How Thermodynamics Drives Wet-out in Adhesive Bonding: Correcting Common Misconceptions

Abstract

The traditional maxim that one should keep the adhesive surface energy below that of the substrate to ensure wet-out is shown to be a false and often counter-productive constraint in designing superior adhesive bonds. The confusion stems from the failure to carefully distinguish the one-substrate problem (coating) from the two-substrate problem (adhesive bonding). When two substrates are bonded the adhesive will almost invariably be driven to wet them out, even if the adhesive is of much higher surface energy than the substrates. The driving force is the elimination of the condensed phase/air interfaces in favor of condensed phase/condensed phase interfaces. Experiments are conducted to demonstrate that high surface energy liquids and adhesives will easily wet low surface energy substrates when sandwiched between them. The standard mathematical formalism for the free energy change of coating, bonding liquid adhesives, and bonding PSA tapes is developed to clarify the thermodynamic distinctions between these very different processes. It is shown that for spontaneous, thermodynamically driven, wet-out the contact angle the adhesive must make with the substrate is <90° for a liquid adhesive and <180° for a pressure sensitive adhesive (PSA) tape. These forgiving criteria are readily met in practice, and are far less stringent than the 0° contact angle required for spontaneous spreading in a coating process.

Keywords:

• WET-OUT
• WETTING
• WICKING
• SURFACE ENERGY
• EXCESS FREE ENERGY