Dielectric strength of polymers

The dielectric breakdown voltage or dielectric strength of polymers may be determined by ASTM-D149. The dielectric breakdown voltage is the maximum applied voltage that a polymer can withstand for 1 min divided by the thickness of the polymer. 

The induced dipole moment of a polymer in an electric field is proportional to the strength of the field, and the proportionality constant is related to the polarizability of the atoms in the polymer. The dielectric properties of polymers are affected adversely by the presence of moisture, and this effect is greater in hydrophilic than in hydrophobic polymers. 

Table III lists some of the physical properties of polymers which contain ethylenebis [tris (2-cyanoethyl) phosphonium bromide]. This additive caused an increase in the dissipation factor and dielectric constant and lowered the dielectric strengths of polyethylene and poly (methyl methacrylate). The effects on mechanical properties were mixed. Obviously, lower concentrations of phosphonium halides would have less effect on mechanical and electrical properties. At levels of 1-3% very little change in properties would be expected. It was surprising that the phosphonium salts were compatible with such a range of polymers. We did not observe any tendency for the phosphonium salts to plate out of or exude from the polymer. In all cases homogeneous blends were obtained.

The average value of the dielectric strength of pure polymers in kV/cm is 200. Chlorinated polymers show values up to 500, polymers containing aromatic rings are on the low side (about 160). 

The dielectric strength of polyethylene has been measured as a function of additive concentration while in stabilised polymers the depth of degradation was found to be uniform . HALS have been found to be effective stabilisers for UV cured coatings and do not influence the cure rate . Bleached wood pulps can be effectively stabilised by ascorbic acid and wool by hydroxy-benzotriazoles and HALS . Hydroxybenzotriazole stabilisers also protect wood pulp and polyurethanes by co-reaction . The spectroscopic properties of monomeric and polymeric benzotriazoles have also been compared . UV absorbers are effective in clear coats and when grafted to wood . Dihydroxybenzophenone stabilisers inhibit the chain scission in the photodegradation of poly(methoxyacrylophenone) while tin stabilisers have also been found to photostabilise PVC . 

Cationic polymerizations are usually conducted in solutions and Irequently at temperatures as low as -80 to -100°C. Polymerization rates at these low temperature conditions are usually fast. The cation and the cormterion in cationic polymerization remain in close proximity. If the intimate association between the ion pair is too strong, however, monomer insertion during propagation wiU be prevented. Therefore the choice of solvent in cationic polymerization has to be made carefully a linear increase in polymer chain length and an exponential increase in the reaction rate usually occur as the dielectric strength of the solvent increases. 

PTFE has outstanding insulation properties over a wide range of temperatures and frequencies. The volume resistivity exceeds 10 ohm-m. The power factor (<0.003 at 60 Hz and <0.0003 at 10 Hz) is negligible in the temperature range — 60°C to - -250°C. PTFEs low dielectric constant (2.1) is unaffected by frequency. The dielectric strength of the polymer is 16-20 kV/mm (short time on 2-mm thick sheet). 

The dielectric strength of PES, as measured in oil by the short term tests ASTM D 149, is 800 KV/em for a 0.23 mm section and 158 KV/cm for 3 2.3 mm. The tracking behaviour of PES when subjected to high voltages is very complex and this phen< nenon is currently under investigation. There are indications that electrical breakdown associated with carbonisation tends to occur across the surface of the material. However, results obtained are similar to those found for other amorphous aromatic polymers like polycarbonate. 

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