Thermoplastic Linear Polyurethane Elastomers

The effect on bulk mechanical properties of these physical forces and states, especially paracrystallinity and hydrogen bonding, is to pseudo- 

Usually an NCO/OH ratio of almost 1 0 is used in the synthesis of TPUs. However post-curable TPUs may have an NCO/OH ratio of 0-98 to 0 99 to provide NCO groups for crosslinking. 

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THERMOPLASTIC (LINEAR) POLYURETHANE ELASTOMERS 247 Calculation of equivalent weights Polycaprolactone... 

If the diol chain extender is used in exact molar proportion to the unreacted isocyanate, then a linear polyurethane elastomer is obtained. The resulting thermoplastic elastomer may be extruded or injection-moulded, and the properties arise from the ability of the hard and soft segments to form semi-crystalline domains that act as virtual crosslinks in the polymer and give it elastomeric properties, as shown in Figure 1.17. 

Soft blocks are composed of linear, dihydroxy poly ethers or polyesters with molecular weights between 600 and 3000. In a typical polymerization of a thermoplastic polyurethane elastomer, the macroglycol is end capped with the full amount of aromatic diisocyanate required in the final composition. Subsequently, the end-capped prepolymer and excess diisocyanate mixture reacts further with the required stoichiometric amount of monomeric diol to complete the reaction. The diol links the prepolymer segments together while excess diol and diisocyanate form short hard-block sements, leading to the (AB)n structure illustrated in Figure 1. Block lengths in (AB)n polymers are frequently much shorter than those in anionically synthesized ABA block copolymers. 

Published information on urethane polymerization detail largely concerns thermoset urethane elastomers systems.4 13 In particular, the work of Macosko et. al. is called to attention. The present paper supplements this literature with information on the full course of linear thermoplastic urethane elastomer formation conducted under random melt polymerization conditions in a slightly modified Brabender PlastiCorder reactor. Viscosity and temperature variations with time were continuously recorded and the effects of several relevant polymerization variables - temperature, composition, catalyst, stabilizer, macroglycol acid number, shortstop - are reported. The paper will also be seen to provide additional insight into the nature and behavior of thermoplastic polyurethane elastomers. 

AA-HD (linear) Adhesives, coatings, thermoplastic polyurethanes, elastomers... 

In the absence of a blowing agent and with conditions favoring the formation of linear chains, thermoplastic polyurethane elastomers can be formed. Materials with different properties can be obtained by altering the ratio of hard to soft blocks, and it is also observed that, in many cases, the hard segments can crystallize. This can give rise to phase-separated structures, shown schematieally in Figure 2.6, which behave as thermoplastic elastomers (see also Section 15.6). 

Disodium hexamethylene bisthiosulfate Polystyrene Polyurethane elastomer, thermoplastic inj. molded goods, bushes Polyethylene, linear low density Polyethylene, low-density... 

Desmocoll 176, 400 and 420 are primarily linear thermoplastic polyurethane elastomer resins of medium, medium-high and high crystallization rates, respectively. They are supplied as small, light brown, talc-covered, pieces. The polymers have a density of 1 2-1-23 g cm . This density, and their stabilization by the hydrolysis stabilizer Staboxal PCD, imply that they are poly(ester-urethanes). They show outstanding adhesion to numerous materials including plastics, rubber, leather, fabrics, wood, metals, etc. 

Reaction polymerization reactions of isocyanates with suitable monomers can he performed in an extruder or in a RIM machine. In the latter reaction thermosets (cross-hnked polymers) are produced. In an extruder usually linear polymers are manufactured. For example from methylene di-p-phenylene isocyanate (MDI), with some macroglycols and 1,4-hutanediol as extenders, segmented polyurethane elastomers are produced in an extruder (6). However, linear condensation polymers are also produced in a vented extruder. For example from MDI, with macrodicarboxylic acids and dicarboxyhc acids as extenders thermoplastic block copolyamide elastomers are produced. The by-product of the condensation reaction, carbon dioxide, is removed in the vented extruder. The polycondensation process can also be performed in solution. For example, MDI can be added to a solution of dicarboxyhc acids in tetramethylene sulfone, with simultaneous removal of the carbon dioxide. Tetramethylene sulfone is the solvent of choice for solution polymerization of isocyanates (7). In addition to dicarboxyhc acids trimellitic acid anhydride and benzophenonetetracarboxylic acid dianhydride (BTDA) are utilized as monomers for condensation polymers. With these monomers poly(amide imides) and poly(imides) are produced. The diisocyanate-derived commercial polycondensation products are listed in Table 1. 

In papers reviewed in this section, the thermoplastic polyurethane elastomer was used [27-52] as a linear component of semi-IPNs in which, as already mentioned in Section 1, the second component should be a chemically crosslinked polymer. As a second component the polyurethane [27], polyepoxyisocyanurate [28-33] and polycyanurate [34-52] networks were used. Once again, it is important to note that if the thermoplastic PU forms a continuous phase, the final semi-IPNs exhibit the properties and behavior of thermoplastic elastomers. 

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