The “traffic leader” in the field of photocurable resins

In the last issue, we introduced that the most widely used resin in the photocuring industry is epoxy acrylate, and the most used and largest used epoxy acrylate is bisphenol A epoxy acrylate.

Bisphenol A epoxy acrylate is prepared by the reaction of bisphenol A epoxy resin and acrylic acid. Bisphenol A epoxy resin is produced by the reaction of bisphenol A and epichlorohydrin under alkaline conditions. By controlling the ratio of the two raw materials and the reaction conditions, bisphenol A epoxy resins with different polymerization degrees can be obtained. The higher the polymerization degree, the larger the molecular weight and the smaller the epoxy value.

As shown in the figure, the variable n in the general formula represents the degree of polymerization, and the value of n is generally between 0-25; the average value of n of low relative molecular mass epoxy resin is generally less than 2, and its softening point is lower than 50°C; the value of n for medium relative molecular mass epoxy resin is between 2-5, and the softening point is between 50-95°C; Values of n greater than 5 is called high relative molecular mass resin, and the softening point is above 100°C. The value of n of commercial bisphenol A epoxy resin can be between 0-12, but in order to obtain bisphenol A epoxy acrylate with a high curing speed, bisphenol A epoxy resin with a high epoxy content and low viscosity is usually selected, so that more acrylate groups can be introduced. Therefore, in most cases, low relative molecular mass bisphenol A epoxy resin models with n≈0 are used. For example, common domestic models are E-51 (epoxy value is 0.51±0.03eq/100g) or E-44 (epoxy value is 0.44±0.03eq/100g).

The process of ring-opening esterification of epoxy groups in bisphenol A epoxy resin and acrylic acid under the action of catalyst is an exothermic reaction. Generally, the epoxy resin is heated to about 80°C, and a mixture of acrylic acid, catalyst and inhibitor is added dropwise. After most of the reaction is completed, the temperature is gradually raised to 100-120°C to complete the reaction. Samples are taken to test the acid value. After the acid value drops below the target value, the temperature is lowered, and the material is discharged. Since bisphenol A epoxy acrylate has a high viscosity at low temperatures, 20% UV monomer (TPGDA, TMPTA) is usually added to dilute and reduce the viscosity to facilitate later feeding.

In the early stage of the reaction of bisphenol A epoxy acrylate, in order to promote the reaction process, a catalyst is generally added. Commonly used catalysts include tertiary amines and quaternary ammonium salts; such as triethylamine, N,N-dimethylbenzylamine, trimethylbenzyl ammonium chloride, triphenyl phosphine, triphenyl antimony, chromium acetylacetonate, tetraethylammonium bromide, etc. Although triethylamine is cheap and easy to obtain, its catalytic activity is relatively low and the product stability is slightly poor; although quaternary ammonium salts have slightly stronger catalytic activity , their cost is slightly higher; triphenyl phosphine, triphenyl antimony, and chromium acetylacetonate have high catalytic activity, and the product has low viscosity, but the color is darker.

Since the reaction temperature of bisphenol A epoxy acrylate is relatively high, a certain amount of polymerization inhibitor must be added to prevent polymerization of acrylic acid (ester) during the heating process. Commonly used polymerization inhibitors include hydroquinone, methyl hydroquinone, p-methoxyphenol , etc.

The above is the synthesis reaction process of bisphenol A epoxy acrylate.