UV Absorber Benzophenone Type (BP-4 / BP-12)

UV Absorber Benzophenone Type (BP-4/BP-12 type, 2-hydroxy-4-methoxybenzophenone, CAS 131-57-7) is the most widely used low-cost UV absorber in UPR gelcoat and composite formulations for general outdoor exposure. The benzophenone UV absorber works through an excited-state proton transfer mechanism similar to benzotriazoles: UV photons are absorbed and the energy is dissipated as heat via intramolecular enol-keto tautomerism, protecting the polymer from photodegradation.

Benzophenone UV absorbers (BP series) absorb primarily in the 280–340 nm range — slightly shorter wavelengths than benzotriazoles — providing complementary UV protection. BP-4 (2-hydroxy-4-methoxybenzophenone) is the simplest and most economical grade, supplied as a white to pale yellow powder, used at 0.2–1.0 phr in UPR gelcoats, casting resins, and laminating systems for general outdoor applications. It has lower molecular weight than benzotriazoles (228 g/mol), providing better cost-efficiency per absorption unit but also higher volatility and potential migration.

In standard UPR gelcoat formulations for boat hulls, garden furniture, and building panels in non-critical outdoor applications, BP-4 at 0.5–1.0 phr provides 3–5 years of effective UV protection. For demanding applications (marine, tropical, long-service-life), benzotriazole types (Tinuvin 328, 384) or a combination of BP-4 + benzotriazole + HALS is preferred to compensate for BP-4's lower persistence. The low cost of BP-4 makes it the standard choice for cost-sensitive markets, with benzotriazoles or HALS added for performance upgrade.