Density Graded Polyethylene Foams Produced by Compression Moulding Using a Chemical Blowing Agent
Density Graded Polyethylene Foams
Produced by Compression Moulding
Using a Chemical Blowing Agent
Jiaolian Yao and Denis Rodrigue*
Department of Chemical Engineering and CERMA, Université Laval, Quebec City, Qc,
G1V 0A6, Canada
Summary
Density graded polyethylene foams were produced using a chemical blowing
agent and compression moulding. To control the local density inside the foams,
the top and bottom plates of a compression moulding set-up were placed at
different temperatures and moulding times in order to produce symmetric and
asymmetric structural foams. Due to different temperature gradients produced
inside the samples, complex density profiles were created. From the samples
obtained, a complete morphological analysis was performed to extract cell size
and cell density across foam thickness in relation with density profiles. Also, to
determine the effect of density profile on the mechanical properties, tensile and
flexural characterizations were performed. The results obtained are discussed in
relation with optimum properties vs. bulk foam density.
INTRODUCTION
Density graded materials (DGM), also called more generally functionally graded
materials (FGM), are composite materials where the local composition is
continuously changing with position. This change can be the result of different
particle structure, morphology, orientation and composition. Initially, the idea
behind the development of functionally graded materials was proposed in a
new composite concept for the Japanese Space Program [1]. Since 1990,
FGM were produced based on different types of matrices (metals, ceramics).
Nevertheless, few studies focused on polymers (thermoplastics, thermosets
and elastomers), their composites [2-8] or sandwich structures [9-11].
Recently, a great deal of effort has been done on modeling the properties of
these FGM [12].
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Jiaolian Yao and Denis Rodrigue
At the beginning, studies on FGM preparation and morphological
characterization were performed. Processing techniques include centrifugal
force fields, fibers stacking, temperature gradient and so on. For example,
functionally graded polyurethane foams were produce using ultrasound to
control porosity [13]. From the definition of functionally graded materials,
polymer structural foams having continuously changing density with position
are a sub-group of FGM. These functionally graded foams (FGF) can be the
result of different cell size, density, geometry or deformation/orientation with
position [14]. In particular, FGF have been shown to be ideally suited for
energy absorption (impact strength) [14].
For FGF production, two main processing approaches have been developed.
The first one is to impose a blowing agent concentration inside the part before
foaming [15-16]. The second option is to impose a temperature gradient inside
the mould [17-18] while foaming the part. Since cell nucleation, growth and
stabilization are functions of temperature and blowing agent concentration,
as well as pressure, a variation with position of any of these parameters will
induce a different local morphology.
In this work, a focus is made on one thermoplastic polymer (polyethylene)
with one chemical blowing agent (azodicarbonamide). Although hollow glass
beads (syntactic foams) [19] and expandable polymer beads [20-22] can be
used to produce FGF, they are not covered in this work. All the foams will be
produced by compression moulding via careful control of the top and bottom
plate temperatures. In particular, the effect of blowing agent content, as well
as mould temperatures and moulding time will be discussed in terms of foam
morphology (cell size and cell density), density profile, as well as flexural and
tensile moduli.
EXPERIMENTAL
Materials
As the matrix, a linear medium density polyethylene (LMDPE) in powder
form was used: HIVAL 103538 (Ashland, Canada). This polymer as a peak
melting temperature of 128°C, a density of 925 kg/m3 and a melt flow index of
3.5 g/10 min. For the chemical blowing agent (CBA), Celogen 754A (Chempoint,
USA) was used. All the materials were used as received.
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Sample Preparation
The samples were produced via compression moulding (Carver hot press).
As presented in earlier works for other materials, the procedure was
divided into three steps [20-22]. The first step is to blend the blowing agent
(Celogen) with the polymer (LMDPE). Since both materials are in a powder
form, a simple dry-mixing technique was used in a high shear mixer to get
homogeneous mixtures (about 2 min). Then, a specific amount of the powder
mixture (35.6 g) was placed inside an aluminium mould having dimensions of
110x110x3 mm3. Then, an unfoamed plate was produced by compression
moulding as the second step. In this case, the temperature was set at 135°C
and a constant force (2 tons) was applied. The heating cycle was: 2 minutes
of pre-heating without pressure, 5 minutes of pressure and 2 min of cooling
by water circulation before demoulding. Finally, the third step consists of
symmetric and asymmetric foam production. The plates prepared in step 2
were placed inside the mould where a constant force (2 tons) was applied.
In this final step (foaming), moulding time and temperature were controlled
as described in Table 1 with sample coding.
In Table 1, sample “LMDPE” is the neat unfoamed polyethylene (without
CBA) which is used for density calibration and reference for mechanical
properties (relative values). As described in Table 1, symmetric and
asymmetric samples were produced for comparison and to determine the
effect of moulding temperature profile on foam morphology and mechanical
properties. For symmetric samples, the upper (Tu) and bottom (Tb) plates of
the compression moulding press were set at the same temperature, while
different temperatures were imposed for asymmetric samples. In the latter
case, the highest temperature (Tu) was always set on the upper plate and the
lowest temperature (Tb) was always set on the bottom plate. Also, moulding
time (time that the sample was hold under pressure) was used to control the
temperature gradient inside the foaming sample.
Density Profile and Foam Density Measurement
Density profiles were determined by a commercial X-ray QMS density profiler
model QDP-01X. Measurements were performed on both foamed and
unfoamed samples (neat polymer). The scanning speed was set at 0.6 mm/s
and the scanning resolution is approximately 0.02 mm.
Overall foams density (rf) was obtained as the mass (M) over volume (V) ratio.
A digital calliper (accuracy of 10-2 mm) was used to measure the length, width
and thickness, while the mass was measured on a Mettler AE200 analytical
balance (accuracy of 10-4 g) according to ASTM D1622.
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Table 1. Moulding conditions and codes for the samples produced
Sample code
Blowing agent content (wt.%)
21
1
Temperature
Tu (°C)
Tb (°C)
160
160
Time
(min)
6
22
170
170
6
23
180
180
6
24
190
190
6
30
200
200
6
15
1
16
200
140
6
200
150
6
13
200
160
6
14
200
170
6
17
25
1
210
160
6
200
150
5.5
26
200
150
6
27
200
150
6.5
28
31
0.5
200
150
7
160
160
6
32
170
170
6
33
180
180
6
34
35
0.5
190
190
6
200
150
5.5
36
200
150
6
37
200
150
6.5
38
46
0.5
200
150
7
200
150
6
47
200
160
6
48
200
170
6.5
135
135
5
LMDPE
0
Foam Morphology
Foam structure was determined by scanning electron microscopy (SEM) on
a JEOL microscope model JSM840A. First, the samples were cryogenically
fractured in liquid nitrogen. Then, the exposed surfaces were coated with a
thin layer of Au/Pd. Finally, micrographs were taken at different magnifications
to get a complete quantitative morphological analysis.
Two parameters were extracted from the micrographs: average cell diameter
(D) and cell density (Nf). From the images taken, image analysis was performed
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using Image-Pro Plus 4.5 (Media Cybernetics). For each cell, the software
calculates the average of 90 diameters taken every 2 degrees from the
geometric centre of each cell area. For cell density, which is defined as the
number of cell (N) per cubic centimetre of foam, the calculations were done
according to the method of Kumar and Weller as [23]:
3/2
N
Nf =  
A
(1)
where A is the area of the micrograph in cm2.
To relate with density profiles and detect changes with position across
thickness, each micrograph was decomposed into five sections of equal
thickness. For each section, the “local” average cell diameter and cell density
were determined [24].
Flexural Modulus
Flexural modulus was determined using a three-point bending geometry
according to ASTM D790. For each moulding condition, the samples
were directly cut (75 mm in length and 12.7 mm in width) in the moulded
plates after 48 h of relaxation. The tests were performed on a universal
mechanical tester (Instron model 5565). Each sample was measured
two times by applying the load on both sides. The measurements were
done with a 50 N load cell and a deformation rate of 2 mm/min at room
temperature (23°C). The span (distance between sample supports) was
fixed at 60 mm. Finally, the flexural modulus was calculated with the slope
in the linear part of the stress-strain curves (less than 2% deformation).
The values reported are the average of a minimum of three measurements
with standard deviation.
Tensile Modulus
Tensile modulus was determined according to ASTM D638 on dog bone
samples (type V) directly cut in the moulded plates after 48 h of relaxation.
An Instron model 5565 mechanical tester was used to perform the tests at a
rate of 2 mm/min and room temperature (23°C) with a 500 N load cell. Tensile
modulus was calculated by the slope in the linear part of the stress-strain
curve (less than 2% deformation). The values reported are the average of a
minimum of three measurements with standard deviation.
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RESULTS AND DISCUSSION
Density Profiles
Typical density profiles for the foams produced are presented in Figures 1-3.
The effect of blowing agent concentration, moulding temperature and time
are discussed next.
Figure 1 presents the effect of moulding temperature for symmetric samples
(same temperature imposed on both sides of the mould) produced at constant
moulding time (6 min). First, it is clear that the density profiles are uniform within
experimental uncertainty. Second, blowing agent concentration and mould
temperature both have a substantial impact on average foam density. This
was expected since higher CBA content produced more gas to drive bubble
growth, while increasing temperature leads to lower bubble growth resistance
(lower polymer melt viscosity) and higher pressure inside the bubble. For the
range of conditions tested, the average foam densities obtained for symmetric
samples were between 342 kg/m3 for sample 30 (highest CBA content and
mould temperature) and 796 kg/m3 for sample 46 (lowest CBA content).
Figure 1. Density profiles for structural foams produced with 0.5% (32 and 33) and 1% (24
and 30) blowing agent at different moulding temperatures (see Table 1 for description).
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Figure 2 presents typical density profiles for asymmetric samples with constant
moulding time (6 min). In this case, it is clear that CBA content and moulding
temperature difference have substantial effects on density profiles. In all
cases, density decreases continuously form the cold side (position 0 mm) to
the hot side of the mould. For example, sample 46 (0.5% CBA) has a density
profile between 751 and 899 kg/m3 with an average density of 796 kg/m3. On
the other hand, sample 15 (1% CBA) has a density profile between 590 and
820 kg/m3 with an average density of 650 kg/m3. For the range of conditions
tested, sample 15 had the highest density difference (230 kg/m3) because
of its higher CBA content (1%) and higher temperature difference across
thickness (60°C).
Figure 2. Density profiles for structural foams produced with 0.5% (46) and 1% (13,
14, 15 and 17) blowing agent at different moulding temperatures (see Table 1 for
description)
Figure 3 presents the effect of moulding time for a fixed temperature gradient
(150-200°C) and CBA content (1%). As expected, local density is decreasing
with time since higher temperature (more time for heat transfer to occur) led to
lower bubble growth resistance. Once again, density decreases from the cold
plate to the hot side of the mould, but a minimum in density is observed in
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these profiles. This minimum can be explained by two independent phenomena.
First, there is the possibility of local higher temperature resulting from the
exothermic decomposition of the CBA (heat of reaction). But at low CBA content
as used here (0.5-1%), this is probably negligible. The second possibility is
a change in local morphology. In this case, a careful morphological analysis
is needed as presented in the next section. Nevertheless, similar results, but
with higher density values, were obtained at 0.5% CBA. A last observation
on Figure 3 is the fact that structural foams with constant skin density, but
of different values on both sides, can be produced by a careful selection of
the moulding conditions at fixed composition.
Figure 3. Density profiles for structural foams produced with 1% blowing agent at
different moulding times (see Table 1 for description)
Foam Morphology
From the SEM micrographs taken, a complete morphological analysis can be
performed and used to explain the density profiles obtained in Figures 1-3.
From these micrographs, the average cell diameter and cell density can be
calculated. Figure 4 presents typical morphologies obtained. A quick look at
the pictures can give some information about foam structure homogeneity with
respect to symmetric and asymmetric cases. The first row (samples 13,16,17)
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presents the effect of mould temperature difference (asymmetric samples) at
fixed CBA content (1%) and moulding time (6 min). It is clear that cell size and
cell density are changing with position across thickness. On the other hand,
the second row (samples 21, 24, 30) presents symmetric samples moulded
at constant time (6 min) and CBA content (1%). In this case the structure is
very homogeneous across thickness with the exception of sample 30 where a
bimodal cell size distribution is observed. This is probably related to conditions
(high moulding temperature and time) leading to cell coalescence. Larger cells
observed close to the sample’s mid-plane can explain the local minimum
density observed in Figure 3. This confirms that cell coalescence is the origin
of lower local density. Finally, the last row presents the effect of temperature at
fixed CBA content (0.5%) and moulding time (6 min) for symmetric (samples
32, 34) and asymmetric (sample 36) samples. Once again, a clear variation
in cell size and cell density is observed with respect to moulding conditions.
Figure 4. Typical SEM micrographs of selected structural foams (see Table 1 for
description)
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In order to get more quantitative information about foam morphology, cell size
and cell density profiles across thickness was estimated by decomposing
each micrograph into five sections of equal thickness to perform the analysis
independently. The results are presented in Table 2 for all the samples tested.
This information can be directly related to the foam structure with respect to
the density profiles obtained. For example, sample 34 (symmetric sample) has
a constant cell size (between 82 and 88 microns) and cell density (between
17 and 26x108 cells/cm3) across thickness within experimental uncertainty.
On the other hand, sample 13 (asymmetric sample) has substantial variation
in both cell size (between 98 and 152 microns) and cell density (between 6
and 20x108 cells/cm3) across thickness. Thus local changes in morphology
are directly related to density variation for FGF.
Now, the effect of average foam density and density profile is presented in
terms of flexural and tensile moduli.
Flexural Modulus
To differentiate between symmetric and asymmetric samples, each sample
was tested on both sides to determine its apparent flexural modulus and
the results (average and standard deviation S.D.) are presented in Table 3
with the average densities calculated from the density profiles. As expected,
symmetric samples have a modulus ratio (ratio between the apparent modulus
measured on both sides) close to unity within experimental uncertainty which
is estimated as 5% here; i.e. any ratio between 0.95 and 1.05 is assumed to
be unity. On the other hand, most of the asymmetric samples have apparent
flexural modulus ratio outside this range. For example, sample 14 has a ratio
as low as 0.68, which is much lower than any reported value so far [18,2022,24-26]. In fact, flexural strength is a function of load direction for asymmetric
samples because the neutral axis is no longer at the geometric center of the
sample. Since the amount of material being under tensile and compressive
stress is not the same, and that tensile and compression moduli are not equal
for polymer foams, the resistance to flexural load will depend on the way the
material is distributed across thickness [26]. From the results obtained, the
modulus ratio seems to be a complex function of average foam density and
density gradient. Nevertheless, the highest apparent flexural modulus is always
obtained when the load is applied on the side having the highest density as
reported several times in the literature [18, 20-22, 24-26]. Finally, it is clear
that average density alone is not enough to predict with high precision the
mechanical properties of foams having complex density profiles. For example,
samples 22, 32, 35 and 36 have almost the same average density (around
720 kg/m3), but their apparent flexural modulus can change substantially from
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one sample to the other (between 339 and 401 MPa). On the other hand,
samples 23, 25 and 34 have very different average densities (509, 684 and
553 kg/m3), but similar apparent flexural modulus (around 260 MPa). These
results clearly indicate that a complex relationship between structure and
flexural properties exists and a complete morphological analysis is needed to
relate density profile with macroscopic properties. These results also indicate
that possible optimization can be made as flexural rigidity or strength can be
maximized under constant weight and thickness. This can be concluded from
Table 3 where the modulus ratio can be higher or lower than unity depending
on density distribution and applied load side.
Table 2. Average cell diameter (D, microns) and cell density (Nf, 108
cells/cm3) for each section (see Table 1 for description)
Sample
code
Section
1
2
D
Nf
D
13
152.2
5.7
143.3
14
168.2
4.8
140.2
3
Nf
D
4
Nf
5
D
Nf
D
Nf
7.0 117.1 12.0
98.8
19.9
97.9
19.5
7.5 138.8
104.9
14.0
84.1
22.3
7.4
15
117.2
11.9
122.5 11.1 118.1 12.7
111.1
13.9
115.7
9.7
16
120.2
10.5
129.2
105.5
17.4
104.4
16.8
17
112.9
12.3
113.3 13.4 133.4
8.0
159.8
4.7
153.1
5.6
21
110.9
10.8
108.5 12.4 111.4 11.2
112.3
10.2
124.0
4.5
22
98.1
11.3
92.3
16.1 87.1
19.9
88.1
20.7
86.7
20.2
23
96.7
16.1
90.4
19.6 86.5
24.1
87.7
23.1
83.9
24.7
30
118.2
9.5
125.9
8.2 115.4 10.2
122.5
8.1
115.1
8.5
31
145.5
2.4
140.2
2.6 140.8
2.2
178.3
0.8
245.4
0.2
32
144.5
3.8
126.6
6.7 122.9
8.8
119.4
9.5
119.6
9.5
33
120.4
7.3
105.1 12.7 104.8 14.4
97.9
17.8
100.9
15.0
9.4 113.9 14.4
34
87.4
22.0
86.0
24.0 86.2
23.1
81.6
25.6
88.6
17.4
35
109.8
6.4
108.9
9.4 104.6 11.3
102.8
12.6
94.8
16.6
36
95.1
14.2
102.5 14.1 104.7 12.2
106.1
11.0
113.3
6.9
37
111.3
9.8
114.1 10.7 114.0 10.4
117.0
9.6
125.1
6.3
38
121.9
7.4
108.4 10.9 103.9 12.5
96.7
16.2
88.1
18.7
46
126.1
5.3
123.4
6.0 129.9
5.2
142.9
3.1
181.3
0.9
47
100.6
8.6
119.6
5.6 139.8
4.2
161.2
2.4
164.5
1.8
48
110.5
5.5
109.6
6.9 117.4
5.4
124.4
4.8
135.4
2.9
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Table 3. Average density (kg/m3) and apparent flexural modulus
(MPa) of the foams produced (see Table 1 for description)
Sample
Code
(kg/m3)
Density
Flexural Modulus
Side 1 ± S.D.
Side 2 ± S.D.
Ratio
LMDPE
923
518.0 ± 14.0
536.3 ± 15.6
0.97
13
565
99.0 ± 4.9
113.4 ± 3.0
0.87
14
464
70.7 ± 3.2
103.8 ± 2.7
0.68
15
650
120.7 ± 5.4
152.2 ± 6.2
0.79
16
582
91.3 ± 3.5
116.6 ± 6.4
0.78
17
401
66.5 ± 2.7
80.0 ± 2.5
0.83
21
792
359.9 ± 10.4
345.4 ± 16.0
1.04
22
723
374.7 ± 40.7
357.3 ± 41.3
1.05
23
509
264.6 ± 34.2
247.0 ± 36.4
1.07
24
433
185.8 ± 25.4
179.7 ± 28.9
1.03
25
684
262.7 ± 9.6
266.9 ± 14.2
0.98
26
666
237.9 ± 14.5
245.1 ± 11.8
0.97
27
621
226.4 ± 13.8
221.1 ± 15.0
1.02
28
542
236.7 ± 7.6
227.1 ± 5.9
1.04
30
342
102.5 ± 11.1
95.3 ± 8.0
1.07
31
779
478.1 ± 9.1
475.3 ± 14.8
1.01
32
719
339.0 ± 25.9
349.2 ± 32.1
0.97
33
572
249.4 ± 14.4
245.5 ± 16.5
1.02
34
553
262.3 ± 8.4
259.6 ± 11.7
1.01
35
717
392.3 ± 10.3
393.6 ± 10.4
1.00
36
721
395.3 ± 6.1
401.2 ± 6.5
0.99
37
665
273.6 ± 28.9
282.7 ± 26.6
0.97
38
626
275.2 ± 27.9
280.4 ± 11.3
0.98
46
796
359.1 ± 76.9
344.7 ± 72.0
1.04
47
699
354.2 ± 30.8
336.4 ± 30.3
1.05
48
647
285.7 ± 25.4
266.4 ± 33.1
1.07
Tensile Modulus
All the samples of Table 1 were also tested under tensile deformation to get
their apparent Young’s modulus as described in the Experimental section.
Table 4 presents the results obtained as the average and standard deviation
(S.D.). As expected, tensile modulus is generally decreasing with increasing
density reduction. Nevertheless, it is clear once again that average foam density
alone is not enough to completely explain the tensile modulus variation and
complete density profile must be accounted for.
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Table 4. Average density (kg/m3) and apparent tensile modulus (MPa)
of the foams produced (see Table 1 for description)
Sample Code
Density
Tensile Modulus
(kg/m3)
Average ± S.D.
LMDPE
923
137.1 ± 4.1
13
565
35.5 ± 1.9
14
464
25.3 ± 0.7
15
650
39.6 ± 2.9
16
582
28.0 ± 1.0
17
401
22.4 ± 0.8
21
792
80.1 ± 6.0
22
723
54.6 ± 2.8
23
509
39.3 ± 0.8
24
433
25.9 ± 1.2
25
684
62.9 ± 0.3
26
666
53.0 ± 1.8
27
621
41.3 ± 2.9
28
542
43.5 ± 0.9
30
342
21.2 ± 1.3
31
779
96.3 ± 2.1
32
719
72.7 ± 6.8
33
572
50.3 ± 4.3
34
553
50.1 ± 1.7
35
717
73.7 ± 2.3
36
721
72.2 ± 1.9
37
665
61.4 ± 4.1
38
626
69.8 ± 1.3
46
796
66.6 ± 1.5
47
699
72.3 ± 2.7
48
647
61.8 ± 3.9
For example, samples 22 and 32 have similar densities (720 kg/m3), but very
different tensile modulus: 55 and 73 MPa, respectively. Since both samples
are symmetric, the difference must be coming from the structure of the foams.
Based on the results of Table 2, sample 22 has lower average cell sizes (90 vs.
120 microns) and higher cell density (20 vs. 8x108 cells/cm3) than sample 32.
On the other hand, samples 15 (asymmetric) and 23 (symmetric) have similar
moduli (39 MPa), but very different average densities (650 vs. 509 kg/m3). Both
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observations indicate that foam morphology (cell size and cell density), as
well as type of structure (symmetric or asymmetric) have substantial effects
on tensile moduli of polymer foams.
CONCLUSIONS
This work presented a simple method to produce different density profiles
using compression moulding with LMDPE as the matrix and azodicarbonamide
as the blowing agent. By independently controlling the temperature on both
sides of the mould, and by careful control of moulding time and blowing agent
content, the final density profile can be adjusted as a result of modifications
in terms of cell size and cell density which can vary continuously across
thickness. Based on the samples produced, which covered a range between
342 and 796 kg/m3 in average density, several conclusions can be obtained:
• Mould temperature and blowing agent concentration have substantial
effect on foam morphology as an increase in both parameters, together or
independently, leads to lower foam density which can be directly related
to larger cells. Nevertheless, too high values produce cell coalescence.
• Symmetric and uniform foams can be produced by imposing the same
temperature on both sides of the compression mould. On the other
hand, imposing a temperature gradient (both sides controlled at different
temperatures), will produce a temperature profile inside the part being
moulded which in turn will produce a continuously changing density
profile across part’s thickness.
• Density gradient inside asymmetric foams is directly related to the
temperature difference imposed in the moulding process. Nevertheless,
moulding time is also important to control the heat transfer inside the
molten polymer as it is foaming.
• Samples having asymmetric density profiles were shown to have flexural
strength (apparent modulus) which depends on the side the load is
applied on. The results indicate that the apparent modulus is always
higher when the load is applied on the side having the highest density.
• Average foam density is not a sufficient parameter to completely predict
the tensile and flexural moduli of polymer foams. Although density profile
has an important effect on flexural modulus, tensile moduli seem to be
highly influenced by cell size distribution which has a direct effect on
density profile.
Finally, more experimental and theoretical works are needed to improve our
understanding of the relations between foam structure (microscopic properties)
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and mechanical properties (macroscopic properties) of functionally graded
foams (FGF) in order to predict with high accuracy their behaviour under
different types of loading and to be able to optimize their strength over weight
ratio for design purposes.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support of NSERC (Natural
Sciences and Engineering Research Council of Canada) and technical support
from CERMA (Research Centre on Advanced Materials). WES Industries Inc.
(Princeville, Canada) supplied the polyethylene samples for this work.
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