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.

California moves to close lid on polystyrene containers

The Californian Senate has passed a landmark bill that could see the use of polystyrene packaging banned across the entire state.

The legislation builds on tens of district ordinances already prohibiting the use of the material, and could affect a wide range of different packaging techniques.

Bill SB 568, sponsored by Long Beach Democratic Senator Allen Lowenthal, calls for a ban on food vendors' use of polystyrene by 1 January 2014, and a ban on school districts using the material a year later.

Polystyrene does not biodegrade for hundreds of years, making it a flagship environmental issue for many conservation groups. The material can also float on water and has been widely blamed for harming marine wildlife.

However, polystyrene is recyclable and the bill makes a provision for school districts to continue to use the material where they have access to suitable recycling facilities.

A number of senators spoke out against the ban. District 14 Senator Tom Berryhill argued that the legislation would cost jobs, while restaurant owners maintained that polystyrene is an inexpensive insulator for food.

However, supporters of the bill insist that manufacturers of polystyrene could easily retool their factories to produce biodegradable alternatives.

In addition to being banned in tens of Californian municipalities, polystyrene has also been prohibited in Portland, Seattle and San Francisco.

The bill was passed 21-15 by senators this week, but still has to get through the Assembly before becoming law.