This invention relates to curable compositions comprising an epoxide resin and an acid containing four carboxyl groups, particularly powder coating compositions, and to the use of such compositions.
Techniques for coating articles with a plastics material in powder form are well known. A typical method is fluidised bed powder coating, in which a preheated article is dipped briefly into a fusible plastics powder which is kept in a fluidised state by an ascending current of gas. On contact with the hot article, the powder melts and clings to the surface and, if it is thermosettable, may be so cured. Another widely used method is electrostatic powder coating. In this method the object to be coated is earthed, and the coating particles are given an electrostatic charge and are projected onto the earthed object. Once the object has been coated, it is heated to fuse the coating and, when a thermosettable resin is used, to cure the resin.
Formulation of fusible, thermosettable powder coating compositions requires a material of sufficiently high softening point that it may be milled to a free-flowing powder. At the same time, it is highly desirable that the composition melts initially and then cures at a relatively low temperature, in order that the article to be coated does not have to be heated to a very high temperature. The choice of curing agent is therefore somewhat restricted, if compliance with these criteria is to be achieved.
A particularly successful curing agent for epoxide resins is trimellitic anhydride (benzene-1,2,4-tricarboxylic acid 1,2-anhydride), fulfilling the requirements given above. It is, however, unpleasant to handle in the finely powdered state, being irritant to the skin and mucous membranes. It is also hygroscopic, and special precautions must be taken to exclude water during the milling of this anhydride if its partial conversion into trimellitic acid is to be avoided.
Attempts have been made to overcome these difficulties by reaction of the acid or the anhydride group of trimellitic anhydride with certain diols or polyols. Such attempts have been only partially successful. In British patent specifications Nos. 963,557, 1,019,568, and 1,318,926, for example, such materials are described but they suffer from some of the drawbacks of trimellitic anhydride itself. Some are sensitive to moisture whilst others form only sticky solids which cannot readily be converted into free-flowing powders. Others have a high melting point, which prevents them from forming smooth coatings unless heated to a very high temperature.
There is therefore a need for curing agents, suitable for use in epoxide resin powder coating compositions, which have the desired softening properties, are neither hygroscopic nor irritant when powdered, and which cure the resin to give a hard, glossy finish.