Image Above: Q-Lab Florida – testing of QUALICOAT approved powders
There still seems to be confusion in the correct specification of powder coating for any given project. Suggesting that you should seek guidance on the correct specification are wise words but not all Powder Coaters are equal in the UK. Some Powder Coaters are what are known as ‘general’ Powder Coaters’ and not specialists in the architectural coating market and as such, often lack specialist equipment for treating the aluminium. Correctly specified, architectural powder coating can last the lifetime of the building.
Specifying QUALICOAT and seeking advice from a QUALICOAT approved applicator is a good start, but, to become a QUALICOAT licenced applicator, each member must have a high standard of equipment and experience in coating architectural aluminium which is often been exposed to severe weathering.
To suggest that a thicker powder coating should be applied to the aluminium to resist the ravages of the British weather is simply wrong and this article will explain why this is the case.
There are basically two fundamental weathering concerns when it come to architectural powder coating. First we have the issue of potential corrosion of the aluminium under the powder coating which will result in blistering under the powder coated surface. Secondly, we have the performance of the coating, basically a resistance to UV. This is where the coating can fade or chalk over time.
Powder coating is not impermeable, water and air can, and will, penetrate its thickness, so we have to ensure that the aluminium is very carefully pretreated. This pretreatment consists of cleaning and etching the aluminium prior to applying a pretreatment system, chrome based or chrome-free, which will completely seal the aluminium surface and offer a good ‘key’ for the powder coating to adhere to. A quality pretreatment is crucial to getting a long lasting powder coat finish. Pretreatment systems can be immersive or sprayed, but the plant investment and space to do this correctly is excessive hence the reason why ‘general’ powder coaters do not have this as a standard process.
More recently, we have seen the availability of flash anodised pretreatment systems become available in the UK. This system of pretreatment can cope with the harshest of environments and can be specified where access to finishes make it difficult to offer regular maintenance.
There is what is known as a ‘Marine Class’ of coating, or ‘Seaside’ Class under QUALICOAT. This is where powder coating is going to be placed near to coastal regions and offers a deeper etch of the aluminium for a more robust pretreatment application. Flash anodising can also be specified in these more severe weather rated areas.
Once powder coating is applied and cured correctly, the powder coating on the project will be subject to UV (Ultra Violet) attack over time, this can become evident in two ways. First, ‘chalking’ on the surface can occur, these are which deposits and are as a result of free-radical UV degradation of the polymer matrix. As the polymer matrix gradually breaks down it exposes the pigment and inorganic fillers contained within the paint film. Importantly this can be relatively easily removed from the film by adopting a regular cleaning regime.
Fade on the other hand, whilst also UV catalysed, is a different mechanism. Generally bright, clean organic pigmented colours such as bright yellow, orange & reds are more susceptible to pigment fade which will occur on almost any applied finish.
Powder coatings are now available in three Classes, 1, 2 & 3. Most low rise city projects are coated in Class 1 powders, but there is a growing increase in the specification of Class 2 powders in the UK. Class 2 powders have a higher resistance to attack from UV and have reduced UV degradation as a result.
So in closing, for a long architectural powder coating life, make sure your Powder Coater has the pretreatment system and experience in place, or just talk to, a QUALICOAT member and specify for peace of mind.