Fillers for Paints E-Book

Detlef Gysau

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Serie: European Coatings LIBRARY
• Sprache: Englisch

Seitenzahl: 282

Veröffentlichungsjahr: 2017

Beschreibung

Explained clearly and simply, without sacrificing scientific expertise or attention to detail, this book enables the reader to speedily gain a comprehensive overview of the working mechanisms and possible application areas of the most common fillers. Now available in its 3rd edition, the book has been updated with the latest market data, information about new fillers and an outlook on future trends, such as sustainability and light weight fillers.

Leseprobe

Detlef Gysau

Fillers for Paints

Fundamentals and Applications

3rd Revised Edition

Cover: Tiberius Gracchus/Fotolia

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Detlef Gysau

Fillers for Paints, Fundamentals and Applications, 3rd Revised Edition

Hanover: Vincentz Network, 2017

European Coatings Library

ISBN 978-3-86630-525-0

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European Coatings Library

Detlef Gysau

Fillers for Paints

Fundamentals and Applications

3rd Revised Edition

For Jacqueline and Gian-Flurin and Mica-Ladina and also Rambo and Fuchur

There is no debt more pressing than the expression of gratitude.

Marcus Tullius Cicero

Foreword

The topic of fillers for use in paints and varnishes is an old one, so one might ask why there has been no comprehensive book on the subject to date. Could it be something to do with the earlier prevailing perception of fillers as cheap materials for bulking up profits? Are fillers even worth writing about? Certainly! The sheer number of mineral end-products, the frequently underestimated effort that goes into their manufacture, the testing done to characterise their diverse properties, their wide-ranging applications – that is an awful lot of information to pack into a single work without diluting its focus.

Simply to consider the spectrum of professions involved in producing and using fillers – geologists, mineralogists, mechanical engineers, machine operators, chemists, paint and varnish specialists – highlights the extent of hidden technical activity. Fillers are instrumental in many properties of coating materials and films: their rheology, content of volatile organic compounds, solids content, brightness, opacity, reflectivity, adhesion, anti-corrosion characteristics, mechanical and chemical resistance… the list goes on. The bottom line is, proper use of fillers calls for a great deal of knowledge.

The present book sets out to convey that knowledge in a straightforward and understandable manner, without compromising scientific objectivity and rigour. Special attention has been given to clear topical division and structuring, to facilitate finding pertinent information, fast. That having been said, the gamut of available fillers is so vast that there would be insufficient space to cover all the materials out there, some of them quite exotic. Instead, this book concentrates on fillers in regular current use, with numerous figures and tables to illustrate their properties and applications. All the same, this book cannot claim to be exhaustive in scope. Readers wishing to obtain further information and details will be served by the extensive bibliographic references provided.

This book is intended for anyone who is in any way professionally involved with fillers used in coating materials. Beginners and students will gain a comprehensive overview of the field, while experienced developers will find practical details of immediate relevance to solving their everyday problems.

In 2016 I was notified that also the second edition of “Fillers for Paints” is going to be sold out soon as well. I am more than delighted to learn that also the second edition found so many new readers. The continued interest in my book is also judged by manifold feedbacks which I received since 2006. All of them expressed to me their thanks and congratulations by filling a knowledge gap in raw materials for paints.

In particular, I appreciate that the book supports training for all different kind of groups, either in industry or science. The third edition allowed me to place small corrections, update market and filler data and add more sub chapters about new fillers and nevertheless an outlook about the future, for example sustainability and light weight fillers.

Detlef Gysau

Oftringen/Switzerland, January 2017

Contents

1 Introduction

1.1 Historical overview

1.2 Filler market

1.3 Definition of fillers and pigments

1.4 Classification of fillers

1.5 References

2 Mineralogy

2.1 Carbonates

2.1.1 Calcium carbonate

2.1.2 Dolomite

2.2 Silicas

2.2.1 Quartz

2.2.2 Cristobalite

2.2.3 Kieselguhr

2.3 Silicates

2.3.1 Talcum

2.3.2 Kaolin

2.3.3 Mica

2.3.4 Feldspar

2.4 Barium sulphate

2.5 References

3 Production of fillers

3.1 Production of natural fillers

3.1.1 Prospecting

3.1.2 Mining

3.1.3 Processing

3.2 Synthetic fillers

3.2.1 Precipitated calcium carbonate

3.2.2 Precipitated barium sulphate

3.2.3 Modified calcium carbonate

3.2.4 Synthetic silicic acids

3.2.5 Precipitated aluminium silicate

3.3 Surface treatment of fillers

3.4 References

4 Characterisation of fillers

4.1 Filler testing

4.1.1 Optical properties

4.1.2 Morphology

4.1.3 Physical properties

4.1.4 Chemical properties

4.2 Filler analytics

4.2.1 Scanning electron microscopy

4.2.2 Spectroscopy

4.2.3 Chromatography

4.2.4 Further methods

4.3 References

5 Properties of fillers

5.1 Carbonates

5.1.1 Natural calcium carbonate

5.1.2 Precipitated calcium carbonate

5.1.3 Modified calcium carbonate

5.1.4 Dolomite

5.2 Silicates

5.2.1 Talcum

5.2.2 Kaolin

5.2.3 Mica

5.2.4 Feldspar

5.2.5 Precipitated aluminium silicate

5.3 Silicas

5.3.1 Quartz

5.3.2 Cristobalite

5.3.3 Diatomaceous earth

5.3.4 Pyrogenous silicic acid

5.3.5 Precipitated silicic acid

5.4 Barium sulphate

5.4.1 Natural barium sulphate

5.4.2 Precipitated barium sulphate

5.5 Aluminium hydroxide and other mineral fillers

5.6 Organic fillers

5.7 References

6 Applications of fillers

6.1 Importance of fillers in paints and coatings

6.2 Important formulation parameters

6.2.1 Non-volatile matter

6.2.2 Spreading rate

6.2.3 Pigment volume concentration

6.2.4 Critical pigment volume concentration

6.2.5 Pigment/filler loading

6.2.6 Packing density

6.3 Filler influences on coating materials

6.3.1 Dispersibility

6.3.2 Rheology

6.3.3 Wet hiding power

6.3.4 Storage stability

6.4 Filler influences on coatings

6.4.1 Hiding power

6.4.2 Colour properties

6.4.3 Reflectivity

6.4.4 Mechanical properties

6.4.5 Chemical resistance

6.4.6 Outdoor durability

6.5 References

7 Trends

7.1 Nanotechnology

7.2 Forms of delivery

7.3 Sustainability

7.4 Light weight fillers

7.5 References

Examples for guide formulations

List of filler examples

Author

Index

1 Introduction

1.1 Historical overview

Paints and varnishes have a history that goes back around 100,000 years, to the time when stone age peoples applied red body-paint as part of their cultish rituals[1]. The first paintings on cave walls date back to the late Stone Age, their origins still somewhat shrouded in mystery. Many thousands of years later, in the 4th century B.C., the intermingling of ancient Egyptian and Greek civilisations brought remarkable developmental advances through “Hagia Techné” or “Alchimia” – hallowed arts practiced by the high priests of the day. Their discoveries about the secrets of paint making remained influential well beyond the 16th century A.D. As the industrial revolution started in the 18th century, paints and varnishes came into widespread use for many different applications. Early 20th century triumphs of chemistry and technology in particular signified a clear departure from empiricism, to science.

The history of fillers can be traced back almost as far as paints and varnishes. Pigment analysis has revealed the presence of filler materials in early cave paintings[2, 3], the oldest identifiable specimens dating from 20,000 to 30,000 years ago, see Table 1.1p.14.

However, the first people to systematically use fillers for their cave paintings were the ancient Egyptians, and the Mediterranean cultures that succeeded them. The most frequent materials were chalk and gypsum, both white mineral fillers. Clays, or crushed mollusc shells, were also used on occasion. As history progressed, the ancient Greeks began using a mineral that was whiter still: white lead. Because of its rare occurrence in nature, they developed an intricate process to obtain the pigment synthetically. Contemporary demand for greater opacity and brightness evidently made the effort worthwhile. The Roman historians Pliny and Vitruvius respectively reported eight and five white pigments then in use, although only three were of real significance: the minerals melinum, paraetonium and cerussa (white lead).

During the period of the Roman Empire, there was a marked increase in the consumption of fillers, which were used in paints for murals, panels and frescoes. But filler production collapsed along with the Roman Empire, and artists subsequently resorted to local minerals. There were large chalk deposits in England, France, the Netherlands and Germany. Even in Spain, chalk grew prevalent under the name of Spanish white. In Italy, though, gypsum predominated. That was the situation until the 19th century, when the industrial revolution came into full swing.

The enormous increase in consumption of raw materials during the industrial revolution also brought a sustained rise in demand for fillers. Semi- and fully-automatic dressing processes were developed to address this demand, as well as to meet the steadily advancing requirements of industry. High-power machinery like crushers, grinding drums and classifiers came into use. The end of the Second World War brought even greater demand for fillers, which was a motor for further modernisation by the filler industry. The resulting technical developments led to ever-finer natural fillers and tailor-made synthetic fillers, some with surface coatings, see Figure 1.1p.14.

Table 1.1: Natural and synthetic white minerals used by the ancients

Ancient name

Modern name

Mineral composition

Cerussa(1,2)

White lead, Krems White, etc.

Basic lead carbonate, made from metallic lead and vinegar

Creta anularia(1,2)

Crete white

Chalk mixed with powdered glass

Cimolia creta(1)

Kimolos chalk

Chalk or clay-like material

Creta eretria(1)

Probably a white talc, named after a place on the southwest coast of Euboa

Creta selinusia(1,2)

Selinus chalk

Chalk or chalk clay, named after a place on Sicily

Melinum(1,2)

Melian white

Bianca San Giovannini or white clay

Paraetonium(1,2)

White sepiolite

Limestone chalk with some magnesium phosphate, silicic acid and clay, named after a place in Libya

Creta argentaria(1)

Argentiferous chalk

Chalk

(1) Pliny, Natural History, XXXV (2) Vitruvius, Ten Books on Architecture, VII

Figure 1.1: Overall European production of paints and varnishes, in millions of tons

source: The Chemical Economics Handbook – SRI Consulting 2009; World Paint & Coatings Industry Association (WPCIA) – Annual Figures

1.2 Filler market

The market often underrates fillers, on account of their relatively low price compared to the other raw materials used for making paints and varnishes. Overall Global and European show a continuous growth since 1997. Once believing the prognosis for the global paint and coatings market, then the number will almost double from 1997 to 2018 to approx. 47 Mio tons. Despite the growth for the production in Europe, its global share drops from 32.0 % in 1997 versus a much stronger growth in emerging markets such as Asia to 23 % in 2018.

If one compares the four million-plus tons of fillers consumed in 2003 with the quantity of paints and varnishes produced in that year, their 42 percent statistical share makes it clear that fillers are the dominant class of raw materials used in paint and varnish production, see Figure 1.2p.15.

The chart of mineral fillers in current use reveals another dominance: natural calcium carbonate is the basis for three quarters of all the fillers used in paints and varnishes. Carbonate fillers together have an 85 percent aggregate share. This profile of mineral filler consumption is essentially repeated on other continents as well, see Figure 1.3, p. 16.

Figure 1.2: Statistical share of fillers in the overall composition of European paints and varnishes

An analysis of application areas reveals that most fillers go into architectural paints, in particular emulsion paints. This group of paint systems is far and away the largest, at around 60 percent of overall paint and varnish production. Empirically speaking, classical and contemporary coating systems both tend to use considerably less fillers, or indeed dispense with them altogether. These systems generally are formulated below the critical pigment volume concentration (CPVC), which necessitate a higher proportion of pigments in order to achieve sufficient opacity.

1.3 Definition of fillers and pigments

There are numerous differences in the properties of fillers and pigments. Yet they can also overlap, depending on the application. Therefore, it is important to draw a clear distinction between these two groups of raw materials. Help is provided by the sets of standard specifications published by the German standards institution (DIN)[4, 5], the European Committee for Standardisation (CEN) and the International Organisation for Standardisation (ISO)[6].

According to DIN 55943, EN 971-1 and ISO 3262 part 1, “a filler is a substance consisting of particles which is practically insoluble in the application medium and is used to increase volume or to improve technical properties and/or to influence optical properties.” The standards discourage the use of terms like “extender”, “extender pigment”, or “pigment extender”, instead stating that “on this basis, whether a substance should be regarded as a filler or a pigment is determined by its application.”

Pigments are defined in the German standards DIN 55943 and DIN 55945: “A pigment is a substance consisting of particles which is practically insoluble in the application medium and is used as a colorant or by virtue of its corrosion-inhibiting or magnetic properties.” Pigments may be described more precisely, for example as inorganic or organic pigments, coloured pigments, white pigments, effect pigments, anti-corrosion pigments, magnetic pigments, etc. depending on their chemical composition, optical or other technical properties.

Figure 1.3: Percent share of fillers in Europe, categorised by mineral

Table 1.2: Summary of filler categories and materials for coating applications

Filler primary category

Natural fillers

Synthetic fillers

Carbonates

Natural calcium carbonate (GCC) Dolomite Barium carbonate

Precipitated calcium carbonate (PCC) Modified calcium carbonate (MCC)

Silicas (silicon dioxides)

Quartz Cristobalite Kieselguhr Diatomaceous earth Glass powder Ground pumice

Pyrogenous silicic acid Precipitated silicic acid

Silicates

Talcum Pyrophyllite Chlorite Kaolin Mica Feldspar Wollastonite Ground shale Amphibole Perlite

Precipitated aluminium silicate Precipitated calcium silicate Precipitated calcium-aluminium silicate Precipitated sodium-aluminium silicate

Sulphates

Natural barium sulphate Calcium sulphate

Precipitated barium sulphate

Oxides

Aluminium hydroxide Magnesium hydroxide

Organic fillers

Wood flour Cork flour

Micro hollow spheres Cellulose fibres

They are categorised according to DIN 55944. Older terms like “colouring body”, “lake dye”, “earth pigment” and “mineral pigment” should no longer be used.

Practically speaking, material constants like the refractive index often determine whether a substance is acting as a pigment or a filler. This is usually apparent from the optical effect of the substance as a component of the coating material. If the substance helps to increase opacity, then it has the characteristics of a pigment. If it behaves transparently, though, it is considered to be a filler. In general, materials with a high refractive index (≥ 1.7) are pigments. All other mineral materials with a similar refractive index, like organic polymers, belong in the category of fillers.

1.4 Classification of fillers

Given the diversity of mineral fillers, it is helpful to divide them into various categories. Categories like carbonates, silicates, silicas (silicon dioxides), sulphates, oxides and organic fillers include well known as well as more obscure materials. In addition to this type of categorisation, fillers are also grouped according to their natural versus synthetic origin.

Not all of the fillers listed in Table 1.2 are (as yet) industrially significant. Although they have been listed here for the sake of completeness, they will not be covered in subsequent chapters of this book.

1.5 References

[1] Pietsch, E., Altamira und die Urgeschichte der chemischen Technologie, Deutsches Museum Abhandlungen und Berichte 31, booklet 1, p. 15, 1963

[2] Science News 125, 348, 1984

[3] Tegethoff, F. W., Calcium Carbonate – From the Cretaceous Period into the 21st Century, p. 55 ff, Birkhäuser Verlag, Basel, 2001

[4] DIN-Taschenbuch 49, Farbmittel, 1. Pigmente, Füllstoffe, Farbstoffe, DIN 5033-1 to DIN 55929: Normen, Beuth, Berlin, 1993

[5] DIN-Taschenbuch 157, Farbmittel, 2. Pigmente, Füllstoffe, Farbstoffe, DIN 55943 to DIN 66131: Normen, Beuth, Berlin, 1993

[6] ISO Standards Handbook, Paints and varnishes, Vol. 3 – Raw materials, ISO 150 to ISO 14900, International Organisation for Standardisation, Geneva, 2002

2 Mineralogy

Current estimates put the age of planet Earth at 4.55 to 4.66 billion years. Industrial minerals are extracted from the Earth’s continental crust, which exhibits a complex, planetwide pattern of belts and mountains with deformed rock series. Somewhat younger than the planet itself are the Earth’s minerals. This chapter discusses them in a competent, concise and understandable way.

2.1 Carbonates

Constantly changing continental surfaces, and the rate of the oceans’ spread, all bore directly on the sedimentation rate of carbonates. Calcium and magnesium were increasingly exchanged between the oceanic crust and seawater as new areas of seabed emerged around the oceanic ridges. Tectonic movement, metamorphosis and volcanic activity also aided the process, making the Earth’s atmosphere progressively warmer and richer in carbon dioxide. This climatic change accelerated the carbonate weathering process, releasing calcium. Tectonic movement also created shallow seas on the continents, which in turn encouraged carbonate sedimentation. Conditions for the formation of carbonate rocks were optimal during the Cambrian period, from the late Devonian to the early Carboniferous, the Permian, Triassic, Jurassic and Cretaceous periods. The largest carbonate formations date from these eras in geological time.

Figure 2.1: The carbonate cycle

Table 2.1: Physical properties and identification numbers of principal carbonates used by the filler industry

Mineral

Chemical formula

CAS no.

Density [g/cm3]

Refractive index

Mohs hardness

Natural calcium carbonate

CaCO3

1317-65-3

2.7 to 2.9

1.59

3 to 4

Dolomite

CaMg(CO3)2

16389-88-1

2.85

1.62

3.5 to 4

The carbonates in principal use as industrial fillers are calcium carbonate and magnesium carbonate. Other carbonates and hydrogen carbonates also exist in association with potassium, sodium and ammonium cations. They are generally used by the food industry as additives, and will not be discussed further here.

2.1.1 Calcium carbonate

Calcium carbonate[1 – 5] has the chemical formula CaCO3. It is the primary constituent of limestone, which is formed by sedimentation of particulate matter. In fact, limestone can be formed in three distinct ways: by chemical precipitation (usually in fresh water), by a biochemical process, or by organogenic sedimentation. The latter two occur in the oceans, due to the high concentration of salt in the water. Various crystal modifications may arise, depending on the kind of organisms involved and the water temperature. The solubility of calcium carbonate in water depends very much on its temperature, and the amount of dissolved carbon dioxide present. Warm ocean water contains little dissolved calcium carbonate: higher temperatures cause the carbonate to precipitate out of solution, eventually forming reefs. In northern latitudes as well as in deep waters, ocean temperatures are low, so the carbon dioxide content is considerably higher. That means the concentration of dissolved calcium carbonate can rise by a hundredfold and more. Chalk shells dissolve entirely beyond a certain depth,3.5 to 5 km depending on latitude. This is known as the carbonate compensation depth, or CCD.

Equation 2.1: Equilibrium reaction for solubility of calcium carbonate in water

Natural calciumcarbonateis largely formed byaprocess of organogenic sedimentation, also known as bioclastic sedimentation. Here, the inorganic remains of invertebrate species like molluscs, corals, sponges, ectoprocta, echinoderms, foraminiferida and algae form deposits on the seabed. The deposited particles can vary greatly in size, depending on the timescale of the deposition process and the degree of compression. The smallest particles are coccoliths measuring just a few micrometers; at the other extreme are whole mollusc shells, several centimetres in size. Pressure causes the sedimentary lime sludge to solidify on ocean floors and form friable sedimentary rock – in other words, chalk. ISO 3262 part 4 defines chalk as a soft, sedimentary rock form of calcium carbonate that results from the chalk formation process. Chalk is typically comprised of microcrystalline calcite crystals with a diameter of around one micrometer. A significant feature that persists even after many millions of years is the mass of shell fragments and skeletons of microscopic marine life forms like coccoliths and foraminifera that chalk contains.

Figure 2.2: Scanning electron microscope photo of ray-like calcite particles constituting the disk-shaped plate of a coccosphere, magnified 6,500 times

source: Omya AG

In warm oceans especially, chalk deposits built up a variety of structures including massive flat reefs up to several hundred metres thick. One such reef limestone belt was formed some 150 million years ago in the late Jurassic period and stretches from the Paris basin all the way to southern Germany. Yet the earliest limestone formations are much older still, with sedimentary deposits dating back as far as the Precambrian era, three billion years ago. Table 2.2 provides a historical perspective.

When a carbonate sediment is formed, it is gradually compacted by the build-up of pressure from layers above, and becomes less porous. The second step is cementation: under increasing pressure, countless pores begin to fill up with precipitated calcite. These two steps are collectively known as diagenesis, a term used to describe the transition from sedimentary sludge to microcrystalline chalk to microcrystalline, dense limestone over many millions of years. Limestone with fine crystalline cement is known as micrite, or microcrystalline calcite. Geologists also call coarse crystalline calcite a sparite. Diagenesis can also produce dolomitisation (see Chapter 2.1.2). Deposits of red chalk, sandstone, quartz, phosphate, pyrite and carboniferous chalk are the result of mineral impurities mixed in with the pure limestone. Limestone purity is therefore rated by categories A through D, classified according to ISO 3262 part 5.

Figure 2.3: Crystalline structure of a single calcite rhombohedron

Table 2.2: Geological timeline for limestone formations

The formation of marble requires radical conditions, which can only be achieved under metamorphosis. The transmutation from chalk or limestone to marble takes place under very high pressures in excess of 1,000 bar, coupled with high temperatures between 200 and 500 °C. Limestone melts under these conditions, before returning to the solid state in a very slow recrystallisation process. Transmutation incidentally increases the purity of the calcium carbonate. The resulting fine to coarse-grained metamorphic rock contains over 50 percent by volume of calcite. Perhaps the best-known deposit of extremely white marble is at Carrara, in Italy. The marble mountains date back some 220 million years, a time when the approaching European and African continents caused thick formations from the Tuscan Nappe to slide over an existing limestone deposit. The deposit was thus covered to a depth of five to ten kilometres and subjected to temperatures of around 300 °C over a long period. Recrystallisation ended just 15 million years ago. A final tectonic squeeze caused what are perhaps the finest marble mountains anywhere to bulge back up to the Earth’s surface. Not only did metamorphosis cause the limestone to transmute into marble, it also transformed the impurities present into new minerals: wollastonite (a form of calcium silicate), quartz, muscovite and phlogopite (both mica), amphibole, diopside, serpentine, graphite, pyrite, marcasite, chalcopyrite and feldspar, to name some examples.

Calcium carbonate occurs in three distinct crystal modifications, so it is also referred to as a polymorphous, variform compound. The three crystal modifications are: calcite[6], aragonite and vaterite. Calcite, with its trigonal crystal system, is predominant in nature. It also happens to be the most multi-faceted, multi-shaped mineral of all, with a crystal lattice of elementary cells in the form of a rhombohedral prism. In other words, a calcite crystal is delineated by six equivalent (rhombic) surfaces, and appears like a diagonally stretched cube. It was this observation that led to the discovery of the fundamental laws of mineralogy by Abbé Haüy in the 18th century. Calcite crystals occur most frequently as hexagonal prisms and scalenohedrons, whereas the fundamental rhombohedralform is rarer in nature.

Aragonite[7], the scarcer rhombic crystal modification, is named after the northern Spanish region of Aragon. It occurs as a cyclic polycrystal in three characteristic crystalline forms, twins and pseudohexagonal trillings being the most common. Aragonite has a characteristic needle or slab shaped appearance, often with pointed ends.

Figure 2.4: Crystalline forms of calcite – a) rhombohedron, b) scalenohedron, c) prism

Vaterite, the third crystal modification of calcium carbonate, is named after the German mineralogist and chemist Heinrich Vater. Its hexagonal crystals are unstable, and hence extremely rare in nature. However, they can be encouraged to form by synthetic precipitation under certain controlled conditions. Vaterite crystals are normally small and fibrous; fine, microscopic platelets are a rarer variant.

As mentioned previously, calcium carbonate is an abundantly occurring mineral. Therefore, only selected examples of well known, industrially viable deposits are listed here. Chalk is found in the Champagne, near Lille and Saint-Omer in France; in the eastern British Isles; Mons in Belgium; Fakse on Jutland in Denmark; near Malmö in Sweden; in Lägerdorf and on the Baltic island of Rügen in Germany; as well as in Poland and Russia. There are limestone deposits in Spain near Zaragossa, Belchite and Granada; on the Greek Ionian islands; at Orgon near Avignon in southern France; in the Friulian Alps and the Abruzzi region of Italy; in Germany’s Swabian and Frankish Albs; as well as on Java in Indonesia. Marble deposits exist in Tautavel in the Spanish Pyrenees; Vizarron in Mexico; Kanwon in South Korea; Carrara, Massa, Laas and Sterzing in Italy; Macael in Spain; Estremoz in Portugal; Middlebury and Danby in Vermont, Cristo in California (USA); Gummern and Graz in Austria; Elnesvagen and Bodö-Fauske in Norway; Pargas and Lappeenranta in Finland; in Sweden; and in the Chinese provinces of Liaoning and Jilin.

Figure 2.5: Crystalline forms of aragonite – a) monocrystal, b) twin, c) hexagonal trilling

2.1.2 Dolomite

Dolomite[8], named after the French mineralogist Dolomieu, is a carbonatic sedimentary rock that has much in common with calcium carbonate. Chemically, dolomite consists of CaCO3 and MgCO3 in a roughly stoichometric ratio. For this reason, dolomite is occasionally referred to as a double carbonate [CaMg(CO3)2]. There may be a slightly higher proportion of calcite (CaCO3) or magnesite (MgCO3), depending on the source.

Dolomite forms in a diagenetic environment on chalk-containing sludge or organic layers on salt-water ocean beds. In the dolomitisation process, calcium ions in the crystal lattice are exchanged for magnesium ions in an equilibrium reaction. The equilibrium point depends on the concentration of calcium and magnesium in the solid rock as well as the surrounding environment. Cation exchange may occur during sedimentation in magnesium-rich seawater, or in a subsequent, secondary dolomitisation process as magnesium-containing water filters through previously formed rock. Dedolomitisation can occur in just the same way, although this is very rare. These processes also affect the rock’s porosity and density, since the density of dolomite at 2.87 g/cm3 is higher than that of calcite. Dolomite, with a Mohs hardness of 3.5 to 4, is also a harder material. Like calcite, dolomite has a trigonal crystal system. The rhombohedrons are fully fissionable. The crystal aggregate is dense and usually fine to coarse-grained.

Dolomite has other important distinguishing criteria too, analogous to calcite. These include its structural fineness, texture, and type of constituents. The details have already been discussed in Chapter 2.1.1. Like calcium carbonate, dolomite is also classified by its purity. ISO 3262 part 7, defines just three categories from A to C.

In rare cases, magnesium may be partially or fully substituted by other cations to produce distinctive varieties of dolomite like ankerite [CaFe(CO3)2] or kutnahorite [CaMn(CO3)2].

Dolomite deposits can be found across many European countries such as France, Germany, Greece, Italy, Norway, Poland, Spain and Sweden. Dolomite is also processed on other continents – America, Asia, Africa – for example in countries like Indonesia, Nigeria and Argentina.

2.2 Silicas

The second most abundant inorganic compound on planet Earth is silicon dioxide, or silica. The English-speaking world considers it a silicate, although German textbooks regard silica as an oxide. The chemical formula for the principal oxide of silicon is SiO2. Silica occurs in many forms, usually crystalline and occasionally in an amorphous state. Nine separate crystalline forms exist, the most frequently encountered being quartz, cristobalite und tridymite. Siliceous rocks are another form of silica. These contain over 50 percent amorphous and crystalline opals and varieties of quartz, for example siliceous earths, and are industrially significant depending on the extent to which they have been compressed. Diatomite, also known as kieselguhr, is one example of a highly compressed siliceous rock, which generally consists of microquartz.

Table 2.3: Physical properties and identification numbers of selected silicas used by the filler industry

Mineral

Chemical formula

CAS no.

Density [g/cm3]

Refractive index

Mohs hardness

Quartz

SiO2

14808-60-7

2.65

1.54

7

Cristobalite

SiO2

14464-46-1

2.32

1.49

6.5

Kieselguhr

SiO2

61790-53-2

1.95

1.46

5.5

Siliceous earth

SiO2

68855-54-9

2.6

1.55

5.5

Neuburger siliceous earth

SiO2

1020665-14-8

2.6

1.55

7/2.5*

*) Content silica/kaolinite

2.2.1 Quartz

The most important modification of silica is definitely quartz[9–11]. It is found in SiO2-rich magmatic rocks like granite, and metamorphic rocks like gneiss and quartzite. Other deposits exist as gangue quartz (ore) in sediments like silica sand and flint, as well as in sedimentary rocks (siliceous rocks, sandstones). From the visual aspect, quartz is usually colourless or white, from transparent through cloudy to opaque. Reddish, yellowish, brownish and other colorations indicate trace element impurities, created in part by radioactive bombardment. The concentration of trace elements is less than 100 ppm[12] as a rule. Inclusions of other minerals, like golden needles of rutile, tourmaline, goethite and haematite are especially frequent in varieties of quartz such as rock crystal. Milky quartz, rose quartz, aventurin quartz, chalcedony, agate, onyx, jasper and opal are some more well known varieties. Quartz is categorised in purity classes A and B according to ISO 3262 part 13.

Crystalline quartz occurs in two modifications. α-quartz with its trigonal crystal system is the most prevalent. At temperatures above 573 °C it turns into β-quartz. The process is reversible on cooling. This phase change is in fact used as a “geological thermometer”[13–15]. Figure 2.6 shows other stable zones for silicon dioxide modifications. Coesite and stishovite are two high-pressure modifications that could be produced in theory, but are insignificant as raw materials.

The α-quartz tectosilicate consists of SiO4 tetrahedrons interlinked by oxygen bridges to produce pseudohexagonal staggered O-rings. Transition to the β-quartz modification simply involves widening the lattice, in other words the O-rings are stretched. Intergrowths are very frequent in alpha quartz, and these lead to twin crystals. Fibrous quartz is a columnar, dense, grained or fibrous aggregate of quartz.

Figure 2.6: Pressure-temperature diagram for modifications of silica

Diatomaceous earth is one of many quartz varieties. It consists largely of SiO2, but often occurs as a mineral blend of quartz and kaolin. As a result, there are also small quantities of iron oxide, titanium dioxide and calcium/magnesium oxide present. Because of the occasionally high kaolin content, the term diatomaceous earth is used interchangeably for kaolins that contain quartz, for kieselguhr and related sediments. There is a well-known deposit of diatomaceous earth at Neuburg an der Donau in Germany, composed of 60 to 90 % siliceous earth and 40 to 10 % kaolin.

2.2.2 Cristobalite

Cristobalite[16] is a mineral that occurs naturally in volcanic rocks and basalts, for example in the Eifel region of the Rhineland Palatinate, and in lunar basalt. In its natural form, it is insignificant as a filler. Cristabolite is turbid with a milky-white colour, and has a tetragonal crystal system.

Cristobalite for use as a filler is made synthetically by thermal modification of quartz.

Figure 2.7: Quartz twin in a trigonal crystal system (hybrid system)

Figure 2.6 shows the phase change from alpha to beta quartz. By adding further thermal energy, β-quartz turns into β-cristobalite at 1,027 °C. Transformation into α-cristobalite occurs on subsequent cooling to 180 to 270 °C. The resulting SiO2 is at least 99 % pure as a rule, although this also depends on the purity of the original quartz. Traces of aluminium and iron oxide are possible. ISO 3262 part 14 defines a minimum cristobalite content of 60 %, with 98 %-plus silica. A more precise classification, like that in general use for mineral fillers, does not exist.

2.2.3 Kieselguhr

This amorphous mineral goes by a variety of names. Apart from Kieselguhr[17, 18], diatomaceous earth (latin: terra silicea) and diatomite are also in common use. Kieselguhr was formed from the seabed sediment of tiny algae remains (diatoms) in the Triassic period. These microscopic life forms, less than 300 µm in size, have highly shaped silicic acid frameworks like disks, cylinders and stars, with numerous deepenings, channels and very fine grooves. This rather special framework gives kieselguhr its low density and finegrained, loose, chalk-like appearance. The framework is often voluminous and porous, which also results in a high specific surface area of 10 to 20 m2/g. Natural deposits of kieselguhr consist of 70 to 90 % amorphous opal A, 3 to 12 % water, plus organic admixtures. Analysis may also reveal traces of iron, aluminium, calcium, magnesium, manganese, titanium,

sodium, potassium, phosphorus and sulphur. The naturally occurring powder is not especially white, and is often light grey in colour. It is also found with a greenish or reddish tint. For this reason, kieselguhr is frequently calcined or flux calcined to give a whiter, purer product. However, thermal processing does not affect its external structure, chemical inertness or porosity, which is due to capillaries and cavities in the silicic acid framework. ISO 3262 part 22 provides information on the requirements for flux calcined kieselguhr. Economically viable deposits are currently being worked in certain states of the USA, for example Lompoc in California. Other mining areas include Queretario in Mexico, the Auvergne region of France, Siebenburgen in Romania, Spain, Denmark, the CIS states, and South Korea.

Figure 2.8: Cristobalite crystal system

Table 2.4: Structural classes of silicates, with examples

Silicate class

Structure

Example

Chemical formula

Nesosilicates

Island silicate with one anion [SiO4]

Phenakite Olivine Zircon

Be2[SiO4] (Mg,Fe)2[SiO4] Zr[SiO4]

Sorosilicates

Group silicate: [SiO4] tetrahedron with defined groups

Thortveitite Hemimorphite

Sc2[Si2O7] Zn4[(OH)2/Si2O7] · H2O

Cyclosilicates

Ring silicate: rings of [SİO4] tetrahedrons

Benitoite Beryll

BaTi[Si3O9] Be3Al2[Si16O18]

Inosilicates

Chain silicate, belt silicate: unlimited single to quintuple chains and belts of anions

Tremolite (amphibole) Sodium waterglass

Ca2Mg5[(OH)2/(Si4O11)2] Na2(Si03)∞

Phyllosilicates

Sheet silicate, layer silicate: [SiO4] tetrahedrons chained together in a single plane → layer lattice

Talcum Kaolinite Muscovite

Mg3[(OH)2/Si4O10] Al4[(OH)8/Si4O