Attenuated Total Internal Reflection Infrared Microscopy of Multilayer Plastic Packaging Foils

• Published in: Applied spectroscopy Vol. 61; no. 6; pp. 593 - 602
• Main Authors: van Dalen, Gerard, Heussen, Patricia C. M., Den Adel, Ruud, Hoeve, Robert B. J.
• Format: Journal Article
• Language: English
• Published: London, England SAGE Publications 01-06-2007
• Subjects: Food Packaging - classification; Materials Testing - methods; Microscopy - methods; Plastics - chemistry; Spectroscopy, Fourier Transform Infrared - methods; LM; Multilayer films; Hyperspectral; Chemical mapping; Internal reflection element; Microspectroscopy; Synthetics; Attenuated total reflection; Polymers; Multivariate; Scanning electron microscopy; FT-IR spectroscopy; Spectral imaging; Elastomers; Laminates; PCA; Image analysis; Adhesives; Packaging; Light microscopy; Fourier transform infrared spectroscopy; SEM; Spectrophotometry; Plastics; IRE; Barrier thickness; Reflectance; ATR; Principal component analysis
• ISSN: 0003-7028; 1943-3530
• DOI: 10.1366/000370207781269738

Multilayer plastic foils are important packaging materials that are used to extend the shelf life of food products and drinks. Fourier transform infrared (FT-IR) spectroscopic imaging using attenuated total internal reflection (ATR) can be used for the identification and localization of different layers in multilayer foils. A new type of ATR crystal was used in combination with a linear array detector through which large sample areas (400 × 400 μm2) could be imaged with a pixel size of 1.6 μm. The method was tested on laminated plastic packing materials containing 5 to 12 layers. The results of the identification of the different materials using ATR-FT-IR were compared with differential scanning calorimetry (DSC) and the layer thickness of the individual layers measured by ATR-FT-IR was compared with polarized light microscopy (LM) and scanning electron microscopy (SEM). It has been demonstrated that individual layers with a thickness of about 3 μm could be identified in multilayer foils with a total thickness ranging from 100 to 150 μm. The results show a spatial resolution of about 4 μm (measured at wavenumbers ranging from 1000 to 1730 cm−1), which is about a factor of two better than can be obtained using transmission FT-IR imaging. An additional advantage of ATR is the ease of sample preparation. A good correspondence was found between visible and FT-IR images. The results of ATR-FT-IR imaging were in agreement with those obtained by LM, SEM, and DSC. ATR-FT-IR is superior to the combination of these techniques because it delivers both spatial and chemical information.