In this work, a phosphorous-containing flame retardant, phenylphosphonate-based compound (EHPP), is synthesized by alcoholysis and hydrazinolysis of phenylphosphonic dichloride, which is subsequently introduced to ethylene-vinyl-acetate (EVA) copolymer to improve its flame retardant performance. The resultant compound was characterized by Fourier transform infrared (FTIR), 1H NMR, 13C NMR, and 31P NMR. The influence of the EHPP on the combustion behaviors of EVA is studied by limiting oxygen index (LOI), UL-94, and cone calorimeter test. The results show that 1 wt% EHPP can reduce peak heat release rate (PHRR) by 40%. Moreover, 2 wt% EHPP can increase LOI from 20.5% to 25.5%. Thermogravimetric analysis/infrared spectrometry (TGA-FTIR) was used to detect the gaseous products of EVA/EHPP to study the gaseous-phase flame retardant mechanism. The EHPP released phosphorus-containing radicals to capture highly active free radicals to improve the flame retardancy of EVA.
Ethylene-vinyl-acetate (EVA) copolymer is a thermoplastic elastomer extensively used in many fields and industries for its excellent mechanical properties and material compatibility. However, because of its chemical constitution, EVA is inherently flammable. Once ignited, it burns vigorously and thus may cause great detriments to people’s life and property, which extremely restricts its practical application such as home appliances, construction, building materials, and cables (1,2,3,4,5). With the continuous development of polymer material industry, the market demand of EVA material presents a trend of increasing year by year. Therefore, it is imperative to improve the flame retardancy of EVA materials (6,7,8,9).
Till now, to minimize the fire hazards, various flame retardant additives have been developed for creating flame retardant EVA. For example, bromine-based flame retardants were commonly used, but now some of them have been restricted in use because of the toxic and corrosive substances released during burning by many environmental regulations in the EU and Asia Pacific (10,11). The action mechanism of halogen flame retardants is mainly through free radical capturing. Halogen groups capture the active groups of hydrogen and hydroxyl in the combustion zone, thus stopping the oxidation reaction, and thus preventing heat generation. In recent years, materials scientists have developed halogen-free FRs, such as phosphorous-containing, silicone-based, intumescent flame retardant (IFR) and polymeric nanacomposites as they were relatively clean and environment-friendly. Non-halogenated flame retardant additives, such as IFR (12,13), aluminum trihydrate (ATH) (14,15), layered double hydroxides (LDH) (16), carbon nanotubes (17,18), graphene (19), and magnesium hydroxide (MH) (20) are reported to flame retarded EVA.
Among these halogen-free FRs, phosphorous-containing flame retardants (PFRs) have particularly been regarded as kinds of highly efficient FRs for EVA. PFRs can be divided into organic phosphorous and inorganic phosphorous flame retardants. Majority of PFRs are active in the solid phase by promoting char formation during burning, while some PFRs have a mechanism of action in the gas phase (21,22,23,24). The most commonly used inorganic PFRs are red phosphorous and ammonium polyphosphate (APP). Red phosphorous has been reported in previous study to improve the flame retardancy of EVA with synergistic effect of MH nanoparticles (1). Moreover, the influence of the coated modified APP on the flame retardant property and water resistance of EVA was also investigated (25). Organic PFRs, for example, Schiff-base polyphosphate ester (PAB) had synergistic flame retardant effect with organo-montmorillonite (OMMT) (26,27). Besides, it is widely recognized that the oxidation state of phosphorus has a significant effect on the flame retardant mechanism of phosphorus-containing FRs. Generally, the higher the oxidation state of phosphorus, the stronger the condensed phase action; the lower the oxidation state of phosphorus, the stronger the gas phase action. When acting in the gas phase, the free radical of phosphorus oxide can act as a halogen-free radical to quench the high-energy hydrogen and hydroxyl radicals in the combustion zone. This effect is most pronounced for phosphine oxide and wears off with increasing oxidation. Meanwhile, the charring effect in condensed phase is the strongest for phosphate and presents a downward trend with decreasing oxidation state (28,29,30). Herein, a PFR phenylphosphonhydrazide (EHPP) as a phosphine oxide was synthesized to improve the flame retardancy of EVA whose flame retardant mechanism mainly acted in gas phase. The thermal stability and flame retardancy of EVA/EHPP composites were also studied.
2 Materials and methods
Phenylphosphonic dichloride was obtained from Shanghai Aladdin Biochemical Technology Co. Ltd. Trithylamine (AR, 99%), diethyl ether (AR, 99.5%), toluene (AR, 99.5%), hydrazine hydrate (AR, 85%), and ethanol (AR, 99.8%) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).
2.2 Preparation of phenylphosphonhydrazide (EHPP)
All reagents were commercially available and used as supplied without further purification. EHPP was synthesized via alcoholysis and hydrazinolysis of phenylphosphonic dichloride (see Figure 1, ref. (31)). 240 mL ethyl ether and 4.28 mL phenylphosphonyl dichloride were added into a 1,000 mL single-mouth flask, and the solution was cooled in −20°C for 30 min. The reaction was supposed to take place in water-free circumstances. Then 10.76 mL trithylamine and 5.4 mL anhydrous ethanol were added, followed by magnetic stirring at room temperature for about 12 h. Precipitation was filtered and fully washed by ether and yellowish oily liquid. Diethyl phenylphosphonate (DEPP) was obtained by rotary evaporating the filtrate at 36°C. Afterwards, 100 mL anhydrous ethanol and 50 mL hydrazine hydrate were added into DEPP and the mixed solution was placed in oil bath at 108°C for 12 h. After cooled to room temperature, the mixture was evaporated at rotary evaporation at 60°C. Ultimately, white solid EHPP was obtained and was dried at 60°C for 8 h under vacuum.
2.3 Preparation of EVA/EHPP composites
EVA granules were dried in a blast air oven for 6 h before processing. EVA/EHPP composites were fabricated via melt blending in a mixer (ThermalHaakeRheomixer) at 80°C with a rotor speed of 60 rpm for 8 min. The composites obtained by mixing were transferred into a mold and then preheated for 5 min. Different specimens for all tests were produced by hot compression and cold compression for 2 min, respectively. The codes of the samples are based on the addition amount of EHPP, for example, EVA/EHPP1 means that the composites contain 99 wt% EVA matrix and 1 wt% EHPP. Table 1 shows the formulas of EVA samples.
|Sample code||EVA (wt%)||EHPP (wt%)||P (wt%)|
2.4 Characterization and measurements
The Fourier transform infrared (FTIR) spectra were obtained using a Vector-22 FTIR spectrophotometer (IR, Bruker, Germany). NMR spectra were obtained on a Varian unity Inova spectrometer (Bruker, Germany, 1H NMR: 500 MHz, 13C NMR: 125 MHz, 31P NMR: 203 MHz) using d6-DMSO as the solvent. Thermal gravimetric analysis (TGA) was carried out under N2 atmosphere at a heating rate of 20°C/min from 30°C to 750°C via TGA analyzer (209 F1, Netzsch, Germany). The mass used in the TGA was 6.00 ± 0.05 mg. Limiting oxygen index (LOI) was tested with a LOI tester (HC-2, Jiangning Analyzer Instrument, China) according to GB2406-80, and the dimension of samples was 100 × 6.5 × 3 mm. UL-94 vertical burning tests were conducted using a vertical burning instrument (CZF-3, Jiangning Analyzer Instrument, China) with specimen dimensions of 127 × 12.7 × 3 mm according to ASTM D3801-1996. Cone calorimeter tests were performed by cone calorimeter (CONE, Fire Testing Technology, UK) according to ISO-5660. Square specimens (100 × 100 × 3 mm) were irradiated at a heat flux of 35 kW/m2. The morphology of residual char after cone calorimeter test was observed by scanning electron microscope (SEM, S-4800, Hitachi, Japan). Thermogravimetric analysis/infrared spectrometry (TGA-FTIR) was conducted by a TGA analyzer which was coupled with a Thermo Nicolet IS10 FTIR spectroscopy (Thermo Scientific, Germany). About 6.00 mg sample was put in a ceramic crucible.
3 Results and discussions
3.1 Characterization of EHPP
The FTIR spectra of EHPP are shown in Figure 2. For EHPP, the characteristic absorption peak of N–H for NH2 group is located at 3,348 cm−1. Besides, the absorption peaks of P–Ph and P–N groups appear at 1,441 and 1,040 cm−1, respectively.
The 1H NMR, 13C NMR, and 31P NMR spectra of EHPP are shown in Figure 3. 1H NMR has five characteristic peaks, which are respectively 7.85–7.63 ppm (a, 2H), 7.61–7.60 ppm (b, 1H), 7.59–7.51 ppm (c, 2H), 4.06–4.00 ppm (d, 2H), and 1.28–1.25 ppm (e, 3H), belonging to hydrogen atoms in different chemical environments. As shown in Figure 3b, the 13C NMR peaks of EHPP are located at 132.52, 132.50, 132.33, 132.25, 129.25, 129.14, 61.93, and 16.78 ppm, respectively, belonging to eight carbon atoms in different chemical environments. The 31P NMR of EHPP is shown in Figure 3c, which shows that only one peak is located at 15.79 ppm because of one phosphorus atom in EHPP. Therefore, it can be judged that EHPP is successfully synthesized.
3.2 Thermal stability of EVA and its composites
The thermal stability of EVA and its composites were explored by TGA under nitrogen atmosphere. Figure 4 shows the decomposition curves of EVA composites and the relevant thermal degradation data are listed in Table 2. Figure 4 shows that the curve of pure EVA involves two steps, which indicates that the thermal degradation behavior of pure EVA presents a typical two-stage process. The first degradation stage ranging from 290°C to 380°C can be attributed to the remove of acetic acid into gas phase (32). Nevertheless, in the second degradation stage, ranging from 390°C to 510°C, the random segments of the remaining material are removed to form unsaturated gas products. This stage is the carbon degradation stage of EVA skeleton, leading to the major mass loss (33,34). As for EHPP/EVA composites, the first smaller weight lost peak is constantly advanced with the addition of EHPP. Besides, Table 2 shows that the T5% of pure EVA resins is 344°C, whereas the T5% of composites decreases continuously with the increase in EHPP content, which implies that EHPP has an obvious promoting effect on the pyrolysis behavior of EVA in the early stage. It has been reported that the addition of PFRs usually lowers the initial decomposition temperature of polymers (35). However, in the second degradation stage, the weight loss peak does not change much, and the Tmax of composites keeps nearly the same as that of pure EVA. The results suggest that the addition of EHPP has little impact on the second degradation stage of EVA. As shown in Table 1, the char residual of pure EVA in 700°C is 1.10%, and in the case of continuing to increase the EHPP containing, the residual of EVA composites remains relatively low state. Therefore, it can be concluded that EHPP has no significant effect on the catalytic carbonization process of EVA thermal degradation, meaning that EHPP basically has little condensed phase flame retardant effect.
|Sample||T5% (°C)||Tmax (°C)||Residual at 700°C (%)|
|EVA||344 ± 1||482 ± 1||1.10 ± 0.11|
|EVA/EHPP1||323 ± 0||482 ± 0||0.61 ± 0.05|
|EVA/EHPP2||316 ± 0||481 ± 0||0.59 ± 0.08|
|EVA/EHPP5||302 ± 0||482 ± 0||0.47 ± 0.02|
3.3 Flame retardancy of the composites
LOI and UL-94 vertical burning test are two primary methods characterizing the fire hazards of materials, and the corresponding results are shown in Table 3. The data show that the LOI value of pure EVA is only 20.5%, which means pure EVA is easy to burn. The introduction of flame retardant EHPP apparently improves the LOI value of EVA. When 1 wt% of flame retardant is added, the LOI value of the composites turns to be 23.5%, and when the loading increases to 2 wt%, the LOI value of EVA/EHPP2 elevates to 25.5%. The value improves to 26.0% while the dosage of EHPP is further improved to 5 wt%. The LOI increase may be attributed to a combination of enhanced melt flow and some degree of flame inhibition. In the vertical burning tests of neat EVA, dropping phenomenon occurs and molten drops ignited the cotton below. According to the experiment, neat EVA reached the UL-94 V-2 ranking because the molten liquid drops down quickly, carrying away most of the heat, which indicates that EVA is a kind of flammable resins that seriously limits its application scope in extensive industries and fields. Although the addition of flame retardant EHPP can improve the LOI values of EVA, the UL-94 levels of four groups of specimens are all assigned to V-2 because the dripping of flaming particles ignites the cotton at the bottom. Obviously, EVA itself is not inclined to form char and EHPP has no positive charring effect on the combustion of EVA materials, which further confirms that the prime flame retardant mechanism of EHPP do not occur in the condensed phase. Moreover, as shown in Table 3, the combustion time of composites (t1 and t2) has a downward trend with the increasing fraction of EHPP. The values of t1 and t2 of pure EVA were 8.9 and 7.7 s, respectively. In terms of 2 wt% of flame retardant added to the composites, t1 decreases to 4.1 s and t2 decreases to 1.2 s. It is indicated that EHPP has conspicuous inhibitory effect on the combustion time of composites. Like LOI test, the reduced combustion time in the vertical burning test may be because of the flame inhibition or melt flow, or both together. In this study, the addition of EHPP not only reduced the combustion time in vertical combustion, but also increased the LOI values. As a result, the flame retardant elements in EHPP may play a role in EVA matrix, rather than because of the accelerated melting rate. In our system, the flame retardant effect can be obtained by adding a relatively low amount of flame retardant, which is mainly because of the radical-trapping ability of phosphine oxide. Different from phosphate flame retardants (36), phosphine oxide flame retardants usually play a flame retardant role in the gas phase and have flame retardant efficiency to some extent.
|Sample||LOI (%)||t1 (s)||t2 (s)||Dripping||Cotton||Rating|
|EVA||20.5||8.9 ± 0.6||7.7 ± 0.5||Yes||Ignite||V-2|
|EVA/EHPP1||23.5||5.8 ± 0.7||1.9 ± 0.2||Yes||Ignite||V-2|
|EVA/EHPP2||25.5||4.1 ± 1.1||1.2 ± 0.5||Yes||Ignite||V-2|
|EVA/EHPP5||26||6.4 ± 0.2||1.1 ± 0.3||Yes||Ignite||V-2|
The cone calorimeter test results of EVA and its composites are presented in Figure 5 and Table 4. Heat release rate (HRR) that refers to the heat released per unit area is an essential parameter to evaluate the comprehensive fire performance and potential fire hazard of polymers. From Figure 5a, pure EVA resin begins to burn vigorously, showing highly flammable performance and relatively high HRR value after ignition. The HRR value reaches to the maximum (peak HRR [PHRR]) at 200 s, which is 964 kW/m2. Nevertheless, the addition of EHPP decreases the PHRR value. When 1 wt% EHPP is added, the PHRR value of composite decreases by 40%, which illustrates that EHPP has a certain flame retardant effect on EVA. Moreover, HRR curve peak of the composites is significantly narrowed, indicating that the intense combustion time of the composites is shortened. From Figure 5b, the total heat release (THR) curve of pure EVA tends to be flat at 280 s, and the THR value turns out to be 103 MJ/m2. However, the THR was not reduced after the introduction of EHPP. The mHRR refers to the average HRR of materials combustion in the cone calorimeter test. Table 4 shows that the mHRR value of pure EVA is 331 kW/m2, while the mHRR values of EVA composites remarkably decrease to 216 kW/m2 (up to 35%), 223 kW/m2 (up to 33%), and 193 kW/m2 (up to 42%) after adding 1, 2, and 5 wt% flame retardant EHPP, respectively. As can be seen in Table 4, the TTI (time to ignite) values of each sample have a degree of decline. It agrees well with the T5% reduction of EVA composites, meaning the earlier degradation of composites burns more quickly. It has been reported that the addition of phosphorus-containing flame retardants usually reduces TTI (37). In addition, smoke is one of the most serious hazards of fire, which can greatly reduce visibility and suffocate the person in fire disasters. Nevertheless, it is a pity that the value of total smoke release (TSR) increased after adding EHPP, but it also shows that the phosphine oxide of EHPP mainly acts in the gas phase, producing a lot of smoke particles. Figure 5d shows mass loss curves of EVA and its composites; the final mass of each sample tends to be the same, indicating that EHPP has little effect on carbonization in the condensed phase. Average effective heat of combustion (EHC), average CO-Yield (COY), and average CO2Y are used to characterize the flame retardant of EHPP in gas phase. Table 4 shows that the average EHC of EVA sample is 33.90 MJ/kg, and the average EHC values of EVA/EHPP1, EVA/EHPP2, and EVA/EHPP5 (32.96, 32.43, and 31.25 MJ/kg, respectively) are lower than pure EVA sample. Besides, the flame retardant in gas phase of EHPP is further confirmed by average COY and average CO2Y values. With the increasing content of EHPP, the average COY values of EVA/EHPP samples increase while the average CO2Y values decrease. One of the characteristics of efficient radical scavenging effect is that reduction in EHC often goes along with increase in COY. As a result, during combustion, more incomplete product CO and less complete combustion product CO2 are produced, indicating that EHPP displays flame retardant effect in gas phase to a certain degree.
|Sample||TTI (s)||PHRR (kW/m2)||tHRR (s)||mHRR (kW/m2)||THR (MJ/m2)||TSR (m2/m2)||Mean EHC (MJ/kg)||Mean COY (kg/kg)||Mean CO2Y (kg/kg)|
3.4 Char residue analysis of the composites
To further investigate the effect of EHPP on the char formation of EVA composites during combustion, the appearance and morphology of the residual chars after cone calorimeter tests were analyzed by digital images. As shown in Figure 6, there is almost no char left for both pure EVA and EVA/EHPP composites, which definitely confirms the previous mass loss in the TGA and cone calorimeter test analysis. Meanwhile, EHPP leads to a higher smoke production, which points to the application of gas phase flame retardation mechanism.
3.5 Mechanism analysis
Flame retardant mechanism can be divided into condensed phase and gas phase. According to the previous analysis, EHPP may mainly play a role in the gas phase. TG-IR was used to analyze the gas products during the thermal degradation process of EVA/EHPP2 to further study the gaseous-phase flame retardant mechanism (38,39). The IR spectra of gas products of EVA/EHPP2 at different temperatures are shown in Figure 7. The pyrolysis of EVA has not occurred below 250°C. The absorption bands of carbonyl group (1,780 cm−1) and hydroxyl group (1,240 cm−1) derived from the deacetylation process of EVA appear when the temperature grows to 350°C, and they nearly disappear at 450°C. Moreover, the peak at 990 cm−1 can be assigned to the release of NH3. After that, the absorption peaks of methyl and ethyl (2,930 cm−1) occur. It is worth noting that the peak of NO˙ radical 1,380 cm can be observed with strong intensity at 350–400, meaning that NO˙ radicals mainly exert quenching radical effect between 350°C and 400°C. Meanwhile, the characteristic band at 1,180 cm for PO˙ free radicals is detected between 350°C and 450°C. During combustion process of EVA, phosphorus-containing and nitrogen-containing fragments can capture H˙ and OH˙ highly active radicals, which are the prime culprits of burning, and thus suppress the combustion in the gas phase. The TG-IR result confirms NO˙ and PO˙ radicals generated from EHPP are considered to be able to efficiently capture H˙ or OH˙ because of their quenching effect, which is conducive to the flame retardancy of EVA composites. Based on the analysis above, we can propose a possible mechanism as shown in Figure 8.
In this article, EHPP was successfully synthesized and then subsequently introduced into EVA via melt blending. It was found that the incorporation of EHPP increases the value of LOI and decreases the values of PHRR, THR, and total burning time in UL-94, indicating that the flame retardancy of composites is improved. Besides, the flame retardant effect of EHPP occurs mainly in the gas phase, where free radicals released from EHPP capture H˙ or OH˙ generated from the degradation of EVA and thus inhibit the combustion process.
Funding information: This work was supported by the Non-profit Project of Science and Technology Department of Ningbo (Grant Number 2019C50029)] and the General Project of Education of Zhejiang Province (Grant Number Y201738757).
Author contributions: Jiawei Jiang and Ruifeng Guo contributed equally to this work as the co-first author. Shiya Ran designed the experiments; Jiawei Jiang optimized and performed the experiments; Jiawei Jiang and Ruifeng Guo analyzed the data; Haifeng Shen contributed reagents/materials/analysis tools; Jiawei Jiang and Ruifeng Guo wrote the paper.
Conflict of interest: The authors state no conflict of interest.
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