On the Solvent-mediated Stabilization of Phenol-formaldehyde Resole Resin- A Revisit

The aging of phenol formaldehyde resole (PF) resin was monitored from an increase in viscosity, indirectly measured from its flow time though a capillary. A way to arrest the self-condensation reaction responsible for aging, is addition of a solvent so as to retards the kinetics of the crosslinking reaction. Conventionally, ethanol was used and is still preferred as the diluent for PF resin. However, no rationale has been given for this preference. On re-investigation, we too found that alcohols are the best diluents to arrest or retard aging of resoles. A detailed study of the evolution of viscosity with time for different alcoholic solution, concentrations and temperatures vis-à-vis a reference solvent, Methyl, Ethyl Ketone (MEK) led to the conclusion that alcohol causes stabilisation of P-F resins by (i) sharing of the non-bonding electrons on the oxygen of alcohols with the electrophilic carbon of the H2C-OH groups of methylol, which quenches the latter’s reactivity towards another methylol phenol, (ii) the nucleophilicity of the OH group, (iii) the ease of proton transfer from the alcoholic -OH groups that facilitates the formation of benzyl alkyl ether with reduced reactivity towards phenols. The efficacy of the alcohol depends mainly on its nucleophilicity and steric environment. For these reasons, the stabilising effect of the alcohols decreased in the order tertiary> primary > secondary. Though the tertiary alcohols are the best, they are expensive and has, in general, a noxious ordure. The next choice falls on a primary alcohol that is sterically least hindered and has adequate nucleophilicity Though methanol appeared to be a good choice, its high volatility and toxicity impede its preference. The next choice is ethanol which is comparatively a strong nucleophile, is less toxic and volatile, easily available, low priced and is also an excellent solvent for P-F resin. Thus, the choice of ethanol as a preferred stabilising solvent for P-F resin is scientifically rationalized..