Polyurethane adhesives, polyurethane elastomers, and methods of making and using the same

Abstract

The present invention provides a polyurethane adhesive, comprising a raw material and a curing agent; the raw material comprises a first polyurethane resin and / or a second polyurethane resin; or the raw material comprises a first polyurethane resin and / or a second polyurethane resin and an unsaturated reactive monomer. The present invention provides a preparation method of the polyurethane adhesive, a polyurethane elastomer prepared by the method and an application of the polyurethane adhesive.

Description

Technical Field

[0001] This invention relates to the technical field of polyurethane adhesives, specifically to a free radical curing polyurethane adhesive, its preparation method, and its application, particularly in the preparation of composite solid propellants. Background Technology

[0002] Polyurethane adhesives are adhesives whose main molecular chain contains urethane groups (-NHCOO-) or isocyanate groups (-NCO). Due to the presence of urethane groups in their molecular structure, polyurethane macromolecules can form hydrogen bonds between themselves or with the adherends, resulting in strong intermolecular cohesion. Furthermore, the presence of flexible segments in the polyurethane adhesive molecular chain gives it outstanding bonding strength, abrasion resistance, high elasticity, impact resistance, oil resistance, and vibration resistance, making it widely used in various fields.

[0003] Polyurethane adhesives can be classified into polyisocyanate adhesives, two-component polyurethane adhesives, and one-component adhesives. All of these adhesives are cured based on the high reactivity of isocyanate groups. However, due to the high reactivity of isocyanate groups, their sensitivity to moisture, high toxicity, and the frequent presence of toxic solvents, their use requires careful ventilation, and they must be stored in a waterproof and light-protected environment. This limits their application in certain specific fields, such as composite solid propellants.

[0004] Currently used adhesives for composite solid propellants are generally polyurethane adhesives cured based on a hydroxyl / isocyanate curing system. Because isocyanates react with moisture in the environment or raw materials, this not only affects the isocyanate / hydroxyl ratio in the propellant adhesive, but also generates carbon dioxide gas. This reaction can create pores in the propellant grain, severely impacting the quality of the propellant loading. Therefore, stringent requirements are placed on the dryness of raw materials and the humidity of the loading environment, hindering the application of easily hygroscopic high-energy materials such as AND and NH4NO3 in propellant loading production. Furthermore, during propellant loading, the two active groups (-OH, -NCO) must be strictly equal in quantity; even slight fluctuations in metering can significantly affect the quality of the loading. Therefore, developing a novel adhesive curing system with a curing reaction process unaffected by moisture and with insensitive formulation metering is an effective way to improve the production quality of composite solid propellant loading.

[0005] In summary, it is necessary to select appropriate unsaturated polyurethane resins, small molecule monomers, and curing agents, and adjust the adhesive formulation to prepare unsaturated polyurethane adhesives with excellent performance, stable storage, and controllable curing process. This will improve the curing process and performance of polyurethane adhesives, enabling their application in the production of propellant loading for highly hygroscopic materials. Summary of the Invention

[0006] To overcome the problems existing in the application of existing polyurethane adhesive systems in the field of solid propellants, this invention provides a free radical curing polyurethane adhesive that retains the original structure and excellent mechanical properties of polyurethane resin, while avoiding the influence of environmental humidity and raw material moisture on the adhesive curing process. It is also simple to cure and easy to operate.

[0007] The present invention also provides a method for preparing the polyurethane adhesive.

[0008] The present invention also provides an elastomer obtained after the polyurethane adhesive is cured.

[0009] The present invention also provides an application of the polyurethane adhesive.

[0010] This invention is achieved through the following technical solutions. A polyurethane adhesive, comprising raw materials and a curing agent; The raw materials include a first polyurethane resin and / or a second polyurethane resin; or The raw materials include a first polyurethane resin and / or a second polyurethane resin, as well as unsaturated reactive monomers; The structure of the first polyurethane resin is as follows: Where 1≤x≤10, 0≤y≤20, 0≤n≤20; The structure of the second polyurethane resin is as follows: Where, 0≤a≤20, 1≤b≤10, 0≤c≤20, 0≤p≤20; R1 is one of poly(diethylene adipate), polybutadiene, polyhexane adipate, polybutylene adipate, polycaprolactone, polybutadiene, polyethylene glycol, polypropylene oxide, or polytetrahydrofuran. R2 is , , , , , , , , , , , , , or One of them; R3 is , , , , or One of them; R4 is , , , or One of them, where 2≤q≤10; R5 is one of -H, -CH3, -CH2CH3, -CH2CH2CH3 or -CH2CH2CH2CH3; R6 is , , , or One of them.

[0011] The amount of the curing agent added is 0.5%-3.5 wt% of the raw material; The unsaturated reactive monomer accounts for 0-50 wt% of the weight of the raw material.

[0012] The unsaturated reactive monomers include at least one of ethyl acrylate, butyl acrylate, methyl acrylate or methyl methacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, epoxy resin diacrylate, diallyl phthalate, butanediol dimethacrylate, vinyltoluene, methyl methacrylate, hydroxyethyl acrylate, tetraethylene glycol dimethacrylate, dipropylene glycol diacrylate, diethylene glycol dimethacrylate, pentaerythritol triacrylate, triallyl cyanurate, trimethylolpropane trivinyl acrylate, isooctyl acrylate or trihydroxypropane tetraacrylate.

[0013] The raw materials also include saturated non-reactive monomers; The saturated non-reactive monomer accounts for 0-50 wt% of the raw material.

[0014] The saturated non-reactive monomer includes at least one of dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, diisooctyl sebacate, dioctyl adipate, dibutyl sebacate, di-n-butyl adipate, nitroglycerin, ethylene glycol dinitrate, glycerol trinitrate, trimethylolethane trinitrate, diethylene glycol dinitrate, triethylene glycol dinitrate, ethylene glycol diazidoacetate, or glycidyl azidoglycerin polyether.

[0015] The curing agent includes a free radical initiator; or The curing agent includes a free radical initiator and an accelerator; The free radical initiator includes at least one of methyl ethyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide, tert-butyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, dodecyl peroxide, tert-butyl peroxide, and dicumyl peroxide. The accelerator includes at least one of cobalt isooctanoic acid, potassium isooctanoic acid, copper isooctanoic acid, cobalt cycloalkane organic acid, potassium cycloalkane organic acid, copper cycloalkane organic acid, N,N-dimethylaniline, and N,N-diethylaniline.

[0016] A method for preparing the polyurethane adhesive includes the step of mixing raw materials with a curing agent.

[0017] A polyurethane elastomer is obtained by removing air bubbles from the polyurethane adhesive under vacuum and then heating and curing it.

[0018] An application of the aforementioned polyurethane adhesive in the preparation of composite solid propellants.

[0019] The polyurethane adhesive accounts for 10%-30% of the mass of the composite solid propellant.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The free radical curing unsaturated polyurethane adhesive provided by the present invention can adjust the structure and composition of each raw material according to the usage requirements to form an adhesive system with controllable structure and performance. It is simple to operate and has wide applicability.

[0021] (2) The preparation method of free radical curing unsaturated polyurethane adhesive provided by the present invention avoids the influence of humidity during the curing process of polyurethane adhesive, ensures the quality stability of adhesive during the curing process, and at the same time, the required curing time at each temperature can be calculated according to the type of initiator and accelerator, so as to achieve stable and controllable curing and high safety.

[0022] (3) The unsaturated polyurethane adhesive provided by the present invention has suitable viscosity, good mechanical properties and toughness after curing, good low temperature performance, and moderate curing speed and curing temperature, making it suitable for use in composite solid propellants. Detailed Implementation

[0023] Numerous specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and similar modifications can be made by those skilled in the art without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0025] This invention provides a polyurethane adhesive, specifically composed of a first polyurethane resin and / or a second polyurethane resin and a curing agent. Unsaturated reactive monomers can also be added to adjust the properties of the polyurethane adhesive. More specifically, the unsaturated polyurethane resin used in this invention is a polyurethane resin with unsaturated double bonds at the end groups. This polyurethane resin has the following advantages: On one hand, the unsaturated end-group double bonds of the resin undergo polymerization initiated by a free radical initiator during the curing process, thereby achieving the curing of the adhesive system. In the first unsaturated polyurethane resin curing system, the double bonds become the crosslinking points of the cured adhesive, and the molecular weight of the unsaturated polyurethane determines the effective chain length between the crosslinking points of the cured product, further determining the crosslinking density of the cured product. In the second polyurethane resin, i.e., the branched polyurethane resin system, the density of end-group double bonds and the density of branching points jointly determine the crosslinking density of the cured product.

[0026] On the other hand, this unsaturated polyurethane resin is a block copolymer composed of urethane hard segments and soft segments. The urethane hard segments have high polarity and readily form hydrogen bonds, while the soft segments have high flexibility. This creates a microphase separation structure, endowing the cured adhesive with outstanding abrasion resistance, high elasticity, impact resistance, oil resistance, and low-temperature resistance. Therefore, using this unsaturated polyurethane resin retains the excellent properties of polyurethane materials while remaining unaffected by environmental factors and moisture in the raw materials during the curing process. This makes it suitable for use in composite solid propellants in various environments.

[0027] The unsaturated reactive monomers used in this invention serve two purposes. First, they reduce the viscosity of the system, facilitating processing and curing. Since the unsaturated polyurethane resin used in this invention has a high viscosity at room temperature, which is detrimental to processing, adding an appropriate amount of unsaturated monomers can reduce the viscosity of the adhesive system, making it easier to process and use. Second, the unsaturated monomers in the adhesive system can be used to adjust the crosslinking density of the cured product, thereby further controlling the mechanical properties of the cured product. Specifically, when the number of double bonds in the unsaturated monomer is one, only chain extension can occur, and no crosslinking points can be formed, resulting in a decrease in the crosslinking network density. When the number of double bonds in the unsaturated monomer is greater than one, new crosslinking points are formed, increasing the crosslinking density of the cured product. Therefore, the crosslinking network density can be adjusted by controlling the number of double bonds in the added unsaturated monomers. Simultaneously, some unsaturated monomers have longer side groups, which can increase the free volume of the crosslinking network structure and reduce the hardness of the cured product.

[0028] The specific structure of the first polyurethane resin is as follows: Where 1≤x≤10, 0≤y≤20, 0≤n≤20; The structure of the second polyurethane resin is as follows: Where 1≤x≤10, 0≤y≤20, 0≤z≤20, 0≤n≤20, 0≤p≤20; R1 is one of poly(ethylene adipate), polybutadiene, polyhexane adipate, polybutylene adipate, polycaprolactone, polybutadiene, polyethylene glycol, polypropylene oxide, or polytetrahydrofuran. The selected groups are all derived from their corresponding oligomeric diols. The soft-segment molecular chains constituting the unsaturated polyurethane resin have high flexibility and a low glass transition temperature, imparting high low-temperature performance to the material. The selected oligomeric diols have suitable molecular weights and reactivity, enabling them to react fully with diisocyanates.

[0029] R2 is , , , , , , , , , , , , , or One of them; the selected groups are all derived from their corresponding diisocyanates, which, together with low molecular weight diols and triols, form the hard segments of unsaturated polyurethane resins, giving the material high strength. The diisocyanates used are commonly used diisocyanate raw materials on the market and have suitable chemical reactivity.

[0030] R3 is , , , , or One of them; the selected groups are all derived from their corresponding low molecular weight diols, which together with diisocyanates form the hard segment of unsaturated polyurethane resin. The low molecular weight diols used have suitable chemical reactivity and can react fully with diisocyanates.

[0031] R4 is , , , , or One of them, 2≤q≤10; the selected groups are all derived from their corresponding end-capping agents, which contain both hydroxyl groups and unsaturated double bonds, and the double bonds have suitable free radical initiation activity, which can initiate polymerization and achieve curing of unsaturated polyurethane resin.

[0032] R5 is one of -H, -CH3, -CH2CH3, -CH2CH2CH3 or -CH2CH2CH2CH3; the selected groups have a small volume and will not cause significant steric hindrance to the free radical polymerization of the double bond.

[0033] R6 is , , , or One of them. The groups selected above are all derived from their corresponding low molecular weight triols, the branching agents of the second unsaturated polyurethane resin, and the hard segments of the unsaturated polyurethane resin composed of diisocyanate. The low molecular weight diols used are commonly used raw materials on the market and have suitable chemical reactivity.

[0034] In the aforementioned polyurethane adhesive, the unsaturated polyurethane resin primarily determines the mechanical properties of the cured adhesive. The reactive monomers are mainly used to adjust the viscosity of the adhesive and the mechanical properties of the cured adhesive. The specific amount added depends on the required viscosity, performance, and molecular weight and structure of each component. The amount of curing agent added is the same as that used for common free radical initiators. Too low an amount can easily lead to incomplete curing, while too high an amount can cause rapid and intense exothermic reactions, resulting in stress concentration in the cured product, increased shrinkage, and even safety hazards.

[0035] The unsaturated reactive monomers include one of the following: isooctyl acrylate, ethyl acrylate, butyl acrylate, methyl acrylate or methyl methacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, epoxy resin diacrylate, diallyl phthalate, butanediol dimethacrylate, vinyltoluene, methyl methacrylate, hydroxyethyl acrylate, tetraethylene glycol dimethacrylate, dipropylene glycol diacrylate, diethylene glycol dimethacrylate, pentaerythritol triacrylate, triallyl cyanurate, trimethylolpropane trivinyl acrylate, or trihydroxypropane tetraacrylate. The selected unsaturated reactive monomers have a single molecule with at least one double bond, and all double bonds possess a certain degree of free radical polymerization activity, allowing them to participate in the curing reaction of the adhesive system. Simultaneously, the selected reactive monomers have high compatibility with unsaturated polyurethane resins, preventing phase separation, and have low molecular weights, which can reduce adhesive viscosity and benefit the curing process and cured product properties.

[0036] Preferably, the polyurethane adhesive further includes saturated non-reactive monomers; saturated non-reactive monomers not only reduce the viscosity of the adhesive system, but also, after the adhesive has cured, the non-reactive monomers act as plasticizers dispersed in the cured elastomer, increasing the free volume between molecular chains, which can improve the flexibility, impact resistance, and low-temperature performance of the cured product, while reducing the hardness of the polymer. Specifically, the saturated non-reactive monomers include one of the following: dimethyl dicarboxylate, dioctyl phthalate, dibutyl phthalate, diisooctyl sebacate, dioctyl adipate, dibutyl sebacate, di-n-butyl adipate, nitroglycerin, ethylene glycol dinitrate, glycerol trinitrate, trimethylolethane trinitrate, diethylene glycol dinitrate, triethylene glycol dinitrate, ethylene glycol diazidoacetate, or glycidyl azidoglycerin polyether. These monomers exhibit good compatibility with unsaturated polyurethane resins, preventing phase separation during and after curing, which would affect the properties of the cured product. Furthermore, their low molecular weight reduces the viscosity of the adhesive. Additionally, some of the non-reactive monomers possess nitrate ester structures, which can increase the energy density of composite solid propellants when applied in the field of composite solid propellants.

[0037] Specifically, the curing agent is composed of a free radical initiator and an accelerator. The curing agent components used in this invention include a free radical initiator component and an accelerator component, used to initiate free radical polymerization of the end-group double bonds of the unsaturated polyurethane resin and the double bonds of the unsaturated monomers, forming a complete cross-linked network structure and achieving curing of the polyurethane adhesive. The initiator decomposes upon heating to generate a large number of starting free radicals. These starting free radicals act as active centers and undergo chain growth reactions with the double bonds in the system, ultimately achieving curing of the adhesive. The accelerator increases the concentration of active centers (free radicals) in the system, accelerates the initiation and chain growth reactions of the free radical chain reaction, increases the reaction rate, and simultaneously improves the structure and properties of the cured product.

[0038] The free radical initiator includes at least one of methyl ethyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide, tert-butyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, dodecyl peroxide, tert-butyl peroxyvalerate, and dicumyl peroxide.

[0039] The accelerator includes at least one of cobalt isooctanoic acid, potassium isooctanoic acid, copper isooctanoic acid, cobalt cycloalkane organic acid, potassium cycloalkane organic acid, copper cycloalkane organic acid, N,N-dimethylaniline, and N,N-diethylaniline.

[0040] Based on the curing agent provided by this invention, the polyurethane adhesive provided by this invention differs from traditional polyurethane adhesives in its curing mechanism. Traditional polyurethane adhesives cure based on the reaction of isocyanate groups in the system. Isocyanate groups have high reactivity and can react with moisture in the environment and raw material components to generate polyurea, changing the isocyanate to hydroxyl ratio in the system and simultaneously generating carbon dioxide gas, leading to pores in the cured product and affecting its performance. In the polyurethane adhesive system of this invention, the content of isocyanate groups is 0, and curing is based on the unsaturated double bonds in the unsaturated polyurethane resin and unsaturated monomers, unaffected by environmental humidity and raw material moisture.

[0041] This invention also provides a method for preparing the polyurethane adhesive, specifically, mixing a first polyurethane resin and / or a second polyurethane resin with an unsaturated reactive monomer to obtain a mixture; alternatively, a saturated non-reactive monomer may be added. Then, a curing agent is added to the mixture, and the mixture is further mixed to obtain the polyurethane adhesive.

[0042] The present invention also provides a polyurethane elastomer, which is obtained by removing air bubbles from the polyurethane adhesive under vacuum and then heating and curing it.

[0043] The polyurethane adhesive provided by this invention can be applied to the preparation of composite solid propellants. Traditional solid propellants are generally cured based on the condensation reaction of isocyanate groups and hydroxyl groups, which suffers from sensitivity to the environment and raw material moisture, as well as sensitivity to the metering of propellant formulations. To address the problems of current solid propellant adhesives, a free radical curing polyurethane adhesive system is proposed. This system uses polyurethane resin with unsaturated double bonds at the ends as the main resin, combined with appropriate amounts of small molecule monomers (unsaturated reactive monomers and saturated non-reactive monomers) and curing agent components to form a free radical curing polyurethane adhesive system. It utilizes the free radical polymerization mechanism of unsaturated end groups to achieve controllable curing of the adhesive, aiming to solve the problem from a fundamental perspective. The molecular chain structure, branching point, and molecular weight of the main resin in the system directly affect the crosslinking network structure and crosslinking density of the cured product, further influencing its mechanical properties. The small molecule monomers are used to reduce the viscosity of the adhesive system to facilitate subsequent processing and molding, and can also be used to adjust the crosslinking network structure and mechanical properties of the cured product. The curing agent components may include free radical initiators and accelerators to initiate the free radical polymerization of double bonds, allowing for flexible control of the curing speed based on the type and amount of initiator.

[0044] This adhesive system has the advantages of being insensitive to formulation metering and imperceptible to moisture and humidity, which can avoid the impact of metering deviation and environmental humidity on the quality of the propellant. At the same time, the curing speed can be flexibly controlled by the type and amount of initiator, and the structure and amount of unsaturated polyurethane and small molecule monomer components can be adjusted to regulate the mechanical properties of the propellant column and meet the requirements of the type of propellant for process performance, mechanical performance and safety performance.

[0045] Specifically, the polyurethane adhesive accounts for 10%-30% of the mass of the composite solid propellant.

[0046] The present invention will be further described below with reference to specific embodiments.

[0047] The detection methods and equipment involved in the following embodiments are as follows: 1. Nuclear magnetic resonance spectroscopy test: 1 All H-NMR measurements were performed using a Bruker AV2500 NMR spectrometer (Germany), with CDCl3 as the solvent. 2. Molecular weight determination: The molecular weight and molecular weight distribution of the resin were determined using a Waters gel permeation chromatography system. 3. Mechanical property testing: Tested according to national standards GB / T6739-1996, GB / T9286-1998, and GB / T1732-1993 respectively; 4. Fourier Transform Infrared Spectroscopy Analysis: Infrared spectroscopy (FT-IR) was performed on a Nicoleti S10 Fourier Transform Infrared Spectrometer from Thermo Fisher Scientific, USA. Specifically, the sample was prepared using the liquid coating method, and the thin film was tested using attenuated total reflectance.

[0048] 5. Rheological performance testing: The rheological performance was tested using the steady-state rheological performance of the HAKE MARS-60 advanced modular rheometer. The test process used a flat rotor system with a plate gap of 1 mm.

[0049] The present invention will be further described below with reference to specific embodiments.

[0050] Example 1 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1), butyl acrylate (BA), and benzoyl peroxide (BPO). The molecular weight of PUA1 resin is approximately 8000 Da, and the number of double bonds per molecule is 2; the molecular weight of BA is 128, and the number of double bonds per molecule is 1.

[0051] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1 and BA).

[0052] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g of PUA1 resin and 6g of small molecule monomer BA to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0053] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0054] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0055] The structure of PUA1 resin is as follows: In this PUA1 resin, x = 1–3, y = 1–5, and n = 1–3. (Because in the resin, the two types of blocks in parentheses X and Y may be continuous or discontinuous. For example, in the PUA1 monomer resin, the total number of X blocks is approximately 3. If the 3 X blocks are continuous, then x = 3 and n = 1. If the 3 X blocks are not continuous, then in the above resin, 1 < x < 3, 1 < n < 3, and the total number of X blocks is 3. The same applies to Y blocks. Therefore, x / y / n are not given definite values, but rather a range. The same applies below.)

[0056] The structural formula of butyl acrylate (BA) is as follows: Example 2 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1, the same PUA1 resin as in Example 1), butyl acrylate (BA), dipropylene glycol diacrylate (DPGDA), and benzoyl peroxide (BPO). The molecular weight of PUA1 resin is approximately 21,000 Da, with two double bonds per molecule. The molecular weight of dipropylene glycol diacrylate (DPGDA) is 242 Da, with two double bonds per molecule.

[0057] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 20% butyl acrylate (BA), 10% dipropylene glycol diacrylate (DPGDA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1, BA, and DPGDA).

[0058] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g PUA1 resin, 4g small molecule monomer BA and 2g small molecule monomer DPGDA into a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0059] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0060] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0061] The structural formula of dipropylene glycol diacrylate (DPGDA) is as follows: Example 3 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1, the same PUA1 resin as in Example 1), butyl acrylate (BA), dipropylene glycol diacrylate (DPGDA), trimethylolpropane triacrylate (TMPTA), and benzoyl peroxide (BPO). The molecular weight of trimethylolpropane triacrylate (TMPTA) is 296 Da, and the number of double bonds per molecule is 3.

[0062] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 20% butyl acrylate (BA), 5% dipropylene glycol diacrylate (DPGDA), 5% trimethylolpropane triacrylate (TMPTA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1, BA, DPGDA, and TMPTA).

[0063] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g PUA1 resin, 4g small molecule monomer BA, 1g diluted DPGDA and 1g small molecule monomer TMPTA into a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0064] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0065] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0066] The structural formula of trimethylolpropane triacrylate (TMPTA) is as follows: Example 4 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1), butyl acrylate (BA), dipropylene glycol diacrylate (DPGDA), trimethylolpropane triacrylate (TMPTA), isooctyl acrylate (EHA), and benzoyl peroxide (BPO). Isooctyl acrylate (EHA) has a molecular weight of 184 Da and zero double bonds per molecule.

[0067] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 10% butyl acrylate (BA), 5% dipropylene glycol diacrylate (DPGDA), 5% trimethylolpropane triacrylate (TMPTA), 10% isooctyl acrylate (EHA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1, BA, DPGDA, TMPTA, and EHA).

[0068] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g PUA1 resin, 2g small molecule monomer BA, 1g small molecule monomer DPGDA, 1g small molecule monomer TMPTA and 2g small molecule monomer EHA into a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0069] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0070] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0071] This invention investigates the influence of small molecule monomer types on the viscosity and cured properties of polyurethane adhesives through Examples 1, 2, 3, and 4. The results are shown in the table below.

[0072] Table 1. Adhesive viscosity and cured product properties in Examples 1-4 As can be seen from Examples 1-4, the viscosity of the adhesive does not change significantly due to the same proportion of added small molecule monomers. However, as the double bond content in the small molecule monomer combination increases, the elongation at break of the cured product decreases significantly, while the tensile strength gradually increases. This is because the free radical curing polyurethane adhesive system is cured based on the free radical polymerization of double bonds. When the number of double bonds in a small molecule monomer is 1, there are 2 active reaction sites, allowing only chain extension. When the number of double bonds in a single small molecule monomer is greater than 1, crosslinking points can be formed. As the double bond content of the small molecule monomer increases, more crosslinking points are formed after the adhesive cures, resulting in a higher crosslinking density in the cured product, leading to an increase in tensile strength and a decrease in elongation at break.

[0073] Example 5 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA2), butyl acrylate (BA), and benzoyl peroxide (BPO). The molecular weight of PUA2 resin is 13000 Da, and the number of double bonds in a single molecule is 2.

[0074] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA2), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA2 and BA).

[0075] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14 g of PUA2 resin and 6 g of small molecule monomer BA to a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0076] (2) Add 0.2 g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0077] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0078] The structure of PUA2 resin is as follows: In this PUA2 resin, x = 1 to 4, y = 1 to 4, and n = 1 to 4.

[0079] Example 6 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA3), butyl acrylate (BA), and benzoyl peroxide (BPO). The molecular weight of PUA3 resin is 13000 Da, and the number of double bonds in a single molecule is 2.

[0080] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA3), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA3 and BA).

[0081] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14 g of PUA3 resin and 6 g of small molecule monomer BA to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0082] (2) Add 0.2 g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0083] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0084] In this PUA 3 resin, x = 1 to 4, y = 1 to 4, and n = 1 to 4.

[0085] Example 7 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA4), butyl acrylate (BA), and benzoyl peroxide (BPO). The molecular weight of PUA7 resin is 27,000 Da, and the number of double bonds per molecule is 3.4.

[0086] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA4), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1 and BA).

[0087] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14 g of PUA4 resin and 6 g of small molecule monomer BA to a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0088] (2) Add 0.2 g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0089] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0090] In this PUA4 resin, a=1~10, b=1~8, c=1~2, p=1~2.

[0091] Example 8 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA2), unsaturated polyurethane resin (PUA3), butyl acrylate (BA), and benzoyl peroxide (BPO).

[0092] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 50% unsaturated polyurethane resin (PUA2), 20% unsaturated polyurethane resin (PUA3), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA2, PUA3, and BA).

[0093] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 10 g of PUA2 resin, 4 g of PUA3 resin and 6 g of small molecule monomer BA to a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0094] (2) Add 0.2 g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0095] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0096] This invention investigates the effect of unsaturated polyurethane resin structure on the viscosity and cured properties of polyurethane adhesives through Examples 3, 4, 5 and 6. The results are shown in the table below.

[0097] Table 2. Adhesive viscosity and cured product properties in Examples 5-8 As can be seen from the above examples, with the same type and amount of small molecule monomers, the unsaturated polyurethane resin directly affects the viscosity of the adhesive. The viscosity in the table shows that Example 5 > Example 7 > Example 8 > Example 1 > Example 6. This is because the viscosity of the adhesive is simultaneously affected by various factors such as the molecular weight of the resin, the molecular chain structure, and the degree of branching. Since the molecular weight of PUA2 is significantly higher than that of PUA1, the viscosity of Example 4 is significantly greater than that of Example 1. Although the molecular weights of PUA2 and PUA3 are the same, the viscosity of Example 6 is significantly lower than that of Example 5. This is because the soft segment of the PUA2 resin's molecular chain is PPG resin, which has high molecular chain flexibility. In Example 7, although the molecular weight of the polyurethane resin is greater than that of the PUA2 resin, the high degree of branching of the PUA4 resin effectively avoids the entanglement of polymer chain segments, resulting in a lower viscosity in Example 7 than in Example 5.

[0098] Since the added diluents in the above embodiments are all BA, they can only extend the chain during the curing process and cannot form crosslinking points. Therefore, the crosslinking density of the cured adhesive depends on the added unsaturated polyurethane resin. Because the unsaturated polyurethane resin in Example 5 has a higher molecular weight, its crosslinking density is lower, and its elongation at break is greater than that in Example 1. When the molecular weight is the same, due to the low intermolecular interaction between the soft segments of the PUA2 resin molecular chain, its elongation at break and tensile strength are significantly lower than those in Example 5. Although the molecular weight of the polyurethane resin in Example 7 is greater than that of the PUA2 resin, the PUA4 resin has a large number of branching points, which become crosslinking points after curing, increasing the crosslinking density of the cured product and resulting in a lower elongation at break.

[0099] Reference Example 1 The unsaturated polyurethane adhesive in this reference example uses unsaturated polyurethane resin (PUA1), butyl acrylate (BA), and benzoyl peroxide (BPO) as materials, and the curing environment humidity is 60%. The molecular weight of PUA1 resin is 8000 Da, and the number of double bonds per molecule is 2; the molecular weight of BA is 128, and the number of double bonds per molecule is 1.

[0100] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1 and BA).

[0101] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g of PUA1 resin and 6g of small molecule monomer BA to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0102] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0103] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold, place the sample in a constant temperature and humidity phase, set the humidity to 60% and the temperature to 71℃, and cure for 66h to obtain a free radical cured polyurethane adhesive cured product.

[0104] See Example 2 The unsaturated polyurethane adhesive in this reference example uses polypropylene glycol diol (PPG3000), isoflurane diisocyanate (IPDI), 1,4-butanediol (BDO), and dibutyltin dilaurate (DBTB). The molecular weight of the PPG3000 resin is 3000 Da.

[0105] Take 15 g of polypropylene glycol diol (PPG3000), 0.68 g of isophorone diisocyanate (IPDI), 0.32 g of 1,4-butanediol (BDO), and add 0.075 g of dibutyltin dilaurate (DBTB) catalyst. Mix well and pour into a polytetrafluoroethylene mold. Cure at 70°C for 72 h with an ambient humidity of 25% to obtain the cured adhesive.

[0106] See Example 3 The unsaturated polyurethane adhesive in this embodiment uses polypropylene glycol diol (PPG3000), isoflurane diisocyanate (IPDI), 1,4-butanediol (BDO), and dibutyltin dilaurate (DBTB). The molecular weight of PPG3000 resin is 3000 Da.

[0107] Take 15 g of polypropylene glycol diol (PPG3000), 0.68 g of isophorone diisocyanate (IPDI), 0.32 g of 1,4-butanediol (BDO), and add 0.075 g of dibutyltin dilaurate (DBTB) catalyst. Mix well and pour into a polytetrafluoroethylene mold. Cure at 70°C for 72 h with an ambient humidity of 60% to obtain the cured adhesive.

[0108] Table 3 Reference Examples 1-3 Adhesive Viscosity and Cured Product Properties Example 1 and Reference Example 1 have the same adhesive formulation composition, both being free radical curing polyurethane adhesives. Reference Examples 2 and 3 have the same adhesive formulation composition, both being isocyanate-based curing polyurethane adhesive systems. Examples 1 and 2 were cured in a low-humidity environment (20%), while Reference Examples 1 and 3 were cured in a high-humidity environment (60%). The results showed that the mechanical properties of the cured adhesives from Examples 1 and 1 were similar, indicating that ambient humidity did not affect the curing of the free radical curing polyurethane adhesive. However, the elongation at break of the cured adhesive from Reference Example 3 was significantly lower than that from Reference Example 2. This is because during the curing process of the adhesive from Reference Example 3, due to the higher ambient humidity, the isocyanate component in the system reacted with water. Water reacted with isocyanate to generate urea and carbon dioxide. Urea can further react with isocyanate to form biuret branching or crosslinking, which not only affected the ratio of isocyanate to hydroxyl groups in the polyurethane adhesive but also formed a large number of crosslinking points, leading to an increase in the crosslinking density of the cured product and a decrease in the elongation at break.

[0109] Example 9 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA3) and benzoyl peroxide (BPO). The molecular weight of PUA3 resin is approximately 13,000 Da, and the number of double bonds per molecule is 2.

[0110] In this embodiment, the raw materials of the free radical curing polyurethane adhesive, by mass percentage, contain benzoyl peroxide accounting for 1% of the total mass of PUA1.

[0111] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 20g of PUA1 resin to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0112] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0113] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0114] Example 10 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1), dioctyl phthalate, and benzoyl peroxide (BPO). The molecular weight of PUA1 resin is approximately 8000 Da, and the number of double bonds per molecule is 2.

[0115] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA4), 30% dioctyl phthalate and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1 and dioctyl phthalate).

[0116] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g of PUA1 resin and 6g of dioctyl phthalate to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0117] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0118] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0119] Example 11 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA1), butyl acrylate (BA), dioctyl phthalate, and benzoyl peroxide (BPO). The molecular weight of PUA1 resin is approximately 8000 Da, and the number of double bonds in a single molecule is 2.

[0120] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA1), 15% dioctyl phthalate, 15% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA1, BA, and dioctyl phthalate).

[0121] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g PUA1 resin, 3g BA and 3g dioctyl phthalate to a single-necked flask and stir at 30°C for 2 hours to obtain a homogeneous mixture. Then cool the mixture to room temperature.

[0122] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0123] (3) Pour the polyurethane adhesive obtained in step (3) into a polytetrafluoroethylene mold and cure it at 71°C for 66 hours to obtain a free radical cured polyurethane adhesive.

[0124] Table 4. Adhesive viscosity and cured product properties in Examples 9-11 In the above embodiments, both Example 9 and Example 6 used PUA3 as the main resin. Example 6 contained 30% of the active monomer BA, while Example 9 did not add any active monomer. Therefore, the viscosity of the adhesive in Example 9 was significantly higher than that in Example 6. Meanwhile, since BA contains only one unsaturated double bond, it can only undergo chain extension during chain extension and cannot crosslink. This reduces the crosslinking density of the cured product and may also lead to the formation of a large number of suspended chains, reducing the mechanical properties of the cured product. Therefore, the mechanical properties of the cured product in Example 9 were higher than those in Example 6.

[0125] Examples 10, 11, and 1 used the same main resin, PUA1, as in Example 1. In Examples 10, 11, and 1, the diluents were 30% dioctyl phthalate and 15% BA (non-reactive diluent), and 15% dioctyl phthalate and 30% BA, respectively. Because the monomer content was the same, the three formulations had similar viscosities. Meanwhile, since dioctyl phthalate did not participate in the curing reaction but only acted as a plasticizer dispersed in the cured product, its tensile strength decreased significantly, while its elongation at break remained relatively unchanged.

[0126] Example 12 The unsaturated polyurethane adhesive in this embodiment uses unsaturated polyurethane resin (PUA2), butyl acrylate (BA), and benzoyl peroxide (BPO). The molecular weight of PUA2 resin is 13000 Da, and the number of double bonds in a single molecule is 2.

[0127] The raw materials of the free radical curing polyurethane adhesive in this embodiment, by weight percentage, include: 70% unsaturated polyurethane resin (PUA2), 30% butyl acrylate (BA), and benzoyl peroxide (BPO, accounting for 1% of the total mass of PUA2 and BA).

[0128] The raw materials of the composite solid propellant slurry in this embodiment, by mass percentage, include 30% free radical curable polyurethane binder, 60% ammonium perchlorate (AP), 10% Al powder, and N,N-diethylaminoethyl methacrylate as a bonding agent (accounting for 0.1% of the total mass of binder, AP, and Al powder).

[0129] The preparation method of the above-mentioned free radical cured unsaturated polyurethane adhesive and its cured product includes the following steps: (1) Add 14g of PUA2 resin and 6g of small molecule monomer BA to a single-necked flask, stir at 30°C for 2 hours to obtain a homogeneous mixture, and then cool the mixture to room temperature.

[0130] (2) Add 0.2g BPO to the mixture in step (1) and stir quickly until homogeneous to obtain free radical curing polyurethane adhesive.

[0131] (3) Add 40g of ammonium perchlorate (AP), 6.67g of Al powder and bonding agent N,N-dimethylaminoethyl methacrylate (0.1 wt% of the total mass of adhesive, AP and Al powder) to the polyurethane adhesive obtained in step (2), and stir at 30°C for 1 hour to obtain composite solid propellant slurry.

[0132] (4) Pour the composite solid propellant slurry obtained in step (3) into a polytetrafluoroethylene mold, place the sample in a vacuum oven, remove air bubbles under vacuum, and cure at 71°C for 66 hours to obtain a composite solid propellant column.

[0133] The bonding agent, N,N-diethylaminoethyl methacrylate, has the following structural formula: The composite solid propellant slurry prepared in this embodiment has good fluidity and is suitable for the processing and molding of composite solid propellants. The solid propellant grain prepared in this embodiment has an elongation at break of 122% and a tensile strength of 0.69 MPa, which can meet the requirements for the use of solid propellants.

Claims

1. A polyurethane adhesive, characterized in that: Including raw materials and curing agents; The raw materials include a first polyurethane resin and / or a second polyurethane resin; The structure of the first polyurethane resin is as follows: Where, 1≤x≤10, 0≤y≤20, 0≤n≤20; The structure of the second polyurethane resin is as follows: Where, 0≤a≤20, 1≤b≤10, 0≤c≤20, 0≤p≤20; R1 is one of poly(diethylene adipate), poly(ethylene adipate), poly(butylene adipate), polycaprolactone, polybutadiene, polyethylene glycol, or polytetrahydrofuran. R2 is , , , , , , , , , , , , , or One of them; R3 is , , , , or One of them; R4 is , ,or One of them, where 2≤q≤10; R5 is one of -H, -CH3, -CH2CH3, -CH2CH2CH3, or -CH2CH2CH2CH3; R6 is , , , or One of them.

2. The polyurethane adhesive according to claim 1, characterized in that: The amount of curing agent added is 0.5%-3.5wt% of the raw material.

3. The polyurethane adhesive according to claim 1, characterized in that: The raw materials also include unsaturated reactive monomers; the unsaturated reactive monomers account for 0-50 wt% of the weight of the raw materials. The unsaturated reactive monomers include at least one of ethyl acrylate, butyl acrylate, methyl acrylate, tripropylene glycol diacrylate, epoxy resin diacrylate, diallyl phthalate, butanediol dimethacrylate, vinyltoluene, methyl methacrylate, hydroxyethyl acrylate, tetraethylene glycol dimethacrylate, dipropylene glycol diacrylate, diethylene glycol dimethacrylate, pentaerythritol triacrylate, triallyl cyanurate, trimethylolpropane trivinyl acrylate, isooctyl acrylate, or trihydroxypropane tetraacrylate.

4. The polyurethane adhesive according to claim 1, characterized in that: The raw materials also include saturated non-reactive monomers; the saturated non-reactive monomers account for 0-50 wt% of the raw materials.

5. The polyurethane adhesive according to claim 4, characterized in that: The saturated non-reactive monomer includes at least one of dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, diisooctyl sebacate, dioctyl adipate, dibutyl sebacate, di-n-butyl adipate, nitroglycerin, ethylene glycol dinitrate, glycerol trinitrate, trimethylolethane trinitrate, diethylene glycol dinitrate, triethylene glycol dinitrate, ethylene glycol diazidoacetate, or glycidyl azidoglycerin polyether.

6. The polyurethane adhesive according to claim 1, characterized in that: The curing agent includes a free radical initiator.

7. The polyurethane adhesive according to claim 1, characterized in that: The curing agent includes a free radical initiator and an accelerator; The accelerator includes at least one of cobalt isooctanoic acid, potassium isooctanoic acid, copper isooctanoic acid, cobalt cycloalkane organic acid, potassium cycloalkane organic acid, copper cycloalkane organic acid, N,N-dimethylaniline, and N,N-diethylaniline.

8. The polyurethane adhesive according to claim 6 or 7, characterized in that: The free radical initiator includes at least one of methyl ethyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide, tert-butyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, dodecyl peroxide, tert-butyl peroxyvalerate, and dicumyl peroxide.

9. The method for preparing the polyurethane adhesive according to claim 1, characterized in that: This includes the step of mixing the raw material with a curing agent.

10. A polyurethane elastomer, characterized in that: It is obtained by removing air bubbles from the polyurethane adhesive according to claim 1 and then heating and curing it.

11. The application of the polyurethane adhesive as described in claim 1, characterized in that: It is used in the preparation of composite solid propellants.

12. The application of the polyurethane adhesive as described in claim 11, characterized in that: The polyurethane adhesive accounts for 10%-30% of the mass of the composite solid propellant.

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