Improving Combustion Analysis of Extruded Polystyrene via Custom Isolation Methodology

Abstract

This study is dedicated to an in−depth analysis of the combustion characteristics of extruded polystyrene (XPS) as a building insulation material with the aim of accurately assessing its fire risk in the built environment. Innovatively, this research employed a cone calorimeter equipped with a self−designed insulating sample holder to conduct a systematic experimental study. Additionally, it performed a comprehensive analysis of the ignition characteristics, heat release rate, fire hazard, smoke release, and toxic gas emission of XPS materials. The experimental results revealed that the combustion behavior of XPS is influenced by multiple factors, including the content of flame retardants and external heat flux, which significantly affect the fire hazard of XPS. When the thermal radiation intensity escalates from 25 kW/m² to 55 kW/m², the peak heat release rate of XPS−B1 rises from 428 kW/m² to 535 kW/m², marking an increase of 25.00%. Conversely, the peak heat release rate of XPS−B2 surges from 348 kW/m² to 579 kW/m², reflecting a substantial increase of 66.38%. This research not only provides a solid theoretical foundation and detailed experimental data for the fire behavior of XPS materials but also holds significant practical importance for enhancing the fire safety of buildings. Overall, this research contributes to the scientific understanding of XPS insulation materials and supports the development of more effective fire prevention measures in construction.

1. Introduction

Building energy efficiency plays a pivotal role in the sustainable development of contemporary society, with the external wall insulation system serving as the cornerstone for achieving such efficiency [1,2,3]. Extruded polystyrene (XPS), a material widely employed in the realm of building insulation, is highly regarded due to its exceptional insulation properties [4,5]. Specifically, XPS exhibits a thermal conductivity ranging from 0.026 to 0.035 W/(m·K), making it particularly effective in reducing heat transfer. Additionally, its strength coefficient spans from 150 to 1000 kpa, ensuring structural integrity and durability. With a water absorption rate of less than 1%, XPS maintains excellent moisture resistance, further enhancing its applicability [6,7]. However, the flammability of XPS poses a huge risk to building safety, and in recent years, it has caused frequent fire accidents, seriously threatening people’s lives and property safety [8,9,10]. Therefore, in−depth research on the combustion characteristics of XPS is of great significance for improving building fire safety.

XPS, as a thermoplastic polymer, involves complex physical and chemical changes during its combustion process [11]. When a fire occurs, XPS undergoes stages such as melting, thermal decomposition, ignition, and combustion when heated, and the melting behavior accelerates the spread of fire [12,13,14,15]. Its combustion characteristics are influenced by various factors such as flame retardant grade, external heat flux, and ignition source [16]. The combustion performance of XPS varies significantly among different flame retardant grades, and flame retardants can slow down the combustion rate and reduce the fire hazard to a certain extent [17,18]. The variation of external heat flux affects the combustion characteristic parameters such as ignition time, heat release rate, and total heat release of XPS, which in turn affects the development and spread of fires [19,20,21]. At present, research on the combustion characteristics of XPS mainly focuses on laboratory environments, using various experimental equipment and methods to simulate fire scenarios to explore combustion behavior and mechanisms. The cone calorimeter is one of the commonly used equipment [22,23,24]. At present, there is still room for improvement in the insulation method used in cone calorimeter (CCT) testing [25]. The ideal insulation measures should focus on minimizing heat loss at the edges or bottom, concentrating testing on the material itself, and providing valuable reference data for relevant numerical simulations.

Hence, the study aims to undertake an exhaustive investigation into the combustion characteristics of XPS. Utilizing a cone calorimeter equipped with a customized adiabatic sample holder, ignition time, heat release rate, critical heat flux, and other essential combustion parameters of XPS were meticulously measured across various flame retardant grades and external heat flux conditions. Furthermore, the impact of these parameters on fire risk was analyzed. This research integrates experimental outcomes with theoretical analysis to uncover the underlying combustion mechanisms of XPS, thereby establishing a scientific basis for the design of building fire protection, as well as for fire prevention and control strategies. Moreover, the study extends its scope to examine the interactions between XPS and various other construction materials, along with the combustion behavior of XPS under diverse environmental conditions, to achieve a holistic assessment of its fire risk within built environments. It is hoped that the findings of this research will provide actionable insights and recommendations aimed at bolstering building fire safety and fostering the sustainable advancement of building insulation materials.

2. Materials and Methods

2.1. Experimental Materials

The XPS specimens employed in the experiments for thickness are 25 mm and procured from a local supplier in Hefei, China. According to the national standard (GB8624−2012) [26], the samples are classified into two distinct categories: XPS−B1 and XPS−B2. XPS−B1 exhibits a bulk density of 29 kg/m³, whereas XPS−B2 has a slightly denser bulk density of 31 kg/m³. Both types of XPS are treated with hexabromocyclododecane (HBCD) as their flame retardant. The thermal conductivity for these samples is narrowly bracketed between 0.030 to 0.033 W/(m·K), and they are uniformly cut to dimensions of 100 mm × 100 mm to maintain experimental consistency and comparability.

2.2. Experimental Procedure

Before the experiment, the sample is positioned horizontally on an insulated sample holder. The conical heater is secured at a distance of 2.5 cm from the sample surface to ensure precise heat application. The perimeter and reverse sides of the sample are enveloped with aluminum foil to mitigate heat loss and prevent the dripping of melted material. Radiation intensities of 25, 35, 45, and 55 kW/m² are selected to emulate the heat flux encountered under various real−world scenarios. To diminish experimental variability and affirm reproducibility, each trial is conducted a minimum of three times.

2.3. Experimental Equipment

The experiments were conducted employing a FTT cone calorimeter (Britain) in accordance with ISO 5660−1 standard [27], featuring a core component: a meticulously calibrated cone heater. This heater emits uniform and stable thermal radiation, effectively mimicking the heat flux to which materials are subjected in actual fire conditions. In the experimental setup, a custom−insulated sample holder made of high alumina ceramic fibers was utilized, as illustrated in Figure 1. This holder surpasses traditional standard sample holders in insulation capabilities, minimizing heat loss and preventing air infiltration from the sample’s surfaces other than the heated top. This innovation ensures that the experimental environment approximates an ideal state of insulation, thereby enhancing the accuracy and reliability of the combustion data obtained.

In previous studies [28], the combustion performance of a standard steel sample holder and a self−made high alumina ceramic refractory fiber sample holder was compared under a constant radiation heat flux of 35 kW/m², provided by a conical calorimeter. Due to the thermosetting nature of polyurethane (PU), it undergoes pyrolysis and carbonization upon heating. The experimental results indicated that the ignition time for PU was nearly identical under both sample holders, averaging around 3 s, suggesting that ignition time is minimally influenced by the type of sample holder. However, there was a significant disparity in heat release rate (HRR) between the two. As depicted in Figure 2, the HRR for both the standard and self−made sample holders rapidly reached an initial peak, followed by a decrease due to the formation of a carbonization layer and the consumption of the sample. Notably, at approximately 40 s, the HRR of the standard sample holder entered a brief quasi−steady state phase, whereas the homemade sample holder exhibited a second peak later on. This divergence is primarily attributed to the differences in the material and structure of the sample holders. The standard sample holder, made of metal, heats up quickly when subjected to thermal radiation alongside the sample, generating secondary heating feedback onto the sample. In contrast, the self−made sample holder, constructed with high alumina ceramic refractory fibers known for their excellent insulating properties, significantly reduces the environmental impact on the sample’s heating, aside from the upper surface. This design markedly diminishes heat transfer at the sample’s edges, ensuring that the sample receives more uniform thermal radiation solely from the top surface. Consequently, while the two sample holders show similar ignition times, there are notable differences in the dynamic changes of HRR. The self−made sample holder’s reduction in edge heat transfer increases the heat absorbed per unit mass of the sample, thereby shortening the extinguishing time and enhancing the prominence of the second HRR peak. This finding not only underscores the benefits of self−made sample holders in capturing the intrinsic behavior of materials but also offers valuable insights for enhancing standard testing methodologies.

3. Results and Discussion

An experimental investigation was conducted to assess the impact of radiation heat source intensity and flame retardant grade on the ignition behavior of commonly utilized insulation materials, employing a cone calorimeter. This study analyzed and compared experimental outcomes, focusing on the combustion properties of diverse flame−retardant insulation materials. Furthermore, it delved into the similarities and differences in the combustion characteristics of thermoplastic XPS. Based on the unique shrinkage and melting attributes of thermoplastic XPS, the existing ignition model was revised to create a refined version tailored for XPS insulation materials. This tailored model facilitated an in−depth analysis of the fire hazards associated with XPS insulation materials. The combustion process of XPS is illustrated in Figure 3, which highlights two primary pyrolysis reactions: initial decomposition of the foam into melt and gas, followed by the further decomposition of the residual melt into gas [29,30]. Notably, a self−designed Khmer bracket, unlike standard steel brackets, offers superior insulation by minimizing heat loss [28].

3.1. Correction of the Ignition Model

Although some oxidation takes place during the heating of the material before ignition, it is justifiable to disregard the impact of oxidation on the overall decomposition process in the solid state. The rationale behind this assumption is that once the flame becomes self−sustaining post−ignition, all available oxygen will be depleted within the flame zone before it can reach the fuel surface, thus preventing any significant oxidation reactions in the solid phase [31]. The ignition time begins by switching on the heater and spark plug and ends as the visible flame is first observed. In Figure 3, ignition time increases with a higher fire retardant rating. The trend is much more obvious with a higher level of FR or a lower heat flux. Samples of the XPS series show a similar trend, observed from Figure 3. As expected, the increased external heat flux shortens the ignition time of the RPUF series and XPS series, while the ignition time of the XPS series is much higher than that of the RPUF series. XPS is ignited through a melt−to−gas decomposition process, where the heated surface recedes as the material is consumed. This increases the distance between the cone heater and the sample surface, causing a decrease in the external heat flux ${{\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}}^{″}$ received by the surface and an increase in conduction heat ${{\dot{\mathrm{q}}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{c}}}^{\prime }$, influenced by the collapse of the XPS structure into melts. The effective heat flux (${{\dot{\mathrm{q}}}_{\mathrm{n}\mathrm{e}\mathrm{t}}}^{″}$) at the heated surface deviates significantly from theoretical predictions, affecting the ignition time.

The increased distance d from the cone heater to the sample’s heated surface causes uneven heat flux distribution, particularly at the edges and corners of the plaques. The heat flux at the corners is consistently lower than that at the center of the sample. Changing the current in a cone calorimeter can alter the intensity and improve the drawbacks of distance variation. However, this work focuses on the heat flux measured at the center of the sample in relation to ignition time. Schartel et al. [32] found that the heat flux at the center, as measured by the heat flow meter, remains constant for d ranging from 0 to 2.5 cm but decreases linearly as d extends from 2.5 cm to 6 cm, as presented in Figure 4a. This phenomenon is confirmed by An et al. [33]. The function F(d) exhibits a linear relationship with the ratio of heat fluxes.

(1)$\mathrm{F}\left(\mathrm{d}\right)={{\dot{\mathrm{q}}}_{\mathrm{d}}}^{″}/{\dot{\mathrm{q}}}^{″}$

where ${\dot{\mathrm{q}}}^{″}$ denotes the heat flux at the standard horizontal plane and ${{\dot{\mathrm{q}}}_{\mathrm{d}}}^{″}$ represents the heat flux at the actual level, which corresponds to the vertical distance d from the sample’s heated surface to the cone heater.

The function and relationship between the heat flux and heater−to−surface distance are given, respectively:

(2)$\mathrm{F}\left(\mathrm{d}\right)=1.1281-0.0560\mathrm{d}$

(3)${\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}-\mathrm{d}}^{″}=(1.1281-0.0560\mathrm{d}){\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}$

Theoretical heat fluxes obtained from Equation (3) exhibit an approximate linear relationship with the measured heat fluxes in a distance range of 2.5–6 cm. As the thickness of XPS melts is too thin to act as the thermally thick samples, the ignition time ${\mathrm{t}}_{\mathrm{i}\mathrm{g}}$ is proved to have a relation with ${\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}$ [34]:

(4)${\mathrm{t}}_{\mathrm{i}\mathrm{g}}=\mathrm{l}\mathsf{\rho }\mathrm{c}{\displaystyle \frac{{\mathrm{T}}_{\mathrm{i}\mathrm{g}}-{\mathrm{T}}_0}{{\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}-\mathrm{C}\mathrm{H}\mathrm{F}}}$

where $\mathsf{\rho }$ refers to the density; $\mathrm{c}$ denotes the specific heat capacity; $\mathrm{l}$ represents the thickness; ${\mathrm{T}}_{\mathrm{i}\mathrm{g}}$ stands for the ignition temperature; ${\mathrm{T}}_0$ signifies the initial or ambient temperature; ${\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}$ indicates the external heat flux; and CHF represents the critical heat flux for ignition, which will be deduced in the subsequent section.

Thus, the ignition temperature has a linear relationship with the external heat flux ${\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}$, and the modified ignition time $\left({{\mathrm{t}}_{\mathrm{i}\mathrm{g}}}^{\prime }\right)$ shows an inversely proportional relationship with ${\mathrm{t}}_{\mathrm{i}\mathrm{g}}$, as seen in Equations (5) and (6).

(5)${\mathrm{t}}_{\mathrm{i}\mathrm{g}}\propto 1/{\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}$

(6)${{\mathrm{t}}_{\mathrm{i}\mathrm{g}}}^{\prime }=1/\mathrm{F}\left(\mathrm{d}\right){\mathrm{t}}_{\mathrm{i}\mathrm{g}}$

where ${{\mathrm{t}}_{\mathrm{i}\mathrm{g}}}^{\prime }$ is the modified ignition time in consideration of the transformed distance d from the heater to the sample surface as XPS melting. This equation presents the change in the ignition time with various radiant distances.

Figure 4b shows the correction between the predicted and experimental ignition time. The prediction error is within 24% considering the 25 kW/m² condition and approaches 0 without the 25 kW/m² cases, indicating that the predicted ignition time (Equation (6)) is reliable enough for the external heat flux higher than 35 kW/m². The difference between prediction and experiments under a heat flux of 25 kW/m² is reasonable as the repeatability of experiments under low heat flux is weaker than those with high heat fluxes, according to previous studies [35,36]. The reason for this phenomenon is that the heat transfer process under low heat flux density is complex and influenced by multiple factors, resulting in a significant increase in the variability of ignition time. Under high heat flux conditions, heat transfer is mainly carried out through a combination of radiation and convection, resulting in higher heat transfer efficiency and thus achieving more consistent ignition times. However, in the case of low heat flux density, the input rate of heat is not sufficient to bring the material to its self−ignition point, resulting in possible temperature gradients and local heat accumulation phenomena during the experimental process, thereby affecting the determination of ignition time.

3.2. Critical Heat Flux

The ignition characteristics of the material were derived using the method proposed by Janssens [37], which involves plotting the incident heat flux against the reciprocal of the ignition time (as illustrated in Figure 5). The ignition time was found to be related to the critical heat flux (CHF). The ignition temperature is defined as the surface temperature of the material at the point of ignition. Within the cone calorimeter, the ignition temperature appears to be an intrinsic property of the material, as it remains constant regardless of the radiation levels. This suggests that the ignition characteristics of materials are primarily influenced by their inherent physical and chemical properties rather than solely by external radiation conditions. When predicting the ignition time of hot, thin samples, the ignition characteristics of XPS and other materials can be described using Equation (7) [38,39,40,41].

(7)${\mathrm{t}}_{\mathrm{i}\mathrm{g}}={\displaystyle \frac{\mathsf{\pi }}{4}}\mathrm{k}\mathsf{\rho }\mathrm{c}{\left[{\displaystyle \frac{{\mathrm{T}}_{\mathrm{i}\mathrm{g}}-{\mathrm{T}}_0}{{\dot{\mathrm{q}}}_{\mathrm{e}\mathrm{x}}^{″}-\mathrm{C}\mathrm{H}\mathrm{F}}}\right]}^2$

where $\mathrm{k}$ is the thermal conductivity, and CHF is the critical heat flux for ignition. Thus, linear relationships are shown between external heat flux and ${\mathrm{t}}_{\mathrm{i}\mathrm{g}}^{-0.5}$ for RPUF and XPS.

The critical heat flux obtained from the cone calorimeter is a characteristic parameter to describe the ignitability of the materials. Figure 5 represents the relationship between ignition time and external radiance for various fire−retardant levels of XPS. The linear curve fitting is conducted with the square root of the inverse of the ignition time of XPS. It is worth noting that the slope of the linear equation of XPS−B1 is smaller than that of XPS−B2, indicating that the ignition time of XPS−B1 is more significantly affected by external heat flux density compared to other samples. This phenomenon may be related to the material structure and thermal response characteristics of XPS−B1.

The calculated CHF of each sample tested is listed in Table 1. The CHF values are 19.28 kW/m² for XPS−B1 and 20.88 kW/m² for XPS−B2, indicating that the level of RF has limited influence on CHF. The CHF values of XPS are much higher than those of RPUF because of the melting behavior of the endothermic reaction. It may also indicate that the endothermic melting progress of thermoplastic materials needs more heat absorption, which is probably the reason for showing a relatively longer ignition time.

3.3. Heat Release

3.3.1. Heat Release Rate

The heat release rate (HRR) curve of XPS is characterized by distinctive features. During the heating phase, XPS undergoes an initial brief heating stage before swiftly reaching its thermal decomposition temperature. At this juncture, a pronounced peak in HRR emerges, indicating that XPS combusts rapidly in a fire scenario, capable of releasing substantial heat over a short timeframe. The peak heat release rate (PHRR) is primarily due to the rapid decomposition of residual materials within XPS at elevated temperatures, accompanied by the emission of a significant volume of pyrolysis gases.

As depicted in Figure 6 and Table 2, when the thermal radiation intensity escalates from 25 kW/m² to 55 kW/m², the PHRR of XPS−B1 rises from 428 kW/m² to 535 kW/m², marking an increase of 25%. Conversely, the PHRR of XPS−B2 surges from 348 kW/m² to 579 kW/m², reflecting a substantial increase of 66.38%. This suggests that an intensified thermal radiation intensity markedly augments the release of heat during combustion, consequently amplifying the fire risk. In general, the incorporation of suitable flame retardants can effectively mitigate the PHRR peak of XPS to a certain extent, signifying an enhancement in the material’s fire resistance. As illustrated in Figure 6b–d, under identical thermal radiation intensities, the PHRR value of XPS−B1 is notably lower than that of the XPS−B2 sample, attributable to the superior flame retardancy of samples with elevated flame retardant content. However, Figure 6a indicates that the PHRR value of XPS−B1 is unexpectedly higher than that of XPS−B2, which may be attributed to the insufficient low heat flux to activate the flame retardant’s maximum efficacy, leading to a limited suppressive effect on the combustion behavior.

As the intensity of thermal radiation increases, the time for both materials to reach the peak heat release rate (T_PHRR) gradually decreases, reflecting the changes in the combustion rate of the materials under different thermal radiation conditions. When the intensity of thermal radiation increases, the amount of heat absorbed by the material surface rapidly increases, causing the starting temperature of the combustion reaction to reach faster, thereby accelerating the progress of the combustion reaction. When the thermal radiation intensity is 25 kW/m², the TPRR of XPS−B1 is 181.79 s, while when the thermal radiation intensity increases to 55 kW/m², the TPRR decreases to 44.95 s, a reduction of about 75.26%. This indicates that under higher thermal radiation intensity, the material burns faster, and the time to reach the peak heat release rate is shorter.

3.3.2. Total Heat Release Rate

For XPS, the total heat release (THR) is intricately linked to factors such as sample mass loss, the effective combustion heat of volatiles, and combustion efficiency. Empirical evidence suggests that while the THR of the two materials fluctuates under various thermal radiation intensities, the overall pattern remains relatively stable, suggesting that the total heat emitted by the materials during combustion is comparatively consistent. As depicted in Figure 7, an escalation in external heat flux exerts a definable influence on the THR of XPS. At a lower thermal radiation intensity of 25 kW/m², the THR values for XPS−B1 and XPS−B2 are 15.04 MJ/m² and 17.83 MJ/m², respectively, indicating a slower combustion process and more restrained heat release under these conditions. However, with an increase in external heat flux, the THR value incrementally rises, signifying that a higher heat flux hastens the combustion of XPS, diminishes the material’s flame retardant efficacy, and liberates additional heat. This observation can be ascribed to the high heat flux intensifying the melting and thermal decomposition of XPS, leading to a marked upsurge in the production rate of combustible gases and the effective heat of combustion.

3.3.3. Fire Thermal Hazard Assessment

The heat release rate and total heat release are pivotal factors in assessing fire hazards. To encapsulate a comprehensive index of material fire behavior, Petrella [42] introduced a holistic index that accounts for both the fire load and the fire growth rate. By examining the ratio of total heat release (THR) to the product of peak heat release rate (PHRR) and ignition time (t_ig), a more systematic and comprehensive evaluation of fire hazards can be achieved. The fire thermal hazard assessment of XPS series materials under varying heat flux conditions, as illustrated in Figure 8, reveals the fluctuating efficacy of flame retardants under different heat fluxes. The experimental data indicate that the incorporation of flame retardants substantially diminishes the THR value of XPS and significantly retards the fire growth rate, thereby mitigating its overall fire hazard.

The performance of XPS−B1 and XPS−B2 materials diverges under different thermal radiation intensities. As the thermal radiation intensity escalates, the peak heat release rate increases and the ignition time may be reduced, leading to a relative enhancement in the fire hazard of XPS. This phenomenon can be attributed to the accelerated thermal decomposition rate of materials in high heat flux environments, where the efficacy of flame retardants may not entirely counteract the surge in heat release due to expedited combustion. A comparison between XPS−B1 and XPS−B2 reveals disparities in their pHRR/t_ig values under the same thermal radiation intensity, suggesting distinct characteristics in the interplay between peak heat release rate and ignition time for these materials. These discrepancies stem from variations in their chemical compositions and other contributing factors.

3.4. Smoke Release

Furthermore, the emission of smoke during combustion is a critical factor influencing the progression and harmfulness of fires. Studies have demonstrated that the smoke release rate among different XPS materials varies, influenced by the chemical composition of the material, the quantity of flame retardant incorporated, and the combustion conditions. As depicted in Figure 9, the smoke release rate of XPS−B2 samples, which have a lower flame retardancy, tends to escalate prematurely and swiftly attain its peak. However, this trend diverges under high heat flux conditions of 55 kW/m², suggesting that the disparity in smoke release rates is attenuated under such intense heat fluxes. Moreover, with the augmentation of thermal radiation intensity, the Peak Smoke Production Rate (PSPR) for both materials incrementally rises. This escalation is attributed to the fact that heightened thermal radiation intensity accelerates the material’s combustion reaction, leading to increased smoke production during the combustion process. At a thermal radiation intensity of 25 kW/m², the PSPR for XPS−B1 is 0.2460 m²/s, while for XPS−B2, it is 0.2220 m²/s; at an intensified thermal radiation intensity of 55 kW/m², the PSPR for XPS−B1 escalates to 0.3162 m²/s, and for XPS−B2, it increases to 0.3095 m²/s. These findings underscore that at elevated thermal radiation levels, the combustion of materials generates a greater volume of smoke, thereby presenting more formidable challenges to fire management and suppression efforts.

The Total Suspended Particulates (TSPs) of the two materials exhibit variations under different thermal radiation intensities (Figure 10). Specifically, under low thermal radiation conditions, the TSP value of XPS−B1 is merely 6.99 m²/m², whereas the TSP value of the XPS−B2 sample is 11.06 m²/m², aligning with the Smoke Production Rate (SPR) law. Under high radiation conditions, the overall trend remains relatively stable, with the influence of thermal radiation intensity being comparatively weak. Notably, at a thermal radiation intensity of 45 kW/m², the TSP of XPS−B1 is 8.11 m²/m², and the TSP of XPS−B2 is 8.74 m²/m². There is a negligible difference in TSP between the two materials across various thermal radiation intensities. This could be attributed to the combustion mechanism and chemical composition of the materials, which determine that the total amount of smoke generated during the combustion process remains relatively consistent.

3.5. Release of CO and CO₂ Gases

When the human body inhales carbon monoxide, it binds to hemoglobin, thereby preventing oxygen from doing so and leading to hypoxia. Severe carbon monoxide poisoning can result in coma, death, and other dire consequences. Excessive carbon dioxide levels in indoor air can also cause discomfort, such as respiratory distress and dizziness. Therefore, it is crucial to thoroughly consider the quantities of toxic gases and carbon dioxide generated during the combustion process to safeguard human health and environmental safety. As illustrated in Figure 11 and Figure 12, as thermal radiation intensity increases, the production rates of carbon monoxide (PCOY) and carbon dioxide (PCO₂Y) for both materials gradually rise. This phenomenon can be attributed to the acceleration of the combustion reaction resulting from heightened thermal radiation intensity, ultimately resulting in an increased production of carbon monoxide and carbon dioxide throughout the process. Specifically, at a thermal radiation intensity of 55 kW/m², XPS−B1 exhibits a PCOY of 0.0381 g/m and a PCO₂Y of 0.3529 g/m, whereas XPS−B2 demonstrates a PCOY of 0.0418 g/m and a PCO₂Y of 0.3822 g/m. These findings indicate that under higher thermal radiation intensities, the combustion of these materials produces greater quantities of carbon monoxide and carbon dioxide, posing heightened risks to human health and the environment.

3.6. Residue Analysis

In Figure 13, XPS exhibits minimal residue post−combustion, a testament to its high combustion efficiency during fires. The foam structure of XPS may collapse due to melting before initial ignition, leading to a thorough combustion with negligible solid residue left behind. This characteristic implies that XPS is less likely to impede fire spread with residue, allowing flames to propagate more smoothly due to the absence of significant residue interference. Concurrently, this indicates that XPS can effectively convert the majority of its material into thermal energy and flue gas upon combustion. The release of such heat energy can further exacerbate temperature increases in the surrounding environment, creating more favorable conditions for fire propagation. Additionally, the substantial smoke production not only diminishes visibility and poses significant challenges for evacuation and firefighting but also poses a severe threat to human health.

4. Conclusions

This study thoroughly investigated the combustion characteristics and fire risk of extruded polystyrene in building environments through systematic experiments and analysis. Research has found that the combustion behavior and fire hazard of XPS are influenced by the content of flame retardants and external heat flux. The cone calorimeter test conducted using a self−designed insulation sample holder improved the accuracy of the experiment and more accurately reflected the combustion characteristics of XPS. Experimental data show that with the increase in thermal radiation intensity, the peak heat release rate of XPS significantly increases, with XPS−B1 rising by 25.00% and XPS−B2 surging by 66.38%, highlighting the amplifying effect of thermal radiation intensity on fire risk. Furthermore, incorporating appropriate flame retardants effectively mitigated the PHRR peak and enhanced XPS’s fire resistance. However, despite these insights, future research should delve deeper into the limitations of our self−designed sample holder and explore the impact of different XPS formulations and varying radiation intensities. Additionally, while our study underscores the significance of flame retardant content, further investigation into the optimal blend and quantity of flame retardants is needed. Addressing these gaps will not only fortify the theoretical foundation for fire safety design and numerical simulation of XPS materials but also have crucial practical implications for enhancing building fire safety, informing the formulation of more scientific fire regulations, and elevating the overall level of building fire protection.

Author Contributions

Conceptualization, Y.H.; methodology, Y.H. and M.W.; formal analysis, Y.H. and M.W.; investigation, M.W.; writing—original draft preparation, Y.H. and M.W.; writing—review and editing, Y.H., M.W., J.L., F.R. and X.Q.; supervision, J.L. and F.R.; funding acquisition, Y.H., X.Q. and C.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. (a) Standard sample holder and self−made sample holder: (b1) Simulation and (b2) object.

Figure 2. The HRR curves of RPUF along the time under standard and the self−designed specimen holders under heat flux of 35 kW/m2 [28].

Figure 3. Ignition time of XPS under external heating flux.

Figure 4. (a) Relationship between the ratio of actual to theoretical heat fluxes and the vertical heater−to−surface distance at the center of sample holder; (b) predicted and experimental ignition time under different heat fluxes.

Figure 5. Linear relationship of the reciprocal ignition time of XPS with variation of external heat fluxes.

Figure 6. HRR curves for XPS samples under different radiant heat flow with time: (a) 25 kW/m2, (b) 35 kW/m2, (c) 45 kW/m2 and (d) 55 kW/m2.

Figure 7. THR of XPS series varies with external heat fluxes.

Figure 8. THR verbs PHRR/tig to assess fire behaviors under different heat fluxes for XPS samples.

Figure 9. SPR curves for XPS samples under different radiant heat flow with time: (a) 25 kW/m2, (b) 35 kW/m2, (c) 45 kW/m2 and (d) 55 kW/m2.

Figure 10. TSP curves for XPS samples under different radiant heat flow with time: (a) 25 kW/m2, (b) 35 kW/m2, (c) 45 kW/m2 and (d) 55 kW/m2.

Figure 11. CO curves for XPS samples under different radiant heat flows with time: (a) 25 kW/m2, (b) 35 kW/m2, (c) 45 kW/m2 and (d) 55 kW/m2.

Figure 12. CO2 curves for XPS samples under different radiant heat flows with time: (a) 25 kW/m2, (b) 35 kW/m2, (c) 45 kW/m2 and (d) 55 kW/m2.

Figure 13. Morphological comparison of XPS−B1 and XPS−B2 before and after combustion.

Table 1

The revised CHF of series of samples in FR.

Samples	XPS−B1	XPS−B2
${{\dot{\mathrm{q}}}_{\mathrm{c}\mathrm{r}}}^{″} (\mathrm{k}\mathrm{W}/{\mathrm{m}}^2)$	19.28	20.88

Table 2

CCT data of XPS series varies with external heat fluxes.

Samples	XPS−B1				XPS−B2
	25 (kW/m2)	35 (kW/m2)	45 (kW/m2)	55 (kW/m2)	25 (kW/m2)	35 (kW/m2)	45 (kW/m2)	55 (kW/m2)
PHRR (kW/m2)	428	398	439	535	348	439	516	579
TPHRR (s)	181.79	94.98	60.04	44.95	169.89	85.01	64.91	48.01
THR (MJ/m2)	15.04	16.40	15.80	15.83	17.83	18.22	18.62	18.57
PSPR (m2/s)	0.2460	0.2537	0.2677	0.3162	0.2220	0.2317	0.2635	0.3095
TSP (m2/m2)	6.99	9.59	8.11	7.97	11.06	10.22	8.74	8.52
PCOY (g/m)	0.0293	0.0256	0.0295	0.0381	0.023362	0.0292	0.0339	0.0418
PCO2Y (g/m)	0.2949	0.2706	0.2910	0.3529	0.2358	0.2992	0.3447	0.3822

References

1. Kabundu, E.; Mbanga, S.; Botha, B.; Ayesu−Koranteng, E. Relative Comparison of Benefits of Floor−Slab Insulation Methods, Using Polyiso and XPS Materials in South Africa, Subject to the New National Building Energy Efficiency Standards. Energies; 2024; 17, 539. [DOI: https://dx.doi.org/10.3390/en17020539]

2. Khoukhi, M.; Fezzioui, N.; Draoui, B.; Salah, L. The impact of changes in thermal conductivity of polystyrene insulation material under different operating temperatures on the heat transfer through the building envelope. Appl. Therm. Eng.; 2016; 105, pp. 669-674. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2016.03.065]

3. Zhang, T.; Yuan, J.; Pang, H.; Huang, Z.; Guo, Y.; Wei, J.; Yu, Q. UHPC−XPS insulation composite board reinforced by glass fiber mesh: Effect of structural design on the heat transfer, mechanical properties and impact resistance. J. Build. Eng.; 2023; 75, 106935. [DOI: https://dx.doi.org/10.1016/j.jobe.2023.106935]

4. Canbolat, A.S. An integrated assessment of the financial and environmental impacts of exterior building insulation application. J. Clean. Prod.; 2024; 435, 140376. [DOI: https://dx.doi.org/10.1016/j.jclepro.2023.140376]

5. Cárdenas−R, J.P.; Cea, M.; Santín, K.; Valdés, G.; Hunter, R.; Navia, R. Characterization and application of a natural polymer obtained from Hydrangea macrophylla as a thermal insulation biomaterial. Compos. Part B Eng.; 2018; 132, pp. 10-16. [DOI: https://dx.doi.org/10.1016/j.compositesb.2017.07.086]

6. Aktemur, C.; Çakır, M.T.; Çakır, M.F. Optimising of thermal insulation thickness based on wall orientations and solar radiation using heating−degree hour method. Case Stud. Therm. Eng.; 2024; 60, 104725. [DOI: https://dx.doi.org/10.1016/j.csite.2024.104725]

7. Cai, S.; Zhang, B.; Cremaschi, L. Review of moisture behavior and thermal performance of polystyrene insulation in building applications. Build. Environ.; 2017; 123, pp. 50-65. [DOI: https://dx.doi.org/10.1016/j.buildenv.2017.06.034]

8. An, W.; Sun, J.; Liew, K.M.; Zhu, G. Effects of building concave structure on flame spread over extruded polystyrene thermal insulation material. Appl. Therm. Eng.; 2017; 121, pp. 802-809. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2017.04.141]

9. Bonner, M.; Wegrzynski, W.; Papis, B.K.; Rein, G. KRESNIK: A top−down, statistical approach to understand the fire performance of building facades using standard test data. Build. Environ.; 2020; 169, 106540. [DOI: https://dx.doi.org/10.1016/j.buildenv.2019.106540]

10. Zhang, Y.; Huang, X.; Wang, Q.; Ji, J.; Sun, J.; Yin, Y. Experimental study on the characteristics of horizontal flame spread over XPS surface on plateau. J. Hazard. Mater.; 2011; 189, pp. 34-39. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2011.01.101]

11. Zhou, B.; Yoshioka, H.; Noguchi, T.; Ando, T. Experimental study of expanded polystyrene (EPS) External Thermal Insulation Composite Systems (ETICS) masonery façade reaction−to−fire performance. Therm. Sci. Eng. Prog.; 2018; 8, pp. 83-92. [DOI: https://dx.doi.org/10.1016/j.tsep.2018.08.002]

12. An, W.; Li, S.; Yin, X.; Peng, L. Combustion and fire safety of energy conservation materials in building vertical channel: Effects of structure factor and coverage rate. Case Stud. Therm. Eng.; 2021; 24, 100847. [DOI: https://dx.doi.org/10.1016/j.csite.2021.100847]

13. Meng, Q.X.; Zhu, G.Q.; Yu, M.M.; Liang, Z.H. Experimental study on upward flame spread characteristics of external thermal insulation material under the influence of porosity. Case Stud. Therm. Eng.; 2018; 12, pp. 365-373. [DOI: https://dx.doi.org/10.1016/j.csite.2018.05.008]

14. Uygunoğlu, T.; Özgüven, S.; Çalış, M. Effect of plaster thickness on performance of external thermal insulation cladding systems (ETICS) in buildings. Constr. Build. Mater.; 2016; 122, pp. 496-504. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2016.06.128]

15. Zhang, W.; Jia, J.; Zhang, J.; Ding, Y.; Zhang, J.; Lu, K.; Mao, S. Pyrolysis and combustion characteristics of typical waste thermal insulation materials. Sci. Total Environ.; 2022; 834, 155484. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2022.155484]

16. Li, X.; Peng, C.; Liu, L. Experimental study of the thermal performance of a building wall with vacuum insulation panels and extruded polystyrene foams. Appl. Therm. Eng.; 2020; 180, 115801. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2020.115801]

17. An, W.; Sun, J.; Liew, K.M.; Zhu, G. Flammability and safety design of thermal insulation materials comprising PS foams and fire barrier materials. Mater. Des.; 2016; 99, pp. 500-508. [DOI: https://dx.doi.org/10.1016/j.matdes.2016.03.080]

18. Ouyang, D.; Yan, H.; Song, J.; Yang, C.; Jiang, T.; Liu, C. Combustion characteristics and fire hazard of polystyrene exterior wall thermal insulation materials. J. Appl. Polym. Sci.; 2023; 140, e53503. [DOI: https://dx.doi.org/10.1002/app.53503]

19. Hossain, M.D.; Hassan, M.K.; Akl, M.; Pathirana, S.; Rahnamayiezekavat, P.; Douglas, G.; Bhat, T.; Saha, S. Fire behaviour of insulation panels commonly used in high−rise buildings. Fire; 2022; 5, 81. [DOI: https://dx.doi.org/10.3390/fire5030081]

20. Qian, X.; Li, S.; Shi, C.; Ren, F.; Che, H. The study of smoke, heat release and fire spread characteristics of expanded polystyrene under low oxygen condition. J. Therm. Anal. Calorim.; 2023; 148, pp. 821-829. [DOI: https://dx.doi.org/10.1007/s10973-022-11788-5]

21. Sun, H.; Pan, Y.; Wang, J.; Gong, J.; Ding, A.; Jiang, J. Flame spread characteristics of inclined extruded polystyrene thermal insulation material. Procedia Eng.; 2018; 211, pp. 651-658. [DOI: https://dx.doi.org/10.1016/j.proeng.2017.12.060]

22. Liu, H.; Li, M.; Jiang, L.; Xu, Q. Experimental and theoretical study on ignition and combustion characteristics of aging woods by cone calorimetry. J. Therm. Anal. Calorim.; 2023; 148, pp. 10573-10582. [DOI: https://dx.doi.org/10.1007/s10973-023-12311-0]

23. Qian, X.; Liu, Q.; Zhang, L.; Li, H.; Liu, J.; Yan, S. Synthesis of reactive DOPO−based flame retardant and its application in rigid polyisocyanurate−polyurethane foam. Polym. Degrad. Stabil.; 2022; 197, 109852. [DOI: https://dx.doi.org/10.1016/j.polymdegradstab.2022.109852]

24. Wang, Y.; Kang, W.; Chen, C.; Zhang, X.; Yang, L.; Chen, X.; Cui, G.; Zhang, Y.; Zhang, F.; Li, S. Combustion behaviour and dominant shrinkage mechanism of flexible polyurethane foam in the cone calorimeter test. J. Hazard. Mater.; 2019; 365, pp. 395-404. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2018.11.027]

25. Hidalgo, J.P.; Torero, J.L.; Welch, S. Experimental Characterisation of the Fire Behaviour of Thermal Insulation Materials for a Performance−Based Design Methodology. Fire Technol.; 2017; 53, pp. 1201-1232. [DOI: https://dx.doi.org/10.1007/s10694-016-0625-z]

26. GB 8624−2012 Classification for Burning Behavior of Building Materials and Products; Standards Press of China: Beijing, China, 2012.

27. ISO 5660−1 Fire Test—Reaction to Fire—Part 1: Rate of Heat Release from Building Products (Cone Calorimeter Method); International Organization for Standardization: Geneva, Switzerland, 2015.

28. Wang, S.; Zhang, H.; Hou, Y.; Cheng, X. Flammability of Building Thermal Insulation Materials Using Self−designed Adiabatic Specimen Holder in Cone Calorimeter. ESSE 2017; De Gruyter: Berlin, Germany, 2017; pp. 89-100. Available online: https://www.researchgate.net/publication/320849988_Flammability_of_Building_Thermal_Insulation_Materials_Using_Self-designed_Adiabatic_Specimen_Holder_in_Cone_Calorimeter_Proceedings_of_the_International_Conference_on_Environmental_Science_and_Sustain (accessed on 21 October 2024).

29. Baeck, S.; Lee, Y.; Lee, H. Classical and reactive simulations of plastic co−pyrolysis: Effects of polystyrene and its fragments on product yields, aggregation, and reaction mechanisms. Chem. Eng. J.; 2024; 497, 154857. [DOI: https://dx.doi.org/10.1016/j.cej.2024.154857]

30. Valencia, L.B.; Rogaume, T.; Guillaume, E.; Rein, G.; Torero, J.L. New method for simulating the kinetic of toxic gases production of upholstered furniture fire. Proceedings of the 11th International Conference on Fire and Materials; San Francisco, CA, USA, 26–28 January 2009; pp. 685-695.

31. Babrauskas, V. Ignition of Wood: A Review of the State of the Art. J. Fire Prot. Eng.; 2002; 12, pp. 163-189. [DOI: https://dx.doi.org/10.1177/10423910260620482]

32. Schartel, B.; Bartholmai, M.; Knoll, U. Some comments on the use of cone calorimeter data. Polym. Degrad. Stabil.; 2005; 88, pp. 540-547. [DOI: https://dx.doi.org/10.1016/j.polymdegradstab.2004.12.016]

33. An, W.; Jiang, L.; Sun, J.; Liew, K.M. Correlation analysis of sample thickness, heat flux, and cone calorimetry test data of polystyrene foam. J. Therm. Anal. Calorim.; 2015; 119, pp. 229-238. [DOI: https://dx.doi.org/10.1007/s10973-014-4165-9]

34. Quintiere, J.G. The effect of angular orientation on flame spreadover thin materials. Fire Saf. J.; 2001; 3, pp. 291-312. [DOI: https://dx.doi.org/10.1016/S0379-7112(00)00051-5]

35. Hermouet, F.; Guillaume, É.; Rogaume, T.; Richard, F.; El Houssami, M. Experimental determination of the evolution of the incident heat flux received by a combustible during a cone calorimeter test: Influence of the flame irradiance. J. Fire Sci.; 2021; 39, pp. 119-141. [DOI: https://dx.doi.org/10.1177/0734904120970440]

36. Shi, L.; Chew, M.Y.L. Fire behaviors of polymers under autoignition conditions in a cone calorimeter. Fire Saf. J.; 2013; 61, pp. 243-253. [DOI: https://dx.doi.org/10.1016/j.firesaf.2013.09.021]

37. Janssens, M.L. Improved Method of Analysis for the LIFT Apparatus, Part I: Ignition. Proceedings of the 2nd Fire and Materials Conference; Washington, DC, USA, 18 October 1993; pp. 37-46.

38. El Gazi, M.; Sonnier, R.; Giraud, S.; Batistella, M.; Basak, S.; Dumazert, L.; Hajj, R.; El Hage, R. Fire Behavior of Thermally Thin Materials in Cone Calorimeter. Polymers; 2021; 13, 1297. [DOI: https://dx.doi.org/10.3390/polym13081297]

39. Fateh, T.; Kahanji, C.; Joseph, P.; Rogaume, T. A study of the effect of thickness on the thermal degradation and flammability characteristics of some composite materials using a cone calorimeter. J. Fire Sci.; 2017; 35, pp. 547-564. [DOI: https://dx.doi.org/10.1177/0734904117713690]

40. Jelle, B.P. Traditional, state−of−the−art and future thermal building insulation materials and solutions—Properties, requirements and possibilities. Energy Build.; 2011; 43, pp. 2549-2563. [DOI: https://dx.doi.org/10.1016/j.enbuild.2011.05.015]

41. Spearpoint, M.J.; Quintiere, J.G. Predicting the piloted ignition of wood in the cone calorimeter using an integral model—Effect of species, grain orientation and heat flux. Fire Saf. J.; 2001; 36, pp. 391-415. [DOI: https://dx.doi.org/10.1016/S0379-7112(00)00055-2]

42. Petrella, R.V. The assement of full−scale fire hazards from cone calorimeter data. J. Fire Sci.; 1994; 12, pp. 14-43. [DOI: https://dx.doi.org/10.1177/073490419401200102]