A process for producing an azo pigment, which comprises reacting a mixture composed of (1) 2-hydroxynaphthalene-3-carboxylic acid or its salt, (2) β-naphthol or its salt and (3) a phenolic by-product having a molecular weight of about 300 and containing about 1 equivalent of phenolic hydroxy groups obtained during the reaction of an alkali β-naphtholate with carbon dioxide, with a diazonium compound of an aromatic aminosulfonic acid, and if desired, converting the product to its lake.
A resin composition includes polylactic acid resin, a plastic resin superior in heat resistance to the polylactic acid resin, and a nucleating agent that promotes crystallization of the polylactic acid resin, with the nucleating agent being a soluble azo lake pigment. The plastic resin superior in heat resistance to the polylactic acid resin is one which has a flexural modulus of elasticity no lower than 1000 MPa at 80° C.