Spectroscopic analysis of organic contaminants in terephthalic acid: colour implications in poly(ethylene terephthalate) manufacture

Colour measurements and doping experiments on polyethylene terephthalate (PET) show that the compounds in crude terephthalic acid which cause the yellow colour of derived PET polymer are carboxylic acid derivatives of anthraquinone and fluorenone. Fluorene derivatives are found to contribute to the PET polymer yellowness. On the other-hand, PET polymer luminance, was not affected by the small variations in impurity levels investigated in this work. 4-carboxybenzaldehyde (4-CBA) the major impurity in manufactured crude terephthalic acid (CTA), dominates the fluorescence spectrum. The fluorescence emission spectrum of 4-CBA is strongly influenced by the polarity and protic nature of the solvent. Associated dimers and aggregates of 4-CBA are primarily formed via the aldehyde and carboxylic acid groups causing marked red shifts in excitation and absorption spectra. Complexation also occurs between the 4-CBA and PTA groups. (C) 1998 Elsevier Science Limited. All rights reserved.