The Study Of Esterfying Reaction Between Epoxy Resins And Carboxyl Acrylic Polymers

During the last two decades, legislation in many countries on the reduction of the volatile organic content of paints and coatings has resulted in the development of many new water-reducible coatings. Among these is the water-reducible epoxy-acrylic composition. It is suitable for a metal coating, especially for an interior can coating. Several commercially available methods have been developed to combine epoxy resin with carboxyl acrylic polymer together. For example, epoxy-acrylate composition is formed from the epoxy resin by the grafting of addition polymer onto aliphatic backbone carbons of the epoxy resin, such grafting being at carbons that have either one or two hydrogens bonded thereto in the ungrafted state. In another preferred method, self-emulsifiable mixtures are prepared by esterifying epoxy resin with carboxyl acrylic polymer in an organic solvent medium in the presence of tertiary amine. Then the adduct react with a base and render it self-dispersible into water in neutralized form. Compared with the free-radical grafting method, the high molecular weight and complexity of the copolymers formed herein lowers the proportion of curing agent needed, and this provides tougher and more impact resistant cured coatings. Indeed, considerable chemical and water resistance is obtained in the absence of curing agent, so it can be omitted. However, from the standpoint of gelation theory, this esterifying practice normally leads to gel because there is more than one equivalent of oxirane functionality in each difunctional epoxide molecule per molecule of carboxyl-functional polymer. Fortunately, the presence of enough amine prevents gelation to occur till the oxirane functionality is entirely consumed.Although this creative technique involving epoxy resin esterifying with carboxyl acrylic resin in the presence of tertiary amine has many advantages, the extensively industrial utility is limited for the following reasons:1. The selection and proportion of reactants is only based on the experience and, sometimes, improper recipes lead to the useless gelation of reaction system.2. The large proportion of amine functions to avoid gelatin in a fashion which is still not fully understood.The purpose of this article is to highlight the effects of factors on the gelling of esterification system, such as the average molecular weight of epoxy resin, the solid content of system, the type of amine, the mole ratio of the reactants. Firstly, the fixed gel/nongel point and critical gel point experiments are designed for formulating the regulation of esterification reaction between epoxy resin and carboxyl acrylic polymer in the presence of tertiary amine in organic solvent. Ternary diagrams are employed to illustrate experimental results. It is disclosed that some factors, such as mole ratio of the ternary reactants, molecular weight of epoxy resin, even the hydrophilic solvent content of system, play important roles in the gelling effect of esterification reaction. In order to explain these experimental results, a suggested reaction process is put forward and, subsequently, a critical gel equation is derived from Carothers gelation theory. The equation provides an explicit connection between the critical gel composition and the relevant parameters of the reaction system such as average carboxyl functionality of acrylic resin, average molecular weight of epoxy resin, the base intensity of tertiary amine, and the solid content of system. The regressive critical gel curve based on the equation is well consistent with the experiment data. It expresses that the suggested reaction process is reasonable. Then, based on the critical gel equation, the important factors that influence the esterification result are discussed qualitatively. All the achievements are beneficial to understanding the reaction process and avoiding useless gelation in preparing the water-reducible compositions. Finally, to meet the very demanding requirements of end use, a series of aqueous coatings baesd on epoxy-acrylic composition are prepared in different reactants proportion. The composition can be applied satisfactorily by any conventional method known to the coating industry. Spraying, rolling, dipping, flow coating and electrodeposition methods can be used for both clear and pigmented films. The film thickness of such coatings is usually less than about 0.08 mm. A number of testing procedures are employed to define the suitability of a given composition for commercial use, such as good storage stability of the composition, excellent adhesion, water resistance and salt solution resistance. The intresting result is that the favorite recipes are the points which are closed to the critical gel curve in ternary diagram.