An efficient mono-component polymeric intumescent flame retardant for polypropylene: preparation and application

A high molecular weight formaldehyde-free polymer flame retardant made from polyvinyl alcohol for cellulose

Polyvinyl alcohol and phosphoric acid were used as primary raw materials to synthesize a polyvinyl alcohol/ammonium phosphate flame retardant (PVAAP) for cotton fabrics. The limiting oxygen index of the cotton fabric treated with 24% PVAAP was 42.1. After 50 standard laundry cycles, the limiting oxygen index remained relatively high (26.3), suggesting that the 24% PVAAP can be used as a durable flame retardant. The vertical flammability test of the cotton fabric treated with PVAAP exhibited no afterflame and afterglow. The cone calorimetry test indicated that the peak of the heat release rate and total heat release of the cotton fabric treated with 24% PVAAP were significantly lower than those of the control cotton. Thermogravimetric and thermogravimetric-infrared spectroscopy revealed that the initial decomposition temperature of the PVAAP-treated fabric was substantially lower than that of the control fabric, and more residual carbon was generated. The PVAAP altered the thermal decomposition pathway of the treated cotton. The X-ray diffraction patterns and scanning electron microscopy images suggested that the PVAAP treatment did not change the structure of the fibers.

Synthesis of chitosan-based flame retardant and its fire resistance in epoxy resin

A novel flame retardant CCD was synthesized by the condensation between cinnamalde and chitosan, followed by the addition reaction with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO), and CCD was named from the initials of the three raw materials. The intermediate product CC and the target product CCD was then characterized by infrared spectrometry and elemental analysis. The flame retardancy of EP thermosets modified by CCD was dramatically improved. Epoxy resin (EP) with 10wt% CCD passed vertical burning (UL-94) V-0 rating and possessed limited oxygen index (LOI) value of 31.6 %. Cone calorimeter test exhibited the introduction of 3.5g CCD into 35g EP decreased the total heat release by 38.8 % and decreased the total smoke product by 72.0 %. XPS, FTIR, SEM and Raman tests were proceeded to determine the char residue for EP/10 % CCD thermoset, and the results showed that char residue for EP/10 % CCD thermoset possessed dense and compact structure which played a positive effect in blocking the exchange of heat and gas.

Thermal Kinetics of a Lignin-Based Flame Retardant

In order to improve the thermal property of epoxy resin (EP), a lignin-based flame retardant was prepared. Focusing on the lignin-based flame retardant, this paper investigates its pyrolysis behavior and kinetics via a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG–FTIR). Based on the FTIR result, which showed a peak at 1222 cm⁻¹, it was assigned a syringyl structure. Its absorption peak intensity was enhanced and this meant that the phenolization of the lignin was successful. Thermogravimetry/derivative thermogravimetry (TG/DTG) results showed that the carbon residues of F-lignin and F-lignin@APP were reduced to 33.5% and 37.5%, respectively. In addition, the maximum decomposition rate of F-lignin@APP20/EP is 11.8%/min, which is 8%/min and 4.7%/min lower than for EP and Al-lignin, respectively. The char residue of F-lignin@APP20/EP is 32.5%, which is much higher than for EP. Lower decomposition rate and higher char residue indicate the improvement of thermal stability of EP by F-lignin@APP. Moreover, the kinetics of Al-lignin20/EP and F-lignin@APP20/EP were conducted by two kinetic methods: Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). It was concluded that the pyrolysis process of Al-lignin 20/EP and F-lignin@APP 20/EP could be divided into three stages, while the value and growth rate of the activation energy of F-lignin@APP 20/EP were much higher than that of Al-lignin 20/EP in stage III.

Flame Retardant Polypropylenes: A Review

Polypropylene (PP) is a commodity plastic known for high rigidity and crystallinity, which is suitable for a wide range of applications. However, high flammability of PP has always been noticed by users as a constraint; therefore, a variety of additives has been examined to make PP flame-retardant. In this work, research papers on the flame retardancy of PP have been comprehensively reviewed, classified in terms of flame retardancy, and evaluated based on the universal dimensionless criterion of Flame Retardancy Index (FRI). The classification of additives of well-known families, i.e., phosphorus-based, nitrogen-based, mineral, carbon-based, bio-based, and hybrid flame retardants composed of two or more additives, was reflected in FRI mirror calculated from cone calorimetry data, whatever heat flux and sample thickness in a given series of samples. PP composites were categorized in terms of flame retardancy performance as Poor, Good, or Excellent cases. It also attempted to correlate other criteria like UL-94 and limiting oxygen index (LOI) with FRI values, giving a broad view of flame retardancy performance of PP composites. The collected data and the conclusions presented in this survey should help researchers working in the field to select the best additives among possibilities for making the PP sufficiently flame-retardant for advanced applications.

Biomolecules as Flame Retardant Additives for Polymers: A Review

Biological molecules can be obtained from natural sources or from commercial waste streams and can serve as effective feedstocks for a wide range of polymer products. From foams to epoxies and composites to bulk plastics, biomolecules show processability, thermal stability, and mechanical adaptations to fulfill current material requirements. This paper summarizes the known bio-sourced (or bio-derived), environmentally safe, thermo-oxidative, and flame retardant (BEST-FR) additives from animal tissues, plant fibers, food waste, and other natural resources. The flammability, flame retardance, and—where available—effects on polymer matrix’s mechanical properties of these materials will be presented. Their method of incorporation into the matrix, and the matrices for which the BEST-FR should be applicable will also be made known if reported. Lastly, a review on terminology and testing methodology is provided with comments on future developments in the field.

Flame Retardation of Natural Rubber: Strategy and Recent Progress

Natural rubber (NR) as a kind of commercial polymer or engineering elastomer is widely used in tires, dampers, suspension elements, etc., because of its unique overall performance. For some NR products, their work environment is extremely harsh, facing a serious fire safety challenge. Accordingly, it is important and necessary to endow NR with flame retardancy via different strategies. Until now, different methods have been used to improve the flame retardancy of NR, mainly including intrinsic flame retardation through the incorporation of some flame-retarding units into polymer chains and additive-type flame retardation via adding some halogen or halogen-free flame retardants into NR matrix. For them, the synergistic flame-retarding action is usually applied to simultaneously enhance flame retardancy and mechanical properties, in which some synergistic flame retardants such as organo-montmorillonite (OMMT), carbon materials, halloysite nanotube (HNT), etc., are utilized to achieve the above-mentioned aim. The used flame-retarding units in polymer chains for intrinsic flame retardation mainly include phosphorus-containing small molecules, an unsaturated chemical bonds-containing structure, a cross-linking structure, etc.; flame retardants in additive-type flame retardation contain organic and inorganic flame retardants, such as magnesium hydroxide, aluminum hydroxide, ammonium polyphosphate, and so on. Concerning the flame retardation of NR, great progress has been made in the past work. To achieve the comprehensive understanding for the strategy and recent progress in the flame retardation of NR, we thoroughly analyze and discuss the past and current flame-retardant strategies and the obtained progress in the flame-retarding NR field in this review, and a brief prospect for the flame retardation of NR is also presented.

Ultralight Industrial Bamboo Residue-Derived Holocellulose Thermal Insulation Aerogels with Hydrophobic and Fire Resistant Properties

In this study, water-soluble ammonium polyphosphate- (APP) and methyl trimethoxysilane (MTMS)-modified industrial bamboo residue (IBR)-derived holocellulose nanofibrils (HCNF/APP/MTMS) were used as the raw materials to prepare aerogels in a freeze-drying process. Synthetically modified aerogels were confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, and thermal stability measurements. As-prepared HCNF/APP/MTMS aerogels showed themselves to be soft and flexible. The scanning electron microscopy (SEM) analysis showed that the foam-like structure translates into a 3D network structure from HCNF aerogels to HCNF/APP/MTMS aerogels. The compressive modules of the HCNF/APP/MTMS aerogels were decreased from 38 kPa to 8.9 kPa with a density in the range of 12.04-28.54 kg/m³, which was due to the structural change caused by the addition of APP and MTMS. Compared with HCNF aerogels, HCNF/APP/MTMS aerogels showed a high hydrophobicity, in which the water contact angle was 130°, and great flame retardant properties. The peak of heat release rate (pHRR) and total smoke production (TSP) decreased from 466.6 to 219.1 kW/m² and 0.18 to 0.04 m², respectively, meanwhile, the fire growth rate (FIGRA) decreased to 8.76 kW/s·m². The thermal conductivity of the HCNF/APP/MTMS aerogels was 0.039 W/m·K. All results indicated the prepared aerogels should be expected to show great potential for thermally insulative materials.

Synergistic Effects of Two-Dimensional MXene and Ammonium Polyphosphate on Enhancing the Fire Safety of Polyvinyl Alcohol Composite Aerogels

Fire and smoke suppressions of polyvinyl alcohol (PVA) aerogels are urgently required due to the serious fire hazard they present. MXene, a 2D transition-metal carbide with many excellent properties, is considered a promising synergist for providing excellent flame retardant performance. PVA/ammonium polyphosphate (APP)/transition metal carbide (MXene) composite aerogels were prepared via the freeze-drying method to enhance the flame retardancy. Thermogravimetric analysis, limiting oxygen index, vertical burning, and cone calorimeter tests were executed to investigate the thermal stability and flame retardancy of PVA/APP/MXene (PAM) composite aerogels. The results demonstrated that MXene boosted the flame retardancy of PVA-APP, and that PAM-2 (with 2.0 wt% MXene loading) passed the V-0 rating, and reached a maximum LOI value of 42%; Moreover, MXene endowed the PVA-APP system with excellent fire and smoke suppression performance, as the the peak heat release rate and peak smoke production rate were significantly reduced by 55% and 74% at 1.0 wt% MXene loading. The flame retardant mechanism was systematically studied, MXene facilitated the generation of compact intumescent residues via ita catalyst effects, thus further restraining the release of heat and smoke. This work provides a simple route to improve the flame retardancy of PVA aerogels via the synergistic effect of MXene and APP.

An assessment of the toxicity of polypropylene microplastics in human derived cells

Environmental pollution caused by plastic waste is a growing global problem. Discarded plastic products and debris (microplastic particles) in the oceans detrimentally affect marine ecosystems and may impact human. Humans are exposed to plastic debris via the consumption of seafood and drinking water, contact with food packaging, or inhalation of particles. The accumulation of microplastic particles in humans has potential health risks such as cytotoxicity, hypersensitivity, unwanted immune response, and acute response like hemolysis. We investigated the cellular responses of secondary polypropylene microplastics (PP particles) of approximately ~20 μm and 25-200 μm in different condition and size to normal cells, immune cells, blood cells, and murine immune cells by cytokine analysis, ROS assay, polarization assay and proliferation assay. We found that PP particles showed low cytotoxicity effect in size and concentration manner, however, a high concentration, small sized, DMSO method of PP particles stimulated the immune system and enhanced potential hypersensitivity to PP particles via an increase in the levels of cytokines and histamines in PBMCs, Raw 264.7 and HMC-1 cells.

Halogen-Free Flame-Retardant Compounds. Thermal Decomposition and Flammability Behavior for Alternative Polyethylene Grades

The effect of six halogen-free flame retardant (FR) formulations was investigated on the thermal stability of two low-density polyethylenes (LDPE) and one linear low-density polyethylene (LLDPE), by means of thermogravimetric analysis (TGA) under nitrogen and air atmosphere. The relative data were combined with flammability properties and the overall performance of the FRs was correlated with the type of branching in the polyethylene grades and to their processing behavior. The thermal degradation kinetics was further determined based on the Kissinger and Coats-Redfern methods. In terms of flammability, the addition of a triazine derivative and ammonium polyphosphate at a loading of 35 wt. %. was found to be the most efficient, leading to UL 94 V0 ranking in the case of the LDPE grade produced in an autoclave reactor.

A novel organic-inorganic hybrid K-HBPE@APP performing excellent flame retardancy and smoke suppression for polypropylene

To overcome the flammability and severe dripping of polypropylene (PP), a novel organic-inorganic hybrid K-HBPE@APP (microencapsulated APP by a hyperbranched polyester (HBPE) via silane coupling agent (KH-550)) was obtained and used as a high-efficient flame retardant and smoke suppressant. Herein, HBPE acted as the charring agent for APP. 20 wt% of K-HBPE@APP imparted PP excellent flame retardancy, V-0 rating (UL-94 test) and 82.6% decrease in the peak of heat release rate (PHRR). However, PP with 25 wt% of mechanically mixed APP and HBPE achieved V-1 rating (UL-94 test) and 77.3% decrease in PHRR. That is because the direct contact and sufficient interaction between APP and HBPE gives full play to their synergy. Besides, K-HBPE@APP accelerated the formation of cross-linked POC/SiOSi/SiOC/SiOP/POΦ structures, leading to a strong and compact char layer with a result of dramatic reduction in heat release rate and smoke production. Furthermore, K-HBPE@APP was highly water-resistant and has good compatibility with PP matrix. In particular, the flame-retarded PP had similar tensile strength to pure PP and enhanced impact strength.

Simultaneously Improved Flame Retardance and Ceramifiable Properties of Polymer-Based Composites via the Formed Crystalline Phase at High Temperature

Ceramifiable polyolefin materials have an excellent application prospect in high-temperature-resistant wires and cables because of their excellent fire safety performance via a ceramization process under fire conditions. During the ceramization process, the control of the crystalline phase plays a vital role in determining the final fire resistance and ceramifiable properties. In this work, ammonium polyphosphate/zinc borate (APP/ZB) was developed to achieve the highly efficient flame retardance and ceramization of the ethylene-vinyl acetate/mica powder/organo-modified montmorillonite (EVA/MP/OMMT) composite. In the combustion test, the EVA/MP/OMMT/APP/ZB system displayed obvious flame retardance feature, showing much lower total heat release and total smoke production than neat EVA. After treating at high temperatures, rigid ceramic products were formed for EVA/MP/OMMT/APP/ZB. The ceramic that was formed at 900 °C had a flexural strength of 10.3 MPa for EVA/MP/OMMT/APP/ZB containing 23 wt % of APP/ZB (9.9:13.1), increased by 2475.0, 635.7, and 586.7% compared to the corresponding values of EVA/MP/OMMT, EVA/MP/OMMT/ZB, and EVA/MP/OMMT/APP. For the latter two systems, the content of ZB or APP was 23 wt %. APP/ZB showed a remarkable fluxing effect on the ceramization of the MP-based EVA composite. The fluxing mechanism of APP/ZB was revealed by different measurements. Both APP and ZB led to the formation of a glass melt containing α-Zn₃(PO₄)₂ and orthophosphate by increasing the temperature. Successively, the melt crystalline structure cohered the OMMT and MP together, accompanied by the gradual disappearance of the mica phase and the generation of eutectic phenomenon. Finally, a ceramic with high flexural strength was formed, leading to the improved flame retardance and ceramifiable properties of EVA-based composites.

Fire safety enhancement of a highly efficient flame retardant poly(phenylphosphoryl phenylenediamine) in biodegradable poly(lactic acid)

Flame-retarded poly(lactic acid) (PLA) biodegradable materials are viewed as promising as sustainable alternatives to petroleum-based commodity polymers. A new highly efficient flame retardant, poly(phenylphosphoryl phenylenediamine) (PPDA), was synthesized by the condensation of phenylphosphoryl dichloride with p-phenylenediamine and its structure was confirmed by ¹H nulear magnetic resonance and Fourier-transform infrared spectroscopy. When 3 wt% PPDA was incorporated into PLA, the limited oxygen index increased from 20.0% of neat PLA to 25.5% and its UL-94 vertical burning testing achieved V-0 rating. Moreover, the total heat release and peak heat release rate values of PLA/3 wt% PPDA material were decreased from 109.1 MJ/m² and 643.7 kW/m² of PLA to 98.3 MJ/m² and 570.0 kW/m², respectively, and the fire performance index increased from 0.081 of PLA to 0.132 m² s/kW. The high fire safety of PPDA in PLA is mainly attributed to the combined effects of the phosphorous-containing radical inhibition and inert gases and the barrier action of the formed char layer. The addition of less than 3 wt% PPDA has little influence on the tensile and impact properties of PLA. The flame retardant PLA blends have great application potential in electrical casing, automobile interiors and three-dimensional printing materials.

Intumescent Phosphorus and Triazole-Based Flame-Retardant Polyurethane Foams from Castor Oil

Synthesis of a novel phosphorus and triazole-functionalized flame-retardant (FR) monomer (PTFM) using azide-alkyne "click" reaction between triprop-2-ynyl phosphate and 2-azidoethanol that can impart intumescent FR property to polyurethane foams (PUFs) has been reported. Polyurethane triazole foams (PUTFs) were prepared using the as-synthesized PTFM and a hydroxylated castor polyol with a hydroxyl value of ∼310 mg KOH/g for application as reactive FR rigid foams. PTFM and the castor polyol were characterized for structural elucidation using Fourier transform infrared and ¹H, ¹³C, and ³¹P NMR. PUTFs with a varying loading content of PTFM were subjected to the lab-scale flame test, cone calorimetry test, Underwriters Laboratory 94 Vertical burning test (UL 94V), and limiting oxygen index (LOI) test. A significant increase in the char yields, reduction in heat release rates, V-1 rating, and 27% of LOI were observed for PUTFs compared to PUFs and proportional to the percentage loading of PTFM. The cumulative effect of nitrogen and phosphorus in PUTFs on their intumescent behavior was evident from the thermogravimetric analysis and scanning electron microscopy micrographs, which were further supplemented by X-ray photoelectron spectroscopy studies, indicating expulsion of N₂ and overall improvement in compression strength as well. Such environment-friendly reactive FRs can be good replacements to the halogenated ones.

Flame Retardant Polypropylene Composites with Low Densities

Polypropylene (PP) is currently widely used in areas requiring lightweight materials because of its low density. Due to the intrinsic flammability, the application of PP is restricted in many conditions. Aluminum trihydroxide (ATH) is reported as a practical flame retardant for PP, but the addition of ATH often diminishes the lightweight advantage of PP. Therefore, in this work, glass bubbles (GB) and octacedylamine-modified zirconium phosphate (mZrP) are introduced into the PP/ATH composite in order to lower the material density and simultaneously maintain/enhance the flame retardancy. A series of PP composites have been prepared to explore the formulation which can endow the composite with balanced flame retardancy, good mechanical properties, and low density. The morphology, thermal stability, flame retardancy, and mechanical properties of the composites were characterized. The results indicated the addition of GB could reduce the density, but decreased the flame retardancy of PP composites at the same time. To overcome this defect, ATH and mZrP with synergetic effect of flame retardancy were added into the composite. The dosage of each additive was optimized for achieving a balance of flame retardancy, good mechanical properties, and density. With 47 wt % ATH, 10 wt % GB, and 3 wt % mZrP, the peak heat release rate (pHRR) and total smoke production (TSP) of the composite PP-4 were reduced by 91% and 78%, respectively. At the same time, increased impact strength was achieved compared with neat PP and the composite with ATH only. Maintaining the flame retardancy and mechanical properties, the density of composite PP-4 (1.27 g·cm-3) is lower than that with ATH only (PP-1, 1.46 g·cm-3). Through this research, we hope to provide an efficient approach to designing flame retardant polypropylene (PP) composites with low density.

Ammonium Polyphosphate Intercalated Layered Double Hydroxide and Zinc Borate as Highly Efficient Flame Retardant Nanofillers for Polypropylene

We found in our previous study that layered double hydroxides (LDHs) which undergo aqueous miscible organic solvent treatment (AMOST) can tune the hydrophobicity surface of LDHs to be hydrophobic, and then the solvent mixing method can be used to prepare polymer/LDH nanocomposites. However, flame retardant property is not very high if LDHs are only used. In this present work, ammonium polyphosphate (APP) intercalated LDHs and zinc borate (ZB) was incorporated into a polypropylene (PP) matrix using the solvent mixing method. The structures, morphologies, and performance of the composites were characterized carefully. The peak heat release rate (PHRR) reduction of PP containing 10 and 20 wt % APP-LDH reached 27% and 55%, respectively, which increased up to 63% compared with PP/CO₃-LDH. After incorporating 2 wt % ZB in the PP/APP-LDH system, the flame retardant property was further improved. Polypropylene composites with 20 wt % APP-LDH and 2 wt % ZB showed a 58% PHRR reduction. In addition, thermogravimetric analyzer (TGA) results indicated that the addition of APP-LDH and ZB improved the temperature at 50% weight loss (T_50%) and the char formation of the materials significantly.

Design, Preparation, and Application of a Novel, Microencapsulated, Intumescent, Flame-Retardant-Based Mimicking Mussel

A novel microencapsulated intumescent flame retardant (TMAPP) was prepared, in which ammonium polyphosphate (APP) is the core and functions as an acid source, melamine-urea formaldehyde (MUF) resin is in the intermediate layer and functions as a blowing agent, and ferric tannin (mimicking mussel) is in the outermost shell layer and functions as a carbonization agent and also as a smoke inhibitor and surface modifier. TMAPP was prepared by treating MUF-microencapsulated APP with tannin acid and FeCl₃. Its structure is characterized by Fourier transform infrared, elemental analysis, and thermogravimetry (TG). TMAPP in epoxy was evaluated for its effect on flame retardancy and mechanical properties. The flame-retardant and smoke suppression performances of EP/TMAPP were studied using cone calorimetry (CONE) and limit oxygen index, and the flame-retardant mechanism was investigated by scanning electron microscopy and TG. The resultant data indicate that EP/TMAPP has very good mechanical properties, flame retardancy, and smoke suppression, whose char residue and thermal stability are increased, and the initial decomposition temperatures are decreased. Meanwhile, the char residue structures were very intumescent and compact. The char residue effectively prevents the underlying materials of EP/TMAPP from further combustion.

Melamine-containing polyphosphazene wrapped ammonium polyphosphate: A novel multifunctional organic-inorganic hybrid flame retardant

To achieve superior fire safety epoxy resins (EP), a novel multifunctional organic-inorganic hybrid, melamine-containing polyphosphazene wrapped ammonium polyphosphate (PZMA@APP) with rich amino groups was prepared and used as an efficient flame retardant. Thanks to the cross-linked polyphosphazene part, PZMA@APP exhibited high flame retardant efficiency and smoke suppression to the EP composites. Thermogravimetric analysis indicated that PZMA@APP significantly enhanced the thermal stability of EP composites. The obtained sample passed UL-94 V-0 rating with 10.0wt% addition of PZMA@APP. Notably, inclusion of incorporating PZMA@APP leads to significantly decrease on fire hazards of EP, for instance, bring about a 75.6% maximum decrease in peak heat release rate and 65.9% maximum reduction in total heat release, accompanied with lower smoke production rate and higher graphitized char layer. With regards to mechanical property, the glass transition temperature of EP/PZMA@APP10.0 was as high as 184.5°C. In particular, the addition of PZMA@APP did not worsen the mechanical properties, compared to pure APP. It was confirmed that the participation of melamine-containing polyphosphazene could significantly enhance the quality of char layer and thereby resulting the higher flame retardant efficiency of PZMA@APP.