Introduction

Rigid polyurethane foams (PU/PUF) are widely used in insulation, cushioning, packaging, etc., owing to their low thermal conductivity, superior mechanical properties, and low density [1,2,3,4]. However, PU are very combustible and their flame spread is very rapid, which limits their use in stringent fire situations. For example, as building materials, PU not only need to be able to provide good insulation properties, but also have good flame-retardant properties [5,6,7,8,9]. In addition, flame-retardant standards around the world are now increasing. Therefore, the development of PU with high physical–mechanical properties and excellent flame retardancy is urgently required [10,11,12,13,14].

In general, improvements in the combustion behavior are commonly achieved upon the addition of flame retardants, especially halogen-free flame retardants [15, 16]. Halogenated flame retardants have been abandoned because of environmental and safety problems [17,18,19]. Melamine is an important flame retardant, since it is rich in nitrogen and has good foaming properties. On the one hand, the dilution of the flame by the sublimated melamine or its decomposition products (NH3, CO, and CO2, and possibly other volatile products) showed an important effect. On the other hand, the formation of an effective closed char layer, which may involve condensation of melamine to form melam, melem, and related products as well as reactions of the amino and isocyanato groups forming urea derivatives play a key role [20]. For example, it can be used in intumescent flame-retardant systems [21, 22]. The effect of melamine compounds such as melamine polyphosphate and melamine cyanurate on the flame retardant of different polymer materials has been studied, for example, polyamides [23,24,25,26,27,28], polypropylene [29, 30], ethylene–vinyl acetate copolymer (EVA) [31], epoxy compounds [32], and PU [33, 34]. These melamine compounds are halogen-free, non-corrosive, low smoke, heat and light stabilizers, and inexpensive [33]. However, their short comings also need to be considered, i.e., these additive flame retardants are often difficult to process in plastics. In addition, their dispersion in the substrate is poor, so it is difficult to meet the requirements of particle size and distribution. In particular, the addition of a flame retardant will destroy the mechanical properties of the foams.

In principle, an effective way of solving the contradiction between flame retardancy and the physical–mechanical properties is to link the flame retardants to the polymer matrix through chemical covalent linkages [35,36,37]. Hexamethoxy methylene melamine (HMMM) is a melamine derivative, which has low viscosity and can be converted to polyether polyol. The HMMM-based polyether polyol can be used to substitute parts or all of the polyols and can copolymerize with isocyanate to produce melamine-based foams.

In the present paper, 1,2-propylene glycol (PG) was used to react with HMMM to synthesize the melamine-based polyether polyol. The structural characterization of HMMM–PG and the mechanical properties, thermal stability, and flame retardancy of the PU using HMMM–PG were systematically studied.

Experimental

Raw materials

  1. (1)

    HMMM was provided by Jiangsu Guoli Chemical Technology Co. Ltd. (Jiangsu, China). PG: commercial AR. p-Toluenesulfonic acid, used as catalyst, were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China), commercial AR.

  2. (2)

    Polyols: 380A, made from poly(propylene oxide) and a sucrose/glycerin base, were purchased from QingdaoLianmei Chemical Co. (Shandong, China). The main properties were as follows: density, 1.15 g cm−3; typical hydroxyl number, 380A mg KOH/g.

  3. (3)

    Silicone glycol copolymer: 8811, used as a surfactant, was purchased from Shanghai Chemical Reagent Co. (Shanghai, China).

  4. (4)

    Catalyst: Triethanolamine was used as a crosslinking agent for PU.

  5. (5)

    Blowing agent: 1,1-Dichloro-1-fluoroethane (141b) was supplied by Hangzhou Fushite Chemical Industry Co., Zhejiang, China.

  6. (6)

    Polymethylene polyphenylene isocyanate (PM-200) was produced by Yantai WanHua, Shandong, China. Polyurethane; NCO content, 31.3 wt%; viscosity 197 mPa.s−1 (25 °C).

Synthesis of HMMM–PG

HMMM (39.02 g, 0.1 mol) and PG (45.62 g, 0.6 mol) were added to a 250-mL three-neck flask with distillation apparatus under a nitrogen atmosphere. p-toluenesulfonic acid (0.77 g, 0.0045 mol) as a catalyst was added under intense stirring. The flask was kept at 160 °C in an oil bath for 8 h while being constantly stirred, and the methanol generated was removed by distillation. After the reaction was complete, the heating was stopped and the reaction temperature was cooled to 70 °C. Then, distillation under reduced pressure to remove the remaining methanol was performed. Finally, the product was obtained in 91.2% yield (59.68 g). Scheme 1 shows the synthesis of HMMM–PG, where,R1-R6 can be –CH3 or –CH2CHOHCH3. The product is an oligomer that exists as a mixture.

Scheme 1
scheme 1

The synthesis of HMMM–PG

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Foam preparation

PU were prepared using a one-pot and free-rise method. First, the components (2)–(5) and HMMM–PG were mixed by means of a high-speed stirrer (1800 rpm) for 1 min at room temperature until a uniform mixture was achieved. Then, component (6) was added to the reaction mixture, stirred again for an additional 10 s at the speed of 1800 rpm, and the mixture was quickly poured into an open paper mold (250 × 250 × 60 mm3) in order to obtain a free-rise foam. Finally, the PU materials were kept in an incubator at 60 °C for 20 min to accelerate the curing process. After preparation, the samples were cut into the desired shapes and sizes according to the corresponding standards used for the evaluation of the different properties.

The formulations are listed in Table 1. The hydroxyl number for the HMMM–PG polyol is 447 mg KOH/g. For the three different samples, the total number of moles of –OH in the polyether polyol is equivalent.

Table 1 The formulations of each sample
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Measurements and characterization

Fourier transform infrared spectroscopy (FTIR) analysis

FTIR spectroscopy was performed using a Thermofisher 6700 spectrometer at a resolution of 4 cm−1 with a total of 32 scans.

LC–MS analysis

Chromatographic analysis was performed with an Agilent 1260 Infinity apparatus: An Agilent G1312B SL binary pump and Agilent G1329B autosampler connected to a Agilent G1314B VWD detector (Agilent, California, USA). Analytes were separated with a 150-mm ZORBAX RRHT SB-C18 column at a temperature of 20 °C maintained using a Agilent G1316A column oven. Detection was carried out using a microTOF-Q II ESI-Qq-TOF mass spectrometer (Bruker, Germany) that was used to monitor the total ion chromatogram for the mass range from m/z = 0 to m/z = 3000.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was performed using a Netzsch 209 F1 thermal analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere. The temperature range was from room temperature to 800 °C.

Compressive strength

The compressive strength was tested according to GB/T 8813-2008 with sheet dimensions of 100 × 100 × 100 mm3.

Fire behavior

The limiting oxygen index (LOI) was obtained according to the standard GB/T 2406.2-2009. An oxygen index instrument (Rheometric Scientific Ltd.) was used on barrels with dimensions of 10 × 10 × 80 mm3. The gross heat of combustion (HoC) was measured in an oxygen bomb calorimeter (Model 6200, Isoperibol Oxygen Bomb Calorimeter, Parr Instrument Co., Moline, Illinois) according to the standard procedure ISO 1716:2010.

Morphological characterization

Scanning electron microscopy (SEM) experiments were performed using a Hitachi S-4800 scanning electron microscope. The micromorphology of the residual char after combustion with a conductive gold layer was observed by low-temperature fracturing under high vacuum with a voltage of 15 kV.

X-ray photoelectron spectroscopy (XPS)

XPS data were obtained using a Perkin Elmer PHI 5300 ESCA system at 250 W (12.5 kV at 20 mA) under a vacuum as high as 10−6 Pa (10−8 Torr). The char residues were obtained from the LOI tests. Typical results from XPS were reproducible within ±3%, and the reported results are the average of three measurements.

Results and discussion

Characterization of HMMM–PG

Figure 1 shows the FTIR spectra of HMMM and HMMM–PG. The new peak at 3352 cm−1, which was attributed to the stretching vibration of –OH, was noted in the spectrum of HMMM–PG, implying that the reaction of HMMM with PG had occurred. The stretching vibration of C=N in a triazine ring at 1539 cm−1 could be identified in the spectra of both HMMM and HMMM–PG. The peaks around 2879–2971 cm−1 were associated with symmetric stretching vibrations of –CH3 and –CH2. The peak at 2816 cm−1 was assigned to the symmetric stretching vibration of –OCH3. The deformation vibration of –CH2 and anti-symmetric deformation vibration of –CH3 were observed at 1460 cm−1. In addition, the peak at 1376 cm−1 was due to the symmetric deformation vibration of –CH3.

Figure 1
figure 1

A comparison of the FTIR spectra of HMMM and HMMM–PG

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In order to study the composition of the HMMM–PG product, we applied HPLC separation coupled with MS detection using methanol as the mobile phase (Figs. 2, 3). The LC chromatograms of HMMM–PG showed five peaks at retention times of 9.4 min (peak 1), 11.6 min (peak 2), 15.5 min (peak 3), 17.5 min (peak 4), and 18.6 min (peak 5). In addition, Fig. 3 displays the mass spectrometry for the peaks at each retention time above with the peak numbers displayed in the parentheses of Fig. 3 referring to the peak numbers in Fig. 2, which confirmed that the five peaks in the LC chromatograms were attributed to five different ether exchanged products: HMMM–PG2, HMMM–PG3, HMMM–PG4, HMMM–PG5, and HMMM–PG6 whose chemical structure and molecular weight are illustrated in Table 2. Besides, HMMM–PG was detected using only MS (Fig. 4) to complement the results (Table 2). The relative intensity showed that the content in descending order of the five different ether exchanged products was HMMM–PG6, HMMM–PG5, HMMM–PG3, HMMM–PG4, and HMMM–PG2.

Figure 2
figure 2

LC chromatograms of HMMM–PG by LC–MS

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Figure 3
figure 3

The mass spectra of HMMM–PG at a retention time of 9.4 min (peak 1), 11.6 min (peak 2), 15.5 min (peak 3), 17.5 min (peak 4), and 18.6 min (peak 5) from LC–MS

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Table 2 The assignment of each peak detected in LC–MS and MS
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Figure 4
figure 4

The mass spectra of HMMM–PG detected with MS

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Figure 5 shows the TG/DTG curves of HMMM–PG and 380A. 380A is a commercially available polyol. The temperature at 5% mass loss (T 0.05) was taken as the onset degradation temperature, which was observed at 181.1 °C for HMMM–PG and at 140.0 °C for 380A. The residue of HMMM–PG at 800 °C was 17.9%, while 380A decomposed completely at 382.8 °C without any residues. The apparent thermal stability of HMMM–PG higher than ordinary aliphatic polyols was most likely due to the stability of the triazine ring in the HMMM–PG structure [38].

Figure 5
figure 5

TG curves of HMMM–PG and 380A

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The effect of shear rate on the shear viscosity of HMMM–PG at 25 °C is shown in Fig. 6. The shear viscosity of HMMM was in the range of 6.5–5.8 Pa s with a shear rate between 0 and 100 s−1, while the shear viscosity values of HMMM–PG were within the range of 161–188 Pa s. HMMM–PG has higher shear viscosity than HMMM. The viscosity was dominated by the intermolecular hydrogen bonds of the nearby adjacent –OH groups [39]. However, at high shear rates (above 40 s−1), the intermolecular hydrogen bonds are destroyed, leading to a sharp decrease in viscosity. This is helpful for foam processing.

Figure 6
figure 6

Shear viscosity curves for HMMM and HMMM–PG

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Physical–mechanical properties and cell morphology of PU

High compression strength and low thermal conductivity are desirable properties for polyurethane foams. Table 3 shows the compression strength, thermal conductivity, and density of PUFs. The compression strength of the PUF:HMMM–PG sample was 0.15 MPa, which was three times higher than that found for the PUF with a commercial polyether polyol 380A (0.05 MPa). The compression strength of the PUF:380A/50%HMMM–PG sample (0.09 MPa) was between PUF:380A and PUF:HMMM–PG. This may be attributed to the higher degree of hydroxyl functionality, which increased the crosslinking density and the rigid triazine ring structures. In addition, the thermal conductivity of the PUF:HMMM–PG sample decreased when compared with the PUF:380A sample, which was due to the reduction of cell size (Fig. 7). In addition, the thermal conductivity of the PUF:380A/50%HMMM–PG sample (0.0267 W/m K) was between PUF:380A (0.0268 W/m K) and PUF: HMMM–PG (0.0260 W/m K). The density of PUF:380A, PUF:380A/50%HMMM–PG, and PUF: HMMM–PG were 36, 34, and 36 kg/m3, respectively. The difference in the density for each sample was small. The physical–mechanical properties of PUF depend on the rigidity of the polymer matrix, which on the other hand depends on the cell morphology of the foam. Figure 7 shows the SEM images of the prepared foams. The shapes of the cells in PUF:380A as well as in PUF:HMMM–PG are approximately spherical. The PUF:HMMM–PG sample shows a decrease in the average cell size with reference to PUF:380A.

Table 3 The mechanical properties of the prepared foams
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Figure 7
figure 7

SEM images of the prepared foams

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The thermal behavior of PU

Figure 8 shows the TG and DTG curves obtained for PUF:380A, PUF:380A/50%HMMM–PG and PUF:HMMM–PG, and the data summarized in Table 4. The temperature at which a 5 wt% of weight loss takes place is defined as the initial degradation temperature (T onset), and the temperature at which the degradation rate reaches a maximum is defined as T max. The results show that the T onset for PUF: 380A was 140.6 °C and the T max was 350.1 °C, which can be assigned to the degradation of the urethane linkages followed by the degradation of polyol and isocyanate [40]. Finally, the residue char of PUF:380A was 6.1 wt% at 800 °C under an N2 atmosphere. For the PUF:HMMM–PG sample, the T onset was 207.3 °C, while the T max was 275.3 °C with a much weaker peak than that found for PUF:380A. The lower T max can be attributed to the more unstable urethane groups formed by the reaction of HMMM–PG and isocyanate. In addition, the weaker thermal decomposition peak was attributed to the higher thermal stability of HMMM–PG. The peak in the range of 200–350 °C obtained for the PUF: HMMM–PG sample may be assigned to the thermal decomposition of the urethane groups and the removal of PG in HMMM–PG, while the peak in the range 350–480 °C for the PUF: HMMM–PG sample was assigned to the thermal decomposition of polyol HMMM–PG and isocyanate. Besides, the residue at 800 °C for PUF:HMMM–PG sample (19.1 wt%) was much higher than that observed for the PUF:380A sample (6.1 wt%). For the PUF:380A/50%HMMM–PG sample, there are two T max, which can be attributed to the thermal decomposition of polyols 380A-based polyurethane and polyols HMMM–PG-based polyurethane, respectively. In addition, the PUF:380A/50%HMMM–PG sample’s residue char was within the scope of PUF:380A and PUF: HMMM–PG.

Figure 8
figure 8

TG/DTG curves for PUF:380A, PUF:380A/50%HMMM–PG, and PUF:HMMM–PG

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Table 4 TGA and DTG data for the various samples
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The flame retardancy of PU

The LOI value obtained for PUF:380A was 18.8%, while the value for PUF:HMMM–PG was 20.9%. In addition, the LOI value obtained for PUF:380A/50%HMMM–PG was between the values of PUF:380A and PUF:HMMM–PG (19.6%) (Fig. 9). As a contrast, the LOI value of PUF using castor oil polyol was 19.9% [41]. The increased LOI value obtained using the polyether polyol HMMM–PG indicated the enhancing effect on flame retardancy. In detail, the combustion processes of the prepared foams at oxygen concentrations of 20.9% are illustrated in Fig. 10. Figure 11 also shows the digital photographs of combustion residues after the combustion test. For PUF:380A, the sample was ignited quickly with rapid flame propagation. Meanwhile, the char layer after combustion was too loose to keep its original shape, which can be seen in Fig. 11. Thus, the combustion residues curled with the spread of flame. For the PUF:380A/50%HMMM–PG sample, the sample was also ignited quickly with rapid flame propagation, while the combustion residues were strong enough to keep its original shape. However, for the PUF:HMMM–PG sample, during ignition, flame propagation apparently slowed down. Then, the flame gradually became smaller and finally extinguished within 15 s. In addition, the combustion residues also kept their original shape.

Figure 9
figure 9

The heat of combustion and limiting oxygen index values obtained for each sample

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Figure 10
figure 10

The combustion processes of the foams at oxygen concentrations of 20.9% (a PUF:380A, b PUF:380A/50%HMMM–PG, c PUF:HMMM–PG)

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Figure 11
figure 11

Digital photographs of the combustion residues after the combustion test

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The HoC is the heat released by the complete combustion per unit mass of material, which can be used to evaluate the potential fire hazard of the construction materials [42]. The heat of combustion is determined by the elemental composition and the interactions between the components of a material. In general, the higher the content of C, the higher the HoC. Moreover, the higher the content of N, P, and O, the lower the HoC. Figure 9 shows the heat of combustion of each sample. The HoC for PUF:380A was 27.52 MJ/kg, but when substituted by HMMM–PG, it decreased to 26.97 MJ/kg. It could be seen that with the substitution of HMMM–PG, the HoC decreased gradually; the change in the LOI values shows an opposite trend. Therefore, the best fire performance was observed for the PUF:HMMM–PG foams.

Morphological characterization of the char residues

The morphologies of the char residues were characterized by SEM to further elucidate the flame-retardant performance of the char layer, and they are displayed in Fig. 12. As seen in Fig. 12, the char residue of PUF:380A was porous with a lamellar morphology. During combustion, the flame and heat would pass through the gap and spread to the unburned portion. However, when using the polyether polyol HMMM–PG, the holes in the surface of carbon residue disappear and the surface of the carbon residues was continuous and dense. This continuous and dense char layer can hinder heat and mass transfer between the gas and the condensed phases, and prevent the underlying polymeric substrate from further attack by heat flux [36].

Figure 12
figure 12

SEM micrographs of the char formed after combustion

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XPS analysis of the char residues

The XPS data are listed in Table 5. In the combustion process, the N elements in the PU foams not only volatilize with the formation of gas such as NO2, but form relatively stable compounds containing C–N and C = N in the char residues [43]. The higher the relative content of N element in the char residue, the more stable the compounds containing N element after combustion and the higher the thermal stability of the material. It can be seen from Table 5 that the N/C radio of PUF:HMMM–PG (0.048) was higher than that of PUF:380A (0.037), while the N/C radio of PUF:380A/50%HMMM–PG (0.040) was between PUF:380A and PUF:HMMM–PG, which indicates that the use of polyether polyol HMMM–PG can improve the flame-retardant properties of PU foams. Further, the atomic percentage of oxygen in the char residues of PUF:380A/50%HMMM–PG and PUF:HMMM–PG was higher than that found for PUF:380A, which illustrates that foams using HMMM–PG have better char formation. The improvement in fire retardancy of PUF:HMMM–PG was mainly reflected in the condensed phase.

Table 5 XPS results obtained for the char residues
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Conclusions

A new kind of melamine-based polyether polyol HMMM–PG was synthesized to prepare polyurethane foams. HMMM–PG began to decompose at 181.1 °C and had higher residues (17.9%) than the common polyol 380A. Besides, HMMM–PG displayed shear thinning because of the damage to the intermolecular hydrogen bonds with increasing shear rate. The compression strength of PUF:HMMM–PG (0.15 MPa) was 3 times higher than that found for PUF:380A (0.05 MPa), which was attributed to the higher degree of hydroxyl functionality that increased the crosslinking density and the rigid triazine ring structures. In addition, the thermal conductivity of the PUF:HMMM–PG sample decreased when compared with the PUF:380A sample. The flame-retardant properties of the foams could be significantly improved, which was mainly reflected in the condensed phase.