A brief discussion on the causes of aging of adhesive joints

 As bonding technology is used more and more widely, the aging problem of bonded joints has received more and more attention. The causes of aging of bonded joints can be roughly divided into the following aspects: the impact of mechanical load on the joints, natural Environmental effects on joints and migration of adhesive components. Due to space limitations, let’s first talk about how the natural environment aggravates the aging of bonded joints:

1. Temperature and rays

Temperature has a great influence on chemical reactions. The higher the temperature, the more likely a chemical reaction will occur and the faster it will occur. Such reactions lead to poor performance changes in bonded joints. Mechanical stress also leads to accelerated failure of bonded joints at high temperatures. It is generally believed that high temperatures accelerate the rapid aging of bonded joints.

The other aspect is radiation, especially ultraviolet rays in sunlight. The polymer segments in adhesives and plastic substrates are susceptible to UV irradiation, which can cause damage to the molecular chains. Polyurethane glue in particular is very sensitive to UV rays.

2. Water

Moisture can penetrate (diffuse) into the cured adhesive layer and be absorbed by the adhesive layer. These absorbed water molecules reduce the cohesive strength of the glue. Because water acts as a lubricant between polymer chains (plasticizer effect), it makes it easier for the molecular chains to slide between each other. The rate of moisture penetrating into the glue layer is very low, but the persistence is very high. At an ambient temperature of 40°C, the water penetration rate is approximately a few millimeters per month for a polymer with a cross-linked structure. When the bonded joint dries, the cohesive strength rises again and returns to its original level.

The biggest problem with adhesive joints exposed to moisture is that water penetration reduces the adhesion between the substrate and the glue. Water penetrates into the adhesive joint and the adhesion becomes smaller. The weakening of the adhesion between the glue and the substrate is usually a slow process. The chemical bonds formed between the glue and the surface of the substrate are more resistant to moisture than physical interaction forces. Once a chemical bond is broken, its adhesion cannot be restored after drying.

The best way to protect glue from moisture is to stay away from it. If this is not possible, the joint can be sealed.

3. Corrosion

For metal joints, corrosion of the base material is mainly caused by the penetration of water vapor. Corrosion of the substrate causes rapid failure of bonded joints in a short period of time. Salt, acid, alkali, water vapor and other substances accelerate the occurrence of corrosion. These substances can not only corrode the base material, but also slowly penetrate from the outside of the adhesive layer to the inside, thereby affecting the cohesion and adhesion of the adhesive layer. Therefore, on the one hand, we need to protect the base material and adhesive layer as much as possible, and on the other hand, we need to improve the corrosion resistance of the adhesive and base material.

4. Chemical media

Substances such as acid rain, acidic haze in nature, or commonly used detergents will also accelerate the aging of bonded joints. On the one hand, these chemical-containing substances will corrode the surface of the base material of the bonded parts; on the other hand, they will also cause irreversible damage to the adhesion layer and cohesion layer of the adhesive.

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