Epoxy bonded aluminum joints

In other experiments Orman and Kerr(72) showed that epoxy-bonded aluminum joints could show a significant loss in joint strength when exposed in 100% RH conditions during the... 

Chelating agents are sometimes used as scavengers to capture undesirable metal ions. These compounds react directly with the metallic substrate, thereby inhibiting its catalytic effects on oxidation. The effect of several different chelating agents on the resistance of epoxy-phenolic bonded aluminum joints to thermal aging is shown in Table 15.5. 

Tables 15.4 and 15.5 present the adhesion shear strength of epoxy (Epon 815C/Versamide 140)-bonded aluminum joints (anodized) and of PU (EN-4/EN-7)-bonded aluminum joints (anodized), respectively. Both types of adhesively bonded joints were primed with high MW HB PAMAM (AD-102). As is evident from the tables, significant improvements in shear strength were ob-...

Extensive information on the durability of bonded aluminum joints is available in the reviews of Minford. " Figure 4 illustrates some typical results, showing the effect of adhesive variation on joint durability for a marine exposure. Vinyl-phenolics and nitrile-phenolics have an excellent history of joint durability and rank among the most resistant to environmental deterioration. In spite of this, however, the current trend is to use epoxy-based adhesives, which provide easier processing and higher peel... 

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

FIGURE 3.9 Plot of tensile shear strength of an aluminum joint bonded with an elevated-temperature curing epoxy adhesive as a function of test temperature. 

Lin, C. J., and Bell, J. P., The Effect of Polymer Network Structure upon the Bond Strength of Epoxy- Aluminum Joints, Journal of Applied Polymer Science, vol. 16, 1972, p. 1721. 

Representative data are shown in Fig. 10.5 for aluminum joints bonded with an epoxy film adhesive and a standard chromate-containing primer. Up until recently, standard corrosion-resistant primers contained high levels of solvent, contributing to high levels of volatile organic compounds (VOCs) and chromium compounds, which are considered to be carcinogens. As a result, development programs have been conducted on waterborne adhesive primers that contain low VOC levels and little or no chrome. Data are presented on several of these primers in Tables 10.8 and 10.9. 

The hostility of certain moisture environments can be seen in Fig. 15.11 Aluminum joints were bonded with room temperature curing epoxy-polyamide adhesive and aged in a hot, wet (tropical) environment and in a hot, dry (desert) environment. Excellent durability is achieved under dry conditions while significant degradation is caused by the wet conditions. 

FIGURE 15.11 Effect of outdoor weathering on the strength of aluminum joints bonded with epoxy-polyamide.30... 

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. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

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. 

In his test, a thin film of adhesive on a glass microscope slide or a metal coupon is cured and soaked in hot water until the film can be loosened with a razor blade. There is usually a sharp transition between samples that exhibited cohesive failure in the polymer and those which exhibited more of an interfacial failure. Since the diffusion of water into the interface is very rapid in this test, the time to failure is dependent only on interfacial properties and may differ dramatically between unmodified epoxy bonds and epoxy bonds primed with an appropriate silane coupling agent. The time to debond in the hot water for various silane primers differed by several thousandfold when used with a given epoxy. In parallel tests, a thick film of epoxy adhesive on nonsilaned aluminum coupon showed about the same degree of failure after 2h in 70°C water as a silaned joint exhibited after more than 150 days (3600 h) under the same conditions. 

Figures 10.3 and 10.4 show the degradation of the epoxy adhesive bond due to the effect of temperature on the anodic aluminum joint at 95% RH. The plots show that those joints exposed at 23 C and 95% RH degraded rapidly when the stress level exceeded a force of 3914 N, which would be...

Figure 10.7 Environmental S-Log A/curves for nitrile-epoxy film adhesive-bonded 2024-T3 aluminum joints at 1000 cycles/minute. ...

In contrast to the attention given to the bonding of aluminum and wood, very few studies have been reported dealing with the durability of other structurally bonded systems." A limited amount of sustained load durability data is available for steel bonded with epoxies and aluminum bonded with urethanesin recent years, reports have appeared dealing with the environmental resistance of modified acrylics and cyanoacrylates. Brockmann has also reported on the durability of steel joints exposed to hostile environments. " ... 

Ti—Zr-based pretreatment provided improved durability of epoxy-bonded AA6060 aluminum joints relative to an alkaline etch and deoxidation pretreatment but was inferior to a chromate treatment in terms of adhesion and resistance against FFC. The presence of cathodic intermetallic a-Al(Fe,Mn)Si particles in AA6060 is detrimental ... 

Lunder O, Lapique F, Johnsen B, Nisancioglu K. Effect of pre-treatment on the durability of epoxy-bonded AA6060 aluminum joints. Int J Adhes Adhes. 2004 24 107-117. 

Picatinny Arsenal workers also carried out a 3-year weathering program on aluminum joints, using 17 different adhesives [25]. Of the 17 adhesives, 13 were epoxies or modified epoxies, as epoxy types are most widely used in adhesive-bonding applications. In the study, only 5 of the original 17... 

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