Fault free dyeing of polyester fiber

Excerpt: Polyesters in its early days were known as alkyd resins. The word alkyd was a result of combining part of the first word 'alcohol' and part of the word 'acid'.


Fault-free dyeing of polyester fiber

* Murli Ramalingam *
Business Director
Leomine Organics Pvt Ltd,

SINCE ancient times, natural polyesters have been known to mankind. Shellac, a natural polymer secreted by the lac insect, was used by ancient Egyptians for embalming mummies. Berzelius first synthesized polyesters of polybasic acids and polyvalent alcoholsin 1847 by reacting tartaric acid and glycerol. Polyesters in its early days were known as alkyd resins. The word alkyd was a result of combining part of the first word 'alcohol' and part of the word 'acid'. Alkyd resins were first marketed by General Electric Co., USA under the brand name 'Glyptal'.

In the late 1920's, Wallace.H.Carothers made a number of polyesters having molecular weights between 2500 and 5000 by condensation reaction of dicarboxylic acids and diols. This was the advent of the modern process of manufacturing polyesters. Later Carothers along with J.W Hill obtained higher molecular weight polyester upto 25000 by extracting water liberated during the condensation reaction of the step wise polymerization process which he developed. Carothers called polyesters having molecular weights greater than 10000 as 'super polymers' since these polyesters had very different properties than polyesters having molecular weight lower than 10000. Super polyesters as they were later called were tough, opaque solids which on melting at high temperatures became clear viscous flowable liquid. Filaments could be drawn from the molten polyester and on cooling these filaments could be drawn to several times their original length. The drawn filaments were transparent, tough and possessed high tenacity

In 1941, Schlack used terephthalic acid and 1:4 butanediol to produce polyester with melting point of 218° C. At the same time Whinfield and Dickson at Calico Printer's association in Great Britain used terephthalic acid and ethylene glycol to produce polyester with a melting point of 268° C. Patent rights for this process was acquired from Calico by ICI (Great Britain) and Du Pont (USA).

ICI marketed polyester fiber under the trade name 'Terylene' and Du Pont's product was named Dacron. Patent rights from Calico was later acquired by firms in France and Italy

Polyester fiber established itself with outstanding success by virtue of its excellent properties suitable for textiles like :

  • Very good crease recovery and shape retention

  • High bulking capacity

  • Good tear and abrasion resistance

  • Good light fastness

  • Light weight

  • Dries rapidly ; easy care properties

  • Resistance to influences of weather, temperature, acids,
    oxidizing agents and solvents Resistance to insects and micro-organisms

Polyester fibers are marketed in form of continuous filament and staple fiber. The latter is produced by cutting or tearing. They are dyed in the form of tow, stock, slub, yarn or fabric both knitted and woven. Polyester fibers are often processed in blends with cotton, viscose, wool and other synthetics like acrylic etc. Owing to their firm inner structure, relatively large portion of crystalline regions, pronounced hydrophobic behavior and absence of reactive groups like hydroxy and Amino, polyester fibers can only be properly dyed using disperse dyes. The dyeing methods must be designed to facilitate penetration of dyes into the fabric structure.

The first systematic study on dyeing of polyester was carried out by Waters in 1950. He concluded that even though disperse dyes have affinity towards polyester fiber, the slow diffusion of dyestuff is the main cause for dyeing problems with polyester. Therefore in order to achieve deep shades higher temperatures around 130° C during dyeing have to be adopted. At lower temperatures of dyeing, carriers have to be used to induce disperse dye molecules to travel into the fiber.

In order to achieve good shape retention and to prevent the marking of running creases during wet processing, polyester fibers must be stabilized. In other words tensions within the individual fibers and in the fabric are levelled out by relaxation. Setting is carried out at high temperature upto 180° C using dry heat. Relaxation of tensions within the textile material results in shrinkage of fibers. These changes in the physical structure of the fiber affect the dye-ability of the fiber. The dye uptake usually increases with increases in pre-setting temperatures employed. It is important that fibers should not undergo further significant shrinkage during subsequent wet processing particularly during exhaust process

When high temperature dyeing machines were first introduced, it was thought that polyester dyeing would be a simple process using high temperature mechanical process and acids to set the pH of the dyebath so as to prevent fiber damage due to hydrolysis. However it turned out that polyester dyeing is a complicated process that required a number of auxiliaries to achieve level dyeing. The reasons for un-level dyeing is related to disperse dyes and polyester fiber itself

Issues related to disperse dyes is mainly due to availability of concentrated dyestuffs with inadequate dispersing agents which lead to dye aggregation during dyeing. Also the particle size of the disperse dyes are not always uniform due to improper milling during production of dyestuff. Sometimes the de-dusting oils used to blend the disperse dyes and its diluents are not self-emulsifiable leading to problems in dyeing

Issues related to polyester fiber pertain to content of oligomers particularly cyclic trimers which are as high as 2%. These trimers cause uneven dyeing, stick to the insides of dyeing machines and stain yarn & fabric of subsequent lots when they are dyed. Polyester fiber manufacturers may use lower quality spin finishes which are mineral oil based that are not self-emulsifiable. These days polyester fiber is more in the form of micro-fiber. With increased surface area the amount of spin finish on micro-fiber is relatively higher that on conventional polyester fiber

Factors that influence perfect dyeing of Polyester fiber by exhaust process :

Considering the above issues related to dyestuff and polyester fiber, there are four key actions that are vital for success during dyeing of polyester viz. Levelling, dispersing, oligomer control and emulsifying.

a) Levelling:
molecular sizes. Typically Violet, Turquoise blue, Royal blue have larger molecules than others. During the adsorption phase ( 70 – 90° C ) different dyes in the recipe tend to exhaust on to the polyester fiber at different rates due to differences in their molecular size (variation in affinity ). When the temperature is increased to 90 – 110° C during diffusion phase the dyestuff penetrate into the polyester fiber at different rates. These two phenomena leads to un-level dyeing. If surfactant based levelling agents are used to correct the problem, the situation worsens since these levelling agents solubilizes the dyes in the oil core of the surfactant micelles. Since the dyes remain in the dyebath without exhausting fully on to the fiber, the dyeing achieved is not satisfactory Leomine Organics synchronized exhaustion of all dyes used in combination in the recipe via dye-affinitive levelling agent. Leomine Level PES forms adducts with respective dye molecules averaging out the differences in size of individual dye molecules. By doing so every dye molecule will be made to have similar affinity to the fiber and will exhaust in a synchronized manner during the adsorption phase.

Therefore the penetration of dyes are also synchronized during the adsorption phase. The dyeing remains uniform during the entire dyeing cycle.

b) Dispersing:
Index no. are of the same quality. They often show slight differences in shade due to impurities, by-product content and their particle size uniformity. Disperse dyes have a tendency to aggregate due to their intrinsic low solubility in water. The larger the dye particle size more is the tendency to aggregate. Inadequate dispersing agents used during the manufacture of dyestuff adds to the problem. There are two types of dispersing agents available – one based on Napthalene Sulfonate formaldehyde condensate and the other based on surfactants. Surfactant based dispersing agents works on solubilizing action which leads to dye retention mentioned earlier. Surfactant based dispersing agent cannot maintain the dyes in dispersion during the entire dyeing cycle. Napthalene Sulfonate condensate delivers genuine dispersing action on which Leomine Disperse XPD ( powder form ) and Leomine Disperse XPL( liquid form ) are based.

c) Oligomer Control:
Oligomer is a by-product of polyester fiber production and its content ranges from 1.5% to 2.5% ( poor quality polyester fiber ). If left untreated these oligomers tend to deposit on the surface of the fiber and dyeing machines affecting dyeing results. Leomine Clear XPO is an effective auxiliary which solves the issues of oligomer interference. It can also be used as machine cleaner.

d) Emulsifying:
Oil can be introduced to the polyester in the dyebath through de-dusting oil in the dyestuff formulation and spin finishes used on the fiber during its production. If these are not well emulsified then they will deposit as oil droplets on the substrate as oil spots. This will subsequently cause dye spots as disperse dyes selectively dissolve in oil droplets. To ensure level dyeing the levelling agent used should have adequate emulsifying power to emulsify the oils present in the dyebath. Leomine Level PES has a powerful emulsifying component to overcome this problem.

The polyester dyer's demand is for a single all-purpose auxiliary which can solve all the problems mentioned above and provide for trouble free dyeing. Leomine Organics has recently launched Leomine Level PEX which at an application rate of 1% on weight of fabric is robust enough to overcome all challenges like Levelling, dispersing, oligomer control and emulsification – a 4-in- 1, state of art, fourth generation polyester dyeing auxiliary which has no equal.

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