Electrocoat E-Book

Michael Brüggemann

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

Veröffentlichungsjahr: 2020

Beschreibung

Electrophoretic paints, commonly known as electrocoat or -paint, are organic coatings dispersed in water, carrying an electric charge. This enables the paint for deposition onto a metal, which is carrying the opposite charge. Resulting from this special way of application are special needs for formulating its coating: This textbook shows requirements and particularities for the electrocoat process and its troubleshooting. It illustrates the theoretical basics of electro-deposition, paint formulation, manufacturing, application process in addition to failures and countermeasures of the electrocoat process. Newcomers and practitioners alike get and a comprehensive overview over the wide field of electrocoats as well as deeper insights into this technology.

Leseprobe

Michael Brüggemann

Anja Rach

Electrocoat

Formulation and Technology

Cover: lumen-digital – stock.adobe.com

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Michael Brüggemann, Anja Rach

Electrocoat: Formulation and Technology

Hanover: Vincentz Network, 2020

European Coatings Library

ISBN 3-7486-0105-0

ISBN 978-3-7486-0105-0

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European Coatings Library

Michael Brüggemann

Anja Rach

Electrocoat

Formulation and Technology

Inhaltsverzeichnis

Foreword

1 Introduction into electrocoats

1.1 Historical evolution

1.2 Motivation to use electrocoat

1.3 Market size

1.3.1 Electrocoat manufacturer and important players

1.4 References

2 Physicochemical basics of electrophoretic paint deposition

2.1 General physicochemical basics

2.1.1 Anomaly of water

2.1.2 pH-value

2.1.3 Dispersion stability

2.2 Principles of formulation

2.2.1 Film formers („binders“) [3]

2.2.2 Pigments and fillers

2.2.3 Additives

2.3 Electrocoat related physiochemical basics

2.3.1 Electrolysis of water

2.3.2 Diffusion boundary layer and electrophoresis

2.3.3 Chemistry of electrodeposition [4], [5]

2.4 Detailed consideration of film deposition

2.4.1 Throw power

2.4.2 Influences on deposition and film thickness

2.5 References

3 Resin technologies

3.1 Cationic epoxy-based electrocoat resins

3.1.1 Main resin

3.1.2 Catalysts

3.1.3 Resins to adjust film build and flow

3.1.4 Cationic paste grind resins

3.2 Cationic polyacrylate-based electrocoat resins

3.3 Anodic electrocoat resins

3.4 References

4 Electrocoat formulation and performance

4.1 Cationic epoxy electrocoat bath set up and parameters

5.2 Anodic polyester electrocoat bath set up and parameters

4.3 Acrylic electrocoat bath set up and parameters

4.4 Cationic epoxy resin blend formulation

4.5 Cationic pigment paste formulation

4.5.1 Pigment paste manufacturing

4.5.2 Catalyst in cationic pigment paste

4.6 Pigments and fillers used in electrocoat

4.6.1 Carbon black

4.6.2 Titanium dioxide

4.6.3 Coloured pigments

4.6.4 Anti-corrosion pigments

4.6.5 Fillers

4.7 Electrocoat curing

4.7.1 Curing window

4.8 Physical properties of cured electrocoat

4.9 Electrocoat film performance and testing

4.9.1 Corrosion protection

4.9.2 Corrosion tests methods

4.9.3 Photo degradation of electrocoats

4.9.4 Performance comparison

4.9.5 Electrocoat throw power performance

4.9.6 Computer-based simulation of electrocoat process and throw power

4.10 References

5 Electrocoat process and line design

5.1 Pretreatment process

5.2 Electrocoat process

5.2.1 Electrocoat bath

5.2.2 Bath circulation system

5.2.3 Filtration system

5.2.4 Ultrafiltration

5.2.5 Heat exchanger

5.2.6 Rinse stages

5.2.7 Rectifier and electrical installation

5.2.8 Anode cells and anolyte circuit

5.2.9 Conveyor and carrier systems

5.2.10 Electrocoat ovens

5.2.11 Feed system

5.2.12 Complete automotive electrocoat line

5.3 Peculiarity of industrial electrocoat lines

5.3.1 Industrial line design

5.3.2 Electrocoat curing and ovens in the industrial application

5.4 Change of electrocoat line by conversion

5.5 Electrocoat bath control

5.6 References

6 Problem solving

6.1 How to trouble shoot [1]

6.2 Always important

6.2.1 Substrate

6.2.2 Pretreatment

6.3 Electrocoat bath parameters

6.3.1 Solids content

6.3.2 pH-value

6.3.3 MEQ values

6.3.4 Conductivity

6.3.5 Pigment to binder ratio (P/B ratio)

6.3.6 Ash test

6.3.7 Solvent content

6.3.8 Ionic contamination

6.3.9 Deposition voltage

6.3.10 Deposition time

6.3.11 Bath temperature

6.4 System/plant operation

6.4.1 Tank agitation

6.4.2 Filtration

6.4.3 Rinsing

6.4.4 Water quality

6.4.5 Rectifier

6.4.6 Anodes (for cathodic coatings)

6.4.7 Oven

6.4.8 Racking

6.5 Coating characteristics

6.5.1 Thickness control and rupture

6.5.2 Throw power

6.6 Surface defects

6.6.1 Craters

6.6.2 Rupture, pin holing and zinc gassing

6.6.3 Mapping

6.6.4 Hash marks

6.6.5 Contact failure

6.6.6 Roughness

6.6.7 Orange peel

6.6.8 Dirt

6.6.9 L-effect/horizontal settling

6.6.1 0Run-out

6.6.11 Drop marks

6.6.12 Redissolution

6.6.13 Foaming/air entrapment

6.6.14 Gloss variations

6.6.15 Colour variations

6.7 Microbial infestation to electrocoat

6.7.1 Impact of bacteria on electrocoat

6.7.2 Monitoring of microbial infestation

6.7.3 Preventive actions

6.8 Follow-up

6.8.1 Checklists for quick failure detection and remedy

6.8.2 Preventive actions

6.8.3 Documentation of plant specific values

6.8.4 Control of measuring devices

6.8.5 Check of final product

6.9 References

7 Future developments

7.1 Auto deposition

7.1.1 Polymers used for auto deposition

7.1.2 Auto deposition formulation and process

7.1.3 Future development needs in auto deposition

7.2 Future development in electrocoats

7.3 References

Authors

Stichwortverzeichnis

Foreword

Electrocoat is a widely used technology designed to paint all different kind of metal parts and car bodies effectively. It impacts our daily life, but it is not known to the public and even for paint chemists it is a technology for specialist only.

This book gives an overview and access to the paint deposition including the resin and paint technology, the electrocoat process, line design and provides a troubleshooting guideline for end users.

It is written for people looking for an introduction into electrocoat technology, for career-changers, and professionals using the electrocoat process to paint conductive metal parts.

In first chapter the book provides information about the historical context, the markets for use and the economic impact of electrocoat.

The 2nd chapter provides insights into the theory of electrocoat deposition, while Chapter 3 gives an overview about the resin technology of anodic and cationic electrocoats. Basic paint formulation for anodic and cationic electrocoats are explained in Chapter 4 in addition to a description of paint performance. It gives insights into electrocoat cure, into physical properties of the cured films, into corrosion protection given by electrocoats including throw power to protect inner parts of the painted article.

Chapter 5 gives an overview of electrocoat line design including metal pretreatment and line engineering background with an overview of essential parts of the process.

Answers important for running electrocoat lines and guidelines to solve some typical electrocoat problems are given in Chapter 6. It is intended to be practical and to provide some ideas on how to control and to maintain electrocoat bath and quality.

Finally, Chapter 7 gives a foresight of electrocoat technology for the coming years and compares it to paints applied by autophoresis.

The book provides useful information and insights into electrocoat technology and process. It gives an overview of the different technologies and paints to protect and to coat metal parts in a highly automated process. We hope it presents utilization to students, paint developers and professional industrial users.

We would like to thank all colleagues who have provided information on specific topics, figures and structures. Especially Prof. Dr. Thomas Brock being co-author in Chapter 2.

We thank Dr. Greggory McCollum, Dr. Richard Karabin and Dr. Steve R. Zawacky for their advice and for reviewing Chapters 3 to 5, Uwe Kuhlmann and Detlef Hildebrandt for the figures in Chapter 2, Klaus Platte for the recommendations in Chapter 6, and Skip Tornow for his review.

Viersen and Paderborn, January 2020

Michael Brüggemann

Anja Rach

1Introduction into electrocoats

Electrophoretic paints commonly known as electrocoat or electropaint are organic paints dispersed in water, carrying an electric charge. This enables the paint for deposition onto a metal, which is carrying the opposite charge given by an electric field. The deposited paint layer is not furthermore water soluble and after curing between 100 to 200 °C, highly robust paint films are achieved. [1], [2]

Several names are used for the products and for the process, here to name electrocoat, electro-coat, electrocoating, electro-paint, ecoat, anodic, cathodic, cationic, electrophoresis and several other versions in different languages.

The electrophoretic coating process is a highly automated process to coat high quantities of metal parts, in different shapes very uniformly. This process and this paint technology are not in the normal consumer awareness, but electrocoats are impacting everybody. They are everywhere, protecting our cars against corrosion, giving as a one coat excellent performance to protect our washing machines, visible on heavy duty trucks, in garden and lawn products or as a clear coat to protect the brass door opener.

The electrophoretic paints can be sort into different groups of resin chemistry and defined by the electrical pole they are depositing. Named as anodic electrocoat or cationic electrocoats. They can be defined by their chemical nature if based on epoxy originated resins or acrylates or based on maleinized oil/polybutadiene. Described by performance as low cure, high throw or high/low film build electrocoats.

All of these coatings have one thing in common, they provide a thin, uniform paint layer of 15 to 30 µm to protect the part coated. Cationic electrocoats based on epoxy resins is the most common product and represents about 90 % of the market.

Table 1.1: Basic overview on cationic and anodic electrocoats

Type

Resin technology

Characteristics

Cationic electrocoat

Based on epoxy resin

Based on acrylates

High performance corrosion protection, used as primer under top coat

Exterior durability, one coat, wide colour palette

Anodic electrocoat

Based on epoxy

Based on acrylates

Based on maleinized oil/polybutadiene type

Anodics have lost their market penetration

Epoxy systems for corrosion protection and acrylics for coloured one coats available

Might be of interest for aluminium

1.1Historical evolution

Electrophoresis as concept known by end of the 19th century but not used to apply paints. It’s industrial breakthrough was in the beginning of 1960 driven by Ford Motor company introducing 1st anodic electrocoat for wheels, in partnership with companies Glidden and PPG. Before that, car bodies were dip painted in solvent-borne dip primers, later in water-borne systems. The interest in electrodeposition concept was intensified by a devastating fire, at a GM transmission plant in 1953, related to solvent-borne dip primer. As first anodic electrocoats arrived in the market, the dip paint systems were replaced quickly due to better film uniformity, lower amount of waste and better efficiency. In summer 1963 the 1st anodic passenger car line started at Ford Wixom and in October 1963 the 1st tank in Europe was ready to go in production at Ford Halewood in UK.

1st anodic paints were based on maleinized oils combined with phenol resins and melamine resin for cure, neutralized with ammonia and amines. Improved performance in corrosion protection was achieved by combining with polybutadiene resins. The paints were added to the bath as one component and the full solubility was achieved using free ammonia out of the bath itself. Using low neutralized feed material, the amount of ammonia and amines was kept constant via consumption by the low neutralized feed material. By this concept, the amount of base was not continuously increasing during the paint deposition process. The peak of the technology was arrived in middle of the seventies. Anodic paints based on epoxy resin technology and acrylates were common in the industry. Anodic electrocoats were the standard corrosion protection of passenger cars until end of this decade. Beside the chemical development of electrophoretic paints, process improvements invented at that time are reaching into our current processes of today. The separation and removal of free alkalinity out of the bath by electrodialysis and membrane technology was invented by ICI in 1970. The concept is in use today in cationic electrocoat bathes as an anolyte system to control the acid concentration. PPG patented in 1969 the use of ultrafiltration to establish a close loop system and to produce by a membrane separation technique, the rinse liquid out of the electrocoat bath. The coated parts were rinsed, and the rinse material went back to the bath itself. This close loop system was an important driver for the breakthrough of electrocoats in the industry, as it reduced the amount of waste significantly and allows until today a material efficiency of more than 95 % by the electrophorese process.

As anodic electrocoats reached their peak in market penetration, the successive technology cationic electrocoat was on its early stage to drive the next technology change. Anodic electrocoat process has a basic technical disadvantage. The coated part is in the electric circuit the anode, metal dissolves as anode reaction and goes into solution. In case of cold roll steel, Fe2+/Fe3+ is formed and partially incorporated in the electrocoat film. It is leading to discoloration at light colours and limited corrosion protection.

Already 1971 the first industrial cationic electrocoat bath was filled by PPG and it took seven years until the first automotive line for passenger cars went into production.

GM Framingham started in 1978 and was the first passenger car line. From 1978 to 1984 almost all anodic electrocoat lines were changed to cationic electrocoats at automotive industry in North America and Europe.

To make that technology change happen, significant invention in resins and electropaint formulations were implemented. The amine functional cationic resins based on epoxy resin, thermally cured via blocked isocyanates resins, were patented by PPG (US3799854A) in 1970. This invention shaped the industry and to cure cationic epoxy-based resin with blocked isocyanates is the common industrial standard until today. From the 1980’s to today, lot of inventions to improve and to increase market penetration happened. Formulation adaptation to the market needs and environmental challenges are implemented. Lead-free, tin-free electrocoats, in low or high film builds, as low cure, as high throw or with improved UV durability have been developed.

Table 1.2: Markets for electrocoats technologies

Cationic

Anionic

Epoxy

Acrylic

Epoxy

Acrylic

Automotive/Commercial vehicle/spare parts/wheels

X

Agriculture

X

X

Motor cycles

X

X

Appliances/white colour

X

X

X

Consumer electronics

X

X

X

Lawn and garden

X

X

Furniture/storage equipment

X

X

X

X

Brass parts/clearcoats

X

X

1.2Motivation to use electrocoat

The electrophoretic process finds a wide range of users and applications in the industry.

The success of anodic and cationic electrocoats for industrial painting of metals is related to the excellent relationship between performance, efficiency, environmentally friendly process and cost. It is a fully automated immersion paint process and best in class to coat many different parts in different complex shapes, complete passenger cars or truck frames with a high-performance coating.

After cleaning of the metal parts and application of an inorganic immersion layer, like zinc phosphate, the parts are immersed into the electrocoat bath. The bath size depends on the parts coated and can start in the industry from 5 m3 to 500 m3. After electrocoat application with voltages between 100 and 400 V and deposition times between 1 and 5 minutes, the parts are rinsed off adherent uncoated material and are thermally cured at 120 to 200 °C.

Users obtain a defect free electrocoat film of high quality with excellent adhesion, hardness, flexibility, chemical – and corrosion resistance. The film builds are uniform by ± 3 µm and typical range of application thickness is between 15 and 30 µm. In comparison to spray applications, electrocoats are able to apply a paint layer in complex, interior areas of the article. This behaviour is known as throw power and related to the characteristic of self-limiting and increasing film resistance. The deposition is initiated in the electrical field between cathode and anode by current flow. The deposition of the paint on the metal part is increasing the electrical resistance in the circuit. Reaching a certain level of resistivity, the current flow is reducing itself, the increase of outside film build is stopped and inside areas of the parts are getting coated. This characteristic leads to excellent thickness control and allows the coating of passenger cars outside/inside and even in the recessed areas of the inner vehicle. As the rinse media is created out of the bath via ultrafiltration process, a close loop system is recovering the uncoated paint and the paint is flowing back from the rinses to the bath. This allows a transfer efficiency of > 95 %. As ultrafiltration process is in place and the removal of acids released during deposition done via anolyte system, the electrocoat baths are stable and do not need to be replaced. At continuous deposition process the material coated is replaced by feed paint material refreshing the bath with new electrocoat material. The electrocoat process is very efficient and is an environmentally friendly paint process. Minimized amount of waste, low volatile organic compounds (VOC) emission and fulfilling current and future environmental regulations.

The cost of coated parts or coated m2 are low compared to other paint applications. But to build a pretreatment line with 7 to 11 stages and electrocoat line with 5 to 6 stages including the oven is a high capital investment. The users of the electrocoat process are to find in industrial applications coating high volume of parts and surface areas. Due the size and complexity of the installation, colour change is in most applications not foreseen and only possible with high efforts. If electrocoat is used as a one coat system and should provide aesthetic colours like for lawn and garden equipment, then separate electrocoat lines in different colours are used.

Electrocoat advantages

uniform film builds in range 15 to 30 µm

coating lot of parts at same time in same quality every day

coating of complex shapes, even complex interiors

excellent corrosion protection

good chemical resistance

water-based

low VOC

environmentally friendly process

high material efficiency

automated process

low application cost

Electrocoat limitations

application basically on metals only

thermal cure necessary

low flexibility in colour change

low light durability of epoxy-based systems

high capital investment

The cationic epoxy electrocoats used as corrosion protection primer represents about 90 % of the market.[3] Cationic acrylics and anodic systems are for applications with lower corrosion protection requirements. Colour, gloss and durability against sun light are the main characteristics. Anodic epoxy-based systems have a better corrosion protection than its acrylic counterpart. The importance of anodic electrocoats for the industry shrunk over the years and might represent less than 5 % today. There are some niche markets or older lines, which are still running without a need to change to a better performance. The interest in this technology is the capability to formulate low cure systems, which can cure at about 110°C to 130 °C, used for temperature sensitive assemblies. Another interesting aspect is the coating of aluminium. Here the anodic electrophoresis process is anodizing the aluminium substrate and good corrosion protection better or equal to cationic systems can be achieved. Anodic acrylics are used for a variety of applications were UV durability with good colour and gloss control is required. Mainly for interior applications like metal furniture, micro wave, storage equipment, coatings for brass, for air diffuser and other all kind of metal parts which are used inside. [4]

1.3Market size

Electrocoats, especially cationic electrocoats, have a high relevance for industrial applications. Lot of lines are running in all industrial countries over the globe. The total size in sales volume is estimated for 2017 at about 3,100 Mill € and between 8000 to 9000 Mill m2 are coated with electrocoats. The yearly market growth expectation is at 4 to 5 % average with higher growth rates in Asia/Pacific than North Amerika and Europe. [3], [5] The growth depends on the growing global automotive market and for all the other industries in emerging countries. Already Asia/Pacific represents 50 % of the global market for electrocoats, followed by Europe and North America, South America, Middle East and Africa. These numbers reflect the industrial growth of the Asia/Pacific region in the last years. The majority are cationic electrocoats based on amine functional epoxy-based resins thermally cured via blocked isocyanates cross linkers.

Figure 1.1: Electrocoat technology distribution

Cationic electrocoats used as corrosion protecting primer is the main technology in automotive industry to protect the vehicles. In combination with galvanized substrates, automotive OEMs are providing warranties against corrosive substrate perforation of 6 to 12 years and more.

Figure 1.2: Electrocoats markets

Transportation include passenger cars, commercial vehicles, trucks, busses, automotive parts

1.3.1Electrocoat manufacturer and important players

In the early stages of anodic electrocoat in 1960, a lot of players were in the market, reduced by market consolidation in paint industry over the last 30 years. Today, the main suppliers for the electrocoat process are

Electrocoat manufacturer

Axalta Coating System, USA

BASF SE, Germany

Kansai Paint Co. Ltd., Japan

KCC Corporation, Korea

Nippon Paints Holding, Japan

PPG Industries Inc., USA

Sherwin Williams/Valspar Corporation, USA

Metal cleaning and pretreatment

Chemetall GmbH, Germany

Henkel AG & Co. KGaA, Germany

Nippon Paints Holding, Japan

Nihon Parkerizing, Japan

PPG Industries Inc., USA

Engineering and equipment manufacturer

Dürr AG, Germany

Eisenmann SE, Germany

Geicotaili-Sha, Japan-Italy

Munk GmbH, Rectifiers and electrical installations

Wächter GmbH, Rectifiers and electrical installations

1.4References

[1]Ullmann’s Encyclopia of Industrial Chemistry paint and coatings, Chapter 3 Paint systems

[2]J. W. Dini, The material science of coatings and substrate, ISBN 0-8155-1320-8

[3]Markets and Markets, Electro coat Marketreports, www.marketsandmarkets.com

[4]K. Follet, Electrocoat Technologies and where they fit, Electrocoat Conference 2004, Orlando

[5]Coatingsworld, Press release ecoat market growth 2015–2020, (8.17.2015)

2Physicochemical basics of electrophoretic paint deposition

This chapter will give a general explanation, which one must know to understand the electrophoretic paint deposition. Some physicochemical basics are necessary for this understanding.

An important role is played here by the particular properties of the small water molecule, in which it differs fundamentally from solvents. The effects of ions and electric charges – from hydrogen ions to charged polymers – are decisive for the behaviour during production and application as well as for the stability of the material, while in solvent-borne coatings they are almost negligible in most cases.

This is a very demanding challenge at first but offers very different possibilities for chemical and electrochemical influencing and process control. While all of this applies to the known water-borne paints, the electric current is additionally used to tailor the deposition process.

2.1General physicochemical basics

2.1.1Anomaly of water

The chemical formula for a water molecule is H2O. Each molecule contains one oxygen and two hydrogen atoms (Figure 2.1).

Figure 2.1: Structure and dipol character of water molecule

Because the oxygen atom has two free electron pairs the water molecule is not linear. Oxygen has a high electronegativity (pulling binding electrons in a bond), and so water is a polar molecule, with an electrical dipole moment (positive and negative electrical charges will separate). Depending on its surrounding the water molecule loses a proton H+ (nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−. H+, immediately protonates another water molecule to form hydronium, H3O+. This illustrates the amphoteric nature of water, which means, water can act as acid as well as a base.

To a small extent, water molecules dissociate into equal amounts of H3O+ and OH-. Overall the dissociation still results in a neutral solution.

2.1.2pH-value

The logarithmic scale used to specify the acidity or basicity of an aqueous solution is pH. It is the negative of the base 10 logarithm of the activity of the hydrogen ion (H+):

The activity is mainly driven by ther concentration of H+ ions, corrected by factors that depend on the environment, temperature and others.

Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Pure water is neutral, at pH 7 (25 °C), being neither an acid nor a base, because it dissociates in equal parts into H+ and OH- ions:

H2O ⇋ H+ + OH-

Measurements of H+ concentration, so of pH, are important in agronomy, medicine, chemistry, water treatment, and many other applications. Table 2.1 shows some pH-values of common acids and bases.

Table 2.1: pH values of some liquids (at low concentrations and at 25 °C) [1]

Source: wikipedia

Liquid

pH

Hydrochloric acid, 0.1 mol/Lw

1.0

↑ acid

Acetic acid, 1 mol/L

2.4

Pure water

7.0

neutral

Soapsuds

8.2 – 8.7

↓ basic

Ammonia water, 0.1 mol/L

11.0

Soda solution, 0.05 mol/L

11.3

Caustic soda, 0.1 mol/L

13.0

The pH scale is traceable to a set of standard buffer solutions whose pH is established by international agreement. The pH of aqueous solutions can be measured with a glass electrode (mostly used for aqueous binders and paints) as a part of a pH meter, or an indicator.

2.1.3Dispersion stability

Dispersion stability of polymers as well as of pigments and other particles in water is important. Solid or liquid particles are electrostatically stabilized when they have the same electrostatic charges on their surface, generating a repulsive force. Such charges can be generated by addition or removal of charged particles. Examples of such particles are H+ or OH- ions, or else of cations, anions or charged polymers. Figure 2.2. shows two in this way positively charged paint particles, which are prevented of coagulation due to their like charges.

Figure 2.2: Repulsion of two positively charged particlesSource: Rippert Anlagentechnik GmbH

Water-borne coatings (aqueous coating materials) are water-soluble or emulsifiable systems.

Polymers in water may be either fully dissolved (molecularly distributes) or in coarsely dispersed states. In order to be soluble in water, polymer molecules must possess ionic groups like carboxylate or ammonium groups or, alternatively, a considerable number of non-ionic, hydrophilic groups or segments such as hydroxyl, carbonyl, amino, amide groups and/or polyether chains.

If the molecules are not hydrophilic enough to form molecularly disperse (true) solutions, several of them may associate to form larger, colloidal aggregates or secondary dispersions, which are virtually clear. The less hydrophilic the molecules are, the larger the disperse particles become. Sometimes, emulsifiers (surfactants) or protective colloids provide stability, but that can result in a later water sensitivity of the coating. If the disperse phase, e.g. an oligomeric resin, is liquid, the dispersion is an emulsion. Microemulsions are very fine-particle, virtually clear, stable emulsions which form when a “self-emulsifying” oil phase in water is stirred gently.

In the case of ionic polymers (polyelectrolytes) we differentiate between anionic, cationic and zwitterionic types. The anionic types are mostly polycarboxylic acids and are neutralized with amines (Figure 2.3), the cationic film formers are usually polyamines to which simple acids (acetic acid, for example) are added. Figure 2.4 provides a schematic representation of the chemical structures of a cationic system.

Figure 2.3: Example of anodic dissolved film former Source: Rippert Anlagentechnik GmbH

Figure 2.4: Example of cationic dissolved film former Source: Rippert Anlagentechnik GmbH

More details will be explained below in Chapter 2.2.3: Electrodeposition

2.2Principles of formulation

The principles of formulation are described in detail in Chapter 4. Here are some terms to understand the basics. Water-dilutable systems are made very similar to conventional solvent-based systems. Nevertheless, there are a number of additional notable differences in terms of the suitability of the individual coating components.

Emulsion paints [2] consist of:

film forming dispersion (binder)

pigments and fillers (+ wetting and dispersing agents), defoaming agents if required

coalescing agents (if needed)

amines (NR

3

), ammonia (NH

3

), NaOH or similar to adjust the pH value in formulations containing binders with carboxyl groups (-COOH)

carboxylic acids like formic acid (HCOOH), acetic acid (CH

3

COOH) or propionic acid (CH

3

CH

2

COOH) in formulations containing binders with amine groups (-NR

2

)

rheological additives

biocides (often needed)

Individually, these components must meet a lot of important requirements specific to emulsion paints.

2.2.1Film formers („binders“) [3]

As already mentioned only a few highly polar organic film formers – such as polyvinyl alcohols, polyacrylamides, polyethylene glycols, some cellulose derivatives as well as acrylates and polyesters with a very high acid value – are soluble in water. Such film formers naturally remain water-soluble in the dried film as well. For that reason, alternative routes had to be found for the development of water-soluble or water-thinnable film formers as they need to be insoluble after drying.

Electrodeposition coatings need water-thinnable but not strictly water-soluble film formers, consisting of relatively short-chain polymers (Mr < 10,000) with acid or basic functional groups capable of salt formation incorporated into the side chains. They are neutralized with suitable bases or acids, which prevents precipitation of the film former.

Following application, the neutralizing agents leaves the film mainly by the electrophorese process and the resulting coating film loses its salt character and is no longer water-soluble.

Electrocoats are stable at neutralization grades of 30 to 50 %. If the neutralization is too high, the deposition equivalent (C/g) is high, gassing is high and the film may stay water soluble.

2.2.2Pigments and fillers

Apart from a few exceptions – those with limited alkali resistance or sensitivity to hydrolysis, for example – most pigments are suitable for aqueous coating materials. However, a too high content of water-soluble components in the used pigment can lead to binder coagulation during storage and to blistering if the coating is exposed to moisture or water. Incidentally, the pigment dispersion is already largely stabilized (in contrast to emulsions) by the water-soluble film formers.