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Name: Saif
Status: Other
Age: 20s
Location: N/A
Country: N/A
Date: June 2004


Question:
Why does wood NOT melt?



Replies:
To melt something would imply that it can be taken between a liquid and solid state by heating and cooling. Wood, like other plant material, is very complex and takes its form from its cellular structure. In its natural state, wood is roughly 1/4 to 2/3 water by weight, so it consists of large amounts of liquid at room temperature to begin with. Wood is the source of many liquid products, such as latex rubber, turpentine, and maple syrup, to name a very few. It is possible to burn wood and condense the smoke into a liquid (which is actually how "liquid smoke" food seasoning is made), but the physical structure of the wood is destroyed in the process and the resulting material cannot be reconstituted back into the original source.

Andy Johnson


Wood does not melt when the temperature is raised because it decomposes chemically first. That is, the chemical bonds that hold it together come apart first. Also, when wood is heated in air, it (or its components) start to oxidize.

Materials like water, metal, or rock are simple structures that do not go through any large changes when they are heated. These materials usually melt. When metals are heated, he atoms usually reorder themselves into a new arrangement at higher temperatures, but then the new arrangement melts. Materials like wood, paper, concrete, are not simple, and some of the chemical bonds essentially fall apart or reorganize. In concrete, the calcium hydroxide decomposes, and the concrete loses strength. Many plastic or polymer materials will melt before they decompose. Some decompose before they melt. Rocks, on the other hand, will often melt.

Bob Erck


Wood is cellulose, which is a larger, longer molecule whose approximate formula is H-(HCOH)n-H. The OH's in this formula link to each other (bridging from molecule to molecule) more strongly than almost any other "functional group". These "hydrogen bonds" are weaker than the covalent bonds within each molecule, but not by a huge factor. When there are more hydrogen bonds in each molecule, it gets difficult to ever free each molecule enough to move around as they do in liquids.

Listing members of this family from smallest to largest:
n=1: methanol, melting point = -94 degreesC
n=2: ethylene glycol (anti-freeze), mp= -12C
n=3: glycerin, mp= 20 degC
n=6: glucose sugar, mp=90-150C
n=12: sucrose sugar, mp=~185C (and it tries to turn brown while you're melting it)
n=18+: starch mp>200C, decomposes
n>20: cellulose (cotton, wood, paper), mp>250C, blackens, chars, and/or burns

You can see that as the molecules get larger, the melting points keep getting higher. But their threshold temperatures for reaction with oxygen in air are all pretty similar. As is their temperature to suffer molecular breakdown in airless places ( i.e., wood-> carbon(charcoal) + water(steam) ).

So you can see that at some point in the sequence, the melting points will be higher than the "burning points", and you will never get to see melting behavior. It just disintegrates instead.

This is also related to a term: "cross-linking". Suppose you have a pile of slimy worms. Like long-chain molecules, the pile behaves somewhat like a liquid. If you glue each worm to two neighbors, what you have is longer worms. A thicker liquid. But if you glue each worm to three different neighbors, then it is all one big knot or web or lattice. And the permanent sense of shape inherent to a solid is born.

In any "cross-linked" substance, these glue-spots are made of molecular bonds, just like the rest of the molecule. So it is no longer possible to separate the solid bulk cleanly into separate molecules. Some irreversible chemical breakdown would have to occur first. Several un-meltable "thermosetting" plastics are in this category. Cellulose can be a cross-linked substance. One might think of cellulose as the carbon polymer (plastic) most closely related to water.

Jim Swenson



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