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China Clay or Kaolin

8

Roger Rothon

Contents

Definitions....................................................................................... 162

Introduction...................................................................................... 162

Kaolin Occurrence and Extraction.............................................................. 164

Properties........................................................................................ 166

Uses in Polymers................................................................................ 167

Calcined Clays................................................................................... 169

Flash Calcined Clays........................................................................ 171

Uses of Calcined Clays in Polymers........................................................ 172

Cross-References................................................................................ 175

References....................................................................................... 175

Abstract

Clays of various sorts are widely available throughout the world and have been used in polymer composites, especially those based on elastomers, since the early days of their industrial application. While there are a large number of clay materials, the main ones used in polymers are based on the mineral kaolinite and are often referred to as kaolin or china clay.

The china clays themselves have limited application in thermoplastic and thermoset applications. This is due to a number of factors, such as poor color and heat aging, especially in polypropylene. The water of crystallization is also an issue for water-sensitive polymers such as nylon and thermoplastic polyesters, as

R. Rothon (*)

Rothon Consultants and Manchester Metropolitan University, Guilden Sutton, Chester, UK e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

© Springer International Publishing Switzerland 2017

R. Rothon (ed.), Fillers for Polymer Applications, Polymers and Polymeric Composites: A Reference Series, DOI 10.1007/978-3-319-28117-9_80

it can be released during processing. Many of the problems are overcome by calcination, and calcined forms are more widely used for these polymers.

On the other hand, china clay is a widely used white filler in the rubber industry. Depending on particle size, it can be used as a semi-reinforcing filler (hard clay) or a non-reinforcing filler (soft clay). Common elastomer applications include chemical liners, bicycle tires, conveyor belts, shoe soles, gaskets, and flooring.

Keywords

China clay • Kaolin • Metakaolin • Calcined clay • Thermoplastics • Elastomers

Definitions

Clay Although broadly distributed and well known, clays are difficult to define precisely. The term is applied to finely grained natural materials which are plastic when wet and hard and brittle when dried, especially after firing. They are usually complex mixtures of various minerals, with the main component being a platy aluminosilicate.

Clay mineral A group of important hydrous aluminum silicates with a layer structure and very small particle size. They may also contain significant amounts of iron, alkali metals, or alkaline earths.

Kaolinite A specific clay mineral with the formula of Al2O3 .2SiO2 .2H2O.

Kaolin A clay where the principal clay mineral is kaolinite, but significant amounts of other minerals can be present.

China clay Another term for kaolin.

Calcined clay A clay which has been heated to high temperature, bringing about chemical and structural changes.

Metakaolin A dehydroxylated kaolinite produced by partial calcination.

Introduction

Despite their ubiquitous nature and familiarity, clays are complex mixtures of variable composition and not at all easy to fully define.

Clays are just one form of metal silicate, which is called a phyllosilicate. Silicates can have many structures, depending on how the silica lattice is organized. In the phyllosilicates there are infinite sheets of silica tetrahedra with three of the four oxygens of each sharing with other tetrahedra. The basic structural unit is thus Si2O5~2. The sheets are then connected to each other by layers of cations, which balance the charge. These cations are often accompanied by water molecules and other neutral atoms or molecules trapped between the sheets. The nature of this interlayer is the principal differentiating factor between phyllosilicates and has a significant effect on their final structure and properties. The phyllosilicates themselves can be subdivided into various types such as micas, talcs, asbestos (serpentine), chlorites, and clays, many of which have important polymer applications.

The clay minerals are a general group within the phyllosilicates in which the balancing cations are predominately aluminum (in the form of oxide or hydroxide), but which also contain large percentages of water trapped between their silicate sheets. Thus, they are described as hydrous aluminum phyllosilicates. While there are a large number of clay minerals, the most important ones for general polymer applications are based on the kaolinite subgroup; this has three members (kaolinite, dickite, and nacrite) all with the formula of Al2O3 .2SiO2 .2H2O. These three minerals are polymorphs, meaning that they have the same chemistry but different structures. The general structure of the kaolinite group is composed of silicate sheets (Si2O5) bonded on one side to the outside of aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. As a result, a kaolin particle has two different faces, one with surface oxygens linked to silicon and the other surface hydroxyls linked to aluminum. These particles can stack together into “books” by hydrogen bonding between these two faces. This hydrogen bonding is significantly stronger than the van der Waals forces between layers in talc and thus kaolin is a harder mineral.

The remainder of this chapter will focus on kaolinite-based clays (often also referred to just as kaolin). Clays which are rich in kaolinite are frequently referred to as china clays. Ball clays are a subset sometimes met, especially in Europe and some parts of the USA. They are formed from the weathering and transportation by water of parent rocks which are deposited in ancient river basins from where they are extracted. They are still kaolinitic, sedimentary, clays and can contain 20-80% kaolinite, with mica (10-25%) and quartz (6-65%) as major other components. Ball clays are fine-grained and plastic in nature. Ball clays occur in relatively scarce deposits due to the combination of geological factors needed for their formation and preservation. They are mainly mined in parts of the Eastern USA and from sites in South West (SW) England. While they are mainly used for ceramics production, they are also found as fillers for rubber. Ball clays give significantly finer, higher specific surface area, polymer products than do the other kaolin clays.

One of the characteristics of clays is that the bond between the fundamental metal silicate sandwich layers is relatively weak, allowing some cleavage, or exfoliation, to generate plates of varying aspect ratios. Some clay types more readily exfoliate than others, with one type, the montmorillonites, being particularly easily treated in this way. These are the basis of nano-clay technology which has received considerable scientific and commercial interest over the last two decades. This is covered in ? Chap. 23, “Nanofillers”.

 
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