Can Wood Be Melted?

[Yoshiman421]

I am having an argument with a friend about melting wood.

 

She says that wood cannot be melted and cites 

http://www.yalescientific.org/2010/05/everyday-qa-can-you-melt-a-wooden-log/ as a source. 

 

I say wood can be melted because all compounds and elements can be melted if hot enough with no exceptions. The wood may need to be heated up at temperatures that we cannot achieve here on Earth, but that does not mean that it isn't possible.

 

The question is not "Can we, as human beings at our current level of technology, melt wood?" The question is "Can wood be melted?"

Thanks for your help solving this argument.

[Maine farmer]

Wood would ignite at around 300 degrees F (according to a very quick google search), so in the presence of oxygen, it would be reduced to ash, water vapor, and carbon dioxide  if you heated it up above that temperature.  I am certain the ash could be melted.  I don't know what wood would do if heated in the absence of oxygen.  That would be interesting to find out.

Edited March 3, 2017 by Farming guy

[Maine farmer]

You really piqued my interest, so I did a slightly longer google search and found thishttps://www.reddit.com/r/askscience/comments/r7zls/what_happens_when_you_heat_wood_in_a_vacuum/ and apparently, wood could melt if heated in a vacuum.  

 

Thanks for asking that one!

[Yoshiman421]

You really piqued my interest, so I did a slightly longer google search and found thishttps://www.reddit.com/r/askscience/comments/r7zls/what_happens_when_you_heat_wood_in_a_vacuum/ and apparently, wood could melt if heated in a vacuum.  

 

Thanks for asking that one!

 

Yeah its a very interesting topic and we were just sitting at work doing nothing so we did pretty extensive google searches and came up with nothing. I had hypothesized that it was possible to heat it in a vacuum and melt it, or an environment filled with an extremely nonreactive element or compound like xenon or boron and then heated. As long as there is an absence of Oxygen, I am reasonably certain that it is possible to melt wood. It would require a lot of energy though.

[exchemist]

Yeah its a very interesting topic and we were just sitting at work doing nothing so we did pretty extensive google searches and came up with nothing. I had hypothesized that it was possible to heat it in a vacuum and melt it, or an environment filled with an extremely nonreactive element or compound like xenon or boron and then heated. As long as there is an absence of Oxygen, I am reasonably certain that it is possible to melt wood. It would require a lot of energy though.

I don't think it would melt. If you read the link Farming Guy posted, it actually says that the major components in wood (the polysaccharide polymers) would decompose before their theoretical melting point were reached. It says you would ultimately get a mixture of gases and liquids out of it, but these would not have the same chemical composition as the starting material.

 

This is actually quite common with long chain molecules. One can think of what happens as the thermal kinetic energy of the molecules builds up as a long chain molecule is heated. Motion in different parts of the chain becomes more and more violent, and eventually chains start to break, due to this - i.e. the molecule decomposes into smaller molecules, giving rise to different compounds. I used to work in the oil industry, where to distil the "heavier", high molecular weight, fractions (used for luboil) we resorted to vacuum distillation. This was done in order to get boiling to occur at temperatures low enough to avoid "cracking", which is an industry word for decomposition of long chain molecules into smaller fragments.  (Needless to say, there is no oxygen present in a refinery distillation column, otherwise..... BANG!) 

 

I can even give an example of a small molecule that does not melt at normal pressures: calcium carbonate. When you heat this it decomposes and CO2 is given off. If you wanted to melt CaCO3, you would need a very high pressure of CO2 gas around it, to stop that happening.

 

So it is not the case that everything melts eventually. Decomposition is a competing pathway, followed by quite a number of substances. It seems to me wood is one of these. 

[Maine farmer]

Who knew such a simple question could be so interesting?  

 

 

 

I can even give an example of a small molecule that does not melt at normal pressures: calcium carbonate. When you heat this it decomposes and CO2 is given off. If you wanted to melt CaCO3, you would need a very high pressure of CO2 gas around it, to stop that happening.

 

So it is not the case that everything melts eventually. Decomposition is a competing pathway, followed by quite a number of substances. It seems to me wood is one of these. 

Well, what if you applied high pressure as you heated wood?  

[OceanBreeze]

I am fairly certain that wood cannot be melted. It is mostly cellulose.

According to the handy NIOSH Pocket Guide:

Cellulose

Melting Point

500-518°F (Decomposes)

[Maine farmer]

I am inclined to agree,but there lingers a tiny fraction of doubt in my mind.  The challenge is that you have to add enough energy to cause a change of phase without breaking the molecular bonds.  Unlikely possible, I agree, but I would like to find out exactly how much energy we are talking about.  I haven't found that speciffic  knowledge for myself, yet.

[exchemist]

Who knew such a simple question could be so interesting?  

 

Well, what if you applied high pressure as you heated wood?  

That would certainly alter the equilibrium position of any decomposition process that involved the evolution of gas, and thereby inhibit it. But cracking, i.e. the breaking of long chains, would not itself be inhibited by this. So I remain very doubtful that a meaningful melting point of, say,  cellulose, one of wood's major constituents, can be defined - and this seems to be borne out by Ocean Breeze's contribution.

 

I quote below what the Wiki article on cellulose has to say about the effect of heat: 

 

" At temperatures above 350 °C, cellulose undergoes thermolysis (also called ‘pyrolysis’), decomposing into solid char, vapors, aerosols, and gases such as carbon dioxide.^[32] Maximum yield of vapors which condense to a liquid called ‘bio-oil’ is obtained at 500 °C.^[33]

Semi-crystalline cellulose polymers react at pyrolysis temperatures (350 – 600 °C) in a few seconds; this transformation has been shown to occur via a solid-to-liquid-to-vapor transition, with the liquid (called ‘intermediate liquid cellulose’ or 'molten cellulose') existing for only a fraction of a second.^[34] Glycosidic bond cleavage produces short cellulose chains of two-to-seven monomers comprising the melt. Vapor bubbling of intermediate liquid cellulose produces aerosols, which consist of short chain anhydro-oligomers derived from the melt.^[35]

Continuing decomposition of molten cellulose produces volatile compounds including levoglucosan, furans, pyrans, light oxygenates and gases via primary reactions.^[36] Within thick cellulose samples, volatile compounds such as levoglucosan undergo ‘secondary reactions’ to volatile products including pyrans and light oxygenates such as glycolaldehyde.^[37] "

 

What is interesting here is that there IS a "molten cellulose" phase, but (a) it exists for only a fraction of a second before decomposing and (b )it is already an oligomer formed from the original polymer, due to the breaking of bonds in the original long chain, which is what I was talking about.

 

Re your later question about the degree of energy to melt the solid vs that needed to break the bonds, that's interesting but tricky. I suppose if we really want to get into that we need to think in terms of the Activation Energy for a glycoside bond cleavage and compare that with the hydrogen bonding that binds cellulose chains together all the way along their length. Qualitatively it seems to me that the longer the chain the more H-bonds have to break to set the chain free as a liquid, whereas the energy needed to break the chain is the same for a small oligomer and for a long polymer. So at some point the H-bonds win out and the chain breaks first.  But this may be simplistic.......

 

All this reminds me of one the choruses I am practising at the moment  with my local choral society:

 

This is quite tricky to sing - lots of modulations led by us in the bass section, or that is how it feels - but great fun. 

Edited March 3, 2017 by exchemist