bonding 2 pieces of ali?

[blakmax]

Adhesive bonding

Dear me. The level of knowledge about adhesive bonding within the rotorheads community is really low. I suggest that you try reading DOT/FAA/AR – TN06/07, Apr 2007 BEST PRACTICE IN ADHESIVE BONDED STRUCTURES AND REPAIRS. You can get this document by emailing the FAA Tech Center. If you want a soft copy send me a private message.

Firstly, the selection of the adhesive is almost a trivial aspect of the technology. Most adhesive bonds do not fail through the adhesive, they fail through the interface between the adhesive and the metal. That form of failure is driven by the surface preparation prior to bonding. Can I assure you that if you "clean with a degreaser then abraid (abrade?) with emery" you will have a weak bond. Here is why:

Adhesive bonding depends on the development of chemical bonds between the adhesive and the metal. (It has very little to do with surface roughness.) To achive a good bond, the surface must be clean (so that the chemical reactions can occur), but it also must be chemically active, which is why the surface must be etched or abraded. If you do clean with a degreaser then abrade with emery you will get what appears initially to be a good bond. However, the interface between the adhesive and the metal will be susceptible to degradation over time and eventually the bond will fail. The mechanism is that the aluminium oxides formed immediately after abrasion will have an affinity for water and will form a hydrated oxide (bohemite). The chemical bonds formed initially dissociate so that the hydrated oxide can be formed, and this leads to disbonding. Hence it is essential that you treat the surface to produce chemical bonds that are resistant to hydration. Be careful though. There are many adhesive bond primers out there which are virtually useless. I suggest that you contact me or visit my web site Adhesion Associates
If you follow this advice, then virtually any adhesive you select should provide a strong, durable bond. If you don't follow the advice, then it will not make any difference which adhesive you use. The only difference will be the colour of the disbond. Obviously, the amount of effort you wish to expend depends on the importance of the bond to structural integrity and the consequences of failure. If the joint is required to provide ongoing strength in a structure that is critical, then you must do everything you can to assure integrity of the bond. If the bond is nothing more than cosmetic and failure is inconsequential, then use a sealant. That will keep the material together probably better than an adhesive at least for a short period of time.


Regards
Blakmax