An Overview of Polyester and Polyester Dyeing Part-4

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The manufacture of polyester fiber consists of 4 stps:

1. Polymerisation: Using PTA/DMT and MEG on either batch or continuous polymerisation (cp_ - forming final polymer

2. Melt spinning : Here molten polymer is forced thorough spinnerette holes to form undrawn filaments, to which spin finish is applied. 
3. 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

4. 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.

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 at 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.

What can spinning mills do to overcome this problem:
One way is to use a Uster Glow meter which measures the reflectance of fibre samples under UV light. We understand that these values lie between 80 and 120 for samples from different bales. so then divide bales with reflectance values of say 80 to 90 , another 91 to 100, third 101 to 110 and fourth 111 to 120. Then while issuing bales to blow room, issue first group say 80 to 90 then issue the enxt group and so on. Bales from different groups should not be mixed. Second is to use bales from each truck separately. Third is to mix up bales from 4/5 trucks to do blending. 
Changes in DEG: The amount of DEG in fibre is directly proportional to dye pick up or dye ability of the fibre. Higher the DEG, higher is the dye ability, so much so that some filament producers add DEG, but then higher DEG will lower tensile properties. So this practice is not followed for fibre, where tensile properties are critical. So if the CP is run at lower throughout, DEG drops down, so the dyeablity of the fibre goes down. Since fibre production group is keen on maintaining merge, they resort to lowering of annealer temperatures to maintain dye ability but in the process tensile properties suffer, and mills will notice thread strength falling by 5-7% if annealer temperature is lowered from say 210 degree C to 180 Degree C. If fibre production group does not do this, then they will produce fibre with a different merge - which normally accumulates in the warehouse and so is not appreciated by both marketing and top management. Also when CP is run at higher than rated, then higher temperatures have to be used to compensate lower residence time, here "b" colour actually improves It must be emphasized that the "b"colour changes occur not only due to higher / lower thorugh put but there are several other factors such as air leakags in valves / polymer lines, failure of pumps to remove product from one reaction vessel to another etc. There is yet one more problem in CP. It is a sudden increase in oligomer content. When the amount of oligomers increase, it manifests itself in excessive white powder formation on rings and ring rail. Oligomers cause problems in spinning of dyed fibres. The surface oligomer content almost doubles on dying dark and extra dark shades. The only way to control oligomers is to use LEOMIN OR in 1 - 1.5 gms/litre in reduction clearing bath. All oligomers will go into suspension in reduction clearing liquor and get removed when the liquor is drained. Higher annealer temperature also cause higher surface oligomers.

Spinning Process

Melt Spinning : 
The fiber forming material is melted and subsequently passed through the holes of a spinneret. Example ; Polyester, Nylon, Polypropylene etc.

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.
Process of Melt Spinning :
Melt Spinning Process.
Figure : Melt Spinning Process.
Special Feature Of Melt Spinning :
  • High production ( SPG. Speed 1000-2000 m/min)
  • Hazard , Non Toxic
  • No environment pollution.
  • No solvent required .
  • Heat of spg high.
Melt Spinning Polymer :

Melting Point
Nylon 6.6
264 degree Celsius
Nylon 6
220 degree Celsius
167 degree Celsius
264 degree Celsius

Dry Spinning : 
In Dry Spinning, Polymer dissolved in a volatile solvent is introduced into a heated drying chambers, where the solvent is evaporate and solid fiber is obtained. This process may be used for the production of Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandax and Vinyan.

What are Synthetic Fibers : The fibers which are obtained by chemical synthesis called synthetic fibers. Polyester, Nylon, Acrylic, PBI etc are the Synthetic Fibers.

Dry Spinning Process :
Figure : Dry Spinning Process
Control of C.V% of Denier: A good international value of C.V.% of denier is 4 to 5. However some fibre manufacturers get value as high as 10 to 12. Denier is controlled by having uniform flow of polymer through each spinnerette hole. However if a hole is dirty or has polymer sticking to it, its effective diameter is reduced; and the filament that comes out becomes finer. IF the spinneretters have been used for more than say 6 to 7 years , then some of the holes would be worn out more than others and filament emerging out would be coarser Currently sophisticated instruments are available to check the cleanliness and actual hole diametrs of each and every hole automatically, but few producers have them. 
Fused Fibres: These are caused mainly at melt spinning either due to breaks of individual filaments or breakages of all the filaments(ribbon break) and polymer and block temperatures are too high. Tying of broken position in the running thread line should be as near to the broken position as possible, failure to do this will result in trailing end leading to fused fibres. Other reasons could be impurities, choking of polymer filters and non-uniform quenching or cooling of filaments. The only way to control is to ensure that breaks at melt spinning are held at the minimum.

Draw line is the place where the fibre is born. All its major properties denier, tenacity , elongation at break , crimp properties, spin finish, shrinkage and dye ability are all imparted here. For obtaining excellent runnability of the fibre in a blend spinning mill, the two most important properties are - spin finish and crimp. Spin finish: Finish is applied to the undrawn tow at melt spinning stage essentially to provide cohesion and static protection. 
On the draw line, a major portion of this finish is washed away, and a textile spin finish is put on the tow by either kiss roll or a spray station. This textile finish consists of two components, one that gives cohesion and lubrication and the other confers static protection, usually these 2 components are used in 70/30 ratio. These spin finishes are complex and each may contain some 18 chemicals to not only control inter fibre friction ( should be high at 0.35 to 0.40), fibre metal friction (should be low at 0.15-0.20), anti bacterial components, anti foaming compund etc. Finish is made in hot demineralise water and is sprayed on to tow after the crimper by a series of spray nozzles mounted on both sides of the tow. 
The finish is pumped to the spray unit by a motor driven metering pump, which is linked to the draw machine such that when the machine stops, the pump motor stops. The percentage of finish on the fibre is based on spin finish manufacturers recommendations and fine tuned by tech service. Once set, the finish and its percentages are normally not changed.The percentage spin finish is decided by the end use of the fibre. Mills blending polyester with viscose need higher amount of spin finish and also mills running their equipment at high speeds. 60 to 65% of problems faced in mills are due to uneven % of spin finish on the fibre. IF a fibre producer desires to put say 0.120% spin finish on fibre, then ideally the %finish should be maintained @ 0.120 +- 0.005 i.e from 0.115 to 0.125 only; then the fibre will run smoothly.If the finish is on the lower side, card web will show high static, web will lap around doffing rolls, sliver will not pass smoothly through coiler tube - causing coiler choking. Sliver could be bulky and will cause high fly generation during drafting. 
On the other hand if spin finish is on the higher side, fibres will become sticky and lap around the top rollers, slivers will become very compact and could cause undrafted. Thus it is extremely important to hold finish level absolutely constant. The reasons for non uniformity is concentration of spin finish varies; sprayer holes are choked ; the tow path has altered and so the spray does not reach it. Normally fibre producers check spin finish% on the fibre quite frequently- even then in actual practice considerable variations occur. 

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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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