Single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking and preparation method thereof

Abstract

The invention provides a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking and a preparation method thereof, and the single-component solvent-free polyurethane hot melt adhesive comprises the following raw material components in parts by weight: 30-60 parts of a polyester polyol mixture; 10 to 40 parts of a polyether polyol mixture; 10 to 25 parts of isocyanate; 5 to 15 parts of tackifying resin; 0.05 to 0.5 part of a catalyst; 0.2 to 1.0 part of an antioxidant; 0.1 to 2.0 parts of a water removal agent; the polyester polyol mixture is crystalline or semi-crystalline polyester polyol; and the polyether polyol mixture takes low-melting-point or amorphous polyether polyol as a second soft segment. According to the invention, crystalline / semi-crystalline polyester polyol and low-melting-point / non-crystalline polyether polyol are taken as core soft segments, and are matched with special isocyanate and functional aids, so that the polyurethane hot melt adhesive with low initial viscosity, excellent initial viscosity, rapid crystallization / curing capacity, high final bonding strength and durable toughness is synthesized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular adhesives, in particular to a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking and a preparation method thereof. BACKGROUND

[0002] The flocking process is widely used in high-end shoe materials (such as snow boots), clothing and luggage fields, and its principle is to make the flock vertically implanted on the substrate coated with adhesive by using the principle of opposite charges attracting each other. For natural protein fiber substrates such as cow hair flock (two-layer cowhide milling flock), they have very high heat sensitivity. When the processing temperature exceeds 120℃, the collagen fibers inside the cow hair flock will undergo irreversible thermal denaturation, causing the leather plate to shrink, harden, become brittle, and even discolor, which seriously damages the natural feel and value of the leather.

[0003] In the prior art, the flocking adhesive mainly faces the following difficulties: Solvent-based adhesive: although it has good hand feeling and low application temperature, it contains a large amount of organic solvent (such as DMF, toluene), and has high VOC emission, which does not comply with environmental regulations; 2. Traditional PUR hot melt adhesive: although it is solvent-free and has high strength, in order to ensure the initial adhesion, it usually contains a large amount of high-crystalline polyester, which results in high melting point and large melt viscosity, and must be applied at a high temperature of 130-150℃, which is easy to scald the cow hair flock substrate; 3. Water-based adhesive: although it is environmentally friendly, it has poor wettability for hydrophobic cow hair flock, weak initial adhesion, and high drying energy consumption and low production efficiency.

[0004] Therefore, it is a technical problem to be solved in the field to develop an environmentally friendly adhesive that can be applied at a low temperature of 90-120℃, while having high initial adhesion (to prevent the flock from falling), high final strength and soft hand feeling. SUMMARY

[0005] In view of the above-mentioned shortcomings of the prior art, the present application aims to provide a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking and a preparation method thereof, which solves the problem that the adhesive in the prior art is difficult to adapt to the flocking of textile.

[0006] To achieve the above object and other related objects, the present application provides a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking, which comprises, by weight, polyester polyol mixture 30-60 parts, polyether polyol mixture 10-40 parts, isocyanate 10-25 parts, tackifying resin 5-15 parts, catalyst 0.05-0.5 parts, antioxidant 0.2-1.0 parts, and water-removing agent 0.1-2.0 parts, wherein the polyester polyol mixture is crystalline or semi-crystalline polyester polyol, which is used as the first soft segment to provide the physical crystallization initial adhesion of the adhesive after cooling; and the polyether polyol mixture is low-melting or non-crystalline polyether polyol, which is used as the second soft segment to dilute the polyester crystallization zone, reduce the system melt viscosity, and impart low-temperature flexibility to the adhesive layer.

[0007] By adopting the above technical solution, the adhesive is endowed with rapid crystallization ability, high cohesive strength, and heat resistance by the polyester polyol mixture, and the system melt temperature and construction viscosity are effectively reduced, and the low-temperature flexibility is improved by the polyether polyol mixture. With crystalline / semi-crystalline polyester polyol and low-melting / non-crystalline polyether polyol as the core soft segment, special isocyanate and functional additives are matched to synthesize a polyurethane hot melt adhesive with low initial viscosity, excellent initial adhesion, rapid crystallization / curing ability, high final bonding strength, and long-term flexibility.

[0008] In an embodiment of the present application, the mass ratio of the polyester polyol mixture to the polyether polyol mixture is 70:30 to 30:70.

[0009] By adopting the above technical solution, the balance of the melt temperature, open time, and final mechanical properties of the adhesive can be finely adjusted.

[0010] In an embodiment of the present application, the polyester polyol mixture is selected from one or more of poly(1,4-butanediol adipate) diol, poly(1,6-hexanediol adipate) diol, polycaprolactone diol, or poly(dodecanedioic acid) diol; the number average molecular weight thereof is 2000-4000 g / mol, and the melting point thereof is 40-60℃.

[0011] By adopting the above technical solution, the crystalline polyester polyol with low molecular weight and high regularity is selected to endow the adhesive with rapid crystallization ability, high cohesive strength, and heat resistance.

[0012] In an embodiment of the present application, the polyether polyol mixture is selected from one or more of poly(tetrahydrofuran ether) diol, polypropylene glycol, or tetrahydrofuran-oxidized propylene oxide copolyether diol; the number average molecular weight thereof is 1000-3000 g / mol, and the glass transition temperature thereof is lower than -60℃.

[0013] By adopting the technical scheme, the non-crystalline or micro-crystalline polyether polyol with a low glass transition temperature is selected, the melting temperature and the construction viscosity of the system are effectively reduced, and the low-temperature toughness is improved.

[0014] In an embodiment of the present application, the isocyanate is liquefied diphenylmethane diisocyanate.

[0015] By adopting the technical scheme, the liquefied diphenylmethane diisocyanate is selected as the main isocyanate component, the activity is moderate, the stable and controllable prepolymer can be synthesized, the strong polarity hard segment micro area can be formed, and excellent initial adhesion and final bonding strength can be provided.

[0016] In an embodiment of the present application, the tackifying resin is selected from hydrogenated rosin glycerin ester, hydrogenated rosin pentaerythritol ester or hydrogenated terpene resin, the softening point is 85-125 DEG C, the catalyst is an organic bismuth catalyst, and the water removing agent is p-toluenesulfonyl isocyanate.

[0017] By adopting the technical scheme, the hydrogenated terpene resin or hydrogenated rosin ester with good compatibility with polyurethane is selected, the wettability and initial adhesion to cow cashmere and various furs are greatly improved without significantly increasing the melting temperature, a small amount of organic bismuth catalyst is added to balance the storage stability of the prepolymer and the moisture curing speed after sizing, and the p-toluenesulfonyl isocyanate is used as the water removing agent to ensure the stability of the single-component product during storage.

[0018] In an embodiment of the present application, the filler is super-fine precipitated barium sulfate or fumed silica.

[0019] By adopting the technical scheme, the small amount of super-fine precipitated barium sulfate or fumed silica is used as the filler to adjust the rheological property and cost, but it needs to be strictly controlled to prevent the viscosity from being significantly increased and the low-temperature performance from being affected.

[0020] In an embodiment of the present application, the hot melt adhesive has a melting viscosity of less than or equal to 8,000 mPa s at 110 DEG C and has coatability flowability in the temperature range of 90-120 DEG C.

[0021] By adopting the technical scheme, the sizing temperature for flocking is successfully reduced from the conventional 130-150 DEG C to 90-120 DEG C, which is much lower than the scalding threshold of natural leather fibers such as cow cashmere. This fundamentally solves the problems of hardening, embrittlement, discoloration, shrinkage and hand feeling deterioration of the base material caused by high temperature.

[0022] A preparation method of the single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described above comprises the following steps: S1, dehydration pretreatment: the polyester polyol mixture, polyether polyol mixture and tackifying resin are put into a reaction kettle, heated to 110-130℃ to melt, and dehydrated under a vacuum degree of ≤-0.095 MPa for 1-3 hours, with the moisture content controlled to be <300 ppm; S2, prepolymerization, and the temperature is lowered to 70-80℃, the liquefied diphenylmethane diisocyanate is added under dry nitrogen protection, the reaction temperature is controlled to be 80-95℃, and the chain extension reaction is carried out for 1.5-3.0 hours to generate an end-NCO group prepolymer; S3, blending and homogenization, and the temperature is lowered to 70-80℃, the catalyst, antioxidant and water removal agent are added, and they are fully mixed and uniformly distributed under high-speed dispersion; S4, dehydration and filling: the mixed material is transferred to a dehydration / defoaming device, and bubbles, trace amounts of water and low-boiling substances are removed under high vacuum, and finally the finished product is filled into an aluminum foil bag or composite material tube that is moisture-proof under the protection of absolutely dry nitrogen.

[0023] The application of a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking in an electrostatic flocking process of a heat-sensitive substrate, the heat-sensitive substrate being cow fiber flock, leather or microfiber synthetic leather; the application of the adhesive has a gluing temperature of 90-120℃.

[0024] As described above, the single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking and the preparation method have the following beneficial effects: I. Overturning low-temperature processing adaptability, realizing perfect protection of sensitive substrates 1. Substrate "zero damage": successfully reducing the flocking and gluing temperature from the conventional 130-150℃ to 90-120℃, which is much lower than the scalding threshold of natural leather fibers such as cow fiber flock. This fundamentally solves the problems of substrate hardening, embrittlement, discoloration, shrinkage and hand feeling deterioration caused by high temperature.

[0025] 2. Widening the application range: the low-temperature characteristic makes the technology safe to be applied to various high-value materials that are sensitive to heat (such as microfiber synthetic leather, high-grade textile fabrics, and some plastic films), providing an unprecedented degree of freedom for product design.

[0026] II. Excellent and balanced bonding and use performance 1. "Instant initial adhesion, strong final adhesion" bonding characteristics: Excellent initial adhesion and positioning ability: through the synergistic effect of crystalline soft segments and tackifying resins, the adhesive produces initial adhesion force instantly upon cooling, ensuring that the flock can be quickly and uprightly fixed, and the flocking efficiency is high.

[0027] Extremely high final adhesive strength: the crosslinked network formed after moisture curing endows the adhesive layer with extremely high cohesive strength and heat creep resistance, and the peel strength (≥6 N / cm) far exceeds that of traditional processes, ensuring that the pile is firmly and durably attached and difficult to pull out.

[0028] 2. Excellent dynamic durability: Flexibility and resistance to bending: the unique soft segment design allows the cured adhesive layer to maintain excellent flexibility and elasticity, and can be repeatedly bent and stretched without delamination or powdering, resulting in a soft and natural feel of the product.

[0029] Overall environmental resistance: excellent resistance to hydrolysis, washing, dry and wet rubbing, and weathering, allowing the flocked product to adapt to complex use environments and strict cleaning requirements, significantly extending the product's life.

[0030] III. Revolutionary green environmental protection and production efficiency improvement 1. Environmentally friendly: Zero VOC emissions: completely free of solvents, no harmful gas emissions during production and use, completely improving the working environment, and meeting the world's most stringent environmental regulations (such as REACH, EPA); Non-toxic and safe: after curing, it is an inert solid, and the finished product is non-toxic and odorless, suitable for consumer products with high safety requirements (such as automotive interiors, home furnishings, and clothing).

[0031] 2. Production efficiency and economic benefits are greatly improved: Simple process: single component, no mixing, ready-to-use, eliminating time-consuming steps such as solvent-based glue preparation, coating, and drying (waiting for solvent evaporation), shortening the production process by more than 30%; Significant reduction in energy consumption: low-temperature application directly reduces heating energy consumption. At the same time, the absence of solvents means that there is no need for a large drying oven and waste gas treatment system, indirectly reducing energy consumption and equipment investment.

[0032] Optimized overall cost: although the unit price of raw materials may be higher, the savings in solvent costs, waste gas treatment fees, energy costs, insurance costs, and the benefits from reduced scrap rates and improved efficiency make the overall life cycle cost more competitive.

[0033] Clean production: the equipment only needs to be heated for cleaning, and there is no problem of waste liquid treatment, making maintenance simple.

[0034] IV. Excellent product stability and consistency 1. Stable storage and use: through precise water removal and packaging technology, the product's performance is stable within the shelf life, and the batch-to-batch consistency is extremely high, providing a guarantee for large-scale stable production.

[0035] 2. Wide application process window: Optimized rheological design ensures stable viscosity within the target temperature range, is insensitive to fluctuations in equipment parameters, is easy to operate and control, and guarantees the uniformity and reproducibility of flocking quality. Detailed Implementation

[0036] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0037] It should be understood that the structures, proportions, sizes, etc., illustrated in this specification are merely for illustrative purposes to aid those skilled in the art and to facilitate understanding and reading. They are not intended to limit the scope of the invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and purpose of the invention, should still fall within the scope of the technical content disclosed in this invention. Furthermore, the terms such as "upper," "lower," "left," "right," "middle," and "one" used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention's implementation.

[0038] Example 1: This embodiment provides a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking, the raw material components of which, by weight, include: Polyester polyol mixture: 30-60 parts; Polyether polyol mixture: 10-40 parts; Isocyanate: 10-25 parts; Tackifying resin: 5-15 parts; Catalyst: 0.05-0.5 parts; Antioxidants and light stabilizers: 0.2-1.0 parts; Dehydrating agent: 0.1-2.0 parts; The polyester polyol mixture is a crystalline or semi-crystalline polyester polyol, which serves as the first soft segment to provide the initial physical tack after the adhesive cools. The polyether polyol mixture uses low-melting-point or non-crystalline polyether polyols as the second soft segment to dilute the polyester crystalline region, reduce the melt viscosity of the system, and impart low-temperature flexibility to the adhesive layer.

[0039] The polyester polyol mixture as the first soft segment is selected from a crystalline polyester polyol with low molecular weight and high regularity. In this embodiment, one or more of polyhexanedioate-1,4-butanediol glycol, polyhexanedioate-1,6-hexanediol glycol, polycaprolactone glycol, or polydodecanedioate glycol can be used. The number average molecular weight of the polyester polyol is 2000-4000 g / mol, and the melting point is 40-60℃. The polyester polyol mixture can be used as the backbone of the adhesive. The molecular chain of the polyester polyol mixture is regular, and the polyester polyol mixture can quickly crystallize when cooled. During the cooling process after sizing, the crystalline polyester segments quickly rearrange to form crystal regions. This physical hardening provides initial cohesive force (initial adhesion), ensuring that the implanted fluff can stand upright and fixed without slippage before chemical crosslinking is completed.

[0040] The polyether polyol mixture as the second soft segment is selected from a non-crystalline or slightly crystalline polyether polyol with low Tg (glass transition temperature). In this embodiment, one or more of polytetrahydrofuran ether glycol, polypropylene glycol, or tetrahydrofuran- propylene oxide copolyether glycol can be used. The number average molecular weight of the polyether polyol is 1000-3000 g / mol, and the glass transition temperature is lower than -60℃.

[0041] The polyether polyol mixture can be used as a softener for the adhesive. The polyether segment (-C-O-C-) of the polyether polyol mixture has a bond angle that is easy to rotate, good flexibility, and is not easy to crystallize. The polyether segment can act as a viscosity reducer in the molten state, diluting the polyester molecules like a solvent, greatly reducing the intermolecular friction, and making the glue have good fluidity similar to Newtonian fluid at low temperature (90-120℃). After solidification, the polyether soft segment can give the glue layer excellent low-temperature flexibility and resilience, making the flocked product feel soft and full, and avoiding the hardening defect of pure polyester glue.

[0042] The ratio of the first soft segment to the second soft segment is 70:30 to 50:50 by proportion control, and the mass ratio of the polyester polyol mixture to the polyether polyol mixture is 70:30 to 50:50.

[0043] The isocyanate as the hard segment is selected from liquefied diphenylmethane diisocyanate (liquefied MDI) as the main isocyanate component in this embodiment. In this embodiment, modified MDI or MDI prepolymer can be used. The activity of the modified MDI or MDI prepolymer is moderate, which is beneficial to the synthesis of stable and controllable prepolymers and can form strong polar hard segment microregions to provide excellent initial adhesion and final bonding strength.

[0044] The isocyanate can react with the polyol to form a prepolymer, providing a chemical crosslinking active group (-NCO). The use of liquefied MDI introduces asymmetry or branching structure in the molecular structure, so that the prepolymer can remain in a liquid state at a lower temperature.

[0045] Tackifier, catalyst, antioxidant and light stabilizer, water scavenger as functional additive system; In this embodiment, the tackifier is selected from hydrogenated terpene resin or hydrogenated rosin ester which is compatible with polyurethane. The tackifier is selected from hydrogenated rosin glycerin ester, hydrogenated rosin pentaerythritol ester or hydrogenated terpene resin, and the softening point is 85-125℃. The tackifier can improve the wettability of non-polar substrates. The surface of the cow fiber velvet contains natural oil. By reducing the surface tension of the glue solution with the tackifier, the glue solution penetrates into the fiber pores of the leather board, and the peeling strength is improved.

[0046] The catalyst is a trace amount of organic bismuth catalyst, which balances the storage stability of the prepolymer and the moisture curing speed after sizing; The water scavenger is added molecular sieve or p-toluene sulfonyl isocyanate to ensure the stability of the single-component product during storage.

[0047] Antioxidant and light stabilizer: composite antioxidant (such as hindered phenol + phosphite) and ultraviolet absorber to ensure the durability of the adhesive during processing and use; The functional additive system also includes fillers, which can be selected from a small amount of ultra-fine precipitated barium sulfate or fumed silica, which is used to adjust the rheological property and cost, but needs to be strictly controlled to prevent significant increase in viscosity and affect low-temperature performance. Example two

[0048] The present embodiment provides a preparation method of a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described in example one, comprising the following steps: S1, dehydration pretreatment: the polyester polyol mixture, polyether polyol mixture and tackifier are put into the reaction kettle, heated to 110-130℃ for melting, and dehydrated under the condition of vacuum degree≤-0.095 MPa for 1-3 hours, and the moisture content is controlled to be less than 300 ppm; The trace amount of water adsorbed in the raw materials is removed. If the water is not removed, the isocyanate (-NCO) added subsequently will be consumed, generating carbon dioxide bubbles and urea groups, which will cause the viscosity of the glue to rise uncontrollably and even cause explosion.

[0049] S2, prepolymerization, cooling to 70-80℃, adding the liquefied diphenyl methane diisocyanate under dry nitrogen protection, controlling the reaction temperature at 80-95℃, and reacting for 1.5-3.0 hours to generate end-NCO group prepolymer through chain extension reaction; The reaction temperature is controlled below 95℃ to avoid side reactions (such as the generation of urea formate) and ensure the linearity of the prepolymer structure, thereby obtaining low viscosity. The reaction endpoint is determined by the detected -NCO content reaching the theoretical value (usually 2.0-3.0%); S3、 Blending and homogenizing, cooling to 70-80℃, adding the catalyst, antioxidant and water-removing agent, and mixing evenly under high-speed dispersion; S4、 Dehydration and filling: transferring the mixture to a dehydration / defoaming device, removing bubbles, trace water and low-boiling substances under high vacuum, and finally filling the finished product into aluminum foil bags or composite material tubes that are moisture-proof under the protection of absolutely dry nitrogen.

[0050] After inspection, the prepared hot melt adhesive has the following core properties: 1. Application temperature: 90-120°C (much lower than the conventional PUR of 130-150°C), which is suitable for the heat resistance threshold of the suede.

[0051] 2. Open time: 15-45 seconds (adjustable), which meets the positioning and adjustment time requirements of the flocking process.

[0052] 3. Initial viscosity (@110°C): ≤8,000 mPa·s, which ensures good spraying, rolling or scraping coating performance.

[0053] 4. Curing method: moisture curing, moderate curing speed, and consideration of operability and production efficiency.

[0054] 5. Final performance: Peeling strength (suede-standard fluff): ≥6 N / cm.

[0055] Heat resistance: ≥60°C.

[0056] Low temperature resistance: ≤-20°C, no adhesive peeling after folding.

[0057] Washing resistance: passed the standard home washing test (such as 5 times).

[0058] Environmental protection: 100% solid content, VOC content meets the most stringent environmental protection standards (such as the requirements of the European Union ECHA). Example Three

[0059] This example provides an application of the single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described in Example 1 in the electrostatic flocking process of heat-sensitive substrates, such as suede, leather or microfiber synthetic leather. The application temperature for the adhesive is 90-120°C. Example Four

[0060] This example provides a specific formula (by weight) of the single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described in Example One: Polyester polyol mixture (PHNA): 40 parts; Polyether polyol mixture (PPG): 30 parts; Isocyanate (liquefied MDI): 18 parts; Tackifying resin (hydrogenated terpene resin): 10 parts; Catalyst (DMDEE): 0.1 part; Water removal agent (PTSI): 0.5 part; In summary, the present application is endowed with the polyester polyol mixture to give the adhesive fast crystallization ability, high cohesive strength and heat resistance, the polyester polyol mixture effectively reduces the system melting temperature and construction viscosity, and improves the low temperature toughness, to crystallize / semi-crystalline polyester polyol, low melting point / non-crystalline polyether polyol as the core soft segment, with special isocyanate and functional additives, synthesis a kind of low initial viscosity, excellent initial adhesion, fast crystallization / curing ability, high final bonding strength and persistent toughness of polyurethane hot melt adhesive.

[0061] Therefore, the present application effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0062] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the present application should be covered by the claims of the present application.

Claims

1. A single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking, characterized in that, By weight, its raw material components include: Polyester polyol mixture: 30-60 parts; Polyether polyol mixture: 10-40 parts; Isocyanate: 10-25 parts; Tackifying resin: 5-15 parts; Catalyst: 0.05-0.5 parts; Antioxidants and light stabilizers: 0.2-1.0 parts; Dehydrating agent: 0.1-2.0 parts; wherein, the polyester polyol mixture is a crystalline or semi-crystalline polyester polyol, which serves as the first soft segment to provide the initial physical tack of the adhesive after cooling; the polyether polyol mixture is a low-melting-point or non-crystalline polyether polyol, which serves as the second soft segment to dilute the polyester crystalline region, reduce the melt viscosity of the system, and impart low-temperature flexibility to the adhesive layer.

2. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: The mass ratio of the polyester polyol mixture to the polyether polyol mixture is 70:30 to 50:

50.

3. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: The polyester polyol mixture is selected from one or more of poly(1,4-butanediol adipate), poly(1,6-hexanediol adipate), polycaprolactone diol, or polydodecanoic acid diol; its number average molecular weight is 2000-4000 g / mol, and its melting point is 40-60℃.

4. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: The polyether polyol mixture is selected from one or more of polytetrahydrofuran ether diol, polypropylene glycol, or tetrahydrofuran-propylene oxide copolyether diol; its number average molecular weight is 1000-3000 g / mol, and its glass transition temperature is below -60℃.

5. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: The isocyanate is selected from liquefied diphenylmethane diisocyanate.

6. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: The tackifying resin is selected from hydrogenated rosin glycerol ester, hydrogenated rosin pentaerythritol ester, or hydrogenated terpene resin, and its softening point is 85-125℃; the catalyst is an organic bismuth catalyst; and the dehydrating agent is p-toluenesulfonyl isocyanate.

7. The single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking according to claim 1, characterized in that: It also includes fillers, which are ultrafine precipitated barium sulfate or fumed silica.

8. The low-temperature flocking single-component solvent-free polyurethane hot melt adhesive according to any one of claims 1-7, characterized in that: The hot melt adhesive has a melt viscosity of ≤ 8,000 mPa·s at 110°C and has coatable fluidity in the temperature range of 90-120°C.

9. A method for preparing a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Dehydration pretreatment: The polyester polyol mixture, polyether polyol mixture and tackifying resin are put into a reaction vessel, heated to 110-130℃ to melt, and dehydrated for 1-3 hours under vacuum degree ≤-0.095 MPa, controlling the moisture content <300 ppm; S2. Prepolymerization reaction: Cool to 70-80℃, add the liquefied diphenylmethane diisocyanate under dry nitrogen protection, control the reaction temperature at 80-95℃, react for 1.5-3.0 hours, and carry out chain extension reaction to generate terminal-NCO-based prepolymer. S3. Blending and homogenization: Cool to 70-80℃, add the catalyst, antioxidant and dehydrating agent, and mix thoroughly and evenly under high-speed dispersion. S4. Dehydration and Filling: The mixture is transferred to a dehydration / degassing device to remove air bubbles, trace amounts of moisture, and low-boiling substances under high vacuum. Finally, under absolute dry nitrogen protection, the finished product is filled into moisture-proof aluminum foil bags or composite material tubes.

10. The application of a single-component solvent-free polyurethane hot melt adhesive for low-temperature flocking as described in claim 1 in the electrostatic flocking process of a heat-sensitive substrate, characterized in that: The heat-sensitive substrate is cow suede, genuine leather, or microfiber synthetic leather; the application temperature is 90-120℃.