Influence of yarn folding on UV protection properties of hemp knitted fabrics
Authors: Kocic Ana , Popovic Dusan , Stankovic Snezana , Poparic Goran 
Published: July 2015

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 eco-friendly
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 there from. 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 water-treated
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 characterized 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.