General Theory of Dyeing -
There is no generally applicable theory of dyeing capable of explaining all dyeing phenomena satisfactorily however, various models are available which provide a basis for the study of a variety of dye fibre systems.
The theories of dyeing are concerned with the character of dye Diffusion in solid polymeric fibres.
Essentially, they are supported by two important fundamentally different models for dyeing Diffusion in fibre namely the pore Diffusion model and the free volume or mobile segment model.
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| Pore diffusion model |
(A) The pore Diffusion model -
This model represents the fibre as a solid structure with a network of interconnected channels or pores that are crammed with dyeing Liquid which is generally water.
The dissolved dye diffuses through these pores where it may be simultaneously adsorbed on the walls of the pores.
The pore model assumes that the pores are connected to every other also on the external dye bath and their diameter is sufficiently large for the dye molecules to search out space in them.
The diffusion of the dye in cellulose fibres is mostly supported by the pore model. in line with this, a network of pores swollen and stuffed with water is present within the fibre and it's within this network that dye Diffusion and sorption happen.
In the case of reactive dyes, diffusion is followed by chemical reactions between the dye and fibre.
It is well known that the physical makeup of the fibres generally affects how effectively cellulose fibres take the dye. This explains why cotton and rayon dye different, as do different types of regenerated cellulose fibres.
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| Pore diffusion model |
(B) The free volume model -
In contrast to the pore model, the free volume model describes the dyeing process as the Diffusion of the dye through the less ordered or amorphous regions of the polymer structure.
The rate of Diffusion is therefore determined by the mobility of the polymer chain segment in these regions.
The most important support for this theory comes from the observation that the temperature dependence of the speed of dyeing for a particular variety of fibre is a smaller amount above a specific temperature.
The resistance of the solid structure of the fibre to the penetration of the dye is far lower above this Temperature.
This temperature is referred to as the glass transition temperature (Tg) of fibre or more precisely the glass transition temperature of the bre under dyeing conditions since the classical glass transition temperature may be a parameter that's normally measured within the dry state of the bre.
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| Free volume model |
The glass transition temperature plays a big role in dyeing. It corresponds at a molecular level to the temperature at which the amorphous regions of a polymer are converted from a glassy state to a rubbery Visco elastic state.
Above this temperature, segments of the polymer chain within the amorphous regions have increased mobility
Segmental mobility causes changes within the arrangement of the chain molecules in these regions.
Due to the segmental mobility "holes" or free spaces are formed above Tg and disappear again or occur at neighbouring sites of the polymer chain segments involved.
In the Visco elastic State the polymer structure can't be considered in static terms the structure changes constantly, however, as the relatively small dye molecule diffuses through such a structure there's the possibility of their Absorption within the free volume of the fibre structure With the bulk of fibre dye system there are fluid transitions between the pore model and therefore the free volume model as shown in the diagram for various substrates.
An increase in the pore mechanism will decrease the cellulose porous acrylic fibres, regular acrylic fibres, polyamide and polyester which can increase the free volume of the mechanism.
The data potential -
All fibres when immersed in water acquire a charge at their surface. Most classes of water-soluble dyestuffs are anionic in nature and their approach to the fibre surface is resisted.
This barrier is overcome by the addition of neutral electrolytes like common salt or Glaubers salt These salts on dissolution in water produce an oversized number of charged Sodium-ion that is attached by and neutralise the electric charge at the fibre surface.
This follows the coloured dyestuff ions’ free access to the surface of the fibre
Absorption and desorption -
When a bre is placed during a solution of a dyestuff which has Affinity for the fibre the dyestuff is adsorbed into the fibre.
As more and more transferred from the answer to the fibre position of equilibrium will develop and further transfer will cease.
The concentration of dyestuff within the bath and the fibre at equilibrium is going to be during a definite ratio of the one to the opposite.
The exact ratio depends on the actual dyestuff the temperature of the tub and also the concentration of salt within the bath.
If on the opposite hand the dyed fabric is placed during a bath of clear water under similar conditions with or without salt, dyestuff is going to be desorbed from the fibre until eventually, the identical balance is about up.
At this time dye transfer won't have ceased but the rates of change would be identical in both directions bath to fibre and fibre to bath.
In Commercially dyeing it's unacceptable to prolong the dyeing time indefinitely and consequently a State of true equilibrium is never reached.
The rates of dyeing in practice are therefore quite as important because the equilibrium effect with all dyestuffs Substantive to Cellulose some simple rules these variables-
(i) With any given dye and dyeing conditions the upper the temperature the more rapid the speed of dyeing
(ii) The higher the temperature the sooner levelling takes place
(iii) At equilibrium the number of dyestuffs absorbed by the fibre is bigger at low temperatures than at higher temperatures.
The biggest difference is observed with those dyestuffs that possess a low affinity for fibre.
In practice, however, this effect is seen only with dyestuffs that dye most rapidly. True equilibrium is established even more slowly at a lower temperature than that at the upper temperature normally used.
Question -
- What is the free volume model and how it is useful for the textile dyeing process?
- Explain the general theory of dyeing.
- What is The pore Diffusion model?
- What is the free volume model?
References
Dyes and dyeing. (n.d.). Gov.uk. from https://www.hse.gov.uk/textiles/dyes-dyeing.htm
eBook, T. (2021, February 22). Textile dyeing PDF by Dr. N. n. mahapatra. Textile EBook. https://www.textileebook.com/2021/02/textile-dyeing-pdf-by-dr-n-n-mahapatra.html
Introduction to dyeing and dyehouse automation. (2014). In Modelling, Simulation and Control of the Dyeing Process (pp. 1–30). Elsevier.
Rana, M. B. (2015). Textile dyes and their application process. https://www.academia.edu/17283292/Textile_dyes_and_their_application_process
textile - Dyeing and printing. (n.d.). In Encyclopedia Britannica.
Textile dyes and dyeing. (n.d.). Pdfdrive.com. from https://www.pdfdrive.com/textile-dyes-and-dyeing-e176024633.html
(N.d.-a). from http://chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.cottonworks.com/wp-content/uploads/2018/01/Dyeing_Booklet.pdf
(N.d.-b). Researchgate.net. from https://www.researchgate.net/publication/338450346_Fabric_Dyeing_Process
(N.d.-c). Researchgate.net. from https://www.researchgate.net/publication/321085555_Handbook_of_Textile_Coloration_and_Finishing
(N.d.-d). Researchgate.net. from https://www.researchgate.net/publication/257956500_Textile_Dyeing
Further reading,
Part 2 Mixing of colours, the difference between dyes and pigments
Part 3 Basic concepts of colours and their impact on the fabric
Part 4 Classification of Textile Dyes
Part 5 Common concepts or terms used in dyeing
Part 6 Shade percentage of dyeing
Part 7 Dyeing assistants
Part 8 Mechanism of Dyeing and chemical bonds for textile
Part 9 General Theory of Textile Dyeing
Writer - Rushikesh Patil (Textile Engineer)
