A high-biomass-content thermoset polyurethane elastomer and a method for preparing the same

Abstract

The application discloses a thermosetting polyurethane elastomer with high biomass content and a preparation method thereof, and relates to the technical field of bio-based polymer materials. The thermosetting polyurethane elastomer is prepared by mixing vegetable oil, betulin and isocyanate in an ice-water bath environment, and then performing heating and curing. The application further discloses a preparation method of the thermosetting polyurethane elastomer with high biomass content, which comprises the following steps: (1) stirring vegetable oil and betulin in an organic solvent, slowly adding isocyanate in an ice-water bath environment, and rapidly stirring to obtain a polyurethane prepolymer solution; and (2) pouring the polyurethane prepolymer solution obtained in the step (1) into a polytetrafluoroethylene mold, and performing heating and curing to obtain the thermosetting polyurethane elastomer. The polyurethane elastomer preparation method is simple and efficient, the adjustable range of product performance is large, the mechanical property is good, the heat resistance and solvent resistance are good, and the biomass content is high.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of bio-based high molecular materials, and particularly relates to a thermosetting polyurethane elastomer with high biomass content and a preparation method thereof. BACKGROUND

[0002] Polyurethane, abbreviated as PU, is a general high polymer compound with urethane bond as a repeating unit. The so-called elastomer refers to a material with a glass transition temperature lower than room temperature, an elongation at break greater than 50%, and good recovery after external force is removed. Polyurethane elastomer is a special category of elastomers, which has rich raw material types, various formulations, and a wide performance range. The lower limit of hardness and elastic modulus exceeds that of rubber, and the upper limit almost covers plastics, and the application is becoming more and more widespread.

[0003] The earliest polyurethane elastomer was invented by Bayer in Germany in the 1930s using toluene diisocyanate and polyols. After more than 80 years of development, polyurethane elastomer has made significant technical progress. At present, polyurethane elastomers with good elasticity, toughness, dimensional stability, and good oil resistance, wear resistance, low temperature resistance, and aging resistance have gradually entered all aspects of our lives.

[0004] At present, the research of polyurethane materials has entered a new period of high efficiency, green, environmental protection, and safety. However, most of the polyurethane materials including polyurethane elastomers are mainly prepared from petroleum-based raw materials, and the use of a large amount of petrochemical resources has caused serious resource consumption and environmental problems. Therefore, it has become a mainstream to use biomass resources instead of petroleum-based resources for the preparation of polyurethane.

[0005] The Chinese patent document with publication number CN115505085A discloses a quercetin-based degradable thermosetting polyurethane and a preparation method thereof. The invention uses biomass resource quercetin as a chain extender and isocyanate to react to obtain a degradable thermosetting polyurethane elastomer material. However, the preparation process is complex, the energy consumption is high, and the tin catalyst used in the process is not environmentally friendly.

[0006] The Chinese patent document with publication number CN115073697A discloses a bio-based polyurethane elastomer. The invention introduces modified enzymatic lignin and controls the addition amount of each raw material, so that the prepared bio-based polyurethane elastomer has excellent hardness, tensile strength, and heat resistance. However, the preparation process of the modified enzymatic lignin raw material in this method is relatively complex, and the production efficiency is low.

[0007] Therefore, how to efficiently prepare bio-based polyurethane elastomers with different performance ranges and different biomass contents under mild reaction conditions is a problem to be solved at present.

[0008] Betulin, also known as betulonic acid, is an important natural product widely present in birch bark. Both betulin and vegetable oil can be obtained from abundant plant resources, and using them to prepare bio-based polyurethane materials has high practical promotion value and provides certain contribution to promoting the development of bio-based polyurethane materials.

[0009] The Chinese patent document with the publication number CN115521434 A discloses a betulin-based thermoplastic polyurethane elastomer and a preparation method thereof. The invention uses betulin, a hydroxyl-terminated polymer and diisocyanate to react, and by adjusting the soft and hard segment ratio of the polymer main chain, the prepared betulin-based thermoplastic polyurethane elastomer has different mechanical properties. However, the tensile properties and biomass content of the polyurethane elastomer have limited adjustable ranges, and the tin catalyst used in the process is easy to pollute the environment. SUMMARY

[0010] In view of the deficiencies in the prior art, the present application provides a high-biomass-content thermosetting polyurethane elastomer, which is prepared by reacting renewable and environmentally friendly biomass raw materials betulin, vegetable oil and isocyanate without catalyst. The polyurethane elastomer is simple and efficient to prepare, has a large adjustable range of performance, good mechanical properties, good heat resistance and solvent resistance, and high biomass content.

[0011] A high-biomass-content thermosetting polyurethane elastomer is prepared by mixing vegetable oil, betulin and isocyanate in an ice water bath environment, then pre-polymerizing and curing.

[0012] The high biomass content refers to that the biomass content in the thermosetting polyurethane elastomer is not less than 60wt%.

[0013] The vegetable oil includes one or more of soybean oil, rapeseed oil, castor oil, palm oil, bio-glycerol and hydroxylated vegetable oil modified by thiol-ene click reaction and epoxy ring-opening reaction in any proportion combination, and the functionality of the vegetable oil is greater than 2.0.

[0014] The structural formula of the betulin is:

[0015]

[0016] Betulin, as a dihydroxy compound, has hydroxyl groups distributed in its molecular structure, which can provide a basis for the synthesis of polyurethane, and the aliphatic ring in the molecular structure of betulin can provide a certain rigidity for the polymer.

[0017] The main component of vegetable oil is straight-chain high fatty acid glyceride, which is a good and sustainable biomass polyol raw material. The long and flexible fatty chain and rigid betulin structure in vegetable oil can better form microphase separation. The introduction of betulin and vegetable oil into polyurethane materials has a series of advantages such as green environmental protection, sustainability and good application potential.

[0018] The isocyanate includes one or several of diphenylmethane diisocyanate, isophorone diisocyanate, 1,5-pentane diisocyanate, hexamethylene diisocyanate, p-phenylene diisocyanate, 1,5-naphthalene diisocyanate, toluene diisocyanate, dicyclohexyl diisocyanate and hexamethylene isocyanate trimer and toluene diisocyanate trimer in any proportion.

[0019] In the present application, both the vegetable oil and betulin are green biomass sources, and the mass ratio is more than 50%. The biomass content of the thermosetting polyurethane elastomer can be changed on the basis of changing the type and proportion of isocyanate.

[0020] When the isocyanate is toluene diisocyanate or hexamethylene diisocyanate, the biomass content of the thermosetting polyurethane elastomer is more than 75wt%.

[0021] When the isocyanate is biomass-derived 1,5-pentane diisocyanate, the biomass content of the thermosetting polyurethane elastomer is 100wt%.

[0022] In the present application, the hard segment of the thermosetting polyurethane elastomer includes betulin residue and isocyanate residue, and the soft segment includes fatty acid chain structure.

[0023] Further preferably, the vegetable oil is castor oil, which has a simple structure, a hydroxyl group in the main chain and is an inedible oil, mainly serving as the soft segment part of the thermosetting polyurethane.

[0024] The isocyanate is hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), 1,5-pentane diisocyanate (PDI) or diphenylmethane diisocyanate (MDI).

[0025] Betulin has a rigid polycyclic structure, and HDI, PDI with fatty chains and TDI, MDI with aromatic rings and castor oil can be dynamically adjusted according to different use scenarios or requirements under the above conditions to prepare products with different properties.

[0026] When the vegetable oil is castor oil, the thermosetting polyurethane elastomer with the following structural formula is obtained:

[0027]

[0028] wherein DI is isocyanate residue, R is the residue of polymer formed by the reaction of isocyanate and oleic acid chain, to extend the chain of the polymer.

[0029] The mechanical properties of the thermosetting polyurethane elastomer can be regulated by adjusting the ratio of the hydroxyl groups of the vegetable oil and betulin and changing the type of isocyanate, the isocyanate index based on the total hydroxyl content is 1.0, and the ratio of the hydroxyl groups of the vegetable oil and betulin varies between 10:0 and 5:5.

[0030] Preferably, the ratio of the hydroxyl groups of the vegetable oil and betulin is between 6:4 and 8:2, and when the isocyanate is toluene diisocyanate, the thermosetting polyurethane elastomer has better tensile strength, larger strain and higher toughness.

[0031] On the basis of the above, increasing the amount of betulin can improve the tensile strength of the thermosetting polyurethane elastomer, and increasing the amount of vegetable oil can improve the toughness of the thermosetting polyurethane elastomer.

[0032] Preferably, when the isocyanate is hexamethylene diisocyanate, the thermosetting polyurethane elastomer exhibits great toughness.

[0033] Preferably, when the isocyanate is toluene diisocyanate, the vegetable oil is replaced by hydroxylated vegetable oil, and the thermosetting polyurethane elastomer exhibits great rigidity and smaller strain.

[0034] The thermosetting polyurethane elastomer in the present application has a large adjustable range, and the tensile modulus of the thermosetting polyurethane elastomer is 2-673 MPa, the tensile strength is 2-32 MPa, and the elongation at break is 16-1100%.

[0035] The present application also provides a preparation method of a thermosetting polyurethane elastomer with high biomass content, which is simple and efficient, and has a large adjustable range of product performance.

[0036] A preparation method of a thermosetting polyurethane elastomer with high biomass content, comprising:

[0037] (1) After the vegetable oil and betulin are fully stirred in an organic solvent, isocyanate is slowly added under an ice water bath environment, and rapid stirring is performed to obtain a polyurethane prepolymer solution;

[0038] (2) The polyurethane prepolymer solution obtained in step (1) is poured into a polytetrafluoroethylene mold and heated and cured to obtain the thermosetting polyurethane elastomer.

[0039] No organic metal compound or amine catalyst is added in the reaction system.

[0040] In the preparation process, the plant oil, betulin and organic solvent need to be treated to remove water before reaction.

[0041] In step (1), the organic solvent is selected from one or more of tetrahydrofuran, chloroform, dichloromethane, methyl ethyl ketone, acetone and toluene in any proportion.

[0042] Preferably, the organic solvent is selected from a mixture of methyl ethyl ketone and tetrahydrofuran, which has a lower boiling point, facilitating the post-processing process.

[0043] Considering the performance of the product polyurethane elastomer, the feeding sequence in step (1) needs to be added in two steps according to the requirements, that is, after the plant oil and betulin are fully mixed in the solvent, the isocyanate is added in an ice water bath environment.

[0044] The stirring mixing time of step (1) is 10-40 min. By rapid stirring, the mixing time after adding isocyanate should not exceed 5 min.

[0045] Preferably, the said sufficient stirring and rapid stirring are selected by magnetic stirring.

[0046] In step (2), the said heating and curing adopts gradient heating mode.

[0047] Preferably, three-stage gradient heating is adopted: reaction at 30-40℃ for 1-3 hours, reaction at 60-90℃ for 2-3 hours, and reaction at 90-120℃ for 3-4 hours.

[0048] Compared with the prior art, the beneficial effects of the present application are:

[0049] (1) The betulin and plant oil used in the present application are renewable biomass raw materials, which have the advantages of easy availability, cheapness and biodegradability, etc. Not only the biomass content of polyurethane is greatly improved, but also the types of bio-based polyurethane are expanded.

[0050] (2) The performance and biomass content of the thermosetting polyurethane elastomer in the present application can be adjusted in a large range, and a series of products with different performance ranges and different biomass contents can be obtained by adjusting the proportion of hydroxyl groups and the type of isocyanate, so as to meet the different requirements of users;

[0051] (3) The betulin in the present application has a rigid aliphatic ring diol structure, which not only can directly participate in the reaction to construct the polyurethane network, but also can use betulin as a large structure hard segment phase to make the polyurethane material realize better microphase separation, and endow the polyurethane material with better toughness, thermal stability and acid and alkali resistance, etc.

[0052] (4) The synthetic method used in the present application is easy to operate and simple in process, and no organic metal compound or amine catalyst is added in the reaction process. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 Schematic diagram of the synthesis process of the thermosetting polyurethane elastomer in Example 1, Example 2 and Example 3.

[0054] Figure 2 Tensile property curve of the thermosetting polyurethane elastomer in Example 1, Example 2, Example 3, Example 4 and Comparative Example 1.

[0055] Figure 3 Fourier transform infrared spectrum of the thermosetting polyurethane elastomer in Example 1, Example 2, Example 3, Example 4 and Comparative Example 1.

[0056] Figure 4 Thermogravimetric curve of the thermosetting polyurethane elastomer in Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0057] The present application will be further illustrated by the following examples and drawings. It should be understood that these examples are only used to illustrate the present application, and are not used to limit the scope of the present application.

[0058] Betulin was purchased from Shaanxi Lvshengyuan Biological Product Manufacturing Co., Ltd., with a purity of 98%; 1,5-pentane diisocyanate was purchased from Wanhua Chemical Group Co., Ltd., with a purity of 99%.

[0059] In the examples or comparative examples, betulin and vegetable oil were dried in a vacuum oven at 80°C for 12 hours before use, and the solvent was dried by activated molecular sieve. After the thermosetting polyurethane elastomer was laid into a film to obtain a sheet, performance testing was carried out.

[0060] Example 1

[0061] (1) 7 parts of dried castor oil (molecular weight 933, functionality f = 2.7) and 3 parts of dried betulin powder (hydroxyl ratio 7:3) were weighed in mole fraction, then an appropriate amount of tetrahydrofuran solution and methyl ethyl ketone solution (to reduce viscosity) were added, and stirred for 10 min until they were uniformly dispersed. Then 10 parts of toluene diisocyanate was slowly added under ice water bath environment, and quickly stirred for 2 min to obtain a polyurethane prepolymer solution;

[0062] (2) Pour the polyurethane prepolymer solution birch alcohol solution on the polytetrafluoroethylene mold, and place it in a vacuum drying oven, and react at 30°C for 60 min; react at 60°C for 180 min; react at 90°C for 60 min to obtain a thermosetting polyurethane elastomer with a biomass content of about 78wt%. Figure 1 As shown in the synthesis process diagram

[0063] Cut the thermosetting polyurethane elastomer into dumbbell-shaped tensile samples, and test them on a universal sample tensile machine. The tensile property curve is shown in Figure 2 As shown in the synthesis process diagram, the elastomer has a modulus of 140 MPa, a tensile strength of 31.6 MPa, an elongation at break of 213%, and a fracture energy of 33.2 MJ / m 3 .

[0064] Example 2

[0065] Replace 7 parts of the dried castor oil in Example 1 with 8 parts of castor oil, and replace 3 parts of the dried birch alcohol powder with 2 parts of birch alcohol powder (hydroxyl ratio of 8:2). The other parameters and methods are the same as in Example 1, and a thermosetting polyurethane elastomer with a biomass content of about 79wt% is obtained.

[0066] When tested by the same method as in Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 23.6 MPa, a tensile strength of 14.3 MPa, an elongation at break of 257%, and a fracture energy of 16.2 MJ / m 3 . Compared with Example 1, it can be seen that the strength and modulus decrease significantly, while the elongation at break increases.

[0067] Example 3

[0068] Replace 7 parts of the dried castor oil in Example 1 with 6 parts of castor oil, and replace 3 parts of the dried birch alcohol powder with 4 parts of birch alcohol powder (hydroxyl ratio of 6:4). The other parameters and methods are the same as in Example 1, and a thermosetting polyurethane elastomer with a biomass content of about 77wt% is obtained.

[0069] When tested by the same method as in Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 461 MPa, a tensile strength of 26.9 MPa, an elongation at break of 110%, and a fracture energy of 20.8 MJ / m 3 . Compared with Example 1, it can be seen that the modulus increases sharply, which is related to the rigid structure of birch alcohol; while the strength and elongation at break decrease.

[0070] Example 4

[0071] Example 4 10 parts of toluene diisocyanate in Example 1 was replaced by 10 parts of biomass-derived 1,5-pentamethylene diisocyanate in mole fraction, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with biomass content of about 100wt% was obtained.

[0072] When tested by the same method as Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 7.7 MPa, a tensile strength of 7.75 MPa, an elongation at break of 360%, and a fracture energy of 8.5 MJ / m 3 Compared with Example 1, although the biomass content was increased, the overall performance of the polyurethane elastomer was decreased due to the replacement of benzene rings.

[0073] Example 5

[0074] 10 parts of toluene diisocyanate in Example 1 was replaced by 10 parts of hexamethylene diisocyanate in mole fraction, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with biomass content of about 77wt% was obtained.

[0075] When tested by the same method as Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 1.4 MPa, a tensile strength of 1.2 MPa, an elongation at break of 1100%, and a fracture energy of 7.8 MJ / m 3 .

[0076] Example 6

[0077] 7 parts of the dried castor oil in Example 1 was replaced by castor oil after click reaction and increase of hydroxyl value (thiol-ene click reaction of castor oil and thio-glycerol under the condition of photoinitiator, and under the action of ultraviolet light), the hydroxyl ratio of the hydroxylated vegetable oil and betulinol was still 7:3, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with biomass content of about 79wt% was obtained.

[0078] When tested by the same method as Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 673 MPa, a tensile strength of 20.5 MPa, and an elongation at break of 16%. The hydroxyl value of the vegetable oil was high, and under the condition of unchanged total hydroxyl ratio, the amount added was less, i.e. the amount of soft fatty acid chain was less, and the polyurethane material showed great rigidity and small strain.

[0079] Comparative Example 1

[0080] The preparation method of Example 1 was followed, 7 parts of dried castor oil was directly replaced by 10 parts of castor oil (the hydroxyl ratio of castor oil and betulin was 10:0) without adding 3 parts of dried betulin powder, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with a biomass content of about 80wt% was obtained.

[0081] When each performance was tested by the same method as Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 2.1 MPa, a tensile strength of 2.2 MPa, an elongation at break of 109%, and a fracture energy of 1.4 MJ / m 3 Compared with Example 1, it can be seen that the strength, modulus and elongation at break are all significantly reduced. It shows that the performance of the component without betulin is poor.

[0082] Comparative Example 2

[0083] The preparation method of Example 1 was followed, 7 parts of dried castor oil was directly replaced by 10 parts of castor oil (the hydroxyl ratio of castor oil and betulin was 10:0) without adding 3 parts of dried betulin powder, 10 parts of toluene diisocyanate was replaced by 10 parts of biomass-derived 1,5-pentane diisocyanate, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with a biomass content of about 100wt% was obtained.

[0084] When each performance was tested by the same method as Example 1, it was found that the thermosetting polyurethane elastomer in this example had a modulus of 4.14 MPa, a tensile strength of 0.8 MPa, an elongation at break of 24%, and a fracture energy of 0.1 MJ / m 3 Compared with Example 1, it can be seen that the strength, modulus and elongation at break are all significantly reduced, which shows that the performance of the polyurethane elastomer using only a single component and not containing a rigid ring (such as toluene diisocyanate, betulin, etc.) in the structure is poor.

[0085] Comparative Example 3

[0086] The preparation method of Example 1 was followed, 7 parts of dried castor oil was directly replaced by 10 parts of castor oil powder (the hydroxyl ratio of castor oil and betulin was 0:10) without adding 3 parts of dried betulin powder, other parameters and methods were the same as Example 1, and a thermosetting polyurethane elastomer with a biomass content of about 84wt% was obtained. However, due to the low activity of betulin, the inconsistent activity of alcohol hydroxyl, and the large steric hindrance, etc., the reaction product was difficult to form a film, and its mechanical properties, infrared spectrum, etc. could not be tested.

[0087] 1. Test method

[0088] Mechanical property test method: the mechanical property of the sample is analyzed by using a ZWICK tensile testing machine, a tensile clamp and a rubber clamp, the size of the sample is 20.0mm (length) x 2.0mm (width) x 0.5mm (thickness), and the loading speed is 10.00mm / min. For accuracy, each sample is measured three times and the average value is taken.

[0089] Fourier transform infrared measurement is carried out on a Nicolet 6700 FT-IR spectrometer of Agilent Technology Company in the United States. All the spectra are recorded in the range of 400-4000cm -1 , 32 scans are carried out, and the resolution is 4.0cm -1 .

[0090] Thermogravimetric analysis is carried out on a thermogravimetric analyzer (Mettler Toledo). The weight of the test sample is about 5mg, all the tests are carried out at 50-800℃, the heating rate is 20℃ / min, and the test is carried out under nitrogen atmosphere.

[0091] 2. Test results

[0092] As can be seen from Figure 3 , the infrared spectra of the thermosetting polyurethane material described in the application all show similar absorption peaks. The strong absorption peaks at 2930cm -1 and 2870cm -1 are the stretching vibration of -CH2 and -CH3; the absorption peaks at 1640-1720cm -1 are the absorption peaks of the carbon group in the urethane; the wider peak at 3200-3600cm -1 is the asymmetric stretching vibration of -NH- and the stretching vibration of -OH; and the -NCO absorption peak at 2250cm -1 disappears. This shows that each component of the thermosetting polyurethane material is successfully synthesized.

[0093] As can be seen from Figure 4 , the thermal decomposition temperature of the thermosetting polyurethane material prepared in the application is 301-308℃, and the heat resistance is good.

[0094] The above examples have described the technical solutions of the application in detail, and it should be understood that the above description is only a specific embodiment of the application and is not used to limit the application, and any modification, supplement or similar replacement within the principle range of the application should be included in the protection range of the application.

Claims

1. A high biomass content thermoset polyurethane elastomer, which is prepared by mixing vegetable oil, betulin and isocyanate in an ice water bath environment, pre-polymerizing, and then curing by heating.

2. The thermoset polyurethane elastomer according to claim 1, wherein, The vegetable oil includes one or more of soybean oil, rapeseed oil, castor oil, palm oil, bio-glycerol, and hydroxylated vegetable oil modified by thiol-ene click reaction or epoxy ring-opening reaction, in any proportion.

3. The thermoset polyurethane elastomer according to claim 1, wherein, The isocyanate is one or more of hexamethylene diisocyanate, toluene diisocyanate, 1, 5-pentane diisocyanate, diphenyl methane diisocyanate, isophorone diisocyanate, 1, 5-naphthalene diisocyanate, dicyclohexyl diisocyanate, and hexamethylene isocyanate trimer and toluene diisocyanate trimer, in any proportion.

4. The thermoset polyurethane elastomer according to claim 3, wherein, The vegetable oil is castor oil, and the isocyanate is hexamethylene diisocyanate, toluene diisocyanate, 1, 5-pentane diisocyanate, or diphenyl methane diisocyanate.

5. The thermoset polyurethane elastomer according to claim 1, wherein, The isocyanate index based on the total hydroxyl content is 1.0, and the hydroxyl ratio of the vegetable oil to betulin is 8:2 to 5:

5. 6.The thermoset polyurethane elastomer of any one of claims 1-5, wherein the tensile modulus of the thermoset polyurethane elastomer is 2-673 MPa, the tensile strength is 2-32 MPa, and the elongation at break is 16-1100%. 7.A method for preparing the high biomass content thermoset polyurethane elastomer of any one of claims 1-6, comprising: (1) stirring the vegetable oil and betulin in an organic solvent, slowly adding the isocyanate in an ice water bath environment, and rapidly stirring to obtain a polyurethane pre-polymer solution; (2) pouring the polyurethane pre-polymer solution obtained in step (1) into a polytetrafluoroethylene mold and curing by heating to obtain the thermoset polyurethane elastomer.

8. The process for the preparation of a thermoset polyurethane elastomer according to claim 7, characterized in that, In step (1), the organic solvent is selected from one or more of tetrahydrofuran, chloroform, dichloromethane, methyl ethyl ketone, acetone, and toluene, in any proportion.

9. The process for the preparation of a thermoset polyurethane elastomer according to claim 7, characterized in that, In step (2), the heating and curing is performed by gradient heating.

10. The process for the preparation of a thermoset polyurethane elastomer according to claim 9, characterized in that, The gradient heating is performed in three stages: 30-40℃ for 1-3 hours, 60-90℃ for 2-3 hours, and 90-120℃ for 3-4 hours.

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