Plastic as Thermal Insulator
Why is plastic a good thermal insulator? Is this true of all
Plastics, and most other polymers, are not good thermal conductors because
the glassy structure of the polymer chains tend to disperse the motions of
the constituent molecules. Good thermal conductors tend to get the
molecules to all move in the same direction at the same time. That happens
with crystalline substances (metals for example). Thermal conduction is a
complicated subject, so this short answer is about all a format like NEWTON
BBS can handle. You might do a web search on the term "thermal
conductivity" for more information.
In the simplest terms, thermal conductivity is functionally equivalent to
electrical conductivity. Anything that doesn't conduct electricity as freely
as metals do, air is one example, is considered a good thermal insulator.
That's why double paned windows are good thermal insulators.
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