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MATERIALS TESTING
Evaluating
Environmental Stress
Cracking of
Medical Plastics
Eric J. Moskala and
Melanie Jones
When a plastic is exposed to a chemical
environment, the material may undergo numerous
changes.
These can include weight gain
if the plastic absorbs the chemical, weight loss
if the plastic is degraded
by the
chemical or if the chemical extracts low-
molecular-weight components of the plastic,
dissolution
if the chemical is a good
solvent, or other changes such as variations in
opacity or color. If the plastic is
under stress, it may also experience
environmental stress cracking, which can be
defined as the crazing
and cracking
that may occur when a plastic under a tensile
stress is exposed to aggressive chemicals.
The potential for environmental stress
cracking is of paramount concern when plastics are
used in
medical device components such
as luers and stopcocks. In these applications,
chemicals such as
isopropanol and lipid
solutions can initiate
crazes
—
microcracks bridged
by polymer fibrils
—
in the
plastic
and seriously compromise its
mechanical integrity.
Products like medical luers can be
susceptible to crazing when under
stress and exposed to aggressive
chemicals. Photo: Eastman
Chemical
Co.
The proper selection of a medical
plastic requires a thorough analysis and
interpretation of the
phenomenon of
environmental stress cracking. The goal of this
article is to provide a framework for
evaluating the suitability of a plastic
for medical uses in terms of its stress-crack
behavior. Our strategy
will be to
examine, in some detail, the roles of the three
critical components of environmental stress
cracking: the chemical environment, the
plastic, and the tensile stress.
THE CHEMICAL ENVIRONMENT
Chemicals that cause
environmental stress cracking can be divided into
those that swell or wet the
polymer and
those that chemically react with the polymer. An
example of the latter would be the caustic
or aqueous sodium hydroxide that can
hydrolyze poly(ethylene terephthalate)
(PET).
1
The reduction in
polymer molecular weight from the
hydrolysis can lead to crazing and eventual
catastrophic failure
—
a
mechanism that has been identified as
the probable cause for stress-crack failures in
one-piece
carbonated soft-drink
containers.
2
This article, however, will emphasize
chemicals that cause stress cracking simply by
swelling or wetting
the polymer. (Lipid
solutions and isopropanol fall into this
category.) It is the general consensus in the
literature that the majority of stress-
crack failures experienced by end-users results
from this category
of chemical.
< br>3
–
5
Numerous
studies have linked the ability of a solvent to
swell a plastic with its ability to
craze the plastic. Perhaps the best-
known work of this kind is that of Kambour et al.,
who
demonstrated
—
in
studies on polycarbonate,
6
poly(phenylene oxide),
7
polysulfone,
8
and
polystyrene
9
—
tha
t the absorption of the solvent and concomitant
reduction in the polymer's
glass-
transition temperature can be correlated with a
propensity for stress cracking. They also showed
that absorption of the liquid by the
polymer tends to be correlated by the solubility
parameters of the
liquid and polymer.
The solubility parameter,
, is defined
as
where
CED
is cohesive energy
density,
E
vap
is
the heat of vaporization, and
V
m
is molar
volume.
Hansen proposed that the
solubility parameter was composed of contributions
from the three major
types of cohesive
forces, namely dispersive, polar, and hydrogen
bonding, so that
where
d
,
p
, and
h
are the dispersive, polar,
and hydrogen bonding components of the total
solubility
parameter.
10
Values of these parameters for a few
representative chemicals are shown in Table I. If
the
solubility parameter of the solvent
is close to the solubility parameter of the
polymer, the polymer will
probably show
some solubility in the solvent or undergo solvent-
induced crystallization. Experience has
shown that absorption of a liquid by a
polymer may be better correlated by using the
partial solubility
parameters.
Liquid
(MPa
)
(MPa
)
(MPa
)
(cm
< br>3
mol
-1
)
(MPa
?
)
?
?
?
Molar
Volume
d
p
h
Total
Isooctane
Heptane
Cyclohexane
Ethylbenzene
Dioctyl
phthalate
Toluene
166.1
147.4
108.7
123.1
377.0
106.8
14.3
15.3
16.8
17.8
16.6
18.0
0.0
0.0
0.0
0.6
7.0
1.4
0.0
0.0
0.2
1.4
3.1
2.0
14.3
15.3
16.8
17.8
18.2
18.2
Methyl
ethyl
ketone
Chloroform
Tetrahydrofuran
Cyclohexanone
Acetone
o-Dichlorobenzene
1-Pentanol
Nitrobenzene
i-Propanol
Ethanol
Dimethyl
sulfoxide
Methanol
Ethylene glycol
Glycerol
Water
90.1
80.7
81.7
104.0
74.0
112.8
109.0
102.7
76.8
58.5
71.3
16.0
17.8
16.8
17.8
15.5
19.2
16.0
20.0
15.8
15.8
18.4
9.0
3.1
5.7
6.3
10.4
6.3
4.5
8.6
6.1
8.8
16.4
5.1
5.7
8.0
5.1
7.0
3.3
13.9
4.1
16.4
19.4
10.2
19.0
19.0
19.4
19.6
20.0
20.5
21.7
22.2
23.5
26.5
26.7
40.7
55.8
73.3
18.0
15.1
17.0
17.4
15.6
12.3
11.0
12.1
16.0
22.3
26.0
29.3
42.3
29.6
32.9
36.1
47.8
Table I. Solubility
parameters for selected
liquids.
11
Because the solubility
parameter of a polymer cannot be calculated
directly from the heat of
vaporization,
indirect methods such as solvent swelling and
group-contribution approaches are used.
Solvents that swell or dissolve the
polymer most effectively will have solubility
parameters close to the
solubility
parameter of the polymer, in keeping with the
adage that
group-contribution approach,
the solubility parameter is determined by using
the equation
where
F
i
is the molar
attraction constant and
V
i
is the molar
volume for each subsegment of the polymer
repeating unit (as demonstrated for PET
in Table II).
Table II. Estimation of solubility
parameter for polyethylene terephthalate (PET)
using the
group-contribution
approach.
12
PET molecular
structure shown at top.
The
ability of the solubility parameter approach to
correlate the absorption behavior of plastics has
been
demonstrated by the authors using
PET, PCTG (a copolyester), and polycarbonate.
Pieces of 3-mil-thick
amorphous,
unoriented, extruded film were suspended in sealed
jars above a few milliliters of liquid.
The films were removed every two weeks
for weighing until they reached an equilibrium
weight. The
results, listed in Table
III, indicate that all three plastics appear to
show a broad peak with a maximum
in
liquid absorption at a solubility parameter of
approximately 20 MPa
?
. The
solubility parameters for
PET, PCTG,
and polycarbonate, as determined by the group-
contribution approach, are 23.5
MPa
?
, 22
MPa
?
, and 21.9
MPa
?
, respectively, which
fall into the range of the broad maximum in liquid
absorption.
(The alcohols are
exceptions to this trend, as seen in Table I,
presumably because of their strong
hydrogen bonding characteristics.)
These results highlight the difficulty in using
the solvent-swelling
technique for
determining the solubility parameter of a polymer
and in using the total solubility
parameter to correlate a polymer's
absorption behavior.
Liquid
PET
PCTG
Polycarbonate
(%)
(%)
(%)
(MPa
?
)
14.3
15.3
16.8
17.8
18.2
19.0
19.0
0
0
0
0
0
0
0
0
1
Total
Isooctane
Heptane
Cyclohexane
Ethylbenzene
Toluene
Methyl ethyl
ketone
Chloroform
6
12
13
57
17
21
15
D
27
31
25
D
Tetrahydrofuran
Cyclohexanone
Acetone
o-Dichlorobenzene
1-Pentanol
Nitrobenzene
i-Propanol
Ethanol
Dimethyl
sulfoxide
Methanol
Ethylene glycol
Glycerol
Water
19.4
19.6
20.0
20.5
21.7
22.2
23.5
26.5
26.7
29.6
32.9
36.1
47.8
18
21
12
25
0
33
28
13
50
1
D
65
24
60
0
28
0
1
50
0
4
58
1
7
16
3
0
0
0
14
5
0
0
0
40
8
1
1
2
Table III. Percentage weight gain for
PET, PCTG
copolyester, and
polycarbonate film in various liquids. D =
dissolved. Values in italics indicate
that the film became
opaque.
THE
PLASTIC
All
plastics can be classified as either amorphous or
crystalline materials. In amorphous plastics such
as
polystyrene and poly(methyl
methacrylate), the polymer chains are randomly
configured, displaying no
significant
order. In crystalline plastics such as
polyethylene and nylon 6/6, the polymer chains are
aligned or ordered into crystallites.
Crystalline plastics,
however, are never completely crystalline, but
rather contain regions of amorphous
material. A few plastics, among them
PET and polycarbonate, can be entirely amorphous
or
semicrystalline, depending on
processing conditions. At room temperature, the
thermodynamically
favored state for
these plastics is the crystalline form; however,
if they are cooled rapidly enough from
the melt to below their glass-
transition temperatures
(T
g
), they will remain in
their amorphous forms.
Under normal
injection molding conditions, parts made from such
plastics are clear, indicating the
absence of crystallinity. If the
finished parts are heated to above
T
g
or are exposed to strong
solvents,
they will crystallize. The
latter phenomenon is often called solvent-induced
crystallization, and was
observed
during the absorption studies discussed above.
The nominally amorphous
PET, PCTG, and polycarbonate films turned opaque
upon exposure to certain
solvents (see
italicized data in Table III), indicating that
crystallization had occurred. Absorption of
these liquids decreased the
T
g
's of the plastics to at
least ambient conditions, giving the polymer
chains
sufficient mobility to align and
crystallize. It was noted that crystallinity
developed much more quickly
in PET and
PCTG than in polycarbonate, primarily because the
copolyesters have much lower
T
g
's
(approximately 80°
C) than
does polycarbonate (approximately
150°
C), and therefore needed to absorb
less liquid before
T
g
was depressed to ambient
temperature. Solvent-induced crystallization may
have
a pronounced effect on stress-
cracking behavior, as will be discussed later.
Although both amorphous and
crystalline plastics are susceptible to
environmental stress cracking, it is
generally recognized that amorphous
plastics tend to be more at risk.
3
–
5
The closely packed
crystalline
domains in crystalline
plastics act as barriers to fluid penetration and
therefore tend to resist
environmental
stress cracking.
THE
TENSILE STRESS
Plastics will exhibit environmental
stress cracking when exposed to an aggressive
chemical
environment if and only if a
tensile stress is present. The tensile stress may
be applied externally or may
simply be
a consequence of residual, or molded-in, stresses.
Residual stresses can be minimized
through the use of proper design
guidelines and the control of critical variables
in the injection molding
process.
13
Externally applied stresses can result from
subassembly processes, shipping and storage
conditions, or improper packing. An
externally applied tensile stress may also be part
of the intended
end-use of the device.
A female luer, for example, may be subjected to
extremely high hoop stresses
upon
insertion of the male
luer.
14
Obviously, the most reliable method for
evaluating the stress-crack resistance of a
plastic in a given
application is to
analyze its performance under simulated end-use
conditions. Alternatively,
stress-crack
resistance can be determined by some type of
standard testing procedure whose results
can be related to the stress and strain
levels observed in end-use conditions. A few of
the numerous
tests that have been
developed to evaluate environmental stress-crack
resistance are listed in the box
on
page 41. The tests differ primarily in the way the
external stress is applied.
ASTM D 1693 describes a test for
evaluating the stress-crack resistance of ethylene
plastics in
environments such as soaps,
wetting agents, oils, or detergents. Strips of a
plastic, each containing a
controlled
defect, are placed in a bending rig and exposed to
a stress-cracking agent. The number of
specimens that crack over a given time
is recorded.
ISO 4600
details a ball or pin impression method for
determining stress-crack resistance. In this
procedure, a hole of specified diameter
is drilled in the plastic. An oversized ball or
pin is inserted in the
hole and the
plastic is exposed to a stress-cracking agent.
After exposure, tensile or flexural tests may
be performed on the specimen.
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