ing that any organic chemical with a χ value less
the hardness readings of the samples that were
than 0.5 can dissolve PVC, the true χ value for
exposed to solutions with a relative solubility of
methylene chloride remains undefined but is less
0.05 were not significantly different from the con-
than 0.5.
trols.
In a previous study (Parker and Ranney 1994),
We proposed that only partial softening occurs
we found that after one week of exposure, PVC
at the lower relative solubilities (0.1 to 0.4) and
pieces exposed to aqueous solutions of methyl-
that these samples will never actually become rub-
ene chloride with a relative solubility of 0.6 had
bery or pliable. Examination of Berens' isotherms
hardness readings that were significantly lower
(Fig. 1) indicates that some samples do become
than the control samples, and these samples were
partially softened but never actually become rub-
rubbery in that they could easily be bent by hand.
bery. We concluded that a longer-term study was
(In all our studies, we use a Barcol Impressor to
necessary to resolve whether softening progresses
determine hardness.) Samples exposed to solu-
further with time, and the lowest relative solubil-
tions with a relative solubility of 0.8 were also
ity at which softening occurs.
similarly affected, with significant changes in the
hardness readings after only four hours. While
Trichloroethylene studies
these findings agree with Berens' (1985) predic-
We conducted another series of studies using
tions for PVC exposed to aqueous solutions with
trichloroethylene (TCE). TCE is a common envi-
an activity of 0.8, Berens would not predict that
ronmental pollutant and is a relatively good swell-
PVC exposed to solutions with an activity, or rela-
ing agent of PVC. There is close agreement be-
tween the χ value of Berens (1985), 0.88, and that
tive solubility, of 0.6 would become rubbery un-
less the true value of χ was very close to 0. We
of Vonk (1985), 0.90. When we (App. A, Ranney
were not able to find in Berens' paper how he
and Parker 1995) subjected small pieces of PVC
determined softening.
well casing to neat (undiluted) TCE, we found
In a subsequent study (Parker and Ranney
that the test pieces showed signs of swelling and
1994), we were able to determine that there were
softening after only one day. However, TCE did
significant changes in the hardness readings of
not dissolve the PVC pieces. Thus, our experi-
test pieces that were exposed to aqueous solu-
mental work appears to confirm that TCE is a
tions of methylene chloride with relative solubili-
good swelling agent of PVC but that it is not a
ties of 0.4, 0.2, and 0.1 for 20 weeks. However,
solvent.
none of these samples were actually rubbery or
We conducted two studies where pieces of PVC
pliable. We concluded that it appeared that soft-
were exposed to aqueous solutions of TCE with a
ening had begun to occur at much lower relative
range of relative solubilities (0.2 to 1.0) (Parker
solubilities, or activities (0.1 to 0.4), than had been
and Ranney 1994). Samples that were exposed to
previously reported or predicted by Berens (1985).
solutions with relative solubilities of 0.8 and 1.0
Because we conducted our own solubility tests to
had Barcol readings that were significantly lower
confirm the aqueous solubility of methylene chlo-
than the controls. The samples that were exposed
ride, we do not feel that our test results can be
to the solutions with a relative solubility of 0.8
explained away by assuming the solubility value
never became pliable or rubbery, while those ex-
we used is incorrect.
posed to the 1.0 relative solubility solutions were
According to Berens (1985), diffusion of or-
pliable by the end of the study (77 days). Gener-
ganic permeants is slowest (Fickian) at activities
ally, the results for TCE agree with what would
less than 0.25. However, Vonk (1985, 1986) felt
be predicted using Berens' isotherms (Fig. 1): PVC
that diffusion would be Fickian for the poorer
would be softened by TCE when the activity, or
PVC swelling agents (e.g., alkylated aromatics) at
relative solubility, is 0.8 or greater and would
activities less than 0.25, but for the solvents and
become rubbery if the relative solubility was 0.95
(at 30C).
the better swelling agents (i.e., anilines, chlori-
nated hydrocarbons, ketones, and nitrobenzenes)
Samples that were exposed to solutions with a
the activity should be less than 0.1. If this is the
relative solubility of 0.6 had hardness readings
case, we would not expect to observe any changes
that were significantly different from the controls
in the hardness readings of samples exposed to
in the first study (68 days) but not in the second
solutions of methylene chloride (a solvent) with
study (77 days). We were not certain why the
relative solubilities less than 0.1. The results from
results for the two studies differed. We thought
this study tend to support this hypothesis since
that if the unsoftened samples had been given
3
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