Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.1 Standard methods of solvent analysis 1059
control equipment, and a voltmeter. The impulse method 44 uses highly divergent field under
impulse conditions. The breakdown voltage of fresh oil decreases as the concentration of aromatic
hydrocarbons increases. The method can be used for quality control of fresh oil, and
determining the effect of service aging, and effect of impurities. The material is placed in
test cell containing electrodes which are supplied from an impulse generator controlled by
voltage control equipment.
15.1.14 ENVIRONMENTAL STRESS CRAZING
Crazing is a group of surface fissures which appear as small cracks after the material has
been exposed to solvent and stress. Crazes are usually oriented perpendicular to stress and
their appearance depends on the index of refraction and on the angle of viewing. A suitable
light source must be used. Transparent plastic materials can be directly tested for crazing. 45
Two variations of method are used to determine stress crazing of transparent plastics:
determination of stress required to cause cracking or determination of craze development
along the time of stress application. In the first method specimens are exposed to solvent by
the direct contact of specimen surface with filter paper wetted with solvent. By selecting
different values of stress (using each time new specimen) the range of two stress forces is
searched for the largest stress under which specimen does not craze and the smallest stress
under which it does craze. In the second variation of the method, sample is tested first
without solvent to assure that it does not craze under the selected load. Solvent is then
applied and specimen inspected in 15 min time intervals taking note of location of craze
front as crazing progresses. If the sample does not craze higher load is selected and vice
versa. Similar to the first variation the range is determined within which specimen crazes
and does not craze.
The crazing effect can be indirectly determined by testing chip impact resistance of
specimens which crack either because of weathering or environmental stress cracking. 46
15.1.15 EVAPORATION RATE
The evaporation rate of a solvent is determined to obtain relative value to some standard, selected
solvent. The solvent selection depends on reasons for solvent use and the type of solvent
and it is usually agreed upon between interested parties. In Europe, diethyl ether is the
most frequently used reference solvent and in the US butyl acetate. The evaporation rate of
other solvents is determined under identical conditions and the resultant values are used to
rank solvents. The most obvious requirement is that the determination is done without excessive
drafts and air currents. The evaporation rate is the ratio of the time required to evaporate
a test solvent to the time required to evaporate the reference solvent under identical
conditions. The results can be expressed either as the percentage evaporated within certain
time frame, the time to evaporate a specified amount, or a relative rate. Relative rate is the
most common.
For halogenated solvents used in cleaning applications, the relative evaporation rate is
compared either to xylene or perchloroethylene. 47 The determination is done on a test panel
using 10 ml of solvent. The relative evaporation rate is calculated.
15.1.16 FLAMMABILITY LIMITS
These tests cover the methods of determination of the minimum temperature at which
vapors in equilibrium with liquid solvent are sufficiently concentrated to form flammable
mixtures with air at atmospheric pressure and concentration limits of chemicals. Flammable
(explosive) limits are the percent levels, volume by volume, of a flammable vapor or gas
mixed in air between which the propagation of a flame or an explosion will occur upon the