Pigments are some of the main components in paint formulation. The most important pigment used in paint making is titanium dioxide, due primarily to three important physical characteristics. Titanium dioxide has high refractive index, strong absorption in the UV region of light spectrum and strong reflectance in the visible spectrum thereby giving it a very high light scattering properties which, when combined with its particle size creates an effective pigment for brightness and opacity.
It also has good compatibility with other pigments, tinters and other additives. In most cases, pigments serve three main functions in paint formulations. These include the optical function of providing colour, opacity and gloss; a protective function with regard to the surface underneath the paint and the binder which can be destroyed by UV; and a reinforcing function for the paint itself helping the binder to stick.
Pigments for paint formulations are submicron particles which must possess certain properties such as good opacifying strength (commonly called hiding power) and brightness. Other important features are excellent resistance to chemical attack, good thermal stability and resistance to UV degradation.
Speaking to LEADERSHIP, Mr Okpe Monday of the Manufacturers Association of Nigeria (MAN), explained that pigments can be classified into three main groups which are inorganic, extender pigments, and anti-corrosive and special function pigments.
According to him, inorganic pigments are of mineral origin and are usually metal oxides, and many metal oxides are found naturally such as red and yellow iron oxides.
He said extender pigments are usually naturally occurring minerals such as clay, mica, talc and limestone because they are often used as a cheap way of reducing the cost of paint while anti-corrosive pigments are very important as steel requires protection against corrosion.
“Of all the various pigments, titanium dioxide is regarded as the most important. Amongst the inorganic pigment, titanium dioxide is the most important pigment in the world accounting for approximately 70 per cent of total volume. About 96 per cent of titanium produced is used as feedstock for titanium dioxide pigment.
“Currently, the major producing feedstock regions are Australia, South Africa, Canada and India, with Australia being the largest producer. The cost of titanium dioxide at present is about six times the cost of calcined kaolin products. Therefore, as a result of increasing demand, its price has been increasing. The price rose by about 35 per cent in 2015 and is expected to increase at an average annual rate of about 7 per cent till 2021.
“The high demand and increasing cost is fueling the need to reduce cost of paint making. For this reason, the search for an alternative to substitute titanium dioxide as paint pigment is vigorously being pursued. One of the most prominent alternatives is calcined kaolin which has physical and chemical properties that compare favorably with those of titanium dioxide. Apart from the cost reduction initiative, the use of calcined kaolin has other advantages. The production process of calcined kaolin is environmentally friendly and requires less capital investment,” he said.
Okpe further said studies had shown that kaolin, when thermally heated at specific temperatures gives products of industrial significance called “calcined kaolin”, an anhydrous aluminium silicate form of kaolin with characteristic properties such as high brightness, hydrophobicity, abrasive nature and improved electrical properties, adding calcinations also alter both the size and shape of kaolin particles, stating the calcinations occur in two stages, which are from 450oC to 700oC. At 700oC, the dehydration of kaolin is completed forming metakaolin mostly used to improve the dielectric properties of PVC.
“Nevertheless, for kaolin to be suitable for use as a pigment, mineralogical composition and fundamental structure of the kaolin are very important since they control the resultant physical and chemical properties, and, therefore the industrial application. Calcination increases the silica and alumina content of kaolin. The silica content determines the texture of the paint while aluminium content determines the brightness and opacity of the paint.
“The ideal composition of silica and alumina in calcined kaolin for paint pigment is in the range of 45 to 47 per cent and 37 to 39 per cent respectively, while that of iron oxide content should be less than 2 per cent. Silica content higher than the above range increases the abrasion of the calcined kaolin pigment.
“As a result of this, it has become necessary to promote the use of Nigeria’s huge reserve of kaolin which is estimated at about 3 billion metric tonnes in different parts of the country as a substitute to titaniue dioxide in paint production. This is important as there are at least 45 known deposits of kaolin in Nigeria and there is no state in the country without at least one deposit,” he added.
He maintained the market for kaolin in Nigeria at present is large, sustainable and expanding because of the numerous applications of the product, adding currently, there is an outright ban on the importation of kaolin into the country in order to stimulate the growth of the local industry.
According to him, some of the major kaolin deposits in the country are those found in Nahuta, Kankara, Onibode, Ifon, Major Porter and these deposits have the potentials to meet industrial quality as pigment because their alumina, silica and iron contents almost fall within the required range.
Speaking to LEADERSHIP, the director-general of the Raw Materials Research and Development Council (RMRDC), Prof. Hussaini Doko Ibrahim, said the global market for pigments had been forecast to reach 9.0 million MT valued at $40.2 billion by 2025, adding this increase in market growth was expected to be fuelled by growth in the paints, coatings, ink and construction industries.
He averred that the paint industry in Nigeria present is highly import dependent as over 70 per cent of the raw materials for the sector are imported, saying the major reason for non-availability of local raw materials for the sector is low capacity utilization in the petrochemical industry and in most of the mineral beneficiating plants.
He said the country spent about N7.6b on the importation of paint pigment in 2015 and this figure rose to N19.9b in 2019 and as a result, the council initiated a project aimed at producing calcined kaolin that would serve as 40 per cent substitute for titanium dioxide in paint making on the basis of the above.
The RMRDC boss said this is expected to lead to reduction in importation of titanium dioxide by about 40 per cent, pointing out, in addition, it would assist in the development of simple technology for production of calcined kaolin and stimulate interest in investment of the solid mineral sector.
He said the first phase of the project had been completed, adding the kaolin deposit in Alkaleri was used, pointing out that raw kaolin was calcined without beneficiation and it produced 5 per cent substitution of titanium dioxide after industrial test.
He further said the second phase of the project involved the calcination of kaolin from seven deposits across the six geo-political zones of the country and 20 per cent substitution was achieved after industrial test, adding the third phase of the project which is aimed at addressing the issues identified in the second phase had also been completed.
“The council designed, developed and test run a gas suspended Calciner. The company has completed the fabrication of the calciner. The cooling section has been installed along with calciner riser column. The installation of the preheating cyclones is on-going after which, the system will be tested without heat to ascertain the zero – solid dynamic behaviour. Also the base of the calciner riser column had been modified to accommodate heating with LPG gas.
“The fabrication of all component parts has been completed. The council presented the calciner to stakeholders in November, 2019 and a number of investors showed interest in the production of calcined kaolin in Nigeria using the technology. The council is working with the investors to work out the modalities for establishment of such facilities near relevant kaolin deposits in the country. When the projects are eventually established, the cost of paint production in the country will reduce by about a quarter while at the same time employing a good number of skilled and unskilled workers,” he added.