How does rubber act as an electrical insulator?

Many of the usual materials we use are electronic conductors, meaning that the electrical charge carriers, when they conduct, are electrons.

The electrical conductivity of these materials is due to the guided movement of free electrons in their matrix in the influence of an applied electrical potential.

The electrical behavior of many materials as to why some are good conductors, some are semiconductors and some others insulators, is now well explained on the basis of ‘Band Theory'.

It states that substances, in which molecules have only strong covalent (sigma) bonds or molecules with non-overlapping weak (pi) bonds, would have most of their electrons in the so-called, ‘valence band' of lower energy, leaving no electrons for the next higher energy band, ‘ conduction band', and that such materials would serve as electrical insulators.

Rubbers, plastics, glasses, dry wood, oils, etc are insulators on this count.

Rubber has additional advantage of mechanical flexibility and can be drawn into shapes, such as gloves, bags, and other required forms of electrical insulators.

Natural and the routine synthetic rubbers are, essentially polymers of a raw material, isoprene, which is chemically 2-methyl-1,3-butadiene, (with a structure of CH{-2}=(CH{-3})CH-CH=CH{-2}. Presence of single and double-bonds, alternatively in a molecule, like in isoprene, is called, ‘conjugation'.

Conjugated molecules possessing pi bonds at their 1,3,5,...positions or at 2,4,6,...positions exhibit resonance, in which Zwitters (entities with both plus and minus charges coexisting at two locations) form by overlapping of pi bonds.

Thus, isoprene which has overlapping (alternating) pi bonds and would be a good conductor if condensed, as such. However, when made into rubber, half of the pi bonds of isoprene molecules are tailored into intermolecular (polymeric) strong sigma bonds to knit into a mechanically flexible but electrically rigid chains.

Thus, in rubber, the pi bonds are not at alternative positions as 1,3,5,...or as 2,4,6,....but as 2,6,10,....or as 1,5,9.... This kind of conversion of otherwise overlapping pi bonds into sigma bonds and non-overlapping pi bonds, causes a huge energy gap between the valence band and the conduction band and thereby resulting in the filling up of all the bonding electrons in the valence band alone with no free electrons available in the conduction band.

When (ordinary) electrical potentials are applied at the opposite ends of such rubber, the electrons of the valence band are hardly elevated into the conduction band and hence no flow of electrical charge is possible across the rubber matrix. That is how rubber acts as an electrical insulator.