Materials scientists and engineers would be the first to tell you that there are primarily two kinds of polymers in the world: thermoplastics and thermosets. Thermoplastics are polymers that can “soften” when heated then harden when cooled; these polymers make up most of the consumer plastics that we know of, like the polyethylene (PE) in plastic bags and the poly(ethylene terephthalate) (PET) in your soda bottles.
Now, thermosets on the other hand are polymers who undergo a “curing” process; unlike their thermoplastic cousins, these polymers do not “soften” when heated, and instead just decompose. This is also usually an irreversible process, meaning these polymers can rarely be recycled through simple heating and remolding. These polymers include the rubber in your car’s tires, and the epoxy resin in your nearest composite glass window.
These thermosets often need the presence of a curing agent, like a catalyst or even just heat, to begin the crosslinking process of a desired prepolymer resin. Crosslinking meshes ang entangles the long chains of polymer molecules together, much like the metal mesh of a chain link fence; it’s this process that makes thermosetting polymers very strong and durable, and imparts upon them properties like chemical resistance.
You see, in contrast to thermoplastic polymers that really just rely on intermolecular forces to stick together, thermosets link their polymers with one another by chemical bonding; this is why your plastic bag simply “melts” when heated, while your rubber tire burns to an irreparable state. Heating can permanently break the chemical bonds of most thermosets.
Or, at least, that’s how things would go in the world if a group of researchers from Stockholm University didn’t have anything to say about it. You see, these particular scientists devised an ingenious thermosetting polymer that cures and performs just the way you’d expect a thermoset to, but with a catch—this thermoset is based off of lignin, an organic polymer that forms most of the support structures in plants. Their novel findings were published in the journal ACS Applied Materials & Interfaces.
The researchers combined lignin with a “non-toxic crosslinking chemical” derived from ethylene glycol, and was mixed then simply cured to allow crosslinking to occur. The lignin, surprisingly enough, didn’t need any chemical modification to allow for this process—it was just simply heated together with the crosslinking agent.
The result is a black polymer that allows for “[tuning] the material properties, [which] opens many opportunities to commercial applications,” according to co-author Mohammad Morsali. For example, the new thermoset was used as an adhesive by modifying the lignin-to-crosslinking-agent ratio to be 50% lignin by weight prior to curing. The result was an adhesive that was capable of bonding even two metal plates of aluminum.
The icing on this thermosetting cake is the fact that the polymer can simply be broken down thermally, and can then be reused multiple times, and was said to retain strength that’s comparable to those of common engineering plastics despite the reforming.
Said co-author Professor Mika Sipponen: “The material that we developed is perfectly in line with the current move towards sustainable circular materials. Owing to its design consisting of so-called dynamic covalent bonds, the material can be formed over and over again by relatively mild heating.”
Fellow co-author Dr. Adrian Moreno continued: “We were amazed by the performance of the new materials, and one of the striking aspects of our results is the simplicity and material-efficiency of the synthetic process. In contrast to earlier examples we do not need any chemical modification or fractionation of the lignin but can simply cook it with the cross-linker in a one-pot reaction.”
(For more polymer news, check out our piece on the true dangers of microplastics in our infants, as well as polymers being used as a basis for “flexible” supercapacitors.)
References
- Boegård, C. (2021, November 24). New lignin based material to replace fossil plastics and adhesives—Stockholm University Center for Circular and Sustainable Systems. Stockholm University. https://www.su.se/stockholm-university-center-for-circular-and-sustainable-systems/news/new-lignin-based-material-to-replace-fossil-plastics-and-adhesives-1.584455
- Coxworth, B. (2021, November 24). Wood waste used in renewable, recyclable thermoset plastic alternative. New Atlas. https://newatlas.com/materials/lignin-renewable-recyclable-thermoset-plastic-alternative/
- Moreno, A., Morsali, M., & Sipponen, M. H. (2021). Catalyst-free synthesis of lignin vitrimers with tunable mechanical properties: Circular polymers and recoverable adhesives. ACS Applied Materials & Interfaces. https://doi.org/10.1021/acsami.1c17412
