Towards Sustainable Food Packaging: Mechanical Recycling Effects on Thermochromic Polymers Performance

Abstract

Integrating thermochromic pigments (TPs) into food packaging offers significant benefits for monitoring temperature variations, improving food safety, and reducing waste. However, the recyclability of such materials remains underexplored, particularly regarding the retention of their optical and mechanical properties after repeated recycling. Addressing this gap, this research aims to evaluate how mechanical recycling affects key properties of polypropylene (PP) blends containing varying TP concentrations. Three formulations, PP100/TP0 (0% TP), PP98/TP2 (2% TP), and PP92/TP8 (8% TP), were subjected to five recycling cycles, with changes in thermal stability, color transition behavior, mechanical integrity, and surface morphology analyzed. The results indicate that PP100/TP0 maintained its mechanical integrity with minimal degradation (6% absolute crystallinity loss; color difference ΔE*_ab = 1.45) across recycling cycles. However, blends containing TPs exhibited progressive deterioration. P98/TP2 displayed moderate reductions in mechanical strength (-10.8%) and thermochromic efficiency (color change ΔE*_ab = 6.52), while PP92/TP8 showed significant degradation, including increased activation temperatures (+3.8 °C) and color vibrancy loss (42.9% loss in saturation). These effects were attributed to polymer breakdown, pigment aggregation, and altered crystallinity. Despite the limitations of recyclability, this study provides critical insights into the feasibility of TPs in sustainable, intelligent food packaging. Further research is required to enhance TP stability during reprocessing, ensuring long-term functionality in circular packaging systems.

Keywords: food packaging; mechanical recycling; polymer degradation; sustainability; thermochromic pigments.

Figure 1

Granulated virgin PP (PP100/TP0) after 0, 1, 2, 3, 4, and 5 recycling steps: (A) control; (B) recycled × 1; (C) recycled × 2; (D) recycled × 3; (E) recycled × 4; (F) recycled × 5.

Figure 2

Granulated PP98/TP2 blends after 1, 2, 3, 4, and 5 recycling steps: (A) recycled × 1; (B) recycled × 2; (C) recycled × 3; (D) recycled × 4; (E) recycled × 5.

Figure 3

Granulated PP92/TP8 blends after 1, 2, 3, 4, and 5 recycling steps: (A) recycled × 1; (B) recycled × 2; (C) recycled × 3; (D) recycled × 4; (E) recycled × 5, highlighting the increasingly pale appearance of high-pigment blends due to thermochromic pigment degradation. Quantitative data extracted from these images are presented in Table 2.

Figure 4

Effect of recycling on tensile test specimens of PP blends: (A) PP100/TP0 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps; (B) PP98/TP2 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps; (C) PP92/TP8 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps.

Figure 5

Visual comparison of impact specimens before and after recycling: (A) PP100/TP0 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps; (B) PP98/TP2 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps; (C) PP92/TP8 tensile test specimens after 0, 1, 2, 3, 4, and 5 recycling steps.

Figure 6

Optical microscope images of PP92/TP8 at 200× magnification using the VHX-S750E optical microscope: (A) 0 recycling step; (B) 1 recycling step; (C) 2 recycling steps; (D) 3 recycling steps; (E) 4 recycling steps; (F) 5 recycling steps.

Figure 7

Scanning electron microscope (SEM) micrographs of PP100/TP0: (L) 50× magnification; (R) 5000× magnification. (A) 0 recycling steps; (B) 1 recycling steps; (C) 5 recycling steps.

Figure 8

SEM micrographs of PP98/TP2: (L) 50× magnification; (R) 5000× magnification. (A) 0 recycling steps; (B) 1 recycling steps; (C) 3 recycling steps; (D) 5 recycling steps.

Figure 9

SEM images of the impact fracture of PP92/TP8: (L) 50× magnification; (R) 5000× magnification. (A) 0 recycling step; (B) 1 recycling step; (C) 3 recycling steps; (D) 5 recycling steps.