Coloring agents are essential components of certain cosmetic products, especially color cosmetic formulations. Most cosmetic colorants are synthetic and are regulated globally. In the US, they are regulated by FDA with monographs for each and all located in Title 21 of the Code of Federal regulations, Parts 73 and 74. In the EU, allowed cosmetic colorants are listed in Annex IV, Regulation 1223/2009/EC on Cosmetic Products, as corrected by Corrigendum to Commission Regulation (EU) 2021/850, 17 June 2021. Many colorants on that list are also used in food and have corresponding E (Europe) numbers such as E-171 for TiO2 and E-172 for iron oxides. In such cases, the specifications for food colorants are used for cosmetic application. Therefore, it is common to see a note in technical datasheet for a cosmetic colorant stating it complies with the 21CFR and E number (like E-172 for iron oxides) specifications.
Although many of us formulate with colorants frequently, we seem to need help on gaining complete clarity on certain aspects of them. In this blog, we will go over some fundamentals and a few common confusions about certain pigments. Let’s first start with some terms that we often hear.
Dye: It is a material that imparts a color and is soluble in the vehicle or substrate in which it is dispersed.
Pigment: It is a material that is insoluble in the vehicle or substrate in which it is dispersed. True pigments are colorants completely insoluble based on their chemical structure and constituent groups. They typically do not contain the normal substitution groups that promote water solubility, such as sulfonates (-SO3), carboxylic acid (-COOH) or hydroxyl groups (-OH). Hence, there is no bleeding in hydrous systems. There are only two examples of true pigments used in cosmetics: D&C Red No. 30 and D&C Red No. 36.
For leave-on cosmetic applications, pigments instead of dyes are often used because dyes are hard to remove after use, thus, stain the skin. Dyes are more commonly used in rinse-off products such as shampoo and mouth rinse. Now let’s go a little further:
Toner: It is a pigment that is produced by precipitating a water-soluble dye as a metal salt. Typical metals used for this precipitation are sodium, calcium, barium and strontium. e.g., D&C Red 7 Ca Salt. (be aware that it is not a lake)
Lake: It is a pigment produced by absorbing a water-soluble dye or a primary toner onto an insoluble substrate. All the lakes are pigments.
F, D and C codes in the names of a colorant stands for its approved use in Food, Drug and Cosmetics. A colorant must meet its purities requirements to ensure its safety. FDA separates color additives into two categories:
- Colorant subject to certification: they are derived primarily from petroleum and are known as coal-tar Most synthetic, organic colorants fall in this category. They must be batch certified by the FDA. They are further divided into two categories:
- Certifiable Primary Colors: They are pure color which contain no extenders or diluents. They have color names and numbers assigned such as FD&C Yellow 5, D&C Red 6 and Ext. D&C Violet 2.
- Certifiable Color Lakes: Lakes follow the same restrictions as the primary colors with the additional rule that they must have the name of the precipitating metal and the word “lake”. An example would be FD&C Yellow 5 Al Lake.
- Colorant exempt from certification: These are natural organic colorants and synthetic inorganics obtained largely from mineral, plant, or animal sources. Although batch certification is not required, purity must be tested by the manufacturer to meet FDA specifications. Examples are Titanium dioxide and Iron oxides.
Now that we have gone over the general terms and regulatory aspects of colorants, let’s look at common ambiguities about a few specific pigments:
- Rutile and anatase
First, they both are TiO2, but refer to two crystalline structures. It is like using Coke or Pepsi to represent carbonated soft drinks. Anatase is slightly softer and less abrasive than rutile. This makes little difference to the skin feel but can make a big difference in TiO2 production process. Rutile is so abrasive that it can wear out the equipment that processes hundred to thousands of tons per campaign. Consequently, rutile is often surface treated with alumina to extend the useful life of the equipment in addition to provide other benefits.
Rutile has a slightly higher refractive index than anatase, and thus, it can scatter light more effectively. So, does it mean that Rutile is more opaque? Not quite. Opacity is the result of scattering which depends as much on the size and size distribution of the pigment particles as on its refractive index. In reality, it is rare to find a rutile and an anatase that have the same particle size, let alone size distribution. Therefore, being rutile or anatase does not necessarily indicate a higher or lower opacity.
Commercial anatase is usually made to have a small primary particle size, in a range of about 140 – 170 nm. That of rutile is often bigger, roughly 170 – 250 nm. Due to its smaller size, anatase scatters blue light slightly more, and thus, imparts a blueish undertone. This is the reason that anatase is often said to be bluer than rutile.
Lastly, the production processes, chloride and sulphate, are often brought into discussion about TiO2. In Chloride process, TiCl4 is vaporized and burnt into rutile. In sulphate process, Ti(SO4)2 is neutralized with base to generate anatase. If aluminum salt is used as the inducer, rutile TiO2 can also be made via the sulphate process. TiO2 made from a Chloride process often has a lower level of contaminants, which translates into high purity and clean color. This had been indeed the case in the past, but not so much anymore since the sulphate process technology has been greatly improved over the years.
- Carbon black
Carbon black can be made via several processes. In the US, carbon black as a cosmetic color additive is called D&C Black No. 2, a high-purity carbon black prepared by the oil furnace process.1 It is manufactured by the combustion of aromatic petroleum oil feedstock and consists essentially of pure carbon, formed as aggregated fine particles with a surface area range of 200 to 260 m2/g.
JSCI monograph requires Carbon black to be obtained by incomplete combustion of natural gas or liquid hydrocarbon. Such carbon black is often called channel black and is not approved by the FDA. This, unfortunately, adds unnecessary complexity to formulating for the global market.
- Chromium oxide and Chromium hydroxide green
Hexavalent Chromium (Cr6+) is known to be carcinogenic, thus, it should not be present or at a very low level in cosmetics products. However, its presence is unavoidable due to the chemistry and manufacturing process. For both pigments, the FDA set a limit of 2% NaOH extract, not more than 0.1% as Cr2O3 (based on sample weight). 2 This limit is equivalent to 684 or 513 ppm maximum Hexavalent Chromium, respectively. The actual level of Cr6+ in a commercial grade needs to be tested for calculating the final level in a finished formulation.
- Mica and Pearlescent pigments
A common restriction people often talk about is the size limit of 150 mm. Mica is an approved colorant for drug use, and the FDA has imposed a size limit on it. Mica can also be used as a colorant for cosmetic applications for which the FDA does not list any size limit in the monograph. Moreover, mica can be used in cosmetics as filler, a category that the FDA does not regulate with specific requirements.
Efforts have been made to list Mica-based pearlescent pigments as approved colorants for cosmetic purposes, but this has not happened yet. as of now, such pigments have been approved as colorants only for drug use, and the corresponding specifications require that the mica meets the colorant specifications for drug use. This is likely the reason that we hear the 150-mm size limit in our industry. As mentioned above, mica-based pigments are not approved colorants for cosmetic use. Consequently, the composition has to be expressed as a mixture of individual components such as, for instance, mica and titanium dioxide. Each of these ingredients needs to meet the corresponding FDA specification if applicable. The size limit on mica for drug use may not be observed.
- Zinc oxide
Zinc oxide is a long approved cosmetic colorant, though its use as opacificer in cosmetics is limited. That main reason is that its opacity is much lower than TiO2 due to its lower refractive index (2 vs. 2.7). Roughly 3 times more ZnO is needed to achieve the same degree of opacity of TiO2. Moreover, ZnO is slightly soluble in water, resulting in the pH of formulations containing ZnO to be above 7.5.
As of August 7, 2022, the use of TiO2 as food colorant has been banned in the EU, directly affecting its use in lip and oral products. Respirable TiO2 is considered carcinogenic, according to Proposition 65 of the state of California, affecting the use of TiO2 in some powder and spray formulations. TiO2 is difficult to replace because of its unique performance and inertness. In light of the regulatory restriction, ZnO with the right size and high opacity has gained attention recently, especially for anhydrous formulations.
- Red 6 lake and red 7 lake in Japan
Most FDA approved colorants can be used in Japan. Red 6 lake, widely used in lip products, is a notable exception. The reason is that Red 6 lake is Red 6 Barium salt laked on barium sulfate, but Red 6 Barium salt is not an approved colorant in Japan. On the other hand, Red 7 lake is Red 6 Calcium salt laked on barium sulfate but Red 6 Calcium salt is approved in Japan. In the case that the shade cannot be achieved without Red 6, Red 6 sodium salt can be used. However, it must be noted that red 6 sodium salt is water soluble, which is opposite to Red 6 lake.
Currently, the FDA has approved 64 color additives for cosmetic use, each of which has its merits and drawbacks due to their unique chemistry and production process.3 The knowledge is important not only for formulating the right color shade, but also for troubleshooting instability and especially, regulatory compliance. The author hopes that this blog will contribute to your learning of cosmetic colorants.