Another thing I never heard of before

[smorris]

http://www.hammerzone.com/archives/s...e_shavings.htm

Urethane shavings self-ignite. I'm familiar with the dangers of oil finishes and such but never thought about urethane.

[Mr__Bill]

Thanks for the link. That is something that is good to know. I would never have thought that would have happened. Now I have something new to worry about.



Bill

[cabinetman]

Regular varnish with the same conditions could do the same thing. Two part epoxy and fiberglass resin mixtures with catalyst should be watched and waste material disposed of properly.
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[phi1l]

Ahem...

regular varnish won't do the same thing. I can't..

Urethane is different from traditional finishes in that it is a polymerized plastic. It cures & doesn't dry like a varnish. A varnish can be re-dissolved & removed urethane is just a layer of plastic on the surface.

IIRC polyurethane needs oxygen for the polymerization process to take place. If you put on too think of a coat the surface cures at the normal rate but under the surface the curing process takes place much slower because oxygen can't get where it's needed.

What happened to that guy was that he sanded down to the uncured resin, exposing it to oxygen with a very high surface area to volume ratio. That rapidly accelerating the exothermic curing process. Notice that the "saw dust" had solidified & couldn't be removed easily from the plastic cup?

The heat generated in curing is normally not noticed because the curing rate usually limited by.

Normal wood saw dust can get hot & spontaneously combust too, of course, but that is from decomposition.

The lesson to be learned here is, there is a reason they say apply ploy in thin coats, & leave plenty of time to "dry" (cure actually) between coats.

[cabinetman]

Ahem...

regular varnish won't do the same thing. I can't..

How do you know...have you tried a test? The next time you get the chance to scrape off uncured varnish and collect it, check to see if it heats up.
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[phi1l]

How do you know...have you tried a test? The next time you get the chance to scrape off uncured varnish and collect it, check to see if it heats up.
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Varnish doesn't cure, it dries. The drying process is a endothermic (it requires energy to convert the solvent from the liquid state to a vapor. So if you were to scrape or sand off incompletely dried varnish it will cool as the varnish finishes drying.

Varnishes & paints consists of what is termed a vehicle and (in the case of paint) pigments dissolved in a solvent. when paint/varnish is applied the solvent evaporates leaving the solid vehicle/pigments on the surface absorbed into the wood.

Polyurethane is actually isocyanate monomer suspended in the solvent. I remembered incorrectly, it is actually the water vapor in the air that is required for the polymerization process to take place. What is left behind when the solvent evaporates is the polymerized plastic. I can't be converted back.


Now if you take a really dry varnish & sand it to small enough particle size you will get a high enough surface area to volume ratio that you can get spontaneous combustion (as you could with any organic dust), But that is an entirely different process.


Chemical Engineers know these things.

[cabinetman]

Varnish doesn't cure, it dries. The drying process is a endothermic (it requires energy to convert the solvent from the liquid state to a vapor. So if you were to scrape or sand off incompletely dried varnish it will cool as the varnish finishes drying.

I'm not arguing the semantics of dried or cured. I'm just asking if you tried it?
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[herb fellows]

Thanks for this. I'm sure I have dumped sanding dust from poly into a garbage can and never thought about this, good to know.

[woodturner]

regular varnish won't do the same thing.

Agreed. Finishes that dry (solvent evaporation - not an exothermic reaction) instead of cure (an exothermic chemical reaction) can't spontaneously ignite.

The challenge with varnishes may be in knowing what type of oil was used to make the varnish. Tung oil is a catalyzing oil, for example, and varnish made with tung oil could be prone to spontaneous combustion.

Finishes that cure solely through solvent evaporation are probably the safest in this aspect. I keep trying to tell everyone that shellac really is the perfect finish . In any event, it does not spontaneously combust.

[woodturner]

have you tried a test?

In this case, it's not practical to test all the possible variations, so a single or small number of tests would be of limited value, essentially anecdotal evidence.

For example, I could easily conduct a test to show that poly does not spontaneously combust - but obviously, based on the link posted, those results would be incomplete and misleading.

The challenge in testing is to develop a practical test set that covers enough of the possibilities to support reasonable conclusions. Developing that test set is very challenging.

[David D]

That stuff can definitely spontaneously combust. Had some neighbors across the street who had refinished their floors and laid the poly on too thick. Started scraping the stuff back off and put it into a trash can on the back screened in porch. Wife woke up in the middle of the night to get a drink of water and saw the flames. Ended up having to gut and rebuild the entire house. Lucky for them, she woke up thirsty. The fire started right below their daughter's bedroom! They all got out in time, but lost most everything in the house. I am VERY cautious with anything even remotely combustible.

[cabinetman]

In response to the expectation of what chemicals can, can't or could do, the real world of working with materials and chemicals is a bit different than textbook explanations. Any solvent or oil based finish material has the propensity to be combustible, or ignite, heat up, under the right conditions.

From Wiki: Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent.

All drying oils, certain alkyds, and many single-component polyurethanes produce heat during the curing process.

I'm not going to debate the many meanings of "dry" and "cure".

For those that use varnishes and polyurethanes and urethanes, should not assume that there is no danger. Proper care in their use and disposal is of great importance. I just don't agree with statements that have "can't" or "don't" in them, when it involves the potential for danger. I trust what I experience personally, and the warnings that accompany the products.
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[woodturner]

From Wiki: Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent.

All drying oils, certain alkyds, and many single-component polyurethanes produce heat during the curing process..

That's one of the problems with wiki's - they are not reliable. Anyone can post anything to the wiki's, and unless there is a more knowledgable person reviewing it, the misinformation remains.

The distinction between "drying" and "curing" is significant - drying oils cannot spontaneously combust, curing oils can.

If you follow the wiki link for "drying oils" we find:
"The term "drying" is actually a misnomer - the oil does not harden through the evaporation of water or other solvents, but through a chemical reaction in which the components crosslink by the action of oxygen. "

So it appears whoever wrote the drying oils wiki page understands the difference.

[cabinetman]

That's one of the problems with wiki's - they are not reliable. Anyone can post anything to the wiki's, and unless there is a more knowledgable person reviewing it, the misinformation remains.

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I agree with you...just like anyone that can make a post to a public forum concerning what may be hazardous and what isn't. I continue to respect and urge any users to pay attention to warnings and information that accompany products that are hazardous, flammable, toxic, or just plain dangerous.
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[Black wallnut]

I agree with you...just like anyone that can make a post to a public forum concerning what may be hazardous and what isn't. I continue to respect and urge any users to pay attention to warnings and information that accompany products that are hazardous, flammable, toxic, or just plain dangerous.
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I think the distinction Cman is making here is that regardless of the science behind finishes and how they act it is reasonable and prudent to read and follow the labels of the products you use. For most it is probabaly smarter to treat all as if there was a spontanous combustion hazard because if you guess wrong there can be lots at stake.