Film Formation E-Book

Peter Mischke

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Serie: European Coatings TECH FILES
• Sprache: Englisch
• Veröffentlichungsjahr: 2014

Seitenzahl: 338

Beschreibung

This eBook of Peter Mischke helps to acquire an extensive understanding of Film Formation. From a brief explanation of the main coating concepts over methods of application up to the physical aspects of drying, Film Formation provides a holistic description of the basic principles. It covers also fundamental polymer and physicochemical aspects and in addition to that it contains a brief overview of test methods. It is suited for everyone aspiring to a deeper understanding of the mechanisms of Film Formation. For newcomers, career-changers, students and professionals who wish to deepen their theoretical knowledge of technique and coatings chemistry. Also for experts who wish to tackle specific problems beyond their daily practice.

Leseprobe

Vincentz Network GmbH & Co KG

Peter Mischke

Film Formation

in Modern Paint Systems

Translated by Ray Brown

Cover: BASF SE, Ludwigshafen, Germany

Bibliographische Information der Deutschen Bibliothek

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

Mischke, Peter

Film Formation in Modern Paint Systems

Hannover: Vincentz Network, 2010

(European Coatings Tech Files)

ISBN 3-86630-803-5

ISBN 978-3-86630-803-9

© 2010 Vincentz Network GmbH & Co. KG, Hannover, Germany

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This work is copyrighted, including the individual contributions and figures.

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ISBN 3-86630-803-5

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European Coatings Tech Files

Peter Mischke

Film Formation

in Modern Paint Systems

Translated by Ray Brown

Foreword

This book is all about how coating materials, especially paints, form films. A student asked me recently: “How is it possible to write a book on a topic like that?” Her question was right on the mark. There can be no doubting that common textbooks on surface coatings devote very few pages to the topic of film formation. And yet, close examination shows that film formation is a theme that surfaces and resurfaces in the expert literature and in technical discussions on coating technology, without ever being specifically addressed. And that leads on to the problems inherent in tackling the topic. These include how to isolate the subject matter from the related areas of application methods, chemistry/formulation, and metrology, and the rest of current coatings literature; the extent to which the latest specialist advances be taken into account. And last but not least: A university teacher in a broad sweep of disciplines ranging from chemical principles to colloid and polymer chemistry through to the science of binders, adhesives and paint chemistry, my purpose in writing this book is to present the basics of film formation in a broad canvas while striking the right balance of topics and not delving too far into other fields. I have drawn on my own expertise and university teaching experience as well as a wealth of information from monographs directly accessible to me and periodicals such as “Farbe und Lack”.

The book commences with a brief explanation of the main coating concepts, before presenting methods of application as the first step towards a finished coating. This is followed by the physical aspects of drying. The middle section of the book deals at length with fundamental polymer and physicochemical aspects. It is intended to enable readers who lack an academic background in chemistry to understand the more specific content of the later chapters, without first having to study a comprehensive textbook on the subject. The second half of the book covers the fundamental film-forming principles and coating systems. A brief overview of test methods used to study film formation is presented in Annex 4.

So who should read this book and why? The intended readership includes:

•third-level students of coating technology wishing to deepen and round out their knowledge of pure paint technology on one hand and pure paint chemistry on the other;

•newcomers and career-changers seeking a readable textbook that is not overloaded with individual facts and/or specialist material

•skilled personnel in paintshops wondering why the various effects they observe as they go about their daily work actually occur and how they might intervene to solve particular problems that arise;

•finally, the book may be particularly instructive to laboratory staff and technicians seeking to gain a deeper understanding of film-forming mechanisms.

My thanks to everyone from the Department of Coatings Technology at Niederrhein University of Applied Sciences who kindly provided thoughtful suggestions, information and hands-on support. I would also like to thank Ilia Korolinskij, a graphics expert, who converted numerous sketches into a print-ready form. Finally, I am forever indebted to my family for sacrificing a great deal of quality time together during the year-and-a-half which I spent writing the book.

Willich/Germany, August 2009

Peter Mischke

Foreword

1Introduction

1.1Basic concepts

1.2General composition of coating materials

1.3General formulation data

1.4Basic physical properties

1.4.1Density

1.4.2Viscosity and flow behaviour

1.4.3Surface tension

1.5Classification of film formation

2Application methods

2.1Spreading, flow coating

2.1.1Brush and roller

2.1.2Roller coating

2.1.3Curtain coating

2.1.4Flow coating

2.2Dip-coating (dipping)

2.2.1Conventional dip-coating

2.2.2Electrodeposition

2.2.3Autophoresis

2.3Spray-painting

2.3.1Air-spraying

2.3.1.1High-pressure spraying

2.3.1.2Low-pressure spraying

2.3.2Airless spraying

2.3.3Hot-spraying

2.3.4Spraying with supercritical carbon dioxide

2.4Electropainting

2.4.1Electrostatically assisted conventional spraying

2.4.2High-speed electrostatic atomisers

2.4.3Purely electrostatic methods

2.4.4Special effects of electropainting

2.5Other application methods for liquid and pasty materials

2.6General application conditions

2.7Powder coating application

2.7.1Powder coating

2.7.2Electrostatic powder spraying

2.7.3Powder-sintering methods

3Substrate wetting, levelling, sagging, edge pulling

3.1Substrate wetting

3.2Levelling

3.3Sagging

3.4Edge pulling

4Physical principles of paint drying and curing

4.1Convection drying

4.2Heat transfer by means of infrared radiation

4.2.1Physical principles

4.2.2Industrial infrared lamps

4.2.3Radiation input into the object for coating

4.3UV irradiation

4.3.1General information on UV radiation

4.3.2UV emitters

4.3.3UV radiant intensity and dosage

4.4Further industrial drying and curing methods

5General principles of film formation

5.1Polymers

5.1.1Basic definitions

5.1.2Homopolymers and copolymers

5.1.3Average molecular weight

5.1.4Basic types of polymer

5.1.5Molecular coils

5.1.6Intermolecular forces and aggregates

5.1.7Polymer networks

5.1.8Glass transition

5.2Solvents and polymer solutions

5.2.1Solvents

5.2.1.1Definition and classification

5.2.1.2Volatility of solvents

5.2.2Polymer solutions

5.2.2.1General

5.2.2.2Affinity between polymer and solvent; and solubility parameters

5.2.2.3Vapour pressure of a solvent in a polymer solution

5.2.2.4Viscosity of polymer solutions

5.3Aqueous-disperse systems

5.3.1Basic terms and classification

5.3.2True and colloidal solutions

5.3.3Primary dispersions

5.3.4Secondary dispersions and emulsions

5.4Diffusion

5.5Basic principles of organic reactions

5.5.1General

5.5.2Timing in organic reactions

5.5.3Chemical kinetics

5.5.4Polar reactions

5.5.5Free-radical reactions

5.5.6Pericyclic reactions

6Physical drying

6.1Physical drying from solutions

6.1.1Solvent transfer from film to ambient air, and heat balance

6.1.2Sequence of events during drying

6.1.2.1Evaporation, diffusion and solvent retention

6.1.2.2Influence of layer thickness

6.1.3Solvent selection

6.1.4Evaporation processes in aqueous systems

6.1.5Film formers for physically drying paints

6.2Physical film formation from dispersions

6.2.1Waterborne primary dispersions

6.2.1.1Qualitative aspects of film formation

6.2.1.2Physical models

6.2.1.3Minimum film-forming temperature and coalescing agents

6.2.1.4Other ways of lowering the MFFT

6.2.1.5Pigmented dispersion coating materials

6.2.2Waterborne secondary dispersions

6.2.3Emulsions

6.2.4Non-aqueous disperse systems

7Oxidative crosslinking

7.1General information and types of binders

7.2Mechanisms of oxidative drying

7.2.1Isolated double bonds

7.2.2Conjugated double bonds

7.3Siccatives

7.4Preventing skinning

7.5Influences on oxidative drying

8Curing of liquid coatings by step-growth reactions

8.1General

8.2Polyaddition and polycondensation

8.2.1Formal principles of molecular enlargement and crosslinking

8.2.2Fundamental physicochemical principles of crosslinking

8.3Important crosslinking reactions

8.4Selected crosslinking reactions in detail

8.4.1Formation of polyurethane systems

8.4.1.1Conventional 2-pack curing

8.4.1.2Waterborne 2-pack curing

8.4.1.3Moisture curing

8.4.1.4Stoving of blocked polyisocyanates

8.4.2Crosslinking of epoxy resins

8.4.2.1Epoxy resins and their reactivity

8.4.2.2Additional information about curing with amines

8.4.2.3Waterborne epoxy-amine systems

8.4.2.4Additional information on curing with polyanhydrides

8.4.2.5Curing via the OH groups

8.4.3Curing of resin polyols with formaldehyde condensation resins

8.4.4Crosslinking of silicic acid esters and sol-gel materials

8.4.5General information on high solids paints

9Film formation by coating powders

9.1General information on coating powders and how they form films

9.2Binders and how they crosslink

9.2.1Hybrid powders

9.2.2Epoxy powders

9.2.3Polyester powders

9.2.4Acrylic powders

9.2.5General information on gelation and crosslinking

9.3Physico-chemical aspects of film formation

9.3.1Physical principles

9.3.2Effect of additives, matting

10Film formation by polymerisation

10.1General polymerisation mechanisms

10.1.1Free-radical polymerisation

10.1.2Cationic polymerisation

10.2Radiation curing

10.2.1Free-radical curing with UV radiation

10.2.1.1Initiation

10.2.1.2Free-radical polymerisation

10.2.1.3Oxygen inhibition

10.2.2Electron beam curing (EBC)

10.2.3Ionic UV curing

10.2.3.1Initiation

10.2.3.2Polymerisation

10.2.4Waterborne binders

10.2.5Dual cure

10.3Curing of unsaturated polyesters

10.3.1Polymerisation

10.3.2Initiation, acceleration

10.3.3Oxygen inhibition

Annex 1: The Orchard equation

Annex 2: The WLF equation

Annex 3: The gel-point theory of Flory and Stockmayer

Annex 4: Test methods used in film formation

Author

Index

1Introduction

1.1Basic concepts

The act of applying a solid, adhering, organic chemical layer or film to any kind of surface or substrate is called coating (note that inorganic coatings, such as distempers, are not the subject of this book) and the outcome is properly called a coating system. Depending on the coating material employed, the coating may be decorative, protective or otherwise functional and may be a varnish, an emulsion paint, a floor-coating compound or a filler. Before and immediately after application, the coating material is a liquid, a paste, or a powder (as in coating powders). The transition to finished coating requires the film of coating material to solidify and is called film formation, or, from an application point of view, drying. These terms and others employed in coating technology are defined in the EN ISO 4618 and also in the German standard DIN 55945 (2007-03).

In order for a coating material to form a film at all, it must contain a substance which is capable of forming a film by itself, i.e. without the presence of other chemical components. Such substances are called film-forming agents or binders.

Older standards, such as DIN 55945 (1988-12 and earlier), clearly distinguished between the terms “binder” and “film-forming agent”. However, this distinction never really took hold in practice, and seems to have largely been abandoned.

According to DIN 55945 (1999-07),

•the film-forming agent is “the binder which is needed in order that the film may form”

In EN ISO 4618, we find:

•binder is the nonvolatile part of a medium, where medium is defined as all constituents of the liquid phase of a coating material.

In keeping with general parlance, in this book we will use the term “binder” even when, according to the traditional definition, a film-forming agent is meant.

Binders generally, and in this book, are the most important class of substance present in all coating materials. Every coating material contains a binder, which frequently is a mixture of several substances (resins, etc). We can say therefore that:

•the binder determines the fundamental properties of the coating material and the coating.

The job of the binder (or film-forming agent) is to form a more or less solid film and, where other ingredients are present, to embed them or to bind them to each other. Binders are generally organic substances that vary considerably in molecular size. Depending on their composition, degree of dissolution/dilution or manufacturing process, they are called either resins, e.g. alkyd resins, epoxy resins, and melamine resins, which can be present in 100 % concentrated, dissolved or dispersed form or polymer dispersions.

Binders composed of small to medium-sized molecules, equivalent to (mean) molecular weights ranging from several hundred to 10,000 g mol-1, must be chemically transformed into large molecules or molecular networks during film formation. This is called hardening or curing. Binders composed of large or long molecules (macromolecules) can form sufficiently solid films without chemical reaction; essentially, the solvent or dispersing agent in the formulation simply evaporates during film formation and the long thread-like molecules become matted, rather like felt. This type of film-forming process is called physical drying. The qualifier “physical” must be used in this case because “drying” can also include curing. Most films which are formed by curing additionally undergo physical drying or surface drying, especially in the early stages. Figure 1.1 shows a schematic diagram of the two basic film-forming mechanisms.

Figure 1.1: Film formation by physical drying (left) and curing (right)

1.2General composition of coating materials

Once a specific coating material has been selected, it is a simple matter of referring to the instructions or technical leaflet to find out how it should be applied and dried. The leaflet or container will even contain general information about the chemical composition, possible health hazards and methods of disposal.

The composition of coating materials will be discussed in a very brief, generalised form below. Details and technical information are provided in this book as and when they are needed for an understanding of film formation. Table 1.1 presents an overview of the general composition of coating materials.

It should be pointed out that not all coating materials contain all classes of substance listed in Table 1.1. Thus, a clearcoat contains neither pigments nor extenders (in the classical sense), and a coating powder contains no solvents. Apart from binders, the ingredients mentioned in Table 1.1 will now be described in brief.

Pigments

These are very finely divided powders which are practically insoluble in the application medium, i.e. the liquid ingredients of the coating material, and/or act as colorants and/or prevent corrosion and/or have other functions. Examples: titanium dioxide (white pigment), carbon black (black pigment), quinacridone pigment (coloured pigment), pearlescent pigment (effect pigment), zinc phosphate (corrosion-protection pigment).

Fillers

Fillers, too, are powders which are virtually insoluble in the application medium. They impart volume or “build” to the coating material and provide or improve certain technological properties. They are typically used to modify flow properties, suppress cracking (provide reinforcement), improve ease of sanding, adhesive strength and weather resistance, matting, etc. The effect exerted by a filler depends critically on its particle size and particle shape (whether isometric, lamellar, or acicular). Examples: chalk, talcum, fibre fillers.

Additives

Additives, traditionally called “adjuvants”, are substances which are added to the formulation in relatively small quantities and which, during manufacture and/or application of the coating material, engender or enhance certain properties or selectively improve the film properties. Additives vary in volatility, and will either remain in the coalescing film or escape from it more or less completely. Examples: Dispersing additives (nonvolatile), slip additives (nonvolatile), catalysts (mostly nonvolatile), defoamers (mostly nonvolatile), anti-skinning agents (volatile), film-forming agents (volatile).

Solvents

This term refers to liquids that are truly capable of dissolving a binder to create a molecular dispersion. Solvents (in the broader sense) are organic liquids, but can also be water. Examples: xylene (aromatic hydrocarbon), butyl acetate (ester), methyl isobutyl ketone (ketone), butyl glycol (ether alcohol), butanol (alcohol).

Table 1.1: General composition of coating materials

Coating material

nonvolatile content

volatile content

binders

solvents and dispersing agents

pigments

volatile additives

film-forming agents

nonvolatile additives

(usually volatile cleavage products during stoving)

If the solvent is added to the coating material to adjust its viscosity prior to processing, it is called a diluent or thinner. Reactive diluents are diluents which are almost completely chemically incorporated into the coalescing film and are therefore classed as binders.

Dispersing agents

Unlike solvents, these liquids (most often water) do not yield a molecular dispersion of a binder but rather contain it in the form of submicroscopic particles or droplets. The overall system is called a dispersion and – in contrast to a solution – usually has some cloudiness. A dispersing agent, too, may act as a diluent. Thus, it is quite common to lower the viscosity of an interior emulsion paint with a little bit of water for application by roller.

1.3General formulation data

The simple paint formulation presented in Table 1.2 will now be used to explain the most important general data relating to formulations.

The nonvolatile matter (also called solids content) is the

•mass fraction of coating material remaining after a specified drying time.

The content of nonvolatile matter is determined in accordance with ISO 3251 by accurately weighing out, e.g., 2 g of the coating material into a lid, drying it under defined (or agreed) conditions, e.g., one hour at 130 °C in a paint drying oven, and then reweighing it. If chemical reactions during drying cause minor products to be released, the nonvolatile values can vary substantially with the temperature and duration of drying.

The theoretical content of nonvolatile matter in the formulation shown in Table 1.2 is calculated as follows:

The additives are ignored if they are present in small quantities.

The content of nonvolatile matter is crucial for application because it indicates the mass of the film that will remain on an object after the coating has dried.

The pigment-binder ratio in the sample formulation is

The pigment volume concentration, PVC, is the volume fraction of pigments and fillers in the dry film volume.

The PVC is calculated as follows:

VP is the pigment volume, VF is the filler volume, VB is the binder volume

Table 1.2: Simple paint recipe (white house paint); from [3]

Ingredient1)

Mass fraction [%]

alkyd resin, 75 %, in white spirit

60.0

titanium dioxide (white pigment)

27.0

cobalat octoate (10 % Co)

0.2

zirconium complex (6 % Zr)

0.5

calcium octoate (5 % Ca)

1.7

white spirit

10.6

100.0

1) Figures quoted in percent are mass fractions (wt.%)

The binder volume here comprises the volume of all nonvolatile matter in the dry film without pigments and fillers, i.e. it also includes curing agents, reactive diluent and, perhaps, plasticiser.

The volumes have to be calculated from the mass fractions in the formulation and the corresponding densities, which are included in the leaflets supplied with the raw materials. In the example in Table 1.2, the density of the resin is 1.04 g cm-3, and the pigment density is 4.1 g cm-31)

The PVC is therefore:

The critical pigment volume concentration, CPVC, is the

•PVC at which the binder volume is just sufficient to fill the voids between the pigment and extender particles.

To calculate the CPVC, the size of the void volume of the pigment or pigment/filler in the formulation must be known. This is commonly measured by the oil absorption value (EN ISO 787-5), which is the quantity of linseed oil needed for converting 100 g of pigment or extender (or a mixture of both) into a coherent, non-lubricating paste. In practice, chemists determine these values direct from the binder solution or water used, instead of from linseed oil. Although oil absorption values have poor reproducibility, the oil absorption value remains important because of its direct use in calculating and developing the formulation.

The oil absorption value can be used to estimate the CPVC from the following, easily derived equation.

ρP

is the density of the pigment/extender

OA

is the oil absorption value of the pigment/filler

ρL

is the density of the linseed oil (0.935 g cm-3)

From this formula, the CPVC of the example formulation is

Where several pigments or extenders are present, the mean density of the mixture is used for ρP; the oil absorption value must be determined experimentally from the entire mixture, since the degree of packing (void filling) cannot be predicted from the individual data.

The Q value is the

•Quotient of PVC/CPVC (·100%).

In the example, the Q value computes to

Formulations having Q-values of less than 100 % are said to be subcritical. They contain more binder than is necessary for filling the voids. The dry film is therefore not porous, i.e. it resembles a typical paint. Coating materials having Q values over 100 % are said to be supercritical. The films are porous, i.e. absorbent, and typical examples are interior emulsion paints and zinc dust paints. Figure 1.2 illustrates the qualitative relationship between Q value, coating properties and the type of coating material.

1.4Basic physical properties

1.4.1Density

1.4.2Viscosity and flow behaviour

The science of flow is called rheology. The most important rheological parameter is the dynamic viscosity, η, of a flowable substance (fluid). It is defined as the ratio of shear stress, τ, to shear rate (velocity gradient) D or 2):

Figure 1.2: Various properties of coating materials, along with the corresponding Q value ranges from [3]

If the viscosity is independent of the shear rate, i.e. stays constant when the shear rate changes, the flow behaviour is said to be Newtonian. Coating materials usually exhibit more or less non-Newtonian behaviour, i.e. the viscosity is dependent on the shear and perhaps on the duration of shearing. The following types of viscosity are distinguished (see also Figure 1.4):

Structural viscosity (pseudoplasticity, shear thinning): the viscosity falls as the shear rate rises. The viscosity change is wholly reversible and responds almost instantaneously to the change in the shear rate.

Dilatancy (shear thickening) is the opposite of structural viscosity, i.e. the viscosity increases with increase in shear. Generally undesirable, dilatant behaviour is sometimes exhibited by very highly concentrated pigment or polymer dispersions.

Thixotropy exists when the viscosity at constant shear rate undergoes a fairly rapid asymptotic decrease and is restored when the shear stress is removed. Thixotropy originates from the reversible build up and degradation of loose gel structures in the fluid, and can be deliberately adjusted with appropriate rheological additives.

Rheopexy is the opposite of thixotropy, i.e. the viscosity increases under constant, weak shearing. Rheopectic materials are very rare.

A yield point is exhibited by a fluid which flows only after a minimum shear stress has been applied; it is nearly always observed in combination with structural viscosity or thixotropy. The presence of a yield point is also called plasticity and, if pronounced, leads to thickening. A yield point cannot be measured directly with simple viscometers, but rather only by extrapolation. Yield points are exhibited especially by products with high volume fractions of pigmentitious components, such as sealants and trowelling compounds, mastics, plastisols, emulsion paints and gels, whose non-sag properties can be seen with the eye.

The flow properties of coating materials are critical. For film formation, the two most important processes governed by rheology are levelling, which is generally desirable, and sagging, which is undesirable and takes the form of “runs” (or “tears”, “curtains”, etc; see Chapter 3).

1.4.3Surface tension

The surface tension of a liquid, σ1, or a solid (material), σs, is the work per unit area dW/dA 3) needed to enlarge the surface (at constant mass) by dA, where “surface” is understood to be the interface to the adjacent gas phase or vacuum. The SI unit is Nm-1 (or mN m-1).

Figure 1.3: Thought experiment for defining dynamic viscosity

The word “tension” reflects the fact that the definition of σ above is based on another force-based equivalent:

For practical purposes, we can conceive of the surface tension generally as a force that acts along every actual or imaginary line in or at the boundary of a surface. For the purposes of film formation, substrate wetting and flow depend on the surface tension of the coating.

Numerous other, often undesirable effects such as edge pulling, cratering and foaming, are also associated with surface tension (see Chapter 3).

1.5Classification of film formation

Film formation – depending on the type of coating material – comprises various individual physical and chemical processes, which sometimes overlap and influence each other. The various types of film formation and drying undergone by coating materials can be classified as follows.

Figure 1.4: Basic rheological behaviour of coating materials: 1) Newtonian, 2) structurally viscous (shear thinning, pseudoplastic), 2’) structurally viscous with yield point (in the τ/D chart), 3) dilatant (shear thickening), 4) thixotropic

•Physically drying coating materials

–solvent or dispersion-based (binders genuinely dissolved or dispersed)

–film formation solely through evaporation of the solvent/water at different temperatures and, perhaps, air humidities

–examples: cellulose nitrate, and chlorinated rubber paints, emulsion paints.

•Chemically curing coating materials

–solvent-free or emulsion-free, yet flowable (“liquid, 100 % systems”)

–film formation by spontaneous, chemical crosslinking reaction between base paint and curing agent (two-component or 2-pack systems) or through activation of crosslinking by means of heat, UV radiation or electron beams (1-pack systems)

–examples: 2-Pack materials based on urethane or epoxies for high-build coating in masonry and corrosion protection, heat-curing 1-pack polyurethane coating compounds, radiation-curing wood/furniture coatings based on acrylates.

•Physically (surface) drying and chemically curing coating materials

–binders genuinely dissolved or dispersed

–film formation initially by partial evaporation of the solvent/water, but mainly by chemical crosslinking, cold-curing 2-pack or heat-curing 1-pack

–examples: most industrial and automotive finishes.

•Oxidatively curing or moisture-curing coating materials

–solventless or solventborne 1-pack coating materials that – perhaps after surface physical drying – crosslink with atmospheric oxygen or air humidity at room temperature or slightly above

–examples: Alkyd resin house paints, moisture-curing polyurethane masonry paint.

•Coating powders

–thermoplastic powders: film formation by purely physical fusion on the preheated objects

–thermosetting (curing) powders: film formation by chemical crosslinking at elevated temperature after fusing and coalescence.

Figure 1.5: Surface tension of a liquid lamella in a wireframe (the area dA applies to the front and rear)

In summary, it should be noted that all types of drying entail an exchange of energy and, often, material with the surroundings. Thus, the external conditions during drying, i.e. the state of the environment around the object covered with the coating, determine the course of the film forming process. This particularly applies to the type and intensity of the energy input into the freshly applied layer (see Chapter 4).

2Application methods

Strictly speaking, application of a coating material is not part of film formation, but it is both a prerequisite and a direct precursor. To be sure, the type of application and the resulting structure of the wet film and layer of coating powder have no effect on the basic mechanisms of film formation; however, they do affect the outcome of the coating process, i.e. the perfection of the dried film.

What now follows is intended merely as an overview of the application methods, with some specific remarks. Much more detailed descriptions can be found in [3, 7, 11, 15], upon which most of this chapter is based.

2.1Spreading (brush, roller), flow coating (roller coating, curtain coating, dipping)

2.1.1Brush and roller

Spreading of paint materials by brush still has its place in coating technology. The reasons are as follows:

•simple process

•high versatility as regards parts shape

•little or no masking of surfaces that are not to be coated

•good wetting of the substrate and incorporation of the coating into voids (cracks, holes, etc.)

•high efficiency of application (also known as transfer efficiency)4)

•wide range of materials processable without the need for precision adjustment

These are offset by weaknesses, such as

•low coverage rate

•uneven thickness

•uneven surface in the form of brush marks

Different types of rollers, such as longhaired or short-haired nylon wool or foam rollers yield much higher coverage rates and smoother surfaces. However, their use is largely limited to flat surfaces.

Figure 2.1: Roller coating; from [3]

2.1.2Roller coating

In roller coating the material is applied by mechanical rolls at a correspondingly high coverage rate. The coating material is applied to flat, panel-like or coil-shaped substrates by a more or less flexible roller. A distinction is made between classic forward roller coating and the more common reverse variant (see Figure 2.1). Whereas in forward roller coating, film breakdown between the roller and the substrate surface leads to brush marks that will not flow out completely where the coating thickness exceeds approx. 12 μm, the reverse variant can yield smooth coatings between 3 and 100 μm thick. All kinds of material viscosities can be present in reverse roller coating, too. Two rollers connected in series can be used to process two-component (2-pack) materials.

Figure 2.2: Curtain coating; from [3]

2.1.3Curtain coating

In curtain coating, the paint falls through a horizontal slot as a very wide, thin curtain onto a virtually flat object carried on a divided conveyor belt. Excess paint is collected in a trough and is pumped back to the head (see Figure 2.2).

Curtain coating yields very flat films of 50 to 500 μm thickness. Since the paint only falls onto the panel with low kinetic energy, and is not, as it were, worked into the surface, good surface pretreatment or priming is essential for good wetting and adequate adhesion. 2-Pack coating is possible by arranging two heads in succession.

2.1.4Flow coating

In flow-coating, the paint is either poured over or sprayed through jets onto complex and bulky articles, such as radiators, rough machinery parts, and roof tiles. The excess paint drains into a sump. To reduce inevitable sagging, the articles may be moved or rotated. This method is only suitable for low-quality, rough-and-ready coatings.

2.2Dip-coating (dipping)

2.2.1Conventional dip-coating

Conventional dip-coating does not make use of electrochemical processes to apply the paint to the surface of the object, unlike the case for electrodeposition (see Chapter 2.2.2) and autophoresis (Chapter 2.2.3). For dip-coating, coatings based on organic solvents can be used if provision is made for adequate encapsulation and extraction of fumes and explosion protection. Coatings based on non-flammable trichloroethylene are no longer in use for toxicological reasons. Advantageous for dip-coating is the use of waterborne paints, although there can be problems with foaming, bath instability due to pH shifts, and entrainment of contaminants (e.g. from pretreatment).

The comments made about flow-coating apply to wet-film quality and application areas. Unwanted sagging can be prevented by removing excess paint with high-voltage electrodes or by centrifuging.

A special type of dip-coating is called barrelling. In this, articles, such as buttons, hooks, and toy figures, are loaded into a slowly rotating barrel filled with a certain amount of paint. After the excess paint has drained away or been removed by centrifuging, the articles are heat-dried either on a separate wire screen or in the barrel itself.

2.2.2Electrodeposition

Industrial use of electrodeposition began in the automotive sector in 1960s – firstly in the form of anodic electrodeposition and then increasingly as cathodic electrodeposition5).

The basic principle of electrodeposition can be summarised as follows:

The pretreated metal parts for coating or priming are dipped in a sufficiently large tank which has been filled with a thin, relatively low-solids (10 to 20 % nonvolatiles) waterborne coating and are connected up as anodes (positive) or cathodes (negative) relative to the tank or separate counter electrodes. The voltage varies with the process and the time elapsed during the dipping phase from 150 to 400 V. A paint layer is deposited, which is almost dry and adheres strongly due to electroendosmosis (see below). The parts are removed from the tank, rinsed and stoved.

The primary electrochemical process in electrodeposition is electrolysis of water, which leads to the formation of protons and oxygen at the anode and hydroxide ions and hydrogen at the cathode.

The protons (or hydronium ions) formed at the anode migrate away from the surface into the bulk of the solution and cause the pH to fall by up to approximately 5 units at the diffusion boundary layer, which is a layer some 100 μm thick on the electrode. Accordingly, the pH at the cathode rises by about 5 units due to the formation of the hydroxide ions (Figure 2.3).

An anodic electrodeposition paint contains a resin bearing several carboxyl groups per molecule which are present in the bath as ions neutralised with amines and/or ammonia and so give rise to a dispersion of negative paint particles.

Conversely, a cathodic electrodeposition resin is an amino oligomer which has been neutralised in the bath with lower carboxylic acids (mostly acetic acid), and forms positive paint particles (Figure 2.4).

As a result of convection in the bath, the negatively charged colloidal particles of the anodic electrodeposition coating enter the boundary layer of the positive anode to which they are attracted and at which they are protonated. Consequently, the resin molecules lose their ability to be diluted by water, and precipitation, also known as electrocoagulation, occurs on the substrate surface. Precisely the opposite occurs in cathodic electrodeposition: the positive paint particles enter the cathode boundary layer, where they coagulate as a result of deprotonation (Figure 2.4). Since the deposited paint layer has poor electrical conductivity, the current quickly diminishes where most of the paint is deposited (at free surfaces facing the counter electrode). Layer growth comes to a standstill at approx. 20 μm thickness (value for cathodic electrodeposition), and more paint is increasingly deposited at electrically less favourable places, a phenomenon known as wrap around.

Figure 2.3: pH shift at the electrodes during the electrolysis of water (in anodic electrodeposition, the workpiece is the anode, while in cathodic electrodeposition it is the cathode)

Figure 2.4: Neutralisation and precipitation (coagulation) of electrodeposition resins on the substrate surface

The main effects observed as regards film formation are:

Gas formation on the substrate surface causes blistering of the primary coating layers and renders them porous (see Figure 2.5). These structures must disappear during subsequent stoving. The layer is dewatered by electroosmosis (electroendosmosis). This is generally accepted (in simple terms) as the flow of electrolyte solutions through capillaries or porous bodies in an externally applied electric field. The effect stems from the fact that the protons (in anodic electrodeposition) and hydroxide ions (cathodic electrodeposition) which are emitted by the substrate electrodes entrain water envelopes with them as they pass through the paint. The solids content of the coating thus increases to up to approx. 90 %.

Connecting up the parts as anodes in anodic electrodeposition leads to significant formation of metal ions from the substrate, e.g. in the form of iron (III) ions, and maybe even to yellowing of the coating. Slight dissolution of aluminium or zinc in the form of hydroxo complexes has been demonstrated in cathodic electrodeposition.

For electrodeposition, as is the case for many conventional, higher-quality metal coating methods, the parts are usually phosphated, chromated, or otherwise furnished with organic conversion layers. These increase paint adhesion, and greatly improve protection against infiltration during subsequent corrosive attack. The conversion layers must be porous and/or electrically conducting enough to allow the high current flows needed for paint deposition.

Electrodeposition is used for high-throughput priming or single-layer coating of metal articles which are always given the same treatment (colour, etc.). Anodic electrodeposition is much simpler than cathodic electrodeposition, but is not now as widespread as its cathodic counterpart primarily because of the lower corrosion protection. Car bodies are now primed almost exclusively by means of cathodic electrodeposition.

2.2.3Autophoresis

This dip-coating method, which has been known for some considerable time, has so far been unable to establish itself in Europe, but is now becoming a bit more widespread because the underlying chemistry has been optimised. The method ranks between conventional dip-coating and electrodeposition and its benefits include much greater simplicity over electrodeposition.

Low-viscosity liquid autophoresis paint is highly acidic and contains an acid-stabilised resin dispersion. Degreased, but otherwise unprocessed iron and steel parts are pickled by the acid after dipping. This causes the release of iron (II) ions, which in turn leads to destabilisation of the anionic dispersion and, in the presence of oxidants, oxidation to trivalent ions. The paint particles thus coagulate on the surface in the form of a porous and, owing to a lack of electrical osmosis, highly watery layer that is then thermally compacted and perhaps stoved.

Figure 2.5: Micrograph of an uncured cathodic electrodeposition film; from [7]

Die gebildeten Lackschichten sind optisch oft nicht vollwertig, da sich alle Inhomogenitäten des Grundmetalls wegen der fehlenden Vorbehandlung abzeichnen. The resultant paint coats are frequently not of a high optical quality, because all the inhomogeneities in the base metal show up due to the lack of pre-treatment. Where this is not a problem, e.g., hidden surfaces, autophoresis is an attractive way of effecting one-coat painting of steel parts.

2.3Spray-painting

Spray-painting methods can be divided into electrostatic and non-electrostatic variants, with the latter in some cases merely being an extension of the conventional methods (see Chapter 2.4). All spraying methods apply a cloud of droplets (spray mist) incompletely onto the object as a “mountain of droplets” (see Figure 3.3) which levels out quickly when the drops coalesce.

2-Pack coatings can be spray-applied as well. The base paint and hardener are either mixed prior to application or, where throughputs are high and/or the mixtures have short processing times, they are mixed in situ inside or in front of the gun with the aid of additional devices, such as static mixers (“Kenics” mixer) and metering equipment.

2.3.1Air-spraying

2.3.1.1High-pressure spraying

Conventional paint spraying with air at a pressure of between 2 and 8 bar (high-pressure spraying) is performed with a spray gun fitted at the front with an atomiser, as shown in Figure 2.6 (details may vary from brand to brand).

When the trigger is squeezed, compressed air from an air line is released from the centric, annular jet at the head of the gun, and rapidly expands. The negative pressure thereby created in front of the jet sucks and entrains the paint, which is emanating from the gun under slight pressure. The forces6) resulting from the exchange of momentum between the air flow and the paint flow cause the jet of paint to be atomised with a wide droplet size distribution. This spray jet now impinges at high speed (up to 30 m s-1) on the object. For the most part, the larger droplets are deposited on the surface of the object. However, the finer ones are dragged away by the air current as it is deflected by, or sweeps past, the surface. This latter phenomenon is called overspray.

A quantitative description of the highly complex atomisation process is beyond the scope of this book. However, Figure 2.7 provides an overview of the influence of key parameters on the mean droplet diameter, the optimum value of which is 30 to 40 μm. The optimum initial viscosity of paint applied by high-pressure spraying is equivalent to a flow time of 20 s from a DIN-4 cup (DIN 53211, withdrawn in 1996) or 55 s from the ISO cup with 4-mm nozzle (DIN EN ISO 2431); this is equivalent to a kinematic viscosity of about 7 · 10-5 m2 s-1 or 70 mm2 s-1.