Phenol-formaldehyde (PF) resin is a high performance adhesive, but has not been widely developed due to its slow curing rate and high curing temperature. To accelerate the curing rate and to lower the curing temperature of PF resin, four types of metal-mediated catalysts were employed in the synthesis of PF resin; namely, barium hydroxide (Ba(OH)₂), sodium carbonate (Na₂CO₃), lithium hydroxide (LiOH), and zinc acetate ((CH₃COO)₂Zn). The cure-acceleration effects of these catalysts on the properties of PF resins were measured, and the chemical structures of the PF resins accelerated with the catalysts were investigated by using Fourier transform infrared (FT-IR) spectroscopy and quantitative liquid carbon-13 nuclear magnetic resonance (13C NMR). The results showed that the accelerated efficiency of these catalysts to PF resin could be ordered in the following sequence: Na₂CO₃ > (CH₃COO)₂Zn > Ba(OH)₂ > LiOH. The catalysts (CH₃COO)₂Zn and Na₂CO₃ increased the reaction activity of the phenol ortho position and the condensation reaction of ortho methylol. The accelerating mechanism of (CH₃COO)₂Zn on PF resin is probably different from that of Na₂CO₃, which can be confirmed by the differences in the differential thermogravimetric (DTG) curve and thermogravimetric (TG) data. Compared to the Na₂CO₃-accelerated PF resin, the (CH₃COO)₂Zn-accelerated PF resin showed different peaks in the DTG curve and higher weight residues. In the synthesis process, the catalyst (CH₃COO)₂Zn may form chelating compounds (containing a metal-ligand bond), which can promote the linkage of formaldehyde to the phenolic hydroxyl ortho position.
Keywords: PF resins; ion-polymer; metal catalysts; phenol ortho position; synthesis mechanism.