Dimer acid refers to the dimer of linear unsaturated fatty acid or unsaturated fatty acid ester with linoleic acid of natural oil as the main component, which is self-condensed by Diels-AldeChemicalbookr cycloaddition reaction under the catalysis of white clay. It is a mixture of multiple isomers, in which the main components are dimer, a small amount of trimer or polymer and a trace amount of unreacted monomer.