1. MECHANICAL RESPONSE OF AGEING AND ANNEALING ON INJECTION
MOLDED HIGH DENSITY POLYETHYLENE
Reaj U Ahmed, Niagara Bottling, LLC, Ontario, CA
Jay C Hanan, Oklahoma State University, Tulsa, OK
Abstract
Short term stress relaxation tests were performed to
observe mechanical effects of ageing and annealing on
injection molded High Density Polyethylene (HDPE).
The investigation revealed no definite relationship
between ageing time and relaxed stress. Two different
annealing parameters were used to investigate the effect
of annealing on injection molded samples. Samples
annealed for longer time showed more consistency and
higher relaxation modulus than the samples annealed for
shorter time and the control samples. The effect of ageing
on annealed samples was also studied in this work.
Introduction
High Density Polyethylene (HDPE) is a thermoplastic
and represents the largest portion of PE produced. Due to
its higher density and crystallinity, HDPE is much stiffer
and stronger than LDPE. HDPE is efficient for injection
molding, blow molding, and extruded items, which makes
it a preferred material in manufacturing products like
personal care, household industrial containers, and bottles.
HDPE is universally recognized as a ‘safer’ plastic and
meets the U. S. Food and Drug Administration (FDA)
requirements for direct food contact applications. HDPE
is more often used in the container of liquids that are
stored at atmospheric pressure [1,2,3]. After production,
these containers are stored in plants or distribution center
warehouses for extended periods and so time dependent
properties like “ageing” can become significant.
Moreover, in production processes like injection molding,
parts are subjected to rapid cooling which introduces
thermal residual stress. These thermal stresses are
expected to relax with increasing ageing time [4].
Removal of waste is a driving force in industry, which
results in removal of unnecessary materials from existing
designs and adds value to alternative approaches for
strengthening materials. Among different approaches,
annealing is one of the popular approaches to modify
polymers as an annealing unit can be incorporated easily
to existing manufacturing processes. Annealing is
assumed to improve the final properties of a polymer via
reduction or elimination of solidification defects, residual
stress, and strain. Motivated by all these reasons, ageing
and annealing in HDPE were studied here with an
emphasis on mechanical performance. Most research
work on HDPE have been done on samples fabricated by
compression molding in lab scale equipment. As the
properties of polymers are highly process dependent, the
behavior of HDPE parts manufactured by different
laboratory processes may be incomplete compared to
industry practice. In this work, HDPE samples were
collected from one piece HDPE closures produced by an
industry scale injection molding machine. Stress
relaxation was set as criteria to determine the effect of
annealing and ageing. Two different annealing
parameters were used to investigate the effect of
annealing on two sets of samples. Samples were air
cooled and the effect of ageing on annealed samples was
also studied.
Ageing of polymers has been studied by several
researchers. Iacopi and White [5] showed the effect of
ageing on injection molded polystyrene bars and found
higher density of the bar with more ageing time. Struik
[6] studied the physical ageing of amorphous materials
and examined creep behavior of materials in different
states of ageing. Struik’s data and analysis were in good
agreement with the engineering properties. Several
researchers have concentrated on the ageing of
semicrystalline polymers. Besides the contribution of the
amorphous phase of semi-crystalline polymers, secondary
crystallinity [7,8] and molecular rearrangement can be a
potential source of ageing in semi-crystalline polymers.
This has been observed and analyzed by White [9],
Siegmann and Kenig [10]. McCrum [11] considered the
effect of crystallinity in his investigation, but Struik [12]
claimed that secondary crystallization is not strong
enough to explain his ageing observation with
polypropylene. In some of his other papers [13, 14, 15,
16] Struik concentrated on the behavior of the amorphous
phase to interpret the physical ageing of semi-crystalline
polymers. He explained the effect of the presence of the
crystal unit in the amorphous phase on molecular
relaxation [17]. A model describing the effect of physical
ageing on the stress relaxation behavior of HDPE at room
temperature was developed by Kubat et al [18]. The
model was based on the separation of the amorphous
phase into two parts as suggested by Struik [21]. Kubat et
al [19] studied the viscoelastic properties of clay filled
and unfilled HDPE after different ageing times on
samples annealed for 8 hours at 120o
C. They used
compression molded samples and found ageing time had a
pronounced influence on the viscoelastic properties of
HDPE. They found the internal stress (evaluated from
stress relaxation data) was increased markedly when the
ageing period was extended. In another paper Kubat et al.

2. [20] studied the effect of ageing on internal stress of
HDPE filled with glass spheres. They found the internal
stress increased with the degree of interaction of the
filler/matrix interface and the ageing time. They found
the surface treatment of the filler had a significant effect
on creep behavior at high applied stress levels and on
ageing behavior of the composites. Welander et al. [21]
studied the influence of high stress on the ageing behavior
of HDPE. They found high applied stress and strain erase
the effect of ageing time on the creep and stress relaxation
behavior of HDPE. They observed change due to ageing
was not same at low and high applied stresses. All of the
previous work regarding ageing of HDPE were
concentrated on samples aged for longer times. The
change of properties at shorter time intervals after
solidification was not studied fully, which with industry’s
trend toward just in time production is important for
understanding the compatibility of different final
products. This motivated us to investigate the change of
HDPE properties at times relevant for many modern
production processes.
Annealing of polyethylene involves reheating of the
polymers for a period of time after they have been
processed. Polymeric molecules solidify in a stressed
position during the manufacturing process such as
injection molding or extrusion. A sudden drop of
temperature during molding freezes the molecules at non-
equilibrium positions influenced by the melt inside the
mold. Annealing relaxes these frozen or residual stresses.
Crystallinity also changes by annealing. Crystals can
grow over time until reaching equilibrium at the annealing
temperature. Relaxation of the residual or frozen stresses
can improve tensile strength, impact strength, and slow
crack growth characteristics [22]. While rapid cooling
suppresses the formation of crystals and gives tough, clear
products, annealing or slow cooling provides relatively
brittle and hazy products [23]. Annealing at high pressure
also leads to the formation of large crystallites with
relaxation of stressed chains and, hence, increases
molecular mobility in amorphous regions [24].
Tiemprateeba et al. [25] studied the effect of annealing on
neat and CaCO3 filled HDPE, and reported that melting
temperature, crystallinity, modulus, and tensile strength
increase with increasing annealing temperature. They
observed the increase of these properties stabilizes at an
annealing temperature of 135-140o
C, and further
increasing of temperature did not significantly alter the
properties. The modulus and yield stress of the samples
increased, at annealing temperatures (Ta) greater than
120o
C. In their tests, yield strain did not show relevance
with annealing temperature. They found for the filled
composites, the degree of crystallinity, and melting
temperature did not depend on the percentage of filler
materials, but depended on annealing temperatures
(increased as Ta increases). Annealing showed a much
higher modulus and tensile strength for the filled
composites. Suwanprateeb [26] used vickers
microhardness and thermal analysis (DSC) for studying
the annealing condition of HDPE and found increasing
annealing temperature increased properties like
crystallinity, melting temperature, tensile modulus, tensile
yield strength, and microhardness. They found
microhardness can be employed as an alternative to
tensile testing in studying the annealing conditions for
HDPE. Tsuruta et al. [27] studied the annealing
characteristics of HDPE fibers extruded at different
constant extrusion draw ratios. Annealing can improve
the modulus more than radiation effects. Suwanprateeb et
al. [28] showed that thermal ageing (annealing) can
improve the modulus for both neat HDPE and
hydroxyapatite (HA)-HDPE composites, while gamma
radiation did not show such significant increases in
modulus values. They observed around a 4% increases in
crystallinity due to annealing of the samples. All these
studies regarding annealing motivates an investigation
into the effect of annealing on stress relaxation of
injection molded samples.
Experimental
Materials and Samples
Samples for this research work were cut from
injection molded closures. High Density Polyethylene
(HDPE) with a melt flow index 11.0 g/10 min and a
density of 0.951 g/cc were used as resin. A Husky
Hylectric 300 was used for injection molding of the
closures. Resin was processed at 201o
C barrel
temperature and 225o
C mold temperature with 0.7 sec
cooling time. Closures were collected immediately after
production. Uniform rectangular samples with
dimensions of 15mm×5 mm were cut from the closures
for observation by DMA.
Ageing
For investigation of the ageing effects, a group of
closures were collected just after injection molding and
short term stress relaxation tests were performed after
increasing lengths of time of 15 to 30 min each, as
indicated in the results section.
Annealing
For annealing, two different annealing parameters
were set for two groups of samples. The first group was
annealed at 125o
C for 30 minutes and the second group
was annealed at 125o
C for 1 hour in an oven. Both groups
of samples were air cooled after annealing.

3. Stress Relaxation
ASTM D 4065 was followed and a tension film
clamp was used in the DMA measurements. Stress
relaxation tests were conducted at 0.9% static strain with
0.001 N static force for 15 to 20 minutes. All of the tests
were performed at 30o
C with a 1 minute soak time.
Results and Discussion
Ageing
The ageing effect on HDPE was investigated for both
non-annealed and annealed samples.
Non-annealed samples
Figure 1 and Figure 2 shows stress relaxation curves
for non-annealed samples. A wide range of relaxed stress
is observed in Figure 2. No definite relationship was
found between the ageing time and the relaxed stress.
Figure 3 shows the stress at different times of stress
relaxation tests for the aged samples. The reason for
getting a wide range of stress relaxation curves can be
explained by the concept of molecular equilibrium. As
samples were collecected from injection molded caps,
some residual stress could be trapped during rapid cooling
at demoulding and molecules of the polymers were not at
equilibrium. As a result, a wide variety of stress
relaxation behavior was observed.
31
88
159
230
315
492
562
1972
0
0.5
1
1.5
2
2.5
3
3.5
1 1 1 1 1 2 2 2 3 4 5 7 10 14 19
Stress(MPa)
Figure 1. 3D plot of relaxed stress in non-annealed sample
having increased age.
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Stress(MPa)
Relaxation Time (min)
31
61
88
119
159
194
230
282
315
463
492
522
562
1944
1972
Age (minute)
Figure 2. 2D plot of relaxed stress in non-annealed sample
having increased age
0
0.5
1
1.5
2
2.5
3
3.5
4
0 500 1000 1500 2000
Stress(MPa)
Ageing Time (min)
10 sec
2 min
4 min
9 min
15 min
Figure 3. Stress vs. ageing time at different test time
(Non-annealed sample).
Annealed Samples
Annealed for 30 minutes
Figure 4 and Figure 5 show the stress relaxation curves
for 30 minute annealed samples. Figure 6 depicts the
relaxed stress as a function of ageing time at different test
time of stress relaxation tests.
57
115
307
358
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 4 5 5 6 7 8 10 12 14 16 19 21
Stress(MPa)
Figure 4. 3D plot of relaxed stress in annealed (30min)
sample having increased age.

4. 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 2 4 6 8 10 12 14 16 18 20 22 24
Stress(MPa)
RelaxationTime (min)
57
85
115
166
307
331
358
Age (minute)
Figure 5. 2D plot of relaxed stress in annealed (30 min)
samples having increased age.
0
1
2
3
4
5
0 100 200 300 400
Stress(MPa)
Ageing Time (min)
10 sec
1 min
2 min
5 min
9 min
15 min
Figure 6. Stress vs. ageing time at different test time
(symbol). Samples were annealed at 1250
C for 30
minutes.
Annealed for 1 hour
Figure 7 and Figure 8 show stress relaxation for
different age samples with 1 hour of annealing. Figure 9
shows the effect of ageing time on relaxed stresses at
different test times. Unlike non-annealed and 30 minute
annealed samples, the stress relaxation curves for 1 hour
annealed samples are not widely distributed.
39
94
225
332
0
0.5
1
1.5
2
2.5
3
3.5
4
1 1 1 1 1 2 2 2 3 4 5 7 10 14
Stress(MPa)
Figure 7. 3D plot of relaxed stress in annealed (1 hour)
sample having increased age.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Stress(MPa)
RelaxationTime (min)
39
63
94
121
225
252
332
Age (minute)
Figure 8. 2D plot of relaxed stress in annealed (1 hour)
sample having increased age.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 100 200 300 400
Stress(MPa)
Ageing Time (min)
10 sec
2min
5 min
9 min
15 min
Figure 9. Stress vs. ageing time at different test time.
Samples were annealed at 125o
C for 1 hour.
Annealing accelerates ageing. These results suggest
annealing at 1 hour provided sufficient time and energy to
relax more residual stress letting the molecules of the
polymer settled in more equilibrium states than the non-
annealed and 30 minute annealed samples. However,
ageing after 1 hour annealing did not show a significant
influence on the magnitude of stress relaxation. On the
other hand, the samples which were annealed for 30
minutes had less time and energy to reach the equilibrium
then the samples which were annealed for 1 hour. As a
result, they were further from equilibrium and still some
residual stress could exist. So additional ageing after
annealing showed effects on the stress relaxation tests of
these samples. The samples which were not annealed at
all, showed a much wider range of stress relaxation curves
as their polymer chains were not in equilibrium. Figure
10 shows the standard deviation of stress as a function of
annealing time after 15 min of stress relaxation tests. The
standard deviation of average stress was found 32% less
for 1 hour annealed HDPE than the non-annealed HDPE.

5. 0
0.05
0.1
0.15
0.2
0.25
1
StandardDeviation(MPa)
Annealing time (hour)
0 0.5 1
Figure 10. Standard deviation of stress among different
samples as a function of annealing time after 15 min of
stress relaxation.
Annealing
Figure 11 shows the stress relaxation curves for
annealed and non-annealed samples. From the figure, the
relaxation modulus for 1 hour annealed HDPE is
significantly higher than that of non-annealed, and 30
minute annealed HDPE. Figure 12 depicts the
comparison of average stresses at different time of stress
relaxation tests for annealed and non-annealed HDPE.
HDPE annealed for 1 hour showed higher stress than the
other two shorter times throughout the entire stress
relaxation test. After 15 minutes of stress relaxation,
HDPE annealed for 1 hour held 24% higher average stress
than the non-annealed HDPE. At the beginning of the
relaxation tests, 30 minute annealed samples showed
slightly higher values of stresses than the non-annealed
samples. But as tests went on, stresses for 30 minute
samples were found lower than non-annealed samples
(Figure 12).
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Stress(MPa)
RelaxationTime (min)
1 H_A 94 min old
30 m A 358 min old
Non-Annealed 119 min old
Annealed for 1 hour
Annealed for 30 minutes
Figure 11. Plot of stress relaxation curves for annealed
and non-annealed HDPE.
0
1
2
3
4
1
Stress(MPa)
0
1
2
3
4
1
Stress(MPa)
b)
0
1
2
3
4
1
Stress(MPa)
c)
1 0.5 0
0
1
2
3
4
1
Stress(MPa)
d)
1 0.5 0
a)
AnnealingTime (hours) Annealing Time (hours)
Figure 12. Comparison of stresses for non-annealed and
annealed samples at 4 test times of stress relaxation
tests; a) after 10 seconds, b) after 2 minutes, c) after 9
minutes, d) after 15 minutes.
From Figure 11 and Figure 12 it is evident that 1 hour
annealing increases the relaxation modulus of neat HDPE.
The evidence here suggests that after 1 hour annealing,
residual stresses were generally relaxed as the polymer
chains had enough time to reach equilibrium. As
annealing can also increase the crystallinity of HDPE
[27], this could be another reason for the increase in
relaxation modulus.
For high speed manufacturing this one factor may be
the most important one uncovered in the present work.
Annealing reduces variation at least through minimizing
residual stresses. Reduced variation is important in
manufacturing. Less variation means fewer jams or other
inconsistencies during automated handling and application
of parts. Along with more consistent leak pressures,
increased stress relaxation should lead to increased seal
strength for HDPE used as closures.

6. Conclusions
The mechanical effect of short term ageing and
annealing was investigated on injection molded HDPE.
Short term changes in polymer mechanical properties
were speculated from industry experience but have now
been experimentally confirmed. Stress relaxation tests
were used to measure relevant changes in mechanical
properties from ageing and annealing. While, no definite
relationship was found between the ageing time and the
relaxed stress. Samples were also annealed at 125o
C for
30 minutes and 1 hour. Annealing for 1 hour showed
more consistency in stress relaxation than shorter times.
Consistency is critical for high speed manufacturing. In
addition to more consistency, after 15 minutes of stress
relaxation, HDPE annealed for 1 hour held a 24% higher
average stress than non-annealed HDPE. While
crystallization can also play a role, residual stress may be
the most significant source of change for these short
times. Future work is needed to measure the role of
crystallization and also to link these shorter time studies
with longer time scales.
Acknowledgement
The authors would like to acknowledge Dr. Elizabeth
J. Orwin, Harvey Mudd College for use of the DMA
Q800 instrument. The authors also acknowledge Niagara
Bottling, LLC for sponsoring the project and providing
the samples. This work is part of an industry sponsored
research program at Oklahoma State University’s
Helmerich Research Center.
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