Metal bonding issues 2

[avior]

I'm trying to bond two 17”x17” aluminum plates with 3M AF163-2 structural adhesive. The plates are modified sulfuric acid anodized, primed with 3M EC 3960 adhesive primer within 4 hrs. after anodization , adhesive film applied and cured 250F/ 45psi 90min.
Lap shear test per ASTM D1002 gives 4700-5000psi but climbing drum peel test ASTM 1781 gives VERRRY bad results (4-5 lb/in) or very good . . . 80-85 lb/in. The worst is that on the same plate from which I cut 5 test samples I may have 3 disastrous failures and 2 strong bonds . . . I can't understand what is happening. There aren't medium values . . . it fail miserably or it pass far high on the same panel.
I have read Boeing, Bombardier and DeHavilland specifications concerning metal bonding and I suppose that every little detail is controlled, adhesive primer is cured, no contamination, I have plates from 2 supplier, anodized in 2 different shops . . . I already run around 40 tests pieces i don't know what to do anymore.
Does anybody has heard about such contrary comportment on samples cut from the same panel?
What I'm doing wrong? Does anodization process lives residues on plates?, or hydration after anodization may developed a kind of local shield so fast???

 

[SWComposites]

What is the difference in failure modes (adhesive, cohesive) and failure surfaces between the high and low results?

 

[avior]

Cohesive failure, adhesive on both surfaces in high results.
On the others samples it looks like primer doesn't stick on the anodized surface, the strip is clean, primer traces on it, most of the primer and all adhesive is on the outer strip . . . like release agent was used instead of anodize treatment (unsealed sulfuric acid anodize).

 

[Compositepro]

When you get large and sudden changes like that it is usually due to a change in failure mode from cohesive to adhesive. This is clearly visible by looking at the specimens. So what do you see? With adhesive failures you really shouldn't even care what the numbers are. The fact that the failure mode is adhesive is a failure in itself. But I have seen people average these numbers to pass. Your problem could be surface treatment, type of carrier in the adhesive, voids in the bondline, or bondline thickness control. Carrier type can cause big differences in peel testing. Knit carrier will give differnt results in different directions. Thin bondlines will give low peel and bondlines are often thin at the edges of panels. If not bagged properly the bondine at the edges can be almost zero and the andherand will distort causing the bonline in the center to be thicker than the film adhesive. The resin flow that occurs in the process will cause air bubbles to flow to and coellesce in the center of the test panel resulting in large voids.

 

[avior]

What I see is that each strip that fails - has between .040-.090” cohesive failure at the very beginning and than changes to adhesive failure within next .020”. I just checked bonding thickness, it seems to be .003-.005” almost uniform on each sample. Carrier I used mat and knit (knit tested in 45*) less low values using knit carrier but still are miserable failures. Low peel values are not related to strip location on the middle or side . . . There are few strips that have voids (2-3) .020-.040” diameter. (center voids no voids at extremity). I cured some panels between plates – not much differences.
Thanks for your answer I'll check panel preparation prior to priming process. Also I'll cure them only between plates to ensure uniform adhesive thickness.
I have also noticed that a bit of adhesive primer may be removed from the surface using a rag with plenty of MEK after primer cure . . . . it is not full cured?

 

[rodneybennet]

You may also check / post the forum at

http://www.composite-agency.com/materials-forum.htm

for additional information (such as proper modelling and intrinsic failure modes) on interfacial strength between different sorts of materials.

Regards,
Rodney

 

[Compositepro]

Primer should be cured per supplier's direction. Most adhesive primers are designed for partial cure. If over- or under-cured peel could be low. A light MEK wipe should not remove primer but strong rubbing will. Also primer thickness is critical. Primers are relatively brittle and if applied too thickly you will get low peel. Target is usually about 0.0002" thick. Primer is for corrosion protection and temporary protection of surface treatment prior to bonding. It does not improve bonding.

 

[spudpeeler]

What you are describing is associated with excess primer thickness. Primer thickness should be .00007" to .00012". Too thin and the primer will not cure, too thick and you get "tiger striping" peel failures with very low results.

Shear results are not affected by thick primer but peel strength is severely lowered if the primer is too thick. This is the most common problem associated with adhesive bonding in the aircraft industry. It is good practice to use the same vendor for adhesive and primer but you might consider using BR127 or BR6747-1 from Cytec. Both are more forgiving in terms of thickness. The 3M primer does work well if thickness is maintained in the correct range.

It is best to measure thickness using a capacitance-type thickness probe. Alternatively, color matching can also be used to determine thickness by fabricating a panel with different thickness levels (cover 4/5 of the panel and spray a box coat; then 3/5; continue to get 5 levels).

 

[avior]

I'm measuring primer thickness using elcometer 345 probe. On flat, bare metal panels primer tickness is between 0.15 mils and 0.17mils, that is actual paint gun setup. On anodized panels I can't measure the thickness because of anodized layer, even to make “0” is impossible, the error is around 0.05mils. Anyway I'm using the same paint gun setup and I hope I have the good thickness. What is killing me is the fact that primer may be wiped with solvent after primer cure cycle, not all but most of it . . . if insist. On other hand the humidity in paint booth is never more than 20%, I don't have equipment to maintain humidity between spec's limits (40-60%).
I don't know why but I have doubts about rinsing after anodization . . . they are using tap water to rinse my panels, the practice is to use demineralized, deionized or distiled water. Does chlorides, or minerals in tap water my live traces on my panel surface, or may react with remanent anodizing salts during rinsing?
By the way I tested with similar results Cytec BR127 primer with FM73 Cytec adhesive . . .

 

[rerig]

1) Could be impurities in the Tap Water.
2) Could be the "Paint Gun" was cleaned with MEK (oil based) and is spraying contaminate along with the primer.
3) Could be someone rinsed a mold released tool in the final Hot Rinse and contaminated the metal. (No Kidding)

Sounds like the primer is not being continuously shaken/stirred while applying and the volatiles are accounting for thickness with no primer strength.

 

[corrosionman]

Avior
Sometimes we seek a complicated reason for what we do not understand, but sometimes the answer is quite simple. How do you ensure 100% contact of the coating with the 17"square plates. also can your problem result from the different expansion coefficients for the adhesive and the aluminium plates - - there will certainly be a sizeable stress on the bond after cooling. Elsewhere in this Composites forum there is an interesting topic relating to such shrinkage,
Corrosionman

 

[avior]

I have to tell you that I received four plates phosphoric anodized, I bonded them in the same conditions, same material, all samples had high peel results, |I was able to reach 89 pound per linear inch . . . it may be sulphuric anodized surface not appropriate to bonding, I just had luck with phosphoric anodize? The fact that I changed the supplier makes the difference and not the process?

 

[blakmax]

Ho avior and others. I am new to this forum, but I have 37 years experience as a scientist in adhesive bonded repairs for aircraft so I have a sound understanding of adhesive bonding technology. I have also seen the results of bad processes and let me say that sulfuric acid anodizing is one of those processes. Sealed chromate anodize is even worse.

The worst aspect of bonding technology is the test methods used to demonstrate bond integrity for surface preparation processes. Strength tests such as ASTM D 1002 are useless at preventing adhesive bond degradation in service.

To understand why strength (and fatigue) tests are useless, it is necessary to understand how adhesive bonds function. Adhesives (and also primers) form chemical bonds to the metallic substrate. These are predominantly covalent and ionic bonds with some attractive (van der Waals) bonds as well. To enable these chemical reactions to occur the surface must be clean (we all know that). However it must also be chemically active so corrosion passivation coatings such as alodine are not appropriate. However, even if these conditions are met, then there is a third aspect that determines the longer term bond durability.

The most common form of bond failure is by adhesion failure at the interface. The chemical bonds formed at the time the adhesive is cured often degrade over time. For aluminum, the most common degradation mechanism is by hydration of the oxide film as Al2O3 changes to Al2O3.2H20. To achieve this, the chemical bonds formed at cure of the adhesive dissociate so that the hydrate can be formed. That results in adhesion failure of the bond. THIS IS TIME DEPENDENT so a strength test which captures the bond strength at one given time may not necessarily capture such degradation. Hence, strength and fatigue tests as mandated by the FAA's regulations for aircraft certification (the FARs) will not necessarily prevent structural failure.

Reliance on strength tests to demonstrate bond integrity is a fallacy.

Testing should be done using the wedge test ASTM D3762 where 1: wide by 6" long specimens are wedged apart by a standard wedge whilst the specimen is subjected to a hostile environment (usually 120F to 140F in 95% RH) BUT ignore that standard's statement of acceptable performance of the test. The standard states that an acceptable result provides a maximum of 0.75 in and an average of 0.5 in growth after 1 hour of exposure. Such a result is a total disaster. Use acceptance criteria of 0.20 in growth in 24 hrs and 0.25 in growth in 48 hrs AND less than 5% interfacial failure in the test zone. My former organization used those criteria and reduced our repeat repair rate from 43% in 1992 to three failures out of about 4000 repairs since then and in those cases there were demonstrated technician errors in the processes.

So, Avior, my advice to you is to keep in mind the chemical nature of the process and look for any factors thast would reduce chemical activity at the time of bonding. Contamination is one possiblity, but also the time between surface preparation and application of the primer may be another. Then cosider if you should be using wedge testing instead of strength tests.

These discussions are contained in DOT/FAA/AR – TN06/07, Apr 2007 which is available throught the FAA Tech Center Library actlibrary@faa.gov however if you are not a US citizen, you will need to send a request to the library at that address. (I am the author and I can't access my work!)

Other material is on my web site adhesionassociates.com

One must really ask why if adhesive bonds can fail interfacially, the tests mandated by the FARs do not prevent one of the more common forms of bond failure. I thiught the FARs were intended to prevent structural failures? Maybe we can start a discussion on this issue?

Regards

blakmax