Electrochemical Impedance Spectroscopy ? A Tool to Characterize the Interface

There are a number of analytical tools that are useful in characterizing adhesives systems. Perhaps the most important is thermal analysis which consists of thermogravimetric analysis, differential scanning calorimetry, thermomechanical analysis, differential mechanical analysis, and dielectric analysis. These have been well discussed in previous SpecialChem articles.¹ However, thermal analysis tools generally only characterize the bulk adhesive material. A significant need has existed for a similar tool that could characterize the adhesive / substrate interface as this is also critical to the durability of adhesive joints.

Electrochemical impedance spectroscopy (EIS) may be a solution to this problem. EIS is a technique that has rarely been used in adhesive bonding studies being primarily relegated to the paint industry. However, it is well suited to determine the environmental degradation of all adhesively bonded systems including structural as well as non-structural adhesive joints.

In the paint industry, EIS is sometimes used to study the degradation of painted metals during immersion in electrolytes such as 5% NaCl. Essentially the technique measures the total impedance versus frequency of a sheet metal coated with a film of high resistivity. On moisture absorption, the impedance spectra changes shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude depending on the adhesive and experimental conditions.

By various methods of measurement and data manipulation that are too detailed to be described in this editorial, electrical properties can be broken down into individual components for the metal, the interface, and the bulk material. Parameters measured as a function of immersion time can be used to calculate the percentage of water uptake, diffusion rates of ions and liquid water, porosity, corrosion reactions, blister resistance, and so forth. This analysis conventionally requires the adhesive or coating to be exposed directly to the immersion medium as in the case of a coating on a metallic substrate.

However, several researchers have adapted EIS to detect initial degradation of a bondline resulting from the ingress of moisture in an adhesive joint with adherends on both sides (e.g., the adhesive is buried between two adherends).² The EIS measurement can then be correlated with mechanical property degradation after similar periods of immersion.

As a research and development tool, EIS appears to be useful for the study of metal pretreatments as well as ingredients used in adhesive formulations to improve moisture resistance such as adhesion promoters, corrosion resistant additives, and hydrophobic modifiers. Because the EIS methodology is fast and relatively straight forward, it can also be used for quality control. The EIS sensor technology enables continuous or periodic monitoring of bondline health to provide input into a condition-based maintenance predictive assessment scheme. In-situ corrosion sensors based on EIS technology has also been used to detect moisture ingress into conventional adhesive bonds. ³

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