Cross-links allophanate

A variety of polymeric subunits is used to make polyurethanes. These include polyesters and polyethers. The major interchain linkages are molecular forces such as hydrogen bonding and the London force. Depending on the type of chain extender and processing temperature, there also may be biuret or allophanate cross-links. 

Subsidiary chemical reactions can take place. The major of these is the formation of an allophanate cross-link, as illustrated in Figure 2.6. This reaction normally needs a temperature of between 120 and 140°C to take place. The presence of a urea group at 100°C can react with the isocyanate group to form a biuret linkage. This is shown in Figure 2.7. 

NN. - dimethyl diaminodiphenyl methane. In the study presented here, the materials were synthesized with the diisocyanate DBDI, and the polyesters PEA or PBA. To avoid crystallization, DEG has been used as a chain extender. The molar ratio of the components was diisocyanate macrodiol chain extender 4 1 3. The stoichiometric proportions used in these polymers (I = 100) means that were truly thermoplastic. They did not have the potential for further reaction with ambient humidity to produce chain lengthening and allophanate cross-linking, seen in similar polymers but with excess isocyanate groups (e.g. I = 110) [17, 60, 61]. 

Flexible polyurethane foams have good dry heat stability, withstanding temperatures up to 150°C for long periods without serious loss of strength. Above this temperature the formation of biuret and allophanate cross-links is reversed and there is a reduction in load bearing properties. Urethane links are stable up to about 200°C. Exposure to ultraviolet light causes discoloration but no apparent deterioration in physical properties. 

Uncatalyzed, these reactions are quite slow below 80-100 C and may be reversed in the range 150 -200 C. Urea groups are normally much more reactive towards isocyanates than are urethane groups so that biuret cross-linking will be more favoured than allophanate cross-linking. 

Moulded cushions tend to have slightly better fire retardancy and better resilience properties than slab-stock products. Hot-cure and, more recently, cold-cure moulding techniques have allowed better control of the side reactions (such as formation of isocyanurate and allophanate cross-links). One-shot processing with low-pressure dual-component dispensing equip-... 

Linear thermoplastic polyurethanes may be obtained by using componnds with two reactive groups such as diisocyanate and diols. Crosslinkable systems can be formed in one of two ways (i) using an excess of isocyanate in the reactions where the excess isocyanate can react with nrethane groups in the polymer to form allophanate cross links, or (ii) nsing multifnnctional polyols or isocyanates. Crosslinking is particularly important for structural adhesives to stop them creeping under load. 

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