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

## TL;DR

This study explores how the thickness of plastic films affects their identification using a specific infrared spectroscopy technique.

## Contribution

The study establishes minimum detectable film thicknesses for polyamide 6 and polyethylene terephthalate using O-PTIR.

## Key findings

- The O-PTIR signal for plastics decreases with thinner film layers.
- PET and PA6 films remained detectable down to 0.18 μm and 0.29 μm, respectively.
- The drop deposition process limits achieving thinner layers for testing.

## Abstract

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.