The Adhesive Joint

The adhesive joint (AJ) is primarily composed of five parts (see Fig. 12.1) the two solid materials bonded (A and A ), the two interfaces or boundary layers between the materials and the adhesive (B and B ), and the bulk adhesive (C). A strong AJ implies (1) strong boundary layers, (2) strong interfacial bonds, and (3) a strong or hard-set adhesive. Weak boundary layers have been attributed by Bikerman to be responsible for most weak joints, fii the case of elastomers or plastics, it is possible to show that even if interfacial or surface bonding of the adhesive is strong, a weak boundary layer results in a weak joint as the materials are separated and the surface molecules are pulled out of the 

The surface of a solid is usually contaminated by adsorbed gases and vapors, and as a result, the adhesives may form a poor or weak contact with the actual surface. The surface adsorbed vapors can be displaced by a liquid adhesive which wets the solid. In some cases, a surface-active agent may be added to an adhesive to facilitate the wetting process (Fig. 12.2). 

These compounds which have alkoxide groups will hydrolyze on a hydroxylated surface to form an alcohol and the surface-bonded organic residue. For example. 

The organic amino group can react further with the adhesive and contribute to the bond strength of the joint. 

Another method used to improve the strength of a bond is to introduce the chemical reactive functional groups directly on the surface of the material. For example, the bonding of Teflon or polyethylene can be improved by introducing OH groups onto the plastic surface by sparking moist air next to the surface to be joined. This is best done by means of a Tesla coil or a Corona discharge. 

Any adhesive joint comprises two substrates, which may or may not be of the same material, which are joined hy means of an adhesive. 

Although aircraft structures comprise thousands of components produced from a myriad of basic materials, the most common substrates for structural adhesive bonding are wood, aluminium, titanium, stainless steel and the composite materials such as bonded sandwich structures, fibre reinforced plastic (FRP) laminates and fibre-metal laminates (FML) where the metal is usually aluminium. 

In post-war applications, wood is nearly always used as bonded pljrwood. 

The aluminium alloys most frequently encountered are the 2(X)0, 7000 and, occasionally, the 6000 ranges with 2024 and 7075 grades being the most common aluminium-lithium alloys such as 8090C are also used in special applications. 

Occasionally, aerospace applications utilise commercially pure titanium hut the most commonly encountered substrate is an alloy of titanium which is designated Ti 6A14V that contains 6% aluminium and 4% vanadium. 

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By testing the adhesive joints for the shift, the loading curve appears to be almost linear until the destruction point. The acoustic noise curves for the weak samples are describing an increased activity at tbe initial loading moment, and just before tbe destruction. The strong samples are acoustically little active up to the start of the macro-destruction. 

The analysis of the test results shows that non-defect adhesive joints of the carbon plastic are acoustically less active than the glued main material. This can be explained by absence of plasticization effect of the die (adhesive layer). The value of the breaking point ("C ) at the adhesive joints shift is 9,6 M Pa. 

By working trough the method of the AE diagnostics, and as it was with carbon plastic case, the adhesive joint were tested by the step- and two-multiple loading. 

The test with the step loading shows that acoustic activity of the solid adhesive joints in the tested carbon plastic is quite low. The maximum on the endurance area was fixed at the predestructive moment. The last is evidence to the fact that the prevailing defect of the adhesive joints is starting its development at the loading level, which is close to the destruction point. 

To examine the accumulation effect activity ( A ZT) in the adhesive joints of the carbon plastic, the artificial defects were made. The samples were loaded up to the stress of 0,6"Zf. The test has showed (table 2) that in the weak samples the acoustic emission, at the repeated loading, will start at the point, which is smaller, than initial loading. While, the weaker sample, the bigger value of the "S. ... 

Table 1 provides the approximate load bearing capabiUties of various adhesive types. Because the load-bearing capabiUties of an adhesive are dependent upon the adherend material, the loading rate, temperature, and design of the adhesive joint, wide ranges of performance are Hsted. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

The path of failure of an adhesive joint can give information about the mechanism of failure if analysis of the elemental and chemical composition can be conducted along the path. Several authors have performed such analyses by loading the adhesive joint until it fractures and then using XPS to analyze each side of the fracture. 

To be effective, there must be a certain minimum concentration of inhibitor at the interface to be protected. Therefore, there must be sufficient inhibitor in the primer, and these inhibitors need to be soluble enough in water to enable transport of inhibitor to the oxide surface as water permeates the adhesive joint. However, too high of a solubility will rapidly deplete the primer layer of inhibitor resulting in a loss of protection. One of the fortuitous properties of zinc and strontium chromates is the limited solubility of these compounds in water (about 1.2 g/1 at 15°C [33]). 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

Patel et al. [70] in a recent publication have explored the adhesive action of the mbber-siUca hybrid nanocomposites on different substrates. The rubber-silica hybrid nanocomposites are synthesized through in situ silica formation from TEOS in strong acidic pH within acryhc copolymer (EA-BA) and terpolymer (EA-BA-AA) matrices. The transparent nanocomposites have been apphed in between the aluminum (Al), wood (W), and biaxially oriented polypropylene (PP) sheets separately and have been tested for peel strength, lap shear strength, and static holding power of the adhesive joints. 

Solutions of different carboxylic acids (fiimaric acid [FA], maleic acid, acrylic acid, succinic acid, and malonic acid) in ethanol have been effectively used as primers to increase the adhesion of synthetic vulcanized SBRs. The increase in the adhesion properties of SBR treated with carboxylic acid is attributed to the elimination of zinc stearate moieties and the deposition of acid on the rubber which migrates into the solvent-borne polyurethane adhesive layer once the adhesive joint is formed. The nature of the carboxylic acid determines the rate of diffusion into the adhesive and the extent of rubber-adhesive interfacial interaction. 

Hot-melt adhesives the molten adhesive wets the surfaces of the plastics to be assembled and interlocks them while solidifying again on cooling. It is necessary for the materials to be assembled to tolerate the temperature of the molten adhesive. Joining is sensitive to the temperature, which involves the melting of the adhesive joint. However, some hot-melt adhesives crosslink after joining and become less sensitive to heat. 

Non-reactive solution adhesives the solvent wets the surfaces to be assembled, then evaporates involving the cohesion of the parts to be assembled by the adhesive joint. The heat behaviour is generally moderate. If the solvent swells the materials to be assembled, there can be migration of materials and subsequent cracking by residual internal stress relaxation. 

Reactive adhesive after wetting the surfaces to be assembled, there is polymerization of the adhesive joint. The heat behaviour can be better than with the prece ng methods. 

Combine your results to calculate the spreading coefficient, Sb/a- WUl the epoxy wet the PVC Assuming that complete wetting can occur (and that a sufficiently thin layer of adhesive is applied), how will the adhesive joint fail when stressed i.e., will the epoxy or the PVC/epoxy interface fail first ... 

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

V. The adhesion joint should not prevent (ideally it should contribute to) the formation of connective tissue scars and regeneration of the tissues. 

VII. The physico-chemical and mechanical properties of the adhesion joint should correspond to the given surgical situation wherein the adhesive is used. 

The adhesion joint should feature satisfactory strength and tightness. It should be asked what parameters are suitable for estimating the strength of glueing before using the adhesive in a clinic. 

The adhesive joint should have a structure which favourably contributes to the healing process. 

The fine porous structure of the adhesive joint causes an early degradation by the cellular mechanism. For KL-3, under experimental conditions, the cellular mechanism of degradation manifests itself two weeks later. Intensive degradation of the adhesive by the hydrolytic mechanism is due to the highly expanded surface under formation of cavities and subsequently of gigantic cells of foreign bodies. 

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