The rate at which a solvent evaporates is affected by four variables: temperature, vapor pressure, surface-to-volume ratio, and air velocity over the surface. Relative humidity is also an important factor if water is used as the volatile component.
The temperature mentioned here is the surface temperature of the object to be coated. The surface temperature of the substrate may be the same as the surrounding environment, but there is a significant difference in sunlight or shade. The volatilization of the solvent will lower the surface temperature, and at the same time, the heat inside and around the paint film will cause the temperature to rise. If the heat spreads quickly, the surface temperature will not drop much. If it is slow, the surface temperature will increase. A sharp decline. For solvents with a high heat of vaporization, and in an environment with fast volatilization, the surface temperature will drop rapidly.
The vapor pressure of the solvent during evaporation is very important. North and South Tide Tips: The boiling point of a solvent is not always proportional to the vapor pressure at other temperatures. For example, at 25°C, n-butyl acetate (boiling point 126°C) volatilizes faster than n-butanol (boiling point 118°C).
The effect of the surface area to volume ratio is also evident. For example, the volatilization rate of the solvent of the paint after opening the barrel must be slower than that of the film during construction. Solvent evaporation rates during spray application are rapid because the atomized particles have a large surface area to volume ratio. For the same reason, compared with the thinner wet film after construction, the solvent volatilization is slower, that is to say, the increase of concentration and viscosity will be slower.
The change of the air velocity on the surface of the coated object will also affect the volatilization of the solvent, and the partial vapor pressure of the air solvent on the air/solvent interface determines the volatilization rate of the solvent. This is why sometimes different construction methods need to add different thinners or different ratios of thinners.
The volatilization rate varies greatly due to different application methods, spraying is very fast, brushing is medium, and flow coating and tape dipping have the slowest volatilization rates. If applied by dipping or flow coating, the rate of evaporation will affect the sagging of the coating.
The difference between air spraying and airless spraying is that air spraying is sprayed to the surface of the object after atomization is driven by air flow. Therefore, the solvent evaporation of air spraying is obviously greater than that of airless spraying, which is why there is more paint mist in air spraying.
When the same paint is applied outdoors and indoors, the solvent volatilization rate is obviously much faster when applied outdoors, unless there is forced ventilation indoors.
Under normal circumstances, the relative humidity has little effect on the volatilization of solvents, but the importance of the effect on water is needless to say. As the relative humidity increases, the evaporation of water slows down. However, very high relative humidity can also inhibit the rate of solvent evaporation. This is one of the reasons why we need to control the relative humidity during construction, no matter for water or solvent.
