The colour chemistry behind this season’s cosmetics trend
Manganese violet has long been used in cosmetics, where its vivid hues, stability and excellent safety in use are much appreciated; it has all requisite FDA and European approvals. In addition, its manufacture is safe, with minimal harmful by-products or toxic substances. Dave Masterman, Technical Support Manager for Holliday Pigments, explains the manufacturing process that perfects the pastel and more vibrant shades of violet so virulent on this spring’s fashion scene.
Manganese Violet (MnV) is a synthetic, inorganic pigment which was first introduced around 1900. It is essentially a covalent combination of ammonia, manganese and phosphate. The standard chemical description is ammonium manganese pyrophosphate.
The chromophore which gives the pigment its bright violet colour is tri-valent manganese, which is stabilised within the compound structure by strong co-ordination with the phosphate and ammonia ions.
Clean manufacture
The process of manufacture is essentially the same today as 100 years ago, involving a suitable manganese salt slurried with di-ammonium hydrogen phosphate and some form of phosphoric acid. By using high quality ingredients, it is a very clean process without harmful residues and the resulting product is pure enough to be used in the most sensitive of applications, such as cosmetics.
Correct selection of raw materials is important because the exact shade and purity of the MnV pigment is determined by the ratio of initial reactants and the initial source of manganese. However, the variation in hue that can be achieved is restricted and, in practice, a blue-shade violet and a red-shade violet are the two ends of the MnV colour range. Intermediate shades can be obtained by blending these two.
 Figure 1: Reflectance comparison of bluer and redder extremes of MnV production
Application processes
To achieve the desired grade of powder to be incorporated in cosmetics, or the other main applications, artists’ colours and white plastic goods (to disguise yellowing), the MnV is ground down to the specified particle size.
The manufacturing process involves high temperatures of around 315˚, but MnV is unusual amongst synthetic inorganic pigments in that it is not calcined. It is neither as hard nor abrasive as most other pigments of its type, but retains many other similarities while exhibiting some properties of its own which are important to appreciate.
Desirable properties
MnV’s general advantages are its good heat stability, light fastness and weathering characteristics. It is easy to disperse, making processing easy. Since it is insoluble and of relatively large particle size, MnV does not migrate and can be safely used in sensitive applications such as food contact plastics. In addition, it is excellent for colour correction applications, such as in white and also clear plastics, as it has high transparency.
The general specifications are shown in Table 1:
| Pigment | Violet 16 |
| Colour Index Number | 77742 |
| CAS Registry Number | 10101-66-3 |
| Formula | NH4MnP2O7 |
| Composition by weight | Mn 22.25%; NH4 7.31%; P 25.08%;
O 45.36% |
| Other names | Permanent Violet, Nuremburg Violet, Mineral Violet |
International cosmetic approval
However, MnV’s major application in terms of volume is in cosmetics, accounting for between 70 and 80% of the total world production, which is around 200 tonnes. There is such high demand for MnV because of the red tone it achieves and the delicate tints that can be created using blends of talc.
The FDA approval 21CFR Part 73.2775 for MnV covers externally applied skin contact cosmetics including lipsticks and those used in the area of the eye; no certification is necessary. It is also listed under reference no. 18 in Annex III part 2 (d) of the EEC Directive 76/768/EEC as a pigment which may be used without restriction.
Eyeliner, mascara and eye shadow are principal uses, while rouges, blushers, nail lacquers and toners are other products typically containing the pigment. Its high light fastness, non-migrating characteristics and stability, with good transparency and purity of shade, make it an ideal choice, and its heat stability and good dispersal characteristics should also aid manufacture.
Other applications
In addition to its main use in cosmetics, MnV is widely used in coatings and plastics. Special grades with improved heat stability have been specifically developed for plastics. MnV also meets the purity requirements for use in food contact plastics, for example AP(89) in Europe and 21CFR Part 178.3297 in the USA.
In both plastics and coatings, MnV is particularly useful to enhance whites. MnV strongly absorbs the yellow light which is responsible for the unattractive yellowish tinge in many white or clear articles. In this application its moderate tinting strength is a particular advantage, as the additional level required is usually much less than 0.1%. |