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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
"The pair had been working on making the chemical inert to dispose of it" and melting bucket = they mixed up too much and it ran away thermally, and dumped a whole bunch of foul decomposition products in the air.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
It's good for amateur epoxy mixers to think about unexpected consequences, especially when going big for the first time, or the 10th time. What happens if you pick the wrong can, or get the proportions backwards? What will you do if bucket starts melting? What if hot chemicals erupt & you're not wearing your PPE?
The Stangenes company for sure has procedures in place for using large-ish volumes of epoxy. Their standard epoxy-cast isolation transformers go up to 1 kVA 50/60 Hz with 50 kV isolation. Standard "high voltage" isolation transformers go up to 44 kVA, with from 150 to over 500 kV isolation using epoxy, oil, and SF6 insulation.
Conkbot, I wouldn't be surprised if you read it right -- that the attempt to "make stuff inert" happened at the beginning instead of the end of the incident. Hadn't occurred to me. Maybe the techs had to dispose of some gallon lots that were past their official expiration date, and decided to turn all of the old Part A's and Part B's (and Part C's?) on the shelf into one solid lump. Remember the TEPCO uranium criticality accident?
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
If you mix up a large batch of the sort of epoxy that's intended for use as glue- in order to use it for potting- it will overheat. The same would happen if you did so to dispose of out of date stock of glue (as speculated earlier). Potting compounds are chosen for low exotherms. Glues aren't. There may, of course, be stacks of other factors that didn't make it into the report..
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Bored Chemist wrote ...
If you mix up a large batch of the sort of epoxy that's intended for use as glue- in order to use it for potting- it will overheat. The same would happen if you did so to dispose of out of date stock of glue (as speculated earlier). Potting compounds are chosen for low exotherms. Glues aren't. There may, of course, be stacks of other factors that didn't make it into the report..
Even some potting compounds have their limits. Once I work with frequently is glass microbead filled, and functions pretty good as an insulator. A small sample will take 24 hours to get reasonably solid and 48-72 to reach full hardness. Mix up 400+grams of resin with the prescribed amount of hardener in a cup of typical proportions, and in 5-10 minutes you'll have a sticky clump in the middle where it overheated itself.
However I was sorely disappointed when I had an entire gallon of off-spec (too viscous) of resin to dispose of. Assuming I ended up light on hardener from not having something to tare out the paint can and just estimating, and the epoxy wasnt warm from pre-mixing like it normally is, and it was a relatively cool day inside. It expanded ever so slightly and had some bubbles on top, but took its sweet time curing. If the resin was hot, and warm inside, it probably would have kicked off and cured in an hour or so. All of this was in a fume hood with a pan under it. (safety first-ish)
2-part anything can be quite temperature sensitive. Smooth-on 2-part silicone (for special effects, mold making etc) is meant to go on skin, and they have a 5 and a 20 minute version. But used on objects instead of people, it takes 20 or 50 minutes to get firm, because its in a 70-75F room instead of on 80-90 degree skin.
klugesmith wrote ...
It's good for amateur epoxy mixers to think about unexpected consequences, especially when going big for the first time, or the 10th time. What happens if you pick the wrong can, or get the proportions backwards? What will you do if bucket starts melting? What if hot chemicals erupt & you're not wearing your PPE?
The Stangenes company for sure has procedures in place for using large-ish volumes of epoxy. Their standard epoxy-cast isolation transformers go up to 1 kVA 50/60 Hz with 50 kV isolation. Standard "high voltage" isolation transformers go up to 44 kVA, with from 150 to over 500 kV isolation using epoxy, oil, and SF6 insulation.
Conkbot, I wouldn't be surprised if you read it right -- that the attempt to "make stuff inert" happened at the beginning instead of the end of the incident. Hadn't occurred to me. Maybe the techs had to dispose of some gallon lots that were past their official expiration date, and decided to turn all of the old Part A's and Part B's (and Part C's?) on the shelf into one solid lump. Remember the TEPCO uranium criticality accident?
Step number one for epoxies : read the TDS, read the TDS read the TDS. Most should give warnings, and a specification for the pot life measurement. (I.e. pot life, 90 minutes, 100 grams in a cup) etc. Some non epoxy resins (polyesters I think? ) use a relatively small amount of hardener, and you can vary the cure time by adjusting how much you put it. Or you vary the hardener depending on what thickness of resin you're using. (I.e. 1/8" thick use 6 drops /oz, 1/4" thick use 5 drops/oz, 1/2" thick use 4 drops/oz, etc)
In general with modern adhesives, you can do pretty amazing things, you just have to select the right adhesive, and use it correctly. 5 minute doesnt work for everything ;)
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
I must admit I've done a few silly things with epoxy. However, we also made many small samples prior to making mistakes that were not documented in the MSDS or manufacturers recommended practice documents. Things I learned:
* Low viscosity epoxies tend to have less filler agents, and or contain solvents that affect curing & drying times (alcohols will often make the material seem like a rubbery gel for several weeks).
* Half hour transparent casting epoxy catalysts are often rather poisonous, and reaction speeds can rapidly change. We were using the same standard plastic additive mix ratio as usual, and for various reasons it flash-potted in about a minute with one brand. Indeed, there would have been a bigger problem with a larger volume. Note, even fabric softeners may contain Triethylenetetramine, and many suppliers may not include a full list of ingredients.
* Some manufacturers recommend a layered pour in the fine print, where each 1" thick layer of material must cure to a tack surface prior to mixing and pouring in a new layer.
* Slow cure (1 week) or oven cure epoxies tend to be stronger.
@Bored Chemist Who knew plaster was just as dangerous as lawn darts.
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Epoxy potting accident
