The effect of chemical factors on bonding strength

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The chemical factors that affect adhesion strength mainly include molecular polarity, molecular weight, molecular shape (number and size of side groups), molecular weight distribution, molecular crystallinity, the stability of molecules in relation to the environment, as well as the properties of other components in the adhesive and the substrate, pH value, etc. 　　 1. Polarity: Generally speaking, the polarity of the adhesive molecules and those of the substrate affects the bonding strength, but this does not mean that an increase in the polarity of these molecules will necessarily lead to an increase in bonding strength. 　　 2. Molecular weight: The molecular weight (or degree of polymerization) of a polymer directly affects the forces acting between polymer molecules. Generally speaking, the relationship between molecular weight and adhesion strength is limited to the case of branched-free linear polymers, which include two types. In the first type, cohesive failure of the adhesive occurs across the entire range of molecular weights; in this case, the bonding strength increases as the molecular weight increases, but remains constant once the molecular weight reaches a certain value. The second type has different degradation patterns due to differences in molecular weight. 3. Side chains: The side groups on long-chain molecules are important factors that determine the properties of polymers. From the perspective of intermolecular forces, the effect of polymer branches is such that when the branches are small, increasing their length reduces the intermolecular forces. When the branch length reaches a certain level, crystallization begins; increasing the branch length enhances the intermolecular forces, which should be the reason for either reducing or increasing the adhesion strength. 　　4. Cross-linking: The cohesive strength of polymers increases as the cross-linking density rises; however, when the cross-linking density is too high, the polymer becomes hard and brittle, which reduces its impact resistance. The strength of cross-linked polymers is closely related to the number of cross-linking sites and the length of the cross-linking molecules. 5. Solvents and plasticizers: The bonding strength of solvent-based adhesives is certainly affected by the amount of residual solvents in the adhesive layer. When the amount of solvent is high, although the infiltration property is good, the cohesive strength decreases due to the reduced cohesion within the adhesive. Plasticizers have a similar effect to solvents; sometimes, even when sticking does not occur, adding an appropriate amount of plasticizer can enable sticking.