Phthalic acid. Molecular model of the industrial chemical phthalic acid (formula: C8H6O4). The atoms are spherical and colour-coded: carbon (blue), hydrogen (white) and oxygen (red). Some derivatives of phthalic acid (the phthalates) are used as plasticisers in the plastics industry. These are chemicals added to a plastic to make it less rigid. Some plastics, such as very flexible plastic film, may be around 50% plasticiser.