Sudhin Datta is a retired Senior Research Associate at ExxonMobil Chemical, in the polymers business. He holds a PhD in organometallic chemistry from Harvard University and postdoctoral engagements at the University of Toronto and the University of Chicago. His work on polyolefin elastomers has been honored in both 2011 and 2014 by ExxonMobil Chemical Outstanding Patent awards, which recognize technology commercialization with widespread utility. In addition, he has been honored with the 2015 Charles Goodyear award of the Rubber Division of the American Chemical Society. Dr. Sudhin can speak with authority on the topic of recycling thermosets. Here, he discusses the processes and challenges of thermoset recycling.
“There is a move to recycle or to reconstitute almost all the polyurethane foam insulation. My sense is no more than 20% of the world’s production of polyurethane foam is actually recycled.”
The most important classical thermosets that are recyclable are polyurethanes, epoxies, and silicones. Additionally, there are materials which behave like thermosets in the recycling process, such as PVC, Teflon, and PEX, cross-linked polyethylene.
The three classical thermosets are recycled for different purposes:
- Polyurethanes are recycled because there is a very large volume in the world in the low-density form. There is inherent value in the materials that come out of polyurethane recycling, and the process only takes a couple of hours. It is not being done in North America and Western Europe, as the companies in such regions would much rather export that waste polyurethane foam to lower cost countries in Asia.
- Epoxies have inherently no value, but reinforced epoxies are recycled for carbon fiber recovery, which are 10 times more expensive than the epoxy itself.
- Silicones are recycled because silicone monomers are very expensive.
Other materials face more economic barriers, such as Teflon and PVC:
- Thermal recycling turns Teflon and PVC into dark intractable solids while releasing toxic acid gases which damage the equipment.
- Teflon recycling is hampered because typically it is present in small quantity by weight and recovering and recycling is economically unjustifiable.
- Typical PVC pipes for city water are composed of filled PVCs. So whatever recycling process should first remove the filler, which is a toxic waste that corresponds to around 40% of the volume.
The thermoset recycling process is not mechanical, so is it always chemical? What about thermal recycling – pyrolysis?
That’s correct. What is called “mechanical recycling” is actually a down cycling, basically just grinding it and feeding it into low-value products such as asphalt or something related. This is not recycling; it is just hiding the problem.
For thermosets, one would not go for pyrolysis. That is what thermosets are. If you heat them up by themselves, nothing happens, they just burn.
The processes are chemical catalyzed reactions (glycolysis and alcoholysis), and they can be understood based on their chemistries:
- Polyurethanes: Polyurethanes are soaked, and then a glycolysis process is carried out by heating up ethylene glycol (at around 280°C) for about four or five hours and breaking the big molecules down to smaller molecules, which can be distilled and recovered. It is claimed a 95% efficiency of whatever output material as free monomers. The process is fairly well understood.
- Epoxies: Reinforced epoxies are recycled via alcoholysis, or there is typically a catalyzed degradation of the process. The epoxies come off and the catalyst is washed off, so the carbon fibers are recovered. The chemistry is well understood, but there is some work to be done to understand the catalyst.
- Silicons: Silicones are recycled in a similar way to polyurethanes, but the molecules are broken down to polydimethylsiloxane (PDMS).
When it comes to thermoplastic materials, there’s a lot of discussion on post-consumer and post-industrial recycling. Is there a parallel for thermosets as well?
The recycling processes of polyurethanes, epoxies, and silicones are basically breaking them down into monomers. So the sourcing of the material is not very important, as long as it has consistent composition.
So thermoset recycling is more like break down the polymer and turn that back into useful building blocks which you can reuse in some other way or probably in a better way.
Silicones are a good example. After recycling, even if you don’t make silicone products, you will make silicone liquids, which have actually higher value because they can be used in shampoo formulations as an anti-foam agent, for instance.
It looks like most of the silicone recycling is done on a small scale by silicone users, particularly in China and India.
Could you comment on societal hindrances and regulations in all plastics recycling?
There are two sets of societal customs and regulations which have to be considered. One is the interest of the waste recyclers. For waste recyclers or those who haul waste away, losing materials to side products is a loss of business and they oppose it vehemently. They oppose the classification of polyolefin or polyurethane, for instance, as a raw material rather than a waste.
In in-kind recycling, where you just take a waste plastic and put it back into its intended use — for example, a polyethylene foam going back into a polyethylene — that doesn’t work very well because consumers are very concerned about odor, color, and history of the plastics used. So there is a huge amount of consumer resistance which always happens whenever you try to bring in in-kind recycling.
The one that seems to get around all these issues is breaking down into monomers and sending to the process and reusing.
So for the regulations, there are politics, there is human psychology, and there are capital and economics. It is not a clean answer.
“Recycling of rubber products is a massive issue. By far, this is a blight of humanity, used tires which no one quite knows what to do with and is not widely discussed.”
