Fiberboards 



2781 



urea resins with low tack were used as well. These binders, which must be 

 introduced after the fibers are dried (fig. 23-26 bottom), are applied in short- 

 retention continuous blenders (fig. 23-27). The retention time in such blenders is 

 1 to 3 seconds. The resin is injected into the fiber mass through radially arranged 

 injection tubes. Typically, resin contents range from 8- to 10-percent resin 

 solids, based on dry fiber weight. 



The pH of the dry fiber furnish is generally not controlled. However, pH of 

 the furnish is taken into consideration in binder formulation, since it affects 

 curing rates. Meyers (1977) found that modification of the pH of dry fiber 

 furnish by spraying it with either a 1- to 2-percent solution of sulfuric acid for 

 downward adjustment or with a 5- to 10-percent solution of sodium bicarbonate 

 for upward adjustment, had significant effects on mechanical and physical 

 properties. He suggested that such treatments could counteract the refractory 

 gluing properties of oaks and certain other species. 



23-8 WET-PROCESS FIBERBOARD MANUFACTURE 



Pulping procedures in fiberboard plants vary significantly according to board 

 product selected for manufacture. Other manufacturing procedures also vary 

 with product. Thus, a fiberboard plant is designed to produce either wet- or dry- 

 formed board, and is generally limited to one or two classes of product. Wet and 

 dry processing differ from forming through pressing. This section describes the 



Figure 23-27. — Short-retention continuous blender. 



