Always Powder Coating (APC) operates a powder coating facility in Melbourne's eastern suburbs and specialises in powder coating of metal chair frames, bike frames and other similar metal components. An important part of powder coating is the cleaning and pre-treatment of the metal. Cleaning is required to remove residual oil and dirt, and phosphate pre-treatment is commonly used to enhance paint adhesion and corrosion resistance.
The most common iron phosphating system uses a multi-stage water-based process. In the case of small items, this involves hanging the parts on racks or hooks suspended from an overhead conveyor. The parts then pass through the phosphating machine, where various solutions are sprayed over the parts. The process usually consists of the following distinct stages:
- alkaline degreaser;
- iron phosphating solution;
- passivating solution (optional, but usually chrome based); and
The solutions are recirculated through storage tanks located next to each stage. Some of the solutions require heating to approximately 60 degrees Celsius. There is significant sludge generation associated with the iron phosphating solution, which necessitates frequent desludging of the tank. In addition, all the solutions require periodic dumping, make-up water is required to replace evaporation losses, and there is a continuous overflow from the rinse stages.
In some operations, solvent-based (usually trichloroethylene) vapour degreasing systems replace the need for an alkaline degreasing stage. Vapour losses from such systems can be quite high, and even when totally enclosed there will still be some vapour loss.
Cleaner Production Initiative
APC have installed a phosphate treatment system that uses an organic phosphating process. The system uses a single solution of ECOPHOR A/477 that acts to both degrease the parts and provide a phosphate coating. The ECOPHOR solution is based on an inorganic polyphosphate dissolved in a low toxicity organic solvent medium. The solution has a very low vapour pressure and is therefore suited to conventional spray application.
The ECOPHOR solution works by initially degreasing the work piece and then depositing a thin layer of iron phosphate. A film forming polymer is deposited co-currently, and this is chemically linked with the metal via the phosphate. This polymer has the ability to absorb oils and greases, thus allowing the solution to be used continuously, without any requirement for periodic dumping. Degreasing of the metal surface, with the aid of the organic solvent, is followed by the adsorption of the oils and greases by the polyphosphate. The polyphosphate polymer will absorb a maximum of 6 per cent of its weight in oils and greases. This represents an average oil loading of 1.5 grams per square metre of treated metal surface. The final structure of the organic polyphosphate surface comprises a very thin layer of amorphous organic modified iron phosphate covered by a polymeric film. This is in contrast to a conventional phosphate process that produces a porous phosphate coating.
Heat curing is required to set the phosphate coating, as it is this process that drives off the remaining solvent and causes the free acid groups in the polymer to react with the substrate to produce a three dimensional structure.
The ECOPHOR solution can be applied by either spray (overhead conveyor) or dip techniques. Both systems have been installed by APC. Small parts (chair frames, etc.) are processed through a tunnel, which comprises a single ECOPHOR application, followed by an air knife to remove excess solution, and finally a curing oven. Large parts are treated in a converted vapour degreasing tank, where the solution is simply sprayed over the part, which is then oven cured. In both systems the parts are sprayed with the ECOPHOR solution for 60 seconds, followed by drying at 90-120 degrees Celsius for between 4 - 6 minutes.
Although the spray system equipment is similar in appearance to a conventional iron phosphating tunnel there are a number of differences that are critical to the successful operation of the system, including:
- operating at reduced spray pressure;
- reducing the number of risers; and
- using stainless steel for all tanks.
The solutions are tested fortnightly for oil content, resin content (polymer) and phosphoric acid. Additions are made where required. This is in contrast to a conventional phosphate process where rigorous process control testing is required (at least once daily).
Advantages of the Process
The adoption of this process has eliminated the generation of wastes normally associated with phosphate treatment systems. This has led to considerable savings. The benefits of the system to APC are:
- no trade waste discharge;
- no sludge generation and subsequent waste disposal costs;
- no down time required for desludging of tanks;
- no water make-up to tanks;
- daily process control monitoring is not required; and
- solution heat-up is not required, which means that the line can be started more quickly, with a consequent saving in labour costs.