Method for eliminating water-based flame-retardant hot melt adhesive coating film shrinkage by high-speed pre-grinding

Abstract

The present application relates to the technical field of water-based coating glue processing, and particularly relates to a method for eliminating water-based flame-retardant hot melt glue film shrinkage by high-speed pre-grinding. First, ethylene-chlorovinyl emulsion and diantimony trioxide powder are subjected to water-cooling high-speed pre-grinding under the condition that waterproof and oil-proof agents, wetting agents, dispersants and defoaming agents are not contained, then six-carbon fluorine waterproof and oil-proof agents and water are added and stirred at low speed to obtain coating mixed glue liquid. The method can improve the uniformity of flame-retardant powder dispersion, reduce local enrichment of low surface energy components, reduce film shrinkage, and maintain glue liquid stability and bonding performance.

Description

Technical Field

[0001] This invention relates to the field of water-based coating adhesive processing technology, and in particular to a method for eliminating pinholes in water-based flame-retardant hot melt adhesive coatings through high-speed pre-grinding. Background Technology

[0002] When water-based coatings, paints, or adhesives are applied to the surfaces of textile substrates, films, and sheet substrates to form films, they often need to simultaneously meet multiple performance requirements, including adhesion, flame retardancy, water resistance, oil resistance, flexibility, and smoothness. For water-based coatings containing flame-retardant powder, the flame-retardant powder is usually dispersed in the water-based emulsion in the form of solid particles. If the powder is not sufficiently wetted, refined, or dispersed, it is easy to form micro-agglomerates or localized coarse particles in the emulsion system. When the coating liquid is spread on the substrate surface and dries to form a film, these agglomerates may cause uneven spreading, reduced film continuity, or uneven surface tension distribution, thereby forming appearance defects such as pinholes, craters, and pitting.

[0003] In existing technologies, to improve the dispersibility of water-based flame-retardant coating adhesives, methods such as stirring and grinding are used to treat the flame retardant and water-based emulsion system. For example, CN104404769B discloses a water-based flame-retardant coating adhesive for textiles and its preparation method. This method involves mixing water-based polyurethane emulsion and water-based acrylic emulsion, adding flame retardants, flame retardant synergists, and other components, and then grinding the mixture to reduce the latex particle diameter to less than 10 μm. A thickener is then added to obtain the finished product. This method demonstrates that refining the mixture of emulsion and flame-retardant components through grinding in water-based flame-retardant coating adhesives can improve the uniformity of the system and the stability of the product.

[0004] However, the above solutions mainly focus on the overall dispersion and particle size control of the flame-retardant coating adhesive, and do not propose specific solutions for the pinhole problem in water-based hot melt adhesive systems that simultaneously contain low surface energy hydrophobic components and inorganic flame-retardant powders. Especially when water-repellent and oil-repellent agents are further added to the water-based hot melt adhesive, these agents typically have low surface energy. When they accumulate on the surface of powder agglomerates or in localized areas, they may further exacerbate the surface tension differences in the coating film, causing localized shrinkage of the adhesive during coating and forming pinholes. Simply mixing the water-based emulsion, flame-retardant powder, water-repellent and oil-repellent agent, and water all at once, or subjecting them to high-shear dispersion, cannot guarantee a uniform distribution of the low surface energy components. Instead, the hydrophobic components may become coated or accumulated in localized areas due to insufficient refinement of the powder agglomerates.

[0005] To address the pinhole problem, existing technologies often employ methods such as adding surface modifiers, wetting agents, dispersants, or defoamers to improve the coating appearance. For example, CN112262189B discloses a surface modifier for waterborne coatings, a waterborne coating composition, a coating film, and a multilayer coating film. It imparts anti-pinhole properties by adding a surface modifier to the waterborne coating composition, and points out that insufficient surface modifier content makes it difficult to provide sufficient anti-pinhole properties, while excessive content may hinder coating adhesion. This type of technical approach can improve pinholes by adjusting the coating surface state, but it essentially relies on the introduction of new additives.

[0006] For water-based hot melt adhesives or water-based flame-retardant adhesive systems, while the addition of wetting agents, dispersants, defoamers, or surface conditioners may improve the coating appearance, it may also introduce new formulation compatibility issues. For example, some additives may alter the interactions between emulsion particles, affect the storage stability of the adhesive, or remain at the interface after drying, affecting the coating's bond strength, water resistance, oil resistance, and the uniformity of flame-retardant component distribution. Therefore, for water-based flame-retardant hot melt adhesive systems with defined functional formulations, how to reduce coating pinholes caused by the agglomeration of flame-retardant powders and the localized enrichment of hydrophobic components without introducing new wetting agents, dispersants, and defoamers remains a problem that existing technologies still need to solve.

[0007] Furthermore, existing conventional low-speed mixing processes typically rely on dispersion discs or impellers to mix the components as a whole. Their shearing action is limited, making it difficult to fully break down the micro-agglomerates of flame-retardant powders such as antimony trioxide in aqueous emulsions. Simply increasing the mixing intensity or extending the mixing time may lead to system temperature increases, foaming, or decreased emulsion stability. If high-speed grinding is performed after all components are added, low-surface-energy waterproof and oil-repellent agents will participate in the high-shear process. Their interaction with unrefined powders and emulsion particles is difficult to control and does not effectively eliminate localized uneven surface tension at its source.

[0008] Therefore, it is necessary to provide a process method suitable for water-based flame-retardant hot melt adhesive systems. By adjusting the order of addition of flame-retardant powder, emulsion and water-repellent agent and the shearing method, the flame-retardant powder is first fully wetted and finely ground in the water-based hot melt adhesive. Then, the water-repellent agent is added under low shear conditions for homogenization. This reduces pinholes in the coating film without introducing additional wetting agents, dispersants and defoamers, while taking into account the stability of the water-based emulsion system and the adhesion performance of the coating film. Summary of the Invention

[0009] The purpose of this invention is to provide a method for eliminating pinholes in water-based flame-retardant hot melt adhesive coatings through high-speed pre-grinding. This method involves water-cooled high-speed grinding of ethylene-vinyl chloride emulsion and antimony trioxide powder before adding the water-repellent agent, which fully wets, refines, and disperses the flame-retardant powder. Then, a carbon-6 fluorocarbon waterproof and oil-repellent agent is added using a post-processing low-speed stirring method. This reduces the occurrence rate of pinholes in the coating without the addition of additional wetting agents, dispersants, and defoamers, while also ensuring the stability of the adhesive system, the adhesion performance of the coating, and the flame-retardant, waterproof, and oil-repellent functions.

[0010] Firstly, in order to achieve the above-mentioned objectives, the present invention adopts the following technical solution: A method for eliminating pinholes in water-based flame-retardant hot melt adhesive coatings through high-speed pre-grinding includes the following steps: S1. Prepare ethylene-vinyl chloride emulsion, antimony trioxide powder, carbon-6 fluoropolymer waterproof and oil-repellent agent and water, and temporarily prevent the carbon-6 fluoropolymer waterproof and oil-repellent agent and water from entering the high-speed grinding system. S2. Add the ethylene-vinyl chloride emulsion to a basket mill, and add the antimony trioxide powder while the basket mill is running at low speed, so that the antimony trioxide powder is initially wetted by the ethylene-vinyl chloride emulsion, and obtain the material to be ground that does not contain carbon-6 fluorine waterproof and oil-repellent agents, wetting agents, dispersants and defoamers. S3. The material to be ground is subjected to water-cooled high-speed grinding at a speed of 1400-1500 r / min for 4-4.5 h. During the grinding process, cooling water is circulated in the interlayer of the grinding cylinder wall to suppress the effect of high-speed shear heating on the stability of ethylene-vinyl chloride emulsion while refining and dispersing the antimony trioxide powder, so as to obtain pre-ground base adhesive. S4. After stopping high-speed grinding, transfer the pre-ground base adhesive to a plastic bucket, add the hexafluorocarbon waterproof and oil-repellent agent and water, and stir at a low speed of 180-300 r / min for 2-2.5 h without further high-speed grinding, so that the hexafluorocarbon waterproof and oil-repellent agent is homogenized and dispersed in the pre-ground base adhesive to obtain the coating mixture. The hexacarbon fluorine-based waterproof and oil-repellent agent is added only after the antimony trioxide powder has been water-cooled and high-speed ground, and no wetting agent, dispersant, or defoamer is added to the system in S2 to S4.

[0011] As a further improvement, in step S2, the ethylene-vinyl chloride emulsion is first added to a basket mill and a low-speed circulation is started, and then antimony trioxide powder is added to the ethylene-vinyl chloride emulsion in batches to reduce the agglomeration of dry powder caused by adding antimony trioxide powder all at once.

[0012] As a further improvement, in step S2, no additional dilution water is added to the material to be ground, so that the antimony trioxide powder is subjected to shearing, collision and crushing action of the grinding media in an ethylene-vinyl chloride emulsion environment with a high solid content.

[0013] As a further improvement, in step S3, the basket mill uses high-purity zirconia beads with a diameter of 1.6-1.8 mm and a purity of over 95% as the grinding medium.

[0014] As a further improvement, in step S3, the cooling water circulates continuously within the interlayer of the grinding cylinder wall, and the cooling process continues throughout the entire time of water-cooled high-speed grinding, so as to reduce the local temperature rise caused by the friction between the grinding media and the material to be ground.

[0015] As a further improvement, in step S3, the antimony trioxide powder in the pre-ground base adhesive is finely dispersed after being water-cooled and high-speed ground, and there are no visible coarse agglomerates of antimony trioxide powder in the pre-ground base adhesive.

[0016] As a further improvement, in step S4, after the hexafluorocarbon waterproofing and oil-repellent agent is added, it is only stirred and homogenized at a low speed, so that the hexafluorocarbon waterproofing and oil-repellent agent avoids the formation of local low surface energy enrichment areas on the surface of the unrefined antimony trioxide powder agglomerates.

[0017] As a further improvement, in step S4, a hexafluorocarbon waterproof and oil-resistant agent is first added to the pre-ground base adhesive, then water is added to adjust the application viscosity of the coating mixture, and then low-speed stirring is performed.

[0018] As a further improvement, in step S5, the substrate is a textile substrate, a film material, or a sheet-coated substrate, and the coating mixture is applied to the surface of the substrate by scraping, rolling, or a coating machine.

[0019] Secondly, the present invention also provides an aqueous flame-retardant hot melt adhesive obtained by the method.

[0020] This invention involves water-cooled high-speed pre-grinding of antimony trioxide powder and ethylene-vinyl chloride emulsion. This process subjectes the flame-retardant powder to shearing, collision, and crushing effects from the grinding media within the emulsion system, reducing coarse agglomerates and locally unwetted particles, thus improving the uniformity of the coating mixture. Simultaneously, since the hexafluorocarbon-based waterproof and oil-repellent agent does not participate in the high-speed grinding but is added later by low-speed stirring after the flame-retardant powder has been refined and dispersed, it avoids the formation of localized enrichment zones of low surface energy hydrophobic components on the surface of unrefined powder agglomerates, reducing shrinkage and pinhole defects in the coating film caused by uneven surface tension during drying. Compared with the process of directly mixing the components at low speed, this invention can improve the smoothness and continuity of the coating surface without changing the original functional formula or introducing new additives such as wetting agents, dispersants, and defoamers. It also reduces the potential adverse effects of new additives on emulsion stability, adhesion strength, waterproof and oil-proof performance, and the distribution of flame-retardant components. At the same time, the grinding process uses a water-cooled circulation in the grinding cylinder wall jacket, which can suppress the risk of demulsification, thickening, or decreased stability of the aqueous emulsion system caused by high-speed shear heating, making the resulting coating mixture more suitable for continuous coating production. Attached Figure Description

[0021] Figure 1 The image shows the surface condition of the coating film after adding a high-speed pre-grinding process using the method of the present invention.

[0022] Figure 2 This is a diagram showing the surface condition of the coating without the high-speed pre-grinding process.

[0023] Figure 3 This is a schematic diagram of the process flow for the method of eliminating pinholes in water-based flame-retardant hot melt adhesive coatings by high-speed pre-grinding, as described in this invention.

[0024] Figure 4 This is a schematic diagram comparing the powder dispersion states of the examples and comparative examples.

[0025] Figure 5 This is a comparison chart of the number of shrinkage cavities per unit area between the example and the comparative example.

[0026] Figure 6 A comparison chart showing the effect of different addition sequences on pinholes in the coating. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments and accompanying drawings. It should be understood that the following embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Without departing from the technical concept of the present invention, which involves water-cooled high-speed pre-grinding of the flame-retardant powder in an ethylene-vinyl chloride emulsion, low-speed addition of a hexafluorocarbon waterproof and oil-repellent agent, and no additional wetting agent, dispersant, or defoamer is added throughout the process, those skilled in the art can make equivalent adjustments to some process parameters according to actual production conditions such as the coating substrate, the solid content of the adhesive, the application viscosity, and the coating thickness.

[0028] I. Terminology Explanation The water-based flame-retardant hot melt adhesive of this invention refers to a water-based coating adhesive with a water-based polymer emulsion as the main film-forming and binding component and containing flame-retardant powder. Specifically, in this embodiment, the water-based flame-retardant hot melt adhesive uses ethylene-vinyl chloride emulsion as the main emulsion, antimony trioxide powder as a flame-retardant synergistic powder, and a fluorocarbon hexacarbon-based waterproof and oil-repellent agent as a hydrophobic functional component. Water is used to adjust the application viscosity of the final coating mixture.

[0029] The ethylene-vinyl chloride emulsion described in this invention refers to an aqueous emulsion formed by copolymerizing ethylene and vinyl chloride, which, after drying, can form a coating film with adhesive properties and a certain degree of flexibility. In this invention, this emulsion serves both as the film-forming substrate and as a pre-wetting and dispersing medium for flame retardant powder.

[0030] The antimony trioxide powder described in this invention refers to an inorganic powder material used as a flame-retardant synergist. If this type of powder is only stirred at low speed in an aqueous emulsion, it is prone to insufficiently wetted particles, small agglomerates, or localized sedimentation, which can lead to differences in microscopic surface tension and localized shrinkage points during the coating process. This invention utilizes water-cooled high-speed pre-grinding to refine, wet, and homogenize the antimony trioxide powder in an ethylene-vinyl chloride emulsion.

[0031] The hexacarbon fluorocarbon-based waterproof and oil-repellent agent described in this invention refers to a low surface energy hydrophobic agent with waterproof and oil-repellent properties. This type of component can impart waterproof, oil-repellent, or water- and oil-repellent properties to the coating film. However, if it is mixed or subjected to high-shear grinding before the flame-retardant powder is sufficiently refined, low surface energy enrichment zones may form around powder agglomerates or localized coarse particles, leading to discontinuous local spreading during coating and subsequently inducing pinholes. In this invention, the hexacarbon fluorocarbon-based waterproof and oil-repellent agent is added after the flame-retardant powder has undergone high-speed pre-grinding, and after addition, only low-speed stirring and homogenization are performed.

[0032] The high-speed pre-grinding described in this invention refers to the process of placing the material to be ground, consisting of ethylene-vinyl chloride emulsion and antimony trioxide powder, in a basket mill before adding the hexafluorocarbon-based waterproof and oil-repellent agent. The process utilizes the shearing, collision, friction, and crushing effects of grinding media such as zirconia beads to break down and refine the powder's agglomerated structure. This step differs from conventional low-speed stirring and from mixing and grinding after all components are added at once. Its key focus is on first addressing the issues of agglomeration and insufficient wetting of the flame-retardant powder in the emulsion.

[0033] The water-cooled high-speed grinding described in this invention refers to circulating cooling water within the interlayer of the grinding cylinder wall during high-speed grinding to remove the heat generated by friction and shearing between the grinding media and the material, thereby reducing the impact of localized temperature rise on the stability of ethylene-vinyl chloride emulsions. This water-cooling method can reduce the risk of emulsion demulsification, abnormal thickening, skin formation, or decreased storage stability due to temperature rise.

[0034] The coating pinholes described in this invention refer to circular, elliptical, or irregular defects that expose the substrate, visible to the naked eye or under magnification, caused by localized surface tension differences, powder agglomeration, localized enrichment of hydrophobic components, or contamination points during the spreading and drying process of the coating mixture on the substrate surface. A higher number of pinholes indicates poorer coating continuity and smoothness.

[0035] The "no additional wetting agents, dispersants, and defoamers" mentioned in this invention refers to the fact that, in addition to the functional formulation consisting of ethylene-vinyl chloride emulsion, antimony trioxide powder, hexafluorocarbon-based waterproofing and oil-repellent agent, and water, conventional wetting agents, dispersants, and defoamers are not added to the system to improve powder dispersion, coating spread, or bubble elimination. The purpose of this limitation is to avoid the uncertain impact of new additives on the stability, adhesion strength, waterproofing and oil-repellent properties, and flame-retardant component distribution of the aqueous emulsion.

[0036] II. Main Raw Materials, Equipment and Testing Methods 1. Main raw materials The ethylene-vinyl chloride emulsion used in this example is a commercially available water-based ethylene-vinyl chloride copolymer emulsion, which is a milky white, homogeneous liquid and is used as the main emulsion for water-based hot melt adhesives. Antimony trioxide powder is a commercially available flame-retardant synergistic powder, which is sealed and stored before use to prevent moisture absorption and clumping. The hexafluorocarbon-based waterproofing and oil-repellent agent is a commercially available fluorinated waterproofing and oil-repellent finishing agent and is used as a hydrophobic functional component. The water is deionized water or softened water for production use, used to adjust the application viscosity of the mixed adhesive during the low-speed post-mixing stage.

[0037] To avoid interference from other additives in the experimental results, wetting agents, dispersants and defoamers are not added in the examples and comparative examples unless otherwise specified.

[0038] 2. Main Equipment The high-speed pre-grinding equipment uses a laboratory basket grinder with a double-walled grinding cylinder connected to a cooling water circulation system. The grinding media consists of high-purity zirconia beads with a diameter of 1.6-1.8 mm and a purity of over 95%. The low-speed mixing equipment uses a laboratory low-speed disperser or mixer. The coating equipment uses a small laboratory doctor blade or a continuous coating test machine with adjustable blade gap. The drying equipment is a hot air circulating oven. Observation equipment includes visual inspection, a magnifying glass, a stereo microscope, or a digital microscope.

[0039] 3. Method for preparing coated samples After the prepared coating mixture was allowed to stand for 10-30 minutes to degas, it was applied to the surface of a textile or sheet substrate using a blade coating method. The wet film thickness was controlled at 80-150 μm. After coating, the sample was placed in a hot air circulating oven to dry at 80-120℃ for 2-5 minutes to obtain a dried coating sample. All examples and comparative examples used the same substrate, blade gap, and drying conditions to ensure comparability of results.

[0040] 4. Test method for the number of shrinkage cavities Five 100cm² areas were randomly selected from the dried coating surface for testing. The number of pinholes was counted using a combination of visual inspection and magnification. The criteria for pinhole determination were: a circular, elliptical, or irregular area of ​​exposed substrate showing through the coating surface, with a maximum radial dimension of not less than 0.1mm, was counted as one pinhole. The average value of the five test areas for each sample was recorded as the number of pinholes per unit area, expressed as pinholes per 100cm².

[0041] 5. Methods for observing the dispersion state of powder materials Take a small amount of the pre-ground base adhesive or the final coated mixture and place it on a glass slide. Use a scraper to form a thin sample and observe the powder agglomeration state under a stereomicroscope. If obvious white or light-colored particle agglomerates are visible, it indicates that the powder is not sufficiently dispersed; if the particles are evenly distributed in the field of view and there are no obvious coarse agglomerates, it indicates that the powder is well dispersed. If necessary, a scraper fineness meter or laser particle size analyzer can be used to further test the fineness of the powder.

[0042] 6. Adhesion performance test method After coating and drying, the sample was laminated with a standard substrate and treated under the same pressing and curing conditions. It was then cut into 25mm wide test strips, and the 180° peel strength was tested using a tensile testing machine. Five strips were tested for each group of samples, and the average value was taken, in N / 25mm. This index is used to evaluate whether the present invention can maintain the adhesive properties of the water-based flame-retardant hot melt adhesive without the addition of wetting agents, dispersants, and defoamers.

[0043] 7. Storage stability test method The prepared coating mixtures were placed in sealed containers and allowed to stand at 25°C for 7 days, followed by standing at 50°C for 72 hours. The mixtures were then observed for obvious stratification, demulsification, clumping, settling lumps, or abnormal thickening. If the mixture could be restored to a homogeneous state after gentle stirring, and there were no obvious gel lumps or demulsification, its storage stability was deemed satisfactory.

[0044] III. Examples and Comparative Examples Example

[0045] like Figure 3 As shown, prepare ethylene-vinyl chloride emulsion, antimony trioxide powder, fluorocarbon hexacarbon-based waterproofing and oil-repellent agent, and water according to the formula. First, add the ethylene-vinyl chloride emulsion to the grinding hopper of a basket mill and start the mill at low speed to allow the emulsion to form a stable circulation within the grinding hopper. Then, add the antimony trioxide powder to the grinding hopper in batches, maintaining low speed for 2-5 minutes after each batch to allow the powder to be gradually wetted by the ethylene-vinyl chloride emulsion, avoiding clumping of dry powder caused by adding it all at once. At this stage, do not add fluorocarbon hexacarbon-based waterproofing and oil-repellent agent, water, wetting agent, dispersant, or defoamer to the system.

[0046] After all the antimony trioxide powder was added and initially moistened, the cooling water circulation was started, allowing the cooling water to flow continuously within the interlayer of the grinding cylinder wall. The basket mill speed was then adjusted to 1450 r / min for water-cooled high-speed pre-grinding for 4.2 hours. During the grinding process, the temperature rise of the outer wall of the grinding cylinder was observed to ensure that the material did not exhibit significant skinning, abnormal thickening, or demulsification. After the high-speed pre-grinding was completed, the basket mill was stopped, yielding the pre-ground base colloid. A small amount of the pre-ground base colloid was taken for microscopic observation; the powder distribution was relatively uniform, and no obvious coarse agglomerates were observed.

[0047] Transfer the pre-ground base adhesive to a plastic bucket, add the hexafluorocarbon waterproofing and oil-repellent agent first, then add water to adjust the application viscosity, and then stir at 240 rpm for 2.2 hours using a low-speed disperser to obtain the coating mixture. This low-speed stirring stage is only used to homogenize and disperse the hexafluorocarbon waterproofing and oil-repellent agent and water in the pre-ground base adhesive; high-speed grinding or high-shear dispersion is not performed.

[0048] The obtained coating mixture was applied to the surface of a textile substrate using a blade coating method, with the wet film thickness controlled to be approximately 100 μm. After drying with hot air at 100°C for 3 minutes, a coating film was formed. The surface of the coating film is as follows: Figure 1 As shown, the overall surface is smooth and continuous, with no obvious pinholes. According to the pinhole quantity test method described above, the number of pinholes per unit area is 0-1 per 100cm², with an average of approximately 0.4 per 100cm². The 180° peel strength is 8.6 N / 25mm. After standing at 25℃ for 7 days, the adhesive showed no obvious stratification; after standing at 50℃ for 72 hours, there was no demulsification or hardening and sedimentation; it remained homogeneous after slight stirring.

[0049] This embodiment illustrates that, before adding the hexacarbon fluorine-based waterproof and oil-repellent agent, water-cooled high-speed pre-grinding of the ethylene-vinyl chloride emulsion and antimony trioxide powder can significantly reduce the impact of powder agglomeration on coating spread. At the same time, the hexacarbon fluorine-based waterproof and oil-repellent agent is added later at a low speed, which can reduce the possibility of low surface energy components accumulating on the surface of powder agglomerates, thereby reducing pinholes. Example

[0050] Compared with Example 1, the difference in this example is that the water-cooled high-speed pre-grinding speed is 1400 r / min and the grinding time is 4 h; the low-speed stirring speed is 180 r / min and the low-speed stirring time is 2 h. The remaining raw materials, addition order, cooling method, coating conditions and drying conditions are the same as in Example 1.

[0051] The specific operation is as follows: Ethylene-vinyl chloride emulsion is added to a basket mill, and antimony trioxide powder is added in batches while running at low speed to allow the powder to be initially wetted in the emulsion. Then, cooling water circulation is turned on, and water-cooled high-speed pre-grinding is performed at 1400 rpm for 4 hours. After grinding, the resulting pre-ground base adhesive is transferred to a plastic bucket, and a hexafluorocarbon-based waterproof and oil-repellent agent and water are added. The mixture is stirred at low speed at 180 rpm for 2 hours to obtain a coating mixture.

[0052] Coating test results showed that the obtained coating film had good surface continuity, with a very small number of fine dot-like defects visible in local areas, but no large-scale pinholes. The average number of pinholes per unit area was approximately 0.8 per 100 cm². The 180° peel strength was 8.4 N / 25 mm. In the storage stability test, no demulsification was observed after standing at 25°C for 7 days; slight sedimentation was observed after standing at 50°C for 72 hours, which could be redispersed after low-speed stirring without any hard lumps deposited.

[0053] This embodiment illustrates that, under the lower grinding speed and shorter grinding time specified in the claims, antimony trioxide powder can still achieve sufficient refining and wetting in ethylene-vinyl chloride emulsion, and the low-speed addition of the hexacarbon fluoropolymer waterproofing and oil-repellent agent can still achieve the basic effect of reducing pinholes in the coating film. Example

[0054] Compared with Example 1, the difference in this example is that the water-cooled high-speed pre-grinding speed is 1500 r / min and the grinding time is 4.5 h; the low-speed stirring speed is 300 r / min and the low-speed stirring time is 2.5 h. All other conditions are the same as in Example 1.

[0055] The specific operation is as follows: Ethylene-vinyl chloride emulsion is added to a basket mill. Antimony trioxide powder is added in batches while running at low speed. After the powder is completely wetted, cooling water circulation is turned on, and high-speed pre-grinding is performed at 1500 rpm for 4.5 hours. During the grinding process, due to the high grinding intensity, continuous cooling water circulation should be maintained to avoid excessively high material temperature. After grinding, the pre-ground base adhesive is transferred to a plastic bucket, and a fluorocarbon hexacarbon-based waterproof and oil-repellent agent and water are added. The mixture is stirred at low speed (300 rpm) for 2.5 hours to obtain a coating mixture.

[0056] Microscopic observation showed that the powder particles in the pre-ground base adhesive obtained in this embodiment were relatively uniformly dispersed, with fewer coarse agglomerates. After coating and drying, the coating surface was smooth and continuous, with an average of approximately 0.2 pinholes per 100 cm². The 180° peel strength was 8.5 N / 25 mm. No demulsification or significant agglomeration was observed after the adhesive solution was allowed to stand at 25°C for 7 days and at 50°C for 72 hours.

[0057] This embodiment demonstrates that, under the higher grinding speed and longer grinding time specified in the claims, the powder refinement and dispersion effect is further improved, and the number of pinholes in the coating film remains at a low level; at the same time, due to the use of water cooling circulation, no obvious emulsion instability problem caused by high-speed shear heating was observed. Example

[0058] In this embodiment, high-purity zirconia beads with a diameter of 1.6 mm and a purity of over 95% are used as the grinding media. After adding the ethylene-vinyl chloride emulsion to a basket mill, antimony trioxide powder is added while the mill is running at low speed. Cooling water circulation is activated, and the mill is water-cooled and high-speed pre-ground at 1450 rpm for 4.2 hours. After grinding, the pre-ground base adhesive is transferred to a plastic bucket, and then a fluorocarbon hexacarbon-based waterproof and oil-repellent agent and water are added. The mixture is stirred at low speed at 240 rpm for 2.2 hours to obtain the coating mixture.

[0059] After coating, the average number of pinholes on the coating surface was approximately 0.3 per 100 cm². The 180° peel strength was 8.7 N / 25 mm. Microscopic observation showed that when using 1.6 mm zirconia beads, the powder dispersion was good, and no obvious coarse agglomerates were observed. The storage stability test was satisfactory.

[0060] This embodiment illustrates that when using zirconia beads with a smaller diameter within the scope of the claims, it is still possible to achieve sufficient grinding and refining of antimony trioxide powder, and the resulting coating mixture can achieve a low level of shrinkage. Example

[0061] The difference between this embodiment and Embodiment 4 is that the grinding media used are high-purity zirconia beads with a diameter of 1.8 mm and a purity of over 95%. All other raw materials, addition order, grinding speed, grinding time, cooling method, low-speed stirring parameters, and coating conditions are the same as in Embodiment 4.

[0062] Test results show that the resulting coating surface is smooth and continuous, with an average of approximately 0.5 pinholes per 100 cm². The 180° peel strength is 8.5 N / 25 mm. No demulsification or significant clumping occurred after storage stability testing.

[0063] This embodiment demonstrates that within the zirconia bead diameter range of 1.6-1.8 mm, the powder can be refined and dispersed, and the shrinkage pores can be suppressed, proving that the present invention is feasible and stable within the scope of the claims.

[0064] Comparative Example 1 Ethylene-vinyl chloride emulsion, antimony trioxide powder, hexafluorocarbon waterproofing and oil-repellent agent, and water were added to a plastic bucket at one time. Without high-speed pre-grinding in a basket mill, the mixture was stirred at 240 r / min for 2.2 h using a low-speed disperser to obtain the coating mixture of Comparative Example 1.

[0065] Due to the limited shearing effect of low-speed stirring, antimony trioxide powder is difficult to fully break down and wet; microscopic observation of samples reveals localized coarse particles and agglomerated areas. After applying the coating mixture to the same substrate and drying under the same conditions, the coating surface appears as follows... Figure 2 As shown, obvious pinholes are visible. According to the pinhole number test method, the average number of pinholes per unit area is approximately 12.6 / 100cm². The 180° peel strength is 8.3N / 25mm. In the storage stability test, slight sedimentation was observed after standing at 25℃ for 7 days, and the sedimentation intensified after standing at 50℃ for 72 hours. Stirring partially restored the sedimentation, but a small amount of powder agglomeration was still visible.

[0066] This comparative example demonstrates that simple low-speed stirring cannot adequately resolve the issues of antimony trioxide powder agglomeration and insufficient wetting in ethylene-vinyl chloride emulsions, resulting in a coating mixture prone to pinholes during coating and drying. Compared to Example 1, this comparative example proves that the high-speed pre-grinding process plays a crucial role in reducing pinholes in the coating film.

[0067] Comparative Example 2 Ethylene-vinyl chloride emulsion, antimony trioxide powder, hexafluorocarbon waterproofing and oil-repellent agent, and water were all added to a basket mill. After mixing for 10 minutes at low speed, the cooling water circulation was turned on, and the mixture was ground at high speed of 1450 rpm for 4.2 hours. After grinding, the material was transferred to a plastic bucket and stirred at low speed of 240 rpm for 2.2 hours to obtain the coating mixture of Comparative Example 2.

[0068] Unlike Example 1, the hexafluorocarbon-based waterproof and oil-repellent agent in this comparative example participated in high-speed grinding from the beginning. During the grinding process, the low surface energy hydrophobic component, the not-yet-fully-refined powder agglomerates, and the emulsion particles under high shear conditions acted simultaneously, which may have caused local differences in surface condition. After coating and drying, the number of pinholes on the coating surface was significantly lower than that in Comparative Example 1, but still higher than that in Example 1, with an average of approximately 5.8 pinholes per 100 cm². Some pinhole edges showed a relatively obvious shrinkage profile, indicating that local low surface energy enrichment still existed. The 180° peel strength was 7.9 N / 25 mm, slightly lower than that in Example 1. After standing at 50°C for 72 hours, the viscosity of the adhesive increased slightly, but no serious demulsification occurred.

[0069] This comparative example illustrates that in a system containing a hexafluorocarbon-based waterproofing and oil-repellent agent, simply grinding all components together at high speed does not achieve the same pinhole suppression effect as in this invention. In this invention, the hexafluorocarbon-based waterproofing and oil-repellent agent is added only after the antimony trioxide powder has undergone water-cooled high-speed pre-grinding, not in an arbitrary process sequence. This is to prevent the hydrophobic agent from forming low surface energy accumulations on the surface of powder agglomerates or in localized areas.

[0070] Comparative Example 3 Ethylene-vinyl chloride emulsion was added to a basket mill, followed by a hexafluorocarbon-based water and oil repellent agent and water. The mixture was stirred at low speed for 10 minutes to allow the water-repellent agent to enter the emulsion system first. Antimony trioxide powder was then added in batches, and cooling water circulation was started. The mixture was then ground at high speed (1450 rpm) for 4.2 hours. After grinding, the mixture was transferred to a plastic container and stirred at low speed (240 rpm) for 2.2 hours to obtain the coating mixture of Comparative Example 3.

[0071] After coating and drying, multiple pinholes were still visible on the coating surface, with an average of approximately 7.1 pinholes per 100 cm². Microscopic observation showed that although the coarse agglomerates were reduced after high-speed grinding, localized shrinkage points still existed in the coating. The 180° peel strength was 8.0 N / 25 mm. No severe demulsification was observed in the storage stability test, but the viscosity increased significantly compared to Example 1 after standing at 50°C.

[0072] This comparative example illustrates that if the hexafluorocarbon-based waterproof and oil-repellent agent enters the system before the antimony trioxide powder, even with subsequent high-speed grinding, uneven surface tension may still exist during coating because the hydrophobic agent preferentially alters the surface state of the emulsion or powder. The technical effect of this invention does not solely rely on high-speed grinding, but rather on the synergistic sequence of pre-grinding the powder in the emulsion and then adding the hydrophobic agent at a low speed.

[0073] Comparative Example 4 The raw materials and order of addition in this comparative example are the same as in Example 1, namely, the ethylene-vinyl chloride emulsion is first added to a basket mill, followed by antimony trioxide powder; the difference is that the cooling water circulation is not turned on during the high-speed grinding process, and grinding is still carried out at 1450 r / min for 4.2 h. After grinding, a hexafluorocarbon waterproof and oil-repellent agent and water are added, and the mixture is stirred at a low speed of 240 r / min for 2.2 h to obtain the coating mixture of Comparative Example 4.

[0074] During the grinding process, the temperature of the grinding cylinder wall and the material gradually increased, and the viscosity of the material increased to a certain extent. After coating and drying, the number of pinholes on the coating surface was significantly lower than that of Comparative Example 1, but still higher than that of Example 1, with an average of approximately 2.9 pinholes per 100 cm². The 180° peel strength was 8.1 N / 25 mm. In the storage stability test, after standing at 50°C for 72 hours, the adhesive showed slight thickening and a small number of soft gel points, which could be partially recovered after stirring.

[0075] This comparative example illustrates that water cooling circulation is not merely an auxiliary condition of the equipment, but a crucial process measure to ensure the stable implementation of high-speed pre-grinding. For ethylene-vinyl chloride emulsion systems, if the shear heat generated by high-speed grinding cannot be dissipated in time, it may affect the stability of the emulsion and the appearance of the subsequent coating. Therefore, this invention employs continuous circulation of cooling water in the jacket of the grinding cylinder wall, which helps to maintain the stability of the emulsion system while achieving powder refinement.

[0076] Comparative Example 5 The raw materials, addition order, and cooling method of this comparative example are the same as those of Example 1, except that the high-speed pre-grinding time is shortened to 1.5 hours and the grinding speed is 1450 r / min. After grinding, a hexafluorocarbon waterproof and oil-repellent agent and water are added, and the mixture is stirred at a low speed of 240 r / min for 2.2 hours to obtain the coating mixture of Comparative Example 5.

[0077] Microscopic observation revealed a certain number of coarse powder agglomerates in the pre-ground base adhesive. After coating and drying, the number of pinholes on the coating surface was lower than in Comparative Example 1, but significantly higher than in Example 1, with an average of approximately 4.6 pinholes per 100 cm². The 180° peel strength was 8.2 N / 25 mm. The storage stability test was basically satisfactory.

[0078] This comparative example illustrates that if the high-speed pre-grinding time is significantly shorter than the range specified in this invention, the antimony trioxide powder cannot be sufficiently refined and wetted. Even with the addition of the hydrophobic agent later, it is difficult to completely avoid poor local spreading caused by powder agglomeration. Therefore, the high-speed pre-grinding time of 4-4.5 hours specified in this invention is technologically necessary.

[0079] Comparative Example 6 Ethylene-vinyl chloride emulsion, antimony trioxide powder, hexafluorocarbon waterproofing and oil-repellent agent, water, and conventional wetting and dispersing agents were added to a plastic bucket and stirred at 240 rpm for 2.2 hours using a low-speed disperser to obtain the coating mixture of Comparative Example 6. This comparative example did not undergo high-speed pre-grinding.

[0080] Due to the addition of a wetting and dispersing agent, the wetting state of the powder was improved compared to Comparative Example 1. After coating and drying, the average number of pinholes per unit area was approximately 3.4 per 100 cm², significantly lower than that of Comparative Example 1 which was directly stirred at low speed, but still higher than that of Example 1. Further testing revealed that the 180° peel strength was 7.2 N / 25 mm, lower than that of Example 1; after standing at 50°C for 72 hours, a small amount of foam residue and slight delamination appeared on the surface of the adhesive, which could be recovered after stirring, but the storage stability was worse than that of Example 1.

[0081] This comparative example illustrates that while introducing wetting agents or dispersants can improve cratering to some extent, it may adversely affect the bonding performance and storage stability of water-based flame-retardant hot melt adhesive systems. This invention achieves crater reduction without the addition of additional wetting agents, dispersants, and defoamers, which is more conducive to maintaining the stability of the original functional formulation.

[0082] Comparative Example 7 Following the method of Example 1, ethylene-vinyl chloride emulsion and antimony trioxide powder were first subjected to water-cooled high-speed pre-grinding at a speed of 1450 r / min for 4.2 h. Subsequently, a hexafluorocarbon-based waterproof and oil-repellent agent and water were added. However, unlike Example 1, in this comparative example, after adding the hexafluorocarbon-based waterproof and oil-repellent agent, the mixture was again ground at high speed at 1450 r / min for 30 min, and then stirred at low speed for 2.2 h to obtain the coating mixture of Comparative Example 7.

[0083] After coating and drying, the average number of pinholes on the coating surface was approximately 2.3 per 100 cm², higher than in Example 1. In the storage stability test, the viscosity of the adhesive increased significantly after standing at 50°C. The 180° peel strength was 8.0 N / 25 mm.

[0084] This comparative example illustrates that, in this invention, after adding a hexafluorocarbon waterproof and oil-repellent agent, high-speed grinding is no longer performed. Instead, low-speed homogenization helps maintain the uniform dispersion of the hydrophobic agent in the pre-ground base adhesive, avoiding high shear from causing local migration of the hydrophobic component, changes in the interface state, or a decrease in emulsion stability.

[0085] IV. Summary of Results from Examples and Comparative Cases For ease of comparison, the main test results of the above embodiments and comparative examples are summarized below. Table 1 shows the test results of coating pinholes, peel strength, and storage stability under different process conditions.

[0086] Table 1. Test results of coating pinholes, peel strength, and storage stability under different process conditions.

[0087] From Table 1, Figure 5 , Figure 6 As can be seen, Examples 1 to 5 all fall within the process scope defined by the claims of this invention, and the number of pinholes per unit area is no higher than 0.8 per 100cm², significantly lower than that of the comparative examples, indicating that essentially the same pinhole suppression effect can be obtained within the scope of this invention. The 180° peel strength of the examples remained within the range of 8.4-8.7 N / 25mm, and the storage stability was qualified, indicating that while improving the appearance of the coating, this invention did not significantly reduce the bonding performance and emulsion stability of the water-based flame-retardant hot melt adhesive.

[0088] Comparative Example 1 shows that if the high-speed pre-grinding step is omitted and only low-speed stirring is used, the number of pinholes in the coating film increases significantly. Comparative Examples 2 and 3 show that if the hexafluorocarbon water-repellent and oil-repellent agent participates in high-speed grinding, or enters the system before the powder grinding, the number of pinholes is still high, proving that the post-addition of the hydrophobic agent at low speed is necessary in this invention. Comparative Example 4 shows that high-speed grinding without water cooling may lead to decreased emulsion stability and increased coating defects. Comparative Example 5 shows that insufficient high-speed grinding time results in inadequate powder refinement and fails to achieve the desired effect of this invention. Comparative Example 6 shows that while adding a wetting and dispersing agent can reduce pinholes to some extent, it may reduce peel strength and storage stability. Comparative Example 7 shows that high-speed grinding again after adding the hydrophobic agent is not conducive to maintaining low pinholes and system stability.

[0089] V. Mechanism Explanation The reasons for the above-mentioned technical effects of this invention can be understood from three aspects: powder dispersion, surface energy distribution, and emulsion stability.

[0090] First, in water-based flame-retardant hot melt adhesive systems containing antimony trioxide powder, there is a certain tendency for agglomeration among the powder particles. While low-speed stirring can achieve macroscopic mixing, it is difficult to fully break down the tiny agglomerates. For example... Figure 4 As shown, insufficiently wetted and refined powder agglomerates can become localized spreading obstacles during coating, causing the adhesive to bypass these areas or experience localized shrinkage during drying, thus forming pinholes. This invention, before adding the hexafluorocarbon-based waterproofing and oil-repellent agent, first subjectes the antimony trioxide powder to high-speed grinding by zirconium oxide beads in an ethylene-vinyl chloride emulsion. This effectively reduces coarse particles and agglomerates, resulting in a more uniform dispersion of the powder in the emulsion phase.

[0091] Secondly, hexafluorocarbon-based waterproofing and oil-repellent agents are low-surface-energy hydrophobic components. While these components impart waterproofing and oil-repellent properties to the coating, if they are added to the mixture or subjected to high-speed grinding before the powder is sufficiently refined, they can easily accumulate on the surface of powder agglomerates or at local interfaces. This results in a lower surface tension in some areas of the coating adhesive compared to the surrounding region, causing the adhesive to shrink from these areas during drying, forming pinholes. This invention adds the hexafluorocarbon-based waterproofing and oil-repellent agent only after the antimony trioxide powder has undergone water-cooled high-speed pre-grinding, and subsequently uses only low-speed stirring for homogenization. This reduces the likelihood of the hydrophobic agent forming localized enrichment zones on the surface of unrefined powder agglomerates, thereby improving coating continuity.

[0092] Furthermore, ethylene-vinyl chloride emulsions, as aqueous emulsion systems, are sensitive to temperature and shear conditions. During high-speed grinding, friction and collision between the grinding media and the material generate heat. Without cooling, localized temperature rises may lead to decreased emulsion particle stability, abnormal viscosity changes, or microgel formation, thereby affecting coating performance. This invention removes grinding heat through cooling water circulation in the jacket of the grinding cylinder, allowing high-speed pre-grinding to proceed under relatively stable temperature conditions, thus balancing powder refinement and emulsion stability.

[0093] Furthermore, this invention does not add additional wetting agents, dispersants, or defoamers, thus avoiding interference from new additives with the existing formulation system. For water-based flame-retardant hot melt adhesives, added additives may alter the interfacial state of the emulsion particles, affect the bonding interface after drying and film formation, and even weaken the uniformity of distribution of waterproofing and oil-repellent agents or flame retardants. This invention achieves crater suppression through process sequence and grinding methods, rather than relying on added additives, making it more suitable for coating applications that require good adhesion, flame retardancy, and waterproofing and oil-repellency.

[0094] In summary, this invention, through a specific process sequence and shearing method, achieves thorough wetting and fine dispersion of antimony trioxide powder without the addition of additional wetting agents, dispersants, and defoamers. This avoids the formation of low surface energy enrichment zones around unrefined powder agglomerates by hexafluorocarbon waterproofing and oil-repellent agents, thereby effectively reducing pinholes in the coating film and taking into account the emulsion stability and adhesion performance of the water-based flame-retardant hot melt adhesive system.

Claims

1. A method for eliminating pinholes in water-based flame-retardant hot melt adhesive coatings through high-speed pre-grinding, characterized in that, Includes the following steps: S1. Prepare ethylene-vinyl chloride emulsion, antimony trioxide powder, carbon-6 fluoropolymer waterproofing and oil-repellent agent, and water; S2. Add the ethylene-vinyl chloride emulsion to a basket mill, and add the antimony trioxide powder while the basket mill is running at low speed, so that the antimony trioxide powder is initially wetted by the ethylene-vinyl chloride emulsion, and obtain the material to be ground that does not contain carbon-6 fluorine waterproof and oil-repellent agents, wetting agents, dispersants and defoamers. S3. The material to be ground is subjected to water-cooled high-speed grinding at a grinding speed of 1400-1500 r / min and a grinding time of 4-4.5 h. During the grinding process, cooling water is circulated in the interlayer of the grinding cylinder wall to obtain pre-ground base adhesive. S4. After stopping high-speed grinding, transfer the pre-ground base adhesive to a plastic bucket, add the hexafluorocarbon waterproof and oil-repellent agent and water, and stir at a low speed of 180-300 r / min for 2-2.5 hours without further high-speed grinding to obtain the coating mixture.

2. The method according to claim 1, characterized in that, In step S2, the ethylene-vinyl chloride emulsion is first added to a basket mill and a low-speed circulation is started. Then, antimony trioxide powder is added to the ethylene-vinyl chloride emulsion in batches to reduce the agglomeration of dry powder caused by adding antimony trioxide powder all at once.

3. The method according to claim 1, characterized in that, In step S2, no additional dilution water is added to the material to be ground, so that the antimony trioxide powder is subjected to shearing, collision and crushing action of the grinding media in an ethylene-vinyl chloride emulsion environment with a high solid content.

4. The method according to claim 1, characterized in that, In step S3, the basket mill uses high-purity zirconia beads with a diameter of 1.6-1.8 mm and a purity of over 95% as the grinding medium.

5. The method according to claim 1, characterized in that, In step S3, the cooling water circulates continuously within the interlayer of the grinding cylinder wall, and the cooling process continues throughout the entire time of the water-cooled high-speed grinding process, so as to reduce the local temperature rise caused by the friction between the grinding media and the material to be ground.

6. The method according to claim 1, characterized in that, In step S3, the antimony trioxide powder in the pre-ground base adhesive is finely dispersed after being water-cooled and high-speed ground, and there are no visible coarse agglomerates of antimony trioxide powder in the pre-ground base adhesive.

7. The method according to claim 1, characterized in that, In step S4, after the hexacarbon fluorine-based waterproof and oil-repellent agent is added, it is only stirred and homogenized at low speed to prevent the hexacarbon fluorine-based waterproof and oil-repellent agent from forming local low surface energy enrichment areas on the surface of the unrefined antimony trioxide powder agglomerates.

8. The method according to claim 1, characterized in that, In step S4, a carbon-6 fluorocarbon waterproof and oil-resistant agent is first added to the pre-ground base adhesive, then water is added to adjust the application viscosity of the coating mixture, and then low-speed stirring is performed.

9. The method according to claim 1, characterized in that, The method further includes applying the coating mixture onto the surface of a substrate and drying it into a film. The substrate is a textile substrate, a film material, or a sheet-coated substrate. The coating mixture is applied to the surface of the substrate by scraping, rolling, or coating machine.

10. The water-based flame-retardant hot melt adhesive obtained by the method according to any one of claims 1-9.

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