Fiberboards 



2853 



0.49 (1 part cell wall, 2 parts pore volume) the volumetric expansion is 13 

 percent; in the case of hardboard with a specific gravity of 0.97 (1 part cell wall, 

 Vi-part pore volume), it would be 27 percent, and in the case of a paper with no 

 pore volume at all, the volumetric expansion would be 40 percent. 



Densified products, generally, swell in the direction of densification, which in 

 fiberboard is in the direction of board thickness. Swelling in the plane of the 

 board is very small, due to the mutual restraint of the "cross laminated" fibers. 



Figure 23-81. — Relationship between specific gravity and volumetric swelling of solid 

 wood and densified wood products. (Drawing from Suchsland and Woodson 1985.) 



In addition to the increased relative expansion resulting from the reduction of 

 pore volume, densification causes swelling by springback, due to swelling 

 forces causing partial failures of bonds between fibers, and creating additional 

 void space. Part or all of this additional void space created during the swelling 

 process is permanent and will not disappear upon redrying of the board (figs. 23- 

 82 and 23-83). This adds substantially to the swelling of densified board such as 

 particleboard and hardboard and is often accompanied by a permanent strength 

 reduction. 



Heat treatment reduces swelling in two ways; it reduces water absorption by 

 the cell wall and helps resist the creation of voids during swelling by improving 

 fiber bonds. High temperatures are more effective than low (fig. 23-84). 



Linear expansion, i.e. , the dimensional changes in the plane of the board upon 

 moisture content change, and its modification are more complicated. Figure 23- 

 85 shows the linear changes of a wet-process hardboard being exposed to a 



