Process-property relation of prepreg trim waste composites via sheet molding compound (SMC)
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Carbon fiber reinforced polymers (CFRP) are used in aerospace industries due to their high strength to weight ratio and increased fuel efficiency. Thus, the increasing demand results in the production of large amount of CF prepreg scrap and CF composites. Both the scrap and the composites at the end of their life are currently disposed in landfills. Disposal of the scrap prepreg is hazardous to the environment, which also incurs cost for landfilling. Recently, automotive industries have shown interest in the use of recycled materials for the manufacture of vehicles. The focus of this research is recycling the post-industrial prepreg waste to produce light weight polymer composites suitable for automotive industries and to understand their process-property relationships. The progression of the research was performed in three stages including employing sheet molding compound (SMC) technique for recycling the prepreg, aging condition assessment of the incoming prepreg waste and mechanical performance analysis of the CF prepreg composite. The SMC technique was used to chop the prepreg tapes and to form a sheet in the SMC line. The standard SMC line was engineered to process the CF prepregs by developing prepreg spool holding system, guiding systems for prepreg tapes, spool-core to shaft friction reduction system, and by introducing the venturi equipment for removing backup film of tapes. Since cutting and SMC preparation of the prepreg wastes depend on their thermal history, a systematic assessment of their aging condition was performed using differential scanning calorimetry (DSC). The DSC analysis revealed that aging advances the cure of the resin in prepregs resulting in non-tacky prepregs which can be cut completely in the SMC line. The non-tacky aged prepregs can be obtained either by oven aging or room temperature aging. The cure kinetic parameters such as thermal transitions, degree of cure, activation energy etc. were studied to investigate the thermal history of the prepregs. To study the mechanical performance of the recycled prepregs, composites with minimum void contents were fabricated from the tacky and the non-tacky prepregs using compression molding. The mechanical properties of the composites, including tensile modulus, tensile strength and impact strength were determined according to the ASTM standards as a function of the composite’s fabrication conditions. The following values were found for the average tensile strength (150 MPa), tensile modulus (40 GPa) and impact strength (144 KJ/m2) of the fresh (minimum out time) prepreg composites. The composites from the aged prepreg also showed comparable properties to that of the fresh (minimum out time) prepreg composites. A micromechanical model for randomly oriented discontinuous fiber composite system was used to calculate theoretical modulus of the CF prepreg composite. The calculated modulus was 69 GPa. In addition to the manufacturing defects such as voids and resin rich and fiber curled area in the composite, the assumption of discontinuous fibers used for the model, which is different from our discontinuous prepreg chips, might also contribute to the modulus discrepancy. The recycled CF prepreg composites are 18% lighter, and are two times higher in tensile modulus and tensile strength compared to the commercial glass fiber SMC composites which are currently used in automotive industries. Furthermore, failure analysis indicated that the recycled composites retained its integrity upon failure under tensile loading. Thus, this thesis suggests that the post-industrial prepreg composites processed by SMC technique is a promising lightweight material for applications in automotive industries.