Evaluation of phenolic resins from one-pot microwave synthesis

Abstract

The characteristics of oxalic acid catalyzed phenolic resins, synthesized using the conventional heating method and by a one-pot microwave process, were evaluated and compared. Phenolic resins were produced in an autoclave with reflux times of 3, 5 and 7 hours, and by one-pot microwave synthesis at different power levels (176.364, 315.092, 453.820, 592.548W) for reaction times of 15–90 minutes. UV-vis spectroscopy and FT-IR analyses showed similarities in the characteristics of the resins synthesized by both methods. Concentration of residual phenol in the products decreased with reaction time. For the conventional method the decrease in phenol after an initial 3hrs of reaction were 4.3% (in 5hrs) and 22.3% (7hrs), while in microwave synthesis at a power level of 176.364W and an initial reaction time of 30mins the phenol decreased by 33.05% (in 60mins) and 91.61% (in 90mins). At a power level of 315.092W and an reaction time of 15mins it decreased by 36.3% (in 20mins), 16.5% (in 25mins) and 10.8% (in 30mins). For resins from the conventional method, melting points increased with increasing reaction time 83C, 97C and 109C for respectively 3hrs, 5hrs and 7hrs, and between 95C and 103C for resins from the microwave method at various power levels and reaction times. The FT-IR spectra analyses for the products showed strong similarity in characteristics, while the melting point data supported the GC-MS results that for the conventional method mostly the degree of polymerization depends on reaction time. The substitution of phenol with cresol results in resins with improved hardness and melting points, but these differences were not obvious in the spectral characteristics. It was also observed that for the conventional heating method the melting points of the CF pre-polymers produced were higher than those of the PF prepolymers (119oC and 127oC for 3hr and 5hr CF). One-pot microwave synthesis was found to reduce the polymerization time significantly and produced resins with shorter gel times. Closest similarities in characteristics were found between resins heated in an autoclave for 7hrs and the resins made by one-pot microwave synthesis at reduced reaction times.