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Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams
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Original Research Paper
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Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams
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Gang Tang a,b,⇑, Xinliang Liu a, Lin Zhou b, Ping Zhang b,⇑, Dan Deng c, Haohao Jiang a
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a
School of Architecture and Civil Engineering, Anhui University of Technology, 59 Hudong Road, Ma’anshan, Anhui 243002, China State Key Laboratory of Environment-friendly Energy Materials & School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China c Department of Polymer Science and Engineering, Jiaxing University, Jiaxing 314001, Zhejiang, China b
a r t i c l e
i n f o
Article history: Received 11 July 2019 Received in revised form 17 October 2019 Accepted 22 October 2019 Available online xxxx Keywords: Rigid polyurethane foam Steel slag waste Melamine pyrophosphate Flame retardancy Thermal property
a b s t r a c t To explore the potential application of industrial waste, steel slag powder in combination with melamine pyrophosphate (MPP) was adopted to improve the flame retardancy of rigid polyurethane foam (RPUF). The incorporation of steel slag slightly reduced the thermal conductivity of the resulting flame-retardant RPUF samples. The addition of MPP and/or steel slag did not significantly alter the thermal stability in terms of T-10% and Tmax but did obviously increase the T-50% value, suggesting the improved thermal resistance of the residues. The coaddition of MPP and steel slag into RPUF resulted in higher LOI values and lower peak heat release rates than the samples incorporating either MPP or steel slag alone. The superior flame retardancy could be attributed to MPP promoting char formation, which then acted as a barrier at the beginning of RPUF thermal decomposition; simultaneously, the thermally stable inorganics in the steel slag powder strengthened the thermal resistance of this char layer. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Steel slag (SS) is an industrial waste from the steel production process, and it is estimated that over 400 million tons are produced annually [1]. The issue of steel slag disposal has been a persistent problem in the steel industry. Although steel slag has been successfully applied as a filler in ultra-high performance concrete in the construction industry [2–4] and as an absorbent in wastewater treatment [5,6], a large amount of steel slag is still stockpiled. Therefore, it is necessary to explore other potential applications
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Abbreviations: MPP, Melamine Pyrophosphate; APP, Ammonium Polyphosphate; DBTDL, Dibutyltin Dilaurate; DTG, Derivative TGA; EDS, Energy Dispersive XRay Spectroscopy; EG, Expandable Graphite; FIGRA, Fire Growth Rate; HRR, Heat Release Rate; LOI, Limiting Oxygen Index; SS, Steel Slag; TED, Triethylene Diamine; PHRR, Peak of Heat Release Rate; POSS, Polyhedral Oligomeric Silsesquioxane; RPUF, Rigid Polyurethane Foam; SEM, Scanning Electron Microscope; TEOA, Triethanolamine; THR, Total Heat Release; Tmax, maximum mass loss temperature; tPHRR, Time to PHRR; TTI, Time to Ignition; T-10%, 10% mass loss temperature; T50%, 50% mass loss temperature; XRD, X-ray diffraction. ⇑ Corresponding authors at: School of Architecture and Civil Engineering, Anhui University of Technology, 59 Hudong Road, Ma’anshan, Anhui 243002, China (G. Tang). E-mail addresses: [email protected] (G. Tang), [email protected] (P. Zhang).
of steel slag, which could facilitate environmental protection and boost the reuse of this industrial waste in the context of sustainable development. Rigid polyurethane foams (RPUFs) have been used in various fields; they are particularly applied as thermal insulation panels in the building and construction industry owing to their relatively low thermal conductivity [7]. However, the high flammability is a major drawback of this foamy material. RPUF is an ignition source with a very rapid fire propagation rate, accompanied by a large amount of toxic gas and smoke release during combustion [8– 10]. The high fire hazard of RPUFs can induce severe fire accidents, constituting a large threat to life and property. For example, the ignition of RPUFs triggered a severe high-rise residential fire on November 15, 2010 that resulted in 58 deaths. Therefore, it is indispensable to endow RPUFs with flame retardancy to ensure their safe application. Today, there are a wide variety of approaches to develop flameretardant RPUFs. These approaches can be divided into two main categories: reactive-type and additive-type flame retardants. Reactive flame retardants focus on the synthesis of polyols that have flame-retardant elements such as phosphorus- and nitrogencontaining polyols [11–13]. These reactive-type approaches typically have the advantages of non-migration and high flame retar-
https://doi.org/10.1016/j.apt.2019.10.020 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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dant efficiency, but they have an increased cost and processing complexity, which restricts their application [14]. Additive flame retardants primarily involve the physical addition of flame retardants into RPUFs, including expandable graphite (EG) [15], pentaerythritol phosphate and EG [16], phosphate esters and EG [17], EG@aluminum hydroxide [18], ammonium polyphosphate (APP) and EG [19], titanium dioxide-modified EG and APP [20], and intumescent flame retardants [21]. In addition to these traditional flame retardant additives, several nano-fillers, including montmorillonite [22], POSS [23] and silica nanospheres/graphene oxide hybrids [24], have also been incorporated into RPUFs as synergists to improve the flame retardancy. However, to achieve a satisfactory flame-retardant effect, a high loading of additive flame retardants is usually required, which negatively affects the mechanical strength and thermal stability of the RPUFs. It is therefore imperative to develop combined additive flame retardant systems for RPUFs to improve the flame retardant efficiency. The basic chemical components in steel slag are SiO2, CaO, Fe2O3, Al2O3, and MnO [25]. Ferric oxide (Fe2O3) can serve as a charring catalyst by promoting crosslinking reactions during polymer decomposition [26]. SiO2, CaO, Fe2O3, Al2O3, and MnO have been used as effective flame retardant additives in various polymeric materials [27–31]. However, to the best of our knowledge, the use of steel slag as a flame retardant additive in polymers has been rarely reported. To broaden the application of steel slag wastes, in this work, we prepared flame-retardant RPUF samples with the coaddition of steel slag powder and melamine pyrophosphate (MPP). The influence of steel slag and MPP loadings on the compression strength, thermal stability and flame retardancy of RPUFs, especially the role of steel slag in the flame retardant mechanism, was investigated.
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2. Experimental
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2.1. Materials
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Polyaryl polymethylene isocyanate (PM-200, NCO content: 30.2–32.0%, average functionality: 2.7) was obtained from Wanhua Chemical Group Co., Ltd. (Yantai, China). Polyether polyol (LY4110, OH content: 430 mg KOH/g), triethylene diamine (TED), dibutyltin dilaurate (DBTDL), silicone surfactant, and triethanolamine (TEOA) were of industrial grade and purchased from Lvyuan New Material Company (Nantong, China). MPP was supplied from Shandong Xiucheng Chemical Industry Co., Ltd. (Ji’nan, China). Steel slag was kindly gifted from Masteel Group Co., Ltd. (Maanshan, China), and its main components are listed in Table S1 (see supporting information). All the raw materials were used as received.
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2.2. Preparation of samples
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The RPUF samples were prepared by a free-rise method according to the formulations listed in Table 1. Taking RPUF-2 as an example, LY-4110, TED, DBTDL, water, silicone, TEOA, SS powder and MPP were added to a 500 ml plastic container. The mixture was vigorously stirred at 3000 rpm at room temperature for 15 min to obtain a uniform slurry. Subsequently, PM-200 was added into the mixture under vigorous mechanical stirring (3000 rpm for 15 s). The mixture was immediately poured into an open mold (200 mm * 200 mm * 10 mm), and the mold was placed in an air convection oven at 80 °C for 12 h to complete the polymerization process. The samples were cut into different shapes with suitable sizes prior to evaluation of the various properties. Other samples were prepared by a similar procedure, adjusting the SS and/or MPP loadings.
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2.3. Characterization
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The density of the samples was determined according to ASTM D 1622-2008. The average value was reported from at least five measurements. The microstructure and morphological information of the samples were observed using a SU8220 scanning electron microscope (SEM) (Hitachi, Japan) equipped with an Aztec X-Max 80 Energy dispersive X-ray spectroscope (EDS) (Oxford, UK). The samples were precoated with a conductive gold layer prior to SEM observation. Thermal conductivity of the samples was measured by a TC3000E thermal conductivity tester (XIATECH, China) according to ASTM C1113-09. The size of the samples was 100 mm (length) by 100 mm (width) by 10 mm (thickness). At least five measurements were repeated for each sample and the average value was reported. The compressive strength was obtained from a CMT6104 electromechanical universal testing machine (MTS Systems co., ltd, China) according to GB/T 8813-2008. The crosshead speed was 5 mm/min, and the size of the samples was 100 mm (length) by 100 mm (width) by 30 mm (thickness). The average value was obtained from at least five parallel tests. The thermal decomposition process of the samples was recorded by a Q50 thermal analyzer (TA Instruments, USA) under an air atmosphere. The heating rate was 20 °C/min and the air flow was 50 ml/min. The limiting oxygen index (LOI) was obtained by a HC-2 oxygen index meter (Nanjing Jiangning Analytical Instrument Co., China) according to ASTM D2863-2017a. The size of the samples used was 100 mm (length) by 10 mm (width) by 10 mm (thickness). At least five measurements were repeated for each sample and the average value was reported. The combustion behavior of the samples was measured by a CCT cone calorimeter (MOTIS, China) according to ISO 5660-1. Samples with a size of 100 mm 100 mm 20 mm were wrapped
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Table 1 Sample formulations. Component
RPUF
RPUF-1
RPUF-2
RPUF-3
RPUF-4
RPUF-5
LY-4110 (g) TED (g) DBTDL (g) H2O (g) Silicone (g) TEOA (g) PM-200 (g) SS (g) MPP (g)
100 1 0.5 2 2 3 135 0 0
100 1 0.5 2 2 3 135 0 27.1
100 1 0.5 2 2 3 135 6.78 20.32
100 1 0.5 2 2 3 135 9.03 18.07
100 1 0.5 2 2 3 135 13.55 13.55
100 1 0.5 2 2 3 135 27.10 0
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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with aluminum foil and placed into the sample holder frame. The heat flux was 35 kW/m2. At least three measurements were repeated for each sample and the average value was reported. X-ray diffraction (XRD) patterns of the samples were recorded by a Rigaku Dmax/rA diffractometer (Japan) using Cu Ka radiation (k = 0.15405 nm). The operation voltage and current were 40 kV and 20 mA, respectively. Raman spectra were collected by a LABRAM-HR confocal Raman microscope (France) with a laser at 514.5 nm.
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3. Results and discussion
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3.1. Structural characterization
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The influence of the steel slag and MPP loadings on the density of the RPUF samples was investigated, and the results are summarized in Table 2. The apparent density of the original RPUF was determined to be 62.6 kg/m3, whereas that of RPUF-1 was increased to 73.4 kg/m3. Since the apparent density of MPP (approximately 500 kg/m3) was much higher than that of the original RPUF, the addition of MPP led to the increased density. Furthermore, the partial replacement of MPP by steel slag resulted in a decreased apparent density of the resultant RPUF samples. The presence of steel slag increased the expansion ratio of the RPUF samples, and this ratio increased with increasing steel slag loading (Fig. S1 in supporting information), indicating that steel slag served as a catalyst during the preparation of the RPUF samples. Because the total weight was fixed, the higher expansion ratio resulted in a lower apparent density. The physical-mechanical properties of RPUFs are closely related to the size and uniformity of their cellular microstructures [11,24]. Fig. 1 presents SEM micrographs of the RPUF samples. It can be seen that the original RPUF displayed a relatively uniform cell size and a closed cellular polyhedron shape (Fig. 1a). The addition of MPP did not significantly change the cell size or shape (Fig. 1b). However, the presence of steel slag induced cell collapse and an irregular cellular shape (Fig. 1c and d). With higher loadings of steel slag, the cell walls of RPUF-4 and RPUF-5 were clearly destroyed (Fig. 1e and f); this resulted from poor interactions between the RPUF matrix and the steel slag. The energy dispersive X-ray analysis (EDS) spectra were taken to determine the elemental distribution of the samples. The original RPUF presented carbon, oxygen and nitrogen distributions in the selected region (Fig. S2). In addition to carbon, oxygen and nitrogen, RPUF-4 also showed phosphorus, calcium, silicon, iron and aluminum, which were homogeneously distributed throughout the cell walls (Fig. S3). The phosphorus element originated from MPP, while calcium, silicon, iron and aluminum originated from steel slag, implying that MPP and steel slag fillers were uniformly distributed within the RPUFs.
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3.2. Compression
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Compression is a very crucial parameter in RPUF applications. Table 2 lists the compressive property of the original RPUF and
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flame-retardant RPUF samples. The compressive stress at 10% strain decreased in the flame-retardant RPUF samples compared to the original RPUF. Furthermore, the compressive stress at 10% strain decreased gradually with an increased steel slag loading. These results can be explained by the power law, where the compressive strength increased with the increase in RPUF density [32]. The specific compressive strength was also calculated by dividing the compressive strength by the density to evaluate the compression. The specific compressive strengths of all the flame-retardant RPUF samples were similar (3.4–3.7 kNm/kg) but lower than that of the original RPUF (4.3 kNm/kg). This result could be attributed to the poor interaction between the RPUF matrix and the steel slag fillers. The poor interaction resulted in the cell collapse, as observed by SEM.
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3.3. Thermal conductivity
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As one of the most popular thermal insulation polymeric materials, low thermal conductivity is desirable for RPUF samples. Table 2 lists the thermal conductivity values of the original RPUF and the flame-retardant RPUF samples. The thermal conductivity value of the original RPUF was 0.0335 W/(mK), which is consistent with previously reported values [33–35]. The incorporation of MPP and/or steel slag slightly reduced the thermal conductivity of the flame-retardant RPUF samples. The lowest thermal conductivity (0.0285 W/(mK)) was observed in the case of RPUF-5, which resulted from the smallest apparent density of RPUF-5 [23]. These results demonstrated that the flame-retardant RPUF samples exhibited comparable or even better thermal insulation compared to the original RPUF.
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3.4. Thermal decomposition behavior
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The TGA and derivative TGA (DTG) profiles of the original RPUF and flame-retardant RPUF samples are presented in Fig. 2, and the TGA and DTG data are summarized in Table 3. The relative thermal stability of the samples was assessed at typical decomposition temperatures, such as the 10% mass loss temperature (T-10%), the 50% mass loss temperature (T-50%) and the maximum mass loss temperature (Tmax). Fig. 2 shows that the thermal decomposition process of the original RPUF was composed of two steps. The first step occurred between 200 and 350 °C (approximately 50% mass loss) and was assigned to the decomposition of the macromolecular chains into small molecular volatiles. The second step occurred from 450 to 700 °C (approximately 98% mass loss) and was ascribed to further thermal oxidation of the char residues. At the end of the studied temperature range (800 °C), only 2.1% of the residue was left, indicating the complete decomposition of RPUF under an air atmosphere. All of the flame-retardant RPUF samples displayed a similar two-step thermal decomposition behavior. The addition of MPP and/or steel slag did not lead to significant change in the T-10% and Tmax values of the resultant flame-retardant RPUF samples but did cause an obvious increment in T-50%. The T-50% for all of the samples was located between the first and second decomposition steps. The higher T-50% value indicated the delayed decom-
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Table 2 Density, compressive property and thermal conductivity of original RPUF and flame retardant RPUF samples. Sample
Density (kg/m3)
Compressive Strength (kPa)
Specific Compressive Strength (kNm/kg)
Thermal Conductivity (W/(mK))
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
62.6 ± 1.5 73.4 ± 4.3 68.4 ± 2.7 58.2 ± 1.8 56.1 ± 0.9 52.3 ± 2.1
270.5 ± 10.3 263.9 ± 8.5 255.1 ± 13.1 213.3 ± 16.8 192.1 ± 9.3 192.7 ± 13.5
4.3 3.6 3.7 3.7 3.4 3.7
0.0335 ± 0.0012 0.0324 ± 0.0015 0.0323 ± 0.0010 0.0314 ± 0.0009 0.0296 ± 0.0011 0.0285 ± 0.0013
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Fig. 1. SEM micrographs of RPUF samples: (a) RPUF, (b) RPUF-1, (c) RPUF-2, (d) RPUF-3, (e) RPUF-4 and (f) RPUF-5.
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position of the char residues that formed at the end of the first decomposition step. Specifically, the T-50% value of RPUF-2 was 401 °C, which was increased by approximately 50 °C compared to the original RPUF, implying an improved thermal resistance of the char by the coaddition of MPP and steel slag. Furthermore, the residue percentage of the flame-retardant RPUF samples was increased in comparison to that of the original RPUF. The residue percentage increased with increasing steel slag loading because the steel slag was composed of various inorganic metal oxides that were thermally stable up to 800 °C. 3.5. Flame retardancy The flame retardancy of the samples was evaluated by measuring their LOI value, as listed in Table 4. The LOI value of the original RPUF was 17%, indicating a very easily ignited polymeric material. The addition of MPP and/or steel slag increased the LOI of the resultant RPUF samples. The highest LOI value (24.0%) was observed for RPUF-2, and further increasing the steel slag loading did not increase the LOI value. The highest LOI of RPUF-2 could be attributed to the best thermal resistance of the char that formed after ignition, which retarded the flame propagation in the LOI testing. A cone calorimeter was further employed to investigate the influence of MPP and/or steel slag on the flame-retardant behavior of the original RPUF and the flame-retardant RPUF samples. Several typical results are summarized in Table 4, including the time to
ignition (TTI), peak of heat release rate (PHRR), time to PHRR (tPHRR) and total heat release (THR). It can be seen from Table 4 that the addition of MPP and/or steel slag had almost no effect on the TTI value. All of the samples showed a relatively low TTI value (3 s), indicating that the RPUF samples were very easily ignited owing to their cellular microstructure that was filled with air. The heat release rate (HRR) versus time plots of the original RPUF and flame-retardant RPUF samples are shown in Fig. 3. The original RPUF reached the maximum HRR value immediately after ignition, with a PHRR value of 216 kW/m2. The incorporation of MPP and/or steel slag resulted in reduced PHRR values, suggesting a flame-retardant effect of MPP and steel slag. Specifically, the PHRR values of RPUF-2 and RPUF-3 significantly dropped to 114 and 103 kW/m2, corresponding to a 47% and 52% reduction, respectively, compared to that of the original RPUF. The fire growth rate (FIGRA) is often used to quantitatively assess a building product’s reaction to fire and is calculated as the ratio between PHRR and tPHRR [36–38]. The original RPUF showed a relatively high FIGRA value of 43.2 kW/m2s, which is indicative of a high-potential fire hazard. As expected, all of the flame-retardant RPUF samples displayed obviously decreased FIGRA values. Moreover, the lowest FIGRA value of 16.3 kW/m2s was observed for RPUF-2, implying a significantly suppressed fire risk for this material. The THR versus time plots of the original and flame-retardant RPUF samples are presented in Fig. 4. The reduced trend in THR was similar to the PHRR. The THR values were much lower for the flame-retardant RPUF samples than for the original RPUF.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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Fig. 3. Heat release rate versus time curves of RPUF samples.
Fig. 2. (a) TG and (d) DTG curves of RPUF samples under air atmosphere.
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Specifically, the THR value of RPUF-2 was reduced to 9.5 MJ/m2, compared to 18.9 MJ/m2 for the original RPUF—a reduction of nearly half of the THR. This significant THR reduction could be attributed to synergism between MPP and steel slag. The phosphate species derived from MPP catalyze the charring of RPUF to
Fig. 4. Total heat release versus time curves of RPUF samples.
Table 3 TGA and DTG data of original RPUF and flame-retardant RPUF samples. Sample
T-10% (°C)
T-50% (°C)
Tmax,1 (°C)
Tmax,2 (°C)
Residues (%) at 800 °C
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
252 250 255 256 256 246
352 371 401 390 368 396
303 295 297 300 302 289
545 535 535 533 535 535
2.1 4.8 5.0 5.6 7.2 13.2
Table 4 LOI values and cone calorimeter data of original RPUF and flame-retardant RPUF samples. Sample
LOI (%)
TTI (s)
PHRR (kW/m2)
tPHRR (s)
FIGRA (kW/m2s)
THR (MJ/m2)
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
17.0 ± 0.5 23.0 ± 0.5 24.0 ± 0.5 23.0 ± 0.5 22.0 ± 0.5 20.0 ± 0.5
2±1 2±1 2±1 2±1 3±1 2±1
216 ± 18 143 ± 10 114 ± 7 103 ± 6 139 ± 13 155 ± 11
5±1 5±1 7±2 5±1 5±1 5±1
43.2 28.6 16.3 20.6 27.8 31.0
18.9 ± 1.9 9.8 ± 0.4 9.5 ± 0.7 10.8 ± 0.5 10.5 ± 0.7 10.6 ± 0.3
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Fig. 6. Raman spectra of the RPUF sample residues.
Fig. 5. (a) XRD profile of raw steel slag and (b) XRD profiles of RPUF-1, RPUF-2 and RPUF-5 residues.
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form a protective barrier, and the metal oxides derived from steel slag strengthen this barrier effect. Thus, the total heat evolved from combustion of the degradation products was blocked. On the basis of the LOI and cone calorimeter results, the coaddition of MPP and steel slag into RPUF showed a superior flameretardant effect compared to the samples with only MPP or steel slag addition. This superior flame-retardant effect could be attributed to co-contribution of MPP and steel slag, where the MPP promoted char formation, which then acted as a barrier in the beginning of the thermal decomposition of the RPUF. Simultaneously, the thermally stable metal oxides in the steel slag strengthened the thermal resistance of this char layer.
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3.6. Residues analysis
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To explore the flame retardant mechanism, the residues of the samples after cone calorimeter testing were analyzed by XRD and Raman spectroscopy. Fig. 5 shows the XRD profiles of the raw steel slag and the residues of RPUF-1, RPUF-2 and RPUF-5. As shown in Fig. 5a, the XRD profile of the raw steel slag exhibited a very complicated composition with several overlapping peaks resulting from various minerals, including Ca(OH)2, Ca2Fe2O5, Ca3Mg(SiO4)2, and Ca2SiO4. Similar XRD patterns for raw steel slag
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have been previously reported [39,40]. In Fig. 5b, the XRD profile of the RPUF-1 residue showed a broad band centered at 2h = 24.5°, which was attributed to the amorphous phase of carbonaceous residues after combustion. The XRD profile of the RPUF-2 residue displayed partial diffraction peaks derived from steel slag due to the high thermal stability of several inorganic species. With the increase of steel slag loading, the XRD profile of the RPUF-5 residue presented more diffraction peaks compared to raw steel slag. The new diffraction peaks at 2h = 38.6 and 44.8° in the RPUF-5 residue XRD profile were assigned to the (1 1 1) and (2 0 0) bands of CaO [41]. The absence of the diffraction peak at 2h = 18.0°, together with the appearance of the diffraction peaks of CaO, confirmed that Ca(OH)2 was converted into CaO under high temperatures. During this reaction, water vapor was released to dilute the flammable volatiles during combustion. Raman spectroscopy is a very useful tool to explore the graphitization degree for char residues. Generally, the graphitization degree is evaluated by the area ratio between the G band and D band (IG/ID) obtained from the Raman spectra [35,42,43]. The G band is associated with a crystalline phase consisting of graphited carbon atoms, while the D band is related to the amorphous phase consisting of disordered carbon atoms [42,44]. The Raman spectra of the residues of all samples are shown in Fig. 6. It can be seen that the Raman spectra of all samples displayed two predominant bands in the investigated wavenumber range: a G band at approximately 1590 cm 1 and a D band at approximately 1360 cm 1. The IG/ID ratio followed the sequence of RPUF-2 (0.41) > RPUF-1 (0.40) > RPUF-3 (0.37) > RPUF-4 (0.36) > RPUF (0.34) > RPUF-5 (0.26). All the flame-retardant RPUF samples except RPUF-5 showed higher IG/ID ratios than the original RPUF, suggesting the higher graphitization degree in the residues. The absence of MPP in RPUF-5 led to poor char formation ability, and most of the residue was inorganic species from the steel slag, which were responsible for the lowest IG/ID ratio. By comparison, the higher IG/ID ratio of RPUF-2 suggested a higher content of graphitized carbons in the residue, which mainly contributed to the better thermal resistance of this residue. The flame-retardant effect strongly depends upon the morphology of the residue during combustion. Fig. 7 displays SEM images of the residues of RPUF, RPUF-1, RPUF-2 and RPUF-5. The residues of the original RPUF and RPUF-1 showed a honeycomb-like structure on the surface containing many holes and cracks. This residue cannot effectively retard the decomposition of volatiles to escape from pyrolysis, thereby feeding the flame. The residue of RPUF-2
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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also exhibited a porous structure, but this residue was more compact than those of RPUF and RPUF-1. Under high magnification the surface was unbroken, with many particles. RPUF-5 presented a cracked residue with many holes on the surface and was composed of almost all the inorganic species. In contrast to RPUF, RPUF-1 and RPUF-5, the residue of RPUF-2 could better serve as a protective barrier to slow the escape of the decomposition volatiles, which accounted for its superior flame-retardant effect.
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4. Conclusion
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In this work, steel slag powder was combined with MPP and applied as a flame retardant additive in rigid polyurethane foams (RPUFs). The addition of MPP increased the density, whereas the presence of steel slag decreased the density of the resultant RPUF
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samples. SEM micrographs revealed that the addition of steel slag lowered the cell size and led to cell collapse owing to the poor interaction between the steel slag and RPUFs. As a result of the low density and poor interaction, the compressive strength of the resultant RPUF samples decreased gradually with the increase of steel slag loading. The TGA results demonstrated that the addition of MPP and/or steel slag slightly altered the onset thermal degradation temperature of the RPUFs but significantly delayed their thermal oxidative decomposition. The coaddition of MPP and steel slag resulted in better flame-retardant properties than the addition of either MPP or steel slag alone. Specifically, the LOI value increased to 24.0% for RPUF-2 (2.5 wt% steel slag, 7.5 wt% MPP) from 17.0% for the original RPUF, and the PHRR value for RPUF-2 was reduced by 47% compared to the original RPUF. The flame retardant mechanism was further explored through analyzing the residues by
Fig. 7. SEM micrographs of the residues of (a, e) RPUF, (b, f) RPUF-1, (c, g) RPUF-2, (d, h) RPUF-5 under different magnifications.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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XRD, Raman and SEM characterization. These findings demonstrated that the superior flame retardancy of RPUF-2 was attributed to its high-quality residue that had a compact and continual surface and a high degree of graphitization, enabling it to serve as an effective barrier during combustion. The inorganic oxides in steel slag played an important role in the flame retardant system by promoting graphitization and enhancing the thermal resistance of the residues.
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Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
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This research was supported by the National Natural Science Fund of China (No. 51403004) and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 17kffk14).
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.10.020.
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Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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Original Research Paper
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Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams
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Gang Tang a,b,⇑, Xinliang Liu a, Lin Zhou b, Ping Zhang b,⇑, Dan Deng c, Haohao Jiang a
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a
School of Architecture and Civil Engineering, Anhui University of Technology, 59 Hudong Road, Ma’anshan, Anhui 243002, China State Key Laboratory of Environment-friendly Energy Materials & School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China c Department of Polymer Science and Engineering, Jiaxing University, Jiaxing 314001, Zhejiang, China b
a r t i c l e
i n f o
Article history: Received 11 July 2019 Received in revised form 17 October 2019 Accepted 22 October 2019 Available online xxxx Keywords: Rigid polyurethane foam Steel slag waste Melamine pyrophosphate Flame retardancy Thermal property
a b s t r a c t To explore the potential application of industrial waste, steel slag powder in combination with melamine pyrophosphate (MPP) was adopted to improve the flame retardancy of rigid polyurethane foam (RPUF). The incorporation of steel slag slightly reduced the thermal conductivity of the resulting flame-retardant RPUF samples. The addition of MPP and/or steel slag did not significantly alter the thermal stability in terms of T-10% and Tmax but did obviously increase the T-50% value, suggesting the improved thermal resistance of the residues. The coaddition of MPP and steel slag into RPUF resulted in higher LOI values and lower peak heat release rates than the samples incorporating either MPP or steel slag alone. The superior flame retardancy could be attributed to MPP promoting char formation, which then acted as a barrier at the beginning of RPUF thermal decomposition; simultaneously, the thermally stable inorganics in the steel slag powder strengthened the thermal resistance of this char layer. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Steel slag (SS) is an industrial waste from the steel production process, and it is estimated that over 400 million tons are produced annually [1]. The issue of steel slag disposal has been a persistent problem in the steel industry. Although steel slag has been successfully applied as a filler in ultra-high performance concrete in the construction industry [2–4] and as an absorbent in wastewater treatment [5,6], a large amount of steel slag is still stockpiled. Therefore, it is necessary to explore other potential applications
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Abbreviations: MPP, Melamine Pyrophosphate; APP, Ammonium Polyphosphate; DBTDL, Dibutyltin Dilaurate; DTG, Derivative TGA; EDS, Energy Dispersive XRay Spectroscopy; EG, Expandable Graphite; FIGRA, Fire Growth Rate; HRR, Heat Release Rate; LOI, Limiting Oxygen Index; SS, Steel Slag; TED, Triethylene Diamine; PHRR, Peak of Heat Release Rate; POSS, Polyhedral Oligomeric Silsesquioxane; RPUF, Rigid Polyurethane Foam; SEM, Scanning Electron Microscope; TEOA, Triethanolamine; THR, Total Heat Release; Tmax, maximum mass loss temperature; tPHRR, Time to PHRR; TTI, Time to Ignition; T-10%, 10% mass loss temperature; T50%, 50% mass loss temperature; XRD, X-ray diffraction. ⇑ Corresponding authors at: School of Architecture and Civil Engineering, Anhui University of Technology, 59 Hudong Road, Ma’anshan, Anhui 243002, China (G. Tang). E-mail addresses: [email protected] (G. Tang), [email protected] (P. Zhang).
of steel slag, which could facilitate environmental protection and boost the reuse of this industrial waste in the context of sustainable development. Rigid polyurethane foams (RPUFs) have been used in various fields; they are particularly applied as thermal insulation panels in the building and construction industry owing to their relatively low thermal conductivity [7]. However, the high flammability is a major drawback of this foamy material. RPUF is an ignition source with a very rapid fire propagation rate, accompanied by a large amount of toxic gas and smoke release during combustion [8– 10]. The high fire hazard of RPUFs can induce severe fire accidents, constituting a large threat to life and property. For example, the ignition of RPUFs triggered a severe high-rise residential fire on November 15, 2010 that resulted in 58 deaths. Therefore, it is indispensable to endow RPUFs with flame retardancy to ensure their safe application. Today, there are a wide variety of approaches to develop flameretardant RPUFs. These approaches can be divided into two main categories: reactive-type and additive-type flame retardants. Reactive flame retardants focus on the synthesis of polyols that have flame-retardant elements such as phosphorus- and nitrogencontaining polyols [11–13]. These reactive-type approaches typically have the advantages of non-migration and high flame retar-
https://doi.org/10.1016/j.apt.2019.10.020 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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dant efficiency, but they have an increased cost and processing complexity, which restricts their application [14]. Additive flame retardants primarily involve the physical addition of flame retardants into RPUFs, including expandable graphite (EG) [15], pentaerythritol phosphate and EG [16], phosphate esters and EG [17], EG@aluminum hydroxide [18], ammonium polyphosphate (APP) and EG [19], titanium dioxide-modified EG and APP [20], and intumescent flame retardants [21]. In addition to these traditional flame retardant additives, several nano-fillers, including montmorillonite [22], POSS [23] and silica nanospheres/graphene oxide hybrids [24], have also been incorporated into RPUFs as synergists to improve the flame retardancy. However, to achieve a satisfactory flame-retardant effect, a high loading of additive flame retardants is usually required, which negatively affects the mechanical strength and thermal stability of the RPUFs. It is therefore imperative to develop combined additive flame retardant systems for RPUFs to improve the flame retardant efficiency. The basic chemical components in steel slag are SiO2, CaO, Fe2O3, Al2O3, and MnO [25]. Ferric oxide (Fe2O3) can serve as a charring catalyst by promoting crosslinking reactions during polymer decomposition [26]. SiO2, CaO, Fe2O3, Al2O3, and MnO have been used as effective flame retardant additives in various polymeric materials [27–31]. However, to the best of our knowledge, the use of steel slag as a flame retardant additive in polymers has been rarely reported. To broaden the application of steel slag wastes, in this work, we prepared flame-retardant RPUF samples with the coaddition of steel slag powder and melamine pyrophosphate (MPP). The influence of steel slag and MPP loadings on the compression strength, thermal stability and flame retardancy of RPUFs, especially the role of steel slag in the flame retardant mechanism, was investigated.
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2. Experimental
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2.1. Materials
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Polyaryl polymethylene isocyanate (PM-200, NCO content: 30.2–32.0%, average functionality: 2.7) was obtained from Wanhua Chemical Group Co., Ltd. (Yantai, China). Polyether polyol (LY4110, OH content: 430 mg KOH/g), triethylene diamine (TED), dibutyltin dilaurate (DBTDL), silicone surfactant, and triethanolamine (TEOA) were of industrial grade and purchased from Lvyuan New Material Company (Nantong, China). MPP was supplied from Shandong Xiucheng Chemical Industry Co., Ltd. (Ji’nan, China). Steel slag was kindly gifted from Masteel Group Co., Ltd. (Maanshan, China), and its main components are listed in Table S1 (see supporting information). All the raw materials were used as received.
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2.2. Preparation of samples
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The RPUF samples were prepared by a free-rise method according to the formulations listed in Table 1. Taking RPUF-2 as an example, LY-4110, TED, DBTDL, water, silicone, TEOA, SS powder and MPP were added to a 500 ml plastic container. The mixture was vigorously stirred at 3000 rpm at room temperature for 15 min to obtain a uniform slurry. Subsequently, PM-200 was added into the mixture under vigorous mechanical stirring (3000 rpm for 15 s). The mixture was immediately poured into an open mold (200 mm * 200 mm * 10 mm), and the mold was placed in an air convection oven at 80 °C for 12 h to complete the polymerization process. The samples were cut into different shapes with suitable sizes prior to evaluation of the various properties. Other samples were prepared by a similar procedure, adjusting the SS and/or MPP loadings.
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2.3. Characterization
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The density of the samples was determined according to ASTM D 1622-2008. The average value was reported from at least five measurements. The microstructure and morphological information of the samples were observed using a SU8220 scanning electron microscope (SEM) (Hitachi, Japan) equipped with an Aztec X-Max 80 Energy dispersive X-ray spectroscope (EDS) (Oxford, UK). The samples were precoated with a conductive gold layer prior to SEM observation. Thermal conductivity of the samples was measured by a TC3000E thermal conductivity tester (XIATECH, China) according to ASTM C1113-09. The size of the samples was 100 mm (length) by 100 mm (width) by 10 mm (thickness). At least five measurements were repeated for each sample and the average value was reported. The compressive strength was obtained from a CMT6104 electromechanical universal testing machine (MTS Systems co., ltd, China) according to GB/T 8813-2008. The crosshead speed was 5 mm/min, and the size of the samples was 100 mm (length) by 100 mm (width) by 30 mm (thickness). The average value was obtained from at least five parallel tests. The thermal decomposition process of the samples was recorded by a Q50 thermal analyzer (TA Instruments, USA) under an air atmosphere. The heating rate was 20 °C/min and the air flow was 50 ml/min. The limiting oxygen index (LOI) was obtained by a HC-2 oxygen index meter (Nanjing Jiangning Analytical Instrument Co., China) according to ASTM D2863-2017a. The size of the samples used was 100 mm (length) by 10 mm (width) by 10 mm (thickness). At least five measurements were repeated for each sample and the average value was reported. The combustion behavior of the samples was measured by a CCT cone calorimeter (MOTIS, China) according to ISO 5660-1. Samples with a size of 100 mm 100 mm 20 mm were wrapped
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Table 1 Sample formulations. Component
RPUF
RPUF-1
RPUF-2
RPUF-3
RPUF-4
RPUF-5
LY-4110 (g) TED (g) DBTDL (g) H2O (g) Silicone (g) TEOA (g) PM-200 (g) SS (g) MPP (g)
100 1 0.5 2 2 3 135 0 0
100 1 0.5 2 2 3 135 0 27.1
100 1 0.5 2 2 3 135 6.78 20.32
100 1 0.5 2 2 3 135 9.03 18.07
100 1 0.5 2 2 3 135 13.55 13.55
100 1 0.5 2 2 3 135 27.10 0
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with aluminum foil and placed into the sample holder frame. The heat flux was 35 kW/m2. At least three measurements were repeated for each sample and the average value was reported. X-ray diffraction (XRD) patterns of the samples were recorded by a Rigaku Dmax/rA diffractometer (Japan) using Cu Ka radiation (k = 0.15405 nm). The operation voltage and current were 40 kV and 20 mA, respectively. Raman spectra were collected by a LABRAM-HR confocal Raman microscope (France) with a laser at 514.5 nm.
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3. Results and discussion
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3.1. Structural characterization
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The influence of the steel slag and MPP loadings on the density of the RPUF samples was investigated, and the results are summarized in Table 2. The apparent density of the original RPUF was determined to be 62.6 kg/m3, whereas that of RPUF-1 was increased to 73.4 kg/m3. Since the apparent density of MPP (approximately 500 kg/m3) was much higher than that of the original RPUF, the addition of MPP led to the increased density. Furthermore, the partial replacement of MPP by steel slag resulted in a decreased apparent density of the resultant RPUF samples. The presence of steel slag increased the expansion ratio of the RPUF samples, and this ratio increased with increasing steel slag loading (Fig. S1 in supporting information), indicating that steel slag served as a catalyst during the preparation of the RPUF samples. Because the total weight was fixed, the higher expansion ratio resulted in a lower apparent density. The physical-mechanical properties of RPUFs are closely related to the size and uniformity of their cellular microstructures [11,24]. Fig. 1 presents SEM micrographs of the RPUF samples. It can be seen that the original RPUF displayed a relatively uniform cell size and a closed cellular polyhedron shape (Fig. 1a). The addition of MPP did not significantly change the cell size or shape (Fig. 1b). However, the presence of steel slag induced cell collapse and an irregular cellular shape (Fig. 1c and d). With higher loadings of steel slag, the cell walls of RPUF-4 and RPUF-5 were clearly destroyed (Fig. 1e and f); this resulted from poor interactions between the RPUF matrix and the steel slag. The energy dispersive X-ray analysis (EDS) spectra were taken to determine the elemental distribution of the samples. The original RPUF presented carbon, oxygen and nitrogen distributions in the selected region (Fig. S2). In addition to carbon, oxygen and nitrogen, RPUF-4 also showed phosphorus, calcium, silicon, iron and aluminum, which were homogeneously distributed throughout the cell walls (Fig. S3). The phosphorus element originated from MPP, while calcium, silicon, iron and aluminum originated from steel slag, implying that MPP and steel slag fillers were uniformly distributed within the RPUFs.
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3.2. Compression
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Compression is a very crucial parameter in RPUF applications. Table 2 lists the compressive property of the original RPUF and
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flame-retardant RPUF samples. The compressive stress at 10% strain decreased in the flame-retardant RPUF samples compared to the original RPUF. Furthermore, the compressive stress at 10% strain decreased gradually with an increased steel slag loading. These results can be explained by the power law, where the compressive strength increased with the increase in RPUF density [32]. The specific compressive strength was also calculated by dividing the compressive strength by the density to evaluate the compression. The specific compressive strengths of all the flame-retardant RPUF samples were similar (3.4–3.7 kNm/kg) but lower than that of the original RPUF (4.3 kNm/kg). This result could be attributed to the poor interaction between the RPUF matrix and the steel slag fillers. The poor interaction resulted in the cell collapse, as observed by SEM.
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3.3. Thermal conductivity
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As one of the most popular thermal insulation polymeric materials, low thermal conductivity is desirable for RPUF samples. Table 2 lists the thermal conductivity values of the original RPUF and the flame-retardant RPUF samples. The thermal conductivity value of the original RPUF was 0.0335 W/(mK), which is consistent with previously reported values [33–35]. The incorporation of MPP and/or steel slag slightly reduced the thermal conductivity of the flame-retardant RPUF samples. The lowest thermal conductivity (0.0285 W/(mK)) was observed in the case of RPUF-5, which resulted from the smallest apparent density of RPUF-5 [23]. These results demonstrated that the flame-retardant RPUF samples exhibited comparable or even better thermal insulation compared to the original RPUF.
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3.4. Thermal decomposition behavior
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The TGA and derivative TGA (DTG) profiles of the original RPUF and flame-retardant RPUF samples are presented in Fig. 2, and the TGA and DTG data are summarized in Table 3. The relative thermal stability of the samples was assessed at typical decomposition temperatures, such as the 10% mass loss temperature (T-10%), the 50% mass loss temperature (T-50%) and the maximum mass loss temperature (Tmax). Fig. 2 shows that the thermal decomposition process of the original RPUF was composed of two steps. The first step occurred between 200 and 350 °C (approximately 50% mass loss) and was assigned to the decomposition of the macromolecular chains into small molecular volatiles. The second step occurred from 450 to 700 °C (approximately 98% mass loss) and was ascribed to further thermal oxidation of the char residues. At the end of the studied temperature range (800 °C), only 2.1% of the residue was left, indicating the complete decomposition of RPUF under an air atmosphere. All of the flame-retardant RPUF samples displayed a similar two-step thermal decomposition behavior. The addition of MPP and/or steel slag did not lead to significant change in the T-10% and Tmax values of the resultant flame-retardant RPUF samples but did cause an obvious increment in T-50%. The T-50% for all of the samples was located between the first and second decomposition steps. The higher T-50% value indicated the delayed decom-
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Table 2 Density, compressive property and thermal conductivity of original RPUF and flame retardant RPUF samples. Sample
Density (kg/m3)
Compressive Strength (kPa)
Specific Compressive Strength (kNm/kg)
Thermal Conductivity (W/(mK))
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
62.6 ± 1.5 73.4 ± 4.3 68.4 ± 2.7 58.2 ± 1.8 56.1 ± 0.9 52.3 ± 2.1
270.5 ± 10.3 263.9 ± 8.5 255.1 ± 13.1 213.3 ± 16.8 192.1 ± 9.3 192.7 ± 13.5
4.3 3.6 3.7 3.7 3.4 3.7
0.0335 ± 0.0012 0.0324 ± 0.0015 0.0323 ± 0.0010 0.0314 ± 0.0009 0.0296 ± 0.0011 0.0285 ± 0.0013
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Fig. 1. SEM micrographs of RPUF samples: (a) RPUF, (b) RPUF-1, (c) RPUF-2, (d) RPUF-3, (e) RPUF-4 and (f) RPUF-5.
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position of the char residues that formed at the end of the first decomposition step. Specifically, the T-50% value of RPUF-2 was 401 °C, which was increased by approximately 50 °C compared to the original RPUF, implying an improved thermal resistance of the char by the coaddition of MPP and steel slag. Furthermore, the residue percentage of the flame-retardant RPUF samples was increased in comparison to that of the original RPUF. The residue percentage increased with increasing steel slag loading because the steel slag was composed of various inorganic metal oxides that were thermally stable up to 800 °C. 3.5. Flame retardancy The flame retardancy of the samples was evaluated by measuring their LOI value, as listed in Table 4. The LOI value of the original RPUF was 17%, indicating a very easily ignited polymeric material. The addition of MPP and/or steel slag increased the LOI of the resultant RPUF samples. The highest LOI value (24.0%) was observed for RPUF-2, and further increasing the steel slag loading did not increase the LOI value. The highest LOI of RPUF-2 could be attributed to the best thermal resistance of the char that formed after ignition, which retarded the flame propagation in the LOI testing. A cone calorimeter was further employed to investigate the influence of MPP and/or steel slag on the flame-retardant behavior of the original RPUF and the flame-retardant RPUF samples. Several typical results are summarized in Table 4, including the time to
ignition (TTI), peak of heat release rate (PHRR), time to PHRR (tPHRR) and total heat release (THR). It can be seen from Table 4 that the addition of MPP and/or steel slag had almost no effect on the TTI value. All of the samples showed a relatively low TTI value (3 s), indicating that the RPUF samples were very easily ignited owing to their cellular microstructure that was filled with air. The heat release rate (HRR) versus time plots of the original RPUF and flame-retardant RPUF samples are shown in Fig. 3. The original RPUF reached the maximum HRR value immediately after ignition, with a PHRR value of 216 kW/m2. The incorporation of MPP and/or steel slag resulted in reduced PHRR values, suggesting a flame-retardant effect of MPP and steel slag. Specifically, the PHRR values of RPUF-2 and RPUF-3 significantly dropped to 114 and 103 kW/m2, corresponding to a 47% and 52% reduction, respectively, compared to that of the original RPUF. The fire growth rate (FIGRA) is often used to quantitatively assess a building product’s reaction to fire and is calculated as the ratio between PHRR and tPHRR [36–38]. The original RPUF showed a relatively high FIGRA value of 43.2 kW/m2s, which is indicative of a high-potential fire hazard. As expected, all of the flame-retardant RPUF samples displayed obviously decreased FIGRA values. Moreover, the lowest FIGRA value of 16.3 kW/m2s was observed for RPUF-2, implying a significantly suppressed fire risk for this material. The THR versus time plots of the original and flame-retardant RPUF samples are presented in Fig. 4. The reduced trend in THR was similar to the PHRR. The THR values were much lower for the flame-retardant RPUF samples than for the original RPUF.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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Fig. 3. Heat release rate versus time curves of RPUF samples.
Fig. 2. (a) TG and (d) DTG curves of RPUF samples under air atmosphere.
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Specifically, the THR value of RPUF-2 was reduced to 9.5 MJ/m2, compared to 18.9 MJ/m2 for the original RPUF—a reduction of nearly half of the THR. This significant THR reduction could be attributed to synergism between MPP and steel slag. The phosphate species derived from MPP catalyze the charring of RPUF to
Fig. 4. Total heat release versus time curves of RPUF samples.
Table 3 TGA and DTG data of original RPUF and flame-retardant RPUF samples. Sample
T-10% (°C)
T-50% (°C)
Tmax,1 (°C)
Tmax,2 (°C)
Residues (%) at 800 °C
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
252 250 255 256 256 246
352 371 401 390 368 396
303 295 297 300 302 289
545 535 535 533 535 535
2.1 4.8 5.0 5.6 7.2 13.2
Table 4 LOI values and cone calorimeter data of original RPUF and flame-retardant RPUF samples. Sample
LOI (%)
TTI (s)
PHRR (kW/m2)
tPHRR (s)
FIGRA (kW/m2s)
THR (MJ/m2)
RPUF RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5
17.0 ± 0.5 23.0 ± 0.5 24.0 ± 0.5 23.0 ± 0.5 22.0 ± 0.5 20.0 ± 0.5
2±1 2±1 2±1 2±1 3±1 2±1
216 ± 18 143 ± 10 114 ± 7 103 ± 6 139 ± 13 155 ± 11
5±1 5±1 7±2 5±1 5±1 5±1
43.2 28.6 16.3 20.6 27.8 31.0
18.9 ± 1.9 9.8 ± 0.4 9.5 ± 0.7 10.8 ± 0.5 10.5 ± 0.7 10.6 ± 0.3
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Fig. 6. Raman spectra of the RPUF sample residues.
Fig. 5. (a) XRD profile of raw steel slag and (b) XRD profiles of RPUF-1, RPUF-2 and RPUF-5 residues.
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form a protective barrier, and the metal oxides derived from steel slag strengthen this barrier effect. Thus, the total heat evolved from combustion of the degradation products was blocked. On the basis of the LOI and cone calorimeter results, the coaddition of MPP and steel slag into RPUF showed a superior flameretardant effect compared to the samples with only MPP or steel slag addition. This superior flame-retardant effect could be attributed to co-contribution of MPP and steel slag, where the MPP promoted char formation, which then acted as a barrier in the beginning of the thermal decomposition of the RPUF. Simultaneously, the thermally stable metal oxides in the steel slag strengthened the thermal resistance of this char layer.
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3.6. Residues analysis
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To explore the flame retardant mechanism, the residues of the samples after cone calorimeter testing were analyzed by XRD and Raman spectroscopy. Fig. 5 shows the XRD profiles of the raw steel slag and the residues of RPUF-1, RPUF-2 and RPUF-5. As shown in Fig. 5a, the XRD profile of the raw steel slag exhibited a very complicated composition with several overlapping peaks resulting from various minerals, including Ca(OH)2, Ca2Fe2O5, Ca3Mg(SiO4)2, and Ca2SiO4. Similar XRD patterns for raw steel slag
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have been previously reported [39,40]. In Fig. 5b, the XRD profile of the RPUF-1 residue showed a broad band centered at 2h = 24.5°, which was attributed to the amorphous phase of carbonaceous residues after combustion. The XRD profile of the RPUF-2 residue displayed partial diffraction peaks derived from steel slag due to the high thermal stability of several inorganic species. With the increase of steel slag loading, the XRD profile of the RPUF-5 residue presented more diffraction peaks compared to raw steel slag. The new diffraction peaks at 2h = 38.6 and 44.8° in the RPUF-5 residue XRD profile were assigned to the (1 1 1) and (2 0 0) bands of CaO [41]. The absence of the diffraction peak at 2h = 18.0°, together with the appearance of the diffraction peaks of CaO, confirmed that Ca(OH)2 was converted into CaO under high temperatures. During this reaction, water vapor was released to dilute the flammable volatiles during combustion. Raman spectroscopy is a very useful tool to explore the graphitization degree for char residues. Generally, the graphitization degree is evaluated by the area ratio between the G band and D band (IG/ID) obtained from the Raman spectra [35,42,43]. The G band is associated with a crystalline phase consisting of graphited carbon atoms, while the D band is related to the amorphous phase consisting of disordered carbon atoms [42,44]. The Raman spectra of the residues of all samples are shown in Fig. 6. It can be seen that the Raman spectra of all samples displayed two predominant bands in the investigated wavenumber range: a G band at approximately 1590 cm 1 and a D band at approximately 1360 cm 1. The IG/ID ratio followed the sequence of RPUF-2 (0.41) > RPUF-1 (0.40) > RPUF-3 (0.37) > RPUF-4 (0.36) > RPUF (0.34) > RPUF-5 (0.26). All the flame-retardant RPUF samples except RPUF-5 showed higher IG/ID ratios than the original RPUF, suggesting the higher graphitization degree in the residues. The absence of MPP in RPUF-5 led to poor char formation ability, and most of the residue was inorganic species from the steel slag, which were responsible for the lowest IG/ID ratio. By comparison, the higher IG/ID ratio of RPUF-2 suggested a higher content of graphitized carbons in the residue, which mainly contributed to the better thermal resistance of this residue. The flame-retardant effect strongly depends upon the morphology of the residue during combustion. Fig. 7 displays SEM images of the residues of RPUF, RPUF-1, RPUF-2 and RPUF-5. The residues of the original RPUF and RPUF-1 showed a honeycomb-like structure on the surface containing many holes and cracks. This residue cannot effectively retard the decomposition of volatiles to escape from pyrolysis, thereby feeding the flame. The residue of RPUF-2
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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also exhibited a porous structure, but this residue was more compact than those of RPUF and RPUF-1. Under high magnification the surface was unbroken, with many particles. RPUF-5 presented a cracked residue with many holes on the surface and was composed of almost all the inorganic species. In contrast to RPUF, RPUF-1 and RPUF-5, the residue of RPUF-2 could better serve as a protective barrier to slow the escape of the decomposition volatiles, which accounted for its superior flame-retardant effect.
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4. Conclusion
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In this work, steel slag powder was combined with MPP and applied as a flame retardant additive in rigid polyurethane foams (RPUFs). The addition of MPP increased the density, whereas the presence of steel slag decreased the density of the resultant RPUF
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samples. SEM micrographs revealed that the addition of steel slag lowered the cell size and led to cell collapse owing to the poor interaction between the steel slag and RPUFs. As a result of the low density and poor interaction, the compressive strength of the resultant RPUF samples decreased gradually with the increase of steel slag loading. The TGA results demonstrated that the addition of MPP and/or steel slag slightly altered the onset thermal degradation temperature of the RPUFs but significantly delayed their thermal oxidative decomposition. The coaddition of MPP and steel slag resulted in better flame-retardant properties than the addition of either MPP or steel slag alone. Specifically, the LOI value increased to 24.0% for RPUF-2 (2.5 wt% steel slag, 7.5 wt% MPP) from 17.0% for the original RPUF, and the PHRR value for RPUF-2 was reduced by 47% compared to the original RPUF. The flame retardant mechanism was further explored through analyzing the residues by
Fig. 7. SEM micrographs of the residues of (a, e) RPUF, (b, f) RPUF-1, (c, g) RPUF-2, (d, h) RPUF-5 under different magnifications.
Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020
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XRD, Raman and SEM characterization. These findings demonstrated that the superior flame retardancy of RPUF-2 was attributed to its high-quality residue that had a compact and continual surface and a high degree of graphitization, enabling it to serve as an effective barrier during combustion. The inorganic oxides in steel slag played an important role in the flame retardant system by promoting graphitization and enhancing the thermal resistance of the residues.
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Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
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This research was supported by the National Natural Science Fund of China (No. 51403004) and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 17kffk14).
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.10.020.
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Please cite this article as: G. Tang, X. Liu, L. Zhou et al., Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.020