Effect pigments can help formulators spice up the appearance of cosmetics. Katrin Steinbach and Ulrich Schmidt suggest how to use them to produce colour cosmetics that are irresistable.
METAL EFFECT PIGMENTS
Almost any cosmetics application might benefit from the common features of metal effect pigments, which are brilliance, opacity and the ease of aligning in thin layers with a metallic appearance. Metal effect pigments represent as thin platelet-like particles composed of pure metals, such as aluminium, copper or metal alloys like bronze (copper/zinc). While aluminium pigments show a very elegant sliver-like effect, pure copper powder pigments exhibit a strong reddish-brown colour. Copper alloys with up to 30% w/w zinc content enable broadening of the colour scope towards greenish-gold tones.[1]
In order to meet the demands of cosmetic regulations globally, metal effect pigments designed for colour cosmetics consist of high purity metals. During the manufacturing process, only approved additives are used in combination with appropriate processing that is suitable to meet the specific demands of the cosmetic and personal care market.
The optical properties of the most common group of metallic effect pigments, aluminium pigments, are influenced mainly by particle shape and size. Compared to conventional cornflake metallic pigments used widely in industrial applications, the silver dollar pigments stand out because of their improved optical properties achieved by a rounder shape of the pigment platelets. This leads to a greater brightness and brilliance of silver dollar pigments.[2]
As proper safety is essential for handling aluminium pigment powders, special silica encapsulated metallic pigments [eg Visionaire grades from Eckart] have been developed. The proprietary silica treatment increases both the mechanical and chemical stability of the aluminium pigments and affords greater flexibility in anhydrous and some aqueous formulations as well as better durability of the pigments in processing.
A unique new coarse pigment fraction of aluminium pigments with a 55µm average particle size offers versatile advantages to the formulators. This silica encapsulated aluminium pigment is the first cosmetic approved grade with coarse particle size and enables the addition of an even higher brilliance and sparkle to applications than the traditional aluminium pigments. In addition to the stunning optical effects due to the large particle dimensions this pigment also offers higher stability that can be of great benefit, particularly in regard to water containing systems and pigment handling.
Coloured aluminium pigments extend the colour spectrum even further towards metallic red and metallic blue. The shade created through the pigment coating with colorants such as CI 15880 (red 34), CI 15850 (red 7) and CI 77510 (ferric ferrocyanide) is extremely colour intense, significantly more efficient than a simple physical blend of the respective aluminium pigment with the same colorants. Formulation-wise these unique coloured aluminium pigments enable vibrant metallic shades of red and blue.
For a better understanding of the diverse design capabilities of metallic pigments, a common consideration of particle size and colour is helpful. Depending on the particle size fraction of an effect pigment, it is possible to achieve very brilliant and bright effects when using particles smaller than 70µm and, in contrast to this, very sparkling and glittery effects when using pigments with mean particle sizes up to 95µm. The final visual effect in the cosmetic application not only depends on the chosen particle size, but also on the pigmentation level as well as on the characteristics of the application system. Comparing opaque metallic pigments with transparent pearlescent pigments generally shows that the use level of metallics in cosmetic formulations is significantly lower due to their strong coverage and brilliance.
OPTICAL EFFECTS WITH PEARLESCENT PIGMENTS
The optical effect of transparent pearlescent pigments originates from a multiple light refraction on thin metal oxide layers.[3] Depending on the composition and type of the metal oxides used, various colour and gloss effects can be created. The most important group of pearlescent pigments, the interference pigments, are produced by coating natural mica platelets with titanium dioxide, which enables them to imitate the whole spectrum of rainbow colours. Furthermore, additional gold and earth tone colour shades can be achieved by mica coatings with iron oxide as well as combinations of iron oxides and titanium dioxide.
These days synthetic substrates are also used for pearlescent pigments, such as borosilicate platelets. In principal their colouring options are comparable to mica-based pigments, but thanks to their controlled and homogeneous particle thickness and high transparency, they can exhibit much better colour purity and gloss, especially in transparent systems like lip glosses and gels.
As they are physiologically inert, pearlescent pigments are well suited for use in any kind of colour cosmetic or personal care application. The available pigment range offers a huge array of visual effects and to benefit from this versatility particle sizes and colours should again be jointly considered. Depending on the particle size it is possible to achieve silky and satin effects when using particles smaller than 25µm and in contrast to this very brilliant and glittery effects when using particle sizes up to 150µm. As mentioned for metallic pigments, the final visual effect in the cosmetic application depends not only on the chosen particle size but also on the pigmentation level as well as on the characteristics of the application system. Low pigmentation levels (0.01% - 0.6%) will lead to enhanced product aesthetics whereas higher pigment concentrations (1% - 5%) will enable visible effects on skin and hair. In this respect, the characteristics of the application system play an important role in such a way that transparent systems will require significantly lower pigment use levels than opaque systems or emulsions.
PIGMENT APPLICATIONS
Wax systems (eg lipsticks)
For lipstick and lip gloss applications, the appropriate choice of effect pigments depends on the country regulations; aluminium is permitted for lip products in the EU, Japan and China but is not currently permitted for lip products in the US. Apart from these considerations pearlescent pigments based on mica and/or borosilicates as well as the metallic pigment range can be used. The mixture of metallic pigments with organic pigments or with a combination of organic and pearlescent pigments will give unique and attractive polychromatic effects. Coloured pearl pigments such as carmine coated pearl pigments can be used to colour the lipstick mass and thus the addition of iron oxides might be avoided. The formulator will benefit from the ease of handling as no pre-dispersion is needed. When used as the only colorant, a loading level of 5% to 15% of effect pigments is common depending on the desired effect and colour shade.
Powder applications (eg eye shadows)
The overall use level of effect pigments in powder products like eye shadows is considerably higher than in any other colour cosmetic application. Starting from 15% their use levels may reach up to 70% or more. In general for pressed powder applications, the oil absorption properties of the powder components used play a significant role for the formulation stability and pressability. In this respect, it is important to know that metallic pigments due to their lower oil absorption values are much easier to compact than pearlescent pigments. But metallic pigments can be more than just visual additives in powder products. These pigments impart a highly luxurious sensory aspect to cosmetic products, especially when used in high concentrations such as in pressed powder applications. The unique cold metallic feel and easy gliding properties are inimitable in the field of cosmetic raw materials and enhance the value of metallic pigments in cosmetic formulations even more.
Foundations
Depending on the pigment loading and chosen particle size fraction, pearlescent pigments coated with iron oxides (so-called Earth Tones) are especially well suited to provide either high coverage or sheer coverage in make-up formulations. In contrast to iron oxides, Earth Tone pigments offer semi-transparent skin tone colours, letting the skin shine through without a masking effect. This leads to a homogeneous and natural look. Thanks to their subtle effects and light reflecting properties, these pigments can also be used to refresh the appearance of the skin and thus create a younger looking skin, making them valuable ingredients for anti-ageing related make-up formulations.
Emulsions, creams & lotions
To create shimmer or sparkling effects in emulsions, silvery pearlescent pigment shades are the first choice. Using small particle sizes adds opacity to the system in combination with a light, pearly shine. Big particle size pigments are very transparent on the skin and add sparkling effects to a lotion. In accordance with the latest trends in body and face care, particles between 1-15µm are particularly recommended.
A light colour shimmer can be achieved by using interference colours such as gold, orange, red, violet, blue and green. It is possible to achieve an almost invisible light colour shimmer to refresh the skin tone when using small particle sizes or to add highlights to the skin when using bigger particles. Particularly recommended colour tones for skin care products are orange and red interference colours thanks to their very neutral colour applied on the skin. Furthermore green interference shades can be used to neutralize skin redness in face cream applications.
To achieve more glamorous effects, borosilicate pigments are recommended. Borosilicates offer a higher brilliance and gloss compared to conventional mica-based pearl pigments. Their transparency makes them predestined to create exceptional effects in emulsions. Due to their synthetic origin these pigments show a pure white powder colour which does not influence the colour of the final emulsion. However, the pigment loading strongly depends on the opacity of the emulsion and should be adjusted accordingly. Low concentrations are to be used in more transparent systems and higher pigment concentrations should be used in opaque systems. Use of scattering ingredients such as titanium dioxide pigments can destroy the shimmer effects.
Clear cosmetic gels
Water-based and water containing gel systems can be visually improved by using mica or borosilicate supported pearlescent pigments. Both pigment types are easy to use in such applications and can be easily dispersed in the water phase before or after adding the thickener. The beauty of working with clear gels and effect pigments is the extremely low use level of the respective pigment. In most cases levels as low as 0.01% will exhibit eye catching effects.
Nail polish
For nail enamel applications, ready to use pigment dispersions or pastes of non silica encapsulated aluminium-based silver dollar, platinum dollar and PVD aluminium pigments are available. These can be added to the formulation without any dusting and are easy to disperse. Generally the use of silver dollar or platinum dollar pigments will add flop effects, metallic brilliance and brightness as well as coverage. Furthermore the addition of holographic pigments obtained by embossing technology provides an angle dependant colour travel that imparts a rainbow effect.[3] This unique effect is achieved by a three dimensional diffractive structure on the surface of the pigment particles.
From a formulator’s point of view, the utilisation of pigment dispersions is very comfortable and only a few handling rules have to be considered. The most important issue is to avoid high shear in order to prevent deformation and degradation of the metallic flakes. It is particularly important to follow these application guidelines when using the very thin and therefore shear sensitive PVD metallic pigments to ultimately generate luxury, eye-catching effects.[4]