Synthesis and Characterisation of Dihydroxystearic Acid-Estolides from Palm-Based Dihydroxystearic Acid

Estolide is a polyfunctional oligomer that contains ester linkages on the alkyl backbone of the molecule, which is formed by the esterification reaction between fatty acids. These ester linkages are more resistant to hydrolysis than those of triglyceride. The unique structure of the estolide makes it has superior physical properties if compared to the mineral and vegetable oil in certain applications. Estolides can be potentially used in a variety of applications, such as surfactants, ingredient or additive in cosmetic and inks formulations. An attempt was made to synthesize dihydroxystearic acid-estolides from palm-based DHSA through a condensation reaction. The hydroxyl and carboxyl groups in DHSA reacted intermolecularly to form DHSA-estolides. The synthesis of DHSA-estolides was successfully conducted via condensation reaction at 180oC without any acid catalyst under atmospheric pressure. The condensations of DHSA at various reaction periods (2 h, 4 h, 6 h and 8 h) yield product mixtures with high saponification values but low acid values indicating fatty acid has been successfully converted into the ester. As the reaction proceeds, the powdery DHSA changes to a sticky and hardly pourable paste. This is due to the homo-oligomerization of DHSA produces DHSA-estolide with higher molecular weight than that of the starting material. The increased average molecular weights (calculated based on the neutralization equivalent, NE and end-group analysis) of the product mixtures obtained indicate the repeating unit or molecular weight of the DHSA-estolides produced increase as the reaction period is increased. The low peroxide values of DHSA-estolides show they are oxidatively stable. Analyses of GPC and HPLC show that the product mixtures obtained are actually mixtures of DHSA and DHSA-estolides of different repeating unit/ molecular weight. Both FT-IR and NMR analyses confirm the formation of estolide. The ester transmittance peak (1742 cm-1 -1733 cm-1) with an acid shoulder (1716 cm-1 -1713 cm-1) is observed in FT-IR spectrum. The ester methine signal (4.84 ppm) and 2 α-methylene protons (α-acid, 2.36 ppm; α-ester, 2.30 ppm) are the distinctive features of DHSA-estolide in 1H-NMR. These DHSA-estolides are found applicable as anti-rust additive in cutting fluid. DHSA-monoestolide and DHSA-diestolide with the acid value of 60 – 90 mgKOH/ g are found compatible in shampoo formulation and improves the performances of the shampoo. DHSA-triestolide and DHSA-pentaestolide with the acid value of 20 -40 mgKOH/ g are found fully function as emulsifier and thickener in water-in-oil emulsion for cosmetic formulations.

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