Reprocessable cross-linked polyurethane and preparation method thereof

Abstract

The invention discloses reworkable cross-linked polyurethane and a preparation method thereof, relates to the technical field of polyurethane, and solves the problems of complex preparation process and limited forming efficiency and size of the existing reworkable cross-linked polyurethane. The cross-linked polyurethane is prepared from the following raw materials in parts by weight: 100 parts of an isocyanate-containing prepolymer; 1 to 20 parts of a cross-linking agent; 0.1 to 3 parts of a catalyst; the cross-linking agent is a thiol cross-linking agent, and the catalyst is a tin catalyst or a zinc catalyst. The preparation method comprises the following steps: adding an isocyanate-containing prepolymer and a cross-linking agent into a reactor, putting the reactor into a spin coater, and uniformly mixing; adding a catalyst into the reactor, and continuously mixing in the spin coater; and pouring the mixture into a mold, carrying out compression molding, and demolding to obtain the molded reprocessable cross-linked polyurethane. The preparation process provided by the invention is simple, the prepared cross-linked polyurethane has the characteristics of plasticity, repairability, recoverability and the like, and the contradiction between high mechanical strength and repeated processing performance is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of polyurethanes, and particularly relates to a reprocessable crosslinked polyurethane and a preparation method thereof. Background Art

[0002] Crosslinked polyurethanes are favored in the industrial field due to their better solvent resistance, shear resistance, and resistance to permanent deformation. However, the nature of traditional permanent crosslinked networks results in high requirements for material forming and non-reprocessability, greatly increasing the cost of crosslinked polyurethanes. Therefore, crosslinked polyurethanes with reversible dynamic covalent bonds have been developed. Commonly introduced dynamic covalent bonds include disulfide bonds (CN202310419866.5), borate ester bonds (CN202310313114.0), Diels-Alder reactions (CN201310006811.8), etc. For example, Chinese Patent Document CN116444768A (publication date: July 18, 2023) discloses a highly efficient damage self-healing and degradable dual-dynamic crosslinked Vitrimer resin, which solves the defect of non-repairable damage of traditional epoxy resins by introducing disulfide bonds; Chinese Patent Document CN116396463A (publication date: July 7, 2023) discloses a reprocessable high-toughness crosslinked polyurethane and a preparation method thereof, which realizes the reprocessing of polyurethane at room temperature by introducing borate ester bonds; Chinese Patent Document CN116444768A (publication date: April 24, 2013) discloses a novel reversible covalently crosslinked thermoplastic polyurethane. A dienophile crosslinking agent is mixed into a polyurethane material containing a novel chain extender or a mixed chain extender with a dienophile, and a Diels-Alder reaction is initiated to crosslink the originally thermoplastic polyurethane, and this crosslinked covalent bond can reverse the reaction within a specific temperature range, enabling the material to be repeatedly processed and used. However, the construction of these dynamic bonds requires the introduction of additional molecular structures, and the preparation process is complex.

[0003] Chinese Patent Document CN116875040A (publication date: October 13, 2023) discloses a reprocessable high-strength biodegradable elastomeric material, and Chinese Patent Document CN112358596A (publication date: February 12, 2021) discloses a cardanol-based shape memory polymer and a preparation method thereof. Both achieve dynamic bonding of urethane bonds by adding an additional catalyst. Crosslinked polyurethanes with urethane bonds as dynamic bonds require a large amount of catalyst (>1 wt%) during the preparation process, which makes the reaction rate too fast during the material forming process. Therefore, a diluent often needs to be added during the preparation process to reduce the reaction rate. Especially for polyurethanes containing aromatic isocyanates, a diluent must be added. However, the diluent needs to be completely discharged from the material after forming, which greatly limits the forming efficiency and forming size of the material. Summary of the Invention

[0004] In order to solve the problems of complex preparation process, limited forming efficiency and size of the existing reprocessable crosslinked polyurethane, the present invention proposes a reprocessable crosslinked polyurethane and its preparation method.

[0005] The technical solution of the present invention is as follows:

[0006] A reprocessable crosslinked polyurethane is prepared from the following raw materials in parts by weight:

[0007] Isocyanate prepolymer, 100 parts; crosslinking agent, 1 part to 20 parts; catalyst, 0.1 part to 3 parts;

[0008] The crosslinking agent is a thiol crosslinking agent, and the catalyst is a tin catalyst.

[0009] Preferably, the thiol crosslinking agent is one or a mixture of at least two of trimethylolpropane tris(3-mercaptopropionate), trimethylolpropane tris(2-mercaptoacetate), pentaerythritol tetra(3-mercaptopropionate), and pentaerythritol tetra(2-mercaptoacetate).

[0010] Preferably, the catalyst is one or a mixture of at least two of dibutyltin dilaurate, stannous octoate, and stannous chloride.

[0011] Preferably, the isocyanate prepolymer is prepared by the following process:

[0012] Add the macromolecular diol into the reaction kettle, vacuum dehydrate at 110°C to 140°C for 1 h to 3 h, then pass nitrogen to restore normal pressure, and cool down to 60°C to 80°C;

[0013] Add the aromatic isocyanate into the reaction kettle and stir to completely react to obtain the isocyanate prepolymer;

[0014] Among them, the mass ratio of the macromolecular diol to the aromatic isocyanate is 100:1 to 50.

[0015] Preferably, the macromolecular diol is one or a mixture of at least two of polyethylene oxide, polypropylene oxide, polytetrahydrofuran, hydroxyl-terminated polylactic acid, and hydroxyl-terminated polyester.

[0016] Preferably, the aromatic isocyanate is one or a mixture of at least two of toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate (PPDI), and naphthalene diisocyanate (NDI).

[0017] The present invention also provides a preparation method of the above-mentioned reprocessable crosslinked polyurethane, including the following steps:

[0018] Add the isocyanate prepolymer and the crosslinking agent into a reactor, and place the reactor in a spin coater to mix evenly.

[0019] Add a catalyst into the reactor, and continue to mix in the spin coater.

[0020] Pour the mixture into a mold, demold after molding by compression, and obtain the formed reprocessable crosslinked polyurethane.

[0021] Preferably, in step S1, the rotation speed of the spin coater is 1800 rpm to 2100 rpm, and the mixing time is 2.5 min to 3 min.

[0022] Preferably, in step S2, the rotation speed of the spin coater is 1900 rpm to 2200 rpm, and the mixing time is 40 s to 50 s.

[0023] Preferably, in step S3, the temperature of the mold is 130 °C to 160 °C; the pressure for molding by compression is 9 MPa to 12 MPa, and the time is 0.8 h to 1 h.

[0024] Compared with the prior art, the specific beneficial effects of the present invention are as follows:

[0025] 1. By utilizing the difference in the catalytic efficiency of tin-based catalysts for different reactive groups and the poor catalytic efficiency of isocyanates and mercapto groups, a thiol crosslinking agent is used instead of the commonly used alcohol crosslinking agent, reducing the reaction rate during the chain extension of the aromatic reprocessable crosslinked polyurethane prepolymer, thus avoiding the use of diluents, reducing the energy consumption and cost of material molding, improving the molding efficiency of the material, and enabling more free selection of the material molding size.

[0026] 2. The preparation process provided by the present invention is simple, and the prepared crosslinked polyurethane has characteristics such as plasticity, reparability, and recyclability, solving the contradiction between high mechanical strength and repeated processing performance. Detailed Embodiments

[0027] To make the technical solutions of the present invention clearer, the technical solutions in the embodiments of the present invention will be described clearly and completely below. It should be noted that the following embodiments are only used to better understand the technical solutions of the present invention and should not be construed as a limitation to the present invention.

[0028] Example 1.

[0029] (1) Add 100 parts of polytetrahydrofuran into a reaction kettle, remove water under vacuum at 130 °C for 2 h, then introduce nitrogen, restore normal pressure and cool down to 70 °C; add 25 parts of diphenylmethane diisocyanate into the reaction kettle and stir until the NCO value obtained according to the hydrochloric acid - dibutylamine titration method is the same as the theoretical value, proving that the reaction is complete, and obtain the isocyanate prepolymer.

[0030] (2) Add 100 parts of the isocyanate prepolymer and 10 parts of pentaerythritol tetrakis(3-mercaptopropionate) to a cylindrical reactor. Place the cylindrical reactor in a spin coater and mix at a rate of 2000 rpm for 3 min. Subsequently, continue to add 3 parts of dibutyltin dilaurate to the cylindrical reactor and continue to mix in the spin coater at a rate of 2000 rpm for 45 s. Pour the mixture into a mold at 130 °C, mold it under a pressure of 10 MPa for 1 h, and then demold to obtain the molded polyurethane material.

[0031] Example 2.

[0032] In this example, 4 parts of dibutyltin dilaurate are used in step (2), and the remaining preparation method is the same as that of Example 1.

[0033] Example 3.

[0034] In this example, the temperature of the mold into which the mixture is poured in step (2) is 160 °C, and the remaining preparation method is the same as that of Example 1.

[0035] Example 4.

[0036] In this example, 4 parts of dibutyltin dilaurate are used in step (2), and the temperature of the mold into which the mixture is poured is 160 °C, and the remaining preparation method is the same as that of Example 1.

[0037] Comparative Example 1.

[0038] (1) Add 100 parts of polytetrahydrofuran to a reaction kettle, remove water under vacuum at 130 °C for 2 h, then introduce nitrogen, restore to normal pressure and cool to 70 °C. Add 25 parts of diphenylmethane diisocyanate to the reaction kettle and stir until the NCO value obtained according to the hydrochloric acid-dibutylamine titration method is the same as the theoretical value, proving that the reaction is complete, and obtain the isocyanate prepolymer.

[0039] (2) Add 100 parts of the isocyanate prepolymer and 2 parts of glycerol to a cylindrical reactor. Place the cylindrical reactor in a spin coater and mix at a rate of 2000 rpm for 3 min. Subsequently, continue to add 0.1 part of dibutyltin dilaurate to the cylindrical reactor and continue to mix in the spin coater at a rate of 2000 rpm for 45 s. The polymer cures in the cylindrical reactor and cannot be poured into the mold subsequently.

[0040] Comparative Example 2.

[0041] (1) Add 100 parts of polytetrahydrofuran to a reaction kettle, remove water under vacuum at 130 °C for 2 h, then introduce nitrogen, restore to normal pressure and cool to 70 °C. Add 25 parts of diphenylmethane diisocyanate to the reaction kettle and stir until the NCO value obtained according to the hydrochloric acid-dibutylamine titration method is the same as the theoretical value, proving that the reaction is complete, and obtain the isocyanate prepolymer.

[0042] (2) Add 100 parts of the isocyanate prepolymer and 10 parts of pentaerythritol tetrakis(3-mercaptopropionate) to a cylindrical reactor. Place the cylindrical reactor in a spin coater and mix at a rate of 2000 rpm for 3 min. Subsequently, continue to add 0.1 part of dibutyltin dilaurate to the cylindrical reactor and continue to mix in the spin coater at a rate of 2000 rpm for 45 s. Then pour the mixture into a mold at 130 °C, mold it under a pressure of 10 MPa for 1 h and then demold to obtain the molded polyurethane material.

[0043] Comparative Example 3.

[0044] The preparation method is the same as that of Comparative Example 2, except that the temperature of the mold poured in step (2) is 160 °C.

[0045] Effect Example 1.

[0046] According to the above experiments, the polyurethane with glycerol as the cross-linking agent cannot be synthesized by the bulk method, and the polyurethane with pentaerythritol tetrakis(3-mercaptopropionate) as the cross-linking agent can be synthesized by the bulk method.

[0047] Test the processing performance of the materials in each example and comparative example:

[0048] Cut the material into a cuboid of 1 mm×2 mm×30 mm. Use a dynamic thermomechanical analysis device to heat the material to 160 °C and keep it constant for 5 min. Then stretch the material by 5% strain and monitor the change of the material stress with time. The time when the material stress drops to 1 / e of the initial stress is defined as τ, which is used to characterize the processing performance of the material. The lower τ is, the easier it is to process.

[0049] The τ values in Examples 1-4 are measured to be 41.5 s, 46.5 s, 22.5 s, and 15.5 s respectively, and the τ values of the materials in Comparative Examples 2 and 3 are 1178.5 s and 532.5 s respectively.

[0050] Effect Example 2.

[0051] Test the tensile strength of the materials in each example and comparative example according to GB / T528-2009. The tensile strengths of the materials prepared in Examples 1-4 are 4.19±0.62 MPa, 4.90±0.86 MPa, 11.32±0.67 MPa, and 10.60±1.64 MPa respectively. The tensile strengths of the materials prepared in Comparative Examples 2 and 3 are 2.74±0.37 MPa and 3.60±0.27 MPa respectively.

[0052] It can be determined from the above test results that as the catalyst content increases to more than 1 part, the material has processability, and at the same time, the material strength increases with the increase of the preparation temperature and the catalyst content. When the preparation temperature is increased to 160 °C and the catalyst content is increased to 4 parts, the material strength no longer increases.

Claims

1. A reprocessable crosslinked polyurethane, characterized in that, It is prepared from raw materials including the following parts by weight: Isocyanate prepolymer, 100 parts; crosslinking agent, 1 part to 20 parts; catalyst, 0.1 part to 3 parts; The crosslinking agent is a thiol crosslinking agent, and the catalyst is a tin catalyst.

2. The reprocessable crosslinked polyurethane according to claim 1, wherein The thiol crosslinking agent is one or a mixture of at least two of trimethylolpropane tris(3-mercaptopropionate), trimethylolpropane tris(2-mercaptoacetate), pentaerythritol tetrakis(3-mercaptopropionate), and pentaerythritol tetrakis(2-mercaptoacetate).

3. The reprocessable crosslinked polyurethane according to claim 1, wherein The catalyst is one or a mixture of at least two of dibutyltin dilaurate, stannous octoate, and stannous chloride.

4. The reprocessable crosslinked polyurethane according to claim 1, wherein The isocyanate prepolymer is prepared by the following process: Add macromolecular diol into the reaction kettle, remove water under vacuum, then introduce nitrogen to restore normal pressure, and cool down to 60°C to 80°C; Add aromatic isocyanate into the reaction kettle and stir until the reaction is complete to obtain the isocyanate prepolymer; The mass ratio of the macromolecular diol to the aromatic isocyanate is 100:1 to 50.

5. The reprocessable crosslinked polyurethane according to claim 4, wherein The macromolecular diol is one or a mixture of at least two of polyethylene oxide, polypropylene oxide, polytetrahydrofuran, hydroxyl-terminated polylactic acid, and hydroxyl-terminated polyester.

6. The reprocessable crosslinked polyurethane according to claim 4, wherein The aromatic isocyanate is one or a mixture of at least two of toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate (PPDI), and naphthalene diisocyanate (NDI).

7. A method for preparing a reprocessable crosslinked polyurethane according to any one of claims 1 to 6, characterized in that, It includes the following steps: Add the isocyanate prepolymer and the crosslinking agent into the reactor, and place the reactor in a spin coater to mix evenly; Add the catalyst into the reactor and continue to mix in the spin coater; Pour the mixture into a mold, demold after molding under pressure to obtain the molded reprocessable crosslinked polyurethane.

8. The preparation method of the reprocessable crosslinked polyurethane according to claim 7, characterized in that, In step S1, the rotation speed of the spin coater is 1500 rpm to 2500 rpm, and the mixing time is 2 min to 4 min.

9. The preparation method of the reprocessable crosslinked polyurethane according to claim 7, characterized in that, In step S2, the rotation speed of the spin coater is 1500 rpm to 3000 rpm, and the mixing time is 25 s to 55 s.

10. The preparation method of the reprocessable crosslinked polyurethane according to claim 7, characterized in that, In step S3, the mold temperature is 130°C to 160°C; the pressure for molding under pressure is 1 MPa to 20 MPa, and the time is 0.5 h to 2 h.