2752 



Chapter 23 



-~V 



Figure 23-6. — Orientation of fibers in fiberboard. A. Random in plane of board, no 

 vertical component. B. Random in plane of board, small vertical component. C. Ori- 

 ented in y direction, small x component, no vertical component. (Drawing from Suchs- 

 land and Woodson 1985.) 



23-6 PULPING PROCESSES 



For manufacture into fiberboard, hardwood chips are pulped mechanically. 

 The pulping process may be aided by thermal softening of the lignin-rich middle 

 lamella, but no chemicals are added to "dissolve lignin or other wood compo- 

 nents. Thermal softening may, however, increase water solubility of hemicellu- 

 loses, thus lowering pulp yield for wet-process board to values significantly less 

 than 100 percent. High temperatures or prolonged thermal treatment, while 

 more effectively promoting natural bonding during fiber mat consolidation, 

 increase dissolved sugars in process effluent water. 



Mechanical pulping is energy intensive, typically accounting for about half of 

 the total energy expended in fiberboard manufacture. It is generally accom- 

 plished in two stages. In the first, and most energy intensive stage, chips are 

 reduced to fiber bundles; the second stage completes fiberization and reduces 

 variations in resulting pulp. 



Pulping method and applied energy affect degree of fiberization; incomplete 

 fiberization yields fiber bundles, while over refining may cause broken or split 

 fibers. Desired characteristics of the pulped fiber vary with fiberboard product. 

 The three primary pulping methods appropriate for hardwood fiberboard pro- 

 duction are as follows: 



• Masonite explosion process 



• Atmospheric disk refining 



• Pressurized disk refining 



