Urethane polymers isocyanates

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

Almost all TDA use is as a chemical intermediate, mosdy in polyurethanes. Toluenediamine derivatives are found as all three components of urethanes isocyanates, chain extenders, and polyols (see Isocyanates, organic Urethane polymers). 

Isocyanates. The commodity isocyanates TDI and PMDI ate most widely used in the manufacture of urethane polymers (see also Isocyanates, organic). The former is an 80 20 mixture of 2,4- and 2,6-isomers, respectively the latter a polymeric isocyanate obtained by phosgenation of aniline—formaldehyde-derived polyamines. A coproduct in the manufacture of PMDI is 4,4 -methylenebis(phenyHsocyanate) (MDI). A 65 35 mixture of 2,4- and 2,6-TDI, pure 2,4-TDI and MDI enriched in the 2,4 -isomer are also available. The manufacture of TDI involves the dinitration of toluene, catalytic hydrogenation to the diamines, and phosgenation. Separation of the undesired 2,3-isomer is necessary because its presence interferes with polymerization (13). 

Low viscosity urethane polymers have been prepared from castor od and polymeric isocyanates (82). These low mix viscosity systems are extremely usehd for potting electrical components where fast penetration without air voids, and fast dispensing cycles are desirable. Very low viscosity urethane systems containing castor polyols have been prepared for use in reclaiming water-logged buried telephone cable and for encapsulating telephone cable sphces (83—86). 

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

Alkyl carbamates (urethanes) are formed from reaction of alcohols with isocyanic acid or urea (see URETHANE POLYMERS). 

Urethane adhesives take their name from the product of the most common step-growth polymerization reaction used to generate the adhesive polymers. Isocyanates react with hydroxyl groups to create urethane (or carbamate) linkages ... 

In the majority of cases the addition product is stable, but in some special cases it is only moderately stable and may either dissociate to form the initial reactants again or decompose to other products. Secondary reactions of isocyanates that are important in the formation of urethane polymers are those with urea and urethanes. These reactions result in the formation of biuret and allophanate, respectively (Figure 2.19). The relative reaction rates of active-hydrogen-bearing compounds with isocyanate are given in Table 2.7. 

Flexible Foams CO2 obtained in situ by the reaction of water with isocyanate has been the chief blowing agent for all commercially produced flexible urethane foams. The amount of water and tolylene diisocyanate (TDI) used determines foam density, providing most of the gas formed is used to expand the urethane polymer. Because water participates in the polymerization reactions leading to the expanded cellular urethane polymer, it has a very pronounced influence on the properties of foams. For better control of the foaming process most foam manufacturers employ distilled or deionized water (16). 

Although employed, at one time, primarily as a war gas, phosgene is now an important chemical intermediate for the synthesis of a large number of commercial materials. Worldwide, it is used mainly in the manufacture of isocyanates (for urethane polymers and organic intermediates), polycarbonates (for speciality polymers), and monomeric carbonates and chloroformates (largely for the synthesis of pharmaceuticals and pest control chemicals). The established large-scale, commercial applications of phosgene are summarized in Fig. 4.7. 

There are a number of syntheses leading to the formation of urethane polymers. However, the most important commercial route is the isocyanate addition polymerization, the reaction between di- and polyfunctional hydroxyl compounds such as hydroxy-terminated polyethers or polyesters and di- or polyisocyanates. When difunctional reactants are being used, linear polyurethanes are produced and the reaction can be schematically represented as follows ... 

Both MDI and TDI, being aromatic diisocyanates, yield urethane polymers that tend to yellow on prolonged exposure to sunlight, presumably due to oxidation of some terminal aromatic amine (derived from these isocyanates). MDI also possesses a methylene group that is susceptible to oxidation via a proton abstraction mechanism involving autoxidation of the aromatic urethane groups to a quinoneimide structure as proposed by Schollenberger et al. (24. 25). 

Other types of curing systems for allyl-terminated urethane polymers were described by Kehr and Wszolek (176). Polyether and polyester urethane-ene systems were prepared by reaction of polyester or polyether dlols with allyl isocyanate or by reaction of isocyanate-terminated prepolymers with allyl alcohol. These urethane-ene systems can be cured by branched monomeric polythiols such as pentaerythritol tetrakis(thioglycolate) C(CH20C0CH2SH)4. 

Urethane polymer resins that still contain unreacted isocyanate groups react with atmospheric moisture and continue curing. The reaction is a two-step process. Eirst, water reacts with some of the free isocyanate groups producing an amine and... 

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