A fluorine-free super-hydrophobic acrylate emulsion with core-shell crosslinking density gradient, a preparation method and applications thereof

Abstract

The application discloses a fluorine-free super-hydrophobic acrylate emulsion with a core-shell crosslinking density gradient, a preparation method and application, and belongs to the technical field of water-based polymer materials and environment-friendly coating. The application solves the technical problems of insufficient hydrophobic performance, difficult balance between mechanical strength and flexibility, and poor dispersion stability of nanoparticles of the existing acrylate emulsion waterproof coating. The emulsion has a core-shell structure, the core layer comprises hard monomers and crosslinking monomers, and the shell layer comprises soft monomers, hydrophobic monomers, a silane coupling agent and crosslinking monomers. The amount ratio of the crosslinking monomers in the core layer to the crosslinking monomers in the shell layer is 5:1-20:1, so that a gradient structure with gradually reduced crosslinking density from the core to the shell is formed. The nano-SiO2 modified by the silane coupling agent is chemically anchored on the surface of the shell layer of the latex particle by a covalent bond, so that the fluorine-free super-hydrophobic effect with a water contact angle of greater than or equal to 150 degrees and a rolling angle of less than or equal to 10 degrees is achieved, and the hydrophobic performance retention rate of the coating is greater than or equal to 90% after 100 times of reciprocating bending.

Description

Technical Field

[0001] This invention belongs to the field of water-based polymer materials and environmentally friendly coating technology, specifically relating to a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, its preparation method, and its application. Background Technology

[0002] With the continued tightening of global plastic restriction and ban policies, and increasingly stringent environmental control standards for per- and polyfluoroalkyl substances (PFAS), the development of fluorine-free, recyclable water-based paper-based waterproof coatings has become an important development direction for the packaging industry. Acrylic emulsions are widely used in paper-based coating fields due to their advantages such as good film-forming properties, environmental friendliness, non-toxicity, and mature processing technology.

[0003] However, existing research on acrylic emulsion waterproof coatings still faces the following technical challenges: First, insufficient hydrophobicity. Conventional acrylic emulsion coatings typically have a water contact angle between 70° and 90°, which is insufficient to meet the high waterproofing requirements (contact angle ≥150°) of high-end paper-based packaging. Existing superhydrophobic coatings often rely on fluorinated compounds to provide low surface energy, but PFAS are being gradually restricted by the EU, US FDA, and other agencies due to their environmental persistence and bioaccumulation. Second, it is difficult to balance coating mechanical strength and flexibility. Increasing crosslinking density to improve waterproofing often leads to brittleness and reduced flexibility; conversely, reducing crosslinking density to ensure flexibility results in insufficient coating density and deteriorated waterproofing. This contradiction is particularly pronounced in high-speed paper coating processes (800~1200 m / min). Third, poor dispersion stability of inorganic nanoparticles. While nano-SiO2 and other inorganic particles can effectively build surface roughness and improve hydrophobicity, they tend to aggregate and settle in emulsion systems, making long-term stable dispersion difficult. In existing technologies, nano-SiO2 is mostly dispersed in the system as a core structure or through simple physical mixing, lacking strong chemical bonds with the polymer matrix. Fourth, the crosslinking design of existing core-shell emulsions lacks gradient. Although there are numerous reports on core-shell acrylic emulsions, the distribution of crosslinking agents (such as EGDMA) in the core and shell layers is mostly uniform or involves single-region crosslinking, without forming a systematic crosslinking density gradient design.

[0004] Therefore, it is evident that there is currently a lack of a waterborne acrylic emulsion and its preparation method that can simultaneously achieve "controllable core-shell crosslinking density gradient, chemical anchoring of SiO2 shell, fluorine-free superhydrophobicity, and high-speed paper-based coating compatibility". Summary of the Invention

[0005] One of the objectives of this invention is to provide a method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, which solves the technical problems of insufficient hydrophobicity, difficulty in balancing mechanical strength and flexibility, and poor dispersion stability of nanoparticles in the existing acrylate emulsion waterproof coating.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a core-shell crosslinked density gradient type fluorine-free superhydrophobic acrylate emulsion includes the following steps: a) preparing modified nano-SiO2 with vinyl functional groups on its surface; b) mixing a hard monomer, a first crosslinking monomer EGDMA, a first emulsifier, and a portion of deionized water, and shearing and emulsifying at a certain speed for 30-60 min to obtain a core layer pre-emulsion; wherein the hard monomer is 20-40 g, and the mass of the first crosslinking monomer EGDMA is 2-5% of the mass of the hard monomer; c) mixing a soft monomer, A hydrophobic monomer, a silane coupling agent, a second crosslinking monomer EGDMA, a second emulsifier, and the remaining deionized water are mixed. The modified nano-SiO2 is then added to the resulting mixture. The mixture is ultrasonically dispersed for 25–35 min and sheared and emulsified at a certain speed for 30–60 min to obtain a shell pre-emulsion. The soft monomer is 10–25 g, the hydrophobic monomer is 15–25 g, the silane coupling agent is 5–15 g, and the mass of the second crosslinking monomer EGDMA is 0.1–0.5 g of the total mass of the soft monomer and the hydrophobic monomer. 5%; the mass ratio of the first crosslinking monomer EGDMA to the second crosslinking monomer EGDMA is 5:1 to 20:1; d. Take a portion of the core layer preemulsion obtained in step b, prepare three portions of initiator, add a portion of the core layer preemulsion together with the first portion of initiator to the reactor, and keep the reaction at 75-85℃ for 30-60 min to form a seed emulsion; e. Add the remaining core layer preemulsion from step b together with the second portion of initiator dropwise to the reactor, and add dropwise at 78-82℃ for 1-2 h to form a highly crosslinked emulsion. f. Add the shell pre-emulsion obtained in step c and the third initiator solution dropwise into the reactor, controlling the reaction temperature at 78~82℃ and the dropwise addition time at 2~3 hours, so that the shell monomers polymerize on the surface of the core layer, while the modified nano-SiO2 participates in copolymerization through its vinyl functional groups and is chemically anchored to the surface of the latex particles shell layer; g. Heat the reaction solution obtained in step f to 85~90℃ and keep it at that temperature for 1~2 hours to ensure the reaction is complete; then cool it down to below 40℃, adjust the pH to 7~8 with ammonia water, filter and discharge the material to obtain the final product.

[0007] The above-mentioned method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, in step a, the preparation step of modified nano-SiO2 is as follows: nano-SiO2 is dispersed in a mixed solvent of ethanol and water, a silane coupling agent is added thereto, the mixture is stirred at 60~80℃ for 4~8 hours, and then centrifuged, washed and dried to obtain the product.

[0008] In the above-mentioned method for preparing a core-shell crosslinked density gradient type fluorine-free superhydrophobic acrylate emulsion, the silane coupling agent added in step a is vinyltriethoxysilane VTES or methacryloxypropyltrimethoxysilane KH-570.

[0009] In the above-mentioned method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, in step b, the hard monomer is selected from one or more of methyl methacrylate, styrene, and acrylonitrile; the emulsifier is a compound system of anionic and nonionic emulsifiers, wherein the anionic emulsifier is selected from sodium dodecyl sulfate and sodium dodecylbenzene sulfonate, and the nonionic emulsifier is selected from alkylphenol polyoxyethylene ether OP-10 or fatty alcohol polyoxyethylene ether, and the mass ratio of anionic to nonionic emulsifier is 1:1 to 3:1.

[0010] In the above-mentioned method for preparing a core-shell crosslinked density gradient type fluorine-free superhydrophobic acrylate emulsion, in step c, the soft monomer is selected from one or more of butyl acrylate, isooctyl acrylate, and ethyl acrylate; the hydrophobic monomer is selected from one or more of octadecyl methacrylate, dodecyl methacrylate, and octadecyl acrylate.

[0011] In the above-mentioned method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, in step c, the amount of modified nano-SiO2 added is 3-10g, and the modified nano-SiO2 is anchored to the surface of the latex particle shell layer through chemical bonds.

[0012] In the above-mentioned method for preparing a core-shell crosslinked density gradient type fluorine-free superhydrophobic acrylate emulsion, all three initiators are potassium persulfate or ammonium persulfate. The amount of the first initiator is 0.3 to 1% of the mass of a portion of the core layer pre-emulsion; the amount of the second initiator is 0.3 to 1% of the mass of the remaining core layer pre-emulsion; and the amount of the third initiator is 0.3 to 1% of the mass of the shell layer pre-emulsion.

[0013] Another objective of this invention is to provide a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, which is prepared by the above-described preparation method. The core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion has a solid content of 35-50%, an average particle size of 100-250 nm, and a particle size distribution PDI ≤ 0.1.

[0014] Another object of the present invention is to provide an application of the above-mentioned core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, wherein the application is as follows: the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion is directly or after dilution coated onto the surface of a paper base, with a coating amount of 5~15 g / m², and dried and cured into a film at 80~120℃ to obtain a recyclable paper base coating material.

[0015] The above-mentioned core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion is applied in the paper-based packaging field. The coating is suitable for high-speed paper machine online coating process with a coating speed of 800~1200 m / min.

[0016] Compared with the prior art, the present invention brings the following beneficial technical effects: (1) Performance Synergy Achieved Through Crosslinking Density Gradient Design. This invention achieves a density gradient structure with a highly crosslinked core and a low-crosslinked shell by precisely controlling the different amounts of EGDMA used in the core layer (2-5%) and the shell layer (0.1-0.5%). The high crosslinking of the core layer imparts rigidity and density to the latex particles, providing excellent mechanical strength and barrier properties; the low crosslinking of the shell layer allows the polymer chain segments in the shell layer to maintain sufficient mobility, which is beneficial for the full migration of hydrophobic monomers and organosilicon components to the coating surface during film formation, thereby maximizing low surface energy. The synergistic effect of these two aspects resolves the contradiction in the prior art where high crosslinking leads to increased brittleness and low crosslinking leads to insufficient waterproofing.

[0017] (2) Chemical anchoring of the SiO2 shell solves the problem of dispersion stability. This invention uses a silane coupling agent to modify the surface of nano-SiO2, giving it vinyl functional groups that can participate in copolymerization. During the shell polymerization process, it is chemically anchored to the surface of the latex particle shell through covalent bonds. This is fundamentally different from the existing technology that uses SiO2 as a core or simply mixes SiO2 physically. Chemical anchoring not only fundamentally solves the problem of nanoparticle aggregation, but also firmly fixes SiO2 to the coating surface, providing a durable micro-nano rough structure, which, together with the low surface energy of the organosilicon component, achieves a superhydrophobic effect.

[0018] (3) Completely fluorine-free and environmentally friendly. This invention does not rely on any fluorine-containing compounds, but only provides low surface energy through long-chain alkyl hydrophobic monomers (such as octadecyl methacrylate) and organosilicon components (VTES / KH-570), combined with the micro-nano rough structure of nano-SiO2, to achieve a superhydrophobic effect with a water contact angle ≥150°. It fully complies with the global environmental trend of PFAS ban.

[0019] (4) Strong adaptability to high-speed paper coating. The emulsion of this invention has excellent rheological properties and film-forming properties, and is suitable for high-speed online coating processes on paper (800~1200 m / min). A coating amount of 5~15 g / m² can achieve excellent waterproof effect, and the coating is firmly bonded to the paper fiber. After 100 cycles of bending, the hydrophobicity retention rate is ≥90%.

[0020] (5) The paper base is recyclable. The coating material of the present invention does not contain non-recyclable plastic components, and the coated paper can be recycled through conventional pulping processes, which meets the requirements of the circular economy. Attached Figure Description

[0021] Figure 1 This is a transmission electron microscope (TEM) image of the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion prepared in Example 1 of the present invention.

[0022] Figure 2 This is a photograph of the water contact angle test of the coated paper obtained in Example 1 of the present invention. Detailed Implementation

[0023] This invention proposes a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, its preparation method, and its application. To make the advantages and technical solutions of this invention clearer and more explicit, the invention will be further described below with reference to specific embodiments.

[0024] All the raw materials mentioned in this invention can be purchased through commercial channels.

[0025] The technical concept of this invention lies in the following: by constructing a synergistic system integrating a core layer with high cross-linking enhancement (2~5%), a shell layer with low cross-linking hydrophobic enrichment (0.1~0.5%), and a SiO2 shell layer with chemical anchoring and roughening, it achieves for the first time a unified system in the same water-based acrylic emulsion that features controllable cross-linking density gradient, fluorine-free superhydrophobicity, and high-speed coating compatibility with paper substrates. This provides a high-performance, environmentally friendly, and industrially scalable waterproof coating solution for the paper-based packaging industry.

[0026] Example 1: The specific preparation steps of the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion of the present invention are as follows: Step 1: Surface modification of nano-SiO2: 10 g of nano-SiO2 with an average particle size of 20 nm was dispersed in 100 mL of ethanol / water (volume ratio 4:1) mixed solvent. 2 g of KH-570 silane coupling agent was added, and the pH was adjusted to 4-5 with glacial acetic acid. The mixture was stirred at 70 °C for 6 hours. After the reaction, the nano-SiO2 was centrifuged (8000 rpm, 15 minutes), washed three times with ethanol, washed three times with deionized water, and dried in a vacuum drying oven at 60 °C for 12 hours to obtain modified nano-SiO2 with vinyl functional groups on its surface.

[0027] Step 2: Preparation of the core layer preemulsion: Add 40 g of deionized water, 1.2 g of sodium dodecyl sulfate (SDS), and 0.6 g of OP-10 to a 500 mL beaker and stir to dissolve. Then add 25 g of methyl methacrylate, 10 g of styrene, and 1.5 g of EGDMA (accounting for 4.3% of the total mass of the 35 g core layer monomers), and emulsify in a high-speed disperser at 3000 rpm for 40 minutes to obtain the core layer preemulsion.

[0028] Step 3: Preparation of the shell preemulsion: Add 50 g of deionized water, 1.5 g of SDS, and 0.8 g of OP-10 to a 500 mL beaker and stir to dissolve. Then add 15 g of butyl acrylate (BA), 20 g of octadecyl methacrylate (SMA), 8 g of VTES, 0.1 g of EGDMA (0.23% of the total mass of the shell monomers 43 g), and 5 g of the modified nano-SiO2 prepared in Step 1. Sonicate the mixture for 30 minutes, and then emulsify it at 3000 rpm for 40 minutes in a high-speed disperser to obtain the shell preemulsion.

[0029] The ratio of core EGDMA (1.5 g) to shell EGDMA (0.1 g) was 15:1.

[0030] Step 4: Seed Emulsion Polymerization: Add 30 g of deionized water and 0.15 g of potassium persulfate (KPS) to a 1 L four-necked flask equipped with a stirrer, condenser, thermometer, and two dropping funnels. Stir and heat to 80°C. Take 15% (approximately 12 g) of the core pre-emulsion as seeds and quickly add it to the flask. Incubate the reaction at this temperature for 30 minutes. The system will exhibit blue light, indicating seed formation.

[0031] Step 5, Core Layer Polymerization: Transfer the remaining core layer preemulsion to the first dropping funnel. Dissolve 0.25 g of KPS in 15 g of deionized water as the initiator solution. Add the core layer preemulsion dropwise at 1.5 mL / min at 80°C, with the initiator solution added in two portions (1 hour apart). The total dropping time is 1.5 hours, and the reaction temperature is maintained at 80±1°C.

[0032] Step Six: Shell Polymerization: After core polymerization, the shell pre-emulsion is transferred to the first dropping funnel. 0.3 g of KPS is dissolved in 15 g of deionized water as the initiator solution. The shell pre-emulsion is added dropwise at 80°C at a rate of 2.0 mL / min, with the initiator solution added in three portions (1 hour apart). The total dropping time is 2.5 hours, and the reaction temperature is maintained at 80±1°C. During this process, modified nano-SiO2 participates in copolymerization through its vinyl functional groups, chemically anchoring itself to the surface of the latex particle shell.

[0033] Step 7, Incubation and Post-treatment: After the addition is complete, heat to 85℃ and maintain the temperature for 1.5 hours to ensure complete reaction. Cool down to below 40℃, adjust the pH to 7.5 with 25% ammonia, and filter through a 200-mesh sieve to obtain a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion.

[0034] The following performance tests were performed on the emulsion and coating prepared in this embodiment: (1) Basic properties of the emulsion: The solid content was 44.2% as tested according to GB / T 11175; the average particle size was 152 nm and the particle size distribution PDI was 0.072 as tested by dynamic light scattering (DLS); the gelation rate was 0.3% and the conversion rate was 97.2%.

[0035] (2) Characterization of emulsion particle morphology: The morphology of emulsion particles was observed using transmission electron microscopy (TEM). Figure 1 The results showed that the latex particles had a regular spherical core-shell structure, and nano-SiO2 particles were uniformly distributed on the shell surface with an average particle size of about 150 nm and a narrow particle size distribution.

[0036] (3) Coating preparation: The emulsion was diluted to a solid content of 30%, and coated onto the surface of 200 g / m² white cardboard using a wire bar coater with a coating amount of 10 g / m². The coated paper sample was dried in an oven at 100℃ for 2 minutes to obtain the coated paper sample.

[0037] (4) The water contact angle was tested according to GB / T 30447, and the result was 153°; the roll-off angle was tested according to GB / T 30693, and the result was 8°. The Cobb value (60 seconds) was tested according to GB / T 1540, and the result was 18 g / m². The water contact angle test photos of the coated paper obtained in this embodiment are shown below. Figure 2 As shown.

[0038] (5) Test on the retention rate of hydrophobic properties after bending: After bending the coated paper sample back and forth 100 times in the same direction, the water contact angle was 142° and the retention rate was 92.8%.

[0039] (6) Adhesion test: The cross-cut adhesion test was conducted according to GB / T 9286-1998, and the result was grade 0 (no coating peeling).

[0040] (7) Recyclability test: The coated paper sample was cut into pieces and placed in an alkaline aqueous solution with pH=10, and stirred at 60°C for 2 hours. The results showed that the coating was completely peeled off from the paper base surface, and the pulp fiber recovery rate was about 95%.

[0041] Comparative Example 1: The difference from Example 1 is that the ratio of EGDMA used in the core layer to the shell layer was adjusted to investigate the effect of the crosslinking density gradient on the performance.

[0042] Core layer: MMA 30 g, St 5 g, EGDMA 1.0 g (accounting for 2.9% of the total mass of the core layer monomers, which is 35 g).

[0043] Shell: BA 12 g, SMA 22 g, VTES 10 g, EGDMA 0.4 g (0.91% of the total mass of the shell monomers, which is 44 g).

[0044] The ratio of core EGDMA (1.0 g) to shell EGDMA (0.4 g) is 2.5:1 (not within the preferred range of 5:1 to 20:1 of this invention, but is used as a comparative reference).

[0045] The remaining formulation and process conditions are the same as in Example 1.

[0046] The emulsion and coating prepared in Comparative Example 1 were subjected to the following performance tests: Solid content 43.8%, average particle size 165 nm. Water contact angle 135°, roll-off angle 25°, Cobb value 32 g / m². After 100 bends, the water contact angle was 112°, with a retention rate of 83.0%. The results indicate that when the core-shell EGDMA ratio is too low (2.5:1), the shell crosslinking is too high, hindering the migration of hydrophobic monomers to the surface and significantly reducing hydrophobic properties.

[0047] Comparative Example 2: The difference from Example 1 is that EGDMA is not added to the shell (resulting in a larger crosslinking density gradient).

[0048] Core layer: MMA 30 g, St 5 g, EGDMA 1.8 g (accounting for 5.1% of the total mass of the core layer monomers, which is 35 g).

[0049] Shell: BA 15 g, SMA 18 g, VTES 10 g.

[0050] The remaining formulation and process conditions are the same as in Example 1.

[0051] The emulsion and coating prepared in Comparative Example 2 were subjected to the following performance tests: Solid content 44.5%, average particle size 148 nm, water contact angle 156°, roll-off angle 6°, Cobb value 15 g / m². After 100 bends, the water contact angle remained at 148°, with a retention rate of 94.9%. The results indicate that without EGDMA in the shell layer, the shell chain segments exhibited the strongest mobility, allowing for sufficient migration of hydrophobic components to the surface, resulting in optimal hydrophobic performance. However, the complete lack of cross-linking in the shell layer may slightly reduce the solvent resistance of the coating.

[0052] Example 2: The difference from Example 1 is that the hydrophobic monomer SMA is replaced with dodecyl methacrylate (LMA) and VTES is replaced with KH-570.

[0053] Core layer: MMA 28 g, St 8 g, EGDMA 1.6 g (4.4% of the total mass of the core layer monomers, which is 36 g).

[0054] Shell: BA 15 g, LMA 20 g, KH-570 8 g, EGDMA 0.1 g (0.23% of the total mass of the shell monomers 43 g), modified nano-SiO2 5 g

[0055] The ratio of core EGDMA (1.6 g) to shell EGDMA (0.1 g) was 16:1.

[0056] The remaining formulation and process conditions are the same as in Example 1.

[0057] The emulsion and coating prepared in Example 2 were subjected to the following performance tests: Solid content 43.5%, average particle size 160 nm, water contact angle 151°, roll-off angle 9°, Cobb value 20 g / m². After 100 bends, the water contact angle was 139°, with a retention rate of 92.1%. The results show that LMA and KH-570 can also effectively construct fluorine-free superhydrophobic surfaces.

[0058] Example 3: The difference from Example 1 is that: In step two, the hard monomers are selected from 25 g of methyl methacrylate and 10 g of acrylonitrile, and 0.7 g of EGDMA (accounting for 2% of the total mass of the core layer monomers of 35 g). In step three, the shell material consists of 15 g of butyl acrylate, 20 g of ethyl acrylate, 8 g of VTES, 0.43 g of EGDMA (accounting for 1.23% of the total mass of the soft monomer 35 g, which is outside the preferred range of 0.1~0.5% in this invention), and 5 g of modified nano-SiO2.

[0059] The ratio of core EGDMA (0.7 g) to shell EGDMA (0.43 g) is approximately 1.6:1 (not within the preferred range of 5:1 to 20:1 of this invention).

[0060] The remaining formulation and process conditions are the same as in Example 1.

[0061] The emulsion and coating prepared in Example 3 were subjected to the following performance tests: solid content 43.1%, average particle size 168 nm, PDI = 0.085, water contact angle 121°, roll-off angle 32°, Cobb value 42 g / m², water contact angle 98° after 100 folds, retention rate 81.0%, adhesion grade 1, and pulp fiber recovery rate approximately 92%.

[0062] The results showed that when the amount of EGDMA in the core layer was too low (2%), the cross-linking density of the core layer was insufficient, the core rigidity of the latex particles was not enough, and it could not provide sufficient structural support for the shell layer. The mechanical strength and density of the coating decreased, and the hydrophobic properties were significantly lower than those in Example 1.

[0063] Example 4: The difference from Example 1 is that: In step two, the hard monomers are selected from 25 g of methyl methacrylate and 10 g of acrylonitrile, and 1.75 g of EGDMA (accounting for 5% of the total mass of the core layer monomers of 35 g). In step three, the shell material consists of 15 g of butyl acrylate, 20 g of ethyl acrylate, 8 g of VTES, 0.215 g of EGDMA (0.6% of the total mass of the soft monomer and hydrophobic monomer 35 g), and 5 g of modified nano-SiO2.

[0064] The ratio of core EGDMA (1.75 g) to shell EGDMA (0.215 g) was approximately 8.1:1.

[0065] The remaining formulation and process conditions are the same as in Example 1.

[0066] The emulsion and coating prepared in Example 4 were subjected to the following performance tests: solid content 44.8%, average particle size 146 nm, PDI = 0.068, water contact angle 152°, roll-off angle 7°, Cobb value 17 g / m², water contact angle after 100 folds 141°, retention rate 92.8%, adhesion grade 0, and pulp fiber recovery rate approximately 96%.

[0067] The results show that when the EGDMA content in the core layer is 5%, the crosslinking density of the core layer is high, and the core rigidity and compactness are excellent. The core-shell EGDMA ratio (8.1:1) is within the preferred range of this invention, and the hydrophobic performance is comparable to that of Example 1. A comparison with Example 3 shows that the crosslinking density of the core layer is a key factor affecting the final hydrophobic performance. In summary, this invention, by precisely controlling the ratio of EGDMA in the core layer (2-5%) to the shell layer (0.1-0.5%) (5:1-20:1), forms a crosslinking density gradient structure. Simultaneously, silane coupling agent-modified nano-SiO2 is covalently anchored to the surface of the latex particle shell layer. These three elements synergistically achieve a fluorine-free superhydrophobic effect with a water contact angle ≥150° and a roll-off angle ≤10°. Furthermore, the hydrophobic performance retention rate of the coating is ≥90% after 100 cycles of repeated bending. This invention provides a high-performance, environmentally friendly, and industrially scalable fluorine-free superhydrophobic and waterproof coating solution for the paper-based packaging industry.

[0068] Any parts not mentioned in this invention can be achieved by referring to existing technologies.

[0069] Those skilled in the art should recognize that the above embodiments are merely illustrative of this application and are not intended to limit this application. Any appropriate changes and variations made to the above embodiments within the essential spirit and scope of this application should fall within the scope of protection of the claims of this application.

Claims

1. A method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, characterized in that, Includes the following steps: a. Preparation of modified nano-SiO2 with vinyl functional groups on the surface; b. Mix the hard monomer, the first crosslinking monomer EGDMA, the first emulsifier, and a portion of deionized water, and shear emulsify at a certain speed for 30-60 minutes to obtain a core layer pre-emulsion; the hard monomer is 20-40g, and the mass of the first crosslinking monomer EGDMA is 2-5% of the mass of the hard monomer; c. Mix the soft monomer, hydrophobic monomer, silane coupling agent, second crosslinking monomer EGDMA, second emulsifier, and the remaining deionized water. Add the modified nano-SiO2 to the resulting mixture, ultrasonically disperse for 25–35 min, and shear emulsify at a certain speed for 30–60 min to obtain a shell pre-emulsion. The soft monomer is 10–25 g, the hydrophobic monomer is 15–25 g, the silane coupling agent is 5–15 g, and the mass of the second crosslinking monomer EGDMA is 0.1–0.5% of the total mass of the soft monomer and the hydrophobic monomer. The mass ratio of the first crosslinking monomer EGDMA to the second crosslinking monomer EGDMA is 5:1–20:

1. d. Take a portion of the core layer preemulsion obtained in step b, prepare three portions of initiator, add a portion of the core layer preemulsion together with the first portion of initiator into the reactor, and keep the reaction at 75~85℃ for 30~60min to form a seed emulsion; e. Add the remaining core layer preemulsion from step b along with the second initiator dropwise into the reactor at 78-82°C for 1-2 hours to form a highly cross-linked core. f. Add the shell pre-emulsion obtained in step c and the third initiator solution dropwise into the reactor, control the reaction temperature at 78~82℃, and the dropwise addition time at 2~3 hours, so that the shell monomers polymerize on the core surface. At the same time, the modified nano-SiO2 participates in copolymerization through the vinyl functional groups on its surface and is chemically anchored to the surface of the latex particles shell. g. Heat the reaction solution obtained in step f to 85~90℃ and keep it at that temperature for 1~2 hours to ensure the reaction is complete; then cool it down to below 40℃, adjust the pH to 7~8 with ammonia water, filter and discharge the product.

2. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 1, characterized in that: In step a, the preparation steps of modified nano-SiO2 are as follows: nano-SiO2 is dispersed in a mixed solvent of ethanol and water, a silane coupling agent is added, and the mixture is stirred at 60~80℃ for 4~8 hours. After centrifugation, washing and drying, the modified nano-SiO2 is obtained.

3. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 2, characterized in that: The silane coupling agent added in step a is vinyltriethoxysilane VTES or methacryloxypropyltrimethoxysilane KH-570.

4. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 1, characterized in that: In step b, the hard monomer is selected from one or more of methyl methacrylate, styrene, and acrylonitrile; The emulsifier is a compound system of anionic and nonionic emulsifiers. The anionic emulsifier is selected from sodium dodecyl sulfate and sodium dodecylbenzene sulfonate, and the nonionic emulsifier is selected from alkylphenol polyoxyethylene ether OP-10 or fatty alcohol polyoxyethylene ether. The mass ratio of anionic to nonionic emulsifier is 1:1 to 3:

1.

5. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 3, characterized in that: In step c, the soft monomer is selected from one or more of butyl acrylate, isooctyl acrylate, and ethyl acrylate; the hydrophobic monomer is selected from one or more of octadecyl methacrylate, dodecyl methacrylate, and octadecyl acrylate.

6. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 5, characterized in that: In step c, the amount of modified nano-SiO2 added is 3-10g, and the modified nano-SiO2 is anchored to the surface of the latex particle shell through chemical bonds.

7. The method for preparing a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 1, characterized in that: All three initiators were potassium persulfate or ammonium persulfate. The amount of the first initiator was 0.3 to 1% of the mass of a portion of the core preemulsion; the amount of the second initiator was 0.3 to 1% of the mass of the remaining core preemulsion; and the amount of the third initiator was 0.3 to 1% of the mass of the shell preemulsion.

8. A core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 7, wherein the solid content of the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion is 35 to 50%, the average particle size is 100 to 250 nm, and the particle size distribution PDI is ≤ 0.

1.

9. The application of the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 8 in the field of paper-based packaging, characterized in that: The application is as follows: a core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion is directly or diluted and coated onto the surface of a paper base with a coating amount of 5~15 g / m², and dried and cured at 80~120℃ to form a film, thereby obtaining a recyclable paper base coating material.

10. The application of the core-shell cross-linked density gradient type fluorine-free superhydrophobic acrylate emulsion according to claim 9 in the field of paper-based packaging, characterized in that: The coating is suitable for online coating processes on high-speed paper machines, with a coating speed of 800~1200m / min.

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