Fiberboards 285 1 



direction. Most particleboards and fiberboards randomize fiber orientation so 

 tiiat board properties in the plane of the board are the same in all directions. This 

 randomization sacrifices part of the strength associated with aligned fibers, 

 however. 



Mechanical orientation of elongated particles and flakes, so that their con- 

 figuration in the product is similar to that in solid wood, has reached commercial 

 application (Elmendorf 1965; Snodgrass et al. 1973). An electrical field can 

 also provide aligning forces to orient small fibers and fiber bundles in manufac- 

 turing dry-formed, high- and medium-density fiberboard. A fiber in a uniform 

 electrical field acts like a dipole (fig. 23-79). The force couple acting on the 

 dipole develops a torque tending to rotate the fiber axis to a position parallel to 

 the electric field. The torque is largest at 6 = 45°. At angles near 90° the induced 

 charge separation does not occur in the direction of the dipole axis, and at 

 smaller angles the moment arm diminishes. An alternating field has a similar 

 aligning effect when the frequency is low enough to allow a reversal of the 

 charge separation between field changes. Frequencies of less than 100 cycles/ 

 second are most effective. Other important variables are the geometry of the 

 particle or fiber, the moisture content and the strength of the field. 



Talbott and Logan (1974) described a forming machine in which resin coated 

 fibers having a moisture content of 9 to 15 percent fall through a series of 

 closely-spaced vibrating strings and enter an electric field of 60 cycles/second 

 and 1,500 to 3,750 volts/inch, causing substantial alignment of the descending 

 fibers. The air-fiber mixture moves through the field at about 50 to 100 feet/ 

 minute. At the bottom of the forming box the fibers are filtered out of the air 

 stream by a screen. Properties of experimental dry formed aligned fiberboards 

 are shown in figure 23-80. 



Industrial application of this method would benefit siding products (reduced 

 linear expansion in the long dimension of the product) and might encourage the 

 use of fiberboard in structural applications. 



23-10 HEAT TREATMENT, TEMPERING, AND 

 HUMIDIFICATION 



Heat treatment by exposure to dry heat, and tempering (heat treatment 

 preceded by adding drying oils to the pressed board) are optional processing 

 steps following hot pressing to improve dimensional stability and to enhance 

 mechanical board properties. These treatments are used only in the manufacture 

 of thin medium- and high-density fiberboards. Insulation board, thick medium- 

 density fiberboard, and particleboard are neither heat treated nor tempered. 



Humidification adds water to bring fiberboard moisture content into equilib- 

 rium with expected air conditions in service, and follows heat treatment or 

 tempering. In general, heat treating and tempering are more effective on wet- 

 formed than on dry-formed board. Heat-treated, and particularly tempered, 

 boards are substantially more expensive than untreated boards. 



