The influence of mixing on the morphology and properties of blends of natural and nitrile rubber

The overall objective of this research is to develop natural rubber/acrylonitilebutadiene rubber (NRlNBR) blends having physical properties superior to NBR compounds and a tolerable resistance to swelling in oils and fuels. This would increase consumption of NR by replacing NBR used in various engineering applications with less costly NRlNBR blends. The rheology of blend components was studied in detail in order to choose mixing conditions and interpret the morphology of NRlNBR blends filled with 20 phr N660 carbon black. It was found that a high shear rate/high temperature combination results in similar apparent viscosities for the two elastomers. Blends were prepared according to single-stage and masterbatch mixing techniques with an intermeshing rotor internal mixer. Rheological, cure and physical properties of the blends were measured and related to mixing conditions, morphology and carbon black distribution. The filled 40/60 NRlNBR single-stage blends prepared at a high rotor speed had a finer morphology than the blends prepared at a low rotor speed. The fine textured blends showed higher tensile strength, lower abrasion resistance and improved compression set at elevated temperatures over those of coarse textured blends. Also, the fine textured blends showed a positive synergism of tensile strength. Modulus and tear strength were highest with most of the carbon black in the NBR phase, whereas ASTM oil and toluene uptake were lowest with most of the carbon black in the NR phase. However, percentage compression set was lowest with carbon black equally distributed between the phases. It was concluded that an NBR (26.6% acrylonitrile) compound could be replaced by the single-stage blends, particularly the fine textured ones, with regard to physical properties such as moduli, hardness, tear strength and compression set in engineering applications, where 2-4 % (by mass) oil swell is tolerable. Oil swell of the NRlNBR single-stage blends is about 5 % (by mass) lower than the predicted.