viscose fiber manufacturing process

Viscose fiber:

It is a semi-synthetic fiber which was earlier known as viscose rayon. Viscose Rayon made from natural sources of regenerated cellulose; such as wood, bamboo and related agricultural products. It has the same molecular orientation like cellulose. Rayon fabrics used widely and in huge demand showing to its special characteristics, like high strength with soft and comfortability.

­n = 175, cellubiose unit for Viscose

n = 250, cellubiose unit for Cuprammonuim

n = 300, cellubiose unit for Polynosic

History behind rayon production:

Famous British chemist Charles Frederick Cross and his collaborators Edward John Bevan and Clayton Beadle patented their artificial silk in the year of 1894. They named it as “viscose”, because its production is associated with an intermediate state of a highly viscous solution. The Viscose rayon was first commercially produced by the U.K. Company Courtaulds Fibers in 1905. In 1910, American Viscose, a division of Courtaulds fibers was formed to produce their formulation in the United States. The name “rayon” was accepted in 1924, from this time  viscose being used for the viscous organic liquid used to make rayon.

What is Regenerated fiber?

The fiber which we obtained by conversion of natural fiber through chemical processes is termed as regenerated fiber. The regenerated fibers are classified into the following four groups —­

I)                   Cellulosic fibers – Viscose Rayon.

II)                Cellulosic esters – Acetate, triacetate.

III)              Protein fibers – Casein.

IV)             Miscellaneous – Alginate, Rubber.

Chemical reaction involves in viscose rayon production: Regeneration sequence:

Step-1: At first wood are chopped into small pieces and processed into cellulose pulp. Then the cellulose pulp is treated with a 17.5% solution of NaOH which converts it into soda-cellulose.

 Cell – OH + NaOH                 Cell – ONa + H₂O

     (Cellulose)                     (Soda cellulose)

Step-2: The soda cellulose reacts with carbon disulphide (CS₂) to form sodium cellulose xanthate [C₆H₇O₂(OH)₂OCS₂Na]

                Cell – ONa + CS₂                   Cell – O – CS₂Na

                  (Soda cellulose)                  (Sodium cellulose xanthate)

Step-3: Sodium cellulose xanthate as dissolved in a dilute solution of caustic soda and its form orange honey like liquid.

Step-4: This liquid extruded into sulphuric acid (H₂SO₄) which regenerates the cellulose now in the form of long filaments called viscose rayon.

Cell – O – CS – SNa + H₂SO₄          Cell – OH + Na₂SO₄

     (Sodium cellulose xanthate)           (Regenerated cellulose)

This point key modification such as changing the shape and size of the cross section well co-operative attitude can be made.

Viscose fiber manufacturing process:

1.  Wood pulp preparation:

At first the bark of the wood is removed and chipped into small pieces about inches. Then the chips are treated with Calcium bi sulphite [Ca(HSO)2] and the treated chips then processed with steam under pressure for about 14 hours. These treatments do not highly affect the cellulose but it decomposes and solubilizes the smearing substances. After that purification of the cellulose. Finally bleached with hypochlorite and converted into boards or sheets of paper.

2.  Steeping & Pressing:

The boards or sheets of paper contain about 90 to 94% of pure cellulose. Primarily conditioning in a room at a definite humidity and temp. This conditioning is compulsory. Vertically stacking and pressing, the boards or sheets are soaked in a 17.5% solution of caustic soda for approximately 1-4 hrs. This process is known as Steeping. Then a hydraulic ram in the press for the excess alkali processing.

3.  Shredding:

The shredding m/c comprises a drum inside which whirl a pair of blades with serrated edges. The shredders are cooled with water and will take 200 lbs of pressed soda cellulose at a time. In approximately two or three hours they break it up into very fine “Crumbs” (break into very small parts).

4.  Ageing:

With the completion of shredding, the crumbs are aged. They are preserved in a galvanized vessel with a cap and through oxidation with atmospheric oxygen, degradative changes took place. Some depolymerization occurs and the degree of polymerization falls from about 780 to 360 in this process. With the increase in the temperature, the aging goes rapidly – that’s the process of de-polymerization.

5.  Churning:

After completion of ageing, the soda cellulose (Cell – ONa) crumbs are initiated into rotating air-tight hexagonal churns. Approximately 10% of their own (crumbs) weight of carbon disulphide (CS2) is introduced and the crumbs and disulphide are churned up together into a deep orange, gelatinous mass of  [C6H7O2(OH)2OCS2Na] sodium cellulose Xanthate is formed. Churning is continued for approximately 3 hrs, the churns revolving at about 2 rpm.

6.  Mixing (Solution):

Through the mixing of sodium cellulose xanthate [C₆H₇O₂(OH)₂OCS₂Na] with dilute caustic soda for approximately 4 to 5 hrs by stirring, the vessel being cooled. The xanthate dissolves to a clear-brown, viscous liquid, like honey. This liquid is known as “Viscose” but is still too impure, and too young to spin.

7.  Ripening:

Storing of the viscose solution for 4 to 5 days at 10^o-18^oC and during this time it ripens. Meanwhile the viscosity at first falls, but afterward rises, so that by the time the solution is ready to spin. The viscose ripening test for spun ability is given below –                                                                                  

Acetic acid test: The viscose solution will dissolve in 40% acetic acid solution before it ripens, but when it is ripe the viscose will precipitate in this solution.

8.  Spinning:

Basically Two types of spinning process. They are: 

1)  Discontinuous: (a) Pot/cake spinning

 (b) Bobbin spinning

2)  Continuous: (a) Nelson spinning

                       (b) American process of industrial rayon corporation.

Solution preparation: Solution for spinning comprises with the following substances-

·         Water(H₂O)-69%

·         Zinc sulphate(ZnSO₄)-1%

·         Sodium sulphate(Na₂SO₄)-18%

·         Glucose (C₆H₁₂O₆) -2%

·         Sulphuric acid (H₂SO₄)-10%

Sodium sulphate (Na₂SO₄) precipitates the cellulose xanthate [C₆H₇O₂(OH)₂OCS₂Na]. The sulphuric acid (H₂SO₄) transforms xanthate into cellulose, carbon disulphide (CS₂) and sodium sulphate (Na₂SO₄). Glucose (C₆H₁₂O₆) provides softness and pliability and zinc sulphate (ZnSO₄) contributes in strength addition.

Filtration: Filtration for the removal of undissolved materials that might cause disruption in smooth spinning process.

Wet spinning: At first, the ripened viscose solution passes through a centrifugal pump. After some processing the solution is forced through a spinneret mostly made of platinum alloys and having many fine holes with diameters ranging from 0.05-0.1 mm. The jet of viscose enters a coagulating bath containing the composition of mixture. The operation in coagulating bath follows two routes to convert regenerated cellulose-

(1)   Main viscose feed (2) Pump (3) Candle filter (4) Glass rounder end (5)Bath for coagulation

(6) Acid replenishing pipe selection (7) Jet spinneret (8) Bottom godet roller (9) Top godet roller

(10) Funnel for smooth processing (11) Topham take up box (12) Drive for take up

(13) Traversing device (14) Air Inlet valve (15) Fumes suction valve (16) Sliding segment

(17) Cake

After the spinning bath operation, it passed through two rollers to the topham box. Topham box is the take up device for cake spun viscose rayon.

9.  Purification:

The collected yarn in the Topham Box is not pure and relatively weak. So it has to be purified. Purification process basically comprises of 5 operations-

I)                   Washing

II)                Desulfurizing

III)              Bleaching

IV)             Conditioning

V)                Drying

The first wash is with water, the second is with Sodium sulphide (Na₂S) solution to remove Sulphur and possible compounds of Sulphur residual from the Xanthate. For complete whitening, so it is treated with a slightly alkaline solution of a sodium hypochlorite to bleach it. This is followed by a wash with dilute hydrochloric acid for conditioning and a final wash with water. After that, the cakes are dried. Finally coning of viscose rayon done and go through the process of shipping.

Quality of viscose rayon depends on the following factors:

1.Spinning bath temperature.

2. Spinning bath composition.

3.Coagulation speed of the filaments.

4. Immersion speed of the filaments in the spinning bath.

5. Spinning speed.

6. Imparted stretch to the filaments.

Physical properties of viscose rayon:

1.  Moisture absorption:

Viscose rayon absorbs moisture more than cotton; the moisture content of viscose rayon is about 13% at 65% R.H. and 70^oF Temp.

2.  Tensile strength:

The fiber in dry condition has more strength than the wet condition. The tensile strength of viscose rayon is 1.5 to 2.4 GPD (gram per denier) in the dry state and 0.7-1.2 GPD in the wet state. For high tenacity viscose rayon, the corresponding values are 3-4.6 GPD and 1.9-3.0 GPD respectively.

3.  Elasticity:

Elasticity of viscose rayon is less than 2-3% after which creep develops and permanent elongation sets in until the yarn breaks.

4.  Elongation at break:

Ordinary viscose rayon has 15-30% elongation at break while high tenacity rayon has only 9-17% elongation at break when the yarn is wet; it is easy to stretch and has higher elongation – 20-35% for ordinary and 14-20% for high tenacity viscose.

5.  Density: 1.5 gm/cc.

6.  Specific gravity: 1.5.

7.  Melting temp: Do not decompose approximately at (176^o-204^o) C.               

8.  Resiliency: Low.

9.  Dimensional stability: High for HWM.

10.  Flammability: Flammable and burning is like cotton.

Chemical properties of viscose fiber:

1.  Effect of acid: Acids like Sulphuric acid, Hydrochloric acid etc. reacts with the viscose rayon and produce hydrocellulose. Cold acid solutions for short time do not attack viscose rayon. If the temp is not high, acids like euphoric, hydrochloric and nitric acid can also be used in strong concentrations. It is attacked by hot dilute or cold concentrated mineral acids which diminishes and deteriorate the fiber.

2.  Effect of alkali: Weak alkali does not disrupt viscose, whereas concentrated alkali will cause swelling and reducing the strength.

3.  Effect of solvent: Resistance to organic solvents.

4.  Effect of bleaching: Though viscose rayon can withstand both oxidizing and reducing bleaches, it can be attacked by strong oxidizing bleaches.

5.  Biological agents: (Action of microorganism)  Microorganisms like molds, mildew, bacteria etc. impact on the color, strength, dyeing property and luster of rayon. Clean and dry viscose rayon is rarely attacked by mold and mildew.

6.  Action of light: Prolonged exposure to sunlight weakens viscose rayon. This weakening is due to moisture and Ultra Violet light of the sunlight and % loss of strength or exposure to Ultra Violet light.

7.  Action of heat: Viscose rayon becomes yellowish color at long exposure to high temperature. Approximately at 300⁰F (150⁰C) or more, viscose rayon loses its strength and begins to crumble at 350⁰ -400⁰F.

8.  Dye ability: Better affinity for dyes than cotton. Applied dyes – Vat, Direct, and Sulphur.

9.  Conductivity: Good conductor of heat.

10.  Flammability: Burns rapidly.

Commercial Name of Viscose: 

·         Zantral

·         Avril

·         Fibro HSO

·         Grasim

High Tenacity viscose rayon: Research of the spinning of rayon has been intensive and unceasing. Every aspect of viscose production and spinning has been studied and increased understanding of the factors involved has made possible the production of rayons with characteristics that are suited to particular application. The most important of these are the high tenacity rayon.

High tenacity viscose rayons are –

I)             Tenasco

II)          Cordura

III)        Durafil

IV)   Forfisan      

Process of high tenacity viscose rayon:

1)      Maintaining a higher than usual degree of polymerization in the cellulose.

2)      Frequent change of filter cloth for better filtration.

3)      Stretching the fiber before it is set, usually as much as 150% as maximum stretch.

4)      The coagulating bath containing more zinc than usual. The presence of high concentration of zinc ions imparts coagulation of the xanthate on the skin and this hamper regeneration of the cellulose. Thus giving time for the stretching to be carried out.

5)      Because color and dye ability are unimportant in type yarn, removal of sulphur from the yarn is unnecessary in fact, it the sulphur is left in it may help the bonding of fibre to rubber.

6)      Use modifiers in spun bath.

7)      Two spinning bath twice.

Special viscose:

Evlan: A viscose rayon staple fiber used for carpet trade. The cross-section is smoother than standard viscose. Rayon has a higher crimp usual. It has a higher strength than usual and this gives durability. It has been widely used since 1962. Evlan is used for Carpets.

Topel: A cross-section cellulosic fiber. It swells very little, as it has much less affinity for water. Good resistance to caustic alkali. Useful in package dyeing. Blending with cotton gives better appearance, handle and drape.

End Uses of Viscose rayon:

The softness, drape and the luster in the fiber make it a preferred choice for ladies garments and menswear. The most common uses of viscose are in apparel like-

·         Dresses

·         Suits

·         Sari    

·         Lingerie

·         Children cloths

·         Umbrella cloths

·         Protective cloths

·         Sewing embroidery threads

·         Curtains

·         Cushions

·         Beach wear

·         Sports wear

·         Nonwoven fabrics of baby wipes, wet wipes and napkins etc.  

Differences between Polyester and Viscose:

While many consider the viscose and polyester to be similar, there are a lot of differences between the two materials.

1.      Both polyester and rayon are made from long fibers but polyester is a synthetic fiber whereas viscose is semi-synthetic that is using natural fibers but chemicals in the process.

2.      Polyesters are more moisture wicking while viscose is more absorbent.

3.      Polyester dries faster and does not wrinkle as easily as viscose.

4.      Polyester is stronger and does not shrink.

5.      Viscose is more likely to pill, where polyester resists abrasion.

6.      Polyester is made from petroleum oil; while viscose is made from plants.

7.      Viscose fiber is biodegradable while most of the polyester fibers are not biodegradable.

8.      Polyester production and use contribute to pollution around the world.      

MUSFIQUR RAHMAN