Investigation of graphene growth on different natural substrates from aromatic plastic wastes and its systematic life cycle assessment

Official URL: https://risc01.sabanciuniv.edu/record=b2766969_(Table of contents)

Plastic pollution has emerged as one of the widespread environmental issues and causes a series landfilling problem and the accumulation of the plastics in the environment and an increase in the GHS emissions. Especially recycling aromatic plastics, such as polyethylene terephthalate (PET) and polystyrene (PS), is complex and strongly sensitive to the process design and conditions due to the presence of polycyclic aromatic hydrocarbons and interunit C–O and/or C–C linkages. Herein, upcycling process becomes crucial to attain high value-added products from aromatic plastics and the conversion of carbon source in wastes into carbon nanomaterials provides prominent advantages over conventional recycling methods. This thesis aims to grow graphene structures on different natural substrates such as talc and organically modified montmorillonite (OMMT) from waste PS and PET sources by applying a sustainable, affordable, and environment-friendly upcycling technique. This promising method is to promote the formation of 2D or 3D graphene structures from waste PS and PET and provides dimension-controlled graphene growth by tailoring the substrate type and size, surface composition of substrate and the degree of aromaticity in polymer. In addition, the effect of polymer processing techniques, such as twin-screw extrusion and thermokinetic mixing, was investigated on the development of graphene grown hybrid additives by analyzing degree of crystallinity. Regarding the developed thermal based upcycling technology, talc substrate with a D50 particle size up to 10 μm promoted the growth of two-dimensional (2D) graphene sheets while talc with a size less than 2 μm assisted the production of three-dimensional (3D) graphene spheres by using PS source. Especially talc treated by iron catalyst triggered carbon accumulation on substrate and increased the number of graphene layers. Instead of talc, OMMT used as a substrate was used to enhance the degradation of PS polymer and indicate the effect of substrate type on graphene growing mechanism. It was possible to attain sheet like graphene on OMMT surface without using any catalyst in the presence of PS waste. In addition to waste PS, PET was used as a carbon source to grow graphene on the surface of iron treated micron sized talc and PS showed better performance than PET polymer since the polymer degredation of PET was more complex than PS. Prior to thermal upcycling, it was observed that the shear rate had a direct effect on the exfoliation of filler. The structural characterization results showed that high shear rate mixer led to the change in crystalline planes of talc whereas conventional twin screw extrusion preserved the structural properties of talc. Furthermore, a systematic life cycle assessment was conducted to evaluate the CO2 footprint of upcycled graphenes grown on talc and OMMT substrates compared to graphene produced by conventional techniques. Upcycled graphene structures obtained by direct carbonization and even catalyst impregnated natural substrate-based graphene growth process with PS or PET source have comparably lower CO2 emission than graphene produced by chemical exfoliation of graphite. To conclude, these newly developed and upcycled hybrid additives offers beneficial insights to preserve graphene dimension by rigid substrate in polymer blending process and keeping the structural integrity of graphene by adopting a circular economy model.