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 tr