Rayon solvent systems

Different. solubility behaviors were observed in organic solvent systems. Regenerated cellulose did not dissolve in the SOj-amine-DMSO system which, however, readily dissolved the native and mercerized samples [15-17]. Similarly, rayon and mercerized cellulose, unlike native cellulose, were insoluble in the dimethylformamide (DMS)-chloral-pyridine solvent system 114]. Thus, the morphology of the amorphous component in addition to its content plays a significant role in the overall reactivity of cellulose. 

Table 1 summarizes some representative solvents or solvent systems for cellulose. The xanthate system has been used for a longtime to make viscose rayon and sponge. However, rayon production, where H2S is released to varying amounts during the recovery system of spent solvent, is shrinking because of environmental issues. On the other hand, cellulose is soluble in aqueous NaOH alone under limited conditions. Microcrystalline cellulose with DP 200—300 is soluble in 6—9% NaOH by freezing and defrosting procedures. Pretreatments of cellulose such as steam explosion are necessary for complete dissolution in the aqueous NaOH for normal bleached wood pulp and cotton linters with DP of more than 500. ... 

Complementary to the traditional xanthate system used to make viscose rayon, films, and sponges, or the use of aqueous NaOH to dissolve cellulose under limited conditions, many nonaqueous solvent systems for cellulose have subsequently been developed. Apart from trifluoroacetic acid, which is the only volatile solvent known for cellulose, the others consist of reagents that react with the hydroxyl groups of cellulose in a polar aprotic solvent, such as dimethyl sulfoxide or A,A-dimethylacet-amide (DMAC). 

Manufactured cellulose fibers are the first man-made fibers ever produced starting with Rayon fibers and the viscose process invented in late 1800s. Currently, manufactured cellulose fibers are produced by either derivative methods or direct methods. In derivative methods, cellulose polymer chains are ehemically modified to form cellulose derivatives, which then are dissolved and spun into fibers. In the direct methods, special solvent systems are used without the chemical modification of the cellulose chains. 

I he simplest is the partition of a solute between two immiscible solvents. In this case [0] /[Z)], = K, where K is the partition coefficient. This equilibrium is often referred to as the Nernst distribution. When [Z)], is plotted against [Z)], at constant temperature the curve is a straight line which terminates at the point when both the fibre and the dyebath are saturated. There are slight deviations from the linearity of the curve, particularly as the solutions become more concentrated. This system is probably exhibited in its ideal form when dyeing cellulose acetate rayon from an alcoholic dye solution, but it is also essentially true in the case of the application of disperse dyes in aqueous suspension to cellulose acetate, because the dyes are all soluble in water to a very limited extent and the undissolved particles act as a reservoir to maintain the concentration of the solution. The curve for this isotherm is shown in Fig. 12.14. 

In 1937, Schweizer [91] discovered that cellulosic fibers such as cotton and hemp readily dissolve in copper hydroxide and ammonium hydroxide solutions. His system is recognized as the Schweizer reagent. The Bemberg Rayon Industry later utilized this solvent for the industrial production of cuprammonium fibers (or cuprammonium rayon) and developed the Bemberg process or cuprammonium process [92]. Kamide and Nishiyama [93] have recently published an excellent review on the history and science of cuprammonium technology. 

Wet-spun fibers are formed by extruding a highly viscous polymer solution through a spinneret into an appropriate liquid bath, where it is solidified. The solidification is brought about by a diffusional interchange between the extruded polymer filaments and the bath. In this process, called coagulation, one or more components from the bath diffuse into the fiber while, in turn, solvent leaves the forming filaments. The net result is a solid fiber. In some fiber systems, such as viscose rayon, there is also a chemical reaction superimposed on the diffusional process [60-62]. In others, the situation is strictly diffusional. Only the diffusional process will be considered in this treatment. 

WEL 8-hour TWA 10 ppm. Carbon disulphide is an inorganic solvent used mainly in the manufacture of viscose rayon fibres. It is absorbed through the Ixmgs and skin and is a multi-system poison affecting the brain, peripheral nerves and the heart. Monitoring is of urinary metabolites. 

The operations used to either wet- or dry-spin acrylics are essentially the same as those already described for rayon and acetate, respectively. The polymer must be completely dissolved in solvent and the solution filtered to remove any impurities that would cause spinnerette blockage. Because acrylic polymers are not soluble in common nonpolar solvents, polar substances such as dimethylfonnamide, dimethylacetamide, or aqueous solutions of inorganic salts such as zinc chloride or sodium thiocyanate are required. Only wet spinning is possible with the latter. Dimethyl formamide boils at 152.8°C and exerts a vapor pressure of 3.7 mm of Hg at 25°C compared with acetone (used in dry spinning of cellulose acetate), which has a vapor pressure of 228.2 mm of Hg at 25 C. It follows that, unlike acetone which requires an activated-carbon system for recovery, dimethylformamide may be condensed directly from the gas stream... 

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