Introduction

Because of the good chemical resistance, mechanical strength, and affordable cost, polyurethane sealants (PUS) have attracted more and more attentions in recent years. It has been widely used as a sealing material in electronic components, construction, and automobile industry [1–3]. Among them, castor oil (CO)-based PUS (CO-PUS) play a dominant role due to its excellent electric insulating properties, strength, and less environmental pollution [4, 5]. However, high flammability of CO-PUS limits its applications. Thus, some research is required to improve the flame retardancy of CO-PUS for their comprehensive applications.

Over the past decades, additive-type flame retardants have been found to be an effective method to improve the flame retardancy of PUS [6]. However, the high concentration of the additive-type flame retardants in PUS has many drawbacks, such as poor compatibility, easy leaching, and a declining mechanical properties [7, 8]. Thus, more attentions have been paid to develop reactive-type flame retardants to replace additive-type flame retardants. In general, the reactive-type flame retardants with phosphorus, nitrogen, or silicon elements were introduced onto the molecule backbone of PUS to improve the flame retardancy of resins by chemical reaction [9–12].

In addition to phosphorus-based reactive-type flame retardants, phosphorus-nitrogen flame-retardant synergism system was reported in many literature and exhibited better flame-retardant efficiency than those only containing phosphorus, due to the synergistic effect between phosphorus and nitrogen. Chen et al. used 2-carboxyethyl(phenyl)phosphinic acid with melamine to synthesize phosphorus and nitrogen-containing flame-retardant additive for polyurethane foam [13]. Qian et al. synthesized phosphorus and nitrogen-containing epoxy acrylate resin by the chemical reaction between phosphorus oxychloride (POCl3), 2-hydroxyethyl acrylate (HEA), and piperazine. All the LOI values were found to be improved, which result from the synergistic effect of phosphorus and nitrogen [14–18]. However, phosphorus and nitrogen-containing compounds were rarely introduced onto the backbone of PUS and the synergistic effect between phosphorus and nitrogen in the condensed phase and gas phase during the combustion was rarely investigated.

In this study, a new flame-retardant polyurethane prepolymer (FRPUP) containing phosphorus and nitrogen was successfully synthesized and characterized. It was used as curing agent and reactive-type flame retardant for PUS (FRPUS). The flame retardancy of FRPUS was characterized by the LOI and CCT, and the thermal behavior of the FRPUS was studied by thermogravimetric analysis (TGA). In addition, TG-IR, RTIR, and XPS were used to analyze the flame-retardant mechanism and the degradation mechanism of FRPUS in both the gas phase and the condensed phase.

Experimental

Materials

Diethanolamine, formalin(37 % in water), Diethyl phosphite (DEPP), Dibutyltin dilaurate (DBTDL), Ethyl acetate (analytical grade) were purchased from Nanjing Chemical Reagents Co., Ltd. (Jiangsu, China). Castor oil (CO, hydroxyl value = 163; molecular weight = 933; average functionality = 2.7) was supplied by Nanjing Qianyue Chemicals Co., Ltd. (Jiangsu, China) and dried at 110 °C under vacuum for 2 h before use. Methylene diphenyl diisocyanate (MDI-50, NCO% = 32.6) was obtained from Yantai Wanhua Group Co., Ltd. (Shandong, China). BF-5(moisture scavenger) was supplied by Shanghai DEYUDE Trade Co., Ltd. (Shanghai, China). Defom 5500 (defoamer) was obtained from Guangzhou Shenggao Chemicals Co., Ltd. (Guangdong, China).

Synthesis of flame-retardant polyurethane prepolymer (FRPUP)

The FRPUP was prepared according to the previous report [16, 19]. Diethanolamine and formalin were added into a three-necked flask and stirred at 40 °C for 2 h. Then the mixture was heated to 80 °C to remove the generated water during the reaction under vacuum. Finally, DEPP was added into the above flask and stirred for another 2 h at 60 °C, the product (BHAPE) was obtained after being dried in a vacuum at 100 °C.

MDI-50 and a proper portion of DBTDL were dissolved in 30 mL of dried Ethyl acetate and charged into a four-necked round-bottomed flask and heated slowly to 70 °C under N2 atmosphere. Then a solution of the BHAPE and 20 mL of dried ethyl acetate was slowly added into the above flask within 30 min, and the mixture was stirred for 2 h (The molar ratio of NCO and OH is 4.5:1, the NCO% of FRPUP is 20.3 %). The obtained product was referred as FRPUP. The synthesis route is illustrated in Scheme 1.

Scheme 1
scheme 1

Synthesis route of FRPUP

Full size image

Preparation of flame-retardant PUS composites

The preparation of FRPUS composites was as follows: Firstly, a certain proportion of CO, DBTDL, BF-5, and Defom 5500 was added into a 500 mL plastic beaker at room temperature and stirred for 10 s. Then FRPUP (curing agent) was added into the mixtures in a certain ratio and stirred for another 10 s. Finally, the obtained viscous mixture was poured into a mold quickly and cured at 70 °C for 6 h. The compositions of FRPUS composites are summarized in Table 1. Other samples were prepared by the similar procedure.

Table 1 Formulations of PUS
Full size table

Characterization

Fourier transform infrared (FTIR) spectroscopy

FTIR spectra of samples were recorded on a FTIR in a range of wave numbers from 4000 to 400 cm−1, using attenuated total reflection Fourier transform infrared (ATR-FTIR) method on a Nicolet (USA) IS10 instrument.

Nuclear magnetic resonance spectroscopy (NMR) analysis

Nuclear magnetic resonance (NMR) measurement was performed on an AVANCE 400 Bruker spectrometer at room temperature. The solvent was CDCl3.

Thermogravimetric analyses (TGA)

Thermogravimetric analyses was performed using a TGA Q500 (TA Instruments) instrument. About 4 mg of each sample was scanned from room temperature to 700 °C at a heating rate of 10 °C/min under nitrogen gas at a flow rate of 100 mL/min.

Limiting oxygen index (LOI)

LOI was carried out with a JF-3 oxygen index instrument (Jiangning Analysis Instrument Factory, Jiangsu, China), and the test was measured according to ASTM D2863. The samples used for the test were 120 × 10 × 10 mm3.

Thermogravimetric analyses/infrared spectrometry (TG-IR)

TG-IR was performed using a TGA Q500 IR thermogravimetric analyzer, which was interfaced to the Nicolet (USA) IS10 FTIR spectrophotometer. About 4 mg of each sample was scanned from room temperature to 700 °C at a heating rate of 10 °C/min in nitrogen atmosphere.

Cone calorimeter testing (CCT)

The cone calorimeter tests of PUS were performed using an FTT2000 cone calorimeter instrument according to ISO 5660 standard. Each PUS specimens (100 × 100 × 10 mm3) were irradiated at a heat flux of 35 kW/m2.

X-ray photoelectron spectroscopy (XPS)

XPS was used to analyze the residual char, and it was carried out using a PHI 5000 VersaProbe (UlVAC-PHI Co., Ltd., Japan) with Al Ka excitation radiation in ultrahigh vacuum conditions.

Result and discussion

Characterization

The FRPUP was prepared according to the previous report [16, 19]. The chemical structure of FRPUP was confirmed by FTIR, 1H NMR, and 31P NMR. As shown in Fig. 1, some specific absorption peaks are observed from FRPUP. The absorptions at 3300 and 2250 cm−1 is assigned to the N–H and -NCO, respectively. The absorption at 1728 cm−1 is attributed to the urethane bond. The absorption peaks at 1218 and 1018–960 cm−1 corresponding to the P=O stretching and the stretching vibrations of P-O-C, are clearly observed.

Fig. 1
figure 1

FTIR spectra of FRPUP

Full size image

The 1H NMR spectra of FRPUP are shown in Fig. 2, confirming the expected chemical structure. The 31P NMR spectrum of the FRPUP is shown in Fig. 3. The peak at 24.9 ppm belongs to the chemical shift of 31P in the FRPUP structure [6]. These results suggest the successful synthesis of the FRPUP.

Fig. 2
figure 2

1H NMR spectra of FRPUP

Full size image
Fig. 3
figure 3

31P NMR spectra of FRPUP

Full size image

Thermal degradation behaviors

The thermal stability and thermal degradation of CO-PUS and FRPUS were investigated by TGA. The TGA curves mainly include the release of decomposition products and the formation of chars. Figure 4 shows their TGA thermograms in N2, and the corresponding data are labeled in Table 2.

Fig. 4
figure 4

TGA and DTG curves of the CO-PUS and FRPUS in N2

Full size image
Table 2 TGA date for CO-PUS and FRPUS in N2
Full size table

CO-PUS displays two main steps of degradation in N2. The first stage occurred between 250 and 350 °C with a peak temperature of 328 °C, which may be due to the degradation of urethane bond [20, 21]. The second stage occurred between 350 and 500 °C and the maximum degradation rate happened at 408 °C, which is related to the degradation of PUS hydrocarbon chains [22]. Compared with CO-PUS, one step occurring between 250 and 350 °C in FRPUS is attributed to the decomposition of P-O-C bond at lower temperature. This phenomenon is probably due to the lower stability of P-O-C bond than C–C bond [23]. The temperatures of the maximum DTG peaks (T max) for FRPUS are slightly higher than that of CO-PUS with the decreased maximum mass loss rate, indicating that FRPUS have higher thermal stability than CO-PUS [24].

Furthermore, Table 2 depicts the char residue of CO-PUS and FRPUS. It can be seen that the char residue of CO-PUS at 700 °C is only 1.5 %, while the char residue of FRPUS at 700 °C increases to 7.2 %. This phenomenon is probably due to the phosphorus and nitrogen elements in FRPUP acting as char-forming catalyst and synergistic agent, respectively. They can accelerate the formation of char during the decomposition process, indicating that FRPUP could enhance the char yield and protect the underlying materials from further degradation [1, 17].

Burning behaviors

To evaluate the flame-retardant property of PUS, LOI was conducted, and the corresponding data are given in Table 3. From LOI data, CO-PUS has a LOI value of 18.3, while FRPUS exhibit a higher LOI value (23.1) [5, 25]. This indicates that incorporating phosphorus and nitrogen elements into the backbone of FRPUP effectively enhanced the flame retardancy of PUS [14].

Table 3 Cone calorimeter date for CO-PUS and FRPUS
Full size table

In order to further evaluate the flammability behavior of PUS, cone calorimeter tests were performed for CO-PUS and FRPUS [10, 18]. Cone calorimeter is able to bring quantitative analysis to the flammability of materials by investigation parameters such as the peak heat release rate (PHRR), the heat release rate (HRR), the total heat release (THR), the rate of smoke release (RSR), the total smoke production (TSP), the average effective heat of combustion of volatiles (Av-EHC), and the average mass loss rate (Av-MLR).

The HRR, THR, RSR, and TSP curves of CO-PUS and FRPUS are shown in Figs. 5 and 6, and the corresponding data are presented in Table 3. From Fig. 5 and Table 3, it is observed that the CO-PUS burned rapidly and its PHRR and THR are 1294 kW/m2 and 386 MJ/m2, respectively. However, the HRR and THR curves of FRPUS increase rapidly at first and then decrease with much lower PHRR and THR values of 673 kW/m2 and 333 MJ/m2, which reduce by 48 and 14 % compared with that of CO-PUS. The reduction of the HRR and THR could probably be attributed to the char formation promoted by phosphorus element in FRPUR, which slows heat and mass transfer between the gas and condensed phases. It can be seen from Fig. 7a and b that the FRPUS have more char residue than that of CO-PUS, in which almost no char was generated. Figure 6 shows that the RSR curves have the similar trend to the HRR curves, and the TSP value of FRPUS increased to 77.6 from 70.7 m2 for CO-PUS, indicating that FRPUS produced more smoke than did CO-PUS. This is probably that the much char layer protected the underlying material from further burning, which resulted in insufficient combustion and more smoke is produced [26].

Fig. 5
figure 5

HRR and THR curves of CO-PUS and FRPUS

Full size image
Fig. 6
figure 6

RSR and TSP curves of CO-PUS and FRPUS

Full size image
Fig. 7
figure 7

Photographs of char residues of CO-PUS (a) and FRPUS (b)

Full size image

Table 3 shows that the Av-EHC of FRPUS is higher than that of CO-PUS, which represented the greater amount of gases decomposed from FRPUS; the result is in accord with that of TSP. The Av-MLR of FRPUS is lower than that of CO-PUS, and they are 0.062 and 0.082 g/s, respectively. This phenomenon demonstrates that FRPUP mainly plays a role in the condensed phase.

Structural characterization of volatilized products

The gaseous thermal degradation products of CO-PUS and FRPUS were analyzed by TG-FTIR.

Figure 8 shows the 3D TG-FTIR spectra of gas phase in thermal degradation of CO-PUS (a) and FRPUS (b) at heating rate 10 °C/min in nitrogen atmosphere [13]. From Fig. 8a and b, it can be seen that peaks in the regions of 3600–4000, 2700–3100, 2100–2400, and 600–780 cm−1 are noted. Some of the gaseous decomposition products are unambiguously identified by characteristic FTIR signals, such as aromatic compounds (650–780 cm−1), CO (2100–2200 cm−1), CO2 (2300–2400 cm−1), and hydrocarbons (2850–3000 cm−1) [20, 27].

Fig. 8
figure 8

The 3D TG-IR spectra of gas phase in the thermal decomposition of a CO-PUS and b FRPUS

Full size image

FTIR spectra of pyrolysis products of CO-PUS and FRPUS at the maximum release rate are shown in Fig. 9. The main gas decomposition products of CO-PUS are compounds containing –OH (such as H2O; 3600–3900 cm−1), –CH3 or –CH2 (2929, 2856 cm−1), CO2 (2356 cm−1), CO (2132 cm−1), and aromatic rings (1450–1700, 750 cm−1). The FRPUS release similar decomposition products to that of CO-PUS. Additionally, the new absorption bonds of FRPUS at 1221 and 1095 cm−1 appears, which are due to the structures of P=O and P–O–P, respectively. So, it can be concluded that the FRPUP in FRPUS decomposed to form polyphosphate and phosphoric derivatives, which could promote the formation of a protective char layer [28, 29].

Fig. 9
figure 9

The TG-IR spectra of gas phase in the thermal degradation of CO-PUS and FRPUS at the maximum decomposition rate

Full size image

Analysis of the residual chars

To further investigate the combustion behavior of FRPUS and the flame-retardant mechanism, the structure of the char after CCT was examined by FTIR and XPS.

The FTIR spectrum of FRPUS char is shown in Fig. 10. It can be observed that-CH3or -CH2 groups (2974, 2927 cm−1) C=O groups (1735 cm−1), P=O groups (1269 cm−1), P-O-C groups (1047 cm−1), and P-O-P groups (1095, 883 cm−1) existed in the char. These results show that the phosphate segments in FRPUP probably decomposed cross-linked phosphoric acid derivatives, which could promote the formation of char residues [7] (Fig. 10).

Fig. 10
figure 10

FTIR spectrum of FRPUS char residue

Full size image

The XPS spectra of C1s, N1s, and P2p of FRPUS are shown in Fig. 11. The residual char of FRPUS mainly contains carbon, nitrogen, oxygen, and phosphorus from FRPUP. The peak at 284.6 eV is attributed to C–C and C-H in aliphatic and aromatic species. The peak at 286.3 eV should be assigned to C-O in C-O-P. The peak at 400 eV can be assigned to the N 1s of nitrogen compounds in the residual char. The peak at 133.4 eV from P 2p spectrum is attributed to the pyrophosphate and/or polyphosphate, which could promote the carbonization of FRPUS [5, 9, 26, 30, 31].

Fig. 11
figure 11

a XPS spectrum and b C1s, c N1s and d P2p XPS spectra of residual char of FRPUS

Full size image

According to the results of TG, TG-IR, FTIR, and XPS, the possible thermal degradation mechanism of FRPUS is shown in Scheme 2. First, from room temperature to 308 °C, the P-O-C and P–C bonds in FRPUP were broken and formed phosphoric acid and some H2O, CO2. Second, at 308–378 °C, the urethane groups formed between CO and FRPUP are decomposed into isocyanate and hydroxyl compounds. With the temperature increase, isocyanate and hydroxyl compounds can further form carbodiimides and hydrocarbon compounds (alkane, alkene) with the evolution of H2O and CO2. At the same time, the phosphoric acid compounds can form polyphosphoric acid, which could accelerate the formation of char as catalyst. Finally, at 378–459 °C, a stable char layer containing phosphorus species and polyaromatic structures is obtained in the presence of polyphosphoric acid, and the volatilized CO and CO2 are released [7, 11, 15, 16, 26].

Scheme 2
scheme 2

Possible thermal degradation mechanism of FRPUS

Full size image

Mechanical properties

The mechanical properties are characterized by universal testing machine according to the GB/T2411-2008 and GB/T 13022-91 standards. The mechanical properties of the PUS including hardness, tensile strength, and elongation at break are listed in Table 4 [26]. FRPUS exhibit lower hardness and tensile strength than CO-PUS. However, FRPUS show a higher value for elongation at break than CO-PUS. This phenomenon can be attributed to the fact that FRPUP has some longer molecular chain than MDI-50, which resulted in the lower cross-linking density than CO-PUS [32, 33].

Table 4 Mechanical and flame properties of CO-PUS and FRPUS
Full size table

Conclusions

In this work, a novel phosphorus- and nitrogen-containing polyurethane prepolymer (FRPUP) was successfully synthesized and applied in PUS. The chemical structure of FRPUP was confirmed by FTIR, 1H NMR, and 31P NMR. The resulting FRPUS demonstrated significant enhancements in thermal properties, flame retardancy, and combustion properties. The thermal degradation of FRPUS was studied by TGA and TG-IR. It was found that FRPUP can greatly enhance the char residues of PUS, which was caused by the synergistic effect of P and N elements in FRPUP. The study of flammability of FRPUS revealed that FRPUP could increase the LOI value and decrease the PHRR and THR. The residues were analyzed by FTIR and XPS, and the results indicated that phosphorus elements in FRPUP could catalyze the cross-linking reaction and form a stable char at high temperature. Furthermore, FRPUP can enhance the toughness of PUS.