Differentiated double-layer polyurethane adhesive and injection packaging method thereof

Abstract

The application belongs to the technical field of head potting materials of hollow fiber membrane modules, and particularly discloses a differential double-layer polyurethane glue and a glue injection packaging method. The differential double-layer polyurethane glue comprises inner-layer sealing polyurethane glue and outer-layer high-toughness hydrolysis-resistant structure polyurethane glue, both of which are two-component polyurethane systems. The inner-layer sealing polyurethane glue is added with a silane-modified diphenyl methane diisocyanate prepolymer component, and the outer-layer high-toughness hydrolysis-resistant structure polyurethane glue is added with polyether polyol and a nano-mica and boron nitride composite filler. A layered and stepwise glue injection, low-frequency intermittent vibration and gradient negative pressure integrated forming process is adopted. Through precise regulation and control of the inner-layer semi-cured window period, the double-layer glue interface can be controlled to be micro-crosslinked, the interlayer bonding performance is significantly improved, and the head hydraulic alternating impact resistance is improved.

Description

Technical Field

[0001] This invention relates to the field of encapsulation materials for hollow fiber membrane modules, and in particular to a differentiated double-layer polyurethane adhesive and its encapsulation method. Background Technology

[0002] Polyvinylidene fluoride (PVDF) hollow fiber immersed membranes are the core filtration elements in wastewater treatment systems. The quality of the polyurethane potting of the end caps directly determines the reliability of the membrane fiber seal, the strength of the end cap structure, and the overall service life of the membrane element. Current mainstream encapsulation solutions still have several shortcomings, hindering improvements in product quality and production efficiency.

[0003] Existing encapsulation processes achieve controlled adhesive creep by pre-coating soft adhesive with a glycerin-based separator, but this results in high separator loss, difficult recycling, lengthy processes, and high costs for auxiliary materials and labor. Another mainstream solution uses molding clay to seal the membrane fiber ends, but after the adhesive cures, excess end cap material must be cut off, generating a large amount of solid waste. The cutting process can also easily damage the membrane fibers, leading to product scrap. Currently, most membrane modules on the market use a single-formula polyurethane monolithic encapsulation. The adhesive properties cannot simultaneously meet the requirements of sealing and structure. The inner layer is difficult to achieve a low-leakage seal, and the outer layer lacks high-strength hydrolysis-resistant structural properties. When immersed in sewage environments for a long time, it is prone to water absorption, softening, hydrolysis, and cracking, ultimately leading to problems such as end leakage and membrane fiber detachment failure.

[0004] In addition, a few double-layer encapsulation products on the market use the same polyurethane formulation for multiple pours, achieving only physical layering. The physicochemical properties of the inner and outer layers are indistinguishable, resulting in weak interlayer adhesion. Under alternating aeration and hydraulic scouring, interface cracking and delamination easily occur. This fails to meet the differentiated application needs of inner layer sealing and outer layer structural reinforcement, leading to insufficient encapsulation stability and persistent leakage and cracking risks during long-term operation. Consequently, it is difficult to meet the production requirements of high-quality, long-life PVDF membranes. In summary, there are currently no publicly available reports of double-layer encapsulation systems and processes that simultaneously solve the aforementioned technical problems. Summary of the Invention

[0005] In view of this, the present invention provides a differentiated double-layer polyurethane adhesive and its injection encapsulation method to solve the defects of existing methods, such as easy peeling between double-layer adhesives, single-layer adhesive with limited performance, poor PVDF bonding stability, and weak hydrolysis resistance of the adhesive. Relying on the differentiated design of the double-layer formula and the controllable cross-linking molding process between layers, it simultaneously solves multiple industry problems such as sealing, structural reinforcement, hydrolysis resistance, and water delamination resistance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A differentiated double-layer polyurethane adhesive, comprising an inner sealing polyurethane adhesive and an outer high-toughness hydrolysis-resistant structural polyurethane adhesive. The mass ratio of the inner sealing polyurethane adhesive to the outer high-toughness hydrolysis-resistant structural polyurethane adhesive is 2.8~3.2:36~40; The inner sealing polyurethane adhesive includes inner sealing polyurethane adhesive component A and inner sealing polyurethane adhesive component B. The inner sealing polyurethane adhesive component A includes polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer; The inner sealing polyurethane adhesive component B includes short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer. The outer high-toughness hydrolysis-resistant structural polyurethane adhesive includes component A and component B of the outer high-toughness hydrolysis-resistant structural polyurethane adhesive. The outer high-toughness hydrolysis-resistant polyurethane adhesive component A includes polymethylene polyphenyl polyisocyanate. The outer high-toughness hydrolysis-resistant polyurethane adhesive component B includes hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organic bismuth catalyst and organic silicone defoamer.

[0007] Preferably, the mass ratio of component A to component B of the inner layer sealing polyurethane adhesive is 0.8~1.2:1. The mass ratio of component A to component B of the high-toughness, hydrolysis-resistant polyurethane adhesive for the outer layer is 0.8~1.2:1.

[0008] Preferably, the mass ratio of polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer in component A of the inner sealing polyurethane adhesive is 100:15~20. The silane-modified diphenylmethane diisocyanate prepolymer includes γ-glycidoxypropyltrimethoxysilane-modified diphenylmethane diisocyanate prepolymer with a silane grafting rate of 5-6%.

[0009] Preferably, the mass ratio of short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer in component B of the inner sealing polyurethane adhesive is 48~52:2~2.5:1.5~2:0.02~0.05:0.01~0.03. The hydroxyl value of the short-chain modified polyether polyol is 350~380 mgKOH / g.

[0010] Preferably, the mass ratio of hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer in component B of the outer high-toughness hydrolysis-resistant polyurethane adhesive is 25~30:28~42:12~15:1.7~2.3:1.2~1.8:0.03~0.05:0.02~0.04; The hydroxyl value of the polyether polyol is 320~360 mgKOH / g.

[0011] Preferably, the oxazolidine latent curing dehydrating agent includes one or more of 3-ethyl-2-methyl-2-(3-methylbutyl)-1,3-oxazolidine, 3-ethyl-2-methyl-2-(4-methylpentyl)-1,3-oxazolidine and 3,3'-diethyl-2,2'-dimethyl-2,2'-di(3-methylbutyl)-1,1'-bisoxazolidine; The organic bismuth catalyst includes one or more of bismuth neodecanoate, bismuth isooctanoate, and bismuth naphthenate. The organosilicon defoamer includes one or more of polydimethylsiloxane, polyether-modified polysiloxane, and fluorosiloxane; The hydrogenated castor oil polyester polyol includes one or more of the following: hydrogenated castor oil and adipic acid condensate, hydrogenated castor oil and phthalic anhydride condensate, and hydrogenated castor oil and dimethyl terephthalate transesterification / condensation. The nanomica and boron nitride composite filler is one or more of the mechanically ground composite of nanomica and hexagonal boron nitride, the solution blended composite of nanomica and hexagonal boron nitride, and the surface grafted modified composite of nanomica and hexagonal boron nitride.

[0012] Another object of the present invention is to provide a method for encapsulating a differentiated double-layer polyurethane adhesive, comprising the following steps: 1) Arrange the polyvinylidene fluoride hollow fiber membrane bundles neatly, ensuring that the end faces of the membrane fibers are flush, and load them into the potting mold, controlling the distance between the bottom end of the membrane fibers and the bottom surface of the mold to be 2.8~3.2mm; 2) Pour the inner layer of sealing polyurethane into the mold, and use intermittent vibration combined with gradient negative pressure control for treatment. Then let it stand to allow the inner layer of sealing polyurethane to be in a semi-cured state. 3) The outer layer of high-toughness, hydrolysis-resistant structural polyurethane adhesive is injected between the inner layer of sealing polyurethane adhesive and the polyvinylidene fluoride hollow fiber membrane bundle in a semi-cured state. After injection, pressure boosting, vibration and curing treatments are carried out in sequence to obtain polyvinylidene fluoride hollow fiber immersion membrane module.

[0013] Preferably, the molding thickness of the inner sealing polyurethane adhesive in step 2) is 2.8~3.2mm; The molding thickness of the outer high-toughness hydrolysis-resistant polyurethane adhesive described in step 3) is 36~40mm.

[0014] Preferably, the frequency of the intermittent vibration in step 2) is 3~4Hz, the amplitude is 1~3mm, each cycle of the intermittent vibration is 2~4min of vibration followed by 1~3min of rest; the number of cycles of intermittent vibration is 2~3. The initial pressure of the gradient negative pressure regulation is -0.01 to -0.03 MPa, the holding time is 2 to 5 minutes, and the final pressure is -0.06 to -0.03 MPa. The settling time is 90-110 minutes, and the pressure during settling is the final pressure.

[0015] Preferably, the infusion pressure in step 3) is -0.02 to -0.05 MPa; The pressure after the pressurization process is -0.04 to -0.08 MPa, and the time is 3 to 8 minutes. The vibration treatment has a frequency of 3~8Hz, an amplitude of 1~3mm, and a duration of 3~6min; The curing process takes 72 to 82 minutes.

[0016] This invention employs a functionally partitioned, dual-layer polyurethane system: the inner layer is a low-viscosity, high-adhesion sealant with added silane-modified MDI prepolymer. The polar silane groups anchor the hydrophobic PVDF membrane surface, achieving high-strength sealing and adhesion without plasma activation pretreatment. The outer layer is a high-toughness, hydrolysis-resistant structural adhesive, using a composite base of long-chain hydrolysis-resistant polyether polyol and hydrogenated castor oil polyester polyol, combined with nano-mica and boron nitride composite inorganic fillers. This significantly improves the structural strength and resistance to wet water hydrolysis of the end cap, resists long-term sewage erosion, and enhances the end cap's resistance to cracking and fiber shedding.

[0017] The supporting process employs low-frequency intermittent vibration combined with gradient negative pressure layered injection molding. It precisely controls the semi-cured state of the inner layer before injecting the outer layer, allowing for slight intermingling and cross-linking at the interface of the two adhesive layers to form a dense transitional adhesive layer. This effectively improves the delamination defects at the interface of the two adhesive layers and enhances the end cap's resistance to the impact of alternating aerated water flow. The entire process effectively controls the glue creep height, ensuring good encapsulation consistency and balancing mass production efficiency with a high finished product yield.

[0018] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: 1. The dual-layer formula has a clear function, breaking through the technical limitations of traditional single-layer casting and double-layer casting with the same adhesive. The inner layer focuses on low viscosity and high strength bonding to achieve reliable sealing; the outer layer focuses on high toughness structure, hydrolysis resistance and corrosion resistance, taking into account both sealing and long-term structural stability, and is suitable for the long-term operation of PVDF hollow fiber immersion membrane wastewater treatment.

[0019] 2. Constructing an interlayer cross-linking transition structure, utilizing the semi-cured state of the inner layer to achieve micro-fusion bonding of the inner and outer adhesives, significantly improving the delamination and cracking problem of conventional double-layer adhesives, and greatly enhancing the end cap's resistance to hydraulic fatigue and aeration impact.

[0020] 3. Membrane bonding without pretreatment: The inner layer silane-modified formula can directly bond hydrophobic PVDF membranes without the need for plasma activation equipment, reducing production line equipment investment and energy consumption, and simplifying the production process.

[0021] 4. Excellent hydrolysis resistance: The outer layer is combined with hydrolysis-resistant polyether polyol and barrier inorganic filler, which significantly improves the colloid's resistance to damp heat and hydrolysis. It is not easy to soften, crack or detach when soaked in sewage for a long time, effectively extending the service life of the membrane module.

[0022] 5. High molding precision and excellent yield rate. Relying on low-frequency intermittent vibration and gradient negative pressure for precise glue control, it significantly improves the problem of misalignment and disorder of film fibers, the glue climbing height is controllable, the product packaging consistency is good, and it is easy to carry out large-scale industrial production. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the end cap structure of the polyvinylidene fluoride hollow fiber immersed membrane module of the present invention. Detailed Implementation

[0025] This invention provides a differentiated double-layer polyurethane adhesive, comprising an inner sealing polyurethane adhesive and an outer high-toughness hydrolysis-resistant structural polyurethane adhesive; the mass ratio of the inner sealing polyurethane adhesive to the outer high-toughness hydrolysis-resistant structural polyurethane adhesive is 2.8~3.2:36~40, preferably 2.9~3.1:37~39, and more preferably 3:38.

[0026] In this invention, the inner sealing polyurethane adhesive comprises inner sealing polyurethane adhesive component A and inner sealing polyurethane adhesive component B; inner sealing polyurethane adhesive component A comprises polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer; inner sealing polyurethane adhesive component B comprises short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer.

[0027] In this invention, the room temperature mixing viscosity of the inner sealing polyurethane adhesive is 7000~9000 mPa·s, specifically 7200 mPa·s, 7500 mPa·s, 7800 mPa·s, 8000 mPa·s, 8200 mPa·s, 8500 mPa·s, and 8800 mPa·s; below 7000 mPa·s, the fiber height is greater than 0.6 cm, which easily contaminates the effective film area; above 9000 mPa·s, the fluidity is insufficient, and the sealing layer is not fully formed; after complete curing, the Shore D hardness is 82~85, specifically 83, 84, and 85.

[0028] In this invention, the outer high-toughness hydrolysis-resistant structural polyurethane adhesive comprises an outer high-toughness hydrolysis-resistant structural polyurethane adhesive component A and an outer high-toughness hydrolysis-resistant structural polyurethane adhesive component B; the outer high-toughness hydrolysis-resistant structural polyurethane adhesive component A comprises polymethylene polyphenyl polyisocyanate; the outer high-toughness hydrolysis-resistant structural polyurethane adhesive component B comprises hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst, and organosilicon defoamer.

[0029] In this invention, the room temperature mixing viscosity of the outer high-toughness hydrolysis-resistant polyurethane adhesive is 4000~5000 mPa·s, specifically 4200 mPa·s, 4400 mPa·s, 4500 mPa·s, 4600 mPa·s, or 4800 mPa·s; and the Shore D hardness after complete curing is 86~89, specifically 86, 87, 88, or 89.

[0030] In this invention, the mass ratio of component A to component B of the inner sealing polyurethane adhesive is 0.8~1.2:1, preferably 0.9~1.1:1, and more preferably 1:1; the mass ratio of component A to component B of the outer high-toughness hydrolysis-resistant structural polyurethane adhesive is 0.8~1.2:1, preferably 0.9~1.1:1, and more preferably 1:1.

[0031] In this invention, the mass ratio of polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer in component A of the inner sealing polyurethane adhesive is 100:15~20, preferably 100:16~19, and more preferably 100:17~18.

[0032] In this invention, the silane-modified diphenylmethane diisocyanate prepolymer includes γ-glycidyl etheroxypropyltrimethoxysilane-modified diphenylmethane diisocyanate prepolymer, with a silane grafting rate of 5-6%, specifically 5.2%, 5.4%, 5.5%, 5.6%, and 5.8%.

[0033] In this invention, the mass ratio of short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst, and organosilicon defoamer in component B of the inner sealing polyurethane adhesive is 48~52:2~2.5:1.5~2:0.02~0.05:0.01~0.03, preferably 49~51:2.1~2.4:1.6~1.9:0.02~0.04:0.01~0.02, and more preferably 50:2.2~2.3:1.7~1.8:0.02:0.01.

[0034] In this invention, the hydroxyl value of the short-chain modified polyether polyol is 350~380 mgKOH / g, specifically 355 KOH / g, 360 KOH / g, 365 KOH / g, 370 KOH / g, or 375 KOH / g.

[0035] In this invention, the mass ratio of hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer in component B of the outer high-toughness hydrolysis-resistant polyurethane adhesive is 25~30:28~42:12~15:1.7~2.3:1.2~1.8:0.03~0.05:0.02~0.04, preferably 26~29:30~40:13~14:1.8~2.2:1.4~1.6:0.03~0.04:0.02~0.03, and more preferably 27~28:35:13.5:2:1.5:0.03:0.02.

[0036] In this invention, the hydroxyl value of the polyether polyol is 320~360 mgKOH / g, specifically 325 KOH / g, 330 KOH / g, 335 KOH / g, 340 KOH / g, 345 KOH / g, 350 KOH / g, or 355 KOH / g.

[0037] In this invention, the oxazolidine latent curing dehydrating agent comprises one or more of 3-ethyl-2-methyl-2-(3-methylbutyl)-1,3-oxazolidine, 3-ethyl-2-methyl-2-(4-methylpentyl)-1,3-oxazolidine, and 3,3'-diethyl-2,2'-dimethyl-2,2'-di(3-methylbutyl)-1,1'-bisoxazolidine; the organobismuth catalyst comprises one or more of bismuth neodecanoate, bismuth isooctanoate, and bismuth naphthenate; and the organosilicon defoamer comprises polydimethylsiloxane. The polysiloxane is selected from one or more of the following: alkanes, polyether-modified polysiloxanes, and fluorosiloxanes; the hydrogenated castor oil polyester polyol includes one or more of the following: hydrogenated castor oil and adipic acid condensate, hydrogenated castor oil and phthalic anhydride condensate, and hydrogenated castor oil and dimethyl terephthalate transesterification / condensation product; the nano-mica and boron nitride composite filler includes one or more of the following: a mechanically ground composite of nano-mica and hexagonal boron nitride, a solution blend composite of nano-mica and hexagonal boron nitride, and a surface grafting modified composite of nano-mica and hexagonal boron nitride.

[0038] In this invention, the preparation methods of the inner sealing polyurethane adhesive, inner sealing polyurethane adhesive component A, inner sealing polyurethane adhesive component B, outer high-toughness hydrolysis-resistant structural polyurethane adhesive, outer high-toughness hydrolysis-resistant structural polyurethane adhesive component A, and outer high-toughness hydrolysis-resistant structural polyurethane adhesive component B are independently to obtain the corresponding components by uniformly mixing them.

[0039] This invention also provides a method for injection encapsulation of a differentiated double-layer polyurethane adhesive. This invention precisely controls the semi-cured nodes of the inner sealing polyurethane adhesive, allowing for minute inter-dissolution and cross-linking at the contact surfaces of the inner and outer adhesives to form an integrated transitional adhesive layer. This significantly improves the interlayer bonding performance of the double-layer adhesive and enhances the end cap's resistance to alternating hydraulic impact. Specifically, the method includes the following steps: 1) Arrange the polyvinylidene fluoride hollow fiber membrane bundles neatly, ensuring that the end faces of the membrane fibers are flush, and load them into the potting mold. Control the distance between the bottom end of the membrane fiber and the bottom surface of the mold to be 2.8~3.2mm, specifically 2.9mm, 3mm, or 3.1mm; 2) Pour the inner layer of sealing polyurethane into the mold, and use intermittent vibration combined with gradient negative pressure control for treatment. Then let it stand to allow the inner layer of sealing polyurethane to be in a semi-cured state. 3) The outer layer of high-toughness, hydrolysis-resistant structural polyurethane adhesive is injected between the inner layer of sealing polyurethane adhesive and the polyvinylidene fluoride hollow fiber membrane bundle in a semi-cured state. After injection, pressure boosting, vibration and curing treatments are carried out in sequence to obtain polyvinylidene fluoride hollow fiber immersion membrane module.

[0040] In this invention, after vibration treatment in step 3), the two layers of colloids slightly dissolve and cross-link at their contact surfaces to form a transitional adhesive layer, thereby achieving integrated molding of the end cap.

[0041] In this invention, the molding thickness of the inner sealing polyurethane adhesive in step 2) is 2.8~3.2mm, specifically 2.9mm, 3mm, or 3.1mm; the molding thickness of the outer high-toughness hydrolysis-resistant structural polyurethane adhesive in step 3) is 36~40mm, specifically 37mm, 38mm, or 39mm.

[0042] In this invention, the frequency of the intermittent vibration in step 2) is 3~4Hz, specifically 3.2Hz, 3.5Hz, or 3.8Hz; below 3Hz, the venting is insufficient, and the residual bubble rate in the colloid is greater than 5%; above 4Hz, the membrane fiber amplitude is too large, leading to misalignment; the amplitude is 1~3mm, specifically 1.2mm, 1.5mm, 1.8mm, 2mm, 2.2mm, 2.5mm, or 2.8mm; each cycle of the intermittent vibration is 2~4min, specifically 2.2min, 2.5min, 2.8min, 3min, 3.2min, 3.5min, or 3.8min; resting for 1~3min, specifically 1.2min, 1.5min, 1.8min, 2min, 2.2min, 2.5min, or 2.8min; the number of cycles of intermittent vibration is 2~3.

[0043] In this invention, the initial pressure of the gradient negative pressure regulation is -0.01 to -0.03, specifically -0.015 MPa, -0.02 MPa, or -0.025 MPa; the holding time is 2 to 5 minutes, specifically 2.5 minutes, 3 minutes, 3.5 minutes, 4 minutes, or 4.5 minutes; and the final pressure is -0.06 to -0.03 MPa, specifically -0.05 MPa or -0.04 MPa.

[0044] In this invention, the settling time is 90-110 min, specifically 92 min, 95 min, 98 min, 100 min, 102 min, 105 min, or 108 min; the pressure during settling is the final pressure.

[0045] In this invention, after standing, the inner layer adhesive (inner layer sealing polyurethane adhesive) reaches a semi-cured state—that is, the adhesive is surface dry but still has reactive activity. At this time, pouring the outer layer adhesive (outer layer high-toughness hydrolysis-resistant structural polyurethane adhesive) can cause a small amount of mutual solubility and cross-linking at the interface of the two adhesives. If the standing time is less than 90 minutes, the inner layer adhesive is too soft, and excessive mutual solubility at the interface will damage the inner layer sealing structure. If it is greater than 110 minutes, the inner layer adhesive is over-cured, and the interface cannot form chemical cross-linking, resulting in a decrease in interlayer peel strength of more than 50%.

[0046] In this invention, the injection pressure in step 3) is -0.02 to -0.05 MPa, specifically -0.03 MPa or -0.04 MPa.

[0047] In this invention, the pressure after the pressurization process is -0.04 to -0.08 MPa, specifically -0.05 MPa, -0.06 MPa, or -0.07 MPa; the time is 3 to 8 minutes, specifically 4 minutes, 6 minutes, 5 minutes, or 7 minutes.

[0048] In this invention, the vibration treatment frequency is 3~8Hz, specifically 4Hz, 5Hz, 6Hz, or 7Hz; the amplitude is 1~3mm, specifically 1.2mm, 1.5mm, 1.8mm, 2mm, 2.2mm, 2.5mm, or 2.8mm; and the time is 3~6min, specifically 3.5min, 4min, 4.5min, 5min, or 5.5min.

[0049] In this invention, the curing time is 72~82 min, specifically 74 min, 75 min, 76 min, 78 min, or 80 min.

[0050] The encapsulation method described in this invention does not require plasma surface pretreatment and can directly perform double-layer encapsulation, with a peel strength between the film fibers and the colloid ≥65N.

[0051] The adhesive fiber height of the polyvinylidene fluoride hollow fiber immersed membrane module of the present invention is ≤0.6cm.

[0052] A schematic diagram of the end cap structure of the polyvinylidene fluoride hollow fiber immersed membrane module of the present invention is shown below. Figure 1 As shown.

[0053] The present invention also provides a polyvinylidene fluoride hollow fiber immersion membrane module obtained by the above-described encapsulation method.

[0054] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] The following are the component sources used in all embodiments of the present invention, which are not intended to limit the present invention: Polymethylene polyphenyl polyisocyanate: Wanhua Chemical, PM-200; γ-glycidyl etheroxypropyltrimethoxysilane modified diphenylmethane diisocyanate prepolymer: Polypropylene glycol (PPG-1000, hydroxyl value 112 mgKOH / g) and polymethylene polyphenyl polyisocyanate (PM-200, Wanhua Chemical) were added to a reactor at a mass ratio of 1:2.5. Under nitrogen protection, the reaction was carried out at 75℃ for 2.5 hours to obtain a terminal isocyanate prepolymer with an isocyanate mass fraction of 12%. The temperature was lowered to 60℃, and KH-560 (5.5% by mass of the prepolymer) was added. The reaction was carried out at a constant temperature for 1.5 hours to obtain the target product with a silane grafting rate of 5%. Short-chain modified polyether polyol: Using glycerol as an initiator, in the presence of a dimetallic cyanide (DMC) catalyst, the mixture was added to a high-pressure reactor. After three nitrogen purgings, the temperature was raised to 125°C, and propylene oxide was continuously introduced to carry out an alkoxylation reaction. The reaction pressure was controlled at 0.4 MPa until the hydroxyl value of the reactant dropped to near the target value. The mixture was then cooled to 90°C and subjected to reduced pressure to remove unreacted monomers and low-boiling substances. The discharged material was filtered to obtain a short-chain polyether polyol with a hydroxyl value of 365 mg KOH / g. Hydrophobic fumed silica: Shandong Hongruitong, HD171; Oxazolidine-based latent solidification dehydrating agent: Quzhou Deyu Technology, DY-201; Organic bismuth catalyst: Shaoxing Yihe Technology, E-20; Organosilicon defoamer: Shanghai Huiyan New Materials, HY-141; Hydrogenated castor oil polyester polyol: Hydrogenated castor oil was added to a reaction flask and vacuum dehydrated for 1.5 hours at 115°C and a vacuum degree of -0.095 MPa. The temperature was then lowered to 80°C, and phthalic anhydride (molar ratio of hydrogenated castor oil to phthalic anhydride was 1:1.8) was added. Using xylene as a solvent, the mixture was heated to 145°C and refluxed for 4.5 hours. After the reaction was completed, xylene was removed by vacuum distillation to obtain the hydrogenated castor oil polyester polyol. (Long-chain hydrolysis resistant) polyether polyol: WANOL® R2305; Nano-mica and boron nitride composite filler: Hexagonal boron nitride powder (particle size D50 = 5 μm) and wet-processed mica powder (particle size D50 = 10 μm) were mixed at a mass ratio of 5:3 and dispersed in deionized water. After stirring evenly, the mixture was ultrasonically dispersed for 30 min to allow water molecules to fully penetrate the interlayer of the powder. Then, it was rapidly frozen in a -20℃ freezer for 12 h. After removal, it was naturally warmed to room temperature to thaw, and the above freeze-thaw cycle was repeated 4 times. The resulting dispersion was ultrasonically treated at 300W power for 3 h, and then centrifuged at 3000 r / min for 10 min. The supernatant was collected, filtered, and dried to obtain the nano-mica and boron nitride composite filler.

[0056] Example 1 (Standard Formula for Municipal Wastewater)

[0057] Inner layer sealing polyurethane adhesive: Component A is obtained by mixing 100 parts of polymethylene polyphenyl polyisocyanate with 18 parts of γ-glycidyl etheroxypropyltrimethoxysilane modified diphenylmethane diisocyanate prepolymer; Component B is obtained by mixing 50 parts of short-chain modified polyether polyol, 2.2 parts of hydrophobic fumed silica, 1.8 parts of oxazolidine latent curing dehydrating agent, 0.02 parts of organic bismuth catalyst, and 0.01 parts of organic silicone defoamer; Components A and B are mixed in a mass ratio of 1:1 to obtain the inner layer sealing polyurethane adhesive.

[0058] The outer layer of high-toughness, hydrolysis-resistant polyurethane adhesive consists of: Component A being polymethylene polyphenyl polyisocyanate; Component B being composed of 28 parts hydrogenated castor oil polyester polyol, 40 parts (long-chain hydrolysis-resistant) polyether polyol, 13 parts nano-mica and boron nitride composite filler, 2 parts hydrophobic fumed silica, 1.5 parts oxazolidine latent curing dehydrating agent, 0.03 parts organic bismuth catalyst, and 0.02 parts organic silicone defoamer; Components A and B are mixed in a mass ratio of 1:1 to obtain the outer layer of high-toughness, hydrolysis-resistant polyurethane adhesive.

[0059] Encapsulation method: Membrane bundle assembly: Arrange the rolled PVDF hollow fiber membrane bundles neatly, ensuring that all membrane fiber ends are flush, and vertically insert them into the filling mold, controlling the distance between the bottom end of the membrane fiber and the bottom surface of the mold to 3mm.

[0060] Bottom sealing layer casting: Pour the prepared inner sealing polyurethane adhesive into the mold to form a bottom sealing adhesive layer with a thickness of 3mm; adopt a 4Hz intermittent vibration process with an amplitude of 2mm, vibrate for 3min per cycle, rest for 2min, repeat 2 cycles, and simultaneously use gradient negative pressure control, with an initial negative pressure of -0.02MPa, which is increased to -0.04MPa after 3min; maintain the pressure and let it stand at room temperature for 110min to make the inner sealing polyurethane adhesive semi-cured.

[0061] Outer structural layer casting: The outer high-toughness hydrolysis-resistant structural polyurethane adhesive is injected between the semi-cured inner sealing polyurethane adhesive and the polyvinylidene fluoride hollow fiber membrane bundle. The overall filling depth of the end cap is controlled at 40mm. During the injection stage, a negative pressure of -0.03MPa is maintained. After the injection is completed, the pressure is increased to -0.06MPa and held for 5 minutes. Then, continuous vibration at 5Hz is used for 4 minutes and then the machine is stopped. The amplitude is 2mm. The two layers of adhesive slightly dissolve and cross-link at the contact surface to form a transition bonding layer, realizing the integrated molding of the end cap.

[0062] Curing and demolding: Curing naturally at room temperature for 82 minutes, demolding after the colloid reaches the demolding strength, and continuing to place at room temperature until fully cured to obtain the finished polyvinylidene fluoride hollow fiber immersed membrane module.

[0063] Test data: Inner layer adhesive Shore D hardness 83, outer layer adhesive Shore D hardness 87; overall fiber height 0.5cm; after continuous immersion in boiling water at 100℃ for 200h, the outer layer adhesive hardness remained at D83 without softening or cracking; the peel strength between the membrane fiber and the adhesive was 70.2N, indicating excellent sealing performance; no interlayer cracking after 500 cycles of hot and cold cycling.

[0064] Example 2 (Formula for High-Salinity Industrial Wastewater)

[0065] The only difference between this embodiment and Example 1 is that the γ-glycidyl etheroxypropyltrimethoxysilane modified diphenylmethane diisocyanate prepolymer in component A of the inner sealing polyurethane adhesive is adjusted from 18 parts to 20 parts, while the remaining raw material ratios and production process parameters remain unchanged.

[0066] Molding test: The PVDF membrane can be stably bonded without pretreatment, and the peel strength of the membrane fiber colloid is 71.5N; after being soaked in high-salt sewage for 6 months, the end cap colloid showed no hydrolysis, debonding, or water seepage, demonstrating outstanding salt resistance and hydrolysis resistance, and no stratification was observed in the aeration circulation test.

[0067] Example 3 (Economic Industrial Wastewater Formula)

[0068] Based on cost optimization in Example 1, the composite filler of nano-mica and boron nitride was adjusted to 15 parts, while the proportions of other raw materials and production process parameters remained unchanged.

[0069] The finished product meets all the standards for hardness, bonding strength and hydrolysis resistance, and is suitable for conventional industrial wastewater treatment. The raw material cost is 13% lower than that of traditional single potting compounds, making it cost-effective and suitable for mass production.

[0070] Comparative Example 1 (Traditional single-instrument + clay sealing process)

[0071] Commercially available polyurethane adhesive (Sriman PU1037) was selected. Clay was rolled into a uniform 1.0mm sheet, cut to the same size as the membrane fiber bundle, and pressed onto the end face of the membrane fiber bundle. The clay adhering to the outer wall was removed, and the bundle was air-dried for 6 hours. The membrane fiber bundle with plugged holes was then placed into a potting mold and vibrated at a frequency of 10Hz for 5 minutes. Polyurethane adhesive was poured in under continuous vibration. During the pouring process, a vacuum was drawn to -0.05MPa, and after pouring, the vacuum was increased to -0.1MPa and held for 4 minutes. After curing at room temperature, the plugged end section was cut off to open the membrane fiber pores. When the same PVDF immersion membrane was encapsulated using this traditional process, after soaking in sewage for 3 months, the end cap adhesive softened due to water absorption, the fiber climbing height was 3.4cm, local water seepage and micro-cracks in the adhesive were observed, indicating poor product stability.

[0072] Comparative Example 2 (Commonly available double-layer casting process using the same materials)

[0073] The same polyurethane adhesive (Sriman PU1037) without silane modification was used for layered casting with no differentiated formula design. After the polyurethane was mixed evenly, it was poured into the mold to a thickness of about 3.0 mm. 4Hz intermittent vibration was used to assist in venting, and the negative pressure was controlled by a synchronous gradient (initially -0.02MPa, increased to -0.04MPa after 3 minutes). It was left to stand at room temperature for 100 minutes to reach a semi-cured state. The same polyurethane adhesive was poured between the cured first layer of adhesive and the hollow fiber membrane bundle, and the overall filling depth of the end cap was controlled at 40 mm. The negative pressure was maintained at -0.03MPa during the injection stage. After the injection was completed, the pressure was increased to -0.06MPa and held for 5 minutes, followed by 5Hz continuous vibration for 4 minutes. After natural curing at room temperature for 77 minutes, it was demolded and left to stand until fully cured. After 6 months of use, the membrane fiber and adhesive interface showed delamination gaps, and the double-layer adhesive interface delaminated and cracked, posing a risk of leakage. The adhesion and hydrolysis resistance were significantly lower than the solution of this invention.

[0074] Comparative Example 3 (Inner layer adhesive does not contain silane-modified components)

[0075] The only difference between this comparative example and Example 1 is that the silane-modified diphenylmethane diisocyanate prepolymer is not added to component A of the inner sealing polyurethane adhesive.

[0076] Test results: The peel strength between the PVDF membrane fibers and the colloid was 38.2 N, which was 45.6% lower than that in Example 1; after soaking in boiling water for 100 h, interfacial debonding occurred, indicating that the silane-modified component is the key to achieving high-strength PVDF bonding without treatment.

[0077] Comparative Example 4 (outer layer adhesive does not contain nano-mica and boron nitride composite filler)

[0078] The only difference between this comparative example and Example 1 is that the outer high-toughness hydrolysis-resistant polyurethane adhesive component B does not contain nano-mica and boron nitride composite filler.

[0079] Test results: After soaking in boiling water for 200 hours, the hardness retention rate of the outer layer adhesive was only 72%, which was 24% lower than that of Example 1, and microcracks appeared on the surface, indicating that the inorganic filler made a significant contribution to the hydrolysis resistance.

[0080] Comparative Example 5 (Outer layer poured after inner layer is fully cured)

[0081] The only difference between this comparative example and Example 1 is that the standing time after the inner layer of sealing polyurethane glue is poured is extended to 240 minutes (when the inner layer is fully cured) before the outer layer of glue is poured.

[0082] Test results: There was no chemical cross-linking at the interface of the two adhesive layers. After 100 cycles of hot and cold cycling, obvious interlayer cracking appeared. The peel strength decreased by 52% compared with Example 1, indicating that precise control of the semi-curing window period is the key to interlayer bonding.

[0083] Comparative Example 6 (using high-frequency vibration instead of intermittent vibration)

[0084] The only difference between this comparative example and Example 1 is that the intermittent vibration is adjusted to a continuous vibration of 10 Hz.

[0085] Test results: The misalignment rate of membrane filament arrangement increased from 0.5% to 8.2%, and the filament climbing height increased from 0.5cm to 1.3cm, indicating that low-frequency intermittent vibration plays an irreplaceable role in controlling membrane filament arrangement and adhesive climbing height.

[0086] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-6

[0087] As can be seen from Table 1: 1. It is impossible to achieve both high-strength adhesion and long-term hydrolysis resistance by simply modifying the inner silane or improving the outer hydrolysis-resistant filler (Comparative Examples 3 and 4). 2. Relying solely on a double-layer structure without formula differentiation cannot solve the problems of weak interlayer bonding and poor PVDF adhesion (Comparative Examples 2 and 5). 3. Relying solely on formula improvements while employing conventional processes cannot achieve low fiber height and neat fiber arrangement (Comparative Example 6).

[0088] This invention achieves a comprehensive performance improvement that cannot be achieved by a single technical solution or simple combination through the synergistic effect of "inner layer silane modification + outer layer hydrolysis-resistant filler + semi-curing interface crosslinking process".

[0089] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0090] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A differentiated double-layer polyurethane adhesive, characterized in that, The differentiated double-layer polyurethane adhesive includes an inner sealing polyurethane adhesive and an outer high-toughness hydrolysis-resistant structural polyurethane adhesive. The mass ratio of the inner sealing polyurethane adhesive to the outer high-toughness hydrolysis-resistant structural polyurethane adhesive is 2.8~3.2:36~40; The inner sealing polyurethane adhesive includes inner sealing polyurethane adhesive component A and inner sealing polyurethane adhesive component B. The inner sealing polyurethane adhesive component A includes polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer; The inner sealing polyurethane adhesive component B includes short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer. The outer high-toughness hydrolysis-resistant structural polyurethane adhesive includes component A and component B of the outer high-toughness hydrolysis-resistant structural polyurethane adhesive. The outer high-toughness hydrolysis-resistant polyurethane adhesive component A includes polymethylene polyphenyl polyisocyanate. The outer high-toughness hydrolysis-resistant polyurethane adhesive component B includes hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organic bismuth catalyst and organic silicone defoamer.

2. The differentiated double-layer polyurethane adhesive according to claim 1, characterized in that, The mass ratio of component A to component B of the inner layer sealing polyurethane adhesive is 0.8~1.2:

1. The mass ratio of component A to component B of the high-toughness, hydrolysis-resistant outer layer polyurethane adhesive is 0.8~1.2:

1.

3. The differentiated double-layer polyurethane adhesive according to claim 2, characterized in that, The mass ratio of polymethylene polyphenyl polyisocyanate and silane-modified diphenylmethane diisocyanate prepolymer in component A of the inner sealing polyurethane adhesive is 100:15~20. The silane-modified diphenylmethane diisocyanate prepolymer includes γ-glycidyl etheroxypropyltrimethoxysilane-modified diphenylmethane diisocyanate prepolymer with a silane grafting rate of 5-6%.

4. The differentiated double-layer polyurethane adhesive according to claim 3, characterized in that, The mass ratio of short-chain modified polyether polyol, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst, and organosilicon defoamer in component B of the inner sealing polyurethane adhesive is 48~52:2~2.5:1.5~2:0.02~0.05:0.01~0.

03. The hydroxyl value of the short-chain modified polyether polyol is 350~380 mgKOH / g.

5. The differentiated double-layer polyurethane adhesive according to claim 4, characterized in that, The mass ratio of hydrogenated castor oil polyester polyol, polyether polyol, nano-mica and boron nitride composite filler, hydrophobic fumed silica, oxazolidine latent curing dehydrating agent, organobismuth catalyst and organosilicon defoamer in component B of the outer high-toughness hydrolysis resistant polyurethane adhesive is 25~30:28~42:12~15:1.7~2.3:1.2~1.8:0.03~0.05:0.02~0.

04. The hydroxyl value of the polyether polyol is 320~360 mgKOH / g.

6. A differentiated double-layer polyurethane adhesive according to any one of claims 1 to 5, characterized in that, The oxazolidine-based latent curing dehydrating agent includes one or more of 3-ethyl-2-methyl-2-(3-methylbutyl)-1,3-oxazolidine, 3-ethyl-2-methyl-2-(4-methylpentyl)-1,3-oxazolidine and 3,3'-diethyl-2,2'-dimethyl-2,2'-di(3-methylbutyl)-1,1'-bisoxazolidine; The organic bismuth catalyst includes one or more of bismuth neodecanoate, bismuth isooctanoate, and bismuth naphthenate. The organosilicon defoamer includes one or more of polydimethylsiloxane, polyether-modified polysiloxane, and fluorosiloxane; The hydrogenated castor oil polyester polyol includes one or more of the following: hydrogenated castor oil and adipic acid condensate, hydrogenated castor oil and phthalic anhydride condensate, and hydrogenated castor oil and dimethyl terephthalate transesterification / condensation. The nanomica and boron nitride composite filler is one or more of the mechanically ground composite of nanomica and hexagonal boron nitride, the solution blended composite of nanomica and hexagonal boron nitride, and the surface grafted modified composite of nanomica and hexagonal boron nitride.

7. A method for encapsulating a differentiated double-layer polyurethane adhesive according to any one of claims 1 to 6, characterized in that, Includes the following steps: 1) Arrange the polyvinylidene fluoride hollow fiber membrane bundles neatly, ensuring that the end faces of the membrane fibers are flush, and load them into the potting mold, controlling the distance between the bottom end of the membrane fibers and the bottom surface of the mold to be 2.8~3.2mm; 2) Pour the inner layer of sealing polyurethane into the mold, and use intermittent vibration combined with gradient negative pressure control for treatment. Then let it stand to allow the inner layer of sealing polyurethane to be in a semi-cured state. 3) The outer layer of high-toughness, hydrolysis-resistant structural polyurethane adhesive is injected between the inner layer of sealing polyurethane adhesive and the polyvinylidene fluoride hollow fiber membrane bundle in a semi-cured state. After injection, pressure boosting, vibration and curing treatments are carried out in sequence to obtain polyvinylidene fluoride hollow fiber immersion membrane module.

8. The method for encapsulating a differentiated double-layer polyurethane adhesive according to claim 7, characterized in that, The molding thickness of the inner sealing polyurethane adhesive mentioned in step 2) is 2.8~3.2mm; The molding thickness of the outer high-toughness hydrolysis-resistant polyurethane adhesive described in step 3) is 36~40mm.

9. The method for encapsulating a differentiated double-layer polyurethane adhesive according to claim 8, characterized in that, The frequency of the intermittent vibration described in step 2) is 3~4Hz, the amplitude is 1~3mm, each cycle of the intermittent vibration is 2~4min, followed by 1~3min of rest; the number of cycles of intermittent vibration is 2~3. The initial pressure of the gradient negative pressure regulation is -0.01 to -0.03 MPa, the holding time is 2 to 5 minutes, and the final pressure is -0.06 to -0.03 MPa. The settling time is 90-110 minutes, and the pressure during settling is the final pressure.

10. A method for encapsulating a differentiated double-layer polyurethane adhesive according to claim 8 or 9, characterized in that, The infusion pressure mentioned in step 3) is -0.02 to -0.05 MPa; The pressure after the pressurization process is -0.04 to -0.08 MPa, and the time is 3 to 8 minutes. The vibration treatment has a frequency of 3~8Hz, an amplitude of 1~3mm, and a duration of 3~6min; The curing process takes 72 to 82 minutes.

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