Thickeners in Vat Dye Textile Printing: Rheology and Morphology

Abstract

This paper is a study of the thickeners used in two-phase vat dye textile printing. Throughout this complex process, the thickener dispersion undergoes several transformations, especially spreading on the textile material, drying, and coagulation. Empirical knowledge prescribes several requirements in terms of viscosity but also concerning the dried thickener. But, a better understanding of the phenomena is necessary to determine the origin of possible failure if the basic recipe is modified. Three different thickeners are considered: guaranate, alginate, and modified starch, as well as a combination of these three polymers. Their rheological properties are characterized in both permanent and oscillatory shear flow. Use of a mixture of polymers instead of a simple one avoids undesirable effects of thixotropy, yield stress, and elasticity, for a given value of the viscosity. The process involves the gelation of the thickener by cross-linking of guaranate chains with borate ions. The mechanical properties of such gels strongly depend on the pH, and their stability is affected by the presence of salt. Additionally, thickener films are dried under different conditions, and the morphology of the film is observed at different scales. It is shown that rapid drying at high temperature prevents crystalline structures obtained in the case of slow drying under ambient conditions. This is propitious to a good flexibility of the polymer film, suitable for fabric handling, good swelling in the steamer, and easy washing off.