Silicon and Nanotechnology for Coatings E-Book

Stefan Sepeur

• Herausgeber: Vincentz Network
• Kategorie: Fachliteratur
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

Beschreibung

The Mission: Total protection, aesthetics, perfection, maximum functionality – not only are the demands imposed on coatings extremely high, but the base products must also meet sustainability considerations. The key? A fusion of silicon chemistry and nanotechnology. This powerful combination unlocks innovative, high-tech coatings formulations that are poised to conquer entirely new application markets in the future. The Audience: Those engaged in the development, production, testing, and marketing of coatings raw materials will benefit from this deep dive into nanostructured coatings, which promise to break new ground in sustainable coatings technology. The Value: This reader-friendly exploration delves into coatings formulated with silicon-based, inorganically and organically modified binder systems, providing practical insights and step-by-step explanations. Whether you are an expert or a practitioner seeking guidance on nanostructured coatings, this overview of various silane-based materials will equip you for the future of coatings.

Leseprobe

Stefan Sepeur | Gerald Frenzer | Frank Groß

Silicon- andNanotechnologyfor Coatings

Basics and Applications

Cover: celiaphoto – stock.adobe.com

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Stefan Sepeur, Gerald Frenzer and Frank Groß

Silicon- and Nanotechnology for Coatings

Hanover: Vincentz Network, 2024

European Coatings Library

ISBN 3-7486-0730-XISBN 978-3-7486-0732-8

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Foreword

You are reading a copy of our book “Silicon- and Nanotechnology for Coatings”. In it, we present the exciting possibilities that silicon chemistry affords for coatings and a glimpse of what this interesting field of research holds in store for the future, along with of the novel properties which it offers. Silicon-based raw materials have been used in the coatings industry for some time but have often been confined to niche markets.

Since the early 1990s, sol-gel technology and organic modifications of inorganic particles, especially silicon dioxide particles, have provided a way to modify organic binders for coatings by introducing inorganic elements. The field of nanoparticles, in particular, offers an opportunity to incorporate new functions into coating systems, without compromising the transparency of the coatings. We explained this in our first book, “Nano-technology”, and we have integrated that content into this book.

We live in a time of change: climate crisis, soaring prices, dwindling resources and a clear need to transition away from oil. These new conditions pose new global challenges in terms of energy, the economy and alternative sources of raw materials. Against this background, the use of silicon-based materials in coatings systems is growing in importance.

In this book, we classify the different types of silicon-based binder and present examples that are close to implementation and products that have already been commercialised. We show the composition and chemical structures of purely silicon-based, inorganically and organically modified binders, how they are produced and examples of their applications. Step by step, we present the basics of various areas of chemistry, such as glass, ceramics, nanotechnology and sol-gel technology, that make up silicon technology.

Join us, too, on a journey into a new raw materials world that might not answer all future questions but will surely be widely adopted in the next generation of coatings.

In this book, we share many interesting suggestions and ideas as we explore the world of silicon technology, and we hope it will inspire you and convey in some measure the hold that silicon chemistry has over us.

Please feel free to contact us with any questions, suggestions or feedback on the topics covered in the book.

Stay innovative – This book will support you.

Saarbrücken, January 2024

Stefan Sepeur, Gerald Frenzer und Frank Groß

[email protected]

1Introduction to silicon technology

It took a long period of intense development before silicon chemistry or silane technology could be applied to paints and coatings. Key to understanding the possibilities afforded by these technologies is the element silicon. A semi-metal and a member of the carbon group in the periodic table, it has very special properties: in coatings chemistry, silicon serves as a bridge between organic and inorganic chemistry. It provides a very exciting and comprehensive way of uniting different areas of chemistry, such as glass, ceramics, organics and nanotechnology in a single material that possesses multifunctional properties.

Silicon dioxide, SiO2, has been used down the centuries as a coating material in glass-making and, later, in enamelling processes. Today, pure SiO2 coatings are either deposited at high temperatures via PVD (physical vapour deposition) or CVD (chemical vapour deposition) or obtained by melting at temperatures in the range > 850 °C.

Water glasses, as the water-soluble alkali silicates are also called, are sealants for concrete and concrete blocks and binders for silicate paints. Further developments in this area have led to the creation of new coating materials called nano-enamels.

Another special feature of silicon is that it can form stable covalent bonds with carbon. The resulting compounds are called silanes. The basic technology for stabilising silanes and working with them is that of sol-gel technology.

Back in 1990, in their book “Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing”, Brinker and Scherer[1] published a comprehensive tome on how to transfer reactive silanes into coating solutions via the sol-gel process. Such coatings are normally very brittle and are still used today as scratch-resistant paints, for example. Sol-gel technology is the basic technology used to convert silanes into reactive, coatable condensates, which are usually also present in nano-particulate phases.

This brings us to nanotechnology, a so-called cross-over technology. It can be used to target coating systems that possess new and unique properties, but only if the size range is carefully chosen.

Until the early 21st century, incompatibilities between sol-gel materials and known organic-based coatings limited the applications, especially for coatings chemistry. Water and alcohol content, very high pH values and heat sensitivity were long responsible for the fact that the coatings chemist's justified question of “Can I mix this into my paint...?” always had to be answered with “No”.

The late 20th century saw the emergence in the adhesives industry of silane-modified or silane-terminated polymers in parallel with sol-gel technology. They are largely produced by adding isocyanosilane to polyether. Curing is done via atmospheric oxygen and usually in the presence of an acid catalyst [2].

It is also possible, in the case of organically modified silanes, to selectively perform an inorganic polymerisation with a precipitation process, for example, and only then, in a second step, to achieve final curing by means of organic polymerisation [3].

The combination of all these possibilities leads to a new class of materials. Through the correct application of silicon technology, there is obtained a new raw material base that, complementarily, and also on its own, is opening up a new perspective for the materials of the future – a future that has transitioned away from petroleum, a future based on renewable resources combined with 100 % recycling and usually much better durability. If these very possible combinations are to be achieved, we need first of all to really understand each individual building block.

1.1Literature

[1]

C.J. Brinker, G.W. Scherer Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press Inc., Boston 1990

[2]

l. Miszke, L. Zander „Neue silanterminierte Polyether-Polymere“ Adhäsion Kleben & Dichten 59, 7-8 (2015)

[3]

NANO-X GmbH WO 02/50191 A2 „Lösungsmittelarme Sol-Gel Systeme“, 27.06.2002

2Reaction principles of silicon

The fact that silicon is a semi-metal puts it in a unique position of being able to form both ionic and covalent bonds. Coatings chemists can exploit this ability to alter the properties in various ways. Two reaction principles make silicon useful for coatings systems [1]:

Reactions involving or yielding inorganic compounds

Reactions involving or yielding organic (carbon) compounds

Silicon thus serves as the bridge atom between inorganic and organic reaction principles (see Figure 2.1).

Figure 2.1: Model of an organosilane

Inorganic-type reactions usually proceed via the sol-gel process. Silanes are capable of reacting variously with themselves (in an atomic redox reaction), with metal ions (ionic reaction) or with particle surfaces (adhesion reaction).

In the case of organic-type reactions, the organic side-chains render a binder, for example, more flexible, hydrophobic or hydrophilic (network converters). If the side-chains contain functional groups, they may react with themselves or as a copolymer in an organic network (network formers; see Figure 2.1).

Figure 2.2: From silicon to high-performance coating

Figure 2.2 shows the various reactions of which silicon is capable in paints and coatings.

Silicon is the starting point for a plethora of raw materials, known as silanes, that act as intermediates in the synthesis of binders, paint raw materials, functional layers, particles, and finished coatings. The most important silane intermediates for coating raw materials are:

Hydrosilanes

Chlorosilanes

Polysilazanes

Alkoxysilanes

Organoalkylsilanes

Functionalised organosilanes

As explained in Chapter 1, silicon, like carbon, is a special case within the periodic table. It can form bonds with inorganic networks and can also be incorporated into an organic network by reaction with carbon chains. Interestingly, both types of bonding can occur on the same atom. In other words, a silicon atom with its four bonding sites can form both ionic and covalent bonds in parallel. Naturally, this is accompanied by interactions and mutual influencing. These in turn can be used to influence macroscopic variables, such as the curing temperature.

These raw materials can now follow different reaction pathways, separately or in parallel (see Figure 2.2).

Products resulting from these reactions include:

Alkali-modified glass networks

Metal-oxide-modified glass networks

Silanised particles, including metal particles

Particles synthesised in any size, ranging from micrometres to nanometres

Organically-modified glass networks

Silicones

Silane-terminated polymers (“silixanes”)

Functional polysiloxanes; crosslinking into polymer or coating networks

…

As we can see, silane chemistry comes with many processes and possibilities. These are not all necessarily new. Some draw on longstanding connections found in the various fields described in the literature. In other words, if we want to functionalise something, it makes sense to know which established chemical processes will lead to which properties. The first step, then, is to cover the basics of silicon chemistry.

We will follow that with a brief journey through the chemistry of glass, ceramics, and water glass, the sol-gel processes for producing coating materials from silanes, nanotechnology, silicone chemistry and finally coatings chemistry and corrosion protection.

This will afford us a basic grasp of the various possible reactions of silanes and their uses in coatings chemistry. Our goal is to know how to make and use the right multifunctional coatings for any given application. We will describe how silane-based coatings raw materials are synthesised and explain how they complement or serve as substitutes in conventional coatings chemistry.

Taking silicon as our starting point, we show in the next couple of chapters how key coatings raw materials can be synthesised from sand, i.e. silicon dioxide. But first, we need to look at elemental silicon.

2.1Literature

[1]

S. Sepeur, Nanotechnologie: Grundlagen und Anwendung, Vincentz Network, 2008

3Silicon

Figure 3.1: The element silicon

The core atom of any glass or silane is silicon (Si), whose name is derived from the Latin word silex, meaning flint. Silicon lies directly beneath carbon in the 4th main group of the periodic table and has the atomic number 14. Silicon is a classic semi-metal. In its pure form, it is greyish-black and has a typical metallic lustre that often ranges from bronze to bluish [1].

Figure 3.2: Metallic silicon

Elemental silicon is not found in nature. However, it is a constituent of many compounds, especially sand, glass, many types of stone, and minerals. As silicon is the fundamental element underpinning silicon technology, we will now look at it in more detail.

3.1Properties and occurrence of silicon

Silicon is the most common element after oxygen. It makes up some 15 percent of the earth's entire mass. It has a high affinity for oxygen and so never occurs in elemental form in nature, but only bound up in the form of the salts of various silicic acids. These salts are called silicates and have the general formula m SiO2 * n H2O. The anhydride of silicic acids is SiO2, which occurs naturally in various forms, e.g. beach sand, quartz, rock crystal and amethyst. The general characteristics along with the physical and chemical properties of silicon are shown in Table 3.1[2].

Table 3.1: General properties of silicon

General

Name, symbol, atomic number

Silicon, Si, 14, semi-metal

Group, period, block

14; 3; p

Appearance

Dark grey with bluish tint

Mass fraction in the earth's crust

25.8 %

Atomic

Atomic mass

28.085 (28.084 – 28.086) u

Atomic radius (calculated)

110 (111) pm

Also of interest are its physical and chemical properties (see Table 3.2).

Table 3.2: Physical and chemical properties of silicon

Physical

Physical state

Solid

Crystal structure

Diamond

Density

2.336 g/cm3 (20 °C)

Mohs hardness

6.5

Magnetism

Diamagnetic (Xm = -4.1 x 10−6)

Melting point

1683 K (1410 °C)

Specific heat capacity

703 J/(kg K) at 298 K

Electrical conductivity

2.52 x 10−4 A/(V m)

Chemical

Oxidation states

-4, (2) +4

Oxides (basicity)

SiO2 (amphoteric)

Electronegativity

1.90 (Pauling scale)

Elemental silicon is used to make semi-conductors and solar cells and is also the base material for various sensors and other micro-mechanical systems (e.g. the lever arm in an atomic force microscope). Although solar cells have nothing to do with coatings at this juncture in the book, it is important to look at how they work. This is because later sections deal with conductive layers based on TiO2/SiO2, which could well prove to be the starting point for sprayable solar cells in the future.

3.2Silicon as a raw material for solar cells

Silicon is the constituent material of the transistors found in microchips – and solar cells operate on a similar principle. Silicon is a semi-conductor, which means that it does not usually conduct electricity, but can be made to do so in certain circumstances. Silicon is the semi-conductor material used to make the photo-active layer of solar cells. These require high-purity solar silicon, which is fabricated by purifying raw silicon [3]. It may may also be produced by purifying monosilane and decomposing it either on a heated surface or in a fluidised bed reactor (see Equation 3.1).

SiH4 → 2 H2 + Si

Equation 3.1

The resulting polycrystalline silicon (known as polysilicon) is over 99.99 % pure. Each silicon atom is surrounded by four neighbouring silicon atoms in a stable crystal structure. The cohesive forces acting between neighbouring atoms stem from the fact that one electron from each atom forms a shared electron pair with one electron from a neighbouring atom.

Figure 3.3: Silicon crystal lattice with electron pairs (blue)

The regular arrangement of the Si atoms gives rise to a lattice structure (see Figure 3.3). The upper and lower layers of the solar cell have different properties because they are doped with different atoms. A conventional solar cell consists of a n-doped layer (n-layer) which is roughly 0.001 mm thick and is incorporated into a p-doped silicon substrate (p-substrate), which is roughly 0.6 mm thick.

Figure 3.4: n-doped silicon created by doping with phosphorus

Some silicon atoms in the upper layer may be replaced by, e.g. a phosphorus atom (see Figure 3.4). Phosphorus has five electrons in its outer shell. As it can only form a pair bond with four silicon atoms in the crystal lattice, it has a spare electron. This electron is therefore only held very loosely by the phosphorus atom. In fact, the bond is so loose that it breaks at room temperature. The silicon doped with phosphorus therefore has free electrons (negative charges) and is known as the n-doped layer.

Figure 3.5: p-doped silicon created by doping with boron

The lower layer of the solar cell is doped with boron in a similar way (see Figure 3.5). Boron has three electrons in its outer shell, each of which forms a pair bond with the neighbouring silicon atoms. However, the electron needed for forming the fourth bond is missing. This missing electron is also known as an electron hole. At room temperature, an electron can "jump" into this hole from a neighbouring Si atom: the hole looks as if it is migrating. The conductivity of this type of doped silicon is therefore based on the mobility of the holes (positive charges). This zone is known as the p-doped layer. When p- and n-doped layers are in contact with each other, there is formed a p-n junction, at which some electrons from the n-doped layer migrate into the p-doped layer to replace missing electrons in the pair bond. As a result of this exchange of electrons, a specific quantity of negative charge is transferred from the n-layer to the p-layer.

Figure 3.6: A p-n junction

Due to the migration of the electrons from the n-doped layer, it is missing some electrons and so is positively charged. The p-doped layer has a few excess electrons and is negatively charged (see Figure 3.6). This process is restricted to a thin boundary layer only, because the growing negative charge of the adjacent p-doped layer hinders further transfer of free electrons; this is an example of like electric charges repelling each other. The change in the charge ratios in the boundary layer creates an electric field between positive and negative charge carriers. As the charge carriers are fixed in place locally, no current flows. The electric field is represented by parallel field lines extending from the positively to the negatively charged boundary layer (see Figure 3.7).

In a solar cell, the n-doped layer faces the sun. It is kept very thin, compared with the p-doped layer, to allow the energy-laden photons of light to penetrate through to the p-n junction (see Figure 3.7).

Figure 3.7: What happens when light strikes a solar cell

When light strikes the space charge region, it may eject an electron from an atom. The “rump” atom is left positively charged, because it has an electron deficiency, i.e. a hole. This process is known as the internal photoelectric or photovoltaic effect. If it occurs in a region where there are no acting external electric forces, the electron will soon return to the rump atom. In that event, the electron and the hole are said to recombine.

If, however, the photovoltaic effect occurs in the space charge region or its immediate vicinity, the electron may be permanently ejected. The p-layer becomes positively charged due to a lack of electrons while the n-layer acquires a negative charge. If the circuit is now closed, electrons start flowing through the externally connected conductor and charge equalisation takes place. As long as there is incident radiation, electric current can flow. Attached to the upper side of the solar cell is a metal contact strip bearing a number of small contact fingers (negative pole). The lower side has an adhesively bonded continuous metal layer acting as contact (positive pole). The contact strip and the metal layer form the electrical poles of the solar cell (see Figure 3.7).

In the coatings field, the oxides and organic variants of silicon are much more important.

3.3From quartz sand to silane

In the earth's crust, silicon essentially occurs in the form of silicate minerals or as pure silicon dioxide.

Figure 3.8: Rock crystal of SiO2

Silicon dioxide is found in both crystalline and amorphous form. The most common exemplar is quartz, of which there are many natural crystalline variants (see Figure 3.9).

Figure 3.9: Quartz-based gemstones (from left to right), garnet (island silicate), amethyst (purple quartz), bloodstone (heliotrope, quartz with fibrous structure).

Silicon forms silicates with many metals. Silicates are the most extensive class of inorganic compounds, in terms of not only quantity, but also the number of different compounds. This is because, on one hand, the silicon in rocks may be substituted (e.g. aluminosilicates, borosilicates or beryllosilicates) and, on the other, different types of silicate structures exist (e.g. layered, island, ring, chain). Natural waters are also an enormous reservoir of silicon: Considerable quantities of it in the form of monomeric silicic acid are found dissolved in rivers and seawater in low concentrations (see Figure 3.10) [4].

Figure 3.10: Silicic acid

In other words, in terms of occurrence, good durability and natural degradation processes, silicon constitutes an alternative to petroleum and carbon. It is already an integral part of modern coatings chemistry and is set to play a growing role there in the future.

There are various methods for transforming silicon into a useful raw material for coatings. The most common of these will be presented in this book, with examples.

The production of silanes entails laboriously processing quartz gravel to elemental silicon. The gravel is liquefied in an electric melting furnace and reduced with carbon at about 2000 °C. The carbon removes the oxygen bound to the silicon dioxide, forming carbon monoxide.

SiO2 + 2 C → Si + 2 CO

Equation 3.2

Liquid silicon metal remains as the main product.

The raw silicon still needs to be refined. This is done by injecting oxygen or adding slag formers to completely remove inhibitors, such as lead, chromium, and nickel. These elements would severely impair the Müller-Rochow synthesis [see Chapter 4.2.1] for the production of silanes [5].

The liquid silicon must be at least 99 % pure. It is poured onto silicon sand, where it is allowed to cool and solidify. The resulting silicon lumps are ground into particles ranging from 10 to 360 µm. This is the raw silicon that is the starting material for the synthesis of silanes.

3.4Literature

[1]

A.F. Hollemann, N. Wiberg, Anorganische Chemie, De Gruyter, 2016

[2]

www.webelements.com (Silicon)

[3]

W. Vorsatz "Große gewinnen", Photovoltaik, 07 (2009)

[4]

http://webmineral.com/chem/Chem-Si.shtml#.XYm6nrnwBFo

[5]

C. Elschenbroich, Organometallics. VCH, Weinheim, 1992

4Silicon-based raw materials

Silicon-based raw materials are so versatile in terms of their possible reactions that we need to initially narrow the discussion to the paints and coatings sector. Here, there are both technical limitations and market constraints at play. We will therefore focus on processes and compounds that are relevant to applied coatings chemists.

Availability of raw materials

Silanes now enjoy commodity product status and are available for purchase from many reputable manufacturers. With the implementation of the EU regulation on chemicals (REACH), certain types of silane are being withdrawn from the market. The same applies to nanoparticles and other organic molecules. In this book, we will present affordable, available raw materials and, if necessary, illustrate any issues using practical examples.

REACH authorisation

The introduction of REACH has caused many products to disappear from the market because of the high registration costs. As a result, some products are now in short supply. Unfortunately, this is a reality for commodity chemicals, too.

Chemists are attempting to mitigate the tough market conditions by drawing on their creative powers, while highlighting potential problems in the case of certain compounds as a precautionary measure.

The price of raw materials

Silanes and the particles used in conjunction with them command a higher price than other binder systems, because their production is often a complex affair. It always makes sense, therefore, to carry out a cost-benefit comparison and not to lose sight of the sale price of the end product.

Our first look into the “new world” of silane chemistry starts with the simplest raw materials: silanes (hydrosilanes).

4.1Hydrosilanes (silanes) as raw material

Figure 4.1: Tetrahydrosilane or monosilane

Silicon, like carbon, forms a large number of hydrogen compounds of general formula:

SinH2n+2

These compounds may or may not possess branched chains or have ring-shaped structures. Ring-shaped silicon-hydrogen compounds are called cyclosilanes (general molecular formula: SinH2n) [1].

The nomenclature follows that of alkanes. Each name ends with the suffix -ane. The number of silicon atoms is included in the name in the form of a Greek number word, e.g. monosilane (one silicon atom), disilane (two silicon atoms), trisilane etc.

If a silane contains four or more silicon atoms, different configurations or, more accurately, constitutions are conceivable. This is known as constitutional isomerism.

Although silanes are the silicon homologues of the carbon-based alkanes, the number of possible silanes is much lower than that of the hydrocarbons.

Table 4.1: Examples of linear, unbranched silanes [2]

Silane

Molecular formula

Melting pointin [°C]

Boiling pointin [°C]

Densityin[g/cm3]

Molar massin [g/mol]

Number ofisomers

Monosilane

SiH4

-184.7

-112.1

0.00135

32.12

0

Disilane

Si2H6

-129.4

-14.8

0.00266

62.22

0

Trisilane

Si3H8

-116.9

+52.9

0.739

92.3200

0

Tetrasilane

Si4H10

-91.6

+108.4

0.795

122.4214

2

Pentasilane

Si5H12

-72.2

+153.2

0.827

152.5228

3

Hexasilane

Si6H14

-44.7

+193.6

0.847

182.6242

5

Heptasilane

Si7H16

-30.1

+226.8

0.859

212.7255

9

The simplest silanes – monosilane and disilane (Si2H6) – are gaseous. From trisilane (Si3H8) onwards, the silanes are liquids. Decasilane (Si10H22) is a solid.

Unlike the homologous alkanes, silanes are highly unstable. They can be synthesised only in the absence of air. The low silanes, i.e. those possessing one to four silicon atoms, are very unstable and can autoignite, explode, and spontaneously combust in air to form silicon dioxide and water.

Figure 4.2: Reaction of tetrahydrosilane with water and oxygen to form SiO2

This reactivity is very high in the case of monosilane but decreases as the chain gets longer. Pentasilane, for example, does not react spontaneously with atmospheric oxygen. Silanes from heptasilane onwards do not undergo spontaneous auto-ignition.

An unusual property of silanes is that, at elevated temperatures of about 1900 °C, they also react with atmospheric nitrogen to form silicon nitride and water, releasing a great deal of energy [3].

Silanes decompose in water at pH > 7 to silicic acid and hydrogen (Figure 4.3).

Figure 4.3: Reaction of tetrahydrosilane with water at pH > 7 to form silicic acid and hydrogen.

In alkaline medium, however, monosilane spontaneously forms alkali silicates.

Silanes, and chlorosilanes, are the starting point and raw materials for alkoxysilanes. They can also be used to synthesise special silanes or for silane-terminated polymers (“silixanes”). Chlorosilanes rank second to silanes as the most important class of raw materials for the synthesis of alkoxysilanes.

4.2Chlorosilanes

As just mentioned, after hydrosilanes, chlorosilanes are the second important class of raw materials for producing complex silane compounds or organosilanes. The simplest chlorosilane is silicon tetrachloride[4].

Figure 4.4: Silicon tetrachloride

sepeur_abb_4_04

Silicon tetrachloride, also called tetrachlorosilane, is a colourless, volatile liquid that fumes in moist air and consists of the elements silicon and chlorine. It is usually obtained by the chemical reaction of chlorine with hot silicon:

Si + 2 Cl2 → SiCl4

Equation 4.1

When this is further processed to silicon dioxide, hydrogen chloride is generated. Like chlorine, it can also be used to make silicon tetrachloride. This offers an integrated production method that conserves raw materials:

Si + 4 HCl → SiCl4 + 2 H2

Equation 4.2

The resulting hydrogen can be used to produce pyrogenic silica.

Silicon tetrachloride is highly reactive. Unlike the haloalkanes, it hydrolyses in damp air (see Equation 4.3):

SiCl4 + 2 H2O → SiO2 + 4 HCl

Equation 4.3

ilicon tetrachloride reacts with oxidising agents, acids, alcohols, bases, ketones, aldehydes and many other compounds. It is highly corrosive and causes severe irritation of the skin, eyes and lungs. In industry, chlorosilanes are mostly produced by the Müller-Rochow synthesis.

4.2.1The Müller-Rochow synthesis of chlorosilanes

Figure 4.5: Dichlorodimethylsilane

The Müller-Rochow synthesis converts elemental silicon into chlorosilanes. These serve as precursors for the synthesis of organosilanes and polysiloxanes, also known as silicones.

The most important subgroup within the silanes is that of the methylchlorosilanes. These are formed from elemental silicon and oxygen-free chloromethane by direct synthesis [5]. This is a gas/solid reaction that is conducted on an industrial scale in continuously operated fluidised bed reactors. Copper serves as catalyst. It forms active centres where the actual reaction takes place. The raw silicon, catalyst and other promoters are mixed and dosed into the fluidised bed reactor. Chloromethane is then introduced. It acts in concert with the pressure and the temperature to initiate fluidisation of the silicon.

The silicon reacts to afford a mixture of silanes in the form of crystal-clear, colourless liquids.

Table 4.2: Products of the Müller-Rochow synthesis [6]

Silane

Molecular formula

Mass fraction [%]

Boiling point [K]

Dimethyldichlorosilane

(CH3)2SiCl2

75 – 94

343

Methyltrichlorosilane

(CH3)SiCl3

3 – 15

339

Trimethylchlorosilane

(CH3)3SiCl

2 – 5

330

Methyldichlorosilane

(CH3)HSiCl2

0.5 – 4

313

Dimethylchlorosilane

(CH3)2HSiCl

0.1 – 0.5

308

Tetrachlorosilane

SiCl4

< 0.1

331

Tetramethylsilane

(CH3)4Si

0.1 – 1

299

Trichlorosilane

HSiCl3

< 0.1

305

Disilane

(CH3)xSi2Cl6-x

2 – 8

> 380

The target product is primarily dimethyldichlorosilane for the production of silicones. All reaction parameters and the temperature control (257 °C to 357 °C) are optimised to ensure that it is obtained in the highest-possible yield (up to 94 % mass fraction). This is converted to silicone fluids and rubbers in downstream processes.

The Müller-Rochow synthesis yields dimethyldichlorosilane in vast quantities, but there are several other ways to synthesise chlorosilanes, e.g. by chemical addition or substitution [7].

Without going into further detail, Table 4.3 provides an overview of possible substituents and the corresponding substitution reactions [8].

Table 4.3: Substitution reactions at silicon

Starter product

Conversion to

Typical reaction

Si-Cl

Si-C

Reaction with carbanions

Si-H

Si-C

Reaction with carbanions or organic halogen compounds

Si-C

Si-Cl

Catalytic conversion with hydrogen chloride

Si-H

Si-Cl

Catalytic conversion with hydrogen chloride

Si-Cl

Si-H

Reduction with hydridic hydrogen

4.2.2Uses of chlorosilanes

Chlorosilanes have myriad uses. Trichlorosilane is an intermediate in the production of high-purity silicon for integrated circuits (microchips). Pyrogenic silicas, which can be produced from chlorosilanes and chloroalkylsilanes by reaction in an oxyhydrogen flame, are important fillers for plastics and popular raw materials for the coatings industry where they are employed as matting agents and for adjusting viscosity. They also serve in combination with organofunctional silanes as raw materials for coatings.

Chlorosilanes are furthermore raw materials for the production of organosilicon compounds.

4.3Organosilicon compounds

Figure 4.6: Methyltrimethoxysilane

Replacement of the hydrogen in the silanes by organic groups affords organosilicon compounds, which are deemed by IUPAC to be derivatives of silicon. Organosilicon compounds were described and studied back in the mid-19th century [9, 10]. Compared with their carbon counterparts, the silanols and siloxanes, too, are highly stable silicon compounds.

As silicon comes below carbon in the periodic table, the alkylsilanes, which are derived from silicon hydrogen Si-H, behave similarly to their hydrocarbon analogues. And, similarly, the carbon can be linked to the silicon via oxygen, nitrogen or sulphur atoms [11]. Organosilicon compounds are described by the general formula:

RnSiX4−n

(n has a value of 1 to 4)

where R represents various organic groups, such as aliphatics, aromatics and heterocycles, and X stands for different groups (see Table 4.4) [11–13].

Table 4.4: Examples of organosilicon compounds

Substituent X

Substance group

Remarks

H or R

Organosilanes

E.g. tetramethylsilane, used in NMR spectroscopy as an (internal) standard to determine the chemical shift

OH

Organosilanols

E.g. trimethylsilanol, a water-proofing agent

Cl

Organochlorosilanes

Serve, among other things, as water-proofing agents in the construction chemistry; organohalosilanes are ofindustrial importance as starting materials for silicone polymers

Si–O,alternating

SiloxanesSilicones

E.g. hexamethyldisiloxane, (H3C)3Si−O−Si(CH3)3

Si–N,alternating

Polysilazanes

E.g. hexamethyldisilazane,(H3C)3Si–NH–Si(CH3)3

Si–C,alternating

Carbosilanes

E.g. (H3C)3Si–CH2–Si(CH3)3

Si-O-R

Alkoxysilanes

E.g. methyltrimethoxysilane

The application areas listed in the table are not the only ones in which organosilicon compounds are used [14]. The resistance to heat and cold as well and the high chemical resistance of organosilicon compounds stem from the inorganic moiety by virtue of its relatedness to SiO2. The organic moiety confers polymer-like behaviour, e.g. plasticity and water-repellency. Specialty silanes, called functional organosilanes, are used to functionalise surfaces in a process known as silanisation.

Organosilanes, silicones and polysilazanes in particular serve as raw materials for modern functional coating systems.

4.3.1Polysilazanes

Polysilazanes are polymeric compounds composed of Si-N structural units [15]. Often, each silicon atom is bonded to two nitrogen atoms and each nitrogen atom is bonded to two silicon atoms, so that molecular chains and rings having the formula

[R1R2Si–NR3]n

are formed. These materials can be converted into ceramics, such as Si3N4, SiON, SiCN, SiCNO or SiC, by pyrolysis at temperatures above 400 °C. The degree of conversion depends on the chemical composition of the polymer and the atmosphere. As a result of this ability, polysilazanes serve as precursors for ceramic coatings. Polysilazanes also possess excellent properties, however, that allow them to serve as coatings in the polymeric or pre-ceramic stage. Polysilazanes offer the following advantages over most organic polymers:

Better thermal and chemical stability

Greater hardness and scratch resistance

Stronger coating adhesion than typical organic coatings

Very high heat resistance: polysilazanes withstand up to 1000 °C without substantial loss of mass – unlike silicone resins, which lose substantial mass above 400 °C

Water and dirt repellency

Corrosion prevention

Generally speaking, there are two classes of polysilazanes (Figure 4.7).

Figure 4.7: Polymer structures for PHPS and OPSZ, where R is CH3-, CH2=CH- or other organic groups

One is inorganic perhydropolysilazanes (PHPSs) that feature H atoms on both Si and N atoms. The other is organic polysilazanes (OPSZs) that have organic functional groups attached to Si atoms. PHPS is an important gap filler and planarisation material in the semi-conductor industry. OPSZ has emerged as a leading technology for durable surface protection [16]. Both OPSZ and PHPS are commercially available [17].

The synthesis of polyorganosilazanes was first described by Krüger and Rochow in 1964 [18]. The reaction is carried out with dichlorosilanes and ammonia in conditions that depend on whether PHPS or OPSZ is to be produced.

Like all polymers, polysilazanes are composed of one or more basic units (monomers). These monomers can be strung together to afford chains of different sizes, rings, and three-dimensionally crosslinked macromolecules having a more or less broad molecular weight distribution. The monomer also serves to describe the chemical composition and the linkage of the atoms (the coordination sphere), but that information reveals nothing about the macromolecular structure [15].

In polysilazanes, each silicon atom is bound to two nitrogen atoms and each nitrogen atom is bound to at least two silicon atoms. When all the remaining valences are saturated by hydrogen atoms, there is obtained perhydropolysilazane [H2Si–NH]n. Organopolysilazanes feature at least one organic group attached to the silicon. The number and type of groups exert a significant influence on the macromolecular structure of these polysilazanes [19].

Although polysilazanes have been known for a long time and were recognised at an early stage as offering great application potential, only a few products have managed to reach market maturity. Likely one of the reasons is the high development costs involved in using these comparatively “expensive” chemicals. The unpleasant, pungent smell of ammonia released during curing naturally is another.

Polysilazanes can serve as coating materials and some of them exhibit outstanding properties in certain applications [20]. However, the toxic ammonia released during curing and the intrinsic expense of the raw material certainly limit the range of applications considerably. Nevertheless, it is an interesting raw material with highly promising prospects. In the next section, we will look at the most important class of silane-based raw materials: the alkoxysilanes.

4.3.2Alkoxy and acetoxysilanes

Alkoxysilanes or acetoxysilanes[21] are usually produced from silanes or chlorosilanes by reaction with the corresponding alcohols.

Figure 4.8: Tetrachlorosilane reacts with methanol to form tetramethoxysilane and HCl

The usual commercial silanes are methanolates (see Figure 4.8), ethanolates, and acetates.

Figure 4.9: Overview of alkoxides and acetates of tetrasilane available on the market

The three silane derivatives shown in Figure 4.9 are usually available on the market. In subsequent sections, we will focus on the methoxy variant when discussing the various silanes, as it is the most common. The corresponding chlorosilanes can, of course, also be used directly. They are not usually employed for coating purposes, on account of their very high reactivity and the HCl by-product. Chlorosilanes are the most reactive with water, followed by TAS, TMOS and, finally, TEOS. The longer the alcohol chain and the more sterically branched it is, the less reactive it is. Aromatic alcohols, in turn, afford materials that have very high hydrolytic stability [22] (see Chapter 5.6.1).

Table 4.5: Selection of commercially available alkoxysilanes

Silane,

Abbreviation,

CAS No.

Structure

Molar mass

[g/mol]

Solids

(as oxide)

Tetramethoxysilane

TMOS

681-84-5

152.22

39.5 %

Methyltrimethoxysilane

MTMS

1185-55-3

136.22

49.3 %

Dimethyldimethoxysilane

DMDMS

1112-39-6

120.22

61.7 %

Propyltrimethoxysilane

PTMS

1067-25-0

164.28

58.0 %

Isobutyltrimethoxysilane

IBTMS

18395-30-7

178.3

61.3 %

Isooctyltrimethoxysilane

IOTMS

34396-03-7

234.41

70.6 %

Octyltrimethoxysilane

OTMS

3069-40-7

234.41

70.6 %

1H,1H,2H,2H-Perfluorooctyltrimethoxysilane

PFTMS

85857-16-5

468.28

86.5 %

Hexadecyltrimethoxysilane

HDMS

16415-12-6

346.68

80.1 %

PhenyltrimethoxysilanePhTMS

2996-92-1

198.3

65.2 %

The silanes can perform different roles in networks. TMOS and TEOS, mentioned above, are raw materials for glass networks or for surface-modification of particles. For more information, see Chapter 5.6 (sol-gel technology).

Organic modification with a carbon chain affects paint systems in different ways. The products are used for flexibilisation, as network converters and for water-proofing surfaces and particles. Commercially available compounds are listed in Table 4.5, along with their properties.

This selection of common silanes can serve as network converters. All of them, without exception, only condense into the inorganic network, onto particle surfaces or with themselves.

Apart from those silanes which have one or more carbon or fluorine chains, there exist a large number of silanes whose carbon side-chains contain functional groups.

Table 4.6: Alkoxysilanes featuring functional groups in the side-chain

Silane,

Abbreviation,

CAS No.

Structure

Molecular weight

[g/mol]

Solids

(as oxide)

Aminopropyl-trimethoxysilane

APTS

13822-56-5

179.29

61.5 %

N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane

DAMO

1760-24-3

222.36

69.0 %

3-Mercaptopropyl-trimethoxysilane

MPTMS

4420-74-0

196.34

64.9 %

3-Isocyanopropyl-trimethoxysilane

ICTMS

15396-00-6

205.28

45.9 %

3-(Acryloxy)propyl-trimethoxysilane

AcPTMS

4369-14-6

234.32

70.6 %

3-Methacrylopropyl-trimethoxysilane

MPTS

2530-85-0

248.35

72.2 %

3-Glycidyloxypropyl-trimethoxysilane

GPTS

2530-83-8

236.34

70.8 %

3-(Trimethoxysilyl) propyl succinic anhydride

156088-53-8

262.23

73.7 %

A selection of the methoxy variants of common functional alkoxysilanes is listed in Table 4.6. In reality, the choice of raw materials here is very much greater. Functional alkoxysilanes are the perfect raw materials for producing silane-terminated polymers, in particular.

Apart from surface functionalisation, use as adhesion promoter, and particle stabilisation, the clear goal is to use these silanes as binder components for paints and coatings. In the next chapter, we will dive deeper into the chemo-technical basics.

4.4Literature

[1]

B. Arkles, Silanes Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, John Wiley & Sons Inc, Vol. 22, 38–69 (2016)

[2]

www.chemie.de/lexikon/Silane.html

[3]

C.J. Brinker, G.W. Scherer Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press Inc., 1990

[4]

K. D. Linsmeier, Handbuch technische Keramik, Verlag Moderne Industrie, 2010

[5]

GESTIS-Stoffdatenbank, siehe http://gestis.itrust.de/nxt/gateway.dll/gestis_de/002720.xml?f=templates$fn=default.htm$3.0

[6]

Pachaly, Achenbach, Herzig, Mautner; Winnacker/Küchler: Chemische Technik: Prozesse und Produkte, Vol. 5: Organische Zwischenverbindungen, Polymere. Page 1105. Wiley-VCH Verlag GmbH & Co. KGaA

[7]

C. Elschenbroich, Organometallics. VCH, Weinheim, 1992

[8]

C.J. Brinker, G.W. Scherer Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press Inc., 1990

[9]

J. J. Ebelmen, Untersuchung der Verbindungen der Borsäure und der Kieselsäure mit den Aethern., Journal für praktische Chemie. 37(1), (1846) 347–376.

[10]

C. Friedel, J. M. Crafts: Ueber einige neue organische Verbindungen des Siliciums und das Atomgewicht dieses Elements, Annalen der Physik und Chemie 127 (1863), 28–32.

[11]

H.U. Reißig, Siliciumverbindungen in der organischen Synthese, Chemie in unserer Zeit, 2.(1984) 46–53, doi:10.1002/ciuz.19840180203

[12]

R. Tacke, Bioaktive Siliciumverbindungen, Chemie in unserer Zeit. 14(6) (1980) 197–207, doi:10.1002/ciuz.19800140605.

[13]

J. Ackermann, V. Damrath, Chemie und Technologie der Silicone II. Herstellung und Verwendung von Siliconpolymeren, Chemie in unserer Zeit 23(3) (1989) 86–99, doi:10.1002/ciuz.19890230304.

[14]

Franz A.K., Wilson S.O. Organosilicon molecules with medicinal applications, J. Med. Chem. 56(2) (2013) 388–405. doi:10.1021/jm3010114. PMID 23061607

[15]

M. Weinmann „Polysilazanes“ in Inorganic Materials, Roger DeJaeger, Mario Gleria, 2007, ISBN 978-1-60021-656-5

[16]

AZ Electronic material USA Corp., US 8470924 B2 “Color-pigmented paint composition having high covering powder, increased scratch resistance, and easy-to clean properties, 8.12.2009.

[17]

www.merckgroup.com/de/brands/pm/durazane.html

[18]

C.R. Krüger, E.G. Rochow, J. Polym. Sci. Vol. A2 (1964) 3179-318

[19]

G. Soraru, R. Riedel, A. Kleebe, P. Colombo Polymer Derived Ceramics, , DEStech publications, Inc. 2010

[20]

S. Brand, M. Mahn, F. Osterod, Farbe und Lack 116, (2010) 25–29

[21]

M. Tausch, M. von Wachtendonk, CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993)

[22]

NANO-X GmbH WO 2010/130241 A3 "Phenolatesterverbindungen", 18.11.2010

5Chemo-technical basics

Multi-functional coating materials acquire their functionality through combinations of various materials and effects. A coating network may be soft, elastic, rubbery or hard. Fillers may exert physical effects by virtue of their specific crystal structure, with the size of specific functional clusters ranging from that of coarse structures down to the nano-scale range. For an appreciation of the various options and to be able to combine the right functionality with the right chemistry, it is important to have a basic understanding of what silane chemistry is capable of.

Silane-based raw materials are the starting point for a host of reactions. As silane chemistry is complex, again, a basic knowledge of the various modules (see below) will help with understanding the various reactions. The modules that can assist with explaining these multi-component networks are:

Glass chemistry: When silanes are linked together via inorganic condensation processes, the outcome is a silicate network, whose macroscopic properties are modified by co-condensation with metal alkoxides (e.g. Al, Zr, Ti ...). These compounds are particularly important in enamelling.

Water glass chemistry: Water-soluble alkali salts, especially Li, Na, and K silicates, serve as the starting materials for a plethora of products.

Ceramic materials technology: This entails sintering ceramic particles to obtain the functionality specific to solids.

Sol-gel chemistry: Affords a basic understanding of the reactions by which silanes form polymeric networks and provides an option for uniting inorganic and organic networks in a covalent network via silane bridges.

Nanotechnology: This is primarily concerned with the production and functionality of particles, especially nanoparticles, the effects of different crystal lattices, the influence of the particles' size, particle stabilisation, especially the surface modification of nanoparticles, and their properties.

Silicone chemistry: This is the use of short organic side-chains and condensation catalysts to fine-tune the length of the chains and the type and form of the condensates.

Paints and coatings chemistry: The focus here is on functionalisation by means of silane-based materials. From modification with nanoparticles through to the use of inorganic-organic binders, there are numerous ways in the functional principles employed in coatings technology can be used in production.

Metallic coatings/corrosion protection: The basic concepts of corrosion protection and a knowledge of how galvanising works, for example, are important for using silicon-based materials to protect a substrate against corrosion.

The literature on each of these specific areas of chemical and materials science now fills entire libraries. The paints and coatings sector here is interested in those modifications which can serve as a basis for further functions to model paint systems that possess special properties and which will be the focus of this book.

5.1Basics of glass chemistry

A “glass” is generally understood to be an amorphous (i.e. it has solidified without undergoing crystallisation), supercooled melt which softens only gradually when heated and whose atoms possess short-range order, but no long-range order. The amorphous state of glass is said to be metastable. This means that, on being heated, glass strives to attain the lower-energy crystalline state and in so doing turns hazy. Glasses, like the alkali silicates (water glass; see Chapter 5.1.4) and pottery, are technically classified as silicates. The main component of most glasses is therefore silicon dioxide. That there are so many different types of glass is due to the different compositions and the process by which the amorphous melts are made.

5.1.1SiO2 as glass and crystal

Figure 5.1: Quartz glass

Crystal glass is a term given to colourless glass that refracts light more strongly than normal glass and, depending on its quality, is capable of generating special colour effects. A familiar example to us is crystals made of quartz glass (see Figure 5.1). The distinction between glass and crystal glass is regulated by EC Directive (69/493/EEC). It defines glassware containing less than 4 % lead oxide as “glass”. Glassware containing more than 10 % lead is considered “crystal”, glassware with more than 24 % lead is called “lead crystal” while glassware with more than 30 % is dubbed “high-lead crystal”. In Germany, these classifications are also regulated by the Crystal Glass Labelling Act (“Kristallglas-Kennzeichungsgesetz”).

Plastics are macromolecular polymers that are solid at room temperature. If the polymer is amorphous and has a glass transition temperature, it is known as an organic glass or synthetic glass. The base monomers of highly transparent synthetic glasses are diethylene glycol bis(allyl carbonate) (DAC), polycarbonate (Bayer “Makrolon”), polyurethanes (“Trivex”) and acrylics (“Plexiglas”).

Apart from synthetic glasses, there are also natural glasses, such as amber, a fossilised tree resin. Obsidian and pumice, too, occur naturally and are volcanic in origin. Tektites and fulgurites are impact glasses formed by meteorite and lightning strikes. Trinitite glass is an unfortunate by-product of nuclear explosions and köfelsite is formed in landslides. All these glasses are formed by the melting of sand. One such example is moldavite (see Figure 5.2).

Figure 5.2: Natural glass: moldavite; the green colour stems mainly from iron oxide present in the molten sand

Synthetic glasses are mostly made by fusing mixtures of raw materials. A totally different way of making functional glasses is afforded by the sol-gel process (see Chapter 5.6), which can be used to produce thin films or aerogels.

5.1.1.1Network formers and network modifiers

The main constituent of glass is usually silicon dioxide, which forms a disordered three-dimensional network of [SiO4] tetrahedra, the corners of which are connected to each other by siloxane bridges, Si-O-Si. In the idealised case, the resulting structure is that of a pure SiO2 quartz glass. Also capable of vitrification from the melt are mixtures of acidic oxides of silicon dioxide, boron trioxide, aluminium trioxide and phosphorus pentoxide, which form a three-dimensional network. In these oxide or silicate glasses, as they are known, these components are called network formers. Since SiO2 is always the main component, these glasses are named by their second most common oxide. The terms phosphate glass, borate glass or aluminosilicate glass thus characterise the basic structure of the glasses, each of which has its own specific properties. The incorporation of trivalent (boron, aluminium) or pentavalent (phosphorus) elements cleaves some of the Si-O-Si bridges of the main component, so that disruption points, such as [O3Al -O-SiO3] are built into the homogeneous glass network, thus increasing the negative charge or, in the case of phosphorus, decreasing it. These points disrupt the glass's rigid lattice structure and so influence the softening and melting points. Glasses having only one component,i.e. the network former, are called monocomponent glasses. Bicomponent and tricomponent glasses contain two and three metal oxides, respectively. Other oxides that can be incorporated into the network to form further disruption points are alkali-metal and alkaline-earth metal oxides, as well as lead and zinc. These are known as network modifiers and are built into the framework formed by the network former. Instead of forming an atomic bond with the silicon, the oxygen forms a much weaker ionic bond in the network with, e.g. an alkali ion. One of the effects of this is a lowering of the melting temperature.

Further additives for glass are:

Fluxes for lowering the melting point:

Potassium oxide, zinc oxide, thallium oxide

Additives for altering the refractive index:

Barium oxide, lead oxide (also absorbs gamma or X-rays)

Opacifying agents:

Tin dioxide, calcium phosphate, fluoride for opal glass, zirconium dioxide

Other functionalities:

Cerium is used for rendering glass resistant to radioactivity and x-rays. Boron oxide is an additive for modifying thermal and electrical properties. Aluminium oxide increases the breaking strength.

Intermediate oxides, such as aluminium oxide, can act as network formers and modifiers, i.e. they can strengthen (stabilise) a glass network or, just like the network modifiers, weaken it. Their specific effect in a glass is always dependent on a number of factors. However, intermediate oxides alone are incapable of glass formation.

The most common raw materials used in mass production of glass are:

Quartz sand

is an almost pure SiO

2

carrier for network formation. It must have a low content of Fe

2

O

3

(< 0.05 %), as otherwise white glass will acquire a green hue. At more than 70 wt%, this raw material makes up the bulk of the mixture.

Soda (Na

2

CO

3

)

serves as a carrier of sodium oxide, which acts as a network modifier and flux and lowers the melting point of the SiO

2

. Carbon dioxide is released in the melt and dissolves out of the glass in the form of a gas. Soda is the most expensive raw material in the field of mass-glass production, because it does not readily occur in nature. Sodium can also be added to the melt in the form of nitrate or sulphate (sodium sulphate is used as a fining agent for reducing the bubbles content).

Potash (K

2

CO

3

)

provides potassium oxide for the melt and, like sodium oxide, serves as a network modifier and flux.

Feldspar (NaAlSi

3

O

8

)

introduces alumina (Al

2

O

3

), in addition to SiO

2

and Na

2

O, into the mixture. This boosts the glass's chemical resistance to water, food and environmental influences. ZrO

2

has a similar function in the network.

Lime