Sunday, November 10, 2019

RAYON FIBRE ( ARTIFICIAL SILK), RAYON FIBRE PRODUCTION METHOD, PHYSICAL PROPERTIES OF RAYON FIBRE, CHEMICAL PROPERTIES OF RAYON FIBRE

RAYON FIBRE (ARTIFICIAL SILK):

Rayon is a regenerated fibre. It is also known as artificial silk. It is produced from reconstructed cellulose. “Rayon was the first fibre to be manufactured by humans” it is produced from naturally occurring polymers and therefore it is not a synthetic fibre, but a regenerated cellulosic fibre. The fibre was sold as artificial silk until the name “rayon” was adopted in 1924. In Europe, it is known as “viscose”. “It is regenerated from lignin-free cellulose sources through some chemical processes”. It can be manufactured by adopting the following various methods:

Viscose rayon :

In this method of production of rayon fibres, the cellulose obtained from cotton lints is used. This cellulose may contain impurities like wax, sugar and fats etc. These impurities are removed by different chemical processes. In this way, a purified form of cellulose is obtained. “This cellulose is used to manufacture the viscose solution”. Finally, rayon fibres are generated by passing it through spinnerets using wet spinning.

Acetate rayon:

In this method of production of rayon fibres, cotton lints are used as a source of cellulose. “Cellulose is reacted with acetic anhydride”. The cellulose acetate results after this reaction. Since triacetate is soluble in chloroform only so that cost of production is very high. After the discovery of different polar solvents such as acetone, which hydrolyses cellulose acetate made the production of acetate rayon fibres was more cost-effective and efficient.

Cupramonium rayon:

In this method of manufacturing rayon fibres, “the purified cellulose reacts with copper and ammonia(Schweizer's reagent)”. Cotton lints are used as a source of cellulose.   The rayon fibres generated by this method possess similar characteristics to that of acetate rayon. This method is not eco-friendly. “Because of bad environmental effects of this production method, cuprammonium rayon is not produced in the United States America at present”.




PRODUCTION OF RAYON FIBRE:

The rayon fibres may be manufactured by using any one of the following three methods:

Viscose rayon

Cupramonium rayon

Acetate rayon

VISCOSE RAYON PRODUCTION METHOD:



In this method of production of rayon fibres, cotton lints are used as a source of cellulose. Purified cellulose is obtained from cotton lints. The cotton lints are dissolved in sodium hydroxide solution. An alkali cellulose results after the reaction. This alkali cellulose is passed between pressing rollers to eliminate the excess amount of sodium hydroxide. The sheet of alkali cellulose results after pressing it. The whole process completes in the following steps:

Shredding or crumbling of alkali cellulose sheet:

The sheet of alkali cellulose is chopped into very small pieces mechanically with the help of a crumbling machine. The surface area of alkali cellulose is increased maximum after breaking the cellulose sheet. The increased surface area of cellulose pieces makes it easier to process the cellulose. This chopped or shredded cellulose is often referred to as “white crumb”.

Aging of white crumbs:

The white crumbs are allowed to stand in contact with the oxygen present in the surrounding air. Since the white crumbs have high alkalinity so that the cellulose is partially oxidized and degraded to lower molecular weight. This degradation is controlled very carefully to produce chain length short enough to give desirable viscosity to the spinning solution, but still long enough to impart good physical properties to the fibre.

Xanthation:

The aged white crumbs are kept into mixing vessels after ageing. The carbon disulphide gas is allowed to pass in the mixing vessel. The CS2(carbon disulphide) gas gets reacted with alkali cellulose. In this reaction, the formation of xanthate ester groups takes place. The carbon disulphide also reacts with the alkaline medium and result in inorganic impurities at the same time. These impurities make the cellulose mixture a yellowish colour. This material is known as “yellow crumb” often. Because access to the CS2 is greatly restricted in the crystalline areas of the alkali cellulose, the yellow crumb is essentially a block copolymer of cellulose and cellulose xanthate.

Dissolution of yellow crumbs:

“The yellow crumbs are dissolved in aqueous sodium hydroxide 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 un-xanthated 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”.

Ripening of viscose solution:

The viscose solution is allowed to stand for a period of time for “ripening”. “Two reactions take place during ripening (Redistribution and loss of xanthate groups)”. The reversible xanthation reaction takes place in some of the xanthate groups.  Cellulosic hydroxyls result 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, areas are gradually broken down and a more complete solution is obtained. The CS2 that is lost reduces the solubility of the cellulose and facilitates regeneration of the cellulose after it is formed into a filament.

Filteration of viscose solution:

The viscose solution is filtered to remove undissolved materials which may interrupt the spinning process or maybe the cause of defects in the rayon filament.

Degassing of viscose solution:

The bubbles of air present in the viscose solution are removed before extrusion because air bubbles may create serious defects in rayon filaments such as voids or weak places. Now viscose solution is ready for extrusion.

The spinning of viscose rayon filaments (Wet Spinning method)

The viscose solution is forced through a spinneret with the help of a high-pressure pump. Spinneret is a device like a shower head that has many thin holes similar in diameter. Each thin hole produces a fine filament of viscose. As the viscose filament comes out from the thin hole of the spinneret, it comes in contact with a solution of sulphuric acid, sodium sulphate and, usually, Zn++ ions. Several reactions take place at this stage, which causes the cellulose to be regenerated and precipitate from the solution. Water diffuses out from the extruded viscose solution to increase the concentration in the filament beyond the limit of solubility. The xanthate groups form complexes with the Zn++ which draw the cellulose chains together. The acidic spin bath converts the xanthate functions into unstable xanthic acid groups, which spontaneously lose CS2 and regenerate the free hydroxyls of cellulose. (This is similar to the well-known reaction of carbonate salts with acid to form unstable carbonic acid, which loses CO2). The result is the formation of fine filaments of cellulose or rayon.

Drawing of viscose filaments:

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. As the chains become more parallel, interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibres.

Washing:

The freshly regenerated rayon contains many salts and other water-soluble impurities which need to be removed. Several different washing techniques may be used.

Cutting:

If the rayon is to be used as a staple (i.e., discreet lengths of fibre), the group of filaments (termed “tow”) is passed through a rotary cutter to provide a fibre that can be processed in much the same way as cotton.


CUPRAMMONIUM RAYON METHOD:

In this method of production of rayon, first of all, the purified cellulose is dissolved into cuprammonium hydroxide. This reaction is carried out at low temperatures in a nitrogen atmosphere. This solution in sodium hydroxide is extruded through a spinneret and the cellulose is generated in the hardening bath ( sulphuric acid) that removes the copper and ammonia. 


This bath also neutralises sodium hydroxide. “The cost of production is more than that of viscose rayon”. Its fibre cross-section is almost round.
Cuprammonium rayon is usually made in fine filaments that are used in lightweight summer dresses and blouses, sometimes in Combination with cotton to make textured fabrics with clubbed, uneven surfaces. 

ACETATE RAYON METHOD:

Acetate Rayon is produced from cellulose acetate by saponification process. First of all reaction of purified cotton is carried out with glacial acetic acid to make it more reactive. “It is then acetylated with an excess of glacial acetic acid and acetic anhydride in the presence of sulphuric acid”. Sulphuric acid acts as a catalyst and increase the speed of the reaction. As a result of this reaction, the cellulose triacetate is formed. When the triacetate is hydrolysed, it is converted into diacetate. 


The resultant mixture is poured into the water which precipitates the “cellulose acetate”. This cellulose acetate is dissolved in acetone and then filtered, deaerated. cellulose acetate is dissolved in acetone into a viscose resin for extrusion through spinnerets (which resemble a showerhead). As the filaments emerge, the solvent is evaporated in hot air by using the dry spinning method. A high degree of orientation is given to the fibre by “drawing” because of the fact that cellulose acetate is more plastic in nature. Its fibre cross-section is almost round but lobed.              

PROPERTIES OF RAYON FIBRE:

Rayon fibre has the following properties:

Physical Properties of Viscose Rayon:


Chemical properties of rayon fibre:

The rayon fibre poses the following chemical properties:

Effect of Acids:

The resistance of regenerated cellulose rayon’s to acids is generally less than that of cotton to the same concentrations of the same acids. Therefore, acid treatments must not be too drastic with respect to concentration, temperature and time. Organic acids can be safely used in 1 to 2 per cent concentrations without injury to the fibre. Inorganic acids such as hydrochloric & nitric can be used in surprisingly strong concentrations provided the temperatures are not too high and the treatment is brief. Oxalic acid for removal of iron stains is not recommended except at temperatures lower than 150°F. At high temperatures and concentrations, all acids will destroy or carbonize regenerated rayon’s. No harmful action will result if applied at .5 to 3 per cent solution at room temperature.

Effect of solvents:

Textile solvents can be used on Viscose rayon without any deteriorating effect. Viscose rayon dissolves in cuprammonium hydroxide solution.

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.

Effect 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.


USES OF RAYON FIBRE

Rayon fibre has the following major uses:
Apparel uses:
blouses, linings, millinery, slacks, lingerie, sport shirts, jackets, dresses, suits, ties, etc. 
Home Furnishings uses:
curtains, blankets, draperies, bed sheets, upholstery, tablecloths, Sleeping bags. 
Industrial Uses:
 medical surgical products, tire cord, nonwoven products,  other industrial products,
Other Uses:
Feminine hygiene products.

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1 comment:

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