Factors Affecting Adhesive Bonding Strength
- surface roughening
When adhesives fully spread the surface of the material, surface roughening helps improve the spread performance of adhesives and increases the density of connected places between the adhesive and the material, this will enhance the bonding strength. On the contrary, if the adhesive fails to spread the material properly, surface roughening will reduce the bonding strength.
- Surface treatment
Surface preparation before bonding is key to successful bonding. It aims to achieve strong and durable joints. Due to the presence of weak boundary layers formed by oxide layers like rust, chrome plating layers, phosphating layers, release agents, etc. on the bonding materials, the surface treatment of the adherent will affect the bonding strength. For example, polyethylene surfaces can be treated with hot chromic acid oxidation to improve bonding strength.
- Penetrate
After bonding, the joint is often affected by the environment, small molecules like water or solvents can penetrate into the glue layer. For example, in wet conditions or underwater, water molecules enter into the glue; in organic solvents, solvent molecules do the same. These molecules first cause the glue layer to change their form, then reach the interface between the glue and the surface. This will weak the bond, eventually causing failure. Penetration does not only start from the edges of the glue layer. If the material being bonded is porous, small molecules can also get in through their gaps, pores, or cracks, then reach the interface and decrease the bonding strength. This penetration not only reduces the physical strength of the joint but can also cause chemical changes at the interface, such as rust, which makes the bond completely useless.
- Movement
Bonded materials contain plasticizers, such as PVC. Since these small molecules do not mix well with polymer molecules, they easily move out from the material’s surface or bonding interface. If the migrated small molecules stay together at the interface, they will stop the adhesive from sticking to the material, making the bond to fail.
- Pressure
When sticking, press pressure to the surfaces. This helps the glue fill small holes on the material easily, even deep holes and tiny tubes, and reduce bad stickers. For weak bonding glues, pressing will make them flue too much and leave not enough glue. So wait until the glue become more strength in bonding before pressing. This also pushes air out of the surface of the material and reduces air bubbles in the gluing area. For thick or solid glues, pressing is necessary when gluing. In these cases, you often need to heat them properly to make them thinner or turn liquid. For example, making insulating pressing layer is done under heat and pressure. To get a strong bond, use different pressure for different glues. And normally, use high pressure for solid or thick glues, and low pressure for thin glues.
- Thickness of the glue layer
Thicker glue layers easily get air bubbles, flaws and early breaks, so you should make the glue layer as thin as possible to get a stronger bond. Also, when thick glue layers get heated, their expansion creates more heat stress in the joint area, which makes the joint break more easily. The stresses on real joints are complex, including shear stress, peel stress and repeated stress. First, shear stress: when off-center pulling force is applied, stress builds up at the ends of the bond. Besides shear force, there is also pulling force along the joint and tearing force across the joint. When a joint is under shear stress, the thicker the material being glued, the stronger the joint. Second, peel stress: this happens when the material being glued is soft. Both pulling and shear forces act on the joint, and all the force concentrates on the glue-material surface, so the joint breaks very easily. Because peel stress is very damaging, you should avoid joint designs that create it when designing. Third, repeated stress: the glue in the joint slowly wears out from repeated stress and breaks at much lower levels than normal static stress. Tough and stretchy glues, like some rubbery ones, handle repeated stress well.
- Internal stress
First, shrinkage stress: When glue cures, it shrinks in volume due to evaporation, cooling and chemical reactions, which causes shrinkage stress. When the shrinkage force is stronger than the adhesion force, the apparent bond strength will drop a lot. Also, uneven stress distribution around the bond edges or gaps in the glue causes stress concentration, which increases the chance of cracks forming. Crystalline glues shrink more when they cure because of crystallization, which also creates internal stress in the joint. If you add a certain amount of rubbery materials that can crystallize or change crystal size, you can reduce the internal stress. Adding tougheners to thermosetting resin glues is the best example. For example, for phenolic-acetal glues, when the acetal content is below 40%, the joint only has interface failure; when it is above 40%, it has cohesive failure, and the bond strength increases a lot. Second, thermal stress: When melted resin cools and cures from high temperatures, it shrinks in volume. The bond holds it in place, which creates internal stress at the interface. If the molecular chains can slide past each other, the internal stress will go away. The main factors that affect thermal stress are thermal expansion coefficient, room temperature, temperature difference and difference in stiffness. To reduce thermal stress caused by different thermal expansion coefficients, you should make the glue’s thermal expansion coefficient close to that of the material being glued. Adding fillers is a good way—you can add powder of the same material, or fibers and powder of other materials.
Post time: 2026-06-01 10:00:41