Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.1 Introduction to diffusion, swelling, and drying 353
Initial evaporation rate, % s -1
Table 7.1.1 shows the effect of cosolvent
addition on the evaporation rate of solvent
mixture.
The initial rate of evaporation of
solvent depends on both relative humidity
and cosolvent presence (Figure 7.1.25). As
relative humidity increases the initial
evaporation rate decreases. The addition
of cosolvent doubles the initial evaporation rate.
In convection drying, the rate of solvent evaporation depends on airflow, solvent partial
pressure, and temperature. By increasing airflow or temperature, higher process rates
can be achieved but the risk of skin and bubble formation is increased. As discussed above,
Vrentas-Duda free-volume theory is the basis for predicting solvent diffusion, using a small
number of experimental data to select process conditions. The design of a process and a
dryer which uses a combination of convection heat and radiant energy is a more complex
process. Absorption of radiant energy is estimated from the Beer’s Law, which, other than
for the layers close to the substrate, predicts: 32
where:
0.12
0.1
0.08
[ ( ) ]
Q () ξ = I αexp−α βh −ξ
r
0
H 2 O
H 2 O+BuOH
Table 7.1.1. Experimentally determined
initial evaporation rates of waterborne
coating containing a variety of solvents at
5% level (evaporation at 25 o C and 50% RH)
Co-solvent
Qr radiant energy absorption
ξ distance from substrate
I0 intensity of incident radiation
α volumetric absorption coefficient
β fractional thickness of the absorbing layer next to the substrate
h thickness
Initial evaporation
rate, μg cm -2 s -1
0.06
none 3.33
methyl alcohol 4.44
0.04
ethyl alcohol 3.56
0.02
0 10 20 30 40 50 60 70
n-propyl alcohol
n-butyl alcohol
4.00
3.67
Relative humidity, %
i-butyl alcohol 3.67
Figure 7.1.25. Initial evaporation rate from n-amyl alcohol 3.33
waterborne coating vs. relative humidity for two
solvent systems. [Data from S Kojima, T Moriga, n-hexyl alcohol 3.22
Polym. Eng. Sci., 35, No.13, 1098-105 (1995).] ethylene glycol
mono-butyl ester
3.22
ethylene glycol
mono-hexyl ester
3.33
butyl carbinol 3.11
methyl-i-butyl ketone 4.89
[7.1.17]