Molecular weight polyether diols

The high reactivity of the isocyanate moiety has been exploited to yield polyurethane/urea hydrogels. A polyisocyanate prepolymer obtained by fiinctiontdizing a moderate molecular weight polyether diol or triol with a diisocyanate will crosslink on exposure to an aqueous environment due to partial hydrolysis and urea formation (55). A similar hydrogel results by reacting a polyisocyanate... 

In addition, polyester polyols are made by the reaction of caprolactone with diols. Poly(caprolactone diols) are used in the manufacture of thermoplastic polyurethane elastomers with improved hydrolytic stabiHty (22). The hydrolytic stabiHty of the poly(caprolactone diol)-derived TPUs is comparable to TPUs based on the more expensive long-chain diol adipates (23). Polyether/polyester polyol hybrids are synthesized from low molecular weight polyester diols, which are extended with propylene oxide. 

Just as diisocyanates can be substituted for acids in the alkyd structure to produce a urethane alkyd or oil, so a simpler structure similar to a polyester can be made. Diols such as those described in the next section as being suitable for use in polyesters, and lower molecular weight polyethers, are reacted with diisocyanates to form linear high molecular weight polyurethanes. These are used as clear lacquers and applied as very low solids solutions on flexible substrates such as plastics and wood. 

Polyester and polyether diols are used with MDI in the manufacture of thermoplastic polyurethane elastomers (TPU). The polyester diols are obtained from adipic acid and diols, such as ethylene glycol, 1,4-butanediol, or 1,6-hexanediol. The preferred molecular weights are 1,000 to 2,000, and low acid numbers are essential to ensure optimal hydrolytic stabihty. Also, caprolactone-derived diols and polycarbonate diols are used. Polyether diols are... 

Poly(tetramethylene oxide) polyols (PTMEG) are high performance polyethers that are crystalline waxes at molecular weights above 650 and liquids at lower molecular weights. They are only available as diols, but they produce adhesives with good hydrolysis resistance and moisture resistance, which is why these adhesives are even used in medical devices, blood bags, catheters, and heart-assist devices [25]. Certain thermoplastic polyurethane adhesives and solvent-borne adhesives are also based on PTMEG s. 

TPU is usually made from hydroxyl-terminated polyether or polyester diols, diisocyanates, and bifunctional chain extenders. Since the composition, the synthetic method, molecular weight, and its distribution are all changeable, there are numerous types of TPUs available, and their prices and properties vary significantly. 

The hot curing process normally uses polyether diol precursors with molecular weights of 3,000 to 5,000 g/mole. We can control the stiffness of the foam by adjusting the average number of isocyanate groups on the chain extender molecules. The higher the functionality of the isocyanate molecules, the more crosslinked, and hence stiffer, will be the product. 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

T inear polyurethane block copolymers have been objects of considerable -L/ research interest over recent years. These materials are composed of three parts, a diisocyanate, a low molecular weight chain extender (usually a diol but sometimes a diamine), and a low molecular weight rubbery polymer, frequently a polyether or polyester. The soft segment consists of the rubbery polymer while the diisocyanate and the chain extender form the hard segment These materials are classified as thermo-... 

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