Basic concepts of Light and colour -
Colour normally visible to us is a combination of the effect of -
(1) Properties of the coloured substances.
(2) Properties of the light falling on it and getting reflected or transmitted.
(3) Properties of our eyes on which the reflected or transmitted light falls.
The sun is the main and most important source in our solar system. It emits energy in the form of light.
The light travels in the form of waves at tremendously High velocity i.e 3*108 m/s.
Colour | Wavelength |
Violet | 390-430 |
Indigo(blue) | 430-460 |
Blue(blue-green) | 460-500 |
Green | 500-570 |
Yellow | 570-590 |
Orange | 590-610 |
Red | 610-700 |
Only light with a wavelength between 400 and 700 nm is visible to the human eye out of all the light that makes up sunlight.
The distribution of the wavelength of the radiation in the visible region of sunlight and their corresponding colours are given below. Light is electromagnetic radiation.
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| Visible light and electromagnetic spectrum |
Colour and chemical constitution -
Colour consist of two groups-
(ii) Auxochrome Explanation -
(i) Chromophore - (colour-bearing group in dye structure)
Nature must be unsaturated for colour development. Coloured compounds have double bonds, while colourless molecules do not
It is recognised that for compounds to be coloured, they must have unsaturated chemical groups known as chromophore or colour-bearing groups.
Chromogen is a chemical that contains a chromophore, although it is not a real dye.
Types of Chromophores -
(b) Independent
Dependent | Independent |
More than one colour or one Chromophore sufficient to impart colour Ex: N2, N2O, NO2 | The single chromophore is sufficient to impart colour Ex: Ethene |
(ii) Auxochromes -
● Auxochromes, which are particular groups' contributions to colour deepening, or
Auxochromes are particular colour-enhancing or colour-intensifying groupings.
● Auxochromes aid in the development of colour but are not colour themselves.
● Auxochromes can be anionic or cationic and can be of the acidic or basic variety.
Several significant auxochromes are -
Ex: -OH,-SO3,-NHR,-NH2,-OSO3H,-COOH,-NR2
● Auxochromes form salts either with acids or Alkalies and also Hydrogen bonds with certain groups like the hydroxyl (-OH) group in cellulose and the amino(-NH2) groups in wool and silk.
● When the presence of chromophore in a substance is accompanied by an auxochrome, it becomes a dye.
● Ex: Azo-benzene containing only chromophore is not a dye but, when an -NH2 group is introduced it becomes p-amino azo benzene which is a dye.
In addition to chromophores and auxochromes, certain solubilizing groups like Sodium sulphonate may also be present in the dyestuffs.
The function of auxochromes -
(i)They increase the intensity of the colour
(ii)They make chromogen a dye which helps for fixing chromogen to the fibre
Types of Auxochromes -
(a) Batho-chrome or redshift
(b) Hypo-chrome or blue shift
Batho-chrome or Redshift | Hypo-chrome or Blueshift |
By incorporation of a certain group in di-molecule shifts the absorption maxima from a lower wavelength to a higher wavelength | Any group when incorporated in a dye structure it brings changes in Absorption maxima from longer wavelength to shorter wavelength |
Groups that deepen the colour of dye or increase the depth of colour are known as Batho-chrome or redshift | It lightens or reduces the intensity of colour or decreases the depth of dye known as Hypo-chrome |
Ortho and para position groups have to deepen the colour | Ex: -OCH3, -OCOCH3 |
Meta position will decrease depending on the colour. | Acylation of phenolic group |
Alkali, arile and halogens are Batho-chrome |
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| Basic concepts of colours and their impact on the fabric |
Questions -
- Describe concepts of colours.
- Which two groups are chemical colours contained?
- Explain Chromophore and Auxochrome colour groups.
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)
