Fourier Transform-Infrared Spectroscopy (FT-IR) is a powerful technique for fast and non-destructive analysis of plastic films.
Mathieu Coté and Frédéric Despagne, ABB Analytical
Fourier Transform Infrared Spectroscopy (FT-IR) is a powerful technique for fast and nondestructive analysis of plastic films. It can measure chemical characteristics and provide detailed compositional information like additive content and co-monomers composition. It also allows to measure plastic films physical characteristics such ase density or rheological attributes.
We present an application of FT-IR spectroscopy to identify chemical structure of plastic packaging film coatings. This is particularly appropriate for detection of counterfeited materials or unknown material deformulation.
Four samples were analysed. Samples 1, 2, and 4 were clear transparent polymers. Sample 3 was a laminated type of film made of a metal layer coated on both surfaces with transparent polymer.
Spectral acquisitions were performed on both sides of each sample with the ABB MB3000 FT-IR spectrometer (Figure 1), using a MIRacle Attenuated Total Reflectance (ATR) accessory. This interface allows to determine film coatings over a few microns.
In order to identify coating components, unknown sample spectra were submitted to a search against a reference polymer library from ST Japan, using the MB3000 "Horizon MB Library" search engine. The "Scalar Product" criterion was selected. The algorithm calculates the angle between the query spectrum intensity vector and all library spectra intensity vectors as the scalar product of the two intensity vectors. The closer the angle is to zero, the better the matching between query and library spectrum. A spectral match Quality Index (QI) is then returned, ranging from 0 (no match) to 100 (perfect match).
Sample infrared spectra are presented in Figure 2 and grouped according to their general shapes for clarity. For each sample film, both sides were arbitrary labeled "a" and "b."
In all cases (with the exception of sample 4), samples are different on both sides, indicating that the polymer films are made of several materials. It is important to note that the spectrum of one side of sample 1 (a) is different from the other sample spectra. Using spectral search, the following materials were identified: polypropylene (sample 1 (a), QI 98.91), polyethylene terephtalate (samples 1 (b), 2 (a) and 3 (a), QI 95.67 to 96.93), ethylene vinyl acetate (samples 2 (b) and 3 (b), QI 92.35 to 92.89) and polyethylene (samples 4 (a) and 4 (b), QI 98.00 to 98.14).
We can therefore conclude that the film coatings are made of the following polymers:
Sample 1: Polypropylene and polyethylene terephtalate
Sample 2 and 3: Polyethylene terephtalate and ethylene vinyl acetate
Sample 4: Polyethylene
FT-IR technology is extremely reliable and can be used off-line as a fast tool in replacement of time-consuming laboratory techniques. It can also be implemented as an in-process control tool during plastic film production to monitor product quality and detect process drifts.