Now, this got me kinda interested,being as I have seen a couple burnt boards over the years…well,decades…
Polystyrene( PS ) is a thermoplastic: It can be heated,melted, reformed and recycled,so that the melting temperature ( circa 240 °C ) causes it to become a liquid, taking up a lot less room than as a foam and demonstrating that kind of ‘wet bread’ look as in http://www.photobox.co.uk/shared/photo.html?c_photo=1324431554
Okay, this is indeed interesting, as it’s indicating that a temperature > 240°C may have been reached in a few spots.This has some implications for use with Polyurethane ( PU ) foams.
Polyurethane is a thermosetting plastic. That is,it doesn’t melt,it burns, given sufficient heat and asupply of oxidiser. And, according to http://www3.interscience.wiley.com/cgi-bin/abstract/113507128/ABSTRACT
Quote:
Abstract[/url]: Oxidation and ignition of flexible polyurethane foams have been investigated by observing the effects of internal and external heating. <span style="font-weight:bold">External temperatures of some 190°C are required to induce combustion</span>. <span style="font-weight:bold">Internal temperatures of 250 -350°C initiate a self-propagating internal reaction which results in foam ignition when the reaction reaches the foam surface</span>. The stability of a polyurethane foam to such heating increases with the age of the foam.
Boldface added for clarity-
Basicly, if you get it hot enough, urethane foam will catch fire.And if you heat it up enough, it’ll burst into flame when it gets some oxidiser,like,oh, for instance, air.
This in turn raises a couple of questions;
Can you set fire to a board by using too hot a batch of resin for a large fding repair/filler?
Yes, you can. It’s not just a surfing urban legend.
How does one slow down the reaction?
In addition to using less catalyst,plasticisers ( like cabosil and q-cell) ,pigments, etc in polyester resins, you can also do it in a cool,shaded place. Also,chilling the resin might help, as the original temperature of the resin should have quite an effect on how fast it initially goes off,the rate of heat generation by the resin should be slower and the ability of the resin to lose heat fast enough to keep it from ‘overreacting’ might be preserved.
It might be useful to run a fan over the spot too, but if it does catch fire, well, the term ‘fanning the flames’ comes immediately to mind.
Adding less catalyst ( hardener) isn’t an option with epoxies, but plasticisers, pigments and such can’t hurt, along with chilling the resin components. That has also been shown to add to pot life and working time without damaging the ability of the epoxy to harden. I wouldn’t go putting the stuff in the freezer, but in a regular refrigerator at around 42° F ( 10° C) could help a lot
Hope that’s of use
doc…