Influence Factors Of Surface Treatment On Bonding Strength

Because bonding is mainly based on the adhesion of adhesive to the adhesive surface, the surface treatment of adhesive material may become the main factor determining the strength and durability of adhesive joints.

However, due to the different degree of oxide, rust, oil, adsorbate and other impurities on the surface of the adhesive material during the processing, pportation and storage, the bonding strength will be directly affected.

There are metallic materials as well as non-metallic materials; there are clean and polluted surfaces; there are smooth surfaces with rough or porous surfaces; according to the thermodynamic point of view, there are high energy surfaces and low energy surfaces; in terms of chemical structure, there are also active surfaces and inert surfaces.

The main influencing factors are cleanliness, roughness and surface chemical structure. Three aspects are introduced below.

Generally, pure metal surfaces have high surface free energy.

Organic adhesives are mostly polymer compounds with low surface free energy.

According to thermodynamic principles, they can infiltrate well.

But in fact, metals are not pure metal surfaces. On their surface, they often have a rust scale or oxide, and organic or inorganic pollutants adsorbed in the process of metal manufacturing, cutting, forming and heat treatment.

The cohesive strength of pollutants formed by these pollutants is very low, and their existence generally reduces the bonding strength.

For example, for aluminum, when the surface of the dirt is removed, the contact angle is greatly reduced to zero. It can be considered that the hydrophobic pollutants covered on the surface of the aluminum surface have been replaced by the adsorption layer with higher surface free energy.

Therefore, the contact angle is the smallest and the adhesive strength is the highest.

It can be seen that the relationship between cleanliness and adhesive strength is determined by the method of measuring contact angle, which has important reference value for selecting the best conditions for surface treatment.

The bonding strength can be improved by grinding the sand or sandblasting to treat the bonding material and roughening the surface properly.

However, roughness should not exceed a certain limit.

The surface is too rough, but it will reduce the bonding strength.

Because the rough surface can not be well infiltrated by the adhesive, the residual air in the concave part is unfavorable for adhesive bonding.

For example, the abrasive strength of sandblasting is higher than that after polishing, and the bonding strength of the sharp abrasive is higher than that of the ball abrasive.

Among them, the surface chemical structure can not only cause changes in the physical and chemical properties of the surface, but also lead to changes in the cohesive strength of the surface layer.

This is because the solid state oxidation-reduction reaction occurs on the stainless steel surface, resulting in a significant decrease in high temperature thermal aging properties.

However, if the surface of the steel is coated with zinc naphthenate, the thermal aging properties of the adhesive will be greatly improved.

Therefore, changing the surface atomic properties that accelerate the cracking of polymers will have a significant effect on the thermal oxidation of the steel joints.

However, after the treatment of sodium naphthalene tetrahydrofuran solution, the tetrafluoroethylene was fractured and some of the fluorine atoms on the surface were torn off and a thin black brown carbon layer was formed on the surface.

In this way, the chemical structure of the surface is changed, and the surface free energy is increased.

The surface suitable for bonding should have a stable, rough and compact oxide layer.

If a small amount of sodium sulfide and other reducing substances are added into the treatment fluid, the durability can be increased by more than 5 times.