Some Factors Affecting the Glue Bond Quality of Hard-Grained Douglas-Fir Plywood

Typical hard-grained 1/8-inch-thick sheets of Douglas-fir heartwood and sapwood veneer were collected from two mills (A & B). Half of each sheet was dried at the mill of origin, the rest dried at the other mill. Drying temperatures at mill A were 330? – 360? F. Those at B were 235? – 360? F. Veneers at 3.4 ? 0.7 percent moisture content were pressed at 285?F. and 200 psi into 12 by 12-inch, 5/8-inch by 5-ply, panels. Press time insured complete cure of three types of glue used (not identified). One panel was pressed for each combination of four factors making a total of 40 panels: [2 mills x 2 drying conditions x 2 veneer types (heartwood & sapwood) x 5 glues]. Ten standard shear specimens from each panel were subjected to the two-cycle boil test in Commercial Standard 45-48; their performance was evaluated by percent of wood failure. Glues did not all behave the same but all gave best results if used with “tighter” veneer (mill A). Veneers dried at lower temperatures (mill B) usually made the best plywood regardless of source at A or B, or whether heartwood or sapwood. Hard-grained heartwood generally made better plywood, but equally good plywood was sometimes produced from hard-grained sapwood.

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