Surface treatment and bonding strength

The surface treatment of the bonding material is one of the most important processes in the entire bonding process and one of the keys to the success and failure of the bonding. Since the adhesive bonding mainly depends on the adhesion of the adhesive to the surface of the adhesive material, the surface treatment of the adhesive material may become a major factor in determining the strength and durability of the adhesive joint. However, due to a series of processing, transportation and storage processes, the adhesive material will have different levels of oxides, rust, oil, adsorbent and other impurities on the surface, which directly affects the bonding strength.

Adhesive materials and their surfaces are varied. There are metallic and non-metallic materials; there are clean and contaminated surfaces; there are smooth and rough or porous loose surfaces; according to the thermodynamic point of view, there are high-energy surfaces and low-energy surfaces; chemical structures, There are also active and inert surfaces.

In order to obtain a glued joint with high bonding strength and good durability, it is required that the prepared surface layer and the base material and the adhesive must be combined firmly, and this combination is not affected by or less affected by environmental conditions.

The role of surface treatment mainly has the following three aspects:

(1) Remove surface dirt and loose layers that prevent adhesion;
(2) increase surface energy;
(3) Increase the surface area.

The quality of the surface treatment will directly affect the bonding strength of the bonding material. Its main influencing factors are cleanliness, roughness and surface chemical structure, which are described below.

First, cleanliness

To obtain good bond strength, the necessary condition is that the adhesive completely wets the surface of the bonding material. In general, pure metal surfaces have high surface free energy. The organic adhesives are mostly high molecular compounds with low surface free energy. According to the principle of thermodynamics, they can infiltrate well. However, the metals that are actually obtained are not pure metal surfaces, and often have a layer of rust or oxide on their surfaces, as well as organic or inorganic contaminants that are adsorbed during the manufacture, cutting, forming, and heat treatment of metals. The contaminants formed by these contaminants have very low cohesive strength and their presence is generally reduced by the strength of the bond.

To obtain good bonding strength, the contact angle on the surface of the bonding material should be small or even zero. For aluminum, for example, when the dirt on the surface is removed, the contact angle is greatly reduced to zero, and it can be assumed that the hydrophobic contaminants covered on the aluminum surface have been replaced by the adsorption layer with higher surface free energy. Now. Therefore, the contact angle is the smallest and the bonding strength is also the highest. From this, it can be seen that the relationship between the degree of cleanliness and the bonding strength expressed by the method of measuring the contact angle is an important reference value for selecting the optimum conditions for surface treatment (see Table 1).

Table 1 Contact angle and bonding strength before and after surface treatment Adhesive bonding material treatment method Contact angle (°) Shear strength (MPa) Aluminum untreated 67 17.2 Degreasing 67 19.3 Chemical treatment 0 26.6 Stainless steel untreated 50-75 36.6 Degreasing 67 44.3 Chemical treatment 10 49.7 Titanium untreated 50-75 9.5 Degreasing 61-71 22.4 Chemical treatment 10 43.2

Second, roughness

It has long been known that mechanical grinding can increase the bonding strength of metals. Whether sanding or sandblasting is used to treat the bonded material and the surface is properly roughened, the bonding strength can be increased. However, the roughness must not exceed a certain limit. The surface is too rough and will reduce the bonding strength. Because too rough surfaces cannot be well-infiltrated by the adhesive, air remaining in the recesses, etc. is detrimental to the bonding.

In addition, the bonding strength is not only related to the surface roughness, but also has a close relationship with the different surface geometries produced by the roughening method. For example, sandblasting is more effective than grinding and then mechanically roughening the bonded joints; sharper abrasives have higher bonding strength than spherical abrasives.

The reason why the surface roughness of the bonding material improves the bonding strength is because the process of mechanical roughening undoubtedly causes the surface to be purified; secondly, it also changes the physical and chemical state of the surface to form a new surface. Layer; Finally, the difference in roughness will also affect the stress distribution at the interface, so as to obtain better bonding strength.

Third, the surface chemical structure

The chemical composition and structure of the bonding material surface have important influences on the bonding performance, durability performance, heat aging performance, etc.; and the influence of the surface structure on the bonding performance is often achieved by changing the cohesive strength, thickness, porosity of the surface layer. , activity, surface free energy, etc. Among them, the surface chemical structure can not only cause the change of the physical and chemical properties of the surface, but also cause the change of the cohesive strength of the surface layer, and thus has a significant effect on the adhesion performance.

For example, when the stainless steel and aluminum adhesive joints glued with phenolic resin are respectively put on the 2880C under heat aging treatment for 50 minutes and 100 minutes, the stability of the aluminum adhesive joints is still good, and the stainless steel adhesive joints almost lose all the bonding strength. . This is because a solid-state redox reaction occurs on the surface of the stainless steel, resulting in a significant reduction in high-temperature heat aging properties. However, if a layer of zinc naphthenate is coated on the surface of the steel, the heat aging performance of the adhesive can be greatly improved. Therefore, changing the atomic properties of the surface that accelerate the cracking of the polymer has a significant effect on the thermal oxidation resistance of the steel joint.

For another example, PTFE is an inert polymer material with a very low surface energy, and ordinary adhesives cannot be firmly bonded. However, after treatment with a sodium-naphthalene-tetrahydrofuran solution, tetrafluoroethylene was broken and some of the fluorine atoms on the surface were torn off and a very thin black-brown carbon layer was formed on the surface. In this way, both the chemical structure of the surface is changed and the surface free energy is also increased, thereby improving the bonding performance.

For another example, titanium and titanium alloys treated by different methods have very different bonding strength and durability. The surface suitable for bonding should have a stable, rough, tight oxide layer. If you add a small amount of reducing substances such as sodium sulfide in the treatment solution, the durability can be increased by more than 5 times.

Source: 21st Century Fine Chemicals Network