Elettra | SYRMEP, IUVS, SISSI-Bio Design of sustainable plastics: a multi-technique characterization of tannin-furanic foams It is now a well-established fact that plastic materials derived from fossil fuels constitute a significant ecological problem. One solution to this problem lies in the research and design of new eco-sustainable materials that can perform the same functions as traditional plastics. Among these, tannin-based foams, particularly tannin-furanic foams, are worth mentioning. A robust scientific literature attributes physical properties such as density, compressive strength, thermal conductivity, and fire resistance to these materials, comparable to those of traditional plastics like polystyrene and polyethylene. Notably, it has been demonstrated that the co-polymerization of condensed mimosa tannin with poly-furfuryl alcohol generates a solid biobased foam, potentially viable for industrial applications. Proper modifications in the synthesis process can potentially be used to tailor the foam properties. To achieve this, it is crucial to understand how specific modifications in the synthesis process affect the final product. To this end, a thorough chemical and morphological characterization of the synthesized foam is necessary. Unlike traditional plastics, the synthesis process yields a heteropolymeric structure consisting of different building blocks interconnected by chemical bonds often arranged randomly. Since the chemical structure significantly influences the material's morphology, a comprehensive characterization that correlates chemical and morphological aspects is essential to guide the synthesis design process. In this work, we employed a multitechnique approach to obtain a comprehensive characterization of tannin-furanic foams produced using different synthesis processes. Specifically, we focused on three tannin-furanic foams, all prepared from condensed mimosa tannin and furfuryl alcohol: a first sample, where bubble production was achieved through mechanical agitation assisted by the presence of polysorbate 80, and two further samples, where bubble growth was supported by a blowing agent, using sulfuric and nitric acid, respectively, to facilitate the co-polymerization process. On one hand, techniques such as x-ray micro-tomography and scanning electron microscopy (SEM) were used to characterize the samples from a morphological point of view. In parallel, spectroscopic techniques such as UV resonant Raman (UVRR), Fouriertransform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance (NMR) measurements were used to characterize the chemical structure of the heteropolymer. Density functional theory (DFT) calculations were also carried out to support the spectral analysis. Additionally, microFTIR was used to provide insights into the chemical heterogeneity in foams obtained from different syntheses. The results were compared with the measured macroscopic structural properties of the foams, such as density, compression resistance, and fire resistance, evaluated through thermogravimetric analysis. This allowed us to identify correlations between these properties, morphology, and chemical constitution of the foams. Our analysis shows that when only mechanical agitation is used, the final foam is denser and more resistant, with smaller pore dimensions compared to those obtained in sulfuric and nitric foams, as reported in Fig. 1 (a). The spectroscopic analysis reveals chemical heterogeneity within the foam due to the significant presence of unreacted Polysorbate 80, as evidenced by the characteristic peaks (2852 cm-1, 2875 cm-1, and 2922 cm-1) in the FTIR spectra of the mechanical foam, reported in Fig. 1 (b). Additionally, this foam has a thermal conductivity comparable to that of wood-fiber insulation boards and possesses high compression resistance compared to other bio-based foams, making it suitable as a building material or core material for sandwich panels. The use of a catalyst agent yields clearly different results depending on whether sulfuric acid or nitric acid is used. In the case of sulfuric acid, chemical analysis reveals that the final foam has a fragmented heteropolymeric structure, characterized by an incomplete cross-linking process. This behavior is reflected in the macroscopic properties of the material, which exhibits significantly lower compressibility compared to the foam obtained through the mechanical process, although the morphological structure and average pore size do not differ significantly from the latter. The synthesis carried out with nitric acid, on the other hand, generates a foam with compressibility at least 3 times higher than that obtained with sulfuric acid, despite the morphological analysis revealing a larger average pore size and comparable density. The analysis of the chemical constitution shows a more complete polymerization process, with good cross-linking of the heteropolymeric structure, facilitated by better polymerization of the furanic component. This behavior is clearly visible in the UVRR spectra of the foams in Fig. 1 (c), where the characteristic peaks of poly-furfuryl alcohol at 967 cm-1 and 1018 cm-1 are more pronounced in the nitric foam compared to the sulfuric one. All these properties make the foam synthesized with nitric acid the best candidate for thermal insulation applications, as a replacement for materials like extruded polystyrene. This work demonstrates how, through a proper comparison between spectroscopic and morphological experimental data, it is possible to achieve a deep morpho-chemical characterization of three differently synthesized tannin-furanic foams. This highlights that the synthesis design process in tannin-furanic foams can be guided to approach the properties of traditional plastics like polystyrene and polyethylene. Elettra Highlights 2024–2025 30
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