Fig. 12. The FIGRA of epoxy/APP10 was remarkably reduced by about 30% compared with that of neat epoxy. Similarly, the FIGRA of epoxy/mAPP10 was slightly lower than that of epoxy/APP10 due to surface modification. Moreover, the FIGRA of epoxy/mAPP20 and epoxy/mAPP30 were also proportionally reduced with increasing mAPP contents. These results describe that mAPP has dramatically influenced the mass loss and the suppression of the heat and smoke during the combustion.
Fig. 10. LOI values of neat epoxy and epoxy composites.
ðNH4 PO3 Þn → ðHPO3 Þn þ n NH3
Sample
The released phosphoric acid was reacted with the epoxy chain and formed the char, which significantly improved the thermal stability observing residual weight at 800 � C shown in Table 5. Furthermore, the epoxy composites containing mAPP showed higher residual weights due to the attached DDM contributed to form the char structures, which lead to the higher thermal stability with the increase of the mAPP contents. 3.5. Flame retardation property of epoxy composite
3.6. Char morphology of epoxy composite
The LOI and CC test were performed to evaluate the flame retarda tion properties of a series of epoxy composites. The LOI value is the minimum fraction of oxygen in the oxygen-nitrogen mixed atmosphere in which is sufficient to maintain the combustion after the materials ignited. The LOI value relatively measures the combustibility of the materials. Higher LOI value indicates that the material is more difficult to burn and maintain the combustion. In general, the material is eval uated as a self-extinguishing material when the LOI value is over 26 [39]. The LOI values of epoxy composites are shown in Fig. 10 and Table 6. The LOI value of epoxy/APP10 was slightly increased compared with that of neat epoxy by decomposition of the APP. On the contrary, the LOI value of epoxy/mAPP10 was more increased than epoxy/APP10 due to attached DDM on the APP, which acted effectively as charring agents during the combustion. Interestingly, the LOI values of epox y/mAPP20 and epoxy/mAPP30 were drastically increased to 43.0% and 44.9%, respectively. This drastic increase was a result of the improved dispersion state of the APP by the curing reaction of attached DDM on
The char morphology was usually analyzed to obtain important in formation about flame retardation property of polymeric material. In Fig. 13, the char morphologies for neat epoxy, epoxy/APP10, and epoxy/mAPP10 are observed to investigate the changes in char struc tures by the addition of the APP and mAPP compared with neat epoxy. The char morphology of neat epoxy shows both broken carbon layers and numerous pores, which confirms that the flame and heat transfer did not prevent during the combustion. However, the char morphology of epoxy/APP10 shows denser carbonaceous char layers than that of neat epoxy due to the decomposition of the APP, but a partially rough carbonaceous char layer was also observed. Moreover, the char morphology of epoxy/mAPP10 shows compact carbonaceous char layers due to the attached DDM on the APP, which acted as charring agents. This compact char structures block the flame and heat transfer during the combustion, which led to the higher flame retardation properties. 26
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Composites Part B 170 (2019) 19–30
Fig. 11. CC data of neat epoxy and epoxy composites. (a) HRR, (b) THR, (c) SPR, and (d) TSP.
Raman spectra have been importantly used to compare flame retar dation property by observing the microstructure of the carbon layer after the epoxy composite was combusted. In Raman spectrum, two characteristic bands were mainly observed, D and G band appearing at approximately 1338 and 1572 cm 1, respectively. The D band appeared by the disorder of the sp2 structure of graphene sheets. The G band appeared by the in-plane tangential stretches of C-C bonds in the sp2 structure of graphene sheets [42,43]. Raman spectra of char residues of neat epoxy, epoxy/APP10, and epoxy/mAPP10 are shown in Fig. 14. The in-plane microcrystalline size was known to be inversely propor tional to the relative intensity ratio of D band over G band (ID/IG). The higher the ID/IG ratio, the smaller the microstructure size of the carbon layer [44]. The ID/IG ratio of epoxy composites was increased from 0.81 of neat epoxy to 0.97 of epoxy/APP10 by addition of APP. Moreover, the ID/IG ratio of epoxy/mAPP10 is 1.02, which is slightly higher than that of epoxy/APP10 due to attached DDM acting as charring agents. This result was consistent with the aforementioned flame retardation prop erties of epoxy composites and FE-SEM images of the carbonaceous char structures formed after the CC test.
can be calculated from the theory of rubber elasticity using storage modulus in the rubbery plateau region. The shear modulus E’ of the cured epoxy is calculated by equation (7). � � r2 dRT 2Mc E’ ¼ 12 1 (7) rf Mc Mn where d is the density, Mn is the number-average molecular weight of polymer chain backbone, and Mc is the molecular weight between crosslinks. Moreover, r21 =r2f is the ratio of the mean square end-to-end
distance of polymer chain to the same quantity in a randomly coiled chain, which was assumed to be unity. The correction factor for chain ends, ð1 2Mc =Mn Þ, also known to be negligible in case of highly crosslinked thermoset resin system ðMn ≫Mc Þ [45]. Therefore, equation (7) could be summarized in equation (8). E’ ¼
dRT Mc
(8)
where E’ is the storage modulus in the rubbery plateau region at Tg þ 40 C. The Mc calculation was known as an indirect method to compare the crosslink density of thermoset resin [45–47]. The crosslink density was higher when the thermoset resin had a lower Mc value. Generally, high crosslink density is known to be beneficial for improving the flame
�
3.7. Dynamic mechanical analysis of epoxy composite Flame retardation properties of composites correlate with the crosslink density. The crosslink density of the cured epoxy composite 27
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Composites Part B 170 (2019) 19–30
Fig. 12. FIGRA curves of neat epoxy and epoxy composites.
Fig. 13. FE-SEM images of char structures of (a, a’) neat epoxy, (b, b’) epoxy/APP10, and (c, c’) epoxy/mAPP10 formed after the CC test. 28
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Composites Part B 170 (2019) 19–30
Fig. 14. Raman spectra of carbonaceous chars of neat epoxy, epoxy/APP10, and epoxy/mAPP10.
retardation properties of thermoset resins [48]. The storage modulus (E’ ) and loss factor (tan δ) curves of epoxy composites are shown in Fig. 15. The Mc values of epoxy composites calculated by equation (6) were shown in Table 3. The Mc value of epoxy/mAPP10 was further decreased than that of epoxy/APP10 due to the attached DDM on the APP. Therefore, this Mc value has a good agreement with the flame retardation properties and the carbonaceous char structures formed after the CC test. The Mc value was proportionally decreased as increasing mAPP contents, because the charring agent gets involved with the additional curing reaction of epoxy resin. This data was also consistent with the curing behavior of epoxy composites.
mAPP were fabricated by hand lay-up process, then their curing behavior and flame retardation property were investigated, focusing on the influences of attached DDM on the APP. The surface modification of the APP contributes to the accelerated curing reaction of epoxy com posites, which is confirmed by the calculated the activation energy by the Friedman method. Moreover, the mechanical property and the flame retardation properties of epoxy composites were also maintained the certain level and considerably improved simultaneously due to the surface modification of the APP. This is because the attached DDM on the APP acted as charring agent, contributed to form the crosslinked structure and promoted the formation of the compact carbonaceous char structure. Also, the compact carbonaceous char structures had a rela tionship with the crosslink density, which was calculated by the theory of rubber elasticity. The crosslink density was proportionally decreased with increasing mAPP contents because the charring agent gets involved with the additional curing reaction of epoxy resin. These data were in good agreements with curing behavior and flame retardation properties of epoxy composites.
4. Conclusions In this study, the surface of the APP was chemically modified by cationic ion exchange with DDM as an efficient charring agent to obtain both fast curing behavior and improved flame retardation properties of epoxy composites. A series of epoxy composites containing APP and
Fig. 15. DMA curves of neat epoxy and epoxy composites. 29
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Acknowledgements
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