Fiberboards 



2765 



OPERATION VARIABLES PULP PROPERTIES 



(CHIP QUALITY) 

 (PLATE PATTERN) 

 (PLATE AGE) 

 (CONSISTENCY) 

 THROUGHPUT RATE 



I / 



SPECIFIC ENERGY 



SPECIFIC SURFACE 



FREENESS (DRAINAGE TIME) ^ 



SHEET PROPERTIES 



BURST INDEX 



■♦BREAKING LENGTH 



FIBER LENGTH 4- 



■♦ TEAR INDEX 



L FACTOR 



Figure 23-18. — Interrelationships between specific energy and other pulp and sheet 

 properties. (Drawing adapted from Johnson and El-Hosseiny 1978.) 



Freeness of pulp on the forming machine is the primary indicator for control- 

 Ung refiner operation. Freeness can be increased by opening the plate gap, which 

 also increases throughput and decreases specific energy. Typical specific ener- 

 gies for three major classes of fiberboard are about as follows: 



Product Specific energy 



Dry-formed fiberboard 



Insulation board 



SIS and S2S wet-formed hardboard 



Hp day si ton of pulp, ovendry basis 

 10-11 

 20-30 

 20-30 



Pressurized refining. — Pressurized refiners, of which the Asplund defibra- 

 tor (fig. 23-19) is the prototype, refine chips in an atmosphere of saturated steam 

 under pressure. They are comprised of preheater, disk mill, and infeed and 

 outfeed devices that maintain internal steam pressure. Figure 25-20 depicts the 

 significant drop in power required to fiberize hardwood and softwood chips 

 when internal refiner temperature exceeds 300 to 340°F. This reduction of power 

 is attributed to softening of lignin, easing mechanical separation of fibers along 

 the middle lamella. 



Process sequence in an Asplund Defibrator (refer to fig. 23-19 for machine- 

 component numbers) begins at the chip infeed chute (1) where green chips (2) 

 are screw fed (3) to form a moving conical non-rotating, compact plug (4) 

 sufficiently dense to block escape of steam from the preheater (6). The plug falls 

 apart as it enters the steam atmosphere of the preheater which is about 10 feet 

 high with inside diameter of about 2 feet. After a minute or less in the preheater, 



