An Overview of Polyester and Polyester Dyeing Part-3

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Polyester Fiber Manufacturing Process:
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 spinneret 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 spinneret and is instantly solidified by the flow of cool dry air. This process is called quenching. The filaments from each spinneret 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.
Polyester fiber manufacturing process
Polyester fiber manufacturing process
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.


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

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

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 staple fibre that can be worked in a mill is 0.7 D.

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.

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

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.

Crimps are introduced to give cohesion to the fibre assembly and apart from crimps/cm. Crimp stability is more important criterion and this value should be above 80% to provide trouble free working. A simple check of crimp stability is crimps/inch in finisher drawing sliver. This value should be around 10 to 11, if lower, the fibre will give high fly leading to lappings and higher breaks at winding. Spin finish also gives cohesion, but cohesion due to crimp is far superior to the one obtained by finish. To give a concrete example, one fibre producer was having a serious problem of fly with mill dyed trilobal fibre. Trilobal fibre is difficult to crimp as such, so it was with great difficulty that the plant could put in crimps per inch of 10 to 11. Dyeing at 130 degrees C in HTHP dying machine reduced the cpi to 6 to 8. Mills oversprayed upto 0.8% did not help. Card loading took place yet fly was uncontrolled, ultimately the fibre producer added a steam chest to take the two temperature to 100degrees plus before crimping and then could put in normal cpcm and good crimp stability. Then the dyed fibre ran well with normal 0.15 to 0.18 % added spin finish.

Several types of spin finishes are available. There are only few spin finish manufacturers - Takemoto, Matsumoto, Kao from Japan, Henkel, Schill &Scheilacher, Zimmer & Schwarz and Hoechst from Germany and George A.Goulston from USA. It is only by a mill trial that the effectiveness of a spin finish can be established. A spin finish is supposed to give high fibre to fibre friction of 0.4 to 0.45, so as to control fibre movement particularly at selvedges , low fibre-metal friction of 0.2 to 0.15 to enable lower tensions in ring spinning and provide adequate static protection at whatever speed the textile machine are running and provide enough cohesion to control fly and lapping tendencies and lubrication to enable smoother drafting.

Spin finish as used normally consists of 2 components - one that gives lubrication / cohesion and other that gives static protection. Each of these components have upto 18 different components to give desired properties plus anti fungus, antibacterial anti foaming and stabilisers. Most fibre producers offer 2 levels of spin finishes. Lower level finish for cotton blends and 100% polyester processing and the higher level finish for viscose blend. The reason being that viscose has a tendecy to rob polyester of its finish. However in most of the mills even lower spin finish works better for low production levels and if the production level is high, high level spin finish is required if it is mixed with viscose. For OE spinning where rotor speeds are around 55000 to 60000 rpm standard spin finish is ok, but if a mill has new OE spinning machines having rotors running @80000 rpm, then a totally different spin finish which has a significantly lower fibre - fibre and fibre - metal friction gave very good results. The need to clean rotors was extended from 8 hours to 24 hours and breaks dropped to 1/3rd. In conclusion it must be stated that though the amount of spin finish on the fibre is only in the range 0.105 to 0.160, it decides the fate of the fibre as the runnability of the fibre is controlled by spin finish, so it is the most important component of the fibre. Effectiveness of spin finish is not easy to measure in a fibre plant. Dupont uses an instrument to measure static behaviour and measures Log R which gives a good idea of static cover. Also, there is s Japanese instrument Honest Staticmeter, where a bundle of well conditioned fibre is rotated at high speed in a static field of 10000 volts. The instrument measures the charge picked up by the fibre sample, when the charge reaches its maximum value, same is recorded and machine switched off. Then the time required for the charge to leak to half of its maximum value is noted. In general with this instrument , for fibre to work well, maximum charge should be around 2000 volts and half life decay time less than 40 sec. If the maximum charge of 5000 and half life decay time of 3 min is used , it would be difficult to card the fibre , especially on a high production card.

Normally measured at 180 degree C for 30 min. Values range from 5 to 8 %. With DHS around 5%, finished fabric realisation will be around 97% of grey fabric fed and with DHS around 8% this value goes down to 95%. Therefore it makes commercial sense to hold DHS around 5%. L and B colour: L colour for most fibres record values between 88 to 92. "b" colour is a measure of yellowness/blueness. B colour for semidull fibre fluctuates between 1 to 2.8 with different fibre producers. Lower the value, less is the chemicals degradation of the polymer. Optically brightened fibres give b colour values around 3 to 3.5. This with 180 ppm of optical brightner.

Each fibre producer has limits of 100 +- 3 to 100+-8. Even with 100+-3 dye limits streaks do occur in knitted fabrics. The only remedy is to blend bales from different days in a despatch and insist on spinning mills taking bales from more than one truck load.

The right way to measure is to card 10 kgs of fibre. Collect all the flat strips(95% of fused fibres get collected in flat strips). Spread it out on a dark plush, pick up fused and undrawn fibres and weigh them. The upper acceptable limit is 30mgm /10kgs. The ideal limit should be around 15mgm/10kgs. DUpont calls fused/undrawn fibres as DDD or Deep Dyeing Defect.

Polyester fibres are available in
  • bright : 0.05 to 0.10 % TiO2
  • Semil dull : 0.2 to 0.3 % TiO2
  • dull : 0.5 % TiO2
  • extra dull : 0.7% TiO2 and
In optically brightened with normally 180 ppm of OB, OB is available in reddish , greenish and bluish shades. Semi dull is the most popular lustre followed by OB (100 % in USA) and bright.

  • DENIER: 0.5 - 15
  • TENACITY : dry 3.5 - 7.0 : wet 3.5 - 7.0
  • %ELONGATION at break : dry 15 - 45 : wet 15 45
  • CRIMPS PER INCH: 12 -14
  • %DRY HEAT SHRINKAGE: 5 - 8 (at 180 C for 20 min)
  • SPECIFI GRAVITY: 1.36 - 1.41
  • % ELASTIC RECOVERY; @2% =98 : @5% = 65
  • Softening temp : 230 - 240 degree C
  • Melting point : 260 - 270 degree C
  • Effect of Sunlight : turns yellow, retains 70 - 80 % tenacity at long exposure
  • ALKALI RESISTENCE: damaged by CON alkali
  • ACID RESISTENCE: excellent
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