Depolymerization using supercritical water oxidation (SCWO)

In most cases depolymerization of plastics is conducted by pyrolysis and gasification pathways (thermal depolymerization) or chemical depolymerization, both described above. Somewhat overlooked is the technology of plastic depolymerization using SCWO. While this technique is described later in the context of POPs-contaminated plastic destruction, it has also been modified for depolymerization.

Plastics that are developed via the condensation polymerization process including PET and nylon can be depolymerized to monomers relatively easily by supercritical waters or supercritical methanol. Cross-linked polymers can be subject to selective decrosslinking reactions in SCWO without significant loss of the backbone chains . PET can be depolymerized to high-purity monomers at over 99% efficiency with either SCWO or supercritical methanol and both pilot and commercial plants have been developed.

Polyurethanes are produced through a reaction of polyisocyanate with polyalcohol (polyol). Tolylenediamine (TDA) and polyol can be produced from decomposition of polyurethane foam in subcritical water, corresponding to the initial isocyanate required for polyurethane production .

On this basis Kobe Steel, Ltd. (Japan) developed a supercritical water recycling process as far back as 1997, using subcritical water to convert heavy distillation residues of Tolylenediisocyanate (TDI) to TDA. The plant has operated at 10 tons/day since 1998. The TDI residues were normally incinerated, but this process allowed recovery of 99.5% pure TDA for polyurethane production . A separate 20 tons per day-plant to process TDI by subcritical water was established in Korea in 2007 by Hanhwa Chemical . In 2002, Panasonic also developed a process to recover and recycle high strength glass fiber-reinforced plastics (GFRPs) from GFRP waste using subcritical water hydrolysis. GFRP is a complex laminated composite material containing polyester resin with glass fibre and filler. Carbon fiber-reinforced plastics (CFRP) were also successfully recovered using similar processes.

Depolymerization using bacteria

While this technique is still largely at an experimental level, one company, Carbios, is scaling up and predicts it will have an industrial-scale facility within five years. The technique uses a bacterial hydrolase enzyme to reduce PET to monomers. The bacterial enzyme is based on a naturally occurring bacteria that has subsequently been modified by scientists to process PET more efficiently, claiming a 90% depolymerization within 10 hours . Carbios have teamed up with an enzyme production company, Novozymes, to scale up bacterial production using fungi to an industrial level.

This enzyme reportedly has a high efficiency producing 16.7 grams of terephthalate per liter per hour and the enzyme costs only 4% of the cost of virgin plastic made from oil. There still remain questions as to how the bacteria deal with additives and contaminants, and the hazardous nature of the waste stream generated after the PET has been removed from the plastic waste. The long lead time to commercial availability and the results of scaled-up trials may not reflect the early promise of the technique. Other bacterial approaches have been developed, including the use of Pseudomonas bacteria to decompose polyurethane, and fungi that can break down PET, but all of these techniques appear to be years, if not decades, from any form of commercial activity.