Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Comparison between tensile behavior of epoxy and polyester matrix composites reinforced with eucalyptus fibers
Caroline G. de Oliveiraa, Frederico M. Margemb, Sergio N. Monteiroc,, , Felipe Perissé Duarte Lopesc
a State University of Northern Rio de Janeiro – Av. Alberto Lamego, 2000, Pq. Califórnia, 28013-602 Campos dos Goytacazes, RJ, Brazil
b Faculdade Redentor, BR-356, 25 – Pres. Costa e Silva, 28300-000 Itaperuna, RJ, Brazil
c Military Institute of Engineering, Praça Gen. Tibúrcio, 80 – Praia Vermelha, 22291-270 Rio de Janeiro, RJ, Brazil
Received 15 June 2017, Accepted 09 August 2017
Abstract

Investigations and uses of composites reinforced with natural fibers have been growing over the last decades. These composites offer economical, technical, societal and environmental advantages. As a consequence, they became promising alternatives to replace synthetic fibers from non-renewable sources. In this way, the study of the properties of these natural composites is of utmost importance to enable its use. The present work compares the tensile mechanical behavior of polymeric composites reinforced with natural eucalyptus fibers. Two matrices widely applied in the industry were used: epoxy and polyester. The results showed that in both cases the introduction of eucalyptus fibers had minor effect on the matrix reinforcement.

Keywords
Eucalyptus fiber, Polymeric composites, Tensile stress
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1Introduction

The current economic growth and technological development are motivating the search for new materials to meet modern technological challenges and, at the same time, preserve the environment. Thus, a demand for high-performance and environmentally friendly materials has been growing in recent decades. Polymeric matrix composites are one of the most important materials nowadays, with applications ranging from sports to automobile and aeronautic industries. These composites are usually reinforced with synthetic fibers, such as glass and carbon fibers [1]. However, these composite materials are obtained from non-renewable resources, which are related to inherent environmental disadvantages. In particular, glass fiber composites are associated with intense processing energy using non-renewable fossil petroleum derivatives [2,3]. By contrast, the modern society requires materials that preserve the environment. Natural lignocellulosic fibers have been studied as composite reinforcement alternative, due to their lower density, lower cost, renewability, biodegradability and relevant mechanical properties [4–16].

The use of natural fibers as reinforcement to polymeric matrix is of interest for several industries. Indeed, natural fiber composites are today used in automobile parts, building panels and furniture [17–20]. In order to find new possibilities to this research field, the eucalyptus fibers have been recently studied as an alternative of reinforcement material and showed significant potential for polymer composites [20–22]. These fibers are extracted from the bark of the Eucalyptus citriodora tree, which is extensively cultivated in Brazil [23]. A more detailed investigation on the mechanical properties of eucalyptus fiber composites is still needed for possible engineering applications. Therefore, the objective of this work is to investigate the tensile behavior of epoxy and polyester matrix composites reinforced with continuous and aligned eucalyptus fibers.

2Experimental procedure

The material used in this work was untreated eucalyptus fibers. The fibers (Fig. 1b) were obtained from the bark (Fig. 1a) of the eucalyptus tree. Eucalyptus plantations are common in Brazil and barks were supplied from a paper industry located in the state of Minas Gerais.

Fig. 1.
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Eucalyptus tree bark (a) and eucalyptus fibers (b).

Composites with different volume fractions (30, 40 and 50vol%) were confectioned by laying the fibers in a steel mold and pouring liquid resin, either DEGEBA/TETA epoxy or methyl-ethyl ketone hardened orthophtalic polyester, under pressure of 3MPa. The specimens were cured for 24h at room temperature and then tested in an universal Instron machine, model 5582, at 298K (25°C). Finally, the fracture surface was analyzed by scanning electron microscopy (SEM) under an acceleration voltage of 15kV.

3Results and discussion

Fig. 2 shows some examples of the load vs. elongation curves obtained from the Instron machine software. These curves display a typical elastic line followed by a sudden fracture for all the compositions, which discloses the brittle behavior of the matrix as well as of the composites. It is important to mention that the polyester matrix composites presented similar behavior.

Fig. 2.
(0.12MB).

Typical load vs. elongation curves: (a) epoxy matrix, (b) 30vol%, (c) 40vol% and (d) 50vol% of eucalyptus fibers.

The tensile strength and elastic modulus results for epoxy and polyester composites with different percentages of eucalyptus fibers are shown in Figs. 3 and 4, respectively. It is possible to note that, within the standard deviation, the presence of eucalyptus fibers did not affect the tensile strength but increased the elastic modulus of epoxy matrix composites. Indeed, the value of elastic modulus increased approximately 50% for epoxy matrix composites. By contrast, for polyester matrix composites, within the error bars, the introduction of eucalyptus fibers slightly decreased the tensile strength, while the elastic modulus remains constant.

Fig. 3.
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Tensile strength (a) and elastic modulus (b) of epoxy matrix composites with different volume fractions of eucalyptus fiber.

Fig. 4.
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Tensile strength (a) and elastic modulus (b) of polyester matrix composites with different volume fractions of eucalyptus fiber.

The decrease observed in the tensile strength for both epoxy and polyester matrix composites is due to the very low adhesion between the matrix and the fibers. It is worth noting that the natural fibers, eucalyptus particularly, have high amounts of water on their surfaces, aggravating this effect.

Figs. 5 and 6 show by SEM the fracture surfaces of the epoxy and polyester matrix composites, respectively, after tensile testing. In these figures, it is possible to observe some voids in the interface between the fiber and the matrix, evidencing the detachment of the fiber from the matrix. It occurred to both matrices. As mentioned, this is attributed to the low adhesion between both matrix and fibers, which is frequently observed in natural fibers composites [15]. It is a consequence of high amounts of water in the natural fiber surface and their hydrophilic nature, contrasting with the hydrophobic nature of the polymeric matrix [24]. An alternative to overcome this problem is to apply a pre-treatment to the fibers, which is beyond the objective of this work.

Fig. 5.
(0.18MB).

SEM fractograph of a tensile ruptured epoxy composite specimen with 40vol% of eucalyptus fibers: (a) 200× and (b) 400×.

Fig. 6.
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SEM fractograph of a tensile ruptured polyester composite specimen with 30vol% of eucalyptus fibers: (a) 200× and (b) 400×.

Despite the small effect on tensile properties, it is known that eucalyptus fibers are extremely effective as reinforcement for impact and bending stresses [20]. This feature is extremely important for the production of pieces for the automotive industry [17–19]. Thus, it is worth to continue studying these fibers as an alternative to the use of synthetic ones. It is believed that natural fibers started a new era in the field of composite materials.

4Conclusions

  • The introduction of eucalyptus fibers, within the standard deviations, did not change the tensile strength of epoxy matrix composite and slightly decreased the tensile strength of polyester matrix composite.

  • The introduction of eucalyptus fibers increased the elastic modulus in both composites, despite the relatively high dispersion of values.

  • SEM fractographs revealed a poor adhesion between both the epoxy and polyester matrices with the eucalyptus fiber, which contributes to a small decrease in the composites tensile strength.

  • A pre-treatment can be an alternative to make eucalyptus fibers better reinforcement material. Although, it implies in higher costs and chemical waste disposal to the environment.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank the support to this investigation by the Brazilian agencies: CNPq, CAPES and FAPERJ.

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Paper was a contribution part of the 3rd Pan American Materials Congress, February 26th to March 2nd, 2017.

Corresponding author. (Sergio N. Monteiro snevesmonteiro@gmail.com)
Copyright © 2017. Brazilian Metallurgical, Materials and Mining Association