I. Ultramarine Pigments

Naturally occurring mineral lapis lazuli is found in very few places worldwide, with the best quality obtained from Afghanistan and Chile. The name "Ultramarin," meaning "from beyond the sea," was given to the pigment obtained from grinding the mineral. The deep blue color has been used by many artists and can be seen in Giotto's chapel in Padua, in Duccio's painting Virgin and Child, and in Renoir's painting The Umbrella. The cost of transporting the mineral and subsequent processing meant that the pigment was more expensive than gold. As a result, the French Government in the 1820s launched a prize competition to develop the first synthetic and economical process for producing synthetic ultramarine blue. The competition ended controversially. However, according to general consensus, Guimet in France and Gmelin in Germany independently designed similar processes for synthetic preparation in 1828. Ultimately the prize was awarded to Guimet and he is frequently cited as the first person to produce ultramarine blue on a commercial scale.

Synthetic ultramarines are inorganic powder pigments commercially available in three colors:

1. Reddish blue, C.I. Pigment Blue: 29: 77007 [CAS: 57455-37-5] 2. Violet, C.I. Pigment Violet: 15: 77007 [CAS: 12769-96-9] 3. Pink, C.I. Pigment Red: 259: 77007 [CAS: 12769-96-9] The ratios of chemical components may vary, but the typical repeating formula unit of a blue ultramarine pigment is Na6.9Al5.6Si6.4O24S4.2. The violet and pink variants derive from the blue, differing primarily in the oxidation state of sulfur groups. This is reflected in somewhat lower sodium and sulfur content.

Chemical Structure

Ultramarine is essentially a three-dimensional aluminosilicate cage surrounded by sodium ions and ionic sulfur groups. A simplified structure is shown in Figure 1. The cage has a sodalite structure with a cubic unit cell size of 9.10 Å. In synthetic ultramarine obtained by calcination from Chinese kaolin, the cage distribution of silicon and aluminum ions is disordered. This contrasts with the ordered arrangement in natural ultramarines, creating a difference. In the simplest ultramarine structure, there are equal numbers of silicon and aluminum ions and the basic cage unit, as needed with a net ionic charge of zero for structural stability, is Na6Al6Si6O24 or (Na+)6(Al3+)6(Si4+)6(O2-)24.

Figure 1: Simplified structure of ultramarine blue

The nature of the sulfur groups responsible for color and their incorporation into the sodalite structure has been reviewed. In ultramarine blue, there are two types of sulfur groups, S3- and S2-, and both are held in the cage as balanced free radicals. In the dominant S3- species, the spacing between three sulfur atoms is 0.2 nm and the angle between them is 10°. While S3- absorbs a broad energy band centered at 600 nm in the visible green-yellow-orange region, S2- absorbs in the ultraviolet region at 380 nm (Figure 2).

Figure 2: Reflection spectrum of ultramarine blue.

The basic cage (Na+)6(Al3+)6(Si4+)6(O2-)24 is derived from Si12O24 by replacement of six of the silicon ions by aluminum. Each Al3+ must be accompanied by a Na+, so that the overall ionic charge of the structure is zero. Therefore, six of the eight sodium sites are always filled with sodium required for cage stability, and the remaining two sites are filled with sodiums associated with ionic sulfur groups. This means that with subsequent oxidation to S3-, leading to loss of one of the accompanying sodium ions, only one S32- polysulfide ion can be placed within the cage (as Na2S3). This gives ultramarine a basic cage formula of Na7Al6Si6O24S3. To increase sulfur content and thus improve color quality, the aluminum content of the cage can be reduced by adding high-silica feldspar to the production formula. This reduces the number of sodium ions required for cage stabilization and leaves more for sulfur group equivalency. A typical product would be Na6.9Al5.6Si6.4O24S4.2 with a simpler, more colorful, more blue tone. An alternative explanation has been proposed for the improved color obtained by adding feldspar to the production formula. In violet (Figure 3) and pink (Figure 4) ultramarines, the cage structure changes very little, but the sulfur chromophores are oxidized, probably to S3Cl-, S4, or S4-.

Figure 3: Reflection spectra of ultramarine violet.

Ultramarines are zeolites, but cage pores are limited by channels of 0.4 nm diameter. Sodium ions can be exchanged with other metal ions (for example silver, potassium, lithium, copper). Exchange with potassium ions can produce a somewhat darker reddish ultramarine blue tone. The basic ultramarine color is a strong, bright reddish blue; the red-green tone varies with chemical composition. Violet and pink derivatives are weaker, less saturated colors (see Figures 2–4 for reflection spectra). The color quality of commercial pigments has been improved by grinding to reduce particle size and thereby increase coloring power. Average particle size typically ranges from 0.7 to 5.0 μm. Figure 5 shows particle size distribution comparing coarse particle types, typically used in laundry applications, with fine particle types, more commonly used in technical applications such as inks and plastics.

Although fine particle pigments have lighter and more greenish color tones than those with thicker particles, when reduced with white they produce brighter and more saturated colors. Figure 4 Reflection spectra of ultramarine pink. Figure 5: Particle size distribution of ultramarine blue.

  With a refractive index close to 1.5, similar to paints and plastic binders, ultramarine blue gives a transparent blue color in bright paints and transparent plastics. Opacity is achieved by adding a small amount of white pigment. Increasing amounts of white produce lighter tones, and very small amounts of ultramarine blue added to white enhance whiteness and color consistency. In many applications, ultramarine blue is stable up to approximately 400°C, violet up to 280°C and pink up to 220°C. All three have excellent light fastness at 7–8 degrees on the International Blue Wool Scale (in full and reduced tones). Color fading due to light exposure or color fading under moderate heat is almost always caused by acid attack. Ultramarines react with all acids and if sufficient acid is present, the pigment completely decomposes, losing all its color, forming silica, sodium and aluminum salts, sulfur and hydrogen sulfide. The release of hydrogen sulfide with acids is a useful test for ultramarine. There are also temporary acid-resistant types obtained by protecting pigment particles with an impermeable silica coating. Blue and violet types are stable under slightly alkaline conditions, but the pink type tends to turn to a violet tone. Ultramarines are insoluble in water and organic solvents, therefore these colors do not bleed or migrate from the polymer. This has led to approval of ultramarine pigments in a wide range of food contact applications. At the macromolecular level, fine ultramarine particles are at high surface energy and are a cohesive compound. The finer types, having larger surface areas, are more difficult to disperse than types with thicker particles and some of them can be used with their surfaces treated to reduce this high surface energy and improve dispersibility. Ultramarines absorb moisture on the outer particle surface and in the internal surface of the zeolite structure. External surface moisture (1–2% depending on particle size) is removed at 100–105°C, but an additional 1% internal moisture content requires 235°C for complete removal. Ultramarine particles are hard and are known to cause wear in equipment using dry or paste pigments. Its specific gravity is 2.35, but the bulk density of pigment powder, proportional to particle size ranging between 0.5 and 0.9 g cm-3, is much lower. Specific surface area varies with particle size and ranges between 1 and 3 m2g-1. Oil absorption also varies with particle size (typically 30–40 g). pH values range from 6 to 9. Ultramarine pigments are largely odorless, non-flammable and non-combustion-supporting pigments.   M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.