Factors Affecting Accelerated Aging and Service Life of Adhesives

Aging of an adhesive is a change in the properties of the material or bonding head over time. Factors affecting adhesive aging include: mechanical stress, temperature, radiation, and media. Aging is caused by a combination of several factors, but these factors are difficult to measure and predict thoroughly. Some substances contained in the adhesive itself may also cause changes in adhesion properties. In addition to being affected by various factors, the location of the aging that occurs during bonding will also vary. Aging can occur on cohesive layers, adhesive layers, or substrate materials.


1.Effects of solvents on adhesives


For metal substrates, the joint is closely related to the absorption of water vapor is the corrosion of the substrate. Corrosion of the substrate results in rapid failure of the bonded joint in a short period of time, so corrosion is also a critical aspect. Such as: salt, acid, alkali and other solvent environments. In addition, moisture can also penetrate (diffuse) into the cured glue layer and be absorbed by the glue layer. This absorbed moisture reduces the strength of the glue. Because water acts as a lubricant between polymer chains, making it easier for molecular chains to slide. The rate of water penetration into the adhesive layer is low, but it is continuous. At a temperature of 40 ° C, the penetration rate of water for the cross-linked structure polymer is about several millimeters per month. When the bonded joint is dried, the cohesive strength increases again, but it cannot return to the original level. The biggest problem with adhesive joints exposed to moisture is reduced adhesion between the substrate and the adhesive. Adhesion weakens when water penetrates the bonded joint. Weakening the adhesion of the adhesive to the substrate is also usually a slow process. The chemical bond formed between the glue and the surface of the substrate is better than the water resistance of the physical interaction force. Once the chemical bond is broken, the adhesion cannot be restored after drying. The best way to protect the bonded joint from moisture is to stay away from water. If this is not possible, seal the joint.


2.Environmental effects on adhesives


The most important factor affecting chemical reactions is temperature. The higher the temperature, the easier the chemical reaction occurs and the faster the reaction speed. Such reactions often lead to changes in the properties of the adhesive. Under high temperature conditions, the damage of adhesive joints will be greatly accelerated, so in general, as the use temperature increases, the adhesive will accelerate the aging rate. Radiation is also one of the factors that cause the aging of adhesives, such as ultraviolet rays in sunlight. Polymer chains in adhesives and plastic substrates are susceptible to radiation damage. In particular, polyurethane is very sensitive to ultraviolet rays, so long-term exposure of the adhesive to the outdoor will cause the adhesive to age quickly, and degumming and shrinking will occur.


Effect of mechanical stress on adhesives


3.The adhesive bonding surface may be under the following mechanical stresses:

A. Peel stress

B, tensile stress

C. Shear stress

D, torsional stress

Peeling stresses result in higher wire forces at the bonded joint, which stresses far exceed the strength of most adhesives. When designing a bonded joint structure, it must be ensured that the bonded joint is not subjected to such peeling stress. When subjected to a tensile load, the force is perpendicular to the bonding surface. The force is transmitted through the entire bonding surface. In principle, there is no problem with the tensile stress of the bonded joint. Tensile stress is generally encountered only in butt joints. Butt joints usually have small bonding surfaces, and the bonding area can only be increased by changing the thickness of the substrate. Because the strength of the substrate generally exceeds the tensile strength of the glue, a design that makes full use of the strength of the substrate is not possible. The best type of stress to which a bonded joint is subjected is shear stress, because this stress is parallel to the bonding surface. Shear stress is present in bonded lap joints and usually has a sufficiently large bonding area. If the existing bonding area cannot support a sufficient load, it is also relatively easy to increase the load that the bonding can bear by increasing the bonding area. Torsional stress is similar to shear stress, and bonded joints are not sensitive to compressive stress. Over time, any type of mechanical stress will cause the aging of the bonded joint, such as: permanent deformation (creep) or crack formation may be caused, which exacerbates the effects of other aging factors.