2778 Chapter 23 



weight). Rosin has a higher melting point than wax, and requires higher dryer 

 temperatures to effectively coat fiber surfaces. 



Starch ( 1 to 2 percent of dry fiber weight) is slurried in water and pumped into 

 insulation board pulp just ahead of the forming machine; starch imparts strength 

 to insulation board, but is attractive to rodents and insects. 



SIS wet-process fiberboard. — Standard additives in SIS board manufacture 

 are phenolic resins and wax. Resin is added in quantities of Vi to 1 percent, in 

 some cases up to 2 percent, depending on length of press cycle. At minimum 

 press cycles resin cure is incomplete. It can be completed during subsequent heat 

 treatment, but the board will suffer substantial springback (immediate thickness 

 expansion upon removal from press), particularly along the board edges, where 

 temperatures in the press are lower than that at the board center. This can be 

 compensated for by increasing the resin content. There is, therefore, a trade-off 

 between resin content and press time, i.e., between resin cost and productive 

 capacity. When resin costs are high, SIS boards will remain in the press longer 

 to minimize springback due to lowered resin content. 



Abitibi Corp., at their Alpena, Mich, plant, uses ferric sulphate as a precipi- 

 tant in place of alum. Ferric sulphate is more corrosive than alum, but produces a 

 dark grey color in the board which is a desirable background for printed paper 

 overlays applied directly to the wet mat in their wall-panel manufacturing line 

 (Eustis^). 



Preparation of furnish for forming machines traps air and causes the stock to 

 foam, particularly at higher temperatures. Tiny air bubbles, beaten into the stock 

 in refiners and mixers, can reduce freeness by 10 to 15 percent, cause tanks to 

 overflow, and interfere with the forming process. In insulation board and wet 

 S2S plants, where efficient water removal on the forming machine is very 

 critical because of the high energy cost of removing the remaining water in the 

 dryers, this air is removed from the stock in deculators, which are vacuum tanks 

 in which the air is boiled off and removed. 



SIS lines do not normally use deculators because water remaining in the mat 

 after forming can be squeezed out mechanically in the first part of the hot press 

 cycle. Instead these plants add defoamers just ahead of the forming machine, to 

 break up air bubbles in the stock. In the past kerosene was used. Currently, 

 special defoamers are available, which reduce the surface tension of the water 

 and cause the air bubbles to break. Too much defoamer, however, interferes 

 with sizing of the sheet (Eustis^). 



S2S wet-process fiberboard. — Wet-formed S2S boards cannot be bonded 

 with phenolic resin binders, because they are thermosetting and would cure 

 during drying of the mat prior to hot pressing. Binders in these boards are in-situ 

 lignin, drying oils, or thermoplastic resins, depending on the final use of the 

 product. 



Masonite makes wet-formed S2S interior-application boards with the addition 

 of wax size only. Masonite siding is made by the SIS process and uses phenolic 

 resin. Other "interior" wet-S2S boards (Abitibi, U.S. Gypsum) are made with 

 the addition of drying oils, linseed oil being the most common one. These oils 

 are emulsified by stirring them with caustic soda; they are added to the stock at 



