Studies on the effect of blend ratio and crosslinking system on thermal, X-ray and dynamic mechanical properties of blends of natural rubber and ethylene-vinyl acetate copolymer

Abstract

Differential scanning calorimetry (d.s.c.), X-ray, dynamic mechanical and microscopy studies were carried out using a series of blends of natural rubber (NR) and ethylene-vinyl acetate copolymer (EVA) in the crosslinked and uncrosslinked states. The crosslinking system (sulfur, peroxide and mixed) and blend ratio do not influence significantly the melting temperature ( T m) of EVA. The decrease in percentage crystallinity of EVA with NR content is related to the incomplete crystallization, as indicated by the lower heat of fusion ( ΔH). The glass transition temperature ( T g) of NR in the uncrosslinked blend did not change substantially and this indicated incompatibility. However, in the case of crosslinked systems, depending on the type of crosslinking agents used, the T g of the NR phase in the blend was shifted to higher temperatures. This has been explained with reference to the predominant crosslinking of the NR phase. From X-ray diffraction patterns, the interplanar distances ( d values) were calculated. With the addition of NR there was a tendency for the d values to increase in all systems. The crystallinity was measured by X-ray and the results were in line with those of d.s.c. measurements. The dynamic mechanical properties such as tan δ, storage modulus and loss modulus of the blends were evaluated. The existence of separate T g values in dynamic mechanical thermal analyser studies indicates that the blends are incompatible. The morphology of the uncrosslinked blends has been studied by scanning electron microscopy.