An Overview of Polyester and Polyester Dyeing Part-5

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Properties of Polyester Yarn:
Ture and heat of fusion of the samples were measured with a differential scanning calorimeter (Perkin-Elmer DSC-7) at heating rate of 10 oC/min and in a temperature range from 30to 350 oC under a nitrogen atmosphere. A 5% alkali aqueous solution was heated to its boiling point,and then a round sample was added to this solution. The solution was stirred, washed with distilled water, and finally dried in a vacuum oven. The weight loss of the sample was calculated by the following equation.
Where W0 and W1 are the weight of the sample before and after alkali treatment, respectively. The fibers were shaped into a hoop form and hung on a 0.7 g clip, and then the length (L1) of the hoop was measured. The hoop was placed in a glass tube in a damp contraction percentage measurement device and dipped at 100 ± 1 oC for 30 min. The hoop was taken out of the tube, and after 2 h, the length (L2) of the hoop was measured. The shrinkage was calculated according to the following equation.
A tensile test was conducted using an Instron mechanical tester (Fafegraph-M and Textecho Co., Germany) at a temperature of 25 oC and a relative humidity of 65%. The specimen length was 100 mm and the tensile speed was 200mm/min. All mechanical property values were obtained by averaging ten experimental values.The birefringence was measured using a polarization microscope (Zeiss, Germany). The angle between the fiber and the polarized light ruler was 45o, and the light wavelength was 546mm. The birefringence was calculated using the following equations:
Where R is the natural value, λ is the wavelength, θ is the rotation angle of the analyzer, and d is the thickness of the specimen.The density of the specimen was measured using a density grade tube (Shibayama density gradient column, Japan) using carbon tetrachloride (density: 1.59)/hexane (density: 0.68) as a mixture solvent at 23 oC.To evaluate the dyeing quality, Lumacron Black SE-3G was used as a dye. A 2% dye (fiber weight was used as a base) was diluted to a liquid ratio of 1:20. The samples were dyed at 100 oC for 30 min and then washed with distilled water at 80 oC for 10min. The surface color difference (K/S) of the samples was calculate dusing equation (6).
Figure 1. Tenacity and elongation (a), birefringence (b), wet shrinkage ratio (c), density and crystallinity (d), and thermal properties of polyester FDY as a function of spinning speed.
Crimp: It is the most important to spin finish for smooth running of fibre. There are 3 aspects of crimp. no of crimps per inch or per cm - usually 12 - 14 crimps per inch crimp stability - be 80% plus and crimp take up - be 27% on tow crimps per inch can be measured by keeping a fibre in relaxed state next to a foot ruler and counting the no of crimps or arcs.

Crimp stability refers to % retention of crimps after subjecting fibre to oscillating straightening and relaxing. We can get an indication on how good crimp stability is in a spinning mill by measuring crimps per inch in fibre from finisher drawing sliver. The crimps per inch of drawing sliver should be at least 10 to 11, if below this, then the crimps stability is poor , so to compensate may be a cohesive compound like Nopcostatt2151 P or Leomin CH be used in the over spary. Fibres like trilobal and super high tenacity fibres are difficult to crimp. Trilobal because of its shape and super high tenacity due to very high annealer temperature (220 degree C) used which makes the fibre difficult to bend. Also fibre dyeing particularly dark and extra dark shades reduces crimps per inch from 14 to 10 - 11 and in trilobal, as it is crimps per inch in fibre is 11 to 12, after dyeing it goes further down to 8 to 9. In dyed trilobal fibre, crimps per inch in fibre at finisher drawing may be around 6 to 7 so necessitating using almost 50% of cohesive compound in the over spray.

Crimp take up is % difference between relaxed length and straightened length of fibre in fibre stage. Normally this difference is around 18 to 20%. If the difference is much smaller, then it means the crimps are shallow and would have lower cohesion. After the tow is crimped , the crimps are set by passing tow through a hot air chamber. If crimp per inch is low, then that could be due to lower stuffer box pressure, but if crimp stability and/or crimp take up is low, it means the steam supply to crimper steam box is low.

Undrawn fiber: As the draw line, 1.6 to 3.0 million filaments are drawn or pulled, if a filament had a break at spinning and this is fed as a trailing end to the drawing, then that end cannot be drawn fully, and causes plasticises and fused fibres. Undrawn fibres are generated if the draw point is not uniform i.e not in a straight line.

Plasticised fiber: When drawline is running and if some filaments breaks then these broken filaments wrap themselves around a rotating cylinder, since most of these cylinders are steam heated, the wrapped portion solidifes. The operator then cuts out the solid sheet and throws it away as waste but then usually picks up the plastic end and uses it to thread the material and so a small piece of plastic material goes into the cutter and falls into the baler.

Tenacity / Dye ability: Both these properties are controlled by acutal draw ratio and annealer temperature. Draw ratio does not change in running, but annealer temperature can fall due to problem of condensate water removal. Most drawlines have temperature indicators - but then some buttons have to be pressed to see the temperatures so if the annealer temperature falls, tenaciy will fall and dye ability will increase which could lead to a change in merge.

Nail Head / Tip Fusion: In the cutting process, a highly tensioned tow is first laid over sharp blades and the pressed down by a Pressure Roll, resulting in filaments being cut. However if some blades become blunt, then the pressing of tow on to those blades creates high temperature and so tips of neighbouring fibres stick to each other and so separating this cluster becomes impossible. If it is not getting removed in Lickerin it will go into the yarn and cause a yarn fault. The tip fusion occurs when the blade is fully blunt. If the blade is not very sharp, it does not give a straight edge, there could be some rounding at the cut edge. Such fibres are called nail heads.

Tungsten carbide blades give sharp cut Opening of fibre cluster after opening: When fibres are cut, they fall down by gravity into the baler. Because of crimping clusters get formed; and so those need to be opened out; otherwise these can cause choking either in blowroom pipes or in chute feed. This opening is obtained by having a ring of nozzles below the cutter through which high pressure air jets are pointed up; and these jets open up fibre clusters.

Over length / Multilength: Over length fibres are those whose length is greater than the cut length plus 10mm and are casued by broken filaments which being broken cannot be straightened by tensioning at the cutter. Multilength are fibre whose length is exactly 2 or 3 times the cut length and are caused by nicks in neighboring blades. 

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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