Conclusions on Aluminum Adhesive Bonding Tests

[MalcolmW]

Gary;

here's is an abbreviated version of a piece I posted previously;

Aluminum Cleaning:

To properly adhesive bond aluminum, the surface must be clean and chemically treated. The highest performance adhesive bonding includes degreasing and chromate conversion. Improper surface preparation is the major cause of adhesive bond failure.

Remove ink and markings from the surface of the aluminum with a light wash using MEK or acetone and white paper towels, followed by a degreasing (vapor phase degreasing preferred).

However a rinse using brake parts cleaner works well, which comes in spray cans and is available from most auto supply stores). Perform all of the above operations in a well-ventilated area (outside or fan blowing) and wearing a respirator.

Etch or chromate convert the surface using an Alodyne solution or a chromate conversion solution made from Iridite 14-2 (same thing). Immerse aluminum part for twenty minutes (at 70F or higher) or until the aluminum becomes a golden-brown color.

Rinse the metal thoroughly with distilled water and dry well. Do not use compressed air, because there is oil in most air compressors which will contaminate the clean surface.

If you do not wish to work with chromate solutions (I can provide a suggestion for safe disposal), you can use a phosphoric acid cleaner (Jasco Prep & Prime) to etch the aluminum surfaces which then passivates the aluminum with dichromate. Clean aluminum as above (no abrading) and immerse the aluminum in the Prep & Prime for thirty minutes (check time). Remove, rinse with distilled water (rain water – see note below) and air dry. All cleaned parts MUST NOT be touched by hand; use fresh, white cotton (mickey mouse style) gloves.

For best results, parts should be coated or bonded immediately after cleaning or chromate conversion. If you do not plan to bond the parts together within twenty-four hours, wrap in fresh ‘butcher’ paper to keep clean.
There are commercial companies that perform chromate conversion processing, and will provide this service on a tolling basis. Prior arrangements must be made to ensure the processed parts remain clean and suitable for adhesive bonding. This may be a way to go if a builder wants to use adhesives and obtain excellent corrosion protection.

Rinsing: Distilled water produces best results. However, rainwater caught from the roof is very low in mineral ions and is an acceptable substitute (discard the first five minutes of runoff – that is the roof rinse water & is dirty).

When working with chromic acid or Iridite solutions, wear nitrile rubber gloves (Harbor Freight – low cost). Also a long apron and shop face shield – safety first and last.

ALTERNATIVES:

Jasco Prep & Prime – caution, this contains both phosphoric acid and dichromates. Use full strength on aluminum at room temperature for thirty minutes, rinse well with distilled water and air dry.

Dupont Quick-Prep - Auto paint supply stores which sell Dupont paint. Quick Prep is a mixture of phosphoric acid and alcohol. When spread over bare metal it reacts with the surface to form a light coat of aluminum phosphate that prepares the surface for bonding. Most finishes can be applied directly over a surface that has been coated with metal prep. Quick-Prep is soluble in water and is generally only mildly toxic unless vaporized by heat. It is always advisable to wear nitrile rubber gloves and protect eyes from the irritating effects of contact.

Sherwin Williams Metal Prep - Auto paint supply stores sell Sherwin Williams paint. Metal Prep is a cleaner and conditioner for bare metal. It is composed of phosphoric acid and glycol ether. See Quick-Prep above for use directions.
Anko Phosphoric Acid Aluminum Brightener – This is available over the internet and is a cleaner and brightener for bare metal. It is composed of phosphoric acid and glycol ether.

All the best & fly safe,

MalcolmW

 

[GESchwarz]

Thanks for the clarification. I have a slight color vision problem, so I guess what looks one color to you is a bit different for me, but I can assure you that what I was looking at was not zinc chromate primer.

 

[GESchwarz]

Holy Smokes, that Iridite at Aircraft Spruce is pricy...$32/qt, $35/gal. I picked up the Jasco product for $6/qt.

Stay tuned, I will have test results within a few weeks, depending on winds of life.

 

[GESchwarz]

Malcolm,

Just this evening, at 60 degrees F., I submerged some coupons in the Jasco product. I got some good fizzing, but after 20 minutes there was no color change, perhaps because of the lower temperature. So I kept the coupons in for a total of 1 hour, still with no color change, however the acid did produce an etched finish, that is, the surface now has a satin appearance, as opposed to the original reflective finish. So I washed the coupons with distiled water and will wait until morning (12 hrs), at which point I will do the bonding.

That I'm not bonding immediately following wash, although it may not be optimal, it better represents how it is likely to happen during actual aircraft assembly. Because I'll be building on evenings and Saturdays, it is unlikely that I will always be able to etch and bond in the same work session. My test results should help to decide just how important that really is.

The results I have come up with so far shows that even without etching, I am able to produce bonds that are stronger than the conventional general aviation materials and fasteners. All coupons of .023" thickness have torn and coupons of .065" thickness have been able to shear groups of three AN -4 rivets.

I guess the concern now is that the aluminum must receive this etch/passivation/chromic treatment to preserve the integrity of the bond over time and exposure to the environment. Any comments on this Malcolm? Just how vulnerable are bonds to degredation over time, without this acid prep process?

Should the Jasco be producing the golden brown color or do you only get that from the iridite process? What effect will the lower temperature have on the etch process? I would imagine that the etch process is a function of temperature.

One other note. In my research I came across an industry survey of aerospace companies that do adhesive bonding. One of the key process variables that all tried to maintain during bonding was Low Humidity.

 

[MalcolmW]

Hello, Gary;

I wish I could offer first hand experience with the Jasco product, however, its recommendations come from others. That being said, it is my understanding and belief that aluminum must be clean (very clean) to provide a good substrate for an adhesive bond.

The Jasco prep and prime does do this - it is what it is intended for. Since the Jasco product is not Alodine, it will not produce the characteristic color of alodining. A satin finish sounds like an 'etched' surface to me.

Etching temperature: Most etching solutions should be at room temperature or above. Raising the temperature speeds up the reaction (rule of thumb - for every 10C raise in temperature - reaction rate doubles, or time to complete etch is cut in half).

In addition to cleanliness, for an adhesive bond to aluminum to have environmental durability, the aluminum must be passivated. Humidity can be a problem, and that can be overcome by using a heat lamp to (slightly) warm the aluminum prior to the adhesive application.

In light of the cost factor and availability, the Jasco product sounds like an acceptable substitute. Recognize that my adhesive development experience was for aircraft that faced extreme conditions (including high temperatures well beyond anything experienced by general aviation), which meant optimizing every adhesive bonding parameter. Far tougher than your requirements.

If you test the Jasco prep and prime with the adhesives that you are considering and it provides similar or better performance, this may provide you with an acceptable surface preparation agent that is readily available at a reasonable cost.

I believe your thorough and cautious approach will result in a strong and safely constructed aircraft. I hope that my comments have helped, and I'm happy to share what little knowledge I have on the subject.

All the best and fly safe,

MalcolmW

 

[GESchwarz]

"What little you know"? Are you kidding? You have a wealth of information that I take very seriously. I appreciate all your help. I am currently bringing my materials up to temperature from the cold shop temp, and will be applying the Partite 7350 Methacrylate to the coupons. I'll be making several coupons of a few different variables. The coupons will then be aged at elevated temperature and thermal cycled for a few days prior to failure testing.

I still have some coupons that I was unable to break. Those will undergo some mechanical and thermal abuse, then tested to failure.

 

[BBerson]

Gary,
The etch and alodine is a two step process.
The first step is cleaning with any phosphoric etch chemical (Jasco will work)

The second step is treatment with chromic acid (Alodine) conversion. The "conversion process" provides a durable corrosion resistant surface with a better bond than a freshly sanded surface. The Alodine leaves a light gold color when done properly according to directions.


BB

 

[MalcolmW]

Hello, Gary, BB;

Chromate conversion of aluminum is widely accepted as an excellent passivation / corrosion protection process that also produces a surface to which adhesives bond well. Two suppliers of Iridite 14-2 (concentrate) from which the ‘alodine’ solution is made are listed below.

As expected, there are a variety of methods for using alodine, depending upon the scale of use and equipment available. Some methods suggest abrasive cleaning, others do not and others recommend an acid etch instead. However, all require aluminum to be very clean prior applying the alodine solution (for varying lengths of time).

For more reading than I normally recommend, see these sources (which also sell Iridite 14-2):

Pegasus - Iridite 14-2 Chromate Coating for Alum., 10 lb, Pickup Only

And for an even more extensive discussion on application methods:

http://galladechem.com/specs/macdermid/78659rev2.pdf (has lots of recommendations ranging from simple to industrial scale).

And if you wish to be very precise in following methods for aluminum passivation / corrosion protection currently recommended for military aircraft (which have changed from when I was so involved), please see Mil-DTL-5541.

Please note in all cases the emphasis upon rinsing well after completing the 'alodining' dip / immersion in all recommended procedures.

Gee, I hope I haven't confused the issue...

Incidentally, your comment:

"The results I have come up with so far shows that even without etching, I am able to produce bonds that are stronger than the conventional general aviation materials and fasteners. All coupons of .023" thickness have torn and coupons of .065" thickness have been able to shear groups of three AN -4 rivets."

I find this very reassuring that you have done an exemplary job of testing before applying, which gives me confidence that you will construct a very strong and safe aircraft structure. You are to be commended for sharing the results of your efforts.

All the best & fly safe,

MalcolmW

 

[GESchwarz]

Thanks again for your valuable input. I would imagine that there is a curve of diminishing returns on the ability of each added process to increase bond strength in the following order of process steps: degrease, MEK wash, abraid if so desired, acid etch, and finally alodine.

My understanding about etch is that it opens up the crystalline structure to make for a good bond. And that the alodine is a corrosion barrier. Is there an error in my understanding? Why is alodine considered critical to adhesion? Because corrosion can cause debond, or does the methacrylate bond better to the chromate layer? I don't recall the manufacturer requiring a chromic treatment as part of the prep for methacrylate.

Next question...Orion indicated that methacrylate is acidic, that therefore begs the question, can methacrylate corrode the very material it's bonded to or is that acid neutralized in the bonding process? Also, is the adhesive a suitable sealant against moisture that would cause corrosion in the bonded surfaces?

I'm wanting to minimize the number of process steps if at all possible, but not at the expense of safety and reliability which of course are my highest priorities.

I'm going to see what the manufacturer has to say about all this and report what I find.

 

[orion]

Epoxy and Methacrylate materials will most likely be sufficient moisture barriers for GA application. The only area where caution might need to be exercised is in the applications of the bonding agents to wet wings. Epoxies are apparently fine but the Methacrylates are not recommended for fuel exposure.

Regarding the acidic nature of the Methacrylates, this is something that just came across my desk through a friend of mine, but contained no hard data. I just brought it up as a point of consideration, with the hope that someone here would maybe have a more definitive answer.

 

[GESchwarz]

Okay, I just got off the phone with the sales rep at Parson Adhesives. He said that their published lap shear strength 3000 psi is based on an isopropyl alcohol wash. He went on to say that abraision, etch, and alodyne would each improve bond strength. I asked if he had strength data for each of these and he said "No". He is basing his claim on experience. He said that his product is not acidic. He said that after 1000 hrs immersion in gasoline, it lost only 5% of lap shear strength and did not exhibit any other detectible physical degredation.

All that from the salesman. Without a certificate of conformance to the sort of spec requirements that we in aviation would like to see, I guess we just have to do our own testing to make sure.

 

[pwood66889]

"I just got off the phone with the sales rep at Parson Adhesives. He said that their published lap shear strength 3000 psi is based on an isopropyl alcohol wash."

Good data point, Gary. Isopropyl Alcohol is available in town, even this one. The biggest point I have against bonded construction is keeping it bonded. Keep up the good work!

And did the nice sales rep say where one could get this Wonder Water? Maybe I can just google their web site.

Percy

 

[GESchwarz]

FYI...The Parson Adhesive website has not been updated lately and therefore does not have info on the product I'm using Partite 7350. To find the data for this material Google the following: Partite 7350 Parson Adhesives. It has a good working time of about 30 or 40 minutes and the tensile elongation is 50% which does a great job against vibration and helps with load or strain distribution (I don't know the materials engineering terminology for this; can anybody help me?).

Percy, The biggest problem with the adhesives we've had in the past is their brittleness. This 7350 and others like it have elasticity, which is equivalent to toughness. Some have tensile elongation of up to 200%

Another interesting thing about some of these methacrylates...when the two parts are mixed the material gets stringy which is characteristic of long chain polymers and protein. It does not run, sag or drip so it's very easy to work with. It's a little like working with the marshmallow cream which is sold in jars or hot taffy. It holds it's shape really well when spread with a trowel (hacksaw blade). It has good holding strength when in it's fluid state which makes it easy to hold parts in place until clamped. It's like no other material I've ever worked with.

I'm not sure what you're refering to with "Wonder Water".

 

[MalcolmW]

Hello, Gary;

it sounds as though you have found an adhesive that will meet your requirements ( Partite 7350), and you may not require extensive surface preparation for application. However, I encourage you to consider 'passivating' the aluminum by chromate conversion (alodining) for protection against corrosion.

If you test the Partite on a substrate that has been 'alodined,' you may see a slight increase in strength. Also consider an environmental test (immersion in warm water for an extended period of time, followed by a retest of peel and tensile testing).

Also, the Parson's representative is probably correct in saying that 'gasoline' doesn't significantly affect the strength of their acrylic adhesive. However, he is referring to MOGAS. The 100LL gas contains aromatics (benzene and toluene) which is part of the package to get the octane rating (without using gobs of tetraethyl lead), and these aromatics are great solvents, which WILL cause the cured acrylic to swell and soften. Regular gasoline does not have much in the way of aromatics.

So, be cautious about assembling a fuel tank or wet wing with acrylic adhesives. A better choice for this role would be a polysulfide (ProSeal), which has excellent chemical resistance and has been used as an adhesive for aircraft assembly (Bede).

Ethanol containing gasoline should not be a problem to acrylic adhesives or polysulfides.

Incidentally, your 'vibration test' is more like a crash survival test - pretty extreme. However, it does provide peace of mind that your adhesive will hold up under the most severe conditions. As Percy said, keep up the good work - I'm most impressed.

All the best & fly safe,

MalcolmW

 

[GESchwarz]

Malcolm,

Again, thanks for continuing to set me straight. Yes I do plan on going the wet wing route. I am familiar with Bede and his use of ProSeal. The ProSeal has only a fraction of the strength of Partite, so to achieve the same level of stiffness as the partite bonded structure around it I will have to increase the surface area of the bond joints by an appropriate amount. Or, do you think that it would be okay to bond the tanks with Partite and seal the wetted surfaces and edges of the joints with Proseal as a barrier to avgas intrusion? That plan isn't really feasible seeing as how much of it may not be accessible after assembly. :|

 

[MalcolmW]

Gee, Gary;

I'm not setting you straight - you're the one who is doing all the hard work (conscientiously, too). All I'm offering are some memories and knowledge from when I worked in adhesive - and glad to do it.

Wet wing or gas tank sealing; I've never done any of that, all I can relate is from a visit to Bede and finding that he assembles the wings (BD4 & others) using ProSeal. Apparently, if ProSeal is strong enough to hold a wing together in flight, I suspect that it's strong enough to hold a gas tank together, especially if is surrounded by other structure bonded with a stronger adhesive. Perhaps a minor change in design could give you the flexibility to use both adhesives.

As for caulking seams, since this technique has been used successfully by others on gas tanks built with mechanical fasteners, if you caulk over an acrylic bonded seam (sealing the acrylic from contacting the 100LL) with ProSeal, it should work.

I have read that there are gas tank sealers which are liquid and are 'sloshed around' inside the tank and poured out, and allowed to dry (I believe that it is a vinyl acetate or polyvinyl alcohol). I don't know how something like this would fit into your construction plans / technique, but could be another possibility.

Anyway, it sounds as though you are on the right track and have done your homework.

All the best & fly safe,

MalcolmW

 

[GESchwarz]

That's where you're wrong. You and several others have set me straight and raised flags every step of the way. I'm just the grunt doing the work, guided by you guys who have knowledge well beyond my own. The only thing special about me is that I have a belief that it's possible, and that all it's going to take is some first-hand R&D.

Something clicked when I was typing my last post, when I wrote that the ProSeal is only a fraction of the strength of the Partite 7350, and not only is it stronger than what has worked for Bede, but it also has an interfacial reliability to it that epoxy does not have. Peel strength is still a problem that can only be solved by good joint design. At this point I feel confident that I can go ahead with my plans to attach all my skins using this adhesive. I have a few more coupons to pull that will compare etched and alodine treated surfaces to the abraided surfaces.

If this attachment method proves successful, it could significantly reduce build time and make home building more achievable for more people. I am also seriously considering using 6061 T6 for all my skins seeing as how the modulus of elasticity is just about the same as the 2024 T3. I'm thinking that the superior tensile strength of the 2024 is limited by the stress concentration at the rivets. My theory is that bonding puts 6061 on the same playing field as 2024 for actual assembled tensile strength. If this is true, then we can bring material cost down too.

So I guess I'm just going to have to prove out my theory! Off the top of my head, I guess I'll compare both materials of common skin thicknesses. I'll be comparing riveted 2024 T3 to bonded 6061 T6 in lap shear to failure.

Does anyone have any comments, suggestions, specifications, or expectations regarding this proposed test?

 

[pwood66889]

"I have read that there are gas tank sealers which are
liquid and are 'sloshed around' inside the tank and
poured out, and allowed to dry (I believe that it is a
vinyl acetate or polyvinyl alcohol). I don't know how
something like this would fit into your construction
plans / technique, but could be another possibility."

I would stay away from "Sloshing Compound." It was a
very popular repair, but can come loose over time due
to improper application/gasoline formulation. One of
my Ercoupe buddies had a forced landing when a chunk
of it broke loose. I redid my `coupe tanks when I saw
the slosh coming loose. Have a web page on it if any
one is interested.

And, "Wonder Water" is a prejoritive term... I am
interested in cost and availability of Partite 7350, tho.

"If this attachment method proves successful, it could
significantly reduce build time and make home building
more achievable for more people."

I agree, and strongly applaud your work Gary! I have
looked at square alum. tube that was Hysol'd together
with gussets for aircraft construction.

Just a few rivets were used; blind ones. Looked like it
would go together faster. With your tests, surety of
joint can be quantified. Thanks again!

Regarding proposed tests. Please adhere to the AC 43-13
rivet spacing and other acceptable data.

Percy in NM, USA

 

[BBerson]

My concern is long term durability. I don't know how that can be determined in advance.
It would require using industry standards I think. Otherwise you are experimenting in the unknown. Thats OK, I do plenty of experimenting as well, as long as you know that things might happen.

I just recall that Monnett had tried bonding on the Moni design and switched to pop rivets after an accident.
And Richard Schreder, the glider designer, reported that he had a large wing skin debond in a forum at Oshkosh. I think it was from corrosion, but I am not sure.

Bonds can degrade over time from freeze/thaw, vibration, corrosion, flight cycles etc.
Thats my concern. Good luck.
BB

 

[GESchwarz]

I just left a message with the Parson sales department asking for price and quantities. Will follow up tomorrow. Good call on the AC 43-13. I hope to have the 6061 by Friday.

I must get in touch with Monnett and Richard Schreder to find out all they know. How can I do that?

The cause of the failure may be easy to determine, or more time consuming. All of these degrading factors can be accelerated. They may have started with a inappropriate materials to begin with. Of the 6 different adhesives I started with (2 epoxies and 4 methacrylates). only one (1) stood above all the rest. The one thing that seems to have made the difference was its elasticity, which makes it most similar to ProSeal, which is very elastic, but not as strong.

So what we need now is the input from Monnett and Schreder.

Is that RICHARD E. SCHREDER, September 25, 1915 - August 2, 2002 ?

If so, I'll move on to Monnett.