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POLYESTER  FIBRE:

Fibre manufacturing process:

polyester fibre production

Today over 70 to 75% of polyester is produced by CP( continuous polymerisation) process using PTA(purified Terephthalic Acid)  and MEG. The old process is called Batch process using DMT( Dimethy Terephthalate) and MEG( Mono Ethylene Glycol). 

Catalysts like 5b3O3 (ANTIMONY TRIOXIDE) are used to start and control the reaction. TiO2 (Titanium di oxide) is added to make the polyester fibre / filament dull. Spin finishes are added at melt spinning and draw machine to provide static protection and have cohesion and certain frictional properties to enable fibre get processed through textile spinning machinery without any problem.

PTA which is a white powder is fed by a screw conveyor into hot MEG to dissolve it. Then catalysts and TiO 2 are added. After that Esterification takes place at high temperature. Then monomer is formed . Polymerisation is carried out  at high temperature (290 to 300 degree centigrade) and in almost total vacuum.  Monomer gets polymerised into the final product, PET (Poly ethylene Terephthalate).

This is in the form of thick viscous liquid. This liquid is them pumped to melt spinning machines. These machines may be single sided or double sided and can have 36/48/64 spinning positions. At each position , the polymer is pumped by a metering pump-which discharges an accurate quantity of polymer per revolution ( to control the denier of the fibre) through a pack which has sand or stainless steel particles as filter media and a spinnerette which could be circular or rectangular and will have  a specific number of holes depending on the technology used and the final denier being produced. Polymer comes out of each hole of the spinnerette and is instantly solidified by the flow of cool dry air. This process is called quenching. The filaments from each spinnerette are collected together to form a small ribbon, passed over a wheel which rotates in a bath of spin finish: and this ribbon is then mixed with ribbon coming from other spinning positions, this combined ribbon is a tow and is coiled in cans. The material is called undrawn TOW and has no textile properties.

At the next machine ( the draw machine), undrawn tows from severl cans are collected in the form of a sheet  and passed through a  trough of hot water to raise the temperature of polymer to 70 degrees C which is the glass transition temperature of this polymer so that the polymer can be drawn. In the next two zones, the polymer is drawn approximately 4 times and the actual draw or the pull takes place either in a steam chamber or in a hot water trough. After the drawing is complete, each filament has the required denier, and has all its sub microscopic chains aligned parallel to the fibre axis, thereby improving the crystallinity of the fibre structure and imparting certain strength.

Next step is to set the strength by annealing the filaments by passing them under tension on several steam heated cylinders at temperatures 180 to 220 degrees C. Also the filaments may be shrunk on the first zone of annealer by over feeding and  imparting higher strength  by stretching 2% or so on the final zone of the annealer. Next the fibre is quenched in a hot water bath, then passed through a steam chest to again heat up the tow to 100 degree C so that the crimping process which takes place in the stuffer box proceeds smoothly and the crimps have a good stability. Textile spin finish is applied either before crimping by kiss roll technique or after crimping by a bank of hollow cone sprays mounted on both sides of the tow. The next step is to set the crimps and dry the tow fully which is carried out by laying the tow on a lattice which passes through a hot air chamber at 85degree C or so.

 

The two is guided to a cutter and the cut fibres are baled for despatch. The cutter is a reel having slots at intervals equal to the cut length desired 32 or 38 or 44 or 51mm. Each slot has a sharp stainless steel or tungsten carbide blade placed in it. The tow is wound on a cutter reel, at one side of the reel is a presser wheel which presses the tow on to the blades and the tow is cut. The cut fibre falls down by gravity and is usually partially opened by several air jets and finally the fibre is baled. Some,  balers have a preweighting arrangement which enables the baler to produce all bales of a pre determined weight.

The bale is transported to a ware house where it is "matured" for a minimum of 8/10 days before it is permitted to be despatched to the spinning mill.

FIBRE SPECIFICATION:

DENIER: Usually the actual denier is a little on the finer side i.e for 1.2 D, it will be 1.16 and for 1.4 , it could be 1.35. The tolerance normally is +- 0.05 and C.V% of denier should be 4 to 5%. Denier specifies the fineness of fibre and in a way controls the spinning limit. Theory tells us that in order to form yarn on ring spinning (and also in air jet) there must be minimum of 60 to 62 ifbres in the yarn cross section. Therefor the safe upper  spinning limit with different denier is

DENIER COUNT(Ne)
1.0 90
1.2 80
1.4 62
2.0 40
3.0 32

The limit is for 38 mm fibre. The limit rises for a longer fibres.

When spinning on open end system, the minimum no of fibres in the yarn cross section is 110. So all the fibre producers recommend finer denier fibres for OE spining . Here the safe upper spinning limit is

 

DENIER COUNT(Ne)
1.0 50
1.2 40
1.4 30
2.0 24
3.0 16

However in actual practice , 30s is an upper limit with OE AND 1.2 Denier is being used, in  USA and other countries, even for 10s count in OE.

Deniers finer than 1.0 are called   micro-denier and commercially the finest polyester  stple fibre that can be worked in a mill is 0.7 D.

CUT LENGTH: Cut lengths available are 32, 38, 44, 51 and 64mm for cotton type spinning and a blend of 76, 88 and 102 mm - average cut length of 88m for worsted spinning. The most common cut length is 38 mm.

For blending with other manmade fibres, spinners preferred 51mm to get higher productivity, because T.M. will be as low as 2.7 to 2.8 as against 3.4 to  3.5 for 38mm fibre. If the fibre legnth is more, the nepping tendency is also more , so a crompromise cutlength is 44 mm. With this cut length the T.M. will be around 2.9 to 3.0 and yarns with 35 to 40% lower imprfections can be achieved compared a to similar yarn with 51 mm fibre. In the future spinners will standardise for 38 mm fibre when the ringspinning speed reaches 25000 rpm for synthetic yarns.

For OE spinning , 32 mm fibre is preferred as it enables smaller dia rotor(of 38mm) to be used which can be run at 80000 to 100000 rpm.

Air jet system uses 38 mm fibre.

TENSILE PROPERTIES: Polyester fibres are available in 4 tenacity levels.

  • Low pill fibres- usuall in 2.0 / 3.0 D for suiting enduse with tenacities of 3.0  to 3.5 gpd(grams per denier). These fibres are generally used on worsted system and 1.4D for knitting
  • Medium Tenacity - 4.8 to 5.0 gpd
  • High tenacity 6.0 to 6.4 gpd range and
  • Super high tenacity  7.0 gpd and above

Both medium and high tenacity fibres are used for apparel enduse. Currently most fibre producers offer only high tenacity fibres. Spinners prefer them since their use enables ring frames to run at high speeds, but then the dyeablity of these fibres is 20 to 25% poorer, also have lower yield on wet processing, have tendency to form pills and generally give harsher feel.

The super high tenacity fibres are used essentially for spinning 100% polyester sewing threads and other industrial yarns. The higher tenacities are obtained by using higher draw ratios and higher annealer temperatures upto 225 to 230 degree C and a slight additional pull of 2% or so at the last zone in annealing.

Elongation is inversely proportional to tenacity e.g

  TENACITY ELONGATION AT BREAK T10 VALUES
LOW PILL 3.0 - 3.5 45 - 55% 1.0 - 1.5
MEDIUM 4.8 - 5.0 25 - 30% 3.5 - 4.0
HIGH 6.0 - 6.4 16 - 20% 5.2 - 5.5
SUPER HIGH 7.0 plus 12 - 14% 6.0 plus

All the above values of single fibre. Testing polyester fiber on Stelometer @ 3mm guage is not recommended.

The T10 or tenacity @ 10% elongation is important in blend spinning and is directly related to blend yarn strength. While spinning 100% polyester yarns it has no significance. Tenacity at break is the deciding factor.

 

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