Acrylics, like their other plastic cousins, have always posed a tricky cost-benefit question. Seen in everything from paints to personal care products, acrylate-based polymers are incredibly useful, but are also tough on the planet. Most are non-biodegradable, and often end up in landfills or the open environment, making the end-of-life fate of acrylics a major dilemma for manufacturers and consumers alike.
An approach that is being studied is to copolymerize the acrylate building blocks with cyclic monomers that ring-open during manufacturing. The result is a large molecule chain with weak ester links that help with its degradability and could make those polymers more easily recyclable. This process, though, is inefficient and wasteful, often yielding heterogeneous products and leaving a lot of the monomers unreacted.
Led by Alexander van Herk and Praveen Thoniyot, Principal Scientist and Senior Scientist at A*STAR’s Institute of Chemical and Engineering Sciences (ICES), a team of A*STAR researchers turned to semi-batch polymerization in hopes of optimizing this process.
Rather than having both reactants be completely present right from the start, the team gradually added the acrylate units to the cyclic monomers as the reaction progressed. To figure out the rate at which acrylate should be fed into the reaction, they used a predictive software called Monomer Addition Profiles (MAP), which takes into account the tendency of either monomer to attach to the growing end of the polymer chain.
The idea was that because acrylate units are much more reactive than cyclic monomers, controlling its entry into the reaction would prevent it from getting used up all at once, and instead allow both reactants to incorporate and intersperse evenly throughout the polymer.
The team saw that when they broke down the semi-batch polymers through alkaline hydrolysis, the pieces were roughly of the same size, suggesting that the weak links had indeed been integrated homogeneously. On the other hand, fragment sizes from the degradation of batch-processed polymers were much more varied, evidence of a haphazard assembly of the monomers.
According to van Herk, this homogeneity comes with many benefits. Small fragment sizes, for instance, are much more readily biodegradable, and having oligomers of roughly the same size makes it easier to repurpose and recycle them into other polymers.
Moving forward, the team wants to see how their approach could help solve the plastic crisis. “For some specific copolymers such as polyacrylates, we are also looking at biodegradability; for others like polystyrene we are looking at recyclability. A very exciting step is that we are now also looking at polyethylene (LDPE), an important polymer for packaging,” said van Herk.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Chemical and Engineering Sciences (ICES), the Institute of High Performance Computing (IHPC) and the Institute of Materials Research and Engineering (IMRE).