Polyurethane Fibres ( Spandax, Lycra)

Polyurethane Fibres ( Spandax, Lycra)

Polyurethane is produced by action of butanediol and hexamethylene diisocyanate.

The polyurethane thus formed has rubber like properties. It gives an elastomeric fibre, which displays elasticity associated with natural rubber and hence can be stretched several times its original length and on releasing the stretching loads it will snap back quickly to recover its original length almost completely. Therefore polyurethane fibres are called snap back or elastomeric fibres.

Different Steps in Fiber Manufacture

Prepolymer Production:

The soft segments of the final polymer are formed in this step. The segments are the source of amorphous regions which permit unfolding of the molecular chains leading to the extension of the fibre under tensile stresses. These segments are made by normal condensation polymerisation techniques. These segments have hydroxy groups at the end.

Reaction Between prepolymers and Diisocyanate

The first prepolymer is reacted with excess of diisocyanate to form urethane groups in the molecular chains.

Segmented polyurethane production

In this step the hard segment is created by chain extension in which second prepolymer is treated with glycols or diamines.

Spinning

When the final polymer contain essentially linear macromolecules then it is dissolved in the solvent ( eg. DMF- Dimethyl Formamide) and extruded through spinnerettes into a coagulating bath ( water) as in wet spinning or into an atmosphere to remove the solvent as in dry spinning.

Properties

Strength: 0.55-1.0 gpd

Extension at Break: 520-610 %

Specific Gravity: 1.20-1.25

Set % at 600% stretch: 70%

Moisture Regain: 0.8-1.2

It is a thermoplastic fibres which sticks at 170 deg C and melts at 230 deg C

It has an excellent resistance to sunlight

It is resistant to insects and microorganisms.

It is resistant to common solvents such as dry cleaning solvents and saturated hydrocarbons.

Chemical Properties

It has good resistance to cold dilute Acids, Hot concentrated acids slightly yellow it.

It has a good resistance to weak and cold alkalies. It has good resistance to cosmetic oils and lotions. Chlorites and hypochlorites attack the fibre.

When heated the fibres fuse and do not shrink from the flame. They burn and produce soft fluffy black ash.

Polypropylene Fibres- Manufacturing Process

Polypropylene Fibres

Propylene is one of the constituents obtained from thermal or catalytic cracking of petroleum. Under suitable polymerising conditions, propylene produces fibres forming polypropylene.

Polymerisation: It is done by dissolving propylene in heptane using TiCl3Al(C2H5)3 catalyst system at about 100 deg C under a pressure of 30 Atm for 8 hours. The polymer has a molecular weight of about 80000.

Spinning : Polypropylene is melt spun. The filaments are extruded at 100 deg C above the melting point, cooled in air chamber and collected on bobbins. The filaments are hot drawn (polyethene- cold drawn) and twisted into yarns.


Properties:

1. PP fibres are colorless and have a smooth surface, with round cross section.

2. Tenacity- 4.5-6 gpd
Elongation at Break: 17-20 %
Elastic Properties at 2% extenstion: Instantenous
Stretch for 30 Seconds: 91%, delayed - 9%
Moisture Regain: Nil

3. Boiling water shrinks PP by about 15-20% in 20 minutes

4. Specific Gravity: 0.85-0.92

5. Softening point- 150 deg C, Melting Point: 160-170 deg C

6. PP is also attacked by atmospheric oxygen in presence of sunlight

7. It has excellent resistance to common organic solvents

8. It is resistant to insects and microorganisms

9. PP is generally resistant to common chemicals.

Polyethylene Fibres

Polyolefin fibres

Fibres made from polymers or copolymers of olefin hydrocarbons such as ethylene, propylene are called polyolefins.

Polyethylene: Of all the fibre forming polymers, polyethylene (made by addition polymerisation) Ch2==Ch2 has the simplest structure.

Manufacture: Ethylene is the principal raw material for producing polyethylene fibres. Ethylene gas is obtained by cracking petroleum.

Polymerisation: Ethylene is polymerised under severe conditions in autoclaves at 200 deg C and 1500 atmospheric pressure in the presence of traces (0.01%) of oxygen acting as a catalyst. The polymer resembles paraffin wax and is characterised by low density.

Spinning : Spinning of polyethylene is carried out by melt spinning. The polymer with a molecular weight of about 15,000 is spun from the melt at about 205 deg C and extended through a spinnerette of 0.1 mm diameter into a current of cooling gas. The filaments are cooled to 15 -60 deg C and stretched 4 to 10 times their original length. The drawn monofilaments are wound on spools.

Properties of polyethylene

a. Polyethylene fibre has a round cross section and has a smooth surface. Fibres made from low molecular weight polyethylene have a grease like handle.

b. Specific Gravity- 0.92
Tenacity - 1.0-1.5 gpd
Elongation at Break %- 45-50
Tensile Strength psi - 15000
Softening Range: deg C- 85-90

c The moisture regain of polyethylene is practically nil and hence moisture does not affect the mechanical properties of the fibres.

d. Polyethylene is insoluble in most of the common organic solvents at room temperature.

e. Polyethylene fibres have a high degree of resistance to acids and alkalies at all concentrations even at high temperature.

f. The fibre is generally inert and is resistant to wide range of chemicals at ordinary temperatures. They are attacked by oxidising agents.

Manufacturing Process and Properties of PVA

Polyvinyl Alcohol Fibres

Polyvinyl alcohol (water soluble compound) can be described as a polyhydric, having secondary alcoholic groups on alternate carbon atoms of an aliphatic macromolecule.

Because of the presence of a large number of hydroxy groups, in its molecular structure, it is soluble in water. This is solublised in water by a treatment with formaldehyde.

Manufacture of Polyvinyl Alcohol


1. Production of acetic acid from acetylene

For this purpose, limestone is calcinated to give quicklime (CaO) which is treated with coke at elevated temperature to form calciium carbide. Acetylene is generated by treating calcium carbide with water. A part of acetylene is converted into acetic acidby combined hydration and oxidation.

Synthesis of Vinyl Acetate

The acetic acid formed in the above step is reacted with acetylene in the presence of zinc acetate catalyst when vinyl acetate is formed.

Polymerisation of Vinyl Acetate

A solution of vinyl acetate in methanol is used for the polymerisation of vinyl acetate in the presence of a peroxide or azo compound as a catalyst.

Conversion of PVAcetate into PVA

NaOH is added in PV Acetate solution in methanol, when alcoholysis of the acetate groups takes place.

Spinning

The precipitated PVA as obtained in the preceding step is pressed and dried. It is then dissolved in water to give a 15% solution of the polymer. This solution is extruded into a spinning bath containing sulphuric acid ( 20%), Glauber's Salt ( 25%), formaldehyde (5%) and water (50%)

Properties

Shrinkage Properties: 10% at 220-230 deg C.

At 220 deg c, It begins to turn yellow and shrinks.

The fiber is inert to animal, vegetable and mineral oils and to most common organic solvents.

It has good resistance to acids under normal conditions, Hot or concentrated mineral acids cause swelling and shrinkage. Its resistance to alkali is generally good. Strong alkalies cause yellowing without affecting the tenacity.

Fabrics made from this fibre do not get solied easily. They are easy to wash and quick to dry. They have good crease retention.


Specific Gravity: 1.28

	Staple	Filament
Tenacity ( GPD)
Dry	3.8-6.2	6.0-8.5
Wet	3.2-5.0	5.0-7.6
Elongation at Break
Dry	13-26%	9-22%
Wet	14-27%	10-26%
Elastic Recovery	65-85%	70-90%
Moisture Regain	4.5-5%	3-5%

Polyvinyl Chloride- Manufacturing Process and Properties

Polyvinyl Chloride (Vinyon)

Fibre Manufacture:

Vinyl Chloride is the principal raw material from which polyvinyl chloride is made by addition polymerisation. There are two methods commonly used for the production of vinyl chloride:

1. Ethylene+ chlorine--> Ethylene Dichloride--600 deg C--> Vinyl chloride +HCl

or

Cl-CH2-CH2-Cl--300deg C +Charcoal--> Vinyl Chloride + HCl

or

Cl-CH2-CH2-Cl--CH3OH+NaOH (60 deg C)--> vinyl Chloride + NaCl+ H2

2. Acetylene +HCL--150 deg C, HgCl--> CH2=CHCl (Vinyl Chloride)

Polymerisation

the vinyl chloride monomer is polymerised in the emulsion form in an autoclave at a pressure of 50 Atm and at a temperature of 65 deg C. A suspension of the polymer is obtained which is then spray dried.

Spinning

This may be done by dry spinning or wet spinning.

1. Dry Spinning: In the dry spinning process the polymer is dissolved in a mixture of CS2 and acetone, filtered and pumped at 70 deg to 100 deg through spinnerettes into a chamber, provided with heated walls, and into which air is introduced. The solvent evaporating from the extruded filaments is carried away by the air. At the bottom of the chamber the solvent free filaments are removed through a fine orifice and wound on a bobbin. The solvent is recovered and used again. the filaments are stretched to ensure that the molecular chains get oriented and the fibres become stronger and attain less extension at break, increased brightness, transparency etc.

2. Wet Spinning: In the wet spinning process, PVC is dissolved in THF (Tetra Hydro Furon) to give a highly concentrated solution, which is spun into water, through a stretch spinning funnel. The filaments ar stretched and cut into staple fibres.

Properties

1. Tenacity: Wet or Dry: 2.7-3 gpd
2. Elongation at BreaK: Wet or Dry: 12-20 %
3. Moisture Content: 0
4. Specific Gravity: 1.4

v. Effect of Heat: It contracts at temperatures above 78 deg C and shrinks to half its original length at 100 deg C.

vi. It has an excellent resistance to sunlight. It is completely resistant to insects and microorganisms. It is inherently non-flammable.

vii. It is exceptionally resistant to caustic soda, nitric acid and sulphuric acid. It has outstanding resistance to many chemicals including bleaching agents, reducing agents.

Acrylic- Manufacturing Process and Properties

Polyacrilonitrile ( Acrylic)

vinyl Cyanide, more commonly known as acrylonitrile, can under go addition polymerisation to form polyacrylonitrile.

Raw Material

Acrilonitrile is the main main raw material for the manufacture of acrylic fibres. It is made by different methods. In one commercial method, hydrogen cyanide is treated with acetylene:

acetylene + Hydrogen cyanide --> Acrilonitrile

2nd Method

Ethylene--Air Oxidation--> Ethylene oxide + HCN--> Ethylene cyanahydrin--Dehydration at 300 deg C (catalyst)--> Acrylonitrile

In a continuous polymerisation process, 95% acrylonitrile and 6% methyl acrylate (400 parts) 0.25% aqueous solution of K2S2O8(600 parts), 0.50 % Na2S2O5 solution ( 600 Parts) and 2N sulphuric acid (2.5 Parts) are fed into the reaction vessel at 52 deg C under nitrogen atmosphere giving a slurry with 67% polymer. The slurry is continuously withdrawn, filtered and washed till it is free from salts and dried.

Acrilonitrile is dry spun. The material is dissolved in dimethyl formamide, the solution contains 10-20 polymers. It is heated and extruded into a heated spinning cell. A heated evaporating medium such as air, nitrogen or steam moves counter current to the travel of filaments and removes the solvent to take it to a recovery unit. The filaments are hot stretched at 100 to 250 C depending on the time of contact in the hot zone, to several times their original length.

Properties of Acrylic Fibres

1. Acrylic has a warm and dry hand like wool. Its density is 1.17 g/cc as compared to 1.32 g/cc of wool. It is about 30% bulkier than wool. It has about 20% greater insulating power than wool.

2. Acrylic has a moisture regain of 1.5-2% at 65% RH and 70 deg F.

3. It has a tenacity of 5 gpd in dry state and 4-8 gpd in wet state.

4. Breaking elongation is 15% ( both states)

5. It has a elastic recovery of 85% after 4% extension when the load is released immediately.

6. It has a good thermal stability. When exposed to temperatures above 175 deg C for prolonged periods some discolouration takes place.

7. Acrylic shrinks by about 1.5% when treated with boiling water for 30 min.

8. It has a good resistance to mineral acids. The resistance to weak alkalies is fairly good, while hot strong alkalies rapidly attack acrylic.

9. Moths, Mildew and insects do not attack Acrylic.

10. It has an outstanding stability towards commonly bleaching agents.

Uses

1. Knit Jersey, Sweater, blankets
2. Wrinkle resistant fabrics.
3. Pile and Fleece fabrics
4. Carpets and rugs.

Properties of Polyester

Tenacity (gpd)	High Tenacity	Normal Tenacity	Staple
Dry	6-7	4.5-5.5	3.5-4
Wet	6-7	4.5-5.5	3.5-4
Elongation (%)
Dry	12.5-7.5	25-15	40-25
Wet	12.5-7.5	25-15	40-25
Density	1.38	1.38	1.38

Moisture Regain

At 65% RH and 70 deg F--> 0.4%

Because of low moisture regain, it develops static charge. Garments of polyester fibres get soiled easily during wear.

Thermal Properties

Polyester fibres are most thermally stable of all synthetic fibres. As with all thermoplastic fibres, its tenacity decreases and elongation increases with rise in temperature. When ignited, polyester fibre burns with difficulty.

Shrinkage

Polyester shrinks approx 7% when immersed in an unrestrained state in boiling water. Like other textile fibres, polyester fibres undergo degradation when exposed to sunlight.

Its biological resistance is good as it is not a nutrient for microorganisms.

Swelling and Dissolving

The fibre swells in 2% solution of benzoic acid, salycylic acid and phenol.

Alcohols, Ketones, soaps, detergents and drycleaning solvents have no chemical action on polyester fibres.

Chemical Resistance

Polyester fibres have a high resistance to organic and mineral acids. Weak acids do not harm even at boil. Similarly strong acids including hydrofluoric acids do not attack the fibres appreciably in the cold.

Uses of Polyester

1. Woven and Knitted Fabrics, especially blends.
2. Conveyor belts, tyre cords, tarpaulines etc.
3. For filling pillows
4. For paper making machine
5. Insulating tapes
6. Hose pipe with rubber or PVC
7. Ropes, fish netting and sail cloth.

Manufacturing Process of Polyester

Manufacture of Polyester

These fibres are also known as Terylene, Terene, Dacron etc.

These fibres are synthetic textile fibres of high polymers which are obtained by esterification of dicarboxylic acids, with glycols or by ester exchange reactions between dicarboxylic acid esters and glycols.

Thus Terylene is made by polymerising using ester exchange reation between dimethyl teraphthlate and ethylene glycol.

Raw Materials

The main raw materials required for the manufacture of Terylene polyester fibres are p-xylene ethylene glycol and methanol.

or Dacron ( Du Pont ) is produced by polycondensation reaction using Teraphthaleic Acid (TPA) and Ethylene Glocol

Manufacture of TPA

P-xylene-- Air, nitric Acid-->P-Toluic Acid--> Teraphthaleic Acid

Manufacture of DMT

p-xylene--Air 200 degC, co-toluate--> Toluic Acid--Ch3OH--> Monomethyl toluate--oxidation--> Monomethyl teraphthalate--CH3OH--> DMT

The use of Dimethyl Teraphthalate is preferred instead of Teraphthalic acid as the purity of the reacting chemicals is essential and it is easier to purify DMT than teraphthalic acid.

Manufacture of Ethylene Glycol

Ethylene--Oxidation with air-->Ethylene Oxide--Hydrolysis-->Ethylene Glycol
or
Ethylene--Hypochlorous Acid HOCl--> Ethylene Chlorohydrin--Alkaline Hydrolysis--> Ethylene Glycol

Production


The polymer is made by heating teraphthalic acid with excess of ethylene glycol ( Both of high priority) in an atmosphere of nitrogen initially at atmospheric pressure. A catalyst like hydrochloric acid speeds up the reaction.

The resulting low molecular weight ethylene glycol teraphthalate is then heated at 280 deg C for 30 minutes at atmospheric pressure and then for 10 hours under vacuum. The excess of ethylene glycol is distilled off. the ester can polymerise now to form a product of high molecular weight. The resulting polymer is hard and almost white substance, melting at 256 deg C and has a molecular weight of 8000-10000. Filaments are prepared from this.

Spinning of Polyester Fibres

The polymer is extruded in the form of a ribbon. This ribbon is then converted into chips.

The wet chips are dried and fed through a hopper, ready for melting. This molten polymer is then extruded under high pressure through spinnerettes down to cylinder.

Each spinnerette contains 24 or so holes. A spinning finish is applied at this stage as a lubricant and an antistatic agent. The undrawn yarn is then wound onto cylinders.

This yarn goes to the drawing zone, where draw twist machines draw it to about four times their original length. This is hot drawn in contrast to cold drawing of nylon filaments.

For the production of staple fibres, the filaments are first brought together to from a thick tow. These are distributed in large cans. The tow is drawn to get correct strength. Then it is passed through a crimping machines, the crimps being stabilized by heating in ovens. It is then cut into specified lengths and baled ready for despatch.

Properties of Nylon 6

Properties of Nylon 6

Nylon 6 has certain advantages over Nylon 6,6,. Firstly the systheisi fo caprolectum is easier than that of Hexamethylene Diamine and Adipic Acid. Therefore it is cheaper to make Nylon 6 than Nylon6,6. Secondly Nylon 6 has greater affinity for acid dyes than Nylon6,6,

Mechanical Properties

Density: 1.14 g/cc
Tenacity: Dry= 4.2-5.8 gpd, Wet=4.0-5.3 gpd
Elongation at Break--> Dry = 24-40, Wet=28-43
Elastic Recovery at 4% extension= 100%
Moisture Regain= 4%
Because of low MR, wet nylon dries quickly.
Melting Point= 215 deg C ( Nylon 66-250 deg C)
It is weakened by prolonged exposure to sunlight.

Chemical Properties

1. It is resistant to most organic acids such as benzene, chloroform, acetone, esters ethers etc.

2. It dissolves in phenol, cresol and strong mineral acids.

3. good resistant towards alkalies.

4. Resistant to inorganic acids

These fibres are cylinderical in shape, with smooth surfaces and without having any markings. The fibres are unifrom in diameter and appear round in cross section.

Uses


a. Tyre Cord Manufacturing
b. Fishing Lines
c. Luxury Yachts
d. Stockings with good fit, sheerness, quick washing and drying properties.

Manufacturing Process of Nylon 6

Manufacturing Process of Nylon 6

Nylon Manufactured in India at present is of this type. This is made from Caprolactum which is made by a series of reactions using products obtained from coal tar

Coal Tar--> Benzene--Chlorine--> Chlorobenzene--> Sodium Phenate--HCL--> Phenol--H2 (Nickel)-->Cyclohexanol--Oxidation Air Fe, Zn Catalyst--> Cyclohexanone--> Cyclohexanone Oxime--H2SO4--> Caprolectum

Polymerisation

Caprolectum is a white flaky solid, melting at 68 deg C and is soluble in water. the polymerisation is carried out in stainless steel cylinders.

Hot Caprolectum is mixed with a suspension of pigment, acid promotor and acid chain stopper. The extent of polymerisation depends upon the temperature of polymerisation. The purpose of acid chain stopper is to stop furthur polymerisation so that a desired density of molten polymer may be obtained.

The molten polymer is extruded into ribbons and cut into chips. These chips are used for the production of continuous filaments.

Melt Spinning

Continuous filaments are made by melt spinning. Dry polymer chips are fed to a melt spinning apparatus, wherein one section of the chips fall, into a melting region where they are heated electrically to 250-260 deg C. The molten polymer flows into a conical section to form a pool, which feeds a spinning pump and spinnerette. The pool is kept under an atmosphere of nitrogen to prevent decomposition by air.

The molten polymer leaving the pump is filtered before entering the spinnerette which is a stainless steel disc having a number of holes, the number and diameter of which determine the type of yarn formed. Before reaching the machine in which cheese is build up, the filaments are moistened with water to ensure dimensional stability of the final packages.

The yarn thus formed is not strong enough and has a very high extensibility. the yarn contains a large number of macro molecules which are unoriented and these must be oriented so as to lie parallel to the length of the fibre to develop full strength. This is done by stretching the yarn to 3-4 times its original length.

Properties of Nylon 6,6

Properties of Nylon 6,6

Strength

The most outstanding property of nylon is its strength and elasticity. The tenacity varies from 8.8-4.3 gpd while corresponding elongation at break varies from 18-45%. The wet strength of nylon is 80-90% of its dry strength and the elongation at break increases by 5-30% on wetting.

Density: 1.14 g/cc

Elastic Recovery

When nylon yarn is stretched 1,2 and 4% with a load of 0.25 gpd for 30 seconds and then released the recovery after 60 seconds is 38, 63 and 73% respectively.

Moisture Regain

Nylon has a moisture regain of about 4% at 65% RH and 70 deg F.

Action of Light

Like other fibres, nylon undergoes degradation and weakens when exposed to lights.

Appearance

Nylon is dull and semi opaque before cold drawing, but on orientation its lustre is greatly incresed. Delustering is done by adding TiO2 in the polymerisation mixture.

Action of Heat

Nylon melts at 262 deg C in an atmosphere of Nitrogen and at 250 deg C in air. When a very hot iron is used for ironing nylon garments, sticking or even fusion may take place. Therefore ironing should not be done above 180 deg C. Permanent set may be applied to Nylon by heat setting with 25 psi pressure with saturated steam. The pleats thus set remian on wearing and washing even in hot water.

Chemical Properties

Nylon is extremely stable chemically. For example dry cleaning solvents, alcohols, aldehydes, ketones, ethers, hydrocarbons, chlorinated hydrocarbons, soaps and synthetic detergents and water including sea water do not affect Nylon.

Also it has got a remarkable stability towards alkali.

Biological Properties

Nylon is not a nutrient for Mildew or bacteria and is not eaten up by moth larvae. But they bite their way up when imprisoned in nylon cloth. It is harmless to human skin.

Manufacturing Process of Nylon 6,6

Manufacturing Process of Nylon 6,6

Nylon 6,6 is made from Hexamethylene diamine and adipic acid as shown in the figure below.



Spinning of Nylon 6,6



The chips of nylon polymer are fed through a hopper A, into a spinning vessel B, on an electrically heated grid ( perforated plate) C. The perforations are so small that the chips do not pass through, but when melted, the liquid can pass.

The molten nylon collects as a pool D, at the bottom of the vessel. This liquid should not come into contact with oxygen or air and hence nitrogen is introduced into the vessel. The molten polymer is kept at a temperature of about 288 deg C and sucked by a pump F, into a spinnerette E. The molten polymer solidifies as soon as it emerges out of the spinnerette. The filament thus formed pass through a colloing zone, in which cold air G circulates directed towards the filaments. The filaments are then passed through a steam chamber H, to wet them before winding on the bobbin L.

Drawing

Nylon filaments as obtained are not very strong. They have to tbe drawn 4-7 times their original length. This is done by cold drawing. The yarn in pulled off from bobbin L through guides M and N, between a pair of rollers O. The speed of rotation of these rollers determines the initial speed. The yarn then goes over a deflector P, and two to three times around roller Q, running at five times the speed than that of O. The yarn subsequently courses through another guide R, and wound on another bobbin which rotates at very high speed, to impart twist in the yarn before being wound.

Properties of Acetate Rayon

Properties of Acetate Rayon

55/20/3s means 55 denier yarn, 20 filament and 3 TPI S side.

Moisture content of sec. Cellulose acetate is 6.5% at 70 deg F and 65% RH.
( Moisture Content= Wt of water in a material /Total wt of material) ( Moisture Regain= wt of water in a material/ oven dry wt of material)
( RH= actual humidity/ humidity of air saturated in water).

Tenacity of Acetate rayon is 1.4 gpd at dry state and 0.9 gpd at wet state.

Elongation at break is 25% in dry state and 35% in wet state

Acetate Rayon is more sensitive to heat. It begins to weaken at 93 deg C. At 175 deg C it becomes sticky and melts at 260 deg C. Like nylon and polyester it is thermoplastic. Thus permanent crimp, pleats and creases can be imparted to the garment under carefully controlled conditions.

Acetate rayon is soluble in acetone, methyl ethyle ketone etc.

Some degeneration takes place when this fiber is exposed to light but not very serious.

It is stable to hot water.

It can also withstand treatment with soap or alkali solution having a pH of not more than 9.5 at temp upto 100 deg C. Therefore it can undergo normal scouring and dyeing operations without affecting the lustre.

It is unaffected by dilute solutions of weak acids but attacked by strong acids. Concentrated organic acids cause swelling

It is resistant to attack by bacteria and fungi. Its low moisture content contributes to resistance to mildew.

It is non toxic and non irritating to skin

Only a few striations ( 2-3) are present in the fibre as can be seen from the longitudinal view. The cross section of the fiber have individual lobes and are round and smooth. It is the smaller number of lobes or serrations of acetate fibres that distinguish the fibre from more numerous serrations of viscose rayon.

Manufacturing Process of Acetate Rayon

Acetate Rayon

We know that

Alcohol + Acid --> Ester

If the cellulose is treated with acetic acid under certain conditions the free hydroxyl groups of cellulose are converted into ester groups.

Manfacture of cellulose acetate

Unlike inthe case of viscose rayon and cuprammonium rayon, where cellulose is dissolved and regenerated, cellulose acetate is manufactured by converting cellulose into a chemical compound of cellulose ( or chem modified cellulose) which is then dissolved in a suitable solvent ( chloroform or acetone) and spun by evaporating the solvent. Thus while viscose and cuprammonium rayons are regenerated fibres, acetate rayon is regenerated modified fibre.

Raw Material

Cotton linters and wood pulp are the most common employed raw materials for the manufacture of acetate rayon

Acetylation Process



The pretreated purified cotton linters are fed into an acetylator ( closed vessel) containing a mixture of acetic anhydride, glacial acetic acid and a small amount of concentrated sulphuric acid. For every 100 kg of cotton linters, 300 kg of glacial acetic acid, 500 kg of acetic anhydride and 8-10 kg of concentrated su;phuric acid may be used. The acetylator consists of a metal tank having a circular door at the top. The door is sealed after adding the mixture of chemicals and cotton linters. A stirrer having many blades rotates in the acetylator to mix the ingredients thoroughly. The acetylation reaction is an exotherimic reation. Heat is removed by circulating cold water through a jacket fitted to the acetylator. The acetylation reation is completed in 7-8 hours at 25-30 deg c. Triacetate is formed at this stage and it is in the form of a suspension in the acetylation mixture called the acid dope.

Hydrolysis ( Partial Deacetylation)

The acid dope from the above process is stored in jars for ageing. Acetic acid, water and sulphuric acid are added and allowed to stand for 10-20 hours. During this period, called ripening period, partial conversion of acetate groups to hydroxy groups takes place. The mixture is then diluted with water and stirred continuously when white flakes of acetate rayon get precipated. The flakes are placed in a centrifuge and the excess water is forced out of the cage through perforations. The flakes are then dried.

Spinning Solution or Dope

Acetate rayon is manufactured by dry spinning. It is dissovled in a volatile solvent (Acetone) to form the spinning solution or dope. This solution is forced through a spinnerette into a chamber in which hot air is circulated. The solvent evaporates leaving filaments of acetate rayon.

The details are as follows. Dried acetate flakes are mixed with three times the weight of acetone in enclosed tanks which are provided with powerful stirrers. The acetate dissolves slowly in the solvent. It takes about 24 hours for the complete dissolution to give a thick clear liquid called dope. The solution is filtered and deareated.

Spinning Process



The dope is spun into acetate rayon filaments on the dry spinning process. The dope is fed from a spinning tank into spinning cabinets. The dope coming out of the spinnerette travels a distance of 2-5 meters vertically downwards to a feed roller, from where it is guided on to a bobbin at a much greater speed than the speed of spinning. This imparts twist to the filaments.

Properties of Cuprammonium Rayon

Properties of Cuprammonium Rayon

1. The one important characteristic of these fibres is their extreme fineness. Filaments as fine as 1.33 deniers are produced regularly ( as compared to viscose rayon which have a usual denier of around 2.5). This increased fineness is due to the stretch that is applied to the filaments during spinning.

2. Because of its fineness, cuprammonium rayons produce a soft silk like handle.

3. It has all the properties of cotton except that the average DP is lower and a larger portion of this fibre is occupied by amorphous regions. Hence the rayon swells to a greater extent and hence chemical reactions take place faster in the case of rayon than in case of cotton.

4. Like viscose rayon it burns rapidly and chars at 180 deg C. It is degraded and weakened by exposure to sunlight in the presence of oxygen and moisture. On ignition, it leaves behind ash containing copper.

5. The average tensile strength of cuprammonium rayon is 1.7-2.3 in dry and 0.9-2.5 in wet state.

6. It has an elongation at break of 10-17% when dry.

7. Moisture content at 70 deg F and 65% RH is about 11% as in case of Viscose Rayon.

8. Dye absorption power for direct dyes of cuprammonium rayon is greater and shades obtained are deeper than viscose rayon.

9. The filaments appear uniform with surfaces having no markings, in the longitudinal view. Cross sections are round and smooth with occasionally slightly oval.

Manufacturing Process of Cuprammonium Rayon

Cuprammonium Rayon


Like Viscose Rayon, cuprammonium rayon is also a regenerated cellulose fibre. Cotton linters are used as the source of cellulose for this rayon.

Ammonical copper oxide solution is also known as cuprammonium hydroxide solution. Cuprammonium hydroxide solution is a solvent for cellulose. When a solution of cellulose in cuprammonium hydroxide is diluted with water or treated with dilute sulphuric acid, the cellulose is regenerated or reprecipitated. By using a spinnerette, filaments of this regenerated cellulose can be produced.

Manufacture of Cuprammonium Rayon

The source of cellulose for this rayon is cotton linters, the purification of cotton linters is carried out in two stages:

a. Mechanical Treatment
b. Chemical Treatment

Mechanical Treatment

The cotton linters are transported in bales in highly compressed state and the object of the mechanical treatment is to loosen them and to remove mechanically admixed and loosly bound impurities such as dust sand, seed residues etc.

Chemical Tratment

The mechanically opened and purified cotton linters are boiled under pressure for several hours with dilute sod ash ( Na2Co3) solution (2%) to which a little amount of caustic soda may be added. The natural fatty matter present in the cotton is converted into soluble substance by the action of soda ash and thus removed from cotton linters.

Dissolution of Cellulose

In this, a solution of hydrated copper sulphate in 300-400 liters of water is introduced in a vessel at ordinary temperature with stirring. Some sugar is also added followed by caustic soda solution to form copper hydroxide.

Ground linters suspended in water are added to the above mixture to form copper cellulose.

The copper cellulose is filtered to remove the liquid, well ground and dissolved in a solution of ammonia in water.

Spinning Solution

By adding certain compounds to the cuprammonium cellulose solution, the solution is made more suitable for spinning. These compounds include glycerine, glucose, tartaric acid, citric acid, oxalic acid, can sugar etc.

Stretch Spinning

In the spinning process, the cuprammonium cellulose solution is discharged through nozzles ( spinnerette) into a solution of sulphuric acid in the form of relatively thick threads which are subsequently pulled( stretched ) to very fine filaments.

Properties of Viscose Rayon

Properties of Viscose Rayon

Moisture Absorption

It absorbs more moisture than cotton. Moisture Content of Coton is 6% at 70 deg F and 65% RH, and for Viscose Rayon it is 13% under the same conditions.

Tensile Strength

The Tensile Strength of the fibre is less when the fibre is wet than when dry. It is 1.5-2.4 gpd in the dry state and 0.7-1.2 gpd in the wet state. For high tenacity variety the values are 3-4.6 gpd and 1.9 to 3.0 gpd.

Elasticity

The elasticity of Viscose Rayon is less than 2-3%. This is very important in handling viscose yarns during weaving, stentering etc when sudden tensions are applied.

Elongation at Break

Ordinary Viscose rayon has 15-30% elongation at break, whule high tenacity rayon has only 9-17% elongation at break.

Density

The density of Viscose rayon is 1.53 g/cc. Rayon filaments are available in three densities: 1.5, 3.0 and 4.5

Action of Heat and Light

At 300 deg F or more, VR loses its strength and begins to decompose at 350-400 deg F. Prolonged exposure to sunlight also weakens the fibre due to moisture and ultraviolet light of the sunlight.

Chemical Properties

Viscose rayon consists of cellulose of lower DP than cotton cellulose. Also amorphous region of Viscose rayon is present to a greater extent, therefore, Viscose rayon reacts faster than cotton with chemicals. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form oxycellulose. Cold acid solutions for a short time do not attack viscose rayon.

Action 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.

Action 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.

Longitudinal View

The longitudinal view of these fibres show many striations running parallel to the long axis of the fibre. The cross section of viscose has striated periphery, having many sharp indentations, and cross sectional contours vary from circular and oval to ribbon-like forms.

Manufacturing Process of Viscose Rayon

Viscose Rayon

It is a regenerated cellulosic fibre and cellulose is the raw material for producing this man made fibre.

The raw material is obtained from a special variety of wood called spruce.

Manufacturing Process

a. Purification of Cellulose:

The manufacture of viscose rayon starts with the purification of cellulose. Spruce trees are cut into timber. Their barks are removed and cut into pieces measuring 7/8" x 1/2" x 1/4". These pieces are treated with a solution of calcium bisulphite and cooked with steam under pressure for about 14 hours.
The cellulosic component of the wood is unaffected by this treatment, but the cementing material called lignin, which is present in the wood, is converted into its sulphonated compound which is soluble in water. This can be washed off, thereby purifying the remaining cellulose. This cellulose is treated with excess of water. After this it is treated with a bleaching agent (sod hypochlorite) and finally converted into paper boards or sheets. This is called wood pulp, which is normally purchased by the manufacturers of viscose rayon.

b. Conditioning of Wood Pulp:
The pulp sheets are cut by a guillotine to the required dimension and are kept in a special room. Air moves freely among the divisors by means of ventilatorys, the temperature is maintained at 30 deg celcius. In this way the desired moisture content can be had.

c. Steeping Process:
The conditioned wood pulp sheets are treated with caustic soda solution ( about 17.5%). It is called mercerising or steeping. The high DP cellulose (1000) is converted into soda cellulose. The sheets are allowed to soak (steep0 until they become dark brown in colour. This takes about 1-14 hours. The caustic soda solution is drained off and sheets are pressed to squeeze out excess caustic soda solution. 100 kg of sulphite pulp gives about 310 kg of soda cellulose.

4. Shredding or cutting process:
The wet, soft sheets of soda cellulose are passed through a shredding machine which cuts them into small bits. In 2-3 hours the sheets are broken into fine crumbs.

5. Ageing Process:
To obtain almost ideal solution of cellulose, the soda cellulose is stored in small galvanised drums for about 48 hours at 28 deg C. This process is called ageing process.The ageing process is essential. During This process, the DP od soda cellulose is decreased from 1000 to about 300 by oxygen present in the air, contained in the drum.

6. Churning Process or Xanthation:
After ageing, the crumbs of soda cellulose are transferred to rotating, air tight, hexagonal churners or mixers. Carbon disulphide ( 10% of the weight of the crumbs) is added to the mixer and churned together for 3 hours by rotating the mixers at a slow speed of 2 rev per minutes. Sodium cellulose xanthate is formed during this process and the colors of the product changes from white to reddish orange.

7. Mixing or dissolving Process:
The orange product i.e. sod.cell.xanthate is in the form of small balls. These fall into a mixer called dissover which is provided with a stirrer. A dilute solution of caustic soda is added, and the contents are stirred for 4-5 hours and at the same time, the dissovler is cooled. The sod.cell.xan. dissovles to give a clear brown thick liquor, similar to honey. This is called 'viscose' and it contains about 6.5% caustic soda and 7.5% cellulose.

8. Ripening Process:
This viscose solution requires to be ripened to give a solution having best spinning qualities. Ripening is carried by storing the viscose solution for 4-5 days at 10 to 18 deg. The viscosity of the solution first decreases and then rises to its original value. The ripened solutoin is filtered carefully and is now ready for spinning to produce viscose rayon filaments.

9. Spinning Process:

The viscose solution is forced through a spinnerette, having many fine holes ( 0.05-0.1mm) diameter. The spinnerette is submerged into a solution containing the following chemicals.
10% --> sulphuric acid, 18%- Sod sulphate, 1% - Zinc sulphate, 2% glucose, 69% water.

The spinning solution is kept at 40-45 deg celcius.

Sodium sulphate precipitates the dissoved sod. cell.xanthate. Sulphuric Acid converts xanthate into cellulose, carbon disulphide and sod. sulphate. the glucose is supposed to give softness and pliability to the filaments whereas zinc sulphate gives added strength.

The quality of viscose rayon filament formed depends upon:

1. The temperature of the spinning bath
2. The composition of the spinning bath.
3. The speed of coagulation
4. The period of immersion of the filament in the spinning bath.
5. The speed of spinning.
6. The stretch imparted to the filaments.

As a number of filaments emerge from the spinnerette, they are taken together to an eye at the surface of the spinning bath and then guided to two rollers from where they are wound on to a spindle.

Common Yarn Faults in Manmade Fibres

Common Yarn Faults in manmade fibres

1. Slubs:

Slub like thick faults seriously mar the appearance of fabrics made from manmade fibres. The following measures can be taken

A. In Blends with cotton

a. properly select the cotton component
b. ensure proper grinding of wirepoints at cards
c. regularly check the ringframe drafting system.

B. In 100% manmade fibres

a. Ensure adequate number of doublings
b. avoid too wide a roller setting and inadequate weighting on rollers.
c. Select correctly the fibres in regard to their compatibility in length.

2. Crackers

This defect is characterised by the cracking sound produced when the yarn is pulled. The sound is produced due to sudden rupture of fibres curled around the yarn.

- Crackers are caused mainly by the presence of very long fibres due to improper cutting of the two.

- They can also be caused due to high vairability in the elongation of the constituent fibres in the blend.

- Ensure wider roller setting in the back zone, adequate roller weighting and avoid too narrow a spacing between the aprons.

- It is helpful to have low roving twist and higher spinning tension through the use of heavier traveller.

3. Neps

This can also mar the appearance of a fabric

- In man made fibres longer and finer fibres tend to produce more neps.
- Other reasons of neps are
- Excessing beating of fibres in the blow room
- Loading of licker-in or cylinder at card
- Blunt wire points on various carding elements
- excessive lap weight

4. Fluffy Yarn

In general presence of short fibres and proneness to static accumulation tend to produce this defect.

The fault can be corrected by maintaining proper atmospheric conditions and reducing the fluff on roving.

5. Smoky Yarn

- The yarn containing synthetic fibres get smoky through long exposure of the running bobbin in a dirty atmosphere in the ring spinning system.

- Installation of smoke filters in H-plant can correct the problem

- Use of roving build can check this defect.