ELECTROCOATING
© Copyright 1999/2004 Mario S Pennisi Principal Consultant - Penlia & Co
mariopen@coatfab.com or mariopen@ worldconferencesystems.com

What is electrocoating?
Electrocoating is another of the more recent coatings systems. It was originally developed as an automotive body primer in the early 1960's and used by Ford in Australia from that time. The technology has grown to become an economically efficient and environmentally friendly process.

Today, more than 98% of all car bodies produced globally use an electrocoat primer. New end uses for electrocoat are found annually, for example, automotive parts and accessories; appliances; heat exchangers; decorative plated objects and heat-sensitive components.

Preparation
Surfaces have to be chemically clean prior to electrocoating. A 7-stage phosphating sequence is usual.

How is it done?
The basic principle of electrocoat is that materials with opposite electrical charges are attracted to each other. In an electrocoat bath a specific charge is applied to the part that is immersed in paint particles that have an opposite charge.

The paint particles are attracted to the part and are deposited on the part to form an even, continuous film over every surface until the coating reaches the desired thickness. Electrocoating is complete when attraction stops.

Electrocoating is classified as either anodic or cathodic depending on the polarity of the charge.

Anodic In anodic electrocoating, the part to be coated is made the anode (a positive electrical charge) and attracts the negatively charged paint particles in the paint bath.

Cathodic
In cathodic electrocoating, the workpiece is made the cathode (a negative charge) and attracts the positively charged paint particles

Uses
Today the electrocoat industry serves much more than the automotive market. It is specified before wet coatings and powder coating because of the tremendous advantages in application techniques, efficiency, automation and environmental compliance.

Anodic type coatings are mainly used for products in interior or moderate exterior environments. Anodic coatings are economical systems that offer excellent colour and gloss control.

Low cure anodic epoxy coatings that have high corrosion resistance in exterior applications are used for agricultural implements, automotive parts and structural steel.

Anodic acrylic coatings provide good colour and gloss control. They are economical and used for interior applications such as metal office furniture, air diffusers, shelving, wire screens and hangers.

Cathodic coatings are mainly high-performance coatings with excellent corrosion resistance and can be formulated for exterior durability.

Cathodic epoxy coatings provide good chemical and corrosion resistance and are used for automotive parts, transformer and appliances.

Cathodic acrylic electrocoatings have high UV durability, corrosion resistance and good colour control. They are used for lawn and garden furniture, agricultural implements, and automotive wheels and trim appliances.

Advantages of Electrocoat
The major advantage of electrocoat over most other systems is its covering ability. It provides total coverage of complex-shaped parts with film uniformity. All recessed areas and sharp edges can be covered and it provides excellent film uniformity and precise film build control without sags, runs or drips. Thickness is easily controlled using simple voltage adjustment. Also, parts can be assembled prior to coating.

Electrocoating lines can be highly automated, closed-loop systems with excellent productivity and low operating costs so no direct labour is required. It allows for high productivity with fast line speeds and high part-racking densities. Parts pass very quickly through the production line, resulting in lower unit costs. The process has high transfer efficiency, approaching 100%, with low energy requirements since no drying or flash-off time is required. Minimal exhaust and air make-up is needed as well as reduced cure times and temperatures.

Electrocoat systems are low maintenance with minimal hook cleaning and no "booth" maintenance. Insurance rates are minimal because there are no specific fire or health hazards.

There are a number of environmental advantages to electrocoating as well. Electrocoating emits low or zero VOCs and HAPs. Operators are not required to wear any special personal protection. The system is totally enclosed and has low air and wastewater emissions. The process produces minimal solid waste and since it is a closed-loop system it minimises water loss. As a waterborne system, fire hazards are reduced.

Properties
In general electrocoats can provide high gloss; good colour control; weatherability; barrier, corrosion, chemical and stain resistance; chip resistance; good edge coverage and hardness. They are FDA-Approved and have no effect on flavour.

Cathodic acrylic electrocoats can provide the best combination of corrosion resistance, exterior durability (high UV resistance) and colour control. In the appliance market single-coat white cathodic acrylic can replace previous applications of primer and topcoat, leading to economic advantages for the end user. New low-VOC and HAPS-free acrylics are in the marketplace, targeted at high-performance applications with improved environmental characteristics.

A two-coat electrocoat process is available for extreme performance environments. The first coat is a conductive epoxy electrocoat, which, when baked, provides a surface that can receive a second coat of acrylic electrocoat. The two-coat system is capable of providing long-term exterior durability and corrosion resistance in excess of 2,000 salt spray hours.

Wet coatings and powder coatings have a tendency to flow away from sharp edges while still in the liquid phase so that corrosion protection of objects such as body panel edges and fins on radiators has proven difficult. Advances in cathodic epoxy electrocoat formulations have resulted in the ability to control film thickness over sharp objects, greatly enhancing corrosion resistance.