Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
In the 50th anniversary year of the Jost Report which defined ‘tribology’ it is appropriate to consider polymer tribology. Great attention has been given to studying metal/metal and metal/ceramic contact tribology; less attention has been given to metal/polymer and polymer/polymer contacts. Polymer components are increasingly replacing metals in structures, housings, flexures and bearings, particularly in automotive weight reduction applications. They are readily formed, by injection moulding, casting or machining with minimal waste, to give components which are lighter, cheaper, corrosion-resistant, quieter and do not require maintenance.
Polymer tribology, as their friction and wear mechanisms, is more complex than for metal and less well understood. Whereas there are well-established ‘Laws of Friction’ for the tribology of metal and ceramic contacts in relative motion, polymer/metal contacts generally do not follow these Laws. The reasons for this are several, including the relative softness of polymers compared to metals, their much lower thermal conductivities associated with heat generation in contacts and also significantly lower melting points. If these issues are not appreciated then polymer applications of rolling, sliding or bearing components will be ‘problematic’.
The Variety of Polymers: A confusing issue is the number of polymers and their available formulations. Whereas over 300 individual polymers are characterized in the ‘Handbook of Chemistry and Physics’, 96th Edn., in that their physical properties and Tg
(glass transition temperatures) have been
determined, a useful working distinction between ‘Synthesised’ and ‘Commercial’ polymers is based on:
- are a polymer’s individual physical properties suitable for applications?,
- is this polymer uniquely useful against the properties of another?,
- the cost of raw materials and production, - the ease of initially producing the polymer and subsequent forming it into a product,
which reduces the number of commercially significant polymers to around 30.
32 LUBE MAGAZINE NO.135 OCTOBER 2016
The commodity polymers of PS (polystyrene), PVC (polyvinyl chloride), PP (polypropylene), and PE (the various forms of polyethylene) are relatively weak materials with Tensile Strengths, Ts
, of the order of 20MPa. They are high volume (Mt/pa), with limited temperature operating ranges up to 100o (<£1/kg), materials which are produced internationally.
C and cheap, Figure 1. The ‘Polymer Pyramid’
No.106 page 1 Polymer Tribology
Professor Malcolm Fox, Royal Academy of Engineering Visiting Professor, School of Mechanical Engineering, University of Bradford BD7 1DP UK,
M.Fox@
bradford.ac.uk R&D Manager, Nylacast Ltd, Leicester LE5 0HL,
malcolm.fox@
nylacast.com
In addition, individual polymers can be produced with a range of molecular weights or with distinctive spatial structures which affect their physical properties, particularly for polyethylene and polypropylene. Further, to improve performance polymers are offered with various additives such as glass or carbon fibre at various treat rates to enhance physical properties, mainly for physical strengths and modulii. Furthermore, polymers can be offered as either physical mixtures of different polymers or chemically combined as co-polymers. The range of available polymers is therefore immense, suitable for varied operating conditions of strength, operating temperature range, resistance to environmental degradation, friction coefficients and wear rates. Necessarily, this article is confined to the plain polymers!
The Polymer Pyramid: Polymers can be arranged on the basis of their physical properties, operating temperature ranges and cost, the ‘Polymer Pyramid’, Figure 1.
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