A thermal expansion microsphere foaming agent and a preparation method thereof

Abstract

The present application belongs to the technical field of microcapsule encapsulation, and particularly relates to a thermal expansion microsphere foaming agent and a preparation method thereof. The method comprises the following steps: 1) uniformly mixing synthetic rubber and an anionic surfactant to prepare a continuous phase solution; 2) using unsaturated nitrile compounds and ester compounds as base materials to prepare a dispersed phase solution; 3) uniformly mixing the continuous phase solution and the dispersed phase solution in a certain proportion to perform a thermal expansion microsphere polymerization reaction and prepare a base material; 4) performing gradient curing on the base material to prepare a precursor; and 5) performing functional modification on the precursor to prepare the thermal expansion microsphere foaming agent. The present application ensures that the thermal expansion microspheres have good stability and dispersibility through a double crosslinking network and three-phase surface energy control, and simultaneously optimizes the foaming ratio-foaming strength flatness of the thermal expansion microsphere foaming agent, so as to ensure that the foamed spheres have good mechanical stability and thermal stability.

Description

Technical Field

[0001] This invention belongs to the field of microcapsule encapsulation technology, specifically relating to a thermally expanding microsphere foaming agent and its preparation method. Background Technology

[0002] Thermally expandable microspheres (TEMs), as a class of functional microcapsule materials with a core-shell structure, have gained wide application in lightweight composite materials, functional coatings, and precision manufacturing due to their unique physicochemical properties. The core structure of TEMs consists of a thermoplastic polymer shell encapsulating a low-boiling-point alkane, forming a core-shell microcapsule. Their working principle is based on a thermodynamic phase transition process: when the temperature rises to near the polymer's glass transition temperature, the outer shell softens, while the inner alkane reaches its vapor pressure expansion point, causing a significant increase in microsphere volume. This process can be described as the polymer shell transitioning from a glassy to a viscoelastic state while the inner alkane transitions from a liquid to a gaseous state, resulting in pressure-driven shell expansion. The limitation on the expansion ratio of traditional microspheres stems from the inherent contradiction of the shell material: increasing the expansion ratio requires a more flexible shell, while improving thermal stability requires a more rigid shell. This trade-off between polymer chain mobility and structural stability leads to significant volume shrinkage in most microspheres above 150 °C, with chain rearrangement and elastic recoil occurring at high temperatures. Existing traditional technologies for thermally expandable microspheres still have technical defects. First, some microsphere materials have insufficient expansion rate and stability, and low expansion rate will make it difficult for the material to meet foaming requirements. Second, poor dispersibility and flowability can easily lead to local over- or under-foaming. Third, low-temperature microspheres are prone to deterioration in expansion performance due to shell leakage during storage, and may even cause dust explosions. Finally, expanded microspheres may undergo secondary expansion or contraction at high temperatures, affecting the long-term stability of the product. Summary of the Invention

[0003] The present invention addresses the problems of expansion rate-stability contradiction, poor dispersibility, low-temperature leakage and high-temperature secondary deformation of traditional thermal expansion microspheres by providing a thermal expansion microsphere foaming agent and its preparation method.

[0004] The main objective of this invention is to: 1. Improve the expansion ratio and high-temperature stability of thermally expandable microspheres.

[0005] II. Improve the dispersibility and flowability of microspheres.

[0006] Third, it achieves narrow control of the foaming temperature window to avoid storage leakage and secondary expansion.

[0007] To achieve the above objectives, the present invention adopts the following technical solution.

[0008] A method for preparing a thermally expanding microsphere foaming agent, the method comprising: 1) mixing synthetic rubber and anionic surfactant evenly to prepare a continuous phase solution.

[0009] 2) Using unsaturated nitrile compounds and ester compounds as substrates, a dispersed phase solution was prepared.

[0010] 3) Mix the continuous phase solution and the dispersed phase solution in a certain proportion to carry out the polymerization reaction of thermally expanded microspheres to prepare the substrate.

[0011] 4) Gradient curing of the substrate to prepare the precursor.

[0012] 5) Functionalize the precursor to produce a thermally expanding microsphere foaming agent.

[0013] Preferably, the synthetic rubber in step 1) is polyvinyl alcohol; the anionic surfactant in step 1) is sodium dodecyl sulfate; and the continuous phase solution in step 1) consists of 0.5–1.2 wt% polyvinyl alcohol and 0.05–0.1 wt% sodium dodecyl sulfate, with the balance being deionized water.

[0014] Preferably, the process of preparing the continuous phase solution in step 1) is as follows: stirring for 2 to 3 hours under environmental conditions of 68 to 72 ℃ and 500 to 800 rpm.

[0015] Preferably, the unsaturated nitrile compound in step 2) is acrylonitrile; the ester compound in step 2) is glycidyl methacrylate.

[0016] As a preferred embodiment, the specific steps for preparing the dispersed phase solution in step 2) are as follows: unsaturated nitrile compounds and ester compounds are mixed evenly at a mass ratio of (10-15):3, and stirred for 10-20 min under nitrogen atmosphere, temperature of 25-30 ℃, and rotation speed of 300-500 rpm to form a premixed monomer.

[0017] 1–2 wt% of trimethylolpropane trimethacrylate and 1–2 wt% of azobisisobutyronitrile were mixed evenly and added to the premixed monomer. The mixture was stirred for 30–40 min under a nitrogen atmosphere, at a temperature of 33–37 °C, a rotation speed of 300–400 rpm, and a dropping rate of 1 mL / min to add a polyethyleneimine ethanol solution (polyethyleneimine dissolved in 3–4 times its weight of anhydrous ethanol). Finally, the mixture was stirred for 5–10 min under a temperature of 4–6 °C, a rotation speed of 300–400 rpm, and a dropping rate of 2 mL / min to add a composite foaming agent.

[0018] The composite foaming agent is prepared by mixing isobutane and n-pentane in a mass ratio of 7:(2.8-3.2).

[0019] Preferably, the continuous phase solution and the dispersed phase solution in step 3) are mixed uniformly at a mass ratio of (8.5 to 9.5):1.

[0020] Preferably, the polymerization reaction in step 3) is carried out in a microfluidic emulsification device with a channel diameter of 180–220 μm and a shear rate of 4800–5200 s⁻¹. -1 The substrate was obtained by filtration under nitrogen atmosphere, temperature of 70–74 °C, and pressure of 0.28–0.32 MPa for 6–8 hours.

[0021] Preferably, step 4) of gradient curing includes: a first stage: circulating hot air at a heating rate of 2 °C / min to 68–72 °C for 1.75–2.25 h.

[0022] Second stage: Increase the temperature to 120-130 ℃ and vacuum dry for 1.5-2 hours.

[0023] The third stage: cooling to 30-40 °C in a nitrogen atmosphere at a rate of 5 °C / min.

[0024] Preferably, the functional modification in step 5) includes three stages; the first stage is to atomize and spray 1.8 to 2.5 wt% of vinyltriethoxysilane onto the surface of the precursor at an ambient temperature of 55 to 65 °C and an atomization pressure of 0.2 to 0.3 MPa.

[0025] Second stage: Using 0.9-1.4 wt% of tridecafluorooctyltriethoxysilane as a precursor, vapor deposition was performed on the precursor that had completed the first stage of functional modification in an environment of 145-155 °C.

[0026] In the third stage, 0.8–1.0 wt% of hydrophobic fumed silica was mixed with the precursor that had undergone the second stage of functional modification. The mixture was stirred for 35–55 min at an ambient temperature of 23–27 °C and a rotation speed of 12–18 rpm.

[0027] A thermally expanding microsphere foaming agent.

[0028] The core of this invention lies in the synergistic effect of three dimensions: high-performance cross-linking network design, surface energy gradient regulation, and narrow distribution response control. This solves the technical problems of traditional thermally expandable microspheres in terms of expansion rate, thermal stability, and dispersibility, and constructs a performance-balanced system of "high expansion-thermal stability-excellent dispersion".

[0029] This invention develops a unique dual-mode crosslinking system that achieves a balanced optimization of high-temperature morphological stability and controllable deformation capability of the microsphere shell through the synergistic effect of a rigid main network and a flexible auxiliary network. The main crosslinking network is based on the ring-opening addition reaction of polyethyleneimine and glycidyl methacrylate epoxy groups to form β-hydroxyamine structural connection points. The main network provides the skeletal rigidity of the microspheres, while the auxiliary crosslinking network is formed through the free radical polymerization of trimethylolpropane trimethacrylate, and its branched structure creates local flexible regions. The synergistic effect of the two networks is manifested as a complementary mechanism of "rigid constraint-elastic deformation". This structure improves the storage modulus retention rate at 150 °C compared to traditional single networks; at the same time, the loss factor is maintained within the ideal range, ensuring sufficient viscoelastic response. The dual-mode structure forms a typical heterogeneous network: the main network forms a rigid skeleton, and the auxiliary network creates flexible regions. This "rigid-flexible" alternating structure enables the microspheres to achieve isotropic deformation without breaking during expansion. The structure exhibits low deformation rate during continuous treatment at 180 °C, and the microspheres maintain their morphology, demonstrating excellent high-temperature structural stability.

[0030] To address the dispersion problem of traditional microspheres in polymer matrices, this invention develops a three-phase surface energy modulation strategy. Through precise design of surface molecules and nanostructures, it optimizes the interface matching between microspheres and the matrix. The first phase structure is based on the hydrolysis and condensation of vinyltriethoxysilane, forming a dense Si-O-Si primary network, providing a basic platform for surface modification. The second phase structure employs tridecafluorooctyltriethoxysilane grafting, significantly reducing surface energy through the directional arrangement of long-chain fluorocarbon groups. The third phase introduces hydrophobically modified nano-silica, creating nanoscale surface roughness and forming a gradient transition from silane layers to fluorosilane layers from the inside out. This gradient structure ensures energy barrier matching with the epoxy resin matrix, achieving near-ideal interfacial compatibility. Another innovation in surface morphology engineering lies in the "deliberately irregular" distribution of nano-silica particles, with the surface roughness parameter Ra controlled within the range of 18–22 nm, forming "uniform irregularity" at the microscopic level. This special surface morphology significantly increases the spatial barrier effect between microspheres, effectively suppressing the tendency to aggregate.

[0031] To address the issue of the wide temperature response range of traditional microspheres, this invention achieves highly controllable microsphere foaming behavior through a synergistic strategy of polymer composition homogenization and precise core alkane distribution. First, controlled free radical polymerization technology is used to precisely regulate the reactivity of comonomers, keeping the reactivity ratio of acrylonitrile and glycidyl methacrylate within a tight range. This ensures high uniformity of the copolymer composition along the chain, reducing the molecular weight distribution index of the polymer shell. This directly leads to a compression of the glass transition temperature distribution width, resulting in an exceptionally narrow transition range for the microsphere shell material, almost approaching the ideal characteristics of a single-component polymer. Core alkane distribution control employs innovative microfluidic emulsification technology. Through a flow focusing chip, the size and uniformity of the droplets are precisely controlled. The optimized product exhibits near-"on / off" expansion behavior: almost no deformation before the initial temperature, and most of the microsphere volume expansion within the temperature window after reaching the initial temperature. This narrow temperature response characteristic is crucial for precise matching with the thermosetting resin curing process, avoiding structural defects caused by delayed foaming behavior.

[0032] This invention successfully constructs a highly synergistic technical system: a dual-mode crosslinking network provides the mechanical basis of the microspheres, ensuring structural stability under high-temperature conditions; three-phase surface energy regulation solves the problem of dispersion uniformity, ensuring the independent existence of the microspheres in the matrix; and synergistic temperature response control makes the foaming behavior precise and controllable, meeting the application requirements under complex process conditions. The integration of these three dimensions breaks through the technical bottleneck of traditional thermally expanding microspheres, which struggle to simultaneously achieve "high expansion rate, thermal stability, and precise responsiveness," thus realizing comprehensive performance optimization. In particular, surface energy regulation not only improves dispersibility but also indirectly enhances foaming uniformity by reducing interparticle forces; the dual-mode crosslinking network not only enhances thermal stability but also improves the accuracy of temperature response by optimizing the mechanical response characteristics of the shell.

[0033] The beneficial effects of this invention are: this invention ensures that the thermally expanded microspheres have good stability and dispersibility through a double cross-linking network and three-phase surface energy control, while optimizing the foaming ratio-foaming strength of the thermally expanded microsphere foaming agent to ensure that the foamed spheres have good mechanical stability and thermal stability. Detailed Implementation

[0034] The present invention will be further described clearly and in detail below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0035] Unless otherwise specified, all raw materials used in the embodiments of the present invention are commercially available or obtainable by those skilled in the art; unless otherwise specified, all methods used in the embodiments of the present invention are methods mastered by those skilled in the art.

[0036] Example 1: A method for preparing a thermally expandable microsphere foaming agent, the method comprising: 1) mixing 0.5 wt% polyvinyl alcohol and 0.05 wt% sodium dodecyl sulfate, with the remainder being deionized water, and stirring for 3 h at a temperature of 68 ℃ and a rotation speed of 500 rpm to prepare a continuous phase solution.

[0037] 2) Acrylonitrile and glycidyl methacrylate were mixed evenly at a mass ratio of 10:3 and stirred for 20 min under a nitrogen atmosphere at 25 ℃ and a rotation speed of 300 rpm to form a premixed monomer. 1 wt% of trimethylolpropane trimethacrylate and 1 wt% of azobisisoheptane nitrile (azobisisoheptanenitrile) of the glycidyl methacrylate were mixed evenly and added to the premixed monomer. Under a nitrogen atmosphere at 33 ℃ and a rotation speed of 300 rpm, a 1 wt% polyethyleneimine ethanol solution (polyethyleneimine was dissolved in 3 times its mass of anhydrous ethanol and mixed thoroughly as a solvent, then 1 wt% of glycidyl methacrylate was dissolved) was added and stirred for 40 min. Finally, 70% of the glycidyl methacrylate was added at a temperature of 4 ℃, a rotation speed of 300 rpm, and a drop rate of 2 mL / min. A dispersed phase solution was prepared by stirring a wt% composite foaming agent for 10 minutes under ambient conditions. The composite foaming agent was prepared by mixing isobutane and n-pentane in a mass ratio of 7:2.8.

[0038] 3) The continuous phase solution and the dispersed phase solution were mixed uniformly at a mass ratio of 8.5:1, and the polymerization reaction of the thermally expanded microspheres was carried out in a microfluidic emulsification device with a channel diameter of 180 μm and a shear rate of 4800 s⁻¹. -1 The filter substrate was subjected to nitrogen atmosphere, temperature of 70℃ and pressure of 0.28 MPa for 8 hours.

[0039] 4) Gradient curing of the substrate: First stage of gradient curing: heating to 68 ℃ with hot air circulation at a heating rate of 2 ℃ / min for 2.25 h; Second stage: heating to 130 ℃ and vacuum drying for 2 h; Third stage: cooling to 30 ℃ in a nitrogen atmosphere at a cooling rate of 5 ℃ / min to prepare the precursor.

[0040] 5) Functional modification of the precursor: In the first stage, 1.8 wt% of vinyltriethoxysilane was atomized and sprayed onto the surface of the precursor at 55 ℃ and 0.2 MPa. In the second stage, 0.9 wt% of tridecafluorooctyltriethoxysilane was vapor-deposited onto the precursor after the first stage of functional modification at 145 ℃. In the third stage, 0.8 wt% of hydrophobic fumed silica (AEROSIL® R 812, the same below) was mixed with the precursor after the second stage of functional modification and stirred for 55 min at 23 ℃ and 12 rpm to prepare a thermally expandable microsphere foaming agent.

[0041] The materials prepared in the examples were subjected to performance testing, and the specific characterization results are as follows.

[0042] Average particle size and particle size distribution: Referring to ISO 13320 standard, the material prepared in this example was measured using a laser diffractometer. The sample was ultrasonically dispersed in deionized water for 1 min, and the opacity was controlled at 10-15%. The uniformity of particle size distribution was quantitatively characterized by the coefficient of variation CV = (standard deviation / average particle size) × 100%.

[0043] Expansion ratio: Referring to GB / T30775 standard, the material prepared in this example was measured using a thermomechanical analyzer. 10 mg of sample was placed in the sample cell and heated to 130 ℃ at 10 ℃ / min under a nitrogen atmosphere. The diameter change was recorded. Expansion ratio = (expanded diameter - initial diameter) / initial diameter × 100%.

[0044] Thermal shrinkage rate: The material prepared in this example was kept in an oven at 200 ℃ for 30 min and then cooled to room temperature. Its diameter change was measured. Thermal shrinkage rate = (diameter after expansion - diameter after thermal shrinkage) / diameter after expansion × 100% to evaluate the thermal stability of the microspheres.

[0045] Angle of repose: Referring to ISO 4324 standard, the geometric parameters of the cone formed by the free flow of microspheres in the material prepared in this example were measured using a special measuring instrument, and the angle of repose of the material was measured.

[0046] Foaming temperature window: The material prepared in this example was analyzed by differential scanning calorimetry. A 5 mg sample was heated in a nitrogen atmosphere at a rate of 10 °C / min, and the difference between the endothermic start and end temperatures was measured.

[0047] Leakage rate: After storing the material prepared in this example in a constant temperature environment of 40 ℃ for 90 days, the weight loss was measured by a thermogravimetric analyzer. Leakage rate = (initial weight - weight after storage) / initial weight × 100%.

[0048]

[0049] Analysis of the characterization results shows that the thermally expandable microsphere foaming agent prepared in Example 1 exhibits excellent comprehensive performance. Its average particle size is 32.7 μm, and the coefficient of variation for particle size distribution is 6.8%, indicating highly uniform microsphere size, meeting the design goal of narrow distribution response control. This result is attributed to the precise control of microfluidic emulsification technology. The foaming agent achieves an expansion ratio of 48.5 times, significantly exceeding traditional products, verifying the effectiveness of the dual-mode crosslinking network (synergistic effect of rigid main network and flexible auxiliary network) in enhancing expansion capacity. The thermal shrinkage rate is only 4.1%, demonstrating excellent high-temperature structural stability. This characteristic is attributed to the mechanical optimization of the shell material and the gradient curing process. The angle of repose is 30.7°, confirming excellent flowability and good dispersibility of the microspheres. This performance is directly related to the three-phase surface energy control strategy (gradient transition from silane layer to fluorosilane layer and nano-silica roughness design). The foaming temperature window is narrow to 119–123 °C, showing precise temperature response characteristics. This characteristic stems from the homogenization of the polymer composition and the precise distribution of the core alkane. With a leakage rate as low as 0.8 wt%, it proves that its sealing performance is reliable and ensures long-term storage stability. The thermal expansion microsphere foaming agent of the present invention achieves synergy in terms of expansion ratio, thermal stability and dispersibility.

[0050] Example 2: A method for preparing a thermally expandable microsphere foaming agent, the method comprising: 1) mixing 0.85 wt% polyvinyl alcohol and 0.07 wt% sodium dodecyl sulfate, with the remainder being deionized water, and stirring for 2.5 h at an ambient temperature of 70 ℃ and a rotation speed of 650 rpm to prepare a continuous phase solution.

[0051] 2) Acrylonitrile and glycidyl methacrylate were mixed evenly at a mass ratio of 13:3 and stirred for 15 min under a nitrogen atmosphere at 27 ℃ and a rotation speed of 400 rpm to form a premixed monomer. 1.5 wt% of trimethylolpropane trimethacrylate and 1.5 wt% of azobisisoheptane nitrile were mixed evenly and added to the premixed monomer. Under a nitrogen atmosphere at 35 ℃ and a rotation speed of 350 rpm, a 1.5 wt% polyethyleneimine ethanol solution (polyethyleneimine dissolved in 3.5 times its mass of anhydrous ethanol and mixed as a solvent, then used to dissolve 1.5 wt% of glycidyl methacrylate) was added and stirred for 35 min. Finally, 75 wt% of glycidyl methacrylate was added at a temperature of 5 ℃, a rotation speed of 350 rpm, and a drop rate of 2 mL / min. A dispersed phase solution was prepared by stirring a wt% composite foaming agent for 8 min under environmental conditions. The composite foaming agent was prepared by mixing isobutane and n-pentane in a mass ratio of 7:3.

[0052] 3) The continuous phase solution and the dispersed phase solution were mixed uniformly at a mass ratio of 9:1, and the polymerization reaction of the thermally expanded microspheres was carried out in a microfluidic emulsification device with a channel diameter of 200 μm and a shear rate of 5000 s⁻¹. -1 The filter substrate was subjected to nitrogen atmosphere, temperature of 72 ℃ and pressure of 0.3 MPa for 7 hours.

[0053] 4) Gradient curing of the substrate: First stage of gradient curing: heating to 70 ℃ with hot air circulation at a heating rate of 2 ℃ / min for 2 h; Second stage: heating to 125 ℃ and vacuum drying for 1.75 h; Third stage: cooling to 35 ℃ in a nitrogen atmosphere at a cooling rate of 5 ℃ / min to prepare the precursor.

[0054] 5) Functional modification of the precursor: In the first stage, 2.3 wt% of vinyltriethoxysilane was atomized and sprayed onto the surface of the precursor at 60 ℃ and 0.25 MPa. In the second stage, 1.2 wt% of tridecafluorooctyltriethoxysilane was vapor-deposited onto the precursor after the first stage of functional modification at 150 ℃. In the third stage, 0.9 wt% of hydrophobic fumed silica was mixed with the precursor after the second stage of functional modification and stirred for 45 min at 25 ℃ and 15 rpm to prepare a thermally expandable microsphere foaming agent.

[0055] The materials prepared in the examples were subjected to performance testing, and the specific characterization results are as follows.

[0056] Average particle size and particle size distribution: Referring to ISO 13320 standard, the material prepared in this example was measured using a laser diffractometer. The sample was ultrasonically dispersed in deionized water for 1 min, and the opacity was controlled at 10-15%. The uniformity of particle size distribution was quantitatively characterized by the coefficient of variation CV = (standard deviation / average particle size) × 100%.

[0057] Expansion ratio: Referring to GB / T30775 standard, the material prepared in this example was measured using a thermomechanical analyzer. 10 mg of sample was placed in the sample cell and heated to 130 ℃ at 10 ℃ / min under a nitrogen atmosphere. The diameter change was recorded. Expansion ratio = (expanded diameter - initial diameter) / initial diameter × 100%.

[0058] Thermal shrinkage rate: The material prepared in this example was kept in an oven at 200 ℃ for 30 min and then cooled to room temperature. Its diameter change was measured. Thermal shrinkage rate = (diameter after expansion - diameter after thermal shrinkage) / diameter after expansion × 100% to evaluate the thermal stability of the microspheres.

[0059] Angle of repose: Referring to ISO 4324 standard, the geometric parameters of the cone formed by the free flow of microspheres in the material prepared in this example were measured using a special measuring instrument, and the angle of repose of the material was measured.

[0060] Foaming temperature window: The material prepared in this example was analyzed by differential scanning calorimetry. A 5 mg sample was heated in a nitrogen atmosphere at a rate of 10 °C / min, and the difference between the endothermic start and end temperatures was measured.

[0061] Leakage rate: After storing the material prepared in this example in a constant temperature environment of 40 ℃ for 90 days, the weight loss was measured by a thermogravimetric analyzer. Leakage rate = (initial weight - weight after storage) / initial weight × 100%.

[0062]

[0063] Analysis of the characterization results shows that the thermally expandable microsphere foaming agent prepared in Example 2 exhibits superior overall performance. Its average particle size is 32.5 μm, and the coefficient of variation in particle size distribution has been further reduced to 6.1%, indicating that the uniformity of microsphere size has been continuously improved under optimized process conditions. This is closely related to the precisely controlled channel diameter and shear rate in the microfluidic emulsification device. The expansion ratio of this foaming agent has increased to 48.8 times, confirming the significant effect of adjusting the monomer ratio and composite foaming agent content on improving expansion capacity. The thermal shrinkage rate is as low as 4.0%, reflecting better high-temperature structural integrity, which is attributed to the fine optimization of the first-stage temperature and the second-stage vacuum drying time in the gradient curing process. The angle of repose has decreased to 30.3°, confirming further enhanced microsphere flowability and dispersibility. This is directly attributed to the synergistic adjustment of the amount of vinyltriethoxysilane coating, the amount of tridecafluorooctyltriethoxysilane vapor deposition, and the amount of hydrophobic vapor-phase silica added in the functionalization modification step, optimizing the surface energy gradient and roughness. The foaming temperature window was further narrowed to 118–122 °C, highlighting more precise temperature response characteristics. This characteristic stems from the effective promotion of polymer composition homogenization and precise distribution of core alkane by setting the stirring speed, temperature, and initiator dosage of the premixed monomer. The leakage rate was reduced to 0.7 wt%, demonstrating its excellent long-term sealing performance. The results of Example 2 further validate the highly synergistic technical system constructed in this invention: the dual-mode crosslinking network (achieved through a 1.5 wt% trimethylolpropane trimethacrylate crosslinking agent) provides stronger mechanical support for the microspheres; the three-phase surface energy regulation strategy, under the improvement of multi-stage modification parameters, more effectively solves dispersion and interface problems; and the synergistic temperature response control achieves a more precise match between foaming behavior and the thermosetting resin curing process under optimized process parameters, breaking through traditional technical bottlenecks and realizing synergistic optimization and comprehensive improvement of performance indicators.

[0064] Example 3: A method for preparing a thermally expanding microsphere foaming agent, the method comprising: 1) mixing 1.2 wt% polyvinyl alcohol and 0.1 wt% sodium dodecyl sulfate, with the remainder being deionized water, and stirring for 2 h at an ambient temperature of 72 ℃ and a rotation speed of 800 rpm to prepare a continuous phase solution.

[0065] 2) Acrylonitrile and glycidyl methacrylate were mixed evenly at a mass ratio of 10:3 and stirred for 10 min under a nitrogen atmosphere at 30 ℃ and a rotation speed of 500 rpm to form a premixed monomer. 2 wt% of trimethylolpropane trimethacrylate and 2 wt% of azobisisobutyronitrile (azobisisobutyronitrile) of the glycidyl methacrylate were mixed evenly and added to the premixed monomer. Under a nitrogen atmosphere at 37 ℃ and a rotation speed of 400 rpm, a 2 wt% polyethyleneimine ethanol solution (polyethyleneimine was dissolved in 4 times its mass of anhydrous ethanol and mixed thoroughly as a solvent, then 2 wt% of glycidyl methacrylate was dissolved) was added and stirred for 30 min. Finally, 80% of the glycidyl methacrylate was added at a temperature of 6 ℃, a rotation speed of 400 rpm, and a drop rate of 2 mL / min. A dispersed phase solution was prepared by stirring a wt% composite foaming agent for 5 minutes under environmental conditions. The composite foaming agent was prepared by mixing isobutane and n-pentane in a mass ratio of 7:3.2.

[0066] 3) The continuous phase solution and the dispersed phase solution were mixed uniformly at a mass ratio of 9.5:1, and the polymerization reaction of the thermally expanded microspheres was carried out in a microfluidic emulsification device with a channel diameter of 220 μm and a shear rate of 5200 s⁻¹. -1 The filter substrate was subjected to nitrogen atmosphere, temperature of 74℃ and pressure of 0.32 MPa for 6 hours.

[0067] 4) The substrate is subjected to gradient curing. The gradient curing process is as follows: first stage: heating to 72 ℃ at a heating rate of 2 ℃ / min and circulating hot air for 1.75 h; second stage: heating to 130 ℃ and vacuum drying for 1.5 h; third stage: cooling to 40 ℃ in a nitrogen atmosphere at a cooling rate of 5 ℃ / min to prepare the precursor.

[0068] 5) Functional modification of the precursor: In the first stage, 2.5 wt% of vinyltriethoxysilane was atomized and sprayed onto the surface of the precursor at 65 ℃ and 0.3 MPa. In the second stage, 1.4 wt% of tridecafluorooctyltriethoxysilane was vapor-deposited onto the precursor after the first stage of functional modification at 155 ℃. In the third stage, 1.0 wt% of hydrophobic fumed silica was mixed with the precursor after the second stage of functional modification and stirred for 35 min at 27 ℃ and 18 rpm to prepare a thermally expandable microsphere foaming agent.

[0069] The materials prepared in the examples were subjected to performance testing, and the specific characterization results are as follows.

[0070] Average particle size and particle size distribution: Referring to ISO 13320 standard, the material prepared in this example was measured using a laser diffractometer. The sample was ultrasonically dispersed in deionized water for 1 min, and the opacity was controlled at 10-15%. The uniformity of particle size distribution was quantitatively characterized by the coefficient of variation CV = (standard deviation / average particle size) × 100%.

[0071] Expansion ratio: Referring to GB / T30775 standard, the material prepared in this example was measured using a thermomechanical analyzer. 10 mg of sample was placed in the sample cell and heated to 130 ℃ at 10 ℃ / min under a nitrogen atmosphere. The diameter change was recorded. Expansion ratio = (expanded diameter - initial diameter) / initial diameter × 100%.

[0072] Thermal shrinkage rate: The material prepared in this example was kept in an oven at 200 ℃ for 30 min and then cooled to room temperature. Its diameter change was measured. Thermal shrinkage rate = (diameter after expansion - diameter after thermal shrinkage) / diameter after expansion × 100% to evaluate the thermal stability of the microspheres.

[0073] Angle of repose: Referring to ISO 4324 standard, the geometric parameters of the cone formed by the free flow of microspheres in the material prepared in this example were measured using a special measuring instrument, and the angle of repose of the material was measured.

[0074] Foaming temperature window: The material prepared in this example was analyzed by differential scanning calorimetry. A 5 mg sample was heated in a nitrogen atmosphere at a rate of 10 °C / min, and the difference between the endothermic start and end temperatures was measured.

[0075] Leakage rate: After storing the material prepared in this example in a constant temperature environment of 40 ℃ for 90 days, the weight loss was measured by a thermogravimetric analyzer. Leakage rate = (initial weight - weight after storage) / initial weight × 100%.

[0076]

[0077] Analysis of the characterization results shows that the thermally expandable microsphere foaming agent prepared in Example 3 exhibits synergistic optimization effects in terms of expansion ratio, thermal stability, and dispersibility. The average particle size is 32.7 μm, and the coefficient of variation for particle size distribution is 6.9%, indicating that the uniformity of microsphere size is further improved by optimizing the channel diameter and shear rate of the microfluidic emulsification device, effectively controlling the droplet dispersion and aggregation process. The expansion ratio reaches 48.9 times, exceeding the 48.8 times of Example 2. This is attributed to the adjustment of the monomer ratio and the content of the composite foaming agent, which strengthens the mechanical support of the bimodal crosslinking network and promotes the efficient expansion of the core alkane. The thermal shrinkage rate is as low as 4.3%, demonstrating excellent high-temperature structural integrity. This is due to the precise control of the first stage of heating to 72 ℃ and hot air circulation for 1.75 h in the gradient curing process, and the second stage of vacuum drying at 130 ℃ for 1.5 h, which optimizes the crosslinking density of the shell polymer. The angle of repose was 31.2°, confirming the good flowability and uniform dispersion of the microspheres. This is related to the synergistic effect of the vinyltriethoxysilane coating amount, tridecafluorooctyltriethoxysilane vapor deposition, and the addition of hydrophobic vapor-phase silica in the functionalization modification, achieving optimization of surface energy gradient and roughness. The foaming temperature window was narrow to 119–125 °C, highlighting precise temperature response characteristics. This is due to the stirring speed and temperature setting of the premixed monomers, ensuring the homogeneity of polymer composition and accurate distribution of core alkane. The leakage rate was maintained at 0.7 wt%, demonstrating reliable long-term storage stability. The sealing performance was enhanced by the ratio of polyethyleneimine ethanol solution and composite foaming agent. Overall, Example 3, through fine-tuning of process parameters, broke through the traditional technical bottlenecks and achieved a balanced improvement in expansion ratio, thermal shrinkage rate, and dispersibility through a dual-mode crosslinking network, three-phase surface energy regulation, and temperature response synergistic mechanism.

[0078] Comparative Example: Based on Example 2, this example only modifies the dispersed phase solution preparation process; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0079]

[0080] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0081]

[0082] Analysis of the characterization results showed a significant deterioration in the performance of Comparative Examples 1, D1-1 and D1-2. The expansion ratio of group D1-1 was only 35.7 times, a substantial decrease compared to Example 2, and the heat shrinkage rate increased sharply to 28.9%, indicating that the acrylonitrile monomer could not construct an effective epoxy crosslinking network. The crosslinking density decreased to 0.9 × 10⁻⁶. -4 mol / cm 3This study confirmed that insufficient cross-linking of the shell polymer led to high-temperature leakage of the core alkane and collapse of the microsphere structure. The foaming temperature window widened to 98–126 °C, a span of 28 °C, reflecting the disordered foaming behavior caused by uneven distribution of the core alkane and defects in the shell cross-linking.

[0083] Although the expansion ratio of group D1-2 was slightly higher than that of D1-1, it was still lower than that of Example 2, with a heat shrinkage rate of 26.8%. The crosslinking density was only 0.7 × 10⁻⁶. -4 mol / cm 3 This indicates that without the ionic crosslinking interaction between polyethyleneimine and epoxy groups, the free radical crosslinking network of trimethylolpropane trimethacrylate (TMP) cannot independently maintain structural strength. The abnormally widened foaming temperature window of 94–120 °C further confirms that the deterioration of shell density leads to the uncontrolled diffusion rate of core alkane. The two sets of comparative data fully demonstrate that the epoxy groups provided by glycidyl methacrylate are the necessary carriers for achieving bimodal crosslinking (free radical + ionic crosslinking), while polyethyleneimine, as an ionic crosslinking agent, plays an irreplaceable role in increasing crosslinking density and inhibiting thermal shrinkage.

[0084] Comparative Example 2: Based on Example 2, this example only modifies the functionalization modification process; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0085]

[0086] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0087]

[0088] Analysis of the characterization results showed a comprehensive deterioration in the performance of Comparative Example 2, D2-1. The angle of repose reached 53.6°, significantly higher than the 30.3° in Example 2, indicating a severe decrease in microsphere flowability. This is directly attributed to the synergistic effect of the missing functionalization modification steps: vinyltriethoxysilane coating, tridecafluorooctyltriethoxysilane vapor deposition, and the addition of hydrophobic vapor-phase silica. This resulted in an imbalance between surface roughness and energy barriers, leading to microsphere aggregation. The epoxy resin dispersion coefficient of variation increased dramatically to 41.3%, far exceeding that of Example 2, confirming that the unmodified surface could not effectively control interfacial tension, and the dispersion uniformity deteriorated sharply. This is directly related to the increased surface energy to 38.7 mN / m; the high surface energy exacerbated the hydrophilic-hydrophobic imbalance, promoting interparticle adsorption. The leakage rate increased to 7.8 wt%, a tenfold increase compared to 0.7 wt% in Example 2, demonstrating a severe degradation in the shell sealing performance. This was due to the lack of surface treatment after gradient curing, which prevented the formation of a dense hydrophobic barrier, leading to a significant release of core alkane after 90 days of storage at 40 °C. The performance degradation of group D2-1 fully highlights the core role of the functional modification process in the three-phase surface energy regulation strategy: vinyltriethoxysilane coating optimized the primary adhesion, tridecafluorooctyltriethoxysilane vapor deposition enhanced hydrophobicity, and the addition of hydrophobic vapor-phase silica regulated the micro-roughness. The three synergistically reduced surface energy and suppressed thermally induced leakage. The absence of any one of these steps resulted in a systematic collapse of the microspheres in key indicators such as flow, dispersion, and sealing performance, further demonstrating that the surface energy gradient optimization achieved through multi-stage modification parameters in this invention is indispensable for overcoming the bottlenecks of traditional technologies.

[0089] Comparative Example 3: Based on Example 2, this example only modifies the substrate preparation process; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0090]

[0091] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0092]

[0093] Analysis of the above characterization results shows that the performance of group D3-1 exhibits a systematic deterioration. Specifically, the particle size distribution variation coefficient is as high as 34.7%, significantly higher than the 6.1% of Example 2. This indicates that the mechanical stirring emulsification process cannot precisely control the uniformity of droplet size, leading to uneven microsphere dispersion and exacerbated aggregation. The expansion ratio is only 36.2 times, a significant decrease from 48.8 times in Example 2. This is attributed to the shell polymerization defects caused by uncontrolled shear rate, which weakens the mechanical support of the bimodal crosslinking network and inhibits the efficient expansion of the core alkane. The foaming temperature window widens to 108–132 °C, spanning 24 °C, reflecting the disordered distribution of the core alkane and insufficient shell density. This confirms that mechanical stirring, instead of microfluidic emulsification, cannot maintain the optimized conditions of a channel diameter of 220 μm and a shear rate of 5200 s⁻¹, resulting in a coordinated loss of control over the temperature response. The shell defect rate increased to 18.3%, a significant increase from 2.2% in Example 2, demonstrating that uneven emulsification under mechanical stirring led to increased shell cracks and porosity, severely deteriorating structural integrity and thermal stability at high temperatures. This result highlights the core role of microfluidic emulsification in achieving monodispersity of droplets and controlling the uniformity of the polymerization reaction: the microfluidic device ensures monodisperse polymerization of droplets by precisely controlling the shear rate and channel size, providing a defect-free substrate for gradient curing. The absence of this step directly leads to a complete collapse of key indicators such as expansion ratio, thermal shrinkage rate, and foaming accuracy, further demonstrating the indispensability of microfluidic emulsification in this invention for overcoming traditional technical bottlenecks and improving high-temperature stability.

[0094] Comparative Example 4: Based on Example 2, this example only modifies the substrate; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0095]

[0096] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0097]

[0098] Analysis of the above characterization results shows that the performance of group D4-1 has significantly deteriorated. The thermal shrinkage rate is as high as 17.5%, a substantial increase compared to 4.0% in Example 2, indicating severe degradation of the microsphere's high-temperature structural integrity. The crosslinking point density has decreased to 1.2 × 10⁻⁶. -4 mol / cm 3 This is significantly lower than the 1.8 × 10⁻⁶ in Example 2. -4 mol / cm 3This study confirmed that the substitution of glycidyl methacrylate for glycidyl methacrylate resulted in the absence of epoxy groups, preventing the construction of an effective bimodal crosslinking network. The swelling degree in acetone solution increased to 82.2 wt%, a significant increase from 26.5 wt% in Example 2, demonstrating that insufficient shell crosslinking density led to increased solvent penetration and deteriorated sealing performance. This result highlights the crucial role of glycidyl methacrylate in providing epoxy groups and supporting ionic crosslinking: it synergistically enhances crosslinking density with polyethyleneimine, inhibiting thermal shrinkage and solvent swelling. The absence of this component directly leads to weakened crosslinking structure and thermal stability collapse, demonstrating that the selection of components in this invention is indispensable for overcoming the bottlenecks of traditional technologies.

[0099] Comparative Example 5: Based on Example 2, this example only modifies the substrate composition; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0100]

[0101] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0102]

[0103] Analysis of the above characterization results shows that the performance of Comparative Example 5, D5-1, exhibits systematic degradation. The expansion ratio is only 27.3 times, significantly lower than the 48.8 times of Example 2, while the thermal shrinkage rate is as high as 18.7%, far exceeding the 4.0% of Example 2. The shell fracture strength drops to 42.1 MPa, and the SEM defect rate rises to 32.6%, indicating that excessive polyethyleneimine causes an imbalance in the crosslinking structure. Specifically, increasing the addition amount to 150 wt% leads to excessive ionic crosslinking, disrupting the synergistic dynamics of free radical crosslinking and ionic crosslinking. This increases the rigidity of the shell polymer but also its brittleness, inhibiting the efficient expansion of the core alkane and exacerbating structural stress cracking at high temperatures. This result highlights the crucial role of precise control of polyethyleneimine content in the dual-mode crosslinking network: appropriate addition optimizes crosslinking density and mechanical flexibility, while excessive addition leads to densification defects in the crosslinking network, weakening high-temperature stability and expansion efficiency. This conversely demonstrates that the crosslinking optimization achieved by balancing the polyethyleneimine ratio in this invention is indispensable for overcoming the bottleneck of synergistic effects between thermal shrinkage and expansion ratio.

[0104] Comparative Example 6: Based on Example 2, this example only modifies the gradient curing process; the remaining steps are the same as in Example 2. The specific settings are as follows:

[0105]

[0106] The performance testing method for the comparative product is completely consistent with that of Example 1. Partial performance characterization was performed, and the characterization results are shown in the table below.

[0107]

[0108] Analysis of the above characterization results showed a significant deterioration in the performance of Comparative Example 6, D6-1. The leakage rate increased to 12.4 wt%, nearly 17 times higher than 0.7 wt% in Example 2, confirming that the cancellation of the gradient curing process led to uncontrolled formation rate of the shell crosslinking network, resulting in increased micro-defect density and sealing barrier failure. The material resilience index dropped to 0.32, indicating limited mobility of the shell polymer chain segments. This was attributed to the intense concentration of crosslinking reactions under single-stage high-temperature curing, which disrupted the dynamic synergy between free radical crosslinking and ionic crosslinking, leading to a loss of stress relaxation ability. The secondary expansion rate at 180 °C was as high as 8.7%, reflecting the collapse of the thermal stability of the shell structure without gradient heating and the intensified secondary diffusion of the core alkane. This is directly related to the shell thickness variation coefficient of 28.7%, indicating that single-stage curing cannot achieve orderly control of polymer phase separation, resulting in uneven shell thickness and local weak areas. This result fully demonstrates the core value of the gradient curing process: the first stage, hot air circulation at 72 ℃ for 1.75 h, ensures the gentle ring-opening of epoxy groups to form a primary cross-linking network; the second stage, vacuum drying at 130 ℃ for 1.5 h, enhances the density of ionic cross-linking. The two stages synergistically control the cross-linking reaction kinetics, achieving uniform shell thickness and optimized stress distribution. The absence of this process directly leads to a systemic collapse of key performance characteristics such as sealing, thermal stability, and structural integrity.

Claims

1. A method for preparing a thermally expanding microsphere foaming agent, characterized in that, The method includes: 1) Mix polyvinyl alcohol and sodium dodecyl sulfate evenly to prepare a continuous phase solution. The continuous phase solution consists of 0.5-1.2 wt% polyvinyl alcohol and 0.05-0.1 wt% sodium dodecyl sulfate, with the balance being deionized water. 2) Using unsaturated nitrile compounds and ester compounds as substrates, a dispersed phase solution was prepared; 3) The continuous phase solution and the dispersed phase solution are mixed evenly in a certain proportion to carry out the polymerization reaction of thermally expanded microspheres and prepare the substrate; 4) Gradient curing of the substrate is performed to prepare the precursor; 5) Functionalize the precursor to produce a thermally expanding microsphere foaming agent; Step 2) The unsaturated nitrile compound is acrylonitrile; Step 2) The ester compound is glycidyl methacrylate; Step 2) The specific steps for preparing the dispersed phase solution are as follows: Unsaturated nitrile compounds and ester compounds are mixed evenly at a mass ratio of (10-15):3, and stirred for 10-20 min under a nitrogen atmosphere, a temperature of 25-30 °C, and a rotation speed of 300-500 rpm to form a premixed monomer. 1–2 wt% of trimethylolpropane trimethacrylate and 1–2 wt% of azobisisobutyronitrile were mixed evenly and added to the premixed monomer. The mixture was stirred for 30–40 min under a nitrogen atmosphere, at a temperature of 33–37 °C, a rotation speed of 300–400 rpm, and a dropping rate of 1 mL / min. Finally, the mixture was stirred for 5–10 min under a temperature of 4–6 °C, a rotation speed of 300–400 rpm, and a dropping rate of 2 mL / min. The composite foaming agent is prepared by mixing isobutane and n-pentane in a mass ratio of 7:(2.8-3.2); Step 3) The polymerization reaction is carried out in a microfluidic emulsification device with a channel diameter of 180–220 μm and a shear rate of 4800–5200 s⁻¹. -1 The substrate was obtained by filtration after being kept in a nitrogen atmosphere at a temperature of 70–74 °C and a pressure of 0.28–0.32 MPa for 6–8 hours. Step 4) The gradient curing includes: First stage: Heat to 68-72℃ at a heating rate of 2℃ / min and circulate hot air for 1.75-2.25 h; Second stage: Increase the temperature to 120-130 ℃ and vacuum dry for 1.5-2 hours; The third stage: cooling to 30-40 °C at a rate of 5 °C / min in a nitrogen atmosphere; Step 5) describes a functional modification process that includes three stages; First stage: Vinyltriethoxysilane at a precursor mass of 1.8–2.5 wt% is atomized and sprayed onto the precursor surface under environmental conditions of 55–65 ℃ and atomization pressure of 0.2–0.3 MPa; Second stage: Using 0.9-1.4 wt% of tridecafluorooctyltriethoxysilane as a precursor, vapor deposition was performed on the precursor that had completed the first stage of functionalization modification in an environment of 145-155 °C. In the third stage, 0.8–1.0 wt% of hydrophobic fumed silica was mixed with the precursor that had undergone the second stage of functionalization modification. The mixture was stirred for 35–55 min at an ambient temperature of 23–27 °C and a rotation speed of 12–18 rpm.

2. The method for preparing a thermally expanding microsphere foaming agent according to claim 1, characterized in that, Step 1) describes the process of preparing a continuous phase solution as follows: Stir for 2–3 hours at an ambient temperature of 68–72 °C and a rotation speed of 500–800 rpm.

3. The method for preparing a thermally expanding microsphere foaming agent according to claim 1, characterized in that, Step 3) The continuous phase solution and the dispersed phase solution are mixed evenly at a mass ratio of (8.5 to 9.5):

1.

4. A thermally expanding microsphere foaming agent prepared by any one of claims 1 to 3.

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