A spectroscopic analysis of carbons derived from heat treated phenolic-resins

The aim of this study is to determine the degree to which the aromaticity of carbons produced from heat-treated (to 1000°C) acid-catalysed phenolic resins is altered when the resin precursors are combined with furfuryl-alcohol, paraformaldehyde and hexamine. Of fundamental importance to the research was the use of four novel techniques to provide the required information; Fourier Transform Infrared Spectroscopy, Solid State Nuclear Magnetic Resonance Spectroscopy, Laser Raman Spectroscopy and X-ray Photoelectron Spectroscopy. To this end, the techniques themselves are described in great detail to indicate their importance as analytical tools. Between 180 and 350°C, paraformaldehyde is seen to provide a linearised ortho-ortho resin structure which greatly assists in cross-linking, and lattice formation at higher temperatures. In the 350 - 750°C range, the release of water is seen as the predominant pyrolysis reaction mechanism, followed by CH aliphatic bridge formation to 400°C. Below 400°C, a combination of furfuryl alcohol and paraformaldehyde creates the most cross-linked structure due to both resin additives providing methyl groups to the phenolic rings, thereby aiding cross-polymerisation. The cross-linking of the furfuryl-paraformaldehyde system is aided by the rapid removal of phenolic hydroxyl groups near 400°C. Double bonded carbon-carbon formation commences in all resins at ca. 550°C, and extensive polycondensation occurs at 600°C due to the removal of the CH and CH2 aliphatic bridges. Rapid ring condensation occurs between 600 and 700°C and is mostly facilitated by the release of aromatic unsubstituted hydrogens (ring protons) below 700°C. A more extensively formed poly-aromatic lattice exists in the furfuryl- paraformaldehyde system, than in the other resins at 700°C, as shown by the ring cluster diameter, D; D700 125 A for LFP, D700 -=--; 50 A for LFH, and D700 20 -40 A for HFH, HFP, NFH and NFP. As many functional groups are retained in the non-furfuryl containing resins, NFH and NFP, the poly-aromatic nuclei of these particular resins are observed as being small and poorly formed. The surface chemistry of all the resins to 1000°C suggests that the surface reactions were very similar to the bulk reactions, such that there were no specific surface phenomena on these resins, and therefore these carbons show surface behaviour similar to glassy carbons. Between 900 and 1000°C the proportion of structural (lattice twisting) defects present in all resins is reduced, although an increase in defects due to the presence of hetero-atoms is shown. To 1600°C, the degree of lattice refinement is marginally favoured by the level of furfuryl-alcohol and paraformaldehyde, and by a higher pyrolysis heating rate. Between 1600 and 2200°C, these factors have little effect on degree of lattice refinement. The recommendation is that a combination of paraformaldehyde and furfuryl-alcohol in a phenolic novolac resin matrix will provide for a highly cross-linked polymeric structure. Although non-graphitising, this furfuryl-paraformaldehyde-phenolic system will achieve a greater degree of lattice ring formation when heat treated to temperatures of at least 1000°C, than will a novolac cured resin containing only hexamine or paraformaldehyde.