Functional Coatings E-Book

Volkmar Stenzel

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

Seitenzahl: 329

Veröffentlichungsjahr: 2014

Beschreibung

This eBook teaches adhesive and sealant formulation in two steps. Each section first describes the application and chemical basis of the type of adhesive or sealant concerned. This is followed by formulation advice and - if possible - an analysis of existing recipes. This analysis includes a calculation of the important characteristic values of the formulations. All calculations based on recipes and formulations are worked through step by step and should therefore be intelligible to beginners, too.

Leseprobe

Vincentz Network GmbH &Co KG

Volkmar Stenzel | Nadine Rehfeld

Functional Coatings

Cover: Fraunhofer-IFAM

Bibliographische Information der Deutschen Bibliothek

Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Volkmar Stenzel, Nadine Rehfeld

Functional Coatings

Hanover: Vincentz Network, 2011

EUROPEAN COATINGS TECH FILES

ISBN 3-86630-808-6

ISBN 978-3-86630-808-4

© 2011 Vincentz Network GmbH & Co. KG, Hanover

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ISBN 3-86630-808-6

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EUROPEAN COATINGS TECH FILES

Volkmar Stenzel | Nadine Rehfeld

Functional Coatings

Foreword

Just about every article or object we encounter in daily life has a surface that has been treated in some way during its manufacturing process. The surface is what we first recognise when we see or touch an article or object. Surface treatment is responsible for the decoration, surface feel and protection of the surface, including corrosion protection. New developments (e.g. nanotechnology, encapsulation technologies, etc.) have opened up new opportunities for the integration of new surface functions, in addition to decoration and protection.

One of the best known functional surfaces is the surface of lotus leaves. These surfaces are extremely dirt-repellent due to their hydrophobic properties. A large amount of research is also being conducted on other, equally interesting surface properties, e.g. anti-fouling, drag-reducing, and self-healing, and is striving to transfer the underlying surface principles into concepts for coatings. Interdisciplinary work groups in research organisations and industrial companies are actively exploring the enormous potential of functional coatings.

This book summarises the vast number of new developments and the results of this work.

The present book gives users of paint technology an overview of the most promising developments and approaches. It also gives readers an idea of what is currently possible and what is likely to be possible in the near future. It highlights the current status of development or market introduction of the different technologies and novel surface functions that go beyond decoration, corrosion protection and surface protection.

We naturally had the difficult task of selecting information from a huge number of publications and we had to focus on the most important aspects for the users of paints and coatings, paint chemists, and paint manufacturers. We chose to focus on organic coatings (including organic-inorganic hybrid coatings) and have not addressed PVD/CVD methods, galvanic methods and other non paint/lacquer related topics.

Bremen, Germany in October 2010

Volkmar Stenzel & Nadine Rehfeld

[email protected]

[email protected]

Contents

1Introduction

1.1Motivation

1.2Content and aim of this book

2Tool-box

2.1Paint surfaces with defined microtopography and nanotopography

2.1.1Examples from nature

2.1.2Influence of surface topography on wetting

2.1.2.1Ideal surfaces

2.1.2.2Non-ideal surfaces

2.1.3Creation of stochastic surface structures in paint films

2.1.4Creation of deterministic surface structures

2.1.5Literature

2.2Microcapsules

2.2.1Introduction

2.2.2Microencapsulation techniques

2.2.2.1Interfacial polycondensation, polyaddition or radical polymerisation

2.2.2.2Coacervation

2.2.2.3Mini-emulsion polymerisation

2.2.2.4Spray drying

2.2.3Release of active agents

2.2.4Perspectives for using microcapsules in functional coatings

2.2.5Literature

2.3Functional nanoparticles/nanofillers

2.3.1Nanotechnology – What does “nano” really mean?

2.3.2Nanoparticles and nanocomposites – examples

2.3.2.1Fibre-like nano-particles (for example carbon nanotubes (CNTs))

2.3.2.2Layered nanoparticles, for example nanoclays or layered double hydroxides (LDHs)

2.3.2.3Spherical nanoparticles, for example nanosilica and other nanooxides

2.3.2.4Porous nanoparticles as hosts for active agents

2.3.3Safety, health, and environmental aspects

2.3.4Literature

2.4Sol-gel process

2.4.1Principle of the sol-gel process

2.4.2Manufacture of functional coatings

2.4.3Literature

2.5Polymer brushes

2.5.1Principles of polymer brushes

2.5.2“Grafting to” process

2.5.3“Grafting from” process

2.5.4Practical application of polymer brushes

2.5.4.1Reversible switching between super-hydrophilicity and super-hydrophobicity

2.5.4.2Surfaces with controlled behaviour against biological systems

2.5.4.3Reversible switching between super-hydrophilic and oleophobic properties

2.5.4.4Outlook for the industrial application of polymer brushes

2.5.5Literature

2.6Biologically inspired coatings

2.6.1Recovery of biofunctional molecules or production of synthetic analogs

2.6.2Chemical attachment of biofunctional molecules to surfaces

2.6.2.1Generation of reactive functional groups: surface activation and linker molecules

2.6.2.2Attachment of bioactive compounds (immobilisation)

2.6.2.3Adhesion promoters mimicking nature

2.6.2.4Example applications

2.6.3Attachment of biofunctional molecules to particles

2.6.4Literature

3Examples for functional coatings

3.1Self-cleaning/dirt-repelling surfaces

3.1.1Hydrophobic surfaces, superhydrophobic surfaces, lotus effect

3.1.2Photocatalytic surfaces

3.1.2.1The photocatalytic effect

3.1.2.2Manufacture of photocatalytic coatings

3.1.3Literature

3.2Anti-icing coatings

3.2.1Ice and its properties

3.2.1.1Physical properties of ice

3.2.1.2Physico-chemical properties of ice

3.2.2Anti-icing/de-icing technologies

3.2.2.1Active anti-icing coatings

3.2.2.2Passive anti-icing coatings

3.2.3Anti-ice evaluation

3.2.4Literature

3.3Self-healing coatings

3.3.1Aesthetic self-healing

3.3.2Structural self-healing

3.3.2.1Microencapsulation of crosslinkable healing agents

3.3.2.2Self-healing anti-corrosion coatings with active agents

3.3.2.3Other approaches for self-healing coatings

3.3.3Industrial application and outlook

3.3.4Literature

3.4Drag-reducing surfaces

3.4.1Laminar and turbulent flow

3.4.2Riblet surfaces – artificial sharkskin structures

3.4.3Hydrophobic and superhydrophobic surfaces

3.4.4Air-trapping surfaces

3.4.5Compliant coatings (Kramer-type coatings)

3.4.6Literature

3.5Anti-fouling coatings

3.5.1Concepts for anti-fouling coatings

3.5.2Biocide-containing coatings

3.5.2.1Biocides and mode of action

3.5.2.2Alternative approaches

3.5.3Foul-release coatings

3.5.3.1Surface free energy

3.5.3.2Elastic modulus

3.5.3.3Surface roughness/topography

3.5.3.4Other approaches

3.5.4Conclusions

3.5.5Literature

3.6Bio-mimetic surfaces

3.6.1Introduction

3.6.2Structure-inspired functions

3.6.2.1Anti-fouling surfaces

3.6.2.2Further application fields

3.6.3(Bio)chemical-inspired functions

3.6.3.1Anti-fouling surfaces

3.6.3.2Biomimetic anti-icing surfaces

3.6.3.3Coatings containing living organisms

3.6.3.4Further application fields

3.6.4Conclusions

3.6.5Literature

4Outlook

Abbreviations

Authors

Acknowledgements

Index

1Introduction

The term “functional coating” has been widely used in recent years and is a buzz-word, as were “nano” and “bio” previously. Accordingly, a very large number of scientific papers have been published that present old and new approaches to functional surfaces and describe a huge number of potential applications for functional coatings.

What, though, do we mean by “functional coating”? There is in fact no general definition for this term. According to DIN EN ISO 4618:2006 a coating is in general a “...continuous layer formed from a single or multiple application of a coating material to a substrate”. Furthermore, a coating material is defined as a “...product in liquid, paste or powder form which, when applied to a substrate, forms a film possessing protective, decorative and/or other specific properties”. Functional coatings have special properties and so give the surface new additional functions. This meaning of the term is reflected in the content of this book.

So how can we utilise the great wealth of information? We chose to focus on organic coatings (including organic-inorganic hybrid coatings), and present the most important aspects for the users of paints and coatings, paint chemists, and paint manufacturers. It also gives readers an idea of what is currently possible and what is likely to be possible in the near future. It highlights the current status of development or market introduction of the different technologies and novel surface functions that go beyond decoration, corrosion protection and surface protection.

1.1Motivation

Surface technology is used in all goods-producing sectors of industry. The added value from surface technology is approximately 3 to 7 %. Surface treatment, coating, and finishing usually focus on necessary functions such as corrosion protection, decoration, design, and surface protection. Additional surface functions can create additional value for industrial products, but what are the most important functions and which are most desired?

In order to answer these questions, the German Society for Surface Treatment (Deutsche Gesellschaft für Oberflächenbehandlung e.V.) carried out a survey (“Forschungsagenda Oberfläche”, DFO Service GmbH, Neuss, 2007) to identify the needs of German industry in the area of surface technologies. More than 300 technical experts from about 100 companies, 30 institutes, colleges etc. and several industrial associations participated in this survey. Despite the fact that the survey only covered Germany, we believe the results also apply to most other advanced industrial countries.

From the many (more than 100) technical topics and ideas that were signalled by the survey, 3 clusters were formed:

•knowledge-based quality improvement

•efficient processes

•multifunctional surfaces

From all the topics that were indicated, nine so-called flagship topics were identified. These flagship topics represent the research areas which are thought to have the highest impact on the competitiveness of manufacturing industry. The flagship topics were defined taking particular account of economic, environmental, and social sustainability.

The flagship topics are as follows:

•active layers (e.g. photovoltaic technology, catalytic surfaces)

•switching surfaces (e.g. switching between hydrophobic/hydrophilic behaviour, colours, electrical conduction/non-conduction)

•anti-fouling surfaces (e.g. lotus effect, photocatalytic self-cleaning, non-toxic maritime anti-fouling)

•self-repairing surfaces (long-term surface protection, e.g. wind turbines, heavy-duty corrosion protection)

•precision manufacturing via model-based control and regulation

•digital factory

•hybrid materials with complex morphology (e.g. anti-reflective glass coatings)

•rapid testing for degradation and corrosion

•brand protection

The bold letters indicate topics that are of relevance for the development of functional surfaces.

Further important findings of the survey were:

•The added value of surface and coating technology in Germany amounts to about 20 billion Euros per annum.

•Higher added value due to the introduction of new surface functions can considerably increase competitiveness. It has been estimated that a 5 % increase in added value due to innovative surface technology compensates a 20 % lower cost of manufacturing in foreign countries.

These two findings are motivation enough to be engaged in the development, manufacturing and selling of products with functional surfaces.

1.2Content and aim of this book

This book outlines recent developments in the field of functional coatings, with the focus on organic-based materials. The first part describes so-called toolbox methods which are currently available to developers. These cover a large number of methods, ranging from the manufacture of specific topographies on the microscale and nanoscale to the use of microcapsules and nanoparticles, and customised surface immobilisation of molecules. The functionalities themselves are at the fore in the second part of this book. These can, for example, be structure-based (e.g. drag-reducing) or chemical-based (e.g. self-healing) and can be produced via very different routes (e.g. anti-fouling, anti-icing).

It is important to stress that most of the examples described here are not yet ready for widespread industrial implementation. Indeed, the functionalities that are described are at different stages of development. Some merely represent promising first laboratory results (e.g. surface binding of anti-freeze proteins), others are at the production concept stage (e.g. paint structuring for drag-reducing surfaces), and some are already in use (e.g. self-healing car lacquer systems). The book gives an indication of the stage of current developments in the area of organic-based coatings and what future technical applications can be expected.

2Tool-box

2.1Paint surfaces with defined microtopography and nanotopography

The topography of a surface plays a major role in determining a variety of properties of that surface. Besides obvious properties such as the degree of gloss/mat, others such as wettability drag, adhesion, and light reflection are highly influenced by the surface topography. This fact can be utilised in order to generate functional paint surfaces. Indeed, nature has utilised this for millions of years, as illustrated by the following examples.

2.1.1Examples from nature

Nature displays a wealth of different surface topographies in plants and animals, giving rise to surfaces having specific functions (see also Chapter 3.6). The fact that nature displays a great variety of functions via the creation of microstructures and nanostructures on surfaces demonstrates that there are opportunities for developing surfaces with similar functions for technical applications. Examples of natural functional surfaces based on microstructures and nanostructures are described in the following.

Lotus leaf

The self-cleaning effect of the lotus leaf [1] is certainly the best known example of a natural, functional surface. The same effect also occurs in other plants, for example lady’s mantle and kohlrabi (albeit “kohlrabi effect” would sound far less interesting than lotus effect). The effect is based on chemical hydrophobicisation due to secreted waxes coupled with a microstructure and nanostructure (for mechanism see Chapter 2.1.2).

Skin of fast-swimming sharks

Decades ago it was observed that the scales of fast-swimming species of sharks have parallel riblets in the direction of flow around of the body. It is now known that this microstructure reduces the drag of the water by a few percentage points and so helps the shark save energy for motion (see Chapter 3.4).

Figure 2.1: Surface of a lotus leaf (computer animation)

source: S. Sepeur [2]

Figure 2.2: Surface structures on fast-swimming sharks [3]

source: Wolfram Hage, DLR

2.1.2Influence of surface topography on wetting

The wetting properties of a surface are very important for many surface functions. For example, the wetting properties very much determine the soiling, easy-to-clean properties, ice formation and adhesion, ability to be coated, and drag (in water). The surface topography at the microlevel and nanolevel in turn has a big influence on the wetting. For this reason, it is worth exploring the relationship between topography and wettability at this point in greater detail.

2.1.2.1Ideal surfaces

The ability of a liquid to wet a smooth surface can be determined, for example, by measuring the contact angle. This is done by placing a drop of liquid on the surface and measuring the contact angle at the solid-liquid-air (or vapour) phase boundary using a special microscope (see Figure 2.3).

The contact angle of a liquid on a surface depends on the relationship between the surface energies (= surface tensions) of the relevant phases at the drop boundary (solid, liquid, gas). The relationship for an ideal smooth and homogenous surface is described by Young’s equation [4]:

where γs is the free surface energy of the solid, γl is the surface tension of the liquid, and γsl is the interfacial energy between the solid and liquid.

Figure 2.3 Definition of the contact angle θ

Figure 2.4: Different wetting behaviour of surfaces

If the contact angle θ is very small (< 10°), then the surface can be very effectively wetted (Figure 2.4). If the contact angle is close to 0, there is said to be complete wetting. If water is the wetting liquid, there is talk of ultrahydrophilicity. This occurs when the surface energy of the solid is equal to or larger than the surface tension of water. Under conditions when condensation can arise, no water droplets can form on such surfaces and they act as anti-fog surfaces.

Conversely, surfaces having a water contact angle of 140° or higher are termed superhydrophobic, namely extremely water-repelling (Figure 2.4). The surface energy of such surfaces is very much smaller than the surface tension of water. Water virtually no longer wets the surface at all. For water contact angles upwards of 160° the term lotus effect is often used. Such surfaces are also called easy-to-clean surfaces as they allow good run-off of water and often show reduced soiling.

Details and examples of such surfaces can be found in the nanotechnology book by Stefan Sepeur, which has also been published in this series [2].

2.1.2.2Non-ideal surfaces

The aforementioned simple relationships only apply for ideal surfaces, namely surfaces that are ideally smooth and chemically homogenous. In the real world we generally deal with surfaces that do not meet these criteria. This becomes clear if we measure the contact angle in a dynamic way rather than statically. This can be done by measuring the contact angle as a function of advancing drop diameter (namely addition of liquid) and receding drop diameter (namely removal of liquid), see Figure 2.5.

Figure 2.5: Advancing and receding contact angle; contact angle hysteresis

Figure 2.6: Drop on inclined surface; left: low contact angle hysteresis, right: high contact angle hysteresis

The contact angle for an advancing drop diameter is generally greater than for a receding drop diameter. This difference is called contact angle hysteresis and usually amounts to between 5 and 20°, but can be considerably higher [5].

Contact angle hysteresis is particularly evident when drops run off an inclined surface, with on the one side an advancing contact angle and on the other side a receding contact angle (Figure 2.6).

The reasons for the differences in the contact angle with advancing and receding contact angle include [6]:

•Topographic surface roughness. When the drop meets a bump on the surface, it jumps into a position having the same contact angle as previously attained. At this point the spreading out of the drop is hindered until it is large enough to overcome the bump. The same effect causes the reduction in size of the drop, and the result is the described hysteresis.

•Chemical heterogeneity of the surface. When the drop spreads out on such a surface, the three-phase boundary line is kept in place (pinned) by relatively liquid-repelling (lyophobic) regions, and when the drop recedes the boundary line is kept in place by liquid-attracting (lyophilic) regions. The result is again hysteresis.

•On non-rigid surfaces (e.g. polymers) the forces that act on the three-phase boundary line can be so large that the surface deforms, and this can also lead to contact angle hysteresis.

In reality these effects often occur in combination, and this can result in considerable intensification of the effect. The roughness has a particularly large effect on contact angle hysteresis, and for this reason this is considered in detail below.

In the past, two essentially empirical laws were put forward for describing the wetting behaviour of rough surfaces. These also represent the two cases that occur in nature [7] (see Figure 2.7).

Wenzel regime

Wenzel described an average contact angle on a rough, chemically homogenous surface [8]. He defined the contact angle on a rough surface θ* as a function of the Young contact angle θ (see above):

Wenzel introduced the roughness coefficient (r’) which represents the relationship between the actual surface and the geometric projected surface. The roughness coefficient (r’) is always larger than 1. Hence in this wetting regime, when considering water as the wetting liquid, both the hydrophilicity and the hydrophobicity are enhanced by the roughness.

Figure 2.7: Schematic representation of the two different cases for wetting rough surfaces, (a) the Wenzel regime and (b) the Cassie-Baxter regime

source: Ulrike Mock [6]

The other case is the Cassie-Baxter regime (see Figure 2.7) which has originally been described for smooth, chemically heterogeneous surfaces [9], but which also applies to rough surfaces where air is trapped under the wetting liquid in indentations. For this type of surface (comprising a solid and air), the contact angle θ* is the average contact angle of the drop on air (assumed to be 180°) and on the (ideally smooth) solid (θ). If a surface has a wetted fraction ϕs (that is the fraction of the surface on which the liquid is in direct contact with the solid), the following equation applies:

The two wetting regimes have very different behaviour, in particular regarding contact angle hysteresis. When a Wenzel regime prevails, the drops have relatively large contact angle hysteresis and a larger run-off angle (the angle relative to the horizontal above which the drop runs off). In contrast, when a Cassie-Baxter regime prevails, the hysteresis and also the run-off angle are small.

The surface chemistry and the roughness largely determine which of the two regimes prevails. The typical behaviour of a given surface with low surface energy is as follows: First of all, the wetting with water follows the Wenzel regime. With increasing roughness (or roughness factor), the contact angle and hysteresis increase. Once a certain roughness value is exceeded, the contact angle increases further but the hysteresis decreases considerably. At this roughness value above which the hysteresis decreases, the system changes over from the Wenzel regime to the Cassie-Baxter regime, namely the fraction of air at the boundary increases considerably.

In the region of moderate hydrophobicity (Young contact angle up to 120°), the Cassie-Baxter regime is metastable. External pressure (in the simplest case, touching the water drop) can lead to the system switching irreversibly into the Wenzel regime. That means that the liquid now follows the surface contour and there is no longer any air between the water and the surface. The contact angles of both regimes are similar in this region, but the hysteresis in the Wenzel regime is 10 to 20 times higher. In the region of the Wenzel regime, the drops hence adhere much more strongly to the base surface.

Due to this behaviour, it often makes more sense to measure the run-off angle than the contact angle in order to characterise the wetting behaviour. In particular for surfaces whose functions are based on their wetting behaviour (e.g. easy-to-clean layers or special anti-icing layers), it is vital to consider both the contact angle and run-off angle when characterising the surface properties.

Customising the hydrophobicity or hydrophilicity of a surface with a given chemistry by introducing a special microstructure or nanostructure is essential for manufacturing surfaces with particular functional properties. The highest known water contact angle of a (smooth) material is 120° [10]. This involves hexagonal close packing of CF3- groups. If one wants to achieve higher contact angles, such a surface must be combined with a suitable topography. Using this principle (combining hydrophobic surface chemistry with suitable microtopography and nanotopography – the lotus principle), superhydrophobic surfaces with contact angles > 150° can be produced. Here, microstructures are often combined with nanostructures, as is the case in nature with lotus leaves.

The ability of a suitable topography to make hydrophilic surfaces more hydrophilic and also hydrophobic surfaces more hydrophobic is also utilised for switchable surfaces. For example, the switching amplitude of the water contact angle for a surface modified with polymer brushes (see Chapter 2.5) was increased from 20° for a smooth surface to 150° when the surface had a suitable microstructure and nanostructure [11].

Microstructures or nanostructures can be either stochastic (namely random and without a recognisable uniform pattern) or deterministic. Deterministic here means that the surface has a very special, uniform pattern that is responsible for the specific function of the surface.

2.1.3Creation of stochastic surface structures in paint films

The creation of surfaces with rough topographies is a classical technique for making mat paints, amongst other things. In the simplest case the paint contains solid, particulate matting agents. After the drying process, which involves loss of solvent and curing shrinkage of the binder and is accompanied by a reduction in volume of the paint film, the result is microscopic spots sticking out of the paint film. Figure 2.8 shows the principle.

Figure 2.8: Left: wet paint film, freshly applied; right: paint film after drying

Figure 2.9: Stochastic surface topography Created by microparticles

source: Fraunhofer IFAM

The size of the particles determines the scale of the surface topography. Depending on the particle size, surface structures can be produced on a microscale or nanoscale. Figure 2.9 shows a surface produced in this way.

Using the same principle it is also possible to manufacture hierarchical surface topographies, for example those that combine a microstructure with a nanostructure. The relevant particle sizes are combined in the paint film for this purpose (Figure 2.10).

Figure 2.10: Left: wet paint film, freshly applied; right: paint film after drying

Figure 2.11: Surface topography created using a two-stage UV-curing method

source: Fraunhofer IFAM

By suitable choice of the particles, the coating matrix, and the volume concentration of the pigment, it is possible to produce paint surfaces having a water contact angle of >140°. So-called lotus effect coatings or paints are usually produced in this way.

The customisation of the topography using microfillers and nanofillers is generally successful for paints that shrink significantly in volume on drying. For paints where this is not the case, other methods are available.

UV-cured coatings generally do not shrink in volume due to solvent loss on curing. There is solely curing shrinkage due to the crosslinking reaction, which can amount to up to 10 %. In order to adjust the surface structuring in such systems, the curing can be undertaken in two stages with different wavelengths. The first curing stage (small wavelength, low penetration depth) leads to crosslinking of the uppermost microns of the paint film. Due to the curing shrinkage in the uppermost layer, which almost floats on the uncured liquid coating below, small folds form and these create the desired surface topography. The subsequent curing at higher wavelength leads to full curing of the coating film. By varying the process parameters (layer thickness, wavelengths, curing times), the topography can be customised over broad limits. Figure 2.11 shows a surface produced using this principle.

This method is much used in the graphics industry for producing mat printing inks.

2.1.4Creation of deterministic surface structures

The before-described methods are very suitable for producing stochastic surface structures, namely random surface structures. In order to introduce specific roughness, for example to customise the wetting properties, these methods are excellent. Other methods must, however, be used when defined, regular structures are required for specific surface functions.

Figure 2.12: Left: principle of simultaneous embossing and curing; right: microstructure in a UV-cured paint

source: Fraunhofer IFAM

Figure 2.13: Principle of the roller-tool for paint application, embossing, and curing [12]

In the simplest case one can imagine the paint film being embossed with a suitable tool during the drying process. This assumes that during the drying process the paint film has a time window when the paint is free of solvent but is not so fully cured that cannot be embossed, and that after removing the tool the structure remains, is not tacky and does not stick to the tool.

A viable time window is generally not available for standard paints. For this reason, an alternative is to carry out the curing and embossing of the surface in a single step, so that the paint loses its tackiness during the curing and there is no reflow of the structures after removal of the tool. This can be effectively achieved using radiation curing paints which can be formulated without solvents (with the exception of reactive thinners). The tool here is preferably made of a material that is transparent to the radiation used for the curing and a material that adheres as little as possible to the cured paint film. The principle is depicted in Figure 2.12.

A stamp method, as described above, can naturally be used for small surfaces that can be stamped by hand or by machine. For larger surfaces, for example facade elements, aircraft components, or ship hulls, a continuous process must be employed. Figure 2.13 shows the principle of a continuous paint application and curing process which is currently being developed for industrial use.

A paint film consisting of an (at least partially) UV-curing polymer system is applied using a wide slit nozzle (4) to a moving UV-transparent belt (1) bearing the negative of the desired structure. When the tool is rolled across a surface, the paint film ends up below a soft roller (3) and is transferred to the surface to be coated. When the paint then comes into the field of the UV lamp (2), it is crosslinked until at least the paint film does not run anymore and is touch-dry. Decisive is that the paint loses its tackiness and does not adhere to belt. The tool therefore leaves behind a microstructured or nanostructured paint film on any desired surface. The application tool can, for example, be controlled using a robot (see Figure 2.14).

An advantage of this method, compared to the application of films, is that single or multiple curved surfaces can also be coated, because the embossing film and the soft rollers can be customised in a very versatile way (see Figure 2.15).

Figure 2.14: Application of microstructured or nanostructured paint films using a robot

source: Fraunhofer IFAM

Virtually any desired nanotopography or microtopography can be applied to large surfaces using this method. With careful selection of the material used for the embossing tool, undercuts can even be reproduced to a certain degree. Figure 2.16 shows two examples of paint surfaces produced with radiation curing paint systems using the aforementioned principle.

Other examples of paint surfaces produced using this method are depicted in Figure 2.17.

The method described here can obviously only be used for reproducing microstructures and nanostructures. The manufacture of the relevant master molds must be undertaken with other, sometimes complex, methods including micro-machining [13], lithographic methods [14] and laser technologies [15].

Figure 2.15: Application of a structured paint film on double-curved surfaces

source: Fraunhofer IFAM

Figure 2.16: Two examples of microstructures. Left: original structure (prepared by Berliner Elektronenspeicherring-Gesellschaft für Synchrotron-Strahlung m.b.H. (BESSY)); right: reproduction on a paint surface (Fraunhofer IFAM)

Figure 2.17: Left: hologram in a clearcoat (master-mold courtesy of topac GmbH); top right: riblet structure for drag reduction (see Chapter 3.4), bottom right: anti-reflective nanostructure (master-mold courtesy of Holotools GmbH)

2.1.5Literature

[1]Barthlott, W.; Scanning electron microscopy of the epidermal surface in plants, Systematics Association’s Special, 41, 1990, S. 69–94

[2]Sepeur, S.; Nanotechnology, Technical Basics and Applications, Vincentz Network, 2008

[3]Hage, W.; Zur Widerstandsverminderung von dreidimensionalen Riblet-Strukturen und anderen Oberflächen Dissertation Berlin 2004

[4]Young, T.; Philosophical Transactions of the Royal Society of London, 95, 1805, 65–87

[5]Butt, H.-J.; Graf, K.; Kappl, M.; Physics and Chemistry of Interfaces, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003

[6]Mock, U.; Dissertation „Über das Benetzungsverhalten polymermodifizierter Grenzflächen“, Freiburg, 2004

[7]McHale, G.; Cassie and Wenzel: Were they really so wrong?, Langmuir, 23, 2007, 8200–8205

[8]Wenzel, R. N.; Journal of Physical and Colloid Chemistry, 53, 1949, 1466–1467

[9]Cassie, A. B. D.; Baxter, S.; Transactions of the Faraday Society, 40, 1944, 546–551

[10]Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y.; The lowest surface free energy based on −CF3 alignment, Langmuir, 15, 1999, 2551–2558

[11]Uhlmann, P.; Houbenov, N.; Ionov, L.; Motornov, M.; Minko, S.; Stamm, M.; Oberflächen passen sich an – bürstenartige Polymermoleküle an Oberflächen mit schaltbaren Eigenschaften, Wissenschaftliche Zeitschrift der Technischen Universität Dresden, 56, 2007, 47–52

[12]Patents DE_10346124_B4 and DE_102006004644_B4

[13]Brinksmeier, E.; Gläbe, R.; Riemer, O.; Twardy, S.; Potentials of precision machining processes for the manufacture of micro forming molds, Mikrosystem Technologies, 14 (12), 2008, 1983–1987

[14]www.holotools.de

[15]Römer, G. R. B. E.; Huis in’t Veld, A. J., Meijer, J.; Groenendijk, M. N. W.; On the formation of laser induced self-organizing nanostructures, Manufacturing Technology, 58, 2009, 201–204

2.2Microcapsules

2.2.1Introduction

Industrial use of microcapsules has grown very fast over the last 30 years. One of the first industrial applications for microcapsules was carbonless copy paper. In 1974 approximately 500,000 tons of this paper was produced, corresponding to 50,000 tons of microcapsules [1].

Microcapsules are used in the food industry (e.g. microcapsules containing flavours in chewing gum), in the detergent industry (e.g. microencapsulated perfume in fabric-softeners which is released weeks after the washing process when the clothes are touched and worn), and in the pharmaceutical industry. The reasons for using microcapsules in these fields are:

•Protection of sensitive materials against environmental attack, enhancement of shelf-life;

•Controlled, triggered, or delayed release of active agents;

•Masking of smell or taste;

•Improved processability (flow properties, handling of toxic materials, and handling of liquids as solids).

In the above-mentioned industries, microcapsules are very common raw materials. Using microcapsules must, however, not increase product prices for the consumer too much. In the food industry, for example, a maximum cost for the microencapsulation can be roughly estimated at €0.1/kg [2]. This figure shows that microencapsulated raw materials for potential use in functional coating materials must be within an acceptable price range.

2.2.2Microencapsulation techniques

The technique of microencapsulation aims at enveloping liquids (in special cases gases and solids too) in a finely dispersed form. The particle diameter can range between 1 and 5000 μm. Typical capsule diameters for different applications are as follows [3]:

•1 to 10 μm for carbonless copy paper;

•30 to 50 μm for pesticides;

•10 to 50 μm for perfumes.

The thickness of the shell typically ranges from 50 nm to several μm. The shell can be formed from synthetic polymers and from biologic materials as well. The external shape of the capsules is mostly spherical. In special cases, grape-like clusters or irregular structures appear. The nature of the wall material and wall thickness determines the ability of the microcapsule to protect the core material or to release it.

The capsule wall separates the content from the external surroundings, and to release it the shell must be opened or must be permeable. The opening may occur via mechanical stress from the outside (shearing, crushing) or via effects from the inside (heating above the boiling point, melting, explosion). Some examples are given below.

For the microencapsulation of agents, numerous preparation technologies are available. In general, microencapsulation techniques can be divided into chemical and physical methods. The Table 2.1 gives an overview of current technologies that are widely used on a laboratory scale and for the industrial production of microcapsules [4].

For application of microcapsules in paints and coatings, the particle size should not exceed approximately 50 μm. For many applications (primers, spray-coatings, etc.) the capsule size should be between 1 and 20 μm. Some applications require even smaller capsules. In the following chapters several examples of encapsulation techniques that can produce capsules in the relevant size range are explained in more detail.

Table 2.1: Overview of microencapsulation technologies

Chemical processes

Physico-chemical processes

Physico-mechanical methods

Interfacial polycondensation

Emulsion polymerisation

Coacervation

Layer-by-layer assembly Sol-gel encapsulation

Supercritical CO2-assisted microencapsulation

Spray dryinq

Dipping or centrifuging techniques

Co-extrusion

Fluidized bed technology

2.2.2.1Interfacial polycondensation, polyaddition or radical polymerisation

A very versatile method for encapsulation of active agents that form a separate, water-immiscible phase in water is interfacial polycondensation or polyaddition. The basis for the microencapsulation process is an emulsion of the oil-in-water (o/w) type. During the microencapsulation process a wall forms around the oil-phase droplets from appropriate monomers via a crosslinking process. The process is outlined in Figure 2.18.

The microencapsulation process comprises the following steps:

Choice of the organic (dispersed) phase

The organic phase contains the active agent for the function that is going to be introduced to the coating material via the microcapsules. Examples of active agents are given below (Chapter 2.2.4). The organic phase must have very low solubility in water and should have a viscosity that allows the preparation of an emulsion in water at a suitable temperature.

Preparation of the water (continuous) phase

The continuous phase contains water-soluble components (additives) such as:

•pH-regulators

•Defoamers

•Agents that influence the wall stability or transparency

•Components of the capsule wall

Figure 2.18: General principle of microencapsulation by interfacial reaction

Figure 2.19: Poly(urea-formaldehyde) capsule material

Preparation of the emulsion

The emulsion is the basis for the microencapsulation process. The droplet size that is achieved during the emulsification determines the size of the microcapsules. The droplet size mainly depends on the following parameters:

•Stirring conditions (speed, type of mixer, mixing geometry)

•Type of emulsifier

•Amount of emulsifier

If stirred by conventional means, the size of the microcapsules ranges from approximately 2 μm up to several hundred microns. By employing a sonication device during the emulsification process, particle diameters down to 220 nm can be achieved [5]. Another method for achieving fine particles with a narrow size distribution is the Shirazu porous glass (SPG) emulsification technique [6].

Wall-building

After adding the monomers and adjusting the appropriate reaction conditions (e.g. pH, temperature) the wall-building process starts. The wall-building reaction can be a polycondensation, polyaddition or radical-polymerisation process. The monomers, crosslinking agents, catalysts, etc. must be soluble in at least one of phases. The polymeric wall-material has to be insoluble in both phases. The achieved wall thickness depends on the concentration of monomers and the reaction conditions. Typical values for the wall thickness of microcapsules produced by interfacial reaction are between 50 and 150 nm.

Several quite different chemical reactions have been used to create microcapsules via an interfacial polymerisation process, e.g.:

•Poly(urea-formaldehyde) (PUF) [7],[8]

The wall-building reaction involves polycondensation of urea and formaldehyde. This reaction is widely used for encapsulation of resins or monomers for self-healing systems (see Chapter 3.3). The reaction takes place under moderate temperatures (<80 °C) and leads to a capsule-polymer as shown in Figure 2.19.

•Poly(melamine-formaldehyde) (PMF) [9]

In this case the wall-building reaction involves polycondensation of melamine and formaldehyde. The process and the chemical nature of the polymer are quite similar to PUF. Capsule walls made of PMF have different mechanical properties in terms of their strength and their interactions with polymeric matrices of coating systems.

•Polymerisation products of epoxy resins and carboxylic acids [10]

A versatile process for the formation of capsule walls is the reaction of an epoxy resin with bi- and tri-functional carboxylic acids. The choice of different acids allows optimisation of the properties of the polymer for the desired application. A microencapsulation process using this reaction has been reported recently, where a diglycidyl ether of bisphenol A was crosslinked with decanedioic acid and 1,3,5-benzenetricarboxylic acid. This process led to stable microcapsules with diameters ranging from 10 to 400 μm depending on the reaction conditions.

•Polyurea [3], [11]

The reaction of difunctional isocyanates with bi- or trifunctional amines or with hydrolysed isocyanates leads to the formation of polyurea. In this case, microcapsules are formed in an emulsion where the organic (dispersed) phase contains the isocyanate, e.g. hexamethylene-1,6-diisocyanate (HMDI) or toluene-2,4-diisocyanate (TDI); and the continuous phase contains the amine component, e.g. hexamethylene-1,6-diamine (HMDA) or diethylenetriamine (DETA). The reaction occurs rapidly at ambient temperature and leads to microcapsules in the range of 0.5 to 50 μm.

Figure 2.20: Principle of microencapsulation by coacervation. A: Dispersed core material in a continuous phase which contains the wall materials (e.g. gelatin/gum arabic) in solution; B: Onset of coacervation by precipitation of fine-particle microcoacervate from solution; C: Precipitation of microcoacervate on core material droplets; D: Coalescence of the microcoacervate on the wall material phase[1]

A microencapsulation process for water-soluble active agents is also possible. In this case an inverse process can be used, in which the dispersed phase is water-soluble and the continuous phase is organic. In this case the microencapsulation takes place in a w/o-type emulsion. One example is given below:

•Crosslinking of hydrophilic polymers e.g. poly(vinyl alcohol) [12]

The crosslinking of hydrophilic polymers, for example the reaction of poly(vinyl alcohol) with glutaraldehyde, can be used to produce semipermeable microcapsules that can be used for controlled release of active agents. The capsule size for this process ranges from 2 to 55 μm.

The microencapsulation via interfacial polymerisation is a very versatile method because the mechanical properties of the capsules, permeability, interaction with the coating matrix and the transparency of the capsules can be tailored for the specific application.

2.2.2.2Coacervation

Coacervation is a classical method for producing microcapsules and was used for instance for manufacturing the microcapsules for carbonless copy paper. The size of microcapsules produced by this method range from approximately 1 to several hundreds of microns. The principle for preparing microcapsules by coacervation is shown in Figure 2.20.

The process of coacervation follows basically three steps [13].

Step 1: Formation of three immiscible chemical phases

The immiscible phases are (i) a continuous phase, (ii) a core material phase, and (iii) a capsule material phase. To form these phases, a dispersion of the core material in a solution of the capsule material polymer in the continuous phase is formed (A in Figure 2.20). The capsule material phase is formed by precipitation of the capsule material (B in Figure 2.20