Ask A Scientist© Environmental Earth Science Archive

name         Nadia S.
status       student
age          13

Question -   What real-life situations involve using an inhibitor?
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Presuming you mean a polymerization-inhibitor chemical additive, there are lots of resins you 
want to buy as liquids, in a bottle, then cure.
The molecules of these resins or glues almost always have double bonds (-C=C-), which are very 
susceptible to half-breaking-open and cross-linking to other molecules, spontaneously 
solidifying all the liquid in your bottle.  when one double-bond breaks
open, one of its dangly ends attaches to another double bond, making it break too, and so on.   
A not-very-useful chain reaction.  Some glues would do it in one month, so the makers add 
~0.1% inhibitors to increase the shelf-life of the bottle to
half-year or more.  Inhibitors "use up" occasional dangly bond ends before they can find 
another double bond to link with, so the chain-reaction is stopped wherever it tries to 
start.


Ordinary casting resin is a fine example.  There are polyester molecules, but there are also a 
large percentage of styrene molecules (H2C=CH-C6H5, might be described as phenyl ethylene or 
vinyl benzene).  This styrene is very reactive, and it's a small
molecule which evaporates easily and gives casing resin it's chemical stink.  (I think styrene 
stink, and hence many casting resins, are a bit more hazardous to health than is usually
 emphasized.)   If small molecules polymerize, many new bonds are made,
releasing lots of heat.  Styrene content is why polyester resin can overheat itself and crack
 and turn brown if it cures (polymerizes) too fast.
    I hope I am not rambling.  My point is, I once bought a quart can of 100% styrene monomer 
	liquid.  Without inhibitors, it would solidify in 10 seconds to 1 minute, and get so hot 
	I wouldd have to drop the bottle.  "Hazardous polymerization" is even one of
the hazards listed on every MSDS chemical safety sheet, right after flammability and such.
    When you add "catalyst" to casting resin, it is really an oxidizer.  It uses up the 
	inhibitor, then what is left attacks a few double bonds, starting polymerization.
   Oxygen in air often acts as an accidental inhibitor.  I think this is why the surface of 
   some resins remains tacky, incompletely cured.
Some glues, like Loctite screw-lockers, use this oxygen inhibition to stay liquid.  Then, 
when the glue is really buried in solid parts, the dissolved air slowly gets used up, and 
suddenly the glue cures in place.   Lots of tricks like that.
   Cyanoacrylate "super-glue" probably needs inhibitor component.  If they add too much, it 
   is reluctant to harden when you put the parts together.  If they add too little, it hardens 
   in the bottle too soon or too easily.
   There is also the industrial solvent Tri-Choro-Ethylene,  (Cl2C=CHCl), which is fine at 
   rinsing grease away and we never want to polymerize it.  A little inhibitor is added to 
   every bottle of that, too.
    Hydrogen peroxide (H-O-O-H) has no double-bonds, but the single-bond between the oxygens 
	is vulnerable to chain reactions which change it to plain water plus oxygen bubbles.  It 
	always has some kind of inhibitor too.  Without it, your drug-store 3%
peroxide would fade to useless in 1 week, and a gallon jug of 30% peroxide would boil out 
dangerously at any moment, and the 90% peroxide used for a few things like back-pack rocket-
suits would be completely impossible.
    Acetylene welding gas has not a double but a triple(!) bond (H-C=-C-H).  Used all the time for cutting and welding heavy steel parts.
Polymerization of this would be extremely hazardous, because it would definitely make enough heat to burst its bottle and catch fire. To suppress it, they dissolve the acetylene 50% in acetone (CH3-CO-CH3).  Seems to me there better be more inhibitors
than catalysts in there, too.  But I have never heard it mentioned.

Hope that pin-points it, Nadia.
Jim Swenson
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