The random discontinuous fiber composite has uniform properties in all directions. The wetlay process is an efficient method to manufacture random discontinuous thermoplastic preform sheets that can be molded into random composite plaques in the hot-press. Investigations were done on the molding parameters that included the set-point mold pressure, set-point mold temperature and cooling methods. The fibers used in the study included glass and carbon fiber. Polypropylene (PP) and Polyethylene Terephthalate (PET) were used as the matrix. Glass/PP and Glass/PET plaques that had fiber volume fractions ranging from 0.05 to 0.50 at an increment of 0.05 were molded. Both tensile and flexural tests were conducted. The test results showed a common pattern, i.e., the modulus and strength of the composite increased with the fiber volume fraction to a maximum and then started to descend. The test results were analyzed to find out the optimal fiber volume fraction that yielded the maximum modulus or strength. Carbon/PET composites plaques were also molded to compare their properties with Glass/PET composite at similar fiber volume fractions. Micrographs were taken of selected specimens to examine the internal structure of the material. Existing micromechanics models that predict the tensile modulus or strength of random fiber composites were examined. Predictions from some of the models were compared with test data. / Master of Science

Fiber hybridization involves the blending of high and low performance fibers in a common matrix to yield a composite with a balance of properties that cannot be achieved by using either fiber alone. In this study, the random wetlay process was used as a compounding method to investigate the effects of fiber hybridization on the mechanical, viscoelastic, and sorption characteristics of steam-exploded wood (SEW) and lyocell (high performance regenerated cellulose) fiber-reinforced polypropylene (PP) composites. The two fiber types were blended in varying proportions within a fixed total fiber content of 50 wt. % and compared with non-hybrid lyocell- and SEW-PP controls. Using PP matrix as basis, it was observed that moduli of all composites generally increased with increasing lyocell concentration, ranging from a minimum 66 % for SP 50 (SEW/PP control) to a maximum 233 % for LP 50 (lyocell/PP control). Ultimate strengths on the other hand, declined for SP 50 but increased with the inclusion of lyocell fibers. Comparisons of hybrid (having 5 - 20 wt % lyocell) with non-hybrid (having 25 - 50 wt. % lyocell) composites revealed a surprisingly greater strength and modulus-building efficiency (by as much as 2.6 times) in the hybrid composites. This observation indicated possible synergism between lyocell and SEW. Analyses of composite property gains as a function of fiber cost also showed greater cost benefits (highest for tensile modulus) in favor of hybridization. The advantages of fiber hybridization on composite properties were again evident under dynamic mechanical analysis where no significant differences in the storage moduli were found between a hybrid composite with 20 wt. % lyocell and a non-hybrid composite with 50 wt. % lyocell loading. Application of the time-temperature superposition principle (TTSP) made it possible to predict storage moduli over extended frequencies for PP and its composites. Comparison of shift factor versus temperature plots revealed decreasing relaxation times of PP with increasing lyocell concentration, which indicated that PP interacted better with lyocell than with SEW fibers. Finally, it was observed from sorption tests that hybrid composites absorbed less moisture than non-hybrid counterparts of either fiber type. The reasons for this observation were not apparent. It is however possible that moisture transport mechanisms within the composites may have been modified as a result of hybridization. / Master of Science