FRICTION AND WEAR | FORMULATION
Careful selection is the key to defeating wear
Successfully managing wear in plastics compounds requires careful selection of base polymers and lubricating additives. Peter Mapleston finds out more
Use of compounds with enhanced wear and friction properties requires sensitivity in the selection of appropriate polymers and suitable fillers. Additive and filler systems play a significant role in meeting demanding requirements on sliding friction properties and long service life of plastics in tribological applications; Additive suppliers and compounders alike are working hard to meet those challenges. At the same time, new solutions are being devel- oped to conform with evolving regulations covering PTFE, which is one of the best-known and longest established lubricating additives. At Mocom (previously the compounding division of Albis Plastic but since last year a standalone operation), Product Specialist Karolina Smrczkova says sensitivity in the choice of polymer and fillers is required when developing tribologi- cally modified compounds. She notes, for example, that in general semi-crystalline polymers have better tribological properties than amorphous ones. “The crystalline structure increases the surface hardness and thus improves sliding and wear behaviour,” she says. The operation temperature and required
stiffness and strength are the most important criteria when selecting suitable polymers for a specific sliding friction application, she advises. As far as fillers are concerned, attention needs to be paid not only to their effect on tribological proper- ties, but also on mechanicals (Figure 1). “For example, PTFE significantly decreases the elonga- tion and impact strength of a compound. Particles
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randomly spread throughout the polymer matrix act like tiny defects, which result in micro cracks, causing the embrittlement of the compound,” says Smrczkova. Silicones on the other hand soften the com- pound and significantly increase elongation and this softening effect can be used to reduce brittleness caused by PTFE addition, she says. “This enables Alcom POM 770/1 PTFE SI2 NC to be used as a sliding block in a gearbox. In this application, it has two sliding partners: a PBT/PET blend and steel. From the first gearshift, the silicone used on both sliding partners ensures a perfect sliding action. The silicone is transferred directly to the surface, creating a smooth sliding film without any inflow. PTFE re-applies the sliding film again after inflow with each gearshift, even in long operation,” Smrczkova explains. The different modes of action are shown in Figure 2. Smrczkova also points to the importance of
processing parameters when moulding parts. An injection mould that is too cold, for example, can significantly impair the tribological properties of a
February 2021 | COMPOUNDING WORLD 43
Main image: Gear wheels are a prime application for wear resistant, low friction plastics, reducing cost and frequently improving NVH performance over metal
IMAGE: SHUTTERSTOCK
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