It has been suggested that wood extractives have effects on the curing reaction of adhesive. The pH and buffer action of wood may affect the gelation of urea-formalde -hyde resin adhesive(UF) because the rate of curing of this resin is closely dependent on hydrogen ion concentration.
In this paper, the influence of the wood extractives with cold water, boiling water or alcohol-benzene on the gelation time of UF and the compressive strength of setting material are investigated. The gelation time was measured by using the gel timer which indicated the resistance to movement of disc through the gelating liquid. The disc is oscillated up and down in the gel with its plant horizontal.
The results of this experiment are as follows :
1) The gelation time of UF to which wood extractives were added increases with the increased amount of wood extractives with cold and boiling water. On the cold water extractives, a certain relationis observed between the gelation time and the pH of wood, that is, the gelation time decreases in connection with the decreasing of hydrogen-ion concentration of each wood. Although the extractives with alcohol-benzene also has a decreasing effect on gelation time, it needs longer than the gelation time of UF only.
2) The compressive strength and the ratio of plastic region to elastic region of set gel decrease with the increasing gelation time.