Polymers are central materials for our present and future society which have enabled us to improve the quality of all aspects of our daily life. However, our lifestyle also generates serious problems due to the ways in which we use polymers. Most polymers are discarded after use, rarely recycled, and end up in landfills, rivers, and oceans. This fate is a typical example of the current linear and fossil-based produce-use-discard value chain that causes enormous environmental pollution. Therefore, a fundamental transition is required in the field of polymeric materials from synthesis out of fossil-based feedstock and one time use to continuous re-use of polymeric products. The focus of our research is on existing polymers as well as on designing novel polymeric structures more amenable to recycling. The key is incorporation of suitable degradable functional groups or smart monomers into the polymers in their design stage. Our approach entails the use of various imine, acetal, triazine and hexahydro-s-triazine derivatives as cleavable groups within the polymer network. [1-5] Most importantly, we demonstrate chemical depolymerization, the subsequent separation and reuse of recovered building blocks, and closed-loop recycling or upcycling of novel polymers. Our studies are expected to contribute to the development of a circular plastic economy and drive research on environmentally friendly materials forward.

Poly(lactic acid) (PLA), polyhydroxyalkanoates (PHAs) and succinate polymers are examples of biobased materials industrialization with their production covering in Europe 1 % of total plastics. Τhis share is anticipated to rise the next years, encouraged also by EU legislation, where there are targets to increase the use of biobased feedstock in plastic packaging. Our group has been studying end-of-life management options for biobased aliphatic polyesters exploring high-quality recycling depending on the waste degradation degree; remelting-restabilization and solid state polymerization (SSP) processes have been evaluated for PLA and PBS as approaches to safeguard against further degradation or even restore recyclate performance. More specifically, for low degradation extent, remelting-restabilization was examined and reprocessing in the presence of appropriate antioxidant systems was suggested for PBS so that the polymer structure and properties are protected and closed-loop recycling be achieved. For PLA, the hydrolysis-based degradation mechanism was efficiently hindered through the addition of readily available aliphatic and aromatic carbodiimides prolonging in parallel the service lifetime of the biopolyester. On the other hand, for highly degraded polyesters, SSP was examined as a molecular weight (MW) repairing tool for PLA and for PBS, reaching MW rebuild higher than 30 % at reaction temperature lower than 130 °C. In parallel, vitrimerization has been explored as an upcycling approach for low-MW PBS: dynamic crosslinking with bisphenol A diglycidyl ether (DGEBA) or glycerol was achieved using a transesterification catalyst and tunable crosslink densities and viscoelastic behavior were attained by adjusting the molar ratios, crosslinker type and temperature.

Polymers have penetrated almost every aspect of our daily lives with applications ranging from packaging and healthcare to automotive parts and construction materials. With a market share of over 60%, polyolefins emerge as the most widely used synthetic polymers. Consequently, the global use of polyolefins contributes to large volumes of plastic waste and a growing concern about how to effectively manage pollution and polymers accumulating in the environment. Although reutilizing polyolefins seems to be a logic choice, their recycling level remains disappointingly low. This is mainly due to the lack of large-scale availability of efficient and inexpensive compatibilizers for mixed polyolefin waste, typically consisting of HDPE and iPP that despite their similar chemical hydrocarbon structure are immiscible.
Here we describe an unconventional approach of using poly-pentadecalactone, a straightforward and simple to produce aliphatic polyester, as a compatibilizer for iPP/HDPE blends, especially the brittle iPP-rich ones [1]. The surprisingly effective compatibilizer transforms brittle iPP/HDPE blends into unexpectedly tough materials that even outperform the reference HDPE and iPP materials. This simple approach creates opportunities for upcycling poly-mer waste to valuable products.

Acknowledgement: special thanks go to SABIC for funding this scientific work.

Mussels are a sustainable and affordable protein source that naturally filters nutrients from seawater [1]. However, the accidental dispersion of plastic “socks” used for their farming on the seabed and along beaches represents a significant environmental problem. Recovery and recycling of polypropylene (PP) may represent an adequate solution for the management of plastic waste generated by mussel farming. Due to the presence on the surface of incrustation of salty biological elements and fouling, currently, a European Waste Code (CER 020104) of special waste is associated with the mussel socks: their correct disposal involves significant expenses (0.25–0.30 EUR/kg). The EU-funded LIFE MUSCLES project was established to develop a new treatment method for the recovery of polypropylene from mussel nets [2]. Through experiments performed on a laboratory scale, it was verified that it is possible to effectively remove biofilm (98%) from the polymeric material using pressurized water (25-30 bar). The spectroscopic, thermal, morphological, and mechanical characterization of the socks shredded and washed for 30 minutes demonstrated that the treatments undergone by the polymer had a non-significant impact on the PP physical properties, thus allowing its reuse in the same supply chain. In particular, the regenerated PP had an elastic modulus value (0.697±0.058 GPa) comparable to that of pristine PP (0.758±0.069 GPa). Therefore, the Life MUSCLES [3] project adopts a circular economy model, emphasizing material recycling through a mobile pilot plant capable of processing 300 kg of mussel nets daily, reducing costs for new nets and minimizing environmental impact.

We report the influence of loaded and unloaded silica particles on the ultimate behavior of polymeric blends based on 30 % mass fraction of virgin and 70 % mass fraction recycled polypropylene, (PP). The final recycled materials include a certain amount of polyethylene, (PE) because it was not fully separated from the PP in recyclates. The silica composites show greater thermal stability than the neat PP blends. A clear separation of crystallization processes of the two components, PP and PE, is noticed when cooling was performed at 2 °C/min. Moreover, a constant location is observed in the crystallization peak for the PE component with the silica content, while that for the PP shows a significant nucleating effect compared with the neat blend. This nucleation effect of the silica is also rather evident from the rheological results. Polarized optical microscopy (POM) images collected during crystallization experiments also revealed the effect of the particle amount on the crystal orientation under shear flow. The application shear flow also affects the nucleating ability, indicating that there is an obvious flow-induced crystallization (FIC) which depends on both the kind of silica and on its composition. Thus, a change from a quasi-simple to a complex rheological behavior is also deduced from the values of the frequency of crossover between the storage and loss moduli. Oxidation induction time (OIT) values show the great improvement of thermal stability of the recycled polypropylene resins by the incorporation of increasing contents of silicas at the same antioxidant concentration.

With increasing demand for sustainable packaging, post-consumer resin (PCR) high-density polyethylene (HDPE) presents challenges due to its variable performance compared to virgin HDPE. This study addresses these challenges by extensively characterising PCR HDPE resins to identify structural and performance features critical for applications in sustainable bottle packaging. Using a dataset of 24 HDPE resins (20 PCR and 4 virgin controls), our approach employs a comprehensive suite of analytical methods—including Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, rheology, colour analysis, and mechanical testing—to develop a feature-rich dataset. Data science techniques such as multivariate analysis are applied to reveal the relationships that underpin PCR quality, allowing for a detailed comparison to virgin HDPE. [1]

This research provides a robust pipeline for assessing and predicting the suitability of new PCRs as substitutes for virgin plastic in packaging. By identifying structure-property relationships, it highlights factors influencing mechanical and aesthetic properties in PCR HDPE, contributing to the advancement of circular plastic use in packaging. These insights support industry efforts to achieve sustainable material cycles by selecting and enhancing PCRs that meet the demands of high-quality applications in a circular economy framework.