Preface: This “Ingredient Profile” column is intended to educate the audience of Cosmetics & Toiletries magazine about the chemistry, manufacture and technology associated with the ingredients employed in cosmetics and personal care products. Although the column is geared primarily toward formulators and product developers, the content will also benefit those in roles such as analytical sciences, process engineering, product stewardship, regulatory compliance, quality assurance, purchasing, marketing and sales.
Each month, “Ingredient Profile” will feature an ingredient according to its International Nomenclature Cosmetic Ingredient (INCI) name. The column will emphasize ingredients that are the products of chemical manufacturing processes to educate readers about the feedstocks and synthesis routes employed to produce cosmetic and personal care ingredients. Key chemical and physical properties of ingredients, reported ingredient functions and product applications also will be presented. Finally, the column will aim to provide readers with concise, well-referenced explanations of the mechanisms of action by which the highlighted ingredients achieve their desired functions.
Chemistry and Manufacture
PEG-150 distearate (INCI: PEG-150 Distearate) is defined as the polyethylene glycol (PEG) diester of stearic acid, wherein the PEG has an average degree of polymerization of 150 oxyethylene repeat units corresponding to an average molecular weight (MW) of 6,600 g/mol. The overall average MW of PEG-150 distearate is 7,170 g/mol—sufficiently high enough to categorize the ingredient as a low MW polymer. PEG-150 is a telechelic polymer, i.e., it is functionalized with a hydrophobic stearate ester on both the α and ω ends of the hydrophilic PEG chain so that both ends of the polymer exhibit surface and interfacial activity.
PEG diesters typically are produced by one of three routes: 1) ethoxylation of fatty acids, 2) direct esterification of PEG with fatty acids to produce water as a by-product, or 3) transesterification of fatty acid esters with PEG to produce alcohols as byproducts. In the case of PEG-150 distearate, only Routes 2 and 3 are desirable since Route 1 leads to mixtures of mono- and difunctional PEG esters and unmodified PEG.
Figure 2 shows the synthesis of PEG-150 distearate via Route 2 in the presence of an acidic esterification catalyst and an antioxidant (to prevent PEG degradation) under heat and vacuum. Critical commodity feedstocks for the production of PEG-150 distearate are ethylene oxide, derived from ethylene, which is used to produce the PEG-150; and stearic acid for direct esterification, or methyl stearate for transesterification.