Fiberboards 



2755 



• The Canadian Standard Freeness Tester (CSF), standard in North Amer- 

 ica for paper pulp and described by TAPPI Standard T-227, is a device 

 into which the pulp sample (3 g in 1,000 ml of water) is placed. The 

 mechanism is devised so that water from the pulp will overflow through a 

 side orifice into a calibrated container yielding a numerical value from 

 to 1,000 ml; the higher the number, the faster the pulp. 



• Insulation board and hardboard plants in the United States generally use 

 the TAPPI Standard SFMC draining tester, which measures the number 

 of seconds to drain a pulp sample (10.6 g in 1 ,000 ml of water) through a 

 40-mesh screen. The higher the number the slower the pulp. Water has a 

 drainage time of 1 .5 seconds, SIS hardboard pulp 15 to 20 seconds, and 

 insulation board pulp 50 to 60 seconds. 



Freeness can be correlated, by experiment in each mill, to processing charac- 

 teristics; it is therefore an important parameter useful to control product quality. 

 In general, the larger the surface area of a weight of pulp (specific surface), the 

 slower the pulp. Pulp specific surface can be increased by more extensive 

 refining to make fibers more ribbon-like and to fibrillate (shred) their ends. 

 Fibrillation of fibers yields a slow pulp but promotes hydrogen bonding, impor- 

 tant in the manufacture of insulation board. Wet-formed hardboards do not rely 

 on hydrogen bonding for their strength, and because of their much thicker mats, 

 require fast-draining or free pulps. 



Dry fiberboard processes do not require draining water from mats, and for 

 these processes freeness is not monitored. 



MASONITE PULPING PROCESS 



The Masonite pulping process, originally used on southern pine but now 

 applied to southern hardwoods at the company's Laurel, Miss, plant, uses the 

 heat of steam to soften the chips and its explosive force to defibrate them. Only 

 the Masonite Corporation uses this process for the manufacture of pulp for 

 fiberboard. (See concluding paragraphs of sect. 26-7 for use of a similar process 

 to make animal food.) In all other mechanical pulping processes, fiberization is 

 accomplished by abrading or cutting tools. 



The Masonite gun. — The Masonite gun — so called because of the explosive 

 nature of the defiberizing phase — is shown in cross section in figure 23-9 as it 

 was described by W. H. Mason in 1927. The gun has not changed much in the 

 intervening years, although its operation is now automated. It is a pressure 

 vessel measuring about 5 feet high and 20 inches in inside diameter with a 

 capacity of about 10 cubic feet to accommodate about 200 pounds of chips 

 admitted from the top. The tapered bottom end is equipped with a slotted port 

 and quick-opening discharge valve. 



After charging with chips, low-pressure steam (350 psi, 450°F) is admitted to 

 heat the chips to about 375°F, and maintain them at this temperature for 30 to 40 

 seconds. High-pressure steam (1 ,000 psi, 540°F) is then admitted over a 2- or 3- 

 second period and pressure held for about 5 seconds. The hydraulically actuated 

 discharge valve is then quickly opened and the chips explode due to sudden 



