I have started using epoxy, the 5 minute kind, as an adhesive to mount some motor mounts.

I like the fact that it gives me some time to move the mount into position without 'siezing' halfway in like white/yellow glue can do at times.

However I had a centering ring let go in a rocket from the body tube I used epoxy on. It got me to wondering, can the heat from an engine degrade an epoxy joint?

What about silicone? Any ever use that for building?

Five minute hobby epoxy is not the best choice for motor mounts. Perfectly mixed it will soften with heat, but the problem is, you almost never get a perfect mix and that's even less heat resistant.

Anything up thru 29 millimeter is well within the strength of aliphatic resin.

I wold not consider any form of silly-seal for any rocket application.

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I use 30 minute epoxy. It flows a bit better and penetrates the surface much better before it sets up into the bt. Fins, also.
The extra weight is a pittance.

Allen.

It's all a matter of preference. 30 minute epoxy, white glue, yellow glue all work great, and all have advantages and disadvantages.

Epoxy....

The good:
Doesn't shrink and cause banding to show on the outside of the body tube.
Doesn't grab.
Holds just fine as long as you use longer setting stuff as Jetlag mentioned.

The bad:
Can eventually cause an allergic reaction, even if you've used it dozens of times without issue.
Has to be mixed fairly accurately each time you need it.
Smells.
Cleans up with alcohol.

Various white and yellow glues.....

The good:
Cheap.
Simple to use. No mixing, just open and squirt.
Cleans up with water. If on your hands, just rub them together.
Haven't heard of anyone being allergic to it.

The bad:
Shrinks (most types anyway) and causes banding on the outside of the body tube.
Can grab if parts fit a little tight and not enough glue is used to make sure it stays wet as the part slides in. Some specific versions are worse than others.

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I love sanding.

The Epoxy I use(Araldite) softens at 60degrees C(140 f), big warning on the packaging. It will depend on brand/formulation but I suspect it won't change much, a few tens of degrees maybe.

I use Epoxy everywhere EXCEPT for MMT's, that is where JB Weld is king, which softens above 315degrees C.

Fom memory, motors will not pass certification if their case temperature exceeds 260deg C, which is well above Epoxy softening temperature but below JB Weld softening temp.

Also never use CA, It's too brittle. I only use it to tack pieces together before going back over with epoxy fillets.

White glue (PVAc) is water soluble and softens in high humidity(tested, proven)
Yellow glue (Aliphatic resin) also water soluble, is apparently not affected by moisture/humidity after it has cured. I have not tested this. Not sure about effects of elevated temperatures.

Agree with all this... and tbzep's post...

Generally speaking, the longer the cure time, the better and stronger the epoxy. So the five minute stuff is the lousiest, followed by the 15 minute stuff, which isn't as good as the 30 minute stuff, etc.

Heat resistance shouldn't be a problem; tube couplers are exposed to heat for such a short period of time it shouldn't be a factor. Motor mounts, which are prone to heat-soak from being in direct contact or close proximity to the hot motor casing(s).

I like white and yellow glue for a lot of things, but tube coupling isn't their strong suit-- too prone to freezing. It CAN be done, but the tubes/couplers have to be a smooth, easy fit (not too tight) and you want to make the joint SLOPPY WET with glue to ensure they don't stick, and assemble the joint with one smooth, STOP FREE motion... The reason that tube couplers tend to stick using white or yellow glues is that when smeared in a very thin layer, as when a coupler slides inside a tube, with the fibrous and water-absorbent properties of the paper tubing materials, the paper outer layers of the coupler and inner layers of the tube itself tend to near-instantly wick the moisture out of the very thin layer of glue between the two parts, causing the glue to 'set up' or "lock up" very quickly. Using a "sloppy wet" thick layer of glue, preferably between a tube and coupler that are not extremely tight fitting, is the best way to ensure success. A VERY tight-fitting coupler/tube combination and a thin layer of glue is almost certain to lock up before the tubes and couplers are in position. Using a thick layer of glue helps "lubricate" the joint with lots of wet glue, but means a little more cleanup later... though wiping off wet glue isn't much of a problem...

Of course, epoxy, not relying on solvents/water evaporation for curing, eliminates that problem, but as mentioned, with problems of its own, like the smell, mixing waste, and eventual sensitivity (allergies).

Later! OL JR

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The other variable to consider is the mix itself. Perhaps your resin/hardener ratioi was off. Probably a long shot but possible. I've had good luck securing motor mounts with Gorilla polyurethane glue. No mixing, longer working time than white/wood glue and no chance of puckering thin walled cardboard tubes.

Never considered silicone for model rocket construction.

I use white glue for the centering rings to the motor tube, BSI's 30 minute Epoxy for the motor mount to the body tube, and then use TTW fins to lock everything in place. My fins tabs are glued in with white glue, in direct contact with the forward CR, then I use internal fillets (epoxy) to strenthen the bond, and epoxy the aft CR in place after the fillets are finished.

With this method, it'd take a lot of things going wrong for the mount to break loose, and by then, I'd likely not want to launch that rocket again (without major repairs).
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Dreaming of making the rockets I dreamed of as a kid (and then some).

"The Guide says there is an art to flying", said Ford, "or rather a knack."
"The knack lies in learning how to throw yourself at the ground and miss."

Launching is Optional... Landing? That Depends on Trees.

I once bought a launch set off of EvilBay that included a partially built model, the builder used electrical tape to try to create a centering ring for the motor mount. Needless to say, it didn't work.

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Dreaming of making the rockets I dreamed of as a kid (and then some).

"The Guide says there is an art to flying", said Ford, "or rather a knack."
"The knack lies in learning how to throw yourself at the ground and miss."

Launching is Optional... Landing? That Depends on Trees.

Thanks for the input everyone. Makes a lot of sense.

I like the suggestion of JB weld. I have some and have used it in the past. Only downside is the cure time.

Pretty sure I got the mix correct and mixed thoroughly, I've been working with epoxies for what seems like 100 years on various modeling activities so pretty familiar with working with it. Oh never developed an allergy to it either /shrug.

Only brought up Silicone because of its very high heat handling capabilitys but now that I think of it, it probably would not bond to the tubes very well. Just thought I would throw it out there!

Anyhow thanks again, vast amount of experience out there!