Driven by the huge burden they place on the environment, scientists are continually looking for new ways to manufacture plastic materials that make them easier to recycle, and MIT researchers are now claiming a significant breakthrough in this area. By modifying the makeup of a common type of plastic, the team has produced a version of equal strength that is far easier to break down after use, allowing it to be rebuilt and recycled to take on an entirely new life.
The work was carried out by chemistry researchers at MIT and focuses on a major class of plastics known as thermosets. Separate from thermoplastics that make up plastic bags and food wrappers, thermosets include materials like epoxies and polyurethane and rubber, are often used in car parts and electrical appliances.
Thermosets are made in a slightly different way to thermoplastics, which means that they aren’t so easily recycled. This is because while thermoplastics can be melted down into liquid and remolded into new shapes, the bonds within thermosets are very difficult to break down, meaning they often burn up under heat rather than return to a liquid form.
“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 team may have found a solution to this dilemma by intervening in the formative stages of the material. It built on some of its earlier work in degradable polymers for drug delivery, in which incorporating silyl ether monomers caused the bonds in the material to break down.
Applying this method to their plastics research, the scientists found success by adding the silyl ether monomers to the liquid precursors that form a type of thermoplastic called polydicyclopentadiene (pDCPD). Having the monomers makeup between 7.5 and 10 percent of the material, the team found that it retained its mechanical strength, but could be easily reduced to a powder form after use through 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.”
By then dissolving that powder in the precursor solution, the team was able to use it to form entirely new pDCPD thermosets. This material is often used for body panels in trucks and buses, but importantly, the team believes the approach could be used in other types of thermosets, too.
“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 study.
The research was published in the journal Nature.
Source: MIT