World of coatings #3 – Coating

Coating begins as soon as the surface pre-treatment - which we described in the last issue of World of Coatings - has been completed. You would think so, but that's not the case. In fact, we start much earlier: as early as the first customer enquiry. Although no physical coating is applied at this stage, the coating system is selected here. And this is at least as important for the result as the correct application later on.

Some customers specify the coating system, and then we check whether we think it fits. We do this on the basis of the manufacturer's specifications, some of which are some 80 pages long.

Other customers ask for a proposal from our side. We then create this based on the requirements, the application, the substrate, the geometry and, of course, the medium and the pressure with which the coating will come into contact. In this way, together with the customer and sometimes with the involvement of the coating material manufacturer, we arrive at an optimally suitable coating system. We also make sure that the manufacturers operate worldwide if necessary, as this means that customers can also be supplied with material directly on site so that improvements can be carried out quickly if the component is used somewhere else in the world.

A Coating Procedure Test (CPT) is often required for particularly critical components. This means that we coat a standardised component as a test, on which a whole series of tests are then carried out. Only when the coating system has been selected and the CPT has been completed and approved, can we finally start with the actual coating.

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The coating itself begins

We work using the airless process. This means we have less overspray and fewer losses. This is good for the employees, the environment and the coating material ends up exactly where it should be. Another advantage is that the higher pressure we work with to make the material flowable means we can use significantly less solvent.

?Such a coating system usually consists of three layers: the base layer on an EP basis with zinc dust or zinc phosphate for corrosion protection, the middle layer in a high layer thickness, also on an EP basis. This intermediate layer contains so-called micaceous iron oxide, which stacks on top of each other like scales, naturally at nanometre level in the material. This scale structure lengthens the path of contamination through the layers. The thereby achieved barrier effect significantly increases the service life of the coating. Finally, a layer of PU top coat is applied. This protects the underlying layers from UV radiation and ensures that the component looks good, for example in the colour of the end customer. For better atomisation and a more homogeneous surface, we use the airmix process for the final layer.

For all three layers, we pay very close attention to compliance with all possible parameters. This involves the pressure during spraying and the temperature of the coating material, which allows us to adjust the viscosity without using more solvent, which ultimately changes the solid volume of the coating, which is not desirable. The nozzle also plays a major role: a large angle for a high area coverage and a smaller angle to be able to work better in filigree structures, because here too the coating material should be applied to the component and not just anywhere. Of course, we monitor the environmental conditions such as temperature and humidity, but also the component temperature and that of the coating material itself. Incidentally, to avoid cross-contamination of these coating materials, we have a separate spray system for each layer. This means that we can keep the materials on the machines unmixed and also save ourselves the time-consuming cleaning between the layers.

It′s in the details: Roughness, thickness and intervals

By the way: The layer thickness depends largely on the surface quality. The roughness is determined by the so-called peak-to-valley value. This is specified by the manufacturers of the coating systems: This must be 50 μ, for example. However, if a greater roughness is created during blasting (see here), this does not mean that the coating will be better. On the contrary: it tends to become too thin at particularly high peaks. This is then a disadvantage for the service life of the coating in use.

Another very critical point is the reworking intervals. Ideally, one layer should be dry before the next is applied. This makes it possible, for example, to determine the layer thickness per layer, which is only possible with a dried layer. However, it is also important not to wait too long, as after a certain time the layers no longer react with each other. This reduces the chemical adhesion of the top layer to the layer below, which is not what you want. Not too early and not too late, but just right is the motto here. Our modern drying chamber helps us with this. It enables us to achieve the optimum drying time and thus significantly reduce throughput times. As a rule of thumb, 10°C more temperature reduces drying times by 50% without any loss of quality, at least in the range between 10°C and 50°C.

Check each step!

At the end, we carry out detailed quality checks again according to the customer's specifications. One required parameter is often the total layer thickness. However, as each layer of the system has a task and must therefore be present in the required thickness, it would be a disaster if the entire layer ended up being too thin. Simply spraying another layer of topcoat on top would produce the required overall layer thickness, but the system as such would not work because the layers underneath would be too thin. That's why we measure after each individual layer, because then you can add more exactly in the layer where this is necessary. That's why we never actually have a total layer thickness that is too thin.

As you can see, there is a lot more to coating than just bluntly holding a syringe to a surface. And that's exactly what we enjoy: creating perfect coatings precisely and carefully, even when things get tricky. ?