The influence of the bleaching and dyeing processes on the comfort properties of knitted fabrics containing elastane

Abstract

Comfort properties of knitted fabrics depend on various fabric characteristics such as chemical composition, knitted structure (fabric thickness and weight), air permeability and water vapour resistance. The purpose of this investigation is to study the influence of different elastane content and different processing stages (bleaching and dyeing) on the comfort properties of single jersey knitted fabrics. Samples made of 100% cotton (20 tex ring spun yarn) and elastane yarn (22 dtex multifilament yarn) in alternating courses (half plating) and elastane yarn in every course (full plating) were scoured, bleached or dyed. Bleaching was done with 50% H2O2 and dyeing in dark blue shade with reactive dyes. In comparison with 100% cotton knitted fabrics, cotton/elastane fabrics have increase fabric thickness for about 5-16%, fabric weight for about 8-21%, and water vapour resistance for about 11- 30%, but decrease air permeability for about 28-51%. Increased fabric thickness is a result of plating which means the simultaneous formation of one loop from two threads, so that one thread will lie on the face of the fabric, while the other thread is fed to the needles in such a way that it forms the back or reverse of the final fabric. In this case, the largest distance between two points of the cross section of the knitted fabric will increase because over cotton yarn is elastane yarn. With the increase of the fabric thickness, fabric weight increases too. 100% cotton knitted fabrics, being smoother and coarser as compared to cotton/elastane assist easier passage of water vapour and air through it, resulting in lower water vapour resistance and higher air permeability. The increased water vapour resistance and reduced fabric air permeability can be attributed to the higher thickness, fabric weight and compactness accompanied with a higher elastane content, which hard segments offer limited water vapour and air flow through knitted fabrics. Insignificant differences in comfort properties (excluded air permeability) were found after different finishing stages. Bleached knitted fabrics have about 4-10% lower thickness, 4-11% lower fabric weight, 2-8% lower water vapour resistance, and 8-30% higher air permeability compared to dyed knitted fabrics. There are many factors that affect comfort properties, such us concentration of chemical agents, pH, temperature and duration of the treatment. Scouring and bleaching were done in high alkaline conditions with strong oxidative agent. In such conditions the cotton became stiffer, wax components were saponified, the fibers in yarns became closer, which resulted in a lower yarn crosssection and reduction of the fabric thickness, fabric weight and water vapour resistance. As the loop is clearer, the air permeability of bleached knitted fabrics is higher. On the other hand, dyeing in dark shade was done in milder conditions, which resulted in higher fabric thickness, fabric weight and water vapour resistance, and lower air permeability in regard to bleached knitted fabrics.