Reinforcement of ethylene-propylene-diene rubber/samarium borate and ethylene-propylene-diene rubber/Sb-doped SnO₂ composites by three types of ethylene-acrylic acid: Assessment of acrylic acid content on crystallinity, cure, mechanical, and electric properties

The aim of this study is to investigate the effect of acrylic acid (AA) content of ethylene-acrylic acid copolymers (EAA) and pH level of filler particles on reinforcement of peroxide-cured ethylene-propylene-diene rubber (EPDM)/samarium borate (SmBO₃) and EPDM/ Sb-doped SnO₂ (ATO) composites. It was found that EAA could reinforce EPDM by both crystallization and hydrogen bonds formed among carboxyl groups at room temperature, whereas at 180^°C EAA could melt and hydrogen bonds would dissociate, both of which could enhance fluidity of gum and facilitate the processing. During vulcanization, carboxylic acid groups could dehydrate among molecules to increase crosslink density. Moreover, the alkaline SmBO₃ could neutralize the carboxyl acid and makes dicumyl peroxide more likely to decompose into radicals than in the presence of acidic ATO, which endows EPDM with more crosslink density. After vulcanization, reinforcement of EPDM could be affected by the interaction of crystallinity of EAA, the formed hydrogen bonds and the amount of crosslink points. Surface and volume resistivity of EPDM composites would be reduced because of more conductive channels caused by the rising AA content. Dielectric properties would be affected by the interaction of polar groups and crosslink density.