4 Regenerated fibres

B. A. Muralidhar

1.    Introduction

In general, the Man-made fibres can broadly be categories into the following two groups, depending on the origin of the fibre forming substance namely (a) Natural polymer fibres (b) Synthetic fibres.In the natural polymer fibres, the fibre forming material is of natural origin where as in synthetic fibres the fibre forming materials are suitable synthetic polymers.

During the latter part of 19th Century, many attempts were made to use crude cellulose as raw material for a textile fibre. The problem resolved itself into finding a way to dissolving cellulose, and extruding the liquid through tiny holes and hardening it into fibre. Cellulose however, would not dissolve in any suitable liquid. However in 1846 it was discovered that cellulose could be turned into nitrocellulose, when treated with nitric acid, its discovery marked the beginning of first man-made fibre industry. Semi-synthetic polymers are, in fact chemically modified natural polymers where only the side groups are partly, significantly or fully modified by a chemical process. The natural built-in structure of the polymer backbone remains by and large intact, despite minor degree of chain scission.

• Semi synthetic fibres are broadly classified as:
• Regenerated cellulose fibres
• Viscose rayon
• Cuprammonium
• Polynosic
• Cellulose ester fibres
• Protein fibres
• Miscellaneous natural polymer fibres

2. Viscose rayon

The first manufactured cellulosic fibre was invented in France in 1884, however first commercial production was in 1910. In 1892, a new process of making regenerated cellulose fibre was invented, in which the natural cellulose are converted into cellulose xanthate, then dissolved to form a viscose solution, which are extruded through fine jets into an acid coagulating bath, where the cellulose is regenerated in a filament forms called viscose rayon. Rayon was first sold as artificial silk. Some of the common rayon are viscose rayon, acetate rayon and cuprammonium rayon. Some of the natural materials based fibres are casein fibre, alginate fibre, vicara, soyabin fibre. Amongst, the various rayon, viscose rayon is most frequently commercially manufactured rayon and finds wide application in textile, tire cord industry. Viscose rayon, has excellent properties suitable for woven and nonwoven textile applications.

Viscose rayon is produced both in the form of continuous filament and staple fibres, they are available in a wide range of properties such as hollow, shaped and surface modified rayon’s. Various other types of viscose rayon include, high tenacity rayon, high wet modulus (HWM) rayon, flame retardant rayon and super adsorbent rayon. Similarly the chemical modification have resulted in other modified viscose fibres, like the cross-linked, grafted rayon and basified.

2.1 Process Details of Viscose Rayon Manufacturing

Raw materials

Raw materials for the manufacture of viscose rayon are either wood pulp (or) cotton linters. The wood pulp obtained from the eucalyptus, southern pine or other timber are purified by boiling with NaOH or sodium bisulphite solution it is further bleached, washed and converted in to sheets like thick blotting paper.

Production of cellulose:

Viscose rayon is produced from cellulose obtained from wood pulp, cotton lint. The wood pulp sheet is then steeped in 18-20% NaOH solution at 20-22oC for about 1 – 4 hours. During this process the cellulose is converted to alkali cellulose and the lower molecular weight hemi-cellulose are dissolved. Following this pressing is done to remove excess liquid.

Then the alkali cellulose obtained is shred to produce white crumb which are fluffy. These crumbs are then aged in oxygen to bring down the degree of polymerization and to improve viscosity.

Xanthation:

In this process, the aged white crumbs of alkali cellulose are mixed with carbon disulphide (CS2)at about 25-30oC for 2-3 hours in an air tight revolving drum. During this process the white crumbs gradually turns yellow and then orange as sodium cellulose xanthate is formed. This is further mixed with dilute solutions of caustic soda, which leads to the formation of a thick oranges brown solution.

At this stage it is possible to control the lustre of viscose rayon. If rayon is produced directly by this sodium cellulose xanthate, the rayon will have a silk like lustre and sheen. However, often a duller rayon is preferred and this is achieved by adding titanium dioxide to the spinning dope.

Ripening:

In the ripening process the sodium cellulose xanthate solution is allowed to ripen at a carefully controlled temperature with repeated filtration. Initially the viscosity of the solution falls sharply but on further standing the viscosity of the solution begins to rise again. Depending on fibre type the solution is allowed to stand for 5-7 days at 10-20oC,soon the xanthate solution reaches a state suitable for spinning. At this stage, the ripened solution is again filtered and degassed to remove air bubbles or other gases which could interfere in the flow of the spinning solution.

Spinning:

In the spinning process, the filtered viscose solution is forced through spinnerets made of gold, palladium, platinum or other corrosion resisting metals. Each spinneret may contain up to20,000 holes with holes measuring between 0.005 to 0.0125 mm in diameter. Viscose rayon is spun using wet spinning process. Here the degassed, filtered viscose solution is forced through a spinneret, submerged in sulphuric acid bath. As the jet of viscose enters into the coagulation bath instantly regeneration of cellulose and coagulation takes place simultaneously. The coagulation bath containing an aqueous solution of Sulphuric acid (H2So4)10-12parts, sodium sulphate18-20 parts, zinc sulphate 1-2 parts and remaining parts water. Further, processes include rinsing and drying.

Drawing and cutting

In the drawing process the filaments are stretched in successive stages as they pass between successive pairs of godet wheels. Stretching enhances the tenacity of the filament. Further, the continuous filament depending on end use requirements can be cut in to short staple fibres for the manufacture of viscose spun yarn.

Properties

The viscose rayon filaments are smooth and straight without any convolutions as in cotton fibres. Its specific gravity is about 1.50 g/cm3, with a moisture regain of about 13% under standard conditions. The wet strength of viscose rayon is a poor, it loses almost half its strength when wet and swells twice its original thickness. Viscose rayon is attacked by hot dilute and cold concentrated mineral acids, whereas it has high resistance to dilute and strong alkalis. It is not effected by most organic solvents.

1.      Viscose rayon has superb drape, feel and retains its rich brilliant colours.

2.      It has high moisture absorbency and easily dyed.

3.      Viscose rayon developed for apparel has tenacity around 2-2.5 g/dinear.

4.      High tenacity viscose rayon has tenacity around 3.5-6 g/dinear.

5.      Breaking elongation – Dry = 9-10% and Wet = nearly 20%

6.      Wet strength of viscose rayon is poor

7.      Viscose rayon readily absorbs water

8.      There is no static built up, and has moderate resistance to acid and alkali.

9.      Viscose is flammable, it loses strength above 149o C and decomposes at 177-244oC

Applications

Viscose rayon in its variety of forms is used in almost every branch of textile industry. It is used in men’s, women’s and children’s clothing, furnishings, household textiles, carpets and medical textiles.

1.      Used in clothing, apparel, embroidery, chenille. Novelty yarn applications

2.      Used in industrial textiles tyre cords, hoses, conveyor belts and braided cord, agricultural textiles

3.      Used in the production of cellophane, feminine hygiene and

4.      Used in outerwear fabrics, suiting, lining of coats and other blends

3.  Cuprammonium rayon

In 1890, a new method for the manufacture of artificial fibres was invented, where the cellulose could be dissolved in a cuprammonium liquor and spun into fibres. This fibre was called cuprammonium rayon. Cuprammonium rayon also known as “cupro” is a manufactured regenerated cellulose fibre obtained by dissolving cellulose in a solution of copper salts and ammonia.“Cupro”is now used widely to distinguish cuprammonium rayon from other viscose rayon.In this process, cellulose is dissolved in ammoniacal solution of cupric salts and forced through the extruder into a coagulation bath where the regeneration of cellulose into continuous filament form takes place. The process is very similar to that of manufacture of rayon, its raw materials are cotton linters and wood pulp. Chemically cuprammonium rayon are greatly similar to viscose rayon, however largely more expensive than other regenerated cellulose fibres. Cupro is most silk-like of all regenerated yarns, because of its inherent properties such as fineness, smooth surface, lustre and good draping qualities. Its moisture regain is around 12% and specific gravity is about1.54 g/cm3. Cupro is not effected by weak oxidising agents, dilute alkalis or by beaches. However, hot dilute, cold concentrated mineral acids and strong alkalis cause fibres swelling and disintegration. Much of this material is used in the manufacture of dress materials, chiffons, satins, nets, lining fabrics, fine drapery and upholstery fabrics.

4. High wet modulus rayon (Polynosic)

A regenerated cellulose fibre which is categorised by high initial wet modulus with a relatively low degree of swelling in NaOH solution. This type of regenerated cellulose is also called modal fibres, characterized by both high tenacity and high wet modulus. The high wet modulus fibres have the following properties:

1.      Resistance to extension when wet.

2.      Increased wet breaking strength

3.      Resistance to swelling by NaOH

4.      Higher degree of polymerization (Dp) of cellulose and

5.      A micro-fibrillar structure.

Polynosic fibres are typically of circular cross-section. It would swell much less than an ordinary rayon yarn and would with stand mercerization conditions. Polynosic fabrics are strong, dimensionally stable, hard wearing suitable for virtually every field of application where cotton is used e.g. dress materials, drapery, curtains, blends with – cotton, synthetic, wool and flax.

5. Cellulose ester fibres

Unlike in viscose and cuprammonium fibres, where the starting materials – cotton lint and wood pulp is converted back into cellulose which structurally resembles cotton fibres here in cellulose ester fibres the cellulose is changed to render it soluble and after spinning, the filaments are left in the changed chemical form. Cellulose acetate fibre is manufactured fibre in which the (–OH) hydroxyl groups are replaced by acetate groups. The secondary acetate are soluble in relatively cheap solvent – Acetone and offered great prospects of success. As such the cellulose triacetate is partially hydrolysed to produce secondary cellulose acetate I which some of the acetate groups of triacetate are reconverted to hydroxyl group as in cellulose. The starting material for the manufacture of cellulose ester is similar to viscose and cuprammonium rayon i.e either cotton linters or purified wood pulp. The fibres are produced in continuous filament and staple fibre forms and are usually crimped artificially. Its specific gravity is about 1.30 g/cm3, moisture regain under standard conditionsabout 6.5%. Strong alkalis and oxidising agents effects the fibres, whereas weak acids do not affect acetate fibres. Cellulose acetate fibres are extensively used in women’s dresses, bathing suits, lingerie’s, linings, ties, socks, sportswear, pyjamas, blended into suiting, shirting materials, furnishing materials and are also used in electrical industry because of its good electrical insulation properties. Cellulose triacetate is a manufacture fibre in which not less than 92% of the hydroxyl groups are acetylated, this the general description used to differentiate secondary acetate fibres from the triacetate.

6. Protein fibres

Nature makes use of the long chain molecules present in protein material to produce protein fibres such as the silk, wool and other animal fibres. It has long been realised that non-fibrous proteins could be manufactured into fibrous form by denaturing the globular proteins and then dissolving the protein in a suitable solvent and then extruding, the solution through the spinneret. Some of the protein by-products which could be considered for being spun in to fibres are:

• Casein obtained from skimmed milk.
• Zein obtained from maize and
• Arachin obtained from groundnut.

Of the above source, only casein fibre is commercially produced. Casein fibre is produced in the form of tow, top or staple fibres only a small amount of casein fibre is used for 100% casein goods. Most of the casein fibres are blended with cotton, rayon, nylon, wool and other synthetic fibres. The fibre surface is smooth with faint striations, cross-section is bean-shaped to circular. Its specific gravity is around 1.30 g/cm3, moisture regain under standard conditions about 14%. Casein is sensitive to alkalis, are stable to moderate strength acids, and the organic solvents do not damage them. Casein fibres exhibit warmth, resilient, fullness and soft handle. It prolongs the whiteness of fabrics made from these yarns. Casein blends are used in knitted fabrics, felts, carpets, resilient fillings, paddings etc.

7.  Miscellaneous fibres 7.1 Alginate fibres

Alginate fibres are made from a natural polymer – sodium alginate extracted from the brown seaweeds. The first step in the fibre manufacture is to convert the alginic acid present in seaweeds to sodium alginate using sodium carbonate and NaOH. Then sodium alginate is wet spun into a coagulation bath containing hydrochloric acid and calcium chloride, where filaments of calcium alginate are formed. Alginate fibres have a folded appearance, and its cross section is circular to oval. Its specific gravity is about 1.78 g/cm3. Alginate fibres are insoluble in water but their wet strength is poor. Alginate fibres readily dissolve in dilute alkali solution, even in soap water. However, the most valuable property of alginate fibre is their non-flammability.

Alginate fibres are used in hosiery industry, where after knitting the fibres are dissolved to produce light weight fabrics. They are used in wound dressings as sodium/calcium alginate are non-toxic and absorbable. They are also used to plug dental cavities.

7.2 Natural rubber fibre

Natural rubber fibres are manufactured fibre in which the fibre forming substance is a natural rubber. Natural rubber is produced by certain species of rubber trees, particularly Heveabrasiliensis. “Latex” the natural polymer obtained from the rubber plants is elastic in nature, which on heating becomes plastic. This rubber polymer is processed in a powerful mills mixed with sulphur and kneaded to render them more thermoplastic and destroy its elasticity. The raw rubber further mixed with vulcanizing and other agents is passed through calender rolls to produce thin sheet, these thin sheets are then cut into fine filaments before or after vulcanization. Vulcanization is a method of setting the molecular structure in its moulded form, to improve its elasticity. Vulcanized rubber when stretched extends many times its original length and snaps back when the force is removed.

Natural rubber filament are usually round or square in cross-section its elastic recovery under normal circumstances is 100%, natural rubber show a gradual decrease in elasticity with increasing time. The specific gravity of natural rubber is about 0.96 to 1.06. Its resistant to alkali is good, however mineral acids, chlorine and organic solvents causes degradation of rubber and must be avoided.

Natural rubber threads are used in knitted swimwear, knitted garments, knitted surgical stockings, lingerie, gloves, braided draw strings, footwear and woven fabrics. Fabrics designed with rubber yarns tend to improve the overall garment fit.

8. Conclusions:

In this division, on regenerated fibres, we have briefly introduced you the salient features of the natural polymer fibres like the regenerated cellulose fibres, cellulose ester fibres, protein fibres and other important miscellaneousnatural polymer fibres. However, because of large scale developments and the successful acceptance of viscose rayon worldwide the manufacturing process of viscose rayon alone has been covered in this module.

References

1. Gordon Cook, J (2005) Handbook of Textile Fibres, Woodhead Publishing Limited, Cambridge England.
2. Premamoy Ghosh (2004) Fibre Science and Technology, Tata McGraw-Hill Publishing Company Limited, New Delhi.