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Electrodeposition of coatings--anodic ED.

As mentioned last month in the introduction to this series (CoetingsTech, p 52, September 2011), there are two different electrodeposition processes: anodic electro-deposition (AED) and cathodic electrodeposition (CED). Anodic electrodeposition involves the deposition of negatively charged polymers along with pigments and additives onto anodes that are the objects being painted. The first commercial ED coating was anodic (1962) as were all ED coatings for a number of years. Anodic products, especially epoxies and acrylics, currently are used for a number of industrial applications, including fasteners, lawn and garden equipment, household products, and agricultural equipment. Solids tend to be lower than for cathodic ED, usually being in the range of 12-15% weight solids. Typical polymers are anionic polyelectrolytes with carboxylic acid groups (acid number 50-80 mg KOH/g resin). Neutralization with amines, ammonia, or alkali metal hydroxides makes them dispersible in water. There are various polyelectrolytes that are capable of crosslinking or reacting with crosslinkers as well as being water dispersible. Since the anodic polymers have carboxyl groups and films are acidic, crosslinkers and catalysts used for conventional waterborne coatings usually work for anodic ED as well. Some AED coatings cure at temperatures as low as 180 [degrees] F, which is very useful if you are trying to cure coatings on large objects such as earthmovers or combines.

Some of the earliest commercial anodic ED coatings were automotive primers based on natural oils with conjugated double bonds (such as dehydrated castor oil and linseed oil) that were reacted with maleic anhydride. Cure was by autooxidation. Cis-1, 4 liquid polybutadienes also can be maleinized to yield electrodepositable polymers. Since these do not have saponifiable backbones, they provide better corrosion resistance than the natural oil products. Maleated epoxy esters give excellent adhesion and hydrolytic stability along with outstanding toughness and corrosion resistance. Crosslinking is through the drying oil fatty esters and/or external crosslinkers such as melamine-formaldehyde resins. Epoxy ethers can be made from diepoxides and allyl alcohol with subsequent copolymerization with acrylic acid and styrene. Alkyds cured with phenolic resins have been used for automotive and other parts. Polyacrylics crosslinked with melamines make excellent one-coat finishes and are the choice whore excellent outdoor durability is needed.

Mechanism

Resin solubilization: R-COOH (insoluble) + KOH (or an amine) [right arrow] R-[COO.sup.-][K.sup.+] (soluble) + [H.sub.2]O

The negatively charged polymer deposits on the anode where the reactions are:

Electrolysis of water: 2 [H.sub.2]O [right arrow] 4[H.sup.+] + [O.sub.2] [up arrow] + 4[e.sup.-]

Metal deposition: M [right arrow] [M.sup.n.sup.+] + n[e.sup.-]

Film deposition/coagulation: R-[COO.sup.-] + [H.sup.+] [right arrow] R-COOH [down arrow]

Followed by crosslinking via the carboxylic acid groups

[Electrolysis also occurs at the cathode: 2 [H.sub.2]O + 2[e.sup.-] [right arrow] [H.sub.2] [up arrow] + 2O[H.sup.-]

Advantages of anodic ED coatings:

* Excellent value: costs are lower than for cathodic ED. This is because the resins and crosslinkers are less expensive and tanks and other equipment can be made of less expensive materials.

* The bath operates at moderate pH (6-8), so tanks and other equipment can be made of carbon steel without fear of attack and dissolution by the bath.

* Inert cathodes are not necessary because metal dissolution does not occur there.

* Volatile counterions can be used, minimizing control problems.

Disadvantages of anodic ED coatings:

* The substrate is subjected to highly acidic conditions (pH 2-4) that may dissolve zinc phosphates and other conversion coatings.

* Many anodic coatings have poor saponification resistance, which means poor resistance to corrosion products.

* There is considerable metal dissolution, which varies with the metal being coated. Steel dissolution is as much as 40X that for cathodic deposition and is likely to discolor the coating.

* Bath pH may drift due to C[O.sub.2] pick-up.

I have yet to find a really good article or paper on anodic ED coatings. However, there are two old publications that still are relevant--a review on resins for anodic [H.U. Schenck, H. Spoor, and M. Mark, Prog. Org. Coal, 7, 1-77 (1979)] and a paper comparing anodic to cathodic [H.U. Schenck and J. Stoelting, J. Oil Col. Chem. Assoc, 63, 482-491 (1980)]. The literature on the subject is mainly in the form of patents.

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By Clifford K. Schoff, Schoff Associates
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Title Annotation:COATINGS CLINIC
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Oct 1, 2011
Words:724
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