Influence of the synthesis conditions on the curing behavior of phenol-urea-formaldehyde resol resins

The curing behavior of synthesized phenol-urea-formaldehyde (PUF) resol resins with various formaldehyde /urea/phenol ratios was studied with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicated that the synthesis parameters, including the urea content, formaldehyde/phenol ratio, and pH value, had a combined effect on the curing behavior. The pH value played an important role in affecting the shape of the DSC curing curves, the activation energy, and the reaction rate constant. Depending on the pH value, one or two peaks could appear in the DSC curve. The activation energy was lower when pH was below 11. The reaction rate constant increased with an increase in the pH value at both low and high temperatures. The urea content and formaldehyde/phenol ratio had no significant influence on the activation energy and rate constant. DMA showed that both the gel point and tan delta peak temperature (T-tandelta) had the lowest values in the mid-pH range for the PUF resins. A different trend was observed for the phenol-formaldehyde resin without the urea component. Instead, the gel point and T-tandelta decreased monotonically with an increase in the pH value. For the PUF resins, a high urea content or a low formaldehyde/phenol ratio resulted in a high gel point. The effect of the urea content on T-tandelta was bigger than that on the gel point because of the reversible reaction associated with the urea component. Too much formaldehyde could lead to more reversible reactions and a higher T-tandelta value. The effects of the synthesis conditions on the rigidity of the cured network were complex for the PUF resins. (C) 2005 Wiley Periodicals, Inc.