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COLD FLOW AND JOINT CREEP Plastic lined piping systems are only as good as the flanged joints holding them together. The ability of the joint to stay tight is dependent on the mechanical properties of the plastic lining. Tensile strength is a very important factor in controlling the amount of cold flow or movement of the plastic faces. The higher the tensile, the more resistant the plastic is to movement, cold flow, or joint creep.
Plastic |
P V D F |
H al a r  ® |
T e fz el  ® |
F E P |
P T F E |
P F A |
MF A |
Tensile Strength at break in PSI |
4 5 0 0 t o 6 5 0 0 |
6 0 0 0 t o 6 7 0 0 |
6 5 0 0 t o 6 7 0 0 |
2 7 0 0 t o 3 1 0 0 |
2 5 0 0 t o 3 0 0 0 |
4 0 0 0 t o 4 5 0 0 |
35 00 to 44 00 |
As you can see from the above table, and Fig.1 and 2, PTFE has the least tensile strength of any plastic. PTFE is the poorest plastic for keeping joints tight. PTFE and PFA piping systems have essentially the same chemical resistance, PFA, however, provides superior cold flow and creep resistance which keeps the joints tight and maintains the integrity of the plastic lined piping system. Due to the poor mechanical properties of PTFE be concerned with any use over 350° F especially in cycling applications. PFA is rated to 500° F maximum use temperature. In applications 311° F and below you may be able to use one of the lower cost fluoropolymers such as TEFZEL®, HALAR®, or PVDF. They have superior resistance to joint creep and cold flow, resulting in a much more trouble free system. CF Fig.1&2 |
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