"This work unveils a fundamental design principle that we believe is general to any kind of thermoset with this basic architecture."Jeremiah Johnson, MIT
Thermoset plastics, which include epoxies, polyurethanes and rubber used for tires, are found in many products that have to be durable and heat-resistant, such as cars or electrical appliances. One drawback to these polymer materials is that they typically cannot be easily recycled or broken down after use, because the chemical bonds holding them together are stronger than those found in other polymer materials, such as thermoplastics.
Chemists at Massachusetts Institute of Technology (MIT) have now developed a way to modify thermoset plastics with a chemical linker that makes them much easier to break down, but still allows them to retain the mechanical strength that makes them so useful.
In a paper in Nature, the researchers describe producing a degradable version of a thermoset plastic called pDCPD, breaking it down into a powder and then using this powder to create more pDCPD. They also proposed a theoretical model suggesting that their approach could be applicable to a wide range of plastics and other polymers, such as rubber.
"This work unveils a fundamental design principle that we believe is general to any kind of thermoset with this basic architecture," says Jeremiah Johnson, a professor of chemistry at MIT and the senior author of the paper.
Thermosets are one of the two major classes of plastics, along with thermoplastics. Thermoplastics include polyethylene and polypropylene, which are used to produce plastic bags and other single-use plastics like food wrappers. These materials are made by heating up small pellets of plastic until they melt, then molding them into the desired shape and letting them cool back into a solid.
Thermoplastics make up about 75% of worldwide plastic production, and can be recycled by heating them up again until they become liquid and then remolding them into a new shape.
Thermoset plastics are made by a similar process, but once they cool from a liquid into a solid, it becomes very difficult to return them to a liquid state. That's because the bonds that form between the polymer molecules are strong chemical attachments called covalent bonds, which are very difficult to break. When heated, thermoset plastics will typically burn before they can be remolded, Johnson says.
"Once they are set in a given shape, they're in that shape for their lifetime," he says. "There is often no easy way to recycle them."
The MIT team wanted to develop a way to retain the positive attributes of thermoset plastics – their strength and durability – while making them easier to break down after use.
In a paper published last year, Johnson's group reported a way to create degradable polymers for drug delivery, by incorporating a building block, or monomer, containing a silyl ether group, which is randomly distributed throughout the polymer. Exposing the polymer to acids, bases or ions such as fluoride causes the the silyl ether bonds to break, leading the polymer to break down.
The same type of chemical reaction used to synthesize those polymers is also used to make some thermoset plastics, including polydicyclopentadiene (pDCPD), which is used for body panels in trucks and buses.
Using the same strategy from their 2019 paper, the researchers added silyl ether monomers to the liquid precursors that form pDCPD. They found that if the silyl ether monomer made up between 7.5% and 10% of the overall material, pDCPD would retain its mechanical strength but could now be broken down into a soluble powder upon exposure to fluoride ions.
"That was the first exciting thing we found," Johnson says. "We can make pDCPD degradable while not hurting its useful mechanical properties."
In the second phase of the study, the researchers tried to reuse the resulting powder to form a new pDCPD material. After dissolving the powder in the precursor solution used to make pDCPD, they were able to make new pDCPD thermosets from the recycled powder.
"That new material has nearly indistinguishable, and in some ways improved, mechanical properties compared to the original material," Johnson says. "Showing that you can take the degradation products and remake the same thermoset again using the same process is exciting."
The researchers believe that this general approach could be applied to other types of thermoset chemistry as well. In this study, they showed that using degradable monomers to form the individual strands of the polymers is much more effective than using degradable bonds to ‘cross-link’ the strands together, which has been tried before. They believe that this cleavable strand approach could be used to generate many other kinds of degradable materials.
If the right kinds of degradable monomers can be found for other types of polymerization reactions, this approach could be used to make degradable versions of other thermoset materials, such as acrylics, epoxies, silicones or vulcanized rubber, Johnson says.
The researchers are now hoping to form a company to license and commercialize this technology. MIT has also granted Millipore Sigma a non-exclusive license to manufacture and sell the silyl ether monomers for research purposes.
Patrick Casey, a new product consultant at SP Insight and a mentor with MIT's Deshpande Center for Technological Innovation, has been working with Johnson’s group to evaluate the technology, including performing some preliminary economic modeling and secondary market research.
"We have discussed this technology with some leading industry players, who tell us it promises to be good for stakeholders throughout the value chain," Casey says. "Parts fabricators get a stream of low-cost recycled materials; equipment manufacturers, such as automotive companies, can meet their sustainability objectives; and recyclers get a new revenue stream from thermoset plastics. The consumers see a cost saving, and all of us get a cleaner environment."
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.