The characteristics of different cresol and urea modified phenol formaldehyde (PF) resins synthesized by conventional heating and microwave irradiation were evaluated. Phenol-formaldehyde (PF), cresol-formaldehyde (CF) and urea/cresol (20, 50 and 80 percent) modified PF resins were polymerized in autoclave for three, five and seven hours with oxalic acid catalyst. The syntheses were repeated by microwave irradiation at different power levels (176.5, 315.1, 454.0, and 592.7W) for reaction times between 15 and 90 minutes. Equimolar amounts of the pre-polymerized PF and CF resins were further heated at various heating times in a post-polymerization process.

Analysis with FT-IR, GC-MS and uv/visible spectroscopy showed strong similarity in characteristics of the products synthesized by microwave irradiation and conventional methods. The concentration of unreacted phenol in the resins decreased with reaction time by 3.8% (in 3hrs), 7.4% (in 5hrs) and 22.1% (in 7hrs), while in microwave synthesis significant decrease in phenol concentration was observed, particularly at higher power levels. Similar observations were made on the modified PF resins containing different ratios of phenol to urea/cresol (80:20, 50:50 and 20:80).

The melting points and gel time of the post-polymerized PF-CF resins were lower and decreased with longer heating time. Microwave irradiation process reduced the polymerization time significantly and produced resins with shorter gel times, improved yield and similar physical and chemical characteristics. The FT-IR spectra analyses for the products showed strong similarity in their chemical structures. Resins of PF-CF post polymerization process showed significant improvement in chemical and thermal stability than the urea/cresol modified PF resins.