Mechanically robust self-healing and recyclable flame-retarded polyurethane elastomer based on thermoreversible crosslinking network and multiple hydrogen bonds

Journal Pre-proofs Mechanically Robust Self-healing and Recyclable Flame-retarded Polyurethane Elastomer based on Thermoreversible Crosslinking Network and Multiple Hydrogen Bonds Shiwen Yang, Shuang Wang, Xiaosheng Du, Zongliang Du, Xu Cheng, Haibo Wang PII: DOI: Reference:

S1385-8947(19)32959-6 https://doi.org/10.1016/j.cej.2019.123544 CEJ 123544

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

5 September 2019 5 November 2019 18 November 2019

Please cite this article as: S. Yang, S. Wang, X. Du, Z. Du, X. Cheng, H. Wang, Mechanically Robust Self-healing and Recyclable Flame-retarded Polyurethane Elastomer based on Thermoreversible Crosslinking Network and Multiple Hydrogen Bonds, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123544

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Mechanically Flame-retarded

Robust

Self-healing

Polyurethane

and

Elastomer

Recyclable based

on

Thermoreversible Crosslinking Network and Multiple Hydrogen Bonds Shiwen Yanga,1, Shuang Wang

a,1,

Xiaosheng Dua, Zongliang Dua, Xu Chenga, and

Haibo Wanga,b* a

College of Biomass Science and Engineering, Sichuan University, Chengdu 610065, PR

China. b

The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education,

Sichuan University, Chengdu 610065, PR China.

*Corresponding Author H. Wang (E-mail: [email protected]). Tel: +86-28-85401296. 1S.

Yang and S. Wang contributed equally to this paper.

Graphical Abstract

Abstract: Self-healing polymers have drawn significant attention because of their great potential for applications in many fields. It is highly desirable to

prepare materials capable of self-healing and with excellent mechanical properties. Additionally, the flammability of polymers make them unsafe during use. A novel strategy is proposed to manufacture a multifunctional polyurethane elastomer with high tensile strength (37.11±1.89 MPa) and excellent self-healing efficiency (91.8 %), containing a thermo-reversible crosslinking network and multiple hydrogen interactions. The furan-terminated phosphorus-based

monomer,

tri(2-furyl)

phosphoramide

(TFP),

was

successfully synthesized and then conjugated into a maleimide-terminated linear segmented polyurethane (MPU) backbone to prepare self-healing, recyclable, and flame-retarded polyurethane (MPUF). Due to its synergetic dual reversible bonds, the multifunctional polyurethane elastomer possessed ultrahigh strength, and excellent self-healing, shape recovery, and reprocessing properties. The phosphorus containing polyurethane elastomer exhibited improved thermal stability a limiting oxygen index value of 28.5%, and a 12.3% decrease in the peak heat release rate (pHRR) relative to those of unmodified polyurethane. This work demonstretes an effective strategy to prepare multifunctional polyurethane elastomer with both high self-healing efficiency and excellent mechanical properties allowing application for this material in the fields of fire-control coatings and building materials. Keywords: Dual reversible bonds, Self-healing, Recyclable, Mechanical properties, Flame retardant. 1. Introduction

With increased environmental pollution and limited resources, there is increased interest in self-healing polymers based on non-covalent bonds (hydrogen bonds1, 2, ionic interactions3, etc.) or dynamic covalent bonds (reversible Diels-Alder reactions4, 5,

transesterification6, disulfide exchange7, boroxine/boronic acid equilibrium process8,

Schiff base9, etc.). Self-healing materials show great potential for use in many fields such as wearable electronics, electronic skin, and smart coatings. In recent years, hydrogen bonds have been applied to build non-covalent polymer due to their features of reversibility, directionality, and sensitivity10-14. The dense hydrogen bonds assembled in polymer offer an efficient exchange to accelerate the healing process, which is essential for H-bonds system15-17. The mobility of polymer chains is typically required to achieve higher healing efficiency, resulting in weak mechanical properties18. Therefore, the practical applications of those materials have been limited by inherent poor mechanical properties. To improve the mechanical properties, covalent crosslinking networks can be incorporated into polymer materials to construct a dual-network structure, but this approach reduces recyclability19. Additionally, irreversible chemical crosslinking will limit the movement of molecular chains and thus compromise the repair efficiency20. To overcome these limitations, an effective approach is the introduction of reversible physical crosslinks, such as electrostatic interactions21, ionic bonds22, and coordination bonds23, as sacrificial bonds to replace chemical crosslink. However, due to the dynamic exchange of reversible physical crosslinking at room temperature, the tensile strength of polymer may be insufficient to meet industrial needs. Hence, there

is a need to balance healing efficiency and suitable mechanical performance. Introducing dynamic covalent crosslinking networks into polymer materials provides a new strategy to take advantage of the synergistic effect between the dynamic covalent bonds and hydrogen bonds given their unique potential for reprocessability, recyclability, and to prolong the service lifespan of materials24-27. For example, Song et al. prepare a dual cross-linked self-healing conductive adhesive composite materials based on dynamic imine bonds and hydrogen bonding28. To the best of our knowledge, despite significant brilliant advances, few high mechanical strength polymer materials based on thermo-reversible crosslinking networks and H-bonds have been described. However, the typical thermoreversible Diels-Alder (DA) reaction is a preferred candidate to construct thermal-induced dynamic cross-linking networks with good thermal reversibility, minimal side reaction, mild reaction condition, and high yield29-33. Moreover, DA bonding is stable at room temperature and the structural stability of the polymer is well maintained at the use temperature, thus ensuring maintenance of the mechanical properties of the polymer at room temperature. It is desirable to integrate a dual dynamic network of dynamic covalent and hydrogen bonds into self-healing polymers with high healing efficiency and improved mechanical properties. Unfortunately, traditional self-healing polymer materials are intrinsically flammable and produce hazardous waste and corrosive fumes during burning, restricting applications as coatings for construction or wood materials34,35. Thus, there is a significant need to improve the fire retardancy of self-healing and recyclable

polyurethane elastomer materials. Phosphorus-containing flame retardants are of recent

interest

as

effective,

halogen-free,

and

nontoxic

materials36,

37.

Phosphorus-containing flame retardants can form a foamed charring layer beneath the fire hazard to limit the transfer of oxygen and heat. One potential strategy is to utilize phosphorus-based flame retardant to provide multi-functional materials with flame resistance. To achieve excellent self-healing and mechanical properties of a polyurethane elastomer with both enhanced flame retardancy and recyclability, we designed a synergetic dual reversible network of polyurethane by introducing a synthetic phosphorus-based flame retardant as the crosslinking agent. The thermal and combustion performances of the prepared cross-linked flame-retarded polyurethane elastomer (MPUF) were investigated by thermogravimetric analysis (TGA), cone calorimeter test (CCT), and limiting oxygen index (LOI) tests. Additionally, the recyclability and self-healing ability of the MPUF were characterized. Finally, the mechanisms underlying the self-healing and flame retardancy of MPUF films were studied systematically. 2. Experimental Section Materials. Phosphorus oxychloride (POCl3), dichloromethane, furfuramine (FA), 1,4-butanediol (BDO), and triethylamine (TEA) were obtained from Huaxia Chemical Industry Co., Ltd (Chengdu, China). Polypropylene glycol (PPG 2000, Mn = 2000 g/mol), isophorone isocyanate (IPDI), and dibutyltin dilaurate (DBTDL, catalyst) were provided by Dragon Clan Co., LTD. (Fujian, China). Methyl ethyl

ketone (MEK), anhydrous sodium sulfate, sodium chloride, and trimethylolpropane (TMP) were purchased by Kelong Reagent Co., LTD. (Chengdu, China). In addition, all samples were dehydrated at 120 ºC under vacuum for 2 h before use. Synthesis of tri(2-furyl) phosphoramide (TFP). TFP was prepared according to previous literature38. Briefly, FA (19.4 g, 0.20 mol), TEA (21.6 g, 0.21 mol), and 100 mL dichloromethane were charged into a 250 mL round-bottom flask with ice-salt bath. Then, POCl3 (10.5 g, 0.07 mol) dissolved in 80 mL dichloromethane was dripped into the flask gently. The reaction was kept at 0°C for 4 h and then 50 °C for 4 h. After that, the reaction product was filtered and then washed with saturated sodium chloride solution twice. After drying over anhydrous sodium sulfate, the faint yellow liquid was collected under reduced pressure to evaporate the dichloromethane. Synthesis of maleimide end-capped polyurethane elastomer (MPU). PPG2000 (24.75 g, 0.012 mol), IPDI (20.25 g, 0.091 mol), and DBTDL (0.05 g) were placed in a 250 mL three-neck round-bottom with a mechanical stirrer, thermometer, and condenser, and allowed to heat to 85 °C with moderating stirring for 2 h to obtain NCO end-capped PU prepolymer. After that, the temperature was cooled to 75 °C and BDO (5.72 g, 0.063 mol) dissolved in 5 mL distilled MEK was added dropwise slowly into the flask to extend the prepolymer chain. The reaction was kept for 3 h until the theoretical NCO content was reached. Then, HEPD (2.54 g, 0.018 mol) in 2 mL MEK was dropped into the stirring reaction to obtain a maleimide-terminated PU prepolymer, the capping reaction of the NCO-terminated prepolymer with HEPD was

continued until the NCO-content reached zero. For comparison, the polyurethane prepolymer (pre-PU) without maleimide modified was prepared as the same steps of maleimide end-capped polyurethane elastomer process. Preparation of crosslinking flame-retarded polyurethanes (MPUF). Mix MPU and TFP uniformly according to the stoichiometric ratio of maleimide and furan to get a series of solution (the ration of maleimide and furan were 3/1, 3/2, 3/3, 3/4). Denoted as MPUF1, MPUF2, MPUF3, and MPUF4. The composite films were fabricated by casting the solution on horizontal PTFE mold at room temperature for 48 h. After that, the films were dried at 60 °C for 24 h before characterization. Besides, pre-PU was mix with TMP according to the equal molar ratio of -NCO and -OH to get uniformly solution, and filmed the solution in the same way as described above to prepare a sample (namely PU) without DA bond for comparison. Synthesis route of MPUF was shown in Scheme 1. Characterization. 1H and

31P

NMR were recorded using Bruker AV 400

spectrometers (400 MHz, Germany) with DMSO-d6 as a solvent. FTIR spectra were recorded using a Nicolet 560 infrared spectrometer at room temperature, and the transmittance mode was used. TGA was carried using a TGAQ50 TA instrument under nitrogen atmosphere from 50 to 800 °C with a heating rate of 10 K/min, respectively. Qualitatively evaluation of the self-healing ability of all samples was executed under a polarizing optical microscope (POM, Leica, DM2500P),

equipped with a temperature programmed heating stage, by observing the cracks made by blades on the surfaces of samples mending process. Stress-strain curves were measured for quantitatively evaluating the healing efficiency and mechanical properties of all samples by using an Instron 4465 testing machine. Elongation rate was set to 50 mm/min. The dimensions of dumbbell-shaped specimens were 20 mm (length)×4 mm (width)×0.5 mm (thickness). Healing efficiency was calculated as the ratio of the tensile strength at break of the healed specimen to the original specimen. LOI tests were measured with an HC-2C oxygen index meter (Nanjing Jiangning Analytical Instrument CO., LTD, China) according to the ASTM-D 2863-2009 standard. The dimensions of the duplicated specimens were 127 mm (length)×10 mm (width)×3 mm (thickness). CCT tests were carried out using an FTT cone calorimeter (Grinstead, UK) at a heat flux of 35 kW/m2, according to the ISO 5660-1 standard. The dimensions of the specimens were 100 mm (length)×100 mm (width)×3 mm (thickness). SEM and EDX analysis employed a Quanta 250 scanning electron microscope (FEI, USA) to analyze the morphologies of char residues and the element distributions. The char residues were obtained from samples after CCT and were made electrically conductive by sputtering with gold.

Scheme 1. Schematic illustration of the preparation of the MPUF.

3. Results and Discussion 3.1. Chemical structure. The chemical structure of synthesized TFP was confirmed by 1H,

31P

NMR, and FTIR spectra. In the spectrum of 1H NMR (Fig. S1a), the peaks at 7.5, 6.2, 6.4 ppm corresponded to the H atom of Ar-H; the peaks at 3.8-4.0 ppm corresponded to the H atom of -CH2-; and the peak at 4.32 ppm corresponded to the H atom of -NH-. In addition, TFP showed a unique peak at 15.8 ppm in the

31P

NMR spectrum, as shown in Fig. S1b, confirming the structure and

purity of the TFP. The above analysis confirmed the successful synthesis of TFP. In the FTIR spectrum of TFP (Fig. S2), the peak at 1221 cm-1 was assigned to the P=O bond; the peak around at 1088 cm-1 was assigned to the P-N bond;

and the strong vibration absorption peak at 734 cm-1 was assigned to the furan ring, further confirming the structure of TFP. The characteristic peak assigned to the urethane bond both was detected in the spectra of PU and MPUF3. The peak at 3340 cm-1 was attributed to the stretching vibration of the N-H bond; the peak at 1530 cm-1 was attributed to the bending vibration of the N-H bond; and the peak at 1710 cm-1 corresponded to the stretching vibration of the C=O bond of the ester group, which confirmed the successful synthesis of polyurethane. In addition, the spectrum of MPUF included two new peaks at 1772 and 734 cm-1, indicating the existence of the DA adduct and the furan ring component. These results confirmed the successful synthesis of the cross-linked flame retardant polyurethane elastomer via Diels-Alder reaction. The carbonyl region in the FTIR spectrum of polyurethane is widely used to study the hydrogen bond structure of polyurethanes39. The fraction of H-bonded carbonyls group is used as an indicator of micro-phase separation. Fig. 1 shows the deconvolution results of the PU and MPUF in the carbonyl groups region by applying Gaussian function. The three peaks in the curve fitting were attributed to free carbonyl groups, disordered H-bonded carbonyl groups, and ordered H-bonded carbonyl groups. The quantitative details are summarized in Table 1. In the MPUF, with increasing TFP content, the ratio of the order H-bonded carbonyl groups increased that of the free carbonyl groups decreased, and the location of ordered H-bonded carbonyl groups moved to lower wavenumber. The above result are consistent with the formation of

H-bonding between N-H in the hard segment urethane group of the polyurethane and N-H of the TFP, while the H-bond interactions between hard segments increased. With increased TFP content, the ordered phase structure of the cross-linked membrane increased, leading to aggregation of the hard segments of the cross-linked membranes, causing cross-linked films to form an ordered, hard chain segment, microcrystalline structure. The micro-phase separation between the hard and soft phase of cross-linked films increased. The degree of micro-phase separation is one of the key factors affecting the mechanical properties of the polyurethane elastomer with a two-phase separation structure. Basically, the hard segments of the polyurethane elastomer serve as a physical cross-linking point, and the soft segments provide flexibility. In this study, the Diels-Alder reaction between TFP and maleimide-terminated polyurethane resulted in additional chemical crosslinking points for cross-linked membranes. The synergistic effects of both chemical and physical crosslinking points affect the overall mechanical property of the prepared MPUF films.

Fig. 1. FT-IR spectra and deconvolution results of PU and MPUF in the carbonyl groups region.

Table 1 Deconvolution calculated results of the crosslinking membranes in the C=O region. Wavenumber (cm-1)

Sample

Integrated intensity (%)

C=OFree

C=OHB, disorder

C=OHB, order

C=OFree

C=OHB, disorder

C=OHB, order

PU

1724

1696

1661

16.5

51.8

31.7

MPUF1

1722

1695

1663

19.2

49.6

31.2

MPUF2

1723

1696

1659

17.7

50.2

32.1

MPUF3

1723

1696

1658

13.3

47.8

38.9

MPUF4

1724

1696

1658

10.2

49.6

40.2

3.2.

Mechanical analysis Generally, mechanical properties are pivotal indexes to expand the

potential applications of polymer materials. Tensile tests were carried out at 25

°C with a test rate of 50 mm/min, as shown in Fig. 2. The results are summarized in Table 2. The tensile stress first surged and then declined beyond the optimum point, while elongation at break increased gradually with increased rise in loading percentage of TFP. This phenomenon was attributed to the increased chemical cross-linking points and H-bonding density near the cross-linker. With further increase of crosslinking density, TFP could not completely react with the maleimide-terminated polyurethane due to steric hindrance, while a linear polyurethane structure produced in cross-linked membranes further increased elongation at break, with a slight decline in tensile strength. All samples of prepared MPUF showed a remarkable change in the slope of the stress-strain curves above 50% strain. The samples exhibited a significant improvement in strength and modulus, consistent with effects of strain hardening phenomena. This likely reflects the initiation of the hard segments, including H-bonds, to orient and rearrangement in the stretching direction. The tensile strength of MPUF3 increased dramatically, from 14.84 to 37.11 MPa, with a negligible decline of the strain from 781 to 702% compared with neat PU. This change was attributed to the increase of H-bonding, as described above. These results demonstrated that the synergistic effects of a covalently cross-linked network formed by DA reaction and H-bonding of hard segments significantly enhanced the mechanical properties of the material.

Fig. 2. The tensile stress-strain curves of PU and MPUF. Stress-strain curves of PU and MPUF under stretching.

Table 2 The details of tensile results of PU and MPUF.

3.3.

Sample

PU

MPUF1

MPUF2

MPUF3

MPUF4

Stress (MPa)

14.84±0.49

6.63±0.32

22.09±1.25

37.11±1.89

27.89±1.08

Elongation (%)

781±3.59

561±3.98

634±6.48

702±6.99

827±5.24

Self-healing performance To observe the repair process of MPUF films, a crack was applied to the

surface of the sample via a surgical blade and then sample was then placed on a heating stage. As shown in Fig. 3, after incubation at 120 °C for 10 min, the cracks on all samples were basically healed, macroscopically demonstrating the occurrence of crosslinking polyurethane film repair. However, the PU sample without DA cross-linking diminished to some extent after heating at 120 °C for 10 min, but did not repair the crack as did the MPUF under the same conditions

(Fig. S3). Therefore, DA reaction was essential for the system to complete the healing process. Tensile measurements with duplicate specimens were conducted to quantitatively evaluate healing efficiency. Sample processing was performed as follows. All the samples were scratched to leave a 100 μm wide mark, placed at 120 °C for 4 h, and then the retro-DA (rDA) reaction occurred, and the cross point was undone. The temperature was then cooled to 60 °C, and maintained for 12 h to re-form the crosslinked structure through sufficient DA reaction. The healing efficiency was defined as the ratio of the tensile stress of the repaired material over the tensile stress of original samples. The stress-strain curves are exhibited in Fig. 4, and the details are summarized in Table 3. The healing efficiencies of MPUF1, MPUF2, MPUF3, and MPUF4 were measured as 81.3, 84.9, 91.8, and 91.6%, respectively. The higher the TFP content, the higher the healing efficiency, which may be related to the higher amount of free-floating TFP present in the crack after the sample was heated. The superior healing efficiency (MPUF3 and MPUF4 were higher than 90%) of this system was partly attributed to the rapid exchange equilibrium of hydrogen bonds. To further verify whether the sample healing efficiency was related to the healing conditions, we selected MPUF3 and healed samples at 120 °C for 1, 2, 3, and 4 h. The materials were then incubated at 60 °C for 8 h before performing a stress-strain test. As shown in Fig. 4a, the healing efficiencies for the samples incubated at 120 °C for 1, 2, 3, and 4 h were 75.8, 81.8, 86.8, and

91.8%, respectively. With extended repair time was prolonged, the rDA reaction at the crack of the crosslinked membrane was more complete, releasing more free-floating polyurethane chain and TFP, which showed improved diffusion and entangled more sufficiently at the crack, resulting in a more easily conjugated function of maleimide and furan. The subsequent maintenance at 65 °C for 12 h guaranteed the reconnection of the DA reaction, restoring the mechanical performance. The self-healing mechanism of the thermo-triggered self-healing of cross-linked PU elastomer is illustrated in Fig. 3a. As described in Fig. 3a, after mixing TFP and MPU, the film was crosslinked at 60 °C by the DA reaction. The mechanism to heal the cross-linked film is as follows: First, the DA crosslink points were cleaved via retro-DA reaction at higher temperature. Then, free-floating polyurethane chains and TFP were allowed to diffuse, entangling with each other to cover the scratch at the macro level. Finally, DA bonds reconnected at lower temperature and, equilibrium hydrogen bonds were exchanged to restore the mechanical property of MPUF. As shown in Fig. 5, the self-healing performance of materials, particularly healing efficiency and stress at break, are represented by different colors. By comparing our data with that previously reported for self-healing polymer materials, our prepared MPUF elastomer exhibits both superior healing efficiency and high mechanical strength.

Fig. 3. (a) Mechanism of the thermo-triggered self-healing of cross-linked PU elastomer. POM images of the incisions on the surfaces of MPUF1 (b and b1), MPUF2 (c and c1), MPUF3 (d and d1), MPUF4 (e and e1) before and after thermo-triggered self-healing.

Fig. 4. (a) Stress-strain curves of original MPUF3 and repaired samples under different time. (b) Self-healing efficiency of MPUF. (c-f) Stress-strain curves of original and repaired specimens of MPUF. Table 3 The detail information of stress-strain curves.

Original

Repaired

Sample

Healing efficiency (%) Stress (MPa)

Elongation (%)

Stress (MPa)

Elongation (%)

MPUF1

6.63±0.32

561±3.98

5.39±0.58

469±5.98

81.3

MPUF2

22.09±1.25

634±6.48

18.77±1.32

622±4.56

84.9

MPUF3

37.11±1.89

702±6.99

34.42±1.54

675±6.32

91.8

MPUF4

27.89±1.08

827±5.24

24.55±1.08

736±5.45

91.6

Fig. 5. A summary of polymer materials with different healing efficiency and mechanical strength. 3.4.

Reprocessing recyclability The shape recovery process of MPUF3 is depicted in Fig. 6a. In this

experiment, the specimen was twisted into another shape at 120 °C, and fixed after cooling down. The deformed shape could recover to the initial one after subsequent heating again. The results indicated that cross-linked membranes

could be twisted into complex shapes at a certain temperature and fixed temporarily by freezing, achieving the shape recovery due to the DA/rDA reaction. Malleability is a pivotal index for self-healing materials, allowing reprocessability and shape changeability. After confirming the shape memory of cross-linked films, recyclability properties was also investigated. As shown in Fig. 6b, the membrane of MPUF3 was cut into fragments and remolded into a circular shape by subjecting fragments to a pressure of 10 MPa at 130 °C for 10 min. The specimen was then maintained at 60 °C for 8 h to ensure the recombination of the DA bonds. The stress-strain curves in Fig. 7a demonstrated that the mechanical performance of recycled sample decreased only slightly compared to that of the original sample, proving its excellent recyclability. Infrared curves of samples at different times during the recycling process were recorded to obtain information about the recycling process (Fig. 7a). The FTIR spectra of MPUF3 at different conditions showed no difference except for the characteristic absorption peak of DA adducts at 1772 cm-1 and the peaks of maleimide at 696 and 827 cm-1. The original sample exhibited a characteristic peak of DA adducts but no peaks corresponding to maleimide. However, heating the sample to 60 °C and then maintaining the sample for 10 min resulted in a spectra with a characteristic peak of maleimide. Additionally, the peak at 1772 cm-1 for DA adducts of the sample reduced significantly

compared with the original sample, verifying the occurrence of retro-DA reaction for this condition. The FTIR of the recycled sample exhibited the peak at 1772 cm-1 of DA adducts, but the peaks of maleimide disappeared, demonstrating the restoration of the DA cross-links from the free-floating maleimide and furan groups. Based on this result, the main determinant of the cross-linked

membranes

recycling

was

the

reversible

nature

of

the

DA/retro-DA reaction, probably accompanied by a dynamic equilibrium of hydrogen bonds.

Fig. 6. (a) Self-recovery of MPUF after shape deformation at 120oC for 10 min. (b) recyclability photos of MPUF.

Fig. 7. (a) Stress-strain curves of MPUF3 during the recycling process; (b) FTIR spectra of original, 130oC, and recycled MPUF3 films. 3.5.

Thermal and flammability characteristics The thermal performances of TFP and MPUF were examined by TGA in

nitrogen and air atmosphere to assess the thermal degradation behavior. As shown in Fig. 8a, TFP began to thermally degrade at 193.7 ºC and exhibited two main stages as shown in Fig. 8b. The char residual of TFP at 800 oC reached 37.9 wt%. As listed in Table 4, the 5 wt% degradation temperature (Td5) values of the PU, MPUF1, MPUF2, MPUF3, and MPUF4 films were 267.0, 280.9, 274.9, 272.5, and 261.2 ºC, respectively, which decreased gradually due to the lower degradation temperature of O=P–O bond. The Td5 of MPUF films increased compared to the measured Td5 for pure PU of 267.0 ºC, indicating that the addition of TFP remarkably improved the thermal stability of PU. The char yields of PU, MPUF1, MPUF2, MPUF3, and MPUF4 were measured as are 6.8, 10.7, 11.9, 13.7, and 15.5 wt%, suggesting that the conjugation of TFP into PU backbone would significantly increase the amount of char residue and enhance the thermal stability of PU films at a higher temperature.

Fig. 8. TGA (a) and DTG (b) curves of TFP, PU, and MPUF in N2.

Table 4 Related Thermal Degradation Data of TFP, PU, and MPUF in N2. Sample

Td5 (°C)

Tmax1 (°C)

Tmax2 (°C)

Char residues at 800 oC (wt%)

TFP

193.7

229.9

563.5

37.9

PU

267.0

322.0

389.4

6.8

MPUF1

280.9

302.6

376.3

10.7

MPUF2

274.9

300.9

376.7

11.9

MPUF3

272.5

290.8

375.0

13.7

MPUF4

261.2

324.6

383.5

15.5

The flame-retardant properties of the prepared MPUF samples were assessed by LOI tests in the presence of TFP and the results are presented in Fig. 9. With the addition of 8 wt% TFP, the LOI value of MPUF4 increased from 19.5 to 28.5%. CCT tests were employed to evaluate the combustion behaviors of the MPUF samples. The curves of the heat release (HRR) and total heat rating (THR) and digital photographs of the residual chars for PU and MPUF4 after CCT are depicted in Fig. 10. In Fig. 10, PU (without TFP being added) burned out after ignition, with pHRR, THR, and char residual values were 650.9 kW/m2, 54.5 MJ/m2, and 0.76 wt%, respectively. When TFP was

added into the PU, the pHRR, and THR values of the MPUF4 significantly decreased to 572.5 kW/m2 and 50.62 MJ/m2, and the residual weight increased to 4.07 wt%. Compared to pure PU, the addition of 8% TFP brought a 9.0% enhancement in LOI value and 12.0% reduction in pHRR. All the results supported the conclusion that the introduction of phosphorus-based TFP into PU films effectively improved their flame retardancy. SEM with EDX was used to analyze the surface morphologies and the chemical composition of the external char layer. As shown in Fig. 11a, PU (without additional TFP) possessed a brittle surface with fragments. In comparison, the external surface of MPUF4 exhibited a compact carbonaceous layer covered with uniform pores. This contributed to the production of nonflammable gases produced by nitrogen-based TFP, which could reduce the oxygen concentration near a fire hazard. Additionally, the compact carbonaceous layer could restrain the diffusion of heat and toxic gases. As shown in Fig. 11a2, there were 67.83 wt% carbon and 32.17 wt% oxygen in the external residual char for PU. In contrast, there were 35.71 wt% carbon and 47.81 wt% oxygen in the MPUF4 residues with a remarkable phosphorus content of 16.47 wt%. The above results indicated that the phosphorus-based material derived from the phosphate groups in TFP during combustion promoted the formation of phosphorus-containing protective layers, which then contributed to flame-retardancy of the condensed phase.

Fig. 9. LOI values of PU and MPUF Films.

Fig. 10. THR (a) and HRR (b) vs time curves of PU and MPUF4. Digital photos of char residues of (c) pure PU and (d) MPUF4 after CCT.

Fig. 11. SEM images of external char residues from (a and a1) PU and (b and b1) MPUF4 at 100 and 1000 SE after CCT. EDX spectra of the char residue at 100 SE for PU (a2), MPUF4 (b2). 4.

Conclusion In summary, we reported a novel strategy to manufacture multifunctional

polyurethane elastomer with high tensile strength (37.11±1.89 MPa), excellent self-healing efficiency (91.8 %), and with improved fire-resistance and recyclability. phosphoramide

Furan-terminated (TFP),

was

phosphorus-based successfully

monomer,

synthesized

by

tri(2-furyl) phosphorus

oxychloride and furfuramine, and then conjugated into maleimide-terminated linear segmented polyurethane (MPU) backbone to prepare crosslinking self-healing and flame-retarded polyurethane (MPUF). The MPUF films exhibited efficient self-healing (over 90% after heat treatment), significant

enhancement of tensile strength, exceptional shape recovery, and good reprocessing ability. Results of TGA, LOI, and CCT tests indicated that TFP displayed remarkable residual char formation of 37.9 wt% and that TFP-modified MPUF achieved a LOI value of 28.5% and a 12.3% decrease in heat release rate (HRR). These results demonstrated that the introduction of the phosphorus-based flame retardant as a crosslinking agent into PU elastomer suggests a new strategy to prepare polyurethane elastomer with high self-healing efficiency and excellent flame-retardancy.

Author Information Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgments We acknowledge the funding support from the National Natural Science Foundation of China (NO. 51773129, 51903167), Support Plan of Science and Technology

Department

of

Sichuan

Province,

China

(2018SZ0174,

2019YFG0257), Supported by the Opening Project of Key Laboratory of Leather Chemistry and Engineering, (Sichuan University), Ministry of Education

(20826041C4159),

International

Science

and

Technology

Cooperation Program of Chengdu (2017-GH02-00068-HZ), Postdoctoral research foundation of Sichuan University (2018SCU12049). We also appreciate He Yi from the Analytical & Testing Center of Sichuan University for his help with SEM characterization.

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