Studies On Toughening Modification Of Acetal Copolymer

Polyoxymethylene (POM) is a kind of important industrial material and becomethe third largest engineering plastic. However, the poor notched impact strength ofPOM restrains its applications. Its toughening modification is a focus topic, but theindustrial application of toughening products remains in the theoretical stage. Thispaper focuses on toughening method of acetal copolymer. Elastomers (PNBR, TPU),ultra-high molecular weight polyethylene (UHMWPE) and acetal homopolymer(HPOM) were used for toughening agents respectively. A series of CPOM compositematerials were prepared by melt blending. The effects of properties of acetalcopolymer on three modification agents were discussed. The properties of compositematerials were characterized and analysed by differential scanning calorimetry (DSC),polarized light microscopy (PLM), thermal gravimetric analyzer (TGA), charpyimpact testing machine, universal testing machine, scanning electron microscopy(SEM) etc.By comparing processing properties and mechanical properties, vulcanizedpowder nitrile rubber (PNBR-A) has a better toughening effect than uncured powdernitrile rubber (PNBR-B). Moreover, under the same content of toughening agent,PNBR-A was far superior to thermoplastic polyurethane (TPU) by comparing withCPOM/TPU. There were differences in toughening machanism between PNBR-A andTPU. Powder nitrile rubber existed in the form of rubber belt in matrix and TPU wasstill in the form of particles when the content was more than10%.It was showed by DSC and TG analysis that PNBR-A can restrain the movementof molecular chain segments, so that the molecular chains arranged orderly in a smallscale. Therefore, the crystallinity of CPOM was improved and melting peak shifted toa higher temperature. Moreover, adding powder nitrile rubber can increase thedecomposition temperature of the materials significantly. T5%and Tpincreased byabout36℃and69℃respectively.UHMWPE was used as impact and nucleation modifier to prepare blends by meltextrusion. When the content of UHMWPE was1wt%, the notched impact strength ofthe material was1.24folds that of neat CPOM and the tensile strength of material was52.8MPa. The comprehensive properties of material stay at a high level. Avrami,Ozawa, and Mo’s theories were performed to analyze the nonisothermalcrystallization kinetics of CPOM and CPOM/UHMWPE (100/1) blend. The result showed that two parts of UHMWPE affected the blend melts. One part had highentanglements with the molecular chains of CPOM, which hindered the movement ofthe molecular chains of CPOM. The other part existed as a form of solid particles andthis one induced heterogeneous nucleation. UHMWPE can slow down thecrystallization rate of composite material, prolong the time of the crystallization andimprove the crystallization of CPOM. Activation energy was calculated by theKissinger method. Addition of1wt%UHMWPE decreased the activation energy ofCPOM significantly.For CPOM/HPOM composites, polyoxymethylene homopolymer improved theoverall performance of materials. When the ratio was50/50, the notched impactstrength of material increased by1.5times and the tensile strength was54.8MPa.However, thermal decomposition temperature of the composite materials appeared anobvious downtrend. Therefore, the effective system of thermal stability is worthfurther exploration.