Pastes

Pastes are basically ointments into which a high percentage of insoluble particulate solids have been added—as much or more than 50% by weight in some instances. This extraordinary amount of particulate matter stiffens the systems through direct interactions of the dispersed particulates and by adsorbing the liquid hydrocarbon fraction within the vehicle onto the particle surfaces. Insoluble ingredients such as starch, zinc oxide, calcium carbonate, and talc are used as the dispersed phase. Pastes make particularly good protective barriers when placed on the skin for, in addition to forming an unbroken film, the solids they contain can adsorb and thereby neutralize certain noxious chemicals before they ever reach the skin. This explains why they are used to ameliorate diaper rash, for when spread over the baby's bottom, they adsorb irritants formed by bacterial action on urine. Like ointments, pastes form an unbroken, relatively water-impermeable film on the skin surface and thus are emollients; unlike ointments, the film is opaque and therefore an effective sun block. Thus, skiers apply pastes around the nose and lips to gain a dual protection. Pastes are actually less greasy than ointments because of the adsorption of the fluid hydrocarbon fraction to the particulates.

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