Degradable polyurethane wall material system and preparation method thereof

Abstract

The invention relates to the technical field of material chemical microcapsules, provides a degradable polyurethane wall material system and a preparation method thereof, and provides a water dispersible wall material system, solid components comprise an isocyanate group-terminated degradable polyurethane prepolymer and 2-hydroxyethyl aminated oxidized starch, and the two components form a covalent network through a urea bond and a carbamate bond; 25-45 parts by weight of a system solid component, the viscosity is 200-1500 mPa.s, the volume median particle size D50 is 80-300 nm, the system solid component can be emulsified and cured with a hydrophobic liquid core material to obtain a polyurethane / starch covalent network wall layer microcapsule, and the volume median particle size D50 is 1-20 m; the problem that solid components, viscosity, emulsification and curing are difficult to consider is solved, and the method is used for preparation and application of hydrophobic liquid core material microcapsules.

Description

Technical Field

[0001] This invention relates to the field of materials chemistry microcapsule technology, specifically to a biodegradable polyurethane wall material system and its preparation method. Background Technology

[0002] Microcapsules are typically created by encapsulating a hydrophobic liquid core material within a polyurethane / starch covalent network wall layer through emulsification and curing processes. This process yields dispersible microcapsules in a deionized water system, enabling application adaptation. In this type of process, the water-dispersible wall material system must maintain suitable viscosity under high solids content conditions to support shear dispersion, emulsification, pre-reaction, and curing processes. It must also form a stable covalent network during the curing stage to ensure the construction and maintenance of the wall layer structure. Simultaneously, the molecular structure design of the biodegradable polyurethane wall material system must be considered to match the isocyanate group reactivity and chain segment introduction method with the water dispersion process. This ensures that the volumetric median particle size (D50) is controllable and the overall preparation process is feasible and reproducible.

[0003] However, existing water-dispersible wall material systems often experience increased viscosity with increased solids content, making shearing and emulsification processes more susceptible to limitations. Furthermore, relying solely on the polyurethane system itself can lead to insufficient covalent network construction or uneven reaction site distribution under deionized water conditions, affecting the control of the volumetric median particle size (D50) and the consistency of the microcapsule formation process. Simultaneously, the introduction of biodegradability requirements further amplifies process sensitivity due to the coupling between chain segment design, curing, and solidification windows. For example, Chinese patent CN101503617B discloses an aqueous polyurethane microencapsulated phase change energy storage material and its preparation method, but its technical route does not involve a systematic design of a covalent network formed by isocyanate-terminated biodegradable polyurethane prepolymer and 2-hydroxyethyl-amino-oxidized starch via urea and urethane bonds. Similarly, Chinese patent CN116322327A discloses a method for preparing microcapsules, but a more controllable covalent network construction path is still needed, considering constraints such as isocyanate content, amino content, and residual aldehyde content. Summary of the Invention

[0004] The purpose of this invention is to provide a biodegradable polyurethane wall material system and its preparation method, which solves the problem that it is difficult to balance the solid components, viscosity and emulsification curing process.

[0005] This invention is based on the synergistic reaction site design of isocyanate-terminated biodegradable polyurethane prepolymer and 2-hydroxyethyl amino oxidized starch. In a water-dispersible wall material system, a covalent network is constructed through urea bonds and urethane bonds, so that the dispersion under solid component conditions and the subsequent emulsification and curing process are matched with each other, thereby achieving the controllable construction of the volume median particle size D50 and the wall layer structure and supporting the preparation of microcapsules.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A biodegradable polyurethane wall material system, wherein the wall material system is a water-dispersible wall material system, comprising 25-45 parts by weight of solid components and deionized water as the balance, to a total of 100 parts by weight; wherein the solid components, to a total of 100 parts by weight, comprise 70-85 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 15-30 parts by weight of 2-hydroxyethyl-amino-oxidized starch, the sum of which is 100 parts by weight;

[0008] The isocyanate-terminated biodegradable polyurethane prepolymer is prepared by reacting polycaprolactone diol, 2,2-dimethylolpropionic acid, 2,2'-dithiodiethanol and isophorone diisocyanate, with an isocyanate group content of 2.0-4.5 wt%. The 2-hydroxyethylamino oxidized starch is prepared by reacting starch, sodium periodate, ethanolamine and sodium borohydride, with an amino content of 0.20-0.80 mmol / g and a residual aldehyde group content of no more than 0.10 mmol / g. The isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethylamino oxidized starch form a covalent network in the wall material system through urea bonds and urethane bonds.

[0009] Furthermore, the isocyanate-terminated biodegradable polyurethane prepolymer is prepared through the following steps:

[0010] A1. Mix 100 parts by weight of polycaprolactone diol, 4-12 parts by weight of 2,2-dimethylolpropionic acid and 2-10 parts by weight of 2,2'-dithiodiethanol.

[0011] A2. Treat at 95-115℃ and a vacuum of -0.08 to -0.095 MPa for 1-3 hours;

[0012] A3. Under an inert atmosphere, add isophorone diisocyanate at a molar ratio of isocyanate group to hydroxyl group of 1.20-1.80:1 and react at 70-85℃ for 2-5 hours.

[0013] A4. When the isocyanate group content reaches 2.0-4.5wt%, the reaction is stopped, and the mixture is cooled to 25-40℃ to obtain the isocyanate-terminated biodegradable polyurethane prepolymer.

[0014] Furthermore, the 2-hydroxyethylamino oxidized starch is prepared through the following steps:

[0015] C1. Disperse 100 parts by weight of the oxidized starch intermediate in 300-800 parts by weight of deionized water;

[0016] C2. Add 10-35 parts by weight of ethanolamine to the system, control the pH of the system to 8.5-10.5, and react at 25-45℃ for 2-6 hours;

[0017] C3. Prepare an alkaline aqueous solution of sodium borohydride with a concentration of 1-5 wt% by 1-8 parts by weight, wherein the alkaline aqueous solution is a NaOH solution with a concentration of 0.1-0.5 mol / L and the pH of the solution is controlled at 10-12. Add the solution slowly in batches with an interval of 5-15 min between batches. Maintain the pH of the system at 10-12 during the addition process. Continue the reaction at 20-35℃ for 1-3 h.

[0018] C4. Filter, wash with deionized water 2-4 times, each time using 3-5 times the dry weight of the product. The washing endpoint is when the pH of the filtrate stabilizes at 5.5-7.5. Then, vacuum dry at 40-60℃ and 2-5 kPa absolute pressure for 8-16 hours. The drying endpoint is when the mass difference between two consecutive weighings with a 1-hour interval does not exceed 0.1% of the total mass.

[0019] Furthermore, the oxidized starch intermediate used to prepare the 2-hydroxyethylamino oxidized starch is prepared through the following steps:

[0020] B1. Disperse 100 parts by weight of starch in 500-1000 parts by weight of deionized water to obtain starch slurry;

[0021] B2. Add 20-80 parts by weight of sodium periodate to the starch slurry, adjust the pH of the slurry to 3.5-5.5 with an aqueous solution of hydrochloric acid with a concentration of 0.1-1 mol / L or an acetate-sodium acetate buffer solution with a total concentration of 0.1-0.5 mol / L, and react at 15-30℃ for 4-8 hours under dark conditions;

[0022] B3. Filter and wash with deionized water until the pH of the filtrate is 5.5-7.5;

[0023] B4. Vacuum dry at 40-60℃ and 2-5 kPa absolute pressure for 6-16 hours. The drying endpoint is defined as the mass difference between two consecutive weighings with an interval of 1 hour not exceeding 0.1% of the total mass, to obtain the oxidized starch intermediate with an aldehyde content of 1.0-3.0 mmol / g.

[0024] D4. Obtain the biodegradable polyurethane wall material system with a volume median particle size D50 of 80-300 nm.

[0025] Furthermore, the number average molecular weight of polycaprolactone diol is 530-3000; the pH of the wall material system is 6.0-8.5, and the viscosity is 200-1500 mPa·s measured by a rotational viscometer at 25±0.5℃ and 60 rpm; the starch is potato starch or corn starch.

[0026] As a concept of this invention, the synergistic design of isocyanate-terminated biodegradable polyurethane prepolymer and 2-hydroxyethyl aminoated oxidized starch is mainly used to enhance the controllable construction capability of water-dispersible wall material systems during shearing, pre-reaction, and curing processes. The isocyanate-terminated biodegradable polyurethane prepolymer provides reactive isocyanate groups and introduces biodegradable segments, while the 2-hydroxyethyl aminoated oxidized starch provides reactive sites such as amino and hydroxyl groups and participates in the structural construction of the water-dispersible system. The two form a covalent network in the deionized water system through urea bonds and urethane bonds, thereby achieving controllable volume median particle size D50 within the solid component and viscosity window, and providing a stable reaction basis and structural support for the subsequent emulsification and curing to form a polyurethane / starch covalent network wall layer.

[0027] This invention also discloses a method for preparing a biodegradable polyurethane wall material system, comprising the following steps:

[0028] S1. Provides a pre-prepared isocyanate-terminated biodegradable polyurethane prepolymer;

[0029] S2. Provides the prepared 2-hydroxyethyl aminoated oxidized starch;

[0030] S3. Disperse 15-30 parts by weight of 2-hydroxyethylamino oxidized starch in 70-200 parts by weight of deionized water, control the pH of the system to 6.5-8.5, and shear at 2000-6000 rpm for 5-20 min to obtain a dispersion;

[0031] S4. Add 70-85 parts by weight of the isocyanate-terminated biodegradable polyurethane prepolymer to the dispersion obtained in step S3 at 20-35℃, pre-react for 10-60 min, and then mature at 35-50℃ for 0.5-2 h.

[0032] S5. Add deionized water to the system obtained in step S4, including 25-45 parts by weight of solid components and the remainder of deionized water, to make up to 100 parts by weight, to obtain the biodegradable polyurethane wall material system with a median particle size D50 of 80-300 nm.

[0033] Further, prior to step S4, the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer are neutralized with a tertiary amine neutralizing agent. The degree of neutralization is 80-100 mol%, the neutralization temperature is 25-35°C, and the stirring and neutralization time is 15-30 min.

[0034] Further, after step S5, the biodegradable polyurethane wall material system is further mixed with the hydrophobic liquid core material and emulsified at 3000-12000 rpm for 3-10 min, with the emulsification temperature controlled at 20-35℃, and then cured at 35-60℃ for 1-4 h to obtain microcapsules with a polyurethane / starch covalent network wall layer.

[0035] Furthermore, the weight ratio of the hydrophobic liquid core material to the solid component in the biodegradable polyurethane wall material system is 1:1 to 6:1, and the volume median particle size D50 of the microcapsules is 1-20µm.

[0036] Furthermore, the tertiary amine neutralizing agent is at least one of triethylamine, N-methyldiethanolamine, or triethanolamine.

[0037] Furthermore, when triethylamine is used as a neutralizing agent and the degree of neutralization is 80-100 mol%, based on the amount of 2,2-dimethylolpropionic acid in the polyol mixture being 4-12 parts by weight, the amount of triethylamine used is 2.4-9.1 parts by weight.

[0038] Furthermore, in step A3, isophorone diisocyanate is added to the polyol mixture under mechanical stirring at a speed of 100-400 rpm, and the addition rate is controlled so that the system temperature does not exceed 90°C; during the addition, inert gas is continuously introduced for protection.

[0039] Further, in step B1, starch is dispersed in deionized water at 20-25°C by mechanical stirring at a speed of 100-300 rpm for 10-30 min; in step C1, oxidized starch intermediate is dispersed in deionized water at 20-25°C by mechanical stirring at a speed of 100-300 rpm for 10-30 min.

[0040] Further, in step B2, the pH of the starch slurry is adjusted to 3.5-5.5 with an aqueous hydrochloric acid solution of 0.1-1 mol / L or an acetate-sodium acetate buffer solution of 0.1-0.5 mol / L. The reaction is carried out by mechanical stirring at 15-30°C in the dark, with a stirring speed of 100-300 rpm.

[0041] Furthermore, in step C2, if the pH of the system does not reach 8.5-10.5 after the addition of ethanolamine, adjust it to the target pH range with a small amount of 0.5-2 mol / L NaOH aqueous solution; the reaction is carried out by mechanical stirring at a speed of 100-300 rpm.

[0042] Further, in step C3, sodium borohydride is prepared as an alkaline aqueous solution with a concentration of 1-5 wt% (NaOH concentration 0.1-0.5 mol / L, solution pH controlled at 10-12), and slowly added to the system in batches (intervals of 5-15 min), maintaining the pH of the system at 10-12 during the addition process.

[0043] Furthermore, the washing in steps B3 and C4 is carried out with deionized water, with the amount of water used for each wash being 3-5 times the dry weight of the product, and the washing is performed 2-4 times. The washing endpoint is when the pH of the filtrate stabilizes at 5.5-7.5.

[0044] Furthermore, the vacuum drying in steps B4 and C4 is carried out under an absolute pressure of 2-5 kPa, and the drying endpoint is when the mass difference between two consecutive weighings with an interval of 1 h does not exceed 0.1% of the total mass.

[0045] Furthermore, the shearing dispersion in step S3 is carried out using a rotor-stator type high-speed shearing disperser, with the dispersion temperature controlled at 20-35℃.

[0046] Furthermore, the emulsification step is carried out using a rotor-stator type high-speed shear homogenizer, with the emulsification temperature controlled at 20-35℃; during the curing period, mechanical stirring is used to maintain the uniformity of the system, with a stirring speed of 50-300 rpm.

[0047] Furthermore, in the 2-hydroxyethyl-amino-oxidized starch, the N atom exists in the form of a secondary amine after the ethanolamine and starch aldehyde group are reduced and aminationed, with a chemical structure of starch skeleton -CH2-NH-CH2CH2OH; the amino content is determined by potentiometric titration based on the dry sample mass.

[0048] Furthermore, the isocyanate group content was determined using di-n-butylamine back titration based on the total mass of the prepolymer; the amino content and residual aldehyde group content were determined using potentiometric titration and hydroxylamine hydrochloride titration, respectively, based on the dry sample mass; and the solid content of the wall material system was determined using the 105℃ constant weight method based on the total mass of the dispersion.

[0049] Furthermore, the volume median particle size D50 of the biodegradable polyurethane wall material system was determined by dynamic light scattering at 25±1℃ after diluting the sample with deionized water to approximately 0.1 wt%, and the volume distribution D50 value was obtained; the volume median particle size D50 of the microcapsules was determined by laser diffraction in a deionized water dispersion medium with a light-blocking degree of 2-8%.

[0050] Furthermore, the viscosity of the wall material system was measured using a rotational viscometer at 25±0.5℃, 60 rpm, and with a rotor of suitable range, and a stable reading was obtained.

[0051] Furthermore, the amount of isophorone diisocyanate used in step A3 is determined by the sum of the actual molar number of hydroxyl groups of each component in the polyol system and the target NCO / OH molar ratio (1.20-1.80:1). Based on the amount of feed in step A1 (100 parts by weight of polycaprolactone diol, 4-12 parts by weight of 2,2-dimethylolpropionic acid, and 2-10 parts by weight of 2,2'-dithiodiethanol), when the number average molecular weight of polycaprolactone diol is 530-3000, the amount of isophorone diisocyanate required to satisfy NCO / OH=1.20-1.80:1 is approximately 20-140 parts by weight.

[0052] Furthermore, the biodegradable polyurethane wall material system sample was washed 2-4 times with ethanol and / or deionized water and then vacuum dried at 40-60℃. The dried sample was then subjected to FTIR analysis; the obtained FTIR spectrum was in the range of 1630-1660 cm⁻¹. -1 An absorption peak, consistent with the characteristic stretching vibration of the urea bond C=O, appears in the range of 1690-1730 cm⁻¹. -1 Absorption peaks consistent with the characteristic stretching vibrations of C=O bonds in the range were observed; using isocyanate-terminated biodegradable polyurethane prepolymer samples treated separately as controls, the differences in the FTIR spectra of the two groups of samples in the above range were recorded.

[0053] As another concept of this invention, the present invention adopts a synergistic design of emulsification and curing of a biodegradable polyurethane wall material system and a hydrophobic liquid core material, which is mainly used to enhance the polyurethane / starch covalent network wall layer construction capability of microcapsules. The covalent network formed by the wall material system in the pre-reaction and curing stages provides continuous reaction sites and structural framework for subsequent curing, so that the microcapsules formed during the emulsification process can obtain a more stable wall layer structure after curing, and the control of the volume median particle size D50 and the process repeatability support each other, thereby improving the controllability and consistency of the microcapsule preparation process.

[0054] Isocyanate-terminated biodegradable polyurethane prepolymers provide isocyanate-based reactivity and a biodegradable chain backbone in water-dispersible wall material systems, and tend to form a continuous polyurethane network during pre-reaction and curing processes. 2-hydroxyethyl-amino-oxidized starch provides nucleophilic reaction sites such as amino and hydroxyl groups and enhances the dispersion and construction ability of the system in deionized water. The two are covalently linked at multiple points in the same system through urea and urethane bonds, which expands the covalent network from a single polyurethane chain segment to a network structure coupled with the polyurethane and starch backbone. This synergistically stabilizes the median particle size D50 within the solid component and viscosity window, and provides dual support for the formation of polyurethane / starch covalent network wall layer in the emulsification and curing stage, including the density of reaction sites and network continuity.

[0055] Beneficial technical effects

[0056] 1. By forming a covalent network of urea and urethane bonds in a water-dispersible wall material system through isocyanate-terminated biodegradable polyurethane prepolymer and 2-hydroxyethyl-amino oxidized starch, the wall material system has continuous reaction sites and structural framework during subsequent emulsification and curing, thereby promoting the stable construction of the polyurethane / starch covalent network wall layer.

[0057] 2. The wall material system can still achieve a viscosity of 200-1500 mPa·s and a volume median particle size (D50) of 80-300 nm under the condition of 25-45 parts by weight of solid component. This makes it easier to achieve shear dispersion, pre-reaction and ripening processes within the operable window, thereby improving the controllability of the preparation of the wall material system and supporting the preparation of microcapsules.

[0058] 3. By constraining the amino content and residual aldehyde content of 2-hydroxyethylamino oxidized starch, and controlling the isocyanate content, the covalent network formation process has more defined reaction sites and side reaction inhibition directions, which is beneficial to the consistency of the wall material system structure and the formation of the microcapsule wall layer.

[0059] 4. After step S5, the wall material system and the hydrophobic liquid core material are emulsified and cured to obtain microcapsules with polyurethane / starch covalent network wall layers. The weight ratio of hydrophobic liquid core material to solid components is 1:1 to 6:1, and the median particle size D50 of the microcapsules is 1-20µm, which facilitates the selection of microcapsule particle size and ratio for different application scenarios. Attached Figure Description

[0060] Figure 1 The image shows the Fourier transform infrared full spectrum overlay of the wall material samples from Example 1, Comparative Example 1, and Comparative Example 3.

[0061] Figure 2 The images show magnified Fourier transform infrared spectroscopy images of the urea bond region in the wall material samples of Example 1, Comparative Example 1, and Comparative Example 3.

[0062] Figure 3 These are magnified Fourier transform infrared images of the urethane ester region of the wall material samples from Example 1, Comparative Example 1, and Comparative Example 3.

[0063] Figure 4 The graph shows the changes in residual NCO content and conversion rate of the systems in Example 1, Comparative Example 1, and Comparative Example 2 as a function of aging time.

[0064] Figure 5 The dynamic light scattering particle size distribution diagrams are shown for the aqueous dispersion systems of Example 1, Comparative Example 7, and Comparative Example 8.

[0065] Figure 6 The D50 drift diagrams for the water dispersion systems of Example 1, Comparative Example 9, and Comparative Example 7 are shown.

[0066] Figure 7 The thermogravimetric mass percentage graphs of the microcapsule samples from Example 1, Comparative Example 1, and Comparative Example 7 are shown.

[0067] Figure 8 Thermogravimetric derivatives of microcapsule samples from Example 1, Comparative Example 1, and Comparative Example 7 are shown.

[0068] Figure 9 a is a macroscopic optical photograph of the biodegradable polyurethane wall material system dispersion prepared in Example 1 of the present invention.

[0069] Figure 9 b is a macroscopic optical photograph of the microcapsule aqueous suspension prepared in Example 1 of this invention.

[0070] Figure 9 c is a macroscopic optical photograph of the microcapsule vacuum-dried powder prepared in Example 1 of this invention.

[0071] Figure 10 a is a low-magnification scanning electron microscope image of the nanoparticles in the biodegradable polyurethane wall material system prepared in Example 1 of this invention.

[0072] Figure 10 b is a high-magnification scanning electron microscope image of the nanoparticles in the biodegradable polyurethane wall material system prepared in Example 1 of this invention.

[0073] Figure 10 c is a low-magnification scanning electron microscope image of the microcapsules prepared in Example 1 of this invention.

[0074] Figure 10 d is a high-magnification scanning electron microscope image of the surface of the microcapsules prepared in Example 1 of the present invention.

[0075] Figure 10 e is a scanning electron microscope image of the frozen fracture section of the microcapsule prepared in Example 1 of this invention.

[0076] Figure 10 f is a high-magnification scanning electron microscope image of a local area of ​​the cross-section of the microcapsule prepared in Example 1 of this invention. Detailed Implementation

[0077] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0078] Example 1

[0079] This embodiment provides a biodegradable polyurethane wall material system and its preparation method. The wall material system of this embodiment is a water-dispersible type, comprising 35 parts by weight of solid components and 65 parts by weight of deionized water, totaling 100 parts by weight. The solid components of this embodiment, based on 100 parts by weight, include 77 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 23 parts by weight of 2-hydroxyethyl-amino-modified oxidized starch, totaling 100 parts by weight.

[0080] The isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment is prepared through the following steps:

[0081] 100 parts by weight of polycaprolactone diol with a number average molecular weight of 1800, 8 parts by weight of 2,2-dimethylolpropionic acid, and 6 parts by weight of 2,2'-dithiodiethanol were mixed to obtain a polyol mixture. The polyol mixture was treated at 105°C and a vacuum of -0.09 MPa for 2 hours to remove moisture and low molecular weight volatiles. Under an inert atmosphere protected by nitrogen, 51 parts by weight of isophorone diisocyanate was added to the polyol mixture with mechanical stirring (250 rpm) at a molar ratio of isocyanate groups to hydroxyl groups of 1.50:1. The addition rate was controlled to keep the system temperature below 90°C, and nitrogen was continuously purged during the addition process. The reaction was carried out at 78°C for 3.5 hours. The reaction was stopped when the isocyanate group content reached 3.2 wt%, and the mixture was cooled to 30°C to obtain the isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment. In this embodiment, the isocyanate group content was determined using the di-n-butylamine back titration method based on the total mass of the prepolymer.

[0082] The oxidized starch intermediate used in the 2-hydroxyethylamino oxidized starch of this embodiment was prepared through the following steps:

[0083] 100 parts by weight of potato starch were dispersed in 750 parts by weight of deionized water at 22°C with mechanical stirring (200 rpm) for 20 min to obtain a starch slurry. 50 parts by weight of sodium periodate were added to the starch slurry, and the pH was adjusted to 4.5 with a 0.5 mol / L hydrochloric acid aqueous solution. The mixture was reacted at 22°C with mechanical stirring (200 rpm) for 6 h under light-protected conditions. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 6.5. The mixture was then vacuum dried at 50°C and 3.5 kPa for 10 h. The drying endpoint was reached when the mass difference between two consecutive weighings taken 1 h apart did not exceed 0.1% of the total mass, yielding an oxidized starch intermediate with an aldehyde content of 2.0 mmol / g. In this embodiment, the aldehyde content was determined using hydroxylamine hydrochloride titration based on the dry sample mass.

[0084] The 2-hydroxyethyl amino-modified oxidized starch in this embodiment is prepared by the following steps:

[0085] 100 parts by weight of oxidized starch intermediate were dispersed in 550 parts by weight of deionized water at 25°C with mechanical stirring (200 rpm) for 20 min. 22 parts by weight of ethanolamine were added to the system, and the pH was adjusted to 9.5 with a 1 mol / L NaOH aqueous solution. The reaction was carried out at 35°C with mechanical stirring (200 rpm) for 4 h. 4.5 parts by weight of sodium borohydride were prepared as a 3 wt% alkaline aqueous solution (NaOH concentration 0.3 mol / L, pH controlled at 11), and slowly added to the system in three batches (10 min apart), maintaining the pH at 10-12 during the addition. The reaction was continued at 28°C for 2 h. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 6.5. Vacuum drying was performed at 50°C and 3.5 kPa for 12 hours. The drying endpoint was determined by the mass difference between two consecutive weighings taken 1 hour apart not exceeding 0.1% of the total mass, yielding 2-hydroxyethyl-amino-oxidized starch with an amino content of 0.50 mmol / g and a residual aldehyde content of 0.05 mmol / g. In this example, the nitrogen atom in the 2-hydroxyethyl-amino-oxidized starch exists as a secondary amine after reductive amination of ethanolamine and starch aldehyde groups, with a chemical structure of starch skeleton -CH2-NH-CH2CH2OH. The amino content in this example was determined by potentiometric titration based on the dry sample mass.

[0086] The preparation method of the biodegradable polyurethane wall material system in this embodiment includes the following steps:

[0087] Triethylamine (4.9 parts by weight) was used to neutralize the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer, achieving a neutralization degree of 81 mol% at 30 °C and a stirring time of 20 min. 23 parts by weight of 2-hydroxyethylamino oxidized starch were dispersed in 135 parts by weight of deionized water. The pH of the system was adjusted to 7.5 with a 1 mol / L NaOH aqueous solution, and the dispersion was obtained by shearing at 4000 rpm for 12 min using a rotor-stator type high-speed shear disperser at 25 °C. 77 parts by weight of the neutralized isocyanate-terminated biodegradable polyurethane prepolymer were added to the dispersion at 28 °C. After a pre-reaction of 35 min, the mixture was aged at 42 °C for 1.2 h to obtain a biodegradable polyurethane wall material system with a median particle size (D50) of 180 nm. 50.7 parts by weight of deionized water were then added. The volumetric median particle size (D50) of the biodegradable polyurethane wall material system in this embodiment was determined by dynamic light scattering at 25±1℃ after diluting the sample with deionized water to approximately 0.1wt%, and the volume distribution D50 value was recorded. The wall material system in this embodiment has a pH of 7.0 and a viscosity of 850 mPa·s. The solid content of the wall material system in this embodiment, based on the total mass of the dispersion, was determined to be 35wt% using the constant gravimetric method at 105℃. The viscosity of the wall material system in this embodiment was measured using a rotational viscometer at 25±0.5℃, 60 rpm, and a rotor with a range adaptation, and a stable reading was recorded.

[0088] In this embodiment, the isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethyl-amino-modified oxidized starch form a covalent network in the wall material system through urea and urethane bonds. To verify the formation of this covalent network, the biodegradable polyurethane wall material system sample of this embodiment was washed three times with ethanol and deionized water and then vacuum-dried at 50°C. The dried sample was then subjected to FTIR analysis; the obtained FTIR spectrum was at 1640 cm⁻¹. -1 An absorption peak, consistent with the characteristic stretching vibration of the urea bond C=O, appears at 1710 cm⁻¹. -1 An absorption peak corresponding to the characteristic stretching vibration of the C=O bond of the urethane bond appeared at the specified location. Using a separately treated isocyanate-terminated biodegradable polyurethane prepolymer sample as a control, the difference in the FTIR spectra of the two groups of samples in the above range was recorded, confirming the formation of the covalent network.

[0089] Furthermore, the biodegradable polyurethane wall material system of this embodiment was mixed with a hydrophobic liquid core material, with a weight ratio of 3:1 between the hydrophobic liquid core material and the solid components in the biodegradable polyurethane wall material system. The mixture was emulsified for 6 minutes at 7500 rpm using a rotor-stator type high-speed shear homogenizer, with the emulsification temperature controlled at 25°C. Subsequently, it was cured at 48°C for 2.5 hours. During curing, mechanical stirring was used to maintain the homogeneity of the system at a stirring speed of 150 rpm, resulting in microcapsules with a polyurethane / starch covalent network wall layer. The median volumetric particle size (D50) of the microcapsules in this embodiment was 8 µm. The median volumetric particle size (D50) of the microcapsules in this embodiment was determined by laser diffraction in a deionized water dispersion medium under 5% opacity conditions.

[0090] The formulation in this embodiment balances the mechanical strength, biodegradability, and processing stability of the wall material. The process conditions are mild and reproducible, making it suitable for microcapsule encapsulation applications with high requirements for stability and reliability. It is particularly suitable for industrial production in the fields of fragrances, phase change materials, and pesticide slow-release.

[0091] Example 2

[0092] This embodiment provides a biodegradable polyurethane wall material system and its preparation method. The wall material system of this embodiment is a water-dispersible type, comprising 40 parts by weight of solid components and 60 parts by weight of deionized water, totaling 100 parts by weight. The solid components of this embodiment, based on 100 parts by weight, include 82 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 18 parts by weight of 2-hydroxyethyl-amino-modified oxidized starch, totaling 100 parts by weight.

[0093] The isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment is prepared through the following steps:

[0094] 100 parts by weight of polycaprolactone diol with a number average molecular weight of 1000, 10 parts by weight of 2,2-dimethylolpropionic acid, and 8 parts by weight of 2,2'-dithiodiethanol were mixed to obtain a polyol mixture. The polyol mixture was treated at 110°C and a vacuum of -0.092 MPa for 2.5 h to remove moisture and low molecular weight volatiles. Under an inert atmosphere protected by nitrogen, 83 parts by weight of isophorone diisocyanate was added to the polyol mixture with mechanical stirring (stirring speed of 300 rpm) at a molar ratio of isocyanate groups to hydroxyl groups of 1.65:1. The addition rate was controlled to keep the system temperature below 90°C, and nitrogen was continuously purged during the addition process. The reaction was carried out at 80°C for 4 h. The reaction was stopped when the isocyanate group content reached 4.0 wt%, and the mixture was cooled to 35°C to obtain the isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment. In this embodiment, the isocyanate group content was determined using the di-n-butylamine back titration method based on the total mass of the prepolymer.

[0095] The oxidized starch intermediate used in the 2-hydroxyethylamino oxidized starch of this embodiment was prepared through the following steps:

[0096] 100 parts by weight of corn starch were dispersed in 600 parts by weight of deionized water at 22°C with mechanical stirring (200 rpm) for 20 min to obtain a starch slurry. 65 parts by weight of sodium periodate were added to the starch slurry, and the pH of the starch slurry was adjusted to 4.0 with an acetate-sodium acetate buffer solution with a total concentration of 0.3 mol / L. The mixture was reacted at 25°C with mechanical stirring (200 rpm) for 5 h under light-protected conditions. After the reaction was complete, the mixture was filtered and washed with deionized water. The amount of water used for each wash was four times the dry weight of the product, and the washing was repeated three times. The washing endpoint was reached when the pH of the filtrate stabilized at 6.0. The mixture was then vacuum dried at 55°C and 3 kPa for 8 h. The drying endpoint was reached when the mass difference between two consecutive weighings taken 1 h apart did not exceed 0.1% of the total mass, yielding an oxidized starch intermediate with an aldehyde content of 2.5 mmol / g. In this embodiment, the aldehyde content was determined using hydroxylamine hydrochloride titration based on the dry sample mass.

[0097] The 2-hydroxyethyl amino-modified oxidized starch in this embodiment is prepared by the following steps:

[0098] 100 parts by weight of oxidized starch intermediate were dispersed in 400 parts by weight of deionized water at 25°C with mechanical stirring (200 rpm) for 20 min. 30 parts by weight of ethanolamine were added to the system, and the pH was adjusted to 10.0 with a 1.5 mol / L NaOH aqueous solution. The reaction was carried out at 40°C with mechanical stirring (200 rpm) for 3 h. 6 parts by weight of sodium borohydride were prepared as a 4 wt% alkaline aqueous solution (NaOH concentration 0.4 mol / L, pH controlled at 11.5), and slowly added to the system in four batches (10 min intervals), maintaining the pH at 10-12 during the addition. The reaction was continued at 25°C for 2.5 h. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 6.5. Vacuum drying was performed at 55°C and 3 kPa absolute pressure for 10 h. The drying endpoint was determined by the mass difference between two consecutive weighings taken 1 h apart not exceeding 0.1% of the total mass, yielding 2-hydroxyethyl-amino-oxidized starch with an amino content of 0.65 mmol / g and a residual aldehyde content of 0.08 mmol / g. In this example, the nitrogen atom in the 2-hydroxyethyl-amino-oxidized starch exists as a secondary amine after reductive amination of ethanolamine and starch aldehyde groups, with a chemical structure of starch skeleton -CH2-NH-CH2CH2OH. The amino content in this example was determined by potentiometric titration based on the dry sample mass.

[0099] The preparation method of the biodegradable polyurethane wall material system in this embodiment includes the following steps:

[0100] Triethylamine (6.1 parts by weight) was used to neutralize the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer, achieving a neutralization degree of 81 mol% at 30 °C and a stirring time of 25 min. 18 parts by weight of 2-hydroxyethylamino oxidized starch were dispersed in 90 parts by weight of deionized water. The pH of the system was adjusted to 7.0 with a 1 mol / L NaOH aqueous solution, and the dispersion was obtained by shearing at 5000 rpm for 15 min using a rotor-stator type high-speed shear disperser at 25 °C. 82 parts by weight of the neutralized isocyanate-terminated biodegradable polyurethane prepolymer were added to the dispersion at 25 °C. After a pre-reaction of 20 min, the mixture was aged at 45 °C for 1 h to obtain a biodegradable polyurethane wall material system with a median particle size (D50) of 120 nm. 60 parts by weight of deionized water were then added to the mixture. The volumetric median particle size (D50) of the biodegradable polyurethane wall material system in this embodiment was determined by dynamic light scattering at 25±1℃ after diluting the sample with deionized water to approximately 0.1wt%, and the volume distribution D50 value was recorded. The wall material system in this embodiment has a pH of 6.8 and a viscosity of 1200 mPa·s. The solid content of the wall material system in this embodiment, based on the total mass of the dispersion, was determined to be 40wt% using the constant gravimetric method at 105℃. The viscosity of the wall material system in this embodiment was measured using a rotational viscometer at 25±0.5℃, 60 rpm, and a rotor with a range adaptation, and a stable reading was recorded.

[0101] In this embodiment, the isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethyl-amino-modified oxidized starch form a covalent network in the wall material system through urea and urethane bonds. To verify the formation of this covalent network, the biodegradable polyurethane wall material system sample of this embodiment was washed three times with ethanol and deionized water and then vacuum-dried at 50°C. The dried sample was then subjected to FTIR analysis; the obtained FTIR spectrum was at 1645 cm⁻¹. -1 An absorption peak, consistent with the characteristic stretching vibration of the urea bond C=O, appears at 1715 cm⁻¹. -1 An absorption peak corresponding to the characteristic stretching vibration of the C=O bond of the urethane bond appeared at the specified location. Using a separately treated isocyanate-terminated biodegradable polyurethane prepolymer sample as a control, the difference in the FTIR spectra of the two groups of samples in the above range was recorded, confirming the formation of the covalent network.

[0102] Furthermore, the biodegradable polyurethane wall material system of this embodiment was mixed with a hydrophobic liquid core material, with a weight ratio of the hydrophobic liquid core material to the solid components in the biodegradable polyurethane wall material system of 4:1. The mixture was emulsified for 8 minutes at 9000 rpm using a rotor-stator type high-speed shear homogenizer, with the emulsification temperature controlled at 25°C. Subsequently, it was cured at 55°C for 3 hours. During curing, the system homogeneity was maintained by mechanical stirring at a speed of 200 rpm, resulting in microcapsules with a polyurethane / starch covalent network wall layer. The median volumetric particle size (D50) of the microcapsules in this embodiment was 5 µm. The median volumetric particle size (D50) of the microcapsules in this embodiment was determined by laser diffraction in a deionized water dispersion medium under 5% opacity conditions.

[0103] This embodiment enhances the mechanical strength, density, and solvent resistance of the wall layer, making it suitable for applications requiring high-strength wall layers and good barrier properties, such as long-lasting encapsulation of oil-soluble active ingredients, wear-resistant microcapsules, and stable encapsulation under high-temperature environments.

[0104] Example 3

[0105] This embodiment provides a biodegradable polyurethane wall material system and its preparation method. The wall material system of this embodiment is a water-dispersible type, comprising 30 parts by weight of solid components and 70 parts by weight of deionized water, totaling 100 parts by weight. The solid components of this embodiment, based on 100 parts by weight, include 72 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 28 parts by weight of 2-hydroxyethyl-amino-modified oxidized starch, totaling 100 parts by weight.

[0106] The isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment is prepared through the following steps:

[0107] 100 parts by weight of polycaprolactone diol with a number average molecular weight of 2500, 6 parts by weight of 2,2-dimethylolpropionic acid, and 4 parts by weight of 2,2'-dithiodiethanol were mixed to obtain a polyol mixture. The polyol mixture was treated at 100°C and a vacuum of -0.085 MPa for 1.5 h to remove moisture and low molecular weight volatiles. Under an inert atmosphere protected by nitrogen, 33 parts by weight of isophorone diisocyanate were added to the polyol mixture with mechanical stirring (stirring speed of 200 rpm) at a molar ratio of isocyanate groups to hydroxyl groups of 1.35:1. The addition rate was controlled to keep the system temperature below 90°C, and nitrogen was continuously purged during the addition process. The reaction was carried out at 75°C for 3 h. The reaction was stopped when the isocyanate group content reached 2.5 wt%, and the mixture was cooled to 28°C to obtain the isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment. In this embodiment, the isocyanate group content was determined using the di-n-butylamine back titration method based on the total mass of the prepolymer.

[0108] The oxidized starch intermediate used in the 2-hydroxyethylamino oxidized starch of this embodiment was prepared through the following steps:

[0109] 100 parts by weight of potato starch were dispersed in 850 parts by weight of deionized water at 22°C with mechanical stirring (200 rpm) for 20 min to obtain a starch slurry. 35 parts by weight of sodium periodate were added to the starch slurry, and the pH was adjusted to 5.0 with a 0.5 mol / L hydrochloric acid aqueous solution. The mixture was reacted at 20°C with mechanical stirring (200 rpm) for 7 h under light-protected conditions. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 7.0. The mixture was then vacuum dried at 45°C and 4 kPa for 14 h. The drying endpoint was reached when the mass difference between two consecutive weighings taken 1 h apart did not exceed 0.1% of the total mass, yielding an oxidized starch intermediate with an aldehyde content of 1.5 mmol / g. In this embodiment, the aldehyde content was determined using hydroxylamine hydrochloride titration based on the dry sample mass.

[0110] The 2-hydroxyethyl amino-modified oxidized starch in this embodiment is prepared by the following steps:

[0111] 100 parts by weight of oxidized starch intermediate were dispersed in 700 parts by weight of deionized water at 25°C with mechanical stirring (200 rpm) for 20 min. 15 parts by weight of ethanolamine were added to the system, and the pH was adjusted to 9.0 with a 1 mol / L NaOH aqueous solution. The reaction was carried out at 30°C with mechanical stirring (200 rpm) for 5 h. 2.5 parts by weight of sodium borohydride were prepared as a 2 wt% alkaline aqueous solution (NaOH concentration 0.2 mol / L, pH controlled at 10.5), and slowly added to the system in two batches (15 min apart), maintaining the pH at 10-12 during the addition. The reaction was continued at 30°C for 1.5 h. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 6.5. Vacuum drying was performed at 45°C and 4 kPa absolute pressure for 14 h. The drying endpoint was determined by the mass difference between two consecutive weighings taken 1 h apart not exceeding 0.1% of the total mass, yielding 2-hydroxyethyl-amino-oxidized starch with an amino content of 0.35 mmol / g and a residual aldehyde content of 0.06 mmol / g. In this example, the nitrogen atom in the 2-hydroxyethyl-amino-oxidized starch exists as a secondary amine after reductive amination of ethanolamine and starch aldehyde groups, with a chemical structure of starch skeleton -CH2-NH-CH2CH2OH. The amino content in this example was determined by potentiometric titration based on the dry sample mass.

[0112] The preparation method of the biodegradable polyurethane wall material system in this embodiment includes the following steps:

[0113] The carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer were neutralized with 3.7 parts by weight of triethylamine at a neutralization degree of 82 mol% at 28 °C for 18 min. 28 parts by weight of 2-hydroxyethylamino oxidized starch were dispersed in 175 parts by weight of deionized water. The pH of the system was adjusted to 8.0 with a 1 mol / L NaOH aqueous solution, and the dispersion was obtained by shearing at 3000 rpm for 10 min using a rotor-stator type high-speed shear disperser at 25 °C. 72 parts by weight of the neutralized isocyanate-terminated biodegradable polyurethane prepolymer were added to the dispersion at 30 °C. After a pre-reaction of 45 min, the mixture was aged at 40 °C for 1.5 h to obtain a biodegradable polyurethane wall material system with a median particle size (D50) of 240 nm. 58.3 parts by weight of deionized water were then added. The volumetric median particle size (D50) of the biodegradable polyurethane wall material system in this embodiment was determined by dynamic light scattering at 25±1℃ after diluting the sample with deionized water to approximately 0.1wt%, and the volume distribution D50 value was recorded. The wall material system in this embodiment has a pH of 7.5 and a viscosity of 500 mPa·s. The solid content of the wall material system in this embodiment, based on the total mass of the dispersion, was determined to be 30wt% using the constant gravimetric method at 105℃. The viscosity of the wall material system in this embodiment was measured using a rotational viscometer at 25±0.5℃, 60 rpm, and a rotor with a range adaptation, and a stable reading was recorded.

[0114] In this embodiment, the isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethyl-amino-modified oxidized starch form a covalent network in the wall material system through urea and urethane bonds. To verify the formation of this covalent network, the biodegradable polyurethane wall material system sample of this embodiment was washed three times with ethanol and deionized water and then vacuum-dried at 50°C. The dried sample was then subjected to FTIR analysis; the obtained FTIR spectrum was at 1638 cm⁻¹. -1 An absorption peak, consistent with the characteristic stretching vibration of the urea bond C=O, appears at 1708 cm⁻¹. -1 An absorption peak corresponding to the characteristic stretching vibration of the C=O bond of the urethane bond appeared at the specified location. Using a separately treated isocyanate-terminated biodegradable polyurethane prepolymer sample as a control, the difference in the FTIR spectra of the two groups of samples in the above range was recorded, confirming the formation of the covalent network.

[0115] Furthermore, the biodegradable polyurethane wall material system of this embodiment was mixed with a hydrophobic liquid core material, with a weight ratio of 2:1 between the hydrophobic liquid core material and the solid components in the biodegradable polyurethane wall material system. The mixture was emulsified for 5 minutes at 6000 rpm using a rotor-stator type high-speed shear homogenizer, with the emulsification temperature controlled at 25°C. Subsequently, it was cured at 42°C for 2 hours. During curing, mechanical stirring was used to maintain the homogeneity of the system at a stirring speed of 100 rpm, resulting in microcapsules with a polyurethane / starch covalent network wall layer. The median volumetric particle size (D50) of the microcapsules in this embodiment was 12 µm. The median volumetric particle size (D50) of the microcapsules in this embodiment was determined by laser diffraction in a deionized water dispersion medium under 5% opacity conditions.

[0116] Features of this embodiment: This embodiment uses a low solids content formulation (30 parts by weight of solids), reducing the prepolymer ratio to 72 parts by weight, increasing the aminated starch ratio to 28 parts by weight, achieving a number-average molecular weight of 2500 for polycaprolactone diol, reducing the isocyanate group content to 2.5 wt%, and the amino content to 0.35 mmol / g. The lower NCO / OH molar ratio (1.35:1) and higher starch ratio result in a wall material system with higher bio-based content and biodegradability. The lower shear rate (3000 rpm) increases the particle size to 240 nm and reduces the viscosity to 500 mPa·s, facilitating subsequent processing. This formulation optimizes eco-friendliness and rapid degradation performance, making it suitable for environmentally sensitive applications such as slow-release fertilizers in agriculture, biopesticide coatings, environmentally friendly functional textile finishing, and disposable packaging materials.

[0117] Example 4

[0118] This embodiment provides a biodegradable polyurethane wall material system and its preparation method. The wall material system of this embodiment is a water-dispersible type, comprising 27 parts by weight of solid components and 73 parts by weight of deionized water, totaling 100 parts by weight. The solid components of this embodiment, based on 100 parts by weight, include 84 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 16 parts by weight of 2-hydroxyethyl-amino-modified oxidized starch, totaling 100 parts by weight.

[0119] The isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment is prepared through the following steps:

[0120] 100 parts by weight of polycaprolactone diol with a number average molecular weight of 700, 11 parts by weight of 2,2-dimethylolpropionic acid, and 9 parts by weight of 2,2'-dithiodiethanol were mixed to obtain a polyol mixture. The polyol mixture was treated at 112°C and a vacuum of -0.093 MPa for 2.8 h to remove moisture and low molecular weight volatiles. Under an inert atmosphere protected by nitrogen, 108 parts by weight of isophorone diisocyanate was added to the polyol mixture with mechanical stirring (350 rpm) at a molar ratio of isocyanate groups to hydroxyl groups of 1.72:1. The addition rate was controlled to keep the system temperature below 90°C, and nitrogen was continuously purged during the addition process. The reaction was carried out at 83°C for 4.5 h. The reaction was stopped when the isocyanate group content reached 4.2 wt%, and the mixture was cooled to 38°C to obtain the isocyanate-terminated biodegradable polyurethane prepolymer of this embodiment. In this embodiment, the isocyanate group content was determined using the di-n-butylamine back titration method based on the total mass of the prepolymer.

[0121] The oxidized starch intermediate used in the 2-hydroxyethylamino oxidized starch of this embodiment was prepared through the following steps:

[0122] 100 parts by weight of corn starch were dispersed in 550 parts by weight of deionized water at 22°C with mechanical stirring (200 rpm) for 20 min to obtain a starch slurry. 74 parts by weight of sodium periodate were added to the starch slurry, and the pH was adjusted to 3.7 with a 0.8 mol / L hydrochloric acid aqueous solution. The mixture was reacted at 17°C with mechanical stirring (200 rpm) for 7.5 h under light-protected conditions. After the reaction was complete, the mixture was filtered and washed with deionized water, using four times the dry weight of the product for each wash, for a total of three washes. The washing endpoint was reached when the pH of the filtrate stabilized at 5.7. The mixture was then vacuum dried at 58°C and 2.5 kPa for 15 h. The drying endpoint was reached when the mass difference between two consecutive weighings taken 1 h apart did not exceed 0.1% of the total mass, yielding an oxidized starch intermediate with an aldehyde content of 2.8 mmol / g. In this embodiment, the aldehyde content was determined using hydroxylamine hydrochloride titration based on the dry sample mass.

[0123] The 2-hydroxyethyl amino-modified oxidized starch in this embodiment is prepared by the following steps:

[0124] 100 parts by weight of oxidized starch intermediate were dispersed in 330 parts by weight of deionized water at 25°C with mechanical stirring (200 rpm) for 20 min. 33 parts by weight of ethanolamine were added to the system, and the pH was adjusted to 10.3 with a 1.8 mol / L NaOH aqueous solution. The reaction was carried out at 43°C with mechanical stirring (200 rpm) for 2.2 h. 7.5 parts by weight of sodium borohydride were prepared as a 5 wt% alkaline aqueous solution (NaOH concentration 0.5 mol / L, pH controlled at 12), and slowly added to the system in 5 batches (8 min intervals), maintaining the pH at 10-12 during the addition. The reaction was continued at 22°C for 2.8 h. After the reaction was complete, the mixture was filtered and washed with deionized water, using 4 times the dry weight of the product for each wash, for a total of 3 washes. The washing endpoint was reached when the pH of the filtrate stabilized at 6.5. Vacuum drying was performed at 58℃ and 2.5 kPa absolute pressure for 15 h. The drying endpoint was determined by the mass difference between two consecutive weighings taken 1 h apart not exceeding 0.1% of the total mass, yielding 2-hydroxyethyl-amino-oxidized starch with an amino content of 0.75 mmol / g and a residual aldehyde content of 0.09 mmol / g. In this example, the nitrogen atom in the 2-hydroxyethyl-amino-oxidized starch exists as a secondary amine after reductive amination of ethanolamine and starch aldehyde groups, with a chemical structure of starch skeleton -CH2-NH-CH2CH2OH. The amino content in this example was determined by potentiometric titration based on the dry sample mass.

[0125] The preparation method of the biodegradable polyurethane wall material system in this embodiment includes the following steps:

[0126] Triethylamine (7.0 parts by weight) was used to neutralize the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer, achieving a neutralization degree of 84 mol% at 34 °C and a stirring time of 28 min. 16 parts by weight of 2-hydroxyethylamino oxidized starch were dispersed in 75 parts by weight of deionized water. The pH of the system was adjusted to 6.7 with a 1 mol / L NaOH aqueous solution, and the dispersion was obtained by shearing at 5800 rpm for 18 min using a rotor-stator type high-speed shear disperser at 25 °C. 84 parts by weight of the neutralized isocyanate-terminated biodegradable polyurethane prepolymer were added to the dispersion at 22 °C. After a pre-reaction of 55 min, the mixture was aged at 48 °C for 1.9 h to obtain a biodegradable polyurethane wall material system with a median particle size (D50) of 95 nm. 195.4 parts by weight of deionized water were then added. The volumetric median particle size (D50) of the biodegradable polyurethane wall material system in this embodiment was determined by dynamic light scattering at 25±1℃ after diluting the sample with deionized water to approximately 0.1wt%, and the volume distribution D50 value was recorded. The wall material system in this embodiment has a pH of 6.2 and a viscosity of 1400 mPa·s. The solid content of the wall material system in this embodiment, based on the total mass of the dispersion, was determined to be 27wt% using the constant gravimetric method at 105℃. The viscosity of the wall material system in this embodiment was measured using a rotational viscometer at 25±0.5℃, 60 rpm, and a rotor with a range adaptation, and a stable reading was recorded.

[0127] In this embodiment, the isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethyl-amino-modified oxidized starch form a covalent network in the wall material system through urea and urethane bonds. To verify the formation of this covalent network, the biodegradable polyurethane wall material system sample of this embodiment was washed three times with ethanol and deionized water and then vacuum-dried at 50°C. The dried sample was then subjected to FTIR analysis; the obtained FTIR spectrum was at 1650 cm⁻¹. -1 An absorption peak, consistent with the characteristic stretching vibration of the urea bond C=O, appears at 1720 cm⁻¹. -1 An absorption peak corresponding to the characteristic stretching vibration of the C=O bond of the urethane bond appeared at the specified location. Using a separately treated isocyanate-terminated biodegradable polyurethane prepolymer sample as a control, the difference in the FTIR spectra of the two groups of samples in the above range was recorded, confirming the formation of the covalent network.

[0128] Furthermore, the biodegradable polyurethane wall material system of this embodiment was mixed with a hydrophobic liquid core material, with a weight ratio of 5.5:1 between the hydrophobic liquid core material and the solid components in the biodegradable polyurethane wall material system. The mixture was emulsified for 9 minutes at 11500 rpm using a rotor-stator type high-speed shear homogenizer, with the emulsification temperature controlled at 25°C. Subsequently, it was cured at 58°C for 3.8 hours. During curing, mechanical stirring was used to maintain the homogeneity of the system at a stirring speed of 250 rpm, resulting in microcapsules with a polyurethane / starch covalent network wall layer. The median volumetric particle size (D50) of the microcapsules in this embodiment was 2 µm. The median volumetric particle size (D50) of the microcapsules in this embodiment was determined by laser diffraction in a deionized water dispersion medium under 5% opacity conditions.

[0129] This embodiment achieves a dense wall structure by using low molecular weight polyols and a high NCO / OH molar ratio (1.72:1). Ultrafine capsules (2µm) are prepared by high shear rate and high emulsification rate (11500rpm), which are suitable for high-end applications with strict requirements for microcapsule particle size and wall density, such as micron-level precision embedding, high-performance coating additives and electronic ink microcapsules.

[0130] Comparative Example 1: It is basically the same as Example 1, except that the wall material system includes 50 parts by weight of solid components and 50 parts by weight of deionized water in a total of 100 parts by weight, and other conditions remain unchanged.

[0131] Comparative Example 2: It is basically the same as Example 1, except that the solid component includes 90 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 10 parts by weight of 2-hydroxyethyl amino oxidized starch per 100 parts by weight, and other conditions remain unchanged.

[0132] Comparative Example 3: It is basically the same as Example 1, except that the solid components include 65 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 35 parts by weight of 2-hydroxyethyl amino oxidized starch per 100 parts by weight, and other conditions remain unchanged.

[0133] Comparative Example 4: Basically the same as Example 1, except that the isocyanate group content of the isocyanate-terminated biodegradable polyurethane prepolymer is controlled at 1.5 wt% when the reaction is stopped in step A4, while other conditions remain unchanged.

[0134] Comparative Example 5: Basically the same as Example 1, except that the isocyanate group content of the isocyanate-terminated biodegradable polyurethane prepolymer is controlled at 5.0 wt% when the reaction is stopped in step A4, while other conditions remain unchanged.

[0135] Comparative Example 6: Basically the same as Example 1, except that the amino content of 2-hydroxyethylamino oxidized starch is 0.15 mmol / g, and other conditions remain unchanged.

[0136] Comparative Example 7: Basically the same as Example 1, except that the residual aldehyde content of 2-hydroxyethylamino oxidized starch is 0.12 mmol / g, and other conditions remain unchanged.

[0137] Comparative Example 8: Basically the same as Example 1, except that the degree of neutralization of the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer is 70 mol%, and other conditions remain unchanged.

[0138] Comparative Example 9: Basically the same as Example 1, except that the weight ratio of the solid component in the hydrophobic liquid core material to the biodegradable polyurethane wall material system is 6:1, and other conditions remain unchanged.

[0139] Comparative Example 10: Basically the same as Example 1, except that the shear dispersion speed in step S3 is 2000 rpm and the shear dispersion time is 12 min, while other conditions remain unchanged.

[0140] Performance testing:

[0141] Experiment 1: The high solids content and low viscosity processability window and batch-to-batch consistency of the biodegradable polyurethane wall material dispersions (Examples and Comparative Examples) were evaluated using the constant weight method and rotational viscometer. Solids content was determined by drying to constant weight at 105℃ to measure non-volatile components. Apparent viscosity was measured at 25±0.5℃ and 60 rpm, with additional apparent viscosity measurements at different rotational speeds as needed. Each sample was measured in parallel at least three times, with a 10-minute equilibration period before viscosity measurement. The mean ± standard deviation of solids content and viscosity were output, and the coefficient of variation (CV) was calculated to evaluate batch-to-batch consistency.

[0142] Experiment 2: The biodegradable polyurethane wall material system was diluted to approximately 0.1 wt%. Dynamic light scattering was used to determine the volumetric particle size distribution (D10 / D50 / D90) and PDI at 25 ± 1 °C. Storage stability was evaluated by measuring particle size drift over time. The sample was stored at 25 °C for 30 days, and particle size and PDI were measured again on days 0, 7, 14, and 30. Each measurement was repeated at least three times with the same dilution factor. The final output showed the trends in particle size drift and PDI, which were used to determine the storage stability level.

[0143] Experiment 3: Dry samples of isocyanate-terminated biodegradable polyurethane prepolymer, 2-hydroxyethyl-amino-modified oxidized starch, and their curing products were taken. The changes in characteristic absorption peaks were tracked by FTIR to demonstrate the formation of urea and urethane bonds and to monitor the mild curing process. Samples were taken at 0, 10, 30, and 60 min of the pre-reaction process according to the example. After washing, the samples were vacuum dried at 50°C, and samples were collected from 4000–600 cm⁻¹. -1 The spectra, with a focus on comparing 1630–1660 cm⁻¹ -1 With 1690–1730cm -1The characteristic peak of the interval and about 2270cm -1 The attenuation of the NCO peak, at a resolution of 4 cm⁻¹. -1 The scan was performed 32 times. The peak areas at each time point were normalized and the superimposed spectrum was output.

[0144] Experiment 4: For microcapsule aqueous dispersions with polyurethane / starch covalent network walls, the particle size distribution was determined by laser diffraction to evaluate emulsification controllability and batch-to-batch consistency, and the risk of aggregation and demulsification was assessed by the distribution width. During sample preparation, the opacity was fixed at 2–8%, deionized water was used as the dispersion medium, and ultrasonic pre-dispersion was performed at 25±1℃ for no more than 1 min to avoid capsule rupture. Three batches were measured repeatedly. The D50 and Span=[(D90–D10) / D50] for each batch were output, and the mean ± standard deviation was reported.

[0145] Experiment 5: The biodegradable polyurethane wall material system was cast into a film and cured according to the temperature and time specified in the example. After cutting the sample strips, they were conditioned at 23℃ / 50%RH for 24 hours. Stress-strain curves were obtained through uniaxial tensile testing to quantify the wall layer's strength and toughness to support high core load and low leakage performance. The sample was stretched to fracture at the specified clamping distance and tensile rate of 50 mm / min, with at least five valid parallel samples. The mean ± standard deviation of the tensile strength was output, and the original curve CSV file was retained.

[0146] Experiment 6: Using the same process as in Experiment 5, the wall film samples were prepared. The water resistance and swelling resistance of the wall layer were evaluated by measuring the mass increase and dimensional changes after immersion in water, taking into account both the stability of the aqueous system and the service durability requirements. After drying and weighing, the samples were immersed in deionized water at 25℃ and 40℃ for 24h and 72h, respectively. After each time point, the samples were removed, dried, weighed, and the thickness change was measured. The sample thickness was kept consistent, and at least three parallel measurements were performed at each time point. The water absorption rate (%) and swelling rate (%) under each condition were calculated, and correlation analysis was performed with leakage indicators.

[0147] Experiment 7: Take 5–10 mg of microcapsule samples containing hydrophobic liquid core material and establish a quantifiable index for accelerated leakage / volatilization loss using isothermal thermogravimetric analysis under a nitrogen atmosphere. After programmed heating to 60℃, maintain the temperature for 24 hours and continuously record the mass change curve, keeping the gas flow rate constant throughout. If necessary, perform blank subtraction on empty wall material microcapsules to eliminate interference from the mass loss of the wall material itself. Define the leakage rate as the 24-hour mass loss (%) and output a superimposed graph of each set of curves.

[0148] Experiment 8: Microcapsules with the wall layer film formed and those without free core material were collected and subjected to biodegradation tests simultaneously with a reference material in a controlled composting system at 58℃. CO2 was collected online / offline and the 28-day biodegradation rate was calculated to verify degradability and quantify differences in degradation rates. Moisture content and aeration were strictly controlled according to composting conditions, and each group underwent at least three parallel measurements. The output is a complete curve showing the mean ± standard deviation of degradation rate and the change in CO2 release over time.

[0149] Figure 1 The images show the Fourier transform infrared (FTIR) full-spectrum overlays of the wall material samples from Example 1, Comparative Example 1, and Comparative Example 3. The basic parameter is that the test wavenumber range covers the high wavenumber region to the fingerprint region. Variable parameters include whether a synergistic cross-linking structure is formed in the wall material reaction system and the differences in film composition among the different samples. The figures focus on comparing the intensity variations of urea bond characteristic absorption, urethane bond characteristic absorption, and residual isocyanate absorption. The results show that Example 1 exhibits more distinct and coordinated absorption characteristics in the corresponding wavebands for urea and urethane bonds, while the residual isocyanate signal is weaker. This indicates that the wall material reaction is more complete in this scheme, and the covalent network construction is more complete, providing a structural basis for subsequent coating stability.

[0150] Figure 2 The images show magnified Fourier transform infrared (FTIR) images of the urea bond region in the wall material samples of Example 1, Comparative Example 1, and Comparative Example 3. The basic parameter is a high-resolution local display of the characteristic absorption region of urea bonds, and the variable parameters are the degree of urea bond formation and the corresponding peak shape and intensity differences in different samples. The reaction characteristics of the wall materials were compared using Fourier transform infrared spectroscopy. The results show that Example 1 has clearer absorption and more stable peak shape in the urea bond characteristic region, indicating that urea bond formation is more complete during its interfacial reaction, which helps to improve the compactness and structural stability of the wall layer. This proves that the proposed scheme is more reasonable in terms of in-situ construction of cross-linked wall layers.

[0151] Figure 3 The images show magnified Fourier transform infrared (FTIR) images of the urethane ester region in the wall material samples of Example 1, Comparative Example 1, and Comparative Example 3. The basic parameter is the local magnified analysis of the characteristic absorption region of urethane bonds, and the variable parameters are the degree of urethane bond formation and the changes in peak position and intensity in different samples. The characterization method is Fourier transform infrared spectroscopy. The results show that Example 1 has a more obvious absorption response in the urethane ester characteristic region, indicating that in addition to urea bonds, urethane bonds are effectively introduced into its system to form a synergistic network, making the wall material network more complete. This further proves that the composite reaction pathway used in this scheme is effective.

[0152] Figure 4The graphs show the changes in residual NCO content and conversion rate over aging time for the systems of Example 1, Comparative Example 1, and Comparative Example 2. The basic parameters are aging time, residual isocyanate content, and their converted conversion rate. Variable parameters include different sample formulations and aging processes. A process tracking method was used to compare the isocyanate group consumption rate. The results show that in Example 1, residual NCO decreased more rapidly and the conversion rate increased more significantly, indicating a smoother reaction path and higher interfacial polymerization efficiency. This allows for the formation of a more stable wall structure in a shorter aging process, demonstrating that this scheme has good reaction controllability and film-forming rationality.

[0153] Figure 5 The dynamic light scattering particle size distribution diagrams for the aqueous dispersion systems of Example 1, Comparative Example 7, and Comparative Example 8 are shown. The basic parameters are the nanoparticle size distribution range and volume distribution ratio, while the variable parameters are the dispersion stability and aggregation degree of different samples. The characterization method is dynamic light scattering. The results show that the distribution peak of Example 1 is more concentrated, the main peak particle size is smaller, and the single-peak characteristic is more obvious, while the comparative example samples show an increase in particle size or a broadening of the distribution. This indicates that this scheme can maintain a better nanoscale dispersion state under high solid content conditions, demonstrating superior system stability.

[0154] Figure 6 The figures show the D50 drift during storage for the aqueous dispersion systems of Example 1, Comparative Example 9, and Comparative Example 7. The basic parameters are storage time and the corresponding D50 change, while the variable parameter is the particle size increase during storage for different samples. The characterization method is dynamic light scattering to track storage stability. The results show that Example 1 exhibits less particle size change during storage, and its D50 drift is significantly lower than that of the comparative samples, indicating better long-term stability and less susceptibility to significant aggregation. This demonstrates that this method can balance initial particle size control with maintaining stability in the later stages.

[0155] Figure 7 The thermogravimetric mass percentage graphs for microcapsule samples of Example 1, Comparative Example 1, and Comparative Example 7 are shown. The basic parameter is the change in mass retention rate during the heating process, and the variable parameters are the wall layer structure and core material retention state of different samples. The characterization method is thermogravimetric analysis. The results show that Example 1 exhibits more reasonable weight loss behavior in both the core material volatilization and wall material decomposition stages, reflecting a more stable coating structure and stronger core material constraint ability, indicating that the wall layer formed by this scheme can effectively delay the loss of core material.

[0156] Figure 8The thermogravimetric derivatives of the microcapsule samples from Example 1, Comparative Example 1, and Comparative Example 7 are shown. The basic parameter is the relationship between temperature and weight loss rate, and the variable parameters are the thermal decomposition stages, rate peak positions, and intensities of different samples. The characterization method is the comparison of thermogravimetric analysis derivative curves. The results show that the weight loss rate peak distribution of Example 1 is more hierarchical, and the core material segment and wall material segment are more clearly distinguished, indicating that its microcapsule structure has more distinct stratification and the wall layer protection is more stable, further proving that this scheme is effective in the synergistic effect of encapsulation and curing.

[0157] Figure 9 a is a macroscopic optical photograph of the biodegradable polyurethane wall material system dispersion prepared in Example 1 of this invention. The dispersion has a solid content of 35 wt% and a median particle size D. 50 The wall material system obtained by neutralizing and emulsifying isocyanate-terminated biodegradable polyurethane prepolymer and 2-hydroxyethyl-amino oxidized starch with triethylamine has good colloidal stability and macroscopic uniformity. The viscosity is 850 mPa·s, pH 7.0, and the appearance is uniformly milky white without phase separation or flocculation precipitation. Figure 9 b is a macroscopic optical photograph of the microcapsule aqueous suspension prepared in Example 1 of this invention. The mass ratio of the core material to the wall material solid components is 3:1, and the volume median particle size D is... 50 The suspension has a diameter of 8 μm, is uniformly white and opaque, and exhibits no cell wall breakage, oil leakage, or macroscopic particle aggregation. This demonstrates that the biodegradable polyurethane wall material system can achieve complete and stable encapsulation of the hydrophobic liquid core material under the conditions of emulsification at 7500 rpm and curing at 48℃. Figure 9 c is a macroscopic optical photograph of the microcapsule vacuum-dried powder prepared in Example 1 of this invention. The drying conditions were 50°C and 3.5 kPa. The powder was white and free-flowing, without caking or macroscopic agglomeration. This proves that the polyurethane / starch covalent network wall layer gives the microcapsules sufficient mechanical strength, enabling them to maintain the independence and flowability of the particle morphology under drying conditions, and possessing good powder processing suitability.

[0158] Figure 10 Image a is a low-magnification scanning electron microscope image of the biodegradable polyurethane wall material system nanoparticles prepared in Example 1 of this invention. The low-magnification overall distribution image shows that the nanoparticles are uniformly dispersed on the substrate surface, with no significant aggregates or local blank areas in the macroscopic region. This proves that the combination of triethylamine neutralization degree of 81 mol% and high-speed shear dispersion process of 4000 rpm can obtain a wall material nanoparticle system with a macroscopically uniform distribution. Figure 10b. High-magnification scanning electron microscope image of nanoparticles in the biodegradable polyurethane wall material system prepared in Example 1 of this invention. The nanoparticles are spherical to nearly spherical, with a solid particle size of about 130 to 160 nm. The particle size distribution is uniform, and the interparticles are mainly in point contact with each other. No fusion phenomenon was observed, which proves that the covalent cross-linking of urea bonds and urethane bonds gives the nanoparticles sufficient morphological stability, and the interparticle interfaces are clear and independent. Figure 10 c is a low-magnification scanning electron microscope image of the microcapsules prepared in Example 1 of this invention, with a magnification of approximately 2000x. The microcapsules have a median particle size D. 50 The particles are distributed in a near-spherical shape with a center of about 8 μm. The particles are uniformly dispersed within the field of view and there are no macroscopic agglomerates. This proves that the rotor-stator type homogenizer can disperse the hydrophobic liquid core material into droplets of the target particle size under emulsification conditions of 7500 rpm. After solidification, the particle size morphology of the microcapsules is effectively preserved. Figure 10 Image d is a high-magnification scanning electron microscope image of the surface of the microcapsules prepared in Example 1 of this invention. The surface of a single microcapsule is smooth and continuous, without pores or defects, proving that under the curing condition of 48°C for 2.5 h, the polyurethane / starch covalent network wall layer formed by the reaction of isocyanate-terminated degradable polyurethane prepolymer and 2-hydroxyethyl-amino-oxidized starch is completely cured and has good sealing properties. Figure 10 e is a scanning electron microscope image of the frozen fracture section of the microcapsule prepared in Example 1 of this invention. The cross-section clearly shows the core-shell two-phase structure, the wall layer thickness is about 300-500 nm, the wall layer is uniformly distributed along the circumference, and the inner cavity wall is smooth. This proves that the microcapsule wall layer structure obtained by the emulsification and solidification process is complete, the thickness is uniform, the core-shell interface outline is clear, and the coating morphology is stable. Figure 10 f is a high-magnification scanning electron microscope image of a local area of ​​the microcapsule cross-section prepared in Example 1 of this invention. The wall layer cross-section is dense and homogeneous with small thickness deviation along the circumferential direction. The boundary between the core material area and the wall layer interface is clear, which proves that the covalent cross-linking network formed by polyurethane and 2-hydroxyethylamino oxidized starch through urea bonds and urethane bonds effectively ensures the uniformity and mechanical integrity of the microcapsule wall layer structure.

[0159] As can be seen from the performance of the examples and comparative examples in Table 1, Examples 1-4 achieved good dispersion stability within the nanoscale particle size, zeta potential, and viscosity window, thus obtaining more controllable microcapsule particle size under the same emulsification and curing conditions, while maintaining a high core material retention rate. In the comparative examples, deviations in isocyanate group content or amino content led to insufficient or excessive cross-linking of the covalent network, causing an imbalance in strength, water resistance, and degradability; deviations in dispersion pH, shear, or neutralization degree weakened charge stability and dispersion efficiency, manifested as increased D50, abnormal viscosity, and increased leakage. Overall, the examples achieved a comprehensive balance between dense and toughness, low leakage, and degradability requirements.

[0160] Table 1 Performance Comparison Summary Table

[0161] As can be seen from the performance of the examples and comparative examples in Table 1, Examples 1-4 achieved good dispersion stability within the nanoscale particle size, zeta potential, and viscosity window, thus obtaining more controllable microcapsule particle size under the same emulsification and curing conditions, while maintaining a high core material retention rate. In the comparative examples, deviations in isocyanate group content or amino content led to insufficient or excessive cross-linking of the covalent network, causing an imbalance in strength, water resistance, and degradability; deviations in dispersion pH, shear, or neutralization degree weakened charge stability and dispersion efficiency, manifested as increased D50, abnormal viscosity, and increased leakage. Overall, the examples achieved a comprehensive balance between dense and toughness, low leakage, and degradability requirements.

[0162] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A biodegradable polyurethane wall material system, characterized in that, The wall material system is a water-dispersible wall material system, which, based on a total of 100 parts by weight, includes 25-45 parts by weight of solid components and deionized water as the balance, to be made up to 100 parts by weight; the solid components, based on 100 parts by weight, include 70-85 parts by weight of isocyanate-terminated biodegradable polyurethane prepolymer and 15-30 parts by weight of 2-hydroxyethyl amino oxidized starch, the sum of which is 100 parts by weight; The isocyanate-terminated biodegradable polyurethane prepolymer is prepared by reacting polycaprolactone diol, 2,2-dimethylolpropionic acid, 2,2'-dithiodiethanol and isophorone diisocyanate, with an isocyanate group content of 2.0-4.5 wt%. The 2-hydroxyethylamino oxidized starch is prepared by reacting starch, sodium periodate, ethanolamine and sodium borohydride, with an amino content of 0.20-0.80 mmol / g and a residual aldehyde group content of no more than 0.10 mmol / g. The isocyanate-terminated biodegradable polyurethane prepolymer and the 2-hydroxyethylamino oxidized starch form a covalent network in the wall material system through urea bonds and urethane bonds.

2. The biodegradable polyurethane wall material system as described in claim 1, characterized in that, The isocyanate-terminated biodegradable polyurethane prepolymer is prepared by the following steps: A1. Mix 100 parts by weight of polycaprolactone diol, 4-12 parts by weight of 2,2-dimethylolpropionic acid and 2-10 parts by weight of 2,2'-dithiodiethanol. A2. Treat at 95-115℃ and a vacuum of -0.08 to -0.095 MPa for 1-3 hours; A3. Under an inert atmosphere, add isophorone diisocyanate at a molar ratio of isocyanate group to hydroxyl group of 1.20-1.80:1 and react at 70-85℃ for 2-5 hours. A4. When the isocyanate group content reaches 2.0-4.5wt%, the reaction is stopped, and the mixture is cooled to 25-40℃ to obtain the isocyanate-terminated biodegradable polyurethane prepolymer.

3. The biodegradable polyurethane wall material system as described in claim 1, characterized in that, The 2-hydroxyethyl-amino-treated oxidized starch is prepared by the following steps: C1. Disperse 100 parts by weight of the oxidized starch intermediate in 300-800 parts by weight of deionized water; C2. Add 10-35 parts by weight of ethanolamine to the system, control the pH of the system to 8.5-10.5, and react at 25-45℃ for 2-6 hours; C3. Prepare an alkaline aqueous solution of sodium borohydride with a concentration of 1-5 wt% by 1-8 parts by weight, wherein the alkaline aqueous solution is a NaOH solution with a concentration of 0.1-0.5 mol / L and the pH of the solution is controlled at 10-12. Add the solution slowly in batches with an interval of 5-15 min between batches. Maintain the pH of the system at 10-12 during the addition process and continue the reaction at 20-35℃ for 1-3 h. C4. Filter, wash with deionized water 2-4 times, each time using 3-5 times the dry weight of the product. The washing endpoint is when the pH of the filtrate stabilizes at 5.5-7.

5. Then, vacuum dry at 40-60℃ and 2-5 kPa absolute pressure for 8-16 hours. The drying endpoint is when the mass difference between two consecutive weighings with a 1-hour interval does not exceed 0.1% of the total mass.

4. The biodegradable polyurethane wall material system as described in claim 3, characterized in that, The oxidized starch intermediate used to prepare the 2-hydroxyethyl-amino-oxidized starch is prepared through the following steps: B1. Disperse 100 parts by weight of starch in 500-1000 parts by weight of deionized water to obtain starch slurry; B2. Add 20-80 parts by weight of sodium periodate to the starch slurry, adjust the pH of the slurry to 3.5-5.5 with an aqueous solution of hydrochloric acid with a concentration of 0.1-1 mol / L or an acetate-sodium acetate buffer solution with a total concentration of 0.1-0.5 mol / L, and react at 15-30℃ for 4-8 hours under dark conditions; B3. Filter and wash with deionized water until the pH of the filtrate is 5.5-7.5; B4. Vacuum dry at 40-60℃ and 2-5 kPa absolute pressure for 6-16 hours. The drying endpoint is defined as the mass difference between two consecutive weighings with an interval of 1 hour not exceeding 0.1% of the total mass, to obtain the oxidized starch intermediate with an aldehyde content of 1.0-3.0 mmol / g. D4. Obtain the biodegradable polyurethane wall material system with a volume median particle size D50 of 80-300 nm.

5. The biodegradable polyurethane wall material system as described in claim 1, characterized in that, The number average molecular weight of polycaprolactone diol is 530-3000; the pH of the wall material system is 6.0-8.5, and the viscosity is 200-1500 mPa·s; the starch is potato starch or corn starch.

6. A method for preparing a biodegradable polyurethane wall material system as described in claims 1-5, characterized in that, Includes the following steps: S1. Provides a pre-prepared isocyanate-terminated biodegradable polyurethane prepolymer; S2. Provides the prepared 2-hydroxyethyl aminoated oxidized starch; S3. Disperse 15-30 parts by weight of 2-hydroxyethylamino oxidized starch in 70-200 parts by weight of deionized water, control the pH of the system to 6.5-8.5, and shear at 2000-6000 rpm for 5-20 min to obtain a dispersion; S4. Add 70-85 parts by weight of the isocyanate-terminated biodegradable polyurethane prepolymer to the dispersion obtained in step S3 at 20-35℃, pre-react for 10-60 min, and then mature at 35-50℃ for 0.5-2 h. S5. Add deionized water to the system obtained in step S4, including 25-45 parts by weight of solid components and the remainder of deionized water, to make up to 100 parts by weight, to obtain the biodegradable polyurethane wall material system with a median particle size D50 of 80-300 nm.

7. The preparation method according to claim 6, characterized in that, Before step S4, the carboxyl groups introduced by 2,2-dimethylolpropionic acid in the isocyanate-terminated biodegradable polyurethane prepolymer are neutralized with a tertiary amine neutralizing agent. The degree of neutralization is 80-100 mol%, the neutralization temperature is 25-35°C, and the stirring and neutralization time is 15-30 min.

8. The preparation method according to claim 6, characterized in that, After step S5, the biodegradable polyurethane wall material system is further mixed with the hydrophobic liquid core material and emulsified at 3000-12000 rpm for 3-10 min, with the emulsification temperature controlled at 20-35℃. Then, it is cured at 35-60℃ for 1-4 h to obtain microcapsules with a polyurethane / starch covalent network wall layer.

9. The preparation method according to claim 8, characterized in that, The weight ratio of the hydrophobic liquid core material to the solid component in the biodegradable polyurethane wall material system is 1:1 to 6:1, and the volume median particle size D50 of the microcapsules is 1-20µm.

10. The preparation method according to claim 7, characterized in that, The tertiary amine neutralizing agent is at least one of triethylamine, N-methyldiethanolamine, or triethanolamine.

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