Kaolin (China Clay) is a hydrated aluminium silicate crystalline mineral, formed over many millions of years by the hydrothermal decomposition of granite rocks. Rocks that are rich in kaolin or kaolinite are known as china clay or white clay. Kaolin can contain very small traces of uranium and thorium, and is therefore useful in radiological dating. Kaolin has also seen some use in organic farming, as a spray applied to crops to deter insect damage, and in the case of apples, to prevent sun scald.
Paper
Kaolin is the most extensively used additive in the manufacturing of paper, primarily as a filler and coating. Kaolin is combined with wood pulp or fiber to generate substantially more paper filler, and thus more paper. It is a less expensive alternative to all wood paper filler, which has become more expensive as demand for paper increases and supply of wood decreases. Kaolin's primary competition in fillers is calcium carbonate which, though more affordable, has less dramatic results. The particle size of Kaolin plays an important role in fillers and coatings, as larger particles provide added strength to paper. Kaolin coatings improve paper appearance, giving it varying degrees of gloss, smoothness, brightness, opacity, and most importantly printability. Three aspects of kaolin that shape these characteristics are purity, rheology, and particle geometry. Kaolin high in titanium and iron oxide impurities tends to be duller, and particle shape and size (rheology) are responsible for kaolin's efficiency as a coating. Kaolin improves printability by providing added ink absorption, ink pigment holdout, lowering linting tendency, and increasing roughness. Kaolin can be processed in different ways to omit varying degrees of these qualities. In some cases it is necessary to blend kaolin with other pigments such as calcium carbonate, talc, titanium dioxide and satin white to produce the desired effect in the final paper product.
Paint
White kaolin high in silica and alumina is widely used in the manufacturing of paint, often as a less expensive alternative to titanium dioxide. Kaolin is essential for improving the brightness of colors in paint, as well as controlling the texture for dispersment and coatability. The silica composition determines texture by absorbing oil in paint mixtures. Levels of alumina determine color brightness, and kaolin free of iron and titanium oxides improves brightness. Generally speaking, the ideal composition of kaolin for paint production is 47% silica, 39% alumina, 13% water (allowing 1% for other elements). Kaolin also improves paint in many other ways such as providing less weight per canister, significantly extending the product life, and improving opacity, gloss, color and viscosity. Calcined kaolin is particularly useful in creating crack resistant paint with high gloss and opacity. Particle size plays a large role in determining qualities opacity, gloss, color and viscosity in paint. Generally speaking the smaller particle size of finer kaolin powder will produce brighter color and more opacity, where coarser kaolin with a small particle size will produce proportionately duller colors and less opacity. Alternatively, coarser kaolin with larger particle size has a positive effect on gloss, and higher viscosity.
Ceramics
Kaolin has a wide range of application is the ceramic industry, and is commonly used in whiteware products, insulators, and refractories. In whitewares, kaolin viscosity improves the molding ability of pre-heated materials, offers a stronger finished product, and a smooth surface finish such as those found in tile and porcelain. Kaolin is becoming more prevalent in porcelain insulation due to its high dielectric strength and as an alternative to the scarcer resources high in potash feldsparic materials. Kaolin is used in refractories due to its extremely high fusion point, low water content, and high green strength. Ceramics produced with kaolin have a wide variety of uses, mainly as a metal substitute. Ceramics are becoming more prevalent in aerospace due to their refractory strength. Ceramic knives stay sharper longer and have better durability than metal. Toyota has created an all ceramic car engine capable of running at much higher heats. Recently there have also been advances in bio-ceramics that may lead to new and better teeth and bone replacements.
Rubber
Kaolin is an ideal additive to rubber due to its "booklet" particle structure which adds strength and stiffness to the product through enhanced stress transfer. Kaolin is a low cost substitute for silica, and improves properties of calcium carbonate found in rubber solutions. Fine kaolin with smaller particles produces a stiffer rubber due to increased surface area, while providing more abrasion resistance. Coarse kaolin with larger particle size has a lower reinforcing affect and produces a more abrasive product used in applications such as rubber flooring where more abrasion is necessary. Hard clays are used to produce fine grain for stronger applications while soft clays produce larger flakes and lower reinforcing effects. 80% of the clays used in rubber production are Air Float hard clays, which are air separated to remove impurities. Soft clays tend to incorporate more easily and therefore have higher loading point and quicker extrusions. Kaolin may be weakened further through the process of delaminating if necessary. Calcined kaolin, or water washed clay, is applied in certain wire and cable manufacturing as it improves conductivity.
Cement
Kaolin serves an important function in the cement industry as highly calcinated, pulverized kaolin adds compressive strength, flexural strength and water permeability to cement. In general 5-15% activated kaolin mixed with cement creates a superiorly strong mortar when hardened. Water permeability is useful in prolonging the durability of concrete and reduces weakening as a whole; calcined kaolin adds flexibility, which is often preferred to the usually brittle finished product. High performance concrete (cement with kaolin additives) can be modified to meet a variety of applications. In particular its shrinkage, strength when compressed, and water permeability makes high performance concrete useful in bridge building.
Adhesives
Kaolin's application in the adhesives industry is based almost entirely on the ability to control its rheology. A unique challenge in the adhesive and sealant industry is the necessity of differing levels of viscosity and shearing rates during the mixing phase by the producer and the application phase by the end user. For instance the producer will often require low viscosity during production, and the end product will need to be firmer so it does not spread. Once again particle size determines the rheology of the end product. The relationship between viscosity and shear rates is critical. Low, medium and high viscosity needs to be paired with low, medium and high shear rates based the product in question, and again these relationships may change from production to end use.
Dyes
Kaolin use in dyes has much the same relationship present in paint production. Kaolin acts as an inexpensive filler or extender of the product while increasing color and absorbability. It has been proven to enhance the catalytic ozonation process necessary in some dyes. As in paint, fine kaolin improves brightness, opacity and color, while coarse kaolin improves viscosity.
Cosmetics and Soaps
Kaolin is widely used in the cosmetic industry in mud masks for its absorbent qualities and its ability to control rheology. Calcined kaolin when applied to the face in a mud mask actually draws out impurities and toxins from the skin. This is possible because of the insolubility of kaolin which retains its absorbent qualities when mixed with water. As in most cases, kaolin is an ideal additive because it helps to control the thickness of the mud solution. One rare quality in kaolin mud masks possess is removing oil, dirt and toxins without leaving areas of the face, mainly the nose, shinny. These are essentially the same qualities that make kaolin useful in soap, although it is considered the least absorbent of the clays commonly used in soap. Therefore it is used most often in soaps for less oily, dryer skin. Kaolin is also used in deodorants, tooth paste, anti-inflammatory creams (poultices), and scrubs.
