Mechanical Strength of Polyester Matrix Composites Reinforced with Coconut Fiber Wastes
Sergio N. Monteiro a, Luiz Augusto H. Terronesa, Felipe P. D. Lopesa, José Roberto M. d’Almeidab
a State University of the Northern Rio de Janeiro, UENF, Advanced Materials Laboratory, LAMAV, Av. Alberto Lamego, 2000, 28013-602, Campos dos Goytacazes, Brazil,*
b Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro, PUC-Rio, Rio de Janeiro, Brazil.
The structural characteristics and mechanical strength of polyester matrix composites incorporated with coconut fruit fibers (also known as coir fibers) wastes were evaluated. The coir fibers were used in two distinct forms; as a pressed mat or a tangled mass. Composites with amounts of coir fiber wastes up to 80 wt.% were fabricated. The as received coir fiber was characterized by scanning electron microscopy coupled with X-ray dispersion analyzer. The composites were prepared with two molding pressures, corresponding to loads of 5 and 10 ton, during the resin cure. The results obtained permitted to compare the technical performance of these composites with other conventional materials. It was found that, in principle, coir fiber reinforced polyester composites could, technically, replace wooden boards or gypsum panels, depending on the amount of incorporated fibers.
Keywords: coconut, coir fiber, composites, polyester matrix, mechanical strength.
Natural lignocellulosic fibers are, in many tropical and temperate regions, important items in the economy and a significant source of jobs for developing countries . Natural fibers from cultivated plants such as cotton, flax and sisal have been used in a large variety of products from clothes to house roofing. Today these fibers are appraised as environmentally correct materials owing to their biodegradability and renewable characteristics . Besides those plants that are cultivated with the main purpose of using the fiber, in other plants the fiber has secondary or no commercial interest and is, usually, regarded as a waste. This is the case of the coconut fruit, which is cultivated for its milk and pulp. The leftover coconut fruit shell or crust is formed by layers of fibers, and is of limited use, being normally discarded as garbage.
The common practice of considering a natural fiber as an undesirable waste, results in its burning or disposal on landfills. In any case, these practices will contribute to pollution. Therefore, in order to preserve the environment, it is necessary to find economically feasible solutions to the increasing amount of natural fiber wastes. This can be achieved through the understanding of natural fibers as recyclable materials, which could be used for different applications, ranging from handicrafts to reinforcement elements for composite materials .
The fibers from the coconut fruit crust that are nowadays disposed as an unwanted waste, might be seen as a recyclable potential alternative to be used in polymeric matrix composite materials [4-6]. The coconut palm tree (Cocos nucifera) is a multivalent fiber producer. Its fiber can be extracted from any part of the tree, including the long leaf sheath, the midribs of the leaves, the bark of the stalk and the fruit crust . Coconut fruit fibers, also known as coir fibers, are extracted from both the exocarp and the endocarp of the fruit. These coir fibers are currently been used in the form of mats in applications such as substrates for ornamental plants, soil support and seat cushion for automobiles. Figure 1 illustrates the seat cushions of cars fabricated in Brazil .
Figure 1: Fiber coir seat cushions for Mercedes Benz automobiles.
In spite of these possible uses, countries like Brazil, with a high production and consumption of coconut, are facing a problem to conveniently dispose the leftover crusts, which have a relatively long degradation time. Figure 2 shows a leftover pile of coconut fruit crust that was formed after the milk and the pulp have been consumed.
Figure 2: Pile of leftover coconut fruits.
Since the application of natural fibers is of current interest around the world and a commercial potential market already exists, the present work investigates the use of coir fibers as a possible reinforcement for polyester matrix composites.
2 Experimental Procedure
The coir fibers were used in two distinct types: individually loosen fibers in a tangled mass or pressed in mats with 1.0 cm of thickness. No special treatment was applied to the fibers, except a room temperature drying. Figure 3 illustrates the two types of coir fibers, tangled mass and pressed mats, used in this work.
A commercially available unsaturated orthoftalic polyester resin with 1 wt.% of metil-ethil-ketone as initiator, was used as matrix for the composites; no accelerator was applied for the cure. After being thoroughly mixed, the resin was poured onto the cavity of a steel mold, which was previously filled with one of the coir fiber types. Since the fibers in any of the two types of arrangement tangle or mat, did not have a preferred orientation, the composites in the present work were considered as randomly oriented .
Composites with amounts of coir fibers ranging from 10 to 80 wt.% were manufactured at two compacting pressure levels: 2.6 and 5.2 MPa, corresponding to molding loads of 5 and 10 ton. Composite plates were set for one week to be cured at room temperature before cut into six specimens per plate. Rectangular specimens 122 mm long, 25 mm wide and 10 mm thick were bend tested, using the three point flexural procedure, on a 100 kN capacity testing machine at a crosshead speed of 5 mm/min, which corresponds to a strain rate of 1.6 x 10-2 s-1. The span to depth ratio was maintained at 9 and the minimum number of specimens used for each test condition and each type of coir fiber was 6.
Figure 3: The two types of coir fibers used in the present work: (a) pressed mat; (b) tangled mass.
The coir fibers were also analyzed by scanning electron microscopy (SEM). The analysis was performed on gold sputtered samples in a Jeol microscope, coupled with EDS, operating at a voltage of 15kV.
SHAPE \* MERGEFORMAT
3 RESULTS AND DISCUSSION
The aspect of a coir fiber observed by SEM is shown in Fig. 4 with different magnifications. One should notice that the fiber surface is covered with protrusions and small voids. In principal, these aspects can facilitate the resin impregnation onto the fiber. Figure 5 shows an EDS spectrum performed at a micro region of the fiber’s surface in Fig. 4. This spectrum reveals that the coir fiber is essentially composed of carbon, as any organic matter, as well as oxygen and calcium. The calcium is associated with the protrusions in Fig. 4. The gold peaks in the spectrum correspond to the sputtered metal used to make the fiber surface an electrical conductor.
Figure 4: SEM images of a coir fiber with (a) 100x and (b) 500x of magnification.
Figure 5: EDS spectrum for a coir fiber.
Table 1 presents the maximum strength obtained in stress-strain curves from three point bend tests for the two types of coir fibers, tangle and mat, polyester composites cured at the two different molding pressures. In this table, the average strength and corresponding standard deviation are given for composites made with 10 to 80 wt.% of coir fibers. It is worth mentioning that composites with less than 50 wt.% of fibers are stiff and relatively hard, while those with more than 50 wt.% are soft and easy to deform. Therefore, in terms of mechanical behavior, it appears to be two distinct materials. Up to 50 wt.% of coir fibers, the polyester composites are rigid like wood. By contrast, above this percentage, the polyester resin does not properly impregnate the fibers, even for a molding pressure of 5.2 MPa. As a consequence, the material becomes flexible and easy to bend, performing like an agglomerate.
Table 1: Flexural rupture modulus for the coir fiber polyester composites
Figures 6 and 7 show the strength variation with the amount of coir fibers (tangle and mat) in the composites cured at the two molding pressure levels. In these graphs, obtained from the data in Table 1, it is important to note the following points. First, for both types of coir fibers and different compacting pressure, the strength tends to decrease with the amount of fibers. This reveals that the randomly oriented coir fibers are not providing a reinforcement effect in polyester matrix composites. Since the coir fiber alone has a tensile stress comparatively high, 106 to 270 MPa [2, 9, 10], compared to that of the bare polyester matrix, it is rather surprising that no reinforcement effect is obtained. However, other works on the subject [4, 5, 11] presented similar results that were attributed to the low modulus of the coir fiber. In fact, for an efficient stress transfer from the matrix to the reinforcing phase, the ratio between the elastic moduli of the fiber and of the matrix must be maximized .
Figure 6: Variation of the composite mechanical strength with the amount of pressed mat coir fibers.
Figure 7: Variation of the composite mechanical strength with the amount of tangled mass coir fibers.
A second point is that each value of strength has a relatively high dispersion, as given by its standard deviation. This is a consequence of the intrinsic variability found on natural fibers that ranges from their non-uniform cross-section, Fig.4, to their mechanical properties. Another point is that, for both types of fiber, there is a tendency for composites cured at higher molding pressure to present corresponding higher strength values. This is due to the more effective impregnation of the fibers by the resin, which certainly occurred as more pressure is applied during the polyester cure. However, the curves for both levels of molding pressure in Fig. 6 and 7 fall approximately within the statistical error related to the standard deviation. Consequently, one cannot be sure that the mold pressure has an actual influence, but just a tendency to improve the composite strength.
In terms of practical interest, the coir fiber composites may be regarded as valid alternatives to replace some conventional construction materials. For example, the rigid composites with less than 50 wt.% of coir fibers have mechanical strength above 10 MPa, Table 1, which is higher than that of a low-density wood particle board with 5-10 MPa  Composites with amounts of coir fibers higher than 50 wt.%, by contrast, are flexible and could be used in applications where structural resistance is not of importance. In spite of their relatively low strength, Table 1, these composites are stronger than gypsum board  and can be considered for panels or ceilings. The fact that the coir fiber composites are impervious to humidity and still support deformation, represent advantages in comparison with the relatively brittle gypsum board, which deteriorates in contact with water.
From the experimental results obtained it can be concluded that:
l Random oriented coir fiber-polyester composites are low strength materials, but with flexural strength high enough to be use as non-structural building elements;
l The lack of an efficient reinforcement by coir fiber was attributed to their low modulus of elasticity, in comparison to that of the bare polyester resin;
l With the fabrication route used, two different products were obtained, namely: rigid composites, for fiber loading less than 50%wt, and agglomerates – when the fiber loading was higher than 50%wt.
The authors thank the following Brazilian Agencies for supporting this investigation: CNPq, CAPES, FAPERJ and FENORTE/TECNORTE.
 Peijs, T., “Natural Fiber Base Composites”, Mater. Technol., v. 15, pp. 281-285, 2000.
 Bledzki, A.K., Gassan, J., “Composites Reinforced with Cellulose-Based Fibres”, Prog. Polym. Sci., v. 24, pp. 201-274, 1999.
 Schuh, T.G., Gayer, U., “Utilization of Natural Fibrs in Plastic Composites”, In: Leão, A.L., Carvalho, F. X., Frollini, E. (eds.), Lignocellulosic Plastic Composites, pp. 181-195, UNESP Publisher, Botucatu, SP, Brazil, 1997.
 Satyanarayana, K., Pillai, C.K.S., Sukumaran, K., et al., “Structure Property Studies of Fibre from Various Parts of the Coconut Tree”, J. Mater. Sci., v.17, pp. 2453-2462, 1982.
 Venkataswamy, K.G., Pillai, C.K.S., Prasad, V.S. et al., “Effect of Weathering on Mechanical Properties of Midribs Coconut Leaves”, J. Mater. Sci., v. 22, pp. 3167-3172, 1987.
 Calado, V., Barreto, D.W., d’Almeida, J.R.M., “The Effect of a Chemical Treatment on the Structure and Morphology of Coir Fibers”, J. Mater. Sci. Letters, v. 19, pp. 2151-2153, 2000.
 Callister Jr., W.D., Materials Science and Engineering – An Introduction, pp. 513, 5 ed., New York, John Wiley & Sons, 2000.
 Satyanarayana, K., Sukumaran, K., Kulkarni, A.G. et al., “Fabrication and Properties of Natural Fibre-Reinforced Polyester Composites”, Composites, v. 17, pp. 329-333, 1986.
 Baley, C., d’Anselme, T., Guyader, J., “Mechanical Properties of Coir Fiber Reinforced Polymeric Composites”, Composites, v. 37, pp. 28-33, 1997.
 Prasad, S.V., Pavithram, C., Rohatigi, P.K., “Alkali Treatment of Coir Fibres for Coir-Polyester Composites”, J. Mater. Sci., v.18, pp. 1443-1454, 1983.
 Agarwal, B.D., Broutman, L.J., Analysis and Performance of Fiber Composites, 1 ed., New York, John Wiley: 1980.
 Youngquist, J.A., “Natural Fiber Reinforced Composites”, In: Gayson M.(eds), Encyclopedia of Composite Materials and Components, pp. 661, 3 ed., chapter 5, New York, John Wiley & Sons, 1986.
 Carter, G.F., Paul, D.E., Materials Science and Engineering, 2 ed., Metals Park, USA, ASTM International, 1991.