Polyisobutene

Polyisobutene is an inert, non-volatile synthetic emollient/film-former used to improve slip and occlusivity (commonly a few percent up to higher levels in lip and balm-type products). Human irritation and patch-test data generally show very low irritancy and sensitization potential, with reactions being uncommon and typically related to product occlusion or co-ingredients rather than the polymer itself. For highly reactive or barrier-compromised patients, I still score it as exceptionally gentle rather than perfectly inert because heavy, occlusive formulations can occasionally sting on fissured skin even when the ingredient is not intrinsically irritating. Safety Notes: Polyisobutene is used at very low levels (~0.1–1%) in leave-on emulsions (creams/lotions, sunscreens, primers) as a slip/occlusivity and texture aid, and at a few percent in color cosmetics and cleansing balms to improve spread and water resistance. At the high end, it can be a primary base in anhydrous consumer products—especially lip glosses, lip oils/balms, and some long-wear makeup—where polyisobutene (often HPIB) commonly reaches ~30–70% to provide gloss, tack, and film-forming. No specific EU/FDA concentration cap is typically applied; practical upper limits are driven by viscosity/feel and product format (leave-on anhydrous vs emulsion/rinse-off).