Lubrication effects on die plateout in lead stabilised rigid poly(vinyl chloride) extrusion

Plateout is an extrusion defect that has caused formulators and processors major problems throughout the history of poly(vinyl chloride) (PVC) processing. It is an unwanted deposit that forms on the die, calibration and degassing sections of extrusion equipment that can cause unacceptable streaking and scoring on the final product through continued deposition. Extrusion companies must therefore periodically strip their equipment to stop excessive formation, but the downtime associated with this cleaning procedure is obviously costly. The purpose of this thesis is to develop an understanding of the lubrication mechanisms within lead stabilised rigid PVC compounds using polyethylene (PE) wax as a source for external lubrication, in order to make formulation recommendations to minimise plateout. Plateout samples supplied by commercial processors were analysed to identify deposit prone additives. Of the techniques investigated, laser ionisation mass analysis (LIMA), gas chromatography/mass spectrometry and infra-red spectroscopy by the diffuse reflectance infra-red Fourier transform (DRIFT) method identified calcium carbonate, titanium dioxide and dibasic lead phosphite as the bulk of die plateout. Additive interactions were investigated by differential scanning calorimetry (DSC) and hot-stage microscopy. Majority of the additives in a lead stabilised window profile formulation did not interact, but molten calcium stearate and normal lead stearate did not recrystallise when mixed together. To simulate commercial processes, a laboratory-scale twin-screw extruder equipped with a plateout sensitive die was used to analyse window profile formulations containing various homopolymer grades of PE wax and oxidised PE or high-density PE lubricants. Structural properties that influence physical properties such as melting point and viscosity were related to their tendency to plateout. The influence of calcium stearate and normal lead stearate dosage on lubrication and plateout performance was also investigated using experimental design.