Dry and wet dual-purpose cold-mixed asphalt mixture cold-mixed liquid as well as preparation method and application thereof

Abstract

The invention relates to a dry and wet dual-purpose cold-mixed asphalt mixture cold-mixed liquid which comprises the following components in parts by mass: 100 parts of matrix asphalt, 10-30 parts of diesel oil, 2-4 parts of an additive and 2-4 parts of a polyurethane prepolymer, the additive consists of epoxy resin and a curing agent; the asphalt mixture cold-mixing liquid can enhance the adhesive force between a repairing material and a pavement, improve the stability and durability of a repairing layer, improve the water resistance of the repairing material, reduce the erosion of water to the repairing layer and enhance the durability of the pavement, so that the prepared cold-mixing asphalt mixture has good construction workability and pavement performance; particularly, the water damage resistance is excellent.

Description

Technical Field

[0001] The invention relates to the technical field of road maintenance, and in particular to a dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid and a preparation method and application thereof. Background Art

[0002] Due to the increase in traffic volume and vehicle load, as well as natural factors such as temperature changes and water erosion, various damage problems will occur in asphalt pavements during use, such as potholes, cracks, and cracks, which affect driving safety and comfort. Traditional asphalt pavement repair methods include hot patching and cold patching. The hot patching method usually uses hot-mix asphalt mixtures, which require construction at high temperatures, are complex in process and consume a lot of energy. The cold patching method uses cold-mix asphalt mixtures, which have the advantages of low construction temperature requirements and simple construction process compared to the hot-mix method.

[0003] With the enhancement of environmental awareness and technological progress, cold-mix asphalt repair technology has gradually been developed and applied. Cold-mix asphalt mixtures do not require preparation and construction under high temperature conditions, have low energy consumption and are environmentally friendly, so they have received more and more attention and application. In the research and application process of cold-mix asphalt repair technology, the material formula is continuously improved and the process technology is innovated to improve the repair effect, extend the service life of the repaired road surface, and reduce construction costs and environmental impact.

[0004] Cold mix asphalt is a road material prepared and constructed at room temperature, usually used to repair pavement or as a new pavement overlay. Wet and dry cold mix asphalt is a cold mix asphalt mixture with special properties, which can be used in dry or wet conditions. This type of mixture has certain particularities in design and formulation, aiming to ensure that it can perform well in different environmental conditions.

[0005] At present, existing cold-mix asphalt mixtures generally have the problem of being unable to be constructed in winter and rainy seasons and generally having low strength. Summary of the invention

[0006] The present application provides a dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid and a preparation method thereof, which solves or partially solves the technical problems that the existing cold-mix asphalt mixture is generally unable to be constructed in winter and rainy seasons, has low initial strength, and has poor water stability and frost resistance; the prepared cold-mix asphalt mixture has good construction ease and road performance, especially excellent performance in water damage resistance, and effectively solves the technical problem that the traditional cold-mix asphalt mixture cannot be constructed in rainy days and winter.

[0007] The present application provides a dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid, which is characterized by comprising the following components: 100 parts by mass of base asphalt, 10 to 30 parts of diesel, 2 to 4 parts of additives, and 2 to 4 parts of polyurethane prepolymer; the additives are composed of epoxy resin and curing agent.

[0008] Based on the same inventive concept, the present application also provides a method for preparing a cold-mix liquid of a dry-wet dual-purpose cold-mix asphalt mixture, which is used to prepare the cold-mix liquid of the dry-wet dual-purpose cold-mix asphalt mixture, comprising the following steps:

[0009] The diesel is used as a solvent-type diluent and mixed with the base asphalt to obtain a diluted asphalt;

[0010] The additive is mixed with the diluted asphalt and stirred sufficiently to obtain an asphalt intermediate;

[0011] The dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid is obtained by mixing the polyurethane prepolymer and the asphalt intermediate and stirring them fully.

[0012] Preferably, during the mixing process of the diesel and the matrix asphalt, the temperature is controlled at 100-120°C and the time is controlled at 5-10 minutes;

[0013] During the mixing process of the additive and the diluted asphalt, the temperature is controlled at 60-80°C and the time is controlled at 5-10 minutes;

[0014] During the mixing process of the polyurethane prepolymer and the asphalt intermediate, the temperature is controlled at 80-100° C. and the time is controlled at 5-10 minutes.

[0015] Based on the same inventive concept, the present application also provides a dual-purpose wet and dry cold mix asphalt mixture, comprising the following components: in parts by mass, 4 to 6 parts of the dual-purpose wet and dry cold mix asphalt mixture cold mix liquid, and 100 parts of aggregate and mineral powder.

[0016] Based on the same inventive concept, the present application also provides a method for preparing a dry-wet dual-purpose cold-mix asphalt mixture, which is used to prepare the dry-wet dual-purpose cold-mix asphalt mixture, comprising the following steps:

[0017] Preheating the cold mix liquid of the dry and wet dual-purpose cold mix asphalt mixture and the aggregate and mineral powder respectively;

[0018] The preheated cold mix liquid of the dual-purpose cold mix asphalt mixture is mixed and stirred evenly with the aggregate and the mineral powder to obtain the dual-purpose cold mix asphalt mixture.

[0019] Preferably, the preheating temperature of the cold mix liquid of the dual-purpose cold mix asphalt mixture is controlled at 80-100° C., and the preheated cold mix liquid of the dual-purpose cold mix asphalt mixture is in a molten state;

[0020] The preheating temperature of the aggregate and mineral powder is controlled at 70-80°C, and the preheating time is controlled at 2-6h;

[0021] During the mixing and stirring process of the cold mix liquid of the dry-wet dual-purpose cold mix asphalt mixture with the aggregate and the mineral powder, the temperature is controlled at 50-80° C. and the time is controlled at 0.5-6 minutes.

[0022] One or more technical solutions provided in the embodiments of the present application have at least the following technical effects or advantages:

[0023] The dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid of the present application adopts diesel as a solvent-type diluent, and epoxy resin and curing agent as additives for diluting asphalt, which can enhance the adhesion between the repair material and the road surface, and improve the stability and durability of the repair layer; adding epoxy resin and curing agent to the diluted asphalt can improve the water resistance of the repair material, reduce the erosion of the repair layer by moisture, and can be adjusted according to different climatic and environmental conditions, and is suitable for dry and wet road repair work; at the same time, the curing agent reacts with the epoxy resin to form a curing system with high strength and durability, accelerate the hardening process of the repair material, shorten the construction period, and improve the construction efficiency.

[0024] The cold mix liquid formula of this application has been carefully designed to ensure that the repaired road surface has good durability, stability and water damage resistance, and improve the quality of the repair. The cold mix liquid preparation technology used, on the one hand, uses environmentally friendly raw materials to reduce the adverse impact on the environment, and on the other hand, can significantly reduce the energy demand in the production process, which is conducive to saving energy resources.

[0025] The dry-wet dual-purpose cold-mix asphalt mixture prepared in the present application can adjust the proportion of cold-mix liquid added according to different climate and environmental conditions, and can be well adapted to dry and wet road repair work. The preparation method is simple and easy, and can effectively improve construction efficiency and save manpower and time costs. DETAILED DESCRIPTION

[0027] The first aspect of the present invention provides a method for preparing a dry-wet dual-purpose asphalt mixture cold mix liquid, the method comprising the following steps:

[0028] (1) using diesel as a solvent-type diluent for base asphalt to obtain solvent-type diluted asphalt;

[0029] (2) adding an additive consisting of an epoxy resin and a curing agent to the diluted asphalt obtained in step (1) and stirring the mixture;

[0030] (3) Adding the polyurethane prepolymer to the product obtained in step (2) and stirring them to prepare a dry-wet dual-purpose asphalt mixture cold mix liquid.

[0031] In the present method, the mass ratio of diesel to base asphalt is (10-30):100, specifically, for example, it can be 10:100, 15:100, 20:100, 25:100 or 30:100.

[0032] In the present method, in step (1), the dilution conditions include: a temperature of 100-120°C, specifically, for example, 100°C, 105°C, 110°C, 115°C or 120°C, and a time of 5-10 min, specifically, for example, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0033] In the present method, in step (2), the mass ratio of the additive consisting of epoxy resin and curing agent to the base asphalt is (2-4): 100. Specifically, for example, it can be 2:100, 3:100 or 4:100.

[0034] In the present method, in step (2), the stirring conditions include: a temperature of 60-80°C, specifically, for example, 60°C, 65°C, 70°C, 75°C or 80°C, and a time of 5-10 min, specifically, for example, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0035] In the present method, in step (3), the mass ratio of the polyurethane prepolymer to the base asphalt is (2-4):100. Specifically, for example, it can be 2:100, 3:100 or 4:100.

[0036] In the present method, in step (2), the stirring conditions include: a temperature of 60-80°C, specifically, for example, 60°C, 65°C, 70°C, 75°C or 80°C, and a time of 5-10 min, specifically, for example, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0037] The second aspect of the present invention provides a dry-wet dual-purpose asphalt mixture cold mix prepared by the above method.

[0038] The third aspect of the present invention provides a method for preparing a cold-mix asphalt mixture, wherein the above-mentioned dry-wet dual-purpose asphalt mixture cold-mix liquid, aggregate and mineral powder are preheated separately, and then the preheated dry-wet dual-purpose asphalt mixture cold-mix liquid, aggregate and mineral powder are mixed.

[0039] In the method for preparing the cold mix asphalt mixture of the present invention, the aggregate may be selected conventionally in the art.

[0040] In the method for preparing the cold mix asphalt mixture of the present invention, the mineral powder may be a conventional choice in the art.

[0041] In the preparation method of cold-mix asphalt mixture of the present invention, in a preferred embodiment, the ratio of the total weight of the wet-dry asphalt mixture cold mix to the aggregate and the mineral powder is (4-6):100, specifically, for example, 4:100, 4.5:100, 5:100, 5.5:100 or 6:100.

[0042] In the preparation method of cold-mix asphalt mixture of the present invention, in a specific implementation, the preheating temperature of aggregate and mineral powder is 70-80°C, and the preheating time is 2-6h; the cold-mix liquid of dry-wet dual-purpose asphalt mixture is preheated to a molten state at a temperature of 80-100°C.

[0043] In the preparation method of cold-mix asphalt mixture of the present invention, in the specific operation process, the preheated dry-wet dual-purpose asphalt mixture cold-mix liquid and mineral powder are sequentially added to the preheated aggregate for mixing, and the specific steps are as follows:

[0044] (1) Mix the preheated aggregate at 80°C for 1 to 2 minutes;

[0045] (2) Add the cold mix of dry and wet dual-purpose asphalt mixture into the aggregate and continue mixing for 3 to 4 minutes;

[0046] (3) The preheated mineral powder is added to the mixture of the cold mix liquid of the dry-wet dual-purpose asphalt mixture and the aggregate and the mixture is mixed for 3 to 4 minutes to obtain a cold mix asphalt mixture.

[0047] The fourth aspect of the present invention provides a cold mix asphalt mixture prepared by the above method.

[0048] The following examples further illustrate the preparation method and application of a dry-wet dual-purpose asphalt mixture cold mix liquid of the present invention. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples.

[0049] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, are all commercially available.

[0050] Example 1

[0051] The wet and dry dual-purpose asphalt mixture cold mix liquid in this embodiment is prepared from diesel, epoxy resin and curing agent, polyurethane prepolymer and base asphalt, and the cold mix asphalt mixture is prepared from the wet and dry dual-purpose asphalt mixture cold mix liquid, natural aggregate and natural mineral powder.

[0052] (I) Preparation of dry and wet dual-purpose asphalt cold mix liquid:

[0053] (1) Add 100 g of diesel to 1 kg of base asphalt at 100° C. to dilute, and stir for 5 min to obtain diluted asphalt;

[0054] (2) After the diluted asphalt is heated to a molten state at 60-80° C., 20 g of an additive consisting of an epoxy resin and a curing agent (the mass ratio used is 2:1) is added, and the mixture is stirred for 5 minutes to prepare an epoxy asphalt mixture cold mix;

[0055] (3) Add 20 g of polyurethane prepolymer to the product obtained in step (2), and stir for 5 minutes to prepare a dry-wet dual-purpose asphalt mixture cold mix.

[0056] (II) Application of dry and wet dual-purpose asphalt cold mix liquid:

[0057] Preparation of cold-mix asphalt mixture: select the AC type dense gradation commonly used in highway pavement, weigh 20 kg of aggregates and mineral powder of corresponding sizes, and preheat them in an oven at 80°C for 4 hours. Preheat the cold-mixed asphalt mixture prepared in Example 1 at 80°C to a molten state, then add the preheated aggregate to a stirring pot at 80°C and stir for 1 to 2 minutes, then add 0.96 kg of molten cold-mixed asphalt mixture to the stirring pot and continue stirring for 3 to 4 minutes. Finally, add the preheated mineral powder and continue stirring for 3 to 4 minutes until no more white material appears in the mixture, and the cold-mixed asphalt mixture is prepared.

[0058] Example 2

[0059] The method of Example 1 is followed, except that when preparing the dry and wet dual-purpose asphalt mixture cold mix, the amount of the additive consisting of epoxy resin and curing agent (the mass ratio used is 2:1) is 30g.

[0060] Example 3

[0061] The method of Example 1 is followed, except that when preparing the dry and wet dual-purpose asphalt mixture cold mix, the amount of the additive consisting of epoxy resin and curing agent (the mass ratio used is 2:1) is 40g.

[0062] Example 4

[0063] The method of Example 1 is followed, except that when preparing the dry and wet dual-purpose asphalt mixture cold mix, the amount of polyurethane prepolymer used is 30 g.

[0064] Example 5

[0065] The method of Example 1 is followed, except that when preparing the dry and wet dual-purpose asphalt mixture cold mix, the amount of polyurethane prepolymer used is 40 g.

[0066] Comparative Example 1

[0067] The method of Example 1 is followed, except that when preparing the dry and wet dual-purpose asphalt mixture cold mix, diesel is replaced with white oil, and the amount of polyurethane prepolymer used is 20 g.

[0068] Test Case

[0069] Initial stability of cold mix: Taking the Marshall specimen height of 63.5±13mm as the control index, 1050g of cold mix was taken and placed in a 25℃ constant temperature box for 2h. Subsequently, the cold mix was taken out and loaded into the test mold, and the front and back sides of the specimen were compacted 75 times each with a Marshall compactor, and then the specimen was demoulded and placed in a 25℃ constant temperature water tank for 30min. Further, according to the test method of "Testing Procedures for Asphalt and Asphalt Mixtures for Highway Engineering" (JTG E20-2011 T 0709), the Marshall stability tester was used to perform a Marshall stability test on the above cold mix asphalt mixture specimens to evaluate the initial stability of the cold mix asphalt mixtures of the embodiments and comparative examples. The larger the value, the higher the initial strength.

[0070] Cold mix molding stability: Taking the Marshall specimen height of 63.5±13mm as the control index, take 1050g of cold mix and place it in a 25℃ constant temperature box for 2h. Subsequently, take out the cold mix and put it into the test mold, compact the front and back sides of the specimen with a Marshall compactor 50 times each, and then place it in a 60℃ constant temperature box for curing for 24h. After taking it out, compact the front and back sides with a Marshall compactor 25 times each, and cure at room temperature for 24h. After the room temperature curing is completed, water bath in a 60℃ constant temperature water tank for 30min. Further, according to the test method of "Testing Procedures for Asphalt and Asphalt Mixtures for Highway Engineering" (JTG E20-2011 T 0709), the Marshall stability tester is used to test the Marshall stability of the above cold mix asphalt mixture specimens, so as to evaluate the molding stability of the cold mix asphalt mixtures of the embodiments and comparative examples. The larger the value, the higher the molding strength.

[0071] Freeze-thaw splitting test: One group of Marshall cold patch material specimens were placed in a constant temperature drying oven at 60℃ for 24h. Then they were immersed in a constant temperature water tank at 25℃ for 2h. After the time was up, a loading test was immediately carried out to measure the maximum load value; another group of Marshall cold patch material specimens were placed in a vacuum condition of 97.3-98.7kPa for 15min and then immersed in water at normal pressure for 30min. Then the cold patch material was taken out and placed in a plastic bag containing 10mL of water, and then placed in a -18±2℃ constant temperature refrigerator for freezing for 16±1h, and then placed in a water bath at 60℃ constant temperature water tank for 24h. After the time was up, it was placed in a constant temperature water tank at 25℃ for 2h. Finally, a loading test was carried out to measure the maximum load value.

[0072] Marshall residual stability after immersion: According to the test method of "Testing Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG E20-2011 T 0709), the Marshall stability tester was used to test the Marshall stability of the cold-mix asphalt mixture specimens of the embodiment and the comparative example. The difference from the Marshall residual stability test of the hot-mix asphalt mixture is that the water bath temperature of the cold-mix specimens is corrected to 25°C, and the temperature conditions of the two groups of specimens are controlled the same. One group of specimens was placed in a 25°C constant temperature drying oven for 48 hours, and then placed in a 25°C constant temperature water tank for immersion for 30 minutes before stability testing. Another group of specimens was immersed in a 25°C constant temperature water tank for 48 hours before stability testing. Further, the percentage of the ratio of the Marshall stability after immersion to the Marshall stability before immersion was taken as the Marshall residual stability after immersion, which was used to evaluate the water stability of the above-mentioned cold-mix asphalt mixture. The larger the value, the better the water stability.

[0073] The initial stability and molding stability of the cold mix asphalt mixtures prepared in Examples 1-5 and Comparative Example 1 were measured respectively, and compared with the standard requirements for the working performance and road performance of cold mix asphalt mixtures in the field. The results are shown in Table 1.

[0074] Table 1

[0075] Initial stability (kN) Forming stability (kN) Example 1 6.2 7.8 Example 2 7.5 9.8 Example 3 8.9 12.5 Example 4 7.6 8.9 Example 5 9.2 13.2 Comparative Example 1 4.8 5.9 Standard requirements ≥2.5kN ≥4kN

[0076] As can be seen from Table 1, when the dry-wet dual-purpose asphalt mixture cold mix liquid prepared by the present invention is used to prepare a cold mix asphalt mixture, in terms of initial stability and molding stability, as the amount of additives composed of epoxy resin and curing agent increases, the strength of the cold-patch asphalt mixture is also improved. At the same time, as the amount of polyurethane prepolymer increases, the stability of the cold-patch asphalt mixture is improved. In terms of early strength and molding strength (initial stability and molding stability), the additives and polyurethane prepolymers have a significant effect on improving the early strength and molding strength of the cold-patch asphalt mixture.

[0077] The freeze-thaw splitting test and the water-immersion Marshall residual stability of the cold-mix asphalt mixtures prepared in Examples 1-5 and Comparative Example 1 were respectively measured, and the results were compared with the standard requirements for the working performance and road performance of cold-mix asphalt mixtures in the field, and the results are shown in Table 2.

[0078] Table 2

[0079] Freeze-thaw splitting strength ratio (%) Marshall residual stability after immersion in water (%) Example 1 81.23 86.35 Example 2 83.56 88.96 Example 3 85.29 92.34 Example 4 82.87 87.69 Example 5 84.57 89.23 Comparative Example 1 75.65 79.35 Standard requirements ≥80 ≥85

[0080] According to the data in Table 2, it can be seen that the present invention uses diesel as a diluent for the base asphalt, adds additives consisting of epoxy resin and curing agent, and polyurethane prepolymer into the solvent-based diluted asphalt for stirring, prepares a wet-dry cold mix liquid, and uses the cold mix liquid to prepare a cold-mix asphalt mixture. When the freeze-thaw splitting strength ratio and the residual Marshall stability after immersion in water are measured to evaluate the water stability, both meet the road standard requirements (≥80 and ≥85%). In general, when the wet-dry asphalt mixture cold mix liquid prepared by the present invention is used to prepare a cold-mix asphalt mixture, when used on a wet road surface, increasing the amount of the wet-dry asphalt mixture cold mix liquid added can also show better water damage resistance and meet the road performance standards of the cold-mix asphalt mixture.

[0081] In summary, the cold mix liquid prepared by the present application optimizes and improves the performance of the asphalt mixture, so that the final cold-patch asphalt mixture has better durability, stability and water resistance, which helps to improve the effect of pavement maintenance and repair and extend the service life of the pavement. Cold mix liquid technology provides an advanced method for the maintenance of asphalt pavements, making it possible to maintain roads under a wider range of climatic and environmental conditions, and helps to promote the innovation and development of asphalt pavement material maintenance technology. This has important scientific significance for the improvement of energy-saving, environmentally friendly high-performance cold-patch asphalt mixture production technology and asphalt pavement material maintenance technology, and is expected to produce positive social and economic benefits in the field of road maintenance.

[0082] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A dry and wet dual-purpose cold mix asphalt mixture cold mix liquid, characterized in that: The invention comprises the following components: by weight, 100 parts of base asphalt, 10-30 parts of diesel, 2-4 parts of additives and 2-4 parts of polyurethane prepolymer; the additives are composed of epoxy resin and curing agent.

2. The dry-wet dual-purpose cold mix asphalt cold mix liquid as claimed in claim 1, characterized in that: The mass ratio of the epoxy resin to the curing agent in the additive is 2:

1.

3. A method for preparing a cold mix asphalt mixture of a dry and wet dual-purpose type, characterized in that: The method for preparing the dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid according to claim 1 or 2 comprises the following steps: The diesel is used as a solvent-type diluent and mixed with the base asphalt to obtain a diluted asphalt; The additive is mixed with the diluted asphalt and stirred sufficiently to obtain an asphalt intermediate; The dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid is obtained by mixing the polyurethane prepolymer and the asphalt intermediate and stirring them fully.

4. The method for preparing a cold mix asphalt mixture of dry and wet dual-purpose according to claim 3, characterized in that: During the mixing process of the diesel and the matrix asphalt, the temperature is controlled at 100-120° C. and the time is controlled at 5-10 minutes; During the mixing process of the additive and the diluted asphalt, the temperature is controlled at 60-80°C and the time is controlled at 5-10 minutes; During the mixing process of the polyurethane prepolymer and the asphalt intermediate, the temperature is controlled at 80-100° C. and the time is controlled at 5-10 minutes.

5. A dry and wet dual-purpose cold mix asphalt mixture, characterized in that: The invention comprises the following components: by weight, 4 to 6 parts of the dry-wet dual-purpose cold-mix asphalt mixture cold-mix liquid according to claim 1 or 2, and 100 parts of aggregate and mineral powder.

6. A method for preparing a dry and wet dual-purpose cold mix asphalt mixture, characterized in that: The method for preparing the dry-wet dual-purpose cold-mix asphalt mixture according to claim 5 comprises the following steps: Preheating the cold mix liquid of the dry and wet dual-purpose cold mix asphalt mixture and the aggregate and mineral powder respectively; The preheated cold mix liquid of the dual-purpose cold mix asphalt mixture is mixed and stirred evenly with the aggregate and the mineral powder to obtain the dual-purpose cold mix asphalt mixture.

7. The method for preparing a wet and dry dual-purpose cold mix asphalt mixture as claimed in claim 6, characterized in that: The preheating temperature of the cold mix liquid of the dry and wet dual-purpose cold mix asphalt mixture is controlled at 80-100° C., and the preheated cold mix liquid of the dry and wet dual-purpose cold mix asphalt mixture is in a molten state; The preheating temperature of the aggregate and mineral powder is controlled at 70-80°C, and the preheating time is controlled at 2-6h; During the mixing and stirring process of the cold mix liquid of the dry-wet dual-purpose cold mix asphalt mixture with the aggregate and the mineral powder, the temperature is controlled at 50-80° C. and the time is controlled at 0.5-6 minutes.