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To curtail flammability risks and improve material properties, flame retardants (FRs) and fillers are mixed into rubbers. High loadings of aluminum trihydroxide (ATH) and carbon black (CB) are the most used FRs and reinforcing additive, respectively, in rubbers. To reduce loading without losing mechanical properties, partial substitution of ATH as well as CB by low amounts of multilayer graphene (MLG) nanoparticles is investigated. The high aspect ratio MLG is made of ten graphene sheets. In polybutadiene/chloroprene (BR/CR) nanocomposites 3 phr MLG replaced 15 phr CB and/or 3 phr ATH. Material and mechanical properties as well as fire behavior of the nanocomposites are compared to BR/CR with 20 phr CB both with and without 50 phr ATH. MLG appears as a promising nanofiller to improve the functional properties: replacement of CB improved rheological, curing, and mechanical properties; substitution of ATH improved nanocomposite properties without affecting flame retardancy.
Valorizing “non-vegan” bio-fillers: Synergists for phosphorus flame retardants in epoxy resins
(2022)
Sustainable, biogenic flame retardant adjuvants for epoxy resins are receiving increased focus. Zoological products like insects, bone meal, and eggshells are available in large quantities, but remain uninvestigated as functional fillers to epoxy resins, although they are potential synergists to flame retardants. The efficacy and flame retardancy of “non-vegan” additives in combination with flame retardants is investigated and the fire behavior and thermal decomposition of bio-sourced epoxy resin composites is characterized. By comparing the fire performance of composites containing flame retardants or fillers at varying loadings (5, 10, and 20%), their role as synergists that enhance the function of organophosphorus flame retardants in bio-epoxy composites is identified and quantified. Peak heat release rates were 44% lower in composites containing both filler and flame retardant versus those containing only flame retardants, and fire loads were reduced by 44% versus the pure resin, highlighting the ability of “non-vegan” fillers to function as synergists.
We successfully synthesized multifunctional P-based hyperbranched polymeric flame retardants (hb-FRs) with varying oxygen-to-nitrogen (O : N) content and characterized them via 1H and 31P NMR and GPC. Their miscibility in epoxy resins (EP) and impact on glass-transition temperatures (Tg) were determined via differential scanning calorimetry (DSC). Using thermogravimetric and evolved gas Analysis (TGA, TG-FTIR), pyrolysis gas chromatography/mass spectrometry (Py-GC-MS), hot stage FTIR, flammability tests UL-94 and LOI, fire testing via cone calorimetry, residue analysis via scanning electron microscopy (SEM) and elemental analysis, detailed decomposition mechanisms and modes of action are proposed. hb-polymeric FRs have improved miscibility and thermal stability, leading to high FR performance even at low loadings. Polymeric, complex FRs increase flame retardancy, mitigate negative effects of low molecular weight variants, and can compete with commercial aromatic FRs. The results illustrate the role played by the chemical structure in flame retardancy and highlight the potential of hb-FRs as multifunctional additives.
We synthesized a library of phosphorus-based flame retardants (phosphates and phosphoramides of low and high molar mass) and investigated their behavior in two epoxy resins (one aliphatic and one aromatic).
The pyrolytic and burning behavior of the two resins (via TGA, TG-FTIR, Hot stage FTIR, Py-GC/MS, PCFC, DSC, LOI, UL-94, Cone calorimeter) are analyzed and compared to the results of flame retardant (FR)-containing composites. A decomposition pathway incorporating the identified modes of action and known chemical mechanisms is proposed. The overlap of decomposition temperature (Tdec) ranges of matrix and FR determines the efficacy of the system. Low molar mass FRs strongly impact material properties like Tg but are very reactive, and high molar mass variants are more thermally stable. Varying PeO and PeN content of the FR affects decomposition, but the chemical structure of the matrix also guides FR behavior. Thus, phosphates afford lower fire load and heat release in aliphatic epoxy resins, and phosphoramides can act as additives in an aromatic matrix or a reactive FRs in aliphatic ones. The chemical structure and the structure-property relationship of both FR and matrix are central to FR performance and must be viewed not as two separate but as one codependent system.
Hyperbranched polyphosphoesters are promising multifunctional flame retardants for epoxy resins. These polymers were prepared via thiol-ene polyaddition reactions. While key chemical transformations and modes of actions were elucidated, the role of sulfur in the chemical composition remains an open question. In this study, the FR-performance of a series of phosphorus-based flame retardant additives with and without sulfur (thioethers or sulfones) in their structure are compared. The successful synthesis of thio-ether or sulfone-containing variants is described and verified by 1H and 31P NMR, also FTIR and MALDI-TOF. A decomposition process is proposed from pyrolytic evolved gas analysis (TG-FTIR, Py-GC/MS), and flame retardancy effect on epoxy resins is investigated under pyrolytic conditions and via fire testing in the cone calorimeter. The presence of sulfur increased thermal stability of the flame retardants and introduced added condensed phase action. Likely, Sulfur radical generation plays a key role in the flame-retardant mode of action, and sulfones released incombustible SO2. The results highlight the multifunctionality of the hyperbranched polymer, which displays better fire performance than its low molar mass thio-ether analogue due to the presence of vinyl groups and higher stability than its monomer due to the presence of thio-ether groups.
A rigid aromatic phosphorus-containing hyperbranched flame retardant structure is synthesized from 10-(2,5 dihydroxyphenyl)-10H-9-oxa-
10-phosphaphenanthrene-10-oxide (DOPO-HQ), tris(4-hydroxyphenyl)phosphine oxide (THPPO), and 1,4-terephthaloyl chloride (TPC). The resulting poly-(DOPO-HQ/THPPO-terephthalate) (PDTT) is implemented as a flame retardant into an epoxy resin (EP) at a 10 wt% loading. The effects on EP are compared with those of the monomer DOPO-HQ and triphenylphosphine oxide (OPPh3) as low molar mass flame retardants. The glass transition temperature, thermal decomposition, flammability (reaction to small flame), and burning behavior of the thermosets are investigated using differential scanning calorimetry, thermogravimetric analysis, pyrolysis combustion flow calorimetry, UL 94-burning chamber testing, and cone calorimeter measurements.
Although P-contents are low at only 0.6 wt%, the study aims not at attaining V-0, but at presenting a proof of principle: Epoxy resinswith PDTT show promising fire performance, exhibiting a 25% reduction in total heat evolved (THE), a 30% reduction in peak heat release rate (PHRR) due to flame inhibition (21% reduction in effective heat of combustion (EHC)), and an increase in Tg at the same time. This study indicates that rigid aromatic hyperbranched polymeric structures offer a promising route toward multifunctional flame retardancy.
Leather is among the most ancient, widely used materials worldwide. Industrial-scale leather production produces large quantities of organic waste attained during shaving and buffing steps during processing. In this study, leather wastes (LW) are used as fillers in flame retarded polymer composites. LW is investigated as a multifunctional bio-filler that enhances the fire performance of flame retarded poly(ethylene–vinyl acetate) (EVA) containing phosphorus flame retardants (P-FRs) ammonium polyphosphate (APP) or a melamine-encapsulated APP (eAPP). Using LW from tanneries as adjuvants to enhance P-FRs in EVA reduces industrial wastes that otherwise require costly waste management solutions. Materials are characterized multi-methodically via mechanical tests, electron microscopy, rheology, thermogravimetric analysis, evolved gas analysis, and condensed phase FTIR, also reaction-to-small-flames and cone calorimeter tests. EVA containing 10 wt-% LW and 20 wt-% P-FRs achieve 20% reductions in fire loads versus EVA, and up to 10% reduction in effective heats of combustion versus EVA with equal (30 wt-%) P-FR loadings. Enhanced char stabilization of EVA composites with LW and P-FRs lowered peaks of heat release rates up to 53% compared to EVA, and up to 40% compared to equal P-FRs loadings. Synergisms between LW and P-FRs in EVA are quantified. A chemical decomposition mechanism is proposed.
Elastomers are usually reinforced by large amount of fillers like carbon black (CB) or silica in order to improve various mechanical properties, such as Young’s modulus, hardness, tear resistance, abrasion resistance, and gas barrier properties. In recent years, such improvements were also obtained by using nanoparticles at significantly lower filler loadings. Graphene is a twodimensional (2D) sheet of a thickness in the atomic scale, composed of a honeycomb structure of sp2 carbon atoms. Besides significant mechanical reinforcement, graphene harbors the potential to be used as a multifunctional filler, as it can also increase the conductivity and weathering stability of elastomer matrices. Ultraviolet (UV) irradiation and oxidative agents can lead to the degradation of elastomers due to a multistep photooxidative process, including the formation of radicals. Carbon-based fillers have an influence on these reactions, as they can absorb UV radiation and act as radical scavengers.
This chapter summarizes the results of our larger project on multilayer graphene (MLG)/elastomer nanocomposites, previously published, which present a comprehensive case study of MLG as a multifunctional nanofiller in elastomer/graphene nanocomposites. Different elastomeric matrices are compared in order to demonstrate the outstanding impact of MLG as a general benefit. The dependency of this effect on concentration is discussed in detail. Taking into account the key role of dispersion, different mixing procedures are compared, evaluating a facile implementation of graphene nanocomposites into conventional rubber processing. Finally, the most probable commercial uses of MLG nanofillers in combination with conventional CB are studied. The nanocomposites were prepared in the kg scale in order to obtain enough specimens to investigate various properties of the uncured and vulcanized rubbers at the highest quality level, including rheology, curing, morphology, several mechanical properties, abrasion, conductivity, gas permeation, burning behavior, and weathering stability. The structure property relationships are asserted and questioned, for example, by investigating the radical scavenging ability or aspect ratio of the MLG. This chapter illustrates the state of the art of graphene/rubber nanocomposites targeted for commercial mass applications.
Polyurethane foams (PUF) are generally flammable, so they are limited in some applications due to strict fire safety requirements. In this study, three distinct industrial benchmark polyurethane foams containing synergistic combinations of expandable graphite (EG) and phosphorous flame retardants (P-FR) were investigated one by one for their fire performance and smoke behavior. This paper aims to substantiate the hypothesis that the combination of EG and P-FR used in polyurethane foams yields a top-notch composite in terms of flame retardancy and smoke behavior by meeting the demanding requirement of low maximum average heat emission (MARHE) and smoke emission in a variety of applications, like advanced materials in construction, lightweight materials for railways, and more.
A series of flexible polyurethane foams (FPUFs) were prepared with single and different combinations of flame retardants and additives. Expandable graphite (EG), phosphorous polyol (OP), copper (II) oxide (CuO), and/or castor oil (CAS) were added to FPUF during the foam preparation in a one-step process. The purpose of the study is to evaluate the synergistic effects of the flame retardants, additives, and the presence of bio-based content on the mechanical properties, flame retardancy, and smoke behavior of FPUFs. The combination of 10 wt % EG and 5 wt % OP in FPUF significantly improves the char yield. In the cone calorimeter experiment, the char yield is nearly three times higher than that with 10 wt % EG alone. The smoke behavior is additionally evaluated in a smoke density chamber (SDC). Comparing the samples with a single flame retardant, 10 wt % EG in FPUF considerably reduces the amount of smoke released and the emission of toxic gases. Replacing the amount of 10 wt % polyether polyol in FPUF with CAS maintains the physical and mechanical properties and fire behavior and enhances the bio-based content. The presence of 0.1 wt % CuO in FPUF effectively reduces the emission of hydrogen cyanide. As a result, this study proposes a multicomponent flame retardant strategy for FPUF to enhance the biomass content and address the weaknesses in flame retardancy, smoke, and toxic gas emissions. A starting point is disclosed for future product development.