Adhesion failures

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

LI. I. The meaning of adhesion, adhesive failure and interfacial strength... 

Another distinction to be made is illustrated with the peel test shown in Fig. 1. Application of stress may cause the joint to fail either adhesively or cohesively . Adhesive failure, shown in Fig. la, is thought ideally to correspond to a perfect... 

Fig. 1. (a) Adhesive vs. cohesive failure, (b) Close-up view of adhesive failure in the pre.sence of an interphase. The locus of failure may be adjacent to or within the interphase (as shown), and particles of material may be ejected during the debonding process. 

There is second shear adhesion test that is now being reported with increasing frequency, the so-called SAFT, or shear adhesion failure temperature test. It is particularly popular among block copolymer PSA developers. In this test [15], a shear specimen with an overlap area of 2.54 cm x 2.54 cm is prepared and suspended in a circulating air oven. A 1-kg weight is attached to the tape and the oven temperature is raised continuously 5.5°C per 15 min until failure. An industry wide standard has not yet been written for this test. 

Loci of failure A, adhesion failure C, cohesive failure of canvas F, failure between films. 

Terpene phenolic resins can also be added to polychloroprene latex without great reduction in hot strength as the resin content is increased, but contactability is reduced. However, an adhesion failure is obtained, even at the 50 phr level. Furthermore, terpene phenolic resins have relatively poor tack and impart the best... 

An investigation of the mechanism of adhesive failure of polydimethylsiloxane elastomers was conducted [75]. The study showed that the total adhesive failure energy could be decomposed into energies for breaking chemical bonds, breaking physical bonds and deforming the bulk viscoelastic elastomer. 

It is difficult to generalize regarding adhesion properties and physical characteristics of urethanes, because both the adhesion properties and physical properties can vary widely. One two-part urethane adhesive may be an outstanding adhesive for bonding SMC/SMC. Another urethane adhesive may fail miserably using the same SMC, due to delamination at the interface or adhesive failure. 

Vinyls Vinyl chloride co-polymer resins were developed in the USA in the late 1930s. They have better weather and slightly more chemical resistance than chlorinated rubber paints. They are generally resistant to crude oil but application is more critical. For example, they are particularly sensitive to moisture present on a surface during painting and this can lead to adhesion failure. They are also more prone to solvent entrapment than chlorinated rubber paints. 

Concave surfaces are of industrial importance, in relation to the internal surface of bores, holes and pipes, but are not found on typical solid testpieces and have received much less discussion. The stress patterns will tend to be the opposite of those found on convex surfaces for example, an oxide growing by cation diffusion should be in tension at the metal interface. Bruce and Hancock have discussed the oxidation of curved surfaces and show how the time to adhesive failure of the oxide can be predicted if its mechanical properties are known. 

A continuous intact film of water-resistant paint forms an effective electrical resistance to the flow of a corrosion current (a resistance of over lO flcm through the film is easily achieved). Underfilm corrosion can then only occur if a channel of electrolyte connecting anode and cathode can be established by local adhesion failure between the coating and the metal substrate. 

Localised adhesion failure occurs most easily where broken scale or rust, or deposits of salts, have impeded wetting of the metal substrate by the film-forming constituents of the paint. 

Each of the multilayered materials of Table II, in pouch form, met the retortability requirements. Examination of the pouches after this test showed that no delamination occurred among the layers. However, microscopic examination of specimens used for bond strength tests showed that adhesive failure rather than cohesive failure occurred be-... 

The deposition temperature is above 1200°C and the deposit usually consists of an outer layer of MoSi2 and an intermediate layer of MoSi.PlP l Such reactions are difficult to control and often result in mechanical stresses and voids at the interface, which may cause adhesion failure. The direct deposition of the silicide is often preferred. This is accomplished by reacting a gaseous silicon compound with a gaseous metal compound, as shown in the following sections. 

FIGURE 27.2 T-peel sfrength values of sulfuric acid-treated styrene-butadiene rubber (SBR)/polyurethane adhesive joints as a function of the immersion time in sulfuric acid. A = adhesion failure R = cohesion failure in the rubber. (From Cepeda-Jimenez, C.M., Pastor-Bias, M.M., Ferrandiz-Gomez, T.P., and Martm-Martmez, J.M., J. Adhes., 73, 135, 2000.)... 

FIGURE 27.9 T-peel strength values of styrene-butadiene rubber (SBS) treated with chloramine T aqueous solutions with different pH/waterbome polyurethane adhesive/roughened leather joints, as a function of the pH value of the chloramine T aqueous solutions. A adhesion failure to the rubber, M cohesive failure in tbe rubber. (From Navarro-Banon, M.V., Pastor-Bias, M.M., and Martm-Martinez, J.M., Proceedings of the 27th Adhesion Society, Wilmington, NC.)... 

The physical properties of automotive enamels are in large part determined by the crosslink structure developed in the paint films during the baking process. Enamels which are not cured sufficiently (undercured) are generally sensitive to humidity and solvents. In addition, they may be prone to chipping and cold cracking. Faints which have been baked excessively (overcured) exhibit intercoat adhesion failure. That is, subsequent coats... 

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