| dc.description.abstract | Plastics revolutionized the world since the beginning of their mass production in the 1950’s. Packaging, construction, and many other industries successfully substituted materials for plastics, or found in these novel materials a new business application. Plastic production – as well as demand – has continuously grown in the past decades. Consequently, the amount of waste generated has also increased. Without an effective way to give new life to plastic waste, discarded plastic has accumulated in earth and marine environments. Accumulation of such waste is detrimental to many forms of life, and a better way to address waste is necessary.
Mechanical recycling efforts began in the 1970’s, consisting of sorting, washing, grinding, melting, and reshaping waste plastic. Although a good alternative for well-sorted polymer waste, it is not applicable to materials such as multilayer films or thermoset polymers. As such, mechanical recycling currently corresponds to less than 10% of the total plastic production. On top of that, this thermal process lowers the quality of the final products in such a way that products can only be recycled a limited number of times before their properties do not meet minimum standards, which inevitably leads to waste. Therefore, it has become clear that mechanical recycling alone is not able to tackle the challenge of waste plastics.
In order to complement mechanical recycling technologies, a new approach called chemical recycling is currently being developed. This process consists in selectively cleaving polymer molecules back into their building blocks, which are called monomers, that can then be manufactured into pristine plastic. There are many advantages involving the use of chemical recycling routes, such as that they yield products with identical performance to polymers manufactured from traditional feedstocks. There is even more potential to be unraveled as chemical upcycling routes are also being investigated. As opposed to converting polymers back to monomers, this process consists in decomposing polymer molecules into products which are more valuable than monomers, such as gasoline, diesel or alkyl-aromatics. Still, there are many challenges to be overcome, such as the fact that performance additives are often present in plastics, augmenting the recycling complexity.
These depolymerization reactions may be carried out in the absence or presence of catalysts. However, it is important to highlight that most catalyst technologies available today were originally designed to convert molecules which are orders of magnitude lower in molecular weight than plastic waste feedstock. Hence, there is a need to understand how this new feedstock will interact with traditional catalysts, and to design new catalysts to optimize polymer conversion.
This work is dedicated to improving understanding of these novel chemical recycling and upcycling reactions. Here, we discuss the impact that common polymer additives may have when they are present in a polyolefin melt undergoing pyrolysis or catalytic decomposition. This is fundamental to grasp how real-world polymer products will behave when subject to these processes, since all commercial plastic products inherently contain additives. Next, we unveil how a common additive may deposit and alter the activity of different pore-sized catalysts. Additionally, we investigate the role that polymer structures may play in the decomposition of polymers over porous heterogeneous catalysts. Finally, new catalyst design approaches are discussed to improve polymer-catalyst interaction in order to increase perceived rates of reaction. Insights from this work may help inform the industry and be one more step towards the development of optimized chemical recycling processes, which will allow for a more circular plastics economy. | en_US |
