- 1 Manufacturing Of Rayon Fibres
- 2 General Properties of Viscose Rayon
- 3 Types of Viscose Rayon
- 4 Applications of Viscose Rayon
- 5 Modal
Rayon fibres are made from cellulose that has been re-formed or regenerated and is identified as regenerated cellulose fibres.
Viscose rayon is made from cotton fibre or wood pulp usually obtained from spruce, hemlock and pine trees.
Manufacturing Of Rayon Fibres
The process of manufacturing viscose rayon consists of the following steps:
(1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7) Ripening, (8) Filtering, (9) Degassing, (10) Wet Spinning, (11) Drawing, (12) Washing, (13) Cutting.
Cellulose pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a temperature in the range of 18 to 259 C in order to swell the cellulose fibres and to convert cellulose to alkali cellulose
The swollen alkali cellulose mass is pressed to a wet weight equivalent of 2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of alkali to cellulose.
The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called “crumbs”. This step provides an increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow.
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 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.
In this step, the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulfide under controlled temperature (20 to 30° C) to form cellulose xanthate.
The yellow crumb is dissolved in an aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to the solution of the otherwise insoluble cellulose.
Because of the blocks of unxanthated cellulose in the crystalline regions, the yellow crumb is not completely soluble at this stage.
Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed “viscose”.
The viscose is allowed to stand for a period of time to “ripen” Two important processes 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 hydroxyls 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.
The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament.
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.
Production of Viscose Rayon Filament: The viscose solution is metered through a spinnerette 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.
Stretching and decomposition are vital for getting the desired tenacity and other properties of rayon. Slow regeneration of cellulose and stretching of rayon will lead to greater areas of crystallinity within the fibre, as is done with high tenacity rayons.
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 fibre.
Sodium and zinc sulfates control the rate of decomposition (of cellulose xanthate to cellulose) and fibre formation Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fibre axis.
The freshly regenerated rayon contains many salts and other water-soluble impurities which need to be removed. Several different washing techniques may be used.
If the rayon is to be used as a staple (i.e, discrete lengths of fibre), the group of filaments (termed “tow”) is passed through a rotary cutter to provide a fibre which can be processed in much the same way as cotton.
General Properties of Viscose Rayon
- The luster of the fibres can vary from very bright to very dull and can be controlled by the addition of delustrants to the Viscose solution.
- The tensile strength of viscose rayon is greater than that of wool, but it is only about half as great as that of silk. Viscose rayon is also weaker than cotton and linen and its strength is reduced by 40% to 70% when wet.
- Viscose rayon has greater elasticity than cotton or linen but less than that of wool or silk. The elasticity of the fibre can be modified during manufacturing; fibres of high extensibility and low tenacity or of low extensibility and high tenacity can be manufactured.
- Viscose rayon creases easily although special crease resistant finish can be applied.
- It is a good conductor of heat and is therefore appropriate for summer clothing.
- Viscose rayon is one of the most absorbent of all textiles. The moisture regains of rayon fibres is slightly higher than that of other natural cellulosic fibres. This characteristic of the fibres provides a higher absorbency rate that facilitates dyeing and finishing processes.
- The fibres stretch easily during yam and fabric manufacture, followed by relaxation shrinkage after laundering. This is mostly the result of fabric construction than of fibre properties; tightly woven fabrics will exhibit less size change because of the compact arrangement of yams and fabrics. Finishes are used on most rayon fabrics to control dimensional stability.
- Viscose rayon because of its smoothness produces fabrics which are resistant to soiling.
- Viscose rayon is cellulosic and it will burn in much the same manner as cotton. High temperatures during ironing may scorch rayon fabrics; laundering temperatures should generally not exceed 60°C.
- The fibres are not damaged by weak solutions of alkalis but strong alkalis cause swelling of the fibres and loss of strength.
- Hot and cold concentrated inorganic acids cause the fibres to disintegrate Viscose rayon fibre is resistant to dry-cleaning solvents and stain removal agents.
- Rayon products have good resistance to ageing and will keep for several years if stored in proper conditions. They should be stored in dry, clean and relatively dark areas
- Mildew will damage all types of rayon; viscose rayon is not attacked by moths, its resistance to insects is similar to cotton and it can be attacked by silverfish. Viscose rayon fibre has generally good resistance to sunlight but will deteriorate when left for long periods in the sun.
Types of Viscose Rayon
Rayon fibres are engineered to possess a range of properties to meet the demands for a wide variety of end uses. Some of the important types of fibres are briefly described.
1 High Wet Modulus Viscose Rayon
These fibres have an exceptionally high wet modulus of about 1 g/den and are used as parachute cords and other industrial uses Fortisan fibres made by Celanese (saponified acetate) has also been used for the same purpose.
2 Polynosic Viscose Rayon
These fibres have a very high degree of orientation, achieved as a result of very high stretching (up to 300 %) during processing. They have a unique fibrillar structure, high dry and wet strength low elongation (8 to 1 1 %), relatively low water retention and very high wet modulus
3 Flame Retardant Viscose Rayon
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 rayons have the additives distributed uniformly through the interior of the fibre and this property is advantageous over flame retardant cotton fibres where the flame retardant concentrates at the surface of the fibre
4 Super-Absorbent Viscose Rayon
This is being produced in order to obtain higher water retention capacity (although regular rayon retains as much as 100 % of its weight).
These fibres are used in surgical nonwovens. These fibres are obtained by including water- holding polymers (such as sodium polyacrylate or sodium carboxymethyl cellulose) in the viscose prior to spinning, to get a water retention capacity in the range of 150 to 200 % of its weight.
5 Micro-Denier Viscose Rayon
Rayon fibres with deniers below 1.0 are now being developed and introduced into the textile market. These can be used to substantially improve fabric strength and absorbent properties.
Applications of Viscose Rayon
Rayon fibres are widely used in apparel and home furnishings fabrics as woven fabrics; they are also commonly applied for industrial uses in products such as reinforcing yams, tires, brake hose or radiator hose.
Non-woven rayon fabrics are also used for personal care products, medical and surgical products and a wide variety of wipes. Blends with other fibres are common such as rayon/ cotton, rayon/polyester, or rayon/wool.
Rayon is a versatile fibre since it can be processed according to the end user’s needs and offers more variety than natural cellulosic fibres.
Through control of fibre size, yarn number, fabric construction techniques, dyes and finishes, fabrics can be produced that are sheer to heavy, soft to firm, stiff to limp, and in all colors.
Modal is a cellulosic fiber produced from wood. Modal has excellent easy care properties and thus stays soft after many washes.
It is very absorbent (absorbing up to 50% more moisture than cotton) and thus very comfortable to wear as it allows the skin to breathe.
The depth of the shade is much more in Modal fibers. It takes more color up colorfast, deep and permanent, bringing out the color’s natural brilliance.
Due to its outstanding breathability and easy-care properties, Modal is enjoying ever-increasing popularity in fabrics for ladies’ outerwear, lingerie, sportswear, and home textiles.
Modal fibers’ manufacturing is similar to Viscose rayon fibers.
- Very Soft handle.
- Excellent Comfort.
- Friendly to skin /Contamination free.
- The high degree of brilliance /whiteness.
- Excellent choice for sports/ladies wear.
- More comfortable than cotton.
- Very Easy to wash.
- Softer and smoother, after repeated wash.
Applications of Modal Fibre
- As a fabric for shirts, blouses, and dresses.
- For ladies’ men’s and children’s clothing.
- As sportswear and leisure-wear.
- For t-shirts, polo shirts, sweatshirts, and sweaters.
- As day and night lingerie, including lace.
- As soft denim for blouses, jackets, and jeans.
- For table cloths and bed linen.
- For towels and bathrobes.
- For socks and stockings, as well as in technical applications, such as tire cord, abrasive ground fabric, rubber cloths, and other coating supports.
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