Recycling in Polyurethane Materials

Polyurethane Material Recycling Rising water and soil pollution, the possibility of raw material depletion, and the resistance of plastics to physical and chemical factors have increased the importance of synthetic polymer waste recycling, recovery, and environmentally friendly disposal methods. Polyurethanes (PU) are versatile synthetic polymers with a wide range of applications. In this research article, we have compiled the recycling process of polyurethane materials and studies conducted in this field. Various methods tested and applied in recent years have proven that PU waste processing can be economically and ecologically beneficial. Currently, mechanical recycling and glycolysis recycling methods are among the most important. Additionally, the biodegradation of polyurethanes is quite promising for both post-consumer and post-production waste. For biological methods, one possibility is the synthesis of PU materials that are sensitive to biodegradation. In conclusion, current research suggests that in the future, disposal of unprocessed polyurethane waste will be possible. Polyurethanes constitute approximately 8% of plastics, meaning they are the 6th most used polymer in the world. Polyurethanes; can be classified as foams and CASE (Coatings, adhesives, sealants, elastomers). Polyurethane foams are also subdivided into flexible and rigid types. Looking at their fields of application, flexible polyurethane sponges are used in mattresses and automotive seats, while rigid polyurethane foams can be used in building insulation and refrigerators. CASE materials are mostly used as part of sports shoes, athletic tracks, electronic products and ship structures. According to conducted research, when examining the Europe, Middle East and Africa final polyurethane products market; flexible foams occupy 36%, rigid foams 32% and CASE 32%; in other words, they have been observed to have nearly equal production ratios. Because they offer superior properties (flexibility, high heat resistance, strength, etc.) and these materials can be utilized in various applications, increasing amounts of polyurethane waste are generated each year. Such waste includes end-of-life (EOL) and post-consumer (PC) products, as well as residues from polyurethane production. Waste can also be generated due to production and processing methods. Two different methods can be used in polyurethane recycling; one of these methods is mechanical recycling, the other is chemical recycling.

Polyurethane Waste Recycling Methods

Mechanical Recycling

Mechanical recycling is the easiest and most basic way to recycle PU. It involves converting solid waste into flakes, granules or powder. This recycling can be achieved through grinding, milling, cutting or shredding. 1. Rebond Flexible Foam: In this method, shredded flexible polyurethane foams are glued together with a binder to create products such as carpet underlayment and mats. 2. Grinding or Powdering: Polyurethane pieces are ground at the end of their service life and converted into a fine powder. The resulting powder is mixed with pure materials to create new polyurethane foam or injection molding reaction. 3. Pressing/Bonding: This recycling method is used on polyurethane materials found in high amounts in automotive, white goods and industrial waste. Polyurethane materials are ground and compressed with a binder or adhesive under heat and pressure to form sheets or different molding shapes. The resulting product can be used in sound insulation applications, floor coverings requiring moisture and elasticity, and waterproof furniture. 4. Compression Molding: In this recycling process, molded injection reaction (RIM) and reinforced RIM parts are ground into fine particles. Subsequently, high temperature and pressure are applied to achieve 100% recycling. This resulting material shows properties similar to the original material.

Chemical Recycling

The other raw material recycling method is chemical recycling. This method is generally a type of polymer reprocessing that leads to the transformation of polymer chains into smaller molecules through chemical processes. Chemical recycling methods include hydrolysis, glycolysis, pyrolysis, gasification, and hydrogenation. An important feature of polyurethanes is the possibility of reversing the polymerization process, which allows recovery of building blocks. Considering costs, applied temperature and additional surfaces, chemical recycling is much more complex than mechanical recycling.

1. Glycolysis

Today, glycolysis is the most widely used chemical recycling method for rigid and flexible polyurethane. It is based on a transesterification reaction in which a hydroxyl group from glycol replaces a carbonyl carbon of an ester group containing a urethane bond. This reaction produces polyols with properties that can be controlled to some degree and may be similar to those of the original material. They can be used in polyurethane production. Glycolysis has two main aspects. The first leads to the recovery of polyols for flexible polyurethane foam production. Second, solid-phase glycolysis (SPG), which results in rigid and flexible polyols. The chemical functions of these resulting polyols are close to the original polyol material and can be used in many different applications.

2. Hydrolysis

Hydrolysis is the first chemical method developed to recycle polyurethane waste, particularly flexible foams. In this process, a new reaction is created between the polyurethanes used and water. As a result of the reaction, polyols and various intermediate chemicals are produced. The resulting polyols contain amine intermediates and carbon dioxide. These polyols are used as fuel and intermediate materials, and as raw materials for polyurethanes. The most important advantage of hydrolysis is that it provides application possibilities for both production waste and post-consumer waste. The process is carried out in an anaerobic environment and at high temperature (above 150–320°C). The biggest disadvantage of hydrolysis is that it requires high energy input to the reactor to heat the batch or apply high pressure, and this process is not economical.

3. Pyrolysis

In this process, polyurethanes are broken down in an oxygen-free environment to produce gas and oil. Pyrolysis is the thermal degradation of long polymeric chains into less complex molecules under anaerobic conditions at high pressure. Among the advantages are the small amount of waste remaining after the pyrolysis process and the possibility of using the obtained products in other petrochemical processes. Unfortunately, this is not always possible because the proportions of individual products cannot be quantitatively determined and it is difficult to obtain products with the desired properties. Hydrogenation: Hydrogenation is a process similar to pyrolysis. Heat, pressure and a hydrogen mixture produce gas and oil from the polyurethane used. It leads to gas and oil formation. The fundamental difference between this process and pyrolysis is the use of high-pressure hydrogen instead of inert gas. For this method to be useful, two important issues must be addressed as in the previous process: the composition of the obtained oil and gas and the cost of converting them into functional products that can subsequently be used as energy materials and substrates in chemical processes. Gasification: Gasification is a highly exothermic reaction of partial oxidation of carbonaceous materials. Its main products are "syngas" (mainly a mixture of carbon monoxide and hydrogen) and ash. This is a highly exothermic reaction. One of the most important advantages of gasification is that waste separation is not necessary. Additionally, polyurethanes mixed with other materials can also be used in the process. According to research, gasification processes produce significant amounts of toxic hydrogen cyanide and nitrogen dioxide. Adding an appropriate catalyst can reduce these emissions to some extent.

Biodegradation

Biodegradation means the breakdown of organic matter by living organisms or their enzymes. It causes the shortening of polymer chains and the elimination of some parts. This leads to a reduction in molecular weight and, under appropriate conditions, can even cause complete mineralization of the degraded material. However, complete degradation of larger polymers generally requires the cooperation of several different organisms. It can consist of several stages: the breakdown of the polymer into monomers, their reduction to simpler compounds, and (under anaerobic conditions) final degradation to carbon dioxide, water and methane. Biodegradation is generally more environmentally friendly than chemical degradation as it does not require high temperatures and complex reagents. Additionally, it can also be applied to the degradation of post-consumer waste. The literature contains numerous records of polyurethane biodegradation. Unfortunately, most consist of research that explains a limited portion of this topic. They are generally applicable to only one type of microorganism or polyurethane. There are many ways to recover polyurethane waste, but improvements and research on this subject continue. The frequently used method is mechanical recycling. This recycling method is relatively inexpensive, but has many limitations. It also produces products of much lower quality than the original polyurethane material. Chemical and raw material recycling require high temperatures and aggressive reagents, and currently only one of these processes is being applied on a larger scale. Biodegradation requires a reasonable temperature and does not require any hazardous chemicals, but research continues for its application at a technological scale.

New Method Found in US Research

One of the research studies on recycling polyurethane materials was conducted at the University of Illinois. A team at the university developed a method to break down polyurethane waste and convert it into other useful products. Polyols are generally not degradable because they are petroleum-based. However, the team intervened in the polyol by adding an acetal, a chemical unit that is more easily degraded. Since polyurethanes are water-resistant, researchers invented an acetal solution that degrades in solvents other than water. As a result of the research conducted, when a combination of trichloroacetic acid and dichloromethane was added to the compound, they recorded that the material swelled at room temperature and began to degrade rapidly. As a result of their study, the researchers found that the products resulting from the process could be used as raw materials for new materials. For example, they converted elastomers, a type of polyurethane used in rubber bands, packaging and automobile parts, into an adhesive. The researchers also emphasized that the greatest challenge in this method is the high cost of the starting material and stated that they are working to find a better, more cost-effective solution. Source: https://mag.turkishplastics.net/index.php/tr/poliuretan-atiklarina-ikinci-bir-sans http://rustempolat.com/makale-detay/poliuretan-geri-donusumu Polyurethane Recycling and Disposal: Methods and Prospects by Aleksandra Kemona and Małgorzata Piotrowska Prepared and compiled by: Nilsu Kotil