How to formulate adhesives with improved bond strength?

Regardless of the type of material or application, the following minimum requirements contribute to successful adhesive joints: 

• Cleanliness of the substrate surface and elimination of weak boundary layers: Foreign materials such as dirt, oil, moisture, and weak oxide layers must be removed from the substrate surface. In case not removed, the adhesive or coating will bond to these weak boundary layers rather than to the substrate. Accidental contamination of the surface is the most common source of a weak boundary layer. 

• Wetting (intimate contact or spreading of the applied material onto the substrate surface): There are a variety of forces (ionic, static, polar, van der Waals, etc.) acting between the adhesive and the substrate that ensure good bonding. Good wetting provides a larger area of contact where these forces may act. Consequently, good wetting is crucial for good bond formation². 

• Solidification of the adhesive: Nearly all polymeric materials (including adhesives and coatings) shrink during solidification. Occasionally, adhesives may shrink due to solvent evaporation, resulting in a reduced mass within the bond line. Even 100% reactive polymers, such as epoxies and urethanes, experience some shrinkage because their solid polymerized mass occupies less volume than the liquid reactants. The result of such shrinkage is again internal stresses at the interface and the possible formation of cracks and voids within the bond line itself. 

• Formation of a "joint" structure (applied material, interface region, and substrate) that is resistant to the operating stress and environment.

• Selection and control of all materials and manufacturing processes.