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Polyester Fibre manufacturing process - 2

polyester

CRIMP PROPERTIES: 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.

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.

DRY HEAT SHRINKAGE: 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.

DYE TAKE UP: 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.

FUSED FIBRES: 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.

LUSTRE: 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.

PHYSICAL AND CHEMICAL PROPERTIES OF POLYESTER FIBRE:

  1. DENIER: 0.5 - 15
  2. TENACITY : dry 3.5 - 7.0 : wet 3.5 - 7.0
  3. %ELONGATION at break : dry 15 - 45 : wet 15 45
  4. %MOISTURE REGAIN: 0.4
  5. SHRINKAGE IN BOILING WATER: 0 - 3
  6. CRIMPS PER INCH: 12 -14
  7. %DRY HEAT SHRINKAGE: 5 - 8 (at 180 C for 20 min)
  8. SPECIFI GRAVITY: 1.36 - 1.41
  9. % ELASTIC RECOVERY; @2% =98 : @5% = 65
  10. GLASS TRANSITION TEMP: 80 degree C
  11. Softening temp : 230 - 240 degree C
  12. Melting point : 260 - 270 degree C
  13. Effect of Sunlight : turns yellow, retains 70 - 80 % tenacity at long exposure
  14. RESISTANCE TO WEATHERING:  good
  15. ROT RESISTENCE: high
  16. ALKALI RESISTENCE: damaged by CON alkali
  17. ACID RESISTENCE: excellent
  18. ORGANIC CHEMICAL RESISTENCE: good

 

PROBLEMS WHICH OCCUR DURING MANUFACTURE OF POLYESTER STAPLE FIBRE:

The manufacture of polyester fibre consists of 4 stps:

  • Polymerisation:Using PTA/DMT and MEG on either batch or continuous polymerisation (cp_ - forming final polymer
  • Melt spinning :Here molten polymer is forced thorough spinnerette holes to form undrawn filaments, to which spin finish is applied and coiled in can
  • Drawings: in which several million undrawn filaments are drawn or pulled approximately 4 times in 2 steps, annealed, quenched, crimped and crimp set and final textile spin finish applied and
  • Cutting: in which the drawn crimped tow is cut to a desired 32/38/44/51 mm length and then baled to be transported to a blend spinning mill.

1.PROBLEMS FACED IN   POLYMERISATION:

properties of Polymer: The polymer formed is tested mainly for intrinsic viscosity (i.v), DEG content, % oligeomers and L and b colours. Intrinsic viscosity is an indirect measure of degree of polymerisation and this value is around 0.63 for polymer meant for apparel fibres. DEG or Di Ethylene Glycol gets formed during polymerisation and varies from   1.2 to 1.8%. Oligomers are polymers of lower molecular weight and vary in quantity from 1.2 to 1.8 %. L and b are measures of colour. L colour signifies whiteness as a value of 100 for L is a perfect value. Most fibres have L colour values around 88 to 92. b colour denotes yellowness/blueness of polymer. the positive sign for b colour indicates yellowness whilst negative sign shows blueness, only polymer which contain optical brightener has b of 3 - 3.5 whilst all semil dull polymers show b values of 1.0 to 2.4. Higher values indicate  more yellowness, which indirectly shows chemical degradation of the polymer.

Running a CP @ lower / higher throughput: Every CP is designed for a certain throughput per day. Like say 180 tons/day or 240 tons/day. Sometimes due to commercial constraints like high buildup of fibre stocks etc. , the CP may have to be operated at lower capacityies. In that case the polymer that is produced has a higher "b" colour and a lower DEG content.

Higher "b" colour of say 1.5 against  normal value of 1.0 will show fibre to be yellowish and has a little more chemical degradation; which gives higher fluorescence under UV light. Most spinning mills have a practice of checking every cone wound under UV lamp  to find out whether there has been any mixup.

However if a mill is consistently receiving fibre with a "b"colour of say 1.0 and then if one despatch comes of "b"colour of say 1.5 then in winding, ring bobbins of both "b" colours will be received, and when cones are wound and checked under UV lamp, then higher "b" colour material will give higher fluorescence compared to that of lower "b" colour materials, and will cause rings under UV lamp. Fortunately a minor difference in "b" colour of 0.4 to 0.5 does not give variation in dyeability.

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