Shrinkage, wood

Urea—Formaldehyde and Urea-Based. In the 1970s and early 1980s, urea materials were in general use particularly for direct field retrofitting of cavity wall constmction of wood frame and masonry. However, because of formaldehyde odor and excess shrinkage under specific conditions, this ceUular plastic has limited use as an insulation. 

Wood While fairly inert chemically, wood is readily dehydrated by concentrated solutions and hence shrinks badly when subjected to the action of such solutions. It is also slowly hydrolvzed by acids and alkalies, especially when hot. In tank construction, if sufficient shrinkage once takes place to allow ctystals to form between the staves, it becomes very difficult to make the tank tight again. 

Shrinkage block jig A metal, wood, plastic, etc. shaped block against which parts are held under light or no pressure while cooling to reduce warpage and distortion. 

Stamm, A.J. and Hansen, L.A. (1937). Minimizing wood shrinkage and swelling. Effect of heating in various gases. Industrial and Engineering Chemistry, 29(7), 831-833. 

Powell in 1904 found that if wood was treated with sucrose solution and subsequently dried at a high temperature, it did not shrink. By this process Powell claimed to strengthen the fibers of the wood. Rice believes that the treatment reduces the cell spaces and stabilizes the wood, thereby minimizing shrinkage and the possibility of rot. An example of the process is as follows ... 

Stamm has made a thorough examination of the effect of sucrose and invert sugar on the relevant physical properties of wood and has found that shrinkage is definitely retarded by the deposition of the sugar in the wood structure. Invert sugar seems superior in this effect to sucrose itself. 

A program of work on cement pastes [45] showed that subsequent rewetting and drying in the presence of neutralized wood resins showed no increase in shrinkage in comparison to a plain paste. The results are given in Fig. 3.37. 

The specimens are then immersed in a molten wax or natural resin and the Cellosolve slowly distilled off. This step invariably involves some shrinkage. Antishrink efficiencies of 80% are, however, obtainable in this way with mixtures of beeswax and rosin. This treatment appears suitable for the preservation of wood artifacts. Christensen (43) has treated wood artifacts by replacing the water with tertiary butanol and this with polyethylene glycol-4000. 

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