| Polyoxymethylene (POM) is an important engineering plastic. Its regular conformation causes POM to crystallize easily and to get large spherical crystals and high crystallinity (approximately 60-80％) in the melting process. As a result, POM possesses good mechanical properties but shows notch sensitive with poor impact strength, high contractibility in melting process and low thermo-degradation temperature.To improve its notched impact strength, POM was modified with thermoplastic polyurethane. It was found that the toughening effect of TPU on POM is related to TPU type, flexible segment/stiff segment ratio in the TPU molecular chain, TPU crosslinkingdegree, processing parameters, and viscosity ratio of TPU to POM, etc. The polycaprolactone based thermoplastic polyurethane with a Shore A hardness of 80 was selected to toughen POM. The results show that this type of TPU can effectively enhance the notched impact strength of POM, depending on the content of the TPU. When the TPU content is 10 wt％, the related notched impact strength of the blend is 14kJ/m2, 130％higher than that of pure POM, and when TPU content increases to 40％, the notched impact strength of blend abruptly increases to 75kJ/m2 because of the formation of the co-continues phase structure. Meanwhile, the elongation at break increases, however the tensile strength and modulus decrease with increasing TPU content.The influence of TPU on the crystallization of POM was also studied. It was found that TPU can change some crystallization parameters of POM such as the crystal nuclei number, the spherical crystal size, the growing rate of the spherical crystal, the nucleating time and the crystallizing rate. The influence of TPU content in POM/TPU blends on the crystallization of POM weakens with decreasing the cooling rate of the blends. It was also shown that the style of POM crystal growth does not chang with the addition of TPU during the main crystallization period.To furthermore improve the impact strength of POM/TPU blends, the influence of processing temperature on the domain size of TPU as dispersed phase was studied in a twin-screw compounding extruder. It was found that the formation of the TPU morphology in POM matrix bended in twin-screw extruder can be well explained by the abrasion mechanism, accordingly the morphology of TPU in POM matrix can be effectively controlled and the toughening effect of TPU on POM can be further intensified. When POM and TPU were blended in extruder at lower temperature (175℃), the blending effect proved to be better than that of master batch method. The notched impact strength of POM/TPU blend can be further improved through adjusting the viscosity ratio of TPU to POM (P=ηTPU/ηPOM) around 1 at the processing temperature (Tp) of 190℃, so as to obtain fine morphology in injection moulding process.The influence of TPU on the thermal stability of POM was studied. It was found that the addition of TPU into POM can effectively improve the thermal stability of POM and increases the thermo-degradation temperature by 10～20℃.The addition of inorganic filler into POM can improve its stiffness, depress its cost and increase its size stability in processing. The influences of nano-CaCO3, spindly CaCO3 and talc on mechanical properties of POM were compared. It was found that these three kinds of fillers can improve the stiffness of POM and the talc is the best one. Nano-CaCO3 can toughen and stiffen POM, while spindly CaCO3 and especially talc decrease the toughness of POM. To maintain the impact strength of POM, POM is filled with three kinds of fillers with the different shape and size in a certain proportion. The comprehensive mechanical properties of the prepared composites are better than that of the composite only filled with talc: the notched impact strength increases 1.8 times higher, the elongation at break increases 2 times higher, and the flexural modulus increases by 400MPa.TPU effectively improves the notched impact strength of POM at expense of the stiffness of POM. It is well known that the stiffness of POM/TPU blends can be improved by introduction of inorganic fillers such as spindly CaCO3, talc and glass fiber etc. Then, the mechanical properties of POM/TPU/inorganic filler composites were investigated. It was found that talc and glass fiber can both effectively improve the stiffness of POM/TPU blends and the latter has a better effect. For POM/TPU(10/90) system, when the filler content is 10％, the stiffening effect of talc was found to be the best, and the impact strength and the elongation at break of composites containing Talc decrease much less than those of composite containg CaCo3 and glass fiber do. Addition of 10％talc to POM matrix toughened with 10％TPU can maintain the stiffness of POM and can simultaneously enhance its impact strength effectively.
|
PDF Full Text Request