This article reviews the history of this important material, its current uses and future research directions.

Bismuth has been used since antiquity but was only isolated as a specific element around 1753. Today bismuth in its elemental form has many uses, including recent developments as a nano wire. Bismuth compounds are well known in pharmaceutical applications both as medicines and, more recently, as radio-opaque agents. Bismuth compounds are also commonly used as pigments.

Bismuth vanadate and bismuth vanadate/molybdate, basic bismuth nitrate and bismuth oxychloride all have been used as pigments. The Bi-V pigments are absorption pigments appearing yellow to red-orange. Since bismuth oxychloride (BiOCl) and basic bismuth nitrate (BiONO₃.H₂O) are both white and can be grown as platelets, they can be pearlescent. Bismuth nitrate tends to recrystallize, leading to loss of pearlescence, and has not found significant application. Therefore, our interest is limited to bismuth oxychloride.

Evolution of Bismuth Oxychloride as a Pigment

Bismuth oxychloride is routinely used in cosmetics because it has the global regulatory status of “approved color additive.” This gives cosmetic formulators great flexibility in using it as a white pigment/filler. Engelhard Corp.’s Mearlite® LBU™, shown as an electron micrograph, illustrates why bismuth oxychloride makes a good white pigment/filler. The rosette form of crystal growth contains thin platelets all growing at different angles; there is no smooth, well-defined surface. Its whiteness is due to its index of refraction, its transparency in the visible region and the irregular surfaces and edges that can scatter light.

Bismuth oxychloride pigments also have been developed to provide pearlescent effects. There has always been motivation to develop synthetic forms of pearlescent pigments instead of relying on the natural pearl derived from fish scales. The requirement is a substance that does not absorb visible light and that can be grown in a platy form. Basic lead carbonate (2Pb(CO₃).Pb(OH)₂ ) was successfully grown in a flat, crystalline form and was the first synthetic pearlescent pigment. However, concerns over the effects of lead compounds drove development of crystals based on other chemistries, including bismuth oxychloride.

An early pearlescent form of bismuth oxychloride is Mearlite GBU™. This product originally was developed in the 1960s. It is available as a dry powder or in a castor oil dispersion suitable for lipstick. The pearlescence of this material is limited by the way the crystal grows (see Figure 2). Several platy crystals grow together, and secondary growth occurs as well. Thus the surface is not very smooth, and there are many edges and steps. Reflectance from a preparation of Mearlite GBU is less than that obtained from titanium dioxide-coated mica.

Development work on more pearlescent forms continued, with highly pearlescent forms becoming commercially available in the 1980s. The bismuth crystals are very thin, as is evident from Figure 3, which shows the face and edge of high-grade crystals. The thickness at the edge is 0.07 microns, or 70 nm. This is the same order of magnitude as the thickness of newer grades of aluminum manufactured by a physical vapor deposition (PVD) process. It is believed [Seubert and Fetz] that when pigment particles are this thin, they have increased mobility in a liquid coating that enhances their ability to orient parallel to the surface. In addition, the very thin edges cause less light scattering. On a weight basis, and even accounting for the high density of bismuth oxychloride, there are many more platelets —and therefore more faces — per unit weight with high-grade bismuth oxychloride than with titanium-dioxide-coated mica.

All high grades of bismuth oxychloride crystals are formulated as pastes because of the tendency of the crystals to agglomerate face-to-face. These formulations are also adjusted so that the bismuth oxychloride, which is quite dense (specific gravity 7.7), does not hard pack, and the crystals are readily redispersed.

These “high grade” forms of bismuth oxychloride compete with titanium dioxide-coated mica as synthetic pearlescent pigments. Coated micas cost less, but they do not have all of the appearance attributes offered by high-grade bismuth oxychloride pigments. In mica-based pigments, the mica acts as a substrate on which a smooth film of titanium dioxide can be grown. It is the titanium dioxide layer that is primarily responsible for the pearlescent appearance. In the case of bismuth oxychloride, there is no substrate; it is the thin, platy form of each crystal that is responsible for its pearlescence. A crystal of a highly reflective pearlescent form of bismuth oxychloride is compared to a titanium dioxide-coated mica platelet in Figure 4. This comparison points out that both are platelets, but the edge of the platelet is much more regular with bismuth oxychloride than with mica.

A close-up comparison (see Figure 5) emphasizes the extreme smoothness of the bismuth oxychloride crystal. Essentially no surface features are apparent on the bismuth oxychloride crystal, whereas the titanium dioxide-coated mica shows a cobblestone microstructure.

The overall result of these differences is that bismuth oxychloride pigments provide much higher reflectance than can be obtained from titanium dioxide-coated micas. At the same time, the pearlescent bismuth oxychlorides give a much smoother appearance than mica-based pigments. In white formulations, the clean bulk color of bismuth oxychloride is an additional advantage. These appearance attributes are summarized in Table 1.

Industrial Applications

Usage of bismuth oxychloride pigments evolved from applications where lead carbonate was the standard and mica-based pearlescents could not meet the desired level of brightness. Examples of these early end products include fishing lures, beads, cosmetic jewelry and buttons. This improved brightness, or luster, has also led to inroads where previously only aluminum pigments were used. Bismuth oxychloride pigments come closest to aluminum’s reflectance and have the benefit of being non-reactive when used in a waterborne system. In addition, their fine and extremely narrow particle size distribution has led to numerous opportunities in printing ink, where equipment parameters limit the size of the pigment. Engelhard’s bismuth oxychloride pigments are primarily provided as dispersions for specific coating or ink systems. The platelets are inherently very fragile and need proper wetting and dispersion to remain intact. However, dry forms of pearlescent bismuth oxychloride are available for the experienced formulator.

Today there are numerous newer applications using bismuth oxychloride pigments. The trends toward smoothness and increased whiteness with a brighter finish have led to the surge in opportunities for bismuth oxychloride. The marble effect and the ability to create “veins” by simply stirring the dispersion in a medium have resulted in applications in solid surface markets such as countertops and spas. Bismuth pigments also can be found as base coatings within the many layers of transparent stains employed in wood finishing. Even the newest wave in writing utensils, the gel pen, could benefit from the brightness and metallic appearance of bismuth oxychloride pigments. Table 2 is intended as a basic guide for the most popular Engelhard pearlescent bismuth oxychloride products, sold under the Mearlite trademark.

Automotive Applications

In the 1980s, styling work with high-grade bismuth oxychloride indicated that the attributes of bismuth oxychloride were fully apparent in automotive applications with basecoat-clearcoat technology. However, an impediment to the use of bismuth oxychloride in this application has always been its susceptibility to photolysis. Bismuth (III) is reduced to Bi (0) by the action of light [Poznyak & Kulak], resulting in severe darkening. While there are many applications where this darkening can be tolerated or is insignificant on the time scale of the application, automotive applications presented a significant challenge. To use bismuth oxychloride in automotive finishes, the crystal had to be stabilized from photolysis.

The first generation of pearlescent bismuth oxychloride with improved light stability was released in the early 1990s [Eberts et al.]. The stabilization is due to a cerium hydroxide post treatment. Two formulations, one for solventborne systems (STL) and one universal formulation (SUQ), were released. These formulations are about 60% crystal and additionally contain resin (6%), additives (1%) and co-solvent.

Several colors were originally styled with this first generation product, and one reached commercial production with a major automotive manufacturer: Black Slate. In this case, the color was dark enough so that the main attribute that bismuth oxychloride contributed to the overall effect was smooth texture with exceptional sheen. This color was quite popular and eventually was made available on a luxury and a standard sedan, as well as an SUV.

Most recently, a second generation of stabilized bismuth oxychloride was released as an experimental product. In this case, in addition to the cerium post treatment, the crystallization process has been modified (see Table 3). This product is expected to allow bismuth oxychloride to be used in the medium shades, where both the brightness and texture of the material can contribute to the overall effect of an automotive finish.

Pearlescent bismuth oxychloride also is capable of providing a liquid-metal look. This is of great interest considering the current design trends and shapes that incorporate this look. The colors that will take best advantage of all attributes of pearlescent grades of bismuth oxychloride — brightness, whiteness and smooth texture — are light shades, including whites. Extensive development work aimed at making bismuth oxychloride crystals more light stable is encouraging and will continue.

The Future of Bismuth as a High-Performance Pigment

The most critical issue for bismuth oxychloride is darkening from exposure to light. In addition, there are concerns about the risk of crystal breakage and settling. These issues have largely been solved, however. Settling concerns can be addressed with appropriate thixotropes, and the risk of crystal breakage is very much dependent on handling method, so that its effect can be minimized.

Interest in bismuth as a pigment has remained high because of the excellent attributes offered by this material. Today, bismuth oxychloride pigments are used in a variety of applications from cosmetics to automotive finishes. Expanded use of bismuth in high-performance pigments will depend upon efforts to develop improved forms. Engelhard, among other pigment manufacturers, has taken up this challenge.

Acknowledgement

One picture is worth a thousand words; the electron micrographs, prepared by Dr. Lydia Rivaud of Engelhard’s H.L. Mattin Laboratories, are gratefully acknowledged.

This article was originally presented as a paper at High Performance Pigments 2000 in Berlin, sponsored by Intertech.

For more information on Bismuth oxychloride, contact Engelhard Corp., Appearance and Performance Technologies, 101 Wood Ave., P.O. Box 770, Iselin, NJ 08830-0770; phone 732/205.5000; fax 732/321.0250; e-mail info@engelhard.com; visit www.engelhard.com.