TECHNICAL FIELD

The present invention relates to a resin composition that cures rapidly, has a low viscosity, and is excellent in long-term viscosity stability, the resin composition serving as a matrix resin material for a fiber-reinforced composite material that provides a molded product having high heat resistance and high toughness at the time of its curing while being easy to handle because of its low tackiness at the time of handling at room temperature.

BACKGROUND ART

A fiber-reinforced composite material has been widely applied as a structural material for an aircraft, an automobile, civil engineering and construction, sporting goods, and the like because the material includes reinforcing fibers, such as glass fibers, aramid fibers, or carbon fibers, and a thermosetting matrix resin, such as an unsaturated polyester resin, a vinyl ester resin, an epoxy resin, a phenol resin, a benzoxazine resin, a cyanate resin, or a bismaleimide resin, has a light weight, and is excellent in mechanical properties, such as a strength, corrosion resistance, and fatigue resistance.

The fiber-reinforced composite material is molded by an approach such as: an autoclave molding method including using prepregs each obtained by impregnating the reinforcing fibers with the thermosetting matrix resin in advance; or a filament winding molding method or a resin transfer molding method including a step of impregnating the reinforcing fibers with a matrix resin that is liquid and a molding step based on heat curing. In the autoclave molding method out of those methods, the prepregs each obtained by impregnating the reinforcing fibers with the resin in advance are laminated, and then a pressure is reduced to bring the respective layers into close contact with each other, followed by the pressurization and heating of the resultant in an autoclave. Thus, a high-quality molded product can be obtained. However, the method has a drawback in that productivity is low because the pressure-reducing step and the curing step in the autoclave are long. In view of the foregoing, a prepreg compression molding method (PCM method) of obtaining the fiber-reinforced composite material as described below has been developed as an approach to improving the productivity. The prepregs laminated in advance are heated and pressurized in a mold so that the prepregs may be molded and the resin may be cured.

In the PCM method, prepregs are each produced by preliminarily impregnating reinforcing fibers, such as carbon fibers, with a thermosetting resin that thermally cures in a short time period, and the prepregs are subjected to pattern cutting, lamination, and preform shaping in advance in accordance with the shape of a molded product. After that, the preform is molded with a high-output hydraulic pressing machine at high pressure and high temperature for a short time period. Thus, a desired fiber-reinforced composite material can be obtained.

With regard to various characteristics that the prepregs to be used in the PCM method are required to have, the prepregs are of course required to be excellent in general mechanical properties. At the same time, each of the prepregs has been required to cure in a short curing time at a curing temperature at the time of its molding while being excellent in storage stability in order that its handling may be facilitated. This is because the shortening of the curing time can improve productivity in limited production facilities.

The PCM method includes a step of laminating the prepregs at room temperature in an operation to be performed before the press molding. However, when the surfaces of the prepregs each have strong tackiness (surface stickiness) in the step, it becomes difficult to make a correction in registration at the time of the lamination, thereby deteriorating workability. Further, when the prepregs adhere to unintended portions to cause a wrinkle or a lamination failure, a defective product is obtained, and hence a yield reduces. Accordingly, a material having low tackiness has been required for improving the productivity.

In addition, in the PCM method, the prepregs are each impregnated with the resin, and a thermosetting resin is used as the resin. The thermosetting resin has been required to have the storage stability and the rapid curability described in the foregoing. Further, the viscosity of the resin at the time of its curing is required to be free from excessively reducing. This is because of the following reason. When the viscosity of the resin is reduced by a temperature at the time of the press molding, the smoothness of the surface of the molded product is improved. However, when the time period for which the viscosity reduces is long, the resin leaks from a mold to make it impossible to obtain a desired molded product. In addition, when the curing reaction proceeds before the reduction in resin viscosity is completed, the resin does not sufficiently flow into the surface, and hence a problem in that the smoothness is impaired occurs. Accordingly, the achievement of a balance between the resin viscosity and a curing rate is important for the shortening of the curing time.

Further, in the PCM method, the molded product is required to be free from deforming at the time of its release from the mold after the press molding. In the PCM method, a molding temperature of from 140° C. to 150° C. is used, and at this time, in the case where the glass transition temperature of the molded product is lower than the temperature of the mold, the resin softens, and is hence liable to cause the deformation of the molded product at the time of the mold release in which the molded product is released from the mold. Accordingly, the use of a thermosetting resin having a glass transition temperature higher than the molding temperature is important for suppressing the deformation of the molded product at the time of the mold release.

In the press molding of a composite material using a mold, a thermosetting resin, such as an unsaturated polyester resin, a vinyl ester resin, or an epoxy resin, has heretofore been used. Each of the unsaturated polyester resin and the vinyl ester resin each having radical polymerizability has a low viscosity and is excellent in rapid curability, but involves a problem in that the mechanical properties of a molded product, such as heat resistance, a strength, and toughness, are relatively low. Meanwhile, the epoxy resin provides a molded product having high heat resistance, a high strength, and high toughness, but involves a problem in that the viscosity of the resin is relatively high. It is important that the viscosity of the resin to be molded by the PCM method at the time of its impregnation and molding be reduced.

In Patent Literature 1, there is a description that there is provided a one-component thermosetting epoxy resin composition that can be cured at low temperature in a short time period, and is excellent in storage stability, the epoxy resin composition including: an epoxy resin containing a bisphenol A-type epoxy resin and a urethane-modified epoxy resin; a curing agent containing dicyandiamide; a first curing accelerator containing 3,4-dichlorophenyl-1,1-dimethylurea; and a second curing accelerator containing an imidazole compound containing a triazine ring. However, when a compound having a urea structure, such as 3,4-dichlorophenyl-1,1-dimethylurea, is used as a curing accelerator, the glass transition temperature of the cured product tends to be liable to reduce. Accordingly, the composition has a drawback in that when the composition is applied to the PCM method, the deformation of a molded product is liable to be caused at the time of its release from a mold.

In Patent Literature 2, there is a description that the addition of a tetraglycidyl amine allows a cured product of resin composition to express a high glass transition temperature. However, many glycidyl amines are each poor in storage stability, and hence each have the following drawback. A blended product thickens during its production, or the resin composition, and by extension, a prepreg impregnated with a resin thicken during their storage, and in the worst case, a curing reaction proceeds to make it impossible to obtain a molded product.

In Patent Literature 3, there is a disclosure of an epoxy resin composition for a carbon fiber-reinforced composite material using an epoxy resin and a phenoxy resin. An epoxy resin containing a polynuclear body is used as the epoxy resin used in the literature. The amount of a polynuclear body component in the composition increases, and hence an increase in glass transition temperature of the cured product along with an increase in crosslink density thereof can be expected. In contrast, the epoxy resin has a drawback in that its viscosity is liable to increase, and hence the fluidity of the resin at the time of its molding is liable to be insufficient.

In Patent Literature 4, there is a disclosure of a resin composition including, as an essential component, 2,4-di (N, N-dimethylureido) toluene serving as a curing accelerator. However, while the curing temperature of the composition is 150° C., its glass transition temperature is about 140° C., and hence the composition has a drawback in that when the composition is applied to the PCM method, the composition is liable to deform at the time of its release from a mold at a molding temperature of 150° C.

In Patent Literature 5, there is a disclosure of a resin composition including, as an essential component, a dicyclopentadiene-type epoxy resin for the purpose of adjusting its tackiness at the time of handling. However, a cured product of the composition has a glass transition temperature of from 110° C. to 130° C., and hence the composition also has a drawback in that when the composition is applied to the PCM method, the composition is liable to deform at the time of its release from a mold at a molding temperature of 150° C.

CITATION LIST

Patent Literature

[PTL 1] JP 2014-185256 A

[PTL 2] JP 2016-504472 A

[PTL 3] JP 2010-248479 A

[PTL 4] JP 2003-128764 A

[PTL 5] JP 2005-225982 A

SUMMARY OF INVENTION

The present invention provides a resin composition for a fiber-reinforced composite material that has low tackiness at room temperature, and in particular, can improve productivity in a PCM method while achieving both of rapid curability and storage stability.

The inventors of the present invention have made an investigation to solve the problems, and as a result, have completed the present invention by blending an epoxy resin, a polymer component having a molecular weight of 10,000 or more, a curing agent, and a curing aid in an epoxy-based resin composition according to a specific formulation.

That is, according to one embodiment of the present invention, there is provided a resin composition for a fiber-reinforced composite material, including, as essential components: a liquid epoxy resin (A) that is liquid at 23° C.; a novolac-type epoxy resin (B); a polymer component (C) that is solid and has a weight-average molecular weight (Mw) of 10,000 or more; dicyandiamide (D); and an imidazole-based curing aid (E), wherein the resin composition includes 10 parts by mass to 35 parts by mass of the liquid epoxy resin (A), and 10 parts by mass to 25 parts by mass of the polymer component (C) out of 100 parts by mass of a total of the components (A) to (E), and has a glass transition temperature of 0° C. or more before curing thereof.

The resin composition for a fiber-reinforced composite material according to the one embodiment of the present invention desirably satisfies one or more of the following conditions:

(1) the polymer component (C) is a phenoxy resin, or is a bisphenol A-type epoxy resin that is solid, or contains both of the resins;

(2) the novolac-type epoxy resin (B) is a phenol novolac-type epoxy resin represented by the following general formula (1), and contains, in measurement in gel permeation chromatography (GPC), a dinuclear body at a ratio of 15% by area or less and a trinuclear body at a ratio of from 15% by area to 60% by area:


where “m” represents an integer of 0 or more;
(3) the imidazole-based curing aid (E) has a triazine ring structure in a structure thereof; and
(4) the resin composition has a viscosity of from 1,000 mPa·s to 10,000 mPa·s at 90° C.

According to another embodiment of the present invention, there is also provided a fiber-reinforced composite material, which is obtained by blending the above-mentioned resin composition for a fiber-reinforced composite material with reinforcing fibers. In the fiber-reinforced composite material, a volume content of the reinforcing fibers is preferably from 25% to 75%.

According to yet another embodiment of the present invention, there is also provided a molded body, which is obtained by molding and curing the above-mentioned fiber-reinforced composite material by a prepreg compression molding method.

