Adherend preparation

In early studies, the durability of Ti-6A1-4V bondments was determined as a function of surface preparation for several adherend preparations and several adhesives... 

BS5350, Methods of Test for Adhesives Adherend Preparation, British Standards Institution, 1976, part Al. 

In the following sections, we first discuss briefly some of the analytical techniques used in the study of adherend surface preparation. Aluminum adherend preparations are discussed next in Section 2 and titanium adherends in Section 3, including processing, oxide morphology and chemistry, and bond durability. Section 4 contains a discussion of the preparations that have been used with steel adherends to improve surface morphology for bonding. 

As shown in Table 5, in the mode I test, the thicknesses of the residual adhesive layer on the failure surfaces were about 250 xm for all the specimens with different surface preparations, which indicated that the failures all occurred in the middle of the adhesive layer in the test regardless of the surface preparation method since the total thickness of the adhesive of the specimens was 0.5 mm. When the phase angle increased as in the asymmetric DCB test with h/H = 0.75, which contains 3% of mode II fracture component, a layer of epoxy film with a thickness of around SO xm was detected on the failure surfaces of all the specimens. Although the failure was still cohesive, the decrease in the film thickness on the metal side of the failure surfaces indicated that the locus of failure shifted toward the interface due to the increase in the mode mixity. On the other hand, because the failure was still cohesive, no significant effect of interface properties on the locus of failure was observed. When the mode mixity increased to 14% as in the asymmetric DCB test with h/H = 0.5, where the mode mixity strongly forced the crack toward the interface, the effect of interface properties on the locus of failure became pronounced. In the specimen with adherends prepared with acetone wipe, a 4-nm-thick epoxy film was detected on the failure surfaces in the specimen with adherends treated with base/acid etch, the film thickness was 12 nm and in the P2 etched specimen, a visible layer of film, which was estimated to be about 100 nm, was observed on the failure surfaces. This increasing trend in the measured film thickness from the failure surfaces suggested that the advanced surface preparation methods enhance adhesion and displace failure from the interface, which also confirmed the indications obtained from the XPS analyses. In the ENF test, a similar trend in the variation of film thickness was observed. 

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. 

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

The surface preparation must enable and promote the formation of bonds across the adherend/primer-adhesive interface. These bonds may be chemical (covalent, acid-base, van der Waals, hydrogen, etc.), physical (mechanical interlocking), diffusional (not likely with adhesive bonding to metals), or some combination of these (Chapters 7-9). 

Fig. 9. Wedge test results of aluminum adherends with the following surface preparations FPL, PAA, and FPL followed by an NTMP treatment. Adapted from Ref. [42].

Direct bonding. In many high-volume production applications (i.e., the automotive and appliance industries), elaborate surface preparation of steel ad-herends is undesirable or impossible. Thus, there has been widespread interest in bonding directly to steel coil surfaces that contain various protective oils [55,56,113-116], Debski et al. proposed that epoxy adhesives, particularly those curing at high temperatures, could form suitable bonds to oily steel surfaces by two mechanisms (1) thermodynamic displacement of the oil from the steel surface, and (2) absorption of the oil into the bulk adhesives [55,56]. The relative importance of these two mechanisms depends on the polarity of the oil and the surface area/volume ratio of the adhesive (which can be affected by adherend surface roughness). 

Effects on lap shear strength of EB-cured epoxy adhesives from different surface preparations on aluminum and composite 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. 

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