Flame Retardancy of Short Flax Fibers Modified by Radiation-Induced Grafting of Phosphonated Monomers: Comparison between Pre- and Simultaneous Irradiation Grafting.

Short flax fibers have been modified by radiation-induced grafting using methacrylate monomers containing phosphorus to give them a flame-retardant character. Two methodologies, namely pre-irradiation and simultaneous irradiation grafting, were examined. Certain parameters, notably the dose and the irradiation source (e-Beam and γ rays), were evaluated. The grafting efficiency, in terms of phosphorus content (mass percentage), was measured by X-ray fluorescence spectrometry (XRF). Using simultaneous irradiation, 2.39 wt% phosphorus could be obtained from 10 kGy, compared to 100 kGy in pre-irradiation. Furthermore, for similar phosphorus levels, the location of the grafted polymer chains was different for the two methodologies. The effect of phosphorus content on thermal properties and fire behavior was evaluated on a microscopic scale using a pyrolytic flow combustion calorimeter (PCFC) and on a laboratory scale using a cone calorimeter. It was then pointed out that flammability was linked to the phosphorus content and likely its location, which is associated with the radiation-induced grafting methodology, showing that the grafting conditions influence the final fire properties. Simultaneous irradiation, thus, proved to be more interesting in terms of efficiency and final properties.

A One-Step Approach for a Durable and Highly Hydrophobic Coating for Flax Fabrics for Self-Cleaning Application.

Highly hydrophobic flax fabrics with durable properties were prepared using the "dip-coating" method for self-cleaning application. Flax fabrics were coated with a polysiloxane coating via a hydrosilylation reaction with a Karstedt catalyst at room temperature. The coated fabrics displayed highly and durable hydrophobic properties (contact angle and sliding angle of about 145° and 23°, respectively) with good self-cleaning ability for certain pollutants and excellent durability. Moreover, the influence of the coating process on the mechanical properties of fabrics was investigated. A decrease in E modulus and an increase in tensile stress at maximum force and elongation at maximum force has been observed. Furthermore, this influence of the coating process can be easily controlled by adjusting the proportion of curing agent in the treatment solution.

Flammability of Thick but Thermally Thin Materials including Bio-Based Materials.

The fire reaction of various types of flammable lightweight materials is investigated using a cone calorimeter. The influences of parameters such as sample density, sample mass, effective heat of combustion and heat flux on the mass loss after exposition are discussed. Interpretations of the hemp fibers' tests results lead us to propose a phenomenological model able to calculate the peak of heat release rate (pHRR) of such thermally thin materials, with or without flame retardant. A database gathering the whole results of tests performed on a large set of materials including fibers, bio-resources panels, bio-based concretes and fabrics is used to validate the proposed model. Interestingly, the model is found to be relevant also for denser wood specimens. The model is based on the distinction of the contributions of the exposed top layer and the deeper layer to the combustion. Indeed, in such materials, the heat conduction is limited (either by the intrinsic properties of the material or by the formation of an insulating char) and therefore the pHRR only depends on a limited volume of materials directly absorbing the heat flux from the radiant cone. Accuracy and limitations of the model are discussed.

One-Step Multifunctionalization of Flax Fabrics for Simultaneous Flame-Retardant and Hydro-Oleophobic Properties Using Radiation-Induced Graft Polymerization.

This study concerns the one-step radiografting of flax fabrics with phosphonated and fluorinated polymer chains using (meth)acrylic monomers: dimethyl(methacryloxy)methyl phosphonate (MAPC1), 2-(perfluorobutyl)ethyl methacrylate (M4), 1H,1H,2H,2H-perfluorooctyl acrylate (AC6) and 1H,1H,2H,2H-perfluorodecyl methacrylate (M8). The multifunctionalization of flax fabrics using a pre-irradiation procedure at 20 and 100 kGy allows simultaneously providing them with flame retardancy and hydro- and oleophobicity properties. The successful grafting of flax fibers is first confirmed by FTIR spectroscopy. The morphology of the treated fabrics, the regioselectivity of grafting and the distribution of the fluorine and phosphorus elements are assessed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). The flame retardancy is evaluated using pyrolysis combustion flow calorimetry (PCFC) and cone calorimetry. The hydro- and oleophobicity and water repellency of the treated fabrics is established by contact angle and sliding angle measurements, respectively. The grafting treatment of flax irradiated at 100 KGy, using M8 and MAPC1 monomers (50:50) for 24 h, allows achieving fluorine and phosphorus contents of 8.04 wt% and 0.77 wt%, respectively. The modified fabrics display excellent hydro-oleophobic and flame-retardant properties with water and diiodomethane contact angles of 151° and 131°, respectively, and a large decrease in peak of heat release rate (pHRR) compared to pristine flax (from 230 W/g to 53 W/g). Relevant results are also obtained for M4 and AC6 monomers in combination with MAPC1. For the flame retardancy feature, the presence of fluorinated groups does not disturb the effect of phosphorus.

New Polymer Composites with Aluminum Phosphates as Hybrid Flame Retardants.

Polymeric aluminum organophosphates are a class of nanostructured aluminum-based compounds that can be considered organic and inorganic hybrid materials. Aluminum phosphates have attracted considerable interest due to their ability to enhance composite materials' mechanical characteristics, lightweight, and thermal properties. Extensive studies have shown the potential of aluminum organophosphates as a component in the development of fire-retardant materials. Aluminum-organophosphorus hybrid (APH) materials have been prepared by reacting aluminum oxide hydroxide (boehmite) with alkyl and aryl phosphoric acids and used to prepare composites with epoxy resin. Boehmite is an aluminum oxide hydroxide (γ-AlO(OH)) mineral, a component of the aluminum ore bauxite. In this work, the composites based on epoxy resin Epidian 601 and commercial curing agent IDA were obtained. Pure boehmite and APH hybrids were added as flame retardants. FTIR and TGA analysis showed that obtained APH possesses a hybrid structure, high thermostability, and various morphologies. These new APH were incorporated into epoxy resin. The infrared spectroscopy confirmed the structure of hybrids and composites. Pyrolysis combustion flow calorimetry (PCFC) and cone calorimeter analyses were performed to assess the flame retardant properties of the composites. The results showed that the incorporation of 17 wt% APH allows a reduction of heat release rate but to a limited extent in comparison to pure boehmite, which is due to the different decomposition mechanisms of both boehmite and hybrids. The cone calorimetry test showed that residue contents correspond quite well to the mineral fraction from boehmite only. The hybrid APHs appear no more efficient than pure boehmite because the mineral fraction in APH is reduced while phosphate fraction cannot promote significant charring.

Influence of Phosphorus Structures and Their Oxidation States on Flame-Retardant Properties of Polyhydroxyurethanes.

This article focuses on the synthesis of polyhydroxyurethane (PHU) materials containing novel phosphorus flame retardants (FR). Four different phosphorus compounds were grafted onto cyclic carbonate: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), diethyl phosphite (DEP), diphenyl phosphite (DPP) and dibenzo[d,f][1,3,2]dioxaphosphepine 6-oxide (BPPO). Thus, three novel phosphorus reactive cyclic carbonates which have never been reported so far were synthetized. Phosphorus FR containing PHU materials were characterized by FTIR to evidence the total conversion of the cyclic carbonate. Moreover, the gel contents up to 80% confirmed the formation of the polymer network. Then, the thermal stability and the flame-retardant properties were investigated by thermogravimetric analyses, cone calorimeter and pyrolysis combustion flow calorimeter. The mode of action of phosphorus compounds, depending on the oxidation state, was especially highlighted. Phosphonate (+III) provided better action in a condensed phase than phosphinate thanks to a more efficient char formation. Among phosphonates, differences were observed in terms of char-formation rate and expansion. DEP provided the best flame-retardant properties, with a reduction of 76% of pHRR with 2 wt% of phosphorus in cone calorimeter analysis. Therefore, this article highlighted the different modes of action of phosphorus flame retardants, depending on the oxidation state of phosphorus, in PHU materials.

Effect of Hygrothermal Ageing on the Mechanical and Fire Properties of a Flame Retardant Flax Fiber/Epoxy Composite.

In engineering applications, natural fiber composites must comply with fire requirements including the use of flame retardant. Furthermore, biocomposites are known to be water sensitive. Whether flame retardants affect the water sensitivity and whether water absorption affects the fire behavior and the mechanical performance of biocomposites are the two main topics addressed in this work. In this study, a flax fiber/epoxy composite flame retardant with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or aluminum diethyl phosphinate (AlPi) was aged in humid atmosphere or by immersion in water. Water absorption kinetics revealed that DOPO induces an increase in equilibrium water content by approximately a factor of 2 due to its intrinsic hygroscopicity and/or its plasticizing effect on the epoxy matrix. In contrast, AlPi does not significantly change the water sensitivity of the biocomposite. Mechanical testing highlighted that, whatever the FR, the evolution of mechanical properties with ageing is governed by the moisture content. The drop of elastic modulus was attributed to a decrease in fiber rigidity due to plasticization, while the increase in tensile strength was assigned to an increase in fiber/matrix friction due to fiber swelling. As regards flame retardancy, only the highest water contents modified the fire behavior. For the AlPi containing biocomposite, the water release resulted in an increase by 50% of the time to ignition, while for the DOPO flame retardant biocomposite the water release was mainly postponed after ignition.

Phosphorus Modified Cardanol: A Greener Route to Reduce VolaTile Organic Compounds and Impart Flame Retardant Properties to Alkyd Resin Coatings.

Novel phosphorylated cardanol molecules based on phosphonate (PO3CR) and phosphate (PO4CR) functions were synthetized. Those molecules have two main actions which are described in this article: the reduction in volatile organic compounds (VOC) and the development of flame retardant (FR) properties conferred on alkyd resins used as coatings for wood specimen. Phosphorylated cardanol compounds have been successfully grafted by covalent bonds to alkyd resins thanks to an auto-oxidative reaction. The impact of the introduction of PO3CR and PO4CR on the film properties such as drying time and flexibility has been studied and the thermal and flame retardant properties through differential scanning calorimeter, thermogravimetric analysis and pyrolysis-combustion flow calorimeter. These studies underscored an increase in the thermal stability and FR properties of the alkyd resins. In the cone calorimeter test, the lowest pHRR was obtained with 3 wt% P of phosphate-cardanol and exhibited a value of 170 KW.m-2, which represented a decrease of almost 46% compared to the POxCR-free alkyd resins. Moreover, a difference in the mode of action between phosphonate and phosphate compounds has been highlighted. The most effective coating which combined excellent FR properties and good coating properties has been obtained with 2 wt% P of phosphate-cardanol. Indeed, the film properties were closed to the POxCR-free alkyd resin and the pHRR decreased by 41% compared to the reference alkyd resin.

Novel Foaming-Agent Free Insulating Geopolymer Based on Industrial Fly Ash and Rice Husk.

This study highlights the synthesis of a new thermal insulating geopolymer based on the alkaline activation of fly ashes. A porous geopolymer material can be prepared without the addition of a foaming agent, using high ratio solution/ashes (activating solutions used are water, sodium or potassium hydroxide). In order to increase the porosity of the material and to make it more ecological, rice husks are incorporated into the formulation. The geopolymer materials were prepared at room temperature and dried at moderate temperature (105 °C) by a simple procedure. The microstructural characteristics of these new porous geopolymers were assessed by optical microscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and X-ray fluorescence (XRF). Infrared spectroscopy (FTIR) was used to confirm the geopolymerisation. The effect of the ratio solution/ashes and the percentage of the rice husk addition on thermal and mechanical analysis was evaluated. An insulating material for a solution/ashes ratio of 0.9 and a rice husk content of 15% having a λ value of 0.087 W/(m·K), a porosity of 61.4% and an Rc value of 0.1 MPa was successfully prepared.

Fire Behavior of Thermally Thin Materials in Cone Calorimeter.

In this study, a representative set of thermally thin materials including various lignocellulosic and synthetic fabrics, dense wood, and polypropylene sheets were tested using a cone calorimeter at different heat fluxes. Time-to-ignition, critical heat flux, and peak of heat release rate (pHRR) were the main parameters considered. It appears that the flammability is firstly monitored by the sample weight. Especially, while the burning rate of thermally-thin materials does never reach a steady state in cone calorimeter, their pHRR appears to be mainly driven by the fire load (i.e., the product of sample weight and effective heat of combustion) with no or negligible influence of textile structure. A simple phenomenological model was proposed to calculate the pHRR taking into account only three parameters, namely heat flux, sample weight, and effective heat of combustion. The model allows predicting easily the peak of heat release rate, which is often considered as the main single property informing about the fire hazard. It also allows drawing some conclusions about the flame retardant strategies to reduce the pHRR..

Correction: Sonnier, R., et al. Influence of Density on Foam Collapse under Burning. Polymers 2020, 13, 13.

The authors wish to make the following corrections to this paper [...].

Influence of Density on Foam Collapse under Burning.

The fire behaviour of flexible polyurethane foams was studied using a cone calorimeter, with a special emphasis on the collapse step. Only one peak of heat release rate, ranging from 200 to 450 kW/m2, is observed for thin foams, depending on the foam density and the heat flux. On the contrary, heat release rate (HRR) curves exhibit two peaks for 10 cm-thick foams, the second one corresponding to the pool fire formed after foam collapse. In all cases, the collapse occurs at a constant rate through the whole thickness. The rate of the recession of the front was calculated using digital and infrared cameras. Interestingly, its value is relatively constant whatever the heat flux (especially between 25 and 35 kW/m2), probably because of the very low heat conductivity preventing heat transfer through the thickness. The rate increases for the lightest foam but the fraction of burnt polymer during collapse is constant. Therefore, the pool fire is more intense for the densest foam. A simple macroscopic model taking into account only the heat transfer into the foam leads to much lower front recession rates, evidencing that the collapse is piloted by the cell walls' rigidity.

Suitability and Modification of Different Renewable Materials as Feedstock for Sustainable Flame Retardants.

Due to their chemical structure, conventional flame retardants are often toxic, barely biodegradable and consequently neither healthy nor environmentally friendly. Their use is therefore increasingly limited by regulations. For this reason, research on innovative flame retardants based on sustainable materials is the main focus of this work. Wheat starch, wheat protein, xylan and tannin were modified with phosphate salts in molten urea. The functionalization leads to the incorporation of phosphates (up to 48 wt.%) and nitrogen (up to 22 wt.%). The derivatives were applied on wood fibers and tested as flame retardants. The results indicate that these modified biopolymers can provide the same flame-retardant performances as commercial compounds currently used in the wood fiber industry. Besides, the flame retardancy smoldering effects may also be reduced compared to unmodified wood fibers depending on the used biopolymer. These results show that different biopolymers modified in phosphate/urea systems are a serious alternative to conventional flame retardants.

Fire behavior of innovative alginate foams.

A new biosourced composite foam (AF, associating foamed alginate matrix and orange peel filler) is successfully tested for fire-retardant properties. This material having similar thermal insulating properties and density than fire-retardant polyurethane foam (FR-PUF, a commercial product) shows promising enhanced properties for flame retardancy, as assessed by different methods such as thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC) and a newly designed apparatus called RAPACES for investigating large-scale samples. All these methods confirm the promising properties of this alternative material in terms of fire protection (pHRR, THR, EHC, time-to-ignition, flame duration or production of residue), especially for heat flux not exceeding 50 kW m-2. At higher heat flux (i.e., 75 kW m-2), flame retardant properties tend to decrease but maintain at a higher level than FR-PUF. The investigation of the effect of AF thickness shows that the critical thickness (CT) is close to 1.5-1.7 cm: heat diffusion and material combustion are limited to the CT layer that protects the underlying layers from combustion. A multiplicity of factors can explain this behavior, such as: (a) negligible heat conduction, (b) low heat of combustion, (c) charring formation, and (d) water release. Water being released from underlying layers, dilutes the gases emitted during the combustion of superficial layers and promotes the flame extinction.

Flame Retardant-Functionalized Cotton Cellulose Using Phosphonate-Based Ionic Liquids.

Cellulose from cotton fibers was functionalized through a dissolution-regeneration process with phosphonate-based ionic liquids (ILs): 1,3-dimethylimidazolium methylphosphonate [DIMIM][(MeO)(H)PO2] and 1-ethyl-3-methylimidazolium methylphoshonate [EMIM][(MeO)(H)PO2]. The chemical modification of cellulose occurred through a transesterification reaction between the methyl phosphonate function of ILs and the primary alcohol functions of cellulose. The resulting cellulose structure and the amount of grafted phosphorus were then investigated by X-ray diffraction, ICP-AES, and ¹³C and ³¹P NMR spectroscopy. Depending on the IL type and initial cotton / IL ratio in the solution, regenerated cellulose contained up to 4.5% of phosphorus. The rheological behavior of cotton cellulose/ILs solutions and the microscale fire performances of modified cellulose were studied in order to ultimately prepare flame retardant cellulosic materials. Significant improvement in the flame retardancy of regenerated cellulose was obtained with a reduction of THR values down to about 5-6 kJ/g and an increase of char up to about 35 wt%.

Flame Retardancy of Wood Fiber Materials Using Phosphorus-Modified Wheat Starch.

Biopolymer-based flame retardants (FR) are a promising approach to ensure adequate protection against fire while minimizing health and environmental risks. Only a few, however, are suitable for industrial purposes because of their poor flame retardancy, complex synthesis pathway, expensive cleaning procedures, and inappropriate application properties. In the present work, wheat starch was modified using a common phosphate/urea reaction system and tested as flame retardant additive for wood fibers. The results indicate that starch derivatives from phosphate/urea systems can reach fire protection efficiencies similar to those of commercial flame retardants currently used in the wood fiber industry. The functionalization leads to the incorporation of fire protective phosphates (up to 38 wt.%) and nitrogen groups (up to 8.3 wt.%). The lowest levels of burning in fire tests were measured with soluble additives at a phosphate content of 3.5 wt.%. Smoldering effects could be significantly reduced compared to unmodified wood fibers. The industrial processing of a starch-based flame retardant on wood insulating materials exhibits the fundamental applicability of flame retardants. These results demonstrate that starch modified from phosphate/urea-systems is a serious alternative to traditional flame retardants.

Exploring the Contribution of Two Phosphorus-Based Groups to Polymer Flammability via Pyrolysis-Combustion Flow Calorimetry.

From a set of around 100 phosphorus-containing polymers tested in pyrolysis-combustion flow calorimetry, the contributions to flammability of two phosphorus-containing pendant groups (called 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and PO3) were calculated using an advanced method previously proposed and validated. The flammability properties include total heat release (THR) and heat release capacity (HRC) measured in standard conditions, i.e., anaerobic pyrolysis and complete combustion. The calculated contributions are in good agreement with the main modes of action of both phosphorus groups, i.e., flame inhibition for DOPO and char promotion for PO3. Moreover, the results provide first conclusions about the cooperative interaction between phosphorus and nitrogen, as well as the influence of the architecture of tested co-polymers.

Cardanol and Eugenol Based Flame Retardant Epoxy Monomers for Thermostable Networks.

Epoxy materials have attracted attention for many applications that require fireproof performance; however, the utilization of hazardous reagents brings about potential damage to human health. Eugenol and cardanol are renewable, harmless resources (according to ECHA) that allow the achievement of synthesis of novel phosphorylated epoxy monomers to be used as reactive flame retardants. These epoxy building blocks are characterized by 1H NMR and 31P NMR (nuclear magnetic resonance) and reacted with a benzylic diamine to give bio-based flame-retardant thermosets. Compared to DGEBA (Bisphenol A Diglycidyl Ether)-based material, these biobased thermosets differ by their cross-linking ratio, the nature of the phosphorylated function and the presence of an aliphatic chain. Eugenol has led to thermosets with higher glass transition temperatures due to a higher aromatic density. The flame-retardant properties were tested by thermogravimetric analyses (TGA), a pyrolysis combustion flow calorimeter (PCFC) and a cone calorimeter. These analyses demonstrated the efficiency of phosphorus by reducing significantly the peak heat release rate (pHRR), the total heat release (THR) and the effective heat of combustion (EHC). Moreover, the cone calorimeter test exhibited an intumescent phenomenon with the residues of phosphorylated eugenol thermosets. Lastly, the higher flame inhibition potential was highlighted for the phosphonate thermoset.

Influence of Ammonium Polyphosphate/Lignin Ratio on Thermal and Fire Behavior of Biobased Thermoplastic: The Case of Polyamide 11.

Flame retardancy of polymers is a recurring obligation for many applications. The development trend of biobased materials is no exception to this rule, and solutions of flame retardants from agro-resources give an advantage. Lignin is produced as a waste by-product from some industries, and can be used in the intumescent formation development as a source of carbon combined with an acid source. In this study, the flame retardancy of polyamide 11 (PA) is carried out by extrusion with a kraft lignin (KL) and ammonium polyphosphate (AP). The study of the optimal ratio between the KL and the AP makes it possible to optimize the fire properties as well as to reduce the cost and facilitates the implementation of the blend by a melting process. The properties of thermal decomposition and the fire reaction have been studied by thermogravimetric analyzes, pyrolysis combustion flow calorimetry (PCFC) and vertical flame spread tests (UL94). KL permits a charring effect delaying thermal degradation and decreases by 66% the peak of heat release rate in comparison with raw PA. The fire reaction of the ternary blends is improved even if KL-AP association does not have a synergy effect. The 25/75 and 33/67 KL/AP ratios in PA give an intumescence behavior under flame exposure.

Fire behavior of halogen-free flame retardant electrical cables with the cone calorimeter.

Fires involving electrical cables are one of the main hazards in Nuclear Power Plants (NPPs). Cables are complex assemblies including several polymeric parts (insulation, bedding, sheath) constituting fuel sources. This study provides an in-depth characterization of the fire behavior of two halogen-free flame retardant cables used in NPPs using the cone calorimeter. The influence of two key parameters, namely the external heat flux and the spacing between cables, on the cable fire characteristics is especially investigated. The prominent role of the outer sheath material on the ignition and the burning at early times was highlighted. A parameter of utmost importance called transition heat flux, was identified and depends on the composition and the structure of the cable. Below this heat flux, the decomposition is limited and concerns only the sheath. Above it, fire hazard is greatly enhanced because most often non-flame retarded insulation part contributes to heat release. The influence of spacing appears complex, and depends on the considered fire property.