Textiles serve as both a barrier and a transporter of heat, air and moisture from one environment to another. In the case of clothing, apparel fabrics act as a boundary between the micro environment immediately surrounding the body and the outer environment. The development and innovation in various man-made fibers are to simulate the use of cotton in clothing applications.
Keywords: wicking, wetting, texturizing, physical bulk, moisture transportation, comfort, capillary action.
Moisture flow through textiles is an important parameter governing the comfort properties. Human body perspires in two forms insensible and sensible perspiration and to be in comfortable state, the clothing which will be worn should allow both the type of perspiration to transmit from the skin to the outer surface. Generation of metabolic sweat is a natural outcome of a person involved in a strenuous activity. When human skin perspires, the main objective is to lose excessive heat from the body. This excessive heat is mainly lost through evaporation of the moisture from the surface of the skin. Moisture present on the skin surface of a clothed human can often produce uncomfortable sensations such a prickle and wet-cling. The build of humidity in clothing microclimate or the air space between a clothing layer and the sweat wetted skin is known to contribute to sensations of dampness and clamminess, especially during cooling period that follows intervals of sweat generating exercise. Therefore moisture transport in textile fabrics is one of the critical factors affecting physiological comfort. Thus, in order for clothed human to remain comfortable, the moisture needs to be removed from skin surface mechanically. This is achieved through the use of fibres and fabrics which absorb the moisture and thus remove it from the skin surface .
Figure 1: Real field application of textured yarns
Texturizing processes were originally applied to man-made fibres to reduce such characteristics as transparency, slipperiness, and the possibility of pilling (formation of small fibre tangles on a fabric surface). Texturizing processes make yarns more opaque, improve appearance and texture, and increase warmth and absorbency. The production of textured yarns stemmed from the need to broaden the area of use of synthetic fibers; applications of such fibers had previously been limited by the fibers to low hygroscopicity and smooth surfaces, which had an unpleasant, glassy sheen. Texturing improves the use characteristics and hygienic properties of synthetic yarns.
Textured yarns are used for manufacturing a wide variety of textile products: hosiery, knitted underwear and outer wear, and shape-retaining knitted fabrics for men’s and women’s suits and overcoats. They are also used in the production of artificial fur, carpets, blankets, and drapery and upholstery fabrics. As of 1975, world production of textured yarns was approximately 1.5 million tons a year.
2. Importance of wicking in Moisture Transportation
Wicking is a spontaneous transport of a liquid driven into a porous system by capillary forces. It is nothing but the ability to sustain the capillary flow. For wicking to take place the fibre should have the ability to get wet by the liquid. Wettability describes the initial behaviour of fiber, yarn and fabric when bought into contact with water. In fact it is balance of forces involved in wetting the fibre surface that drives the wicking process. A liquid that does not wet the fibers cannot wick into a fabric. When a fibre is wetted by a liquid the existing fibre air interface is displaced by a new fibre liquid interface.
In the absence of external forces the transport of liquids into fibrous assemblies is driven by capillary forces that arise from the wetting of the fibre. Contact angle decides the wettability of the fibres. In case of contact angles above 90o liquid in a capillary is depressed below the surface instead of rising above it. For the wicking process to take place spontaneously, the balance of energy has to be such that energy is gained as the liquid advances into the material. The wicking rate is dependent on capillary dimensions of the fibrous assembly and the viscosity of the liquid. When wicking takes place in a material whose fibres can absorb liquid the fibres may swell as liquid is taken up, so reducing the capillary spaces between the fibres, potentially altering the rate of wicking.
3. Kinetics of wicking
Most textile processes are time limited, and the rate of wicking is therefore important. However, the wicking rate is not solely governed by interfacial tensions and the wettability of the fibers, but by other factors as well. The wicking rate depends on the capillary dimensions of the substrate and the viscosity of the liquid. For a theoretical treatment of capillary flow in fabrics, the fibrous assemblies are usually considered to consist of a number of parallel capillaries. The advancement of the liquid front in a capillary can be visualized as occurring in small jumps. The advancing wetting line in a single capillary stretches the meniscus of the liquid until the elasticity of the meniscus and the inertia of flow are exceeded. The meniscus contracts, pulling more liquid into the capillary to restore the equilibrium state of the meniscus. The movement of the liquid in a non-homogeneous capillary system, such as a fibrous assembly, is discontinuous for another reason as well. The wetting front advances into the capillary system in small jumps, because the irregular capillary spaces have various dimensions.
Wicking is affected by the morphology of the fibre surface, and may be affected by the shape of the fibres. The common belief that fibre shape does not affect wetting is valid only for the wetting of single fibres. The shape of fibres in an assembly such as yam or fabric affects the size and geometry of the capillary spaces between fibres, and consequently the rate of wicking. The flow in a capillary may stop when geometric irregularities allow the meniscus to reach an edge and flatten.
However, the wicking rate is not solely governed by interfacial tensions and the wettability of the fibers, but by other factors as well. The wicking rate depends on the capillary dimensions of the substrate and the viscosity of the liquid. Wicking is affected by the morphology of the fibre surface, and may be affected by the shape of the fibres as well. The common belief that fibre shape does not affect wetting is valid only for the wetting of single fibres. The shape of fibres in an assembly such as yam or fabric affects the size and geometry of the capillary spaces between fibres, and consequently the rate of wicking.
Cotton was the most preferred material to be used as clothing especially as next to skin garment since it can absorb the sweat from the body quickly. But it creates a sensation of dampness and clamminess once it is fully wet and makes the wearer feel discomfort especially during the cooling period. Therefore more and more researches are carried out in the field of man-made fibers to simulate the use of cotton in clothing applications. The production of textured yarns stemmed from the need to broaden the area of use of synthetic fibers; applications of such fibers which had previously been limited to low hygroscopicity, smooth surfaces, unpleasant and glassy sheen.Texturing is defined as, the process in which durable crimps, curls, loops or other fine distortions are imparted along the length of the continuous filament yarns; so as to increase their bulkiness. Texturing gives filament yarn the desired volume and elasticity. Texturing improves the use characteristics and hygienic properties of synthetic yarns.
4. Wicking in yarns
Yarns are basically of two types, Spun yarn and filament yarns. Spun yarn is made by twisting or otherwise bonding staple fibres together to make a cohesive thread. Filament yarn consists of filament fibres (very long continuous fibres) either twisted together or only grouped together.
4.1 Wicking in Spun yarn
Spun yarn structure
Spun yarn is made by twisting or otherwise bonding staple fibres together to make a cohesive thread, or “single”. Twisting of fibres into yarn is done in the process called spinning and yarn spinning was one of the very first processes to be industrialized. Spun yarns may contain a single type of fibre, or be a blend of various types. Combining synthetic fibres (which can have high strength, lustre, and fire retardant qualities) with natural fibres (which have good water absorbency and skin comforting qualities) is very common. The most widely used blends are cotton-polyester and wool-acrylic fibre blends. There are various types of spun yarn available based on the manufacturing method and principle of spinning. The figure below shows the structure of almost all types of spun yarn technology.
One aspect of structure is the visual appearance, created solely by the peripheral layer of the yarn, and a second aspect is the internal and external make-up. Yarn structures are very variable. The differences are partly deliberately caused, depending on the intended use of the yarn, but for the most part they are predetermined by the means available.
Among the different yarns made from different spinning technologies, maximum wickability is found in conventional ring spun warp yarns as compared to compact and hosiery yarns. Yarn construction features such as twist, diameter, crimp and fibre denier are related to rate of water transport in fibre assemblies. Different fiber has different wicking capacity. Water transport in yarns is slightly influenced by wetting properties of individual fibre materials and depends mainly on wetting behaviour of whole yarn. The increase in yarn roughness due to random arrangement of its fibres gives rise to a decrease in rate of water transport. Yarns with more twist exhibited a reduced wicking but they maintain the trend of sudden increase in wicking performance at higher twist levels due to spiral wicking. Pore size, shape and orientation affects the penetration of liquid into the yarn structure. Increase in yarn roughness due to random arrangement of its fibres gives rise to a decrease in rate of water transport. Overall, yarn twist, fiber type and yarn count are the three main primary factors that effects the wicking performance of the spun yarns. For the good wicking to take place, the fiber has to wet first. Therefore the fibers (natural fibres) with higher moisture regain show better wicking performances. This is the reason why Cotton is the most preferred fiber in fabricating efficient moisture transport textile materials among all the commercially available natural fibres.
4.2 Wicking in Filament yarns
Almost all synthetic filaments are cylindrical in shape, with smooth surfaces, unpleasant and glassy sheen and are limited to low hygroscopicity. Synthetic yarns are either made by twisting certain number of filaments or by just holding them in the form of bundle.
Number of filaments, yarn tension and twist in synthetic yarns again has a significant effect on its wicking performance. The pore size, shape and orientation of the filaments effects penetration of liquid into yarn structure. A mathematical model to predict the wicking behaviour of yarn was developed based on the macroscopic force balance method which considered different parameters like fiber denier, yarn denier, fiber cross sectional shape, number of filaments in the yarn and yarn twist. The yarn having same linear density but made of more number of fibers leads to higher wicking rate and also with the increase in fibre shape factor the wickability of the yarn increases.
4.3 Wicking in Texturized yarns
These yarns are made by a process, which combines multiple filament yarns into a yarn with some of the characteristics of spun yarns. In textured yarns, False- twist textured and Air-Jet textured yarns are most commonly used for many applications.
Air-Jet texturisation: with this method overfeeding of filaments at high speed into a chamber is also employed, but instead of using heat to facilitate the texture profile, compressed air is blown into the chamber and this causes the loose lengths of the filaments in the yarn to spread apart and form entangled random loops. The entanglement retains the texture of random loops
False Twist texturisation: in this method the CF yarns are twisted and heated simultaneously, and then untwisted when cold, thereby loosely retaining the heat-set helical shape of the Twist.
Figure 5: Air-jet textured filament yarn
Figure 6: False twist textured filament yarn
Wickability of filament yarns increases after texturising. During texturing process, the inter filament spaces created in the core of textured yarns improves its bulkiness. This aids to better capillary penetration of the liquid in the inter filament spaces as compared to the feed yarns. It is not only about the interfilament spaces, but there is also a significant difference in the physical bulk and loop frequency of textured yarns that matters for the wickability of the fibers. Higher physical bulk causes more interfilament spaces for capillary action and more surface loops create rough surfaces. The rough surface created by the loops reduces the capillary action of the liquid. Therefore the textured yarn with hydrophobic nature, higher physical bulk and less surface loops shows good wicking properties. Physical bulk, loop frequency and moisture regain are the 3 main factors that decides the wickability of a particular textured yarn.
Comparing between the 3 familiar synthitic yarns i.e polyester, nylon and viscose, Polyester has a very good wicking property after texturisation. Nylon and viscose though having better regain than polyester show less wickability, since they undergo swelling in wet condition due to moisture absorption causing reduction in inter filament spaces available for capillary penetration. But in case of polyester, it is hydrophobic in nature which does not swell and shows good wickability. However, the wicking rate of PET decreases after belnding it with viscose/nylon due to decrease in physical bulk and increased loop frequency (roughness).
Therefore, surface roughness and the bulk properties of the filament yarn along with liquid properties (i.e. viscosity, surface tension, volatility and stability) play a significant role during wicking. And hence bulk yarns are most preferably used in designing clothing for improved moisture management.
5. Wicking of liquid in fabrics
Liquid transportation and drying rate as two vital factors affecting physiological comfort of sports garments. Sorption is an important performance property of apparel fabrics. Although liquid transport studies have been carried out on different fabrics, the influence of fabric structural features has not been fully explored. Liquid transport studies consider textile assemblies as single capillaries, even though these materials consist of capillaries that vary in diameter and length and are interconnected in a complex manner. The phenomenon of liquid migration during sorption from one yarn to another yarn and back to the first yarn is often overlooked despite the fact that it is an important part of the sorption process in fabrics 
Figure 7: Different fabric structures
Wicking is because of forces of capillarity, originating from the surface and interfacial tensions at the phase interfaces of the system. In any system where capillarity is causing relative motion between solid and a liquid, the shape of the solid is an important factor governing the rate and direction of liquid flow. Thus, the complex contours formed by fibres in yarns and yarns in fabrics constitute the boundaries of channels along which the liquid flows. The construction of textile fabrics provides three distinct capillary system one in warp direction in the plane of the fabric, second in filling direction in the plane of the fabric and third extending very short distances through the thickness of the fabric. When a liquid enters a textile, it enters all three capillary systems and the rate of liquid migration depends on relative geometrical characteristics. Yarn twist, plying of yarn, connecting weave of the fabric affects wicking behaviour. Fabrics without connecting weave have smallest wicking coefficient, when connecting crossing is increased wicking coefficient increases. The yarn intersection in the fabric structure acts as a new reservoir which feeds the branches equally and guides the liquid migration. The migration of liquid in the inter-yarn spaces proceeds as per the law of capillarity from larger spaces to smaller spaces.
The thickness of the fabric also plays an important role because amount of water wicked from one layer to another depends on pore sizes and their corresponding volumes which is a dependent factor of yarn liner density (yarn diameter). It has been observed eventually that, moisture evaporation and drying time in case of filament yarns than spun yarn (natural fibers). Therefore filament yarns are most preferred to carry out the research in exploring their possibilities of being used effectively for developing a good moisture management system.
It is obvious that the wicking rate will be more in case of textured yarn fabrics as compared to flat yarn fabrics. The increase in the bulkiness of the yarn provides fine capillary for the transport of water. With further increase in bulkiness, the yarns in the fabric may come closer than the minimum distance required for capillary action to take place which actually ceases the wicking action in them. In case of woven fabrics especially plain weave, the point of intersection of yarns in the fabric acts as new reservoir for the transport of water. As, the textured yarns have higher bulkiness these reservoirs may be larger and thus will hold large amount of water. Thus, these larger reservoir will also be leading to a good wicking property of that particular fabric. Also, it was found that textured yarn fabrics had considerably higher air permeabilitywhich may also be one of the reasons for higher wicking heights in textured yarn-fabrics. In comparison with woven and knitted fabrics, the wickability is better in knitted fabrics than wove fabrics. wickability increases with structural cell stitch length. Knitted fabrics have higher structural cell (loop) stitch length as compared to single pique, double pique, honeycomb or plain woven fabrics and hence they show better performance of wickability and absorption of water when drop is placed on the fabric.
Surface roughness and the bulk properties of the fiber along with liquid properties (i.e. viscosity, surface tension, volatility and stability) play a significant role during wicking. Physical bulk is an important characteristic of textured yarns and is effected by yarn surface characteristics, fibre properties and dimensions and frequency of the loop protruding out of the core. The wicking rate of the filament increases after texturing which adds up to its moisture transport properties.
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