Polymers, especially thermoplastics often show poor adhesive properties. Physical pre-treatment methods for structural adhesive bonding of polymers are commonly named as suitable to increase the adhesive bond strength. In particular, a chemical “activation” (creation of functional groups) as well as an increased polarity and wetting behavior were identified in literature by many authors as the main reason for good adhesion.
This talk aims to give answers to the question, what kind of surface characteristics (e.g. roughness, functional groups, surface free energy, etc.) a polymer surface must show to enable high-strength and durable structural adhesive joints. Focusing the need of a chemical surface functionalization after physical pre-treatment, different polymers (thermoplastic and thermoset) were modified with common pre-treatment methods, like plasma and vacuum-UV (VUV) radiation and with different process gases, like air, nitrogen, argon and fluorine. In addition, selected samples were rinsed after treatment with different polar solvents (water or ethanol). By doing so, most different surfaces were created, regarding both the chemical composition and the wetting behavior (hydrophobic and hydrophilic surfaces). The results show, that physical pre-treatment processes create “Low Molecular Weight Oxidized Material” (LMWOM) for many polymers, which covers the surface. As LMWOM is loose and removable from surface, it has no impact on the formation of adhesion forces. Therefore, analytically measured surface properties cannot be directly related to the adhesive bond strength, if LMWOM has not been removed beforehand. Furthermore, the rinsed as well as the investigation with the per-fluorinated and super hydrophobic samples show that neither a specific chemistry nor an increased polarity or improved wetting behavior wetting is needed for enhanced adhesion on polymers.
In contrary to literature and the published textbook opinion, this means that adhesion does not take place through the formation of covalent bonds but mainly through Van der Waal’s (VdW) interactions. In order to allow enough VdW interaction a surface enlargement on a nm/ molecular scale is needed. Obviously, a physical pre-treatment like plasma or VUV is able to create an appropriate molecular surface structure/morphology, caused through the chemical etching process initiated on the polymer surface. However, a specific adjusted chemistry is not needed or plays a subordinate, negligible role for adhesion on polymers.
Co-authors: J. Holtmannspötter, H.-J. Gudladt