Thermal and combustion behavior of ethylene-vinyl acetate/aluminum trihydroxide/Fe-montmorillonite composites.
Flame retardancy is commonly introduced to polymers by adding metal hydroxide fillers, such as aluminum trihydroxide (ATH). Typically about 60 wt% of filler is required, however, and this creates problems in compounding and high density end products with poor flexibility (1). Because of the difference in chemical structure between the polymer matrix and the ATH particles, the ATH particles usually must be modified to decrease the degree of interfacial tension between the matrix and filler particles. Coaling agents such as fatty acids are commonly applied for modification of the ATH particles (2-4).
In the past two decades, polymer/layered inorganic nanocomposites (PLN) have attracted considerable attenlion because a small (<5 wt%) amount of nanolillers added into the composites can apparently enhance mechanical, thermal, dimensional, and barrier performance properties (5). It has been reported that Fe-OMT is one of the novel clays in the flame-retardant fields (6), (7). Fe-OMT can also be used as synergistic (lame retardant in ethylene-vinyl acetate copolymer/magnesium hydroxide (EVA/MH) and polypropylene/aluminum tri hydroxide (PP/ATH) composites (8), (9). As far as we are aware, no work has been done on the synergistic effect of Fe-OMT in EVA/ATH compounds. In the present work, the effects of Fe-OMT and ATH on the flame-relardant and thermal properties of the EVA/ATH/Fe-OMT composites have been studied using limiting oxygen index (LOI). UL-94 test, cone calorimeter, microscale combustion calorimeter (MCC), and thermal analysis (TGA).
EVA-14 (containing 14 wt% vinyl acetate) was bought from Sumitomo Chemical (Japan). ATH, with decomposition temperature of about 220 [degrees] C and average particle size of about 2 [micro]m was supplied by Martin. Germany. Fe-montmorillonite with cation-exchange capacity of 102 mequiv/l00 g was synthesized as follows: hydrous oxide was prepared by mixing [[Na.sub.2]Si.sub.3] * [9H.sub.2]O with [FeC.sub.3] * [6H.sub.2]O and [[Zn(COOCH.sub.3]).sub.2] * [2H.sub.2]O solutions to set the atomic ratio at Si/Fe/Zn = 4:1.7:0.3 (refer lo previous reports for a detailed method of synthesis (6), (7). Then the Fe-montmorillonite was treated with /V-hexadecyltri-methylammonium bromine (CTAB) to obtain Fe-OMT (6). The formulations are listed in Table 1.
TABLE 1. The formulations of the EVA/ATH/Fc OMT composites. Sample code EVA/wl% ATH/wt% Fe-OMT/wt% LOI UL 94 EVA 100.0 -- -- 17.0 No rating Fe-OMT-0 50.0 50.0 -- 30.0 No rating Fe-OMT-0.5 50.0 49.5 0.5 30.6 No rating Fe-OMT-1.0 50.0 49.0 1.0 31.8 No rating Fe-OMT-2.0 50.0 48.0 2.0 33.0 V-0 Fe-OMT-3.0 50.0 47.0 3.0 36.0 v-0
All the samples were prepared by using a Haake Rheo-mix Banbury mixer with the same procedures. EVA was added into the mixer with rotational speed of 20 rpm at 160 [degrees]C. ATH tiller and Fe-OMT were added after the EVA polymer melted and the mixing was carried out at 50 rpm for 10 min. The obtained composites were finally compression molded at 160 [degrees]C for 10 min under 10 MPa into sheets of suitable thickness. Samples for testing were cut from the compressed sheets according to the standards mentioned in the following part.
Limiting Oxygen Index
Limiting oxygen index (LOI) was measured according lo ASTM D 2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China). The specimens used for the test were of dimensions 100 x 6.5 x 3 [mm.sup.3].
UL 94 Testing
The vertical test was carried out on a CFZ-2-type instrument (Jiangning Analysis Instrument Company, China) according to the UL 94 test standard. The specimens were of dimensions 100 x 13 x 3 [mm.sup.3].
The cone calorimeter (Stanton Redcroft, UK) tests were performed according to ISO 5660 standard procedures. Each specimen of dimensions 100 x 100 x 3 [mm.sup.3] was wrapped in aluminum foil and exposed horizontally to an external heat flux of 35 [kW/m.sup.2].
Microscale Combustion Calorimeter
Approximately 4-8 mg of each sample was weighed with a microbalance and placed in a Pyroprobe (CDS Analytical Model 2000) for rapid pyrolysis in a pyrolysis-combustion flow calorimeter. A special probe was placed into a heated interface continuously purged with pure nitrogen. The system was programmed to heat at 1 [degrees]C/s from 90 to 600 [degrees]C and to hold the temperature there for 30 s. During pyrolysis, the volatilized decomposition products are transferred in the stream of nitrogen to a high-temperature combustion furnace where pure oxygen is added and the decomposition products are completely combusted. The amount of oxygen consumed is measured with an oxygen analyzer and used to calculate a heal release rate (HRR).
Thermogravimetric analysis was carried out in nitrogen on a STA 409C TGA apparatus (Netzsch Company, German) with ceramic crucible sample holders, at a heating rate of 10 [degrees]C/min.
RESULTS AND DISCUSSION
LOI and UL 94 Raring
The LOI and UL-94 tests are widely used to evaluate flame-retardant properties of polymer composites. Table 1 lists the related LOI and UL-94 data obtained from different loading of Fe-OMT. It can be seen from Table 1 that the LOI value of Fe-OMT-0 with 50 wt% ATH increases to 30.0 from 17.0 of original EVA without ATH. The LOI values of samples (Fe-OMT-0 to Fe-OMT-3.0) gradually increase to 36.0 with increase in the loading of Fe-OMT in the formulation. The results obtained from the UL-94 tests show that 2 wt% Fe-OMT can make the EVA/ATH/Fe-OMT composites pass the UL 94 test. These results indicate that the addition of a suitable amount of Fe-OMT can increase the llameretardance of EVA/ATH compounds.
OMT, used alone in polymer, is usually considered to be ineffective for LOI and UL 94 test (10). However, the above data indicate that the (lame-retardant performances of EVA/ATH compounds are enhanced by partly substituting ATH with Fe-OMT. The mechanism of the enhancement in LOI and UL 94 ratings is mainly due to the physical and chemical process in the solid phase, as reported in the literature (7-9). The addition of Fe-OMT also increases the polymer melting viscosity, which favors the LOI and UL-94 test (11). Furthermore, iron in Fe-OMT can capture radicals in the combustion process (7-9).
Heat Release Rate
Cone calorimeter has widely been used to evaluate the flammability characteristics of polymer materials. Although a cone calorimeter test is in a small-scale, the obtained results have been found to correlate well with those obtained from a large-scale lire test and can be used to predict the combustion behavior of materials in a real lire 112]. The HRR measured by cone calorimeter is a very important parameter as it expresses the intensity of a lire.
The changes of HRR as a function of burning time for different samples are shown in Fig. 1. It can be found from Fig. 1 that pure EVA burns very fast after ignition. A very sharp HRR curve appears at the range of 70-250 s, whereas Fe-OMT-0 with 50 wt% ATH shows a dramatic decline of the HRR curve and its combustion is prolonged to 390 s from the 250 s of the control EVA. The HRR of Fe-OMT-0 shows many small peaks during burning, which indicates the gradual burning of the specimen through the thickness alter the initial charred layers were formed. This combustion feature of multiple HRR peaks has also been reported by Grexa (13) and Fu (14). The HRR values of other samples (Fe-OMT-0.5 to Fe-OMT-2.0) decrease with increasing loading of Fe-OMT. and their multiple peak features is not as obvious as for Fe-OMT-0. This indicates that there is a uniform char residue on the surface of these samples. And, their burning time was also prolonged to 500-700 s. Fe-OMT-2.0 with 2 wt% Fe-OMT shows the lowest HRR value among all the samples. The heal release rate of Fe-OMT-3.0 increases compared with Fe-OMT-2.0. This result can be illustrated by the char residue after cone calorimeter test (Fig. 2). In the case of Fe-OMT-3.0, there is a collapse in the char residue, which indicates the char residue is loose and shows week strength.
OMT is usually considered to be an effective additive in cone calorimeter test in flame-retardant systems. The mechanism of the reduction in heat release rate is mainly due to the physical processes instead of chemical process in the condensed phase, as reported in the literature (6). OMT lends to migrate near the regressing sample surface without sinking through the polymer melt layer during the gasification/burning process (15), (16). In this EVA/ATH/ Fe-OMT composite, it has been found the flame-retardant mechanism is not only based on the physical effect, but also on the chemical effect. On the one hand, the accumulated Fe-OMT consequently formed a charred layer by interacting with the compounds from ATH, which acts as a heat insulation barrier. On the other hand, the carbonization by the accumulated Fe-OMT on the surface of the sample provides an important effect, which will be illustrated by the photographs of char residues after cone calorimeter (Fig. 2). This charred layer prevented heat transfer and transportation of degraded products between melting polymer and surface, thus reduced the HRR and related parameters.
Digital Photographs of Char Residue
Figure 2 are digital photos of residues of EVA/ATH/ Fe-OMT series. It can be seen that a coherent and dense char can be formed only at a suitable ratio of Fe-OMT to ATH. For EVA/ATH/Fe-OMT series, the samples with 0.5, 1.0, 2.0 wt% Fe-OMT can form good and coherent char residue after burning, as shown in Fig. 2. This result can interpret that the residual mass of Fe-OMT-1.0 with 1.0 wt% is higher than others from 200 to 750 s in Fig. 3. From the char structure, we can explain the combustion phenomenon of the flame-retardant EVA composites. It can be illustrate the reduced rate of mass loss with the addition of Fe-OMT in the mass loss curves from 200 to 750 s (Fig. 3). As a result, HRR values are strongly reduced, as shown in Fig. 1. Furthermore, there is a collapse in the char residue of Fe-OMT-3.0 with 3.0 wt9f Fe-OMT, which can be used to explain Fe-OMT-3.0 shows high heat release rate (Fig. 1) and low char residue weight (Fig. 3).
Figure 3 shows the weight of the char residues during cone calorimeter test. Il can be seen that the char residues weight of the samples with Fe-OMT is lower than that of the sample without Fe-OMT between 100 and 180 s. This phenomenon can be illustrated by the low decomposition temperature of Fe-OMT, which has been reported in literatures (5), (6). However, from 180 s, the mass of the samples with Fe-OMT is higher than that of Fe-OMT-0 without Fe-OMT. During combustion, Fe-OMT may migrate onto the surface of the burning and catalyze carbonization, creating a physical protective barrier on the surface of material, as shown in Fig. 6. The char on the surface would act as a protective barrier and can thus limit the oxygen diffusion to the substrate or give a less disturbing low volatilization rate. In this study, it was found that the char residue of samples with Fe-OMT is similar.
Total Heat Release
Figure 4 presents the total heat release (THR) for all the samples. The slope of THR curve can be assumed as representative of fire spread (17). From Fig. 4, it can be seen that the THR is decreased by Fe-OMT. It is very clear that the flame spread of samples (Fe-OMT-0-Fe-OMT-3.0) has decreased, and the flame spread of samples Fe-OMT-2.0 is comparatively the lowest. It is also suggested there is a synergistic effect of flame retardant between Fe-OMT and ATH.
Carbon Monoxide and Carbon Dioxide Production Rates
Figures 5 and 6 show the CO and [CO.sub.2] production rates from EVA and flame-retardant EVA under a heat flux of 35 [kW/m.sup.2]. The incomplete combustion of flame-retardant composite systems can be seen in the CO production rate. Compared with pristine EVA, the CO production rate of flame-retardant systems is highly decreased throughout the whole range of fire in the experiments. It has been reported that inorganic hydroxides usually act as catalysts for the oxidation of the carbonaceous residues reducing the [CO/CO.sub.2] ratio (18), (19). Furthermore, with the addition of Fe-OMT, the CO production rate decreases.
It is very interesting that the carbon monoxide production rate is greatly decreased with the addition of Fe-OMT. In the case of pristine EVA, there is CO production rate peak between 5:0 and 80 s, which attributes to the decomposition of EVA resin before ignition. After ignition, there is strong CO production rate peak. This peak is the incomplete combustion of a lot of pyrolysis products from EVA in short time. For the sample Fe-OMT-0, the first CO production rate peak between 40 and 160 s attributes to incomplete combustion led by water from decomposition of [AI(OH).sub.3] The second CO production rate peak between 160 and 370 s attributes to incomplete combustion of" a lot of combustible gas led by the cracks in char residue. The third CO production rate peak between 370 and 460 s is caused by incomplete combustion because of flame is out. For the samples with Fe-OMT, the first CO production rate peak decreased greatly than the sample Fe-OMT-0. Furthermore, it is decreased with the addition of Fe-OMT. More importantly, the initial time prolongs with the addition of Fe-OMT. This indicates that Fe-OMT has a very excellent ability to inhibit the formation of carbon monoxide. And. the last CO production rate peak of the sample with Fe-OMT is caused by incomplete combustion because of flame is out. And, the Fe-OMT-3.0 with 3.0 wt% Fe-OMT shows the lowest carbon monoxide production rate among all samples. The above phenomena can be illustrated in the following. Firstly, Fe-OMT can catalyze EVA carbonization (Fig. 2). That is, there is more compact char residue formed on the surface of the sample with Fe-OMT. The compact char residue can restrain combustible gases, so the released flammable gases can be complete combustion, which leads to the little carbon monoxide production rate. Second, the carbon monoxide can be catalyzed to carbon dioxide by Fe-OMT which can migrate onto the surface of ihe sample (15), (16).
The [CO.sub.2] production rates of the flame-retardant systems significantly decrease because the compact char residue prevent combustible gases from being diffused into air, resulting in a complete combustion of the released flammable gases. The [CO.sub.2] production rates data essentially mirror the HRR data.
Microscale Combustion Calorimeter
The dynamic flammability data detected by microscale combustion calorimeter of EVA and flame-retardant EVA samples are shown in Fig. 7. Compared with EVA, the peak HRR values decrease with addition of Fe-OMT. In the case of Fe-OMT-0, its peak HRR value is 446.5 J g [K .sup.-1], which is obviously lower than that of EVA (478.5 J g [K.sup.-1]) as shown in Fig. 7. As for Fe-OMT-0.5, peak HRR value is 408.1 J g [K .sup.-1]. lower than that of Fe-OMT-0. Furthermore, the peak heal release rate decreases further with the increase of Fe-OMT. And, the Fe-OMT-3.0 sample shows the lowest peak heat release rate. It is believed that stable char forming by the catalyzing carbonization of Fe-OMT is responsible for the improvement of flame retardant. This result is not well in line with the results from cone calorimeter (Fig. 1). The reason is that there is no collapse phenomenon in the MCC test, which needs only 4-8 mg sample. However, the strength of the char residue decreases greatly when the content of Fe-OMT is raised to 3.0 wt%, and there is a collapse in the char residue.
It is very interesting that the heat release rate decrease with the addition of ATH at the temperature between 340 and 380 [degrees]C. This can be illustrated by the fact that ATH decompose at about 250 [degrees]C to release some incombustible gas, such as water, which leads the decrease of heal release rate in microscale combustion calorimeter test. However, the heat release rate increases with the addition of Fe-OMT at the same temperature range compared with Fe-OMT-0 without Fe-OMT. The above phenomena can be illustrated in the following. The first reason is that some combustible gas may be released from the decomposition of the surfactant in Fe-OMT at low temperature; The second reason is the fact that the - OH groups on the Fe-OMT can catalyze EVA to decomposition (20), (21); the third reason is due to montmorillonite-catalyzed elimination of acetic acid as discussed in TGA results and in agreement with literature data (22).
TGA and DTG curves for EVA and its composites are shown in Fig. 8. EVA undergoes two degradation steps as shown in Fig. 8. The first decomposition step is due to the loss of acetic acid and the second involves random chain scission of the remaining material, forming unsaturated vapour species, such as butane and ethylene (23), (24).
From Fig. 8, it can be found that Fe-OMT-0 shows lower decomposition rate in the second step but higher in the first step than EVA. The incorporation of ATH lowers the decomposition rate of the second step but accelerates the loss of acetic acid. It is obvious that the - OH groups on the fillers can assist [beta]-hydrogen leaving. That is to say that the loss of acetic acid can be catalyzed by ATH. To our surprise, ternary composites, which contain both Fe-OMT and ATH, show similar degradation behavior with Fe-OMT-0. However, the char residue increases with the addition of Fe-OMT. Polymer/clay nanocomposites have been studied widely and Costache reported similar catalyzing function of OH groups on the edges of montmorillonite layers (19), (20), resulting in high char residue. These results are well corresponding with the heat release rate results in the MCC test.
Fe-OMT has a good flame-retardant synergistic effect with ATH in the EVA/ATH/Fe-OMT compounds. A suitable amount of Fe-OMT can increase the LOI value and UL-94 rating.
The cone calorimeter test data reveal that the values of HRR, Mass, and THR of EVA/ATH/Fe-OMT compounds apparently decrease with increasing amount of Fe-OMT. Fe-OMT is very effective to reduce the carbon monoxide production rate. The optimum amount of Fe-OMT in the compounds is 2 wt%. The microscale combustion calorimeter results indicate that Fe-OMT can decrease the heat release rate and catalyze the decomposition of EVA. The TG data confirmed the cone calorimeter results, in which Fe-OMT can catalyze carbonization.
The synergistic flame-retardant mechanism of Fe-OMT with ATH in the EVA compounds is due to its physical and chemical effect in the condensed phase.
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Xilei Chen, Chuanmei Jiao, Jun Zhang
College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao 266042, People's Republic of China
Correspondence to: Chuanmei Jiao; e-mail: jiaoehm@qusLedu.cn
Contract grant sponsor: The National Natural Science Foundation of China; contract grant number: 50876048.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2011 Society of Plastics Engineers
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|Author:||Chen, Xilei; Jiao, Chuanmei; Zhang, Jun|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2012|
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