Aluminum adhesive interface

Chrome complexes have been developed as adhesion promoters by the reaction of chromium chloride with methacrylic acid. The chromium oxide portion of the adhesion promoter reacts with a substrate while the methacrylic portion reacts with a free radical curing outer layer. Chrome-based adhesion promoters are commonly used as a primer for aluminum foil to increase the strength and durability of aluminum/polyethylene interfaces.26... 

For reasons as described above, some metal adhesive interfaces are chemically capable of accelerating the rate of oxidation. For example, it has been found that nearly all types of structural adhesives exhibit better thermal stability when bonded to aluminum than when bonded to stainless steel or titanium (see Fig. 15.3). 

Pig. 33. EE from T-peel testing of Aluminum/epoxy interface for adhesive and adhesive plus cohesive failure. 

Abstract Proper treatment of an adherend surface is one of the most important factors in assuring high initial strength and extended durability of high-performance adhesive joints. There are several requirements for a good surface preparation (1) The surface must be cleaned of any contamination or loosely bound material that would interfere with the adhesive bond. (2) The adhesive or primer must wet the adherend surface. (3) The surface preparation must enable and promote the formation of chemical and/or physical bonds across the adherend/ primer-adhesive interface. (4) The interface/interphase must be stable under the service conditions for the lifetime of the bonded structure. (5) The surface formed by the treatment must be reproducible. In this chapter, high-performance surface treatments for several metals and other materials are discussed. Surface treatments of aluminum and other metals are used to illustrate how proper surface preparations meet these requirements. 

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

The XPS analysis of the failure surfaces of the wedge samples primed at 51% RH and bonded with PES and immersed in DI water at 100°C are reported in Table 3. The concentration of sulfur in all of the primed samples bonded with PES was about 5% and suggests that failure occurred primarily within the adhesive. The atomic concentrations of aluminum, silicon, and titanium were below 0.5%, precluding assignment of failure within the alkoxide layer. The control sample failed at the interface between the steel and the adhesive. 

Another way moisture can degrade the strength of adhesive joints is through hydration or corrosion of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron, can undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very inadequate bonding to their base metals. Thus, the adhesive or sealant used for these materials must be compatible with the firmly bound layer of water attached to the surface of the metal oxide layer. 

Strong chemical bonds between the adhesive and adherend help stabilize the interface and increase joint durability. Aluminum joints formed with phenolic adhesives generally exhibit better durability than those with epoxy adhesives. This is partially attributable to strongly interacting phenolic and aliphatic hydroxyl groups that form stable primary chemical bonds across the interface. 

In selecting a pretreatment process for aluminum or any other substrate, both the initial strength and the permanence in a specific operating environment must be considered. Mechanical abrasion is a useful pretreatment in that it removes the oxide and exposes bare aluminum. When this is done, however, many of the benefits of the protective oxide layer are lost. For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraded metal, but it will deteriorate rapidly when exposed to a harsh environment such... 

Copper substrates are commonly bonded with epoxy adhesives in the microelectronics and marine industries. Compared to aluminum substrates, copper when bonded with epoxy adhesives provides lower initial strength. Depending on the adhesive and the type of test used, this can be as much as 50 percent lower. Similar to aluminum joints, copper joints bonded with epoxy adhesives can show poor durability in moist environments unless the interface is protected. 

Kim, G., and Ajersch, F., Surface Energy and Chemical Characteristics of Interfaces of Adhesively Bonded Aluminum Joints, Journal of Materials Science, vol. 24,1994, pp. 676-681. 

Pigments used in barrier coatings should also be hydrophobic in nature, and possess good adhesion across pigment-binder interfaces. Some typical pigments used are flat platey aluminum flakes, glass flakes, stailness steel flakes and micaceous iron oxide. 

---