Coatings Formulation E-Book

Bodo Müller

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2023

Seitenzahl: 565

Beschreibung

The Mission: A single book covering the practical and scientific basics underpinning the strategic formulation of modern paint and coatings systems – from physicochemical concepts to the recipes themselves. This book explains and elaborates in some depth on the key principles of coatings formulation. Indispens-able for formulators. The Audience: Trainees, students and newcomers to the profession who are seeking to acquire a solid grounding in coatings formulation, along with experienced formulators wishing to deepen, extend or refresh their knowledge. A knowledge of chemistry and basic knowledge of binders, pigments and additives are required. The Value: Coatings formulation explained step by step. The book opens with a look at the composition of coatings, placing special emphasis on the base binder in each type. Advice on specific formu- lations is then given before formulation guidelines are analysed. Throughout, the focus is on coatings formulation and how to arrive at the final recipe. A special feature of the book is its detailed index, which allows the reader to conduct targeted searches for specific aspects of coatings formulation.

Leseprobe

Markus Schackmann

Bodo Müller

Coatings Formulation

An International Textbook

4th Completely Revised Edition

Cover: cakeio – stock.adobe.com

Bibliographische Information der Deutschen Bibliothek

Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.d-nb.de abrufbar.

Schackmann, Markus and Müller, Bodo:

Coatings Formulation: An International Textbook

4th Completely Revised Edition

Hanover: Vincentz Network 2023

European Coatings Library

Print ISBN 978-3-7486-0424-2

Online ISBN 978-3-7486-0660-4

© 2023 Vincentz Network GmbH & Co. KG, Hanover

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Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems.

Exclusion of liability

It should be noted that this book reflect the authors’ personal views, based upon their own knowledge. This does not absolve readers of the responsibility of performing their own tests with respect to the uses and applications of various processes or products described herein, and/or of obtaining additional advices regarding the same. Any liability of the authors is excluded to the extent permitted by law, subject to all legal interpretations.

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Foreword

The authors would like to thank Prof. Bodo Müller and Ulrich Poth for their extensive and well-founded preparatory work. After the unexpected death of Prof. Poth in 2018 and Mr. Müller's retirement in 2017, the new authors have been given the opportunity to take over the baton and are delighted about the nomination.

There is no other book on the market that describes formulations in such detail. Although there are numerous helpful guideline formulations from raw material manufacturers and partly developed at Esslingen University, there are usually hardly commercializable for economic, ecological or technical reasons and best a rough starting formulation. Therefore, the further development of paints and coatings, as well as building materials and adhesives, is an essential aspect to ensure the competitiveness of the manufacturing companies. In this book, important relationships between raw materials and their interaction are described and explained. Although the phenomenological experience of people working in the laboratory is invaluable, a sound knowledge of raw materials and their composition is an important supplement to make it easier to apply the knowledge gained to new tasks.

In addition to the book "Understanding Additives", the reader receives a comprehensive picture of the coating raw materials and, above all, their interaction.

The book at hand will teach paint formulation in two steps. In each chapter, the chemical composition and in particular the binders of the coatings presented are first described. This will then be followed by formulation advice and an analysis of existing recipes (e.g. starting formulations). This analysis consists in calculating the important characteristic values of coatings, such as the pigment/binder ratio, pigment volume concentration and, as necessary, the hardener addition level. Finally, examples of how to develop a real-life paint formulation are provided in the case of the most important types of coatings. All calculations based on recipes and formulations are worked through step by step and should therefore be intelligible to beginners, as well.

The skills acquired in dealing with these recipes can also be employed in other applications, such as adhesives and sealants. This book focuses on the paint formulation itself, and how to arrive at it.

Of the many various paint and coating systems available, the selection provided in this textbook features the most important types. The formulations have been developed mostly from starting formulations or patent examples and cannot be used to produce paints without further ado. Patent restrictions or registered trademarks (™ or ®) are not mentioned explicitly. Furthermore, it should be noted that product and trade names may change as a result of mergers and acquisitions. Nonetheless, most of the raw materials described herein or their equivalents should be available worldwide.

This textbook seeks to familiarize laboratory assistants, engineers and chemists with the practice of formulating paints. It presupposes a basic knowledge of chemistry, binders, pigments and additives. It will also serve as a reference work for all readers interested in paints and coatings.

Esslingen, Germany in November 2022

Markus Schackmann

[email protected]

Contents

Foreword

Part I Basics

1 Introduction

1.1 Preliminary remarks

1.2 Comments on environmental protection

1.3 Paints and coatings as high-tech products

1.4 Definitions

1.5 Coatings

1.5.1 Solidification of paints

1.5.2 Phase boundaries in coatings

1.6 Adhesion

1.6.1 Wetting of substrates

1.6.2 Adhesion forces and mechanisms

1.6.3 Adhesion promoters/adhesion-promoting layers

1.6.4 Corrosion inhibitors, anticorrosive pigments, corrosion protection additives

1.7 Systematic classification

1.8 References

2 Pigment dispersions

2.1 Fundamentals of disperse systems

2.2 Stabilization of dispersions

2.2.1 Electrostatic stabilization

2.2.2 Entropic stabilization

2.3 Wetting and dispersing agents

2.3.1 Dispersing agents

2.3.2 Wetting agents (surfactants)

2.4 Wetting of pigments

2.5 References

3 Paint formulation

3.1 Ratio of binder to solid particles

3.1.1 Pigment/binder ratio and pigment volume concentration

3.1.2 Oil adsorption value

3.2 Influence of pigments on the properties of coatings

3.3 Development of paint formulations

3.4 Multi-coat systems

3.5 References

Part II Solvent-borne paints

1 Paints that form films at ambient temperatures

1.1 Physically drying paints

1.1.1 Paints based on cellulose nitrate

1.1.2 Physically drying paints based on acrylic resins

1.1.3 Paints based on rubber modifications

1.2 Oxidatively curing paints

1.2.1 Oxidative-curing reactions

1.2.2 Binders for oxidative curing

1.2.3 Siccatives and anti-skinning additives

1.2.4 Oil varnishes

1.2.5 Alkyd resin paints

1.2.6 Paints based on epoxy esters

1.3 Two-component systems

1.3.1 Two-component polyurethane paints

1.3.2 Two-component epoxy systems (2C-EP)

1.4 Coatings based on silane-terminated prepolymers

1.4.1 Silane-terminated polyurethanes

1.4.2 Silane-terminated epoxy resins

1.5 References

2 Stoving enamels

2.1 Definitions

2.2 Stoving enamels based on amino resins

2.2.1 Chemical structure of amino resins

2.2.2 Types and properties of amino resins

2.2.3 Combination partners for amino resins

2.2.4 Crosslinking reactions

2.2.5 Catalysis of crosslinking reactions

2.2.6 Formulation of stoving enamels based on amino resins

2.3 Stoving enamels based on thermosetting phenolic resins (resols)

2.4 Stoving enamels based on blocked polyisocyanates

2.4.1 Structure and properties of blocked polyisocyanates

2.4.2 Combination partners for blocked polyisocyanates

2.4.3 Comparison of blocked polyisocyanates and amino resins in stoving enamels

2.4.4 Formulation of stoving enamels based on blocked polyisocyanates

2.5 Other solvent-borne stoving enamels

2.5.1 Self-crosslinking acrylic resins

2.5.2 Self-crosslinking polyesters

2.5.3 Reaction of epoxy groups with acid derivatives.

2.5.4 Siloxanes in stoving enamels

2.6 Fastness to re-coating

2.7 References

Part III Water-borne paints

1 Solubility and dispersibility of paint resins in water

1.1 Exceptional position of water as a paint solvent

1.2 Distributions of polymers in water

1.3 Dispersions and emulsions of paint resins and polymers

1.3.1 Primary dispersions

1.3.2 Emulsions of liquid paint resins

1.3.3 Secondary dispersions

1.4 Aqueous solutions of paint resins

1.4.1 Water-solubility of paint resins

1.4.2 Neutralizing agents

1.4.3 Cosolvents

2 Water-borne paints and coatings that dry/cure at ambient temperatures

2.1 Physically drying paints

2.1.1 Film formation by primary dispersions

2.1.2 Latex gloss enamels

2.2 Facade coatings

2.2.1 Latex paints

2.2.2 Silicone resin paints

2.2.3 Silicate paints (two-components)

2.2.4 Latex silicate paints (one-component)

3 Water-borne paints that cure at ambient temperatures

3.1 Water-borne paints that cure oxidatively

3.1.1 Water-borne paints based on alkyd resins

3.1.2 Further oxidatively curing binders

3.1.3 Hybrid systems

3.2 Two-component, water-borne systems

3.2.1 Two-component, water-borne polyurethane paints

3.2.2 Water-borne two components epoxy paints

3.3 References

4 Water-borne stoving enamels

4.1 Guidelines for water-borne stoving enamels

4.2 Water-borne stoving enamels based on amino resins

4.3 Thermosetting coatings based on phenolic resins (resols)

4.4 Electrodeposition paints

4.4.1 Electrodeposition processes

4.4.2 Anionic deposition paints

4.4.3 Cationic electrodeposition paints

4.5 References

Part IV Solvent-free coatings

1 Two-components systems

1.1 Two-components polyurethane coatings

1.2 Two-components epoxy coatings

1.3 Coatings based on unsaturated polyester resins

2 Radiation curing

2.1 Definitions

2.2 UV curing

2.2.1 Principles of UV curing

2.2.2 UV coating process

2.2.3 UV initiators and sensitizers

2.2.4 Resins for UV coatings

2.2.5 Reactive diluents for UV coatings

2.2.6 Properties and application of UV coatings

2.2.7 Typical UV coatings

2.3 Electron beam curing

3 Powder coatings

3.1 Development of powder coatings

3.2 Production of powder coating materials and general properties

3.3 Application of powder coatings

3.3.1 Fluid bed sintering

3.3.2 Electrostatic spray application

3.4 Composition of powder coatings and special properties

3.4.1 Thermoplastic powder coatings

3.4.2 Crosslinkable powder coatings

3.4.3 Prospects for powder coatings

3.5 References

Part V Design of experiments

1 Experiments – past, present and future

2 DoE in a nutshell

3 Rules and good practice

4 The path to DoE in coatings formulation

4.1 The planning phase

4.2 Execution

4.3 Analysis and interpretation of the results

5 Above and beyond

6 References

Literature

Authors

Stichwortverzeichnis

Part I Basics

1Introduction

1.1Preliminary remarks

Paints are semifinished products (intermediates); the end products are the coated objects. Paints are used in a wide variety of applications (Figure I-1.1).

Figure I-1.1 shows that protective paints for buildings accounted for the largest proportion, followed by paints for general industry and other applications. All these different paint systems (with exception of printing inks) will be described in this textbook, especially with regard to challenges of paint formulation.

1.2Comments on environmental protection

The general public unfortunately has a negative image of paints because organic solvents are emitted into the atmosphere when solvent-borne paints are applied. In the past, most decorative or protective paints were solvent-borne. In this chapter, we would like to redress this negative image by describing the emissions-lowering measures that have been taken to improve the environmental safety performance of coatings.

In Germany, emissions into the atmosphere are regulated by “TA Luft”, which limits the level of organic solvents that may be emitted during paint application. This legislation therefore actively contributes to environmental protection. There are three ways to lower the level of organic solvents emitted from paint formulations see next page:

Figure I-1.1: Analysis of paint and printing ink selling in Germany in 2019

Source: VDL e.V.

Use high-solids paints (paints with a high content of non-volatile content and therefore low content of organic solvents)

Replace organic solvents by water

Water-borne paints

Latex paints

Use solvent-free systems

Two-component systems

Radiation-curing coatings

Powder coatings

Table I-1.1: Classification of solvent-borne (pigmented) paints

paints type

nonvolatile content (wt.%)

low-solids

< 30

normal-solids

30 to 60

medium-solids

60 to 70

high-solids

> 70(sometimes > 80)

ultra-high-solids

> 90

All these low-emission, ecologically beneficial paints will be described in this book.

For the sake of clarity, the solids contents of paints are classified in Table I-1.1 while ultra-high-solids with a solid content of > 90 % have recently been added.

The level of organic solvents emitted can also be lowered by certain application methods, e.g. those which have a high transfer efficiency, such as electrostatic spraying, and those which adsorb the solvent as the paint is being applied.

The trend to replace more and more solvent-based paints with water-based paints since the introduction of emission guidelines is weakening in some application areas. Especially in industrial coatings or in the automotive industry, drying ovens are used to cure the paint. Since water requires an extraordinary amount of energy and time to evaporate, the operation of such plants is extremely energy-intensive. This is illustrated by the growing focus on energy management (e.g. according to ISO 50001). Therefore, the industry is increasingly pursuing the development of high-solids and solvent-free coatings.

The reduction in the levels of organic solvents emitted during automotive painting (OEM) is shown in Figure I-1.2[1]; there has been much progress in this regard over the last two decades.

Figure I-1.2: Total emissions (g) of organic solvents during painting of vehicles (OEM), expressed in terms of surface area (m2) of the vehicle bodies (average values; years may vary with vehicle producer)

Corrosion protection as a means of protecting the environment

The second measure taken to protect the environment is not immediately apparent and will be illustrated with the example of iron corrosion (unalloyed steel; Figure I-1.3).

Figure I-1.3: Corrosion protection as a means of protecting the environment (highly simplified equations)

Thermodynamically, the atmospheric corrosion of iron or unalloyed steel (formation of rust: Fe2O3 · H2O) is the opposite of its production. The production of iron (in a blast furnace) uses iron ore (e.g. Fe2O3) and coal or coke (C) and generates the greenhouse gas, carbon dioxide (CO2; Figure I-1.3). Corrosion protection extends the working life of corrodible metallic materials (e.g. steel). This means that raw materials and energy are conserved and there is less of a burden placed on the environment. Besides preserving value, any corrosion protection measure, e.g. coating, therefore also serves to protect the environment. For example, only 5 to 10 kg paint is required for coating a 1,000 kg vehicle; but this coating greatly extends the working life.

Integrated view of paints and coatings

Figure I-1.2 is an integrated view of paints and coatings, ranging from the recovery of resources (e.g. oil production) to the manufacture of raw materials and paints. It also includes the disposal of coated objects when their working life is over [2].

Any assessment of the ecological impact of a paint system must consider all the production steps shown in Figure I-1.4.

Figure I-1.4: Integrated view of paints and coatings. By recovery of resources is meant, e.g., oil recovery. Key chemicals are, e.g., ethylene, propylene. Examples of chemical intermediates are acrylic acid, epichlorohydrin.

1.3Paints and coatings as high-tech products

As already mentioned in chapter Chapter I-1.2, paints often have a negative image because of solvent emissions. Measures that lower levels of solvent emissions, such as the introduction of water-borne paints and powder coatings (Figure I-1.2), are not reported in the mass media yet. On the other hand, there is much public debate about so-called future or key technologies, especially information technology and biotechnology. More recently, nanotechnology has been mentioned in this context. Unlike coatings technology, terms such as “technology of the future” and “key technology” have a positive image in this public debate.

A sub-area of nanotechnology is that of nano-particles, which are particles with diameters of 100 nm or less [3], [4]. Coatings experts prick up their ears since many disperse pigments and fillers are nothing other than nanoparticles. Long-established paint raw materials, such as carbon blacks and pyrogenic (fumed) silica (see Table I-2.3), are now considered as nanoparticles [3].

In addition, nanostructures have generated further innovative ideas in coatings technology. Recent examples are the chemical incorporation of nanoscale silica into paint resins [5] and the pretreatment of metals by nanostructured layers of silica [6]; both nanostructures are made by sol-gel processes [7]. A current review of nanomaterial technology applications in coatings is presented in [25]. In conclusion, there are many links between nanotechnology (with its special public image) and coatings technology (and its negative public image) that have gone unnoticed in the past. Paints and coatings are high-tech products and their public image should benefit from a greater general awareness of these considerations.

1.4Definitions

The following necessary definitions and abbreviations are mostly in accordance with DIN and EN [8], [9].

Coating material is the generic term for liquid-to-pasty or powdery materials which consist of binders and, if necessary, additionally of pigments, other colourants, fillers, solvents and additives.

Coating materials can be subdivided as follows:

Paints

Coating materials for plasters

Knifing fillers

Special coating materials (e.g. for floors)

The paints or coating materials described in this book will be classified in Chapter I-1.7.

The binder is the non-volatile content of a coating material without pigments and fillers but including plasticizers, driers and other non-volatile additives. The binder bonds the pigment particles to each other and to the substrate.

To simplify matters, driers and other non-volatile additives may be neglected in calculations of characteristic coating values of coatings (Chapter I-3.1), without risk of major error.

Colourant is the generic term for all colour-bearing substances; they may be further subdivided as shown in Figure I-1.5.

Figure I-1.5: Colourants

Pigments are particles which are virtually insoluble in the paint or coating composition. Synthetic inorganic pigments (Figure I-1.5) can be subdivided into white, black, coloured and effect pigments. Anisometric effect pigments are, e.g. metal effect and pearlescent pigments. In Figure I-1.5 , only colourants and effect pigments are presented (in many cases, effect pigments are coloured too). Luminescent pigments have been neglected here for the sake of simplicity. Functional inorganic pigments, such as corrosion protection pigments, flame-retardant pigments, conductive pigments, and magnetic pigments are beyond the scope of this systematic presentation and are therefore not included in Figure I-1.5.

Fillers (extenders) are powdery materials (particles) which are virtually insoluble in the paint or coating composition. They are mostly used to extend the volume (lower the price), to confer or to improve technical properties (e.g. abrasion or stone-chip resistance) and/or to influence optical properties.

It should be mentioned that fillers have only minor colour-bearing properties; in some cases, (e.g. latex paints, see Chapter III-2), they are used like white pigments to increase hiding power. Mostly, fillers are inorganic substances.

The solvent is a liquid that mostly consists of several components and dissolves binders without chemical reaction. Solvents must be volatile when solidification of the paint (film formation) takes place. If the binder is not dissolved but dispersed, the liquid phase (often water) is called the dispersion medium (see Chapter 1, Part III).

Additives are substances added in small proportions (usually less than 5 wt.-%) to coating compositions to modify or improve properties of the liquid paint (e.g. rheology) or of the solid coating (e.g. gloss).

The following helpful abbreviations for describing the binders of coatings will be used in this book (Table I-1.2).

1.5Coatings

1.5.1Solidification of paints

Solidification (also called film formation) is the transition of an applied paint from the liquid to the solid state. A distinction is made between (physical) drying and (chemical) curing, and they can take place simultaneously or one after another. Solidification transforms the intermediate paint product into the final product, which is the solid coating adhering on the substrate.

Physical drying

Table I-1.2: Abbreviations for some binders

letter symbol

binder (paint resin)

AK

alkyd resin

SP

saturated polyester

UP

unsaturated polyester

AY

acrylic resin

CAB

cellulose acetobutyrate

CN

cellulose nitrate

EP

epoxy resin

EPE

epoxy ester resin

MF

melamine resin

PF

phenolic resin

UF

urea resin

PUR

polyurethane

PVAC

polyvinyl acetate

PVB

polyvinyl butyrale

PVC

polyvinyl chloride

RUC

chlorinated rubber

RUI

cyclized rubber

Physical drying of an applied paint is the transition from the liquid to the solid state by evaporation of solvents (including water).

The dissolved binder molecules first form polymer coils flooded by solvents (solvates) which are mobile in a phase of free solvent (Figure I-1.6). The bound and free solvents are in equilibrium. The vapour pressure of a solvent in a binder solution is lower than that of the pure solvent. On account of its vapour pressure, the solvent (the free solvent initially) will always evaporate far below its boiling point if an adequate volume of air prevents the establishment of an equilibrium vapour pressure in the gas phase above the paint film. For complete physical drying, adequate ventilation is necessary.

Figure I-1.6: Simplified diagram of a diluted binder solution

Because some of the solvent evaporates, the polymer coils (binder) approach each other; simultaneously there is a decrease in the level of bound solvent. Finally, the solvates come into close contact. At this stage, there are no longer any boundaries between the solvates and the first stage of solidification is reached. Further solvent evaporation becomes progressively slower. The film shrinks and further solidification occurs. Small fractions of solvent may remain in the film for a long time, even in stoving coatings; this phenomenon is called solvent retention.

In organosols [dispersions of binders in organic solvents (NAD or non-aqueous dispersion)], most of the solvent used as dispersion medium evaporates first of all. Ultimately, the polymer dispersion turns into a solution, namely in that part of those solvents which evaporate slowly and are good solvents for the polymer.

Plastisols (dispersions of polymers in plasticizers) solidify by gelation at higher temperatures; i.e. the polymers dissolve in the plasticizers (or vice versa), which become a solid film on cooling.

Physical drying of aqueous polymer dispersions (latices) is a special case and is described in Chapter III-1.

Another type of physical solidification is the cooling of melted thermoplastic powder coatings (see Chapter IV-3.4.1).

Chemical curing

Chemical curing of an applied paint is the transition from the liquid to the solid state accompanied by an increase in molar mass and crosslinking. Therefore, “oxidative drying” of alkyd resins must be called “oxidative curing” [8].

A basic requirement for any chemical reaction (including curing) is adequate mobility of the reacting molecules. This is the case in gases and liquids, but in solids it occurs only at the interfaces.

For this reason, liquid binders (see Chapter IV-1) and melts (see Chapter IV-3) solidify well by means of chemical reactions.

Binder solutions are particularly advantageous. In solution, binder molecules that would otherwise be solids are capable of reacting with each other. The first precondition is that the binder molecules be dispersed by the solvent and form solvates as described above. These solvates should be mobile in the free solvent. Second, the binder molecules must even be mobile when curing takes place in order that reactive functional groups may react chemically with each other.

In curing paints, mostly two different types of binders (or binder and crosslinking agent) have to be made to react with each other. A basic requirement for homogeneous chemical reactions between two different oligomers or polymers is compatibility.

Binders or binder and crosslinking agent are compatible when they contain similar structural parts or at least structural parts of similar polarity or solubility. If binders of inadequate compatibility have to be combined, the mixture of the two can be pre-reacted. In most cases, this is effected by chemical reactions at elevated temperatures. Typical pre-reactions are pre-condensation of saturated polyesters with either urea or silicone resins. A pre-adduct of an epoxy resin and a resol is described in detail in Chapter III-4.3.

The various molecules come together and react with each other by means of diffusion processes which should be sufficiently rapid. High-molecular, immobile molecules crosslink less effectively than low-molecular, mobile ones.

Curing (crosslinking) is employed especially in industrial paint applications because it can be accelerated to yield ready-to-use coatings within a relatively short time. Industrial cycle times determine the paint formulator’s choice of crosslinking reactions. On one hand, these must be effective within a given time. On the other, the paints must offer as much storage stability as possible. Consequently, the paint systems used crosslink only at elevated temperatures (stoving paints) or start curing after the addition of crosslinkers, initiators or catalysts (e.g. acid-curing paints). Moreover, it is possible to separate the reactive components in storage (two-components paint systems).

Dispersions can contain very high-molecular binders. This is advantageous if the paints are to undergo physical drying only. Chemical reactions in dispersions are more difficult to effect. Dispersed binders can be cured with added crosslinking agents which can be dissolved or dispersed too. These crosslinking agents must diffuse into the phase boundary and further into the centre of the dispersed binder particles if chemical curing reaction is to take place.

Often, dispersed binders are only crosslinked at the phase boundary. If the dispersed particles consist of a (pre)mixture of binder and crosslinking agent, the binder system can be cured effectively. Another possibility is to combine a polymer dispersion with a dissolved self-crosslinking binder; in this case the dispersed binder phase will not be crosslinked.

Powder coatings are cured effectively only if binder and crosslinking agents are mixed efficiently (polymer melt in the extruder) before the powder is ground (see Chapter IV-3.2).

The term emulsion needs to be explained here (especially in contrast to dispersions). Emulsions are two-phase systems with a liquid disperse phase (e.g. binder) and mostly water as the dispersion medium. Through physical drying only, binder emulsions will form permanently tacky films (e.g. the coatings on flycatchers). Solid coatings can be formed by binder emulsions only through curing (chemical reactions); examples are emulsions of long oil (oxidatively curing) alkyd resins or emulsions of liquid epoxy resins (Chapter III-3.2.2).

Dispersions, however, are two-phase systems with a solid disperse phase (e.g. binder) and mostly water as the dispersion medium (see above). Through physical drying only, dispersions can form solid films – in contrast to the case for emulsions (e.g. latex gloss enamels and latex paints; Chapter III-2.1.2 and III-2.2.1).

Organic binders as polymeric materials

At this point, we will now discuss coatings from the point of view of plastics technology and polymer science; this is unusual, but offers interesting insights. Organic binders in coatings are nothing more than polymeric plastic materials and can be classified according to the rules of plastics technology (Figure I-1.7 and Figure I-1.8).

Figure I-1.7: Classification of polymeric plastic materials

Figure I-1.8: Schematic diagram of polymeric plastic materials

Coatings (films) formed by physical drying are plastomers. Coatings formed by chemical curing reactions are thermosets. Elastomers are not commonly used in coatings technology and can be neglected.

Plastomers are thermoplastic and soluble in suitable solvents. The rate of dissolution of physically dried coatings is usually low because of the low surface area (planimetric surface area); this is advantageous but should not be confused with solubility. Thermosets are unmeltable; at elevated temperatures they decompose by chemical degradation. Moreover, thermosets are insoluble; sometimes cured coatings show some swelling which may lead to (mostly reversible) softening of the coating.

1.5.2Phase boundaries in coatings

Non-pigmented clearcoats may have heterogeneous film structures caused by, e.g. nonuniform crosslinking or separation of binders [10]. The consequence of this heterogeneity could be large inner phase boundaries originating from various binder phases. Furthermore, certain functional groups of the binder (e.g. carboxyl groups) can be oriented towards polar substrates (e.g. metal oxides), and that also causes heterogeneity [10].

Generally, film structures containing pigments and fillers are heterogeneous and have large phase boundaries (so-called phase-boundary-dominated systems).

Coatings exhibit as may as four different phase boundaries (Figure I-1.9).

The first phase boundary is the binder/substrate interface; this phase boundary is equal to the planimetric surface area of the substrate (may be four times as large because of the roughness of the substrate surface). The function of this interface is adhesion (and, on metal substrates, corrosion protection).

The second phase boundary is the binder/pigment or filler interface; this phase boundary can be very large because of the large specific surface area of pigments (up to 100 m

2

/g; see

Chapter I-2

). This interface is responsible for the internal cohesion of the coating and may influence its mechanical properties (such as stone-chip resistance). A pigmented coating may therefore be viewed as a (very thin) composite material. Moreover, this interface can influence the corrosion protection properties of the coating

[20]

.

The third phase boundary is the binder/atmosphere interface (in the case of coatings for hydraulic steel constructions, a binder/water interface). This phase boundary is approximately equal to the planimetric surface area. Of great practical interest is the fact that weathering of coatings takes place at this interface (perhaps also in the layers below).

Furthermore, there may be binder/binder phase boundaries if the binder consists of different phases.

As a rule, the binder structure at all phase boundaries is different from the polymer structure in the bulk phase, which has been discussed above.

Figure I-1.9: Simplified diagram of coating below the critical pigment volume concentration on a substrate (cross-section)

1.6Adhesion

Adhesive strength is a measure of the resistance of a coating to mechanical removal from the substrate; the usual unit of measurement in coatings technology is force/area (N/mm2; MPa).

Permanent adhesion (under wet conditions, too) of the coating on the substrate and in a multi-coat system is a basic prerequisite for the protective effect (e.g. corrosion protection).

One exception is strippable coatings, which are temporary coatings for protecting goods during transport.

1.6.1Wetting of substrates

A necessary (but not sufficient) prerequisite for good adhesion is adequate wetting of the substrate by the liquid paint during application. The substrate/air interface (surface) is converted into a substrate/liquid interface (an interface between two condensed, immiscible phases). During subsequent drying or curing, the film solidifies (Figure I-1.10).

Figure I-1.10: Interfaces on solid substrates (diagram not true to scale)

The term wetting is important in coatings technology because pigment particles also have to be wetted (Chapter I-2.3). Thus, wetting needs to be discussed in more detail.

Surface and interfacial tension

In a liquid such as water, all molecules in the bulk phase are uniformly surrounded by their neighbouring molecules. Thus, the attractive forces acting on these molecules extend equally in all directions in space and cancel each other out (Figure I-1.11). At the water/air interface, things change dramatically because there is a water molecule is surrounded by other water molecules only at the interface and in the direction of the bulk phase. Thus, the forces of attraction do not cancel each other; a force acts on the water molecule in the direction of the inner phase of the liquid (Figure I-1.11). The effect of this force is that the liquid’s surface becomes as small as possible. Therefor a droplet in gravity-free space is spherical – the sphere has a minimal surface area combined with maximum volume.

Figure I-1.11: Force diagram to explain surface tension

To enlarge a surface area, molecules have to move from the bulk to the surface. Thus, forces of attraction have to be overcome and work needs to be done or energy expended.

The following mathematical conversion shows that energy/area is equal to force/length. In other words, the surface tension is also a measure of surface energy:

energyareaJm2+m∙nm2Nmforcelength;interfacial or surface tension

Typical surface tensions of liquids are presented in Table I-1.3.

The higher the surface tension, the greater is the cohesion in the respective phase and the stronger are the forces of interaction between the atoms or molecules in that phase. Water molecules are strongly associated with each other by hydrogen bonds, which generate a high level of cohesion and a high surface tension. The water surface behaves like a skin. The less polar the liquids are, the lower is their surface tension.

Wetting

A simplified diagram of the wetting of a solid surface (substrate) by a liquid is presented in Figure I-1.12.

Figure I-1.12: Wetting of a solid substrate by a liquid.

γS

Surface tension of the solid substrate

γL

Surface tension of the liquid

γSL

Interfacial tension between solid substrate and liquidt

Θ

Contact angle of the liquid on the substrate

A measure of wetting is the contact angle Θ between the solid substrate and the applied liquid drop (Figure I-1.12). The smaller the contact angle Θ, the better is the wetting.

Wetting is mathematically described by Young’s equation:

Table I-1.3: Surface tensions [mN/m] of liquids

liquid

surfacetension [mN/m]

remark

mercury

500

liquid metal

water

73

epoxy resins

45 to 60

melamine resins

42 to 58

alkyd resins

33 to 60

acrylic resins

32 to 40

butyl glycol

32

xylene

29 to 30

white spirit

26 to 27

butyl acetate

25

butanol

23

white spirit

18 to 22

free of aromatic hydrocarbons

hexane

18

Table I-1.4: Critical surface tensions [mN/m] of solid substrates

solid substrates

surfacetension [mN/m]

glas

73

phosphated steel

43 to 46

poly(vinyl chloride)

39 to 42

tin-plated steel*

approx. 35

aluminium*

33 to 35

aluminium*

32 to 39

polypropylene

28 to 29

steel (untreated)*

29

polydimethylsiloxane

19

polytetrafluorethylene

19

* Solid metal (oxide) surfaces! Melted (liquid) metals have much higher surface tensions (see mercury in Table I-1.3)

In different references one may find a slight variation of these values.

In the literature, surface tensions of up to a few hundred mN/m are frequently given for metals (see mercury in Table I-1.3); however, these values refer to the liquid metals at the respective melting temperatures [12]. For wetting, however, the surface tension (at room temperature) of solid metal surfaces coated with oxide and adsorbate layers is important (Table I-1.4).

The critical surface tension of solids (glass, metal, plastic) can be measured indirectly determine via wetting tests (measurement of contact angles, Zismann method) and is given in Table I-1.4.

The critical surface tensions of solids (Table I-1.4) can be measured indirectly in wetting experiments [26], [27].

Possible changes in the surfaces of substrates (e.g. oxide layers, mould-release agents) have to be considered because there may be a great effect on the surface tension (see below).

Metal surfaces

If the prerequisite for sufficient wetting is γL < γS, then water should not spread on metal surfaces (see relatively low surface tensions of solid metals in Table I-1.4). This is borne out by measurements of contact angles of water on various pre-treated aluminium sheets (Table I-1.5); while the water does wet (Θ < 90°), no spreading occurs (Θ > 0°).

Table I-1.5: Contact angle of water on various aluminium surfaces

aluminium surface

contact angle Θ (± 5°)

rolled

63

pickled with a commercial caustic

22

pickled with NaOH

31

Figure I-1.13 is a schematic diagram of an aluminium surface [11] that would be better referred to as an aluminium oxide surface; similar considerations apply to all commonly used metals. If the model in Figure I-1.13 fully describes reality, the hydrated aluminium oxide surface would be wetted by water completely; but this is not observed (Table I-1.5). Figure I-1.13 is a simplification of the real situation. ESCA/XPS measurements show that various pre-treated aluminium surfaces have, in addition to aluminium and oxygen, a significant amount of carbon. Presumably, the carbon is adsorbed from the atmospere (e.g. carbon dioxide or hydrocarbons) [12]. Thus, while there is still no universal model that describes the structure of metal surfaces, it is certain that metal oxides are present.

Figure I-1.13: Hydrated aluminium (oxide) surface (very simplified model). Similar considerations apply to the Fe/Fe2O3 system

Surfaces of plastics

Even more complicated are the structures of surfaces of engineering plastics[11]. The problem here is that the bulk properties of the polymer are different from the surface properties. Theses differences may be caused by the composition of the plastic or by the production or processing conditions. Mostly the surfaces have low energies (low surface tension), which leads to poor wetting.

Composition of plastics

Many plastics contain low-molecular components, such as additives (e.g. stabilizers), residues of solvents and sometimes plasticizers. All these components can impair adhesion if they are on the surface. Many low-molecular components tend to migrate to the surface and accumulate there. Thus, there may be an anti-adhesive layer on the surface of plastics (Figure I-1.14).

Production and processing conditions of plastics

Mould-release agents

When moulded or compressed plastic parts have to be released from the mould, internal and external mould-release agents are used. Internal mould-release agents are mixed into the plastic pellets and are distributed completely in the plastic material; therefore, there is no point in sanding the plastic surface. Internal mould-release agents generate plastic surfaces that are either unwettable or barely wettable. External mould-release agents are sprayed into the open injection mould; they are based on paraffins, soaps and oils (including silicone oils). Because of the processing conditions, external mould-release agents are found not only in the surface layer, but also in the layers below.

Surface properties caused by processing conditions

Injection moulding or compression creates surface properties that differ from those of the bulk polymer. The dense surface layers of moulded plastic materials are very smooth (oriented layers;

Figure I-1.15

).

Figure I-1.14: Surface of engineering plastics

Improvements in wetting

In the case of water-borne paints especially, γL > γS and leads to poor wetting. There are two ways to improve wetting:

Lower the surface tension of the water-borne paint (γ

L

) by adding wetting agents (see

Chapter I-2.2.3

).

Increase the surface tension of the substrate (γ

S

).

The surface tension of metals may be increased, for example, by phosphating (see Table I-1.4). Oxidation of plastic surfaces (e.g. by flame treatment) generates polar functional groups (e.g. -OH, -COOH) on the surface and increases γS. A detailed description of pre-treatment processes for different plastics is presented in [11]. Sometimes sanding or rubbing with emery or cleaning with organic solvents or water-borne cleaners will improve the wettability of plastic surfaces. Furthermore, adhesion-promoting primers may be applied before the plastics are painted. For example, polyolefins (low surface tensions; Table I-1.4) can be coated with chlorinated polymers, which increase the surface tension.

1.6.2Adhesion forces and mechanisms

Until today there is no adhesion theory covering all phenomena; only partial areas can be explained theoretically. However, an only partial theoretical interpretation of phenomena such as adhesion or delamination is still better than none at all.

Adhesion/cohesion

Adhesion is defined as the effect of forces of attraction at the interface of two different solid phases. Adhesion is expressed in units of energy/area (compare surface tension). In contrast, the units of adhesive strength are force/area: N/mm2 (MPa).

The counterpart to adhesion is cohesion. Cohesion is the effect of forces of attraction within the same phase (e.g. in the solid coating). Cohesion is the state in which particles (molecules) of a single substance are held together; it is a special instance of adhesion in which only molecules of the same kind adhere to each other.

Adhesive failure

Loss of adhesion by coatings can lead to the following fracture patterns, which vary with the level of adhesion and cohesion.

Adhesive failure:

Adhesion < cohesion

Cohesive failure:

Adhesion > cohesion

(desired)

Both types of fractures:

Adhesion ≈ cohesion

Fracture of the substrate:

Adhesion resp. cohesion > strength of the substrate (rare)

It should be pointed out that adhesive failure is often cohesive failure in a weak layer of the coating close to the interface (“weak boundary layer”) that goes unrecognized. The chemical composition of the coating or the arrangement of the polymer molecules in the boundary layer often differs from that in the bulk coating. For example, a zone of reduced strength can be formed between the chemisorbed polymer on the substrate (monolayer) and the bulk polymer. For simplicity, this unwanted fracture pattern is usually called adhesive failure too.

Theories of adhesion

In general, a distinction is made between specific adhesion(interaction of interfaces independent of the geometrical shape of the surface) and mechanical adhesion (Figure I-1.15).

Figure I-1.15: Mechanisms of adhesion

Mechanical adhesion (Figure I-1.16) takes place when liquid paint enters into cavities (voids, roughness) of the substrate and the cured coating is anchored mechanically therein.

Figure I-1.16: Mechanical adhesion

Rough or even porous substrates are frequently encountered, e.g. crystalline and therefore rough, phosphated metal surfaces (see Figure I-3.11 later) and porous wood surfaces; Figure I-1.17 clearly shows the pores in beech wood.

Figure I-1.17: Scanning electron micrograph of beech wood

Prerequisites for efficient mechanical adhesion are adequate wetting of the substrate by the paint and a low paint viscosity.

A consequence of Table I-1.6 is that primary valence bonds between coating and substrate lead to optimal adhesion. Thus, primary valence bonds will be discussed first.

Table I-1.6: Types of chemical bonds and bond energies [13]

type of chemical bond

bond energy [kJ/mol]

ionic linkage (salt links)

covalent bonds

600 to 1000

60 to 700

permanent dipoles (Keesom)

induced dipoles (London)

< 20

< 2

dispersion forces (London)

hydrogen bonds

< 40

< 50

In different references one may find a slight variation of these values.

Ionic bonds

Ionic bonds (e.g. salt formation) are formed especially on mineral substrates, such as metal oxide layers, phosphated metal surfaces, concrete and so on (Figure I-1.18).

Figure I-1.18: Salt formation on metal oxide surfaces

Figure I-1.18 shows an acid-base reaction between a hydroxyl group at the surface and a carboxyl group from a binder and a metal salt or complex. The disadvantage of ionic bonds is that permeating water can break them and that can lead to loss of adhesion under wet conditions.

Chelate complexes on surfaces

Chelate complexes on metal (oxide) surfaces should be largely stable to hydrolysis and lead to improved adhesion in wet conditions. Experience shows that epoxy resins cured with amines and cured phenolic resins (resols) exhibit very good adhesion on metals, even under wet conditions; both observations can be explained by assuming that chelate complexes are formed (Figure I-1.19 and Figure I-1.20) [14], [15].

Figure I-1.19 (left above): Adhesion of 2-components epoxy coatings by the formation of chelate complexes (model)

Figure I-1.20 (right above): Adhesion of resols by the formation of chelate complexes (model)

Chelates forming 6-member rings are known to be stable (Figure I-1.20) [15]. Recent molecular calculations [16] suggest that a second resol-metal chelate (8-member-ring chelate in Figure I-1.20) may be formed. As a rule, chelates forming 8-member rings are less stable than their 6-member counterparts but should also be considered in this special case.

Covalent bonds

A further possibility is the formation of covalent bonds on polymer substrates, such as primers and plastics, by chemical reaction between the binder and suitable functional groups on the polymer substrate. There is a rule in the coating of curing primers with topcoats that the primers should not be completely cured before the application of the topcoat because otherwise the binder of the primer will not have any reactive functional groups left for reaction with the topcoat. In contrast, oxidation of plastic surfaces (e.g. flame treatment, Chapter I-1.7) generates reactive functional groups (e.g. -OH, -COOH) that may react chemically with the binder of a coating.

Secondary valence bonds

The less effective secondary valence bonds (Table I-1.6) are the more common type of specific adhesion. A prerequisite for this is close contact between coating and substrate because secondary valence bonds have only a limited radius of action. The following types of secondary valence bonds exist:

Dipole forces (dipole-dipole interaction)

Prerequisite: Polar groups (permanent dipoles) on the surface of the substrate and in the coating.

Hydrogen bonds

A disadvantage of hydrogen bonds (

Figure I-1.21

) is that they can be broken by water and that may decrease the level of adhesion under wet conditions.

Van-der-Waals-forces are of universal importance and relatively weak non-covalent interactions between atoms and molekules. Whose interaction energy for spherical particles drops to about the sixth power of the distance. According to today's understanding, the Van der Waals forces can thus be divided into three components:

The Keesom interaction between two dipoles (dipole-dipole forces).

The Debye interaction between a dipole and a polarisable molecule (dipole-induced dipole forces).

The London dispersion interaction (London forces) between two polarizable molecules (induced dipole-induced dipole forces). The London forces are often referred to as Van-der-Waals-forces in the narrower sense.

Dispersion forces are of universal importance, but not very specific. Dispersion forces increase with increase in molar mass. In nonpolar materials (e.g. polyethylene), dispersion forces are virtually the only forces present.

The attractive forces are not very strong, but very numerous, therefore these forces are not negligible.

Figure I-1.21 (below): Hydrogen bonds on metal (oxide) surfaces (model)

Further adhesion theories

Adhesion by means of covalent bonds could alternatively be called chemisorption (adsorbate and substrate react chemically) while adhesion by secondary valence bonds could be termed physisorption (neither adsorbate nor substrate is chemically involved) – this is the so-called adsorption theory. Other adhesion theories exist:

Diffusion theory

This theory applies to polymer substrates such as plastic, or previous polymer layers like primers. The coating adheres by diffusion of oligomer chains of the binder into the surface layer of the polymer substrate; this leads to a mechanical linkage. Diffusion takes place especially above the glass transition temperature Tg of polymers. Swelling of the polymer substrate by organic solvents also improves diffusion because Tg is lowered (plasticization).

For example, the adhesion of water-borne paints on plastics can be improved by adding a slowly evaporating cosolvent, such as N-methylpyrrolidone (NMP; see Chapter III-1.4.3), or by rubbing the plastic with a suitable organic solvent. Both actions causes temporary swelling of the polymer substrate.

Electrostatic adhesion

This effect is not very important in coatings technology. Electrostatic adhesion occurs by close contact of two different polymers or a polymer (e.g. binder) and a metal. Because of different electronic emission energies, electron transfer can occur between the two materials. Electron transfer creates an electrical double layer that induces electrostatic attraction.

1.6.3Adhesion promoters/adhesion-promoting layers

1.6.3.1Silane adhesion promoters

Silane adhesion promoters are well established in adhesives technology [11], [17], [27]. They can also be called functional trialkoxysilanes (Figure I-1.22).

Figure I-1.22: Typical silane adhesion promoters

Function of silane adhesion promoters

Hydrolysis of alkoxy groups by atmospheric moisture yields reactive silanol groups (Figure I-1.23). Silanol groups can react by condensation with the hydroxyl groups on the surfaces of metals (Me-OH), glass (Si-OH), mineral fillers and construction materials (e.g. Si-OH, Ti-OH, Al-OH) and with each other (Si-OH). The reaction with hydroxyl groups on surfaces effects a change in the functionality of the inorganic surface (Figure I-1.23). Functional group X, with a corresponding functionality of the binder, can react with the binder to produce a covalent bond between the binder and the inorganic surface. Figure I-1.24 shows an example of this type of covalent bonding produced by the reaction of aminosilane with epoxy resin.

Figure I-1.23: Reaction of silane adhesion promoters

Figure I-1.24 (left above): Reaction between silane adhesion promoter and epoxy resin

Silane adhesion promoters are also called coupling agents (Figure I-1.25) [18]. The effectiveness of silane adhesion promoters strongly depends on the substrate (Figure I-1.26) [11].

Figure I-1.25 (left below): Silane adhesion promoters as coupling agents

Figure I-1.26 (right): Effectiveness of silane adhesion promoters for various substrates

Applications of silane adhesion promoters

Coupling of adhesives and sealants (significant)

[11]

.

Adhesive bonding and sealing of glass, metals and mineral construction materials.

Coupling of binders and mineral fillers

Inorganic fillers such as talc may be surface-treated with silane adhesion promoters. Choosing an appropriate silane for the respective binder can improve the bond between the (cured) binder and filler. Thus, water-vapour diffusion through the coating can be decreased and corrosion protection improved

[20]

.

Coupling of coatings (so far of minor significance); aimed especially at improving adhesion in wet conditions

[19]

.

It should be noted that silane adhesion promoters in paints may react with traces of water in solvent-borne paints or with atmospheric moisture (

Figure I-1.27

). These side-reactions (hydrolysis + condensation,

Figure I-1.27

) produce polysiloxanes which may cause cratering. Before silane adhesion promoters are added, traces of water should be removed with the appropriate additives (e.g. molecular sieves); overly high humidity should be avoided if possible.

Figure I-1.27: Side-reactions of silane adhesion promoters

Figure I-1.28: Composition of an adhesive layer of polyacrylic acid on steel

1.6.3.2Thin polymeric adhesive layers

Thin layers of poly(meth)acrylic acids can improve adhesion of cured coatings (e.g. AK/MF or two-components PUR) on steel in wet conditions [21]. These layers are produced by dipping steel sheets into dilute (about 1 wt.-%) aqueous solutions of poly(meth)acrylic acids (for 1 to 2 min); pH about 3. Iron is converted into iron(II) to yield an organic conversion coating.

The thickness of these very thin layers is 20 to 30 nm. For polyacrylic acid, the optimum efficiency was established to be a molar mass of about 100,000 g/mol [21]. A rough chemical composition of such adhesive layers is presented in Figure I-1.28. Phenolic resins (resols) and epoxy resins may also generate thin adhesive layers [13].

1.6.4Corrosion inhibitors, anticorrosive pigments, corrosion protection additives

Adhesion of a coating is a necessary but on its own insufficient requirement for corrosion protection. Additional protective measures should be considered. Thus, corrosion inhibitors, anticorrosive pigments, corrosion protective additives will be discussed briefly in this chapter.

Anticorrosive pigments can be classified as follows:

Chemically active anticorrosive pigments

(e.g. zinc oxide) bind corrosion stimulators such as chloride or sulphate by forming insoluble compounds and/or stabilizing the pH value of a coating in contact with the corrosion medium. Therefore, slight solubility in the corrosion medium is necessary.

Electrochemically active anticorrosive pigments

(e.g. zinc chromate, zinc phosphate) passivate metal surfaces by forming thin layers, such as chromate or phosphate layers. Again, slight solubility in the corrosion medium is necessary. The distinction from chemically active pigments a) is sometimes unclear because anticorrosive pigments exist which can act in both ways (e.g. zinc phosphate).

Active, cathodic protective anticorrosion pigments

(e.g. zinc dust) act as sacrificial anodes and protect the metal substrate (see

Chapter II-1.2.6

). They are “classic” pigments and insoluble.

Passive anticorrosive pigments

, barrier pigments (e.g. micaceous iron oxide, see

Figure I-3-8

) lengthen the diffusion pathways for corrosion stimulators and therefore improve the corrosion protection of a coating (see

Chapter I-3.2

). Again, they are “classic” pigments, and insoluble.

Anticorrosive pigments can be classified with respect to their solubility in corrosion media as slightly soluble (a, b) and completely insoluble (c, d).

Some highly active anticorrosive pigments (e.g. the carcinogenic zinc chromates) should not be used anymore on account of ecological and toxicological reasons. But relatively ecologically safe active anticorrosive pigments (e.g. phosphates) are comparatively less efficient. Thus, where possible, non-toxic organic additives are needed to improve the corrosion protection afforded by coatings. Such additives are commonly termed corrosion inhibitors[21], [23]; a more correct term is corrosion protection additives[24]. The difference between the two terms is explained below.

Corrosion inhibitors in particular are substances dissolved in a liquid medium that concentrate at the metal (oxide)/medium interface; i.e., a protective layer at the interface is generated by adsorption. This layer prevents or decreases corrosion (see electrochemically active anticorrosive pigments). This definition characterizes the primary property of corrosion inhibitors used in the water of heating and cooling circuits.

For application in anticorrosive paints, corrosion inhibitors have to meet additional requirements such as:

Adequate solubility in the liquid paint

Compatibility with the binder(s)

Low water solubility in the dried or cured coating

No negative side effects, such as discolouration or increased hydrophilicity of the coating

These demands have led to the development of corrosion inhibitors that possess secondary properties specifically for coatings [22], namely corrosion protection additives. Corrosion protection additives are a thus subset of corrosion inhibitors (Figure I-1.29). In other words, a corrosion protection additive is a corrosion inhibitor but the converse is not true.

Figure I-1.29: Sets of corrosion inhibitors, corrosion protection additives and different anticorrosive pigments (simplified diagram)

Figure I-1.29 tries to visualize with the aid of set theory the differences between corrosion inhibitors, corrosion protection additives and the four different types of anticorrosive pigments (a - d, see above).

1.7Systematic classification

Coatings can be classified according to very different criteria. In the present book they are divided as follows:

the use of a solvent (e.g. solvent-borne, water-borne or solvent-free),

the type of film formation (e.g. drying or curing),

the chemical composition of the binder (e.g. acrylic or cellulose nitrate paint),

the number of paint components,

the application method (e.g. dipping or spraying),

the function (e.g. primer or topcoat) or

the designated use (e.g. industrial coating or façade paint)

[28]

,

[29]

.

The objective of this section is to classify paints additionally with respect to film formation and in some cases with respect to the chemical composition of the binder with a view to highlighting further interesting relationships.

Usually, film formation occurs when an applied paint transitions from the liquid to the solid state. In certain instances, it might be better to call this transition solidification, as will be shown later.

Classification by type of film formation

Figure I-1.30 is an attempt to classify coatings systematically by the different mechanisms of film formation. Solidification at elevated temperatures means temperatures above 100 ºC; these stoving enamels (thermosetting coatings) are mostly one-component systems. In spite of its complexity, Figure I-1.30 only shows the most common types of coatings; specialities such as three-component systems are not included. Moreover, there are also major coating systems that do not fit into this systematic classification – namely thermoplastic powder coatings and sintering powders. These solidify physically and reversibly by cooling of a polymer melt. In these two specific cases, the term film formation is inappropriate and it is better to call this process solidification. The same applies to the solidification (gelation) of plastisols; this is also a purely physical process which, however, is irreversible, in contrast to the case for thermoplastic powder coatings. Plastisols serve, e.g., as automotive underbody coatings; for more details on plastisols, see [27].

One-component oxidatively and moisture curing coatings (Figure I-1.30: see solidification at ambient temperature → one-component → chemical curing) are strictly speaking two-component systems in which the second component is atmospheric oxygen or humidity. This fact is important for moisture curing, which proceeds extremely slow in a desert environment (where humidity is very low).

Water-borne latex paints are a major type of coating that is used in particular for protecting buildings (Figure I-1.30 see solidification at ambient temperature → one-component → physical drying). As physical drying yields solid coatings, the binder (disperse polymer particles) must be solid, and so this type of coating is a dispersion (solid disperse phase).

It would also be possible to further sub-classify the various types of coating in Figure I-1.30 in terms of the use of solvent, i.e. solvent-borne, water-borne and solvent-free. Of course, not every type of coating exists in all three variants. Interesting in this context are powder coatings, which by definition are solvent-free. However, there are also so-called powder-slurries, composed of an aqueous dispersion of a super-fine powder coating. This raises the question now as to whether a powder slurry is a powder-coating or a water-borne coating system? This conundrum shows that systematic classifications also have their limits.

Figure I-1.30: Systematic classification of film formation by coatings

2C: two-component systems – 1C: one-component systems

Classification by chemical composition of the binder

The classification of coatings by chemical composition of the binder naturally includes the type of film formation by the corresponding coating systems. It also needs to take account of classification on the basis of solvent use. In the following section, we concentrate on three important binder systems.

We start in Figure I-1.31 with a manageable example, namely coatings based on phenolic resins (PF). In the past, it was easy to differentiate between resols (chemically curing) and novolaks (physically drying). However, as modern novolaks (polyphenols) are also used in thermosetting powder coatings, classification is now more complicated.

Figure I-1.31: Systematic classification of coatings based on phenolic resins

More complex are coatings based on epoxy resins (EP), presented in Figure I-1.32. Here the epoxy resins are based on bisphenol A (with proportionate amounts of bisphenol F, as needed). These are classified as one-component (1C) and two-component (2C) systems. Two-component epoxy coatings are usually cured at ambient temperature. The one-component types can be divided into oxidatively curing epoxy ester resins and various types of stoving enamels based on epoxy resins (Figure I-1.32).

Figure I-1.32: Systematic classification of epoxy coatings (EP) based on bisphenol A epoxy resins (and proportionate amounts of bisphenol F, where necessary)

2C: two-component systems – 1C: one-component systems

The most complex case, coatings based on polyurethanes (PUR), is presented in Figure I-1.33. Here, too, the systems may be classified as one-component (1C) and two-component (2C) types. Two-component polyurethane systems are usually cured at ambient temperature but also at elevated temperatures [(e.g. an automotive, two-component polyurethane high-solids thermosetting clearcoat (OEM)]. One-component systems that solidify at ambient temperature are physically drying latex gloss enamels based on polyurethane dispersions (see second formulation in Chapter III-2.1.2) as well as the oxidatively and moisture-curing systems. The other one-component systems cure at elevated temperatures, e.g. “classic” stoving enamels.

Figure I-1.33: Systematic classification of polyurethane coatings (PUR); in spite of its complexity, this overview does not describe all polyurethane systems

2C: two-component systems – 1C: one-component systems

1.8References

[1]

A. Goldschmidt, H.-J. Streitberger, BASF Handbook on Basics of Coatings Technology, Vincentz Network (2003) and Perizonius, I-Lack 62 (1994) p. 82 ff

[2]

M. Harsch, Lacktechnologien ganzheitlich optimieren, Lack im Gespräch, Informationsdienst Deutsches Lackinstitut, No. 62, März 2000

[3]

A. Rössler, G. Skillas, S. E. Pratsinis, Chemie in unserer Zeit 35 (2001) No. 1, p. 32–41

[4]

C. Göbbert, M. Schichtel, R. Nonninger, Farbe & Lack, 108 (2002) No. 7, p. 20–24

[5]

T. Adebahr, Europ. Coat. Journ. No. 4, p. 144-149 (2001)

[6]

A. J. Vreugdenhil, V. N. Balbyshev, M. S. Donley, Journ. Coat. Technol. 73 (2001) No. 915, p. 35–43

[7]

A. C. Pierre, Introduction to Sol-Gel Processing, Kluwer Academic Publishers, 2nd ed. (2002)

[8]

DIN-Term, Beschichtungsstoffe, Vincentz Verlag, Hannover, 1st ed., 2001

[9]

Römpp Lexikon, Lacke und Druckfarben, Hrsg. U. Zorll, Thieme Verlag, 1998

[10]

W. Funke, U. Zorll, defazet, 29 (1975) p. 146 ff

[11]

E. M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, New York (2000), Chapter 6, 7 and 16 as well as appendix D-2.

[12]

W. Brockmann, S. Emrich, Adhäsion, 44 (2000) No. 9, p. 40–44

[13]

J. Gähde, Farbe & Lack 101 (1995) p. 689 ff

[14]

H. Kollek, C. Matz, Adhäsion 12/1989, p. 27 ff

[15]

B. Müller, kleben & dichten, Adhäsion, 46 (2002) No. 6, p. 34–38 and Surface Coat. Int. Chapter B, 85 (2002) p. 111-114

[16]

V. Strehmel, B. Strehmel, Modification of Phenol-Methanal Network Polymers, in Polymer Modification – Principles, Techniques and Applications, Marcel Dekker New York - Basel, p. 575–705 (2000) especially p. 599

[17]

E. P. Plueddemann, Silane Coupling Agents, Plenum Press, New York (1982) und U. Deschler et al., Angew. Chem. 98 (1986) p. 237-253

[18]

B. Meyer-Roscher, S. Wellman, H. Brockmann, kleben & dichten, Adhäsion, 39 (1995) No. 4, p. 34–37

[19]

W.-D. Kaiser, A. Rudolf, S. Pietsch, Farbe & Lack 103, No. 7 (1997) p. 80–88

[20]

N. Wamser, E. Urbino, Farbe & Lack 95 (1989) p. 109 ff

[21]

Z. Gao, H. Yamabe, B. Marold, W. Funke, Farbe & Lack, 98 (1992) p. 917 ff

[22]

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