High-cavity-stability acrylic hollow sphere emulsion and preparation method thereof

Abstract

The application discloses a kind of high cavity stability acrylic hollow sphere emulsion and preparation method thereof.To solve the problems such as shell layer rupture and cavity collapse of acrylic hollow sphere emulsion.The method comprises the following steps: preparing core layer latex particles containing acidic functional monomers;forming shell layer outside the core layer latex particles to obtain core-shell type acrylic emulsion;cooling the core-shell type acrylic emulsion to a predetermined neutralization temperature, and adding a redox catalyst containing a transition metal ion source and a chelating agent;preparing a neutralizing agent aqueous solution, and gradually adding the obtained system in stages, controlling the pH of the system to gradually rise to the target pH in stages, and the total rising time is controlled within a predetermined time range;after reaching the target pH, adding an oxidizing agent and a reducing agent for post-treatment;heating the obtained system for post-heat aging;cooling and filtering to obtain a cavity stable acrylic hollow sphere emulsion.The cavity stability and cavity retention rate of the emulsion are improved.

Description

Technical Field

[0001] This invention relates to the field of waterborne polymer emulsions and functional polymer microsphere materials. Specifically, it relates to a method for preparing an acrylic hollow sphere emulsion with high cavity stability, and an acrylic hollow sphere emulsion with high cavity stability. Background Technology

[0002] Acrylic hollow sphere emulsions are a class of polymer emulsions with core-shell or multilayer structures. They typically swell under alkaline conditions through a core layer containing carboxyl groups or other hydrophilic groups, forming a hollow structure confined by a relatively rigid or cross-linked shell. These emulsions possess characteristics such as low density, good hiding power, good matting properties, good thermal insulation, and the ability to reduce the amount of inorganic fillers required. Therefore, they have wide application value in water-based coatings, paper coatings, inks, leather finishing, and functional coatings.

[0003] Existing acrylic hollow sphere emulsions are generally prepared using seed emulsion polymerization, core-shell emulsion polymerization, or multi-stage emulsion polymerization methods. The basic preparation approach involves first preparing a core layer containing acidic functional monomers, then polymerizing this core to form a shell layer with a certain strength and cross-linking degree. Finally, an alkaline neutralizing agent is added to cause the core layer to swell, resulting in polymer latex particles with a hollow structure. However, in actual experiments and production processes, the cavity formation process of hollow sphere emulsions is quite sensitive to neutralization conditions, shell strength, particle size distribution, and system stability. If the neutralizing agent is added too quickly, the pH rises too rapidly, the neutralization temperature is unsuitable, or the shell strength is insufficient, it can easily lead to problems such as rapid expansion of latex particles, sudden increase in particle size, shell rupture, cavity collapse, widening of particle size distribution, emulsion thickening, sedimentation and stratification, or decreased storage stability. Especially after a period of storage, some hollow sphere emulsions may exhibit continued particle size changes, stratification, precipitation, decreased hiding power, or indistinct hollow structure after drying, affecting their stable application in coatings, paper coatings, and other water-based coating systems.

[0004] Therefore, it is necessary to provide a preparation method that can stably control the neutralization and expansion process of hollow sphere emulsion, reduce the risk of cavity collapse, reduce particle size fluctuations, and improve storage stability. Summary of the Invention

[0005] To address the technical problems of existing alkali swelling methods in preparing acrylic hollow sphere emulsions, such as shell rupture, cavity collapse, wide particle size distribution, poor storage stability, and cavity shrinkage after drying, this invention provides a method for preparing an acrylic hollow sphere emulsion with high cavity stability, as well as an acrylic hollow sphere emulsion with high cavity stability. The method reduces the initial expansion sensitivity of the core layer through cooling pretreatment, and achieves slow and uniform water absorption and expansion of the core layer through staged neutralization, effectively avoiding shell rupture and cavity collapse. Combined with oxidation-reduction post-treatment and post-heat preservation curing, the cavity structure is permanently stabilized, significantly improving the particle size uniformity, storage stability, and cavity retention rate after drying of the emulsion.

[0006] In a first aspect, the present invention provides a method for preparing an acrylic hollow sphere emulsion with high cavity stability, comprising the following steps: Step S1: Prepare core-layer latex particles containing acidic functional monomers; Step S2: Polymerize the outer layer of the core latex particles to form a shell layer, thereby obtaining a core-shell acrylic emulsion; Step S3: Cool the obtained core-shell acrylic emulsion to a predetermined neutralization temperature and add a redox catalyst, wherein the redox catalyst comprises a transition metal ion source and a chelating agent; Step S4: Prepare a neutralizing agent aqueous solution and add it to the system obtained in step S3 in stages, controlling the pH of the system to rise to the target pH in stages, and controlling the total pH rise time within the predetermined time range. Step S5: After reaching the target pH, add oxidant and reducing agent to the system for post-treatment; Step S6: Heat the system obtained in step S5 and then keep it at that temperature for further curing. Step S7: Cool and filter to obtain cavity-stabilized acrylic hollow sphere emulsion.

[0007] Furthermore, the preparation of core-layer latex particles containing acidic functional monomers includes: Add deionized water and emulsifier to the reactor, stir and heat to 75~85℃, add part of the initiator, and then dropwise add a core layer monomer mixture consisting of acidic functional monomers, soft monomers, hard monomers and optional crosslinking agents. The dropwise addition time is 1.5~3 hours, and after the dropwise addition is completed, keep the reaction at the temperature for 0.5~1.5 hours. The acidic functional monomer is one or both of methacrylic acid and acrylic acid, the soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, and isooctyl acrylate, and the hard monomer is selected from one or more of styrene, methyl methacrylate, and acrylonitrile.

[0008] Further, by mass, the acidic functional monomer is 15-50 parts, the soft monomer is 20-50 parts, the hard monomer is 10-40 parts, and the crosslinking agent is 0-8 parts; the theoretical acid value of the core layer is 100-350 mgKOH / g; the particle size range of the core layer latex particles is 100-350 nm, and the PDI is less than or equal to 0.10.

[0009] Furthermore, the shell layer is formed by copolymerization of shell hard monomers and shell soft monomers, wherein, by mass, the shell hard monomers are 60-95 parts and the shell soft monomers are 5-40 parts. The shell layer also contains a shell crosslinking agent, the amount of which is 0-5 parts. The mass ratio of the shell monomers to the core monomers is 1:1 to 5:1. The theoretical glass transition temperature Tg of the shell polymer is ≥70℃.

[0010] Furthermore, the shell hard monomer is selected from one or more of styrene and methyl methacrylate, the shell soft monomer is selected from one or more of butyl acrylate and ethyl acrylate, and the shell crosslinking agent is selected from one or more of divinylbenzene and ethylene glycol dimethacrylate.

[0011] Further, the predetermined neutralization temperature is 50~70℃; the transition metal ion source is selected from one or more of ferrous chloride, ferrous ammonium sulfate, ferrous sulfate, copper sulfate, cobalt acetate, manganese sulfate, and cerium ammonium nitrate; the chelating agent is selected from one or more of ethylenediaminetetraacetic acid or its sodium salt, citric acid or its sodium salt, tartaric acid or its sodium salt, and gluconic acid or its sodium salt.

[0012] Further, the mass percentage concentration of the neutralizing agent aqueous solution is 3-20%; the neutralizing agent is selected from one or more of ammonia, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and 2-amino-2-methyl-1-propanol.

[0013] Furthermore, The neutralizing agent aqueous solution is added gradually in three stages, and the control parameters for each stage are as follows: First stage: Adjust the pH of the system to 2.5~4.5, with a pH increase rate of 0.3~1.0 pH units / hour, and add a neutralizing agent accounting for 15~30% of the total neutralizing agent; Second stage: Adjust the pH of the system to 4.5~7.0, with a pH increase rate of 0.5~1.5 pH units / hour, and add 35~55% of the total amount of neutralizing agent; The third stage: Adjust the pH of the system to 7.0~9.5, with a pH increase rate of 0.2~0.8 pH units / hour, and add 20~40% of the total amount of neutralizing agent; The total time for pH rise during the entire process is 3 to 10 hours.

[0014] Further, the oxidant is selected from one or more of tert-butyl hydroperoxide, hydrogen peroxide, and potassium persulfate; the reducing agent is selected from one or more of sodium styrax, ascorbic acid, sodium bisulfite, and sodium metabisulfite; the molar ratio of the oxidant to the reducing agent is 1:0.5~2; The amount of oxidant used is 0.05~0.5wt% of the total solid content of the emulsion.

[0015] Furthermore, the predetermined neutralization temperature is 55~60℃; the post-heat preservation and curing temperature is 60~90℃, and the time is 1~4 hours.

[0016] Furthermore, the particle size polydispersity index (PDI) of the cavity-stabilized acrylic hollow sphere emulsion is ≤0.15, and the particle size change rate after storage at 50°C for 30 days is ≤8%; after drying, the proportion of latex particles with identifiable cavity structures in the cavity-stabilized acrylic hollow sphere emulsion is ≥80%.

[0017] Secondly, the present invention provides an acrylic hollow sphere emulsion with high cavity stability, prepared by the preparation method as described in any one of the first aspects; the latex particles of the acrylic hollow sphere emulsion have a core layer and a shell layer, the latex particles have a hollow cavity structure, the core layer contains copolymer units of acidic functional monomers, the theoretical acid value of the core layer is 100~350mgKOH / g; the theoretical glass transition temperature Tg of the shell layer is ≥70℃; the particle size polydispersity index (PDI) of the emulsion is ≤0.15, the particle size change rate after storage at 50℃ for 30 days is ≤8%, and the proportion of latex particles with identifiable cavity structures after drying is ≥80%.

[0018] Beneficial effects

[0019] This invention establishes a complete process system from expansion process control to cavity structure shaping, solving a series of problems caused by the one-time rapid addition of neutralizing agent in the traditional alkali swelling method.

[0020] First, the pre-cooling treatment before neutralization reduces the mobility of the polymer chain segments in the core layer. Combined with the phased and slow addition of the neutralizing agent and precise control of the total pH rise time, the carboxyl groups in the core layer are gradually ionized, the osmotic pressure gradient is gently established, and the shell layer obtains sufficient stress relaxation time. This fundamentally avoids shell rupture and abrupt particle size changes caused by shock expansion. The latex particles expand uniformly, have a concentrated particle size distribution, and exhibit excellent batch reproducibility.

[0021] Secondly, after neutralization, an oxidation-reduction post-treatment is immediately carried out to induce a mild micro-crosslinking reaction inside the shell, building an elastic skeleton that resists cavity shrinkage and greatly improving the structural stability of latex particles during long-term storage.

[0022] Finally, post-heat curing promotes relaxation and rearrangement of the ionized chain segments in the core layer and eliminates the internal stress accumulated in the shell layer during expansion, enabling the cavity morphology to achieve permanent "thermal setting" at the molecular chain level. These three effects work synergistically to ensure that the emulsion has stable particle size and no sedimentation or stratification during storage, and that the cavity structure remains intact after drying and film formation, giving the coating excellent hiding power and lightweight effect.

[0023] The acrylic hollow sphere emulsion prepared by this invention possesses excellent cavity structure stability. Its core layer contains acidic functional monomer copolymer units with an appropriate acid value range, while the shell layer has a high glass transition temperature to provide sufficient structural constraint. Together, these factors ensure the integrity and regularity of the hollow cavity structure within the latex particles. Furthermore, the emulsion exhibits a concentrated particle size distribution without significant changes during storage, and shows no sedimentation or stratification. After drying, the cavity structure maintains a high proportion of its original structure. Therefore, it can be directly used as a functional microsphere material in fields such as opaque coatings, matte coatings, lightweight coatings, paper coatings, architectural coatings, and water-based inks, providing downstream applications with stable and reliable high-performance hollow polymer microsphere products. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 A schematic flowchart illustrating a method for preparing a high-cavity-stability acrylic hollow sphere emulsion according to an embodiment of the present invention; Figure 2 The effect diagram of the printing test is shown by applying the acrylic hollow sphere emulsion of the present invention to the coating material and coating it on printing paper. Figure 3 This is a dynamic light scattering particle size distribution diagram of the acrylic hollow sphere emulsion obtained in Example 1 of the present invention; Figure 4 This is a dynamic light scattering particle size distribution diagram of the acrylic hollow sphere emulsion obtained in Example 2 of the present invention; Figure 5 This is a dynamic light scattering particle size distribution diagram of the acrylic hollow sphere emulsion obtained in Example 3 of the present invention; Figure 6 This is a scanning electron microscope (SEM) image of the acrylic hollow sphere emulsion obtained in Example 1 of the present invention after drying. Figure 7 Another scanning electron microscope (SEM) image of the acrylic hollow sphere emulsion obtained in Example 1 of the present invention after drying; Figure 8 This is a dimensional diagram of a single acrylic hollow sphere from Embodiment 1 of the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0028] It should be noted that if any directional indication (such as up, down, left, right, front, back, etc.) is involved in the embodiments of the present invention, the directional indication is only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0029] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0030] To better understand the above technical solutions, the present invention will be further described below in conjunction with specific implementation methods and embodiments, but the scope of protection of the present invention is not limited thereto.

[0031] This invention provides a method for preparing a hollow acrylic sphere emulsion with high cavity stability, such as... Figure 1 As shown, it includes the following steps: Step S1: Prepare core-layer latex particles containing acidic functional monomers; Step S2: Polymerize the outer layer of the core latex particles to form a shell layer, thereby obtaining a core-shell acrylic emulsion; Step S3: Cool the obtained core-shell acrylic emulsion to a predetermined neutralization temperature and add a redox catalyst, wherein the redox catalyst comprises a transition metal ion source and a chelating agent; Step S4: Prepare a neutralizing agent aqueous solution and add it to the system obtained in step S3 in stages, controlling the pH of the system to rise to the target pH in stages, and controlling the total pH rise time within the predetermined time range. Step S5: After reaching the target pH, add oxidant and reducing agent to the system for post-treatment; Step S6: Heat the system obtained in step S5 and then keep it warm for maturation. Step S7: Cooling and filtering to obtain cavity-stabilized acrylic hollow sphere emulsion.

[0032] In the specific embodiments of the present invention, all raw materials used can be obtained through commercial channels or prepared according to methods known in the art.

[0033] The acidic functional monomers, soft monomers, hard monomers, shell hard monomers, shell soft monomers, and crosslinking agents are all industrial-grade polymerization products and do not require purification before use. If the monomers contain polymerization inhibitors, they can be removed by conventional alkaline washing or vacuum distillation. The emulsifiers are industrial-grade and can be used directly. The initiators, neutralizing agents, oxidizing agents, reducing agents, transition metal ion sources, and chelating agents are all analytical grade or industrial-grade and can be used directly.

[0034] Specifically, step S1 involves the preparation of core-layer latex particles. Deionized water and emulsifier are added to a reactor, and the mixture is stirred and heated to 75-85°C. The emulsifier is a compound system of anionic and nonionic emulsifiers. The anionic emulsifier is selected from one or more of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate, and the nonionic emulsifier is selected from one or more of octylphenol polyoxyethylene ether-10. The total amount of emulsifier is 1.0-4.0% of the total mass of the core-layer monomers. After adding a portion of the initiator, a mixture of core-layer monomers, consisting of acidic functional monomers, soft monomers, hard monomers, and optional crosslinking agents, is added dropwise. The initiator is selected from potassium persulfate or ammonium persulfate, and the amount used is 0.2-1.0% of the total mass of the core-layer monomers.

[0035] The amounts of each component in the core layer monomer mixture, by mass, are: 15-50 parts acidic functional monomer, 20-50 parts soft monomer, 10-40 parts hard monomer, and 0-8 parts crosslinking agent. The acidic functional monomer is methacrylic acid and / or acrylic acid, and its amount determines the theoretical acid value of the core layer. Controlling the amount of acidic functional monomer ensures a theoretical acid value of 100-350 mgKOH / g for the core layer. If the acid value is below 100 mgKOH / g, the core layer lacks sufficient hydrophilicity, resulting in weak water absorption and swelling drive after alkali neutralization, making it difficult to form a fully developed cavity structure. Ultimately, the latex particles have small pore sizes and insufficient hollowness, and the covering power and thermal insulation performance do not meet expectations. If the acid value is too high, the core layer is too hydrophilic, absorbing too much water too quickly during neutralization and swelling. The shell layer cannot withstand excessive volume expansion and is prone to cracking, leading to severe cavity collapse. Therefore, controlling the acid value within the range of 100~350mgKOH / g ensures sufficient expansion driving force while avoiding shell damage caused by excessive expansion.

[0036] The soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, and isooctyl acrylate, which imparts appropriate flexibility to the core layer and prevents cracking due to excessive brittleness during drying. The hard monomer is selected from one or more of styrene, methyl methacrylate, and acrylonitrile, which imparts a certain degree of rigidity to the core layer. The crosslinking agent is selected from one or more of divinylbenzene, ethylene glycol dimethacrylate, and trimethylolpropane triacrylate, which is used to control the crosslinking density of the core layer.

[0037] The core-layer monomer mixture is added dropwise over 1.5 to 3 hours, and then kept at a constant temperature for 0.5 to 1.5 hours after addition to ensure complete monomer conversion. After the reaction is complete, core-layer latex particles containing acidic functional monomers are obtained, with a particle size range of 100 to 350 nm, preferably 200 to 300 nm, and a polydispersity index (PDI) of less than or equal to 0.10.

[0038] Step S2 involves the polymerization and formation of the shell layer. Specifically, a mixture of shell monomers, consisting of hard shell monomers, soft shell monomers, and an optional shell crosslinking agent, is added dropwise to the core layer latex particle dispersion obtained in step 1, while the remaining initiator is added dropwise simultaneously. The dropwise addition of the shell monomer mixture takes 2 to 4 hours, and after the addition is complete, the reaction is maintained at this temperature for 0.5 to 1.5 hours.

[0039] The hard shell monomer is selected from one or more of styrene and methyl methacrylate, preferably a combination of styrene and methyl methacrylate in a mass ratio of 1:0.5~2. The soft shell monomer is selected from one or more of butyl acrylate and ethyl acrylate. The shell crosslinking agent is selected from one or more of divinylbenzene and ethylene glycol dimethacrylate.

[0040] By weight, the shell hard monomer comprises 60-95 parts, the shell soft monomer comprises 5-40 parts, and the shell crosslinking agent comprises 0-5 parts. The "0-5 parts" here means: the shell crosslinking agent can be omitted (0 parts), in which case the shell relies solely on the rigidity of the high Tg monomer to provide structural strength; or it can be added up to 5 parts. The crosslinking agent participates in copolymerization during the shell polymerization process, forming crosslinks between polymer chains, further enhancing the structural constraint of the shell during expansion and preventing shell rupture caused by excessive water absorption by the core layer. The shell crosslinking agent is an optional component, depending on the specific application's requirements for shell strength.

[0041] The mass ratio of shell monomer to core monomer is 1:1 to 5:1, preferably 1.5:1 to 3:1. The theoretical glass transition temperature (Tg) of the shell polymer is ≥70℃, preferably 80 to 120℃. The Tg of the shell polymer is determined by the Fox equation. This Tg range ensures that the shell remains in a glassy state under the temperature conditions of expansion within the core, possessing sufficient rigidity to constrain the volume expansion of the core and prevent excessive expansion and rupture of the cavity. After polymerization, a core-shell acrylic emulsion is obtained, with latex particles exhibiting a "soft core, hard shell" structural characteristic.

[0042] Step S3 is a pretreatment before neutralization, in which the core-shell acrylic emulsion obtained in step S2 is cooled to a predetermined neutralization temperature. The predetermined neutralization temperature is 50~70℃, preferably 55~60℃. During the cooling process, the mobility of the polymer chain segments in the latex particle shell gradually decreases, preparing conditions for the subsequent mild and controllable expansion process.

[0043] After cooling and before neutralization begins, a redox catalyst is added to the system. The redox catalyst comprises a transition metal ion source and a chelating agent.

[0044] The transition metal ion source is selected from one or more of ferrous chloride, ferrous ammonium sulfate, ferrous sulfate, copper sulfate, cobalt acetate, manganese sulfate, and cerium ammonium nitrate, preferably ferrous chloride or copper sulfate. As the catalytically active center for redox reactions, transition metal ions can undergo Fenton-type or Fenton-like reactions with oxidants added in subsequent steps (such as hydrogen peroxide or tert-butyl hydroperoxide) to generate highly reactive free radical species (such as ·OH, t-BuO·). Specifically, Fe²⁺ is used as the catalytically active center for redox reactions. + For example, Fe² + It reacts with H2O2 to generate ·OH radicals and Fe³⁺. + This reaction can be carried out at relatively low temperatures; Fe³ + It can then be reduced to Fe²⁺ by reducing agents (such as sodium thiosulfate and ascorbic acid). + This enables catalytic cycling.

[0045] The chelating agent is selected from one or more of ethylenediaminetetraacetic acid or its sodium salt, citric acid or its sodium salt, tartaric acid or its sodium salt, and gluconic acid or its sodium salt, with citric acid or its sodium salt being preferred. The chelating agent functions in three ways: First, it stabilizes transition metal ions in solution through coordination, preventing hydrolysis and precipitation under alkaline conditions and ensuring the catalyst maintains its catalytic activity throughout the neutralization process. Second, the coordination structure formed between the chelating agent and the metal ions can regulate the redox potential of the metal ions, maintaining their catalytic activity at an optimal level within the target pH range and avoiding drastic fluctuations in catalytic activity as the pH increases. Third, the presence of the chelating agent allows trace amounts of metal ions to be uniformly dispersed in the emulsion system in a complexed state, ensuring that each latex particle is surrounded by a catalyst, thus achieving uniform initiation of the redox reaction in the emulsion system.

[0046] The unique feature of this pre-embedded catalyst is that it can initiate subsequent redox reactions under mild conditions (50~70℃) without the need for additional high-temperature heating. Since the latex particles have not yet begun to expand (the neutralization process has not yet started), the catalyst can be uniformly pre-placed near the surface of the latex particle shell. After the oxidant and reductant are added subsequently, the reaction can proceed at a lower temperature, avoiding the thermal shock and damage to the newly formed, thermodynamically metastable cavity structure caused by the high temperature (75~85℃) required for traditional thermal initiation.

[0047] Cooling pretreatment effectively reduced the initial swelling activity of the core polymer, making the water absorption and swelling process after neutralization more gentle and controllable. Catalyst pre-embedding ensured that subsequent redox reactions could be uniformly initiated at low temperatures, avoiding thermal shock to the cavity structure caused by high-temperature post-treatment.

[0048] Step S4 involves staged neutralization and expansion. The neutralizing agent is prepared as an aqueous solution with a mass percentage concentration of 3-20%, preferably 5-15%. The neutralizing agent is selected from one or more of ammonia, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and 2-amino-2-methyl-1-propanol, with sodium hydroxide being preferred. Dilution of the neutralizing agent is a prerequisite for staged neutralization—after adding a relatively dilute aqueous solution to the system, the local alkali concentration is lower, avoiding localized over-alkali impact caused by concentrated alkali droplets contacting latex particles.

[0049] The neutralizing agent aqueous solution is added to the system in stages, controlling the pH of the system to rise gradually in stages. Preferably, the neutralizing agent aqueous solution is added in three stages. The control parameters for each stage are as follows: First stage (slow initiation stage): Adjust the pH of the system to 2.5~4.5, with a pH rise rate of 0.3~1.0 pH units / hour (preferably 0.5~0.8 pH units / hour), add 15~30% of the total amount of neutralizing agent, maintain a temperature of 25~50℃, and stir at a speed of 150~250 rpm.

[0050] In the first stage, the system is in a strongly acidic environment, and the carboxyl groups (-COOH) on the surface of the core layer begin to react with OH groups. - A neutralization reaction occurs, producing a carboxylate (—COO) - Due to the extremely low pH (2.5~4.5), only the outermost exposed carboxyl groups are neutralized; the carboxyl groups inside the core layer have not yet reacted. The carboxylate groups are strongly hydrophilic, attracting water molecules into the surface region of the core layer, establishing a moderate osmotic pressure gradient on the outermost layer. The key technical feature of this stage is the extremely low pH rise rate (0.3~1.0 pH units / h), allowing the osmotic pressure gradient to build slowly rather than instantaneously. The initial osmotic pressure is extremely low, causing almost no impact on the shell layer. A relatively high stirring speed (150~250 rpm) ensures rapid and uniform dispersion of the neutralizing agent after addition, preventing localized over-concentration.

[0051] Second stage (uniform expansion stage): Adjust the pH of the system to 4.5~7.0, with a pH increase rate of 0.5~1.5 pH units / hour (preferably 0.8~1.2 pH units / hour), add 35~55% of the total amount of neutralizing agent, maintain the temperature at 25~50℃, and stir at 120~200 rpm.

[0052] When the pH rises above 4.5, more carboxyl groups inside the core are neutralized, the osmotic pressure gradient increases significantly, and water molecules permeate into the core at a higher rate, driving the core to absorb water and expand uniformly. This stage is the main stage of cavity formation—the polymer chains in the core expand under the plasticizing effect of water, increasing in volume, and the shell undergoes uniform deformation. The technical characteristics of this stage are that a moderate pH rise rate (0.5~1.5 pH units / h) provides sufficient expansion driving force, while the appropriate reduction in stirring speed (120~200 rpm) avoids high shear forces damaging the expanding latex particles. The second stage adds the largest proportion of neutralizing agent (35~55%) because this stage requires the neutralization of the largest number of carboxyl groups and has the greatest degree of expansion.

[0053] The third stage (endpoint adjustment stage): Adjust the pH of the system to 7.0~9.5 (preferably 8.0~9.0), with a pH increase rate of 0.2~0.8 pH units / hour (preferably 0.3~0.6 pH units / hour), add 20~40% of the total amount of neutralizing agent, maintain a temperature of 25~50℃, and a stirring speed of 100~180 rpm.

[0054] The target pH range for the third stage is 7.0–9.5. Too high a pH (>9.5) will cause excessive expansion or even dissolution of the core layer, leading to rupture of the shell layer due to insufficient internal pressure; too low a pH (<7.0) will result in incomplete neutralization of carboxyl groups, incomplete expansion of the core layer, and insufficient cavity size. The pH rise rate is reduced to a minimum (0.2–0.8 pH units / h) to ensure precise neutralization of residual carboxyl groups and to release stress in the expanded core layer—the internal stress accumulated during the rapid expansion in the second stage is gradually relaxed during the "slow rise" in the third stage, preventing localized collapse caused by stress concentration. The stirring speed is further reduced (100–180 rpm) to minimize disturbance to the already formed cavity structure.

[0055] The total pH rise time should be controlled within 3 to 10 hours, preferably 4 to 8 hours. The "total pH rise time" refers to the total time from the addition of the first drop of neutralizing agent until the pH reaches the final target value, not the holding time after reaching the target pH.

[0056] Each stage must ensure that the pH target of the previous stage has been achieved and the system pH has stabilized before proceeding to the next stage. Specifically, when the system pH changes by no more than 0.05 pH units within 10 minutes, the system is considered stable and can proceed to the next stage. Throughout the process, the pH changes are monitored in real time using an online pH meter, and the rate of neutralizing agent addition is controlled by a peristaltic pump or a constant flow pump to achieve precise control of the pH rise rate.

[0057] The three-stage neutralization method achieves a dynamic balance between the stepwise neutralization of carboxyl groups in the core layer, the establishment of an osmotic pressure gradient, the gradual infiltration of water, and the stress relaxation of the shell layer. This avoids the shell rupture and cavity collapse caused by excessive instantaneous osmotic pressure in the traditional "one-step" neutralization method. The latex particles exhibit stable particle size growth during expansion, resulting in a concentrated particle size distribution. The PDI is controlled below 0.15, demonstrating excellent batch-to-batch repeatability.

[0058] Step S5 is a post-oxidation / reduction treatment.

[0059] After reaching the target pH in step 4, an oxidant and a reducing agent are added to the system for redox post-treatment. The oxidant is selected from one or more of tert-butyl hydroperoxide, hydrogen peroxide, and potassium persulfate, with hydrogen peroxide being preferred. The reducing agent is selected from one or more of sodium thiosulfate, ascorbic acid, sodium bisulfite, and sodium metabisulfite, with sodium thiosulfate or ascorbic acid being preferred.

[0060] The molar ratio of oxidant to reducing agent is 1:0.5~2, preferably 1:0.8~1.2. The amount of oxidant used is 0.05~0.5wt% of the total solid content of the emulsion, preferably 0.1~0.3wt%.

[0061] The oxidation-reduction post-treatment has a triple synergistic effect in this invention: Firstly, residual monomers are eliminated (post-polymerization). The oxidant and reductant react to generate highly reactive free radicals (such as ·OH, t-BuO·). These free radicals undergo addition reactions with unreacted monomers remaining in the system (such as St, BA, MMA, etc.), initiating chain growth and incorporating small molecule monomers into the polymer chain. This step can significantly reduce the VOC content of the system and increase the total monomer conversion rate. If a catalyst has been pre-embedded in step 3, this step can be carried out at 50~70℃, resulting in high free radical generation efficiency and a mild and controllable process.

[0062] Secondly, the shell layer exhibits mild micro-crosslinking. In addition to initiating the polymerization of residual monomers, the free radicals generated by redox reactions directionally abstract active hydrogen atoms from the shell polymer chains (such as the benzylic hydrogen in styrene units or the tertiary hydrocarbons in polyacrylate segments), generating polymer chain free radicals (P·). When two adjacent P· chain free radicals meet, coupling termination occurs (P1· + P2· → P1-P2), forming new CC crosslinks between the shell polymer chains. This micro-crosslinking density is controlled at a low level—not completely solidifying the shell, but constructing a sparse chemical crosslinking network between the shell molecular chains. After the formation of sparse CC crosslinks, the relative slippage between the shell polymer chains is restricted, and the expanded core layer is more tightly wrapped by this network, significantly improving the mechanical stability of the cavity structure.

[0063] Third, it provides skeletal support for subsequent heat preservation and curing. The shell layer, reinforced by micro-crosslinking, can better withstand stress changes during the thermodynamic relaxation process without creep deformation in the subsequent S6 heat preservation and curing step. The presence of the crosslinking network enables the shell layer to maintain dimensional stability at high temperatures, making the high-temperature "heat setting" effect of the cavity structure more complete and longer-lasting.

[0064] The redox post-treatment constructs an elastic framework that resists cavity shrinkage through shell micro-crosslinking, while simultaneously achieving deep removal of residual monomers at low temperatures. The synergistic effect of these three actions is reflected in the long-term storage stability of the emulsion—the shell crosslinking network effectively counteracts the tendency for cavity shrinkage caused by molecular chain creep during storage.

[0065] Step S6 is post-heating and curing. After step 5 is completed, the system is heated to 60~90℃, preferably 70~85℃, and cured for 1~4 hours, preferably 1.5~3 hours. The stirring speed is maintained at 80~160 rpm.

[0066] Post-curing and heat treatment achieve permanent thermal setting of the cavity structure at the molecular chain level through chain segment relaxation and internal stress elimination. Hollow sphere emulsions treated in this way exhibit significantly higher cavity retention rates after coating and drying compared to untreated samples.

[0067] Step S7: Cooling and Filtration. Cool the emulsion obtained in Step 6 to room temperature (15~35℃). Cooling can be done naturally or with circulating water. The cooling rate should not be too fast (not exceeding 2℃ / min) to avoid thermal stress concentration inside the latex particles due to rapid cooling. Stirring should be maintained during the cooling process at a speed of 60~120 rpm.

[0068] After cooling to room temperature, the mixture is filtered through a 200-400 mesh filter to remove any small amounts of coagulated material or mechanical impurities, resulting in a cavity-stabilized acrylic hollow sphere emulsion product.

[0069] In this invention, steps S3 to S6 constitute a complete process chain, and there are temporal coordination and functional superposition relationships between the steps: Cooling pretreatment "passivates" the polymer chain segments in the core layer, creating a temperature window for staged neutralization; catalyst pre-embedding enables the redox reaction to be initiated immediately at low temperature after neutralization, achieving a seamless connection between shell micro-crosslinking and cavity shaping. Staged neutralization completes the gentle construction of the cavity, but at this time the cavity structure is in a thermodynamic metastable state; redox post-treatment introduces a chemical crosslinking network in the shell layer, "locking" the metastable cavity; post-heat preservation curing eliminates internal stress through physical chain segment relaxation, completing the "permanent shaping" of the cavity morphology.

[0070] In this invention, the amounts of each component are expressed in "parts by mass," which only indicates the relative proportions between the components and does not limit the sum of the amounts of all components to 100 parts by mass. Those skilled in the art can scale up or down the amounts of each component according to the actual production scale.

[0071] The theoretical acid value of the core layer is calculated based on the amount of acidic functional monomers used in the core layer formulation; the pH rise rate refers to the change in the pH value of the system per unit time, which is controlled by adjusting the addition rate of the neutralizing agent aqueous solution; the particle size polydispersity index (PDI) is measured by a dynamic light scattering instrument at 25°C, and all of these can be processed using existing methods, so they will not be elaborated here.

[0072] Through the above-mentioned five-in-one process control system of "cooling pretreatment - staged neutralization - total pH rise time control - oxidation-reduction posttreatment - post-heat preservation and ripening", the present invention has achieved the following technical effects: Regarding particle size uniformity, since the neutralizing agent is added in stages and the pH rise rate is precisely controllable, the stepwise neutralization of the carboxyl groups in the core layer and the water absorption and swelling of the core layer are carried out simultaneously, avoiding the problem of uneven expansion caused by pH surge in the traditional one-time alkali addition method. As a result, the particle size distribution of the latex particles is highly concentrated.

[0073] Regarding long-term storage stability, the redox post-treatment constructs a sparse chemical cross-linking network in the shell layer, effectively counteracting the cavity shrinkage trend caused by polymer chain creep during storage. At the same time, it eliminates residual monomers and VOCs, and eliminates particle size drift caused by the continuous polymerization of residual monomers during storage. Post-heat preservation and curing allow the core layer chain segments to fully relax and the internal stress of the shell layer to be completely released, eliminating the thermodynamic driving force of cavity shrinkage. The synergistic effect of the two ensures that the cavity structure remains stable during long-term storage, avoiding problems such as particle size increase, sedimentation and stratification, and thickening and gelation.

[0074] Regarding the retention of cavities after drying and film formation, post-heat curing achieves thermal sizing of the cavity morphology at the molecular chain level through chain segment relaxation and internal stress elimination. Combined with the skeletal support of the shell micro-crosslinked network, even if the moisture completely evaporates during the subsequent coating and drying process, the cavity structure can still maintain its regular shape without collapsing, thus enabling the coating to have excellent hiding power, whiteness, matting properties, or lightweight effect.

[0075] This invention also provides a hollow acrylic spherical emulsion with high cavity stability, wherein the latex particles have a three-layer structure of core layer-shell layer-hollow cavity. The core layer is located inside the latex particle and contains copolymer units of acidic functional monomers, forming a closed hollow cavity inside the core layer; the shell layer covers the outside of the core layer, forming the outer shell of the latex particle, and its theoretical glass transition temperature Tg≥70℃ provides structural constraint during the expansion of the core layer.

[0076] The theoretical acid value of the core layer is 100~350 mg KOH / g. This acid value range ensures that the core layer has sufficient hydrophilic expansion driving force to form a fully developed cavity, while avoiding excessive expansion and shell rupture caused by excessively high acid values. A shell Tg ≥ 70℃ ensures that the shell is in a glassy state under neutralization expansion temperature conditions, possessing sufficient rigidity to resist the expansion pressure of the core layer and form a regular cavity structure.

[0077] In terms of product performance, the polydispersity index (PDI) of the emulsion particle size is ≤0.15, indicating that the latex particle size is highly uniform; the particle size change rate after 30 days of storage at 50℃ is ≤8%, indicating that the cavity structure does not shrink significantly during long-term storage; and the proportion of latex particles with identifiable cavity structure after drying is ≥80%, indicating that the cavity structure of the latex particles can still remain intact after the drying process.

[0078] The acrylic hollow sphere emulsion of the present invention can be directly used in fields such as opaque coatings, matte coatings, lightweight coatings, paper coatings, architectural coatings and water-based inks, and can be mixed with other components in the formulation without additional treatment. Figure 2 The image shows the effect of applying the acrylic hollow sphere emulsion of the present invention to printing paper and then printing a test coating.

[0079] To further illustrate the effect of the dosage of each key component in this embodiment on the performance of the acrylic hollow sphere emulsion, acrylic hollow sphere emulsions prepared in Examples 1 to 5 and Comparative Examples 1 to 4 are provided below and tested.

[0080] Example 1

[0081] Step (1): Preparation of core layer latex particles

[0082] In a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet pipe, 120 parts of deionized water, 1.2 parts of sodium dodecyl sulfate, and 0.8 parts of octylphenol polyoxyethylene ether-10 (OP-10) were added, and the mixture was stirred and heated to 80°C. After adding 0.3 parts of potassium persulfate, the core layer monomer mixture was added dropwise. The core layer monomer mixture consisted of 35 parts of methacrylic acid, 35 parts of butyl acrylate, 25 parts of styrene, and 3 parts of divinylbenzene, and the dropwise addition time was 2 hours. After the dropwise addition was completed, the reaction was maintained at this temperature for 1 hour to obtain core layer latex particles. The average particle size of the core layer latex particles was determined to be 220 nm, the PDI was 0.08, and the theoretical acid value of the core layer was 228 mg KOH / g.

[0083] Step (2): Polymerization to form a shell

[0084] In the core-layer latex particle dispersion obtained in step (1), a shell-layer monomer mixture was added dropwise. The shell-layer monomer mixture consisted of 50 parts styrene, 35 parts methyl methacrylate, 15 parts butyl acrylate, and 2 parts ethylene glycol dimethacrylate. The addition time was 3 hours, and 0.3 parts potassium persulfate was added dropwise simultaneously. After the addition was complete, the reaction was maintained at this temperature for 1 hour. The mass ratio of shell-layer monomer to core-layer monomer was 2.5:1, and the theoretical Tg of the shell polymer was 95℃.

[0085] Step (3): Pretreatment before neutralization

[0086] Cool the core-shell acrylic emulsion obtained in step (2) to 58°C and add 0.02 parts of ferrous sulfate and 0.05 parts of sodium citrate.

[0087] Step (4): Neutralize the expansion in stages

[0088] Sodium hydroxide was prepared into a 10% (w / w) aqueous solution and added to the system in three stages: First stage: Adjust the pH of the system from about 2.0 to 3.5 at a rate of 0.6 pH units / hour, add 22% of the total amount of neutralizing agent, control the temperature at 58℃, and stir at 200 rpm. Second stage: Adjust the pH of the system from 3.5 to 6.0 at a rate of 1.0 pH units / hour, add 45% of the total amount of neutralizing agent, control the temperature at 58℃, and stir at 160 rpm. The third stage: The pH of the system was adjusted from 6.0 to 8.5 at a rate of 0.4 pH units / hour. The added neutralizing agent accounted for 33% of the total amount of neutralizing agent. The temperature was controlled at 58℃ and the stirring speed was 130 rpm.

[0089] The total time for pH rise was 6 hours.

[0090] Step (5): Post-oxidation and reduction treatment

[0091] After reaching the target pH (8.5), 0.15 parts of hydrogen peroxide (based on the total solid content of the emulsion, the same below) and 0.12 parts of sodium thiosulfate were added to the system. The molar ratio of oxidant to reducing agent was 1:1. The mixture was stirred at 58°C for 30 minutes.

[0092] Step (6): Post-heat preservation and ripening

[0093] Heat the system to 80°C and keep it warm for 2 hours while stirring at 120 rpm.

[0094] Step (7): Cooling and filtering

[0095] The emulsion obtained in step (6) was naturally cooled to room temperature and filtered through a 300-mesh filter to obtain a cavity-stabilized acrylic hollow sphere emulsion.

[0096] Example 2

[0097] Other conditions are the same as in Example 1, except that the neutralizing agent in step (4) is added in two stages: in the first stage, the pH of the system is adjusted to 4.0, the pH rise rate is 0.8 pH units / hour, and 40% of the total amount of neutralizing agent is added; in the second stage, the pH of the system is adjusted to 8.5, the pH rise rate is 0.6 pH units / hour, and 60% of the total amount of neutralizing agent is added. The total pH rise time is 6 hours.

[0098] Example 3

[0099] Other conditions are the same as in Example 1, except that the total pH rise time in step (4) is 4 hours (the pH rise rate is increased accordingly in each stage: 0.9 pH units / hour in the first stage, 1.5 pH units / hour in the second stage, and 0.6 pH units / hour in the third stage).

[0100] Example 4

[0101] The other conditions are the same as in Example 1, except that the post-heating and curing temperature in step (6) is 70°C and the time is 3 hours.

[0102] Example 5

[0103] The other conditions are the same as in Example 1, except that the cooling temperature in step (3) is 50°C (instead of 58°C) and the neutralization temperature in step (4) is 50°C.

[0104] Comparative Example 1

[0105] Other conditions are the same as in Example 1, except that the cooling pretreatment and catalyst pre-embedding in step (3) are omitted. That is, after step (2) is completed, neutralization and expansion are carried out directly at 80°C (without cooling, ferrous sulfate and sodium citrate are not added); the neutralizing agent in step (4) is added to the target pH 8.5 at once (without stages), and the addition time is controlled within 30 minutes; the oxidation-reduction posttreatment in step (5) is carried out at 80°C; the post-heat preservation and aging in step (6) is omitted, and the neutralization is completed by direct cooling and filtration.

[0106] Comparative Example 2

[0107] Other conditions are the same as in Example 1, except that the phased neutralization operation in step (4) is omitted, that is, the neutralizing agent is added to the target pH 8.5 at once, and the addition time is controlled within 30 minutes (but the cooling pretreatment and catalyst pre-embedding in step 3, the oxidation-reduction post-treatment in step 5 and the post-heat preservation and ripening in step 6 are still retained).

[0108] Comparative Example 3

[0109] Other conditions are the same as in Example 1, except that the post-heat preservation and curing in step (6) is omitted, that is, after step (5) is completed, the temperature is directly cooled and filtered (but the cooling pretreatment and catalyst pre-embedding in step 3, the phased neutralization in step 4 and the oxidation-reduction posttreatment in step 5 are still retained).

[0110] Comparative Example 4

[0111] Other conditions are the same as in Example 1. The difference is that the oxidation-reduction post-treatment in step (5) is omitted. That is, after step (4) is completed, the process directly proceeds to step (6) for post-heat preservation and ripening (but the cooling pretreatment and catalyst pre-embedding in step 3, the phased neutralization in step 4 and the post-heat preservation and ripening in step 6 are still retained).

[0112] The test solutions obtained from each embodiment and comparative example were subjected to the following tests: (1) Particle size and particle size distribution (PDI) determination The Z-mean particle size and PDI of latex particles were determined using a dynamic light scattering instrument (such as Malvern Zetasizer Nano ZS). Test conditions: The samples were diluted with deionized water to a solid content of approximately 0.01~0.05%, equilibrated at 25℃ for 5 minutes, and then tested at a 90° angle. Each sample was measured in triplicate, and the average value was taken.

[0113] (2) Storage stability test at 50℃

[0114] Referring to the accelerated test method for "Storage Stability" in GB / T 20623-2006 "Emulsions for Architectural Coatings", the emulsion samples were placed in sealed glass bottles and stored in a constant temperature oven at 50±1℃ for 30 days. Samples were taken before and after storage, and the Z-mean particle size of the samples was measured using a dynamic light scattering instrument. The particle size change rate was calculated using the following formula: Particle size change rate (%) = |D 30 -D0| / D0×100%, where D0 is the initial Z-average particle size, D 30 The Z-average particle size is the particle size after storage at 50℃ for 30 days. Simultaneously, observe and record changes in the sample's appearance (whether stratification, precipitation, clumping, or thickening / gelling occurs).

[0115] (3) Observation and statistics of cavity structure

[0116] The emulsion sample was diluted with deionized water to an appropriate concentration. The diluted emulsion was then used to directly observe the aqueous sample using a cryo-transmission electron microscope (cryo-TEM), which accurately reflects the original morphology of the emulsion in its liquid state. The accelerating voltage was 80–120 kV. Five to ten different fields of view were randomly photographed, and the cavity structures of at least 300 latex particles were identified and statistically analyzed. Latex particles with a clearly defined empty cavity in the central region were counted as having an "identifiable cavity structure." The cavity retention rate (%) was calculated as: (Number of latex particles with identifiable cavity structures / Total number of latex particles) × 100%.

[0117] (4) Coating opacity measurement

[0118] The emulsions prepared in the examples and comparative examples were respectively prepared into coatings, and the contrast ratio (coverage) of the coatings was determined in accordance with GB / T 23981.1-2019 "Determination of hiding power of paints and varnishes - Part 1: Determination of contrast ratio of white and light-colored paints".

[0119] The measurement results of each embodiment and comparative example are summarized in Table 1 below.

[0120] Table 1: Summary of measurement results for each example and comparative example.

[0121]

[0122] Figure 3The figure shows the dynamic light scattering particle size distribution of the acrylic hollow sphere emulsion obtained in Example 1 of this invention (the left side of the figure shows the particle size and light intensity distribution, and the right side shows the correlation curve). The test was conducted using a dynamic light scattering instrument at 25°C. The sample was diluted with deionized water to a suitable concentration before measurement. As shown in the figure, the particle size distribution exhibits a single sharp peak with a peak average of 1406 nm and an area ratio of 100%. No secondary peaks such as aggregates or small particle fragments were observed. The right side shows that the correlation coefficient decreases smoothly over time, indicating that the latex particles are well dispersed in the system without agglomeration. Statistical results show that the Z-mean is 1358 nm, and the polydispersity index (PDI) is 0.01059. These results demonstrate that the acrylic hollow sphere emulsion prepared in Example 1 has a highly monodisperse particle size distribution, verifying the effectiveness of the staged neutralization and expansion process of this invention in achieving synchronous and uniform expansion of latex particles.

[0123] Figure 4 This is a dynamic light scattering particle size distribution diagram of the acrylic hollow sphere emulsion obtained in Example 2 of the present invention. As shown in the figure, the particle size distribution also exhibits a single sharp peak with a peak average of 1487 nm and an area ratio of 100%; the correlation coefficient shows a smooth decreasing trend. The Z-mean is 1368 nm, and the polydispersity index (PDI) is 0.03712.

[0124] Figure 5 This is a dynamic light scattering particle size distribution diagram of Embodiment 3 of the present invention. Figure 3 , Figure 4 The same results show that the particle size distribution exhibits a single sharp peak, and the correlation curve decays smoothly, proving that this batch of samples also has monodisperse particle size distribution characteristics, and there are no aggregates or small particle fragments in the samples.

[0125] Figure 6 This is a scanning electron microscope (SEM) image (magnification ×7.00k, scale bar 5.00μm) of the dried acrylic hollow sphere emulsion obtained in Example 1 of this invention. The sample was observed using a scanning electron microscope after gold sputtering treatment at an accelerating voltage of 3.0kV and a working distance of 5.5mm. Figure 6 As can be seen, the latex particles are distributed in a monodisperse state, with intact spherical shapes and smooth surfaces, without obvious cracks, adhesions, or large-area collapses. Most of the latex particles in the figure have a clearly identifiable cavity structure, and slight indentation features can be observed on the surface of some latex particles. This is a typical manifestation of the capillary force generated by water evaporation during the drying process, and the hollow spheres maintaining their shape due to the shell support during the drying process. This further proves that the hollow sphere emulsion prepared in this invention has excellent cavity structure stability.

[0126] Figure 7This is a scanning electron microscope (SEM) image (magnification ×3.50k, scale bar 10.0μm) of the dried acrylic hollow sphere emulsion obtained in Example 1 of this invention. The observation conditions are the same as those of the present invention. Figure 6 Same. At lower magnification, the distribution of latex particles can be observed over a larger field of view. Figure 7 It can be seen that the latex particles are monodisperse as a whole, with intact spherical shapes and uniform particle size distribution, which is highly consistent with the DLS detection results (Z-mean 1358~1368nm, PDI 0.01059~0.03712). No obvious cracks, aggregations or abnormal deformations of latex particles were observed in the field of view. The hollow spheres have a high degree of hollowness and uniform size and structure, indicating that the acrylic hollow sphere emulsion prepared by this invention has excellent morphological regularity and dispersibility. Figure 8 This is a dimensional diagram of a single acrylic hollow sphere, with an outer diameter of 1485 nm and an inner diameter of 831.8 nm. These dimensions can be used to determine the hollowness of the particles.

[0127] Referring to Table 1, Comparative Example 1 used a traditional one-time alkali addition method without cooling pretreatment, catalyst pre-embedding, and post-heat preservation curing. Its PDI was as high as 0.24, indicating that the latex particles underwent significant widening of the particle size distribution during expansion, with some particles over-expanding while others under-expanded. After 30 days of storage at 50℃, the particle size change rate was as high as 28.6%, indicating that the cavity structure underwent severe shrinkage or collapse during storage. The cavity retention rate was only 35%, with most latex particles losing their cavity structure after drying. The coating coverage was only 0.72. In contrast, Example 1 had a PDI reduced to 0.01, a particle size change rate of only 3.2%, a cavity retention rate as high as 92%, and a coating coverage of 0.95. These data fully demonstrate the synergistic effectiveness of the five-in-one process system of "cooling pretreatment—staged neutralization—oxidation-reduction post-treatment—post-heat preservation curing" of this invention.

[0128] Comparative Example 2 retained the cooling pretreatment, catalyst pre-embedding, redox post-treatment, and post-heat preservation curing, but the neutralizing agent was still added in a single step, with a PDI of 0.19, a particle size change rate of 18.3%, and a cavity retention rate of 52%. This indicates that even with other process methods, without the core step of staged neutralization, the core layer will still experience a widening of the particle size distribution and instability of the cavity structure due to instantaneous impact expansion. This further demonstrates the crucial role of staged neutralization in achieving uniform expansion and cavity stability.

[0129] Comparative Example 3 retained the staged neutralization and other steps, but omitted the post-heat curing process. Its particle size change rate was 16.7%, and its cavity retention rate was 58%. This indicates that without post-heat curing, the internal stress of the latex particles was not fully released, and the cavity morphology was not thermally set. Therefore, the cavities still shrink significantly during storage and drying. Post-heat curing, through chain segment relaxation and internal stress elimination, achieves permanent thermal setting of the cavity morphology, which is a crucial step in ensuring long-term storage stability and cavity retention rate after drying.

[0130] Comparative Example 4 retained the staged neutralization and post-heat curing processes but omitted the redox post-treatment. Its particle size change rate was 12.4%, and its cavity retention rate was 68%. This indicates that even after post-heat curing releases internal stress, without the chemical framework support provided by the shell micro-crosslinking, the cavity will still slowly shrink due to molecular chain creep during long-term storage. The redox post-treatment, by constructing a sparse chemical crosslinking network in the shell, provides skeletal support for heat setting during the post-heat curing process; both are indispensable.

[0131] The performance of Example 2 (divided into 2 stages) was slightly lower than that of Example 1 (divided into 3 stages), indicating that the more refined the stages (3 stages are better than 2 stages), the more precise the control of the expansion process, and the better the cavity stability. The performance of Example 3 (total pH rise time of 4 hours) was slightly lower than that of Example 1 (6 hours), indicating that appropriate slow expansion is beneficial to the full stability of the cavity structure. The performance of Examples 4 (curing temperature of 70°C) and 5 (cooling temperature of 50°C) was comparable to that of Example 1, indicating that each step of the present invention has a wide process window, which is beneficial to the stability of industrial production.

[0132] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

[0133] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, preferred embodiments are described in order to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art can make other changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of this invention.

[0134] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of this invention; if these modifications and variations of this invention fall within the scope of the claims of this invention and their equivalents, then this invention also intends to include these modifications and variations.

Claims

1. A method for preparing an acrylic hollow sphere emulsion with high cavity stability, characterized in that, Includes the following steps: Step S1: Prepare core-layer latex particles containing acidic functional monomers; Step S2: Polymerize the outer layer of the core latex particles to form a shell layer, thereby obtaining a core-shell acrylic emulsion; Step S3: Cool the obtained core-shell acrylic emulsion to a predetermined neutralization temperature and add a redox catalyst, wherein the redox catalyst comprises a transition metal ion source and a chelating agent; Step S4: Prepare a neutralizing agent aqueous solution and add it to the system obtained in step S3 in stages, controlling the pH of the system to rise to the target pH in stages, and controlling the total pH rise time within the predetermined time range. Step S5: After reaching the target pH, add oxidant and reducing agent to the system for post-treatment; Step S6: Heat the system obtained in step S5 and then keep it at that temperature for further curing. Step S7: Cool and filter to obtain cavity-stabilized acrylic hollow sphere emulsion.

2. The preparation method according to claim 1, characterized in that, The preparation of core-layer latex particles containing acidic functional monomers includes: Add deionized water and emulsifier to the reactor, stir and heat to 75~85℃, add part of the initiator, and then dropwise add a core layer monomer mixture consisting of acidic functional monomers, soft monomers, hard monomers and optional crosslinking agents. The dropwise addition time is 1.5~3 hours, and after the dropwise addition is completed, keep the reaction at the temperature for 0.5~1.5 hours. The acidic functional monomer is one or both of methacrylic acid and acrylic acid, the soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, and isooctyl acrylate, and the hard monomer is selected from one or more of styrene, methyl methacrylate, and acrylonitrile.

3. The preparation method according to claim 2, characterized in that, By weight, the acidic functional monomer is 15-50 parts, the soft monomer is 20-50 parts, the hard monomer is 10-40 parts, and the crosslinking agent is 0-8 parts; the theoretical acid value of the core layer is 100-350 mgKOH / g; the particle size range of the core layer latex particles is 100-350 nm, and the PDI is less than or equal to 0.

10.

4. The preparation method according to claim 1, characterized in that, The shell layer is formed by copolymerization of shell hard monomers and shell soft monomers. By mass, the shell hard monomers are 60-95 parts and the shell soft monomers are 5-40 parts. The shell layer also contains a shell crosslinking agent, and the amount of the shell crosslinking agent is 0-5 parts. The mass ratio of the shell monomers to the core monomers is 1:1 to 5:

1. The theoretical glass transition temperature Tg of the shell polymer is ≥70℃.

5. The preparation method according to claim 4, characterized in that, The shell hard monomer is selected from one or more of styrene and methyl methacrylate, the shell soft monomer is selected from one or more of butyl acrylate and ethyl acrylate, and the shell crosslinking agent is selected from one or more of divinylbenzene and ethylene glycol dimethacrylate.

6. The preparation method according to claim 1, characterized in that, The predetermined neutralization temperature is 50~70℃; the transition metal ion source is selected from one or more of ferrous chloride, ferrous ammonium sulfate, ferrous sulfate, copper sulfate, cobalt acetate, manganese sulfate, and cerium ammonium nitrate; the chelating agent is selected from one or more of ethylenediaminetetraacetic acid or its sodium salt, citric acid or its sodium salt, tartaric acid or its sodium salt, and gluconic acid or its sodium salt.

7. The preparation method according to claim 1, characterized in that, The neutralizing agent aqueous solution has a mass percentage concentration of 3-20%; the neutralizing agent is selected from one or more of ammonia, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and 2-amino-2-methyl-1-propanol.

8. The preparation method according to claim 1, characterized in that, The neutralizing agent aqueous solution is added gradually in three stages, and the control parameters for each stage are as follows: First stage: Adjust the pH of the system to 2.5~4.5, with a pH increase rate of 0.3~1.0 pH units / hour, and add a neutralizing agent accounting for 15~30% of the total neutralizing agent; Second stage: Adjust the pH of the system to 4.5~7.0, with a pH increase rate of 0.5~1.5 pH units / hour, and add 35~55% of the total amount of neutralizing agent; The third stage: Adjust the pH of the system to 7.0~9.5, with a pH increase rate of 0.2~0.8 pH units / hour, and add 20~40% of the total amount of neutralizing agent; The total time for pH rise during the entire process is 3 to 10 hours.

9. The preparation method according to claim 1, characterized in that, The oxidizing agent is selected from one or more of tert-butyl hydroperoxide, hydrogen peroxide, and potassium persulfate; the reducing agent is selected from one or more of sodium styrax, ascorbic acid, sodium bisulfite, and sodium metabisulfite; the molar ratio of the oxidizing agent to the reducing agent is 1:0.5~2. The amount of oxidant used is 0.05~0.5wt% of the total solid content of the emulsion.

10. The preparation method according to claim 1, characterized in that, The predetermined neutralization temperature is 55~60℃; the post-heat preservation and curing temperature is 60~90℃, and the time is 1~4 hours.

11. The preparation method according to claim 1, characterized in that, The cavity-stabilized acrylic hollow sphere emulsion has a particle size polydispersity index (PDI) ≤ 0.15 and a particle size change rate ≤ 8% after storage at 50°C for 30 days; the cavity-stabilized acrylic hollow sphere emulsion has a latex particle ratio of ≥ 80% after drying with identifiable cavity structure.

12. A high cavity stability acrylic hollow sphere emulsion, characterized in that, The acrylic hollow sphere emulsion is prepared using the preparation method described in any one of claims 1 to 11; the latex particles of the emulsion have a core layer and a shell layer, the latex particles have a hollow cavity structure, the core layer contains copolymer units of acidic functional monomers, the theoretical acid value of the core layer is 100~350 mgKOH / g; the theoretical glass transition temperature Tg of the shell layer is ≥70℃; the particle size polydispersity index (PDI) of the emulsion is ≤0.15, the particle size change rate after storage at 50℃ for 30 days is ≤8%, and the proportion of latex particles with identifiable cavity structures after drying is ≥80%.

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