1. Field of the Invention
The present invention relates to amino-functional polyurethane prepolymers, a process for their preparation, and to their use for modifying and preparing epoxy resins and polyurethanes.
2. Description of the Prior Art
Coatings formed from epoxy resins are distinguished by high hardness, good chemical resistance and outstanding metal adhesion. Solvent-free epoxy resin formulations are generally prepared using low molecular weight epoxides in combination with monomeric diamines, since components of higher molecular weight would result in a viscosity that is much too high for processing. Because of the high crosslinking density, however, the coatings produced from these epoxy resin formulations are mostly too brittle and unsuitable for practical use.
The brittleness of the aforementioned epoxy resins can be reduced by adding suitable plasticizers, but the result of their addition is a ternary system, associated with a proportionately higher complexity in formulation. Also, the monomeric diamines customarily employed have a volatility which makes them substances difficult to handle, requiring particular protective measures.
Proposals have therefore been made to flexibilize these epoxy resins by using amino-functional polyurethane prepolymers, which can be obtained, for example, by hydrolyzing the corresponding ketimines (U.S. Pat. No. 3,993,708, DE-A 2 546 536). Other processes for preparing amino-functional polyurethane prepolymers include the hydrolysis of isocyanate-functional polyurethane prepolymers in the presence of strong bases (DE-A 2 948 419) or in mixtures of water with specific solvents such as dimethylformamide (DMF) (EP-A 0 219 035); the reaction of NCO-functional prepolymers with tertiary alcohols and subsequent thermal breakdown (DE-A1 270 046); and the hydrogenolysis of O-benzyloxyurethanes (DE-A 3 035 639).
The aforementioned processes have the disadvantage, however, that coupling products, such as ketones in the ketimine process, must first be removed, which is costly and inconvenient, or the actual product must be extracted from an aqueous phase (hydrolysis process). The thermolysis of tertiary alkylurethanes is not a very gentle method and leads to strongly coloured products. The blocking agent in the hydrogenolysis of O-benzyloxyurethanes is decomposed to form toluene and carbon dioxide and thus cannot be used again in the reaction.
JP-A 61215397 describes reaction products of organic isocyanates (e.g. tolylene diisocyanate) and arylsilanols such as triphenylsilanol that can be used in the customary fashion as (reversibly) blocked isocyanates.
JP-A 62074919 describes aliphatic isocyanates (e.g. hexane 1,6-diisocyanate), also blocked with triphenylsilanol, which can be used, in combination with defined catalysts, as hardeners for epoxides. It does not report the in situ formation or preparation of amines.
The possibility of the hydrolytic breakdown of the silylurethanes described (i.e., the reaction of the silylurethanes with water) involving elimination of silanol and CO2, was not recognized in the two stated applications.
It has now been found that amino-functional polyurethane prepolymers are obtained smoothly and under mild conditions from the corresponding prepolymers containing silylurethane groups by means of hydrolytic breakdown. The resulting products are notable for minimal color and low viscosity.