5. Stress Conditions of the Bonded Body

Adhesives undergo substantial changes in cohesive strength properties as they transition from flexible, wet materials to tough, rigid solids, with a range of 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 factor in the generation of internal stresses within the bonded body.

In some cases, the adhesive may only need to play a temporary bonding role, such as fixing parts during positioning and locking. Considering bonding stresses is necessary in applications with specific strength requirements for adhesives, particularly the nature and magnitude of the stresses generated, and the application conditions of the bonded components. After selecting the adhesive, the performance of the adhesive in the joint also depends on many factors, most importantly the joint design, the condition of the surfaces to be bonded, the bonding technique used, the thickness of the adhesive layer, and the strength, thickness, or shape of the adhered parts.

The type and magnitude of stresses provided by the adhesive largely depend on the joint design. The bonded body may be subjected to shear forces, tensile or compressive forces, cleavage or peeling forces, or any combination of these stresses. Most adhesives exhibit good compressive strength; some may have low peel strength but high shear strength, or vice versa. Often, the required joint strength can be achieved, 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 adhesive film in the joint is of particular significance in selecting the appropriate adhesive to meet the required strength. Using high-modulus adhesives results in higher tensile and shear strengths when the adhesive film thickness is small. Thermosetting resins typically achieve optimal strength with an adhesive film thickness of 0.03-0.12mm; below 0.03mm, the strength usually decreases. Strength is related to the smoothness of the adhered surfaces, and lack of adhesive in the joint is dangerous. On the other hand, when using elastic adhesives, increasing the adhesive film thickness results in higher peel strength. Especially when the adhesive film exceeds 0.13mm in thickness, optimal strength is usually achieved. To reduce joint stresses, adhesives that are harder than the adhered materials after curing are not used.

The conditions under which the bonded body is subjected to external stresses must be specified in detail. Not all adhesives perform equally well under loads that the bonded joint may support, such as intermittent or vibratory loads. Some adhesives form hard, brittle bonded layers that are prone to cracking under vibratory loads, while others can withstand intermittent loads but not continuous loads. Increasing the loading rate significantly increases the bond strength (e.g., impact or shear strength) for many adhesives, which is also a factor worth considering.

6.  Compatibility Between Adherend and Adhesive

When the adherend is incompatible with the adhesive, it will lead to bond failure of the bonded component. This is also true even when one of the components of the adhesive is incompatible with the adherend. For example, metal parts may be corroded by acidic (or alkaline) adhesives; plasticizers in flexible plastics migrate to the adhesive, leading to interfacial bond failure; solvents or volatiles in the adhesive affect plastic films.

Whenever possible, detailed specifications of the adhesive properties should be provided along with the adhesive sample, which is undoubtedly beneficial to both the adhesive manufacturer and the implementer of the bonding process. For electronic components and printed circuit boards, it is typically required that the adhesive does not corrode copper and other component materials under use or storage conditions. When bonding explosives or similar pyrotechnic materials, other chemical reactions may occur that can disrupt the bond or even adversely affect the explosive (i.e., sensitization or desensitization).

7. Requirements of the Bonding Process

The conditions under which the adhesive bonds are also important criteria for selecting the correct adhesive. In factory or assembly line production, the determined assembly environment may limit the selection of adhesives for bonded products. The operational performance of the adhesive considered at this time may often override the potential interests of the user.

Typical factors associated with the bonding process include: the form of the adhesive, the preparation and application method of the adhesive, the shelf life of the adhesive, the pot life of the adhesive, the means or equipment necessary for using the adhesive, the variability of the bonding process, the allowed time between coating and bonding (lamination), the time and temperature for drying the adhesive layer, the curing temperature and service temperature of the adhesive layer, the rate of change in bonding strength at different temperatures, special requirements and precautions such as odor, flammability, and toxicity, etc.

The selection method for applying adhesive to workpieces depends not only on the physical properties of the adhesive but also on the size and shape of the parts, the number of assembled components to be coated, and the dimensions of the parts. The form of the adhesive ranges from thin liquid to paste and solid states. Different application means are used for different forms. For example, thin liquids are sprayed, brushed, or rolled on, while paste adhesives are applied using glue machines or scraper-type applicators.

For adherends, the adhesiveness of the adhesive is usually important. Adhesion or tackiness plays a decisive role in parts that will be assembled after being coated with adhesive. During assembly, the tack time range determines the time interval between coating and assembly of the bonded parts. Therefore, the adhesive properties will determine the necessary conditions for using the adhesive (i.e., the form of the adhesive, mass transfer rate, mixing time, and application method). In contrast to thermoplastic adhesives, thermosetting adhesives generally have less adhesion. Adhesion varies greatly, depending on the molecular structure and aggregated form of the adhesive. Latex adhesives become tacky only when their liquid dispersion medium (carrier) is removed (evaporated); solvent-based rubbers become tacky even when containing significant amounts of solvent. Both types of adhesives exhibit good adhesion. For certain bonded parts, the curing temperature of the adhesive affects the selection of the adhesive. Many thermosetting adhesives require heating and pressing to form bonded parts, and when these conditions cannot be achieved during processing, cold-curing adhesives are typically used.

The selection of an adhesive can also be determined by the geometry and arrangement of the components of the bonded part. Generally, loosely fitted components require gap-filling adhesives, whereas tightly fitted components require low-viscosity adhesives.