Mechanical properties aged and unaged polybutadiene-novolak blend as a function of rubber concentration
Adams, Hussein Sheikh
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Mechanical properties of polybutadiene - Novolak blends were studied in the composition range of 0 to 100 weight percent of the rubber component. The blends were prepared using direct blending method. The mechanical properties studied include; ultimate tensile strength, Youngs modulus of elasticity, elongation at breaks and impact strength. The effects of exposure to Ultraviolet (UV) light on the above properties were also investigated using an ageing chamber designed at Kenyatta University. The results obtained show that both the elongation at break and the impact strength increase with increasing rubber concentration in the blend. A reverse effect was observed or both ultimate tensile strength and young’s modulus of elasticity. This increase in the toughness properties of the blend is explained in terms of the stress concentration effects of the rubber particles. The irradiation of the blends with UV light was found to lower the toughness of the polyblend. This is explained as being due to the formation of peroxide compounds which introduce 'Oxygen-bridges' between the two polymers thereby cross linking them. As a major finding of this work, the effect of the embitterment of the blend system on exposure to UV light was found to increase with dose for the fist fifteen days only to remain almost a constant thereafter. These phenomena are explained in terms of the nature of peroxides, which increases with initial dose, but levels off soon after. Using known models for rubber-toughened plastics, the theoretical values of the various parameters studied were calculated. The theoretical values of both impact strength and Young's modulus of elasticity closely fitted experimental results. The ultimate tensile strength model predicted values, which were at variance with the experimental data. The explanation for this is given onto he basis of the foundation on which the model is established. This model does not take into consideration the stress concentration effects of the rubber particles, a factor that is apparently central to the theories of rubber toughening mechanism.