Physical factors affecting adhesive bonding strength (paint adhesion can be used for reference)
The physical factors that affect adhesive strength mainly include the following aspects:
1. Surface roughness
When the adhesive is well wetted onto the surface of the adhered material (contact angle θ< 90 °), surface roughness is beneficial for improving the wetting degree of adhesive liquid on the surface, increasing the contact point density between the adhesive and the adhered material, and thus improving the bonding strength. On the contrary, when the adhesive has poor wetting of the adhered material（ θ> 90 °), surface roughness is not conducive to the improvement of adhesive strength.
2. Surface treatment
The surface treatment before bonding is the key to successful bonding, with the aim of obtaining a strong and durable joint. Due to the existence of "weak boundary layer" formed by the oxide layer (such as rust), chromium plating layer, phosphating layer, release agent, etc. of the adherend material, the surface treatment of the adherend will affect the bonding strength. For example, the surface of polyethylene can be treated with hot chromic acid oxidation to improve bonding strength. When heated to 70-80 ℃ for 1-5 minutes, a good adhesive surface can be obtained. This method is suitable for polyethylene sheets, thick walled pipes, etc. When polyethylene film is treated with chromic acid, it can only be carried out at room temperature. If carried out at the above temperature, the surface treatment of the thin film shall be carried out using plasma or micro flame treatment.
When treating the surfaces of natural rubber, styrene butadiene rubber, nitrile rubber, and chloroprene rubber with concentrated sulfuric acid, it is hoped that the rubber surface will undergo mild oxidation. Therefore, in a relatively short period of time after acid application, the sulfuric acid must be thoroughly washed away. Excessive oxidation actually leaves more fragile structures on the rubber surface, which is not conducive to bonding.
When bonding locally on the surface of vulcanized rubber, the surface treatment should remove the release agent, and it is not advisable to use a large amount of solvent to wash to prevent the release agent from spreading to the treated surface and hindering bonding.
The surface treatment of aluminum and its alloys aims to generate alumina crystals on the aluminum surface, while the naturally oxidized aluminum surface is a very irregular and loose alumina layer, which is not conducive to bonding. So, it is necessary to remove the natural alumina layer. But excessive oxidation can leave a weak layer in the bonding joint.
3. Penetration
Bonded joints are often infiltrated by other low molecular weight substances due to the influence of environmental atmosphere. For example, when the joint is in a humid environment or underwater, water molecules penetrate into the adhesive layer; The polymer adhesive layer is in an organic solvent, and solvent molecules penetrate into the polymer. The penetration of low molecular weight substances first deforms the adhesive layer and then enters the interface between the adhesive layer and the adherend. Reduce the strength of the adhesive layer, resulting in damage to the bonding
Permeation not only starts from the edge of the adhesive layer, but also for porous adherends, low molecular weight substances can penetrate into the adherend from its pores, capillaries, or cracks, and then invade the interface, causing defects or even damage to the joint. Penetration not only leads to a decrease in the physical properties of the joint, but also results in chemical changes at the interface due to the penetration of low molecular weight substances, creating rust areas that are not conducive to bonding and causing complete failure of the bonding.
4. Migration
Containing plasticizers, adhesive materials such as PVC materials are prone to migration from the surface or interface of polymers due to their poor compatibility with polymer macromolecules. If the migrated small molecules gather at the interface, they will hinder the adhesion between the adhesive and the adhered material, causing adhesion failure.
5. Pressure
During bonding, pressure is applied to the bonding surface, making it easier for the adhesive to fill the potholes on the surface of the adhesive, and even flow into deep holes and capillaries, reducing bonding defects. For adhesives with low viscosity, excessive flow will occur during pressure, resulting in a lack of adhesive. Therefore, pressure should be applied when the viscosity is high, which also promotes the escape of gas on the surface of the adhesive and reduces the porosity in the bonding area.
For thicker or solid adhesives, applying pressure during bonding is an essential means. In this case, it is often necessary to increase the temperature appropriately to reduce the viscosity of the adhesive or to liquefy the adhesive. For example, the manufacturing of insulation laminates and the formation of aircraft rotors are all carried out under heating and pressure.
In order to achieve higher adhesive strength, different pressures should be considered for different adhesives. Generally, high pressure is applied to solid or high viscosity adhesives, while low pressure is applied to low viscosity adhesives
6. Adhesive layer thickness
A thicker adhesive layer is prone to bubbles, defects, and early fracture, so the adhesive layer should be made as thin as possible to achieve higher adhesive strength. In addition, the thermal expansion of the thick adhesive layer after heating also causes greater thermal stress in the interface area, which is more likely to cause joint failure.
The stresses acting on actual joints are complex, including shear stress, peel stress, and alternating stress.
(1) Shear stress: Due to the eccentric tension effect, stress concentration occurs at the bonding end. In addition to shear stress, there are also tensile forces in the same direction as the interface and tearing forces perpendicular to the interface direction. At this point, under the action of shear stress, the thicker the adhesive, the stronger the joint.
(2) Peeling stress: When the adhesive is a soft material, the effect of peeling stress will occur. At this point, there are tensile and shear stresses acting on the interface, which concentrate on the bonding interface between the adhesive and the adherend, making the joint easily damaged. Due to the high destructive nature of peel stress, it is advisable to avoid using joint methods that generate peel stress during design.
(3) Alternating stress: The adhesive on the joint gradually fatigue due to alternating stress, and fails under conditions far below the static stress value. Strong and elastic adhesives (such as some rubber based adhesives) have good fatigue resistance.
7. Internal stress
(1) Shrinkage stress: When the adhesive solidifies, the volume shrinks due to evaporation, cooling, and chemical reactions, causing shrinkage stress. When the shrinkage force exceeds the adhesive force, the apparent adhesive strength will significantly decrease. In addition, the uneven distribution of stress around the adhesive ends or gaps in the adhesive also results in stress concentration, increasing the possibility of cracks. When crystalline adhesives are cured, they cause significant volume shrinkage due to crystallization, which also causes internal stress in the joint. If a certain amount of rubbery substance that can crystallize or change the size of the crystal is added to it, internal stress can be reduced. Adding toughening agents to thermosetting resin adhesives is the best explanation. For example, phenolic acetal adhesive, when the acetal content is less than 40%, the joint undergoes simple interface damage; At over 40%, it is cohesive failure and the bonding strength is significantly enhanced.
(2) Thermal stress: At high temperatures, when molten resin cools and solidifies, volume shrinkage occurs, resulting in internal stress at the interface due to bonding constraints. When there is a possibility of slip between molecular chains, the internal stress generated disappears.
The main factors affecting thermal stress include coefficient of thermal expansion, temperature difference between room temperature and Tg and elastic difference
In order to alleviate the thermal stress caused by the difference of coefficient of thermal expansion, the coefficient of thermal expansion of the adhesive should be close to the coefficient of thermal expansion of the adherent. Adding filler is a good method. The powder of this material or the fiber or powder of other materials can be added.
