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Name: Nadia S.
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: February 2004


Question:
What real-life situations involve using an inhibitor?



Replies:
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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