Cell wall specific gravity

If there are no major disruptions, depolymerizations, or decrystallization of cell wall polymers during the reaction chemistry to modify solid wood, there are no statistical differences in mechanical properties of chemically modified wood as compared to nonmodified wood. There are so many differences in moisture levels, specific gravities, and fibers per unit cross-section in control vs. modified woods that no definitive conclusions can be made. If the reaction chemistry used to modify solid wood does result in major disruptions, depolymerizations, or decrystallization of cell wall polymers, then there are major statistical differences between control and chemically modified solid wood. Losses in mechanical properties can vary from large decreases in all properties to complete loss of cell wall structure and wood is converted to a thermoplastic film. 

For simplicity, we assume that no filling of the cell wall is provoked by the treatments, so that Vp = 0, as well as no change of lumen sizes and specific gravities or Young s moduli of microfibrils. Only the matrix volume, specific gravity, and mechanical properties are modified by the treatment. 

PO), WPG 22%, resulted in bonded cell wall bulking as in the case of (A), except the introduced group was hydrophilic. This made a big difference as the RH increased. At 0% RH, (PO) had slightly lower E /y because of the increase in specific gravity and tan 8 was lower than that for untreated wood. At 35% RH, tan 8 was almost the same as that for untreated wood, but at 60% RH and 85% RH, tan 8 was much larger. This was due to the flexibility introduced into the cell wall because the hydrophilic ether allowed water to act as a plasticizer. 

WPC), WPG 138%, indicates that a small amount of this polymer may enter the cell wall, but in general this treatment only fills the lumen space. The increase in specific gravity decreases E /y. Water can still enter the cell wall and act as a plasticizer, so that tan 8 increases as the RH increases. 

Any procedure for wood specific gravity or density based on measuring weight and volume can be considered accurate only if the wood sample has been first extracted with suitable organic solvents to remove extraneous resins, oils, fats, gums, etc. (27). These materials bulk cell walls, block potential sites for water adsorption/ab-sorption, alter potential wood swelling/shrinkage, and thereby interfere with the accurate characterization of wood tissue. 

For the purpose of the discussion that follows the density of the dry cell wall will be taken as 1.5 g/cm, and its specific gravity GJ as 1.5. 

The specific gravity G of the cell wall at any moisture content M is given by... 

The thermal conductivity of wood also increases greatly with its moisture content, as well as with other factors such as temperature and specific gravity. A number of proposed empirical equations indicate that the thermal conductivity of wood increases with increasing moisture content. These equations have been summarized by Siau (45) who proposed that the primary effect of water was to swell the cell wall and, thus, to provide a larger heat conduction area for a given wood. 

After consideration of published chemical results, Grattan and Mathias (26) concluded that for routine conservation of small amounts of waterlogged wood, chemical analysis is unnecessary. They felt that the use of specific gravity to calculate loss of cell wall material was more economical and less time consuming to perform. This conclusion led to the development of the standard freeze-drying treatment method used at the Canadian Conservation Institute. 

Specific gravity is an indicator of pOTosity or void volume, since the specific gravity of the solid wood cell wall can be measured at 1.46 -1.54 depending upon measurement technique or approximated at 1.50 [15]. On a dry basis, the porosity, or fractional void volume, can be expressed as follows ... 

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