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Wetting Envelope
Development of a method for an easy and fast estimation of adhesion in bonded systems using Wetting Envelope
Project duration
From: 01.05.2021 To: 31.10.2023Description
Although it is known that controllable surface properties, such as surface energy or wettability and surface roughness, are decisive for the adhesion build-up, there is still no established method for an easy and quick prediction of the adhesion of an adhesive and joining partner in bonding technology. The aim of this research project was therefore to transfer the wetting envelope model, which is already established in the coatings industry and is based exclusively on wettability, to adhesives. The surface energy of liquids and adhesives is usually determined by measuring the contact angle on a pendant drop. However, this method reaches its limits above a certain adhesive viscosity, which is why alternative measurement methods for determining the surface energy (i.e. inverse gas chromatography) were used and evaluated in the project.
In the course of the project, it was possible to show how the wetting properties are influenced not only by the surface energy but also by the rheology and curing of the adhesive. Based on these two influencing variables, a holistic view of the wetting behavior can be derived, which goes significantly beyond the original model of the wetting envelope. Overall, there is no simple functional relationship between the surface energies of the adhesive and the surface of the bonded part on the one hand and the wetting behavior on the other, so that the wetting envelope theory cannot be transferred directly to adhesive bonding technology. Similarly, a clear correlation between the measured strengths or adhesion work and the respective calculated values for the adhesion work from surface energy measurements cannot be established for all adhesives. No adhesion prediction could be made for a 1C PU system used in the research project, whereas the calculated work of adhesion correlated directly with the bond strength for a 2C epoxy adhesive.
Acknowledgments
The IGF project 21696 N from the Research Association FSKZ e.V. was supported by the Federal Ministry of Economic Affairs and Climate Action through the German Federation of Industrial Research Associations (AiF) as part of the programme for promoting industrial cooperative research (IGF) on the basis of a decision by the German Bundestag.
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