Handbook of Solvents - George Wypych - ChemTech - Ventech!

50 George Wypych
The vapor pressure of solvent was found in the previous section to depend on its molecular
weight and temperature. Figure 2.3.3 shows that the evaporation rate of a solvent
may be predicted based on knowledge of its boiling point and Figure 2.3.4 shows that the
specific heat of solvent also relates to its boiling point. The boiling point of solvent also depends
on its molecular weight as does enthalpy and heat of vaporization. But there is not a
high degree of correlation among these quantities because molecular associations exist
which cannot be expressed by universal relationship. For this reason experimental values
are used to compare properties of different solvents. The two most frequently used reference
solvents are: diethyl ether (Europe) and butyl acetate (USA). The evaporation rate of
other solvents is determined under identical conditions and the solvents are ranked
accordingly. If diethyl ether is used as a reference point, solvents are grouped into four
groups: high volatility < 10, moderate volatility 10-35, low volatility 35-50, and very low
volatility > 50. If butyl acetate is used as the reference solvent, the solvents are grouped into
three classes: rapid evaporation solvents > 3, moderate 0.8-3, and slow evaporating solvents
< 0.8.
In some applications such as coatings, casting, etc., evaporation rate is not the only important
parameter. The composition must be adjusted to control rheological properties, prevent
shrinkage, precipitation, formation of haze, and to provide the required morphology.
Solvents with different evaporation rates can address all existing requirements.
Both the surface tension of mixture and solvent diffusion affect the evaporation rate.
This becomes a complex function dependent not only on the solvents present but also on influence
of solutes on both surface tension and diffusion. These relationships affect the real
evaporation rates of solvents from the complicated mixtures in the final products. In addition,
solvent evaporation also depends on relative humidity and air movement.
2.3.7 FLASH POINT
Flash point is the lowest temperature, corrected to normal atmospheric pressure (101.3
kPa), at which the application of an ignition source causes the vapors of a specimen to ignite
under the specific conditions of the test. Flash point determination methods are designed to
be applied to a pure liquid but, in practice, mixtures are also evaluated. It is important to understand
limitations of such data. The flash point of a solvent mixture can be changed by
adding various quantities of other solvents. For example, the addition of water or
halogenated hydrocarbons will generally increase the flash point temperature of mixture.
The flash point can also be changed by forming an azeotropic mixture of solvents or by increasing
the interaction between solvents. At the same time, the flash point of single component
within the mixture is not changed. If conditions during production, application, or in a
spill allow the separation or removal of a material added to increase the flash point, then the
flash point will revert to that of the lowest boiling flammable component.
An approximate flash point can be estimated from the boiling point of solvent using
the following equation:
Flash point = 0.74T b
[2.3.7]
Figure 2.3.5 shows that there is often good correlation between the two but there are instances
where the relationship does not hold. The correlation for different groups of solvents
varies between 0.89 to 0.96.
Flash point can also be estimated from vapor pressure using the following equation: