Effects of Bark, Density Profile, and Resin Content on Medium-Density Fiberboards from Southern Hardwoods

Pressure refined barky fibers of hickory (Carya tomentosa Nutt.), sweetgum (Liquidambar styraciflua L.), and southern red oak (Quercus falcata var. falcata) had a greater percentage of fines than did refined bark-free fibers of these species. Inclusion of bark decreased tensile and bending strengths of fiberboards by 16 to 18 percent, MOE by 10 to 14 percent, and internal bond by 8 percent. Density profile of boards strongly influenced their bending properties. Specimens with greatest density variation from face to core were 37 to 59 percent higher in MOR and 32 to 50 percent higher in MOE than specimens with no density contrast from face to core. At average density of 0.68 g/cc (when equilibrated to 50 percent R.H.), 3/4-inch boards of greatest MOE (556,000 psi) and MOR (5,159 psi) were bark-free sweetgum with faces about 50 percent denser than cores. This density profile resulted from pressing with platen pressure of 480 psi and platen temperature of 335?F. Increasing resin level from 8 to 10 percent improved bending and tensile properties by only 4 to 10 percent, but internal bond was increased by 38 percent. Linear expansion (50 to 90 percent R.H.) was greatest in high density boards and was not affected significantly by inclusion of bark. It averaged about 0.29 percent. Thickness swell was significantly less in southern red oak boards than in sweetgum or hickory. Inclusion of bark decreased thickness swell of southern red oak and hickory boards but increased it in sweetgum boards. Thickness swell in boards of about 0.7 g/cc density averaged about 8.9 percent. Face hardness was shown by the modified Janka ball test to be highest for uniform density boards, suggesting that this test more truly evaluates core properties than face properties.

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