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.
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.
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