Elastomers virtually crosslinked

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. 

Of the available polyurethane types, the virtually crosslinked thermoplastic polyurethane elastomers would seem to be best and ideally suited for supply and use in powdered form. Their uses include adhesive applications which exploit the advantages of the powder form. 

The concept of physical or virtual crosslinks was first introduced by Scollen-berger et al. [2,3] when they demonstrated that hnear segmented ester-based thermoplastic polyurethanes exhibited high extensibility and elasticity in a non-vulcanized form. These materials behaved as thermoplastic elastomers (TPE), as opposed to chemically crosslinked elastomers, such as vulcanized natural rubber, which cannot be thermally reprocessed. 

Scollenberger C S, Scott H and Moore G R (1962) Polyurethane VG, virtually crosslinked elastomer. Rubber Chem Tech 35 742-752. 

Thus, in alkyd, polyester, and epoxy coatings applied to CRS, phosphatized steel, and aluminum, the use of ZAs APG (aminofunctional) and CPG (carboxy-functional) has allowed for the virtual elimination of blister formation and corrosion after 300 h of salt fog exposure. The use of multifunctional ZAs in a Kraton base adhesive has allowed for a 52% increase in T-peel strength on EPDM rubber when compared with the same adhesive containing aminofunctional silane. Incorporation of mercaptofunctional ZA into crosslinkable elastomers has... 

Liquid polymer While virtually all elastomers can be prepared in sufficiendy low molecular weights to exhibit fluid behavior, only those liquid polymers that can be converted to an elastomeric solid will be reviewed. It might appear that this could be accomplished by crosslinking, but this is not the case. While some crosslinking is desirable, the primary reaction required is one of chain extension that is the end-to-end attachments of the low molecular weight chains via reactive terminal groups. 

Curatives include all those chemicals and additives that are added to a rubber compound during mixing to enable the compound to cure (vulcanize) when elevated temperatures are applied. Chemically, curing (vulcanization) Involves the generation of crosslinks between the giant elastomer molecules. Without curatives present, a rubber compound normally cannot cure. The useful cured properties of virtually any rubber are obtained only after vulcanization has taken place. 

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