HWM rayon

The characteristics and properties of HWM rayon have been described by Goldenberg [281]. The HWM fiber production is increasing worldwide, mainly by conversion of regular rayon production lines but in some cases by new plants or spinning lines built specifically for the purpose. 

The HWM staple fibers have essentially all of the best attributes of regular rayon except for a few important differences. They swell less in water, are somewhat stiffer due to higher cellulose DP (IV) and orientation, and are almost twice as strong and resist dimensional change. Fabrics made from HWM fibers can be dyed and later finished (cross-linked) by much the same techniques as those used for cotton fabrics. In 50-50 blends of polyester and HWM rayon, fabrics can be made that are virtually indistinguishable from some cotton counterparts [282]. 

Process conditions that favor chemical crimp formation are similar to those used for improved tenacity staple (2inc/modifier route). However, spin bath temperature should be as high as possible (ca 60°C) and the spin-bath acid as low as possible (ca 7%). Attempts have been made to overcome some of the leanness of high strength rayons by increasing the crimp levels. ITT Rayonier developed the Prim a crimped HWM fiber (36) and made the process available to their customers. Avtex developed Avdl 111. Neither remain in production. 

Fig. 12.8. Cross-sectional morphologies of some of the rayon fibers, (a) High wet modulus (b) regular rayon (c) crimped HWM (d) hollow (e) cuprammonium (f) trilobal. (Sources All except trilobah Turbak, A., "Rayon" in Encyclopedia of Polymer Science and Engineering, 2nd ed., Vol. 14, p. 55, copyright John Wiley Sons, Inc., New York, 1985 and used with permission of the copyright owner trilobal photo Gupta, B. S. and Hong, C.T., INJ, 7(1), 38 (1995).)...

Viscose and modified viscose are composed of cellulose and like cotton they are polymer of anhydroglucose unit. The significant physical differences between various regenerated cellulose and cotton polymers are listed in Table 1.14. HWM viscose rayon may appear nearly round in cross-section. Viscose polymers are very amorphous and have high moisture absorption capacity of 11 to 16%. Vis-... 

An example of the first type is the use of low-acid, low-salt, and low-temperature spin-baths, which slow down the cellulose regeneration sufficiently to yield HWM polynosic rayon [190]. Another example is the Lilienfeld process for which the viscose, made from unaged alkali cellulose with excess carbon disulfide and only a short ripening, is spun into a cold spin-bath containing 50-85% of sulfuric acid. This is a case of stabilizing the xanthic acid. Rayon produced in this way has tenacities greater than 5 g/den. 

The composition of the viscose is determined by the amount of cellulose xanthate and caustic soda used. For regular rayon, the cellulose content may vary from 8 to 9.5% and the caustic soda from 5 to 6%, depending on process specifications and the efficiency of the equipment at specific facilities. Viscoses required for high-performance rayon, HWM, and tire yarn use a higher DP cellulose and richer (more alkali) compositions with 6.0-7.5% cellulose and 6.0-7.5% NaOH. Generally, from 27.5% to 29.5% CS2, based on the cellulose in... 

The high-performance rayons overcome this disadvantage. The HWM fiber has cotton like mechanical properties and a caustic resistance that allows mercerization. It is compatible in blends with all grades of cotton where it adds strength, improved luster and appearance, and a softer hand. In blends with nylon, polyester, acrylics, and triacetate, it has good strength retention after resination, and the blended fabrics have superior wash-and-wear performance and resistance to pilling. 

FIGURE 10.86 Rayon fibers showing fibrillation on treatment with nitric acid (a) high wet-modulus (b) crimped HWM and (c) polynosic. 

Another important advantage of true HWM fibers is their better resistance to caustic soda. Although garments made of rayon are not normally subjected to exposure to strong caustic solutions, fabric preparation, such as dyeing, bleaching, etc., sometimes requires caustic treatment. In HWM blends with cotton, fabrics are sometimes mercerized or treated... 

While HWM fibers demand and get a premium (they require better purity wood pulp), the added expenses of modifiers and production rates are much lower. As a result, in some instances due to economic pressures, compromises in quality have been made. Unfortunately, some rayon fibers are sold as HWM that have actual wet-moduli somewhat lower than the 0.5 g/den (shown by Szego) to be necessary for dimensional stability. Such fibers also have reduced resistance to caustic soda, as is evident by higher Ss-s values. 

In general, fabrics made from HWM fibers can be prepared, dyed, and finished in the same manner as those made from regular rayon with a few exceptions. For durable-press effects, less resin or cross-linking agents may be used than for regular rayon but slightly more than required for 100% cotton fabrics. 

A number of new cotton like rayons shown in Figure 10.91 was introduced to the textile market during the 1970s. One of these developments in HWM technology was the result of research at ITT Rayonier Research Laboratories in Whippany, New Jersey [284-287]. A chemically crimped fiber called Prima, with an unusual lobed cross section, HWM properties, low 56 5, and more than adequate strength gives more bulk and cover to fabrics. It can be blended with cotton or polyester or in heavier denier with wool. It is also used 100% in the tube-sock hosiery business in direct competition with cotton [288]. 

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