The coating of microparticles with pigment tends to be approached as a physical immobilisation process, rather than as an in-situ synthesis of the desired coating product on the surface of the particle (the major exception, of course, being TiO2).
In large part, this is because the specific conditions required to synthesise the pigment in the form required are not conducive to its deposition on the ‘scaffold’ particle surface. However, MTL has established a number of routes by which ready formed pigment particles may be immobilised on other particles in a robust fashion in order to produce coloured particles with additional functionality over and above that of the pigment itself.
It is important to appreciate fully the distinction made between a pigment, which is insoluble in the vehicle (resulting in a suspension), and a dye, which either is itself a liquid or is soluble in its vehicle (resulting in a solution). MTL has developed distinct technologies for the immobilisation of dyes on particulate surfaces.
The properties of a colour, such as its saturation or lightness, may be determined by the other substances that accompany pigments either in bulk or in a suspension. Binders and fillers added to pure pigment chemicals also have their own reflection and absorption patterns, which can affect the final spectrum. Likewise, in pigment/binder mixtures, individual rays of light may not encounter pigment molecules, and may be reflected as is. These stray rays of source light contribute to the saturation of the colour. Pure pigment allows very little white light to escape, producing a highly saturated colour. A small quantity of pigment mixed with a lot of white binder, however, will appear desaturated and pale, due to the high quantity of escaping white light. This is a major consideration when immobilising pigments on the surface of other particles, which themselves may contribute to the final colour saturation of the composite material.
Selection of a pigment for a particular application is determined by cost, and by the physical properties and attributes of the pigment itself. For example, a pigment that is used to colour glass must have very high heat stability in order to survive the manufacturing process whilst in artistic paint, heat stability is less important, while light fastness and toxicity are greater concerns.
The following are some of the attributes of pigments that determine their suitability for particular manufacturing processes and applications:
- Lightfastness and sensitivity for damage from ultra violet light
- Heat stability
- Magnetic component
- Tinting strength
- Opacity or transparency
- Emissivity (Infra-Red Reflectivity)
- Resistance to alkalis and acids
- Reactions and interactions between pigments
There are a number of these potential application areas which could be approached using technologies to immobilise pigments on a low density support structure. MTL has evolved a number of different pigment coating technologies in response to opportunities identified in the diagnostics, aerospace, textile, defence and cosmetics sectors. As indicated, these application areas demand a particular functionality or specification.