Tuning the Nanoscale Properties of Phosphorylated Cellulose Nanofibril-Based Thin Films To Achieve Highly Fire-Protecting Coatings for Flammable Solid Materials

A biomimetic closed-loop recyclable, long-term durable, extreme-condition resistant, flame-retardant nanocoating synthesized by reversible flocculation assembly

Flame-retardant coatings have attracted increasing attention in mitigating the fire threat of flammable polymer materials. Their durable application inevitably provides high resistance to various complex environments, however, discarded stable materials will turn into another man-made waste disaster. The paradigm shift toward a sustainable future is to combine durability and recyclability of coatings. Herein, we demonstrate a biomimetic coating that reversibly captures active flame-retardant nanomaterials by flocculation assembly using anionic polyacrylamide covering the polyurethane foam surface. Strong hydrogen bonding and microstructural interlocking provide the coating with high durability under complex harsh conditions (underwater, chemical exposure, hydrothermal aging, long-term external extrusion, etc.). Meanwhile, the disassembly/reorganization of the coating can be easily repeated in response to pH stimulation with a recycling rate of 97%. The experiments and theoretical calculations reveal the mechanism of the reversible flocculation assembly. This biomimetic strategy of responsive flocculation assembly opens the way for functional coatings with integrated durability and recyclability.

Composite Films of Nanofibrillated Cellulose with Sepiolite: Effect of Preparation Strategy

Cellulose nanofibrils (CNFs) are nanomaterials with promising properties to be used in food packaging and printed electronics, thus being logical substitutes to petroleum-based polymers, specifically plastics. CNFs can be combined with other materials, such as clay minerals, to form composites, which are environmentally friendly materials, with acceptable costs and without compromising the final properties of the composite material. To produce composite films, two strategies can be used: solvent casting and filtration followed by hot pressing. The first approach is the simplest way to produce films, but the obtained films may present some limitations. In the present work, CNFs produced using enzymatic or TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation pretreatments, followed by high-pressure homogenization, or only by mechanical treatment (homogenization), were used to produce films by both the available procedures. The films obtained by filtration + hot pressing presented higher tensile strength and Young’s modulus compared with those obtained by solvent casting. In general, a decrease in the values of these mechanical properties of the films and a decrease in elongation at break, with the addition of sepiolite, were also observed. However, for the TEMPO CNF-based films, an improvement in tensile strength could be observed for 10% of the sepiolite content. Furthermore, the time necessary to produce films was largely reduced by employing the filtration procedure. Finally, the water vapour barrier properties of the films obtained by filtration are comparable to the literature values of net CNF films. Thus, this technique demonstrates to be the most suitable to produce CNF-based composite films in a fast way and with improved mechanical properties and suitable gas barrier properties.

A highly efficient chemical approach to producing green phosphorylated cellulosic macromolecules

The introduction of phosphate groups into cellulosic fibers allows for the tuning of their fire resistance, chelating and metal-adhesion properties, enabling the development of flame-retardant adhesive and adsorbent materials. Toward that end, the major challenge is developing a novel efficient and environmentally friendly phosphorylation route that offers an alternative to existing methods, which can achieve the targeted properties. For this purpose, cellulosic fibers were chemically modified herein using solid-state phosphorylation with phosphoric acid and urea without causing substantial damage to the fibers. The morphological, physicochemical, structural and thermal characterisations were examined using FQA, SEM, EDX, FTIR, ¹³C/³¹P NMR, conductometric titration, zeta potential measurement and thermogravimetric analysis. All the characterisations converge towards a crosslinked polyanion structure, with about 20 wt% grafted phosphates, a nitrogen content of about 5 wt% and a very high charge density of 6608 mmol kg⁻¹. Phosphate groups are linked to cellulose through a P–O–C bond in the form of orthophosphate and pyrophosphates. Furthermore, thermal properties of the phosphorylated cellulosic fibers were investigated and a new degradation mechanism was proposed.

The introduction of phosphate groups into cellulosic fibers allows for the tuning of their fire resistance, chelating and metal-adhesion properties, enabling the development of flame-retardant adhesive and adsorbent materials.

A facile method for the phosphorylation of cellulosic fabric via atmospheric pressure plasma

Green chemistry approach for phosphorylation of cellulose, under atmospheric pressure plasma was investigated and compared with conventional thermal method. The attachment of the phosphate groups was evaluated by ³¹P and ¹³C solid state NMR spectroscopy and XPS. The thermal method led to the formation of monophosphate of cellulose along with a side product of polymerized phosphate, whereas the plasma method produced only the monophosphate, without any side products. Unlike with the thermal treatment, the appearance and the mechanical properties of the viscose fabric remained nearly same after the plasma treatment. Also, the dyeability of the plasma modified fabric remained unchanged, whereas it decreased significantly in the thermally modified fabric. The amount of phosphate quantified by phosphomolybdate assay was found to be 2.88 ± 0.06 and 4.09 ± 0.19 % in the plasma and the thermal methods, respectively. This method has the potential to replace the existing methods of phosphorylation of cellulose.

Composites of nanofibrillated cellulose with clay minerals: A review

Biopolymers-based composites are, in general, environmentally friendly materials, which can be obtained from renewable sources. Some of them can also present promising properties to be used in food packaging and electronic devices, being thus logical substitutes to petroleum-based polymers, specifically plastics. Cellulose nanofibrils (CNF) obtained by chemical/enzymatic pre-treatments followed by a mechanical treatment appear as a new suitable biomaterial. However, CNF are still quite expensive materials, due to the required chemicals/equipment/energy involved, and additionally, they present some limitations such as high hydrophilicity/high water vapour permeability. The combination of CNF with clay minerals, such as montmorillonite or kaolinite, as widely available geo-resources, represents an excellent way to reduce the amount of CNF used, enabling the production of valuable materials and reducing costs; and, at the same time it is possible to improve the characteristics of the formed materials, such as mechanical, gas barrier and fire retardancy properties, if appropriate conditions of preparation are used. Nevertheless, to obtain hybrid CNF/clay composites with superior properties it is necessary to ensure a good dispersion of the inorganic material in the CNF suspension and a good compatibility among the inorganic and organic components. To fulfil this goal, several strategies can be considered, including physical treatments of the suspensions, CNF and clay surface chemical modifications, and the use of a coupling agent. In this review article, the state-of-the-art on a new emerging generation of composites (films, foams or coatings) based on nanofibrillated cellulose and nanoclay, with focus on strategies for their preparation and most relevant achievements is critically reviewed, bearing in mind their potential application as substitutes for common plastics. A third component has been eventually added to these organic-inorganic hybrids, e.g., chitosan, carboxymethylcellulose, borate or epoxy resin, to enhance specific characteristics of the material. Some general background on the production of different types of CNF and their main properties is previously provided.