Viscose yarn manufacturing process

Viscose yarn manufacturing process

viscose rayon is a regenerated form of cellulose. In this process, purified cellulose is mercerized with NaOH and xanthate with CS, after which it is dissolved in NaOH to form a spinning solution, from which cellulose is regenerated by the action of acid.

Chemical Reaction

When cellulose is converted into viscose it functions as alcohol. Cellulose xanthate is made exactly in the principle of the xanthate of simple alcohol, Only one of the three OH, groups known to be present in each glucose unit of cellulose is xanthates.


Steps in the viscose process

In general, the viscose process should have the following principle:

1. Purification of cellulosic material

2.Formation of soda-cellulose

3. Formation of cellulose-xanthate

4. Preparation of spinning solution

5. Fibre formation

Viscose Fibre formation the process consists of the following structural modifications:

a) Swelling of cellulose I chain with the hydrates of Sodium hydroxide.

b) Breakage of crystalline and amorphous parts.

c) Replacement of the hydroxyl group by the Xanthate group.

(P Removal of Xanthate group in spinning and then Sodium groups by hydroxyl groups.

c) Formation of Vander Waals forces and Hydrogen bonding.

(} Formation of cellulose II structure.

Viscose manufacturing technology mainly consists of the following three

a) Purification of Cellulosic material

b) Preparation of Cellulose Xanthate solution.

c) Fibre formation and regeneration of Cellulose.

Purification of cellulose material

The raw material for the viscose process should have high cellulose content. Softwoods are selected as raw material for viscose fibre which is having around 25 - 32%. Purification i.c., pulping is a process id which the lignin is dissolved in chemical reagents and other forms of cellulose are eliminated. The pulping operations are a) Wood preparation, b) digestion  and c) washing.

The woods in the form of sizeable logs are supplied to the mill. The size varies from 1.5 to 2.5-meter lengths. The preparation into this log form is joined by renovating the bark by means of a high-pressure jet of water perpendicularly to the axis of the log. The log is then reduced into chips of the size of T.6-2.2 cm in length, 0.2-0.4 cm in thickness, and 1,3- 25 cm in width. These chips are then treated with pulping reagents to remove óther forms of cellulose, hemicellulose. lignocellulose, coloring matter, resinous matter, mineral compounds.

The common pulping reagents arc: a) Sulfurous acid and calcium or magnēesium bisulfite, b) Soda or Sodium sulfide, sodium hydroxide, and sodium carbonate. The temperature and time of treatment can be varied from 100 - 140°C and 8- 14 hours. The pulp forms a muddy and pasty appearance with light yellow to deep brown color. The pulp is diluted with water and screens through a coarse screen or knotter to remove uncooked chips. The diluted pulp is pressed into boards and is further purified with bleaching powder. These boards contain about 90-94 % cellulose. They can be transferred to a rayon mill and can be used after conditioning with the exact amount of moisture take-up.

Preparation of cellulose xanthate solution

There are several steps in the manufacture of Cellulose Xanthate solution. These steps are

1. Reaction with a strong alkali, known as steeping and shredding into small crumbs or lumps.

2.Aging of alkali cellulose for lowering the degree of Polymerisation.

3.Reaction with carbon disulfide for conversion to alkali-soluble Cellulose Xanthate.

4.Dissolution in dilute alkali.  

5. Ripening of Viscose to control the solution viscosity.

6. Filtration of the viscose for the removal of foreign particles.

7. Deaeration.

After deaeration, the spinning solution should contain 6.5 to 9.0% Cellulose, 5.5 to 6.5% Sodium hydroxide and 2.0 to 2.5% total Sulphur.

Steeping

The first step in the viscose process is the treatment of the pulp for mercerisation with NaOH at mercerizing strength. The pülp sheets are steeped or immersed in 16-%-NaOH, present in a big tank. Considerable swelling of the cellulose will take place. All the soluble products will come out from the whiteboard After the pulp has been thoroughly impregnated, The material is subjected to very high pressure. The object of this is to press out the excess liquid until the material weighs those times as much as the original pulp. The product of this reaction is known as alkali cellulose. After the pressing, the alkali which comes out can be used again after caustic recovery.

Shredding

The sheets of alkali cellulose are transferred to a shredding machine. This machine consists of two sets of revolving blades rotating in the opposite direction at high speed. The machine is provided with a jacket through which water can be circulated so as to control the temperature and keep it about 20° C. The maintenance of the perfect temperature is very important in all the viscose operations. The shredding machine crumbles them and Tears and putts them apart (shredding) but does not grind them Thealkali cēllulose sheets are converted into a light fluffy mass.

Aging

The crumbs are then transferred to steel containers in which they are stored under controlled temperature conditions between 21 to 23°C. The time can be from 3 to 72 hours depending upon the catalysts and alkali,

Presence of air in the presence of alkali reduced the chain length of cellulose (depolymerization) resulting in a decrease in viscosity. This is really desirable to make the desired spinning solution. When the right viscosity is obtained, these pieces are all transferred to the drum in an inert atmosphere and kept at low temperatures.

Xanthation

The aged crumbs are then transferred to large rotating drums or crumbs.

This is a hexagonal drum horizontally placed by an axis. The drum is usually double jacketed and can be cooled with the introduction of ice water during the operation. Liquid carbon disulphide is introduced inside the drum slowly until it equals to 30-40 % of the weight of the original wood pulp. In this condition the hexagonal drum rotates along the axis at a very slow speed; 2-3 turns per minute for 3 hours. The formation of cellulose xanthate is accompanied by some rise in temperature, which will be avoided by cooling.

During the reaction, the color of the crumbs changes from light yellow to deep yellow to orange and to deep orange-brown. This is an indication that chemical reactión is complete. Alkali cellulose has now been converted into cellulose xanthate. The xanthate becomes pasty and begins to stick at the surface of the drum in small lumps.

Ripening

The xanthate, thus obtained, is quite big in molecular size which is not easy to spin by a spinnerette. So it is depolymerized in the ripening process. During this ripening period, chemical changes take place which determines the spinning quality of the solution. The ripening time is usually four to five days with temperature-controlled between 15° to 20°C. During this process, the decrease in viscosity can be measured from time to time and some. a special test is generally carried out to determine the exact condition.

Preparation of spinning solution of viscose 

Cellulose xanthate is soluble dilute NaOH. A solution is made of cellulose xanthate with NaOH which can be extruded into filaments. The temperature is reduced to 17°C and a dilute solution of NaOH is added such that the change will contain about 6.5% NaOH and 7.5% cellulose in the form of xanthate. This operation requires 3 to 6 hours. During this time the xanthate dissolves to give a thick viscous solution known as viscose. During this operation, certain mineral pigments are added if a dull yarn is desired.

Filtration

Beforé spinning, insoluble impurities must be removed. The spinning solution is pumped through several filters and at the same time deaerated with a vacuum to remove all air bubbles. The presence of insoluble particles will break the continuity of the filament and the presence of small air bubbles would cause weak spots in the final yarns.

Spinning process

The conversion of the viscose solution into the solid fiber is generally referred to as spinning. The spinning of viscose fibre consists of extrusion, coagulation, stretching, and take-up or collection. The solution after filtration by the candle filter is extruded in the coagulation bath, stretched, and collected in the Topham box.

The viscose solution should have the viscosity of 30-50 poise. It is pumped to the spinning machine by means of metering pumps which ensures accurate feeding of the solution to each spinning head. The spinning solution is filtered in the candle filter and extruded under uniform pressure through spinnerettes into an acid coagulating or hardening bath, known as a spin bath. A spinnerette is a cap or jet, provided with a number of fine funnel-shaped openings. The spinnerettes are made of precious metals


such as platinum, platinum-gold. Each orifice forms an individual filament, and so size and number of the orifices determine the number of filaments in the final yam and its denier. Each hole diameter is between 40 to 80 microns.

All of the spinnerettes are immersed in a long trough through which the coagulating solution flows. The coagulating solution chemicals and conditions are mentioned in Table.

 Coagulating bath conditions

1.  H2SO4                   8-10%  or 145 GPL
2.  Na2SO4                16-24%  280 GPL
3.  ZnSO4                  1-2%  or 12 GPL
4.  temperature         45 - 50
5.  Spinning speed   120 m/min

As the viscose solution extrudes through the spinnerette, it comes into contact with it. spin bath chemicals. It passes for a particular time in the spin bath. Sodium sulphate precipitates sodium cellulose xanthate from the *viscosé into the form of filaments. Sulphuric acid converts the xanthate into cellulose. Because of the reaction with the spin bath chemicals, the solution coagulates in the form of threads of Sodium Cellulose Xanthate. Further, it reacts with an acid to form cellulose.

This reaction is a slow process that takes a few hours for completion. The streams of cellulose xanthate solution, after spinnerette under the surface of the acid bath, coagulate and become hard. They are then pulled under a guide to the bottom godet rollers. The filaments pass from the bottom godet round another guide, made of acid - resisting material, generally glass round the top godet.

The tap godet is driven faster than the bottom godet. The filaments are stretched to about 100% between the bottom and sapos do For the yarn collection, a centrifugal method of spinning is generally utilized as the yarn is partially plastic. After stretching, the yam then passes through a traversing glass funnel to the revolving spin pot known as 'Topham Box', after its inventor C.F.Topham.

These pots rotate at about 5000 to 10000 rpm. The yarn is thrown against the side of the hox with the aid of centrifugal force and it is laid in the form of a 'cake". The speed of the box, yarn speed and rate of traverse of the glass funnel determine the çake density. Also because of the rotation of the hox, the yarm is automatically twisted. The direction of twist depends upon the direction of the rotation of Topham box. The cake is a stable package, which can be removed when the rotation of the box stops. The weight of the cake is around 1.5 Kg. Collection of the cake i.e., doffing is usually done at every fixed interval determined by the yarn delivery rate and denier of the yarn.

The yarn thus collected is full of acid and it is stored in humid chambers for cértain time. After storage, the cakes are wrapped in protective clothes, washed, purified, and dried for further processing.

The factors which affect the quality of viscose yarn is:

1. Temperature of coagulating bath
2. Composition of bath
3. Speed of coagulation
4. Length of immersion
5. Speed of spinning
6. Stretch imparted between the godets

Purification of viscose fibre

The purification of viscose fibre process consists of four operations. a) Desulphurising: The yams are washed with dilute sodium sulfide solution- at 50°C to remove residual sulphur

 b) Washing: the yarn is then washed thoroughly 

c) Bleaching: the yarn is then bleached with hypochlorite bleach liquor at room temperature 

d) Washing: small amounts of residual bleach are removed by an antichlor, after which the yam is well rinsed and dried. 

The skin solidifies, while the core still contains solvent. So the cross-section collapses and results in a serrated structure.

Purification of viscose fibre

Some importance properties of given bellow

 Physical properties of viscose yarn

Tenacity and elongation

Ordinary viscose rayon is reasonably strong. Its tenacity is about 2.6 gms/den. This differs from cellulose acetate which has a dry tenacity of only 1.3-1.7 gms/den. The wet strength of the viscose rayon is about 1.4 gms/ den. The elongation at break (dry) is about 15% and (wet) 25%.

Moisture content

Under standard conditions (65% R.H. and 22°C) the moisture content of viscose rayon is 11 - 13%. The lower the humidity of the atmosphere, the lower the moisture content of the yarn and vice versa. At 20% R.H.,
the moisture is 5%, 7.5% at 30% R.H., 10.5% at 50% R.H., 17% at 80% R.H. and 22% at 90% R.H.

Absorbency

Viscose rayon is highly absorbent and takes up water readily without the aid of any assistants like wetting agents. Oven dry cellulose is extremely hygroscopic and comparable to the best drying agents. When water is absorbed by viscose-rayon, 0.4-7.0% axial swelling occurs in normal viscose rayon, and 0.7-2.0% in case of highly' oriented rayon. The presence of water in regenerated cellulose increases the penetration of reagents into the cellulose, increases the electrical conductivity, reduces the breaking strength and changes other mechanical characteristics, Cellulose is wet by all types of oils and when oil is imparted as a dulling agent, it is held very tenaciously. Its removal is not always easy because of uneven application and absorption.

Creep

The elasticity of viscose rayon is not high. If stretched and then released

from strain, it does not return to its original length, although for some time afterward, it continues to shrink towards, but not completely t. its original length. This phenomenon has been variously described as 'delayed elasticity' 'creep' and 'plasticity'. The effect of this' behavior is that if ends of yarn during weaving are exposed to sudden strains, they may be permanently stretched and will result in streaky dyeing.

Density

The density of viscose rayon is 1.52 gm/cc, the same as that of cotton.

Electrical properties of viscose

Owing to its high moisture absorption, viscose rayon does not lend itself particularly well to insulation purposes. When quite dry, it is a good insulator, but the moisture that it inevitably picks up considerably reduces its value for electrical use. Viscose is not so liable to develop static charges in textile working as is cellulose acetate.

Chemical properties of viscose

Degradation

Since cellulose is extremely sensitive to the action of acid and also to oxidation, acids produce hydro cellulose and oxidizing agents produce ox cellulose. In either case, a breakdown of the molecular chain is brought about i.e., degradation. In the case of acids, the cellulose chain is attacked at the O-linkage whereas oxidizing agents attack the two secondary OH groups. Hydro-cellulose and ox cellulose are weaker than cellulose and their formation is a destructive process. Photocellulose is another type created by partial oxidation with light.

Effect of light

Light has a deteriorating influence on all regenerated cellulosic products, and degradation takes place on the surface exposed to sunlight. It is due both to water and to the UV rays of the sun. The damage to viscose rayon is slightly greater in an atmosphere of 45% R.H. than in one of 65 or 90% R.H. Viscose rayon loses little strength when exposed to UV rays, and loses more when exposed to daylight. This is due to the formation of photocellulose.

The action of dry heat

Most regenerated celluloses, under the influence of heat as well as a light show, rapid loss in strength, these changes being accompanied by an increase in copper number and alkali solubility. The quality index of viscose fibres decreases either as the temperature increases or as the moisture content of the surrounding atmosphere increases. Both the breaking strength and fluidity of viscose rayon appear to be functions of the R.H. to which samples are exposed. *

Degradation of cellulose is slower in the absence of O2 Continued heating in the absence of O2 leads to deterioration of the cellulose. If cellulose is exposed to relatively high temperatures, drastic degradation of the material occurs. Short heating at high temperatures, as at 140°C is less harmful than long heating at low temperatures.

Action of acids

The resistance of regenerated cellulose rayons to acids are generally less than that of cotton to the same the concentration of the same acids,

a) Organic acid (acetic and formic) can be safely used in 1-2 % concentration (dry) without damage to the fiber.

b) Inorganic acids as H2SO4, HNO3 or HCI can be used in a surprisingly strong concentration, provided the temperatures are not too high and the treatment is brief.

In all cases, acids must be neutralized thoroughly and must certainly not be allowed to dry on the material or serious weakness will result. Oxalic acid for removal of Iron stains is not recommended except at temperature lower than 65°C.

At high temperature and concentration, all acids will destroy or carbonize regenerated rayon’s. NaHSO3 is applied to regenerated cellulose rayon as an antichlor and to remove MnO2, from permanganate bleached goods. Acids in contact with yarn ease rayon to become hard and brittle. Acids tend to swell of rayon filaments.

The action of oxidizing agents

As regenerated fibers are made from bleached pulp and are bleached by the producers, it is not necessary to use bleaching agents to restore their whiteness. Peroxide solution can weaken this at 65°C. Hence H2O2 is applied below 55°C. Na202 is not suited for this purpose. NaOCL in acid solution has a destructive bleaching action and can only be applied cold and in great dilution. Alkaline NaOCI solutions are much milder in their action than the acid. KMNO4, bleach should be used only in mild acid solution as the formation of MnO2 prevents the bleaching action from proceeding. Hydro-sulfite compounds as Na-hydrosulphite, Na- sulphoxylates formaldehyde, basic or normal Zn-alkoxylate formaldehyde, as well as other reducing compounds, form small quantities of hydro cellulose in regenerated cellulose yarns when treatment is too drastic.

Action of soap

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 yams. When soap alone is used, there is a tendency for the ionized fatty acid from the soap to adhere tenaciously to the individual rayon filaments. During the drying of such materials and subsequent storage, the free fatty acid radical is very likely to turn rancid to give the goods an objectionable odor. This is prevalent on oil-delustered rayons, because of the fatty ac radical of the soap adheres tenaciously to the minute oil globules in the structure of the yarn. The fatty acid radical will produce 'scroop' on the fabric or fibre after a long duration. Rancidity can be prevented by a final rinse in hard water.

Action of solvent

Textile solvents as pine oil, hydrogenated hydrocarbons, benzene, toluene, xylene, gasoline and carbon tetrachloride can be safely used on regenerated rayons. They are employed as spotting agents before or during scouring process or as additions to the occurring boil off the bath.

Effect of iron

Fe(OH), tends to weaken rayon yarns directly. In the presence of air, moisture, carbonic acid, iron is transferred and is readily absorbed by rayon. On exposure to air, Fe(OH), absorbs O, and forms Fe(OH), At this time Fe-salt is very active and may act as a catalyst under certain conditions by converting cellulose into oxycellulose by taking the air. This results in tendering Staining, making or touching of rayon to iron or iron surfaces as occurs in tỉnting, boil-off, throwing and dyeing must be avoided. All traces of the stains can be removed in 5-15 mins by 1-2 % of oxalic acid at 65° C or below. But this treatment is not used in the case of regenerated fibers as it is a harsh treatment.

Dyeing properties of viscose yarn

Viscose fibres dye readily with all dyestuffs which are substantive to cotton. The dyeing should be carried out at low term nature, with the presence of retarding agents and lower concentration of electrolytes for a good affinity and better exhaustion from the dyebath. Physical variations in rayon yarn arising during manufacture become more apparent after dyeing and result in a difference in dye uptake of different filaments.

 Biological properties of viscose yarn

The influence of moths, mildew on viscose causes discoloration and stains in rayön material. It will affect strength, dye-affinity, and luster. Dry viscose rayon is rarely attacked. The presence of moths and mildews depends upon the type of warp size, temperature, and humidity of storage place.

Use of viscose yarn

Viscose rayon is suitable for all normal textile needs including those of apparel. It is required in curtains, furniture coverings, transport furnishings, table cloths, cushions, bedspreads, quilt covers, lace, fine fabrics, sportswear, and other dresses and tire-cords. It is not suitable as sea-ropes, fishing nets, insect netting and other fields related to chemical contacts.