Synergetic enhancement of mechanical and fire-resistance performance of waterborne polyurethane by introducing two kinds of phosphorus–nitrogen flame retardant

A Comprehensive Review of Reactive Flame Retardants for Polyurethane Materials: Current Development and Future Opportunities in an Environmentally Friendly Direction

Polyurethanes are among the most significant types of polymers in development; these materials are used to produce construction products intended for work in various conditions. Nowadays, it is important to develop methods for fire load reduction by using new kinds of additives or monomers containing elements responsible for materials' fire resistance. Currently, additive antipyrines or reactive flame retardants can be used during polyurethane material processing. The use of additives usually leads to the migration or volatilization of the additive to the surface of the material, which causes the loss of the resistance and aesthetic values of the product. Reactive flame retardants form compounds containing special functional groups that can be chemically bonded with monomers during polymerization, which can prevent volatilization or migration to the surface of the material. In this study, reactive flame retardants are compared. Their impacts on polyurethane flame retardancy, combustion mechanism, and environment are described.

Mechanically Tough and Highly Stretchable Hydrogels Based on Polyurethane for Sensitive Strain Sensor

Hydrogels with flexible and stretchable properties are ideal for applications in wearable sensors. However, traditional hydrogel-based sensors suffer from high brittleness and low electrical sensitivity. In this case, to solve this dilemma, a macromolecular polyurethane crosslinking agent (PCA) was designed and prepared; after that, PCA and two-dimensional (2D) MXene nanosheets were both introduced into a covalently crosslinked network to enhance the comprehensive mechanical and electrochemical properties of the hydrogels. The macromolecular polyurethane crosslinking agent promotes high-tensile strength and highly stretchable capacity by suitable covalent crosslinking. The optimized hydrogel, which exhibited maximum tensile strength and maximum elongation at break, had results of 1.21 MPa and 644%, respectively. Two-dimensional MXene nanosheets provide hydrogel with high electrical conductivity and strain sensitivity, producing a wearable device for the continuous monitoring of human movements and facial microexpressions. This study demonstrated an efficient structure design strategy for building mechanically tough, highly stretchable, and sensitive dual-mode MXenes-based wearable sensors.

Robust, Flame-Retardant, and Anti-Corrosive Waterborne Polyurethane Enabled by a PN Synergistic Flame-Retardant Containing Benzimidazole and Phosphinate Groups

Waterborne polyurethanes (WPUs) have attracted great interest owing to their environmentally friendly properties, and are wildly applied in production and daily life. However, waterborne polyurethanes are flammable. Up to now, the challenge remains to prepare WPUs with excellent flame resistance, high emulsion stability, and outstanding mechanical properties. Herein, a novel flame-retardant additive, 2-hydroxyethan-1-aminium (2-(1H-benzo[d]imidazol-2-yl)ethyl)(phenyl)phosphinate (BIEP-ETA) has been synthesized and applied to improve the flame resistance of WPUs, which has both phosphorus nitrogen synergistic effect and the ability to form hydrogen bonds with WPUs. The WPU blends (WPU/FRs) exhibited a positive fire-retardant effect in both the vapor and condensed phases, with significantly improved self-extinguishing performance and reduced heat release value. Interestingly, thanks to the good compatibility between BIEP-ETA and WPUs, WPU/FRs not only have higher emulsion stability, but also have better mechanical properties with synchronously improved tensile strength and toughness. Moreover, WPU/FRs also exhibit excellent potential as a corrosion-resistant coating.

Synthesis of DOPO-Based Phosphorus-Nitrogen Containing Hyperbranched Flame Retardant and Its Effective Application for Poly(ethylene terephthalate) via Synergistic Effect

To obtain industrialized poly(ethylene terephthalate) (PET) composites with highly efficient flame retardancy, a phosphorus-nitrogen (P-N) containing hyperbranched flame retardant additive was synthesized by 9,10-dihydro-9-oxa-10-phospho-phenanthrene-butyric acid (DDP) and tris(2-hydroxyethyl) isocyanurate (THEIC) through high temperature esterification known as hyperbranched DDP-THEIC (hbDT). The chemical structure of the synthesized hbDT was determined by FTIR, ¹H NMR, ¹³C NMR, and GPC, etc. Subsequently, hbDT/PET composites were prepared by co-blending, and the effects of hbDT on the thermal stability, flame retardancy, combustion performance, and thermal degradation behavior of PET were explored to deeply analyze its flame retardant mechanism. The test results showed that hbDT was successfully synthesized, and that hbDT maintained thermal stability well with the required processing conditions of PET as retardant additives. The flame retardant efficiency of PET was clearly improved by the addition of hbDT via the synergistic flame-retardant effect of P and N elements. When the mass fraction of flame retardant was 5%, the LOI of the hbDT/PET composite increased to 30.2%, and the vertical combustion grade reached UL-94 V-0. Compared with pure PET, great decreased total heat release (decreased by 16.3%) and peak heat release rate (decreased by 54.9%) were exhibited. Finally, the flame retardant mechanism of hbDT/PET was supposed, and it was confirmed that retardant effect happened in both the gas phase and condensed phase. This study is expected to provide a new idea for the development of low toxic, environment-friendly and highly efficient flame retardant additive for polyesters in an industry scale.

Fabrication of Highly Efficient Flame-Retardant and Fluorine-Free Superhydrophobic Cotton Fabric by Constructing Multielement-Containing POSS@ZIF-67@PDMS Micro-Nano Hierarchical Coatings

The facile construction of a cotton fabric with excellent flame-retardant and water-proof abilities is of great interest for multitask requirements. Herein, a nonfluorine, highly efficient, and cost-effective multifunctional cotton fabric was fabricated via sequentially depositing a novel multielement-containing flame-retardant phosphorylated octa-aminopropyl POSS (PPA-POSS) and a fluorine-free superhydrophobic coating of zeolitic imidazolate framework-67@poly(dimethylsiloxane) (ZIF-67@PDMS). Influences of the PPA-POSS concentration and ZIF-67@PDMS formula on the fire retardancy and water repellency of treated cotton were systematically investigated. The optimized flame-retardant sample CTF3 with 6.2 wt % PPA-POSS exhibited a high limiting oxygen index (LOI) of 34% and self-extinguishing ability. CTF3 was further modified with a properly formulated superhydrophobic ZIF-67@PDMS coating. CTF3-PHB2 displayed enhanced thermal stability, flame retardancy, and outstanding superhydrophobicity. Thermogravimetric analysis (TGA) results demonstrated that CTF3-PHB2 presented a high char residue of 35.9%, which was 220.5% higher than that of the control cotton (11.2%). More importantly, the heat release rate (HRR), total heat release (THR), and average effective heat of combustion (av-EHC) values of CTF3-PHB2 were significantly reduced by 51.4, 56.2, and 68.4%, respectively, compared with those of a pure cotton fabric. Moreover, CTF3-PHB2 showed superhydrophobicity (WCA > 159.3°) and good mechanical abrasion resistance. In addition, CTF3-PHB2 also showed protective abilities such as antifouling, self-cleaning, and water/oil separation performances even for strong acid/alkali mixtures. Thereby, it is believed that the PPA-POSS@ZIF-67@PDMS coating is promising for application in multifunctional textile materials.

Emerging trends in flame retardancy of rigid polyurethane foam and its composites: A review

Owing to the superior thermal insulating attributes of rigid polyurethane foam (RPUF) compared to other insulating materials (expanded and extruded polystyrene, mineral wool), it remains the most dominant insulating material and most studied polymer foam. Like other polyurethane foam, RPUF is highly flammable, necessitating the incorporation of flame retardants (FR) during production to lower combustibility, promoting its continuous use as insulation material in construction, transportation, and others. The popular approaches for correcting the high flammability of RPUF are copolymerization and blending (with FR). The second method has proven to be most effective as there are limited trade-offs in RPUF properties. Meanwhile, the high flammability of RPUF is still a significant hindrance in emerging applications (sensors, space travel, and others), and this has continuously inspired research in the flame retardancy of RPUF. In this study, properties, and preparation methods of RPUF are described, factors responsible for the high flammability of PUF are discussed, and flame retardancy of RPUF is thoroughly reviewed. Notably, most FR for RPUF are inorganic nanoparticles, lignin, intumescent FR systems of expandable graphite (EG), ammonium polyphosphate (APP), and hybridized APP or EG with other FR. These could be due to their ease of processing, low cost, and being environmentally benign. Elaborate discussion on RPUF FR mechanisms were also highlighted. Lastly, a summary and future perspectives in fireproofing RPUF are provided, which could inspire the design of new FR for RPUF.

Halogen-free instinct flame-retardant waterborne polyurethanes: composition, performance, and application

Ideal halogen-free instinct flame-retardant waterborne polyurethanes have high flame-retardant efficiency, environmental friendliness, fine compatibility, and good thermostability. Phosphorus flame-retardants are currently widely used in halogen-free instinct flame-retardant waterborne polyurethanes (HIFWPU), especially those with phosphorous-nitrogen co-structures. Phosphorous-nitrogen HIFWPU have become a hotspot because their co-structures provide higher flame-retardance as compared to waterborne polyurethanes. This review introduces three main types of HIFWPU based on composition, performance and application. HIFWPU not only have improved flame-retardance but also satisfy the various requirements for functionality. HIFWPU have been widely developed in textile, furniture, automobile, and aerospace applications.

An eco-friendly wood adhesive based on waterborne polyurethane grafted with gelatin derived from chromium shavings waste

An environmentally friendly wood adhesive developed from waterborne polyurethane (WPU) grafted gelatin (G) was investigated in this research. First, the G was extracted from chromium shavings waste, and then mixed with a prepolymer emulsion of WPU to synthesis the graft copolymer (WPUG) via a solvent-free emulsion copolymerization. The synthesized copolymer was characterized using the mechanical properties test, TGA, FT-IR, and other analysis technology. The results indicated that the WPUG had a good overall performance. Specifically, the contact angle reached 111.5°, the tensile strength reached 32.91 MPa, the temperature of the maximum weight loss was greater than 350 °C. The WPUG adhesive had excellent bonding power and mechanical properties; the dry bonding strength reached 4.21 MPa when the ratio between free amino groups of the G and isocyanate-groups of the WPU (the R value) was 1.5. This preparation of the graft copolymer not only satisfies the need of environment-friendly wood adhesives, but it also effectively improves the recyclability of chromium shavings waste.

Highly transparent, mechanical, and self-adhesive zwitterionic conductive hydrogels with polyurethane as a cross-linker for wireless strain sensors

Zwitterionic hydrogels have attracted a myriad of research interests for their excellent flexibility and biocompatibility as flexible wearable sensors. It is desired to create E-skins that integrate high mechanical strength, sensory sensitivity, and broad adhesion, possessing potential in the fields of intelligent robots and bionic prostheses. In this work, a novel macromolecular cross-linker (MPU) based on waterborne polyurethane (WPU) was designed and applied to synthesize multifunctional conductive hydrogels (PASU-Zn hydrogels). Importantly, in the presence of MPU, the hydrogels exhibited well-balanced mechanical properties (elongation at break 1193%, tensile strength 1.02 MPa, outstanding puncture resistance, and self-recovery abilities). When assembled as wireless strain sensors, PASU-Zn sensors displayed distinguished sensing characteristics to detect mechanotransduction signals of human movements in real-time. Specifically, owing to the dipole-dipole interaction and hydrogen bonding of zwitterions and MPU, the hydrogels have remarkable self-adhesion properties to various surfaces of wood, PDMS, and pigskin, allowing them to stick to skins by themselves without using any adhesive tapes when used. It is deemed that the as-designed zwitterionic hydrogels show great promise for wearable devices and bionic skins.

Effect of Sepiolite-Loaded Fe2O3 on Flame Retardancy of Waterborne Polyurethane

In this study, a kind of inorganic composite flame retardant (Sep@Fe2O3) was prepared by combining solution deposition and calcination methods using sepiolite microfiber material as carrier. This inorganic compound flame retardant was combined with waterborne polyurethane (WPU) through layer-by-layer method to prepare WPU composites. The SEM and EDS, TEM, and XRD were used to characterize the microscopic morphology and crystal structure of WPU composites. Thermogravimetric analysis tests confirmed the good thermal stability of WPU/Sep@Fe2O3 composites; at the temperature of 600°C, the carbon residual percentage of WPU/Sep, WPU/Fe2O3, and WPU/Sep@Fe2O3 composites is 7.3%, 12.2%, and 13.4%, respectively, higher than that of WPU (1.4%). Vertical combustion tests proved better flame-retardant property of WPU/Sep@Fe2O3 composite-coated cotton than noncoated cotton. The microcalorimeter test proved that the PHRR of WPU/Sep@Fe2O3 composites decreased by 61% compared with that of WPU. In addition, after combining with Sep@Fe2O3, the breaking strength of WPU increased by 35%.

Synergistic Effects of Ladder and Cage Structured Phosphorus-Containing POSS with Tetrabutyl Titanate on Flame Retardancy of Vinyl Epoxy Resins

The cage and ladder structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) have been synthesized through the hydrolytic condensation of 9,10-dihydro-9-oxa-10-phosphenanthrene-10-oxide (DOPO)-vinyl triethoxysilane (VTES). The unique ladder and cage–ladder structured components in DOPO-POSS endowed it with good solubility in vinyl epoxy resin (VE), and it was used with tetrabutyl titanate (TBT) to construct a phosphorus-silicon-titanium synergy system for the flame retardation of VE. Thermal stabilities, mechanical properties, and flame retardancy of the resultant VE composites were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), three-point bending tests, limiting oxygen index (LOI) measurement, and cone calorimetry. The experimental results showed that with the addition of only 4 wt% DOPO-POSS and 0.5 wt% TBT, the limiting oxygen index value (LOI) increased from 19.5 of pure VE to 24.2. With the addition of DOPO-POSS and TBT, the peak heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) were decreased significantly compared to VE-0. In addition, the VE composites showed improved thermal stabilities and mechanical properties comparable to that of the VE-0. The investigations on pyrolysis volatiles of cured VE further revealed that DOPO-POSS and TBT exerted flame retardant effects in gas phase. The results of char residue of the VE composites by SEM and XPS showed that TBT and DOPO-POSS can accelerate the char formation during the combustion, forming an interior char layer with the honeycomb cavity structure and dense exterior char layer, making the char strong with the formation of Si-O-Ti and Ti-O-P structures.

Flame retardant and smoke-suppressant rigid polyurethane foam based on sodium alginate and aluminum diethylphosphite

In order to improve the flame-retardant effect and thermal behaviour of rigid polyurethane foam (RPUF), the flame retardancy of sodium alginate (SA), aluminium diethyl phosphite (ADPO₂) and expandable graphite (EG) were proposed. First, the structures of RPUF with or without flame retardancy were confirmed by scanning electron microscopy (SEM). Additionally, the combustion behaviours and thermal performance of the flame-retardant polyurethane were evaluated through thermogravimetric analysis (TGA), limiting oxygen index (LOI) tests, and UL-94 tests. Finally, the cone calorimeter results reveled the RPUF/5ADPO₂/7.5SA/7.5EG exhibit excellent thermodynamic properties. The results of the heat release rate (HRR), total heat release (THR), total smoke production (TSP), and smoke production rate (SPR) could demonstrate the smoke-suppressant and flame-retardant of polyurethane. The system of RPUF/ADPO₂/SA/EG showed excellent flame-retardant in polyurethane.

Flame-retardant ethylene vinyl acetate composite materials by combining additions of aluminum hydroxide and melamine cyanurate: Preparation and characteristic evaluations

There is a great interest to develop efficient fire-resistant materials. While ethylene vinyl acetate (EVA) is a widely used material, it suffers from the problem of relatively high inflammability which seriously hinders its usage as the product material with a high flame-retardant requirement. In this study, a strategy to combine aluminum hydroxide (ATH) and melamine cyanurate (MCA) with EVA was proposed to prepare the EVA composite materials with high flame resistance. It was found that slight addition of MCA could increase the lubricity of EVA and raise the compatibility between EVA and ATH. Thermogravimetric analysis (TGA) indicated that the thermal stability of EVA was improved via adding MCA, which was evidenced by the delayed thermal decomposition temperature. Moreover, the combustion results indicated that the EVA composite with 60 parts per hundred (phr) ATH and 40 phr MCA addition (EVA-60-40) displayed the optimal isolated layer favoring the fire resistance. In addition, the highest limiting oxygen index (LOI) value (27.5%) and V-0 rating of the EVA-60-40 as compared with other components indicated its incombustible nature. These results suggested the synergetic effect of ATH and MCA additions, the high efficiency of the proposed strategy and the wide application prospect of the produced EVA-ATH-MCA composite materials.

High-performance water-borne fluorescent acrylic-based adhesive: synthesis and application

Water-borne adhesives have immense importance in cellulose-based materials, where their durability, handling, and strength remain to be a major concern. The present work demonstrates the development of three water-borne adhesives, namely, poly(1-vinyl-2-pyrrolidone-co-acrylic acid), poly(acrylonitrile-co-acrylic acid), and poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) applicable for cellulose-based materials. These acrylic-acid based adhesives were characterized by Fourier-transform infra-red spectroscopy, thermogravimetric analysis, X-ray diffraction, gel permeation chromatography, and universal testing machine. The synthesized polymer adhesives can be stored in the powder form for a longer period, thus utilizing less space. In order to use as adhesives, suitable formulations can be prepared in water. The adhesives show thermal stability up to 300 °C. Our studies show that poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) showed higher lap shear strength (ASTM D-906) than commercially available adhesives. In addition, these adhesives, being fluorescent in nature, can be detected under UV light and thus are applicable for the detection of fractured joints of any specimen. This property also helps in anti-counterfeiting applications, thus adding further to their utility.

Synthesis and application of a water-borne fluorescent acrylic adhesive, which can be stored as a powder for long-term use.