Does perturbing firm epoxy affect bond strength?

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rocketsam2016

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I've had a few cases recently where I wanted to perturb some epoxy that was very firm but not yet hard. For example: fine-tuning tilt of a fin or adjusting the shape of a fillet. Assuming I'm not trying to get the epoxy to attach to a new part of the surface, does pushing on the shape of the fillet or slightly changing the angle of a set piece meaningfully degrade the final bond strength?
 

GregGleason

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IMHO, no. Now if you pulled it off and then reattached it, then yes. But in this case you should be ok.

Greg
 

Bat-mite

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Along with Greg, I'm guessing no. The bond point is critical (epoxy gets into cracks, crevices, and crannies); and as Greg wrote, if you pull it off, you lost that mechanical bond. But I don't believe the outer shape has anything to do with the integrity.

Someone will be along with a real answer shortly.
 

SCP

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I had done some empirical testing at work on a related situation (threadlockers and cylindrical fit bonding agents - aka Loctite). There was a noticeable reduction in strength when joints were moved in mid-cure. I am not a chemist or anything, but it was just an empirical test we did a few years back in the middle of a supplier quality crisis. My explanation is something like: If you think of polymer chains beginning to form together and cure, and you disturb them, they cannot re-bond...again I am not a chemist, and did not have to go further with the test after we learned to just not move the particular joint that was failing. Maybe epoxy is different?
 

Steve Shannon

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I had done some empirical testing at work on a related situation (threadlockers and cylindrical fit bonding agents - aka Loctite). There was a noticeable reduction in strength when joints were moved in mid-cure. I am not a chemist or anything, but it was just an empirical test we did a few years back in the middle of a supplier quality crisis. My explanation is something like: If you think of polymer chains beginning to form together and cure, and you disturb them, they cannot re-bond...again I am not a chemist, and did not have to go further with the test after we learned to just not move the particular joint that was failing. Maybe epoxy is different?
I would expect exactly the same thing from epoxy. The bond (peel strength) may not be compromised, but the strength of the epoxy, meaning its ability to withstand stress, could easily be weakened.
 

FredA

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Absolutely weakened - you are breaking chains that are forming and the more solid state gives less freedom to re-form.
 

rocketsam2016

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Absolutely weakened - you are breaking chains that are forming and the more solid state gives less freedom to re-form.
So how does one know when it's not OK to adjust things? Perhaps the pot-life is the limit? For rocketpoxy that would be 30-40 minutes. A lot of guides online though recommend waiting 30-45 minutes before even applying it for external fillets, which would definitely exceed this time.
 

Steve Shannon

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So how does one know when it's not OK to adjust things? Perhaps the pot-life is the limit? For rocketpoxy that would be 30-40 minutes. A lot of guides online though recommend waiting 30-45 minutes before even applying it for external fillets, which would definitely exceed this time.
I would try not to move it after the pot life time. I never have agreed with letting it partially cure to increase viscosity. I simply add sufficient thickening amendments when I mix it. Those amendments , unless micro balloons, increase the strength or toughness anyway.



Steve Shannon
 

rocketsam2016

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A little research suggests that "gel time" may be the time after which the epoxy strength is compromised by movement. I just sent an email to glenmarc asking about this and I'll report whatever they tell me back to this thread. The spec sheet only lists pot life (30-40m), and does recommend letting the epoxy sit 15-30 minutes before use for making fillets.
 

NateLowrie

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Pot life is defined as the amountof time it takes for an initial mixed viscosityto double, or quadruple for lower viscosityproducts (<1000 cPs). Timing starts from themoment the product is mixed, and is measuredat room temperature (23&#730;C). Variations in temperature, the amount of epoxy mixed, and when the epoxy is applied can affect curing time drastically.

Working life or gel time is the generally accepted time in which you need to have all of your movement done and parts in position. There is no universally accepted measure of working time. Even if there was, it is still highly dependent on site conditions like external temperature, how long it's in the pot, size of the pot, etc. For example, if you lower the ambient temperature from 80 degrees to 65 degrees, most epoxies will take twice as long to cure.

The processing parameters I settled on were: 1) control the temperature in the room to keep it at 70 degrees 2) Only work with the epoxy for the listed pot life 3) If mixing more than 75g of epoxy, apply it all in the first few minutes after mixing to prevent premature cook off and 4) Cure for 2 times the listed set time.

Those rules pretty much cover almost all my basis. Eventually I am going to build an oven to cure at elevated temperatures to better hone in the system. If your re-positioning because the epoxy is running, try a thixotropic agent like fumed silica to firm it up while at rest.
 

rocketsam2016

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I got a response back from Glenmarc saying it should be OK (great customer service to respond so fast!):

My email to them:
If I'm using rocketpoxy for a fillet application, is it safe to adjust the contour of the fillet after the rocketpoxy is firm but still malleable? It obviously wouldn't adhere to any surface area not in the initial bond, but what about manipulating the shape of the fillet? And in general, how long a window is there from mixing to when the epoxy shouldn't be mechanically perturbed at all?
Their response
Yes you can manipulate the shape of the fillet really up to the point it gets hard with little to none known bad issues. I am attaching a data sheet, if you follow the section on fin fillets it will give you great strength and great looking fin fillets.


The attached fillet instructions suggest waiting 15-30 minutes after mixing to apply, and then waiting another 15-30 minutes before then optionally touching up the edges.

So, seems like I'm in the clear for some nudging I did earlier this week to better shape a sagging fillet between a bulkhead and a coupler using my finger, but going forward I'll certainly still try and be cautious about it based on what folks in this thread have said.
 

jolevich

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Just wanted to clarify this. "you can manipulate the shape of the fillet really up to the point it gets hard with little to none known bad issues" I'm meaning that you can shape the outward edges and/or top of the fillets and it will be OK. I have built lots of very fast (Mach 3) and big N motor rockets and do this all the time and have never had a problem. As I know how things can get carried away very fast here on the forum I will explain this better. I am not advocating that you wait until the Rocketpoxy is very, very stiff and almost completely hard and then you completely lift it off, knead it and re-position it, that probably would not be a good thing for bonding strength.

But even if it is getting stiff and you wet your finger and/or tool with isopropyl alcohol and push around the top and/or edges to get a better shape you are looking for, I have yet to see this do anything that is detrimental to the bond for the rockets I have built and launched over many years, and I do this quite often. I am attaching a Rocketpoxy data sheet which may be helpful in the correct procedure in making fin fillets and general use.

View attachment RP_G5000_DATASHEET_P3.pdf
 
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rocketsam2016

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Thanks John for the detailed response, hope you didn't mind me quoting you :)

Yeah all I'm talking about is mild shaping without affecting the interface between epoxy and fiberglass.

It sounds like tilting a fin even slightly with stiff epoxy isn't gauranteed disaster but it's probably a terrible idea.
 

jolevich

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Hi Sam, no problem at all. Actually I appreciate discussions like this as it can be very helpful to others who probably have the same questions, and I am always very glad to help. While yes my company makes Rocketpoxy I am also a very active level 3 rocketeer including being the president/prefect of Michiana Rocketry (a high power Tripoli rocket club in the Midwest) so my more important goal is to help others who have questions and try and make sure high power rockets are being built in the safest manner possible.

I too must admit that I have been guilty of slightly tilting a fin (re-positioning to make sure it is perpendicular) after the Rocketpoxy was setting up and really have not found any ill effects from this. I'm not advocating this but sometimes a jig my slip a little and you catch it later than sooner and make the best of it. I think there is enough overkill in Rocktpoxy strength that if you plan and build your rockets correctly a few small items like this really will not do it in, at least that is what I have experianced.
 
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ksaves2

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My favorite albeit heavy epoxy that unfortunately went out of production was PowerPoxy Weld. Not a fancy high temp epoxy but I found one could get to
re-liquefy with a heat gun a couple of times. Some epoxies you hit with heat and it just sits there and cures. This stuff one would be able to get it to liquefy
even if it was modestly gelled to reshape and more importantly to say soak into the plywood fins.

I had a 4" Loc tubed rocket come in ballistic early on in my pursuing of the hobby. Did a post-mortem on the fincan and of course the external fillets pulled off
the cardboard tube but I had used 80 grit on the fillet/root contact point and could not snap the fillet off the plywood period. That stuff really bonded to the
wood and it took a screwdriver/chisel to get it to break off the wood. I did the heat gun treatment thereafter until the epoxy went out of production.

Kurt
 
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