An Overview of Polyester Staple Fiber
This organic synthetic fiber is formed by the polycondensation of a diacid (terephthalate acid) and alcohol (glycol-ethylene). The used process to obtain this polymer is the melt spinning with 400% heat stretching.
Polyester is a category of polymer whose monomer contains the ester functional group. The most common polyester for fiber purposes is poly (ethylene terephthalate), or simply PET. This is also the polymer used for many soft drink bottles and it is becoming increasingly common to recycle them after use by remelting the PET and extruding it as fiber. This saves valuable petroleum raw materials, reduces energy consumption, and eliminates solid waste sent to landfills.
PET is made by reacting ethylene glycol with either terephthalic acid or its methyl ester in the presence of an antimony catalyst. The reaction is carried out at high temperature and vacuum to achieve the high molecular weights need to form useful fibers. PET is melt spun.
Today over 70 to 75% of the polyester is produced by CP (continuous polymerization) process using PTA(purified Terephthalic Acid) and MEG. The old process is called the Batch process using DMT (Dimethyl Terephthalate) and MEG( Mono Ethylene Glycol). Catalysts like 5b3O3 (ANTIMONY TRIOXIDE) are used to start and control the reaction.
TiO2 (Titanium dioxide) is added to make the polyester fiber/filament dull. Spin finishes are added at melt spinning and draw machines to provide static protection and have cohesion and certain frictional properties to enable fiber to get processed through textile spinning machinery without any problem.
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
- Resistance to Weathering: good
- Rot Resistance: high
- Alkali Resistance: damaged by CON alkali
- Acid Resistance: excellent
- Organic Chemical Resistance: good
Physical properties of Polyester Fibers:
- Thickness : 1.2D, 1.5D , 2.0D
- Color: white
- Length: Variable cut lengths
- Density : 1.39 g/cc
- Tenacity : high, 40 to 80 cN/tex
- Moisture regain : 0.4 % (at 65% R.H and 20°C)
- Elongation: high, 15 to 45%
- Flame reaction: melts, shrinks, black fumes
- Melting point: 260°C
Characteristics of Polyester Fibers:
- Good strength
- Low absorbency
- Resistant to stretching and shrinking
- Resistant to most chemicals
- Easy to wash – Quick-drying
- Crisp and resilient when wet or dry
- Wrinkle and abrasion-resistant
- Retains heat-set pleats and creases
End Uses of Polyester Fibers:
- Clothing: blouses, shirts, children’s wear, dresses, neckties, lining, lingerie and underwear, permanent press garments, slacks, suits.
- Home furnishings: carpets, curtains, bedsheets, and pillow covers.
- Technical textiles: floor coverings, V-belts, ropes and nets, tire cord.
Production Process of Polyester Fiber
Manufacturing Process of Polyester Fiber
Polymerization – Manufacturing Polyester
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 temperatures. Then monomer is formed. Polymerization is carried out at high temperatures (290 to 300 degrees centigrade) and in an almost total vacuum. Monomer gets polymerized into the final product, PET (Polyethylene Terephthalate).
Melt Spinning – Manufacturing Polyester
This is in the form of thick viscous liquid. This liquid is then 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 fiber) through a pack that 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 towed and is coiled in cans. The material is called undrawn TOW and has no textile properties.
Polymer Drawing and Cutting – Manufacturing Polyester
At the next machine ( the draw machine), undrawn tows from several cans are collected in the form of a sheet and passed through a trough of hot water to raise the temperature of the 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 submicroscopic chains aligned parallel to the fiber axis, thereby improving the crystallinity of the fiber structure and imparting certain strength. The 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 overfeeding and imparting higher strength by stretching 2% or so on the final zone of the annealer. Next, the fiber is quenched in a hot water bath, then passed through a steam chest to again heat the tow to 100 degrees C so that the crimping process which takes place in the stuffer box proceeds smoothly and the crimps have good stability. The 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 that passes through a hot air chamber at 85degree C or so. The tow is guided to a cutter and the cut fibers 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 onto the blades and the tow is cut. The cut fiber falls by gravity and is usually partially opened by several air jets and finally, the fiber 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 warehouse where it is “matured” for a minimum of 8/10 days before it is permitted to be despatched to the spinning mill.
Recycled polyester is sustainable or not?
Recycled polyester, also known as rPET, is obtained by melting down existing plastic and re-spinning it into new polyester fiber. While much attention is given to rPET made from plastic bottles and containers thrown away by consumers. In reality, polyethylene terephthalate can be recycled from both post-industrial and post-consumer input materials. But, just to give an example, five soda bottles yield enough fiber for one extra-large T-shirt.
Let’s make a pros and cons of recycled polyester.
PROs of Recycled Polyester:
1. Keeping plastics from going to landfill and the ocean
Plastic has been found in 60 percent of all seabirds and 100 percent of all sea turtle species because they mistake plastic for food.
Taking plastic waste and turning it into a useful material is very important for humans and our environment
2. rPET is just as good as virgin polyester but takes fewer resources to make
Recycled polyester is almost the same as virgin polyester in terms of quality, but its production requires 59 percent less energy compared to virgin polyester, according to a 2017 study by the Swiss Federal Office for the Environment.
CONs of Recycled Polyester
1. Recycling has its limitations
Many garments are not made from polyester alone, but rather from a blend of polyester and other materials. In that case, it is more difficult, if not impossible, to recycle them.
Even clothes that are 100 percent polyester can’t be recycled forever.
After all, even though rPET takes 59 percent less energy to produce than virgin polyester, it still requires more energy than hemp, wool, and both organic and regular cotton, according to a 2010 report from the Stockholm Environment Institute
2. The process of recycling PET impacts the environment, too
The chips generated by mechanical recycling can vary in color: some turn out crispy white, while others are creamy yellow, making color consistency difficult to achieve. Inconsistency of dye uptake makes it hard to get good batch-to-batch color consistency and this can lead to high levels of re-dyeing, which requires high water, energy, and chemical use
3. Recycled polyester releases microplastics
A paper published in 2011 in the journal Environmental Science Technology found that microfibers made up 85 percent of human-made debris on shorelines around the world. It doesn’t matter if garments are from virgin or recycled polyester, they both contribute to microplastics pollution.
Recycled polyester, often called rPet, is made from recycled plastic bottles. It is a great way to divert plastic from our landfills. The production of recycled polyester requires far fewer resources than that of new fibers and generates fewer CO2 emissions.
There are 2 ways to recycle polyester: For mechanical recycling, plastic is melted to make new yarn. This process can only be done a few times before the fiber loses its quality. Chemical recycling involves breaking down the plastic molecules and reforming them into yarn. This process maintains the quality of the original fiber and allows the material to be recycled infinitely, but it is more expensive.
A recycled polyester is definitely a sustainable option for our wardrobe. However, we need to be aware that it is still non-biodegradable and takes years to disappear once thrown away.