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There are a number of pre-treatment chemical and paint alternatives available. It is very often said that the sucess of a coil coater is due to how closely he marries his operation with pre-treatment and paint suppliers. There should be constant communication at all levels to be sure that the most economical and suitable quality is being achieved. Those that do not practice this can soon fall behind the market.
No matter what the feed coil type, the strip must be pre-treated before paint is applied to obtain any acceptable level of prolonged paint adhesion and corrosion resistance.
Prior to pre-treatment the feed is cleaned to remove oil and any other surface contaminants. This is done in high pressure spray tanks with contact times of between 2 and 5 seconds. The solution is normally hot (60-70 deg C) alkali although acid based cleaners are also available. Following the cleaning tanks usually two hot (50-60 deg C) water spray tanks are used to be sure that the alkali from cleaning is not carried through to the pre-treatment stage. Between each spray tank rubber squeegees also prevent carry-though of the solution from one tank to the next.
Following cleaning, the pre-treatment is applied. This is usually chromic or phosphoric acid based although chromium free systems are now available. The pre-treatment performs a dual function.
Due to the move toward maintaining the environment, chemical companies are developing chromium free pre-treatment chemicals. A number of these are now commercialised and are being used globally.
Generally, there is an ability to include chemicals which can pre-treat a range of substrates in the one line. However, it is very important when designing a line to, very early, discuss and decide on the markets being targeted, thus dictating which base feed will be used. In some cases the pre-treatment supplier may need to be provided with typical product so that he correctly nominate which cleaners and pre-treatment should be recommended. Often, it is the cleaner section that is important.
For example, in the case of uncoated steel, the base needs to be abraded using “Scotchbrite” brushes between the two alkaline cleaning stages. The pre-treatment is usually a mixed oxide type system, for example, Henkels Bonderite 901). This results in the best paint adhesion. In the case of galvanised steel, sometimes phosphate based systems, for example, Henkel’s Bonderite 37S is used. This type gives the best corrosion resistance under most conditions and in special cases, like in whitegood appliances, may be the preferred choice. However, in other cases like normal building and construction, chromium, or recently developed chromium free systems will be adequate.
Pre-treatments can be applied by spray, dunk or roll coater systems. The final choice is dependent on the desired line speed and the capital available. Bronx International have engineered and installed all three types of pre-treatment systems.
However, the most common approach is to use roller coating systems. Any of the following systems may be used:
a) horizontal pull through undriven rolls – solution is applied to the strip, as a pool just in front of the rolls
b) horizontal pull through driven rolls – solution is transferred from a tray to a pick up roll and then to a rubber applicator roll which then applies the solution to the strip.
c) standard S wrap coaters.
In general type a) is the least expensive and used on low speed (20 – 40 m/min) lines, b) is used on medium speed lines (40 – 90 m/min) and c) on higher speed lines.
The pre-treatment is applied very thinly, typically 1 – 3 um and requires complete drying before application of the primer. As the drying is carried out at a strip temperature of 60 – 70 degC the strip is cooled by a cold air blast before priming.
Primers can be used between the pre-treatment and the top coat to enhance paint adhesion, but mainly to improve the corrosion protection for the base metal. The primer paint is rich in chromium and provides excellent corrosion resistance. The primer is applied thinly at 5 – 7 um dry film thickness. The primer must be fully cured at approximately 220 – 235°C.
Previously, the most common type of primer used was an epoxy primer. This paint is typically supplied at 35 – 40% solids. However, polyester primers are now being used and are increasing in popularity, due to their cost effectiveness and improved adhesion and corrosion performance. Water based primers are also available, but should only be used with water based top coats. Generally water based paint systems are not very popular as they tend to be a little more expensive and operationally a little more difficult. However, they do have very good adhesion properties.
Special High Build Primers (often polyurethanes) are also available where the dry film thickness of the primer paint is increased significantly. These high build primers provide much greater corrosion protection particularly in severe environment conditions such as marine applications.
There are a number of different top coats available. The most popular is a polyester system, typically used for external roofing and walling applications. Other systems are acrylic, polyurethane, plastisol, water based, silicon modified polyesters, vinyls and alkyds. Each system has its advantages depending on performance requirement and price. The top coat and the primer must be compatible for best results. The top coat can be applied to any base metal.
There is also an expanding range of highly durable top coats available. The fluorocarbon paints are very high quality and provide excellent protection against adverse environmental conditions. These are the top coat of choice for high build primer systems but are also commonly used over normal primers and give excellent extra corrosion resistance as well as protection against weathering. These are, however the most expensive systems available.
Pattern coatings are also gaining popularity. Patterns can be designed at will for whatever the customer requires. The process involves applying the ink over a base coat (either with or without primer, depending on final application) by either ‘rotogravure’ or ‘flexographic” techniques. For best wearing characteristics a thin (5um ) layer of clear coating is applied over the pattern coat.
Whenever a top coat is applied to the strip, some form of backing coat is also recommended. If a backing coat is not applied, there is a very high risk of the top coat being ‘scuffed’ by the unpainted bottom surface as the strip is recoiled. It is best that the gloss level of the backing coat matches the gloss level of the top coat. The backing coat is normally applied at 5 – 7um dry film thickness. The backing coat is normally applied at the same time as the top coat and cured simultaneously with the top coat as the strip passes through the curing oven. Backing coats can be any type of paint, but for economical reasons alkyd or polyester paints are usually used. On aluminium, often the backing coat is a clear or coloured lacquer.
Both chemical and paint companies have been working on providing a solution that performs the functions of the pre-treatment chemical and the primer paint within one solution. Obviously this could lead to much lower equipment costs for a paint line. Bronx International are monitoring these ongoing developments.
Disclaimer: This information is for general information purposes only and should be viewed as such. For detailed, precise information for your upcoming coil coating line, its best to speak to a Bronx Technologist.
Download this article: Pre-Treatment and Paint Options – Joe Parkinson