The Dielectric Properties of Phenolic Resins and Molded Compositions

The dielectric constant and power factor of purified phenol‐formaldehyde Novolak were determined at room temperature over a frequency range of 1,000 cycles to 45,000,000 cycles. The effect of substituents in the para position of the phenol or in the methylene bridge connecting the phenolic nuclei were studied. The non‐curing type Novolaks were mixed with pure phenol‐formaldehyde Novolaks and cured by the addition of hexamethylenetetramine. The effect of low molecular weight products on the dielectric properties of Novolaks and the effect of the hydroxyl group were also determined. Dielectric data are presented for phenolic molding materials compounded with mica. The power factors of phenol‐formaldehyde Novolaks are increased at low frequencies and decreased at higher frequencies through curing the Novolak by heating with hexamethylenetetramine. The dielectric constant of the resin in either the uncured or cured state decreased gradually with an increase in frequency indicating a very wide range of dielectric absorption. Curing the Novolak increases its dielectric constant by approximately 9% over the entire range of frequency studied. The dielectric constant and power factor of mixed Novolaks, when tested in either the cured or uncured state, are found to be practically directly proportional to the percentage by weight of each component present in the mixture. The power factor and dielectric constant of phenol‐formaldehyde Novolaks can be reduced considerably by placing an alkyl or aryl group as a substituent in the methylene bridge. An even greater reduction in dielectric properties is obtained by placing the substituent group in the para position in the phenol nucleus. The indications are that the power factors of Novolaks are not materially affected by low molecular weight products contained in them. In all cases, the dielectric constant remains unchanged within the limits of the accuracy of the test method. The power factor of a phenol‐formaldehyde Novolak is materially reduced over the whole range of frequency by benzylation of the hydroxyl group. The greatest improvement is obtained at the higher frequencies. The dielectric constant is lowered by approximately 23% through benzylation of the hydroxyl groups.