Cohesion Mechanism of Adhesive Resin
Hotmelt adhesive is a complicated physicochemical process. adhesive strength depends not only on the chemical structure of adhesive resin and material to be bonded but also on the bonding process conditions. According to the application practice of adhesive resin, the cohesion mechanism caused in the surface of adhesive resin and material to be bonded can be divided as below.
3.1 Surface wettability mechanism
Hotmelt adhesive resin property is usually closely related to the surface wettability between two different materials. When there is excellent surface wettability between adhesive resin and the material to be bonded, these two materials can tightly adsorb each other. Few spaces are left and air is discharged from the material surface, which leads to forming of strong intermolecular interaction to improve the adhesive strength of adhesive resin. Infiltrating degree of adhesive resin on the surface of other materials can be denoted using contact angle θ as shown in Figure 2. With the decrease of the contact angle θ, the infiltrating degree and the adhesive strength increased.
Figure 2. Schematic diagram of contact angle.
Chemical bond mechanism
When the surfaces of adhesive resins and material to be bonded touched each other, the active functional groups on different surfaces can react to form chemical bond, such as ionic bond, covalent bond, hydrogen bond and coordinate bond (Figure 3), which leads to the bonding between these two different materials. Furthermore, it is confirmed that the more chemical bonds, the higher adhesive strength and the stronger anti-stress corrosion cracking property for adhesive interface.
Figure 3. Ionic bond, covalent bond, hydrogen bond and coordinate bond on adhesion interface.
Molecular diffusion mechanism
For the interface between two different materials, when the environment temperature is higher than the glass transition temperature or melting point both of these two polymers, with the increase of thermal motion of polymer chains, can diffuse and interweave with each other. The interface will gradually fuse, and a stable homogenous phase formed finally, which can reflect the adhesive strength of adhesive resin.
Static electricity mechanism
There is much different types of charge that distribute in the surfaces of adhesive resin and material to be bonded. When these two materials touched each other, it is easy to lose electrons for one material surface, meanwhile, for another material surface is easy to obtain electrons. Finally, the structure of the electric double layer formed by transferring of charge between two material surfaces. Electrostatic attraction caused from the electric double layer makes important contribution for adhesive strength of adhesive resin.
Weak boundary layer mechanism
When composite material such as aluminum-plastic composite pipe bonded by adhesive resin was destroyed, it is usually recognized that destruction took place on the interface of two materials. In fact, destruction often took place at material inner or weak boundary layer, which may exist in adhesive resin, material to be bonded or both of them. If there was impurity near interface, weak boundary layer will form in adhesive resin layer and material to be bonded layer. When the composite material was destroyed, destruction must take place at weak boundary layer, rather than the interface of two materials.
Anchoring effect mechanism - adhesive resin for aluminum-plastic composite pipe
It is rough for the surface of all the materials, and there are a lot of tiny holes and grooves. Adhesive resin melt can get into these interspaces by flowing, infiltrating or extruding. Adhesive resin will leave in interspaces after cooling and solidifying, which can form anchoring effect with the material to be bonded. The resulting mechanical force will offer adhesive strength for adhesive resin.
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