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MECHANICAL PROPERTIES EVALUATION OF GLASS FIBER AND
HOLLOW GLASS BUBBLE REINFORCED POLYAMIDE 6
COMPOSITES
T. R. M. Ferreira1, F. Dias2, A. B. da Silva3
Av. Amazonas, 5253, Belo Horizonte / MG, CEP: 30.421-169
Centro Federal de Educação Tecnológica de Minas Gerais1,3, Fiat Chrysler
Automobiles2
ABSTRACT
This work presents a comparative study of mechanical properties and densities of
polyamide 6 (PA 6) composites reinforced with glass fiber and hybrid composites
of PA 6 reinforced with glass fibers and hollow glass bubble. The composites
studied was inject molded to obtain composites of PA 6/glass fiber (70/30) wt%
and hybrid composites PA 6/glass fiber/hollow glass bubble, in which the mass
fraction of fillers was maintained 30 wt%, with the hollow glass bubble content
varying between 3-10 wt%. The PA 6/ glass fibers/ hollow glass bubble hybrid
composites obtained in this study have density much smaller than traditional PA
6/ glass fibers composites, with a slight reduction in the mechanical properties,
i.e., tensile and impact strength were slight reduced with increase of hollow glass
bubbles in the composition, although, even with these reductions, the hybrid
composites studied have good mechanical properties.
Keywords (05): Composite Materials, Glass Fiber, Hollow Glass Bubble,
Polyamide 6, Density.
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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1. INTRODUCTION
In the last decades, the automotive industry is investing in researches in order to
reduce the weight of vehicles [1, 2]. Initially, weight reduction was concentrated
in replacing metal to plastic components. However, it is known that polymers, in
general, have mechanical properties inferior to metals, compromising its
application in structural parts, therefore it was perceived the need to incorporate
reinforcing materials to plastic components, such as fibers (glass, carbon, aramid,
etc.) and inorganic fillers (talc, calcium carbonate, etc.), although, these fillers
may result in increased density to the final composite, justifying the studies on
hollow glass bubbles [2, 3].
The hollow glass bubbles are spherical thin-walled glass (0.5-2.0 µm) made of
outer stiff glass and inner inert gas, with a diameter between 10-200 µm, which
combine very low weight, high resistance to uniform compression, good thermal
properties, acoustic insulation and good dielectric properties [2]. Based on these
properties the hollow glass bubbles has been used to produce polymeric
composites with corresponding properties, i.e, light weight, thermal insulation,
smooth surface and stiffness for different applications [1, 4]. These microspheres,
due to the spherical shape and the aspect radio equal to 1, can be incorporated into
the polymer in a high volume loading without resulting increased viscosity to the
composite, which could adversely affect subsequent processing and molding
operations [3,4]. However, considering reinforcement characteristics, anisotropic
fillers are more efficient compared to isotropic fillers, [2]. The glass fibers and
hollow glass bubbles combination constitutes an excellent solution to combine
lower density, dimensional stability and good mechanical properties. Hollow glass
bubbles offer the benefit of weight reduction and the glass fibers are able to
compensate the reduction in tensile strength and flexural strength caused by the
addition of these microspheres. Polyamide 6 (PA6) is a thermoplastic polymer
that has good chemical resistance to nonpolar solvents and excellent thermal and
mechanical properties. Polyamide-reinforced composites may be used to
advantage in several fields including automotive industries [2]. In this context,
this work proposes a study of the mechanical properties of hybrid composites PA
6/glass fiber/hollow glass bubble, aiming the application in automotive
components. For the selection of the composites formulations, the composite PA
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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6/glass fiber (70/30) wt% was considered as the base composite for comparison,
since it is a material widely used today in automotive industries.
2. MATERIALS AND METHODS
2.1 MATERIALS
PA 6 with density of 1.14 g/cm3 , melting temperature of 220.7°C (measured by
DSC), glass fibers with density of 2.45 g/cm3, average length between 3-4.5 mm,
compatibilized with organosilanes and hollow glass bubbles S42XHS (3MTM)
with density of 0.42 g/cm3.
2.2 METHODS
i) Composites preparation
The PA 6 / glass fibers composites and PA 6/glass fibers / hollow glass bubble
hybrid composites were produced by melting blending using a Thermo Scientific
twin-screw extruder, with side feeder. The twin screw speed, barrel temperature
and melt pressure at the die were 200 rpm, 250ºC, respectively. To mixture
preparation, the glass fibers and holow glass bubble were added in the side feeder,
in order to prevent its break. First of all, PA6/glass fibers composites with 30 wt%
of glass fibers were prepared this was the reference sample. To prepare the
PA6/glass fibers / hollow glass bubbles hybrid composites, the total amount of
fillers was kept at 30 wt%, changing the hollow glass bubble contents, between 3
and 10 wt%.
The processing was performed in two steps, according to the following
description: in the first stage, only PA 6 and glass fibers were introduced in the
extruder (it was used shear screw profile before and after the mixing zone); in the
second step, the pellets of the first stage and hollow glass bubbles were mixed,
using shear screw profile only before the mixing zone to minimize the breakage of
the microspheres. Both fillers were introduced in the side feeder.
After extrusion, tensile specimens were obtained by injection molding, according
to ASTM D638, using a Thermo Scientific equipment. The contents of all
formulations studied are summarized in Table 1.
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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Tab.1: Samples formulation studied.
Sample Formulation
A 70 wt% PA6 + 30 wt% GF*
B 70 wt% PA 6 + 27 wt% GF + 3 wt% HGB*
C 70 wt% PA 6 + 25 wt% GF + 5 wt% HGB
D 70 wt% PA 6 + 22.5 wt% GF + 7.5 wt% HGB
E 70 wt% PA 6 + 20 wt% GF + 10 wt% HGB
*The abbreviations GF and HGB refer to glass fiber and hollow glass bubble, respectively.
ii) Characterization
To perform the filler characterization, a calcining experiment was performed,
based on ISO 3451-1 (method A). The calcining process consisted in heating the
samples in a methane flame (natural gas) until it was extinguished, leaving only
inorganic filler (white residue) and some organic load (black residue). Then, the
samples were placed in a furnace at 750ºC, for 30 minutes. After this process, the
filler morphology was evaluated by scanning electron microscopy (SEM) using a
Shimadzu equipment, SSX-550 superscan model, and the average diameter of the
fillers was obtained measuring 100 fillers using the IMAGE Pro-Plus 4.5 software
(sample A and B after calcining processes were used). The amount of inorganic
filler in the formulations was calculated, using the Eq. A.
(A)
For samples characterization, the specimens were previously placed in an oven for
24 hours at 95ºC. This procedure aims to reduce the materials humidity, which
could affect the results, since PA 6 is a highly hygroscopic polymer. The
composites morphology were evaluated by SEM using the same equipment
described above. After the cryogenic fracture with liquid nitrogen, the samples
were metallized with gold through the sputtering technique.
The samples densities were obtained according to the Archimedes principle. For
mechanical characterization, the following tests were performed: tensile strength
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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test, according to ISO 527 and Izod impact resistance test, according to ISO 180.
For all tests, five specimens of each sample were used and, then, the arithmetic
mean of the values and their standard deviations was calculated.
The tensile strength test were performed in an Instron universal testing machine,
model 4467, and the following parameters were considered: speed of 50 mm/s,
extensometer of 50 mm and a load cell of 30 kN. The Izod impact was carried out
in a Ceast equipment 6545/000 model and the following parameters were
considered: pendulum of 2.75 J and preloading of 0.011 J. All mechanical tests
were performed at room temperature.
3. RESULTS AND DISCUSSION
3.1 FILLERS CHARACTERIZATION
Figure 1 (a) shows the SEM images to sample A, after the calcining process, and
Figure 1 (b) shows the diameter distribution of the glass fibers.
12 14 16 18 20 220
10
20
Fre
qu
en
cy (
%)
Diameter (m)
Fig. 1: SEM images of sample A, after calcining processes.
It was observed that glass fibers present a smooth and uniform surface and
average diameter between 16 ± 2 m. It was not possible to obtain the average
length of the fibers, due to the high breakage after extrusion. Figure 2 (a) shows
the SEM images to sample B, after the calcining process; through of this analysis
was possible to evaluated hollow glass bubbles, it have also a smooth surface and
average diameter between 115 ± 76 m.
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50 100 150 200 250 300
0
10
20
Diâmetro (µm)
Fre
qu
ên
cia
(%
)
Diâmetro
Fig. 2: SEM images of sample B, after calcining process.
3.2 COMPOSITES CHARACTERIZATION
The images obtained by SEM are showed below in fig. 3–7.
Fig. 3: SEM image of sample A. Fig. 4: SEM image of sample B
By analyzing the sample A (fig. 3), it was observed that the glass fibers show
good dispersion and distribution in the PA 6 matrix. However, it was possible to
visualize some small holes, resulting from the extraction of fibers during the
cryogenic fracture. For sample B (fig. 4), it was observed similar behavior, as it
also has good fillers distribution in the matrix. In this figure, it can be seen that the
glass fibers are more adhered to the matrix than the hollow glass bubbles.
It can be noted that samples C, D and E, despite of having a larger amount of
hollow glass bubbles than samples A and B, do not show agglomerates and
maintain good fillers distribution and dispersion in the matrix. These samples also
have holes due to cryogenic fracture. It was observed that the microspheres are
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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not well adhered to the matrix, evidencing an adhesion problem between filler and
polymer.
Fig. 5: SEM image of sample C. Fig. 6: SEM image of sample D.
Fig. 7: SEM image of sample E. Fig. 8: SEM image of sample D.
On the other hand, in fig. 8, it can be seen that the PA 6 matrix “wets” the glass
fibers. Therefore, the fibers have a rough surface, unlike the smooth surface
observed after calcining test; the microspheres remain with the smooth surface.
Furthermore, the hollow glass bubbles are more brittle than the glass fibers,
probably due to the poor adhesion with PA 6 matrix. Such behavior can be
justified by the presence of an organosilane compatibilizer on the glass fibers
surface, which promotes improved chemical interaction between polymer and
fiber. The microspheres, however, were not compatibilized.
Samples densities obtained according to the Archimedes principle, and the density
reduction with the increase of amount of hollow glass bubble, in comparison to
the PA6/ glass fibers with 30 wt% of filler are showed in table 2.
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Tab. 2: Density test results and analyses of density reduction.
Sample Density (g/cm³) Density Reduction (%)
A
(70/30) 1.32 ± 0.004 -
B
(70/27/3) 1.28 ± 0.003 3.0
C
(70/25/5) 1.22 ± 0.009 7.6
D
(70/22.7/7.5) 1.20 ± 0.001 9.1
E
(70/20/10) 1.12 ± 0.024 15.1
The density test shows a great reduction in the composites density, in comparison
to the reference sample, with the increasing fraction of hollow glass bubbles in the
material. The mechanical properties, i.e, Young’s modulus, tensile strength
values, elongation at break, obtained by tensile strength test, and absorbed impact
energy, obtained by Izod impact resistance test are summarized in table 3.
Tab. 3: Mechanical properties of samples.
Sample Young’s modulus
(MPa)
Tensile strenght
(MPa)
Elongation at
break (%)
Impact
Strenght (J)
A
(70/30) 5565.0 ± 399.2 86.8 ± 11.6 3.1 ± 1.2 4.3 ± 0.2
B
(70/27/3) 6139.2 ± 178.9 97.3 ± 1.3 3.7 ± 0.6 3.8 ± 0.1
C
(70/25/5) 5663.8 ± 275.2 89.8 ± 2.2 2.4 ± 0.1 3.3 ± 0.2
D
(70/22.7/7.5) 5483.4 ± 313.1 84.7 ± 1.3 2.6 ± 0.6 3.0 ± 0.1
E
(70/20/10) 5380.7 ± 298.4 79.3 ± 2.7 2.0 ± 0.1 2.5 ± 0.1
The mechanical properties of hybrid composites was slight reduced with increase
of amount of hollow glass bubble in its composition, in comparison to the
reference sample. According to the table 1, considering the standard deviations
the Young’s modulus was the same to all samples; the tensile strength, elongation
at break and impact resistance maximum reduction, between the studied
formulations, was about 8.6, 35.4 and 41.8%, respectively, to the PA6 / glass
fibers / hollow glass bubble (70 / 20/ 30) wt% (sample E), in comparison to the
PA6 / glass fibers (70 / 30) wt% (sample A),
22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais06 a 10 de Novembro de 2016, Natal, RN, Brasil
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It is possible to see that was a very low reduction, considering the lowest amount
of glass fibers in the sample E, remembering that glass fibers is the filler that
conferees better mechanical properties for these composites [2,3]. Nevertheless,
the sample B presented improved properties compared to the sample A. This
result could be attributed to a better dispersion and/or dispersion of fillers in the
thermoplastic matrix during processing for sample B.
Analyzing the weight reduction of samples, table 2 shows that a maximum
reduction of 15% was achieved (sample E). In all composites studied, a good
combination of weight reduction and mechanical properties maintenance was
successfully obtained. For example, by replacing five percent of fibers glass by
hollow glass bubbles in the composition, it was possible to reduce the composite
density in almost 8%, while the mechanical properties was practically the same
observed to the reference sample, only to the elongation at break and impact
strength it was observed a reduction about 22.6 and 23.6%, respectively, which
was not so high.
CONCLUSION
Young’s modulus, tensile strength, elongation at break, absorbed impact energy
and density of hybrid composites of PA 6 reinforced with glass fibers and hollow
glass bubble were studied thought the comparison of its properties with the
traditional PA / glass fibers composites. By observing the composites SEM
images, it could be verified a marked break of the microspheres. In addition, it
was noted that these fillers show a smooth surface, demonstrating the poor
adhesion between them and the polymer. However, the PA 6 matrix showed good
wettability with the glass fibers, therefore they have rough surfaces. It happened
because only the glass fiber surfaces were compatibilized with organosilane.
The mechanical properties of the hybrid composites were slight reduced with
increase of amount of hollow glass bubble in its studied formulations; however,
even with these reductions, the hybrid composites still exhibit a good set of
properties. On the other hand, the hybrid formulations showed a great reduction in
its densities and consequently decrease in weight. Thus, the materials developed
in this work combine low density and acceptable mechanical behavior, especially
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the formulations with up to 5 wt% of hollow glass bubbles (samples B and C).
Thus, these composites can be successfully used to replace some automotive
components, which are currently made by the composite PA 6/glass fiber (70/30)
wt%, providing a considerable weight reduction for these materials.
ACKNOWLEDGMENT
The authors thank FAPEMIG to the financial support, FCA to the work
motivation and the suppliers LANXESS and 3MTM to providing the raw materials
and to the important assistance during the project.
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