Effects of Temperature and Humidity on Application Performance and Film Quality of Waterborne Coatings
The application performance and final film quality of waterborne coatings are highly dependent on and strictly controlled by the temperature and humidity conditions during application. Improper control will easily lead to defects such as poor drying, insufficient adhesion, blistering and cracking.
Core Influencing Factors: Temperature and Humidity
Temperature and humidity affect coating application mainly through two critical processes: water evaporation and reaction of film-forming substances.
Effects of Temperature
Recommended range: The suitable application temperature for waterborne coatings is generally between 10℃ and 35℃.
Low temperature (< 5–10℃)
- Slow water evaporation, resulting in difficult film drying.
- The temperature may fall below the minimum film-forming temperature (MFFT) of emulsion polymers, preventing the formation of a continuous, dense coating film.
- For two-component waterborne coatings (e.g., epoxy and polyurethane systems), chemical reactions will slow down significantly or even stop, leading to failed curing.
High temperature (> 35℃)
- Rapid water evaporation may cause rapid skin formation on the film surface while trapped internal moisture cannot escape, resulting in blistering and cracking.
- The flowability of the coating will be impaired, bringing about appearance defects such as brush marks and craters.
Effects of Humidity
Recommended range: The ideal relative humidity (RH) for application is 40% to 80%.
High humidity (RH > 80–85%)
- Saturated moisture in air severely hinders water volatilization from the coating film, multiplying drying time.
- Un-evaporated condensed water may be encapsulated inside the film to form pores, greatly reducing the corrosion resistance and mechanical strength of the coating.
- For polyurethane coatings, the isocyanate component may react with water vapor to generate carbon dioxide, causing pinholes in the film.
- Long-term exposure to high humidity tends to trigger mold growth on the coating surface.
Low humidity (RH < 40%)
- Excessively fast water evaporation leaves insufficient time for film-forming substances to level and fuse uniformly, which may result in incomplete films, low gloss or reduced adhesion.
Key Indicator: Dew Point Temperature
For outdoor application or environments with fluctuating temperature and humidity, dew point temperature is a more critical indicator than temperature or humidity alone. It refers to the temperature at which water vapor in air starts to condense into dew.
Core principle: The surface temperature of the substrate to be coated must be at least 3℃ higher than the ambient dew point temperature.
Why it matters: If the substrate surface temperature is close to or lower than the dew point, water vapor will condense on the substrate to form an invisible thin water film. Applying waterborne coating under such conditions traps the water film beneath the coating, inevitably causing loss of adhesion, blistering and premature substrate corrosion.
Key Application Control Points
To guarantee coating quality, the following points shall be comprehensively controlled:
- Monitoring and measurement: Use thermo-hygrometers and dew point meters (or thermo-hygrometers with dew point calculation function) to monitor ambient conditions. Continuous monitoring is required before, during and after coating application.
- Environmental conditioning: For indoor application, adjust temperature and humidity to the range specified in the product data sheet by air conditioners, dehumidifiers, hot air blowers and other equipment.
- Substrate inspection: Ensure substrates (such as wood and metal) are dry, clean and meet the required temperature.
- Process adjustment: Under non-ideal environmental conditions, adopt thin-coat multi-pass application. Apply the next coat only after full drying of the previous layer.
Summary
Successful waterborne coating application relies on precise control of three environmental parameters: temperature, humidity and dew point. Prior to large-scale application, always check the specific requirements in the product technical datasheet and conduct a small-area trial test.
