Study Of The Effects Of Ultrasound On The Structure And Properties Of Polypropylene And Its Composites

Mixing is an important method of polymer processing operations to prepare polymer composites with high performances, and melt compounding is a widely used method among many mixing methods becasuse it is simpler and cheaper. However, how to improve the dispersion of modifiers in polymer matrix through melt compounding is an urgent problem to be solved by many reseachers. In this study, ultrasound was introduced into melt compounding to enhance the dispersion of modifiers in polymer matrix for the excellent dispersing ability so as to improve the performances of the composites.A set of ultrasonic device for polymer melt was set up to study the effects of ultrasonic treatments on the degradation and crystallization of polypropylene (PP). Ultrasonic degradation was investigated by means of the intrinsic viscosity and gel permeation chromatography (GPC). The degradation kinetics model was built up. The effects of ultrasonic power, ultrasonic frequency, sonication time, melt temperature and sonication distance were studied, and the degradation mechanism was proposed. The crystallization of polypropylene (PP) under ultrasonic irradiation was characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and polarizing microscope. The rheological and mechanical properties were also investigated.The ultrasonic disintegration behavior of polystyrene-block-poly(ethylene-co-butylen-es)-block-polystyrene (SEBS) in PP matrix was studied by means of scanning electron microscope (SEM). The disintegration kinetics model was derived, and the influencing factors on ultrasonic disintegration were investigated. In situ compatibilization of PP/SEBS blends in the presence of ultrasonic irradiation was investigated by means of infrared spectroscopy (IR) and dynamic mechanical analysis (DMA). The rheological and mechanical properties were also investigated.The ternary nanocomposites of PP/SEBS/OMMT were prepared by continuous mixing in a two-rotor continuous mixer with the aid of ultrasound. The effects of ultrasound and PP-g-MA on the microscopic dispersion morphology of the composites were studied by means of XRD, transmission electron microscope (TEM) and SEM. The crystallization of the composites was characterized by XRD, and the rheological and mechanical properties were also investigated. The main conclusions are as follows:(1) The reduction in the intrinsic viscosity and the molecular weight could be attributed to the collapsing of ultrasonically driven bubbles, which were observed in the treated samples. And the amount of bubbles increases with the increase of sonication time and ultrasonic power, indicating that the acoustic cavitation bubbles are more easily formed in polymer melt treated by ultrasound with higher intensity. In addition, it could be found that more cavitation bubbles were observed in the sample ultrasonically treated at 20 KHz. Therefore, the increase of sonication time and ultrasonic power could accelerate the degradation of PP, and PP could be more easily degradated under ultrasonic treatment with a frequency of 20 KHz.(2)?-form PP could be induced by ultrasonic irradiation, and its amount increased as the sonication time and ultrasonic power increased to a certain value and then decreased as the time and power further increased due to the degradation of PP.(3) A disintegration kinetics model of the dispersed phase in polymer matrix was proposed to explain the ultrasonic disintegration kinetics, and it is in accordance with the equation: The disintegration rate kinetics model was also proposed: The increase of ultrasonic power could enhance the dispersion of SEBS, and SEBS particles were more easily disintegrated at 20 KHz. Moreover, the disintegration extent decreased with the increase of sonication distance or melt temperature.(4) The compatibility of PP/SEBS blends could be improved due to the formation of a copolymer of PP and SEBS by the combination of different macroradicals induced by ultrasonic oscillation, while the compatibilizing effect would be weakened when the blends were ultrasonically treated at a high intensity due to the substantial degradation of polymers.(5) PP/SEBS/OMMT nanocomposites of varying concentrations of maleic anhydride-grafted polypropylene (PP-g-MA) were prepared by continuous mixing assisted by ultrasonic oscillation for the first time. The impact strength of the nanocomposites prepared by the ultrasonically assisted compounding was improved by 80.9% and 7.9% compared with the pure PP and the untreated composite respectively, and reached to 49.2J/m. The tensile modulus of the nanocomposites with 15 ph PP-g-MA prepared by the ultrasonically assisted compounding was improved by 24.8% and 84.8% compared with the pure PP and the untreated composite respectively, and reached to 913.6MPa.