Preparation method of gum dipping canvas

Abstract

The invention discloses a preparation method of gum dipping canvas, and relates to the technical field of gum dipping canvas preparation. During use, the impregnated canvas is firstly dried at the low temperature of 100 DEG C and then is respectively dried at the temperature of 150 DEG C, 130 DEG C and 75 DEG C in sequence, so that the situation that when high-temperature drying is directly adopted firstly, a glue film is formed on the surface layer of the canvas firstly, a glue solution on the inner layer of the canvas still contains a large amount of water, and the water on the inner layer is converted into water vapor in the heating process, so that the canvas is damaged is avoided. The inner-layer glue solution flows and gathers in the water vapor evaporation direction to break through the surface-layer glue film, and when the weak points of the outer-layer glue film are broken through, the cloth cover pocking marks are finally formed; according to the method, the drying temperature in the initial stage is reduced, the situation that the surface layer is rapidly dried to form a glue film is avoided, then water stored in the inner layer can be effectively removed in the drying process, and pocking marks are prevented from being formed on the surface after the gum dipping canvas is dried.

Description

Technical Field

[0001] The invention relates to a preparation technology of dipped canvas for producing adhesive tape, in particular to a preparation method of the dipped canvas. Background Art

[0002] The transmission belt is the transmission medium in the transmission belt system. It is a composite product of elastomer and fabric used for power transmission and is used in the field of industrial production. Dipped canvas is an important component of the transmission belt, and the dipping solution is an important raw material for preparing the dipped canvas. Traditional dipping solution uses solvent chemicals such as toluene, xylene, gasoline, acetone, etc. as the main raw materials, which have certain flammable and explosive properties. The volatilization of chemicals will also have an impact on the environment. After the dipped canvas is vulcanized and formed, powder will fall off during use, polluting the environment.

[0003] For example, CN103665493B discloses a dipping solution for transmission belts and a preparation method thereof, and a dipping cloth and a preparation method thereof. The dipping solution is a mixture of mixed liquids A, B, C, D and rubber adhesive RC: A contains hydroquinone, formaldehyde and deionized water; B contains butylpyrrolidone latex and deionized water; C contains adhesive A and deionized water; D contains blocked isocyanate, deionized water, paint black slurry or water-soluble carbon black and adhesive RP; rubber adhesive RC; the dipping solution contains hydroquinone, formaldehyde, etc., which are easily volatilized during use and have an impact on the environment; at the same time, during use, after the dipping is completed using the dipping solution, the outer surface of the dipped canvas is prone to quality pitting; when the pitting on the cloth surface is serious, it will affect the adhesive performance of the tape. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing dipped canvas. When in use, the dipped canvas is first dried at a low temperature of 100°C, and then dried at 150°C, 130°C and 75°C respectively, thereby solving the problem of quality pitting on the outer surface of the dipped canvas in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] The present invention is a method for preparing a dipped canvas, comprising:

[0007] Step 1: Prepare a dipping solution and put the obtained dipping solution into a dipping solution tank:

[0008] Step 2: Control the canvas to be dipped in glue to pass through the dip tank:

[0009] Step 3, drawing out the canvas from the dipping solution tank, and drying the obtained canvas to obtain a canvas blank;

[0010] Step 4, the obtained canvas blank is subjected to fiber opening, napping and cooling to obtain the dipped canvas and then rolled up;

[0011] In step 1, the temperature of the dipping solution is controlled to be between 45° C. and 50° C.;

[0012] In the step 3, the canvas is cooled at a temperature of 25±3°C for 3s-5s, and dried at 100±5°C, 150±5°C, 130±5°C and 75±5°C for 3-5min, 4-8min, 5-7min and 8-20min respectively; the total drying time in the step 3 is 30-35min.

[0013] Furthermore, in step 2, the canvas passes through the dipping liquid tank for 2-5 minutes; and a stirring device and a heating device are arranged in the dipping liquid tank, so as to keep the canvas in the dipping liquid tank during use.

[0014] Furthermore, in the step 2, a pair of guide roller groups are provided at the bottom of the dipping liquid tank for guiding the canvas introduced into the dipping liquid tank body, and the contact time of the canvas with the dipping liquid when passing through the tank body is adjusted during use; further, a guide roller group with a top higher than the liquid level of the dipping liquid in the dipping liquid tank is provided in the tank body located between the two guide roller groups.

[0015] Furthermore, the canvas is selected from one of cotton canvas, polyester-cotton canvas or polyester canvas; and when the canvas is polyester-cotton canvas, it is pre-dried and heated before dipping in glue and then introduced into the dipping liquid tank.

[0016] Furthermore, the dipping solution comprises, by weight, 50-60% of an aqueous polyurethane resin, 5-10% of a surfactant, and the remainder of deionized water;

[0017] The preparation method of the dipping solution comprises:

[0018] Stp01, put the waterborne polyurethane resin into the stirring device, and slowly add 50%-60% of deionized water during the stirring process;

[0019] Stp02, control the temperature in the stirring device to rise to 50℃-55℃;

[0020] Stp03, add surfactant into the stirring device during stirring, stir evenly and add 10%-15% of deionized water heated to 45°C into the stirring device;

[0021] Stp04. Slowly add the remaining 25%-40% of deionized water into the stirring device, stirring while adding.

[0022] Furthermore, the surfactant comprises, by weight, 9-20% polyethylene glycol monomethyl ether, 7-15% diisocyanate, 2-4% sodium hydroxide and 61%-83% deionized water;

[0023] The preparation method of the surfactant comprises:

[0024] Stp11, distilling polyethylene glycol monomethyl ether under reduced pressure at 100° C.-120° C. and a vacuum degree of 0.09 MPa for 5 hours to remove water;

[0025] Stp12, add polyethylene glycol monomethyl ether, sodium hydroxide and 80% of the diisocyanate into the container, stir in a constant temperature water bath at 60℃-80℃, and react for 10-15h;

[0026] Step 13, filter out the sodium hydroxide solid, add the remaining amount of diisocyanate, and continue the reaction for 1-2 hours;

[0027] Stp14, cooling the reaction system of Stp13 to 40°C-50°C;

[0028] Stp15, slowly adding deionized water into the reaction system obtained in Stp14, and stirring to obtain a surfactant.

[0029] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0030] The present invention first dries the dipped canvas at 100°C and then sequentially dries it at 150°C, 130°C and 75°C to avoid the situation where a glue film is first formed on the surface of the canvas when high-temperature drying is used directly first, while the glue in the inner layer of the canvas still contains a large amount of water. During the heating process, the moisture in the inner layer is converted into water vapor, and the glue in the inner layer flows and gathers in the direction of evaporation of the water vapor to break the surface glue film, and finally forms pockmarks on the cloth surface when the weak points of the outer glue film are broken. That is, in the present invention, the drying temperature in the initial stage is lowered to avoid the surface layer from being dried rapidly to form a glue film, so that the moisture in the inner layer can be effectively removed during the drying process. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0032] Figure 1 The present invention is a flow chart of the method for preparing the dipped canvas. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] In the production process of adhesive tape, usually the mixed rubber and the dipped canvas are compounded to form a tape blank, which is then vulcanized and formed. In the traditional preparation process, when the dipped canvas is directly dried at high temperature, quality pitting is prone to occur on the outer surface of the dipped canvas. The presence of these quality pitting affects the bonding strength between the dipped canvas and the rubber, and easily causes the adhesive tape to debond.

[0035] In order to solve the problem that the outer surface of the dipped canvas is prone to quality pitting, Example 1 of the present invention provides a new method for preparing the dipped canvas, comprising (see Figure 1 ):

[0036] Step 1: Prepare a dipping solution and put the obtained dipping solution into a dipping solution tank:

[0037] Step 2: Control the canvas to be dipped in glue to pass through the dip tank, and control the canvas to pass through the dip tank for 3 minutes. Under the premise of keeping the canvas able to obtain the best amount of glue as much as possible, improve the dipping efficiency and avoid the dip time being too long to affect the production efficiency:

[0038] Step 3, drawing out the canvas from the dipping solution tank, and drying the obtained canvas to obtain a canvas blank;

[0039] Step 4, the obtained canvas blank is subjected to fiber opening, napping and cooling to obtain the dipped canvas and then rolled up;

[0040] In step 1, the temperature of the dipping solution is controlled to be between 45° C. and 50° C.;

[0041] In the step 3, the canvas is cooled at 25°C for 4s, and dried at 100°C, 150°C, 130°C and 75°C for 4min, 6min, 6min and 18min respectively.

[0042] In the present invention, the canvas is polyester-cotton canvas, but in some other possible implementations, cotton canvas and polyester canvas may also be used.

[0043] In the above, the canvas that has been impregnated with glue is first dried at a low temperature of 100°C, and then dried at 150°C, 130°C and 75°C respectively; the drying temperature in the initial stage is lowered during use to avoid rapid drying of the impregnated surface layer to form a glue film, thereby effectively removing the moisture in the inner layer during the drying process.

[0044] It can be known that in the present invention, the dipping solution is maintained between 45°C and 50°C. The dipping solution at this temperature has good fluidity, that is, when the canvas to be dipped is controlled to pass through the dipping solution tank during use, the dipping solution can be controlled to repeatedly soak the canvas and adhere to both sides of the canvas, and the dipping solution can be kept evenly adhered to any length position of the canvas passing through the dipping solution tank.

[0045] In order to verify the preparation method of the dipped canvas, in comparative example 1, steps 1-2 and step 4 are the same as those in embodiment 1, and only in step 3, the canvas is dried at 150°C, 130°C and 75°C for 8min, 8min and 18min respectively.

[0046] The relevant test results are shown in Table 1 below:

[0047] project Appearance Adhesion strength Fiber peeling and fracture Example 1 No pitting 6.2 Partial break Comparative Example 1 There are pitting 4.5 A small amount of breakage

[0048] As can be seen from Table 1 above, when in use, the dipped canvas prepared by the present invention has no pitting on its outer surface and has a good appearance; and by comparing Example 1 with Comparative Example 1, the bonding performance between the dipped canvas and the rubber coating prepared in Example 1 is better, which shows that the presence of pitting will affect the bonding strength between the dipped canvas and the rubber coating.

[0049] Specifically, in the above technical solution, the dipping solution used includes 55% of aqueous polyurethane resin, 7% of surfactant, and the balance of deionized water by weight, and the preparation method of the dipping solution includes:

[0050] Stp01, put the waterborne polyurethane resin into the stirring device, and slowly add 50% of the deionized water during the stirring process;

[0051] Stp02, control the temperature in the stirring device to rise to 50°C;

[0052] Stp03, add surfactant into the stirring device during stirring, stir evenly, add 15% of deionized water heated to 45°C into the stirring device;

[0053] Stp04. Slowly add the remaining 35% of deionized water into the stirring device, stirring while adding.

[0054] Furthermore, deionized water is added slowly during use, so that the waterborne polyurethane resin preparation process can be kept from experiencing drastic temperature changes in a short period of time, thereby avoiding uneven dispersion of the components in the waterborne polyurethane resin.

[0055] That is to say, when in use, a dipping solution formed by mixing an aqueous polyurethane resin, a surfactant and deionized water is used, and the dipping solution does not contain hydroquinone, formaldehyde and the like, so as to avoid volatilization of hydroquinone, formaldehyde and the like in the dipping solution at high temperature when the dipped canvas is dried. After the volatilization of hydroquinone, formaldehyde and the like, the volatilization will have an impact on the environment.

[0056] In the present invention, the waterborne polyurethane resin is a reaction product of diisocyanate, polyester diol, propane pyridinium sulfonate and dimethylol propionic acid, and the obtained waterborne polyurethane resin contains a pyridine ring structure; that is, when used, the pyridine group is introduced into the system, and the polar amino atoms on the pyridine ring can react with the organic groups in the rubber or polyester fiber to form hydrogen bonds and dipole bonds, thereby improving the bonding strength, so that the dipped canvas has good film-forming performance when bonded to the rubber layer or rubber covering adhesive, and the adhesive film is flexible and has excellent flexural resistance.

[0057] In Comparative Example 2, nylon is used instead of polyester, and the canvas dipping operation is carried out according to steps 1 to 4. After the canvas dipping operation is completed, the obtained dipped canvas is tested including appearance and performance.

[0058] The relevant test results are shown in Table 2 below:

[0059] project Appearance Adhesion strength Fiber peeling and fracture Example 1 No pitting 6.2 Partial break Comparative Example 2 No pitting 5.6 A small amount of breakage

[0060] From the above results, it can be seen that the polyester canvas obtained by dipping in rubber has better interlayer bonding strength than the dipping in rubber nylon canvas.

[0061] The properties of polyester fiber and nylon fiber of polyester canvas and nylon canvas were tested respectively; the test results are shown in Table 3 below.

[0062] project Polyester Nylon Softening point (℃) 238 180 Melting point (℃) 260 220 Breaking strength (cN / tdex) 8.1 8.2 Elongation at break (%) 14.5 24.5 Shrinkage rate (%) 2.6 7.5 Thermal decomposition starting temperature (℃) 438 410 Thermal decomposition termination temperature (℃) 468 444 Weight loss rate (%) 83.5 92.4

[0063] The starting and ending temperatures of polyester are slightly lower than those of nylon 6. This is because the fiber will be thermally degraded under the action of heat, causing a decrease in molecular weight and changes in composition. Under the action of heat, the crystallization of the fiber will disintegrate and the orientation will decrease. The presence of benzene ring structure in polyester molecules makes it difficult to thermally crack. The CN bond in the amide group-CO-NH- and the C-0 bond in the main chain of the macromolecule in polyamide fibers are easily broken after heating, which reduces the degree of polymerization of the macromolecule. Therefore, when used, polyester canvas has better heat resistance than nylon canvas. In addition, due to the small number of methylene groups in nylon macromolecules, more hydrogen bonds are formed, and the interaction between macromolecules is strong, resulting in the bonding performance of nylon canvas with rubber layer or rubber covering glue being lower than that of polyester canvas with rubber layer or rubber covering glue when used.

[0064] In order to provide a better dipping effect during use, a stirring device and a heating device are arranged in the dipping liquid tank, so as to keep the dipping liquid tank in a flowing state and within a constant temperature range during use, thereby ensuring the dipping effect of the canvas.

[0065] In an achievable technical solution, the provided surfactant comprises, by weight, 9-20% polyethylene glycol monomethyl ether, 7-15% diisocyanate, 2-4% sodium hydroxide and 61-83% deionized water; the specific surfactant comprises, by weight, 9% polyethylene glycol monomethyl ether, 8% diisocyanate, 3% sodium hydroxide and 80% deionized water; or the surfactant comprises, by weight, 15% polyethylene glycol monomethyl ether, 12% diisocyanate, 3% sodium hydroxide and 70% deionized water; or the surfactant comprises, by weight, 20% polyethylene glycol monomethyl ether, 15% diisocyanate, 4% sodium hydroxide and 61% deionized water.

[0066] Specifically, the preparation method of the surfactant includes:

[0067] Stp11, distilling polyethylene glycol monomethyl ether under reduced pressure at 100° C.-120° C. and a vacuum degree of 0.09 MPa for 5 hours to remove water;

[0068] Stp12, add polyethylene glycol monomethyl ether, sodium hydroxide and 80% of the diisocyanate into the container, stir in a constant temperature water bath at 60℃-80℃, and react for 10-15h;

[0069] Step 13, filter out the sodium hydroxide solid, add the remaining amount of diisocyanate, and continue the reaction for 1-2 hours;

[0070] Stp14, cooling the reaction system of Stp13 to 40°C-50°C;

[0071] Stp15, slowly adding deionized water into the reaction system obtained in Stp14, and stirring to obtain a surfactant.

[0072] The surfactant provided above is synthesized by a two-step method during the synthesis process. The synthesis process is simple, does not require group protection, does not use organic solvents, and is environmentally friendly. At the same time, compared with traditional surfactants, the product is easier to adsorb on the gas-liquid interface, can more effectively reduce surface tension, and has a lower lower limit temperature for use.

[0073] Furthermore, when using the surfactant, the deionized water is added slowly, so that the surfactant preparation process can avoid drastic temperature configuration in a short period of time, thereby avoiding uneven dispersion of the components in the surfactant.

[0074] Different from Example 1, in Comparative Example 3, the polyester-cotton canvas to be introduced into the dipping solution tank is pre-dried and heated, and the temperature of the polyester-cotton canvas area corresponding to the moment the polyester-cotton canvas contacts the dipping solution in the dipping solution tank is kept higher than 35°C.

[0075] Keep following steps 1 to 4 to carry out the canvas dipping operation, and after completing the canvas dipping operation, conduct appearance and performance tests on the obtained dipped canvas.

[0076] The relevant test results are shown in Table 3 below:

[0077] project Appearance Adhesion strength Fiber peeling and fracture Example 1 No pitting 6.2 Partial break Comparative Example 3 No pitting 6.6 A lot of breakage

[0078] The test results show that due to the strong water absorption of polyester-cotton canvas, when in use, the polyester-cotton canvas comes into contact with the air and absorbs some water. Therefore, when in use, the polyester-cotton canvas should be pre-dried before dipping in glue to remove the moisture absorbed by the polyester-cotton canvas from the air, so as to increase the amount of glue attached to the canvas and avoid pitting on the cloth surface.

[0079] Comparative Example 4 is different from Comparative Example 3. In Comparative Example 4, step 3 sequentially controls the polyester canvas to be dried at temperatures of 150° C., 130° C., and 75° C. for 8 min, 8 min, and 18 min, respectively.

[0080] After the canvas dipping operation is completed, the obtained dipped canvas is tested for appearance and performance. The relevant test results are shown in Table 4 below:

[0081] project Appearance Adhesion strength Fiber peeling and fracture <![CDATA[Adhesive application amount (g / m 2 )]]> Example 1 No pitting 6.2 Partial break 220 Comparative Example 3 No pitting 6.6 A lot of breakage 265 Comparative Example 4 A small amount of pitting 5.7 Partial break 265

[0082] According to the above content control, and combined with the experimental results of comparative example 1, it can be known that by pre-drying and heating the polyester canvas before dipping, the number of pits in the dipped canvas after dipping can be reduced, and the interlayer bonding strength after dipping can be improved. And when in use, heating the polyester canvas before dipping can increase the amount of adhesive attached to the canvas during the dipping process of the polyester canvas.

[0083] In Comparative Example 5, the time for the canvas to pass through the dipping solution tank is adjusted, and the rest is carried out in the manner of Example 1. That is, on the basis of Example 1, the time for the canvas to pass through the dipping solution tank is adjusted from 3 minutes to 4 minutes in Comparative Example 5; the obtained dipped canvas is tested for appearance and performance, and the relevant test results are shown in Table 5 below.

[0084] project Appearance Adhesion strength Fiber peeling and fracture <![CDATA[Adhesive amount (g / m 2 )]]> Example 1 No pitting 6.2 Partial break 220 Comparative Example 5 No pitting 6.6 Partial break 267

[0085] Based on the above experimental data, it can be known that by increasing the amount of adhesive attached to the polyester canvas during the dipping process, the interlayer bonding strength of the product can be improved.

[0086] In order to adjust the time for the canvas to pass through the dipping liquid tank in step 2, the present invention provides a pair of guide roller groups at the bottom of the dipping liquid tank for guiding the canvas in the dipping liquid tank body, and adjusts the contact time of the canvas with the dipping liquid when passing through the tank body when in use, that is, by controlling the traction speed of the canvas to remain unchanged during use, the spacing between the two guide roller groups is adjusted to adjust the time for the canvas to pass through the dipping liquid.

[0087] In Comparative Examples 6-8, in order to improve the dipping effect, a guide roller group with a top higher than the liquid level of the dipping liquid in the dipping liquid tank is arranged in the tank body located between the two guide roller groups, so that when in use, the canvas with the dipping liquid attached is controlled to protrude from below the liquid level of the dipping liquid during the dipping process, and then when cooled by air at a wind speed lower than 0.5m / s, the canvas is controlled to be sent back to below the liquid level of the dipping liquid after cooling to complete the second dipping.

[0088] Compared with Example 1, in the two schemes, by adjusting the distance between the two guide roller groups, the total contact time between the canvas and the dipping solution is maintained for 3 minutes in Example 1 and Comparative Examples 6-10; and synchronously controlling in Comparative Examples 6-10, the canvas sticks out from above the dipping solution surface and then re-enters below the dipping solution surface, and the duration of this intermittent stage is controlled to be 10s, 20s, 25, 30s and 40s respectively, and synchronously the dipped canvases obtained in Example 1 and Comparative Examples 6-10 are subjected to relevant performance tests, and the relevant performance test results are shown in Table 6 below.

[0089] project Appearance Adhesion strength Fiber peeling and fracture <![CDATA[Adhesive amount (g / m 2 )]]> Example 1 No pitting 6.2 Partial break 220 Comparative Example 6 No pitting 6.6 Partial break 240 Comparative Example 7 No pitting 6.7 Partial break 265 Comparative Example 8 No pitting 6.7 Partial break 271 Comparative Example 9 No pitting 6.6 Partial break 274 Comparative Example 10 No pitting 6.6 Partial break 275 Comparative Example 11 No pitting 6.3 Partial break 222

[0090] It can be seen from the above data that by controlling the operation of the canvas to stick out from above the liquid surface of the dipping liquid and then re-enter below the liquid surface of the dipping liquid, the bonding strength can be improved while increasing the amount of glue attached to the canvas; and combined with the experimental data of Comparative Example 5, it can be known that by controlling the operation of the canvas to stick out from above the liquid surface of the dipping liquid and then re-enter below the liquid surface of the dipping liquid, the bonding strength can be improved while obtaining the same amount of glue attached.

[0091] At the same time, through the analysis and control of the experimental data of comparative examples 6-10, when in use, by controlling the duration of the intermittent stage in which the canvas protrudes from above the liquid surface of the dipping liquid and then re-enters below the liquid surface of the dipping liquid to be between 20s and 25s, it can not only obtain more glue attached to the canvas, but also simultaneously improve the bonding strength.

[0092] Similarly, in the above-mentioned comparative example 11, by shortening the interval between the two guide roller groups, that is, on the basis of Example 1, the time for the canvas to pass through the dipping liquid tank in comparative example 11 is adjusted from 3 minutes to 2.7 minutes; in comparative example 11, by controlling the duration of the intermittent stage in which the canvas protrudes from above the dipping liquid surface and then re-enters below the dipping liquid surface is 25 seconds; and thus, when in use, the amount of adhesive attached to the canvas obtained in comparative example 11 is controlled to be equivalent to the amount of adhesive attached to the canvas in Example 1. Furthermore, through analysis of the above data, it can be seen that in the present invention, under the premise of the same amount of adhesive attached to the canvas, dipping is carried out by secondary dipping, which can improve the bonding strength of the product after dipping.

[0093] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a dipped canvas, characterized in that: The steps include: Step 1: Prepare a dipping solution and put the obtained dipping solution into a dipping solution tank: Step 2: Control the canvas to be dipped in glue to pass through the dip tank: Step 3, drawing out the canvas from the dipping solution tank, and drying the obtained canvas to obtain a canvas blank; Step 4, the obtained canvas blank is subjected to fiber opening, napping and cooling to obtain the dipped canvas and then rolled up; In step 1, the temperature of the dipping solution is controlled to be between 45° C. and 50° C.; In the step 3, the canvas is cooled at a temperature of 25±3°C for 3s-5s, and dried at 100±5°C, 150±5°C, 130±5°C and 75±5°C for 3-5min, 4-8min, 5-7min and 8-20min respectively; the total drying time in the step 3 is 30-35min.

2. The method for preparing a dipped canvas according to claim 1, characterized in that: In step 2, the canvas passes through the dipping tank for 2-5 minutes.

3. The method for preparing a dipped canvas according to claim 1, characterized in that: The dipping solution comprises 50-60% by weight of an aqueous polyurethane resin, 5-10% by weight of a surfactant, and the balance of deionized water; The preparation method of the dipping solution comprises: Stp01, put the waterborne polyurethane resin into the stirring device, and slowly add 50%-60% of deionized water during the stirring process; Stp02, control the temperature in the stirring device to rise to 50℃-55℃; Stp03, add surfactant into the stirring device during stirring, stir evenly and add 10%-15% of deionized water heated to 45°C into the stirring device; Stp04. Slowly add the remaining 25%-40% of deionized water into the stirring device, stirring while adding.

4. The method for preparing a dipped canvas according to claim 3, characterized in that: The surfactant comprises, by weight, 9-20% polyethylene glycol monomethyl ether, 7-15% diisocyanate, 2-4% sodium hydroxide and 61%-83% deionized water; The preparation method of the surfactant comprises: Stp11, distilling polyethylene glycol monomethyl ether under reduced pressure at 100° C.-120° C. and a vacuum degree of 0.09 MPa for 5 hours to remove water; Stp12, add polyethylene glycol monomethyl ether, sodium hydroxide and 80% of the diisocyanate into the container, stir in a constant temperature water bath at 60℃-80℃, and react for 10-15h; Step 13, filter out the sodium hydroxide solid, add the remaining amount of diisocyanate, and continue the reaction for 1-2 hours; Stp14, cooling the reaction system of Stp13 to 40°C-50°C; Stp15, slowly adding deionized water into the reaction system obtained in Stp14, and stirring to obtain a surfactant.

5. The method for preparing a dipped canvas according to claim 3, characterized in that: During the cooling process of step 4, the canvas blank is cooled at room temperature with the aid of a fan blowing airflow directly toward the canvas blank.

6. The method for preparing a dipped canvas according to claim 1, characterized in that: A stirring device and a heating device are arranged in the dipping liquid tank, so as to maintain the dipping liquid in the tank during use.

7. The method for preparing a dipped canvas according to claim 1, characterized in that: The canvas is selected from one of cotton canvas, polyester-cotton canvas or polyester canvas.

8. The method for preparing a dipped canvas according to claim 7, characterized in that: When the canvas is polyester-cotton canvas, it is pre-dried and heated before dipping in glue and then introduced into the dipping liquid tank.

9. The method for preparing a dipped canvas according to claim 1, characterized in that: In step 2, a pair of guide rollers are provided at the bottom of the dipping liquid tank for guiding the canvas introduced into the tank body of the dipping liquid tank, and the contact time between the canvas and the dipping liquid is adjusted when in use.

10. The method for preparing a dipped canvas according to claim 9, characterized in that: A guide roller group with a top higher than the liquid level of the dipping solution in the dipping solution tank is arranged in the tank body between the two guide roller groups.

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