textile natural fibres

8/10/2019 textile natural fibres

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UNIT 3

There are several advantages and disadvantages for both synthetic and natural fibres.

Advantages and Disadvantages of Synthetic Fibre

Advantages

1. Synthetic fibres are strong so they can take up heavy things easily.

2. Synthetic fibres retain their original shape so it's easy to wash and wear.

3. Synthetic fibres are generally soft so they are used in clothing materials.

4. Varieties of colours are available as they are manufactured.

5. Clothes made by synthetic fibres are generally cheaper than those made by natural fibres.

Disadvantages

1. Does not absorb moistures

Synthetic fibres do not absorb sweat, trapping heat in our body.

2. Rough feel

Synthetic fibres may give rough feel, making it unsuitable for pyjamas, underwear, etc.

Advantages and Disadvantages of Natural Fibre

Advantages

1. Comfortable

Clothes made by natural fibres are more comfortable than those made by synthetic fibres.

2. Environment

Producing materials from natural fibres are less harmful to our environment.

3. Fire resistant

Natural fibres are resistant to fire but polymer based fibres will melt.

Disadvantages

1.

ExpensiveMaterials produced by natural fibres are generally expensive as synthetic fibres can be made

easily by manufacturing.

2. Shrink

Natural fibres might shrink due to aggressive washing.

Spinning for Man Made Fibre : The conversion of polymer into fibre, filament is called spinning,

from the manmade fibre view. The fibre forming substance is made temporal fluid extruded through

spinneret and then return to solid state (by solidifying) in fibre form.

Most synthetic and cellulosic manufactured fibres are created by extrusion forcing a thick, viscous

liquid (about the consistency of cold honey) through the tiny holes of a device called a spinneret to form

continuous filaments of semi-solid polymer. In their initial state, the fibre-forming polymers are solids

and therefore must be first converted into a fluid state for extrusion. This is usually achieved by melting,


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if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated), or by dissolving

them in a suitable solvent if they are non-thermoplastic cellulosic. If they cannot be dissolved or melted

directly, they must be chemically treated to form soluble or thermoplastic derivatives. Recent

technologies have been developed for some specialty fibres made of polymers that do not melt, dissolve,

or form appropriate derivatives. For these materials, the small fluid molecules are mixed and reacted to

form the otherwise intractable polymers during the extrusion process

Advantage of man-made fibre technology:

Production does not depend on climate & location.

Quality & quantity can be controlled during production.

Length & thickness can be controlled as per requirement.

Staple length can be controlled as per requirement.

Smoothness and high luster.

Fibre processing is easier.

High productivity and low cost.

Disadvantage of man-made fibre technology:

Fabrics made of MMF are difficult to stitch. Low air permeability.

Uncomfortable.

Unhygienic.

Hydrophobic.

Man Made Fibres are produced in three forms

1. Filament:Filament fibres are spun from spinnerets with 350 holes or less is determined by the size

of the yarn to be made

Monofilament: This filament is made only of a single filament.

Multifilament: These filaments are made of more than one filament.

2. Staple:Staple fibre is of limited and relatively short length

3. Filament tow. An assemble of twist free fibre

The SpinneretThe spinnerets used in the production of most manufactured fibres are

similar, in principle, to a bathroom shower head. A spinneret may have fromone to several hundred holes. The tiny openings are very sensitive to

impurities and corrosion. The liquid feeding them must be carefully filtered

(not an easy task with very viscous materials) and, in some cases, the

spinneret must be made from very expensive, corrosion-resistant metals.

Maintenance is also critical, and spinnerets must be removed and cleaned on

a regular basis to prevent clogging.

As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a

rubbery state and then solidified. This process of extrusion and solidification of endless filaments is

called spinning, not to be confused with the textile operation of the same name, where short pieces of

staple fibre are twisted into yarn. Number of holes in spinneret: For monofilament: 1-3, Formultifilament: 100-1000, Filament tow: up to 50,000

Shape of holes:


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There are typically three typesof spinning for polymers: Melt, Dry andWet.

Melt spinning (Fig. 1)is used for polymers that can be melted easily.

Dry spinning (Fig. 2)involves dissolving the polymer into a solution that can be evaporated.

Wet spinning (Fig. 3)is used when the solvent cannot be evaporated and must be removed by

chemical means.


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Melt spinning

In melt spinning the fibre-forming material is melted and extruded through spinnerets, and the

jets harden into solid filaments as they cool on emerging from the spinneret. Nylon is a melt-

spun fibre. The process of spinning by which fibre or filament is produced from melted polymer

chips by extruding through spinneret continuous filament produced here.

Number of filament depends on number of holes in spinneret.

It is the most critical operation in the production of nylon polyester, and polyolefin, poly

propylene fibres as number of fibre properties such as uniformity, crystallinity, and orientation

are imparted to the yarn during processing.

Requirements for melt spinning

The polymer should not be volatile

The polymer should not decompose in the molten state and the melting point.

Polymer should be 30 degree centigrade less than its decomposition temp

Advantage:

Can be used for both staple and continuous filament. Direct and simple process.

No environment pollution.

No solvent required.

Non toxicity and no risk of explosion.

High production speed (25003000 ft/min)

Low investment cost.

Disadvantage:

Required more proper maintenance of the m/c.

Heat of input is high.


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Typical melt spinning:

Polymer Melting point

Nylon6, 6 264oC

Nylon6 220oC

PET 264oC Polypropylene 167

oC

Poly ethylene 125oC

Dry spinning

In dry spinning the fibre-forming substance is dissolved in a solvent before the solution is

extruded. As the jets of solution emerge from the spinneret, a stream of hot air causes the solvent

to evaporate from the spinning solution, leaving solid filaments. Acetate is dry spun by extruding

acetone solutions of cellulose acetate into hot air.

In dry spgn, the fibre forming polymer dissolved in a volatile solvent is introduced into a heateddrying chamber where the solvent is evaporated and solid fibre is obtained through spinneret.


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Properties of solvent:

Solvent should not be volatile.

It should be organic.

It should have low boiling point.

It should be comparatively cheap.

It should be thermally stabilized.

It should be non toxic

Advantage:

Suitable for producing fine denier fibres.

No need of wash of fibre.

Disadvantage:

Investment cost is high.

Toxic and risk of explosion.

Heat input is very high.

Can not be used for staple fibre production.

Wet spinning

In wet spinning the solution of fibre-forming material is extruded into a coagulating bath thatcauses the jets to harden as a result of chemical or physical change.

Viscose, for example, is wet spun. The solution of cellulose xanthate is extruded into an aqueous

solution of acids and salts, in which the cellulose is regenerated to form solid filaments.

In wet spinning a suitable solvent is used to prepare fibre forming solution and another coagulantis used in coagulating bath.

Solution is extruded through spinneret and in contact of coagulant cellulose is regenerated.


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Advantage:

Suitable for staple fibre.

Both organic and inorganic solvent can be used.

Low temp reqd.

Higher production due to continue spgn.

Softener process. Investment cost is low.

Disadvantage:

Slow spgnspeed.

Washing and bleaching of fibres are needed.

Toxic.

Spinning: Difference

Parameter Melt spgn Dry spgn Wet spgn

1. Investment cost Low High Low

2. Hazard Non toxic Toxic Toxic

3. Heat of spgn High Very high Low

4. Spinneret hole 2many

thousands

300900 2000075000

5. Spgn speed 25003000

ft/min

25003000

ft/min

150300 ft/min

6. Productivity High Low High

7. Application Filament or

staple

Filament Filament or

staple

8. Solvent Not required Only volatile

organic solvent

Both organic and

inorganic solvent

can be use.


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Comparative features of melt , dry and wet spinning

Features Melt Dry Wet

Investment Cost Low High Low

Hazard Non-toxic Toxic

(Risk of explosion)

Toxic

Heat of Spinning High High Low

Spinneret Hole 2 to many thousand 300-900 20,000-75,000

Spinning Speed 2500-3000 ft/min 2500-3000 ft/min 150-300 ft/min

RAYON FIBRES

Rayon is the oldest commercial manmade fibre. The process used to make viscose can either be a

continuous or batch process. The continuous process is the main method for producing rayon. Three

methods of production lead to distinctly different rayon fibres: viscose rayon, cuprammonium rayon and

saponified cellulose acetate.

Viscose Rayon

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. 1, 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 25C in order to swell the cellulose fibres and to convert cellulose to alkalicellulose.

(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 30C) in order to depolymerize the cellulose to the desired degree of polymerization. In this step


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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.

Figure 1: Process of manufacture of viscose rayon fibre

(5) Xanthation: In this step the aged alkali cellulose crumbs are placed in vats and are allowed to reactwith carbon disulphide under controlled temperature (20 to 30C) 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 fibre material) and

is deaerated.

(6) Dissolving: The yellow crumb is dissolved in aqueous caustic solution. The large xanthate

substituent on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing


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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 CS2can thenescape 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 CS2that 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 spinnerette 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 fibre, as is done with high-tenacity rayon.

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 fibre. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to

cellulose) and fibre formation.

Variables in spinning are:

Temperature and composition of bath,

Speed of coagulation

Length of immersion

Speed of spinning

Stretch imparted

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 fibre axis. As the chains become more parallel,

interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibres[.

(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].

(13) Cutting: If the rayon is to be used as staple (i.e., discreet lengths of fibre), the group of filaments(termed "tow") is passed through a rotary cutter to provide a fibre which can be processed in much the

same way as cotton


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Cellulose Acetate

Acetate ismanufactured by treating purified cellulose refined from cotton linters and/or wood pulp with

acetic anhydride in the presence of a catalyst. The resultant product, cellulose acetate flake, is

precipitated, purified, dried, and dissolved in acetone to prepare the spinning solution. After filtration,

the highly viscous solution is extruded through spinnerets into a column of warm air in which the

acetone is evaporated, leaving solid continuous filaments of cellulose acetate. The evaporated acetone is

recovered using a solvent recovery system to prepare additional pinning solution. The cellulose acetate

fibers are intermingled and wound onto a bobbin or shippable metier cheese package, ready for use

without further chemical processing. In the manufacture of staple fiber, the filaments from numerous

spinnerets are combined into tow form, crimped, cut to the required length, and packaged in bales.


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Process flow chart of manufacturing of acetate

Acetylation Process

The pre-treated 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 sulphuric 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

exothermic reaction. Heat is removed by circulating cold water through a jacket fitted to the acetylator.

The acetylation reaction 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 hydroxyl groups takes place. The mixture is then diluted with water and

stirred continuously when white flakes of acetate rayon get precipitated. 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 dissolved 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 deaerated.


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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

greaterspeed than the speed of spinning. This imparts twist to the filaments.

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