3082 Chapter 25 



Reeves (1979b) enumerated several reasons for using hardwood pulps in 

 papermaking: 



• The hardwood is readily available. 



• Use of hardwoods will improve resource utilization and facilitate good 

 silvicultural practices. 



• Hardwoods are generally denser than softwoods, so more weight of 

 hardwood than softwood can be charged in digesters. 



• Lignin in hardwoods is easier to digest than softwood lignin and hard- 

 woods generally give higher pulp yields than softwoods; this permits 

 more throughput with improved yield and less expenditure of energy and 

 chemicals. 



• Hardwood pulpwood costs less than softwood pulpwood. 



• Many paper properties are improved by the addition of substantial 

 amounts of short, fine hardwood fibers. Hardwood fibers generally con- 

 tribute to improved formation, opacity, and surface properties of a sheet. 

 Since the fibers do not require extensive refining to obtain uniform 

 dispersions, the hardwood pulps have a beneficial effect on papers (e.g., 

 tissue products) requiring bulk, softness, and absorbancy. Papers con- 

 taining appreciable amounts of hardwood pulps also are more stable and 

 curl less with moisture changes. Although hardwood fibers are not as 

 strong as softwood fibers, these positive contributions of hardwood pulps 

 to paper properties have made them essential to many grades of paper. 



Reeves (1979b) noted that offsetting these advantages are some disadvan- 

 tages. Hardwoods are more difficult to debark (see chapter 17), and the amount 

 of bark available for fuel is thereby reduced. This disadvantage is negated to 

 some degree because barks of many southern hardwoods have a significant fiber 

 content (table 13-48). Hardwoods, being denser than softwoods, require more 

 power to chip. Southern pine has a significant content of resin, which yields 

 turpentine and rosin. Hardwoods, however, contain fatty resins in lesser 

 amounts than the resin in pine, and these fatty resins do not yield turpentine and 

 rosin; byproducts from hardwood kraft pulping are therefore less valuable than 

 those from southern pine. 



Hardwoods, but not softwoods, contain a substantial proportion of vessel 

 elements (table 5-3). These short, wide, and thin-walled elements (figs. 5-52 

 bottom and 5-99 with related discussion) bond less well than fibers in the paper 

 sheet and have a tendency to be picked off the surface as viscous, tacky printing 

 inks split between the paper and applicator surfaces. This tendency varies with 

 species; white oak vessels are particularly subject to picking. Use of starch or 

 other adhesives applied at a size press can completely control picking, and any 

 procedure which improves internal bonding in a sheet tends to improve picking 

 resistance (Reeves 1979b). Thus gyratory refining of oak pulps reduces their 

 picking tendency (Byrd and Fahey 1969). Most manufacturers of fine paper, 

 especially papers for offset printing, use a variety of hardwood pulps with no 

 apparent difficulty. 



