Hot Press Gluing of Hardboards to Douglas-Fir Veneers with Phenolic Resin Adhesives

Three types of hardboard, one each of the smooth two side-treated (I), screen back-treated (II), and screen back-standard board types (III), were selected. All hardboards and the Douglas-fir (Pseudotsuga taxifolia Britt.) face veneer were 1/10-inch thick. The construction used was 3/4-inch 5 ply made from 1/10-inch faces and backs and 3/16-inch centers and cores. By alternating moisture contents (2 and 8 percent) of hardboard of Douglas-fir face and core and center stock, four moisture combinations were obtained. The two glues selected were representative of exterior phenolic adhesives used in the Douglas-fir plywood industry. To find out the relative rates of temperature increase in the innermost glue line of hardboard-faced plywood as compared to all Douglas-fir construction, innermost glue line temperature curves were obtained. From these curves, the time required to reach 220?F. (sufficient cure temperature) for each panel was determined. Panels reaching 220?F. in the shortest time indicated the shortest total press time. Both the types of face material and the moisture content combination significantly affected the time required to reach 220?F. Panels made from hardboards II and III reached 220?F. faster than the panels with hardboard I or Douglas-fir faces. The 8 percent face-2 percent core and center construction reached 220?F. in a significantly shorter time than any others. As indicated by the two-cycle boil shear test, the average percent wood and/or fiber failure was higher for the panel made with hardboard III face and back than for the corresponding all Douglas-fir panels. Among the hardboards tested, hardboards II and III produced better bond quality. Poorest results were obtained with the 8 percent hardboard-8 percent veneer moisture content combination. The trend was that the lower the moisture content the better the bonding quality. In the knifing test, the specimen was a 1- by 9-inch strip cut from the panel to be tested with the direction of the green in the veneer next to the hardboard parallel with the long dimension of the test piece. The veneer was knifed away from the hardboard and the glue line was evaluated from an inspection of both surfaces to determine the percent wood and/ or fiber failure. Comparison was made between this technique and the shear test, but revealed no significant difference. Because of its simplicity, the knifing technique was recommended for further study as a routine mill quality control test.

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