Pigments such as titanium dioxide and iron oxides and fillers like talc, mica and starch are the essential parts of colour cosmetic formulations. The pigments provide the colour and shades.
Fillers provide the necessary body and texture, and improve the feel during application. However, pigments and fillers are hydrophilic. There are hydroxyl groups and absorbed water on the surface can form hydrogen bonding resulting in attractive force that causes agglomeration. The pigments have primary particle sizes usually in the sub-micron range and have large surface areas. Therefore, they always show a great tendency to agglomerate due to the van der Waals force. As a result, there are several well-known drawbacks associated with uncoated pigments or fillers:
- The feel is often coarse and poor due to high friction among large agglomerates. Even when they are milled to small size, re-agglomeration often occurs.
- Pigments/fillers are difficult to disperse in an oil phase and to develop maximum colour strength.
- Pigments can migrate from an oil phase to an aqueous phase in an emulsion, which results in formulation instability.
- The surface of some metal oxides can be chemically reactive and cause degradation of other ingredients.
The drawbacks were noticed early. Surface treatment was introduced in the mid- 1970s and has gained much popularity since the early 1980s. Filler pigments and coloured pigments may be treated with a wide range of compounds to modify their physical and chemical characteristics. Surface treatment technology virtually revolutionised the colour cosmetic formulation by providing major improvements in skin feel, application, pigment wetting and formula stability.
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