All these elements can have a dramatic effect on the sensory and functional properties of the resulting ester.
Skin migration defines the ester's natural propensity to spread, independently. Skin migration values are calculated based on the extent of spreading of the emollient on skin. A low skin migration value may be beneficial in all areas of colour cosmetics, whereas a high skin migration value indicates emollients more suited to formulations such as self-tanning where enhanced coverage is required.
Skin play defines how easily and for how long an ester can be massaged over the skin. It is assessed using a panel of sensory evaluators. High skin play means that the material will move freely over the skin's surface during application and will remain on the skin long enough to facilitate even spreading and coverage, this is important in many skin care applications. On the other hand, low skin play means that a material will not move freely over the skin and could be either a highly tenacious ester that does not spread readily or a light ester that is quickly absorbed into the skin.
Refractive index can be used to indicate an ester's propensity to convey shine on skin or hair. Esters with high refractive indexes will impart glossing benefits and are incorporated into various colour cosmetics and hair care products. Esters with low refractive indexes can be used to impart a mattifying effect for colour cosmetics and skin care applications.
Many emollient esters are classed as low tack and this would be considered beneficial in most applications. However, in some applications, a degree of tack can be advantageous. For example, esters with high tack can be used to enhance pay-off in stick applications, like deodorants, and enhance film durability in lip products.
Skin feel comprises of two parts: emollience and afterfeel. Emollience can be classified as low, medium or high and afterfeel can be classified as silky or cushiony.
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