Structural Flakeboards and Composites 2979 



Table 24-12 shows MOR values obtained by Price (1978) from 4- by 8-foot 

 panels of this near-optimum design; these larger panels had slightly lower MOR 

 values than the small panels described in figure 24-38. 



INTERNAL BOND STRENGTH 



Internal bond strength (IB) is tensile strength perpendicular to panel faces. It 

 is a measure of the cohesiveness of flake-to-flake bonds, and is evaluated at 

 board equilibrium moisture content at 72°F and 50 percent RH. Section 24-2 

 noted that for 2-MW waferboard and 2-MF flakeboard, the National Particle- 

 board Association requires an IB of at least 50 psi. Performance specifications of 

 the American Plywood Association for APA RATED STURDI-FLOOR and 

 APA RATED SHEATHING panels are discussed in section 24-2 under the 

 paragraph heading Bond durability. 



Flake orientation, which has a strong effect on MOE and MOR, has only 

 minor effect on IB (Price 1978). The remaining factors that were shown to 

 influence MOE and MOR, also influence IB in flakeboards made of southern 

 hardwoods. Lei and Wilson (1980) concluded that internal bond strength of 

 flakeboards can be increased by eliminating or reducing the size of voids — a 

 concept not easy to achieve in practice. 



There is some evidence, from work by W.E. Johns and W.L. Plagemann at 

 Washington State University in 1983, that temperatures at which flakes are dried 

 influence IB strengths of boards made from the flakes; a moderate dryer tem- 

 perature (e.g. 150°C) appears to promote higher IB strengths in southern hard- 

 wood flakeboards than higher (350°C) or lower (20°C) temperatures. 



Wood density and species. — Hse (1975c) made panels 0.5-inch thick from 

 3-inch-long, 0.015-inch-thick, 3/8-inch-wide veneer flakes of 9 species of hard- 

 woods commonly found on southern pine sites. Low-density species compacted 

 readily when pressed; resulting good flake contact yielded panels with high IB at 

 acceptable panel density (tables 24-9 and 24-1 1). He found that IB was linearly 

 related to compaction ratio (fig. 24-40). Average IB strengths ranged from 51 

 psi for white oak at board density of 44.5 Ib/cu ft to 385 psi for black tupelo at 

 49.5 Ib/cu ft (table 24-9). IB strength increased with panel density for all species 

 except sweetbay. IB strength for the four densest species (i.e., hickory and the 

 oaks) was significantly lower than for the other species, even though their resin 

 coverage was greater. At 44.5 Ib/cu ft all four species yielded panels with IB 

 values less than the target value of 70 psi. At panel density of 49.5 Ib/cu ft, 

 however, satisfactory bond strength was obtained. Table 24-11 summarizes 

 Hse's findings in terms of minimum board densities at which a target IB strength 

 of 70 psi can be met. 



On average, for the 44.5- and 49.5 Ib/cu ft boards, black tupelo flakeboards 

 had the highest IB strengths (312 psi) and were followed in decreasing order by 

 red maple (299 psi), sweetbay (248 psi), white ash (209 psi), sweetgum (183 

 psi), southern red oak (100 psi), post oak (89 psi), hickory (87 psi), and white 

 oak (69 psi). 



