Temperature, humidity and related climatic parameters are important to many industrial processes and the application of paint and coatings is no exception to that rule. Measuring and controlling climatic parameters such as relative humidity, air-temperature, dewpoint and moisture is vital to a successful application of high-end coating systems. Incorrect climatic conditions during pre-treatment and application often lead to premature failing of coatings systems.
Temperature and relative humidity during application
Almost each coating has its own requirements regarding climatic conditions. Most two component epoxies require a relatively high temperature (often more than 15°C) combined with a low relative humidity. Moisture curing coatings need a high humidity level in order to cure. Water based coatings cannot be applied at very low temperatures and too low humidity causes the paint to dry too fast while a high humidity slows the curing process down. Surface tolerant coatings accept a wider range of climatic conditions and some coatings can even be applied underwater.
Each paint manufacturer will mention in its application guidelines or MDS (material datasheet) specifically what type of climatic conditions are required. Wrong environmental conditions can lead to problems and defect such as:
- Solvent Popping
- Solvent detention
For explanations and extensive information about paint defects check the “Fitz Atlas of Coating Defects”.
A special parameter which is of high importance in the coating industry is DewPoint. Wikipedia describes Dewpoint as ” The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapour to condense into water. The condensed water is called dew. The dew point is a saturation temperature.
The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.”
In the surface treatment dewpoint is one of the most critical climatic parameters. Dewpoint always has to be observed in combination with the surface temperature. A surface temperature that is below the dewpoint temperature of its surrounding air will immediately show condensation. On a rough, blast cleaned surface, the small water drops of dew are hardly visible. However when this moisture is trapped under a paint film severe problems will arise over time. Corrosion under the paint, poor adhesion of insufficient curing are some of the possibilities.
Since the dewpoint temperature (Td) depends on the relative humidity (RH) and air temperature (Ta) it is quite unstable. Also the surface temperature may vary. This is why most paint manufacturers specify a Delta T (difference between Surface temperature and Dewpoint) of +3°C (+5°F).
Picture for example the complex micro climate inside a ballast tank of a ship that carries a warm cargo (eg. crude oil), sailing in a cold sea in humid air. One wall of the tank might be degrees above dewpoint while the other could be well below.
Dewpoint under 0°C is not possible. As soon as that temperature is reached we speak about a “Frost Point”.
When should you measure the climatic conditions?
Basically the conditions should be observed prior to the pre treatment process until the coating reached a full cure. When the surface temperature of steel is below dewpoint temperature the bare steel will corrode instantly after blast cleaning.
Temperature of the coating or substrate
The coating temperature is important during application but also during storage. Most coatings will loose their performance once they have been frozen before/during their shelf life. Viscosity of paint is in direct relationship with the temperature of the paint which affects the application.
Extreme substrate temperatures also cause their share of problems. High substrate temperatures may cause the paint to dry too fast which could lead to pinholes or pores for example. Low surface temperature may prevent the paint to form a film as many coatings know a so called “Minimum Film Forming Temperature” or “MFFT” value.
Moisture in substrate
The moisture content of substrates to be coated is a critical factor as well. Fresh moist concrete will not hold any paint or applying a powder coating to a damp wooden panel will cause steam to be created when the panel passes through the curing oven, thus causing damage to the coating.
Check the MOISTURE section for more information and products about this topic.
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