Quality Control of PVC foils

PVC (polyvinyl chloride) foils with a PVDC (polyvinylidene chloride) coating are often used for high performance packaging films like pharmaceutical blister packs or in food packaging. In multi-layer blister films, the PVC serves as the thermoformable backbone structure, whereas the PVDC coating acts as a barrier against moisture and oxygen. The Water Vapor Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR) are influenced by the composition and the thickness of the coating.

A fast way to monitor PVDC coating thickness is with near-infrared spectroscopy. Results are provided in a few seconds, indicating when adjustments in the polymer production process are necessary.

Figure 1. DS2500 Solid Analyzer

Several 250 μm PVC foils coated with a PVDC layer of varying thickness (40 g/m² , 60 g/m² , 90 g/m²) were measured on the DS2500 Solid Analyzer. The measurements were carried out in transflection mode using the NIRS gold diffuse reflector with 1 mm pathlength. This ensures that the spectral pathlength is constant while enhancing the spectral signal. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.

**Table 1.** Hardware and software equipment overview
Equipment	Metrohm number
NIRS DS2500 Solid Analyzer	2.922.0010
Vision Air 2.0 Complete	6.6072.208
NIRS gold diffuse reflector, 1 mm	6.7420.000
NIRS mini sample cup	6.7402.030

All 68 measured Vis-NIR spectra (Figure 2) were used to create a prediction model for quantification of PVDC layer thickness. The quality of the prediction model was evaluated using correlation diagrams, which display a very high correlation between Vis-NIR prediction and the reference values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.

Figure 2. Vis-NIR spectra of PVC foils with different PVDC layer thicknesses measured on a DS2500 Solid Analyzer.

Figure 3. Correlation diagram for the prediction of PVDC layer thickness using a DS2500 Solid Analyzer.

**Table 2.** Figures of merit for the prediction of PVDC layer thickness using a DS2500 Solid Analyzer.
Figures of merit	Value
R²	0.992
Standard error of calibration	1.7 g/m²
Standard error of cross-validation	1.9 g/m²

This application note demonstrates the feasibility of differentiating PVC foils coated with different PVDC layer thickness (40, 60, 90 g/m² PVDC on 250 μm PVC foils). The thickness of the PVDC layer could be successfully determined with NIR spectroscopy with an average difference with respect to the reference data of 2%. Vis-NIR spectroscopy enables a fast determination without any sample preparation, and therefore represents a suitable method to measure PVDC layer thickness.

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