Additional Studies on the Softening of Rigid PVC
by Aqueous Solutions of Organic Solvents
LOUISE V. PARKER AND THOMAS A. RANNEY
Table 1 gives the χ values Berens derived for a
INTRODUCTION
number of organic solvents (at 30C). Table 2 gives
values for χ that were determined (at 20C) by
Materials used in groundwater monitoring
wells must be able to withstand exposure to a
Vonk (1985). PVC is only softened by solvents with
a χ value less than 1 (Berens 1985) or 1.2 (Vonk
variety of groundwater conditions. While it is com-
monly known that certain neat solvents (pure
1985); the lower the number, the greater the sol-
vent or swelling power. A neat solvent with a χ
product) will degrade PVC pipe, the effects of
high concentrations of these solvents in aqueous
value less than 0.5 can completely dissolve PVC
solution is not well understood.
(Berens 1985). According to Vonk (1985), PVC is
nearly inert to organic solvents if their χ value is
This study examined whether an aqueous so-
lution that contained 18 organic chemicals that
greater than 3 (e.g., alcohols, aliphatic hydrocar-
are either solvents or good swelling agents of PVC
bons and organic acids).
are able to cause softening when combined in a
Berens observed that the rate of penetration
solution, with each solute at an activity of 0.05.
of an organic solute in PVC changed by many
This study is one of a series designed to deter-
orders of magnitude as the activity of the pen-
mine the ability of PVC to withstand long-term
etrant in the environment changed. He noted that
exposure to various organic contaminants in aque-
at ambient temperatures, rigid PVC would only
ous solutions.
be softened by solvents or strong swelling agents
of PVC at activities greater than 0.5. The less ag-
gressive the solvent (i.e., the higher the χ value),
the higher the activity required to cause soften-
BACKGROUND
ing. Figure 1 shows the isotherms Berens derived
using the FloryHuggins equation for various val-
Work by two researchers (Berens 1985, Vonk
ues of χ and the approximate softening range for
1985, 1986) on the permeation of organic solutes
PVC at 30C. Berens predicted that for lower tem-
through PVC pipe has shed some light on this
peratures (such as those commonly encountered
topic. In these studies PVC was exposed to a range
in groundwater), slightly higher activities would
of concentrations, or activities, of organic solvents.
Berens and Vonk noted that the solvent interac-
be needed to cause softening.
tion parameter χ used in the FloryHuggins equa-
For aqueous solutions the activity of a solvent
tion* could be used as a measure of the solvent
can be approximated by dividing the concentra-
or swelling power of a particular organic solvent.
tion of the compound in solution by its solubility
in water. Thus, according to Berens, if aqueous
solutions of solvents are going to soften rigid PVC,
the pure (or neat) solvent must be able to soften
* The FloryHuggins equation relates activity a to the
or dissolve PVC and must exist in solution at a
volume fraction V1 of the organic chemical in the poly-
concentration that exceeds one-half its solubility
mer as follows:
in water (activity > 0.5).
ln (a) = ln V1 + (1 V1) + χ (1 V1) 2
One criticism of Berens's work is that the vari-
where χ is the FloryHuggins interaction parameter.
ous test (activities) solutions he used in his ex-
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