Adhesive adherend interface

The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should be used only when justified. The most likely occasions for a primer to be used are when (1) the adhesive or sealant cannot be applied immediately after surface preparation, (2) the substrate surface is weak or porous, or (3) the adhesive-adherend interface requires additional protection from service environments such as moisture. 

Moisture degradation of adhesive bonds occurs within the bulk adhesive material, at the adhesive-adherend interface, and within certain substrates. These degradation mechanisms are discussed below. Particularly insidious is the effect of the combined elements of moisture, stress, and temperature. Unfortunately, this synergistic effect occurs at relatively low temperatures, and such a service environment is common to many adhesive applications. For these reasons, this combined environment is given special focus in Sec. 15.5.2. 

Internal degradation within the bulk adhesive or sealant occurs primarily by absorption of water molecules into the polymer structure. All polymers will absorb water to some extent. Moisture can also enter by wicking along the adhesive-adherend interface or by wicking along the interfaces caused by reinforcing fibers and the resin. Deterioration may occur more quickly in a 100 percent relative humidity (RH) environment than in liquid water because of more rapid permeation of the vapor. 

Most structural adhesives are, therefore, formulated to provide the best compromise between environmental resistance and the desired mechanical properties. Experience has generally revealed that although the moisture ingress of the adhesive or sealant does affect the durability, it is seldom the dominant factor. Generally, of greater importance is how the moisture influences the adhesive-adherend interface region. Table 15.16 summarizes the moisture resistance and performance properties of some of the more common structural adhesives. 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

Adhesive Failure - Failure of an adhesive bond at the adhesive-adherend interface. An example is an adhesive failure that leaves adhesive all on one adherend, with none on the other. Adhesive failure is less desirable than cohesive failure because it is indicative of a joint with lower adhesive strength. See also Cohesive Failure. 

Besides the environmental factors mentioned above, the materials involved in a structural joint also influence bond strength and durability. The factors in the material category include the adherends the adhesive the design of the joint absence of surface contamination (including contamination with wood extractives) stability of the adherend surface the ability of the adhesive to wet the surface, and entrapment of air/volatiles. Thus, the condition of the adhesive/adherend interface becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint [31]. 

Given that the adhesive itself should determine the strength of a bonded joint, the stress required to rupture a joint is, nevertheless, not a well-defined materials constant. When two materials are bonded, the resultant composite has at least five elements, namely the adhesive itself, two adhesive/adherend interfaces, and two adherends. If a primer is applied to both substrate surfaces, the number of elements increases to at least nine. These elements involving a metallic adherend are depicted schematically in Fig. 3.2. Note that the adhesive (or primer in this case) is in contact with the metal surface oxide layer, and not with the metal itself. 

Moisture resistance. The adhesive should be formulated to minimise moisture transport through the adhesive itself. The equilibrium water content (A/ ) should not exceed 3% by weight after immersion in distilled water at 20 °C. The permeability, obtained from the product of the coefficient of diffusion (D) and My, should not exceed 5 x 10 " m-/s at 20 °C (see Fig. A.l). A film of adhesive, approximately 1 mm thick cast in polytetrafluoroethylene-lined moulds and weighing at least 3 g, is suggested for this test(3). This requirement is to enhance the potential for a durable adhesive/adherend interface, even if moisture uptake is not deleterious to the adhesive itself. 

This theory proposes that adhesion takes place due to electrostatic effects between the adhesive and the adherend. An electron transfer is supposed to take place between the adhesive and the adherend as a result of unlike electronic band structures. Electrostatic forces in the form of an electrical double layer are thus formed at the adhesive-adherend interface. These forces account for the resistance to separation. This theory gains support from the fact that electrical discharges have been noticed when an adhesive is peeled from a substrate. 

Weak boundary layers can occur in the adhesive or adherend if an impurity concentrates near the bonding surface and forms a weak attachment to the substrate. When failure takes place, it is the weak boundary layer that fails, although failure appears to take place at the adhesive-adherend interface. 

Failure, adhesive—Rupture of an adhesive bond at the interface between the adhesive and the adherend. Rupture of an adhesive bond in which the separation appears visually to be at the adhesive/adherend interface. 

Fatigue—condition of stress from repeated flexing or impact force upon the adhesive-adherend interface weakening of material caused by repetitive loading and unloading. 

Adhesive failure is a rupture of an adhesive bond, such that the separation is at the adhesive—adherend interface. This failure is mainly due to a material mismatch or inadequate surface treatment. Adhesive failure should be avoided. 

Failure, adhesive n. The rupture of an adhesive bond, such that the separation appears to be at the adhesive adherend interface. Note—Sometimes termed failure in adhesion. Skeist I (ed) (1990) Handbook of adhesives. Van Nostrand Reinhold, New York. 

Weak boundary layer theory. According to the weak boundary layer theory, when bond failure seems to be at the interface, usually a cohesive break of a weak boundary layer is the real event. Weak boundary layers can originate from the adhesive, the adherend, the environment, or a combination of any of the three. When bond failure occurs, it is the weak boundary layer that fails, although failure seems to occur at the adhesive-adherend interface. Figure 9.16 shows examples of certain possible weak boundary layers for a metallic substrate. For plastic substrates there are many more opportunities for weak boundary layers, such as mold release, plasticizer migration, and moisture migrating to the interface. Certain weak boundary layers can he removed or strengthened by various surface treatments. 

Electrostatic The electrostatic theory states that electrostatic forces are formed at the adhesive—adherend interface. These forces account for resistance to separation. 

Two aspects of the effects of environmental aging deserve mention here. The first concerns the locus of joint failure, i.e., the path followed by the fracture surface during the breaking of the joint. If structural adhesive joints are prepared correctly, then failure invariably occurs by cohesive fracture through the adhesive layer. However, it is commonly found that after environmental exposure the locus of failure is at, or very close to, the adhesive/adherend interface.As pointed out by Kinloch, this change in... 

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