What is talc?
The term “talc” has four complementary meanings:
- Talc is a mineral: it is hydrated magnesium silicate.
- Talc is a rock known as steatite or soapstone, which is composed of varying proportions of the mineral talc, often combined with other minerals such as chlorite and carbonate.
- Talc is an industrial raw material in powder form, which is used in a wide range of applications.
- Talc is a cosmetic powder. This is its most widely known application—but one of the smallest in terms of consumption.
GEOLOGY AND OCCURRENCE
Talc deposits always result from the transformation of existing rocks under hydrothermal activity. Through this process, the components (MgO, SiO2, H2O) required for forming the parent rock into talc are brought by hydrothermal water. The size and the geometry of the final deposit depend upon the size and nature of the parent rock, and the intensity and scale of the phenomenon. The geological context required for such a transformation to occur is known as a low to medium temperature and pressure metamorphism. Tectonic movements always play a major role: earth movements allow the hydrothermal fluid to penetrate the mother-rock, generating permeability that makes reactions inside the rock mass possible. The surrounding pressure, either at the time of the transformation or at a later stage, determines the mineral’s degree of platyness, otherwise known as lamellarity (low pressure/low lamellarity, high pressure/high lamellarity).
The type of mineralisation depends essentially on the nature of the parent rock. Talc deposits are classified according to the parent rock from which they derive. There are four types of talc deposits:
- Deriving from magnesium carbonates. More than half of world production comes from this kind of deposit, found in ancient metamorphosed carbonate sequences. This talc is generally pure and white.
- Deriving from serpentines. This type of deposit provides about 20% of the world’s talc. Here, the crude ore is always grey but can be up-graded—generally using flotation techniques—to improve mineralogy and whiteness.
- Deriving from alumino-silicate rocks. About 10% of world production comes from these deposits. They are sometimes found in combination with magnesium carbonate deposits. The crude ore is generally grey due to the presence of chlorite (another phyllosilicate), but no up-grading is generally necessary as chlorite, like talc, is a functional mineral conferring benefits to a number of industrial applications.
- Deriving from magnesium sedimentary deposits. Talc is formed by direct transformation of magnesium clays. No deposits belonging to this category are currently being mined.
- This wide diversity of origin and types naturally gives rise to a wide variety of ores and product grades, which differ according to their mineralogical composition, colour and crystalline structure (micro-crystalline or lamellar).
Talc is a natural mineral (Mg3Si4 O10(OH)2 – molecular weight 379.26). As shown in the opposite figure, its elementary sheet is composed of a layer of magnesium-oxygen/hydroxyl octahedra, sandwiched between two layers of tetrahedral silica. The main surfaces, known as basal surfaces, of the elementary sheet contain neither hydroxyl groups nor active ions, making talc both hydrophobic and inert.
The size of an individual talc platelet i.e. a few thousand elementary sheets, can vary from approximately 1 micron to over 100 microns depending on the conditions of formation of the deposit. It is this individual platelet size that determines a talc’s lamellarity. A highly lamellar talc will have large individual platelets whereas a microcrystalline talc will have small platelets.
The elementary sheets are stacked on top of one another, like flaky pastry, and because the binding forces (known as Van de Waal’s forces) linking one elementary sheet to its neighbours are very weak, the platelets slide apart at the slightest touch, giving talc its characteristic softness.
Talc is odourless. It is insoluble in water and in weak acids and alkalis. Although talc has a marked affinity for certain organic chemicals, it generally has very little chemical reactivity. It is neither explosive nor flammable. Above 900°C, talc progressively loses its hydroxyl groups and above 1050°C, it re-crystallises into different forms of enstatite (anhydrous magnesium silicate). Talc’s melting point is at 1500°C.
Talcs differ according to their mineralogical composition, i.e. the type and proportion of associated minerals present. The most common mineral found with talc is chlorite, which is structurally and chemically very similar. Dolomite and magnesite are also often present. As we have seen above, talcs also differ in their degree of lamellarity.
People always think of talc as white but it can also be grey, green, blue, pink and even black.
Basically, talc is many talcs.
Talc shows an affinity for certain organic chemicals. It is therefore organophilic. In the case of talc’s affinity for polypropylene, a likely explanation is that the position of the oxygen atoms on the surface of the talc platelet corresponds to the carbon bonds on the surface of the polypropylene crystal.
Talc’s organophilic properties are used to great benefit in a number of applications, for instance, it helps polypropylene to crystallise. In papermaking, the talc particles attract the undesirable resin droplets, i.e. organic chemicals, in the pulp onto their surface. This is known as pitch control. In body powder applications, perfume, also organic, is adsorbed onto the talc surface and retained.
FUNCTIONS AND APPLICATIONS
The five main properties of talc that make it a unique substance for industrial and domestic applications are:
- Lamellarity (composed of platelets that slide off each other)
- Softness (unctuous and non abrasive)
- Chemical inertness
- Affinity for organic chemicals
Talc is used as an anti-sticking agent, an anti-caking agent, a lubricant, a carrier, a thickener, a strengthening filler, a smooth filler, and an adsorbent.
In many industrial processes, talc is used to prevent two materials from sticking together. Talc powdering of moulds is widely used for example in tyre manufacturing or foundry applications. Certain foodstuffs or conveyor belts are also powdered with talc, to excellent effect. Similarly, chip boards are powdered to stop them from sticking together in storage.
Talc is extensively used in animal feed. It wraps each feed particle, setting up a natural barrier that stops moisture escaping from or entering the treated feed. Talc also improves the fluidity of the material, which helps to keep equipment clean (minimising bacteria proliferation), lowers energy consumption, reduces production stoppages, and makes the operator’s job safer.
Talc is used as a lubricant in pharmaceuticals for example. As well as not reacting with active ingredients, it facilitates the ingestion of solid drugs (pills, powders, etc). Talc also helps mould and demould pills. It is widely used as a lubricant in the transport of dry materials.
The chemical inertness of talc is of obvious interest for carrier applications. Excipients in pharmaceuticals and cosmetics (dry or moist) are a classic example. Apart from its inertness, talc also has the advantage of not generating bacterial growth and of releasing fragrances progressively. Carriers are also critical in applications such as fertilisers and plant protection. Adding talc to active ingredients results in a well-managed release, allowing the practical and safe handling of very small and very large quantities of active substances.
Talc is able to influence the viscosity of water- and solvent- based formulations. In paints for example, it increases covering power, improves flow and prevents settling. It also improves the adhesion and final mechanical properties of the coating. Talc enables paints, with the necessary low viscosities, to be formulated with significantly less organic solvents. The use of talc in the formulation of a range of liquid products (paint, cosmetics, glues, etc.) also contributes to controlling sedimentation rates during storage.
Talc is used to reinforce various types of resins, principally polypropylene (PP). Although the talc used in such applications is finely ground, it nevertheless maintains its lamellar structure. This gives the PP the best trade-off between rigidity and impact strength. Applications include domestic appliances, food packaging films and, above all, automotive components such as bumpers and dashboards. Talc is also used as a semi-reinforcing filler in rubber. Talc’s properties bring benefits to ceramics (household, construction, refractory and technical). It reduces firing time and temperature, improves vitrification and consequently the resistance of the ceramic. It also improves thermal shock resistance, particularly important in catalytic exhausts.
Talc’s smoothness is appreciated in products that require suppleness and smoothness. A typical example is colouring pencils which, for a smooth and regular effect, must be robust, but at the same time softer than the paper. In terms of volume, talc is also the principal ingredient in putties, particularly polyester putties where it improves adhesion and sandability.
Talc’s adsorption properties, i.e. adsorbing onto the surface only, are key in a number of applications, particularly pitch control in paper making where talc is used to absorb organic impurities (pitch, unwanted anions) which are sticky. Talc also greatly improves printing runnability. As a filler in paper, it increases smoothness and machinability, reduces friction, abrasion and porosity. In paper coating, it helps to improve ink transfer, finish feel and legibility of printing. Talc’s adsorption properties are also valuable in the treatment of waste water by the activated sludge method. Providing an adequate support surface, talc platelets actually “ballast” the bacteria used in such treatments, thereby improving sedimentation and avoiding the release of bacteria in the final clean effluent.
The following table gives a non-exhaustive list of the main registries in which talc and chlorite are listed. Depending on the registries, substances may be given an inventory number or simply be listed. Talc Chlorite CAS 14807-96-6 1318-59-8 EINECS 238-877-9 215-285-9 AICS (Austrailia) yes no CEPA (DSL/NDSL) (Canada) yes (DSL) yes (DSL) ECL Serial No (Korea) 1-686 9312-1666 ENCS/MITI (Japan) no no NEPA (China) – – PICCS (Phillipines) yes yes TSCA (USA) yes yes