Thermosets Built To Break Down

If plastics had personalities, thermosets would have a reputation for being strong and stubborn. Unlike thermoplastics, which make up water bottles, strawberry containers, and other recyclable plastics, thermosets can’t be remolded with heat or chemicals. Once hardened, a thermoset is locked into its shape because of chemical cross-linking. This makes thermosets useful in applications where the plastic will encounter high temperatures or harsh chemicals but also means thermosets can’t be recycled. So they ultimately wind up in landfills.

Now, thanks to a new poly­merization reaction, chemists have made thermosets that can be returned to their constituent diamine monomers via exposure to low pH (Science 2014, DOI: 10.1126/science.1251484).

A team led by Jeannette M. García and James L. Hed­rick of the IBM Almaden Research Center developed the reaction, which condenses a diamine monomer with paraformaldehyde. At low temperature, the reaction forms a hemiaminal dynamic covalent network. Turn up the heat, and this material will cyclize to form a poly(hexahydrotriazine).

To make the thermosets, the researchers use 4,4ʹ-oxydianiline—a common monomer used to make polyimides, such as DuPont’s Kapton. With this diamine, the hemiaminal network is as strong as fiberboard, despite the fact that almost one-third of the material is composed of water and N-methylpyrrolidone solvent. The resulting poly(hexahydrotriazine) is even stronger and is resistant to solvents and cracking from environmental stress.

“There aren’t a lot of new polymer-forming reactions,” Hedrick points out. Thermosets made via this new condensation, he says, have promising microelectronic, automotive, and aerospace applications. “You can take the material back down to the monomer after its useful lifetime,” he says. “That has huge ramifications. When you think about working on a complex automotive or aerospace part, the ability to rework and refabricate it if you make a mistake is priceless.”

“This is an elegant use of readily available starting materials and easy-to-do chemistry,” comments Stuart J. Rowan, a polymer expert at Case Western Reserve University. “Given the simplicity of the reaction conditions and the wide variety of amine-containing monomers that are commercially available, one can imagine using this chemistry to obtain a wide range of interesting materials.” Indeed, the IBM researchers show they can make organogels with self-healing properties using the same reaction on a different diamine monomer.

García tells C&EN that she discovered the polymerization when she was trying to make a known thermoset material and accidentally left out one of the starting materials. What she got was a plastic plug at the bottom of her reaction flask. “I had to break the flask with a hammer to get it out,” she recalls. Once out, she couldn’t grind the polymer up, so she took a hammer to it too. “Since we were after high-strength materials anyway, I decided to go ahead and try to figure out what exactly I’d made.” With help from the computational chemists at IBM, García figured out that she had prepared a poly(hexahydrotriazine).

In developing the thermosets, the group was concerned with costs. The 4,4ʹ-oxydianiline is inexpensive and commercially available. Also, García says, “there’s no catalyst involved in the polymerization, so you don’t have to worry about metal contamination and you don’t have to worry about purification of your product. That keeps costs down.” IBM hopes to partner with a chemical maker to develop the materials further.