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Polyurea (PU) elastomers have become increasingly popular in recent years due to their exceptional blast mitigation, ballistic protection, and wear resistance properties. However, their applications as protective materials are often limited as a result of their high flammability. In this study, aromatic dianhydrides (ArDAs) were employed to capture the thermal decomposing products of PU during combustion and facilitate the generation of highly heat-resistant aromatic polyimides (i.e., end group capturing effect), thus enhancing the material's char-forming ability and improving its flame retardancy. The ArDAs were found to significantly reduce the peak heat release rate (PHRR), total heat release (THR), total smoke release (TSR), and total CO release (TCO) of PU, and meanwhile increased the amount of char residue. At 20 wt% content of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), PHRR, THR, TSR and TCO of PU were reduced by 70.6%, 32.5%, 44.7% and 58.8%, respectively, the char residue was increased by 405%, and the mechanical properties of PU were well maintained (88.5% tensile strength and 91.0% elongation at break of pure PU). These findings demonstrate that ArDAs could serve as a new kind of halogen/phosphorus-free flame retardant for PU elastomers. [Display omitted] • Novel halogen/phosphorus-free flame retardants were designed for PU elastomers. • The flame retardant mechanism is end group capturing by aromatic dianhydrides (ArDAs). • PHRR, THR, TSR and TCO were simultaneously reduced, and char residue was improved. • Both excellent flame retardant property and high mechanical property were achieved. [ABSTRACT FROM AUTHOR]

Enhancing simultaneously the flame retardancy and mechanical properties of polymer materials is a good expectation but always a tough challenge. In this work, two multifunctional compounds: phosphaphenanthrene/vinylsiloxane (DDVS) and phosphaphenanthrene/phenylsiloxane (DDPS) were synthesized, and both effectively improved the flame-retardant properties, toughness and UV shielding performance of epoxy resins (EPs) with high transparency. It was further found that the flame retardancy and toughness of EPs were greatly affected by simply changing the substituents from phenyl to vinyl groups on Si atom. Compared with DDPS, DDVS with vinyl group on Si showed more efficiency in improving flame retardancy (higher LOI value), self-extinguishing properties (higher UL 94 grade) and higher quality retention (more carbon residue) in EP. For example, DDVS was added in two-thirds of the amount of DDPS to make EPs achieve UL94 V-0 rating. This attributed to the high-temperature self-crosslinking of unsaturated carbon-carbon double bonds in DDVS, which can increase the charring ability of EPs and form more high-temperature stable char residues. Meantime, this also inhibited the release of volatile phosphorus-containing pieces from DDVS decomposition and retained more P in condense phase, thus improving the compactness and graphitization degree of char layers. The introduction of unsaturated bonds strengthened the condense-phase flame-retardant function of DDVS in EP, while retaining the original gas-phase flame-retardant actions of DOPO groups. The mechanism of gas-phase and condensed-phase mechanisms cooperated with each other, obtaining better flame retardant effect. Meanwhile, DDVS possessed greater efficiency than DDPS in increasing the fracture toughness of matrix due to the difference of steric hindrance between the substituents on silicon. Furthermore, the introduction of DDVS and DDPS did not significantly affect the transparency of EPs and the high visual transparency of materials was still maintained. In brief, the coordination design of phosphaphenanthrene and vinylsiloxane groups provided a theoretical basis to simultaneously improve the flame retardancy and toughening of EPs without losing glass transition temperature, while maintaining the high visual transparency of EPs with good UV shielding performance. [ABSTRACT FROM AUTHOR]

To simultaneously improve the storage stability of epoxy/imidazole systems and flame retardancy without sacrificing the thermal, mechanical and dielectric properties of thermosets, a novel multifunctional polyetherimide containing benzimidazole and phosphorus (mPI) was synthesized and used as a hardener for diglycidyl ether of bisphenol A (DGEBA). In addition to retaining the effective curing behavior of imidazole, mPI, a macromolecular hardener and flame retardant, could endow the mixture of DGEBA and mPI (EP/mPI) with the long latent period and thermosets with the high performance. The results showed that mPI was unreactive at ambient temperature and underwent the rapid curing reaction at 170 °C that is a little higher than its glass transition temperature (T g) of 164.7 °C, and EP/mPI showed the long thermal latent period of 70 days or more. Meanwhile, the cured EP/mPI attained the UL-94 V-0 rating with the LOI value of 31.2%. Besides, EP/mPI thermoset showed a 42.9 °C, 47.6 °C, 18.2 °C, 135.4% and 60.6% increase in the T g , heat deflection temperature, temperature at 5% mass loss, storage modulus and Young's modulus, respectively, as well as a 3.6% decrease in the dielectric constant at 100 MHz, in comparison with those of the traditional cured DGEBA/benzimidazole (EP/BIM). This work provides a facile strategy to prepare one-pot epoxy/imidazole formulations and flame-retarded epoxy thermosets with high performance by using the well-designed multifunctional polyetherimides as a hardener and flame retardant, which has good application prospects in electronics. [Display omitted] [ABSTRACT FROM AUTHOR]

To simplify the manufacture and address the intrinsic flammability of epoxy resin (EP) products, the single-component, fire-safe EP systems are increasingly in demand. However, current design strategy is difficult to achieve the simultaneous improvement in storage stability, flame retardancy and smoke suppression. Herein, a phosphorus/imidazole-containing, aluminum-based complex (ADIM) was synthesized, and applied as a latent curing agent for EP. The shelf life of EP/ADIM system reached 30 days, and its gel time at 150 °C was only 42 min, indicative of great thermal latency and fast curing rate. EP/ADIM thermoset showed superior flame retardancy, as confirmed by a limiting oxygen index (LOI) of 35.0% and a UL-94 V-0 classification. In addition, the total smoke production and maximum smoke density of EP/ADIM thermoset were 38.7% and 22.3% lower than those of EP thermoset cured by imidazole (EP/IM), demonstrating obviously-enhanced smoke suppression. ADIM inhibited the heat release and smoke generation of the EP matrix via radical capturing and catalytic charring. Moreover, EP/ADIM thermoset exhibited comparable thermal resistance and mechanical properties to EP/IM thermoset. Hence, this work offers an integrated design strategy for creating high-performance, fire-safe, single-component EP with well-preserved mechanical and thermal properties. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, a phosphorus-containing bio-based monomer (MBDPP) was first synthesized from renewable magnolol, and subsequently photo-cured with tri-thiol (TPTMP) or tetra-thiol (PETMP) monomers via thiol–ene polymerization. The synthesized MBDPP/TPTMP and MBDPP/PETMP membranes exhibited high char residues at 700 °C in the TGA tests, which were 19.8 wt% and 24.2 wt% for nitrogen while 16.5 wt% and 20.2 wt% for air, respectively, indicating superior charring capacity of both membranes. DMA results indicated that both the higher stiffness and the T g of the cured MBDPP/PETMP membrane was ascribed to its higher cross-linking density than that of the cured MBDPP/TPTMP owing to the tetra-functional groups of PETMP. In the open flame test, both MBDPP/TPTMP and MBDPP/PETMP membranes self-extinguished immediately when removing the flame source, manifesting excellent intrinsic anti-flammability. The cone calorimeter results further demonstrated that MBDPP/TPTMP and MBDPP/PETMP membranes showed lower PHRR (419 and 388 kW/m2, respectively) and THR (41.38 and 38.98 MJ/m2, respectively) values than most of the thiol-ene photo-polymerized counterparts previously reported. The excellent anti-flammability of MBDPP/TPTMP and MBDPP/PETMP membranes originated from the formation of expandable char residues that played an efficient role in hampering the transfer of flammable pyrolysis products to flame zone as well as insulating the heat irradiation during combustion. Additionally, both MBDPP/TPTMP and MBDPP/PETMP membranes showed excellent transparency (transmittance >85% in the visible region) and flexibility (breaking elongation around 40%). Taking the intrinsic anti-flammability, exceptional transparency and high flexibility into account, the MBDPP/TPTMP and MBDPP/PETMP systems are suitable as intumescent flame retardant coating materials. [Display omitted] [ABSTRACT FROM AUTHOR]

The aggregation and plasticizing of most 9,10-dihydro-9-oxa-10-phosphaphenanthrene −10-oxide (DOPO) derived additives often result in deteriorating the glass transition temperature and mechanical properties of epoxy resins (EP). To address this issue, an imide-DOPO derivative (BMP) with bulky aromatic structure was successfully synthesized. Benefitting from the space hindering and intermolecular interaction (π-π stacking) of BMP, it does not lead to the loss of glass transition temperature, and only a minor damage of tensile and impact strengths is caused. Additionally, with only 0.78 wt% loading of phosphorus (10 wt% BMP), the EP/BMP 10% achieves UL-94 V-0 and high LOI value of 35%. Simultaneously, both the heat release rate and total smoke production are markedly reduced. More excitingly, BMP exhibits aggregation-induced emission enhancement of fluorescence in the N, N -dimethylformamide/water mixed solution as well as epoxy resin. This work provides a new insight in simultaneously imparting flame retardancy and fluorescence to epoxy resins without losing glass transition temperature. [Display omitted] • Imide-DOPO derivative exhibits aggregation-induced emission enhancement. • Imide-DOPO derivative endows epoxy resin with excellent flame retardancy. • No loss of T g while the improvement of flame retardancy. • Fluorescence is imparted to the flame retardant epoxy resin. [ABSTRACT FROM AUTHOR]

Epoxy resins (EPs) are a kind of widely used engineering materials, because of their excellent mechanical, chemical and physical properties, but a severe problem they faced is their poor flame retardancy. Single molecule flame retardants have met the bottleneck after the massive researches of the past decade, so, with the development of the polymer technology, hyperbranched flame retardants have been researched and applied, owing to their better and more efficient flame retardancy. Herein, a facile thermal treating procedure was applied to the cured EP thermosets which contained a crosslinkable additive flame retardant to in-situ form a hyperbranched flame retardant. The single molecule flame retardants with N/P elements and crosslinkable nitrile groups were synthesized through an easy method, and physically mixed with EP matrix. After the curing reaction of EP matrix, the thermosets were further treated with a thermal treating procedure, while transferring the single molecule flame retardant into hyperbranched flame retardant, the semi-interpenetrating network was formed at the same time. The flame-retardant properties have been measured by limiting oxygen index (LOI) tests, UL-94 vertical burning tests and cone calorimeter tests, the results showed the flame retardants were extremely efficient and greatly elevated the flame retardancy of EPs, since the 0.5 wt% content could make modified EP samples pass the UL-94 V-0 rate, and with 20 wt% content the LOI value reached 49.5 % and the peak heat release rate reduced for 71.6 % compared with pure EPs. [ABSTRACT FROM AUTHOR]

Scientific researches have been focused on finding halogen-free solutions as effective as halogenated ones owing to environmental and health problems in recent years. This fact is the main driving force for upcoming academic works in the flame retardancy field. Boron compounds, are high potential flame retardant additive in polymer based materials owing to their low toxicity, molecular diversity, and various ways of flame retardant action. They have been used solely or as an adjuvant in combination with mineral fillers, intumescent systems, phosphorus, halogen, nitrogen, silicone and carbon containing additives. This review provides a comprehensive description of the flame retardant effect of boron compounds with general flame retardant mechanisms in natural polymers, biocomposite applications, thermoplastics, elastomers, and thermosets as well as the synergistic effect studies with other flame retardant additives. This review covers the related studies until the end of 2020. Remarkable future research opportunities and some challenges are also suggested. [Display omitted] [ABSTRACT FROM AUTHOR]