- Email: [email protected]
polyester composites
ARTICLE IN PRESS Polymer Testing 27 (2008) 591– 595
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Mechanical performance of coir fiber/polyester composites S.N. Monteiro a, L.A.H. Terrones a, J.R.M. D’Almeida b, a
Science and Technology Center, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000-Horto-28015-820, Campos, RJ, Brazil ´lica do Rio de Janeiro, Materials Science and Metallurgy Department, Pontifı´cia Universidade Cato ´vea-22453-900, Rio de Janeiro, RJ, Brazil ˆs de Sa ˜o Vicente,225-Ga Rua Marque
b
a r t i c l e in fo
abstract
Article history: Received 24 January 2008 Accepted 10 March 2008
The structural characteristics and mechanical properties of coir fiber/polyester composites were evaluated. The coir fibers were obtained from disregarded coconut shells that if not properly processed constitute an environmental hazard. The as-received coir fiber was characterized by scanning electron microscopy coupled with X-ray dispersion analysis. Composites prepared with two molding pressures and with amounts of coir fiber up to 80 wt% were fabricated. Up to 50 wt% of fiber, rigid composites were obtained. For amounts of fiber higher than this figure, the composites performed like more flexible agglomerates. The results obtained for flexural strength allowed comparison of the technical performance of the composites with other conventional materials. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Coir fiber Composites Flexural strength Micromechanics
1. Introduction Natural fibers such as cotton, flax and sisal have been used since historical times in a large variety of products, ranging from clothes to house roofing. Today, these fibers are appraised as environmentally correct materials owing to their biodegradability and renewable characteristics. Moreover, lignocellulosic fibers are neutral with respect to the emission of CO2 [1]. This is an extremely important aspect, and puts lignocellulosic fibers as materials in context with the Kyoto protocol. In addition to plants that are cultivated with the main purpose of using the fiber, in other plants the fiber has secondary or no commercial interest at all. This is the case of the banana trees, which are cultivated for the fruits. From the leftover leaves and bark of banana trees, fibers with good mechanical properties can be obtained [2]. However, these fibers are seldom used since the tree is normally discarded as garbage after the fruits are collected.
Corresponding author. Tel.: +55 21 3527 1789; fax: +55 21 3527 1236.
E-mail address: [email protected] (J.R.M. D’Almeida). 0142-9418/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.03.003
Another crop with a similar characteristic is coconut (Cocos nucifera). Plantations of coconut are spread all over the world in tropical and sub-tropical regions, and are an important item in the economy of many of these regions. The annual world production of coconut is about 42 million MT, which would equate to almost 50 billion coconuts [3]. The main use of coconut is for culinary purpose, and after extraction of the copra and/or of the liquid endosperm that fills the interior of the fruits the fruit shell is disregarded. Transformation industries and regions where consumption of coconut is high have, therefore, a large problem to conveniently dispose of this waste, since the fruit shell has a long decay time. The coconut fruit is, in fact, adapted for being dispersed by seawater, and can float for months without rotting. Growing attention is nowadays being paid to coconut fiber. Fibers extracted from the husk of the nut, known as coir fiber, are now being commercially used, blended with natural rubber latex in the production of seat cushion parts in automobiles [4]. These fibers are extracted from the external layer of the exocarp and from the endocarp of the fruit. The coconut palm can, in fact, be regarded as an integral fiber producer because fibers can be extracted from many parts of the palm, such as from the leaf sheath, the bark of the petiole or from the midribs of leaves [5,6].
ARTICLE IN PRESS 592
S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
Many aspects of the use of coir fibers as reinforcement in polymer–matrix composites are described in the literature. Coir fiber–polyester composites were tested as helmets, as roofing and post-boxes [7]. These composites, with coir loading ranging from 9 to 15 wt%, have a flexural strength of about 38 MPa. Coir–polyester composites with untreated and treated (PMMA and PAN grafted) coir fibers, and with fiber loading of 17 wt%, were tested in tension, flexure and notched Izod impact [8]. The results obtained with the untreated fibers show clear signs of the presence of a weak interface—long pulled-out fibers without any resin adhered to the fibers—and low mechanical properties were obtained. Although showing better mechanical performance, the composites with treated fibers present, however, only a moderate increase on the values of the mechanical properties analyzed. Alkali treatment is also reported for coir fibers [9,10]. Treated fiber–polyester composites, with volume fraction ranging from 10% to 30%, show better properties than composites with untreated fibers, but the flexural strength of these composites was consistently lower than that of the bare matrix. A maximum value of 42.3 MPa is reported against a value of 48.5 MPa for the neat polyester. Acetylation of coir fibers increases the hydrophobic behavior, increases the resistance to fungi attack and also increases the tensile strength of coir–polyester composites [11,12]. However, the fiber loading has to be fairly high, 45 wt% or even higher, to attain a significant reinforcing effect when the composite is tested in tension. Moreover, even with high coir fiber loading fractions, there is no improvement in the flexural strength [12]. From these results, it is apparent that the usual fiber treatments reported so far did not significantly change the mechanical performance of coir–polyester composites. Since most data in literature with few exceptions [12] usually cover only a specific loading fraction of fibers, and remembering that the increase of cost due to the treatment of the fibers must be a point of concern, this work was aimed at analyzing the flexural mechanical behavior of untreated coir–polyester composites covering a larger range of weight fractions. The effect of the molding pressure on the flexural strength of the composites was also evaluated.
2. Experimental procedures and materials Individually loose coir fibers were used in two distinct forms: tangled or pressed in mats with a thickness of 1.0 cm. The fibers were used untreated, except that they were dried at 50 1C for 24 h. Fig. 1 illustrates the two kinds of coir fibers, tangled mass and pressed mats, used in this work. A commercially available unsaturated orthoftalic polyester resin with 1 wt% of methyl-ethyl-ketone as catalyst was used as matrix for the composites. After being thoroughly mixed, the resin was poured into the cavity of a steel mold, which was previously filled with a suitable amount of coir fiber. Composites with amounts of coir fibers ranging from 10 to 80 wt% were manufactured at two pressure levels, namely: 2.6 and 5.2 MPa. The cure was done under pressure at room temperature.
Fig. 1. Aspect of the coir fibers used: (a) tangled fibers and (b) pressed mat.
As the fibers in any of the two configurations (tangled or mat) did not have a preferred orientation, the composites fabricated in the present work are considered as randomly oriented. Rectangular specimens 122 mm long, 25 mm wide and 10 mm thick were bend tested, using the three-point bending procedure, on a 100 kN capacity testing machine. The velocity of the test was 5 mm/min, which corresponds to a strain rate of 1.6 102 s1. The span-to-depth ratio was maintained constant at 9, and the minimum number of specimens used for each of the test conditions and coir fiber arrangements was 6. Before their incorporation in the composites, the coir fibers were analyzed by scanning electron microscopy (SEM). The analysis was performed on gold-sputtered samples in a microscope, coupled with EDS, operating at a beam voltage of 15 kV. 3. Experimental results and discussion The characteristic surface aspect of a coir fiber observed by SEM is shown in Fig. 2. As reported previously [10,13], one should notice that the fiber surface is covered with protrusions and with voided areas left by detached
ARTICLE IN PRESS S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
593
Fig. 2. Surface morphology of coir fibers, showing the characteristic array of protrusions (’) found at the surface of the fibers.
protruded material. In principle, and similar to what was observed for piassaba fibers [14], these morphological aspects can facilitate the resin impregnation onto the fiber. Fig. 3 shows an EDS spectrum performed at the fiber’s surface. The spectrum reveals, besides the obviously carbon and oxygen common to any organic matter, the presence of calcium and other alkaline and alkaline earth elements. Calcium was associated with the protrusions shown in Fig. 2. This result was surprising, since silicon-rich protrusions were identified in a previous work [13]. It is suggested that this marked difference could be due to differences in the soil where coconuts were raised. A detailed analysis needs to be carried out on this specific topic. The gold peaks in the spectrum correspond to the sputtered metal used to avoid charging at the microscope chamber during the analysis. Table 1 presents the average flexural strength and corresponding standard deviation obtained in the threepoint bend tests for the two coir fiber arrangements, tangled or mat, polyester composites cured at the two different molding pressures. As a first comment, one should say that composites with less than 50 wt% of fibers were found to be stiff and relatively hard, while those with more than 50 wt% were soft and deformable. Therefore, with respect to the mechanical behavior, the composites manufactured act as two completely distinct materials. Up to 50 wt% of coir fibers, the manufactured composites are rigid, structural-like, materials. By contrast, above this percentage, the polyester resin does not properly impregnate all fibers, even for a molding pressure of 5.2 MPa. As a consequence, the material becomes flexible and easy to bend, performing like binderless agglomerates [15]. Fig. 4 shows the strength variation with the amount of coir fibers for the composites fabricated 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 fiber and different compaction pressures, the strength tends to decrease with the amount of fiber. This reveals that the randomly
Fig. 3. EDS spectrum of the fiber’s surface.
Table 1 Flexural strength of the coir fiber– polyester composites Weight % of coir fiber
Molding pressure Pressed mat
10 20 30 40 50 60 70 80
Tangled mass
2.6 MPa
5.2 MPa
2.6 MPa
5.2 MPa
25.775.2 18.973.6 14.574.5 9.671.1 6.070.9 4.371.7 3.071.2 0.970.6
31.276.7 22.671.2 21.473.5 11.473.3 11.971.5 5.971.2 4.672.6 1.070.4
29.176.8 28.272.1 22.579.9 20.775.1 15.577.9 6.775.1 5.473.5 3.672.2
32.873.8 29.573.5 24.773.3 23.975.9 21.178.4 14.375.8 8.873.6 6.173.0
oriented coir fibers are not reinforcing the polyester matrix at all. Previous data showing the non-reinforcing behavior of coir fibers in composites submitted to bending do not show a steady decrease of the flexural strength as observed here. The results obtained by Hill and coworkers [12] present a maximum around 50 wt%. Since in the present work a smaller span-to-depth ratio (S/d ¼ 9) was used in relation to that of Hill and coworkers (S/d ¼ 16), and knowing that the smaller the span-to-depth ratio the greater the contribution of the shear stress, one could assert that the results obtained are not directly comparable. The main conclusion of both works, however, is the non-reinforcing behavior of coir fibers. In fact, the reinforcing behavior of coir fibers in polyester matrix is expected to be minimized due to the low modulus of the coir fiber. Values as low as 4.7 GPa are reported [12], and for an efficient partition of the load applied to the composite between the matrix and the fibers, the ratio of the fibers’ elastic modulus to that of the matrix must be
ARTICLE IN PRESS 594
S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
1 0.7
40
Vf = 0.1
20
10
0.5
Molding Pressure 2.6 MPa 5.2 MPa
0 10
20
40 30 50 60 70 Amount of Coir Fiber (wt.%)
0
80
1
50 Ef / Em
100
Fig. 5. Variation of the ratio of the load carried out by fibers with respect to the load applied to the composite as a function of the elastic moduli ratio and volume fraction of fibers.
40 Flexural Rupture Strength (MPa)
Vf = 0.3
30 Pf / Pc
Flexural Rupture Strength (MPa)
0.5
TANGLED MASS
PRESSED MAT 30 Molding Pressure 2.6 MPa 5.2 MPa
20
10
0 10
20
30 40 50 60 70 Amount of Coir Fiber (wt.%)
80
Fig. 4. Variation of the flexural strength with the mass fraction of coir fibers and molding pressure. (a) tangled fibers; (b) pressed mat.
maximized [16]. Fig. 5 shows that for fiber/matrix modulus ratio close to 1, as would be the case for the composites analyzed here, only a fraction of the load applied to the composite (Pc) will be carried by the fibers (Pf). Namely, 0:1pðP f =P c Þp0:7 for volume fractions of fiber ranging from 0.1 to 0.7. Therefore, the matrix is always under high loading and failures under low stress levels are to be expected. Another point is that the experimental results have 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 nonuniform cross-section to their mechanical properties. A further point is that, for both types of fiber used, 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 a higher pressure is applied during the matrix setting. However, the curves for both levels of molding pressure, Fig. 4a and b, fall approximately within the statistical error related to the
standard deviation. Consequently, one cannot be sure that for the molding pressures used in this work pressure has an actual influence, but just a tendency to improve the composite strength. In fact, the molding pressure was observed to have only a secondary effect on the flexural mechanical behavior for piassava–polyester composites [17] and chopped bagasse–polyester composites [18]. In terms of practical interest, the coir fiber composites may be regarded as valid alternatives to replace some conventional materials nowadays used by the building industry, and also as furniture. For example, the rigid composites with less than 50 wt% of coir fibers can be tailored to have tensile strength above 10 MPa, Table 1, which is higher than that of a low-density wood particle board with 5.5–9.7 MPa [19]. 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 fact, and in spite of their relatively low strength, Table 1, these composites are stronger than gypsum board [19] and can be used in panels or ceilings. Moreover, the fact that coir fiber composites are impervious to humidity represents a clear advantage in comparison with the relatively brittle gypsum board, which deteriorates in contact with water. 4. Conclusions From the experimental results obtained, the following conclusions can be made:
Random oriented coir fiber–polyester composites are
low-strength materials, but can be designed to have a set of flexural strengths that enable their use as nonstructural building elements. The lack of an efficient reinforcement by coir fibers is attributed to their low modulus of elasticity, in comparison with that of the bare polyester resin.
ARTICLE IN PRESS S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
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.
Acknowledgment The authors acknowledge the financial support from the Brazilian Agency CNPq. References [1] A.K. Mohanty, M. Misra, L.T. Drzal, Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world, J. Polym. Environ. 10 (2002) 19. [2] A.G. Kulkarni, K.G. Satyanarayana, P.K. Rohatgi, K. Vijayan, Mechanical properties of banana fibres (Musa sepientum), J. Mater. Sci. 18 (1983) 2290. [3] J.G. Ohler, The coconut palm and its environment, in: J.G. Ohler (Ed.), Modern Coconut Management—Palm Cultivation and Products, FAO and Intermediate Technology Publications Ltd., London, 1999, pp. 12–156. [4] T.G. Schuh, U. Gayer, Automotive applications of natural fiber composites, in: A.L. Lea˜o, F.X. Carvalho, E. Frollini (Eds.), Lignocellulosic–Plastics Composites, Unesp Publishers, Botucatu, Brazil, 1997, pp. 181–195. [5] M.A. Venkataswamy, C.S.K. Pillai, V.S. Prasad, K.G. Satyanarayana, Effect of weathering on the mechanical properties of midribs of coconut leaves, J. Mater. Sci. 22 (1987) 3167. [6] K.G. Satyanarayana, C.K.S. Pillai, K. Sukumaran, S.G.K. Pillai, P.K. Rohatgi, K. Vijayan, Structure property studies of fibres from various parts of the coconut tree, J. Mater. Sci. 17 (1982) 2453. [7] K.G. Satyanarayana, K. Sukumaran, A.G. Kulkarni, S.G.K. Pillai, P.K. Rohatgi, Fabrication and properties of natural fibre-reinforced polyester composites, Composites 17 (1986) 329.
595
[8] J. Rout, M. Misra, A.K. Mohanty, S.K. Nayak, S.S. Tripathy, SEM observations of the fractured surfaces of coir composites, J. Reinf. Plast. Compos. 22 (2003) 1083. [9] J. Rout, M. Misra, S.S. Tripathy, S.K. Nayak, A.K. Mohanty, The influence of fibre treatment on the performance of coir–polyester composites, Comp. Sci. Technol. 61 (2001) 1303. [10] S.V. Prasad, C. Pavithram, P.K. Rohatgi, Alkali treatment of coir fibres for coir–polyester composites, J. Mater. Sci. 18 (1983) 1443. [11] C.A.S. Hill, H.P.S.A. Khalil, The effect of environmental exposure upon the mechanical properties of coir or oil palm fiber reinforced composites, J. Appl. Polym. Sci. 77 (2000) 1322. [12] C.A.S. Hill, H.P.S.A. Khalil, Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites, J. Appl. Polym. Sci. 78 (2000) 1685. [13] V. Calado, D.W. Barreto, J.R.M. d’Almeida, The effect of a chemical treatment on the structure and morphology of coir fibers, J. Mater. Sci. Lett. 19 (2000) 2151. [14] J.R.M. d’Almeida, R.C.M.P. Aquino, S.N. Monteiro, Tensile mechanical properties, morphological aspects and chemical characterization of piassava (Attalea funifera) fibers, Comp. Part A 37 (2006) 1473. [15] J.E.G. van Dam, M.J.A. van den Oever, W. Teunissen, E.R.P. Keijsers, A.G. Peralta, Process for production of high density/high performance binderless boards from whole coconut husk, Ind. Crops Prod. 19 (2004) 207. [16] B.D. Agarwal, L.J. Broutman, Analysis and Performance of Fiber Composites, Wiley, New York, 1980. [17] J.F. de Deus, S.N. Monteiro, J.R.M. d’Almeida, Effect of drying, molding pressure, and strain rate on the flexural mechanical behavior of piassava (Attalea funifera Mart) fiber–polyester composites, Polym. Test. 24 (2005) 750. [18] M.V. de Sousa, S.N. Monteiro, J.R.M. d’Almeida, Evaluation of pretreatment, size and molding pressure on flexural mechanical behavior of chopped bagasse–polyester composites, Polym. Test. 23 (2004) 253. [19] S.N. Monteiro, R.J.S. Rodriguez, M.V. de Souza, J.R.M. d’Almeida, Sugarcane bagasse waste as reinforcement in low cost composites, Adv. Perform. Mater. 5 (1998) 183.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Mechanical performance of coir fiber/polyester composites S.N. Monteiro a, L.A.H. Terrones a, J.R.M. D’Almeida b, a
Science and Technology Center, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000-Horto-28015-820, Campos, RJ, Brazil ´lica do Rio de Janeiro, Materials Science and Metallurgy Department, Pontifı´cia Universidade Cato ´vea-22453-900, Rio de Janeiro, RJ, Brazil ˆs de Sa ˜o Vicente,225-Ga Rua Marque
b
a r t i c l e in fo
abstract
Article history: Received 24 January 2008 Accepted 10 March 2008
The structural characteristics and mechanical properties of coir fiber/polyester composites were evaluated. The coir fibers were obtained from disregarded coconut shells that if not properly processed constitute an environmental hazard. The as-received coir fiber was characterized by scanning electron microscopy coupled with X-ray dispersion analysis. Composites prepared with two molding pressures and with amounts of coir fiber up to 80 wt% were fabricated. Up to 50 wt% of fiber, rigid composites were obtained. For amounts of fiber higher than this figure, the composites performed like more flexible agglomerates. The results obtained for flexural strength allowed comparison of the technical performance of the composites with other conventional materials. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Coir fiber Composites Flexural strength Micromechanics
1. Introduction Natural fibers such as cotton, flax and sisal have been used since historical times in a large variety of products, ranging from clothes to house roofing. Today, these fibers are appraised as environmentally correct materials owing to their biodegradability and renewable characteristics. Moreover, lignocellulosic fibers are neutral with respect to the emission of CO2 [1]. This is an extremely important aspect, and puts lignocellulosic fibers as materials in context with the Kyoto protocol. In addition to plants that are cultivated with the main purpose of using the fiber, in other plants the fiber has secondary or no commercial interest at all. This is the case of the banana trees, which are cultivated for the fruits. From the leftover leaves and bark of banana trees, fibers with good mechanical properties can be obtained [2]. However, these fibers are seldom used since the tree is normally discarded as garbage after the fruits are collected.
Corresponding author. Tel.: +55 21 3527 1789; fax: +55 21 3527 1236.
E-mail address: [email protected] (J.R.M. D’Almeida). 0142-9418/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.03.003
Another crop with a similar characteristic is coconut (Cocos nucifera). Plantations of coconut are spread all over the world in tropical and sub-tropical regions, and are an important item in the economy of many of these regions. The annual world production of coconut is about 42 million MT, which would equate to almost 50 billion coconuts [3]. The main use of coconut is for culinary purpose, and after extraction of the copra and/or of the liquid endosperm that fills the interior of the fruits the fruit shell is disregarded. Transformation industries and regions where consumption of coconut is high have, therefore, a large problem to conveniently dispose of this waste, since the fruit shell has a long decay time. The coconut fruit is, in fact, adapted for being dispersed by seawater, and can float for months without rotting. Growing attention is nowadays being paid to coconut fiber. Fibers extracted from the husk of the nut, known as coir fiber, are now being commercially used, blended with natural rubber latex in the production of seat cushion parts in automobiles [4]. These fibers are extracted from the external layer of the exocarp and from the endocarp of the fruit. The coconut palm can, in fact, be regarded as an integral fiber producer because fibers can be extracted from many parts of the palm, such as from the leaf sheath, the bark of the petiole or from the midribs of leaves [5,6].
ARTICLE IN PRESS 592
S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
Many aspects of the use of coir fibers as reinforcement in polymer–matrix composites are described in the literature. Coir fiber–polyester composites were tested as helmets, as roofing and post-boxes [7]. These composites, with coir loading ranging from 9 to 15 wt%, have a flexural strength of about 38 MPa. Coir–polyester composites with untreated and treated (PMMA and PAN grafted) coir fibers, and with fiber loading of 17 wt%, were tested in tension, flexure and notched Izod impact [8]. The results obtained with the untreated fibers show clear signs of the presence of a weak interface—long pulled-out fibers without any resin adhered to the fibers—and low mechanical properties were obtained. Although showing better mechanical performance, the composites with treated fibers present, however, only a moderate increase on the values of the mechanical properties analyzed. Alkali treatment is also reported for coir fibers [9,10]. Treated fiber–polyester composites, with volume fraction ranging from 10% to 30%, show better properties than composites with untreated fibers, but the flexural strength of these composites was consistently lower than that of the bare matrix. A maximum value of 42.3 MPa is reported against a value of 48.5 MPa for the neat polyester. Acetylation of coir fibers increases the hydrophobic behavior, increases the resistance to fungi attack and also increases the tensile strength of coir–polyester composites [11,12]. However, the fiber loading has to be fairly high, 45 wt% or even higher, to attain a significant reinforcing effect when the composite is tested in tension. Moreover, even with high coir fiber loading fractions, there is no improvement in the flexural strength [12]. From these results, it is apparent that the usual fiber treatments reported so far did not significantly change the mechanical performance of coir–polyester composites. Since most data in literature with few exceptions [12] usually cover only a specific loading fraction of fibers, and remembering that the increase of cost due to the treatment of the fibers must be a point of concern, this work was aimed at analyzing the flexural mechanical behavior of untreated coir–polyester composites covering a larger range of weight fractions. The effect of the molding pressure on the flexural strength of the composites was also evaluated.
2. Experimental procedures and materials Individually loose coir fibers were used in two distinct forms: tangled or pressed in mats with a thickness of 1.0 cm. The fibers were used untreated, except that they were dried at 50 1C for 24 h. Fig. 1 illustrates the two kinds of coir fibers, tangled mass and pressed mats, used in this work. A commercially available unsaturated orthoftalic polyester resin with 1 wt% of methyl-ethyl-ketone as catalyst was used as matrix for the composites. After being thoroughly mixed, the resin was poured into the cavity of a steel mold, which was previously filled with a suitable amount of coir fiber. Composites with amounts of coir fibers ranging from 10 to 80 wt% were manufactured at two pressure levels, namely: 2.6 and 5.2 MPa. The cure was done under pressure at room temperature.
Fig. 1. Aspect of the coir fibers used: (a) tangled fibers and (b) pressed mat.
As the fibers in any of the two configurations (tangled or mat) did not have a preferred orientation, the composites fabricated in the present work are considered as randomly oriented. Rectangular specimens 122 mm long, 25 mm wide and 10 mm thick were bend tested, using the three-point bending procedure, on a 100 kN capacity testing machine. The velocity of the test was 5 mm/min, which corresponds to a strain rate of 1.6 102 s1. The span-to-depth ratio was maintained constant at 9, and the minimum number of specimens used for each of the test conditions and coir fiber arrangements was 6. Before their incorporation in the composites, the coir fibers were analyzed by scanning electron microscopy (SEM). The analysis was performed on gold-sputtered samples in a microscope, coupled with EDS, operating at a beam voltage of 15 kV. 3. Experimental results and discussion The characteristic surface aspect of a coir fiber observed by SEM is shown in Fig. 2. As reported previously [10,13], one should notice that the fiber surface is covered with protrusions and with voided areas left by detached
ARTICLE IN PRESS S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
593
Fig. 2. Surface morphology of coir fibers, showing the characteristic array of protrusions (’) found at the surface of the fibers.
protruded material. In principle, and similar to what was observed for piassaba fibers [14], these morphological aspects can facilitate the resin impregnation onto the fiber. Fig. 3 shows an EDS spectrum performed at the fiber’s surface. The spectrum reveals, besides the obviously carbon and oxygen common to any organic matter, the presence of calcium and other alkaline and alkaline earth elements. Calcium was associated with the protrusions shown in Fig. 2. This result was surprising, since silicon-rich protrusions were identified in a previous work [13]. It is suggested that this marked difference could be due to differences in the soil where coconuts were raised. A detailed analysis needs to be carried out on this specific topic. The gold peaks in the spectrum correspond to the sputtered metal used to avoid charging at the microscope chamber during the analysis. Table 1 presents the average flexural strength and corresponding standard deviation obtained in the threepoint bend tests for the two coir fiber arrangements, tangled or mat, polyester composites cured at the two different molding pressures. As a first comment, one should say that composites with less than 50 wt% of fibers were found to be stiff and relatively hard, while those with more than 50 wt% were soft and deformable. Therefore, with respect to the mechanical behavior, the composites manufactured act as two completely distinct materials. Up to 50 wt% of coir fibers, the manufactured composites are rigid, structural-like, materials. By contrast, above this percentage, the polyester resin does not properly impregnate all fibers, even for a molding pressure of 5.2 MPa. As a consequence, the material becomes flexible and easy to bend, performing like binderless agglomerates [15]. Fig. 4 shows the strength variation with the amount of coir fibers for the composites fabricated 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 fiber and different compaction pressures, the strength tends to decrease with the amount of fiber. This reveals that the randomly
Fig. 3. EDS spectrum of the fiber’s surface.
Table 1 Flexural strength of the coir fiber– polyester composites Weight % of coir fiber
Molding pressure Pressed mat
10 20 30 40 50 60 70 80
Tangled mass
2.6 MPa
5.2 MPa
2.6 MPa
5.2 MPa
25.775.2 18.973.6 14.574.5 9.671.1 6.070.9 4.371.7 3.071.2 0.970.6
31.276.7 22.671.2 21.473.5 11.473.3 11.971.5 5.971.2 4.672.6 1.070.4
29.176.8 28.272.1 22.579.9 20.775.1 15.577.9 6.775.1 5.473.5 3.672.2
32.873.8 29.573.5 24.773.3 23.975.9 21.178.4 14.375.8 8.873.6 6.173.0
oriented coir fibers are not reinforcing the polyester matrix at all. Previous data showing the non-reinforcing behavior of coir fibers in composites submitted to bending do not show a steady decrease of the flexural strength as observed here. The results obtained by Hill and coworkers [12] present a maximum around 50 wt%. Since in the present work a smaller span-to-depth ratio (S/d ¼ 9) was used in relation to that of Hill and coworkers (S/d ¼ 16), and knowing that the smaller the span-to-depth ratio the greater the contribution of the shear stress, one could assert that the results obtained are not directly comparable. The main conclusion of both works, however, is the non-reinforcing behavior of coir fibers. In fact, the reinforcing behavior of coir fibers in polyester matrix is expected to be minimized due to the low modulus of the coir fiber. Values as low as 4.7 GPa are reported [12], and for an efficient partition of the load applied to the composite between the matrix and the fibers, the ratio of the fibers’ elastic modulus to that of the matrix must be
ARTICLE IN PRESS 594
S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
1 0.7
40
Vf = 0.1
20
10
0.5
Molding Pressure 2.6 MPa 5.2 MPa
0 10
20
40 30 50 60 70 Amount of Coir Fiber (wt.%)
0
80
1
50 Ef / Em
100
Fig. 5. Variation of the ratio of the load carried out by fibers with respect to the load applied to the composite as a function of the elastic moduli ratio and volume fraction of fibers.
40 Flexural Rupture Strength (MPa)
Vf = 0.3
30 Pf / Pc
Flexural Rupture Strength (MPa)
0.5
TANGLED MASS
PRESSED MAT 30 Molding Pressure 2.6 MPa 5.2 MPa
20
10
0 10
20
30 40 50 60 70 Amount of Coir Fiber (wt.%)
80
Fig. 4. Variation of the flexural strength with the mass fraction of coir fibers and molding pressure. (a) tangled fibers; (b) pressed mat.
maximized [16]. Fig. 5 shows that for fiber/matrix modulus ratio close to 1, as would be the case for the composites analyzed here, only a fraction of the load applied to the composite (Pc) will be carried by the fibers (Pf). Namely, 0:1pðP f =P c Þp0:7 for volume fractions of fiber ranging from 0.1 to 0.7. Therefore, the matrix is always under high loading and failures under low stress levels are to be expected. Another point is that the experimental results have 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 nonuniform cross-section to their mechanical properties. A further point is that, for both types of fiber used, 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 a higher pressure is applied during the matrix setting. However, the curves for both levels of molding pressure, Fig. 4a and b, fall approximately within the statistical error related to the
standard deviation. Consequently, one cannot be sure that for the molding pressures used in this work pressure has an actual influence, but just a tendency to improve the composite strength. In fact, the molding pressure was observed to have only a secondary effect on the flexural mechanical behavior for piassava–polyester composites [17] and chopped bagasse–polyester composites [18]. In terms of practical interest, the coir fiber composites may be regarded as valid alternatives to replace some conventional materials nowadays used by the building industry, and also as furniture. For example, the rigid composites with less than 50 wt% of coir fibers can be tailored to have tensile strength above 10 MPa, Table 1, which is higher than that of a low-density wood particle board with 5.5–9.7 MPa [19]. 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 fact, and in spite of their relatively low strength, Table 1, these composites are stronger than gypsum board [19] and can be used in panels or ceilings. Moreover, the fact that coir fiber composites are impervious to humidity represents a clear advantage in comparison with the relatively brittle gypsum board, which deteriorates in contact with water. 4. Conclusions From the experimental results obtained, the following conclusions can be made:
Random oriented coir fiber–polyester composites are
low-strength materials, but can be designed to have a set of flexural strengths that enable their use as nonstructural building elements. The lack of an efficient reinforcement by coir fibers is attributed to their low modulus of elasticity, in comparison with that of the bare polyester resin.
ARTICLE IN PRESS S.N. Monteiro et al. / Polymer Testing 27 (2008) 591–595
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.
Acknowledgment The authors acknowledge the financial support from the Brazilian Agency CNPq. References [1] A.K. Mohanty, M. Misra, L.T. Drzal, Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world, J. Polym. Environ. 10 (2002) 19. [2] A.G. Kulkarni, K.G. Satyanarayana, P.K. Rohatgi, K. Vijayan, Mechanical properties of banana fibres (Musa sepientum), J. Mater. Sci. 18 (1983) 2290. [3] J.G. Ohler, The coconut palm and its environment, in: J.G. Ohler (Ed.), Modern Coconut Management—Palm Cultivation and Products, FAO and Intermediate Technology Publications Ltd., London, 1999, pp. 12–156. [4] T.G. Schuh, U. Gayer, Automotive applications of natural fiber composites, in: A.L. Lea˜o, F.X. Carvalho, E. Frollini (Eds.), Lignocellulosic–Plastics Composites, Unesp Publishers, Botucatu, Brazil, 1997, pp. 181–195. [5] M.A. Venkataswamy, C.S.K. Pillai, V.S. Prasad, K.G. Satyanarayana, Effect of weathering on the mechanical properties of midribs of coconut leaves, J. Mater. Sci. 22 (1987) 3167. [6] K.G. Satyanarayana, C.K.S. Pillai, K. Sukumaran, S.G.K. Pillai, P.K. Rohatgi, K. Vijayan, Structure property studies of fibres from various parts of the coconut tree, J. Mater. Sci. 17 (1982) 2453. [7] K.G. Satyanarayana, K. Sukumaran, A.G. Kulkarni, S.G.K. Pillai, P.K. Rohatgi, Fabrication and properties of natural fibre-reinforced polyester composites, Composites 17 (1986) 329.
595
[8] J. Rout, M. Misra, A.K. Mohanty, S.K. Nayak, S.S. Tripathy, SEM observations of the fractured surfaces of coir composites, J. Reinf. Plast. Compos. 22 (2003) 1083. [9] J. Rout, M. Misra, S.S. Tripathy, S.K. Nayak, A.K. Mohanty, The influence of fibre treatment on the performance of coir–polyester composites, Comp. Sci. Technol. 61 (2001) 1303. [10] S.V. Prasad, C. Pavithram, P.K. Rohatgi, Alkali treatment of coir fibres for coir–polyester composites, J. Mater. Sci. 18 (1983) 1443. [11] C.A.S. Hill, H.P.S.A. Khalil, The effect of environmental exposure upon the mechanical properties of coir or oil palm fiber reinforced composites, J. Appl. Polym. Sci. 77 (2000) 1322. [12] C.A.S. Hill, H.P.S.A. Khalil, Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites, J. Appl. Polym. Sci. 78 (2000) 1685. [13] V. Calado, D.W. Barreto, J.R.M. d’Almeida, The effect of a chemical treatment on the structure and morphology of coir fibers, J. Mater. Sci. Lett. 19 (2000) 2151. [14] J.R.M. d’Almeida, R.C.M.P. Aquino, S.N. Monteiro, Tensile mechanical properties, morphological aspects and chemical characterization of piassava (Attalea funifera) fibers, Comp. Part A 37 (2006) 1473. [15] J.E.G. van Dam, M.J.A. van den Oever, W. Teunissen, E.R.P. Keijsers, A.G. Peralta, Process for production of high density/high performance binderless boards from whole coconut husk, Ind. Crops Prod. 19 (2004) 207. [16] B.D. Agarwal, L.J. Broutman, Analysis and Performance of Fiber Composites, Wiley, New York, 1980. [17] J.F. de Deus, S.N. Monteiro, J.R.M. d’Almeida, Effect of drying, molding pressure, and strain rate on the flexural mechanical behavior of piassava (Attalea funifera Mart) fiber–polyester composites, Polym. Test. 24 (2005) 750. [18] M.V. de Sousa, S.N. Monteiro, J.R.M. d’Almeida, Evaluation of pretreatment, size and molding pressure on flexural mechanical behavior of chopped bagasse–polyester composites, Polym. Test. 23 (2004) 253. [19] S.N. Monteiro, R.J.S. Rodriguez, M.V. de Souza, J.R.M. d’Almeida, Sugarcane bagasse waste as reinforcement in low cost composites, Adv. Perform. Mater. 5 (1998) 183.