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Rubber materials and various plastics are
used as sealing materials. For a detailed overview we
recommend the “Handbuch Dichtungspraxis” (“Handbook of
sealing practice”) by Wolfgang Tietze, Vulkan Publishing
House Essen.
Here we will restrict ourselves to a few
highlights amongst the rubber materials (elastomers) as well
as a few selected plastics, in particular PTFE.
Elastomers are used to make O rings,
standard rotary shaft seals, wipers and many other sealing
elements. They all share permanent elastic behaviour. They
vary in their chemical, thermal and mechanical durability.
Here is a short introduction to the various types.
Download (pdf-files):
[
Elastomers in seal technology ]
[
Chemische Beständigkeit von Elastomeren ]
[
Chemische Beständigkeit von Kunststoffen in der
Dichtungstechnik ]
Selected plastics
PTFE
Partly crystalline polytetrafluorethylene is a polycarbon
almost entirely enveloped by fluorine atoms. These
carbon-fluorine bonds are one of the strongest bonds known
in organic chemistry. This is why PTFE has such a high
chemical and thermal durability.
PTFE compounds
The addition of fillers, for example the production of PTFE
compounds, leads to an improvement in resistance to abrasion
and heat conduction i.e. a reduction of cold conductance and
thermal expansion. In addition to this, the electrical
conductance and the abrasion behaviour of the counterpart
can be influenced.
Chemical durability
Neither solvents nor acids alter the properties of PTFE.
Use with cooling agents leads to a reversible gain in weight
from 4-10%.
The only chemical reaction occurs with molten or dissolved
alkali metals.
At higher temperatures and pressures PTFE reacts with
elemental fluorine and chlortrifluoride.
Monomers such as styrene, butadine or acrylnitride can
penetrate PTFE in small quantities, which may lead to
swelling.
Durability to light and weather influences
PTFE shows an excellent durability to light and weather
influences.
Energy-rich radiation
PTFE is not one of the materials able to withstand radiation.
Flammability
Fluoropolymers are the most flame resistant of the plastics.
In an independent flame the gaseous products of
decomposition ignite at 500-560°C.
Water uptake
Even after a long time in water, no water uptake (according
to DIN 53472/8.2) can be determined.
Physiological properties
PTFE is physiologically inert. Durability to super-heated
steam allows for good sterilisation in the pharmaceutical
and food industries.
Coefficients of friction
PTFE has the lowest coefficient of friction of all solid
materials. Furthermore the static and dynamic coefficients
of friction are almost identical, which rules out a
“stick-slip“ effect.
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