Phthalic Anhydride (PA)

Phthalic Anhydride (PA) is a high-melting-point aromatic anhydride primarily used as a raw material in the manufacture of unsaturated polyester resins (UPR), alkyd resins, and certain epoxy anhydride formulations. In UPR production, PA is condensed with diols (glycols) to form the ortho-phthalate backbone of orthophthalic UPR, which is the most cost-effective standard grade of UPR for general composites. PA is a white crystalline solid with a melting point of 130–131°C, requiring heated mixing and processing equipment for handling. In epoxy curing applications, PA is used as a solid anhydride curing agent for high-temperature applications, providing moderate Tg (80–100°C) and good chemical resistance. The aromatic ring structure contributes to heat resistance but also causes yellowing under UV exposure, limiting PA-epoxy applications to non-UV-exposed environments. PA has a relatively low anhydride equivalent weight (AEW ~148 g/eq) due to its high molecular density. For most practical epoxy casting applications, liquid anhydrides (MTHPA, MHHPA) are preferred over PA due to processing convenience; PA is primarily important as a UPR raw material.