Weathering degradation mechanism in polyester powder coatings

The replacement of terephthalic acid (TPA) by isophthalic acid (IPA), an isomer of TPA, in the polyester backbone produces a significant increase in the weathering resistance of powder coatings exposed to tropical climate. The study of weathering of IPA- and TPA-based powder coatings in accelerated weathering chamber has shown with support of FTIR spectroscopy, the formation of photo-byproducts that are proposed to be generated by a photo-induced oxidation converting –CH2– in β-position to CO of esters into anhydride groups. Subsequent hydrolysis of these anhydrides is responsible of polymeric chain cleavage generating acidic groups and coating bleaching. FTIR analysis of exposed panels in Florida has demonstrated presence of same photo-byproducts.