PROCESSING | PURGING COMPOUNDS
Right: Rapid- Purge says it has identified a mineral filler that provides ‘near ideal’ purging characteristics
application process. “Initial testing has been very successful and significant production obstacles have been overcome. We intend to launch this product by mid-2022,” he says. The company has also recently introduced a
product called RPM6000, which is a highly glass- filled, viscous purge that works with a surfactant and antioxidant package. The glass fibre content in the compound scours the screw and barrel surfaces while the surfactants help to soften residues to achieve complete removal.
Compound options While RapidPurge produces a wide variety of purging compounds it is its chemical line of purge compounds — first introduced 40 years ago — that remain the most popular with compounders and masterbatch producers. “The main challenge they [compounders] typically face is that it is hard to clean a machine when there is little pressure and agitation, like you would get in an injection moulding machine,” says Serell. “Our chemical cleaners actually depolymerise the resident resin, which makes removal easy. Pressure and agitation are not necessary, nor is a long soak cycle. These products also utilise foaming agents that deliver the chemical cleaning to low-flow areas, including the backs of screw flights.” RapidPurge also emphasises the importance of
Below: Asaclean’s Plus and EX purging compounds target change- over and contamination in compound production
appropriate filler selection for use in purging compounds. Serell says the most commonly used mineral in purging compound formulation is calcium carbonate. It is inexpensive and abundant but it is not ideal for removing resins and contami- nants. With a Mohs hardness of 3–4, calcium carbonate is simply not hard enough to remove the most burnt-on contaminants.
Another commonly used mechanical purge filler is glass fibre. With a Mohs hardness in the range of 5–7, glass fibres are excellent at removing resin and
contaminants. The shape of the fibres is also conducive to removing contaminants as the ends of the fibres ‘scrape’ the metal surfaces in the machine; Serell says it is probably the best filler for removing carbon. However glass fibres have a downside. There is a risk of agglomeration as the fibres can get tangled, causing blockages. And glass fibre filler can be sensitive to prolonged residence times — if left in the machine too long the fibres can migrate out of the polymer causing the screw to ‘freeze up’. The most common issue of concern with glass
fibres is probably machine wear. However, while processing glass-fibre filled resins is likely to result in measurable wear, it requires a significant quantity of material and time to occur. Glass-filled purge compounds are typically in a machine for just a few minutes and represent only a few barrel capacities. Serell says that even daily use would llikely not be enough to cause a screw or barrel to fail earlier than its normal lifetime. He says that in many years of testing purging compounds, including glass filled products, RapidPurge has not measured any accelerated wear.
Mineral alternatives Other minerals can be used, though. In addition to hardness, the shape of the mineral particles when ground is important — particles with a smooth or rounded surface will typically be less-effective cleaners than those that have angular edges. RapidPurge claims to have found what it believes is a near ideal mineral for use in purging. The undisclosed mineral has a high Mohs hardness and a naturally angular particle shape when ground. It is not otherwise used in the plastics industry, but is said to be ‘perfect’ for this application — the sharp edges of the hard particles are said to cleave off
50 COMPOUNDING WORLD | September 2021
www.compoundingworld.com
IMAGE: ASACLEAN
IMAGE: SHUTTERSTOCK
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