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Abstract

This study proposes a novel strategy for the value-added utilization of converter OG sludge by incorporating it as a flame-retardant additive in rigid polyurethane foam (RPUF). A series of RPUF/OG composites were fabricated via a one-step all-water foaming technique. The effects of OG sludge on the thermal stability, combustion behavior, and flame retardant properties of RPUF were systematically investigated using multiple analytical methods, including limiting oxygen index (LOI) test, vertical burning test (UL-94), thermogravimetric analysis (TG), and cone calorimeter test (CCT). The results show that OG sludge can significantly enhance the fire safety of the composites. While traditional metrics like the limiting oxygen index showed limited improvement, CCT results indicate that the peak heat release rate (pHRR), total heat release (THR), and total smoke production (TSP) of the RPUF/OG50 composite decrease significantly by 16.5%, 24.1%, and 43.6%, respectively. This confirms the effectiveness of OG sludge in suppressing fire intensity and smoke generation. TG reveals that OG sludge improves the thermal stability of RPUF composites, with a residual char yield of up to 21.7 wt% at 800°C. Scanning electron microscopy (SEM) and Raman spectroscopy analyses demonstrate that the complex metal oxides in OG sludge enhance the compactness of the char residue of RPUF composites, forming a stable barrier layer that inhibits the transport of heat and substances during the combustion of the composites. This study provides a new approach for the high-value and sustainable utilization of OG sludge, and offers directions for the development of flame-retardant RPUF composites for building thermal insulation applications.

Keywords

OG sludge rigid polyurethane foam flame retardant composites thermal stability

Introduction

The iron and steel sector produces a diverse range of solid wastes. Currently, most of the dust and sludge is disposed of simply by landfilling. Not only does this way consume a huge quantity of land resources, but it also constitutes a substantial threat to human health and the natural environment.¹ At present, the reuse and utilization of converter OG sludge in China mainly make use of a number of technologies. They are reverse sintering; processing zinc - containing dust and sludge with the help of rotary hearth furnaces, rotary kilns, and cold briquetting technology; and the digestion approach of adding lime powder to converter OG sludge.^2–4 Most of these technologies require in depth treatment of OG sludge, and their added value needs to be enhanced. Therefore, promoting the high value utilization of dust and sludge through green and low - carbon methods has become an urgent task, and it is also a key issue that the current metallurgical industry urgently needs to address.⁵

Converter OG sludge is mainly composed of oxides formed by the oxidation of elements such as Fe, Ca, Si, and P in metallurgical raw materials, impurities introduced during the steel making process, and furnace lining materials that erode and fall off. Its main components are low valence iron oxide, calcium oxide, and a small amount of zinc oxide, phosphorus pentoxide, etc.¹ The metal oxide components discussed earlier have been extensively employed as flame retardant synergists within polymer composites. The origin of the flame retardant effect of metal oxides lies in their various functional characteristics, including physical shielding,⁶ catalytic carbonization,⁷ and chemical capture.⁸ These mechanisms are essential for enhancing the flame retardant characteristics of materials, lowering smoke production, and reducing toxicity levels.⁹ Therefore, being a blend of the aforementioned components, converter OG sludge holds extensive application potential in the realm of flame retardancy.²

Notably, the utilization of various industrial solid wastes as fillers or flame retardant additives in polymers has gained significant attention.^10,11 For instance, modified iron tailings have been used synergistically with ammonium polyphosphate (APP) to enhance the flame retardancy of thermoplastic polyurethane (TPU).¹² Similarly, sludge residue from wastewater treatment (digestate) has been incorporated into Poly (Lactic Acid) (PLA) as a synergistic additive with APP, improving fire performance.¹³ In particular, fly ash (FA), another abundant industrial by-product, has been successfully incorporated into polymers like epoxy and polybutylene adipate terephthalate (PBAT) to improve flame retardancy, often showing synergistic effects with conventional flame retardants.^12,14 These studies typically report the formation of stable protective char layers containing new crystalline phases, which enhance fire resistance. However, despite its complex metal oxide composition resembling a pre-designed synergistic system, the direct application of untreated converter OG sludge as a flame retardant filler in polymers, particularly in rigid polyurethane foam (RPUF), remains largely unexplored and represents a significant research gap. Therefore, exploring the feasibility and effectiveness of utilizing this unique, untreated OG sludge to address the flammability issue of RPUF is of great interest.

Rigid polyurethane foam (RPUF) is formed by the addition polymerization reaction of polyols and isocyanates.^15,16 It has advantages such as good thermal insulation performance, low mass, good mechanical properties, and a simple preparation process.¹⁷ It finds extensive application in various areas such as building exterior wall insulation, pipeline heat preservation, and the insulation of refrigerators, freezers, and water heaters.^18,19 Nevertheless, RPUF shows weak fire resistant properties, with a relatively low limiting oxygen index (around 18 vol%). Additionally, it emits toxic gases like HCN and CO when burning.²⁰ These deficiencies have significantly hampered the widespread use of RPUF.²¹ Consequently, enhancing the flame retardancy of RPUF and decreasing the emissions during its combustion are crucial problems that the industry urgently needs to solve.

Presently, the reactive type and the additive type serve as two commonly adopted strategies for enhancing the flame retardancy of RPUF.²² Reactive flame retardants are made by chemically bonding flame retardant groups (hydroxyl, amino or epoxy groups, etc.) to the polyurethane molecular chain, so that the flame - retardant performance becomes an intrinsic property of the material. This can endow polyurethane foam with excellent flame - retardant properties during the foaming process.^23–25 However, the preparation procedure of this approach is intricate and expensive, and the number of reactive flame retardants in the market is relatively small, with a limited range of choices.¹⁹ Additive flame retardants usually refer to halogenated flame retardants, phosphorus flame retardants,^26,27 metal hydroxides,²⁸ intumescent flame retardants,²⁹ expanded graphite (EG),^30–32 organoclay³³ and other flame retardant substances are incorporated into polyurethane via the blending approach to enhance its flame retardant characteristics. Additive flame retardants do not engage directly in the foaming and curing reactions, instead, they provide a wider range of structural variations compared to reactive flame retardants.³⁴ In addition, their synthesis procedures are much simpler, and additive flame retardants have drawn significant interest within the industry due to their simplicity, dependability, and cost effectiveness.

In this work, we propose a novel and high-value strategy by employing untreated converter OG sludge directly as an additive flame retardant for RPUF. To the best of our knowledge, this is the first comprehensive study to systematically investigate the use of converter OG sludge in RPUF for enhancing flame retardancy. RPUF/OG sludge composites were prepared through a full water foaming technique. The limiting oxygen index (LOI), vertical combustion (UL-94), cone calorimetry (CCT), and scanning electron microscopy (SEM) were employed to investigate the flame retardant properties, combustion characteristics, and morphological changes of the RPUF/OG sludge composites. This study not only offers a novel approach for the high-value utilization of converter OG sludge but also demonstrates the fabrication of flame-retardant RPUF composites using metallurgical solid waste, contributing to both waste management and material science.

Experimental

Materials

Polyether polyol (LY-4110) and Triethylenediamine (A33) was obtained from Jiangsu Lvyuan New Material Technology Co., Ltd. Polymethylene polyphenyl isocyanate (PM-200) was obtained from Wanhua Chemical Group Co., Ltd. Silicone oil foam stabilizer (AK-8805) was obtained from Jining Hengtai Chemical Co., Ltd. Triethanolamine (TEOA) was obtained from Sinopharm Chemical Reagent Co., Ltd. Dibutyltin dilaurate (LC) was obtained from Air Products and Chemicals, Inc. The converter OG sludge was obtained from MaSteel (Group) Holding Co., Ltd. The as-received sludge was first dried in an oven at 105°C for 24 hours to remove moisture. Subsequently, it was manually ground using an agate mortar and pestle to break down agglomerates and obtain a fine powder suitable for blending with the polyurethane precursors. Distilled water was produced internally within the science lab.

Synthesis n of RPUF composites

The composition ratios of raw materials for the RPUF/OG sludge composites are detailed in Table 1. Polyether polyol LY-4110, LC, A33, AK-8805, TEA, converter OG sludge, and deionized water were placed in a beaker with a volume of 500 mL at one time according to the set proportion. After being thoroughly mixed with a stirrer, the accurately weighed PM-200 was put in, and the mixture was rapidly agitated for a period of 10 seconds. When white bubbles emerged from the mixture, it was rapidly poured into an open mold to allow for free-rise foaming. After approximately 15-20 seconds of free foaming, the mold was transferred to a forced draft drying oven set at 80°C for 8 hours to complete the curing process. Then, it could remain undisturbed at ambient temperature for 24 hours. Afterwards, the samples were sectioned and prepared for testing in accordance with the experimental requirements.

Table 1.

Composition of raw materials of RPUF and RPUF/OG sludge composites (unit: g).

Sample	LY-4110	PM-200	LC	AK-8805	A33	TEOA	Water	OG
RPUF	100	135	0.5	2	1	3	2	0
RPUF/OG10	100	135	0.5	2	1	3	2	10
RPUF/OG20	100	135	0.5	2	1	3	2	20
RPUF/OG30	100	135	0.5	2	1	3	2	30
RPUF/OG40	100	135	0.5	2	1	3	2	40
RPUF/OG50	100	135	0.5	2	1	3	2	50

Characterization

Limiting Oxygen Index (LOI): The LOI of the composite materials was assessed with a JF-3 type oxygen index analyzer (produced by Nanjing Jiangning District Instrument Analysis Factory) in accordance with the ASTM D2863 standard. The specimens had dimensions of 10 × 10 × 127 mm³.

Vertical Combustion (UL-94): The vertical combustion behavior of the test samples was evaluated with a horizontal and vertical combustion tester (model CZF-3, made by Nanjing Jiangning District Instrument Analysis Factory) following the GB/T 2408 standard. The test specimens were characterized by dimensions of 127 × 13 × 10 mm³.

X-ray Diffraction (XRD): The mineral composition of the OG sludge was tested using a D8 ADVANCE X-ray single-crystal diffractometer (Bruker, Germany). The designated material employed was a Cu target, operating at a power level of 3 kW. The test angle scope spanned from 5° to 80°, and the precision of the 2θ angle was no more than 0.02^°.

X - ray Fluorescence Spectroscopy (XRF): To study the chemical makeup of the converter OG sludge, an ARL Advant’X Intellipower TW3600 scanning X-ray fluorescence spectrometer (Thermo Fisher Scientific, USA) was put into use.

Mechanical Properties: A DCS - 5000 universal material testing equipment (Jinan Puye Electromechanical Technology Co., Ltd) was used to evaluate the compressive strength of the samples. The dimensions of the specimen material were 50 × 50 × 50 mm³. Every specimen underwent testing five times, and the arithmetic mean was calculated.

Thermal Conductivity Test: A TC3000 E thermal conductivity meter (produced by Xi’an Xiaxi Electronic Technology Co., Ltd) was employed to measure the thermal conductivity of the samples. Every specimen had a size of 50 × 50 × 25 mm³. Every specimen underwent testing three times, and the arithmetic mean was computed.

Thermogravimetric Analysis (TGA): The thermal stability of the samples (5-10 g) in a nitrogen atmosphere was studied using a DTG-60H thermogravimetric analyzer (Shimadzu, Japan). The analysis proceeded at a heating velocity of 20°C every minute, spanning a temperature range from ambient temperature to 800°C.

Cone Calorimeter Test (CCT): A VOUCH 6810 Cone Calorimeter (produced by Suzhou Yangyi Woerqi Testing Technology Co., Ltd) was employed to measure the combustion performance of the samples wrapped in aluminum foil in accordance with the ISO 5660-1 standard. Every specimen had a size of 100 × 100 × 25 mm³. The test was carried out under a radiant heat flux of 35 kW/m² and an air flow rate of 24 L/s.

Scanning Electron Microscopy (SEM): The morphologies of the specimens and their carbonaceous residues were analyzed by virtue of a JSM - 6490LV scanning electron microscope (JEOL, Japan). The samples were subjected to surface conductive layer - spraying treatment before testing, and the acceleration voltage was 15 keV.

Laser Confocal Raman Spectroscopy (Raman): The specimens underwent heat treatment within a muffle furnace adjusted to a temperature of 600°C and maintained for 10 minutes to yield the carbon residue. During the examination of the carbon residues, a Lab. RAM HR Evolution (HORIBA Scientific) was utilized. The wavelength span was between 800 cm⁻¹ and 2000 cm⁻¹, and the resolution stood at 1 cm⁻¹.

Fourier-Transform Infrared Spectroscopy (FTIR): Utilizing the Nicolet 6700 infrared spectrometer of Thermo Fisher Scientific from America, the FTIR spectra of the samples were obtained. The spectral range covered wavelengths from 4000 cm⁻¹ to 400 cm⁻¹, with a resolution of 4 cm⁻¹.

Results and discussion

Characterization of OG sludge

The chemical and mineral make-up of converter OG sludge was probed through the use of XRF and XRD. From Table 2, it is plain that the chief chemical components of converter OG sludge include CaO, MgO, SiO₂, Fe₂O₃, and Al₂O₃. Figure 1 shows the XRD pattern of OG sludge. By comparing with the characteristic diffraction of standard cards, it was found that the mineral composition of OG sludge mainly consists of Fe (PDF#: 06-0696), FeO (PDF#: 06-0615), Al_0.5Fe₃Si_0.5 (PDF#: 45-1205), Al₁₃Co₄ (PDF#: 50-0786), etc. In the context of using OG sludge in RPUF composites, iron oxides can act as active components in the flame retardant mechanism. They are capable of catalyzing the carbonization process of the polyurethane matrix when it is burning. This catalytic influence facilitates the formation of a more robust and compact char layer. Such a char layer is of great significance in enhancing the flame-retardant characteristics of the composite material.^35,36

Table 2.

Chemical composition of converter OG sludge.

Chemical composition	CaO	SiO₂	MnO	Fe₂O₃	Al₂O₃	MgO	ZnO	Other
Amount/wt%	12.06	2.57	0.64	80.78	0.54	2.02	0.228	1.162

Figure 1.

XRD pattern of OG sludge.

Cell structure

The SEM was conducted to examine how OG sludge affects the cellular architecture of rigid polyurethane foam. As illustrated in Figure 2 (a), the original RPUF specimen exhibits a clear-cut closed-cell honeycomb configuration. There are small holes on certain parts of the honeycomb structures, and the overall cell size is relatively consistent. Figure 2 (b)-(d) show the cell structures of RPUF/OG sludge composites with different additions of OG sludge. As is evident from Figure 2, upon the addition of OG sludge, the cell size of the composite diminishes gradually. This may be because the addition of OG sludge reduces the free energy that is necessary for nucleation within the composite.³⁷ At the same time, it was found that with the addition of OG sludge, the integrity and uniformity of the cells decline to some extent, leading to an increase in the proportion of open-cell structures. This issue might arise from the poor matching between the OG sludge particles and the cellular composition of the RPUF matrix.³⁸

Figure 2.

SEM images of RPUF and RPUF/OG composites: (a) RPUF; (b)RPUF/OG10; (c)RPUF/OG30; (d) RPUF/OG50.

Physical property

The physical characteristics, mechanical properties, and thermal traits of polymeric foam materials are substantially impacted when it comes to their visible density, thermal conductivity, and resistance to compression. These key properties greatly affect the overall performance and behavior of these materials. As is evident from Table 3, with the rise in the amount of OG sludge, the compressive strength of the RPUF composite diminishes progressively. The pure RPUF exhibits the highest compressive strength, reaching 0.325 MPa, whereas the compressive strength of RPUF/OG50 is the lowest, amounting to 0.140 MPa. This observed decrease in intensity is consistent with the research trends reported for other polymer composites incorporated with rigid inorganic fillers.^39–41 There may be two main reasons for this observed decrease in strength. First, the rigid OG sludge particles may act as stress concentration points within the polymer matrix, initiating microcracks under load. Second, and more critically, there is a lack of strong chemical bonding between the RPUF and the OG sludge particles, resulting in weak interfacial adhesion. This weak interface is a major limiting factor for the mechanical properties of filled polymer systems.^41,42 Contrary to the variation trend of compressive strength, the incorporation of OG sludge has enhanced the thermal insulation performance of RPUF composites. The thermal conductivity decreased from 0.0372 W·(m·K)⁻¹ for pure RPUF to 0.0321 W·(m·K)⁻¹, representing a reduction of approximately 13.7%. This can be attributed to the fact that the dispersed solid particles of OG sludge increase the tortuosity of heat transfer paths inside the foam, forcing heat to travel through more complex paths with higher resistance—an effect known to lower the thermal conductivity of porous materials.⁴³ Additionally, the numerous interfaces between OG sludge and the RPUF matrix act as sites for phonon scattering, which can effectively impede the propagation of thermal energy through the solid phase of the composite.⁴⁴ At the same time, the apparent density of the RPUF composites showed a moderate change (ranging from 41.6 kg·m⁻³ to 45.3 kg·m⁻³, with a maximum decrease of approximately 8.2%). This is because OG sludge may act as a cell nucleating agent during the foaming process to refine the foam structure; meanwhile, the increase in its content also increases the viscosity of the system, restricts the full expansion of the foam, thereby reducing the polyurethane density and improving the foaming efficiency.⁴⁵ The apparent density of the composites did not show a consistent changing trend with the addition of OG sludge, but fluctuated within a certain range. This further confirms that the enhancement of thermal insulation mainly stems from the inherent ability of the filler to hinder heat transfer, rather than a substantial change in foam density.

Table 3.

Physical properties of RPUF and RPUF/OG composites.

Sample	λ/(W·(m·k)⁻¹)	Compressive Strength/(MPa)	ρ/(kg·m⁻³)
RPUF	0.0373	0.325	45.3
RPUF/OG10	0.0335	0.324	41.6
RPUF/OG 20	0.0335	0.267	43.8
RPUF/OG 30	0.0330	0.211	44.3
RPUF/OG 40	0.0322	0.145	44.8
RPUF/OG 50	0.0326	0.140	44.9

Combustion behavior and flame retardant properties

The CCT serves as a crucial testing approach for researching the combustion traits of polymer composites.^46,47 CCT was applied to examine the heat release rate (HRR) and total heat release (THR) of RPUF and RPUF/OG composites. The curves of RPUF composites, along with the relevant data, are illustrated in Figure 3(a)-(b), and the relevant data are tabulated in Table 4. According to Table 4, the time to ignition (TTI) of RPUF, RPUF/OG10, and RPUF/OG30 are all between 1 and 3 s, which is chiefly associated with the organic substances and porous architecture of RPUF.²⁵ However, the TTI of RPUF/OG50 reaches 9 s, mainly due to the following reasons. Firstly, with the increase in OG sludge addition, as a non - combustible component, OG sludge effectively reduces the RPUF content through the filler effect. Secondary, the tightly packed char layer that develops productively restrains the expulsion of flammable components produced by the cracking of RPUF. The peak heat release rate (PHRR) of RPUF reaches 410.5 kW·m⁻². This implies that while the burning of RPUF is taking place, a significant amount of heat can be liberated, leading to severe thermal risks. As OG sludge is added, the PHRR of RPUF composites shows a gradual decline. Once the quantity of added OG sludge attains 50 parts, the PHRR descends to 342.9 kW·m⁻², signifying a 16.5% reduction in comparison to RPUF. This demonstrates that the incorporation of OG sludge is advantageous for inhibiting the heat liberation during the combustion of the composite and enhancing its fire - safety characteristics. At the same time, it is found that OG sludge also has markedly decreases the THR of the composite material. As OG sludge increases, the THR of RPUF/OG composites slowly decreases. When 50 parts of OG sludge are added, the THR of the RPUF/OG50 composite drops to 23.3 MJ·m⁻², a 24.1% decrease compared with the pure sample (30.7 MJ·m⁻²). The average effective heat of combustion (Av-EHC) reflects the combustion extent of volatiles in the gas phase during the combustion process.⁴⁸ The Av-EHC value of the pure sample amounts to 137.38 MJ·kg⁻¹. When 10 parts of OG sludge are added in, the Av-EHC of RPUF/OG10 descends to 60.86 MJ·kg⁻¹, representing a 55.7% decrease relative to the pure sample. This demonstrates that OG sludge has a definite gas-phase flame retarding function. This is mainly because the metal oxide components contained in OG sludge have a certain quenching effect on H· and HO· gas phase radicals.⁴⁹

Figure 3.

(a) HRR, (b) THR, (c) SPR, (d) TSR, (e)Mass and (f) CO production curves of RPUF and RPUF/OG composites.

Table 4.

Cone calorimeter data of RPUF and RPUF/OG composites.

Sample	RPUF	RPUF/OG10	RPUF/OG30	RPUF/OG50
TTI (s)	3	3	3	9
Tp (s)	18	22	18	17
Td (s)	86	111	111	146
pHRR (kW·m⁻²)	410.5	395.1	368.1	342.9
THR (MJ·m⁻²)	30.7	27.3	27.4	23.3
TSR (m²·m⁻²)	654.1	545.8	472.8	369.1
Av-EHC(MJ·kg⁻¹)	137.38	60.86	115.48	86.8
FPI((m²·s)·kW⁻¹)	0.0073	0.0075	0.0081	0.0216
FGI (kW·(m²·s) ⁻¹)	22.80	17.95	20.44	20.17
SF(MW·m⁻²)	2684.9	2156.4	1740.2	1265.5

Figure 3(c)-(f) illustrates the curves regarding the smoke release and mass of RPUF as well as RPUF/OG composites. As is evident from Figure 3(c), the smoke production rate (SPR) of the unadulterated sample reaches its peak (≈0.38 m²·s⁻¹) 50 s after combustion, and then drops to zero at around 75 s. Upon the addition of OG sludge, the SPR of the RPUF/OG composites experiences a substantial reduction, and the addition of 50 parts of OG sludge is the most obvious, indicating that OG has obvious smoke - suppressing characteristics for RPUF/OG composites. This may be because the metal oxides within the OG sludge facilitate the aromatization and subsequent carbonization of the small molecule flammable constituents resulting from the thermolysis of the RPUF matrix, thereby decreasing the smoke particles discharged into the gas phase. Based on Figure 3(d), the total smoke release (TSR) for pure RPUF amounts to 654.1 m²·m⁻², while the TSR of RPUF/OG50 reduces to 369.1 m²·m⁻², signifying a 43.6% reduction relative to the unadulterated specimen. The smoke factor (SF) is the outcome of PHRR multiplied by TSR, providing a comprehensive measure of both heat release and smoke production.⁵⁰ As is also evident in Table 4, the SF of the pure specimen amounts to 2684.9 MW·m⁻², and the SFs of RPUF/OG10, RPUF/OG30, and RPUF/OG50 are all significantly lower compared to the pure sample. Among these, the SF of RPUF/OG50 reduces to 1265.5 MW·m⁻², signifying a 43.5% reduction when contrasted against the unadulterated specimen. The aforementioned results demonstrate that OG sludge exerts an outstanding smoke suppressing influence on RPUF composites. Figure 3(e) presents the variation in mass throughout the test. As is evident from the figure, upon the incorporation of OG sludge, the ultimate char yield of the RPUF composites exceed the pure specimen in this regard. The final char yield of the composite with 50 parts of OG sludge added is the largest, which amounts to 17.9 wt%, demonstrating that OG sludge is capable of enhancing steadiness of the RPUF composite when exposed to elevated temperatures. This outcome is in excellent agreement with the thermogravimetric test. Figure 3(f) presents the CO generation curves during the combustion of RPUF and RPUF/OG sludge composites. It is observable that pristine RPUF continuously generates CO during the combustion process, and the CO generation amount remains at 0.003 g·s⁻¹ after 100 s until the end. After adding OG, the CO concentration of the RPUF/OG composite is equivalent to that of RPUF in the early stage. At around 130 s, its CO release amount rapidly decreases to 0.001 g·s⁻¹. This can be ascribed to the fact that the low-valence Fe²⁺ in the OG sludge can promote the conversion of CO to CO₂ during the combustion process through the oxidation reduction cycle.⁵¹ The aforementioned data suggest that OG sludge can significantly decrease CO emissions while RPUF composites are burning and boost their fire safety characteristics.

On this basis, the Fire Growth Index (FGI) and Fire Performance Index (FPI) were introduced to quantitatively evaluate the impact of OG sludge on the fire safety of the composites.⁴ FGI is formulated as the quotient of PHRR and the time needed to attain this peak value (Tp), whereas FPI is computed as the quotient of TTI and the PHRR. A greater FPI value and a reduced FGI value signify an improved level of fire safety for the composite material. As is shown in Table 4, the FPI and FGI of the pure sample are 0.0073 (m²·s)·kW⁻¹ and 22.8 kW·(m²·s)⁻¹, separately. Upon the addition of OG sludge, the FPI of the RPUF/OG composites shows a positive correlation with the quantity of added OG sludge, whereas the FGI exhibits a negative correlation with the amount of added OG sludge. Finally, the FPI of RPUF/OG50 is 0.0216 (m²·s)·kW⁻¹ and the FGI is 20.17 kW·(m²·s) ⁻¹, indicating that OG sludge is capable of remarkably enhancing the fire safety of RPUF/OG composites.

The flame retardant characteristics of RPUF as well as RPUF/OG composites are evaluated by means of LOI and UL - 94 testing methods. As indicated in Table 5, the RPUF exhibits an LOI value of 18.0 vol%. In the UL-94 test, it burned violently with a relatively severe dripping phenomenon, and its combustion rating was determined as non-rated (NR). With the addition of OG sludge, the LOI value of the composites showed a slight increase, reaching a maximum of 20.3 vol% in the RPUF/OG30 composite. This indicates that OG sludge alone is insufficient to significantly improve the flame retardant properties of RPUF, which may be attributed to the fact that this system primarily functions through the filler effect of OG sludge. This is consistent with the combustion behavior of many polymer composites containing only a single component of non-intumescent inorganic fillers: without the use of a synergistic flame retardant system, the improvement of flame retardant properties by physical fillers is relatively weak.^38,52,53 However, it is noteworthy that in the UL-94 test, the addition of OG sludge completely suppressed the melt dripping phenomenon of all composites. This is mainly because the incorporation of OG sludge increases the viscosity of the composite’s melt during combustion, which can effectively prevent the spread of burning droplets.⁵⁴ This result demonstrates that although OG sludge itself cannot endow RPUF with a high-level flame retardant rating, it can address the critical fire hazard of “dripping”, effectively inhibit the spread of flames during the combustion of the composites, and improve their fire safety level.

Table 5.

LOI and UL-94 test results of RPUF and RPUF/OG composites.

Sample	LOI/vol%	UL-94
Sample	LOI/vol%	t₁/t₂^a (s)	Dripping	Rating
RPUF	18	BC	Y	NR^b
RPUF/OG10	19.2	BC	N	NR
RPUF/OG20	19.4	BC	N	NR
RPUF/OG30	20.3	BC	N	NR
RPUF/OG40	19.5	BC	N	NR
RPUF/OG50	19.4	BC	N	NR

Note: t₁ and t₂ represents the burning duration subsequent to the first and second ignition respectively; BC - burns to the clamp; NR - non rated.

Thermal stability

The TG and DTG curves of the RPUF/OG composites are presented in Figure 4. As can be seen from Figure 4 (a) (b), the thermal decomposition process of the RPUF composites mainly consists of two stages. The initial stage takes place in the temperature interval of 220 - 400°C and is related to the breakdown within the polyurethane molecular chain’s hard segments. During the second stage, one can observe the existence of the temperature region from 400 to 600°C, which is in line with the decomposition of the flexible components within the polyurethane molecular structure.⁵⁵ In the case of the RPUF/OG30 and RPUF/OG50 composites, a third decomposition phase emerges at around 625°C. This phenomenon can be ascribed to the additional dehydration and stabilization of the char layer generated in the initial phase in high temperature environments, resulting in the formation of a more compact char layer.⁵⁶ As presented in Table 6 for the pure RPUF composite, the temperature associated with a 5% reduction in mass (T_-5%), the temperature associated with a 50% reduction in mass (T_-50%), the peak temperature during the first stage decomposition (T_max1), and the peak temperature during the second stage decomposition (T_max2) are 253°C, 357°C, 355°C, and 620°C respectively. When 10 parts of OG sludge is added, the T_-5% value of the RPUF/OG10 composite reaches a peak of 264°C, indicating that the incorporation of a minor quantity of OG sludge is capable of enhancing the thermal steadiness of the RPUF/OG composite. When the amount of OG sludge is increased further, the T_-5% values of RPUF/OG30 and RPUF/OG50 are 248°C and 259°C, respectively. By contrast, the T_max1 and T_max2 values of the RPUF/OG composites exhibit no substantial alteration in comparison to those of the pure RPUF composite. This implies that the OG sludge primarily functions as a filler and does not engage in a chemical reaction in the RPUF matrix when at elevated temperatures.⁵⁷ Nevertheless, as the amount of added OG sludge rises, the T - 50% value of the RPUF/OG composites gradually goes up. Additionally, the char yield at 800°C attains 21.7 wt%, being significantly superior to that of the unmodified RPUF composite. This suggests that OG sludge can significantly increase the steadiness of RPUF/OG composites under high-temperature conditions.

Figure 4.

(a) TG and (b) DTG curves of RPUF and RPUF/OG composites.

Table 6.

TG and Cone calorimeter data of RPUF and RPUF/OG composites.

Sample	RPUF	RPUF/OG10	RPUF/OG30	RPUF/OG50
T_-5wt%/°C	253	264	248	259
T_-50 wt%/°C	357	358	360	361
T_max1/°C	355	358	358	349
T_max2/°C	620	628	630	628
Char residue at 800°C/wt%	14.7	18.6	19.5	21.7

Carbon residue analysis

The SEM was utilized to explore the microscopic architecture of the carbon residues derived from RPUF composites following calcination in a muffle furnace. As is evident from Figure 5 (a), the carbon residue of the pure specimen features a substantial quantity of pore structures. The carbon layer is both thin and fragile, and such a structure fails to facilitate the reduction of heat transfer and the suppression of flame spread during combustion. From Figure 5 (b), it is noticeable that with the addition of 10 parts of OG sludge, the compactness of the carbon residue of RPUF/OG10 increases, and the pore structures are considerably decreased. When 30 parts of OG sludge are added, the thickness of the carbon layer of RPUF/OG30 is further increased, and more granular substances precipitate on its surface, as shown in Figure 5 (c). This is because during the combustion procedure, the OG sludge particles relocate to the outer layer of the carbon material. Consequently, the density of the carbon residue is increased, thereby inhibiting the transmission of heat and substances in the course of the composite’s combustion. Upon adding 50 parts of OG sludge, the carbon residue of RPUF/OG50 maintains the skeletal structure of RPUF during combustion, but its compactness decreases, as depicted in Figure 5 (d).

Figure 5.

SEM images and Raman spectra of RPUF and RPUF/OG composite carbon residue: (a),(e) RPUF; (b),(f) RPUF/OG10; (c),(g) RPUF/OG30; (d),(h) RPUF/OG50.

For a more in depth analysis of the char forming characteristics of RPUF and RPUF/OG composites, Raman spectroscopy was utilized to analyze the carbon residues of RPUF and RPUF/OG composites. As depicted in Figure 5(e)-(f), all curves exhibit a double peak pattern. The D peak, which is detected at around 1360 cm⁻¹, is associated with non-crystalline carbon atoms. In contrast, the G peak, identified at about 1580 cm⁻¹ has a connection with carbon atoms in the crystalline phase.⁵ The ratio of the peak areas of the D peak to the G peak (I_D/I_G) serves to depict the degree of graphitization in carbon materials. A decreased ID/IG ratio implies a greater level of graphitization within the carbon residue, resulting in superior heat-resistant properties. The I_D/I_G ratio of the unadulterated sample is 2.33. With the addition of OG sludge, the I_D/I_G value of the RPUF/OG composites decreases, indicating that the metal oxides present in OG are capable of facilitating the carbonization and graphitization processes of the polyurethane molecular chain. Once 30 parts of OG sludge are incorporated, the I_D/I_G value of the carbonaceous residue in RPUF/OG30 decreases further to 2.01. This implies that the addition of a suitable quantity of OG sludge is capable of facilitating the generation of a graphitized carbon stratum inside the RPUF matrix. When the amount of OG sludge added reaches 50 parts, the I_D/I_G value rises to 2.07 again, indicating that the addition of an excessive amount of OG sludge is not conducive to the graphitization process of the RPUF matrix.

Mechanisms of flame retardation and smoke suppression

As a result of the aforesaid analysis, the mechanism by which the RPUF/OG composite retards flames is described in Figure 6. It can be concluded that the OG sludge exerts its effect mainly through the following ways: Firstly, it can reduce the content of RPUF through the filler effect, thereby reducing the combustibility of the RPUF composite. Secondly, the metal oxides within the OG sludge are capable of facilitating the carbonization and graphitization of the decomposition products of the polyurethane molecular chain. They augment the density of the carbonaceous layer and efficiently impede the exchange of substances and heat in the course of the combustion process. Thirdly, the metal oxides within the OG sludge are able to scavenge HO· and H· in the gas phase, thereby attaining the goal of gas phase flame retardancy.⁵⁸ Therefore, the OG sludge demonstrates its flame retardant effect primarily through the synergistic mechanism between the gas and condensed phases.

Figure 6.

Diagram illustrating the flame retardant mechanism of RPUF/OG composites.

Conclusion

To address the dual challenges of environmental pressure from metallurgical solid waste OG sludge and the high flammability of RPUF, this study innovatively utilized OG sludge as a flame retardant additive and successfully prepared RPUF/OG composites via a one-step all-water foaming technique. The effects of OG sludge content on the thermal stability, combustion performance, char residue morphology, and graphitization degree of RPUF/OG were systematically investigated. The research demonstrates that the incorporation of OG sludge effectively enhances the overall fire safety performance of the composites. Although the improvement in the LOI is limited, it significantly suppresses dripping and slows down flame spread. CCT indicate that the peak heat release rate of the RPUF/OG composites decreases with the addition of OG sludge, and the time to peak is advanced. When the OG sludge content reaches 50 parts, the PHRR and TSR of the RPUF/OG50 composite are reduced by 16.5% and 43.6%, respectively, compared to pure RPUF. This result clearly indicates that OG sludge effectively suppresses both the fire intensity and smoke production of the composites. TG reveals that the addition of OG sludge primarily functions in the condensed phase through a physical filling mechanism. While it does not alter the main thermal degradation pathway of the RPUF matrix, it significantly enhances the high temperature stability and char forming ability of the material. The residual char yield of the RPUF/OG50 composite at 800°C increases to 21.7 wt%, effectively improving the high-temperature stability of the RPUF/OG composites and forming a more stable protective barrier. SEM and Raman spectroscopy results of the char residues indicate that an appropriate amount of OG sludge can guide the formation of a denser, more highly graphitized protective char layer, thereby effectively inhibiting heat transfer during combustion. However, excessive addition can compromise char layer integrity due to filler agglomeration. This work opens a new pathway for the high-value resource utilization of metallurgical process solid waste OG sludge and provides insights for enhancing the fire safety performance of RPUF building insulation materials.

Footnotes

Acknowledgements

This research was supported by National Natural Science Foundation of China (No.22075053), the Natural Science Foundation of the Anhui Higher Education Institutions (No.2022AH050288), Anhui Student Research Training Program (No.S202310360230), Anhui Provincial Nature Science Foundation (No.2108085ME178), Anhui Province Key Laboratory of Environment-friendly Polymer Materials (No.KF-202307).

Author contributions

Yani Zhang: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Writing - original draft. Zhen Dai: Data curation; Formal analysis; Investigation; Writing – original draft. Xunyi Wang: Data curation; Funding acquisition; Investigation; Writing - review & editing. Xinjie Huang: Funding acquisition; Investigation; Supervision; Formal analysis. Zimin Liu: Formal analysis; Project administration; Supervision; Writing - review & editing. Qiang Wu: Writing - review & editing, Formal analysis, Supervision. Kang Dai: Supervision; Writing - review & editing; Funding acquisition. Xiuyu Liu:Writing - review & editing; Funding acquisition. Gang Tang: Supervision, Project administration, Writing - review & editing; Funding acquisition; Methodology.

Declaration of conflicting interests

The authors declare that they have no known competing financial interests or personal relation-ships that could have appeared to influence the work reported in this paper.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by National Natural Science Foundation of China (No.22075053), the Natural Science Foundation of the Anhui Higher Education Institutions (No.2022AH050288), Anhui Student Research Training Program (No.S202310360230), Anhui Provincial Nature Science Foundation (No.2108085ME178), Anhui Province Key Laboratory of Environment-friendly Polymer Materials (No.KF-202307).

ORCID iD

Gang Tang

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, upon reasonable request.*

References

Gajdzik

Wolniak

. Transitioning of steel producers to the steelworks 4.0—literature review with case studies. Energies 2021; 14: 4109.

Long

Jia

Liang

, et al. Effect of parameters on reduction behaviour of preheated converter sludge pellets in grate-rotary kiln process. Ironmak Steelmak 2018; 45: 672–677.

She

Wang

Xue

, et al. Basic properties of steel plant dust and technological properties of direct reduction. Int J Miner Metall Mater 2011; 18: 277–284.

Zhu

Kremenakova

Wang

, et al. An analysis of effective thermal conductivity of heterogeneous materials. Autex Res J 2014; 14: 14–21.

Abbaspour

Kalbasi

Shariatmadari

. Effect of steel converter sludge as iron fertilizer and soil amendment in some calcareous soils. J Plant Nutr 2004; 27: 377–394.

Zang

Gao

Yao

, et al. Real-time monitoring of charcoal layer resistance for investigating the synergistic flame retardant mechanism of zinc oxide and intumescent flame retardants in SBS. Mater Today Commun 2024; 39: 109374.

Long

Song

Luo

, et al. In situ construction of iron-rich metal-organic frameworks on wool surface for enhanced flame retardancy and low smoke generation. Colloids Surf 2024; 692: 134034.

Shi

Xie

, et al. Organic copper phosphate-decorated layered double hydroxide to enhance the flame retardancy and smoke suppression of epoxy resin. Polym Degrad Stabil 2024; 220: 110664.

Wang

Sun

, et al. Staggered distribution structure Cu-Mn catalysts for mitigating smoke and gas toxicity in combustion: unravelling mechanistic insight through operando studies. Compos Commun 2024; 52: 102142.

10.

Liu

, et al. High-value and environmentally friendly recycling method for coal-based solid waste based on polyurethane composite materials. Polymers 2024; 16: 2044.

11.

Yang

Liu

Tang

, et al. Fire retarded polyurethane foam composites based on steel slag/ammonium polyphosphate system: a novel strategy for utilization of metallurgical solid waste. Polym Adv Technol 2022; 33: 452–463.

12.

Yang

Liu

Tao

, et al. Metallurgical solid waste modified thermoplastic polyurethane composites: the thermal stability and combustion properties. J Appl Polym Sci 2023; 140: e53434.

13.

de la Vega

Vázquez-López

Wang

D-Y

. Enhancement of poly(lactic acid) fire retardancy through the incorporation of sludge residue as a synergistic additive. Polymers 2025; 17.

14.

Wang

Liu

Zhang

, et al. Advanced converter sludge utilization technologies for the recovery of valuable elements: a review. J Hazard Mater 2020; 381: 120902.

15.

Krämer

Zammarano

Linteris

, et al. Heat release and structural collapse of flexible polyurethane foam. Polym Degrad Stabil 2010; 95: 1115–1122.

16.

Zhang

Macosko

Davis

, et al. Role of silicone surfactant in flexible polyurethane foam. J Colloid Interface Sci 1999; 215: 270–279.

17.

Long

Yang

, et al. Thermoplastic polyurethane composite modified by piperazine pyrophosphate: flame retardancy, thermal stability and combustion properties. J Thermoplast Compos Mater 2024; 37: 1327–1343.

18.

Akindoyo

Beg

MDH

Ghazali

, et al. Polyurethane types, synthesis and applications – a review. RSC Adv 2016; 6: 114453–114482.

19.

Zhu

Liu

, et al. Recent advances in fire-retardant rigid polyurethane foam. J Mater Sci Technol 2022; 112: 315–328.

20.

Kaya

. Determination of chemical structure, mechanical properties and combustion resistance of polyurethane doped with boric acid. J Thermoplast Compos Mater 2022; 35: 720–739.

21.

Naldzhiev

Mumovic

Strlic

. Polyurethane insulation and household products – a systematic review of their impact on indoor environmental quality. Build Environ 2020; 169: 106559.

22.

Kirbaş

. Investigation of the internal structure, combustion, and thermal resistance of the rigid polyurethane materials reinforced with vermiculite. J Thermoplast Compos Mater 2022; 35: 1561–1575.

23.

Rao

, et al. Persistently flame-retardant flexible polyurethane foams by a novel phosphorus-containing polyol. Chem Eng J (Amsterdam, Neth) 2018; 343: 198–206.

24.

Borreguero

Velencoso

Rodríguez

, et al. Synthesis of aminophosphonate polyols and polyurethane foams with improved fire retardant properties. J Appl Polym Sci 2019; 136: 47780.

25.

Wang

Zhao

Rao

, et al. Inherently flame-retardant rigid polyurethane foams with excellent thermal insulation and mechanical properties. Polymer 2018; 153: 616–625.

26.

Yuan

Shi

, et al. Highly-efficient reinforcement and flame retardancy of rigid polyurethane foam with phosphorus-containing additive and nitrogen-containing compound. Mater Chem Phys 2018; 211: 42–53.

27.

Wang

Qiao

Guo

, et al. Preparation of cobalt-based metal organic framework and its application as synergistic flame retardant in thermoplastic polyurethane (TPU). Composites Part B 2020; 182: 107498.

28.

Gao

Zheng

Zhou

, et al. Synergistic effect of expandable graphite, melamine polyphosphate and layered double hydroxide on improving the fire behavior of rosin-based rigid polyurethane foam. Ind Crops Prod 2013; 50: 638–647.

29.

Zhang

Feng

Han

, et al. Effect of ammonium polyphosphate/cobalt phytate system on flame retardancy and smoke & toxicity suppression of rigid polyurethane foam composites. J Polym Res 2021; 28: 407.

30.

Akdogan

Erdem

Ureyen

, et al. Synergistic effects of expandable graphite and ammonium pentaborate octahydrate on the flame-retardant, thermal insulation, and mechanical properties of rigid polyurethane foam. Polym Compos 2020; 41: 1749–1762.

31.

Gao

Zhou

. A flame retardant rigid polyurethane foam system including functionalized graphene oxide. Polym Compos 2019; 40: E1274–E1282.

32.

Shi

Xie

, et al. Flame retardancy of different‐sized expandable graphite particles for high‐density rigid polyurethane foams. Polym Int 2006; 55: 862–871.

33.

Kong

Zhou

, et al. Effect of surface modification of montmorillonite on the properties of rigid polyurethane foam composites. Chin J Polym Sci 2010; 28: 615–624.

34.

Aydoğan

Yurtseven

Usta

. Investigation of the combustion, thermal and mechanical characteristics of rigid polyurethane foam added with talc and intumescent flame retardant. J Thermoplast Compos Mater 2023; 36: 2789–2814.

35.

Gao

Zhao

. Effect of nano-fillers on the thermal conductivity of epoxy composites with micro-Al2O3 particles. Mater Des 2015; 66: 176–182.

36.

Peng

Niu

Qin

, et al. Study on flame retardancy and smoke suppression of PET by the synergy between Fe2O3 and new phosphorus-containing silicone flame retardant. High Perform Polym 2020; 32: 871–882.

37.

Shi

Wang

, et al. Utilization of super-hydrophobic steel slag in mortar to improve water repellency and corrosion resistance. J Clean Prod 2022; 341: 130783.

38.

Zhu

Yang

Liu

, et al. Flame-retarded rigid polyurethane foam composites with the incorporation of steel slag/dimelamine pyrophosphate system: a new strategy for utilizing metallurgical solid waste. Molbank 2022; 27: 8892.

39.

Tang

Zhang

Wang

, et al. Improvements in the flame retardancy, smoke suppression, mechanical strength and thermal insulating properties of rigid polyurethane foams modified with DOPO-Grafted cyclosiloxane and diatomite. Constr Build Mater 2025; 495: 143620.

40.

Nguyen

Van Ngo

. Research on mechanical properties and fire retardancy of epoxy composites reinforced by fly ash of thermal power plant. Vietnam J Chem 2023; 61: 97–108.

41.

Liu

Wang

Chow

, et al. Effects of inorganic fillers on the thermal and mechanical properties of poly(lactic acid). Int J Polym Sci 2014; 2014: 827028.

42.

Tang

Liu

Yang

, et al. Phosphorus-containing silane modified steel slag waste to reduce fire hazards of rigid polyurethane foams. Adv Powder Technol 2020; 31: 1420–1430.

43.

Kuang

Chen

Zhai

, et al. Microstructure, thermal conductivity, and flame retardancy of konjac glucomannan based aerogels. Polymers 2021; 13: 258.

44.

Jia

, et al. Castor oil-based polyurethane ternary composites with enhanced thermal conductivity and mechanical properties by using liquid metal as second filler. Polym Compos 2024; 45: 6184–6197.

45.

Maamoun

Mahmoud

Naeim

, et al. Effect of density and thickness of flexible polyurethane foam on the performance of triboelectric nanogenerators. Mater Adv 2024; 5: 6132–6144.

46.

Huang

Jiang

Liang

, et al. A green highly-effective surface flame-retardant strategy for rigid polyurethane foam: transforming UV-cured coating into intumescent self-extinguishing layer. Composites, Part A 2019; 125: 105534.

47.

Zhang

Cheng

, et al. Effects of hexaphenoxycyclotriphosphazene and glass fiber on flame-retardant and mechanical properties of the rigid polyurethane foam. Polym Compos 2020; 41: 3521–3527.

48.

Jin

Qian

Qiu

, et al. High-efficiency flame retardant behavior of bi-DOPO compound with hydroxyl group on epoxy resin. Polym Degrad Stabil 2019; 166: 344–352.

49.

Homa

Wenelska

Mijowska

. Enhanced thermal properties of poly(lactic acid)/MoS2/carbon nanotubes composites. Sci Rep 2020; 10: 740.

50.

Jiao

Wang

Chen

. Fire safety of thermoplastic polyurethane based on pyromellitic dianhydride. Fire Mater 2018; 42: 454–462.

51.

Wang

Mou

, et al. Catalytic CO oxidation by gas-phase metal oxide clusters. J Phys Chem A 2019; 123: 9257–9267.

52.

Zhang

, et al. Coal fly ash reinforcement for the property enhancement of crude glycerol-based polyurethane foam composites. Waste Dispos Sustain Energy 2022; 4: 271–282.

53.

Lenża

Merkel

Rydarowski

. Comparison of the effect of montmorillonite, magnesium hydroxide and a mixture of both on the flammability properties and mechanism of char formation of HDPE composites. Polym Degrad Stabil 2012; 97: 2581–2593.

54.

Nguyen

, et al. Effect of metal oxide nanoparticles and aluminum hydroxide on the physicochemical properties and flame-retardant behavior of rigid polyurethane foam. Constr Build Mater 2022; 356: 129268.

55.

Zhang

Yang

Han

, et al. Preparation and properties of melamine phytic acid modified rigid polyurethane foam. J Nanjing Univ Technol (Nat Sci Ed) 2021; 43: 197–203.

56.

Yuan

Wang

Shi

, et al. The influence of highly dispersed Cu2O-anchored MoS2 hybrids on reducing smoke toxicity and fire hazards for rigid polyurethane foam. J Hazard Mater 2020; 382: 121028.

57.

Shao

Shi

, et al. Synergistic effect of activated carbon, NiO and Al2O3 on improving the thermal stability and flame retardancy of polypropylene composites. Polymers 2023; 15: 2135.

58.

Friederich

Laachachi

Ferriol

, et al. Tentative links between thermal diffusivity and fire-retardant properties in poly(methyl methacrylate)–metal oxide nanocomposites. Polym Degrad Stabil 2010; 95: 1183–1193.

Fabrication and properties investigation of flame retardant rigid polyurethane composites based on converter OG sludge (a metallurgical by-product): A novel strategy for solid waste utilization

Abstract

Keywords

Introduction

Experimental

Materials

Synthesis n of RPUF composites

Characterization

Results and discussion

Characterization of OG sludge

Cell structure

Physical property

Combustion behavior and flame retardant properties

Thermal stability

Carbon residue analysis

Mechanisms of flame retardation and smoke suppression

Conclusion

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

ORCID iD

Data Availability Statement

References