Surface treatment of the bonding material to the entire bonding process, the most important step, is also one of the key successes and failures of bonding. Since the main means of adhesion bonding adhesive on the surface of the bonding material, the surface treatment of the bonding material strength and durability could become a major factor in determining glued joints. However, due to the bonding material in a series of processing, transport, storage process, there will be varying degrees of surface oxides, rust, oil, and other impurities adsorbent, which directly affect the bonding strength.
Bonding material and its surface is varied. There are also non-metallic materials of metal materials; there has clean a contaminated surface;
There is also smooth rough or porous loose surface; in accordance with the thermodynamic point of view, there are high-energy and low-energy surface of the sub-surface; from the chemical structure considered, but also the active surface and the inert surface of the points.
In order to obtain a high bonding strength, durability and good adhesive joints, the surface layer and the substrate material and the adhesive must be prepared with a solid, and this combination is not less affected by environmental conditions.
The effect of surface treatment mainly in the following three aspects:
(1) to remove loose dirt and impede the surface layer glued;
(2) increasing the surface energy;
(3) increase the surface area.
The quality of the surface treatment will directly affect the bonding strength of the bonding material. The main factor is the cleanliness, roughness and surface chemical structure of these three aspects, the following were to be introduced.
To obtain good bonding strength, the necessary conditions are completely wet the surface of the adhesive bonding material. Typically, pure metal surface having a high surface free energy. The mostly organic binder having a low surface free energy of the polymer compound. According to the principles of thermodynamics, it can be a good wetting between them. But in fact the resulting metal is not pure metal surface, on their surfaces often with a layer of oxide or rust, and adsorption in the manufacture of metal cutting, forming, heat treatment process of organic or inorganic contaminants. These contaminants within the contaminated layer consisting of cohesive strength is low, their presence is generally required to reduce the bonding strength.
To obtain a good bonding strength, the contact angle of the surface of the bonding material should be small or even zero. For example, to aluminum, after removing the dirt on the surface, as well as greatly reduce the contact angle to zero, it is believed at this time covered on the aluminum surface is hydrophobic contaminants adsorbed layer having a high surface free energy of the substituted . Therefore, the contact angle minimum, maximum bonding strength. It can be seen, the contact angle was measured by the method to represent the relationship between cleanliness and bonding strength, the conditions are selected as the best surface treatment is an important reference value.
It has long been known to use mechanical grinding method can increase metal bonding strength. Whether sanding or by sand-blasting the bonding material handling, appropriately roughened surface, can improve the bonding strength. However, the roughness can not exceed certain limits. Surface is too rough and it will reduce the bonding strength. Because it is too rough surfaces can not be well adhesive infiltration, the remaining air pockets and so on Bonding is unfavorable.
In addition, the bonding strength is not only related to the surface roughness, and different surface roughening methods resulting geometry is also closely related. For example, more than a good sandblasting polishing after mechanical roughening process after bonding strength; sharp abrasive than spherical abrasive processing bonding strength.
Bonding material surface roughening reason will increase bonding strength, First, because the mechanical roughening the surface of the process no doubt has been purified; secondly, because it changes the physical and chemical state of the surface, forming a new surface layer; and finally, different roughness also affects the interface stress distribution, so as to obtain a better bonding strength.
3、Surface chemical structure
Chemical composition and structure of the surface of the bonding material bonding performance, durability, heat aging properties have a significant impact; The influence of surface structure on the bonding properties tend to be by changing the surface layer of cohesive strength, thickness, porosity, surface free energy and activity realized. Wherein the surface can cause changes in the chemical structure of the surface of the physical and chemical properties, it can also cause changes in the surface layer of cohesive strength, resulting in a significant impact on the adhesion properties.
For example, phenolic resin bonding of stainless steel and aluminum pieces were glued on the 2880C heat aging 50min and 100min, the stability of the aluminum bonding member is still good, and the stainless steel glue unions lost almost all bonding strength. This is because, on the stainless steel surface occurs solid phase redox reaction, resulting in high-temperature heat aging performance declined significantly. However, if the coating layer on the steel surface zinc naphthenate, heat aging performance bonding member can be greatly improved. Thus, changes can accelerate atomic properties of the polymer surface cracking, with obvious impact on those oxidation resistant steel joints right.
Another example, is a low surface energy PTFE inert polymer materials, adhesives generally can not be firmly glued. However, sodium - naphthalene - tetrahydrofuran solution treatment, tetrafluoroethylene faulting occurred, a fluorine atom on the surface of the part was pulled, and produces a thin layer of carbon black-brown on the surface. In this way, changing the chemical structure of the surface, but also increases the surface free energy, thus improving the bonding performance.
Again, titanium and titanium alloys of different treatment methods on bonding strength and durability can be very different. Suitable for bonding surfaces should have a stable, rough, tight oxide layer. If a small amount of sodium sulfide and other reducing substances in the treatment solution, the durability can be increased more than 5 times.
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