Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 401
Figure 7.3.16. Phase diagram of MEK-toluene-VAGH system with the different initial solvent compositions. The
concentration is measured by rapid scanning FT-IR method. [Adapted, by permission, Suzuki et al., Proceedings
of 9th International Coating Science and Technology Symposium, Delaware, USA, May 17 - 20, 1998, pp. 21-24].
example of PVAc-methanol system is suitable for the FT-IR spectroscopy because of their
separated bands, it is difficult to extend FT-IR technique to the other solvent systems.
7.3.3.3 High-airflow drying experiment using flame ionization detector (FID)
total hydrocarbon analyzer
The gravimetric method is limited to a certain air velocity level due to the oscillation of balance
in the high airflow stream. However most of the industrial drying process accomplished
by passing the coating under high air velocity jet of hot air. Thus to simulate the
industrial drying conditions and according drying phenomena better, high airflow drying
experiment setup (HADES) was suggested by Cairncross et al. 52,57
Low air velocity results in low heat transfer coefficient, the heat transfer coefficient of
conventional laboratory drying experiments ranges from 1 to 10 J/m 2 s o C, while that of industrial
dryer is ranges about 20 to 200 J/m 2 s o C. The high heat and mass transfer at the evaporation
surface may result in ‘trapping skinning’ because the surface evaporation is too high
in compared with the rate of diffusion of solvent within the coating. The heat transfer coefficient
of HADES is up to 26.4 J/m 2 s o C which is equivalent with that of usual single-side impingement
dryers, 52 and it was reported that the HADES successfully simulates the skinning
phenomena with the solution of PMMA-toluene. 52
HADES measures solvent concentration of exhaust gas from the sample chamber
where the solvent is evaporated from the coating. Then the rate of evaporation is equal to the