Polyurethanes, reversible dissociation

The interest in blending of thermoplastics with thermoplastic polyurethanes (TPUs) is equivocal some properties of TPUs, such as erosion and abrasion resistance are excellent [85,86], which already justify the blending efforts, but the urethane linkage (-NH-C0-0-) in TPUs is unstable at high temperatures. Isocyanate and hydroxyl groups form via reversible dissociation of the urethane bond, and they are very reactive with the usual functional groups (amine, epoxy. 

The analysis of pyrolysate shown in Table 14.1.2 indicates only derivatives for the polyether sequences from the polymer. No traces of methylaniline were seen in the pyrogram, and the elution of an amino derivative of benzylbenzene is not likely to happen in the conditions used for this separation. Therefore, the use of pyrolysis results for the identification of this type of polyurethane is not conclusive. At lower temperatures three main paths for decomposition were indicated. One first process is the dissociation of the urethane linkage, taking place as the reverse of urethane formation, as shown below ... 

Similarly, an aqueous treatment for wool uses an isocyanate blocked with a bisulfite. In this case, the by-product bisulfite salt can be removed, and possibly recycled, by washing the fabric with water at the end. The dissociation temperature decreases in the following order alcohol > lactam > phenol > ketoxime > active methylene compound. e-Caprolactam is a typical lactam, ethyl acetoacetate a typical active methylene compound. This thermal reversibility has been used to prepare a polyurethane from 1,4-butanediol, 4,4 -diphenyldiiso-cyanate, and 4,4 -isopropylidenediphenol (bisphenol A) (2.30) that can be recycled just by remolding.76... 

Since 1-propanol is monofunctional and reacts with "matched dissociation isocyanate" (at internal urethane chain positions) as well as "unmatched isocyanate" (at polymer chain terminal positions) its effects include cleavage of some polyurethane chains in the process of generating more urethane groups. As a result, polymer DP, melt viscosity, and torque drop until the shortstop is consumed, or escapes the mixture by volatilizaton. Figure 9 shovjs that the more 1-propanol used to shortstop the polymerizations, the more pronounced the polymer reversion was. 

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