Exploiting Waste towards More Sustainable Flame-Retardant Solutions for Polymers: A Review

The development of sustainable flame retardants is gaining momentum due to their enhanced safety attributes and environmental compatibility. One effective strategy is to use waste materials as a primary source of chemical components, which can help mitigate environmental issues associated with traditional flame retardants. This paper reviews recent research in flame retardancy for waste flame retardants, categorizing them based on waste types like industrial, food, and plant waste. The paper focuses on recent advancements in this area, focusing on their impact on the thermal stability, flame retardancy, smoke suppression, and mechanical properties of polymeric materials. The study also provides a summary of functionalization methodologies used and key factors involved in modifying polymer systems. Finally, their major challenges and prospects for the future are identified.

Rigid polyurethane foam composites based on iron tailing: Thermal stability, flame retardancy and fire toxicity

In order to explore the potential utilization value of iron tailings, the typical solid waste-iron tailings was introduced into rigid polyurethane foam (RPUF) as a flame retardant filler in this paper. The flame retardant performance, combustion performance, gas-phase products and char residue’s related properties of RPUF/ITS composites were systematically investigated by limiting oxygen index, thermogravimetric (TG), cone calorimetry (CCT) and thermogravimetric-infrared spectrometry (TG-FTIR). The results showed that ITS improved the overall thermal stability of the composites, and the T-5%, T-50%, Tmax1, Tmax2 and char residue rates were all higher than those of the pure samples. The CCT indicated that ITS had a certain effect on smoke suppression and heat release reduction. The peak heat release rate of RPUF-6 was reduced by 22.75% compared with that of the pure sample, and the total smoke release of RPUF-2 was reduced by 25.36%. Smoke factor (SF), fire growth rate index and fire performance index indicated that ITS reduced the fire risk of RPUF/ITS composites. TG-FTIR showed that ITS inhibited the decomposition of RPUF/ITS composites, and the release intensity of hydrocarbons, CO2, isocyanate compound, CO, aromatic compounds and esters decreased significantly. TG, MCC, scanning electron microscope and Raman implied that ITS promoted the formation of a dense char layer in RPUF and improved the heat resistance of the char layer.

Preparation and Study on the Flame-Retardant Properties of CNTs/PMMA Microspheres

In this paper, carbon nanotubes (CNTs)/poly(methyl methacrylate) (PMMA) composites with excellent thermal stability and flame retardancy were prepared by in situ polymerization. The morphology, structure, transmittance, thermal stability, flame retardancy, and mechanical properties of the materials were characterized with scanning electron microscopy (SEM), thermogravimetric analysis (TGA), cone calorimetry, etc. According to the results, the initial decomposition temperature of CNTs/PMMA prepared using carbon nanotubes with a concentration of 2 mg/mL increases from 175 to 187 °C when compared with pure PMMA, and the weight loss ratio decreases significantly at the same time. In addition, the maximum limiting oxygen index (LOI) value of CNTs/PMMA composites is 22.17, which is 26.9% higher than that of PMMA. SEM images of residues after LOI tests demonstrate that when CNTs/PMMA is heated, a dense and stable interconnected network structure (i.e., carbon layer) is formed, which can effectively inhibit the combustion of pyrolysis products, prevent the transfer of heat and combustible gas, and finally interrupt the combustion of composite materials. However, a 25% decrease in the transmittance of CNTs/PMMA composites is observed in the Ultraviolet-visible (UV-vis) spectra. Although the addition of CNTs reduces the transparency of PMMA, its tensile and impact strength are all improved, which illustrates that CNT is a competitive flame retardant for PMMA.

Effect of the B:Zn:H2O Molar Ratio on the Properties of Poly(Vinyl Acetate) and Zinc Borate-Based Intumescent Coating Materials Exposed to a Quasi-Real Cellulosic Fire

In order to investigate an influence of the B:Zn:H₂O molar ratio on the fire protection efficiency of poly(vinyl acetate)-based thermoplastic intumescent coating materials (ICs), systems containing ammonium polyphosphate, melamine, pentaerythritol and different types of zinc borates (ZBs) were tested in a vertical position in quasi-real fire conditions. 3ZnO·2B₂O₃·6H₂O (ZB6), 2ZnO·3B₂O₃·3.5H₂O (ZB3.5) or 3ZnO·2B₂O₃ (ZB0) were added in amounts of 1–10 wt. parts/100 wt. parts of the other coating components mixture. Char formation processes and thermal insulation features were investigated using an open-flame furnace heated according to the cellulosic fire curve. Thermogravimetric features (DTG), chemical structures (FTIR) and mechanical strength of the ICs and the chars were analyzed as well. It was revealed that the type and dose of the ZBs significantly affect thermal insulation time (TIT) (up to 450 °C of a steel substrate) and sagging (SI) of the fire-heated coatings as well as the compressive strength of the created chars. The highest TIT value (+89%) was noted for the sample with 2.5 wt. parts of ZB3.5 while the lowest SI (−65%) was observed for the coatings containing 10 wt. parts of the hydrated borates (i.e., ZB3.5 or ZB6). The best mechanical strength was registered for the sample filled with the anhydrous modifier (3 wt. parts of ZB0). The presented results show that the ICs with the proper ZBs can be used for effective fire protection of vertically positioned steel elements.

Biodegradable Flame Retardants for Biodegradable Polymer

To improve sustainability of polymers and to reduce carbon footprint, polymers from renewable resources are given significant attention due to the developing concern over environmental protection. The renewable materials are progressively used in many technical applications instead of short-term-use products. However, among other applications, the flame retardancy of such polymers needs to be improved for technical applications due to potential fire risk and their involvement in our daily life. To overcome this potential risk, various flame retardants (FRs) compounds based on conventional and non-conventional approaches such as inorganic FRs, nitrogen-based FRs, halogenated FRs and nanofillers were synthesized. However, most of the conventional FRs are non-biodegradable and if disposed in the landfill, microorganisms in the soil or water cannot degrade them. Hence, they remain in the environment for long time and may find their way not only in the food chain but can also easily attach to any airborne particle and can travel distances and may end up in freshwater, food products, ecosystems, or even can be inhaled if they are present in the air. Furthermore, it is not a good choice to use non-biodegradable FRs in biodegradable polymers such as polylactic acid (PLA). Therefore, the goal of this review paper is to promote the use of biodegradable and bio-based compounds for flame retardants used in polymeric materials.

Estimation of Mechanical Performance, Thermal Stability and Flame Retardancy of High-Impact Polystyrene/Surface-Modified APP/Carboxylic-Functionalized MWCNTs Nanocomposites

An ammonium polyphosphate (APP) surface-modified by silane coupling agent was used as flame retardant in high-impact polystyrene (HIPS). A series of HIPS nanocomposites containing different mass fractions of APP (k-APP) surface-modified by silane coupling agent (3-aminopropyl triethoxysilane, KH 550) and carboxylic-functionalized MWCNTs (COOH–MWCNTs) were prepared by the melt blending method. A composite only containing APP was also prepared as a reference material. Scanning electron microscopy (SEM) was employed to investigate the dispersion of the fillers into the HIPS matrix, and it was found the hydrophobic groups on the k-APP surface would greatly enhance the dispersion and prevent agglomerations compared with APP. Furthermore, the COOH–MWCNTs also showed good dispersibility into the matrix. Mechanical tests of the nanocomposites revealed that k-APP exhibits a more beneficial effect on both tensile and flexural properties compared with APP. Thermogravimetric analysis (TGA) and cone calorimeter tests (CCT) were conducted to probe the thermal and flammability properties of the nanocomposites, respectively. The synergistic effects of k-APP and COOH–MWCNTs on mechanical, thermal and flammability properties were examined as well.

Novel Core-Shell Hybrid Nanosphere towards the Mechanical Enhancement and Fire Retardance of Polycarbonate

It is a huge challenge to achieve highly efficient fire retardance with no mechanical damage to polymers. In our current research, a novel core-shell titanium dioxide@diphenylphosphinic (TiO₂@DPP) nanosphere was first synthesized through a hydrothermal reacting process, and applied in simultaneously enhancing the fire retardance and mechanical properties of polycarbonate (PC). The well-designed TiO₂@DPP exhibited a significant effect on combustion performance and mechanical reinforcement of PC. At only 0.10 wt % of TiO₂@DPP, PC/TiO₂@DPP passed the UL-94 V-0 rating, and its oxygen index value rose to 29.3%. Moreover, the peak value of the heat release rate was remarkably decreased by 34.1% in the combustion test, accompanied by the formation of more compacted char layer and the release of more incombustible gas. Equally important another aspect is that the PC containing only 0.10 wt % of TiO₂@DPP possessed higher elongation at break and higher tensile strength than pure PC, correspondingly increased by 27.7 and 14.7%. The analysis of the flame-retardant mechanism revealed that the improved fire retardance of PC is primarily ascribed to the barrier action of a cross-linking network containing phosphorus and titanium, the dilution of nonflammable gases such as H₂O, and the quenching effect of free radicals which are from the phosphorous group in the gas phase. All these experimental results demonstrate that the core-shell hybrid TiO₂@DPP may achieve a simultaneous significant improvement in fire retardance and mechanical properties of PC.

Preparation of a Novel Intumescent Flame Retardant Based on Supramolecular Interactions and Its Application in Polyamide 11

The flammability and melt dripping of the widely used bio-based polyamide 11 (PA 11) have attracted much attention in the last decade, and they are still a big challenge for the fire science society. In this work, a novel single macromolecular intumescent flame retardant (AM-APP) that contains an acid source and a gas source was prepared by supramolecular reactions between melamine and p-aminobenzene sulfonic acid, followed by an ionic exchange with ammonium polyphosphate. The chemical structure of AM-APP was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. AM-APP and TiO₂ were then introduced into PA 11 by melt compounding to improve the fire resistance of the composite. The fire performance of PA 11 composites was evaluated by the limiting oxygen index (LOI), vertical burning (UL-94), and cone calorimetry tests. The results showed that the presence of 22% AM-APP and 3% TiO₂ increased the LOI value from 22.2 to 29.2%, upgraded the UL-94 rating from no rating to V-0, completely eliminated melt dripping, and significantly decreased the peak heat release rate from 943.4 to 177.5 kW/m². The thermal behaviors were investigated by thermogravimetric (TG) analysis and TG-FTIR. It is suggested that AM-APP produces an intumescent char structure and releases inert gases, whereas TiO₂ may consolidate the char layers, leading to the improvement in the fire resistance of PA 11.

Experimental Investigation on Mechanical and Thermo-Mechanical Properties of Alumina Filled Polypropylene Composites Using Injection Molding Process

In the present study, Polypropylene composites were fabricated by varying different weight percentages (0 to 20%) of alumina powder by using Injection Molding Technique. Then the fabricated composites were characterized by physical characterization such as void content test, chemical characterization such as Fourier-transform infrared spectroscopy, mechanical characterization such as Shore hardness test, flexural test, compressive strength, Izod impact test, thermo-mechanical characterizations such as Dynamic Mechanical Analysis, Thermo-gravimetric analysis and morphological characterizations such as Scanning Electron Microscopy (SEM) and Atomic force Microscopy (AFM). The results indicate that the shore hardness values increase up to 25% with the increase in alumina filler up to 20%. The highest flexural strength and flexural modulus achieved are 59.26 MPa and 2126 MPa respectively with 20% filler loading. Further, it was also revealed that the storage modulus of the composites was found to be higher than that of the virgin PP because filler increases the stiffness of the composites. Thermo-gravimetric analysis (TGA) measurements indicate that both the initial degradation temperature and end degradation temperature increase with increasing filler content. SEM exhibits that alumina particles were uniformly and finely dispersed though some aggregates and agglomerates are identifiable. AFM results indicate that morphology of alumina in the PP matrix is characterized by a chainlike branched structure.

Photocatalytic Surface-Initiated Polymerization on TiO2 toward Well-Defined Composite Nanostructures

We demonstrate the use of TiO2 nanospheres as the photoinitiator for photocatalytic surface-initiated polymerization for the synthesis of various inorganic/polymer nanocomposites with well-defined structures. The excitation of TiO2 by UV-light irradiation produces electrons and holes which drive the free radical polymerization near its surface, producing core/shell composite nanospheres with eccentric or concentric structures that can be tuned by controlling the surface compatibility between the polymer and the TiO2. When highly porous TiO2 nanospheres were employed as the photoinitiator, polymerization could disintegrate the mesoporous framework and give rise to nanocomposites with multiple TiO2 nanoparticles evenly distributed in the polymer spheres. Thanks to the well-developed sol-gel chemistry of titania, this synthesis is well-extendable to the coating of the polymers on many other substrates of interest such as silica and ZnS by simply premodifying their surface with a thin layer of titania. In addition, this strategy could be easily applied to coating of different types of polymers such as polystyrene, poly(methyl methacrylate), and poly(N-isopropylacrylamide). We expect this photocatalytic surface-initiated polymerization process could provide a platform for the synthesis of various inorganic/polymer hybrid nanocomposites for many interesting applications.

Multifunctional cyclotriphosphazene/hexagonal boron nitride hybrids and their flame retarding bismaleimide resins with high thermal conductivity and thermal stability

A novel hybridized multifunctional filler (CPBN), cyclotriphosphazene/hexagonal boron nitride (hBN) hybrid, was synthesized by chemically coating hBN with hexachlorocyclotriphosphazene and p-phenylenediamine, its structure was systemically characterized. Besides, CPBN was used to develop new flame retarding bismaleimide/o,o'-diallylbisphenol A (BD) resins with simultaneously high thermal conductivity and thermal stability. The nature of CPBN has a strong influence on the flame behavior of the composites. With the addition of only 5 wt % CPBN to BD resin, the thermal conductivity increases 2 times; meanwhile the flame retardancy of BD resin is remarkably increased, reflected by the increased limited oxygen index, much longer time to ignition, significantly reduced heat release rate. The thermogravimetric kinetics, structures of chars and pyrolysis gases, and cone calorimeter tests were investigated to reveal the unique flame retarding mechanism of CPBN/BD composites. CPBN provides multieffects on improving the flame retardancy, especially in forming a protective char layer, which means a more thermally stable and condensed barrier for heat and mass transfer, and thus protecting the resin from further combustion.