Лако-красочные материалы — производство

Although the degradation of a bond is likely to be inevitable, there are means by which to slow down the process; some of which were discussed above. For convenience of the discussion, these methods can be classified as environment, materials, and design related.

Changing the environment to which a bond is exposed is probably the most effective means of ensuring good durability; bonded structures are not likely to degrade at mod­erate temperatures and in low humidity. Unfortunately, this usually is not a viable option. However, it may be possible to protect the bond from its external environment, at least for a period of time. Ways to design a joint to do this are discussed below.

Material selection and preparation are perhaps more feasible options than chan­ging the environment. Although not an economical solution, substitution of titanium for aluminum would solve many moisture-related problems. Selection of more water- resistant adhesives and/or corrosion-resistant primers is more common and was illus­trated in Figs. 9 and 10 [12,18]. Here, selection of a water-resistant adhesive (Cytec FM-300) decreased the final crack length in the wedge test by 1-2 cm for both FPL and PAA surfaces compared to a water-wicking adhesive (FM-123). The use of a chro­mate-containing (corrosion-resistant) primer (Cytec BR-127) further decreased the final crack length (Section III. B).

Surface preparation is another commonly used means to increase durability. We have already seen that the durability of microrough surfaces is superior to smooth surfaces or to surfaces with only larger-scale roughness (Fig. 1) and that hydration-resistant alu­minum surfaces provide further improvements (see Section III. A). Figures 9 and 10 also illustrated this enhancement. For a given adhesive, PAA surfaces that are more hydration resistant show less crack growth than FPL surfaces and, for both FPL and PAA surfaces, treatment with a hydration inhibitor (NTMP) gives superior durability over untreated surfaces. This improvement of PAA surfaces over FPL surfaces has also been demon­strated in the field, most notably in Vietnam, where FPL-treated joints suffered a large number of disbonds whereas PAA-treated joints were significantly more reliable [10].

Proper design of a joint or structure is also necessary to maximize durability. Although moisture cannot be prevented from reaching a bondline, it can be slowed or reduced in quantity. One way is to prevent pooling or other accumulation of water by designing the geometry to promote runoff, or including adequate drain holes. Maintenance is then required to ensure that the holes do not become plugged. Sealants are also used to slow down moisture ingress from joint edges and seams [116]. Again, proper application is necessary to prevent moisture accumulation and to ensure the absence of an easy path to the interface. One category of sealants, water-displacing corrosion inhibitors (WDCIs), can even creep under existing water films, displacing the moisture and eliminating the corrosive environment [116].

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 creates 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 reach a critical area—the same general principle by which sealants work.

IV. SUMMARY

Long-term durability is one of the most important properties of many adhesive bonds. Although it can be difficult to achieve in aggressive environments, modern materials and processes have proven successful in increasing durability. Moisture is the cause of most environmentally induced bond failures. It can weaken or disrupt secondary (dispersion — force) bonds across the adhesive-adherend interface, especially those involving high — energy surfaces such as metals; as a result, the joint may need to rely solely on primary (covalent or ionic) or physical (mechanical interlocking) bonds. More severe degradation can subsequently occur with hydration or corrosion of the adherend surface. At this point, the joint will fail regardless of the type of bonding at the interface.

Most means of improving durability involve slowing down the degradation mechan­isms or providing additional bonding schemes, e. g., primary and/or physical bonds that are less susceptible to degradation. Surface preparations that provide physical bonds and a hydration-resistant surface are typical examples. The use of coupling agents, phenolic-based adhesives (with aluminum adherends) and sol-gel treatments are other examples where stable chemical bonds are formed in the interphase and slow down bond degradation.