Effects of Magnesium Hydroxide on the Flame Retardancy of Ethylene-Vinyl Acetate Copolymers/Nitrile Rubber Blends

The fire-resistant noncorrosive ethylene-vinyl acetate copolymers with vinyl acetate (VA) content > 40 wt.% (EVM)/acrylonitrile-butadiene rubber (NBR)/magnesium hydroxide (MDH) composites were prepared, and the influence of MDH contents on flame retardancy, thermal stability, filler dispersion, and mechanical properties were studied. The flame-retardant effect of three different flame retardants [micro-sized MDH (mMDH), nano-sized MDH (nMDH), and micro-sized aluminum hydroxide (mATH)] was also investigated by cone calorimetry, thermal gravimetric analysis, and rubber process analysis in this paper. The decrease of the heat release rate and total heat release, the increase of residual mass, and the enhancement of thermal stability of the composites were all due to the flame-retardant effect of the MDH. The EVM/NBR vulcanizates had the best flame retardancy when the mMDH content was 180 phr. It was also found that the nMDH, mMDH, and mATH at the same loading had no obvious influence on the limiting oxygen index, while the combustion behaviors measured by the cone calorimeter had significant differences. The addition of ATH prolonged the time to ignition. The mMDH showed a better flame retardancy for EVM/NBR vulcanizates than the nMDH.

Keywords: aluminum hydroxide; flame retardancy; magnesium hydroxide; polymer composite; rubber; thermal properties

Polymeric materials have been used widely during the past few decades. But most of them are easy to ignite under continuous heat and release some toxic gases and smoke during combustion, which increases the danger of fires. Therefore, study of fire-resistant noncorrosive (FRNC) polymeric materials continues to be of great concern all over the world.[[1]–[3]] Compared with halogenated flame retardants, inorganic hydroxide fillers such as magnesium hydroxide (MDH) and aluminum hydroxide (ATH) play more and more important roles as excellent nontoxic and smoke-suppressing additives.[[4],[5]] These hydroxide fillers release crystal water during thermal degradation, followed by an endothermic reaction to reduce the temperature, form an oxide layer on the surface of the material, and dilute the flammable gases, all of which are beneficial for the improvement of the flame-retardant properties of the materials.[[6],[7]] However, their low effectiveness makes it necessary to incorporate them at a very high content to gain a satisfying flame retardancy, which leads to a poor dispersion and damage of the material's mechanical properties.[[8]] What's more, the particle size and types of flame retardants are also of great importance in flame-retarding behavior. For instance, Li et al.[[9]] has reported that the flame-retardant effect of ATH was better than that of MDH for similar shapes and particle sizes in ethylene-vinyl acetate.

In recent years, ethylene-vinyl acetate copolymer with high VA content (EVM) has been one of the popular rubbers, because it has excellent heat/oil/weather resistance, good physical properties, and filler acceptance.[[10],[11]] Shi et al.[[12]] have reported that EVM with VA content of 70 wt.% could be a good choice as damping materials and in EVM/ acrylonitrile-butadiene rubber (NBR)/polyvinyl chloride (PVC) blends; the addition of PVC, which was partially miscible with the EVM/NBR blends, remarkably widened the effective damping temperature range. Meisenheimer[[13]] found that a linear relationship existed between the vinyl acetate content and heat of combustion. As the VA content was increased, less heat was released. Therefore, EVM was used as the matrix to impart high-performance FRNC in EVM/NBR blends with high content of inorganic flame retardants. The research described here was mainly devoted to studying the influence of MDH content and three different flame retardants [micro-sized ATH (mATH), micro-sized MDH (mMDH), and nano-sized MDH (nMDH)] on the flame retardancy of EVM/NBR vulcanizates, aiming to optimize the flame retardancy.

Sample formulations are shown in Table 1. EVM (Levaprene® 800, VA = 80 wt.%) and NBR (Perbunan®2840, acrylonitrile = 28 wt.%) were supplied by Lanxess Co., Germany. The mATH (particle size distribution, D50  ≤  1.3 μm) was supplied by Nabaltec GmbH, Germany. The mMDH (D50  ≤  2μm) and nMDH (D50  ≤  95 nm) were supplied by Shandong Helon Co., Ltd, China.

Table 1 Formulation of flame-retardant EVM/NBR composites

Compounding was carried out on an open mill with initial temperature at 25°C. EVM was initially added in the mill and blended with the NBR component, followed by the other ingredients in this order: activators, flame retardants, and peroxide cure system [dicumyl peroxide (DCP)/triallyl isocyanurate (TAIC)]. The blends were cured in a press at 170°C for 10 minutes.

Tensile properties were determined on a tensile testing machine (Zwick BT1-FR005TN A50, Germany) using dumbbell specimens, according to ASTM D 412-06a. All the tests were carried out at 25 ± 2°C.

Limiting oxygen index (LOI) was determined using a HC-2 type instrument (Nanjing Analytical Instrument Factory, China) according to ASTM D 2863-77. The dimensions of the sheets were 120 × 6.5 × 3 mm3.

The combustion behavior was investigated using a FTT Standard Cone Calorimeter (FTT Fire Testing Technology, England) at a heat flux intensity of 35 kW/m2 according to ISO 5660-1993. The samples (100 mm × 100 mm × 6 mm) were cut out from the press-cured sheets.

Thermogravimetric analysis data were obtained in air at a heating rate of 10°C/min from room temperature to 800°C using a TG209 thermogravimetric analyzer (Netzsch, Germany).

Filler dispersion was characterized by a Rubber Processing Analyzer (RPA 2000, Alpha Technology, USA) in the strain range of 0.28%–1000% at 60°C and frequency of 1 Hz with the uncured samples.

The assessment frequently used of the combustion behavior of laboratory specimens is the LOI. LOI is defined as the minimum fraction of oxygen in the mixture of oxygen and nitrogen when the sample can sustain combustion after ignition. The LOI values not only represent the ability to resist fire but the susceptibility to oxygen. The higher the LOI is, the better the flame retardancy of the materials.

Figure 1 shows the influence of different contents of mMDH on the LOI of EVM/NBR vulcanizates. As expected, the LOI values increased with the increase of mMDH contents, which demonstrated that the use of flame retardant mMDH was beneficial for the improvement of the flame retardancy of EVM/NBR blends. The flame-retardant principle of MDH mainly comes from the endothermic reactions of its decomposition and the formation of a surface carbon layer, which consequently decreases the surface temperature. The more the mMDH content the blend has, the greater the endotherm the composite has.

Graph: Figure 1 The influence of mMDH contents on the LOI of EVM/NBR.

The cone calorimeter is a modern device used to study the fire behavior of small samples of various materials in the condensed phase. It gathers data regarding the ignition time, mass loss, combustion products, heat release rate (HRR), and other parameters associated with the sample's burning properties.[[14]] The device usually allows the fuel sample to be exposed to different heat fluxes over its surface. The principle for the measurement of the HRR is based on the Huggett's principle that the gross heat of combustion of any organic material is directly related to the amount of oxygen required for combustion.[[15]]Table 2 shows the cone calorimeter data of EVM/NBR vulcanizates with different contents of mMDH.

Table 2 Cone calorimeter data of EVM/NBR vulcanizates with different contents of mMDH

The HRR is defined as the rate of combustion heat release per unit area after the material is ignited by preset thermal radiation heat flux. The total heat release (THR) is defined as the total combustion heat release of the materials per unit area during the combustion process. The higher the HRR and THR values, the more heat reaction over the material surface, which will accelerate the thermal degradation rate and the spread of fire and increase the fire hazard. The peak of heat release rate (PHRR) is also one of the important combustion parameters, implying the combustion process of materials. Usually it contains one or two peaks, and the initial peak reflects the typical combustion characteristics. The materials which are easy to form a carbon layer usually show two peaks. The formation of a carbon layer will weaken the heat transfer into the inside of the material and restrain the flammable gases to the flame zone, which will cause a decrease of the HRR height. The fire performance index (FPI) is defined as the ratio between time to ignition (TTI) and the PHRR. It can be used to evaluate fire hazards and has a certain correlation with the time to flashover. Thus, it is generally accepted that when the value of FPI of a material is smaller, its fire risk is higher.

It can be seen in Fig. 2 and Table 2 that the sample without flame retardant showed the highest HRR and the shortest TTI. The sharp HRR curve also revealed the material was very easy to be ignited. However, the HRR of the samples with mMDH dramatically decreased with two widely separated peaks, which implied the combustion manner changed from severe burning to the gradual heat permeation into the interior of the rubber. It can also be seen in Fig. 2 that the two main PHRRs decreased in height with the increase of mMDH content from 100 to 180 phr and the times when both PHRR occurred were prolonged. This was mainly due to the formation of a char layer which was closely related to flame-retardant content.[[16]] The char layer would protect interior of the rubber against the effect of radiation heat and flame. So it is necessary for hydroxide flame retardants to have a certain load so that the char layer can be formed. In this study, all the samples filled with mMDH formed a char layer when the mMDH content was more than 100 phr.

Graph: Figure 2 Curves of HRR versus burning time for EVM/NBR filled with different contents of mMDH.

The samples with flame retardants formed a stiffer surface of char layer, even before ignition, which can hinder the spread of radiation heat inside, so the materials can't be ignited quickly; as a result, the TTI was prolonged. The TTI of the EVM/NBR/mMDH samples increased as the MDH content increased, in Fig. 2 and Table 2. The TTI increased from the 52 s to 119 s when the mMDH content increased from 0 to 180 phr, but it decreased to 103 s when the mMDH content was 200 phr, although it had the longest total burning time of about 900 s (shown in Fig. 2). It was also observed that the smoke and residue obviously reduced in the experiments with the increase of mMDH content.

Figures 3 and 4 are the dynamic curves of THR and residual mass as a function of burning time for the EVM/NBR/MDH composites. When the char layer wasn't strong enough to resist the accumulation of heat, especially at the end of burning, the char surface was gradually destroyed until only some residue remained. So the residual mass changes can be used to reflect the intensity of char layer which further affects the heat release of the materials. The EVM/NBR sample without mMDH had the most THR and least residual mass, while the composite with 180 phr mMDH had the least THR. In addition, the THR and the loss rate of the residual mass both gradually decreased as the mMDH content increased for the EVM/NBR/mMDH samples, but there was nearly no difference in the THR and residual mass when mMDH content between 180 and 200 phr.

Graph: Figure 3 THR versus burning time for EVM/NBR filled with different contents of mMDH.

Graph: Figure 4 Residual mass versus burning time for EVM/NBR filled with different contents of mMDH.

In this study, the effect of char layer on heat insulation barrier became excellent with the addition of mMDH for the EVM/NBR/mMDH composites, and the FPI of 0.82 was the highest when 180 phr mMDH was used. So it was concluded that EVM/NBR blends with 180 phr mMDH had the best flame retardancy.

Thermal gravimetric analysis is a microscopic characterization method to analyze the thermal degradation behavior of a material. The thermogravimetric curves of EVM/NBR composites with various mMDH loadings are shown in Fig. 5. The pure mMDH decomposed in one step from 340°C to 440°C. The corresponding DTG peak occurred at 440°C, while the maximum thermal weight loss was 31%. The degradation of the EVM/NBR blend was more complex; it decomposed from 320°C to 665°C in several steps, with a maximum thermal weight loss of nearly 93%. The degradation included VA's breaking off from the main chain of EVM, the decomposition of NBR, breakage of the main chains, and burning of the residual carbon. Compared with the EVM/NBR blend, the EVM/NBR/mMDH composites showed a two-step degradation process. In addition, the addition of MDH increased the initial decomposition temperature of the composites and the combustion residual mass also significantly increased due to the increasing char weight.

Graph: Figure 5 TG (a) and DTG (b) curves for EVM/NBR/mMDH composites.

The comparison of mMDH's flame-retardant effects is shown in Table 3. For Mg(OH) 2, the actual residual mass ratio equaled the theoretical residual mass ratio (wt. of MgO), which means the overall mass loss of MDH came from the release of crystal water. Then we calculated the theoretical residual mass ratios for EVM/NBR/140 phr MDH and EVM/NBR/200 phr MDH by regarding 6.95% and 69.0% as the theoretical residual mass ratios for the EVM/NBR blend and Mg(OH) 2. It was found that the actual residual mass ratio for EVM/NBR/140 phr mMDH was almost the same as the theoretical one, which meant the Mg(OH)2 completely decomposed. However, for EVM/NBR/200 phr mMDH, the actual residual mass ratio was 2.3% lower than the theoretical one, which was presumed to be due to serious aggregation of the Mg(OH)2 caused by the high loading worsening the material's decomposition and weakening its flame retardancy. This corresponds to the fact that the 200 phr MDH loaded composite had poorer flame retardancy than the 180 phr mMDH loaded one (see Table 2).

Table 3 The comparison of MDH's flame-retardant effect

When the fillers form a network structure in the rubber compounds, the storage modulus of material will significantly increase. Generally the storage modulus G′ presents a typical nonlinear decline with increasing strain amplitude, which is called the Payne Effect. Here, ΔG′, defined as the difference between G′ at 0.4% strain and G′ at 100% strain, can be used to tell the degree of the Payne Effect. Larger ΔG′ means a stronger Payne Effect, which implies a poor dispersion of fillers in the compounds.[[17],[18]] The effects of mMDH contents on the storage modulus as a function of strain are shown in Fig. 6. The G′ at 0.4% strain gradually increased as the MDH content increased. When the strain was gradually increased, the G′ decreased, which indicated the filler network was being destroyed. The greater the mMDH content, the faster the rate of G′ reduction. The 200 phr MDH loaded compounds had the maximum ΔG′, which implied the poorest dispersion for the highest loaded mMDH in the EVM/NBR blends. This further proved that the presumption about aggregation of the MDH for the 200 phr sample for Table 3 was correct.

Graph: Figure 6 Effects of mMDH contents on storage modulus G′ of composites.

The effects of mMDH contents on the mechanical properties of EVM/NBR/mMDH composites are shown in Fig. 7. The elongation at break decreased as the mMDH content increased. This was because the aggregation of highly loaded mMDH was easy and formed stress concentration points and the interface defects between matrix and powder also increased. When the composites were subjected to external force, they were easily broken since these weak interfaces could not transfer the stress. The tensile strength of the composites decreased gradually as the mMDH content increased, but the tensile strength of 180 phr and 200 phr MDH filled composites instead increased. It is suggested that the 180 phr and 200 phr mMDH filled composites were broken at low elongation of less than 200%, where the filler-filler networks formed by the mMDH aggregates had not completely been destroyed and consequently contributed to the tensile strength.

Graph: Figure 7 Effects of mMDH content on the mechanical properties of EVM/NBR composites.

As is known, different flame retardant types result in different combustion behavior in composites; even for the same flame retardant types, the particle size is an important structural parameter which can affect the mechanical properties and flame retardancy. Many researchers have reached different conclusions about the effect.[[19]–[21]] In this study, the influence of nMDH, mMDH, and mATH on the LOI of the EVM/ NBR vulcanizates was characterized with the results shown in Fig. 8. The flame retardancy of the composites was better after adding 120 phr flame retardants. However, the LOIs did not differ significantly between the three types of flame retardants. During the experiments it was observed that the mATH filled composites had melt dripping while the MDH filled composites didn't. The residue from the MDH filled composites after combustion kept a regular shape, reflecting that MDH was superior to ATH in the formation of char layer.

Graph: Figure 8 Effects of flame retardants types on LOI of EVM/NBR vulcanizates.

The cone calorimeter data of EVM/NBR vulcanizates with different types of flame retardants are shown in Figs. 9 and 10. The flame retardant mATH obviously prolonged the TTI, but the HRR of the mATH filled composite was higher than the two MDH filled in the later combustion period. The THR curves had a similar trend. This phenomenon is attributed to the stronger char layer of MDH than mATH. Comparing the mMDH with the nMDH, it can be seen that HRR of the latter was higher than that of former after 250 s, as well as THR curves. It was presumed that the char layer caused by the mMDH was harder to destroy than that caused by the nMDH in the later stage of combustion. The TTI and burning time for both were similar, which indicted the MDH with different particle size had the same flame-retardant efficiency because it mainly came from the early release of water.

Graph: Figure 9 HRRs versus burning time for the composites filled with different kinds of flame retardant.

Graph: Figure 10 THRs versus burning time for the composites filled with different kinds of flame retardant.

Figure 11 shows the changes of residual mass as a function of burning time for the EVM/NBR composites with 120 phr of the different flame retardants. The EVM/NBR composites with 120 phr mMDH had the most residual mass. So it was concluded that mMDH showed a slightly better flame retardancy in EVM/NBR vulcanizates than nMDH, with both being better than mATH.

Graph: Figure 11 Residual mass versus burning time for the composites with different flame retardants.

The FRNC EVM/NBR/MDH composites demonstrated significant potential for flame-retardant applications. The test results showed that the flame retardancy of EVM/NBR composites was improved by adding both micro- and nano-sized flame retardant MDH. The decrease of HRR and THR, the increase of residual mass, and the enhancement of the thermal stability of the composites were all due to the flame retardant effect of MDH. The EVM/NBR composite had the best flame retardancy when the mMDH content was 180 phr. It was also found that the nMDH, mMDH, and mATH, at the same loading, had no obvious influence on LOI, while the combustion behaviors measured by cone calorimeter had a significant difference. The addition of ATH can obviously prolong the TTI. The mMDH showed a better flame retardancy for EVM/NBR vulcanizates than the nMDH. Therefore, the use of mMDH has advantages as flame retardants in the EVM/NBR blends.

The authors thank Lanxess Deutschland GmbH and Qingdao Science and Technology Development Program (no. 11-2-4-3-7-jch) for their support, without which the work presented in this article would not have been possible.