Investigation of pyrolytically produced condensates of phenol-formaldehyde resins, in relation to their structure and decomposition mechanism

Abstract

Phenol-formaldehyde resins (PFRs) were synthesized under various conditions. The structure of the PFRs obtained, whether long chain or highly branched, relates directly to three controlling factors: (i) phenol to formaldehyde mole ratio, (ii) phenol to NaOH mole ratio and (iii) the temperature and duration of the heating. The kinetic behaviour of the PFRs and the analysis of the condensed pyro-products, formed at selected isothermal pyrolysis temperatures, are presented. A fluidized-bed reactor, monitored with a flame ionization detector (FID) or equipped with a liquid nitrogen cooled trap, was employed. The kinetic behaviour was studied using the FID profile, whereas the analytical study of the pyro-products was performed by GC and GC/MS/C (gas chromatography/mass spectrometry/computer). These analyses yielded the relation between structure and pyro-products. The condensed pyro-products were separated into two boiling range groups; low boiling range, i.e. methylbenzene, dimethylbenzenes and trimethylbenzenes and the corresponding phenols, and high boiling-range, i.e. methyl-substituted xanthenes and methyl-substituted diphenylmethanes. The comparison between the chemically controlled thermal reactions of bis(2-hydroxyphenyl)methane (2BHM) and bis(4-hydroxyphenyl)methane (4BHM) and the pyrolysis products of p- and o-cresol resins leads to the following results. (i) The location of the hydroxyl groups in the resin structure and the pyrolysis temperature affect considerably the characteristic distribution and composition of the condensed pyro-products. (ii) In long-chain PFR, the methylene bridge that links the phenols is located at the ortho and para positions (one of the ortho positions is free). (iii) Dimers of the methyl-substituted phenols, such as methyl-substituted dihydroxydiphenylmethanes, which were expected to appear in the high boiling range, are thermally unstable and decompose to methyl-substituted phenols, or through cyclization form methyl-substituted xanthenes. Experiments conducted in an autoclave under inert or hydrogen atmospheres indicate that the cyclization is through water removal and not by hydrogen extraction. The decomposition mechanism of the PFRs is initiated by bond rupture, yielding free radicals. The stabilization of the free radicals is by extraction of hydrogen atoms from the resin structure and results in a hydrogen-depleted spent (decrease in the H/C aromatic ratio).