Durability joints

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Extensive information on durability of adhesive joints is more available on aluminum than on other substrates. Figure 15.17 illustrates typical results showing the effect of adhesive variations on joint durability during marine exposure. Vinyl-phenolics and nitrile-phenolics have... 

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. 

Differences in performance are also noted depending on the specific nature of the environment. Differences can be expected in joint durability when bonds are exposed to an aggressive wet-freeze-thaw cycle, marine seacoast, or inland environments. Outdoor weathering conditions are often classified by one of the following exposure conditions ... 

Minford, J. D., in Permanence of Adhesive Bonded Joints, Durability of Structural Adhesives, A. J. Kinloch, ed., Applied Science Publishers, London, 1983, p. 135. 

Minford, J. D., Comparison of Aluminum Adhesive Joint Durability as Influenced by Etching and Anodizing Treatments of Bonded Surfaces, Journal of Applied Polymer Science, Applied Polymer Symposia, vol. 32, 1977, pp. 91—103. 

Table II Effect of HTTHG Treatment on Joint Durability...

A. Freydin, Adhesive Joints Durability and Strength, Khimiya, Moscow, p. 272 (1981). 

In Chapter 3, theoretical aspects of adhesion are reviewed with the object of discussing why adhesives stick, before addressing practical aspects of the surface pretreatment of a number of common construction materials, It is shown that merely establishing interfacial contact between adhesive and adherend is often not sufficient in itself to ensure satisfactory performance. Particular, and sometimes elaborate, pretreatment procedures are found to be necessary for maximising joint durability, and this subject is further elaborated in the following chapter. 

Optimisation of surface pretreatment is the key to maximising joint durability. The adhesive influences the surface oxide layer and the surface oxide layer influences the boundary layer polymer matrix the whole must therefore be viewed as a unique system for every adherend-adhesive combination. The interplay of chemical bonding... 

Comyn, J. The relationship between joint durability and water diffusion. Chapter 8 in Developments in Adhesives - 2 (Ed. Kinloch, A.J.), Applied Science Publishers, 1981. 

J.D.Minford, "Joint Durability Studies with Abraded, Etched, Coated and Anodized Aluminum Adhesives," ACS, ORPL Preprints, Kansas City, MO, 9/1982. 

The phosphoric acid-anodized process provides markedly improved stressed-bond joint durability and retards bond-line crevice corrosion (started at an edge) in severely corrosive environments when compared to chromic acid-anodized and FPL etched. (See Chapter 9 for description.)... 

Stressed-bond joint durability is markedly affected by the adherend prebond surface treatment and the adhesive/primer system in contact with it. This is evidenced by the poor performance of FM 123-L/BR 123 (non-CIAP) adhesive/primer system on FPL-etched and chromic acid-anodized 2024-T3 aluminum alloy, clad and bare, and the superior performance of the same systems when BR 127 (corrosion-inhibiting adhesive primer (CIAP)) is substituted for BR 123 (non-CIAP). 

The wedge test method is discriminatory and provides a relative ranking for many of the parameters that affect bond-joint durability. 

Simple adhesion tests will always be informative. They can show, for example, whether or not a surface is sufficiently firm to be painted satisfactorily, or allow the progressive effects of weathering to be monitored (see Weathering of adhesive joints. Durability). 

Wedge test D G DIXON Use of test for comparing joint durability... 

The application of these principles to an actual adhesively bonded joint is anything but straightforward. One problem is the lack of pertinent information on the performance of adhesive joints. Most test data generated by adhesive producers are only useful for comparison purposes and are of limited use to the design engineer. Also, there is only limited information available on the performance of bonded joints exposed to service environments, while subjected to static or dynamic stresses. Adequate adhesive characterization and prediction of joint durability remain goals for the future. 

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... 

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