Intrepid Industries Pty Ltd is principally a powder coating operation, specialising in coating outdoor furniture and lattice work made from die cast aluminium.
The parts to be coated were given a phosphate treatment prior to coating, to remove any residual contaminants on the work piece and to provide a phosphate coating to enhance corrosion protection. The powder coat was then applied to the parts, followed by oven curing to produce a durable finish.
The parts are suspended from an overhead conveyor by hooks as they pass through the phosphate pre-treatment system and spray booth. The hooks also get coated along with the parts. As the hooks are reused, they are recoated every time they pass through the paint booth. Because the hooks must provide an effective earth to enhance the attraction of the powder for the workpiece, the paint hooks require frequent cleaning.
The former process used for paint stripping was to immerse the parts in a heated tank containing a methylene chloride based stripping solution. A sealing layer of paraffin oil on the surface of the solvent was used to minimise evaporative losses. There were a number of significant concerns with this method of paint stripping, including:
- a residue was left on the parts, which had to be removed by water washing;
- it was a dirty process;
- there was a concern with both flammability and occupational health; and
- there was a high cost associated with the methylene chloride solution, which had to be dumped periodically at a cost of $1,000-1,500 per batch.
Cleaner Production Initiative
Intrepid Industries replaced the methylene chloride paint stripping process with an oven that removes the paint by a controlled process called pyrolysis.
The oven has a main combustion chamber, into which the parts are loaded. After loading, the oven is heated in an oxygen deficient atmosphere to a temperature at which the paint is broken down through pyrolysis. The pyrolysis products are ducted to an afterburner to achieve full combustion, prior to discharge to the atmosphere. The oven operates at a temperature of around 400 degrees Celsius, and takes about four hours per cycle.
An ash residue is left on some of the parts, which requires removal. For paint hooks and other components which are used by Intrepid Industries, this ash residue is removed in the cleaning stage of the phosphate treatment process. Intrepid Industries also cleans paint hooks, wire shelving and similar components for other companies.
Advantages of the Process
Intrepid Industries does not have any information on the cost savings from utilisation of the pyrolysis furnace technology, principally because the methylene chloride stripping system was significantly under utilised. The parts were not cleaned as frequently as required because of the reluctance to use the methylene chloride system. As the oven system is much cleaner, it is used on a more frequent basis. This has resulted in a reduction in rejects and improvement in overall coating efficiency. Staff morale has increased considerably because they no longer have to handle the stripping chemicals and operate the former process.
As well as the advantages over the more traditional paint stripping techniques, Intrepid Industries believes that the system has an advantage compared to other thermal paint removal techniques such as sand furnaces, because it has a slower rate of heating which does not affect the heat treatment of the metal parts.
Cleaner Production Incentive
The main reasons that Intrepid Industries investigated oven stripping were concerns over occupational health and safety, and environmental problems created by the existing methylene chloride based process.
In 1990, when Intrepid Industries was first investigating the possibility of using oven technology for paint stripping, lack of local knowledge was a significant barrier.
The only technical information at that time was from US technical journals. The company initially tried, unsuccessfully, to get an oven built locally, but ultimately had to import an oven from the USA. This delayed installation of the unit by 12 months.