Adhesive/adherend compatibility

The selection procedure of Section 5.2 depends largely on adhesive/adherend compatability. Information included there gives a good insight into the interdependence of each on the another. However, the following general review of the more common materials may be helpful. 

Consult Table 5.3 (Adhesive/adherend compatibility) and, on a copy of Table 5.5 (Assessment), enter the status - R, S, P for reject, secondary and primary - of each adhesive for each adherend surface. Reject here indicates a basic adherend/ adhesive incompatibility which renders the adhesive unsuitable. A primary adhesive/adherend combination will function well, right up to the limit of the adhesive s capability. Secondary indicates that performance may not be so predictable. It should be appreciated, however, that an inherently high strength adhesive rated as secondary on a given adherend may well produce a far stronger bond than a weak adhesive rated as primary for the same material. 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

Initial intimacy of contact between the adhesive and adherend must of course precede the formation of a diffusion interphase, but in contrast to contact adhesion, the issue which is dominant is not the maximization of the work of adhesion but instead must be some appropriate measure of the phase compatibility, in the sense of mutual solubility. 

S.2.2.2. Composite adherends. Composite adherends are bonded in both the cured and uncured states. Wherever possible the adhesive and all adherends are cured simultaneously to avoid the added cost of additional autoclave cure cycles. In many cases this is not practical due to part size and complexity. Cured parts can be bonded to uncured parts, which is known as cobonding, and fully cured parts can be bonded together, which is known as secondary bonding. Adhesives for composites are formulated to be compatible with matrix resins in either cured or uncured states. 

Epoxy adhesives command a large portion of the structural adhesives market. Many people are probably familiar with epoxy structural adhesives, as these are the typical two-part adhesives found in hardware stores and supermarkets. Epoxy adhesives owe their popularity with both the general public and industry to their ease of use, their relative safety, and their compatibility with many adherends (Fig. 14.3). The various chemical reactions involving the epoxy ring provide a fertile field for the development of a wide range of properties.29 Even a half century after the first epoxy patents were issued,30 new patents on epoxy adhesive technology continue to appear every month (Fig. 14.1.)... 

If bonded joints have to last when stressed by forces, the choice of the adequate adhesive is just as important as the bonding-compatible arrangement of the adherends in the glueline. Here, some basic rules have to be observed ... 

An important criterion for determining the chemical compatibility between an adherend and an adhesive in a solvent is the solubility parameter, 6. The solubility parameter is the square root of the cohesive energy density, CED ... 

For adhesive applications, thermal stability alone is insufficient. Processability of high-temperature polymers is of great importance. These polymers should be processed under moderate conditions with no volatile evolution. The adhesives should be compatible with adherends and should have good, durable properties. For broader uses, the manufacturing cost should be low because cost, so far, has been the major obstacle to developing high-temperature adhesives. 

P(7) The material compatibility between the adhesive and the adherends shall be checked in cooperation with the adhesive manufacturer or by testing, as the possible material incompatibility may significantly reduce the adhesion. 

The adhesive and adherend must be compatible, if their union is to last. 

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