JP7512064B2 - CURABLE RESIN COMPOSITION AND TOW PREPREG USING SAME - Google Patents

Description

The present invention relates to an epoxy resin composition that exhibits high fracture toughness, heat resistance, and low water absorption upon curing, and a tow prepreg using the same.

Fiber-reinforced composite materials are composed of reinforcing fibers such as glass fibers, aramid fibers, and carbon fibers, and thermosetting matrix resins such as unsaturated polyester resins, vinyl ester resins, epoxy resins, phenolic resins, benzoxazine resins, cyanate resins, and bismaleimide resins. They are lightweight and have excellent mechanical properties such as strength, corrosion resistance, and fatigue resistance, and are therefore widely used as structural materials for aircraft, automobiles, civil engineering and construction, sporting goods, and other applications.

Manufacturing methods for fiber-reinforced composite materials include autoclave molding, which uses prepregs in which the reinforcing fibers have already been impregnated with a thermosetting matrix resin, press molding, and other methods, such as wet layup molding, pultrusion molding, filament winding molding, and RTM, which include a process of impregnating the reinforcing fibers with a liquid matrix resin and a molding process by thermosetting.

One of the filament winding molding methods is the dry method, which uses tow prepregs in which the reinforcing fibers have already been impregnated with resin. The dry method has the advantage of being able to shorten the winding speed and having excellent stability in the resin ratio, making it highly productive and stable in quality for fiber-reinforced composite materials, and is particularly used as a manufacturing method for high-pressure gas tanks.

In the dry method, in order to improve the quality of the tow prepreg, a matrix resin with a preferred viscosity range and a small rate of increase in viscosity is used to ensure stable impregnation and easy handling during winding. In addition, it is desirable for the cured molded body to have high fracture toughness and high heat resistance to improve the impact resistance and fatigue resistance of the fiber-reinforced composite material, as well as a low water absorption rate for long-term reliability.

The use of aliphatic epoxy resins as a method for reducing the viscosity of matrix resins is described in Patent Documents 1 and 2. However, these methods result in a decrease in heat resistance. Therefore, in order to achieve both low viscosity and heat resistance, it is necessary to pay attention to components other than aliphatic epoxy resins.

There are various methods to improve the heat resistance of matrix resins, such as the use of multifunctional epoxy resins and curing agents that provide high crosslink density (Patent Documents 3 and 4). However, although these methods can increase the glass transition temperature (Tg), they also increase the resin viscosity and cause a decrease in toughness due to the high crosslink density.

Fiber-reinforced composite materials obtained by hardening the matrix resin gradually absorb water over long-term use, causing a decrease in strength and glass transition temperature. The initial glass transition temperature can be improved by increasing the crosslink density, but the crosslink sites created by ring-opening of the epoxy resin contain hydroxyl groups, which increase the water absorption rate and decrease the glass transition temperature. In order to reduce the water absorption rate, the crosslink density must not be increased too much.

In order to reduce the water absorption rate of molded products, it is important to reduce the amount of hydroxyl groups contained in the structure, and one effective method is to reduce the amount of hydroxyl groups generated by the reaction between the epoxy groups of the epoxy resin and the active hydrogen groups of the hardener, i.e., to reduce the crosslink density.

However, lowering the crosslink density leads to a decrease in heat resistance due to a drop in the glass transition temperature of the cured product, so a method is needed to increase heat resistance even when the crosslink density is low.

Regarding the matrix resin of fiber-reinforced composite materials, attempts have been made to improve the heat resistance of molded products by using multifunctional epoxy resins and hardeners, but a method that can also achieve low viscosity and low water absorption is desired.

Japanese Patent Application Laid-Open No. 11-302507 JP 2019-189750 A JP 2019-526650 A JP 2013-159618 A

The present invention aims to provide a resin composition that can be used as a matrix resin for tow prepregs to obtain fiber-reinforced composite materials with excellent long-term reliability because it has low viscosity, excellent impregnation into reinforcing fibers, and high heat resistance and low water absorption of the molded products obtained by curing.

As a result of investigations to solve the above-mentioned problems, the inventors of the present invention focused on the equivalent ratio of a specific aliphatic epoxy resin and a curing agent used, and discovered that it is possible to obtain a resin composition that has low viscosity and gives molded products high heat resistance and low water absorption, thus completing the present invention.

That is, the present invention is a curable resin composition comprising, as essential components, a bisphenol-type epoxy resin (A), an aliphatic epoxy resin (B) having, on average, two or more epoxy groups in the molecule, dicyandiamide or a derivative thereof (C), and a solid aromatic urea compound (D), characterized in that the ratio (H/E) of the number of moles of active hydrogen groups contained in component (C) to the number of moles of epoxy groups contained in the curable resin composition is 0.25 to 0.50, the amount of aliphatic epoxy resin (B) is 3 to 15 parts by mass per 100 parts by mass of the total of components (A), (B), (C), and (D), and the viscosity at 25°C measured with an E-type viscometer is in the range of 4 to 32 Pa·sPa·s.

The aliphatic epoxy resin (B) having an average of two or more epoxy groups in the molecule is preferably an epoxy resin having a structure derived from a polyhydric hydroxy compound selected from glycerol, trimethylolpropane, or pentaerythritol having an epoxy equivalent of 100 to 250 g/eq, and having a viscosity of 1000 mPa·s or less at 25°C as measured by an E-type viscometer.

In addition to the bisphenol-type epoxy resin (A), the aliphatic epoxy resin (B) having an average of two or more epoxy groups in the molecule, dicyandiamide or its derivative (C), and the solid aromatic urea compound (D), the composition contains core-shell rubber particles (E), and it is preferable that the amount of the core-shell rubber particles (E) is 1 to 6 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), (D), and (E).

The preferred form of the tow prepreg in the present invention is one in which the volume content of reinforcing fibers is 48 to 72%.

Another aspect of the present invention is a fiber-reinforced composite material obtained by molding a tow prepreg in which reinforcing fibers are blended with the above-mentioned resin composition using a filament winding molding method.

The curable resin composition of the present invention has low viscosity and excellent impregnation into reinforcing fibers. The molded product obtained by curing a tow prepreg using the composition exhibits high fracture toughness, heat resistance, and low water absorption, and is particularly suitable for use in fiber-reinforced composite materials obtained by the filament winding molding method.

Hereinafter, an embodiment of the present invention will be described in detail.
The curable resin composition of the present invention contains as essential components a bisphenol type epoxy resin (A), an aliphatic epoxy resin (B) having two or more epoxy groups on average in the molecule, dicyandiamide or a derivative thereof (C), and a solid aromatic urea compound (D). Hereinafter, the bisphenol type epoxy resin (A), the aliphatic epoxy resin (B), the dicyandiamide or a derivative thereof (C), the solid aromatic urea compound, or the solid imidazole compound (D) will also be referred to as the (A) component, the (B) component, the (C) component, and the (D) component, respectively.

In the present invention, the ratio (H/E) of the number of moles of active hydrogen groups contained in component (C) to the number of moles of epoxy groups contained in the curable resin composition is 0.25 to 0.50. If H/E is less than 0.25, the crosslinks will be sparse, resulting in a cured product with a low Tg and brittleness. If H/E exceeds 0.50, the crosslinks will be dense, resulting in a large amount of hydroxyl groups, a high water absorption rate, and poor long-term reliability.

In the present invention, the amount of aliphatic epoxy resin (B) contained in the curable resin composition is 3 to 15 parts by mass per 100 parts by mass of the total of components (A), (B), (C), and (D). If the amount of component (B) is less than 3 parts by mass, no improvement in heat resistance is observed, and if the amount of component (B) exceeds 15 parts by mass, the toughness of the cured product decreases.

The curable resin composition of the present invention has a viscosity at 25°C measured with an E-type viscometer in the range of 4 to 40 Pa·s, preferably in the range of 5 to 25 Pa·s. Within this range, the composition has good impregnation into the reinforcing fibers, and the appearance of the fiber-reinforced composite material obtained after molding is good.

In the present invention, the aliphatic epoxy resin (B) contained in the curable resin composition and having an average of two or more epoxy groups in the molecule is preferably an epoxy resin having an epoxy equivalent of 100 to 250 g/eq, a structure derived from a polyhydric hydroxy compound selected from glycerol, trimethylolpropane, or pentaerythritol, and a viscosity of 1000 mPa·s or less at 25°C as measured by an E-type viscometer, since this reduces the viscosity of the curable resin composition while increasing heat resistance.

The curable resin composition of the present invention may contain an epoxy resin other than the bisphenol-type epoxy resin (A) or the aliphatic epoxy resin (B) having, on average, two or more epoxy groups in the molecule, so long as the amount is less than 20 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), and (D).

In addition to components (A) and (B), other epoxy resins that can be used include, for example, glycidyl ethers of alkylene oxide adducts of bisphenols having two or more epoxy groups in one molecule, novolac epoxy resins such as phenol novolac epoxy resins, cresol novolac epoxy resins, and bisphenol A novolac epoxy resins, alicyclic epoxy resins such as 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, and 1-epoxyethyl-3,4-epoxycyclohexane, glycidyl esters such as phthalic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, and dimer acid glycidyl ester, and glycidyl amines such as tetraglycidyldiaminodiphenylmethane, tetraglycidyldiaminodiphenylsulfone, triglycidylaminophenol, triglycidylaminocresol, and tetraglycidylxylylenediamine. These epoxy resins may be used alone or in combination of two or more.

The resin composition of the present invention may contain bisphenol-type epoxy resin (A), an aliphatic epoxy resin having two or more epoxy groups in an average molecule, dicyandiamide or a derivative thereof (C), a solid aromatic urea compound (D), and core-shell rubber particles (E), and the amount of the core-shell rubber particles (E) is preferably 1 to 6 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), (D), and (E). Within this range, a fiber-reinforced composite material with excellent strength and enhanced fracture toughness can be obtained without reducing the elastic modulus of the cured product.

The core-shell type rubber particles (E) are composed of a core portion and a shell portion that forms the outer layer of the core portion. The core portion is preferably made of a polymer whose main component is an elastomer or a rubber-like polymer, and the shell portion is preferably made of a polymer graft-polymerized to the core portion. The addition of the core-shell type rubber particles has the effect of improving toughness and tackiness of the prepreg, and the average particle diameter is preferably 1 to 500 nm, and more preferably 30 to 300 nm, in terms of volume average particle diameter.

In the resin composition of the present invention, dicyandiamide or a derivative thereof (C) is used as a curing agent. Dicyandiamide is a curing agent that is solid at room temperature and is hardly soluble in epoxy resins at room temperature, but when heated to 180°C or higher, it dissolves and reacts with epoxy groups, making it a latent curing agent with excellent storage stability at room temperature. In addition, as a derivative thereof, an N-substituted dicyandiamide derivative such as N-hexyldicyandiamide described in JP-A-11-119429 can be used.

The amount of cyandiamide or its derivative (C) contained in the curable resin composition of the present invention is such that the ratio (H/E) of the number of moles [H] of active hydrogen groups contained in component (C) to the number of moles [E] of epoxy groups in the entire epoxy resin including components (A) and (B) is 0.25 to 0.50, more preferably 0.30 to 0.40 equivalents. If H/E is less than 0.25, the crosslinking becomes sparse and the Tg becomes low. If H/E exceeds 0.50, the crosslinking becomes dense and the cured product becomes brittle. From another perspective, the amount of component (C) used is preferably in the range of 2.0 to 6.0 parts by weight per 100 parts by weight of the curable resin composition.

As the solid aromatic urea compound (D), those which act as a curing accelerator and which not only have satisfactory impregnation properties into the reinforcing fibers during mixing but also have satisfactory heat resistance during curing are preferred.
Examples of solid aromatic urea compounds include 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 1,1'-(4-methyl-1,3-phenylene)bis(3,3-dimethylurea), N-phenyl-N',N'-dimethylurea, N-(4-chlorophenyl)-N',N'-dimethylurea, N-(3,4-dichlorophenyl)-N',N'-dimethylurea, N-(3-chloro-4-methylphenyl)-N',N'-dimethylurea, N-(3-chloro-4-ethylphenyl)-N', Examples of the compound include N'-dimethylurea, N-(3-chloro-4-methoxyphenyl)-N',N'-dimethylurea, N-(4-methyl-3-nitrophenyl)-N',N'-dimethylurea, 2,4-bis(N',N'-dimethylureido)toluene, and methylene-bis(p-N',N'-dimethylureidophenyl), and among these, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 1,1'-(4-methyl-1,3-phenylene)bis(3,3-dimethylurea) are preferred. These compounds may be used alone or in combination of two or more, and are not limited to the above, so long as they are chemically stable and do not dissolve in epoxy resins at room temperature.
The amount of the solid aromatic urea compound (D) used is preferably 0.01 to 7 parts by weight relative to 100 parts by weight of the curable resin composition. More preferably, it is 1 to 5 parts by weight. If it exceeds 7 parts by weight, the powder component increases, which causes a problem that voids tend to increase. If it is less than 0.01 part by weight, it causes a problem that fast curing cannot be achieved.

Additives such as defoamers and leveling agents can be added to the curable resin composition of the present invention to improve surface smoothness. These additives can be blended in an amount of 0.01 to 3 parts by mass, preferably 0.01 to 1 part by mass, per 100 parts by mass of the resin composition.

The curable resin composition of the present invention is produced by uniformly mixing the above-mentioned components (A), (B), (C), and (D), and other components as necessary. The resulting resin composition has a viscosity in the range of 4 to 32 Pa·s, as measured at 25°C using an E-type cone-plate type viscometer. Within this range, the impregnation of the reinforcing fibers is good, and the appearance of the fiber-reinforced composite material obtained after molding is good, and a fiber-reinforced composite material with few voids is obtained.

In addition, the curable resin composition of the present invention can further contain other curable resins. Examples of such curable resins include, but are not limited to, unsaturated polyester resins, curable acrylic resins, curable amino resins, curable melamine resins, curable urea resins, curable cyanate ester resins, curable urethane resins, curable oxetane resins, and curable epoxy/oxetane composite resins.

The curable resin composition of the present invention can be blended with coupling agents, conductive particles such as carbon particles and metal-plated organic particles, thermosetting resin particles, inorganic fillers such as silica gel, nanosilica, alumina fiber, and clay, and conductive fillers. The use of conductive particles and conductive fillers can improve the conductivity of the resulting cured resin material and fiber-reinforced composite material.

Conductive fillers include carbon black, carbon nanotubes, fullerenes, and metal nanoparticles, and may be used alone or in combination. Among these, the incorporation of carbon nanotubes in particular is widely known for not only improving electrical conductivity but also increasing the impact strength of fiber-reinforced composite materials even when incorporated in amounts of less than 1 wt % of the fiber-reinforced composite material, and is therefore preferably used.

The curable resin composition of the present invention is impregnated into reinforcing fibers or bundles to form a tow prepreg. The method for forming the tow prepreg may be a known method. The tow prepreg thus obtained is suitable for use in fiber-reinforced composite materials obtained by a filament winding molding method.

The method of processing the curable resin composition of the present invention into a tow prepreg and producing a fiber-reinforced composite material is not particularly limited, but is preferably used as a method for producing a pressure vessel by the filament winding method. A molded product in which a layer of fiber-reinforced composite material is formed so as to cover the liner is obtained by winding the tow prepreg around a metal or resin liner and then heat curing. The liner may then be removed as necessary. The present invention is also preferably used as a method for producing a hollow cylindrical fiber-reinforced composite material, for example, a shaft or roll-shaped molded product, by the filament winding method. A molded product is obtained by winding the tow prepreg around a metal or resin mandrel and heat-molding it, and the mandrel may be removed depending on the application.

The reinforcing fibers used in the tow prepreg of the present invention are selected from glass fibers, aramid fibers, carbon fibers, boron fibers, etc., but it is preferable to use carbon fibers in order to obtain a fiber-reinforced composite material with excellent strength.

In a tow prepreg made from the curable resin composition of the present invention and reinforcing fibers, the volume content of the reinforcing fibers is preferably 48-72%, and more preferably in the range of 55-68%, which results in a molded product with few voids and a high volume content of the reinforcing fibers, resulting in a molding material with excellent strength.

In the present invention, it is more preferable that the cured product obtained by curing the curable resin composition for tow prepregs at a temperature of 160°C for 1 hour has a flexural modulus of 2.0 GPa or more as measured in accordance with JIS K7171 and a glass transition temperature (Tg) of 120°C or more as measured in accordance with JIS K7121.

Next, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. Parts indicating the blend amount are parts by mass unless otherwise specified. The unit of epoxy equivalent is g/eq.

The abbreviations for the components used in the Synthesis Examples and Examples are as follows.
(A) Component YD-128: Bisphenol A type epoxy resin, epoxy equivalent 187 (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
YDF-170: Bisphenol F type epoxy resin, epoxy equivalent 170 (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
(B) Component YH-300: glycidyl ether of trimethylolpropane, epoxy equivalent 144, viscosity 149 mPa·s, average number of epoxy groups: 2.2 (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
EX-411: glycidyl ether of pentaerythritol, epoxy equivalent 227, viscosity 819 mPa·s, average number of epoxy groups: 2.9 (manufactured by Nagase ChemteX Corporation)
(B') Component PG-207: glycidyl ether of polypropylene glycol, epoxy equivalent 298, viscosity 44 mPa·s (manufactured by Nippon Steel Chemical & Material Co., Ltd.), average number of epoxy groups: 1.7
EX-211: glycidyl ether of neopentyl glycol, epoxy equivalent 140, viscosity 22 mPa·s (manufactured by Nagase ChemteX Corporation), average number of epoxy groups: 1.8
(C) Component DICY: dicyandiamide, active hydrogen group equivalent 21 g/eq
(D) Component DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea TDU: 1,1'-(4-methyl-1,3-phenylene)bis(3,3-dimethylurea)
Other components MX-154: Bisphenol A type epoxy resin containing 40 wt% core-shell type rubber particles (manufactured by Kaneka Corporation), epoxy equivalent 297
(A) Component CSR-MX: Core-shell type rubber particle component in MX-154 (E) Component EP-MX: Bisphenol A type epoxy resin component in MX-154, epoxy equivalent 187

Example 1
85 parts of YD-128 as the component (A), 9 parts of YH-300 as the component (B), 3.2 parts of DICY as the component (C), and 2.9 parts of TDU as the component (D) were placed in a 150 mL plastic container and mixed with stirring at room temperature for 5 minutes using a vacuum mixer “Awatori Rentaro” (manufactured by Thinky Corporation) to obtain a curable resin composition.

(Viscosity Measurement)
The viscosity value at 25° C. was measured using an E-type cone-plate type viscometer. A curable resin composition was prepared, and 0.8 mL of the composition was used for the measurement. The value 60 seconds after the start of the measurement was taken as the viscosity value.

(Preparation of molded plates for measuring glass transition temperature, fracture toughness, and water absorption rate)
The curable resin composition was poured into a mold having a length of 60 mm and a width of 240 mm and having a 4 mm-thick spacer cut into a flat plate shape, and cured at 160°C for 2 hours to obtain a molded plate for measurement. This was used to measure the glass transition temperature, mechanical properties, and water absorption rate, which will be described later.

(Measurement of Glass Transition Temperature)
The obtained molded plate was cut into a size of 3 mm x 3 mm using a benchtop band saw, and further polished to a thickness of approximately 1.0 mm using a belt disk sander. Measurement was performed using a differential scanning calorimeter under a nitrogen atmosphere at a temperature increase rate of 10°C/min, and the glass transition temperature Tg was determined as the intersection of the tangent at the inflection point of the DSC curve and the temperature at which the inflection begins, i.e., the tangent in the temperature range 20 to 30°C lower than the inflection point.

(Measurement of flexural modulus and flexural strength)
The obtained molded plate was cut into a size of 80 mm x 10 mm using a bench band saw, and a bending test was performed on the bending test pieces at a temperature condition of 23°C using a method in accordance with JIS 7171, and the bending modulus and bending strength were calculated.

(Measurement of fracture toughness)
The obtained molded plate was cut into a size of 80 mm x 10 mm using a bench band saw.
After processing into test pieces conforming to ASTM 5045, a fracture toughness test was carried out at a temperature of 23° C., and the fracture toughness value was calculated.

(Measurement of water absorption rate)
The obtained molded plate was cut into a size of 40 mm x 10 mm using a benchtop band saw, and the weight before the water absorption test was measured and used as the initial weight. Next, the test piece was allowed to absorb water by standing for 240 hours in a thermohygrostat SH-641 (manufactured by Espec Corporation) under conditions of 85 ° C.-85% RH. Thereafter, the test piece was taken out and left to stand for 88 hours or more under conditions of 23 ° C.-50% RH, after which the weight was measured and used as the weight after water absorption. The water absorption rate was calculated using the following formula.
Water absorption rate (%)=(weight after water absorption−initial weight)/initial weight×100

Examples 2 to 10, Comparative Examples 1 to 7
Curable resin compositions were prepared in the same manner as in Example 1, except that the raw materials (A) to (E) were used in the compositions shown in Tables 1 and 2.
Using this curable resin composition, the glass transition temperature, mechanical properties, and water absorption were measured in the same manner as in Example 1.

The test results are shown in Tables 1 and 2, respectively.

Claims (4)

A curable resin composition comprising, as essential components, a bisphenol-type epoxy resin (A), an aliphatic epoxy resin (B) having, on average, two or more epoxy groups in the molecule, dicyandiamide or a derivative thereof (C) as a curing agent, and a solid aromatic urea compound (D) , wherein the aliphatic epoxy resin (B) has a structure derived from a polyhydric hydroxy compound selected from glycerol, trimethylolpropane, and pentaerythritol having an epoxy equivalent of 100 to 250 g/eq, and has a viscosity of 25° C. as measured by an E-type viscometer. the ratio (H /E) of the number of moles of active hydrogen groups contained in the entire curing agent of component (C) to the number of moles of epoxy groups contained in the curable resin composition is 0.25 to 0.50; the amount of aliphatic epoxy resin (B) is 3 to 15 parts by mass per 100 parts by mass of the total of components (A), (B), (C), and (D); and the viscosity at 25°C measured with an E-type viscometer is in the range of 4 to 32 Pa s. The curable resin composition according to claim 1, further comprising: a bisphenol-type epoxy resin (A), an aliphatic epoxy resin (B) having, on average, two or more epoxy groups in the molecule, dicyandiamide or a derivative thereof (C), a solid aromatic urea compound (D), and in addition, core-shell rubber particles (E), wherein the blending amount of the core-shell rubber particles (E) is 1 to 6 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), (D), and (E). A tow prepreg comprising reinforcing fibers blended into the curable resin composition according to claim 1 in an amount of 48 to 72% by volume. A molded article obtained by molding the tow prepreg according to claim 3 by a filament winding molding method.