Kaolin
Mineralogy
Kaolin (china clay) is a hydrated aluminium silicate crystalline mineral
(kaolinite) formed
over many millions of years by the hydrothermal decomposition of granite rocks.
Hydrous kaolin is characterised by its fine particle size, plate like or
lamellar particle shape and chemical inertness.
Kaolin is named after the hill in China (Kao-Ling) where it was first mined, for many years before it’s discovery in Europe (Cornwall, England) in 1745 by William Cookworthy who was looking for a source of material to produce white chinawares.
Particle Shape: Platy, with typical aspect ratio of 20:1
Specific Gravity: 2.6
Refractive Index: 1.56
MOH Hardness: 2.5
Moisture (max %): 1.5
pH: 5-7.5
Applications
Kaolin is now used in ceramics, paper, paint, plastics, rubber, sealant, adhesives and cosmetics.
Formulated into paint, fine kaolin such as Supreme™ can act as a functional extender of TiO2, reducing formulation cost and improving rheology. Coarser kaolins, Polwhite™ B & Polwhite™ E do not offer so much potential to extend TiO2 but enhance the dry film properties of primers and undercoats. Polsperse™ 10 offers paint makers the possibility to extend TiO2 in solvent based gloss paints processed using high speed dissolvers rather than bead mills traditionally associated with this type of paint – cutting processing time and increasing efficiency.
Rubber formulations benefit from the use of fine kaolin, Supreme™ and Speswhite™, give semi-reinforcing properties and good barrier properties, particularly useful in automotive hose applications. The coarser kaolin, Polwhite™ E, can be used in flooring compounds to increase abrasion resistance as well as making the compound cheaper.
In sealants and adhesives, kaolin is used to modify rheological properties.
Calcined kaolin
Mineralogy
Calcined Kaolin is an anhydrous aluminium silicate produced by heating ultrafine natural kaolin to high temperatures in a kiln. The calcination process increases whiteness and hardness, improves electrical properties, and alters the size and shape of the kaolin particles.
Mineralogy: Metakaolin or amorphous, aluminium silicate (defect spinel)
Particle Shape: Irregular, with surface voids
Specific Gravity: 2.6
Refractive Index: 1.56
MOH Hardness: 4-5
Moisture (max %): 0.5
pH: 6-8
Production
Calcination occurs in two steps.
At 700°C the dehydroxylation of the kaolin is complete forming a poorly crystalline metakaolin. Products produced at this temperature include PoleStar™ 450 which can improve the dielectric properties of PVC cable insulation, and MetaStar™ which acts as a pozzolanic concrete additive.
At 980°C an amorphous defect spinel is formed which undergoes a transformation, with mullite recrystallising in an amorphous glass at temperatures above 1100°C. Products produced within this temperature range provide excellent properties in rubber compounds, especially when coated with silane, improving mechanical properties and chemical resistance. Silane treatment further enhances rigidity, toughness and dimensional stability in polyamide mouldings.
Markets for Calcined Kaolin
When formulated into film compounds Imerys calcined kaolin improves the thermal properties of agricultural films, giving the potential to reduce heating costs, reduce the planting to cropping time and increase the length of the growing season. In film that requires antiblock additive, calcined kaolin in the form of Infilm™ 200 offers an ideal and cost effective balance of antiblock, haze and clarity performance.
In plastics the use of calcined kaolin is in PVC cable to improve electrical performance and in its coated form as Polarite™ 102A, as a functional filler in engineering thermoplastics.
PoleStar™ 200P is a very effective opacifying extender in paint, partially replacing TiO2, reducing the overall cost of paint formulations but maintaining paint film quality.
