Rheological properties, morphology, and oil resistance in natural rubber (NR)/nitrile rubber (NBR) blends were investigated as functions of blending conditions and viscosity ratios of the blends. As for the blending condition effects, Mooney viscosity of the blends depended more strongly on blending time than rotor speed. Size of the NR dispersed phase was approximately independent of rotor speed but decreased with increasing blending time up to 25 min. As blending time further increased, NR dispersed phase size increased. The results of relative tensile strength, which is an indicator for oil resistance, in this study were in agreement with those of the blend morphology, indicating that the oil resistance in 20/80 NR/NBR blend depended strongly on the phase morphology of the blend. The smaller the size of NR dispersed phase was, the higher was the resistance to oil of the blend. However, a decrease in the size of the dispersed phase by the modification of the viscosity ratio via the use of low-molecular-weight rubber (i.e., liquid natural rubber and epoxidized liquid natural rubber) did not result in an improvement in the oil resistance.

ABSTRACT:The phase morphology and oil resistance of 20/80 NR/NBR blends filled with different types of fillers and copolymers were investigated. In the case of filler effect, N220, N330, and N660 carbon blacks with different particle sizes were used. Additionally, the blends filled with nonblack-reinforcing fillers, that is, precipitated and silanetreated silica, were investigated. To study the compatibilization effect, maleated ethylene propylene diene rubber (EPDM-g-MA) and maleated ethylene octene copolymer (EOR-g-MA) were added to the blends. The results revealed that the addition of filler, either carbon black or silica, to the blend caused a drastic decrease in NR dispersed phase size. Carbon blacks with different particle sizes did not produce any significant difference in NR dispersed phase size under the optical microscope. Silica-filled blends showed lower resistance to oil than did the carbon black-filled blends. In addition, it was determined that neither EOR-g-MA nor EPDM-g-MA could act as a compatibilizer for the blend system studied. The oil resistance of the blends with EPDMg-MA is strongly affected by the overall polarity of the blend. In the case of EOR-g-MA, the oil resistance of the blends is significantly governed by both overall polarity of the blend and phase morphology.