The dyeing of polyester fibers is problematic because the rate of diffusion of disperse dyes through the fiber is so low at temperatures where dye uptake is very low, making the process unfeasible. The problem is due to the high degree of crystallinity of polyester fibers and its low glass transition temperature, Tg. Whilst dyeing can be carried out at temperatures above the Tg, that is, at about 130°C, an alternative is to apply dyes at temperatures at the boil in the presence of a chemical called a carrier.
A carrier is a small molar mass compound that is added to a disperse dyebath in order to accelerate the rate of uptake of the dyes within PES (and other hydrophobic) fibers. Although several theories have been proposed to explain the mechanism by which carriers promote dye uptake, it is generally held that carriers plasticise the fiber and lower the glass transition, Tg, of the substrate.
The effect of the carrier on the polymer chains of the polyester fibers is similar to that of a raised temperature, allowing more rapid ingress of dye and the development of deep shades. Their smell is objectionable, however, and unless they are removed completely from the fiber, they can lower the light fastness of the dyeing.
The types of chemicals used as carriers are non‐ionic aromatic organic compounds of fairly low molecular weight (around 150–200). Typical of the carriers used fall into four main chemical groups:
- Phenols, such as o‐phenylphenol
- Hydrocarbons, such as biphenyl
- Chlorinated hydrocarbons, such as dichlorobenzene
- Esters, such as butyl benzoate
Phenols and chlorinated hydrocarbons are no longer marketed due to concerns about aquatic toxicity. Indeed such is the concern about the use of carriers that they are now little used in polyester dyeing, the high‐temperature method being preferred. An exception, however, is their use in the dyeing of polyester in its blends with wool, since wool is easily damaged above boiling temperature, especially at 130°C.
The effect of plasticisation is also observed in fibers where the free volume model is applicable. Plasticising chemicals enter into the amorphous domains and thereby lower the glass transition temperature. As a result the flexibility of the polymers chain increase, which, for example, is used to soften polyvinyl chloride based material. In polyester (polyethylen terephthalate) dyeing, high temperature and increased pressure (HT-dyeing, thermosol dyeing) are required to achieve sufficient rate of dyestuff uptake, or the carriers are added to the dyebath to lower the dyeing temperature below 100°C. The carrier molecules reduce Tg of the polyester to such a extent that an acceptable dyeing rate is observed at the boil and permits use atmospheric dyeing apparatus.
It has been established that certain hydrocarbons, phenols, amino acids, amides, alcohols, esters, ketones, nitriles etc. accelerate the rate of dyeing polyester fiber with disperse dyes from aqueous medium at temperature up to 100°C. These dyeing assistants alter the dispersing properties of the dyes and the physical characteristics of the fiber so that more dye can be transferred from the dye bath to the fiber. These are called carriers and are necessary for dyeing polyester fibers at the normal pressure and temperature below 100°C to increase the dyeing rate and to permit dye migration within the fiber. Level dyeing of disperse dyes depend on the migration power of the dye which is affected by nature and amount of carrier, dyeing time, temperature and the shade.
Properties of Carriers in Dyeing:
- Low molecular weight.
- Organic compound.
- Mainly aromatic compound of low water soluble.
- Anionic emulsifying agent.
- Lowering Tg thus promote chain movement and create free volume.
Some Commercial Name of Carriers:
|Palanil Carrier A
|Chlorinated aromatic compound
Factors Considered for Selecting a Carrier:
- High carrier efficiency.
- Availability at low cost.
- Little or no effect on light fastness of final dyeing.
- Absence of unpleasant odor.
- Non toxicity.
- No degradation or discoloration of fiber.
- Ease of removal after dyeing.
- High stability under dyeing conditions.
- Compatibility with dyestuffs.
- Ease of dispersion in the dye bath.
- Low volatility of the carrier including low volatility in the steam.
- Uniform absorption by the fiber.
Mechanism of Carrier Action in Polyester Dyeing:
In carrier method of polyester dyeing, carrier is used. Carriers swell the fiber and ultimately cause relaxation. They may operate by opening up the internal fiber structure and allow the dye molecules to diffuse more rapidly. They act as molecular lubricants reducing inter-molecular forces operating in the fiber, thereby following the dye molecule to force its way in. Its action may be described as below:
- It creates dye film on fiber surface.
- Carrier takes dye inside the fiber from dye carrier association.
- It increases the solubility of dye in the dye bath.
- Carriers penetrate inside the fiber polymer chain and thereby reduce inter-chain attraction. Thus polymer chains become movable and so dye molecules may enter the polymer system of fiber.
- It increases fiber swelling.
- The absorbed carrier increases the rate of dye uptake by creating liquid co-fiber.
- It increases the absorbency power of fiber.
- It lubricates the thermally agitated fiber molecules.
- 2-10 gm/lit carrier is used depending on material and liquor ratio and depth of shade.
- The automatic portion of carrier is postulated to have Van Der Waal’s force and attraction for hydrophobic group of it attracts water.
- With increasing molecular weight the carrier efficiency also increases up to a certain limit.
- An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
- Textile Chemistry by Thomas Bechtold, Tung Pham
- Physico-chemical Aspects of Textile Coloration by Stephen M. Burkinshaw
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.