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Combustion characteristics and synergistic effects of red phosphorus masterbatch with expandable graphite in the flame retardant HDPE/EVA composites.

INTRODUCTION

High-density polyethylene (HDPE) is one of the most widely used polyolefins due to its balanced mechanical properties, chemical resistance, and ease processing advantages [1-4]. To improve its flexibility, toughness, environmental stress cracking resistance and transparency, one of the most used attempts, such as the blending of HDPE with poly(ethylene-co-vinyl acetate) (EVA), has been widely performed [5-7], However, the inherent flammability of HDPE/EVA blends has limited its applications in many fields where good flame-retardant property is required. To solve this problem, some flame retardants were added into HDPE/EVA blends to improve the flame retardancy of materials [8-10]. Traditional halogen-containing flame retardant materials produce a great deal of smoke and toxic gases during the combustion. It is therefore worthwhile to investigate halogen-free flame retardants instead of the halogen-containing flame retardants [11-14].

In recent years, intumescent flame retardants (IFRs) have been extensively investigated, and exhibit excellent flame retardancy and good char-forming ability [15-19]. Expandable graphite (EG) is being used in a growing number of applications as one of IFRs additives, as a char-forming agent, as a blowing agent, and as a smoke suppressant [20-22]. EG is one kind of graphite intercalation compounds that have been treated with an agent ([H.sub.2] S[O.sub.4] or HN[O.sub.3]) which is intercalated into the crystal structure of the graphite. The intercalated particles under heat source can expand in the direction perpendicular to the carbon layers in the crystal structure [23, 24], These intumescent structures can rely on heat-induced decomposition to produce a char layer that insulates the substrate from the heat source and oxygen. Therefore EG has been successfully used as a flame retardant for polymer materials and the satisfactory properties can be obtained. Thirumal et al. [25] prepared water-blown rigid polyurethane foam (PUF) with two different particle sizes (180 and 300 [micro]m) of EG. It was found that the flame-retardant properties of PUF were improved with increasing EG loading level. PUF filled with the larger EG particles showed better mechanical properties and fire-retardant properties than the PUF filled with smaller ones. Svoboda and co-workers [26] aslo studied the effect of different weight ratios of EG on the flame retardancy of ethylene-octene copolymer composites. In addition, EG particles were also prepared by the [H.sub.2] [O.sub.2]-hydrothermal method. The product was called as [H.sub.2] [O.sub.2]-HEG, and the flame-retardant properties of the composites were investigated by Kuan et al. [21]. It was found that the [H.sub.2] [O.sub.2]-hydrothermal method was more efficient on improving the expanded volume of EG, and the HDPE/[H.sub.2] [O.sub.2]-HEG composites possessed excellent thermal stability and flame-retardant property.

However, flame-retardant efficiency of polyolefin with only EG as a flame retardant is relatively low, to obtain required flame retardancy, so the addition of synergistic flame retardants is necessary. Cai and co-authors [27] prepared a flame retardant shape-stabilized phase change material made up of paraffin, HDPE, EG and ammonium polyphosphate (APP) and zinc borate (ZB). The results indicated that the synergistic effects between EG and APP contributed to a higher thermal stability, however, the anti-synergistic effects between EG and ZB occurred, leading to a low thermal stability. Flame retardancy and thermal decomposition properties of flexible PUF filled with EG, ZB, Mg[(OH).sub.2] et al. were investigated and discussed. The improvement of fire resistance of the composites was observed on the basis of their own mechanism [28]. The flammability of EVA/IFR [APP/pentaerythritol (PER)/ZB system] and EVA/IFR/Synergist (CaC[O.sub.3], natural graphite, or EG) composites was investigated by Wu et al. [29]. The data showed that the heat release rate (HRR), total heat release (THR), and total smoke release of EVA/1FR/EG composites decreased to about 62.1, 76.2, and 44% compared with that of pure EVA, respectively. This suggested that the fire resistance of the materials was considerably improved by the synergistic effects of IFR with EG. Ge et al. [20] also reported a synergistic effect of APP with EG on the flame retardancy of acrylonitrile-butadiene-styrene (ABS). It was found that the limited oxygen index (LOI) reached 31% and the UL-94 test passed V-0 rating at a weight ratio of 3:1 for EG and APP when the total loading level of flame retardants was fixed at 15 wt%. TGA data indicated that the addition of EG and APP (3:1 by weight) to ABS led to an increase in the amount of high-temperature residues by 11.8 wt%, and a decrease of mass loss rate by 0.7%/[degrees]C compared with pure ABS.

As far as we are aware, however, no work has so far been reported on the synergistic effects of red phosphorus master-batch (RPM) with EG in the flame-retardant HDPE/EVA blends. In our previous work [9], it was found that EG treated by surface modifier could improve the mechanical properties. Although the peak of HRR decreased sharply in a cone calorimeter experiment, however, the flame-retardant HDPE/EVA composites was still burning quickly when exposed to a fire and the UL-94 V-0 rating could not be achieved. In this study, attempts have been made to develop high efficiency EG-based flame retardancy in HDPE/EVA blends. The synergistic effects and mechanism of RPM with EG on flame retardancy and thermal properties of HDPE/EVA/EG/RPM composites have been investigated by using limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CCT), thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) and scanning electron microscopy (SEM).

EXPERIMENTAL

Materials

High-density polyethylene (HDPE, 5000S, MFR = 0.923 g/ 10min, ASTM D1238-04) was obtained from Daqing Petrochemical Company, China Petroleum. Ethylene vinyl-acetate copolymer (EVA, 7240M, VA= 15 wt%, MFR = 1.5 g/10 min, ASTM D1238-04) was supplied by TAISOX, China Taiwan. EG (average particle size: 45 /un) was supplied by Qingdao Kangboer Graphite Company. RPM, RPM660P, made of 40 wt% polypropylene and 60 wt% red phosphorus, was supplied by Chenguang Chemical Research and Design Institute Co., LTD. Silane coupling agent ([gamma]-methacryloxypropyl trimethoxy silane) was a commercial product supplied by Nanjing Shuguang Chemical Group.

Sample Preparation

The HDPE/EVA (70/30, wt/wt) composites with the desired amounts of EG and RPM were compounded in a twin-screw extruder (Model HFB-150/3300, made in Nanjing Ruiya Polymer Processing Equipment Co., China) at the range of 100-170[degrees]C and screw speed of 150 rpm. The extrudate was cut into pellets. And then all the dried samples were hot-pressed under 8 MPa for 10 min at 170[degrees]C into sheets of suitable thickness. The sheet size and the thickness were dependent on the testing methods. The formulations of the samples used in the present study were listed in Table 1.

Measurements and Characterization

Limiting Oxygen Index (LOI). The LOI value was measured using a JF-4 type instrument (Manufactured by Jiangning Analysis Instrument Factory, Nanjing, China) on sheets 120 x 6.5 x 3 [mm.sup.3] according to the standard oxygen index test (ISO 4589).

UL-94 Test. The UL-94 vertical burning test was performed with a vertical burning instrument (made in Jiangning, China). The specimens for test were of dimensions of 127 x 12.7 x 3 [mm.sup.3] according to the American National UL-94 test (ASTM D3801).

Cone Calorimeter Test (CCT). The cone calorimeter (Stanton Redcroft, UK) tests were carried out according to ISO 5660 standard procedures. Each specimen of dimensions 100 X 100 x 3 [mm.sub.3] was wrapped in aluminium foil and exposed horizontally to an external heat flux of 35 kW/[m.sup.2].

Thermogravimetric Analysis (TGA). The thermal properties of the composites were examined by TGA using NETZSCH TG209F1 Thermal Analyzer. In each case, about 8-10 mg specimens were heated from room temperature to 700[degrees]C with a linear heating rate of 10[degrees]C/min under [N.sub.2] atmosphere.

Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra were recorded using a Thermo Nicolet 6700 FTIR spectrometer. All the samples were maintained at a series of temperatures isothermally for 15 min in the muffle furnace. The charred residues left after degradation of samples at different temperatures were mixed with KBr powders, and then the mixture was compressed into plates for FTIR analysis.

Morphology Observation. SEM observations of char residues were performed by using a field emission scanning electron microscopy (FE-SEM, JEOL JSM-7500F). The charred residues of the samples were obtained from after UL-94 burning tests. The gold-coated samples to avoid accumulation of charges were analyzed at an accelerating voltage of 20 kV.

RESULTS AND DISCUSSION

Flammability Behaviors: LOI And UL-94 Test

The flammability properties of HDPE/EVA composites containing different flame retardants are analyzed by LOI and UL-94 vertical burning test, and the results are listed in Table 1. The LOI value of HDPE/EVA is only 18.5%, which indicates its flammable nature. However, the flame-retardant properties of HDPE/EVA are greatly improved due to the addition of RPM with EG. The LOI value increases with increasing the RPM content. It reaches a maximum as the weight ration of EG and RPM is 2:1, and then decreases when the content of RPM is higher than that of EG. For example, the LOI value of the HDPE/EVA/EG composites (sample HEEM0) is 25.0%, however, the LOI values of the HEEM2 and HEEM4 samples with 6.7 and 13.3 phr RPM are 28.4 and 27.3%, respectively. The data suggest that the increase of LOI of the composites is attributed to the better synergistic effects of suitable amount of RPM with EG in the flame-retardant HDPE/EVA. But more RPM will cause an anti-synergistic effect for RPM and EG in the flame-retardant HDPE/EVA. The UL-94 test also shows the same trend. The HDPE/EVA and HEEM0 samples burn quickly, and even the flame reached the clamp, thus resulting in the failure of classification (listed in Table 1). In addition, HDPE/EVA burned with serious dripping, but the dripping phenomenon disappeared due to the presence of EG for HEEM0. The HDPE/ EVA composites only with individual 20 phr EG do not pass the UL-94 level. To solve this problem, RPM is selected as a synergist of EG to improve the flame retardancy of the composites, where total amount of flame retardants including both EG and RPM is kept at 20 phr. The addition of RPM has a great effect on the flammability of HDPE/EVA/EG composites. At the loading level of only 5 phr RPM, the stringent rating UL-94 V-0 is achieved for the HEEM1 sample. At the same time, it can be found from Table 1 that HDPE/EVA/RPM (HEEM6 sample) only achieves the UL-94 V-2 rating and the LOI value decreases to 23.2%. The results further manifest that a favorable synergistic effect exists between suitable amount of RPM and EG on enhancing the flame retardancy of HDPE/EVA.

Figure 1 shows the photographs of the samples of HDPE/ EVA, HEEM0, and HEEM2 collected after UL-94 test. The melted-form residues of HDPE/EVA are improved by the addition of EG and RPM. The pointed shape formed at the ignition side due to flame dripping is gradually changed to an obvious swollen char residue. There is not any charred residue left for HDPE/EVA after combustion. However, when EG is used alone as a flame retardant, the HDPE/EVA/EG composites (HEEM0) fails in UL-94 test because the fluffy and brittle charred residues has formed on the surface of materials. The char layer of HEEM2 is obviously much denser and more compact than that of HEEM0. The high-quality char layer can efficiently prevent the underlying polymeric materials from burning further.

Dynamic Flammability: Cone Calorimeter Test

CCT based on the oxygen consumption principle has been widely used to evaluate the flammability characteristics of polymeric materials [30-32]. Some parameters, including the time to ignition (TTI), the HRR, the peak heat release rate (PHRR), the THR, mass loss, CO and C[O.sub.2] production rate, can be obtained from cone calorimeter of which the most important parameters are HRR and PHRR.

Figure 2 shows the dynamic curves of HRR versus time for HDPE/EVA, HEEM0, HEEM2, and HEEM6 samples. The detailed data of the aforementioned series of samples are listed in Table 2. It is clearly observed that HDPE/EVA bums out within 500 s after ignition. A very sharp HRR peak with a PHRR of 672.7kW/[m.sup.2] for HDPE/EVA appears at the range of 100-500 s. However, the HEEM0 sample with only 20 phr EG presents a dramatic decrease of the HRR values and its flammability time is prolonged to 985 s from the 500 s of HDPE/EVA. The PHRR values of HEEM0 and HEEM6 samples dramatically decreases to 132.2 and 232.8 kW/[m.sup.2], which are only 19.7 and 34.5% of that of HDPE/EVA, respectively. In addition, the HRR values of the HEEM2 sample with 13.3 phr EG and 6.7 phr RPM are lower than those of HEEM0 and HEEM6 samples during the whole period, and the PHRR value is only 101.1 kW/[m.sup.2]. This indicates a better fire resistance for this sample. The results reveal that the combination of RPM with EG presents a favorable flame-retardant synergistic effect in flame-retardant HDPE/ EVA/EG/RPM composites.

Fire performance index (FPI) is defined as the ratio of TTI to PHRR, which is generally used to design the escape time for firefighters in a real fire [30]. The value of FPI is also used to predict whether a polymer material easily develops drastic combustion after ignition. Therefore, the higher the FPI value, the better is the fire resistance. It is found from Table 2 that the TTI value of HEEM0 increases to 38 s from the 32 s of HDPE/ EVA. In addition, the TTI value is further elongated to 50 s for the HEEM2, which is larger than those of the other three materials. This indicates that the combination of EG and RPM additives can decrease the thermal conductivity of the composites. A varied TTI can be due to quite a number of effects such as decomposition temperature, decomposition kinetics and the combustibility of decomposition products [33], The FPI value of HDPE/EVA is 0.0476, whereas the FPI values of HEEM0 and HEEM2 samples increase to 0.2874 and 0.4946, which are about 6.0 and 10.4 times of that of HDPE/EVA, respectively. But the FPI decreases for HEMM6 with only 20 phr RPM. Generally, the higher FPI and lower PHRR values mean that the better fire resistance can be obtained, which can reduce the loss and casualty in a real fire [30], This indicates that the flame retardancy of the HDPE/EVA blend is improved greatly by EG. Moreover, HEEM2 has the highest FPI value among four samples, which means the best fire resistance is obtained for the composites with 6.7 phr RPM. The above results further indicate the good synergistic effects of RPM with EG in the flame retarded HDPE/EVA composites.

The THR curves versus time for the above samples are shown in Fig. 3. The changes of THR curves are similar to those of HRR curves and their values significantly decrease for the flame-retardant composites compared with HDPE/EVA. The THR values of HDPE/EVA/EG (HEEM0) and HDPE/EVA/ RPM (HEEM6) decrease to 68.9 and 73.4 MJ/[m.sup.2] from the 77.1 MJ/[m.sup.2] of HDPE/EVA after burning, respectively. And the THR value of HDPE/EVA/EG/RPM (HEEM2) is only 50.3 MJ/[m.sup.2]. The phenomenon can be explained that an intumescem char layer is formed on the surface of the matrix while burning, which makes a thermal insulation, provokes the extinguishment of the flame, prevents combustible gases from feeding the flame, and separates oxygen from burning materials [20], In addition, the THR values of HEEM2 are lower than those of HEEM0 and HEEM6 during the whole combustion, indicating that the RPM has good synergistic effect of flame-retardant property with EG in the flame retarded HDPE/EVA composites.

Figure 4 displays the dynamic mass loss curves versus time for HDPE/EVA, HEEM0, HEEM2, and HEEM6 samples. It is clearly observed from Fig. 4 that HDPE/EVA loses its weight quickly after ignition and a dramatic decrease of mass occurs at the range of 200-400 s in a short time, whereas the mass losses of HEEM0, HEEM2 and HEEM6 are lower than that of HDPE/ EVA, and the mass decreases slowly during the whole combustion. Moreover, the mass loss slows down further due to the synergistic effect of RPM with EG in the flame-retardant HDPE/EVA. The charred residues of the HEEM0 and HEEM2 samples are about 28.7 and 33.5% after flammability, respectively. This is attributed to char formation and its morphological structure on the surface of the polymer matrix, and the favorable flame-retardant synergistic effect of RPM with EG [29]. During the combustion, the compact charred residues may occur on the surface of the burning materials creating a physical protective barrier. But the charred residues of the HDPE/EVA composites with only 20 phr RPM (HEEM6) decrease compared with those of HEEM0 and HEEM2.

Polymer materials can release a large amount of toxic gases and smoke during the combustion process, and the C[O.sub.2] and CO along with HRR also play a critical role in fire conditions. The C[O.sub.2] and CO production rate curves for the HDPE/EVA, HEEM0, HEEM2, and HEEM6 samples were shown in Figs. 5 and 6, respectively. It is evidenced from Fig. 5 that the changes of C[O.sub.2] emission are similar to those of HRR and the C[O.sub.2] emission values of the flame-retardant composites drastically decrease compared with that of HDPE/EVA. And the HEEM2 sample has the lowest C[O.sub.2] emission among the four samples. These data indicate again that the synergistic effects of RPM with EG significantly reduce the gases production of HEEM2. However, CO emission values for the above samples are not parallel to those of HRR. The incorporation of RPM leads to more CO emission compared with HEEM0 sample. The possible reason is that the products of phosphorus oxide and phosphoric acid will consume more oxygen, which causes less C[O.sub.2] and more CO emission for the HEEM2 sample. In addition, the HDPE/EVA/RPM (HEEM6) produces less C[O.sub.2] and more CO emission, which is in accord with the above discussion.

Figure 7 shows the photographs of the residue samples collected after CCT. It can be found from Fig. 7A that the combustion of HDPE/EVA is very complete and no charred residue is left. However, the intumescent charred residues are formed in the flame retardant composites, as shown in Fig. 7B and C. At the same time, HEEM0 left loose and brittle charred residues, whereas the charred residues of HEEM2 are more compact and integrated. This suggests that the incorporation of RPM with EG can improve the structures of the charred layers and reinforce thermal stability of the charred layers. The compact and consolidated charred layers, as excellent barriers, can efficiently protect the underlying polymer materials from further burning, and reduce heat transfer and air incursion. This exhibits that HEEM2 has excellent flame-retardant performance.

Thermal Stability

TGA and derivative thermogravimetric (DTG) curves of EG and RPM flame retardants under a flow of nitrogen at a heating rate of 10[degrees]C/min are illustrated in Fig. 8, and the correlative data are listed in Table 3. As shown in Fig. 8, the TGA curves of the two flame retardants only display one-step thermal degradation process, and RPM has better thermal stability than EG. The charred residue of EG is more than that of RPM. EG begins to expand at 180[degrees]C and the maximum weight loss occurs at 210[degrees]C accompanied with 51.6 wt% expansion of the charred residues at 700[degrees]C. However, RPM begins to decompose at 408[degrees]C and the maximum weight loss occurs at 465[degrees]C, leaving 10.3 wt% charred residues at 700[degrees]C.

Figure 9 shows the TGA and DTG curves of HDPE/EVA, HEEM0 and HEEM2 samples, and the detailed data are given in Table 3. The thermal decomposition behaviors of the above three samples mainly display a two-step process. The first stage occurring at the range of 300-400[degrees]C is related to the expansion of EG and the deacylation of the EVA phase through radical and ionic [beta]-elimination mechanism [34, 35]. The second degradation stage which takes place at the temperature range of 400-520[degrees]C involves the degradation of the HDPE phase accompanied with the decomposition of the polyacetylene-ethylene chains and volatilization of the residual polymer [35]. And there is not any charred residue left for the HDPE/EVA sample, suggesting that the degradation of HDPE/EVA is very complete. However, the charred residues of the HEEM0 and HEEM2 samples after degradation are kept by 18.8 and 10.1 wt% at the high temperature of 700[degrees]C, respectively. This possible reason is that EG expands and generates voluminous insulative layers to protect the underlying polymeric matrix when exposed to heat [29]. The addition of RPM significantly reduces the charred residue of the HEEM2 sample, and this is determined by the intrinsic characteristics of RPM. As a synergistic flame retardant, RPM itself has hardly charring capacity (as shown in Fig. 8) during burning process at a high temperature under [N.sub.2] because its flame-retardant mechanism is gas-phase and condensed-phase flame retardancy. Only the decomposition of RPM occurs at high temperatures, resulting in lower charred residues. Compared with CCT data, the charred residues of TGA are inconsistent with those of CCT. We think that the difference may be caused by the different testing methods and conditions. For CCT, the charred residues were obtained after combustion under air. However, for TGA, the residues were obtained after degradation in [N.sub.2].

The onset degradation temperature ([T.sub.onset]), the first maximum weight loss temperature and the second one ([T.sub.max2]) as well as the maximal weight loss rate and its corresponding temperature ([D.sub.pk] and [T.sub.pk]) from the TGA and DTG curves are listed in Table 3. There are more weight losses and more quick weight loss rate for the flame-retardant HDPE/EVA composites than the HDPE/EVA matrix at the range of 200440[degrees]C, which can be attributed to the thermal decomposition of the additives and polymer matrix. [T.sub.onset], of HEEMO is lower than that of HDPE/EVA by 35[degrees]C. When EG is exposed to a heat source over 180[degrees]C, it starts to expand and decompose because of the presence of sulfuric acid or nitric acid, which results in the low [T.sub.onset] for HEEMO. The thennal degradation of HEEM2 was slightly slower than that of HEEMO at the range of 200-440[degrees]C, whereas HEEM2 shows much faster degradation at the range of 440-500[degrees]C. However, the corresponding [T.sub.onset], [T.sub.max1] and [T.sub.max2] of HEEM2 increase to 393, 375 and 495[degrees]C, which are 15, 2 and 2[degrees]C higher than those of HEEMO, respectively. The apparent improvement in the thermal stability of HEEM2 can be explained that the stable charred layers act as barriers hindering the transport of degradation products, which can efficiently prevent the underlying polymer materials from being degraded.

Therrmal-Oxidative Degradation Behaviors

Chemical alterations in the solid residue at different pyrolysis temperatures are detected by FTIR. The changes of FTIR spectra of the HEEMO and HEEM2 samples at different degradation temperatures are showed in Fig. 10. It was found from Fig. 10A that some characteristic peaks of HEEMO sample disappear gradually with the increase of pyrolysis temperature and all peaks nearly disappear at 500[degrees]C, indicating that the sample decomposes completely. The intensities of the peaks at 2918 and 2848 [cm.sup.-1] assigning to the C[H.sub.2] asymmetric and symmetric vibrations and at 1463 [cm.sup.-1] corresponding to the C[H.sub.2] or C[H.sub.3] deformation vibration of aliphatic groups decrease gradually with increasing the thermal oxidative degradation temperature [29]. Moreover, the relative intensities of the characteristic peaks at 1740, 1380, and 1237 [cm.sup.-1] (corresponding to the stretching vibration of C=0, C[H.sub.3] and C-O stretching vibration, respectively [36-38]) decrease and then disappear with increasing degradation temperature. In addition, the absorption peak at 719 [cm.sup.-1] due to the deformation vibration in [(-C[H.sub.2]-).sub.n (n [greater than or equal to] 4) groups disappear with the increase of temperature [29, 36]. The absorption peak at 1639 [cm.sup.-1] assigning to the stretching vibration of C=C still remains at different pyrolysis temperatures, which indicates that the formation of carbon-carbon double bonds results from the cleavage of the main chains after thermal oxidation degradation [36]. The above changes of these bonds indicate that the breakdowns of the HDPE and EVA main chains take place during the thermal degradation.

It is observed from Fig. 10B that the thermo-oxidation degradation of the HEEM2 sample is significantly affected by the addition of RPM with EG. Several weak peaks between 1000 and 1200 [cm.sup.-1] are the characteristic peaks of RPM, which are attributed to P--O--P, P--O--C and P[O.sub.3] complex structures [39]. The location of peak at 1018 [cm.sup.-1] is drifted and the peak at 1121 [cm.sup.-1] is widened obviously, which shows that RPM has been predominantly oxidized to various phosphoric derivatives. These derivatives can react with resin to form more stable structure containing P--O--P and P--O--C complexes. This proves the previous conjecture of TGA further. These thermo-oxidative data give positive evidence of the flame-retardant mechanism: the RPM can promote the formation of compact char layers in the condensed phase during the burning of polymer materials. These compact char layers can slow down heat and mass transfer between the gas and condensed phases, and prevent the underlying polymeric substrate from attacking by heat flux in a flame further.

Morphology of Charred Residues

Figure 11 displays the SEM photographs of the charred residues of HEEMO and HEEM2 samples after UL-94 burning tests. It is clearly found from Fig. 11A and B that there are lots of expanded "worm-like" structures on the surface of charred residues, which is a typical of EG layer structure. The charred residues of the HDPE/EVA/EG composites with only 20 phr EG (HEEMO) are very loose. At the same time, there are many holes and crevasses on the surface of the charred residues for the HEEMO sample. The possible reason is that the heat and flammable volatile gases formed from the pyrolysis of the HEEMO sample penetrate the loose charred layers during the combustion. However, almost no holes or flaw can be seen in the charred residues of the HEEM2 sample containing only 6.7 phr RPM, as shown in Fig. 11B. Obviously the charred residue layers of the HDPE/EVA composites containing RPM with EG are solider and more compact than that of the HDPE/EVA composites with EG alone. The transfer of heat and flammable gases and volatiles is effectively retarded by this insulative compact layer, and thus the flame-retardant performances of the sample are improved. The SEM observations exhibit further evidence that the synergistic mechanism of RPM with EG in the flame-retardant HDPE/EVA/ EG/RPM composites can be ascribed to the formation of compact charred residues promoted by RPM with EG as effective barriers and thermal insulation layers.

Through the analysis of TGA, FTIR, and SEM, and combined with the other previous studies [35, 36], the synergistic mechanism is proposed for RPM with EG in the flame-retardant HDPE/ EVA/EG/RPM composites. In the gas phase, the nonflammable gases including CO2, S02, and H20 release during the decomposition of the composites can dilute the combustible gases. In the condensed phase, RPM and EG play an important role in different periods. Initially, the "worm-like" char structure is formed by the inflation of EG at the range of 180-250[degrees]C, but the charred layers cannot effectively endure heat flux for a long time at a high temperature. Then phosphoric derivatives generated from RPM cover the surface of the underlying polymer, which can strengthen the char barrier for its strong adhesion effect.

CONCLUSIONS

The flame resistance, thermal stability, and synergistic effects of RPM with EG in the flame-retardant HDPE/EVA composites were investigated. Significant improvement in the flame-retardant properties was observed for the HDPE/EVA composites with EG and RPM additives. This was attributed to the solider, stable, and compact charred residues and the synergistic effect of RPM with EG in the flame-retardant HDPE/EVA/EG/ RPM composites. The thermal stability of the HDPE/EVA blends was enhanced by the addition of flame retardants. The quality and quantity of the charred residues were improved for HDPE/EVA/EG/RPM composites due to the addition of RPM.

ABBREVIATIONS

ABS      Acrylonitrile-butadiene-styrene
APP      Ammonium polyphosphate
CCT      Cone calorimeter test
DTG      Thermogra v imetric
EG       Expandable graphite
EVA      Ethylene vinyl-acetate
FE-SEM   Field emission scanning electron microscopy
FPI      Fire performance index
FTIR     Fourier-transform infrared
HDPE     High-density polyethylene
HRR      Heat release rate
LOI      Limiting oxygen index
PER      Pentaerythritol
PHRR     Peak heat release rate
PUF      Polyurethane foam
RPM      Red phosphorus masterbatch
SEM      Scanning electron microscopy
TGA      Thermogravimetric analysis
THR      Total heat release
TTI      Time to ignition
ZB       Zinc borate


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Mengqi Tang, (1) Man Chen, (1) Yang Xu, Xiaolang Chen, (1,2) Zhidan Sun, (3) Zhibin Zhang (3)

(1) Key Laboratory of Advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

(2) The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

(3) School of Life Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

Correspondence to: X. Chen; e-mail: chenxl612@sina.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51003088, 51373139; contract grant sponsor: Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University); contract grant number: sklpme2014-4-21; contract grant sponsor: Fundamental Research Funds for the Central Universities; contract grant number: SWJTU12CX009.

DOI 10.1002/pen.24180

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Formulations and flame retardancy of several samples.

                                                  UL-94
            (HDPE/    EG/    RPM/
Samples    EVA)/phr   phr    phr    LOI/%   Rating   Dripping

HDPE/EVA     100       0      0     18.5    Failed      Y
HEEM0        100      20      0     25.0    Failed      N
HEEM1        100      15      5     27.5     V-0        N
HEEM2        100      13.3    6.7   28.4     V-0        N
HEEM3        100      10     10     28.0     V-0        N
HEEM4        100       6.7   13.3   27.3     V-0        N
HEEM5        100       5     15     26.3     V-0        N
HEEM6        100       0     20     23.2     V-2        Y

HDPE: EVA = 70: 30; EG treated by 2 wt% silane coupling agent.

TABLE 2. Cone calorimeter data of HDPE/EVA/EG/RPM composites.

                            PHRR (b)         AHRR (c)
Samples    TTI (a) (s)   (kW/[m.sup.2])   (Kw/[m.sup.2])

HDPE/EVA       32            672.7            216.8
HEEM0          30            132.2             91.4
HEEM2          50            101.1             74.8
HEEM6          35            232.8            152.7

                FPI (d)
Samples    ([m.sup.2] x s/kW)   Residues (%)

HDPE/EVA         0.0476             5.1
HEEM0            0.2269             29.1
HEEM2            0.4946             30.5
HEEM6            0.1503             24.6

(a) Time to ignition.

(b) Peak of heat release rate, expressing the intensity of a fire.

(c) Average HRR within the front 300s from heat radiation.

(d) Fire performance index, the ratio of TTI and PHRR.

TABLE 3. Detailed data from TGA and DTG of several samples.

           [T.sub.onset]   [T.sub.max 1]   [T.sub.max 1]
Samples    ([degrees]C)    ([degrees]C)    ([degrees]C)

EG              180             --              297
RPM             408             --              515
HDPE/EVA        413             371             482
HEEMO           378             373             493
HEEM2           393             375             495

            [T.sub.pl]    [D.sub.pk]
Samples    ([degrees]C)    (%/min)

EG             210           4.6
RPM            465           21.1
HDPE/EVA       468           31.0
HEEMO          470           20.6
HEEM2          468           23.0
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Title Annotation:high-density polyethylene/ethylene vinyl-acetate
Author:Tang, Mengqi; Chen, Man; Xu, Yang; Chen, Xiaolang; Sun, Zhidan; Zhang, Zhibin
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Dec 1, 2015
Words:6177
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