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Occlusive agents are usually oily substances that coat the stratum corneum (SC) rendering an emollient effect as well as the ability to decrease transepidermal water loss (TEWL). Two of the best occlusive ingredients currently available are petrolatum and mineral oil. Petrolatum, for example, has a water vapor loss resistance 170 times that of olive oil.1 However, petrolatum and mineral oil have a greasy feeling on the skin, leaving them cosmetically undesirable, and have further lost popularity because of the greater awareness of the environmental effects of processing these products. Other synthetic agents commonly used as occlusive ingredients include paraffin, squalene, dimethicone, and propylene glycol.2 Lanolin is an example of a natural occlusive ingredient. Organic occlusive ingredients have also become increasingly popular and include argan oil, beeswax, borage seed oil, safflower oil, olive oil, jojoba oil, and tamanu oil. Occlusive ingredients seem to be most effective when placed over damp skin and are only effective while present on the skin because once removed, TEWL returns to the normal level. Occlusives are usually combined with humectant ingredients.
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INFLUENCES ON THE OCCLUSIVE EFFECT3
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In order for an ingredient to impart an occlusive effect, the molecules in that ingredient must be able to align to form a tight barrier (or palisade). Short straight chains of the same length are the most efficient at aligning in this manner. When a product contains various compounds with diverse types of chains or with different chain lengths, it is inherently a less effective occlusive than an agent with a consistent type of molecule with a straight chain. An example of a well-structured occlusive is mineral oil, which contains straight alkyl chains but of varying lengths. This contributes to its ability to form a tight palisade on the skin and exhibit a strong occlusive effect (Figure 8-1). If mineral oil had straight alkyl chains of the same length, its occlusive effect would be even stronger. In contrast, the molecules in botanical oils can be extremely diverse, making it difficult for the molecules to tightly align. For this reason, many botanical oils make poor occlusive agents.
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Another factor that determines occlusive ability is ingredient substantivity, or the ability of the ingredients to stay on the skin’s surface. If the molecules rapidly penetrate into the SC, then the result is a weaker palisade or occlusive effect, which is known as lower substantivity. Therefore, ingredients with larger molecules and a comparatively lower capacity to penetrate the SC, thus higher substantivity, may display greater occlusive activity. Viscosity can also affect ...