Fiberboards 



2777 



PRIM. REFINER 



STOCK CHEST 



STOCK CHEST 



STOCK CHEST 



I 



STOCK CHEST 



I 



SEC. REFINER 



»^.°.^„o-^"f "lyp-^i^^^ 



j PUMP 



H20 H20 H20 



Figure 23-25. — Schematic of stock flow in a wet-process fiberboard plant, showing pH, 

 stock consistency, and introduction of additives. (Drawing from Suchsland and 

 Woodson 1985.) 



In the next stock chest the consistency is reduced to about 3.5 percent. The 

 diluting water can be fresh water, but generally is machine white water (water 

 removed from the stock during sheet forming on the Fourdrinier machine). 

 Recirculation of machine white water reduces water treatment requirements and 

 recovers and reintroduces chemical additivies not retained in the mat. Size, 

 binder, and other chemicals are added in this stock chest and mixed. 



In the following stock chest the pH is reduced to about 4.5 by the addition of 

 alum, to precipitate the chemical additivies. Secondary refining occurs next, 

 followed by further dilution of the stock to forming machine consistency (2 

 percent). In the last stock chest prior to the forming machine, sulfuric acid may 

 be added to finally adjust pH. 



Application of extra resin (up to 5 percent of dry fiber weight) into mat edges 

 during forming firms edges, reduces springback, and allows minimal press 

 cycles (Eustis^). Steinmetz and Fahey (1968) found that addition of extra resin to 

 wet mat surfaces improved dimensional stability in experimental boards, but 

 resin levels required were very high. 



Wax emulsion for improving water resistance is added to S IS furnish in very 

 small quantities, from 1/10- to 1/2-percent based on dry fiber. Wax emulsion 

 may also be added in diluted form by spraying it on the surface of the mat on the 

 forming machine. The emulsion spray acts as a defoamer as it breaks surface 

 bubbles and is then sucked into the mat by vacuum. In other cases, molten wax is 

 added to very hot stock (180°F) or applied to the chips. However, this may cause 

 the formation of wax drops which appear on the board surface as spots that 

 interfere with finishing operations. Bark of some species, e.g., Douglas-fir 

 (Pseudotsuga menziesii (Mirb) Franco) contains sufficient wax that its incorpor- 

 ation into SIS wet-process hardboard may preclude need for additional size. 



Insulation board. — Insulation board is sized with wax in emulsified form 

 (0.75 to 1 .25 percent dry wax based on dry fiber weight), sometimes in combi- 

 nation with rosin, and with powdered asphalt ( 10 to 15 percent based on dry fiber 



