2948 Chapter 24 



For other blender designs, see Maloney (1977, p. 438-457). Efficient resin 

 application will be a key to successful manufacture of panels from southern 

 hardwoods, and more research on the subject is needed. Kasper and Chow 

 (1980) described a method of using X-ray spectrometry to determine resin 

 distribution in flakeboard that should be useful during the conduct of such 

 research. 



24-6 MAT MOISTURE CONTENT, CONSTRUCTION, 

 AND FORMATION 



MAT MOISTURE CONTENT 



Amount and distribution of moisture through the thickness of a formed mat 

 significantly affect properties of pressed panels. By increasing moisture content 

 in face layers, and decreasing it in the core layer, surfaces can be densified 

 during pressing, thereby increasing panel modulus of elasticity and modulus of 

 rupture. Below mat moisture levels at which boards blow apart when the hot 

 press is opened, manipulation of mat moisture content can considerably alter 

 board properties. 



Laboratory trials with mats of species mixtures of southern hardwood flakes 

 blended with liquid phenol-formaldehyde resin, indicate that for press tempera- 

 tures in the range from 290°F to 345°F and compaction ratios from 1 . 1 to 1 .2 

 mat moisture content for 1/2-inch-thick panels should average about 10 percent; 

 an average moisture content of 12 percent should not be exceeded if panel blows 

 are to be avoided. 



With the alloy of isocyanate and phenol-formaldehyde resin described in table 

 24-10, mat moisture content for 1/2-inch-thick panels may be as high as 14 

 percent without delamination at press opening. 



SPECIES COMPOSITION 



About 47 percent of pine-site hardwood volume is oak, and hickory comprises 

 a significant portion (8.5 percent). Single-species flakeboards of dense southern 

 hardwoods tend to be excessively heavy when fabricated to have acceptable 

 mechanical properties (table 24-11). Flakes of these, and other dense woods, 

 must therefore be mixed in controlled proportions with those of less-dense 

 woods such as sweetbay, red maple, yellow-poplar, and sweetgum, which as a 

 group comprise about 25 percent of the total volume of pine-site hardwoods. 

 Laboratory attempts to concentrate dense oak and hickory flakes in core layers 

 and to place low-density sweetgum, red maple, yellow-poplar, and red maple on 

 faces of 1/2-inch-thick sheathing panels have not been particularly successful. 



Researchers experienced in constructing sheathing panels from southern hard- 

 woods conclude that the same species mix (recipe) should be used uniformly 

 throughout all board layers, and that the species mix should be controlled. This 



