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Calcined Clays

As mentioned above, kaolin clays have a number of limitations for thermoplastic and thermoset applications. These can be reduced or eliminated by careful heat treatment (calcination). Among the benefits of this process are a significant reduction in water content and dielectric constant, better whiteness, and much reduced tendency to cause cure interference in elastomer formulations.

A number of products are produced in this way depending on the calcination conditions, especially temperature. Often these products are all called calcined clays, but there are two important types of calcined product for polymer applications, metakaolin and an amorphous product generally referred to just as calcined clay or calcined kaolin. In this article the use of the term calcined clay is restricted to the higher temperature amorphous product.

Metakaolin can be regarded as a partly calcined product and is produced by endothermic dehydroxylation which removes the water of crystallization. Above 500 °C kaolinite starts to lose its water of crystallization, and, by 650 °C, approximately 90% of this dehydroxylation is complete, leaving residual hydroxyl groups randomly distributed but isolated so that further condensation will not readily occur. This product is known as metakaolin, it still retains some crystalline structure, but X-ray diffraction patterns are very diffuse and weak. The aluminum, which was originally in sixfold octahedral sites, now occupies four- and fivefold sites almost equally.

Due to the structural changes, metakaolin is much more chemically reactive than the kaolinite from which it was formed (and also has a very reactive surface (Newman 1987). This reactivity is exploited for its pozzolanic activity in building materials, such as high-strength cements and mortars.

Metakaolin is stable up to 980 °C, when a defect spinel structure, which is virtually amorphous, forms exothermically. Products produced between 980 and 1100 C are known as calcined clays. Above 1100 °C there is a slow transformation of the defect spinel with mullite forming in an amorphous silica matrix. This extremely hard and inert refractory material only has minor use in polymer applications.

During calcination, kaolinite plates tend to fuse face-to-face and so calcined clays are coarser than the feed clays from which they are produced and also have significantly lower aspect ratios, approximately 3:1. (Compare Fig. 3 with Fig. 2.)

Fig. 3 A typical calcined clay (reproduced with kind permission from Imerys)

Table 4 Important properties of calcined products compared with uncalcined kaolin

Property

Fine kaolin

Metakaolin

Calcined (amorphous)

Specific surface area (m2/g) BET

16-30

12

7-12

Surface hydroxyl concentration (nm~2)

8

1

1

Moisture content (at equilibrium%)

1.5-2.0

0.3

0.2

Accelerator and peroxide adsorption

High

Low

Very low

Dielectric constant

2.6a

1.3

1.3

aNote that this is for pure, thoroughly dried kaolinite; most kaolin clays will be higher

Table 5 Dielectric constant (relative permittivity) for clays and some other common fillers (note some impurities can significantly affect these values)

Filler

Dielectric constant

Kaolin

2.6

Metakaolin

1.3

Calcined kaolin

1.3

Calcium carbonate

6.1

Talcs

5.5-7.5

Micas

2.0-2.6

Wollastonite

6.0

Aluminum hydroxide

7.0

Crystalline silicas

3.8

Metakaolin and the amorphous aluminosilicate produced at 980-1100 °C have approximately the same Mohs hardness of 4.0, specific gravity 2.6, and refractive index 1.6. Other important characteristics of typical products compared with kao- linite clay are given in Table 4. One of the principal attractions of kaolin fillers over other common types is that they have a much lower dielectric constant (more correctly referred to as relative permittivity). This is demonstrated in Table 5. The dielectric constant is important for applications such as cable insulation, as it controls energy losses and heat buildup in the insulation. Low values give the least losses. Such effects are negligible in low voltage applications, but become increasingly important as voltage increases.

The residual hydroxyls on the surface of calcined clays make them very receptive to organo-silane coupling agents with which they are widely used in polymer composites. These silanes are chosen to suit the polymer matrix, with vinyl, amino, and sulfur functional silanes being most often used. Some manufacturers offer calcined clays pretreated with silane, but the “in situ” method of adding the silane during composite production is also employed.

 
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