Natural rubber/clay nanocomposites: effect of organic modifier of clay on cure characteristics and mechanical properties

Abstract

Over the recent past, nanomaterials have emerged as potential reinforcing and functional materials for rubber composites. Montmorillonite (MMT) clay, which belongs to the 2:1 smectites family, is the widely investigated nanomaterial for improving reinforcement of rubber composites. Improved reinforcement is achieved with a lower concentration of these clay minerals compared to that of traditional fillers such as carbon black and silica. However, hydrophilic clay needs to convert into organophilic since clay minerals are not easily intercalated when they are combined with conventional non-polar rubber such as natural rubber (NR). In general, MMT clay is organically modified with alkyl ammonium cation (known as organic modifier) to make them more compatible with non-polar polymeric materials.

This research study investigated the effect of different types of alkyl ammonium modifiers on cure characteristics and mechanical properties of natural rubber/clay nanocomposites in view of developing a suitable curing system for the nanocomposite. A natural rubber (NR)-clay  nanocomposite (NRCN) was prepared by melt compounding of NR with organically modified montmorillonite (OMMT) clay and other compounding chemicals using a laboratory scale internal mixer. Vulcanization kinetics of the NRCN was evaluated in comparison to NR gum compound, with a moving die rheometer (MDR). Processing safety and optimum cure time showed a marked reduction whilst indicating a significant effect on thermo-oxidative degradation as measured by reversion time when clay was modified with different modifiers, in comparison to unmodified clay, used in the rubber compound formulations. The results of cure characteristics have been interpreted with reference to the types of organic modifier and degree of exfoliation of clay. A vulcanization system with improved processing safety (delayed onset curing) and a reduced cure rate for NRCN was developed by incorporating a pre-vulcanization inhibitor.

Mechanical properties of NRCN vulcanizates showed a significant reinforcement; especially stiffness and strength characteristics, when NR was melt compounded with organically modified clay compared to that of unmodified montmorillonite clay. However, the highest reinforcement was achieved with montmorillonite clay modified with quaternary aliphatic ammonium ion containing hydrogenated tallow.