Ultrasound Induced Physical And Chemical Reactions Of Blending Polymer Melt And Structure Development During Injection

At present, the processing of polymer materials is regarded as a key course controlling quality and property rather than a pure physical course. New processing method and morphology controlling of polymer during processing are hot research topics in polymer science and engineering. Ultrasonic extrusion, a high efficient and green clean processing method, has been developed to enhance the proceesibility of polymer materials in recent years. The ultrasonic oscillations with high frequency and low amplitude can affect the structure of polymer during ultrasonic extrusion, further improve the properties of polymers. On the basis of previous studies, the effects of ultrasonic irradiation on compatibility of polymer blends and properties of polymer/clay nanocomposites were studied in this thesis. The useful potential of ultrasonic extrusion was greatly exploited. Furthermore, ultrasonic degradation and viscosity drop were studied and their mechanisms were also established in this thesis. In addition, the discrepancies between near and far part from gate at injection molded parts were discussed. The main results are stated as follows. (1) Ultrasonic degradation of PA6, EPDM and PS melts was conducted using the static ultrasonic irradiation reactor specially designed in our lab. For EPDM and PS melts, the experimental results shown that the molecular weight or intrinsic viscosity decreased with ultrasonic irradiation time and approached to a limiting value, below which no further degradation took place. The ultrasonic degradation reaction includes only rupture of macromolecular chains, which occurs at C-C bond. For PA6 melt, chain extension also exists in ultrasonic degradation due to reactive end group. Some surprising experimental results show that for low molecular weight PA6 (LPA6), the viscosity-average molecular weight of LPA6 gets increased in the presence of ultrasonic oscillations due to extending reaction of end groups (-COOH and -NH2), while for high molecular weight PA6 (HPA6), the viscosity-average molecular weight of HPA6 decreases with ultrasonic irradiation time, and passes through a minimum, then increases with ultrasonic irradiation time, up to a limiting molecular weight. FTIR analysis and microtitration method confirm that the chain scission of HPA6 under ultrasonic irradiation occurs at the location of C-N bond. The initial product of ultrasonic degradation in PA6, EPDM and PS melts is macroradical, which is confirmed by FTIR spectra of samples ultrasonically treated and untreated. There are two kinds of chain rupture modes in the presence of ultrasonic irradiation. In first stage, the ultrasonic rupture is a random process, and the molecular weight distribution broadens. After that, ultrasonic rupture of macromolecular chains becomes a non-random process, and the molecular weight distribution becomes narrow. (2) The degradation model of polymer melts was proposed to explain the ultrasonic degradation of polymer melts, and ultrasonic degradation of polymer melts are regarded as a mechanochemical process. Ultrasonic degradation kinetics of polymer melts obeys the equations: M t = M∞+Ae?kt or [η]t = [η]∞+([η]0 ?[η]∞)e?kt. The limiting value is independent of the initial molecular weight. The distance of ultrasonic action, ultrasonic intensity and temperature of polymer melts can affect the rate, extent and limiting molecular weight of ultrasonic degradation. (3) The effects of the ultrasonic oscillations on mechanical properties, phase morphology, as well as size distribution fractal dimension of dispersed particle were studied. The experimental results indicated that ultrasonic treatment could improve the compatibility of PS/EPDM blends. This is attributed to the formation of copolymer of PS and EPDM by combination of different marcoradicals, resulting from hemolytic cleavage of polymer chains induced by ultrasonic irradiation. Thedisproportional termination of the macroradicals leads to the degradation of PS and EPDM. Mechanical properties were affected by both ultrasonic compatibilization and degradation. These properties depended strongly on the ultrasonic intensity, ultrasonic treatment time and number of extrusion. (4) The variations of linear viscoelastic properties of PS/EPDM and HDPE/PA6 blends can further confirm the ultrasonic degradation and compatibilization during ultrasonic extrusion. The proper ultrasonic intensity can increase the complex viscosity η*, storage modulus G′and loss modulus G′′of PS/EPDM (80/20). Conversely, if ultrasonic intensity is too high, the value of η*, G′and G′′would decrease. The η*, G′and G′′of HDPE/PA6 (80/20) measured at 235 °C increases with the ultrasonic treatment. The opposite manner was observed, due to greater extent of degradation PA6, if the content of PA6 in HDPE/PA6 blends was high. However, if the experimental temperature was 210 °C, i.e. PA6 particles were not melted and the structure of HDPE/PA6 resembled that of particle-filled system, ultrasonic compatibilization led to the reduction of η*, G′and G′′. Similary to a compatibilizer, the compatibilization of PS / EPDM and HDPE/PA6 blends induced by ultrasonic irradiation can cause the increasing slope of logG ' versus logG " plots in the terminal region. Using emulsion model, the interfacial tension between the matrix and the dispersed phase can be estimated. The results showed that ultrasonic irradiation can decrease the interfacial tension, which attributes to the ultrasonic compatibilization. (5)The extrusion processing behaviors of PS, EPDM and PS/EPDM (80/20) were studied by using a special ultrasonic oscillations extrusion system developed in our lab. The die pressure and volume flow rate were measured at different ultrasonic intensity and screw rotation speed. The relationship between ultrasonic intensity and die pressure, apparent viscosity of polymers, volume flow rate, as well as die swell at the same screw rotation speed was investigated. The effects of screw rotation speed on the processing behaviors of polymers and their blends at the same ultrasonic intensity were also studied. The experimental results showed that, in the presence of ultrasonic irradiation, the proceesibilities of polymers and their blends wereimproved. Their possible mechanism was discussed in this article. The change of apparent viscosity during ultrasonic extrusion should be reversible physical course, therefore the apparent viscosity of melts would increase to initial value when no ultrasonic irradiation. The decrease of apparent viscosity of melts during ultrasonic extrusion could not be interrelated with the ultrasonic degradation. (6) Two kinds of polymer/OMMT nanocomposites, PECN and PA6CN, were prepared by ultrasonic extrusion. This method has great advantages over either in situ intercalative polymerization or polymer solution intercalation. First, it is environmentally benign due to the absence of organic solvents. Second, it is compatible with current industrial process, such as extrusion. WAXD analysis and TEM observation indicated that ultrasonic irradiation can't change the structure of PECN, but can affect the microstructure of PA6CN, causing more regions of exfoliated clay, due to higher compatibility. Our experimental results can also show the great effects of ultrasonic irradiation on crystal structure, crystallinity, size and distribution of spherulite of PA6 in PA6CN. The crystallizing behaviors of HDPE in PECN were changes slightly during ultrasonic extrusion due to insignificant changes in structure of PECN. SEM observation implied that the size of particles of OMMT dispersed in polymer matrix decrease and its distribution become narrow after ultrasonic extrusion. Due to the better dispersion of clay, the elongation at break of PA5CN and PECN ultrasonically treated got greatly increased by 67.9 and 51.9%, meanwhile ultrasonic oscillations also improved their other mechanical properties, such as tensile and impact strength. (7) The development of morphology and structure of injection part along the direction of melt flow was studied. The melt viscosity and injection conditions can affect greatly the morphology and structure of injection part. It can be found that the morphology and structure of injection part varies gradually along the direction of melt flow, causing that the impact strength of near part is higher greatly than that of far part. Near part of H2C5 shows a very visible interface of skin and core layer, through which the core layer can be removed. The extent of orientation of core layer is lower than that of skin layer. When subjected to impact load, the core layer of nearpart of H2C5 is separated from the skin layer and the core layer is not broken off. The area of interface of skin and core layer increases with the decrease of distance from gate. Polarizing infrared ray pattern shown that the extent of orientation of near part of H2C5 is higher greatly than that of far part, causing that the fracture occurs in the far part of injection molded H2C5. SEM observation indicated more and less particles elongated along the direction of melt flow. And degree of deformation of dispersed particles increases with the decrease of distance from gate.