The clearing of dyed polyester. Part 1. A comparison of traditional reduction clearing with treatments using organic reducing agents

Abstract

Reduction clearing using aqueous sodium dithionite under alkaline conditions at elevated temperatures is commonly carried out as an aftertreatment to remove deposits of disperse dye and other impurity residues from the surface of dyed polyester. In this paper, the effect of conventional reduction clearing on the colour and fastness properties of polyester dyed with a series of selected commercial disperse dyes at a range of depths of shade is established, and an understanding of the scientific principles is developed. The results correlate closely with an assessment of surface dye removal by cold acetone extraction of the dyed samples. Because of certain environmental, technological, and economic disadvantages associated with traditional reduction clearing using sodium dithionite, there is industrial interest in alternatives. Thus, this paper also describes a study of the use of two organic reducing agents, thiourea dioxide and hydroxyacetone, and the relative merits of the three processes are discussed. The outcome of clearing varies with the particular dye. Mechanistic explanations are proposed, based on individual characteristics of the molecular structures of the dyes. Scanning electron microscopic investigations of the surface of dyed samples before and after reduction clearing were qualitatively consistent with the assessments of the technical performance. This paper is the first in a series in which we will subsequently report further comparative studies, based on similar methodology, exploring alternative clearing processes.