Fiberboards 



2819 



density range by a process almost identical to that for medium-density thin 

 boards, but are governed by a different commercial standard and are sold in 

 entirely different markets. 



Any comparison of wet- and dry-process fiberboard manufacturing must 

 therefore be limited to thin boards. Besides the two important attributes men- 

 tioned above, the dry process has the following technological limitations and 

 advantages: 



Limitations of dry-process thin boards. — 



• elimination of hydrogen bonds 



• elimination or substantial reduction of lignin bonds 



• absolute necessity of resin binder addition 



• difficult control of uniform fiber deposition 



• furnish handling and storage difficulties due to low bulk density 



• greater fire danger 



• air pollution problems 



• inferior board surface 



• greater linear expansion 



Advantages of dry-process thin boards. — 



• S2S surfce 



• higher yield 



• possibility of making multi-layer board 



• reduced sensitivity to species characteristics 



• possibility of using automatic thickness and density control devices 



• absence of bias (difference in properties in the two principal directions) 



• high internal bond strength 



None of these limitations and advantages are decisive enough to cause a shift 

 from wet process to dry process or vice versa, since most can be compensated 

 for, where necessary, by adjusting process variables and by improved proc- 

 essing equipment. 



Once expected to dominate future fiberboard manufacture in North America, 

 the dry process has encountered growing costs and power needs for air-pollution 

 abatement. Also, increases in oil prices have substantially increased costs of 

 fiber drying and resin. At the same time, the wet-process manufacturers have 

 reduced their effluent disposal problem by better white water circulation sys- 

 tems. These developments have kept the wet-process competitive (FAO 1976). 



A direct comparison of capital and manufacturing costs of wet- and dry- 

 process plants, manufacturing !/8-inch hardboard and 7/16-inch medium density 

 siding is given in tables 23-6 and 23-7. An analysis of the manufacturing cost 

 estimates indicates the following: 



• Chemical costs are signifcantly higher for the dry process than for the wet 

 process. 



• The wet process has slightly higher power costs but lower fuel 

 requirements. 



