98 THE PLANT CELL WALL 



rigidity or tensile strength. The effect of orientation (and conse- 

 quently increased interchain force) is well illustrated in synthetics 

 or modified polymers which may be prepared either in bulk or 

 fibrous form (Table 2). The elastic modulus (reversible deformation) 

 is increased from two-to four-fold by orientation, and tensile 

 strength (breakage) may be increased by a factor of 10 or more. 

 The effect of orientation is evident even in the case of polyethylenes, 

 whose hydrocarbon chains are held together only by weak van 

 der Waals forces. 



Wood, as a complex of cells and tissues, contains polymers 

 varying widely in crystallinity, together with amorphous polymers 

 and variously ordered small molecules. The weakness of wood 

 relative to the fibers themselves must then reflect the higher pro- 

 portion of imperfectly aligned or weakly bonded surfaces in the 

 former. 



Wood; Chemical 



The chemical technology of wood encompasses its utility both 

 as a fuel and raw material. Recognizing that the combustion 

 properties of wood must be taken into account when wood is 

 used as a construction material or as a fuel, we proceed to a con- 

 sideration of its chemical utilization. 



The thermal decomposition of woods has long been employed to 

 obtain useful chemicals. Before the introduction of coking methods, 

 the decomposition of wood provided the charcoal required in iron 

 production. Two major by-products of charcoal manufacture are 

 methanol and acetic acid. The importance of wood distillation 

 as a source of alcohol has declined since the development of effi- 

 cient catalystes for the direct syntheses of methanol from carbon 

 monoxide and hydrogen. Acetic acid derived from wood is now 

 largely supplanted by fermentation methods and its synthesis from 

 acetylene. 



Although wood distillation yields only a few substances in 

 large quantities the diversity of minor products is impressive. 

 More than one hundred compounds have been identified and 

 include members of the following groups: carboxylic acids; aliphatic 

 alcohols; aliphatic aldehydes and furfurals; acetone and other 



