Nondestructive Characterizations of Polyethylene/Nylon Laminates by Near-Infrared Spectroscopy

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Abstract
The use of near-infrared (NIR) spectroscopy for the rapid and nondestructive analysis of food packaging laminates containing polyethylene (PE), polyamide-6 (PA-6), and ethylene vinyl alcohol (EVOH) layers is demonstrated. The method of Pathlength Correction with Chemical Modeling (PLC-MC) is used to estimate the total laminate thickness, and Principal Component Regression (PCR), is used to estimate the thickness percentages of PE and EVOH in the laminates, from NIR reflective-transmission spectra in the region 1500–2500 nm. Results indicate that the NIR method can be used to determine the total laminate thickness within 2–4 μm, the PE layer thickness percentage within 0.7–1.8%, and the EVOH layer thickness within 0.7–0.8 μm. In addition, detailed observation of the PCR models indicates that the NIR method is also sensitive to the absorbed water content, the morphology of the polymers, and perhaps the amount of polyurethane adhesive in the laminates. The usefulness of PCR outlier detection, for identification and characterization of strange samples, and principal component rotation, for improvement of PCR model interpretability, is also demonstrated.