High performance polymers
 
Classification of high performance polymers (compounds).
Internal lubricants and reinforcements
Listing of "high performance" and "engineering" polymers
Physical properties
 Criteria for material selection and combinations.
Temperature effects
Closure
Classification of high performance polymers (compounds):
  • hardwearing.
  • high temperature resistant and high mechanical strength.

High Performance polymers have a thermal resistance >150°C. Examples of hard wearing high performance polymers are:

  • PEEK - Polyetheretherketon
  • PES - Polyethersulfon 
  • PI - Polyimide

Engineering polymers are classified by a temperature resistance within 100°C and 150°C. Examples of engineering polymers are

  • PA6.6 - Polyamide
  • PA6 G - Cast Polyamide (LFX with AF lubricant)
  • POM - Poly Oxy Methylene (acetal)
  • PETP - Poly Ethylene Terephtalate (PET)
  • UHMWPE - Ultra High Molecular Weight Polyethylene
  • PBTP - 

Standard polymers have a thermal resistance below <100°C and less suitable for sliding / rolling surfaces. Examples of these polymers are:

  • HDPE - High Density Poly Ethilene
  • ABS
  • PMMA
  • PVC

Most high performance polymers (compounds) are reinforced by
     fibres or/and filled with internal anti friction lubricants. 

Traditional reinforcements and internal AF lubricants are:

  • PTFE
  • Silicone oil
  • Graphite
  • MoS2
  • Aramide
  • Carbon fibre
  • Glass fibre
  • Alloy- / blend technology e.g. HWPE

Effects of reinforcements and internal AF lubricants are:

  • PTFE reduces the coefficient of friction. Is effective at high pressure. Creates a PTFE film between the compound and the counterface.
  • Silicone oil reduces coefficients of friction. Migrates to the wear surface. Offers lubricity at start up and at high speeds. Not effective at high surface pressures. 
  • PTFE with silicone oil improves tribological properties over a broad velocity range. Excellent for oscillatory motion (less "slip stick"). Considerable improvement in LPV at high speeds.
  • Molybdenum Sulphide (MoS2) enhance the crystallization of PA (surface hardening). Reduction of "slip stick" effect. Moderate improvement of wear factor.
  • Graphite Powder as boundary lubricant often used in aqueous moisture environment.
  • Aramide for improvement of the wear factor. Low counter face wear against soft metal, e.g. Cu, Al. Low plastic-on-plastic wear (identical partner). Low noise. Low abrasive wear. Dimensional stability.
  • Carbon fibres improve mechanical performance. Results in higher LPV-value and in reduction of wear factor. Reduce wear of both surface and mating surface as compared to glass fibres. Statically dissipate / conductive.
  • Glass fibres improve mechanical properties. Higher LPV-value. Reduction of wear factor. Increase wear of mating surface. 
The listing below is restricted to polymers that are "well" described by industrial suppliers. Both the physical properties as well as the composition is described. Unfortunately, tribology data is still limited.

Listing of "high performance" commercial polymers

  • PEEK
  • PEEK-BG (zwart), Carbon Fibre, PTFE and Graphite
  • PEEK-GF30, 30% Glass Fibre
  • PEEK-CA30, 30% Carbon Fibre (not listed)
  • PPS+, fibre reinforced and solid AF lubricants
  • PES
  • PI+15% Graphite

Listing of "engineering" commercial polymers

  • PA6.6, Polyamide (Nylon)
  • PA6.6 GF30, 30% Glass Fibre reinforced
  • POM C, Copolymer, Acetal
  • POM H, Homopolymer, Acetal, Delrin
  • POM H-TF, Homopolymer + Teflon
  • PETP
  • PETP TX

Criteria for material selection

  • wear factor
  • dynamic and static coefficients of friction
  • limiting PV value
  • counter face wear
  • material costs

Material combinations and consequences.

  • polymer - metal
  • polymer - polymer

Internal lubricated compounds versus metal

  • Up to 90% of frictional heating is conducted by the metal part.
  • fibre reinforced polymers wear the metal surface.

Plastics against plastics

  • Dissimilar polymers results in a reduction of the static coefficient of friction.
  • Use of PTFE lubrication dramatically reduces wear rates in both similar and dissimilar resins.
  • Against a fibre reinforced compound, the mating material should contain PTFE lubrication
  • Aramide compounds have excellent wear characteristics.
Impression of friction and wear for different combinations for comparison purposes only.
mat.1 mat.2 k1 k2 µstat. µdyn.
PA6 Steel 4 - 0.xx 0.28
POM Steel 1.3 - 0.14 0.21
POM+20%PTFE Steel 0.3 - 0.07 0.15
PA6 PA6 50 22 0.06 0.07
POM POM 280 200 0.19 0.15
POM+20%PTFE POM+20%PTFE 11 12 0.19 0.17
POM PA6 1.2 1.0 0.04 0.06
POM+20%PTFE PA6 0.4 0.7 0.05 0.06
POM+20%PTFE PA6+20%PTFE 0.5 0.24 0.03 0.04
* data from LNP, with permission. Thrust Washer measurements,
  mat.1: moving sample, mat.2 stationary counter face, k·10-15 m2/N.
* Data presented for impression, values strongly depends on pressure, velocity,
  temperature, roughness, macro geometry etc.
Temperature effects

Else than for metals where the mechanical strength and the tribological properties are constant  (<250°C) the properties of polymers change much with the temperature.
 

Coefficient of friction; PA6.6-steel, Ra=0.1µm, v=0.1 m/s, p=1.5 MPa.
Honselaar e.a., TNO, Constructeur 1989/8

The curve of PA6+15%PTFE is very smooth compared to the neat PA6.6.
Closure:

High performance polymers can have excellent tribological properties however, disappointment may occur if the fundamentals are not fully understood. For example, glass fibre reinforcements wear the steel counter surface. A combination of dissimilar polymers is necessary to have acceptable wear resistance and low static friction. A  bad heat conduction results in higher contact temperatures with degradation of the tribological aspects. For some combinations a very smooth counter surface is required. Running in conditions must be taken into account etc. 

Fibre reinforced polymer versus steel

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