Part 2: Stability of Exempt Colors
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The second general class of color additives is the comprised of the ones exempt from FDA certification. This class includes both inorganic and non-synthetic organic colors. The major ones used are all pigments and find lesser use in toiletry products. The stability of the exempt colors will be review based on their chemical structures.
Carmine –Carmine is the aluminum lake of carminic acid. The carminic acid is the processed extract from dried, female Cochineal Beetles. The Beetles are harvested predominately in Peru, although some do come from the Canary Islands. For a partially natural colorant, remember it is an aluminum lake although it is not included in the INCI name, Carmine exhibits good stability. It is stable in most organic solvents and has reasonable light and heat stability. Being a chemical lake, it can be broken down in strong acids and bases, liberating free carminic acid.
Iron Oxides – From a chemical stability standpoint, the iron oxides are almost ideal colorants for cosmetic products. They are relatively easy to disperse, while being inert to water, acids, alkalis and solvents. They are opaque and exhibit excellent light stability. The iron oxides come in a variety of shades from yellow to red and black.
The one note of caution with the iron oxides is heat. Yellow iron oxide is a hydrated ferric oxide while the red is its anhydrous version. The red is produced by calcination of the yellow at about 800°C, and is very heat stable in the relative temperature range where cosmetic products are produced. The yellow, however, will shed some of its water of hydration at temperatures as low as 125 – 150°C, causing some shift in color that can be seen. The black iron oxide is a mixture of ferric and ferrous oxides. Like the yellow, it will change color, becoming redder, at temperatures of 125 – 150°C. It is also important to note that the black iron oxide is magnetic and will coat iron or mild steel containers.
Ferric Ammonium Ferrocyanide – Ferric Ammonium Ferrocyanide is also known as Iron Blue. Ferric Ammonium Ferrocyanide is very stable to heat and light and it also has good resistance to solvent. In aqueous systems, it is stable in acid, but decomposes at pH above 7. It is also very difficult to disperse.
Chromium Greens – Chromium Oxide Green and Chromium Hydroxide Green are typical mineral pigments with excellent lightfastness, heat stability and bleed resistance. The hydrated version is brighter, cleaner and bluer than the anhydrous one. Both colors are also very stable in alkalis and acids, well as solvents. In some formulations, the hydrated green will react with perfume oils, resulting in an odor.
Ultramarines – The Ultramarines are excellent colors, available in a wide range of shades including green, pink and the most popular, blue. This class of colors exhibits very good stability to light, heat and most solvents. The Ultramarines do have one drawback, they decompose in the presence of acid and evolve hydrogen sulfide. H2S is the smell associated with rotten eggs, not a very pleasant fragrance in cosmetic products.
Manganese Violet – Manganese Violet is a manganese ammonium pyrophosphate complex. For an inorganic pigment, Manganese Violet is bright and clean. It has excellent light stability, is resistant to organic solvents and has good heat stability in the range of cosmetic product manufacture. It is stable in acids, but will decompose in neutral to alkaline systems. Depending upon the pH, the color will disappear or turn black.
Titanium Dioxide – Titanium Dioxide (TiO2) is almost a perfect pigment. There are virtually no global regulatory restrictions on its use and it exhibits excellent stability. It is resistant to acids and alkalis and is stable in organic solvents. It has excellent lightfastness and is heat stable in all cosmetic products.
You can find out more about selecting the right colors for your cosmetic products at SunCROMA® for Cosmetics