DYNAMIC-MECHANICAL PROPERTIES OF THERMOSETTING RESIN ADHESIVES .1.

The effects of preheating (120 or 160 degrees C) treatments on the dynamic thermomechanical properties of an aqueous phenol-formaldehyde (PF) resol resin was studied by using a dynamic mechanical analysis (DMA). The results and conclusion are summarized as follows: It appeared that the untreated liquid state of PF resin has a distinct gel point with sudden changes of storage modulus (E'), loss modulus (E ''), and tan delta, whereas, vacuum dried PF resin exhibited different E', E '', and tan delta curves during measurements at elevated temperatures. As the preheating time was prolonged, the initial (20 degrees C) E' generally increased with a drop of the peak in tan delta but independent of the preheating temperature. The gel point of the resin preheated for a few minutes at 120 degrees C appeared lower than that of resin preheated for the same times at 160 degrees C. These results suggested that the curing reactions of PF resin were dependent on heating temperature, and the result of the partially cured resin preheated for few minutes at 120 degrees C was unstable at an increasing temperature. It also was observed that the resin preheated for 5 min at 160 degrees C cured completely with the disappearance of the tan delta peak, whereas resin preheated at 120 degrees C cured incompletely with a tan delta peak independent of the preheating time. In addition, the authors have proposed a method of determining the degree of cure for PF resin in mechanical aspects, The method is based on the change of the E' of preheated resin during DMA measurements. Compared with the conventional methods, such as methanol extraction, this method is affected by a solvent including resin and is expressed seemingly as a mechanical cure.