Paint Finishing on Commercial Products
Polyester Powder Coated Aluminium is the most common choice for the finishing of aluminium windows and doors for commercial projects. The process of polyester powder coating has been used in construction for over forty years and the benefits to this sophisticated way of coating windows and doors are widely acknowledged.
The powder coating process.
Aluminium which is powder coated has finely divided polyester powders which are given a polarised electric charge from a high voltage generator. The particles, being similarly charged repel each other and seek an earth (the article to be coated). The coated components are heated to 200°C for 10 minutes causing the powder to melt, flow and chemically react to form a continuous film which is firmly anchored to the substrate. Electrostatic coating, as the process is known, provides good coverage as the powder tends to ‘wrap-around’ the profile. Colours should be chosen from the extensive RAL or BS colour ranges and are available in matt, satin or high gloss finishes.
Aluminium, even when thoroughly cleaned, does not present an ideal surface for painting. Extrusions emerging from the die at about 500oC immediately oxidise on contact with the air. The oxides of the alloying elements (magnesium, silicon, copper, manganese, etc.), swarf and extrusion ‘fines’ (mechanical linishing can be offered as an additional service), damage caused during handling and organic materials (dirt and oils) must all be removed to ensure adequate paint adhesion. This cleaning process is an essential part or polyester powder coating and must in all cases be applied by a qualified company meeting the current British Standards.
Products from a reputable supplier should all meet both RAL and Syntha Pulvin Finishes, complying with BS6496 and 6497 across the full range of colours. The paint guarantees offered for powder coated finishes are:
Syntha Pulvin 15 years for Syntha Pulvin Gloss and Satin
25 years for Syntha Pulvin Plus
RAL & BS 25 years for Gloss Satin or Matt.
The correct specification of powder coated aluminium is vital when dealing with aluminium windows, doors, curtain walling, extrusions, aluminium panels and associated products. If you specify such products you will need to be sure that you are specifying correctly.
Polyester powder coating from aluminium window manufacturers is the most common method of coating fenestration products. Modern finishes range from standard solid colours in satin, matt or gloss and in recent years there has been the addition of metallic and textured effects.
A Brief History of Powder Coating.
Powder coating was introduced into the UK in the early 1970’s and is the youngest of the surface finishing techniques in common use today. The finish is commonly used in various applications including white goods, automotive, and engineering as well as in construction.
What it involves.
Powder coating involves the application of a dry paint powder which is charged electrically. The aluminium extrusion being coated is earthed. With the tecnique, the powder is attracted to the extusion. This, and a combination of static electricity ensures adhesion. Following this process, the extusion is placed in a special oven and the subsequent high temperature melts the polyester, allows the finish to flow and cure. At the end of this process the aluminium extrusion has cooled and is complete.
Just prior to this process the aluminium extrusions will undergo cleaning in a variety of solutions to prepare the metal for coating which is explained further below.
Powder coating differs from the wet paint process. Wet paint will normally use a solvent where the solids are in suspension in a liquid carrier which must fully evaporate before the finish is ready to use.
Environmetally, polyester powder coating is friendlier as no solvents are used.
The Various Processes.
Pre-Treatment is vital in the powder coating of aluminium extrusions. Following the extrusion of an aluminium section from the press the metal will be contaminated with a variety of substances such as oil which unless cleaned will affect the application and lifespan of the coating. Mill finish aluminium (as aluminium straight from the press is referred to) oxidises naturally. Effective treatment prior to painting is essential therefore.
Qualicoat, one of the leading authorities on powder coating say that:
The vast majority of powder coat failures can be put down to poor or inconsistent pre-treatment methods.
Therefore pre-treatment will ensure a good clean surface that the powder relies on for good contact and adhesion.
The leading Association in this treatment methods recommends a basic procedure of pre-treatment which is carried out in immersion tanks or vertical methods which will involve:
Clean, Rinse, Etch, Rinse, Desmut, Rinse, Chromate or Chrome Free Conversion, Rinse, Demin Rinse, Dry.
The Coating of Extrusions.
Following the pre-treatment process and dependent upon what lines (horizontal or vertical) the powder coater uses the aluminium is transferred into a booth. Special powder coating guns will then coat the extrusionwith a fine surface thickness of powder. Any over-sprayed powder has the facility to be re-used.
The extrusion is then transferred into the baking oven at temperatures between 160 to 210oC.
BS EN 12206-1:2004‘Paints and varnishes – Coating of aluminium and aluminium alloys for architectural purposes’ is the latest standard which has replaced the commonly known and used BS6496:1984. This standard covers the pre-treatment process and in line with many such standards it will involve sample testing.
The correct specification to cover polyester powder coating today will require the coating to be carried out by an approved supplier to the “Qualicoat” Standard.
Marine and Chlorine Environments.
Where installations are coastal or in Chlorine environments such as swimming pools there are additional categories of coating for such installations.
All polyester powder coated extrusions will age and be affected by contaminants in the air and during its lifetime. Routine cleaning is essential to prolong the life of the finish. There are powders which can be used for extreme environments or special applications where routine cleaning would be difficult.