CN112778499A - Preparation method of low free meta-xylylene isocyanate polyurethane prepolymer - Google Patents
Preparation method of low free meta-xylylene isocyanate polyurethane prepolymer Download PDFInfo
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- C08G18/7642—Polyisocyanates or polyisothiocyanates cyclic aromatic containing only one aromatic ring containing at least one isocyanate or isothiocyanate group linked to the aromatic ring by means of an aliphatic group containing at least two isocyanate or isothiocyanate groups linked to the aromatic ring by means of an aliphatic group having a primary carbon atom next to the isocyanate or isothiocyanate groups, e.g. xylylene diisocyanate or homologues substituted on the aromatic ring
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Abstract
The invention discloses a preparation method of a low free-space mXDI polyurethane prepolymer, which comprises the following steps: step 1): mixing mXDI, polyhydric alcohol and a high-boiling point solvent, and reacting to obtain a polyurethane prepolymer reaction solution; step 2): and evaporating the polyurethane prepolymer reaction liquid to obtain a light component and a heavy component, wherein the light component comprises the high-boiling-point solvent and mXDI, and the heavy component is low-free mXDI polyurethane prepolymer. According to the invention, through a thin film distillation device, the collected components are polyurethane prepolymer, no solvent is contained, and only a very small amount of free mXDI monomer is contained; the solvent and the redundant monomers used in the invention can be completely recycled, no waste liquid is discharged, and the method is economical and environment-friendly.
Description
Technical Field
The invention belongs to the technical field of prepolymer preparation, and particularly relates to a preparation method of a low-free meta-xylylene isocyanate (mXDI) polyurethane prepolymer.
Background
Polyurethane is a kind of hard-segment and soft-segment alternating multi-block copolymer containing repeated carbamate structure on the molecular chain. The strong polarity of the carbamate group and the hydrogen bond action among polyurethane molecules endow the polyurethane material with the advantages of good toughness, excellent elasticity, wear resistance, solvent resistance, corrosion resistance and the like. The alternating micro-phase separation structure of the soft and hard sections of the polyurethane enables the polyurethane material to be widely applied to the fields of metal industry, artificial intelligence, biomedicine, building engineering, aerospace, new materials and the like.
Diisocyanate is one of the essential main monomer raw materials for synthesizing polyurethane materials, and determines the properties of the polyurethane materials. Currently, the diisocyanate monomers commonly used include Hexamethylene Diisocyanate (HDI), Toluene Diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate (PPDI), 1, 5-Naphthalene Diisocyanate (NDI), m-xylylene isocyanate (mXDI), and the like. Wherein MDI, PPDI and NDI are aromatic diisocyanate, solid at normal temperature and toxic; aliphatic HDI and aromatic TDI are liquid at normal temperature and are toxic; and mXDI is aliphatic diisocyanate containing benzene rings, is liquid at normal temperature and normal pressure and has a boiling point of 88-90 ℃. The mXDI-based novel polyurethane product has the characteristics of almost no yellowing, no toxicity, high adhesion and the like.
Polyurethane prepolymer which is generally adopted when polyurethane is prepared by casting, processing and molding is used as a raw material, and the polyurethane prepolymer is mixed with a chain extender for reaction and then molded. The prepolymer is prepared by reacting an excess of diisocyanate with an oligomeric polyol such as polyester or polyether under certain conditions. The polyurethane prepolymers prepared in this way inevitably contain free diisocyanate monomers, typically in the range of about 5 to 10%. When the isocyanate is used and processed, the residual diisocyanate monomer volatilizes to pollute the polyurethane processing environment, and the diisocyanate which is contacted and absorbed can cause great harm to human bodies. Corresponding environmental protection and sanitation rules are provided at home and abroad, and a certain allowable range is regulated for the concentration of isocyanate existing in the working environment. In addition, during storage, residual diisocyanate is susceptible to hydrolysis, which increases the viscosity of the prepolymer and affects the storage stability of the prepolymer and the properties of the polyurethane. Finally, the high content of diisocyanate monomer can affect the overall properties of polyurethane products such as hard segment content, elasticity, strength, etc.
Low free polyurethane prepolymers offer advantages over conventional polyurethane prepolymers. Firstly, the low free polyurethane prepolymer can greatly improve the operation environment, and is safer and more sanitary; the low free polyurethane prepolymer has lower viscosity and molding processing fluidity; the service life in the kettle is longer, so that the operation time is longer; the curing speed is high, the demolding time is shortened, and the production efficiency can be improved. And secondly, the polyurethane prepared by adopting the low free polyurethane prepolymer has a more regular structure and better dynamic mechanical property.
The currently known methods for removing unreacted diisocyanate monomer from prepolymers include: chemical reaction, extraction, adsorption, thin film evaporation, and short path distillation. Wherein the chemical reaction method, the solvent extraction method and the adsorption method can easily realize that the mass fraction of the monomer is less than 1 percent, but the mass fraction of the free monomer is difficult to realize less than 0.1 percent. Therefore, these several methods are mainly used for preparing conventional polyurethane prepolymers. The film evaporation method and the short-path distillation method can ensure that the material has short retention time in the high-temperature process, and are more favorable for removing the free monomer in the prepolymer. Compared with an extraction method and an adsorption method, the membrane distillation method is simple to operate, high in separation efficiency and more economical. However, with the decrease of the monomer content, the viscosity of the prepolymer increases, the difficulty of monomer removal gradually increases, and the single thin film distillation method and the short path distillation method are difficult to realize the one-time efficient monomer removal.
Based on a thin film evaporation method and a short path distillation method, the equilibrium relation of the concentration of free monomers in the separation process can be improved by introducing an inert solvent with high boiling point, so that the free diisocyanate monomers in the prepolymer can be efficiently removed, and the low-free polyurethane prepolymer can be obtained.
Disclosure of Invention
Aiming at the defects and defects of the prior art, the invention aims to provide a preparation method of low-free m-xylylene isocyanate (mXDI) polyurethane prepolymer, which is simple and can be used for continuous production, and the prepared prepolymer has low free mXDI content.
The technical scheme adopted by the invention for solving the technical problem is as follows:
a method of preparing a low free meta-xylylene isocyanate (mXDI) polyurethane prepolymer comprising:
step 1): mixing mXDI, polyhydric alcohol and a high-boiling point solvent, and reacting to obtain a polyurethane prepolymer reaction solution;
step 2): and evaporating the polyurethane prepolymer reaction liquid to obtain a light component and a heavy component, wherein the light component comprises the high-boiling-point solvent and mXDI, and the heavy component is low-free mXDI polyurethane prepolymer.
In the above method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the molar ratio of the m-xylylene isocyanate to the polyol is 2:1 to 10:1 (e.g., 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1), preferably 2:1 to 6: 1; when the molar ratio is less than 2: at 1, the prepolymer obtained contained more dimer and had an NCO value lower than the theoretical value (i.e., NCO value of mXDI-polyol-mXDI polymer). However, if the molar ratio is greater than 2:1, the dimer component will be reduced and the NCO value of the prepolymer will approach the theoretical value. When the molar ratio exceeds 10:1, the theoretical NCO value can be obtained, but it is difficult to remove too much free mXDI monomer.
In the above-mentioned method for producing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in said step 1), the mass ratio of the sum of the mass of said mXDI and polyol to the high boiling point solvent is 0.5:1 to 2.0:1 (e.g., 0.7:1, 0.9:1, 1.1:1, 1.3:1, 1.5:1, 1.7:1), preferably 0.8 to 1.6; preferably, the high boiling point solvent is a single or mixed high boiling point solvent; if the amount of the solvent is too small, XDI is not distilled off cleanly and remains in a large amount, and if the amount of the solvent is too large, the solvent may remain in the prepolymer.
In the above-mentioned method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the temperature of the reaction is 25 to 80 ℃ (e.g., 30 ℃, 40 ℃, 50 ℃, 60 ℃, 70 ℃) and the time of the reaction is 2 to 10 hours (e.g., 3 hours, 5 hours, 7 hours, 9 hours).
In the above-mentioned method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the reaction is performed with stirring.
In the above method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, the step 1) is performed under nitrogen protection.
In the above preparation method of the low free meta-xylylene isocyanate polyurethane prepolymer, as a preferred embodiment, the step 1) is specifically:
step a): mixing mXDI and a high boiling point solvent to obtain mXDI solution,
step b): and adding polyol or a mixed solution of polyol and a high-boiling point solvent into the mXDI solution, and reacting to obtain a polyurethane prepolymer reaction solution.
In the above-mentioned method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in said step 1) b), said polyol, or a solution of said polyol and a high boiling point solvent is added dropwise to said mXDI solution obtained in step a), wherein said dropwise addition is performed for a period of 1 to 3 hours, and said dropwise addition is performed at a temperature of 20 to 70 ℃.
In the above-mentioned method for preparing a low free mXDI prepolymer, as a preferred embodiment, in said step 1), the boiling point of said high boiling point solvent at normal pressure is 190-300 ℃ (e.g., 200 ℃, 220 ℃, 240 ℃, 260 ℃, 280 ℃); the choice of the high boiling solvent of the present invention is based on the consideration of the interaction force of the mXDI with the solvent at a certain temperature, one of which deviates from raoult's law.
In the above-mentioned method for producing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the high boiling point solvent includes an aliphatic solvent such as methyl benzoate, ethyl benzoate, propyl benzoate, isopropyl benzoate, butyl benzoate, isobutyl benzoate, t-butyl benzoate, dimethyl phthalate, dimethyl isophthalate, diethyl phthalate, diisobutyl phthalate, diethyl isophthalate, diisopropyl succinate, diisopropyl glutarate, diisopropyl adipate, diisobutyl adipate, diisopropyl sebacate, DBE dibasic ester, butyrolactone, valerolactone, triacetin and caprolactone, and other types of solvents, such as one or more of N, N-diethyl acetamide, N-methyl pyrrolidone and sulfolane; preferably, the high boiling point solvent is selected from at least one of dibasic ester DBE, dimethyl phthalate, glyceryl triacetate, diisopropyl adipate and diisobutyl adipate.
In the above method for preparing a low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the polyol is a polyether polyol, a polycarbonate polyol or a polyester polyol; preferably, the polyether polyol is at least one of polypropylene oxide ether polyol, polytetrahydrofuran ether glycol and polypropylene glycol; the polycarbonate polyol is at least one of polyhexamethylene glycol carbonate dihydric alcohol and polypentanediol carbonate dihydric alcohol; the polyester polyol is at least one of polybutylene adipate diol, polyhexamethylene adipate diol, polycaprolactone diol and polyhexamethylene adipate diol; preferably, the polyol is selected from at least one of polytetrahydrofuran polyol PTMEG1000, polycarbonate polyol PCD2000 and polycaprolactone polyol PCL 2000.
In the above-mentioned preparation method of the low free mXDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the molecular weight of the polyol is 500-5000g/mol, preferably 1000-3000 g/mol.
In the above method for producing a low free mXDI polyurethane prepolymer, as a preferred embodiment, the temperature of the evaporation is 80 to 160 ℃ (for example, 80 ℃, 100 ℃, 120 ℃, 140 ℃) and the degree of vacuum (gauge pressure) at the time of the evaporation is 0.1 to 2.0mmHg (for example, 0.1mmHg, 0.2mmHg, 0.4mmHg, 0.6mmHg, 0.8mmHg, 1.0mmHg, 1.2mmHg, 1.4mmHg, 1.6 mmHg).
In the above-mentioned method for producing a low free mXDI polyurethane prepolymer, as a preferred embodiment, the light fraction obtained in step 2) may be used as a raw material in step 1) for recycling.
In the above-mentioned method for producing a polyurethane prepolymer having a low free mXDI content, as a preferred embodiment, in said step 2), said evaporation is conducted in a thin film distillation apparatus.
The reaction formula of the invention is as follows:
compared with the prior art, the invention has the following beneficial effects:
(1) according to the invention, through a thin film distillation device, the collected components are polyurethane prepolymer, no solvent is contained, and only a small amount of free mXDI monomer is contained.
(2) The prepolymer prepared by the method has the advantages of low viscosity, low free isocyanate content, stable performance and long storage period.
(3) The light components collected by the invention are solvent and mXDI, are transparent clear liquid at room temperature, can be repeatedly used, and are used for preparing prepolymer.
(4) The solvent and the redundant monomers used in the invention can be completely recycled, no waste liquid is discharged, and the method is economical and environment-friendly.
Drawings
FIG. 1 is an IR spectrum of a low free mXDI-PTMEG1000 polyurethane prepolymer prepared in example 3 of the present invention.
FIG. 2 is a flow chart of the reaction process of the present invention.
Detailed Description
In order to highlight the objects, technical solutions and advantages of the present invention, the present invention is further illustrated by the following examples, which are presented by way of illustration of the present invention and are not intended to limit the present invention. The technical solution of the present invention is not limited to the specific embodiments listed below, and includes any combination of the specific embodiments.
Determination of the NCO value of the prepolymer: the NCO content of the polyurethane prepolymer is determined by the titration method of di-n-butylamine-hydrochloric acid according to the content of isocyanate groups in the standard polyurethane prepolymer in the HG/T2409-1992 chemical industry, and the same method is adopted in the examples.
Determination of free mXDI in prepolymer: for the low heavy component obtained, i.e. the polyurethane prepolymer, the free mXDI content of the polyurethane prepolymer was determined by gas chromatography. And repeatedly washing a quantitative prepolymer sample in quantitative n-hexane to extract free mXDI. The quantification is carried out by adopting an external standard method, and the detection is carried out by using a gas chromatograph.
Example 1
Melting polytetrahydrofuran polyol PTMEG1000, and keeping the temperature at 60 ℃ for later use; 282.3g of mXDI and 500g of glyceryl triacetate were added to the reaction vessel under nitrogen protection, mixed well with stirring, and the temperature was maintained at 60 ℃. Then, 500g of polyol PTMEG1000 liquid is dropwise added into the reaction vessel under the stirring condition, and the dropwise adding time is 2 hours; after further stirring and reacting for 5h at 60 ℃, mXDI-PTMEG1000 polyurethane prepolymer reaction solution is obtained.
The resulting mXDI polyurethane prepolymer reaction solution was fed into a thin film evaporator at 80 ℃ with the thin film temperature set at 95 ℃, the vacuum degree of 0.1-0.2mmHg, and the condensation temperature of 20-25 ℃, and the light and heavy fraction distillates were collected, respectively, to give a low free mXDI polyurethane prepolymer with an NCO value of 4.86% (theoretical 6.03%). The free mXDI content of the polyurethane prepolymer was determined to be 0.1%. The light component is glyceryl triacetate containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 2
Melting 500g of polytetrahydrofuran polyol PTMEG1000 at 60 ℃, and uniformly mixing with 300g of glyceryl triacetate in another container, keeping the temperature at 25-30 ℃ for later use; under the protection of nitrogen, 282.3g of mXDI and 500g of glyceryl triacetate were added to the reaction vessel, stirred and mixed uniformly, and the temperature was maintained at 25 ℃. The above polyol solution was slowly added to a reaction vessel containing a mXDI solution with stirring and nitrogen for 2 h. After the addition, 200g of glyceryl triacetate was added. After the reaction is continued for 18h at 25 ℃, mXDI-PTMEG1000 polyurethane prepolymer reaction solution is obtained.
The resulting polyurethane prepolymer was fed to a thin film evaporator at 60 ℃ under a film temperature of 100 ℃ under a vacuum of 0.1 to 0.2mmHg and a condensation temperature of 20 to 25 ℃ to isolate a low-free mXDI-PTMEG1000 polyurethane prepolymer having an NCO value of 4.88%. The free mXDI content of the polyurethane prepolymer was determined to be 0.1%. The light component is glyceryl triacetate containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 3
Melting polytetrahydrofuran polyol PTMEG1000, and keeping the temperature at 60 ℃ for later use; under the protection of nitrogen, 376.4g of mXDI and 300g of glyceryl triacetate are added into a reaction vessel, stirred and mixed uniformly, and the temperature is maintained at 70 ℃; then 500g of polytetrahydrofuran polyol PTMEG1000 and 300g of glyceryl triacetate are mixed uniformly in another container and added into the reaction container, and the dropping time is 1.5 h. After the addition, 200g of glyceryl triacetate was added, and the reaction was continued at 70 ℃ for 4 hours to obtain a mXDI-PTMEG1000 polyurethane prepolymer reaction solution.
Adding the obtained polyurethane prepolymer reaction liquid into a film evaporation device at the temperature of 80 ℃, setting the film temperature to be 100 ℃, the vacuum degree to be 0.2-0.5mmHg and the condensation temperature to be 20-25 ℃, separating out low-free mXDI-PTMEG1000 polyurethane prepolymer, and determining the NCO value to be 5.87%; the free mXDI content of the mXDI-PTMEG1000 polyurethane prepolymer was determined to be 0.09%. The light component is glyceryl triacetate solution containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 4
A mixed solution of 800g of the light fraction distilled out in example 3, i.e., triacetin solvent and mXDI was titrated, and 169.5g of mXDI was determined as the light fraction. 207g of mXDI was added to the above light fraction solution at 20-25 ℃ and mixed well with stirring. Then 500g of the melted polytetrahydrofuran polyol PTMEG1000 is mixed with the mXDI solution at 60 ℃ and the liquid adding time is 2 h. After the addition, the reaction was carried out at 60 ℃ for 5 hours to obtain a mXDI-PTMEG1000 polyurethane prepolymer reaction solution.
At 80 deg.c, the obtained polyurethane prepolymer reaction liquid is fed into a film evaporator at 100 deg.c, vacuum degree of 0.2-0.5mmHg and condensing temperature of 20-25 deg.c to separate low-free mXDI-PTMEG1000 polyurethane prepolymer with NCO value of 5.91%. The free mXDI content of the polyurethane prepolymer was determined to be 0.1%. The light component is glyceryl triacetate solution containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 5
Melting polytetrahydrofuran polyol PTMEG1000, and keeping the temperature at 60 ℃ for later use; under the protection of nitrogen, 564.6g of mXDI and 500g of dibasic ester DBE are added into a reaction vessel, stirred and mixed uniformly, and the temperature is maintained at 60 ℃; and then 500g of polytetrahydrofuran polyol PTMEG1000 and 400g of dibasic ester DBE are uniformly mixed in a container and added into the reaction container, and the dropping time is 2 hours. After the addition, 100g of the dibasic ester DBE was added, and the reaction was continued at 60 ℃ for 4.5 hours to obtain a mXDI-PTMEG1000 polyurethane prepolymer reaction solution.
At 80 deg.C, the obtained polyurethane prepolymer is fed into a film evaporator, the film temperature is set at 100 deg.C, the condensation temperature is 20-25 deg.C, the vacuum degree is 0.5-1.0mmHg, the low-free mXDI-PTMEG1000 polyurethane prepolymer is separated, and the NCO value is 5.95%. The free mXDI content of the polyurethane prepolymer was determined to be 0.15%. The light component is dibasic ester DBE containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 6
Melting polytetrahydrofuran polyol PTMEG2000, and keeping the temperature at 60 ℃ for later use; 376.4g of mXDI and 400g of dibasic ester DBE were added to the reaction vessel under nitrogen protection, stirred and mixed well, and the temperature was maintained at 60 ℃. Then 1000g of polytetrahydrofuran polyol PTMEG2000 and 500g of dibasic ester DBE are mixed evenly in another container and then added into the reaction container, and the dropping time is 2.5 h. After the addition, 100g of dibasic ester DBE is added again, and after the reaction for 6 hours at the temperature of 60 ℃, mXDI-PTMEG2000 polyurethane prepolymer reaction solution is obtained.
The resulting polyurethane prepolymer reaction solution was fed into a thin film evaporator at 80 ℃ and the film temperature was set at 105 ℃, the condensation temperature was 20-25 ℃ and the degree of vacuum was 0.5-1.0mmHg, and the low-free mXDI-PTMEG2000 polyurethane prepolymer was isolated and the NCO value was 3.35% (theoretical 3.54%). The free mXDI content of the polyurethane prepolymer was determined to be 0.13%. The light component is dibasic ester DBE containing mXDI, and can be continuously used for preparing the next batch of prepolymer.
Example 7
Melting polycarbonate polyol PCD2000, and keeping the temperature at 60 ℃ for later use; 376.4g of mXDI and 400g of glyceryl triacetate were added to the reaction vessel under nitrogen protection, stirred and mixed well, and the temperature was maintained at 60 ℃. Then 1000g of polycarbonate polyol PCD2000 and 600g of glyceryl triacetate are mixed evenly in another container, and then the mixture is added into the reaction container, and the dropping time is 2.5 h. After the addition, the reaction was carried out at 60 ℃ for 5 hours to obtain a mXDI-PCD2000 polyurethane prepolymer reaction solution.
The resulting polyurethane prepolymer reaction solution was fed into a thin film evaporator at 80 ℃ with the film temperature set at 105 ℃, the condensation temperature at 20-25 ℃ and the vacuum degree at 1.0-1.5mmHg, and the low-free mXDI-PCD2000 polyurethane prepolymer was isolated with an NCO value of 3.31% (theoretical 3.54%). The free mXDI content of the polyurethane prepolymer was determined to be 0.1%. The light component contains mXDI glyceryl triacetate, and can be continuously used for preparing the next batch of prepolymer.
Example 8
Melting polycaprolactone polyol PCL2000, and keeping the temperature at 50 ℃ for later use; under the protection of nitrogen, adding 470.5g of mXDI and 500g of solvent glyceryl triacetate into a reaction vessel, stirring and mixing uniformly, and maintaining the temperature at 50 ℃; then, 1000g of polycaprolactone polyol PCL2000 and 1000g of glyceryl triacetate are uniformly mixed in a reactor and added into the reaction container, and the dripping time is 3 hours; after 6h of reaction at 50 ℃ a mXDI polyurethane prepolymer reaction solution was obtained.
The resulting polyurethane prepolymer reaction solution was fed into a thin film evaporator at 80 ℃ with the film temperature set at 110 ℃, the condensation temperature at 20-25 ℃ and the vacuum at 1.0-1.5mmHg, and the low-free mXDI-PCL2000 polyurethane prepolymer was isolated with NCO value of 3.25% (theoretical 3.54%). The free mXDI content of the polyurethane prepolymer was determined to be 0.1%. The light component contains mXDI glyceryl triacetate, and can be continuously used for preparing the next batch of prepolymer.
Although the present invention has been described in detail herein with reference to the general description and the specific embodiments, the foregoing examples are intended to be illustrative of the invention, and the invention is not limited to the specific embodiments shown, and modifications may be made thereto without departing from the spirit and scope of the invention. It will thus be appreciated that modifications and variations may be made to the invention without departing from the principles thereof, and within the scope of the invention as claimed.
Claims (10)
1. A method for preparing a low free-space mXDI polyurethane prepolymer, which is characterized by comprising the following steps:
step 1): mixing mXDI, polyhydric alcohol and a high-boiling point solvent, and reacting to obtain a polyurethane prepolymer reaction solution;
step 2): and evaporating the polyurethane prepolymer reaction liquid to obtain a light component and a heavy component, wherein the light component comprises the high-boiling-point solvent and mXDI, and the heavy component is low-free mXDI polyurethane prepolymer.
2. The method of preparing a low free mXDI polyurethane prepolymer in accordance with claim 1, wherein in step 1), the molar ratio of mXDI to polyol is 2:1 to 10:1, preferably 2:1 to 6: 1.
3. The method of claim 1, wherein the mass ratio of the sum of the mXDI and the polyol to the high boiling solvent is 0.5:1 to 2.0:1, preferably 0.8 to 1.6.
4. The method for preparing low free mXDI polyurethane prepolymer according to claim 1, wherein in the step 1), the temperature of the reaction is 25-80 ℃, and the time of the reaction is 2-10 h;
preferably, the reaction is carried out with stirring;
preferably, the step 1) is performed under the protection of nitrogen.
5. The process for the preparation of low free mXDI polyurethane prepolymer according to claim 1, wherein step 1) is in particular:
step a): mixing mXDI and a high boiling point solvent to obtain mXDI solution,
step b): and adding polyol or a solution of polyol and a high-boiling point solvent into the mXDI solution, and reacting to obtain a polyurethane prepolymer reaction solution.
6. The method for preparing a low free mXDI polyurethane prepolymer as claimed in claim 5, wherein in the step 1) of b), the polyol, or the solution of the polyol and the high boiling point solvent is added to the mXDI solution obtained in the step a) in a dropwise manner, wherein the dropwise addition time is 1-3 hours, and the temperature of the dropwise added liquid and the mXDI solution is 20-70 ℃.
7. The method for preparing a low free mXDI polyurethane prepolymer as claimed in any one of claims 1 to 6, wherein, in said step 1), said high boiling point solvent has a boiling point of 190-300 ℃ at normal pressure;
preferably, the high boiling point solvent is selected from one or more of methyl benzoate, ethyl benzoate, propyl benzoate, isopropyl benzoate, butyl benzoate, isobutyl benzoate, tert-butyl benzoate, dimethyl phthalate, dimethyl isophthalate, diethyl phthalate, diisobutyl phthalate, diethyl isophthalate, diisopropyl succinate, diisopropyl glutarate, diisopropyl adipate, diisobutyl adipate, diisopropyl sebacate, dibasic ester DBE, butyrolactone, valerolactone, glyceryl triacetate, caprolactone, N-diethylacetamide, N-methylpyrrolidone and sulfolane; preferably, the high boiling point solvent is selected from at least one of dibasic ester DBE, dimethyl phthalate, glyceryl triacetate, diisopropyl adipate and diisobutyl adipate.
8. The method for preparing a low free mXDI polyurethane prepolymer in accordance with claim 7, wherein in said step 1), said polyol is a polyether polyol, a polycarbonate polyol or a polyester polyol; preferably, the polyether polyol is at least one of polypropylene oxide ether polyol, polytetrahydrofuran ether glycol and polypropylene glycol; the polycarbonate polyol is at least one of polyhexamethylene glycol carbonate dihydric alcohol and polypentanediol carbonate dihydric alcohol; the polyester polyol is at least one of polybutylene adipate diol, polyhexamethylene adipate diol, polycaprolactone diol and polyhexamethylene adipate diol; preferably, the polyol is selected from at least one of polytetrahydrofuran polyol PTMEG1000, polycarbonate polyol PCD2000 and polycaprolactone polyol PCL 2000;
preferably, the polyol has a molecular weight of 500-5000g/mol, more preferably 1000-3000 g/mol.
9. The method for preparing a low free mXDI polyurethane prepolymer as claimed in claim 8, wherein in the step 2), the temperature of the evaporation is 80 to 160 ℃, and the vacuum degree during the evaporation is 0.1 to 2.0 mmHg.
10. The method for preparing a low free mXDI polyurethane prepolymer in accordance with claim 1, wherein the light fraction obtained in step 2) is used as a raw material in step 1) for recycling; preferably, the evaporation is processed in a thin film distillation apparatus.
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