Studies of the interactions of resin acids with Pigment Yellow 13

Abstract

The surface interaction between Pigment Yellow 13, a classical diarylide pigment, and abietic acid type resin acids, used commercially to coat pigment products, was investigated by the joint techniques of solid-state ¹³CP/MAS NMR and powder X-ray diffraction. The initial objective of the study was to synthesise ¹³C labelled resin acid analogues to provide a handle for the NMR study. Attempts were made to label the carboxylic acid carbon of abietic acid, via a Barton reaction and subsequent Grignard reaction and carboxylation. Esterification of various resin acid substrates with ¹³C labelled diazomethane was also attempted. These routes proved difficult, due to the sterically hindered nature of the acid substrates. Labelled adducts were formed from ¹³C-malcic anhydride and isomerised abietic acid, but were found to be lost from the pigment during resination. Attempts to form a more stable adduct with N-methylmaleimide were unsuccessful. A series of experiments was carried out to coat Pigment Yellow 13 with incremental amounts of abietic acid, maleopimaric acid and dihydroabietic acid. The maleopimaric acid was lost during processing. The pigment samples resinated with abietic acid and dihydroabietic acid were analysed by the techniques mentioned previously. The presence of resin acids during pigment processing was found to enhance the crystallinity of the pigment particles, as opposed to the heat treatment during processing which promotes particle size. The resin acids were found to be more amorphous than the starting resin when reprecipitated on the pigment particles, ¹³C CP/MAS NMR shows that there is no chemical interaction between the pigment molecule and the carboxylic acid of the resin.