2992 Chapter 24 



percent. Linear stability of APA RATED STURDI-FLOOR and APA RATED 

 SHEATHING panels must be such that they can satisfy one of three alternative 

 specifications (see paragraph Linear Expansion in text related to table 24-3). 



The optimum 1/2-inch-thick, small, phenolic-bonded flakeboards of mixed 

 southern species described by Hse et al. (1975), and the 4- by 8-foot 1/2- and 

 5/8-inch-thick panels of mixed southern woods made by Price (1978) according 

 to the procedure of Hse et al. — see table 24-12 for fabrication details — meet 

 these specifications. 



As noted by Kelly (1977) particleboards composed offtakes are most linearly 

 stable; thin flakes (less than 0.012 to 0.015 inch) and flake lengths of 1 inch and 

 longer favor linear stability. If resin content is sufficient for adequate internal 

 bond strength, additional resin only slightly improves linear stability. Addition 

 of wax to the furnish reduces dimensional changes under cyclic relative humid- 

 ity conditions and short term liquid water exposure. The literature is not clear 

 about the relationship between flakeboard density and linear expansion (Kelly 

 1977). Geimer (1982), however, concluded from research on Douglas-fir flake- 

 board that boards saturated by vacuum-pressure-soak treatment had linear ex- 

 pansion positively correlated with specific gravity, i.e., dense boards had 

 greater linear expansion than less dense boards. 



SINGLE-SPECIES FLAKEBOARDS 



Boards made of veneer flakes. — Hse (1975c) made single-species flake- 

 boards from 3-inch-long, 0.015-inch-thick, 3/8-inch wide veneer flakes of nine 

 hardwoods commonly found where southern pines grow and measured their 

 linear expansion under three exposure conditions (table 24-9). As panel density 

 increased, percentage of water absorbed declined in all but two species (sweet- 

 bay and sweetgum) during the 5-hour boil test, and declined in all but one 

 species (sweetbay) during the ovendry- vacuum-pressure-soak test (OD-VPS). 

 Hse found that, on average, panels of high-density species (hickory, southern 

 red oak, post oak, and white oak) absorbed a slightly lower weight percentage of 

 water. In the 50-90 percent RH exposures, species and panel densities had no 

 effect on moisture gain. Water absorption varied significantly with exposure, as 

 follows: 



50-90 percent RH 13-16 percent 



5-hour boil 68-125 percent 



OD-VPS 75-1 16 percent 



Ranges for average linear expansions in the three exposure tests were: 



50-90 percent RH 0.068-0.35 1 percent 



5-hour boil 045- .443 percent 



OD-VPS 027- .480 percent 



