Plastics do not readily break down naturally, so there has been no great way to dispose these materials in an environmentally friendly manner. Advances such as the development of recycling streams have improved plastic waste management, but it's not always easy to find uses for the recycled material.

One weakness of the current recycling infrastructure is the reliance on separate recycling streams for different materials. This careful sorting of materials is necessary due to differences in chemical structures of polymers that make them poorly compatible. For example, if we were to combine two of the most ubiquitous plastics in the world, they wouldn't even mix when in a liquid phase. Recently, however, an interdisciplinary team of researchers has developed an additive that enables these two polymers to be recycled together.

The plastics in question are polyethylene (PE) and isotactic polypropylene (iPP). Materials produced from a mix of these polymers exhibit two distinct phases; at the interface of the phases, the polymers adhere poorly to one another, resulting in materials that are mechanically weak compared to the individual components. General municipal waste typically has a 70/30 ratio of PE to iPP, so there's a significant amount of material to separate out.

The researchers developed an additive composed of connected blocks of PE and iPP (a material called a block copolymer). In essence, it was a single polymer chain, with side branches that could contain a variable number of either of the main polymers. Even if the two main polymers remained separate, the block copolymer could bridge and link them together, since it had side branches that were compatible with both PE and iPP.

The team focused on optimizing its reaction with the two plastics through improved catalysts. Their new catalysts had longer lifetimes, enabling production of higher molecular weight PE/iPP copolymers. This allowed the team to control the length of these copolymers by altering the reaction conditions.

When synthesizing the ethylene blocks, a constant ethylene feed was used, and the reaction time was varied to control molecular weight. By contrast, to produce the polypropylene blocks, the precursor:catalyst ratio was varied to control molecular weight.

The researchers evaluated the influence of the block copolymer additive on the adhesion between laminates of PE and iPP using a simple peel test. Rectangular sheets of PE/iPP were laminated at 180 degrees Celsius with or without inclusion of a 100 µm thick adhesive layer in between the sheets. The materials were then pulled apart perpendicular to the laminate layers.

The team found that the laminates that did not contain the block copolymers easily peeled apart, as expected due to the poor interfacial adhesion. By contrast, incorporation of the block copolymers increased the peel strength. In particular, a specific form of the block copolymer, which incorporated four separate functional units, exhibited a very considerable adhesive strength.

Evaluation of several different block copolymers revealed that the length of the side chains on the block copolymer influenced the strength of the interface between PE and iPP as well as the resulting failure mechanisms. The researchers think that larger block sizes enable enhanced entanglement between the chemically identical blocks attached to the copolymer and within the bulk plastic material.

Next, the researchers evaluated the mechanical properties of the materials. These studies revealed that inclusion of as little as one percent of the block copolymer increased the strain at break point to 450 percent, up from about 12 percent for a typical unmodified PE/iPP blend. Though previous strategies to overcome this compatibility issue have also included additives, the additives have typically been required in much higher percentages (over 10 percent).

The results suggest that we can tune the block copolymer architectures and molecular weights to enable welding of different mixtures of commercial PE and iPP. If we added this approach to the recycling streams today, we could be able to reuse significantly more plastic with enhanced efficiency. Follow-up studies evaluating the use of simpler random copolymers could lead to the development of additives that are easier to produce and are potentially less expensive.

Science, 2017. DOI: 10.1126/science.aah5744 (About DOIs).