Layer-by-Layer Assembly of Efficient Flame Retardant Coatings Based On High Aspect Ratio Graphene Oxide and Chitosan Capable of Preventing Ignition of PU Foam

The layer-by-layer (LbL) technique is adopted for the construction of multilayers encompassing chitosan and graphene oxide (GO) platelets capable of improving the flame retardant properties open cell PU foams. The LbL assembly follows a linear growth regime as evaluated by infrared spectroscopy and yields a multilayer structure where GO platelets are embedded within a chitosan continuous matrix. 3 and 6 bi-layers (BL) can efficiently coat the complex 3D structure of the foam and substantially improve its flame retardant properties. 3BL only add 10% to the original mass and can suppress the melt dripping during flammability and reduce both the peak of heat release rate by 54% and the total smoke released by 59% in forced combustion tests. Unprecedented among other LbL assemblies employed for FR purposes, the deposition 6BL is capable of slowing down the release of combustible volatile to the limits of non-ignitability thus preventing ignition in half of the specimens during cone calorimetry tests. This has been ascribed to the formation of a protective coating where the thermally stable char produced by chitosan serves as a continuous matrix embedding GO platelets, which control volatile release while mechanically sustaining the PU foam structure

La "misteriosa" scrittura grande: paleografia e storia

Nel 1952 Jan Olof Tjäder riusciva finalmente a decifrare la cosiddetta misteriosa scrittura grande che caratterizza la prima riga di quattro papiri ravennati contenenti protocolli dei gesta municipalia (504-625 d. C.) e a individuarne la base grafica nella corsiva romana antica. Questo permetteva quindi al Tjäder di dimostrare che l’antica scrittura comune romana non era sopravvissuta, dopo il IV secolo, solo nelle litterae caelestes riservate ai documenti imperiali (cfr. Th.9.19.3, 367 d. C.), ma che ad esse si affiancavano anche le scritture iniziali di alcuni papiri egiziani contenenti processi verbali d’udienza e, appunto, quella dei protocolli dei gesta ravennati. Il presente contributo si propone dunque una verifica della teoria del Tjäder circa il ruolo che la misteriosa scrittura grande ravennate ha avuto nella storia della scrittura latina, soprattutto alla luce delle più recenti edizioni che hanno reso accessibile un sempre maggior numero di fonti per i secoli III-VII. Quindi si è tentato di riconnettere il dato grafico che emerge dai gesta ravennati ad un quadro storico-istituzionale più ampio, in particolare attraverso il confronto tra le fonti documentarie, le fonti legislative (relativamente a quegli interventi normativi che interessano direttamente la scrittura dei documenti pubblici) e la storiografia di carattere paleografico

Effects of Graphite Oxide Nanoparticle Size on the Functional Properties of Layer-by-Layer Coated Flexible Foams

The exploitation of self-assembled coatings comprising graphite oxide (GO) nanoplates has been recently demonstrated as a promising route to improve the fire safety of flexible polyurethane (PU) foams. However, limited knowledge has been gathered on the correlations between the physical and chemical properties of different GO grades and the performance obtained in this application. This work addresses the effects of the nanoparticle dimensions on the layer-by-layer (LbL) assembly and flame-retardant properties of GO-based coatings deposited on PU foams. To this aim, three GO bearing different lateral sizes and thicknesses were selected and LbL-assembled with chitosan (CHIT). Coating growth and morphology were evaluated by FTIR and FESEM, respectively. The resulting CHIT/GO assemblies were demonstrated to be capable of slowing down the combustion of the PU both in flammability and forced combustion tests. In addition, compressive stress/strain tests pointed out that the LbL-coated foams (22–24 kg/m3) could easily replace denser commercial PU foam (40–50 kg/m3) with weight reduction potentials in the transport field. These results are correlated with the properties of the employed GO. The production of assemblies characterized by a high density of CHIT/GO interfaces is identified as the main parameter controlling the FR efficiency and the mechanical properties of the coatings

Graphene Oxide Exoskeleton to Produce Self‐Extinguishing, Nonignitable, and Flame Resistant Flexible Foams: A Mechanically Tough Alternative to Inorganic Aerogels

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Polyelectrolytes Enabled Reduced Graphite Oxide Water Dispersions: Effects of the Structure, Molecular Weight, and Charge Density

The polyelectrolyte (PE)-based water dispersion of graphene-related materials (GRMs) represents an interesting intermediate for the development of advanced materials by sustainable processes. Although the proof of concept has been demonstrated, there is a lack of knowledge for what concerns the effects of parameters typical of PEs such as functionalization, molecular weight, and charge density. In this work, we evaluate the effects of such parameters on the quality and long-term stability of reduced graphite oxide (rGO) dispersion in aqueous media prepared by ultrasound sonication in the presence of different PEs. Four PEs were evaluated: polyacrylic acid (PAA), branched poly(ethylenimine) (BPEI), sodium carboxymethyl cellulose (CMC), and poly(sodium 4-styrenesulfonic acid) (PSS). The prepared dispersions were thoroughly characterized by means of UV-visible spectroscopy, thermogravimetric analysis, dynamic light scattering, and Raman spectroscopy. The highest concentrations of rGO were achieved by BPEI with a molecular weight of 25,000 and 270,000 Da (33 and 26 mu g/mL, respectively). For other PEs, the rGO concentration was found to be independent of the molecular weight. The PAA-based dispersions displayed the best through-time stability while yielding homogeneous dispersion with a smaller average size and narrower size distribution

Facile and Low Environmental Impact Approach to Prepare Thermally Conductive Nanocomposites Based on Polylactide and Graphite Nanoplatelets

In this work, the preparation of nanocomposites based on poly(l-lactide) PLLA and graphite nanoplatelets (GNP) was assessed by applying, for the first time, the reactive extrusion (REX) polymerization approach, which is considered a low environmental impact method to prepare polymer systems and which allows an easy scalability. In particular, ad hoc synthesized molecules, constituted by a pyrene end group and a poly(d-lactide) (PDLA) chain (Pyr-d), capable of interacting with the surface of GNP layers as well as forming stereoblocks during the ring-opening polymerization (ROP) of l-lactide, were used. The nanocomposites were synthesized by adding to l-lactide the GNP/initiator system, prepared by dispersing the graphite in the acetone/Pyr-d solution, which was dried after the sonication process. DSC and X-ray diffraction measurements evidenced the stereocomplexation of the systems synthesized by using the pyrene-based initiators, whose extent turned out to depend on the PDLA chain length. All the prepared nanocomposites, including those synthesized starting from a classical initiator, that is, 1-dodecanol, retained similar electrical conductivity, whereas the thermal conductivity was found to increase in the stereocomplexed samples. Preferential localization of stereocomplexed PLA close to the interface with GNP was demonstrated by scanning probe microscopy (SPM) techniques, supporting an important role of local crystallinity in the thermal conductivity of the nanocomposites

Cellulose nanocrystals polyelectrolyte complexes as flame retardant treatment for cotton fabrics

In this work, polyelectrolyte complexes (PECs) are employed as an efficient way for the deposition of functional flame retardant coatings based on cellulose nanocrystals (CNCs). To this aim, CNCs have been combined with branched polyethyleneimine (BPEI) obtaining gel-like PECs to be deposited on cotton by an easy doctor-blading approach. The morphology of the coated fabrics was investigated by scanning electron microscopy. The thermal stability was evaluated by thermogravimetric analyses while the achieved flame retardant properties were assessed by horizontal flammability tests. The deposition of the CNCs/BPEI PECs produces a homogeneous coating capable of self-extinguishing the flame with only 8 % of weight added to the fabric. Post combustion residue investigations highlighted how these CNCs/BPEI PECs can produce a swelled charred barrier consisting of polyaromatic structures embedded within an amorphous carbon. The results reported in this paper open up to a practical and industrially viable strategy for the exploitation of CNCs in the field of flame retardant coatings

Recyclable nanocomposites of well-dispersed 2D layered silicates in cellulose nanofibril (CNF) matrix

Nanocomposites based on components from nature, which can be recycled are of great interest in new materials for sustainable development. The range of properties of nacre-inspired hybrids of 1D cellulose and 2D clay platelets are investigated in nanocomposites with improved nanoparticle dispersion in the starting hydrocolloid mixture. Films with a wide range of compositions are prepared by capillary force assisted physical assembly (vacuum-assisted filtration) of TEMPO-oxidized cellulose nanofibers (TOCN) reinforced by exfoliated nanoclays of three different aspect ratios: saponite, montmorillonite and mica. X-ray diffraction and transmission electron micrographs show almost monolayer dispersion of saponite and montmorillonite and high orientation parallel to the film surface. Films exhibit ultimate strength up to 573 MPa. Young's modulus exceeds 38 GPa even at high MTM contents (40–80 vol%). Optical transmittance, UV-shielding, thermal shielding and fire-retardant prop- erties are measured, found to be very good and are sensitive to the 2D nanoplatelet dispersion

Layer-by-Layer-Coated Cellulose Fibers Enable the Production of Porous, Flame-Retardant, and Lightweight Materials

New sustainable materialsproduced by green processing routes arerequired in order to meet the concepts of circular economy. The replacementof insulating materials comprising flammable synthetic polymers bybio-based materials represents a potential opportunity to achievethis task. In this paper, low-density and flame-retardant (FR) porousfiber networks are prepared by assembling Layer-by-Layer (LbL)-functionalizedcellulose fibers by means of freeze-drying. The LbL coating, encompassingchitosan and sodium hexametaphosphate, enables the formation of aself-sustained porous structure by enhancing fiber-fiber interactionsduring the freeze-drying process. Fiber networks prepared from 3 Bi-Layer(BL)-coated fibers contain 80% wt of cellulose and can easily self-extinguishthe flame during flammability tests in vertical configuration whiledisplaying extremely low combustion rates in forced combustion tests.Smoke release is 1 order of magnitude lower than that of commerciallyavailable polyurethane foams. Such high FR efficiency is ascribedto the homogeneity of the deposited assembly, which produces a protectiveexoskeleton at the air/cellulose interface. The results reported inthis paper represent an excellent opportunity for the developmentof fire-safe materials, encompassing natural components where sustainabilityand performance are maximized

Fire-retardant and transparent wood biocomposite based on commercial thermoset

Transparent wood (TW) biocomposites combine high optical transmittance and good mechanical properties and can contribute to sustainable development. The safety against fire is important for building applications. Here, a “green” bleached wood reinforcement is impregnated by water soluble and flame-retardant melamine formaldehyde (MF) in a scalable process, for a wood content of 25 vol%. FE-SEM is used for characterization of optical defects and EDX to examine MF distribution at nanoscale cell wall pore space. Curing (FTIR-ATR), mechanical properties, optical transmittance (74% at 1.2 mm thickness) and flame-retardant properties are also characterized (self-extinguishing behavior and cone calorimetry), and scattering mechanisms are discussed. The fire growth rate of transparent wood was less than half the values for neat wood. Transparent wood/MF biocomposites show interesting wood-MF synergies and are of practical interest in building applications. Critical aspects of processing are analyzed for minimization of optical defect