Not only are most thermosetting resins made from non-renewable ingredients, but they're also difficult to recycle. Swedish scientists have set about addressing those problems, by developing a new lignin-based, thermoset-like material that can be reused several times.
Lignin is a type of organic polymer that makes up much of the support tissue in plants, including trees. It's also a byproduct of the pulp and paper industry, and has previously been put forward for use in applications such as cheaper batteries, waste-derived carbon fiber and stronger concrete.
Researchers at Stockholm University have now combined lignin with a non-toxic cross-linking chemical derived from ethylene glycol, to create a material with characteristics much like those of traditional thermosetting resins. There's no need to chemically alter the lignin beforehand – it's simply "cooked" along with the cross-linker.
The resulting black material can be formed into numerous shapes via conventional casting or injection molding techniques.
Additionally, by varying the lignin-to-cross-linker ratio, it's possible to tweak the material's characteristics – it can be soft and tough, hard and brittle, or even take the form of a strong adhesive. In all cases, it can also be thermally broken back down and reused multiple times, retaining a mechanical strength that is described as being comparable to that of existing engineered plastics.
"The material that we developed is perfectly in line with the current move towards sustainable circular materials," says Asst. Prof. Mika Sipponen. "Owing to its design consisting of so-called dynamic covalent bonds, the material can be formed over and over again by relatively mild heating."
The research is described in a paper that was recently published in the journal ACS Applied Materials & Interfaces.
Source: Stockholm University
