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Friction and wear tests showed a reduction in wear of up to 85% on plastic-steel pairings, according to researchers.

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An astonishing 23% of global energy consumption can be attributed to friction losses. Components with reduced friction, therefore, represent an important contribution to conserving resources and achieving climate protection targets.

In the case of plastics, reduced friction can also reduce the presence of microplastics in the environment. With the development of microcapsules filled with liquid lubricants for plastics, the Potsdam Fraunhofer Institute for Applied Polymer Research IAP and the Plastics Center SKZ in Würzburg are supporting these goals. Their self-lubricating plastics achieve up to 85% less wear.

Whether it's sliding doors, plastic gears, or other moving components, a variety of applications subject materials to friction. Plastics manufacturers sometimes incorporate solid lubricants into plastics to reduce component wear. However, a relatively small number of solid lubricants are suitable for plastics processing. By contrast, there is a much wider range of liquid lubricants, some of which are more effective. In a cooperative effort, the two research institutes have succeeded in encapsulating liquid lubricants in such a way that they can be incorporated into polymers as functional substances and later develop all the advantages of a liquid lubricant in the component.

High-temperature mixing without damaging capsules was a challenge

"We managed to incorporate Fraunhofer IAP‘s microcapsules filled with liquid lubricant into thermoplastics using a twin-screw extruder,” explains Moritz Grünewald, researcher in the Materials Development Group at the SKZ Plastics Center. “The challenging task was to mix the microcapsules with thermoplastics under high temperatures without damaging the capsules. Only when friction occurs in the final component should the capsules break and release the lubricant. This allows the component to lubricate itself automatically. Our friction and wear tests showed a reduction in wear of up to 85% on plastic-steel pairings. Thus, components last significantly longer and generate less microplastic," said Grünewald.

Based on these results, the material system is being optimized further for potential applications. The development is now focusing on improved mechanical and thermal properties of the self-lubricating plastics.

Testing reinforcing materials

The additional use of reinforcing materials such as fibers is intended to make the self-lubricating plastics more mechanically stable. In the project, the researchers investigated which type of fiber is best suited for this purpose and how the microcapsules can be optimally bonded to the plastic matrix. In addition, the capsules may also be incorporated into higher-melt plastics in order to further widen the technical application possibilities. To this end, even more stable capsule wall materials are being developed in close cooperation with project partners.

"The main challenge is that the capsules should not break until they come into frictional contact in the final application. However, if we introduce additional fibers and increase the temperatures, the stress on the capsules increases during the mixing process," explains Dr. Alexandra Latnikova, micro-encapsulation specialist at Fraunhofer IAP. Her team develops the microcapsule systems.

Numerous inquiries from industry highlight the need for novel plastics with optimized friction and wear properties. Micro-encapsulation technology has major advantages for companies in this regard — the wide range of liquid and advanced lubricants can now be used as internal lubricants with on-demand release properties. The project is accompanied by a committee that includes companies from all sectors of the plastics industry, lubricant manufacturers, and micro-encapsulators. The project is open to further partners.

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