Polyester based knitted fabric means any fabric has polyester fibre as composition in the fabric. It may vary from 5 % to 100% . Based on polyester % in the fabric the processing method will change . As fabric may have polyester in the form spun fibre or filament . Processing of the fabric will depend on this also.
Polyester knitted fabrics are used on a large scale for outer wear of all types. The main reasons for their success are the wide patterning potential, the good durability and crease resistance, and the excellent easy-care properties. Polyester knitted fabrics are composed almost exclusively of texturized yarn. To avoid creases and breakages, knitted fabric that have not been set should always be stored or treated in full width.
Polyester fabrics are made using the following yarns
- Filament yarns.
- Textured yarns.
- Spun yarns.
Filament yarns are made in monofilament and multifilament forms.
Textured Yarns are produced of polyester multifilament. they are given a texturizing either in conjunction with the drawing process or subsequently as part of the throwing and texturizing process in producing the finished yarns.
Spun yarns , means it is made from the fibre . It may be 100% polyester or blend with any other fibre as like Viscose, Cotton , Modal etc .
Properties of polyester fabrics – Fabrics made of regular tenacity polyester filament yarns are very strong and durable. They do not have a high degree of elasticity which means it is characterized as having a high degree of stretch resistance, which means polyester fabrics are not likely to stretch out of shape too easily. This property makes polyester suited for knitted garments, sagging and stretching that would ordinarily occur are reduced.
Polyester fabrics have good dimensional stability. It has got a high degree of resilience. Polyester fabric resist wrinkling when dry, it also resists wrinkling when wet. For example, a suit of polyester will keep its pressed appearance after many wearing, even after exposure to rain or moist, humid weather. Fabrics of polyester filament yarn have satisfactory draping qualities. The trilobal filament type is more supple and imparts better drapability. spun yarns are also more flexible and softer, thereby imparting the draping quality.
Polyester fabrics are better conductors of heat than acrylic fabrics. Polyester fabrics have low absorbency as a result it will dry very rapidly since almost all the moisture will lie on the surface rather than penetrate the yarns. So polyester fabrics are well suited for water –repellent purposes, such as rain wear. This low absorbency means that polyester fabrics will not stain easily except oil which has got affinity for polyester, and it is difficult to remove.
The main disadvantage due to low absorbency is polyester fabrics are clammy and uncomfortable in humid weather because they will not absorb perspiration or atmospheric moisture. This property can be improve by using multifilament Polyester filament yarn. As it helps to evaporate water fast by spreading water in larger area .
There is essentially no water shrinkage of polyester fabrics; therefore, shirts, blouses and even slacks may be safely laundered. When ironing polyester fabrics, it is best to use low to medium heat. excessive heat will cause polyester to melt. The wrinkle resistance of polyester is extremely good. Polyester fabrics made from spun polyester yarns tend to pill.
Polyester fabrics shrink as much as 20 % during wet – finishing operations and they are generally heat –set in later treatments. Polyester fabrics are better suited for outdoor use because it has good resistance to degradation by sunlight. They are resistant to mildew.
Scouring of Polyester based fabric –
It is required for all type of polyester knitted qualities. It is carried out as Single bath Scouring & Weight Reduction in HTHP Dyeing machine. It involves following steps
1- Loading of fabric at water which is pre heated up 50 deg C, and having no chemicals . As warm wash will help to remove Knitting oils from the polyester based fabric easily . It is in general seen if we add wetting agent then it becomes some times difficult to remove oil completely . We might have listen several times from knitting oil seller that our oil is water soluble , We should focus on this line. Hence first give only 10 min warm wash .
2- Weight Reduction of Polyester fabric
The weight reduction of polyester is nothing but Saponification of Terephthalic ester, caused by sodium hydroxide. Under predetermined conditions, alkaline hydrolysis involves the progressive peeling of polyester surface resulting in loss of weight without altering its favorable properties.
The weight reduction (WR) process is generally carried out to reduce the rigidity of polyester as well as to increase the bounce suppleness of the polyester fabric.
A std recipe for the starting of scouring may be
4 % NaOh
1% wetting agent
1% oligomer remover ( Surfactant or solvent based oil remover )
We can H2 O2 , Stabliser if it has Cotton also in the blended fabric . It will help to clean Polyester and cotton both together
The treatment need minimum 90 Deg C temperature if we need Weight reduction . it may be taken up 110 deg C temperature also for 30 minutes minimum .
If cotton content is more than poly then it can be avoided as first step , It can be done after polyester dyeing also which will help as Reduction clearing also .
But if polyester content is more then it is always advisable to do this Treatment first then do heatsetting and take for dyeing after that . As by this method we can control Weight of fabric , can avoid crease marks during high temperature dyeing .
4- For polyester rich fabric or 100 % polyester filament based fabric neutralization with ozalic acid will help to avoid any stains on the fabric . For cotton rich polyester fabric can be neutralize with acetic acid .
Process Sequence
The basic finishing processes for 100% polyester filament yarn fabrics may be arranged in the following three sequences.
- Scour –Heatset – Dye
- Heat set –Scour— Dye.
- Scour—Dye—Heat set.
Following are the precautions taken for the above process.
- Circular knitted fabrics should be slit as soon as possible after knitting (within 24 hours). If they cannot be further processed immediately, they must be rolled flat, completely free from creases. If the goods are stored in tubular form for some time, the folds will become fixed.
- Prestabilizing is necessary if the fabric is liable to become creased during precleaning or if the selvedges tend to curl. This is done by running the fabric in a tensionless state through a steaming chamber ((saturated steam ,100deg c.); the fabric relaxes, and fibres begin to crimp.
- Prior to dyeing, all spin finish, knitting machine oil and other impurities must be removed. At the same time washing develops the crimp which is lost in knitting.
- Scouring is carried in continuous full width washing ranges using Soda Ash and good detergent. In which the fabric is able to relax completely without longitudinal tension. The shrinkage is often over 20 %. It is very important for the Poly Lycra Fabric and 100 % Polyester fabric. Do not load directly such fabric in soft flow , Otherwise it will give hard creases in the fabric , which will not be removed later. And if spun oil is not removed from the fabric before heat treatment , then there may be moir type of visual effect in the fabric after final finishing . t is only oil which has been fixed in uneven form in fabric .
- After washing, rinsing, hydroextraction uniform drying is essential because variations in the residual moisture content are liable to cause uneven fixation, resulting in unevenness in dyeing.
- Heat setting is done in stenter at 150-180 deg c for 20-30 seconds. The higher the setting temperature, the better the dimensional stability of the treated fabrics., but the handle becomes flatter and harsher if the setting temperature is too high.
- It is very critical to decide heat setting temperature of fabric . It depends based on so many factor . Such as Type of yarns used in the fabric , Elastomeric Type , Type of blended fibre , Is the yarn used is pre dyed .
- Heatsetting temperature decides the stretch property of fabric also . If we cross the temperature 160 deg C in polyester based fabric we are compromising with the stretch property . It becomes more stiff as soon as we crossed temperature 160 Deg C . If the fabric has Elastane fibre then minimum required temperature is 185 Deg C for the heatsetting .
- If fabric is manufactured from the Dyed yarn or Melange yarn which has Disperse dyes , then it will start loosing bonding as we cross temperature 145 deg C, Unfixed dyes will come on the surface of fabric , It will result in poor fastness property of the finished fabric . If we are trying to manufacture yarn dyed poly fibre based fabric which has elastomeric fibre also , then it is advisable to use EASY SETTING ELASTANE yarn . which can be heatset at temperature 150 Deg C, in place of 185 Deg c
- Texturized polyester knitted fabrics are normally dyed at 130 deg c in HTHP machine. it gives a full, bulky handle to the goods using medium to high sublimation fastness disperse dyes. If fabric has to go for higher temperature treatment after Dyeing process as like printing and curing , Heatsetting , Drying at high temperature then it is advisable to use High sublimation fast dyes.
One thing should always keep in mind that there is no straight line in the processing of the fabric , It is combination of applications and it depends on the person who is using these process.