Cellulose xanthic acid

Uses and Physiological Properties of Carbon Disulphide.—Besides its employment as a solvent (see p. 260), carbon disulphide is used extensively in the manufacture of viscose silk. Viscose is a solution of the sodium salt of the cellulose ester of thiolthioncarbonic acid (p. 268) in water or dilute aqueous sodium hydroxide, or it may be described as an aqueous solution of the sodium salt of cellulose xanthic acid. For its production cellulose is steeped in concentrated sodium hydroxide solution and then pressed, the product being called alkali-cellulose and the formula CeH10O5.NaOH assigned to it. This is converted into viscose by treatment with carbon disulphide, when the colour changes to golden yellow ... 

Viscose was discovered by Cross and Sevan in 1892 during a programme of research on the general properties of cellulose. It is the sodium salt of cellulose xanthic acid. The acid, which does not exist in the free state, is ethyl hydrogen dithiocarbonate (1)... 

Cellulose xanthic acid therefore has the structure (2) and the sodium salt is represented by (3)... 

In the manufacture of rayon the sodium salt of cellulose xanthic acid is obtained by the action of carbon disulphide on alkali cellulose... 

Salts of the series of xanthic acids of the general formula ROCSSH. Certain xanthates such as ZIX are ultra accelerators for mbber. Cellulose xanthate is the intermediate product in the manufacture of viscose. See Viscose Rayon. Xanthogen Sulphide... 

Xanthx Acid.—Another solvent of cellulose is xanthlc acid, also called xanthonic or xanthogenic acid. Xanthic acid is the ethyl ether of -di-thio-carbonic acid. Its formula is HS-CS-OC2H5. When heated with water to 500° under pressure cellulose is dissolved and undergoes decomposition. 

Viscose Silk.—III. From Cellulose Xanthate. We have referred to the solvent action of xanthic acid, which is the ethyl ether of di-thio-carbonic acid, viz., HS-CS-OC2H5. When sodium cellulose is dissolved in xanthic acid the cellulose is in the form of sodium cellulose xanthate. A solution properly prepared by treating cellulose with sodimn hydroxide and carbon di-sulphide in the presence of benzene or carbon tetra-chloride, in which polymerization of the cellulose compound is effected, is decomposed by forcing capillary streams of the solution into a solution of ammonium sulphate. The cellulose is thus obtained as in the other processes in the form of fine filaments of a hydrated cellulose possessing silk-like properties. Artificial silk of this type is known as viscose silk and is made in large quantities. In 1914 about 20,000,000 pounds of artificial silk were made, of which about 3,000,000 pounds were made in the United States. Most of this product was viscose silk. 

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. 

Coagulation of xanthate is performed in 10% aqueous sulphuric acid. The xanthic acid derivative that is formed is not stable and is decomposed, and cellulose is regenerated. Viscose rayon fibres and cellophane films have been produced by this method. 

The influence of acidity on the decomposition of cellulose xanthate is illustrated in Figure 10.59. In strong acid such, as the Lilienfeld-type spin-bath, the xanthic add (Ft) is protonated and stabilized, leading to a slower regeneration of the cellulose. Steric effects associated with the group R also affect the course of the reaction. Thus when R is a bulky l-butyl group, protonation is hindered and t-butyl xanthate decomposes very rapidly, even in strong acid [177]. 

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