New blue pigment

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2010

An unexpected process was found to develop a new, more environmentally friendly blue.

KEY CONCEPTS
• The current blue pigments have been used for a long time but are either environmentally hazardous or suffer from durability issues.
• An unexpected process has been found to develop a new blue pigment by incorporation of manganese dioxide into other metal oxides.
• These manganese-based oxides are not toxic, although they are quite stable and display good durability in initial testing.

In many cases, the last feature looked at in a lubricant is its color. At times we are so busy making sure that a specific product has sufficient lubricity, extreme pressure characteristics, corrosion inhibition and foam control to worry about such an aesthetic as appearance.

But the color of a lubricant can be important to the end-user. For example, an end-user working with metalworking fluids prefers to have products that are blue in color. If a supplier decides to have the identical fluid dyed red and evaluated by the end-user, the chances are the red-colored metalworking fluid will fail because of the preference for blue.

Typically, lubricant companies work with oil- and water-soluble dyes that are used in small quantities. In some cases, complaints are received about the durability of the dye, as it may change color during use.

The current types of blue pigments have been available for some time. For example, Egyptian blue (calcium copper silicate) has been known since the time of the pharaohs. Other blue pigments include Cobalt blue, Han Chinese, Maya blue, Ultramarine, Prussian blue and Azurite.

Mas Subramanian, Milton-Harris professor of materials science at Oregon State University in Corvallis, Ore., says, “Many of the currently used blue pigments are either environmentally hazardous or suffer from durability issues. Cobalt is highly toxic and Prussian blue evolves hydrogen cyanide under acidic conditions. Ultramarine and Azurite are not stable under high temperature and acidic conditions.” 

Development of a new, more environmentally friendly blue pigment would be welcomed. Such a pigment has just been prepared.

YITTRIUM INDIUM MANGANESE OXIDES
Subramanian found that preparation of solid solutions of yttrium oxide, indium oxide and manganese dioxide at temperatures between 1,100 C and 1,200 C produces a bright blue solid. He says, “This product is an unexpected result as yttrium oxide is a white solid, indium oxide is a yellowish white solid and manganese dioxide is a black solid.”

Subramanian indicates that the blue solid was developed by accident. He explains, “We were originally looking to develop materials that display both ferroelectric and ferromagnetic properties. Materials that can be both photoelectric and magnetic simultaneously are not known.” 

Subramanian relates that a graduate student took a bright blue solid out of a hot furnace as he was walking through the lab. The intensity of the blue color made him realize that something unique had just been prepared.

As the stoichiometry of the metal oxides is changed, the color of the resulting pigment can change from blue to black. Figure 3 shows a number of images of the yttrium indium manganese oxides at various concentrations of manganese. In moving from left to right, the manganese concentration increases, leading the color of the solid to darken. Finally, yttrium manganese oxide is found to be black, as noted by the image on the extreme right of Figure 3.


Figure 3. Addition of manganese oxide into yttrium indium oxide leads to the preparation of a blue pigment. As the concentration of manganese increases in moving from left to right, the color of the pigment darkens until it becomes black as yttrium manganese oxide. (Courtesy of Oregon State University)

The intense blue color of the yttrium indium manganese oxides is due to the coordination of manganese atoms into an unusual trigonal bypyramid structure. Subramanian says, “This gives rise to splitting in energy levels in manganese’s d orbitals that result in strong absorption in the red/green region of the visible spectrum and changes the color of the samples to intense blue.” 

Initial evaluation of the yttrium manganese oxides has been in pigment and paint applications. Subramanian says, “We know that this new type of blue pigment is heat stable, but it is also not attacked by acids and bases. The color also appears not to fade after durability testing.” 

Subramanian indicates that he knows of no conditions under which this new blue pigment fades. These metal oxides are not soluble in water but potentially could be dispersed in this medium. He speculates that some of the paint companies are using this approach.

The intense blue pigment can be prepared by introducing manganese into other metal oxides to prepare the trigonal bipyramid structure. Among the examples are scandium aluminum magnesium oxide, lutetium gallium magnesium oxide and indium gallium magnesium oxide. Subramanian adds, “We needed to add 5% manganese to these oxides to generate the intense blue color.”

Another benefit of using this combination of metal oxides is that they are all more environmentally friendly than other blue dyes. Subramanian says, “Indium oxide, yttrium oxide and manganese dioxide are not toxic.” 

Future work will involve development of less expensive metal oxides that can act as blue pigments. Subramanian says, “We are particularly interested in preparing metal oxides that are more costeffective by limiting the indium content.”

With the development of this new series of pigments, the possibility exists in the future that they can be utilized to provide an intense blue color that will be enjoyed by lubricant formulators and users.

More information on this work can be found in a recent paper (1) or by contacting Subramanian at mas.subramanian@oregonstate.edu. 

REFERENCE
1. Smith, A., Mizoguchi, H., Delaney, K., Spaldin, N., Sleight, A. and Subramanian, M. (2009), “Mn3+ in Trigonal Bipyramidal Coordination: A New Chromophore,” Journal of the American Chemical Society, 131 (47), pp. 17084–17086.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.