Electrocoat
Formulation and Technology
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
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.
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Table 1.1: Basic overview on cationic and anodic electrocoats |
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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/ |
Anodics have lost their market penetration Epoxy systems for corrosion protection and acrylics for coloured one coats available Might be of interest for aluminium |
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.
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Table 1.2: Markets for electrocoats technologies |
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Cationic |
Anionic |
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Epoxy |
Acrylic |
Epoxy |
Acrylic |
|
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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 |
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.
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]
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
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
[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)
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.
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.
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+):
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pH = − log [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:
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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.
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Table 2.1: pH values of some liquids (at low concentrations and at 25 °C) [1] Source: wikipedia |
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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.
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
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:
Individually, these components must meet a lot of important requirements specific to emulsion paints.
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.
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.
Because they are surface-active substances, water-thinnable film formers and any dispersing agents that may be present can increase the tendency towards foam formation, which means that defoaming agents may need to be added to the mill base. The surface tension of coating materials containing water-soluble film formers is high itself – firstly because of the high content of polar groups in the film former molecules and secondly because of the high surface tension of water – often too high even to enable the substrate to be thoroughly wetted. While the surface tension can be sharply reduced by using suitable co-solvents, it may need to be reduced even further by addition of flow-control agents (defoamers). Biocides are necessary if the content of organic solvents is not sufficiently high to prevent attack by microorganisms.
Electrodeposition of electrocoats is initiated by the electrolysis of water. The increasing H+ and OH- concentration at the electrodes change the pH and neutralize the colloidally dispersed paint particles. The paint particles then coagulate and precipitate at the electrode surface. The paint layer gets tightly packed and insulates the electrode. This insulation causes the electrolysis of water to stop, no or very low amounts of ions (H+ or OH-, depending on the electrode) are produced and therefore no further particles are precipitated, thus stopping the coating process. In this chapter you find the four main physiochemical processes that finally form the electrocoat layer:
The electrolysis of water decomposes water into oxygen and hydrogen gas as a function of the electric current. The electrocoating process uses a DC rectifier to create this electric current between a conductive part (workpiece) and counter-charged electrodes.
The reduction of hydrogen ions (H+) in water occurs at the cathode and the oxidation of OH–ions occur at the anode (see Figure 2.5).
Cathode: Reduction of hydrogen ions by electron uptake at the negative charged electrode to hydrogen gas and and enrichment of OH– ions.
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H2O ⇋ H+ + OH- |
|
2 H+ + 2 OH- + 2 e- → 2 OH- + H2 |
Anode: The electrolytic dissociation of the water molecule by release of electrons at the positive charged electrode to oxygen
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2 H2O - 4 e- → 4 H+ + O2 |
For the electrolysis of water the gas formation is the main process. In the electrocoat application the enrichment of hydroxyl ions OH- (cathode) and hydrogen ions H+ (anode) is the decisive process. These permanently accumulating ions lead to changed pH-value at the electrodes. The cathode so has an alkaline and the anode an acidic environment.
Figure 2.5: Schematic representation of water electrolysisSource: Rippert Anlagentechnik GmbH
Electrophoresis is the migration of electrically charged particles (macromolecular type or solid particles) under the influence of an electric field. As opposite charges attract each other the direction of migration is determined by the direction of the electric field and the charge of the particles (Figure 2.6).
If an aqueous dispersion of cationic binders (positively charged polymer) is used, the polymer molecules (cations), together with pigment particles, migrate towards the cathode (negative charge). For anodic binders the polymer has a negative charge and the workpiece has a positive charge.
Figure 2.6: Migration of ions in an electrical field
Fe: Electrical force
FR: Frictional force Source: Rippert Anlagentechnik GmbH
In charge and size different particles migrate at different speeds in an electric field.
In electrochemistry, the diffusion layer (also: Nernst’s diffusion layer or diffusion boundary layer) is a region in the vicinity of an electrode where the concentrations are different from their value in the bulk solution. The diffusion boundary layer is a thin film of liquid adhering to the (metal) surface. In contrast to the interior of the bulk, there is no convection in this layer. The mass transfer occurs only by (slower) diffusion. The generic term for both (convection and diffusion) is migration.
The definition of the thickness of the diffusion layer is arbitrary because the concentration approaches asymptotically the value in the bulk solution, see Figure 2.7.
Figure 2.7: Assymptotic curve describing concentration in Nernst´s diffusion layer [3]
C: bulk concentration
Ce: concentration at metal surface
C0: concentration at outer surface of diffusion layer
δ: thickness of the Nernst diffusion layer Source: Rippert Anlagentechnik GmbH
The thickness depends on the flow velocity inside the bath and also on the diffusion coefficient (D) of the material. Relevant to cyclic voltammetry, the diffusion layer has negligible volume compared the volume of the bulk solution.
In the case of a parallel flowed flat surface, the relationship arises in the case of a laminar flow [3]:
Equation 2.1
|
δ N = 3 l1/2 vBad -1/2 ν 1/6 D 1/3 |
withδ N:thickness of Nernst´s diffusion layer
l:coordinate along the flow
vBad:flow velocity inside the bath
ν:kinematic viscosity of bath
D:diffusion coefficient of transported particle
In ED paint (ED = electrodeposition) baths the diffusion layer thickness is about 100 µm.
Regarding the conditions inside and outside the diffusion layer, at first Figure 2.8 shows schematically the paint particles distribution in the bulk before applying the voltage.
Figure 2.8: Paint particles without current flow
Source: Rippert Anlagentechnik GmbH
After applying the DC voltage the electrocoat binder („P“) deposits in the areas closest to the counter electrode (Figure 2.9) first. After these areas are isolated the electric field lines are forced into more recessed areas and start to deposit a paint film there. In this way even Faraday cages can be coated. This described process is responsible for good throw power (see Chapter 2.3.1) and is an important advantage of the electrocoat process.
Figure 2.9: Paint particles after applying DC voltage
COH - (crit.): The hydroxyl ion concentration above which the polymer ions begin to coagulate
Source: Rippert Anlagentechnik GmbH
The highly liquid electrodeposition bath must be continuously stirred or agitated to prevent pigments and fillers from settling out, even outside the deposition period. Amongst other things, this means that strictly speaking the commonly used term “electrophoresis” and also “anaphoresis” or “cataphoresis” does not apply since no significant bulk migration of particles occurs in the electrical field. Instead, the coating particles that are deposited are principally those that are close to the surface of the workpiece as a consequence of diffusion within the diffusion layer.
The changes in concentration of the H+ ions (cH+) or of the OH- ions, as well as those of the corresponding acids and bases, are restricted to the diffusion boundary layer at the electrodes, as already stated no more than 100 μm thick. Figure 2.10 provides a schematic view of these concentration gradients in the bulk and in the boundary layers. The migration of charged particles in the large, central part of the bath, made turbulent by stirring is negligibly slow in comparison to the stirring speed (convection).
Figure 2.10: Diffusion boundary layers and H+ concentrations in the ED bath during current flow [2]
Source: Rippert Anlagentechnik GmbH
At CEC (cationic electrocoat) the cations moving towards the cathode by electrophoresis pass through regions of increasing pH in the Nernst diffusion layer, so that they coagulate by contact with more and more hydroxyl ions on their way to the electrode. Since the polymer ions in a cationic electrodeposition bath undergo coagulation in the range above pH ≈ 10 to 12, this process will therefore begin somewhere within the region of the boundary layer. The corresponding process occurs in an anodic electrodeposition bath when the pH value falls below 4.
As we have learned, the electrolysis causes an alkaline environment at the cathode and an acidic environment at the anode, followed by paint particle precipitation of the suspension and built up of a paint layer on a conductive workpiece. In cationic electrocoat systems we are using binders where a neutralizing acid creates the positively charged polymer. The polymer solid is then deposited onto the part. In anodic electrocoat systems, the hydrogen ions neutralize the amine groups of a negatively charged polymer.
The electrodeposition process solely works through the change in pH value. The binder is chemically changed. The current is used only for the electrolysis of water, not for the status change of the binder.
The following reaction equations describe – in anodic electrocoat – first the solution reaction during bath production and then the reaction during the deposition under the effect of the applied DC voltage.
Reaction of binder resolution
Equation 2.2
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|
Deposition at the anode after application of DC voltage
Equation 2.3
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|
Anodic electrocoat has negatively charged paint particles that are deposited onto the positively charged metal substrates (anode). Looking at the schematic pictures of the electrolysis, in anodic electrocoat the oxidation occurs at the workpiece. As many metals can be oxidized in this process (see Equation 2.3), metal ions will migrate into the paint film. Applied on the workpiece as a protective coating, these incorporated ions will interact with moisture. The result will be some discoloration (on lighter colours) and a general weaker corrosion resistance compared to cationic electrocoat. The performance properties of anodic systems on steel parts are limited due to this mechanism.
Equation 2.4
|
Fe – 2e- → Fe2+ |
Iron oxidation at the anode, producing and emitting Fe ions.
The main use of anodic electrocoat is for interior products or moderate exterior environments. As most anodic systems are acrylic binders, they offer high UV resistance and excellent colour and gloss control. The corrosion resistance is certainly also a function of the binder composition used, but even with binder formulations, that have a better corrosion resistance than acrylics the Fe ions are limiting the corrosion resistance.
Analogously, in the following, the resolution and deposition in the cationic electrocoat (CEC):
Resolution of a CEC binder
Equation 2.5
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|
The resolution reaction of the cationic binder is a reversible process, which precipitates the dissolved binder again by abstraction of H+ (deprotonation) with hydroxyl ions.
Deposition at the cathode after application of DC voltage
Equation 2.6
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|
Cationic electrocoat has positively charged paint particles that are deposited onto the negatively charged workpiece (cathode). No iron ions will be in the deposited film and therefore the corrosion resistance is significantly better than in anodic systems.
Cationic electrocoat is commonly used in applications where corrosion resistance is very important. They are formulated for exterior durability. Due to the paint chemistry mainly used (epoxy resin) they have very bad weathering durability and have to be topcoated if they are used in exterior environment.
Specially in the agriculture industry more and more cationic acrylics or acrylic/epoxy mixed formulations are used to combine “the best of both worlds”. Finally, these systems provide a reasonable corrosion and UV resistance for the desired use.
Details of the different resin technologies used can be found in Chapter 3.
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