The melting point of 'as polymerised' PTFE increases with increasing molecular weight, and PTFE also superheats, ie. the apparent melting point increases with increasing heating rate.
The influence of molecular weight on melting point is much reduced after the polymer has been sintered (once melted). Most sintered polymers melt in the range 325 to 330°C(617-626°F) when reheated at 16 deg C/minute (28.8 deg F/minute).
The way in which the melting point of sintered PTFE varies with applied pressure are:
1 atmosphere 324°C(615°F)
69 atmospheres 335°C(635°F)
207 atmospheres 356°C(673°F)
615 atmospheres 419°C(786°F)
The graphical representation of the Specific volume/temperature relationship for highly crystalline fabricated PTFE clearly reveals the presence of the transition point which occurs at 19°C(66°F) with PTFE
Therefore a more useful way of indicating thermal expansion is to express it as a percentage increase in length between two temperatures. Values obtained parallel to and at right angles to the direction of the moulding pressure are quoted separately as they were found to be slightly different.
Temperature range |
Parallel to the direction of moulding pressure |
Perpendicular to the direction of moulding pressure % |
°C |
°F |
% |
% |
15 to 30 |
59 to 86 |
0.4 |
0.4 |
30 to 50 |
86 to 122 |
0.3 |
0.3 |
30 to 100 |
86 to 212 |
0.8 |
0.8 |
30 to 150 |
86 to 302 |
1.5 |
1.5 |
30 to 200 |
86 to 392 |
2.4 |
2.3 |
30 to 250 |
86 to 482 |
3.4 |
3.6 |
Over the temperature range 20-35°C(68-95°F) it has found the thermal conductivity of PTFE to be 6 x 10-4 cal/cm s deg C. This result may be expressed in a variety of units:
6 x 10-4 cal/cm s deg C
2.2 x 10-3 kcal/m h deg C
2.6 x10-3 joule/cm s deg C
1.7 Btu in/ft2 h deg F
Thermal conductivity of PTFE is fairly constant (measured at 0,20,50 and 70°C(32,68, 122, 158°F), with a slight tendency to rise at the higher temperatures
- Specific Heat And Heat of Fusion
The specific heat of PTFE has been determined by Marx and Dole. For temperatures above 40°C(104°F) they give the relationship:
Cp= 0.227 + (2.50 x 10-4) T cal/g deg C.
Within its normal range of working temperatures, the upper limit of which is generally quoted as 260°C(500°F), PTFE suffers no degradation. Indeed weight losses observed between 260 and 360°C(500 and 680°F) will be exceedingly small and due to the loss of minute amounts of moisture or gas absorbed in the polymer.
At processing temperatures of about 380°C(716°F) the rate of decomposition of PTFE is still very low and it is only at temperatures in excess of 400°C(752°F) that thermal decomposition of pure PTFE becomes significant.
Pyrolysis of PTFE in a vacuum at temperatures from 423.5 to 513°C(794 to 955°F) shows the decomposition rates of 0.00152% per minute and 1.264% per minute respectively at these temperature.
The thermal degradation of PTFE was relatively little affected by oxidising conditions.
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