Phthalic anhydride is the cheapest polybasic organic acid and therefore the most widely used in the production of alkyd resins. The rapid rise in consumption of surface coatings and finishes made from these resins presages greater demand for phthalic anhydride (and glycerin) in the future, particularly if this type of outdoor finish for wood is successful.
The world-wide shortage of naphthalene, with attendant sharp increases in price, raises the question of whether there may not be partial or complete replacement of phthalic anhydride by other polybasic acids in certain types of alkyd resins. The probability of such replacement seems remote unless the use of other polybasic acids, at present much higher priced, so improves the properties of the resins as to give a superior product. Approximately 100 pounds of naphthalene are required to produce 109 pounds of phthalic anhydride. Naphthalene at 3 cents per pound gives phthalic anhydride a raw material cost of 2.75 cents per pound as compared with 1.45 cents per pound when naphthalene was 1.55 cents per pound. In other words, the increase of 1.5 cents per pound in naphthalene, meant an increase of only about 1⅓ cents per pound in the raw material cost of phthalic anhydride, and only approximately ¼ cent per pound in the raw material cost of an alkyd resin surface coating containing about 20-percent phthalic anhydride.
POLYBASIC ACIDS OTHER THAN PHTHALIC ANHYDRIDE
Maleic acid and anhydride.
Maleic anhydride is obtained as a byproduct in the manufacture of phthalic anhydride and as a major product by the vapor phase catalytic oxidation of benzene. Domestic production, still small compared with phthalic anhydride, has increased many fold during the past two or three years. In 1937 there were three producers of maleic anhydride, with an output totaling 2,114,176 pounds. The uses of maleic acid derivatives other than in making resins are minor.
Malic acid and malomalic acid.
Malic acid is widely distributed in the vegetable kingdom, occurring especially in unripe apples. Commercially it is obtained by synthesis. Domestic production was reported for the first time in 1935. Malomalic acid is formed by heating malic acid. United States Patent No. 1,091,627 covers a resin made from malic acid and glycerin which will increase the toughness of phthalate resins. United States Patent No. 1,667,198 suggests the use of malomalic acid to form resins of glass-like appearance.
Adipic acid.
Adipic acid is made by oxidation of cyclohexanol. When condensed with glycerin it yields an alkyd resin which is soft and rubbery and which does not harden when heated. Numerous patents have been granted on the preparation of adipic acid and its resins. Commercial production of adipic acid was first reported in 1935, and the output increased in 1936 and in 1937.