Thermal Conductivity

Thermal conduction is the transfer of heat from one part of a body to another with which it is in contact. The thermal insulating capacity of plastics is rated by measuring thermal conductivity.

Thermal insulation of polymers (thermoplastics, foam or thermoset) is essential to:
 

• Understand the processing of the material into the final product,
• Establish appropriate applications of the material. For e.g., polymeric foams for insulation.

Note: PUR and PIR can be molded as board materials. They can be used as insulation foams in roofs, plastered walls, sandwich walls, and floors. Unlike metals, plastics do not have free electrons available for conduction mechanisms. Hence, they are poor conductors of heat.

 

Thermal conductivity is denoted by the letter k. It is calculated using the formula:
 

Where,
 

• k is the thermal conductivity of the material [in W/(m·K)]
• Q is the quantity of heat passing through a base area of the sample [in Watts]
• A is the base area of the sample [in square meters]
• d is the distance between two sides of the sample [in meters]
• T₂ is the temperature on the warmer side of the sample [in Kelvin or Celsius]
• T₁ is the temperature on the colder side of the sample [in Kelvin or Celsius]

Structural changes in polymers

Thermal conduction in polymers is based on the mechanism of movement of molecules. This occurs across:
 

• intramolecular bonds and
• intermolecular bonds.

Crosslinking in thermosets & elastomers increases thermal conductivity. This is because the Van der Waals bonds are replaced by valence bonds. Alternatively, a decrease in thermal conductivity can be due to:
 

• decreasing interbond path length,
• factors causing increased disorder, or
• free volume in polymers.
  
   

Crystallinity in polymers
 

 

The crystallinity of polymers leads to improved packing of molecules. This in turn increases thermal conductivity. 

 

Amorphous polymers
 

• They show an increase in thermal conductivity with increasing temperature. This increase is up to the glass transition temperature (Tg).
• Above Tg, the thermal conductivity decreases with an increase in temperature.

Note: For amorphous plastics at 0-200°C, the thermal conductivity lies between 0.125-0.2 Wm^-1K^-1.

 

Semi-crystalline thermoplastics
 

• They have a higher thermal conductivity in the solid state than in the melt state. This is due to an increase in density upon their solidification.
• But, in the melt state, the thermal conductivity of these polymers reduces to that of amorphous polymers.

Note: Semi-crystalline thermoplastics have ordered crystalline regions. Hence, they have better conductivity.


 

1. Thermally Conductive Thermoplastics — View All Products
2. Thermally Conductive Rubbers — View All Products
3. Thermally Conductive Thermosets — View All Products
4. Thermally Conductive TPEs/TPVs — View All Products

Filler or fiber content

Organic plastics are very good insulators. Thermal conductivity of polymers increases with increasing:
 

• volumetric filler content or
• fiber content up to 20% by volume fraction.

» Inorganic fillers — They have high thermal conductivity. Thus, they increase the thermal conductivity of filled polymers.

» Gaseous fillers — The incorporation of gaseous fillers in the structure decreases heat conduction. This happens in polymeric foams. This is due to the increase in the number of closed cells in the foam minimizes heat conduction. 

 

Hydrostatic pressure
 

The thermal conductivity of melts increases with hydrostatic pressure.

 

Compression of plastics
 

Compression of plastics imposes the opposite effect on thermal insulation as it increases the packing density of molecules.

 

Other Factors

The thermal conductivity increases with an increase in:
 

• density of material
• moisture of material
• ambient temperature

The guarded hot plate apparatus is used to measure the thermal transmission properties of homogeneous insulation materials. This is done in the form of flat slabs.

Procedure
 

1. A solid sample of material is placed between two plates.
2. One plate is heated and the other is cooled or heated to a lesser extent.
3. The temperature of the plates is monitored until they are constant.
4. Thermal conductivity is calculated by using the:
   • steady state temperatures,
   • thickness of the sample, and
   • heat input to the hot plate.

Thermal conductivity of plastics is generally measured by:
 

• ASTM C177 — It measures thermal transmission properties. It also measures steady-state heat flux. The measurement is by means of the guarded-hot-plate apparatus.
   
• ISO 8302 — It determines steady-state thermal resistance and related properties. Guarded hot plate apparatus is used in this case.

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