Sustainable Flame Retardant Research Articles

Sustainable lignin-based high-efficiency flame-retardant epoxy resins with excellent mechanical properties

As an abundant natural aromatic polymer, how to realize the high-value utilization of lignin (LA) is still a difficult problem. Herein, a sustainable flame retardant (LA@APP) was synthesized through hydrogen bonding interactions between LA and ammonium polyphosphate (APP), then flame retarded alone epoxy resin (EP/LA@APP). With loading of 6 wt% LA@APP, the LOI of EP/LA@APP6 rose to 27.9 % and achieved UL-94 V0 rating. Meanwhile, EP/LA@APP6 presented a 63.5 % reduction in peak of heat release rate (pHRR) and an 51.3 % decrease in peak of smoke production rate (pSPR). LA@APP could promote the EP composites to form the denser and more continuous expanded charring residuals than APP during the combustion process, effectively shielding the underlying epoxy substrate and thus enhancing fire safety. Additionally, the shell layer of LA@APP, abundant in hydroxyl groups and methoxy groups, effectively improved the compatibility and interfacial interaction with EP. Therefore, the tensile, flexural and impact strength of EP/LA@APP6 reached 50.8 MPa, 86.1 MPa and 7.81 kJ/m2, higher than those of pure EP at 48.3 MPa, 81.5 MPa and 7.45 kJ/m2. This flame retardant with simultaneously enhanced fire safety and mechanical properties of epoxy resin was expected to realize the high-level utilization of lignin.

Development of sustainable flame-retardant bio-based hydrogel composites from hemp/wool nonwovens with chitosan-banana sap hydrogel

Flame retardant (FR) finishing is crucial for developing protective textiles, traditionally relying on halogen, phosphorus, and phosphorus-nitrogen chemistries, which have limitations like toxicity and fabric stiffening. Innovative approaches such as nanotechnology, plasma treatments, and natural resource-based finishes are being explored to achieve sustainable FR textiles. This study presents the development and comprehensive characterization of hydrogel composites made from nonwoven fabrics composed of various hemp/wool blends (70/30, 80/20, and 90/10). The nonwoven fabrics were treated with a chitosan hydrogel incorporating banana sap to enhance their properties. Scanning electron microscope (SEM) examined the surface morphology and structural integrity of the composites, while Fourier transform infrared spectroscopy (FTIR) identified chemical interactions and functional groups. Differential scanning calorimeter (DSC) revealed thermal properties, water absorbency tests demonstrated hydrophilicity, mechanical testing assessed tensile strength, and vertical flammability tests evaluated fire resistance. SEM and FTIR revealed a successful coating of chitosan hydrogel with banana sap inclusions onto the hemp/wool nonwoven fabric, forming a composite structure. DSC analysis suggests higher chitosan content and hemp fiber ratio (like 70/30) lead to increased thermal stability of hydrogel composites. Higher chitosan concentrations in the hydrogel significantly improve the flame-retardant properties of hemp/wool nonwoven fabrics by reducing char length and enhancing protective char layer formation, with banana sap further promoting charring. These results indicate that the developed composite can be effectively used in flame-retardant textiles.

Bio-functionalization of fillers with natural phytic acid for sustainable flame-retardant agents for natural rubber composites

The current challenge for sustainable flame retardants has led to innovative approaches, including the utilization of natural substances to enhance the fire resistance of elastomer composites. This paper presents the design of eco-friendly hybrid flame-retardant additives based on mineral fillers (hydrotalcite (LDH) and sepiolite (SEP)) modified with aminosilane (APTS) and phytic acid (PA) for natural rubber (NR) biocomposites. The synergistic effects of the PA and silanized mineral fillers resulted in NR composites with improved thermal, mechanical, and barrier properties, as well as increased flame retardancy. Importantly, it was proven that direct in-situ application of PA into elastomer cured with sulphur-based crosslinking systems is ineffective, due to its negative effect on the curing process (significantly reducing torque moment). Binding the PA onto silanized fillers enabled the creation of hybrid additives that leverage the flame-retardant benefits of PA without compromising the other properties of the polymer. The addition of PA-modified fillers to NR reduced the heat release rate (HRR) by around 40 %, indicating outstanding flame retardancy. This work makes a significant contribution and expands the understanding of using eco-friendly natural compounds to reduce the flammability of sulfur-cured elastomers.

Wastewater phosphorus enriched algae as a sustainable flame retardant in polylactide

Revolutionizing our polymer industry for adaption to a sustainable carbon circular economy has become one of today's most demanding challenges. Exploiting renewable resources to replace fossil-fuel—based plastics with biopolymers such as poly(lactic acid) (PLA) is inevitable while using waste streams as a raw material resource at least is promising. When it comes to using PLA as technical polymer, its high flammability must be addressed by flame retardants compatible with the thermoplastic processing of PLA and its compostability. This study proposes microalgae enriched with phosphorus from wastewater (P-Algae) as an elegant way towards a kind of sustainable organophosphorus flame retardant. The concept is demonstrated by investigating the processing, pyrolysis, flammability, and fire behavior of PLA/P-Algae, while varying the P-Algae content and comparing P-Algae with four alternative bio-fillers (phosphorylated lignin, biochar, thermally treated sewage sludge, and metal phytate) with different P-contents as meaningful benchmarks.

Keratinous Natural Fibres as Sustainable Flame Retardants and Reinforcements in Polymer Composites

Natural fibres have been used as fibre reinforcements in composites as they offer eco-friendly and economic advantages, but their susceptibility to deterioration when exposed to heat and flames has limited their practical application in fibre-reinforced polymeric composites. Fire-reaction properties have been explored in reasonable detail for plant fibres, but a gap exists in the understanding of animal fibre-reinforced composites. Understanding the thermal and fire reactions of these keratin-rich animal fibres is crucial for material selection and advancing composite product development. The current paper critically discusses the existing research landscape and suggests future research directions. The use of keratinous fibres in composites can definitely improve their thermal stability and fire performance, but it also appears to adversely affect the composite’s mechanical performance. The main part of this paper focuses on the flame-retardant treatment of keratinous fibres and polymer composites, and their behaviour under fire conditions. The final part of this paper includes a brief look at the environmental impact of the treatment methods; the overall processing of keratinous fibre-reinforced composites is also presented to gain further insight.

Simple and environmentally friendly preparation strategy of zeolite supported iron diphenylphosphate for improving the fire safety of EP

The design of efficient epoxy resin (EP) flame retardants and the exploration of low toxicity and sustainable flame retardants strategies remain a great challenge. This work proposed a novel Zeolite@DPA-Fe flame retardant was prepared by a simple and environmentally friendly aqueous solution method using natural zeolite, iron chloride and diphenyl phosphonic acid as the main raw materials, which was used to improve the fire safety of EP. The results showed that Zeolite@DPA-Fe can endow EP with good flame retardant properties. After adding 5 wt% Zeolite@DPA-Fe, the LOI of EP composites was 32.2 %, reaching UL-94 V-0 grade. In addition, the catalytic charring of Zeolite@DPA-Fe promoted the graphitization of the char layer and significantly reduced the rate of heat release and smoke production during composite combustion. Hence, this study provides a promising strategy for the preparation of highly efficient and environmentally friendly flame retardant EP composites.

Exploiting Waste towards More Sustainable Flame-Retardant Solutions for Polymers: A Review.

The development of sustainable flame retardants is gaining momentum due to their enhanced safety attributes and environmental compatibility. One effective strategy is to use waste materials as a primary source of chemical components, which can help mitigate environmental issues associated with traditional flame retardants. This paper reviews recent research in flame retardancy for waste flame retardants, categorizing them based on waste types like industrial, food, and plant waste. The paper focuses on recent advancements in this area, focusing on their impact on the thermal stability, flame retardancy, smoke suppression, and mechanical properties of polymeric materials. The study also provides a summary of functionalization methodologies used and key factors involved in modifying polymer systems. Finally, their major challenges and prospects for the future are identified.

Bio-derived polyphosphazene modified halloysite nanotubes as eco-friendly flame retardants to significantly reduce smoke release and enhance the fire safety of epoxy resin

At present, designing efficient bio-based flame retardants for epoxy resin (EP) and exploring low-toxicity and sustainable flame retardant strategies is still a great challenge. In this work, a novel bio-based hybrid flame retardant was successfully prepared by constructing a resveratrol-based cyclic cross-linked polyphosphazene (PPZ) shell on the surface of halloysite nanotube (HNT). The enhancement effects of HNT@PPZ on the fire safety of EP composites were investigated by thermal degradation, simulated fire performance (cone calorimeter test), gas-phase product analysis, and condensed-phase char residue analysis. Compared to pure EP, he total heat release (THR), total smoke production (TSP) and the average yield of carbon monoxide (Av-COY) of EP/3HNT@PPZ were reduced by 47.8 %, 46.5 % and 48.8 % respectively. Furthermore, the EP composites containing HNT@PPZ had a stable flame retardancy even after 6 months of natural aging. It was found the barrier effect of HNT@PPZ in condensed phase was further strengthened by the formation of new product aluminum phosphate, which formed a multiple protective char layer. Additionally, PPZ shell improved compatibility and dispersion of HNT in EP matrix, the mechanical properties of the composite with HNT@PPZ were improved significantly. This work proposed a feasible solution for the preparation of highly efficient bio-based flame retardants with excellent heat reduction and smoke toxicity reduction ability, which could be expected to broaden the application of biomass resources in the development of highly efficient flame retardants.

Chemical, pyrolysis, combustion properties and mechanism analysis of wood treated with biomass-based carrageenan-collagen modified ammonium polyphosphate

Biomass-based flame retardants are characterized by readily available raw materials and are environmentally friendly, making the design of green and efficient biomass-based flame retardants a critical direction in the development of flame-retardant technology. In this study, a novel biomass-based flame retardant (Acc) was developed using ammonium polyphosphate (APP) as a raw material in combination with the biomass materials carrageenan (CAR) and collagen (COL). Flame-retardant wood was prepared via ultrasonic impregnation. Via various analytical and testing methods, such as the Limiting Oxygen Index (LOI), thermogravimetric analysis, scanning electron microscopy, Raman spectroscopy, cone calorimetry, and thermogravimetric–infrared coupling, the combustion performance of biomass-based flame retardants on wood was investigated to elucidate the flame-retardant mechanism. The results indicated that the flame retardant with a CAR:COL:APP ratio of 1:1:3 exhibited the best flame-retardant effect, with an LOI of 31.8 %. The peak heat release rate was reduced to 53.8 % compared with that of untreated wood, and both the peak smoke release rate and total smoke release were reduced by 63.2 % and 49.2 %, respectively, demonstrating a significant smoke-suppression effect. Analysis of the mechanism revealed that APP, COL and CAR generated non-combustible gases such as ammonia and gases containing P and S when exposed to fire. These gases dilute oxygen and eliminate •H and •HO radicals in the gas phase, thereby exerting flame-retardant effects. Acc promotes the formation of a dense crosslinked carbon layer containing N=C, N-C/P-N, N-H, and C-O-P. This carbon layer acts as a physical barrier, isolating oxygen (O2), blocking gas emissions, and slowing heat exchange. This study provides a new avenue for the development of green and sustainable flame retardant wood.

Fabrication of sustainable flame retardant cotton fabrics via foam finishing with sodium polyborate

The conventional process of imparting flame retardancy to cotton fabrics requires the consumption of large amounts of water, chemicals, and energy, which usually involves halogen-containing and phosphorus-containing chemicals. In this work, an eco-friendly sodium polyborate (SPB) foam was creatively introduced onto the cotton fabric surface by a bladed coater to improve the fire resistant performance. For the cotton sample with only 6.7% weight gain (WG) of SPB coating, the LOI value of cotton fabric (SPB treated cotton-1) was enhanced to 32.6% from 18.5%, the damaged length was decreased to 5.7 cm, and the sample self-extinguished in the vertical burning test. The peak of heat release rate value, total heat release value, and total smoke production (TSP) value decreased by 87.0%, 72.7%, and 22.2% respectively compared with those of the control sample. The TSP was further decreased by 80.6% when the WG of SPB was 16.7%. Besides, the SPB treated cotton showed enhanced antibacterial activities against S.aureus and E.coli. Particularly worth mentioning is in the SPB foam finishing procedure, no organic solvent and P or Cl-containing chemicals were involved, and the processing time was only around 3 min with obviously reduced water consumption, compared with dip-padding finishing. More importantly, SPB treated cotton kept good softness, whiteness, water vapor permeability, and air permeability. The SPB foam finishing in this work shows considerable potential in realize good flame retardancy on cotton fabrics by eco-friendly and high-efficient strategy.

Polyaniline/chitosan coating as the novel and sustainable flame retardant and UV protection route for silk fabric

The growing awareness of human health and environmental protection stimulates the development of more sustainable flame retardant (FR) approaches. In this study, polyaniline (PANI) with a nitrogen-containing aromatic structure was synthesized as a novel functional compound to improve the FR and UV protection performance of silk fabric. Biobased chitosan was applied to develop the PANI/chitosan coating to improve the binding capability and dispersion uniformity of PANI on the silk surface. The surface morphology, thermal resistance, heat generation capacity, flame retardancy and UV protection performance were explored. The PANI/chitosan coating with 3 wt% PANI endowed the silk fabric with effective flame retardancy. The coated silk fabric self-extinguished during vertical flammability test and possessed a decreased char length of 9.5 cm versus 30 cm of the pristine silk fabric; it could also withstand 20 launderings, showing good laundering resistance. The coated silk fabric displayed markedly decreased heat release behavior. A combined condensed phase and gas phase FR action was suggested for the PANI/chitosan coating on silk fabric. The coated silk fabric also exhibited excellent UV protection with the UPF of 50+ because of the UV-shielding of PANI.

Development of a process for flame retardant coating of textiles with bio-hybrid anchor peptides

The fire protection of materials has an important role in our everyday life and covers a highly diverse spectrum of substances, materials, and fields of application. Important fields of application for fire protection, especially in public areas, are construction and transport, electronic devices, furnishings, and textiles (e.g., applications for occupational safety, carpets, curtains, upholstery, insulation, and technical applications in outdoor areas). The efficient and durable finishing of materials with flame retardant additives is crucial to ensure effective fire protection. Many of the flame retardant additives currently used are based on bromides, chlorides, phosphates, or antimony. However, these flame retardants are harmful to the environment and/or health (not phosphates). Therefore, the use of these flame retardants is already being restricted by EU directives (e.g., REACH regulation), and it is foreseeable that they will be further restricted in the future. To keep up with this development, innovative and sustainable solutions must be developed in the short term. The amount of flame retardant additives that are harmful to the environment and health must be reduced. In the medium term, these harmful additives must be completely replaced by sustainable flame retardant additives that are not harmful to the environment and health. This paper describes research results to reduce the amount of additives in the short term. In order to reduce the amount of additives used, an innovative refinement process is being developed. In a first step, the flame retardant additives are combined with bio-based adhesion promoters (anchor peptides). Anchor peptides bind with high selectivity, binding strength, and occupancy density to a broad portfolio of materials (e.g., synthetic polymers, metals, ceramics, and natural materials) and enable the finishing of the materials with a broad spectrum of functional units (e.g., flame retardant additives). Material functionalization by anchor peptides is energy-efficient and resource-saving at room temperature in aqueous solution and is scalable in its production. Based on these developments, in this paper, a finishing process is presented with which flame retardant textiles can be equipped with bio-based anchor peptides. A requirements’ outline for the new finishing process is described. Established processes (e.g., foulard, coating machine, and roller application) are compared with each other and evaluated with regard to the requirements and their suitability. The most suitable process is then designed, and a laboratory scale as well as an industry scale concept are presented.

Sustainable Flame Retardant Coatings Derived from Chinese Lacquer Blend with Synergistic Flame Retardant Silicon-Modified Ammonium Polyphosphate

The intumescent flame retardant has a high efficiency of carbonization, oxygen barrier, and thermal interruption effect, which is considered to be a promising green and environmentally friendly flame retardant. Herein, we demonstrated a new approach to improve the flame retardant performance of Chinese lacquer films by adding silicon modified ammonium polyphosphate (SAPP), leading to superior flame retardant. SAPP drastically raised the limiting oxygen index (LOI) from 20.1% to 30.6%, and at the same time, the impact resistance was increased from 7 cm to 20 cm without changing the hardness of the lacquer films. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy were used to investigate the chemical components and morphology of the residual char of Chinese lacquer and SAPP composite films. The thermal degradation process of films was detected and analyzed by thermogravimetry and microcalorimetry, and the possible flame retardant mechanism was discussed. This simple approach opens up a new way to design for improved flame retardancy of organic film polymers.

Rethinking the pathway to sustainable fire retardants.

Flame retardants are currently used in a wide range of industry sectors for saving lives and property by mitigating fire hazards. The growing fire safety requirements for materials boost an escalating demand for consumption of fire retardants. This has significantly driven both the industry and scientific community to pursue sustainable fire retardants, but what makes a sustainable flame retardant? Here an overview of recent advances in sustainable flame retardants is offered, and their renewable raw materials, green synthesis and life cycle assessments are highlighted. A discussion on key challenges that hinder the innovation of fire retardants and design principles for creating truly sustainable yet cost-effective fire retardants are also presented. This short work is expected to help drive the development of sustainable, cost-effective fire retardants, and expedite the creation of a more sustainable and safer society.

Sustainable composites for lightweight and flame retardant parts for electric vehicles to boost climate benefits: A perspective

The global automotive sector is moving towards zero-emission transportation, which has led to the rapid growth of the electric vehicles (EVs) market, projected to reach USD 1318 billion in 2028. The two key challenges that EVs are facing are their heavy curb weight (∼ 1000 lbs. heavier than their petroleum-based counterparts) due to their battery packs, and their fire threat, caused by thermal runaway of batteries. Further, growing concerns about the sustainability of EV technology has created pressure on automotive manufacturers to drive innovation of new materials with less environmental impacts. These challenges can be addressed by developing high-performance sustainable polymer composites for lightweight components for EVs with enhanced flame retardancy without compromising their performance. Manufacturing and integration of automotive parts from sustainable composites target and support the “Circular Economy” of EVs. Lightweighting EV components will increase travel ranges, minimize crash impacts, lower braking distances, and reduce the wear and tear of the tires. In sustainable and renewable composite manufacturing, “engineering plastics” including recycled carbon fibers and sustainable resource derived advanced biocarbon will have an edge over “traditional plastics”. In addition, the inclusion of sustainable flame-retardants in the EV components will increase the flashover duration, resulting in better thermal management and delay in thermal runaway. Thus, in this perspective, the recent acceleration in automotive weight reduction strategies is discussed through the utilization of high-performance lightweight and sustainable composites with superior flame retardancy to address the criteria for the circularity of materials towards climate benefits.

Hexachlorocyclotriphosphazene functionalized lignin as a sustainable and effective flame retardant for epoxy resins

Lignin, a low-cost and abundant polyphenol biomass, has considerable potential to develop sustainable flame retardants for polymers due to its aromatic skeleton and high char-forming capability after combustion. In order to integrate phosphorous and nitrogen elements, herein lignin was facilely modified by hexachlorocyclotriphosphazene through a one-step nucleophilic substitution reaction. The functionalized lignin (Lig-HCCP) was used as an additive to improve the flame retardancy of epoxy resins. The effects of Lig-HCCP with different loading levels on the curing behaviors, rheological characteristics, as well as thermal, mechanical, and flame-retardant properties of the resulting epoxy composites were studied in detail. Notably, the epoxy composite containing 20 wt% Lig-HCCP showed the highest limited oxygen index (LOI) of 28.2% and achieved a UL-94 V-0 rating in the vertical burning test. Furthermore, the results of cone calorimeter tests indicated that the peak heat release rate, heat release rate, and total smoke production of the epoxy composite were decreased by 58.9%, 38.5%, and 63.1%, respectively, compared with the control resin. The flame retardant mechanisms could be ascribed to the P-N synergy and abundant aromatic structures in Lig-HCCP, which contributed to the formation of intumescent and compact char layers. This work demonstrates a simple and promising approach to utilize lignin feedstocks for value-added flame retardants.

Waterborne hybrid polyurethane coatings containing Casein as sustainable green flame retardant through different synthesis approaches

Waterborne polyurethane (WPU) dispersions were prepared for flame retardant coatings. Specifically, alkoxysilane-capped polycaprolactone-based WPUs were synthesized employing the acetone process, and Casein, as a green and sustainable flame retardant additive, was added by two different methods (in situ and ex situ). These two strategies made possible to evaluate the effect of the Polyurethane/Casein interaction in the final properties of the dispersions and films. FTIR and solid-state 29Si NMR, confirmed the formation of the siloxane network during film generation process. The addition of Casein during the synthesis (in situ) resulted in a covalent bonding between the polyurethane and Casein, which significantly increased the particle size. However, the incorporation after phase inversion of the WPU (ex situ), did not change the particle size. Tensile tests revealed that the covalent bond promoted an increase in the brittleness of the material compared to ex situ approach due to a better dispersion of the Casein in the system. TGA results showed that Casein increased the thermal stability of all the coatings, especially of those obtained by the ex situ route. Moreover, and according to the microscale combustion calorimeter (MCC) and vertical burning test (UL-94) measurements Casein delayed the combustion of the material. Consequently, due to their characteristics, these Casein-WPU dispersions could potentially be used as combustion retardant coatings, where good physicochemical properties are essential for effective performance.

Development of a New Type of Flame Retarded Biocomposite Reinforced with a Biocarbon/Basalt Fiber System: A Comparative Study between Poly(lactic Acid) and Polypropylene.

A new type of partially biobased reinforcing filler system was developed in order to be used as a flame retardant for polylactic acid (PLA) and polypropylene (PP)-based composites. The prepared materials intended for injection technique processing were melt blended using the novel system containing ammonium polyphosphate (EX), biocarbon (BC), and basalt fibers (BF). All of the prepared samples were subjected to a detailed analysis. The main criterion was the flammability of composites. For PLA-based composites, the flammability was significantly reduced, up to V-0 class. The properties of PLA/EX/BC and PLA/EX/(BC-BF) composites were characterized by their improved mechanical properties. The conducted analysis indicates that the key factor supporting the effectiveness of EX flame retardants is the addition of BC, while the use of BF alone increases the flammability of the samples to the reference level. The results indicate that the developed materials can be easily applied in industrial practice as effective and sustainable flame retardants.

A bio-based flame retardant coating for improving flame retardancy and anti-dripping performance of polyamide 6 fabric

Eco-friendly and sustainable flame retardant (FR) approaches derived from natural products for polyamide 6 (PA6) fabrics are of interest. Therefore, a novel biomass phytate taurine salt (PTS) was synthesized using phytic acid and taurine, and then it was coated onto PA6 fabric. The chemical structure of PTS, and the flame retardant performance and mechanism, the washing durability and physical properties of coated PA6 fabrics were explored. The damaged length of coated PA6 fabrics reduced to 12.0 cm without droplets during the vertical burning test, achieving a UL94 V-0 level. The LOI increased from 23.0% to 31.8% after PTS coating. The results of cone calorimetry showed that the pHRR of PA6-PTS sample decreased by about 11.0% with a char residue of 12.0%. The investigations on violates and char residues confirmed that PTS coating improved the flame retardancy of PA6 fabric through a combined gas-phase and condensed-phase action. In addition, the coated PA6 fabrics could still maintain V-0 level after 15 washing cycles. The tensile strength of PA6 fabrics was also slightly improved after PTS coating. In general, this work developed a durable and sustainable PTS coating with excellent flame retardancy for PA6 fabric.

Sustainable modification of polyamide 6 fabric by Maillard reaction product for mitigating fire hazards and molten-dripping

Developing durable and sustainable flame retardant (FR) polyamide 6 (PA6) fabric is of great interest. Herein, an eco-friendly Maillard reaction product (MRP) was developed by carbonyl-amino condensation between glucose and thiourea. Then, the functional PA6 fabric was developed through an exhaustion process. We further explored the anti-dripping ability, flame retardancy and washing durability, FR mechanism and physical properties of MRP-modified PA6 fabric. The modified PA6 fabric possessed a high LOI of 31.6% and a low damaged length of 11.3 cm without dripping action. The smoke emission was also reduced by 38.2% after MRP modification, although the Caramel skeleton exerted a scaffold effect with PA6. The MRP macromolecule inhibited PA6 flammability through gas-phase FR mechanism. In addition, the MRP macromolecule could firmly adsorb on PA6 fabric, corresponding to superior washing durability, and the V-0 rate was also reached after 40 washings. The modified PA6 fabric also had a yellowish-brown color and improved tensile strength.