Pretreated adherend

Adhesive system Type Curing conditions Pretreatment Adherend 1 Adherend 2... 

The often used FPL etdi of an aluminum-lithium alloy bonded with polysulfone leads to interfacial (at the metal oxide/polymer interface) failure (38) which is a surprisingly uncommon type of failure. The results leading to this assignment are shown as XPS C Is and O Is narrow scan spectra in Figure 15. This definitive assignment of failure mode is based on the fact that one failure surfece has an oi gen photopeak similar to the pretreated adherend before bonding and the other failure surfece has an 0 gen photopeak similar to the adhesive. 

A brief review of each technique will be followed by a discussion of results illustrating the application of the particular technique to adhesion. Examples are given of surface characterization of pretreated adherends, adhesive/adherend interactions, failure surface analysis, and correlation of these results to bond performance. Good summaries of the four major surface analytical techniques, namely XPS, AES, ISS, and SIMS, are given by Hercules(5) and Hofmann.( ) (See also Chapter 6 by Davis.)... 

Thus, AES can yield chemical information about the surface composition of pretreated adherends as well as compositional information below the surface with depth profiling. Correlations of the surface composition with bond performance can be made. 

Johnsen et al. found that the durability of the bonded joints, assessed by the Wedge Test [40] (Fig. 16), almost irrespective of electrolyte used, to be nearly as good as obtained using conventionally pretreated adherends, which had been CSA pickled and then DC anodised in phosphoric acid [38]. Bjprgum et al., however, found that an AC PAA pretreatment yielded joints which failed cohesively whereas the AC SAA joints tended towards adhesion failure [39]. 

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. 

Another means of providing a hydration-resistant surfaee is its treatment with a hydration inhibitor [41]. Fig. 9 shows wedge tests results for a Forest Produet Laboratory (FPL) bond [43], an FPL bond pretreated with nitrilotrismethylenephos-phonie (NTMP) aeid [42,44,45], and a PAA bond. The monolayer eoverage of NTMP stabilizes the FPL surfaee against hydration and provides wedge test bond performanee similar to that of PAA-treated adherends. 

Since the choice of surface pretreatment prescribed for a metallic adherend has a direct effect on the performance of a joint in humid conditions, four types of commonly utilized automotive surface preparations were examined. The effects upon durability of no cleaning, alkaline cleaning, lubricating or zinc phosphating were examined. Accordingly, adherends were prepared using one of the four methods detailed below. 

Initial bond strength depended heavily upon substrate type rather than surface preparation. Regardless of pretreatment, initial bond strength was highest when using Al-Tl adherends and lowest when the adherends were galvanized steel. 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

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. 

On the assumption of a clean and, respectively, pretreated surface, the next step will be the application of the adhesive. It has to be ensured, however, that in areas with adhesive forces emanating from the adherend surface, the adhesive molecules are really able to get closer. Only then can the adhesive distribute itself on the surface, that is, wet the surface despite a, more or less, existing roughness. Furthermore, sufficient flowability of the adhesive is important. A complete and equal wetting of the surface to be bonded is therefore an indispensible prerequisite for the production of a strong bonded joint. Figure 6.5 demonstrates the difference between a low-viscosity and high-viscosity adhesive. 

Following the surface preparation, it is the task of surface pretreatment to generate the adhesive forces on the adherend surfaces required for the development of a strong bonded joint. Since almost all materials interesting for bonding have the property to cover the surfaces with impurity layers (oxides, rust, dust, greases), those layers have to be completely removed prior to adhesive application, since otherwise failures in the development of the adhesive forces will occur (Figure 7.5). 

Operation (partly) in vacuum. Reactive gases (oxygen, hydrogen, fluorine), which are transferred into an energy-rich state (plasma)by microwave stimulation with the possibility of chemical surface modification, are fed into the plasma chamber with the adherends to be pretreated. 

Surface treatment Generic term for processes applied to surfaces to achieve a surface condition of the adherends suitable for a bonded j oint or to optimize them in view of their adhesiveness. Such processes are divided into surface preparation, pretreatment and post-treatment. 

The adhesive thickness was about 0.1 mm. The dimensions of the bonded area were 6 in xl in (15 cmx2.5 cm). AU adherends (for shear and peel) were aluminum 2024-T3 alloy pretreated by unsealed chromic anodization. The substrates were cleaned with trichloroethylene prior to bonding. The shear and peel tests were conducted on an Instron 4481 tester. 

Table 33.1 Adhesive systems, adherends, and pretreatments of the adhesive joints studied in this work.

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

Plastic adherend Pretreatment/preparation Adhesive types... 

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. 

Together with the literature detailing practical information on pretreatment procedures(25-33), there exist many research publications concerned with the optimisation or development of methods directed towards particular adherends. A visual appreciation of the effects of pretreatment has been facilitated in particular by electron microscopy in its various forms (e.g. Figs 3.6 and 3.7). 

---