By: M. Arslan Sohail

(TS1-A)To: Sir Imran Raza

Malik

VISCOSE RAYON

Viscose RayonIntroduction:Viscose Rayon is the oldest commercial man-made fiber. Viscose rayon is the naturally regenerated cellulosic fiber that can be made from naturally occurring cellulosic based material. (such as cotton linters, wood pulp etc).It can be found in cotton-like end uses as well as silk-like end uses. Due to its fine silk-like properties, it is also known as“Artificial Silk”.

Viscose RayonChemical Structure:Chemical structure of viscose is same as that of cotton i.e. Cellulose, But the polymer chains are much shorter. The degree of polymerization value of rayon is 400 – 700 while that of cotton is 5000.

ManufacturingThe process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: Steeping, Shredding, Aging, Xanthation, Dissolving, Ripening, Filtering, Degassing, Spinning, Drawing.

1. Steeping:Cellulose pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a temperature in the range of 18 to 25°C in order to swell the cellulose fibers and to convert cellulose to alkali cellulose.(C6H10O5)n + nNaOH —> (C6H9O4ONa)n + nH2O

2. Shredding: The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called “crumbs”. This step provides increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow.

3. Ageing: The alkali cellulose is aged under controlled conditions of time and temperature (between 18 and 30° C) in order to depolymerize the cellulose to the desired degree of polymerization. In this step the average molecular weight of the

4. Xanthation: In this step the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulphide under controlled temperature (20 to 30°C) to form cellulose xanthate.(C6H9O4ONa)n + nCS2 —-> (C6H9O4O-SC-SNa)n

original pulp is reduced by a factor of two to three. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration.

5. Dissolving: The yellow crumbs of cellulose xanthate is dissolved in aqueous caustic solution. Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed “viscose”.

6. Ripening:The viscose is allowed to stand for a period of time to “ripen”. Two important process occur during ripening: Redistribution and loss of xanthate groups. The reversible xanthation reaction allows some of the xanthate groups to revert to cellulosic hydroxyls and free CS2. This free CS2 can then escape or react with other hydroxyl on other portions of the cellulose chain. In this way, the ordered, or crystalline, regions are gradually broken down and more complete solution is achieved. The CS2 that is lost reduces the solubility of the cellulose and facilitates regeneration of the cellulose after it is formed into a filament.(C6H9O4O-SC-SNa)n + nH2O —> (C6H10O5)n + nCS2 + nNaOH 7. Filtering: The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament.

8. Degassing: Bubbles of air entrapped in the viscose must be removed prior to extrusion or they would cause voids, or weak spots, in the fine rayon filaments.

Spinning – (Wet Spinning): Production of Viscose Rayon Filament: The viscose solution is metered through a spinneret into a spin bath containing sulphuric acid (necessary to acidify the sodium cellulose xanthate), sodium sulphate (necessary to impart a high salt content to the bath which is useful in rapid coagulation of viscose), and zinc sulphate (exchange with sodium xanthate to form zinc xanthate, to cross link the cellulose molecules). Once the cellulose xanthate is neutralized and acidified, rapid coagulation of the rayon filaments occurs which is followed by simultaneous stretching and decomposition of cellulose xanthate to regenerated cellulose.

The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fibre formation.(C6H9O4O-SC-SNa)n + (n/2)H2SO4 –> (C6H10O5)n + nCS2 + (n/2)Na2SO4

In standard viscose of 30-50 poise viscosity made with 32% CS2 is spun into an aqueous acid salt spin bath of the following type at a temperature of 40-50 0c . H2SO4 = 8-10%Na2SO4 = 16-24%ZnSO4 = 1-2%

Modified Viscose Rayons1. Tyre yarn:

A viscose solution of viscosity 100 poise containing modifiers 1-3% by weight of cellulose and with a CS2 content of 40% is spun underripe (salt index-6-15) into a aqueous spinning bath containing – H2SO4 8-10%Na2SO4 16-24%ZnSO4 6%The spin bath temperature is kept around 550c and the spinning speed is between 40 and 60 m/min. The stretch applied is 75-125%

2. High wet-modulus yarns (HWM):In this process the ageing of alkali cellulose is eliminated and dissolving cellulose xanthate in water rather than in caustic soda also eliminate the ripening of cellulose xanthate. Lower the concentration of acid used in spinning bath with little salt. These are known high wet modulus which means to have greater resistance to deformation when wet.

3. Polynosic fibre Polynosic is similar rayon fibre but difference in process of manufacturing than viscose rayon. Since the manufacturing process is different so their morphological structure also different. Generally polynosic fibre has high crystallinity and high orientation. This give high mechanical strength and chemical resistant, high wet modulus and more dimension stable.NOTE- In polynosic process we eliminated the Ageing stage, Ripening stage ,Diluted acid concentration & zinc sulphate.

4. Super high wet modulus rayon:By adding 1% formaldehyde to spin bath or to the viscose substantially increases the toughness and plasticity of viscose gel. We can get the stretch of 500-600% .Disadvantage of this compound is that it is very toxic.

Viscose rayon consists of cellulose of lower DP than cotton cellulose. Also amorphous region of Viscose rayon is present to a greater extent, therefore, Viscose

rayon reacts faster than cotton with chemicals.

1. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose. 2. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form

oxycellulose. 3. Cold acid solutions for a short time do not attack viscose rayon.

Chemical Properties

4. Action of Solvents: Textile solvents can be used on Viscose rayon without any deteriorating effect. Viscose rayon dissolves in cuprammonium hydroxide solution.

5. Effect of Iron: Contact with iron in the form of ferrous hydroxide weakens viscose rayon yarns. Therefore staining, marking or touching of rayon to iron or iron surface should be avoided.

6. Action of Microorganisms: Microorganisms ( moulds, mildew, fungus, bacteria) affect the colour, strength, dyeing properties and lustre of rayon. Clean and dry viscose rayon is rarely attacked by moulds and mildew.

7. Action of Soaps: Ordinary soaps in usual textile concentration have no direct effect on regenerated cellulose materials. Improper use of soap or use of poorly made soap results in rancidity and odor in rayon fabrics or yarns.

Biological Properties: 1. Rayon resist all insects except silverfish.2. Regular rayon fibers are subject to harm by rot-producing bacteria3. High wet modulus fibers are fairly resistant to bacteria.

Physical PropertiesShape:Colour:Luster:Tenacity:• Dry• WetElongation:Elastic recovery:Density:Moisture Content:Dimensional Stability:Resistance against:• Alkali• Acid• SunlightThermal:• Heat• Flame

ControllableControllableControllable

1.5 – 3.01.1 – 1.520%Good1.5 g/c cm 10 – 15 %Good

AveragePoorAverage

Not above 150°CBurn

IdentificationMicroscopic View:

Longitudinal: Uniform diameter with striation running parallel to the fiber.

Cross-sectional: highly servated (nearly round for polynosic)

BURN TEST:Burns slowly without flame with slight melting and smell of burnt paper, leaves soft black ash.

Solubility test:Soluble: 80% Sulphuric acid (m/m) at room temperature for 15 min.

Insoluble:• Nitric acid (conc.) at room temperature for 15 min.• Acetone at room temp.• Chlorine bleach 5%.• 90% Phenol or m-cresol at room temp.• Formic acid 90%.

USESBlends: Regular rayon and high wet modulus usually blend with many fibers like polyester, acrylic, nylon, acetate, cotton, flax, wool and ramie.

With polyester, nylon and acrylic:Rayon contributes absorbency, comfort, and softness when blended with them.With cotton: It alters appearance to create soft luster.With wool:It will decrease the cost.With flax and ramie: It can help produce appearance that are typically associated with linen like fabrics.

Specialty rayon:1. Flame retardant fibers:Flame retardance is achieved by the adhesion of the correct flame- retardant chemical to viscose. Examples of additives are alkyl, aryl and halogenated alkyl or aryl phosphates, phosphazenes, phosphonates and polyphosphonates. Flame retardant rayon have the additives distributed uniformly through the interior of the fiber and this property is advantageous over flame retardant cotton fibers where the flame retardant concentrates at the surface of the fiber.

2. Super absorbent rayons:This is being produced in order to obtain higher water retention capacity (although regular rayon retains as much as 100 % of its weight). These fibers are used in surgical nonwovens.

3. Micro denier fibers: Rayon fibers with deniers below 1.0 are now being developed and introduced into the market. These can be used to substantially improve fabric strength and absorbent properties.

4. Cross section modification: Modification in cross sectional shape of viscose rayon can be used to dramatically change the fibers aesthetic and technical properties. For example: Viloft: A flat cross sectional fiber sold in Europe, which gives a unique soft

handle, pleasing drape and handle. Fiber ML(multi limbed): Another modified cross section fiber that has a very

well defined trilobal shape. Fabrics made of these fiber have considerably enhanced absorbency, bulk, cover and wet rigidity all of which are suitable for usage as nonwovens.

Union Fabrics:Viscose is also used in constructing a union fabric with other yarns. One great example is Viscose Rayon with Eri silk.Viscose Rayon as warp with Eri silk of three different counts viz., 2/40s, 2/60s and 2/80s as weft were interwoven on a semi-automatic power loom to produce union fabrics. Its is also used with cotton fibers to make union fabric.Special colour effect of Viscose Union fabrics (stain test):When viscose is used in union fabric with other fiber it gives special contrast when dyed with same dye due its different stain test. Lets take example of different dye mixtures (shirlastains):

Shirlastain DFiberCottonViscose

ColourBright BlueBright green

Shirlastain A

FiberCottonViscoseDiacetateTriacetateBombyx SilkTussah silkRegenerated proteinWoolNylonEtc.

ColourPale purpleBright pinkGreenish yellowOff whiteDark brownChestnut brownYellow – OrangeBright yellowCream to yellow