46 SKIN CARE
Wetting pigments
The first step in the dispersion process of dispersion is wetting by displacing the air surrounding pigment particles, which facilitates the separation of aggregates and agglomerates. Vehicles are often used in formulation for other purposes and have inherent wetting properties. Ricinus communis (castor) seed oil and mineral oil have historically been used to wet pigments for anhydrous dispersions because of their ubiquity and low cost.
Natural oils, including castor oil, are subject to oxidation resulting in off odours. Selection oils with good oxidative stability such as Limnanthes alba (meadowfoam) seed oil and the use of antioxidants is recommended when using naturals.4,5 Branched esters are excellent wetting agents and experimentation with levels is important in emulsion systems where they can have significant effects on the viscosity of the final formulation.6 The degree to which each pigment absorbs oil varies. Pigments with higher oil absorption need a higher oil-to-pigment ratio to ensure there is enough of the vehicle to fill the spaces around the pigments. This differential in oil absorption is also important in balancing the amount of oils and fillers across different shades of a formulation so that they maintain the same characteristic aesthetics and stability. The use of lecithin or high-HLB surfactants aids grinding pigments into water and talc is used almost exclusively for dry powder applications.2,3
Pre-dispersed pigments Many cosmetic companies purchase dry pigment and disperse it in their factories. However, pigments can be purchased pre- dispersed in a wide variety of vehicles including castor oil, synthetic waxes, and water. Pre-dispersed pigments are convenient and provide consistency in colour, especially when a manufacturer does not have the know-how and proper equipment for producing a good dispersion.
PERSONAL CARE ASIA PACIFIC
Surface treated pigments Most pigments and powders used in cosmetics have a hydrophilic surface comprised of polar hydroxyl groups where water is adsorbed. Consequences of hydrophilic surface in formulation include agglomeration of particles contributing to poor skin-feel, poor dispersion/wetting in cosmetic fluids, and poor stability. Surface treated pigments overcome these challenges and provide additional benefits. They are far easier to disperse, leading to more consistent colour and quality in finished goods, and provide long wear, improved sensory attributes or other characteristics.
Oil absorption is also reduced, making it easier to balance formulations across different skin tones. Surface treatments can also improve suspension, hindering flocculation and settling. Surface treated pigments were developed when more women began to powder their faces in the early 20th century and found the need to ‘touch up’ during the day. They were confronted with the problems of carrying loose powder around in their handbag.
Early loose powder containers suffered
from numerous issues which were sources of annoyance, including bulkiness, the need for frequent filling, as well as leaks and spills. In an effort to improve the durability of decorative cosmetics, silicone treated pigments were developed and introduced in Japanese formulations in 1976. These early treatments paved the way for the development of ‘two-way’ makeup cakes, which could be applied dry, or with a wet sponge.
Higher in bulk density, metal soap
treatments were introduced in 1977 to improve compressibility and impart a creamy skin-feel. Amino acid treatments were introduced in 1979. This treatment was particularly useful in the formulation of long wearing liquid foundation. The combinations of surface treatments and substrates have
grown dramatically, including innovative natural and super hydrophobic options.3,7 Below are some currently available
surface treatment types and their benefits:
Disodium stearoyl glutamate This amino acid based treatment is well suited for liquid makeup. It enhances the perception of moisturisation, improves compressibility, and has good compatibility with skin. It is also naturally derived. The slight acidity of this treatment renders the pigments more compatible with skin pH and thus helps preserve the delicate chemistry of the stratum corneum.
Dimethicone Dimethicone treated pigments provides long wear and waterproofing properties, and improves dispersion and compatibility with silicones used in emulsions for foundations and other liquid makeup. This treatment also imparts a smooth, creamy feel during application.
Alkyl silane Trimethoxycaprylylsilane treatment provides water repellency and is more lipophilic than silicone treatments. It is good for long-lasting, waterproof, transfer resistant cosmetics. Silane treatment improves compressibility in powders and gives a velvety skin-feel.
Methicone This treatment exhibits strong hydrophobicity for long wear and high performance applications, such as the current ‘athleisure’ makeup trend. Methicone treatment allows for easy dispersion of pigments in oils and silicones. Methicone gives a soft skin-feel and reduces drag of pigments on application to skin. The choice of surface treatment type will depend on the application and what particular properties are desired or need to be improved in the finished formulation.
Pearlescent effect pigments Nacreous, or pearlescent, pigments are
November 2017
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