Adhesive adhesion theory

Stresses Due to Contraction Caused by Differences in Thermal Expansion:

When a liquid adhesive transitions to a solid state, the theoretical strength of the bond decreases due to the concentration of internal stresses typically present in the structure. The most common cause of internal stresses is the difference in the coefficients of thermal expansion between the adhesive and the substrate. These stresses must be considered when the adhesive or sealing material solidifies at a temperature higher than the normal service temperature. This issue is usually unavoidable under service conditions.

The stresses caused by differences in thermal expansion can be significant. For example, consider a bearing where a polyamide-imide part is placed inside a steel part. Suppose the adhesives used are cured at 250°F. At this temperature, all substrates and adhesives are in equilibrium. As the temperature decreases, stresses are induced within the adhesive due to the greater shrinkage of polyamide-imide compared to steel. At room temperature, these stresses may be significant but not large enough to cause adhesive failure. Now, suppose the bearing is used in an environment with temperatures ranging from 250°F to -40°F. At 250°F, the internal stresses due to thermal expansion differences return to equilibrium.

However, when the service temperature drops to -40°F, the difference in thermal expansion creates internal stresses that add to the curing-induced stresses, potentially leading to failure.

A simple example with a sample graph of bonds formed with high-temperature curing adhesives can illustrate this. The bond strength increases with temperature up to a maximum limit and then decreases. This is similar to the case where internal stresses decrease with increasing service temperature. When the temperature rises to a point where internal stresses are completely relieved, the bond strength reaches its maximum. The temperature at which this occurs is close to the curing temperature. At higher temperatures, additional stresses are created, or thermal degradation effects become evident, reducing the bond strength.

Several solutions exist for the issue of coefficient of thermal expansion mismatch. One approach is using flexible adhesives that deform with substrate changes, although flexible adhesives typically have lower cohesive strength. Another method is matching the thermal expansion coefficient of the adhesives closer to the substrate by selecting a different adhesive or modifying the adhesive formulation. A third solution involves coating substrates with a material like a primer or coupling agent that provides a relief mechanism for the thermal expansion coefficient differences.

Curing Stresses Created by Adhesive or Sealant Shrinkage:

Almost all polymeric materials shrink during solidification. Sometimes, they lose weight due to solvent evaporation. Even 100% reactive adhesives, such as epoxies and urethanes, exhibit some shrinkage because the polymerized mass occupies less volume than the original liquid reactants. Table 1 shows typical volumetric shrinkage for reactive adhesive systems during curing.

The result of this shrinkage is the creation of internal stresses, leading to the formation of cracks and voids in the bond lines. Elastic adhesives deform under internal stresses and are less affected by shrinkage. Formulation specialists can often adjust the final hardness of the adhesive or sealant to minimize stress during shrinkage.

Performance During Service:

When the solidification mechanism is complete, the bond usually undergoes working conditions. The service environment may include high temperatures, stresses, chemicals, radiation, and other factors. It is important for the bond to resist environmental conditions to maintain operational strength throughout the adhesive's useful life. The impact of service conditions on the adhesive bond is primarily the creation of concentrated stresses or aging due to environmental factors.

Short-Term Effects:

Concentrated stresses are mainly due to immediate temperature effects and differences in expansion coefficients. The effect of these expansion coefficient differences on internal stresses formed during curing will be described in subsequent sections. However, thermal stresses can easily develop during the bond's useful life.

To reduce such stresses, flexible materials are often used, or the expansion coefficients are matched closely. The expansion coefficient of adhesives and substrates should be as close as possible to limit stress generation. Polymeric adhesives generally have larger thermal expansion coefficients than metals. The thermal expansion coefficient of adhesives can be modified using various fillers. An elastic adhesive can endure internal stresses within its structure.

When an adhesive bond is formed and placed in service conditions, other forces can weaken this bond. The type of stress, its origin, and the rate at which it is applied are important. When a bond is continuously separated, it is due to breaking strong bonds within the molecular chains or breaking weak bonds between these chains. Bond separation can occur slowly or rapidly.

Cracking occurs when concentrated stress becomes large enough to physically separate adjacent sections. Crystalline or cross-linked polymers usually tend to crack and are less prone to creep. When bond separation appears at the time of bond failure, it is usually due to cracking on the interface or the replacement of the adhesive or substrate by a chemical. Cracks can be caused by internal or external stresses.

Creep occurs when sufficient force is applied to a linear mass of molecules to disrupt their crystalline order. Creep is a slow process accelerated at temperatures above the adhesive’s glass transition temperature (T_g). Polymers with low T_g cannot be used where large forces are applied to the bond at moderate temperatures. Cross-linking reduces creep because polymer parts are fixed by a network structure and cannot be easily moved.

Long-Term Effects:

Destructive external stresses may originate from mechanical, thermal, or chemical sources. Temperature, humidity, salt spray, liquids, gases, mechanical loads, radiation, and vacuum are common corrosive and destructive environmental conditions. They may cause enough degradation to lead to early failures. Environmental resistance required depends on the application, and these requirements must be considered before final bond configuration and material selection.

Sustained loads can lead to early failures, although unstressed bonds may also lack sufficient strength in such environments. The simultaneous effects of different types of stress (mechanical and chemical) often overlap, causing failure. Thermal cycling and cyclical loads can reduce bond durability.

There are times when a stress-free, cured bond fails without a clear reason. This issue is due to adhesive aging, which may occur under moderate or low loads. When the behavior of this type of bond is considered, the causative factor is often referred to as "displacement." This term means the replacement of the adhesive from the surface by chemical agents or this problem occurring in the substrate. In a bond to the substrate, a dynamic equilibrium exists between the adhesive and other molecules. Water, solvents, plasticizers, and various materials can compete with these bonds. Moisture is the most common interfering factor. Bonds are resistant to displacement when the adhesive-substrate bond is thermodynamically or kinetically favorable. Some common coupling agents form a thin layer between the substrate and adhesive, having high affinity with both materials, and can easily replace unwanted molecules.