تهیه استرهای گلیکول از اسیدهای چرب روغن سویا و پتانسیل آن ها به عنوان عامل انعقاد در فرمولاسیون های رنگی Preparation of Glycol Esters of Soybean Oil Fatty Acids and Their Potential as Coalescent Aids in Paint Formulations

During the past decade, water-borne coatings have found broad acceptance in architectural as well as industrial applications and are expected to have continued good growth rates in the coating industry. The driving forces behind this trend are based upon both environmental and economic concerns to reduce volatile organic contents (VOC) of most coating materials. Traditionally, latex coatings, based upon small particles of a synthetic resin such as acrylic polymers, have required the use of a coalescent agent in substantial quantities (1). The coalescent aid in latex coatings is added to improve the filming properties of the coatings (2,3). The function of the coalescent aid is to soften the latex particles so they can flow together and form a continuous film with optimal film properties after the water has evaporated. Without the coalescent aid, the latex coatings may crack and not adhere to the substrate surface when dry at ambient temperatures (4–۹). Conventionally, alcohol esters and ether alcohols, such as ethylene glycol monobutyl ether and TEXANOL® (۲,۲,۴- trimethyl-1,3-pentanediol monoisobutyrate; Eastman, Kingsport, TN), are widely used as coalescent aids in the latex coatings industry. These coalescent aids are counted as VOC, which are considered to cause smog and air pollution. Vegetable oils, such as linseed oil and soybean oil, are used as drying oil in oil-modified alkyd resins (10) or as base fluids for environmentally safe lubricants (11). In Europe, vegetable oils have been used as diesel fuel, either neat or blended with diesel oil (12,13). Soybean oil, because of its abundance, comparatively lower prices, and environmental safety, has been considered in making a new additive for coatings. Addition of neat soybean oil, without surfactant, to polyacrylic latex will cause a surface problem such as cratering because soybean oil does not dissolve mutually with polyacrylic latex. Soybean oil glycol esters therefore are prepared by transesterification of soybean oil and glycol with an alkaline catalyst at high temperature. Attempts have been made to replace the glycerin part of the oil with glycol to enhance the solubility of the derivatives, making them more miscible with water than neat oil. Soybean oil, when transesterified with excess glycol, yields a mixture of fatty acid alcohol monoesters along with glycerol. The hydroxyl and ester functional groups in the reaction product enhance the hydrophilicity of fatty acid and increase miscibility in water. Moreover, the longchain fatty acid portion of the reaction product not only is a nonvolatile organic compound but also can be potentially oxidized, resulting in oligomerization or cure, and can help harden the coating after coalescence. This paper reports on the preparation of several glycol esters, and their specific physiochemical properties, that may be used as coalescent aids, such as ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol, in latex paint formulations. The new coalescent aids were evaluated on their ability to lower the minimum film formation temperature (MFFT) of latex polymer so that it would allow a uniform polymer film to be formed at typical ambient temperatures for painting. RESULTS AND DISCUSSION To get complete transesterification of soybean oil without having side products, i.e., mono- and diglycerides, the reaction time was monitored by TLC. TLC results gave an Rf value of the starting material of 0.97, while the Rf values of all derivative products were 0.3 at approximately 10 h of reaction time. A 10-h reaction time, therefore, was chosen to ensure that all triglycerides were converted to the desired products. In the purification process, the interfacial turbid layer was discarded to expedite a rapid production. The yield of the product after purification therefore was approximately 85–۹۰% based on soybean oil. The typical yield of transesterification, however, is nearly quantitative based upon mass balance. It should be noted that a small amount of diester was seen in ethylene and diethylene glycol systems. The diester was typically less than 6%. GC was used to analyze the composition of the reaction products. The retention times and compositions of reaction products are shown in Table 2. The GC results showed two main broad peaks in each soybean oil derivative. The first peak of each derivative, approximately 10–۱۴%, was the saturated fatty ester, and the second peak, approximately 86–۹۰%, was the unsaturated fatty ester derivative. These results corresponded to the amounts of saturated fatty acid (mainly palmitic acid) and unsaturated fatty acid (mainly linoleic acid and oleic acid) in soybean oil of approximately 14 and 86%, respectively.