How much Micro Balloons are too much Micro Balloons?

[amiliv]

So, I have this regular, relatively thin epoxy. I mixed it with some Micro Balloons to make it thicker, adding stuff until it was paste like, a bit more thick than e.g. RocketPoxy. I made a test fillet on my "gluing the fins testing jig" (piece of plywood pretending to be body tube, and popsicle stick pretending to be fin). I spread extra mix on the paper towel and let it cure.

Good news is that I was able to pull almost perfectly smooth and rounded fillet that would require no sanding. Bad news, the thing was rather soft. The cured epoxy was much less brittle compared to pure (no fillers) epoxy, and was even decently flexible. However, it is also easy to tear, and i could make indentation in it with my fingernail...

So, OK. I added too much Micro Balloons into it. They might have been wrong type of filler for thickening the epoxy (as far as I understand, they are mostly used to make it easier to sand). Should I have used mix of Silica and Microballoons instead? What fillers do you use? Micro Balloons only, silica only, more balloons than silica or the other way around?

Yes. I know. I could have just used something that already has fillers in it to thicken it, like JB Weld or RocketPoxy, instead of mixing my own. But where's fun in that? ;-)

 

[dhbarr]

Glass microspheres should be harder than acrylic or phenolic -- you used the latter, the red ones?

And yup, a touch of fumed silica should stiffen the mix up. It's thixotropic so go easy -- when you're mixing it will thin out but when you stop it will stiffen again.

Reminder: the silica is floaty/fluffy, bad to breathe, and if you add very much will turn your epoxy rock hard, extremely difficult to sand.

Talc can be used to thicken as well but it's not very hard.

 

[cwbullet]

I usually add enough to make it have the consistency of peanut butter.

 

[amiliv]

Thanks for replies. The box says: Hollow "glass" spheres, particle size 20 to 200 microns. The stuff is white. Not sure why they put "glass" in quotes. Looking at https://www.tapplastics.com/uploads/pdf/Product_Bulletin_14-2016.pdf after the fact, they do recommend combining silica with micro balloons for fillet application. I'll try that, and go easier on fillers this time. I've box of West Systems silica, while epoxy and micro balloons are made by TAP. But I guess silica is silica, doesn't matter what brand it is.

 

[ep29030]

I've been using phenolic beads as a modifier for fin fillets. I add them about 3:1 (beads/epoxy) and mix it to about a peanut butter consistency. Once dried, they sand easily. Never had a fillet crack or deform during a flight. I'm sure you can overdo the modifiers, but I've not gone that far yet.

I've used micro balloons in the past, but I prefer the phenolic beads for fillets.

 

[Rschub]

Micro balloons are not a structural filler, they are designed to make sanding easier. Using them is basically adding millions of air voids in the epoxy.

I know dozens of people will say they do this with no problem. YMMV

 

[conneryc]

I usually add enough to make it have the consistency of peanut butter.

This!

 

[conneryc]

Micro balloons are not a structural filler, they are designed to make sanding easier. Using them is basically adding millions of air voids in the epoxy.

I know dozens of people will say they do this with no problem. YMMV

For TTW fins, I always use phenolic microballoon additive for external fillets, but for internal fillets to both the fin/motor mount and fin/inner body I use colloidal silica additive.

I had my RDS Northstar fall from 5,000' on just the drogue after the main failed to deploy and the picture below shows the only damage to the rocket. One fin broke....and it broke ABOVE the external microballoon fillet.

 

[shockie]

When microballons become giant Chinese balloons, then that's too much.

 

[SolarYellow]

I've been planning the jump to structural epoxies. Figure US Composites 635. Have also figured having on hand Q-cells (lighter, easier sanding), Cabosil (tougher, denser, harder sanding), and 1/8" chopped carbon fiber (gives the fillet more structural integrity than either of the others without adding a lot of weight). Blend the fillers per the requirements of the application.

 

[rharshberger]

U

I've been planning the jump to structural epoxies. Figure US Composites 635. Have also figured having on hand Q-cells (lighter, easier sanding), Cabosil (tougher, denser, harder sanding), and 1/8" chopped carbon fiber (gives the fillet more structural integrity than either of the others without adding a lot of weight). Blend the fillers per the requirements of the application.

US Composites 635 is a laminating epoxy, and it takes a fair amount of both fumed silica/cabosil and microsperes glass or phenolic to thicken but it is not a "structural epoxy" the US Composites 150 is thicker and used for bonding so it requires less additives to make it even thicker. I use both frequently and am a proponent of the US Composite products.

 

[boatgeek]

Did you have some unfilled epoxy from the same batch? I'd almost suspect that there was a ratio or mixing problem--I've never had microballoons set up so soft that you can mark it with a fingernail. If anything, they usually insulate the epoxy so it sets up a bit faster than unfilled epoxy.

As an add-on to the other advice above, I usually make a small-ish fillet with wood flour filler for structure. Wood flour makes a nice peanut-butter consistency (aka thixotropic) paste that's easy to spread and stays put. It's somewhat stronger than the microballoons. After that has set, I clean it up a little bit with a coarse round file and lay some epoxy with microballoons for the final fillet surface. That makes a surface that's easy to sand clean. YMMV, other options work too, etc. etc.

 

[Rschub]

"Structural" epoxy is slightly misnamed, these epoxies are meant to connect pieces of structure together, and thus are often modified to be less stiff than the laminating epoxy that makes up the actual structure of the composite part. This is also a simplification because the stiffness of a part depends on things besides the epoxy stiffness. Making fillets out of pure or filled epoxy is not very structurally efficient anyway.
This probably doesn't matter for 95% of the rocket fillets out there, but if you are pushing the envelope, you should have a good understanding of the difference.

 

[dr wogz]

West systems has a good list / guide of additives & thickeners:

https://www.westsystem.com/filler-selection-guide/
I'm a big fan of their 407 fairing filler.

You can also use balsa dust or baby powder as a filler.. sands easy (and smells 'baby fresh'!)

as for strength, some have added 'milled fiber' and/or strands of fiberglass / carbon fiber to add structure (kinda like adding 'rebar' to concrete) Just lay the strands in the wet epoxy..

as a side note: microballons also reduce weight as it thickens.. Remember, it is 'bubbles', so you are making an 'Aero Bar' fillet / a foam fillet..

 

[waltr]

Micro balloons are not a structural filler, they are designed to make sanding easier. Using them is basically adding millions of air voids in the epoxy.

I know dozens of people will say they do this with no problem. YMMV

That is how I use micro balloons.
Great as a filled/top coat after epoxy down glass cloth to fill and smooth.

Also mix micro balloons with acrylic urethane primer as a filling primer on Balsa and to fill spirils in tubes.

Makes sanding much easier.