The use of recycled plastic material in packaging is now a well-established reality in the sector. Legislation requires it and, in Spain, starting in 2025, PET packaging must contain at least 25% recycled material in accordance with Royal Decree 1055/2022. However, despite the widespread use of recycled content, certain applications that require monitoring and quantifying its presence still pose challenges.
This is the case of three-layer coextruded PET sheets with an A/B/A structure for the thermoforming of trays, where A is virgin PET and the intermediate layer B is recycled PET. These sheets present a problem when attempting to verify the presence of this multilayer structure or even the thickness ratio. Due to the transparency of the materials and the coextrusion process itself, it is impossible to confirm the multilayer structure using conventional methods. A cross-section of the film, obtained with a microtome and observed under an optical microscope, shows no distinction between the three layers (see Figure 1).
Since sample preparation and optical microscopy offer a fast way to observe layer distribution in multilayer films, it is frustrating when this technique cannot be used for these types of samples.
To address this problem, several tests were carried out in the AIMPLAS characterisation laboratories using different film samples, with cross-section images acquired at various temperatures. Using a special optical-microscopy accessory, a heating stage, it is possible to apply a controlled temperature scan, from room temperature up to higher values (not exceeding 180°C), while observing the film cross section. The PET/RPET/PET sheet is heated in a controlled manner and undergoes the typical thermal transitions of this material.
The aim of heating the sample is to determine whether the two types of PET behave differently, revealing optical changes that could be used to distinguish them. Observing the sample during the temperature increase provides micrographs of the different stages of PET heating, and it is during cold crystallisation that the three-layer structure becomes visible due to the different nature of the two PET grades used to produce the sheets (see Figure 2).
As shown, the three-layer structure becomes clearly distinguishable at 140°C, allowing verification of the presence of two different materials and even enabling control of their thickness ratio.
Through this approach, AIMPLAS provides a solution to the monitoring challenge presented by this type of film. The method can be useful both for production control and for assessing the quality of the manufactured sheets.
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