2746 



Chapter 23 



Table 23-3. — Medium-density fiherboard plants (MDF-dry) in the United States, 1983 

 (Dickerhoof and McKeever 1979, updated by authors, 1983) 



State 



City 



Company name 



Annual 

 production capacity 



Alabama Eufaula 



Arkansas Malvern 



California Rocklin 



Montana Columbia Falls 



North Carolina Moncure 



Spring Hope 



Oklahoma Broken Bow 



Oregon Medford 



South Carolina Holly Hill 



Marion 



Virginia Bassett 



New Mexico Las Vegas 



Total 



'Under construction, 1983; not included in total. 



23-3 SOME CHEMICAL ASPECTS 



Pulping processes are classified as mechanical (sect. 25-5), semichemical 

 (sect. 25-6), or full chemical (sect. 25-7). Only mechanical pulps, in which 

 fibers are separated by frictional force or by steam pressure, are used to make 

 fiberboards; no chemicals are added to dissolve intercellular lignin. Several 

 important chemical reactions do occur, however, during mechanical pulping and 

 subsequent manufacturing steps. 



Acid hydrolysis during the pulping stage causes a breakdown and subsequent 

 loss of part of the hemicelluloses. Acidity necessary for such hydrolysis is 

 developed by simultaneous formation of acetic and formic acids from wood 

 carbohydrates. The hydrolysis reaction not only reduces pulp yield, but loads 

 process water with biodegradable sugars which may create pollution problems. 



Condensation, in which two or more molecules combine, with the separation 

 of water, is important in the curing of phenolic resins used in fiberboard manu- 

 facture. This reaction takes place at elevated temperature in the hot-pressing 

 phase of manufacture, to form a permanently bonded three-dimensional network 

 in the fiberboard. Even without resin binders, cell wall components may under- 

 go condensation reactions to form bonds within the fiberboard. 



During fiberboard pressing and heat treatment, pyrolysis releases volatiles, 

 and further degrades products of hydrolysis from the pulping stage, and subse- 

 quently forms condensation products. These reactions darken the board and 

 reduce its hygroscopicity. The volatiles may cause pollution problems. 



Control of pH is necessary at two stages of wet-process fiberboard manufac- 

 ture. Mechanical pulp leaving a refiner has a pH of less than 4 (slightly acidic); 

 to more efficiently remove dissolved solids, pH in the pulp washer is increased 

 to about 5 by adding fresh water or caustic soda. After addition of sizing 

 chemicals such as waxes and asphalt to resist water, and phenolic resins and 



