Adhesive interface

It must also be recognized that adhesive interfaces are not static entities, but may deteriorate or even strengthen over time, and often it is the time course of interfacial strength or durability under different conditions and in different environments that is of greatest concern [4]. As important as durability issues are, they too will not be a direct concern of this chapter. 

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

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). 

Solvent wiping. Rubbers tend to swell by application of solvents and the mechanical interlocking of the adhesive is favored. Although chlorinated hydrocarbon solvents are the most effective, they are toxic and cannot be used toluene and ketones are currently the most common solvents. The treatment with solvents is effective in the removal of processing oils and plasticizers in vulcanized mbbers, but zinc stearate is not completely removed and antiozonant wax gradually migrates to the mbber/polyurethane adhesive interface. Table 27.1 shows the moderate increase in adhesion produced in SBR by MEK wiping. 

Arnold M, Cavalcanti-Adam EA, Glass R, Bliimmel J, Eck W, Kantlehner M, Kessler H, Spatz JP (2004) Activation of integrin function by nanopattemed adhesive interfaces. ChemPhysChem 5 383-388... 

These samples demonstrated exceptional durability regardless of surface preparation, due to the moisture resistant chemical and physical interlocking of the paint/adhesive interface. In addition, the zinc particles within the paint are encapsulated by the organic resin. Since few, if any, metal oxide sites are available for hydrolysis by moisture at the interface, more durable joints are the end result. 

While a non-phosphated topcoat/adhesive interface provided an excellent, moisture resistant, occlusive seal even under the most severe cycle testing, phosphated ZM adherends did not prove to be as durable in comparison (Figure 11). The reason for this lies in the fact that phosphate coverage on Zincrometal is incomplete. Partially crystalline phosphates are non-uniformly interspersed on randomly exposed zinc dust spheres at the surface. Consequently, the moisture resistance normally provided at the adhesive/topcoat interface was reduced due to the incomplete sealing between the topcoat/ adhesive surfaces. This became apparent as most of the failures examined after aging in these environments were concentrated at the adhesive/phosphate/paint interface. Results obtained on these samples were similar to those obtained for phosphated CRS joints, indicating that the locus of failure occurred at phosphate crystal sites. Note, however, that the durability of these joints was still considered to be very good in comparison to other metallic oxide/ adhesive interfaces. 

Apparently, the chemical bonding present at the paint/adhesive interface is much stronger than that occurring at either the phosphate/adhesive or the phosphate/topcoat interfaces. In the case of ZM, phosphating to improve durability is not necessary, and in fact, was proven to be detrimental. The paint provides a moisture resistant barrier layer which reduces the activity of water at the interface providing for a surface receptive to the chemical and physical bonds necessary to promote good adhesion and enhance durability. 

The dentin-adhesive interface has been studied using a Raman microprobe technique [199], which shows the formation of resin-reinforced dentin and the penetration of resin into dentin substrate to a depth of 5-6 microns. Further study of the interface showed that only small molecules such as MMA, 4-MET (hydrolyzed 4-META) or oligomers infiltrated the dentin, and that all of the resin in the dentin originated from the monomer solution [200]. SEM and TEM studies of the ultrastructure of the resin-dentin interdiffusion zone showed a 2 micron zone with closely packed collagen fibrils running parallel to the interface [201]. 

E. Adhesive Interface Integrity Ultra Sonic Reflection or Transmission... 

I For the case of copper, a mixture of cuprous and cupric oxides is present on the copper surface which acts as a defect semiconductor. Therefore, electrons can readily be transported from copper to its oxide surface allowing oxidation to continue at the metal oxide/adhesive interface ls. This continued oxidation reaction which involves the base metal can interfere with adhesion between the oxide and the adhesive. Hence, the underlying metal atoms can effect the adhesion forces in some cases 171... 

Find et al. [42] developed a nickel-based catalyst for methane steam reforming. As material for the micro structured plates, AluchromY steel, which is an FeCrAlloy (see Section 2.10.7) was applied. This steel forms a thin layer of alumina on its surface, which is less than 1 pm thick. This layer was used as an adhesion interface for the catalyst. I ts formation was achieved by thermal treatment of micro structured plates for 4 h at 1 000 °C. 

Although none of these studies of polymer adsorption help us determine the relative importance of intermolecular attraction in adhesion, they collectively do establish that this mechanism is exceedingly important as a first step, and in some instances the only step, in the formation of wood-adhesive interfaces. 

When a corrosive medium contacts the edge of a bonded joint and finds an extremely active surface, such as that produced by a fresh acid treatment of the metal substrate to improve adhesion, corrosion at the metal-adhesives interface can occur. This initial corrosion and the subsequent penetration can take several forms. 

Predicting the service life of adhesives is a risky business. The most difficult question ever put to an adhesive consultant is, How long will the adhesive joint last in service The problem is that an adhesive joint is not made up of just one element. It contains several elements, and some of them interact. In fact, in most adhesive joints at least five elements must be considered substrate A, interface A, the adhesive, interface B, and substrate B. To understand and predict the rate of degradation of each of these elements is challenging, but it can be done. The most difficult failure situations to predict are those that result from interactive effects. 

For reasons as described above, some metal adhesive interfaces are chemically capable of accelerating the rate of oxidation. For example, it has been found that nearly all types of structural adhesives exhibit better thermal stability when bonded to aluminum than when bonded to stainless steel or titanium (see Fig. 15.3). 

Hydrolysis of some chemical bonds breaking of bonds within the adhesive molecule and at the substrate-adhesive interface... 

Many film adhesives have a supporting carrier or reinforcement fabric incorporated into the adhesive to improve handling of the film and provide control of bond line thickness. The carriers are usually glass, polyester, or nylon fabrics of knitted, woven, or nonwoven construction. The difficulty with such carriers is that they can provide an effective way of moisture entering the bulk of the adhesive. Moisture can wick along the fiber-adhesive interface. Nylon carriers should especially be reviewed since they have a strong tendency to absorb moisture. 

EXPERIMENTAL CHARACTERIZATION OF CARBON-FIBER/CONCRETE ADHESIVE INTERFACE FOR RETROFITTING OF CONCRETE BRIDGE STRUCTURES... 

Concrete structure. Retrofitting, Carbon fiber sheet. Adhesive interface, Mixed mode. Fracture toughness. Fracture criterion... 

Experimental Characterization of Carhon-Fiher/Concrete Adhesive Interface... 

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