Material
 General guide tp PTFE Materials (Polytetrafluoroethylene)
This is the pure form of PTFE.
Standard sheets are cut from a moulded block.
Moulded sheets are bought to customer order.
PTFE has excellent resistance to most mineral and organic chemicals such as acids, bases and solvents.
Can be used to about 240 centigrade (464 farenheit).
Can release dangerous fumes if heated above 260 centigrade (500F.).
Tends to flow under pressure.
Relatively poor recovery under varying loads
Special grades are available with:
Additional lubrication for heavy duty bearing uses (eg bridge bearings).
Fillers such as glass, carbon fibre, bronze, graphite.
Expanded types are also available for applications needing a softer material.
Generally used in the presence of aggressive chemicals.
Can be supplied as a bore liner to shield pipe gaskets from the chemical attack.
These are supplied to fit the pipe type and bore.
They are known as PTFE Envelopes.
 Polytetrafluoroethylene (PTFE) is a polymer of tetrafluoroethylene. This non-elastic material is characterized by a series of outstanding properties:
» The surface is slippery and repels most media. Its use is favored in all cases where the retention of residues is to be avoided.
» PTFE is non-toxic at working temperatures up to +200 °C.
» The coefficient of friction is very low against most opposing surfaces made of other materials. Stiction and friction are almost the same.
» The electrical insulating properties are outstandingly good. They are almost independent of frequency, temperature and weathering effects.
» The chemical resistance exceeds that of all other thermoplastics and elastomers. Thus the resistance to swelling is good in almost all media.
» Liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures and pressures.
The temperature tolerance is between -200 °C and +260 C. Even at -200 °C, PTFE still has some elasticity and therefore it can be used for seals and constructional parts, for example, with liquefied gases.
When using parts of pure PTFE, the following points should be noted:
» Above a certain level of stress, the material continues to deform due to creep or cold flow.
» The abrasion resistance is low.
» Thermal expansion, like most plastics, is 10 times that of metals.
» The thermal conductivity is low so that removal of heat from bearings and from other moving seals can lead to problems.
» The material is not elastomeric but is hard like polyethylene.
For these reasons, elastomeric seals in assemblies cannot simply be replaced by PTFE seals. With lip seals, provision must be made for a constant additional contact pressure by use of springs or by other means. PTFE is filled with graphite, glass fibers, bronze and carbon to achieve special properties. |
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