Butylene glycol is a commonly used ingredient that plays multiple roles in cosmetics, including as a humectant, texture enhancer, solvent, and penetration enhancer. Research shows that butylene glycol not only helps increase the water content of the skin (also known as hydration), but can also help reduce the roughness of the skin's surface (depending on how it is combined in the formula). Although not known as a preservative per se, some of the properties of butylene glycol can help increase a formula's stability against microorganisms that can damage the skin's surface.
As a raw material, butlyene glycol is a clear and viscous liquid. It is similar to propylene glycol but has a lighter texture. It is used in a wide range of concentrations with reports of up to 50%, although many suppliers limit it to 30%. The minimum amount tends to be around 0.5%, in which case it is typically part of a mixture with plant extracts and/or preservatives.
The Cosmetic Ingredient Review Board has evaluated numerous toxicology tests and other research on butylene glycol over the years and has determined that it is safe within the wide concentration range currently used in cosmetic products. The US Food and Drug Administration (FDA) has even determined that butylene glycol is safe as a food additive. https://www.paulaschoice.com/ingredient-dictionary/ingredient-butylene-glycol.html?q=Butylene%20Glycol&fdid=ingredients