New trends in pigment modification

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.  
Common surface treatment  A surface coating agent can be applied onto the surface of a particle either by blending with powder using various mechanical means or precipitation from a solution. Other methods can be used but actual use is limited. The coating is then attached to the surface through two mechanisms: 
• Physical bonding – mainly van der Waals force. This bonding is often weak and the coating can be stripped off due to dissolution, melting or physical abrasion.
• Chemical bonding – through a chemical reaction to form covalent or ionic bonds. This bonding is strong and stable in formulation processes.  

In general, complete, chemically reacted treatments are preferred when permanent physical or chemical modification is the goal, particularly when the pigment or filler is to be dispersed in a liquid vehicle. Many types of surface treatment have been developed since the 1980s and the most common coatings are listed in Table 1. Silicones and silane coatings have excellent hydrophobicity and are widely used in two-way powder foundations. Use increased greatly, as W/Si emulsions became highly popular. Metal soap and organo-titante treatments have good skin affinity and excellent pressability, and are popular in powders. Perfluoroalcohol phosphates treatment repels both water and oil and can prevent shine on skin and breakage of make-up. It has been widely used by premium brands in long wear foundation and make-up. All these treatments have greatly improved the aesthetics of cosmetic products and provided the basis for novel formulation. However, there are limitations for each of these treatments. For example, metal soap and organo-titanate treatment are not stable enough in acidic conditions and the hydrophobicity is unsatisfactory. Silicone treatment in general has poor wettability in hydrocarbons and esters. Fluoro compounds render pigments both hydrophobic and lipophobic but are very difficult to disperse in most cosmetic liquids.