Journal of Minerals & Materials Characterization & Engineering, Vol. 6, No.1, pp 1-16, 2007
jmmce.org Printed in the USA. All rights reserved
1
Effect of Changing Environments
on Microstructure of HDPE Polymer
S. Bal1, D. Mahesh2, T.K. Sen2 and B.C. Ray1
1: Department of Metallurgical and Materials Engineering, N. I. T, Rourkela -769008,
INDIA
2: Department of Chemical Engineering, N. I. T, Rourkela
Address for Correspondence: Dr. Smrutisikha Bal, B/17, N. I. T. Campus, Rourkela-8,
PIN-769008, Orissa, INDIA, Email:
ABSTRACT:
The analysis of environmental effects on microstructure of HDPE polymers were
done according to their mechanical behavior obtained from tensile, impact and
microhardness testing and thermograms obtained from DSC. The samples were
conditioned under three different kinds of humidity and temperature environment. With
increase in conditioning time, the variations of tensile strength, impact, microhardness
were observed to be nonlinear in nature. The high values are possibly due to
polymerization such as cross-linking. The low values may be attributed by
depolymerization mechanism such as chain scission and bond breakage. The variation of
data is due to more physical damage of the polymer because of unequal expansion of the
surface. The refrigeration also affects the crystalline regions by changing free volume.
Again rearrangement of end groups in the lamella through partial melting and
recrystallization are observed due to thermal conditioning and thermal shock.
Keywords: HDPE, Thermal cycling, Humidity cycling, Tensile, Impact,
Cross-linking, Degradation.
1. INTRODUCTION
Among the thermoplastic polymers, High Density Polyethylene (HDPE) is an
attractive material for scientific and technological studies due to its low cost, good
properties and versatility. It is also a material with potential for application in outdoor
exposure. For this reason, many studies have been conducted over the years to investigate
various aspects of its weathering behavior [1–8]. Environmental parameters acting on a
polymer such as temperature, humidity, chemical exposure, radiation, biological agents,
and their combinations significantly influence the strength of a polymer through
intervening structural changes. The effect of temperature on the stiffness of polymers is
probably most important with regard to the design and use of polymers for load bearing

2 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
engineering applications. The better mechanical properties of HDPE extend their use into
larger shapes, the sheet materials on the interior of appropriately designed vessels,
packing in columns, and solid containers to compete with glass and steel.
Humidity affects the performance of the polymer, as the polymer absorbs
moisture from air humidity and holds water molecules rather firmly by hydrogen
bonding, which causes a slow variation of properties like electrical characteristics,
mechanical strength and dimensions [9]. This information will prove to be useful to
product design engineers, mold designers, failure analysts and general plastics
practitioners in all phases of product design and development [10]. In general, the
permeability of plastics depends on crystallinity, molecular orientation, chain stiffness,
free volume, cohesive energy density, temperature, and moisture sensitivity [11-13]. The
presence of moisture in a polymer often accelerates the creep process, presumably
through a plasticization effect. Previous studies on polymer products have indicated that
the creep and dynamic loss can be significantly increased for conditions of changing
moisture content. This suggests that not only the moisture content, but also the sorption
history are important for determining the viscoelastic properties of polymeric fibers [14].
The moisture content of the polyethylene plays an important role in the formation and
transport of space charge caused by injected electrons and holes or ionization of
impurities, including by-products of cross-linking agents and catalyst residues. Polymer
structures, such as straight chain, branch chain, co-monomer and crystallinity, also
influence space charge formation and transport in various ways [15].
Thermal energy tends to lead to disorder and thus, strong secondary bonds are
necessary to attract polymer molecules in order to produce crystalline arrangements.
Semi-crystalline polymers like polyethylene maintain some rigidity beyond glass
transition temperature (Tg), though there is still a drop in modulus as the melting
temperature is approached.
Measurement of physical properties of materials by analytical techniques is being
used to investigate how the polymer is changing during the aging process. Tensile and
micro hardness tests measure the bulk property of the material that results from
crystallinity, lamella thickness and crystal size [16]. Tensile tests are used to measure the
force response when a sample is strained, compressed or sheared at a constant rate. These
provide a means to characterize the mechanical properties of a polymer in terms of
modulus, strength and elongation to failure which in turn depend on the entanglement
between molecules, their orientation and voids within that act as stress raisers initiating
cracks and premature failure. Impact tests measure the energy required for a sample to
fail under different loading histories [17]. Hardness is an important property as it is easily
and rapidly measured based on resistance to penetration by an indentor into the plastic
under constant load. Apart from quality assurance tests, hardness tests have been shown
to be able to determine changes in morphology and microstructure of polymers [18,19].
Micro hardness measurements have been used to correlate with properties like Young’s
modulus and compressive yield stress [20].

Vol.6, No.1 Effect of Changing Environment 3
On view of the above fact, in the present work, the effect of environmental
parameters such as temperature, moisture and thermal conditioning have been studied
macroscopically by tensile, impact and micro hardness testing on the HDPE polymers.
The objective of this work is to understand the change observed in the microstructure of
these polymers due to ageing by correlating them with the changes obtained in their
mechanical properties.
2. EXPERIMENTAL
2.1 Sample preparation
Raw materials of HDPE in the form of dried plastic pellets were subjected to
high-pressure injection molding machine to obtain the specimens for mechanical tests as
per ASTM standard. The samples were exposed to different thermal and humidity cycles
in a humidity chamber respectively. Another conditioning was done by subjecting
samples in a hot air circulating oven.
2.2 Environmental treatment
2.2.1 By changing temperature at specific humidity
One complete thermal cycle is defined as exposure of sample at 250C and 95%
humidity for 1 hour and at 500C and 95% humidity for another hour. In each case two
samples of the material were exposed, where one is tested directly and other sample is
frozen at -200C and then subjected to testing.
2.2.2 By changing humidity at specific temperature
One complete humidity cycle is defined as exposure of sample at 500C and 50%
humidity for 1 hour and at 500C and 95% humidity for another hour. In each case two
samples of the material were exposed, where one is tested directly and other sample is
frozen at -200C and then subjected to testing.
2.2.3 Thermal conditioning followed by quenching
The samples were then put into hot air circulating oven at the specified
temperature of 1000C for 5, 10, 20, 30, 40, 50 and 60 minutes, respectively. When the
required time was reached the oven was turned off and the specimens are immediately
dipped in water maintained at a temperature less than 50C. Two specimens of the material
are used as the standard specimens without conditioning.
2.3 Mechanical test
2.3.1. Tensile Testing
In the present study, tensile tests were performed on INSTRON 1195 in all the
three cases at a fixed crosshead speed of 10mm min-1. Samples were prepared according
to ASTM D 638 and tensile strength of standard and conditioned samples was calculated.

4 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
2.3.2. Impact Testing
In this test a fast moving hammer strikes a notched specimen (ASTM D 256) and
the energy absorbed in breaking a fast moving specimen is measured and impact strength
is noted from the calibrated scale in J.
2.3.3. Micro hardness testing
The sample was indented using Durometer following ASTM procedure No.
D2240 and the reading is noted from the calibrated scale.
2.4. Thermal analysis by Differential scanning calorimetry (DSC)
Thermal analysis using DSC (Netzsch STA 409C) is done only for samples
(2.2.3). Each composition was weighed and put into aluminum pans and are heated from
200C to 2000C at heating rate of 200C min-1. Data were collected continuously and then
analyzed by the DSC analysis programme. Melting temperature was detected as the peak
temperature of the main endotherm from the DSC graph.
3. RESULTS AND DISCUSSION:
With increase in number of thermal cycles at a specific humidity, the variations of
mechanical properties were studied. Both the unfrozen & frozen specimens when
exposed to less number of thermal cycles show an increase in tensile strength [Fig.1(a) &
Fig.1(b)]. This may be due to increase in inter-chain interaction that attributes cross-
linking of the carbon chain. When the number of thermal cycles increases, the elongation
increases while the tensile strength and modulus diminish. Impact strength of the polymer
samples shows a down trend initially because voiding may occur during crystallization
and also domination of thermomechanical stress due to thermal cycling, but in the later
case the difference of the data obtained. The reason is increase in free volume due to
refrigeration that affects the impact strength [Fig.1(c) & Fig. 1(d)]. The microhardness
variation [Fig.1(e) & Fig.1(f)] suggest that the aging process initially leads to a closer
packing of the material resulting in harder and denser products but prolonged aging
shows decrease in microhardness that lead to a brittle, mechanically weaker m aterial [21] .
The polymer specimens lose weight in first cycle as the desorption dominates when
compared to the absorption of the moisture, but as the number of cycles increases, the
gradual increase in the amount of moisture absorbed is observed [Fig.1(g) & Fig.1(h)].

Vol.6, No.1 Effect of Changing Environment 5
15
17
19
21
23
25
27
29
024681012
NO O F CYCLE S
TENSIL E STRENGT H(N/mm ^2 )
FIG 1(a). Effect of thermal cycling on Tensile strength of HDPE
21.5
22
22.5
23
23.5
24
24.5
024681012
NO OF CYCL ES
TENSI L E STRENGT H( N/ mm ^2 )
FIG 1(b). Effect of thermal cycling cum refrigeration on Tensile strength of HDPE

6 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
0
0.5
1
1.5
2
2.5
3
3.5
4
024681012
No OF CYCL ES
I MPACT ST RENGTH(J )
FIG 1(c). Effect of thermal cycling on Impact strength of HDPE
1
1.2
1.4
1.6
1.8
2
2.2
024681012
No OF CYCL E S
I MPACT ST RENGT H( J )
FIG 1(d). Effect of thermal cycling cum refrigeration on Impact strength of HDPE

Vol.6, No.1 Effect of Changing Environment 7
40
45
50
55
60
65
70
024681012
Time(min)
Microhardness
FIG 1(e). Effect of thermal cycling on Microhardness of HDPE
52
54
56
58
60
62
64
024681012
Time(min)
Microhardness
FIG 1(f). Effect of thermal cycling cum refrigeration on Microhardness of HDPE

8 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
-0.002
-0.001
0
0.001
0.002
0.003
0.004
0.005
024681012
No OF C YCLE
CHANGE IN WT
FIG 1(g). Amount of moisture absorbed by HDPE Impact specimens
-0.001
-0.0005
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
024681012
No OF CYCL E
CHANGE IN WT
FIG 1(h). Amount of moisture absorbed by HDPE tensile specimens

Vol.6, No.1 Effect of Changing Environment 9
With increase in number of humidity cycles at a specified temperature, the
mechanical properties such as tensile, impact and microhardness data shows a greater
variation with respect to refrigeration samples [Fig. 2(a) – Fig. 2(f)]. This is due to the
fact that the moisture is absorbed only on the surface of the specimen, which is not
penetrated into the core of the specimen, thus leading to the unequal expansion of the
polymer and damaging the sample physically [22]. In order to explain the data more
clearly this needs further investigation. It is observed that after the first humidity cycling
the specimens lose in weight of samples but on prolonged exposure the amount of
moisture induced in the sample increases [Fig. 2(g) & Fig. 2(h)].
20
21
22
23
24
25
26
024681012
Time(min)
Tensile strength(N/mm^2)
FIG 2(a).Effect of humidity cycling on Tensile strength of HDPE
15
17
19
21
23
25
27
024681012
Time(min)
Tensi le s t r e ngt h(N/ m m ^ 2 )
FIG 2(b).Effect of humidity cycling cum refrigeration on Tensile strength of HDPE

10 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
0
0.5
1
1.5
2
2.5
024681012
Time(min)
I m pact st r e ngt h( J)
FIG 2(c). Effect of humidity cycling on Impact strength of HDPE
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
024681012
Time(min)
I m pact st r engt h( J)
FIG 2(d). Effect of humidity cycling cum refrigeration on Impact strength of HDPE

Vol.6, No.1 Effect of Changing Environment 11
55
56
57
58
59
60
61
62
63
64
024681012
Time(min)
Micohardness
FIG 2(e).Effect of humidity cycling on Microhardness of HDPE
60
61
62
63
64
65
66
67
68
69
70
024681012
Time(min)
Microhardness
FIG 2(f). Effect of humidity cycling cum refrigeration on Microhardness HDPE

12 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
-0 .001
-0.0005
0
0.0005
0.001
0.0015
0.002
0.0025
024681012
Time(min)
Increase in weight(g)
FIG 2(g).Amount of moisture absorbed by HDPE impact specimen
-0.0004
-0.0002
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.0018
024681012
Time(min)
I ncrease in weight( g)
FIG 2(h).Amount of moisture absorbed by HDPE tensile specimens
The effect of thermal conditions on tensile, impact, microhardness and melting
temperature was obtained. In initial cycles, tensile strength increases because of increase

Vol.6, No.1 Effect of Changing Environment 13
in Tg value due to chain stiffness [Fig. 3(a)]. So the polymer becomes brittle since the
ductility is lost due to heavier crosslinking [23]. Both the microhardness and melting
temperature curves [Fig. 3(c) & Fig. 3(d)] show increasing trend at higher conditioning,
indicating crystallinity as well as Young’s modulus increase [24]. As the conditioning
time increases the depolymerization phenomena dominates, which leads to bond
breakages, chain scission and thus reducing the tensile strength of polymer. The impact
data shows a decrease in impact strength [Fig.3(b)] since the crystallinity becomes
sensitive to thermal shock, which produces a discontinuous change in free volume.
22
22.5
23
23.5
24
24.5
25
25.5
26
26.5
0 10203040506070
TIME (min)
TENS IL E S TRE NGT H(N / mm^2 )
FIG 3(a).Effect of thermal conditioning on Tensile strength of HDPE
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
0 10203040506070
Ti me ( m i n)
I m pact st r ngt h( J)
FIG 3(b).Effect of thermal conditioning on Impact strength of HDPE

14 S. Bal, D. Mehesh, T.K. Sen and B.C. Ray Vol.6, No.1
133.5
134
134.5
135
135.5
136
136.5
137
137.5
138
0 10203040506070
Time(min)
Melting Temperature('c)
58
58.5
59
59.5
60
60.5
61
61.5
0 10203040506070
TIME (min )
MIC ROHAR DNE SS
FIG 3(c).Effect of thermal conditioning on Microhardness of HDPE
FIG 3(d).Effect of thermal conditioning on Melting temperature of HDPE
4. CONCLUSIONS

Vol.6, No.1 Effect of Changing Environment 15
With the increase in the thermal and humidity cycling, the variations of tensile
strength, impact strength and microhardness are observed to be inconsistent. The
recurrence of the high and low values of these quantities is due to changes in the
organization of the system and morphological changes in the material. The high values
indicate an increase in the polymer’s crystallinity and/or the occurance of cross-linking
that includes entanglement of chains. The low values are due to depolymerization, which
includes either/both thermal degradation and moisture degradation that were attributed to
chain breaking. The refrigeration also affects the free volume of the polymer thus leading
to mechanical stress in the polymer. Slow cooling from melt or annealing just below
melting point produces thicker lamellae. Where long molecules emerge from the lamellae
they may crystallize in one or more adjacent lamellae thereby forming tie molecule. In
rapid cooling, some side branches may become incorporated as crystal defects in the
crystalline regions. Thermal condition data show a different behavior than the other two
cycles. The variation in the microhardness is attributed to the changes in surface because
of configurational rearrangement of end groups and by rapid changes induced by the
thermal shock at the surface. Near the melting point, the regular good lamellar stacking is
destroyed through partial melting and recrystallization, yielding a larger proportion of
new thin lamellae which contribute to the drastic hardness depression observed. At late
stages of aging voiding may occur during crystallization process resulting brittleness.
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