Résultats d'une analyse d'impédancemétrie sur un ordinateur

Electrochemical impedance spectroscopy

The Electrochemical Impedance Spectroscopy (EIS) is a technique for characterizing electrochemical systems.

The principle of this technique consists of applying a low amplitude sinusoidal signal to the electrochemical system and measuring its response at different frequencies.

In the case of an electrochemical impedance measurement applied to the aerosol generators, the test method consists of bringing the coating or varnish (container) into contact with the product which will serve as electrolyte (content).

The test is based on the application of a low alternative potential at the input, by means of a reference electrode (RE) and a counter electrode (CE) connected to a potentiostat* and the measurement of the ouput current, between the reference electrode and the working electrode (WE), which will allow the impedance (alternating current electrode) to be calculated. In this case, it is the metal aerosol can (or a container, metal bottle) which constitutes the working electrode.

In order to be able to measure an output current, it is necessary that the prodcut – the electrolyte – has a minimum conductivity. This measurement is carried out at different frequencies during the test, typically between 100 mHz and 100 kHz.

The physical model used to quantify the response to the product, the electrolyte, is a RLC model: resistance, inductance, capacity (see figure below). To quantify the quality of the coating used, we are interested by the resistance and capacity values and and their changes over time. In this goal, several measurements are carried out at regular intervals, over a period of one month, to characterize the stability of the coating to extended contact with the product.

Schéma de la spectroscopie d'impédance électrochimique

The Electrochemical Impedance Spectroscopy (EIS) is a technique that allows a rapid and reliable initial assessment of the compatiblity of a coating towards a given product. This method is complementary to storage tests for the evaluation of container-content compatibility, as it is not possible to evaluate compatibility in the gas phase with this technique.

This is an effective method to compare different varnishes between them and to anticipate faster incompatibilities. Thanks to the new generation of potentiostat*, with which the LEREM is equipped, it is possible to measure very low output signals (currents), which makes it possible to obtain a response with resistance and capacity values even with very poorly conductive formulations.

* Potentiostat: accurate device that generates and measures low electrical signals with electrodes for electrochemical studies.

Electrochemical studies include other techniques as tin rate measurements on tinplate sheets as well as corrosion current measurements by amperometry. These studies will be the subjects of a future article.

General information on impedancemetry

IMPEDANCEMETRY IS A SCIENCE

A brief history of impedancemetry

The first tests relating to impedancemetry date back to Georg Simon OHM (1788-1854), a famous physicist, who established the fundamental relationships between current, voltage and electrical resistance. This science has become popular in recent years, particularly in the medical or paramedical fields. However, it should be noted that this science has been implemented in packaging studies since the 80’s. LEREM, for its part, has been practicing these studies since early 90’s.

Imagine that you can materialize a set of elements through a mathematical model and that these elements can give you physical quantities that you can study in absolute way but also in a relative way. For example, how they evolve over time…

This is what happens with modern person scales, which can read your muscle mass index in addition to your fat mass index, or even from one measurement to another can read how your diet changes your muscle mass…

It is one of many examples of impedance measurement applications. The popularized results of these measurements have gone through mathematical modelling. The mathematical models were calibrated based on the results of measurements of very weak and very precise electrical signals at different frequencies. These results were then used to represent your body in an equivalent electrical model of an RLC circuit (resistive, inductive, capacitive).

Very weak electric power, without any danger, cross your body through the feet and hands or through the measuring points which are connected to your body. You now know the basic principle of impedance measurements works.

USAGE IN OUR FIELD

How can we now apply this science to metal packaging?

It will be done the same way as the bathroom scale, by applying weak electrical signals of great precision at different frequencies to the packaging.

We will seek, depending on the results, to create an equivalent RLC model. We will not try to find the packaging muscle mass but try to find a combination of things. That combination is made of internal coating, substrate and filled formula that present a mathematical model showing a high resistance and a very low capacity. What’s more, the measurements will be repeated at regular intervals so it can confirm, or not the stability of the results over time.

For example, it will thus be possible over a period of 4 weeks to reliably confirm that a combination of varnish, coated on a metallic substrate with a formulation is not stable and therefore possible to anticipate bad storage test results (or compatibility)

These measurements can only be applied if the packaged formula is sufficiently conductive. Certain formulas are not conductive enough and therefore do not allow studies by impedancemetry.

Evaluating the conductivity of formulas can be done easily by using a conductivity meter. Weak signal generators called potentiostat are also intended to go into safety mode if the formula is not compatible with the means of measurement. Note that these studies cannot replace storage compatibility studies. This is a reliable method of knowing more quickly what will not be compatible at the end. This is not a substitute for storage compatibility testing.

In addition, equipment’s and systems for processing measurements have evolved. Regarding this point, LEREM will be equipped with brand new state of the art equipment from December 2021. It will be covered in a future article.

With more than thirty years’ experience in this field and equipped with state-of-the-art equipment, LEREM remains at your disposal to carry out compatibility studies containing content using impedancemetry.

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