Prepreg and fiber-reinforced composite material

Abstract

The purpose of the present invention is to provide a prepreg which satisfies one of a)-c): a) having superior storage stability and capable of providing a fiber-reinforced composite material having superior mechanical properties; b) capable of providing a fiber-reinforced composite material having superior mechanical properties, and the obtained fiber-reinforced composite material has superior appearance quality; and c) having superior storage stability, generating less amount of heat when cured, and enabling the cure extent and the viscosity in the B-stage state to be flexibly controlled. To achieve the purpose, the present invention provides a prepreg including reinforced fibers and an epoxy resin composition which contains an epoxy resin and a curing agent represented by a specific chemical formula, and which satisfies a specific condition.

Claims

1. A prepreg comprising: an epoxy resin composition that contains a curing agent A represented by Chemical formula A and an epoxy resin, and satisfies one of (i) to (iii) below; and a reinforcing fiber: ##STR00003## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each independently represent hydrogen, methyl, ethyl, or n-propyl: (i) satisfying Conditions 1 and 2; (ii) satisfying Conditions 1 and 3; and (iii) satisfying Condition 4 where Conditions 1 to 4 are as follows: Condition 1: a bending modulus of a resin cured product obtained by curing the epoxy resin composition at 220 C. for 120 minutes is 3.60 GPa or more; Condition 2: Tg change of the epoxy resin composition between before and after storage at 40 C. and 75% RH for 10 days is less than 10 C.; Condition 3: a temperature at which the epoxy resin composition exhibits a minimum viscosity when heated at a rate of 5 C./min from 40 C. to 250 C. in dynamic viscoelastic measurement is 150 C. or more and 200 C. or less; and Condition 4: the epoxy resin composition contains a combination of the curing agent A and a curing agent B or a combination of the curing agent A and a curing agent C as a curing agent: Curing agent B: 4,4-dimethyl-3,3-diaminodiphenyl sulfone; and Curing agent C: 3,3-diaminodiphenyl sulfone.

2. The prepreg according to claim 1, wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 in Chemical formula A are all hydrogen.

3. The prepreg according to claim 1, having a total content of a component X below of 20 to 50% by mass relative to 100% by mass of a total mass of the epoxy resin: Component X: at least one epoxy resin selected from the group consisting of [x1], [x2], and [x3] below: [x1]: a trifunctional or higher functional epoxy resin having a benzene ring; [x2]: a bisphenol F epoxy resin; and [x3]: an isocyanuric acid epoxy resin.

4. A fiber-reinforced composite material, which is a cured product of the prepreg according to claim 1.

Description

EXAMPLES

(1) The present invention will be described more specifically with reference to examples below, but the present invention is not limited to the description of these examples.

(2) The components used in examples are as follows.

(3) <Material Used>

(4) Curing Agent A

(5) [A]-1 2,2-diaminodiphenyl sulfone

(6) (Synthesis) At room temperature, 2,2-diaminodiphenylsulfide (1.1 kg, 5.1 mol, manufactured by Chengzhou Harvestchem) was dissolved in N,N-dimethylformamide (DMF) (10.1 L), potassium peroxymonosulfate (4.7 kg, 7.6 mol) was added thereto, and the mixture was stirred at room temperature for 20 hours. Subsequently, water (22 L) and toluene (22 L) were added to the reaction liquid, and the mixture was stirred for 30 minutes and then filtered through Celite. The filtrate was washed with toluene (10 L). The filtrate was separated, and the aqueous layer was extracted with toluene (10 L). The obtained organic layer was washed with water (10 L), saturated aqueous sodium thiosulfate solution (10 L), and saturated saline (10 L) in this order, and concentrated under reduced pressure to give a crude product.

(7) (Purification) The obtained crude product was dissolved in ethanol (2.5 L), then water (0.8 L) was added thereto, and the precipitated solid was collected by filtration. Subsequently, the solid obtained by filtration was dissolved in ethyl acetate, silica gel (150 g) was added thereto, the mixture was stirred for 30 minutes, and then filtered under reduced pressure on 150 g of silica gel, and the filtrate was concentrated to give a crude product. Further, methanol (0.8 L) was added to the obtained crude product, and the mixture was stirred for 30 minutes. Then, the solid was collected by filtration and dried at 40 C. under reduced pressure to give 2,2-diaminodiphenyl sulfone (0.43 kg).

(8) [x1]: a trifunctional or higher functional epoxy resin having a benzene ring

(9) [x1]-1 SUMI-EPDXY (registered trademark) ELM 434 (a diaminodiphenylmethane epoxy resin, manufactured by Sumitomo Chemical Industry Company Limited)

(10) [x1]-2 Araldite (registered trademark) MY0600 (an aminophenol epoxy resin, manufactured by Huntsman Advanced Materials Co., Ltd.)

(11) [x1]-3 jER (registered trademark) 154 (a phenol novolak epoxy resin, manufactured by Japan Epoxy Resins Co., Ltd.)

(12) [x2]: a bisphenol F epoxy resin

(13) [x2]-1 EPICLON (registered trademark) Epc830 (a bisphenol F epoxy resin, manufactured by Dainippon Ink & Chemicals, Inc.)

(14) [x2]-2 Epotohto (registered trademark) YDF-2001 (a bisphenol F epoxy resin, manufactured by Tohto Kasei Co., Ltd.)

(15) [x2]-3 jER (registered trademark) 4010P (a bisphenol F epoxy resin, manufactured by Mitsubishi Chemical Corporation)

(16) [x3]: an isocyanuric acid epoxy resin

(17) [x3]-1 TEPIC (registered trademark) S (an isocyanuric acid epoxy resin, manufactured by Nissan Chemical Industries, Ltd.)

(18) epoxy resins [x4] other than epoxy resins [x1], [x2], and [x3]

(19) [x4]-1 jER (registered trademark) 828 (a bisphenol A epoxy resin, manufactured by Mitsubishi Chemical Corporation)

(20) [x4]-2 EPICLON (registered trademark) EXA-1514 (a bisphenol S epoxy resin, manufactured by Dainippon Ink & Chemicals, Inc.)

(21) [x4]-3 Epikote (registered trademark) YX4000H (a biphenyl epoxy resin, manufactured by Japan Epoxy Resins Co., Ltd.)

(22) Curing Agents Other than the Curing Agent A

(23) Curing agent B 4,4-dimethyl-3,3-diaminodiphenyl sulfone

(24) Curing agent C 3,3DAS (3,3-diaminodiphenyl sulfone, manufactured by MITSUI FINE CHEMICALS, Inc.)

(25) Seikacure-S (4,4-diaminodiphenyl sulfone, manufactured by SEIKA CORPORATION)

(26) The curing agent B was prepared by the method described below.

[Step 1] Manufacturing Step of 4,4-dimethyl-3,3-dinitrodiphenylsulfone

(27) In concentrated sulfuric acid (2.3 L, 4.2 mol), 4,4-dimethyldiphenylsulfone (1.4 kg, 5.7 mol, manufactured by Sigma-Aldrich Co. LLC) was dissolved, and then the solution was cooled to 4 C. Concentrated nitric acid (0.76 L, 17.1 mol) was added dropwise thereto over 4 hours while maintaining the temperature of the reaction solution at 11 C., and the mixture was stirred at room temperature overnight. Subsequently, the reaction solution was cooled to 6 C. and ice water (1.4 L) was added thereto over 2 hours while maintaining the temperature at 15 C. or less. The precipitated solid was collected by filtration and the filtrate was washed with water. The obtained solid was dried under reduced pressure at 50 C. to give 1.8 kg of a white solid. The obtained white solid was dissolved in chloroform (25 L) and the solution was stirred. Then, heptane (25 L) was added thereto, the mixture was stirred for 30 minutes, and then the precipitated solid was collected by filtration. The solid obtained by filtration was dried under reduced pressure to give 1.6 kg of 4,4-dimethyl-3,3-dinitrodiphenylsulfone.

[Step 2] Manufacturing Step of 4,4-dimethyl-3,3-diaminodiphenyl Sulfone

(28) In methanol (5.0 L), 4,4-dimethyl-3,3-dinitrodiphenylsulfone (0.55 kg, 1.64 mol) was dissolved, and the air inside the system was replaced with argon gas. In another container, 5% palladium carbon (0.13 kg, 50% wet) was added to methanol (1.0 L) degassed with argon gas to prepare a suspension of palladium carbon. The suspension was added to 4,4-dimethyl-3,3-dinitrodiphenylsulfone in methanol, and methanol (0.6 L) was further added thereto. Subsequently, the air inside the reaction system was replaced with hydrogen gas, and the mixture was stirred for 2 days while replenishing hydrogen. Thereafter, the reaction solution was filtered through Celite, and the filtrate was washed with methanol (13.0 L). The same operation was repeated three times, and methanol was distilled off under reduced pressure to give a solid (1.1 kg).

(29) The obtained solid was suspended in ethyl acetate (6.4 L), the suspension was stirred for 5 minutes, then heptane (25.0 L) was added thereto, and the mixture was stirred for 20 minutes. The precipitated solid was collected by filtration and dried under reduced pressure to give 4,4-dimethyl-3,3-diaminodiphenyl sulfone (1.08 kg).

(30) Other Components [x5]

(31) [x5]-1 SUMIKA EXCEL (registered trademark) PES5003P (polyether sulfone, manufactured by Sumitomo Chemical Company, Limited)

(32) <Method for Preparing Epoxy Resin Composition>

(33) The predetermined amounts of components other than the curing agent were placed in a kneader, the mixture was heated to 150 C. with being kneaded, and kneaded at 150 C. for 1 hour to give a transparent viscous liquid. The temperature of the viscous liquid was lowered to 60 C. with the liquid being kneaded, then the curing agent was added thereto, and the mixture was kneaded at 60 C. for 30 minutes to give an epoxy resin composition.

(34) <Method for Evaluating Storage Stability of Epoxy Resin Composition>

(35) The storage stability of the epoxy resin composition was evaluated as follows. The initial epoxy resin composition (3 g) obtained by the above method was weighed out in an aluminum cup and allowed to stand in a thermo-hygrostat bath under the environment at 40 C. and 75% RH for 10 days. The glass transition temperature after storage was defined as T.sub.1, and the initial glass transition temperature was defined as T.sub.0. The change amount of the glass transition temperature was defined as Tg=T.sub.1T.sub.0, and the storage stability was judged by the value of the Tg. The glass transition temperature was measured by weighing out 3 mg of the epoxy resin after storage in a sample pan and heating the resin at 5 C./min from 20 C. to 150 C. using a differential scanning calorimeter (Q-2000: manufactured by TA Instruments). The midpoint of the inflection point of the obtained heat generation curve was obtained as Tg.

(36) <Method for Measuring Temperature at Which Epoxy Resin Composition Exhibits Minimum Viscosity>

(37) The temperature at which the epoxy resin composition exhibits a minimum viscosity was obtained as follows. The initial epoxy resin composition obtained by the above method was collected and subjected to dynamic viscoelastic measurement in the temperature range of 30 to 250 C. under conditions of a torsional vibration frequency of 0.5 Hz and a heating rate of 5 C./min using a viscoelasticity measuring device (ARES, manufactured by TA Instruments Inc.). The minimum point of the viscosity curve was taken as the temperature at which the epoxy resin composition exhibits a minimum viscosity.

(38) <Method for Evaluating Bending Modulus of Resin Cured Product>

(39) The epoxy resin composition was defoamed in vacuum, and then cured for 2 hours at a temperature of 220 C. in a mold set to give a thickness of 2 mm with a 2 mm thick Teflon (registered trademark) spacer to give a plate-like resin cured product having a thickness of 2 mm. From the resin cured product, a specimen having a width of 10 mm and a length of 60 mm was cut out and subjected to three point bending at a span of 32 mm and a cross head speed of 100 mm/min using an Instron universal testing machine (manufactured by Instron) to measure the bending modulus. The average of the values obtained by measuring five specimens cut out from the same resin cured product was taken as the value of bending modulus.

(40) <Method for Preparing Precured Product>

(41) The predetermined amounts of components other than the curing agent were placed in a stainless steel beaker, and the mixture was heated to 150 C. with being appropriately kneaded with a spatula to give a transparent viscous liquid. The temperature of the viscous liquid was lowered to 60 C., then the curing agent was added thereto, and the mixture was kneaded at 60 C. for 30 minutes to give an epoxy resin composition. The prepared epoxy resin composition (about 3 g) was weighed out in an aluminum cup, placed a hot air oven preheated to 140 C., allowed to stand for 2 hours, taken out from the oven, and cooled to room temperature to give a precured product.

(42) <Method for Measuring Resin Properties of Precured Product>

(43) (1) Method for Evaluating Storage Stability

(44) The storage stability of the precured product was determined as follows. The precured product (3 g) obtained by the above method was weighed out in an aluminum cup and allowed to stand in a thermo-hygrostat bath under the environment of 40 C. and 75% RH for 6 days. The glass transition temperature after storage was defined as T.sub.1, and the initial glass transition temperature (before the precured product was allowed to stand in the thermo-hygrostat bath) was defined as T.sub.0. The change amount of the glass transition temperature was defined as Tg=T.sub.1T.sub.0, and the storage stability was judged by the value of the Tg. The glass transition temperature was measured by weighing out 3 mg each of the precured products before being allowed to stand in the thermo-hygrostat bath and after being allowed to stand for 6 days in a sample pan, and heating the precured products at 5 C./min from 20 C. to 150 C. using a differential scanning calorimeter (Q-2000: manufactured by TA Instruments). The midpoint of the inflection point of the obtained heat generation curve was taken as the glass transition temperature Tg.

(45) (2) Method for Measuring Degree of Curing and Amount of Heat Generation

(46) The prepared epoxy resin composition (3 mg) was weighted out in a sample pan, and subjected to measurement under conditions of the constant rate heating of 5 C./min from 30 C. to 300 C. using a differential scanning calorimeter (Q-2000: manufactured by TA Instruments). The amount of heat generation was calculated from the obtained DSC curve according to JIS K0129 (1994). The amount of heat generation of the precured product was also measured in the same manner as above. The degree of curing of the precured product was calculated from (the amount of heat generation of the precured product)/(the amount of heat generation of the epoxy resin composition)100.

(47) <Method for Evaluating Bending Modulus of Resin Cured Product (Additional Curing)

(48) The epoxy resin composition was defoamed in vacuum, then cured for 2 hours at a temperature of 140 C. in a mold set to give a thickness of 2 mm with a 2 mm thick Teflon (registered trademark) spacer, and allowed to stand for 24 hours at room temperature to give a precured product. The obtained precured product was additionally cured at a temperature of 220 C. for 2 hours to give a plate-like resin cured product having a thickness of 2 mm. From the resin cured product, a specimen having a width of 10 mm and a length of 60 mm was cut out and subjected to three point bending at a span of 32 mm and a cross head speed of 100 mm/min using an Instron universal testing machine (manufactured by Instron) to measure the bending modulus. The average of the values obtained by measuring five specimens cut out from the same resin cured product was taken as the value of bending modulus.

(49) <Method for Measuring Exothermic Onset Temperature of Epoxy Resin Composition>

(50) The prepared epoxy resin composition (3 mg) was weighted out in a sample pan, and subjected to measurement under conditions of the constant rate heating of 5 C./min from 30 C. to 300 C. using a differential scanning calorimeter (Q-2000: manufactured by TA Instruments). The baseline of the DSC curve was set according to JIS K0129 (1994), and the point at which the tangent of the rise of the DSC curve due to heat generation crosses the baseline obtained by the above method was taken as the exothermic onset temperature.

(51) <Method for Producing Prepreg>

(52) The epoxy resin composition prepared according to the above <Method for Preparing Epoxy Resin Composition> was applied on release paper using a film coater to produce a resin film with a predetermined mass per unit area. The mass per unit area of the resin film was adjusted to be 39 g/m.sup.2.

(53) This resin film was set in a prepreg making instrument, and a sheet-shaped carbon fiber TORAYCA (registered trademark) T700S (manufactured by TORAY INDUSTRIES, INC., mass per unit area: 150 g/m.sup.2) aligned in one direction was impregnated with the resin film from both sides thereof by heating and pressing to give a prepreg. The resin content of the prepreg was 35% by mass.

(54) <Method for Evaluating Properties of Fiber-Reinforced Composite Material>

(55) (1) Method for Evaluating 0 Bending Strength and 0 Bending Modulus of Fiber-Reinforced Composite Material (Mechanical Properties of Fiber-Reinforced Composite Material)

(56) The fiber direction of the unidirectional prepreg prepared by the above <Method for Producing Prepreg> was aligned, and 13 plies of the prepregs were laminated and molded at a temperature of 220 C. for 120 minutes under a pressure of 0.6 MPa at a heating rate of 1.7 C./min in an autoclave to prepare a CFRP of a unidirectional material having a thickness of 2 mm. From this laminated board, a specimen having a width of 15 mm and a length of 100 mm was cut out and subjected to three point bending using an Instron universal testing machine (manufactured by Instron) according to JIS K 7074 (1988). The 0 bending strength and the 0 bending modulus were measured at a span of 80 mm, a crosshead speed of 5.0 mm/min, an indenter diameter of 10 mm, and a fulcrum diameter of 4.0 mm. The converted values at the fiber content of 60% by mass were calculated from the average of the values obtained from the six specimens cut but from the same laminated plate, and were taken as 0 bending strength and 0 bending modulus.

(57) (2) Method for Evaluating Appearance of Fiber-Reinforced Composite Material

(58) The fiber direction of the unidirectional prepreg produced by the above <Method for Producing Prepreg> was aligned, and 13 plies of the prepregs were laminated to give a laminate. The laminate was placed in a single-sided mold, the periphery of the single-sided mold was covered with a sealing material (the cover film and the mold were joined together to seal the inside of the mold), and then a bleeder made of a thick nonwoven fabric (which plays a role of a spacer that provides a passage of air and the resin) was placed in the circumference of the laminate. A tube was placed as a suction port on the bleeder, and then the sealing material and the cover film were joined together to cover the single-sided mold with the cover film. As the cover film, a film having flexibility was used. A vacuum pump was connected to the tube as a suction port to suck air in a molding space (a space formed by the single-sided mold and the cover film and including the laminate), thereby the pressure in the molding space was reduced. Then, the single-sided mold was placed in the oven controlled at 220 C. and held for 120 minutes to produce a fiber-reinforced composite material. After the fiber-reinforced composite material was held for 120 minutes, the single-sided mold was taken out to demold the fiber-reinforced composite material.

(59) The appearance of the fiber-reinforced composite material was visually judged from the viewpoint of smoothness of the surface and the presence or absence of blurs and pinholes. The material having a smooth surface without blurs or pinholes was evaluated as A, the material confirmed to have one or two of irregularities, blurs, and pinholes on the surface was evaluated as B, the material confirmed to have irregularities on the surface, blurs and pinholes was evaluated as C.

Example 1

(60) An epoxy resin composition was prepared using 100 parts by mass of jER (registered trademark) 828 as the epoxy resin other than the component X and 32.9 parts by mass of 2,2-diaminodiphenyl sulfone as the curing agent A, according to the above <Method for Preparing Epoxy Resin Composition>.

(61) This epoxy resin composition was evaluated according to the <Method for Evaluating Storage Stability of Resin Composition> and the <Method for Measuring Temperature at Which Epoxy Resin Composition Exhibits Minimum Viscosity>. The composition had a Tg of 3 C. and thus had good storage stability, and exhibited the minimum viscosity at 183 C.

(62) The epoxy resin composition was cured by the above method to prepare a resin cured product, and the bending modulus thereof was measured and found to be 3.61 GPa.

(63) A prepreg having a resin content of 35% by mass was produced from the obtained epoxy resin composition using a carbon fiber TORAYCA (registered trademark) T700S (manufactured by Toray Industries, Inc.) as a reinforcing fiber according to the <Method for Producing Prepreg>.

(64) Mechanical properties of the fiber-reinforced composite material obtained from the prepreg were measured. The material had a 0 bending strength of 1580 MPa and a 0 bending modulus of 127 GPa, and thus showed good mechanical properties.

(65) The fiber-reinforced composite material obtained by holding the laminate of the prepregs in a reduced pressure state in an oven controlled at 220 C. for 120 minutes had a good surface appearance and was evaluated as A.

Examples 2 to 17

(66) An epoxy resin composition, an epoxy resin cured product, and a fiber-reinforced composite material were prepared in the same manner as in Example 1 except that the resin composition was changed as shown in Tables 1-1 and 1-2. As in Example 1, all of the obtained resin compositions had good storage stability and a good bending modulus of the cured product, and the temperatures at which the epoxy resin composition exhibits a minimum viscosity were in an appropriate temperature range.

(67) The 0 bending strength, 0 bending modulus, and surface appearance of the fiber-reinforced composite materials were good.

Comparative Example 1

(68) An epoxy resin composition and an epoxy resin cured product were prepared in the same manner as in Example 1 except that 4,4-dimethyl-3,3-diaminodiphenyl sulfone was used instead of the curing agent A. The resin composition and evaluation results are shown in Table 2. The bending modulus was 3.40 GPa, and thus insufficient. In addition, the Tg was 27 C., and thus storage stability was insufficient. The temperature at which the epoxy resin composition exhibits a minimum viscosity was 108 C., and thus low.

(69) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was 1480 MPa, and thus insufficient. The surface appearance was poor.

Comparative Example 2

(70) An epoxy resin composition and an epoxy resin cured product were prepared in the same manner as in Example 1 except that 3,3-diaminodiphenyl sulfone was used instead of the curing agent A. The resin composition and evaluation results are shown in Table 2. The bending modulus was 3.82 GPa, and thus good. However, the Tg was 25 C., and thus storage stability was insufficient. The temperature at which the epoxy resin composition exhibits a minimum viscosity was 134 C., and thus low.

(71) The 0 bending properties of the fiber-reinforced composite material were good, but the surface appearance was poor.

Comparative Example 3

(72) An epoxy resin composition and an epoxy resin cured product were prepared in the same manner as in Example 1 except that 4,4-diaminodiphenyl sulfone was used instead of the curing agent A. The resin composition and evaluation results are shown in Table 2. The Tg was 9 C., and thus storage stability was good, but the bending modulus was 3.22 GPa, and thus insufficient. The temperature at which the epoxy resin composition exhibits a minimum viscosity was 142 C., and thus low.

(73) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was 1470 MPa, and thus insufficient. The surface appearance was poor.

Comparative Example 4

(74) An epoxy resin composition and an epoxy resin cured product were prepared according to the method described in Example 1 of Patent Document 1. The resin composition and evaluation results are shown in Table 2. The bending modulus was 3.50 GPa, and thus good, but the Tg after storage at 40 C. and 75% RH for 10 days was 28 C., and thus storage stability was insufficient. The temperature at which the epoxy resin composition exhibits a minimum viscosity was 146 C., and thus low.

(75) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was 1500 MPa, and thus insufficient. The surface appearance was poor.

Example 18

(76) Measurement of |T2T1|

(77) An epoxy resin composition was prepared according to the above <Method for Preparing Epoxy Resin Composition> by adding 80 parts by mass of jER (registered trademark) 828, 20 parts by mass of SUMI-EPDXY (registered trademark) ELM 434 (manufactured by Sumitomo Chemical Industry Company Limited), and 4.1 parts by mass of 4,4-dimethyl-3,3-diaminodiphenyl sulfone as a curing agent based on 100 parts by mass of the total of the epoxy resin. A prepreg having a resin content of 35% by mass was produced from the obtained epoxy resin composition using a carbon fiber TORAYCA (registered trademark) T700S (manufactured by Toray Industries, Inc.) as a reinforcing fiber according to the <Method for Producing Prepreg>.

(78) T1 of the obtained epoxy resin composition was measured according to the <Method for Measuring Exothermic Onset Temperature of Epoxy Resin Composition>.

(79) An epoxy resin composition was prepared according to the <Method for Preparing Epoxy Resin Composition> by adding 80 parts by mass of jER (registered trademark) 828, 20 parts by mass of SUMI-EPDXY (registered trademark) ELM 434 (manufactured by Sumitomo Chemical Industry Company Limited), and 33 parts by mass of 2,2-diaminodiphenyl sulfone as a curing agent based on 100 parts by mass of the total of the epoxy resin.

(80) T2 of the obtained epoxy resin composition was measured according to the <Method for Measuring Exothermic Onset Temperature of Epoxy Resin Composition>.

(81) The obtained value of |T2T1| was 88 C.

Evaluation of Properties in Example 18

(82) An epoxy resin composition was prepared according to the above <Method for Preparing Epoxy Resin Composition> by adding 80 parts by mass of jER (registered trademark) 828, 20 parts by mass of SUMI-EPDXY (registered trademark) ELM 434 (manufactured by Sumitomo Chemical Industry Company Limited), and 33 parts by mass of 2,2-diaminodiphenyl and 4.1 parts by mass of 4,4-dimethyl-3,3-diaminodiphenyl sulfone as a curing agent based on 100 parts by mass of the total of the epoxy resin.

(83) A precured product was prepared from the obtained epoxy resin composition according to the <Method for Preparing Precured Product>.

(84) The storage stability of the obtained precured product was evaluated. The Tg increased only by 0.7 C. after storage at 40 C. and 75% RH for 6 days, and thus, the precured product had sufficient storage stability. The degree of curing of the precured product was 24%.

(85) The precured product was additionally cured by the above method to prepare a resin cured product, and the resin cured product was subjected to three point bending test. The resin cured product had a bending modulus of 3.90 GPa, and thus showed good mechanical properties.

(86) The storage stability of the obtained precured product was evaluated. The Tg increased only by 0.7 C. after storage at 40 C. and 75% RH for 6 days, and thus, the precured product had sufficient storage stability. The degree of curing of the precured product was 24%.

(87) The amount of heat generation of the precured product was evaluated and found to be 199 J/g.

(88) The precured product was cured by the above method to prepare a resin cured product, and the resin cured product was subjected to three point bending test. The resin cured product had a bending modulus of 3.90 GPa, and thus showed good mechanical properties.

(89) Mechanical properties of the fiber-reinforced composite material obtained from the prepreg including the epoxy resin composition were measured. The material had a 0 bending strength of 1640 MPa and a 0 bending modulus of 128 GPa, and thus showed good mechanical properties.

Examples 19 to 22

(90) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were prepared in the same manner as in Example 18 except that the amount of the curing agent added was changed as shown in Table 3-1. |T2T1| was evaluated in the same manner as in Example 18. |T2T1| showed almost constant values of 85 to 89 C. The storage stability of each precured product was evaluated in the same manner as in Example 18. All the precured products had good storage stability.

(91) The amounts of heat generation of the precured products in Examples 19 to 22 were 170, 156, 130, and 110 J/g, respectively.

(92) The degree of curing of the precured product increased as the compounding ratio of 4,4-dimethyl-3,3-diaminodiphenyl sulfone was increased. Specifically, the degrees of curing in Examples 19 to 22 were 37, 44, 58, and 65%, respectively, and the relationship between the compounding ratio between 4,4-dimethyl-3,3-diaminodiphenyl sulfone and 2,2-diaminodiphenyl sulfone and the degree of curing showed linearity.

(93) All the resin cured products had good values of the bending modulus.

(94) Both the 0 bending strength and the 0 bending modulus of all the fiber-reinforced composite materials were good.

Examples 23 to 27

(95) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were produced in the same manner as in Examples 18 to 22 except that 2,2-diaminodiphenyl sulfone and 3,3-diaminodiphenyl sulfone were used as the curing agents. |T2T1| was evaluated in the same manner as in Examples 18 to 22. The values of |T2T1| were 57 to 60 C. The storage stability of each precured product was evaluated in the same manner as in Example 18. All the precured products had good storage stability.

(96) The amounts of heat generation of the precured products in Examples 23 to 27 were 198, 177, 163, 142, and 120 J/g, respectively.

(97) The degree of curing varied from 39 to 75% depending on the compounding ratio between the curing agents.

(98) All the resin cured products had good values of the bending modulus.

(99) Both the 0 bending strength and the 0 bending modulus of all the fiber-reinforced composite materials were good.

Examples 28 to 30

(100) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were prepared in the same manner as in Example 18 except that the resin compositions were changed as shown in Table 3-3. |T2T1| was evaluated in the same manner as in Examples 18 to 22. The values of |T2T1| in Examples 28 to 30 were 85, 88, 59 and 44 C., respectively. The storage stability of each precured product was evaluated in the same manner as in Example 18. All the precured products had good storage stability. The degrees of curing of the precured products are shown in the table.

(101) The amounts of heat generation of the precured products in Examples 28 to 30 were 164, 167, 155, and 115 J/g, respectively.

(102) All the resin cured products had good values of the bending modulus.

(103) Both the 0 bending strength and the 0 bending modulus of all the fiber-reinforced composite materials were good.

Comparative Examples 5 to 9

(104) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were produced in the same manner as in Examples 18 to 22 except that 4,4-diaminodiphenyl sulfone and 3,3-diaminodiphenyl sulfone were used as the curing agents. |T2T1|, storage stability of the precured product, the degree of curing, and the amount of heat generation were also evaluated in the same manner as in Examples 18 to 22.

(105) The values of |T2T1| were 11 to 15 C., and thus low.

(106) When the ratio between the curing agents used in combination was changed, the change in the degree of curing was 51 to 59%, and thus small. Therefore, when the resin composition is used in a prepreg, the tackiness and drapability cannot be controlled. In Comparative Examples 8 and 9, Tgs were large and the storage stability was insufficient.

(107) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was insufficient.

Comparative Examples 10 to 14

(108) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were produced in the same manner as in Examples 18 to 22 except that 4,4-diaminodiphenyl sulfone and triethylenetetramine were used as the curing agents. |T2T1|, storage stability of the precured product, the degree of curing, and the amount of heat generation were also evaluated in the same manner as in Examples 18 to 22.

(109) The values of |T2T1| were 115 to 120 C., and remarkably large.

(110) Since triethylenetetramine having high reactivity with the epoxy resin was used, the Tg was increased and the storage stability remarkably decreased. The mechanical properties were also insufficient.

(111) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was insufficient.

Comparative Examples 15 to 19

(112) An epoxy resin composition, a precured product, a resin cured product, and a fiber-reinforced composite material were produced in the same manner as in Examples 18 to 22 except that diethyltoluenediamine and triethylenetetramine were used as the curing agents. |T2T1|, storage stability of the precured product, the degree of curing, and the amount of heat generation were also evaluated in the same manner as in Examples 18 to 22.

(113) The values of |T2T1| were 96 to 98 C., and remarkably large.

(114) Tgs were remarkably large, and storage stability was insufficient. The mechanical properties were also low.

(115) Since the bending modulus of the resin cured product was low, the 0 bending strength of the fiber-reinforced composite material was insufficient.

(116) TABLE-US-00001 TABLE 1-1 Example Example Example Example Example Component 1 2 3 4 5 Epoxy resin jER 828 Bisphenol A epoxy resin 100 80 70 80 70 EPICLON EXA-1514 Bisphenol S epoxy resin Epikote YX-4000H Biphenyl epoxy resin x1: SUMI-EPOXY Diaminodiphenylmethane 20 ELM 434 epoxy resin x1: Araldite MY0600 Aminophenol epoxy resin 30 x1: jER 154 Phenol novolak epoxy resin x2: EPICLON 830 Bisphenol F epoxy resin 20 30 x2: Epotohto YDF-2001 Bisphenol F epoxy resin x2: jER 4010P Bisphenol F epoxy resin x3: TEPIC-S Isocyanuric acid epoxy resin Curing agent Seikacure-S 4,4-diaminodiphenyl sulfone 3,3DAS 3,3-diaminodiphenyl sulfone 2,2-diaminodiphenyl 32.9 36.6 38.8 33.7 34.1 sulfone Properties Tg change after storage at C. 3 1 1 2 2 of resin 40 C. and 75% RH for composition 10 days Properties Bending modulus GPa 3.61 3.80 4.11 3.90 3.92 of resin cured product Properties Temperature at which the C. 183 180 179 182 180 of resin resin composition exhibits composition the minimum viscosity when heated from 40 C. at a rate of 5 C./min Properties 0 Bending strength MPa 1580 1611 1660 1633 1640 of fiber 0 Bending modulus GPa 127 128 128 129 129 reinforced composite material Properties Surface appearance A A A A A of fiber reinforced composite material Example Example Example Example Component 6 7 8 9 Epoxy resin jER 828 Bisphenol A epoxy resin 50 80 70 50 EPICLON EXA-1514 Bisphenol S epoxy resin Epikote YX-4000H Biphenyl epoxy resin x1: SUMI-EPOXY Diaminodiphenylmethane 25 ELM 434 epoxy resin x1: Araldite MY0600 Aminophenol epoxy resin x1: jER 154 Phenol novolak epoxy resin x2: EPICLON 830 Bisphenol F epoxy resin 50 x2: Epotohto YDF-2001 Bisphenol F epoxy resin x2: jER 4010P Bisphenol F epoxy resin x3: TEPIC-S Isocyanuric acid epoxy resin 20 30 25 Curing agent Seikacure-S 4,4-diaminodiphenyl sulfone 3,3DAS 3,3-diaminodiphenyl sulfone 2,2-diaminodiphenyl 34.9 38.7 41.6 44.9 sulfone Properties Tg change after storage at C. 2 2 2 0.5 of resin 40 C. and 75% RH for composition 10 days Properties Bending modulus GPa 3.85 4.03 4.21 4.32 of resin cured product Properties Temperature at which the C. 177 175 177 180 of resin resin composition exhibits composition the minimum viscosity when heated from 40 C. at a rate of 5 C./min Properties 0 Bending strength MPa 1630 1650 1700 1720 of fiber 0 Bending modulus GPa 129 128 129 128 reinforced composite material Properties Surface appearance A A A A of fiber reinforced composite material

(117) TABLE-US-00002 TABLE 1-2 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Component ple 10 ple 11 ple 12 ple 13 ple 14 ple 15 ple 16 ple 17 Epoxy resin jER 828 Bisphenol A epoxy resin 70 70 50 50 50 50 70 70 EPICLON EXA-1514 Bisphenol S epoxy resin Epikote YX-4000H Biphenyl epoxy resin x1: SUMI-EPOXY Diaminodiphenylmethane 30 30 50 10 ELM 434 epoxy resin x1: Araldite MY0600 Aminophenol epoxy resin x1: jER 154 Phenol novolak epoxy resin x2: EPICLON 830 Bisphenol F epoxy resin x2: Epotohto YDF-2001 Bisphenol F epoxy resin 50 x2: jER 4010P Bisphenol F epoxy resin 50 40 30 30 x3: TEPIC-S Isocyanuric acid epoxy resin Curing agent Seikacure-S 4,4-diaminodiphenyl 7.7 4.7 sulfone 3,3DAS 3,3-diaminodiphenyl 7.7 4.7 sulfone 2,2-diaminodiphenyl 30.8 30.8 33.8 23 17.2 22.2 18.8 18.8 sulfone Properties Tg change after storage at C. 6 2 3 1 2 2 5 2 of resin 40 C. and 75% RH for composition 10 days Properties of Bending modulus GPa 3.81 3.75 4.00 3.89 3.98 3.98 3.81 3.61 resin cured product Properties Temperature at which the C. 181 183 185 185 182 181 170 172 of resin resin composition exhibits composition the minimum viscosity when heated from 40 C. at a rate of 5 C./min Properties 0 Bending strength MPa 1633 1600 1640 1600 1645 1644 1580 1570 of fiber 0 Bending modulus GPa 127 128 128 127 129 129 126 126 reinforced composite material Properties Surface appearance A A A A A A B B of fiber reinforced composite material

(118) TABLE-US-00003 TABLE 2 Comparative Comparative Comparative Comparative Component Example 1 Example 2 Example 3 Example 4 Epoxy resin jER 828 Bisphenol A epoxy resin 100 80 80 EPICLON EXA-1514 Bisphenol S epoxy resin 5 Epikote YX-4000H Biphenyl epoxy resin 25 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 ELM 434 epoxy resin x1: Araldite MY0600 Aminophenol epoxy resin x1: jER 154 Phenol novolak epoxy resin 35 x2: EPICLON 830 Bisphenol F epoxy resin 35 Thermoplastic SUMIKA EXCEL Polyether sulfone 15 resin PES 5003P Curing agent Seikacure-S 4,4-diaminodiphenyl 36.6 sulfone 3,3DAS 3,3-diaminodiphenyl 36.6 34 sulfone 4,4-dimethyl-3,3- 32.9 diaminodiphenyl sulfone 2,2-diaminodiphenyl sulfone Properties Tg change after storage at C. 27 25 9 28 of resin 40 C. and 75% RH for composition 10 days Properties of Bending modulus GPa 3.40 3.82 3.22 3.50 resin cured product Properties Temperature at which the C. 108 134 142146 of resin resin composition exhibits composition the minimum viscosity when heated from 40 C. at a rate of 5 C./min Properties 0 Bending strength MPa 1480 1520 1470 1500 of fiber 0 Bending modulus GPa 125 125 126 126 reinforced composite material Properties Surface appearance C C C C of fiber reinforced composite material

(119) TABLE-US-00004 TABLE 3-1 Example Example Example Example Example Component 18 19 20 21 22 Epoxy resin jER 828 Bisphenol A epoxy resin 80 80 80 80 80 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 20 20 20 ELM 434 epoxy resin Curing agent 2,2-diaminodiphenyl 33 25.6 18.3 11 3.7 sulfone 4,4-dimethyl-3,3- 4.1 12.2 20.4 28.5 36.7 diaminodiphenyl sulfone Properties of T2 T1 C. 88 87 89 85 89 uncured resin Properties of Tg change after storage C. 0.7 1.3 1.5 3 5 resin after at 40 C. and 75% RH for precuring 6 days Degree of curing 24 37 44 58 65 Amount of heat generation J/g 199 170 156 130 110 Properties of Bending modulus GPa 3.90 3.85 3.85 3.75 3.75 resin cured product Properties 0 Bending strength MPa 1640 1631 1640 1600 1610 of fiber 0 Bending modulus GPa 128 128 129 129 128 reinforced composite material

(120) TABLE-US-00005 TABLE 3-2 Example Example Example Example Example Component 23 24 25 26 27 Epoxy resin jER 828 Bisphenol A epoxy resin 80 80 80 80 80 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 20 20 20 ELM 434 epoxy resin Curing agent 2,2-diaminodiphenyl 33 25.6 18.3 11 3.7 sulfone 3,3DAS 3,3-diaminodiphenyl 3.7 11 18.3 25.6 33 sulfone Properties of T2 T1 C. 60 58 57 57 58 uncured resin Properties of Tg change after storage at C. 0.4 0.9 1.6 3.1 3.5 resin after 40 C. and 75% RH for precuring 6 days Degree of curing 39 45 56 69 75 Amount of heat generation J/g 198 177 163 142 120 Properties of Bending modulus GPa 3.90 3.95 3.95 4.05 4.10 resin cured product Properties 0 Bending strength MPa 1643 1650 1655 1700 1695 of fiber 0 Bending modulus GPa 127 128 128 129 129 reinforced composite material

(121) TABLE-US-00006 TABLE 3-3 Example Example Example Component 28 29 30 Epoxy resin x2: EPICLON 830 Liquid bisphenol F epoxy 60 80 60 resin x1: SUMI-EPOXY ELM 434 Diaminodiphenylmethane epoxy 20 resin x1: jER 154 Phenol novolak epoxy resin 20 20 x1: Araldite MY0600 Aminophenol epoxy resin 20 20 Curing agent 3,3DAS 3,3-diaminodiphenyl sulfone 14.6 2,2-diaminodiphenyl sulfone 27.8 27.9 19.8 4,4-dimethyl-3,3- 13.2 13.3 diaminodiphenyl sulfone Properties of T2 T1 C. 85 88 59 uncured resin Properties of Tg change after storage at 40 C. C. 2 1.8 1.9 resin after and 75% RH for 6 days precuring Degree of curing % 37 36 55 Amount of heat generation J/g 164 167 155 Properties of Bending modulus GPa 4.10 4.00 3.95 resin cured product Properties of 0 Bending strength MPa 1697 1670 1683 fiber 0 Bending modulus GPa 129 128 128 reinforced composite material

(122) TABLE-US-00007 TABLE 4-1 Comparative Comparative Comparative Comparative Comparative Component Example 5 Example 6 Example 7 Example 8 Example 9 Epoxy resin jER 828 Bisphenol A epoxy resin 80 80 80 60 80 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 20 20 20 ELM 434 epoxy resin Curing agent Seikacure-S 4,4-diaminodiphenyl 33 25.6 18.3 11 3.7 sulfone 3,3DAS 3,3-diaminodiphenyl 3.7 11 18.3 25.6 33 sulfone Properties of T2 T1 C. 14 15 13 12 11 uncured resin Properties of Tg change after storage at C. 4.5 5.8 8.3 11.2 13.2 resin after 40 C. and 75% RH for precuring 6 days Degree of curing 51 51 54 54 59 Amount of heat generation J/g 181 163 148 130 112 Properties of Bending modulus GPa 3.30 3.45 3.55 3.70 3.75 resin cured product Properties 0 Bending strength MPa 1450 1500 1510 1533 1540 of fiber 0 Bending modulus GPa 127 128 127 129 128 reinforced composite material

(123) TABLE-US-00008 TABLE 4-2 Comparative Comparative Comparative Comparative Comparative Component Example 10 Example 11 Example 12 Example 13 Example 14 Epoxy resin jER 828 Bisphenol A epoxy resin 80 80 80 80 80 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 20 20 20 ELM 434 epoxy resin Curing agent Seikacure-S 4,4-diaminodiphenyl 33 25.6 18.3 11 3.7 sulfone Triethylenetetramine 1.4 4.3 7.2 10.1 12.9 Properties of T2 T1 C. 119 115 116 120 110 uncured resin Properties of Tg change after storage at C. 22 24.7 31.1 35 39 resin after 40 C. and 75% RH for precuring 6 days Degree of curing 54 58 58 58 59 Amount of heat generation J/g 174 140 115 77 46 Properties of Bending modulus GPa 3.20 3.12 3.05 2.87 2.80 resin cured product Properties 0 Bending strength MPa 1470 1430 1430 1400 1401 of fiber 0 Bending modulus GPa 125 125 124 124 124 reinforced composite material

(124) TABLE-US-00009 TABLE 4-3 Comparative Comparative Comparative Comparative Comparative Component Example 15 Example 16 Example 17 Example 18 Example 19 Epoxy resin jER 828 Bisphenol A epoxy resin 80 80 80 80 80 x1: SUMI-EPOXY Diaminodiphenylmethane 20 20 20 20 20 ELM 434 epoxy resin Curing agent Triethylenetetramine 1.4 4.3 7.2 10.1 11.2 Diethyltoluenediamine 23.7 18.4 13.1 7.9 2.3 Properties of T2 T1 C. 96 98 97 98 98 uncured resin Properties of Tg change after storage at C. 20.5 25.7 33.1 36.1 38.1 resin after 40 C. and 75% RH for precuring 6 days Degree of curing 61 62 64 64 65 Amount of heat generation J/g 75 69 57 46 35 Properties of Bending modulus GPa 2.90 2.92 2.88 2.78 2.70 resin cured product Properties 0 Bending strength MPa 1410 1400 1395 1380 1370 of fiber 0 Bending modulus GPa 126 126 126 125 124 reinforced composite material

(125) The unit of each component in the tables is parts by mass.