Influence of yarn folding on UV protection properties of hemp knitted fabrics

Abstract

In the last years the media have highlighted the damage of the ozone layer and the resulting increase of ultraviolet radiation (UVR) reaching the Earth’s surface. Prolonged and repeated, both occupational and recreational, sun exposure of the population causes some detrimental effects. Clothing is considered to be one of the most important tools for UV protection. It is generally accepted that synthetic fibres provide a high UV protection capability of textiles, while cellulose fibres (cotton, linen, hemp, viscose) have a low UV absorption capacity. However, natural pigments, pectin and waxes in natural cellulose fibers, and lignin in hemp fibers, act as UV absorbers having a favorable effect on UPF of grey-state fabrics. Bearing in mind the trend of reintroduction of hemp fibers as a source of ecofriendly textiles, there is a serious lack of study about the potential of hemp materials in terms of UV protection. Folded yarn is a complex yarn composed of two or more component yarns arranged parallel and twisted together to make a "new quality" yarn. Folding of yarns is an operation undertaken in order to modify single-yarn properties to an appreciable degree. There are very few investigations concerning the relationship between the yarn properties and UV protection effectiveness of the fabric made therefrom. In addition, there is no any result in the scientific literature about the influence of yarn folding on UV protection properties of textile materials. Having this in mind, for our research the idea was to evaluate the effect of yarn folding in this regard. The plain knitted fabrics composed of single or two-folded hemp yarn were compared in terms of UV protection properties. The Ultraviolet Protection Factor (UPF), as the quantitative measurement of the material effectiveness to protect the human skin against UVR, was determined for the textile materials by in vitro test method according to the European standard EN 13758. The knitted fabrics construction and physical properties were also determined. Bearing in mind that plain knitted fabrics are particularly susceptible to relaxation, they were subjected to relaxation and shrinkage by wetting process, and testing procedure was repeated on the watertreated samples. The results obtained indicated that the folding operation influences UV protection properties of knitted fabrics through an influence on a loop configuration, i.e. the fabric openness. Relaxation and shrinkage of the knitted fabrics due to wet relaxation caused the reduction of macro-porosity increasing the UPF of the knitted fabrics. Although the knitted fabric produced from single hemp yarn was characterised by higher UPF, the UVR transmittance of the folded hemp yarn knitted fabric after wet relaxation placed it in the "excellent UV protection category" (according to European Standard EN 13758-2). This fact together with the better thermal comfort manifested itself in higher air permeability, confirmed the potential of folding operation in terms of UV protection properties of textile materials.