1. Field of the Invention
The present invention relates to a process for preparing a polyurethane emulsion.
2. Description of the Prior Art
Various polyurethane emulsions have been proposed and have been used in various applications such as coating agents, binders, impregnating agents and the like because of their excellent characteristics. Recently, polyurethane emulsions have been used in civil engineering fields as coagulants, soil stabilizers and the like. However, in the preparation of polyurethane emulsions, polyurethane compounds are usually hydrophobic and thus it is impossible to form stable emulsions by uniformly dispersing the polyurethane compounds in water. Accordingly, it has been proposed (I) to use a surfactant in the emulsion system or (II) to introduce a hydrophilic group into the polyurethane molecule in order to easily disperse the polyurethane molecules in water. When method (I) has been used, only unstable emulsions have been obtained.
The following methods are prior art techniques which use the procedure of method (II): U.S. Pat. No. 3,479,310 and Japanese Patent Publication No. 9076/1968 show a method of forming stable polyurethane emulsions by introducing a hydrophilic group into a polyurethane molecule. Japanese Patent Publication No. 27904/1969 shows a method of introducing a salt forming group in a polyurethane molecule by forming a microgel of polyurethane. Japanese Patent Publication No. 27349/1965 shows a method of dispersing a polyurethane having tertiary amino nitrogen atoms in an acidic solution. U.S. Pat. No. 3,412,054 shows a method of introducing a carboxyl group into a polyurethane molecule by using a compound having the formula ##STR1## which is prepared by oxidizing an aldehyde. In all of these methods, there are various difficulties which arise during the preparations. For example, oxymonocarboxylic acids such as lactic acid, glycolic acid, .alpha.-oxypropionic acid, and .beta.-oxypropionic acid are thermally unstable so that they easily dehydrate in the reaction between a hydroxyl group and an isocyanato group. Oxypolycarboxylic acids such as oxymalonic acid, malic acid, tartaric acid and citric acid each have a melting point higher than 130.degree.C and are easily decomposed upon heating to generate carbon dioxide gas. The mutual miscibility of an oxypolycarboxylic acid and a polyisocyanate is usually not very good. It is hard to introduce a carboxyl group into the polyisocyanate component by reacting it stoichiometrically with the polyisocyanate. A carboxylic acid such as salicylic acid or resorcinol carboxylic acid has a phenolic hydroxyl group. In order to introduce the carboxylic group into the polyisocyanate, it is necessary to react the isocyanato group with a phenolic hydroxyl group at a temperature higher than 100.degree.C. At these temperatures, the reaction of a carboxyl group with the isocyanato group also abnormally increases the viscosity thereof or solidifies the product. Moreover, in order to introduce a carboxyl group into the polyisocyanate with a polycarboxylic acid such as isophthalic acid or maleic acid, the carboxyl group reacts with the isocyanato group to eliminate carbon dioxide gas at a temperature higher than 130.degree.C, which greatly increases the viscosity and in many cases, causes solidification. It is clear from what is known about the reaction that the introduction of a carboxyl group into the polyisocyanate component is very difficult when applied to the preparation of polyurethane emulsions.
The carboxylic acid modified isocyanates are usually high viscosity products. If a large amount of an organic solvent is used to dilute the product, it may be possible to form an aqueous dispersed state. However, serious toxicity and fire problems arise. Many different molecules having nitrogen atoms in the polyisocyanate component have been proposed to form emulsions of cationic type polyurethanes. These products have been produced by reaction of an isocyanate and a nitrogen atom containing compound. The products still present various industrial difficulties in that the reaction is very fast, control of the reaction is difficult and side-reactions easily occur. Also, the increase of viscosity is substantial, large amounts of solvent have to be added, and storage stability is poor.
A need therefore continues to exist for an improved method for preparing polyurethane emulsions.