Impact of Biodegradable Polymers and/or Nanoparticles’ Presence on the Thermal Depolymerization Kinetics and Mechanism of Non-biodegradable Polymers Used in Packaging

Plastic packaging usually consist of non-biodegradable polymers, such as poly(ethylene terephthalate) (PET), polypropylene (PP), or polystyrene (either rigid PS, or expanded polystyrene, EPS). The short-term applications of these products together with their large consumption have resulted recently in huge amounts of plastic wastes reaching each year the final recipients. Therefore, appropriate recycling techniques should be developed. In the framework of sustainable development, thermo-chemical recycling seems to be the best choice for handling this vast waste stream. Thermal depolymerization in an inert atmosphere (also referred as pyrolysis) has long being studied as such technique leading to a variety of products depending on the process conditions and polymer type. In current years, a great deal of interest has been observed on the use of biodegradable and biobased polymers as an alternative to the petrochemicals-based polymeric materials for packaging that will lead to reduced landfills by waste plastics. Biopolymers, such as polyhydroxybutyrate (PHB), thermoplastic starch (TPS) and poly(lactic acid) (PLA) are increasingly and most commonly used polymers in the food processing and packaging industry. However, the presence of such biodegradable and biobased packaging polymers in traditional recycling channels, could act as impurities for conventional plastics thus affecting the thermal degradation properties. Moreover, a new class of nanocomposite materials has provided a viable option for improving the antimicrobial, mechanical and barrier properties of plastics used in packaging. The most common class of nanoparticle materials used as layered inorganic solids chiefly clay minerals (i.e. montmorillonite), metal oxides or silver nanoparticles. Again, the presence of such materials in the polymer-recycling stream may cause a serious problem. In this project, for the first time, a systematic study is proposed for the depolymerization of commodity, non-biodegradable, polymers in the presence of either biodegradable polymers or nanoparticles. Blends from the polymers reported above or nanocomposite polymers will be formed and their depolymerization kinetics will be thoroughly investigated. Experimental techniques, such as thermogravimetric analysis or pyrolysis connected with GC/MS for the identification of the products, will be employed. Isothermal or non-isothermal conditions will be used. In addition, theoretical mathematical models will be developed to simulate the depolymerization kinetics. Both, detailed mechanistic models based on the degradation mechanism and isoconversional approaches to provide the variant of the activation energy with the degree of degradation, will be developed and applied to neat polymers, polymer blends and polymer nanocomposites. For the first time reconciliation of these two modeling approaches will be attempted. Finally, thermal depolymerization will be investigated in the presence of specific catalysts and their effect on the product distribution will be explored. The main scope of this research originates from the following question: How the presence of biodegradable and biobased polymers, (such as PHB or TPS or PLA) or nanoparticles (such as nano-clay, Ag nanoparticles or metal oxides) interfere the recycling process by pyrolysis of conventional polymers used in plastic packaging, such as PP, PET or PS? Therefore, understanding the thermal degradation kinetics and the reaction mechanism in a molecular level could help us to design improved large-scale processes for the thermo-chemical recycling of plastic packaging materials targeting in specific value-added products.