1. Field of the Invention
This invention relates to a new process for the preparation of novel air drying, thixotropic binders containing free isocyanate groups by the reaction of selected prepolymers or semi-prepolymers containing free isocyanate groups with selected aromatic diamines, the thixotropic binders obtainable by this process and their use for the preparation of coating and sealing compounds which are air drying i.e. capable of hardening under the influence of atmospheric moisture.
2. Description of the Prior Art
Polyurethane based binders containing free isocyanate groups and capable of hardening under the influence of atmospheric moisture are easily processed and therefore widely used, in particular for the preparation of coating and sealing compounds.
Thixotropic flow properties of such coating and sealing compounds would be desirable for many purposes, for example for the formation of thick coatings.
It was therefore an object of the present invention to provide air drying binders with thixotropic properties containing free isocyanate groups.
It was surprisingly found that this problem could be solved by means of the process according to the invention described below. The invention is based on the surprising observation that reaction products containing urea groups obtained from (i) isocyanate prepolymers or semi-prepolymers based on polyisocyanate mixtures of the diphenylmethane series containing a high proportion of 2,4'-diisocyanatodiphenylmethane and (ii) certain alkyl substituted aromatic diamines are suitable for rendering the above-mentioned binders thixotropic. Analogous reaction products based on conventional diisocyanates such as toluene diisocyanate are not suitable for this purpose. This finding is surprising since it is known, for example from DE-AS No. 1,805,693 or DE-AS No. 2,360,019, that reaction products of toluene diisocyanate with mono- and/or polyamines are very suitable for imparting thixotropic properties to other binders, for example those based on alkyd resins.