Silicon and Nanotechnology for Coatings: 2nd edition

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

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

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].

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

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).

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.

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

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:

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.

The homologous series of linear, unbranched silanes has the general formula H-(SiH2)n-H, where n = 1, 2, 3, ... . The simplest member is tetrahydrosilane or monosilane (SiH4), a colourless gas which spontaneously combusts in air to form SiO2.

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].