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Background:

According to this Wikipedia article:

Wood warping costs the wood industry in the U.S. millions of dollars per year. Straight wood boards that leave a cutting facility sometimes arrive at the store yard warped. This little-understood process is finally being looked at in a serious way. Although wood warping has been studied for years, the warping control model for manufacturing composite wood hasn't been updated for about 40 years.

Factors contributing to the warping of wood include plant species, temperature and temperature changes, humidity and humidity changes and wet-dry cycles.

Discussion:

I'm not interested in which species are more prone to warping, how to prevent warping by treatments, etc. Nor am I interested in the biological processes of living or freshly cut wood. I'm interested in the processes happening over time on a molecular level that lead to macro-scale warping of wood. Water, in particular, seems to play a major role in the warping of wood, implying that possibly the lignin and cellulose matrix of wood is disturbed by the formation of hydrogen bonds. But if this is the case, or whatever the case, why would the microscale effects not cancel out rather than causing macro scale warping in a particular direction? This may be more of a physical than a chemical question, however.

Question:

What are the chemical process responsible for the warping of processed (as opposed to living or freshly cut) wood?

Edit 1:
A link to an interesting and relevant video was given in a comment, in which several types of wood were placed in a chamber which was evacuated of air, then pressurized with ammonia gas. The video also gave a link to a patent for this process. The following is an excerpt from the patent:

The rigidity of wood is the result of crosslinking between adjacent cellulose chains by water molecules which hydrogen bond between sites on adjacent cellulose chains. Anhydrous ammonia is extremely reactive with water, and it is believed that anhydrous ammonia scavengers water from wood, breaking the crosslinking between cellulose chains, and permitting the cellulose chains to slide, which renders the wood very flexible so that it can readily be bent or twisted. When the ammonia volatilizes from the treated wood, water vapor reestablishes crosslinking and rigidity.

The reason for this edit is to suggest that the disruption of hydrogen bonding between adjacent cellulose chains, when the wood is dried then rehydrated, for example, may also play a role in "natural" warping. I'm going to look into this more and I hope it gives somebody else a clue to a good answer.

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    $\begingroup$ It's probably both -- the swelling of wood when wet is more of a physical process, but the properties that lead to permanent deformations on subsequent drying likely have some manner of chemical explanation. $\endgroup$ – hBy2Py Apr 24 '17 at 19:34
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    $\begingroup$ Related: youtube.com/watch?v=9Z0SsAyHKzc $\endgroup$ – Curt F. Apr 24 '17 at 19:46
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    $\begingroup$ It's rather a biological question, behavior of dying wood depends on it's structure as a whole not directly on single molecules. $\endgroup$ – Mithoron Apr 24 '17 at 20:06
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    $\begingroup$ Great video @CurtF. (I recommend everyone take a look). Here's what the linked patent said: "The rigidity of wood is the result of crosslinking between adjacent cellulose chains by water molecules which hydrogen bond between sites on adjacent cellulose chains. Anhydrous ammonia is extremely reactive with water, and it is believed that anhydrous ammonia scavengers water from wood, breaking the crosslinking between cellulose chains, and permitting the cellulose chains to slide, which renders the wood very flexible so that it can readily be bent or twisted." Then it hardens when water returns. $\endgroup$ – airhuff Apr 25 '17 at 2:00
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    $\begingroup$ Since this, as u say, has already been studied at length I don't think I can give a better answer. As u say its the water that cross links the cellulose chains. My speculation is that once wood dries considerably the cellulose fibers are released to deform into a form that they would have grown into to compensate for the stresses they were exposed to in nature. I believe there is a lot of unevenness in the cellulose chains in the wood. Actually, I believe rived wood doesn't warp because it is split along the grain of the wood. Sawn boards warp because they are cut across the grain in many plac $\endgroup$ – discountbrainsurgery Apr 29 '17 at 21:07
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Warping (be it a bow, crook, cup, or twist); as well as loosening of knots, checking, splitting, and collapse; of wood is caused by uneven shrinkage.

To explain uneven shrinkage chemically, consider wood as 50% cellulose. Cellulose, which is a polymer consisting of linked glucose monomers, is arranged into fibrils. Fibrils are part of larger structures which make up the cell walls of wood fiber. Binding these cell walls together is lignin, which is a phenylpropanol polymer. Cellulose and lignin (along with hemicellulose) are the major structural compounds in wood.

Cellulose in wood is a tangle of rigid crystalline cellulose and flexible amorphous cellulose. While the structure of crystalline cellulose makes it water insoluble, amorphous cellulose has sites like its base monomer glucose which are capable of absorbing water. When water is absorbed both the shape and volume of the amorphous cellulose change. While amorphous cellulose changes its shape, the crystalline cellulose (as well as lignin) retains its shape and volume.

Cellulose may also change shape and volume due to other factors such as temperature fluctuation and biological degradation. Because (1) wood is not uniform; (2) moisture, temperature, and biological changes are rarely uniform throughout the bulk of a piece of wood; and (3) most wood is exposed to some degree of these changes over time; uneven shrinkage occurs. Because of uneven shrinkage in wood, it warps.

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  • $\begingroup$ These phenomena affects degradation of wood fibers in historical documents as well; however, I figured this is outside the scope of the question. $\endgroup$ – Agriculturist Sep 7 '17 at 19:26

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