Bonding technology is a bonding method that closely combines materials or components through adhesives, which is widely used in various manufacturing industries. Compared with traditional mechanical connections, such as bolt connection and riveting, bonding not only has the advantages of light weight and low cost, but also can effectively avoid the stress concentration problem at the connection, so as to improve the strength and stability of the overall structure. In addition, bonding can provide good sealing and corrosion resistance, especially when dealing with the connection of heterogeneous materials, showing its unique advantages.

When the surface quality of the adhered object is poor, the bonding effect will be negatively affected. Therefore, in order to improve the bonding effect, it is necessary to perform plasma surface treatment on the surface of the adhered object. Plasma surface treatment can not only clean the surface of the object to remove pollutants, but also enhance the interaction between the adhesive and the surface of the object.

Plasma processing

Plasma is an electrically neutral, gas-like substance composed of molecules, atoms, electrons, ions or free radicals. It is considered to be the fourth state of matter and exists widely in the universe. When the material after gasification continues to be heated or subjected to a strong electric field, the electrons break away from the constraints of the nucleus ( this process is also called 'ionization ' ) and become free electrons. At this time, the material becomes a plasma composed of the nucleus ( with positive charge ) and the free electrons ( with negative charge ). According to the temperature difference between electrons and ions in the plasma system, it can be divided into high temperature plasma and low temperature plasma. In high temperature plasma, the temperature of ions and free electrons is extremely high. In low temperature plasma, the temperature of free electrons is extremely high, while the temperature of neutral particles or ions is much lower, even the same as room temperature. Due to the high energy of the electrons in the low-temperature plasma, the surface and molecules of the material can be ionized or cracked under high-speed bombardment, and the temperature of the whole system is still in the normal temperature state, so it is especially suitable for the surface modification of polymer materials. At the same time, low-temperature plasma treatment technology is an environmentally friendly dry surface treatment technology. The treatment process has no reagent residue, and is often used to improve the wettability, interfacial adhesion, hydrophilicity, biocompatibility and other properties of polymer materials.

Why plasma treatment can increase the adhesion between materials

After plasma treatment, it is easy to introduce polar groups or active points on the surface of the material, so that the non-polar surface is transformed into a polar surface, thereby increasing the interaction between the binder and the surface of the material and improving the adhesion. The mechanism of this improvement in adhesion may be as follows :

( 1 )The experimental results show that the surface energy of the material surface is improved after plasma treatment, and the increase of surface energy and the introduction of polar groups increase the intermolecular dipole force. 

( 2 ) The possibility of forming chemical bonds between the binder and the introduced polar groups at the bonding interface was enhanced.

( 3 ) The surface roughness of the material is increased. Through AFM observation, the surface roughness of the material is improved after plasma treatment, which effectively improves the adhesion between the materials and improves the mechanical properties of the material as a whole.

( 4 ) The weak boundary layer on the surface of the material can be removed by plasma treatment, so that the surface energy can be increased and the influence of the weak boundary layer with poor mechanical properties after bonding can be avoided.