Development of Polyoxymethylene Composites

ポリオキシメチレン複合材の開発

2017-09-25

2017-09-25

Good balance of mechanical, thermal, chemical and wear properties is a strength point of the engineering thermoplastic as Polyoxymethylene (POM). However, the application of POM has a limitation as a low toughness. Thus, this is the one drawback that several researchers try to overcome. In this research the material and processing were designed to introduce the new function and possibility of production for POM. The adaptation of blending technique with a biodegradable thermoplastic as Poly (lactic acid) or PLA was the first candidate. The super ductility was generated from this blend system. The elongation of POM/PLA was over 300% under tensile testing speed of 10 mm/min. It was noted that the PLA content should be limited in the range of 30 – 40 wt.% to remain the ductile property. Furthermore, the limit of POM/PLA fabrication was found. With high PLA content, the in-completed distribution of POM rich phase was found and resulted on poor mechanical properties. However, the poor distribution could be improved by increasing of injection speed. That also improved mechanical properties too. Short glass fiber reinforced POM composite by direct fiber feeding injection molding (DFFIM) process was the second candidate for improving mechanical properties of POM and POM/PLA blend. The various matrix feeding speed, screw rotational speed and also number of fibers roving related to the fiber loading content and fiber. The decreased matrix feeding speed and increased screw rotational speed led to the increasing of fiber loading content. For product quality, the tensile properties of POM and POM/PLA blend were successfully improved by reinforcing short glass fiber. The tensile modulus and tensile strength of all materials increased with the increasing of fiber loading content. However, at high fiber loading content, the agglomeration and misalignment of fiber appeared on the fracture surface of POM and POM blend composites and resulted the non-increasing of tensile strength. The strength prediction was used to estimate the maximum strength of those materials. The strength of POM can be improved at least 18 %, if the fiber distribution and fiber orientation are modified. The single edge notch was marked on the sample to simulate the failure of material under the crack and compared to un-notched sample. The fracture toughness of each material depended on the load that was applied to open crack. The materials that presented the brittle crack initiation resulted the higher fracture toughness when compared to the initiation of crack as ductile or crazing behavior. While the sensitive on notch depended on the fracture transformation between un-notched and notch samples. In the case of POM, the plastic deformation still occurred in the initial crack even finally fractured as brittle. At the same time, POM/PLA blend presented the low fracture toughness, but less sensitivity on the notch. The percent crystallinity at crack root affected the fracture behavior of the super ductility of the POM/PLA blend. The low crystallinity at 1000 um of crack depth led to the super ductile fracture whereas the crack depth at 500 um with high crystallinity presented only the small crazing and propagated to brittle fracture. The fracture behavior under the open hole notch was also simulated. The stress whitening relates the low sensitivity on the open hole notch of materials. Moreover, if the materials have ductility, the crack propagates slowly. On the contrary, if materials deform without the sign of stress whitening, their failure is quite serving and rapid. In the case of POM composite, the increasing of glass fiber loading content had an influence on the increasing sensitivity of open hole notch. Crazing of matrix, stress whitening and fiber pull out were found on GF/POM fracture surface, whereas the low sensitivity on notch as neat POM presented large stress whitening zone and fanlike crack pattern. Finally the effect PLA on wear performance of the POM/PLA blend was investigated by the sample sliding against carbon steel condition. The various pressured loads were applied to sample surface to examine the friction coefficient value of each sample. The increasing of an applied load led to the decreasing of coefficient of friction and increasing of wear weight loss, whereas the increasing of PLA content led to the increasing of coefficient of friction. The high sensitive on heat generation of PLA caused the occurring the thermal softening and melting behavior and the high coefficient of friction and this phenomenal also affected on POM/PLA blends.

博士（学術）

京都工芸繊維大学

14303甲第855号

http://hdl.handle.net/10212/2431

eng

ETD

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Thesis