Effects of consolidation parameters on creep, fatigue and dynamic mechanical behaviour of self-reinforced polypropylene composites
Wanyama, Paul Simiyu
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Environmental concerns, production costs, reprocessability, and weight of traditional polymer-based composites are major issues that have heightened research in the field of materials science. Thus research on the development of new classes of composites called self-reinforced polymer composites has intensified. Self-reinforced polypropylene composites prepared by compression moulding were examined in this study. This study produced composites (SRPPCs) reinforced with polypropylene fibers of different draw ratios (Dr: 11, 8, 5, 2) and different fiber weight fractions (Fw; 15, 12, 9, 6 and 3). Composites were fabricated at consolidation temperatures in the range 163 to 175oC, and consolidation time in the range 30 to 130 seconds. This study investigated the effects of consolidation temperature, consolidation time, fiber draw ratio and fiber weight fraction on dynamic mechanical properties, fatigue behaviour, creep deformation and thermal stability of SRPPCs. The samples were subjected to dynamic mechanical analysis (DMA) in the temperature range 30 to 110oC at frequency of 1Hz. From the DMA results, it was observed that storage modulus was greatest at optimum consolidation temperature of 170oC, stiffness increased with consolidation time, fiber draw ratio and fiber weight fraction. Fatigue test was also performed on the samples at constant cyclic strain amplitude of 20μm, frequency of 5Hz, and temperature 35oC. Fatigue resistance and fatigue endurance was improved significantly at consolidation temperature of 170oC, consolidation time of 130 seconds, fiber draw ratio of 11, and fiber weight fraction of 15wt%. Short-term isothermal creep tests were carried out at different temperatures ranging from 30 to 90oC under an applied stress of 5MPa. Remarkable improvement on creep resistance occurred when consolidation temperature, consolidation time, fiber draw ratio and fiber weight fraction were maintained at 170 oC, 130 seconds, 11 and 15wt% respectively. Burgers and Findley power law models could satisfactorily be applied to analyse the short-term creep behaviour of the composites as well as predict the creep deformation beyond the experimental time of 12 minutes. Creep master curves were created using time-temperature superposition principle (TTSP). Using the master curves, creep deformation of order 108 minutes could be predicted. The temperature dependence of the shift factors could best be described by the Arrhenius equation. All these positive gains made in mechanical properties are attributed to fiber drawing which enhances molecular orientation (crystalline regions) and minimizes segmental movement of the chain molecules. Thermal decomposition tests on the samples exhibited improved thermal stability of the self-reinforced polypropylene composites.