Overall Review of Surface Modification Technology

(Reprint from PCI Asia 2000)

　

Taizo Miyoshi, Ryo Ohara, and Kazu Abe*

Miyoshi Kasei, Inc. and Cosmo Trend Corp.*

　

Key Words:

Surface Coating, Pigments, Water Repellency, Sunscreen, Two-way Foundation

1. Introduction

A wide variety of pigments and substrates are used within regulations throughout the world for color cosmetics, and all possess inherent drawbacks for topical cosmetic applications. Inorganic colorants such as titanium dioxide and iron oxides, for instance, exhibit hydrophilic properties, and thus are prone to wash off upon contact with perspiration and humidity. These pigments also have primary particle sizes in the sub-micron region, which maximizes its coloring effects; however show a great tendency to agglomerate. When these particles agglomerate, they lose their intended brightness or opacity in cosmetic formulations, thereby nullifying its original intention of maximizing its color. Substrates such as talc or mica have problems dealing with overall wear properties, since these substrates are hydrophilic by nature and do not adhere to the skin. Recent utilization of microfine pigments such as microfine titanium dioxide and zinc oxide as sunscreens, also shed light on the photo-oxidative property (1) associated with microfine pigments, and minimizing such negative effects on other cosmetic ingredients in the formulation is greatly needed.

These properties aforementioned, stems from the interaction-taking place at the surface of the pigments and substrates, and thus a chemical modification of the surface has been proposed. Surface modification by inorganic oxides such as silica or aluminum compounds, or organic compounds such as reactive silicones or metallic soap have been found effective in improving water repellency, adhesion to the skin, and dispersion properties in various color cosmetic products. It must be pointed out that the introduction of two-way foundation in Japan coincides with the development of hydrophobic surface treatments, notably by silicone, in the late 1970's.

This paper describes the properties of overall organic surface modification technology presently available. Surface modification today, use organic compounds applied to various cosmetic inorganic pigments such as titanium dioxide and iron oxides as well as substrates such as talc, mica, and spherical silica. Surface treated pigments and substrates show dramatic improvement in texture, water repellency, dispersion ability, and wear properties.

2. Recent Trend in Surface Modification Technology

Converting hydrophilic pigments and substrates to hydrophobic ones, has been the main focus of cosmetic chemists in the early stages of surface treatment technology. The simplest way of surface treatment is to merely mix the hydrophilic powders with an appropriate oil. Adequate mixing will ensure that each particle be covered with the oil, and thereby cover all the "hydrophilic points" on the pigment. This will then facilitate its dispersion into various oils, since the oil layer around each particle acts as a hydrophobic (which in this case is the same as being lipophilic) layer, and make the pigment appear to repel water. However, this sort of surface treatment technique, which is a coating by only mechanical means, does not usually result in satisfactory water repellency. Any sort of mechanical shearing easily rubs the oil coating off, thereby making the pigment hydrophilic again.

One of the breakthroughs in the early stage of surface treatment came from coating by methylhydrogenpolysiloxane (methicone), which forms an interlocking fishnet-like silicone film around the hydrophilic particles. This then modifies the surface of the pigment, from a hydrophilic nature to a hydrophobic one. Methicone coating also dramatically improves many other important parameters such as texture, wear properties, and dispersion quality of these pigments into various oil ingredients. Surface treatment of inorganic pigments by silicone compounds, however, does not increase the affinity with the skin, which directly affects the overall wear properties of cosmetics, to satisfactory levels. Thus, coatings by other compounds such as amino acid (2) (3), hydrogenated lecithin (4), collagen, and metallic soap have been proposed. Each of these surface treatments enhances the wear properties of the pigments onto the skin, while still maintaining its hydrophobic character. Each treatment offers unique properties in texture and skin adhesion. Amino acid treatment in particular is known for its consistent skin adhesion and has been heavily used with inorganic colorants in liquid foundations. Hydrogenated lecithin treatment, which utilizes a hydrogenation process to minimize oxidation of lecithin, has been the benchmark for creamy texture in various pressed products including eye shadows. This treatment has been consistently found to be popular for foundations requiring high slip quality in their final products.

Over the past ten years, the cosmetic industry has ventured toward a new direction in surface treatment technology, by employing more sophisticated compounds. One such complex compound is the use of fluorinated compound as a surface treatment, either by fluorocarbons or fluoro-silicones. Surface treatment by fluorocarbons in other industries (besides cosmetics) has been known and used since the early 1960s (5). However, it has taken twenty years before being fully accepted by the cosmetic industry (6). Fluorocarbon coating offers both hydrophobic and lipophobic (oil repellent) properties. With the use of fluorocarbon coated pigments, formulators can offer shine-free foundations, since these pigments do not get wetted by neither perspiration nor sebum. Recent trends in lipophobic coating include coating by fluoro-silicone, notably perfluorosilane (7), (8). This treatment forms a siloxane bonding on the surface of pigments to provide greater oil repellency.

So-called silicone treatment has also seen new developments in recent years. One of the new generation silicone coatings include surface treatment by reactive dimethicone (9). This straight chain reactive dimethicone possesses a reaction group at one side of its chain that bonds onto the surface of pigments by a chemical reaction. The bonded dimethicone coating offers an extremely silky texture, since the bonded dimethicone remains in the liquid form while adhering onto the surface of the pigment, and thereby exhibits properties of normal dimethicone oil. This then makes the powder feel very dewy and silky. Moreover, bonded dimethicone does not have any problems dealing with unreacted hydrogen residue found in typical methicone coatings. Surface treatments by alkyl silane, which is represented by SIR_n(RÕ)_4-n where R is an alkyl group, and RÕ being a reaction group (such as an alkoxy group), have also become popular (10), (11).

One of the main obstacles in the utilization of these custom-made compounds as a surface treatment of pigments, is trying to conform to various international, environmental, transportational, and obviously, cosmetic regulations. The cost and time required, to obtain all necessary information for distribution and use, sometimes overweigh the market potential for "new" surface treatments. One unique approach to avoid such issues is with the use of "hybrid" surface coatings. "Hybrid" surface coating mentioned here, utilizes an initial layer of coating on the pigment as a base on which an additional layer of coating is reacted. Though hybrid coating consists of two different layers of chemical compounds, these two layers are synthesized (reacted) to be basically one compound that surrounds the base material. The merit of hybrid coating can best be illustrated by utilization of polymethylsiloxane as a base layer (12). Since polymethylsiloxane treatment on most pigments contains residual unreacted Si-H groups, even by using a chemical vapor deposition method (CVD), the residual hydrogen gas generation has become a primary concern to formulators. However, modification of Si-H groups of polymethylsiloxane with alkenyl compounds by hydrosilylation., has been found to result in numerous new coatings, with entirely different surface properties; some of which include a hydrophilic tail on an already hydrophobic pigment (13). Different from "hybrid" surface coatings, multiple layered surface coatings which are recently beginning to gain attention, also provides similar potential and benefits as well (14).

Surface coating has broadened its potential in other cosmetic applications in the past several years as well. In the area of sunscreen products, inorganic sunscreens, such as untreated microfine titanium dioxide and zinc oxide, exhibit very high photo-oxidative properties when irradiated with UV radiation. The application of surface coating on microfine pigments has been found to be very effective in minimizing such negative property (15). In addition, surface coating improves the dispersion properties, and helps prevent agglomerations, which correlates to high transparency on the skin, as well as increasing overall SPF values. Organic pigments and lakes are also being coated with various materials, including silicones as well. The application of silicone coating on organic pigments, for instance, facilitates the ease of dispersion during the manufacturing process, and also improves texture in lipstick products.

3. Evaluation of Surface Treated Pigments

Since powder ingredients typically determine the overall performance of color cosmetic products such as pressed foundations, liquid foundations, eye shadows, and blushers, choosing the right coating on the powder ingredients is vital. Moreover, understanding the characteristics of various surface treatments on substrates and pigments is the most important aspect of understanding surface modification technology. This section focuses on the techniques associated with the evaluation of powder materials, specifically for use in color cosmetics.

3.1 Evaluation of Surface Treated Samples

Specific substrates and pigments were used to evaluate the performance of surface treated powders. For this evaluation, some of the most commonly used substrates and pigments were used. The same starting raw material lots (Table-1) have been used to apply various surface treatments (Table-2). The evaluation performed for this paper were kinetic friction coefficient, water repellency, oil absorption, and wear properties of a pressed foundation using the above treated pigments.

Substrates	Properties
Talc	Jet-milled talc with average particle size of about 10 microns
Mica	Average particle size of about 10 microns
Sericite	Classified by proprietary process for tight particle size distribution for enhanced texture and adhesion. Average particle size of about 10 microns
Titanium Dioxide/talc	Mixture of rutile titanium dioxide (70 - 80 %) and jet-milled talc for improved dispersion property and texture

Table-1: Types and Properties of Substrate Samples

Types of Surface Treatment	INCI Designation
Amino acid	Pigment (and) Disodium stearoyl glutamate (and) Aluminum hydroxide Pigment (and) Disodium stearoyl glutamate (and) Aluminum hydroxide
Metal soap	Pigment (and) Aluminum dimyristate Pigment (and) Aluminum dimyristate
Hydrogenated lecithin	Pigment (and) Hydrogenated lecithin (and) Aluminum hydroxide Pigment (and) Hydrogenated lecithin (and) Aluminum hydroxide
Methicone	Pigment (and) Methicone Pigment (and) Methicone
Adsorbed silicone	Pigment (and) Dimethicone Pigment (and) Dimethicone
Fluoro-compound	Pigment (and) DEA C8-18 perfluoroalkylphosphate ester (and) Aluminum hydroxide

Table-2: Types of Surface Treatments

3.1.1 Kinetic Friction Coefficient

Evaluation of powder ingredient texture is very subjective and difficult. Though actual human sensory evaluation is still very popular, evaluation of the kinetic friction coefficient (16), which measures friction using electronic signals, enables the user to impart numerical values to frictional qualities. The type of equipment used for this evaluation uses a sensory arm that oscillates over a specific area of powder, which is spread over a double-sided tape. As the arm moves back and forth, the machine picks up the resistance of the drag over the powder and places a number on the friction felt. This oscillation was repeated five times, and the fifth value was printed as the powder's friction coefficient. The frictional values obtained from this machine correlated very well to human sensory evaluation, as confirmed with panel testing.

Lower figures in the table below indicate less friction, and thus better slip property.

Talc

	Mica	Sericite	TiO2/Talc
Control (uncoated)	0.496	0.600	0.510	0.709
Amino acid	0.459	0.403	0.525	0.549
Metal soap	0.532	0.338	0.398	0.444
Hydrogenated Lecithin	0.517	0.465	0.405	0.521
Methicone	0.562	0.610	0.558	0.597
Adsorbed silicone	0.502	0.499	0.485	0.655
Fluoro-compound	0.732	0.610	0.657	0.670

Table-3: Kinetic Friction Coefficient of Surface Treated Samples

From the data above, it seems that metal soap treatment offers the best slip with mica, sericite, and TiO₂, while amino acid treatment works better with talc. It should be clarified that the machine measures only the frictional drag felt from the powder, whereas the human touch can sense other factors, such as moisture content (dewy or silky feeling), and affinity towards the skin (ability to blend better onto the skin). This means that although the frictional coefficient values found here offers a fine background as to differentiate powder slip, final selection of powders usually needs human evaluation.

3.1.2 Water Repellency

Water repellency, or hydrophobicity, directly affects the wear properties of final cosmetic products. Water repellency was evaluated by using the contact angle method. Each surface treated powder was pressed into a pan at high pressure and placed in a contact angle meter. A water droplet of 1.5mm f is placed on the pressed pan and was allowed to sit for 10 seconds, after which the contact angle was measured. Higher numbers indicate better water repellency.

	Talc	Sericite	TiO2/Talc
Control (uncoated)	0	0	0
Amino acid	129	130	143
Metal soap	137	141	151
Hydrogenated Lecithin	120	137	152
Methicone	140	138	149
Adsorbed silicone	137	129	154
Fluoro-compound	142	146	153

Table-4: Contact Angle Results of Surface Treated Samples (degrees)

While untreated pigments showed absolutely no water repellency (absorption of the water droplet), almost all of the treated pigments showed a contact angle of over 130 degrees. While fluoro-compound treatment showed the best water repellency in talc and sericite, adsorbed silicone showed the best water repellency with TiO₂.

3.1.3 Oil Absorption

Oil Absorption of samples was evaluated by simulating ASTM D281-45 method, by using dimethicone oil (20 cst). Oil absorption greatly affects the tactile aspect of powder ingredients. Moreover, high oil absorbing surface treatments such as amino acid can, and is frequently used in sebum-control type formulations, together with specific absorbent materials such as spherical porous silica.

	Talc	Sericite	Mica	TiO2/Talc
Control (uncoated)	85	78	129	42
Amino acid	84	87	87	32
Metal soap	82	63	79	29
Hydrogenated Lecithin	72	59	87	26
Methicone	91	78	131	33
Adsorbed silicone	77	59	93	28

Table-4: Oil Absorption of Surface Treated Samples (using dimethicone oil)

As the reader may have noticed Fluoro compound treatment is not mentioned here, because of obvious reasons that Fluoro compound treatment repels oil and so treated pigments do not absorb it. Aside from the amino acid treatment, most treatments either increase (compared to the control) or decrease its amount of oil absorption. Amino acid treated sericite and methicone treated titanium dioxide behave uniquely according to the rest of the table.

3.1.4 Wear Properties

Pressed foundation (Table-5) using each treated sample were prepared, and 0.25 mg per cm² of each sample was applied to the back of human subjects by a sponge applicator. The site was then evaluated by use of a video-microscope for skin adhesion performance. Then, the subjects were exposed in a sauna at 90 centigrade for 5 minutes and allowed to perspire before evaluation of wear properties by a video-microscope at the same area.

Talc	77.16 (%)
Sericite	10.0
Titanium dioxide / talc mixture	5.00
Yellow iron oxide / talc mixture	2.70
Red iron oxide / talc mixture	1.00
Black iron oxide / talc mixture	0.30
Dimethicone	1.48
Squalane	1.18
Octyldodecyl oleate	1.18

Table-5: Sample Formulation

Among the surface treatments evaluated for adhesion property, amino acid, metal soap and dimethicone (bonded) showed excellent skin adhesion. Good skin adhesion, means that most of the pigment covers the skin and its wrinkles smoothly in an even layer. Untreated pigments on the other hand, tend to fall into the wrinkles and do not form a clean layer as observed through a video microscope. Due to the higher affinity with skin, both amino acid and metal soap seem to produce the best result. Since bonded dimethicone does not polymerize, the coating layer stays similar to dimethicone oil and thus enhances adhesion properties. Generally, water repellent surface treatments (as shown in 3.1.2) exhibited good wear properties. In particular, metal soap and dimethicone (bonded) treatment showed excellent wear properties. On the other hand, all untreated pigment samples were washed off by perspiration.

4. Conclusion

It is obvious that surface treatments not only conceal the many drawbacks of uncoated pigments, but also enhance characteristics that are almost essential in modern day make-up products. When formulators start on a project on formulations, each think of many concepts that make it unique compared to others. Some formulators place their priority on feeling towards the skin, while others may be more interested in skin adhesion, or water and perspiration repelling characteristics, or its ability to blend well in oil. In each case, surface treatments provide the best answer for formulations, because of its ability to impart any of these characteristics on almost any pigment. Formulators can therefore choose any pigment for their formulation and add many new benefits onto these pigments, while diminishing any negative characteristics, with the use of surface treatments.

5. References

　

(1) Yamamoto et al., Formation of singlet oxygen upon UV-irradiation of microfine oxides and its prevention by surface-coating. Proc. of IFSCC Congress. Cannes, 1998.

(2) US Patent 4,606,914

(3) US Patent 5,326,392

(4) US Patent 4,622,074

(5) US Patent 3,632,744

(6) Japanese Patent Application Tokko-Hei 5-86984

(7) Japan Patent 2597492 and 2597494

(8) Horino M., "Properties of Inorganic Powders treated with Water & Oil Repellent Agent and Application of Cosmetics." Shikizai Kyokaishi 65(8) (1992): 492-499.

(9) US Patent 5,368,639

(10) Aerosil Nippon. Technical Bulletin Aerosil No.13. Aerosil Nippon: Tokyo, 1996.

(11) US Patent 5,486,631

(12) Japanese Patent Application Tokkai-Sho 63-113082

(13) Suhara T. et al., "Characterization of titanium dioxide modified with alcoholic hydroxyl groups." Shikizai Kyokaishi 65(4) (1992): 264-270.

(14) Miyoshi T. et al., Development of Novel Surface-Coating for Inorganic Sunscreens. Proc. of European UV Sunfilters. Paris, 1999. Step Publishing Ltd.: England, 1999.

(15) Miyoshi T. et al., Benefits of Surface-Coating on Micro-fine Oxides. Proc. of European UV Sunfilters. Paris, 1998. Step Publishing Ltd.: England, 1998.

(16) Kato Tech Co., Ltd. KES-SE Friction Coefficient Measuring Instrument. Tokyo: Itochu, 1998.

　USCC Sep. 25, 00