2852 



Chapter 23 



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DENSITY (G/CM^) 



Figure 23-80. — Effect of alignment on properties of fiberboard. Random refers to un- 

 aligned boards, x and y refer to alignment of the fibers in the two principal directions 

 of the board. The x direction is parallel to fiber alignment. (Left) Bending strength at 

 three density levels; values are average of three species. (Right) Linear expansion in 

 fiberboards of three species. (Drawings after Talbott 1974.) 



IMPROVEMENT OF BOARD PROPERTIES 



Dimensional stabilization. — Reduction of dimensional changes of wood 

 products resulting from adsorption and desorption of water in the cell wall (figs. 

 8-16, 8-17, and 8-19) is usually desirable, particularly in high-density products. 

 Volumetric expansion of wood approximately equals the volume of water ad- 

 sorbed by the cell wall. In densified wood products with reduced pore volume, 

 the same amount of water will still be adsorbed under a given exposure condi- 

 tion, resulting in a greater relative volumetric expansion. 



This is illustrated in figure 23-81 , where 1 cm"* of cell wall substance (specific 

 gravity 1 .46) at 100 percent relative humidity adsorbs 28 percent of its weight in 

 water (1 .46 x .28 = 0.40 g). This is the maximum amount of water the cell wall 

 can adsorb. Additional water uptake would fill the pore volume without further 

 swelling. The expansion of the cell wall by 0.4 cm"* results in different relative 

 volumetric expansion values, depending upon the total volume (cell wall plus 

 pore volume) of the product. In the case of solid wood with a specific gravity of 



