Design of environmental-friendly rubber composites driven by biodegradability assessment

BUCCHIERI, DANIELA

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Biocca

Abstract

The rapid increase in goods needed and a parallel waste production, particularly evident with the significant growth in the global population in recent decades, poses a threat to both public health and the environment, jeopardizing the sustainable development of our planet. Projections suggest that global municipal waste generation will reach 3.4 billion tons annually by 2050. Some human activities produce more waste than others, such as the transportation sector, which includes tyre production. Tyre production surpassed 2.5 billion units in 2021, with an estimated 2% growth over the next five years. With more than half of this number (1.5 billion units) becoming yearly waste, the challenges associated with management of tyre’s end of life represent a significant issue. Tyre grinding is one of the most commonly used techniques for recycling this material. However, it has been demonstrated that the ground tyres obtained cannot be incorporated into virgin blends, as the properties of the new compound would be compromised. Additionally, a significant amount of tyre debris is released into the environment during driving, acceleration and braking, due to abrasion with the asphalt. These tyre debris, along with asphalt and dust, are known as tyre and road wear particles (TRWP). Numerous studies have examined characteristics of TRWP, strategies to reduce the release of them, but little is known about their fate. Tyres consist of a blend of highly complex and diverse materials, including elastomers, fillers, plasticizers, stabilizers, vulcanizing agents, antioxidants, textile components, metals, and more. Several bacteria have been studied for their ability to degrade vulcanized rubber, particularly natural rubber blends. However, due to the complexity of the material, it is unlikely that the biodegradation, if possible, is solely attributable to individual microorganisms. In this research project, funded by Pirelli Tyres S.p.A, both aspects of abiotic and biotic degradation were assessed using four different rubber compounds produced using typical tyre tread composites, as well as simpler blends based on natural rubber compounds. As an example of abiotic degradation, we examined photooxidation, while for biotic degradation we focused our attention on the still unexplored potential of natural microbial communities. Natural environments host a multitude of microorganisms that thrive in polluted environments and adapt to use pollutants to their advantage for growth, often producing enzymes capable of metabolizing the pollutants, including polymer such as rubber compounds. Thus, following this concept, microorganisms derived from soil contaminated with tyre debris were isolated and initially tested as single isolates through high-throughput techniques, with the future aim of assessing them for different enzymatic activities. In the meanwhile, the ability of some microorganisms to degrade rubber compounds was assessed both at morphological and at chemical level. Additionally, the biodiversity found in the soil samples was evaluated and compared with the biodiversity of control samples through metagenomic analysis. Overall, the described work lays the foundations for a complete assessment of TRWP end of life. In parallel, as some innovative tyre compounds contain lignin as alternative filler, we concentrated our attention in characterizing a novel laccase from the white-rot fungus Trametes polyzona, which a preliminary screening suggested as promising for specific industrial applications. Indeed, we could demonstrate its peculiar ability to decolorize specific dyes. The future development of this work will be to merge targeted and untargeted approaches to describe TRWP end of life and possibly developing protocols for managing this wasted material in a logic of circularity of resources and of diminishing environmental impact.

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