MODIFICATION | WEAR RESISTANCE
its products at the Dusseldorf event. He explained that metal sulphides such as molybdenum disulphide (MoS2 phide (WS2
) and tin sulphide (SnS2
), tungsten disul- ) are good
lubricants because they have a layer lattice struc- ture, with layers of atoms bonded together by relatively-weak Van de Waals forces able to slide across each other easily. Schmied illustrated his point with test results on various ETPs, including PPA, PPS, PEEK, and PI (polyimide). These results show how critical it is to match the additive to the polymer, but also provide evidence that when the system is optimised, improvements in tribological properties can be considerable. In a compound based on PEEK
containing a new synthetic lubricant – SLS 22, based on tin disulphides and phosphates – it was possible to reduce average (over time) coefficient of friction (CoF) by as much as 40% and to also reduce wear by a similar amount.
Optimised selection Graphite and other solid lubricants for improving friction and wear were discussed by Klaus Rath- berger, Managing Director of Georg Luh, which specialises in graphite and mica additives. He compared the effects of MoS2
with boron nitride
and graphite and emphasised that selection of the optimum solution must be made according to the needs of the application (taking into account electrical and thermal conductivity, cost, colour, etc). “Graphite has high potential for modification according to specific application needs and combination of properties — lubrication + electrical and thermal conductivity,” he said. Researchers and students at Luleå University of
Technology, in Sweden, are investigating the effect of various forms of nano-carbons on the tribologi- cal properties of high-performance plastics. Professor Nazanin Emami reported on some of the results, which showed a clear correlation between friction and wear reduction (Figures 3 and 4).
Figure 1 and 2: Effect of synthetic lubricant SLS 22 on CoF in various samples of PEEK (pin-on-disc tests) Source: Tribotecc
Slippery problems Turning to the topic of lubricant combinations, Cayetano Espejo Conesa, a Teaching Fellow at the UK’s University of Leeds, explained that one plus one does not always equal two when it comes to the use of internal and external types. In plastics-metal systems, polymers can be inherently self-lubricating due to the formation of a thin polymer-based transfer layer on the metal counter-face that improves tribological properties. Polymer self-lubricant properties can also be enhanced through internal lubrication, using such additives as MoS2
, graphite,
silicones, nanophases, and also ionic liquids. However, there is a potential problem when
Figure 3: Averaged CoF and specific wear rates over a test period of 20 hours in water-lubricated conditions for a number of different polymers (Luleå University of Technology)
28 COMPOUNDING WORLD | February 2020
Source: Luleå University of Technology
www.compoundingworld.com
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