may 2007
Qualicoat (UK & Ireland) was set up in January 2001 following the rapid growth in the adoption of the powder coat standard across Europe. Over the last few years the vast majority of the powder coated architectural aluminium being imported from Europe has been coated to the Qualicoat standard. In this two part series Douglas McAlistair, Chairman of Qualicoat, covers two important subjects facing the industry today; how to avoid powder coat failure and the safe disposal of powder coat waste.
The Qualicoat standard is commonplace across Europe since 1986 and now a well known quality standard, the UK’s users and specifiers are now adopting the standard, as it not only offers an exceptional quality finish, but is also independently tested by Bodycote, a UK testing house. Qualicoat members agree to pre-finish and apply powder coatings to a minimum standard using a wide range of materials from various manufacturers. The standard also meets and exceeds the latest British Standard for architectural powder coatings, BS EN 12206-1:2004.
Powder coating of aluminium profiles takes two stages, a chemical pre-treatment, like an ‘undercoat’ which seals the surface of the aluminium and provides a ‘key’ for the second stage powder coating. Dry powder is applied by electrostatic spray to the electrically earthed profiles, which are then subject to a temperature of a little over 200 degrees Celsius when the powder melts, flows, gels and finally chemically reacts and cures. All this takes place on a continuous production line.
Powder coat failure occurs generally for one of two reasons, either poor application or environmental issues such as harsh environments which includes incorrect cleaning methods. Failure of the surface treatment can be determined when the coating lifts from the surface of the metal, or changes in coating colour due to UV light. By far the most common cause of failure is down to poor application which is the result of the powder coater not having the right equipment. Failure becomes evident within the first few years of use as the coating lifts from the surface of the metal. Poor pre-treatment and/or incorrect oven temperatures are normally to blame for this failure and frequent testing of process sample plates should identify the problem and allow the coater to reject or re-coat the affected batch.
The pre-treatment process consists of dipping in tanks or spraying on a vertical powder coat line, in several liquid chemical solutions which primarily de-grease the metal surface and apply a thin chemical coating to seal the surface and provide a ‘key’ for the final finish. Between the applications there is normally a clean water rinse and the final process is that of drying the profile. The treatment is monitored to ensure that the solutions remain at their correct chemical values to ensure that a consistent thickness of pre-treatment is applied. Any areas which are not correctly coated due to contaminants on the surface of the metal, or thinning of the pre-treatment, will reduce the sealing and ‘keying’ of the final powder coating to the surface of the metal.
Thin sample plates are normally put through the powder coat line every few hours to confirm coating quality and these are subject to destructive testing which determine the success of the coating. These plates however do not always give a correct indication of a complete melt and cure of the final coating. If the profile being coated is a heavy profile for example, and is hung on the line very close to other profiles, the time taken to reach optimum temperature for melt and curing of the powder coating may not be met and could show up later as a failure of the finish.
Checking the thickness of the powder coating at the end of the line ensures that the correct amount of powder has been applied and that the thickness meets the standards but does not give any indications of the two potential and latent defects in the material as described above.
Certain environments such as industrial and coastal locations usually require no more than a thicker surface treatment to cater for the powder coat surface being eroded more quickly. Powder coaters generally offer this service at an extra cost once the location has been approved and normally comes with a reduced guarantee period. Usually any guarantee on surface finishes comes with a mandatory cleaning regime which consists of cleaning the surface of the coating at least every six months with a mild detergent solution and details of these are normally provided by the powder coater. Using harsh abrasives to remove marks or stains on the powder coating will often irrevocably damage the surface and lead to future failure of the finish.
Finish failures can be very expensive, ensuring that suppliers of powder coated profiles and fabricated windows, doors and conservatory roofs specify the highest standards cannot be understated. Powder coaters should be using some of the most up to date equipment and importantly the correct equipment for the specialist application of Architectural finishes, as specified by the powder manufacturer. The Qualicoat organisation works with member companies who represent the architectural powder coat supply chain in the UK. The highly respectable international Qualicoat standard helps to reduce powder coat failure in buildings, ensuring that powder coating remains the number one choice for aluminium finishing.
