Structural Flakeboards and Composites 2943 



Table 24-10. — Ejfect of application of polyisocyanate to flakes before addition of liquid 

 phenol-formaldehyde resin on internal bond strength and stability of J /2 -inch-thick flake- 

 board panels of southern red oak and white oak (Hse 198 1)'-^-'' 



Wood species 

 and resin 



Psi Percent 



Southern red oak 



Straight phenol-formaldehyde resin 29 26.0 0.358 



Half phenolic and half polyisocyanate 147 27.2 .370 



Three-fourths phenolic and 



one-fourth polyisocyanate 158 28.0 .343 



White oak 



Straight phenol-formaldehyde resin 30 41.4 .528 



Half phenolic and half polyisocyanate 176 31.8 .329 



Three-fourths phenolic and 



one-fourth polyisocyanate 149 30.8 .375 



'The phenolic resin had molar ratio of formaldehyde to phenol of 1 .9, reaction concentration was 

 47.5 percent, and molar ratio of sodium hydroxide to phenol was 0.45; the commercially available 

 polyphenol isocyanate had functionality of 2.7, and viscosity of 200 to 275 cps at 25°C; resin content 

 of all panels was 5 percent of the ovendry weight of the flakes. 



^Panels were homogenous with randomly oriented flakes cut 3 inches long, 0.015 inch thick, and 

 random width on a shaping-lathe headrig. The flakes were conditioned to 7 percent moisture content 

 before resin application. 



^Panels were pressed at 335°F with 45-second press closing time and 5-minute closed-press time, 

 to a density of 45 pounds per cubic foot (basis of ovendry weight and volume at test). 

 "^After ovendry-vacuum-pressure-soak cycle. 



APPLICATION OF RESIN 



Unlike the waferboard plants of Canada which need contend only with aspen, 

 the manufacturer of hardwood structural flakeboard in the southern pine region 

 must tailor processes to suit a number of species with diverse anatomical charac- 

 teristics. South wide, the most plentiful pine-site hardwoods are s wee tgum (13.2 

 percent), white oak (12.3 percent), hickory sp. (8.5 percent), and southern red 

 oak (8. 1 percent). Flakes from each of these species are distinctive (figs. 1 8-102 

 and 18-274ABC). Sweetgum flakes tend to be wide — some will be folded, 

 hickory frequently yields tubular flakes, and the two oaks form strands. Because 

 white oak has prominent tyloses in the vessels (figs. 5-13 top and 5-22) that 

 inhibit flake separation at vessels, strands of white oak are usually wider than 

 those of the red oaks that lack tyloses in vessels. 



For reasons not entirely clear, probably related to compaction ratios and press 

 times used — but possibly to surface texture of flakes (fig. 24-19) — powdered 

 resins have been successfully applied to aspen flakes but not to flakes of pine-site 

 hardwoods. 



Failing to obtain good results with powdered resin, researchers have favored 

 liquid resins for southern hardwood structural flakeboards. In most laboratories, 

 the flakes are tumbled in a rotating drum (14 rpm) perhaps 3 to 4 feet in diameter 



