Zinc cellulose xanthate

Polyoxyalkylene derivatives are typical of the oxygen-containing viscose modifiers. In this type of modifier, association involving the ether oxygen of the polyoxyalkylene chain is believed to be part of the mechanism by which acid diffusion and cellulose regeneration are retarded. Two possibilities exist, (I) protonation of the ether oxygen [200] retards hydrogen ions from penetration into the filament and facilitates formation of zinc cellulose xanthate and (II) formation of chelate compounds with zinc and zinc salts that increase the stability of the semipermeable membrane [201]. 

Zinc salts are added to the spin bath to slow down the regeneration reaction by forming a less easily decomposed zinc cellulose xanthate intermediate. This allows greater stretch levels to be applied and results in fibers with thicker skins. There is still imcertainty as to whether the zinc cellulose xanthate gel acts by hindering acid ingress or water loss. High levels of zinc in the spin bath allow the production of tough fibers for tire reinforcement and industrial use. 

The next significant strength improvement followed the 1950 Du Pont (19) discovery of monoamine and quaternary ammonium modifiers, which, when added to the viscose, prolonged the life of the zinc cellulose xanthate gel, and enabled even higher stretch levels to be used. Modifiers have proliferated since they were first patented and the list now includes many poly(aIkylene oxide) derivatives (20), polyhydroxypolyamines (21-23), and dithiocarbamates (24). 

Nitrogen-containing modifiers such as DMA, discussed by Deshmukh [196], are generally ineffective without zinc in the spin-bath. It is known that DMA reacts with carbon disulfide in viscose to form dimethyldithiocarbamate (Equation 10.16), which is an effective agent in modifying viscose. The mechanism by which cellulose xanthate decomposition is retarded is believed to involve association of the thiocarbamate with the xanthate group by a bridging zinc atom (Equation 10.17). 

In wet spinning, the polymer solution is spun into a coagulant bath. An example is a 7% aqueous solution of sodium cellulose xanthate (viscose), which is spun into a dilute sulfuric acid bath, also containing sodium sulfate and zinc sulfate (160). The zinc ions form temporary ionic cross-links between the xanthate groups, holding the chains together while the sulfuric add, in turn. 

Spinning Viscous rayon filaments are spun by wet spinning process. The viscous fluid is delivered to the spinneret, which is immersed in a spin bath containing sulfuric acid, sodium sulfate and zinc sulfate. As the rayon filaments rapidly coagulate they are stretched, and cellulose xanthate decomposes to regenerated cellulose. 

This includes reactions of the polymer groups with metallic sites on the particle surface that may result in the formation of stable or insoluble compounds through covalent, ionic or coordination bonding. Carboxyl flocculants such as polyacrylic acid and carboxyl-methyl cellulose can chemisorb on the surface of calcite and sphalerite which have calcium or zinc sites on them. Certain flocculants, such as cellulose and starch with xanthate and polyacrylamide with dithiocarbamate with high chemically active groups, have been found to exhibit selective reaction with sulfide minerals. Such complexing polymers have been investigated for their use in selective flocculation processes. 

Again, irrespective of the hardware the chemistry is consistent. The partially regenerated fiber from the spinning machine is contaminated with sulfuric acid, zinc sulfate, sodium sulfate, carbon disulfide, and the numerous incompletely decomposed by-products of the xanthation reactions. The washing and drying systems must yield a pure cellulose fiber, suitably lubricated for the end use, and dried to a moisture level of aroimd 10%. 

V-Ethylpyridinium chloride/DMF (1 /I), A-ethylpyridinium chloride/pyridine (1 /I), trifluoroacetic acid, aqu. solutions of alkalies (ice-cold), beryllium perchlorate, cdcium thiocyanate, cupriammonium hydroxide, cupriethylenediamine, sodium xanthate, tetramethylammonium hydroxide, zinc chloride/hydrochloric acid (cold), zinc chloride (hot), cone, phosphoric acid, cone, sulfuric acid. See also table on cellulose in this Handbook... 

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