Analysis of aging and long-term stability of adhesives.

Aging refers to changes in the properties of a material or bonding joint over time. Aging is caused by the external environment or by the material's own processing process. Factors are mechanical stress, temperature, radiation, and media such as water

. Some substances contained in the adhesive itself may also cause performance changes in the adhesive joints. In addition to being affected by a variety of factors, the aging positions that occur in the bonding joints will vary. Aging may occur on cohesion, adhesion, or substrate materials. Aging is caused by a combination of several factors, but these factors are difficult to measure and predict thoroughly.mechanical stress the bonding joint may be in the following mechanical stresses: A peel stress, B stretch stress, C shear stress, D torsional stress.of the adhesive results in a higher wire force at the bonding joint, which is much stronger than most adhesives. When designing the bonding joint structure, it is important to ensure that the bonding joint is not subjected to such peeling stress.when under an extruded load, the force is perpendicular to the bonding surface. The transfer of force passes through the entire bonding surface, and in principle there is no problem with the bonding joint to withstand stretch stress. Stretch stress is generally only encountered in the butt joint, butt joint is usually small bonding surface, only by changing the thickness of the substrate to increase the bonding area. Because the substrate strength usually exceeds the stretch strength of the glue, it is impossible to make full use of the substrate strength design.type of stress that a bonding joint can withstand is shear stress, which is parallel to the bonding surface. Shear stress exists in bonded bonding joints and usually has a large enough bonding area. If the existing bonding area cannot withstand enough load, it is also relatively easy to increase the bonding load by increasing the bonding area.torsional stress is similar to shear stress, and bonding joints are insensitive to compression stress. Over time, any type of mechanical stress can cause aging of the bonding joints, such as the possibility of permanent deformation (creep) or cracks. Mechanical stress may also exacerbate other aging factors (e.g., joint damage caused by water vapor).environmental impactthe bonding joints are exposed to different environmental impacts during use. These influences can occur at the same time and have an impact on each other.1, temperature and radiation effects
the most important factors affecting chemical reactions are temperature.
the temperature, the more likely it is to react and the faster the reaction.
reaction usually results in a change in the performance of the connector (usually undesirable).
damage to the bonding joints by mechanical stresses at high temperatures will be greatly accelerated, and it is thought that rising temperatures will accelerate aging.radiation is also one of the factors that causes the aging of bonding joints (including ultraviolet light in sunlight). Polymer chains in adhesives and plastic substrates are susceptible to radiation damage. In particular, polyurethanes are very sensitive to UV rays.2,
moisture can penetrate (diffusion) into the cured adhesive layer, and is absorbed by the glue layer.
absorbed water reduces the strength of the glue.
water acts as a lubricant (plasticizer effect) between polymer chains, making it easier to slide between molecular chains.
water penetrates into the adhesive layer at a very low rate, but is persistent.
at a temperature of 40 degrees C, the penetration rate of water into the crosslinked structure polymer is about a few millimeters per month.
when the bonding joint is dried, the cohesion strength rises again, but does not return to its original level.
the biggest problem with exposure to moisture to adhesive joints is the reduced adhesion between the substrate and the adhesive.
water penetrates into the bonding joint and the adhesion is reduced.
the adhesive and substrate adhesion is usually a slow process.
bonds formed on the surface of the adhesive and substrate are better water resistant than the physical interaction forces.
once the chemical bonds are damaged, the adhesion can no longer be restored after drying.
best way to prevent bonding joints from being affected by moisture is to stay away from water.
can not be reached, the connector can be sealed. 3, corrosion
for the metal substrate bonding joint, and the absorption of water vapor is closely related to the corrosion of the substrate.
the corrosion guide bonding joint of the substrate fails quickly in a short period of time, corrosion is also a critical aspect.
corrosion usually occurs quickly, especially in addition to water, there are some accelerated corrosion of the media (e.g. salt, acid, alkali, etc.), can erode the substrate from the outside or from the glue layer.
but in many cases this effect can be controlled, such as protecting the entire substrate or protecting only the bonding area.
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