When it comes to the end-of-life phase of thermosets – contrary to thermoplastics – the recycling technology of heating or melting cannot be applied. Still, the polyurethane foam industry at large has been working at addressing this challenge over the past years. Many of the past issues have been resolved in the supply chain, the regulatory environment has evolved and recycling technologies have come a long way.
Today there are two main technologies available for the recycling of PU foam efficiently, mechanical and chemical (advanced / enhanced) recycling. The difference between the two is that chemical recycling follows degradation principle – meaning that polyurethane wastes are depolymerized into its primary raw materials.
Mechanical Recycling
Mechanical recycling as a method exists for decades. The PU foam supply chain has made great efforts already over the past decades to improve its resource efficiency at the production stage. Mechanical processing with or without the addition of binder is the easiest and most basic way to recycle PU. Today, almost all PU foam production and conversion waste or scrap is sold by foamers and converters as trim and to produce PU bonded foam used in various products.
PU foam found in comfort applications has the same lifespan as the product itself – up to several decades. On the other hand, the main application of rigid PU foam – sandwich panels – have a much longer average lifespan. This discrepancy results in different quality of mechanically recycled PU foam.
Chemical Recycling
Chemical recycling, also known as enhanced recycling, involves any type of reprocessing technology using chemical agents or processes that directly affect either the formulation of the plastic or the polymer itself. In this process polyurethane foam is broken down into chemical raw materials, namely polyols and diisocyanates. Considering costs, applied temperature and additional substrates, enhanced recycling is much more demanding than the mechanical recycling pathway.
There are two main chemical recycling technologies. The first one is chemolysis which is split in many different subcategories such as acidolysis, hydrolysis, aminolysis or alcoholysis, depending on the chemical reagents used. Today it is possible to recycle the polyols fraction of the PU foam with this technology, and research is taking place to also recycle the diisocyanates in the future. The second technology is thermochemical recycling, in which materials are brought back to a molecular level in oils and gases, which can be used as new raw materials for the petrochemical industry. Using the so-called "mass balance approach", up to 100% of bio-based and/or recycled polyols and isocyanates could be used in foam formulations in the future.
Other Technologies
One technology more commonly used for recycling of rigid PU foam is biodegradation. This technology is characterized by the breakdown of organic substances by living organisms or their enzymes. It results in shortening of polymer chains and the elimination of some of its parts. That leads to the reduction of its molecular weight, and in favourable conditions, it can even result in the complete mineralization of degraded material. However, the complete degradation of larger polymers usually requires the cooperation of several different organisms. It can consist of a few stages: breakdown of the polymer to monomers, their reduction to simpler compounds and final degradation to carbon dioxide, water and methane (under anaerobic conditions).