Flame‐retardant thermoplastic vulcanizates (TPVs) of natural rubber (NR)/polypropylene (PP) (60/40 wt %) blends filled with alumina trihydrate (ATH) were prepared with an internal mixer. To increase the properties of flame‐retardant NR/PP TPV, the new mixing method, stepwise masterbatch mixing (SMB) method was adopted. The effects of SMB method along with different ATH loadings on microstructure and properties of NR/PP TPVs were investigated. Conventional one‐step mixing (CV) method was also studied for comparison. Transmission electron microscopy analysis showed that different processes led to a variation in microstructural homogeneity, which imposed various effects on blend properties. The mechanical properties of TPVs changed with ATH loading, and the strength of the samples obtained from SMB method was higher than those of CV method. LOI and cone calorimetry tests revealed that the flame retardancy of NR/PP blends dramatically increased at higher ATH loading. Furthermore, the increment level of flame retardancy was accelerated in the blends produced particularly through SMB method, resulting from homogeneity of local ATH distribution in NR/PP blend. Greater combustion resistance of blends prepared from SMB route were confirmed by thermogravimetry and pyrolysis‐gas chromatography–mass spectrometry techniques. Finally, a burning mechanism between filler structure and flammability of NR/PP TPVs obtained from CV and SMB methods was discussed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46231.

Flame‐retardant blends of polyethylene‐octene elastomer (POE) and natural rubber (NR) filled with expandable graphite (EG) were melt‐mixed with azodicarbonamide to prepare elastomeric foams. The effect of the NR incorporation on the properties of EG filled POE foam was investigated. Gel content and microstructure observations revealed that the addition of NR into the POE/EG blend induced change in molecular structure, which produced a finer cellular structure during foaming. The compressive strength and elastomeric recovery of the POE/NR/EG blend foams were found to increase with increasing NR content. Based on the limiting oxygen index and horizontal burning measurements, all the POE/NR/EG blend foams were found to have good flame‐retardant properties. On the other hand, these POE/NR/EG blend foams exhibited higher combustion rates with an increasing NR content under high heat radiation and temperature in a cone calorimeter test. This study demonstrates that the combined use of NR as a modifier and EG as a flame retardant in POE foams provides an effective and practical method of obtaining well‐balanced improvements in cellular structure, mechanical properties, and flame resistance.

Rubber nanocomposite foams based on 60/40 ethylene vinyl acetate (EVA)/natural rubber (NR) were melt-mixed with flame retardant silicon dioxide (SiO2) (20 parts per hundred rubber, phr), and foamed by compression molding process. In this study, the effect of mixing phenomena of SiO2 through two different compounding techniques such as direct mixing (DM) and phase selective mixing (PSM) methods on structure, thermal stability, combustility and flame retardancy of EVA/NR blend nanocomposite foams were investigated. DM method is a melt mixing of EVA, NR, layered silicate and SiO2, followed by foaming. PSM is a new method based on pre-mixing EVA with SiO2, then melt mixing of EVA/SiO2 masterbatch with NR and layered silicate, and finally foaming. Based on TEM technique, it was found that the SiO2 was exclusively located in dispersed NR phase for the sample prepared by DM method, and the SiO2 was preferably dispersed in continuous EVA matrix when PSM method was employed. However, the different mixing methods did not significantly alter their cellular structures. The thermal stability and char residue content of foamed samples with SiO2 increased obviously when compared with those of corresponding foams without SiO2. The results based on limiting oxygen index (LOI) test and oxygen bomb calorimetry indicated that the foams combined with SiO2 had better combustion resistance and flame retardancy due to barrier effect of thermally stable silicon-based char layer. Further, the SiO2 filled foamed system obtained from the PSM method showed a higher degree of improvement in thermal stability, combustion resistance and flame retardancy than that of DM method because the homogeneous dispersion of SiO2 in EVA matrix rather than the selective dispersion in NR phase. This resulted in the continuity of flame retardant EVA/SiO2 phase in the 60/40 EVA/NR nanocomposite foams, which exerted more efficient fire barrier of the silicon-based char layer.

Flame retardant rubber foams of ethylene vinyl acetate (EVA)/natural rubber (NR)/layered silicate blends filled with silicon dioxide (SiO2) were prepared by using azodicarbonamide (ADC) as a blowing agent. Specifically, SiO2 was added in EVA/NR blend nanocomposites to produce good flame retardant foams. The properties of EVA/NR blend nanocomposite foams with different SiO2 loading (0, 20, 30, 40 parts per hundred rubber, phr) were investigated through transmission electron microscopy (TEM), scanning electron microscopy (SEM), rheological property test, mechanical property measurement, flammability tests, thermogravimetry analysis (TGA) and pyrolysis-gas chromatography-mass spectrometry (Pyrolysis-GC-MS). Compared with the simple EVA/NR blend nanocomposite, the added SiO2 increased the blend compatibility between EVA and NR phases and melt strength/viscosity of the EVA/NR blend nanocomposites, thus promoting cellular structure of the EVA/NR nanocomposite foams. Increasing SiO2 loading resulted in higher cell density, smaller cell size, and lower volume of void. These improvements caused higher strength and elastomeric recovery. The LOI test results showed that flame retardancy of the EVA/NR blend nanocomposite foams increased at higher SiO2 loading as a result of formation of insulation silicon dioxide-based char. TGA and pyrolysis-GC-MS analyses also validated the finding that the silicon dioxide-based char in the foamed samples containing higher SiO2 loading was more effective on improving thermal stability, which was responsible for lower material combustibility and better flame retardancy. Based on our finding, it was concluded that a good flame retardant EVA/NR blend nanocomposite foam with the best improvement in strength and elastomeric recovery was achieved when combined with 40 phr SiO2.

Blends of natural rubber (NR) and polypropylene (PP) were melt-mixed with phenolic resin crosslinking agent to obtain thermoplastic vulcanizates (TPVs). The blend composition of NR and PP was at 60/40 %wt. Alumina trihydrate (ATH) was incorporated into the blends to improve the flame retardancy, and the effect of ATH incorporation at different levels (20 - 120 PHP, part per hundred polymers) on blend morphology, ATH dispersion, mechanical properties, thermal resistance and flame retardancy was investigated. Analyses by transmission electron microscopy (TEM) and energy dispersive X-ray-scanning electron microscopy (EDX-SEM) revealed the dispersed phase-like morphology of 60/40 NR/PP TPVs. The presence of ATH in the TPVs did not have much influence on morphology of 60/40 NR/PP blends. Increasing ATH content decreased the dispersion level of ATH, which caused a decrease of tensile strength and elongation at break. Furthermore, an addition of ATH into the NR/PP blends resulted in lower elastic recovery, but slight increase of hardness. It was also found that the ATH suppressed the intensity of mass loss rate of NR and PP, resulting slower thermo-oxidative decomposition process of the blend materials. Limiting oxygen index (LOI) and UL94-V rate were enhanced with more incorporated ATH content. According to the results present in this study, the addition level of ATH for development of flame retardant NR/PP blend-based TPVs with acceptable elastomeric properties was recommended at 100 PHP.