Thermal conductivity of plastics
Which materials can conduct heat, how additives improve conductivity and in which industry these plastics are used.
Thermal conductivity describes a material's ability to transfer thermal energy by conduction. It is stated in watts per metre and kelvin (W/mK) and represents a key material property in the technical environment. The higher the value, the faster heat spreads within a component.
In design, mechanical engineering, electrical engineering and vehicle construction, thermal conductivity directly influences component safety, dimensional accuracy and service life. An incorrect assessment can lead to overheating, distortion, stress cracks or loss of function.
Heat conduction simply explained
Thermal conductivity describes exclusively heat transfer through direct material contact within a material. It is one of the three fundamental forms of heat transport alongside convection (heat transfer through moving liquids or gases) and thermal radiation (energy transfer through electromagnetic waves).
Heat conduction takes place in the solid material without the material moving as a whole. The higher the lambda value, the faster the temperature spreads within the component.
What does thermal conductivity mean in concrete terms?
Heat is passed on within a solid body by molecular or electron movement. Metals possess freely moving electrons that transport energy very efficiently, which is why they achieve high conductivity values. Plastics transfer heat predominantly via lattice vibrations in the molecular structure and are therefore significantly lower.
The physical basis is Fourier's law of heat conduction. It describes the heat flow as a function of temperature difference, material thickness and thermal conductivity. In practice this means: not only the material property is relevant, but also wall thickness, temperature gradient and contact area.
Two fundamental objectives emerge for design:
- High values are suitable for heat sinks, heat distributors or temperature-critical components.
- Low values are suitable for insulation, thermal decoupling or protective functions.
Practical example with calculation (Fourier's law)
Fourier's law describes the heat flow Q through a component:
Q = λ · A · (ΔT / d)
Here λ stands for thermal conductivity, A for the area, ΔT for the temperature difference and d for the material thickness.
Example: A 10 mm thick POM sheet (λ ≈ 0.30 W/mK) with 0.01 m² area separates two areas with a 40 K temperature difference.
Q = 0.30 · 0.01 · (40 / 0.01) = 12 watts
Thus 12 watts of heat output flow through the sheet. An aluminium component of the same geometry would transfer around 8000 watts at λ ≈ 200 W/mK. This example illustrates the enormous difference between metal and plastic.
Comparison table: thermal conductivity of typical materials
The following values represent typical guide values at room temperature. Deviations are possible depending on alloy, processing or filler content.
| Material | Thermal conductivity (W/mK) | Typical application |
|---|---|---|
| Copper | approx. 380–400 | Heat transfer, busbars |
| Aluminium | approx. 200–235 | Heat sinks, housings |
| Steel | approx. 45–60 | Construction |
| POM | approx. 0.30 | Sliding and precision parts |
| PEEK | approx. 0.25 | High-temperature components |
| PA6 | approx. 0.25–0.30 | Machine components |
| PTFE | approx. 0.25 | Chemical and sealing technology |
| PVC | approx. 0.16 | Construction and insulation applications |
| GF-reinforced plastics | approx. 0.30–0.40 | Mechanically loaded components |
There is often a difference of several orders of magnitude between metal and plastic. This factor significantly determines the thermal behaviour of a design.
Temperature dependence of thermal conductivity
Thermal conductivity changes as temperature rises. While metals usually show only moderate changes, plastics react more sensitively to temperature increase.
| Material | λ at 20 °C (W/mK) | λ at 80 °C (W/mK) |
|---|---|---|
| POM | approx. 0.30 | approx. 0.27 |
| PA6 | approx. 0.28 | approx. 0.24 |
| PEEK | approx. 0.25 | approx. 0.23 |
| Aluminium | approx. 220 | approx. 205 |
These temperature-dependent changes play a role particularly under continuous load or increased ambient temperature. This factor significantly determines the thermal behaviour of a design.
Plastic thermal conductivity in detail
Plastic thermal conductivity depends strongly on the base polymer, the filler content and the internal structure. Unreinforced standard plastics typically range between 0.15 and 0.35 W/mK.
The crystallinity of the polymer, the molecular orientation through the processing process as well as any fibre reinforcements or particulate fillers have a decisive effect. High crystallinity generally leads to somewhat improved heat transfer, while amorphous structures exhibit lower conductivity values. The alignment of the molecular chains, for example through extrusion or injection moulding, also influences heat conduction in the flow direction.
Additives such as glass fibres, ceramic particles, graphite or metal powder can specifically increase thermal conductivity. The filler content, the particle size and the distribution in the material determine the achievable property value. Special thermally conductive compounds achieve values between 1 and 10 W/mK and are used in LED carriers, power electronics or thermally loaded housings.
Despite these optimisations, plastics generally remain significantly below metallic conductivity values. Their advantage lies in controlled heat management, electrical insulation, low weight as well as high corrosion and media resistance.
In technical practice, thermal conductivity is often also referred to as heat conductivity value or lambda value. The heat conductivity value describes the same physical relationship and is usually stated in data sheets with the symbol λ. The heat conduction of plastics takes place significantly more slowly than in metals, which makes them particularly suitable for insulating or thermally decoupled designs.
Factors influencing thermal conductivity
- Material structure: crystallinity, molecular orientation and fibre content directly influence heat transfer within the material.
- Temperature: as service temperature rises, material properties change and with them thermal conductivity.
- Moisture: hygroscopic plastics such as PA change their physical properties through water absorption and thus the heat conductivity value.
- Fillers and reinforcements: graphite, ceramic, metal particles or glass fibres specifically increase thermal conductivity.
- Density and porosity: air inclusions or microscopic cavities reduce effective heat conduction.
- Component geometry: wall thickness, contact areas and installation situation determine the real heat flow more than the pure table value.
In practice, the actual heat transport always results from the interaction of these factors.
Thermal contact resistance
Alongside the material property, the thermal contact resistance between two components plays a central role. Even materials with high thermal conductivity can show significantly reduced heat transfer due to poor contact surfaces, air gaps or low contact pressure. The contact resistance often influences the overall system more than the pure lambda value of the material.
Clean surface finishing, defined contact pressure or thermally conductive intermediate materials significantly improve real heat transfer.
Practical applications
- Thermal insulation: housings, spacers or protective cladding benefit from low thermal conductivity, as they reduce heat transfer between assemblies.
- Heat management in electronics: LED carriers, power electronics or sensor housings use thermally conductive plastics to dissipate waste heat in a controlled manner.
- Mechanical engineering: guides, bearing housings or covers with defined thermal conductivity stabilise temperature behaviour and support dimensional accuracy.
- Vehicle construction: lightweight plastic components combine thermal decoupling with weight saving and design flexibility.
- Medical technology: plastic components provide thermal insulation of sensitive components and at the same time increase chemical resistance.
- Chemical and process engineering: materials with defined thermal conductivity enable controlled temperature management in aggressive environments.
Design notes on material selection
When selecting a suitable material, a systematic consideration of the entire thermal system is advisable. The temperature range in continuous operation as well as possible short-term peak loads form the basis of every decision. Internal or external heat sources in the component, such as motors, electronic components or friction zones, are equally relevant.
Contact surfaces to metals significantly influence the real heat flow, as different thermal conductivities have a direct effect on temperature distribution. In addition, mechanical requirements such as stiffness, wear behaviour and dimensional accuracy play a role, as thermal expansion and temperature gradients can generate design stresses.
Alongside the technical requirements, economic framework conditions must be taken into account. Material availability, machinability and service life feed into the overall assessment. An isolated consideration of the table value rarely leads to the optimum result. Only the combination of thermal, mechanical and chemical load defines the suitable material.
Thermal diffusivity and distinction
Alongside thermal conductivity, thermal diffusivity plays an important role. While thermal conductivity describes how well a material transports heat, diffusivity indicates how quickly a temperature change spreads within the material. It additionally takes into account density and specific heat capacity.
Materials with high diffusivity react quickly to temperature changes, while materials with low diffusivity heat up or cool down more slowly. For dynamic applications, for example in electronics or with changing load conditions, this parameter is of particular importance.
Measurement methods and standards
Thermal conductivity is determined using standardised test methods. In plastics technology, the laser flash method and the guarded hot plate method are particularly widespread. While the laser flash method measures rapid temperature changes and calculates thermal conductivity from them, the guarded hot plate method works with steady-state temperature fields.
For plastics, DIN EN ISO 22007, among others, forms the normative basis for determining thermal conductivity and thermal diffusivity. The measurement conditions such as specimen thickness, temperature range and moisture state significantly influence the results and must be taken into account when interpreting them.
Material reference from the range
In technical use, typical engineering plastics such as POM or PA6 show conductivity values around 0.25 to 0.30 W/mK and are suitable for mechanically loaded components with moderate heat input. High-performance plastics such as PEEK combine similar heat conductivity values with significantly higher temperature resistance. PTFE additionally offers chemical resistance at comparable thermal conductivity.
By comparison, aluminium achieves around 200 W/mK and often serves as a reference material for heat dissipation. This juxtaposition illustrates why plastics are primarily used for insulation or controlled heat management.
A simplified representation illustrates the difference:
Metal (aluminium): 200 W/mK ──────────────────────────────── Steel: 50 W/mK ───────────── Plastics: 0.2–0.3 W/mK ─
The graphical classification shows the considerable gap between metallic and polymeric materials.
Typical planning errors when assessing thermal conductivity
In practice, incorrect assumptions regularly occur that lead to wrong material decisions. The table value is often considered in isolation, without taking into account geometry, contact conditions or temperature range. Likewise, the thermal contact resistance between components is underestimated.
A further error consists in equating thermal conductivity with thermal insulation capability without analysing the actual temperature gradient. Temperature-dependent changes in the lambda value also often remain unconsidered in early planning phases.
A systematic consideration of the entire heat path prevents these planning errors and increases operational safety.
Material advice and material selection
The selection of the suitable material depends on more than the pure lambda value. Temperature range, mechanical load, chemical environment and economic requirements determine the optimum solution. Engineering plastics such as POM, PA6 or PEEK offer different advantages in heat management depending on the application.
A well-founded material selection always takes into account the real installation situation and the entire system environment. On the basis of concrete requirements, it is possible to decide specifically whether insulation, heat dissipation or controlled heat conduction is the priority.
FAQ on thermal conductivity
What is a high thermal conductivity?
A high thermal conductivity is typically in the range above 50 W/mK. Metals such as aluminium or copper achieve significantly higher values and are suitable for cooling and heat distribution.
How high is plastic thermal conductivity?
Plastic thermal conductivity is usually between 0.15 and 0.35 W/mK. Specially modified compounds with fillers achieve values of 1 to 10 W/mK.
Why do metals possess a higher thermal conductivity than plastics?
Metals have freely moving electrons that transport thermal energy very efficiently. Plastics conduct heat predominantly via molecular vibrations.
What unit does thermal conductivity have?
The unit is watts per metre and kelvin (W/mK). It describes the heat flow through a material at a defined temperature difference.
What role does thermal conductivity play in mechanical engineering?
In mechanical engineering it influences the temperature behaviour of components, housings and guides. A suitable material choice stabilises dimensional accuracy and function.
Can the thermal conductivity of plastics be specifically increased?
Through additives such as graphite, ceramic or metal particles, thermally conductive plastic systems are created for electronics, LED technology and thermally loaded assemblies.
What is a good lambda value?
A good lambda value depends on the intended purpose. For cooling applications, high values are considered advantageous; for insulation or thermal decoupling, low values.
Is 0.3 W/mK high or low?
A value of 0.3 W/mK is considered very low compared with metals, but within engineering plastics it is a typical standard range.
Which plastic conducts heat best?
Special thermally conductive compounds with graphite or ceramic filling achieve the highest conductivity values among plastics and are specifically developed for thermally loaded applications.
