Fire Safety Performance of Typical Silica-Based Flame-Retardant Rigid Polyurethane Foam

Fire Safety Performance of Typical Silica-Based Flame-Retardant Rigid Polyurethane Foam

Introduction

Polyurethane foam (PUF) is widely used in various fields thanks to its corrosion resistance, excellent thermal insulation, aging resistance, low cost, and high strength. However, like other organic polymer materials, rigid polyurethane foam (RPUF) is flammable, which has long limited its application and caused numerous fire accidents. For instance, in the "11/18" Beijing fire in 2017, RPUF combustion in cold storage was the main cause of 27 casualties. To enhance RPUF's fire safety, different flame retardants have been added. Yet, some of these additives release harmful gases during combustion, negatively impact physical properties, and pollute the environment. Therefore, improving RPUF's fire safety is a key research topic in safety engineering.

Tetraethyl orthosilicate (TEOS) has low viscosity and high penetration depth. As a liquid organic compound, it shows good compatibility with RPUF. During combustion, TEOS thermally decomposes into SiO₂, which can catalyze oxidation, improve the char layer structure, and offers high-temperature resistance and excellent thermal conductivity. In this study, TEOS was incorporated into RPUF to prepare RPUF/TEOS composites, and various tests were conducted to evaluate their flammability, thermal degradation, and gas emission characteristics.

I. Experimental Section

1. Experimental Materials

- Tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si, molecular weight: 208.33, density: 0.929 g/cm³, free acid: 0.05%, analytical grade) was supplied by BASF Chemical Co., Ltd., Tianjin.

- Composite polyether polyol (Y4110) and methylene diphenyl diisocyanate (MDI, PM200) were provided by ShunDa Polyurethane Co., Ltd., Yantai, Shandong Province.

2. Experimental Instruments

- Cone Calorimeter, Model 6810, Suzhou Yangyi Wolke Testing Technology Co., Ltd.

- Microcalorimeter, Model MCC-2, Govemark, USA.

- Thermogravimetric Analyzer, Model DT-50, SETARAM Instruments, France.

- Infrared Spectrometer, Model IRAffinity-1, Shimadzu, Japan.

- Oxygen Index Analyzer, Model HC-2G, Nanjing Jiangning Analytical Instruments Co., Ltd.

3. Sample Preparation

The RPUF composite system was prepared by stirring and mixing, with the formulation detailed in Table 1. A specific amount of white component was weighed, and TEOS was added according to the corresponding ratio. The mixture was then quickly placed in a disperser and stirred at 1500 rpm for 0.5 hours. After that, the black component was added, and the mixture was immediately returned to the disperser for high-speed stirring at 1500 rpm for 8 seconds until foaming occurred. The foaming mixture was quickly poured into a mold with a sealed base and allowed to foam naturally at room temperature. After cooling for 0.5 hours, the mold was removed to obtain the rigid polyurethane foam material. The prepared foam was then placed in an 80 °C oven for post-curing for 4 hours. Finally, the foam plastic was cut to appropriate dimensions for further characterization.

4. Characterization and Performance Tests

- Fourier Transform Infrared Spectroscopy (FT-IR) Analysis: To investigate the hydrogen bonding interactions between segments in RPUF, FT-IR spectra were recorded in the range of 4000–500 cm⁻¹ with a resolution of 2 cm⁻¹ and 50 scans.

- Cone Calorimetry (CCT): In accordance with ISO 5660-1, samples were cut to 100 mm × 100 mm × 10 mm. The cone calorimeter was used to assess the flame retardancy and fire safety properties of the samples under a radiation intensity of 25 kW·m⁻².

- Thermogravimetric-Infrared (TG-IR) Analysis: Pyrolysis experiments were conducted using a thermogravimetric analyzer in a nitrogen atmosphere (flow rate: 60 mL·min⁻¹). Samples (9–11 mg) were heated from room temperature to 760 °C at 20 K·min⁻¹. The thermal degradation characteristics were monitored by TG-IR, with a wavelength range of 4000–500 cm⁻¹.

- Microcalorimetry (MCC): To further analyze the combustion characteristics, samples (5–6 mg) were placed in a crucible and exposed to a radiation temperature of 900 °C, with a heating rate of 1 K·s⁻¹, heating from 75 °C to 750 °C.

- Limiting Oxygen Index (LOI): The LOI was measured using an HC-2 oxygen index analyzer to evaluate the heat resistance of the samples, which were sized at 120 mm × 10 mm × 10 mm.

II. Results and Discussion

1. FT-IR Analysis

Figure 1 presents the FT-IR spectra of TEOS, RPUF, and RPUF/TEOS composites.

Characteristic peaks for NHCO (2937 cm⁻¹), NHOR1R2 (2860 cm⁻¹), C=O (1726 cm⁻¹), NH (1527 cm⁻¹), and C-O-C (1226 cm⁻¹) are observed. A distinctive peak at 2978 cm⁻¹ in RPUF/TEOS is absent in pure RPUF but matches a peak in TEOS. Additionally, other characteristic peaks of TEOS, especially the silicon-specific peak at 1105 cm⁻¹, appear in the RPUF/TEOS spectrum. This confirms that TEOS is uniformly dispersed in the RPUF composite.

2. CCT Analysis

Table 2 lists the cone calorimetry data for pure RPUF and RPUF/TEOS composites at a radiation power of 25 kW·m⁻². The following analysis compares the thermal, smoke, and residual char parameters:

• Heat Release Rate (HRR) and Total Heat Release (THR)

Figure 2(a) and (b) illustrate the HRR and THR curves for RPUF. HRR and THR are critical for assessing the thermal hazard of polymers and influence smoke generation and mass loss rates.

From Figure 2(a), RPUF/TEOS composites generally exhibit lower HRR than RPUF. Specifically, RPUF/TEOS0.5 (with 0.5% TEOS) shows a 5.8% reduction in peak HRR (PHRR) compared to RPUF. The ignition time for RPUF/TEOS composites is longer than for pure RPUF, with RPUF/TEOS1.0 igniting seven times slower. This is attributed to TEOS hindering the release of flammable gases under external heat radiation. Before 38 seconds of combustion, RPUF/TEOS composites mostly have lower HRR values than RPUF, with extended ignition and combustion times. This is because the conjugated π-bonds formed between RPUF and TEOS enhance the composite's strength and thermal/oxidative stability. Consequently, TEOS incorporation delays ignition and reduces PHRR. However, Figure 2(b) shows that RPUF/TEOS composites have higher THR than pure RPUF. This may be due to smoldering of internal polymers within the char layer towards the end of combustion, prolonging the burning process and increasing total heat release.

• Smoke Release Rate (SPR) and Total Smoke Production (TSP)

Figure 3(a) and (b) present the smoke release rate and total smoke production curves for RPUF composites. SPR and TSP are key smoke - related parameters.

From Figure 3(a), the RPUF/TEOS composites show a lower SPR than pure RPUF. The peak smoke release rate (PSPR) values for RPUF/TEOS0.5 and RPUF/TEOS1.0 are 23.8% and 22.5% lower than those of pure RPUF, while RPUF/TEOS2.0 shows a significant 35% reduction. This indicates that TEOS addition lowers smoke generation during RPUF combustion by forming a char layer that hinders the release of flammable gases.

From Figure 3(b), the TSP values for samples with different TEOS contents are slightly higher than for pure RPUF. This might be due to smoldering of internal polymers within the char layer towards the end of combustion, prolonging smoke generation and increasing total smoke production. However, considering the SPR curves in Figure 3(a), TEOS addition does reduce the SPR of RPUF/TEOS composites during combustion and extends the burning time. Overall, TEOS incorporation has a smoke - suppression effect on RPUF.

• Mass Analysis

Figure 4 presents the mass curves of RPUF composites. It shows that the final residual char of RPUF/TEOS is slightly less than that of pure RPUF, indicating that TEOS addition doesn't promote char formation during combustion. This might be due to the poor compatibility between TEOS and polyurethane, which may degrade the thermal properties of the composite.

In terms of the decomposition process, RPUF/TEOS starts to decompose slightly later than pure RPUF. As combustion continues, pure RPUF exhibits the highest maximum mass loss rate, while RPUF/TEOS composites have lower maximum mass loss rates than pure RPUF.

• CO and CO₂ Generation Rates and Totals

Figure 5(a) and (b) show the RPUF composite's CO generation rate and total CO output curves. Figure 5(c) and (d) show the CO₂ generation rate and total CO₂ output curves. Table 3 lists the toxicity and fire safety parameters of RPUF composites from cone calorimetry.

From Figure 5(a), it can be observed that the CO generation rate curve for pure RPUF has only one peak, whereas the curve for RPUF/TEOS has two peaks. Comparing the data up to 200 seconds (as the experiment for RPUF ends by then), the Peak CO generation rate (PCO) for RPUF/TEOS is found to be lower than that of pure RPUF (0.0058 g·s⁻¹). This indicates that the addition of TEOS reduces the CO generation rate. However, after 200 seconds, the CO generation rate for RPUF/TEOS starts to rise again, eventually reaching a second peak comparable to that of RPUF.

Figure 5(b) reveals that prior to 100 seconds, the total CO generation for RPUF/TEOS composites is lower than for pure RPUF. Nevertheless, after 100 seconds, there is an increase in total CO generation. This trend is consistent with the observed increase in total smoke generation (as shown in Table 3(b) until the end of combustion. The rise in CO generation rate and total CO generation towards the end of the combustion process is likely due to smoldering of the polymer within the char layer of the composite material.

The CO₂ generation rate peaks for RPUF, RPUF/TEOS0.5, RPUF/TEOS1.0, and RPUF/TEOS2.0 are 0.141, 0.132, 0.130, and 0.134 g·s⁻¹, respectively. TEOS addition reduces the CO₂ generation rate of RPUF/TEOS. Before 79 seconds, RPUF/TEOS composites have lower total CO₂ generation than pure RPUF. However, after 79 seconds, the total CO₂ generation increases, which might be due to the smoldering of internal polymers within the char layer towards the end of combustion. This extends the smoke generation process, increasing CO₂ generation rate and total CO₂ generation.

• Fire Performance Index (FPI) and Fire Growth Index (FGI)

This study focuses on the fire safety performance of RPUF/TEOS composites, using FPI and FGI for further evaluation.

As shown in Table 3, pure RPUF has the lowest FPI (0.012) and the second-highest FGI (10.83). In contrast, RPUF/TEOS composites exhibit higher FPI and mostly lower FGI than pure RPUF. Notably, RPUF/TEOS1.0 has an FPI of 0.089, seven times higher than pure RPUF, and an FGI of 2.56, 24% of pure RPUF's value. This indicates that RPUF/TEOS composites, especially RPUF/TEOS1.0, have better fire safety performance than pure RPUF.

• Charcoal morphology and structure

Cone calorimetry profile test after the end of combustion, carbon slag photos are shown in Figure 6. Figure 6 shows that the carbon layer of RPUF / TEOS composite material is more intact and dense, and the carbon layer is expanded, so it becomes a barrier to oxygen and combustible gases, this structure significantly inhibit the mass and heat transfer, thus effectively slowing down the combustion process of the substrate material.

3. Limiting Oxygen Index (LOI) test

RPUF / TEOS composite material LOI values are shown in Table 4. Table 4 shows that all RPUF / TEOS composite material LOI value is more than 20.8% and are higher than the pure RPUF (20.1%), the experimental process found that the composite material has a self-extinguishing and no molten droplet phenomenon. This is due to the thermal decomposition of TEOS into SiO2, which migrates to the surface of the residual carbon layer and forms a barrier to oxygen and combustible gases.

4. Thermogravimetric analysis (TG)

Figures 7(a) and (b) show the TG and DTG curves of pure RPUF and RPUF/TEOS composites. As shown in Table 5 (T5% indicates the temperature at which the mass loss is 5%, and Tmax indicates the temperature at which the DTG is maximum), from T5% and Tmax, it can be seen that there is not much difference between the T5% and Tmax of RPUF/TEOS composites and pure RPUF, and the residual carbon rate of RPUF/TEOS0.5 composite material is increased by 5.7% compared with that of pure RPUF at 750 ℃, and the residual carbon rate of other RPUF/TEOS composites increases by 5.7% compared to that of pure RPUF. The residual carbon rate of RPUF/TEOS composites was slightly lower than that of pure RPUF; from the viewpoint of mass loss rate, it can be seen that the maximum mass loss rate of RPUF/TEOS composites was lower than that of pure RPUF. It can be concluded that TEOS promotes the process of carbon formation in RPUF, and the carbon layer formed has the functions of oxygen and heat insulation as well as the inhibition of the release of combustible products, and the maximum mass loss rate can be reduced and the charcoal layer formed is not as strong as the charcoal layer formed in RPUF. The maximum mass loss rate and the thermal stability of RPUF/TEOS composites were improved.

5. Thermogravimetric-IR (TG-IR)

The infrared spectra of the gases produced during the thermal decomposition of RPUF are shown in Figure 8. From the infrared spectrum of RPUF, it can be seen that the peak near 3730 cm-1 corresponds to the stretching vibration of the N H bond in the urethane, and the peaks located at 2360 and 2290 cm-1 confirm the presence of CO2 and isocyanate compounds (NCO), which are typical gaseous products of the first stage of RPUF, and the peaks near 1730 and 1510 cm-1 correspond to the carbonyl compounds (C The peaks near 1730 and 1510 cm-1 correspond to carbonyl compounds (C O) and aromatic compounds, respectively; the characteristic peaks at 1260 and 1110 cm-1 are the functional groups C O C, and the water peak is at the peak value of 3725 cm-1, and the characteristic peaks of CH2 and CH3 are at the peak values of 2974 and 2855 cm-1. The characteristic peaks of the above products have been found in the RPUF and the RPUF/TEOS composite, which indicate that TEOS as an additive flame retardant does not change the cleavage of the polyurethane molecular chain. This indicates that TEOS as an additive flame retardant does not change the cleavage process of polyurethane molecular chain, and two degradation processes, which is consistent with TG.

The first stage is the degradation of the hard part of the polyurethane chain, and the main products are isocyanate compounds, amines, hydrocarbons and carbon dioxide. The second stage is the degradation of the soft part, and the main product is CO. The hydrocarbon intensity of RPUF/TEOS is lower than that of pure RPUF, and the biggest difference between them is that the characteristic peak of CO2 at 2360 cm-1 and the peak of CO2 at 2360 cm-1 are not the same as those of pure RPUF.

The biggest difference between the two is the characteristic peaks of CO2 at 2360 cm-1 and carbonyl compounds (C O) at 1730 cm-1. It can be seen that the peaks of pure RPUF and RPUF/TEOS composites are similar before 500 ℃, and after that, between 600 and 800 ℃, the characteristic peaks of CO and CO2 in the pure RPUF welcome new peaks, while the RPUF/TEOS composites keep the stable values (substantially lower than that of pure RPUF). The CO and CO2 characteristic peaks of pure RPUF welcomed new peaks, while the RPUF/TEOS composites maintained stable values (much lower than that of pure RPUF), especially the release of CO, and the RPUF/TEOS composites produced a very small amount of CO and harmful gases at the peak of the thermal decomposition, which was attributed to the fact that the TEOS facilitated the entrance of more hydrocarbons into the condensed phase and reduced the release of flammable and harmful gases, and the result was similar to that of the rate of the generation of CO and CO2 tested by the conical calorimeter.

III. Conclusion

Based on all experiments, the limiting oxygen index (LOI) test shows that ethyl orthosilicate (TEOS) can improve the LOI of RPUF (rigid polyurethane foam)/TEOS composite material. Cone calorimetry test (CCT) results show that TEOS can reduce the peak heat release rate (PHRR) and smoke release rate (SPR) values, prolong the ignition time of RPUF/TEOS. Thermogravimetric analysis (TG) proved that TEOS could reduce the heat loss rate of RPUF/TEOS composite materials, and TEOS could effectively inhibit the release of CO and CO2 in thermogravimetric infrared (TG-IR) test. The results showed that the presence of TEOS improved the flame retardancy of RPUF composites.

In summary, TEOS, as an environmentally friendly flame retardant, was thermally decomposed into SiO2, which migrated to the surface of the material and facilitated the formation of a relatively dense and swollen residual carbon layer. The residual carbon layer becomes a protective barrier against combustible gases and oxygen. The shielding layer significantly inhibited the transfer of oxygen and heat, effectively delayed the combustion of the underlying materials, and improved the fire safety performance of RPUF composites.