2978 Chapter 24 



from a low of 35.9 Ib/cu ft for sweetbay to a high of 57. 1 Ib/cu ft for white oak 

 (table 24-1 1). He also found that bending strength increases proportionately as 

 compaction ratio increases, with close correlation (fig. 24-39). 



Flake quality. — Just as well-cut veneer flakes yield boards with superior 

 MOE values compared to boards made from industrially-cut flakes (fig. 24-32), 

 they also yield boards with superior MOR values. Among major types of indus- 

 trial flakers (drum, disk, shaping-lathe, and ring) evaluated by Price and Leh- 

 mann (1978), the shaping-lathe produced oak and hickory flakes that yielded 

 boards with highest MOR (fig. 24-4). With sweetgum, the disk flaker gave 

 highest MOR values at compaction ratios above 1.3 (fig. 24-10). 



As noted in section 24-4, slope of grain reduces flake tensile strength drasti- 

 cally, e.g., a 10-degree slope of grain can reduce flake tensile strength by 49 

 percent and a 20-degree slope by 97 percent. 



Flake length/thickness ratio. — As with MOE, MOR of flakeboard increases 

 with increasing flake length/thickness ratio, most of the increase being obtained 

 at an L/T ratio of 200 — for example, using 3-inch-long, 0.015-inch-thick flakes 

 in face layers. Three-inch-long core flakes cut 0.025-inch thick also contribute 

 more to panel bending strength than core flakes 1 .5 inches long of this thickness 

 (fig. 24-33 center). 



Flake width, moisture content of mat, and inclusion of southern pine in 

 the furnish. — MOR increases with MOE, therefore refer to the paragraphs with 

 these headings in the previous subsection and to figures 24-35 and 24-36. 



Inclusion of baldcypress in the furnish. — Flakeboards comprised of one- 

 third baldcypress and two-thirds mixed southern hardwoods have modulus of 

 rupture, at 41-43 pounds/cu ft panel density, of about 5,300-5,630 psi (table 24- 

 13). 



Seven-species mix of bottomland hardwoods. — At 42 pounds/cu ft density, 

 optimally fabricated flakeboards of seven hardwood species typical of south- 

 central Louisiana had modulus of rupture of 5,500 psi (table 24-14, fig. 24-33). 



Resin content. — Rice and Carey (1978) found that an increase in resin 

 content significantly increased MOR of yellow-poplar and sweetgum flake- 

 boards, particularly for resin contents between 4 and 6 percent and at board 

 densities greater than 42 pounds/cu ft (fig. 24-37). 



Hse (1975b) found that in southern hardwood flakeboards, all strength proper- 

 ties increased substantially as resin content increased from 2 to 8 percent; with 

 increase from 8 to 10 percent, however, modulus of rupture and modulus of 

 elasticity increased only slightly and internal bond strength decreased because 

 excess moisture, introduced by the additional liquid resin, resulted in less than 

 optimum gluing conditions. The lower limit of resin content (applied as a liquid) 

 to yield adequate bond strength was about 4 percent. 



Optimization of MOR. — ^Text related to figure 24-38 in the previous subsec- 

 tion on MOE, described a near-optimum fabrication procedure for flakeboard 

 comprised of 20 percent each of hickory, white oak, southern red oak, sweet- 

 gum, and southern pine. Small flakeboards of this design had MOR of 6,625 psi 

 when flakes were aligned at board density of 45.5 Ib/cu ft, and 5,300 psi with 

 random-flake placement at a board density of 47.5 Ib/cu ft (fig. 24-38 center). 



