Polyol esters in personal care formulations part 1 – Chemistry
As the quest for natural, renewable, biodegradable and green products continue there has been increased interest in working with a variety of esters and triglycerides. Esters when applied to the skin, not only act as emollients but also provide an aesthetic effect which can be varied by structure.
This article is the first of two that deal with polyol esters themselves, synthesis and properties of the polyol esters. This particular class of polyol esters are based upon specific highly branched polyols namely. neopentyl glycol (2 OH groups), trimethylol propane (3 OH groups) and pentaerythritol (4 OH groups). These esters find their main use in industrial lubrication because not only are they effective lubricants, they are oxidatively stable and have a relatively low viscosity when one considers their molecular weight.
Esters are a class of compounds that have been used as polar oils in many formulations. Esters consist of a carbonyl group (C=O) covalently bonded to –OR, where R is an alkyl group. Esters cover a wide range of products and are classified by the groups connected on either side of the -C(O)O- bond. The majority of esters used in the cosmetic field are produced by the so called “direct Esterification” process”. Direct esterification is a process by which organic acid and an alcohol are directly reacted. This distinguishes them form “trans-esterification esters”, which as the name suggests are made by trans-esterification. Trans-esterification is a process of reacting alcohol with an ester, rather than an acid. A simple example is reacting methyl stearate with decanol, making decyl stearate and methanol. Instead of getting water as a byproduct like direct esterification, the byproduct of a trans-esterification reaction is an alcohol. Figure 1 shows the reaction sequence.
The progress of the reaction can be followed by (1) the change in acid value (which decreases as the acid reacts); (2) the hydroxyl value (which drops a ...