JP7199136B2 - Resin cross-linking agent containing resol type phenolic formaldehyde co-condensation resin and phenols, and rubber composition containing said resin cross-linking agent - Google Patents

Description

TECHNICAL FIELD The present invention relates to a resin cross-linking agent containing a resol-type phenol-formaldehyde cocondensation resin and a phenol having a specific structure, and a rubber composition containing the resin cross-linking agent.

In cross-linking various rubbers to obtain cross-linked rubbers, in addition to the method of cross-linking using sulfur, the method of cross-linking using a resol-type phenol-formaldehyde cocondensation resin as a cross-linking agent is generally performed. In particular, rubber with an extremely low degree of unsaturation, such as butyl rubber and ethylene propylene rubber, is known to have excellent weather resistance, heat resistance, and mechanical properties. , resin cross-linking using a resol-type phenol-formaldehyde cocondensation resin having a cross-linking structure of C—C bond as a cross-linking agent.

On the other hand, when cross-linking is performed using a resol-type phenol-formaldehyde cocondensation resin as a cross-linking agent, the speed of cross-linking is slow with the resin alone. Compounds, halogen-containing elastomers such as chloroprene rubber and chlorosulfonated polyethylene, organic acids, and crosslinking accelerators such as halogenated alkylphenol-formaldehyde cocondensation resins are used in combination.

However, if a compound or resin having a halogen atom or an organic acid is used as a cross-linking accelerator, metal molds may corrode during cross-linking molding of rubber, and when rubber molded products are used in electrical appliances, metal molds may be damaged. There are problems such as corrosion of parts, poor contact, and defects due to elution of components into the electrolyte. Therefore, in the cross-linking of rubber used in fine rubber molded products and electrical products, the above-described cross-linking accelerator cannot be used in combination. being considered.

For example, Patent Document 1 proposes a method of using a mixture of a resole-type phenol-formaldehyde co-condensation resin and a novolak-type phenol-formaldehyde co-condensation resin as a resin crosslinking agent. It is described that the speed of cross-linking is sufficiently improved without using a compound, resin, or organic acid as a cross-linking accelerator.

JP-A-2002-234968

Therefore, when the inventors of the present application repeated the method described in Patent Document 1, although the crosslinking speed was improved, the resin crosslinking agent described in the patent document or the rubber composition containing the resin crosslinking agent (not yet Cross-linked rubber composition) is very sticky, causing sticking to the two rolls in the kneading process, and sticking and staining of rubber pieces, etc. to metal molds during cross-linking in the molding process. , the handling property of the cross-linking agent or the uncrosslinked rubber composition containing the cross-linking agent is remarkably deteriorated, and it has been found that it is difficult to use the resin cross-linking agent industrially.

An object of the present invention is to obtain a sufficient cross-linking speed when cross-linking using a resin cross-linking agent without using a compound or resin having a halogen atom that may corrode metal, or an organic acid as a cross-linking accelerator. It is to provide a resin cross-linking agent.

As a result of intensive studies by the present inventors in order to solve the above problems, they have found that by using the resin crosslinking agent described below, a sufficient crosslinking rate can be obtained without using the above crosslinking accelerator. Specifically, the following inventions are included.

[1]
A resole-type phenol formaldehyde cocondensation resin and the following general formula (1):

（上記一般式（１）中、Ｒ_１及びＲ_２は水素原子、分岐を有してもよい炭素数１～４のアルキル基、フェニル基、又は４－ヒドロキシフェニル基を表す。）
で表される化合物、下記一般式（２）：

(In general formula (1) above, R ₁ and R ₂ represent a hydrogen atom, an optionally branched C 1-4 alkyl group, a phenyl group, or a 4-hydroxyphenyl group.)
A compound represented by the following general formula (2):

（上記一般式（２）中、ｎは１～９の整数を表す。）
で表される化合物、下記一般式（３）：

(In the above general formula (2), n represents an integer of 1 to 9.)
A compound represented by the following general formula (3):

（上記一般式（３）中、Ｒ_３及びＲ_４は水素原子、分岐を有してもよい炭素数１～４のアルキル基又は４－ヒドロキシフェニル基を表す。）
で表される化合物、及びビス（４－ヒドロキシフェニル）スルホンからなる群から選ばれる少なくとも１種のフェノール類とを含む樹脂架橋剤。

(In general formula (3) above, R ₃ and R ₄ represent a hydrogen atom, an optionally branched C 1-4 alkyl group, or a 4-hydroxyphenyl group.)
A resin cross-linking agent comprising a compound represented by and at least one phenol selected from the group consisting of bis(4-hydroxyphenyl)sulfone.

[2]
2. The resin cross-linking agent according to claim 1, which contains 1 to 15 parts by weight of phenols with respect to 100 parts by weight of the resol-type phenol-formaldehyde cocondensation resin.

[3]
A rubber composition comprising rubber and the resin crosslinking agent according to claim 1 or 2.

[4]
4. The rubber composition according to claim 3, which contains 5 to 23 parts by weight of the resin crosslinking agent with respect to 100 parts by weight of the rubber.

[5]
5. A rubber composition according to claim 3 or 4, wherein the rubber is a rubber containing butyl rubber or ethylene propylene rubber.

[6]
The rubber composition according to any one of claims 3 to 5, wherein the content of halogen atoms contained in the rubber composition is 0.1% by weight or less.

According to the present invention, it is possible to provide a resin cross-linking agent capable of obtaining a sufficient cross-linking speed without using a compound or resin having a halogen atom that may corrode metals, or an organic acid as a cross-linking accelerator. Become. In addition, the rubber composition (uncrosslinked rubber composition) of the present invention containing the resin crosslinking agent of the present invention also has characteristics such as controlled adhesiveness and excellent workability and moldability. Furthermore, the crosslinked rubber composition obtained by crosslinking the rubber composition of the present invention has an improved crosslink density compared to rubber crosslinked by a known method, and therefore has excellent swelling resistance and solvent resistance. It becomes possible to obtain a crosslinked rubber composition.

The present invention will be described in detail below. The resin crosslinking agent of the present invention includes a resol-type phenol formaldehyde cocondensation resin, a compound represented by the general formula (1), a compound represented by the general formula (2), and a compound represented by the general formula (3). It is a mixture of a resin and a phenol, containing the represented compound and at least one phenol selected from the group consisting of bis(4-hydroxyphenyl)sulfone.

As the resol-type phenol-formaldehyde cocondensation resin contained in the resin crosslinking agent of the present invention, one synthesized by a known method is used. Examples of the co-condensation resin include resol-type phenol-formaldehyde co-condensation resin, resol-type cresol-formaldehyde co-condensation resin, resol-type alkylphenol-formaldehyde co-condensation resin, resol-type resorcinol-phenol-formaldehyde co-condensation resin, resol-type resorcinol-cresol. - Formaldehyde cocondensation resin etc. are mentioned. Among these cocondensed resins, resol-type alkylphenol-formaldehyde cocondensed resins are preferable from the viewpoint of compatibility with rubber, and resols in which the alkyl group of the alkylphenol is an optionally branched alkyl group having 1 to 20 carbon atoms. Type alkylphenol-formaldehyde cocondensation resins are particularly preferred.

The softening point of the resol-type phenol-formaldehyde cocondensation resin contained in the resin crosslinking agent of the present invention is usually 120°C or less, preferably 80 to 110°C. Depending on the type of phenols constituting the cocondensation resin, by using a resin having a softening point of 120 ° C. or less, dispersibility is improved when preparing a rubber composition containing the resin crosslinking agent of the present invention. The temperature during rubber kneading can be lowered, and a rubber composition containing the resin crosslinking agent of the present invention can be prepared more easily.

Specific examples of the resol-type phenol-formaldehyde cocondensation resin contained in the resin crosslinking agent of the present invention include Takkiroru 201 manufactured by Taoka Chemical Co., Ltd., Hitanol 2501 manufactured by Hitachi Chemical Co., Ltd., Schenectady International Inc.; SP-1044, SP-1045, etc., manufactured by K.K.

The compound represented by the general formula (1) contained in the resin crosslinking agent of the present invention, the compound represented by the general formula (2), the compound represented by the general formula (3), and bis(4- Among at least one phenol selected from the group consisting of hydroxyphenyl)sulfone (hereinafter sometimes referred to as phenols having a specific structure), specific phenols represented by the above general formula (1) For example, bisphenol A (2,2-bis(4-hydroxyphenyl)propane), bisphenol F (4,4'-dihydroxydiphenylmethane), bisphenol B (2,2-bis(4-hydroxyphenyl)butane), bisphenol E (1,1-bis(4-hydroxyphenyl)ethane), bisphenol AP (1,1-bis(4-hydroxyphenyl)-1-phenylethane), bisphenol BP (bis(4-hydroxyphenyl)diphenylmethane), etc. Specific examples of phenols represented by the general formula (2) include bisphenol Z (1,1-bis(4-hydroxyphenyl)cyclohexane), 1,1-bis(4-hydroxyphenyl) cyclododecane and the like, and specific examples of the phenols represented by the general formula (3) include 1,2-bis(4-hydroxyphenyl)ethane 1,1,2,2-tetrakis(4-hydroxy phenyl)ethane and the like.

The resin crosslinking agent of the present invention generally contains 1 to 15 parts by weight, preferably 3 to 10 parts by weight of a phenol having a specific structure per 100 parts by weight of the resol-type phenol-formaldehyde cocondensation resin. By setting the content of the phenol having a specific structure to 1 part by weight or more, it is possible to obtain a sufficient crosslinking rate, and by setting it to 15 parts by weight or less, the dispersibility of the resin crosslinking agent in the rubber is improved. In addition, it is possible to suppress a decrease in the tensile strength of the crosslinked rubber without hindering the original crosslinks between the rubber molecules.

Examples of the method for producing the resin crosslinking agent of the present invention include a method of mixing a resol-type phenolic formaldehyde cocondensation resin and a phenol having a specific structure while pulverizing using a jet mill, a feather mill, or the like, and a method of pulverizing in advance. A method of stirring and mixing a cocondensation resin and a phenol having a specific structure may be used.

Next, the rubber composition containing the resin crosslinking agent of the present invention will be described in detail. The resin cross-linking agent of the present invention can be used as a cross-linking agent as long as it is a rubber having a double bond in the molecule. ), butadiene rubber (BR), butyl rubber (IIR), and ethylene propylene rubber (EPDM). Rubbers containing butyl rubber or ethylene propylene rubber are preferred because they are

Since the rubber composition of the present invention improves the cross-linking speed without using a halogen atom-containing cross-linking accelerator, it is possible to reduce the halogen content in the rubber composition of the present invention to 0.1% by weight or less. is. The halogen content contained in the rubber composition of the present invention is obtained by burning and decomposing the rubber composition in a flask filled with oxygen, and absorbing the inorganic halogen content in the generated gas into the absorption liquid in the flask to obtain an analysis sample, The sample can be quantified by silver nitrate titration, potentiometric titration, ion chromatography, or the like.

The rubber composition of the present invention contains antioxidants, fillers such as carbon black, various inorganic fillers such as calcined clay, zinc white, stearic acid, oils, etc., which are usually blended in rubber, for the purposes of the present invention. They can be appropriately selected and blended within a range that does not interfere, and commercially available products can be suitably used as these blending agents.

If necessary, the resin crosslinking agent and rubber composition of the present invention can be used in combination with a crosslinking agent that is usually blended with rubber. When the resin cross-linking agent of the present invention is used in combination with other cross-linking agents, they can be appropriately selected and blended within a range that does not interfere with the object of the present invention, and commercially available products can be suitably used.

A crosslinked rubber composition obtained by crosslinking the rubber composition of the present invention is obtained by crosslinking the rubber composition of the present invention described above. As a method for cross-linking the rubber composition of the present invention, for example, a method in which the process from molding to the end of cross-linking is performed by a pressure press or the like, or a method in which molding and primary cross-linking are performed by a pressure press for 5 to 15 minutes, and then , A method of performing secondary crosslinking in an oven or an electric furnace for, for example, 30 minutes to 4 hours. From the viewpoint of productivity of the crosslinked rubber composition, a method of performing the primary crosslinking and secondary crosslinking separately. is preferred.

The cross-linking temperature of the rubber composition of the present invention can be applied in the same temperature range as in the case of using a conventional resin cross-linking agent. Specifically, it is, for example, 130 to 230°C, preferably 150 to 210°C, more preferably 160 to 200°C.

A crosslinked rubber composition obtained by crosslinking the rubber composition of the present invention can be used as various rubber products, and can be particularly suitably used as packings, O-rings, and the like.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. The softening point of the resol-type phenol formaldehyde cocondensation resin described below is a value measured by a method based on JIS K2207.

1. Production example of resol-type phenol formaldehyde cocondensation resin <Production example 1: Synthesis of resin-1>
In a four-necked flask equipped with a reflux condenser and thermometer, 300 g (2.00 mol) of p-tert-butylphenol, 130 g (4.00 mol) of 92% paraformaldehyde, 8.33 g of 48% aqueous sodium hydroxide solution ( 0.10 mol) and 150 g of toluene were charged, and the internal temperature was raised to 65°C, stirred at the same temperature for 1 hour, further heated to 88°C, and stirred at the same temperature for 5 hours.
After cooling to 65° C., 84 g of water, 90 g of toluene and 16.4 g of 30% sulfuric acid were added, stirred, allowed to stand, and separated. After that, washing with water was repeated three times.
Subsequently, the toluene layer containing the resin liquid is concentrated under normal pressure for 1 hour until the internal temperature reaches 118 ° C., and then at a reduced pressure of 90 torr and 1 hour until the internal temperature reaches 121 ° C. By performing vacuum concentration, resol type p- 368 g of tert-butylphenol/formaldehyde cocondensation resin (Resin-1) was obtained. The softening point of the obtained cocondensation resin was 110°C.

2. Example of production of resin cross-linking agent containing resol-type phenol-formaldehyde co-condensation resin and phenol having specific structure

<Example 1: Preparation of resin cross-linking agent A-1>
In a mortar, 50 g of Tackirole 201 (softening point: 88°C, manufactured by Taoka Chemical Co., Ltd., hereinafter sometimes referred to as resin 201), which is a commercially available resol-type alkylphenol-formaldehyde cocondensation resin, and 1,1,2, 4.0 g of 2-tetrakis(4-hydroxyphenyl)ethane (hereinafter sometimes referred to as THPE) was charged and mixed with a pestle at 25° C. to obtain a THPE-containing resin cross-linking agent A-1 ( 54 g with a THPE content of 7.4% by weight) were obtained.

<Example 2: Preparation of resin cross-linking agent A-2>
In the same manner as in Example 1 except that 4.0 g of bisphenol A (hereinafter sometimes referred to as Bis-A) was used instead of THPE, a resin cross-linking agent A-2 containing bisphenol A (bisphenol A content 7.4 % by weight) 54 g.

<Example 3: Preparation of resin cross-linking agent A-3>
In the same manner as in Example 1 except that 4.0 g of bisphenol F (hereinafter sometimes referred to as Bis-F) was used instead of THPE, a resin cross-linking agent A-3 containing bisphenol F (bisphenol F content: 7.4 % by weight) 54 g.

<Comparative Example 1: Preparation of Resin Crosslinking Agent A-14>
In the same manner as in Example 1 except that 4.0 g of resorcin (hereinafter sometimes referred to as RES) was used instead of THPE, 54 g of resorcin-containing resin cross-linking agent A-14 (resorcin content: 7.4% by weight) was added. Obtained.

<Comparative Example 2: Preparation of resin cross-linking agent A-15>
In the same manner as in Example 1 except that 4.0 g of phenol (hereinafter sometimes referred to as Ph) was used instead of THPE, 54 g of a phenol-containing resin cross-linking agent A-15 (phenol content: 7.4% by weight) was added. Obtained.

<Comparative Example 3: Preparation of resin cross-linking agent A-16>
In the same manner as in Example 1 except that 4.0 g of p-cresol (hereinafter sometimes referred to as p-Cr) was used instead of THPE, resin cross-linking agent A-16 containing p-cresol (p-cresol content 7.4% by weight) was obtained.

<Comparative Example 4: Preparation of resin cross-linking agent A-17>
In the same manner as in Example 1 except that 4.0 g of maleic anhydride (hereinafter sometimes referred to as MA) was used instead of THPE, resin cross-linking agent A-17 containing maleic anhydride (maleic anhydride content: 7.0 g) was prepared. 4% by weight) 54 g were obtained.

<Example 4: Preparation of resin crosslinking agent B-1>
50 g of the resol-type p-tert-butylphenol/formaldehyde cocondensation resin (Resin-1) produced in Production Example 1 and 2.5 g of THPE were placed in a mortar, and mixed at 25° C. with a pestle while crushing to remove THPE. 53 g of resin cross-linking agent B-1 (THPE content: 4.8% by weight) was obtained.

<Example 5: Preparation of resin cross-linking agent B-2>
A resin cross-linking agent containing bisphenol S was prepared in the same manner as in Example 4, except that 2.5 g of bis(4-hydroxyphenyl) sulfone {bisphenol S (hereinafter sometimes referred to as Bis-S)} was used instead of THPE. 53 g of B-2 (4.8% by weight of bisphenol S) was obtained.

3. Production Examples and Evaluation of Physical Properties of Rubber Compositions Containing Resol-Type Phenols-Formaldehyde Co-Condensation Resins and Phenols Having Specific Structures

<Example 6>
100 parts by weight of butyl rubber (Polycerbutyl 402), 50 parts by weight of carbon black ("Seist S" (SRF grade) manufactured by Tokai Carbon Co., Ltd.), 5 parts by weight of zinc oxide (Seido Chemical Industry Co., Ltd., zinc oxide type 2), A masterbatch rubber (256 g) containing 100 parts by weight of calcined clay (“BURGESS KE” manufactured by Burgess Pigment Co., Ltd.) and 1 part by weight of stearic acid (“Beads stearic acid Tsubaki” manufactured by NOF Corporation) was kept at 20 ° C. It was placed in a 6-inch open roll manufactured by Kansai Roll Co., Ltd. and wound around the roll with a roll width of 2 mm. After that, the resin cross-linking agent A-1 (THPE content: 7.4% by weight, 16.2 g) produced in Example 1 was added, and the cross-linking was carried out three times. Next, the roll width was adjusted to 0.25 mm, and the mixture was kneaded by triangulating 10 times to prepare 272 g of a rubber composition.
Then, the rubber composition was press-molded by heating at 190° C. and 6 MPa for 10 minutes. Next, the press-molded rubber sheet is placed in a vacuum oven heated to 190° C. and heated at 190° C. for 150 minutes under a reduced pressure of 40 mmHg to obtain a cross-linked rubber composition (cross-linked rubber sheet )made.
The rubber composition and the crosslinked rubber composition obtained by the above method were subjected to the following tests. Table 1 shows the results.

<Examples 7 to 10, Reference Examples 1 and 2, Comparative Examples 5 to 8>
A rubber composition and a crosslinked rubber composition were prepared in the same manner as in Example 6 except that the resin crosslinking agent was changed to those shown in Tables 1 and 2 below, and the following evaluation measurements were performed. Results are shown in Tables 1 and 2. In Tables 1 and 2 below, the compounding amount of each component represents the compounding amount (parts by weight) per 100 parts by weight of butyl rubber. In Reference Examples 1 and 2, only Resin 201 (Reference Example 1) and Resin-1 obtained in Production Example 1 (Reference Example 2) were used as resin crosslinking agents.

<Test method for rubber composition and crosslinked rubber composition>
(1) Cross-linking properties Tests were carried out by a method conforming to JIS K6300-2:2001. Specifically, using a rotorless rheometer (oscillation angle ±3.00°), the rubber composition is heated at 190°C for 60 minutes while measuring the torque change, and from the measured value The maximum torque (MH (dN·m)) and vulcanization speeds t(10) and t(90) of the rubber composition were obtained. The vulcanization speeds t(10) and t(90) are the time (t(10) ), and the time to reach 90% (t(90)), and for Examples 9 and 10, the time to reach 10% of the maximum torque value obtained in Reference Example 2 (t(10)), and the time to reach 90% The time (t(90)) to reach the
(2) Hardness A test was performed according to JIS K6253. (Type of tester: type A durometer, test temperature: 25°C)
(3) Tensile test A test was performed in accordance with JIS K6251. (Specimen shape: Dumbbell No. 3, Tensile speed: 500 mm/min, Test temperature: 25°C)
Specifically, test pieces were prepared from crosslinked rubber, and modulus M50 (MPa), M100 (MPa), breaking strength Tb (MPa), and breaking elongation Eb (%) were measured.
(4) SWELL test A set of two test pieces prepared by punching a rectangular parallelepiped with a cutter of 50 mm in length × 5 mm in width × 2 mm in thickness from a crosslinked rubber sheet, immersed in toluene, and volume change after immersion for 72 hours. rate and weight change were measured.

<Evaluation of rubber composition and crosslinked rubber composition>
(1) Evaluation values (numerical values) Regarding the evaluation values (numerical values) of (1) to (4) above, the values of each test result obtained in Reference Example 1 for Examples 6 to 8 and Comparative Examples 5 to 8 was expressed as a relative comparison value when the was set to 100. In addition, for Examples 9 and 10, the value of each test result obtained in Reference Example 2 is set to 100, and the relative comparison values are shown.

(2) Relative evaluation Five items of cross-linking speed, cross-linking density, dispersibility/moldability (Tb/Eb), dispersibility/moldability (appearance), swelling resistance/solvent resistance were evaluated by the following method. Tables 1 and 2 show the results of comparison and evaluation with reference examples. Examples 6 to 8 and Comparative Examples 5 to 8 were compared and evaluated with Reference Example 1, and Examples 9 and 10 were compared and evaluated with Reference Example 2.

(2-1) Evaluation of Crosslinking Speed If t (10), which indicates the initial speed of crosslinking, is 20 points or more smaller than that of each Reference Example, there is a concern that scorch (premature crosslinking) may occur. A decrease of more than a point is considered inferior to each reference example, and t (90), which is a guideline for the end point of crosslinking, indicates that the smaller the value, the faster the crosslinking, so 5 points for each reference example. The decrease above is defined as an improvement over each reference example. Among these two evaluation values, those with better results than those of each reference example were evaluated as ⊚, those with inferior results were evaluated as x, and those with other evaluation values (equivalent to each reference example) were evaluated as ◯.

(2-2) Evaluation of crosslink density Of the hardness and tensile tests, M50 / M100 and the maximum torque (MH) in the rheometer test are compared with Reference Examples, and all values are 3 points or more compared to Reference Examples. An increase is considered better than the reference example, a decrease of 3 points or more is inferior to the reference example, and a difference in the range of ± 2 points is equivalent to the reference example. ⊚ indicates that the results are better than those of Reference Example, x indicates inferior results, and ◯ indicates others (similar to those of Reference Example).
In Comparative Example 5, since the test piece broke before 50% elongation, the M50 and M100 values could not be measured.

(2-3) Dispersibility, Formability (Evaluation by Tb, Eb)
Regarding the dispersibility of the resin cross-linking agent and the moldability of the rubber composition, in the tensile test, Tb and Eb were each compared with the reference example, and both values were reduced by 5 points or more from the reference example. The sample was marked with x as being inferior, and the other samples were marked with ◯ as being equivalent to the reference example.

(2-4) Dispersibility, moldability (visual evaluation)
Visually observe the rubber composition after kneading with rolls, the crosslinked rubber sheet after press molding, and the surfaces of rolls and molds, and check for the presence of rubber agglomerates, foaming, deformation (such as thinning), and the rolls and gold. The presence or absence of contamination of the mold was confirmed, and the cases where rubber aggregates, foaming, deformation, etc. were observed, or the cases where contamination of the rolls, molds, etc.

(2-5) Swelling resistance, solvent resistance SWELL test volume change rate (difference in volume of rubber test piece before and after immersion in toluene / volume of rubber test piece before immersion in toluene) and weight change rate (before and after immersion in toluene The difference in the weight of the rubber test piece/the weight of the rubber test piece before immersion in toluene) was compared with the reference example. A reduction of more than one point is indicated by ⊙, indicating an improvement over the reference example.

As shown in Tables 1 and 2 above, the rubber composition crosslinked with the resin crosslinking agent of the present invention is compared with the case where only the resol-type alkylphenol-formaldehyde cocondensation resin is used as the crosslinking agent (Reference Examples 1 and 2). However, it was found that each physical property described above was improved.

In addition, the rubber composition crosslinked with a resin crosslinking agent containing phenols other than phenols having a specific structure, on the contrary, compared to the case where only a resol-type alkylphenol-formaldehyde cocondensation resin was used as a crosslinking agent, each physical property was found to deteriorate (Comparative Examples 1 to 4).

Claims (4)

A resole-type phenol formaldehyde cocondensation resin and the following general formula (1):

(In general formula (1) above, R ₁ and R ₂ represent a hydrogen atom, an optionally branched C 1-4 alkyl group, a phenyl group, or a 4-hydroxyphenyl group.)
A compound represented by the following general formula (2):

(In the above general formula (2), n represents an integer of 1 to 9.)
A compound represented by the following general formula (3):

(In general formula (3) above, R ₃ and R ₄ represent a hydrogen atom, an optionally branched C 1-4 alkyl group, or a 4-hydroxyphenyl group.)
and at least one phenol selected from the group consisting of bis(4-hydroxyphenyl)sulfone, and a rubber resin cross-linking agent. 2. The resin cross-linking agent according to claim 1, which contains 1 to 15 parts by weight of phenols with respect to 100 parts by weight of the resol-type phenol-formaldehyde cocondensation resin. A rubber composition comprising rubber and the resin crosslinking agent according to claim 1 or 2. 4. The rubber composition according to claim 3, which contains 5 to 23 parts by weight of the resin crosslinking agent with respect to 100 parts by weight of the rubber.