PROPERTIES AND USES OF CELL WALLS 97 



tic. Relative to still air as a reference material, most woods have 

 a conductivity only three-fold greater transverse to the grain and 

 five-fold greater along the grain. Balsa wood, cork, expanded 

 polymers, and loose fibers (glass, cotton, etc.) are among the few 

 superior insulators. 



The effect of temperature on the dimensional stability of wood 

 is also small. The coefficient of linear expansion along the grain 

 is generally lower than the typical values for metals, plastics and 

 glass. Linear expansion of wood transverse to the grain exceeds 

 iron, steel, other metals, ceramics and glasses, but is less than 

 most synthetics. As would be expected from the other properties 

 considered, wood is also an excellent electrical insulator. 



Proper choice of structural material will, of course, depend 

 upon aesthetic requirements and economic factors, including 

 production costs, transportation, etc. In passing, we may won- 

 der at the superior mechanical properties of natural fibers 

 over wood itself, and the general basis for fiber strength. 



The tensile strength of an ideal cellulose fiber made of a single 

 continuous polymer can be calculated from the energies of (work 

 required to break) the C — C, C— H, and C— O bonds in the primary 

 valence chain. This figure, at least, 800 kg/mm 2 far exceeds meas- 

 ured values for fibers. On the other hand the secondary forces 

 holding together the molecules of a carbohydrate crystal (e.g. 

 a sugar) give a breaking strength of only 30 kg/mm 2 . 



Real fibers contain discontinuous and overlapping polymer 

 chains of imperfect and variable crystallinity. Thus, real breaking 

 strength must depend upon secondary rather than primary valence 

 forces. Real fibers range in tensile strength from values approxi- 

 mating the sugar crystal to values two to three-fold higher. Hence, 

 the strength of real fibers derives from resistance to shearing forces 

 which pull apart the polymer chains, by breaking the secondary 

 bonds. Shear, like other elastic properties, is defined in terms 

 of the stress, F/a. Thus, the displacement (strain) imparted by 

 a given force will diminish with increasing surface as the stress 

 itself is reduced. 



It follows, therefore, that better crystallized polymer bundles 

 will have more interchain contact surface, and will exhibit higher 



