Pulp and Paper 3107 



from the pulped fibers, or conversely, the purity of the resulting cellulose pulp. 

 Chemical constitution, as well as fiber morphology, of the wood to be pulped is 

 therefore of great importance when considering pulping alternatives. 



H. A. Schroeder (personal correspondence, December 1981) approximated 

 this chemical constitution in hardwoods and softwoods as follows: 



Typical Typical 



Constituent hardwood softwood 



...Percent of ovendry extractive-free weight... 



Cellulose ." 45 42 



Hemicelluloses 31 29 



Lignin 22 28 



Pectin, ash, etc 2_ 1_ 



100 100 



Summative chemical analyses of stemwood of 18 of the principal pine-site 

 hardwoods are given in table 6-1. Percentages of the components range as 

 follows: 



Component Percent^ 



Cellulose 33.8 - 48.7 



Hemicellulose 23.2 - 37.7 



Lignin 19.1 - 30.3 



Extractives 1.1- 9.6 



Ash .1 - 1.3 



100 



The distribution, as well as the summative analysis, of cellulose, hemicellu- 

 lose, and lignin is of interest to the pulp maker. As noted in section 6-2, mature 

 cell walls of hardwood fibers are complex layered structures (figs. 5-73 and 5- 

 74). The true middle lamella, completely enclosing individual cells and cement- 

 ing them together, is a continuous lignin-rich matrix. The thin outer primary cell 

 wall — difficult to distinguish from the middle lamella — is also highly lignified, 

 but contains hemicellulose, cellulose, pectin, and protein. Together these layers 

 are known as the compound middle lamella. The major portion of the cell wall, 

 the secondary wall, is primarily cellulose and hemicellulose, but contains some 

 lignin. 



No data are available on lignin distribution across fiber walls of southern 

 hardwoods, but Fergus and Goring ( 1970ab) found that about 80 percent of the 

 lignin in fibers of Betula papyhf era Marsh, is in the secondary wall layers; the 

 rest is in the very thin middle lamella regions where it is a principal constituent. 

 They found that the secondary wall is 16-19 percent lignin, the middle lamella 

 30-40 percent, and the cell comer regions 72-85 percent. 



^Hergert, H. H., T. H. Sloan, J. P. Gray, and K. R. Sandberg. The chemical composition of 

 southeast hardwoods. Unpublished data privately communicated to the author December 12, 1977. 

 (See section 6-1.) 



