Bonded joints, improving durability

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. 

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. 

Of primary importance in moist environments is the plasticization, or softening, of the adhesive, a process that depresses Jg and lowers the modulus and strength of the elastomer [89-91]. Plasticization of the adhesive may also allow disengagement from a microrough adherend surface to reduce physical bonding and thus reduce joint strength and durability [37]. On the other hand, it may allow stress relaxation or crack blunting and improve durability [92]. 

Another approach to improve durability involves overdesigning the bond so that the actual stresses experienced are a small fraction of the stresses that the joint is capable of withstanding. Stresses are thereby reduced to below any critical level and the load can be carried even if moisture ereates a disbond over a portion of the joint. Of course, this approach may not be feasible from a cost or weight standpoint. Alternatively, the bond can be designed so that moisture has a long diffusion path to reaeh a eritical area—the same general principle by whieh sealants work. 

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. 

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]. 

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.)... 

In summary, it can be stated that a large number of different surface treatments exist to improve the adhesional properties of PP. It should be stated here that some manufacturers of cyanoacrylate adhesives recommend special chemical primers which produce adhesion on PP without other treatment. In the experience of the author, this type of primer improves the adhesion but produces bonded joints that are insufficiently durable, especially under humid enAuronmental conditions. 

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

Bond enhancement is not required to increase initial strength, but where bonded joints may be subjected to repeated wetting and drying or long periods of exposure to water and high humidity (e.g., with outside applications of externally bonded FRP composites) it may be necessary to improve long-term durability of the structural bonded joint with primers, adhesion promoters and other surface treatments (Custodio et al., 2008, 2009b). 

Surface finish and surface preparation are both key factors in the success of an adhesively bonded joint and, in many applications, roughening the plastic surface can be beneficial to the overall bond strength and the durability. If the adhesive is injection moulded, it is often possible to spark erode the mould tool to give a slightly rougher surface finish at the bond line thus improving the mechanical keying of the adhesive to the surface. A surface finish of between 1 and 2 Ra will invariably improve the adhesion performance of cyanoacrylates to thermoplastics. 

Increasing the bonded area will usually decrease the overall load on the joint and therefore improve the reliability and durability as there is a lower overall stress on the joint. The adhesive will always be stronger in compressive, tensile and shear loads than in peel or cleavage loads and typically the peel load of an adhesive will be less than 5% of the nominal shear strength so ensuring that the bonded joint cannot be subjected to peel or cleavage loads will improve the overall integrity of the joint. 

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 ... 

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. 

The application of corrosion-resistant primers has become standard practice for the structural bonding of aluminum in the automotive and aerospace industries. The adhesive-primer combinations are chosen to provide both maximum durability in severe environments and higher initial joint strength. Improved service life is typically achieved by establishing strong and moisture-resistant interfacial bonds and protecting the substrates surface region from hydration and corrosion. 

Chemical Surface Modification. In considering the interface, one must contemplate not only the possibility of moisture disrupting the bond but also the possibility of corrosion of the substrate. Corrosion can quickly deteriorate the bond by providing a weak boundary layer before the adhesive or sealant is applied. Corrosion can also occur after the joint is made and, thereby, affect its durability. Mechanical abrasion or solvent cleaning can provide adhesive joints that are strong in the dry condition. However, this is not always the case when joints are exposed to water or water vapor. Resistance to water is much improved if metal surfaces can be treated with a protective coating before being bonded. 

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

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