UV Absorber Benzotriazole Type (Tinuvin 328 Type)

UV Absorber Benzotriazole Type (Tinuvin 328 type, CAS 25973-55-1) is 2-(2H-benzotriazol-2-yl)-4,6-bis(1,1-dimethylpropyl)phenol, a high-performance benzotriazole UV absorber with strong absorption in the 300–380 nm UV range relevant to photodegradation of polyester, vinyl ester, and epoxy composites. The benzotriazole chromophore absorbs UV radiation and dissipates the energy as harmless heat through an intramolecular proton transfer mechanism, protecting the polymer matrix from photooxidation-induced chain scission and yellowing.

Tinuvin 328 type is distinguished from simpler benzotriazoles by its large alkyl substituents (bis-1,1-dimethylpropyl groups) on the phenol ring, which reduce volatility, improve compatibility with non-polar resin systems, and decrease extraction by water. This makes it particularly suitable for applications combining UV stability with water exposure — marine gelcoats, outdoor water features, pool surrounds, and irrigation system components — where lower-MW benzotriazoles might leach out over time.

In UPR gelcoat formulations for outdoor composites, Tinuvin 328 type UVA is used at 0.3–1.0 phr, typically combined with HALS (Tinuvin 770 type or polymeric HALS) for synergistic protection. The benzotriazole absorbs the UV photons while the HALS scavenges radicals from the small fraction of UV that penetrates. In clear or lightly pigmented composites, this dual system provides Class 1 weathering performance in QUV accelerated weathering tests and Xenon arc testing up to 2000–4000 hours without significant gloss loss.