Upcycling to sustainable conversion of polypropylene waste to graphene grown talc hybrids as additive for efficient thermoplastic processing and its systematic life cycle assessment

Official URL: https://risc01.sabanciuniv.edu/record=b2708494_(Table of contents)

Plastic waste is a significant environmental and climate concern that threatens the ecosystem and leads to soil and water contamination. Although plastic recycling provides several benefits, the recycled plastics do not have the same performance as virgin plastic composites. Instead of traditional recycling processes, it is possible to produce high valueadded carbon nanomaterials by using a rich hydrocarbon source in plastics. This thesis aims to grow graphene structures on talc substrate by using polypropylene sources and applying an environmentally friendly, sustainable, and cost-effective upcycling technology, and develop lightweight thermoplastic composites with this newly designed hybrid additive by reducing the main reinforcement amount in the target automotive plastics. In addition, comprehensive life cycle assessment was carried out to demonstrate the environmental gains from the proposed innovative upcycling solutions with the benchmarking study by evaluating CO2 emission reductions. Initially, carbon source in polypropylene (PP) wastes were converted into vapor phase and bind to talc surface in the presence of metal catalyst by initiating graphene growth in mild conditions by upcycling technology. Herein, a selective method to obtain 2D and 3D graphene structures was developed by selecting a suitable talc size, activating talc surface, producing different PP blends with talc by using two main polymer processing techniques: high shear mixing with thermokinetic mixer and twin-screw extrusion, and defining suitable pyrolysis and carbonization techniques. The results indicated that talc with a D50 particle size of up to 10 μm enhanced the growth of 2D graphene sheets whereas talc size less than 2 μm increased the formation of 3D graphene spheres. In other words, the developed upcycling method enables an easy and low-cost obtainment of graphene based two-dimensional and/or three-dimensional nano- or submicron-sized structures on talc surfaces. In addition, flash pyrolysis by rotating furnace provided to produce graphene/hybrid additives in a short time by decreasing the defects compared to the hybrid obtained by gradual heating in chamber furnace. There was a significant effect of blending technique on the crystal structure of hybrid additive. Thermokinetic mixer used to prepare talc/PP samples for carbonization process caused complete exfoliation of talc and changed the crystalline planes and decreased the particle size due to the occurrence of blending process at around 4000 rpm. However, twin-screw extrusion preserved the layered structure of talc and did not change its crystal structure since blending was occurred at very low mixing rates of 350 rpm. After the selection of an ideal graphene/talc hybrid additive production, the produced hybrid additives were used as a reinforcement in homopolymer PP and copolymer PP at different loading ratios to reach the automotive standards and decrease the talc content and maintain the mechanical performance of the composites. The most remarkable improvement was achieved by the addition of 5 wt% hybrid additive in CopoPP matrix, flexural modulus and tensile modulus were increased by 88% and 57%, respectively, compared to neat CopoPP. Furthermore, %10 talc reduction was provided with this new graphene/talc hybrid additive and even 6% improvement in flexural modulus was accomplished at 5 wt% hybrid loading compared to the performance of compound having 15% talc used in serial part production. In conclusion, the conversion of polypropylene waste into high valueadded graphene/talc hybrid additive by circular economy targeted upcycling process brings a new insight in graphene manufacturing and the production of lightweight thermoplastic plastic. Also, life cycle assessment approach was adopted to assess the environmental sustainability performances of the new automotive thermoplastic composites in comparison to their conventional equivalents.