textile science
4. PROCESSING OF NATURAL FIBERS-natural fibers
4.1. more info on processing of natural fibers
Man-made fibers can be modified at any stage during the production process (with additives such as pigments) or immediately afterwards (by drawing, crimping or the application of finishes) to suit the end product. The fibers can be created to match the requirements of the textile. Once engineered to fit a given application, fiber characteristics are identical from batch to batch. Alternatively if a desired result cannot be achieved by modifying one type of man-made fiber, different types can be modified to give the required performance.
GENERAL PRINCIPLES
All man-made fibers are produced from polymers. These substances are solids at normal temperatures. The polymers which are the starting products for man-made fibers may well be in form of chips, small particles of perhaps one to two millimetres in diameter and two to three millimetres in length.
In order to make filaments from these polymers, their physical shape must be altered, and the polymer chips must be converted to a liquid form. This is necessary as the polymer needs to be forced or extruded through fine holes to form filaments.
The process of producing a man-made fiber is often termed ‘spinning’. This leads to confusion with the purely mechanical process of assembling staple fibers into a yarn, which is called ‘spinning’. The process of producing a man-made fiber will therefore be described as ‘extrusion’.
There are three methods of extruding fibers: Melt extrusion, Solvent-dry extrusion and Wet extrusion. The method used for each fiber depends upon the ease of conversion of the polymer from solid to liquid state.
· If the polymer can be converted to a liquid form easily just by heating, then melt extrusion can be used.
· If polymer is chemically damaged by heating but can be dissolved in a suitable solvent which will later evaporate, then dry extrusion is used.
· If polymer cannot be heated or dissolved in an evaporative solvent, wet extrusion must be used.
The molecules in the extruded filaments are likely to be randomly orientated within the fiber. For the fiber to have good tenacity and a reasonable low extension at break, the molecules need to be lined up along the fiber axis. In order to do this, a further process known as stretching or drawing is needed. This increases the tenacity and decreases the extension-at-break.
The production of manmade fibers usually includes the following processes:
i. Preparation of polymer (polymerization, chemical modification, etc.)
ii. Preparation of the spinning fluid (polymer melt or solution)
iii. Spinning (extrusion, solidification, and deformation of the spinning line or filament)
iv. Drawing (due to a higher linear speed at the take-up roll relative to that at the die; drawing is used to increase the degree of molecular orientation and improve the tensile strength, modulus of elasticity and elongation of the fibers)
v. Heat treatment
vi. Textile processing (twisting, oiling, dyeing, etc.).
Three processes for man-made fiber production
1. Melt extrusion
This is the simplest fiber production processes. It is used for the production of Polyester, Polyamides, Propylene and Polymers such as nylon, polyethylene, polyvinyl chloride, cellulose triacetate
The polymer is melted and extruded through the fine holes of a spinneret (metal plate with small holes drilled through it). The number of holes in the spinneret determines the size of the yarn, as the emerging fibers are brought together to form a single strand. The molten polymer passes through the holes in the spinneret plate and emerges into a steam of cool air which causes it to solidify. The solidified bundle of filaments is passed around a rotating roller and then forwarded to a collecting device.
The melt extrusion process can therefore be split into three stages heating, extrusion and cooling.
Drawing
The fibers is not yet suitable for use because the polymer molecules are not highly oriented along the fiber axis. To orient them, the fiber is drawn forming areas of crystalline order. In this process the fiber is in a plastic rather than a molten state.
2. Solvent-dry extrusion
If a polymer will not meet the conditions for melt extrusion, an alternative method of converting the solid polymer to a liquid state must be used. If it is possible to use a volatile solvent, that is a solvent which is highly reactive, then the polymer can be extruded into warm air to evaporate the solvent away. This is known as solvent-dry extrusion and is the least used for major fiber production techniques.
The polymer solution is extruded through a spinneret into heated gas so that the solvent is rapidly evaporated from the filaments. The filaments drop through the hot gas steam for five to six metres to a rotating roller and are then fed to the wind-up device.
Cellulose acetate, cellulose triacetate, acrylic, modacrylic, aromatic nylon, and polyvinyl chloride are made by dry spinning.
After treatments
Filaments must be washed to remove any remaining solvent from the extrusion stage. Drawing can also be carried out at this stage, in either air or liquid.
3. Wet extrusion
1. If the polymer does not melt without decomposition and will not dissolve in a volatile solvent, it must be dissolved in a non-volatile solvent to change it from solid to liquid state. There are two steps in wet extrusion: physical process and chemical regeneration. In this the polymer solution (i.e., polymer dissolved in a solvent as in dry spinning) is spun into a coagulating solution to precipitate the polymer (solidify them). This process has been used with acrylic, modacrylic, aromatic nylon, and polyvinyl chloride fibers.
AFTER TREATMENTS
As for solvent-dry spinning, the filaments of viscose must be washed to rid them of any chemicals from the coagulating bath. The filaments are dried then suitable spin finishes are applied.
SYNTHETIC FIBERS
Introduction
They are made up of substances which do not normally form fibers but are synthesized. To synthesize is to build new molecular structures by adding small molecules to give a large, long molecule. This process is known as polymerization. And the large molecule formed is known as polymer.
Made fibers from synthetic polymers are created using polymerization of various chemical inputs to create polymers. Polymerization is the process of combining many small molecules into a large molecule – a polymer. Polymers are simply large molecules composed of repeating structural units. Polymers used for synthetic fibers are produced from intermediates (which in turn have been produced from crude oil) and applying a catalyst. Polymers are the building blocks of synthetic fibers
NYLON (A POLYAMIDE)
ü Is the oldest synthetic fiber, It belongs to the polyamide group polyamides are formed with a recurrent amide group (-CO-NH-) as part of the polymer chain. There are different types of nylons distinguished by the chemical structure of the individual polymer.
ü The original form of nylon is known as 6.6 because it has six carbon atoms.
o Examples of nylon fabrics:
· Brocade
· Damask
· Gabardine
· Lace net
· Satin
· Knitted fabrics (jersey)
It is produced in USA and UK in both filament and staple form
PRODUCTION OF NYLON
It is produced in the following ways:
1. POLYMERIZATION
ü The raw materials are heated , causing the short chain molecules to combine to make long molecules until fibers are formed
ü These fibers are cooled whereby they solidify into a hard ribbon like from.
ü They are then cut into chips ready for spinning.
2. SPINNING
ü The polymer chips are heated to a molten mass.
ü They are passed through tiny holes in a spinneret, cooled and solidified into filament form.
ü They are then twisted together to form a yarn.
3. DRAWING
ü The yarn is stretched or drawn so as to reduce the thickness and lengthen the filaments. This increases the strength and changes the appearance of the fiber from dull and opaque to shiny and clear.
ü If staple fibers are needed the filament fibers are cut and a crimp is added.
Properties of nylon
1. Has a cool, clammy feel when in filament form, however this feel can be altered by creating filaments which are three-sided. In nature to give a smoother and softer fabric.
2. It is lustrous, transparent and has high tensile strength.
3. It is very strong and durable, both wet and when dry.
4. It has high resistance to abrasion and is very elastic, meaning it easily comes back to its original form after any stretching, pull or bending. For this reason it needs little or no ironing.
5. It is also mildews, moth and flame resistance and has low resistance to sunlight.
6. It has good resistance to alkalis but is disintegrated by strong acids.
7. It has very low absorbency i.e. hydrophobic for this reason it dries quickly however, it also makes it build up electrostatic electricity making it very uncomfortable to wear as it clings on the body.
8. It is thermoplastic, this means it is sensitive to heat. This quality is exploited to permanently set pleats in garments.
9. It is easy to wash and doesn’t burn readily, it is one of the safest textile fibers.
10. It is very versatile, it’s produced into various types of yarns from very sheer to very stout.
11. It blend well with cotton, wool, rayon and silk giving them added strength.
12. It also dyes well and has good color fastness however, this quality makes it pick up color or grease from other garments when laundering
NOTE: A distinct disadvantage of nylon made from spun yarns is that it pills badly. Pilling is the formation of a small balls of fiber on the fabric on the fabric due to friction.
Spun yarns are the staple fibers that are cut from the filament fibers in order to produce short fibers.
Advantages of nylon
a. Lightweight
b. Strong and durable
c. Abrasion resistance
d. Easy to wash and dries easily
e. Resist shrinkage and wrinkles
f. Insulating properties
Limitations of nylon
a. Builds up electrostatic electricity making it very uncomfortable to wear as t clings on the body
b. Poor resistance to sunlight
c. Picks up oils and dyes in wash
d. Thermoplastic i.e. sensitive to heat
Identification test for nylon
1. BURNING TES
· Nylon draws away from the flame before igniting.
· It drips and melts in flame producing white smoke.
· It forms a hard bead and smells of fresh celery, chemical or acid.
· It is self-extinguishing.
2. CHEMICAL TEST
It is soluble in formic acid and carbolic acid (90% phenol).
3. MICROSCOPIC TEST
The longitudinal view shows long, smooth filaments
Cross-sectional view shows circular, spotted structures.
Fiber structure may be varied to give different fiber properties e.g. a triobal filament maybe created to produce fabric with more glitter.
USES OF NYLON
1. Used in general apparel, which is sportswear, stockings, foundation garments, overalls, rainwear, lingerie, blouses, skirts and shirts.
2. Used for trimmings such as ribbons and lace.
3. Used in making furnishing such as carpets, bed-sheets, curtains and upholstery.
4. It is also quite useful for industrial uses in making industrial making tires, ropes, tarpaulins and conveyor belts.
5. Widely used in hosiery and household furnishings.
6. Mainly used for outer wear and swim wear fabrics, ski pants and active sportswear.
7. Used for lining material in coats, jackets because of excellent durability however it is expensive
TERYLENE (A POLYESTER)
Terylene is one of the polyesters in the market today. Polyesters are derived from petroleum by products namely; ethylene, glycol and terephalic acid.
Polyester fiber are produced in filament, staple and low lengths.
Other types of polyesters are: Dacron, kodel and vycron.
PRODUCTION OF TERYLENE
Its production is similar to that of Nylon.
Properties of Terylene
1. Shrink resistance and extremely strong, It is popular for apparel fabrics
2. It is cool to wear and quite resilient.
3. It is also crease resistance therefore it needs little or no ironing and is good for travelling.
4. It has a high abrasion resistance.
ACRYLIC (A POLYACRILONITRILE)
Acrylic is produced from acrylonitrile; which is got from either petroleum or natural gas.
True acrylics have over 85% of acrylonitrile while mod acrylics have between 35% - 85%
PRODUCTION OF ACRYLIC
Are production in a similar manner to other synthetics?
Orlon is dry spun and produced in both filament and staple forms, whereas acrilan and courtelle are wet spun and produced in staple form only.
Properties of acrylic
i. Lustrous and has a warm, soft handle
ii. Does not irritate the skin
iii. Light weight, strong and durable
iv. Crease, shrink and stretch resistant.
v. Moth and mildew resistant
vi. It is Thermoplastic
vii. Damaged by alkalis but resist acids.
viii. It is not damaged by sunlight or abrasion
ix. Has low moisture absorbency, therefore Dries quickly
x. It builds up electricity and may even produce sparks for this reason it is used in an operating room as it can ignite ether a gas that is used to make patients sleep so that they do not feel pain during operation.
Identification test
1. Burning test
· Melts in flame, rolls, twists and forms droplets of hot, molten plastic.
· May or may not continue to burn after the flame is removed.
· Produces an acid or chemical oduor
· Forms a hard, thick, black beaded edge.
2. Chemical test
Dissolves in 80% sulphuric acid.
3. Microscopic test
Longitudinal view – shows a fiber which looks look like fine glass rods
Cross-sectional view – shows circular structures
Uses of acrylics
1. Used in making of general apparel such as coats, suits, dresses, blouses and underwear.
2. Used to make carpets, blankets and bed spreads, lining fabrics and limited fabrics
MODACRYLICS
They are modified acrylics. They contain between 35-85% acrylonitrile. Their characteristics are similar to those of acrylics but they are fire resistant, self-extinguishing, do not drip and are easier to dye.
Examples of mod acrylics include Dynel, teklan and kanekalon.
Uses of mod acrylics
Because of their fire resistant characteristic, mod acrylics are widely used for making children’s nightwear, racing drivers, overalls, fireside rugs, electrical insulations, furnishings, public buildings, wigs and hair pieces.
POLYOLEFIN FIBERS
These are fibers that are made from 85% ethylene or propane gas. They are produced by melt spinning and are used for household and industrial purpose due to their low moisture absorbency.
Examples of polyolefin are polypropylene and polythene
Properties of polythene
1. Polyolefin are rot proof
2. Color fast in sunlight
3. Light weight
4. Very strong
5. Relatively cheap
6. Have low moisture absorbency
7. They are not affected by either alkalis or acids but are affected by sunlight.
Uses of polyolefin
· Used for making deck chair covers, ropes, packing papers, fishing nets, filter cloths and carpet backing.
ACTIVITY: why do you think polythene is not popular for making apparel fabrics?
Let us now learn about some fibers that are used minimally in the textile industry, they have much lower production rate but are important all the same. They are referred to as minor fibers.
MINOR FIBERS
SPANDEX (AN ELASTOMERIC FIBER)
Elastomeric fibers are got from polyurethane which is derived from petroleum by products. They have a high elasticity power of between 450%-700% and have complete and instant recovery.
They are often blended with other fibers so as to give them the preferred texture and appearance e.g. spandex, Lycra (UK & USA) and spanzelle (USA).
Properties of spandex
1. Spandex fibers can stretch up to 500%-800% of their length before breaking.
2. They high recovery rate once released therefore, they are used for sportswear such as swim suits.
3. Superior dye ability with brilliant colors.
4. Very good resilience and highly flexibility; Very good abrasion resistance. Good dimensional stability in wet condition; however some types shrink with the exposure to the high temperature.
5. They resist perspiration, this makes them ideal for sportswear and underwear
6. Good resistance to household bleaches and most acids but are damaged by hot alkalis as it causes rapid deterioration. It is resistant to dry cleaning solvents;
7. They dry quickly, easy to launder and blend well with nylon and Terylene.
Spandex in Consumer Use
i. Lycra in swimwear, innerwear and active sportswear means providing fitting clothes with comfort and freedom of movement.
ii. It also improves the quality of Knitted / Woven fabrics as it prevents bagging and accelerates wrinkle recovery.
iii. Lycra in swimwear, innerwear and active sportswear means providing fitting clothes with comfort and freedom of movement.
iv. Make home furnishings such as bed sheets, curtains and seat covers.
v. It also improves the quality of Knitted / Woven fabrics as it prevents bagging and accelerates wrinkle recovery.
POLYVINYL CHLORIDE FIBERS (P.V.C)
These fibers are not used for clothing purposes because they are very sensitive to heat and therefore, cannot be ironed. However, they may be used to bond some needle constructed fabrics.
Properties of P.V.Cs
1. P.V.C are moth, mildew, and chemical and flame resistant but sensitive to heat.
2. They are non-absorbent
Uses of P.V.Cs
a. Are used for industrial purposes such as protective clothing, tarpaulins (lorry covers) and filter cloths
b. They are used for home furnishing such as seat covers, desk chairs and carpets.
MAN-MADE OR REGENERATED FIBERS
We have looked at fibers that occur in either animals or plants as well as fibers that are processed from chemicals that is synthetic fibers. Although synthetic fibers are man-made the “the man-made” refers to all fibers that are processed from both natural raw materials and chemicals. They are produced by the chemical treatment of certain raw material, the main raw materials are wood pulp, cotton linters, petroleum extracts and by products of coal. Other fibers are made from proteins like milk, soya beans as well from maize meal and a natural rubber base.
They can be divided into two main groups namely viscose rayons and acetate or triacetate rayons
VISCOSE RAYONS
It’s the first man-made fiber formally called artificial silk. It was developed by scientists who tried to copy the silk worm by producing a liquid secretion which can solidify into filament. It is mainly produced from cellulose of spruce and eucalyptus trees as well as cotton linters, petroleum extracts and by products of coal e.g. are crepe, damask, flannel, gabardine, jersey, satin, taffeta and velvet
Processing of viscose rayon
It’s processed in the following steps
i. Source
The eucalyptus and spruce trees are felled and the bark is removed. This is done in pulp mills, the cellulose is extracted, pressed and cut into sheets.
ii. Steeping
The cellulose is steeped into caustic soda, which converts it into alkali cellulose which is then shredded into crumbs and left to oxidize.
iii. Xanthation
The alkali cellulose is treated with carbon disulphide to give sodium cellulose xanthate
iv. Dissolving
The sodium cellulose xanthate is dissolved in caustic soda (NaOH) to form the spinning solution known as viscose which is left to ripen or to ferment.
v. Filtration
Any undissolved particles are removed by filtration
vi. Wet spinning and extrusion
The viscose is forced through fine holes in a jet (a spinner) into a coagulating bath of sulphuric acid and salts. This process produces the original cellulose in filament form.
If fine staple fibers are required the filaments from several jets are drawn together and the low is cut into fibers of desired length.
vii. Washing
The yarns are washed to remove all traces of chemicals then they are dried
viii. Twisting and winding
The yarns are given a twist and wound on to bobbins
|
Spruce |
|
Logs |
|
Shredded |
|
Cooked with |
|
Caustic soda |
|
Made into sheets of wood-pulp |
|
( |
|
Alkali cellulose |
|
) |
|
The sheets are shredded |
|
And mixed with carbon |
|
Disulphide |
|
The mix is tipped into a |
|
Caustic soda bath, |
|
Forming viscose |
|
Spinneret |
|
Acid-bath |
|
Fibers or filaments |
|
Spool of |
|
Rayon yarn |
Properties of viscos rayon
i. Rayon is smooth, absorbent and cool to wear as it is a good conductor of heat for this it is quite comfortable to wear.
ii. It is fairly strong but is weaker when wet
iii. It dyes and is highly color fast when spun dyed
iv. It drapes well
v. It burns readily
vi. It is affected by static electricity
vii. Its damaged by acids and alkalis and especially by acetone for this care should be taken during laundry-work and especially when removing stains
viii. It is also damage by friction, twisting and wringing therefore, it should be washed by kneading and squeezing method and should not be tumbled dried.
ix. It may also shrink or stretch unless its specially treated
Identification test for viscose rayon
i. Burning test
Viscose rayon burns easily, flaming, smells of burning paper (like cotton) and leaves grey ash.
ii. Chemical test
It dissolves in acetone (nail vanish remover)
iii. Microscopic test
Longitudinal view shows long smooth fibers
Cross sectional view shows deep striations
Uses of viscose rayon
1. It’s quite versatile and therefore it may be used for all wearing apparel and industrial clothing
2. Household furnishing such as table linen, curtains, carpets, blankets and bedspreads
ACETATE RAYON
It is produced from cotton linters and wood pulp treated with acetic acid. It is the nearest substitute to silk e.g. are satin, taffeta, brocade and knitted fabrics used for making lingerie.
Processing of acetate rayon
i. Source
Cellulose is got from wood pulp and cotton linters, it is treated with acetic acid to make the polymer more active.
ii. Acetylation
More acetic acid is used to replace the hydrogen atoms of the hydroxyl group by the acetyl group to form cellulose acetate, some of the acetyl groups are removed.
iii. Ripening
The cellulose acetate solution is allowed to ripen until acetylation is complete
iv. Precipitation
The cellulose acetate is precipitated by adding water
v. Drying
It is washed, dried and made into white flakes
vi. Spinning
The spinning solution is made by dissolving the flakes in acetone with the addition of a little water.
vii. Filtration
The solution is filtered and pumped through a spinneret, the filaments are solidified by a current of warm air this is known as dry spinning.
Filament and staple yarns can both be produced this way
NB: Acetate can be spun dyed whereby the dye stuffs are added to the acetate solution before it is spun. This ensures the color is inside the fiber thus giving greater color.
Properties of acetate rayon
i. It has an attractive rich appearance
ii. It has very good draping quality
iii. It is absorbent, soft and smooth, it dyes well and blends well with other fibers
iv. Resistant to moth and mildew
v. Thermoplastic heat set pleats are permanent
vi. Shrink and crease resistant
vii. It is slightly weaker when wet
viii. Builds up static electricity when ironing
ix. It also fades color due to sunlight exposure
x. Weakened by long exposure to acids, alkalis and sunlight
xi. Dissolves in acetone
Identification test for acetate rayon
i. Burning test
It melts, rolls and twist to form a thick, black beaded edge. It has low melting point and produces a chemical acidic odour.
ii. Chemical test
It dissolves in acetone
iii. Microscopic test
Longitudinal view shows long smooth fibers like striated rods while the cross-sectional view shows bulb like structures.
Uses of Acetate Rayon
i. Quite versatile and attractive for this they are used for all wearing apparel including lingerie (underwear) and children wear
ii. Furnishing and draperies
iii. Hand knitting yarns as well as cigarette filter tips.
TRIACETATE RAYON
It’s made from wood pulp and oil treated with acetic acid and acetic anhydride. It’s a similar composition to that of acetate rayon but triacetate rayon has not less than 92% acetated hydroxyl group ions e.g. are tricel and arnel.
Processing of Triacetate Rayon
It’s similar to that of Acetate Rayon only the flacking is done the flakes are dissolved in methylene chloride plus alcohol.
The yarn is dry spun but it must be given an anti-static treatment before it is wound on to a package. It is produced in both filament and staple yarns.
Properties of Triacetate Rayon
i. Burning test – the edge melts, rolls, twists and forms droplets of molten plastic. It may or may not continue burning once flame is removed.
It produces a chemical or acidic odour and forms a hard black mass.
ii. Chemical test – it gets swollen by acetone
It dissolves in methylene chloride.
Uses of Triacetate Rayon fabrics
a. Used to make general apparel such as blouses, dresses, scarves, suits, ties, slacks and lingerie
b. Make rainwear
c. Make ribbons and interlining for quilts
METALIC FIBERS
They are made from metal, metal coated with strong polyester, clear plastic or cellulose acetate, a core completely covered by metal. They are produced by slitting very thin sheets of the metal into narrow ribbons since they are quite weak and soft they are used as a wrapping around a strong fiber.
The color is put in the adhesives, which stick the plastic to the metal. Different colored yarns of aluminium are often used because gold and silver tend to discolor. Polyester may be used as a coating for Aluminium fiber.
Metallic fibers are used to add richness, luxury or glitter to fabrics. Latex is an example of a metallic fiber.
Larex metallic fiber
Latex
Properties of Metallic fibers
1. Metallic fibers are quite expensive
2. Weak and soft
3. Quite durable, light and smooth
4. Washable and dry cleanable
5. They crease easily
6. Fairly good elasticity and elongation
Uses of metallic fibers
1. Metallic fibers are used for making apparel
2. Making furnishing fabrics such as those used in car upholstery, theater curtains, spectacle frames as well as radio and television sets.
GLASS FIBERS
This is ordinary glass made into yarns, glass is made up of silica (sand) and limestone. The fibers lack elasticity and flexibility necessary for general clothing, therefore they are used for industrial and household purposes only
Properties of Glass fibers
i. Glass fibers are strong but non absorbent
ii. They do not stretch but are easy to wash
iii. They are flame and wrinkle resistant
iv. They withstand sunlight
v. They resist mildew and cold acids but they are damaged by alkalis.
vi. They break easily
vii. They produce sharp ends on the fabric for this reason they are unsuitable for clothing.
Uses of Glass fibers
i. Glass fibers are useful because of their fire resistant ability hence used to make curtains, firefighting clothes, insulating electrical equipment, fire screen spectacles as well as in fabrics for making restaurant table cloths
ii. In industries plastics are reinforced with glass to make fabrics that are stronger than steel
Reinforced plastic