A preparation method of a polyurethane elastomer

Abstract

The present invention relates to a method for preparing a polyurethane elastomer. First, a Schiff base chain extender (PD) with a disulfide bond and a hydrazone bond is synthesized from 3,3'-dithiobis(propionyl hydrazide) and a benzene ring compound with an aldehyde group, a hydroxyl group and two adjacent functional groups. Subsequently, a waterborne polyurethane elastomer prepared with this chain extender can solve the problem of difficulty in coordinating the excellent mechanical properties and high self-healing rate of typical self-healing polyurethanes. The polyurethane elastomer obtained in the present invention has photoluminescence characteristics, and can track its self-healing process by monitoring the change in fluorescence intensity at the crack, which can effectively avoid the accumulation of cracks and improve the practicality of the elastomer.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyurethane elastomers, and particularly relates to a preparation method of a polyurethane elastomer. Background Art

[0002] Waterborne polyurethanes have excellent mechanical properties and self-curing properties, and can greatly reduce the volatilization of organic solvents, which is very important for environmentally friendly applications. Therefore, they are widely used in furniture, biomedicine, and wearable devices. However, during their use, they will inevitably be damaged to some extent. In particular, internal cracks or local damage will cause the material properties to deteriorate rapidly, thus bringing great potential safety hazards. Therefore, a new generation of polyurethane materials should have good healing and recycling capabilities to reduce the use cost and extend the service life.

[0003] In recent years, disulfide bonds and acylhydrazone bonds have received increasing attention due to their mild reaction conditions and few side reactions. Nevejans et al. prepared thermoplastic polyurethanes with high mechanical properties by introducing a diol with aromatic disulfide bonds, and this diol can induce self-healing at high temperatures (European Polymer Journal, 2019, 112: 411-422.); Schubert et al. reported a covalently cross-linked polymer based on acylhydrazone bond exchange, and this material can heal deep scratches up to 1 cm at 125 °C (Advanced Functional Materials, 2015, 25(22): 3295-3301.). However, both of these require self-healing at high temperatures, and there is a problem of conflict between high mechanical properties and high self-healing efficiency in typical self-healing polyurethanes, that is, the high chain stiffness and entanglement required to improve mechanical strength conflict with the high chain diffusivity and dynamic bond exchange required for repairing damage. Summary of the Invention

[0004] The object of the present invention is to provide a preparation method of a polyurethane elastomer in view of the deficiencies in the current technology. This method first synthesizes a Schiff base chain extender (PD) with disulfide bonds and acylhydrazone bonds by using 3,3'-dithiobis(propionylhydrazide) and a benzene ring compound with one aldehyde group, one hydroxyl group and two adjacent functional groups. The waterborne polyurethane elastomer prepared with this chain extender can solve the problem that it is difficult to coordinate between excellent mechanical properties and high self-healing rate of typical self-healing polyurethanes. The polyurethane elastomer obtained by the present invention has photoluminescence characteristics, and can track its self-healing process by monitoring the change of fluorescence intensity at the crack, which can effectively avoid the accumulation of cracks and improve the practicality of the elastomer.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A preparation method of a polyurethane elastomer, the method comprising the following steps:

[0007] In the first step, 3,3'-dithiobis(propionylhydrazide) (DPH) is dissolved in ethanol and then dropped into compound A; the resulting mixture is stirred and refluxed for 2 - 7 h, washed with ethanol, and dried under reduced pressure at 30 - 100 °C to obtain a white powder, which is a Schiff base (PD) chain extender containing disulfide bonds and hydrazone bonds;

[0008] Wherein, the mass ratio of DPH to compound A is 1:0.3 - 1; 1 - 5 g of DPH is added per 10 - 50 mL of ethanol; the compound A is a benzene ring compound with an aldehyde group, a hydroxyl group, and the two functional groups adjacent;

[0009] In the second step, compound B and compound C are injected into a reactor, then 2,2-dimethylolpropionic acid and compound D are added, and the reaction is carried out for 1 - 4 h under a nitrogen atmosphere. Then, the PD chain extender, compound E, and a catalyst are added, heated to 60 - 90 °C, and the reaction continues for 3 - 8 h; after the reaction temperature is lowered to 20 - 50 °C, triethylamine is added and the reaction continues for 0.5 - 3 h; finally, the obtained prepolymer and deionized water are emulsified under high-speed shear dispersion at 1000 - 2000 rpm for 1 - 4 h to obtain a WPU-PD emulsion;

[0010] Wherein, the mass ratio of compound B, compound C, 2,2-dimethylolpropionic acid, compound D, PD chain extender, compound E, catalyst, and triethylamine is 1:0.2 - 1:0.1 - 0.5:0.3 - 2:0.01 - 0.09:0.01 - 0.09:0.006 - 0.015:0.05 - 0.2; the compound B is one or more of polyethers or polyesters; the compound C is one or more of diisocyanates; the mass ratio of the prepolymer to deionized water is 1:2 - 6;

[0011] The compound D is a solvent; the compound E is a glycol chain extender;

[0012] In the third step, the WPU-PD emulsion is directly dropped onto a substrate, the coating thickness is 40 - 60 μm, dried at room temperature for 24 - 72 h, and then dehydrated at 40 - 90 °C for 8 - 18 h to obtain a polyurethane elastomer.

[0013] The compound A in the first step is one or more of salicylaldehyde, 5-methylsalicylaldehyde, 3-methoxysalicylaldehyde, and 3-ethoxy-o-hydroxybenzaldehyde.

[0014] The compound B in the second step is one or more of polypropylene glycol, polytetramethylene ether glycol, neopentyl glycol adipate, and polyhexyl carbonate diol;

[0015] The compound C is one or more of toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, 1,5-naphthalene diisocyanate, dimethylbiphenyl diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, norbornane diisocyanate;

[0016] The compound D is one or more of tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide;

[0017] The compound E is a chain extender such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,2-propanediol, hexanediol;

[0018] The catalyst is one or more of stannous octoate, dibutyltin dilaurate, zinc naphthenate, tetra-isobutyl titanate.

[0019] The substrate in the third step is one of glass sheet, tinplate, leather or fabric.

[0020] A Schiff base (PD) chain extender containing disulfide bond and acylhydrazone bond, and the structural formula of the chain extender is as follows:

[0021]

[0022] Among them, R 1 is H, -CH3, -CH2CH3 or -OCH3, -OCH2CH3; R 2 is H, -CH3, -CH2CH3 or -OCH3, -OCH2CH3; R 3 is H, -CH3, -CH2CH3 or -OCH3, -OCH2CH3; R 4 is H, -CH3, -CH2CH3 or -OCH3, -OCH2CH3.

[0023] The substantial features of the present invention are:

[0024] By introducing a Schiff base chain extender (PD) with disulfide bond and acylhydrazone bond, the present invention uses the hydrogen bond formed by acylhydrazone as a physical crosslinking point to promote the microphase separation of polyurethane, improve the thermal stability, tensile strength and toughness of the elastomer, integrate various dynamic bonds together, and endow the molecular chain with faster fluidity. The obtained polyurethane elastomer provides a new idea for solving the problem of contradiction between excellent mechanical properties and high self-healing efficiency, and also launches new measures for detecting cracks and monitoring crack healing.

[0025] The beneficial effects of the present invention are:

[0026] First, the present invention prepared a waterborne polyurethane (WPU) elastomer by using the Schiff base (PD) of disulfide bond and hydrazone bond as a chain extender. The hydrogen bonds formed by hydrazone can not only serve as physical crosslinking points to promote the microphase separation of polyurethane, improve the thermal stability, tensile strength and toughness of the elastomer, but also integrate various dynamic bonds as "buckles" to synergistically reduce the activation energy of polymer chain movement and endow the molecular chain with faster fluidity. WPU-PD exhibits excellent mechanical properties at room temperature, such as the tensile strength and fracture energy are 25.91 MPa and 121.66 kJ m -2 , respectively, and has a high self-healing efficiency of 93.7% in a short time under medium heating conditions. It solves the conflict between high mechanical properties and high self-healing efficiency existing in typical self-healing polyurethanes.

[0027] Second, salicylaldehyde hydrazone is a molecule with aggregation-induced emission (AIE) characteristics and can be used as a fluorescent dye. Therefore, the system containing PD can exhibit luminescence behavior. Due to the diffuse reflection of light by the rough surface, under the irradiation of 365 nm ultraviolet light, the fluorescence at the crack is the strongest on the entire film surface, which provides great convenience for diagnosing the damage position on the surface of polyurethane materials. At the same time, combined with the self-healing ability of WPU-PD, this phenomenon can help observe the self-healing process. During the crack healing process, the fluorescence intensity gradually decreases until it completely disappears. This self-monitoring and self-repair of cracks can effectively reduce the accumulation and expansion of cracks in polymer materials and improve their reliability and durability during service life. Description of the Drawings

[0028] Figure 1 is the Fourier transform infrared spectrum of PD obtained in Example 1;

[0029] Figure 2 is the nuclear magnetic resonance hydrogen spectrum of PD obtained in Example 1;

[0030] Figure 3 is the Fourier transform infrared spectrum of PA obtained in Example 2;

[0031] Figure 4 is the nuclear magnetic resonance hydrogen spectrum of PA obtained in Example 2;

[0032] Figure 5 are the stress-strain curves of different samples obtained in Examples 3-10. Among them, under the same test conditions, Figure 5 a is the stress-strain curve of WPU elastomers with different PD contents (Examples 3, 7-10), Figure 5 b is the stress-strain curve of WPU elastomers with different chain extenders (Examples 4-6, 9);

[0033] Figure 6 Photographs of the WPU-PD-0.45 elastomer film obtained in Example 9 resisting nib puncture and the elastomer after puncture;

[0034] Figure 7 Photographs and stress-strain curves of the WPU-PD-0.45 elastomer obtained in Example 9 during the stretching process; among them, Figure 7 a is a photograph of the notched WPU-PD-0.45 elastomer film during the stretching process in Example 9, Figure 7 b is the stress-strain curves of the notched and unnotched WPU-PD-0.45 elastomers in Example 9;

[0035] Figure 8 Loading-unloading cycle curves of the WPU-PD-0.45 elastomer film at 800% strain obtained in Example 9;

[0036] Figure 9 a are photographs of the WPU-PD-0.45 elastomer film before and after 800% strain stretching in Example 9, Figure 9 b is the loading-unloading cycle curve of the WPU-PD-0.45 elastomer film at 200% of the same strain in Example 9;

[0037] Figure 10 Optical microscope images of the surface scratch healing process of different samples in Examples 4-6 and Example 9, among which, Figure 10 a are optical microscope images of the surface scratches of WPU-PD-0.45, WPU-PA-0.45, WPU-SS-0.45, and WPU-PA / SS during the healing process at 80 °C, Figure 10 b in Examples 4-6 and Example 9 are optical microscope images of the surface scratches of WPU-PD-0.45, WPU-PA-0.45, WPU-SS-0.45, and WPU-PA / SS at different healing times at 40 °C;

[0038] Figure 11 Demonstration diagram of the mechanical properties of the healed WPU-PD-0.45 elastomer film in Example 9;

[0039] Figure 12 Stress-strain curves of the original and healed WPU-PD-0.45 samples at different times at 80 °C in Example 9;

[0040] Figure 13 Stress-strain curves of the original WPU samples obtained in Examples 4-6 and the WPU samples healed at 80 °C for 8 h, among which, Figure 13a is the stress-strain curve of the WPU-PA-0.45 (Example 4) elastomer film after healing at 80 °C for 8 h, Figure 13 b is the stress-strain curve of the WPU-SS-0.45 (Example 5) elastomer film after healing at 80 °C for 8 h, Figure 13 c is the stress-strain curve of the WPU-PA / SS (Example 6) elastomer film after healing at 80 °C for 8 h;

[0041] Figure 14 are the stress relaxation curves of different samples obtained in Example 9 and Example 6 at 60, 70, and 80 °C. Among them, Figure 14 a is the stress relaxation curve of the WPU-PD-0.45 elastomer film at 60, 70, and 80 °C (Example 9), Figure 14 b is the stress relaxation curve of WPU-PA / SS at 60, 70, and 80 °C (Example 6);

[0042] Figure 15 is the solution casting recovery process of the WPU-PD-0.45 film and the tensile curve of the recovered sample obtained in Example 9. Among them, Figure 15 a is the recovery process of the WPU-PD-0.45 elastomer film obtained in Example 9 by solution casting, Figure 15 b is the tensile curve of the recovered sample of the WPU-PD-0.45 elastomer film obtained in Example 9;

[0043] Figure 16 is the fluorescence characteristic diagram of the WPU-PD film. Among them, Figure 16 a is the photo of the WPU-0 and WPU-PD elastomer films under a 365 nm ultraviolet lamp, Figure 16 b is the ultraviolet-visible absorption spectrum of the WPU-PD elastomer film (1 mg of WPU-PD is dissolved in 10 mL of methanol solution), Figure 16 c is the fluorescence spectrum of the WPU-PD film;

[0044] Figure 17 a is the schematic diagram of the fluorescence intensity before and after the healing of the "HEBUT" - shaped crack on the WPU-PD film under ultraviolet light irradiation. Among them, Figure 17 b is the picture of the healing process of the "HEBUT" - shaped crack on the WPU-PD-0.45 film at 80 °C. Detailed implementation mode

[0045] Example 1:

[0046] Synthesis of a Schiff base chain extender (PD) containing disulfide bonds and hydrazone bonds. The specific steps are as follows:

[0047] Dissolve 2.38 g of DPH in 30 mL of ethanol and drop it into 3.05 g of salicylaldehyde. Stir the resulting mixture and react it at 60 °C for 4 h, wash it three times with ethanol, and dry it under reduced pressure (negative 0.1 MPa) at 60 °C. The obtained white powder is a Schiff base (PD) chain extender containing disulfide bonds and acylhydrazone bonds. The synthesis route is as follows.

[0048]

[0049] The Fourier transform infrared spectroscopy (FTIR) of the PD chain extender was tested using a German Tensor-27 spectrometer to characterize its structure. Scanning range: 1000 - 4000 cm-1, resolution: 4 cm-1. The results are as Figure 1 shown. The nuclear magnetic resonance hydrogen spectrum (1H-NMR) of the PD chain extender was tested using an AVANCE 400 NMR spectrometer. Scanning range: 0 - 15 ppm, scanning frequency: 400 MHz. The results are as Figure 2 shown. These results indicate that PD was successfully prepared.

[0050] Example 2:

[0051] The synthesis of a Schiff base chain extender (PA) containing an acylhydrazone bond is as follows:

[0052] Other steps for the synthesis of PA are the same as those for the preparation of PD in Example 1, except that APH is used instead of DPH. The synthesis route is as follows.

[0053]

[0054] The Fourier transform infrared spectroscopy (FTIR) test results of PA are as Figure 3 shown, and the nuclear magnetic resonance hydrogen spectrum ( 1 1H-NMR) test results are as Figure 4 shown. These results indicate that PA was successfully prepared.

[0055] Example 3:

[0056] The preparation of an aqueous polyurethane emulsion is as follows:

[0057] First step: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.23 g of 1,4-butanediol and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.29 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain WPU-0 emulsion.

[0058] Second step: Drop the WPU-0 emulsion directly onto a glass slide with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain a waterborne polyurethane elastomer film.

[0059] Test the WPU-0 elastomer film using a CMT-6104 universal testing machine (XinSanSi, Shenzhen, China) according to the GB / T 1040-92 standard. The results are as Figure 5 shown in a. Use a 365 nm ultraviolet lamp to irradiate the film to test its luminescence properties. The results are as Figure 16 shown in a. The WPU-0 elastomer does not emit light. Measure the ultraviolet absorption spectrum of the elastomer film using an ultraviolet-visible spectrophotometer (CARY300, USA) with a wavelength range of 400 to 650 nm. The results are as Figure 16 shown in b. There is no strong absorption peak. Measure the fluorescence emission spectrum of the elastomer film using a fluorescence spectrometer (FLS920, Edinburg, Britain) with an excitation and emission wavelength of 280 nm. The results are as Figure 16 shown in c. There is no emission peak and it does not emit light.

[0060] Example 4:

[0061] Preparation of a waterborne polyurethane elastomer using PA as a chain extender, the specific steps are as follows:

[0062] First step: The same as in Example 2.

[0063] Step 2: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and at a stirring speed of 50 rmp, subsequently add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.17 g of PA obtained in Example 2, 0.19 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue reacting for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.42 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PA-0.45 emulsion.

[0064] Step 3: Drop the WPU-PA-0.45 emulsion directly onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain a waterborne polyurethane elastomer film with PA as the chain extender.

[0065] The results of the static uniaxial tensile experiment of the WPU-PA-0.45 elastomer film are as Figure 5 shown in b. The surface scratch healing test was adopted: Scratch the surface of the film with a blade, and then place the film in an oven at 80 °C / 40 °C for healing. Observe the state changes of the scratches at different healing stages with an optical microscope (DMM-400C, Cai Kang, Shanghai). The results are as Figure 10 shown in 10a (80 °C) and 10b (40 °C). After the WPU-PA-0.45 heals at 80 °C for 30 min, the scratch morphology changes slightly, but there are still obvious visible traces; after the WPU-PA-0.45 heals at 40 °C for 24 h, the scratch morphology changes slightly, and there are very obvious visible traces. The cut healing test was adopted: The sample was completely cut into two pieces and placed together to ensure close contact of the fracture interface; then it was placed at different temperatures for different times, and the stress-strain curve of the healed sample was compared with the original curve to evaluate the healing of the mechanical properties. The healing efficiency was defined as the ratio of the tensile strength of the healed sample to the original tensile strength; the results are as Figure 13 shown. The healing efficiency of the WPU-PA-0.45 healed at 80 °C for 8 h is 61.9%.

[0066] Example 5:

[0067] Preparation of a waterborne polyurethane elastomer with disulfide bonds as the chain extender, the specific steps are as follows:

[0068] Step 1: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and at a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.10 g of 4,4'-dihydroxydiphenyl disulfide, 0.19 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.35 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-SS-0.45 emulsion.

[0069] Step 2: Drop the WPU-SS-0.45 emulsion directly onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain a waterborne polyurethane elastomer film with 4,4'-dihydroxydiphenyl disulfide as the chain extender.

[0070] The results of the static uniaxial tensile experiment of the WPU-SS-0.45 elastomer film are as Figure 5 shown in Figure 10 b. As shown in Figure 13 a, after WPU-SS-0.45 is healed at 80 °C for 30 min, the scratch morphology changes slightly, but there are still obvious visible traces; after WPU-SS-0.45 is healed at 40 °C for 24 h, the scratch morphology changes slightly, and there are relatively obvious visible traces. As shown in

[0071] Example 6:

[0072] Step 1: The same as in Example 2.

[0073] Step 2: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and at a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.17 g of PA obtained in Example 2, 0.10 g of 4,4'-dihydroxydiphenyl disulfide, 0.15 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.48 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PA / SS emulsion.

[0074] In the third step, the WPU-PA / SS emulsion was directly dropped onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, it was dehydrated at 70 °C for 12 h to obtain an aqueous polyurethane elastomer film with PA and 4,4'-dihydroxydiphenyl disulfide as chain extenders together.

[0075] The results of the static uniaxial tensile experiment of the WPU-PA / SS elastomer film are as Figure 5 shown in Figure 10 b. As shown in Figure 13 a, after the WPU-PA / SS healed at 80 °C for 30 min, the scratch morphology changed slightly, but there were still obvious visible traces; after the WPU-PA / SS healed at 40 °C for 24 h, the scratch morphology changed slightly, and there were relatively obvious visible traces. As shown in Figure 14 shown, the healing efficiency of the WPU-PA / SS healed at 80 °C for 8 h was 72.5%. The stress relaxation test of the WPU-PA / SS elastomer film was carried out using a dynamic thermal analyzer (Q800, TA, USA); the rectangular specimen was first straightened under a pre-tightening force of 0.001 N. After equilibration at the target temperatures (60 °C, 70 °C, 80 °C), the test sample was further stretched at a constant strain of 3% to maintain the deformation, and the change of the relaxation modulus with time was recorded. The results are as b shown.

[0076] Example 7:

[0077] The first step is the same as that of Example 1.

[0078] In the second step, 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate were injected into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide were added. After reacting at 80 °C for 1.5 h, 0.07 g of PD, 0.22 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate obtained in Example 1 were added, and it was heated to 85 °C and continued to react for 5 h. After the reaction temperature dropped to 35 °C, 0.60 g of triethanolamine was added to neutralize the mixture for 1 h. Finally, the obtained 13.35 g of prepolymer was emulsified with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PD-0.15 emulsion.

[0079] In the third step, the WPU-PD-0.15 emulsion was directly dropped onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, it was dehydrated at 70 °C for 12 h to obtain an aqueous polyurethane elastomer film with 0.15 mol of PD as a chain extender.

[0080] The results of the static uniaxial tensile experiment of the WPU-PD-0.15 elastomer film are asFigure 5 As shown in Fig. a, no yield phenomenon occurred during the stretching process of the film, and it exhibited typical elastic behavior. As Figure 16 As shown in Fig. b, the strong absorption bands at 280 and 290 nm are attributed to the π-π* transitions of the benzene ring and imino group, respectively, while the absorption band at 320 nm is caused by the n-π* transition of the imino group in the conjugated system. As Figure 16 As shown in Fig. c, the film has an emission peak at 475 nm.

[0081] Example 8:

[0082] The first step is the same as in Example 1.

[0083] The second step: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.13 g of PD, 0.20 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate obtained in Example 1, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.39 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PD-0.3 emulsion.

[0084] The third step: Drop the WPU-PD-0.3 emulsion directly onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain an aqueous polyurethane elastomer film with 0.3 mol of PD as the chain extender.

[0085] The results of the static uniaxial tensile experiment of the WPU-PD-0.3 elastomer film are as Figure 5 As shown in Fig. a, no yield phenomenon occurred during the stretching process of the film, and it exhibited typical elastic behavior. As Figure 16 As shown in Fig. b, the strong absorption bands at 280 and 290 nm are attributed to the π-π* transitions of the benzene ring and imino group, respectively, while the absorption band at 320 nm is caused by the n-π* transition of the imino group in the conjugated system. As Figure 16 As shown in Fig. c, the film has an emission peak at 475 nm.

[0086] Example 9:

[0087] The first step is the same as in Example 1.

[0088] In the second step, 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate were injected into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and at a stirring speed of 50 rmp, 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide were then added. After reacting at 80 °C for 1.5 h, 0.20 g of PD obtained in Example 1, 0.19 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate were added, and the temperature was heated to 85 °C and the reaction continued for 5 h. After the reaction temperature dropped to 35 °C, 0.60 g of triethanolamine was added to neutralize the mixture for 1 h. Finally, the obtained 13.45 g of prepolymer and 50 mL of deionized water were emulsified under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain a WPU-PD-0.45 emulsion.

[0089] In the third step, the WPU-PD-0.45 emulsion was directly dropped onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, it was dehydrated at 70 °C for 12 h to obtain a waterborne polyurethane elastomer film with recoverability, high self-healing rate, high strength, and high toughness using 0.45 mol of PD as a chain extender.

[0090] The results of the static uniaxial tensile experiment of the WPU-PD-0.45 elastomer film are as Figure 5 shown in a. No yield phenomenon occurred during the stretching process of the film, and it had typical elastic behavior. To further verify the hydrogen bond effect, the influence of different chain extenders on the mechanical properties was evaluated. According to Figure 5 b, the mechanical properties of WPU-PD-0.45, WPU-PA-0.45, or WPU-PA / SS were significantly better than those of the WPU-SS-0.45 elastomer, which verified that the hydrogen bond effect could effectively improve the mechanical properties of the WPU elastomer. The toughness of WPU-PD-0.45 was 102.80 MJ m -3 , even higher than that of some common fibers, such as carbon fiber (25 MJ m -3 ) or nylon fiber (80 MJ m -3 ). Using a marker pen to simulate accidental injuries that may occur in daily life, no puncture damage was observed, showing good toughness ( Figure 6 ). The tear resistance of the WPU-PD-0.45 elastomer film was studied through a uniaxial tensile test. As Figure 7 shown in a, during the stretching process, the notch became significantly blunter, and cracks were generated at the front end of the notch. Before fracture, the cracks gradually expanded longitudinally with stretching but hardly expanded transversely, showing good fracture toughness and notch insensitivity. The pre-damaged specimen with a 1-mm notch could be stretched to more than 7 times its original length, and the tensile strength and elongation at break were 11.53 MPa and 734%, respectively ( Figure 7b); The fracture energy calculated by the Greensmith method is 121.66 kJ / m 2 ; The high crack tolerance of the WPU-PD-0.45 elastomer film is due to the fact that the physical crosslinking network can effectively dissipate energy through the rapid fracture and recombination of multiple dynamic bonds, which is conducive to the stress diffusion at the crack to the entire elastomer network and hinders the further expansion of the crack. The self-recovery property was studied by cyclic tensile tests (the samples were tested by stretching to 200% strain at a speed of 50 mm / min, and 10 loading-unloading cyclic tensile tests were carried out without time interval. After standing at room temperature for 24 h, the loading-unloading cycle was carried out again); Figure 8 It shows that the 260% residual strain of WPU-PD-0.45 is difficult to recover immediately after being stretched to 8 times its original length, but it basically recovers after relaxation for a short time ( Figure 9 a); The results are as Figure 9 shown in b. The strain was set to 200%, and 10 consecutive loading-unloading cycles were carried out; the first cycle showed an obvious hysteresis loop because the fracture of weak disulfide bonds dissipated a large amount of energy; in the second cycle, the area of the hysteresis loop and the tensile strength decreased significantly, and the decreasing trend became flat in the subsequent loading-unloading cycles; this is because the sacrificial network broken in the first cycle did not recover immediately, the energy dissipation in the second cycle was much smaller than that in the first cycle, and only a few sacrificial bonds broke in the subsequent loading-unloading cycle tests; however, after standing at room temperature for 24 h, the loading-unloading curve of WPU-PD-0.45 almost coincided with the first one. This indicates that the broken dynamic bonds in the WPU-PD-0.45 film were rapidly recombined, which may be due to the fracture of some disulfide bonds or / and the efficient recombination of adjacent broken hydrogen bonds, showing excellent elasticity and fatigue resistance; this characteristic helps to delay the performance degradation of the elastomer in practical applications and extend its service life. As Figure 10 shown in a, the optical microscope images of WPU-PD-0.45, WPU-PA-0.45, WPU-SS-0.45, and WPU-PA / SS during the healing process are shown respectively; the scratch morphology of WPU-PA-0.45, WPU-SS-0.45, and WPU-PA / SS changed slightly after 30 min, but obvious visible traces were still left, while the scratch of WPU-PD-0.45 completely healed within 30 min. To further study the self-healing ability of the elastic material at a lower temperature, the samples were placed at 40 °C for 24 h, and the scratch of WPU-PD-0.45 almost healed, as Figure 10 shown in b; the experimental results show that the self-healing process can also occur at 40 °C, but it is slower than that at 80 °C. Therefore, the WPU-PD system has better self-healing performance. As Figure 11As shown, the original green film and the dyed red film were completely cut into two independent parts; then, the two films of different colors were reconnected and healed at 80 °C for 6 h; the repaired sample could easily lift a 2-kg object without tearing, which confirmed the excellent self-healing and mechanical properties of the WPU-PD-0.45 elastomer. The self-healing ability of the WPU-PD-0.45 sample was quantitatively evaluated by static tensile testing; as Figure 12 shown, the mechanical properties became better with the extension of the healing time, and the healed samples showed tensile curves similar to those of the original WPU-PD-0.45; after healing for 2 h, the tensile strength and elongation at break were 13.33 MPa and 601%, respectively, and the healing efficiency was 51.4%; after healing for 4 h, the film could be stretched to 782%, the tensile strength reached 18.61 MPa, and the healing efficiency was 71.8%; after healing for 6 h, it increased to 84.2%, and when continued to extend to 8 h, the tensile strength and elongation at break of WPU-PD-0.45 were 24.28 MPa and 928%, respectively, and the healing efficiency reached 93.7%, and the curve almost overlapped with that of the original sample; indicating that the mechanical properties were effectively restored after healing for 8 h; within a longer healing time, the polymer segments interpenetrated more fully at the fracture interface, thus generating a stronger reconstructed crosslinked network. However, the healing efficiencies of WPU-PA-0.45, WPU-SS-0.45, and WPU-PA / SS under the same conditions were much lower than those of the WPU-PD-0.45 elastomer film ( Figure 13 ). Based on the above experimental results, the WPU-PD-0.45 elastomer film showed the best self-healing performance, which was attributed to the synergistic effect of multiple reversible bonds; the hydrogen bonds formed by acylhydrazone served as the anchor points for physical crosslinking, which not only helped to reduce the scratches caused by physical damage and assisted in crack closure, but also provided a fixed appropriate site for the exchange of adjacent disulfide bonds, increasing the collision probability between free radicals and promoting the decomposition reaction of disulfide bonds. To further study the synergistic effect of disulfide bonds and acylhydrazone, we also compared the stress relaxation behaviors of WPU-PA / SS and WPU-PD-0.45; as Figure 14 shown, although WPU-PA / SS contained both disulfide bonds and acylhydrazone bonds and the molar ratio was the same as that of WPU-PD-0.45, the former still showed a higher Ea (82.39 kJ mol -1) and slow stress relaxation behavior; this may be due to the relatively slow dynamic exchange rate of acylhydrazone bonds, and its slow metathesis reaction may limit the exchange reaction of disulfide bonds, resulting in no synergistic effect being exhibited by the simple mixing of chain extenders; therefore, these research results indicate that PD, as an efficient chain extender, has the lowest activation energy and the fastest chain mobility, which is beneficial to the decomposition and rearrangement of polyurethane segments. Due to the reversible characteristics of the dynamic system, the WPU film has another advantage, that is, it can be processed and recycled by solution casting method, such as Figure 15 shown in a; the WPU-PD-0.45 sample was cut into pieces and dissolved in ethanol, and then the solution was dropped onto a horizontally placed film-forming paper to evaporate the solvent, and the reshaped WPU-PD-0.45 sample was obtained after complete drying; the dissolution-remolding cycle was carried out three times, and the obtained film still showed good transparency; from Figure 15 the tensile test results in b, it can be seen that the stress-strain curve of the recycled film is almost the same as that of the original WPU-PD-0.45, only the mechanical properties decreased slightly; indicating that the WPU-PD-0.45 elastomer film has excellent recyclability, which is mainly due to the linear structure of the molecular chain, and there is only physical crosslinking and no chemical crosslinking in the system, making the chain segments move more freely; this recycling performance helps to improve the resource utilization efficiency, reduce environmental pollution, and achieve sustainability. As Figure 16 shown in a, WPU-0 does not emit light, while the WPU-PD film shows strong green fluorescence under ultraviolet light irradiation, indicating that the prepared WPU-PD has good photoluminescence function. Figure 16 b shows the ultraviolet absorption spectrum of the WPU-PD film; the strong absorption bands at 280 and 290 nm are attributed to the π-π* transitions of benzene rings and imino groups respectively, while the absorption band at 320 nm is caused by the n-π* transition of imino groups in the conjugated system. The WPU-PD film has an emission peak at 475 nm, and the fluorescence intensity increases with the increase of PD content ( Figure 16 c), which can be attributed to the increase in the degree of conjugation in WPU. Due to the diffuse reflection of light by the rough surface, under the irradiation of 365 nm ultraviolet light, the fluorescence at the crack is the strongest on the entire film surface, which provides great convenience for diagnosing the damage location on the surface of polyurethane materials. At the same time, combined with the self-healing ability of WPU-PD, this phenomenon can help to observe the self-healing process ( Figure 17 a). As Figure 17 shown in b ((i) 0 minutes, (ii) 10 minutes, (iii) 20 minutes, (iv) 30 minutes), the cracks in the shape of "HEBUT" cut with a scalpel show relatively strong green fluorescence, and during the crack healing process, the fluorescence intensity gradually decreases until it completely disappears; this self-monitoring and self-repair of cracks can effectively reduce the accumulation and expansion of cracks in polymer materials, and improve their reliability and durability during their service life.

[0091] Example 10:

[0092] The first step is the same as that of Example 1.

[0093] The second step: Inject 5.20 g of polytetramethylene ether glycol and 3.42 g of isophorone diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.27 g of PD, 0.17 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate obtained in Example 1, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 13.5 g of prepolymer with 50 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain WPU-PD-0.6 emulsion.

[0094] The third step: Drop the WPU-PD-0.6 emulsion directly onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain an aqueous polyurethane elastomer film with 0.6 mol of PD as the chain extender.

[0095] The results of the static uniaxial tensile experiment of the WPU-PD-0.6 elastomer film are as Figure 5 shown in a. No yield phenomenon occurred during the stretching process of the film, and there was a typical elastic behavior; the WPU film had high tensile properties, with an elongation at break of 800 - 1400%, and the tensile strength increased rapidly with the increase of PD content ( Figure 5 a). First, the presence of phenyl groups makes the elastomer stronger. Second, the increase in PD content is accompanied by an increase in the hydrogen bond content. The hydrogen bonds between chains form a physical cross-linking network, which further enhances the tensile strength of the elastomer. At the same time, the interaction of multiple hydrogen bonds also promotes the aggregation of intermolecular phases and enhances the degree of microphase separation, resulting in better mechanical properties. As Figure 16 shown in b, the strong absorption bands at 280 and 290 nm are attributed to the π-π* transitions of benzene rings and imino groups respectively, while the absorption band at 320 nm is caused by the n-π* transition of imino groups in the conjugated system. As Figure 16 shown in c, the film has an emission peak at 475 nm.

[0096] Example 11:

[0097] The first step is the same as that of Example 1.

[0098] Step 2: Inject 10.10 g of polyhexyl carbonate diol and 2.52 g of hexamethylene diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.27 g of PD obtained in Example 1, 0.17 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue to react for 5 h. After the reaction temperature drops to 35 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 17.5 g of prepolymer with 60 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PD emulsion.

[0099] Step 3: Drop the WPU-PD emulsion directly onto a glass slide, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 12 h to obtain a recyclable, high self-healing rate, high strength, and high toughness polyurethane elastomer.

[0100] Example 12:

[0101] Step 1: The same as in Example 1.

[0102] Step 2: Inject 10.23 g of polypropylene glycol and 2.49 g of 1,4-cyclohexane diisocyanate into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, then add 0.79 g of 2,2-dimethylolpropionic acid and 3.0 g of N,N-dimethylformamide. After reacting at 80 °C for 1.5 h, add 0.27 g of PD obtained in Example 1, 0.17 g of 1,4-butanediol, and 0.05 g of dibutyltin dilaurate, and heat to 85 °C. Continue to react for 6 h. After the reaction temperature drops to 40 °C, add 0.60 g of triethanolamine to neutralize the mixture for 1 h. Finally, emulsify the obtained 17.6 g of prepolymer with 60 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain the WPU-PD emulsion.

[0103] Step 3: Drop the WPU-PD emulsion directly onto leather, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 h, dehydrate at 70 °C for 15 h to obtain a recyclable, high self-healing rate, high strength, and high toughness polyurethane elastomer.

[0104] Example 13:

[0105] Step 1: The same as in Example 1.

[0106] In the second step, 5.20 g of polybutylene adipate diol and 2.61 g of 2,4-xylene diisocyanate were injected into a three-necked flask equipped with a nitrogen inlet. Under a nitrogen atmosphere and a stirring speed of 50 rmp, 0.79 g of 2,2-dimethylolpropionic acid and 4.0 g of N,N-dimethylformamide were then added. After reacting at 80 °C for 1.5 h, 0.27 g of PD, 0.17 g of 1,4-butanediol, and 0.05 g of stannous octoate obtained in Example 1 were added, and the temperature was heated to 85 °C and the reaction continued for 5 h. After the reaction temperature dropped to 40 °C, 0.60 g of triethanolamine was added to neutralize the mixture for 1 h. Finally, the obtained 13.69 g of prepolymer was emulsified with 40 mL of deionized water under high-speed shear dispersion at 1500 rpm for 1.5 h to obtain a WPU-PD emulsion.

[0107] In the third step, the WPU-PD emulsion was directly dropped onto tinplate, and the coating thickness was 40 - 60 μm. After drying at room temperature for 24 h, it was dehydrated at 75 °C for 12 h to obtain a recyclable, high self-healing rate, high strength, and high toughness polyurethane elastomer.

[0108] In summary, we successfully synthesized a Schiff base chain extender containing double covalent dynamic bonds and introduced it into the WPU chain. Compared with other reported self-healing polyurethanes, WPU-PD-0.45 has excellent mechanical properties, such as a tensile strength of 25.91 MPa, an elongation at break of 936%, and a fracture energy of 121.66 kJ m -2 Due to the presence of dynamic bonds, the WPU-PD-0.45 elastomer film has good self-healing performance, where scratches can be eliminated within 30 min, and the completely cut elastomer reaches a healing efficiency of 93.7% within 8 h at 80 °C. The results of stress relaxation studies show that compared with WPU with one type of dynamic covalent bond or a mixture of two types of dynamic covalent bonds, the synergistic effect of multiple dynamic bonds in WPU-PD gives it the lowest activation energy and the fastest chain mobility, thus promoting the flow of molecular chains and enabling the elastomer to have a fast and efficient self-healing ability. In addition, due to the inherent fluorescence characteristics of WPU-PD, we can clearly observe the damage location of the elastomer and track its self-healing process under ultraviolet light, thereby improving the reliability and durability of the elastomer. This work provides guidance for the preparation of tough, self-healing, recyclable, and functional polyurethane elastomers.

[0109] Matters not covered in this invention are well-known technologies.

Claims

1. A preparation method of a polyurethane elastomer, characterized in that the method comprises the following steps: In the first step, 3,3'-dithiobis(propionyl hydrazide) (DPH) is dissolved in ethanol and then dropped into compound A; the resulting mixture is stirred and refluxed for 2 - 7 h, washed with ethanol, and dried under reduced pressure at 30 - 100 °C to obtain a white powder, which is a Schiff base PD chain extender containing disulfide bonds and hydrazone bonds; Among them, The mass ratio of the described DPH to Compound A is 1:0.3 - 1; 1 - 5 g of DPH is added to every 10 - 50 mL of ethanol; In the second step, Compound B and Compound C are injected into the reactor, then 2,2 - dimethylolpropionic acid and Compound D are added, and the reaction occurs for 1 - 4 h under a nitrogen atmosphere. Then, a PD chain extender, Compound E, and a catalyst are added, and the temperature is heated to 60 - 90 °C and the reaction continues for 3 - 8 h. After the reaction temperature drops to 20 - 50 °C, triethylamine is added and the reaction continues for 0.5 - 3 h. Finally, the obtained prepolymer is emulsified with deionized water for 1 - 4 h to obtain the WPU - PD emulsion; Among them, the mass ratio of Compound B, Compound C, 2,2 - dimethylolpropionic acid, Compound D, the PD chain extender, Compound E, the catalyst, and triethylamine is 1:0.2 - 1:0.1 - 0.5:0.3 - 2:0.01 - 0.09:0.01 - 0.09:0.006 - 0.015:0.05 - 0.2; the mass ratio of the prepolymer to deionized water is 1:2 - 6; The described Compound D is a solvent; In the third step, the WPU - PD emulsion is directly dropped onto the substrate, with a coating thickness of 40 - 60 μm. After drying at room temperature for 24 - 72 h, dehydration is carried out at 40 - 90 °C for 8 - 18 h to obtain the polyurethane elastomer; The Compound A described in the first step is one or more of salicylaldehyde, 5 - methylsalicylaldehyde, 3 - methoxysalicylaldehyde, and 3 - ethoxy - o - hydroxybenzaldehyde; The Compound B described in the second step is one or more of polypropylene glycol, polytetramethylene ether glycol, neopentyl glycol adipate, and polyhexyl carbonate diol; The Compound C is one or more of toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, 1,5 - naphthalene diisocyanate, dimethylbiphenyl diisocyanate, phthalic diisocyanate, tetramethylbenzene diisocyanate, and norbornane diisocyanate; The Compound D is one or more of tetrahydrofuran, N - methylpyrrolidone, N,N - dimethylformamide, N,N - dimethylacetamide, and dimethyl sulfoxide; The Compound E is ethylene glycol, propylene glycol, 1,4 - butanediol, 1,2 - propanediol, or hexanediol; The catalyst is one or more of stannous octoate, dibutyltin dilaurate, zinc naphthenate, and tetra - isobutyl titanate.

2. The preparation method of the polyurethane elastomer according to claim 1, characterized in that the substrate in the third step is one of a glass sheet, tinplate, leather or fabric.

3. The preparation method of the polyurethane elastomer according to claim 1, characterized in that the dispersion in the second step is carried out by high-speed shearing at 1000 - 2000 rpm.

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