MECHANICAL PROPERTIES EVALUATION OF GLASS FIBER AND … · i.e., tensile and impact strength were slight reduced with increase of hollow glass bubbles in the composition, although,

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

[email protected]

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


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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.


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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


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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


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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),


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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.

REFERENCES

1- OLDENBOA, M.; FERNBERGC, S.P.; BERGLUND, L.A. Mechanical

behaviour of SMC composites with toughening and low density additives.

Composites A, v. 34, p. 875-885, 2003.

2- FERREIRA, J.A.M.; CAPELA, C.; COSTA, J.D. A study of the mechanical

behaviour on fibre reinforced hollow microspheres hybrid composites.

Composites A, v. 41, p. 345-352, 2010.

3- CAPELA, C.; FERREIRA, J.A.M.; COSTA, J.M.; MENDES, N. Mechanical

Properties of Injection-Molded Glass Microsphere-Reinforced Polyamide.

Journal of Materials Engineering and Performance IN press 2016.

4- LI, J.; LUO, X; LIN, X. Preparation and characterization of hollow glass

microsphere reinforced poly(butylene succinate) composites. Materials and

Design, v. 46, p. 902-909, 2013.


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MECHANICAL PROPERTIES EVALUATION OF GLASS FIBER AND … · i.e., tensile and impact strength were slight reduced with increase of hollow glass bubbles in the composition, although,