The basic function of adhesive is to coat the surface of adherends to connect them. When manufacturing bonded parts, the first thing to consider is selecting the appropriate adhesive from different types of adhesives. Here are 10 factors that influence adhesive selection.

01. Adhesive assembly
    The entire processing of the bonded assembly has a greater or lesser impact on the choice of adhesive. The surface created by the processing before bonding is directly related to the bonding effect. If it cannot be changed, then an adhesive must be selected that matches this surface. After the bonding is completed, the processing method of the entire bonded part (cutting, stamping, heating, freezing, dipping, etc.) must also be considered when selecting the adhesive. If the entire bonded part is a unit on a production line, in order to synchronize with other units, the operating performance of the adhesive must be adapted to the speed of the production line. The form of the adhesive (liquid, paste, film or solid) and the processing method of the bonded parts (manual or mechanical) must also match.

02. Properties of the adhered material
    The physical and chemical properties of the materials to be bonded and the degree of surface preparation required prior to bonding are another important factor to consider when selecting an adhesive. The selection goal at this time is to expand the degree of adhesion on the joint so that the joint fails cohesively when tested and does not cause joint failure at the adhesive/adherent interface. Only in this way can it be possible to achieve the highest strength of the adhesive.

03.Strength of the adhered material
    Low-strength materials, such as fabric, felt, or some wood-like materials, may be weaker than the adhesive, so joint failure occurs within the material. For this type of application, the adhesive chosen allows the joint to be used under any physical conditions that the material can withstand, without the risk of bond failure. At this time, the use of high-strength adhesive will cause the bonding material to exceed specifications or become expensive.

04. Thickness of adhered material
    Another factor that cannot be ignored is the form of the adhesive. Solvent-based adhesives wrinkle the edges of joints when bonding thermoplastic films or laminates, and their action on rigid thermoplastic materials often reduces the need for surface preparation of the material. The special shape of the material being adhered often facilitates the application of special forms of adhesives. The edge connection of aluminum honeycomb structure to flat metal sheets is best bonded with liquid primer and thermosetting adhesive film (fiberglass cloth as lining). Copper fins and aluminum tubes can be easily made into heat exchangers using paste glue. For porous materials, it is more suitable to use high viscosity or paste adhesive.

05. Stress conditions of the bonded body
    The adhesive changes from a flexible, wet material to a tough, hard solid, and its cohesive strength properties change significantly, about several thousand N/cm2. The increase in cohesive strength is the fundamental reason why the adhesive layer can withstand different stresses, but the formation of cohesive strength is also an important reason for the stress in the bonding body.
    In some cases, the adhesive may only be needed for temporary bonding purposes. For example, when positioning and locking parts, the parts are fixed. In applications that have special strength requirements for the adhesive, it is necessary to consider bond stress. In particular, the nature and magnitude of the stress and the application conditions of the bonded parts are particularly important. After the adhesive is selected, the performance of the adhesive on the joint also depends on many factors, the most important of which are the joint design, the condition of the surfaces being joined, the bonding technology used, the thickness of the adhesive layer, and the strength and thickness or shape of the adherend parts.
    The type and magnitude of the stresses provided by the adhesive are largely determined by the joint design. The bonded body may be subjected to shear, tension or compression forces, or cleaving or peeling forces, as well as any combination of these stresses. Most adhesives exhibit good compressive strength; some adhesives may have low peel strength but high shear strength, or vice versa. It is often possible to achieve the required joint strength, even with low strength adhesives. Of course, where large-area joints cannot be designed, the use of high-strength adhesives becomes inevitable.
    The thickness of the joint adhesive film is of special significance in selecting a suitable adhesive to meet the required strength. Using high-modulus adhesive can achieve higher tensile strength and shear strength when the thickness of the adhesive film is small. To obtain the best strength of thermosetting resin, the film thickness is usually 0.03-0.12mm. If the thickness is below 0.03mm, the strength will usually be reduced. The strength is related to the smoothness of the bonded surface. Lack of glue in the joint is dangerous. On the other hand, when using elastic adhesives, increasing the film thickness results in higher peel strength. Especially when the film thickness exceeds 0.13mm, the best strength is usually achieved. In order to reduce joint stress, adhesives that are harder than the adhered material after curing are not used.
    The conditions of external stress to which the bonded body is subjected must be specified in detail. Not all adhesives work exactly the same for the loads that a bonded joint may support, such as intermittent loads or vibration loads. Some adhesives form hard, brittle bonds that break easily under vibration loads, while other adhesives can withstand intermittent loads but cannot support continuous loads. Increasing the load rate will significantly increase the bond strength (such as impact or shear strength) for many adhesives and is a factor worth considering.