Pigment Characteristics
INTRODUCTION
Building upon my article on 'Colourants: Pigments and Dyes', I will be talking about the characteristics of pigments, including lightfastness, tinting strength and hiding strength.
LIGHTFASTNESS
Lightfastness, also known as the permanence of colour, is the chemical stability of a pigment under the exposure to light for a long duration of time. Light is a source of energy which can cause the colour to alter due to chemical changes in pigments. For example, over time, the pigments may become desaturated, tinted (whitened), shaded (darkened) or even completely disappear.
Lightfastness testing was first developed by the American Society for Testing and Materials (ASTM) which conducted these tests with various lights such as sunlight, fluorescent UV lamps, cool white fluorescent etc. However, these experiments were inaccurate as these tests often exposed paint samples to the intense radiation of light which caused a quick change in the appearance of the pigments. Moreover, when testing, the amount of light was unknown or was not kept constant.
In order to solve this problem, the Association of Textile Chemists and Colorists (AATCC) developed a ‘blue wool scale’. The scale uses two blue wool textile fading cards which each consist of eight strips of blue wool that fade at different rates. [1] Ultraviolet radiation in light causes the pigment in blue wool to fade. All of the blue pigments selected for the scale differ as each pigment takes 2-3 times longer to begin fading compared to the pigment selected above. This means that the strip on the top has the least permanence and the strip on the bottom has the most permanence. [2]
One card scale is exposed to a specific wavelength of ultraviolet and the other is exposed to no light. [1] Two samples are also made for the pigment that wants to be tested. One of the samples is also placed in the dark and the other exposed to ultraviolet light at the same time as the blue wool scale. When a reference pigment in the blue wool scale begins to discolour, all the other pigments that have started to discolour after that point are rated as having the level of lightfastness as the reference pigment. The pigment is then given an ASTM (American Society of Testing and Materials) rating. [3]
TINTING STRENGTH
Tinting strength (or known as tinting power) is the measure of the relative power of colouring of a pigment in a form of a paste. [4]
A tint test is used to show the ability of a pigment (in paste form) to maintain its strength when mixed with a white pigment paste. The test reveals the ability of the pigment to maintain its strength (its intense colour) while using the same volume of white paste. The darker/more vibrant the colour is at the end of the test, the higher the tinting strength of the pigment. For example, when comparing phthalocyanine blue with ultramarine blue, phthalocyanine blue has 40 times higher tinting strength than ultramarine blue. This means that if phthalocyanine blue were to be used in high concentrations, it would completely overpower a larger variety of colours mixed with it compared to ultramarine blue. [5, 6]
Tinting strength is always determined relatively, meaning that there is no absolute scale. Hence, when a tint test is performed, information of the medium used must be recorded. A reference pigment would be selected and it is labelled as 100%. Other pigments compared to the reference pigment can have a higher or lower percentage than 100% depending on the tinting strength (the lower the number, the lower the tinting strength). [7]
The tinting strength values for two similar pigments can also be used to calculate the concentration of the pigment required to give the same intensity. If the pigments have a high tinting strength such as phthalocyanine, companies would most likely reduce the concentration within the application substance as a high concentration is not required. [4]
HIDING STRENGTH
Hiding Strength (or known as opacity) is the covering power of pigment when applied to another substance. The opacity of pigment varies due to its chemical structure and size of particles. [7] The hiding strength is heavily influenced by the refractive index, the particle size of the pigment and the dispersibility of the pigment.
Different pigments can absorb different wavelengths of light to create ‘colour’. When light travels through a substance, its velocity is slower, causing the beam of light to be refracted. The refractive index is calculated by the formula of n = (sin i) / (sin r). n represents the refractive index, i is the angle of incidence (the angle between the light beam entering the substance and the line perpendicular to the surface of the substance) and r is the angle of refraction (the angle between the refracted beam of light and the line perpendicular to the surface). [8]
As the refractive index increases, more of the light is scattered, meaning that opacity increases. Though this is the case for many pigments, there is one special case for this relationship which are white pigments. White pigments have little to no absorption of light. This means that the hiding strength of white pigments completely relies on the scattering of the incident light. [9]
The particle size of a pigment is the average size of the particles in the pigment. Most synthetic pigments are manufactured in a range of particle sizes to ensure they can be used for different applications. As the particle size decreases, the tinting strength increases. This is because small particle sizes have a large surface area to volume ratio which produces a more intense colour.
Dispersibility is the measure of how easy a pigment can spread/distribute across an area when in a liquid medium. Some pigments are more susceptible to clumping due to the electrostatic cling of their particles. Due to the clumping, the hiding strength decreases since the dispersity increases. [6]
BIBLIOGRAPHY
[1] MacEvoy, Bruce. “Labeling, Lightfastness & Toxicity.” Handprint, 8 Jan. 2015, www.handprint.com/HP/WCL/pigmt6.html#lightfast/
[2] “The Blue Wool Scale.” Materials Technology Limited, www.drb-mattech.co.uk/uv%20blue%20wool.html/
[3] Johnson, Nicholas. “Blue Wool Lightfastness Standard References.” Tanguay Photo Mag, 10 Apr. 2019, www.tanguayphotomag.biz/digital-printing/blue-wool-lightfastness-standard-references.html/
[4] Briggs, T. R. “The Tinting Strength of Pigments.” The Journal of Physical Chemistry, ACS Publications, 1918, p. 1
[5] Schadler, Koo. “Learn the Characteristics of Pigments.” Artists Network, 31 Aug. 2011, www.artistsnetwork.com/art-mediums/watercolor/learning-the-characteristics-of-pigments/
[6] MacEvoy, Bruce. “The Material Attributes of Paints.” Handprint, 8 Jan. 2015, www.handprint.com/HP/WCL/pigmt3.html/
[7] Teichmann, Günther. “Practical Methods for Determining the Tinting Strength of Pigments in Concrete.” Technical Service Department, www.sept.org/techpapers/40.pdf/
[8] Hodgkins, Leila. “Refraction.” Schoolphysics, 2013, www.schoolphysics.co.uk/age16-19/Optics/Refraction/text/Refraction_/index.html/
[9] O'Hanlon, George. “Why Some Paints Are Transparent and Others Opaque.” Natural Pigments Inc., 6 Dec. 2013, www.naturalpigments.com/artist-materials/transparent-opaque-paints/
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