Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE PTFE
UHMWPE antistatic oil-filled
Nylon 6 Cast Nylon 6 Cast, oil-filled Acetal, homo-polymer
PEEK, unfilled PEEK 30% GF
Polysulphone 0.10
0.12 0.12 0.13
0.26 0.14
0.20
0.18 0.31
0.37 PVC
HDPE LDPE
Polypropylene homo-pol Nylon 66
Nylon 66 ‘Super Tough’ Nylon 66, Extrd’d.
Acetal, co-polymer PEEK 30%GR PEEK, 30% carbon-filled PPO/PS 0.30
0.28 0.60 0.26
0.25 0.28 0.26
0.18
0.30 0.22
0.39 Table 1. Dynamic Friction Coefficients for Some Common Groups of Polymers
of 0.14, greater than µ for PTFE of 0.10 but less than µ for PEEK at 0.18, where in this case:
/EPTFE
The effects of polymer structure and fillers on friction coefficients are separated out in Table 2 which shows that solid fillers usually increase the coefficient of friction. However, wear rates depend upon the particle size and hardness and thus the overall nature of the filler. Both friction coefficients and wear rates for a filled polymer need to be determined under real operating conditions of load and velocity because in some cases they can be opposite, e.g., for PEEK: - µ for unfilled PEEK, µ=0.18, - but PEEK + 30% carbon fibre, µ=0.22 but with a decreased wear rate,
- whereas PEEK + 30% glass fibre, µ=0.30, but an increased wear rate.
Effect of Polymer Structure Polyofefins - UHMWPE
“, antistatic “, oil-filled PP, homo-pol HDPE LDPE
POM/Acetal - Acetal, co-polymer
Acetal, homo-polymer µ
0.12 0.12 0.13 0.26 0.28 0.60
0.18 0.20
Effect of Halogen Elements – Flouro- & Chloro-Polymers PTFE PVDF PVC
0.10 0.24 0.30
Effect of Filler - PEEK, unfilled,
PEEK, 30% carbonfilled PEEK, 30% GR
Effects of Structure and Filler - Nylon 6-Cast, wax-filled, low friction,
Nylon 6-Cast, antistatic, low friction
Nylon, wax-filled,
Nylon 6-Cast, oil-filled, Nylon 6-Cast, unfilled, Nylon 66, 30% GF
0.18 0.22 0.30
0.04
0.05 0.08 0.14 0.26 0.31
No.106 page 3
Reducing Polymer Friction Coefficients: If friction coefficients can be varied upwards, then they should also be capable of being reduced. The main techniques of reducing polymer friction are: - inclusion of solid (soft) fillers, - external lubrication, - intrinsic (internal) lubrication
The inclusion of solid soft fillers into a harder polymer reduces friction coefficients by using the harder polymer as a matrix and the softer filler/polymer as a lubricant, forming a transfer film at the interface between the filled polymer and a steel surface. PEEK/PTFE sintered composites are a good example, the ratio of Young’s Moduli, EPEEK
, being 8:1, giving a friction coefficient - PTFE is the soft lubricant in a harder PEEK support matrix,
- the sintered composites supports higher applied loads than is possible than for PTFE alone,
- there is clear SEM/EDX evidence of a PTFE transfer film onto the steel counterface. But the transfer film tends to be oriented in the direction of relative movement with higher friction coefficients normal to the direction of original movement.
External lubrication of polymer components reduces friction between the polymer and metal surfaces. As most polymers and their physical properties are unaffected by hydrocarbon and synthetic lubricants over a wide range of high temperatures, externally lubricated polymer components are used in many demanding applications. Figure 3 shows a chain element for a thin packaging film stretching machine, for which there are two chains running in parallel, each 1km long, through a 165o
C
furnace at 8m/s. The black plastic slides in each chain element are injection moulded PEK 30, PEK is the simplest PAEK polymer family member, with 30% carbon rod content. The complete chains run on their side in separate baths of synthetic ester lubricant to achieve a friction coefficient of 0.09.
*GF is glass filled Table 2. Effects of Polymer Structure and Fillers on Friction Coefficients 34 LUBE MAGAZINE NO.135 OCTOBER 2016 Figure 3. PEK 30 in Chain Element of Film Stretching Machine
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