Structural Flakeboards and Composites 2999 



As seen from figures 24-30, 24-39, and 24-40, however, considerable com- 

 paction is required to develop acceptable MOE, MOR, and IB strength. Swell- 

 ing of flakeboard could be minimized by achieving a uniform degree of 

 compaction and density across panel width and length; visualize use of sponge- 

 like caul plates that would not unduly compress those areas of the mat that 

 contained too much wood substance. In practice, this concept is difficult to 

 apply; also, such panels must be sanded to uniform thickness. 



Use of uniformly thin flakes (e.g., 0.015 inch) favors thickness stability 

 compared to non-uniform thick flakes. Flake length, in the range from 1 to 3 

 inches, does not strongly affect thickness swell if flakes are thin. 



Increasing resin content of flakeboard increases thickness stability somewhat 

 (fig. 24-9), but the increment of improvement diminishes as resin content 

 increases above levels in normal industrial use (8 percent maximum for liquid 

 phenolic resin). Inclusion of wax, usually about 1 percent by weight, moderates 

 dimensional changes during humidity fluctuations and short-term wetting, but 

 probably does not alter thickness stability during long-term changes in relative 

 humidity. 



Single-species flakeboards vary considerably in thickness stability; for exam- 

 ple, white oak flakeboards have greater thickness swell than boards of most 

 other major southern hardwoods (table 24-9). 



To give the reader some appreciation of the magnitude of thickness swell 

 under various conditions, the balance of this section presents experimental data 

 pertinent to flakeboards made from hardwoods found on southern pine sites. 



EXPERIMENTAL DATA 



Single-species flakeboards made from veneer flakes. — Hse (1975c) mea- 

 sured thickness swell in boards of nine species (table 24-9), comprised of veneer 

 flakes 3 inches long, 0.015 inch thick, and 3/8-inch wide bonded with 4-percent 

 phenol-formaldehyde resin applied in liquid form. He found that ranges of 

 average thickness swelling in three exposure tests were: 



50-90 percent RH 13-32 percent 



5-hour boil 20-1 12 percent 



VPS 20-57 percent 



The vacuum-pressure-soak test (VPS) consisted of soaking 3- by 9-inch speci- 

 mens in water under vacuum (25 inches Hg) for 30 minutes and then under 65 psi 

 pressure (at room temperature) for 24 hours. 



Hse found that average thickness swelling varied from test to test. The 5 -hour- 

 boil consistently resulted in the greatest thickness swelling (average 43.7 per- 

 cent), followed in order by the OD-VPS (average 28.4 percent), and 50 to 90 

 percent RH exposure test (average 18.7 percent). 



In 5-hour-boil and OD-VPS tests, thickness swelling for all species increased 

 with increasing panel density. 



