Durability surface treatments

Primers are required to be resistant to all of the same fluids and environments as the adhesive, and are in addition expected to be compatible with secondary finishes such as corrosion and fluid resistant primers applied to cured bond assemblies. The most commonly used primers for 250°F cured epoxy adhesives also have active corrosion inhibitors themselves to combat corrosion at bondlines. This last requirement is somewhat dated, evolving from the severe corrosion and delamination problems experienced before U.S. airframe manufacturers adopted durable surface treatments. 

Surface preparation of the core foil was originally simple acid etching. As the importance of durable surface treatments became known, a more stable chemical conversion coating with an organic primer-like coating became standard. Still, water ingression into honeycomb structure continued to cause the occasional... 

Local repair of delamination originally caused by non-durable surface treatment is only temporarily successful at best. The surface treatment on the unrepaired portion of the assembly remains susceptible to attack and the area of delamination will likely continue to grow once the assembly is put back into service and exposed to moist conditions. Replacement or complete remanufacture of the component is the only way to permanently address this type of damage. However, time-limited repairs using bonded or mechanical methods can be used to extend the life of the component until a major overhaul is scheduled. In some cases such as widespread disbond of fuselage doublers, mechanical repairs (rivets and fastened doublers) and continued inspection are used to extend the life of the skin indefinitely because of the high cost of replacement. 

A wide variety of special durable surface treatments have been used on manufactured fibers. These include treatments for imparting such characteristics as soil resistance, antistatic behavior, and wearer comfort through moisture wicking and transport. Fiber finishes also have been used successfully in promoting adhesion between two materials, as, for example, between polyester tire cord and rubber, and between glass fiber and polyester resin. 

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

The surface preparation method must be carefully considered, especially if the completed weldbond is to have long-term durability to hostile environments. The surface preparation should provide an optimal surface for both adhesion and welding. Thus, the choice of surface treatment is crucial, and there can be a conflict of requirements. The spot welding process requires a low electrical surface resistance, and many adhesive surface preparation processes provide a high surface resistance because of oxide layer buildup. When it is impossible to harmonize on a surface treatment, current practice tends to favor treatments that yield good weld nuggets at the expense of the adhesive bond. 

However, with the proper combination of surface treatment and adhesive, these differences in durability to aggressive environments can be minimized, as shown in Table 16.3. The initial shear strength and permanence depend on the type of alloy and the pretreatment used. Note that the data presented here show only the relative differences in joint strength for one specific epoxy adhesive and are not representative of other adhesive formulations. There was no attempt to maximize any of these values through choice of the adhesive. 

FIGURE 16.2 Effect of surface treatment on the durability of epoxyaluminum joints exposed to room temperature water immersion. (1) Anodized, (2) grit-blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating.18... 

Devine, A. T., Adhesive Bonded Steel Bond Durability as Related to Selected Surface Treatments, U.S. Army Armament Research and Development Command, Large Caliber Weapon Systems Laboratory, Technical Report ARLCD-TR-77027, December 1977. 

Surface Treatment of Polytetrafluoroethylene Primer, Adhesive, Corrosion Inhibiting, -67 to 200°F Primer, Adhesive, Corrosion-Inhibiting, for High Durability Structural Adhesive Bonding... 

The interphase region is complex, and its composition is usually unknown. Primers or surface treatments often are used to improve control of the interphase and provide increased adhesion, durability, and resistance to aggressive environments. The chemistry of primers and surface treatments is as varied as the chemistry of the adhesives, but they will not be considered further in this chapter. 

Figures 30.17-30.19 depict the durability of the surface treatments examined with ETA-3183, which is the most difficult one to modify the surface effectively among samples employed. Because the water can reach the interface directly through cross-cuts, this accelerated adhesion test was much more severe when cross-cuts were...

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