Effects of Aluminum Hydroxide and Layered Double Hydroxide on Asphalt Fire Resistance

Conjugated Microporous Polymer Aerogels Encapsulated within Hydroxyapatite Nanowires Exhibit Good Thermal Insulation and Flame-Retardant Properties

Aerogels have been widely studied in the field of thermal insulation. Herein, we reported a kind of conjugated micropolymer (CMP) aerogel synthesized by 1,3,5-triethynylbenzene and 2-amino-3,5-dibromopyridine. To enhance the flame-retardant property, we composited hydroxyapatite (HAP) nanowires with a CMP aerogel. Transmission electron microscopy (TEM) analysis revealed that HAP nanowires were encapsulated within nanosized CMP tubes. In addition, the thermal conductivity of HAP2-NCMP aerogel was 0.0251 W m-1 K-1, which possesses good thermal insulation property. In the micro-combustion calorimeter (MCC) test, compared with pure NCMP, the peak heat release rate (pHRR) of HAP2-NCMP decreased from 39.3 to 30.82 W g-1, approximately 21.6% lower. Furthermore, with the increased addition of hydroxyapatite in the HAP-NCMP composite, the pHRR of HAP3-NCMP decreased by about 37.4%. Besides, NCMP possesses good mechanical properties, with a compressive strength of 117.3 kPa at a strain level of 60%. These findings suggest promising application potential for HAP-NCMP in energy-saving and flame-retardant applications.

Effect of Compounded Aluminum Hydroxide Flame Retardants on the Flammability and Smoke Suppression Performance of Asphalt Binders

Compounded aluminum hydroxide (ATH) flame retardants have been widely used for their low cost and environmentally friendly characteristics. However, previous research lacks a systematic and comprehensive comparison. In addition, the combustion characteristics and phase characterization of asphalt binders are not taken into account either. In this work, flame retardants, for instance, APP, Sb2O3, ZB, and LDHs, were compounded with ATH. The flame retardant behavior, together with the smoke suppression behavior, of asphalt binders with compounded flame retardants was determined by LOI and CCT. Furthermore, mechanisms on flame retardants were investigated. It was found that ATH compounded with ZB significantly reduced the heat smoke release and suppressed the formation of toxic volatiles during asphalt combustion. This was because ATH/ZB facilitated the formation of polyaromatic structures and improved the resistance of the char layer. ATH compounded with APP showed an antagonistic effect in the limiting oxygen test because the reaction between ATH and APP inhibited and delayed the decomposition of ATH during asphalt combustion with more aluminum phosphate presenting relatively poor barrier properties produced.

Recent advances in metal-family flame retardants: a review

The use of polymer materials is inextricably linked to our manufacturing life. However, most of them are easily combusted in the air and the combustion process generates a large amount of toxic fumes and dangerous smoke. This can result in injuries and property damage, as well as limiting their use. It is essential to enhance the flame-retardant properties and smoke suppression performance by using multiple flame retardants. Metal-based flame retardants have a unique chemical composition. They are environmentally friendly flame retardants, which can impart good smoke suppression, flame retardancy to polymers and further reduce the production of toxic gases. The differences in the compounds formed between the transition metals and the main group metals make them act differently as flame retardants for polymers. As a result, this study presents the research progress and flame-retardant mechanism of flame-retardant polymers for flame retardants from different groups of metals in the periodic table of elements in a systematic manner. In view of the differences between the main group metals and transition metals, the mechanism of their application in flame retardant polymer materials is carefully detailed, as are their distinct advantages and disadvantages. And ultimately, prospects for the development of transition metals and main group metals are outlined. It is hoped that this paper will provide valuable references and insights for scholars in the field.

Understanding the Flame Retardant Mechanism of Intumescent Flame Retardant on Improving the Fire Safety of Rigid Polyurethane Foam

Combinations of multiple inorganic fillers have emerged as viable synergistic agents for boosting the flame retardancy of intumescent flame retardant (IFR) polymer materials. However, few studies on the effect of multiple inorganic fillers on the flame retardant behavior of rigid polyurethane (RPU) foam have been carried out. In this paper, a flame retardant combination of aluminum hydroxide (ATH) and traditional flame retardants ammonium polyphosphate (APP), pentaerythritol (PER), melamine cyanurate (MC), calcium carbonate (CC), and expandable graphite (EG) was incorporated into RPU foam to investigate the synergistic effects of the combination of multiple IFR materials on the thermal stability and fire resistance of RPU foam. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) revealed that 8 parts per hundred polyols by weight (php) filler concentrations were compatible with RPU foam and yielded an increased amount of char residue compared to the rest of the RPU samples. The flame retardancy of multiple fillers on intumescent flame retardant RPU foam was also investigated using cone calorimeter (CCTs) and limiting oxygen index (LOI) tests, which showed that RPU/IFR1 (APP/PER/MC/EG/CC/ATH) had the best flame retardant performance, with a low peak heat release rate (PHRR) of 82.12 kW/m², total heat release rate (THR) of 15.15 MJ/m², and high LOI value of 36%. Furthermore, char residue analysis revealed that the use of multiple fillers contributed to the generation of more intact and homogeneous char after combustion, which led to reduced decomposition of the RPU foam and hindered heat transfer between the gas and condensed phases.

Thermal Degradation Characteristics of Styrene-Butadiene-Styrene Copolymer Asphalt Binder Filled with an Inorganic Flame-Retarding Agent

Asphalt binder is a complex mixture of dark brown polymers composed of hydrocarbons with generally poor fire resistance. To improve its flame retardancy when used in tunnel asphalt pavements, a new inorganic flame-retardant filler (FR) containing magnesium hydroxide, aluminum hydroxide, inorganic phosphate, and melamine salt was explored. Thereafter, limiting oxygen index (LOI) and smoke suppression tests for the flame-retarded asphalt binder (FRA) mastics mixed with FR and styrene-butadiene-styrene (SBS) copolymer asphalt binder were conducted. Thermogravimetric (TG) and differential scanning calorimetry (DSC) curves for the FRA were correspondingly generated. Based on the TG data, the reaction function g(α), apparent activation energy Ea, and pre-exponential factor A were quantitatively evaluated using kinetic analysis. In addition, a Fourier transform infrared spectrometry (FTIR) test was utilized to assess the effects of the presence of FR on the chemical composition of the asphalt binder. Dynamic shear rheometer (DSR) tests were also performed to evaluate the rheological behavior of FRA. Results show that the presence of the FR significantly reduced the LOI and improved the smoke suppression during combustion of the asphalt binder mastics. The presence of FR was found to increase the Ea and the complexity of the combustion reaction, thereby improving the flame retardancy of the asphalt binder. FTIR analysis indicated that the presence of FR did not induce any strong chemical reactions to significantly impact or alter the functional groups of the asphalt binder. Furthermore, it was also observed that the rutting parameter and critical failure temperature of FRA increased with the addition of FR due to the stiffening effect of the solid FR particles.

Measurements, emission characteristics, and control methods of fire effluents generated from tunnel asphalt pavement during fire: a review

Tunnels are widely used in high-grade roads, particularly in mountainous areas; however, tunnel fires often result in severe economic losses and casualties. The fire effluents produced from asphalt pavement have attracted significant research attention. The main objective of this study is to assimilate information on various aspects of bituminous mixture emissions during fires. In this study, the fume emissions of bitumen and bituminous mixtures during combustion are comprehensively reviewed and summarized. First, the test methods for fire effluents produced by bitumen and bituminous mixtures after combustion are summarized. Second, the factors influencing the fume concentration and composition are determined. In addition, different methods to reduce the emission of fire effluents are compared, particularly for the suppression of toxic gas emissions. Then, reasonable suggestions are proposed to reduce the damage caused by hazardous gases to humans and the environment. This review is beneficial for comprehensively understanding the fume emission behaviour and future research on the smoke suppression of highway tunnel asphalt pavements during fires.

Multi-Objective Optimization and Performance Characterization of Asphalt Modified by Nanocomposite Flame-Retardant Based on Response Surface Methodology

In order to improve the safety of the tunnel asphalt pavement in the event of a fire, and reduce the deterioration of the low temperature crack resistance of the asphalt by the flame retardant. The research uses aluminum hydroxide (ATH) as a smoke suppressant, diethyl aluminum hypophosphite (ADP) as a flame retardant, and halloysite nanotubes (HNTs) as a synergist to modified styrene-butadiene-styrene block copolymer (SBS) modified asphalt (MA). First, the content of ATH, ADP, and HNTs was used as the response variable. The physical properties (Penetration, Softening point, Ductility) and static flame retardant properties (Limiting oxygen index meter, Ignition point) of the asphalt modified by nanocomposite flame-retardant (HNTs-CFRMA) were the response variables. The response surface methodology was used to design the test, and regression models were established to analyze the influence of flame retardants on the performance of asphalt. Then, comprehensively considering the effects of physical properties and flame retardant properties, the normalized desirability function was used to perform a multi-objective optimization design on the components of the nanocomposite flame retardant modifier to obtain the best flame retardant formula. Finally, the rheological properties of MA, conventional flame-retardant modified asphalt (CFRMA), and HNTs-CFRMA were tested based on Dynamic shear rheometer, Multiple stress creep test, Force ductility tester, and Bending beam rheometer. The performance of flame-retardant and smoke suppression were tested by the Cone calorimeter tests. The result shows that ATH, ADP, and HNTs can enhance the high temperature performance of asphalt, reduce the penetration. The addition of HNTs can increase significantly the softening point and reduce the deteriorating effect of flame retardants on the low temperature performance of asphalt; the addition of ATH and HNTs can improve significantly the flame retardancy of asphalt. Based on the desirability function of power exponent, the formulation of the nanocomposite flame retardant with better physical properties and flame retardant properties is ATH:ADP:HNTs = 3:5:1, and the total content is 9 wt%. Nanocomposite flame retardants can improve obviously the high temperature rheological properties of asphalt. The rutting factor and the cracking factor of HNTs-CFRMA improve markedly, and the irrecoverable creep compliance is reduced, compared with MA and CFRMA. Nanocomposite flame retardant can make up for the deterioration of conventional flame retardants on asphalt’s low temperature performance. At the same time, it has better flame-retardant performance and smoke suppression performance.

Research on the Flame Retardancy Properties and Mechanism of Modified Asphalt with Halloysite Nanotubes and Conventional Flame Retardant

The inflammability of asphalt road will promote fire spread in the tunnel and produce lots of toxic smoke. To improve the fire resistance of asphalt pavement, mineral powder flame retardants are generally replaced by flame retardants in equal amounts. In this study, the effects of the synergistic flame retardancy system of halloysite nanotubes (HNTs) and conventional flame retardants (CFR) on the flame retardancy performance and mechanism of asphalt were investigated. Firstly, the flame retardancy properties of the HNTs and CFR composite modified asphalt were investigated based on the Cleveland open cup method (COC), Limiting oxygen index meter (LOI), and Cone calorimeter tests (CCTs). Then, the flame retardancy mechanism of the modified asphalt was studied based on Thermogravimetric analyzer (TGA), Fourier-transform infrared (FTIR), and Scanning electron microscopy (SEM). The results show that adding HNTs could improve the flame retardancy of the CFR modified asphalt binder. When 1 wt % HNTs and 8 wt % CFR were used, the limiting oxygen index of asphalt increased by 40.1%, the ignition temperature increased by 40 °C, while the heat release rate, total heat release, the smoke production rate, total smoke release, and other parameters decreased with varying degrees. Based on TG, FTIR, and SEM, the targeted flame retardancy mechanism and synergistic effect of HNTs/CFR flame retardancy system were revealed and summarized as three stages: (1) Stage 1, aluminum hydroxide (ATH) absorbs heat through thermal decomposition and inhibits the decomposition of lightweight components in asphalt; (2) Stage 2, aluminum diethyl phosphate (ADP) decomposes and produces organic phosphoric acid, which catalyzes crosslinking and ring thickening of asphalt and the quenching effect of phosphorus free radicals to block the combustion; and (3) Stage 3, HNTs plays an important role in increasing the integrity and density of the barrier layer. In addition, the Al₂O₃ produced by the decomposition of ATH, the carbon layer formed by the ADP catalyzed pitch, and HNTs play a significant synergistic effect in the formation of the barrier layer. Thus, the combination of HNTs and CFR has been proved to be a prospective flame retardancy system for asphalt.

Thermal Characteristics, Kinetic Models, and Volatile Constituents during the Energy Conversion of Bituminous SARA Fractions in Air

To understand the thermal characteristics, nonisothermal kinetic models, and volatile constituents during the energy conversion of bituminous materials at the fraction level, differential scanning calorimetry-mass spectrometry tests were performed on bituminous four fractions, including saturates, aromatics, resins, and asphaltenes (SARA). Then, three-dimensional (3D) nonisothermal kinetic models of SARA fractions were established and volatile constituents of SARA fractions were discussed. Results indicate that when the heating rate is increased, the decomposition temperature ranges in each stage increase and the initial decomposition, peak, and burn-out temperatures of each SARA fraction all shift to high temperatures. Also, the whole energy conversion processes of SARA fractions are mainly exothermic reactions. Additionally, the energy conversion mechanism in each stage of saturates and aromatics accords with different nonisothermal kinetic models. However, the energy conversion mechanisms of resins and asphaltenes are similar and both accord with the 3D diffusion models. Further, the established nonisothermal kinetic models in each decomposition stage of SARA fractions are feasible to describe the energy conversion processes of SARA fractions. The released small molecular volatiles from saturates and aromatics increase when the heating rate is increased, but the macromolecular volatiles are decreased. The opposite is true for resins, but all volatiles emitted from asphaltenes are increased. Finally, the heating rate has little influence on the constituents of emitted gaseous products from SARA fractions but shows an effect on the release amount of volatiles from SARA fractions. The main common volatiles of SARA fractions are CO₂, H₂O, methanol, hydrazine, propyne, acetaldehyde, and propane. This study contributes to further reveal the energy conversion mechanisms of bituminous materials.

Flame Retardancy, Fire Behavior, and Flame Retardant Mechanism of Intumescent Flame Retardant EPDM Containing Ammonium Polyphosphate/Pentaerythrotol and Expandable Graphite

The intumescent flame retardant ethylene–propylene–diene rubber (EPDM) was prepared using intumescent flame retardant (IFR), including ammonium polyphosphate (APP) /pentaerythrotol (PER) and expandable graphite (EG), as the flame retardant agent. The effects of IFR and EG on the flame retardancy, fire behavior, and thermal stability of the EPDM were investigated. The results show that IFR and EG have excellent synergistic flame retardant effects. When the mass ratio of IFR to EG is 3:1 and the total addition content is 40 phr, the limiting oxygen index (LOI) value of the EPDM material (EPDM/IFR/EG) can reach 30.4%, and it can pass a V-0 rating in the vertical combustion (UL-94) test. Meanwhile, during the cone calorimetry test, the heat release rate and total heat release of EPDM/IFR/EG are 69.0% and 33.3% lower than that of the pure EPDM, respectively, and the smoke release of the material also decreases significantly, suggesting that the sample shows good fire safety. In addition, the flame retardant mechanism of IFR and EG is systematically investigated by thermogravimetric analysis/infrared spectrometry (TG-IR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), and the results indicate that IFR and EG have only physical interaction. Moreover, the reason why IFR exhibits a poor flame retardant effect in EPDM materials is explained.

Thermal behaviors and harmful volatile constituents released from asphalt components at high temperature

Asphalt binder releases lots of heat and harmful volatiles at high temperature. To further understand thermal behaviors, dynamic release and toxic constituents of emitted volatiles during the combustion of asphalt binder, such fractions as saturates, aromatics, resins and asphaltenes (SARA) were first prepared. Thermal behaviors, volatile constituents and combustion residue microstructures of SARA fractions are discussed. Results indicate that polymerization degree of asphalt binder is high and the content of polycyclic aromatic compounds is large. Combustion processes of resins and asphaltenes only show single-stage exothermic reactions, but other two fractions present obvious multi-stage combustion reactions. As the heating rate is raised, the incomplete combustion of SARA fractions is increased, and more volatiles are released. Main volatiles released from SARA fractions are inflammable, toxic, corrosive or explosive compounds, and such common volatiles as acetaldehyde and propane are released from each SARA fraction. More toxic volatiles are released at combustion stage I, but macromolecular volatiles are mainly released at stage II. Volatile release behaviors of saturates and aromatics are more obviously affected by the heating rate. Combustion residues show more intact morphologies from saturates to asphaltenes, and mainly contain C, O and S elements. Asphalt binder is hazardous material at high temperature.

Evaluation of Asphalt with Different Combinations of Fire Retardants

When a fire takes place in a tunnel, the surface of the asphalt pavement will burn and release a large amount of smoke, which is toxic to human health. Thus, in order to prevent the combustion of the asphalt pavement under fire, it is necessary to propose some methods to retard its physical and chemical reaction under the high temperature. In this study, ten different combinations of fire retardants and a control case where no fire retardant was applied were prepared for evaluation. The thermogravimetric (TG)–mass spectrometry (MS) tests were used to evaluate their effect on the fire retardance from mass and energy perspectives and the Fire Dynamics Simulator (FDS) software was used to evaluate the fire retardance from temperature and smoke distribution perspectives. In experimental analysis, the TG (thermogravimetric) and DTG (differential thermogravimetric) curves were used to analyze the mass loss rate and residual mass of the asphalt and the activation energy was calculated and analyzed as well. In addition, decay rate of mass loss rate and increasing rate of activation energy were proposed to evaluate the ease of combustion of the asphalt with and without fire retardants. The results show that in laboratory experiments, the fire retardant combination which includes 48% aluminum hydroxide, 32% magnesium hydroxide, 5% expanded graphite, and 15% encapsulated red phosphorous would lead to an improved effect of fire retardance. In numerical modeling, the temperature and smoke height distribution over time were adopted to evaluate the fire retardance effect. The temperature distribution was found to be symmetrical on both sides of the combustion point and the same combination as proposed in experimental analysis was found to have the best effect on fire retardance due to the largest decrease in temperature. Additionally, because of the highest smoke height distribution, an improved effect on smoke suppression was also found when this combination was applied.

Special Issue of Environment-Friendly Construction Materials

This special issue, “Environment-Friendly Construction Materials”, has been proposed and organized as a means to present recent developments in the field of construction materials. For this reason, the articles highlighted in this editorial relate to different aspects of construction materials, from pavement materials to building materials, from material design to structural design, from self-healing to cold recycling, from asphalt mixture to cement concrete.

Flame-Retardant Mechanism of Layered Double Hydroxides in Asphalt Binder

The flame retardancy of asphalt binders with layered double hydroxides (LDHs) was investigated using limiting oxygen index (LOI) and cone calorimeter tests. The flame-retardant mechanism of the LDHs was also studied with thermogravimetry and differential scanning calorimetry (TG–DSC), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The cone calorimeter testing results indicated that 2 wt.% of the LDHs can decease the peak heat and smoke release rate of asphalt binders. Because a low dose of LDHs can be well dispersed in asphalt binder and favor the formation of polyaromatic structures during combustion, the thermal oxidation resistance and compactness of the char layer can be improved. The LOI of asphalt binder can be increased and the heat and smoke release during combustion can be decreased with 25 wt.% LDHs. The decomposition of LDHs can absorb the heat release of the initial two stages of asphalt combustion and reduce the burning rate of asphalt. Due to the loss of loosely bound water in the LDHs during the blending process and the decrease of dispersibility at a high LDH dose, the improvement of thermal stability is limited.

Influence of Overheating Phenomenon on Bitumen and Asphalt Mixture Properties

In the course of manufacturing, transport and installation, road bitumens and asphalt mixtures can be exposed to the impact of elevated process temperatures exceeding 240 °C. This mainly applies to the mixtures used for road pavements and bridge deck insulation during adverse weather conditions. The heating process should not change the basic and rheological properties of binders and the asphalt mixtures that to a degree cause the degradation of asphalt pavement durability. The work involved analyzing the properties of non-modified bitumens and SBS polymer modified bitumens, heated at temperatures of 200 °C, 250 °C and 300 °C for 1 h. Next, the asphalt mixtures were heated in the same temperatures. Based on the developed Overheating Degradation Index (ODI) it was demonstrated that polymer-modified bitumens were characterized by higher overheating sensitivity A_(ODI) than non-modified bitumens, which was confirmed by mixture test results. Overheating limit temperatures T_(ODI) were determined, which in the case of polymer-modified bitumens are up to 20 °C lower than for non-modified bitumens. When the temperature increases above T_(ODI), loss of viscoelastic properties occurs in the material which causes, among other effects, a loss of resistance to fatigue cracking.