CELLULOSE

Re: why cellulose burn rather than melt?
Date: Wed Nov 25 13:43:46 2009
Posted By: Werner Sieber, Research Scientist,
Area of science: Chemistry
ID: 1258711594.Ch
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Message:

Hi Awais,

To be precise, cellulose only burns if there is oxygen present. If you heat it in the absence of oxygen, it still doesn't melt, but decomposes, turning black. At a sufficiently high temperature, you are left with carbon.

The formula and structure of cellulose is described here: http://en.wikipedia.org/wiki/Cellulose

As you can see, it is a polymer, and its molecules have many hydroxyl groups sticking out on the sides. These polar groups are able to form hydrogen bonds with neighboring molecules. Even though hydrogen bonds aren't as "strong" as most covalent bonds, their large number makes for strong cohesion among the cellulose molecules, making it very difficult for them to move relative to each other, and melting means movement of molecules relative to each other.

As soon as you chemically modify the cellulose by replacing the hydroxyl groups, for instance with ester or ether groups, you get a meltable polymer, which usually is also soluble in some organic solvents, unlike the native cellulose.

Best regards,
Werner Sieber