Compressibility and Bond Quality of Western Softwood Veneers

Presents and discusses data relating to compressibility of selected western species of softwood veneers listed under group 1 of CS 122-40, when hot pressed under standard manufacturing conditions for plywood. Test species included: Sitka spruce, noble fir, Pacific silver fir, grand fir, Shasta red fir, white fir and Douglas fir. A widespread range in compressibility was evident between different species. For panels bonded with either blood or phenolic glue, laboratory-pressed plywood of five plies of 1/8-inch veneer, western hemlock had the least and Pacific silver fir the most compression during hot pressing and after reconditioning. The two lots of. Pacific silver fir showed the least variability in compressibility among geographical sources while the two lots of Sitka spruce showed the most. The western softwood plywoods compressed from 20 to 85 percent more after bonding with blood glue than did Douglas-fir. Plywood bonded with blood glue compressed more during hot pressing than did similar panels bonded with phenolic glue, but after pressure release, compression was greater within plywood bonded with phenolic glue. Springback of the blood-glued panels was about double that of panels bonded with phenolic glue. It was found that when five plies of 1/10-inch veneer were compared to 5 plies of I/8-inch veneer of the same species and source, after being hot pressed under similar conditions, the final compression of the thinner veneers was from 66 to 78 percent greater. After pressing under similar conditions and starting with veneer of about the same thickness, plywood of western softwood species failed to meet requirements for minimum thickness of WSP 1 sheathing, while Douglas-fir surpasses requirements for minimum thickness of sheathing. As specific gravity increased, compression decreased. All plywood pressed with blood glue passed the test for bond quality of interior-type plywood. In exterior-type plywood. panels from three different lots of veneer failed to meet industry standards. Evidence suggested that mill-produced plywood compressed less than small-sized lab panels of the same species. This was believed due to differences in loss of moisture during hot pressing, effect of knots in reducing compression of mill panels, and basic differences in measuring techniques.

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