5 - Stress Cracking of Nylon Polymers in Aqueous Salt Solutions
5 - Stress Cracking of Nylon Polymers in Aqueous Salt Solutions
5 - Stress Cracking of Nylon Polymers in Aqueous Salt Solutions
Nylon-salt interactions
1. Introduction
Both organic liquids and inorganic salt solutions can
cause environmental stress cracking of nylon polymers [1, 2]. Results discussed in a previous paper [3]
demonstrated the role of polymer-liquid solubility in
the stress cracking of nylon by organic solvents.
Rapidly sorbed liquids were observed to plasticize the
surface, allowing homogeneous stress relaxation to
occur in competition with craze growth. To minimize
this surface plasticization effect a subsequent study of
nylon cracking in aqueous salt solutions employed a
fracture mechanics approach [4]. With this technique
the failure characteristics of razor-cut precracked
specimens of nylon polymers were examined. Preliminary results indicated that salt-induced cracking
occurred only at elevated temperatures (above 50 C).
Moreover, the total failure time was determined
largely by the craze growth kinetics. Crack growth
occurred rapidly only after a crazed region had
propagated across nearly the entire sample crosssection. In addition, the previous results showed that
nylon 6 was more susceptible to cracking than nylon
6,6, while nylon 11 was essentially immune to stress
cracking. Though differences in susceptibility for the
various polymers can be attributed to differences in
crystallinity and/or chemical structure (amide group
concentration), the temperature dependence of the
0022-2461/87 $03.00 + .12 1987 Chapman and Hall Ltd.
1715
2.8.
2.6.
Differential
scanning calorimetry
(DSC)
1716
property
measurements
3. Results
Tensile
100r
0
80
7O
.t
O~C~,
0
5
|
10
,
|
~
15
20
25
Time l j 2 ( h1/2)
,
30
35
40
35
30
t ~
w 25
c
"~
20
{15
10
5
0t
I
25
__
10
15
20
I
30
9
8
A
7
Na CI (50% saturation)
96
03
Na
CI (saturated)
4
3
2
1
0
10 11 12 13
Aqueous
solution
Concentration
(M)
Temperature
( C)
Weightgain
(%)
NaC1
CaC12
LiC1
MgC12
CaC12
LiC1
CaCI2
CaC12
LiC1
MgC12
6.1"
6.7*
15.0"
5.7*
5.7
5.7
3.3
6.7*
15.0*
5.7*
100
100
100
100
100
100
100
85
85
85
4.6
28.5
25.9
23.8
23.0
8.7
12.4
17.4
14.3
22.6
*Saturated solutions.
The weight gains for nylon 6,6 films immersed in
water or aqueous sodium chloride solutions are shown
in Fig. 3. As the concentration of salt increases the
maximum weight gain decreases. Thus the sodium
chloride acts as a diluent lowering the concentration
(and thus the activity) of the water with a resultant
decrease in the equilibrium sorption level of water.
This behaviour differs markedly from that observed fbr
nylon 6,6 films in the calcium chloride (Fig. 2).
Results for nylon 6,6 in lithium and magnesium
chloride solutions were similar to that in the calcium
chloride as shown in Table I. For these saturated
solutions at 100C the amount of absorbed salt and
water was greater than 20% in all cases. This demonstrates that all three of these salt solutions interact
strongly with the nylon in contrast to the results for
sodium chloride.
Solutions having equimolar concentrations were
also investigated by diluting the calcium and lithium
chloride to give 5.7 M solutions, the same as the saturated magnesium chloride. Weight gains for nylon 6,6
equilibrated in these solutions are also shown in
Table I. There is a significant reduction in the weight
gain in the diluted lithium chloride compared to the
saturated solution.
Data for nylon 6,6 in a calcium chloride solution
diluted to 50% of the saturation (3.3 M) is also shown
in Table I. The results suggest that the equilibrium
weight gain is directly proportional to the amount of
calcium chloride in the solution. For example, in 6.7 M
calcium chloride a weight gain of 28.5% was observed,
while in the 3.3 M solution the weight gain was 12.4%.
For direct comparison with craze growth kinetics
data (to be reported [6]), weight gains were a!so
measured at 85C. Compared to the higher temperature, the equilibrium weight gains decreased, with
magnesium chloride being the most soluble followed
by calcium and lithium chlorides. The precision in the
data is insufficient to determine whether the order of
solubility at 100C is reversed compared to 85 C.
Data for nylon 6 films are listed in Table II. The
results indicate that in the 50% saturated (3.3 M) calcium chloride the nylon 6 showed a similar weight
gain to the nylon 6,6 (12.9% compared with 12.4%,
respectively). Also in the equimolar (5.7 M) solutions
considerably less absorption occurred in the lithium
chloride. At 55 C, for the saturated solutions, a significantly higher sorption of aqueous magnesium
1717
Concentration
(M)
Temperature
( C)
Weight gain
(%)
CaC12
CaC12
CaC12
LiC1
MgC12
CaCl 2
LiCl
MgCl2
6.7*
3.3
5.7
5.7
5.7*
6.7*
15.0"
5.7*
i00
100
100
100
100
55
55
55
44.8
12.9
25.0
8.5
26.6
1.6
1.3
13.6
*Saturated solutions.
Temperature
( C)
Weight
gain
Composition (wt %)
(%)
Ca
CaC12
H20
Concentration
in film
(M)
Nylon 6
50
75
100
7.8
78.2
42.0
0.6
4.5
3.1
1.8
12.3
8.5
5.5
31.6
20.4
3.0
3.5
3.9
Nylon 6,6
50
75
100
3.0
22.0
28.5
0.2
2.3
3.1
0.6
6.3
8.5
2.4
11.7
13.8
2.4
4.9
5.6
1718
. . . .
. / r y D nylon
polymers [9], the infrared results suggest that the calcium chloride solution does not simply disrupt this
network, but in fact replaces it with some form of
association of the hydrated ions and amide groups in
the nylon.
The infrared spectra of the salt-equilibrated film
also shows a broad band at 3400cm -1 as a shoulder
on the sharper - N - H absorption band at 3300 cm -j .
In a previous study of a cast nylon film which contained only calcium chloride, (i.e. no water) an intense
broad absorption was also observed at this frequency
[10]. This was attributed to free, or non-hydrogenbonded, N - H groups in accordance with the results of
Trifan and Terenzi [9] for salt-free nylon. However, the
3400 cm-1 absorption reported here may also be attributed to the O - H groups of the absorbed water. A
similar, though less intense, absorption was noted in
this study in the infrared spectra of a nylon 6 film
which was saturated with water. Also, vacuum drying
at 100C decreased the intensity of this band. Assuming that this band at least in part is due to free N - H
groups, the existence of such groups seemingly contradicts the extensive broadening of the 3300 cm -1 band
on the lower-frequency side. That is, the absorbed
calcium chloride would appear to be causing an
increase in both free N - H and more associated N - H .
To avoid this contradiction it is tentatively suggested
that the 3400 cm-I band is entirely due to the large
amount of absorbed water in the nylon films used in
this study.
----.
Dry nylon
,f
6)
0t-
7-Equilibrated
in
,~ aqueous C a C I 2
I[
4000
3500
5108
3000
O0 1600
Wavenumber (ore -1 }
1400
TAB LE IV Analysis of nylon 6,6 films equilibrated in saturated aqueous salt solutions at 100C
Solution
CaClz
LiC1
MgCI2
NaC1
H20
Concentration
(M)
6.7
15.0
5.7
6.1
-
Weight
gain
(%)
Composition (%)
Cation
Salt
Water
28.5
25.9
23.8
4.6
8.5
3.1
0.6
1.1
0
0
8.5
3.4
4.4
0
0
13.8
17.1
t4.8
4.6
8.5
Concentration
in film
(M)
5.6
4.7
3.1
-
Description
M, ( x 104)
2~rw ( x 104)
mz ( 105)
Capron ER20
Zytel 2781
Zytel 2781
Zytel 2781
4.72
4.30
4.32
3.96
~ 60*
14.4
14.5
12.3
~ 800*
4.78
4.67
3.99
*Sample had a high molecular weight tail giving uncertain Mw and ~14z.
1719
Dry nylon6,6
Dry nylon6
SaNt
e:qUt~~
YuIg:at b~ast~ ~ ~
.u_
e-
"0
"0
e.
UJ
Second--heating
. ~ t ~ ~~ ~ ~ ~
' ~ - -
--u
Treatment
Crystallinity (%)
Extractables (%)
Nylon 6 (film)
None
Water,
Water,
CaClz,
CaCI~,
26
28
30
28
33
0
1.4
0.7, 0.7*
3.2, 5.6*
50 C
100 C
75 C
100 C
Nylon 6 (injection-moulded)
None
31
None
Water, 50 C
Water, 100 C
CaClz, 100C
29
34
33
32
0
1.2
0, 3.2*
None
37
*Repeated measurement.
1 720
--
[{a}
. . . .
re~ 130C
v
...I
-100
-50
100
150
200
Temperature (C)
Figure 7 Dynamic mechanical loss factor (tanS) against tem-
T A B L E V I I Tensile properties of nylon films equilibrated in saturated aqueous calcium chloride at 100 C*
Sample
Treatment
Modulus (GPa)
Tensile strength
(MPa)
Nylon 6
Vacuum-dried control
CaC12, 100C for 1.5h
1.06
0.32
42.4
24.8
Nylon 6,6
Vacuum-dried control
CaCI2, 100C for 24h
2.03
0.83
108.1
101.9
Elongation at
break (%)
8.0
18.5
125
178
*Modulus and strength were calculated using dimensions of dry films, not swollen film dimensions.
4. Discussion
The results of this investigation have shown that those
aqueous salt solutions which were previously shown
to cause crazing of nylon are also partially soluble in
the material at elevated temperatures. This includes
calcium, lithium, and magnesium chlorides. Sodium
chloride, which showed no tendency to induce crazing,
was also not absorbed into the nylon structure. The
chemical analysis confirmed that both salt ions and
water are absorbed into the nylon with the partitioning
of the salt and water being dependent on temperature
as well as on nylon type.
The crystallinity measurements of films previously
equilibrated in salt solutions suggested that the salt is
sorbed into the amorphous regions of nylon without
inducing significant changes in crystallinity. Similarly
no degradation in molecular weight was detected as a
result of salt-solution absorption. Thus the evidence
indicates that the sorption process is physically reversible and appears similar in this regard to the swelling
of amorphous polymers by organic solvents.
However, the ionic nature of the salt solution
coupled with the intermolecular hydrogen bonding of
1722
the nylon lead to a much more complicated polymerliquid interaction than the van der Waals type of
bonding in organic solvents. For example, the infrared
spectroscopy results indicated that the absorbed calcium chloride formed an association with the amide
groups in the nylon. This was observed in spite of the
large amount of water in the equilibrated films.
Evidence that the association of the amide groups
with the hydrated salt ions stiffens the nylon chains
was provided by the dynamic mechanical measurements which showed a shift of the glass transition to
higher temperatures. Such a shift is indicative of a
restricted mobility for the nylon polymer chains in the
amorphous phase in the presence of the aqueous salt
solution.
That absorbed salt alone restricts the amorphous
phase mobility is shown by the very high Tg for the
vacuum-dried film. DSC results also indicate an
increased Tg for the second heating of equilibrated
films. These films are also thought to contain very
little water. This confirms the observations previously
reported by others for nylon-salt systems [11]. For the
film equilibrated in the aqueous solution, the loss of
water during the dynamic test leaves some doubt as to
the precise location of T~. Since absorbed water
decreases Tg while salt alone increases it, one expects
an intermediate effect of the salt solution. The fact
that salt solution induced crazing occurs at temperatures just above the Tg of the dry nylon makes the
question of the Tg location of even greater interest.
For example, if absorbed salt raises the Tg, the sorption process should in turn be retarded. On the other
hand, a lowering of Tg is more compatible with solvent crazing models developed for amorphous glassy
polymers. Unfortunately the available evidence does
not allow a resolution of the question of the Tg of the
salt solution equilibrated nylon films.
The tensile property measurements, on the other
hand, indicate that the modulus is decreased as a
result of the sorption process. The inference is that the
Tg may also be reduced or at least substantially
broadened. The reduction in modulus is considered
very significant in terms of proposing a mechanism for
salt-solution-induced crazing. It establishes a parallel
between absorbed salt solution in nylon and absorbed
organic solvents in glassy polymers, since the latter
can also reduce modulus and increase tensile elongation. Thus relaxation controlled craze growth models
developed for solvent crazing of glassy polymers
[14, 15] may be equally applicable to salt solution
induced craze growth in crystalline nylon. This will be
examined in detail in the next paper in this series [6].
The results of the present study are compatible with a
4.
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7.
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9.
10.
11.
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14.
15.
Received 20 June
and accepted 18 August 1986
1723