2774 Chapter 23 



FIBERBOARD BINDERS 



Lignin is potentially the most important binder in fiberboard manufacture. If it 

 is exposed by the pulping process and "activated" in the hot press, additional 

 binders may not be necessary. Masonite, for example, makes wet-formed hard- 

 boards without additional binders. Most fiberboard manufacturers, however, 

 use added binders to either enhance the lignin bond or establish artificial bonds 

 in the absence of lignin bonding, as follows — listed in order from least to most 

 binder content: 



Process and product Primary bond Secondary bond 



Wet process 



Insulation board Hydrogen Starch (com, rye, or potato), 



asphalt 



SIS Masonite Lignin — 



SIS Lignin Phenolic thermosetting, 



thermoplastics 



S2S Lignin Thermoplastics, drying oils 



MDF Lignin Thermoplastics, drying oils 



Dry process 



S2S Thermosetting phenolics — 



MDF Thermosetting ureas — 



Wet-formed S2S fiberboards cannot use thermosetting resins such as pheno- 

 lics because in wet-forming the mat must be dried at high temperature before it is 

 hot pressed, and thus would be precured before mat consolidation. Instead, 

 thermoplastic resins such as Vinsol (derived from pine rosin) and Gilsonite 

 (from naturally occurring asphalt) are employed. Drying oils such as linseed, 

 tung, tall, and soybean oils are also used as binders in the wet S2S process — 

 alone or in combination with thermoplastic resins. 



In SIS wet-formed hardboard, water is first pressed from the mat without 

 application of heat, and then the mat is consolidated in a hot press with a screen 

 on one side of each board so steam can escape. For such boards a water-soluble, 

 highly condensed thermosetting phenolic resin of high pH can be precipitated on 

 fiber surfaces to form bonds during hot pressing. 



Dry-formed hardboard and medium density fiberboards rely entirely on added 

 resin binders, since their processes do not provide conditions under which the 

 lignin bond can be utilized. These resins are cured during hot pressing. 



Resin binders and drying oils not only bond fibers, but also size them. Drying 

 oils size by surface modification; resins additionally impart stability by restrain- 

 ing swelling through improved bonding. Penetrating resins further enhance 

 stability. Thickness swelling can be almost completely restrained by addition of 

 sufficient resin (fig. 23-24), but only at levels uneconomic for standard hard- 

 board manufacture. Normal phenolic resin levels are about 1 or 2 percent in wet- 

 formed hardboard, and up to 5 or 6 percent in dry-formed hardboard. Most 

 strength properties and soprtion characteristics improve little at resin content in 

 excess of 3 percent (American Marietta Company n.d.). Urea-bonded medium- 

 density fiberboard made by the dry process commonly requires 8 to 11 percent 

 resin content. 



