Fiberboards 2779 



rates ranging from 1/2 to 1 Vi percent. The lower the board density, the more oil is 

 added, as follows (Eustis^): 



Board density Oil addition 



Pounds/cubic foot Percent of dry fiber weight 

 65-70 3/4 



50-55 1.5-2 



Both U.S. Gypsum Corp. and Abitibi Corp. use ferric sulphate, which is more 

 corrosive than alum, to precipitate the oil emulsions, producing a characteristic 

 grey pulp which turns to grey-brown in the hot press. It also results in bending 

 strengths at least 10 percent higher than boards in which the oils are precipitated 

 with alum. The fixation of the oils on the fiber seems to be purely mechanical. 

 Losses are therefore great and closed water systems (recirculation of machine 

 white water) become imperative (Eustis^). Much higher oil quantities are used in 

 so-called slush ovelays — thin layers of highly refined pulp applied on top of the 

 regular mat by means of a secondary headbox. Oil contents can be as much as 6 

 percent, but only about 1 Vi percent is retained. In closed white water systems the 

 lost additives build to a constant level and circulate, only retained quantities 

 being added. 



Other mills vary considerably in their use of binders. Temple East-Tex pro- 

 duces wet S2S siding with only linseed oil and size. No resin is added. Boise 

 Cascade at International Falls, Minn, produces wet S2S siding (Insulite) using 5 

 percent, based on dry fiber weight, of a high melting point thermoplastic resin 

 derived from naturally occurring asphalt (Gilsonite); in this operation wax is 

 used as size and alum as precipitant. Weyerhaeuser Co. at Craig, Okla. produces 

 a wet-process S2S siding with a similar thermoplastic binder. The siding pro- 

 duced by Abitibi Corp. incorporates a thermoplastic resin made from pine rosin 

 (Vinsol); it is an SIS product. 



To prevent these thermoplastic resins from sticking to hot-press platens, 

 release agents such as diesel fuel, kerosene, silicones, and urea are applied to the 

 mat. 



Dry-formed hardboard and medium-density fiberboard. — The standard 

 binder for dry-formed hardboard is phenol-formaldehyde liquid resin, added 

 in proportions greater than in wet-formed hardboard where phenolics and other 

 binders play a secondary role. Wax is the usual sizing agent. As an example, 

 Weyerhaeuser Co. at Klamath Falls, Oregon produces dry-formed siding with 6- 

 percent phenolic resin content and V^-percent wax (based on dry fiber weight) 

 applied before the dryer (fig. 23-26 top). In other mills, liquid phenolic resin is 

 introduced through the disk refiner hallow shaft, so that it is applied to the chips 

 as they are fiberized between the plates. Binder added to dry furnish must not 

 have tack (stickiness) or the fiber will lump together causing uneven distribu- 

 tion so that deposition of the furnish in the forming machine is difficuU or 

 impossible. 



The development of the dry medium-density fiberboard process in the 

 1960's was based in part on a so-called in-situ resin which was a combination 

 melamine-urea formaldehyde resin of low molecular weight, low tackiness, and 

 low viscosity which condensed after application to the furnish. Later, standard 



