# UV-Induced Aging in Thermochromic Pigment-Integrated Food-Grade Polymers: A Performance Assessment
The proprietary nature of many thermochromic systems, particularly those involving microencapsulated leuco dyes, presents regulatory hurdles due to incomplete disclosure of chemical composition. Because these additives are usually left in the polymer matrix and not eliminated during reprocessing, mechanical recycling could be more difficult. In closed-loop applications, leftover pigments might impact recycled materials' look, thermal characteristics, or compliance if separation or stabilization processes are used. Recyclability and regulatory compatibility remain key challenges limiting the widespread adoption of thermochromic food packaging systems. Exposure to UV light in indoor and outdoor settings may impact the structural and functional integrity of plastic food packaging [30; 31; 32; 33]. External UV exposure commonly occurs during transportation, storage, and especially retail display--particularly when products are placed near windows, under fluorescent or LED lighting, or in open-air markets exposed to sunlight [34]. Internally, UV exposure may occur in food processing and packaging facilities where UV sterilization is used for hygiene control [35] or from UV-emitting light sources within refrigerators, display cases, or vending machines [36]. Prolonged exposure to UV light can weaken the polymer chains in plastic, resulting in embrittlement, staining, loss of barrier properties, and photofading or reduced responsiveness of functional additives such as thermochromic pigments [37; 38]. This underscores the importance of UV resistance in plastic packaging materials for long-term use or light-sensitive applications. UV light is the electromagnetic spectrum's 200-400 nm region. Short-wave UV light (UVC) is between 200 and 280 nm, medium-wave UV light (UVB) is between 280 and 320 nm, and long-wave UV light (UVA) is between 320 and 400 nm [39]. The wavelengths at which light is released from gas discharge depend on the excitation, ionization, and kinetic energy of the constituent elements and the gas itself [40]. A mixture of excited and non-excited atoms, cations, and electrons produced by applying a high voltage across a gas volume is known as gas discharge [41]. UV light has great promise for the food processing industry as an alternative to conventional thermal processing [42]. It can be used to pasteurize juices, sanitize food contact surfaces, extend the shelf life of fresh products, and provide post-lethality treatment for meats, ensuring safety after cooking [43]. Previous studies have addressed recyclability [44], chemical resistance [45], and mechanical behavior of thermochromic materials in food contact environments. However, little information exists on the photostability of TPs [46]. The processes of UV degradation are well-established in polymer science [47; 48; 49]. They typically involve chromophores within the polymer or dye molecule absorbing high-energy UV photons, which initiate free radical reactions that irreversibly alter the chemical structure [50]. Even minor structural defects or impurities can act as initiation sites in semicrystalline polyolefins such as polypropylene [51]. UV photon absorption leads to excited-state formation, generating alkyl or peroxy radicals in the presence of oxygen [52]. These radicals propagate degradation through hydrogen abstraction and oxidation reactions, resulting in chain scission, crosslinking, and the formation of carbonyl and hydroxyl groups on the surface [53]. In TPs, this may lead to loss of thermochromic reversibility, loss of overall color, and complete failure of the indicator functionality. Thermochromic pigment systems, particularly microencapsulated leuco dyes, are especially vulnerable--UV exposure can disrupt the internal dye-developer-solvent equilibrium, degrade capsule walls, or decompose active dye species [54]. Adding UV stabilizers or protective coatings could help reduce the impact of UV on TPs [55]. Adding TPs into plastic food contact packaging has the potential to reduce both food and packaging waste while enhancing food safety.