Environmental Conditions tures should be measured to avoid applica-
5.1 Environmental Conditions tures should be measured to avoid applica-
tion problems should either become too hot Environmental, or ambient, conditions can or too cold for satisfactory film formation greatly affect all phases of a coating opera- ( Figure 5.2 ). Application at incorrect tem- tion. This chapter examines: (1) specific peratures can cause defects such as: environmental conditions, (2) testing equip-
ment, and (3) methods that are most likely to
• Blistering
be of concern to the coating inspector ( Fig-
• Pin holing
ure 5.1 ).
• Cratering • Dry spray
5-2 Environmental Testing
The coating manufacturer should specify the
5.1.2 Relative Humidity
maximum and minimum surface tempera- Relative humidity is a measure of the tures for coating application.
amount of moisture in the air compared to saturation level (the amount it can hold at a given temperature) and may affect the coat- ing operation because of either too much or not enough relative humidity ( Figure 5.3 ). Many coating specifications restrict coating application when the relative humidity is expected to be too high. Typically, there are solvents in the coating film that need to evaporate and, if the relative humidity is too
Figure 5.2 Surface Temperatures
high, there is no available space for the sol- vent to evaporate. If this happens, the sol-
ASTM D3276, Standard Guide for Painting vents that remain in the coating cause Inspectors (Metal Substrates), states that the problems with the film such as solvent minimum surface temperature for coating entrapment and lack of curing. application is usually 40°F (5°C). It may be
as low as 0°F (-18°C) for cold-curing one- or two-component systems or 50°F (10°C) for conventional two-component systems. Coating specifications may further state that coating should not be undertaken when the temperature is dropping and within 5°F (3°C) of the lower limit.
The maximum surface temperature for coating application is typically 125ºF
Figure 5.3 Relative Humidity Changes Relative to a
(50ºC), unless otherwise clearly specified. A
Parcel of Air (yellow) as Temperature Increases
surface that is too hot may cause the coating Dew Point is the temperature at which mois- solvents to evaporate so fast that application ture begin to form on a steel surface. It is the is difficult, blistering takes place, or a temperature to which a volume of air must porous film results. Most epoxy formula-
be cooled in order to reach saturation. It is a tions stop the curing process below 50°F function of air temperature and the relative (10°C). humidity. A good illustration is a simple
Also of note, we should only apply coatings glass of ice water. The moisture condenses to a substrate when that substrate is at least
on the outside of the glass because the 5°F (3°C) above the determined dew point
glass’s surface temperature is below the dew in order to prevent moisture on the surface
point.
from being coated.
Environmental Testing 5-3
The infrared thermometer is useful for mea- Wind speed can adversely affect a coating
5.1.3 Wind Speed
suring temperature under circumstances in job in several ways. It can:
which thermocouples or other probe-type sensors cannot be used or do not produce
• Blow abrasives past the boundaries of the abrasive-blast work area to an area where
accurate data for a variety of reasons.
coatings are being applied • Cause excessive drift or overspray of
sprayed coatings • Accelerated solvent evaporation after
application • Contribute to the formation of dry spray The coating inspector should also stay alert
to possible affects wind-blown contaminants (e.g., sea spray, salt, blast media, dust, or sand) might have on the coating job. If the wind speed is determined to be harmful, the coating inspector (if possessing the author-
Figure 5.4 Infared Thermometers
ity) should immediately advise the supervi- Proper use of the equipment is very simple:
sor and the contractor. point, shoot, and read. There are three basic
measuring techniques: