The function of a Pigment Used in Coating
Paints, coatings, and inks all contain pigments, which are essential components. They are used in paint and coating compositions to give the wet or dry film colour, bulk, or a desired physical and chemical quality.
Colour of Pigment
The chemical structure of a pigment determines its colour, which is defined by the selective absorption and reflection of various wavelengths of light at the pigment's surface.
Coloured pigments absorb a portion of all light wavelengths. Consider the following scenario:
- Blue pigment reflects the input white light's blue wavelengths while absorbing all other wavelengths. As sodium light contains almost no blue component, a blue automobile in orange sodium light appears black.
- Almost all of the light is absorbed by black pigments.
- White pigments reflect almost all of the visible light that strikes them.
- The property of fluorescent pigments is intriguing. They absorb light in places outside the visible spectrum (ultraviolets that the human eye cannot perceive), split the energy up, and re-emit it in the visible spectrum, in addition to having high reflection in certain portions of the visible spectrum.
As a result, they appear to produce more light than the light reflected back to them, resulting in their dazzling colour.
When selecting a pigment, consider the colour strength (or tinctorial strength). When a coloured pigment is combined with another pigment, its colour strength refers to how well it retains its original colour. The less pigment required to obtain a typical depth of shade, the higher the colour strength.
One of the factors that affect a pigment's colour strength is its chemical structure.
- The Colour strength of organic pigments is determined by their capacity to absorb specific wavelengths of light. Colour strength is increased in highly conjugated and aromatic compounds.
- Coloured inorganic pigments with metals in two valency states provide a high colour strength. Those with a cation trapped in a crystal lattice, on the other hand, are all just weakly coloured.
The colour strength of a pigment is also influenced by particle size. With smaller particles, colour strength is increased. The key factor that impacts the particle size of pigment crystals is the manufacturing conditions. Manufacturers of pigments have a critical role. They are able to:
- By limiting crystal formation during synthesis, you can reduce the particle size.
- Colour strength can be increased by effective dispersion.
The colour strength of the paint is also influenced by pigment dispersion. Actually, it gives the finer particles colloidal stability, preventing flocculation and allowing them to use their full intrinsic colour power.
At temperatures often associated with coatings, few pigments break down. Pigments become more soluble at higher temperatures, and discolouration can occur. As a result, heat stability is tightly linked to solvent resistance in organic pigments. Pigments that work well at a given stoving temperature may not work as well in an application that requires 10°C greater.
At high temperatures, chemical stability is also likely to be crucial. In powder coating systems, this is frequently the case.
Coil coatings are another important topic because metal complex pigments can react with stabilisers at high temperatures, generating significant shade shifts. When pigments are exposed to high temperatures, changes can occur in their crystal structure.
Heat resistance is usually better in highly crystalline pigments than in polymorphic pigments, where different crystal modifications may react differently to heat. Inorganic pigments are more heat stable than organic pigments, with the exception of yellow iron oxide, which loses water from the crystal at high temperatures.
Heat stability is system-dependent, and any test must reflect this. All tests analyse the colour difference between the sample in issue and a standard that has been processed at the lowest temperature at various temperature intervals.
The entire pigmented system, not just the pigment, is assessed for lightfastness. Because the binder provides various degrees of protection to the pigment, the same pigment in a polymer will have more lightfastness than it will in paint.
Pigments will almost always have a substantially lower lightfastness in a printing ink system, where there is less resin to cover the pigment and where there is a double impact of light travelling through the pigmented layer, being reflected by the substrate, and then travelling back through the pigmented layer.
In a pigmented system, other pigments may have an impact on lightfastness. These are some of them:
- Most organic pigments degrade faster when exposed to titanium dioxide. As a result, increasing titanium dioxide ratios resulted in lower lightfastness.
- Because iron oxide is an effective UV absorber, it can improve the light fastness of organic pigments.
A synergistic effect occurs when the combination of two pigments improves lightfastness. An opposing effect occurs when the light fastness obtained is lower.
Although some inorganic pigments are unaffected by light, the majority of pigments, including all organic pigments, are altered in some way: darkening or full fading can happen.
A pigment's capacity to resist light is influenced by its chemical composition, as well as pigment concentration, crystal modification, and particle size distribution. Furthermore, environmental factors such as water and chemicals in the atmosphere, as well as the paint system, can have a significant impact on the outcome.
Only in the final formulation and application can a pigmented system's lightfastness be truly verified, and lightfastness testing must be conducted under carefully controlled test settings.
Colour pigments should be chosen for outdoor applications based on their weather resistance qualities. Pigments will almost always have a substantially lower lightfastness in a printing ink system, where there is less resin to cover the pigment and where there is a double impact of light travelling through the pigmented layer, being reflected by the substrate, and then travelling back through the pigmented layer.
The selection of pigments for outdoor application is influenced by the following factors:
- Required outdoor performance (lifetime, climatic region/Kilo Langley)
- Binder type
- The concentration of the pigment
- Presence of titanium dioxide (which typically accelerates fading)
- Concentration and type of light stabilisers used
The surface of the painted object, as well as the processing heat history, can affect performance.
After the above variables have been identified, the best technique to determine weathering resistance in service is to conduct outdoor exposure experiments in the affected climatic region(s). This isn't always possible; thus, rapid testing is a popular alternative. Machines, as well as a xenon lamp, are available, with wet cycles sandwiched between lengthier dry cycles. The 1-5 Grey Scale is used to rate weatherability. A score of 5 indicates no change, whereas a score of 1 indicates a significant change.