Hot melt pressure-sensitive adhesive relies mainly on physical adsorption and mechanical anchoring for bonding.
Physical adsorption
Physical adsorption forces can be categorized from weak to strong as van der Waals forces, dipole-dipole interactions, hydrogen bonding, and ionic bonding. Practical application tests confirm that adhesives with high polarity can induce a significant difference in interface polarity on the adhered surfaces, leading to mutual adsorption and improved adhesion. Therefore, selecting chemically polar materials to synthesize hot melt pressure-sensitive adhesives can enhance adhesive strength and increase the affinity for the adhered surface.
However, highly polar chemical components (elements) often exhibit high reactivity and are prone to aging through interaction with oxygen. Additionally, highly polar chemical components usually have deeper colors, which can affect the appearance of the adhesive or bonded material, thereby reducing the product's value.
For materials with low surface energy or low polarity, such as PE and PP, theoretically, adhesives with extremely low polarity should be chosen to minimize the interface contact angle or maximize the contact area, thus achieving optimal van der Waals forces. However, practical experience indicates that the contribution of van der Waals forces to the overall adhesive strength of hot melt pressure-sensitive adhesives is not as significant as the dipole interactions generated by the polarity difference between two materials at the interface.
Mechanical anchoring
Regardless of the color and polarity of the adhesive, all adhesives have unique viscoelastic properties. The viscous part allows the adhesive to flow, deform, and permanently extend, while the elastic part enables the adhesive to undergo instantaneous deformation, rebound, resist tensile strength, and exhibit heat resistance. The viscoelastic changes of the adhesive are closely related to the types, molecular weights, component ratios, and compatibility of high molecular weight resins, thickeners, plasticizers, and various additives. Changes in temperature, speed, time, and pressure can also alter the viscoelastic properties of the adhesive.
Factors influencing the adhesive strength of hot melt pressure-sensitive adhesives:
Surface roughness of the adhered surface
For a completely smooth adhered surface, regardless of the adhesive's flowability and mechanical anchoring properties, the maximum contact surface area that can be obtained is 100%. However, as surface roughness increases, less flowable adhesives may achieve a contact surface area less than 100%, while more flowable adhesives can achieve over 100%. A larger contact surface area between the adhesive and the adhered material on a unit projection area requires higher energy for detachment.
Interface fracture mode
The fracture mode of the adhesive layer interface is closely related to the strength of the adhesive and the adhered material. If the adhered material must be torn apart after bonding, the cohesive strength and interface energy of the adhesive must be greater than the cohesive strength of the adhered material itself.
