2960 Chapter 24 



controlled and manipulated to considerable degree by controlling fabrication 

 variables. In general, the fabricator's objective is attainment of acceptable 

 mechanical properties at the lowest practical panel density. 



Discussion that follows in this section is limited to data on structural flake- 

 board bonded with phenol-formaldehyde resin and comprised of the principal 

 hardwoods found on southern pine sites. Broader reviews of engineering and 

 physical properties of particleboard, principally softwood, bonded with both 

 urea and phenolic resins, have been provided by McNatt (1973) and Kelly 

 (1977). Data on tropical hardwood flakeboards can be found in Haygreen and 

 French (1971) and Vital et al. (1974). 



DENSITY 



Densities of structural flakeboard of southern hardwoods for sheathing and 

 floor decking generally are in the range from 42 to 50 pounds per cubic foot 

 (ovendry basis) depending on the percentage of oak and hickory they contain and 

 on the mechanical properties required. Inclusion of significant proportions of the 

 dense woods, and/or requirement of high strength, result in dense panels. Densi- 

 ty profile across panel thickness varies with pressing cycle (figs. 24-27 and 24- 

 28). The relationships between panel density and seven mechanical properties 

 are discussed in the following sub-sections. 



MODULUS OF ELASTICITY 



Structural flakeboard panels must support, without undue deflection, loads 

 incurred in service — such as snow loads on roofs and concentrated loads on floor 

 decking — and also loads imposed during construction, e.g., workmen carrying 

 bundles of shingles. Panel stiffness is a function of modulus of elasticity (MOE) 

 as well as panel thickness. Section 24-2 noted that modulus of elasticity (MOE) 

 in bending of construction exterior grade plywood in the United States is about 

 1 ,200,000 psi along the grain of face veneers and 500,000 psi across the grain. 

 For 2-MW waferboard and 2-MF flakeboard, the National Particleboard Associ- 

 ation (1980) requires an MOE of at least 450,000 and 500,000 psi, respectively 

 (see table 24-2). Deflection specifications for APA RATED STURDI-FLOOR 

 panels are given in table 24-3 and related discussion; those for APA RATED 

 SHEATHING in table 24-5 and related discussion. 



MOE of flakeboard panels is affected by a number of variables including 

 wood species, flake dimensions, flake orientation, resin content, mat moisture 

 profile, pressing procedure, and density profile. Hse et al. (1975) examined 

 these relationships as they relate to phenol-formaldehyde-bonded flakeboard 

 made from pine-site hardwoods; their conclusions and other pertinent literature 

 are summarized in the following paragraphs. 



Flake orientation. — Price (1974) provided basic information that confirmed 

 results of other workers and showed that MOE and ultimate tensile strength of 



