8.  Conditions of Use

For each type of assembly component, when selecting an adhesive, it is crucial to consider the strength that the bonded parts must maintain throughout their expected service life under the conditions of use. Therefore, to specify a suitable adhesive, designers must understand all the conditions that need to be met.

Typically, strength and durability requirements are important factors, which have been discussed in the context of stress conditions in bonded bodies. Different types of adhesives exhibit widely varying responses to different stresses and stress rates. Thermoplastic adhesives are unsuitable for structural applications because they tend to fail when supporting relatively low loads and soften when heated. Thermoplastic adhesives cannot withstand prolonged vibration stress, although they may demonstrate greater strength than thermosets in short-duration tests. Thermoplastic rubber-based adhesives usually have high peel strength but relatively low tensile and shear strength. In contrast, thermoset resins are often used as the basic component of structural adhesives. Structural adhesives become relatively rigid adhesive layers at room temperature and retain most of their strength. Therefore, in general, the use of thermoset structural adhesives is preferable in situations requiring high strength and high resistance to failure. Adhesives based primarily on thermoset resins or rubber/resins are also suitable for supporting vibration loads but have relatively poor strength when supporting peel or cleavage stresses. Under impact loads, elastomeric adhesives will prove more satisfactory than brittle thermoset adhesives.

Another important factor to consider is the effective temperature range required for the adhesive. Generally, the operating temperature of bonded parts refers to the temperature of the adhesive layer, not the ambient temperature. The temperature of bonded parts is often lower than the ambient temperature, especially when they are intermittently or briefly exposed to heated environments. At high temperatures, all adhesives will lose strength to varying degrees, and some may soften or decompose into useless substances. Many types of thermoset and thermoplastic adhesives can be used below 70°C; however, at 120°C, only a few thermoplastic adhesives are suitable.

Service Life of Adhesives

Typically, adhesives are required to have a service life, which is the useful life of the bonded parts. In practice, the bonding validity period or life of the adhesive should be longer than that of the bonded parts. For example, the adhesive used to bond a sharpening stone to a metal base must maintain satisfactory bonding until the sharpening stone is worn out.

Of course, in some cases, a short service life is another requirement for adhesives. In assemblies, adhesives are sometimes used to temporarily adhere components together to facilitate installation using other means, or to "clamp" workpieces to the workbench during processing, such as when grinding glass, stone, magnetic heads, or metals. Bonding ceramic powder before sintering and binding foundry sand materials are two other examples of temporary bonding. There is a special requirement for binding foundry sand, which is that the bonding fails after the cast piece takes shape, and the sand mold disintegrates and no longer adheres to the cast piece. Waxes, sealing waxes, shellac, and others are often used as temporary adhesives.

9. Storage Requirements for Bonded Parts

The operating conditions of bonded parts are usually given attention, but the storage conditions of bonded parts are often overlooked. The correct approach is to consider the potential exposure of bonded parts to abnormal extreme temperatures and impact loads during transportation and storage when selecting an adhesive. For example, jet engines operate under hot conditions but may be stored or transported in environments below -40°C. Ignoring storage and transportation conditions when selecting an adhesive can lead to damage to bonded parts before use.

10. Cost

When considering the bonding process as a whole, selecting the appropriate properties of an adhesive is more important than the cost of the adhesive. In addition to the price of the adhesive, it is necessary to consider the production efficiency and other factors associated with the selected adhesive. Merely adopting the lowest-cost adhesive without considering the following factors may not yield good economic results:

(1) The overall efficiency of bonding (related to bonding area and number of components);

(2) The ease of using the equipment required for the application or processing process (equipment includes molds, fixtures, pressure tools, heating ovens, applicators, etc.);

(3) The time required for the process (including assembly time, preparation time for the bonded components, curing time, etc.);

(4) Labor costs for assembling and inspecting the bonded components;

(5) The amount of waste compared to other joining methods.

Usually, it is economical to adopt fast-setting adhesives with a simple bonding process, even if the price of the adhesive is slightly higher, as long as complex assembly fixtures or molds are not required.