Viscose Rayon


Viscose is a viscous organic liquid used to make rayon and cellophane. Viscose is becoming synonymous with rayon, a soft material commonly used in shirt, coats, jackets, and other outer wear.

Viscose fibres are made from regenerated cellulose. The process of dissolving pulp (the very viscose solution of pulp gave the name to the fibre product) was an achievement of the industrial revolution of the 19th century. And this innovation triggered the discovery of full synthetic fibres during the 20th century. Many other cellulose dissolving and regeneration processes like cuprammonium rayon, which was the first process made available for manmade fibres and was already developed in the 1850s, or cellulose derivatives (like acetate) were in competition with the viscose process, but viscose proved to be superior based on process and product performance. Its booming period ended after World War 2 with the introduction of the synthetic competitive products. In the last decade, the production has stabilised at approximately 2.7 million tonnes worldwide (Europe
600000 tonnes).

It is preferably supplied to market end-uses (textile products and nonwovens) where the hydrophilic (moisture absorbing) properties of the material, for instance in direct contact with the skin or with mucous membranes, are relevant.

Currently (2005), about 85 % of the total viscose fibre production is produced as staple fibres and about 15 % as filaments.

It has to be mentioned that a substantial amount of regenerated cellulose in the form of films (cellophane) based on the viscose process are still applied in sausage encasings and other packaging foils.

In recent history in Europe, textile viscose filament end-uses are receiving increased competition (resulting in phasing out of capacity) by cheaper competitive yarns based on polyester and polyamide, whereas viscose staple fibre and viscose tyre cord keep a strong position.

Viscose currently is becoming less common also because of the polluting effects of carbon disulphide and other by-products of the process, forcing some factories to close.

  • Processes (staple fibres and filament yarn)

When producing viscose fibres, the pulp – which is mainly cellulose from wood – is dissolved and subsequently precipitated under controlled conditions. The most important process worldwide is the so-called ‘viscose process’ where the alkaline pulp is treated with carbon disulphide (CS2) and dissolved by adding sodium hydroxide solution. A viscous orange-brown solution called ‘viscose’ is formed which is ripened, degassed and then pressed through spinnerets into a highly acidic spinning bath. Here, the cellulose precipitates when CS2 and the by-product H2S is released. After this, the cellulose is stretched, washed and then undergoes further processing.

At this point, a distinction has to be made between staple fibres and filament yarn:

• Staple fibres are cut into short pieces after the spinning bath. These short fibres, which are each approximately 4 cm long, are spun into textile yarns or processed into ‘non-woven’ products later on.
• In contrast, filament yarns are spun into endless fibres which can be used immediately.

Viscose products for textile usage with certain improved product characteristics are called ‘modal fibres’.

Regular rayon (or viscose) is the most widely produced form of rayon. This method of rayon production has been utilized since the early 1900s and it has the ability to produce either filament or staple fibers. The process is as follows:

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Figure 1.  gives a schematic view of both staple fibre and filament yarn production.

Cellulose: Production begins with processed cellulose

Immersion: The cellulose is dissolved in caustic soda: (C6H10O5)n + nNaOH —> (C6H9O4ONa)n + nH2O

Pressing: The solution is then pressed between rollers to remove excess liquid

White Crumb: The pressed sheets are crumbled or shredded to produce what is known as “white crumb”

Aging: The “white crumb” aged through exposure to oxygen

Xanthation: The aged “white crumb” is mixed with carbon disulfide in a process known as Xanthation, 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: (C6H9O4ONa)n + nCS2 —> (C6H9O4O-SC-SNa)n

Yellow Crumb: Xanthation changes the chemical makeup of the cellulose mixture and the resulting product is now called “yellow crumb”

Viscose: The “yellow crumb” is dissolved in a caustic solution to form viscose

Ripening: The viscose is set to stand for a period of time, allowing it to ripen:

(C6H9O4O-SC-SNa)n + nH2O —> (C6H10O5)n + nCS2 + nNaOH

Filtering: After ripening, the viscose is filtered to remove any undissolved particles

Degassing: Any bubbles of air are pressed from the viscose in a degassing process

Extruding: The viscose solution is extruded through a spinneret, which resembles a shower head with many small holes

Acid Bath: As the viscose exits the spinneret, it lands in a bath of sulfuric acid, resulting in the formation of rayon filaments:

(C6H9O4O-SC-SNa)n + ½nH2SO4 —> (C6H10O5)n + nCS2 + ½nNa2SO4

Drawing: The rayon filaments are stretched, known as drawing, to straighten out the fibers

Washing: The fibers are then washed to remove any residual chemicals

The basic process ends here. Depending on the desired product it is continued optional by

Cutting: The filaments are cut down when producing staple fibers

Spinning: Filament yarns are spun into endless fibres Figure

  • Production of filament yarns

Until the spinning step, the process is very similar to the production of staple fibres

Long fibred pulp is used as the raw material. For the first step, it is treated with diluted sodium hydroxide solution (approximately 15 %). Afterwards, the liquid is removed by pressing and then it is recycled back into the process together with fresh NaOH. Next, the pulp sheets are defibrated, pre-ripened and put into CS2 for chemical conversion to xanthate. After the addition of aqueous NaOH, the viscose emerges which is ripened and degassed in vacuum prior to spinning.

Depending on the quality of the fibres, the spinnerets have different numbers of holes ranging from 30 to more than 2000. The spinning bath is sulphur acidic and contains high concentrations of sodium sulphate (Na2SO4) and zinc sulphate (ZnSO4).

Three different spinning methods are used:

  1. pot spinning – the viscose is pressed directly into the spinning bath. This is possible for threads from the size of 67 to 1330 dtex*
  2. continuous spinning – the viscose is pressed through the spinneret into a spinning tube where the flowing spinning bath picks up the coagulating fibre. This is again possible for threads from the size of 67 to 1330 dtex
  3. bobbin spinning – this process is similar to continuous spinning, but the fibre is fully coagulated. In order to achieve this, it is let into a second spinning bath where the coagulation is finished. This technique is possible for threads from the size of 1220 to 2440 dtex.

After spinning, the fibres are washed, finished, dried and spooled.
Currently, there are installations with integrated as well as batch washing

Blow Room


  • Introduction

Basic operations in the blowroom:

  1. opening
  2. cleaning
  3. mixing or blending
  4. micro dust removal
  5. uniform feed to the carding machine
  6. Recycling the waste

Blow room installations consists of a sequence of different machines to carry out the above said operations.Moreover Since the tuft size of cotton becomes smaller and smaller, the required intensities of processing necessitates different machine configuration.

  • TECHNOLOGICAL POINTS IN BLOWROOM

· Opening in blowroom means opening into small flocks.Technological operation of opening means the volume of the flock is increased while the number of fibres remains constant. i.e. the specific density of the material is reduced

· The larger the dirt particle , the better they can be removed

· Since almost every blowroom machine can shatter particles, as far as possible a lot of impurities should be eliminated at the start of the process.Opening should be followed immediately by cleaning, if possible in the same machine.

· The higher the degree of opening, the higher the degree of cleaning. A very high cleaning effect is almost always purchased at the cost of a high fibre loss. Higher roller speeds give a better cleaning effect but also more stress on the fibre.

· Cleaning is made more difficult if the impurities of dirty cotton are distributed through a larger quantity of material by mixing with clean cotton.

· The cleaning efficiency is strongly dependent on the TRASH %. It is also affected by the size of the particle and stickiness of cotton. Therefore cleaning efficiency can be different for different cottons with the same trash %.

· There is a new concept called CLEANING RESISTANCE. Different cottons have different cleaning resistance.

· If cotton is opened well in the opening process, cleaning becomes easier because opened cotton has more surface area, therefore cleaning is more efficient

· If automatic bale opener is used, the tuft size should be as small as possible and the machine stop time should be reduced to the minimum level possible

· If Manual Bale openers are used, the tuft size fed to the feed lattice should be as small as possible

· Due to machine harvesting , cotton contains more and more impurities, which furthermore are shattered by hard ginning. Therefore cleaning is always an important basic operation.

· In cleaning, it is necessary to release the adhesion of the impurities to the fibres and to give the particles an opportunity to separate from the stock. The former is achieved mostly by picking of flocks, the latter is
achieved by leading the flocks over a grid.

· Using Inclined spiked lattice for opening cotton in the initial stages is always a better way of opening the cotton with minimum damages. Ofcourse the production is less with such type of machines. But one should bear in mind that if material is recycled more in the lattice, neps may increase.

· Traditional methods use more number of machines to open and clean natural fibres.

· Mechanical action on fibres causes some deterioration on yarn quality, particularly in terms of neps . Moreover it is true that the staple length of cotton can be significantly shortened .

· Intensive opening in the initial machines like Bale breaker and blending machines means that shorter overall cleaning lines are adequate.

· In a beating operation, the flocks are subjected to a sudden strong blow. The inertia of the impurities accelerated to a high speed, is substantially greater than that of the opened flocks due to the low air resistance
of the impurities. The latter are hurled against the grid and because of their small size, pass between the grid bars into the waste box, while the flocks continue around the periphery of the rotating beater.

· By using a much shorter machine sequence, fibres with better elastic properties and improved spinnability can be produced.

· Air streams are often used in the latest machine sequence, to separate fibres from trash particles by buoyancy differences rather than beating the material against a series of grid bars.

· There are three types of feeding apparatus in the blowroom opening machines

  1. two feed rollers( clamped)
  2. feed roller and a feed table
  3. a feed roller and pedals

· Two feed roller arrangements gives the best forwarding motion, but unfortunately results in greatest clamping distance between the cylinders and the beating element

· feed roller and pedal arrangement gives secure clamping throughout the width and a small clamping distance, which is very critical for an opening machine

· In a feed roller and table arrangement, the clamping distance can be made very small. This gives intensive opening, but clamping over the whole width is poor, because the roller presses only on the highest points
of the web. Thin places in the web can be dragged out of the web as a clump by the beaters

· Honeydew(sugar) or stickiness in cotton affect the process very badly. Because of that production and quality is affected. Particles stick to metal surfaces, and it gets aggravated with heat and pressure. These deposits change the surface characteristics which directly affects the quality and running behaviour. · There are chemicals which can be sprayed to split up the sugar drops to achieve better distribution. But this system should use water solutions which is not recommended due to various reasons.

· It is better to control the climate inside the department when sticky cotton is used. Low temperature ( around 22 degree celcius) and low humidity (45% RH). This requires an expensive air conditioning set up.

· The easiest way to process sticky cotton is to mix with good cotton and to process through two blending machines with 6 and 8 doublings and to install machines which will seggregate a heavier particles
by buoyancy differences.

· General factors which affect the degree of opening , cleaning and fibre loss are,

  1. thickness of the feed web
  2. density of the feed web
  3. fibre coherence
  4. fibre alignment
  5. size of the flocks in the feed (flock size may be same but density is different)
  6. the type of opening device
  7. speed of the opening device
  8. degree of penetration
  9. type of feed (loose or clamped)
  10. distance between feed and opening device
  11. type of opening device
  12. type of clothing
  13. point density of clothing
  14. arrangement of pins, needles, teeth
  15. speeds of the opening devices
  16. throughput speed of material
  17. type of grid bars
  18. area of the grid surface
  19. grid settings
  20. airflow through the grid
  21. condition of pre-opening
  22. quantity of material processed,
  23. position of the machine in the machine sequence
  24. feeding quantity variation to the beater
  25. ambient R.H.%
  26. ambient temperature

· Cotton contains very little dust before ginning. Dust is therefore caused by working of the material on the machine. New dust is being created through shattering of impurities and smashing and rubbing of fibres. However removal of dust is not simple. Dust particles are very light and therefore float with the cotton in the transport stream.Furthermore the particles adhere quite strongly to the fibres. If they are to be eliminated they are to be rubbed off.The main elimination points for adhering dust therefore, are those points in the process at which high fibre/metal friction or high fibre/fibre friction is produced.

· Removal of finest particles of contaminants and fibre fragments can be accomplished by releasing the dust into the air, like by turning the material over, and then removing the dust-contaminated air. Release of dust into the air occurs wherever the raw material is rolled, beaten or thrown about.Accordingly the air at such positions is sucked away. Perforated drums, stationary perforated drums, , stationary combs etc. are some instruments used to remove dust

Digg This

Properties of Cotton Fibre that needed for cotton spinning


The following properties a cotton fibre should have to contain of:-

1. Fibre Fineness: Fineness is one of the most important parameter determining the yarn quality. This is the best cotton properties that almost all of the cotton importers wants. Cotton  Fibre fineness influences the number of fibre in cross section. The finer fibre results the higher no. of fibre in yarn cross-sections. Fibre fineness influences primarily-

a) Spinning limit.

B) Lustre. C) Yarn Strength. D) Handle E) Yarn Evenness F) Productivity.

2. Maturity: The maturity of cotton fibre defines in terms of the development of the cell wall. A fully mature fibre has a developed cell wall. On the other hand, an immature fibre has a very thin cell wall. A fibre is to be considered as mature fibre when the cell wall of moisture swollen fibre represents 50% to 80% of the round cross section, as immature fibre represents 30% to 45% and as dead when it represents less than 25%.

Immature fibres lead to –a) napping b) loss of yarn strength c) high proportion of short fibre d) varying dyeability. So good cotton properties is to said as matured cotton fibre.

3. Fibre Length: The average length of spinnable fibre is called staple length. Staple length is one of the most important fibre characteristics. The quality, count, strength, etc depend on the staple length of the fibre. Higher the staple length, higher the yarn quality. Staple length influences –a) yarn evenness b) lustre of the product c) spinning limit d) yarn strength e) yarn hairiness f) handle of the product

The following length grouping are currently used in stating the trade staple or basic cotton properties-

Short Staple: 1″ or less.

Medium Staple: 1 and 1/32 inch to 1 and 1/8 inch

Long Staple: 1 and 5/32 inch to 1 and 3/8 inch.

Extra Long Staple: 1 and 13/32 inch and above.

4.  Cotton Fibre Strength: Toughness of fiber has a direct effect on yarn and fabric strength.  Cotton Properties means such type of physical behavior of a fibre. The higher the fibre strength, the higher the yarn and fabric strength. Very weak cotton tends to rupture during processing both in blow room and carding, creating short fibres and consequently deteriorate yarn strength and uniformity. Some significant breaking strength of fibres are-

Polyester >>35 to 60 CN/Tex

Cotton>>>>15 to 40 CN/Tex

Wool>>>>12 to 18 CN/Tex

5.  Cotton Fibre Cleanness: In addition to usable fibre, cotton stocks contain various kinds of foreign matters. Some vegetable matters are- a) husk portion, b) seed portion c) stem portion d) leaf portion e) wood portion.

Some mineral matters are- a) earth b) sand c) dust.

Some other foreign materials are- a) metal fragments b) cloth fragments c) packing materials.

6. Cotton Fibre Elongation: Elongation is specified as a percentage of the starting length. Textile products without classify would hardly be usable. They must be deformed and also return to the original shape. The fibre elongation should be at least 1 to 2%. The greater crease resistance of wool compared with cotton arises due to difference in theirelongation.