Synergistic effect of MXene on the flame retardancy and thermal degradation of intumescent flame retardant biodegradable poly (lactic acid) composites

Carbon-based Flame Retardants for Polymers: A Bottom-up Review

This state-of-the-art review is geared toward elucidating the molecular understanding of the carbon-based flame-retardant mechanisms for polymers via holistic characterization combining detailed analytical assessments and computational material science. The use of carbon-based flame retardants, which include graphite, graphene, carbon nanotubes (CNTs), carbon dots (CDs), and fullerenes, in their pure and functionalized forms are initially reviewed to evaluate their flame retardancy performance and to determine their elevation of the flammability resistance on various types of polymers. The early transition metal carbides such as MXenes, regarded as next-generation carbon-based flame retardants, are discussed with respect to their superior flame retardancy and multifunctional applications. At the core of this review is the utilization of cutting-edge molecular dynamics (MD) simulations which sets a precedence of an alternative bottom-up approach to fill the knowledge gap through insights into the thermal resisting process of the carbon-based flame retardants, such as the formation of carbonaceous char and intermediate chemical reactions offered by the unique carbon bonding arrangements and microscopic in-situ architectures. Combining MD simulations with detailed experimental assessments and characterization, a more targeted development as well as a systematic material synthesis framework can be realized for the future development of advanced flame-retardant polymers.

Recent Progress in Two-Dimensional Nanomaterials for Flame Retardance and Fire-Warning Applications

Graphene-like 2D nanomaterials, such as graphene, MXene, molybdenum disulfide, and boron nitride, present a promising avenue for eco-friendly flame retardants. Their inherent characteristics, including metal-like conductivity, high specific surface area, electron transport capacity, and solution processability, make them highly suitable for applications in both structural fire protection and fire alarm systems. This review offers an up-to-date exploration of advancements in flame retardant composites, utilizing pristine graphene-like nanosheets, versatile graphene-like nanosheets with multiple functions, and collaborative systems based on these nanomaterials. Moreover, graphene-like 2D nanomaterials exhibit considerable potential in the development of early fire alarm systems, enabling timely warnings. This review provides an overview of flame-retarding and fire-warning mechanisms, diverse multifunctional nanocomposites, and the evolving trends in the development of fire alarm systems anchored in graphene-like 2D nanomaterials and their derivatives. Ultimately, the existing challenges and prospective directions for the utilization of graphene-like 2D nanomaterials in flame retardant and fire-warning applications are put forward.

Recent Advances and Outlook in 2D Nanomaterial-Based Flame-Retardant PLA Materials

Poly (lactic acid) or polylactide (PLA) has gained widespread use in many industries and has become a commodity polymer. Its potential as a perfect replacement for petrochemically made plastics has been constrained by its extreme flammability and propensity to flow in a fire. Traditional flame-retardants (FRs), such as organo-halogen chemicals, can be added to PLA without significantly affecting the material's mechanical properties. However, the restricted usage of these substances causes them to bioaccumulate and endanger plants and animals. Research on PLA flame-retardants has mostly concentrated on organic and inorganic substances for the past few years. Meanwhile, there has been a significant increase in renewed interest in creating environmentally acceptable flame-retardants for PLA to maintain the integrity of the polymer, which is the current trend. This article reviews recent advancements in novel FRs for PLA. The emphasis is on two-dimensional (2D) nanosystems and the composites made from them that have been used to develop PLA nanocomposite (NCP) systems that are flame retarding. The association between FR loadings and efficiency for different FR-PLA systems is also briefly discussed in the paper, as well as their influence on processing and other material attributes. It is unmistakably established from the literature that adding 2D nanoparticles to PLA matrix systems reduces their flammability by forming an intumescent char/carbonized surface layer. This creates a barrier effect that successfully blocks the filtration of volatiles and oxygen, heat and mass transfer, and the release of combustible gases produced during combustion.

Recent advances in flame retardant and mechanical properties of polylactic acid: A review

The large-scale application of ecofriendly polymeric materials has become a key focus of scientific research with the trend toward sustainable development. Mechanical properties and fire safety are two critical considerations of biopolymers for large-scale applications. Polylactic acid (PLA) is a flammable, melt-drop carrying, and strong but brittle polymer. Hence, it is essential to achieve both flame retardancy and mechanical enhancement to improve safety and broaden its application. This study reviews the recent research on the flame retardant functionalization and mechanical reinforcement of PLA. It classifies PLA according to the type of the flame retardant strategy employed, such as surface-modified fibers, modified nano/micro fillers, small-molecule and macromolecular flame retardants, flame retardants with fibers or polymers, and chain extension or crosslinking with other flame retardants. The functionalization strategies and main parameters of the modified PLA systems are summarized and analyzed. This study summarizes the latest advances in the fields of flame retardancy and mechanical reinforcement of PLA.

Flame-Retardant and Recyclable Soybean Oil-Based Thermosets Enabled by the Dynamic Phosphate Ester and Tannic Acid

The demands of safety and sustainability have driven the development of intrinsic flame-retardant biobased polymers from renewable materials. Herein, a mechanically robust, good flame-retardant, and recyclable thermoset was developed from renewable epoxidized soybean oil (ESO) by using 2-hydroxyethyl methacrylate phosphate (HEMAP) as the reactive flame retardant and tannic acid (TA) as the charring agent. The flame resistance of the obtained ESO-based thermoset achieved the highest UL-94 of V-0 rating and a limited oxygen index value of 26.7% due to the synergistic flame-retardant effect of phosphate and TA. The flame-retardant mechanisms of the gaseous phase and condensed phase were fully investigated by thermogravimetric infrared, scanning electron microscopy-energy-dispersive spectrometry, X-ray photoelectron spectroscopy, and Raman spectra. It is confirmed that the incorporation of phosphate and TA could effectively promote the formation of dense carbon layers and delay the pyrolysis of long aliphatic chains. The ternary crosslinking of ESO, HEMAP, and TA via free-radical polymerization and epoxy-ring opening reaction resulted in a rigid network with a high crosslink density, bestowing the thermoset with superior tensile strength (20.0 MPa), flexural strength (36.3 MPa), and bonding strength (16.7 MPa on steel). Moreover, the ESO-based thermoset exhibited a fast stress relaxation behavior due to the transesterification of dynamic β-hydroxyl phosphate esters, which enables the network with thermal-healing ability and recyclability. This study explores a feasible method to prepare an intrinsic flame-retardant polymer from commercially available and renewable vegetable oils and natural polyphenols.

MOF-derived 3D petal-like CoNi-LDH array cooperates with MXene to effectively inhibit fire and toxic smoke hazards of FPUF

The toxic smoke produced by the combustion of flexible polyurethane foam (FPUF) may not only caused casualties, but also polluted the environment. Here, double metal hydroxide derived from ZIF-67 (MOF-LDH) modified Ti₃C₂T_X (Ti₃C₂T_X@MOF-LDH) was innovatively designed to solve the serious smoke and fire hazards of FPUF. The FPUF nanocomposite containing 6 wt% Ti₃C₂Tx@MOF-LDH achieved a 16.1% reduction in total smoke production (TSP) along with 22.2% reduction in peak smoke production rate (PSPR), which greatly reduced the hazard of smoke. At the same time, toxic gases, such as carbon monoxide (CO), carbon dioxide (CO₂), and aromatic compounds, showed the same reduction pattern. In addition, the heat release of FPUF nanomaterials was also suppressed. In particular, the FPUF/Ti₃C₂Tx@MOF-LDH 3.0 achieved 110.4% and 76.1% increase in compressive strength and tensile strength, respectively, confirming the effective mechanical enhancement. Therefore, this work provided a new reference for the preparation of high-performance FPUF nanocomposites with low smoke, low fire hazard and excellent mechanical properties.

Thermal Stability and Flammability Studies of MXene-Organic Hybrid Polystyrene Nanocomposites

Polystyrene (PS) is widely used in the plastics industry, but the application range of PS is limited due to its inherently high flammability. A variety of two-dimensional (2D) nanomaterials have been reported to impart excellent flame retardancy to polymeric materials. In this study, a 2D nanomaterial MXene-organic hybrid (O-Ti3C2) was applied to PS as a nanofiller. Firstly, the MXene nanosheets were prepared by acid etching, intercalation, and delamination of bulk MAX (Ti3AlC2) material. These exfoliated MXene nanosheets were then functionalized using a cationic surfactant to improve the dispersibility in DMF. Even with a small loading of functionalized O-Ti3C2 (e.g., 2 wt%), the resulting PS nanocomposite (PS/O-Ti3C2) showed good thermal stability and lower flammability evidenced by thermogravimetric analysis (TGA) and pyrolysis-combustion flow calorimetry (PCFC). The peak heat release rate (pHRR) was significantly reduced by 32% compared to the neat PS sample. In addition, we observed that the temperature at pHRR (TpHRR) shifted to a higher temperature by 22 °C. By comparing the TGA and PCFC results between the PS/MAX and different weight ratios of PS/O-Ti3C2 nanocomposites, the thermal stability and 2D thermal- and mass-transfer barrier effect of MXene-organic hybrid nanosheets were revealed to play essential roles in delaying the polymer degradation.

Fabrication of thermoplastic polyurethane with functionalized MXene towards high mechanical strength, flame-retardant, and smoke suppression properties

Recently, two-dimensional MXene demonstrated promising advantages to improve the flame-retardant performance of composites; however, its compatibility with polymer matrix is a great concern. In this study, MXene was first functionalized with phosphorylated chitosan (PCS) to obtain the PCS-MXene nanohybrid. The resulting nanohybrid was introduced into the thermoplastic polyurethane (TPU) matrix via solution mixing followed by the hot-pressing method, affording TPU/PCS-MXene nanocomposite. The resulting nanohybrid exhibited superior compatibility with the TPU matrix, enhancing mechanical performance of the TPU/PCS-MXene nanocomposite compared to the pristine TPU and TPU/MXene nanocomposite. Besides, the flame-retardant performance of TPU/PCS-MXene nanocomposite was greatly enhanced, while the smoke emission was effectively suppressed. As only 3 wt% PCS-MXene was introduced, peak heat release rate, total heat release, and total smoke production of the composite decreased by 66.7%, 21.0%, and 27.7%, respectively, compared to the pristine TPU. Systematical characterization was then carried out to investigate the enhancement mechanism of PCS-MXene, highlighting the crucial role of PCS combined with the catalytic effect of MXene. In brief, the compatibility issues of MXene were effectively addressed, and its flame-retardancy enhanced greatly via the PCS modification, the bio-based characteristic of which, in turn greatly benefits the further development of MXene-polymer composite.

A Study of Thermal Degradation and Fire Behaviour of Polymer Composites and Their Gaseous Emission Assessment

The use of polymer composite materials in the aeronautics and automotive sectors has increased dramatically, and their fire behaviour has become a critical parameter in terms of fire safety. On this premise, it is critical to demonstrate that these composite materials constitute elements whose safety justifies a high level of confidence. This is based on their combustibility and the rate at which flammable and toxic gaseous species are emitted. Thus, strict fire safety regulations are enforced by the relevant authorities concerned because of their potential fire risk. This study analysed papers published between 1970 and 2021 that described the devices used to characterise the thermal behaviour of composite materials at various scales. The objective was to highlight the thermophysical phenomena, making it possible to accurately assess the flammability and thermal stability of polymer composite materials. The results of this research reveal that the small-scale facilities provide detailed understanding and mastery of the thermal reaction properties of the composites. While with the medium scale, the extended fire reaction parameters, which are the key indicators of the fire safety performance, can be determined. On a large scale, the tests were carried out using devices such as the NexGen burner recommended by the FAA. Therefore, with such assays, it is possible to assess the rates of thermal degradation as well as quantified pyrolysis gases. However, compared to other scales, there were very few works on a large scale. In addition, by focusing on the polluting nature of synthetic composite materials, there is also few research studies aimed at designing new polymer composite materials from biological sources.

Miniaturized Salinity Gradient Energy Harvesting Devices

Harvesting salinity gradient energy, also known as "osmotic energy" or "blue energy", generated from the free energy mixing of seawater and fresh river water provides a renewable and sustainable alternative for circumventing the recent upsurge in global energy consumption. The osmotic pressure resulting from mixing water streams with different salinities can be converted into electrical energy driven by a potential difference or ionic gradients. Reversed-electrodialysis (RED) has become more prominent among the conventional membrane-based separation methodologies due to its higher energy efficiency and lesser susceptibility to membrane fouling than pressure-retarded osmosis (PRO). However, the ion-exchange membranes used for RED systems often encounter limitations while adapting to a real-world system due to their limited pore sizes and internal resistance. The worldwide demand for clean energy production has reinvigorated the interest in salinity gradient energy conversion. In addition to the large energy conversion devices, the miniaturized devices used for powering a portable or wearable micro-device have attracted much attention. This review provides insights into developing miniaturized salinity gradient energy harvesting devices and recent advances in the membranes designed for optimized osmotic power extraction. Furthermore, we present various applications utilizing the salinity gradient energy conversion.

A novel multifunctional flame retardant MXene/nanosilica hybrid for poly(vinyl alcohol) with simultaneously improved mechanical properties

A new MXene-based flame retardant and a new strategy for the synthesis of multifunctional polymers.