When manufacturing a paint, it is very important to know how to choose all its components so that it retains its properties when used in the application for which it has been designed. One of these properties, perhaps one of the most important, is undoubtedly the colour.
The components that determine the colour of a paint are the pigments. There is a great variety of them which differ not only in colour but also in other properties that are very important for the paint. The list of paint pigments available on the market is so long that the choice is by no means simple. The question to be asked is: How to choose paint pigments?
Types of paint pigments
Paint pigments are mainly divided into two main groups according to their chemical structure: inorganic pigments and organic pigments.
Inorganic pigments, based on minerals (usually metals and metallic salts), have been used since ancient times. They were already used by our ancestors for cave painting in their caves. Compared to organic pigments (whose use is much more recent) they offer higher opacity and better light fastness, both weather and high temperature resistance. They are also generally less expensive than organic pigments. However, the latter offer more vivid colours.
However, advances in technology nowadays allow us to obtain paint pigments with better properties than those that would correspond to their chemical nature.
There is a standard classification for pigments: the colour index. Pigments are thus grouped according to their colour: Blue Pigments (PB); Black Pigments (PBk); Brown Pigments (PBr); Green Pigments (PG); Orange Pigments (PO); Red Pigments (PR); Violet Pigments (PV); Yellow Pigments (PY); White Pigments (PW). This index depends on the chemical nature of the pigment which determines its performance based on several parameters.
What parameters should we look for when choosing paint pigments?
There are several parameters that determine how paint pigments will perform in different applications.
The colour of paint pigments depends mainly on their chemical structure that absorbs and reflects different wavelengths when light strikes the surface of the pigment.
Colour strength (or tinting strength) is the facility with which a pigment retains its colour characteristics when mixed with another pigment. The higher the strength, the less pigment is required to achieve a given shade of colour.
In organic pigments, colour strength depends on the ability to absorb certain wavelengths of light. Highly conjugated and highly aromatic molecules show higher intensities.
Inorganic pigments that have metals with two valence states show a high intensity, while those with a cation trapped in a crystal lattice show a weaker colour.
Particle size also influences the colour strength of a pigment. In general, smaller particles give a higher colour strength. The manufacturing process of pigments influences the particle size of their crystals. The dispersion of the pigment also plays an important role in the colour strength of the paint.
Depending on the paint application, it is important to consider how pigments react to temperature. In general, paint pigments become more soluble at higher temperatures, so shading may occur.
Pigments with a highly crystalline structure are generally more heat resistant than polymorphic pigments, where different crystalline modifications may respond differently to heat. Usually, inorganic pigments have improved thermal stability, although there’s an exception with yellow iron oxide which loses water from the crystal at high temperatures.
All tests performed evaluate the colour of pigments at several temperature ranges, and evaluate the difference in colour between the sample in question and a standard processed at the minimum temperature.
Light is an element that affects the behaviour of pigments in a coating. Some inorganic pigments are unchanged by exposure to light, but most pigments and especially organic pigments may undergo darkening or complete colour fading.
The ability of a pigment to resist light is influenced by the chemical composition, pigment concentration, crystal modification and particle size distribution. In addition, environmental factors can drastically affect the results, such as the presence of water and chemicals in the atmosphere or in the paint system.
Depending on the application, it is very important to choose pigments with high light fastness.
For outdoor applications, it is very important to consider the weather stability of the pigments. Weather stability, which is related to light fastness, also depends on atmospheric conditions (including sea salt, waste gases from industrial areas or the very low humidity of desert conditions).
Weather stability depends on the required outdoor performance (shelf life, climatic region…), the pigment concentration, the presence of titanium dioxide (which may reduce or accelerate fading depending on the grade used), and the concentration and type of light stabilisers used.
A pigment must be insoluble in the vehicle (the medium in which it is dispersed) and must not react with any of the paint components. Once in the dry film, the pigment must also not be affected by the substrate and the agents it comes into contact with, including water, which may simply be present in the form of condensation, or acidic industrial atmospheres. Under certain conditions, pigments may dissolve and cause application problems.
The solubility of a pigment gives rise to the following problems:
If the pigment dissolves in the solvent, as the paint dries, the solvent comes to the surface and evaporates leaving crystals of the pigment on the surface in the form of a fine powder. As solubility increases with temperature, this phenomenon worsens at elevated temperatures.
Plate out is similar to blooming but occurs with plastics and powder coatings. However, it is not due to the pigment dissolving, but to the pigment surface not being adequately wetted. It occurs mainly with complex pigments, and once cleaned from the surface, it does not reappear.
Pigments in a dried paint film may dissolve in the solvent contained in a new coat of paint applied over the original film. If the topcoat is of a different colour, especially a white or pale colour, the result can be disastrous. Again, high temperatures worsen the problem.
This phenomenon can occur in paints intended for bead mills. During the grinding stage, heat is generated and dissolves part of the pigment. Over a period of time, the dissolved “pigment” starts to precipitate, loses gloss and colour strength. This becomes especially noticeable in the case of paints containing two pigments of different colours that have different solubility characteristics. The more soluble pigment dissolves, and then, as it comes out of the solution and precipitates, the paint will take on the shade of the second pigment. Recrystallisation can occur even in aqueous systems. It can be avoided by using less soluble pigments and/or by controlling the temperature during the dispersion process.
Opacity or hiding power
The hiding power is the ability of a pigmented coating to hide the surface on which it has been applied. It depends on the film’s ability to absorb and scatter light. It is not only related to colour, because while dark and saturated colours, such as blacks and dark blues, are usually opaque, and light colours more transparent, there are organic blue pigments that are transparent, and light pigments, such as titanium dioxide, that are opaque.
A key factor in the opacity of a pigment is its refractive index (RI) which measures the ability of a substance to bend light. The opacifying effect is proportional to the difference between the refractive index of the pigment and that of the medium in which it is dispersed.
Inorganic pigments have a high refractive index, and organic pigments have much lower values. Consequently, most inorganic pigments are opaque, while organic pigments tend to have higher transparency.
The particle size distribution of the pigment is another factor that also plays an important role in opacity. Considering that the refractive index of a compound cannot be changed, the pigment manufacturer can have an influence on the particle size of the pigments; consequently, particle size selection has become one of the major advances in pigment technology in recent years.