PROBLEMS & THEEIR REMEDIES IN POLYSTER PROCESSING

Problems and Their Remedies in Polyester Processing-

 

 

 

 

Introduction

 

The estimated polyester fibre production, in 2007, was 34 million tons (approx)-39.7% of the whole textile
fibre production-while, cotton production was 28 million tons (approx) and other cellulosic fibres, 2.8 million
tons (approx). On a positive note, polyester fibres are estimated to grow by 5% annually, even as other
textile fibres stagnate or decrease.

 

What is PET?

 

The development of fibres from polyester have come a full circle since scientists at ICI, UK, first produced
dimethyl terephthalate from ethylene glycol and terephthalic acid. Subsequent years saw the arrival of this
new fibre in the market, chemically known as polyethylene terephthalate (PET).

 

Independent of this research, DuPont (US) and Hoechst (Germany) also developed this synthetic fibre. By
today's standards, it is the most versatile synthetic fibre ever invented. It lends itself beautifully alone, as a
textile fibre, or as blends with cotton, viscose, polynosic, linen, silk, wool or hemp. The chief characteristics
of this fibre include:

 

• Good crease recovery
• Good shape retention
• High bulking capacity
• Good abrasion
• Tear resistance
• Good dye ability
• Acceptable moisture absorption
• No strength loss during wetting and other chemical treatments
• Resistance to microorganism, insects, mildew, etc.

 

Structure and modifications

 

Mainly manufacture by the direct reaction of terephthalic acid and ethylene glycol, polyester is devoid of
functional groups.

 

Polyester modifications are made to alter the inner structure of the fibre. These modifications are physical,
using special methods, or they contain copolymers or different chemical components, giving the desired end
results like low pilling, dyeing behavior to anionic and cationic dyes, etc. An interesting modification is the
process known as ‘texturising’, where the molten fibre is first stretched and then cooled. It gives the fabric a
luxurious bulkiness, softness and extra shine.

 

Various types of additions and modification also affect the performance of the fibre/ fabric. For example,
delustering agents (usually titanium dioxide) results in dull polyester, while adding optical whitener to the
fibre before spinning produces white fibre.

 

Physical Properties

 

• Density: 1.38 - 1.5
• Melting point: 252 - 256οC
• Softening point: 230 - 250οC
• Glass transition temperature: 68οC

 

Drawbacks

 

The major drawback of the low moisture regain polyester fibres is its wearing comfort. Hence, 67/ 33%
blends with cotton result in excellent fabrics having the best properties of both the fibres. Another
disadvantage is that oils, fats, grease, etc. can easily soil polyester fabrics because of the electrostatic
charge build up and the fibres tend to pill. Hence, adding a singeing process takes care of this problem by
burning off the surface fibre.

 

Dream come true

 

In the 90’s the polyester micro fibre was a commercial breakthrough in the world of synthetic textiles. A
polyester fibre of exceptional soft feel, comfort, and rich appearance, was a synthetic fibre technologist’s
dream. Micro fibres made it a reality.

 

What is so special about micro fibre?

 

• Excellent softness
• High drape
• Silky handle
• Good breath ability
• Super comfort
• Rich appearance

 

Thus its use is rapidly growing in the fashion apparel sector as well as the technical textile sector – which
itself is growing at rate of 35% in India. Considering these trends and the versatile uses of polyester
microfilament fabrics, studying aspects of wet processing and problems in coloration of microfilament textiles
is worthwhile.

 

Right first time

 

In textile wet processing problems and their remedies are very important. As a ‘wrong first time’ is followed
by harsh consequences like:

 

• Extended processing time
• Substandard fabric
• Extra cost
• Additional energy
• Late delivery
• Late delivery of following batches

 

 Preparatory chemical processing for polyester

 

 

Singeing

 

Singeing is a pretreatment for PET-cellulosic blend fabrics (not 100% PET fabric). The modern gas singeing machines have efficient and computer controlled burners, which can control the width, height, angle and intensity of the flames.

 

Problems encountered:

• Any uneven heat transfer is liable to cause variations in the setting of PET fibres, resulting in uneven, dyeing of fabric.
• In singeing protruding PET fibres tend to form globules, which are difficult to remove.
• The globules dye deeper shade than the original fibre when dyeing is carried out by exhaust method.

 

Remedies:

 

• Singeing as a pretreatment should only be done if the continuous thermosol method is used for dyeing.
• Singeing effect, if one or more passages are given, can reduce the pilling effect on the fabric. Leading to better usage and thus in such cases singeing acts as post treatment machine.

Setting of polyester

 

In order to achieve good shape retention and prevent running creases from marking during wet processing, PET fibres and their blends must be stabilised. In other words, tension within the individual fibres and in the fabric is leveled out by relaxation. Heat setting of PET is carried out at 180 C, to ensure 0% residual shrinkage (natural shrinkage) and completion of primary crystallization.

 

Problems:

 

• Measurement of surface temperature of fabric
• Speed uniformity in machine
• Non-uniformity in setting effects for large run of fabrics
• Incomplete removal of residual shrinkage (0% shrinkage never achieved)
• Excessive shrinkage causes problems in subsequent processes

 Remedies:

 

•  Accurate and precise measurement of fabric surface temperature using infrared sensing thermometers
•  Measuring residual shrinkage in lab (should not be more than 0.5%)
•  Achieving precise relaxation of fabric by over feed arrangement
•  Using modern hot air stenters for accurate speed adjustment, air humidity measurement and overfeed

 Chemical preparation

 

 Processing aids are applied during sizing of microfilament yarns in order to help and protect the delicate polymer fabric during processing. The common feature of all washing processes is the exchange of substances in an aqueous media.

 

 Problems:

•  Temperature variations in the batch cause variations in the degree of bleaching
•  Poor temperature control can cause fibre tendering
•  Variations in reaction time between ends of the cloth on the batch can cause ‘tailing’ in dyeing
•  Efficient and optimal removal of sizes and other processing aids
•  Creasing and abrasion marks in the fabric during chemical pretreatments

 Remedies:

• To minimize creasing, it is important to control shrinkage of the fabric i.e. relaxation should occur under minimum tension
• To reduce creasing, use of suitable lubricating agent is recommended
• Latest Harish JT10 type of jiggers and lately atmospheric pressure jiggers, it is possible to have heat efficiency and liquor spray and D.C. motor drives for uniform treatments
• Variations of these with micro process controls and uniform speeds from beginning to end and with HTHP arrangements can be used for this process
•  For the removal of large amounts of oils or fats like oils and spin finishes, pre-cleaning with organic solvents (perchloroethylene) has established itself
• Weight reduction process carried out by caustic soda of concentration 22-25% by weight at elevated temperature of 90 - 95°C under controlled time gives polyester a silk like appearance adapted successfully in India

Printing

  

Printing of polyester is a specialized subject that needs elaborate study. Polyester can be printed with the following class of dyestuffs:

•  Indigo sol colours
•  Pigment dyes
•  Disperse dyes

 Disperse printing of polyester consists of following fundamental principles:

•  Thickener-acid stable
•  Disperse dyes
•  An acid donor or catalyst
• An auxiliary to enhance the dye uptake as and when required

 The methods of fixation after printing and drying are:

 

• Pressure steam fixation method
• High temperature steam fixation method
• Thermo fixation method

Thickening agent is the most important component of the printing paste and it should be cost effective. The fact is that since the printing paste is acidic the thickener should have very good stability in the pH range of 2-5 for long time. The most favoured thickener the world over is guar gum. Others include:

• Modified guar gum
• Carboxyl methyl cellulose
• Sodium alginate
•  Synthetic thickeners based on polymeric organic compounds

The disperse dyes usually become dull under alkaline conditions. To improve this, the pH of the print paste is controlled to pH of 4.5-5 by addition of citric or tartaric acid – the more common being citric acid. However, tartaric acid is selective in some real cases where the acidic conditions are more stable than citric acid.

 

Problems:

 

• Splashing printing paste during printing due to vibration of blotting paper
• Sticking and threading property of gum when screen is lifted causes sharp lines losing their sharpness
• Yellowing and uneven prints of the printed fabric cloth causing poor sharpness, brightness, colour value
• Bleeding at the time of printing and steaming, caused by the use of excess urea. Coarse screen mesh and excess dyestuffs may cause bleeding as well
• Changing tone due to high time lapse between printing and drying. improper heating in the ager also results in tone variation due to uneven diffusion of disperse dyes
• Hallowing/ mark off due to sublimation of disperse dyes having low sublimation fastness
• Changing colour value due to incompatibility of dye combinations. The presence of impurities in water like calcium, magnesium and iron forms a complex with the dyestuffs, lowering the colour value and giving a dull effect
• Staining during after treatment if proper sequence of rinsing is not followed, because superfluous disperses dyes get re-deposited on the unprinted cloth during hot water rinsing

Remedies:

 

• Proper viscosity of print paste. Guar gum and its percentage with proper cooking method will eliminate the splashing problem. A gum paste having higher solid content and certain plasticity is recommended
• Fabric should be free from auxiliaries like size, coning oil, non-ionic emulsifiers and cotton impurities, to eliminate the problem of yellowing and uneven prints of printed fabric
• Selecting suitable thickener, moderate strength of the dyestuffs, appropriate dosage and small motifs in printing excludes bleeding. Low viscosity of paste may cause bleeding
• Tone change can be eliminated by uniform heating and quick drying. Auxiliaries, like print developer TX, have a synergistic action on the different types of dyes used in mixing and brings about synchronization of diffusion rates of individual dyes
• Hallowing/ mark off can be negated by selecting proper dyes and good quality wrapper (cotton wrapper) for pressure aging
• Colour value change can be barred by maintaining appropriate pH of 5-6. Compatibility of dyes i.e. having diffusion temperature in the same range is selected. Use of sequestering agent in print paste is important or else usage of soft water is emphasized
• After-treatment staining is reduced by following the sequence of: Cold wash 􀃆 Reduction 􀃆 Hot water 􀃆 Soaping 􀃆 Clearing􀃆Rinsing
• Using a good auxiliary like catalyst PC helps in removal and destroying unfixed superfluous disperse dyes and also helps in breaking baked-dried film of printing gum faster  

Washing

 

 Good printing can be spoiled by bad or improper washing techniques. Essentially, the best washing sequence has to follow the following steps:

 

• Soak the printed fabric well to swell the gum of the print paste for easy removal
• Remove the swollen gum by an efficient washing arrangement in the open soaper or winches
• Continue washing in 2-3 compartments in cold and agitating conditions, till the wash water is free from contaminated colours
• Add 2-3gpl of good quality detergent with dispersing functions-0.5-1gpl caustic soda, 1-2gpl sodium hydrosulphite-to remove all unfixed dyestuffs efficiently
• Treatment with 2-3gpl detergent and 1-2gpl of soda ash at boil, in two compartments followed by drying in drying ranges after hot wash.