A New, Recyclable Thermoset Plastic

The recent development of reversible thermosets by IBM scientists means recycling may be possible now for materials previously unrecyclable.

The art of simplicity is a puzzle of complexity.
-Douglas Horton

Sometimes it’s good to redefine the rules. For example, there are certain rules in recycling. Recyclable plastic and the process of recycling deal with thermoplastics, which are melted and reshaped. Pyrolysis, on the other hand, returns plastics to their original, crude oil form, after which they can be used for fuel. But what about chemical recycling, depolymerization, or going back to the monomers? Yes, this is possible, too, now with thermosets.

How can a thermoset plastic be recycled? IUPAC defines a thermoset plastic as one that changes irreversibly into an insoluble polymer network by curing with temperature or radiation. The irreversibility of the network formation prevents recycling. But, if you were to make a reversible polymer network, that would change the whole story!

Reversible Thermosets

A team of scientists from IBM has come up with two novel reversible network thermosets based on trifunctional crosslinking molecules of triazine, which is formed in situ from a diamine (dianiline) and formaldehyde. While the resulting polymers were very rigid, characterized by high Young’s moduli (a measure of stiffness, calculated as a ratio of tensile stress over extensional strain), they could be converted back to monomers in very acidic conditions. Substituting dianiline with a hydrophilic diamine of polyethylene glycol, researchers obtained self-healing hydrogels (this video shows a piece of gel obtained by connecting pieces of two water-soluble, self-healing gel samples (colored and clear).

The scientists from IBM reported their findings in Science:

We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4′-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young’s moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.

Simplicity of Polymerization

The work has received a lot of attention due to the simplicity of the polymerization and the impressive strength of the resulting polymers. According to the feature article in Science, “the use of multifunctional amine for making high-performance triazine polymeric networks is unprecendented.” And while thermo-, photo- and stress-reversible thermoset polymers have been previously reported, a pH-reversible thermoset polymer is an important scientific advancement with great potential.

As green, biodegradable, and recyclable polymers are gaining popularity among environmentally-minded consumers, many industries, from the medical to the automotive, increasingly are using such polymers in their products. Now we know that even very resilient polyimide polymers, used to make a variety of products from electronics to aerospace materials, can be chemically recycled. And as for water-soluble self-healing hydrogels, one can predict a range of biomedical drug delivery and tissue-specific applications.

Image by rrraven/123RF.
Source: “Recyclable, Strong Thermosets and Organogels via Paraformaldehyde Condensation With Diamines,” by J.M. García et al., Science, May 16, 2014;344(6185):732-5. doi: 10.1126/science.1251484.
Source: “Materials Science. Toward Recyclable Thermosets. Long TE,” Science, May 16, 2014;344(6185):706-7. doi: 10.1126/science.1254401.