Article by: Kelly Dobos
Part 1: Stability of Certified Colors
An important consideration in the color additive selection process is stability. Once the regulatory requirements for the market(s) where a cosmetic or toiletry product will be sold are satisfied, it must be determined that the colorants used will be stable when used in formulation. This is typically done via stability testing.
Colors must not react with other chemicals in the formula and must be stable under the conditions to which they will be subjected to during the manufacturing process, including factors such as temperature, time, concentration and pH.
Additionally color additives must be stable under the conditions to which they will be exposed to during storage, display, and use. These conditions include
- changes of temperature.
In order to study the stability of color additives, they are divided into two general categories. These categories are the synthetic organic colorants, defined by the US FDA certifiable colors, and the FDA designated exempt colors. This post will cover certifiable colors and part two will address the exempt colors.
Certifiable cosmetic colors
The certifiable colors can be put into subcategories based on their chemistry. The colorants in each of these categories exhibit similar stability characteristics. These categories are as follows:
Non-soluble Azo Colors – The most common colorant in this category is D&C Red No. 36. Double bonded nitrogen atoms form the azo linkage, which is one of the earliest, and still most important, means of producing colored compounds. These double bonded nitrogen atoms combining two aromatic structures set up a resonance in the molecule, which results in the perceived color.
Azo colors, as color additives with this type of structure are called, are the most important class of synthetic organic colorants used in cosmetic and toiletry products. D&C Red No. 36 contains no water solubilizing groups, such as SO3 or COOH. At the same time, even though it is completely organic, it is not soluble in normal organic oils or hydrocarbons because of its structure and charges.
Colors of this type are hydrophobic, bleed resistant to water and alkali and are quite lightfast. They do have a slight tendency to bleed in oil or organic solvents. Because of this, they should not be used in products that contain strong solvents like nail polish. One other caution with D&C Red No. 36 is its tendency to bleed slightly in the oils and waxes used in lipstick. This bleed causes it to crystallize and darken upon continued re-heating of the bulk. When used in a single heat bulk lipstick, it is not normally problematic.
Soluble Azo Colors – A typical soluble azo color is D&C Red No. 33. Again, double bonded nitrogen atoms, connecting the aromatic rings, form the azo group. The two SO3 groups make this colorant water-soluble. All of the color additives in this group are water soluble dyes that are used to color toiletry products. They are precipitated as chemical lakes to form the pigments that are used in decorative cosmetics.
The dyes are resistant to both acid and alkali and have reasonable light stability. However, their pigment forms (lakes) tend to bleed in water and are not stable in either acid or alkali. Under these conditions, the lakes break down and release some of the soluble dye, which is the cause of the bleed.
The lakes of these colorants are stable in oils, waxes and aromatic solvents. In addition, they exhibit good heat stability up to 100° C. As a result of these properties, these lakes are widely used in lipsticks and nail polish. Some of the other important colorants in the soluble azo category are FD&C Yellow No. 5, FD&C Yellow No. 6 and FD&C Red No. 40.
Slightly Soluble Azo Colors – This class of colorants is the most important of the synthetic organic colors. A typical member of the category is D&C Red No. 6. This color additive, even though it has both an SO3 group and a COOH group, is not very water soluble because of its structure.
These color additives exhibit good light stability, are heat stable to the 105°C range and are resistant to bleed in oils and solvents. This makes them very useful in powders, lipsticks and nail polish. Unfortunately, they do not have the same stability properties in aqueous products. In strong alkali, they turn yellow and in strong acid they bluish red.
The other two colors worth noting in this category are D&C Red No. 31 and D&C Red No. 34. They are more limited in use because of FDA regulatory restrictions. Both colorants are external only colors. In addition, D&C Red No. 31 has a slight tendency to bleed in alcohol, which further limits its usefulness.
Soluble non-Azo Colors – Most of the important colorants in this category are in the Fluorescein family. A typical example is D&C Red No. 22. It is the sodium salt of Tetra Bromo Fluorescein. The Fluorescein family of dyes all act as pH indicators and, as such, will change colors accordingly. All of them will stain the skin and, therefore, are used make long-wearing lipsticks.
The Fluorescein pigments are all classical chemical lakes and are used because of their unique clean shades, not because of their stability properties, which are pretty poor. They can only be kept in the 100°C for short periods of time. They bleed slightly in aqueous systems and heavily in solvents, making them completely unsuitable for nail polish.
Finally, they are quite fugitive to light; natural, incandescent and fluorescent. In order to use these colorants in clear packaging, UV absorbers must be incorporated into the plastic packaging. Other important color additives in the soluble non-azo category are FD&C Blue No. 1 and D&C Yellow No. 10. Like the Fluorescein color additive family, these colors have very pure clean shades.
Unfortunately, they also exhibit similar stability characteristics. Their lightfastness is poor, they bleed in solvents and their lakes tend to have a slight water bleed. They cannot be exposed to 100° C for long periods of time.
Miscellaneous Colors – The most useful color additive in this category for decorative cosmetics is D&C Red No. 30. . It is an excellent color for use in most type of decorative cosmetics. It has very good stability to light, heat (100°C range), acids, alkalis and most organic solvents. The one stability problem with D&C Red No. 30 is its slight tendency to bleed in acetone. As a result of this characteristic, the colorant finds only limited use in nail polish. For toiletry products, the two colors in this category are D&C Green No. 5 and D&C Green No. 8.
Both are dyes and are soluble in water and useful in aqueous systems. Neither colorant is acid or alcohol soluble, but both are exhibit good solubility and stability in alkalis. The lightfastness of D&C Green No. 8 is much better than that of the D&C Green No. 5.
In part 2, we review exempt FDA cosmetic colors
You can find out more about selecting the right colors for your cosmetic products at SunCROMA® for Cosmetics