Phosphine oxide for reducing flammability of ethylene-vinyl-acetate copolymer
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e-Polymers 2021; 21: 299–308
Research Article
Jiawei Jiang, Ruifeng Guo, Haifeng Shen, and Shiya Ran*
Phosphine oxide for reducing flammability of
ethylene-vinyl-acetate copolymer
https://doi.org/10.1515/epoly-2021-0027
received November 27, 2020; accepted February 20, 2021
1 Introduction
Abstract: In this work, a phosphorous-containing flame Ethylene-vinyl-acetate (EVA) copolymer is a thermoplastic
retardant, phenylphosphonate-based compound (EHPP), elastomer extensively used in many fields and industries
is synthesized by alcoholysis and hydrazinolysis of phenyl- for its excellent mechanical properties and material com-
phosphonic dichloride, which is subsequently intro- patibility. However, because of its chemical constitution,
duced to ethylene-vinyl-acetate (EVA) copolymer to improve EVA is inherently flammable. Once ignited, it burns vig-
its flame retardant performance. The resultant compound orously and thus may cause great detriments to people’s
was characterized by Fourier transform infrared (FTIR), 1H life and property, which extremely restricts its practical
NMR, 13C NMR, and 31P NMR. The influence of the EHPP on application such as home appliances, construction, building
the combustion behaviors of EVA is studied by limiting materials, and cables (1–5). With the continuous develop-
oxygen index (LOI), UL-94, and cone calorimeter test. The ment of polymer material industry, the market demand of
results show that 1 wt% EHPP can reduce peak heat release EVA material presents a trend of increasing year by year.
rate (PHRR) by 40%. Moreover, 2 wt% EHPP can increase Therefore, it is imperative to improve the flame retardancy of
LOI from 20.5% to 25.5%. Thermogravimetric analysis/ EVA materials (6–9).
infrared spectrometry (TGA-FTIR) was used to detect the Till now, to minimize the fire hazards, various flame
gaseous products of EVA/EHPP to study the gaseous-phase
retardant additives have been developed for creating flame
flame retardant mechanism. The EHPP released phos-
retardant EVA. For example, bromine-based flame retar-
phorus-containing radicals to capture highly active free
dants were commonly used, but now some of them have
radicals to improve the flame retardancy of EVA.
been restricted in use because of the toxic and corrosive
Keywords: phosphorus-based compound, ethylene-vinyl- substances released during burning by many environ-
acetate copolymer, thermal stability, flame retardancy, mental regulations in the EU and Asia Pacific (10,11). The
free radical trapping action mechanism of halogen flame retardants is mainly
through free radical capturing. Halogen groups capture
the active groups of hydrogen and hydroxyl in the combus-
tion zone, thus stopping the oxidation reaction, and thus
preventing heat generation. In recent years, materials scien-
tists 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)
* Corresponding author: Shiya Ran, Laboratory of Polymer Materials
(14,15), layered double hydroxides (LDH) (16), carbon nano-
and Engineering, NingboTech University, Ningbo 315000, China,
e-mail: Ranshiya@nit.zju.edu.cn tubes (17,18), graphene (19), and magnesium hydroxide
Jiawei Jiang: Laboratory of Polymer Materials and Engineering, (MH) (20) are reported to flame retarded EVA.
NingboTech University, Ningbo 315000, China; College of Chemical Among these halogen-free FRs, phosphorous-con-
and Biological Engineering, Zhejiang University, Hangzhou 310000, taining flame retardants (PFRs) have particularly been
China
regarded as kinds of highly efficient FRs for EVA. PFRs
Ruifeng Guo: Borg Warner Emissions Systems, Ningbo 315000,
China
can be divided into organic phosphorous and inorganic
Haifeng Shen: Laboratory of Polymer Materials and Engineering, phosphorous flame retardants. Majority of PFRs are active
NingboTech University, Ningbo 315000, China in the solid phase by promoting char formation during
Open Access. © 2021 Jiawei Jiang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
300 Jiawei Jiang et al.
burning, while some PFRs have a mechanism of action in dichloride (see Figure 1, ref. (31)). 240 mL ethyl ether and
the gas phase (21–24). The most commonly used inorganic 4.28 mL phenylphosphonyl dichloride were added into a
PFRs are red phosphorous and ammonium polyphosphate 1,000 mL single-mouth flask, and the solution was cooled
(APP). Red phosphorous has been reported in previous in −20°C for 30 min. The reaction was supposed to take place
study to improve the flame retardancy of EVA with syner- in water-free circumstances. Then 10.76 mL trithylamine and
gistic effect of MH nanoparticles (1). Moreover, the influence 5.4 mL anhydrous ethanol were added, followed by mag-
of the coated modified APP on the flame retardant property netic stirring at room temperature for about 12 h. Precipita-
and water resistance of EVA was also investigated (25). tion was filtered and fully washed by ether and yellowish
Organic PFRs, for example, Schiff-base polyphosphate oily liquid. Diethyl phenylphosphonate (DEPP) was obtained
ester (PAB) had synergistic flame retardant effect with by rotary evaporating the filtrate at 36°C. Afterwards, 100 mL
organo-montmorillonite (OMMT) (26,27). Besides, it is widely anhydrous ethanol and 50 mL hydrazine hydrate were
recognized that the oxidation state of phosphorus has a sig- added into DEPP and the mixed solution was placed in oil
nificant effect on the flame retardant mechanism of phos- bath at 108°C for 12 h. After cooled to room temperature,
phorus-containing FRs. Generally, the higher the oxidation the mixture was evaporated at rotary evaporation at 60°C.
state of phosphorus, the stronger the condensed phase Ultimately, white solid EHPP was obtained and was dried
action; the lower the oxidation state of phosphorus, the at 60°C for 8 h under vacuum.
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 2.3 Preparation of EVA/EHPP composites
most pronounced for phosphine oxide and wears off with
increasing oxidation. Meanwhile, the charring effect in con- EVA granules were dried in a blast air oven for 6 h before
densed phase is the strongest for phosphate and presents a processing. EVA/EHPP composites were fabricated via
downward trend with decreasing oxidation state (28–30). melt blending in a mixer (ThermalHaakeRheomixer) at
Herein, a PFR phenylphosphonhydrazide (EHPP) as a phos- 80°C with a rotor speed of 60 rpm for 8 min. The compo-
phine oxide was synthesized to improve the flame retar- sites obtained by mixing were transferred into a mold and
dancy of EVA whose flame retardant mechanism mainly then preheated for 5 min. Different specimens for all tests
acted in gas phase. The thermal stability and flame retar- were produced by hot compression and cold compression
dancy of EVA/EHPP composites were also studied. 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%
2 Materials and methods
2.1 Materials
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 sup-
plied without further purification. EHPP was synthesized
via alcoholysis and hydrazinolysis of phenylphosphonic Figure 1: Synthetic process of EHPP.
Phosphine oxide for reducing flammability of EVA copolymer 301
EVA matrix and 1 wt% EHPP. Table 1 shows the formulas NH2 group is located at 3,348 cm−1. Besides, the absorp-
of EVA samples. tion 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
2.4 Characterization and measurements are shown in Figure 3. 1H NMR has five characteristic
peaks, which are respectively 7.85–7.63 ppm (a, 2H),
The Fourier transform infrared (FTIR) spectra were obtained 7.61–7.60 ppm (b, 1H), 7.59–7.51 ppm (c, 2H), 4.06–4.00 ppm
using a Vector-22 FTIR spectrophotometer (IR, Bruker, (d, 2H), and 1.28–1.25 ppm (e, 3H), belonging to hydrogen
Germany). NMR spectra were obtained on a Varian unity atoms in different chemical environments. As shown in
Inova spectrometer (Bruker, Germany, 1H NMR: 500 MHz, Figure 3b, the 13C NMR peaks of EHPP are located at
13
C NMR: 125 MHz, 31P NMR: 203 MHz) using d6-DMSO 132.52, 132.50, 132.33, 132.25, 129.25, 129.14, 61.93, and
as the solvent. Thermal gravimetric analysis (TGA) was 16.78 ppm, respectively, belonging to eight carbon atoms
carried out under N2 atmosphere at a heating rate of in different chemical environments. The 31P NMR of EHPP
20°C/min from 30°C to 750°C via TGA analyzer (209 is shown in Figure 3c, which shows that only one peak is
F1, Netzsch, Germany). The mass used in the TGA was located at 15.79 ppm because of one phosphorus atom in
6.00 ± 0.05 mg. Limiting oxygen index (LOI) was tested EHPP. Therefore, it can be judged that EHPP is success-
with a LOI tester (HC-2, Jiangning Analyzer Instrument, fully synthesized.
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 spe- 3.2 Thermal stability of EVA and its
cimen dimensions of 127 × 12.7 × 3 mm according to ASTM composites
D3801-1996. Cone calorimeter tests were performed by
cone calorimeter (CONE, Fire Testing Technology, UK) The thermal stability of EVA and its composites were
according to ISO-5660. Square specimens (100 × 100 × 3 mm) explored by TGA under nitrogen atmosphere. Figure 4
were irradiated at a heat flux of 35 kW/m2. The mor- shows the decomposition curves of EVA composites and
phology of residual char after cone calorimeter test was the relevant thermal degradation data are listed in Table 2.
observed by scanning electron microscope (SEM, S-4800, Figure 4 shows that the curve of pure EVA involves
Hitachi, Japan). Thermogravimetric analysis/infrared two steps, which indicates that the thermal degradation
spectrometry (TGA-FTIR) was conducted by a TGA ana- behavior of pure EVA presents a typical two-stage pro-
lyzer which was coupled with a Thermo Nicolet IS10 FTIR cess. The first degradation stage ranging from 290°C to
spectroscopy (Thermo Scientific, Germany). About 6.00 mg 380°C can be attributed to the remove of acetic acid into
sample was put in a ceramic crucible. gas phase (32). Nevertheless, in the second degradation
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
Table 1: Formulas of EVA samples
Sample code EVA (wt%) EHPP (wt%) P (wt%)
EVA 100 0 0
EVA/EHPP1 99 1 0.18
EVA/EHPP2 98 2 0.36
EVA/EHPP5 95 5 0.91
Figure 2: FTIR spectra of EHPP.
302 Jiawei Jiang et al.
Figure 3: 1H NMR (a), 13
C NMR (b), and 31P NMR (c) spectra of EHPP.
stage, ranging from 390°C to 510°C, the random segments (33,34). As for EHPP/EVA composites, the first smaller
of the remaining material are removed to form unsatu- weight lost peak is constantly advanced with the addition
rated gas products. This stage is the carbon degradation of EHPP. Besides, Table 2 shows that the T5% of pure EVA
stage of EVA skeleton, leading to the major mass loss resins is 344°C, whereas the T5% of composites decreases
Figure 4: TG (a) and DTG (b) curves of EVA and its composites under nitrogen conditions.
Phosphine oxide for reducing flammability of EVA copolymer 303
Table 2: Data of TG and DTG for EVA and its composites under improved to 5 wt%. The LOI increase may be attributed to
nitrogen conditions a combination of enhanced melt flow and some degree of
flame inhibition. In the vertical burning tests of neat EVA,
Sample T5% (°C) Tmax (°C) Residual at 700°C (%) dropping phenomenon occurs and molten drops ignited
EVA 344 ± 1 482 ± 1 1.10 ± 0.11 the cotton below. According to the experiment, neat EVA
EVA/EHPP1 323 ± 0 482 ± 0 0.61 ± 0.05 reached the UL-94 V-2 ranking because the molten liquid
EVA/EHPP2 316 ± 0 481 ± 0 0.59 ± 0.08 drops down quickly, carrying away most of the heat,
EVA/EHPP5 302 ± 0 482 ± 0 0.47 ± 0.02
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
continuously with the increase in EHPP content, which retardant EHPP can improve the LOI values of EVA, the
implies that EHPP has an obvious promoting effect on the UL-94 levels of four groups of specimens are all assigned
pyrolysis behavior of EVA in the early stage. It has been to V-2 because the dripping of flaming particles ignites
reported that the addition of PFRs usually lowers the the cotton at the bottom. Obviously, EVA itself is not
initial decomposition temperature of polymers (35). How- inclined to form char and EHPP has no positive charring
ever, in the second degradation stage, the weight loss effect on the combustion of EVA materials, which further
peak does not change much, and the Tmax of composites confirms that the prime flame retardant mechanism of
keeps nearly the same as that of pure EVA. The results EHPP do not occur in the condensed phase. Moreover,
suggest that the addition of EHPP has little impact on the as shown in Table 3, the combustion time of composites
second degradation stage of EVA. As shown in Table 1, (t1 and t2) has a downward trend with the increasing
the char residual of pure EVA in 700°C is 1.10%, and in fraction of EHPP. The values of t1 and t2 of pure EVA
the case of continuing to increase the EHPP containing, the were 8.9 and 7.7 s, respectively. In terms of 2 wt% of flame
residual of EVA composites remains relatively low state. retardant added to the composites, t1 decreases to 4.1 s
Therefore, it can be concluded that EHPP has no signifi- and t2 decreases to 1.2 s. It is indicated that EHPP has
cant effect on the catalytic carbonization process of EVA conspicuous inhibitory effect on the combustion time of
thermal degradation, meaning that EHPP basically has composites. Like LOI test, the reduced combustion time
little condensed phase flame retardant effect. 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
3.3 Flame retardancy of the composites in vertical combustion, but also increased the LOI values.
As a result, the flame retardant elements in EHPP may
LOI and UL-94 vertical burning test are two primary play a role in EVA matrix, rather than because of the
methods characterizing the fire hazards of materials, accelerated melting rate. In our system, the flame retar-
and the corresponding results are shown in Table 3. dant effect can be obtained by adding a relatively low
The data show that the LOI value of pure EVA is only amount of flame retardant, which is mainly because of
20.5%, which means pure EVA is easy to burn. The intro- the radical-trapping ability of phosphine oxide. Different
duction of flame retardant EHPP apparently improves the from phosphate flame retardants (36), phosphine oxide
LOI value of EVA. When 1 wt% of flame retardant is flame retardants usually play a flame retardant role in the
added, the LOI value of the composites turns to be gas phase and have flame retardant efficiency to some
23.5%, and when the loading increases to 2 wt%, the extent.
LOI value of EVA/EHPP2 elevates to 25.5%. The value The cone calorimeter test results of EVA and its com-
improves to 26.0% while the dosage of EHPP is further posites are presented in Figure 5 and Table 4. Heat
Table 3: Detailed data of LOI and UL-94 vertical burning test
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
304 Jiawei Jiang et al.
Figure 5: HHR (a), THR (b), SPR (c), and mass loss (d) curves of EVA and its composites.
release rate (HRR) that refers to the heat released per unit on EVA. Moreover, HRR curve peak of the composites is
area is an essential parameter to evaluate the comprehen- significantly narrowed, indicating that the intense com-
sive fire performance and potential fire hazard of poly- bustion time of the composites is shortened. From Figure 5b,
mers. From Figure 5a, pure EVA resin begins to burn the total heat release (THR) curve of pure EVA tends to be
vigorously, showing highly flammable performance and flat at 280 s, and the THR value turns out to be 103 MJ/m2.
relatively high HRR value after ignition. The HRR value However, the THR was not reduced after the introduction
reaches to the maximum (peak HRR [PHRR]) at 200 s, of EHPP. The mHRR refers to the average HRR of mate-
which is 964 kW/m2. Nevertheless, the addition of EHPP rials combustion in the cone calorimeter test. Table 4
decreases the PHRR value. When 1 wt% EHPP is added, shows that the mHRR value of pure EVA is 331 kW/m2,
the PHRR value of composite decreases by 40%, which while the mHRR values of EVA composites remarkably
illustrates that EHPP has a certain flame retardant effect decrease to 216 kW/m2 (up to 35%), 223 kW/m2 (up to 33%),
Table 4: Cone calorimeter data of EVA and its composites
Sample TTI (s) PHRR tHRR (s) mHRR THR TSR Mean EHC Mean COY Mean CO2Y
(kW/m2) (kW/m2) (MJ/m2) (m2/m2) (MJ/kg) (kg/kg) (kg/kg)
EVA 58 964 200 331 103 1,463 33.90 0.037 2.160
EVA/EHPP1 39 578 175 216 97 1,978 32.96 0.042 2.024
EVA/EHPP2 41 611 175 223 97 2,076 32.43 0.048 1.981
EVA/EHPP5 36 626 160 193 96 2,275 31.25 0.054 1.900
Phosphine oxide for reducing flammability of EVA copolymer 305
Figure 6: Digital photos of the char residues of EVA (a) and its composites (b).
composites; the final mass of each sample tends to be the
same, indicating that EHPP has little effect on carboniza-
tion 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 characteris-
Figure 7: FTIR spectra during the thermal degradation process of
tics of efficient radical scavenging effect is that reduction
EVA/EHPP2. 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, indi-
and 193 kW/m2 (up to 42%) after adding 1, 2, and 5 wt% cating that EHPP displays flame retardant effect in gas
flame retardant EHPP, respectively. As can be seen in phase to a certain degree.
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 degra-
dation of composites burns more quickly. It has been 3.4 Char residue analysis of the composites
reported that the addition of phosphorus-containing
flame retardants usually reduces TTI (37). In addition, To further investigate the effect of EHPP on the char
smoke is one of the most serious hazards of fire, which formation of EVA composites during combustion, the
can greatly reduce visibility and suffocate the person in appearance and morphology of the residual chars after
fire disasters. Nevertheless, it is a pity that the value of cone calorimeter tests were analyzed by digital images.
total smoke release (TSR) increased after adding EHPP, As shown in Figure 6, there is almost no char left for both
but it also shows that the phosphine oxide of EHPP pure EVA and EVA/EHPP composites, which definitely
mainly acts in the gas phase, producing a lot of smoke confirms the previous mass loss in the TGA and cone
particles. Figure 5d shows mass loss curves of EVA and its calorimeter test analysis. Meanwhile, EHPP leads to a306 Jiawei Jiang et al.
Figure 8: Hypothetical flame retardant mechanism of EVA and its composites.
higher smoke production, which points to the application generated from EHPP are considered to be able to effi-
of gas phase flame retardation mechanism. ciently 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
3.5 Mechanism analysis propose a possible mechanism as shown in Figure 8.
Flame retardant mechanism can be divided into con-
densed phase and gas phase. According to the previous
analysis, EHPP may mainly play a role in the gas phase. 4 Conclusion
TG-IR was used to analyze the gas products during the
thermal degradation process of EVA/EHPP2 to further In this article, EHPP was successfully synthesized and
study the gaseous-phase flame retardant mechanism then subsequently introduced into EVA via melt blending.
(38,39). The IR spectra of gas products of EVA/EHPP2 at It was found that the incorporation of EHPP increases the
different temperatures are shown in Figure 7. The pyro- value of LOI and decreases the values of PHRR, THR, and
lysis of EVA has not occurred below 250°C. The absorp- total burning time in UL-94, indicating that the flame
tion bands of carbonyl group (1,780 cm−1) and hydroxyl retardancy of composites is improved. Besides, the flame
group (1,240 cm−1) derived from the deacetylation pro- retardant effect of EHPP occurs mainly in the gas phase,
cess of EVA appear when the temperature grows to where free radicals released from EHPP capture H˙ or OH˙
350°C, and they nearly disappear at 450°C. Moreover, generated from the degradation of EVA and thus inhibit
the peak at 990 cm−1 can be assigned to the release of the combustion process.
NH3. After that, the absorption peaks of methyl and ethyl
(2,930 cm−1) occur. It is worth noting that the peak of Funding information: This work was supported by the
NO˙ radical 1,380 cm can be observed with strong inten- Non-profit Project of Science and Technology Department
sity at 350–400, meaning that NO˙ radicals mainly exert of Ningbo (Grant Number 2019C50029)] and the General
quenching radical effect between 350°C and 400°C. Project of Education of Zhejiang Province (Grant Number
Meanwhile, the characteristic band at 1,180 cm for PO˙ Y201738757).
free radicals is detected between 350°C and 450°C. During
combustion process of EVA, phosphorus-containing and Author contributions: Jiawei Jiang and Ruifeng Guo con-
nitrogen-containing fragments can capture H˙ and OH˙ tributed equally to this work as the co-first author. Shiya
highly active radicals, which are the prime culprits of Ran designed the experiments; Jiawei Jiang optimized
burning, and thus suppress the combustion in the gas and performed the experiments; Jiawei Jiang and Ruifeng
phase. The TG-IR result confirms NO˙ and PO˙ radicals Guo analyzed the data; Haifeng Shen contributedPhosphine oxide for reducing flammability of EVA copolymer 307
reagents/materials/analysis tools; Jiawei Jiang and (14) Cavodeau F, Sonnier R, Otazaghine B, Lopez-Cuesta J-M,
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