Bonding to aluminum can be a difficult task. Many engineers will search for a single and simple remedy in the form of a low-temperature adhesive for making structural joints without welding. A number of manufacturers offer adhesives intended to get the job done. VHB foam tapes have also been studied as a solution, depending upon application requirements such as shear, peel resistance, service temperature, and energy absorption. However, all adhesive-based remedies and their performance specifications will be compromised if surface preparation is overlooked.
First, almost all aluminum is annealed to produce an application-appropriate surface, and to achieve high-speed processing properties, surface wettability, adhesion, and chemical resistance. Prior to rolling, aluminum is sprayed with rolling oils. These oils are typically comprised of paraffinic and naphthenic hydrocarbons to which various alcohols and esters are added. Practically speaking, these oils increase rolling process life and reduce mill power consumption. However, these rolling oils will become contaminated with other oils and hydraulic fluids that can impact surface quality such as the creation of staining.
After rolling, an amorphous oxide layer is immediately formed as a result of a chemical reaction between oxygen and humidity (in ambient air) and the rolled metal surface. During the annealing process, this oxide forms a barrier that grows thicker as a result of both the increased diffusion of oxygen through the oxide layer, and the chemical reaction of the aluminum under heat. The heat of annealing also promotes water loss and a more compact oxide, essentially creating a barrier layer on top of the aluminum. To complicate matters further, the process of annealing aluminum coils creates a big difference from the outside edge to inside core of the coil roll because air (O2 and H2O) is present at the edges. So there’s a thicker oxide layer at the edges, as opposed to the middle of the coil. As a result, aluminum surfaces will vary considerably because of variations in oil and oxide contamination concentrations. The problem remains of how adhesion across aluminum surfaces can be uniform and predictable.
Rather than introducing wet chemical surfactants that require effluent waste disposal, consider dry surface treatment techniques. By using these techniques to volatize and vaporize rolling oil deposits and break down oxides, more interfacial surface energy can be created to a level defined by “watt density.” This dry surface modification (of flat or 3D topographies) can be accomplished a number of ways, including the use of specific designs of corona treatment (air plasma) systems, flame treatment systems, plasma treatment systems, or combinations of these approaches.
Aside from contamination removal, the use of these atmospheric pressure surface modification systems can also improve aluminum grain definition, contribute a specified chemical functionalization, and promote mechanical/ chemical bond strength. Moreover, the level of corona, flame, or plasma treatment can be specified to achieve a surface wettability, usually at <5° contact angle, which can assure consistent adhesive adhesion results relative to MIL specs (see table). As such, it is highly recommended that representative samples of aluminum sheets or parts be evaluated for surface tension variations, then trialed with corona, flame, or plasma treatment technologies to define the required process technique and watt density level to properly prepare aluminum for adhesive bonding success.
This work was done by Rory Wolf, Technology Director of Atmospheric Plasma Systems at Enercon Industries Corporation. For more information, visit http://info.hotims.com/40438-122.