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Chemistry in Pictures: Elastic wood

by Manny I. Fox Morone
September 11, 2020


Credit: Liangbing Hu’s research group
Credit: Liangbing Hu’s research group

It looks like wood, and it’s even made from wood. But what’s special about the so-called elastic wood made in Liangbing Hu’s lab at the University of Maryland, College Park, is that it can be squished—and it bounces—as if it were made of rubber. The recipe for elastic wood is fairly simple, but it’s harsh: Boil balsa wood in a solution of sodium hydroxide and sodium sulfite for several hours, freeze it for a couple of days, and then freeze-dry it for another day. The procedure breaks down the rigid structure of the wood by chemically chopping up the long molecular chains of lignin and hemicellulose inside the cell walls. This makes the cell walls thinner, and it also makes the cell walls release cellulose fibrils, which get tangled up and form a gel network inside the wood’s natural pores. The thinner cell walls make the wood squishy, and the water-filled gel gives the material the springiness that makes it flexible and bouncy, so bouncy that University of Maryland postdoc Chaoji Chen can bounce an elastic wood sphere like a Super Ball (below).

Credit: Liangbing Hu’s research group. Read the paper here (ACS Nano 2020 DOI: 10.1021/acsnano.0c04298).

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peter arendovich (September 16, 2020 11:28 AM)
It is impressive and the technique more so..

I wonder what will be the outcome if sew dust or wood-chips would be submitted to the to the process .
Is the product water dispersible ,, does the property gets destroyed in water ?
Robert Gartner (September 28, 2020 11:47 AM)
Cooking in sodium hydroxide & sodium sulfide is basically Kraft cooking. Only here, obviously, it is not done to the degree, that the lignin & hemicellulose fragments become soluble. Plus it uses balsa wood, which is lighter, i.e. contains more air and thus more pores. I suppose, freezing would pierce the cell walls by the growth of ice crystals to create interconnection of the pores, to control maybe the mobility of water / air throughout the structure. From the upper video, it is visible, that the given sample contains at least some water. As they desire to form a cellulose gel in some of the pores, some water appears to be desired. But I guess, there has to be a limit, as completely water-filled pores would make the material heavy and non-squishy (as water is in compressible & would be squeezed out under compression. So they remove the "excess" water by freeze drying.
The question would indeed be, how prone this material is to reabsorb water & loose its special elastic property.
Kent Robarge (September 30, 2020 8:09 PM)
The article states sodium sulfite, not sulfide. This is essentially a form of alkaline sulfite pulping not the kraft pulping process. Alkaline sulfite is a very niche player in the pulp and paper industry (the majority of what small amount of sulfite pulping that still remains is acid or neutral).

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