IZOD IMPACT TESTS WITHPOLYESTER MATRIX REINFORCED WITH BURITI FIBERS

Michel Picanço Oliveira(2),Verônica Scarpini Candido(2), Giulio Rodrigues Altoé(1), Frederico Muylaert Margem(1), Isabela Leão Amaral da Silva(1), Sergio Neves Monteiro(2)

(1) IME - Military Institute of Engineering, Rua Marechal Rondon, 282, Vinhosa, Itapeurana-RJ, 28300-000, . (2) State University of the Northern Rio de Janeiro, UENF.

Abstract

The buriti palm tree (Mauritia flexuosa) has in its petiole natural fibers with great potential for reinforcement of polymer composites. This work attempts to evaluate the impact resistance of this type of fiber reinforcing polyester matrix. The fibers were mixed with polyester resin under pressure in a metallic mold, and cured at room temperature for 24 hours. Specimenswere prepared with fibers percentages varying from 0 to 30% in volume.These specimens were tested in an Izod impact pendulum and the fracture surfaces were examined by scanning electron microscopy, SEM. The impact resistance increased substantially with the relative amount of buriti fiber reinforcing the composite. This performance was associated with the resistance imposed by the fibers and the crack propagation behavior.

Keywords: buriti fibers, impact izod, polyester, composites, SEM.

INTRODUCTION

Natural fibers are steadily substituting synthetic fibers, particularly the common glass fiber, as the reinforced phase of polymeric composites in many engineering applications such as automobile interior components, cyclist helmets, housing panels and windmill fins(1-4). The lignocellulosic fibers obtained from vegetables offer societal, economical, environmental and technical benefits(5,6) in comparison to the glass fiber as composite reinforcement. In particular, the impact resistance of a naturally flexible lignocellulosic fiber is a technical advantage over the brittle glass fiber in a situation of a crash event. This is the case of automobile parts such as the head-rest and the interior front panel that should not have a brittle rupture during an accident. In fact, the parts should be soft and able to absorb the impact energy without splitting in sharp pieces, to avoid injuring the passengers (6).

Lignocellulosic fibers such as coir, flax, jute, ramie, curaua and sisal are currently being used in automobile composite parts that require both strength and toughness (4). The buriti fiber, although strong and flexible (7) has not yet been applied in composites for automobile components. Actually, the fibers obtained from the petiole of the buriti palm tree commonly used to fabricate ropes and baskets owing to its high strength. This has motivated the study of the mechanical characteristic of the fibers.

In spite of existing works on the properties of buriti fiber composites(8-9), the impact resistance of continuous and aligned fiber reinforcing polymeric composites has yet to be evaluated. Therefore, the objective of the present work was to access the toughness through the energy absorbed by notched Izod impact specimens of polyester composites reinforced with different amounts of continuous buriti fibers.

MATERIALS AND METHODS

Petioles extracted from buriti palm tree (Mauritia flexuosa) were supplied by Prof. Nubia S.S. Santos from the University of Para. Fibers were longitudinally cut from the petiole with a sharp razor. A random lot of one hundred of these fibers were statistically evaluated in terms of equivalent diameter distribution. Figure 1 shows the histogram corresponding to six arbitrary intervals of diameter. From this histogram, an average equivalent diameter of 0.58 mm was calculated.

Figure 1. Distribution histogram for six diameter intervals.

Several other buriti petiole fibers were cleaned in water and dried at 60°C in a stove to be used as composite reinforcement. Continuous and aligned fibers were laid down, in separate amounts of 10, 20, and 30 % vol, mixed with still fluid unsaturated orthophtalic polyester resin with 0.5% hardener (ethyl-methyl-ketone), in a metallic mold with 100 x 140 mm, under 10ton of pressure to ensure the correct impregnation between the resin and fibers. A 24 hours cure at room temperature was allowed for these composite samples. After unmolded, the samples were cut following the ASTM D256 standard.Ten specimens for each percentage of buriti fiber composite were impact tested in aPANTECpendulum, Fig. 1, set in the Izod configuration.

Figure 1: Pantec impact pendulum.

RESULTS AND DISCUSSION

Figure 3 shows the macrostructural appearance of broken specimens with different amounts of buriti fibers, from 0 to 30% in volume. In this figure, it is important to note that only the pure polyester specimen was completely separated in two parts. For the composite specimens, the impact failed to separate the sample in two parts. This indicates that the polyester matrix without the addition of buriti fiber is brittle and the impact-generated crack propagates without being arrested until the specimen separates. However, for any proportion of buriti fiber the initial propagating crack is blocked and the rupture migrates to the fiber/matrix interface. The specimen then bends upon impact of the hammer, but does not separate due to the flexibility of the fibers that are not broken.

Figure 2: Typical specimens of polyester matrix composites with different volume fractions of buriti fiber, broken by Izod impact

The impossibility to completelybreak the specimen, as seen in Fig.2, indicates that samples with 10%, 20% and 30% of fiber are underestimated to obtain the value of the composite toughness. If all the fibers were broken, causing the specimen separation in two parts, the energy absorbed by the fiber would be even greater.

There are important factors related to the impact fracture characteristic of polymeric reinforced with long and aligned natural fibers. The relatively low interface strength between a hydrophilic natural fiber and a hydrophobic polymeric matrix contributes to an ineffective load transfer from the matrix to a longer fiber. This results not only in a relatively greater fracture surface but also a higher impact energy needed for the rupture. The impact energy obtained in the Izod impact tests of polyester matrix composites reinforced with different volume fractions of fibers buriti are shown in Table 1.

Table 1 - Izod Impact energy for polyester matrix composites reinforced with buriti fibers.

Fiber Content (%) / Impact Energy (J/m)
0 / 13.00  2.00
10 / 165.70  29.70
20 / 231.10  35.30
30 / 303.91  42.50

From the data in Table 1, the graph of the energy absorbed in the Izod impact vs. the corresponding volume fraction of buriti fibers in the polyester matrixwas plotted, as shown in Fig. 4.This figure shows a significant increase in the Izod impact energy with the amount of buriti fibers.

Figure 3: Variation of the Izod impact energy with the amount of fiber in the composites.

Other lignocellulosic fibers present the same behavior(4), which is due to the heterogeneous nature of these fibers, causing substantial dispersion in the composites properties. Even considering the error bars, it is possible to interpret the increase of impact energy, as following a linear relationship.

The values obtained for the impact energyare the second highest obtained so far for Izod configuration with polyester matrix, overcome only by the ramie fiber, which reached 353 J/m with 30% fiber in polyester matrix(3). Indeed, for most lignocellulosic fibers, the increase inthe Izod impact energy is directly related to the increase in the fiber volume fraction(9).

The fact that buriti fibers remain unbroken after the impact, as shown for the composites in Fig. 3, is an indication that cracks had propagated along the fiber/matrix interface causing the fiber separation from the polyester but not the fiber rupture. This effect increasesthe cracks trajectory through the composite, creating a greater impact energy. Similar behavior was also observed in the pullout tests(10). Thus, the composites absorb relatively larger amounts of energy, leading to an increase in the impact resistance.

Figure 4 shows the typical fracture surface of a 30% buriti fiber compositecaused by an Izod impact test. With lowmagnification, Fig 4(a), the surface region where the fibers are bending, instead of breaking, can be observed. With higher magnification, Fig 4(b), one sees the interface fiber/matrix where a crack is propagating. The crack “river pattern” is observed at the left side of Fig. 4(b).

Figure 4: SEM micrograph of the fracture for a 30% buriti composite. (a) 30 x; (b) 1600 x.

Figure 4 (b) also shows a detail of the interface between the polyester matrix and a buriti fiber, especially signs of adhesion between them. Some cracks may be observed, however, propagating through the fiber/matrix interface.This behavior confirms the mechanism of rupture between the buriti fiber and polyester matrix associated with low interfacial resistance, resulting in greater impact energy.

CONCLUSIONS

  • There is a significant increase in energy absorbed in Izod impact tests with the incorporation of buriti fibers in a polyester matrix composite.
  • The weak interface between the buriti fibers and the polyester matrix contributes greatly to increase the impact energy by changing the cracks trajectory in the composite.
  • Most of this increase in toughness is apparently due to the low buriti fiber/polyester matrix interfacial shear stress. This results in a higher absorbed energy as a consequence of a longitudinal propagation of the cracks throughout the interface, which generates larger rupture areas, as compared to a transversal fracture.

ACKNOWLEDGEMENTS

The authors thank the support to this investigation by the Brazilian agencies: CNPq, CAPES, and FAPERJ.It is also acknowledged the donation of the buriti petiole used in this work by Prof. Nubia S.S. Santos from UFPA as well as the permission to the use of the SEM microscope by the PEMM from COPPE/UFRJ.

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