Product

Phthalic anhydride is a bicyclic aromatic compound consisting of a benzene ring fused to a five-membered cyclic anhydride ring with the expanded molecular formula C₆H₄(CO)₂O, and it represents the principal commercial form in which phthalic acid is sold and used industrially. Phthalic anhydride has the expanded molecular formula C₆H₄(CO)₂O, CAS number 85-44-9, and a molecular weight of 148.12 g/mol. It was historically notable as the first anhydride of a dicarboxylic acid to be used commercially.

Physical Properties

Phthalic anhydride appears as a white to colorless lustrous solid, typically supplied as flakes, needles, or powder with a mild, distinctive odor.

• Melting point: 130–134°C

• Boiling point: ~284–285°C

• Density: 1.53 g/cm³

• Flash point: ~152°C

• Solubility: Soluble in ethanol and acetone; slightly soluble in ether; decomposes/hydrolyzes in contact with water to form a corrosive phthalic acid solution

Chemical Behavior

As a reactive cyclic anhydride, phthalic anhydride readily undergoes esterification with alcohols to form phthalate esters, and it reacts with water to hydrolyze back to phthalic acid. It also reacts with primary amines and is used as a synthetic intermediate for producing various downstream chemicals, resins, and dyes. May cause corrosion to metals and is classified as capable of causing skin, eye, and respiratory irritation, along with potential organ damage upon repeated exposure.

Production

Phthalic anhydride is manufactured industrially by catalytic vapor-phase oxidation of either ortho-xylene or naphthalene with air, over a vanadium pentoxide (V₂O₅) fixed-bed catalyst at elevated temperature. Ortho-xylene has become the dominant feedstock globally due to its greater availability from catalytic reforming compared to naphthalene.

Applications

Historical EPA data indicates roughly 50% of phthalic anhydride production is directed to plasticizers (via phthalate esters for PVC), 25% to alkyd resins, 20% to unsaturated polyester resins, and the remainder to miscellaneous uses and exports. It also serves as a monomer for synthetic resins such as glyptal resins, and is used in synthesizing primary amines and various agricultural and dye intermediates. Commercial suppliers such as Lanxess market phthalic anhydride specifically as a "high purity crystalline molten PAN" building block for plasticizer, alkyd, and unsaturated polyester resin manufacturing across automotive, construction, and electronics sectors.

Health and Safety

Phthalic anhydride is classified as harmful if swallowed or inhaled, and it causes skin irritation, serious eye damage, and may trigger allergic skin or respiratory reactions (asthma-like symptoms) upon inhalation. Prolonged or repeated exposure is associated with organ damage, and the substance carries an aquatic toxicity classification (harmful to aquatic life with long-lasting effects). It requires storage under inert gas in a cool, dark location due to moisture sensitivity, and standard PPE (gloves, eye protection) is recommended when handling.