Electrophoretic Deposition - Types of EPD Chemistries For Electrophoretic Painting

Types of EPD Chemistries For Electrophoretic Painting

There are two major categories of EPD chemistries: anodic and cathodic. Both continue to be used commercially, although the anodic process has been in use industrially for a longer period of time and is thus considered to be the older of the two processes. There are advantages and disadvantages for both types of processes, and different experts may have different perspectives on some of the pros and cons of each.

The major advantages that are normally touted for the anodic process are:

1. Lower costs compared to cathodic process.
2. Simpler and less complex control requirements.
3. Fewer problems with inhibition of cure of subsequent topcoating layers.
4. Less sensitivity to variations in substrate quality.
5. The substrate is not subjected to highly alkaline conditions, which may dissolve phosphate and other conversion coatings.
6. Certain metals, such as zinc, may become imbrittled from the hydrogen gas which is evolved at the cathode. The anodic process avoids this effect since oxygen is being generated at the anode.

The major advantages that are normally touted for the cathodic processes are:

1. Higher levels of corrosion protection are possible. (While many people believe that cathodic technologies have higher corrosion protection capability, other experts argue that this probably has more to do with the coating polymer and crosslinking chemistry rather than on which electrode the film is deposited.)
2. Higher throwpower can be designed into the product. (While this may be true with the currently commercially available technologies today, high throwpower anodic systems are known and have been used commercially in the past.)
3. Oxidation only occurs at the anode, and thus staining and other problems which may result from the oxidation of the electrode substrate itself is avoided in the cathodic process.

A significant and real difference which is not often mentioned is the fact that acid catalyzed crosslinking technologies are more appropriate to the anodic process. Such crosslinkers are widely used in all types of coating applications. These include such popular and relatively inexpensive crosslinkers such as melamine-formaldehyde, phenol-formaldehyde, urea-formaldehyde, and acrylamide-formaldehyde crosslinkers.

Melamine-formaldehyde type crosslinkers in particular are widely used in anodic electrocoatings. These types crosslinkers are relatively inexpensive and provide a wide range of cure and performance characteristics which allow the coating designer to tailor the product for the desired end use. Coatings formulated with this type of crosslinker can have acceptable UV light resistance. Many of them are relatively low viscosity materials and can act as a reactive plasticizer, replacing some of the organic solvent that otherwise might be necessary. The amount of free formaldehyde, as well as formaldehyde which may be released during the baking process is of concern as these are considered to be hazardous air pollutants.

The deposited film in cathodic systems is quite alkaline, and acid catalyzed crosslinking technologies have not been preferred in cathodic products in general, although there have been some exceptions. The most common type of crosslinking chemistry in use today with cathodic products are based on urethane and urea chemistries.

The aromatic polyurethane and urea type crosslinker is one of the significant reasons why many cathodic electrocoats show high levels of protection against corrosion. Of course it is not the only reason, but if one compares electrocoating compositions with aromatic urethane crosslinkers to analogous systems containing aliphatic urethane crosslinkers, consistently systems with aromatic urethane crosslinkers perform significantly better. However, coatings containing aromatic urethane crosslinkers generally do not perform well in terms of UV light resistance. If the resulting coating contains aromatic urea crosslinks, the UV resistance will be considerably worse than if only urethane crosslinks can occur. A disadvantage of aromatic urethanes is that they can also cause yellowing of the coating itself as well as cause yellowing in subsequent topcoat layers. A significant undesired side reaction which occurs during the baking process produces aromatic polyamines. Urethane crosslinkers based on toluene diisocyanate (TDI) can be expected to produce toluene diamine as a side reaction, whereas those based on Methylene diphenyl diisocyanate produce diaminodiphenylmethane and higher order aromatic polyamines. The undesired aromatic polyamines can inhibit the cure of subsequent acid catalysed topcoat layers, and can cause delamination of the subsequent topcoat layers after exposure to sunlight. Although the industry has never acknowledged this problem, many of these undesired aromatic polyamines are known or suspected carcinogens.

Besides the two major categories of anodic and cathodic, EPD products can also be described by the base polymer chemistry which is utilized. The are several polymer types that have been used commercially. Many of the earlier anodic types were based on maleinized oils of various types, tall oil and linseed oil being two of the more common. Today, epoxy and the acrylic types predominate. The description and the generally touted advantages are as follows:

1. Epoxy: Although aliphatic epoxy materials have been used, the majority of EPD epoxy types are based on aromatic epoxy polymers, most commonly based on polymerization of diglycidal ethers of bis phenol A. The polymer backbone may be modified with other types of chemistries to achieve the desired performance characteristics. Generally, this type of chemistry is used in primer applications where the coating will receive a topcoat, particularly if the coated object needs to withstand sunlight. This chemistry generally does not have good resistance to UV light. However, this chemistry is often used where high corrosion resistance is required.
2. Acrylic: These polymers are based on free radical initiated polymers containing monomers based on acrylic acid and methacrylic acid and their many esters which are available. Such polymers often also include styrene as a monomer. Generally, this type of chemistry is utilized when UV resistance is desirable. These polymers also have the advantage of allowing a wider color palette since the polymer is less prone to yellowing when compared to epoxies.