MATERIALS IN DESIGN & PROTOTYPING FEATURE U
sed in all manner of environments – from electronics, aerospace and
healthcare, to cookware and industrial applications – PTFE is produced by the free-radical polymerisation of tetrafluoroethylene. It is a thermoplastic polymer which is white at room temperature and has one of the lowest co-efficiencies of friction recorded in a solid material. Consisting wholly of high-bonded carbon and fluorine, this material has a high molecular weight and is almost completely non-reactive. Added to this, the material is
hydrophobic. This combination of features mean it has long been used as a tribological material to reduce energy consumption in friction-intensive machinery, as well as reactive and corrosive applications. However, the material does have
poor wear properties, low strength and modulus, inferior thermal expansion and conductivity ratings, and a tendency to creep, makes it less appealing for particularly harsh environments.
FILLED PTFE To overcome the drawbacks, manufacturers are now producing filled PTFE. By combining a carefully balanced mixture of alternative compounds and embedding it within the PTFE matrix, it is possible to reduce the performance limitations and create a polymer suitable for 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, carbon and graphite. So let us explore a number of key filler compounds currently on the market.
CARBON AND GRAPHITE FILLED PTFE
The addition of carbon and 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
HOW TO... SPECIFY filled PTFE materials
Polytetrafluoroethylene (PTFE) is an engineered polymer that is widely used in the manufacture of component parts. To meet more aggressive environments, however, manufacturers are embedding other compounds within the PTFE matrix. Phillip Charlton from Morgan Advanced Materials explains
and 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
“Understanding the variables at play
during the manufacture of filled PTFE materials will
high carbon and 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.
however, that the quality of a particular additive can alter the performance of the material, impacting its suitability 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 and graphite is a great example of this. A high carbon
undoubtedly benefit any specifier looking to identify or commission the design of component parts for highly hazardous environments”
BRONZE A bronzed filled PTFE cannot compare with its carbon-graphite counterpart for
wear resistance, but 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 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.
UNDERSTAND THE VARIABLES 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.
Morgan Advanced Materials
www.morganadvancedmaterials.com
DESIGN SOLUTIONS | NOVEMBER 2016 39
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