This invention relates to a process for the preparation of polyurethanes from diisocyanates containing uretdione rings.
It is known in the art (DE-PS No. 1,153,900) to react diisocyanates containing uretdione groups with relatively high molecular weight polyhydroxyl compounds and optionally difunctional chain lengthening agents to produce polyurethanes. In order to achieve sufficient polyaddition of the uretdione-containing diisocyanates (which are generally high melting and only sparingly soluble), it is usually necessary to employ reaction temperatures above 100.degree. C. Use of such temperatures may however result in the opening of the uretdione rings and the initially straight chained polyurethanes may undergo a much greater degree of cross-linking than desired.
Substantial lowering of the solidification temperature and shortening of the solidification time in the curing process by using catalysts of the kind conventionally used in polyurethane chemistry, e.g. organic metal salts or tertiary amines have been attempted. However, the density of cross-linking is found to be undesirably increased and the mechanical properties of the polyurethane are seriously impaired.
The same problems are encountered when solid uretdione diisocyanates with delayed reactivity such as those which have been described in DE-OS No. 3,230,757 are used for the preparation of storage stable mixtures of so-called one component systems for the production of polyurethanes. In these cases, again, the use of organic lead or tin compounds as the preferred catalysts cannot completely prevent opening of the uretdione rings and the attendant undesirable side reactions.