Viscose

VISCOSE

Synonyms

Rayon, regenerated cellulose.

Source

Viscose is a viscous orange-red aqueous solution of sodium cellulose xanthogenate obtained by dissolving wood pulp cellulose in sodium hydroxide solution and treating with carbon disulphide.

Preparation

The starting material is cellulose prepared from coniferous wood (spruce), or scoured and bleached cotton linters. The wood is delignified similar to cellulose wadding. It reaches the rayon manufacturers as boards of white pulp, containing 80–90% of cellulose and some hemicellulose (mainly pentosans). The hemicellulose being alkali-soluble, are removed in the first stage of the process by steeping in sodium hydroxide solution. The excess alkaline liquor is pressed out and alkali-cellulose (sodium cellulosate) remains. This is dissolved by treatment with carbon disulphide and sodium hydroxide solution to give a viscous solution of sodium cellulose xanthate. After ‘ripening’ and filtering, the solution is forced through a spinneret, a jet with fine nozzles, immersed in a bath of dilute sulphuric acid and sodium sulphate, when the cellulose is regenerated as continuous filaments. These are drawn together as a yarn, which is twisted for strength, desulphurized by removing free sulphur with sodium sulphide, bleached, washed, dried and conditioned to a moisture content of 10%.

Description

The rayon is a white, highly lustrous fibre. Its tensile strength varies from two-third to one-and-a-half times that of cotton. When wetted, it loses about 60% of its tensile strength. It has a proportionately greater loss than is found with cotton. The fabric is a water-repellent (e.g. cotton crepe bandage).

Chemistry

Viscose rayon is a very pure form of cellulose. Its ash contains sulphur. The cellulose molecules of the original natural material are more separated from one another in the viscose solution than in the vegetable material and in the regenerated fibres is still less closely packed. The side-to-side aggregation of the long-chain molecules is different from that in natural celluloses. The size of the molecules is also reduced. Wood cellulose has molecules of the order of 9,000 glucose residue units, whereas those of viscose rayon have only about 450.

Chemical Tests

1.     The fibres give the general tests for vegetable and regenerated carbohydrate fibres.

2.     On ignition they behave like cotton; distinction from acetate rayon and alginate yarn, wool, silk, nylon, and glass.

3.     With N/50 iodine and sulphuric acid, 80%, they give a blue colour similar to that given by cotton; distinction from acetate rayon, alginate yarn, jute, hemp, wool, silk, and nylon.

4.     With ammoniacal copper oxide they behave like absorbent cotton; distinction from acetate rayon, jute, wool, and nylon.

5.     Cold sulphuric acid, 60% w/w, dissolves the fibre; distinction from cotton, oxidized cellulose, alginate yarn, flax, jute, hemp, and wool.

6.     Warm (40°C) hydrochloric acid does not dissolve the fibre; distinction from acetate rayon, silk, and nylon.

7.     It is insoluble in boiling potassium hydroxide solution (5%); distinction from oxidized cellulose, wool, and silk.

8.     Shirla stain A produces a bright pink; distinction from cotton, oxidized cellulose, acetate, rayon, wool, silk, and nylon.

9.     Phloroglucinol and hydrochloric acid produce no red stain; distinction from jute, hemp, and kapok.

10. The fibres, like cotton, are insoluble in acetone, formic acid 90% or phenol 90%; distinction from acetate rayon and nylon.

Uses

Viscose rayon is used to manufacture fabrics, surgical dressings, absorbent wool, enzyme, and cellophane.