Fibers shrink resistance

Dimensional Stability. The wet heat resistance of PVA fiber is indicated by the wet softening temperature (WTS) at which the fiber shrinks to a specified ratio. At one time, the WTS was not more than 95°C for nonacetalized PVA fiber, but improvement of WTS has been achieved by improvement in heat-drawing and -treating techniques other methods proposed include suppression of polymerization temperature of vinyl acetate (36) and employment of alkafi spinning (37). 

Textile Finishing. Polyethyleneimine-A/-methylolurea derivatives improve the crease and wear resistance of cotton (429,430). The adhesion between individual wool fibers is improved by pretreatment with amines, which leads to improved shrink resistance (431). An antimicrobial finish can be appHed to cotton by using a combination of PEI and ureas to bind zinc pyrithione to the fabric (432). After wool has been provided with a flameproof finish using fluorozirconate or fluorotitanate, the wool can be neutralized with PEI (433). Conventional neutralizing agents caimot be used for this purpose since they impair the flameproof characteristics of the impregnated fabric. 

Fig. 8. Electron micrograph of Merino wool fibers in a fabric that have been treated with a typical shrink-resistance polymer, showing fiber—fiber bond...

Urea—formaldehyde and melamine—formaldehyde reagents are resin formers, which not only cross-link cotton but also copolymerize with themselves. These have been used both as simple cross-linkers or prepolymer systems. If too much of the polymerization is concentrated on the fiber surface, the fabric may be sufficiendy stiffer that it takes on a boardy character. As such, the finisher must control the action of agent to give the desired crisp hand but prevent the development of boardiness. Melamines have been recommended for applications when complete shrink resistance is required. However, both finishes were rejected for the white-shirt market because of loss of strength when hypochlorite bleach is used on account of vulnerable NH groups and the ensuing discoloration (37). 

Properties of fibers can be altered by carrying out interfacial polymerizations on their surfaces. Thus the shrink resistance of wool can be improved by immersing the fiber first in a solution containing one component of a condensation polymer and then immersing it in another solution containing theother component. Polyamides, polyurethanes, polyureas, and other polymers and copolymers may be grafted on wool in this manner. 

Figure 1.27. A heat-shrinkable tubing package for protecting spliced fibers. Shrinking is accomplished by means of an internally heated resistance rod. (Reproduced with permission from reference 40.)...

Interfacial and solution polycondensations are commercially important. For example, an unstirred interfacial poly condensation reaction is utilized in the production of polyamide fibers. Another important application of interfacial polycondensation is the enhancement of shrink resistance of wool. The wool is immersed first in a solution containing one of the reactants and subsequently in another solution containing the other reactant. The polymer resulting from the interfacial reaction coats the wool and improves its surface properties. 

Nytril fiber n. A manufactured fiber containing at least 85% by weight of a long chain polymer of vinylidene dinitrile [-CH2-C (CN)2-] and having the vinylidene dinitrile group in no less than every other unit in the polymer chain (FTC definition). Nytril fibers have a low softening point so they are most commonly used in articles that do not require pressing such as sweaters and pile fabrics. They are also blended with wool to improve shrink resistance and shape retention. 

Industrial Shrink-Resist Treatments. Mechanistically, shrink-resist treatments can be divided into degradative and additive types. Degradative treatments use an oxidizing agent to eliminate or reduce the differential friction effect of the fiber simface by modifying or removing the scales. In some cases, a polymer is also applied to the surface to mask the modified scales. The additive approach... 

Additive Shrink-Resist Treatments. The principal additive shrink-resist treatment for wool fabrics uses the polymer Synthappret BAP (Bayer AG). This is a poly(propylene oxide) polyurethane, containing reactive carbamoyl sulfonate (bisulfite adducts of isocyanate) groups, ie —NHCOSOa" Na" ". An aqueous solution of this polsrmer is padded onto woven fabrics. The polymer cross-links on drying to form flexible linkages between fibers and yams (101). Other polymers may be applied at the same time to modify the handle. 

From 1910 onward waste filament yam had been chopped into short lengths suitable for use on the machinery designed to process cotton and wool staples into spun yams. In the 1930s new plants were built specifically to supply the staple fiber markets. During World War II the production of staple matched that of filament, and by 1950, staple viscose was the most important product. The new spun-yam oudets spawned a series of viscose developments aimed at matching the characteristics of wool and cotton more closely. Viscose rayon was, after all, silk-like. Compared with wool it lacked bulk, residence, and abrasion resistance. Compared to cotton, it was weaker, tended to shrink and crease more easily, and had a rather lean, limp hand. 

The second substrate utilized was a high loft polyester, representative of material used in air filters, furniture padding, and automobile interior pads. The material used here is constructed of 1 1/2 denier fiber, weighs 1 1/2 ounces per square yard, and is relatively open in construction. Polyester as a material is water resistant and flammable. This fiber ignites at around 520°C, but melts at about 265°C. In many flame retardant tests, the polyester will melt and shrink" away from the flame, giving spurrious results. Once again, not all binders have an affinity for this substrate, and flame retardancy is affected. 

Cellulose acetate fibers are defined by the Textile Fiber Products Identification Act as manufactured fibers in which the fiber-forming substance is cellulose acetate in which not less than 92% of the hydroxyl groups are acetylated. Cellulose triacetate is employed to make many tricot fabrics and sportswear. Cellulose triacetate textile is shrink- and wrinkle-resistant and easily washed. 

---