Rayon Fiber: Types, Properties, Manufacturing Process and Uses
Ashish Kumar Dua
M.Tech, Dept. of Textile Engineering,
Indian Institute of Technology (IIT), Delhi.
Rayon fiber is composed of pure cellulose, the substance of which the cell walls of such woody plants as trees and cotton are largely composed of. They are made from cellulose that has been reformed or regenerated; consequently, these fibers are identified as regenerated cellulose fibers. Rayon is a manufactured regenerated cellulosic fiber. It is the first man-made fiber. It has a serrated round shape with smooth surface. It loses 30–50% of its strength when it is wet. Rayon is produced from naturally occurring polymers and therefore it is not a synthetic cellulosic fiber. The fiber is sold as artificial silk. There are three principal varieties of rayon namely viscose, cuprammonium and acetate rayon.
Rayon is the oldest fiber, is the regenerated cellulose fiber with a wide spectrum properties. Cellulose is to be one of the most useable natural polymers worldwide. It is biodegradable and renewable polymer. The common sources for industrial purpose are wood pulp and cotton lint. Highly purified wood pulp consists of 95 – 99% cellulose. It is called ‘chemical cellulose’ and ‘dissolving pulp’. Those chemical cellulose or dissolving pulps are use to manufacture man made fibers (e.g. viscose rayon, cellulose acetate). The process used to make viscose can either be a continuous or batch process. The batch process is flexible in producing a wide variety of rayons having broad Rayon’s versatility is the result of the fiber being chemically and structurally engineered by making use of the properties of cellulose from which it is made. However, it is somewhat difficult to control uniformity between batches and it also requires high labor involvement. The continuous process is the main method for producing rayon. Three methods of production lead to distinctly different types of rayon fibers, viscose rayon, cuprammonium rayon and saponified cellulose acetate.
The original name for viscose was rayon, and it was created as far back as 1855. George Audemars found that, when he dipped a needle into a thick solution of mulberry bark pulp and gummy rubber, he could make a thread. This process did not become viable until 1884, however, when Hilaire de Bernigaud patented the first commercial ‘synthetic’ fibre which he then manufactured the material known as Chardonnay Silk in 1889; the fabric was highly flammable, though, so it was taken off the market. In 1891, Charles Cross discovered the viscose process and Courtaulds produced the first safe, commercial viscose rayon in 1905 and was known as artificial silk. By 1925, there were two forms of rayon: one from regenerated, pure cellulose and one from a cellulose compound. These became separate entities in the 1950s: rayon and acetate. Rayon became known as viscose in the 1990s when the material became linked to the production process, like many other man-made fibres. The main countries that produce viscose are Italy, Austria, Germany, the United States, Brazil and India.
Its hygroscopicity and easy dyeability are additional assets. Rayon fiber can be produced with wide properties particularly mechanical properties. Method of dissolving cellulose were first discovered in the late 19th century and first fiber were made by dissolving cellulose in cuprammoniumm hydroxide and then forcing the solution through tinny orifice into a bath containing reagent to remove solvent to regenerate cellulose in filament form. Problem associated with lack of stability and considerations of cost competitiveness soon pushed this method into background. With the discovery of the cellulose the cellulosic have been and still are predominantly produced by this process. Due to the strong intermolecular bonds, cellulose does not melt and does not dissolve readily in ordinarily available solvents; chemists have resorted to the derivatization of cellulose to render it soluble and process-able. Specifically, the viscose process was developed. It converted cellulose into sodium cellulose xanthate, which was soluble in a caustic solution, making it possible to wet-spin the polymer into a fiber or film. This technique was accepted worldwide and has prospered. The process, however, consists of multiple steps and causes pollution. As a result, end users have looked for alternate methods of processing cellulose.
2. Different Types of Rayon Fiber
Rayon fiber is engineered to possess a range of properties to meet the demands for a wide variety of end uses. Types of rayon fiber are given below:
- High wet modulus rayon
- Polynosic rayon
- Specialty rayons
- Super absorbent rayons
- Tencel rayon
Some of the important types of rayon fiber are briefly described-
2.1 High wet modulus yarn (HWM):
These rayon fiber has exceptionally high wet modulus of about 1 g/den and are used as parachute cords and other industrial uses. Fortisan fibers made by Celanese (saponified acetate) has also been used for the same purpose.
The high-wet-modulus (HWM) fibers represent the third generation rayon produced by a process differing from the polynosic route, viz. the Toramomen process. The HWM fibers are made by a process which depends on the use of zinc modifier systems; by suitable adjustment of recipes and processing conditions, a wide variation of fiber properties can be achieved starting with fibers of low extensibility, similar to the polynosics, and extending to high extensibility fibers which are less cotton-like in their wet physical properties, but more suitable for blending with synthetic polymer fibers like polyester fiber, where stability can be achieved by resin finishing. The HWM fibers are capable of achieving a high level of crimp that cannot be obtained via the polynosic route; thus, the yarns can be more bulky, and the fabrics more lofty. However, for purposes of blending with cotton, it is desirable to select the low-elongation type of HWM fiber. The high elongation types of HWM fibers could be used with advantage in polyester blends, where the matching of fiber modulus becomes possible, and significant yarn strength improvement obtained over the polyester-cotton equivalent.
2.2 Polynosic rayon:
These rayon fibers 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 11 %), relatively low water retention and very high wet modulus.
The polynosic rayon fi bres represent a fair attempt to reduce the degree of disadvantage suffered by viscose in relation to cotton. By selecting good quality pulp and reducing the severity of chemical treatments a higher molecular chain-length is preserved (500–700 as compared with 240–270 of viscose rayon) and a microfibrillar structure akin to cotton is obtained by controlled precipitation. The polynosic fibers have a round cross-section, while viscose rayon has a serrated cross-section. The advantages achieved by way of higher tenacity, ratio of wet-to-dry tenacity, wet modulus, etc. can be readily seen in Table 4.7. High wet modulus and good elastic recovery are the most important features of polynosics. They differ from ordinary viscose by having better dimensional stability, ability to withstand mercerization, more crispiness, and lower water imbibition. By and large, the wet processing methods used for cotton could be applied to polynosics.
2.3 Specialty rayon fiber:
2.3.1 Flame retardant fibers: Flame retardancy 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.
2.3.2 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. These fibers are obtained by including water- holding polymers (such as sodium polyacrylate or sodium carboxy methyl cellulose) in the viscose prior to spinning, to get a water retention capacity in the range of 150 to 200 % of its weight.
2.3.3 Micro denier fibers: Rayon fiber 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.
2.3.4 Cross section modification: Modification in cross sectional shape of viscose rayon can be used to dramatically change the fibers’ aesthetic and technical properties. One such product is Viloft, a flat cross sectional fiber sold in Europe, which gives a unique soft handle, pleasing drape and handle. Another modified cross section fiber called Fiber ML (multi limbed) has a very well defined trilobal shape. Fabrics made of these fibers have considerably enhanced absorbency, bulk, cover and wet rigidity all of which are suitable for usage as nonwovens .
2.3.5 Tencel rayon: Unlike viscose rayon, Tencel is produced by a straight solvation process. Wood pulp is dissolved in an amine oxide, which does not lead to undue degradation of the cellulose chains. The clear viscous solution is filtered and extruded into an aqueous bath, which precipitates the cellulose as fiber. This process does not involve any direct chemical reaction and the diluted amine oxide is purified and reused. This makes for a completely contained process fully compatible with all environmental regulations.
2.3.6 Lyocel: A new form of cellulosic fiber, Lyocell, is starting to find uses in the nonwovens industry. Lyocell is manufactured using a solvent spinning process, and is produced by only two companies – Acordis and Lenzing AG. To produce Lyocell, wood cellulose is dissolved directly in n-methyl morpholine n-oxide at high temperature and pressure. The cellulose precipitates in fiber form as the solvent is diluted, and can then be purified and dried. The solvent is recovered and reused. Lyocell has all the advantages of rayon, and in many respects is superior. It has high strength in both dry and wet states, high absorbency, and can fibrillate under certain conditions. In addition, the closed-loop manufacturing process is far more environmentally friendly than that used to manufacture rayon, although it is also more costly.
Uses: Some major uses in apparel like as shirts, blouses, blankets, window treatment dresses, jackets, hats, socks, bedsheets and industrial uses such as tire cord, non- woven product, and also medical surgery product and other uses as hygiene product, diapers, towels. Rayon is the major feedstock in the production of carbon fiber.
3. Manufacturing Process of Viscose Rayon Fiber
The process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: (1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7)Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12) Washing, (13) Cutting.
The various steps involved in the process of manufacturing viscose are shown in Fig. and clarified below.
(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) Pressing: 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.
(3) 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.
(4) Aging: 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.
(5) 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
Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution. The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20°C under high-shear mixing conditions to obtain a viscous orange colored solution called “viscose”, which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated.
(6) Dissolving: The yellow crumb is dissolved in aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the inter-chain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to 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”.
(7) 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
(8) Filtering: The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament.
(9) 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.
(10) 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. 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 fiber, 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 fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 –> (C6H10O5)n + nCS2 + (n/2)Na2SO4
Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
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 40-50oC.
Spinning speed may be high as 120m/min.
(11) Drawing: The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fiber axis. As the chains become more parallel, inter-chain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibers.
(12) 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.
(13) Cutting: If the rayon is to be used as staple (i.e., discreet lengths of fiber), the group of filaments (termed “tow”) is passed through a rotary cutter to provide a fiber which can be processed in much the same way as cotton.
4. Structure of Rayon Fiber
In regenerated celluloses, the unit cell structure is an allotropic modification of cellulose I, designated as cellulose II (other allotropic modifications are also known as cellulose III and cellulose IV). The structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than definite polymorphic forms of cellulose which depend on the conditions by which the fiber is made. Rayon fiber properties will depend on: how cellulose molecules are arranged and held together; the average size and size distribution of the molecules.
- Cellulose I is shown in native cellulose.
- Cellulose II has been seen in regenerated cellulose or and mercerized cellulose.
- Cellulose III is produce when treat with liquid ammonia (NH3) or organic amines (RNH2).
- Cellulose IV is generate when we treat cellulose with heat and glycerol (CH2(OH)CH(OH)CH2(OH)).
Many models describe ways in which the cellulose molecules may be arranged to form fiber fine structure. The most popular models of fiber fine structure are the fringed micelle and fringed fibrillar structures. Essentially, they all entail the formation of crystallites or ordered regions.
The skin-core effect is very prominent in rayon fibers. Mass transfer in wet spinning is a slow process (which accounts for the skin-core effect) compared to the heat transfer in melt spinning. The skin contains numerous small crystallites and the core has fewer but larger crystallites. The skin is stronger and less extensible, compared to the core. It also swells less than the core; hence, water retention is lower in the skin than in the core although moisture regain is higher in the skin. This is explained by an increased number of hydroxyl groups available for bonding with water as a result of a larger total surface area of the numerous small crystallites.
When rayon fibers are worked in the wet state,the filament structure can be made to disintegrate into a fibrillar texture. The extent to which this occurs reflects the order that exists in the fiber structure, as a consequence of the way in which the cellulose molecules are brought together in spinning. Another important structural feature of rayon fiber is its cross-sectional shape. Various shapes include round, irregular, Y-shaped, E-shaped, U-shaped, T-shaped and flat.
5. Properties of Rayon Fiber
Variations during spinning of viscose or during drawing of filaments provide a wide variety of fibers with a wide variety of properties. These include:
- Fibers with thickness of 1.7 to 5.0dtex, particularly those between 1.7 and 3.3 dtex, dominate large scale production.
- Tenacity ranges between 2.0 to 2.6 g/den when dry and 1.0 to 1.5 g/den when wet.
- Wet strength of the fiber is of importance during its manufacturing and also in subsequent usage. Modifications in the production process have led to the problem of low wet strength being overcome.
- Dry and wet tenacity extend over a range depending on the degree of polymerization and crystallinity. The higher the crystallinity and orientation of rayon, the lower is the drop in tenacity upon wetting.
- Percentage elongation-at-break seems to vary from 10 to 30 % dry and 15 to 40 % wet. Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
- Thermal properties: Viscose rayon loses strength above 149°C; chars and decomposes at 177 to 204°C. It does not melt or stick at elevated temperatures.
- Chemical properties: Hot dilute acids attack rayon, whereas bases do not seem to significantly attack rayon. Rayon is attacked by bleaches at very high concentrations and by mildew under severe hot and moist conditions. Prolonged exposure to sunlight causes loss of strength because of degradation of cellulose chains.
- Abrasion resistance is fair and rayon resists pill formation. Rayon has both poor crease recovery and crease retention.
6. Role of Zinc in Spinning Bath
In the non-zinc spinning process under normal conditions of acid concentrated in the spinning bath, cellulose xanthate gel is converted into cellulose xanthic acid and then to cellulose. The process is very fast and regeneration takes place before the cellulose molecules can properly oriented. This result in a rather disorientation matrix with poor crystalline organization. The net result is a regenerated cellulose filament with poor dry strength and a very interior wet strength. So the zinc result in a transient zinc cellulose xanthate complex which is more stable against acid induced regeneration. Zinc being bivalent form a transient cross link between the adjustment xanthate groups. Coupled with crosslinkg the strong deswelling action of zinc xanthate gel which can be stretched to a highly oriented structure with small crystal size and relatively large crystals.
7. Spinning with Modifiers
It should be recognized that in the presence of zinc, modifiers enhance the action of zinc in spinning bath. It do not effect the viscose. Its mechanism is the formation of a semipermeable membrane by the combined action of zinc ions by the product trithiocarbamate ions from the viscose and the modifiers. This semipermeable retards the diffusion of both Zn+ and H+ ions in the filament, but actual ratio Zn+ to H+ ion penetration is markedly increased in the presence of modifiers. Hence its act as barrier for proton diffusion. Hence its acidification boundary shift further away from the spinning nozzle in the presence of modifiers.
Types of modifiers:
- Tertiary amine
- Quaternary ammonium salt
- Polyoxyalkylene derivative
- Polyoxyhydroxy polyamide
7.1 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 –
The spin bath temperature is kept around 55oC and the spinning speed is between 40 and 60 m/min. The stretch applied is 75-125%
7.2 Modified high wet-modulus yarns:
The condition of viscose solution and spinning bath composition are generally similar to those tyre yarns.
Spinning bath temperature-35oC is kept lower because it gives more deformable gel necessitating a slower spinning speed 20-40 m/min The result is that gel fibers are stretched at an earlier state of the gel dehydration and decomposition when the gel is more plastic and can be stretched more (125%-150%).
7.3 Polynosic fiber:
Polynosic is similar rayon fiber but difference in process of manufacturing than viscose rayon. Since the manufacturing process is different so their morphological structure also different. Generally polynosic fiber 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 and zinc sulphate.
- 6% cellulose
- 4.4% NaOH
- 500-600 D.P
- 500 poise viscosity
- Spinning speed—->20-30m/min
7.4 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.
8. Fiber Variant for Improved Bulk and Handle
Approaches has been to produce high performance crimped fibers where the bulk is due to the interaction between the fibers, creating bulk in resultant yarns and fabric. The second approach has been to produce an inherently bulky fiber using an inflation technique during fiber production.
Composition for producing high wet performance fibers,
8.1 High performance crimped fibers:
For many years standard crimped fibers have been available which are produced by altering the regeneration condition so that the skin of the fiber burst while still in spin bath. The liquid viscose thus processed is regenerated under slightly different conditions and a bicomponent structure result. The two parts of the fiber shrink differently in subsequently washing and drying processes and the fiber develops a permanent crimp as a result.
8.2 Inflated fibers:
In this a range of fibers cross-section can be produced, but a tubular structure provide the best combination of bulk and handle while still retaining the physical properties and processing performance of standard viscose rayon.
8.3 Spinning specifications:
|Xanthate sulphur (%)||0.8-1.2|
|Salt index (NaCl)||4-6|
|Primary Bath||Secondary Bath|
|Take-up speed (m/min)||30-50||30-50|
8.4 Super absorbent fibers:
These fibers are used in sanitary protection and in surgical dressing. For these end uses high absorbency and purity. For these fiber highly hydrophilic polymers that are also compatible with viscose are added to the spinning solution. Chemicals such as sodium polyacrylate, carboxymetyl cellulose, acrylamide-2-metylpropne sulphonic acid.
8.5 Flame retardant fibers:
Flame retardant additives are added in the viscose dope. Some of compound used are halogenized triaryl, halogenized alkyl phosphate, halogenized alkyl thiophosphate, aloxyphosphopanzenes. A flame retardant fiber using a flame retardant compound is phosphorous. The flame retardant compound is mixed with viscose solution prior to spinning.
Limiting oxygen index (LOI) of more than 26% qualifies a material for flame retardency, Tufban’s LOI value of 30-32% makes it an excellent fire resistant material. It is claimed that normal washing and dry –cleaning do not effect its flame retardant characteristics.
9. Incorporation of Carbon in Viscose Fiber
(a) Incorporation of carbon black for antistatic properties: Electrically conductive carbon is used for the production of electrically conductive viscose fiber. The conductivity increases with decreasing particle size of carbon. For the production of electrically conductive viscose fibers, a slightly alkaline, electric conductive carbon black with a particle size of 20 nm is dispersed in water and mixed in viscose solution prior to spinning. It is found that a loss of about 50% in tenacity of this fiber compared with the tenacity of regular viscose fiber. On the other hand electricity conductive is increased by five orders of magnitude.
(b) Incorporation of activated carbon: Activated carbon containing fiber with outstanding adsorption properties. For the production of activated carbon-containing viscose fibers, activated carbon from coconut with a very fine pores is used. This activated carbon is has to be aground to a very fine particle size and thoroughly dispersed in water in order to be incorporated in viscose fiber in a satisfactory way. These fibers are used in application in such as protective clothing against gases, flat structure for industrial gas adsorportion, shoe insoles odour-absorbing and antimicrobial wound dressing.
(c) Incorporation of graphite: Incorporating 40% of lubricating graphite with a purity 99.5% into viscose yields fibers with excellent lubricating properties which as packing and sealing for crankshafts. Packings from graphite containing viscose fibers may be used to a temperature of 180-200oC. They are stable in the pH range 5-9 and predominantly used to seal pumps and fitting for water, salt solutions, weak organic and inorganic acids.
10. Alternative to the Viscose Process
The viscose process provides inexpensive route to regenerate cellulose in fibrous form it constituent a health hazard due to toxic pollutants. Two major route which are much easier and less polluting than the old viscose process.
10.1 Spinning of solution of cellulose derivatives:
- In which dissolution involves nearly simultaneous derivative formation for example metal-amine solvents, N2O4-DMF, DMSO-PF SYSTEM.
- That produces a derivative from which cellulose can be easily generated for example-liquid ammonia salt system.
10.2 Organic solvent (Direct solvent):
10.2.1 Ammonia-ammonium thiocyanate: The solvent is prepared by condensing NH3 to a predetermined weight with a known amount of NH4SCN. Solution of cellulose in NH3-NH4SCN are prepared by making a slurry of cellulose and solvent and stirring at- 10oC. Fibers have been spun using wet spinning, dry spinning and dry-jet wet spinning containing 14% cellulose at a solvent composition of 24.5:75.5 (wt%) NH3:NH4SCN
10.2.2 N-Methylmorpholine N-Oxide and Water:
- It is the best solvent to dissolve viscose fiber.
- Its having strong oxidant property due to which it can dissolve cellulose.
- During the dissolving step time and temp. are maintained properly otherwise thermal degradation takes place. Generally temp. is around 130oC
- If the temp. is goes above 150oC, DP of cellulose goes down for this we add some phenolic oxidant. It stabilize the solution and finally oxidize the color compound.
Note- Its having high potential to dissolve cellulose up to 50%.
10.2.3 N-N Dimethyl acetamide and lithium chloride:
- It is another solvent which dissolve cellulose. It directly linked with very high reproducible.
- It was observed that fibers from wet spinning process exhibited superior properties.
- It makes an complex with OH-group of cellulose and help to dissolve cellulose.
- Take mixture of DMAc and dried cellulose. The mixture is distilled at 165oC for 30 min. in N2 atmosphere
- Mixture is then cool down at 100oC can add required amount of LiCl stirred at 80oC for 40 min.
- Generally this solvent can dissolve upto 15% (w/w) for DP-130 and 4% (w/w) for DP-1700.
10.2.4 Ionic liquid: Salt that melt at temp. below 100oC called ionic liquid. It can be used as green solvent. Or reaction media. Generally contain imidazolium, pyridinium or organic ammonium salt. The anions could be chloride, bromide. Room temp. have more complex structure. The polymer can be regenerated by precipitation with water. They are able to achieve a very high polymer concentration of up to 25%.
10.2.5 Amine salt: Two component system (amine salt) consisting of hydrazine (NH2-NH2) or ethylenediamine (NH2-CH2-CH2-NH2) and various thiocyanate salt such as LISCN, NASCN or KSCN that dissolve cellulose pump. However a high concentration 40-50%. Salt is generally required to obtain high concentration (upto 18-20% w/w) spinning dope.
Hydrazine is a good solvent swelling agent, similar to ammonia. Its boiling point 113.5oC which is much higher than (-33.4oC ). So that due to high boiling point its offer same potential advantages for investigating the solubility and behavior of cellulose.
|Dry tear strength (cn/tex)||22||22||35||38|
|Wet tear strength (cn/tex)||28||12||20||30|
|Water retention (%)||50||90-100||75||50-70|
|Degree of fibrillation||2||1||1||3|
11. Development in Process Technology
- Old batch-wise process to continuous or semi-continuous system. In the batch wise process the sequence of steeping, pressing and ageing took up to 40 hr. to produce alkali cellulose.
- Difficulties in ensuring contact temp. and equal ageing time, because a large no. of bins involved, frequently resulted in a variable degree of polymerization in the resultant viscose.
- In the modern plants bales of wood pulp are automatically fed into continuously slurry.
- By the use of catalyst and elevated temp during ageing have reduced to time 4-5 hr.
- Xanthation process has been improved with the use of wet churns. In which both xanthation and mixing carried out.
- More recently the introduction of back flush filters with non-woven metal screens has improved the filtration efficiency with the new non- woven metal screens the filtration amount has increased 50 fold. Thus the filtration size could be decreased.
- Completely automatization.
Development in process chemistry:
Reduction in chemical used such as CS2, NaOH and H2SO4. There are number of ways to achieve reduction in the amount of CS2 used in viscose and also allow a substantial reduction in viscose alkali content. Process-SINI process-Its also known as double steeping process operation of aged alkali cellulose at lower alkali concentration (10-12%). A second steeping after ages reduces the amount of free alkali in the crumb with out changing the bound alkali. This reduces the formation of by-product and improves distribution of xanthate group to get a stable viscose.
- This process yield a 30% reduction in CS2 usuage, reducing CS2 emission.
- It is claimed that even interior-grade pulp can be used with this process to yield a good quality of viscose fiber.
- Due to removal of low molecular weight fractions. In the second steeping as well as increased rate of swelling of alkali cellulose which increases the reactivity to CS2 during xanthation. With this process a higher CS2:NaOH ratio (9:4.5) can be used in viscose solution which result in a substantial reduction in H2SO4.
- Activation of cellulose with liquid ammonia prior to xanthation also reduces CS2 consumption by as much as 33%.
- Xanthation in the presence of surfactants like Berol spin decreases CS2 consumption without effecting the quality of rayon produced. The addition of urea to the steeping solution result in change viscosity of viscose, the ripening time decreases and a high degree of xanthate substitution is obtained. It is presumed that complex with the alkali cellulose is formed which control the side reactions occurring during xanthation process.
- A reduction in viscosity of viscose allow for an increases in α-cellulose content and leads to reduction in consumption of H2SO4 and also less amount of energy is required for transport, filtration, deaeration.
12. Uses of Rayon Fiber
Some major rayon fiber uses are given below:
- Apparel: Accessories, blouses, dresses, jackets, lingerie, linings, millinery, slacks, sportshirts, sportswear, suits, ties, work clothes;
- Home Furnishings: Bedspreads, blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery;
- Industrial Uses: Industrial products, medical surgical products, nonwoven products, tire cord
- Development in process technology and process chemistry are much environment friendly
- Polynosic fiber show high crystallinity, high resistant, high dimension stability.
- By different solvent, spinning specifications, modifiers. We can make end use product.
- Solvent is very costly so need to recycle it.
- Ethylenediamine/NaSCN System is the best solvent
 Spinning of cellulose from N-methyl morpholine N-oxide in the presence of additives, polymer 1990,vol 31,march.
 Structure formation of regenerated cellulose materials from NMMO solutions, progress in polymer science 26(2001).
 Life cycle assessment of man-made cellulose fibers, lenzinges berichte 88(2010)
 Modified polynosic fibers.
 Handbook of fiber chemistry, M lewin and E.M pearce.
 V.B Gupta and V.K Kothari, Manufacturing of fiber technology 481-513.
 Fibres to Fabrics by Bev Ashford
 The Substrates – Fibres, Yarn and Fabric by Mathews Kolanjikombil
 Introduction to Textile Fibres by H. V. Sreenivasa Murthy
 Novel cellulose solvent system and dry jet wet spinning of cellulose ED/KSCN solution, by hyun jik cel (thesis)
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.