Chemical-Physical Characterization of Bio-Based Biodegradable Plastics in View of Identifying Suitable Recycling/Recovery Strategies and Numerical Modeling of PLA Pyrolysis

Abstract

AbstractSeveral bio-based and biodegradable polymers have been lately introduced on the market as potential substitutes for conventional plastics in order to decrease the environmental impacts related to plastics manufacturing and especially end of life disposal. The most applied route for the management of these types of bioplastics once they enter the waste stream is co-treatment with biowaste in anaerobic digestion and/or composting plants that may lead to their recycling as digestate and/or compost. Several studies however, have reported the incomplete biodegradation of these materials at lab-scale and/or in conventional treatment plants and the significant content of small inert particles, including microplastics, in the final products. This could represent an obstacle to the agricultural use of the produced digestate and/or compost. It is therefore necessary to study all the possible options for the recycling of these types of materials based on the specific characteristics of the polymers that constitute them. In this study, four different types of bio-based biodegradable plastics were characterized by chemical-physical analysis. In particular, the main properties investigated included the content of volatile and non-volatile phases, crystallinity, main elemental composition, content of different phases by spectroscopic investigation using Fourier Transform InfraRed spectra and of metals and metalloids of potential environmental concern. The results of the thermogravimetry analysis indicated that all of the recycling/recovery options considered (compost production via biodegradation, chemical recycling and energy recovery) could be potentially applicable for the examined bioplastics, since they showed to contain polymers that volatilize below 550 °C. The highest volatile matter contents were measured for PLA cups and starch-based films, while the highest ash contents were found for the other two types of rigid bioplastics, which also showed the highest concentrations of elements of potential environmental concern, that were anyhow quite limited, and reduced higher heating values estimated by elemental analysis compared to PLA or starch-based films. In addition, the rigid bioplastics tested exhibited a higher degree of crystallinity, which could be associated to a lower biodegradability. With regard to chemical recycling processes, the results of the chemical-physical investigations indicated that pyrolysis could be a technically viable process to apply for the treatment of all of the bioplastic samples examined. Thus, PLA, which is manufactured through lactic acid condensation, chemical recycling by rapid depolymerization through pyrolysis was evaluated applying a numerical model implemented in Aspen plus®. Results indicated that the best yields in terms of lactide recovery could be obtained at an temperature of 400 °C and 10 s residence time and that other valuable products may be obtained potentially by fractional condensation. Graphical Abstract