Electrodeposition of coatings--anodic ED.
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.
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.
By Clifford K. Schoff, Schoff Associates
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||COATINGS CLINIC|
|Author:||Schoff, Clifford K.|
|Date:||Oct 1, 2011|
|Previous Article:||Meetings and events.|
|Next Article:||Planning for the future.|