THE UNIVERSITY OF TULSA
DEPARTMENT OF MECHANICAL ENGINEERING
MECHANICAL CHARACTERIZATION OF POLYMERIC MICROSPHERES TO
TRADITIONALLY FILLED VINYL-ESTER EPOXY RESIN
by
Steven Taylor
THE UNIVERSITY OF TULSA
May 1, 2006
Taylor, S.
ABSTRACT
Polymeric microspheres in various weight percents were used as lightweight filler
in a vinyl-ester sheet molding compound (SMC) resin system used in the automotive
industry for structural components. The results of the microspheres were compared to
vinyl-ester resin containing calcium carbonate that is widely used as a low cost filler.
Test plaques were molded and formed into samples to determine tensile properties,
flexural properties, density, thermal expansion along with evaluating the microstructure
and fracture surfaces of the composite. It was found that the fillers used can be
characterized as small voids in the resin decreasing the overall tensile strength of the
composite. In addition, Paul’s theoretical model best predicts the modulus of each
composite.
INTRODUCTION
Polymeric microspheres can be used as a light weight alternative to traditionally
used calcium carbonate as the filler material in SMC. The concept of using hollow
microspheres as filler material comes from the late 1970's when these micro spheres were
used in combination with a resin system for lightweight plastics in marine applications.
As the automotive industry looks for stronger, lighter and inexpensive alternatives to
sheet metal, hollow microspheres are becoming readily available and can provide the
potential weight saving required. With a lower bound density of 0.10 g/cm3,
microspheres can lower the standard density of SMC parts from 1.5 g/cm3 to 1.3 g/cm3 or
lower.
Currently, microspheres are being used in a wide variety of applications, which
include bowling balls and sport and leisure boats. General Motors is presently the only
known automobile manufacturer using microspheres in production on their Chevrolet
Corvette. Ford Motor Company has the goal of producing low density structural SMC
components within the next year and components with class-A surfaces following
shortly. Although the price for microspheres is currently around $7.00 / lb, prices are
expected to drop with an increase in demand and the availability of more efficient
production processes. Potential advantages include lighter parts with as good or better
material properties than those currently in production.
RELEVANT RESEARCH
Tension, compression, flexural and fracture test were conducted on composites
containing various volume fractions of 3M glass bubbles K 15 and K46 and Phenoset
BJO-093 as filler in an Epicote 1006 epoxy resin system was investigated by Wouterson
et al. Mechanical test data were normalized and presented as specific mechanical and
fracture properties. Both tensile and flexural test revealed similar results in modulus,
strength and failure modes. 3M K15 glass microspheres had the largest thickness to
radius ratio and showed an increase in specific modulus with increase in volume fraction
of filler. The 3M K46 glass microspheres showed a constant specific modulus with
increase in volume fraction of microspheres, and a decreasing trend was found in specific
modulus of phenolic microspheres. An increase in tensile strength for all fillers was
found between 0% and 20% volume fractions. Volume fractions of 30% and greater
showed a decreasing trend in tensile strength. A Decrease in Flexural strength was found
with an increase in volume percent of filler for all three fillers. Scanning electron
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Taylor, S.
microscope images of fracture surfaces from the different mechanical tests show plastic
deformation of the epoxy and debonding of microspheres from the resin. The debonding
of microspheres was found to cause a reduction in matrix volume resulting in lowered
material strength.
Dikie reported findings of effective modulus of polymethyl methacrylate
(PMMA) filled with various volume fractions of glass beads and dispersed rubber phase.
The modulus of the material was experimentally found using dog-boned shaped tensile
specimens. The multi-component form of the Kerner model for modulus of the
composite was found to be an inappropriate fit through the data. The report concludes
the predicted modulus may be dependent on other factors not included in the Kerner
model including size distribution, filler particle deformability and filler to matrix
modulus ratios.
Li et al. investigated several models to predict the modulus of asphalt concrete
consisting of a binder and aggregate. The modulus, Poisson’s ratio, and volume fractions
for both the aggregate and binder were all known and assumed to be isotropic. Resilient
modulus test were conducted per ASTM 4123-82. Known raw material properties for
each phase of the composite were used in theoretical models including rule of mixtures,
Hirsh, Hashin composite spheres, and Christensen Lo models. The Hirsh, Hashin, and
Christensen Lo models gave reasonable predictions, but differences could be attributed to
the high volume fraction of large sized aggregate and the bond and interaction between
the different phases of the matrix.
Hseih et al. conducted a study comparing 12 different theoretical models to the
experimental results of the elastic modulus of a two phase material comprised of
aluminum oxide containing 0 to 100% volume percentage of nickel aluminide. The
samples were made using a hot-pressing technique then determining the modulus using
an ultrasonic method. Hsieh showed the Reuss and the Hashin-Shtrikman lower bond
equations gave good modulus predications to the experimental data.
The objective of the research described in this report is to determine the effective
modulus of filler used in vinyl-ester sheet molding compound resin. In particular, several
theoretical models will be compared to a vinyl-ester base resin containing various volume
fractions ranging from 0 – 50% of calcium carbonate, both with published mechanical
properties. Those theoretical models that best predict the modulus of the control will
then be used to determine the effective modulus of the polymeric microspheres. In
addition, the effects on other material properties as a function of filler type and filler
volume, such as percent shrinkage, flexural strength, coefficient of linear thermal
expansion, and density will be evaluated.
EXPERIMENTAL
Materials
The materials used for this study include the dispersion of polymeric
microspheres and calcium carbonate as fillers in a vinyl-ester resin. The microspheres
were Dualite E130-055D which consists of a polyvinylidene chloride copolymer shell
coated with calcium carbonate. The average nominal density of the spheres is 0.130
g/cm3 with an average particle size between 45 – 65 μm. The microspheres strength is
characterized by their ability to withstand pressure commonly referred to as burst
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Taylor, S.
strength. Because of the polymeric nature of the spheres, the spheres burst strength is
both time and temperature dependent.
Calcium carbonate is widely used as low cost filler in a variety of resin systems.
Calcium carbonate came from Korth Kristalle GMBH in a powder form with an average
particle size on the order of 10 μm. The fillers were mixed with Ashland Arotech Q6055
vinyl-ester resin containing appropriate amounts Luperox DDM-9 as a catalyst, and
Westdry Cobalt 6% and Aldrich N, N-Dimethylaniline as accelerators. A fixed two
minute gel time at 90°C established the appropriate ratio of resin to catalyst and
accelerators.
Sample Preparation
Each filler was mixed with the vinyl-ester resin system in volume percents
ranging form 0 to 50% in 10% increments. The contents were stirred slowly to prevent
the formation of air bubbles and to properly distribute the filler throughout the resin. The
mixtures were injection molded into plaques measuring 150mm by 80mm by 3.5mm at
90°C and 275 kPa and allowed 10 minutes to cure. After removal from the mold, the
plaques were subjected to a post cure at 100°C for 60 minutes and stored at standard
atmospheric conditions. Each plaque’s width was measured in five places and compared
to the 80mm width of the mold surface to determine the percent shrinkage from the
molding process, and then cut into test specimens for mechanical testing.
Mechanical Testing
Tensile test and immersion density test were performed with each sample.
Tensile test specimens were cut into 80mm by 19mm blanks and had a 10mm gage length
machined using a high speed router and a tensile specimen jig. Tensile specimens were
tested at standard laboratory conditions, at a crosshead speed of 10mm/min on a
Measurements Technology Incorporated (MTI) universal load frame. A 2000 kg load
cell and MTS 10mm extensometer measured force and strain, which was used to obtain
ultimate tensile stress, ultimate tensile strain and Young’s modulus. A total of 8
specimens were tested for each sample per ASTM D638.
Specimens measuring 80mm long by 13mm wide were cut from plaques and used
for immersion density test. The specimens had cut edges wet sanded with 320 grit sand
paper and width variance kept within 0.05mm. Immersion density measurements were
made in agreement with ASTM 792-00(3). Test specimens were weighed dry using an A
and D Company/Limited electronic balance model number HR-60. Next, the specimens
were placed into a 250mL beaker and buoyant forces were measured using a MettlerToledo density determination kit and an A and D Company/Limited electronic balance
model number HR-60. Using Archimedes principle and the buoyant force and dry weight
of the specimen, the density was calculated. Each sample was comprised of 8 test
specimens.
Microstructure Analysis
Cross sections from each sample of plaques were cold mounted and labeled for
convenient handling and referencing when looking at the microstructure under a
microscope. Cold mounting procedures require little heat and pressure, which minimizes
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Taylor, S.
degradation to the polymer samples. Plastic forms were used to mold a two part
polyester resin and hardener around the test specimen. Cross sections measuring
approximately 25 mm by 10 mm by 3 mm were cut from the center section of test
plaques and placed upright inside the plastic form. The two part polyester resin system
was mixed and poured into the plastic ring and allowed to cure overnight. After the
polyester was cured the specimens were polished smooth for viewing using a Nikon
metallograph microscope at a magnification of 20X. Image J image processing and
analysis software was used to evaluate filler percentage of the microstructure.
Fracture Surface Analysis
Fracture surfaces from the tensile test specimens were prepared for viewing with a
scanning electron microscope (SEM). First, the fracture surface was removed from the
test specimen to allow insertion into the SEM viewing chamber. Next, aluminum tape
was used to adhere the fracture surface to a graphite disk. Due to the lack of electrical
conductivity of the composite, the sample was placed in a sputter coater supplied with a
gold target. The sample was sputter coated for approximately 5 minutes, receiving an
approximate 30 micron thick surface layer of gold. Lastly, the specimen was placed into
a vacuum chamber before being placed on the stage of the scanning electron microscope
for fracture surface analysis.
THEORETICAL MODELING
Numerous theoretical models have been proposed for predicting the properties of
a material such as density, modulus and tensile strength. Among the most common of
these models is the Rule of Mixtures parallel and series model. The parallel model, often
referred to as iso-strain, states that regardless of the load, both the filler and resin will
have equal amounts of strain (Figure 1).
Base Resin
Filler
Base Resin
Figure 1: Parallel (Iso-strain) Model
The parallel model is a function of volume percent of base resin and filler
denoted by Vr and Vf , respectively, and the material property of interest for each the base
resin and filler, in this case Young’s modulus Er and Ef, respectively. Equation 1 gives
the relationship of the predicated composite modulus (Ec) as a function of the individual
phase characteristics.
E c  E r  Vr  E f  V f
(1)
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Taylor, S.
The series model, also referred to as the Iso-stress model, states that each phase of
the composite carries the same amount of stress, but is allowed to have different values of
strain. Using the same nomenclature as Equation 1, the series model is illustrated in
Figure 2 and the mathematical relationship given in Equation 2.
Base Resin
Filler
Base Resin
Figure 2: Series (Iso-stress) Model
Vf
V
1
 r 
Ec Er E f
(2)
The rule of mixtures gives an upper bound (parallel model) and lower bound
(series model) prediction for two-phase materials. Most other theoretical models give a
prediction in between the two rules of mixtures. This investigation looked at a number of
models and found three that give comparable predictions to the experimental vinyl-ester
sheet molding compound – calcium carbonate modulus data. These models include
Hashin-Shtrikman model (Equation 3), Kerner model (Equation 4), and the Paul model
(Equation 5).
Kc  Kr 
Kc 
Vf
31  V f 
1

K f  K r 3K r  4G r
(3)
K r 3K f  4 K r   4GrV f K f  K r 
3K
f


(4)
1  V 
(5)

Er  Er E f  Er V f
2
Ec 
 4Gr   3V f K r  K f
E r  E f  E r V f
2
2/3
2/3
1/ 3
f
These equations use the same nomenclature as that in the rule of mixtures where
V is volume fraction, E is Young’s Modulus, K is bulk modulus and G is shear modulus.
Subscripts c, f, and r denote the composite, filler, and resin, respectively.
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Taylor, S.
RESULTS AND DISSCUSSION
Percent Shrinkage
When components of a subassembly are made from a molding process, it is
important to be able to predict how the component will shrink to create a part that is
within design specification. Figure 3 shows the percent shrinkage of molded plaques
containing calcium carbonate and polymeric microspheres as filler.
5.0%
Percent Shrinkage
4.0%
3.0%
2.0%
1.0%
0.0%
0%
10%
20%
30%
40%
50%
Filler Volume Percentage
Figure 3: Percent Shrinkage versus Filler Volume for (♦) CaCO3, (■) Polymeric
Microspheres, (▲) Neat Vinyl-Ester Resin and (--) Suggested Trend Lines.
Table 1: Percent Shrinkage Statistical Data
Material
No Filler
10% CaCO3
20% CaCO3
30% CaCO3
40% CaCO3
50% CaCO3
10% E130-055D
20% E130-055D
30% E130-055D
40% E130-055D
50% E130-055D
Averagae
0.031
0.023
0.023
0.026
0.019
0.013
0.026
0.033
0.036
0.032
0.033
Stdev
0.012
0.011
0.007
0.001
0.005
0.004
0.011
0.010
0.004
0.007
0.001
COV
0.383
0.489
0.289
0.054
0.287
0.329
0.435
0.310
0.100
0.209
0.033
From Figure 3 and Table 1, it is noticed that there is much scatter for all volume
fractions of calcium carbonate and polymeric microspheres. Although a decrease in
average percent shrinkage can be seen with an increase in volume percentage of calcium
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Taylor, S.
carbonate a trend between percent shrinkage, volume fraction of filler and filler type is
statistically insignificant. All the data do have distinctive upper limits in the scatter as
shown with the suggested trend lines. This indicates that the total percent shrinkage may
not be captured by width measurements taken. The plaques do contain two additional
dimension, length and thickness that were not measured, and percent shrinkage could
possibly be an anisotropic characteristic of the material that is dependent on molding
processes.
Immersion Density
Immersion density tests reveal two key pieces of information, the obvious being
the weight of the material, and the other is a macro-level indication of how well spheres
are distributed compared to theoretical calculations. The densities of both fillers and the
resin system are all known and have been provided by the suppliers. The rule of mixtures
parallel model was used to create the theoretical curve fits though the data (Figure 4).
2.5
Density (g/cm3)
2.0
1.5
1.0
0.5
0.0
0%
10%
20%
30%
40%
50%
Filler Volume Percentage (%)
Figure 4: Density versus Filler Volume for (♦) CaCO3, (■) Polymeric Microspheres,
(▲) Neat Vinyl-Ester Resin and (─) rule of mixture’s curve fit.
There tends to be good correlation between the theoretical and actual values for
all volume percentages of calcium carbonate ranging from 1.55% difference at 10%
calcium carbonate by volume to 3.55% difference at 50% calcium carbonate by volume.
The polymeric microspheres have a downward trend, but have linear increase in percent
difference with increase in filler volume percentage. At 50% microspheres by volume
there is 51% difference between the theoretical value and actual data. Microstructure
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Taylor, S.
analysis samples were prepared to determine the micro-distribution of the spheres. These
results along with the density data will provide further insight into the differences
between the modeled and actual data, and are discussed in a following section.
Tensile Test
Figure 5 shows modulus versus volume percent of calcium carbonate. It is seen
that the parallel and series models give the upper and lower bounds with the remaining
three models falling in between. Table 2 shows the percent difference between
theoretical models and experimental data. The Paul model gives moduli predictions more
accurate to experimental values at larger values of filler volumes with a 3.7% difference
at 50% filler, but has difference of over 15% at lower volume fractions. The Kerner
model has a difference of less than 10% for all cases, and therefore can be considered the
best model for the vinyl-ester / calcium-carbonate material.
14000
Parallel
12000
Paul
Modulus (MPa)
10000
Kerner
8000
Hashin-Shtrikman
6000
4000
Series
2000
0
0%
10%
20%
30%
40%
50%
Volume Percentage of Filler (%)
Figure 5: Young’s Modulus versus Filler Volume of (♦) CaCO3 and (▲) Neat
Vinyl-Ester Resin.
Table 2: Percent Difference of Theoretical Models from Figure 5
% Filler
0%
10%
20%
30%
40%
50%
Actual E
3269
4608
5617
7276
9468
12282
Parallel
0.01%
137.47%
231.42%
261.34%
258.71%
239.01%
Series
0.01%
21.53%
27.99%
36.92%
43.98%
48.86%
Hashin
0.01%
11.01%
8.43%
11.04%
13.33%
14.20%
Paul
0.01%
15.83%
21.25%
15.60%
8.98%
3.74%
Kerner
7.00%
5.00%
2.49%
5.47%
8.08%
9.18%
Modulus versus filler volume for the polymeric microspheres is shown in Figure
6, along with the five theoretical models used to predict the calcium carbonate data. Each
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Taylor, S.
model was independently fit through the data to determine the effective modulus of the
spheres. Table 3 gives the tabulated percent difference between the theoretical models
and experimental data for filler volumes ranging from 0% to 50%.
6000
Kerner
Modulus (MPa)
5000
4000
Paul
3000
Parallel
Series
2000
Hashin-Shtrikman
1000
0
0%
10%
20%
30%
40%
50%
Volume Percentage of Filler (%)
Figure 6: Filler Volume versus Young’s Modulus of (■) Polymeric Microspheres
and (▲) Neat Vinyl-Ester Resin
Table 3: Percent Difference of Theoretical Models from Figure 6
% Filler
0
10
20
30
40
50
Actual E
3269
3616
3216
3190
3169
2579
Parallel
0.01%
10.33%
0.01%
0.01%
0.18%
21.64%
Series
0.01%
10.34%
0.02%
0.02%
0.17%
21.69%
Hashin
0.01%
10.00%
0.23%
0.34%
1.42%
18.62%
Paul
0.01%
10.33%
0.01%
0.04%
0.22%
21.58%
Kerner
50.54%
53.24%
92.45%
115.27%
139.08%
222.58%
Table 4: Theoretical Model’s Effective Modulus
Model
Parallel
Series
Hashin
Paul
Kerner
Effective E (Mpa)
3,005
3,018
2,636
3,005
8,590
From Figure 6 and Table 3, the Kerner model giving the best fit for calcium
carbonate, had the worst fit for the polymeric microspheres. The Paul model, which gave
a slight overestimate of the calcium carbonate data, gives a good prediction for the
modulus of the polymeric microspheres. From Table 4 and the percent difference
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Taylor, S.
between the experimental data and theoretical models in Table 3, the effective modulus
of the polymeric microspheres is 3,000 MPa.
Figure 7 shows the average ultimate tensile stress versus volume percentage of
filler for test specimens containing calcium carbonate and polymeric microspheres.
Ultimate tensile stress decreases with an increase in filler volume percentage for both
fillers. A line which is equal to resin volume percentage times the ultimate stress of the
neat resin was fit through the data. The line fits the decreasing trend of the data,
signifying that the addition of filler creates voids, decreasing the overall ultimate tensile
strength of the material.
80.0
Ultimate Tesnile Stress (MPa).
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0%
10%
20%
30%
40%
50%
60%
Volume Percentage of Filler (%)
Figure 7: Ultimate Tensile Stress versus Volume Percentage of Filler for (♦) CaCO3
and (■) Polymeric Microspheres.
Microstructure Analysis
Images of the microstructure with 100μm scale for all volume fractions of both
fillers were taken using a Nikon metallograph microscope and are shown in Figure 8.
Overall the calcium carbonate samples show a good dispersion of particles and there is
good correlation between theoretical and actual volume fractions of calcium carbonate
using Image J image processing and analysis software. It appears that there are
significantly less microspheres than should be for all volume fractions of polymeric
microspheres. At 10% volume fraction of spheres, Image J indicated less than 1%
coverage of microspheres over the area of the image. The Microstructure at 50% volume
fraction of microspheres had about 21% coverage of microspheres.
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Taylor, S.
10% DL E135-055D
10% CaCO3
20% DL E135-055D
20% CaCO3
30% DL E135-055D
30% CaCO3
40% DL E135-055D
40% CaCO3
50% DL E135-055D
50% CaCO3
Figure 8: Microstructure Analysis
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Taylor, S.
There are a number of explanations as to why the actual volume fraction of
spheres is low. The first is that polymeric microspheres tend to deform and shrink at high
pressures and elevated temperatures as used in the molding process. At 550 KPa and
90°C the supplier says the spheres will have a decrease in size around 30%. The
remainder of the shortage of spheres could be one of two things. First, the spheres could
have a density greater than specified by the supplier causing the volume fraction of
spheres to be lower since the resin and fillers were mixed by appropriate weight.
Secondly, the density of the resin is one order of magnitude greater than that of the
microspheres causing the spheres and resin to separate before or during the molding
process. The spheres were weighed and have a density close to that specified by the
supplier. In addition, the vinyl-ester resin / microsphere matrix was mixed less than 60
seconds before each plaque was molded. Further testingt may be needed to fully
understand and quantify what is happening to the microspheres during the mixing and
molding process. The microstructure images and lack of spheres through the cross
section of the samples explain why the density measurements were greater than predicted
in the immersion density test.
Fracture Surface Analysis
Fracture surface images of a tensile specimen containing 40% polymeric
microspheres were obtained to understand the mechanics for failure. From Figure 9, it
can be seen that the bond between the polymeric microspheres and vinyl-ester resin is
poor, causing there to be whole spheres and pits in the surface. Although hard to see, the
largest sphere had a separation between the outer layer and resin on the bottom right
corner. The 20um sphere in the lower left corner of the image has broken in two, which
is what should be expected for the majority of the spheres in this resin / filler system.
The image supports the idea of both fillers having the effect of small voids through the
cross-section of the material, decreasing ultimate tensile strength. In order to increase the
overall strength of the material, filler sizing effects should be investigated to create a
stronger bond between resin and filler.
Figure 9: 40% Polymeric Microstructure Tensile Surface Fracture Surface
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Taylor, S.
SUMMARY AND CONCLUSIONS
In this study vinyl-ester plaques containing varying volume percents of calcium
carbonate and polymeric microspheres as filler, were compared by percent shrinkage,
density, tensile properties and microstructure analysis. Various theoretical models
including rule of mixtures, Kerner model, Paul Model and Hashin-Shtrikman Model were
all used to predict material behavior. The following conclusions have been made:
1. The Paul theoretical model does best in fitting both the calcium carbonate and
polymeric microspheres data for elastic modulus.
2. The effective modulus of the polymeric microspheres in Ashland Q6055 vinylester resin is approximately 3000Mpa
3. Further testing should be performed to fully explain the lower than expected
percentage of polymeric microspheres in molded plaques for all volume fractions.
4. Scatter in percent shrinkage measurements may be caused by inadequate
measuring techniques.
5. The decrease in ultimate tensile strength with increasing filler amounts is caused
by poor adhesion between the resin / filler matrix
6. Sizing effects should be investigated to create a greater bond strength between
filler and resin to increase the ultimate tensile strength of the material.
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REFRENCES
1.
Wouterson, E. W., Boey, F. Y.C., Hu, X., Wong, S.C., “Specific Properties
and Fracture Toughness of Syntactic Foam: Effect of Foam Microstructures,”
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2.
Dickie R. A., “On the Modulus of Three-Component Particulate-Filled
Composites,” Journal of Applied Polymer Science, Vol. 17, 1973, pp. 25092517.
3.
Li, Yongqi, and Metcalf, John B., “Two-Step Approach to Prediction of
Asphalt Concrete Modulus from Two-Phase Micromechanical Models,”
Journal of Materials in Civil Engineering, July/August 2005, pp. 407-415.
4.
Hsieh, Chin-Lung, Tuan, Wei-Hsing, “Elastic Modulus of Two-Phase
Materials,” Bulletin of the College of Engineering, N.T.U., Vol. 89, October
2003, pp. 35-44.
5.
ASTM D638-03, “Standard Test Method for Tensile Properties of Plastics,”
American Society for Testing and Materials, 2003
6.
ASTM D792-00, “Standard Test Method for Density and Specific Gravity
(Relative Desnity) of Plastics by Displacement,” American Society for
Testing and Materials, 2000
7.
Mettler-Toledo GmbH, Laboratory and Weighing Technologies, "Operating
Instructions, Density Determination Kit," Greifensee, Switzerland, 1998
8.
ASTM D6272-02, “Standard Test method for Flexural Properties of
Unreinforced and Reinforced Plastics and Electrical Insulating Materials by
Four Point Bending,” American Society for Testing and Materials, 2002
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