Isophthalic acid (PIA) has three major uses:

PET (PolyEthylene Terephthalate) copolymer, which is used in bottle resins and to a much lesser extent, for fibers. PIA (Purified Isophthalic Acid) reduces the crystallinity of PET, which serves to improve clarity and increase the productivity of bottle-making.

Unsaturated polyester resins, where the addition of PIA improves thermal resistance and mechanical performance, as well as resistance to chemicals and water.

Polyester/alkyd surface coating resins, where PIA increases resistance to water, overall durability and weather ability.

The largest use of purified isophthalic acid (PIA) is as an intermediate in the production of unsaturated polyester resins, followed by polyester and alkyd resins (mainly for surface coatings) and inks, reinforced plastics and packaging applications. Its fastest growing application is as a comonomer  in polyethylene terephthalate (PET) bottle resins. PIA provides excellent hardness, corrosion and stain resistance, hydrolytic  and thermal stability and low resin colour.