CARBON-FIBER-REINFORCED COMPOSITE MATERIAL AND METHOD FOR PRODUCING CARBON-FIBER-REINFORCED COMPOSITE MATERIAL

Abstract

The present invention provides a carbon-fiber-reinforced composite material having excellent tackiness, excellent compatibility with epoxy resins, and excellent interfacial adhesion while being capable of achieving high mechanical strength, and a method for producing a carbon-fiber-reinforced composite material. Provided is a carbon-fiber-reinforced composite material containing: carbon fibers; an epoxy resin; a curing agent; and a polyvinyl acetal resin, the polyvinyl acetal resin including structural units represented by the following formula (1), each R.sup.1 in the formula (1) being an alkyl group having a carbon number of 1 or greater:

##STR00001##

wherein R.sup.1s may be the same as or different from each other.

Claims

1. A carbon-fiber-reinforced composite material comprising: carbon fibers; an epoxy resin; a curing agent; and a polyvinyl acetal resin, the polyvinyl acetal resin including structural units represented by the following formula (1), each R.sup.1 in the formula (1) being an alkyl group having a carbon number of 1 or greater: ##STR00006## wherein R's may be the same as or different from each other.

2. The carbon-fiber-reinforced composite material according to claim 1, wherein in the polyvinyl acetal resin, R's in the formula (1) comprise an alkyl group having a carbon number of 1 or greater and/or an alkyl group having a carbon number of 3 or greater.

3. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin has a hydroxy group content of 15.0 mol % or more and 45.0 mol % or less.

4. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin has an average degree of polymerization of 2,500 or less.

5. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin has a glass transition temperature of 75 C. or higher.

6. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin includes a structural unit containing an acid-modified group.

7. The carbon-fiber-reinforced composite material according to claim 6, wherein in the polyvinyl acetal resin, the structural unit containing an acid-modified group is contained in an amount of 0.01 to 20 mol %.

8. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin has a hydroxy group content of 16.0 mol % or more and 45.0 mol % or less.

9. The carbon-fiber-reinforced composite material according to claim 1, wherein the polyvinyl acetal resin is contained in an amount of 0.01 parts by weight or more and 40.0 parts by weight or less relative to 100 parts by weight of the epoxy resin.

10. A method for producing a carbon-fiber-reinforced composite material, comprising at least the steps of: forming a resin composition containing an epoxy resin, a curing agent, and a polyvinyl acetal resin; and forming a composite of the resin composition with carbon fibers, the polyvinyl acetal resin including structural units represented by the following formula (1), R.sup.1s in the formula (1) each being an alkyl group having a carbon number of 1 or greater: ##STR00007## wherein R's may be the same as or different from each other.

Description

DESCRIPTION OF EMBODIMENTS

[0152] The present invention is more specifically described in the following with reference to, but not limited to, examples.

Example 1

(Production of Polyvinyl Acetal Resin)

[0153] An amount of 2,700 g of pure water was added to 250 g of a polyvinyl alcohol resin having an average degree of polymerization of 300 and a degree of saponification of 99 mol %, and stirred at 90 C. for about two hours for dissolution. This solution was cooled to 40 C., and to the solution were added 100 g of hydrochloric acid having a concentration of 35% by weight and 90 g of acetaldehyde to perform acetalization, whereby a reaction product was precipitated. Thereafter, the acetalization was completed at 40 C., followed by neutralization, washing with water, and drying by conventional methods. Thus, white powder of a polyvinyl acetal resin was obtained.

[0154] The obtained polyvinyl acetal resin was dissolved in DMSO-d.sub.6 at a concentration of 10% by weight, and 13C-NMR was performed to measure the alkyl acetal group content (degree of acetoacetalization), the hydroxy group content, and the acetyl group content.

(Production of Carbon-Fiber-Reinforced Composite Material [Prepreg])

[0155] To 100 parts by weight of a bisphenol A epoxy resin (JER828, available from Japan Epoxy Resins Co., Ltd.) were added 6 parts by weight of a curing agent (dicyandiamide) and 10 parts by weight of the obtained polyvinyl acetal resin, and they were mixed using Process Homogenizer (available from SMT) at 15,000 rpm to prepare a resin composition.

[0156] Subsequently, the obtained resin composition was impregnated into PAN carbon fibers (available from Toray Industries Inc., T700SC-12000-50C, number of filaments: 12,000, fineness: 800 tex, density: 1.8 g/cm.sup.3) by a hand lay-up method and cured by heating at 150 C. for one hour, whereby a prepreg was produced. Here, 300 parts by weight of the PAN carbon fibers were used for 100 parts by weight of the bisphenol A epoxy resin.

Examples 2 to 10, 17, and 19 to 21 and Comparative Examples 1 to 4

[0157] A polyvinyl acetal resin, a resin composition, and a prepreg were produced as in Example 1 except that a polyvinyl alcohol resin (PVA) and an aldehyde of the types and in the amounts shown in Table 1 were used, and that the resin composition was prepared in accordance with the formulation shown in Table 2.

[0158] The polyvinyl acetal resins obtained in Examples 1 and 2 were the same. The polyvinyl acetal resins obtained in Examples 4 and 5 were the same.

[0159] In Examples 4 to 6 and 8 and Comparative Example 3, two different aldehydes were used.

[0160] In Example 17 and Comparative Example 4, a bisphenol F epoxy resin (NPEF-170, available from Nan Ya Plastics Corporation) was used instead of the bisphenol A epoxy resin (JER828, available from Japan Epoxy Resins Co., Ltd.).

Example 11

(Production of Carboxylic Acid-Modified Polyvinyl Acetal Resin)

[0161] An amount of 100 g of a carboxylic acid-modified polyvinyl alcohol resin was added to 1,000 g of pure water and stirred at a temperature of 90 C. for about two hours for dissolution. This solution was cooled to 40 C., and to the solution were added 90 g of hydrochloric acid (concentration: 35% by weight) and 90 g of acetaldehyde. The solution temperature was cooled to 10 C., and this temperature was maintained to perform acetalization reaction. Thereafter, the solution was held at 40 C. for three hours to complete the reaction, followed by neutralization, washing with water, and drying by conventional methods. Thus, white powder of a carboxylic acid-modified polyvinyl acetal resin was obtained.

[0162] The carboxylic acid-modified polyvinyl alcohol resin included a carboxy group-containing structural unit represented by the formula (4-1) (wherein R.sup.2 is a single bond, R.sup.3 is a methylene group, and X.sup.1 and X.sup.2 are hydrogen atoms) and had an average degree of polymerization of 400, a degree of saponification of 99.0 mol %, and an acid-modified group content of 0.7 mol %. Here, the single bond means an alkylene group having a carbon number of 0.

[0163] A resin composition and a prepreg were obtained as in Example 1 except that the obtained carboxylic acid-modified polyvinyl acetal resin was used.

Example 12

[0164] A carboxylic acid-modified polyvinyl acetal resin, a resin composition, and a prepreg were produced as in

[0165] Example 11 except that in Production of carbon-fiber-reinforced composite material [prepreg], the amount of the polyvinyl acetal resin added was 2 parts by weight.

Example 13

[0166] A carboxylic acid-modified polyvinyl acetal resin, a resin composition, and a prepreg were produced as in Example 11 except that the carboxylic acid-modified polyvinyl alcohol resin used included a carboxy group-containing structural unit represented by the formula (4-1) (wherein R.sup.2 is a single bond, R.sup.3 is a methylene group, and X.sup.1 and X.sup.2 are hydrogen atoms) and had an average degree of polymerization of 400, a degree of saponification of 99.0 mol %, and an acid-modified group content of 2.0 mol %.

Example 14

[0167] A carboxylic acid-modified polyvinyl acetal resin, a resin composition, and a prepreg were produced as in Example 11 except that the carboxylic acid-modified polyvinyl alcohol resin used included a carboxy group-containing structural unit represented by the formula (4-1) (wherein Re is a single bond, R.sup.3 is a methylene group, and X.sup.1 and X.sup.2 are hydrogen atoms) and had an average degree of polymerization of 600, a degree of saponification of 99.0 mol %, and an acid-modified group content of 1.0 mol %, and that the amount of acetaldehyde added was 110 g.

Example 15

(Production of Sulfonic Acid-Modified Polyvinyl Acetal Resin)

[0168] An amount of 100 g of a sulfonic acid-modified polyvinyl alcohol resin was added to 1,000 g of pure water and stirred at a temperature of 90 C. for about two hours for dissolution. This solution was cooled to 40 C., and to the solution were added 90 g of hydrochloric acid (concentration: 35% by weight) and 90 g of acetaldehyde. The solution temperature was cooled to 10 C., and this temperature was maintained to perform acetalization reaction. Thereafter, the solution was held at 40 C. for three hours to complete the reaction, followed by neutralization, washing with water, and drying by conventional methods. Thus, white powder of a sulfonic acid-modified polyvinyl acetal resin was obtained.

[0169] The sulfonic acid-modified polyvinyl alcohol resin had a structure in which a sulfonic acid group was directly bonded to a carbon atom of the main chain, and had an average degree of polymerization of 300, a degree of saponification of 99.0 mol %, and an acid-modified group content of 0.7 mol %.

[0170] A resin composition and a prepreg were obtained as in Example 1 except that the obtained sulfonic acid-modified polyvinyl acetal resin was used.

Example 16

[0171] A carboxylic acid-modified polyvinyl acetal resin, a resin composition, and a prepreg were produced as in Example 11 except that the carboxylic acid-modified polyvinyl alcohol resin used included a carboxy group-containing structural unit represented by the formula (4-1) (wherein R.sup.2 is a single bond, R.sup.3 is a methylene group, and

[0172] X.sup.1 and X.sup.2 are hydrogen atoms) and had an average degree of polymerization of 2,500, a degree of saponification of 99.0 mol %, and an acid-modified group content of 1.0 mol %, and that 38 g of acetaldehyde and 110 g of butyraldehyde were added instead of 90 g of acetaldehyde.

Example 18

[0173] A carboxylic acid-modified polyvinyl acetal resin, a resin composition, and a prepreg were produced as in Example 11 except that in Production of carbon-fiber-reinforced composite material [prepreg], a bisphenol F epoxy resin (NPEF-170, available from Nan Ya Plastics Corporation) was used instead of the bisphenol A epoxy resin (JER828, available from Japan Epoxy Resins Co., Ltd.).

(Evaluation)

[0174] The polyvinyl acetal resins, resin compositions, and prepregs obtained in the examples and the comparative examples were evaluated as follows. Tables 1 and 2 show the results.

(1) Measurement of Glass Transition Temperature (Tg)

[0175] The glass transition temperature of the obtained polyvinyl acetal resins was measured using a differential scanning calorimeter (DSC) at a temperature increase rate of 10 C./min.

(2) Tensile Strain

[0176] The obtained resin compositions were cured by heating at 150 C. for one hour, whereby resin cured plates were produced. Dumbbell specimens (JIS K7161 Feb. 1B type) were cut out from the plates and used to measure the tensile strain using a universal tester (available from Instron) under the following conditions.

Test conditions: JIS K 7161-2
Test rate: 1 mm/min
Gripping distance: 115 mm

[0177] High tensile strain of the resin composition allows the prepreg formed using the composition to achieve similarly high tensile train.

(3) Epoxy Resin Compatibility

[0178] Each of the obtained polyvinyl acetal resins (10 parts by weight) was added to 90 parts by weight of the epoxy resin used in Production of carbon-fiber-reinforced composite material [prepreg] and heated at 150 C. for dissolution, whereby an evaluation sample was produced.

[0179] The produced sample was left to cool to room temperature, and left to stand still at room temperature for 24 hours. In this process, the appearance of the sample was observed at the following three points in time: at 150 C., at 70 C. during cooling, and after standing still at room temperature for 24 hours. The presence or absence of phase separation was determined by whether the solution was clear or turbid, and evaluated in accordance with the following criteria.

oo (Excellent): The compatibility was good and the solution was clear at any of 150 C., 70 C., and room temperature.
o (Good): No phase separation was observed at 150 C. nor at 70 C., but the solution was turbid at room temperature.
(Fair): No separation was observed at 150 C., but the solution was turbid at 70 C. and room temperature.
x (Poor): No dissolution occurred at 150 C., and the solution was turbid.

(4) Adhesiveness (Interfacial Shear Strength Measurement)

[0180] The polyvinyl acetal-containing resin compositions obtained in the examples and the comparative examples were dropped onto carbon fibers and cured by heating at 150 C. for one hour, whereby measurement samples were produced. The carbon fiber/resin interfacial shear strength of the produced samples was measured using Evaluation Equipment For Interfacial Property Of Composite Material (available from Tohei Sangyo Co., Ltd., model HM410) by a microdroplet method (pull-out speed: 0.12 mm/min).

(5) Toughness

[0181] Five sheets of each of the obtained prepreg were stacked. A hole was drilled into the stack, and the opening was observed for the appearance and evaluated in accordance with the following criteria.

oo (Excellent): No delamination occurred at all.
o (Good): Separation occurred at only one sheet.
x (Poor): Separation occurred at two or more sheets.

(6) Tackiness

[0182] The obtained prepregs were evaluated for tackiness by tactile feel.

oo (Excellent): The prepreg had appropriate tackiness and excellent handleability.
o (Good): The prepreg had slightly excessive or slightly insufficient tackiness, but had no problem in the handleability.
x (Poor): The prepreg had significantly excessive or significantly insufficient tackiness and has a problem in the handleability.

TABLE-US-00001 TABLE 1 Acetalization step PVA Aldehyde Polyvinyl acetal resin Average Degree of addition Average Degree of degree of saponification Aldehyde amount R.sup.1 degree of acetoacetalization polymerization (mol %) type (g) type polymerization (mol %) Example 1 300 99 Acetaldehyde 90 CH.sub.3 300 74 Example 2 300 99 Acetaldehyde 90 CH.sub.3 300 74 Example 3 2000 99 Acetaldehyde 110 CH.sub.3 2000 74 Example 4 300 99 Acetaldehyde 30 CH.sub.3 300 20 Butyraldehyde 110 C.sub.3H.sub.7 Example 5 300 99 Acetaldehyde 30 CH.sub.3 300 20 Butyraldehyde 110 C.sub.3H.sub.7 Example 6 300 99 Acetaldehyde 38 CH.sub.3 300 25 Butyraldehyde 110 C.sub.3H.sub.7 Example 7 2300 99 Acetaldehyde 115 CH.sub.3 2300 74 Example 8 2300 89 Acetaldehyde 20 CH.sub.3 2300 10 Butyraldehyde 115 C.sub.3H.sub.7 Example 9 400 99 Acetaldehyde 80 CH.sub.3 400 59 Example 10 800 99 Acetaldehyde 93 CH.sub.3 800 69 Example 11 400 99 Acetaldehyde 90 CH.sub.3 400 72.3 Example 12 400 99 Acetaldehyde 90 CH.sub.3 400 72.3 Example 13 400 99 Acetaldehyde 90 CH.sub.3 400 71 Example 14 600 99 Acetaldehyde 110 CH.sub.3 600 73 Example 15 300 99 Acetaldehyde 90 CH.sub.3 300 72.3 Example 16 2500 99 Acetaldehyde 38 CH.sub.3 2500 25 Butyraldehyde 110 C.sub.3H.sub.7 Example 17 300 99 Acetaldehyde 90 CH.sub.3 300 74 Example 18 400 99 Acetaldehyde 90 CH.sub.3 400 72.3 Example 19 300 89 Acetaldehyde 82 CH.sub.3 300 64 Example 20 300 99 Acetaldehyde 90 CH.sub.3 300 79 Example 21 300 81 Acetaldehyde 75 CH.sub.3 300 55 Comparative 800 93 Formaldehyde 83 H 800 0 Example 1 Comparative 600 99 Benzaldehyde 200 C.sub.6H.sub.5 600 Example 2 Comparative 800 88 Benzaldehyde 93 C.sub.6H.sub.5 800 0 Example 3 Formaldehyde 50 H Comparative 300 99 Butyraldehyde 165 C.sub.3H.sub.7 300 Example 4 Polyvinyl acetal resin Alkyl Other Acid- acetal acetal Hydroxy Acetyl modified Degree of group group group group Acid group butyralization content content* content content modification content Tg (mol %) (mol %) (mol %) (mol %) (mol %) type (mol %) ( C.) Example 1 0 74 0 25 1 0 106 Example 2 0 74 0 25 1 0 106 Example 3 0 74 0 25 1 0 110 Example 4 47 67 0 32 1 0 75 Example 5 47 67 0 32 1 0 75 Example 6 47 72 0 27 1 0 90 Example 7 0 74 0 25 1 0 110 Example 8 52 62 0 27 11 0 76 Example 9 0 59 0 40 1 0 107 Example 10 0 69 0 30 1 0 110 Example 11 0 72.3 0 26 1 Carboxy group 0.7 109 Example 12 0 72.3 0 26 1 Carboxy group 0.7 109 Example 13 0 71 0 25 1 Carboxy group 2 109 Example 14 0 73 0 25 1 Carboxy group 1 100 Example 15 0 72.3 0 26 1 Sulfonic acid 0.7 80 group Example 16 47 72 0 26 1 Carboxy group 1 86 Example 17 0 74 0 25 1 0 106 Example 18 0 72.3 0 26 1 Carboxy group 0.7 109 Example 19 0 64 0 25 11 0 94 Example 20 0 79 0 20 1 0 105 Example 21 0 55 0 25 19 0 86 Comparative 0 0 86.5 6.5 7 0 118.0 Example 1 Comparative 0 0 59 40 1 0 120 Example 2 Comparative 0 47 25 27 1 0 117 Example 3 Comparative 70 70 0 29 1 0 68 Example 4 *The amount of acetal groups other than the structural units represented by the formula (1)

TABLE-US-00002 Evaluation (resin composition) Prepreg formulation (parts by weight) Adhesiveness Resin composition formulation Tensile (Interfacial Polyvinyl strain Epoxy resin shear Epoxy Curing acetal Carbon Evaluation (prepreg) Epoxy resin (%) compatibility strength: MPa) resin agent resin fibers Toughness Tackiness Example 1 Bisphenol A 6.72 62.3 100 6 10 300 Example 2 epoxy resin 6.72 62.3 100 6 2 300 Example 3 7.28 61.2 100 6 10 300 Example 4 4.67 69.3 100 6 5 300 Example 5 4.67 69.3 100 6 2 300 Example 6 5.21 70.8 100 6 20 300 Example 7 7.36 61.9 100 6 10 300 Example 8 6.10 63.5 100 6 5 300 Example 9 6.40 78.3 100 6 10 300 Example 10 6.70 74.4 100 6 10 300 Example 11 7.28 78.1 100 6 10 300 Example 12 7.28 78.1 100 6 2 300 Example 13 7.51 81.3 100 6 10 300 Example 14 7.90 76.2 100 6 10 300 Example 15 6.98 76.2 100 6 10 300 Example 16 6.10 71.1 100 6 10 300 Example 17 Bisphenol F 6.60 60.4 100 6 10 300 Example 18 epoxy resin 7.10 67.5 100 6 10 300 Example 19 Bisphenol A 7.05 61.5 100 6 10 300 Example 20 Epoxy resin 6.60 61.5 100 6 10 300 Example 21 7.05 60.8 100 6 10 300 Comparative Bisphenol A 6.61 60.4 100 6 5 300 x Example 1 epoxy resin Comparative 2.30 x 58.1 100 6 5 300 x x Example 2 Comparative 2.45 x 58.9 100 6 5 300 x Example 3 Comparative Bisphenol F 1.86 x 60.7 100 6 5 300 x x Example 4 Epoxy resin

INDUSTRIAL APPLICABILITY

[0183] The present invention can provide a carbon-fiber-reinforced composite material having excellent tackiness, excellent compatibility with epoxy resins, and excellent interfacial adhesion while being capable of achieving high mechanical strength.