Primer failure of bonded joint 2

[airborne12]

Hi,
We bonded carbon composite (CF-epory) to aluminium with 3M EPX DP490 adhesive (epoxy). A metal primer (epoxy, also from 3M) was used for corrosion protection (galvanic corrosion). On test samples we saw green residue on both the CFRP and the aluminium, indicating that the primer itself had failed. (which also shows that a bond is always as strong as the weakest link!)
First investigation showed that the thickness of the primer was somewhat higher than prescribed by 3M (25micrometer instead of the prescribed 5-17micrometer). Furthermore the primer was cured for 1hr at 60C, where 3M prescribed 1/2hr@80C or 10min@120C. 3M claims that our cure cylce was too long at elevated temperature, which led to a more brittle epoxy primer. I find it hard to believe because the primer will always see the extra cure for the adhesive.
Does anybody have experience with primer failure? Is the cure cycle the problem, or the primer thickness? Or maybe an other explination?

thanx
jasper

 

[Goahead]

I would first try to follow the instructions, both for the thickness and for the cure cycle. Cleanliness and surface finish are also probably contributing factors. But are you sure that the primer is needed at all?

http://www.welding-advisers.com/

 

[ewh]

airborne12 is right to use a primer; without some sort of separation, you would open yourself to galvanic corrosion between the carbon and the aluminum.

 

[corrosionman]

airborn12 when you say it had failed, what failed? The bond between the aluminium and the primer ? or between CF and primer ?? Failed because it did not meet theoretical stress?? Does the maker of the primer offer stress levels ?
What is the green stuff you mention . If you heat cure at high temp then on cooling there is high stress du to different shrinkage because of different expansion rates of CF and Aluminium.Instead of the primer why not just use a thin layer of the same adhesive and partially cure it to actt as insulator. on the other hand do an experiment and make a joint without any primer whatsoever you will certainly find that tthere is no electric contact between the two bonded surfaces. Regards DW

 

[ewh]

Do you have the space to use fiberglass as a sacrificial ply between the CF and the aluminum?

 

[airborne12]

I forgot to mention that the primer was green. Because there was green residue on both the aluminum and the CF part (the fracture surface was completely flat, the adherents didn't fail). Hence we concluded that the primer itself had failed.

The joint is a scarf joint, where both the CF and the aluminium were machined at 5deg angle. It would be possible to place a glassfibre layer in between (when applying the adhesive), but on the other hand it would reduce the strength of the adhesive. Since primers are commonly used in Aerospace industry, we assumed that it would provide sufficient corrosion protection.

I prepared some test samples for today, to check what the strength is for different primer thickness. (unfortunately 3M cannot supply us with strength of the primer for different thickness)

Unfortunately we deviated a little bit from the instructions. It seems that the applying the primer is VERY critical. Does anybody else have experience with primer strength?

 

[rerig]

We had primer problems until we used a Stir-pot Paint gun for primer applications only.
The continuous agitation of the primer was critical to keeping the solids in suspension and a pre-shake with paint shaker allowed too much time to pass, allowing the solids to settle.
We would use the paint shaker on the 5 gal can, then immediatly decant to the stir-pot paint gun, then while continuously agitating, apply the primer. None of this was actually described in the user application information nor the Specification calling out this particular primer.

We had been getting the correct thickness of primer (carrier only!), but there were no solids there for strength.
This was for a 350F metal bond application. (Aluminum-to-Aluminum)
Suggest calling 3M Eng. Tech Support and just ask if this could be a problem.