Pure kaolinite has the idealized chemical composition Al2O3.2SiO2.2H2O. It is a crystalline material with a triclinic form found in microscopic pseudohexagonal plates, which fairly readily undergo cleavage. As a consequence, it has a low Mohs hardness, between 2.5 and 3, depending on the ancillary minerals present. The specific gravity is 2.6 and refractive index is 1.56. As described above, its structure can be regarded as a gibbsite, Al(OH)3, layer bonded to a siloxane (Si2O5) layer. The idealized structure described above is rarely found in nature, with isomorphous substitution of both Si and Al ions by transition metals and particularly by iron, often occurring. This leads to electrical charges on the plates; with edges being positively charged and faces negatively charged. These charges are countered by ions, which surround the particles in a double layer. (See Jepson (1984) and references therein for a more complete discussion of the properties of clay and kaolinite.)
Because of the isomorphous substitution and also because of the occurrence of broken bonds at the edges, there are both Lewis and Bronsted acid sites on the surface of a kaolinite particle. These sites can be very reactive.
One of the important particle properties of clays used in polymer applications is the platiness or aspect ratio of the particles. This has a number of consequences. It raises viscosity, making processing more difficult, but increases stiffness and gas barrier properties. The aspect ratio achieved varies considerably from product to product. While the bonding of the silicate and gibbsite layers is strong, that between the adjacent silicate layers is weak and readily disrupted, allowing thin, hexagonal, plates to be produced. These plates have aspect ratios in the range 5:1 to 50:1 (but very fine clays can have even higher aspect ratios), which are dependent on particle size. Aspect ratios of the plates are dependent on the nature of the clay deposit with, for example, US clays being blockier than clays from SW England; but they are dependent also on the processing used, because kaolinite stacks or “booklets” readily cleave during processing.
Kaolinite reacts only with very strong acids and bases, is not affected by organic solvents, and undergoes ion exchange reactions; but from most points of view, it is an inert mineral. It does undergo a complex series of reactions when heated, which are of commercial importance and will be discussed fully below (section “Uses in Polymers”).
The commercial products sold as kaolin or china clay (and even clay) can vary in actual composition very significantly and, while the main mineral present is usually kaolinite, this is not always the case. Some “china clays” in fact are sold that contain only 25% kaolinite, but normally kaolins will contain 70-99% kaolinite, with the main impurities being mica, quartz, and feldspar. Other silicates, metal oxides and organic matter, are usually found in trace amounts. Ball clays already discussed above are an example of this.
Particle-size distributions (expressed as equivalent spherical diameter, esd) of clays depend on the inherent particle size of the deposit and the amount of refining that has been carried out during production. In primary deposits, the kaolin plates are usually bound together in a book-type structure (Fig. 1), and refining will separate them to a certain extent. Most commercial products for the polymer industries will be degritted at 300 mesh, so the normal top cut is at least 75 pm. More commonly, clays will have been refined to 20 or 10 pm for a range of applications, and speciality products will have a top cut of 5 pm or finer.
Secondary clays are usually very much finer than primary clays and products that are approximately 100% finer than 5 pm can be obtained by a fairly simple air-float or degritting procedure. Air-floated clays are fairly common in some regions of the world, including the USA.
Although kaolin can be regarded as chemically inert, it does have a complicated surface chemistry. Surface hydroxyls either from the gibbsite layer, or from adsorbed gels, readily react with commercial bifunctional coupling agents such as silanes and titanates. (See ? Chap. 2, “Surface Modifiers for Use with Particulate Fillers” for a discussion of coupling agents.) Kaolinite plates have negatively charged faces and positively charged edges and adsorption of both positive and negative ions are of great commercial importance. For example, small amounts of polyanions adsorb on to edges, deflocculating or flocculating the clay depending on the molecular weight of the polymer, and are an essential feature of its use in aqueous media (the paper and paint industries especially).
The presence of Lewis and Bronsted acid sites gives rise to a variety of chemical reactions. Amines, or other Lewis bases, readily adsorb and the use of fatty amines to render the clay organophilic has been applied for many years to modify properties in a number of applications]. Because of these reactive sites, kaolin will enter into organic reactions and, of particular interest for plastics, will catalyze the polymerization of certain monomers. Sometimes depolymerization can occur and they can promote the dehydrochlorination reaction of polyvinyl chloride (PVC).