Thermal Properties of filled PTFE such as Thermal Expansion, Thermal Conductivity are dependent on the Thermal properties of fillers.
The thermal expansion of most fillers is less than that of PTFE and since the expansion of the composite will be somewhere between that of the two constituents, filled PTFE compounds expand less than unfilled PTFE. During fabrication certain fillers, notably fibres and platelets, become preferentially oriented (as do the particles of PTFE to a much lesser degree). The result is that some compounds have a markedly different expansion in the directions parallel and perpendicular to the direction of moulding. As thermal expansion is virtually linear below 15°C (59°F) a coefficient is quoted; but since there is a transition point at about 19°C (66°F) and the expansion is far from linear above 30°C (86°F) changes in dimension are quoted as a percentage for a given temperature range. Figures for some common metals are also quoted in Table 9 (Linear thermal expansion: PTFE and Filled PTFE range). It is very important to note that these figures are actual linear thermal expansions with virtually all stresses removed from the material. Changes in the dimensions of a specified part due to changes in temperature will differ from these figures if stresses are present.
The thermal conductivity of PTFE is very low, making it a good thermal insulating material. Many fillers, notably metals and metal oxides, have high thermal conductivity, but in general they are effectively encapsulated by PTFE so that the conductivity of the compounds is still relatively low. Typical figures for the PTFE and Filled PTFE range are given in Table 10, together with those of some common metals and insulating materials.
Table 10. Thermal conductivities| Material | Thermal conductivity | |
|---|---|---|
| 10-4 cal / cm s deg C |
Btu in / ft2 h deg F |
|
| Unfilled PTFE | 6 | 1.7 |
| 15% Glass Filled PTFE | 8 | 2.3 |
| 25% Glass Filled PTFE | 9 | 2.6 |
| 15% Graphite Filled PTFE | 11 | 3.2 |
| 25% Coke Filled PTFE | 13 | 3.8 |
| 60% Bronze Filled PTFE | 19 | 5.5 |
| 33% Carbon Filled PTFE | 27 | 7.8 |
| 63% Bronze+Graphite Filled PTFE | 30 | 8.7 |
| Aluminium | 4950 | 1400 |
| Brass | 2300 | 650 |
| Iron (cast) | 1520 | 430 |
| Steel (med. carbon) | 1100 | 310 |
| Glass | 18.4 | 5.2 |
| Still air | 0.64 | 0.18 |
| Granulated cork | 1.10 | 0.31 |
| Kapok | 0.85 | 0.24 |
| Units | 15% Glass Filled PTFE | 25% Glass Filled PTFE | 25% Carbon Filled PTFE | 60% Bronze Filled PTFE | 15% Graphite Filled PTFE | 63% Bronze+Graphite Filled PTFE | Unfilled PTFE | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Para | Perp | Para | Perp | Para | Perp | Para | Perp | Para | Perp | Para | Perp | Para | Perp | |
| Coefficient of expansion from -60 to +15°C x 10-5 (-76 to +59°F) per °C |
9.2 | 6.6 | 9.2 | 4.4 | 6.9 | 6.4 | 6.6 | 6.4 | 8.6 | 6.4 | 5.4 | 5.2 | 9.9 | 9.6 |
| Change from: 15-30°C (5°-86°F) % |
0.4 | 0.3 | 0.4 | 0.3 | 0.4 | 0.2 | 0.4 | 0.2 | 0.3 | 0.2 | 0.3 | 0.1 | 0.4 | 0.4 |
| 30-50°C (86-122°F) % | 0.3 | 0.2 | 0.3 | 0.1 | 0.2 | 0.2 | 0.2 | 0.1 | 0.2 | 0.2 | 0.2 | 0.2 | 0.3 | 0.3 |
| 30-100°C (86-212°F) % | 0.9 | 0.6 | 0.8 | 0.4 | 0.6 | 0.5 | 0.6 | 0.5 | 0.7 | 0.6 | 0.5 | 0.5 | 0.8 | 0.8 |
| 30-150°C (86-302°F) % | 1.7 | 1.0 | 1.5 | 0.7 | 1.2 | 1.0 | 1.1 | 0.9 | 1.4 | 1.0 | 0.9 | 0.8 | 1.5 | 1.5 |
| 30-200°C (86-392°F) % | 2.4 | 1.5 | 2.2 | 1.0 | 1.9 | 1.5 | 1.8 | 1.5 | 2.1 | 1.6 | 1.4 | 1.3 | 2.4 | 2.3 |
| 30-250°C (86-482°F) % | 3.5 | 2.2 | 3.2 | 1.4 | 2.7 | 2.4 | 2.5 | 2.2 | 3.2 | 2.3 | 2.1 | 2.0 | 3.4 | 3.6 |
Notes:
1. All measurements made on samples taken from discs 7.30 cm dia x 1.27 cm thick (2 7 / 8 inch dia x 1 / 2 inch), preformed at 700 kgf / cm2 10 000 Ibf/in2 and sintered at 380°C (716°F).
2. Coefficients of expansion (x 10-5 per °C) of some other materials are:
Aluminium 2.4
Brass 1.9
Glass 0.9
Iron (cast) 1.1
Steel (med, carbon) 1.2