Fiberboards 



2747 



drying oils as binders, the pH is reduced (often by addition of sulfuric ascid) and 

 then aluminum sulphate or paper makers, alum (Al2(S04)3) is added as a precipi- 

 tator. The aluminum ion, reacting with the sizing material, forms an insoluble 

 precipitate on the fiber surface which reduces water absorption and improves 

 fiber bonding. 



In dry-process fiberboard manufacture there is no control of fiber pH except 

 that provided by catalysts added to resins just prior to resin application. 



Bonds between fibers in insulation board and in paper, both of which are wet- 

 formed, are enhanced by close contact resulting from water surface tension as 

 the water evaporates; addition of an adhesive to these products is usually not 

 necessary. Most fiberboards, however, require addition of an adhesive, which in 

 liquid form contacts and interacts with surfaces of mating fibers, developing a 

 cohesive strength similar to that of the fibers it bonds together. Resin adhesives 

 such as urea-formaldehyde and phenol-formaldehyde are irreversibly solidified 

 by chemical reactions from a change in pH or the heat of hot pressing, or both. 

 Phenol-formaldehyde resins are used in the manufacture of hardboard, and urea- 

 formaldehyde resins for MDF. Phenol-formaldehyde resins form water-proof 

 and boil-proof glue lines. Urea-formaldehyde bonds have considerable water 

 resistance but are not considered exterior-type. Regardless of glue-line quality, 

 swelling of fibers during exterior exposure can cause permanent strength loss. 



Even without the use of the resin adhesives, bonds can be established between 

 fibers, although the mechanism of such bonding is not clearly understood. 

 Lignin-rich fiber surfaces, perhaps reacting with pentose hydrolysis products, or 

 perhaps softened by interaction of heat and water content, bond in some wet- 

 formed boards. Lignin bonds in dry-formed boards are more difficult to achieve. 



23-4 RAW MATERIAL 

 SPECIES 



Approximately half of the fiberboard plants in the United States are located in 

 the South (tables 23-1 , 23-2, and 23-3). Many of these plants use southern pine, 

 but southern hardwoods are also widely used — notably in several MDF plants 

 and in the large hardboard facility at Laurel, Miss. 



The second insulation board plant in the United States was built in 1931 in 

 Greenville, Miss, and used 100-percent cottonwood (it was later converted to 

 use mineral fibers only). Most industry experts believe that softwoods make 

 better wet-process insulation board than hardwoods, however, although soft- 

 woods require longer chip steaming cycles and more energy for mechanical 

 pulping than do hardwoods. Long, soft coniferous fibers drain well and provide 

 a strong pulp; fiber distribution within insulation board mats is not a problem as 

 it is with hardboards. 



For hardboards made by the wet process hardwoods are preferred because 

 their short fibers cause fewer high-density spots arising from uneven fiber 

 distribution, a defect termed cockle. Very short fiber elements (fines), however, 

 promote development of cockle because they resist water drainage. Also, hard- 

 woods make very springy fibers that release water more quickly than softwood 



