Phenol-formaldehyde electrical conductivity

Ultra thin microporous carbon films are derived via the pyrolysis of phenolic precursors. The latter can be prepared from resorcinol-formaldehyde resins using a base catalyst. After several hours at 50°C of curing, the solution forms a stable polymeric film. Followed by a solvent exchange and ambient pressure drying, the film is pyrolysed in argon atmosphere at temperatures above 800°C. The result is an electrically conducting polymeric carbon film, the structure of which resembles the organic precursor, but shows microporosity in addition. Hereby, films with thicknesses of > 5 microns and sufficient mechanical stability can be made. 

In 1909 the first true plastic was developed by reacting phenolic-formaldehyde (Bakelite) in the presence of a (propriety) catalyst. It was done by L. Hendrik Baekeland [3] who coined the word plastic , a substance he created from coal tar. The material had excellent heat resistance and low electrical conductivity and when blended with mica, clays, asbestos, etc., had considerable strength and resistance. In recent years, these materials have been used as part of manufacture to make casings for clocks, toasters and radios, among other things. 

The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670°C in a N2 atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase... 

Table 16.5. Electrical conductivity and elemental analysis data of polyacenic polymers and phenol-formaldehyde resin...

The maximum recommended film thickness is 25 p.m. At greater thicknesses, volatiles from the curing reaction, mainly water and some formaldehyde and phenol, can cause defects. These coatings have excellent electrical insulation properties, ie, up to 20 V/p.m, because of low moisture absorption and low conductance. The coatings are hard with low flexibiUty, depending on curing conditions and film thickness. 

---