Structural Flakeboards and Composites 2939 



While effective on aspen flakes, powdered resins have been much less suc- 

 cessful when applied to southern hardwoods such as oaks, hickories, and sweet- 

 gum. By the early 1980's, therefore, liquid resin was preferred by those with 

 most experience in fabricating structural panels from southern hardwood flakes. 



Powdered resin probably fails on southern hardwoods because these species 

 are less compacted than aspen in flakeboard pressing, and their cure times are 

 short. Powdered resins applied to flakeboards comprised of 60 percent oaks and 

 40 percent low density southern hardwoods require high compaction ratios, or 

 long cure time, or both, as follows: 



Liquid resins. — Numerous liquid resins of the resol type were prepared and 

 tested by Hse (1975b, 1978) in a search for an economical formulation effective 

 in gluing flakeboards of mixed southern hardwoods. Of those evaluated, the 

 phenol-formaldehyde resin that performed best was formulated as follows (fig. 

 24-18). 



Statistic Value 



Reaction concentration, percent by weight 47.5 



Molar ratio of sodium hydroxide to phenol 0.45 



Molar ratio of formaldehyde to phenol 



(first addition of formaldehyde) 1.5:1 



Molar ratio of formaldehyde to phenol (in second 



addition after 2.5 hours of reaction time) 0.3:1 



Reaction temperature 95°C 



This resin yielded satisfactory bonds in laboratory panels made of mixed south- 

 em hardwoods with: 



• A minimum resin-solids content of 4 percent of weight of ovendry 

 flakes 



• A maximum mat moisture content of 14 percent 



• A hot press temperature of 325°F 



• A minimum hot press time (closed) of 4 minutes 



Subsequently Hse (1975c), using a very similar resin, evaluated the internal 

 bond strength and stability of 1/2-inch-thick panels of nine species of southern 

 hardwoods pressed to three densities. Within the range of the experiment, all 

 species — except white oak and post oak — yielded panels of acceptable internal 

 bond strength and dimensional stability at panel density, ovendry- weight basis, 

 of 44.5 pounds per cubic foot or less (table 24-9). The flakes used in this 

 experiment were identical to those shown in figure 18-264. 



