Article Type: Research Article Article Citation: J.A. dos Santos,
R.C. Tucunduva, and J.R.M. d’Almeida
(2021). MECHANICAL CHARACTERIZATION OF PA 12 AND HDPE PIPES BEFORE AND AFTER
AGING IN WATER AT TWO DIFFERENT TEMPERATURES. International Journal of Research
-GRANTHAALAYAH, 9(1), 248-256. https://doi.org/10.29121/granthaalayah.v9.i1.2021.2973 Received Date: 03 January 2021 Accepted Date: 31 January 2021 Keywords: HDPE PA12 Aging Mechanical
Behavior Glass Transition
Temperature Polymer pipes are being widely used by many industrial segments. Although not affected by corrosion, the mechanical performance of these pipes can be reduced due to exposure to temperature, UV radiation and by contact with various fluids. Depending on the deterioration process, embrittlement or plasticization may occur, and the service life of the pipe can be severely reduced. In this work, the combined action of temperature and water upon the mechanical performance of polyamide 12 and high-density polyethylene pipes is evaluated. Destructive and non-destructive techniques were used and the performance of both materials was compared. Both polymers were platicized by the effect of water. However, for high density polyethylene the effect of temperature was more relevant than for polyamide. This behavior was attributed to the dependence of the free volume with the markedly different glass transition temperature of the polymers and the temperatures of testing.
1. INTRODUCTIONNowadays
polymer pipes are increasingly being used to transport fluids in various
industrial segments. The oil & gas segment is, for example, one where the demand for
plastic pipes has significantly increased over the past years [1]. Another segment in which the polymer tubes are widely used
is the transportation and distribution of drinking water, where it is estimated
that more than 69% of the system is formed by plastic tubes [2]. These
pipes have as main advantages low density and resistance to corrosion. Very relevant too is the fact that the cost of assembly and
maintenance of polymer pipes can be very cost effective when compared to those
of steel pipes. High
density polyethylene (HDPE) and polyamide (PA) are two of the most largely used
polymers used to manufacture pipes. This is due to the good set of properties
these polymers possess, especially in relation to chemical stability over
various chemical compounds, such as gasoline [3]and lubricant oil [4]. However,
the mechanical properties of polymers can deteriorate during their service
lives due to several factors, including prolonged exposure to UV radiation,
chemicals, water and temperature [5]. Combination of these factors can greatly
reduce the expected service life. For example, the predicted life of
polybutylene pipes designed for water transportation was severely reduced from
> 50 years to ~15 years, due to the combined effect of temperature and
chlorine added to domestic water as a disinfectant [6]. In many instances, deterioration results in
embrittlement of the polymer and this can be a severe issue for the performance
of the polymer part or assembly [5]. When pipes are used under the ground, embrittlement
can lead to crack initiation and can cause leakage of the transported fluid,
for example, due to the loads caused by normal traffic on a street under which
the pipe is. On the other hand, the water absorption can plasticize the
polymer, which can cause excessive permanent deformation in the polymer part[7]. In this
work, a study was undertaken to analyze and compare the mechanical behavior of
PA12 and HDPE after exposure of these materials to water aging under different
temperatures. In addition to destructive tests, the evaluation of the mechanical
behavior of these polymers was also performed using non-destructive analyses. 2. MATERIALS AND METHODSCommercial
high-density polyethylene (HDPE) and polyamide 12 (PA12) pipes were used in
this work. The nominal diameter and thickness of the pipes were, respectively,
110 mm and 10 mm for both materials. The PA12 used in this work is designed to
withstand higher operating pressures and has a glass transition temperature (Tg) of 36°C, as specified by its manufacturer. The HDPE
pipe is also designed to operate pressurized and has a Tg
of -110°C. Specimens
were machined from the pipes in order to perform the
mechanical tests, following the geometry and dimensions appropriate to each
test. Table 1 summarizes the tests performed, the corresponding standards and also some remarks regarding specific details of the
geometry of the specimens. Table 1: Summary of the tests performed.
The
tensile, impact and sonic test samples were aged in tap water for a period of 6
months. Part of the samples was aged at room temperature (23 ± 2°C) and part
was aged at 70 ± 1°C. This temperature was chosen because it is close to the
maximum normal working temperature of polymeric pipes. The parallel plate
loading samples were aged at the same experimental conditions for a period of 7
months. The tensile tests were performed using test
equipment with a capacity of 10 kN. The test speed
used was 50 mm/min and 5 specimens were tested in the as-received condition and
after 6 months of immersion in water. The objective of these tests was to
evaluate the variation in the yield stress of the polymers due to aging. Thus,
the tests were performed only up to a deformation of 10%, beyond the yield
stress. Izod impact tests were performed on an equipment with 5.4 J capacity.
Ten samples were tested for each material condition. The
parallel plate loading test was performed on a test equipment with a capacity
of 30 kN. The compressive load was applied at a
constant rate of 12.5 ± 0.5 mm/min. As recommended by ASTM standard D 2412, the
test was stopped when a reduction of 40% of the original pipe diameter is
achieved. Three specimens were tested for each material condition. The elastic
recovery of these samples was evaluated by measuring the variation of the
diameter as a function of time. Periodic measurements were carried out over 4
months. Measurements were performed using a ruler caliper with an accuracy of
0.2 mm. For the first 10 days measurements were taken at 24-hour intervals.
After this period the measurements were carried out with an interval of 10 days
during 4 months. After 4 months, the variations of the
measurements obtained were on the precision scale of the measuring instrument.
Thus, the measurement procedure was ended. The impulse excitation tests were performed using an electret
microphone with a detection range varying from 0.5 to 20 kHz, and an acquisition
time of 0.6 s. The signal acquired was processed by dedicated software (SonelasticÒ). The
samples were first
measured using a ruler caliper within ± 0.01 mm and weighted within ± 0.0001g. The
test configuration used is shown at Fig.1, where the relative position between
the actuator (A) and the microphone (B) corresponds to the flexional-torsional
boundary condition [8].
The results reported are the average of 10 measurements per specimen. Figure 1:
Test configuration of the impulse excitation tests. The actuator (A) and the
microphone (B) are highlighted. The fracture surface of the Izod specimens
was analyzed by scanning electron microscopy (SEM). The analysis was performed
using secondary electrons, with electron beam voltage of 20 kV and current of
40 mA on gold sputtered surfaces. 3. RESULTS AND DISCUSSIONSThe results of the tensile tests are
summarized in Table 2. Although the standard deviation of the results is high,
it is possible to observe that the mean values of the yield stress are reduced
and those of the yield strain are increased due to aging. For HDPE the aging at 70 °C caused the greatest variations on
yield point in relation to the as-received material. For PA 12, the effect of
temperature does not seem to be as effective and the effect of water absorption
seems to be more important. The percentage variation of the mean values is also
presented in parentheses in Table 2. Table 2: Yield stress and strain before and after aging at room temperature (RT) and at 70°C.
Fig.2 shows appearance changes undergone by
the HDPE due to the aging process. Fig.2a shows the as-received material. Figs.
2b and 2c show the material aged at room temperature and at 70 °C, respectively.
It can be seen that the original orange and reflective
surface is turning dark orange and opaque. For PA 12 the visual changes were
much more subtle, and just a reduction of gloss was observed after aging. The difference of behavior between the two
polymers may be related to several factors, which affect the absorption of the
fluid by the polymer, namely: i) the polarity of the
polymeric chain[9];
ii) the degree of crystallinity of the polymer [10],
and iii) the polymer to fluid affinity [11].
Besides, the difference between the glass transition temperature of each
polymer and the aging temperatures can be important because it affects the free
volume of the polymer [12]. In respect to the polarity of the polymer
chain and its influence upon absorption of water, it is expected that polar
polymers are to be more sensitive to the diffusion process than apolar polymers [9],[13]. On the other hand, in relation to the
degree of crystallinity, the higher the value of the degree of crystallinity,
the lower the diffusivity, because the crystalline regions are more compact
than the amorphous regions. Therefore, analyzing only these aspects it would be
expected that HDPE, which is a polymer with a high degree of crystallinity, would
be less affected than polyamide, which in addition to lower crystallinity, is a
polar polymer. Figure 2: Variation of the aspect of HDPE tensile
specimens as a function of aging: a) as-received; b):
aged at room temperature; c) aged at 70°C. The Flory-Huggins
equation can be used to infer the interaction between HDPE and PA 12 with
water. Namely,
(1) where
χ12 is the interaction coefficient that indicates the affinity
of a polymer with a solvent, V is the molar volume of the solvent, R is the
perfect gas constant, T is the temperature (K), δ1 and δ2
are, respectively, the Hidelbrand solubility
parameter of the solvent and of the polymer and β is the entropy term (≅ 0.34) [11],[14]. From this equation, and using the following
Hansen solubility parameters for polyethylene (δ2 = 17.6), for
polyamide (δ2 = 22.8) and for water (δ1 = 47.8)
[14], it can be observed that the
interaction coefficient, χ12, for HDPE-water is higher than the
polyamide-water interaction coefficient, because HDPE > PA12. This
analysis, therefore, indicates that the diffusion of water in HDPE is to be
more difficult than the diffusion of the water in the polyamide, because if the
difference between the parameters is small, the liquid-polymer affinity is high and the diffusion of the liquid will be favored [11]. Thus, the
greater variation of the properties of the HDPE over that of the polyamide cannot
be explained solely on the basis of the water /
polymer affinity or the parameters of crystallinity and polarity. The
relationship between the aging temperatures and the glass transition
temperatures of the two polymers seems to be the main parameter to be
considered. In fact, as previously mentioned, the difference between these
temperatures affects the fraction of the free volume. The glass transition temperature of the
polyamide (35°C to
45°C) is near the aging temperatures, while the Tg
of the polyethylene (-110°C)
is well below both temperatures (i.e., 23 and 70°C). As the free-volume
fraction is proportional to T-Tg, the free volume
available for water diffusion in HDPE will be greater than in the polyamide [11].
This may result in a higher plasticizing effect when aging occurs at 70°C than at room
temperature for HDPE, but not for PA. Therefore, the temperature had a much
greater relevance for HDPE than for PA, including the variation of the visual
appearance of the samples. The results of the Izod impact test are shown
in Fig.3. These results are in agreement with the
trend observed in the tensile test, i.e., the temperature plays an important
role for the aging of HDPE, but not in relation to the PA.
(a)
(b) Figure 3: Izod test: a) HDPE; b) PA12. The overall trend of the results of the
impact tests also agrees with the observation that the material is being
plasticized by water, as expected. Therefore, the impact energy increases after
aging, with the exception of the HDPE samples aged at
room temperature. SEM fractographic analysis of the samples corroborates the
increase of impact strength after aging. The fracture surface of the
as-received specimens shows the usual pattern of mirror, mist
and hackle regions common to polymers, Fig.4a [15],[17].
Figure 4: Fracture surface of the impact specimens:
(a) Overall view, showing the common mirror, mist and
hackle regions (HDPE, 23x); (b) Characteristic features of the mist to hackle
transition region (HDPE, 200x); (c) Idem (PA, 200x); (d) After aging long
striae are present at the mist to hackle region (HDPE, 200x); (e) Idem (PA,
200x). At the mist to hackle transition, incipient
striae can be observed, Figs. 4b and 4c, indicating a certain degree of plastic
deformation capacity [18].
However, after aging very long striae are observed,
Figs.4d and 4e, and, since striae are formed by shear and are indicative of
deformation processes in the polymer [18],
their presence is indicative that the deformability of the polymers was
increased due to water absorption. It
should be noted, however, that with the SEM analysis, it was not possible to
see differences between the striae formed after aging at room temperature and
70°C. As stated
by ASTM D 2412-08 standard, the parallel plate test has as its prime result the absence
of cracks at the surface of the pipe, at the point where the tensile stress is
at its maximum. None of the samples showed cracks, as exemplified in Fig.5. The
trace of the load-elongation curves of the parallel plate loading test are
shown in Fig.6. For PA12 (Fig.6a), the test results show that the curves of the
as-received samples fall above the curve of the samples aged at room
temperature and at 70°C. This result agrees with the plasticizing effect of
water and emphasizes that, for the same applied load, the aged samples will
present larger deformation than the as-received material. Figure 5: Naked
eye inspection of the tensile surface at the parallel plate load test. Figure 6: Parallel-plate test results for: a) PA; b)
HDPE. However,
and unlike what was observed in the tensile and impact results, the samples
aged at 70 °C were more affected than those aged at room temperature. This can
be clearly seen in Table 3, which lists the stiffness factor, SF, calculated according to equation 2,
described in ASTM D 2412-08 standard, namely:
(N.m) (2) Where F/x is the force per unit
length of the pipe, △y is the deflection at a reduction of 40% of the original pipe
diameter, and r is the mean radius of the
pipe. For HDPE,
as can be observed in Fig.6b, the results of the as-received samples and those
aged at room temperature are almost identical. The results of these two samples
fall above the curve of samples aged at 70 °C, indicating, again, that
temperature affects HDPE more severely than PA. The variation of the stiffness
factor for the HDPE is also listed in Table 3. Table 3: Parallel plate test: Stiffness factor (SF) at 40% reduction of the original pipe diameter.
From a
practical point of view, as important as the resistance of a polymer pipe to a
crushing load is its ability to recover its original shape after load removal.
This will involve, for example, resuming fluid flow without considerable energy
loss. Fig.7 shows the measured elastic recovery for the HDPE and PA 12 tubes
after removal of the crushing load. It is worth remembering that the diameter
of the specimens was reduced by 40% during the application of the load. For
both materials it is possible to see that there is a large instantaneous
recovery, greater than 80%, once the load is removed. After 3,300 h (140 days)
the recovery of the original diameter of the PA 12 samples was about 94% and
for HDPE was about 96%. In fact, for both materials the elastic recovery
stabilized after ~500 h. For PA 12
there was no statistically significant difference between the elastic recovery
of the samples as-received and the aged ones (based on the results of unicaudal t-Student
test with 95% confidence level). For HDPE the elastic recovery curves of the
samples as-received and those aged at room temperature
were similar. The curve for the samples aged at 70 ° C presents a slightly
lower elastic recovery, in agreement with the previous results indicating that
temperature had the most significant effect on the HDPE. It should be
emphasized, however, that the differences obtained are not statistically
different, based on Student's t test
results. Figure 7: Elastic recovery after reduction
of the diameter of the specimens by 40%, and removal of the applied load. a)
PA; b) HDPE. For a better visualization different symbols were used at each
aging condition (, D, O). The color indicates a common
aging condition. The
results of the impulse excitation test showed that the modulus of elasticity
decreased after aging. This behavior also agrees with the data so far presented, and indicates that both materials are affected by
their contact with water. However, as already observed PA12 is less affected
than HDPE. For the PA12, the modulus of elasticity variation was of - 4.8% and
- 9.0%, respectively, after aging at room temperature and at 70°C. When HDPE is
considered an increase of 2.0% was measured when the specimens were aged at
room temperature and a decrease of - 35.0% after aging at 70°C. The results
obtained is shown in Fig.8. The
results of the impulse excitation test showed that the modulus of elasticity
decreased after aging. This behavior also agrees with the data so far presented, and indicates that both materials are affected by
their contact with water. However, as already observed PA12 is less affected
than HDPE. For the PA12, the modulus of elasticity variation was of - 4.8% and
- 9.0%, respectively, after aging at room temperature and at 70°C. When HDPE is
considered an increase of 2.0% was measured when the specimens were aged at
room temperature and a decrease of - 35.0% after aging at 70°C. The variation
of the results obtained is shown in Fig.8. 4. CONCLUSIONS AND RECOMMENDATIONSThe
objective of this study was to evaluate the effects of exposure to water and
temperature on PA12 and HDPE pipes. The results indicate that, as expected,
water acts as a plasticizer and that the temperature accelerates the
degradation process. Therefore, the samples aged at 70°C showed a greater
variation of properties. It was shown that the greater variation of the
properties of HDPE in respect to that of PA12 cannot be explained solely on the basis of the water/polymer affinity or the parameters
of crystallinity and polarity. In fact, water diffusion into a polymer is
dependent on the free volume of the polymer, which increases with increasing
temperature, but it is also dependent on the glass transition temperature of
each polymer. Thus, the effect caused by aging at 70 °C on HDPE was more
pronounced than that on PA12, and the difference of behavior between PA12 and
HDPE in respect to the different destructive and non-destructive test results
was explained based not only on the aging test conditions but on the difference
between the glass transition temperature of each polymer and the test
temperatures. The very high elastic recovery showed by the two polymers indicate that under the experimental conditions of this work, physical aging due to diffusion of water is probably the main degradation mechanism. From a practical point of view this implies that after removal of the external load the pipes almost recover their original shape, and fluid flow can be resumed without significant energy loss.
Figure 8: Variation of the elastic modulus of the aged specimens relative to the as-received materials SOURCES OF FUNDINGThis research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. CONFLICT OF INTERESTThe author have declared that no competing interests exist. ACKNOWLEDGMENTThe authors acknowledge the
Brazilian Agencies CNPq (Grant number: 307363/2018-0)
and CAPES (Finance Code 001). REFERENCES
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