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West 105 got really hot. Normal?

AlpineYoda

Well Known Member
Patron
Mixed up a batch of West 105 / 206 (slow) with a very large proportion of cab-o-sil. I've mixed 105/206 several times, but this was a first.

Within about 3 minutes, the mixture got very hot. I was using an old plastic take-out food bowl and it got too hot to hold. I even started to see steam / smoke come out of the bowl. Within 10 minutes, chunks started to form in the mixture. I was using 206 - SLOW hardener - but this hardened up really fast.

I've made 10-12 batches of West before and it has gotten warm, but nothing like this.

I used about 7-8 squirts of both fluids (I use the measured pumps to get my mixture right.)

It is about 95 degrees today. Probably 90+ in the hangar right now.

Is this normal? Or did I make a big mistake somewhere?

The parts I was bonding / filling both appear okay. Warm but not hot. It's a thin layer applied of the mix between fiberglass and aluminum.

I left the bowl outside on a pile of gravel in case it catches fire - still very hot, but no flames.
 
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Mixing large quantities of epoxy will result in it getting very hot. It can “cook off” and melt your container. It may affect the properties of the epoxy, too. A larger surface area, like in a pan, will not be as deep and won’t get as hot. I usually just mix smaller batches, 2-3 pumps. :rolleyes:
 
I didn't realize 8 pumps was a "large" quantity.

How do the pros do it? When Van's makes a cowling or a cabin top, they must be using a heck of a lot more than 8 pumps worth of epoxy.
 
As a general rule, all epoxies exotherm, more so when using a faster hardener. The trick is allowing the generated heat to escape before it further accelerates the cure, which generates even more heat, which rapidly leads to a runaway cure.

In addition to hardener choice, the other factors are ambient temperature, additives, and container size/shape/material.

High shop temperatures require slower hardener.

Additives, particularly micro balloons, tend to serve as built-in insulation. Heat from the center of the batch has trouble reaching the external surface of the container (where it can escape) because the hollow air spaces limit conduction.

A large bucket shaped container also limits heat transfer from the mix. Foam and plastic materials too.

So, any time you mix a large batch of epoxy/micro, or in this case epoxy/cabo, immediately dump the entire contents of the mixing cup out onto a sheet of 4 mil plastic. Spread it out on the sheet so it can stay cool, and the cure rate will be as appropriate for the hardener choice and ambient temp.

Speaking of which, 90F is mighty warm. Pot life is generally rated near 77F/25C.
 
I didn't realize 8 pumps was a "large" quantity.
How do the pros do it? When Van's makes a cowling or a cabin top, they must be using a heck of a lot more than 8 pumps worth of epoxy.

Google the magic word: prepreg.
 
It's the cab-o-sil

I have experienced the same thing when mixing in a large quantity of cab-o-sil.
I repeated it by mixing the same amount of epoxy with no cab-o-sil and it was fine. Kept adding cab-o-sil and at a certain point it got really hot and started smoking.
 
As a general rule, all epoxies exotherm, more so when using a faster hardener. The trick is allowing the generated heat to escape before it further accelerates the cure, which generates even more heat, which rapidly leads to a runaway cure.

In addition to hardener choice, the other factors are ambient temperature, additives, and container size/shape/material.

High shop temperatures require slower hardener.

Additives, particularly micro balloons, tend to serve as built-in insulation. Heat from the center of the batch has trouble reaching the external surface of the container (where it can escape) because the hollow air spaces limit conduction.

A large bucket shaped container also limits heat transfer from the mix. Foam and plastic materials too.

So, any time you mix a large batch of epoxy/micro, or in this case epoxy/cabo, immediately dump the entire contents of the mixing cup out onto a sheet of 4 mil plastic. Spread it out on the sheet so it can stay cool, and the cure rate will be as appropriate for the hardener choice and ambient temp.

Speaking of which, 90F is mighty warm. Pot life is generally rated near 77F/25C.


This was very useful. Thank you for so much thought and effort.

And yeah, 90F is pretty hot today. The hangar has heat, but no AC. I can work on these parts now in this heat, or, checking the forecast... October??
 
Pumps

Also possible the pumps are not dispensing exact quantites but it sounds like a combination of factors resulted in a runaway exotherm. Generally it's considered unusable. I toss the cup in a bucket of cold water.
Like Dan said. Prep everything and pour it as soon as possible or pour it into something so it's more spread out.
 
Great comments from Dan

Question- I have experienced random episodes of faster than normal cure rate with reasonably small quantities. I concluded the common thread was mixing new ingredients in a container with “ work in progress “ material and a wet stir tool from the previous batch. Is it possible for material late in the reaction process to act as “ super “ catalyst ?
 
Question- I have experienced random episodes of faster than normal cure rate with reasonably small quantities. I concluded the common thread was mixing new ingredients in a container with “ work in progress “ material and a wet stir tool from the previous batch. Is it possible for material late in the reaction process to act as “ super “ catalyst ?

Don't know! I use a fresh cup for every batch.
 
Rutan

See Moldless composite aircraft construction video done by Burt Rutan.
This is where it all started, and lots of us learned from it!
Small batches...lots of cups, stir sticks, brushes, etc...
Large layups need one or two mixing and others working to apply it.
The pumps make it nice and easy to dispense for sure.

https://youtu.be/p9GTYsNBiaE
 
epoxy

I found that 3 pumps of resin at a time, in a bowl will work. As has been said, if you need more, spread it out or use multiple bowls.

...and I never use bowls twice, especially when one is much further along in the curing process...
 
Run away heat

Imagine a box/container, with each side being three boxes long. Each side of the large box has nine boxes in it. So the surface of the whole container shows 9 boxes per side x 6 sides = 54 box sides of surface area. There are actually 27 total small boxes in the large box though. If you were to mix in each of these individual boxes instead of the one large box made up of the 27 smaller boxes, you have 162 box sides of surface area. Three times more area to dispense heat if you mix in small batches. Like mentioned before, the heat going up feeds on itself, making the reaction go faster and faster, giving off more heat per unit time. Smaller batches, and cooler room temperatures are the answer…
Jim Baker
RV-6
 
So, any time you mix a large batch of epoxy/micro, or in this case epoxy/cabo, immediately dump the entire contents of the mixing cup out onto a sheet of 4 mil plastic. Spread it out on the sheet so it can stay cool, and the cure rate will be as appropriate for the hardener choice and ambient temp.

I use this method too. It's tough to keep the epoxy from flashing off in the hot summer weather.
 
Question- I have experienced random episodes of faster than normal cure rate with reasonably small quantities. I concluded the common thread was mixing new ingredients in a container with “ work in progress “ material and a wet stir tool from the previous batch. Is it possible for material late in the reaction process to act as “ super “ catalyst ?

Larry,
I am probably reading this wrong, but you should not add new resin and hardener to an existing mixed batch - or even used container.

I learned that in my boat building days the hard way!
 
Every temp increase of 10 deg F cuts the cure time in half. If it is In a pot you minimize the surface area for heat to dissipate so as it gets hotter it speeds up the exotherm, which makes it hotter, which speeds up....etc. If you pour it out on to where you are working quickly and spread it out you won’t have a problem. I never mix more than 4 oz usually. If I am laminating a big part I will do more but then I don’t have to lose time mixing in filler etc. I mix and dump as quick as I can.

Also, and Dan has said this numerous times, the pumps are sketchy. After reading one of his posts I tested mine with scales and the mix was off by 20%. I keep them for dispensing convenience, but I do my mix by weight only. Small digital scales from amazon or horror fraught are very cheap.
 
I mixed up some epoxy on a very hot day last summer and noticed that it cured much faster than normal, and with a great deal of heat and fumes. I seem to remember that the label on the epoxy advised using it at more moderate temperatures.
 
I grew up around my dads business fabricating large fiberglass vessels for the petrochemical industry. The work was done in an open shed in Baton Rouge. So the temps were 90-100 during the work day 4 mo a year. When the temp was over 85 they used a very slow hardener. The mix was limited to 16 oz at a time and as soon as the mix was complete it was poured out thinly on a large visqueen sheet. The materials were wetted and applied. The sheets were rotated as in the epoxy was allowed to dry and a wet roller system would flex and clean the plastic for reuse the next day. They managed the exotherm by getting it spread out as fast as process would permit.

So to the OP I’d say mix what you need just get it spread out and no cup work in more than oz quantities.
 
I have learned my lesson! Thanks everyone for the info.

I had to do a lot more fiberglass work last night - I was bonding the two pairs of door shells together.

I moved everything in the office building attached to my hangar and found an empty conference room in the 70s. I covered everything in drop cloths and cardboard. I mixed similar combinations of 105/206/cabosil as earlier in the day, but I limited it to 2-3 pumps at a time. Bonding the doors meant that I was making 5 batches or so per door, but I kept them small.

Pot life was the expected 30+ minutes. Nothing ever got too hot.

So, lesson learned. Don't work when it is too hot and keep the batches small!
 
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