| Processing aid agents and ultrasonic wave were introduced during processing of ultrahigh molecular weight polyethylene (UHMWPE) with a general single-screw extruder. The mechanism of the conveying, melting, plasticating in the extruder and the effect of ultrasonic wave at the die during extrusion were studied. The mechanical and anti-wear properties of the extrudate, and the mechanism of the anti-wear enhancing owing to the addition of PP were explored. The experimental results show that.1.The addition of polypropylene (PP) improves the processability of UHMWPE, ultrasonic wave at the die during extrusion can effectively reduce the melt viscosity and die pressure, the extrusion efficiency is thus improved. Appropriate amount of PP and ultrasonic wave intensity can improve the mechanical strength, heat-distortion temperature and the anti-wear properties of UHMWPE.2. The mechanism of conveying, melting and plasticating of UHMWPE in the extruder were investigated with screw extraction experiment. FT-IR, DSC, WAXD and AFM show that PP is enriched between the blend and the barrel wall, thus increases the frictional force on the wall, the conveying of the solid down to the channel could be carried out. FT-IR, SEM analysis reveals that during the melting and plasticating of UHMWPE/PP blend in the barrel, PP melted first, melt film was formed on the barrel wall. The melt flows from the melt-film toward the active flight flank to form melt pool, exerting considerable pressure on the UHMWPE powder in the passive flight flanks, the hard-compacted problem ofABSTRACTUHMWPE is thus overcome. In the case of UHMWPE/PE blend, the melted PE penetrates into UHMWPE domains immediately, there is not enough PE melt aggregated on the barrel wall and PE melt pool could not be formed in the active flight flanks, therefore the UHMWPE powder in the passive flight flanks could not get enough pressure to have them compacted, combined with the low friction coefficient and the nonsticeness of UHMWPE to the barrel wall, the extrusion of UHMWPE with general single screw extruder is rather difficult. Microstructure analysis shows that PP located in the amorphous zone (or low crystallimty zone) among UHMWPE crystal domains, the net of UHMWPE chain entanglement is destroyed, the chain entanglement density of UHMWPE is decreased effectively, the flowability of UHMWPE is increased prominently upon the addition of PP. PE could not reduce effectively the chain entanglement density of UHMWPE, the enhancement of processability of UHMWPE by the addition of PE is rather limited.3. Ultrasonic wave at the die during extrusion can reduce the die pressure and the apparent viscosity of UHMWPE/PP blend prominently. Ultrasonic effects also contribute to the increase of output. The effect of ultrasonic wave on the reduction of the apparent viscosity is extremely remarkable to UHMWPE/PP blend. With the decrease of PP content in the UHMWPE/PP blend, the chain entanglement density increased, the effectiveness of the melt viscosity and die pressure reduction increased. The higher the entanglement density, the more the apparent viscosity was reduced. Ultrasonic wave can reduce crystal dimension and crystallinity. With appropriate dose of ultrasonic treatment, the impact strength, young's modulus and anti-wear properties of the extrudate are improved.4. The anti-wear property of UHMWPE can be improved by blending it with appropriate amount of PP. The friction coefficient and wear rate of UHMWPE are much higher than those of UHMWPE/PP blend, the wear rate of UHMWPE is more sensitive to load and wear time than that of the UHMWPE/PP blend. Long scratch grooves and cracks occurred in the worn surface of UHMWPE, while no such serious damage is observed in the worn surface of the UHMWPE/PP blend.IVUnder high sliding speed(400rpm), the temperature of the worn surface is so high, the melt loss is the major cause of UHMWPE wearing, no melting trace could be detected in the worn surface of UHMWPE/PP blend. AFM indicates that friction force between UHM...
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