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Rayon and acetate fibres are not fibrillar in structure because the natural cellulose morphology is eliminated during the manufacturing process of these regenerated fibres. The regenerated fibres are much weaker and more extensible than cotton, reflecting principally the ower degree of structural regularity and perfection and also lower degree of polymerisation (molecular weight) of the cellulose chains.
Rayons are the first man-made fibres. Several types of rayons are being produced currently. Rayons are regenerated cellulose fibres. Viscose and cuprammonium produced by dissolving alkali-cellulose (obtained by reaction of wood pulp or cotton waste with caustic soda) in carbon disulphide and cuprammonium hydroxide respectively., passing the solution through spinnerets and then solidified by precipitation in mineral acid solution.
Viscose is normally dope dyed or mass coloured by adding pigments to the viscose solution before spinning, This is preferred rather than conventional dyeing due to better uniformity of shades, better fastness, lower coloration costs etc. However, the shade range is greatly limited as compared to conventional dyeing processes. Delustrants like titanium dioxide, fire-resistant chemicals etc. can also be incorporated to improve the appearance and properties of the viscose yarn.
In the regenerated fibres prepared by the conventional method, the cellulose molecules are not very highly oriented and the proportion of the amorphous region is large. Consequently, the fibre has poor dry strngth and very poor wet strength.
High-wel-modulus rayons or polynosic rayons are produced from high grade cellulose and the precipitation is carried out under milder conditions to prevent degradation of cellulose. Cellulose sodium xanthate is prepared from unripened alkali-cellulose and dissolved in water insted of caustic soda solution. It is then spun from a dilute sulphuric acid solution containing no additives. The thread is stretched three times its original length durring coagulation. It has a microfibrilliar structure not very different from that of cotton. Polunosic fibres have a very high tensile strength, less extension at break and reduced swelling in water.
The majority of viscose rayon has a serrated surface and irregular cross-section due to skin (higher orientation) formation during precipitation. The viscose fibre is long and straight unless the fibre has been crimped. The fibre is much less crystalline (30%) than cotton (70%). High-tenacity rayons are more crystalline.
Since rayons are essentially cellulose, the properties are very similar to those of cotton. The differences in properties are due to differences in the degree of polymerisation, crystallinity and orientation within the fibre. Unlike cotton, the wet strength is less than the dry strength, and the reduction in strength after wetting is less in case of polynosic fibres.
Rayons possess greater luster than cotton and are often delustered by adding suitable pigments before spinning.
The chemical properties are similar to those of cotton. However, these fibres are less chemical-resistance than cotton. Prolonged exposure to sunlight causes degradation and subssequent loss of strength. The fibre is less expensive and hence is used as a substitute of cotton, especially for blending with synthetic fibres. Rayons are resistant to common household solvents and to light and heat except under extreme conditions.
Both carbon disuplhide and cuprammonium hydroxide cause environmental problems, hence attempts have been made to develop an alternate process. As a result, lyocell fibres have been developed which are made by regeneration of cellulose from its solution with organic solvents, e.g. N-methyl morpholine -N-oxide (NMMO) or from cellulose carbamate solution a reaction product of cellulose and urea. NMMO is recovered after spinning. A number of lyocell fibres are available in the market such as Tencel (Courtaulds, USA). Lyocell (Lenzig, Austria), New cell (Akzo-Nobel, Germany) etc. The viscose fibres have a folded or serrated cross-section, homogeneous and skin-core morphology is as in viscose. The crystallinity and orientation are higher than those of viscose. The dyeing behaviour is, in general, like other cellulosic fibres. The dye yeild for selected dyes are higher than that of viscose, but less than that of mercerised cotton. The substantivity towards dyes is similar to viscose.
Rayons are the first man-made fibres. Several types of rayons are being produced currently. Rayons are regenerated cellulose fibres. Viscose and cuprammonium produced by dissolving alkali-cellulose (obtained by reaction of wood pulp or cotton waste with caustic soda) in carbon disulphide and cuprammonium hydroxide respectively., passing the solution through spinnerets and then solidified by precipitation in mineral acid solution.
Viscose is normally dope dyed or mass coloured by adding pigments to the viscose solution before spinning, This is preferred rather than conventional dyeing due to better uniformity of shades, better fastness, lower coloration costs etc. However, the shade range is greatly limited as compared to conventional dyeing processes. Delustrants like titanium dioxide, fire-resistant chemicals etc. can also be incorporated to improve the appearance and properties of the viscose yarn.
In the regenerated fibres prepared by the conventional method, the cellulose molecules are not very highly oriented and the proportion of the amorphous region is large. Consequently, the fibre has poor dry strngth and very poor wet strength.
High-wel-modulus rayons or polynosic rayons are produced from high grade cellulose and the precipitation is carried out under milder conditions to prevent degradation of cellulose. Cellulose sodium xanthate is prepared from unripened alkali-cellulose and dissolved in water insted of caustic soda solution. It is then spun from a dilute sulphuric acid solution containing no additives. The thread is stretched three times its original length durring coagulation. It has a microfibrilliar structure not very different from that of cotton. Polunosic fibres have a very high tensile strength, less extension at break and reduced swelling in water.
The majority of viscose rayon has a serrated surface and irregular cross-section due to skin (higher orientation) formation during precipitation. The viscose fibre is long and straight unless the fibre has been crimped. The fibre is much less crystalline (30%) than cotton (70%). High-tenacity rayons are more crystalline.
Since rayons are essentially cellulose, the properties are very similar to those of cotton. The differences in properties are due to differences in the degree of polymerisation, crystallinity and orientation within the fibre. Unlike cotton, the wet strength is less than the dry strength, and the reduction in strength after wetting is less in case of polynosic fibres.
Rayons possess greater luster than cotton and are often delustered by adding suitable pigments before spinning.
The chemical properties are similar to those of cotton. However, these fibres are less chemical-resistance than cotton. Prolonged exposure to sunlight causes degradation and subssequent loss of strength. The fibre is less expensive and hence is used as a substitute of cotton, especially for blending with synthetic fibres. Rayons are resistant to common household solvents and to light and heat except under extreme conditions.
Both carbon disuplhide and cuprammonium hydroxide cause environmental problems, hence attempts have been made to develop an alternate process. As a result, lyocell fibres have been developed which are made by regeneration of cellulose from its solution with organic solvents, e.g. N-methyl morpholine -N-oxide (NMMO) or from cellulose carbamate solution a reaction product of cellulose and urea. NMMO is recovered after spinning. A number of lyocell fibres are available in the market such as Tencel (Courtaulds, USA). Lyocell (Lenzig, Austria), New cell (Akzo-Nobel, Germany) etc. The viscose fibres have a folded or serrated cross-section, homogeneous and skin-core morphology is as in viscose. The crystallinity and orientation are higher than those of viscose. The dyeing behaviour is, in general, like other cellulosic fibres. The dye yeild for selected dyes are higher than that of viscose, but less than that of mercerised cotton. The substantivity towards dyes is similar to viscose.
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