DE69326059T2 - PREPREGS, METHOD FOR PRODUCING AND COMPOSITE COATING - Google Patents

Description

The present invention relates to prepregs used to produce fiber-reinforced plastics that have excellent properties in terms of strength, elastic modulus, impact resistance and interlayer strength.

Fiber-reinforced plastics as a type of composite material are anisotropic materials with reinforcing fibers and a matrix resin as essential components; their physical properties in the fiber direction are significantly different from those in the other directions. In general, the strength and elastic modulus are very high in the fiber direction, but low in the other directions. In a generally used method for producing fiber-reinforced plastic material, layers of a film-like precursor called a prepreg (formed by impregnating reinforcing fibers with an uncured thermosetting resin) are laminated, molded and cured to obtain the intended product. Unless otherwise stated, the term "composite material" herein refers to a fiber-reinforced plastic material obtained by laminating layers of a prepreg and by molding and curing the laminate. When a composite material is obtained from a prepreg, a fabric obtained by weaving reinforcing fibers is used in the prepreg, or layers of a prepreg made of unidirectionally arranged reinforcing fibers are laminated with transverse reinforcing fiber directions to make the physical properties in the final product nearly isotropic.

However, it is known that when these methods are used, the impact strength and other properties of the composite material are not significantly improved by increasing the strength of the reinforcing fibers, since they are affected by interlayer fracture. The interlayer region of the composite material is the region near the interface between laminated layers of a prepreg. This region has a low content of reinforcing fibers, and since these are oriented differently on both sides of the interface, fracture often occurs in this region. In particular, a composite material using a thermosetting resin as the matrix resin has insufficient impact resistance because the matrix resin has low strength. When a tensile load is applied to a cross laminate, interlayer separation often occurs at one end of the laminate, so that the latitude in laminate composition is often limited. Various methods have been proposed to improve physical properties (particularly impact resistance and interlayer strength) in directions other than the fiber direction, and many of these methods use a material other than the matrix resin as the interface region to absorb fracture energy.

US-A-4,604,319 discloses placing a thermoplastic resin between fiber-reinforced prepreg layers to obtain higher impact strength. In this case, however, the tackiness and stretchability, advantages of a thermosetting resin, are disadvantageously sacrificed. In US-A-5,028,478, the applicants disclosed a matrix resin containing resin fine particles. In particular, it was disclosed that the localized occurrence of resin fine particles on the surfaces of layers of a prepreg improved the impact strength of the composite material while maintaining the tackiness and stretchability of the prepreg. However, this method also involves the problem that it is not easy to obtain resin fine particles. In addition, fine particles often penetrate into the reinforcing fibers, and the penetration of fine particles into the fibers impairs the physical properties of the composite material. Any attempt to avoid this complicates the prepreg manufacturing process. JP-A-90-32843 discloses a method of improving the interlayer strength of the composite material by bonding fabric to the surfaces of fiber-reinforced prepregs. In general, it is easier to impregnate a resin into fibers than into particles and this method is more advantageous in this respect, but the advantage is lost by the need to weave the fibers. In addition, when manufacturing a fabric, there are limits to the minimum basis weight that can be achieved and it is not possible to obtain an interlayer material with a suitable basis weight.

A paper in "Composite Materials: Testing and Design (Seventh Conference), ASTM STP 893", p. 256, states that interlayer strength can be improved by placing a Kevlar or polyester mat between the layers. However, the manufacture of the mat requires the cutting and matting steps after the production of fibers and cannot take full advantage of the use of fibers. There are also limitations in reducing the basis weight.

P-A-90-32843, 92-292635, 92-292636, 92-292909, 92-325527, 92-325528, 92-93529 and 93-17603 disclose that the interlayer strength of the composite material can be improved by arranging thermoplastic resin fibers in a certain direction on the surfaces of fiber-reinforced prepreg layers. However, the attempt to reduce the basis weight of the thermoplastic resin fibers brings with it the disadvantage that the basis weight and hence the performance in the width direction of the prepreg becomes non-uniform. Furthermore, when using unidirectionally arranged reinforcing fibers, arranging the thermoplastic resin fibers parallel to the reinforcing fibers allows the thermoplastic resin fibers to penetrate into the reinforcing fibers, which impairs the physical properties of the composite material.

These methods have disadvantages in that the improvement in impact strength is not sufficient and the impact strength is improved at the expense of other properties such as interlayer shear resistance and ease of handling. Compared with conventional prepregs, all of these methods have the problem that the manufacturing process becomes complicated.

Another relevant prior art document is JP-A-2063821, which discloses prepregs for use in the manufacture of a laminated board, e.g. for a printed circuit board. A first prepreg is prepared by impregnation A first prepreg is prepared by impregnating a para-amide fiber nonwoven fabric with bisphenol type epoxy resin and drying, while a second prepreg, which is laminated on both sides of the first prepreg to form outer surface layers, is prepared by impregnating a glass fiber nonwoven fabric with the same epoxy resin and drying.

The aim of the invention is to provide prepregs which can be used to produce composite materials which have excellent properties in terms of strength, elastic modulus, impact resistance and interlayer strength and which can be easily and simply produced; furthermore, the invention also aims to provide methods for producing the prepregs.

To achieve the above object, the invention provides in a first aspect a prepreg as defined in claim 1.

To achieve the above object, the invention also provides in a second aspect a process for producing a prepreg as defined in any one of claims 5, 6 or 7.

The invention is described below based on preferred features and embodiments.

Component [A] of the invention comprises long reinforcing fibers and can be selected from various fibers suitable for the particular application of the composite material produced, for example carbon, graphite, aramid, silicon carbide, aluminum oxide, boron, tungsten carbide and glass fibers. It is also possible to use several of these reinforcing fibers in combination.

Of these fibers, carbon and graphite fibers are well suited for the present invention because they have satisfactory specific strength and elastic modulus and contribute greatly to weight reduction. Any kind of carbon and graphite fiber is suitable for the respective applications. High-strength carbon fibers having a tensile elongation of 1.5% or more are suitable for providing high-strength composite materials. High-strength, highly elongated carbon fibers having a tensile strength of 450 kgf/mm² or more and a tensile elongation of 1.7% or more are more preferable; high-strength, highly elongated carbon fibers having a tensile elongation of 1.9% are most suitable. The reinforcing fibers used herein must be long, having a length of 5 cm or more. If the fibers are shorter than 5 cm, the strength of the reinforcing fibers cannot be sufficiently exhibited in the composite material. The carbon and graphite fibers may also be used in a mixture with other reinforcing fibers. The reinforcing fibers are not limited in their shape or arrangement, and those arranged in one direction and in random directions, in sheet, mat, fabric, or braided are suitable. For applications requiring high specific strength and high specific modulus of elasticity, reinforcing fibers aligned parallel in one direction are best, but those arranged in a woven manner and easy to handle are best.

The thermosetting resin composition used in component [B] of the invention consists mainly of a resin which is cured by external energy such as heat, light or electron beam to form an at least partially three-dimensionally crosslinked material. Preferably, a thermosetting thermosetting resin is used. The thermosetting resin particularly suitable herein is an epoxy resin, and is generally used in combination with a hardener and a curing catalyst. In particular, an epoxy resin made of an amine, a phenol or a compound having a carbon-carbon double bond is preferable. The epoxy resin having an amine as a precursor may, for example, be selected from various isomers of tetraglycidyldiaminodiphenylmethane, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol and triglycidylaminocresol. The epoxy resin having a phenol as a precursor may, for example, B. be selected from bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, phenol novolac epoxy resin and cresol novolac epoxy resin. The epoxy Resin having a compound having a carbon-carbon double bond as a precursor can be selected from, for example, alicyclic epoxy resins. Resins obtained by brominating these epoxy resins can also be used. An epoxy resin having an aromatic amine, such as tetraglycidyldiaminodiphenylmethane, has good heat resistance and good adhesion to the reinforcing fibers, which is why it is very suitable for the invention. An epoxy resin can preferably be used in combination with an epoxy hardener. The epoxy hardener used can be any compound having an active group that can react with an epoxy group. A compound having an amino group, acid anhydride group or azido group is preferred. It can be selected from, for example, dicyandiamide, various isomers of diaminodiphenylsulfone, various derivatives of diaminodiphenylmethane and aminobenzoates. Specifically, dicyandiamide is preferably used because it enables long storage life of the prepreg. Various isomers of diaminodiphenyl sulfone are particularly useful in the invention because they impart good heat resistance to the cured material; the alkyl derivatives of diaminodiphenylmethane, particularly 3,3,5,5'-tetraalkyl derivatives, are useful herein because they impart high elongation and low water absorption capacity to the cured material. Preferred aminobenzoates include trimethylglycol di-p-aminobenzoate and neopentylglycol di-p-aminobenzoate, and since they are superior to diaminodiphenyl sulfone in terms of tensile elongation but inferior in heat resistance, they are selectively used for certain applications.

The thermosetting resin used in the component [B] may preferably be selected from maleimide resins, acetylene-terminated resins, nadic acid-terminated resins, cyanate-terminated resins, vinyl-terminated resins and allyl-terminated resins. Any of these resins may also be mixed with an epoxy resin or other resin. A reactive thinner may be used and a modifier such as a thermoplastic resin or elastomer may be mixed in such a way that the heat resistance is not greatly reduced. According to the invention, the component [B] of the prepreg comprises a thermoplastic resin-mixed thermosetting resin composition. A maleimide resin is a compound having an average of two or more maleimide groups per molecule. A bismaleimide of diaminodiphenylmethane is particularly preferred. The maleimide compound can be, for example, B. selected from N,N'-phenylenebismaleimide, N,N'-hexamethylenebismaleimide, N,N'-methylene-di-p-phenylenebismaleimide, N,N'-oxydi-p-phenylenebismaleimide, N,N'-4,4'-benzophenonebismaleimide, N,N'-diphenylsulfonebismaleimide, N,N'-(3,3'-dimethyl)methylenedi-p-phenylenebismaleimide, N,N'-4,4'-dicyclohexylmethanebismaleimide, N,N'-m-(or p-)xylylenebismaleimide, N,N'-(3,3'-diethyl)methylenedi-p-phenylenebismaleimide, N,N'-m-tolylenedimaleimide and bismaleimide of bis(aminophenoxy)benzene and also from the reaction product of maleic anhydride and a polyamine produced by reaction between aniline and formalin. In addition, two or more of these maleimide compounds may also be used as a mixture, and the maleimide compound used may also contain a monomaleimide compound such as N-allylmaleimide, N-propylmaleimide, N-hexylmaleimide or N-phenylmaleimide.

The maleimide resin is preferably used in combination with a hardener. The hardener can be any compound with an active group that can react with a maleimide group. Preferably, a compound with an amino group or an alkenyl group such as allyl, benzocyclobutene, allylnadinimide, isocyanate, cyanate or epoxy group is used. A typical hardener with an amino acid is diaminodiphenylmethane. Typical hardeners with an alkenyl group are O,O'-diallylbisphenol A and bis(propenylphenoxy)sulfone.

A bismaleimide triazine resin (BT resin) consisting of any of the above bismaleimides and a cyanate is also suitable as the thermosetting resin as component [B] of the invention. A preferred resin having cyanate end groups is a cyanate compound of a polyhydric phenol such as bisphenol A. A mixed resin consisting of a cyanate resin and a bismaleimide resin is sold as BT resin by Mitsubishi Gas Chemical Co., Inc. and is suitable for the present invention. These resins generally have better heat and water resistance. ity than epoxy resins, but also lower strength and impact resistance, and are therefore selectively used for certain purposes. The weight ratio between bismaleimide and cyanate is in the range of 0:100 to 70:30. A resin of 0:100 corresponds to a triazine resin, which is also suitable for the invention.

In addition, a thermosetting resin having reactive groups at the ends is suitable as component [B] of the invention. The reactive groups at the ends are preferably nadinimide, acetylene or benzocyclobutene groups.

The thermosetting resin composition of component [B] of the invention may also be selected from thermosetting resins widely used in industry, such as phenol resin, resorcinol resin, unsaturated polyester resin, diallyl phthalate resin, urea resin and melamine resin.

With the thermosetting resin composition of component [B] of the invention, a thermoplastic resin such as polyethersulfone, polysulfone, polyimide or polyetherimide is mixed. Sheets of a prepreg of the invention are laminated and cured by means of heat or light to obtain a composite material. In the composite material, the resin component produced by curing component [B] is referred to as component [D]. Component [B] preferably used for the prepreg of the invention is a resin composition capable of phase separation while heated from 10°C to 250°C. In this case, component [D] of the composite material has a phase-resolved structure. Whether a resin composition can phase separation can be easily judged by observing the just-heated composition using a microscope.

It is preferable that the structurally phase-separated component [D] separates structurally into a microphase consisting primarily of a thermosetting resin and a microphase consisting primarily of a thermoplastic resin.

It is more preferable that the preferred resin composition is a cured resin material which structurally separates into a microphase consisting mainly of a thermosetting resin and a microphase consisting mainly of a thermoplastic resin. The component [D] is obtained by curing the component [B]. It is further preferable that, when the component [B] is cured, the microphase-separated structure has the following feature. In the structure, the structure consisting mainly of a thermoplastic resin exists separately from the structure consisting mainly of a thermosetting resin, and at least the phase consisting mainly of a thermoplastic resin has a three-dimensional continuous network. This structure is obtained when a homogeneously compatible structure consisting of a not yet cured thermosetting resin and a thermosetting resin is once molded and separates into phases while being cured. It is even more preferable that the component [D] is a cured resin material structurally separated into two microphases each of a three-dimensionally continuous network. In some cases, it is also preferable that another dispersed phase is contained in the continuous phase. In particular, the presence of a dispersed rubber preferably greatly contributes to the improvement of strength.

More preferably, the continuous phase composed mainly of a thermoplastic resin is structurally arranged at intervals of 0.01 to 20 µm. If the intervals are smaller than 0.01 µm, the fracture surface is of small roughness depth, and since the fracture paths are short, it is difficult to have high strength. If the intervals exceed 20 µm, the fracture paths are simplified, so that the effect of improving the strength is reduced. More preferably, the intervals are 0.1 to 10 µm.

The phase-separated structure of the cured resin can be viewed in the conventional way under the microscope. It can be viewed through an optical microscope, but it is preferable to stain the material with e.g. osmium tetroxide to make it visible in the Electron microscope. Their presence can be determined when it is clear that different phases exist, that at least one of the phases forms a continuous structure, and that a dispersed phase is present in the continuous phase. When an analyzer such as an X-ray microanalyzer is used with a microscope, the components can be identified.

The ultimate load energy release rate GIC of the cured resin of the invention can be calculated by GIC = KIC²(1 - g²)/E, where KIC is the stress intensity factor according to ASTM E 399-83, g is Poisson's ratio, and E is the flexural elastic modulus according to ASTM D 790.

The thermoplastic resin in components [B] and [D] is thermoplastic resin which is widely used in industry but does not impair the high heat resistance and high elastic modulus typical of the thermosetting resin. Thermoplastic resin belonging to engineering aromatic plastics is preferred. It can typically be selected from the highly heat-resistant thermoplastic resins having an aromatic polyimide skeleton, aromatic polyamide skeleton, aromatic polyether skeleton, aromatic polysulfone skeleton or aromatic polyketone skeleton soluble in thermosetting resins. Particularly, those having an aromatic polyimide skeleton are preferred because they have excellent heat resistance, solvent resistance and strength, for example, polyethersulfone, polysulfone, polyimide, polyetherimide or polyimide having a phenyltrimethylindane structure. An aromatic polyimide suitable as a thermoplastic resin can be synthesized by any method known in the industry. Typically, a tetracarboxylic dianhydride and a diamino compound can be reacted together to synthesize the desired product. The tetracarboxylic dianhydride is preferably selected from aromatic tetracarboxylic dianhydrides such as pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ethertetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, before preferred aromatic tetracarboxylic dianhydrides such as 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride and 3,3,4,4'-diphenyl sulfone tetracarboxylic dianhydride. The diamino compound is preferably selected from aromatic diamino compounds such as diaminodiphenylmethane, m-phenylenediamine, p-phenylenediamine, diaminodiphenyl ether, diaminodiphenyl sulfone, diaminodiphenyl sulfide, diaminodiphenylethane, diaminodiphenylpropane, diaminodiphenyl ketone, diaminodiphenyl hexafluoropropane, bis(aminophenoxy)benzene, bis(aminophenoxy)diphenylsulfone, bis(aminophenoxy)diphenylpropane, bis(aminophenoxy)diphenylhexafluoropropane, diaminodiphenylfluorene, fluorenediamine, dimethyl substitution derivative of diaminodiphenylmethane, tetramethyl substitution derivative of diaminodiphenylmethane, diethyl substitution derivative of diaminodiphenylmethane, tetraethyl substitution derivative of diaminodiphenylmethane and dimethyldiethyl substitution derivative of diaminodiphenylmethane, more preferably from aromatic Diamino compounds such as bis(aminophenoxy)benzene, bis(aminophenoxy)diphenylsulfone, bis(aminophenoxy)diphenylpropane, bis(aminophenoxy)diphenylhexafluoropropane, diaminodiphenylfluorene, dimethyl substitution derivative of diaminodiphenylmethane, tetramethyl substitution derivative of diaminodiphenylmethane, diethyl substitution derivative of diaminodiphenylmethane, tetraethyl substitution derivative of diaminodiphenylmethane and dimethyldiethyl substitution derivative of diaminodiphenylmethane.

Preferably, the thermoplastic resin molecule having a polyimide, polyamide, polyether, polysulfone or polyketone skeleton has a hexafluoropropane skeleton because its solubility in uncured thermosetting resin is improved to form a proper phase-separated structure after curing. This structure significantly lowers the water absorption capacity of the cured resin, thereby improving the environmental resistance of the cured resin.

In terms of compatibility control, it is particularly preferable that the thermoplastic resin in components [B] and [D] is a block copolymer or a Graft copolymer is composed of a chain that is compatible with the thermosetting resin and a chain that is incompatible with the thermosetting resin.

One of the preferred examples is a block or graft copolymer having a chain consisting of a siloxane skeleton which is incompatible with the thermosetting resin in the components [B] and [D], high strength and low water absorption. Particularly preferred is a block or graft copolymer having a polyimide, polyamide, polyether, polysulfone or polyketone skeleton which is compatible with the thermosetting resin in the component [B] (as a part other than the chain consisting of the siloxane skeleton). As the siloxane skeleton, dimethylsiloxane is particularly preferred, but phenylsiloxanes and their copolymers are also preferred.

The increase in resin viscosity due to the addition of a thermoplastic resin having a block copolymerized siloxane skeleton is small compared with that caused by the addition of an aromatic thermoplastic resin having the same molecular weight. Therefore, this causes the handleability to be less restricted and a prepreg using the resin as a matrix resin to have excellent tackiness and stretchability. From another point of view, the amount of the thermoplastic resin added to the component [B] is less restricted and a large amount can be added to the resin composition without impairing the tackiness to achieve higher resin toughness.

The most preferred block or graft copolymer as the thermoplastic resin in the components [B] and [D] has a polyimide chain portion.

In view of solvent resistance and fatigue resistance, it is preferred that the thermoplastic resin in the components [B] and [D] has functional groups which can react with the thermosetting resin in the component [B] at the ends, since they improve the adhesion at the interface. Such a thermoplastic resin can be prepared from, for example, resins having functional groups such as amino, epoxy, Hydroxyl or carboxyl groups at the ends, especially with amino groups at the ends.

Preferably, the amount of the thermoplastic resin in the components [B] and [D] is 5 to 40% by weight based on the weight of all the ingredients in the components [B] and [D]. If the amount is less than this range, the effect of increasing the strength is small, and if it is more than this range, the handleability is significantly reduced. A more preferable range is 8 to 30% by weight.

The thermoplastic resin in component [B] may be previously dissolved or simply dispersed in the uncured thermosetting resin or even partially dissolved or dispersed. The percentage of solution or dispersion can be controlled to adjust the viscosity of the resin and therefore also the tackiness and stretchability as a prepreg. The dispersed thermoplastic resin is mostly dissolved in the thermosetting resin during molding and phase-separates again before curing is complete to help form the correct microphase-separated structure.

The molecular weight of the thermoplastic resin in components [B] and [D] is preferably in a number-average molecular weight range of 2,000 to 20,000 when the thermoplastic resin is to be previously dissolved in the uncured thermosetting resin. If the molecular weight is less than this range, the effect of increasing the strength is small, and if it is more than the range, the viscosity of the resin is significantly increased, so that the handleability is considerably limited. A more preferable range is about 2,500 to 10,000.

Component [C] consists of long fibers made of thermoplastic resin and is randomly distributed near the surface layer of the prepreg. The "long fibers" have a length of 3 cm or more. The "random distribution" means that the fibers are not arranged in such a way that the same pattern is formed at constant intervals. repeated. According to the invention, component [C] of the prepreg is in the form of a nonwoven fabric. The random distribution can be achieved by simple spreading or spraying and does not require a special device such as a weaving machine used in the manufacture of conventional fabrics. Random distribution can also be achieved by using a long-fiber nonwoven fabric. A long-fiber nonwoven fabric has excellent productivity compared with a fabric or a mat because it can be obtained directly from a raw resin without forming filaments therefrom. In addition, according to this manufacturing method, the prepreg of the invention does not have the problem of parallel fiber arrangement that component [C] penetrates into component [A]. Component [C] in the prepreg of the invention is characterized by being irregularly arranged. In order to obtain desired physical properties, it is desirable to keep the basis weight of component [C] as uniform as possible. Component [C] is distributed near the surface layer of the prepreg but does not cover the entire surface so that it can be easily impregnated with the matrix resin to impart the tackiness and stretchability of the matrix resin to the prepreg so that it has excellent handleability. Furthermore, component [C] serves to maintain a certain amount of resin on the surface of the prepreg to improve the tackiness compared with conventional prepregs and to keep the change of the tackiness with time small.

Component [C] is made of a thermoplastic resin. A typical thermoplastic resin for component [C] has bonds selected from carbon-carbon bonds, amide bonds, imide bonds, ester bonds, ether bonds, carbonate bonds, urethane bonds, thioether bonds, sulfone bonds, imidazole bonds and carbonyl bonds in the main chain. Polyamides, polycarbonates, polyacetals, polyphenylene oxide, polyphenylene sulfide, polyarylates, polyesters, polyamideimides, polyimides, polyetherimides, polysulfones, polyethersulfones, polyetheretherketones, polyaramid and polybenzimidazole have excellent impact resistance and are suitable as materials for nonwoven fabrics in the invention. Of these, polyamides, poly imides, polyamideimides, polyetherimides, polyethersulfones and polysulfones have high strength and satisfactory heat resistance; they are particularly suitable for the invention. The strength of polyamides is excellent, while amorphous transparent nylons can also impart heat resistance. Component [C] can also be formed by using long fibers made of a plurality of thermoplastic resins in combination or by using long fibers obtained by spinning a plurality of thermoplastic resins together. These methods are preferable because the properties of the composite material can be improved by using an optimum combination of thermoplastic resins.

Component [C] must be distributed near the surface layer of the prepreg. When the prepreg is made into a composite material, an interlayer region of a certain thickness is formed to locate component [C] between the layers; thus, a composite material with excellent impact resistance can be provided. The distribution near the surface layer means that more than 90% of component [C] is present in a range from the surface of the prepreg to 30% of the thickness of the prepreg. When more than 90% of component [C] is present in a range from the surface of the prepreg to 20% of the thickness of the prepreg (which is preferred), the effect of the invention is more pronounced.

If a large amount of component [C] is present deep inside, away from a surface layer that does not satisfy the above conditions, the interlayer energy absorption is insufficient, so that the effect of improving the composite material in terms of its impact resistance and interlayer strength is reduced; since the arrangement of the reinforcing fibers is disturbed and the percentage of the matrix resin near the reinforcing fibers is reduced, the strength and heat resistance of the composite material may be impaired.

When component [C] is locally distributed on both sides of the prepreg, layers of the prepreg can be laminated freely without having to pay attention to which side is the front or back when the composite material is produced. However, when two layers of a prepreg whose component is distributed only on one side are laminated together and component [C] is held between the two layers, the same effect can be achieved. The prepreg with component [C] distributed only on one side is also within the scope of the invention.

The distribution of component [C] in the prepreg can be determined as follows.

First, a prepreg is held in close contact between two smooth support plates and gradually heated over a long period of time to cure it. It is essential that gelation in this case be carried out at as low a temperature as possible. If the temperature is suddenly raised before gelation is achieved, the resin in the prepreg will flow and proper distribution in the prepreg cannot be determined. After gelation is complete, the prepreg is gradually heated and additional time is spent to cure it. The cured prepreg is cut and the cross section is photographed at 200X or more magnification on a size of 200 x 200 mm. If it is difficult to distinguish between components [B] and [C], one of them should be selectively colored for observation purposes. Either an optical microscope or an electron microscope can be used as required. Photomicrography is used to first determine the average thickness of the prepreg. The average thickness of the prepreg is obtained by taking measurements at five optical points on the photograph and averaging the values. Lines are then drawn parallel to the front surface direction of the prepreg at depths of 30% prepreg thickness from the surfaces in contact with both backing plates. The area of component [C] between the surface in contact with the backing plate and the 30% parallel on both sides are determined as well as the total area of component [C] in the total thickness of the prepreg. The ratio between the determined values is called the rate of component [C] in 30% depth from the surfaces of the prepreg. The area determination can also be done by a weight method or image processing using an image analyzer. To exclude effects of partial dispersion in the distribution, the evaluation procedure should be carried out over the entire width of the obtained photo at five or more optional points on the photo to average the values.

In order to obtain a composite material with higher impact strength, it is preferable that the elastic modulus and yield strength of the material of component [C] are lower than those of the cured resin of component [B]. However, if the elastic modulus of the material of component [C] is as low as an elastomer, component [C] is liable to be deformed during molding due to varying conditions such as pressure, temperature and heating rate, so that the thickness of the interlayer region of the laminate varies and thus the composite material becomes unstable in terms of its physical properties. In order to achieve reproducibly high strength which is insensitive to variations in molding conditions, it is preferable that the elastic modulus of the material as the main constituent of component [C] is in the range of 80 to 400 kg/mm2. Furthermore, for the same reason as above, it is preferable that the tensile elastic modulus of the component [C] in the form of fibers is in a range of 40 to 5,000 kg/mm2. A suitable amount of the component [C] is 2 to 30 wt% based on the total weight of the components [B] and [C] in the prepreg or composite material. If the amount is less than 2 wt%, the intended effect is insufficient, and if it exceeds 30 wt%, the prepreg has significantly impaired properties (stickiness, stretchability). When component [C] is used to increase the interlayer strength of the composite material due to its high elongation at break and strength, while the rigidity of component [B] serves to provide compressive strength of the composite material, it is recommended that the amount of component [C] be present in a rather minor amount in a range of 2 to 20 wt%, more preferably 4 to 13 wt%.

The prepreg described above can be produced by the following processes.

Procedure 1

Component [C] is randomly spread on the surface of [A] (impregnated with component [B]) to form a prepreg. In this state, component [C] is exposed on the surfaces of the prepreg, so that the tackiness is insufficient. Accordingly, it is desirable to heat and press the surfaces with a hot roller, etc. after completion of spreading to impregnate component [C] with component [B].

In a modification of the method, component [C] may be randomly distributed in a planar state on the surfaces of component [A] impregnated with component [B], and release paper applied to component [B] may be adhered to the surfaces and heated and pressed for impregnation. In this case, it is preferable that component [B] impregnated in component [A] differs in composition from component [B] applied to the release paper. Particularly, when component [B] applied to the release paper is stickier than component [B] impregnated in component [A], the resulting prepreg preferably has a higher stickiness.

Procedure 2

Component [C] is randomly distributed in a planar state on the surface of component [B] (formed as a film on a base such as a release paper) and the laminate is bonded to component [A] and heated and pressed to form a prepreg.

In a variation of this method, a similar operation can also be carried out using component [A] partially impregnated with component [B].

Procedure 3

Component [C] is randomly distributed in a planar state on component [A] and the laminate is impregnated with component [B] to form a prepreg. This process is particularly suitable when component [A] can retain its shape as a fabric.

In the above three methods, in order to randomly distribute the component [C] in a planar state, a thermoplastic resin is first spun to obtain monofilaments or multifilaments, which are then sprayed onto an object using compressed air, for example, directly or through a swivel guide or after striking a baffle plate once. In this case, in principle, any spinnable resin can be used to form component [C]. In addition, there are no restrictions on the weight reduction per unit area.

Further, it is possible to randomly distribute the filaments discharged from a nozzle plate in a planar state. In this case, the filaments discharged from a conventional nozzle plate by means of a compressed air stream may be stretched and sprayed onto the article; or the undrawn filaments are sprayed using a melt-blowing nozzle plate. These methods are suitable for easily spinnable resins. In the above methods, the long fibers may also be sprayed once onto a collector such as a screen cloth and then applied to the surface of the article instead of being sprayed directly onto the article. Further, a long fiber nonwoven fabric of component [C] may be prepared in advance by the spunbonding or melt-blowing method and used to prepare a prepreg by any of the following methods.

Procedure 4

Nonwoven fabrics of component [C] are bonded to component [A] impregnated with component [B] to form a prepreg. In this case, since component [C] is exposed on the surfaces of the prepreg in this state and has insufficient tack, it is desirable to heat and press the surfaces by means of a hot roller, etc. after completion of bonding to carry out the impregnation of component [C] with component [B].

In a modification of this method, the nonwoven fabrics of component [C] can be impregnated with component [B] in advance. Furthermore, in this case, it is preferable that component [B] impregnated in component [A] has a different composition than component [B] impregnated in the nonwoven fabric of component [C]. Particularly when component [B] impregnated in the nonwoven fabric of component [C] is stickier than component [B] impregnated in component [A], the resulting prepreg preferably has higher stickiness.

Procedure 5

The component [B] formed as a film on a base such as a release paper, a nonwoven fabric of the component [C] and the component [A] are overlapped in any order or simultaneously (regardless of the relative position when overlapping), and heated and bonded to each other to form a prepreg. In this case, the relative overlapping position keeping the nonwoven fabric of the component [C] between the component [A] and the component [B] formed as a film is preferable because the nonwoven fabric of the component [C] is easily impregnated with the component [B].

In a particularly preferred modification of the process, the nonwoven fabrics of component [C] are overlapped on both sides of component [A] and the formed Films of component [B] are provided for overlapping on both nonwoven fabrics to form a prepreg. In another particularly preferred variation of the method, a nonwoven fabric of component [C] is adhered to the surface of the film-formed component [B], and two layers of the laminate are then overlapped on both sides of component [A] with the nonwoven fabrics of component [C] held on the side of component [A], and heated and pressed by means of a hot roll to form a prepreg.

Examples

The following is a detailed explanation of the invention with reference to examples.

Example 1 of the invention

The following raw materials were kneaded to form a matrix resin composition.

(1) Tetraglydicyldiaminodiphenylmethane (ELM434, a product of Sumitomo Chemical Co., Ltd.), 60 parts by weight

(2) Bisphenol A epoxy resin (Epikote 828, a product of Yuka Shell Epoxy K. K.), 20 parts by weight

(3) Trifunctional aminophenol epoxy resin (ELM100, a product of Sumitomo Chemical Co., Ltd.), 20 parts by weight

(4) 4,4'-Diaminodiphenylsulfone (Sumicure S, a product of Sumitomo Chemical Co., Ltd.), 47.3 parts by weight

(5) Polyethersulfone (PES5003P, a product of Mitsui Toatsu Chemicals, Inc.), 16 parts by weight

An intermediate product was prepared by impregnating the carbon fibers (T800H, a product of Toray Industries, Inc.) with the matrix resin by the drum winding method The amount of carbon fibers per unit area was 190 g/m², the amount of matrix resin was 90.6 g/m².

The intermediate product was sprayed on its side with nylon 66 fibers (five 15 denier filaments) using a blower with a baffle attached to the tip and compressed air to produce a prepreg. The basis weight of the fibers was 13.0 g/m2.

24 layers of the prepreg were laminated in a quasi-isotropic configuration (+45º/0º/-45º/90º) and treated in an autoclave at 180ºC and a pressure of 6 kg/cm² for 2 hours. The resulting cured sheet was cut into 150 x 100 mm specimens. A drop impact test of 1,500 in.lbs/in was carried out on the center of each specimen and the post-impact compressive strength was measured according to ASTM D 695 and was found to be 34.0 kg/mm².

Example 2 of the invention

An intermediate product was prepared by impregnating carbon fibers (T800H, a product of Toray Industries, Inc.) with the same matrix resin as in Example 1 of the invention using the drum winding method. The basis weight of the carbon fibers was 190 g/m², and the basis weight of the matrix resin was 90.6 g/m².

The intermediate product was sprayed on one side with polyetherimide (Ultem 1010, a product of GE Plastics) fibers (13 denier filaments) using a blower with a baffle attached to the tip and compressed air to obtain a prepreg. The basis weight of the fibers was 13.0 g/m2.

A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured in the drop test was 35.8 kg/mm².

Example 3 of the invention

An intermediate product was prepared by impregnating carbon fibers (T800H, a product of Toray Industries, Inc.) with the same matrix resin as in Example 1 of the invention using the drum winding method. The basis weight of the carbon fibers was 190 g/m², and the basis weight of the matrix resin was 90.6 g/m².

The intermediate product was sprayed on one side with nylon 12 fibers discharged from a nozzle plate with an orifice using a blower with a baffle at the tip and compressed air while the fibers were stretched to obtain a prepreg. The basis weight of the fibers was 13.0 g/m2.

A hardened plate was made from the prepreg; the compressive strength measured in the drop test was 36.1 kg/mm².

Comparison example 1

Carbon fibers (T800H, a product of Toray Industries, Inc.) were impregnated with the same matrix resin as in Example 1 of the invention to produce a prepreg containing no thermoplastic resin fibers by the drum winding method. The basis weight of the carbon fibers was 190 g/m², and the basis weight of the matrix resin was 103.6 g/m².

A cured plate was produced from the prepreg as in Example 1 of the invention; The compressive strength measured in the drop test was 19.7 kg/mm².

Example 4 of the invention

Release paper was coated with the same matrix resin composition as in Example 1 of the invention (45.3 g/m², using a reverse roll coater). The resin film was sprayed on its surface with nylon 66 fibers (15 denier, 5 filaments) using a blower with a baffle attached to the tip and compressed air. The basis weight of the nylon fibers was 6.5 g/m².

The resin film sprayed with nylon 66 filaments was fixed on a drum winder, and carbon fibers (T800H, a product of Toray Industries, Inc.) were wound around it. In addition, another resin film sprayed with nylon 66 was bonded to the resin film sprayed with carbon fibers. Both films were pressed for impregnation to obtain a prepreg. The basis weight of the carbon fibers was 190 g/m². A cured plate was produced from the prepreg as in Example 1 of the invention, and the compressive strength measured by the drop test was 34.4 kg/mm².

Example 5 of the invention

Release paper was coated using a reverse roll coater with the same matrix resin composition as in Example 1 of the invention (45.3 g/m2).

The resin film was sprayed onto one surface with polyamide fibers (Glyclamide TR-55, a product of Emser Werke) dispensed from a nozzle plate with an orifice using a blower with a baffle at the tip and compressed air while the fibers were stretched. The basis weight of the polyamide fiber was 6.5 g/m².

The resin film sprayed with glylamide TR-55 fibers was fixed on a drum winder, and carbon fibers (T800H, a product of Toray Industries, Inc.) were wound around it. In addition, another resin film sprayed with glylamide TR-55 fibers was adhered to the resin film wrapped with carbon fibers, and both films were pressed for impregnation to obtain a prepreg. The basis weight of the carbon fibers was 190 g/m².

A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured in the drop test was 33.7 kg/mm².

Example 6 of the invention

A carbon fiber fabric (plain carbon fiber fabric T800H, a product of Toray Industries, Inc.; 196 g/m2 as fiber weight per unit area) was sprayed on one side with nylon 66 fibers (15 denier, 5 filaments) using a blower with a baffle attached to the tip and compressed air. The basis weight of the nylon 66 fibers was 16.0 g/m2. The sprayed fabric was impregnated with the same matrix resin as in Example 1 of the invention to obtain a prepreg. The basis weight of the resin was 130 g/m2.

A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured in the drop test was 29.4 kg/mm².

Example 7 of the invention

Screen fabric was sprayed with polyamide fibers (Glycolamide TR55, a product of Emser Werke) ejected from a single-orifice nozzle plate using a blower with a baffle attached to the tip and compressed air while the fibers were stretched. The fiber layer collected on the screen fabric was heat-bonded to form a Glycolamide TR-55 nonwoven fabric. The basis weight of the fibers was 6.5 g/m².

An intermediate product was prepared by impregnating carbon fibers (T800H, a product of Toray Industries, Inc.) with the same matrix resin as in Example 1 of the invention using the drum winding method. The basis weight of the carbon fibers was 190 g/m², and the basis weight of the matrix resin was 90.6 g/m².

The Glylamide TR-55 nonwoven fabric was bonded to both sides of the intermediate product to produce a prepreg.

A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured in the drop test was 34.0 kg/mm².

Example 8 of the invention

A nylon 6 fiber ejected from a single-orifice die plate was stretched, distributed and collected on screen mesh by a blower with a baffle attached to the tip and compressed air. The fiber layer collected on the screen mesh was hot-bonded by a hot press to produce a nylon 6 fiber web of 6.5 g/m² basis weight of fibers.

An intermediate product was prepared by impregnating carbon fibers (T800H, a product of Toray Industries, Inc.) with the same matrix resin as in Example 1 of the invention using the drum winding method. The basis weight of the carbon fibers was 190 g/m², and the basis weight of the matrix resin was 90.6 g/m².

Two layers of the nylon 6 nonwoven fabric were bonded to both sides of the intermediate product to prepare a prepreg. A cured plate was prepared from the prepreg as in Example 1 of the invention; the compressive strength measured by the drop test was 33.1 kg/mm2.

Example 9 of the invention

Release paper was coated using a reverse roll coater with the same matrix resin composition as in Example 1 of the invention (at 45.3 g/m2).

The same Glylamide TR-55 nonwoven fabric as in Example 7 of the invention was adhered to the resin film, and the laminate was pressed to be fixed with a calender roll. Then, two layers of the laminate were overlapped with the nonwoven fabric kept inside, and collimated carbon fibers (T800H, a product of Toray Industries, Inc.) were adhered to both sides; they were heated and pressed using a hot roll for impregnation to obtain a prepreg. The basis weight of the carbon fibers was 270 g/m². A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured by the drop test was 34.3 kg/mm².

Example 10 of the invention

Release paper was coated using a reverse roll coater with the same matrix resin composition as in Example 1 of the invention (at 45.3 g/m2).

The same nylon 6 nonwoven fabric as in Example 8 of the invention was bonded to the resin film, and the laminate was pressed to be fixed by a calender roll. Then, two layers of the laminate were overlapped while keeping the nonwoven fabric inside, and collimated carbon fibers (T800H, a product of Toray Industries, Inc.) were bonded to both sides; they were heated and pressed using a hot roll for impregnation to obtain a prepreg. A cured plate was produced from the prepreg as in Example 1 of the invention, and the compressive strength measured by the drop test was 33.6 kg/mm2.

Example 11 of the invention

Spun composite fibres ejected from a composite spinneret plate having a core and a sheath and consisting of a portion of polyethylene terephthalate (ejected from the core A portion of nylon 6 (ejected from the casing) was distributed and collected on screen mesh using a blower with a baffle attached to the tip and compressed air. The fiber layer collected on the screen mesh was heat bonded using a hot press to produce a nonwoven fabric having a fiber basis weight of 6.5 g/m². Release paper was coated using a reverse roll coater with the same matrix resin composition as in Example 1 of the invention (45.3 g/m²).

The nonwoven fabric was bonded to the resin film and pressed to be fixed by a calender roll. Then, two layers of the laminate were overlapped while holding the nonwoven fabric inside, and collimated carbon fibers (T800H, a product of Toray Industries, Inc.) were bonded to both sides; they were heated and pressed using a hot roll for impregnation to obtain a prepreg. The basis weight of the carbon fibers was 190 g/m².

A cured plate was produced from the prepreg as in Example 1 of the invention; the compressive strength measured in the drop test was 33.8 kg/mm².

Example 12 of the invention (1) Synthesizing a reactive polyimide oligomer

In a 3000 ml detachable flask equipped with a nitrogen inlet, thermometer, stirrer and water separator, the internal atmosphere of which was replaced with nitrogen, 392 g (0.91 mol) of bis[4-(3-aminophenoxy)phenyl]sulfone (BAPS-M), 39 g (0.11 mol) of 9,9'-bis(4-aminophenyl)fluorene (FDA) and 147 g (0.11 mol) of amino-terminated dimethylsiloxane of 650 NH₂ equivalents (BY-16-853, a product of Toray Silicone) were stirred and dissolved in 2000 ml of N-methyl-2-pyrrolidone (NMP). In addition, 300 g (1.02 mol) of solid biphenyltetracarboxylic dianhydride was gradually added and the mixture was stirred at room temperature for 3 hours and then at 120 °C for 2 hours. The flask was cooled to room temperature and 50 mL of triethylamine and 50 mL of toluene were added. The flask was heated again for azeotropic dehydration to obtain about 30 mL of water. The reaction mixture was cooled and diluted with twice the amount of NMP; the mixture was slowly poured into 20 L of acetone to precipitate amino-terminated siloxane polyimide oligomer as a solid product. The precipitate was dried at 200°C in vacuum.

The number average molecular weight (Mn) of the oligomer was measured by gel permeation chromatography (GPC) using dimethylformamide (DMF) as solvent and was 5,500 (expressed as polyethylene glycol, PEG). The glass transition temperature measured by differential thermal analysis (DSC) was 223ºC. The introduced siloxane skeleton and the amino end groups could be confirmed by NMR spectrum and IR spectrum.

(2) Preparation of a resin for component B and measurement of the physical properties of the resin

20 parts of the siloxane polyimide oligomer synthesized in (1) was added to 41 parts of O,O'-diallylbisphenol A and the mixture was heated at 140°C for 2 hours. Then, 39 parts of diphenylmethanebismaleimide was homogeneously dissolved in the mixture. The vessel was connected to a vacuum pump and kept under vacuum for defoaming. The mixture was poured into a mold preheated to 120°C and treated to enable its separation and subjected to a curing reaction in an oven at 180°C for 2 hours to produce a 3 mm thick cured resin sheet. This was post-cured at 200°C for 2 hours and at 250°C for 6 hours. The Tg of the cured resin obtained was 295°C. The ultimate load energy release rate GIC was 450 J/m², the bending elastic modulus was 380 kg/mm². The resin plate with a size of 60 x 10 x 2 mm was boiled for 20 hours and the water absorption coefficient was determined; it was 2.0%.

The cured resin was polished and the polished surface was stained with osmium tetroxide and the reflection electron image was observed under a scanning electron microscope. It was found that a microphase-separated structure was present, with the continuous phase consisting mainly of the oligomer.

(3) Manufacturing a prepreg and a composite material and measuring their physical properties

First, 20 parts of the siloxane polyimide oligomer synthesized under (1) were dissolved in 41 parts of O,O'-diallylbisphenol A and 39 parts of diphenylmethanebismaleimide were homogeneously mixed with the solution using a kneader.

Release paper thinly coated with a silicone release agent was coated with the resin composition at a constant thickness to obtain a resin film with a basis weight of 47 g/m².

Two layers of a nylon 6 nonwoven fabric, 5 g/m² per unit area, with a basis weight of 190 g/m² were overlapped on both sides of unidirectionally parallel aligned carbon fibers ("Torayca" T800H, a product of Toray Industries, Inc.), and two layers of the resin film were pressed onto the laminate to impregnate the fibers with the resin and prepare a prepreg. The prepreg had good tack and good stretchability.

The prepreg was held between two smooth Teflon sheets and gradually heated to 180ºC over 2 weeks to be cured. Its cross section was observed under a microscope and photographed. The amount of nonwoven fabric in the area from the prepreg surface to 30% prepreg depth was determined and was 100%, so it was found that the nonwoven fabric was sufficiently localized in the surface areas of the prepreg.

The cured prepreg was stained with osmium tetroxide and its reflection electron image was observed in a scanning electron microscope. It was found that the matrix resin formed a microphase-separated structure with an oligomer-rich continuous phase.

24 layers of the prepreg were laminated in a quasi-isotropic configuration (+45º/0º/ -45º/90º)₃₅), and the conventional vacuum autoclave forming method was used to obtain a cured sheet by heating from 25ºC to 180ºC at a heating rate of 1.5ºC/min under a pressure of 6 kg/cm² and heating at 180ºC for 2 hours for molding. The fibers in the cured sheet accounted for 55.4 vol% and the resin for 35.8 wt%. After the molding was completed, the cross section was observed by an optical microscope and it was confirmed that all the nonwoven fabric was present in the interlayer region of the cured sheet. The cured sheet was polished and the polished surface was colored with osmium tetroxide; the reflection electron image was observed in the scanning electron microscope. A microphase-separated structure with a continuous phase consisting mainly of the imide oligomer was observed.

The hardened plate was subjected to a drop test and the measured compressive strength was 33.1 kg/cm2.

The prepregs of the invention contain irregularly arranged long thermoplastic resin fibers near the surface layers and are therefore sticky and exhibit stretchability; the composite materials obtained by heating and molding the prepregs have high elastic modulus, high heat resistance and very high impact resistance and interlayer strength. In addition, the prepregs are easy to manufacture and their materials are widely available, which is a great advantage for industrial production.

Claims (7)

1. Prepreg for use in the manufacture of fiber-reinforced plastics, wherein the prepreg comprises the following components [A], [B] and [C], wherein the component [C], impregnated with the component [B], is located on one side or both sides of the component [A] impregnated with the component [B]: [A]: long reinforcing fibres with a length of 5 cm or more; [B]: Matrix resin of thermosetting resin composition mixed with thermoplastic resin; and [C]: Nonwoven fabric made of randomly arranged thermoplastic resin fibers. 2. The prepreg according to claim 1, wherein the nonwoven fabric of thermoplastic resin fibers [C] is one or more selected from polyamides, polycarbonates, polyimides and polyetheretherketones. 3. The prepreg according to claim 1 or 2, wherein the thermosetting resin composition [B] comprises one or more selected from epoxy resins, maleimide resins, acetylene-terminated resins, nadic acid-terminated resins, cyanate-terminated resins, vinyl-terminated resins, allyl-terminated resins and phenolic resins. 4. Prepreg according to one of the preceding claims, wherein the following formula (1) applies: 2 ? C x 100/(C + B) ? 30 where B is the weight of component [B] per unit area of prepreg (g/m²) and C is the weight of component [C] per unit area of prepreg (g/m²). 5. A method for producing a prepreg according to any one of claims 1 to 4, the method comprising the steps of arranging the component [C] on one side or both sides of component [A] and impregnating the intermediate product with component [B]. 6. A method for producing a prepreg according to any one of claims 1 to 4, the method comprising the steps of impregnating component [A] with component [B] and adhering component [C] to one side or both sides of the impregnated component [A]. 7. A process for producing a prepreg according to any one of claims 1 to 4, wherein the process comprises the steps of impregnating component [A] with component [B] and adhering the component [C] impregnated or bonded with component [B] to one or both sides of the impregnated component [A].