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Chapter 23 



fibers from wet mats, allowing faster line speeds, and shorter drying and press 

 cycles. Dry-formed hardboards and medium-density fiberboards are less sensi- 

 tive to species characteristics than wet- formed boards. 



In the North, aspen is a preferred species for hardboard and MDF. Cotton- 

 wood, willow, and yellow-poplar in the South compare favorably with aspen as 

 fiberboard furnish except that willow makes a darker board. These woods have 

 longer and stronger fibers than most southern hardwoods, yielding strong mats 

 less subject to handling damage during manufacture — a factor particularly im- 

 portant in the S2S, wet hardboard process. 



Medium-density southern hardwoods such as elms, ashes, hackberry, black 

 tupelo, and sweetgum vary considerably in color (figs. 5-4 through 5-16) and 

 when used in mixtures may produce more large fiber bundles (shives) and a 

 greater range in fiber-bundle diameters than a single-species furnish. More 

 uniform defibration of mixtures can be accomplished through adjustment of 

 cooking cycles and increasing energy input to primary disk refiners. Mainte- 

 nance of uniform species composition is required for close control of board 

 properties (fig. 23-4) — an idea easy to express but difficult to execute. 



Oak species are the most difficult to incorporate successfully into fiber- 

 boards — particularly as whole-tree chips. Oak bark is highly acid and interferes 

 with the settling of suspended solids in waste- water treatment systems, and 

 wears out dust pipes in the dry process. Oak chips steam-cook quickly, are 

 readily overcooked, and yield a short-fibered slow-draining pulp. Pure oak 

 hardboards and insulation boards are brittle. Oak is therefore mixed with other 

 species. A hardboard siding, for example, is being successfully manufactured 

 from a 50-50 mix of oak and southern pine. In the North, a 50-50 oak-birch 

 mixture is used to manufacture a paper-overlaid SIS hardboard. 



Hickory species are about as difficult to incorporate as oak. 



WOOD DENSITY AND BOARD PROPERTIES 



Except in insulation board, fiberboard density is determined by the degree of 

 compaction of the mat during pressing. High hardwood specific gravity will 

 generally result in higher bulk density of the fiber and, at a given board density, a 

 lower compaction ratio (board density/wood specific gravity). But low compac- 

 tion ratios lead to poor contact between fibers and lowered strength. In dry- 

 formed hardboard, therefore, strength properties are negatively correlated with 

 wood specific gravity and with mat bulk density (fig. 23-5). Wood specific 

 gravity has little effect on dimensional stability, however. 



FIBER LENGTH 



Fiber length has little effect on the mechanical properties of dry-formed 

 hardboard, but does affect dimensional stability in the plane of the board. Long 

 fibers promote board stability during water absorption or desorption. Long fibers 

 also yield strong mats, which are needed for high-speed handling. 



Length of fibers may also be a factor controlling their orientation in fiber- 

 boards. Short fibers are much more likely to develop a vertical or Z-component 



