1985]). There are no χ values available for the
tested (80% or 10 M) were actually pliable, i.e.,
other three chemicals, but tests by our laboratory
easily bent in half.
indicate that they are all PVC solvents (App. A,
The dimethylformamide solution was the least
Ranney and Parker 1995). In fact, tetrahydrofuran
aggressive solution. During a 20-week study
is commonly used as a component of PVC glues.
(Parker and Ranney 1995), only the test pieces
We (Parker and Ranney 1995) found that the
exposed to the 60 (8.2 M) and 80 percent (10.9 M)
test solutions containing tetrahydrofuran were the
solutions had hardness readings that were sig-
most aggressive. After seven days, a 60% solution
nificantly lower than the controls. The samples
(8.4 M) dissolved the PVC pieces. By the end of a
that were exposed to the 80% solution were also
20-week study that used lower concentrations,
pliable. Because there was no additional weight
the hardness readings of the samples exposed to
gain in any of these samples during the last four
the lowest concentration tested (0.01% or 0.0014
weeks of the study, equilibrium may have been
M) were significantly lower than the controls.
reached.
However, none of these samples were noticeably
It is clear from this study that PVC solvents
softened when tested manually, even the samples
and swelling agents that are completely miscible
exposed to the highest concentration tested (10%
in water can soften PVC when mixed with water.
or 1.4 M). Although there is a lot of variability in
These results are puzzling in that the aqueous
the weight change data, by the end of the study
solutions of acetone were more aggressive than
there was no additional weight gain in the samples
the aqueous solutions of the dimethylformamide
that were exposed to the 1.0, 0.1, and 0.01% solu-
and pyridine, and yet our laboratory studies show
tions. This may indicate that these solutions have
that these two chemicals are more aggressive than
reached equilibrium. If this is the case, then we
acetone (App. A, Ranney and Parker 1995). There-
would not expect any additional softening to oc-
fore, we would expect that the aqueous acetone
cur.
solutions would be the least aggressive solutions.
The acetone solution was the next most aggres-
It appears that the ability of aqueous solutions of
sive solution (Parker and Ranney 1995). After
these chemicals does not completely correlate with
seven days, the hardness readings for samples
the neat chemicals' ability to dissolve or swell
exposed to the 60% (10.2 M) solutions were sig-
PVC. This can be related to other interactions that
nificantly lower than the controls, and the samples
occur in aqueous solution. According to Leggett
(pers. comm. 1995* ) acetone is less basic than the
were pliable when tested manually (i.e., could be
other solutes and would interact less strongly with
bent in half easily). In a longer-term study (20
water, and thus an aqueous solution would have
weeks) (Parker and Ranney 1995), there was no
a higher activity on a molar basis.
significant change in the hardness readings of
samples that were exposed to the 2.5% (0.43 M)
and 5% (0.86 M) solutions. There was a signifi-
cant change in the Barcol readings of samples
PURPOSE OF THIS STUDY
exposed to the 10% (1.7 M), 20% (3.4 M), and 40%
The purpose of this study was to determine:
(6.9 M) solutions. However, none of these samples
1) whether softening progresses further in those
were noticeably softened when tested manually.
solutions already showing a significant change in
Although the weight gain data were highly vari-
hardness readings but are not yet pliable?
able, it did appear that the samples exposed to
2) for solutions with single organic solutes, what
the 40%, 10%, and the 5% solutions had reached
is the lowest relative solubility where softening
equilibrium. If this is the case, we would not ex-
occurs?
pect further softening to occur. However, this is
3) for solutions with multiple organic solutes,
unproven at this time.
what is the lowest relative solubility where soft-
Our initial study (Parker and Ranney 1995)
ening occurs?
showed that aqueous solutions of pyridine were
4) for solutions with multiple organic solutes,
not highly aggressive. Even the 60% solution had
how well does the sum of the relative solubilities
no effect on the hardness readings after seven
of the solutes in the solution correlate with the
days. However, after a 20-week study (Parker and
solution's ability to soften PVC?
Ranney 1995), there was a significant change in
the hardness readings for the samples exposed to
the lowest concentration tested (20% or 2.5 M).
* D. Leggett, Research Chemist, USA Cold Regions Research
Samples exposed to the highest concentration
and Engineering Laboratory, Hanover, New Hampshire.
5
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