Polyurethanes and Polyureas

Most urethane polymers are thermosets [247, 248]. They are mainly used in the production of foams, taking advantage of the reaction of water and of the precipitation of insoluble ureas, which stabilize the foam even before the system 

As with the other thermosets in this chapter, these processes will not be discussed in this section. We will nevertheless give a few hints of the reaction engineering of the production of polymers and macromonomers based on this chemistry other relevant uses are adhesives, binders, coatings, thermoplastic elastomers, and fibers. 

The catalysis of isocyanate reactions has been extensively studied because of its critical importance in many of these processes. Noncatalyzed (or rather, self-catalyzed) reactions may sometimes be fast enough in practice isocyanate reactions with amines are so fast that only recent studies using stopped-flow methods could lead to useful data [255, 256], metallic or tertiary amine catalysts being ineffective in this case. 

As often happens with polymerization reactions, simple rate laws can seldom describe the whole course of reaction because of catalysis or inhibition by the urethane groups formed or by the initial reagents. Self-association of metallic catalysts, or their loose complexation by products or reagents, also prevents correlation of rates of reaction by simple proportionality or even power-law relations. Catalysis of isocyanate reaction with hydroxyls [250, 251] is by far the best understood. 

It might be thought that modeling of polyurethane processes would be relatively straightforward, given that reactions are mostly irreversible and the methods described in Section 3.4.4 should deal with them without difficulties. 

Pol3nirethanes contain the structural characteristics of both potyesters and polyamides, whereas polyureas might be viewed as poty(diamides). 

Their liability to biodegradation is comparable to that of polyesters and polyamides with differences in their biodegradation rates. Generally, biodegradability of polyurethanes is dependent on whether the pre-polymer is polyester or polyether. 

When various molecular weights of aliphatic or aromatic diisocyanates of poly(caprolactonediol)s are treated with different microorganisms, the rate of degradation increases with an increase in the length of polyester segment. Polyurethanes derived from aliphatic diisocyanates are degraded faster than those derived from aromatic diisocyanates. 

The history of polyurethanes begins with Otto Bayer3 at Germany s I. G. Farben-industrie (tire predecessor company of Bayer AG4) in 1937, tire year of tire first disclosure of diisocyanate addition polymerization to form polyurethanes and polyureas. The main impetus for this work was tire success of Wallace Caro titers 

Synthetic Methods in Step-Growth Polymers. Edited by Martin E. Rogers and Timothy E. Long 2003 John Wiley Sons, Inc. ISBN 0-471-38769-X 

TABLE 4.1 Polyurethanes Applications, Properties, and Processing Methods 

Applications Physical States and Properties Processing Methods 

Thermosets Thermoplastics Amorphous or microcrystalline Hard or soft Transparent or opaque High or low Tg Aromatic or aliphatic Hydrophilic or hydrophobic 100% Solids/solvent bome/water borne Resilient or energy absorbing 

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Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

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