Design and Fabrication of Insulating Composite Fire-Resistant Glass

Abstract

With the acceleration of urbanization and the rapid development of high-rise buildings, building fire safety has become an increasingly prominent concern. Traditional fire-resistant materials are no longer sufficient to meet the multifaceted requirements of modern architecture in terms of safety, energy efficiency, and aesthetics. This study innovatively develops an insulating composite fire-resistant glass. Through the design and optimization of the fire-resistant liquid type and the multi-layer composite structure, a synergistic enhancement in fire resistance performance, thermal insulation efficiency, and optical properties is achieved. Experimental results demonstrate that the insulating composite fire-resistant glass exhibits transparency comparable to that of commercial clear glass. More importantly, under standard fire resistance tests, it achieves a fire resistance rating exceeding 2 h, with a temperature rise on the non-fire side ≤ 100 °C. Furthermore, a series of composite fire-resistant glasses with varying fire protection grades can be fabricated as required. Its superior fire resistance is attributed to the unique tightly bonded porous interface structure, which effectively impedes heat conduction, thermal radiation, and heat convection. The design strategy and fabrication methodology presented in this study offer a novel technical pathway for advancing the building industry toward green development, demonstrating broad market application prospects and significant social benefits.

1. Introduction

With the continuous acceleration of economic development and urbanization, the demands for building safety and functionality have concurrently increased. Dense spaces such as high-rise buildings, large-scale complexes, and public transportation hubs are continuously emerging. Consequently, the associated fire risks have become increasingly prominent due to structural complexity and high occupant density [1,2,3]. Fire-resistant glass, as a critical safeguard for fire safety, not only plays a key role in fire isolation, spread control, and ensuring occupant evacuation [4,5], but also demonstrates significant comprehensive value in areas such as green buildings, urban resilience, and critical infrastructure protection. This drives the industry’s evolution from single-function safety protection toward diversification, systematization, and low-carbon development [6,7].

Based on their structural differences, fire-resistant glasses can be categorized into monolithic fire-resistant glass and composite fire-resistant glass. Monolithic fire-resistant glass is a specialized type of glass that exhibits enhanced thermal resistance and impact resistance after subjecting ordinary float glass to specific treatments. Typical silicate-based fire-resistant glass is primarily composed of SiO₂, with minor additions of oxides such as B₂O₃, Al₂O₃, TiO₂, and Li₂O, offering advantages such as high light transmittance, good weatherability, and low density [8]. Increasing the content of these oxides can further improve the thermal shock resistance of silicate-based fire-resistant glass, ensuring its structural integrity under flame exposure. However, the fabrication process for this type of glass is complex and costly, significantly limiting its widespread application. Another type of monolithic fire-resistant glass is wired fire-resistant glass, which incorporates embedded metal wires or mesh within the glass pane. This reinforcement not only provides structural support and enhances the overall impact resistance of the fire-resistant glass but also enables multifunctional integration when connected to electric heating and security alarm systems [9]. Nevertheless, this type of glass inherently lacks thermal insulation performance. When exposed to fire, excessive surface temperatures can lead to spontaneous cracking or shattering, posing a considerable risk of secondary hazards. Additionally, the presence of embedded metal wires or mesh significantly compromises the optical transmittance of the glass.

Emerging composite fire-resistant glass is fabricated by bonding two or more layers of glass, offering superior thermal insulation and compressive properties. It can be categorized into laminated composite fire-resistant glass and grouting-type fire-resistant glass [10]. Laminated composite fire-resistant glass is produced by coating a thin layer of a specialized fire-resistant slurry between two glass panes, which is then cured; multi-layer lamination can also be employed to meet varying fire resistance requirements [11,12]. Currently, the most commonly used interlayer material in laminated composite fire-resistant glass is polyvinyl butyral (PVB), which offers advantages such as aesthetic appeal, heat resistance, high impact strength, and mechanical robustness. However, it is highly susceptible to moisture-induced delamination and exhibits poor sealing performance, significantly limiting its application [13]. Grouting-type fire-resistant glass is manufactured by injecting a fire-resistant liquid gel into the cavity between two or more glass panes, followed by curing [14,15,16]. The fire-resistant liquid gel not only effectively blocks flame propagation but also mitigates the transmission of high temperatures and hazardous smoke, thereby enabling effective control of fire development [17,18,19]. It is regarded as the most ideal building fire protection product [20]. Consequently, grouting-type fire-resistant glass has garnered increasing attention, and the key fire-retardant technology lies in the transparent fire-resistant liquid gel used for grouting. Investigating the types of gel and the number of grouting layers is therefore critical to achieving optimal fire resistance performance [21,22,23].

Recent studies on organic–inorganic flame-retardant systems, particularly phosphorus–nitrogen flame retardants, have shown that phosphazene-based or phosphorus/nitrogen-containing compounds can promote char formation, dilute combustible volatiles, and generate compact protective residues in polymer matrices and intumescent coatings [24,25,26,27]. These findings indicate that the combination of inorganic silicate frameworks with phosphorus–nitrogen flame-retardant chemistry is an effective route for improving thermal stability and flame-retardant efficiency. However, most previous studies focused on epoxy resins, polymer composites, or coating systems, whereas transparent grouting-type fire-resistant glass requires simultaneous optical transparency, interfacial compatibility, and high-temperature insulation. Therefore, exploring fire-resistant liquid gels within multi-layer glass cavities remains necessary for developing building-oriented transparent fire protection products.

This study innovatively develops an insulating composite fire-resistant glass. By optimizing the multi-layer composite structure and the type of fire-resistant liquid gel, a synergistic enhancement in fire resistance performance, thermal insulation efficiency, and optical properties is achieved. Furthermore, through structural and compositional analysis of the composite fire-resistant glass, its fire resistance mechanism is elucidated. This work provides both a conceptual framework and a research foundation for the design and fabrication of high-performance insulating composite fire-resistant glass, offering significant theoretical implications and engineering application value.

2. Methods

2.1. Materials

Tempered glass for architectural use was procured from Anhui Jianke Energy-Saving Building Materials Co., Ltd. (Suqian City, China). The fire-resistant liquid was obtained from two suppliers: Anhui Jianke Energy-Saving Building Materials Co., Ltd. and Fuyang Mingyi Glass Technology Co., Ltd. (Fuyang City, China). Butyl sealant was purchased from Beijing Huali Building Materials Decoration Co., Ltd. (Beijing City, China), and silicone sealant was sourced from Chengdu Guibao Technology Co., Ltd. (Chengdu City, China).

The two fire-resistant liquids used in this work were commercial gel-forming fire-resistant liquids. Their exact proprietary formulations were not disclosed by the suppliers; therefore, only the available component information and the characterization results are reported here. Based on the supplier information, common formulations of transparent grouting-type fire-resistant liquids, and the EDS/SEM observations in this study, the liquids mainly contain water-soluble silicate species, phosphorus–nitrogen flame-retardant additives, carbonaceous/char-forming components, and stabilizing additives. The nanoscale irregular particles observed in the cured fire-resistant liquid layer are mainly attributed to silicate nanoparticles formed through hydrolysis-polycondensation and network aggregation of the silicate components.

2.2. Composite Fire-Resistant Glass Preparation

First, the surfaces of architectural tempered glass panels (length 1200 mm, width 700 mm, thickness 5 mm) were cleaned and dried to remove dust, grease, and other contaminants. Butyl sealant was then applied to laminate multiple glass panels, forming a hollow cavity structure. Subsequently, two different types of fire-resistant liquid were injected into the internal cavities of the glass assemblies. After degassing treatment, the edges of the glass units were sealed with silicone sealant and cured in a homogenizing furnace to obtain insulating composite fire-resistant glass. The composite fire-resistant glass infused with the fire-resistant liquid from Mingyi Company was designated as CFG-1, while that infused with the fire-resistant liquid from Jianke Company was designated as CFG-2. For multi-layer insulating composite fire-resistant glasses of varying thicknesses, the same fabrication procedure was adopted. After filling the internal cavities of the glass assemblies completely with the fire-resistant liquid, each cavity was sealed and encapsulated. The insulating composite fire-resistant glasses with thicknesses of 25 mm and 35 mm were designated as CFG-25 and CFG-35, respectively. Specifically, CFG-25 corresponds to a structure of 5 mm tempered glass + 5 mm fire-resistant liquid + 5 mm tempered glass + 5 mm fire-resistant liquid + 5 mm tempered glass. CFG-35 corresponds to a structure of 5 mm tempered glass + 5 mm fire-resistant liquid + 5 mm tempered glass + 5 mm fire-resistant liquid + 5 mm tempered glass + 5 mm fire-resistant liquid + 5 mm tempered glass.

The 25 mm and 35 mm multi-layer structures were selected as representative configurations commonly used in practical engineering applications rather than as an exhaustive optimization series. The present work therefore compares representative two-liquid-layer and three-liquid-layer structures. Systematic optimization of the number of layers and the thickness of each liquid layer will be carried out in future work to balance fire resistance, transparency, weight, and cost.

2.3. Characterization

The morphology of the insulating composite fire-resistant glass was characterized using scanning electron microscopy (SEM, SU8600, Hitachi, Tokyo, Japan). Specifically, glass samples to be characterized were trimmed to dimensions of 1–2 cm and affixed to the surface of a metal sample stage using conductive adhesive. Subsequently, an ultrathin platinum layer was deposited on the sample surface via a high-resolution sputter coater to impart electrical conductivity. Finally, the sample morphology was observed under a scanning electron microscope at an accelerating voltage of 1–2 kV.

The chemical composition of the insulating composite fire-resistant glass was analyzed using energy-dispersive X-ray spectroscopy (EDS, Hitachi, Tokyo, Japan), an accessory integrated with the SEM system. During morphological observation, the elemental content and distribution at designated points, along line scans, or over specific areas of the sample could be examined as required.

2.4. Fire-Resistant Property Test

As the core performance indicator of fire-resistant glass, the fire resistance performance was tested in accordance with the GB/T 12513-2006 standard [28]. The procedure is as follows: The composite fire-resistant glass specimen, with dimensions of 1200 mm × 700 mm, was mounted onto the opening of an experimental furnace wall. The furnace temperature was regulated to follow the time–temperature curve specified in GB/T 12513-2006. Five temperature sensors were positioned at different locations on the unexposed surface of the glass specimen (at the center and four corners). The temperature on the non-fire side was monitored and recorded three times per minute, and the average value was calculated to determine the temperature rise on the non-fire side. Concurrently, during the heating process, the appearance (transparency) and integrity (e.g., breakage or detachment) of the glass were continuously observed.

GB/T 12513-2006 adopts a standard time–temperature curve that is essentially consistent with ISO 834-1:1999 [29] or fire-resistance testing of building elements. This curve is a standardized severe-fire exposure used to compare building components under reproducible conditions, although real fire scenarios may vary with fuel type, ventilation, fire load, and compartment geometry. For each experimental condition, three parallel specimens were prepared and tested. Five temperature sensors were arranged on each specimen, and the temperature rise values reported in this study represent the average values obtained from the sensors and repeated specimens.

3. Results and Discussion

3.1. Characteristics of Composite Fire-Resistant Glass

In this study, the core design concept centers on the multi-layer composite glass structure formed by infusing a fire-resistant liquid. Accordingly, the micromorphology of the composite fire-resistant glass was first examined. As shown in Figure 1a, the composite fire-resistant glass is observed to consist of outer glass layers and particulate powder derived from the fire-resistant liquid infused within the hollow cavity, forming a typical “sandwich” composite structure. The outer glass layers exhibit uniform thickness and continuous integrity, with some pores present on the cross-section. The fire-resistant liquid particles are compact and dense, completely filling the entire cavity without defects such as cracks or voids.

The interfacial bonding condition of the composite fire-resistant glass significantly influences mass and heat transfer during fire exposure, thereby playing a critical role in its fire resistance performance. Subsequently, the interfacial morphology between the glass layer and the fire-resistant liquid layer was examined using high-magnification SEM (Figure 1b). It was observed that the glass layer and the fire-resistant liquid layer are tightly bonded, with no gaps or delamination. Upon further magnification, a thin and continuous transition zone was identified at the interface, indicating slight physical or chemical diffusion between the two materials during the fabrication process (Figure 1c). This intimate interfacial bonding and the formation of a transition zone effectively facilitate stress transfer and dispersion, significantly enhancing interlaminar adhesion. Consequently, delamination or spalling of the fire-resistant liquid material under thermal shock at elevated temperatures is prevented, preserving the structural integrity of the composite fire-resistant glass.

Moreover, as shown in Figure 1c, the micromorphology of the fire-resistant liquid layer is also observable. It consists of a large number of cross-linked and stacked nanoscale irregular particles, forming a three-dimensional network structure with numerous pores distributed both on the surface and within the interior. This continuous and interconnected porous architecture not only substantially reduces solid-state heat conduction but also effectively impedes thermal radiation and heat convection, thereby conferring excellent thermal insulation properties to the fire-resistant liquid layer. Collectively, these structural characteristics provide the primary structural assurance for the long-term fire resistance performance of the composite fire-resistant glass, demonstrating the feasibility of the fabrication approach and methodology adopted in this study.

It should be noted that the SEM observations in this study are qualitative. Numerical porosity, pore-size distribution, and specific surface area were not calculated from the SEM images because reliable quantification requires additional image analysis and porosimetry techniques. Future work will combine mercury intrusion porosimetry and Brunauer–Emmett–Teller analysis to quantify the porous structure formed at high temperatures.

To further validate the feasibility of the approach and methodology proposed in this study, elemental distribution analysis on the cross-section of the composite fire-resistant glass was conducted using EDS mapping. Figure 1d presents an overlay of multiple elemental distributions, enabling simultaneous observation of the spatial distribution of various elements across the cross-section. It can be observed that distinct elemental regions correspond to different functional layers with well-defined boundaries. For instance, elements such as Si, O, Na, and K exhibit clear demarcation lines between the glass layer and the fire-resistant liquid layer and are predominantly enriched in the glass layers on both sides. This demonstrates that the distribution of the glass layer and the fire-resistant liquid layer governs the elemental distribution patterns. To more clearly visualize the respective distributions of the glass layer and the fire-resistant liquid layer, the spatial distributions of Si (corresponding to the SiO₂ component in the glass layer) and C (corresponding to the carbon black component in the fire-resistant liquid) were separately characterized, as shown in Figure 1e and Figure 1f, respectively. Consistent with the aforementioned analysis, Si is predominantly concentrated within the glass layers, with its distribution contour aligning precisely with that of the glass layers. In contrast, C is distributed across both the glass layers and the fire-resistant liquid layer. More importantly, within the interfacial transition zone, the distribution of C exhibits a concentration gradient, providing evidence of its diffusion behavior at the interface. This diffusion enhances the adhesion of the fire-resistant liquid layer to the glass substrate surface.

The elemental diffusion discussed here is inferred from EDS mapping rather than from EDS line-scan or depth-profile measurements. The mapping results are sufficient to identify the spatial distribution and interfacial transition trend, but quantitative diffusion profiles will require line-scan, depth-profiling, or related characterization in subsequent research.

In summary, the structural and compositional analysis results demonstrate, at the microscopic level, that the design approach and fabrication methodology employed in this study successfully yield an insulating composite fire-resistant glass characterized by a dense structure and excellent interfacial bonding. The unique porous architecture of the infused fire-resistant liquid layer endows the composite fire-resistant glass with highly efficient thermal insulation performance. Furthermore, the robust interfacial bonding formed through diffusion between the glass layer and the fire-resistant liquid layer lays a solid foundation for the stable fire resistance performance of the composite fire-resistant glass under elevated temperature fire exposure.

In addition, the interfacial adhesion between the glass layer and the fire-resistant liquid layer was evaluated qualitatively through SEM morphology. Quantitative adhesion strength, such as pull-off or shear strength, was not measured in this study and will be examined in future mechanical tests.

3.2. Fire-Resistant Properties of Composite Fire-Resistant Glass

Following the design and fabrication of the insulating composite fire-resistant glass, its core performance—fire resistance—was evaluated, and the influence of different fire-resistant liquids on fire resistance performance was investigated. Figure 2a,b presents the macroscopic morphologies of the composite fire-resistant glasses fabricated using the fire-resistant liquid from Mingyi Company and Jianke Company, respectively, before and after standard fire resistance testing. It can be observed that both types of composite fire-resistant glass exhibit flat and smooth surfaces free of visible defects, along with high transparency comparable to that of commercial clear glass, thereby satisfying everyday application requirements. During the 90-min standard fire resistance test, the non-fire side surfaces of both composite fire-resistant glass specimens remained intact and smooth, with virtually no changes in macroscopic morphology. This indicates a fire resistance rating exceeding 90 min, demonstrating excellent fire resistance performance. Meanwhile, the composite fire-resistant glass transitioned from transparent to an opaque white state. This transformation is attributed to the oxidation and carbonization of both types of infused fire-resistant liquid under elevated temperature conditions, resulting in the formation of a carbon black/ash mixture, which lays the foundation for fire resistance and thermal insulation. Furthermore, it can be observed that the resulting carbon black/ash mixture is uniformly distributed throughout the entire cavity of the composite fire-resistant glass, confirming the complete and homogeneous filling of the fire-resistant liquid within the cavity, as well as its uniform oxidation/carbonization under high-temperature exposure.

As another core indicator for evaluating fire resistance performance, thermal insulation performance was quantitatively assessed by monitoring and recording the non-fire side surface temperatures of the two types of insulating composite fire-resistant glass during the standard fire resistance test. As shown in Figure 2c, the furnace temperature rapidly increased to 750 °C within 15 min and further rose to 1100 °C within 120 min. In contrast, the non-fire side surface temperatures of both insulating composite fire-resistant glass specimens remained substantially lower than the furnace temperature, with a temperature difference of 700–950 °C, demonstrating excellent thermal insulation performance and providing enhanced safety for occupant evacuation and property protection. The inset enlarged view reveals that the temperature rise rates on the non-fire side of both insulating composite fire-resistant glass specimens were similarly extremely slow (~1 °C/min), with temperature increases ≤180 °C. However, the insulating composite fire-resistant glass fabricated with the fire-resistant liquid from Jianke Company fractured after 90 min of standard fire resistance testing, whereas the insulating composite fire-resistant glass fabricated with the fire-resistant liquid from Mingyi Company withstood nearly 120 min of standard fire resistance testing, thereby exhibiting superior fire insulation performance. In subsequent tests, the insulating composite fire-resistant glass fabricated with the fire-resistant liquid from Mingyi Company was used exclusively. After investigating the effect of fire-resistant liquid type on the fire resistance performance of the insulating composite fire-resistant glass, the influence of glass thickness—i.e., the number of fire-resistant liquid-infused layers—on fire resistance performance was further examined. Figure 3a,b presents the macroscopic morphologies of composite fire-resistant glasses with thicknesses of 25 mm and 35 mm, respectively, (for detailed structural configurations, see Methods) before and after standard fire resistance testing.

The better performance of CFG-1 compared with CFG-2 is attributed mainly to differences in the proprietary formulations of the Mingyi and Jianke fire-resistant liquids. Although their detailed compositions were not available for direct comparison, the Mingyi fire-resistant liquid is inferred to possess higher thermal stability, more effective char-forming behavior, and stronger interfacial bonding with the glass layer. During fire exposure, these features are beneficial for forming a more compact and continuous porous insulating layer, thereby delaying cracking and maintaining integrity for a longer time. By contrast, the Jianke liquid may form a less compact carbonized layer or undergo earlier thermal degradation, leading to fracture after 90 min. Detailed FTIR, XRF, and TGA comparisons will be carried out in future work to verify this interpretation. Heat-flux density was not calculated in this study because the current experimental setup was not equipped with heat-flux sensors; therefore, the temperature difference is used here as a direct indicator of thermal insulation performance. The absence of sudden cracking or shattering during standard furnace exposure also indicates that the samples had good thermal shock resistance under the present test conditions, although dedicated thermal shock tests are still needed.

It can be observed that increasing the thickness from 25 mm to 35 mm does not affect the transparency of the composite fire-resistant glass, which remains comparable to that of commercial clear glass. Furthermore, the surface of the 35 mm thick composite fire-resistant glass remains flat and smooth, free of any visible defects. During the 60-min standard fire resistance test, both CFG-25 and CFG-35 maintained stable structural integrity without cracking or flame penetration, thereby satisfying the requirements for fire integrity. Meanwhile, the transparency transitioned to a uniformly opaque white state, consistent with the mechanism described above.

Subsequently, the temperature rise on the non-fire side of CFG-25 and CFG-35 was quantitatively measured (Figure 3c). It was found that the non-fire side temperature of CFG-25 increased extremely slowly, with a maximum temperature of only 100 °C, well below the standard critical threshold of 180 °C. This demonstrates that CFG-25 satisfies and exceeds the 60-min requirement for both fire integrity and thermal insulation, and can therefore be classified as insulating fire-resistant glass (Class A). CFG-35 exhibited even superior thermal insulation performance. Its non-fire side temperature rise curve remained consistently lower than that of CFG-25, indicating a slower rate of temperature increase, with the temperature approaching 100 °C only after 120 min of testing. More importantly, its fire resistance duration was twice that of CFG-25 (120 min). Therefore, increasing the number of layers (i.e., thickness) of the composite fire-resistant glass structure significantly enhances both the fire insulation limit and thermal insulation performance. This improvement can be attributed to two main factors. On one hand, the increased composite structure directly extends the heat transfer path from the fire-exposed side to the non-fire side, thereby raising the thermal resistance—particularly due to the insulating effect of the porous structure of the fire-resistant liquid material. On the other hand, the buffering effect of the multi-layer composite structure allows the fire-resistant liquid layers near the non-fire side sufficient time to undergo carbonization and oxidation, forming a porous insulating structure. This enables the full utilization of the fire-resistant and insulating effects of the multi-layer fire-resistant liquid configuration.

The temperature rise values of CFG-25 and CFG-35 are reported as average values calculated from the monitoring points and repeated specimens, rather than as single-point data from an individual sample.

In summary, this work systematically regulated the type of fire-resistant liquid and the thickness of the composite fire-resistant glass and investigated their effects on fire resistance performance, thereby achieving excellent fire integrity and thermal insulation in the insulating composite fire-resistant glass. Compared with commercial non-insulating fire-resistant glass, which offers only fire integrity, the core advantage of the insulating composite fire-resistant glass lies in its superior thermal insulation performance. Furthermore, by tailoring these parameters, a series of composite fire-resistant glasses meeting various fire protection grades (e.g., from Class C 1 h to Class A 1.5 h) can be fabricated to accommodate diverse building fire safety requirements. This demonstrates that the design approach and fabrication methodology adopted in this study offer favorable design flexibility and tunability.

3.3. Fire-Resistant Mechanism of Composite Fire-Resistant Glass

To elucidate the fire resistance mechanism of the insulating composite fire-resistant glass, the structural and compositional evolution of the material after exposure to various elevated temperatures was characterized and analyzed. As shown in Figure 4a, after heat treatment at 400 °C, the liquid components in the gel-state fire-resistant liquid volatilized and underwent oxidation, transforming into irregularly shaped solid powders, including flaky particles, agglomerated blocks, and scattered granules. These solid powder structures were relatively loose and exhibited weak interfacial bonding. At this stage, the interfacial layer in direct contact with the glass retained a relatively flat and dense structure, rather than the porous architecture essential for fire insulation, indicating that this temperature was insufficient to induce complete oxidation and carbonization of the fire-resistant liquid. Subsequently, the temperature was increased to 500 °C (Figure 4b). The initially irregular solid powders transformed into a flocculent structure resulting from oriented carbonization and crystallization of the fire-resistant liquid at elevated temperatures. However, these bundled flocculent structures still lacked a porous morphology. Meanwhile, thermal stress generated at this temperature induced microcracks in the fire-resistant liquid layer within the interfacial region, which not only compromised interfacial adhesion but also provided pathways for heat transfer. Consequently, the structural configuration at this temperature remained inadequate for achieving superior fire insulation performance. Upon further increasing the temperature to 600 °C (Figure 4c), the fire-resistant liquid underwent not only oxidation and carbonization but also sintering of the resulting solid powders due to the excessively high temperature. This caused the fire-resistant liquid layer in the interfacial region to revert to a dense state. Concurrently, the coarse powder structures transformed into fine granular particles with significantly reduced surface roughness. Nevertheless, the porous structure critical for fire insulation performance had not yet developed. When the temperature was raised to 700 °C (Figure 4d), partial sublimation of certain components within the solid powder occurred, leading to melting and structural reconstruction of the fire-resistant liquid layer, accompanied by pore formation. This substantially increased the surface roughness of the fire-resistant liquid layer and initiated preliminary fire insulation performance. However, due to the limited extent of sublimation, the number of pores formed remained relatively low, thereby restricting the degree of improvement in fire insulation performance.

Finally, the heating temperature was further elevated to 800 °C. At this temperature, the fire-resistant liquid layer underwent rapid melting and structural reconstruction, resulting in significantly refined particle sizes. More importantly, the volatilization of certain solid components intensified, thereby forming a highly porous structure with elevated porosity, which laid the foundation for fire insulation performance (Figure 5a). Examination of the interfacial region between the fire-resistant liquid layer and the glass layer (Figure 5b,c) revealed two key phenomena. On one hand, the elevated temperature induced pore formation within the glass layer itself, while the overall structure remained intact and continuous. On the other hand, due to high-temperature melting, reconstruction, and sintering, diffusion occurred between the fire-resistant liquid layer and the glass layer, forming a continuous and tightly bonded interfacial transition zone, which effectively enhanced interfacial adhesion. Corresponding EDS elemental mapping further confirmed these observations. Elements such as K, Na, O, Si (representing the glass component), and C (representing the fire-resistant liquid component) were distributed correspondingly throughout the entire interfacial layer, without distinct elemental boundaries demarcating different material layers. This demonstrates that the two materials diffused and fused together, forming a robust physicochemical bond (Figure 5d–f). This tightly bonded interfacial porous structure not only significantly reduces solid-state heat conduction but also effectively impedes thermal radiation and heat convection, thereby endowing the insulating composite fire-resistant glass with excellent fire integrity and fire insulation performance.

Thus, the heat transfer mechanism proposed in this work is a qualitative interpretation based on morphology, elemental distribution, and temperature-rise behavior. The respective contributions of conduction, convection, and radiation were not quantitatively separated. A coupled heat transfer model and additional thermal-property measurements, such as laser flash analysis and heat-flux monitoring, will be developed in future work.

Figure 6 summarizes the proposed fire-retardant mechanism of the insulating composite fire-resistant glass. During fire exposure, the fire-resistant liquid layer undergoes dehydration, oxidation, carbonization, and thermal reconstruction, gradually forming a tightly bonded porous insulating layer between the glass sheets. This porous network lengthens the heat transfer path and suppresses solid-state conduction, while the multi-layer glass structure restricts convection and attenuates thermal radiation. At the same time, the fused interfacial transition zone strengthens the adhesion between the glass layer and the fire-resistant layer, helping the composite maintain structural integrity and sustain effective fire insulation under prolonged high-temperature exposure.

4. Conclusions

In summary, this study fabricated a transparent insulating composite fire-resistant glass by infusing a fire-resistant liquid into the hollow cavity of multi-layer tempered glass. The effects of the fire-resistant liquid type and the multi-layer composite structure on fire resistance performance were systematically investigated. The resulting glass demonstrated excellent fire integrity and thermal insulation properties, including a fire resistance rating exceeding 2 h and a temperature rise on the non-fire side of ≤100 °C. Moreover, by tailoring the fire-resistant liquid formulation and the multi-layer composite configuration, a series of composite fire-resistant glasses meeting various fire protection grades (e.g., from Class C 1 h to Class A 2 h) can be fabricated to accommodate diverse building fire safety requirements. Through analysis of the structural and compositional evolution of the fire-resistant liquid at elevated temperatures, it was found that at 800 °C, the fire-resistant liquid undergoes oxidation and carbonization, transforming into a tightly bonded porous structure at the interface. This unique structure not only markedly suppresses solid-state heat conduction but also effectively blocks thermal radiation and heat convection. The fire resistance mechanism of the insulating composite fire-resistant glass was thus elucidated, providing a theoretical foundation for the design and fabrication of composite fire-resistant glass and substantially facilitating its practical application.

The developed composite fire-resistant glass is suitable for building applications in which both transparency and fire compartmentation are required, including high-rise buildings, large commercial complexes, hospitals, schools, airports, railway stations, fire-resistant partitions, fire doors and windows, and curtain-wall systems. Future work will focus on: (1) a detailed compositional analysis of commercial fire-resistant liquids and their relationship with fire resistance; (2) systematic optimization of layer number and liquid-layer thickness; (3) comparative validation under international standards such as ISO 834-1 and EN 1363-1; (4) post-fire mechanical strength, impact resistance, flexural strength, and thermal shock testing; and (5) long-term durability evaluation under different service environments.