2772 Chapter 23 



sodium hydroxide or sodium carbonate, can be added directly to the pulp or 

 emulsified (pH 9 or 10) before addition. Water resistance imparted increases 

 uniformly with increasing content of rosin size up to about 2 percent; above 2 

 percent the effect lessens, and size content exceeding 3 percent (ovendry-weight 

 basis) is of little benefit (Swanson et al. 1971). 



The precipitant, paper makers' alum (aluminum sulphate Al2(S04)3) diluted 

 to contain 1 or 2 pounds of dry alum per gallon of water, is added in a quantity 

 sufficient to reduce the pH of the thoroughly mixed pulp and size to about 4.5. 



Wax size. — Waxes, hydrocarbons of high molecular weights (300 to 700) 

 derived from crude oil residuals or distillates, having melting points from 120 to 

 200°F, are insoluble in water, and are inert. Wax sizes are prepared by melting 

 the wax and then emulsifying it in water to yield two types: acid-stable and non- 

 acid-stable (table 23-4). 



In the manufacture of fiberboard, wax sizes are used to improve water resis- 

 tance. For wet-process boards, emulsified and homogenized wax sizes are added 

 to the watery pulp at temperatures below the melting point of the wax, and 

 precipitated with alum. Effectiveness of rosin and wax size increases with 

 increasing drying temperatures applied to the fiber mat (fig. 23-23). 



In dry-process fiberboards wax is added directly to chips or fibers, in molten 

 or emulsion form — sometimes together with liquid resins. Wax sizes in both 

 wet- and dry-process fiberboards lower strength properties more than rosin 

 sizes, particularly when they exceed 0.5 percent of dry fiber weight; wax 

 contents of 0.2 to 0.5 percent, however, have small effect on board strength. 



Asphalt size. — Asphalt is a black to dark brown solid or semi-solid, pre- 

 dominantly bitumen material which gradually liquifies when heated. It occurs 

 naturally or is obtained during petroleum refining. Asphalts used for sizing have 

 a higher resin content, and lower oil content and molecular size than paving 

 asphalts. 



Asphalts are used in emulsion form, added to pulp not exceeding 135°F in 

 temperature, and precipitated by addition of alum. Asphalt-sized pulps in head- 

 boxes of forming machines should be adjusted to a pH of 4.5 to 5.0 and 

 temperatures not exceeding 135°F. Typical specifications for asphalt emulsion 

 size are as follows (Lorenzini 1971): 



Statistic Value 



Asphalt content, percent by weight 57-60 



Emulsion pH 9-11 



Particle size range, |xm 1-5 



Viscosity, Saybolt-Furol, seconds 20-100 



Asphalt softening point (ASTM D 2308), °F 185-210 



Asphalt penetration at 77°F (ASTM D5) 0-10 



Asphalt sizes do not decrease bond strength; in insulation board, tensile and 

 bending strength are increased by asphalt sizing due to smooth sheet formation 

 and improved drainage. Because of the dark color imparted by asphalt sizing, its 

 use is mostly limited to insulation board, where typical content of emulsified 

 asphalt size is 10 to 15 percent of dry fiber weight. 



