Properties of Inorganic Pigments

Before discussing the properties of inorganic pigments, it is necessary to first define pigment and explain where it is used. Pigment is a colored material that is completely insoluble in water. At the same time, pigments can also be defined as metals in powder form, generally obtained in very small form through spraying or chemical precipitation. Pigments are used to colorize paints, inks, plastics, cosmetics, food and other materials. The use of pigments dates back to prehistoric times. The oldest pigments are natural minerals. Going back 30,000 years, we encounter hematite, brown iron ore and other minerals used in cave paintings. Around 2000 B.C., red, violet and black pigments were produced from iron oxide or mixtures created by burning manganese sprouts and were used in pottery. Following developments over the years, the pigment industry began in the 18th century. Pigments differ from one another in terms of chemical composition, physical shape and optical properties. In general, pigments are characterized by heat stability, colorability, transparency or opacity, resistance to light, toxicity, and resistance to alkalis and acids. Colored pigments can be examined in two categories according to their sources. These are organic pigments and inorganic pigments. Organic pigments are pigments that contain hydrogen, nitrogen and oxygen atoms along with carbon in their structure, and natural organic pigments are obtained from animals and plants. Organic pigments are divided into two types within themselves: natural and synthetic. Today, a large portion of organic pigments are synthetic pigments produced from petroleum sources. The color range of organic pigments is very wide and nearly complete.  

Properties of Inorganic Colored Pigments

Inorganic pigments are oxides of metals that do not contain iron or insoluble metal salts in certain metals. This pigment group is cheaper than organic colored pigments, but has a narrower color scale. Inorganic materials can be classified according to the materials they contain. They can be examined as Iron Oxide Pigment, Lead Chromate Pigments, Cadmium Pigments, and Bismuth Vanadate Yellow.  

Iron Oxide Pigments:

Iron oxide pigment is also called English red or colcotar. This pigment is a stable, hard and harmless red pigment. Other iron oxide pigments consist of iron oxide yellow and iron oxide brown. Due to its low cost, it is used in oil paint production, and due to its hardness, it is used in polishing in very fine powder form. Transparent paints are also made from iron oxide red and yellow pigments with small particle sizes. The color intensity of these pigments is not very high, but they can be preferred because they are inexpensive. At the same time, opacity, solvent resistance, chemical resistance and high durability are among the positive properties of iron oxide pigments. Although the use of iron oxide pigments in the paint industry is gradually decreasing, its use still exists.  

Lead Chromate Pigments:

Lead chromate pigments, which are economical and have high opacity, are yellow and orange-red in color. This pigment group consists of light chrome yellow, medium chrome yellow, chrome orange and molybdate orange. When we look at the general properties of lead chromate pigments, we can say that they provide the opportunity to create a wide color range with their vivid colors. At the same time, these pigments can be used in paint processes in many fields by developing both their outdoor durability properties and dispersion ease properties. However, because they are harmful to human health, the use of this group of pigments has been banned in many countries.

Cadmium Pigments:

These pigments, which represent a wide range, have not achieved high usage rates because they are harmful to human health. These pigments are obtained from various cadmium compounds and have higher heat stability and outdoor durability compared to lead chromate pigments. However, because they are both more expensive and cadmium is harmful to human health, these pigments have not become an alternative to lead chromate pigments.  

Bismuth Vanadate / Bismuth Molybdate Yellow:

The color of the pigment, which is light lemon yellow in color, can be modified by changing the molybdate content it contains. This pigment has been used as an alternative to light chrome yellow pigment due to its high heat stability, opacity and outdoor durability properties. We can also describe the inorganic pigments we classified above according to the materials they contain, through their colors. We can classify them as white pigment, black inorganic pigment, blue-ultramarine pigment, yellow pigment, red pigment and green pigment. • White pigments: The most important white pigment is titanium dioxide, which makes up 80 percent of pigments used. Lead and zinc compounds are also used as white pigments, but are not used as frequently as before. • Titanium dioxides maintain the first place in usage rate due to their easy dispersibility, high covering power and demand for white and pastel colors. • Looking at yellow pigments, iron oxide hydroxide contains rust-colored yellow, lead sulfate chromate ranges from lemon yellow to orange, and basic lead oxide lead chromate forms orange-colored pigments. • As for red pigments, among these pigments are iron oxide, lead molybdate, orange-tinted red consisting of lead chromate lead hydroxide mixtures chrome orange pigments, lead chromate mixtures molybdenum reds and cadmium pigments in cadmium selenide composition giving colors from orange to burgundy. • Looking at blue-ultramarine, or cobalt, this pigment was born by binding polysulfur form of sulfur to alkali in sodium, aluminum silicate. The pigment can change color according to the sulfur content it contains. Depending on the amount of sulfur, it creates colors such as greenish blue and purple. • As black inorganic pigment, iron oxide in magnetite structure is used. • As green inorganic pigment, from light to dark, mixtures of chrome yellow and prussian blue with increasing blue ratio can be shown, along with chromium oxide and chromium hydroxide. Among inorganic pigments there are also zinc, aluminum and bronze powders.  

Differences Between Organic and Inorganic Pigments

The colors of inorganic pigments are not as fresh as organic pigments and their color ranges are narrower. Besides this, organic pigments are weak in terms of light resistance and air resistance. However, inorganic pigments have properties such as resistance to weather conditions and high temperature resistance. When we compare their fields of use, organic pigments are used in plastics, rubbers, inks, putties, paints, coatings and other related applications. Inorganic pigments are used in building materials, enamels, ceramics, glass and outdoor coatings.   Sources: 1. Inorganic Pigments, Söğüt B. 2. Pigments and Paints, Eren, M.H 3. Industrial Inorganic Pigments, Gunter Buxbaum 4. https://megep.meb.gov.tr/mte_program_modul/moduller_pdf/Pigmentler.pdf 5. https://stmcoatech.com/organik-pigmentler-nedir/ 6. https://stmcoatech.com/inorganik-pigmentler/