Polyurethane hot melt adhesive for use in flame-retardant fabric composite and preparation method thereof

Abstract

The application provides a kind of polyurethane hot melt adhesive for flame-retardant fabric composite used in clothing and a preparation method thereof, which is prepared by the reaction of the following components in parts by weight: 35-60 parts of flame-retardant polyether polyol; 10-30 parts of polyester polyol; 8-18 parts of diisocyanate; 3-10 parts of intramolecular reaction type flame retardant; 5-12 parts of tackifying resin; 0.1-0.8 parts of hydrolysis-resistant stabilizer; 0.03-0.3 parts of catalyst; and 0.1-0.5 parts of antioxidant. The intramolecular reaction type flame retardant is a halogen-free DOPO-based phosphorus-containing diol oligomer, which is the only reactive flame retardant in the system. In the preparation process of the polyurethane hot melt adhesive, the NCO / OH molar ratio of the system is controlled to be 1.5-2.2, and the free NCO mass content of the finished product at the reaction endpoint is limited to 1.5%-2.5%. The application achieves efficient halogen-free flame-retardant effect, does not produce toxic hydrogen halide and thick smoke during combustion, and greatly improves the ecological safety and fire protection ability of clothing worn close to the body.

Description

Technical Field

[0001] This invention relates to the field of textile flame-retardant composite adhesives, and in particular to a polyurethane hot melt adhesive for use in the composite of flame-retardant fabrics for clothing and its preparation method. Background Technology

[0002] In the textile industry, especially in flame-retardant workwear for high-risk environments such as fire fighting, petrochemicals, and power line inspection, as well as in high-end flame-retardant home textiles and flame-retardant elastic clothing, the durable flame-retardant protective performance and the comfort of wearing the fabric are the core evaluation indicators. The lamination of multi-layered functional fabrics typically relies heavily on adhesives, and the comprehensive physicochemical properties of the adhesives directly determine the quality and lifespan of the finished composite fabric.

[0003] Currently, adhesives used for laminating flame-retardant clothing fabrics face significant generational limitations in technology. Early solvent-based flame-retardant adhesives, using ethyl acetate or N,N-dimethylformamide (DMF) as solvents and adding flame-retardant powders, contain large amounts of volatile organic compounds (VOCs). This not only causes serious environmental pollution during production and coating but also poses potential safety and health hazards due to trace solvent residues deep within the fabric, making it extremely difficult to pass OEKO-TEX and other eco-textile certifications in the high-end apparel market.

[0004] The subsequent development of water-based flame-retardant adhesives, while solving the VOC exceeding standard problem at its source, suffers from drawbacks. Due to the high surface tension and latent heat of vaporization of water molecules, drying energy consumption is high, and water easily carries adhesive particles deep into the flame-retardant fibers. After drying and curing, this leads to fiber adhesion and flattening, causing the fabric to lose its original fluffiness and resilience, resulting in a severely stiff and rigid feel, and sacrificing the drape and soft touch expected of clothing. To overcome the inherent defects of solvent-based and water-based adhesives, solvent-free reactive moisture-curing polyurethane hot melt adhesives (PUR) with 100% solids content have been introduced into the textile composite field.

[0005] However, conventional PUR adhesive layers do not inherently possess flame-retardant properties. To impart flame-retardant performance, the industry commonly employs physical blending, directly adding halogen-containing (such as decabromodiphenyl ethane) or inorganic phosphorus / nitrogen-based (such as ammonium polyphosphate and melamine cyanurate) flame-retardant powders. This physical blending method presents several deep-seated industry challenges: First, the powder addition often needs to reach over 20% of the total system to achieve the desired flame-retardant rating, leading to a dramatic increase in the adhesive's melt viscosity and extremely poor flowability and uniformity during high-temperature coating. Second, there is a significant thermodynamic compatibility barrier between non-reactive inorganic flame-retardant powders and the organic polyurethane matrix. During daily wear, mechanical friction, UV aging, and repeated industrial washing / dry cleaning, free flame-retardant molecules or particles easily migrate and precipitate from the adhesive layer to the fabric surface, resulting in severe "whitening" or "powdering" phenomena. This causes a precipitous decline in the flame-retardant effect of the fabric after several washes. Based on this, some existing technologies have proposed the idea of ​​introducing reactive flame retardants in the polyurethane synthesis stage.

[0006] For example, some patents disclose the use of halogenated phosphate diols as chain extenders in the synthesis of polyurethane prepolymers. However, such approaches have significant drawbacks: halogenated flame retardants release toxic and highly corrosive hydrogen halide fumes during high-temperature combustion, seriously threatening the lives of personnel at fire scenes and causing secondary corrosion to equipment, which is inconsistent with the current environmentally friendly development trend of green halogen-free flame retardants. Other literature (such as CN105315950A and CN117700680B) has attempted to use partially halogen-free phosphorus-containing reactive monomers, but has failed to systematically address the negative impact of these monomers on the physical and mechanical properties of the polyurethane. In particular, the introduction of large-volume or rigid phosphorus-containing groups leads to a sharp increase in the cohesive energy density of the molecular chain, resulting in an imbalance in the proportion of hard segments in the polyurethane. Macroscopically, this manifests as a high modulus of the adhesive layer and loss of elasticity. Furthermore, due to the failure to control the surface energy of the high-temperature melt, the adhesive is easily and unrestrictedly penetrates into the deep pores of the fabric during the hot-pressing process, resulting in "adhesive seepage" or "adhesive permeation." This causes localized stiff patches to form on the composite fabric, severely compromising the comfort of the garment. In addition, because textiles undergo frequent washing, conventional polyester PUR is highly susceptible to ester bond hydrolysis and chain breakage in humid, hot, and slightly alkaline detergent environments, leading to insufficient long-term wash-resistant peel strength.

[0007] In summary, existing flame-retardant adhesive technologies struggle to achieve a balance among four dimensions: halogen-free reactive bulk flame retardancy, low surface energy impermeability, extremely soft fabric feel, and durable wash-resistant peel strength. A novel molecular structure design and formulation synergy solution is urgently needed. Summary of the Invention

[0008] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing and its preparation method, in order to solve the problem in the prior art of achieving a balance among the four dimensions of halogen-free reactive bulk flame retardancy, low surface energy waterproof adhesive, extremely soft fabric feel, and durable wash-resistant peel strength.

[0009] To achieve the above and other related objectives, the present invention provides a polyurethane hot melt adhesive for use in the lamination of flame-retardant fabrics for clothing, which is prepared by reacting the following components in parts by weight:

[0010] 35-60 parts of flame-retardant polyether polyol;

[0011] 10-30 parts of polyester polyol;

[0012] 8-18 parts of diisocyanate;

[0013] 3-10 parts of intramolecular reactive flame retardant;

[0014] 5-12 parts of tackifying resin;

[0015] Hydrolysis-resistant stabilizer: 0.1–0.8 parts;

[0016] Catalyst 0.03–0.3 parts;

[0017] Antioxidant 0.1–0.5 parts;

[0018] The intramolecular reactive flame retardant is a halogen-free DOPO-based phosphorus-containing diol oligomer, which is the only reactive flame retardant in the system.

[0019] In the preparation process of the polyurethane hot melt adhesive, the NCO / OH molar ratio of the system is controlled to be 1.5 to 2.2, and the mass content of free NCO in the finished product at the reaction endpoint is limited to 1.5% to 2.5%.

[0020] By employing the above-mentioned technical solution, DOPO-based glycol oligomers, through the highly active hydroxyl groups at both ends of their molecules, can undergo a stepwise addition reaction with diisocyanate (-NCO) during the synthesis stage, thereby permanently grafting and embedding them into the polyurethane polymer backbone in the form of covalent bonds, forming block copolymers. On the one hand, this molecular-level chemical anchoring completely eliminates the migration, volatilization, and precipitation of small flame retardant molecules during long-term washing, rubbing, or storage of the fabric, endowing the adhesive layer with long-lasting, wash-resistant flame retardant properties with the same lifespan as the matrix. On the other hand, when exposed to high temperatures or open flames, the DOPO characteristic groups segmented on the polyurethane main chain are the first to undergo thermal decomposition, releasing phosphorus-containing free radicals (such as PO·) with extremely strong free radical quenching effects in the gas phase. These free radicals can efficiently capture and consume the high-energy H· and OH· free radicals required to maintain the combustion chain reaction, playing a significant gas-phase flame-retardant role. At the same time, in the condensed phase, the phosphorus-containing decomposition products act as a strong acid source, promoting the rapid dehydration and carbonization of the polyurethane matrix and the surface of the bonded fabric fibers, forming a dense, hard, intumescent, heat-insulating, and oxygen-barrier carbon layer. The synergy of the gas-phase and solid-phase dual flame-retardant mechanisms achieves a highly efficient halogen-free flame-retardant effect, and the combustion process does not produce toxic hydrogen halides or dense smoke, greatly improving the ecological safety and fire protection capabilities of the clothing worn close to the skin.

[0021] In one embodiment of the present invention, the flame-retardant polyether polyol is a phosphorus-nitrogen synergistic reaction polyether polyol, the number-average molecular weight of which is controlled to be 1500-2000 and the hydroxyl value is 56-75 mg KOH / g.

[0022] By adopting the above technical solution and introducing DOPO-based diol, the soft segment structure of the system was precisely compensated and controlled by physical and mechanical means. Since the core structure of the DOPO molecule contains a biphenyl ring with large steric hindrance and extremely high rigidity, its introduction into the polyurethane backbone will inevitably significantly increase the rigidity and cohesive energy density of the polymer chain. If not controlled, this can easily lead to a high modulus and a stiff feel after the adhesive layer is cured. To neutralize this rigidity effect, this invention defines flame-retardant polyether polyols with specific number-average molecular weights (1500–2000) and specific hydroxyl values. Polyether polyols within this parameter range not only provide soft segments rich in phosphorus and nitrogen synergistic flame-retardant elements, but their appropriate molecular chain length also endows the polyurethane network with sufficient chain flexibility and free volume space, cleverly offsetting the hardening negative effect brought by the rigid ring of DOPO. At the same time, the defined hydroxyl value (56–75 mgKOH / g) ensures the uniformity of the crosslinking network density, so that the crosslinked and cured adhesive layer exhibits excellent low modulus, high elongation at break, and high resilience characteristics. From the root of the microscopic molecular structure, this ensures the soft feel and high elasticity of the composite fabric, avoiding local hardening and brittleness.

[0023] In one embodiment of the present invention, the number average molecular weight of the halogen-free DOPO-based phosphorus-containing diol oligomer is 400-800, the mass content of phosphorus in its structure is not less than 8.5%, and it is a high-viscosity liquid or semi-solid at room temperature.

[0024] In one embodiment of the present invention, the tackifying resin is a low softening point hydrogenated petroleum tackifying resin with a softening point range of 80°C to 100°C.

[0025] By adopting the above technical solution, the low softening point hydrogenated petroleum resin has a fully aliphatic cyclic or chain-like nonpolar structure, without polar groups such as hydroxyl and amino groups, and exhibits moderate microphase separation characteristics in the strongly polar hard segment matrix of polyurethane. This thermodynamic microphase separation significantly reduces the surface free energy of the hot melt adhesive in the high-temperature molten coating state on a macroscopic scale. When the low surface energy adhesive is transferred or sprayed onto the surface of porous flame-retardant fibers (such as flame-retardant cotton or aramid) through an anilox roller, it can form an optimized contact angle on the fiber surface. This allows it to quickly spread and wet the substrate surface to form a strong initial physical anchoring point. Furthermore, because the system maintains suitable melt tension and cohesion, it effectively resists the capillary effect that pulls the adhesive into the deep pores of the fabric.

[0026] In one embodiment of the present invention, the hydrolysis-resistant stabilizer is a monomeric polycarbodiimide or a polymeric polycarbodiimide, used to capture the carboxyl chain scission generated by the hydrolysis of polyester polyol to form a stable N-acylurea derivative.

[0027] By adopting the above technical solution, it is found that textile composite materials need to undergo frequent weakly alkaline industrial washing or household high-temperature washing in daily use. The polyester polyol ester bonds in the polyurethane structure, which provide the initial adhesive strength, are prone to hydrolysis and chain breakage under humid and hot environments, generating terminal hydroxyl and terminal carboxyl groups. The newly generated carboxyl groups will act as protic acid catalysts, further accelerating the autocatalytic hydrolysis of the remaining ester bonds, leading to an avalanche-like decline in the adhesive interface strength. The polycarbodiimide molecule contains highly reactive carbodiimide (-N=C=N-) groups, which, as a highly efficient acid scavenger, can keenly capture and react irreversibly with the trace amounts of terminal carboxyl groups generated in the early stage of hydrolysis to generate highly stable N-acylurea derivatives.

[0028] In one embodiment of the present invention, the diisocyanate is selected from one or more mixtures of 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, isoflurone diisocyanate or hexamethylene diisocyanate.

[0029] In one embodiment of the present invention, the polyester polyol is a microcrystalline polyester polyol formed by the condensation polymerization of adipic acid with neopentyl glycol and / or 1,6-hexanediol, and has a number-average molecular weight of 2000 to 3500.

[0030] A method for preparing a polyurethane hot melt adhesive for use in the flame-retardant fabric lamination for clothing as described above includes the following steps:

[0031] S1. Dehydration treatment: The flame-retardant polyether polyol, polyester polyol, and tackifying resin are added to a reaction vessel and stirred for 1.5h to 2.5h under conditions of 105℃~120℃ and vacuum degree ≤-0.09MPa until the moisture content of the system is less than 0.05% by mass.

[0032] S2, Prepolymerization reaction: Remove the vacuum and fill with inert gas for protection, cool the system to 70℃~85℃, add the diisocyanate, and stir the reaction for 1h~1.5h.

[0033] S3. Chain extension and flame retardant reaction: Add the preheated and dehydrated halogen-free DOPO-based phosphorus-containing diol oligomer, hydrolysis-resistant stabilizer, antioxidant and catalyst, and continue the reaction at a constant temperature of 85℃~95℃ for 1.5h~2.5h until the mass content of free NCO in the system reaches the target range of 1.5%~2.5%;

[0034] S4. Degassing and Discharge: Stop heating and remove tiny bubbles from the reaction system for 30-45 minutes under a vacuum of ≤-0.09MPa. Then, seal and discharge the material in a moisture-free environment.

[0035] In one embodiment of the present invention, in step S3, the heating rate of the isothermal reaction process is controlled at 1.5℃ / min to 2.5℃ / min to prevent excessive local heat release from causing gelation of the system.

[0036] By adopting the above technical solution, the urea-urea side reaction between residual moisture in the system and highly reactive isocyanate can be effectively avoided, thus ensuring the purity of the prepolymer. More importantly, placing the sterically hindered DOPO diol oligomers in the chain extension stage with slow heating ensures that these rigid monomers with large steric hindrance can overcome the steric barrier and be fully and uniformly grafted to the end of the polyurethane prepolymer main chain or side chain position. This avoids phase separation or agglomeration caused by excessively rapid local reactions, resulting in a stable and controllable reaction process and high polymer conversion rate.

[0037] An application of a polyurethane hot melt adhesive for laminating flame-retardant fabrics for clothing, as described above, is used for laminating flame-retardant workwear fabrics, flame-retardant home textile fabrics, flame-retardant elastic clothing, or aramid fabrics. The coating amount of the polyurethane hot melt adhesive is 10g / m² to 25g / m². After the laminated fabric undergoes 30 standard industrial washes, the peel strength retention rate of the adhesive layer is ≥85%.

[0038] As described above, the polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing and its preparation method of the present invention have the following beneficial effects:

[0039] 1. Through molecular-level block intercalation of specific DOPO-based phosphorus diol oligomers, true halogen-free bulk reactive long-lasting flame retardancy is achieved. The combustion process exhibits high gas-solid dual-phase flame retardant synergy, low smoke and non-toxicity, meeting stringent safety requirements for high-end special tooling.

[0040] 2. Through a triple physicochemical intervention—specific hydroxyl-value polyether soft segment flexibility compensation, low surface energy microphase separation of hydrogenated petroleum resin, and stringent NCO prepolymer molecular weight locking—the penetration channels of the adhesive into the deep capillaries of the fabric are locked at the micro-rheological scale. After the adhesive is applied to the surface of the flame-retardant fibers, it quickly establishes cohesive shaping without compressing the fiber gaps, giving the composite fabric an extremely excellent soft and supple drape, high breathability, and high elasticity.

[0041] 3. The precise introduction of polycarbodiimide anti-hydrolysis agent, combined with the high-density interfacial chemical covalent bonding of appropriate amount of polar free isocyanate groups, greatly enhances the adhesive layer's ability to resist aging and peeling under humid and slightly alkaline washing environment. The interfacial adhesion strength retains a rate of over 85% after vigorous washing and rubbing, making it fully adaptable to the extremely harsh combat and washing environments such as military protective clothing, heavy industrial work clothes, and professional fire-fighting clothing, and possessing extremely high industrial application value. Detailed Implementation

[0042] 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.

[0043] Example 1:

[0044] This invention provides a polyurethane hot melt adhesive for use in the lamination of flame-retardant fabrics for clothing, which is prepared by reacting the following components in parts by weight:

[0045] 35-60 parts of flame-retardant polyether polyol;

[0046] 10-30 parts of polyester polyol;

[0047] 8-18 parts of diisocyanate;

[0048] 3-10 parts of intramolecular reactive flame retardant;

[0049] 5-12 parts of tackifying resin;

[0050] Hydrolysis-resistant stabilizer: 0.1–0.8 parts;

[0051] Catalyst 0.03–0.3 parts;

[0052] Antioxidant 0.1–0.5 parts;

[0053] The intramolecular reactive flame retardant is a halogen-free DOPO-based phosphorus-containing diol oligomer, which is the only reactive flame retardant in the system.

[0054] During the preparation of polyurethane hot melt adhesive, the NCO / OH molar ratio of the system is controlled at 1.5–2.2, and the free NCO content in the final product at the reaction endpoint is limited to 1.5%–2.5%.

[0055] Flame-retardant polyether polyols are phosphorus-nitrogen synergistic reaction polyether polyols, with their number-average molecular weight controlled to 1500-2000 and hydroxyl value of 56-75 mg KOH / g.

[0056] The number average molecular weight of halogen-free DOPO-based phosphorus-containing diol oligomers is 400-800, the mass content of phosphorus in their structure is not less than 8.5%, and they are in a high-viscosity liquid or semi-solid state at room temperature.

[0057] The tackifying resin is a low-softening-point hydrogenated petroleum tackifying resin, with a softening point range of 80℃~100℃.

[0058] The hydrolysis-resistant stabilizer is a monomeric polycarbodiimide or a polymeric polycarbodiimide, used to capture the carboxyl chain scission generated by the hydrolysis of polyester polyols to form stable N-acylurea derivatives.

[0059] The diisocyanate is selected from one or more mixtures of 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, isoflurone diisocyanate, or hexamethylene diisocyanate.

[0060] Polyester polyols are microcrystalline polyester polyols formed by the condensation polymerization of adipic acid with neopentyl glycol or 1,6-hexanediol, with a number-average molecular weight of 2000 to 3500.

[0061] Example 2:

[0062] This embodiment provides a method for preparing a polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing, including the following steps:

[0063] S1. Dehydration treatment: The flame-retardant polyether polyol, polyester polyol, and tackifying resin are added to a reaction vessel and stirred for 1.5h to 2.5h under conditions of 105℃~120℃ and vacuum degree ≤-0.09MPa until the moisture content of the system is less than 0.05% by mass.

[0064] S2, Prepolymerization reaction: Remove the vacuum and fill with inert gas for protection, cool the system to 70℃~85℃, add the diisocyanate, and stir the reaction for 1h~1.5h.

[0065] S3. Chain extension and flame retardant reaction: Add the preheated and dehydrated halogen-free DOPO-based phosphorus-containing diol oligomer, hydrolysis-resistant stabilizer, antioxidant and catalyst, and continue the reaction at a constant temperature of 85℃~95℃ for 1.5h~2.5h until the mass content of free NCO in the system reaches the target range of 1.5%~2.5%;

[0066] S4. Degassing and Discharge: Stop heating and remove tiny bubbles from the reaction system for 30-45 minutes under a vacuum of ≤-0.09MPa. Then, seal and discharge the material in a moisture-free environment.

[0067] In step S3, the heating rate of the isothermal reaction process is controlled at 1.5℃ / min to 2.5℃ / min to prevent excessive local heat release from causing gelation of the system.

[0068] Example 3:

[0069] This embodiment provides a polyurethane hot melt adhesive for use in flame-retardant fabric lamination for clothing, comprising the following components by weight:

[0070] Flame-retardant polyether polyol 45 parts; polyester polyol 20 parts; diisocyanate (MDI) 12 parts; halogen-free DOPO-based phosphorus-containing diol oligomer 6 parts; hydrogenated petroleum tackifying resin 8 parts; polycarbodiimide hydrolysis-resistant stabilizer 0.5 parts; antioxidant 1010 0.3 parts; catalyst DMDEE 0.05 parts.

[0071] Its preparation method is carried out according to the following steps:

[0072] (1) Dehydration treatment: 45 parts of flame-retardant polyether polyol, 20 parts of polyester polyol, and 8 parts of hydrogenated petroleum tackifying resin were sequentially added to a reactor equipped with an anchor stirrer, a precision temperature control jacket, and a vacuum pump. The temperature was raised to 110°C, and the mixture was kept at a constant temperature and stirred for 2 hours under a vacuum of -0.095 MPa. The Karl Fischer method was used to test the moisture content of the system, and the moisture content had been reduced to 0.03%.

[0073] (2) Prepolymerization reaction: The vacuum in the reactor was released and high-purity nitrogen was introduced for protection. Cooling was started to stabilize the material temperature to 80°C. Then, 12 parts of preheated and molten MDI were quickly added. The theoretical molar ratio of NCO / OH in the system was calculated to be approximately 1.85. Under the protection of nitrogen micro-positive pressure, the reaction was carried out at a speed of 150 r / min for 1.2 hours to form NCO-terminated prepolymer.

[0074] (3) Chain extension and flame retardant reaction: 6 parts of DOPO-based glycol oligomer preheated and dehydrated in a vacuum oven, 0.5 parts of polycarbodiimide, 0.3 parts of antioxidant, and 0.05 parts of catalyst were simultaneously added to the reactor. The heating rate was controlled at 2.0℃ / min, and the temperature was gradually increased to 90℃. The reaction was continued at a constant temperature with stirring for 1.5 hours. Samples were taken every 15 minutes during the reaction, and the free NCO mass fraction was tested by di-n-butylamine back titration until the test results stabilized within the theoretical endpoint range of 2.1%.

[0075] (4) Degassing and discharge: Stop the jacket heating, restart the vacuum pump to a vacuum level of -0.095MPa, and perform high vacuum degassing treatment on the reaction system for 40 minutes to remove microbubbles. Then, under a tight air and moisture barrier, pour the molten transparent adhesive into the aluminum foil composite tube, seal and store for later use.

[0076] In summary, this invention integrates halogen-free DOPO reactive block flame retardant, surface energy-controlled seepage prevention system, ultra-strong hydrolysis stabilization and precise polymerization parameters to achieve a highly efficient halogen-free flame retardant effect. The combustion process does not produce toxic hydrogen halides or dense smoke, greatly improving the ecological safety and fire protection capabilities of clothing worn close to the skin.

[0077] Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0078] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A polyurethane hot melt adhesive for use in the composite of flame-retardant fabrics for clothing, characterized in that, It is prepared by reacting the following components in parts by weight: 35-60 parts of flame-retardant polyether polyol; 10-30 parts of polyester polyol; 8-18 parts of diisocyanate; 3-10 parts of intramolecular reactive flame retardant; 5-12 parts of tackifying resin; Hydrolysis-resistant stabilizer: 0.1–0.8 parts; Catalyst 0.03–0.3 parts; Antioxidant 0.1–0.5 parts; The intramolecular reactive flame retardant is a halogen-free DOPO-based phosphorus-containing diol oligomer, which is the only reactive flame retardant in the system. In the preparation process of the polyurethane hot melt adhesive, the NCO / OH molar ratio of the system is controlled to be 1.5 to 2.2, and the mass content of free NCO in the finished product at the reaction endpoint is limited to 1.5% to 2.5%.

2. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The flame-retardant polyether polyol is a phosphorus-nitrogen synergistic reaction polyether polyol, with its number-average molecular weight controlled to be 1500-2000 and its hydroxyl value to be 56-75 mg KOH / g.

3. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The number-average molecular weight of the halogen-free DOPO-based phosphorus-containing diol oligomer is 400-800, the mass content of phosphorus in its structure is not less than 8.5%, and it is a high-viscosity liquid or semi-solid at room temperature.

4. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The tackifying resin is a low softening point hydrogenated petroleum tackifying resin with a softening point range of 80℃ to 100℃.

5. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The hydrolysis-resistant stabilizer is a monomeric polycarbodiimide or a polymeric polycarbodiimide, used to capture the carboxyl chain scission generated by the hydrolysis of polyester polyol to form a stable N-acylurea derivative.

6. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The diisocyanate is selected from one or more mixtures of 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, isoflurone diisocyanate, or hexamethylene diisocyanate.

7. The polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 1, characterized in that: The polyester polyol is a microcrystalline polyester polyol formed by the condensation polymerization of adipic acid with neopentyl glycol or 1,6-hexanediol, with a number-average molecular weight of 2000-3500.

8. A method for preparing a polyurethane hot melt adhesive for use in composite flame-retardant fabrics for clothing as described in any one of claims 1-7, characterized in that: Includes the following steps: S1. Dehydration treatment: The flame-retardant polyether polyol, polyester polyol, and tackifying resin are added to a reaction vessel and stirred for 1.5h to 2.5h under conditions of 105℃~120℃ and vacuum degree ≤-0.09MPa until the moisture content of the system is less than 0.05% by mass. S2, Prepolymerization reaction: Remove the vacuum and fill with inert gas for protection, cool the system to 70℃~85℃, add the diisocyanate, and stir the reaction for 1h~1.5h. S3. Chain extension and flame retardant reaction: Add the preheated and dehydrated halogen-free DOPO-based phosphorus-containing diol oligomer, hydrolysis-resistant stabilizer, antioxidant and catalyst, and continue the reaction at a constant temperature of 85℃~95℃ for 1.5h~2.5h until the mass content of free NCO in the system reaches the target range of 1.5%~2.5%; S4. Degassing and Discharge: Stop heating and remove tiny bubbles from the reaction system for 30-45 minutes under a vacuum of ≤-0.09MPa. Then, seal and discharge the material in a moisture-free environment.

9. The method for preparing the polyurethane hot melt adhesive for use in flame-retardant fabrics for clothing according to claim 8, characterized in that: In step S3, the heating rate of the isothermal reaction process is controlled at 1.5℃ / min to 2.5℃ / min to prevent excessive local heat release from causing gelation of the system.

10. An application of a polyurethane hot melt adhesive for use in the composite of flame-retardant fabrics for clothing as described in any one of claims 1-7, characterized in that: For use in the lamination of flame-retardant workwear fabrics, flame-retardant home textile fabrics, flame-retardant elastic clothing or aramid fabrics, the coating amount of the polyurethane hot melt adhesive is 10g / m² to 25g / m², and the laminated fabric retains ≥85% of the adhesive layer peel strength after 30 standard industrial washes.