How to use fillers with epoxy?

[mperegrinefalcon]

Hello,
I have recently aquired a gallon kit of Duralco 4461 for some lamination purposed, but I am curious about using it for fillets too. It is way too fluid as is, but I am wonding if using fillers with it could make it useable for structural commponents as well.

I have some milled fibers, fumed silica, and glass microspheres I can add to it. Is there anyone here who has any guidance on how much to add of each?

I can do some testing on some scrap fiberglass, but I do not have equipment for destructive testing and data gathering.

 

[OverTheTop]

I use microballoons. I just add a bit at a time till I get what behaves like toothpaste.

 

[mperegrinefalcon]

I use microballoons. I just add a bit at a time till I get what behaves like toothpaste.

Just in general, or have you used it with Duralco 4461 before?

 

[prfesser]

Yes, you can use fillers with almost any epoxy. Microspheres will increase sand-ability of the fillets, and will also thicken the epoxy...though weight-for-weight, they're not as good at thickening as fumed silica is. Microballoons also weaken the mix but that's not a serious issue. Epoxies are quite a bit stronger than is necessary for *most* rocketry construction.

I would run a few tests on small portions. Add enough microballoons to the resin to give desired sand-ability. It will take a pretty large volume, probably close to the volume of the resin. Then add fumed silica (if necessary) to get the desired thickness. Then add curative, mix, allow to harden.

Here is a bit of nonsense on epoxies that I posted a while back.

 

[mtnmanak]

Theses guidelines work pretty much universally:



This page even gives you recommend mixing ratios for their fillers (Colloidal Silica 406 is my "go to" filler - seems the best all around filler from West):

https://www.westsystem.com/filler-selection-guide/

 

[OverTheTop]

Just in general, or have you used it with Duralco 4461 before?

I use West Systems 105/206.

 

[mperegrinefalcon]

Yes, you can use fillers with almost any epoxy. Microspheres will increase sand-ability of the fillets, and will also thicken the epoxy...though weight-for-weight, they're not as good at thickening as fumed silica is. Microballoons also weaken the mix but that's not a serious issue. Epoxies are quite a bit stronger than is necessary for *most* rocketry construction.

I would run a few tests on small portions. Add enough microballoons to the resin to give desired sand-ability. It will take a pretty large volume, probably close to the volume of the resin. Then add fumed silica (if necessary) to get the desired thickness. Then add curative, mix, allow to harden.

Here is a bit of nonsense on epoxies that I posted a while back.

I forgot to mention in my above post, this is for a minimum diameter 54mm rocket to mach 2.5ish. I plan it as a test run before I build my minimum diameter M3464 rocket to mach 3.1, so I want to be sure of the stength of this fillet. I can put togehter a set of test pieces with various different filler ratios and and different fillers and use them for destructive tests, but I do not have the equipment to gather data from that. Any idea of what I might need to do that kind of testing?

 

[prfesser]

Frankly, I'm not competent to address your concerns in that area; I've flown exactly one machbusting rocket and that was 20 years ago. Advice from some of the rocketnuts who have flown MD mach-breakers would probably be as useful, possibly more so, than data from home-brew testing setups.

Perhaps...rather than working with fillers, use one of the methods that have been described here on TRF to form near-perfect fillets that require little or no sanding. That way you could use the epoxy as-is, or with a little fumed silica as a thickener (doesn't affect strength significantly).

 

[Handeman]

I am NOT an expert in epoxies and have only used US Composites and Rocketpoxy. I have used laminating epoxy with ground fibers and silica as fillers for structural fillets. I won't use that again. I will always use a structural epoxy for fillets. Laminating epoxies are great for laminating but aren't designed for structural use.

 

[tHoagland]

I occasionally use microballoons with epoxy. I have two use cases.

1) I finished my fillets, they're strong, sufficient and ugly. The epoxy with microballoons is easier to sand and shape than structural epoxy and is very useful to cover and smooth blemishes.

2) I need to fill a hole that I drilled and no longer want. Sometimes as an error and sometimes to inject epoxy for internal fillets.

In both cases I tend to use 15 minute epoxy, add enough balloons to get a pretty viscous mix and apply like bonds. For smoother fillets the mix is a bit like peanut butter because I want something that spreads. For filling holes, it's more like drywall mud.

 

[mtnmanak]

Missed that you were looking to do a Mach 2+ flight. I have used West Systems with fillers on rockets under Mach 2 with good success. Above Mach 2, I prefer Cotronics for the adhesive and ablative. If you think about using Cotronics products, make sure you understand the proper cure temps and whether you have the facilities to cure at those temps.

 

[mperegrinefalcon]

Missed that you were looking to do a Mach 2+ flight. I have used West Systems with fillers on rockets under Mach 2 with good success. Above Mach 2, I prefer Cotronics for the adhesive and ablative. If you think about using Cotronics products, make sure you understand the proper cure temps and whether you have the facilities to cure at those temps.

Yeah, I forgot to mention that in the initial post. I have some Duralco 4461 I planned to use for laminating, but I am thinking I will experiment with it for use on fillets as well. I found that if I leave it for about 6 hours it gets to a very nice gel stage that I think I can make some decent fillets with. I still might add some fumed silica to it. I will probably just go for it to test with an L1000 on 4461 fillets with 1 layer of T2T CF vacuum bagged and see how that does.

 

[David Schwantz]

If you are planning a mach 2 + flight do not use that epoxy. Get the stuff that has been proven to work at that speed. Pro line 4500 from Wildman.

 

[ep29030]

for fin fillets, my favorite filler is phenolic beads. You can mix phenolic beads:epoxy in a 3:1 ratio and get a nice peanut-butter consistency. Phenolic beads sand well, but are a bit more substantial than foam microbeads. Great for sport flying rockets. Not great for Mach 2+ flights.

 

[kramer714]

Lots of the fillers people use might give the aesthetic look they are going for but have adverse effects on strength and performance. This can be exacerbated by the type of epoxy used and the mixing process. A proper filler will make the fillet tougher and reduce tensile stresses due to resin shrinkage on cure (the crack you see on unfilled epoxy thick fillets). For fillets you want good toughness and peel strength. Micro balloons (especially hand mixed) will reduce the peel strength (look at T-peel), tensile strength and toughness.

Laminating resins are designed to be thin, wet out dry fabric, and have good inter laminar strength and reasonable modulus. They dont have good toughness, or peel strength.

Structural Adhesives use different hardeners, additives, and fillers than laminating resins. The ingredients depend on their application and can include spherical fillers, short fiber (ground) ceramic, longer fiber or micro-balloons.

Micro balloons are there to lighten weight and to thicken, not to increase strength or toughness. For example we used to vacuum mix micro-balloons with epoxy adhesives for closeouts in the edges of honeycomb panels, or to fix damage in honeycomb panels. One other thing we use micro-balloons for is bondline control. A small amount of micro-balloons mixed into the adhesive maintains a certain minimum bond line (keeps the adhesive from running out).

Silica (or ceramic) filled adhesives are stiff, have good shear strength, low shrinkage during cure, and have poor peel and toughness, They can be thixotropic if that is important. At the day job we use ceramic filled adhesives in areas where we have pure shear, like bonding a cup over a shaft.

High temperature resins have a high degree of cross-linking, good for temperature bad for toughness.

For almost all rocket applications (including fillets) you want a tough adhesive (high elongation), with good peel strength a the temperature you need. A good example is 3m DP-420 Black. it has a T-peel of like 60 lb/in at RT and like 20 lb/in at 180F. This combined with an overlap shear strength of over 3,000 psi at 180F.

My suggestion, for highly loaded rockets, dont use micro balloons in fillets, might look good but not giving you the strength and toughness you need. Use a real toughened resin.

Mike (worked for 30 years in composites) K

 

[kramer714]

From
Determination of Cohesive Parameters
for Aerospace Adhesives
By
Josef G. Hortnag

Masters thesis 2013

He measures and calculates
traction-separation curve (TSC) depicting a
materials’ ability to carry load on a crack before it propagates. TLC are used when determining
cohesive parameters which will be denoted as the cohesive strength σc, the cohesive toughness Γc
and critical crack opening displacement δc.

Look for T-peel strengh as agood idea of cohesive toughgness

 

[mperegrinefalcon]

Lots of the fillers people use might give the aesthetic look they are going for but have adverse effects on strength and performance. This can be exacerbated by the type of epoxy used and the mixing process. A proper filler will make the fillet tougher and reduce tensile stresses due to resin shrinkage on cure (the crack you see on unfilled epoxy thick fillets). For fillets you want good toughness and peel strength. Micro balloons (especially hand mixed) will reduce the peel strength (look at T-peel), tensile strength and toughness.

Laminating resins are designed to be thin, wet out dry fabric, and have good inter laminar strength and reasonable modulus. They dont have good toughness, or peel strength.

Structural Adhesives use different hardeners, additives, and fillers than laminating resins. The ingredients depend on their application and can include spherical fillers, short fiber (ground) ceramic, longer fiber or micro-balloons.

Micro balloons are there to lighten weight and to thicken, not to increase strength or toughness. For example we used to vacuum mix micro-balloons with epoxy adhesives for closeouts in the edges of honeycomb panels, or to fix damage in honeycomb panels. One other thing we use micro-balloons for is bondline control. A small amount of micro-balloons mixed into the adhesive maintains a certain minimum bond line (keeps the adhesive from running out).

Silica (or ceramic) filled adhesives are stiff, have good shear strength, low shrinkage during cure, and have poor peel and toughness, They can be thixotropic if that is important. At the day job we use ceramic filled adhesives in areas where we have pure shear, like bonding a cup over a shaft.

High temperature resins have a high degree of cross-linking, good for temperature bad for toughness.

For almost all rocket applications (including fillets) you want a tough adhesive (high elongation), with good peel strength a the temperature you need. A good example is 3m DP-420 Black. it has a T-peel of like 60 lb/in at RT and like 20 lb/in at 180F. This combined with an overlap shear strength of over 3,000 psi at 180F.

My suggestion, for highly loaded rockets, dont use micro balloons in fillets, might look good but not giving you the strength and toughness you need. Use a real toughened resin.

Mike (worked for 30 years in composites) K

Thank you for the detailed explanation, just what I was looking for. I will not use 4461 for fillets at all.
Since you have so much experience, would you think that Duralco 4461 is even a good epoxy for laminating structural components? The ultimate goal is minimum diameter M to mach 3.1, so I believe I need the heat resistance, and this epoxy cures at room temp. I attached the datasheet for 4461. The reason I picked 4461 is because I have used 4460 before for laminating a rocket before and that flew to mach 2.4 and survived, but 4460 requires 250°f for 4 hours to cure so it is a pain to work with. I figured I would get 4461 because it is pretty much the same, it just cures at room temp. (The first one was not done very well and it still worked)

A guy at the launches I go to recommended I use Loctite E-120HP for my fillets. He has flown minimum diameter rockets to mach 2+ without any lamination, and just uses that epoxy as fillets. I might try that, and laminate with the carbon fiber and vacuum bag as practice. I can also look into proline 4500 as well, since people are recomending that.