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Emerald Group Publishing Limited
Copyright © 1998, MCB UP Limited
The name extender pigments may create the supposition that they hold minor importance and therefore merit only superficial study. In reality, they profoundly affect the performance of most pigmented coatings. Because of the great number of extender types and their extreme diversity of properties, most formulators are not thoroughly informed regarding their properties and most advantageous use. This situation poses a challenge; the opportunity for improvement of finished products by more expert use of extenders.
While extender pigments are mineral compounds of relatively low refractive index, they differ in composition, particle size and shape. They develop very little hiding in gloss and semi-gloss paints, but they contribute "high flat hiding" (air pigmented-interface) to paints at low cost. They are used to control gloss, texture, suspension, viscosity, etc. The main type of extenders are carbonate, silicates, sulphates and oxides.
Extender pigments are available in particle sizes ranging from 0.01 to 44 microns and in a variety of shapes including spheroid, needles, fibres and plates. Particle shape influences pigment packing, flexibility of the film, crack bridging, etc. Particle size and particle size distribution influence hiding, viscosity, film porosity, vehicle and surfactant demand, gloss, fineness, etc. Reactivity of the extender pigments with binder is critical. Salt spray, blister, corrosion and check-crack resistance are dependent on the factor.
Considerable work has been reported on the best way to substitute one extender pigment for another to produce reasonably equal paint properties. It was found that when extender pigments were substituted for one another at equal weight or equal volume they produced substantially different paint properties, such as: consistency, durability, permeability, scrubbability, dirt collection, enamel holdout and others.
This class of extender pigments is the most widely used in both interior and exterior paints to control gloss, mould, consistency and cost. It is available as both natural and synthetic (precipitated) types and in an extremely wide range of particle sizes, from 0.05 to more than 44 microns.
The natural calcium carbonate is used in much greater quantity because it is cheaper and is equally suitable or superior for some purposes. The coarser grades find rather restricted use in putty, caulking compounds, undercoats for metal and in other products when high pigment loading, flow and tooth are desired. These grades have little flatting effect. The grades of intermediate particle size are used in interior flats and semi-glosses and in house paints. The fine particle grades, largely precipitated, are used in printing inks. Frequently, two grades are used in a formula to obtain the desired balance of properties.
The source of the natural calcium carbonate extenders are limestone, marble and English chalk. Most grades contain 95-99 per cent CaCo3. Some of the grades are dry ground and classified by air flotation. The liner grades are wet ground and classified by sedimentation or centrifuging. The many grades produced differ widely in particle shape, average particle size, particle size distribution and purity.
Natural calcium carbonate extenders are also widely used in exterior house paints, alone or in conjunction with talc. In comparison with talc they reduce the chalking rate, improve the colour retention of tints and increase mould resistance. This usage introduces a special requirement of particle size. Fine particle size, especially so-called "fines", impart a tendency to failure by cracking. It has been determined that to be satisfactory the materials should be of large particle size, have low vehicle demand and have low reactivity. These extender pigments are also used in exterior latex house paints to improve colour retention.
Calcium carbonate is alkaline and acts as a buffer in water-based paint systems. Because these extenders are alkaline, they should not be used with alkali sensitive coloured pigments such as iron blue and chrome green; nor where staining by soluble iron or copper salts may be encountered. Calcium carbonate cannot be used in acid catalyzed baked melamine polyester resins systems because they neutralize the catalyst.
As a class, the precipitated or synthetic calcium carbonate pigments are 98 to 99+ per cent calcium carbonate, have small particle size and a narrow range of size distribution, are high in oil absorption and are brighter than the natural products. The better grades vary in particle size from about 0.03 to 8 microns, which puts them in the colloidal range. It is reported that the optimum particle size for hiding power in interior latex paints and paper coatings is about 0.2 micron. Precipitated pigments are used where maximum flatting power is needed. Some grades are surface treated with rosin or fatty acids. This produces greater ease of incorporation and permits use in gloss products without appreciable loss of gloss. In some formulations, it reduces penetration into porous surfaces, thus increasing sealing action and hold-out of top coats.