Plastic as Thermal Insulator
Date: Fall 2010
Why is plastic a good thermal insulator?
Is this true of all polymers?
Good thermal conductivity, such as metals exhibit, results from the
material's valance electrons being loosely coupled to their respective
nuclei. This allows the electrons to absorb thermal energy and move
freely from atom to atom, carrying this thermal energy with them.
Valence electrons in plastic materials are more tightly held to each
plastic molecule. Although there is bound to be an obscure exception,
essentially all polymers are poor thermal conductors.
Electrical conductivity is possible in plastics - in fact, my graduate
thesis was in the production of electrically conductive polymers. However,
such polymers have to be specially designed. In order for an organic polymer
to be electrically conductive, the electrons within the polymer have to be
mobile - they have to be able to move across the polymer chain, and be able
to "jump" across chains. This can be achieved by producing polymers that
have carbon-carbon double bonds. If the double bonds are conjugated
(separated only be one single bond), the electrons can become delocalized
(they do not stay around just one or two carbons) - and as such the electrons
become mobile. This flow of electrons is current.
In the normal or average plastic, the polymer is made up of mostly single
carbon-carbon bonds where the electrons remain around the same carbons. The
electrons are not mobile. As such, there is no good mechanism to allow the
flow of electrons or current.
Greg (Roberto Gregorius)
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Update: June 2012