A textile property of significant consumer interest is the ability of fabrics to maintain their dimensions, whether the textile is used in apparel or home furnishings. Shrinkage is a reduction in fabric length or width; growth is a term used for an increase in a fabric dimension. A fabric that neither shrinks nor grows is said to be dimension-ally stable. When one considers that a garment with a 25-inch waist will decrease by 1% inches if it shrinks only 5 percent, it is understandable that controlling shrinkage has been a goal of textile producers and finishers for many years. Such control has usually involved submitting the fabric to conditions such as moisture or heat that might induce shrinkage, before it is sewn into garments.
Fabrics most susceptible to shrinking during laundering are those that have high moisture absorbance. These include cotton, rayon, linen, and wool. Most synthetics (polyester, nylon, acrylic, and olefin) do not absorb water to a great extent and have higher dimensional stability. Manufacturing of woven or knitted fabrics imposes stresses in the materials as they are stretched and held taut. When the tension is removed and these fabrics are subjected to wetting during laundering, the yarns relax, moving closer together. The amount of relaxation depends on the degree of stretching the fibers underwent during manufacturing and the propensity of the fibers to absorb water and to stretch. Wool and rayon are more extensible than many other fibers and therefore shrink more. Wool has a further problem in that the fibers are covered with scales that can catch and lock together, entangling under conditions of moisture, mechanical action, and heat. This is called felting shrinkage and is the primary reason wool fabrics are not normally washable. These differences in fabrics and fibers point out not only the specification of laundering or dry-cleaning procedures for textiles, but also the need for development of shrinkage-control treatments that are fiber and fabric specific. Compressive shrinkage, the most general control method, is appropriate for 100-percent cotton, linen, or rayon fabrics. In the process, the fabric is dampened and placed on a thick woolen or felt blanket that travels around a small roller. The wet fabric is stretched as it moves around the contour of the roller and then compresses and squeezes as it enters a straight area. Heat is applied to set in the compressed structure. Sanforized and Sanfor-set are trade names for this type of shrinkage-control treatment.
Wool fabrics require special treatments to inhibit both relaxation and felting shrinkage. The former can be controlled by dampening or steaming the fabric and allowing it to dry in a relaxed state. In one variant of this, decating, the fabric is wound on a perforated cylinder and steam is injected. Cold air then sets the relaxed fabric structure. Preventing felting shrinkage allows wools to be laundered. These treatments involve altering the scales on the wool fibers so they do not catch on each other and become highly entangled. One such process degrades the outer scale layer, making the fibers smoother. A preferred method, which runs less risk of degrading the fiber itself, is to mask the scales by applying a thin polymer film.
Synthetic fibers, which are sensitive to heat, can shrink when heat is applied. This phenomenon is apparent when synthetic-fiber fabrics are ironed at too high a temperature. The tendency for heat shrinking can be controlled in finishing by heat setting the fabric at a temperature that allows the molecules in the fibers to relax somewhat. They are therefore less likely to relax further and shrink during the use and care of the textile product.
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