Expanding on E-Coat

Electrocoat is a unique immersion coating process, providing complete coverage at low film builds and excellent film properties as a single coat, as a primer, or with two coats as primer and topcoat. In addition to good performance and reasonable cost, electrocoat is an efficient process and an excellent compliance option because it has very little waste and low VOC.

Electrocoat products and processes continue to evolve, with development focused on lower energy use, very low VOC content and methods to reduce cost per applied square foot. The following is a primer on how a combination of new materials and good process control can be used to achieve the highest possible efficiency.

Anodic System

Electrocoating works on the principle of “opposites attract.” The paint materials may be applied in an anodic or cathodic system, a reference to the charging arrangement of the system.

Anodic electrocoating involves the use of negatively charged paint particles that are deposited onto positively charged metal substrates. During the anodic process, small amounts of metal ions migrate into the paint film and limit the performance properties of these systems. These ions become trapped in the depositing paint film, and, due to their ability to interact with moisture, limit film corrosion performance. Anodic coatings are economical systems that offer excellent color and gloss control. Their main use is for products in interior or moderate exterior environments.

Cathodic System

A cathodic system, where positively charged paint particles are attracted to a negatively charged part, leaves far less iron in the deposited film and provides much better corrosion resistance. Cathodic coatings are preferred for high-performance applications.

Material Selection

Electrocoat materials are available in acrylic, epoxy, and hybrid formulations. Typically, selection depends on the desired quality, performance, cost, and environmental objectives of the end user. Epoxies are known for their corrosion and chemical resistance, acrylics possess good ultraviolet resistance and color control, and hybrids—a mix of epoxy and acrylic polymers—give a combination of properties.

Manufacturers of electrocoat materials are working continuously to develop formulas that can improve system operation, reduce environmental concerns and lower operating costs. Some of the latest enhancements are outlined here.

Cathodic Epoxies

Cathodic epoxy e-coat paints deliver excellent corrosion resistance and provide one of the best primers available. The goal for improvement isn’t to boost performance but rather to reduce cost.

Two important advances that can lead to lower applied cost for this product are low cure temperature for energy savings and improved throwpower/shutdown for reduced film build control, coverage and reduced material use.

High-performance, low-temperature products are available that cure at 325°F today instead of the more traditional 400°F. Manufacturers are working on formulations that will further reduce the cure temperature to 275°F in the future. In a typical, medium-sized electrocoat system with gas cost at $1/100 ft3, annual cost savings for a system that runs 2,000 hr/year would be $13,592.

Throwpower is the ability of the electrocoat material to get into internal recesses in parts. It can be controlled somewhat by the applied voltage, bath solids, conductivity, dwell time in solution, bath temperature, solvent levels and tank agitation.

If throwpower can be improved to occur more quickly and with more uniform precision relative to the rest of the surface, the total coating will be more uniform and held to a thinner number. Table 1 is an analysis showing how a change of coating material with better throwpower and overall film build control reduced material use, increased production and reduced scrap.

But good throwpower is only part of the puzzle. Good shutdown characteristics further improve film build uniformity and help control cost.

If it’s necessary to build up significant film (say 0.9 mils) on the outside of a rack of parts to get an acceptable minimum build (say 0.6 mils) in the middle, a lot of paint will be wasted with heavier than necessary film thickness. A product with good throwpower plus good shutdown properties will provide the best possible film build control and therefore a lower cost per applied square foot.

In one case where a material was changed for another material with superior throwpower/shutdown characteristics, the difference was worth $27,600 a year with three shifts of operation. Consistent film control to avoid heavy film on the outside of the rack can save from 20 to 35% in material usage.

Cathodic Acrylics
Cathodic acrylic e-coat products can be useful for UV resistance, but their corrosion resistance typically isn’t as good as epoxies. In situations that require high-quality corrosion resistance and exceptional UV resistance with an automotive Class A appearance, the usual answer has been a two-coat process using a cathodic epoxy as a primer layer with a topcoat of a higher-performance liquid or powder coating.

The cost implications of a multi-coat system are significant. Material, labor, equipment and maintenance costs are much higher and cycle time is longer. A new product is available that can be applied in a single coat and provide 100% coverage, 750 hr or more of salt spray resistance over zinc, automotive quality exterior durability, and Class A appearance is now available. This provides an excellent cost savings when compared to the multi-step options.

Cathodic Alkyds

In the past, e-coat couldn’t be used on temperature-sensitive parts or on parts that were so heavy that it wasn’t feasible to achieve the required peak metal temperature (PMT) to cure the product in conventional ovens. A recently developed cathodic alkyd solves this problem. This acrylated alkyd resin chemistry is actually an “air dry” e-coat material, although it’s recommended that it be forced dry at temperatures from 180–230°F to make it simpler to process.

Performance of this product is good, and it also has good Faraday penetration, providing corrosion resistance in recesses that are hard to achieve with spray coating. The product is HAP-free as supplied, with a VOC content of 2.8–3.2 lb/gal. It’s available as a single component material and can be custom formulated. Today, this process is used on fully assembled engines and many other temperature-sensitive products.

SPC and Efficiency

Improvements in paint materials, new equipment and effective racking are obvious methods that can improve e-coat operation by reducing costs and increasing quality output. But statistical process control, a formal method of statistical analysis using monitoring data to control the variables that affect the process (SPC), is critical to optimum efficiency.

Its four basic steps include measuring the process, eliminating variances for consistency, monitoring the process, and then improving it to reach its best target value.

The percent of paint solids in the e-coat bath has an impact on many other critical operating parameters. Percent paint solids should be the starting place for development of a proper SPC program.

Addition of paint solids to the bath is based on consumption. Racks, parts or amp-hours can be used to measure consumption and control batch addition intervals. Rack loads can vary, so it’s usually more accurate to use amp-hours as a basis for the volume of additions.

A batch feed system should add to the tank every ten to fifteen minutes to make sure that the correct amount of feed is being delivered. Once the system is stable, the frequency can be lowered to weekly.

Paint tank level is also important. Percent solids in the e-coat bath is inversely proportional to tank level, which is typically controlled by addition of water to rinse system. Water doesn’t have to be added as regularly as paint, but additions should be done more than once per shift.

The amount of water added will depend on the amount of paint addition, and should be charted along with the amount of paint added per day. Water added to control tank level may change if other variables change such as tighter control over paint solids content, seasonal atmospheric changes and rinse temperatures.

The bath temperature affects performance and should be regularly monitored and controlled. A differential controller comes on at a low set point and turns off at a high point, which may allow as much as a 4°F temperature variation—a swing big enough to affect film build.

A differential controller often has too much variability and should be replaced with a proportional controller that will vary the amount of cool water sent to the heat exchanger according to the temperature. The higher the temperature, the larger volume of cool water sent to the heat exchanger. A proportional controller has an adjustable range. The amount of cool water sent can be adjusted per degree of temperature.

It’s a good idea to take bath temperature readings in different areas of the tank to make sure circulation is uniform. Circulation may need to be improved if there are temperature differences within the tank.

If percent solids is controlled and the anolyte system is performing correctly, bath pH will stabilize. Bath pH has historically been fine-tuned by adding acid to the bath, but some newer paint materials may need different adjustment methods. Consult your supplier for the correct adjustment method.

An electrocoat bath is usually operated at a higher temperature and has to be cooled, usually to about 77°F, to measure pH.

Bath conductivity is another variable that should be routinely measured. Like pH, conductivity is temperature-sensitive and a constant bath temperature will help reduce variability of the conductivity readings. Conductivity will also stabilize when the percent solids is in control.

If the drag-in that comes in on parts is highly conductive, it may raise the conductivity in the paint bath. Conductivity of the water dripping off parts can be measured by placing a catch pan under the parts as they exit the washer. If it exceeds 25 mhos, rinsing in the washer should be examined and improved to reduce conductivity.

If bath conductivity is still too high, it may be that an excess amount of paint is being carried out or purged to control the tank level. Purging should be kept to a minimum. If drag-in is low in conductivity and purge volume is stable, bath conductivity should also be stable.

A proper ratio of pigment (paste) to resin should be fed into the system and maintained. The volume of resin and pigment used each day should be checked, and if the ratio doesn’t stabilize the pigment could be settling. This will be evident when the tank is dumped. Circulation needs to be improved if there is settling on the tank bottom.

Conductivity of water from a DI or RO system is typically under 10 micromhos. It should be monitored periodically to ensure proper operation of the parameters in the tank.

Anolyte conductivity is automatically controlled, but it may vary quite a bit. If it does vary, the controller and the position of the probe that senses conductivity and controls the valve should be checked. If the probe is close to the water inlet, incoming water will dilute the liquid around the probe and result in a false reading. The water inlet should be close to the bottom of the tank. If it’s near the top, water runs into the tank and out the drain, never diluting the liquid in the tank.

Ultrafiltration flux rate is charted for maintenance purposes. Filters should be cleaned when it dips to 75% of the original rate. The rate may increase and filter cartridges may last longer if overall bath control improves.

Rinse percent solids and solids to drain provide a good indication of how well the system is performing and should also be charted for maintenance purposes. These charts will change as the system comes into control. Solids to drain should be minimized to avoid excess paint loss.

When system parameters are stable, film builds are more consistent. Changes in film build are a good indicator of out-of-control variables, so monitoring film thickness is an excellent way to see if the system controls are working.

 

New Materials, New Applications

Innovations in e-coat materials lead to better and more flexible products that help to create new markets for e-coat. Here are some examples:

Automotive Industry. Electrocoat’s exceptional corrosion protection has made it a natural fit both as a primer and single coat. Frames that used to be coated with hot wax required shielding around exhaust systems. The use of e-coat has improved performance and reduced noise and weight at a lower applied cost. Applications include truck and trailer frames, engine cradles, hitches, suspension components and underbody components.

Radiators traditionally coated with relatively low-performance spray coatings can now have more thorough coverage and better performance. Special epoxy coatings not only coat the tight recesses in the core of the radiator, they also provide exceptional coverage on sharp edges to further enhance corrosion resistance.

Appliance Industry. Laundry appliances have used a number of expensive processes to provide corrosion resistance under extreme conditions of heat and chemical exposure. A standard coating would be a cationic epoxy primer for corrosion resistance and a high-performance topcoat for decorative appearance. Now a detergent-resistant cationic acrylic e-coat supplies all properties in a single coat.

Heavy Equipment. Materials with ultra low cure temperatures have made it possible to e-coat heavy gauge material traditionally difficult to handle due to higher-temperature cure requirements. This capability can enhance corrosion resistance while reducing applied cost on such products as agricultural and construction equipment.

Decorative Metal Finishing. Spray and dip lacquer is often used to provide protection and gloss over plated parts such as jewelry and decorative metal furniture. New e-coat systems can apply a clear finish that improves performance with excellent material utilization, reduced environmental concerns and fewer defective parts.
 

DuPont Company

PPG