TENSILE AND FLEXURAL PROPERTIES OF
SOLID-STATE MICROCELLULAR ABS PANELS
K. V. Nadella and V. Kumar
Department of Mechanical Engineering
University of Washington
Seattle, WA 98195-2600, USA
ABSTRACT
This research is motivated by the potential of solid-state microcellular panels as novel core materials in sandwich construction.
We report on the tensile and flexure properties of microcellular ABS cores. Microcellular ABS panels with densities in range of
20%-80% of solid density, were produced using CO2 as the physical blowing agent. Following this, the integral skin on each of
the panels was machined off via a milling operation. This was done in order to characterize the mechanical properties of only
the microcellular cores. Once the skin was removed, the panels were further machined and sanded into tensile and bending
test samples. The sample preparation and testing was conducted according to ASTM D638 for tensile and ASTM D790 for
flexural property characterization. It was found that tensile and flexural behavior of the microcellular ABS panels is described
quite well by the Gibson-Ashby cubic cell model [1].
Introduction
Solid-state microcellular foams refer to thermoplastic foams with cells in the range of 1-100 μm in size. Typically these foams
are rigid, closed-cell structures and are produced in a two-stage batch process. In the first stage, the polymer is placed in a
pressure vessel with a high-pressure and non-reacting gas such as CO2. Over time, the gas diffuses into the polymer, and
attains a uniform concentration throughout the polymer specimen. When this specimen is removed from the pressure vessel
and brought to the atmospheric pressure, a “supersaturated” specimen that is thermodynamically unstable due to the
excessive gas dissolved into the polymer is produced. In the second stage, the specimen is heated to what is termed the
foaming temperature. This step is typically carried out in a heated liquid bath with temperature control. The dissolved gas
lowers the glass transition temperature of the polymer [2] and the foaming temperature needs only to be above the glass
transition temperature of the gas-polymer system in order for the bubbles to nucleate and grow. Since the polymer is still in a
solid state, the foams thus produced are called “solid-state foams” to distinguish them from the conventional foams that are
produced in an extruder from a polymer melt. A unique feature of this process is that an integral smooth skin of solid polymer
can be created on the microcellular foam. The thickness of this integral skin can be independently controlled [3]. The
microcellular process was invented at Massachusetts Institute of Technology in early eighties [4, 5], in response to a challenge
by food and film packaging industries to reduce the amount of polymer used in thin-walled articles. Thus was born the idea to
create microcellular foam, where we could have, for example, 100 bubbles across 1 mm thickness, and expect to have a
reasonable strength for the intended applications. An early review of the subject appeared in 1993 [6]. A more recent account
of microcellular foams can be found in the Handbook on Polymer Foams, 2003 [7].
Background
We have adopted the original microcellular process to produce thick, flat panels that could be used as part of a panel system
for housing construction, for example. This is achieved in a Constrained Foaming process illustrated in Fig. 1. The foaming
step is conducted inside the platens of a heated press, where metal shims maintain a gap equal to the desired panel
thickness. The constrained foaming process has been used for producing microcellular ABS panels with densities as low as
10% of solid ABS [5, 6]. Since skin-core structures can be directly produced in the solid-state process, these panels may be
suitable for use in certain applications, such as structural foams, without further modification. On the other hand, the smooth
polymer skin makes these foams ideal as core materials for lamination with stiff outer sheets of higher stiffness. To develop
applications for solid-state microcellular ABS panels it is important to understand the fundamental mechanical properties of
these materials. In this study we characterize the tensile and flexural behavior of skinless microcellular ABS cores made with
the solid-state microcellular constrained foaming process. These properties can then be used to predict the behavior of solidstate microcellular ABS foams with integral skin using the I-beam model of Hobbs for flexural modulus and simple square
power-law of Moore for tensile modulus [12].
Metal Shims
Saturated Polymer Sheet
Hot Platen
Hot Platen
CO2
Unsaturated polymer sheet
Stage I
GAS
SATURATION
Force
Force
Stage II
CONTROLLED
GAS DESORPTION
Microcellular
Polymeric
Foam Sheet
Stage III
CONSTRAINED
FOAMING
Figure 1: Constrained foaming process for microcellular thermoplastic panels
Experimental
Materials
For this study Cycolac HXW extrusion grade acrylonitrile butadiene styrene (ABS) sheets were obtained from General Electric
Plastics. The sheets were 9.53 mm thick with a glass transition temperature of 116 C and a density of 1.037 g/cm3,
respectively. The ABS sheets were machined into 190.5 mm x 254 mm rectangular samples and processed in the as-received
condition. For gas saturation 99.9% pure medical grade CO2 gas was obtained from Airgas.
Equipment
For the gas saturation step, a 0.28 m diameter and 0.635 m long pressure vessel, made by Ken-Weld Co. Inc, and rated for a
maximum pressure of 10.34 MPa at 65 C was used. The pressure inside the vessel was regulated using an OMEGA CN8500
process controller with a resolution of ±0.01 MPa. The constrained foaming step was carried out on an MTP-14 hydraulic press
with heated platens. The press was made by Tetrahedron Associates, Inc. and is rated for use up to a maximum force of 50
tons. To create a gap between the top and bottom platens, two stacks of metal shims each with a stacked height of 13 mm
were used. For machining the microcellular ABS panels into test samples a 2-axis CNC milling machine made by TRAK was
used. A Mettler-Toledo AE240 precision balance with an accuracy of 10 μg was used to measure the density of microcellular
ABS samples as described in the experimental procedure section. For the tensile and flexure testing an Instron Universal
Testing Machine with a 250 kN load cell was used. The Instron machine had automatic control and data acquisition through a
personal computer.
Sample Preparation and Testing Procedure
Twenty four ABS sheets were first interleaved with a porous paper and then placed in a pressure vessel. Once the pressure
vessel was closed CO2 gas at a pressure of 5 MPa and a temperature of 22.2 C was introduced into the chamber for sorption.
The chamber temperature was then maintained at 22.2 C throughout the residence time of the samples in the chamber. The
saturation pressure and saturation temperature were regulated to ± 0.1 MPa and ± 1C, respectively. After 270 hours of
saturation time, the chamber was depressurized to atmospheric conditions and the CO2 saturated ABS sheets were removed
from the pressure vessel. The 24 samples were divided into three sets of 8 samples. The first set was foamed immediately in
the heated hydraulic press. The second and third sets were allowed to desorb before foaming at room temperature for a period
of 3 and 4 days respectively. By desorbing the three sets prior to foaming the gas concentration in the saturated ABS can be
varied thereby allowing the creation of panels with similar density but varying cell size. This will then allow the study on the
effect of cell size on the tensile and flexure properties of these microcellular ABS panels.
For the foaming step two samples from a set were placed between the platens of the heated hydraulic press at each foaming
temperature as shown in Figure 1. Of these two samples one sample was designated for tensile testing and the other for
flexural testing. The platens were preheated to the desired foaming temperature (Tfoam) and were separated by 13 mm thick
metal shims to create a gap in which the saturated sheets were placed for foaming. The shim thickness was chosen to be the
same as that used for prior study [5] where the saturation was carried out at room temperature. After the saturated polymer
sheets were inserted between the heated platens a force of 544 kg was applied on the top and bottom surfaces of the polymer
sheet to constrain its foaming in the thickness direction. The foaming cycle of the saturated polymer sheet in the heated press
consisted of two stages. In the foam generation stage the platens were maintained at the desired foaming temperature for 10
minutes under a force of 544 kg. This ensures ample time for heat to flow through the polymer sheet for uniform foaming to
occur. In the foam stabilization stage the platens were cooled down to the room temperature (22 C) for 10 minutes under a
force of 3629 kg. This ensures that the polymer sheet reaches the room temperature thereby stabilizing the foam structure.
The same procedure was repeated on the second set of eight ABS sheets that were desorbed for 3 days and the third set of
eight ABS sheets that were desorbed for 4 days. The foaming temperatures were in the range of 67.5-115C and was chosen
from a prior study [7-11] based on the target densities.
After foaming in the heated press the microcellular ABS panels were allowed to age at atmospheric pressure and room
temperature for over 100 days so that the remainder of the CO2 gas in the cells diffuses out. Each of the 12 microcellular ABS
samples and one solid ABS sheet designated for tensile testing were then machined using a CNC mill into 5 smaller
rectangles of size 200 mm x 19 mm x 7.25 mm. The other 12 microcellular ABS samples and solid ABS sheet designated for
flexure testing were milled 6 smaller rectangles of size 125 mm x 18.5 mm x 6.5 mm. This machining operation removed the
integral skin on the microcellular ABS panels thereby leaving a cellular structure on the surface of each microcellular ABS core
sample. Following this machining step the densities of the ABS cores were determined using the ASTM D792-91 standard test
method [12]. The samples used for density measurement were approximately 25 mm long and were cut out from the machined
rectangular ABS core samples. The rectangular samples designated for tensile testing were machined to the ASTM D638Type
I geometry using the same milling machine. After the milling operations, each surface of all the test samples was carefully
sanded by hand using 200, 400, 600 grit paper in that sequence. This was done to ensure that the effect of varying surface
finish on the test properties is minimal.
Once sanded all the test samples were conditioned in a laboratory environment with a humidity range of 40-55% RH and a
temperature of 22.2 C for a minimum of 48 hours. Tensile testing was conducted on the Instron machine according to ASTM
D638 at a speed of 5 mm/min which corresponds to an initial strain rate of 0.1 mm/mm/min. Upon testing all the 5 samples at a
given density the tensile strength and tensile modulus values were obtained from the acquired test data. In the case of flexural
testing, a three point bend set-up was used according to ASTM D790. The flexure testing was conducted at a cross-head rate
of 2.667 mm/min, which conforms to the recommendations of ASTM D790. Flexural strength and flexural modulus values were
obtained from the acquired test data. Tables 1 and 2 below show the processing conditions, relative density (ratio of density of
microcellular ABS to the density of solid ABS) and resulting tensile and bending properties respectively. All samples in Tables
1 and 2 were saturated at 5 MPa.
Table 1: Solid-state microcellular process conditions, relative densities and tensile properties for 5 MPa saturated ABS sheets.
Tensile Strength
(MPa)
Sample
ID
Desorb
Time
Foaming
Temperature
(C)
Average
Relative
Density
Mean
Standard
Deviation
Specific
Tensile
Strength
(MPa)
A1-A5
0 Day
67.78
0.66
18.96
0.77
Mean
Standard
Deviation
Specific
Tensile
Modulus
(MPa)
28.58
819.84
66.11
1235.80
1268.69
Tensile Modulus (MPa)
B1-B5
0 Day
80.00
0.49
14.53
0.61
29.64
621.88
34.56
C1-C5
0 Day
92.78
0.36
9.51
0.58
26.78
351.38
19.90
989.25
D1-D5
0 Day
105.00
0.27
6.90
0.89
25.65
259.02
26.09
963.41
E1-E5
3 Day
75.00
0.81
21.53
0.36
26.64
1058.44
33.85
1309.45
F1-F5
3 Day
85.00
0.67
18.11
0.49
26.96
723.54
166.33
1077.31
G1-G5
3 Day
100.00
0.49
12.24
0.76
25.19
469.65
44.67
966.50
H1-H5
3 Day
110.00
0.47
14.01
0.23
29.81
561.56
15.90
1195.38
I1-I5
4 Day
80.00
0.71
19.43
0.37
27.23
850.88
43.06
1192.59
J1-J5
4 Day
90.00
0.67
17.88
0.75
26.78
638.46
72.14
956.27
K1-K5
4 Day
105.00
0.43
13.44
1.10
30.95
554.31
32.49
1276.60
L1-L5
4 Day
115.00
0.53
13.16
2.07
24.73
516.09
112.91
970.17
M1-M5
Solid
NA
1.00
38.50
0.93
38.50
2161.55
37.51
2161.55
Table 2: Solid-state microcellular process conditions, relative densities and flexure properties for 5 MPa saturated ABS sheets.
Flexural Strength
(MPa)
Sample
ID
Desorb
Time
Foaming
Temperature
(C)
Average
Relative
Density
Mean
N1-N6
O1-O6
P1-P6
Q1-Q6
R1-R6
S1-S6
T1-T6
U1-U6
V1-V6
W1-W6
X1-X6
Y1-Y6
Z1-Z6
0 Day
0 Day
0 Day
0 Day
3 Day
3 Day
3 Day
3 Day
4 Day
4 Day
4 Day
4 Day
Solid
67.78
80.00
92.78
105.00
75.00
85.00
100.00
110.00
80.00
90.00
105.00
115.00
NA
0.68
0.51
0.40
0.28
0.81
0.65
0.46
0.52
0.75
0.63
0.45
0.50
1.00
30.94
21.36
11.96
7.94
37.84
24.79
15.72
18.43
29.94
26.09
12.70
15.71
63.73
Flexural Modulus
(MPa)
Standard
Deviation
Specific
Flexural
Strength
(MPa)
Mean
Standard
Deviation
Specific
Flexural
Modulus
(MPa)
1.36
1.78
0.87
1.40
4.59
2.42
1.32
1.14
1.34
1.03
1.39
2.22
0.30
45.36
41.65
29.71
28.68
46.75
38.04
34.17
35.58
40.10
41.72
28.07
31.72
63.73
998.64
656.03
380.26
255.35
1226.27
779.61
412.61
551.58
986.58
822.41
403.72
459.25
2218.28
40.94
59.79
28.79
38.44
144.16
78.65
73.95
66.06
42.47
27.38
44.59
68.72
16.84
1463.98
1279.02
944.30
922.83
1514.93
1196.40
897.16
1064.82
1321.52
1315.39
892.30
926.91
2218.28
Results
Density Results
Figures 2(a) and 2(b) show the relationship between relative density and foaming temperature for the microcellular ABS panels
made as described above. In general, for a given saturation pressure and desorption time, as the foaming temperature
increases the relative density decreases until a minimum is reached beyond which the relative density increases due to bubble
collapse. Also, as desorption time increases the relative density increases for the same foaming temperature. This is due to
reduction in the concentration of CO2 gas that is available for bubble growth. Except for slight variation, the relative density for
a given set of process conditions among the tensile and bending samples is similar.
1.0
1.0
0.9
0.8
Relative Density
Relative Density
0.8
0.7
0.6
0.5
0.7
0.6
0.5
0.4
0.4
0.3
0.3
0.2
65
70
75
80
85
90
95
100
Foaming Temperature
(a)
105
110
115
5 MPa, 0 Day Desorb
5 MPa, 3 Day Desorb
5 MPa, 4 Day Desorb
0.9
5 MPa, 0 Day Desorb
5 MPa, 3 Day Desorb
5 MPa, 4 Day Desorb
120
0.2
65
70
75
80
85
90
95
100
105
110
115
120
Foaming Temperature
(b)
Figure 2: (a) Plot of relative density versus foaming time for microcellular ABS panels made in the constrained foaming
process at various process conditions. These samples were used to determine the tensile properties of microcellular ABS
cores. (b) Plot of relative density versus foaming time for microcellular ABS panels made in the constrained foaming process at
various process conditions. These samples were used to determine the flexural properties of microcellular ABS cores.
45
45
40
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
30
25
Specific Tensile Strength, MPa
Tensile Strength, MPa
35
40
20
15
10
5
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
35
30
25
20
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
15
10
5
0
0.0
1.2
0.1
0.2
0.3
0.4
Relative Density
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Relative Density
(a)
(b)
Figure 3: (a) Plot of tensile strength versus relative density for skinless microcellular ABS cores made in the constrained
foaming process at various process conditions. Note the tensile strength of these cores is lower than and follows the trend of
the rule of mixtures. (b) Plot of specific tensile strength as a function of relative density. Note that the specific tensile strength
of skinless microcellular ABS cores is fairly constant over the whole range of density reductions.
2500
2500
Tensile Modulus, MPa
2000
1750
1500
Specific Tensile Modulus, MPa
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
2250
1250
1000
750
500
250
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
2250
2000
1750
1500
1250
1000
750
500
250
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Relative Density
Relative Density
(a)
(b)
0.9
1.0
1.1
1.2
Figure 4: (a) Plot of tensile modulus versus relative density for skinless microcellular ABS cores made in the constrained
foaming process at various process conditions. Note the tensile modulus of these cores is lower than and follows the trend of
the rule of mixtures. (b) Plot of specific tensile modulus as a function of relative density. Note that the specific tensile modulus
of skinless microcellular ABS cores is fairly constant over the whole range of density reductions.
Tensile Properties
Figures 3(a) and 4(a) below show the behavior of tensile strength and tensile modulus, respectively, as functions of relative
density for skinless microcellular ABS cores. As expected when the relative density increases the tensile properties are higher.
Also, the tensile strength and modulus for a given density are in the range of 25-50% below the value predicted by the rule of
mixtures and this trend holds throughout the range of densities that have been tested. The tensile performance of these
materials can be significantly improved by leaving the solid integral skin. In fact, a thin integral skin enables the production of
microcellular ABS foams of various densities whose tensile strength and modulus closely follows the rule of mixtures as
reported by Weller [13]. The tensile strength to weight and tensile modulus to weight benefits of microcellular ABS cores of
various densities can be determined by the plots shown in Figures 3(b) and 4(b). Both the specific tensile strength and specific
tensile modulus are essentially constant for microcellular ABS cores over the range of densities tested. This trend is similar to
microcellular ABS foams with integral skins as reported by Weller [13]. The specific tensile strength for skinless microcellular
ABS cores with density reductions above 30% is higher than that predicted by the rule of mixtures which is very beneficial in
applications where lightweight is desired without a loss in inherent tensile strength. In the case of tensile modulus the
microcellular ABS cores with relative density below 50% show favorable values for the inherent tensile modulus.
80
80
Flexural Strength, MPa
70
60
50
Specific Flexural Strength, MPa
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
40
30
20
10
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
70
60
50
40
30
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0
0.0
1.2
0.1
0.2
0.3
Relative Density
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Relative Density
(a)
(b)
Figure 5: (a) Plot of flexural strength versus relative density for skinless microcellular ABS cores made in the constrained
foaming process at various process conditions. Note the flexural strength of these cores is lower than and follows the trend of
the rule of mixtures. (b) Plot of specific flexural strength as a function of relative density. Note that the specific flexural strength
of skinless microcellular ABS cores is closely approximated by rule of mixtures.
2500
2500
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
2000
1750
1500
1250
1000
750
500
250
0
0.0
0 Day Desorb
3 Day Desorb
4 Day Desorb
Solid ABS
Rule of Mixtures
2250
Specific Flexural Modulus, MPa
Flexural Modulus, MPa
2250
2000
1750
1500
1250
1000
750
500
250
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Relative Density
Relative Density
(a)
(b)
0.8
0.9
1.0
1.1
1.2
Figure 6: (a) Plot of flexural modulus versus relative density for skinless microcellular ABS cores made in the constrained
foaming process at various process conditions. Note the flexural modulus of these cores is lower than and follows the trend of
the rule of mixtures. (b) Plot of specific flexural modulus as a function of relative density. Note that the specific flexural modulus
of skinless microcellular ABS cores is closely approximated by rule of mixtures.
Flexure Properties
Figures 5(a) and 6(a) below show the behavior of flexural strength and flexural modulus, respectively, as functions of relative
density for skinless microcellular ABS cores. As expected when the relative density increases the tensile properties are higher.
Also, the tensile strength and modulus for a given density are in the range of 30-50% below the value predicted by the rule of
mixtures and this trend holds throughout the range of densities that have been tested. The flexural strength to weight and
flexural modulus to weight behavior of microcellular ABS cores of various densities can be determined by the plots shown in
Figures 5(b) and 6(b). Both the specific flexural strength and specific flexural modulus are closely approximated by the rule of
mixtures over the range of densities tested. The flexural performance of these materials with the solid integral skin will be
significantly better than that shown in Figures 5 and 6, due to the well-known I-beam effect. Also, the solid-state microcellular
process provides the ability to accurately control the thickness of the integral skin by manipulating the residence time in step II
shown in Figure 1. As desorption time increases the thickness of the integral skin increases [3] and this in turn will improve the
flexural performance of microcellular ABS foams. Calculations performed by the authors according to Gibson and Ashby’s
model for closed cell foams with integral skins reveal that it is possible to match the flexural stiffness of the solid with select
combinations of integral skin thickness and bulk microcellular foam thickness.
Conclusions
The conclusions of this study are
1) For a given density tensile strength and modulus of skinless, solid-state microcellular ABS cores is 25%-50% below
that predicted by rule of mixtures.
2) For a given density the specific tensile strength and modulus of skinless, solid-state microcellular ABS cores is
essentially constant and in some cases higher than the values predicted by the rule of mixtures.
3) For a given density tensile strength and modulus of skinless, solid-state microcellular ABS cores is 25%-50% below
that predicted by rule of mixtures.
4) For a given density the specific tensile strength and modulus of skinless, solid-state microcellular ABS cores is
essentially constant and in some cases higher than the values predicted by the rule of mixtures.
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