Structural Flakeboards and Composites 29 1 9 



24-3 REVIEW OF PROPERTIES OF PINE-SITE 

 HARDWOODS 



Inclusion of hardwood bark in flakeboards usually diminishes their modulus 

 of rupture, modulus of elasticity, internal bond strength, and screwholding 

 capability (Murphey and Rishel 1969; Gertjejansen and Hay green 1973; Kehr 

 1979; Starecki 1979; Wisherd and Wilson 1979; Rishel et al. 1980). In most 

 cases, therefore, it is the properties of bark-free stemwood that are of interest to 

 manufacturers of structural flakeboard. 



Any plan for broad usage of the southern hardwood resource for flakeboard 

 manufacture must take into account the species mix. This mix varies significant- 

 ly with region, but south wide averages are indicative of some of the problems. 

 Oaks comprise 47 percent of the 1 2-State volume of hardwoods growing among 

 southern pines. Sweetgum and white oak are the most plentiful species, and 

 hickory is third in abundance. (See discussion related to tables 2-7 through 2- 

 18.) 



In pine-site hardwoods measuring 6 inches in dbh, stemwood specific gravity 

 based on ovendry weight and green volume ranges from 0.40 in yellow-poplar to 

 0.67 in post oak; the average for all hardwood species, weighted by volume of 

 occurrence in the southern pinery, is 0.569. Density of stemwood (weighted 

 ovendry basis) therefore averages 35.5 pounds per cubic foot in these small trees 

 (table 7-7). 



Sweetgum has the highest moisture content at 120.4 percent of ovendry 

 weight, and green ash the lowest at 47.4 percent; weighted average for stem- 

 wood of all the pine-site hardwood species in trees measuring 6 inches in dbh is 

 79.3 percent (table 8-2). 



Anatomy of the many species varies significantly. Impermeable woods such 

 as post oak and white oak differ from more permeable woods in optimum wood 

 moisture content for gluing, glue spread, and assembly time. Researchers have 

 not yet fully explained, however, the reasons why post oak and white oak flakes 

 are so difficult to bond. 



The chemical makeup of stemwood of the pine-site hardwoods also varies 

 significantly (tables 6-1 through 6-8), and stemwood pH varies from 4.35 in 

 northern red oak to 5.74 in American elm (table 6-23). These chemical vari- 

 ations may cause bonding problems. 



Mechanical properties of some of the woods are exceptional. Nine of the 

 species (five oaks and four hickories) have modulus of elasticity values (table 

 10-6) higher than that for loblolly pine — a premium structural wood; modulus of 

 rupture of wood of these nine species is in the range from 12,600 to 20,200 psi. 

 Average modulus of elasticity of stemwood of all 22 of the principal hardwood 

 species found on southern pine sites, weighted by the volumes in which they 

 occur and measured at 12-percent moisture content, is 1,705,0(X) psi. 



From this brief review of wood properties significant in flakeboard manufac- 

 ture, it is evident that special technology is required to incorporate all these 

 species — or a significant number of them — into a satisfactory product. The 

 balance of the chapter describes this technology. 



