Properties of Green Composites based on Polypropylene Reinforced by Bamboo Shoot Culm Sheath Fibers

190 Bamboo fiber MAPP resin Adhesive agent Paper tab Paper tab will be cut when testing Blade Journal of Chemistry, Vol. 45 (5A), P. 190 - 195, 2007 Properties of Green Composites based on Polypropylene Reinforced by Bamboo Shoot Culm Sheath Fibers Received 16 August 2007 Nguyen Huy Tung1 and Toru Fujii2 1Polymer Centre, Hanoi University of Technology - Vietnam 2Mechanical Faculty, Doshisha University - Japan summary Silane and NaOH were used to treatment bamboo shoot culm sheath fiber. After treatment, the interfacial shear strength of fiber with MAPP increased by 24% and 30% respectively. Alkali treatment has much effect on bamboo shoot culm sheath fiber than silane treatment. Washing NaOH treatment bamboo fiber with acetic acid was improved IFSS of bamboo fiber and polypropylene (PP) and strength of composite PP reinforced by bamboo fiber. I - Introduction In the world and also in Vietnam, the “green composite” has been paid more and more attentions because of their properties such as cheap, available, bio-degradable [1, 2]. There were many published papers concerned these materials. However, these papers were only covered few problems on the natural fibers composite [3]. Bamboo fibers were treated with isocyanate silane or sodium hydroxide to improve interfacial shear strength (IFSS) with maleic grafted polypropylene (MAPP) matrix. Micro- droplet test was used to investigate IFSS of fibers with MAPP. II - Composite fabrication and testing method Bamboo culm sheath fiber reinforced polypropylene (BsFRP) was fabricated using hot press method at 190oC and 2 MPa. Interfacial strength was determined by using micro-droplet test under laboratory condition: 21±3oC and 60±5%RH. The tensile speed was 0.5 mm/min. IFSS is calculated by the following equation: IFSS= Fp/(L1*L2) Here, Fp is the maximum load to pull out the fiber from the matrix. L1 is the embedded length of the fiber. L2 is the circumference of the fiber. Fig. 1: Schematic drawing of micro-droplet testing III - Results and discussion 1. IFSS of bamboo shoot culm sheath fiber with MAPP 191 In composite material, IFSS is an important part on mechanical properties. Because the interface plays a major role in transferring the stress from matrix to fiber, it is important to be able to characterize the interphase and the level of adhesion to properly understand the performance of the composite [4]. Fig. 2 shows the increment of IFSS when the fiber diameter goes down. In micro-droplet test, the matrix covered the fiber before it was pulled out. The small diameter fibers have larger contact area with matrix than big diameter fibers do. Hence, the IFSS was higher with small diameter fiber. In Fig. 3, alkali treatment improves the IFSS of fiber and MAPP matrix by 25%. NaOH makes the fibers surface roughness that allows certain mechanical interlocking. NaOH treatment also increase the wetability of fiber surface with matrix and this made the increasing of IFSS. In case of silane treatment, the fiber surface was modified and this modification increases the fiber matrix interaction. These results are agree with several author’s reports [6 - 8]. The silane coupling agent has two types of functional groups. One group calls hydrolysable alkoxy group able to condense with hydroxyl groups that on the bamboo fiber surface. The other group (isocyanate group) can interact with MAPP matrix. Besides the reaction of silane with hydroxyl group of bamboo fiber on the surface, the formation of polysiloxane structures might also occur [9]. According to Mieck et al. the application of alkyl-functional silane does not lead to chemical bonds between the cellulose fibers and the polypropylene matrix. However, it seems to be realistic to assume that the long hydrocarbon chains, provided by the silane application influence the wet ability of the fibers and that the chemical affinity to the polypropylene is improved [10, 11]. a. 600
4`00µ m b. 400
2`00µ m c. Under 200µ m In te rf ac ia ls tre ng th ,M Pa 0 0.5 1 2 2.5 3 a b c 1.5 In te rf ac ia ls tre ng th ,M Pa In te rf ac ia ls tre ng th ,M Pa a. No treat b. Treat with Silane c. Treat with NaOH a b c 0 0.5 1 1.5 2 2.5 3 3.5 . o treat . reat with silane . reat with NaOH In te rf ac ia ls tre ng th ,M Pa Fig. 2: Variation of interfacial strength with different fiber diameter Fig. 3: Variation of interfacial strength with different treated fiber 2. Effect of fiber diameter on tensile strength of bamboo shoot culm sheath fiber reinforced PP (BsFRP) The relationship between tensile strength of BsFRP and bamboo fiber is shown in Fig. 4. Like IFSS, the tensile strength of BsFRP was also affected by the fiber diameter. With small diameter, the tensile strength was higher. It can be attributed to that thin fibers are more flexible than these with bigger diameter fiber. During the pressing process, small fibers are easily and well distributed in the hot matrix. Moreover, the contact area of small fiber with matrix is larger than that of big fiber. Therefore, unexpected voids form in fabricating process was reduced and the strength was improved. This can be seen clearly on the SEM observation of fracture surface of BsFRP (Fig. 6).There were many voids appeared in the composite materials using 192 fiber diameter from 200 - 600 µm. These voids prevent the contact of fiber and matrix. Under the outside load, the crack will start from these void area and make the material fail. The elastic modulus of BsFRP also increased when diameter of fiber decreased. The modulus of BsFRP using under 200 µm diameter fiber was 5 times higher than that of BsFRP using fiber with diameter 400 - 600 µm. 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 a b c Te ns ile st re ng th ,M Pa a . 600
4` 00 µ m b. 4 00
2` 00µ m c. U nd er 2 00 µ m . 600 ~ 4 00 'm . 400 ~ 20 0 µ m . n d er 200 µ m Te ns ile st re ng th ,M Pa a . 6 0 0 ~ 4 0 0 'm b . 4 0 0 ~ 2 0 0 µ m c . U n d e r 2 0 0 µ m El as tic m od ul us ,G Pa 0 2 4 6 8 1 0 1 2 a b c El as tic m od ul us ,G Pa Fig. 4: Effect of fiber diameter on tensile strength of BsFRP Fig. 5: Effect of fiber diameter on elastic modulus of BsFRP 5 0 0 µ m 5 0 0 µ m 5 0 0 µ m Fiber 600  400 µm Fiber 400  200 µm Under 200 µm Fig. 6: SEM observation of fractured surface of BsFRP 3. Effect of treatment on strength of BsFRP The strengths BsFRP using silane and NaOH treated fibers are placed in Fig. 7. It is clear that treatment process improved the strength of composite. NaOH and silane treatment increased the strength by 25% and 20% respectively in comparing to untreated fiber BsFRP. NaOH treatment has more effective on bamboo shoot culm sheath fiber than silane treatment did. The modulus of composite is also increased by about 20% after the treatments. Thus, both sodium hydroxide and silane are modified bamboo fiber surface and improved strength of composite materials. But the mechanism of these treatments is different. Silane may cover bamboo fiber surface and become linking between fiber and matrix by using their different groups. The strength of hydrogen bonding between silane and bamboo fiber surface depends on number of OH group of cellulose on the fiber surface. Bamboo shoot culm sheath is collected from young bamboo. Therefore the percentage of cellulose might lower than that of adult bamboo culm. So the effect of silane on bamboo shoot culm sheath fiber was lower than bamboo fiber. 193 On the other hand, sodium hydroxide (NaOH) modified bamboo shoot culm sheath fiber by dissolving hemi cellulose and lignin and cleaned the fiber surface. After NaOH treatment, bamboo fiber surface were rough. Fibers themselves are more flexible because some lignin was extracted. Hence, PP matrix was more easily to penetrate into fiber during material fabrication. This explains the reason why BsFRP had higher tensile strength after treatment with sodium hydroxide. Although the strength of BsFRP was improved when applying sodium hydroxide treatment was applied, it is still low in comparing to other materials. The reason might be that there is sodium hydroxide stay inside bamboo fiber after washing by fresh water. This excessive sodium hydroxide becomes barrier to prevent fiber contact with matrix. Therefore, in this study, we try to use acetic acid to neutralize the excessive sodium hydroxide after treatment. NaOH treated bamboo shoot culm sheath fibers were put in a 0.01% acetic acid solution for 2 hours. Then fibers were washed with water carefully. Clean fibers were dried at 80oC for 2 hours. The strength of BsFRP using NaOH and acetic acid treated bamboo fiber was measured. From the results in Fig. 9, the IFSS is increase by 25% after clean excessive sodium hydroxide with acetic acid. The tensile strength and modulus are also increased by 25% (Fig. 10 and 11). Thus, NaOH stayed inside fiber after treatment has much effect on strength of BsFRP. The SEM of fractured surface of BsFRP (Fig. 12) also indicates the improvement of interphase in BsFRP. Voids was reduced and fibers were well distributed in matrix. According to these results above, bamboo shoot culm sheath fibers can be used in some application as bamboo fiber. 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 a b c a. N o tre at b . T rea t w i th S ilan e c. T rea t w ith N aO H Te ns ile st re ng th ,M Pa Te ns ile st re ng th ,M Pa 0 1 2 3 4 5 6 7 a b c a. N o tr ea t b . T rea t w ith S ilan e c . T re a t w ith N aO H El as tic m od ul us ,G Pa El as tic m od ul us ,G Pa Fig. 7: Effect of surface treatment on tensile strength of BsFRP Fig. 8: Effect of surface treatment on elastic modulus of BsFRP In te rf ac ia ls tre ng th ,M Pa 0 0 .5 1 1.5 2 2.5 3 3.5 4 4.5 a. N o trea t b . T reat w ith N aO H c. T reat w ith N aO H + A cetic A cid a b c In te rf ac ia ls tre ng th ,M Pa 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 a b c a. N o tr ea t b . T rea t w ith N aO H c . T re a t w ith N aO H + A c e tic A c id Te ns ile st re ng th ,M Pa Te ns ile st re ng th ,M Pa Fig. 9: Variation of interfacial strength of alkali and acetic anhydride treated fiber Fig. 10: Effect of alkali and acetic anhydride treatment on tensile strength of BsFRP 194 0 1 2 3 4 5 6 7 8 a b c El as tic m od ul us ,G Pa a. No treat b. Treat w ith NaO H c. Treat w ith NaOH +A cetic Acid El as tic m od ul us ,G Pa Fig. 11: Effect of alkali and acetic anhydride treatment on elastic modulus of BsFRP 5 0 0 µ m 5 0 0 µ m 5 0 0 µ m Untreated NaOH treated NaOH + acetic acid treated Fig. 12: SEM observation of fractured surface of BsFRP IV - Conclusions 1. IFSS of bamboo shoot culm sheath fiber with MAPP was improved after treating with silane or alkali. 2. Alkali treatment has much effect on bamboo shoot culm sheath fiber than silane treatment has. 3. Bamboo shoot culm sheath fiber can be use for “green” composite as well as bamboo fiber. 4. Washing NaOH treated bamboo fibers by acetic acid was improved IFSS of bamboo fiber and PP and strength of BsFRP. References 1. C. M. Chen, H. C Chen, M. Tracy and Y. Liao. Bonding moso bamboo with copolymer resins made of biomass residue extract with phenol and formaldehyde, Forest Products Journal, Vol. 50, 9B, P. 70 - 74 (2000). 2. T. Fujii, K. Okubo and T. Shito. 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