24th International Colloquium Tribology - January 2024 105 Tribological Behaviour of Polymer Compounds containing Microencapsulated Lubricants Susanne Beyer-Faiss 1* , Regina Wannenmacher 1 , Thomas Witt 1 , Moritz Grünewald 2 1 Dr. Tillwich GmbH Werner Stehr, Horb-Ahldorf, Germany 2 SKZ - KFE gGmbH, Freiburg, Germany * Corresponding author: susanne.beyer.faiss@tillwich-stehr.com 1. Introduction Worldwide, polymer materials are used in millions of components subjected to friction and wear processes. Especially in applications where additional external lubrication has to be avoided, slip modified polymers are used. For example, PTFE is widely used as an additive in polymers that significantly reduces friction and wear and minimizes stick slip effects. In view of the future restriction of fluorine-containing substances through the planned PFAS strategy of the EU, the development of alternative systems that do not contain fluorine, is very important. Such an alternative can be micro-encapsulated lubri-cants, which are incorporated in the polymer matrix, and which shall have a beneficial effect on the friction and wear behaviour even when running dry, but with-out weakening the mechanical properties too much. To assess the tribological properties, various model testing systems with different load collectives were used to explore the application limits of the newly developed polymer materials. The materials used in this study were produced as part of the project „Tribologically effective pseudo-solids for mechanically and thermally highly loaded thermo-plastic components“ (project ID 21707 BG of the Fördergemeinschaft für das Süddeutsches Kunststoff-Zentrum e.V. FSKZ). The project is funded by the Fe-deral Ministery for Economy and Climate Protection (BMWK) via the Consortium of Industrial Research Associations (AiF) in the frame of the programme to support industrial joint research (IGF). 2. Experimental 2.1 Materials Figure 1: Ester oil filled microcapsules Base polymer material is PP, which has been modified with 10% microcapsules. The microcapsules are built with a skin of melamine resin and filled with an ester oil. They have a spherical shape with a diameter around 20-µm. As reference material the neat PP as well as a modification of PP with 30% glass fibers and 15% PTFE has been tested. All tests have been performed under dry running conditions as well as equipped with an initial minimum quantity lubrication 2.2 Tests in model system sphere-on-plate under oscillating motion Friction and wear tests have been performed using the sphere-on-plate model system under oscillating motion using ½” spheres out of steel 1.3505 and the plates out of the polymer materials. Friction coefficients and linear wear were monitored at two load settings (25N and 50N) with a frequency of 4 Hz and an amplitude of 4-mm for 2 hours at ambient temperature (short term tests). It could be found, that the incorporation of 10 % microcapsules improves the wear rates between 50 to 70 %, whereas the friction values nearly stayed on the same level. The excellent wear rates of an optimized PP blend with GF/ PTFE cannot be reached. They are better by one order of magnitude. 2.3 Tests in model system sphere-on-prism under rotating motion Friction and wear tests have been performed using the sphereon-prism (ISO 7148-2) model system under unidirectional rotating motion using ½” spheres out of steel 1.3505 and the prism out of the polymer materials. Friction coefficients were monitored dependent on sliding speed (0-210 mm/ s) and load (1-3-6 N) at 25 °C (short term tests). Figure 2: Under dry sliding conditions the base material PP shows a typical speed dependent behaviour: static friction values of 0.20 rising up to 0.53 at sliding speed 200-mm/ s. Incorporation of microcapsules shows a tendency to higher friction values than the neat material. The reference 106 24th International Colloquium Tribology - January 2024 Tribological Behaviour of Polymer Compounds containing Microencapsulated Lubricants material with PTFE/ GF filler has lower friction values and a lower dependence on sliding speed. The difference of friction coefficients compared to an initial small volume lubrication with the microcapsules´ oil and the base polymer is remarkably lower around one magnitude. 2.4 Tests in model system plain bearing on shaft under rotating motion The injection moulded polymer plain bearings with an inner diameter of 5mm are checked in long-term tests under application-related conditions in start-stop operation. The tests (ISO 7148-2, alternating rotating motion) are performed in sliding combination with a steel 1.3505 shaft (diameter 5 mm) and a bearing load of 15N and 30 N under an intermittently rotating movement with 240 cycles, each with an increasing and decreasing profile of the rotation speed from zero to max. 100 rpm (continuously accelerated within 120s) and reverse (continuously reduced within 10s). In between the cycles a standstill period of 5s is established, symbolizing 240 start-stop conditions. In addition, the linear wear of the bearing is monitored. Under dry sliding conditions the base polymer PP shows high friction values, even under the low test load configuration with 15N bearing load. At 30N test load friction forces measured exceeded the measuring range of the friction sensor. Figure 3: Incorporation of 10% microcapsules improves the friction behaviour of PP remarkably: at 15 N and 30 N test loads the bearings showed a smooth running in behaviour and friction values stabilizing in between 0.2 and 0.3 dependent on sliding speed. Figure 4: The contacting zone with the steel axis in the test bearing after the tests looks like polished. There is no wear visible. Figure 5: Compared to the reference material PP with glass fibers and PTFE fillers the results are different: there is no pronounced running in of the bearing system visible and friction coefficients vary around 0.25 to 0.28. Figure 6: Within the contact zone with the steel axis, significant wear is produced on the PP/ GF/ PTFE bearing. In turning direction of the axis, abrasive wear is visible. The wear particles are disposed in the running-out zone outside the sliding contact. 3. Conclusion The incorporation of oil filled microcapsules within a polymer matrix does not show in all tested cases a positive influence on the friction and wear behaviour compared to the neat polymer. Dependent on the model system, the type of motion and the load-speed setting chosen, the response of the microcapsules has to be differenciated. In some cases, the friction and wear reduction is nearly as good as tested with the reference material containing glass fibers and PTFE fillers. At higher loads friction coefficients stronger increase with accelerating speeds compared to the neat material. The incorporation of oil filled microcapsules to a polymer cannot reach the effect of an external lubrication. Even an initial lubrication with only a minimum quantity of oil improves friction and wear reduction of the base polymer remarkably. In a next step, also a combination of microcapsules in combination with glass fibers shall be considered.