Four-arm cross-linked polyurethane elastomer and preparation method thereof

Abstract

The invention relates to a four-arm cross-linked polyurethane elastomer and a preparation method thereof, and belongs to the technical field of polyurethane. The polyurethane elastomer is a four-arm cross-linked polyurethane elastomer which is obtained by curing and molding ascorbic acid, polycaprolactone glycol, diphenylmethane diisocyanate and a catalyst and has an enol type carbamate bond, wherein the four-arm cross-linked polyurethane elastomer is obtained by curing and molding ascorbic acid, polycaprolactone glycol, diphenylmethane diisocyanate and a catalyst; the polyurethane elastomer has high thermal stability, meanwhile, due to the highly regular structures of the hard chain segment and the soft chain segment, the elastomer has an obvious strain induced crystallization effect during stretching, self-reinforcement and self-toughening of the material are achieved, and the polyurethane elastomer has the characteristics of high strength and high elongation at break.

Description

Technical Field

[0001] This invention relates to a four-arm cross-linked polyurethane elastomer and its preparation method, belonging to the field of polyurethane technology. Background Technology

[0002] Polyurethane elastomers (PUEs) are macromolecular polymers obtained by exothermic reactions of polyols and polyisocyanates to form urethane (-NH-COO-) groups in the molecular chain. Due to their strong structural designability, diverse preparation methods, and excellent overall performance, they have shown significant application value in aerospace, automotive, and high-temperature sealing industries. Their unique alternating soft and hard segment structure forms a microphase separation system through hydrogen bonding of urethane groups. The soft segments impart flexibility and deformation recovery, while the hard segments provide mechanical strength and stiffness. However, as industrial technology expands into extreme environments (such as aerospace thermal protection systems and new energy vehicle power components), the high-temperature stability of materials has become a critical performance indicator. Polyurethane elastomers, being inherently not heat-resistant, thus limit their further applications. The temperature limit of traditional polyurethane elastomers is typically below 250°C. At high temperatures, phenomena such as soft segment melting and hard segment dissociation easily occur, leading to a sharp drop in material strength, accelerated creep, and increased permanent deformation. Especially under dynamic load conditions, the molecular chain breakage caused by thermo-oxidative aging will accelerate material failure, which seriously restricts its application expansion in the field of high-temperature engineering.

[0003] Current research on improving the temperature resistance of polyurethane mainly focuses on introducing rigid aromatic ring structures, increasing crosslinking density, or adding inorganic fillers. However, these methods often lead to a significant decrease in material toughness. Excessive crosslinking can inhibit molecular chain segment movement, increasing the brittleness of the elastomer. Although the introduction of inorganic fillers can improve thermal stability, it is prone to stress concentration and poses a risk of interfacial delamination during dynamic use.

[0004] How to achieve a synergistic improvement in temperature resistance while maintaining the high strength and toughness of materials has become a key technical challenge that urgently needs to be solved in the field of polyurethane materials. Ascorbic acid is a common organic compound, previously used primarily in the biomedical field. Due to the influence of the carbon-carbon double bond on its ring, its four internal hydroxyl groups are divided into two types: enol hydroxyl groups attached to the ring and ordinary hydroxyl groups on the aliphatic ring. Based on this, this invention utilizes the two different properties of hydroxyl groups and a unique polymerization method to design a polyurethane elastomer with high strength, toughness, and high temperature resistance. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a four-arm crosslinked polyurethane elastomer and its preparation method. This polyurethane elastomer has unique enol urethane bonds within its structure. These dynamic chemical bonds maintain a stable crosslinking density within the polymer system, resulting in high thermal stability. Polycaprolactone diol (PCL) and diphenylmethane diisocyanate (MDI) have highly regular and symmetrical structures. When subjected to stress orientation, they change from yellow to white, exhibiting a significant strain-induced crystallization effect, achieving self-reinforcement and self-toughening of the elastomer system, and possessing extremely high true stress.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows.

[0007] A four-arm cross-linked polyurethane elastomer, wherein the polyurethane elastomer is a four-arm cross-linked polyurethane elastomer with enol urethane bonds obtained by curing and molding ascorbic acid (AA), polycaprolactone diol (PCL), diphenylmethane diisocyanate (MDI) and a catalyst.

[0008] Based on the mass of ascorbic acid (AA), polycaprolactone diol (PCL), diphenylmethane diisocyanate (MDI), and catalyst being 100%, the percentage content of each component is as follows:

[0009] Polycaprolactone diol (PCL) 60%–73%;

[0010] Diphenylmethane diisocyanate (MDI) 22%–32%;

[0011] Ascorbic acid 4.5%–8%;

[0012] Catalyst 0.1%–0.2%.

[0013] Preferably, the weight-average molecular weight of the polycaprolactone diol (PCL) is 1900–2000 g / mol.

[0014] Preferably, the molar ratio of -OH in polycaprolactone diol (PCL) to -OH in ascorbic acid is 1:1.5 to 3.

[0015] Preferably, the catalyst is dibutyltin dilaurate (DBTDL).

[0016] A method for preparing the four-arm crosslinked polyurethane elastomer of the present invention includes the following steps:

[0017] (1) Dehydration treatment of polycaprolactone diol;

[0018] (2) The dehydrated polycaprolactone diol, diphenylmethane diisocyanate and catalyst are mixed and stirred at 45-50°C to produce an isocyanate-terminated prepolymer.

[0019] (3) Dissolve ascorbic acid (AA) in N,N-dimethylformamide (DMF) by ultrasonication to obtain an ascorbic acid solution; mix the ascorbic acid solution with the prepolymer and stir at 20-40°C for 5-10 min. After the reaction is completed, a mixture is obtained.

[0020] (4) The mixture is poured into a mold, vacuum degassing is performed, and then cured and molded. After curing, it is cooled to obtain a four-arm cross-linked polyurethane elastomer.

[0021] Preferably, in step (2), the stirring rate is 220-300 r / min and the stirring time is 1-2 h.

[0022] Preferably, in step (3), the ratio of N,N-dimethylformamide (DMF) to polycaprolactone diol (PCL) in step (2) is 40 mL: 10-20 g.

[0023] Preferably, in step (3), the stirring rate is 220-300 r / min.

[0024] Preferably, in step (4), the vacuum degree is less than 0.08 MPa and the degassing time is 5 to 10 minutes during vacuum degassing.

[0025] Preferably, in step (4), the curing temperature is 80-90℃ and the curing time is 3-5 days.

[0026] Beneficial effects

[0027] This invention provides a four-arm crosslinked polyurethane elastomer. This elastomer uses ascorbic acid as a chain extender to extend the chain of a polyurethane prepolymer solution, achieving a unique four-arm crosslinking mode centered on a five-membered ring. The resulting polymer has a stable crosslinking system. Two of the four hydroxyl groups in ascorbic acid are unique enol hydroxyl groups, forming enol urethane bonds with dynamic characteristics not found in conventional urethane bonds. These bonds can be reconstructed under external stimuli (stress, temperature), thus maintaining a stable crosslinking density. Based on this characteristic, this four-arm crosslinked polyurethane elastomer exhibits high thermal stability. Furthermore, due to the highly regular structure of the hard and soft segments, the elastomer exhibits a significant strain-induced crystallization effect during stretching, achieving self-reinforcement and self-toughening of the material, resulting in high strength and high elongation at break.

[0028] This invention provides a method for preparing a four-arm crosslinked polyurethane elastomer. Polycaprolactone diol (PCL) is used as the soft segment, and diphenylmethane diisocyanate (MDI) and ascorbic acid (AA) are used as the compounded hard segments. Solution polymerization is employed, with DMF as the solvent to reduce the viscosity of the reaction system. First, PCL, MDI, and a catalyst are mixed, and the polymerization reaction yields an isocyanate-terminated polyurethane prepolymer. The polymerization temperature is controlled to avoid the problem of rapid polymerization of ascorbic acid. After the temperature decreases, it is mixed with the DMF solution of AA and cured to form the final product. Attached Figure Description

[0029] Figure 1 The image shows the infrared spectrum of the polyurethane described in Comparative Example 1.

[0030] Figure 2 The infrared spectrum of the four-arm cross-linked polyurethane elastomer described in Example 1 is shown.

[0031] Figure 3 Tensile strength test diagram of the four-arm cross-linked polyurethane elastomer described in Comparative Example and Example 1.

[0032] Figure 4 This is a diagram showing the strain-induced crystallization phenomenon during the uniaxial tensile test of the four-arm cross-linked polyurethane elastomer described in Example 1.

[0033] Figure 5 The thermogravimetric curves of the four-arm cross-linked polyurethane elastomers described in Comparative Example 1 and Example 1 are shown in comparison. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments.

[0035] Example 1

[0036] The molar ratio of -OH in PCL to -OH in ascorbic acid is controlled to be 1:1.5.

[0037] A method for preparing a four-arm cross-linked polyurethane elastomer, comprising the following steps:

[0038] 1. Raw material dehydration: Water in polycaprolactone diol (PCL) reacts with isocyanate. To eliminate the interference of water, it is dehydrated. Melt PCL at 80℃, weigh 100g and pour it into a 250ml three-necked flask. Add a magnetic rotor, set the oil bath temperature to 105℃, the rotation speed to 250r / min, and connect a vacuum pump for dehydration for 2 hours, controlling the vacuum degree to be less than 0.08MPa. After dehydration, pour the dehydrated material into a tetrafluoroethylene bottle for later use.

[0039] 2. Preparation of the prepolymer: MDI was melted in an oven at 60℃. 8.40 mmol (16.80 g) of PCL and 21.00 mmol (5.26 g) of MDI were weighed, dissolved in an appropriate amount of DMF, and added to a 250 ml three-necked flask. 0.02 g of DBTDL was added dropwise as a catalyst. The temperature was set at 50℃, the rotation speed at 250 r / min, and the reaction was carried out for 1.5 hours. An isocyanate-terminated polyurethane prepolymer was obtained.

[0040] 3. Chain extension: Weigh 6.30 mmol (1.11 g) of ascorbic acid powder into a glass beaker, add an appropriate amount of DMF and dissolve by sonication. After the ascorbic acid is completely dissolved, drop the solution into the above prepolymer solution. Set the polymerization temperature to 40℃, the speed to 250 r / min, and stir for 5 minutes to obtain the final reaction product.

[0041] 4. Defoaming: Pour the above reaction product into a silicone mold, place it in a vacuum cylinder, control the vacuum degree to be less than 0.08MPa, and defoam for 10 minutes.

[0042] 5. Curing: Place the defoamed product into a forced-air drying oven, set the temperature to 80℃, evaporate the residual solvent in the oven and achieve further reaction, and cure for 3 days.

[0043] Example 2

[0044] The molar ratio of -OH in PCL to -OH in ascorbic acid is controlled to be 1:2.

[0045] In this embodiment, 6.65 mmol (13.30 g) of PCL, 21 mmol (5.26 g) of MDI, and 0.02 g of DBTDL were reacted to obtain the prepolymer. For the chain extension portion, 6.65 mmol (1.17 g) of ascorbic acid was dissolved in an appropriate amount of DMF and mixed with the above prepolymer. The remaining conditions were the same as in Example 1.

[0046] Example 3

[0047] The molar ratio of -OH in PCL to -OH in ascorbic acid is controlled to be 1:3.

[0048] In the preparation of the prepolymer in this embodiment, 5.00 mmol (10.00 g) of PCL, 21 mmol (5.26 g) of MDI, and 0.02 g of DBTDL were reacted to obtain the prepolymer. The chain-extended portion was prepared by dissolving 7.5 mmol (1.32 g) of ascorbic acid in an appropriate amount of DMF and mixing it with the prepolymer. The remaining conditions were the same as in Example 1.

[0049] Comparative Example

[0050] Chain extension was performed using 1,4-butanediol (BDO), a common chain extender.

[0051] 1. Raw material dehydration: Water in polycaprolactone diol (PCL) reacts with isocyanate. To eliminate the interference of water, it is dehydrated. Melt PCL at 80℃, weigh 100.00g and pour it into a 250ml three-necked flask. Add a magnetic rotor, set the oil bath temperature to 105℃, the rotation speed to 250r / min, and connect a vacuum pump for dehydration for 2 hours, controlling the vacuum degree to be less than 0.08MPa. After dehydration, pour the dehydrated material into a tetrafluoroethylene bottle for later use.

[0052] 2. Preparation of the prepolymer: MDI was melted in an oven at 60℃. 9.00 mmol (18.00 g) of PCL and 18.90 mmol (4.73 g) of MDI were weighed, dissolved in an appropriate amount of DMF, and added to a 250 ml three-necked flask. 0.06 g of DBTDL was added dropwise as a catalyst. The temperature was set at 50℃, the rotation speed at 250 r / min, and the reaction was carried out for 1.5 hours. An isocyanate-terminated polyurethane prepolymer was obtained.

[0053] 3. Chain extension: Weigh 9.00 mmol (0.81 g) of BDO into a glass beaker, add an appropriate amount of DMF to dissolve it, and after the BDO is completely dissolved, drop the solution into the above prepolymer. Set the polymerization temperature to 75℃, the rotation speed to 250 r / min, and react for 4 hours to obtain the final product.

[0054] 4. Defoaming: Pour the above reaction product into a silicone mold, place it in a vacuum cylinder, control the vacuum degree to be less than 0.08MPa, and defoam for 10 minutes.

[0055] 5. Curing: Place the defoamed product into a forced-air drying oven, set the temperature to 80℃, evaporate the residual solvent in the oven and achieve further reaction, and cure for 3 days.

[0056] The tensile strengths of the three polyurethane elastomers prepared in Examples 1-3 were 46.6 MPa, 45.1 MPa, and 44.6 MPa, respectively.

[0057] The product obtained in Comparative Example 1 was a transparent film. A small sample was cut for infrared spectroscopy testing, and the results are as follows. Figure 1 As shown.

[0058] The product obtained in Example 1 was a brownish-red four-arm cross-linked polyurethane elastomer, due to the partial conversion of ascorbic acid into brownish-red dehydroascorbic acid. Its infrared spectral results are as follows: Figure 2 As shown. 2271cm -1 The absence of a characteristic absorption peak at 3327 cm⁻¹ indicates that -NCO has reacted completely; -1 and 1155cm -1 The peaks that appear correspond to the stretching vibration peaks of NH in the urethane group and the stretching vibration peaks of COC. The disappearance and appearance of these peaks indicate that the polyurethane was successfully prepared.

[0059] Figure 3 The figures show the tensile strength test results of the four-arm crosslinked polyurethane elastomers obtained in Comparative Example 1 and Example 1. In Comparative Example 1, BDO, as a difunctional chain extender, is unable to support the polymer network structure, resulting in a tensile strength of only 7.35 MPa, which severely limits its application scenarios. In contrast, Example 1 achieved a tensile strength of 46.6 MPa, corresponding to an elongation at break of 1386.6%, comparable to the toughest spider silk in nature, demonstrating a significant advantage over the former.

[0060] Figure 4 The image shows the strain-induced crystallization phenomenon during the stretching process of the four-arm cross-linked polyurethane elastomer obtained in Example 1. The dumbbell-shaped sample radiates upwards and downwards from its center, exhibiting a noticeable whitening phenomenon.

[0061] Figure 5 The thermogravimetric curves of the four-arm cross-linked polyurethane elastomers obtained in Comparative Example 1 and Example 1 are shown in the figure. In the comparative example, the polyurethane elastomer prepared with BDO as a chain extender began to lose weight at 230°C, while the 5% weight loss temperature of the four-arm cross-linked polyurethane elastomer reached 322°C, and the weight was still maintained at more than 80% at 365°C, showing excellent high-temperature resistance.

[0062] In summary, the invention includes, but is not limited to, the above embodiments. Any equivalent substitutions or partial improvements made under the spirit and principles of this invention shall be considered to be within the protection scope of this invention.

Claims

1. A four-arm cross-linked polyurethane elastomer, characterized in that: This polyurethane elastomer is a four-arm cross-linked polyurethane elastomer with enol urethane bonds, obtained by curing and molding ascorbic acid, polycaprolactone diol, diphenylmethane diisocyanate and catalyst. Based on the mass of ascorbic acid, polycaprolactone diol, diphenylmethane diisocyanate, and catalyst being 100%, the percentage content of each component is as follows:

2. The four-arm cross-linked polyurethane elastomer as described in claim 1, characterized in that: The weight-average molecular weight of the polycaprolactone diol is 1900–2000 g / mol.

3. The four-arm cross-linked polyurethane elastomer as described in claim 1, characterized in that: The molar ratio of -OH in polycaprolactone diol to -OH in ascorbic acid is 1:1.5 to 3.

4. A four-arm cross-linked polyurethane elastomer as described in claim 1, 2, or 3, characterized in that: The catalyst is dibutyltin dilaurate.

5. A method for preparing a four-arm crosslinked polyurethane elastomer according to any one of claims 1 to 4, characterized in that: The method steps include: (1) Dehydration treatment of polycaprolactone diol; (2) The dehydrated polycaprolactone diol, diphenylmethane diisocyanate and catalyst are mixed and stirred at 45-50°C to produce an isocyanate-terminated prepolymer. (3) Dissolve ascorbic acid in N,N-dimethylformamide by ultrasonication to obtain an ascorbic acid solution; mix the ascorbic acid solution with the prepolymer and stir at 20-40°C for 5-10 min. After the reaction is completed, a mixture is obtained. (4) The mixture is poured into a mold, vacuum degassing is performed, and then cured and molded. After curing, it is cooled to obtain a four-arm cross-linked polyurethane elastomer.

6. The method for preparing a four-arm cross-linked polyurethane elastomer as described in claim 5, characterized in that: In step (2), the stirring rate is 220-300 r / min and the stirring time is 1-2 h.

7. The method for preparing a four-arm cross-linked polyurethane elastomer as described in claim 5, characterized in that: In step (3), the ratio of N,N-dimethylformamide to polycaprolactone diol in step (2) is 40 mL: 10-20 g.

8. The method for preparing a four-arm cross-linked polyurethane elastomer as described in claim 5, characterized in that: In step (3), the stirring rate is 220-300 r / min.

9. The method for preparing a four-arm cross-linked polyurethane elastomer as described in claim 5, characterized in that: In step (4), during vacuum degassing, the vacuum degree is less than 0.08 MPa and the degassing time is 5 to 10 minutes.

10. The method for preparing a four-arm cross-linked polyurethane elastomer as described in claim 5, characterized in that: In step (4), the curing temperature is 80-90℃ and the curing time is 3-5 days.

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