Kaolin, calcined clay

Organofunctional silanes are used to promote polymer-to-filler bonding with clay or siHca fillers. Vinyl silanes are used in peroxide-cured wire insulation to promote stronger bonding with calcined clay fillers. Mercapto silanes are used to treat kaolin clay in sulfur-cured compounds. 

Fig. 2. (a) Brightness improvement obtained by the use of kaolin as a filler in paper and (b), opacity improvement obtained as pulp fibers are replaced with various kaolins. In both cases, A represents fine-particle calcined clay B, high brightness No. 1 kaolin C, coarse-particle water-washed kaolin and D,... 

Clay minerals are widely used in the filler industry and the production of the various types has been described by Hancock and Rothon [85]. The products available range from unaltered kaolin to products produced by calcination. Kaolin deposits are widespread throughout the world. While simple clay minerals are extensively used as fillers in elastomers, their use in thermoplastics is more restricted and the principal products used are those obtained by calcination of ka-olinite. Thus metakaolinite finds application in PVC and silane treated calcined clay in polyamides. The production of these forms only is discussed below. 

The second material is simply known as calcined clay and is formed when kaolin is heated above 1000 °C. This is an amorphous material with a defect spinel structure. A few isolated hydroxyls are retained on the particle surface and this enables the material to react with materials such as organo-silanes. 

Calcined kaolin Calcined kaolin clay. See Kaolin, calcined... 

Calcined alumina. See Alumina Calcined baryta. See Barium oxide Calcined diatomaceous earth Calcined diatomite. See Cristobalite Calcined kaolin Calcined kaolin clay. See Kaolin, calcined... 

Calcined clay n. China clay (kaolin) that has been heated until the combined water is removed and the plastic character of the clay is destroyed. This produces an air-solid interface within the particle, which increases hiding in he resulting coating. 

Clay n. Any naturally occurring sediment rich in hydrated silicates of aluminum, predominating in particles of colloidal or near-colloidal size. There are many types of clays and clay-like minerals. Those of particular interest to the plastics industry are varieties refined by nature and man to a state of good color and particle-size distribution, such as kaolin (China clay). They are used as fillers in epoxy and polyester resins, PVC compounds, and urethane foams. Calcined clays are those that have been heated to a high temperature to drive off the chemically bound water, sometimes also surface-treated to improve their chemical inertness and moisture resistance. They are used primarily in vinyl insulation. 

Both kaolin and calcined clays are available with surface treatments. Many treatments may be applied but few have any commercial importance. Kaolins are available treated with surfactants and pH adjusters, which produce kaolins that may be dispersed directly into water, (e.g., rubber latices), or with amines to enhance cure performance. Stearic acid treated products may provide ease of dispersion. The most important treatments, technically, are the organo-silanes. 

Metakaolin (calcined clay) is an ultra fine pozzolana, produced from the mineral kaolin at temperature between 700°C and 900"C in that temperature kaolinite loses water. This thermal activation is also referred to as calcining. The obtained product is highly pozzolanic and in recent years there has been an increasing interest in the utilisation of metakaolin used as partial replacement of Portland cement (up to 30%), often as an alternative to the use of SF. 

Kaolin is clay, i.e. a hydrous aluminosilicate. There are two varieties (a) a naturally occurring, hydrous form, and (b) the form obtained when the clay is heated at more than 600°C to remove the water. This is the calcined, anhydrous form. It is harder than the hydrous variety, i.e. 6-8 on the Mohs scale, instead of 2. The average equivalent spherical diameter (esd) of the natural form is 1.5-1.8 pm compared with a wider range (0.9-3) after calcining. 

Paper fillers - High-brightness, low-abrasion airfloated, water-washed and delaminated kaolins are used as pulp extenders in acid-sized paper, where they improve opacity, smoothness and ink receptivity. Partially calcined clay is used as a Ti02 extender. 

Paper Filling kaolins, titanium dioxide, talc, calcium carbonate, calcined clays, synthetic silicas and aluminas. 

Other fillers - Kaolin and Ti02 are used in alkaline papermaking, but to a much lesser extent that in acid papermaking. Kaolins are cheaper in some eases than calcimn earbonate, and satisfy some filling requirements for alkaline. Calcium carbonate does not opacify well so TiOa must be used for some papers. The extender pigments, especially calcined clay and talc, are used and perform better than calcium carbonate when opacifying is needed. 

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