Environmental Impacts
During the usage process, bonded joints are exposed to different environmental impacts. These influencing factors may act simultaneously and interact with each other.
1. Temperature and Radiation Effects
         The most significant influencing factor on chemical reactions is temperature. The higher the temperature, the more likely a chemical reaction is to occur and the faster the reaction rate. Such reactions usually lead to changes in the performance of the joint (usually undesirable). The damage of mechanical stress to the bonded joint is greatly accelerated under high - temperature conditions. Generally, an increase in temperature is considered to accelerate aging.
          Radiation is also one of the influencing factors causing the aging of bonded joints (including ultraviolet rays in sunlight). The polymer chains in adhesives and plastic substrates are vulnerable to radiation damage. In particular, polyurethane is very sensitive to ultraviolet rays.
2. Water
          Moisture can penetrate (diffuse) into the cured adhesive layer and be absorbed by it. This absorbed moisture reduces the strength of the adhesive. Because water acts as a lubricant between the polymer chains (plasticizer effect), making it easier for the molecular chains to slide. The rate of water penetration into the adhesive layer is low but continuous. At a temperature of 40 °C, the penetration rate of water into cross - linked polymers is approximately a few millimeters per month. When the bonded joint is dried, the cohesive strength rises again, but it cannot return to its original level.
     The biggest problem caused by exposing the bonded joint to moisture is the reduction of the adhesion between the substrate and the adhesive. The adhesion weakens after water penetrates into the bonded joint. The weakening of the adhesion between the adhesive and the substrate is usually also a slow process. The chemical bonds formed between the adhesive and the substrate surface are more water - resistant than physical interactions. Once the chemical bonds are damaged, the adhesion cannot be restored after drying. The best way to prevent the bonded joint from being affected by water vapor is to keep it away from water. If this is not possible, the joint can be sealed.
3. Corrosion
       For bonded joints of metal substrates, closely related to the absorption of water vapor is the corrosion of the substrate. Corrosion of the substrate leads to the rapid failure of the bonded joint in a short period, so corrosion is also a crucial aspect. Corrosion usually occurs quickly, especially when there are some media that accelerate corrosion in addition to water (e.g., salt, acid, alkali, etc.), which can erode the substrate from the outside or from inside the adhesive layer. However, in many cases, this effect can be controlled, such as protecting the entire substrate or only protecting the bonded area.