Diisocyanates, urethanes derived from

Dozens of isocyanate functional compounds have been synthesized, but only a few find much use in urethane structural adhesives. The choices are largely dictated by a combination of performance, price, and safety considerations. Most of the materials used in adhesives are derived from the aromatic isocyanates, toluene diisocyanate (TDI) and 4,4 -diphenyl-methane diisocyanate (MDI). 

Most of the commercial polyurethanes are derived from the polymerization of polyesterdiols or polyetherdiols with diisocyanates giving poly(ester-urethanes) and poly(ether-urethanes). 

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 aliphatic dllsocyanates that are being used commercially in urethane coatings are 3-(isocyanatomethy1)-3,5,5-trimethyIcyclohexyl isocyanate (Veba-Chemie A.G.) (28). "dimeryl" diisocyanate derived from dimerized linoleic acid (Henkel Corp.) (29). and xylylene diisocyanate (XDI) (Takeda Chemical Co.) (30). It is interesting to note that no catalysts are required for the reaction of XDI with hydroxyl compounds and that its reactivity is similar to that of TDI. 

Suskind (1A9) described the formation of film-forming cationic urethane latices. The isocyanate-terminated prepolymer derived from either a polyester or polyether diol and tolylene diisocyanate was first chain-extended with an alkyIdiethanolamine to yield a relatively low molecular weight urethane capable of further chainextending reactions ... 

A generic urethane acrylate is shown in Figure 1. It is formed by the reaction of a diisocyanate with a polyol and a hydroxy alkyl acrylate. Two types of substructures are found in urethanes, hard and soft segments. The hard segment is derived from the dllsocyanate and urethane linkages. The soft segment Imparts... 

The first commercial thermoplastic elastomers (TPE) were the thermoplastic urethanes (TPU). Their general structure is A-B-A-B, where A represents a hard crystalline block derived by chain extension of a diisocyanate with a glycol. The soft block is represented by B and can be derived from either a polyester or a polyether. Figure 5.1 shows typical TPU structures, both polyester and polyether types. 

A commercial grade of high-impact (notched Izod > 900 J/m) POM resin (Delrin 100 ST, DuPont) is believed to be a blend of POM with >30 wt% of a thermoplastic poly(ester-urethane) elastomer derived from poly(l, 4-butane adipate) diol and methylene-bis-(4,4 -diphenyl diisocyanate) (MDl) (Hexman 1989). This blend is reported to have a cocontinuous or semi-interpenetrating network of the elastomer in a matrix of the polyacetal (Flexman et al. 1990). The toughening effect in such a blend of IPN-type morphology was interpreted to occur partly through a rubber band mechanism by which the fracture energy is absorbed. The bands of rubbery domains were believed to span the crack and participate in the deformation process. 

For materials based on MDI and polyester SS, the initial decomposition point is 227°C [244]. Much higher decomposition temperature, i.e. 319°C was recorded for polyester PUs achieved with TDI and polyesters derived from adipic acid, ethyl glycol and neopentyl glycol, and trimethylolpropane [245]. Decomposition of the urethane bonds in polyester PUs based on the diisocyanate MDI and chain extended with EG begins at 240°C, and it reaches its maximum rate at 337-356°C [246]. Decomposition of PUs obtained form TDI was observed to start as early as at 200 C and to run through three steps with polybutadiene derived PUs being more stable [247]. 

Polyurethanes consist of flexible polyester or polyether units (blocks) alternating with rigid urethane units (blocks) derived from a diisocyanate, commonly a mixture of 2,4- and 2,6-toluene diisocyanate ... 

There is only one commercial series of isocyanate-cured urethane rubbers, namely the Urepan series of Bayer. Urepan 600 is a polyester derived from diethylene glycol adipic acid and toluene diisocyanate. The ratio of polyester to diisocyanate is such that hydroxyl groups, not isocyanate groups, are situated at the ends of the chains. The chain length is kept... 

Polyurethanes consist of flexible polyester or polyether units (blocks) alternating with rigid urethane units (blocks). The rigid urethane blocks are derived from a diisocyanate, commonly a mixture of 2,4- and 2,6-toluene diisocyanate. The more flexible blocks are derived from low-molecular-weight (MW 1000-4000) polyesters or polyethers with —OH groups at each end of the polymer chain. Polyurethane fibers are fairly soft and elastic and have found use as Spandex and Lycra, the "stretch" fabrics used in bathing suits, leotards, and undergarments. 

In addition, poly(urethane)s and segmented polyfurethane urea)s derived from lysine diisocyanate, can be enzymatically degraded by various proteases. Thiol proteases, such as papain, bromelain, and ficin, show a high activity. In addition, protease K and chymotrypsin hydrolyze the poly(urethane)s (9). 

Foam dressings are typically composed of polyurethane, but can also be constructed from rubber, acrylic, polyethylene, neoprene, etc. The value of urethane foams is derived from their well-established biocompatibility. Further to this, by suitable choice of diisocyanate and polyol combination, they can be tailored to be hydrophilic or hydrophobic. The former is useful in exudate reduction, while the latter may have lower adherence to the wound surface. These foams can be impregnated with pharmaceuticals, such as antibiotics, or activated carbon to reduce odor. 

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