Study On Toughening And Wear-Resistant Modification For Domestic-Manufactured Polyoxyme Thylene

Polyoxymethylene is a well known engineering thermoplastic that has been widely applied in various fields for its excellent mechanical properties and its desirable self-lubrication. However, domestic-manufactured POM industry has a relatively inferior production capacity and a narrow range of industrialized products, compared with foreign POM manufacturing. Moreover, currently there are very few POM products that have been applied in fields related to advanced science and technology such as automobile and electronic appliance, etc. So there is an increasing emphasis on toughening and wear-resistant modification for domestic-manufactured POM.This study can be divided into the following two parts:The first part is about the study on toughening modification for domestic-manufactured polyoxymethylene.POM tends to form spherulite that is related to its inferior izod impact strength so that it can not meet the mechanical requirement in many case. In this study,diversified types of toughening method has been adopted to improve impact toughness of POM,of which there are mainly two system that achieve an advanced impact resistant property:one is adding of elastomer modifiers,the other is collaborative system offered by PTFE fiber and PEO. The method metioned above both are regarded the most effective one and had made a series of large progress. In this study, rubber modifying agents has been studied to improve the impact toughness of polyoxymethylene including thermoplastic polyurethane(TPU), polyethylene-octenepolymer(POE) and polyethylene glycol(PEO).Besides,the incorporation of compatibilizer plays a key role in toughening modification. Novolak has been studied in the blending system involving POM and TPU. Actually, Novolak serves as a new medium to combine POM with the rubber particle. Thus, the toughening agent can be dispersed homogeneously in the matrix which become the stress concentration to make the impact energy exhausted. Therefore, the toughness of POM composite with 3% Novolak increases by 25% compared with that of pure POM.In this study, comprehensively advanced properties are also obtained after we eliminated the loss in stiffness, modulus and yield strength that are caused by the low rigidity of the rubber particles. Stiffness and fracturetoughness are opposing performance parameters and a better balance is required to develop an efficient materials. In order to overcome this problem, the appropriate addition of reinforcing fillers such as PTFE fiber is introduced. The PTFE fiber can advance both the izod impact strength and mechanical properties by the effect of PTFE fiber's pulling out of POM matrix, which absorb a great amount of outside impact energy as well consume it in a short time, and thus prevent the material from serious destroy.The second part is about the study on wear-resistant modification for domestic-manufactured polyoxymethylene.A novel type of polyoxymethylene (POM)-based composites with polytetrafluoroethylene (PTFE) fiber and poly(ethylene oxide) (PEO) were prepared via melt extrusion in order to enhance the frictio lubrication and wear resistance of POM.The tribological experiments demonstrated that the coefficient of friction and wear rate of POM were both reduced by the incorporation of PTFE fiber, and furthermore, the higher the PTFE-fiber loading, the better the tribological properties. The wear mechanism is derived from the thin film of PTFE formed on the contact-surfaces during sliding. The addition of PEO into POM/PTFE fiber composites could enhance the formation of transfer film on the mating surfaces during sliding contact,and thus further improved the friction and wear performance. The wear behaviors also depended on the normal loading and sliding duration, under which the transfer film could form appropriately. In addition, the abrasion of the dispersed PEO domains in matrix is also helpful to the heat dissipation when the counter-surface exerts oscillating forces on the POM surface. It should be emphatically mentioned that the use of PTFE fiber as tribological additive could improve the notched impact strength of POM, and the subsequent incorporation of PEO led to a more significant toughening effect for POM. The morphological investigation reveals that the toughening effect is attributed to the dissipation of impact energy through the pullout of PTFE fiber and the plastic deformation of the POM matrix induced by PEO.Polarized optical microscopy demonstrates that the presence of PEO could interfere in the crystallization of POM and reduced the size of spherulites, and consequently weakened the sensitivity to notch. Although the POM-based composites underwent a different mode of thermal degradation from neat POM with a slight reduction in the temperature at rapid weight loss, their thermal stabilities were maintained well enough to meet the requirement for its application.