Fiberboards 277 1 



process. Elevated temperatures (but not above 140°F) and a pH of about 5 favor 

 efficient washing. Vacuum washers may be arranged in series for countercur- 

 rent washing; in such arrangements the wash water runs in a direction opposite 

 to the pulp so that the filtrate of the last stage is used as wash water and dilution 

 water for the next to last stage, and so on, resulting in a concentrated filtrate 

 discharge from the initial stage. Pulps are discharged from vacuum washers at 

 consistencies of 12 to 14 percent. 



Continuous rotary presses (fig. 23-22 bottom) remove more water from 

 pulp than vacuum washers and discharge at consistencies up to 75 percent. They 

 dewater pulp by forcing it into a conical cage with a tapered screw which applies 

 pressure of several thousand psi. Water, forced through the screen cage into the 

 machine housing, is then removed. 



23-7 CHEMICAL ADDITIVES 



Chemicals are added to fiberboard furnishes for seven reasons, the first four of 

 which are common to most fiberboard processes, as follows: 



• Control of pH 



• Increased water resistance (sizing) 



• Enhancement or establishment of fiber-to-fiber bonds 



• Control of process (defoamers, release agents) 



• Protection of fiber from decay and insect attack 



• Fire protection 



• Coloration 



They are added in small quantities because they are costly and because their 

 presence, while enhancing one desired property, may be detrimental to another. 

 Chemicals that increase water resistance, for example, may interfere with fiber- 

 to-fiber bonding. 



SIZING OF FIBERBOARD 



Sizing is the process by which individual fibers are coated with a hydrophobic 

 chemical to reduce product water absorption, thereby diminishing its linear 

 expansion, thickness swelling, surface deterioration, and strength loss caused by 

 swelling of wood fibers. In dry-process fiberboards, the size is applied directly 

 to either chips or fibers, generally together with the resin binder required in all 

 such boards. 



In the manufacture of all wet-process fiberboards, sizing is accomplished in 

 two steps: first, the sizing chemical is mixed in the water-diluted pulp; then a 

 precipitant is added which causes the size to floe and fix to fiber surfaces. 



Rosin size. — Commonly used in paper manufacture, and to some extent in 

 insulation board, most rosin is obtained from living southern pines (gum rosin), 

 extracted from resin-rich stumpwood (wood rosin), or obtained by fractional 

 distillation of tall oil, a by-product of southern pine kraft pulp mills (tall oil 

 rosin). Rosin size, prepared by saponifying molten rosin through addition of 



