- Email: [email protected]
Thermal stability and heat release effect of flame retarded PA66 prepared by end-pieces capping technology
Composites Part B 167 (2019) 34–43
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Thermal stability and heat release effect of flame retarded PA66 prepared by end-pieces capping technology
T
WenYan Lyu∗, XiuMin Chen, YouBing Li, Shuang Cao, YiMing Han College of Materials Science and Technology, ChongQing University of Technology, 69 Hongguang Avennue BaNan District, ChongQing, 400054, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PA66 Thermal properties Heat release effect End-pieces capping
In order to improve the flame retardancy of PA66, bis-N-benzoguanamine-phenylphos-phamide (MCPO) was grafted to PA66 main chains by end-pieces capping technology. The thermal properties and heat release effect of flame retardant PA66 (FR PA66) composites were investigated by thermogravimetry, differential scanning calorimetry, and cone calorimetry test analyses. In the presence of 8 wt% MCPO in PA66 composites, the initial decomposition temperature (T5) and peak decomposition temperature (Tmax) of PA66 composites were 47 °C and 34 °C higher than that of pristine PA66, respectively. Compared to the traditional flame retardant PA66 composites, the heat release rate, total heat release, total smoke production, and CO release rate significantly reduced by 31.3%, 36.3%, 16.9%, and 14.7% by introducing P containing benzoguanamine derivative, respectively. Moreover, the vertical flame spread test illustrated that the melt dripping of PA66 stopped for higher precursor concentrations in the composites carbonized char, and the limiting oxygen index (LOI) value of PA66 composites reached 29% as evidenced by the LOI test.
1. Introduction As one of the major engineering plastics, polyamide 66 (PA66) featured with excellent thermal stability, weather-shield durability, chemical resistance, high mechanical performance, and easy processing is increasingly used in mechanical and/or structural applications, such as stressed functional automotive parts, bearing structural parts of aeronautics and transportation, packaging materials in electric and electronic industries, and safety parts in sports and leisure [1]. However, the applications of PA66 are highly limited due to its water-absorbing property [2] and flammability of virgin PA66 [3]. Thus, in order to improve the poor flame resistance and enhance the application fields of PA66, flame retardant PA66 composites have been prepared by adding various flame retardants [4–6] such as decabromodiphenyl ether, melamine cyanurate, and polyphosphates. Because the release of toxic and corrosive chemicals during combustion could lead to environmental and health issues, phosphorus- and nitrogen-containing compounds such red phosphorus [7] and melamine cyanurate [8] are often used to replace halogen-based compounds in flame retardant composites. In view of this, a number of studies have been reported on the flame retardancy of PA66 using red phosphorus and phosphorus or nitrogen-containing compounds. For instance, Levchik et al. [9] thoroughly investigated the decomposition and flame
∗
retardancy of polyamides, concluding that phosphate containing compounds play an important role in improving the thermal stability and flame retardancy of polyamides. Overall, these studies suggested that conventional phosphate flame retardants suffer from a few limitations, including achieving higher flame retarding levels at the expense of lower mechanical properties of polyamide materials. In recent years, intumescent flame retardants (IFRs) have been widely used in flammable polymers [10]. Compared to conventional flame retardants, IFRs have several satisfactory advantages such as high efficiency, environmentally friendly, and halogen-free characteristics [11]. Many studies [12,13] reported IFRs to be a mixture of three components, namely, an acid source, a carbon source, and a blowing agent. For instance, Lim et al. [14] introduced in detail about the application of ammonium polyphosphate (APP) as IFR in thermoplastic composites. It was suggested that thermal properties, such as limiting oxygen index (LOI) value together with cone calorimetry and thermogravimetric analysis (TGA) performance could be enhanced by the optimum combination of APP, PER and melamine which function as intumescent flame retardant. Furthermore, Sahyoun et al. [15] reported a new PA66 copolymer modified by silicophosphorylated filler aiming to improve the fire behavior of the matrix. The results show that the nanocomposite exhibits a slightly earlier ignition but the peak of heat release rate (pHRR)
Corresponding author. E-mail address: [email protected] (W. Lyu).
https://doi.org/10.1016/j.compositesb.2018.12.016 Received 22 July 2018; Received in revised form 14 November 2018; Accepted 3 December 2018 Available online 05 December 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
Composites Part B 167 (2019) 34–43
W. Lyu et al.
Scheme 1. The synthetic routes of MCPO.
Scheme 2. The end capping route of flame retardancy PA66.
thermal stability, flame retardancy, and heat release effect of the resulting PA66 composites were investigated, and the results are discussed. The MCPO used in the preparation process was synthesized following our previously reported procedure [16]. MCPO contains three benzene rings, two triazine rings, one P atom, and dual activity endaminos in a molecule. The initial decomposition temperature of MCPO can reach to 300 °C, as indicated by the TGA.
Table 1 The MCPO weight loading of PA66 composites. Samples
PA66/g
MCPO/g
1 2 3 4 5
1000 920 940 920 900
0 40 60 80 100
2. Experimental Table 2 The fame retardancy of FR PA66.
2.1. Materials
Samples
Limit oxygen index
UL94 1.6 mm rating
Cotton indicator ignited by melt drips
Self-extinguish time/s
1 2 3 4 5
21 24 25 27 29
V-2 V-2 V-1 V-0 V-0
Yes Yes Yes No No drips
4 9 13 10 8
The starting materials and solvents were all of commercial grade and used without further purification. Industrial reagent grade MCPO was synthesized in our laboratory. The synthetic route is shown in Scheme 1. PA66 (101 L) was purchased from Dowdupont Co., Ltd. 2.1.1. Preparation of PA66 composites Prior to injection molding, PA66 masterbatches and MCPO were dried at 100 °C for 24 h. PA66 composites were prepared by melt blending using a SHJ-35 twin screw extruder (NanKing ChengMeng Machinery) in the temperatures ranging from 260 to 270 °C. Because of a mass of the end -COOH groups on the PA66 main chains, the -NH2 groups of MCPO could take place in the dehydration reaction with the -OH bonds of the carboxylic acid. The effective flame retardancy elements, benzene rings, P, and triazine rings can be directly and effectively grafted to the tail end of PA66 main chains. Then, the PA66 composites pellets were mold injected using a TX-CA1 machine (HaiTian Plastic Machinery) in the temperature range 260–265 °C to form specimens with different sizes and shapes suitable for various
decreased by approximately 33%. The presence of low concentration of silicon (0.65 wt%) and phosphorus (0.75 wt%) promotes the formation of an expanded char layer that acts as a barrier. A reduction in approximately 18% effective heat of combustion (EHC) value suggests an additional flame inhibition effect of phosphorus in the gas phase. In this study, in an effort to improve the flame retardancy of PA66, FR PA66 was prepared from PA66 and MCPO through blending extrusion by end capping technology due to a wide distribution of active carboxylic acid groups in the terminal pieces of PA66 main chains. The 35
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Fig. 1. The TGA curves of FR PA66.
Fig. 2. The DTG curves of FR PA66.
composites (5 wt% weight loss) and the temperature of the maximum weight loss are denoted as T5 and Tmax, respectively. Differential scanning calorimetry (DSC) investigation was carried out using a Q20 universal testing machine (America TA Instruments Co., Ltd). The samples (3 mg) were placed inside an Al crucible at 25 °C and the measurements were carried out at the heating rate of 10 °C/min following ASTM E794-2001. Other tests related to the LOI (10 × 8 × 4 mm3) following ASTM D2863-2013, horizontal vertical combustion (120 × 13 × 1.6 mm3) following UL94: 2013, cone calorimeter method (100 × 100 × 4 mm3)
tests. A summary of the synthetic route is shown in Scheme 2, and the samples in terms of MCPO loadings are listed in Table 1.
2.2. Characterization TGA was performed using a Q50 thermal analyzer (America TA Instruments Co., Ltd). The samples (5 mg) were placed inside a Pt crucible and heated under nitrogen atmosphere in the temperature ranging from 25 to 600 °C at a heating rate of 10 °C/min following ASTM E2105-2016. The initial degradation temperature of PA66 36
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Fig. 3. The DSC curves of FR PA66（down）.
Fig. 4. The HRR curves of FR PA66.
following ASTM E1354, microphotography of PA66 composites and char layer were examined using a LOI apparatus (K-R2406S, Suzhou KaiTeEr Instrument Equipment Co., Ltd), horizontal vertical combustion apparatus (HVR-JT, Cixi LanTian Measure Equipment Co., Ltd), cone calorimeter (FTT0029, Universal Technology Co., Ltd) and scanning electron microscope (Sigma HDTM, Carl Zeiss AG Co., Ltd).
Table 3 The thermal stabilities of FR PA66. Samples
T5/oC
Tmax/oC
Tg/oC
Tm/oC
1 2 3 4 5
363 368 386 410 411
463 477 484 497 499
228 226 222 216 189
266 264 262 250 235
37
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estimated to be 36 °C larger than that of pure PA66 from 463 °C to 499 °C at 10 wt% MCPO. These observations suggested improved thermal stability of PA66 composites. The DTG curves of PA66 composites clearly indicated that the maximum exothermal peak of PA66 composites shifted to the right side (Fig. 2), demonstrating that the Tmax of PA66 composites was higher than that of the pristine PA66. At MCPO exceeding 8 wt%, the PA66 composites curves depicted two sharp peaks in the temperature ranges 400–450 °C and 450–500 °C, attributed to the heat decomposition of the P–N bonds in MCPO and ester bonds in PA66, respectively. Moreover, the strong conjugation effect of MCPO with rigid groups of numerous benzene rings needed to absorb more heat in the decomposition process [18]. Therefore, the thermal stability of PA66 was significantly improved by introducing the flame retardant MCPO. The DSC curves of PA66 composites obtained under nitrogen atmosphere are shown in Figs. 3 and 4. The incorporation of MCPO decreased the melting point (Tm) and glass transition temperature (Tg), probably due to the barrier effect of rigid benzene rings in the MCPO chains. With increasing number of benzene rings, the barrier effect obviously restricted the movement of the PA66 chains, thereby decreasing the rate of the crystallization of PA66 matrix [19]. Hence, the Tm and Tg, especially of 10 wt% MCPO flame retardant PA66 decreased. Table 3 lists the TGA and DSC data of PA66 composites. The TGA and DSC results indicate that although the T5 and Tmax increased to 411 °C and 499 °C, the Tm and Tg decreased distinctly by 31 °C and 39 °C, respectively, when MCPO contents reach to 10 wt%. In contrast, the T5, Tmax, Tm, and Tg of PA66 composites attain 410, 497, 216, and 250 °C with 8 wt% MCPO. Comparing pure PA66, the PA66 composites containing 8 wt% MCPO possess preferably integrated thermal stabilities.
Table 4 The cone calorimeter data details of FR PA66. Samples
HRR(kW/ m2)
THR(MJ/ m2)
TSP(m2/ kg)
COR( × 10−3 g/s)
ML(g)
1 2 3 4 5
624.9 614.6 559.7 494.1 429.0
94.5 87.3 82.3 73.5 60.2
8.94 8.59 8.12 7.67 7.43
6.81 6.44 6.27 5.95 5.83
1.64 1.72 1.92 2.01 2.20
3. Results and discussion 3.1. Flame retardant properties of PA66 composites Table 2 lists the results of LOI and UL94 with 1.6 mm thickness of PA66 composites, indicating that moderate MCPO contents (8 wt%) induced higher LOI value, and UL94 V-0 rating was achieved at 8 wt% MCPO. This phenomenon could be explained by the flame retarding effect of the PO• free radicals released from MCPO, promoting the crosslinking of PA66 and forming a char layer. In addition, the spread of PO• captured the small molecules of free radicals generated by the thermal decomposition of PA66 to inhibit the continuity of chain combustion reaction. In turn, this prevented the volatilization of the oligomers of PA66 and blocked the transfer of heat and flammable gas, which effectively reduced both the after flame time and flammability of melt drips [17]. 3.2. Thermal stability of PA66 composites The TGA and DTG curves of PA66 composites under nitrogen are shown in Figs. 1 and 2, respectively. As the content of MCPO increased, the T5 of PA66 composites increased than that of pure PA66 (Fig. 1). The T5 of PA66 composites was 411 °C, which was higher than that of pure PA66 (363 °C). Moreover, the Tmax of PA66 composites was
3.3. Heat release effect analysis Cone calorimetry is useful to evaluate the flame retardancy of
Fig. 5. The HRR curves of FR PA66. 38
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Fig. 6. The THR curves of FR PA66.
Fig. 7. The TSP curves of FR PA66.
60.2 MJ/m2, the TSP decreased 1.51 m2/kg obviously from 8.94 m2/kg to 7.43 m2/kg, the COR decreased 14.39% distinctly than that of pure PA66 (6.81 × 10−3 g/s). The HRR, THR, TSP, EHC, SEA, and ML profiles of PA66 composites are shown in Figs. 5–10, respectively. EHC is one of the parameters to measure the heat release of polymer, and is
polymers. The HRR, THR, TSP, ML, and COR data are listed in Table 4, indicating that the HRR, THR, TSP and COR decreased with increasing MCPO content. In the presence of 10 wt% MCPO in PA66 main chains, the peak of HRR declined gradually from 624.9 kW/m2 to 429.0 kW/ m2, the THR reduced from 94.5 MJ/m2 assigned to pure PA66 to 39
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Fig. 8. The EHC curves of FR PA66.
Fig. 9. The SEA curves of FR PA66.
decrease in the smoke production when the mass fraction of MCPO was increased step by step. Table 4 and Figs. 5–10 clearly indicate that the heat release of PA66 composites obviously improved by the addition of MCPO, and the
defined as the specific value between HRR and MLR. Fig. 8 shows the remarkable change that the fluctuation is dwindled gradually with increasing MCPO contents. SEA is another important argument to study the smoke release accompanied by mass loss. Fig. 9 reveals the sharp 40
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Fig. 10. The ML curves of FR PA66.
Fig. 11. The micro-morphology of char layer.
from the reaction between phosphoric acid derivatives with decomposition products of PA66, may insulate the underlying polymer substrate from the heat source and slow down both the heat and mass transfer.
significant decrease in the HRR, THR, TSP, EHC, SEA, and COR implies that the heat storage capacity of PA66 composites enhanced, and the heat release rate, production of heat and smoke release were limited during combustion, which seemed similar to other polyamide flame retardant composites [20]. This phenomenon may be attributed to the release of non-combustion gas NH3 [21], and the PO• free radicals [22–24] issued from MCPO in the PA66 matrix would dilute the combustion gas in the atmospheric environment, inducing the crosslinking of PA66 matrix. Moreover, the stable protective multilayer carbonaceous structure present on the surface of PA66 composites, produced
3.4. Micro-morphology analysis Fig. 11 shows smooth, uniform, and compact stack layers. Significant amounts of char were found in the sample generated from combustion, providing the evidence that poly-phosphoric acid was 41
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Fig. 12. The EDX test pictures of char layer. [3] Liu Y, Wang Q. Melamine cyanurate-microencapsulated red phosphorus flame retardant unreinforced and glass fiber reinforced polyamide 66. Polym Degrad Stabil 2006;91:3103–9. [4] Levchik SV, Costa L, Camino G. Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides: Part II-polyamide 6. Polym Degrad Stabil 1992;36:229–37. [5] Horrocks AR, Smart G, Kandola B, Holdsworth A, Price D. Zinc stannate interactions with flame retardants in polyamides; Part 1: synergies with organobromine- containing flame retardants in polyamides 6 (PA6) and 6.6 (PA6.6). Polym Degrad Stabil 2012;97:2503–10. [6] Levchik SV, Costa L, Camino G. Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides. I. Polyamides 11 and 12. Polym Degrad Stabil 1992;36:31–41. [7] Jou WS, Chen KN, Chao DY, Lin CY, TYeh J. Flame retardant and dielectric properties of glass fibre reinforced nylon-66 filled with red phosphorous. Polym Degrad Stabil 2001;74:239–45. [8] Gijsman P, Steenbakkers R, Fürst C, Kersjes J. Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66. Polym Degrad Stabil 2002;78:219–24. [9] Levchik SV, Costa L, Camino G. Effect of the fire-retardant ammonium polyphosphate on the thermal decomposition of aliphatic polyamides. Part III—polyamides 6.6 and 6.10. Polym Degrad Stabil 1994;43:43–54. [10] Enescu D, Frache A, Lavaselli M, Monticelli O, Marino F. Novel phosphorous–nitrogen intumescent flame retardant system. Its effects on flame retardancy and thermal properties of polypropylene. Polym Degrad Stabil 2013;98:297–305. [11] Patrick Lim WK, Mariatti M, Chow WS, Chow WS, Mar KT. Effect of intumescent ammonium polyphosphate (APP) and melamine cyanurate (MC) on the properties of epoxy/glass fiber composites. Compos B Eng 2012;43(2):124–8. [12] Chen XL, Jiao CM. Synergistic effects of hydroxy silicone oil on intumescent flame retardant polypropylene system. Fire Saf J 2009;44:1010–4. [13] Gao F, Tong LF, Fang ZP. Effect of a novel phosphorous–nitrogen containing intumescent flame retardant on the fire retardancy and the thermal behaviour of poly (butylene terephthalate). Polym Degrad Stabil 2006;91:1295–9. [14] Lim KS, Bee ST, Sin LT, Tee TT, Ratnam CT, Hui D, Rahmat AR. A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites. Compos B Eng 2016;84:155–74. [15] Sahyoun J, Bounor-Legaré V, Sonnier L Ferry R, Sonnier R, Cruz-Boisson FDa, Melis F, Bonhommé A, Cassagnau P. Synthesis of a new organophosphorous alkoxysilane precursor and its effect on the thermal and fire behavior of a PA66/PA6 copolymer. Eur Polym J 2015;66:352–66. [16] W. Y. Lyu, Y. H. Cui, X.J. Zhang, J.Y. Yuan, W. Zhang Synthesis, Thermal stability and flame retardancy of PA66, treated with dichlorophenyl phosphine derivatives, Des Monomers Polym, DOI: 10.1080/15685551.2016.1169375. [17] Zhan ZS, Xu MJ, Li B. Synergistic effects of sepiolite on the flame retardant properties and thermal degradation behaviors of polyamide 66/aluminum diethylphosphinate composites. Polym Degrad Stabil 2015;117:66–74. [18] Cai YB, Wu N, Wei QF, Zhang K, Xu QX, Gao WD, Song L, Hu Y. Structure, surface morphology, thermal and flammability characterizations of polyamide6/organicmodified Fe-montmorillonite nanocomposite fibers functionalized by sputter coating of silicon. Surf Coating Technol 2008;203:264–70. [19] Lin ZD, Guan ZX, Xu BF, Chen C, Guo GH, Zhou JX, Xian JM, Cao L, Wang YL, Li MQ, Li W. Crystallization and melting behavior of polypropylene in β-PP/polyamide 6 blends containing PP-g-MA. J Ind Eng Chem 2013;19:692–7. [20] Guo WW, Yu B, Yuan Y, Song L, Hu Y. In situ preparation of reduced graphene oxide/DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites. Compos B Eng 2017;123:154–64. [21] Lv P, Wang ZZ, Hu KL, Fan WC. Flammability and thermal degradation of flame retarded polypropylene composites containing melamine phosphate and pentaerythritol derivatives. Polym Degrad Stabil 2005;90:523–34. [22] Qiu SL, Ma C, Wang X, X Zhou, Feng X,M, Yuen Richard KK, Hu Y. Melaminecontaining polyphosphazene wrapped ammonium polyphosphate: a novel multifunctional organic-inorganic hybrid flame retardant. J Hazard, Mater 2018;344:839–48. [23] Zhao JJ, Dong X, Huang S, Tian XJ, Song L, Yu Q, Wang ZW. Performance comparison of flame retardant epoxy resins modified by DPO–PHE and DOPO–PHE. Polym Degrad Stabil 2018;156:89–99. [24] Chang QF, Long LJ, He WT, Qin SH, Yu J. Thermal degradation behavior of PLA composites containing bis DOPO phosphonates. Thermochim Acta 2016;639:84–90.
produced from MCPO during the thermal degradation, and it functioned as a dehydration agent and driving force responsible for heatresistant char layer during the carbonization process. The char layer could prevent the heat transmission and diffusion, reduce HRR, and limit the production of combustible gases. Moreover, some micro holes were observed on the char layer surface, as illustrated in Fig. 11(b). This could be due to cracks in the P–N bonds, producing nonflammable NH3. Some studies suggested that the blowing agent formed by nitrogen elements could induce honeycomb-like structure in the char layer. This, in turn, isolated the transfer of combustible gases into the material inner layers [25]. Interestingly, the well-ordered honeycomb structures were observed, as shown in Fig. 11(d), probably attributed to the gas held by the char layer, and heat transmission could be delayed by the honeycomb structures. The mechanism of phosphorus-nitrogen containing flame retardant suggested that the formation of phosphoric acid could act as catalyst in the dehydration of the polymer at temperatures before decomposition [26]. The dehydration process yielded thermally stable double bonds in the polymer chain and produced significant amounts of char depending on the P elements [27]. In addition, the EDX profiles also verified the presence of P derivatives (Fig. 12). Homogeneously distributed phosphorus atoms were observed on the char layer surface after the carbonization process. Consequently, the surface was overlaid by the nonperfect combustion of char. 4. Conclusions In conclusion, the flame retardant and thermal properties of PA66 improved significantly by adding MCPO. The V-0 rating and 29% (LOI) of PA66 composites were achieved following the UL 94 and LOI test results, respectively. The TGA results indicated that the initial and peak decomposition temperature of PA66 composites increased by 47 °C and 34 °C in the presence of 8 wt% MCPO, respectively. Moreover, the heat release rate and production of smoke decreased, as observed by the HRR and TSP results, and gas phase flame retardant mechanism was confirmed by the cone calorimetry and SEM analyses due to the release of NH3 and PO• from MCPO. Acknowledgments This study was supported by the Scientific Enablement Research Funding of ChongQing University of Technology (2017ZD02) and the key development projects of technology innovation and industry application demonstration of Chongqing (cstc2017zdcy-zdyf0046, cstc2018jszx-cyzd0398). References [1] Bellenger V, Tcharkhtchi A, Castaing Ph. Thermal and mechanical fatigue of a PA66/glass fibers composite material. Int J Fatig 2006;28:1348–52. [2] Timmaraju MV, Gnanamoorthy R, Kannan K. Influence of imbibed moisture and organoclay on tensile and indentation behavior of polyamide 66/hectorite nanocomposites. Compos B Eng 2011;42:466–72.
42
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W. Lyu et al. [25] Liu Y, Zhang Y, Cao ZH, Fang ZP. Synthesis and performance of three flame retardant additives containing diethyl phosphite/phenyl phosphonicmoieties. Fire Saf J 2013;61:185–92. [26] Yang S, Wang J, Huo SQ, Cheng LF, Wang M. Preparation and flame retarded of an intumescent flame-retardant epoxy resin system constructed by multiple flame
retardant compositions containing phosphorus and nitrogen heterocycle. Polym Degrad Stabil 2015;119:251–9. [27] Zhang X, Li YB, Zuo Y, Lv GY, Mu YH, Li H. Morphology, hydrogen-bonding and crystallinity of nano-hydroxyapatite/polyamide 66 biocomposites. Compos. Part. AAppl. S 2007;38:843–8.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Thermal stability and heat release effect of flame retarded PA66 prepared by end-pieces capping technology
T
WenYan Lyu∗, XiuMin Chen, YouBing Li, Shuang Cao, YiMing Han College of Materials Science and Technology, ChongQing University of Technology, 69 Hongguang Avennue BaNan District, ChongQing, 400054, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PA66 Thermal properties Heat release effect End-pieces capping
In order to improve the flame retardancy of PA66, bis-N-benzoguanamine-phenylphos-phamide (MCPO) was grafted to PA66 main chains by end-pieces capping technology. The thermal properties and heat release effect of flame retardant PA66 (FR PA66) composites were investigated by thermogravimetry, differential scanning calorimetry, and cone calorimetry test analyses. In the presence of 8 wt% MCPO in PA66 composites, the initial decomposition temperature (T5) and peak decomposition temperature (Tmax) of PA66 composites were 47 °C and 34 °C higher than that of pristine PA66, respectively. Compared to the traditional flame retardant PA66 composites, the heat release rate, total heat release, total smoke production, and CO release rate significantly reduced by 31.3%, 36.3%, 16.9%, and 14.7% by introducing P containing benzoguanamine derivative, respectively. Moreover, the vertical flame spread test illustrated that the melt dripping of PA66 stopped for higher precursor concentrations in the composites carbonized char, and the limiting oxygen index (LOI) value of PA66 composites reached 29% as evidenced by the LOI test.
1. Introduction As one of the major engineering plastics, polyamide 66 (PA66) featured with excellent thermal stability, weather-shield durability, chemical resistance, high mechanical performance, and easy processing is increasingly used in mechanical and/or structural applications, such as stressed functional automotive parts, bearing structural parts of aeronautics and transportation, packaging materials in electric and electronic industries, and safety parts in sports and leisure [1]. However, the applications of PA66 are highly limited due to its water-absorbing property [2] and flammability of virgin PA66 [3]. Thus, in order to improve the poor flame resistance and enhance the application fields of PA66, flame retardant PA66 composites have been prepared by adding various flame retardants [4–6] such as decabromodiphenyl ether, melamine cyanurate, and polyphosphates. Because the release of toxic and corrosive chemicals during combustion could lead to environmental and health issues, phosphorus- and nitrogen-containing compounds such red phosphorus [7] and melamine cyanurate [8] are often used to replace halogen-based compounds in flame retardant composites. In view of this, a number of studies have been reported on the flame retardancy of PA66 using red phosphorus and phosphorus or nitrogen-containing compounds. For instance, Levchik et al. [9] thoroughly investigated the decomposition and flame
∗
retardancy of polyamides, concluding that phosphate containing compounds play an important role in improving the thermal stability and flame retardancy of polyamides. Overall, these studies suggested that conventional phosphate flame retardants suffer from a few limitations, including achieving higher flame retarding levels at the expense of lower mechanical properties of polyamide materials. In recent years, intumescent flame retardants (IFRs) have been widely used in flammable polymers [10]. Compared to conventional flame retardants, IFRs have several satisfactory advantages such as high efficiency, environmentally friendly, and halogen-free characteristics [11]. Many studies [12,13] reported IFRs to be a mixture of three components, namely, an acid source, a carbon source, and a blowing agent. For instance, Lim et al. [14] introduced in detail about the application of ammonium polyphosphate (APP) as IFR in thermoplastic composites. It was suggested that thermal properties, such as limiting oxygen index (LOI) value together with cone calorimetry and thermogravimetric analysis (TGA) performance could be enhanced by the optimum combination of APP, PER and melamine which function as intumescent flame retardant. Furthermore, Sahyoun et al. [15] reported a new PA66 copolymer modified by silicophosphorylated filler aiming to improve the fire behavior of the matrix. The results show that the nanocomposite exhibits a slightly earlier ignition but the peak of heat release rate (pHRR)
Corresponding author. E-mail address: [email protected] (W. Lyu).
https://doi.org/10.1016/j.compositesb.2018.12.016 Received 22 July 2018; Received in revised form 14 November 2018; Accepted 3 December 2018 Available online 05 December 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
Composites Part B 167 (2019) 34–43
W. Lyu et al.
Scheme 1. The synthetic routes of MCPO.
Scheme 2. The end capping route of flame retardancy PA66.
thermal stability, flame retardancy, and heat release effect of the resulting PA66 composites were investigated, and the results are discussed. The MCPO used in the preparation process was synthesized following our previously reported procedure [16]. MCPO contains three benzene rings, two triazine rings, one P atom, and dual activity endaminos in a molecule. The initial decomposition temperature of MCPO can reach to 300 °C, as indicated by the TGA.
Table 1 The MCPO weight loading of PA66 composites. Samples
PA66/g
MCPO/g
1 2 3 4 5
1000 920 940 920 900
0 40 60 80 100
2. Experimental Table 2 The fame retardancy of FR PA66.
2.1. Materials
Samples
Limit oxygen index
UL94 1.6 mm rating
Cotton indicator ignited by melt drips
Self-extinguish time/s
1 2 3 4 5
21 24 25 27 29
V-2 V-2 V-1 V-0 V-0
Yes Yes Yes No No drips
4 9 13 10 8
The starting materials and solvents were all of commercial grade and used without further purification. Industrial reagent grade MCPO was synthesized in our laboratory. The synthetic route is shown in Scheme 1. PA66 (101 L) was purchased from Dowdupont Co., Ltd. 2.1.1. Preparation of PA66 composites Prior to injection molding, PA66 masterbatches and MCPO were dried at 100 °C for 24 h. PA66 composites were prepared by melt blending using a SHJ-35 twin screw extruder (NanKing ChengMeng Machinery) in the temperatures ranging from 260 to 270 °C. Because of a mass of the end -COOH groups on the PA66 main chains, the -NH2 groups of MCPO could take place in the dehydration reaction with the -OH bonds of the carboxylic acid. The effective flame retardancy elements, benzene rings, P, and triazine rings can be directly and effectively grafted to the tail end of PA66 main chains. Then, the PA66 composites pellets were mold injected using a TX-CA1 machine (HaiTian Plastic Machinery) in the temperature range 260–265 °C to form specimens with different sizes and shapes suitable for various
decreased by approximately 33%. The presence of low concentration of silicon (0.65 wt%) and phosphorus (0.75 wt%) promotes the formation of an expanded char layer that acts as a barrier. A reduction in approximately 18% effective heat of combustion (EHC) value suggests an additional flame inhibition effect of phosphorus in the gas phase. In this study, in an effort to improve the flame retardancy of PA66, FR PA66 was prepared from PA66 and MCPO through blending extrusion by end capping technology due to a wide distribution of active carboxylic acid groups in the terminal pieces of PA66 main chains. The 35
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Fig. 1. The TGA curves of FR PA66.
Fig. 2. The DTG curves of FR PA66.
composites (5 wt% weight loss) and the temperature of the maximum weight loss are denoted as T5 and Tmax, respectively. Differential scanning calorimetry (DSC) investigation was carried out using a Q20 universal testing machine (America TA Instruments Co., Ltd). The samples (3 mg) were placed inside an Al crucible at 25 °C and the measurements were carried out at the heating rate of 10 °C/min following ASTM E794-2001. Other tests related to the LOI (10 × 8 × 4 mm3) following ASTM D2863-2013, horizontal vertical combustion (120 × 13 × 1.6 mm3) following UL94: 2013, cone calorimeter method (100 × 100 × 4 mm3)
tests. A summary of the synthetic route is shown in Scheme 2, and the samples in terms of MCPO loadings are listed in Table 1.
2.2. Characterization TGA was performed using a Q50 thermal analyzer (America TA Instruments Co., Ltd). The samples (5 mg) were placed inside a Pt crucible and heated under nitrogen atmosphere in the temperature ranging from 25 to 600 °C at a heating rate of 10 °C/min following ASTM E2105-2016. The initial degradation temperature of PA66 36
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Fig. 3. The DSC curves of FR PA66（down）.
Fig. 4. The HRR curves of FR PA66.
following ASTM E1354, microphotography of PA66 composites and char layer were examined using a LOI apparatus (K-R2406S, Suzhou KaiTeEr Instrument Equipment Co., Ltd), horizontal vertical combustion apparatus (HVR-JT, Cixi LanTian Measure Equipment Co., Ltd), cone calorimeter (FTT0029, Universal Technology Co., Ltd) and scanning electron microscope (Sigma HDTM, Carl Zeiss AG Co., Ltd).
Table 3 The thermal stabilities of FR PA66. Samples
T5/oC
Tmax/oC
Tg/oC
Tm/oC
1 2 3 4 5
363 368 386 410 411
463 477 484 497 499
228 226 222 216 189
266 264 262 250 235
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estimated to be 36 °C larger than that of pure PA66 from 463 °C to 499 °C at 10 wt% MCPO. These observations suggested improved thermal stability of PA66 composites. The DTG curves of PA66 composites clearly indicated that the maximum exothermal peak of PA66 composites shifted to the right side (Fig. 2), demonstrating that the Tmax of PA66 composites was higher than that of the pristine PA66. At MCPO exceeding 8 wt%, the PA66 composites curves depicted two sharp peaks in the temperature ranges 400–450 °C and 450–500 °C, attributed to the heat decomposition of the P–N bonds in MCPO and ester bonds in PA66, respectively. Moreover, the strong conjugation effect of MCPO with rigid groups of numerous benzene rings needed to absorb more heat in the decomposition process [18]. Therefore, the thermal stability of PA66 was significantly improved by introducing the flame retardant MCPO. The DSC curves of PA66 composites obtained under nitrogen atmosphere are shown in Figs. 3 and 4. The incorporation of MCPO decreased the melting point (Tm) and glass transition temperature (Tg), probably due to the barrier effect of rigid benzene rings in the MCPO chains. With increasing number of benzene rings, the barrier effect obviously restricted the movement of the PA66 chains, thereby decreasing the rate of the crystallization of PA66 matrix [19]. Hence, the Tm and Tg, especially of 10 wt% MCPO flame retardant PA66 decreased. Table 3 lists the TGA and DSC data of PA66 composites. The TGA and DSC results indicate that although the T5 and Tmax increased to 411 °C and 499 °C, the Tm and Tg decreased distinctly by 31 °C and 39 °C, respectively, when MCPO contents reach to 10 wt%. In contrast, the T5, Tmax, Tm, and Tg of PA66 composites attain 410, 497, 216, and 250 °C with 8 wt% MCPO. Comparing pure PA66, the PA66 composites containing 8 wt% MCPO possess preferably integrated thermal stabilities.
Table 4 The cone calorimeter data details of FR PA66. Samples
HRR(kW/ m2)
THR(MJ/ m2)
TSP(m2/ kg)
COR( × 10−3 g/s)
ML(g)
1 2 3 4 5
624.9 614.6 559.7 494.1 429.0
94.5 87.3 82.3 73.5 60.2
8.94 8.59 8.12 7.67 7.43
6.81 6.44 6.27 5.95 5.83
1.64 1.72 1.92 2.01 2.20
3. Results and discussion 3.1. Flame retardant properties of PA66 composites Table 2 lists the results of LOI and UL94 with 1.6 mm thickness of PA66 composites, indicating that moderate MCPO contents (8 wt%) induced higher LOI value, and UL94 V-0 rating was achieved at 8 wt% MCPO. This phenomenon could be explained by the flame retarding effect of the PO• free radicals released from MCPO, promoting the crosslinking of PA66 and forming a char layer. In addition, the spread of PO• captured the small molecules of free radicals generated by the thermal decomposition of PA66 to inhibit the continuity of chain combustion reaction. In turn, this prevented the volatilization of the oligomers of PA66 and blocked the transfer of heat and flammable gas, which effectively reduced both the after flame time and flammability of melt drips [17]. 3.2. Thermal stability of PA66 composites The TGA and DTG curves of PA66 composites under nitrogen are shown in Figs. 1 and 2, respectively. As the content of MCPO increased, the T5 of PA66 composites increased than that of pure PA66 (Fig. 1). The T5 of PA66 composites was 411 °C, which was higher than that of pure PA66 (363 °C). Moreover, the Tmax of PA66 composites was
3.3. Heat release effect analysis Cone calorimetry is useful to evaluate the flame retardancy of
Fig. 5. The HRR curves of FR PA66. 38
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Fig. 6. The THR curves of FR PA66.
Fig. 7. The TSP curves of FR PA66.
60.2 MJ/m2, the TSP decreased 1.51 m2/kg obviously from 8.94 m2/kg to 7.43 m2/kg, the COR decreased 14.39% distinctly than that of pure PA66 (6.81 × 10−3 g/s). The HRR, THR, TSP, EHC, SEA, and ML profiles of PA66 composites are shown in Figs. 5–10, respectively. EHC is one of the parameters to measure the heat release of polymer, and is
polymers. The HRR, THR, TSP, ML, and COR data are listed in Table 4, indicating that the HRR, THR, TSP and COR decreased with increasing MCPO content. In the presence of 10 wt% MCPO in PA66 main chains, the peak of HRR declined gradually from 624.9 kW/m2 to 429.0 kW/ m2, the THR reduced from 94.5 MJ/m2 assigned to pure PA66 to 39
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Fig. 8. The EHC curves of FR PA66.
Fig. 9. The SEA curves of FR PA66.
decrease in the smoke production when the mass fraction of MCPO was increased step by step. Table 4 and Figs. 5–10 clearly indicate that the heat release of PA66 composites obviously improved by the addition of MCPO, and the
defined as the specific value between HRR and MLR. Fig. 8 shows the remarkable change that the fluctuation is dwindled gradually with increasing MCPO contents. SEA is another important argument to study the smoke release accompanied by mass loss. Fig. 9 reveals the sharp 40
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Fig. 10. The ML curves of FR PA66.
Fig. 11. The micro-morphology of char layer.
from the reaction between phosphoric acid derivatives with decomposition products of PA66, may insulate the underlying polymer substrate from the heat source and slow down both the heat and mass transfer.
significant decrease in the HRR, THR, TSP, EHC, SEA, and COR implies that the heat storage capacity of PA66 composites enhanced, and the heat release rate, production of heat and smoke release were limited during combustion, which seemed similar to other polyamide flame retardant composites [20]. This phenomenon may be attributed to the release of non-combustion gas NH3 [21], and the PO• free radicals [22–24] issued from MCPO in the PA66 matrix would dilute the combustion gas in the atmospheric environment, inducing the crosslinking of PA66 matrix. Moreover, the stable protective multilayer carbonaceous structure present on the surface of PA66 composites, produced
3.4. Micro-morphology analysis Fig. 11 shows smooth, uniform, and compact stack layers. Significant amounts of char were found in the sample generated from combustion, providing the evidence that poly-phosphoric acid was 41
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Fig. 12. The EDX test pictures of char layer. [3] Liu Y, Wang Q. Melamine cyanurate-microencapsulated red phosphorus flame retardant unreinforced and glass fiber reinforced polyamide 66. Polym Degrad Stabil 2006;91:3103–9. [4] Levchik SV, Costa L, Camino G. Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides: Part II-polyamide 6. Polym Degrad Stabil 1992;36:229–37. [5] Horrocks AR, Smart G, Kandola B, Holdsworth A, Price D. Zinc stannate interactions with flame retardants in polyamides; Part 1: synergies with organobromine- containing flame retardants in polyamides 6 (PA6) and 6.6 (PA6.6). Polym Degrad Stabil 2012;97:2503–10. [6] Levchik SV, Costa L, Camino G. Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides. I. Polyamides 11 and 12. Polym Degrad Stabil 1992;36:31–41. [7] Jou WS, Chen KN, Chao DY, Lin CY, TYeh J. Flame retardant and dielectric properties of glass fibre reinforced nylon-66 filled with red phosphorous. Polym Degrad Stabil 2001;74:239–45. [8] Gijsman P, Steenbakkers R, Fürst C, Kersjes J. Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66. Polym Degrad Stabil 2002;78:219–24. [9] Levchik SV, Costa L, Camino G. Effect of the fire-retardant ammonium polyphosphate on the thermal decomposition of aliphatic polyamides. Part III—polyamides 6.6 and 6.10. Polym Degrad Stabil 1994;43:43–54. [10] Enescu D, Frache A, Lavaselli M, Monticelli O, Marino F. Novel phosphorous–nitrogen intumescent flame retardant system. Its effects on flame retardancy and thermal properties of polypropylene. Polym Degrad Stabil 2013;98:297–305. [11] Patrick Lim WK, Mariatti M, Chow WS, Chow WS, Mar KT. Effect of intumescent ammonium polyphosphate (APP) and melamine cyanurate (MC) on the properties of epoxy/glass fiber composites. Compos B Eng 2012;43(2):124–8. [12] Chen XL, Jiao CM. Synergistic effects of hydroxy silicone oil on intumescent flame retardant polypropylene system. Fire Saf J 2009;44:1010–4. [13] Gao F, Tong LF, Fang ZP. Effect of a novel phosphorous–nitrogen containing intumescent flame retardant on the fire retardancy and the thermal behaviour of poly (butylene terephthalate). Polym Degrad Stabil 2006;91:1295–9. [14] Lim KS, Bee ST, Sin LT, Tee TT, Ratnam CT, Hui D, Rahmat AR. A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites. Compos B Eng 2016;84:155–74. [15] Sahyoun J, Bounor-Legaré V, Sonnier L Ferry R, Sonnier R, Cruz-Boisson FDa, Melis F, Bonhommé A, Cassagnau P. Synthesis of a new organophosphorous alkoxysilane precursor and its effect on the thermal and fire behavior of a PA66/PA6 copolymer. Eur Polym J 2015;66:352–66. [16] W. Y. Lyu, Y. H. Cui, X.J. Zhang, J.Y. Yuan, W. Zhang Synthesis, Thermal stability and flame retardancy of PA66, treated with dichlorophenyl phosphine derivatives, Des Monomers Polym, DOI: 10.1080/15685551.2016.1169375. [17] Zhan ZS, Xu MJ, Li B. Synergistic effects of sepiolite on the flame retardant properties and thermal degradation behaviors of polyamide 66/aluminum diethylphosphinate composites. Polym Degrad Stabil 2015;117:66–74. [18] Cai YB, Wu N, Wei QF, Zhang K, Xu QX, Gao WD, Song L, Hu Y. Structure, surface morphology, thermal and flammability characterizations of polyamide6/organicmodified Fe-montmorillonite nanocomposite fibers functionalized by sputter coating of silicon. Surf Coating Technol 2008;203:264–70. [19] Lin ZD, Guan ZX, Xu BF, Chen C, Guo GH, Zhou JX, Xian JM, Cao L, Wang YL, Li MQ, Li W. Crystallization and melting behavior of polypropylene in β-PP/polyamide 6 blends containing PP-g-MA. J Ind Eng Chem 2013;19:692–7. [20] Guo WW, Yu B, Yuan Y, Song L, Hu Y. In situ preparation of reduced graphene oxide/DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites. Compos B Eng 2017;123:154–64. [21] Lv P, Wang ZZ, Hu KL, Fan WC. Flammability and thermal degradation of flame retarded polypropylene composites containing melamine phosphate and pentaerythritol derivatives. Polym Degrad Stabil 2005;90:523–34. [22] Qiu SL, Ma C, Wang X, X Zhou, Feng X,M, Yuen Richard KK, Hu Y. Melaminecontaining polyphosphazene wrapped ammonium polyphosphate: a novel multifunctional organic-inorganic hybrid flame retardant. J Hazard, Mater 2018;344:839–48. [23] Zhao JJ, Dong X, Huang S, Tian XJ, Song L, Yu Q, Wang ZW. Performance comparison of flame retardant epoxy resins modified by DPO–PHE and DOPO–PHE. Polym Degrad Stabil 2018;156:89–99. [24] Chang QF, Long LJ, He WT, Qin SH, Yu J. Thermal degradation behavior of PLA composites containing bis DOPO phosphonates. Thermochim Acta 2016;639:84–90.
produced from MCPO during the thermal degradation, and it functioned as a dehydration agent and driving force responsible for heatresistant char layer during the carbonization process. The char layer could prevent the heat transmission and diffusion, reduce HRR, and limit the production of combustible gases. Moreover, some micro holes were observed on the char layer surface, as illustrated in Fig. 11(b). This could be due to cracks in the P–N bonds, producing nonflammable NH3. Some studies suggested that the blowing agent formed by nitrogen elements could induce honeycomb-like structure in the char layer. This, in turn, isolated the transfer of combustible gases into the material inner layers [25]. Interestingly, the well-ordered honeycomb structures were observed, as shown in Fig. 11(d), probably attributed to the gas held by the char layer, and heat transmission could be delayed by the honeycomb structures. The mechanism of phosphorus-nitrogen containing flame retardant suggested that the formation of phosphoric acid could act as catalyst in the dehydration of the polymer at temperatures before decomposition [26]. The dehydration process yielded thermally stable double bonds in the polymer chain and produced significant amounts of char depending on the P elements [27]. In addition, the EDX profiles also verified the presence of P derivatives (Fig. 12). Homogeneously distributed phosphorus atoms were observed on the char layer surface after the carbonization process. Consequently, the surface was overlaid by the nonperfect combustion of char. 4. Conclusions In conclusion, the flame retardant and thermal properties of PA66 improved significantly by adding MCPO. The V-0 rating and 29% (LOI) of PA66 composites were achieved following the UL 94 and LOI test results, respectively. The TGA results indicated that the initial and peak decomposition temperature of PA66 composites increased by 47 °C and 34 °C in the presence of 8 wt% MCPO, respectively. Moreover, the heat release rate and production of smoke decreased, as observed by the HRR and TSP results, and gas phase flame retardant mechanism was confirmed by the cone calorimetry and SEM analyses due to the release of NH3 and PO• from MCPO. Acknowledgments This study was supported by the Scientific Enablement Research Funding of ChongQing University of Technology (2017ZD02) and the key development projects of technology innovation and industry application demonstration of Chongqing (cstc2017zdcy-zdyf0046, cstc2018jszx-cyzd0398). References [1] Bellenger V, Tcharkhtchi A, Castaing Ph. Thermal and mechanical fatigue of a PA66/glass fibers composite material. Int J Fatig 2006;28:1348–52. [2] Timmaraju MV, Gnanamoorthy R, Kannan K. Influence of imbibed moisture and organoclay on tensile and indentation behavior of polyamide 66/hectorite nanocomposites. Compos B Eng 2011;42:466–72.
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W. Lyu et al. [25] Liu Y, Zhang Y, Cao ZH, Fang ZP. Synthesis and performance of three flame retardant additives containing diethyl phosphite/phenyl phosphonicmoieties. Fire Saf J 2013;61:185–92. [26] Yang S, Wang J, Huo SQ, Cheng LF, Wang M. Preparation and flame retarded of an intumescent flame-retardant epoxy resin system constructed by multiple flame
retardant compositions containing phosphorus and nitrogen heterocycle. Polym Degrad Stabil 2015;119:251–9. [27] Zhang X, Li YB, Zuo Y, Lv GY, Mu YH, Li H. Morphology, hydrogen-bonding and crystallinity of nano-hydroxyapatite/polyamide 66 biocomposites. Compos. Part. AAppl. S 2007;38:843–8.
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