First evidence that tough plastic can be biodegraded
The first evidence for biodegradation of phenolic resins could have implications for recycling the widely used plastic.
A study posted today on ES&T’s Research ASAP website (DOI: 10.1021/es060408h) shows for the first time that a fungus can break down the phenolic resins widely used in plastic formulations. The finding could make it easier to recycle the synthetic polymers, which are known for their durability and resistance to attacks by termites and fungi.
Phenolic resins are commonly used as industrial adhesives and in heavy-duty automotive parts such as the plastic trim on car bodies and the plastic containers that hold air filters. The resins are formed when phenol and formaldehyde are cured at a high temperature and pressure in the presence of catalysts, so that the molecular chains form an interlocking 3D structure that is hard to break. As a result, they cannot be melted and remolded like other plastics, such as polyethylene and polyvinyl chloride. Instead, most of the 2.2 million tons (t) that are manufactured worldwide every year end up in landfills.
White-rot fungus is known to decompose organic pollutants such as DDT, TNT, PCBs, and dioxins and can be used to clean up these toxins from the environment. The fungus produces enzymes called ligninases, which can break down lignin, the compound that makes up the dry part of wood. Researchers in the department of biology at the University of Wisconsin–La Cross wanted to test whether the fungus could also degrade phenolic resins, which have a molecular structure similar to that of lignin. They used a generic industrial formula to make the polymers and placed resin chips in cultures with the fungi.
The several hundred species of wood-rotting fungi fall into two broad categories, white-rot fungus and brown-rot fungus. The researchers tested 11 different fungi strains, including 5 species of white-rot and 1 species of brown-rot fungus. All of the species used in the new research have been previously studied for their ability to biodegrade pollutants.
The researchers first realized that the white-rot fungus was degrading the phenolic resins when their color changed from yellow to a light pink, the color of the phenol and formaldehyde subunits used to make the resins, says Adam Gusse, a biology graduate student and lead author of the study. They confirmed the presence of those subunits by gas chromatography/mass spectrometry and by locating pockmarks on the surface of the resin chips with scanning electron microscopy.
The U.S. generates about 25 million t of plastic waste every year, of which phenolic resins make up less than 10%. That might not seem like much, Gusse says, but the numbers add up over time because the plastics never biodegrade. Being able to recycle phenolic resins might keep one more type of plastic out of landfills.
Proof that white-rot fungi can degrade phenolic resins is a significant finding that has valuable research implications, but its application for recycling is not very clear, says Jeff Morrell, a professor in the department of wood science and engineering at Oregon State University. He calls this work “a first step” that opens up avenues for fundamental research in understanding the mechanism as well as applied work to look at what conditions affect the process. “A dozen more steps would have to happen before you can make this commercially viable,” he says.
The automotive parts made from pure phenolic resins are likely to be the first products that would be recycled, Gusse says. Jim Wilson, the vice president of the Consortium for Research on Renewable Industrial Materials, points out that it would be hard for fungi to reach the phenolic resins in wood composites; this means the resin would have to be separated from the wood before recycling. Also, he does not see a rationale for recycling the resins. He says that instead of throwing resin-laden wood products into landfills, they could be chopped into pieces and reused to make other products.
Unlike wood products treated with chromated copper arsenate, wood composites made with phenolic resins do not pose environmental hazards, says Charles Frihart, the head of wood adhesives science and technology at the U.S. Department of Agriculture’s Forest Products Laboratory. As a result, it is easy to dump them in landfills. He is nonetheless researching soy-based adhesives as an environmentally friendly alternative to phenolic resin adhesives.
Gusse and his colleagues are currently investigating whether white-rot fungi—known as the only microbes that degrade wood—can degrade a resin and wood mixture. “This would show that we wouldn’t have to remove the resins from the composite first,” Gusse says, “but just grind the composite into smaller pieces and let the fungus go to town on all of it.”
In the future, Gusse says that cost and advanced technologies could be driving factors for recycling phenolic resins. Researchers at Japan’s National Institute for Resources and Environment developed a process to degrade the resins in 1997; however, it takes a lot of heat and chemicals and the price would be high compared with that of virgin phenol, which costs about 80 cents for a pound. Gusse says that combining the Japanese technology with fungal biodegradation could save costs. He says that this will become important as petroleum prices rise, because phenol comes from petroleum.
