An Investigation towards Coupling Molecular Dynamics with Computational Fluid Dynamics for Modelling Polymer Pyrolysis.

Building polymers implemented into building panels and exterior façades have been determined as the major contributor to severe fire incidents, including the 2017 Grenfell Tower fire incident. To gain a deeper understanding of the pyrolysis process of these polymer composites, this work proposes a multi-scale modelling framework comprising of applying the kinetics parameters and detailed pyrolysis gas volatiles (parent combustion fuel and key precursor species) extracted from Molecular Dynamics models to a macro-scale Computational Fluid Dynamics fire model. The modelling framework was tested for pure and flame-retardant polyethylene systems. Based on the modelling results, the chemical distribution of the fully decomposed chemical compounds was realised for the selected polymers. Subsequently, the identified gas volatiles from solid to gas phases were applied as the parent fuel in the detailed chemical kinetics combustion model for enhanced predictions of toxic gas, charring, and smoke particulate predictions. The results demonstrate the potential application of the developed model in the simulation of different polymer materials without substantial prior knowledge of the thermal degradation properties from costly experiments.

A Review on Lithium-Ion Battery Separators towards Enhanced Safety Performances and Modelling Approaches.

In recent years, the applications of lithium-ion batteries have emerged promptly owing to its widespread use in portable electronics and electric vehicles. Nevertheless, the safety of the battery systems has always been a global concern for the end-users. The separator is an indispensable part of lithium-ion batteries since it functions as a physical barrier for the electrode as well as an electrolyte reservoir for ionic transport. The properties of separators have direct influences on the performance of lithium-ion batteries, therefore the separators play an important role in the battery safety issue. With the rapid developments of applied materials, there have been extensive efforts to utilize these new materials as battery separators with enhanced electrical, fire, and explosion prevention performances. In this review, we aim to deliver an overview of recent advancements in numerical models on battery separators. Moreover, we summarize the physical properties of separators and benchmark selective key performance indicators. A broad picture of recent simulation studies on separators is given and a brief outlook for the future directions is also proposed.

MXene/chitosan nanocoating for flexible polyurethane foam towards remarkable fire hazards reductions.

MXene/chitosan nanocoating for flexible polyurethane foam (PUF) was prepared via layer-by-layer (LbL) approach. MXene (Ti3C2) ultra-thin nanosheets were obtained through etching process of Ti3AlC2 followed by exfoliation. The deposition of MXene/chitosan nanocoating was conducted by alternatingly immersing the PUF into a chitosan solution and a Ti3C2 aqueous dispersion, which resulted in different number of bilayers (BL) ranging from 2, 5 and 8. Owing to the utilization of ultra-thin Ti3C2 nanosheets, the weight gain was only 6.9% for 8 BL coating of PUF, which minimised the unfavourable impact on the intrinsic properties of PUF. The Ti3C2/chitosan coating significantly reduced the flammability and smoke releases of PUF. Compared with unmodified PUF, the 8 BL coating reduced the peak heat release rate by 57.2%, alongside with a 65.5% reduction in the total heat release. The 8 BL coating also showed outstanding smoke suppression ability with total smoke release decreased by 71.1% and peak smoke production rate reduced by 60.3%, respectively. The peak production of CO and CO2 gases also decreased by 70.8% and 68.6%, respectively. Furthermore, an outstanding char formation performance of 37.2 wt.% residue was obtained for 8 BL coated PUF, indicating the excellent barrier and carbonization property of the hybrid coating.

Functionalization of MXene Nanosheets for Polystyrene towards High Thermal Stability and Flame Retardant Properties.

Fabricating high-performance MXene-based polymer nanocomposites is a huge challenge because of the poor dispersion and interfacial interaction of MXene nanosheets in the polymer matrix. To address the issue, MXene nanosheets were successfully exfoliated and subsequently modified by long-chain cationic agents with different chain lengths, i.e., decyltrimethylammonium bromide (DTAB), octadecyltrimethylammonium bromide (OTAB), and dihexadecyldimethylammonium bromide (DDAB). With the long-chain groups on their surface, modified Ti3C2 (MXene) nanosheets were well dispersed in N,N-dimethylformamide (DMF), resulting in the formation of uniform dispersion and strong interfacial adhesion within a polystyrene (PS) matrix. The thermal stability properties of cationic modified Ti3C2/PS nanocomposites were improved considerably with the temperatures at 5% weight loss increasing by 20 °C for DTAB-Ti3C2/PS, 25 °C for OTAB-Ti3C2/PS and 23 °C for DDAB-Ti3C2/PS, respectively. The modified MXene nanosheets also enhanced the flame-retardant properties of PS. Compared to neat PS, the peak heat release rate (PHRR) was reduced by approximately 26.4%, 21.5% and 20.8% for PS/OTAB-Ti3C2, PS/DDAB-Ti3C2 and PS/DTAB-Ti3C2, respectively. Significant reductions in CO and CO2 productions were also obtained in the cone calorimeter test and generally lower pyrolysis volatile products were recorded by PS/OTAB-Ti3C2 compared to pristine PS. These property enhancements of PS nanocomposites are attributed to the superior dispersion, catalytic and barrier effects of Ti3C2 nanosheets.

Robust, Lightweight, Hydrophobic, and Fire-Retarded Polyimide/MXene Aerogels for Effective Oil/Water Separation.

Polyimide (PI) aerogels have attracted great attention owing to their low density and excellent thermal stability. However, hydrophobic surface modification is required for PI aerogels to improve their ability in oil/water separation due to their amphiphilic characteristic. Two-dimensional MXenes (transition metal carbides/nitrides) can be utilized as nanofillers to enhance the properties of polymers because of their unique layered structure and versatile interface chemistry. Herein, the robust, lightweight, and hydrophobic PI/MXene three-dimensional architectures were fabricated via freeze-drying of polyamide acid/MXene suspensions and thermal imidization. Polyamide acid was synthesized using N-N-dimethylacetamide and 4,4'-oxydianiline. MXene (Ti3C2Tx) dispersion was obtained via the etching of Ti3AlC2 and ultrasonic exfoliation. Taking advantage of the strong interaction between PI chains and MXene nanosheets, the interconnected, highly porous, and hydrophobic PI/MXene aerogels with low density were fabricated, resulting in the improved compressive performance, remarkable oil absorption capacity, and efficient separation of oil and water. For the PI/MXene-3 aerogel (weight ratio, 5.2:1) without any surface modification, the water contact angle was 119° with a density of 23 mg/cm3. This aerogel can completely recover to its original height after 50 compression-release cycles, exhibiting superelasticity and exceptional fatigue-resistant ability. It also showed high absorption capacities to various organic liquids ranging from approximately 18 to 58 times of their own weight. This hybrid aerogel can rapidly separate the chloroform, soybean oil, and liquid paraffin from the water-oil system. The thermally stable hybrid aerogel also exhibited excellent fire safety properties and outstanding reusability under an extreme environment.

Novel 3D Network Architectured Hybrid Aerogel Comprising Epoxy, Graphene, and Hydroxylated Boron Nitride Nanosheets.

A novel three-dimensional (3D) epoxy/graphene nanosheet/hydroxylated boron nitride (EP/GNS/BNOH) hybrid aerogel was successfully fabricated in this study. This was uniquely achieved by constructing a well-defined and interconnected 3D network architecture. The manufacturing process of EP/GNS/BNOH involved a simple one-pot hydrothermal strategy, followed by the treatment of freeze-drying and high-temperature curing. In comparison with EP/GNS-3, EP/GNS/BNOH-3 demonstrated improvement of 97% for compressive strength at 70% strain. Through compression tests, fracture occurred for EP/GNS-3 at ninth compression cycles, whereas EP/GNS/BNOH-3 retained its original form after twenty compression cycles, with a residual height of 97% (i.e., only 3% reduction). By the addition of BNOH in the polymer matrix, the dynamic heat transfer and dissipation rates of EP/GNS/BNOH aerogels were also considerably reduced, indicating that the aerogel with BNOH additive possessed excellent thermal insulation properties. Thermogravimetric analysis results revealed that the thermal stabilities of EP/GNS and EP/GNS/BNOH aerogels were improved with increasing loading of EP, and EP/GNS/BNOH aerogels exhibited a better thermal stability at high temperatures. Through the elevated levels attained in the compressive strength, superelasticity, and thermal resistance, EP/GNS/BNOH aerogels has the great potential of being a very effective thermal insulation material to be utilized across a board range of applications in building, automotive, spacecraft, and mechanical systems.

Comparative Studies on Thermal, Mechanical, and Flame Retardant Properties of PBT Nanocomposites via Different Oxidation State Phosphorus-Containing Agents Modified Amino-CNTs.

High-performance poly(1,4-butylene terephthalate) (PBT) nanocomposites have been developed via the consideration of phosphorus-containing agents and amino-carbon nanotube (A-CNT). One-pot functionalization method has been adopted to prepare functionalized CNTs via the reaction between A-CNT and different oxidation state phosphorus-containing agents, including chlorodiphenylphosphine (DPP-Cl), diphenylphosphinic chloride (DPP(O)-Cl), and diphenyl phosphoryl chloride (DPP(O3)-Cl). These functionalized CNTs, DPP(Ox)-A-CNTs (x = 0, 1, 3), were, respectively, mixed with PBT to obtain the CNT-based polymer nanocomposites through a melt blending method. Scanning electron microscope observations demonstrated that DPP(Ox)-A-CNT nanoadditives were homogeneously distributed within PBT matrix compared to A-CNT. The incorporation of DPP(Ox)-A-CNT improved the thermal stability of PBT. Moreover, PBT/DPP(O3)-A-CNT showed the highest crystallization temperature and tensile strength, due to the superior dispersion and interfacial interactions between DPP(O3)-A-CNT and PBT. PBT/DPP(O)-A-CNT exhibited the best flame retardancy resulting from the excellent carbonization effect. The radicals generated from decomposed polymer were effectively trapped by DPP(O)-A-CNT, leading to the reduction of heat release rate, smoke production rate, carbon dioxide and carbon monoxide release during cone calorimeter tests.