The use of water-based polyelectrolyte complexes (PECs) is a promising flame retardant (FR) approach capable of delivering functional coatings with an industrially viable number of deposition steps. Here cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) have been used as building block for the preparation of PECs capable of conferring flame retardant properties to cotton fabrics and flexible polyurethane foams (PU). Cotton fabrics were directly coated with a CNC-based complex by a simple doctor blade approach [1]. Scanning Electron Microscopy (SEM) micrographs showed the presence of a dense and compact layer on the fabric texture. This coating easily self-extinguished the flame during flammability tests. The same properties have been maintained even after a washing step thus demonstrating the intrinsic good durability of the coating in these conditions. Flame retarded PU were prepared by single step deposition of a gelatin/phytic acid/CNF PECs. Different CNF concentration have been investigated. SEM observation pointed out the formation of a continuous and homogeneous coating only at low CNF concentration (< 0.25 wt.-%). Flammability test in horizontal configuration showed a self-extinguishing behavior within few seconds after the removal of the blue methane flame. Interestingly, samples coated by PECs with 0.1%wt CNF showed the deposition of a more homogenous coating that resulted in a non-igniting behavior (i.e. total absence of flame spread after 6 s of flame application). In addition, the use of Layer-by-Layer assembled PECs as tool enabling the production of light-weight materials encompassing natural fibers is also presented [2]. Chitosan and sodium polyphosphate have been selected as coating constituents. When applied to the cellulose fibers this assembly produced continuous and homogeneous coatings with only 1,2 and 3 deposited bi-layers (BL). Self-standing 3D structures based only on the coated fibers were achieved by means of freeze- drying. SEM observations pointed out the formation of a porous fiber network where the deposited Layer-by-Layer (LbL) coating is acting as a glue conferring structural integrity. The cellulose fiber content ranged between 80-90 wt.-% highlighting the potentialities of the proposed approach in delivering lightweight porous structure with extremely high fiber content. Flammability tests in horizontal configuration evidenced a self-extinguishing behavior for foams prepared from 1BL coated fibers. At 3BL the foams showed a non- igniting behavior where no flame persisted on the sample after the removal of the methane flame. The same 3BL foams also self- extinguished the flame during flammability tests in vertical configuration. By cone calorimetry the prepared foams showed very low Heat Release Rate, HRR, values (pkHRR < 100 kW/m 2 ) as well as extremely limited smoke production (Total Smoke Release, TSR < 50 m 2 /m 2 ). These results clearly show the FR potentialities of the prepared foams especially if compared with commercially available petroleum-based foams, such as PU, that normally display higher HRR and TSR values under the same testing conditions. The achieved results clearly demonstrate the versatility of PECs in delivering functional materials where flame retardancy and sustainability are optimized.

References

1. L. Maddalena, J.M. Indias, P. Bettotti, M. Scarpa, F. Carosio Polym. Degrad. Stab. 2024, 220, 110646.
2. M. Marcioni, M. Zhao, L. Maddalena, T. Pettersson, R. Avolio, R. Castaldo, L. Wågberg, F. Carosio ACS Appl. Mater. Interfaces 2023, 15, 36811

Acknowledgments The financial support from Italian Ministry of University (MUR) call PRIN 2017 with the project 2017LEPH3M “PANACEA” is acknowledged. The author acknowledges financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 1409 published on 14.9.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title Green cellulose bAsed fire safe and lightweight Insulating mAterials (GAIA) – CUP project (CUP: E53D23017860001- Grant Assignment Decree No. 1389 adopted on 01/09/2023 by the Italian Ministry of Ministry of University and Research (MUR).