Determination of hydroxyl value of vegetable oils-based polyols through ring-preopening, solvent-extraction, and acetylation method

Vegetable oils-based polyols prepared by epoxidation-hydroxylation often have the phenomenon of the coexistence of epoxy and hydroxyl groups due to different ring-opening degrees. The acylation method recommended by the American Society of Testing Materials (ASTM) standard (D1957-86) and the Association of Official Analytical Chemists (AOAC) standard (965.32) is applicable only to fatty alcohols in oils and fats and their derivatives but not to vegetable oils-based polyols since the residual epoxy groups in vegetable oils-based polyols lead the tested hydroxyl value (OHV) to be higher than the actual OHV. A method combining ring-preopening of residual epoxy groups, ether extraction of ring-preopened vegetable oils-based polyols, and subsequent acetylation by acetic anhydride-acetic acid was established for OHV determination of vegetable oils-based polyols. The accuracy of the method was verified by the known theoretical OHVs of short-chain alcohols and simulated vegetable oils-based polyols. The absolute values of relative deviations for OHVs of short-chain alcohols and simulated vegetable oils-based polyols were within 1.0% and 3.1%, respectively, which indicates the feasibility of the established method in eliminating the negative effects of residual epoxy groups in vegetable oils-based polyols on OHV determination as well as accurately determining and evaluating the OHV of vegetable oils-based polyols. The method can broaden the range of OHV determination up to 1800 mg KOH/g. The ratio of acetic anhydride and acetic acid and reaction time are key factors affecting the complete degree of acetylation, which can be regulated mutually in accurately determining the OHV of vegetable oils-based polyols according to the practice. It is the first time, to our best knowledge, to establish a method to eliminate the negative effects of residual epoxy groups in vegetable oils-based polyols on OHV determination. The established method provides a feasible method for polyol industry to accurately evaluate the product quality of vegetable oils-based polyols.