Bond-joint durability

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

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. 

BOND-JOINT DURABILITY AS A FUNCTION OF SURFACE PRETREATMENT... 

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. 

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. 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

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. 

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., Durability of Aluminum Bonded Joints in Long-Term Exposure, International Journal of Adhesion and Adhesives, vol. 2, no. 1, 1982, p. 25. 

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. 

From the above discussion, it follows that it should be possible to improve the durability of bonded joints by the introduction of suitable coupling agents at the interface. If the coupling agent is capable of chemically interacting with the metal or its oxide, displacement of the adhesive at the interface will be prevented. 

The effects of water and temperature on the adhesive itself are also of utmost importance to the durability of bonded structures. In the presence of moisture, the adhesive can be affected in a number of ways, depending on its chemistry and how rapidly the water permeates through and causes significant property changes [51,86-88]. The potential efficacy of moisture penetration on the locus of failure of bonded joints has been discussed in the previous section. As expected, elevated temperature conditions tend to degrade joint strength at a faster rate. 

The moisture content of both the wood and the adhesive affect the fracture behavior of adhesive bonded joints. Wood joints are especially sensitive to moisture effects as a result of the porosity and permeability of wood, which allows ready access by water to both the interior of the wood member and the adhesive layer. Irle and Bolton [57] showed that the superior durability of wood-based panels bonded with an alkaline PF adhesive compared to panels bonded with a UF adhesive was due to the ability of the phenolic adhesive to absorb and be plasticized by water. In the plasticized state, the phenolic adhesive is able to reduce stress concentrations that otherwise fracture the wood or the adhesive in urea-bonded panels. 

In summary, bonded repairs are the preferred approach for manufacturing repairs to both honeycomb sandwich and monolithic secondary stmcture. However, for the FAA and European Airworthiness and Safety Administration, the main reason for withholding certification of bonded repairs for primary structure is the lack of certainty over bond quality as it is not possible to assess strength and durability of bonded joints without destructive testing. 

Despite some recent developments, the exploitation of the fitll potential for on-site bonded joints is mainly restrained at present by the lack of structural design guidance, standards for durability assessment and onsite acceptance testing. 

This article discusses briefly the use of adhesives on the construction site in the context of structural repair and reinforcement the requirements and practical dilficulties in the work on site with regtirds to the strength and durability of the rehabilitated timber structure and the consequent need for queility control. It also highlights the characteristics and requirements that must be fidfilled by structural adhesives and reinforcing materials factors affecting performance and durability of bonded joints tmd ways to improve adhesion and durability. Finally, it points out some research needs and future developments identified by the authors. 

However, if safety is at stake, these approaches are not at all suitable due to the high variability in strength and durability regain provided by such an approach [13]. Not only will the adhesive most likely fail to reach the (closed) very tip of the fissure, but also this type of bonded joint itself is very prone to delamination due to timber dimensional variations. 

Water is often regarded as one of the most distressing agents that can affect the bonded joint performance and durability. Most bonded structures when exposed to water or humidity will lose strength over a period of time and in rare cases they may collapse, although this effect is limited to extreme conditions. 

As moisture, temperature is also an important factor in the durability of structural adhesive joints, since it can affect the creep, fatigue and fire performance of bonded joints. 

Besides the environmental factors mentioned above, the materials involved in a structural joint also influence bond strength and durability. The factors in the material category include the adherends the adhesive the design of the joint absence of surface contamination (including contamination with wood extractives) stability of the adherend surface the ability of the adhesive to wet the surface, and entrapment of air/volatiles. Thus, the condition of the adhesive/adherend interface becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint [31]. 

Another chemical treatment that has resurged recently due to improvements made in its formulation is the hydroxymethylated resorcinol (HMR). HMR has been used successfully with several timbers and adhesives, to promote the exterior durability of their bonded joints [65, 71]. Consequently, this technique seems ready for industrial application, at least for the species and adhesives tested. Nevertheless, studies to clarify some aspects of its action mechanism are still needed [20]. 

---