Handbook of Solvents - George Wypych - ChemTech - Ventech!

396 Seung Su Kim, Jae Chun Hyun
7.3.3 MEASUREMENT OF THE DRYING RATE OF COATED FILM
The drying rate of coating is easily measured by simple experimental equipment. In a controlled
air condition, the weight loss of coating due to the solvent evaporation is measured
by electrical balance and filed at the PC. The weight loss with time is converted to drying
rate of coating per unit area. The commercialized experiment equipment is commonly used
to obtain the drying rate, such as thermo-gravimetric analyzer (TGA). Nowadays commercial
TGA equipped with FT-IR or gas chromatography is available and readily used to obtain
not only the overall drying rates of coatings but also the relative drying rate of each
solvent in a multi-solvent system. However it is difficult to obtain such data at a high air velocity,
because the air stream disturbs the balance and cause to oscillate the balance reading.
Practically the available air velocity of this kind would be 1 m/sec or lower.
Fourier-Transform-Infrared (FT-IR) spectrometer is used to obtain the drying rate at a
higher air velocities. 47,50 FT-IR is commonly used for the analysis of organic materials. Recently,
FT-IR is applied to measure the drying rate of coating. This specially designed
FT-IR with coating apparatus and air blowing system made it possible to measure solvent
content without disturbances of airflow, and moreover it enabled us to find the content of
each solvent with drying proceeded. 47,50 This can be used to study selective evaporation of
solvent and phase separation phenomena in a multi-solvent system. 50
The air velocity of industrial dryer is up to 20 m/s or more and the temperature of oven
is normally up to 200 o C. A specially designed drying chamber was suggested to measure
solvent concentration in these drying conditions. 52,57 The chamber is equipped with flame
ionization detector (FID) total hydrocarbon analyzer, and the oven exit gas which contains
the evaporated solvent flows to the analyzer. The drying rate is calculated by multiplying
the solvent concentration with exit gas flow rate. It provides access to a wider range of drying
conditions that approximate industrial conditions.
However, it is much more difficult to find out the actual drying rate of coating in an industrial
dryer. In a continuous industrial dryer it is impossible to measure the drying rate by
any of the above methods, because the coated substrate is running through the dryer at the
speed of several hundreds meter per minute. To measure the drying rate of coating in such a
condition, the dryer exit gas of each zone is analyzed by gas chromatography. Then the drying
rate of each zone is calculated by multiplying solvent concentration with exit gas flow
rate. From the drying rate of each zone, the evaporated amount of solvent is calculated. Thus
the solvent concentration of coating is found at the point of each zone end. 28,30
7.3.3.1 Thermogravimetric analysis
The drying rate of a coating could be easily found by measuring coating weight loss during
drying in a laboratory. The set-up of experimental equipment is relatively easy, and the
commercial equipment can be readily available such as thermogravimetric analyzer
(TGA). 5,53 As drying proceeded, the weight of coating decreases due to the solvent evaporation.
The amount of solvent loss with time is monitored by the balance. The schematic representation
of the equipment is shown in Figure 7.3.10. The sample, such as coated films or
a tray that contains the coating liquid, is mounted on the balance to be monitored. The temperature
of coating is measured by non-contact infrared thermometer. A thin wire type thermocouple
can be used to measure the temperature, and the thermocouple is attached to the
coating or sample tray. The air is made up by conventional blower and is heated up to a certain
temperature by electrical heater. The air is supplied to the coating surface through