18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS EFFECT OF ALKALI-TREATMENT ON TENSILE PROPERTIES OF KENAF LONG FIBERS Y. Nitta 1* , J. Noda 2 , K. Goda 2 and W-I. Lee 3 1 Graduate School of Science and Engineering, Yamaguchi University , Ube, Japan, 2 Department of Mechanical Engineering, Yamaguchi University , Ube, Japan, 3 Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul, Korea *Corresponding author ( p037ve@yamaguchi-u.ac.jp ) Keywords : Alkali treatment, Kenaf fiber, Tensile strength, Fracture strain, X-ray diffraction cell type natural fibers. In addition, a new cross- 1 Introduction sectional area measurement for alkali-treated kenaf Recently, global environmental problems about fibers is proposed using a laser system. CO 2 emission become serious concerns. Thus, a 2 Experimental number of biomass-based material researches are nowadays being developed. Green composite using 2.1 Materials kenaf fiber and polylactic acid (PLA) is the Kenaf fibers ( Hibiscus cannabinus ), harvested in promising material in the field of composite Vietnam was used in this study. Alkali treatment materials. It has already been used in cellular phones was applied to the fibers in a 10wt%NaOH solution and car interior parts. However, practical kenaf/ for two hours at room temperature or a 15wt%NaOH PLA composites are often produced as an injection for one, two, four or six hours. Hereinafter, molded material, while it has not been enough untreated fibers are denoted as UT, and alkali- reported about the mechanical properties of long treated fibers are denoted as “A” followed by fiber reinforcement. On the other hand, some (concentration) - (treating time). For example, A15- chemical treatments are applied to natural fibers to 2 means alkali-treated fibers in a 15wt%NaOH improve the interfacial properties between natural solution for two hours. fibers and resin. Alkaline treatment is one of the 2.2 Cross-sectional area measurement most typical chemical treatments. Kawahara, et al. [1] explored the mechanical properties of alkali- In general, there are large variation in cross- treated kenaf fiber by changing concentration of the sectional area of natural fibers. Although cross- aqueous solution of NaOH in the relatively low section of kenaf fibers is a complicated shape, cross- range, i.e. 1 to 7 %. The results showed that tensile sectional area of general natural fibers as well as strength of the treated fibers did not change so kenaf has often been evaluated as a circular shape by largely. On the contrary, alkali-treated kenaf fiber measurement of fiber projection width. Circular reinforced composites indicated high flexural shape assumption gives a large difference from strength and modulus [2]. It is guessed from the actual cross-sectional area, and results in bringing above that, although low concentration alkali-treated over- or under- estimation in strength. Cross- kenaf fibers can improve mechanical properties of sectional area of kenaf fibers was evaluated using the composites, this is mainly caused by interfacial the data-base approximation method (hereinafter, it improvement. When single-cell type natural fibers is denoted as DBA) [4]. DBA method is based on the such as ramie are mercerized by high concentration database for a distribution of actual fiber cross- alkali solution, the mechanical properties are sectional area obtained from cross-sectional pictorial dramatically changed, especially in fracture strain[3]. images of natural fibers. As mentioned above, On the other hand, the effect of mercerization has usually the fiber cross-sectional area is calculated not been explored sufficiently for multi-cell type assuming a projection width as a diameter, which is natural fibers such as kenaf. measured by optical microscope or laser system. The purpose of this study is thus to investigate the Fig.1 shows a microphotograph of the cross-section effect of high concentration alkali treatment on the of a kenaf fiber. As observed in the figure, the shape tensile properties of kenaf fibers, which posses a of the fiber cross-section is not circle. Actual fiber relatively high content in lignin, inherent in multi- cross-sectional area of kenaf fibers, as indicated in
120 ° 120 ° 60 ° 60 ° Hexagon Hexagon Table 1 Comparison of actual cross-sectional area with circle- and hexagon-assumption. Assumption 0 ° 0 ° Actual Circle Hexagon A c [ µ m 2 ] 5010 7356 5783 CC ‐ 0.800 0.900 (a)Micrograph of (b)Polygon A C : Cross-sectional area, CC: Coefficient of correlation. cross-section. approximation. Fig.1. Optical micrograph of cross-section on a ! the actual cross-sectional area is stronger than that of kenaf and polygon approximation. the circle-assumption area. Table 1 shows comparision of actual cross-sectional area with circle- and hexagon- assumption. From this table, it Actual cross-sectional area , µ m 2 is found that more exact cross-section area of natural 10000 fibers can be evaluated through hexagon-assumption areas because the coefficients of correlation is higher. More appropriate approximation shape of cross- 5000 section area was examined by selecting a circle, ellipse, hexagon, pctagon, dodecagon or UT A15-2 icositetragon. Correlation coefficient of each shape 0 is shown in Fig. 3. Correlation coefficients after 0 5000 10000 15000 20000 hexagon are around 0.9 and almost constant. Cross-sectinal area of circle , µ m 2 Therefore, the fiber cross-sectional area calculated (a) Circle-assumption from the hexagon assumption in the above is valid. Hexagonal approximation equation obtained here Actual cross-sectional area , µ m 2 10000 was y= 0.68 x+ 1080. Fiber cross-sectional area was evaluated using this equation. Tensile strength calculated from hexagonal approximation is shown in the Table 2. It is considered that the corrected 5000 strength are close to exact values because their UT cross-sectional areas are based on the DBA. A15-2 2.3 Tensile test 0 0 5000 10000 15000 20000 Cross-sectinal area of hexagon , µ m 2 Single fiber tensile tests were carried out using a tension and compression testing machine with a fine (b) Hexagon-assumption load cell. The gauge length of the tensile specimen Fig.2. Relationship of actual cross-sectional area was 25 mm, the tensile speed was 1.0 mm/min and with (a) circle-assumption area, and (b) the number of samples in each condition was ten. hexagon-assumption area for kenaf fibers. The projective width of these natural fibers were measured from three directions of 0, 60 and 120 painted area in black, were measured through an degrees at 0.1 mm interval along their axial direction, image analysis. At each image, the projective widths using a laser scan micro-meter (LSM-500S, from three directions of 0, 60 and 120 degrees were Mitutoyo Corporation). In this experiment the shape measured as shown in Fig. 1. In this case, the cross- of cross-section is assumed as an hexagon. sectional area of the fiber was evaluated by Displacement was measured using a laser assuming the shape of hexagon, composed of 3 displacement meter (KEYENCE, LS-7500). diagonal lines corresponding to 3 projective widths. 3 Results and Discussion The relations between the actual cross-sectional area, and the circle and hexagon-assumed areas for kenaf 3.1 Tensile properties 2(a) and (b) in red point, respectively. It is proved Experimental results are shown in Table 3. Tensile that correlation of the hexagon assumption area to 2
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