Abstract:
Purging compounds allow for rapid colour and material changes in plastics converting machines. They have both a cleaning and a purging action. The cleaning action refers to the removal of contaminants, e.g. carbon deposits, from the die, barrel and extruder-screw surfaces. This mechanism relies on conventional detergency in combination with high wall shear stresses. The purging action refers to the observed narrowing in the residence time distribution and is less well understood. The action of a purge compound may encompass both the cleaning and purging mechanisms in order to achieve the desired effect. In industry the effectiveness of a purge is determined by a visual observation of the extrudate. Therefore, a scientific method was required to quantitatively determine the efficiency of each purging mechanism under a set of fixed experimental conditions. In this study, a method was developed using a laboratory film-blower, which made use of a phthalocyanine blue pigment to impart colour to the film produced. The method was used to test the efficiency of various commercial purge compounds in switching the colour of film from blue to clear. The analysis was achieved by measuring the residual pigment concentration in the blue polymer film using a UV -Visible spectrophotometer. Commercial purge compounds function by means of various mechanisms such as filler abrasion, solvent dissolution, the dislodging of deposits with the aid of surfactants, etc. This study also considered the use of slip additives as an additional purging mechanism. In proposing that slip additives can contribute to a purging action, several slip additives where tested in polyethylene (PE), polypropylene (PP) and acrylonitrile butadiene styrene (ABS). The slip additives polypropylene wax and polyamide showed a significant increase in the MFI of PE whereas the additives polar wax and polyamide exhibited a slight increase in MFI of PP. Three additives, namely, polyamide, polypropylene wax and polar wax had showed noticeable improvement on the MFI of ABS. The slip additives that showed a significant improvement in polyethylene were tested together with six different commercial purge compounds. These compounds were tested for their colour change efficiency using polyethylene and switching from blue to clear in a laboratory film blower. It was found that the slip mechanism contributes very little to the purging action. Polymeric materials tend to adhere to hot metal surfaces. When the material continues to adhere to the metal after cooling down, cleaning of the processing equipment becomes very difficult. A purge manufactured by the CSIR, Pretoria, exhibited this problem. This study also covers an investigation into overcoming the adhesion problem of this purging compound.
