Flame-Retardant Paper from Wood Fibers Functionalized via Layer-by-Layer Assembly

Layer-by-Layer Assembled Graphene Oxide on Carbon Fiber toward Phosphate Bonded Coatings with Excellent Interfacial Bond and Tribological Performance

To enhance the interfacial property, carbon fiber (CF) was modified with graphene oxide (GO) using a layer-by-layer self-assembly method and subsequently incorporated into phosphate bonded coatings as a reinforcement. CF modified with GO (CF-GO) was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractometer, Raman spectroscopy, and thermogravimetric analysis. Additionally, the tribological behavior of phosphate bonded coatings with CF-GO was investigated. The results show that GO is grafted onto the CF surface through electrostatic interactions. Besides, the CF surface becomes rougher due to the modification of GO, leading to a stronger interfacial bond between CF and the coating. Notably, as the content of CF-GO increases, both the friction coefficient and the wear rate of the coating decrease. CF-GO can form a lubricant film on the worn surface, which leads to a decrease in the friction coefficient and wear rate. Moreover, in CF-GO, CF assumes the role of a tree trunk, while GO functions as branches, collaboratively bridging cracks, as well as altering and impeding crack propagation pathways, which can consume the fracture energy and improve the cohesive strength of the coating, further contributing to a lower wear rate. Specifically, the coating with 15 wt % CF-GO exhibits a 34% reduction in the friction coefficient and a 58% decrease in the wear rate compared to those of the coating without CF-GO. These findings highlight the significant potential of CF-GO in enhancing the tribological properties of phosphate bonded coatings, making them more durable for antiwear applications.

Surface coated ZnO powder as flame retardant for wood: A short communication

In the present study, the ability of a coating of zinc oxide (ZnO) powder to improve the fire-safety of wood exposed to radiative heat flux was examined, focusing on the ignition time of the wood. To test ZnO's efficiency on the wood substrate, two different amounts of ZnO (0.5 and 1 g ZnO per dm2) were applied to the wood surface and exposed to radiative heat from a cone calorimeter wherein a pristine piece of wood with no ZnO treatment was taken as control. The experiments were conducted at three different irradiation levels i.e., 20, 35, and 50 kWm-2. The results showed that applying ZnO on the surface of the wood significantly increased the ignition time (TTI). For the three different heat fluxes, using 0.5 g ZnO per dm2 coating on the wood surface increased the TTI by 26-33 %. Furthermore, the application of 1 g of ZnO per dm2 generated a TTI increment of 37-40 %. All three irradiation levels showed similar trends in TTI. The micrographs taken before and after combustion showed no significant disparity in the morphology of ZnO. The agglomerated ZnO particles on the wood surface remained intact after combustion. This study demonstrates a facile method of using ZnO to delay the ignition of wood. This could potentially impart fire-safety to wooden structures/façades in wildland-urban interfaces and elsewhere by reducing flame spread.

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

New sustainable materials produced by green processing routes are required in order to meet the concepts of circular economy. The replacement of insulating materials comprising flammable synthetic polymers by bio-based materials represents a potential opportunity to achieve this task. In this paper, low-density and flame-retardant (FR) porous fiber networks are prepared by assembling Layer-by-Layer (LbL)-functionalized cellulose fibers by means of freeze-drying. The LbL coating, encompassing chitosan and sodium hexametaphosphate, enables the formation of a self-sustained porous structure by enhancing fiber-fiber interactions during the freeze-drying process. Fiber networks prepared from 3 Bi-Layer (BL)-coated fibers contain 80% wt of cellulose and can easily self-extinguish the flame during flammability tests in vertical configuration while displaying extremely low combustion rates in forced combustion tests. Smoke release is 1 order of magnitude lower than that of commercially available polyurethane foams. Such high FR efficiency is ascribed to the homogeneity of the deposited assembly, which produces a protective exoskeleton at the air/cellulose interface. The results reported in this paper represent an excellent opportunity for the development of fire-safe materials, encompassing natural components where sustainability and performance are maximized.

Multifunctional Flame-Retardant, Thermal Insulation, and Antimicrobial Wood-Based Composites

Wood has been used in a variety of applications in our daily lives and military industry. Nevertheless, its flammability causes potential fire risks and hazards. Improving the flame retardancy of wood is a challenging task. Herein, a phytic acid-based flame retardant (referred to as AMPA) was synthesized based on supramolecular reactions between melamine and p-amino-benzene sulfonic acid followed by a reaction with phytic acid using deionized water as the solvent. A composite wood was prepared by removing lignin to tailor the unique mesoporous structure of the material, followed by coating AMPA on the surfaces of wood microchannels. The limiting oxygen index of wood has been improved to 52.5% with the addition of 5.6 wt % AMPA. The peak heat release rate for the prepared composite wood was reduced by 81% compared to that for delignified wood, which demonstrates the excellent flame-retardant performance of the prepared composite wood. Furthermore, AMPA and mesoporous structures endow antimicrobial and thermal insulation functions. Hence, this work provides a feasible method for preparing flame-retardant wood-based materials for diversified applications.

High-Temperature Fire Resistance and Self-Extinguishing Behavior of Cellular Graphene

The ability to protect materials from fire is vital to many industrial applications and life safety systems. Although various chemical treatments and protective coatings have proven effective as flame retardants, they provide only temporary prevention, as they do not change the inherent flammability of a given material. In this study, we demonstrate that a simple change of the microstructure can significantly boost the fire resistance of an atomically thin material well above its oxidation stability temperature. We show that free-standing graphene layers arranged in a three-dimensional (3D) cellular network exhibit completely different flammability and combustion rates from a graphene layer placed on a substrate. Covalently cross-linked cellular graphene aerogels can resist flames in air up to 1500 °C for a minute without degrading their structure or properties. In contrast, graphene on a substrate ignites immediately above 550 °C and burns down in a few seconds. Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric studies reveal that the exceptional fire-retardant and self-extinguishing properties of cellular graphene originate from the ability to prevent carbonyl defect formation and capture nonflammable carbon dioxide gas in the pores. Our findings provide important information for understanding graphene's fire-retardant mechanism in 3D structures/assemblies, which can be used to enhance flame resistance of carbon-based materials, prevent fires, and limit fire damage.

Pyrolysis of Precious Chinese Xuan Paper Containing Ammonium Phytate as a Flame Retardant

Xuan paper with outstanding cultural and artistic values is one of the most precious Chinese handmade papers and is widely used in traditional calligraphy and painting. However, the highly combustible cellulosic raw materials of Xuan paper present potential fire hazards. Ammonium phytate (AP) originating from biosourced phytic acid has been used for the flame-retardant treatment of Chinese Xuan paper by facile spray coating. The limiting oxygen index value of the treated Xuan paper increased to higher than 40%, demonstrating that the flammability of Xuan paper was greatly reduced by this treatment. The excellent flame retardancy afforded by this treatment was confirmed by cone calorimetry. TGA was used to demonstrate that the presence of AP changed the thermal decomposition process to promote char formation during the degradation of Xuan paper. The flame-retardant mode of action of phytate-coated Xuan paper was investigated using TG-FTIR, SEM, and XPS spectra. A P-N cooperative effect was proposed to account for both the condensed phase and gas-phase flame-retardant actions. The phosphorus component promotes char formation in the condensed phase, while the nitrogen component releases inert species to dilute the fuel load in the gas phase. The ink-wetting property of the coated Xuan paper was influenced negligibly by the coating process. The development of fire-resistant Xuan paper using ecofriendly flame retardants through simple and convenient spray coating has been demonstrated.

Progress in research on natural cellulosic fibre modifications by polyelectrolytes

In order to improve the mechanical properties and functionalities of natural cellulosic fibres, this paper first analyzed the characteristics of natural cellulosic fibres and the conventional modification methods of natural cellulosic fibres, and then focused on the polyelectrolytes modified natural cellulosic fibres. The main methods and process parameters of this modification were described in detail; the modification effects of polyelectrolytes on different types of fibres were systematically summarized; the influencing factors on modification of fibres were also discussed in depth; the characterization methods of polyelectrolytes modified fibres were analyzed in detail. Finally, the main application fields of polyelectrolytes modified fibres were systematically summarized.

The use of model cellulose gel beads to clarify flame-retardant characteristics of layer-by-layer nanocoatings

Layer-by-Layer (LbL) assembled nanocoatings are exploited to impart flame-retardant properties to cellulosic substrates. A model cellulose material can make it possible to investigate an optimal bilayer (BL) range for the deposition of coating while elucidating the main flame-retardant action thus allowing for an efficient design of optimized LbL formulations. Model cellulose gel beads were prepared by dissolving cellulose-rich fibers followed by precipitation. The beads were LbL-treated with chitosan (CH) and sodium hexametaphosphate (SHMP). The char forming properties were studied using thermal gravimetric analysis. The coating increased the char yield in nitrogen to up to 29 % and showed a distinct pattern of micro intumescent behavior upon heating. An optimal range of 10-20 BL is observed. The well-defined model cellulose gel beads hence introduce a new scientific route both to clarify the fundamental effects of different film components and to optimize the composition of the films.

Non-Isothermal Thermogravimetry of Selected Tropical Woods and Their Degradation under Fire Using Cone Calorimetry

For selected tropical woods (Cumaru, Garapa, Ipe, Kempas, Merbau), a relationship was established between non-isothermal thermogravimetry runs and the wood weight loss under flame during cone calorimetry flammability testing. A correlation was found for the rate constants for decomposition of wood in air at 250 and 300 °C found from thermogravimetry and the total time of sample burning related to the initial mass. Non-isothermal thermogravimetry runs were assumed to be composed from 3 theoretical runs such as decomposition of wood into volatiles itself, oxidation of carbon residue, and the formation of ash. A fitting equation of three processes was proposed and the resulting theoretical lines match experimental lines.

Improving air barrier, water vapor permeability properties of cellulose paper by layer-by-layer assembly of graphene oxide

A paper-based packaging material with improved air barrier and water vapor permeability (WVP) properties was synthesized based on layer-by-layer assembly consisted of poly(ethylenimine) (PEI)/graphene oxide (GO) on a filter paper substrate. The effect of the pH of GO suspension on the zeta potential and air permeability (AP) of the modified paper was investigated in detail. The results indicated that the pH of GO suspension resulted in significant difference in the AP of the modified paper. Compare with the pristine paper, the AP of the modified paper with (PEI/GO)₁₀ multilayer films synthesized at pH 2.5 decreased by 99.99 %, while the WVP increased by 15.82 %. The modified paper as packaging material could prolong the shelf-life of oyster mushroom, indicating the modified paper has huge potential application on the preservation of agricultural products.

Surface Properties of Spray-Assisted Layer-By-Layer ElectroStatic Self-Assembly Treated Wooden Take-Off Board

Wooden take-off board is easy to crack, deform, discolor, and decay when it is used outdoors, which not only increases maintenance costs but also reduces its service life. Multifunctional coatings with UV-resistant, water-repellent, and flame-retardant properties were successfully obtained on the surface of a wooden take-off board substrate by spray-assisted layer-by-layer self-assembly. The coatings consisted of positively-charged chitosan, Al (OH)3, and negatively-charged sodium phytate through electrostatic adsorption several times. The treated wood exhibited high UV resistance, and the color remained constant after 720 hours of ultraviolet irradiation. The wettability of the wood surface after treatment became superhydrophobic, with initial static contact angles as high as 140°. In addition, limiting oxygen index and air exposure combustion tests were used to verify that chitosan, sodium phytate, and aluminum hydroxide could synergistically confer significant fire resistance to modified wood.

Weak polyelectrolyte-based multilayers via layer-by-layer assembly: Approaches, properties, and applications

Layer-by-layer (LbL) assembly is a nanoscale technique with great versatility, simplicity and molecular-level processing of various nanoscopic materials. Weak polyelectrolytes have been used as major building blocks for LbL assembly providing a fundamental and versatile tool to study the underlying mechanisms and practical applications of LbL assembly due to its pH-responsive charge density and molecular conformation. Because of high-density uncompensated charges and high-chain mobility, weak polyelectrolyte exponential multilayer growth is considered one of the fastest developing areas for organized molecular films. In this article, we systematically review the current status and developments of weak polyelectrolyte-based multilayers including all-weak-polyelectrolyte multilayers, weak polyelectrolytes/other components (e.g. strong polyelectrolytes, neutral polymers, and nanoparticles) multilayers, and exponentially grown weak polyelectrolyte multilayers. Several key aspects of weak polyelectrolytes are highlighted including the pH-controllable properties, the responsiveness to environmental pH, and synergetic functions obtained from weak polyelectrolyte/other component multilayers. Throughout this review, useful applications of weak polyelectrolyte-based multilayers in drug delivery, tunable biointerfaces, nanoreactors for synthesis of nanostructures, solid state electrolytes, membrane separation, and sensors are highlighted, and promising future directions in the area of weak polyelectrolyte-based multilayer assembly such as fabrication of multi-responsive materials, adoption of unique building blocks, investigation of internal molecular-level structure and mechanism of exponentially grown multilayers, and exploration of novel biomedical and energy applications are proposed.

Fire Characteristics of Selected Tropical Woods without and with Fire Retardant

The flammability of tropical woods and the effect of a selected fire protection coating were evaluated using a cone calorimeter at a cone radiancy of 35 kW/m2. Three samples were from the South American continent (Cumaru, Garapa, Ipe), and two were from the Asian continent (Kempas and Merbau). Samples were treated with commercial fire retardant (FR) containing ferrous phosphate as an essential component. The untreated samples were used as reference materials that were of particular interest concerning their flammability. It was shown that there is unambiguous correlation between the effective heat of combustion (EHC) and total oxygen consumed (TOC) related to mass lost during burning for both the untreated and treated samples. In the case of Cumaru and Garapa, there exists an inverse relation between the amount of smoke and carbon residue. The decisive effect on the time of ignition was performed by the initial mass of the sample. This is valid for the spruce and the Cumaru, Ipe, and Kempas, both treated and untreated with retardant, while Garapa and Merbau were found to decline. According to the lower maximum average rate of heat emission (MARHE) parameter, a lower flammability was observed for the treated samples of wood, except for Garapa wood. Fire-retardant treated Garapa and Merbau also have a significantly lower time to ignition than untreated ones.

Flame-Retardant Wood Composites Based on Immobilizing with Chitosan/Sodium Phytate/Nano-TiO2-ZnO Coatings via Layer-by-Layer Self-Assembly

Composite coatings of inorganic nanomaterials with polyelectrolytes are promising materials for wood modification. Endowing wood with flame retardancy behavior can not only broaden the range of applications of wood, but also improve the safety of wood products. In this work, chitosan/sodium phytate/TiO2-ZnO nanoparticle (CH/SP/nano-TiO2-ZnO) composite coatings were coated on wood surface through layer-by-layer self-assembly. The morphology and chemical composition of the modified wood samples were analyzed using scanning electron microscopy and energy dispersive spectrometry. The thermal degradation properties and flame retardancy of the samples treated with different assembly structures were observed by thermogravimetric analysis, limiting oxygen test, and combustion test. Due to the presence of an effective intumescent flame retardant system and a physical barrier, the CH/SP/nano-TiO2-ZnO coatings exhibited the best flame retardant performance and required only approximately six seconds for self-extinguishing. The coated samples had a limiting oxygen index of 8.4% greater than the original wood.

Layer-by-layer modified low density cellulose fiber networks: A sustainable and fireproof alternative to petroleum based foams

Wood-based cellulose fibers were used to prepare porous, low density and wet-stable fiber networks (FN). Multilayer coatings consisting of chitosan (CH), sodium hexametaphosphate (SHMP) and inorganic nanoparticles comprising of either sodium montmorillonite (MMT), sepiolite (SEP) or colloidal silica (SNP) were deposited by the layer-by-layer (LbL) technique onto FNs in an effort to impart flame-retardancy. A simulated fire scenario measured by cone calorimetry showed that five quadlayers (QL) of CH/SHMP/CH/MMT, CH/SHMP/CH/SEP and CH/SHMP/CH/SNP can produce significant reduction in peak heat release rate (pkHRR). In detail, the coating containing SEP showed the largest reduction of the pkHRR by 47% relative to the uncoated FN. MMT and SEP coated FNs were also able to self-extinguish fire and to retain their shapes after direct exposure to a methane flame. This study hence shows that the LbL assembly is a highly effective way to impart flame-retardant properties to this new type of porous FN.

Flame Retardancy of Wood Fiber Materials Using Phosphorus-Modified Wheat Starch

Biopolymer-based flame retardants (FR) are a promising approach to ensure adequate protection against fire while minimizing health and environmental risks. Only a few, however, are suitable for industrial purposes because of their poor flame retardancy, complex synthesis pathway, expensive cleaning procedures, and inappropriate application properties. In the present work, wheat starch was modified using a common phosphate/urea reaction system and tested as flame retardant additive for wood fibers. The results indicate that starch derivatives from phosphate/urea systems can reach fire protection efficiencies similar to those of commercial flame retardants currently used in the wood fiber industry. The functionalization leads to the incorporation of fire protective phosphates (up to 38 wt.%) and nitrogen groups (up to 8.3 wt.%). The lowest levels of burning in fire tests were measured with soluble additives at a phosphate content of 3.5 wt.%. Smoldering effects could be significantly reduced compared to unmodified wood fibers. The industrial processing of a starch-based flame retardant on wood insulating materials exhibits the fundamental applicability of flame retardants. These results demonstrate that starch modified from phosphate/urea-systems is a serious alternative to traditional flame retardants.

Synergistic Fire Hazard Effect of a Multifunctional Flame Retardant in Building Insulation Expandable Polystyrene through a Simple Surface-Coating Method

This work reports a strategy based on γ-aminopropyltriethoxysilane (KH550) and graphene oxide (GO)-functionalized 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to fabricate P-N-Si integrated flame retardant [KDOPO-modified GO (DGO)] through mild Mannich and Silanization reactions to overcome the challenge of single gas-phase fire retardancy of DOPO. DGO-based phenolic epoxy resin (DGO/PER) is manufactured and coated on the surface of expandable polystyrene (EPS) foam plates to achieve fire safety, which is used as the thermally insulating external wall in buildings and constructions. The DGO/PER paintcoat imparts high fire safety to the EPS foam plate, exhibiting a high limiting oxygen index value of 29%, and a UL-94 V-0 classification is achieved with only 300 μm of layer thickness compared with the DOPO/PER paintcoat. Meanwhile, all combustion parameters such as peak heat release rate, heat release rate, total heat release, smoke release rate, total smoke rate, and ignition time present excellent promotions for EPS@DGO compared with EPS@DOPO. These dramatically reduced fire hazards are mainly attributed to the synergistic effects of DGO. Meanwhile, the DGO/PER flame-retardant paintcoat cannot deteriorate the thermal insulation performance of the EPS foam plate.

Formation of Chitosan/Sodium Phytate/Nano-Fe3O4 Magnetic Coatings on Wood Surfaces via Layer-by-Layer Self-Assembly

Magnetic wood would have potential uses in electromagnetic shielding and electromagnetic wave absorption. In this paper, magnetic coatings on a wood surface were synthesized using a layer-by-layer self-assembly method. As the cationic polyelectrolyte carrier, natural macromolecular chitosan was pre-immobilized on a wood surface first, followed by the alternate adsorption of anionic polyelectrolyte sodium phytate and positively-charged Fe3O4 nanoparticles. The concentration of pH-controlled chitosan solution, sodium phytate solution, and Fe3O4 nanoparticle suspension, soaking time, and the number of alternating sedimentary layers varied. The morphology and crystal structure of the Fe3O4 modified wood samples were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The magnetic hysteresis loops showed that the modified wood had magnetic properties which were improved as the number of assembled layers increased.

Temperature-triggered sensitive resistance transition of graphene oxide wide-ribbons wrapped sponge for fire ultrafast detecting and early warning

Fire prevention and safety of combustible materials is a global challenge. To reduce their high fire risk, traditional smoke detectors are widely used indoor via detecting smoke product after combustion; however, they usually show a long response time and limitation for outdoor use. Herein, we report a temperature-induced electrical resistance transition of graphene oxide wide-ribbon (GOWR) wrapped sponges to reliably monitor fire safety of the combustible materials. Novel rectangle-like GOWR sheets are synthesized from unzipping carbon nanofibers and used to fabricate GOWR wrapped melamine formaldehyde sponges with multi-functionalities, e.g. lightweight, good hydrophobicity, reversible compressibility, excellent acidic/alkaline tolerance and flame resistance. The GOWR sheets on the sponge skeleton can be in-situ thermally reduced once encountering a flame attack or abnormal high temperature, inducing a distinct transition in electrical resistance. Consequently, an ultrafast alarm response of ∼2 s to flame attack is triggered, and rapid fire early warning signals to abnormal high temperatures, e.g. ∼33 s at 300 °C, are achieved below ignition temperature of most combustible materials. This method drives substantial motivation and opportunity to develop advanced fire detection and early warning sensors for reducing the high fire risk of various combustible materials in outdoor applications.

Spray-Drying-Assisted Layer-by-Layer Assembly of Alginate, 3-Aminopropyltriethoxysilane, and Magnesium Hydroxide Flame Retardant and Its Catalytic Graphitization in Ethylene-Vinyl Acetate Resin

Alginates (nickel alginate, NiA; copper alginate, CuA; zinc alginate, ZnA) and 3-aminopropyltriethoxysilane (APTES) were alternately deposited on a magnesium hydroxide (MH) surface by the spray-drying-assisted layer-by-layer assembly technique, fabricating some efficient and environmentally benign flame retardants (M-FR, including Ni-FR, Cu-FR, and Zn-FR). The morphology, chemical compositions, and structures of M-FR were investigated. With 50 wt % loading, compared with EVA28+MH, the peak heat release rate, smoke production rate, and CO production rate of EVA28+Ni-FR decreased by 50.78%, 61.76%, and 66.67%, respectively. The metals or metal oxide nanoparticles arising from alginates could catalyze the pyrolysis intermediates of EVA into graphene and amorphous carbon, which could bind the inorganic compounds (the decomposition products of MH and APTES) together and form some more protective barriers. For each M-FR, the flame retardant and smoke suppression efficiency were different, which were caused by the diverse carbonization and graphitization behaviors of three alginates. ZnA generated some ZnO aggregations and could not catalyze the graphitization of intermediates. For CuA, the catalytic graphitization was limited by the tightly binding graphene layer. As for NiA, the configuration of the Ni atom could not provide strong binding of Ni substrate and carbon. The liquid-like Ni nanoparticles could restructure and get out from firm graphene shells, so the catalytic graphitization of NiA was efficient and sustainable. This work displayed the catalytic graphitization mechanism of alginates while exploring a simple and novel strategy for fabricating efficient green flame retardants.

Lignin-Modified Carbon Nanotube/Graphene Hybrid Coating as Efficient Flame Retardant

To reduce fire hazards and expand high-value applications of lignocellulosic materials, thin films comprising graphene nanoplatelets (GnPs) and multi-wall carbon nanotubes (CNTs) pre-adsorbed with alkali lignin were deposited by a Meyer rod process. Lightweight and highly flexible papers with increased gas impermeability were obtained by coating a protective layer of carbon nanomaterials in a randomly oriented and overlapped network structure. Assessment of the thermal and flammability properties of papers containing as low as 4 wt % carbon nanomaterials exhibited self-extinguishing behavior and yielded up to 83.5% and 87.7% reduction in weight loss and burning area, respectively, compared to the blank papers. The maximum burning temperature as measured by infrared pyrometry also decreased from 834 °C to 705 °C with the presence of flame retardants. Furthermore, papers coated with composites of GnPs and CNTs pre-adsorbed with lignin showed enhanced thermal stability and superior fire resistance than samples treated with either component alone. These outstanding flame-retardant properties can be attributed to the synergistic effects between GnPs, CNTs and lignin, enhancing physical barrier characteristics, formation of char and thermal management of the material. These results provide great opportunities for the development of efficient, cost-effective and environmentally sustainable flame retardants.

Superior Flame-Resistant Cellulose Nanofibril Aerogels Modified with Hybrid Layer-by-Layer Coatings

Nanometer thin films consisting of cationic chitosan (Ch), anionic poly(vinylphosphonic acid) (PVPA), and anionic montmorillonite clay (MMT) are deposited on highly porous, wet-stabilized cellulose nanofibril (CNF) aerogels via the layer-by-layer (LbL) technique. Model experiments with silicon oxide surfaces are used to study the details of LbL formation and the multilayer structure. Formation of layers on the aerogels is also investigated as a function of solution concentration by use of polyelectrolyte titration. Thermogravimetric analysis indicates that the LbL coating significantly improves thermal stability of the CNF aerogel. Horizontal flame test shows that aerogels coated with five quadlayers of Ch/PVPA/Ch/MMT, using solutions/dispersion of high concentration, are able to self-extinguish immediately after removal of flame, and LbL-coated aerogels do not ignite under heat flux (35 kW/m²) in cone calorimetry. The LbL-coated aerogel can prevent flame penetration from a torch focused on the surface, achieving temperature drops up to 650 °C across the 10 mm thick specimen for several minutes. LbL treatment is hence a rapid and highly effective way to specifically tailor the surface properties of CNF aerogels in order to confer unprecedented flame-retardant characteristics.

A novel and facile strategy for highly flame retardant polymer foam composite materials: Transforming silicone resin coating into silica self-extinguishing layer

In this study, a novel strategy was developed to fabricate highly flame retardant polymer foam composite materials coated by synthesized silicone resin (SiR) polymer via a facile dip-coating processing. Applying the SiR polymer coating, the mechanical property and thermal stability of SiR-coated polymer foam (PSiR) composites are greatly enhanced without significantly altering their structure and morphology. The minimum oxygen concentration to support the combustion of foam materials is greatly increased, i.e. from LOI 14.6% for pure foam to LOI 26-29% for the PSiR composites studied. Especially, adjusting pendant group to SiOSi group ratio (R/Si ratio) of SiRs produces highly flame retardant PSiR composites with low smoke toxicity. Cone calorimetry results demonstrate that 44-68% reduction in the peak heat release rate for the PSiR composites containing different R/Si ratios over pure foam is achieved by the presence of appropriate SiR coating. Digital and SEM images of post-burn chars indicate that the SiR polymer coating can be transformed into silica self-extinguishing porous layer as effective inorganic barrier effect, thus preserving the polymer foam structure from fire. Our results show that the SiR dip-coating technique is a promising strategy for producing flame retardant polymer foam composite materials with improved mechanical properties.

Nanomaterials-modified cellulose paper as a platform for biosensing applications

Recently, paper substrates have attracted tremendous interest from both academia and industry. Not only is paper highly abundant and portable, it is lightweight, disposable, easy-to-use, and can be rolled or folded into 3D configurations. More importantly, with a unique porous bulk structure and rough and absorptive surface properties, the construction of nanomaterials-functionalized cellulose has enabled cellulose paper to be applied for point-of-care (POC) paper devices with reasonably good performance at low cost. In this review, the latest advances in the modification of nanomaterials on paper cellulose are summed up. To begin with, the attractive properties of paper-based analytical devices are described. Then, fabricating methods for the functionalization of cellulose with diverse materials, including noble metals, bimetals, metal oxides, carbon nanomaterials, and molecular imprinting polymer nanoparticles, as well as their applications, are introduced in detail. Finally, the current critical issues, challenges, and future prospectives for exploring a paper-based analytical system based on nanomaterials-modified cellulose are discussed. It is believed that more strategies will be developed in the future to construct nanomaterials-functionalized cellulose, paving the way for the mass production of POC paper devices with a satisfactory performance.

Strong and tuneable wet adhesion with rationally designed layer-by-layer assembled triblock copolymer films

In this study the wet adhesion between Layer-by-Layer (LbL) assembled films of triblock copolymer micelles was investigated. Through the LbL assembly of triblock copolymer micelles with hydrophobic, low glass transition temperature (T_g) middle blocks and ionic outer blocks, a network of energy dissipating polymer chains with electrostatic interactions serving as crosslinks can be built. Four triblock copolymers were synthesized through Atom Transfer Radical Polymerisation (ATRP). One pair had a poly(2-ethyl-hexyl methacrylate) middle block with cationic or anionic outer blocks. The other pair contained the same ionic outer blocks but poly(n-butyl methacrylate) as the middle block. The wet adhesion was evaluated with colloidal probe AFM. To our knowledge, wet adhesion of the magnitude measured in this study has not previously been measured on any polymer system with this technique. We are convinced that this type of block copolymer system grants the ability to control the geometry and adhesive strength in a number of nano- and macroscale applications.

Layer-by-Layer Assembly of Multifunctional Flame Retardant Based on Brucite, 3-Aminopropyltriethoxysilane, and Alginate and Its Applications in Ethylene-Vinyl Acetate Resin

An efficient and multifunctional brucite/3-aminopropyltriethoxysilane (APTES)/nickel alginate/APTES (B/A/Nia/A) hybrid flame retardant was fabricated via the layer-by-layer assembly technique with brucite, silane coupling agents, nickel chloride, and sodium alginate. The morphology, chemical composition, and structure of the hybrid flame retardant were characterized. The results confirmed the multilayer structure and indicated that the assembled driving forces were electrostatic interactions, dehydration condensation, hydrogen bonds, and coordination bonds. When used in ethylene-vinyl acetate (EVA) resin, the multifunctional flame retardant had better performance than brucite in improving the flame retardancy, smoke suppression, and mechanical properties. With 130 phr loading, the multifunctional flame retardant achieved a limiting oxygen index value of 32.3% and a UL 94 V-0 rating, whereas the brucite achieved only 31.1% and a V-2 rating, respectively. The peak heat release rate and total heat released decreased by 41.5% and 8.9%, respectively. The multifunctional flame retardant had an excellent performance in reducing the smoke, CO, and CO2 production rates. These improvements could be attributed to the catalyzing carbonization of nickel compounds and the formation of more protective char layers. Moreover, the elongation at break increased by 97.5%, which benefited from the improved compatibility and the sacrificial bonds in the nickel alginate. The mechanism of flame retardant, smoke suppression, and toughening is proposed.

Self-Forming 3D Core-Shell Ceramic Nanostructures for Halogen-Free Flame Retardant Materials

The synthesis of aluminum phosphates-based composites has been widely studied during the past decade because of the promising industrial application of these materials. Here we show a simple one-pot heterogeneous precipitation approach to fabricate a sepiolite-phosphate (SepP) composite with adequate control of the size and dispersion of the phosphate nanoparticles. This coupling between aluminum phosphate and sepiolite nanofibers results in the development of a novel three-dimensional rigid supported phosphate structure, which is generated during the thermal treatment. According to our results, this phenomenon can be explained by a migration-coalescence mechanism of phosphate nanoparticles over the sepiolite support, assisted by a liquid phase. It is worth pointing out that this stimulant behavior observed here could have potential technological applications such as halogen-free flame retardant materials.

Bio-inspired writable multifunctional recycled paper with outer and inner uniform superhydrophobicity

One kind of multifunctional superhydrophobic recycled paper from the secondary use of waste paper has been prepared successfully with wonderful self-cleaning, anti-fouling and oil absorption abilities.