Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 407
rate should be equal in ideal case. We can check the measurement error by comparing measured
and theoretical evaporation rate through the whole dryer. ER per is calculated by equation
A in Figure 7.3.21.
per
n
∑
k = 1
per
k
ER − ER
ER
= ε [7.3.12]
In most cases the value of ε is about 0.05 ~ 0.10. ER per is the theoretical evaporation
rate which assumes that all the solvent is evaporated only in the dryer. But in the real situation,
some of the solvent is evaporated before the dryer, such as in the coating head and the
pre-zone (between the coating head and the first dryer zone), and some of the solvent vapor
leaks out of the dryer through the gaps where the substrate running in and out. Therefore
measured ER was less than ER per by about 10%.
This experimental method gives precise results at the early stages of drying where the
concentration of solvent at the exit gas is high. And it is a unique method to measure the solvent
concentration of coating in the production and pilot scale dryer.
7.3.4 MISCELLANEOUS
7.3.4.1 Drying of coated film with phase separation
As a final process of coating process, drying plays an important role for the quality of products.
The structure of coating is determined during the drying process. The structure formation
of coating depends on the history of drying (drying process path) which represents the
composition changes of coating during drying. The drying process path depends on the drying
conditions such as temperature and velocity of hot air, residence time of coating in the
dryer, humidity or solvent concentration of drying air, the initial composition of coating etc.
Figure 7.3.23 shows the two different structures of coating according to the extremely different
drying conditions (cellulose acetate solution of 10 wt% prepared in acetone 80 wt%
Figure 7.3.23. Cross-sectional SEM images of final coating microstructures prepared under slow and fast rates of
external gas-phase mass transfer conditions. [Adapted, by permission, Mahendra et al., Proceedings of 9th International
Coating Science and Technology Symposium, Delaware, USA, May 17 - 20, 1998, pp. 177 - 180].