PE and PP can combine with new method

PLASTICS recycling could one day become much easier, with the discovery of a technique that allows polyethylene (PE) and polypropylene (PP) plastics to combine, something previously not possible.

PE and isotactic PP (iPP) account for two thirds of global plastics use, but are immiscible and so cannot be recycled simultaneously due to their different chemical structures. Just 2% of the world’s plastics are currently recycled. Geoffrey Coates, a professor of chemistry and chemical biology at Cornell University, US, and his lab collaborated with a team at the University of Minnesota to try to solve the problem. They found a way to combine the two into a tetrablock polymer, with alternating PE and iPP segments.

“I recognised that block copolymers with PE and iPP might have the capability of compatibilising these separate polymers, so we simply tried it. Our lab has quite a bit of expertise in making block copolymers,” Coates told The Chemical Engineer.

Coates and the team used a catalyst system based on pyridylamidohafnium with B(C₆F₅)₃. Propylene is added first, and when this has been entirely consumed, the researchers add ethylene. The reaction produces the tetrablock polymer.

The researchers tested the strength of the tetrablock polymer by using it to weld together strips of plastic, and compared it to another type of block copolymer, diblock polymers. The team mechanically pulled apart the strips of plastic. Welds made with the diblock polymers failed very quickly, but the welds made with tetrablock polymers were so tough, the plastic strips broke.

“People have done things like this before, but they'll typically put 10% of a soft material, so you don't get the nice plastic properties, you get something that's not quite as good as the original material,” said Coates. “What's exciting about this, is we can go to as low as 1% of our additive, and you get a plastic alloy that really has super-great properties.”

The improved strength of the combined PE-PP polymer compared to the individual plastics means that items, for example jugs or bottles, could be produced with less plastic. Coates says that this would mean that the product uses less oil, there is less to recycle, and it is lighter and so less fossil fuel is needed to transport it. There would also be financial savings due to reduced plastic separation costs. There is even the possibility that due to the increased mechanical strength of the mixed polymer, the recycled material could be more valuable.

Coates told The Chemical Engineer that the team now plan to try to extend the technology to other polymers.

Science doi.org/b2fg