Improvement of strength of bamboo fiber reinforced composites by freezing filamentization

Transcription

1 Improvement of strength of bamboo fiber reinforced composites by freezing filamentization T. Katayama & M. Ashimori Department of Mechanical Engineering, Doshisha University, Japan Abstract The application ranges of plastic moldings are increasing with recent remarkable improvement of resinoid performance, mold accuracy, and mold-processing technique. They have been applied in our daily commodities, for example, electrical appliances, car components, gear which requires high precision, and optical lenses. The thermoplastics that carry out the melting-solidification molding are more excellent than the thermosetting plastics that are accompanied by curing. Therefore, this can recycle and shorten the molding cycle. Fiber Reinforced thermoplastics (FRTP) have high specific strength and rigidity. For the reinforcement of FRTP, glass fibers and carbon fibers are mainly used. However, the treatment after usage is difficult, and environmental damage is large. Therefore, natural fibers are focused on because they are friendly to the environment. We focused especially on bamboo fibers because these have good mechanical characteristics, can grow very quickly, and are not used as an industrial resource. Therefore, our study is to use bamboo fiber that has excellent mechanical properties and good productivity for reinforcement. Also, we use polypropylene (PP) which is a general-purpose plastic as matrix. We use these materials and examine the mechanical properties of moldings, which were prepared by injection molding. The bamboo fibers that we use are explosion fibers and crushed fibers. Keywords: bamboo fiber, injection molding, freezing, aspect ratio, filamentization.

2 434 High Performance Structures and Materials II 1 Introduction This study makes use of the composites of bamboo fibers and polypropylene (PP) which is a general-purpose plastic as reinforcement for matrix. We used the injection molding to prepare composites and examine the mechanical properties. The types of bamboo fibers are steam explosion fibers which are form bamboo column by steam explosion, and crushed fibers which are commercial bamboo chips. In our previous study, aspect ratio, moisture and dispersion are found to be important factors. Problem in injection molding is fiber breakages caused by the screw. Problem in crushing fibers are the short fiber length; they eventually became powders. The problem with the explosion fiber is that the resin dose not infiltrate because it becomes cotton when setting it in the mixer for filamentization. So, our purpose is to reduce its diameters and that will be freezing the bamboo fiber bundles. In this study, we will filamentise the fibers without using the mixer, but by freezing with the freezer or liquid nitrogen in the very last of the processing. 2 Experimental methods 2.1 Filamentization methods Crushed bamboo fibers Crushed fibers are commercial bamboo chips as original material. Bamboo chips are crushed bamboos in China. We select bamboo fibers in our study. We take out bamboo fibers by sifting. These diameter are 125~21 μm (7~12 Mesh) and 21~425 μm (4~7Mesh) Steam explosion bamboo fibers Explosion fibers are made from bamboo column by steam. We put the bamboo column that node parts has removed in the processing machine, than we send steam and maintain it for one hour with fixed pressure of 175. And we recompress, and send steam and maintain it for five minutes with fixed pressure of 175. We repeated this procedure for ten times Perfect fiber We gathered perfect fibers by riddling the fine powder-like pieces after crushing by the hand. They had not been used as the crushed fibers in our recent study. These diameters are from 4 μm to 12 μm Freezing by liquid nitrogen The bamboo fiber was soaked in the liquid nitrogen, and it was frozen until the liquid nitrogen evaporated for about 2 minutes. The temperature is 196.

3 High Performance Structures and Materials II Freezing by freezer The freezer freezes bamboo fibers for 72 hour. The temperature is Molding methods and conditions The FRTP of fiber weight content 3wt% were made. The fiber and resin (PP) were directly turned on in the hopper of the molding machine, and the injection molding was done. The dumbbell type specimens were cut out and the tensile tests were carried out. Molding conditions were the screw rotational speed of 5rpm, the shooting speed of 1m/s and the cylinder part temperature of Tension tests Tension test of fibers The distance between gauge marks is 5mm and the test speed is 1mm/min, the load is added until the fiber break, and the maximum load is measured Tension test of composites The tensile test use 5566types Instron universal test machine and applied the regulation for JIS K754 correspondingly. Using dumbbell type test piece described above, the distance between gauge marks is 5mm and the test speed is 1mm/min, the load is added until the test piece break, and the maximum load is measured. 2.4 Measurement of fiber diameters and fiber lengths The composites were soften in the furnace of 12 for ten minutes. We take it out from furnace, and make it thinner with a metallic board. The picture has taken by the microscope and digitalize to the computer, and we measured the fiber length and diameter by using the image analysis software. 3 Results and discussion 3.1 Microscopic view of bamboo fiber Figure 1(a) shows the scanning electron microscope (SEM) view of frozen bamboo fiber, and figure 1(b) shows that of non-frozen bamboo fiber. Figures 1(c)-(f) are 21~425 μm, 125~21 μm, 4~12 μm and steam explosion bamboo photographs. From comparison of 2 photographs, frozen bamboo fiber found that there were no wood parts on the surface. However, with the specialists view, the bamboo fiber found that internal lignin was not removed. By removal of wood parts from the surface, filamentization becomes easier, fibers with smaller diameter are obtained, and improvement of aspect ratio is expected. It is thought that wood parts were removed from the surface, because of the expansion of moisture when freezing the bamboo fibers.

4 436 High Performance Structures and Materials II (a) (b) (c) (d) (e) (f) Figure 1: (a) Frozen fiber, (b) non-frozen fiber, (c) , (d) , (e) 4 12, (f) steam explosion fiber.

5 High Performance Structures and Materials II 437 Fiber diameter (μm) ~7mesh(before freezing) 4~7mesh(after freezing) 7~12mesh(before freezing) 7~12mesh(after freezing) steam explosion(before freezing) steam explosion(after freezing) perfect Type of fiber Figure 2: Type of fiber and fiber diameter. tensile strength(mpa) type of fiber 4~7mesh before frozen 4~7mesh after frozen 7~12mesh before frozen 7~12mesh after frozen explosion before frozen explosion arter frozen 12~ before frozen Figure 3: Type of fiber and tensile strength. 3.2 Influence of fiber diameters by freezing Figure 2 shows comparison of fiber diameter before and after of freezing 7 types of fibers with the freezer for 72 hours. Fiber types are 125~21μm and 21~ 425μm of crushed fibers, steam explosion fiber and perfect fiber. From this figure, it is found that the fiber diameter becomes smaller by freezing in the case of explosion fibers and 4 to 7 mesh fibers. The effect of making the fiber diameter smaller will be expected in the case of 7 to 12 mesh as well. And, we expect that by repeating the freezing process, fine fibers of the minimum diameter can be obtained.

6 438 High Performance Structures and Materials II tensile strength (MPa) type of fiber nondreezed with maleic acid liquid nitrogen with maleic acid freezer with maleic acid (a) aspect ratio liquid nitrogen freezer (b) Figure 4: (a) Type of fiber and tensile strength, (b) type of fiber and aspect ratio. 3.3 Influence of fiber strength by freezing Figure 3 shows comparison of fiber strength before and after of freezing 7 types of fibers with the freezer for 72 hours. Fiber types are same as 3.2. This figure shows that there is quite strength fluctuation for all fibers, because fibers has larger strength in the outer-side. From this result, it is found that fiber strength will not be affected in the freezing and processing methods. 3.4 Influence of composites strength by freezing Crushed fibers Figure 4(a) shows comparison of strength of BFRTP using non-frozen fibers after crushing, frozen fibers with liquid nitrogen after crushing, and frozen fibers

7 High Performance Structures and Materials II 439 with the freezer after crushing. Figure 4(b) shows the aspect ratio of fibers used in BFRTP mentioned above. From figure 4(a), the improvement of strength can be expected by freezing. Also, strength of the case with the freezer is larger than strength of the case with liquid nitrogen. From the result, freezing the bamboo fiber gradually can expect the improvement of strength. Freezing the bamboo fiber gradually is more effective than freezing it rapidly for improving strength. From figure 4(b), it is found that there are few differences in the aspect ratio of frozen fibers by the freezer and liquid nitrogen. However, from our past study, the aspect ratio is 1 to 3 when bamboo fibers are not been frozen, so we already knew that the aspect ratio improves by freezing. 35 Tensile strength (MPa) mixer liquid nitrogen freezer Type of fiber (a) Aspect ratio liquid nitrogen freezer Type of fiber (steam explosion) (b) Figure 5: (a) Type of fiber and tensile strength, (b) type of fiber and aspect ratio.

8 44 High Performance Structures and Materials II explosion fibers Figure 5(a) shows comparison of the strength of BFRTP by differences of freezing process of fibers after steam explosion, and figure 5(b) shows the aspect ratio of BFRTP by differences of freezing process of fibers after steam explosion. From figure 5(a), improvement of strength can be expected by freezing better than mixing. Also, freezing the bamboo fiber gradually with the freezer can expect the improvement of strength. From figure 5(b), improvement of strength can be expected by freezing better than mixing. Also, freezing the bamboo fiber gradually with the freezer can expect the improvement of strength. 4 Conclusions o o o o Frozen fibers are found removal of wood parts, because of the expansion of moisture when freezing the bamboo fibers. The fiber diameter becomes smaller by freezing it. Freezing and processing methods will not affect the strength of fiber. Freezing the bamboo fiber gradually with the freezer can expect more improvement of BFTP strength. References [1] Z.Maekawa, J.Seikei-Kakou, Vol.6, No.8, , (1994). [2] D.Morizono and M.Kubota, Doshisha University bachelor s thesis, (22). [3] S.Nagai, Doshisha University master's thesis, (23). [4] W.Tanaka, Matsushita Electric Works technical report, (21).