Recent developments in the chemistry of halogen-free flame retardant polymers

Multifunctional Gelcoats for Fiber Reinforced Composites

Achieving special features in polymer composites, such as flame retardancy and thermal and electrical conductivity, often requires the application of different additives, which might negatively affect other properties of the polymer matrix and the composite structure. Furthermore, the application of solid additives in composites produced by liquid transfer moulding can lead to the filtration of the additive by the reinforcement, which causes a non-uniform particle distribution and an uneven performance. An evident solution to address these issues is to apply the additives in a separate layer on the surface of the composite. As in many applications, gelcoats are used to reach appropriate surface quality, a reasonable progression in the composite industry is the development of multifunctional gelcoats. In this article, after a short introduction to gelcoats and their main base materials (unsaturated polyester, epoxy, and others) multifunctional gelcoats are discussed according to their functionality, in particular water resistance, electric conductivity and flame retardancy. Classical and novel gelcoat preparation methods (application by brush and/or roller, spraying, UV-curing, in-mould gelcoating), as well as common defects that occur during gelcoating are discussed. Finally, the testing methods of multifunctional gelcoats are outlined.

Effect of Graphene on Flame Retardancy of Graphite Doped Intumescent Flame Retardant (IFR) Coatings: Synergy or Antagonism

A comparative study between graphene and modified graphene oxide (mGO) on the flame retardancy of graphite doped intumescent flame retardant (IFR) coatings is preliminarily investigated by cone calorimeter (CC), XRD, and SEM, with the final aim of clarifying the interactions between different graphenes and graphite doped coatings (polyester resin-ammonium polyphosphate-urea-pentaerythritol). The CC results determine that graphene exerts an obviously antagonistic effect on flame resistance, evidenced by the increased peak heat release rate (p-HRR) of 56.9 kW·m−2 for SD8+graphene (sample coating contains graphite with a particle size of 8 μm and 0.5 wt.% graphene as dopant), which increased by 80.6% compared with SD8 (coating contains graphite with a particle size of 8 μm); substitution with graphene or mGO imparts an acceleration of fire growth, because graphene inertness improves the viscosity of melting system, evidenced by the cracked appearance and porous structure of SD8+graphene. However, the higher reactivity of mGO favors the combustion; the barrier effect inhibits the transfer of mass and heat simultaneously, leading to a slight influence on flame retarding efficiency.

A Novel POSS-Based Copolymer Functionalized Graphene: An Effective Flame Retardant for Reducing the Flammability of Epoxy Resin

In this study, a novel copolymer, PbisDOPOMA-POSSMA-GMA (PDPG), containing methacryloisobutyl polyhedral oligomeric silsesquioxane (POSSMA), reactive glycidyl methacrylate (GMA), and bis-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide methacrylate (bisDOPOMA) and derivative functionalized graphene oxide (GO) were synthesized by a one-step grafting reaction to create a hybrid flame retardant (GO-MD-MP). GO-MD-MP was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Flame-retardant epoxy resin (EP) composites were prepared by adding various amounts of GO-MD-MP to the thermal-curing epoxy resin of diglycidyl ether of bisphenol A (DGEBA, trade name E-51). The thermal properties of the EP composites were remarkably enhanced by adding the GO-MD-MP, and the residue char of the epoxy resin also increased greatly. With the incorporation of 4 wt % GO-MD-MP, the limiting oxygen index (LOI) value was enhanced to 31.1% and the UL-94 V-0 rating was easily achieved. In addition, the mechanical strength of the epoxy resin was also improved.

Recent Advances in Bio-Based Flame Retardant Additives for Synthetic Polymeric Materials

It would be difficult to imagine how modern life across the globe would operate in the absence of synthetic polymers. Although these materials (mostly in the form of plastics) have revolutionized our daily lives, there are consequences to their use, one of these being their high levels of flammability. For this reason, research into the development of flame retardant (FR) additives for these materials is of tremendous importance. However, many of the FRs prepared are problematic due to their negative impacts on human health and the environment. Furthermore, their preparations are neither green nor sustainable since they require typical organic synthetic processes that rely on fossil fuels. Because of this, the need to develop more sustainable and non-toxic options is vital. Many research groups have turned their attention to preparing new bio-based FR additives for synthetic polymers. This review explores some of the recent examples made in this field.

Phosphorus-Based α-Amino Acid Mimetic for Enhanced Flame-Retardant Properties in an Epoxy Resin

This work demonstrates the introduction of a phosphonate moiety into a commonly used curing agent, 4,4′-diaminodiphenylmethane (DDM), via an α-aminophosphonate. This compound (DDMP) can be prepared and isolated in analytical purity in under 1h and in good yield (71%). Thermoset polymer (epoxy-derived) samples were prepared using a room-temperature standard cure (SC) and a post-cured (PC) protocol to encourage incorporation of the α-aminophosphonate into the polymer network, with improved flammability properties observed for the latter. Thermogravimetric analysis under a nitrogen atmosphere showed increased char yield at 600°C, and similar observations were made when analysis was conducted in air. Significant reductions in flammability are observed at very low phosphorus content (P%=0.16–0.49%), demonstrated by higher char yields (25.5 from 14.0% in air), decreased burn time from ignition (60 to 24s), and decreased mass loss after ignition (87.6 to 58.5%). Limiting Oxygen Index for the neat polymer (P%=0%, 20.3±0.8%) increased with increasing α-aminophosphonate additive (P%=0.16%, 20.8±0.6%; P%=0.32%, 21.4±0.4%; P%=0.49%, 22.6±0.8%).

High residue bio-based structural–functional integration epoxy and intrinsic flame retardant mechanism study

Research on structural–functional integration of polymers has become an inevitable trend and development orientation in modern materials science. An intrinsic flame-retardant epoxy with superior mechanical properties and reusability is of great application value as a composite matrix and structural material. We newly synthesized two bio-based epoxy resins, VSE and VDE, the Young's modulus of product cured by DDM (4,4-diaminodiphenyl methane) achieve 5013 MPa and 4869 MPa, respectively. The LOI values of VSE and VDE were 38.7% and 34.5% respectively and both meet UL-94 V-0 rating. High char residue at 800 °C (34.5% and 28.0%, respectively) means a superior thermal stability which conventional epoxies are unreachable. Besides, cured VDE have convenient processability which can be re-shape as heating up and retain complete structural performance after cooling to room temperature. Furthermore, thermogravimetric analysis coupled with infrared spectroscopy (TGA-IR) and energy dispersive X-ray spectroscopy (EDS) were used to assist scanning electron microscopy (SEM) to investigate the intrinsic flame-retardant mechanism. In this work, the effect and process of nitrogen–phosphorus synergy on flame retardant is revealed finally. These results indicate the newly prepared epoxy has excellent flame retardancy, mechanical properties and recyclability which opens new possibilities in practical applications of epoxy such as coatings, potting or composite matrix in the near future.

Research on structural–functional integrated polymers has become an inevitable trend in modern materials science. An intrinsic flame-retardant epoxy with superior performance and reusability is of great application value in the near future.

Effects of a Phosphorus Flame Retardant System on the Mechanical and Fire Behavior of Microcellular ABS

The present work deals with the study of phosphorus flame retardant microcellular acrylonitrile–butadiene–styrene (ABS) parts and the effects of weight reduction on the fire and mechanical performance. Phosphorus-based flame retardant additives (PFR), aluminum diethylphosphinate and ammonium polyphosphate, were used as a more environmentally friendly alternative to halogenated flame retardants. A 25 wt % of such PFR system was added to the polymer using a co-rotating twin-screw extruder. Subsequently, microcellular parts with 10, 15, and 20% of nominal weight reduction were prepared using a MuCell^® injection-molding process. The results indicate that the presence of PFR particles increased the storage modulus and decreased the impact energy determined by means of dynamic-mechanical-thermal analysis and falling weight impact tests respectively. Nevertheless, the reduction of impact energy was found to be lower in ABS/PFR samples than in neat ABS with increasing weight reduction. This effect was attributed to the lower cell sizes and higher cell densities of the microcellular core of ABS/PFR parts. All ABS/PFR foams showed a self-extinguishing behavior under UL-94 burning vertical tests, independently of the weight reduction. Gradual decreases of the second peak of heat release rate and time of combustion with similar intumescent effect were observed with increasing weight reduction under cone calorimeter tests.

Plasticizers Derived from Biomass Resources: A Short Review

With rising environmental concerns and depletion of petrochemical resources, biomass-based chemicals have been paid more attention. Polyvinyl chloride (PVC) plasticizers derived from biomass resources (vegetable oil, cardanol, vegetable fatty acid, glycerol and citric acid) have been widely studied to replace petroleum-based o-phthalate plasticizers. These bio-based plasticizers mainly include epoxidized plasticizer, polyester plasticizer, macromolecular plasticizer, flame retardant plasticizer, citric acid ester plasticizer, glyceryl ester plasticizer and internal plasticizer. Bio-based plasticizers with the advantages of renewability, degradability, hypotoxicity, excellent solvent resistant extraction and plasticizing performances make them potential to replace o-phthalate plasticizers partially or totally. In this review, we classify different types of bio-based plasticizers according to their chemical structure and function, and highlight recent advances in multifunctional applications of bio-based plasticizers in PVC products. This study will increase the interest of researchers in bio-based plasticizers and the development of new ideas in this field.

Molecular Firefighting-How Modern Phosphorus Chemistry Can Help Solve the Challenge of Flame Retardancy

The ubiquity of polymeric materials in daily life comes with an increased fire risk, and sustained research into efficient flame retardants is key to ensuring the safety of the populace and material goods from accidental fires. Phosphorus, a versatile and effective element for use in flame retardants, has the potential to supersede the halogenated variants that are still widely used today: current formulations employ a variety of modes of action and methods of implementation, as additives or as reactants, to solve the task of developing flame-retarding polymeric materials. Phosphorus-based flame retardants can act in both the gas and condensed phase during a fire. This Review investigates how current phosphorus chemistry helps in reducing the flammability of polymers, and addresses the future of sustainable, efficient, and safe phosphorus-based flame-retardants from renewable sources.

Fire Alarm Wallpaper Based on Fire-Resistant Hydroxyapatite Nanowire Inorganic Paper and Graphene Oxide Thermosensitive Sensor

Wallpaper with multiple functions, such as fire resistance and an automatic alarm in fire disasters, will be attractive for the interior decoration of houses. Herein, we report a smart fire alarm wallpaper prepared using fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires (HNs) and graphene oxide (GO) thermosensitive sensors. At room temperature, the GO thermosensitive sensor is in a state of electrical insulation; however, it becomes electrically conductive at high temperatures. In a fire disaster, high temperature will rapidly remove the oxygen-containing groups of GO, leading to the transformation process of GO from an electrically insulated state into an electrically conductive one. In this way, the alarm lamp and alarm buzzer connected with the GO thermosensitive sensor will send out the alerts to people immediately for taking emergency actions. After the surface modification with polydopamine of GO (PGO), the sensitivity and flame retardancy of the GO thermosensitive sensor are further improved, resulting in a low responsive temperature (126.9 °C), fast response (2 s), and sustained working time in the flame (at least 5 min). Compared with combustible commercial wallpaper, the smart fire alarm wallpaper based on HNs and GO (or PGO) is superior owing to excellent nonflammability and high-temperature resistance of HNs, which can protect the GO (or PGO) thermosensitive sensor from the flames. The smart fire alarm wallpaper can be processed into various shapes, dyed with different colors, and printed with the commercial printer and thus has promising applications in high-safety interior decoration of houses.

Superior flame retardancy and smoke suppression of epoxy-based composites with phosphorus/nitrogen co-doped graphene

Phosphorus and/or nitrogen doping is an effective method of improving the physical and chemical properties of reduced graphene oxide (rGO). In this work, phosphorus and nitrogen co-doped rGO (PN-rGO), synthesized using a scalable hydrothermal and microwave process, was used as an additive to improve the flame retardancy of epoxy resin (EP) for the first time. Chemical structure and morphology characterization confirmed that the nitrogen and phosphorus atoms were doped into the graphite lattice adopting pyrrolic-N, pyridinic-N, quaternary-N and pyrophosphate and metaphosphate forms. Doping increased the oxidization resistance of rGO and the thermal-oxidative stability of its composites' char, while also improving the catalytic charring ability of polymer. Both effects resulted in the formation of a stable char protective layer during burning and to a significant improvement in flame retardation and smoke suppression in the final composites. The peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) for the EP-based composite (containing 5 wt% PN-rGO) decreased by 30.9%, 29.3% and 51.3%, respectively, compared to neat EP. Our work has produced a promising graphene-based flame retardant additive for the mass production of high-performance composites, also expended the application of heteroatom-doped graphene.

Enhanced thermal conductivity and flame retardancy of polyamide 6/flame retardant composites with hexagonal boron nitride

High performance composite of polyamide 6 (PA6)/flame retardant (FR)/hexagonal boron nitride (hBN) was prepared via twin screw extrusion, followed by injection molding. The heat dissipation of the composite was significantly improved by incorporating 40 vol% of hBN, and the corresponding thermal conductivity was up to 5.701 W/(m·K), nearly 17 times that of the PA6/FR composites. In addition, the combination effect of hBN and FR to the flame retardancy of the composites was observed, and the addition of hBN could dramatically enhance the flame retardancy of composites, achieving a UL94 V-0 rating with a limited oxygen index (LOI) value of 37%. This multifunctional modification would broaden the application field of PA6 composites in light-emitting diode (LED) lamps, electronic products, and so on.

High definition polyphosphoesters: between nucleic acids and plastics

Nucleic acids and synthetic polyphosphoester materials have been distinct fields – this review shows how these areas now comprise a continuum.