Thesis title: Biodegradable food packaging made from recycled cellulose and biobased polymers
Food packaging is the set of products used for the protection of food items, with the purpose of containing, safeguarding, and facilitating their handling and delivery to the consumer. This PhD project aims to develop a new food packaging solution aligned with the principles of a circular economy, aiming to reduce the environmental impact caused by waste from traditional packaging and, consequently, its effects on human health.
The packaging is derived from cellulose used for paper and cardboard, partially or entirely produced from recycled fibers. This material is already employed for contact with certain food products in many European countries. Packaging materials, including recycled paper, must meet a series of basic safety criteria. This means that recycled paper used in food packaging should prevent the potential migration of components that could pose health risks. The innovation focuses on creating advanced polymeric films, derived both from natural sources and from the recycling of additional polymers, intended to coat the inner surface of the recycled packaging, enabling its suitability for food contact applications. In this context, the biobased polymer compounds industry is rapidly growing, driven by economic and environmental concerns related to the constant use of and dependence on non-renewable raw materials, such as crude oil. Biopolymers capable of successfully replacing traditional plastics already exist. However, limitations remain regarding certain properties, such as mechanical and thermal characteristics, which restrict the application of biobased polymers in packaging products.
The possibility of proposing innovative packaging will make it possible to meet the targets set by European directives. In particular, the European Union has approved the law on recycling and reuse of packaging, setting new reuse targets that will be mandatory by 2030 and indicative for 2040. The targets vary depending on the type of packaging used. Starting from 2030, various types of single use plastic packaging will be banned, including packaging for fruits and vegetables weighing less than 1.5 kg, packaging for foods, single portions, and beverages consumed in bars and restaurants, and single use products used in hotels for personal hygiene. The ban will also extend to plastic bags under 15 microns. The regulation aims to reduce the 186 kilograms of packaging waste per capita generated annually in the EU and sets a target to reduce packaging by 5 % by 2030, 10 % by 2035, and 15 % by 2040. All packaging must be minimized in weight and volume and contain a minimum percentage of recycled content.
Poly(urethane acrylate) is an extremely versatile plastic material; for this reason, it is used in various industries, including the food sector. In recent decades, poly(urethane acrylate) (PUA) has been widely employed as an adhesive for textiles and UV coatings due to its unique properties, such as excellent abrasion resistance, adhesion to substrates, light stability, and weather resistance. These characteristics have made it a candidate for the development of innovative polymeric films.
Initially, the synthesis and characterization of the polymer were carried out to meet all the requirements for its use as a food packaging material, thanks to close collaboration with the organic chemistry research group led by Prof. Marcantoni and Prof. Gabrielli at the University of Camerino. Several polyurethane acrylate films were developed to optimize the production cycle for the final large-scale manufacturing of food packaging.
The main component of PUA is urethane acrylate oligomer (UAO), obtained through a two-step polymerization process. In the first step, a polyaddition reaction occurs between a diol, polyethylene glycol (PEG), and a diisocyanate group, isophorone diisocyanate (IPDI), in the presence of a catalyst, to produce a urethane prepolymer with terminal isocyanate groups. In the second step, an end-capping agent, 2-hydroxyethyl methacrylate (HEMA), is added to the prepolymer to obtain the urethane acrylate oligomer. Finally, a photoinitiator (benzophenone) and a co-initiator (methyldiethanolamine) are introduced to enable UV induced photopolymerization. FT-IR analysis was used to monitor the reactions, which was crucial to proceed to the next step and to evaluate the reaction rates. GPC was employed to determine the molecular weight of the polymers, the conversion percentage, and the presence of any residual reagents and solvents.
The feasibility of using a recyclable catalyst was also evaluated. The most used catalysts are tin-based compounds, such as dibutyltin dilaurate (DBLT), but their homogeneous nature makes it impossible to completely remove the catalyst from the polymer matrix. Replacing homogeneous catalysts with heterogeneous ones enables catalyst recovery and reuse. A procedure was developed for synthesizing PUA using a cerium trichloride heptahydrate and sodium iodide system supported on silica, which is inexpensive and non-toxic. The LD50 value of this catalyst is sixteen times higher than that of DBLT. This catalyst can be filtered, washed, vacuum-dried, and reused for seven cycles without any significant reduction in activity. After selecting the best catalytic system, reaction conditions were optimized by varying the amount of catalyst, the isocyanate ratio, and the temperature. The highest molecular weight was obtained after 2 h at 70 °C with 0.1 wt% of CeCl3⋅7H2O–NaI/SiO2 and 1.5 equivalents of IPDI.
The urethane prepolymer obtained was characterized using FT-IR and NMR analysis to confirm its formation. Urethane prepolymers with different molecular weights were investigated to study the effect of molecular weight on UAO formation. It was observed that values exceeding 2000 Da didn’t lead to the formation of UAO. This behavior suggests that increasing the weight average molecular weight (Mw) of the prepolymer makes the NCO-terminated groups less available for further reactions with acrylate compounds. Additionally, attempts to increase the percentage of HEMA resulted in a decrease in the conversion rate.
At this point, attention was focused on the synthesis of bio-based polyurethane. PEG was replaced with oligo(L-lactic acid) (OLLA), synthesized from L-lactic acid, and reacted with IPDI. The data obtained were comparable to those derived from fossil-based polyols. Pursuing the goal of synthesizing a fully biobased urethane prepolymer, OLLA was reacted with L-lysine diisocyanate, and the chemical and thermal properties of the final polymer were compared with those of fossil-based materials.
For the characterization of the polymer film, TGA analysis was performed, which showed two weight losses: one related to the thermal degradation of the hard segments and the other to the soft segments. DSC analysis showed a characteristic thermal behavior of poly(urethane acrylates), and SEM analysis showed that the surface of biobased and fossil-based poly(urethane acrylates) is smooth and homogeneous in which no crack or phase separation are observed, confirming the goodness of polymerization process.
Plastic pollution has become one of the most critical environmental issues. Among polymeric materials, polyethylene terephthalate (PET) has been the most important over recent decades due to its excellent chemical and physical properties for various applications. Its reduction is only possible through chemical and mechanical recycling techniques. For this purpose, a microwave-assisted recycling process for the innovative aminolysis of PET waste has been proposed to synthesize PUA coatings. The first step enabled the complete and efficient depolymerization of PET, forming terephthalamide diols in just one hour. The second step involved the formation of the polymer film through UV radiation polymerization.
The depolymerization approach involves a two-step process, starting with a Henry reaction between nitromethane and organic aldehydes to yield the corresponding β-nitro alcohols, which could then be further reduced to obtain the desired β-hydroxy amines. Yields of up to 95 % were achieved for the first step using only 5 mol% of the catalyst. Regarding reactivity, it was found that linear aliphatic aldehydes were more reactive in this transformation than their branched or aromatic counterparts. After obtaining the β-hydroxy amines, PET aminolysis was carried out using microwave irradiation technology on waste from water bottles. ATR spectra confirmed the success of the depolymerization process and highlighted the importance of the purification step, which eliminates any trace of undecomposed PET.
At this stage, the synthesis of PUA was performed and confirmed by ATR analysis. To study whether the presence of terephthalamide diols could influence PUA properties, a control sample was synthesized using PEG and IPDI, excluding the terephthalamide diol monomer from the polymer structure. GPC analysis revealed that the obtained compound had an Mw value around 2,500 Da. Similar results were observed for bulkier compounds, while others showed a Mw of approximately 3,500 Da, confirming the possibility of increasing the molecular weight of the final prepolymer by modifying the monomer structure.
DSC analysis was used to study the potential microphase separation state of PUA films by evaluating the glass transition temperature (Tg). It was observed that the Tg of the hard segment of all PUA films was above room temperature, indicating that the hard segment portion of these films is in a glassy state, which will influence the resulting mechanical properties. To this end, uniaxial tensile tests were performed in collaboration with the research group in chemical, materials, and environmental engineering led by Prof. Valente at Sapienza University of Rome. These tests showed that a wide range of property combinations could be achieved, governed by multiple factors such as crosslinking density, overall phase-separated morphology, molecular weight, crystallinity, and temperature. Notably, all samples exhibited ductile behavior.
Finally, in collaboration with MS Packaging S.r.l., the elimination of the polymer coupling step to recycled cardboard substrate was evaluated and optimized, testing direct polymerization using a UV lamp. Additionally, the barrier properties of the film for food packaging were tested to ensure impermeability to water, oil, and acidic liquids, which are critical for maintaining food freshness and preventing contamination. Subsequently, polymerization was carried out directly on the recycled cardboard preform to integrate the material's properties into a real packaging solution. This prototype box demonstrated mechanical strength, polymer durability, and the scalability of the technology for diversified packaging applications. Finally, a pilot demonstrator was created to evaluate the applicability of innovative solutions on an industrial scale. The introduction of this demonstrator represents a crucial step toward validating the new approach, enabling practical analysis and process optimization before its eventual large-scale implementation. This first pilot demonstrator not only confirms the validity of the developed technology but also opens new pathways for future innovations in sustainable and high-performance packaging.