Since its accidental invention in 1938, polytetrafluoroethylene (PTFE) has been successfully integrated into a plethora of environments from electronics, aerospace and medicine to cookware and industrial applications. Produced by the free-radical polymerisation of tetrafluoroethylene, this thermoplastic polymer is white in room temperature and widely regarded as having one of the lowest co-efficiencies of friction ever recorded in a solid material—currently ranking third in the world with a co-efficiency rating of 0.05 – 0.10. Consisting wholly of high-bonded carbon and fluorine, this material has a high molecular weight and is almost completely non-reactive.
Couple this low co-efficiency of friction and its inherent non-reactive nature with the fact that PTFE is also completely hydrophobic and it is no surprise that it has long been used as a tribological material to reduce energy consumption in friction-intensive machinery environments as well as reactive and corrosive applications.
PTFE performs significantly better than rival products such as engineering plastics, nylon and acetal and, in many ways, is comparable in performance to ultra-high-molecular weight polythyne (UWHMPE) for the manufacture of application-specific component parts. On the flip side, though, its poor wear properties, inferior thermal expansion and conductivity ratings and tendency to creep makes it less appealing for particularly harsh environments.
In order to meet an increasing requirement for component parts that offer all the key benefits of PTFE, without the associated drawbacks, leading manufacturers of engineered polymers are producing filled PTFE. By combining a carefully balanced mixture of alternative compounds and embedding it within the PTFE matrix, it is possible to reduce PTFE’s performance limitations to create a polymer, which behaves incredibly well in very specific and highly aggressive environments. Some common compound additives used in the development of filled PTFE include glass fibre, bronze, copper, molybdenum disulphide, zinc oxide and carbon graphite (also used as two separate compounds).
Filled PTFE grade materials often have complex molecular formulae, having been fine-tuned for very specific applications, which means specifiers face the unenviable task of identifying which compound mixture best suits their project. While there is no replacement for a well-established relationship with a reputable manufacturer in which materials information can be exchanged on a project-by-project basis, upholding a general working knowledge of the common filled PTFE compound additives and their associated performance benefits is sure to aid the specification process and often the client/specifier relationship, too. Let us explore a number of key filler compounds currently on the market, which satisfy the needs of a number of specialist, often highly aggressive, applications.
Carbon graphite filled PTFE
The addition of carbon graphite to PTFE can increase the material’s wear resistance and thermal expansion properties, making it between two and eight times more effective against thermal expansion, and up to 1,000 times more resilient to wear damage in applications such as air compressors up to a discharge pressure of 20 bar.
It is important to remember, however, that the quality of a particular additive can alter the performance of the material, making it more or less suitable for certain applications. This can make specification even more challenging because there are multiple variables at play when selecting the right product.
The use of carbon graphite is a great example of this. A high carbon graphite filled PTFE offers low coefficient of thermal expansion, making it ideal for the manufacture of water turbine bearings and labyrinth seals. Slightly less carbon graphite (a medium to high filler) won’t offer the same thermal expansion properties, but will yield optimum wear rates, making it more beneficial to air compressor applications.
Additionally, a premium medium to high carbon graphite filled PTFE, which benefits from lower porosity, can be ideal for light gases in lubricated high pressure duties up to 100 bar. Finally, a standard quality medium graphite filled PTFE is ideal for any application requiring flexibility.
A bronzed filled PTFE cannot compare with its carbon graphite counterpart for wear resistance, but nevertheless, still performs well in this area and is more suitable for air compressors where gas exceeds 20 bar. This is particularly the case in air compressors with piston temperatures due to the preferential thermal conductivity of bronze as a compound. Compared to traditional PTFE, a bronze filled alternative can deliver thermal conductivity ratings, which are up to ten times greater.
The addition of a special filler to enhance a medium bronze filled PTFE can improve its wear resistance enough for it to operate effectively in an application, which combines high air pressure and high air temperature.
Glass fibre is used alongside a number of other compounds to produce filled PTFE grades suitable for chemically aggressive environments and those applications requiring a low co-efficiency of thermal expansion.
A medium glass fibre and copper filled PTFE, for example, provides low thermal expansion, while the addition of glass fibre on its own can create a PTFE material, which is almost chemically inert and suitable for oxygen-focused applications.
Understanding the variables at play during the manufacture of filled PTFE materials will undoubtedly benefit any specifier looking to identify or commission the design of component parts for highly hazardous environments. Above all, the most important thing is to maximise relationships with manufacturers, who take the time to understand each of your projects to ensure that the components specified meet the exacting criteria of a complex project brief.
The article is reproduced with courtesy to Morgan Advanced Materials