For over 80 years, polyurethanes have become one of the most dynamically developing groups of polymers that are continuously adding to their range of applications. This is all due to their segmented structure, as polyurethanes consist of both flexible and rigid segments.
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The myriad of application areas for polyurethanes rely on the distinct design of their molecular structure. It is this requirement that imposes limitations on the recycling process.[2] While it is now possible to provide a complete cradle to grave lifecycle for polyurethanes, nevertheless the amount of recycled polyurethanes that can actually be used without interfering with the original design principles for a particular type of polyurethane is limited. It has been the experience of the author that at most about 20% of recycled polyurethane can be used without interference in the final properties.
The worldwide market for polyurethanes has been valued at USD 87.1 billion at the end of 2023 [2] and is expected to grow to USD 91.49 billion by the end of 2024. Polyurethanes have truly become a polymer that accompanies man in every aspect of daily life, whether it is toothbrushes, footwear or the polyurethane foam under the dashboard of your car, polyurethanes have become intertwined with our existence. It is therefore our responsibility to manage this resource responsibly and to recycle it as best we can. In this document a number of strategies will be described.
To understand the degradation mechanism of polyurethanes, it is helpful to look at the two ingredients that react to give the final product (Schematic 1).
As noted by Kemona et al [3], two of the largest markets for polyurethanes remain rigid- and soft foams. These are also markets that can absorb most of the recycled polyurethanes without complicating the basic polymerization reaction as depicted in Schematic 1. As a general rule, unless really valuable low molecular weight polyols can be regained through chemical- or bio-degradation, the end market segments to be targeted is recommended to be soft- and rigid foams.
Schematic 1:
A summary of the various chemical depolymerization strategies are given by Kemona [3] and [4]. There are of course numerous other publications but these two is in the opinion of the author a sound foundation to understand the rest of the literature. In all cases presented here for chemical degradation of polyurethane, it must be realized as ideal situations with concomitant high yields. In truth, most of these processes are probably far from either.
As the name suggests, this depolymerization reaction takes place in water at a high temperature of 150 ºC to 320 ºC. The medium can be liquid or steam. The reaction is probably more productive if it is done under pressure. Both polyols and amines can be recovered and used in most polyurethane applications. The amines are recovered after phosphorylation. The biggest disadvantage is the high energy inputs required and probably also the equipment costs.
Schematic 2:
Hydroglycolysis is an improvement over hydrolysis as both glycols such as crude glycerol and water is used at lower temperatures of 200 ºC using lithium hydroxide as a catalyst. It is not commonly used due to the high cost of the process and the yield which is around 50 %.
Aminolysis starts by grinding the polyurethane to a very fine state to increase the surface area. A high surface area is a given for most recycling options when it comes to polyurethanes and should be included as a unit process(es) in any consideration of polyurethane recycling. The aminolysis reaction is performed at a relatively low temperature of 80 to 190 ºC. Typical catalysts that can be used under an inert atmosphere such as nitrogen are sodium hydroxide (cheapest) as well as aluminium hydroxide and sodium methoxide. Both ammonia or other amines can also be used and the reaction is a transesterification reaction. The type of starting amine will determine the type of final amine produced which can theoretically be used in polyurethanes, melamine resins, epoxy resins, polyesters and polycarbonates. However, this reaction has only reached bench scale or research scale and only proves that the reaction is viable.
Schematic 3. Idealized amino/ammonolysis reaction leading to bi-carbamate/uret structures and polyols.
The catalysts used in this case are phosphoric and phosphonic acids. They lead to the formation of liquid products which form due to the reaction between the urethane group and the ester alkoxy group. The result is a mixture of phosphorus containing oligomers. The products from this can theoretically be used in flame retardant polyurethanes and polyurethanes with improved adhesion to metals as well as better UV resistance. The biggest disadvantage is that the starting products are not recovered, for example the pure polyols.
Schematic 4:
Glycolysis in the presence of glycols is the most successful chemical depolymerization method at present. It is a transesterification process that is most successfully used for rigid and soft polyurethane foams as well as elastomers. This process is the closest chemical depolymerization process to date that gives a semblance of control over the final product. This is probably where enzymatic depolymerization could have an advantage. Two directions stem from glycolysis. In the first one, polyols are recovered for flexible foams and in the second, the so-called split phase glycolysis polyols are recovered for both rigid – and soft foams [5]. Diethylene glycol is used to extract the top layer after the reaction is complete which is used for flexible foams while the bottom layer is used for hard foams. The bottom layer is treated with propylene oxide using sodium hydroxide as catalyst under inert conditions to yield solid polyols. The main disadvantage of this process is the requirement to segregate the waste and to make sure the waste is free of contaminants that may influence the reaction with propylene oxide.
Schematic 5:
The acidolysis of polyurethanes has recently been reviewed
Schematic 6:
Schematic 7:
In Schematic 6, adipic acid was used with the intention of incorporating it into the structure of the final polymer. It was pointed out by Grdadolnik et al, that the degradation of the polyurethane foam was improved with higher concentrations of adipic acid but at the expense of esterification of the hydroxyl groups of the polyols liberated in this way[7]. These carboxylic end-group polyols could however be successfully incorporated into new polyurethane foams. The carboxylic acid end-groups probably contributed to the liberation of CO2 which leads to a system where the polyol also assists with cell-formation in the final polyurethane foam structure. Acidolysis can lead to the formation of more stable amide groups which leads to an irreversible acidolysis reaction.
Catalysis using a strong base is another chemical recycling method
Schematic 8:
Schematic 9:
The hydrolysis of carbamates could and should also be employed as a recycling strategy for the aforementioned glycolysis residue. In this way, the carbamate residue can be decomposed to precious chemicals such as amines, which subsequently are feedstock in the production process of isocyanates. Amines used for catalysts in polyurethane polymerization controls many aspects of the final foam morphology and control the rate of the polymerization reaction.
The methanolysis of polyethylene terephthalate is a good example of alcoholysis which can also be applied to polyurethane depolymerization
Schematic 10:
Schematic 10 depicts the alcoholysis of PET, a polyester that is generally known to be used in, for example water bottles. Correspondingly, the amount of waste generated by PET is vast generating significant interest in recycling.
Schematic 11:
Schematic 11 above depicts the general mechanism associated with alcoholysis [5]. Alcoholysis is a strategy that utilizes transesterification as a means to break down or build up alcohols
Exchanging alcohol species can be achieved with transcarbamoylation [11]. This is akin to transesterification in polyesters.
Schematic 12. Concurrent depolymerization and polycondensation as a means to upgrade recycled polyurethanes.
The transcarbamoylation of polyurethanes is a novel way to add value to recycled polyurethanes. If this method could be practiced on large scale, it would be an environmentally friendly alternative to derive novel polyurethanes since it involves no new raw materials, and one could hopefully control the final product properties by including additional polyols and additives.
A rendition of the polyurethane structure is given by Magnin [12].
Schematic 13:
In the work by Magnin et al, esterase and amidase enzymes are used. It is interesting to note that the combination of both enzymes increased the polyurethane depolymerization yield. It is also of interest that a number of hydrolases are available, notably from Proteus. Amidases used in Magnin et al were obtained from Escherisa Coli strains. A number of enzymes have also been tested by Kemona et al been tested and are reproduced and listed below in Table 1 [3]. Table 1 also impresses the fact that not all enzymes are equal in selectivity with regard to the type of polyurethane being degraded and hence products obtained.
Most of the enzymes identified for polyurethane cleavage appear to belong to the class of hydrolases. It can also be appreciated that the number of enzymes to bring about enzymatic degradation will be limited since polyurethanes do not occur naturally in nature.
Table 1. A list of some of the polyurethanes depolymerized/degraded with corresponding enzymes used to achieve this.
Many of the studies on the use of enzymes have focused on microbial communities found in landfills and sewage water. As mentioned, depending on the type of polyurethane and the type of enzyme used, the type of product of enzymatic degradation may vary a lot. In most cases, when looking at the polyurethane structure, degradation is set to start with the softer segments of the polyurethane.
Schematic 14. Esterase catalyzes the depolymerization of polyesters into carboxylic acid terminated oligomers and polyols (top) while yielding carbamic acid and polyols using polyurethanes (bottom).
| Type of Polyurethane | Type of Enzyme |
| Polyester PU (Impranil) Thermoplastic | Esterase, Lipase, Protease, Cutinase |
| Thermoplastic polyester PU Thermoplastic | Lipases, Esterases, Pancreatin, Polyamidase, Proteases |
| Thermoplastic polyether PU Thermoplastic | Esterase, Chymotrypsin, Proteases |
| Thermoplastic Polycarbonate PU | Cholesterol esterase |
| Thermoplastic poly (ester ether) PU | |
| Thermoplastic poly (ester urea) PU | Lipase, Cholesterol esterase |
| Thermoplastic poly (ether urea) PU | Cholesterol esterase, Elastase, Papain |
| Polyester PU coating | Lipase |
| Polyacryl PU coating | Pancreatin |
| Impranil DLN-SD (Covestro) | Esterase, Amidase [12] |
| Model substrates | Cutinase [13] |
| Impranil DLN | Hydrolase [14] |
| Model substrates | Lippase [15] |
| Polyurethane foams | Laccase [16] |
Most of the promising results with regard to polyurethane biodegradation can be assigned to the hydrolysis of the polyester fraction (COO) of the polyurethane. This is accomplished by esterases. Schematic 14 depicts the idealized outcome of applying esterase as an enzymatic biodegrading option leading to carbamic acid as a product, it is probably more likely to find amines and CO2 due to the breakdown of this unstable carbamic acid species.
Most fungal species used in polyurethane biodegradation belong to the genres Aspergillus, Penicillium, Chaetomium, Cladosporum and Trichoderma. Most fungi identified for polyurethane biodegradation are filamentous and only a few types of yeasts have been identified. The reason for this observation is not quite clear. It could be that the filamentous yeasts through filaments growing in the substrate assist with mechanically increasing the surface area available for degradation while this is possibly not likely for yeasts.
Schematic 15:
Schematic 16:
Since fungi are not the only organisms that can excrete enzymes to degrade polyurethanes, one would expect that bacteria should be able to do same [3]. Some of the bacteria identified include Pseudomonas chloraphis, Bacillus subtills, Comamonas acidovorans and Acinebacter gemeri. The enzymes associated with these bacteria are the lipases and esterases. Probably a lesser known enzyme, generally used in other types of applications such as aerobic polymerization of phenolic structures, is laccase [16]. This multicopper enzyme is commercially available and has generated great interest in polyester biodegradation. It has now been established that laccase can degrade a wide range of polyurethanes.
Another recently discovered enzyme is cutinase [17]. This enzyme is excreted by the fungus Humicola insolens. Again, the primary bonds attacked are ester bonds and significant hydrolysis is observed.
While the chemical processes for recycling polyurethanes usually lead to a wide range of molecular species with different molecular masses and functionality, enzymatic degradation of waste polyurethane could benefit from the selectivity exhibited by enzymes [9]. In this way, either the polyol segment may be left in-tact or the urethane segment. By combining enzymes, both the harder urethane segments as well as the softer polyol segments can be conveniently degraded [12].
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In conclusion the idealized mechanisms for both chemical – as well as enzymatic degradation reactions have been presented as generalized chemical mechanisms. Most of these reactions, both chemical and enzymatic can occur in aqueous media which make it possible to transition one from type of recycling to the next.
The most likely area of application of the end-products of both chemical and enzymatic degradation is in rigid- and soft polyurethane foams. These market segments command a significant combined market segment in the polyurethane industry.
The possible products for chemical and enzymatic degradation have been compared in this document intentionally. While tandem enzymatic systems have already been elucidated by Magnin et al, similarly it should be an important strategy to run in tandem or in series both chemical and enzymatic recycling processes to recover the greatest value from both systems. In the case of enzymes, greater selectivity of the final product could provide additional value.
While chemical and enzymatic methods of polyurethane degradation are compared here to find synergies, it must be realized that chemical means of recycling polyurethanes will continue to dominate in terms of cost effectiveness.
We all know the dashboards of our cars are based on a yellow rigid foam. This is painted or clad with a plastic layer. In addition, the seats olf our cars contain a soft foam, also based on polyurethanes. Polyurethanes are used in coatings and glues and in footwear. Polyurethanes touch every aspect of our lives
The most effective recycling would be mechanical recycling and chemical recycling. Already in South Africa and elsewhere, hydrolysed polyurethane (PU) is used in percentages varying from 5% to 20%. It can be called a reactive filler in new polyurethane products.
As above, mechanical, chemical, biological and gasification.
Mechanical recycling to some extent delays the inevitable. Let’s say that a soft polyurethane foam is cut into small pieces and then glued together under pressure to yield a cheap mattress that is exported to Africa. Eventually, that mattress ends up in a land fill. The expensive components that made the PU flexible in the first place namely the polyols are not recovered and the PU takes anything from 8 to 30 years to decompose. If we could deconstruct the PU foam from the start, we could obtain some of the polyols or sections of the PU back. This can be reacted back into PU products thereby extending the lifecycle of the PU.
Glycolysis is the use of glycols such as diethylene glycol, ethylene glycol, propylene glycol and many other types of alcohols and glycols to react with PU waste through a transesterification reaction. Depending on the level of glycolysis, products from this process may yield low molecule weight PU with hydroxyl functionality or actual polyols as the polyols are liberated form the chemical structure of the original PU. Normally, the level of purity of the final product is cost related so the lowest cost products are made that serve as functional fillers for new PU products.
Acidolysis uses polyfunctional organic acids to transesterify waste PU. This leaves low molecular weight species with organic acid functionality. These molecules can be used to make polyureas by reacting them with a diisocyanate or it can be incorporated into polyesters and other types of condensation polymers.
Unfortunately, the recycled PU can never quite match the original virgin material. Theoretically it is possible to obtain the starting materials used to make the original PU but the cost involved does not make the process viable. For this reason, both PU and polyesters such as PET are chemically recycled to the point where they can perform a valuable role as a functional filler.
Both mechanical and chemical recycling requires the necessary infrastructure to be able to cut and shred the PU and then reactors, mostly 316 steel reactors with heating and mixing capability as well as storage facilities and packaging facilities or unit operations. These requirements require capital outlays. If the equipment are already in place, modification may be necessary and of course maintenance of the equipment. Of course, none of the infrastructure investment will happen without adequate demand. For this reason, mechanical and chemical recycling are best included into the best practices of companies that already produce PU goods.
The concept of a circular economy amongst other things strives to use a product for a longer time in one form or another. If one can recycle PU either mechanically or chemically, one extends the lifetime of the original material. Instead of ending in a landfill the PU is recovered and put to use in a different form. For example, if 20% of a vehicle dashboard is made of recycled PU, the time the recycled PU will spend in the dashboard is extensive. Another important aspect of PU technology is the use of raw materials obtained form natural materials such as plant oils including castor oil and other types of oils. Apart from this, the author has extensive experience in using other raw materials such as starch and lignin that be combined with recycled PU to give products that are much easier to decompose should they be returned to landfills for example.
Currently, one of the more innovative ideas in terms of PU recycling is the use of enzymes. Enzymes are biological catalysts excreted by micro-organisms such as bacteria and fungi. The possibility exist to obtain more pure recycled raw materials such as polyols that can be re-used as actual raw materials and not just functional fillers.
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