3118 



Chapter 25 



Table 25-9. — Properties of handsheets of beaten, unbleached, low-yield Douglas-fir 



kraftpulp dried conventionally compared with those of unbeaten, unbleached, high-yield 



sweetgum and northern red oak kraft pulps press dried (Data from Setterholm')^ 



Douglas-fir Sweetgum Northern red oak 



low-yield kraft high-yield kraft high-yield kraft 



beaten pulp con- unbeaten pulp unbeaten pulp 



Handsheet property ventionally dried"^ press dried'* press dried'' 



Basis weight, gin? 205 205 205 



Specific gravity .85 .91 .92 



Burst, points 156 125 191 



Ring crush, pounds 120 194 205 



Folds, number 1,500 180 462 



Tear, g 375 176 160 



Tensile strength, psi 7,920 8,150 11,110 



Modulus of elasticity, thousand psi 774 940 1,170 



Strain to failure, % 3.30 2.35 3.32 



Edgewise compressive strength, psi 2,700 4,800 5,090 



'Personal correspondence with V. C. Setterholm December 1981 . All paper tested at 7.5 percent 

 moisture content. 



^Temperatures and pressures used in press drying the hardwood pulps are in the range probably 

 achievable in industrial practice. 



•^53 percent yield, beaten to 550 ml Canadian Standard Freeness. 



■^72 percent yield, pressed at 400 psi and 400°F, to percent moisture content. 



^60 percent yield, pressed at 800 psi and 400°F, to percent moisture content. 



25-5 MECHANICAL PULPING 



Methods for making mechanical fiber for hardboard, medium-density fiber- 

 board, and insulation board are described in section 23-6; southern hardwoods 

 are much used in these products. 



For paper and paperboard, however, mechanical pulping of southern hard- 

 woods is limited by the low strength of these pulps and by their high power 

 requirements. Accounts of efforts to make paper and paperboard from dense 

 hardwood mechanical pulps follow. 



GROUNDWOOD 



Stone grinding, in which a bolt of freshly cut wood is pressed against a 

 revolving grindstone, was the first mechanical defibration process developed. It 

 is still widely used to produce mechanical pulp from softwoods to be mixed with 

 chemical pulp for newsprint. Stone groundwood, mostly softwood, is also used 

 for wallpaper, in some toilet tissue, toweling and wrapping paper, and in paper- 

 board for boxboard furnish, bottlecap stock, and matchstick stock. Also, stone 

 groundwood is an important ingredient of groundwood printing papers. For a 

 description of the stone grinding process, and sectional views of a stone grinder 

 and the pulp stone it carries, see Agriculture Handbook 420 (Koch 1972, p. 

 1421-1425). See also references cited on page 2238 of this text. 



