Recycling alginate composites for thermal insulation

Thermal insulation materials represent one the most straightforward, yet effective, technologies for improving the energy efficiency of buildings (and not only) – one of the key strategies for reducing carbon emissions. Natural-based materials and downcycled industrial and agricultural waste, thanks to their potentially reduced environmental footprint, have already made their way up to the market with the aim of limiting the ever-growing waste stream generated by the industrial sector. Research efforts on the topic are currently mainly focused on developing new insulation solutions, in which waste is reconverted as a new valuableresource. Carbohydrates, such as alginate, cellulose or chitosanare currently extensively studied base materials for thermal insulation systems, in the form of aerogels or as low-impact binding agents in waste-filled panels. Unfortunately, little or no attention has been paid to the end-of-life fate of these recycled materials; disposal (or incineration) still represents the only available option. This unprofitable scenario is even more critical in the case of polysaccharide-based composites specifically developed to reuse industrial waste. 
This was the starting point of our work, mainly conducted at the laboratories of the Engineering and Architecture department of the University of Trieste, in collaboration with TomoLab at Elettra. We developed arecycling process for an alginate-based thermal insulation foam, in which the original material is fully recovered and the thermal and acoustic insulation performances are maintained. The original foam is produced via a patented process in which alginate is used as the host poly-anionic matrix for industrial fiberglass waste. Upon addition of a cation (calcium, obtained from CaCO₃ upon acidification with d-glucono-δ-lactone, GDL), the alginate is capable of forming a gel via ionotropic gelation due to electrostatic interaction with its negatively charged backbone. The dried foam is finally obtained by freeze-drying. The recycling process starts with the addition of a water solution of a chelator (Ethylenediaminetetraacetic acid disodium salt, EDTA-2Na) which possesses a higher affinity toward the cation. This results in the cation chelation and therefore in the disassembling of the material’s alginate matrix (original Alginate Foam, oAF); a homogeneous solution is thus obtained. Upon deactivation of the chelator via acidification, the cations become again available to crosslink the polymer. In particular, four different strategies were explored to reobtain alginate gelation: addition of CaCO₃ and GDL (rAF – CG); addition of GDL (rAF – G); addition of CaCO₃, GDL and HCl, at 75 °C (rAF – CG – A); addition of GDL and HCl 6, at 75 °C (rAF – G – A). With a subsequent freeze-drying step, the recycled foam is re-obtained. As a proof of concept, we have used alginate as the polyanion; however, in principle, any composite material based on a matrix made via ionotropic gelation can be recycled with this process. Due to the presence of the chelating agent directly in the recycled material, multiple recycling cycles can be performed, simply by activating it in a basic aqueous environment.
 

Figure 1.  – SEM and μCT images of: (A) (a) oCAF; (B) (b) rCAF–GC–A; (C) (c) rCAF–GC; (D) (d) rCAF–G–A; (E) (e) rAF–G. SEM scale bar equal to 500 μm, μCT scale bar equal to 2 mm.

This work demonstrates the feasibility of producing natural-based, effective thermal insulators that are fully recyclable. This simultaneously tackle the issue of energy efficiency and that of a sustainable sourcing for these materials, thanks to a cradle-to-cradle approach and in the spirit of the circular economy.
We developed arecycling process for an alginate-based thermal insulation foam, in which the original material is fully recovered (Figure 1) and the thermal and acoustic insulation performances are maintained (Figure 2). The original foam is produced with a patented process in which alginate is used as the host poly-anionic matrix for industrial fiberglass waste. Upon addition of a cation (calcium), the alginate is capable of forming a gel via ionotropic gelation due to electrostatic interaction with its negatively charged backbone. The dried foam is finally obtained by freeze-drying. The recycling process start with the addition of a water solution of a chelator (Ethylenediaminetetraacetic acid disodium salt, EDTA-2Na) which possess a higher affinity toward the cation. This results in the cation chelation in the disassembling of the material alginate matrix; a homogeneous solution is thus obtained. Upon deactivation of the chelator via acidification, the cations become available to crosslink the polymer. With a subsequent freeze-drying step, the recycled foam is re-obtained. As a proof of concept, we have used alginate as the polyanion; however, in principle, any composite material based on a matrix made via ionotropic gelation can be recycled with this process. 

Figure 2.  – Thermal conductivity of oCAF, rCAF. (a) average thermal conductivity values are presented, with the corresponding average absolute error. (b) Averaged sound absorption coefficients of oAF and rAF samples, mean absolute deviation for each data-point was calculated. Absorption curve of Rock-Wool has been added as reference from literature [D’Amore et al. 2017, J Clean Prod 165, 1306-1315]. Rock-Wool sample thickness: 10 mm.

Figures are reprinted from Carbohydrate Polymers, 251, Matteo Cibinel, Giorgia Pugliese, Davide Porrelli, Lucia Marsich, Vanni Lughi, Recycling alginate composites for thermal insulation, 116995, Copyright 2021, with permission from Elsevier.

 

This research was conducted by the following research team:

Matteo Cibinel¹, Giorgia Pugliese¹, Davide Porrelli², Lucia Marsich¹, Vanni Lughi¹
 

¹ Department of Engineering and Architecture, University of Trieste, Via Valerio 6/1, 34127, Trieste, Italy
² Department of Medicine, Surgery and Health Sciences, University of Trieste, Piazza dell’Ospitale 1, 34125, Italy

Contact persons:

Vanni Lughi, email: vanni.lughi@dia.units.it
Matteo Cibinel, email: matteo.cibinel@hotmail.it