Detection of barrier films (EVOH) in laminated plastic
In the plastics industry, TRICLOPS can be used for quality control, to characterize plastic films in real time during the production process.
TRICLOPS spectrometer can be configured in transmission mode to quickly and simply characterize the optical absorption of many materials from the visible to the mid-infrared (400–4500 nm). Here we investigate whether TRICLOPS in transmission mode can be used to confirm the presence of a thin inter layer barrier film (EVOH) in a laminated plastic product.
The intensity spectra (each collected in <2 seconds) of the plastic samples are subltely but clearly distinct.
Absorption features given by the EVOH layer are detectable in the SWIR region, in the spectral range ~ 1800 – 2300 nm.
These measurements were used to train an identification model.
Using this model, TRICLOPS can operate as an at-line automatic analyzer, assessing the presence of the EVOH layer with high accuracy
Introduction
Barrier films play a crucial role in extending the shelf life of packaged products by providing a robust barrier against gases, moisture, and other environmental factors.
Among the various materials used for barrier films, ethylene vinyl alcohol (EVOH) stands out due to its exceptional gas barrier properties. EVOH is commonly used in multilayer plastic films in the food packaging industry to protect contents from oxygen permeation and maintain product quality over extended periods.
The detection and characterization of EVOH layers within plastic films are vital for ensuring the integrity and performance of the packaging material. Spectroscopic techniques, particularly those operating in the near-infrared (NIR) and mid-infrared (MIR) regions, offer a non-destructive, rapid, and accurate method for analyzing these barrier films.
Measurements
An experimental analysis was conducted over 6 plastic samples (Figure 1), 3 of them containing an EVOH film and 3 of them without it. 7 measurements per sample were taken on both sides and in different positions.
A simple transmission setup was used for the measurements, using a halogen lamp for the sample’s illumination.
Figure 1: Laminated plastic samples
Experimental results
The spectra were obtained using TRICLOPS in full detection range (400– 4500 nm) with medium resolution averaging 5 scans (<2 seconds total collection time). The corresponding intensity spectra are shown in Figure 2 a).
The EVOH layer (if present) is very thin, and thus the absorption signal we are trying to detect is small and can be obscured by sample-to sample variations in e.g. surface roughness, alignment, and other variables (Figure 2.a).
These variations can be addressed by selecting an appropriate preprocessing pipeline (in this case normalization → baseline correction → wavelength reduction). The resting spectra (Figure 2b) are very consistent and distinctive absorption features given by the EVOH layer are revealed in the spectral region ~ 1800 – 2300 nm.
Figure 2: a) Raw spectra from TRICLOPS b) The same dataset after pre-processing removed variability and thus reveals EVOH signatures
Data analysis
A PLSDA (Partial Least Squares Discriminant Analysis) algorithm was chosen for developing a reliable predictive model that can univocally classify different fabrics. The validation of this model was done by 5-fold cross validation (CV): 5 subsets of the dataset were created to sequentially calibrate the model on 4 of them and evaluate its predictive effectiveness on the remaining one.
An additional analysis of the single wavelengths importance in the regression model was done to reduce the predictors’ set, improving the model performances.
The figures of merit for the model’s evaluation are the accuracy of cross validation and the correlation coefficient R^2. The results are shown in the table.
To visually observe the clustering effectiveness of the PLS algorithm on the different fabrics the PLS scores plot is shown in Figure 3.
Figure 3: PLS scores plot
Conclusion
In the application note TRICLOPS was demonstrated to be a reliable instrument for detecting EVOH barrier layers in plastic film thanks to their unique spectral fingerprints. The reliability, non-destructiveness and velocity of the spectrometer makes TRICLOPS suitable for quality assessment solutions in the packaging industry.
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