2 min read Last modified on:August 26th, 2021

Phthalic acid

Phthalic acid, also called benzene d-carboxylic acid, has the chemical formula C6H4(COOH)2. Different isomers of phthalic acid are known by different names. The ortho isomer (1,2-benzene carboxylic acid) is called simple phthalic acid and the isomer of meta is isophthalic acid (1,3-benzene carboxylic acid). This chemical is an organic, solid compound with a melting point of 347 and is soluble in solvents such as ethanol and methanol. Figure (1) shows the molecular structure of phthalic acid.

Phthalic acid

Fig (1):The molecular structure of phthalic acid

Isophthalic acid is produced by the oxidation of methazylene using oxygen at a rate of about one billion kilograms per year. This process uses a cobalt-manganese catalyst. BP was one of the first companies to commercialize isophthalic acid production technology. In the BP process, a mixture of acetic acid, water, metasylene, catalyst and air is fed into a reactor.

The resulting isophthalic acid crystallizes on site to facilitate mechanical separation from the solvent. To further purify the acid, it is mixed with a solvent, heated, and the solution fed to a reactor. Again, the reactor effluent crystallizes and is mechanically separated from the solvent. The purified acid is then dried for purification. Figure (2) shows the isophthalic acid production in different countries.

Fig (2): The isophthalic acid production in different countries.

The main and industrial applications of pure isophthalic acid are the production of polyethylene terephthalate (PET) and the production of unsaturated polyester and the production of other resins used in the surface coating industry. A mixture of terephthalic acid and isophthalic acid is used for the production of mineral water bottles and food packaging. High efficiency benzoimidazole based polymers are produced from isophthalic acid. Other applications of isophthalic acid include:


  • For the production of insulation and insulation products
  • In the production of fiberglass reinforced plastic for products such as pipes and automobiles
  • Production of industrial coatings for metal appliances and office furniture
  • As an intermediary in the production of unsaturated polyester resins