Epoxy thickening

[Delta Whisky]

Kitfox Pilot brought up the time honored question regarding mixing flox in epoxy (IAW the instruction manual) to thicken it in a recent post. I didn't want to hijack his thread so I'm starting this one.

I'm assuming he was talking about the Hysol or Loctite 9460 that comes in the kit. If that is the case, then the question is: did you add flox (or anything) to thicken it? I didn't because mine didn't show any tendency go anywhere. The data sheet says it is 5 -10% fumed silica and that should be enough to give it the property we're looking for and the first "specific benefit" listed on the TDS is "non-sag slump resistance". Is there any chance that the instructions haven't caught up to the material in the new kits? If you have the time to respond - what is/was your experience?

[efwd]

We may be confusing Flox with micro beads. Flox is Cotton that is used to simply stiffen the HySol so that it doesn't run. Micro beads (glass beads or silica) are intended to be mixed with HySol so that the two pieces remain separated (disimilar metal) as when you install the Jury strut brackets (stainless) onto the wing spar (aluminum).

[Dave S]

Microbaloons, Microbeads, fused silica; while they are pretty much the same base product (solicon dioxide) do have sufficiently different properties to keep any physical chemist occupied. Cotton flox is another thing - basically powdered/bleached bluejeans.


The safe bet is to follow precisely what the kitfox instructions say to use. Be sure the terms being used and product identity are exactly as contained in the Kitfox instructions.

At least the older kits came with two products to be added to the epoxy structural adhesive - those would be cotton flox ( which is fine cotton fibers) used to thicken the epoxy; and Microbaloons ( which are micron sized glass spheres with air inside - microbaloon is a pretty good description) used precisely as EFWD mentioned.


Microbeads are different - come in a wide range of particle sizes and not necessarily containing an air bubble. Many industrial purposes. Larger versions of the stuff is used for bead blasting.

Fused silica is something else - it has a use as a food/feed additive (conditioning agent) and is generally used as a thickening material unlike microbaloons or microbeads due to its branching structure of the particle - tends to intermesh (FYI - microbaloons flow like water - fused silica does not.

Best advice - just stick with the product identity and the kit instructions - it is not beyond possibility that some material may be different than what was used 10 years ago - use and do what is defined in the instructions for the specific kit.

[efwd]

Ah, we learn even more stuff.

[Kitfox Pilot]

Delta Whisky, Thanks for posting this question. And thanks guys for the answers to help us new builders out.

[jrevens]

The "social distancing" & "stay-at-home orders" have afforded me a little time to waste, so I decided to test my theory that adding micro-balloons for the purpose of mitigating galvanic corrosion between dissimilar metals is not effective for that purpose, and is actually detrimental in that the strength of the epoxy joint is weakened. The fact that the joint is weakened compared to using pure epoxy is uncontested... it's a well known fact. What I doubted is just the theory that the balloons insure separation necessary to prevent corrosion. First off, just the fact that steel & aluminum, for instance, are touching each other doesn't create damaging galvanic action. There has to be an electrolyte, like water plus impurities, between the 2 materials for it to occur. If the joint is dry and sealed from moisture and air, galvanic action will not occur. So the epoxy by itself should do that part of the job very well. The second thing is separation... how much difference do the micro-balloons actually make? If there is no electrical conductivity between the 2 metals there is no contact. The pictures show test samples of 6061-T6 Al & 300 series SS, one bonded with 9460 Hysol with a good quantity of micro-balloons and another with just epoxy. One set was also riveted with SS rivets and the other not. The metal was cleaned, abraded with a Scotchbrite pad then cleaned again.Those without rivets were bonded with approximately 20 lbs/sq inch of force during cure. There was no electrical conductivity & very high resistance between those pieces with the non-riveted samples using the Mega-Ohm scale of my meter. As shown, there was no resistance between the metals that were also riveted together... conductivity was very good. It seems reasonable to assume this is because of the rivets. Micro-balloons are never going to prevent metal to metal contact at the rivets,and that point of mechanical connection would be the most vulnerable to possible moisture intrusion and concentrated galvanic corrosion in my opinion. I don't know who came up with the idea to use micro-balloons to begin with but I don't believe it's valid, in my opinion. So 2 things - I'm not suggesting that people should deviate from the factory instructions in any way when building their airplanes, and I could be wrong in my analysis. Also, if you have some nagging doubt that maybe you didn't add enough micro-balloons to the epoxy... don't worry. I don't think it makes a darn bit of difference, other than the more you add the weaker the joint.

[Kitfox Pilot]

Good post John. Anytime we have the corrosion on two connected pieces of metal here on the farm it is almost always starts at the bolt holes where the bolts hold things together so your rivet assumption for letting in the moisture sounds correct.

[SomeDay7]

Guess my idea about

adding micro-balloons is different from the purpose of mitigating galvanic corrosion between dissimilar metals. I think a great test would be to add the micro-balloons to some Hysol in one sample and none in another. Fasten identical plates together with rivets. Have identical arms hooked to a scale that would test the separation strength of the bond after the rivets are removed. I think the micro-balloons actually allow Hysol to remain in the bond area more so than squeezing out of the area with none.

[Delta Whisky]

Good work John. Thanks for the tests and the pictures. As noted in the starting post, Harlan's observation and question was the one that piqued my interest because, as you noted, adding anything to resins weakens the joint. (The only exception I've come across: some modifiers can improve compressive strength but seldom is an epoxy resin used in such a load path.) So how much flox should be added? I've not come across an answer in my manual and Harlan's question resonated with my experience. When I mixed my first batch and was about to add fumed silica (much better than flox as a thixotropic agent) I noticed that it didn't appear to need any - so I didn't add any. Checking the SDS I saw why - it is already there and one of the key features of 9460 is its non-sag attribute. Like yourself, I'm not going to suggest NOT following the manual but this forum is a great place to share ideas and a few facts that could help others.

As to your test results - I'm not surprised that your results with and without micro balloons yielded the same results. The SDS indicates that 9460 contains SiO2 and micro crystalline silica. A couple of nights ago I ran down the associated CAS numbers and found that there is a likelihood that one of them is either a micro balloon or micro bead product. One can't be sure by using only the CAS but the chances are good due to the fact that the performance values in the TDS are for joints of 0.125 mm (0.0049 in). Again, I'm only guessing but normally joint thickness in structural adhesive joints is controlled by micro-beads (they perform better than balloons for that purpose and it only takes a small quantity). SomedDay7 was on to something. Only Coloradans want a dry joint - but that's on 420 day. (Sorry, I just couldn't pass that one up.) So, there is a good chance that is why your test values came out identical - either balloons or beads were already there. Maybe a test starting with a resin that is known to be unmodified is in your neo-virus future.

BTW - I'm going with a theory that the epoxy called out in KF joints, except for the ribs, is for sealing (anti-corrosion) purposes. (Help me here: excepting the ribs, are there any epoxy only joints in the current KF?) And - I agree that the rivets can be an access port to H20; a situation that can be mitigated somewhat by installing the rivets wet (i.e., dip in epoxy before inserting).

Thanks again for the test.

[efwd]

Off the top of my head, Hysol only was used to adhere the leading edge to the spar, and to secure the airfoil shape plastic to the struts. That is where the double barrel syringe system is most appreciated IMO.