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fluoroplastic-Coatings

The idea of ​​industrial fluoroplastic coatings is more than 30 years back. Was that a technical sensation back then? No less sensational today is the fact that the triumph of fluoroplastics is continuing. And it seems that their possibilities are inexhaustible: Year after year their fields of application are widening or new applications are becoming known where fluoroplastic coatings have at least one of their advantages. For example, by making work processes safer or even possible. Or increase benefits and sales opportunities. Or by doing it all together. And yet it would be wrong to sell fluoroplastic coatings as magical, omnipotent miracle drugs.

What do fluoroplastic coatings do for what purpose?
That already says that the "one-for-all coating" does not exist. It is always to be assessed on a case-by-case basis. When making a recommendation, the qualified specialist is guided solely by technical aspects and not by a coating material that is currently "in".
As complex as the solutions to problems with fluoroplastics are, the technician also sees their material-specific properties during processing as complex. So are for example! Coatings for chemical corrosion protection are not to be equated with plastic linings. Because the chemical and thermal resistance of fluoroplastics in their plastic form is by no means identical to a coating made of the same material.

A coating company with ambition will always go to the trouble of testing coated parts for their chemical resistance at specific temperatures. The values ​​that come to light here are quite different from the values ​​of the pure plastic. And less in the resistance to chemicals, but rather in the still possible working temperature.
Although ETFE coatings z. For example, for chemical corrosion protection in the hands of the experienced coater optimal problem solutions within certain limits, working temperatures of over 100 ° C with ETFE or similar materials are absolutely critical.
A very specific problem with fluoroplastic coatings is the ability to allow their pore contents (e.g. moisture, chemicals, oil, etc.) to flow through under normal pressure conditions. This phenomenon is called permeability or hydraulic conductivity. Every plastic has its specific permeability value; one is higher, the other lower. The important factor in this context – because it can be influenced – is the layer thickness. Figure 1 shows the permeability as a function of the layer thickness. It was measured with 35% hydrochloric acid at 60 °C. From this curve it can be clearly seen that the permeability is only no longer of importance from a layer thickness of 600 μm. But especially in chemistry, the wisdom applies that a chain is only as strong as its weakest link. In the case of a coating, this is its thinnest point. Using the example of a coated large container, practice shows that with a minimum value of 800 μl, partial layer thicknesses of 2000 μ and more also occur.

Another specific evil is vapor diffusion. That is, the propensity of gaseous molecules to penetrate the plastic layer and attack the substrate. Fig. 2 illustrates the complex problem using the example of water vapor diffusion. Simplified, the following formula describes the degree of vapor diffusion:
In the case of plastic linings (the problem is primarily solved by increasing the layer thickness. Often up to 5 mm. In the case of coatings, the increase in the factor "L" is, as already mentioned, only possible to a limited extent (approximately up to 1000 u.). That is why one is forced to act on alternative influencing factors.Figure 3 shows that the vapor pressure difference (AP) is exponentially dependent on the temperature difference (AT).For example, a reactor with an operating temperature of 100 °C and an outside temperature of 20 °C has an AT of 80 °C. This means that the coating is at high risk of diffusion. In order to reduce the AT value, it is advisable in this case to insulate the outer wall of the reactor. Tests have shown that an AT value of 60 °C is not should be exceeded.

Layer thicknesses do not grow in the sky
Even the process used to apply the coating must take account of the material-specific properties of fluoroplastics already mentioned. Electrostatic application has proven itself. The material-specific problem here is that plastic has an insulating effect above a certain layer thickness and as a result eliminates electrostatics. For this reason alone, layer thicknesses cannot be increased at will. But even with materials that are applied in powder form and then melted, the physics cannot be “overturned”. Because from a certain thickness, the fluoroplastic follows the laws of gravitation during sintering (i.e. in its melting phase): it flows off the carrier material. However, you can "bend" the physical laws a little with ideas: In order to stabilize the desired thicker layers, the technician installs mechanical supports. If they are designed in such a way that they get the indispensable electrostatics as an additional effect, that is in fact proof of more in-depth know-how.

In principle, almost all geometric shapes can be coated. Nevertheless, it has been shown that it is advantageous if the customer pays attention to certain prerequisites when designing a part. All corners and edges should be rounded and have a radius of approx. 10 mm, but at least 5 mm. Ideally, one should also avoid constructing the carrier of a coating with different wall thicknesses.

Corrosion protection with PFA
This fluoroplastic is used where ETFE is no longer strong enough as a protective shield. PFA coatings can be applied with layer thicknesses of up to 1000 u. It would be nice to conclude that when it comes to high temperature corrosion protection, PFA is the answer to all questions. But here, too, what was said at the beginning applies again: It depends on careful consideration of the conditions of use. Because without know-how, instead of new technology, it's just a muddled process.