Introduction

Ethylene–vinyl acetate (EVA) is a copolymer with good physical and mechanical properties that has been widely applied in many fields. However, like most organic polymers, EVA is also inherently inflammable with a high smoke emission and thus its applications have been restricted due to the terrible fire-resistance [1, 2]. Intumescent flame retardant (IFR) systems have been commonly regarded as an efficient way to enhance the flame-resistance of polymer materials because of the characteristic of low toxicity, low corrosion, low smoke and excellent char-forming ability, which would be a potential candidate for substituting halogen-containing flame retardants [3,4,5].

However, IFRs are always with low flame retardant efficiency and the large addition must be required for obtaining good flame-retardant properties, which certainly decreases polymer materials’ comprehensive performance. To enhance the flame retardant efficiency of IFR in polymers, zeolites [6], transition metal oxides [7], metal compounds [8], organic boron siloxane [9, 10] and the like have been used as flame-retardant synergistic agents. For instance, the synergistic effect of transition metal oxides on polymer/IFR systems have been widely investigated, such as metal oxides of Zn, Ti and Ni in intumescent flame retardant polypropylene system by Cai et al. [11] and Yang et al. [12], oxides of Fe and La in EVA/IFR systems, ZnO in silicone rubber/IFR systems by Chen et al. [13] and so on. These results revealed that transition metal oxides as synergists could effectively promoted forming high-quality char layers with good mechanical strengths and thermal stabilities through catalyzing cross-linked reactions in IFR system.

In the previous work, our lab has prepared a hyperbranched triazine-based char-forming agent (HTCFA) showing good thermal stability, and it has been applied to an innovated IFR system with APP, which showed excellent flame retardancy in PP matrix [14]. In this work, nano-ZnO was selected as a synergist and its impact on combustion behavior and thermal decomposition properties of EVA/IFR system based on HTCFA and APP have been investigated by LOI, UL-94, CCT, TGA and residue analysis (SEM, FTIR, LRS and XPS).

Experimental

Materials

EVA containing 17.6–20.4 wt% vinyl acetate units was bought from BASF-YPC CO., Ltd. (Nanjing, China). APP (n > 1000) was produced by Polyrocks Chemical Co. Ltd, China. HTCFA was prepared in our lab [14] and Scheme 1 shows its structure. Nano-zinc oxide (≥ 99.9% metals basis, 30 ± 10 nm) was provided by Sinopharm Chemical Reagent Co., Ltd., China.

Scheme 1
scheme 1

The structure of HTCFA

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Preparation of Composites

EVA and all the additives were dried in vacuum at 80 °C for 12 h before processing. EVA samples were melt-mixed via a torque rheometer (KX-160, Jiangsu, China) with the process temperature of 130 °C and the rotation speed of 40 r/min. Then, they were hot-pressed to produce sheets with standard sizes of subsequent tests. The formulations are presented in Table 1. The loading of IFR was fixed at 25 wt% with the weight ratio of APP and HTCFA 3:1, which showed the optimum flame retardant performance in our previous work [15].

Table 1 Formulation of neat EVA and EVA composites and catalytic effects of nano-ZnO
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Instruments and Characterization

LOI values were measured using a FTT 0080 LOI instrument (Fire Testing Technology Ltd. (FTT), UK) according to American Society for Testing and Materials (ASTM) D2863-17 and the sample dimension was 130 mm × 6.5 mm × 3.2 mm. UL-94 vertical burning rating tests were performed by an FTT0082 tester (FTT, UK) following the procedures in ASTM D3801-10 (sample dimension: 125 mm × 12.7 mm × 3.2 mm). The forced combustion behaviour was measured by an FTT0007 cone calorimetry (FTT, UK) according to ISO 5660-1 with an external heat flux of 50 kW/m2 (sample dimension: 100 mm × 100 mm × 3 mm).

Fourier transform infrared (FTIR) spectra of residues after cone test were recorded by a Nicolet 6700 FTIR spectrometer (Nicolet, USA) with KBr pellets.

TGA tests were performed on a TGA instrument thermal analyzer (TA Q50, USA). About 2–5 mg of a tested sample was heated from 50 to 700 °C under neat N2 atmosphere at rate of 20 °C/min.

The morphologies of the char were observed in a FEI Quanta 250 FEG field-emission SEM with a voltage of 20 kV. LRS spectra were obtained from a LabRAM HR Evolution Raman spectrometer (HORIBA, Japan) with excitation by a 532 nm helium–neon laser line and scanning in the scope of 50–4000 cm−1. XPS spectra were recorded by a PHI Quantera-II SXM (Ulvac-PHI, Japan) using Al Kα excitation radiation (X-ray power of 2.5 kW).

Results and Discussion

Flammability of EVA Composites

Flammability properties of EVA composites were tested by limiting oxygen index and vertical flammability tests. Figure 1 shows the impact on LOI and UL-94 rating results of EVA/IFR composites when adding diverse contents of nano-ZnO. EVA/IFR composite only attained a LOI value of 25.2% and reached UL-94V-2 rating. However, the flame retardancy of EVA/IFR composites was significantly improved after combining nano-ZnO. It could be observed that the LOI values of EVA/IFR/ZnO systems initially went up rapidly with increasing the nano-ZnO loadings and then decreased with the nano-ZnO concentration more than 0.5 wt%. When adding 0.5 wt% nano-ZnO, LOI reached the highest value of 29.5% and UL-94V-0 rating was obtained. And all the composites containing nano-ZnO could reach UL-94V-0 rating. As a result, it indicated that there was an appropriate loading amount of nano-ZnO, with which IFR and nano-ZnO gave the optimal flame retardant synergistic effect on EVA. The higher content of nano-ZnO might break the harmony between expansion behavior and char-forming function of EVA/IFR/ZnO composites. Therewith the action mechanism would be analyzed subsequently.

Fig. 1
figure 1

Effect of nano-ZnO on the flame retardant properties of EVA/IFR composites

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Catalytic Effects of Nano-ZnO

The catalytic efficiency (CAT-EFF) of nano-ZnO on EVA/IFR composites, defined as the LOI augmenter caused by per weight percent of Zn [16], was calculated based on Formulation provided in Table 1 footnote and it shows the CAT-EFF results. The CAT-EFF values for EVA/IFR/ZnO composites increased at first and then reduced subsequently with increasing the nano-ZnO concentration, which was similar with LOI results. When the nano-ZnO loading was 0.5 wt%, the highest CAT-FEE value (10.75) was obtained, which also possessed the highest LOI value. Therefore, 0.5 wt% loading of nano-ZnO was the most effective for EVA/IFR systems, which would be chosen as the subject to be further investigated.

Forced Combustion Behaviour of EVA/IFR/ZnO Composites

Cone calorimetry was a superior measurement method for studying the combustion behaviour of materials. Table 2 summarized the important parameters which were used to describe the characteristics of pure EVA and EVA composites during combustion.

Table 2 Cone calorimetry data of EVA composites
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Figure 2 shows the heat release rate (HRR) and total heat release (THR) curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO. As shown in Fig. 2, pure EVA burned quickly after ignition, and a HRR peak of 1758 kW/m2 occurred at about 150 s. As far as EVA/IFR and EVA/IFR/0.5ZnO, both HRR values showed two main peaks. The first stage was attributed to the decomposition of the IFR, and thereout cross-linking structures formed a preliminary covering on the surface of matrix material. Its low viscosity and terrible strength were not enough to block the generated gas to make the char layer expand, and the ability to block heat release was limited inevitably. As accumulated, the char layer began to swell, but it was not strong enough to prevent from being undone by a steady stream of gases, so the second phase appeared. Fortunately, as the old char layer was destroyed, the new one swelled immediately. Over time, the expanded carbon layer became stronger and stronger, once again effectively blocking the release of heat. It was obvious that the peak2-HRR value of EVA/IFR/0.5ZnO was smaller than that of EVA/IFR. Furthermore, the THR value of EVA/IFR was much lower than pure EVA (136 MJ/m2) and the value of EVA/IFR/0.5ZnO presented a further lower THR value with the addition of nano-ZnO. Also, it was worth noting that the THR curve of EVA/IFR was steeper than that of EVA/IFR/0.5ZnO from 220 to 350 s which corresponded to the main decomposition process of the composite, and then tended to be flat. That was to say, EVA/IFR without ZnO reached the maximum heat release value earlier than that with nano-ZnO and thus the former presented the stronger combustion intensity. This result could be explained by the fact that the presence of nanometer zinc oxide improved the quality of the char layer which was more continuous and denser, resulting in the longer heat release duration, even if it began to form a little later (Table 3).

Fig. 2
figure 2

HRR (a) and THR (b) curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO

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Table 3 TGA data of IFR with and without nano-ZnO under nitrogen
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From Table 2, it was not difficult to find the time to ignition (TTI) with addition of IFR or IFR/0.5ZnO was obviously shortened. It was reported that the pyrolysis of EVA started from the deacetylation of VA monomers [17], and APP could accelerated the decomposition [18]. Also, the existence of ZnO could further catalyze the pyrolysis process.

Figure 3 gives the smoke production rate (SPR) and smoke release product (TSP) curves of composites. It was found that with adding IFR and IFR/0.5ZnO into EVA, all the peak-SPR (PSPR) values significantly decreased compared to neat copolymer. Especially, it reduced from 0.206 m2/s for EVA to 0.077 m2/s and 0.058 m2/s, with 62.8% and 72.0% reduction when adding IFR and IFR/0.5ZnO and the PSPR of EVA/IFR/0.5ZnO appeared obviously later than EVA/IFR. This indicated that the presence of nano-ZnO in IFR inhibited the smoke production rate of composites. For TSP, the value of EVA/APP/0.5ZnO was slightly lower than EVA/IFR but much lower than neat EVA, which suggested EVA/IFR combining nano-ZnO possessed more excellent smoke suppression properties.

Fig. 3
figure 3

SPR (a) and TSP (b) curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO

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The emission of toxic gas (CO) and CO2 was assessed and Fig. 4 shows the product curves of CO (COP) and CO2 (CO2P) which are similar with the HRR curves. With adding IFR and IFR/0.5ZnO into EVA, all the generations of CO and CO2 were restrained. Still, the combination of IFR and nano-ZnO could more efficiently inhibit forming CO and CO2 during combustion of composites. This also was attributed to the formation of firm charred layer protecting the underlying polymer from burning.

Fig. 4
figure 4

COP (a) and CO2P (b) curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO

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Figure 5 gives the mass loss curves of EVA and EVA composites during burning, and the char residues are also listed in Table 2. Neat EVA lost its mass much faster than EVA/IFR and EVA/IFR/0.5ZnO and remained nothing at the end. Further, it was also obvious that much more residues were obtained (23.4 wt%) after nano-ZnO combined with IFR, which indicated that nanometre zinc oxide could promote the formation of the stable expansive char crust, prohibited transmitting heat and combustible gas generated from burning as mentioned above.

Fig. 5
figure 5

The residue mass curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO composites

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Digital photographs of the residues after cone calorimeter test are displayed in Fig. 6. Obviously, both EVA/IFR and EVA/IFR/ZnO composites formed swollen and dense char, yet a few of tiny holes and cracks were found on the outer surface labelled using ovals in Fig. 6a for EVA/IFR char, bringing the weaker barrier effect of char. It was surprising that the presence of 0.5 wt% nano-ZnO in composites significantly promoted forming homogeneous and compact char residues without visible holes on the surface, as shown from Fig. 6b. High-quality intumescent char layers could play a key role in preventing the transfer of heat/combustible gas, consequently improving the flame resistance of EVA composites.

Fig. 6
figure 6

Digital photos of chars of EVA/IFR (a) and EVA/IFR/0.5ZnO (b)

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Thermogravimetic Analysis

Thermogravimetic analysis was used to rapidly evaluate the thermal degradation behaviour and charring ability of IFR and IFR/0.5ZnO. TGA and DTG curves are present in Fig. 7, and the data are collected in Table 2. It could be seen that IFR possessed great thermal stability, whose initial decomposition temperature was 324 °C based on 5 wt% mass loss (T5%). The thermal decomposition course of IFR was split into three stages. As for the first step, it might be attributed to the release of water, ammonia and cross-linked polyphosphoric acids which occurred at 305 °C. The second one appeared at 446 °C and was the main decomposition process, resulting from the disintegration and formation of char. The last peak occurred at 660 °C and was attributed to the decomposition of char residue. However, the combination with nano-ZnO did affect the degradation behaviour of IFR system and there was an apparent synergistic effect between them. The T5% of IFR system lowered from 324 to 310 °C with the addition of nano-ZnO, which agreed with the previous results based on metal oxides system [11]. This was due to the fact that metal oxides could catalyze the amidation reactions between APP and HTCFA with the latter containing lots of imide groups, promoting forming char residues. In the curves of IFR/ZnO, it was found that the third peak was not evident and there were only two obvious decomposition steps, whose peaks occurred at 305 °C and 416 °C. This indicated that the existence of nano-ZnO could improve the char-forming ability and high-temperature thermal stability of IFR, testified by that the char residue of IFR/ZnO system was 61.8 wt%, 56.6 wt%, and 53.5 wt% at 500 °C, 600 °C, and 700 °C respectively, much higher than the corresponding values of IFR and IFR/ZnO calculation. Also, comparing the experimental thermal analysis curve of IFR/ZnO and the calculation one calculated based on the experimental values and mass fraction in IFR/ZnO system on the basis of Formula 1, some obvious differences occurred, especially under high temperature (about > 450 °C). The third peak in the calculation DTG curve of IFR/ZnO still existed, which disappeared in the experimental curve. This revealed that the introduction of nano-ZnO could change the thermal degradation behaviour and enhance the thermal stability of APP/HTCFA system due to forming stable compounds or fragments under high temperature.

Fig. 7
figure 7

TGA (a) and DTG (b) curves of IFR systems with and without nano-ZnO under N2

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$${W_{{\text{calculation}}}}={W_{IFR}} \times 98.04\% {\text{ }}+{W_{ZnO}} \times 1.96\%$$
(1)

Figure 8 presents the TGA and DTG curves of EVA and EVA composites under N2 atmosphere and Table 4 listed the results. The EVA mass loss curve showed two degradation platforms. The first step could be explained by the elimination of the acetate side groups (deacetylation), leaving the polyenes and polymer chains with unsaturated bonds [19,20,21]. What’s more, the second step could be ascribed to the cleavage of the polyenes in the form of alkyl at high temperatures to achieve complete thermal degradation. EVA began to go through thermal decomposition from 365 °C (T5%), and almost completely degraded up to 500 °C with nothing left at 700 °C. The addition of IFR to EVA at 25 wt% weight fraction led to a slight advance in the initial thermal degradation process of EVA. However, the thermal decomposition was transferred to high temperature and Tpeak moved to 495 °C from 487 °C. Further, the char residues of EVA/IFR composite was significantly enhanced to 8.6 wt% at 700 °C. With the introduction of nano-ZnO into EVA/IFR, both T5% and Tpeak decreased and the char residues were remarkably improved with 15.3 wt% at 600 °C and 14.7 wt% at 700 °C. This indicated the presence of nano-ZnO promoted forming stable char residues at high temperature, effectively inhabiting the transfer of heat and diffusion of oxygen/inflammable volatiles.

Fig. 8
figure 8

TGA (a) and DTG (b) curves of EVA, EVA/IFR and EVA/IFR/0.5ZnO under N2

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Table 4 TGA data of EVA and EVA composites under N2 and air atmosphere
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Besides the thermal decomposition properties of EVA composites under N2, the thermal oxidative degradation behaviour was also evaluated because polymer materials were practically used under air. Figure 9 gives the TGA and DTG curves under air atmosphere and the corresponding data are also listed in Table 4. Pure resin started to compose from 346 °C, and there was only 0.1 wt% char left above 600 °C. After the addition of IFR or IFR/0.5ZnO into EVA, the degradation behaviour changed above about 400 °C with the better thermal stability and more char residues. Moreover, the Tpeak values of composites shifted to high temperature from 480 °C for EVA/IFR to 486 °C for EVA/IFR/0.5ZnO and yet the Rpeak of the latter (1.21%/min) was lower than the former (1.33%/min). Also, the char residues obtained at 700 °C of EVA/IFR/0.5ZnO was 13.5 wt% which was much higher than those of EVA/IFR (10.9 wt%). These results indicated EVA/IFR/0.5ZnO also outperformed EVA/IFR in air atmosphere for the high-temperature thermal stability of residues, which could bring a better protective armor for under matrix in the real fire disaster.

Fig. 9
figure 9

TGA (a) and DTG (b) curves of EVA and EVA composites under air

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Morphology Analysis of Char Residues

The micro-morphologies of the char layers after cone test for EVA/IFR and EVA/IFR/0.5ZnO were observed by SEM to further analyze the action mode of catalytic carbonization effect of nano-ZnO on the char-forming process of EVA composites. As shown in Fig. 10, when magnified 500 times, each sample (regardless of existing nano-ZnO) seemed to generate a continuous and complete surface, which was able to play a role as an isolation barrier in restricting transferring of oxygen/heat from underlying matrix. When magnified 1000 times, EVA/IFR system began to show its weakness, and some holes were clearly visible on the surface of the carbon residue, while EVA/IFR/0.5ZnO composite was still intact and solid. The results suggested that the combination of nano-ZnO into EVA/IFR produced the advantage of forming more cross-linked network structures, thus a more compact and tighter char layer occurred with better quality. Therefore, this kind of char layer contributed to the greater thermal stability, which was also revealed by TGA results very well.

Fig. 10
figure 10

SEM photographs of EVA/IFR (a1: × 500, a2: × 1000) and EVA/IFR/0.5ZnO (b1: × 500, b2: × 1000) char residues

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Structure Analysis of the Char Residue

Figure 11 presents the Raman spectra of EVA/IFR and EVA/IFR/0.5ZnO residues after cone test to disclose the residue graphitization structure. There were two characteristic bands to be noted in the spectra: D band (~ 1360 cm−1) and G band (~ 1580 cm−1), which correspond to the vibration of amorphous or unorganized and graphitic structures of carbon materials, respectively. The integral peak area ratio of D band and G band, R = AD/AG, could well reflect the graphitization degree and was inversely proportional to an in-plane microcrystalline size of carbon materials. The lower R value indicateed a higher graphitization degree of residues and the matrix would obtain a better shield protection from burning [22,23,24]. As seen from Fig. 11, the R values for EVA/IFR and EVA/IFR/0.5ZnO were 2.88 and 2.12 respectively, meaning that EVA/IFR/0.5ZnO residues possessed richer graphitic structure, while EVA/IFR char layer contained less graphitic structure. Raman analysis results further manifested the introduction of nano-ZnO could accelerate the EVA/IFR composite forming higher graphitization-degree char residues, more effectively protecting the composites against burning and hence leading to better flame retardancy for EVA/IFR/ZnO composite.

Fig. 11
figure 11

Raman spectra of EVA/IFR and EVA/IFR/0.5ZnO char residues

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Composition Analysis of Char Residues

FTIR spectra of char residues after cone test for EVA/IFR-3 and EVA/IFR-6 are presented in Fig. 12, which could be used to investigate the chemical composition. The spectra showed typical characteristic peak at 3118 cm−1 belonged to –OH and the absorption peak at about 1638 cm−1 was attributed to polyaromatic C=C structure [25]. Band appeared at 1400 cm−1 was attributed to P–N structure, and meanwhile the absorption peak located at 1002 cm−1 should be the corresponding contribution of the stretching vibration of C–O–P structure [25]. The above FTIR spectra revealed that there were abundant C–O–P and P–N structures existing in residues, indicating that cross-linked reactions took place between APP and HTCFA [26] and consequently phosphorous-containing polyaromatic residues formed during burning. The type of chars could result in a superior barrier to prevent effectively the material from degrading. Furthermore, comparing the two FTIR spectra, the characteristic peaks for EVA/IFR/ZnO at 1400 cm−1 and 1004 cm−1, corresponding to P–N and P–O–C structures, showed quite higher intensity than EVA/IFR, which may be due to the reactions between nano-ZnO and APP, remaining more polyphosphoric acid to react with the char-formation agent HTCFA in condensed phase. This also explained why EVA/IFR/ZnO possessed higher-quality char residues as mentioned above.

Fig. 12
figure 12

FTIR spectra of EVA/IFR and EVA/IFR/0.5ZnO char residues

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XPS spectra could analyze the surface chemical structure of condensed phase [27]. The element concentrations in residues after cone test are listed in Table 5. Obviously, the results showed nano-ZnO was beneficial to lock more P and O in condensed phase during combustion of EVA/IFR composite, which could effectively strengthen the char quality as discussed earlier.

Table 5 Elemental concentrations of EVA/IFR and EVA/IFR/0.5ZnO residues from XPS
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Meanwhile, C1s, O1s, N1s and P2p spectra of EVA/IFR and EVA/IFR/0.5ZnO residues are shown in Figs. 13, 14, 15 and 16 and the fitting results are listed in Table 6. From Fig. 13, the binding energy (BE) of 284.8 eV, 286.3 eV and 289.1 eV for C1s peaks were designated to C–C group in aliphatic and aromatic fragments [28], C–O in P–O–C structures and C=O groups oxidized respectively during combustion [29, 30]. As seen in Table 6, it should be noted the percentage of C–O groups in EVA/IFR/ZnO residues (7.5%) was higher than that in EVA/IFR residues (6.7%) and this suggested combination of nano-ZnO into IFR system could promote composites forming more P–O–C cross-linking structures. Especially for oxidized carbon atoms (C=O), the percentage in EVA/IFR/ZnO residues (0.2%) was nearly 12.5 times lower than that in EVA/IFR residues (2.5%), indicating the char barrier effect of the former with the lower oxidation was enough strong to restrain further oxidation of underlying matrix during combustion.

Fig. 13
figure 13

C1s spectra of char residues of EVA/IFR (a) and EVA/IFR/0.5ZnO (b)

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Fig. 14
figure 14

O1s spectra of char residues of EVA/IFR (a) and EVA/IFR/0.5ZnO (b)

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Fig. 15
figure 15

N1s spectra of char residues of EVA/IFR (a) and EVA/IFR/0.5ZnO (b)

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Fig. 16
figure 16

P2p spectra of char residues of EVA/IFR (a) and EVA/IFR/0.5ZnO (b)

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Table 6 Fitting results of C1s, O1s, N1s and P2p spectra for EVA/IFR and EVA/IFR/0.5ZnO residues
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The O1s fitting curves for the two samples are shown in Fig. 14. The signals at around 531.6 eV showed the existence of C=O of phosphate or carbonyl groups while the BE of 533.0 eV should be assigned to –O– in P–O–C and C–O–C groups [28, 30]. Clearly seen from Table 6, EVA/IFR/ZnO composite possessed the higher percentage of –O– group (84.1%) than EVA/IFR (82.4%) in residues, which revealed that the introduction of nanometre oxide zinc promoted IFR and EVA matrix forming more cross-linked C–O–C and C–O–P structures during combustion. Also, the result was in agreement with C1s spectra fitting results.

As seen in Fig. 15, the N1s spectra at 402.0 eV were corresponded to C–N or C=N in the triazine structures, which suggested nitrogen atoms existed in the form of C–N or C=N in residues.

In regard to the spectra of P2p, a single BE peak at 134.7 eV could be owing to P=O structure in P–O–C group. Nevertheless, as known, the P2p peak in the decomposition products (mainly polyphosphoric acid) of APP appeared at 132.8 eV [31]. Thus the shift of bind energy from 132.8 to 134.7 eV indicated the cross-linked reactions among the components containing APP took place, leading to forming C–O–P=O structures [32] in residues after combustion. It may also be concluded that the cross-linking reactions in EVA/IFR/ZnO composite with the addition of nano-ZnO were more efficient than in EVA/IFR proved by the higher P content in residues (Table 5), which was from the phosphorous-containing acids remained in condensed phase during the char-formation process.

Conclusions

The combination of nano-ZnO with IFR system based on APP and triazine char-forming agent HTCFA improved the flame retardant properties and suppressed the combustion behaviour of EVA with increasing LOI values and reducing the release of heat and smoke by promoting forming superior char layer. Thermal analysis results showed that there was an obvious synergistic effect between nano-ZnO and IFR, and the incorporation of the metal oxide significantly promoted forming more stable char residues of IFR and EVA/IFR. SEM results proved the introduction of nano-ZnO facilitated the formation of a homogeneous and compact intumescent char layer during combustion and this kind of char would play a critical role in condensed-phase flame-retardant mechanism. The residue analysis results indicated nano-ZnO promoted the formation of higher graphitization degree char layer including richer P–O–C and P–N cross-linked structures due to the catalytic carbonization effect of metal oxide and consequently the high-quality char residues more efficiently suppressed the combustion and smoke release of EVA composites. It was concluded that nano-ZnO could be regarded to be an effective synergist enhancing the flame retardant and smoke suppression functions of IFR system based on APP and HTCFA.