Cathodic Electrocoat: Priming the Way to Unsurpassed Product Finishing Performance

The global electrocoat market is estimated to grow from USD 3.08 billion in 2016 to USD 3.80 billion by 2021, at a CAGR of 4.34 percent between 2016 and 2021. This market is witnessing moderate growth because of demand for e-coat from the end-use industries such as automotive and appliances1.

Electrocoating is a process that uses an electrical current to deposit an organic coating from a paint bath onto a part or assembled product. Due to its ability to penetrate recesses and thus coat complex parts and assembled products with specific performance requirements, electrocoat is used worldwide throughout various industries to coat a wide swath of products including those in automotive, appliance, marine and agriculture. Electrocoat, and specifically cathodic electrocoat, has enabled a dramatic improvement in corrosion resistance over that offered by anodic electrocoat or other more conventional methods of coating.

Cathodic electrocoat is available in multiple technology types, most notably is epoxy cathodic and acrylic cathodic.

  • Acrylic cathodic is expected to experience the highest growth in the e-coat market. Cathodic acrylic e-coat is typically used in applications that require UV durability as well as corrosion protection on ferrous substrates. It is also used in applications where light colors are required.
  • Cathodic acrylic e-coat is available in a wide range of glosses and colors to provide both exterior weathering and corrosion protection. Acrylic cathodic is used as a one-coat finish for agricultural implements, garden equipment, appliances and exterior HVAC.

The application of electrocoat involves four steps:

  1. Pretreatment2: After the parts are cleaned, a pretreatment is applied to prepare the metal surface for electrocoating.
  2. Electrocoat Application: Positively charged cathodic paint is deposited on the electrically conductive substrate from a cathodic paint bath using direct current. The positively charged paint is deposited at the negatively charged cathode where reduction takes place.
  3. Post Rinses: Parts are rinsed to reclaim undeposited paint solids.
  4. Bake: Paint is baked to thermally cross-link the paint and volatilize water as well as any residual organic solvents.

In step 2, paint particles are deposited on the surface of the electrically conductive substrate to form an insulating film.  The rate of the film deposited diminishes with time as the conductivity of the paint surface has an insulating effect as the film increases in film thickness. At this point the deposited film has very little water and solvent present so the water post rinses (step 3) do not have a negative effect on the deposited film. The coated substrate is then baked to eliminate water and remaining volatile as well as to crosslink the polymeric film.

Cathodic Electrodeposition - learn more about cathodic electrocoating in the the Prospector Knowledge Center
Figure I: Cathodic Electrodeposition

As figure I indicates, in cathodic electrodeposition, the positively charged paint is attracted to the negatively charged cathode where reduction occurs, resulting in the liberation of hydrogen gas. At the anode, oxidation occurs with the accompanying release of oxygen. The deposited paint film is coalesced into a relatively insoluble paint film and after one or more water rinses, the deposited paint film enters a bake oven to enable the crosslinking of the cathodic paint film.

The advantages and disadvantages of cathodic electrocoat include:

  • Excellent corrosion resistance, even at lower film thicknesses
  • Offers excellent resistance to bimetallic corrosion (when dissimilar metals are in contact)
  • Frequent color change is not practical
Cathodic Electrocoat Deposition example - learn more in the Prospector Knowledge Center
Electron flow in cathodic electrocoat deposition - learn more in the Prospector Knowledge Center.

Many cationic epoxy electrocoat resins are comprised of a Bisphenol A based epoxy resin comprised of amine groups that are neutralized with a low molecular weight acid such as formic, acetic or lactic acid. Since the coating bath has a pH of slightly below 7, bath components are comprised of stainless steel or other corrosion resistant materials to prevent rust formation.

The most common crosslinker is a blocked isocyanate, so once the coating is baked, the blocked isocyanate is activated and reacts with available hydroxyl and amine groups. Other components of a typical electrocoat bath include pigment, filler pigment, water, solvent and a low level of modifying resins such as plasticizers and flexibilizersflow modifiers and catalysts.

Cross section of automotive coating system - learn more in the Prospector Knowledge Center.
Cross section of automotive coating system


Ecoat is used because it provides superior corrosion protection as it coats surfaces that are inaccessible by conventional means. Film thickness is uniform without any defects such as sags, runs or edge beads. Electrocoat is also very cost effective as it provides nearly 100 percent material utilization with good energy efficiency and a relatively low cost per square foot of applied coating.

Throwpower is the ability of an electrocoat to penetrate into “hard to reach” areas, such as the inside of a hollow metal object. Dependent on applied voltage, bath solids, conductivity, deposition time, bath temperature, solvent levels, and proper tank agitation, deposition time, throwpower and coating appearance can be optimized.

A simple dip-applied coating cannot effectively coat the interior of complex shaped parts, as during the bake process, the water/solvent has a washing effect in the interior portions of the part that prevents adequate film build. At the time an electrocoated object is removed from the bath, most of the water and solvent is squeezed from the electrocoat so that during the bake the washing effect is minimal as compared to that of a simple dip-applied coating.

The film build of electrocoat paint is self-limiting as the film becomes more insulative as the thickness of the film approaches its maximum. Higher voltage and longer immersion times will permit higher film builds until the maximum possible film build is reached, which is normally about 1.0 mil and 1.2 mils.

Voltage is normally between 225 and 400 volts. If the voltage is too high, there will be film rupture of the coating applied to the outer surfaces. This is called the rupture voltage. At a sufficiently high voltage, the current will break through the film, leading to gas generation under the film (hydrogen for cathodic and oxygen for anodic).

Other factors that affect film build include bath temperature and conductivity. Immersion times are normally on the order of 2-3 minutes.

In summary cationic electrocoating is expected to grow at a faster rate than that of more conventional product finishing processes as it provides excellent corrosion protection for complex shapes, low volatile organic content and lastly acrylic cationic electrocoat also offers resistance to UV light for experior applications.

To read the rest of the article please click here to head over to UL Prospector.


Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.

Overcoming Paint Film Defects: Causes and Remedies

Paint film defects can appear during or immediately after application or become more apparent after the coating is cured. While there is no standard convention for the nomenclature of film defects, this article will separate film defects into the two categories mentioned above.

Crawling, crafters, crazing - a variety of paint defects can occur after application or curing. Learn the causes and solutions here.
Example of crazing.
Copyright: paylessimages / 123RF Stock Photo

Paint film defect causes

The largest number of paint defects is from dirt particles1embedded in the paint. Most other paint defects are the results of:

  • lack of cleanliness
  • surface preparation
  • application error
  • attention to detail

Surface tension

Many coating defects are related to surface tension issues. Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible. This occurs when the forces at the interface of a liquid differ from those within the liquid, attributed to uneven force distribution of molecules at the surface. A common unit of surface tension is dynes/cm2 (force/unit area).

For example, applying a coating with a higher surface tension than the substrate may cause dewetting, crawling, pinholing, holidays and telegraphing.

Likewise, the difference in surface tension at the paint surface can result in cratering or fisheyes.

Table 1: Surface tension of paint Solvents

Solvent Surface Tension Dynes/cm
Water 72.8
Toluene 28.4
Isopropanol 23.0
n-Butanol 24.8
Acetone 25.2
Methyl propyl ketone 26.6
Methyl amyl ketone 26.1
PM acetate 28.5


Table 2: Liquid surface tension of Polymers used to reduce surface defects

Polymer mj/m2
Poly(dimethylsiloxane) 22.6
Poly nButyl Acrylate 33.7
Poly nButyl Methacrylate 31.2


Highly polar molecules (e.g. water) have a higher surface tension than less polar materials (see Tables 1 and 2). Surface defects can often be reduced or eliminated by using small amounts of additives with low surface tension such as polydimethyl siloxanes (DMS), poly butyl acrylate or poly 2-ethyl hexyl acrylate. These additives tend to migrate to the surface to help flow and leveling.

Table 3: Defects that can occur during or soon after application

           Defect Appearance                Causes          Remedy
Crawling Uneven film thickness, dewetting High surface tension paints applied to a substrate with lower surface tension. For example, paint on steel with oil on the surface
  • Proper surface cleaning of metallic or plastic surfaces
Craters/fish eyes Small round depressions in the surface of the coating Small particles of a low surface tension contaminant (e.g. oil, grease, silicone oil, wax) on the substrate or that embeds in the coating
  • Proper spray booth air filtration and the contaminant elimination.
  • The addition of surface wetting agents such as DMS and/or polyacrylates with a low glass transition (Tg).
Crazing, cracking Small cracks formed in the coating. This can occur on recoat or if coating is applied to solvent sensitive plastics Application of coatings on plastics where the paint contains strong solvent that solvates the underlying coating layer or plastic substrate
  • Use solvent that will not crack or craze the plastic.
  • Test spot resistance of substrate with suitable solvent.
Dirt, contamination Small raised imperfections in the surface of the coating
  • Surface not carefully cleaned.
  • Dirty spray booth and/or booth filters.
  • Pressure in the spray booth too low.
  • Unsuitable work clothes.
  • Inadequate paint filtration
  • Ensure cleanliness of the environment where the coatings are applied
Loss of gloss, blush Areas of low gloss or a white haze Humidity condenses on the wet paint due to the cooling effect of solvent evaporation when the substrate temperature is below the dew point. Causes:

  • Unsuitable reducers
  • Poor air circulation in drying oven
  • Film thickness too high or low
  • Proper humidity control


Mottling Uneven appearance of metallic paints
  • Dirty spray gun nozzle
  • Incorrect air pressure
  • Incorrect reducer
  • Faulty spray technique
  • Incorrect spray viscosity
  • Use proper viscosity cup to obtain spray viscosity.
  • Clean and maintain spray guns on a regular basis.
  • During application maintain spray gun parallel to the substrate and maintain correct distance from gun to substrate.
  • Follow Technical Data Sheets instructions.
Poor hiding · Uneven paint coverage
  • Nonuniform substrate surface
  • Uneven or inadequate paint coverage to mask the substrate color
  • Uniform and sufficient paint application to obtain proper hiding.
Runs and sags Drips and sags
  • Paint applied too thick or too wet to a vertical surface and the force of gravity overcomes the forces resisting the downward flow of paint (viscosity).
  • Temperature too low to enable proper solvent evaporation (solvent born paint), or humidity too high (waterborne paint).
  • Adjust low shear viscosity of paint with appropriate thickener.
  • Use proper reducer and viscosity adjustment for environmental conditions.
  • Adjust spray gun and apply thinner wet coats. If a waterborne paint, apply paint in a lower humidity environment.
Skips/holidays Incomplete paint coverage
  • Paint applied too thin
  • Minute areas on the substrate surface of low surface tension, causing inadequate film flow and coverage.
  • Proper paint application and ensure surface cleanliness.
Striping, banding Stripes of uneven paint appearance (e.g. differing color) Uneven paint application
  • Use proper viscosity cup to obtain spray viscosity.
  • Clean and maintain spray guns on a regular basis.
  • During application, maintain spray gun parallel at the correct distance to the substrate and maintain
Telegraphing Highlighting of the surface of the coated substrate through the coating. Such defects as fingerprints, sand scratches and water spots on the substrate become visible on the coating surface Coating with high surface tension applied to a substrate with lower surface tension. e.g. Fingerprints or silicone oil on a substrate surface.
  • Ensure that the substrate is thoroughly clean and absent of low surface tension oils and fingerprints.
Wrinkling, lifting, aligatoring Upon applying an overcoat, the existing paint film shrivels, wrinkles or swells; may also occur during drying. Solvents in the new paint swell the underlying paint finish.
  • Allow sufficient cure times of underlying paint
  • Ensure that the new paint is compatible with the undercoat
  • Proper application of the new paint (not too wet).


Table 4: Defects that are more apparent after cure

Defect Appearance Causes Remedy
Air entrapment Similar to solvent popping or bubbles Paint pump sucking air when paint level is low. In two component urethanes, moisture present reacts with isocyanate to cause CO2 generation.
  • Proper attention to paint line conditions.
  • Ensure use of urethane grade solvents and proper spray gun air filtration through desicant.
  • Addition of moisture scavenger in paint.
Barnard Cells Hexagonal pattern in the surface of a cured paint film. Convection pattern from pigment segregation as a result of surface tension differentials Adjust formulation to overcome flooding and differential surface tension at surface
Blisters Bubbles near the surface of a film during oven cure that do not break through the surface. Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level.
  • Proper oven staging to enable slow release of solvent.
  • In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release.
  • Increase flash time before bake.
  • Use slower evaporating solvent.
  • For spray application, apply additional thinner coats to build film rather than fewer thick coats.
  • For waterborne coatings, use a dehydration bake lower than the boiling point of water, followed by a second bake to cure.
Orange peel Rough surface that resembles the surface profile of an orange Paint applied at high viscosity or under conditions deleterious to proper flow and leveling.
  • Adjust paint to proper viscosity with correct reducer per technical data sheets.
  • Apply at proper fluid delivery rate and atomizing air pressure.
Solvent pop Broken bubbles at the surface of a film that do not flow out during oven cure Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level. The bubbles break the surface when the solvent volatilizes.
  • Proper oven staging to enable slow release of solvent.
  • In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release.
  • Increase flash time before bake.
  • Use slower evaporating solvent.
  • For spray application, apply additional thinner coats to build film rather than fewer thick coats.
  • For waterborne coatings, use a dehydration bake lower than the boiling point of water followed by a second higher bake to cure.
  • Lastly, the use of lower Tg resins along with lower dry film thickness decrease popping.


Search Prospector for formulating remedies to overcome paint film defects:

Defect Remedy material
Crawling and substrate wetting
Craters and fish eyes
  • PDMS
  • polyalkyl acrylates
Runs and sags
Air entrapment
Solvent pop, blisters For melamine cure systems:


To read more, please click here to head over to UL Prospector.


Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.

New US Patent on Rust Coatings Granted

US Patent on Coatings that Arrest Rust Granted to Chemical Dynamics LLC

US Patent 8,927,649 was awarded to Chemical Dynamics LLC on January 6, 2015. US Patent 8,927,649 covers of a film forming penetrant coating with a high cross-link density that arrests corrosion on rusty substrates.

Patent Abstract: A one part, storage stable polymerizable formulation is provided that includes an ethenically unsaturated polymerizable compound intermixed with a free radical polymerization initiator and an organic solvent. The organic solvent provides storage stability and upon evaporation of the solvent, the rate of polymerization of the compound accelerates independent of addition of a second part. The formulation includes at least 30 total weight percent solids upon cure. Optional additives to the formulation include at least one of a cure accelerator, a filler, a plasticizer, a colorant, and a cure inhibitor. A process for forming a polymerized coating on an article involves the application of this formulation to the substrate of an article and allowing sufficient time for the solvent to evaporate to form the polymerized coating on the article. The substrate of the article forms a corrosion barrier even without prior removal of a native corrosion layer on a surface of the substrate of the article.

Reactive Silanes for Enhancement of Coating Performance

Reactive silanes (EU) can be used in multiple applications to enhance the performance of coatings. They can improve adhesion to inorganic substrates, provide crosslinking, improve pigment dispersion, improve hydrophobicity and scavenge moisture. A silane (EU) that contains at least one carbon silicon bond (CH3 – Si -) is called an organosilane (EU).Reactive silane is the term used to define compounds that have a trialkoxysilyl group and an alkyl group (R) containing a reactive constituent.

Silanes_SM2The trialkoxy silanes that are discussed in this article are those that contain primarily trimethoxy (EU) groups, as these molecules are the most widely used to enhance coating performance in a wide variety of applications, as follows:

  • Adhesion Promoter (EU)– Silanes, when added to paints, can enhance adhesion to inorganic surfaces including metals and glass.
  • Coupling Agent (EU) – Silanes are used for coupling organic polymers to inorganic materials, including pigments and fillers.
  • Crosslinking Agent (EU) – Selective organofunctional alkoxysilanes can react with organic polymers to provide a trialkoxysilyl group into the polymer backbone. In turn, the silane can then react with moisture to crosslink and form a three-dimensional siloxane cross-linked structure.
  • Dispersing Agent (EU) – Used to improve the hydrophobicity of inorganic pigments, flow and the ability to be dispersed in organic polymers and solvents.
  • Moisture Scavenger (EU) – In moisture sensitive formulations, the three alkoxysilane groups can scavenge water by reacting with moisture to form alcohol molecules.

To read the full article about Reactive Silanes for Enhancement of Coating Performance, click here.