O-PTIR Detection of Plastic Films: Influence of Layer Thickness

By Dr. Noopur JainFeb 6 2026

In a recent article published in the journal ACS Measurement Science Au, researchers examined the minimum detectable thickness for two widely used plastics, polyamide 6 (PA6) and polyethylene terephthalate (PET), applied as coatings onto polypropylene (PP) and polyethylene (PE) substrates, respectively.

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Background

The increasing complexity of material compositions in the plastics industry, involving combinations of different plastics, additives, and coatings, demands rapid and dependable analytical techniques. This diversity presents a significant hurdle for plastics recycling, necessitating the development of sensitive methods to accurately identify foreign substances and contaminants. Such advancements are crucial for establishing more effective and efficient sorting, separation, cleaning, and recycling processes. Optical Photothermal Infrared Spectroscopy (O-PTIR) is highlighted as a promising analytical approach in this context.

Previous studies have explored O-PTIR for analyzing thin layers of poly(methyl methacrylate) (PMMA), 15.7 μm diameter PMMA microspheres, microplastics, and paint layers, among others. However, a systematic investigation into the penetration depth of O-PTIR in plastics had not been previously conducted.

The Current Study

The study involved dissolving PA6 and PET granulates separately in hexafluoroisopropanol (HFIP) to create solutions of varied concentrations. Films of varying thicknesses were then produced using a drop deposition method onto PE and PP substrates, followed by solvent evaporation at room temperature. Film thickness was primarily controlled by adjusting the concentration of the plastics in the solution.

Specifically, solutions were prepared with plastic concentrations of 1.0, 0.5, 0.25, 0.125, and 0.0625 wt %. A plastic concentration lower than 0.0625 wt % could not be used with the drop deposition method, as the polymers failed to form a homogeneous layer, instead aggregating into discrete particles or island-like domains upon solvent evaporation.

Reference specimens of pure PA6 and PET were prepared on KBr windows using the highest concentration solution under the same conditions as the tested films.

The manufactured films were characterized using digital microscopy to confirm film quality. The primary measurements were conducted using the O-PTIR microscope, mIRage. For each solution, three film specimens were produced, and each film was measured three times at the same three points. The reported O-PTIR spectra represent the average of nine recorded spectra, normalized such that the highest peak intensity is set to 1.

Following O-PTIR, the specimens were embedded in epoxy resin, cut, and coated with a gold layer of approximately 15 nm thickness to make them conductive for scanning electron microscopy (SEM) analysis.

Results and Discussion

The reference O-PTIR spectra for PE, PP, PA6, and PET showed characteristic peaks at 1473 cm⁻¹ (PE), 1377 cm⁻¹ (PP), 1641 cm⁻¹ (PA6), and 1721 cm⁻¹ (PET). These spectra aligned well with published data for these plastics.

As the thickness of the plastic film decreased, the intensity of the characteristic O-PTIR peak for the coating material (PA6 or PET) declined, while the signal from the substrate (PP or PE) increased. This trend confirmed the expected relationship. However, the primary goal of determining the minimum detectable film thickness could not be achieved because the film production method (drop deposition) reached its technical limit, preventing the creation of sufficiently thin and stable layers that would render the O-PTIR signal undetectable.

The thinnest stable layers achieved were approximately 0.18 μm for PET and less than 0.29 μm for PA6. Importantly, these minimum stable film thicknesses still exhibited clearly recognizable O-PTIR signals.

The relationship between film thickness and the IR signal has been observed in other studies, which found that for thin films (less than about 1 μm), the signal intensity is roughly proportional to thickness, consistent with a low-absorbance regime. Above this thickness (e.g., 1 to 2 μm), the signal increase becomes non-linear, and further increases in thickness yield diminishing incremental signal growth. In this study, O-PTIR measurements were consistent with these findings, as the peak heights for films produced from 0.5 wt % and 1 wt % solutions (thicknesses ≥2.98 μm for PET and ≥1.51 μm for PA6) did not differ significantly. Thinner layers showed an almost linear trend in average peak height across the varied concentrations.

Conclusion

The feasibility study investigated the dependence of O-PTIR signals on film thickness for PA6 and PET films applied to PP and PE substrates. The main finding is that the thinnest layers successfully produced, approximately 0.18 μm for PET and <0.29 μm for PA6, still yielded distinct O-PTIR signals. Future work should also focus on investigating the interaction between the substrate and the coating, studying smaller variations in film thickness to enhance the reliability of the relationship between IR absorption peak heights and thickness, and examining the influence of measurement conditions, such as IR and probe laser powers, on the detectability of the polymer films.

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