Sciencemadness Discussion Board

NO2 ion to create N2

Smetje - 4-11-2013 at 05:39

Hi all

I'm trying to create a nucleating site for a poly-urethane mixture. While adding N2 in the reaction mixture seems logical, this poses quite a challenge in terms of gas-size distribution.

That's what got me thinking towards chemical N2 creation.
Consider the following:
NH4Cl (aq) + KNO2 (aq) ---> N2 (g) + K+Cl- (aq) + 2 H2O (l)

I've chosen the above (theoretical) reaction because of the formation of water and the presence of K+ ions.

My question is quite simple:

Do you foresee any issues with this set-up, specifically the potential formation of NH4NO2 ?

I would like to dissolve the starting solids in glycerine (glycerol), to minimize the total water content. Concentration-wise I would not go over 15-25 mass%.

Thanks for your ideas and remarks.

-Smetjes

Edit small change in nomenclature and/or abbreviations. Excuses for the ambiguity.

[Edited on 4-11-2013 by Smetje]

woelen - 4-11-2013 at 06:04

PU-mixture?
MEG? DEG? Are these mono ethylene glycol and the dimer?

And what do you mean with m% ? Is this mass percentage, or 1/1000th of a percent?

You should be more clear, all these acronyms and notations are not common knowledge.

vulture - 4-11-2013 at 06:21

Nucleating site for what? Precipitation? PU forming reaction?

Are you trying to make foams?

Smetje - 4-11-2013 at 06:35

Quote: Originally posted by vulture  
Nucleating site for what? Precipitation? PU forming reaction?

Are you trying to make foams?


Hi Vulture

yes, the general idea is to make poly-urethane (let's abbreviate to PU) foams.
Reaction of the basic ingredients (polyol + di-isocyanate) would give a dense, coarse solid. Therefore it is typically "blown", by for example water (which reacts with the isocyanate to form carbon dioxide) and other gasses. The solid (PU) consists of decahedronical cells - which I would like to minimize through nucleation sites. Hence trying to create N2 with NO2.
In this set-up, the needed thermal energy would be available through the exothermic character of the PU reaction.

I hope this context is sufficient?

-Smetje

bbartlog - 4-11-2013 at 07:11

Quote:
specifically the potential formation of NH4NO2


Well, I think that's actually what you want ... isn't that on the pathway to N<sub>2</sub> production? KNO<sub>2</sub> doesn't decompose by itself.
Quote:
Concentration-wise I would not go over 15-25 mass%.


That may be quite a lot, no? But I don't know how light your intended foam will be.

Anyway, the system you propose (or one similar, using sodium nitrite plus ammonium chloride) has been used before to generate foams. I don't know whether your proposed mixture will have solubility issues; if it does you may need to use a surfactant to create an emulsion.
A second problem is that this reaction does not proceed all that quickly without a catalyst. Acids are sometimes employed (HCl, acetic acid, citric acid). However, since the reaction is exothermic, too low a pH can also cause problems / runaways. Whether this is a big issue for you may well depend on the size of the foam structures you are trying to create. For larger items I would expect it to be very difficult to maintain a similar temperature throughout the structure.
Finally, it sounds like you will need to make sure that this gas creation reaction runs to completion before your polymerization reaction, but not *too* long before, so that it doesn't result in the destruction of an already solid polymer.

Smetje - 4-11-2013 at 07:42

Thanks for the detailed reply, bbartlog!
Quote:
specifically the potential formation of NH4NO2


I'm slightly worried about the explosive character of this compound...
Quote: Originally posted by bbartlog  

Anyway, the system you propose (or one similar, using sodium nitrite plus ammonium chloride) has been used before to generate foams.

Have you got information on these systems? I've never heard of N2 being used for anything but nucleation.
Quote: Originally posted by bbartlog  

I don't know whether your proposed mixture will have solubility issues; if it does you may need to use a surfactant to create an emulsion.
A second problem is that this reaction does not proceed all that quickly without a catalyst. Acids are sometimes employed (HCl, acetic acid, citric acid). However, since the reaction is exothermic, too low a pH can also cause problems / runaways. Whether this is a big issue for you may well depend on the size of the foam structures you are trying to create. For larger items I would expect it to be very difficult to maintain a similar temperature throughout the structure.


The solubility issue has been taken care of, through the use of commercially available surfactants.
I do not agree with you on the acids as catalysts however. See (wikipedia) Polyurethane catalysts about the tertiary amine catalysts. Lowering pH would inhibit the formation of the RO- anion.
Temperature distribution throughout the foam is an issue, though. However, heated moulds are shown to be effective.
Quote: Originally posted by bbartlog  

Finally, it sounds like you will need to make sure that this gas creation reaction runs to completion before your polymerization reaction, but not *too* long before, so that it doesn't result in the destruction of an already solid polymer.


That is an issue I've been thinking about quite a lot. The only solution I've come up with so far, is to use a virtually non-catalysed PU reaction and adding energy by mixing.

AJKOER - 4-11-2013 at 08:00

Quote: Originally posted by Smetje  
....
That's what got me thinking towards chemical N2 creation.
Consider the following:
NH4Cl (aq) + KNO2 (aq) ---> N2 (g) + K+Cl- (aq) + 2 H2O (l)

................


I think it is more instructive to write the reaction as:

NH4Cl (aq) + KNO2 (aq) ---> KCl + NH4NO2

Now, research the product NH4NO2 and one will find a bit of concern in dealing with Ammonium nitrite in acidic aqueous conditions (explosion hazard). The reason relates to stability issues of HNO2 (formed upon acidifying) which is prone to rapid gaseous decomposition in anything other than cold dilute solutions.

The salt Ammonium nitrite has been described by some as acutely toxic, so keep this in mind.

So, I confirming the concerns expressed above about a runaway reaction plus some.

What about using CO2 in place of N2 via this path.

bbartlog - 4-11-2013 at 09:06

interesting point about the catalysis ... if I read the information right, the polymerization catalysts are all basic? Anyway, the decomposition reaction of ammonium nitrite does not exactly *require* acidic conditions, it just proceeds faster under them. Which suggests that at least you have a useful handle for speeding up one reaction at the expense of the other: if the nitrogen is being produced too fast, you can bump up the pH, and if it's being produced too slowly, you can lower it. Of course it could be that there is no middle ground that will allow both reactions to proceed at a reasonable rate, but maybe that's where temperature comes in.

Anyway, I found a little bit of information on this in "Production Chemicals for the Oil and Gas Industry" by Malcolm A Kelland; p268 is available for preview via Google. A rather different application than yours, of course.

vulture - 4-11-2013 at 11:46

I suspect you will run into trouble by using ammonium salts, since the catalysts are basic. You'll have NH3 formation, deactivation of your catalyst and formation of nitrite salts. Seems like a mess to me.