Polyurethane

Polyurethanes (PU) are present in many aspects of modern life. They represent a class of polymers that have found a widespread use in the medical, automotive and industrial fields. Polyurethanes can be found in products such as furniture, coatings, adhesives, constructional materials, filters, paddings, paints, elastomers and synthetic skins. Polyurethanes are replacing older polymers for various reasons. The United States government is phasing out chlorinated rubber in marine and aircraft and coatings because they contain environmentally hazardous volatile organic compounds ^{[1, 2]}. Auto manufacturers are replacing latex rubber in car seats and interior padding with PU foam because of lower density and greater flexibility^[3]. Other advantages of PUs are that they have increased tensile strength and melting points making them more durable^[4]. Their resistance to degradation by water, oils, and solvents make them excellent for the replacement of plastics^[5]. As coatings, they exhibit excellent adhesion to many substances, abrasion resistance, electrical properties and weather resistance for industrial purposes^[5-7].
Worldwide, more and more attention is being focused on polyurethane recycling due to on-going changes in both regulatory and environmental issues. Increasing landfill costs and decreasing landfill space are forcing consideration of alternative options for the disposal of polyurethane materials^[8]. Polyurethane is successfully recycled from a variety of consumer products, including: appliances, automobiles, bedding, carpet cushion, upholstered furniture^[9]. The polyurethane industry has identified workable technologies for recovering and recycling polyurethane waste materials from discarded products as well as from manufacturing processes. For example, in 2002, 850 million pounds of polyurethane were used to make carpet cushion^[10], of which 830 million pounds were made from scrap polyurethane foam. Of the total scrap used, 50 million pounds came from post-consumer waste. EC Draft directive^[11] for end-of-life vehicles (ELV) disposal reported that in the year 2005, 15.0% of vehicle weight is disposed (maximum) to landfill, and predicted that in the year 2015, only 5.0% of vehicle weight will be disposed (maximum) to landfill. The polyurethane industry is committed to meeting the current needs of today without compromising the needs of tomorrow. The continued development of recycling and recovery technologies^{[12–14]}, investment in infrastructure necessary to support them, the establishment of viable markets and participation by industry, government and consumers are all priorities.

Figure 1. The chemical structure of polyurethane

Polyurethanes were first produced and investigated by Dr. Otto Bayer in 1937. Polyurethane is a polymer in which the repeating unit contains a urethane moiety. Urethanes are derivatives of carbamic acids that exist only in the form of their esters^[15]. The major advantage of PU is that the chain is not composed exclusively of carbon atoms but rather of heteroatoms, oxygen, carbon and nitrogen^[4]. For industrial applications, a polyhydroxyl compound can be used. Similarly, poly-functional nitrogen compounds can be used at the amide linkages. By changing and varying the polyhydroxyl and polyfunctional nitrogen compounds, different PUs can be synthesized^[15]. Polyester or polyether resins containing hydroxyl groups are used to produce polyesteror polyether-PU, respectively^[6]. Variations in the number of substitutions and the spacing between and within branch chains produce PUs ranging from linear to branched and 9exible to rigid. Linear PUs is used for the manufacture of fibers and molding^[6]. Flexible PUs is used in the production of binding agents and coatings^[5]. Flexible and rigid foamed plastics, which make up the majority of PUs produced, can be found in various forms in industry^[7]. Using low molecular mass prepolymers, various block copolymers can be produced. The terminal hydroxyl group allows for alternating blocks, called segments, to be inserted into the PU chain. Variation in these segments results in varying degrees of tensile strength and elasticity. Blocks providing rigid crystalline phase and containing the chain extender are referred to as hard segments^[7]. Those yielding an amorphous rubbery phase and containing the polyester/polyether are called soft segments. Commercially, these block polymers are known as segmented Pus^[16].

Polyurethanes are one of the most versatile materials in the world today. Their many uses range from flexible foam in upholstered furniture, to rigid foam as insulation in walls, roofs and appliances to thermoplastic polyurethane used in medical devices and footwear, to coatings, adhesives, sealants and elastomers used on floors and automotive interiors^[17,18]. Polyurethanes have increasingly been used during the past thirty years in a variety of applications due to their comfort, cost benefits, energy savings and potential environmental soundness. What are some of the factors that make polyurethanes so desirable? Polyurethane durability contributes significantly to the long lifetimes of many products. The extensions of product life cycle and resource conservation are important environmental considerations that often favor the selection of polyurethanes^[19-21]. Polyurethanes (PUs) represent an important class of thermoplastic and thermoset polymers as their mechanical, thermal, and chemical properties can be tailored by the reaction of various polyols and poly-isocyanates.

Polyurethane foams are prepared by the polymerization of polyols with isocyanates. One of the most commonly used reactive isocyanates toluenediisocyanate, TDI. It is made from toluene by nitration and then reduction followed by treatment with phosgene. The isocyanate residue reacts readily with alcohols to give carbamates (urethanes) or amines to give ureas.

After years of production of PUs, manufacturers found them susceptible to degradation. Variations in the degradation patterns of different samples of PUs were attributed to the manyproperties of PUs such as topology and chemical composition^[22]. Enzyme molecules can easily come in contact with water-soluble substrates thus allowing the enzymatic reaction to proceed rapidly. However, the enzyme molecules are thought to have an extremely inefficient contract with insoluble substrates (e.g. PU). In order to overcome this obstacle, enzymes that degrade insoluble substrates possess some characteristic that allows them to adhere onto the surface of the insoluble substrate^[23-25].
The observations made by Akutsu et al. (1998)^[26] for the polyurethanase PudA indicate that this enzyme degrades PU in a two-step reaction: hydrophobic adsorption onto the PU surface followed by the hydrolysis of the ester bonds of PU. The PU esterase was considered to have a hydrophobic-PU-surface binding domain (SBD) and a catalytic domain. The SBD was show to be essential for PU degradation. This structure observed in PudA has also been reported in poly(hydroxyalkanoate) (PHA) depolymerase, which degrades PHA. PHA is insoluble polyester synthesized as a food reserve in bacteria. In PHA depolymerase enzymes, the hydrophobic SBD has been determined by amino acid sequence analysis and its various physicochemical and biological properties^{[24, 27]}. Another class of enzymes that contain a SBD is cellulases. Several cellulase enzymes have been observed to contain three main structural elements: the hydrolytic domain, a flexible hinge region, and a C-terminus tail region involved in substrate binding^[28-30].
Thus far, only two types of PUase enzymes have been isolated and characterized: a cell associated, membrane bound PU-esterase^[26] and soluble, extracellular PU-esterases^[31-33]. The two types of PUases seem to have separate roles in PU degradation. The membrane bound PU-esterase would allow cell-mediated contact with the insoluble PU substrate while, the cell-free extracellular PU-esterases would bind to the surface of the PU substrate and subsequent hydrolysis. Both enzyme actions would be advantageous for the PU-degrading bacteria. The adherence of the bacteria cell to the PU substrate via the PUase would allow for the hydrolysis of the substrate to soluble metabolites that would then be metabolism by the cell. This mechanism of PU degradation would decrease competition between the PU-degrading cell with other cells and also allow for more adequate access to the metabolites. The soluble, extracellular PU-esterase would in turn hydrolyze the polymer into smaller units allowing for metabolism of soluble products and easier access for enzymes to the partially degraded polymer.

1. Hegedus, C.R., Pulley, D.F., Spadafora, S.J., Eng, A.T., Hirst, D.J., 1989. A review of organic coating technology for U.S. naval aircraft. Journal of Coatings Technology61, 31–42.
2. Reisch, M.S., 1990. Marine paint makers strive to meet environmental concerns. Chemical and Engineering News 17, 39–68.
3. Ulrich, H., 1983. Polyurethane. In: Modern Plastics Encyclopedia, Vol. 60. McGraw-Hill, New York, pp. 76–84.
4. Bayer, O., 1947. Polyurethanes. Modern Plastics 24, 149–152.
5. Saunders, J.H., Frisch, K.C., 1964. Polyurethanes: Chemistry and Technology, Part II: Technology. Interscience Publishers, New York.
6. Urbanski, J., Czerwinski, W., Janicka, K., Majewska, F., Zowall, H., 1977. Handbook of Analysis of Synthetic Polymers and Plastics. Ellis Horwood Limited, Chichester, UK.
7. Fried, J.R., 1995. Polymer Science and Technology. Prentice-Hall, PTR, Englewood CliHs, NJ.
8. J. DeGaspari, Mechanical Engineering Magazine (ASME) June 1999.
9. Alliance for the Polyurethanes Industry. .
10. New Forecasts for Polypropylene, Polystyrene and Polyurethane, Gobi International, May 20, 2002.
11. Directive 2000/53/EC of the European Parliament and of the Council on 18 September 2000 on End-of-Life Vehicles.
12. J. Scheirs, Polymer Recycling, John Wiley & Sons, Chichester, 1998, chapter 10.
13. K.C. Frisch, Advances in Plastic Recycling vol. 1, ISBN 1-56676-737-1-Technomic Publishing, 1999.
14. K.C. Frisch, D. Klempner, Advances in Plastic Recycling, vol. 2, ISBN 156676-793-8-Technomic Publishing, 2001.
15. Dombrow, B.A., 1957. Polyurethanes. Reinhold Publishing Corporation, New York.
16. Young, R.J., Lovell, P.A., 1994. Introduction to Polymers, 2nd Edition. Chapman & Hall, London.
17. P.F. Brains, Polyurethanes Technology, John Wiley & Sons, 1969.
18. C. Hepburn, Polyurethane Elastomers, Elsevier Science, England, 1992.
19. Z. Wirpsza, Polyurethane, Chemistry, Technology and Applications, Ellis Harwood, England, 1993.
20. J. Dodge, Polyurethane Chemistry, Second ed., Bayer Corp., Pittsburgh, PA, 1999.
21. A.G. Bayer, Polyurethane Application Research Department, ‘‘Bayer Polyurethanes,” Leverkusen, Germany. Edition January, 1979.
22. Pathirana, R.A., Seal, K.J., 1983. Gliocladium roseum (Bainier), A potential biodeteriogen of polyester polyurethane elastomers. Biodeterioration 5, 679–689.
23. van Tilbeurgh, H., Tomme, P., Claeyssens, M., Bhikhahai, R., Pettersson, G., 1986. Limited proteolysis of the cellobiohydrolase I from Trichoderma reesei. FEBS Letters 204, 223–227.
24. Fukui, T., Narikawa, T., Miwa, K., Shirakura, Y., Saito, T., Tomita, K., 1988. EHect of limited trypic modi)cations of a bacterial poly(3-hydroxybutyrate) depolymerase on its catalytic activity. Biochimica Biophysica ACTA 952, 164–171.
25. Hansen, C.K., 1992. Fibronectin type III-like sequences and a new domain type in prokaryotic depolymerases with insoluble substrates. FEBS Letters 305, 91–96.
26. Akutsu, Y., Nakajima-Kambe, T., Nomura, N., Nakahara, T., 1998. Purification and properties of a polyester polyurethane-degrading enzyme from Comamonas acidovorans TB-35. Applied Environmental Microbiology64, 62–67.
27. Shinomiya, M., Iwata, T., Kasuya, K., Doi, Y., 1997. Cloning of the gene for poly(3-hydroxybutyric acid) depolymerase of Comamonas testosteroni and functional analysis of its substrate-binding domain. FEMS MicrobiologyLetters 154, 89–94.
28. Knowles, J., Lehtovaara, P., Teeri, T., 1987. Cellulase families and their genes. Trends in Biotechnology5, 255–261
29. Bayer, E.A., Setter, E., Lamed, R., 1985. Organization and distribution of the cellulosome in Clostridium thermocellum. Journal of Bacteriology 163, 552–559.
30. Langsford, M.L., Gilkes, N.R., Sing, S., Moser, B., Miller Jr., R.C., Warren, R.A.J., Kilburn, D.G., 1987. Glycosylation of bacterial cellulases prevents proteolytic cleavage between functional domains. FEBS Letters 225, 163–167.
31. Ruiz, C., Main, T., Hilliard, N., Howard, G.T., 1999b. Purification and characterization of two polyurethanse enzymes from Pseudomonas chlororaphis. International Biodeterioration & Biodegradation 43, 43–47.
32. Allen, A., Hilliard, N., Howard, G.T., 1999. Puri)cation and characterization of a soluble polyurethane degrading enzyme from Comamonos acidovorans. International Biodeterioration & Biodegradation 43, 37–41.
33. Vega, R., Main, T., Howard, G.T., 1999. Cloning and expression in Escherichia coli of a polyurethane-degrading enzyme from Pseudomonas fluorescens. International Biodeterioration & Biodegradation 43, 49–55.

polyurethane: A polymer containingthe urethane group –NH·CO·O–,prepared by reacting di-isocyanateswith appropriate diols or triols. Awide range of polyurethanes can bemade, and they are used in adhesives,durable paints and varnishes,plastics, and rubbers. Addition ofwater to the polyurethane plasticsturns them into foams.

A synthetic polymer containing the group –NH–CO–O– linking the monomers. Polyurethanes are made by condensation of isocyanates (–NCO) with alcohols.

Polyurethane is a polymer containing the urethane group. Polyurethanes find a wide variety of applications in many industries. In fact, mollifiable polymers (hydrophobic) like bitumen, polyvinyl acetate and polyurethane form a major class of soil conditioners. Polyurethane resins and the foams are also used as florists' mounting media or plant growth media.