Processability and instabilities of polyolefin-organoclay nanocomposites in a single screw extruder.

摘要

The impact of organoclay on the rheology and extrusion of polyethylenes was studied. Organoclay effect was studied at very low clay loading (≤ 0.1 wt %) while serving as a processing aid. The polyethylenes used in this work were high density polyethylene (HDPE) and linear low density polyethylenes of different branch content. A special design single screw extruder was used in the study of the extrusion melt instabilities. The slit die attached to the extruder has three highly sensitive piezoelectric transducers mounted along its length. Particle Image Velocimetry (PIV) was used in the study to measure wall slip during extrusion of polyethylene while organoclay served as a processing aid. The morphological characterization with X-ray diffractometer (XRD) and Scanning Electron Microscopy (SEM) showed that good dispersion was obtained with master batch-dilution method of polyolefin-organoclay nanocomposites. The rheological results showed that shear-rate dependent viscosity, normal stress difference, extensional strain and stress growth of HDPE were reduced with the addition of organoclay. So, organoclay (≤ 0.1 wt %) has an effect on the shear and extensional rheology of HDPE. The reduction is more pronounced in linear polyethylene. Such effects gradually decrease as the branch content increases. The trend is independent of the type of flow (shear and extensional). It is striking to note that FT-rheology is not effective in explaining the impact of organoclay on polyethylene. The intensity of the melt instability was characterized with both a moment analyses and a distortion factor (DF) from an advanced Fourier transform analysis. Both showed the same trends in the characterization of the pressure fluctuations in the die. Generally, addition of organoclay (≤ 0.1 wt %) to HDPE led to the reduction in DF. The ratio of first and second moment analyses became reduced as well. The results quantified the extent of elimination of gross melt fracture in HDPE by organoclay. Also, the extrusion pressure was reduced with organoclay (≤ 0.1 wt %) inclusion hence more throughput. There was a good correlation between rheology and extrusion. However, the maleated polyethylene added as a compatibilizer did not give substantial synergistic effect. To further understand the mechanism involved during polyethylene-organoclay extrusion, the effect of organoclay on the wall slip of high density polyethylene (HDPE) was investigated with the aid of particle image velocimetry (PIV). The study showed that organoclay did not cause wall slip during low shear testing in a parallel plate rheometer. PIV measurements during continuous extrusion of HDPE showed that organoclay induced more wall slip. So, it was suggested based on these results that in the presence of high shear flow, organoclay aligned in the flow direction and migrated towards the die wall. The alignment and migration affect the bulk properties (like shear thinning) and surface properties (like wall slip) of HDPE. Such effects contributed to the reduction in the extrusion pressure of HDPE and possibly elimination/postponement of melt instabilities in HDPE during continuous extrusion. Furthermore, the rheological tests on the HDPE containing organoclay, boron nitride and fluoropolymer showed that the phase angle of HDPE during frequency sweep reduced below the cross over frequency. All the processing aids eliminated the weak sharkskin-like instability. However, the fluoropolymer did not succeed in eliminating the stick-slip fracture. The gross melt fracture in HDPE was not eliminated by boron nitride and organoclay at apparent shear rate of 114 s-1. The combined organoclay and fluoropolymer did not as well. However, both moment and distortion factor analyzes were able to quantify the visual trends in the extrudates. The quantifying tools indicated that combined organoclay and fluoropolymer as processing aids acted better in the reduction of the pressure fluctuation compared to when both were used individually.