Thermo-Mechanical Chain Branching of Commercial High Density Polyethylene during Extrusion

Abstract

Two different phenomena may affect average molecular weight (Mw) of molten polyethylene during extrusion process. The first is crosslinking, which can be divided to two categories of chain branching and network formation, leading to an increase in Mw. The second is chain scission which leads to a decrease in the average Mw. In this work, chain branching of molten stabilized pipe grade high density polyethylene (HDPE) and unstabilized one, extruded in industrial twin screw extruder, has been studied. Therefore a series of analytical techniques including melt flow rate (MFR), capillary rheometer, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and Fourier transform infrared spectroscopy (FTIR) were employed. The unstabilized samples' MFR were lower than stabilized ones' which showed a higher melt viscosity of unstabilized samples due to their higher Mw. By applying rheometry test in different modes, the unstabilized PE samples showed a higher shear viscosity in comparison with stabilized ones agreeing with MFR results. DSC results showed a difference in degree of crystallinity between samples. This difference was verified by DMA result of solid state which showed a higher shear storage modulus for stabilized samples. Also, DMA results confirmed the obtained results from rheometry test in melt state. Additionally, FTIR results of stabilized and unstabilized samples demonstrated the difference between their chemical structures. Although it seems that the level of chain branching in this grade of HDPE is low howover, all techniques' results are in a good agreement which makes the provided results and data reliable. Moreover, a combination of the applied methods in this work can be helpful to determine the validity and efficiency of antioxidants