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Reversibly photocrosslinked BIO-based composites with barrier properties from industrial by-products

Periodic Reporting for period 1 - ComBIOsites (Reversibly photocrosslinked BIO-based composites with barrier properties from industrial by-products)

Reporting period: 2018-08-01 to 2020-08-31

Plastic waste accumulation is an environmental emergency, as most plastics are not biodegradable; they mostly come from fossil sources, whose exploitation and depletion are also a reason for concern. Packaging is the market sector with the highest demand of plastics in EU. The recycling rate of 39.5% being far away from the target set by the European Commission, 55% by 2025, the packaging and consumer goods industries seek solutions for reducing its environmental impact. Packaging is now one of the largest application fields of bioplastics, i.e. plastics that are biobased and/or biodegradable. The challenge of a sustainable and recyclable gas barrier packaging material still needs to be addressed; currently, oil-based plastic foils or multilayer films that are single-use, neither recyclable nor biodegradable, are mostly used.
ComBIOsites aims at developing recyclable composite materials for flexible packaging, using biobased raw materials and environmentally friendly processes, such as photopolymerization. Cellulose is the most abundant biopolymer on Earth. Nanocellulose (NC) is derived from cellulosic biomass through top-down approaches. NC films are good barriers against gases as oxygen and carbon dioxide, thus appealing for packaging applications. However, their hydrophilicity prevents their use in humid environments. It is therefore convenient to embed NC in crosslinked polymeric matrices, as in traditional composites, guaranteeing high mechanical performances and water/solvent resistance. To prepare the polymeric matrix a chemical process is required. Photocrosslinking is a green process, solvent-free, performed at room temperature and very fast, thus with low energy consumption, so attractive for industrial use. Crosslinking though can hinder recyclability. To tackle this issue, we combine NC with biobased prepolymers that we functionalize with a reversibly photocrosslinkable group, able to ensure the curing of the polymer upon irradiation at a given wavelength, and allow its dismantling at a different wavelength; the reversibility of the curing reaction opens up the possibility of recycling the composite at the end of its life cycle.
The research activity concerned the development of biocomposites combining NC with a reversibly photocrosslinkable biobased polymer.
NC was extracted from cellulosic biomass consisting of disordered hemp bast fibers, byproduct of hemp grown for the production of seeds and not suitable for production of textiles or high-performance composites. In order to have a greener process, bleaching of the fibers, traditionally involving the use of chlorinated products, was not performed. NC was characterized by spectroscopic techniques and electron microscopy to assess fiber size and size distribution, crystallinity, composition. Films were obtained from NC either by filtration or by solvent exchange, and their permeability and mechanical properties were evaluated (See Fig. 1 A). This part of the work was carried out in collaboration with CNR-STIIMA (Biella, IT) and the LGP2 group at INPG - Pagora (Grenoble, FR).
In parallel, as a prework to select suitable monomers and processes, epoxidized and methacrylated biobased monomers (Fig. 1 B) were used as matrices for biobased composites with commercial or in home produced hemp NC, prepared by solvent exchange or direct mixing, and subsequent photocuring. Several exchange students were involved in this work within Erasmus+ projects.
Selected monomers were copolymerized with coumarin derivatives, to obtain waterborne copolymers with different contents of coumarin moieties (see Fig 1 C), in collaboration with the Institut Charles Gerhardt of the Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM, France). Eventually, a copolymer that can be crosslinked upon exposure to UVA light, and shows, upon irradiation with UVC, a degree of reversibility of the crosslinking that depends on the previous UVA exposure time and intensity was obtained. The reversible photocrosslinking was studied by UV-Vis spectroscopy; thermogravimetric analysis and extraction/solubility tests were employed to compare the polymers before and after being irradiated.
Composites were obtained from hemp NC or commercial microfibrillated cellulose and the reversibly crosslinkable copolymers (see Fig 1 D) by direct mixing in water suspensions. The reversible crosslinking within the composites was evaluated by UV-Vis spectroscopy. Their thermal properties and permeability to oxygen and water vapour were evaluated before and after irradiation, in collaboration with the LPAC laboratory at EPFL in Switzerland.
Finally, a model was developed for the prediction of diffusivity of gases in composites with different volume fractions, aspect ratios and orientation of cellulosic fibers.
Design of Experiments for optimization of the materials, and modelling work are underway in the framework of Master thesis projects at Politecnico di Torino.
The work carried out within this project led to the publication of three open access papers, and three more are planned. Furthermore, the project ComBIOsites and its results were presented with posters at two scientific conference, and two conferences addressed to a mixed industrial and scientific audience. One oral presentation has been accepted foran upcoming scientific conference.
In the framework of the project, the fellow had the opportunity to be co-supervisor of five master thesis projects, and of four internships of international students, from Bachelor to PhD level. In addition, the fellow was invited to present the results of the project within two seminars addressed to master students, PhD fellows and researchers (at the University of Parma, Italy and at EPFL, CH) and to share her experience with prospective applicants at the MSCA-IF Infoday at Politecnico di Torino. Participation to the European Researcher's Night, in Milan (Italy) in 2019 and online in 2020, allowed to present the project at the general public, and two outreach communications were published through the online channels of Politecnico di Torino (website, twitter, facebook, online magazine).
The results obtained contribute to the effort of providing sustainable alternatives to oil-based and non-recyclable plastic and composite materials. This was done through a number of actions, leading to the following achievements:
1) synthesis of novel biobased reversibly crosslinkable polymers, previously only demonstrated for oil-based polymers.
2) valorization, as a source of NC, of biomass composed of non-aligned hemp bast fibers through an original chlorine-free process.
3) combination of the biobased polymers and nanofillers to obtain reversibly photocrosslinkable biobased composites, through a solvent-free process.
4) use of a photoinitiator-free reversible photocrosslinking process: this is an additional advantage in food packaging as residual photoinitiator can potentially leach out from the material.
5) photocrosslinking reversibility (although with a non-quantitative conversion), which is promising in view of recycling the composites.
Potential impacts are:
1) the exploitation of non-edible biomass instead of oil-based components for the production of composites
2) the development of packaging made of these composites, that can lead to the reduction of non-degradable and non-recyclable plastic waste.
Figure 1