Effect of fiber length on tensile strength of jute-fiber-reinforced polypropylene with several kinds of interfacial treatment

Transcription

1 Effect of fiber length on tensile strength of jute-fiber-reinforced polypropylene with several kinds of interfacial treatment K. Shima, K. Okubo & T. Fujii Department of Mechanical Engineering, Doshisha University, Japan Abstract Tensile strength was observed for natural jute-fiber-reinforced polymer (JRPP) having different fiber lengths and its surface treatment in order to evaluate the improvement of the mechanical property of natural composite. The composite was fabricated by the hot-pressing method with chopped jute fibers scattered randomly in polypropylene. Three kinds of fiber treatments were applied to change the interfacial strength between fiber and matrix, while the specimen has three different kinds of fiber lengths. Few different changes were observed in fiber strength, for any of the different kinds of treatment. Test results showed that JRPP has the most effective interfacial strength in order to improve tensile strength. The most effective interfacial strength shifted to a hlgh value according to the short fiber length. This means that an excess of interfacial strength rather than a decrease in strength of the composite occurs when using long jute fibers. 1. Introduction Jute-fiber-reinforced polypropylene (JRPP) is an ecological material and many workers reported its mechanical properties [l-71. However, JRPP has poor interfacial strength between fiber and matrix, because polypropylene is an apolar polymer. Jute fibers are also covered with mineral oil that reduces the adhesive strength between fiber and matrix. Therefore, the users always attempt to improve the interfacial strength to obtain the expected tensile strength of JRPP. To increase the composite strength, it might also be effective to use long jute fibers as reinforcement. But the fiber breakages would be caused if the composite were reinforced by long fibers. However tensile strength of jute fiber

2 262 Surface neatment V1 has low tensile strength, that is of that of glass fiber. Therefore, the most effective interfacial strength should be determined by the fiber length for JRPP. As author know, few papers have been published the most effective interfacial strength for JRPP considering fiber length. The purpose of present study is to show the most effective interfacial strength between fiber and matrix for chopped jute fiber reinforced polypropylene. The composite tensile strengths were shown with respect to the interfacial strength and fiber length. 2.1 Materials The matrix used in this investigation was homo-polypropylene (Japan Polychem Corporation), the modified polypropylene with maleic-anhydride (Sanyo chemical Industories). The jute fiber bundles were cut into 1, 5 and l Omm length to reinforce the composite. The jute fiber bundles were separated by a mixture machine to well contact with matrix of PP. 2.2 Fiber treatment The jute fibers were soaked in ethanol in order to remove mineral oil covered with it. Fibers were treated with amino silane-coupling agent (1% of weight content) to improve the interfacial shear strength. Some jute fibers were soaked in mineral oil again after it to decrease interfacial shear strength. 2.3 Preparation of composites Composite plates were fabricated by the conventional hot pressing method at a temperature of 200 for 20 minutes. Composite contained 30 vol% of jute fibers randomly distributed in the sample plate. Table 1 lists all composite formulations with respect to the interfacial strength. The single fiber composites were also prepared according to the following devised procedure: (1) two flat aluminum plates having mirror-polished surface covered with a mold releasing agent were prepared; (2) 10 fibers were carefully aligned within two PP films sandwiched between the aluminum plates; (3) the molds were placed in the hot press machine under the vacuum at a temperature of 200 for 10 minutes; (4) after cooling the thin plate were cut into the small specimens (2~45~0.1 in mm)which contained one single fiber, aligned longitudinally in its centerline. 2.4 Tensile test of JRPP The tensile properties of specimens were determined using 10 samples for each composite with testing machine at a constant cross-speed of 1 &min. The geometry of the specimens for tensile test shows in Fig. l.

3 2.5 Measurement of interfacial shear strength Surface Treatment V1 263 Embedded single fiber test were performed by small tensile tester at a constant cross-speed of 0.5 mm/min. Five samples were tested for each experimental condition. Samples were loaded until fiber breakages were saturate. Mean fiber length Ls was measured by optical microscope after the test. The fiber critical length Lc was considered equal to (413) of Ls where the interfacial shear strength z, was evaluated by following the simplified physical model proposed by Kelly-Tyson, which yields the well-known expression [g-91: q d zrnax =- (1) 2Lc where d is diameter of fiber andof is the mean fiber strength and it was expressed following equation. 1 where m is Weibull shape and a! is scale parameter which were determined by tensile test of fiber. Unit : [mm] Table 1 Treatment on jute fiber. Fig. l Geometry of specimen. 3. Result and discussion 3.1 Effect of surface treatment on fiber strength Material P-material M-material S-material Fiber treatment Ethanol Silane Figure 2 shows the tensile strength distribution of the jute fibers. It was observed that there were large scatter in the tensile strength of jute fiber ,X b 5.0.t g 0.5 v Fiber strength [MPa] Fig.2 Tensile strength distribution of jute fibers.

4 264 Surface Deatment V1 The distribution of fiber strength also slightly shifted to low value, if the silane-coupling agent treated jute fibers. However average strength little changed even for any kinds of the treatments. 3.2 Effect of interfacial strength on strength of JRPP The embedded single fiber test showed that the interfacial strength was higher in order of S, M and P-material. Scannig electron micrographs at fracture surface of P, M and S-material are showed in Fig.3. The fibers of P-material (a) were not covered with PP matrix and the fiber pull-outs were observed in the fracture surface. However the fibers of M (b) and S-material (c) which had high interfacial strength were covered with PP matrix. (a) P-material (Oil) (b) M-material (Ethanol) (c) S-material (Amino) Fig.3 Photographs of fracture surface. Figure 4 shows the tensile strength of JRPP with respect to the interfacial shear strength between fiber and matrix. It is observed that there was the most effective interfacial shear strength to obtain high tensile strength for JRPP using 5 and 10 mm of jute fibers. (d B Y B bl2 d 2 Y m Q) 3.m V) d g Interfacial shear strength [MPa] Fig.4 Variation of tensile strength of JRPP with respect to interfacial shear strength.

5 Surface Treatment V1 265 The tensile strength of JRPP using 1 mm in fiber length increased in the range of the interfacial strength in current study. If higher interfacial strength could be obtained than that in this study, the tensile strength of JRPP using 1 mm in fiber length had maximum at the certain interfacial strength as well as in other fiber lengths. 3.3 Most effective interfacial shear strength To investigate the micro-fracture mechanism around the interface, the crack propagation was observed in an embedded single-fiber specimen with a notch of 1 mm. Figure 5 shows photographs of fracture propagation due to a matrix crack originated from the notch. A plastic deformed zone of PP matrix was observed as a white zone. Figure 6 shows a schematic model explaining the fracture of JRPP. Jute fiber \, pp debonding Fiber (a)p-material (b) M-material (c)s-material ( z,,,=15mpa) ( T,,,=28MPa) ( z,,,=48mpa) Fig.5 Micro-fracture progress of JRPP related to the interfacial shear strength. When interfacial strength was low ((a), pattern-a), the interfacial debonding between fiber and matrix occurred around the fiber. The interfacial debonding prevented from transmitting force to fibers under low applied load. Therefore P-material shows low strength in case of (a). In opposite, when interfacial strength was high ((c), pattern-c), the fiber broke before the crack reached to the jute fiber due to stress consentration at a front of crack tip, because the interfacial debonding was not produced. This fiber breakage promoted the crack propagation under low level of loading.

6 If the specimen had moderate interfacial strength ((b), pattern-b), the fibers did not break before the crack tip reached to the fiber even under high loading level. This means that the stress on fibers might be reached due to localized small interfacial debondings around the fiber compared with that in case of (c). Therefore in this case, the maximum strength of M-material was obtained by that the fiber breakage was caused with the matrix crack. Jute fiber PP Interfacial debonding c length b (ac) > 0 ult (L) r (ac) < ult a<< ac - 5 (4 > ult (L) r (4 < r ult Fiber bre akage Fig.6 Schematic model explaining the fracture of JRPP 3.4 Shift of most effective interfacial strength according to fiber length Figure 7 shows the most effective interfacial shear strength with respect to the fiber length. It is showed that the most effective interfacial shear strength decreased according to long jute fiber length. To explain why the most effective interfacial strength shifted with fiber length, the strength of chopped single fiber was measured by the fiber test. Figure 8 shows the tensile strength of single jute fiber with respect to fiber length. It was observed that tensile strength of fiber increased, if fiber length was short. It was considered that strength of the short fiber was high due to less initial flaws on the fiber. Figure 9 shows the surface of specimen near the fracture point. It was confirmed that fiber length determined whether fiber breakages in the matrix even in the same interfacial strength. In fracture model as shown Fig.6, most

7 Surface Treatment V1 267 effective state (Pattern-B) could be obtained by low interfacial shear strength in the JRPP was fabricated with long jute fibers which has moderate fiber strength Fiber length [mm] Fig.7 Most effective interfacial strength for JRPP fabricated with chopped jute fibers Fiber length [mm] Fig.8 Single fiber strength with respect to fiber length.

8 268 Surface Deatment (a) l mm (M-material) (b) 5 mm (M-material) (c) l0 mm (M-material) 4. Conclusion Fig.9 The surface of specimen near the fracture point. In ths paper, the effect of fiber length on relationship between tensile strength and interfacial strength was investigated for jute fiber reinforced polypropylene and the following results were obtained. 1. JRPP has the most effective interfacial strength in order to improve tensile strength. 2. The most effective interfacial strength shifted to low value according to the long fiber. 3. Excess of interfacial strength rather than decrease the strength of composite having long fiber. References Seema Jain, Rakesh Kumr, U.C.Jinda1, J. Mat. Sci., 27,4598 (1992). Seema Jain, U.C.Jinda1, Rakesh Kumar, J. Mat. Sci. letters, 12, 558 (1993). F.G.Shin, X.-J.Xian, W.-P.Zheng, M.W.Yipp, J. Mat. Sci., 24, 3483 (1989). A.Varada Rajulu, S.Allah Baksh, G.Ramachandra Reddy, K.Narasimha CHARY, J. Reinforced Plastics and Composites, 17, 1507 (1998). Daniel F. Caulfield, Daan Feng, S.Prabawa, R.A.Young, Anand R.Sanadi, Die Angewandte Makromolekulare Chernie, 272, 57 (1999). Bernd Reck, Johannes Tiirk, Die Angewandte Makromolekulare Chemie, 272, 5 (1999). Jarrod J. Schemenauer, Tim A.Osswald, Anand R.Sanadi, Daniel F.Caulfield, Society of Plastics Engineers Annual Technical Conference, 58,2206 (2000). A.N.Netravali, R.B.Henstenburg, S.L.Phoenix, P.Schwartz, Polymer Composites 10-4,226 (1989). Kelly,A., Tyson, W.R., High Strength Materials, John Wiley & Sons, 578 (1965).