madscientist
National Hazard
Posts: 962
Registered: 19-5-2002
Location: American Midwest
Member Is Offline
Mood: pyrophoric
|
|
Liquification of rubber by nitric acid
I have been wondering about the mechanism of nitric acid's liquification of rubber. I believe I have figured out what it is. Rubber is composed of
hydrocarbon polymer chains, which are held together by sulfur atoms. I'm hypothesizing that the sulfur atoms are being oxidized by the nitric acid,
breaking apart the hydrocarbon chains; therefore liquifying the rubber. This would explain why the rubber also bubbles when it is liquified by nitric
acid.
Any thoughts or comments on this hypothesis?
I weep at the sight of flaming acetic anhydride.
|
|
|
Polverone
Now celebrating 21 years of madness
Posts: 3186
Registered: 19-5-2002
Location: The Sunny Pacific Northwest
Member Is Offline
Mood: Waiting for spring
|
|
It is an interesting idea. You might try inquiring further on sci.chem or sci.materials. Or, assuming that you want a "learn by doing" experience, I
could visit the library and look for analytical methods for sulfur. You analyze some rubber before nitric-attack and then after, and see if sulfur's
gone away. Actually, for qualtitative testing, I think I can recall how to do it off the top of my head: lead acetate/ nitrate will yield a dark
precipitate in the presence of hydrogen sulfide (or sulfides of more active metals). Polysulfides (not sure about regular sulfides) will yield a
superfine, very pale yellow precipitate of elemental sulfur in hydrogen peroxide. Now, the question, how to turn that sulfur in rubber into sulfide?
Elemental sulfur readily forms sulfide/ polysulfides with hot aqueous NaOH. Will sulfur- containing rubber do the same? I don't know, but I intend to
find out now! The weather has suddenly turned nasty again here, so it may be a while until I can make some more nitric acid to complete the second
part of the experiment. Perhaps we can mutually verify results of our experiments.
PGP Key and corresponding e-mail address
|
|
|
madscientist
National Hazard
Posts: 962
Registered: 19-5-2002
Location: American Midwest
Member Is Offline
Mood: pyrophoric
|
|
I will go get some NaOH soon and experiment with that idea of yours. I intend to produce at least 50ml of fairly concentrated HNO3 tonight, so I
should be able to do some experimenting soon.
I weep at the sight of flaming acetic anhydride.
|
|
|
Polverone
Now celebrating 21 years of madness
Posts: 3186
Registered: 19-5-2002
Location: The Sunny Pacific Northwest
Member Is Offline
Mood: Waiting for spring
|
|
Well, I let 2 g of rubber (from plain, uncolored rubber bands) boil overnight with NaOH/denatured alcohol (added some water as the alcohol boiled
off). This morning the rubber bands were swollen up like dead worms after a rain shower... I tried adding some of the liquid obtained to lead nitrate
solution, but I just got a white precipitate. Obviously the sodium hydroxide reacting with the lead nitrate... So then I tried silver nitrate. Silver
nitrate crystals turned a very satisfying black when placed in the liquid. Is this a positive test for sulfur? I tried mixing up some more denatured
alcohol/NaOH (no rubber bands) and added some silver nitrate to that. The crystals turned a deep brown (not the black I got with the rubber band
liquid). Different, certainly, but not enough of a difference to inspire confidence in my analytical methods :-).
So I think I am going to try an alternate method. I will get some canning wax and heat it with the rubber bands. I think (hope) that this will convert
sulfur to H2S... And then I can detect this with my nose or filter paper moistened with silver nitrate solution. At least that's the plan right now. I
tried looking last night online to see if I could find information about typical sulfur content in rubber, but found nothing relevant. So I don't even
really know what an appropriate baseline value is.
PGP Key and corresponding e-mail address
|
|
|
PHILOU Zrealone
International Hazard
Posts: 2893
Registered: 20-5-2002
Location: Brussel
Member Is Offline
Mood: Bis-diazo-dinitro-hydroquinonic
|
|
Ag hydroxyde is black! 2AgOH turns fast into black-brown Ag2O by water loss!
AgNO3 is a strong oxydiser, it leaves atomic Ag or molecular Ag2O on organic stuffs, reason why you had brown-black strains.
I think that the best way to test for sulfur is to neutralise a little your alkaline NaOH media with H2SO4; then add some CuSO4...Cu(OH)2 is pale
truquoise blue precipitate while CuS is black very unsoluble precipitate!
Rubber depending on its composition:
Natural is -CH2-C(CH3)=CH-CH2- It is not very strong and to reinforce it, it receive addition of S at high T (process is called vulcanisation): S
makes likages between polymer fiber by addition to the C=C double link. You then end up with -CH2-C(CH3)(-S-)-CH(-S-)-CH2- .The CS- can link to other
-SC making dissulfur links like the ones found in many proteins like hair proteins (permanent hair dressing is based on breaking S-S bonds and
reconnection in shape).
Natural rubber is oxydised on the C=C by HNO3 and makes -CO-, -CHOH- and -CO2H; all those groups favourise dissolution, reason why natural rubber
seems to melt in HNO3.
Vulcanised rubber don't do so, the C-S-S-C are oxydised into -C=S, C-S-C, C-SO-C, C-SO2-C that increases melting point and intermolecular forces (VDW)
and Coulomb forces; then when you look at rubber in HNO3 it looks like it is hardening and become cristalline, breackable. This proces of oxydation
happens also but a little less faster when vulcanised rubber is exposed to oxygen and sunlight; peroxydation occurs and the elasticity is lost; you
get same symptoms as with HNO3 oxydation.
Some perfluororubber are resistant!
Some chlororubber may resist longer than the two first mentionned rubber ...
Silicon rubber ... everything depends on the rubber you use!
PH Z
|
|
|
bfesser
|
Thread Pruned
2-3-2014 at 05:32 |
![[*]](images/xpblue/default_icon.gif){kind=icon}
