2838 



Chapter 23 



178 



BOARD UNIT WT. KG/M^ (LBS./FT.^) 



Figure 23-68. — Press daylight required for fiberboards with density of 47 pounds/cu ft, 

 related to bulk density of the uncompressed fiber. (Drawing after Chryst and Rudman 

 1979.) 



pounds/cu ft). Such pulp is produced only in pressurized refiners (sec. 23-6). 

 Low bulk density of the furnish poses special problems in handling, transporta- 

 tion, and storage, and requires presses with sufficient daylight (maximum clear- 

 ance between plates) to accommodate the thick mats (fig. 23-68). These press 

 openings are dimensioned to accommodate the precompressed mat. Mats before 

 precompression are much thicker (9 to 24 inches). Caulless systems transport the 

 precompressed mat into the press by loader trays; such systems require an 

 additional 3 inches of daylight. 



A second unique element is the binder formulation. Since any resin with a 

 high tack (stickiness) would prevent uniform distribution of the furnish, so- 

 called in situ resin systems were developed by Allied Chemical Company in the 

 early 1960's. Their low tackiness and viscosity make them suitable for applica- 

 tion to bulky dry fiber furnish. The precondensation of these in situ resins is 

 terminated at a very low molecular weight, which reduces their tackiness. The 

 condensation is completed in the hot press. 



