Table 6. Mean relative hardness (Barcol) values and percent
to a solution of methylene chloride (either
relative standard deviations for the mixed-solvent study for
a solvent or a good swelling agent of PVC)
various contact times.
at an activity of 0.05 for 20 weeks. Previ-
ous work by other researchers (Berens 1985,
Treatment
1
3
7
14
21
42 days
Vonk 1985, 1986) also predicts that an aque-
control
ous solution of a PVC solvent or swelling
agent should not be able to soften PVC at
X
80.0
78.6
77.8
78.8
80.0
79.0
this low an activity. However, we found
%RSD
1.3
1.9
1.4
2.4
1.5
2.4
that a solution containing a mixture of 18
test soln
good swelling agents of PVC, each at an
X
activity of 0.05, was able to rapidly soften
74.8*
74.2*
72.0*
68.2*
67.0*
65.4*
%RSD
1.7
2.4
1.4
5.2
5.3
4.1
PVC. Clearly there is an additive effect of
organic solutes, and a solution containing
several organic solutes has a much greater
ability to soften PVC than does a solution
PVC. In this study we saw a statistically signifi-
containing any one of the solutes by itself.
cant change in the Barcol readings of samples ex-
In this study we also found that several or-
posed to the solution of mixed organic solutes
ganic solvents that are either solvents or good
after only one day (Table 6, App. C). After 14 days,
swelling agents of PVC and are totally miscible
several of the test pieces were noticeably softened
in water can soften PVC. Tetrahydrofuran is a sol-
when tested manually, although none became pli-
vent commonly used in PVC glues. After 20 weeks
able. Thus, even solutes at activities as low as 0.05
the lowest concentration we tested (0.01%, or
can soften PVC if enough organic compounds that
0.0014 M) was able to cause a significant change
are good solvents or swelling agents of PVC are
in the hardness readings. Lower concentrations
present.
were not tested but may also have an effect. Ac-
Based on the results from this study, we now
etone is a good swelling agent of PVC and was
wonder whether a mixture of organic solutes at
able to cause a significant change in the hardness
even lower activities could also soften PVC and
readings of test pieces exposed to concentrations
whether chemicals that are solvents and good
of 10% (1.7 M) but not 5% (0.86 M). Dimethylforma-
swelling agents of PVC have any synergistic ef-
mide was the least aggressive solvent tested and
fect on other poorer swelling agents. We also won-
was able to cause a significant change in the hard-
der what happens in solutions that contain sev-
ness readings in 60% (8.2 M) and 80% (10.9 M)
eral different activities and what happens if you
solutions but not at 40% (5.5 M).
have a solution that contains low activities of
Currently we are conducting several additional
several different PVC solvents or swelling agents
longer-term studies (1 year or more) that may allow
plus some dissolved PVC solvent or swelling agent
us to determine:
that is totally miscible in water. We have started
Whether softening progresses further in
several studies to address these issues.
those solutions already showing a signifi-
cant change in hardness readings;
Whether a mixture of good swelling agents
of PVC, at lower activities than we tested
CONCLUSIONS
(0.05), will soften PVC;
Knowing whether the initial softening we have
Whether PVC solvents and good swelling
measured in this study and in previous studies
agents have some type of synergistic ef-
will progress and understanding what happens
fect on poorer swelling agents;
in a solution that contains several organic solutes
What occurs in an aqueous solution that
is essential if we are to develop any guidance for
contains a number of PVC solvents and
predicting the ability of PVC to withstand expo-
good swelling agents at varying activities;
sure to various organic contaminants over the long
and
term. While we still cannot give definitive guid-
Whether lower concentrations of PVC sol-
ance, these studies have increased our knowledge
vents and swelling agents that are totally
and moved us closer to being able to do that.
miscible in water can soften PVC and what
In a previous study (Parker and Ranney 1994),
occurs when these chemicals are also
we found no softening of PVC that was exposed
present in a mixture of organic solutes.
9
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