Kaolin - Talc - Mica

Kaolin

Kaolin is a silicate clay mineral with the formula Al2(SiO5)(OH)4. In its natural state it is a whitish, soft and hydrophilic powder. It consists mainly of hexagonal kaolinite crystals with a typical size between 0.1 micrometers and 10 micrometers. The mineral may contain a variety of other minerals such as feldspar, muscovite, quartz and anatase. There is also raw kaolin which tends to appear yellowish because it is colored by iron hydroxide pigments. Colored kaolin is usually chemically bleached for commercial use to remove iron pigment and other minerals.

It is widely used in coatings, for example in architectural coatings where it increases the strength and opacity of the paint, resistance to rubbing and stains, and prevents cracking in the event of deformation of the substrate.

In automotive coatings, it plays a significant role in giving the paint some excellent qualities such as film smoothness, gloss, corrosion resistance and film uniformity.

Furthermore, it is widely used as a filler and to support more expensive minerals such as titanium dioxide, it has an excellent action in improving the distension of paints and a strong action in keeping the components of the formula in suspension, thanks to its natural thixotropy of clays.

The hardness of kaolin (from 2 to 2.5 Mohs) and its physical qualities contribute to making the paint long-lasting, resistant, fireproof and suitable as a barrier to humidity and exposure to atmospheric agents.

Kaolin has excellent rheological and mixing capacity and is compatible with a vast series of formulations of water-, oil- or solvent-based paints, electrophoretic inks and varnishes and mastics. Finally, it is not harmful to people exposed in the workplace.

The percentages of use vary from 5 to over 20% of the formulation.

Talc

Talc is a natural hydrated magnesium silicate mineral Mg3Si4O10(OH)2 with a lamellar structure. It is hydrophobic and inert to most chemical reagents and does not decompose on contact with acids. It is a poor conductor of electricity, low thermal conductivity and high resistance to thermal shock. It decomposes at temperatures above 900-1000°C and the hardness is 1 Mohs, placing it among the softest minerals.

The high whiteness and natural shine generally makes the use of whiteners unnecessary and, in turn, can act as a whitener. Thanks to its softness, it is ideal for primer coatings for the possibility of sanding and repainting.

Talc has lubricating, anti-adhesive and fluidizing properties, allowing the preparation of formulations with a high dry residue without creating excessive viscosity.

As an additive in polyurethane waterproof coatings, talc powder can not only reduce the volumetric shrinkage of the coating during curing, but also improve the wear resistance and adhesion of the coating, while reducing costs and providing stability to storage and heat. In latex paints it is an indispensable element as it improves their aesthetics, hardness and resistance to corrosion.

The lamellar structure of talc allows for effective leveling of the coating, reducing defects and increasing its physical and decorative performance. In high-temperature resistant coatings, talcum powder can improve bending stiffness while ensuring structural stability.

In architectural coatings it provides excellent brushability, light retention, leveling, covering effect, resistance to humidity and crack formation.

Also often used in wood coatings, usually for primer or top-coat polyurethane paints. The refractive index of talc, being similar to resin, gives the film high transparency, allowing the underlying wooden substrate to be appreciated. Furthermore, the shiny effect of talc makes it possible not to use zinc stearate, which is more expensive.

The percentages of use vary from 5 to over 20% of the formulation, considering however that in the absence of other fillers, talc tends to sediment and/or surface.

Mica

Mica belongs to a group of minerals made up of complex silicates of aluminum and potassium also containing, depending on the type, magnesium, iron, sodium, lithium, fluorine and hydroxyls. It is characterized by easy flaking, flexible and more or less elastic, the morphology of the particles is similar to flakes or platelets. The generic formula is XY2–3Z4O10(OH)2 where: X is mainly represented by K, Na or Ca; Y from Al, Mg, Fe; Z from Si or Al.

Mica has good thermal and electrical insulation and is inert to acids. The hardness is between 2.5 and 3 Mohs. In coatings it is abundantly exploited for the following peculiarities:

Barrier effect: the flake structure forms a substantially parallel orientation in the paint film, reducing the possibility of penetration of water and other corrosive substances. This ability helps make the coating anti-corrosive.

Anti-wear: Due to its hardness, mica adds resistance to abrasion, thus improving the quality of automotive, road, mechanical and wall coatings.

Insulation: It has extremely high thermal resistance and proves to be an excellent insulator. Combined with silicone resin or silicone boron resin, materials suitable for high-risk environments such as mines, tunnels, special buildings, special structures, etc. are obtained.

Fireproof: it is a very valuable flame retardant, in addition to organic halogens, it is possible to prepare specific fireproof coatings.

Anti-UV: Mica has excellent UV and infrared shielding properties. Therefore, the addition of mica powder in outdoor coatings can greatly improve the UV resistance of the paint film and delay its aging.

The percentages of use vary from 5 to over 10% of the formulation, compared to kaolin and talc, it tends to create denser and stickier mixtures.

*The descriptions given represent only a generic overview, there are countless types for each mineral species, suitable for specific areas. They are often combined with each other, together with other fillers, to highlight certain peculiarities and minimize the defects of each species.