While many cities and some states have banned single-use plastics, bags and other polyethylene packaging still clogs landfills and pollutes rivers and oceans. One major problem with recycling polyethylene — which makes up one-third of all plastic production worldwide — is economic. Recycled bags end up in low-value products, such as decks and construction material, providing little incentive to reuse the waste.
A new chemical process converts polyethylene plastic into a strong and more valuable adhesive and could change that calculus. While all recycled plastic — hundreds of billions of pounds of polyethylene are produced each year — can’t be turned into a material with adhesive properties, if some fraction of that can be turned into something that is of high value, that can change the economics of turning the rest of it into something that is of lower value.
For most plastics, recycling means chopping it up and forming it into generic products — in the process, tossing out many of the properties engineered into the original plastic such as pliability and ease of processing. And while new methods of recycling can break down plastics into their chemical constituents for use as fuels or lubricants, these products, too, are low-value and can be environmentally questionable — another fossil fuel to burn — or have a short lifetime. To make recycling more attractive, researchers and the plastics industry have been looking for ways to “upcycle” — convert recycled plastic into something more valuable and longer-lived.
The chemical process keeps many of the original properties of polyethylene but adds a chemical group to the polymer that makes it stick to metal — something polyethylene normally does poorly. The modified polyethylene can even be painted with water-based latex, which easily peels off standard low-density polyethylene (LDPE).
While the process is not yet economical for industrial use, it could be the starting point for adding other properties besides stickiness. The success also hints that other catalysts could work with other types of plastics, such as the polypropylene found in recycled plastic bottles, to produce higher-value products that are economically attractive.
The catalyst would have to work at high temperatures, since the solid recycled plastic has to be melted. Also, it would have to work in a solvent that is nonpolar and thus able to mix with polyethylene, which is nonpolar. This is one reason it does not stick to metals, which are polar, or charged.
Getting polyethylene to adhere to things — including latex paint — opens up many opportunities; for example, artificial hip sockets and knee implants often integrate polyethylene with metal components and could be made to adhere better to metal. Functionalized polyethylene could be used to coat electrical wire, provide the glue that sticks other polymers together — such as in milk cartons — or make more durable composites of plastic and metal such as in toys.