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Author Topic: Bonding Carbon Fiber to Balsa  (Read 2446 times)
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Alan Mkitarian
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« on: August 06, 2008, 10:31:45 PM »

Thought it proper to post this for all to see. It's from Bill Lovins who is the Editor of the Max-Out of the MMM. I found it in the Florida Modelers Association's July-August news letter.
 
 The usual adhesive (CA) used in bonding carbon sheet to balsa does a relatively poor job. So, I decided to try some alternatives. Using a radius of about 3-4", I tried the following adhesives (without thinning). The samples were subjected to heat, cold and humidity. Then moderate pressure was used to try to peel the carbon away from the balsa. Using the scale 1 (poorest) to 10 (best) here's the results:

   Ambroid -3
   Stix-It (a contact cement) -4
   UGL (a contact cement) -5
   CA (medium) -7
   30 Min Epoxy -8
   Krazy Glue (a CA) -9
   Goop -10
   Formula 560 canopy cement -10

Alan Mkitarian
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thymekiller
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« Reply #1 on: August 07, 2008, 08:21:59 AM »

Good info. Iv thought about cf many times. Im not familar with canopy glue. Where can I get it ?

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Alan Mkitarian
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« Reply #2 on: August 10, 2008, 08:21:54 PM »

After I read what I posted I discovered that I had forgotten to include the last Note so here it is:

Note: the carbon sheet I used in the tests was what is usually used for rib cap strips. Dull on one side and shiny on the other. I used strips cut from an .007" x 3 x 36" roll cut in uniform widths. The carbon came from MRL.

Alan Mkitarian
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Mike Taylor
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« Reply #3 on: August 11, 2008, 12:37:21 AM »

I think the high rating of the canopy cement must rely on the open texture of the unidirectional tow/tape. I know that white glues and similar product don't bond well to most cf rods or tubes.
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thymekiller
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« Reply #4 on: August 11, 2008, 12:34:09 PM »

Thank you for the reply. I am not familer with mrl. Can cf be purchased in thin sheets? Is it difficult to cut?

Sorry to ask so many basic questions. Im a newbie.

thymekiller
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Mike Taylor
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« Reply #5 on: August 11, 2008, 04:28:11 PM »

Tim G. at Peck Polymers has a LOT of different kinds of CF - rod, shapes, plates, etc. http://www.peck-polymers.com/store/

If you really want to get into CF, you need to visit http://www.cstsales.com/ I needed a solution to thin, light, rigid ducts, and they had an assortment of resins and hardeners in a system that allowed selection of viscosity, flexiblity and set time, plus very light cloth and tissue, to get long work time and super thin composites. They also have mono-directional tow, which can be cut with scissors.
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thymekiller
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« Reply #6 on: August 11, 2008, 09:01:51 PM »

Thanks guys, for the links. Theres more to it than i thought. Cool!!

thymekiller
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slopemeno
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« Reply #7 on: August 12, 2008, 03:19:24 PM »

That unidirectional tow is some amazing stuff. I've used it to join the broken halves of a very, very heavy (70 ounce) 60" slope sailplanes wing, and it's held up amazingly well. Locally in the San Francisco Bay Area 'Tap Plastics' carries unidirectional cloth, which you can disassemble into the tows.

As far as gluing it, true thin laminating resin is the way to go. You want to make every attempt to get the tow "wetted out" and then blot out as much of the resin as possible. I've found with my slopers that fuselages that were a dry layup ended up being much stronger long term than fuselages that were not.

If you are laminating tows to a spar, vacuum bagging is the way to go. You can make incredibly strong light structures that way, and it's a fairly simple process.

Goop/Shoe Goo/E-6000 are also super handy. We use this stuff on our combat gliders, and we've lost planes for as many as four years, and when we get the plane back the E-6000 is still as flexible as it was when new, even though it sat in the sun and rain for all those years.

Black, or rubberized CA is also getting a good reputation for a flexible bond.
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Pit
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« Reply #8 on: August 17, 2008, 08:02:43 AM »

Tim G. at Peck Polymers has a LOT of different kinds of CF - rod, shapes, plates, etc. http://www.peck-polymers.com/store/
^^^SNIP^^
 They also have mono-directional tow, which can be cut with scissors.

One blurb of caution/tip when using CF tow. Wear some sort of disposable gloves and wrap the ends with masking tape effectively making two 'handels', and cut THROUGH the tape. It's a very good preventative action from getting CF splinters - something you do NOT want!! Makes wetting out the tow LOT'S easier! Also a good idea two have another pair of hands to help.The gloves also prevent getting adhesive on the skin (VERY IMPORTANT when using polyurathanes or ANY two-part adhesive)!! AND, we all know that GOOD ventilation is a must Roll Eyes?

Okay, that was more than one 'blurb' Roll Eyes.

Cheers, Pete
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« Reply #9 on: August 17, 2008, 02:35:48 PM »

I've been bonding carbon to balsa for 25 years or so. The choice of adhesive often depends on several other factors besides just pure peel or shear strength. For example, for indoor models, weight is a big factor. I found that thinned ambroid (or Indoor cement) made an excellent adhesive for carbon tow on hlg bodies and wing reinf. It would not be my choice for larger models though. When the solvent evaporates from the ambroid, you get a decent fiber to glue ratio at a very low weight.

Another criteria that often drove my adhesive choice is temperature resistance. I found that when covering with mylar or other synthetic films, it was sometimes neccessary to generate a fair amount of heat to shrink the covering. Many glues that would otherwise seem suitable to bond cap strips to ribs would fail under the heat of the iron. The caps would pop-up as the glue softened. I chose a high temperature resistant high strength epoxy (3M DP-460) and had no further problems with cap pops.

Incidentally, attaching cap strips with a slow cure epoxy turned out to be faster than with thick CA. I could coat all of the ribs top and bottom (the caps hang from the d-box with a little tail of magic mending tape) and then apply all of the ribs at once. A little ear of tape at the junction of each cap and TE piece stops any sideways movement. I run a strip of 1/8" rubber over every rib (the wing is on a jig) that goes around a small finishing nail set at the LE and TE of each rib location. After an overnight cure, the job is done. A little trick I learned: after sanding and cleaning the carbon with a solvent (acetone or MEK) I lay it down on a sheet of glass and completely cover the surface with strips of magic mending tape. I make sure that I overlap the front and back edges onto the glass. Then, using a straight edge and an olfa knife (ideal because you can snap off the blade every few cuts to maintain sharpness) I slice the carbon sheet (0.1mm from CST or similar) into the required widths (1.5mm for wing ribs etc.). The strips are pulled up from the glass carefully and the tape coating is left on. I use the little era of tape that remains on the front edge of the strip to temporarily attach it to the front of the d-box. The best part is that after bonding, the tape is removed and any glue that might have made it's way to the surface it pulled away with the tape. No need for sanding!

As for thin versus thick epoxy, my experience is that thick epoxy is superior. Thin epoxy is usually a laminating resin that is optimized for hardness and low viscosity, not for high adhesive strength. Thinning a thick epoxy with alcohol (or other solvent) can often reduce the quality of the bond. I have found that if you use a very slow cure epoxy that is fairly thick (with high shear and peel strength), you can spread it out to a very thin film quite easily. Often, I'll spread the epoxy onto a rib, and then remove nearly all of it with a tooth pick before bonding. Incidentally, my records show that it is actually lighter to use epoxy than either thin or thick CA contrary to intuition. No soaking in.

Also, there are many adhesives that exhibit high peel strength. But many exhibit excessive "creep" under load. Not desirable. For speed I'd suggest thick CA. For strength I'd recommend a good epoxy (adhesive not lam resin) or urethane adhesive. They are better in shear, creep and temp resistance than any of the cheaper glues mentioned above.

Tony
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bentodd
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« Reply #10 on: September 26, 2019, 12:04:42 AM »

Not knowing any better, I use thinned Titebond wicked thru uncured unidirectional carbon fiber.  I slit the carbon to the thickness of the rib.  Run a small bead on the rib, stick the strip down to the rib.  Being uncured, it lays on the rib and doesn't spring up.  Hold it down for a moment until the Titebond starts to grab.  Now dribble a little more until the carbon is saturated.  When everything dries you can run your iron over it to really lock it down.

Someday I am going to do some comparisons of the strength of epoxy bonded carbon and Titebond bonded carbon.  I am sure epoxy is better but Titebond might be a close second.

I have one question, where do you start and stop your cap strips on ribs?
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Tmat
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« Reply #11 on: September 26, 2019, 11:09:09 AM »

"I have one question, where do you start and stop your cap strips on ribs?"

Depends on the structure. Generally, the cap strip extends out over the trailing edge (I trim them flush with the trailing edge). If you have a balsa leading edge I extend the cap onto the leading edge. With a carbon D-box, the cap extends about 3 mm onto the dbox.

Tmat
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flydean1
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« Reply #12 on: September 26, 2019, 06:43:59 PM »

2 Questions:

Where do you buy uncured unidirectional carbon fiber?

When using the 460 adhesive, do you just squirt it out and mix it on some sort of card, or do you use the nozzles that mix the glue when it is dispensed?
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« Reply #13 on: September 26, 2019, 07:49:39 PM »

Dean,

The 460 dispenser does not mix the epoxy, it just squirts it out. You will need to mix with toothpick or stirring stick. 

By "uncured unidirectional carbon fiber" do you mean carbon tow? A tow is a flat bundle of carbon fiber with no epoxy. A 1-K tow has a thousand very fine stands of carbon fiber--looks a bit like a black shoelace. Other sizes are available. Spread tow CF, used for wing skins, is made by running a tow through a series of rollers that spread the tow out into wide, very thin ribbon of carbon. This is then attached to a foam substate using laminating epoxy resin (very thin epoxy).

Or are you thinking of pultruded carbon fiber? These are made by pulling carbon fibers wetted out with epoxy resin through a die to make a rod. A variety of sizes and shapes are available from CST Sales. The  rectangular strips are used for spars and the triangular ones for trailing edges. Very thin strips are used for rib caps. Rods and tubes are also available. The pultrusions are, in my opinion much more useful than the tow.

Hope this helps.

Louis
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flydean1
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« Reply #14 on: September 26, 2019, 08:44:14 PM »

Thanks Louis,

In years past, it was possible to get rolls of CF strip good for rib caps.  Is it available now?
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bentodd
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« Reply #15 on: September 26, 2019, 10:39:35 PM »

I buy composite fibers from Soller Composites

https://www.sollercomposites.com/UNI.html  are tapes where the fibers are all perfectly in line and uniform thickness.  Usually there is some schrim or other material to keep the fibers organized.  You can easily slit the tape into thinner strips such as 1/16 for rib cap strips

I also bought a roll of Tow, https://www.sollercomposites.com/tow.html  The fibers not perfectly organized but work well here thickness is not critical.  It is a lot cheaper, a whole roll that will last me a life time for $20
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lincoln
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« Reply #16 on: September 27, 2019, 02:17:28 AM »

Seems like some of the posts here aren't making clear whether they're talking about pre-cured stuff, tow, or uni "cloth". The latter sometimes has a little bit of cross stitching or something which can make it a little less than straight. I know of at least one case where this resulted in significantly less stiffness. Presumably that means less strength too.

If I was doing cap strips and I didn't want to use something pre-cures, I think i'd apply a thin layer of 30 minute epoxy on the wood and let it partially cure first. Then I'd use something meant for wetting out cloth on the carbon itself. I've heard that if you bond epoxy to epoxy within the first couple of days, you get a nice chemical bond. However, I expect the pre-cured stuff would be much stronger and a little stiffer if bonded correctly.

I glued some pre-cured unidirectional strips to basswood, using epoxy meant for wetting out fabric. It had very low peel strength. Later, I did a test piece using 30 minute epoxy with much better results. Generally it was the basswood that failed, not the epoxy. I suppose with balsa this might not matter as much, unless it was old style Guillows wood,  but I'd suggest doing a test first.

IMHO, CA is seldom the best glue to use.
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