Influence of Layer Thickness on the Detection of Plastic Films Using Optical Photothermal Infrared Spectroscopy (O-PTIR)

This study investigates how varying film thickness affects the qualitative identifiability of plastics using optical photothermal infrared spectroscopy (O-PTIR) on examples of common plastics such as polyamide 6 (PA6) and polyethylene terephthalate (PET). The main methodology consists of applying a thin layer of PA6 and PET separately to each substrate, namely, PET to polyethylene (PE) and PA6 to polypropylene (PP), and reducing the thickness of the coatings until the O-PTIR signal of the film is no longer detectable. As expected, the characteristic O-PTIR signal for PA and PET decreased with a decreasing film thickness. However, the results show that the O-PTIR detection limit for the plastics could not be reached, as the characteristic peaks of the substrate plastics are still clearly visible at a layer thickness of approximately 0.18 μm for PET and approximately 0.29 μm for PA6. These are the minimum stable film thicknesses that could be achieved since the selected film production process (drop deposition process) does not allow for thinner layers. As this work is a feasibility study, further factors influencing the O-PTIR measurement of plastic films should be investigated in a subsequent work.