Polyester Manufacturing Process
Fibre 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 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.
Drawing and Cutting
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 tow 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.
Physical and Chemical Properties of Polyester Fiber
Denier: 0.5 – 15
Tenacity : dry 3.5 – 7.0 : wet 3.5 – 7.0
%Elongation at break : dry 15 – 45 : wet 15 45
%Moisture Regain: 0.4
Shrinkage in Boiling Water: 0 – 3
Crimps per Inch: 12 -14%
Dry Heat Shrinkage: 5 – 8 (at 180 C for 20 min)
Specific Gravity: 1.36 – 1.41%
Elastic Recovery @2% =98 : @5% = 65
Glass Transition Temp: 80 degree C
Softening temp : 230 – 240 degree C
Melting point : 260 – 270 degree C
Effect of Sunlight : turns yellow, retains 70 – 80% tenacity at long exposure
Resistence to Weathering: good
Rot Resistence: high
Alkali Resistence: damaged by CON alkali
Acid Resistence: excellent
Organic Chemical Resistence: good