Foamed hot melt adhesive for filters and method for its preparation

Abstract

The application discloses a foaming hot melt adhesive for filters and a preparation method thereof, and belongs to the field of adhesives. The steps comprise the following: adding nano-silicon dioxide and a foaming agent into molten hot melt adhesive base materials, carrying out three-stage shear foaming, discharging and cooling; the foaming agent comprises azodicarbonamide and citric acid; in the three-stage shear foaming, the temperature of the first stage is below 150 DEG C, the temperature of the third stage is 195-210 DEG C, the temperature of the second stage gradually increases from the temperature of the first stage to the temperature of the third stage, and the stirring speed of the second stage > the stirring speed of the first stage > the stirring speed of the third stage. The application realizes staged foaming, combines multi-stage shear rate control and nanoparticle synergistic bubble stabilization, effectively improves the uniformity of foaming, avoids local excessive foaming or under-foaming phenomenon, improves the uniformity of foaming, can obtain stable bubble structure, and can enhance the bonding strength of the adhesive layer.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of foamed hot melt adhesive for filter and preparation method thereof, belong to adhesive field. BACKGROUND

[0002] In filter manufacturing field, hot melt adhesive is widely used in the fixing of filter material and frame, the sealing of filter core and other links as an important adhesive. In order to improve the filtering area and efficiency of filter, and reduce weight, foamed hot melt adhesive emerges as the times require. Uniform and stable cell structure is the key to ensure its bonding strength. However, the prior art faces many challenges in achieving this goal.

[0003] Firstly, in large-scale production process, due to the influence of many factors such as improper control of stirring speed leading to excessive shear force, mismatch between foaming agent decomposition rate and hot melt adhesive system, and low hot melt adhesive melting viscosity, the cell structure is difficult to be uniform, which seriously affects the adhesive strength of adhesive layer. The bubbles cannot exist stably after formation, which leads to difficulty in guaranteeing product quality and consistency.

[0004] The commonly used foaming agent, such as azodicarbonamide (ADCA), although has good foaming effect, but the foaming process is difficult to control, and local over-foaming and local under-foaming phenomenon is easy to occur. Single foaming agent acts at the same time, which leads to too fast decomposition rate, and is not conducive to the formation of uniform cell structure. The selection and amount of foaming agent directly affect the performance and stability of foamed hot melt adhesive, therefore, it is necessary to develop a more controllable foaming agent system. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the present application provides a kind of foamed hot melt adhesive for filter and preparation method thereof, which can effectively control the uniformity of foaming.

[0006] The technical scheme adopted by the present application to solve its technical problems is:

[0007] In the first aspect, the present application provides a kind of foamed hot melt adhesive for filter and preparation method thereof, steps include: hot melt adhesive base material is put into reaction kettle, heating and stirring, nano-silicon dioxide and foaming agent are added and mixed with molten hot melt adhesive base material, three-stage shear foaming is carried out, and the foamed hot melt adhesive for filter is obtained after discharging and cooling.

[0008] The foaming agent includes azodicarbonamide and citric acid.

[0009] In the three-stage shear foaming, the temperature of the first stage is below 150 DEG C, the temperature of the third stage is 195 DEG C-210 DEG C, and the temperature of the second stage gradually rises from the temperature of the first stage to the temperature of the third stage. The stirring speed of the second stage is greater than that of the first stage, and the stirring speed of the third stage is the lowest.

[0010] The process effectively solves the problems of bubble rupture and uneven foaming in the production process of foaming hot melt adhesive by multi-stage shear rate control and nanoparticle synergistic bubble stabilization. In the first stage, a lower shear rate is used, and the temperature is controlled not to be too high, which can avoid the premature decomposition of the foaming agent and ensure its uniform dispersion in the hot melt adhesive matrix. In the second stage, the temperature is gradually increased, and the shear rate is also increased, which can induce the foaming agent to start decomposing. The two foaming components have different decomposition temperatures, and citric acid decomposes first and almost completely in the second stage, producing an appropriate amount of bubble nucleus. At the end of the second stage, azodicarbonamide also begins to decompose. In the third stage, the shear rate is reduced, which can make the bubbles slowly expand and stabilize, avoiding bubble rupture. Nanosilica has high surface energy and can be adsorbed on the bubble surface to form a physical barrier, enhancing the strength of the bubble and preventing it from rupturing. At the same time, nanosilica can also increase the viscosity of the hot melt adhesive, further stabilizing the bubbles.

[0011] Further, the hot melt adhesive matrix is EVA resin, and the content of vinyl acetate is 20%-35%.

[0012] EVA resin as a hot melt adhesive matrix has good flexibility and adhesion. When the content of vinyl acetate is in the range of 20%-35%, a good balance between flexibility and adhesion strength can be achieved. For foaming hot melt adhesive for filters, appropriate flexibility can make the hot melt adhesive adapt to the small deformation between different parts of the filter during assembly and use, reducing the cracking or delamination phenomenon caused by stress concentration. For example, when the filter is subjected to temperature changes or slight vibration, the flexibility of the foaming EVA resin can buffer these external forces and maintain good adhesion to the various parts of the filter. At the same time, such EVA resin has good adhesion strength, which can firmly bond the filter paper, frame and other components of the filter together. EVA resin has good processing performance and is compatible with the three-stage shear foaming process, which can uniformly mix with nanosilica, foaming agent and other components under different temperature and stirring conditions, helping to form a stable and excellent performance foaming hot melt adhesive product.

[0013] Further, in the foaming agent, the mass ratio of azodicarbonamide to citric acid is 2:1.

[0014] The specific mass ratio optimizes the synergistic effect of azodicarbonamide and citric acid. Citric acid decomposes first to form bubble nuclei, and azodicarbonamide decomposes later to continue to expand the bubble nuclei. The foaming agent does not decompose at the same time, so the pores do not expand too quickly and cause the bubble to rupture, nor does it decompose too slowly to affect the formation of the pores. This precise adjustment is conducive to forming a uniform, fine, and stable pore structure. For the foaming hot melt adhesive used in the filter, such a pore structure not only improves the flexibility and cushioning performance of the hot melt adhesive, but also ensures that it has a certain strength. When the filter is impacted or vibrated by the outside world, the uniform pore structure can more effectively disperse stress and avoid local stress concentration, which can cause the hot melt adhesive to fail. At the same time, the pore structure produced under this ratio has a suitable effect on the density of the hot melt adhesive. Under the premise of ensuring the sealing and bonding performance of the filter, the weight of the filter is reasonably controlled, meeting the development needs of high efficiency and light weight of the filter.

[0015] Further, 1.5-2.5 parts by weight of the foaming agent are used per 100 parts by weight of the hot melt adhesive base material.

[0016] This dosage range can ensure foaming effect while maintaining the overall performance of the foaming hot melt adhesive. Too little foaming agent dosage will result in insufficient gas production, which cannot form enough pore structure, making the flexibility and cushioning performance of the foaming hot melt adhesive poor. For example, when the filter is vibrated, the few pores cannot effectively absorb the vibration energy, which may cause the hot melt adhesive to crack or the filter components to loosen. Too much foaming agent dosage will result in too much gas production, and the pores will grow too large or even rupture, reducing the strength and stability of the foaming hot melt adhesive. Within the dosage range of 1.5-2.5 parts, the appropriate amount and size of pores can be formed in the hot melt adhesive. These pores not only give the foaming hot melt adhesive good flexibility, allowing it to adapt to the deformation of the filter under different working conditions, but also ensure a certain strength to withstand the pressure and vibration of the filter during operation.

[0017] Further, 0.2-0.4 parts by weight of the nano-silicon dioxide are used per 100 parts by weight of the hot melt adhesive base material.

[0018] The addition of nano-silicon dioxide can significantly improve the performance of the foaming hot melt adhesive. Within this dosage range, it is beneficial for the nano-silicon dioxide to be uniformly dispersed in the hot melt adhesive base material. When it fills between the pores and the molecular chains of the hot melt adhesive, it plays a reinforcing role, allowing the filter to withstand greater pressure or vibration without the hot melt adhesive easily cracking or losing its adhesive properties. For example, in the air filter of an automobile engine, the vibration generated during engine operation is relatively large, and the appropriately amount of nano-silicon dioxide reinforced foaming hot melt adhesive can firmly bond the filter paper and the frame, ensuring the normal operation of the filter.

[0019] Further, in the three-stage shear foaming, the stirring speed of the first stage is 50 rpm-60 rpm, the stirring speed of the second stage is 130 rpm-150 rpm, and the stirring speed of the third stage is 30 rpm-40 rpm.

[0020] The low stirring speed (50 rpm-60 rpm) of the first stage is conducive to the preliminary mixing of the hot melt adhesive base material, nano-silicon dioxide and foaming agent at a low temperature as much as possible to ensure the melting of the hot melt adhesive base material, and avoids uneven dispersion of components or excessive decomposition of the foaming agent due to too fast stirring. The slow stirring allows sufficient time for the components to contact and fuse with each other, laying a good foundation for the subsequent foaming process. In the preparation of the filter foaming hot melt adhesive, this ensures the consistency of the hot melt adhesive components at each part of the filter, ensuring the stability of the product quality. The high stirring speed (130 rpm-150 rpm) of the second stage, combined with the temperature rise, accelerates the decomposition of the foaming agent and the dispersion of the gas in the hot melt adhesive. The fast stirring forms a large number of fine bubble nuclei, and the low stirring speed (30 rpm-40 rpm) of the third stage is to stabilize the bubble structure at high temperature. The low speed reduces the rupture or combination of the bubbles due to excessive stirring, allowing the bubbles to further grow and stabilize at a suitable temperature, thereby ensuring that the foaming hot melt adhesive can provide good cushioning performance and maintain certain strength and sealing performance in the filter, meeting the actual use requirements of the filter.

[0021] Further, in the three-stage shear foaming, the time length of the first stage is 9 min-12 min, the time length of the second stage is 5 min-8 min, and the time length of the third stage is 13 min-16 min.

[0022] The time length of the first stage 9 min-12 min gives the hot melt adhesive base material, nano-silicon dioxide and foaming agent sufficient time to preliminarily mix uniformly at a low temperature. In this process, the components gradually penetrate and fuse with each other, creating a stable system environment for the subsequent foaming reaction. For example, the nano-silicon dioxide is uniformly dispersed in the hot melt adhesive base material, preparing for enhancing the performance of the foaming hot melt adhesive. The time length of the second stage 5 min-8 min, under the conditions of fast stirring and temperature rise, promotes the rapid decomposition of citric acid to produce a large amount of gas, forms a large number of fine bubble structures, and makes azodicarbonamide begin to decompose. The time length of the third stage 13 min-16 min, at a higher temperature and a lower stirring rate, allows the bubbles to further grow and stabilize. There is sufficient time for the bubbles to adjust the structure in a stable environment, enhancing the strength and stability of the foaming hot melt adhesive, allowing it to withstand long-term external forces such as vibration and pressure in the filter, ensuring the sealing and bonding performance of the filter, and prolonging the service life of the filter.

[0023] Furthermore, the citric acid is a core-shell particle coated with modified rosin resin; in the step of adding the hot melt adhesive substrate into the reactor, heating and stirring, the heating temperature is lower than the softening point of the modified rosin resin.

[0024] Using coated citric acid can effectively reduce corrosion of production equipment and extend its service life. The coating layer can delay the exposure of citric acid, which is beneficial for the uniform dispersion of citric acid in the melt. In addition, the sacrificial coating material itself can act as a tackifier for hot melt adhesives, optimizing the adhesion of hot melt adhesives.

[0025] Furthermore, the preparation steps of the core-shell particles include: dissolving the modified rosin resin in a volatile organic solvent to obtain a modified rosin resin solution, spraying the modified rosin resin solution onto the surface of citric acid powder and drying it, and then sieving it to obtain the core-shell particles.

[0026] A solution of modified rosin resin, dissolved in a volatile organic solvent, is prepared for uniform spraying onto the surface of citric acid powder. The spraying process allows the modified rosin resin to uniformly coat the citric acid, forming a complete and uniform core-shell structure. The drying step removes the organic solvent, fixing the core-shell structure. Sieving removes any small agglomerates formed during spraying, ensuring the uniformity of the citric acid core-shell particles used to prepare the foamed hot melt adhesive. Uniformly sized core-shell particles participate in the foaming reaction at the same reaction rate during the three-stage shear foaming process, which is beneficial for forming a uniform cell structure.

[0027] Secondly, this application provides a foaming hot melt adhesive for filters, which is made by the method for preparing foaming hot melt adhesive for filters described in the first aspect.

[0028] The foamed hot melt adhesive has a more uniform cell structure and improved bonding strength. Nano-silica or polymer microspheres enhance the stability of the bubbles.

[0029] The beneficial effects of this invention are as follows: This invention employs a composite foaming agent system, achieving staged foaming, effectively improving foaming uniformity, and avoiding localized over-foaming or under-foaming. The synergistic effect of citric acid and azodicarbonamide makes the foaming process more controllable and the cell structure more uniform. Multi-level shear rate control and nanoparticle-assisted foam stabilization effectively reduce the bubble breakage rate and improve foaming uniformity, resulting in a stable bubble structure that enhances the adhesive strength of the adhesive layer. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this invention.

[0031] It should be understood that, without conflict, any and all embodiments of the present invention can be combined with technical features of any other embodiment or multiple other embodiments to obtain other embodiments. The present invention includes such combinations to obtain other embodiments.

[0032] Unless otherwise specified, all technical and scientific terms used herein have the standard meaning in the field to which the claimed subject matter pertains. Where multiple definitions exist for a term, the definition herein shall prevail.

[0033] This application provides a method for preparing foamed hot melt adhesive for filters, the steps of which include: putting hot melt adhesive substrate into a reaction vessel, heating and stirring, adding nano-silica and a foaming agent to mix with the molten hot melt adhesive substrate, performing three-stage shear foaming, discharging and cooling to obtain foamed hot melt adhesive for filters.

[0034] The foaming agents include azodicarbonamide and citric acid.

[0035] In the three-stage shear foaming process, the temperature in the first stage is below 150℃, and the temperature in the third stage is 195℃-210℃. The temperature in the second stage gradually increases from the temperature in the first stage to the temperature in the third stage. The stirring rate in the second stage is greater than the stirring rate in the first stage, which is greater than the stirring rate in the third stage.

[0036] The specific ingredients and process flow are as follows:

[0037] Ethylene-vinyl acetate copolymer (EVA): 100 parts by weight;

[0038] Nano-silica: 0.2-0.4 parts by weight;

[0039] Azodicarbonamide (ADCA): 1.0-1.7 parts by weight;

[0040] Citric acid: 0.5-0.85 parts by weight.

[0041] Raw material pretreatment: The hot melt adhesive substrate (ethylene-vinyl acetate copolymer EVA, model: Elvax 260, DuPont) is put into a jacketed reactor, heated to 120°C, and stirred to completely melt the EVA.

[0042] Nanoparticle dispersion: Add bubble stabilizer nano silica (model: AEROSIL 200, Evonik Degussa) to molten EVA at a ratio of 0.2-0.4 wt%.

[0043] Foaming agent mixing: Mix the high-temperature foaming agent azodicarbonamide and the low-temperature foaming agent citric acid at a mass ratio of 2:1, and then add them to the reaction vessel.

[0044] Multi-stage shear foaming:

[0045] First stage (low shear mixing): Set the stirring speed to 50 rpm and stir continuously for 10 minutes to evenly disperse the foaming agent in the hot melt adhesive substrate at a temperature of 120℃-150℃.

[0046] Second stage (medium shear induction): Increase the stirring speed to 150 rpm and continue stirring for 5 minutes. The foaming agent begins to decompose and generate bubbles. The temperature rises to 200℃ at a uniform rate within 5 minutes.

[0047] The third stage (low shear stabilization): reduce the stirring speed to 30 rpm and continue stirring for 15 minutes to allow the bubbles to expand slowly and stabilize. The temperature should be maintained above 200℃ and should not exceed 220℃.

[0048] Cooling and setting: Remove the foamed hot melt adhesive from the reactor and cool it to room temperature to obtain the foamed hot melt adhesive product.

[0049] This application embodiment achieves uniform dispersion of the foaming agent, effective bubble induction, and stable bubble expansion through three stages of stirring speed control, thus preventing bubble rupture. Nano-silica can adsorb onto the bubble surface, forming a physical barrier that enhances bubble strength and prevents rupture. A temperature of 120°C ensures the fluidity of EVA, facilitating subsequent operations. The composite foaming agent system enables staged foaming, improving foaming uniformity. A 2:1 ratio of ADCA (azodicarbonamide) to citric acid balances the effects of high-temperature and low-temperature foaming.

[0050] Multi-stage shear foaming:

[0051] First stage: Low-speed stirring can avoid excessive shear force that could cause the foaming agent to decompose prematurely.

[0052] Second stage: Medium-speed stirring can induce the foaming agent to begin to decompose, producing an appropriate amount of bubble nuclei.

[0053] The third stage: slow stirring allows the bubbles to expand slowly, forming a uniform bubble structure.

[0054] Citric acid is acidic and may corrode production equipment such as reaction vessels under high temperature and moisture conditions, thus shortening the service life of the equipment.

[0055] In a preferred embodiment, citric acid is coated, for example, with modified rosin resin. The coating effectively isolates the citric acid from the equipment surface, reducing its corrosiveness. Simultaneously, the coating can rupture under specific temperature or pressure conditions, releasing the citric acid and allowing it to function as a foaming agent. The specific coating process is as follows.

[0056] Citric acid pretreatment: Pass the citric acid powder through a 200-mesh sieve to remove agglomerated particles and ensure uniform particle size.

[0057] Preparation of coating solution: The modified rosin resin (model: D-135, Shenzhen Xingchengda) was dissolved in anhydrous ethanol to prepare a solution with a concentration of 10wt%.

[0058] Fluidized bed coating: Citric acid powder is added to the fluidized bed coating machine, and the fluidized bed temperature is controlled at 35℃ and the air inlet velocity is 1.5m / s.

[0059] Coating solution application: The coating solution is sprayed onto the citric acid powder in the fluidized bed at a rate of 5 mL / min using a peristaltic pump. During the spraying process, continuous stirring is maintained to ensure that the citric acid powder is uniformly heated and coated.

[0060] Drying: After spraying, continue fluidized bed drying for 30 minutes to allow the ethanol to evaporate completely, resulting in citric acid coated with modified rosin resin.

[0061] Post-processing: Pass the coated citric acid through a 400-mesh sieve to remove any agglomerated particles.

[0062] Fluidized bed coating technology enables uniform coating of citric acid powder, improving coating efficiency and quality. By controlling parameters such as fluidized bed temperature, air inlet rate, and spraying rate, the thickness and uniformity of the coating layer can be precisely controlled. Modified rosin resin effectively isolates citric acid from the equipment surface during initial feeding, preventing localized high-concentration citric acid corrosion. In the first stage, as stirring progresses, the citric acid gradually disperses evenly, while the modified rosin resin coating layer melts and disperses into the hot melt adhesive matrix, acting as a tackifier for the EVA hot melt adhesive.

[0063] Specifically, the coating layer accounts for 5wt%-15wt% of the total mass of citric acid.

[0064] This preferred embodiment reduces the corrosiveness of the foaming agent to the equipment, extending its service life. The modified rosin resin coating can control the release rate of citric acid, further improving the uniformity of foaming. The modified rosin resin also has certain antioxidant properties, which can improve the stability of the foamed hot melt adhesive.

[0065] Implementation Case 1

[0066] Raw material pretreatment: Add 100g of EVA (Elvax 260, DuPont) into a jacketed reactor, heat to 120°C, and start stirring to completely melt the EVA.

[0067] Nanoparticle dispersion: 0.3g of nano silica (AEROSIL 200, Evonik Degussa) was added to molten EVA.

[0068] Foaming agent mixing: Mix 1.3g of azodicarbonamide and 0.65g of citric acid, and then add them to the reaction vessel.

[0069] Multi-stage shear foaming:

[0070] First stage (low shear mixing): Set the stirring speed to 50 rpm and stir continuously for 10 minutes to evenly disperse the foaming agent in the hot melt adhesive substrate at a temperature of 140℃.

[0071] Second stage (medium shear induction): Increase the stirring speed to 150 rpm and continue stirring for 5 minutes (the temperature rises from 140°C to 210°C within 5 minutes), and the foaming agent begins to decompose and generate bubbles.

[0072] The third stage (low shear stabilization): reduce the stirring speed to 30 rpm and continue stirring for 15 minutes to allow the bubbles to expand slowly and stabilize at a temperature of 210℃.

[0073] Cooling and setting: Remove the foamed hot melt adhesive from the reactor and cool it to room temperature to obtain the foamed hot melt adhesive product.

[0074] Performance testing:

[0075] Cell structure uniformity: The cell structure of the foamed hot melt adhesive was observed using a scanning electron microscope (SEM, model: JSM-6390LV, NEC). After sampling, gold sputtering was performed with an accelerating voltage of 10kV. The average diameter and cell size distribution were measured using image analysis software (ImageJ).

[0076] Adhesive strength: Referring to GB / T 7124-2008 "Test Method for Peel Strength of Pressure-Sensitive Adhesive Tapes", foamed hot melt adhesive was applied to a PET substrate and bonded to an ABS substrate. The 180° peel strength was then tested. A universal testing machine (model: CMT6104, MTS) was used for the test, with a tensile speed of 50 mm / min.

[0077] Performance data:

[0078] Cell structure uniformity: The average cell diameter is 150μm, and the cell size distribution ranges from 100 to 200μm.

[0079] Adhesive strength: 8 N / 25 mm at 180° peel strength.

[0080] Implementation Case 2

[0081] Raw material pretreatment: Add 100g of EVA (Elvax 260, DuPont) to a jacketed reactor (two reactors, 100g of EVA added to each), heat to 120°C, and start stirring to completely melt the EVA.

[0082] Nanoparticle dispersion: 0.2 g and 0.4 g of nano silica (AEROSIL 200, Evonik Degussa) were added to molten EVA respectively (0.2 g of nano silica was added to one reactor and 0.4 g of nano silica was added to the other reactor).

[0083] Foaming agent mixing: Mix 1.3g of azodicarbonamide and 0.65g of citric acid, and then add it to the reaction vessel (from this step onwards, the operation of the two reaction vessels is the same, and will not be noted again below).

[0084] Multi-stage shear foaming:

[0085] First stage (low shear mixing): Set the stirring speed to 50 rpm and stir continuously for 9 minutes to evenly disperse the foaming agent in the hot melt adhesive substrate at a temperature of 130℃.

[0086] Second stage (medium shear induction): Increase the stirring speed to 150 rpm and continue stirring for 5 minutes (the temperature rises from 130°C to 205°C within 5 minutes), and the foaming agent begins to decompose and generate bubbles.

[0087] The third stage (low shear bubble stabilization): reduce the stirring speed to 30 rpm and continue stirring for 16 minutes to allow the bubbles to expand slowly and stabilize at a temperature of 205℃.

[0088] Cooling and setting: Remove the foamed hot melt adhesive from the reactor and cool it to room temperature to obtain the foamed hot melt adhesive product.

[0089] Performance testing:

[0090] Cell structure uniformity: The cell structure of the foamed hot melt adhesive was observed using a scanning electron microscope (SEM, model: JSM-6390LV, NEC). After sampling, gold sputtering was performed with an accelerating voltage of 10kV. The average diameter and cell size distribution were measured using image analysis software (ImageJ).

[0091] Adhesive strength: Referring to GB / T 7124-2008 "Test Method for Peel Strength of Pressure-Sensitive Adhesive Tapes", foamed hot melt adhesive was applied to a PET substrate and bonded to an ABS substrate. The 180° peel strength was then tested. A universal testing machine (model: CMT6104, MTS) was used for the test, with a tensile speed of 50 mm / min.

[0092] Performance data:

[0093] When the amount of nano-silica is 0.2g: the average cell diameter is 180μm, the cell size distribution range is 120-250μm, and the 180° peel strength is 7N / 25mm.

[0094] When the amount of nano-silica is 0.4g: the average cell diameter is 120μm, the cell size distribution range is 80-150μm, and the 180° peel strength is 9N / 25mm.

[0095] Implementation Case 3

[0096] Citric acid pretreatment: Pass the citric acid powder through a 200-mesh sieve to remove agglomerated particles and ensure uniform particle size.

[0097] Preparation of coating solution: The modified rosin resin (D-135, Shenzhen Xingchengda) was dissolved in anhydrous ethanol to prepare a solution with a concentration of 10wt%.

[0098] Fluidized bed coating: Citric acid powder is added to the fluidized bed coating machine, and the fluidized bed temperature is controlled at 35℃ and the air inlet velocity is 1.5m / s.

[0099] Coating solution application: A peristaltic pump is used to spray the coating solution onto the citric acid powder in the fluidized bed at a rate of 5 mL / min. During spraying, continuous stirring is maintained to ensure uniform heating and coating of the citric acid powder. The coating layer mass is controlled to be 10% of the total citric acid mass.

[0100] Drying: After spraying, continue fluidized bed drying for 30 minutes to allow the ethanol to evaporate completely, resulting in citric acid coated with modified rosin resin.

[0101] Post-processing: Pass the coated citric acid through a 400-mesh sieve to remove any agglomerated particles.

[0102] Raw material pretreatment: Add 100g of EVA (Elvax 260, DuPont) into a jacketed reactor, heat to 120°C, and start stirring to completely melt the EVA.

[0103] Nanoparticle dispersion: 0.3g of nano silica (AEROSIL 200, Evonik Degussa) was added to molten EVA.

[0104] Foaming agent mixing: Mix 1.3g of azodicarbonamide and 0.71g of citric acid coated with modified rosin resin (inferred from the degree of coating, it contains approximately 0.65g of citric acid), and then add it to the reaction vessel.

[0105] Multi-stage shear foaming:

[0106] First stage (low shear mixing): Set the stirring speed to 50 rpm and stir continuously for 10 minutes to evenly disperse the foaming agent in the hot melt adhesive substrate at a temperature of 135℃.

[0107] Second stage (medium shear induction): Increase the stirring speed to 150 rpm and continue stirring for 5 minutes (the temperature rises from 135°C to 205°C within 5 minutes), and the foaming agent begins to decompose and generate bubbles.

[0108] The third stage (low shear stabilization): reduce the stirring speed to 30 rpm and continue stirring for 15 minutes to allow the bubbles to expand slowly and stabilize at a temperature of 205℃.

[0109] Cooling and setting: Remove the foamed hot melt adhesive from the reactor and cool it to room temperature to obtain the foamed hot melt adhesive product.

[0110] Cell structure uniformity: The cell structure of the foamed hot melt adhesive was observed using a scanning electron microscope (SEM, model: JSM-6390LV, NEC). After sampling, gold sputtering was performed with an accelerating voltage of 10kV. The average diameter and cell size distribution were measured using image analysis software (ImageJ).

[0111] Adhesive strength: Referring to GB / T 7124-2008 "Test Method for Peel Strength of Pressure-Sensitive Adhesive Tapes", foamed hot melt adhesive was applied to a PET substrate and bonded to an ABS substrate. The 180° peel strength was then tested. A universal testing machine (model: CMT6104, MTS) was used for the test, with a tensile speed of 50 mm / min.

[0112] Performance data:

[0113] Cell structure uniformity: The average cell diameter is 140 μm, and the cell size distribution ranges from 90 to 190 μm.

[0114] Adhesive strength: 9.2 N / 25 mm at 180° peel strength.

[0115] Comparison Case: Existing Foamed Hot Melt Adhesives on the Market

[0116] Cell structure uniformity: The cell structure of the foamed hot melt adhesive was observed using a scanning electron microscope (SEM, model: JSM-6390LV, NEC). After sampling, gold sputtering was performed with an accelerating voltage of 10kV. The average diameter and cell size distribution were measured using image analysis software (ImageJ).

[0117] Adhesive strength: Referring to GB / T 7124-2008 "Test Method for Peel Strength of Pressure-Sensitive Adhesive Tapes", foamed hot melt adhesive was applied to a PET substrate and bonded to an ABS substrate. The 180° peel strength was then tested. A universal testing machine (model: CMT6104, MTS) was used for the test, with a tensile speed of 50 mm / min.

[0118] Performance data:

[0119] Cell structure uniformity: The average cell diameter is 200μm, and the cell size distribution ranges from 100 to 300μm.

[0120] Adhesive strength: 7.5 N / 25 mm at 180° peel strength.

[0121] The comparison between the implementation case and the control case shows that the foamed hot melt adhesive prepared by the fluidized bed coating process of modified rosin resin coating citric acid not only helps to inhibit equipment corrosion, but also proves that the bonding strength is slightly improved and the cell structure is more uniform: the cell size distribution range is narrower and the cell structure is more uniform.

[0122] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0123] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing a foamed hot melt adhesive for filters, characterized in that the steps include... include: The hot melt adhesive substrate is put into a reaction vessel, heated and stirred, and nano silica and foaming agent are added and mixed with the molten hot melt adhesive substrate. Three-stage shear foaming is carried out, the material is discharged and cooled to obtain the foamed hot melt adhesive for the filter. 0.2-0.4 parts by weight of the nano-silica are used per 100 parts by weight of the hot melt adhesive substrate; For every 100 parts by weight of the hot melt adhesive substrate, 1.5-2.5 parts by weight of the foaming agent are used, wherein the foaming agent comprises azodicarbonamide and citric acid, and the mass ratio of the azodicarbonamide to the citric acid is 2:1; The hot melt adhesive substrate is EVA resin; In the three-stage shear foaming process, the temperature in the first stage is below 140℃, the temperature in the third stage is 195℃-210℃, the temperature in the second stage gradually increases from the temperature in the first stage to the temperature in the third stage, and the stirring rate in the second stage is greater than the stirring rate in the first stage, which is greater than the stirring rate in the third stage. The stirring speed in the first stage is 50rpm-60rpm, the stirring speed in the second stage is 130rpm-150rpm, and the stirring speed in the third stage is 30rpm-40rpm. The duration of the first stage is 9min-12min, the duration of the second stage is 5min-8min, and the duration of the third stage is 13min-16min.

2. The method for preparing foamed hot melt adhesive for filters according to claim 1, characterized in that, The vinyl acetate content of the EVA resin is 20%-35%.

3. The method for preparing foamed hot melt adhesive for filters according to claim 1, characterized in that, The citric acid is a core-shell particle coated with modified rosin resin; in the step of adding the hot melt adhesive substrate into the reactor, heating and stirring, the heating temperature is lower than the softening point of the modified rosin resin.

4. The method for preparing foamed hot melt adhesive for filters according to claim 3, characterized in that, The preparation steps of the core-shell particles include: dissolving the modified rosin resin in a volatile organic solvent to obtain a modified rosin resin solution, spraying the modified rosin resin solution onto the surface of citric acid powder and drying it, and then sieving it to obtain the core-shell particles.

5. A foaming hot melt adhesive for filters, characterized in that, It is made by the method for preparing foamed hot melt adhesive for filters according to any one of claims 1 to 4.