Abstract
The production of barrier packaging materials, e.g., for food, by physical vapor deposition (PVD) of inorganic coatings such as aluminum on polymer substrates is an established and well understood functionalization technique today. In order to achieve a sufficient barrier against gases, a coating thickness of approximately 40 nm aluminum is necessary. This review provides a holistic overview of relevant methods commonly used in the packaging industry as well as in packaging research for determining the aluminum coating thickness. The theoretical background, explanation of methods, analysis and effects on measured values, limitations, and resolutions are provided. In industrial applications, quartz micro balances (QCM) and optical density (OD) are commonly used for monitoring thickness homogeneity. Additionally, AFM (atomic force microscopy), electrical conductivity, eddy current measurement, interference, and mass spectrometry (ICP-MS) are presented as more packaging research related methods. This work aims to be used as a guiding handbook regarding the thickness measurement of aluminum coatings for packaging technologists working in the field of metallization.
Highlights
As early as 1994, researchers were looking for the absolute minimum of material usage for disposable packaging, pursuing the need for environmental protection
A quantitative identification of atoms in a probe is possible by standard dissolutions, as there a linear relationship between the signal intensities of the ions and the is a linear relationship between the signal intensities of the ions and the concentration of the element [72,73]
It should be considered that this method determines all the aluminum in the sample: aluminum existing as oxide as well as the pure metal, which might explain the deviations from other methods [42]
Summary
As early as 1994, researchers were looking for the absolute minimum of material usage for disposable packaging, pursuing the need for environmental protection. Aluminum foil with a thickness of about 6 to 40 μm has extremely high barrier properties. In order to maintain the high barrier properties of aluminum while simultaneously minimizing material usage, nanometer-thin aluminum coatings are applied on polymers via PVD (vacuum evaporation). In this process, aluminum is heated until it evaporates in a vacuum chamber. The polymer substrate is moved across the aluminum gas cloud so that the metal condenses on the polymer surface In this way, thicknesses of only a few nanometers can be realized.
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