Bonding Mechanism of Hot Melt Adhesive
Published on Apr. 02, 2020
Why hot melt adhesive can produce adhesion, there have been many theories in the theoretical world for this, until today there is no conclusive conclusion. From a practical point of view, there is still no single adhesive theory that can explain all the bonding phenomena. For hot melt adhesives, what theory is used to explain its bonding? Let's first look at what theories are about bonding.
The mechanical theory is also known as the glue nail theory. In popular terms, the glue is solidified by penetrating into the gaps on the surface of the material, forming small glue nails, which produces mechanical bonding, which makes the adhesive mechanically fixed to the substrate after curing. Together. Because the appearance of solids is not completely smooth, the surface of solids under the microscope is composed of numerous irregular peaks and valleys.
The mechanical engagement of the adhesive is the main bonding factor for multi-space-based materials such as foam. For the bonding of non-porous materials, mechanical sanding is much better than untreated. We often use this mechanical sanding method in leather bonding and rubber bonding.
Mechanical roughening, that is, sanding has the following advantages:
1) Form a mechanical lock;
2) a fresh surface is formed;
3) a highly reactive surface is formed;
4) Increased surface area.
The mechanical theory explains the bonding mechanism of rough surfaces well, but it is difficult to explain the bonding mechanism of smooth surfaces, which is its disadvantage.
2. Adsorption theory
Adsorption theory believes that adhesion is caused by molecular contact between two materials and produces surface forces. The process of contact between the adhesive and the surface of the adherend is called "wetting". For the adhesive, its surface tension should be lower than the critical surface tension of the solid to achieve the wetting effect.
Most organic adhesives easily wet metal solids. But many solid organic substrates have less surface tension than commonly used adhesives. Good wetting requires that the adhesive have a surface tension lower than that of the substrate. This fact indicates that organic adhesives (such as epoxy adhesives) have good adhesion to metals, but to untreated polymer substrates such as polyethylene, polypropylene, and fluorocarbons have poor adhesion.
After bookbinding hot melt adhesive contacts the adherend, it can achieve permanent adhesion through the interaction between molecules, and four types of chemical bonds can be generated during adhesion cohesion.
The main valences are ionic bonds; covalent bonds; metal parts. The secondary price keys are Van der Waals.
Bookbinding Hot Melt Adhesive
3. Electrostatic theory
The electrostatic theory believes that the existence of adhesion is due to the existence of an electric double layer in the interface between the glue and the adherend, which results in the formation of electrostatic forces. This electrostatic force is inherently resistant to separation. People have confirmed the rationality of this theory through the phenomenon of discharge when peeling the cured adhesive on the substrate.
4. Diffusion theory
The diffusion theory believes that the existence of adhesive force is caused by the mutual diffusion between the glue and the adherend molecules. This theory is reasonable when both the adhesive and the adherend are polymers, and both have long molecular chains that can move. For example, solvent-based or hot-melt bonding of thermoplastics is considered to be the result of inter-molecular diffusion.
Although the problem of bonding mechanism has not been determined, understanding various bonding theories can help us grasp the basic requirements of bonding. Maybe bonding is not a theory that can explain it. For packaging hot melt adhesive, different materials should be bonded using different bonding theories. For example, bonding on textiles and plastics will use different theoretical models. In general, among the various bonding mechanisms, mechanical theory and diffusion theory are closer to the actual situation of hot-melt adhesive bonding.