Material - inherent Characteristics Affecting the Strength of Adhesives
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Polarity
The polarity of molecules in adhesives and adherends has an impact on the bonding strength. However, an increase in the polarity of these molecules does not necessarily enhance the bonding strength. From the perspective of improving bonding strength, altering the polarity of the surface in the interface region is more effective than changing the polarity of all molecules. For example, after plasma surface treatment of polyethylene, polypropylene, and polytetrafluoroethylene, polar groups such as hydroxyl groups, carbonyl groups, or carboxyl groups are generated on the surface, significantly improving their bondability.
Molecular Weight
The molecular weight (degree of polymerization) of polymers affects the intermolecular forces. These forces determine the melting point and boiling point of substances. For polymers, they determine the glass transition temperature Tg and melting point Tm. Whether it is an adhesive or an adherend, the molecular weight of the polymer influences the bonding strength. For linear polymers without branches, there are two scenarios. In one case, cohesive failure of the adhesive occurs across the entire molecular weight range. Here, the bonding strength increases with the increase in molecular weight until a certain value is reached and then remains constant. In the other case, the failure mode varies with molecular weight. At low molecular weights, cohesive failure occurs. As the molecular weight increases, the bonding strength increases. When the molecular weight reaches a certain value, the cohesive force of the adhesive is equal to the adhesive force, resulting in mixed failure. When the molecular weight is further increased, the cohesive force exceeds the adhesive force, and the wettability deteriorates, leading to interfacial failure. At this point, there is a specific molecular weight corresponding to the maximum bonding strength.
Side Chains
The side groups on long - chain molecules are crucial factors in determining the properties of polymers. Considering intermolecular forces, when the side branches are small, increasing the length of the side branches reduces the intermolecular forces. When the side branches reach a certain length, crystallization begins, and further increasing the length of the side branches increases the intermolecular forces. This can lead to either a decrease or an increase in the bonding strength.
Crystallinity
Polymers with a high degree of crystallinity have a regular state of molecular condensation. If the melting point is not high, heating the crystalline polymer will disrupt the ordered molecular arrangement within the crystalline region, and the molecules start to transition to the molten state. Therefore, polymers with a high degree of crystallinity are suitable for use as hot - melt materials.
