An inherently flame-retardant polyamide containing a phosphorus pendent group prepared by interfacial polymerization

Abstract

Inherent flame retardation has the advantage that it will allow polymers to impart the flame retardancy permanently, and the introduction of even a few weight percent of the flame retardant unit can lead to remarkable improvements in the overall flame retardancy. In this paper, DDP was acyl-chlorinated as the monomer to synthesise the inherent flame-retardant polyamide by interfacial polymerization. Then, the chemical structures, crystalline structures, molecular weight, thermal properties, thermal stabilities and flame retardancy of the resultant polyamides were studied. The results show that the inherent flame-retardant polyamide has good thermal stability, T_d of the samples were all above 310 °C. Furthermore, the incorporation of DDP, just 7.5 mol%, into the molecular chain can significantly improve the flame retardancy of polyamide, causing the pHRR value to be reduced by 37.8%.

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1. Introduction

Polyamides are an important class of engineering thermoplastics that exhibit useful properties such as high thermal and oxidative stability, chemical resistance, excellent mechanical properties and relatively low dielectric constant.^1–3 The demand for polyamides is growing steadily due to their superior performance characteristics that are increasingly expected in aerospace, microelectronics, automobile and other industries.^4–8 However, the application of polyamides is still restricted in many territories where fire safety is important because of its inadequate flame retardance.^9,10 As the result, the improvement of the flame retardance of polyamides has gained much attention in the last decades.

Traditionally, the halogen-based flame retardants comprise the most widely applied method due to the advantages of high efficiency and low cost. Nevertheless, they can emanate toxic and potentially carcinogenic dioxins and furans along with obscuring smoke during combustion, which pollute environment, compromise human health and damage biota. So, the development of environmentally friendly flame retardants has become an important topic in both academic and industrial communities.

Among the hopeful alternative flame retardants, the phosphorus-based compounds are believed to be the most promising candidate owing to their multifold advantages including notable high-efficiency flame resistance, less toxic gases and smoke generation when burning.^11,12 And, they can be divided into two major categories. Additive flame retardants, which are the most widely used, always exhibit heterogeneous dispersion and poor compatibility in the matrix. Reactive flame retardants have the advantage that they are covalently incorporated into the polymeric system, which allows them to impart the flame retardancy permanently.^13,14 Moreover, the introduction of even a few weight percent of the flame retardant unit can lead to remarkable improvements in the overall flame retardancy of the polymer.^15–17

The inherent flame-retardant polyamide was mostly synthesized by melt polycondensation, which need high temperature and vacuum degree. Nevertheless, interfacial polymerization is a facile approach to prepare polyamide,^18–20 which is widely employed for membranes.^21–24 The polymerization occurs simultaneously at the interface between two immiscible solvents. Typically, an amine is dissolved in an aqueous phase and an acyl chloride is dissolved in an organic phase. So, incorporation of the flame-retardant monomer into the skeleton of polyamide by interfacial polymerization is an efficient method.

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives are an attractive phosphorus-based flame retardant for polyamide, which are known to predominantly act by a gas-phase mechanism through the formation of PO· radicals.^25,26 At present, some researches have been reported that 9,10-dihydro-10[2,3-di(hydroxycarbony)propyl]-10-phosphaphenanthrene-10-oxide (DDP), which is a phosphorus-containing diacid monomer, can be used as the flame-retardant monomer to improve the flame retardancy of polyamide and polyester.^27–29 Therefore, DDP can be applied for interfacial polymerization by acyl chlorination.

In the present paper, DDP was acyl-chlorinated by thionyl chloride to synthetise the flame-retardant monomer (DDP–COCl, Scheme 1). Then, the inherent flame-retardant polyamide was synthesized by using hexamethylene diamine in the aqueous phase and DDP–COCl and adipoyl chloride mixture in the organic phase through interfacial polymerization (Scheme 2). In addition, the chemical structures, crystalline structures, molecular weight, thermal properties, thermal stabilities and flame retardancy of the resulted polyamides were studied.

Scheme 1 Synthesis route of DDP–COCl.

Scheme 2 Synthesis route of PA-1 (a) and PA-2–PA-5 (b) by interfacial polymerization.

2. Experimental

2.1. Materials

9,10-Dihydro-10[2,3-di(hydroxycarbony)propyl]-10-phosphaphenanthrene-10-oxide (DDP) was purchased from Zhejiang Chemical Industry Research Institute Co., Ltd. (Hangzhou, China). Adipoyl chloride (AC) was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Hexamethylene diamine (HMDA), thionyl chloride (SOCl₂), N,N-dimethylformamide (DMF), sodium hydroxide (NaOH), dichloromethane (DCM) and formic acid (FA) were totally supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Acyl chlorination of DDP

A mixture of DDP (8.65 g, 0.025 mol), SOCl₂ (70 mL) and DMF (5 drops) was charged into a 250 mL round bottom flask. After that, the mixture was refluxed in a N₂ atmosphere at 70 °C for 4 h, and the residual SOCl₂ was removed by distillation. Finally, the obtained product, DDP–COCl, was dried at 80 °C under vacuum for 12 h.

2.3. Synthesis of polyamides

AC and DDP–COCl was dissolved in 50 mL of DCM, and HDMA and NaOH was dissolved in 50 mL of deionized water (Scheme 3). The detailed formulations of the samples are given in Table 1. Then, the aqueous solution was slowly poured into the organic solution and a thin white layer appeared on the interface. The white layer was drawn out to make the reaction complete. The collected polymer was dissolved in FA solution, then NaOH solution was poured into FA solution to make polymer precipitate and removal the unreacted monomer. At last, the obtained polyamide was filtered, washed with deionized water, and dried at 100 °C.

Scheme 3 Schematic diagram of interfacial polymerization for synthesis of polyamide.

Table 1 Formulation, intrinsic viscosity ([η]) and molecular weight of PA-1–PA-5

Sample	AC (mol%)	DDP–OCl (mol%)	HMDA (mol%)	Content of phosphorus (%)		[η] (dL g^−1)	Molecular weight (10^4 g mol^−1)	LOI (%)	UL-94 (3.2 mm)
				P^a	P^b
a Theoretical phosphorus content of the resulting polymer.b Experimental phosphorus content of the resulting polymer; calculated by ^1H-NMR.
PA-1	50.0	—	50.0	—	—	0.96	2.4	21.0	No rating
PA-2	47.5	2.5	50.0	0.55	0.50	0.84	2.1	25.0	V-2
PA-3	45.0	5.0	50.0	1.10	0.81	0.54	1.3	26.5	V-2
PA-4	42.5	7.5	50.0	1.65	1.13	0.51	1.3	28.0	V-2
PA-5	40.0	10.0	50.0	2.20	1.57	0.49	1.2	29.5	V-1

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2.4. Measurements

Fourier transform infrared analysis (FTIR). The FTIR spectra were recorded with MAGNA-IR 7500 spectrometer (Nicolet Instrument Company, USA) using KBr pellet. The transition mode was used and the wavenumber range was set from 4000 to 400 cm⁻¹.

Nuclear magnetic resonance analysis (NMR). ¹H NMR and ³¹P NMR spectra of samples were conducted on an AVANCE AV 400 Bruker spectrometer (Bruker BioSpin, Switzerland), dimethylsulfoxide-d₆ (DMSO-d₆) or trifluoroacetic acid-d (TFA-d) was used as the solvent.

Intrinsic viscosity and molecular weight. The solution viscosity of the samples were measured with an Ubbelhode viscometer with formic acid according to ISO 307 standard. The intrinsic viscosity ([η]) was estimated using the Solomon–Ciuta equation of a single-point measurement:³⁰
where η_sp is specific viscosity, η_r is the relative viscosity, and c is the concentration (0.005 g mL⁻¹). The molecular weight (M_η) of the samples were estimated from [η] values with the Mark–Houwink–Sakurada equation:

[η] = K(M_η)^a (1)

where K and a are 0.11 mL g⁻¹ and 0.72, respectively.

Differential scanning calorimetry (DSC). DSC analyses were performed in a DSC Q2000 instrument (TA Instruments Inc., USA) using 5–10 mg of each sample under nitrogen atmosphere with a flow rate of 50 mL min⁻¹. Samples were heated from 0 to 300 °C at a rate of 10 °C min⁻¹, and the temperature was maintained at 300 °C for 5 min befor decreased from 300 to 0 °C at a rate of 10 °C min⁻¹, then reheated to 300 °C at a rate of 10 °C min⁻¹ to determine the melting temperature.

Thermogravimetric analysis (TGA). TGA was carried out using a Q5000 IR thermogravimetric analyzer (TA InstrumentsWaters, China) at a linear heating rate of 20 °C min⁻¹ in air atmosphere. The weight of all the samples were kept within 5–10 mg. Samples in an open Pt pan were examined under an airflow rate of 6 × 10⁻⁵ m³ min⁻¹ at a temperature ranging from room temperature to 800 °C.

Limiting oxygen index (LOI). LOI was measured using a HC-2 oxygen index meter (Jiang Ning Analysis Instrument Company, China) on sheets 100 × 6.7 × 3 mm³ according to the standard oxygen index test ASTM D2863-2010.

UL-94 vertical burning test. The vertical burning test was conducted by a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Company, China). The specimens used were 127 × 12.7 × 3 mm³ according to UL-94 test ASTM D3801-2010.

Microscale combustion calorimetry (MCC). Combustion properties of plastics were measured using a microscale combustion calorimeter (MCC-2, Govmark) according to ASTMD 7309-7. 4–6 mg of each sample was heated from 90 to 650 °C at a heating rate of 1 °C s⁻¹ in a nitrogen stream of 80 cm³ min⁻¹. The volatilized decomposition products in the nitrogen stream were mixed with a 20 cm³ min⁻¹ stream of pure oxygen prior to entering a 900 °C combustion furnace, and the decomposition products were completely combusted. Heat release rate (HRR) in watts per gram of sample (W g⁻¹) was calculated from the oxygen depletion measurements.

3. Results and discussion

3.1. Chemical structure of DDP–COCl

The synthesis route of DDP–COCl is shown in Scheme 1, and the flame-retardant monomer was characterized by FTIR and NMR. As shown in Fig. 1, the characteristic absorptions near 808 cm⁻¹ suggest the formation of –COCl. DDP–COCl also shows strong absorptions at 923 cm⁻¹ corresponding to P–O–C (aromatic) stretching, P=O vibration absorption around 1187 cm⁻¹, P–C stretching absorption around 1427 cm⁻¹ and C=O absorption around 1727 cm⁻¹.

Fig. 1 FTIR spectra of DDP–COCl.

The ¹H-NMR spectrum of DDP–COCl is presented in Fig. 2 with DMSO-d₆ as solvent. The chemical shift data of DDP–COCl are: δ(H1) = 2.50 ppm, δ(H2) = 2.68 ppm, δ(H3) = 2.97 ppm. The δ of 7.28–8.17 ppm accounts for chemical shifts of H on the diphenyl. Combining the FTIR and NMR results suggests that we prepared the monomer as depicted in Scheme 1.

Fig. 2 ¹H NMR spectra of DDP–COCl.

3.2. Characteration of PA

Fig. 3 shows the FTIR spectra of PA-1–PA-5 with the characteristic absorptions of –CONH– near 3300 and 1630 cm⁻¹ and the aliphatic chain (–CH₂–) near 2970 and 2860 cm⁻¹. For PA-2–PA-5, the absorptions near 916 cm⁻¹ and 1436 cm⁻¹ were attributed to the vibrations of P–O–C and P-ph, which are the characteristic absorptions of DDP.

Fig. 3 FTIR spectra of PA-1–PA-5.

The chemical structure and the ¹H-NMR spectrum of PA-1 are illustrated in Fig. 4. The peak near 12.1 ppm is attributed to trifluoroacetic acid as solvent. And the other peaks (2.0–4.1 ppm) are attributed to the aliphatic chain (–CH₂–). Meanwhile, the chemical structure, the ¹H-NMR spectrum and the ³¹P-NMR spectrum of PA-5 are shown in Fig. 5. For ¹H-NMR spectrum, the appearance of the peaks at 7.7–8.7 ppm, which account for chemical shifts of H on the diphenyl. For ³¹P-NMR spectrum, the peak at 42.1 ppm, which indicate that DOPO is on the molecular chain of PA-5. According to the characterization above, DDP has been incorporated into the polyamide chain by the interfacial polymerization.

Fig. 4 ¹H NMR spectra of PA-1.

Fig. 5 ¹H NMR (a) and ³¹P NMR (b) spectra of PA-5.

The molecular weight of polyamide was measured by intrinsic viscosity measurements. As shown in Table 1, the [η] values of PA-1–PA-5 ranged from 0.96–0.49 dL g⁻¹ and the viscosity average molecular weight was in the range 2.4 × 10⁴ to 1.2 × 10⁴. This indicates that DDP–COCl participated in the reaction, the molecular weight of polyamide is decrease.

3.3. Crystallization and melting behaviors of polyamide

The melting and crystallization behaviour of polyamide was investigated using DSC, which is plot in Fig. 6 and the relative data are listed in Table 2. The heating scans shown in Fig. 6(a) display that there is only one melting peak in the second heating scan for polyamide. This phenomenon indicated that the inherent flame-retardant polyamide with DDP formed a random copolymer. Observe the endothermic peak carefully, an extra shoulder peak corresponding to the melting of γ-crystalline phase of polyamide before the main endothermic peak appears. By comparison, the melting temperature (T_m) of all the inherent flame-retardant polyamide is lower than that of PA-1. Further, the values of T_m series decrease with an increase in the content of DDP. These findings can be understood on the change of conformation of polyamide. As the monomer of DDP was incorporated into PA66 chains, which occupied a large volume to limit the locomotion of the molecular chain and decreases the density of hydrogen bonding and crystallinity in flame-retardant polyamide, and further make T_m drop down.

Fig. 6 DSC melting endotherms during the second warming cycle (a) and crystallization exotherms (b) during the first cooling cycle of PA-1–PA-5.

Table 2 DSC melting and crystallization results of PA-1–PA-5

Sample	Tm (°C)	Tc (°C)	ΔHc (J g^−1)	Xc (%)
PA-1	262	234	52.2	25.3
PA-2	256	226	67.8	32.9
PA-3	251	219	48.9	23.7
PA-4	236	204	47.4	23.0
PA-5	230	197	44.2	21.5

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The first cooling scan shown in Fig. 6(b) indicates that there is only one crystallization peak for polyamide. For PA-1, the intensity of the peak is sharper than others, indicating that the crystallization ability of polyamide is depressed with the increases in DDP content. Additionally, with increasing the content of DDP from 2.5 to 10 mol%, the values of crystallization temperature (T_c) shift from 234 to 197 °C which imply that the locomotion of the molecular chain was limited. The degrees of crystallinities (X_c) of polyamide was calculated and listed in Table 2. The X_c value of PA-2, 2.5 mol% DDP, increase compared with that of PA-1. With the further increase in the content of DDP, the X_c values decrease. We all know that the crystallization process is governed by two processes: diffusion and nucleation. When the content of DDP is little, it acts as a nucleating agent to increase the nucleation rate and the movement of the molecular chain is not restricted highly. When the content of DDP is too much, it can effectively limit the movement of the molecular chain and hinder the diffusion process of polymer chains to the growing crystallite. These results suggest that T_m and T_c decrease by incorporating DDP into the polyamide chain and the DDP group may have a negative influence on the crystallization.

3.4. Thermal stability of polyamide

The thermal stability of polyamide was characterized with TGA under air atmosphere, as shown in Fig. 7. The initial decomposition temperature (T_d) can be considered as the temperature at which the weight loss is 5 wt%, and the temperature of the maximum mass loss rate (T_max) is defined as the temperature at which the samples present the maximal mass loss rate. So the key values are listed in Table 3. For PA-1, it present T_d at 344 °C and characterizes of three main stages with T_max at 385 °C, 431 °C and 483 °C. Meanwhile, the solid residue at 800 °C is 1.6 wt%. When the content of DDP is little, for PA-2 and PA-3, T_d slightly increases to 361 °C and 372 °C. This phenomenon should been attributed to the change of X_c values. But, with the further increase in the content of DDP, for PA-4 and PA-5, T_d slightly decreases to 339 °C and 319 °C. In addition, with the increasement of DDP, the solid residue at 800 °C was raised from 1.6 to 7.6 wt%. This may result from the phosphorus in DDP forming phosphoric acid and promoting carbonization on the surface of polymer. According to the characterization above, it shown that T_d of the inherent flame-retardant polyamide were all above 310 °C, indicating that these polyamide have good thermal stability.

Fig. 7 TGA (a) and DTG (b) curves of PA-1–PA-5 in air atmosphere.

Table 3 TGA and DTG data of PA-1–PA-5 in air atmosphere

Sample	Td (°C)	Tmax1 (°C)	Tmax2 (°C)	Tmax3 (°C)	Residue (wt%)
PA-1	344	385	431	483	1.6
PA-2	361	426	551	—	3.7
PA-3	372	427	525	—	5.0
PA-4	339	398	419	530	6.3
PA-5	319	387	416	519	7.6

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3.5. Flame retardancy of polyamide

UL-94 and LOI tests are widely used to evaluate the flammability of polymer composites. The LOI values and UL-94 testing results of the polyamide are listed in Table 1. Polyamide without DDP is highly combustible that which presents no rating in the UL-94 test and the LOI value is only 21%. As the concentration of phosphorus is increased from 0.50 to 1.57 wt%, the LOI of the inherent flame-retardant polyamide increase from 25% to 29.5%, and PA-5 can pass UL-94 V-1 rating at 3.2 mm. The improved flame retardancy of the inherent flame-retardant polyamide are result from the incorporation of DDP into the molecular chain.

Microscale combustion calorimeter (MCC) is one of the most effective bench scale methods for investigating the combustion properties of polymer materials that directly measures the heat of combustion of the gases evolved during controlled heating of milligram-sized samples. MCC can provide the important parameter which correlate with the intrinsic flammability properties of polymers. And the variation of the parameter can reflect the transform of the degradation pathway of the inherent flame-retardant polyamide, owing to the chemical changes caused by DDP. The HRR curves are shown in Fig. 8. When the content of DDP is not more than 5.0 mol%, the incorporation of DDP can not effectively improve the flame retardancy of polyamide. However, it is found that the peak heat release rate (pHRR) value of PA-4 and PA-5 have a reduction of 37.8% and 47.8%, respectively, compared with that of PA-1. The reduced pHRR may be attributed to two factors: first, the decomposition of DDP, PO· free radical, captures the H· and HO· free radicals and intercept the radical chain reactions in the flame, leading to a reduction of the heat production until self-extinguishment of the flame; second, the carbonization restrains the flame spread and slows down the decomposition of polyamide. The results clearly indicate that the incorporation of DDP, more than 7.5 mol%, into the molecular chain significantly improves the flame retardancy of polyamide.

Fig. 8 Heat release curves obtained from MCC of PA-1–PA-5.

4. Conclusions

In this study, DDP was acyl-chlorinated as the monomer to synthetise the inherent flame-retardant polyamide by interfacial polymerization. The aforementioned data refer to this paper shown that the inherent flame-retardant polyamide has good thermal stability, T_d of the samples were all above 310 °C. When incorporation of DDP into the molecular chain, the values of T_m and T_c decrease due to that the molecular chain is restricted by the DDP group. However, they also keep high crystallinity. Furthermore, the incorporation of DDP, just 7.5 mol%, into the molecular chain can significantly improve the flame retardancy of polyamide, making the pHRR value has a reduction of 37.8%.

Acknowledgements

The work was financially supported by National Natural Science Foundation of China (51573173, 51323010) and Fundamental Research Funds for the Central Universities (WK2320000032).

References

1. J. Peng, P. J. Walsh, R. C. Sabo, L. S. Turng and C. M. Clemons, Polymer, 2016, 84, 158–166.
2. G. Zhang, Y. X. Zhou, Y. Li, X. J. Wang, S. R. Long and J. Yang, RSC Adv., 2015, 5, 49958–49967.
3. J. L. Xiao, Y. Q. Tan, Y. H. Song and Q. Zheng, RSC Adv., 2016, 6, 41392–41403.
4. W. B. Huang, T. T. Zhang, J. H. Yang, N. Zhang, T. Huang and Y. Wang, RSC Adv., 2015, 5, 101466–101474.
5. R. S. Andre, A. Pavinatto, L. A. Mercante, E. C. Paris, L. H. C. Mattoso and D. S. Correa, RSC Adv., 2015, 5, 73875–73881.
6. A. Bera and S. K. Jewrajka, RSC Adv., 2016, 6, 4521–4530.
7. G. Zhang, Y. X. Zhou, Y. Kong, Z. M. Li, S. R. Long and J. Yang, RSC Adv., 2014, 4, 63006–63015.
8. Y. Ohsedo, M. Oono, K. Saruhashi and H. Watanabe, RSC Adv., 2015, 5, 82772–82776.
9. I. Butnaru, M. P. Fernandez-Ronco, J. Czech-Polak, M. Heneczkowski, M. Bruma and S. Gaan, Polymers, 2015, 7, 1541–1563.
10. A. D. Naik, G. Fontaine, F. Samyn, X. Delva, J. Louisy, S. Bellayer, Y. Bourgeois and S. Bourbigot, RSC Adv., 2014, 4, 18406–18418.
11. H. Y. Ma, L. F. Tong, Z. B. Xu, Z. P. Fang, Y. M. Jin and F. Z. Lu, Polym. Degrad. Stab., 2007, 92, 720–726.
12. H. Ge, G. Tang, W. Z. Hu, B. B. Wang, Y. Pan, L. Song and Y. Hu, J. Hazard. Mater., 2015, 294, 186–194.
13. Y. L. Liu and S. H. Tsai, Polymer, 2002, 43, 5757–5762.
14. W. Z. Hu, J. Zhan, N. N. Hong, T. R. Hull, A. A. Stec, L. Song, J. Wang and Y. Hu, Polym. Adv. Technol., 2014, 25, 631–637.
15. T. Fu, D. M. Guo, J. N. Wu, X. L. Wang, X. L. Wang, L. Chen and Y. Z. Wang, Polym. Chem., 2016, 7, 1584–1592.
16. S. H. Jiang, Y. L. Zhu, Y. Hu, G. H. Chen, X. X. Shi and X. D. Qian, Polym. Adv. Technol., 2016, 27, 266–272.
17. X. F. Yang, Q. L. Li, Z. P. Chen, L. Zhang and Y. Zhou, J. Therm. Anal. Calorim., 2013, 112, 567–571.
18. G. S. Huang, S. X. Zhang, D. S. Li, M. L. Zhang, G. Zhang and J. Yang, Polym. Int., 2013, 62, 411–418.
19. G. Zhang, G. S. Huang, D. S. Li, X. J. Wang, S. R. Long and J. Yang, Ind. Eng. Chem. Res., 2011, 50, 7056–7064.
20. J. Park, M. Goh and K. Akagi, Macromolecules, 2014, 47, 2784–2795.
21. S. Hermans, E. Dom, H. Marien, G. Koeckelberghs and I. F. J. Vankelecom, J. Membr. Sci., 2015, 476, 356–363.
22. R. Oizerovich-Honig, V. Raim and S. Srebnik, Langmuir, 2010, 26, 299–306.
23. G. L. Jadav and P. S. Singh, J. Membr. Sci., 2009, 328, 257–267.
24. J. Y. Sum, A. L. Ahmad and B. S. Ooi, J. Membr. Sci., 2014, 466, 183–191.
25. A. Buczko, T. Stelzig, L. Bommer, D. Rentsch, M. Heneczkowski and S. Gaan, Polym. Degrad. Stab., 2014, 107, 158–165.
26. J. Sahyoun, V. Bounor-Legare, L. Ferry, R. Sonnier, F. Da Cruz-Boisson, F. Melis, A. Bonhomme and P. Cassagnau, Eur. Polym. J., 2015, 66, 352–366.
27. Z. Y. Wei, C. Zhou, Y. Yu and Y. Li, Polymer, 2015, 71, 31–42.
28. C. Zhang, J. Y. Huang, S. M. Liu and J. Q. Zhao, Polym. Adv. Technol., 2011, 22, 1768–1777.
29. H. B. Chen, Y. Zhang, L. Chen, Z. B. Shao, Y. Liu and Y. Z. Wang, Ind. Eng. Chem. Res., 2010, 49, 7052–7059.
30. J. R. Ernzen, F. Bondan, C. Luvison, C. H. Wanke, J. D. Martins, R. Fiorio and O. Bianchi, J. Appl. Polym. Sci., 2016, 133, 43050.

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