Pulp and Paper 3139 



Hardwood and softwood chemical pulps are bleached under similar condi- 

 tions and bleaching sequences. Hardwoods require less chemical for equal 

 brightness. Shrinkage (reduction in yield) during bleaching is usually less for 

 hardwood kraft pulp due to less residual lignin and lower alkaline solubility. 



Strength properties. — Hardwood pulps are inferior to softwood pulps in 

 strength, especially tear, and particularly so in comparison with the high 

 strength values for southern pine. As noted in section 25-3, strength properties 

 of pulps are related to fiber length, strength, and morphology. Short fibers, 

 averaging about one-third the length of softwood fibers, are the primary cause of 

 the poorer strength properties of hardwood pulps (figs. 25-1 1 through 25-13). 



Hardwood pulps are rarely used where strength is of importance unless they 

 are fortified with appreciable amounts of kraft softwood fiber. However, addi- 

 tions of hardwood pulps to southern pine pulps in amounts up to 20 percent can 

 improve paper formation without severe strength reduction. In admixtures to 10 

 percent even a slight increase in tensile and tear strength can result. Mixing the 

 softwood and hardwood chips prior to pulping can achieve similar results in the 

 production of wrapping and sack paper and in food board and linerboard al- 

 though it is more advantageous to pulp them separately and blend during or after 

 beating (Worster and Bartels 1976). 



Press drying of hardwood high-yield kraft pulps holds considerable promise 

 of imparting strength sufficient for their use as linerboard. For a discussion of 

 press drying see section 25-4. 



Hardwood kraft pulps possess many desirable characteristics offsetting their 

 low strength properties. Good sheet formation and surface texture, low porosity 

 and high bulk, opacity, softness, and absorbency qualify them for high-grade 

 products for which hardwood pulps are best used bleached and only slightly 

 beaten. They often compete with softwood sulfite pulps for the same uses (Casey 

 1980). Being absorbent and softer than softwood sulfite pulps, hardwood kraft 

 pulp is well suited for use in tissues. The smooth surface of paper from hard- 

 wood kraft pulps gives it good printability, which together with its high opacity 

 makes it ideal for fine papers, although additives may be required to prevent 

 picking, especially in pulps prepared from ring-porous woods. Hardwood pulps 

 with poorer strength properties are often used for bleached facing on linerboard, 

 or more usually as filler in food board, where strength is not critical. 



Reaction wood. — In kraft pulping as a means of whole tree utilization, 

 hardwoods possess an advantage over softwoods. Reaction wood of hardwood, 

 such as that found in limbs, is poorer in lignin and considerably richer in 

 cellulose than normal wood. In softwoods the reverse is true, and their reaction- 

 wood lignin appears to be more highly condensed than that of normal wood. The 

 pulping of hardwood limbs should present no difficulties and the yield should be 

 higher than that from stemwood at comparable lignin contents. The resultant 

 pulp, however, should have poorer physical properties because of coarser, 

 shorter fibers. Also the extra cellulose in tension wood is present as a gelatinous 

 layer adjacent to the lumen and therefore not normally involved in interfiber 

 bonding during papermaking (Clermont and Bender 1958). 



