2694 Chapter 22 



veneer was 222 cubic feet, the equivalent of 2. 18 square feet of ys-inch panels 

 per board foot of log input, Doyle scale; or 1 .50 square feet of ys-inch panels per 

 board foot of log input, International V4-inch scale. Of the total input log volume 

 (inside bark) of 808 cubic feet, percentage yields on a cubic basis were as 

 follows: 



Product Percent of input volume 



Usable grades C and D veneer 27 



Green chippable waste, including cores 55 



Dry waste 18 



100 



Craft made all-oak plywood in thicknesses of ys-inch to Vs-inch that had satisfac- 

 tory appearance (fig. 22-64) and production economics, but which failed to pass 

 the vacuum pressure soak test for plywood sheathing glue lines. It performed 

 well in field tests, however; after 6 years use in commercial applications, no 

 delaminations were observed (Craft 1975). 



Gluing of oak faces to less dense hardwood inner plys. — ^Jokerst and Lutz 

 (1974) found that Vs-inch northern red oak face and back veneers could be 

 bonded to a Vs-inch eastern cotton wood (Populus deltoides Bartr. ex Marsh.) 

 core in a cure time of 4 minutes — ^possibly less if plywood were hot-stacked 

 (after pressing) for 4 hours, as is the custom with southern pine plywood. Bonds 

 so made with a 70-percent solids, furfural-extended hot-press phenolic adhesive 

 satisfied glue-line requirements for exterior-type plywood. Spread rate was 80 to 

 83 pounds of adhesive per thousand square feet of double glue line; platen 

 pressure was 175 psi, platen temperature 300°F, open assembly time was mini- 

 mal, and closed assembly time was 9 minutes. After a year of exposure to the 

 weather in Madison, Wis., these plywood panels had no delamination; shear 

 strength and wood failure did not change substantially from tests made before 

 exposure (Jokerst et al. 1976). 



For exterior-type plywood, specimens subjected to a standard vacuum-soak 

 test^^ must average 85 percent or more, wood failure in the glueline. Craft (1975) 

 approached, or exceeded, this standard of phenolic glueline quality with red oak 

 and hickory faces and backs glued to hardwoods of lower density, as follows 

 (sweetgum and sweetbay should also have worked well, but were not included in 

 the trials): 



^"^In the vacuum-soak treatment small plywood specimens, prepared to stress gluelines in shear 

 when gripped at the two ends in a tension machine, are submerged in 120°F water and subjected to a 

 vacuum (15 inches of mercury) for 30 minutes. Following release of the vacuum, specimens 

 continue to soak for 15 hours at atmospheric pressure in water. The water is not permitted to cool 

 below 75°F. At the end of the 15-hour soak, specimens are sheared when wet in a tension machine, 

 dried, and the percentage of wood failure evaluated. 



