The effects of ultrasonic waves application during extrusionon the processing parameters and the properties of polyethylene

Abstract

The application of polymers in daily life has increased recently, where extensive energy costs are always an issue to consider during the manufacturing. Competitive markets in plastic industry require regular research and development of the materials and the manufacturing process. This thesis addresses the application of the ultrasonic vibrations in the polymer extrusion with the aim of reducing power consumption of the extrusion process. The effective penetration depth of the ultrasonic vibrations in the polymer melt was determined and used in the design of the ultrasonic horn and the housing block. Two different grades of polyethylene was extruded using a 25mm extruder to investigate the effects of ultrasonic vibrations on the polymer melt during the extrusion of polyethylene strips at 5 different screw speeds. The extruder barrel pressure, die entrance pressure decreased when processing with ultrasonic vibrations and when compared to conventional extrusion. The extruded products’ properties were systematically investigated using tensile, DMA and DSC tests. The results indicated that the power consumption of the extrusion process reduced by the application of ultrasonic vibrations without any adverse effect on the extruded products’ properties. A 50mm extruder with second generation ultrasonic horn was used to study the effects of ultrasonic vibrations on the processing parameters and properties of the strips and pipes. The extrusion trials were carried out using two grades of polyethylene for production of the strips and four different grades of polyethylene for production of pipes. Processing with ultrasonic vibrations showed lower barrel and die entrance pressure when compared to the conventional extrusion. The consumed power for extrusion of the strips and pipes was reduced by application of ultrasound in comparison with conventional extrusion. The mechanical, thermal and rheological properties of the strips and pipes did not change by the application of ultrasonic vibrations. The third generation ultrasonic horn was designed to allow ultrasonic assisted extrusion of pipes using a 90mm industrial extruder. The trials were not successful as a result of ultrasonic horn and generator failure due to change in the horn design characteristics at processing temperatures. New key factors involved in sonotrode design were identifies and raised and based on these factors, the suggestions to improve the performance of the horn was made. As a result of this work, various theories are discussed to explain the effects of ultrasonics on the melt. The lack of change in mechanical, thermal and rheological properties indicated that any modification of the polyethylene structure by this research could be dismissed. Thus, the most possible explanation for the observed effect on the process is that high frequency vibrations could increase temperature locally or reduce friction between polymer melt and the ultrasonic horn. It leads again to the key conclusion that with the correct horn design, total power consumption can be reduced without negatively affecting the polymer and product properties.