Capillary flow of liquid water through plain knitted fabrics

In extreme weather conditions and activity levels of human subjects evaporation of sweat is critical for maintaining the sensorial and thermal comfort. Fabrics, from which clothes worn next to the skin are made, play an important role in facilitating the transfer of body liquid perspiration away from the skin to the environment through the mechanisms of capillary flow and evaporation. This work is a theoretical and experimental investigation of water flow characteristics of plain knitted fabrics with relevance to their structure geometry and constituent fibre chemistry. Plain knitted fabrics were produced by systematically varying different production parameters including fibre type, fibre orientation, yarn folding, yarn twist, yarn linear density, and blend ratio. Cotton and polyester fibres were used. Some commercial fabrics were included in the study. The gravimetric absorbency test system (GATS) was adapted for testing the water areal flow and uptake rates through the fabrics. Yarns taken from the produced fabrics were also tested for horizontal linear flow of liquid water. A theoretical model to predict the capillary flow of liquid water through yarns was proposed. The model is based on the representation of the inter-fibre pores in terms of the hydraulic radius theory.It is established that the plain knitted fabric configuration as interlocking of loops plays an important role in facilitating the capillary flow of liquid water through the fabric. The yarn contact at the crossover points of the knitted loop enables a connected path for liquid flow which increases at higher contact pressure at the crossover points. If the contact pressure increases beyond a certain limit it starts to negatively affect the flow because the higher pressure reduces yarn porosity. When the number of yarns in contact with the liquid source per unit area of the fabric plane increases the capillary flow increases. Fabric compactness, which is controlled by yarn diameter and stitch length, is an important fabric parameter that determines these effects the fabric configuration have on the capillary flow. The experiments and the micro-structural analysis revealed that inter-fibre pores within the plain knitted fabric transfer the capillary driven liquid water through the structure at a faster rate. These pores hold the higher percentage of the fabric total air volume. It was also established that both fibre chemical nature and yarn fine structure geometry have critical effects on the apparent contact angle which is a critical factor controling capillary flow of liquid water. The smooth yarn surface made of filament polyester fibre gave a low apparent contact angle in contrast with the constituent filament which showed a high real contact angle. On the contrary, due to the more disorderly fibre arrangement on the yarn surface, yarns made of the staple polyester fibre showed a similar high contact angle to the constituent fibre. The experimental results of capillary flow of liquid water through yarns showed a strong correlation with the estimated results based on the theoretical model derived from the Kozeny-Carman equation. The model provides theoretical basis for understanding the effects of the geometric and material parameters on the capillary flow through the yarn. The model predicts that as the total fibre perimeter within the yarn cross section increases, or yarn porosity decreases, the velocity decreases, however, the eventual distance the water travels through the yarn increases.