Researchers from the University of California Berkeley created a chemical process that can upcycle polyethylene plastic into a more valuable adhesive.

In recent years, many cities and states have banned single-use plastics. But even with these bans, plastic pollution is still a major problem. Polyethylene, which is often used to create plastic bags, is one-third of plastic production worldwide. In these recycling practices, plastic bags often end up in low-value products, like construction materials. The team’s new adhesive upcycles polyethylene into a new product that is more valuable than the original state.

Using a catalyst based on ruthenium (gold ball, center), UC Berkeley chemists were able to add specific chemical groups. Source: UC Berkeley image by Liye ChenUsing a catalyst based on ruthenium (gold ball, center), UC Berkeley chemists were able to add specific chemical groups. Source: UC Berkeley image by Liye Chen

In typical plastic recycling, plastic is chopped up and formed into genetic products. This process gets rid of many of the properties that were engineered into the original plastic, like its pliability and ease of processing. Newer recycling methods can break down plastics into chemical constituents that can be used as fuel or lubricants. While this is useful, these products have low value, are environmentally sketchy and have a short lifetime.

The new chemical process preserves many of polyethylene's original properties and adds a chemical group to the polymer that makes it stick to metal. Polyethylene does not typically stick well to metal. The team's modified polyethylene can be painted with water-based latex. The latex peels off with standard low-density polyethylene (LPDE). Adhesion was enhanced while preserving the good traits, which is difficult to do.

To create the new catalytic process, the team added small chemical units to large hydrocarbon chains in specific places to create functionalized polymers. The resulting catalyst must work at higher temperatures because solid plastic must be melted. It also had to work in a solvent that is non-polar and can mix with polyethylene. One reason that polyethylene doesn’t typically stick to metal is because metal is polar.

The team created a ruthenium-based catalyst. It meets requirements and could add OH groups to the polymer chain without a highly reactive hydroxyl breaking polymer chain. The reaction produced a polyethylene compound that sticks tightly to aluminum metal. The researchers wanted to further understand the new compound’s adhesion, and they found that a small percentage of alcohol added to the polymer boosted the adhesion by 20 times.

Currently, the process is not economical enough for industrial use. The team believes they can further improve it before releasing it publicly. This research could be the starting point for adding more properties. The team says that other catalysts could work just as well as polyethylene.

A paper on this research was published in Chem.