5. Pressure

During bonding, applying pressure to the bonding surface makes it easier for the adhesive to fill the pits on the adherend surface and even flow into deep holes and capillaries, reducing bonding defects. For adhesives with low viscosity, excessive flowing will occur when pressure is applied, resulting in lack of adhesive. Therefore, pressure should be applied when the viscosity is relatively high, which also promotes the escape of gas from the adherend surface and reduces the air holes in the bonding area. For thick or solid adhesives, applying pressure during bonding is an essential means. In this case, it is often necessary to appropriately increase the temperature to reduce the consistency of the adhesive or liquefy it. For example, the manufacture of insulating laminates and the molding of aircraft rotors are carried out under heating and pressurization. To obtain higher bonding 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 generating bubbles, defects, and early fractures. Therefore, the adhesive layer should be made as thin as possible to obtain higher bonding strength. In addition, the thermal stress caused by the thermal expansion of the thick adhesive layer in the interface area after heating is also relatively large, which makes the joint more likely to be damaged. The stress acting on the actual joint is complex, including shear stress, peel stress, and alternating stress.

• Shear stress: Due to the eccentric tension, stress concentration occurs at the bonding end. In addition to the shear force, there are also tensile forces in the same direction as the interface and tearing forces perpendicular to the interface. At this time, under the action of shear stress, the greater the thickness of the adherend, the greater the strength of the joint.
• Peel stress: When the adherend is a soft material, peel stress will act. At this time, there are tensile stress and shear stress acting on the interface, and the force is concentrated on the bonding interface between the adhesive and the adherend. Therefore, the joint is easily damaged. Due to the great destructiveness of peel stress, joint designs that generate peel stress should be avoided as much as possible.
• Alternating stress: The adhesive on the joint gradually fatigues due to alternating stress and fails under conditions far lower than the static stress value. Tough and elastic adhesives (such as some rubber - like adhesives) have good fatigue resistance.

7. Internal Stress

• Shrinkage stress: When the adhesive cures, its volume shrinks due to volatilization, cooling, and chemical reactions, causing shrinkage stress. When the shrinkage force exceeds the adhesion force, the apparent bonding strength will be significantly reduced. In addition, the uneven stress distribution around the bonding end or the voids in the adhesive also causes stress concentration, increasing the possibility of crack formation. Adhesives with crystallinity have relatively large volume shrinkage during curing, which also causes internal stress in the joint. Adding a certain amount of rubber - like substances that can crystallize or change the crystal size can reduce the internal stress. Adding a toughening agent to thermosetting resin adhesives is a good example. For example, in the phenolic - acetal adhesive, when the acetal content is less than 40%, the joint undergoes simple interface failure; when it is more than 40%, it is cohesive failure, and the bonding strength is significantly enhanced.
• Thermal stress: At high temperatures, when the molten resin cools and solidifies, volume shrinkage occurs, and internal stress is generated at the interface due to the restraint of bonding. When there is a possibility of slippage between molecular chains, the generated internal stress disappears. The main factors affecting thermal stress are the thermal expansion coefficient, the temperature difference between room temperature and Tg, and the elastic difference. To alleviate the thermal stress caused by the difference in thermal expansion coefficients, the thermal expansion coefficient of the adhesive should be close to that of the adherend. Adding fillers is a good method, such as adding the powder of this material or the fibers or powder of other materials.