Effects of Alkali Treatment to Reinforcement on Tensile Properties of Curaua Fiber Green Composites

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1 541 Effects of Alkali Treatment to Reinforcement on Tensile Properties of Curaua Fiber Green Composites Alexandre GOMES, Koichi GODA and Junji OHGI Alkali treatment was performed on Curaua fibers to improve their mechanical properties. Curaua fibers were dipped into 5%, 10%, and 15% concentrated sodium hydroxide solutions for 1 h and 2 h. The effects of solution concentrations and treatment times on the physical and mechanical properties of curaua fibers were evaluated. Tensile tests of untreated and alkali treated curaua fibers were carried out. Those results showed that the tensile strength of the treated fibers decreased in comparison to untreated fibers, whereas the fracture strain of the treated fibers increased greatly in comparison with that of untreated fibers. In addition, green composites reinforced by untreated and alkali-treated curaua fibers were fabricated by a press forming method. Tensile tests were carried out for both composites. Results showed that both tensile strength and fracture strain of the composite using treated fibers increased in comparison with the composite using untreated fibers. Key Words: Curaua, Natural Fibers, Alkali Treatment, Single Fiber Test, Biodegradable Resin, Tensile Strength, Fracture Strain 1. Introduction To help reduce the ever-expanding consumption of petroleum, a nonrenewable resource, ecologically aware studies and their practical applications, such as the use of green materials have garnered increasing interest. Natural fiber composites using kenaf, sisal, and coir have already been applied to produce several industrial products, such as interior parts of automobiles (1), (2). Natural fibers are not only light and strong; they are also inexpensive, renewable, and eco-friendly. Especially, green composites biodegradable resin matrix composites reinforced by natural fibers are expected to be an important material for realizing and maintaining a sustainable productive society. Curaua (Ananas erectifolius), an Amazon forest plant, offers superior strength among natural fibers (3). Curaua fibers are abundant and can be extracted easily from their leaves. Therefore, curaua fibers are highly regarded for their potential use as reinforcing material for natural fiber composites. This study investigated physi- Received 7th May, 2004 (No ) Graduate School of Science and Engineering, Yamaguchi University, Tokiwadai, Ube , Japan. b1280@stu.cc.yamaguchi-u.ac.jp Department of Mechanical Engineering, Yamaguchi University, Tokiwadai, Ube , Japan. goda@yamaguchi-u.ac.jp cal and mechanical properties of as-supplied and alkalitreated curaua fibers using tensile tests and scanning electron microscope observations. This report also presents discussion of the effect of alkali treatment on curaua fiber properties. Moreover, biodegradable resin matrix composites reinforced by treated and untreated curaua fibers were newly developed. Effects of alkali treatments on mechanical properties of such materials were examined through tensile tests. 2. Tensile Properties of Curaua Fiber 2. 1 Experimental procedure This study used curaua fibers (supplied from POEMA of Para Federal University, Brazil) as a test material. Alkali treatment was applied to the curaua fiber surface to improve its mechanical properties (4). Treatment was carried out by dipping small bunches of fibers into 5wt%, 10wt%, and 15wt% concentrated sodium hydroxide (NaOH) solutions. The treatment times were 1 and 2 hours. Such different concentrations of solutions elucidated effects of alkali concentration on the mechanical properties of curaua fibers. After alkali treatment, the fibers were washed for a few minutes using a 1% concentrated acetic acid solution. Subsequently, the fibers were washed with water and dried at room temperature. Finally, we measured the respective densities and weight losses of 5%, 10%, and 15% treated fibers. JSME International Journal Series A, Vol. 47, No. 4, 2004

2 542 Table 1 Mechanical properties of untreated and alkali-treated curaua fibers Fig. 1 Tensile-test specimen Single fibers were extracted from untreated and treated bunches of curaua fibers to carry out tensile tests. Single fibers were bonded to a paper with rectangular holes of 10 mm length, as shown in Fig. 1. Then, 20 single-fiber specimens for each condition, i.e. untreated, 5%, 10% and 15% treated fibers, were prepared according to the testing method for carbon fibers (Japan Industrial Standard, R 7601). The gage length for each specimen was 10 mm. The diameter of each specimen was measured using an optical microscope. The cross-head speed of the tensile testing machine was 0.8 mm/min. The tensile load and displacement were then recorded for each sample using a load-cell system and laser system, respectively Results and discussion Mechanical properties Table 1 shows results of untreated and alkali-treated specimens that were tensile-tested. Therein, all values are averages. Results showed that the diameter of curaua fibers was decreased by alkali treatment, except for fibers treated for 2 h in 5% NaOH. The decrease in fiber diameter is caused by change of morphology of the treated fiber, as shown later. Results also showed that the tensile strength of curaua fibers was decreased markedly by alkali treatment. This fact may be attributable to fiber damage caused by chemical reaction with sodium hydroxide during treatment. That is to say, this damage is considered to be caused by a chemical structure change such that cellulose in the fiber partially changes from crystalline cellulose I into amorphous cellulose II (5). On the other hand, the fracture strain of treated fibers increased greatly in comparison with that of untreated fibers. Regarding the alkali solution s concentration, the tensile strength of treated fibers decreased with increasing sodium hydroxide content. Therefore, the tensile strength of 5% treated fibers is higher than that of 10% treated fibers, followed by that of 15% treated fibers. However, fracture strain increases with an increase in the sodium hydroxide content. Fracture strains of fibers treated in 5%, 10% and 15%NaOH for 2 h reached , , and , respectively. The Young s modulus of treated fibers decreased compared to untreated fibers. Fibers treated using the same concentration of sodium hydroxide showed slightly higher tensile strength for 1 h treatment than those treated for 2 h, except for 5% treated fibers; 2 h treatment provided larger fracture strains than 1 h treatment. Series A, Vol. 47, No. 4, 2004 Table 2 Coefficient of variation of untreated and alkali-treated curaua fibers Table 2 shows coefficients of variation for fiber diameter and tensile strength of the untreated and treated fibers. The coefficients of variation in fiber diameter of the treated fibers show almost identical values as that of the untreated fiber, except for the fibers that had been treated in 15%NaOH for 2 h. On the other hand, the coefficients of variation in strength of the treated fibers decrease greatly in comparison with those of the untreated fibers. These results indicate that the variation in strength of curaua fibers is reduced by alkali-treatment. We anticipate that such improvement of variation in fiber strength can increase the composite strength Stress-strain diagrams Stress-strain diagrams of 1-h and 2-h alkali-treated curaua fibers are shown in Figs. 2 and 3, respectively. These diagrams show the influences of both treatment time and alkali solution concentration on the mechanical properties of treated fibers. Slopes of the stress-strain curves of the treated fibers are lower than that of the untreated fiber. The lower slope of the stress-strain diagram typically implies decreased tensile strength and Young s modulus. However, as mentioned above, the fiber fracture-strains increase drastically after alkali treatment. It is noteworthy that the fracture strain of fibers treated at 2 h with 15% sodium hydroxide was approximately 0.09, which is more than double the fracture strain of the untreated fiber. In addition, the stress-strain behavior of treated fibers appears to be nonlinear and similar to plastic deformation behavior. Therefore, it is expected that any improvement on the fiber fracture strain is capable of generating an increase in fracture toughness of composites that are reinforced with such treated fibers Physical and morphological properties Surfaces of untreated and treated fibers were observed using a scanning electron microscope. Figure 4 (a) and (b) show SEM photographs of the surfaces of untreated fiber and 2-h treated fiber with 15% solution. Observations of the treated fiber surface revealed that the fiber is not truly a JSME International Journal

3 543 Fig. 2 Stress-strain diagram of 1-h alkali treated curaua fibers Fig. 5 Effects of alkali-treatment on the morphological structure of curaua fibers Fig. 3 Stress-strain diagram of 2-h alkali treated curaua fibers fibers. The decrease in density implies that the effect of the fiber weight reduction is greater than that of the fiber diameter. This fact proves definitively that the treated fiber is changed into a porous bundle structure without sufficient lignin. Figure 5 shows a schematic of the morphological change in the fiber caused by alkali treatment. As mentioned above, the original fiber structure is apparently a rod-like shape, similar to the structure of fiber-reinforced composites. Alkali treatment changes the structure into a bundle structure, which comprises monofilaments that are barely bonded to each other by a small amount of lignin. Not only the chemical structure change mentioned above, but also such morphological change brings a decrease in strength of the fibers. 3. Tensile Properties of Curaua Fiber Green Composite Fig. 4 Table 3 SEM photographs of surfaces of (a) untreated and (b) 15% alkali treated curaua fibers Density and loss of weight of treated and untreated curaua fibers monofilament. It is a bundle of monofilaments bonded and covered by lignin. Therefore, alkali treatment is inferred to provoke removal of a great amount of lignin from the untreated fiber surface. The existence of lignin on the untreated fiber gives it a rougher surface than that of treated fibers. Table 3 shows changes of fiber weights and densities after alkali treatment. Data in the table indicate that both the fiber weights and densities decrease with increasing alkali solution concentrations. Removal of lignin reduced the weight, diameter and density of alkali-treated 3. 1 Fabrication of curaua fiber green composite To study whether alkali-treated curaua fibers can function as a reinforcement, a fiber reinforced composite was fabricated, using curaua fibers treated in 10%NaOH solution for 2 h. That composite was then compared with the untreated fiber reinforced composite. A hydrophilic biodegradable resin based on cornstarch (Randy CP-300; Miyoshi Oil and Fat Co., Ltd.) was used as the matrix of both untreated and treated fiber reinforced composites. Table 4 shows the resin properties. The composites were fabricated by inserting the fibers into a metallic mold. Subsequently, resin was poured onto them and the material was pressed with slight pressure at 150 C for 20 min. Then, the heating process was stopped. During the cooling process, when the temperature reached 110 C, pressure of 3.27 MPa was applied to the material until the temperature decreased to approximately room temperature. Figure 6 shows the fabrication process of curaua-fiber reinforced composites. After fabrication, the volume fraction of fibers was calculated using the following equation: V f = 1 W W f ρ m V, (1) where W and V are the weight and volume of the fabricated composite, respectively. W f is the weight of curaua JSME International Journal Series A, Vol. 47, No. 4, 2004

4 544 Table 4 Mechanical properties of biodegradable resin Fig. 7 Table 5 Shape and dimensions of tensile-tested specimens Mechanical properties of curaua / biodegradable resin composites Fig. 6 Fabrication process of curaua fibers reinforced composites fibers included in the composite. ρ m is the density of the biodegradable resin. The volume fraction ranged from 16% to 35% for the untreated fiber composites and from 19% to 34% for the treated fibers composites Tensile properties of curaua fiber green composite Tensile tests were carried out on untreated-fiber and treated-fiber reinforced composites. Aluminum plates were attached with epoxy adhesive on both ends of fabricated composites to perform tensile testing. The gage length and width were 50 mm and 15 mm, respectively. The thickness varied from 1 mm to 2 mm, dependent on the fiber content. A strain gage was attached on the center of the specimens for uniaxial strain measurement. Crosshead speed of the testing machine was 1 mm/min., in accordance to the tensile testing method for carbon fiber reinforced plastics (JIS K 7073). Figure 7 shows the shape and dimensions of tensile-tested specimens. Table 5 shows tensile properties of untreated- and treated-curaua-fiber reinforced composites. All data are shown as averages. The results showed that, for the same volume fraction, tensile strength of treated-fiber composites is slightly higher than that of untreated-fiber reinforced composites, whereas the fracture strain of the former composites is much higher than that of the latter composites. The values of Young s moduli of untreated and treated composites were almost identical. Stress-strain behaviors of untreated and treated composites are shown in Fig. 8. As expected, the increased fracture strain of treated fibers increased the fracture strain of the treated-fiber composites compared to untreated composites. It is inferred that lignin included in the surface of untreated fibers has hydrophobic properties that promote unsuitable interfacial compatibility with the hydrophilic resin. We consider that this effect decreased the interfacial strength of untreatedfiber composites in comparison with treated-fiber composites. In addition, past theoretical models have shown that Series A, Vol. 47, No. 4, 2004 Fig. 8 Stress-strain diagram showing untreated and treated curaua composites composites reinforced by fibers that have a large variation in strength exhibit less strength (6). Table 2 shows that the variation in strength of untreated fibers is larger than that of treated fibers. For that reason, it is inferred that these effects, i.e. interfacial incompatibility and variation in strength engender a decrease in tensile strength of the untreated-fiber composites. In any case, an expectation from the above results exists that the treated-fiber composites obtained in this study could offer a dramatic improvement in mechanical properties such as impact strength and fracture toughness. Figure 9 (a) and (b) show SEM photographs of fracture surfaces of untreated and treated fiber composites. Fibers are not uniformly dispersed in matrix, but remain in a bundle-like manner. It is observed that some fiber bundles were pulled out from the matrix. As shown in the Fig. 9 (b), the degree of fiber breakage on the treated fiber bundles is slightly uneven. Nevertheless, the degrees of roughness of both the whole fracture surfaces are almost identical. For that reason, the difference of interfacial strengths between the fiber bundles and matrix cannot be observed clearly from Fig. 9 (a) and (b). Hence more de- JSME International Journal

5 545 Fig. 9 Fig. 10 SEM photographs of fracture surfaces of (a) untreated and (b) alkali-treated composites Strength and specific strength of electric fiberglass (E- G), high strength and stiffness fiberglass (S-G), alkali resistant fiberglass (Ar-G), untreated curaua fibers (UC), 5% alkali treated curaua fibers (5%C), 10% alkali treated curaua fibers (10%C), and 15% alkali treated curaua fibers (15%C) tailed investigation is required to clarify mechanical interaction between hydrophobic and hydrophilic constituents. Future research efforts must address this point Comparison of curaua fibers and glass fibers Properties of untreated and alkali-treated curaua fibers indicate the possibility of application of curaua fibers in place of glass fibers as reinforcement for composites. Figure 10 shows strength and specific strengths of three types of glass fibers (7), untreated curaua fibers, and curaua fibers treated with 5%, 10% and 15%NaOH solutions for 2 h. The strength level of all types of glass fibers is much higher than that of any type of curaua fiber. However, specific strengths of curaua fibers that are either untreated or treated with 5% solution for 2 h are markedly higher than those of alkali resistant glass fibers. The excellent performance in terms of specific strength of curaua fibers would reduce the weight of composites reinforced using them. Such composites would be lighter than glass-fiber reinforced plastic composites (GFRP). Furthermore, GFRP do not decompose easily. Moreover, their reclamation process generates a large environmental load. Furthermore, it is difficult to establish a suitable method of discarding these materials. Incineration of GFRP generates numerous problems including incinerator damage and generation of black smoke and odors (8) (10). Industrial workers who produce glass fibers and assemble GFRP components often suffer skin irritation and respiratory diseases caused by inhalation of glass-fiber dust. For those reasons, the use of curaua-fiber reinforced composites may solve various problems that accompany use of GFRP. 4. Conclusion This study clarified effects of alkali treatment on tensile properties of curaua-fiber reinforced biodegradable resin-matrix composites. The obtained results are: ( 1 ) Curaua fibers were treated using 5%, 10%, and 15% hydroxide sodium solutions for 1 h and 2 h. Mechanical properties of untreated and alkali treated curaua fibers were investigated using tensile tests and SEM observations. Tensile strength decreased for treated fibers in comparison with untreated fibers, whereas fracture strain increased for alkali-treated fibers. The fracture strain of fibers treated for 2 h with 15%NaOH solution reached nearly 0.09, which is about double that of untreated fibers. ( 2 ) SEM observations revealed structures of untreated and treated curaua fibers. The structure of one untreated single fiber is actually a bundle of small single fibers that are bonded and covered by lignin (composite structure). The alkali treatment action changes the original structure into a bundle of single fibers (bundle structure) that are barely bonded by a very small quantity of lignin. ( 3 ) This study fabricated composites reinforced by untreated fibers and reinforced by fibers treated for 2 h in a 10%NaOH solution. Their mechanical properties were investigated and compared through tensile tests. The results showed that the tensile strength of alkali-treated composites is slightly higher than that of untreated composites, whereas fracture strain is dramatically improved for alkali-treated composites in comparison with untreated composites. ( 4 ) Untreated and 5% alkali treated curaua fibers showed higher specific strength than alkali resistant glass fibers. The superior performance of curaua fibers in terms of specific strength implies that their use as reinforcement can produce lighter composites than use of GFRP. References ( 1 ) Wambua, P., Ivens, J. and Verpoest, I., Natural Fibers: Can They Replace Glass in Fibre Reinforced Plastics, Journal of Composites Science and Technology, Vol.63 (2001), pp ( 2 ) Leao, A., Rowell, R. and Tavares, N., Applications of Natural Fibres in Automotive Industry in Brazil- JSME International Journal Series A, Vol. 47, No. 4, 2004

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