Plastics don’t break down easily naturally, so there was no good way to dispose of these materials in an environmentally friendly way. Advances such as the development of recycling streams have improved the management of plastic waste, but it is not always easy to find uses for the recycled material.
A weakness of the current recycling infrastructure is the dependence on separate recycling streams for different materials. This careful sorting of materials is necessary because of differences in polymer chemical structures 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 in liquid phase. Recently, however, an interdisciplinary team of researchers has developed an additive that allows 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 show two distinct phases; at the interface of the phases, the polymers adhere poorly to each other, resulting in materials that are mechanically weak compared to the individual components. General municipal waste usually has a 70/30 ratio of PE to iPP, so a significant amount of material must be separated.
The researchers developed an additive composed of linked blocks of PE and iPP (a material called a block copolymer). Essentially it was a single polymer chain, with side branches that could contain a variable number of each of the main polymers. Even if the two main polymers were kept separate, the block copolymer could bridge and link them together because it has side branches that are compatible with both PE and iPP.
The team focused on optimizing the reaction with the two plastics through improved catalysts. Their new catalysts had a longer life, enabling the production of higher molecular weight PE/iPP copolymers. This allowed the team to control the length of these copolymers by changing the reaction conditions.
A constant ethylene supply was used in synthesizing the ethylene blocks and the reaction time was varied to control molecular weight. In 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 with a simple peeling test. PE/iPP rectangular sheets were laminated at 180 degrees Celsius with or without the addition of a 100 µm adhesive layer 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 peeled apart easily, as expected due to poor interfacial adhesion. In contrast, integration of the block copolymers increased the peel strength. In particular, a specific form of the block copolymer incorporating four separate functional units showed very high tack.
Evaluation of 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 believe that larger block sizes allow for improved 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 showed that incorporation of only one percent of the block copolymer increased the stress at break point to 450 percent, compared to about 12 percent for a typical unaltered PE/iPP blend. While previous strategies to solve this compatibility problem also included additives, the additives were typically required in much higher percentages (over 10 percent).
The results suggest that we can tune the block copolymer architectures and molecular weights to allow the welding of different blends of commercial PE and iPP. If we added this approach to recycling streams today, we could reuse significantly more plastic with greater efficiency. Follow-up studies evaluating the use of simpler random copolymers could lead to the development of additives that are easier to produce and may be less expensive.
Science2017. DOI: 10.1126/science.aah5744 (About DOIs).