Magnesium silicate or talc

Citation

(1998), "Magnesium silicate or talc", Pigment & Resin Technology, Vol. 27 No. 2. https://doi.org/10.1108/prt.1998.12927baf.011

Magnesium silicate or talc

Magnesium silicate or talc ­ 2MgO.4SiO₂.H₂O

This mineral rock is mined in many parts of the world. The crystal form and resultant physical characteristics vary with the location of the rock. Normally talc rock contains a mixture of types. The paint-making properties of a particular commercial talc pigment are governed by the type of structure or mixture of types, the average particle size and the particle size distribution. Since most grades of talc are predominantly fibrous or platy, it is common practice to classify them on this basis. Fibrous talcs are usually rated best for exterior durability and pigment suspension with some deficiency in flowing and smoothness of film.

Platy talcs impart superior brushability, flow, smoothness of film and enamel hold-out. They also add "slip" to paints and printing inks. The granular talcs, rather few in number, have best adhesive and sanding properties, which indicates their use in sanding sealers and surfaces.

Many grades of talc are manufactured, each designed for specific uses or purposes. They range in particle size from rather coarse, relatively low oil absorption to "superfine" with high oil absorption and Hegman fineness of up to 7.

There is also a surface treated grade of talc with low oil absorption and Hegman fineness of 6-7, permitting use in gloss enamels. Talc has a variety of desirable properties. The dry brightness is high and colour is good in both oil and water systems. Talc wets and disperses very easily in all paint liquids, such as organic solvents, varnishes, alkyds and water, often requiring no more than moderate stirring. It has excellent suspension and helps keep other pigments suspended.

The relatively coarse grades are used where some film roughness or "tooth" is desirable, such as interior wall primers, undercoaters, and texture paints. The smooth grades (about 4 Hegman fineness) are popular for house paint use; they are also general purpose extenders. The superfine talc of 5 to 7 Hegman and higher is used for control of gloss, consistency and sagging of semi-gloss enamel. Talc pigments are used rather extensively in joint cements, crack fillers and caulking compounds. The requirements of the particular product dictate whether coarse or fine grades are used.

Talc pigments are used in a great variety of industrial paints, especially in primers and undercoats. Structural steel primers are fibrous talc as all or part of the extender. It improves selling properties, film strength and recoatability. Many industrial flash bake primers for product and transport finishes are advantageously formulated with talc. The platy type of magnesium silicate which includes soapstone, is useful in undercoats for metal because it tends to improve sanding properties and water resistance. The latter effect may result from the plate like particles lengthening the path of moisture through the film.

The China clay (kaolin) extender pigments are essentially aluminium silicates. The clay is mined in the Carolinas, Georgia and in England. Originally, clay extenders were consumed chiefly by the paper industry, which still uses them in huge quantities. During recent years the clay suppliers have been adapting the pigments to the paint industry. The simple air dried and air floated products have been upgraded by water washing, close control of particle size, calcination and surface treatment. Calcination is the most significant of these innovations, and it is done on a large scale.

The particle shape is mostly laminar, hexagonal shaped plates. The widely used grades have a fine top particle size of about 10 microns. All grades are hydrophilic in varying degrees and are chemically almost neutral. Oil absorption ranges from 25-44 for the non-calcined grades to 48-58 for the calcined grades. The brightness varies with the grades from 80-92 per cent reflectance. Clay pigments weigh about 21.6 pounds per solid gallon, approximately one pound less than calcium carbonate.

Clay pigments tend to impart toughness to coatings because their platy shape reinforces the film. If used in house paints, they tend to chalk faster than talc. They are used extensively in undercoats for metal to promote easy sanding. Calcined clay is finding increased use to flat interior wall paints and produces smooth films. The grades, having extremely fine particle size, are used in coloured semi-gloss and gloss paints to reduce flooding and floating. In solvent thinned paints some clay pigments may exhibit poor suspension but the calcined and surface treated grades tend to overcome this difficulty.

Since clay pigments are hydrophilic, they are good extenders for waterbased paints. They have good suspension properties, are easily dispersed and impart good brushing and levelling. They are generally used in combination with other extenders.

These extender pigments exist in two principal forms :

(1) anhydrite; and

(2) gypsum (or terra alba).

Only the first form is used extensively in the paint industry. Anhydrite may be produced by calcining gypsum. It is very bright and stays in suspension well. It is used in some gloss paints, linoleum and other types of products in which its particular properties are of advantage. Anhydrite calcium sulphate is slightly soluble in water and forms gypsum, as long-needle crystals on storage in latex paints; however, it is used in some latex "project paints" which are used immediately by contractors. Water solubility makes it reactive with some latex systems so its use is quite limited.

Barium sulphate extender pigment is available in two forms:

(1) a ground ore as barytes; and

(2) precipitated as blanc fixe.

Recent advances in grinding and bleaching procedures produce barytes extenders with much improved colour and fineness. Blanc fixe, while possessing excellent colour and fine particle size, is not used in large tonnage in the paint industry because barytes is generally satisfactory and much lower in cost.

The principal use of barytes is in industrial undercoats, including automotive primers. Because of its low oil demand and dense, granular or nodular structure, it imparts good filling or build and good enamel hold-out. It is almost essential in such coatings in which gloss of the top coat is important. The chemical inertness of barytes makes it valuable in some chemical resistance coatings used in highly acidic conditions.

Silica pigments are made in three distinctly different types:

(1) ordinary silica, made by grinding and classifying quartz;

(2) diatomaceous silica; and

(3) synthetic silicas.

Quartz type, or crystalline silica, has quite restricted use. It is difficult to disperse, has poor suspension, and is so abrasive that it discolours badly if ground on roller mills or in steel ball mills. It is the main extender used in wood fillers. For this purpose, it has an outstanding combination of properties, low oil absorption with resultant low shrinkage, inertness toward binders and good tooth.

Diatomaceous silicas, or amorphous silicas, are widely used for their very high flatting efficiency. They take their name from the fact that the diatomaceous earth from which they are obtained is composed of the siliceous skeletons of aquatic diatoms. These skeletons are porous, fragile and have almost every conceivable geometrical form.

Diatomaceous silicas vary from about 83 to 95 per cent SiO₂. Most of the products are white, but some are light grey, light pink or buff coloured. They have a high bulking value and a very high oil absorption. The combination of irregular structure and high oil absorption produces very high flatting effect, high thickening power and good pigment suspension. Diatomaceous silica is widely used in interior flat wall paints. Because of its porosity and friability, it detracts from the ease of stain removal and scubbability of flat wall paints. Moreover, it is more subject than most extenders to developing a polish when scrubbed or washed. It has also been used to flatten exterior house paint and is reported to contribute to all-round durability. The proportion that can be used for this purpose is limited by effects on consistency brushing and flowing properties.

Dispersion should be performed by high-speed mixing or by addition to a pebble mill shortly before the grind is completed. Otherwise, the fragile particles will be broken down, greatly reducing flatting power and suspension properties. Since appreciable reduction in particle size during paint manufacture is impracticable, it is highly important that the mix-in Hegman fineness be tested carefully in selecting a grade of diatomaceous silica.

Synthetic silicas, also amorphous silicas, are synthetically manufactured and have high flatting efficiency. They are used in lacquers, varnishes and enamels to reduce gloss.

Mica comprises a group of several minerals that differ in composition and physical properties. The muscovite type is the usual source of extender pigment. Mica occurs in large sheets of crystals known as books. After the rocks have been delaminated by compressed air, mica pigment is produced by both wet and dry grinding methods. Probably the most widely used grades are wet ground and have relatively large particle size, about 325 mesh. There are also grades with an average particle diameter of only 5-10 micros.

The very plate-like structure of mica is unique and explains the properties that it imparts to paints. It provides mechanical reinforcement of the film and it increases the length of the path that water and moisture must travel to penetrate the film. It has been used in small portion to eliminate cracking in oil house paints. It has also been used in latex house paints to prevent cracking and to improve brushing properties. The proportion of mica is usually low, in the rate of 5 per cent of total pigment. Mica, in small amounts, is a specified ingredient of several Federal Specifications structural steel printers, and is often found in proprietary products of this type. It improves the salt spray resistance and durability of these products. In exterior paints, mica imparts a sparkle to films by reflected sunlight, and this may be considered undesirable.