Adhesive bonding as a joining process of the 21st century
Bonding is often referred to as the joining process of the 21st century. A key advantage of this process is that, in theory, it can join almost any combination of materials. In practice, however, this requires extensive research to select the optimal combination of surface pretreatment and adhesive.
In many cases, such tests are not possible due to time or cost reasons, which can lead to problems with the adhesive bonds. In the past, GWP has proven itself competent in identifying the component that is disrupting adhesion in the event of damage and in testing and optimizing substrate-adhesive combinations in advance.
The following example illustrates how the GWP detects contamination and evaluates the adhesion of adhesives to different substrates.
Contamination as a cause of delamination
Contamination is a common cause of adhesive bonds coming apart. In some cases, the delamination only occurs after a longer period of time. One example of this is the bonding of a polystyrene insulation layer to an aerated concrete substrate using a PU adhesive. Three years after bonding, some delamination occurred and the cause had to be determined.
Contamination detection method: ToF-SIMS
The ToF-SIMS method (Time-of-Flight Secondary Ion Mass Spectrometry) provides detailed information about the atomic and molecular composition of the topmost atomic layer of a surface. In time-of-flight mass spectroscopy, the sample surface is bombarded with a beam of primary ions. These ions cannot penetrate the sample matrix, but transfer their energy to the molecules and atoms on the surface. This creates secondary ions that are charged and are separated in a time-of-flight analyzer according to their mass/charge ratio (m/z).
The result of this analysis is a secondary ion mass spectrum that shows a characteristic pattern for the respective surface.
The main advantages of this method are the high surface sensitivity and the very low detection limit (from concentrations of 10 ppm in a monolayer). In addition, the high mass resolution a reliable identification of chemical components on the surface.
Although the data from ToF-SIMS analysis are fundamentally not quantitative – since the intensity of the secondary ions depends on the matrix effect of the sample – semi-quantitative Results are obtained when the chemical environment of the samples to be compared is similar.
investigation of the interfaces
The ToF-SIMS analysis was carried out on both the intact and the debonded sample, with the investigation taking place on the Styrofoam and the PU side of the adhesive interface. To analyze the interfaces, the intact bond was carefully debonded using tweezers.
Results of the analysis
A comparison of the intact (iO) and the de-adhesive (niO) sample showed that the following elements and compounds were increasingly detected on the de-adhesive sample:
- Inorganic ions: Na, Cl, NH₄ (possibly contaminated by fingerprints)
- Aliphatic hydrocarbons (possibly mineral oil residues)
- silicone fragments
- Oxygen-containing hydrocarbons (possibly fragments of plasticizers)
- sulfates and dodecylbenzenesulfonate (surfactants)
Conclusion
ToF-SIMS analysis has provided significant clues to the causes of debonding by identifying the specific contaminants that compromised the bond. Such detailed investigations make it possible to evaluate the adhesion of adhesives to different substrates and, if necessary, to optimize the surface treatment or adhesive choice.