Diisocyanates allophanate reaction

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

This is an interesting comparison with regard to control of polymer crosslinking by biuret- or allophanate-forming side reactions. For example, 1,5-naphthalene diisocyanate showed the greatest tendency toward alloph-anate formation, and hexamethylene diisocyanate the least. 

A typical polyurethane adhesive may contain, in addition to the urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, and allophanate groups. Polyurethanes are formed by the addition reaction of diisocyanates or polyisocyanates with polyols (Figure 3.12) through a step-growth polymerization... 

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]. 

The initial step to prepare polyurethane polymers for solution wet or dry spinning includes reaction of 1000-3500 molecular weight macroglycol with a diisocyanate at molar ratios of between about 1 1.4 and 1 2.0. Reaction conditions must be carefully selected and controlled to minimize side reactions, eg, allophanate and biuret formation, which can result in trifunctional branched chains and ultimately to insoluble cross-linked poljuners. For the prepolymer reaction, poly(tetramethylene ether) glycol [25190-06-1] and bis(4-isocyanatophenyl) methane [101-68-8] are currently the most commonly used macroglycol and diisocyanate. Several types of polyester-based macroglycols are included in spandex producers product lines, but with the exception of Dorlastan, made by Bayer AG in Germany, the polyester-based products represent only a minor part of their spandex fiber production. 

Because of its relatively low boiling point HDI is currently only supplied in the form of derivatives (trimer, allophanate, biuret, etc) (18). Polyrmeric aliphatic isocyanates are obtained by copolymerization of 2-isocyanatoethyl methacrylate and styrene (19). Hydroxyalkyl acrylates and methacrylates are used as monomers to form hydroxyl group containing polymers, which are cross-hnked with diisocyanates or blocked diisocyanates (20). The elusive parent dusocyanate, 0=C=N—N=C=0, is only stable at —75°C, and therefore it is not suitable as a monomer or comonomer in polymerization reactions (21). 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

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. 

When an excess of diisocyanate is used, crosslinking occurs through formation of an allophanate, which is the reaction product of an already formed urethane group and an isocyanate... 

---