JP2015232062A - Wet friction material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wet friction material exhibiting higher durability.SOLUTION: There is provided a wet friction material manufactured by heating and drying a resin impregnated paper-making body where an impregnation liquid which is a thermosetting resin dissolved in a solvent, is impregnated into a paper making body from a fiber, the impregnation liquid has a viscosity within a range of 5 to 8 cP at a liquid temperature of 18°C to 25°C when a nonvolatile content is 50%.

Description

  The present invention relates to a wet friction material.

  In recent years, automobiles that employ an automatic transmission (hereinafter sometimes abbreviated as AT) as a transmission have become widespread. In general, AT is a combination of a torque converter that transmits torque using an automatic transmission fluid and a multi-stage transmission gear. In general, a torque converter is provided with a lockup clutch for engaging a pump impeller for inputting a driving force from an engine and a turbine liner for driving an output shaft. The lock-up clutch is provided with a wet friction material used in the presence of an automatic transmission fluid. As the pump impeller and the turbine liner are engaged via the lock-up clutch, the transmission efficiency of the driving force is improved.

  As described in Patent Documents 1 to 3, usually, a wet friction material is produced by using a fiber and an additive such as pulp as raw materials, and then impregnating the papermaking body with a thermosetting resin, It is manufactured by heating the paper body after impregnation. The wet friction material is porous and has countless continuous pores on the surface and inside.

  When the wet friction material is deteriorated, the transmission efficiency of the driving force is lowered, so that the wet friction material is required to have high durability.

Japanese Patent Laid-Open No. 6-229434 JP 2009-62514 A JP 2013-32794 A

  An object of this invention is to provide the wet friction material which shows higher durability.

  The present inventor has conducted earnest research focusing on the thermosetting resin used in the production of the wet friction material. And by reducing the viscosity of the thermosetting resin used in the conventional impregnating liquid, the content of the thermosetting resin, which is a non-volatile content in the impregnating liquid, can be made the same, which is required for wet friction materials. The present inventor has found that the durability of the wet friction material is remarkably improved while maintaining the desired friction characteristics, and has completed the present invention.

  That is, the wet friction material of the present invention has a viscosity of the impregnating liquid when the non-volatile content of the impregnating liquid, that is, the content of the remaining thermosetting resin excluding the volatile component solvent contained in the impregnating liquid is 50%. It is characterized by being manufactured by impregnating this impregnating liquid into a papermaking body containing fibers at a liquid temperature of 18 ° C. to 25 ° C. within a range of 5 to 8 cP.

  According to the wet friction material of the present invention, since the viscosity of the thermosetting resin to be impregnated is set low, the inside of the papermaking body can be easily impregnated. From this, it becomes easy to coat the fibers of the papermaking body with the thermosetting resin, and the pores formed inside the papermaking body can be made uniform. As a result, the automatic transmission liquid can be uniformly permeated into the paper body. Therefore, the cooling performance by the automatic transmission fluid is easily applied to the entire paper body, and the deterioration of heat resistance due to the generation of heat spots is suppressed, and the durability is improved. Here, when the viscosity exceeds the specified range, the impregnation performance of the impregnating liquid deteriorates. When the viscosity is smaller than the specified range, the viscosity of the impregnating liquid is determined by the thermosetting resin contained in the impregnating liquid. It becomes difficult to maintain the desired characteristics required for the wet friction material. Therefore, the viscosity of the impregnation liquid is defined from the viewpoint of impregnation performance and maintaining the properties of the thermosetting resin.

It is a graph which shows the result of the slip durability test of the evaluation example 1. (A) The initial microscope picture of the wet friction material of Example 1, (b) The microscope picture of the wet friction material of Example 1 after 12000 cycles. (A) Initial micrograph of wet friction material of Comparative Example 1, (b) Micrograph of wet friction material of Comparative Example 1 after 7600 cycles. 7 is a graph showing the results of a μ-V test in Evaluation Example 2. 2 is a cross-sectional enlarged photograph of a wet friction material of Example 1. 2 is a cross-sectional enlarged photograph of a wet friction material of Comparative Example 1.

  The best mode for carrying out the present invention will be described below. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from the numerical value range can be used as upper and lower numerical values.

  In the wet friction material of the present invention, the viscosity of the impregnating liquid is set within a range of 5 to 8 cP at a liquid temperature of 18 ° C. to 25 ° C. when the non-volatile content is 50%. It is manufactured.

  In this specification, the viscosity means a value measured with a Brookfield type rotational viscometer according to JIS K7117-1.

  Conventionally, the impregnating liquid used for the production of the wet friction material has a viscosity of about 10 cP at a liquid temperature of 18 ° C. to 25 ° C. when the nonvolatile content is 50%.

  Examples of the thermosetting resin include phenol resin, modified phenol resin, melamine resin, and epoxy resin, and phenol resin is particularly preferable. As the thermosetting resin, those described above may be used alone or in combination.

  In order to obtain the viscosity of the present invention, the viscosity can be obtained by changing the molecular weight of the thermosetting resin. For example, in the case of a phenol resin, a resin having a desired viscosity may be synthesized by reacting a phenol derivative and an aldehyde derivative under appropriate conditions with reference to known reaction conditions.

  Examples of the phenol derivative include phenol, phenylphenol, aminophenol, alkylphenols such as cresol, xylenol, ethylphenol, butylphenol and octylphenol, polyhydric phenols such as resorcinol and catechol, bisphenol A, bisphenol F, bisphenol S, bisphenol E, Examples thereof include bisphenols such as thiobisphenol, bis (hydroxyphenyl) ether and dihydroxybenzophenone, halogenated phenols such as chlorophenol and bromophenol, and naphthols such as α-naphthol and β-naphthol. As a phenol derivative, the above-mentioned thing may be used independently and multiple may be used together.

  Examples of aldehyde derivatives include formaldehyde, paraformaldehyde, trioxane, acetaldehyde, propionaldehyde, hexamethylenetetramine, furfural, glyoxal, butyraldehyde, benzaldehyde, crotonaldehyde, and acrolein. As an aldehyde derivative, the above-mentioned thing may be used independently and multiple may be used together.

  By changing the thermosetting resin itself in this way, the viscosity of the impregnating liquid can be reduced without reducing the content of the thermosetting resin, which is the non-volatile content of the impregnating liquid, and the impregnation performance to the papermaking body can be reduced. It becomes possible to improve. In the present invention, a phenol resin is used as the thermosetting resin, and the viscosity of the specified range is obtained by using a phenol resin having a molecular weight lower than that of the conventional phenol resin. In addition, the solvent should just melt | dissolve a thermosetting resin and volatilize, for example, if it is a phenol resin, methyl alcohol, methyl ethyl ketone, etc. will be mentioned.

  Hereinafter, the present invention will be described along the method for producing a wet friction material of the present invention.

  The method for producing a wet friction material according to the present invention includes a step of preparing a paper body made of paper, and a viscosity at a liquid temperature of 18 ° C. to 25 ° C. of 5 to 8 cP when the non-volatile content is 50%. An impregnation step of impregnating the impregnation liquid therein, and a drying step of heating and drying the papermaking body after the impregnation step, that is, the resin-impregnated papermaking body.

  First, the process for preparing a paper body made of fibers will be described.

  Fibers include glass fiber, rock wool (rock wool), slag wool (mineral cotton), titanate fiber, ceramic fiber, silica fiber, alumina fiber, silica-alumina fiber, kaolin fiber, bauxite fiber, cayanoid fiber, boron Fiber, magnesia fiber, inorganic fiber such as metal fiber, carbon fiber, linter pulp, wood pulp, synthetic pulp, polyester fiber, polyamide fiber, aramid fiber, polyimide fiber, polyvinyl alcohol modified fiber, polyvinyl chloride fiber , Polyethylene fiber, polypropylene fiber, polybenzimidazole fiber, acrylic fiber, phenol fiber, nylon fiber, cellulose fiber, or one or more organic fibers such as polyparaphenylene benzoxazole (PBO) fiber. Aramid fibers are preferred from the viewpoints of ease of penetration of automatic transmission fluid, durability, price, versatility and the like.

  By making such a fiber, a skeleton of the paper body is formed. And the papermaking body used as a wet friction material contains additives, such as a friction modifier which controls friction.

  Additives include barium sulfate, calcium carbonate, aluminum hydroxide, magnesium carbonate, potassium titanate, zinc oxide, titanium oxide, diatomaceous earth, cork powder, cashew dust, graphite, carbon fiber, carbon powder, molybdenum disulfide, trisulfide Antimony, aluminum powder, wollastonite, silica, silicon carbide, boron carbide, titanium carbide, silicon nitride, boron nitride, alumina, zirconia, cashew, talc, kaolin, nitrile rubber, acrylonitrile butadiene rubber, styrene butadiene rubber, silicon rubber, An example is fluororubber.

  Such additives are included in the papermaking body at a predetermined ratio in order to obtain desired friction characteristics. A normal papermaking method can be used as a method for producing the papermaking body. That is, the fiber and the necessary additive are prepared in an amount necessary for obtaining a desired friction characteristic, and this is dispersed in water to form a slurry. A paper body is obtained by dehydrating and drying the slurry.

  Next, the impregnation process will be described. In the impregnation step, the papermaking body is impregnated with the impregnation liquid. The paper body impregnated with the impregnating liquid is a resin-impregnated paper body. Here, the impregnating liquid contains a solvent so that the thermosetting resin can easily be impregnated into the paper body. When the amount of the solvent is increased, the viscosity of the impregnating liquid easily decreases, and the thermosetting resin penetrates more into the paper body. It becomes easy. However, when the viscosity is reduced by increasing the amount of the solvent, the amount of non-volatile components in the impregnating liquid, that is, the amount of the thermosetting resin is decreased as a result of increasing the amount of the solvent. For this reason, the amount of the thermosetting resin in the papermaking body is reduced, and a portion of the papermaking body where the coating on the fibers is insufficient tends to remain. As a result, it becomes easy to invite a situation where the wet friction material is insufficient in strength or has characteristics such as desired friction characteristics. For this reason, in the present invention, the viscosity of the thermosetting resin itself is lowered, not reduced by increasing the amount of solvent, and the impregnating liquid is easy to impregnate without causing a decrease in the amount of resin in the papermaking body. In the present invention, by adjusting the molecular weight to be smaller than the conventional one, the liquid temperature during normal impregnation (18 ° C. to 25 ° C.) is maintained even if the non-volatile content (thermosetting resin content) of the impregnation liquid is the same as the conventional one In this case, the impregnating liquid has a lower viscosity than the conventional one. By using such an impregnating liquid having a reduced viscosity, it is possible to produce a wet friction material satisfying desired required characteristics without reducing the amount of the thermosetting resin in the paper body. Here, as the impregnation method, a normal impregnation method can be used, and examples include a method of immersing the papermaking body in the impregnation liquid and a method of dropping the impregnation liquid into the papermaking body.

  Next, a drying process for heating and drying the paper body after the impregnation process will be described. In this drying step, the solvent contained in the impregnation liquid impregnated in the paper body is volatilized to adhere the thermosetting resin to the paper body. Therefore, the solvent only needs to be sufficiently volatilized. For example, the drying temperature may be 150 to 250 ° C. and the drying time may be 30 to 60 minutes.

  Since the wet friction material of the present invention is manufactured using a material having a lower viscosity than the conventional thermosetting resin, it is easy to enter the inside of the papermaking body, and the thermosetting resin is relatively made of the papermaking body. It exists uniformly inside, and it is difficult to partially form a lump of thermosetting resin inside the paper body as in the case of using a conventional thermosetting resin. In addition, it is easy to uniformly coat the paper body with the resin. When a conventional thermosetting resin is used, a resin lump is likely to be partially generated inside the paper body, and the generated lump partially forms a narrow portion in the pores inside the paper body. The pore distribution of the friction material becomes non-uniform. As a result, the automatic transmission fluid that flows into the pores and cools the wet friction material is unevenly present in the wet friction material, and the insufficient portion of the automatic transmission fluid is likely to become a heat spot, causing deterioration in durability. It was. Further, if the coating of the papermaking body on the fibers is not uniform, the strength of the fibers is not constant, and the strength of the uncoated portion is inferior, which also causes the durability to deteriorate. On the other hand, the wet friction material of the present invention is difficult to form a lump of thermosetting resin, and it is easy to uniformly coat the paper body with the resin. Durability performance is improved compared to the material.

  The wet friction material of the present invention is used in the presence of an automatic transmission fluid. Applications include lockup clutches and wet multi-plate clutches.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to the following examples. In the following, unless otherwise specified, “part” means part by mass, and “%” means mass%.

(Example 1)
A slurry was prepared by dispersing 40% by weight of aramid fibers, 10% by weight of an additive such as a friction modifier, and 50% by weight of a filler such as diatomaceous earth in water. The slurry was made into paper and dried to obtain a paper body. In addition, the kind and compounding quantity of compounding materials, such as an aramid fiber contained in a papermaking body, are determined by the characteristic requested | required of a wet friction material, and are not limited to this.

  Next, methanol was used as a volatile component solvent to prepare an impregnating solution of phenol resin having a nonvolatile content of 50% and a viscosity of 20 c at 6.5 cP. Here, although the phenol resin of the conventional impregnation liquid used the thing of average molecular weight (Mn) 220-300, in this invention, by using what was adjusted to average molecular weight (Mn) 100-200, The viscosity of the impregnating liquid is within the specified range. The viscosity is in accordance with JIS K7117-1, using a Brookfield rotational viscometer type A, and a rotational speed of 50 / min. , And measured at room temperature (18 ° C. to 25 ° C.).

  The papermaking body was immersed in an impregnating liquid whose viscosity was adjusted to a specified range to prepare a resin-impregnated papermaking body in which the papermaking body was impregnated with a thermosetting resin. Thereafter, the resin-impregnated papermaking body, which is the papermaking body after impregnation, was dried by heating at 200 ° C. for 60 minutes. In the wet friction material of Example 1, the phenol resin in the wet friction material accounts for 30 to 50% by weight.

(Comparative Example 1)
A wet friction material of Comparative Example 1 was produced in the same manner as in Example 1 except that an impregnating solution of a phenol resin having a nonvolatile content of 50% and a viscosity of 20 c at 10 ° C was used. In addition, in the wet friction material of the comparative example 1, the phenol resin in a wet friction material occupies 30 to 50 weight%.

(Evaluation Example 1) Slip Durability Test In order to evaluate the durability of the wet friction material of the present invention, a slip durability test with one cycle of 5 seconds of slip and 25 seconds of non-slip is performed on the wet friction material under the following conditions. Went. The above cycle was repeated until the friction coefficient μ of the wet friction material decreased by 10% from the initial friction coefficient. The results are shown in FIG.

Testing machine: Low-speed sliding friction test equipment (LVFA) ACS-XI
Wet friction material shape: Record shape with outer diameter 133mm, inner diameter 113.6mm Lubricating oil: Automatic transmission fluid (CVT fluid)
Lubricating oil amount: 1000mL (Immerse all wet friction materials)
Oil temperature: 100 ° C
Plate temperature: 240 ° C
Rotational speed: 1333 rpm.
Torque: 70Nm
Calorific value: 130 J / (s · cm ² )

  As shown in FIG. 1, the friction coefficient μ of the wet friction material of Comparative Example 1 was less than 8000 cycles and decreased by 10% or more from the initial friction coefficient. On the other hand, the friction coefficient μ of the wet friction material of Example 1 was suitably maintained even when the number of cycles exceeded 30000. The wet friction material of Example 1 showed significantly superior slip durability as compared with the wet friction material of Comparative Example 1.

  FIG. 2 shows an initial microphotograph of the wet friction material of Example 1 and a microphotograph after 12,000 cycles. The magnification of each micrograph is 200 times. In the wet friction material of Example 1, no particular change was observed even after 12000 cycles. From FIG. 2, it can be seen that in the wet friction material of Example 1, clogging of pores was not observed, and the lubricating oil movement path in the slip durability test was suitably maintained.

  FIG. 3 shows an initial micrograph of the wet friction material of Comparative Example 1 and a micrograph after 7600 cycles. The magnification of each micrograph is 200 times. In the wet friction material of Comparative Example 1, it can be seen that the surface of the wet friction material was remarkably smoothed after 7600 cycles. That is, it can be said that the friction coefficient of the wet friction material has decreased as a result of the smoothing of the surface of the wet friction material.

  In the wet friction material of Comparative Example 1, a thermosetting resin lump (dama) is observed as shown in FIG. The pores are constricted by this lump, and the automatic transmission fluid with cooling performance flows in and out. As a result, the frictional heat cannot be sufficiently dissipated and the wet friction material becomes hot. It is considered that the deterioration was promoted by this. On the other hand, in the wet friction material of Example 1, since a low-viscosity thermosetting resin is used, a lump is not easily formed as shown in FIG. 5, and an automatic transmission liquid having cooling performance can sufficiently flow in and out. As a result, the durability is considered to be improved.

(Evaluation example 2) μ-V test Further, as an index indicating durability performance, the wet friction material was accelerated from 0 rpm to 210 rpm in 10 seconds under the following conditions, and then decelerated in 10 seconds from 210 rpm to 0 rpm in 1 second. A μ-V test was carried out as a cycle. The friction coefficient μ at 50 rpm and the friction coefficient μ at 200 rpm in each cycle were measured. The value of (friction coefficient μ at 50 rpm) / (friction coefficient μ at 200 rpm) was taken as the μ ratio, and the cycle was repeated until the μ ratio exceeded 1. The results are shown in FIG.

Testing machine: Low-speed sliding friction test equipment ACS-XI
Wet friction material shape: Record shape with outer diameter 133mm, inner diameter 113.6mm Lubricating oil: Automatic transmission fluid (CVT fluid)
Lubricating oil amount: 1000mL (Immerse all wet friction materials)
Oil temperature: 100 ° C
Plate temperature: 240 ° C
Surface pressure: 1.0 MPa

  The μ ratio of the wet friction material of Comparative Example 1 exceeded 1 at about 5000 cycles. On the other hand, the μ ratio of the wet friction material of Example 1 was preferably less than 1 until the number of cycles was 25000. When the μ ratio is smaller than 1, the μ-V characteristic has a positive gradient, and the wet friction material of the present invention exhibits a suitable μ ratio over a long period of time, that is, the wet friction material of the present invention is stable. It was confirmed that the μ-V characteristic shows a positive slope. Thus, it can be said that the wet friction material of the present invention is excellent in anti-shudder properties.

(Evaluation example 3) Oil absorption test To evaluate the ease of inflow of automatic transmission fluid into the wet friction material, 7 μL of the automatic transmission fluid is dropped on the wet friction material, and then the wet friction material causes the automatic transmission fluid to drop. The time until absorption was measured. The results are shown in Table 1. Note that CVT fluid (trade name: Nissan genuine CVT fluid NS-3) was used as the automatic transmission fluid.

  The wet friction material of the present invention has an oil absorption time of 10 seconds or less, and it was confirmed that the wettability with respect to the automatic transmission fluid and the permeability of the automatic transmission fluid were improved as compared with the conventional wet friction material.

(Evaluation example 4) Resin ratio measurement test In order to use as an index whether the automatic transmission fluid to the wet friction material uniformly penetrates into the wet friction material, each wet friction material of Example 1 and Comparative Example 1 The cross section is divided into three equal parts in the thickness direction, and the resin ratio is measured for each of the surface portion (both front and back surfaces) and the inside, and the difference between the resin ratio of the larger surface portion and the internal resin ratio is the internal resin The deviation rate of how much larger than the rate was calculated. That is, the ratio is calculated by the following equation.
Percentage of deviation (%) = (larger resin ratio of the surface portion−internal resin ratio) / internal resin ratio

  The reason for using such a bias is as follows. The resin ratio in the thickness direction of the wet friction material is optimally the same between the inside and the surface portion, but the internal resin ratio for the impregnating liquid to permeate from the surface to the inside tends to be smaller than the surface portion. Therefore, the difference between the resin ratio of the larger surface part and the internal resin ratio is obtained, and the ratio of this difference to the internal resin ratio is calculated, that is, thermosetting is performed by calculating the partial rate. It can be determined whether or not the conductive resin is uniformly impregnated into the paper body. The resin ratio is a value obtained by measuring the weight before impregnation of the paper body and the weight after impregnation heat drying, and dividing the weight after impregnation heat drying by the weight before impregnation. It is an index indicating how much is contained.

  From Table 2, in the conventional comparative example 1, the resin rate of the surface part is high with respect to the internal resin rate, and the deviation rate with respect to the internal resin rate is as large as 73%. The fact that the resin ratio of the surface portion is higher than the internal resin ratio means that the internal porosity is large but the surface portion porosity is small, and the automatic transmission fluid does not easily flow from the surface. On the other hand, in Example 1 of the wet friction material of the present invention, the difference between the resin ratio of the surface portion relative to the internal resin ratio is 28% and 70% or less in terms of the deviation rate. It is greatly reduced from 73% of the above. Therefore, it can be seen that the difference between the internal porosity and the porosity of the surface portion is small, the porosity in the thickness direction is higher than that of the conventional one, and is uniform throughout the thickness direction. . From this, it can be said that the automatic transmission fluid easily flows into the wet friction material uniformly. Further, when the overall state of the papermaking body shown in FIGS. 5 and 6, that is, the ease of formation of the thermosetting resin mass and the coating state on the papermaking body fiber are comprehensively determined, the pores inside the papermaking body are determined. It can be considered that the distribution is more uniform than the conventional one. When the pore distribution is made uniform in this way, the automatic transmission fluid is likely to penetrate uniformly into the entire wet friction material, and the entire wet friction material is easily cooled uniformly by this uniformly penetrated automatic transmission fluid. .

  Summarizing the above evaluation results, the wet friction material of the present invention is difficult to form a lump of thermosetting resin inside the papermaking body from the internal observation of the papermaking body of FIGS. It can be seen that the difference in the resin ratio in the thickness direction decreases to 70% or less, and the entire wet friction material approaches a uniform pore distribution. Due to the pore distribution approaching the uniformity, the transmission time of the automatic transmission fluid is shortened to 10 seconds or less from the result of the oil absorption test, and it is easily absorbed into the wet friction material and is easily discharged. For this reason, it can be said that the cooling performance by the automatic transmission fluid has been improved, and the slip durability and μ ratio measurement have shown good results. Moreover, it was proved from the internal observation of the papermaking body of FIGS. 5 and 6 that the wet friction material of the present invention coats the fibers of the papermaking body more uniformly with the thermosetting resin than the conventional one. This uniform coating suppresses variations in strength of the papermaking body fibers. This means that the strength of the papermaking body constituting the skeleton of the wet friction material is not partially covered by the thermosetting resin. Although it occurs, the strength is improved by reducing such weak parts. This improvement in strength also leads to improved durability.

  As described above, the wet friction material of the present invention is a paper body using a thermosetting resin contained in an impregnating liquid of a paper body, which is obtained by reducing the viscosity of the impregnating liquid to a specified viscosity by controlling the molecular weight or the like. Impregnate. Thus, by adjusting the thermosetting resin itself to a specified viscosity by adjusting the viscosity of the impregnating solution without solvent dilution, the papermaking body can be obtained without reducing the amount of the thermosetting resin contained in the wet friction material. The penetration property of the thermosetting resin into the paper and the covering property to the fibers inside the paper body, that is, the impregnation property are improved. By improving the impregnation property, the porosity distribution of the wet friction material is made more uniform than the conventional one, and the inflow and discharge of the automatic transmission fluid is improved and the cooling efficiency is increased. Further, the vulnerability of the paper body itself is suppressed by improving the covering property to the fibers inside the paper body. The durability is improved by the improvement of the cooling performance and the effect of suppressing the vulnerability.

  Regardless of the thermosetting resin, when the viscosity of the impregnating liquid is decreased by increasing the amount of solvent, the number of impregnations is increased to increase the resin amount of the thermosetting resin impregnated in the wet friction material. Although the amount of resin can be set, the amount of solvent volatilized in the air increases with the increase in the number of impregnations, and the burden on the environment increases, leading to an increase in cost. The present invention is an invention that makes it possible to improve durability without increasing environmental burden and without increasing cost.

Claims (4)

A wet friction material prepared by heating and drying a resin-impregnated papermaking body in which an impregnating solution in which a thermosetting resin is dissolved in a solvent is impregnated in a papermaking body in which fibers are made,
When the impregnating liquid has a nonvolatile content of 50%, the viscosity of the impregnating liquid is within a range of 5 to 8 cP at a liquid temperature of 18 ° C to 25 ° C.   When the impregnation liquid enters the paper body from both sides in the thickness direction of the paper body, the resin ratio of the thermosetting resin in the thickness direction of the paper body is minimized inside the paper body, and 2. The wet friction material according to claim 1, wherein the difference between the inside of the paper body and the surface portion in the thickness direction of the resin-impregnated paper body is 70% or less of the deviation relative to the inside of the paper body.   3. The wet friction material according to claim 1, wherein when the automatic transmission fluid is dropped into the wet friction material, the absorption time of the automatic transmission fluid is 10 seconds or less. Friction material. Preparing a paper body made of fibers;
An impregnation step of impregnating the papermaking body with an impregnation solution obtained by dissolving a thermosetting resin in a solvent;
A method for producing a wet friction material comprising a drying step of heating and drying the paper body after the impregnation step,
When the impregnating liquid has a non-volatile content of 50%, the viscosity of the impregnating liquid is in the range of 5 to 8 cP at a liquid temperature of 18 ° C to 25 ° C.