Thursday, January 10, 2013

Effects of Knit Structure on Knitted Fabric Performance

Knit fabrics are most comfortable, durable and make most flexible effect on human body. I am going to describe the effect of knitted structure on knit fabric performance in details. 


Durability Factors 

Strength of knitted fabrics is considered to be less important for durability than it is in woven goods. Knitted fabrics are easily stretched to accommodate changes of shape as a result of stresses imposed in wear and care. When knits are made from resilient fibers and yarns, the ability to stretch and recover from stretching will be enhanced. In comparison to knitted fabrics, woven fabrics are generally firmer and have less elongation. In knits the loops in the structure can be deformed horizontally or vertically, increasing the stretch in both directions. But, as noted earlier, variations in knitting techniques can increase or decrease extensibility of knitted fabrics. Double knit fabrics and warp knit fabrics are usually less extensible than single knit fabrics.A major problem in the durability of knits is the runs that can develop in weft knits when one of the loops is broken. If stronger fibers and yarns are used in these knits, they will be less likely to run. 


Appearance 
Shear ability, discussed in chapter 15, is an important factor in the appearance of textiles. In general, because of the ways in which yarns are combined, woven fabrics have higher shearability than do knitted fabrics. Knitted fabrics have good flexibility and are easily extended. Warp knits do not shear as easily as weft knits. 

These qualities must be taken into account by designers, although they may not be aware of the technical terminology used to describe the fabric properties. For example, a warp knit tricot fabric with low shearability would not be used to make a bias-cut garment in which the intention is to take advantage of high shearability. Instead, the designer would be likely to use a soft, gathered construction where the high flexibility of the fabric would work to good advantage, but where most of the draping would fall in the vertical direction of the fabric. In general, knits wrinkle less than do other fabrics. This is because loosely constructed fabrics generally allow more fiber redistribution and motion. However, knitted fabrics, because of their greater extensibility, are more likely to lose their shape in laundering. The stresses applied in knitting distort the shapes of the loops rather than just stretching the yarns as in weaving. On relaxation, the loops broaden, shrinking the fabric length and increasing the width. As with woven fabrics, the tighter the structure, the higher the shrinkage until the structure becomes so tight that further shrinkage is not possible. Such fabrics may, however, buckle. 

Knitted fabrics tend to have lower cover than do woven fabrics, with weft knits having substantially more porous structures than warp knits, unless the fabric is deliberately made to have an open, lacy construction as in some Raschel knits. Use of thinner or thicker yarns can increase the cover of knitted fabrics. 


Comfort Factors
Knits usually entrap more air than woven fabrics, although the tightness of the knit is a factor as well. Pile or napped knit constructions are especially good for cold weather because the yarns or fibers perpendicular to the surface provide numerous spaces for dead air. This effect is maximized if such fabrics are worn with the napped or pile surface next to the body, or if they are covered with another layer. The flexibility of knits contributes to a feeling of fabric softness. Fibers and yarns used can enhance or detract from the smoothness of knitted fabrics. 


Elongation and Recovery 
The majority of bathing suits for women, and many for men, are tight fitting, with stretch required for getting the garments on and off. Since knits have much higher elongation than woven or nonwoven fabrics, they are the preferred fabric construction for bathing suits that fit tightly to the body. Fibers too play an important role in achieving tightness of fit. Those with lower modulus and high resilience will enhance the stretch and recovery properties of knits. Not only elastomeric fibers but also nylon have low modulus and good recovery. Nylon and spandex are often chosen for swimwear because of these properties. Polyester is not usually used because it has a higher modulus and therefore does not stretch as easily. Knitted fabrics of nylon blended with elastomeric fibers, such as spandex or rubber, will provide the highest amount of stretch. The elastic fibers also have high recovery from stretch. 


Colorfastness 
Bathing suits are exposed to sunshine, chlorine and other chemicals in swimming pools, and salt water in oceans. Fibers and yarns are both important in maintaining the colorfastness of swimming suits that are often made of bright-colored fabrics. Light-fastness of dyed spandex is fair to good, while that of nylon dyed with acid dyes is good. Spandex normally makes up only a small percentage of the fiber content of swimwear fabrics and is often used in core-spun yarns wrapped with nylon. This helps to protect ir from sunlight. Colorfastness of nylon and spandex exposed to pool and salt water is fairly good but these fabrics will show some loss over time. 


Absorbency 
Since bathing suits are worn while swimming in water, the absorbency of the fabric is a consideration. If a material absorbs and retains a significant amount of water, the weight of the suit will increase, affecting its comfort and function. Synthetic fibers have low water regain and are more appropriate for bathing units than the natural fibers that were used many years ago. 


Environmental Resistance 
Light, chlorine, and salt water can also degrade textile fibers. When this occurs, bathing suit fabrics may lose some of their recovery from stretch. Nylon and polyester are more resistant to ultraviolet light and chlorine than spandex. That is another reason why spandex is usually used in small percentages blended with nylon. The use of rubber as an elastomeric fiber in swimwear has decreased because of its susceptibility to degradation by light and other environmental conditions. Because resistance of nylon to degradation by light is higher for fibers that have not been delustered, brighter nylons are usually used in swimwear.
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