Background of the invention

1. Field of the invention

This invention relates to a surface-treated, water-containing aromatic polyamide pulp (aromatic polyamide is referred to hereinafter as aramid in some cases) and to a process for producing the same. More particularly, it relates to a water-containing aromatic polyamide pulp which is easy to handle when used in beater sheet and which gives excellent mechanical characteristics, and to a process for producing the same.

2. Background art

As a method for improving the adhesiveness of an aromatic polyamide fiber to a rubber, there has been known a method which comprises subjecting the aromatic polyamide fiber to epoxy resin-treatment and thereafter to treatment with a resorcino!-formaldehyde latex ruber (referred to hereinafter as RFL in some cases) as seen in JP-B-50-94,289, JP-B-51-20,983 and the like. In these methods, the fibers to be treated are long fibers of aromatic polyamide in the form of yarns, cords and the like which are used in hoses, transmission belts, conveyor belts, tires and the like, and there have heretofore been known substantially no methods for treating an aromatic polyamide pulp for increasing the adhesiveness thereof to a rubber. Also, almost all the above methods require drying treatment and heating treat ment, so that when said methods are applied to the treat ment of an aromatic polyamide pulp, there are such problems that the opening of pulp becomes insufficient and the dispersibility of pulp is impaired. Thus, in a beater sheet or the like in which the aromatic polyamide in the

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form of pulp is used in combination with a rubber, these treatment methods cannot be used, and hence, there has been desired aromatic polyamide pulp which is excellent in adhesiveness to a rubber and good in dispersibility.

It is now an object of this invention to provide a water-containig aromatic polyamide pulp which is suitable for producing a beater sheet which can be used e.g. as a gasket packing of a fastening portion in

10 an automobile or the like or for the production of cylinder heads, which has good dispersibility and openability of aromatic polyamide pulp and excellent adhesiveness to latex rubber and gives a beater sheet having excellent machanical characteristics.

15 Other objects and advantages of this invention will become apparent from the following description.

Summary of the invention

20 According to this invention, there is provided a water-containing aromatic polyamide pulp which has been surface-treated with an epoxy resin and a resorcinol formaldehyde latex rubber, and which has a water content of 30 to 95% by weight.

r e This invention further provides a process for producing the water- containing aromatic polyamide pulp mentioned above, which comprises a step of dispersing an aromatic polyamide pulp in an aqueous epoxy resin emulsion, a step of mixing the resulting aqueous disper sion with a resorcinol-formaldehyde latex rubber and a component

30 instabilizing the resorcinol-formaldehyde latex rubber and a step of removing the excessive water.

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Detailed description of the invention

The aramid, or aromatic polyamide, used in this invention is such that at least 85 mole % of the amide bonds are obtained from aromatic ring di amine components and aromatic ring dicarboxylic acid components.

Specific examples thereof include polyparaphenylene terephthalamide, poly etaphenylene terephthalamide, polypara-benzamide, poly-4,4'-diaminobenzanilide, polyparaphenylene-2,-naphthalic amide, copolyparaphenylene/4,4'-(3,3'-dime-thyl iphenylene) terephthalamide, copolyparaphenylene/2,5-pyridylene terephthalamide, polyorthophenylene- phthalamide, polymetaphenylene phthalamide, polyparaphenylene phthalamide, polyorthophenylene isophthal amide, polymetaphenylene isophthal amide, polyparaphenylene isophthalamide, polyorthophenylene isophthal amide, poly-l,5-naphthalene phthalamide, poly

4,4'-diphenylene orthophthal amide, poly-4,4'-diphenylene is-ophthalamide, poly-l,4-naphthalene phthalamide, poly

1,4-naphthalene isophthalamide, poly-l,5-naphthalene isophthal a-mide and the like; alicyclic amine-containing aromatic polyami-des, representatives of which are compounds obtained by replacing a part of the benzene nucleus of the aromatic di amines of these aromatic polya ides with pi-perazine, 1,5-dimethylpiper-azine or

2,5-diethylpiperazine and the like; and copoly ers of aromatic polyamides whose aromatic di amines contain two phenyl groups bonded by a group such as ether bond, -S-, -SO2-, -CO-, -NH- or the like such as 3,3'-oxydiphenylenediamine, 3,4-oxydiphenylenediamine or the like, for example, poly-3,3'-oxydphenylene terephthalamide/ polyparaphenylene te-rephthal amide copolymer, poly-3,4 oxydi phenyl ene terephthal ami de/polyparaphenylene terephthalamide copolymer and the like.

The term "aramid pulp" used herein means highly fibrillated aramid fibers having a specific surface area of, preferably, 3 to 25 m/g as

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measured by the BET method and a freeness of, preferably, 40 to 700 ml as measured by the Canadian standard method of JIS P8121 "Method of Testing Freeness of Pulp".

The method of producing the aramid pulp is not critical and there can be used, for example, the method disclosed in J-B-59-03, JP-A-2-200,809 and the like.

The epoxy resin for producing the aqueous epoxy resin emulsion used in this invention is not critical and may be any epoxy resin which can achieve the purpose of this invention. For example, the following can be used: Bisphenol A type liquid epoxy resins such as Sumiepoxy ELA-128 (trade name of Sumitomo Chemical Co., Ltd.) and the like; bisphenol A type solid epoxy resins such as Sumiepoxy ELA-012 (same as above) and the like; orthocresol novolak type epoxy resins such as Su iepoxy ESCN-220 (same as above) and the like; tri-glycidyl aine type epoxy resins such as Sumiepoxy EL-120 (same as above) and the like; and tetraglycidyl amine type epoxy resins such as Sumiepoxy EL-434 (same as above) and the like; etc. Among them, the tetrafunctional tetra glycidyl amine type epoxy resins are preferred in respect of enhancing the ad-hesiveness. Moreover, the epoxy equivalents of the above epoxy resins are preferably 1,000 g/eq. or less. When it is more than 1,000 g/eq., sufficient adhesiveness is not obtained.

In order to produce the aqueous emulsion using the above epoxy resin, methods which have been generally used can be applied as they are. That is to say, for example, an aqueous epoxy resin emulsion (referred to hereinafter as merely the epoxy emulsion in some cases) is obtained by dispersing an epoxy resin in water by high speed stirring in the presence of a nonionc surface active agent such as an ether compound of polyoxyethylene and a higher aliphatic alcohol or the like. In this case, the epoxy resin/surface active agent composition ratio (by

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weight) is varied depending upon the kinds of epoxy resin and surface active agent; however, it is preferably selected from the range of from 97/3 to 70/30 from the viewpoint of stability of emulsion, adhesiveness and the like. Also, it is possible to use a commercially available epoxy emulsion, which is an emulsion of an epoxy resin, such as ANS-1001, ANS-1006 (these are trade names of Takemoto Oil & Fat Co., Ltd.) or the like.

Moreover, when an epoxy resin whose glycidyl groups have been partially hydrolyzed into glycol groups is used, adsorption of the epoxy emulsion on the aromatic polyamide pulp becomes easy.

In order to obtain an emulsion of the epoxy resin whose glycidyl groups have been partially hydrolyzed into glycol groups, there can be used a method by which an epoxy resin which has been hydrolyzed by a general method is used as the starting material to prepare an emulsion dispersion. However, as mentioned below, when an epoxy emulsion dispersion is prepared by a conventional method and thereafter hydro¬ lyzed, it is possible to more easily obtain a uniform and stable emulsion. The hydrolysis of the emulsion can be carried out by various methods depending upon the type of emulsion; however, it is the simplest method to heat treat the emulsion as it is. The hydrolysis is effected for ring-opening a part of the epoxy groups in the epoxy resin into glycols; however, preferably at least 10, more preferably at least 20% but less than 99%, of the originally existing epoxy groups are hydrolyzed. When the hydrolysis is insufficient, the adsorption of the resin on the pulp becomes insufficient and when the hydrolysis is excessive, performances of the treated pulp such as adhesiveness to latex rubber and the like are deteriorated.

Suitable hydrolysis conditions are varied de pending upon the kinds, compositions and concentrations of the epoxy resin and surface active agent; however, in the case of, for example, a nonionic emulsion of

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Sumiepoxy EL-434 which is a tetraglycidylamine type epoxy resin or ANS-1006, hydrolysis is effected as follows: This emulsion is subjected to heat-treatment at a temperature of 65°C for 130 hours to obtain an emulsion of an epoxy resin, the objective glycidyl groups of which have been partially hydrolyzed into glycol groups. By this treatment, the epoxy equivalent is increased from about 120 g/eq. to about 240 g/eq. In this case, the reaction conversion becomes 50%. A similar effect can also be obtained by allowing the emulsion to stand at room temperature for 3 to 6 months. Furthermore, the reaction can be accelerated with a catalyst such as an acid, a base, an amine or the like.

In the treatment of the aramid pulp, first of all, the step of dispering the aromatic polyamide pulp in the aqueous epoxy resin emulsion is carried out. Specifically, the aramid pulp is treated with an epoxy emulsion, preferably the above-mentioned emulsion of an epoxy resin whose glycidyl groups have been partially hydrolyzed, to adhere the epoxy resin to the surface of the aramid pulp. The treating method is not critical; however, for example, the following method can be carried out. As background art in this respect, WO 94/05854 is referred to.

The aramid pulp is first dispersed in water to such an extent that a sufficient fluidity is given. The suitable concentration of the aramid pulp in the dispersion is varied depending upon the specific surface area and freeness of the pulp used; however, it is preferably selected from the range of 0.2 to 5% by weight. In order to effect uniform dispersion, a general propeller stirrer can be utilized. Subsequently, while this dispersion is stirred, the predetermined amount of the epoxy emulsion, preferably the above-mentioned emulsion of an epoxy resin whose glycidyl groups have been partially hydrolyzed, is dropped into the dispersion. The amount of the emulsion dropped is adjusted so that the amount of the poxy resin adhered to the pulp becomes preferably 0.3 to 10% by weight, more preferably 1 to 6% by weight,

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based on the weight of the absolute dry pulp. When the amount is less than 0.3% by weight, no sufficient treatment effect is obtained, and when it is more than 10% by weight, the effect corresponding to the large amount of the emulsion adhered is not obtained and hence said amount is not economical. The adhesion of the epoxy resin to the surface of the pulp starts simultaneously with the drop wise addition of the emulsion. When the dropwise addition of the emulsion has been completed, the stirring is continued as it is for 5 to 30 minutes to allow the epoxy resin to sufficiently adhere on the surface of the pulp. According to the process of this invention, only the above treatment allows about 100% of the epoxy resin to adhere to the surface of the aramid pulp.

Secondly, the step of mixing the aqueous dispersion of the aromatic polyamide pulp surface-treated with an epoxy resin with the resorcinol-formaldehyde latex rubber and a component instabilizing the resorcinol-formaldehyde latex rubber is carried out. In this step, the latex rubber is instabilized to allow the rubber component to adhere to the aromatic polyamide pulp. In general, the condensation product of resorcinol with formaldehyde is water-soluble, and hence, it is difficult to allow the whole of the desired amount of the condensation product to adhere to the pulp without heating and drying treatments. However, the present inventors have found that when RFL prepared by mixing and ageing the latex rubber and the resorcinol-formaldehyde condensation product is used, substantially the whole amount of the RFL added can be allowed to adhere to the pulp by instabilizing the latex rubber.

The method of instabilizing the latex rubber comprises, when the rubber latex is anionic latex rubber, acidifying the system, or adding a cation to lower the ion potential, thereby coagulating the latex rubber. In the case of a cationic rubber latex, adversely thereto, the system is alkalified, or an anion is added to the latex rubber. The

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time at which the system is alkalified or acidified or a cation or an anion is added may be either before or after the RFL is added. The component instabilizing the latex rubber is specifically explained below. In the case of an anionic latex rubber, an inorganic acid or an organic acid such as hydrochloric acid, nitric acid, carbonic acid, phos phoric acid, acetic acid, oxalic acid or the like may be added, or a salt such as aluminum sulfate, copper sulfate, iron trichloride, calcium chloride, aluminum chloride or the like may also be added to instabilize the latex rubber. Aluminumsulfate and the like which are polyvalent metal salts are easily handled and have a large coagulation activity on the latex rubber, so that the adhesion of RFL to the pulp is relatively easy. In the case of a cationic latex rubber, bases such as ammonia, sodium hydroxide, potassium hydroxide, calcium hydroxide and the like and salts such as sodium carbonate, sodium hydrogencarbonate, sodium acetate and the like can. -be used to instabilize the latex rubber.

The mole ratio of the resorcinol to formaldehyde in the RFL is preferably in the range of from 1:0.1 to 1:8, more preferably from 1:0.5 to 1:4. It is also effective to use an initial condensation product obtained by reacting resorcinol with formaldehyde at the desired feed ratio in the presence of a basic catalyst or an acidic catalyst, and the initial condensation product may be used as it is or after reaction with formaldehyde. As the initial condensation product, there may be used compounds derived from halogenated phenols, formaldehyde and phenol derivatives; compounds derived from polyhydric phenols ad sulfur chloride; etc. may be used, and a mixture of two or more of them may also be used.

The solid weight ratio of the resorcinol formaldehyde condensation product to the latex rubber is preferably in the range of from 1:0.1 to 1:10, more preferably from 1:0.5 to 1:4. When the solid weight ratio is less than 0.1, the amount of the latex rubber becomes too

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small to allow RFL to adhere sufficiently to the pulp. When the solid weight ratio is more than 1:10, the amount of the latex rubber becomes too large to obtain a sufficient mechanical strength when the pulp is used in a beater sheet.

The latex rubber used in the treatment with RFL may be the same type as the latex rubber used in the beater sheet, or a mixture thereof with other types of latex rubber such as acrylonitrile-butadiene latex rubber (NBR latex), styrene-butadiene latex rubber (SBR latex) may be used to obtain a sufficient adhesiveness.

The amount of RFL adhered to the pulp is preferably less than 20% by weight, more preferably 3 to 15% by weight, based on the weight of the pulp. When the amount is 20% by weight or more, the dispersibility of the pulp hs a tendency to deteriorate, and then the .amount is less than 3% by weight, the mechanical strength of the beater sheet has a tendency to lower.

After RFL is adsorbed on the pulp, the excessive water is removed. The method of removing water is not critical, and it is possible, for example, to filter the pulp in a conventional manner and then dehydrate the pulp so as to obtain the desired water content, thereby obtaining water-containing aramid pulp surface-treated with an epoxy resin and RFL. The water content of the pulp is adjusted to fall within the range of from 30 to 95% by weight. Preferably, the water content is of from 40 to 90% by weight.

The water content referred to above can be determined by the following calculation formula: Water content [weight%] = {(Wj.- 2)/ ι} x 100% W- weight of pulp in the hydrous state W2: weight of pul,p after absolute drying

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Controlling the water content to 30% by weight or less requires a high pressure and special operations such as heating, drying and the like and hence is not economical. In addition, it follows that the dispersibility of the treated pulp is impaired. When the water content is 95% by weight, the water-containing pulp becomes heavy and hence the operability becomes bad. Also, said water content is not economical in the aspect of transportation or the like. In the application to beater sheet, the above-mentioned water content range is preferred in respect of handling, dispersibility and economy.

EXAMPLES

This invention is explained in more detail below by way of Examples; however, this invention should not be construed to be limited thereto.

In the Examples, the evaluation of the water containing aramid pulp was conducted by the following method.

Evaluation method 1

Evaluation of aramid pulp/inorganic filler/latex in the form of beater sheet model .

(1) Paper-making

Pulp of a weight corresponding to 5 g of pulp in terms of absolute dry weight and 15 to 30 g of diatomaceous earth (controlled so that the pulp/ diatomaceous earth/latex rubber weight ratio after the paper- making becomes 1/3/1 depending upon the filler retaining property of pulp) and 35.4 g of aluminum sulfate (tetradecahydrate to octadecahydrate) were weighed and dispersed in two liters of water at 3,000 rpm for 3 minutes in a 2-liter standard pulper (manufactured by

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Kumagai Riki Kogyo .K.). Subsequently, the beaten pulp was transferred to a 15 liter water tank and water was added thereto to adjust the total weight to 9.3 kg. A latex rubber solution prepared by adding water to 12.5 g of NBR latex [Latex Nipol 152 (trade name of c Nippon Zeon Co., td.)] and 2.0 g of a curing agent [NC811 (a trade name of Nippon Zeon Co., Ltd.)] with stirring by means of an agitator to adjust the total weight to 150 g was added to the pulp dispersion and the resulting mixture was stirred fo 3 minutes. Subsequently, paper making was conducted in a conventional manner using a 25-cm - _n square shaped sheeting machine (manufactured by Kumagai Riki Kogyo K.K.) and a #80-mesh wire net, and 5 thereafter, dried at 50°C for five hours to obtain a 25-cm square ara id/di-atomaceous earth/latex rubber composite paper having a basis weight of 400 g/cm.

15 (2) Curing of sheet

The above sheet was press-molded at 105°C at 50 kg/cm for 30 minutes to cure the sheet, thereby obtaining a beater sheet model.

20 (3) Tensile test

A JIS No. 1 test piece was punched out of the beater sheet model obtained by the above-mentioned method and measured for tensile strength under the following conditions: 5 Test piece size: JIS No. 1 dumbbell Gauge length: 70 mm Crosshead speed: 5 mm/min

2. Evaluation method 2 (Freeness) 0

The freeness of the water-containing aramid pulp was measured according to the Canadian standard method of JIS P8121 "Method of Testing Freeness of Pulp".

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Example 1

In a beaker, 19.88 g of Twaron 1097 (a trade name of Nippon Aramid for polyparaphenylene terephthalamide pulp, specific surface area by the BET method: 6 m ² /g, water content: 6% by weight) was dispersed into two liters of deionized water. While this dispersion was stirred, 5.86 g of. an epoxy emulsion which had been subjected to heat-treatment to hydrolyze the glycidyl groups [ANS 1006, a trade name of Takemoto Oil & Fat Co.,Ltd., WPE (epoxy equivalent): 288 g/eq.] was dropped into the dispersion in 30 seconds, and the stirring was continued at room temperature for 15 minutes. Subsequently, 10 g of aluminum sulfate (tetradecahydrate to octadecahydrate) was added thereto and stirring was continued for 5 minutes. Thereafter, 42 g of an RFL solution (the content was as shown hereinafter) was added and the resulting mixture was stirred for 15 minutes, after which dehydration _^ was conducted until the water content became about 70% by weight to obtain a water containing aramid pulp surface-treated with an epoxy resin and RFL. Performance of this water-containing aramid pulp was evaluated by the above-mentioned evaluation method to obtain the results shown in Table 1. The dispersibility in water and openability of this water- containing aramid pulp were good.

As the RFL solution, a mixture (450 g in total) of the following components was prepared, aged for 24 hours and then used: Resorcinol-formaldehyde initial condensate* ¹ : 6.15 g Formaldehyde (37% reagent): 2.71 g

NBR latex rubber* ² : 63.59 g

0.1% Ammonia water: 404.55 g

Note *1: Sumikanol 700S (a tradename of Sumitomo Chemical Co.,ltd.) *2: Nipol 1562 (a trade name of Nippon Zeon Co., Ltd.)

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Example 2

In the treatment using the same starting materials as in Example 1 in the same amounts as in Example 1, the RFL was added after the addition of the epoxy emulsion, and subsequently, the aluminum sulfate as added to obtain a water-containing aramid pulp surface treated with the epoxy and the RFL. Performance of this water-containing aramid pulp was evaluated by the above-mentioned evaluation method to obtain the results shown in Table 1. The dispersibility in water and open ability of this water-containing aramid pulp were good.

Example 3

In a 200-liter reaction vessel, 1.0 kg of Twaon 1097 (the same as above) was dispersed in 90 liters of deionized water. _^ While this disperseon was stirred, 313 g of a hydrolyzed epxy emulsion [ANS 1006 (a trade name of Takemoto Oil & Fat Co. Ltd., WPE: 300 g/eq.)] was added to the dispersion in two minutes ad the stirring was continued at room temperature for 15 minutes. Subsequently, 350 g of aluminum sulfate (tetradecahydrate to octadecahydrate) was added thereto and stirring was continued for 15 minutes, after which 2,250 g of an RFL solution (described hereinafter) were added thereto and the resulting mixture was stirred for 15 minutes. Thereafter, the mixture was subjected to dehydration until the water content became about 70% by weight to obtain a water-containing aramid pulp surface treated with the epoxy resin and RFL. Performance of this water-containing aramid pulp was evaluated by the above-mentioned evalution method to obtain the results shown in Table 1. The dispersibility in water and openability of this water-containing aramid pulp were good.

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As the RFL solution, a mixture (2,250 g in total) of the following components was prepared, aged for 24 hours and then used: Resorcinol-formaldehyde initial condensate*!: 30.8 g formaldehyde (37% reagent): 13.5 g

NBRlatex rubber* ² : 182.9 g

0.1% Ammonia water . 2,022.8 g

Note: *1: Sumikanol 7005 (a trade name of Sumitomo Chemical Co., Ltd.) *2: Nipol 1562 (a trade name of Nippon Zeon Co., td.)

TABLE 1

Example 1 Example 2 Example 3

Pulp freeness ml 580 580 580

Amount of epoxy resin added (wt.%) 5 5 5

Amount of RFL added (wt.%) 10 10 10

Sheet tensile strength (kg/cm ³ ) 48 46 50

Comparative Example 1

Untreated aramid pulp (Twaron 1097, water-content about 6% by weight) was used as it was, instead of the surface treated water-containing aramid pulp to evaluate the properties by the above-mentioned evaluation method to obtain the results shown in Table 2.

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Comparative Example 2

In a beaker, 19.88 g of Twaron 1097 were dispersed into two liters of deionized water. While this dispersion was stirred, 5.86 g of an epoxy emulsion [ANS 1006 (a trade name of Takemoto Oil & Fat Co., Ltd., WPE: 288 g/eq.)] in which the glycidyl groups had been hydrolyzed was droped into the dispersion in 30 seconds, after which the stirring was continued at room temperature for 15 minutes. Subsequently, the dispersion as filtered and then dehydrated until the water content became about 70% by weight to obtain a water-containing aramid pulp surface-treated with the epoxy resin. Performance of this pulp was evaluated by the above mentioned evaluation method to obtain the results shown 5 in Table 2.

Comparative Example 3

In a beaker, 19.88 g of Twaron 1097 were dispersed into two liters of deionized water. While this dispersion was stirred, 10 9 of aluminum sulfate (tetra dacahydrate to octadecahydrate) was added, and then stir ring was continued for five minutes, after which 42 g of the same RFL solution as in Example 1 was added to the dispersion and the resulting mixture was stirred for 15 minutes and thereafter dehydrated until the water content became about 70% by weight to obtain a water- containing aramid pulp surface-treated with the RFL. Performance of this aramid pulp was evaluated by the above-mentioned evaluation method to obtain the results shown in Table 2.

The water-containing aromatic polyamide pulp of this invention has good dispersibility in water and good openability and is excellent in wettability with and adhesion to a latex rubber added in the process for producing a beater sheet, so that a beater sheet having uniform quality and excellent mechanical strength can be produced.

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TABLE 2

Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3

Pulp freeness (ml) 570 580 580

Amount of epoxy resin added (wt.%) 0 5 0

Amount of RFL added (wt.%) 0 0 10

Sheet tensile strength (kg/cm ³ ) 30 32 36

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