KR102701071B1 - Light-weight prepreg and method for producing the same - Google Patents

Abstract

A lightweight prepreg capable of producing a lightweight long fiber reinforced sheet and a lightweight composite molded product, and a method for producing the same, are disclosed. The lightweight prepreg comprises a matrix resin; and reinforcing fibers impregnated in the matrix resin, wherein the matrix resin is formed of an epoxy resin composition comprising an epoxy resin, a curing agent that hardens the epoxy resin, and a foaming agent. The method for producing the lightweight prepreg comprises a step of impregnating reinforcing fibers into an epoxy resin composition comprising an epoxy resin, a curing agent that hardens the epoxy resin, and a foaming agent; and a step of curing the epoxy resin composition in which the reinforcing fibers are impregnated to form a matrix resin.

Description

{Light-weight prepreg and method for producing the same}

The present invention relates to a prepreg and a method for manufacturing the same, and more particularly, to a lightweight prepreg capable of manufacturing a lightweight long fiber reinforced sheet and a lightweight composite material molded product and a method for manufacturing the same.

Fiber-reinforced composite materials have high specific strength and specific elastic modulus, and are therefore widely used in the manufacture of sports and leisure goods, aviation goods, automobile and railway goods, and general industrial goods. Filament winding, infusion, RTM (Resin Transfer Molding), pultrusion, and press are among the molding methods used to manufacture molded products using fiber-reinforced composite materials. In particular, prepreg, which is an intermediate product in which matrix resin (base material) is pre-impregnated into reinforcing fibers, is used in molding methods such as autoclave, vacuum bag, sheet winding, and bladder molding. Using prepreg makes it easy to increase the fiber content affecting the mechanical properties of the molded product, manage the target fiber content, reduce internal defects such as voids, and easily improve properties such as heat resistance and impact strength.

Recently, a method has been used in which a long-fiber prepreg is manufactured using long fibers arranged in one direction, the manufactured long-fiber prepreg is cut in the longitudinal and vertical directions to manufacture prepreg chips, these are laminated and pressed into a sheet shape to manufacture a long-fiber reinforced sheet, and then the long-fiber reinforced sheet is molded to manufacture a fiber-reinforced composite molded product. Such long-fiber reinforced sheets have the advantages of excellent matrix impregnation and the possibility of molding parts having complex shapes, but there is a problem that the weight of the molded product increases, i.e., there is a limit to weight reduction (see EP 1134314 A1, Patent Publication No. 10-2019-0033686, etc.).

Meanwhile, in the case of sheet molding compounding (SMC) materials, which are made by laminating and pressing resin sheets containing carbon fibers that are usually 2.5 to 5 cm long, and then hardening them, it is possible to mold parts with complex shapes, but when carbon fibers are used, the impregnation of the base material is low, so there is a limit to the increase in mechanical properties, and there is also a limit to weight reduction.

The purpose of the present invention is to provide a lightweight prepreg capable of producing a lightweight long fiber reinforced sheet and a lightweight composite material molded product, and a method for producing the same.

Another object of the present invention is to provide a lightweight prepreg capable of manufacturing a molded product having a complex shape while reducing the weight of the molded product, and a method for manufacturing the same.

To achieve the above object, the present invention provides a lightweight prepreg comprising a matrix resin; and reinforcing fibers impregnated in the matrix resin, wherein the matrix resin is made of an epoxy resin composition comprising an epoxy resin, a curing agent that hardens the epoxy resin, and a foaming agent.

In addition, the present invention provides a method for manufacturing a prepreg, comprising the steps of impregnating reinforcing fibers into an epoxy resin composition comprising an epoxy resin, a curing agent for curing the epoxy resin, and a foaming agent; and the steps of curing the epoxy resin composition impregnated with the reinforcing fibers to form a matrix resin.

In addition, the present invention provides a fiber-reinforced composite material molded product molded with the lightweight prepreg.

By using the lightweight prepreg according to the present invention, not only can a lightweight long-fiber reinforced sheet and a lightweight composite molded product be manufactured, but also a molded product having a complex shape can be manufactured while reducing the weight of the molded product.

Hereinafter, the present invention will be described in detail.

A lightweight prepreg according to the present invention comprises a matrix resin and reinforcing fibers impregnated in the matrix resin, wherein the matrix resin is made of an epoxy resin composition comprising an epoxy resin, a curing agent that hardens the epoxy resin, and a foaming agent.

In the above epoxy resin composition, the epoxy resin functions as a matrix that impregnates the reinforcing fibers of the prepreg, provides reactivity and adhesiveness to the resin composition, and provides heat resistance, toughness, chemical resistance, etc. to the molded product after curing. As the epoxy resin, a conventional curable epoxy resin can be used without any particular limitation, and for example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, biphenyl epoxy resin, novolac type epoxy resin, naphthalene type epoxy resin, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, dicyclopentadiene type epoxy resin, alicyclic epoxy resin, etc. can be used. Preferably, bisphenol A type epoxy resin and bisphenol F type epoxy resin can be used alone or in mixture, and more preferably, bisphenol A type epoxy resin can be used. The above epoxy resin may exist in a solid, semi-solid, liquid or mixed state thereof at room temperature, and may be formed as a mixture of, for example, 40 to 60 wt% of a liquid epoxy resin and 40 to 60 wt% of a solid epoxy resin to control the viscosity of the matrix resin.

The above curing agent is a compound that reacts with an epoxy resin to harden the epoxy resin so that the epoxy resin has a predetermined viscosity. As the curing agent, a typical curing agent that hardens an epoxy resin can be used without any particular limitation. Specific examples of the curing agent include dicyandiamide, dichlorophenyl dimethylurea, and the like.

The above-mentioned foaming agent is foamed during the heat curing process of the epoxy resin, and forms pores inside the prepreg, thereby reducing the density and weight of the prepreg, the long-fiber reinforced sheet manufactured therefrom, and the molded product. The above-mentioned foaming agent may be a heat-expandable microsphere foaming agent made of a thermoplastic resin, specifically, Expancel 920 DU20 and Expancel 920 DU40 from Akzonobel. The heat-expandable thermoplastic microsphere foaming agent includes a thermoplastic shell that encapsulates the foaming agent. The thermoplastic shell is typically made of a thermoplastic polymer, and when heated, the thermoplastic shell softens and the foaming agent retained in the thermoplastic shell evaporates, causing the thermoplastic microspheres to expand, thereby foaming the matrix resin. Such heat-expandable thermoplastic microsphere foams are known, for example, in U.S. Pat. No. 3,615,972, International Patent Publication No. WO2007/091960, and Republic of Korea Patent Publication No. 10-2019-0012225.

In the epoxy resin composition used in the prepreg of the present invention, the content of the epoxy resin is 70 to 96 wt%, preferably 79 to 92 wt%, the content of the curing agent is 1 to 10 wt%, preferably 3 to 6 wt%, and the content of the foaming agent is 3 to 20 wt%, preferably 5 to 15 wt%. If the content of the curing agent is too high, there is a problem that the viscosity of the epoxy resin excessively increases, and if the content of the curing agent is too low, there is a concern that the curing of the epoxy resin is insufficient, making it difficult to form a sheet-shaped prepreg. In addition, if the content of the foaming agent is too high, mixing of the epoxy resin composition may be difficult, and if the content of the foaming agent is too low, the foaming amount may be small, and the density reduction of the prepreg molded product may be insufficient.

As reinforcing fibers impregnated in the above matrix resin, reinforcing fibers for forming prepregs, such as carbon fibers, graphite fibers, glass fibers, and polymer fibers, can be used without special restrictions, and these reinforcing fibers can be used in the form of continuous fibers such as tows and fabrics, chopped or short fibers, long fibers, mats, etc.

In the prepreg according to the present invention, the content of the matrix resin (RC, Resin Content) is not particularly limited as long as it has the rigidity, moldability and viscosity as a prepreg, but is, for example, 20 to 70 wt%, preferably 30 to 60 wt%. If the matrix resin content (RC) is too small, there is a possibility that the reinforcing fibers are not completely impregnated into the resin during the manufacture of the prepreg and voids may remain inside. In addition, the fibers are exposed on the surface after molding, making it difficult to secure excellent surface quality. If the matrix resin content (RC) is too large, the reinforcing fiber content is relatively low, which may lower the mechanical strength such as the specific strength and the specific elasticity, and the excessively included resin may flow a lot during press molding, which may cause problems such as disruption of the arrangement of the reinforcing fibers or resin leakage near the edge of the molded product. In addition, the reinforcing fiber content per unit area of the prepreg (Fiber Areal Weight: FAW) is suitably 50 to 300 g/ ^m2 .

The above prepreg can be manufactured by impregnating reinforcing fibers into the epoxy resin composition, and curing the epoxy resin composition impregnated with the reinforcing fibers to form a matrix resin. As the impregnation method, a dry method (hot melt method) and a wet method (solution method) are generally used. The wet method is a method in which the reinforcing fibers are immersed in an epoxy resin composition solution in which the epoxy resin composition is dissolved in an epoxy resin dissolving solvent such as methyl ethyl ketone (MEK) or acetone, and then the solvent is removed by passing the resulting mixture through a drying oven to manufacture a prepreg. An example of the hot melt method, which is a dry method, is as follows. First, the epoxy resin composition is heated to a temperature of about 60 to 100° C. to ensure fluidity to reduce the viscosity, and then the epoxy resin film is manufactured by coating it on a release paper to a calculated thickness, and the film and the reinforcing fibers spread out in a flat shape are combined or laminated with heat and pressure, thereby impregnating the reinforcing fibers with the epoxy resin. The hot melt method has the advantage of being able to produce prepregs that do not contain residual solvents. As coating methods for producing resin films, comma coating, roll coating, slot die coating, etc. can be used.

The prepreg according to the present invention can be used to manufacture a fiber-reinforced composite molded product. As a method for manufacturing a fiber-reinforced composite molded product using the prepreg of the present invention, a conventional molding method such as autoclave, vacuum bag, or press molding can be used. For example, the prepreg of the present invention can be press molded in a mold at a temperature of 140 to 160° C. and a pressure of 5 to 10 kgf/cm ² for 2 to 5 minutes to manufacture a molded product.

In addition, after manufacturing a long-fiber prepreg using long fibers arranged in one direction as reinforcing fibers, the formed long-fiber prepreg is cut in the longitudinal and vertical directions to manufacture prepreg chips, which are then randomly dispersed and used as a compounding material. In addition, the prepreg chips can be randomly dispersed, laminated in a sheet form, and then compressed to manufacture a long-fiber reinforced sheet. The compounding material and long-fiber reinforced sheet manufactured in this manner can be molded to manufacture a fiber-reinforced composite molded product having a complex shape (see Patent Publication No. 10-2019-0033686, etc.).

Since the prepreg according to the present invention contains a foaming agent in the matrix resin, when the prepreg is molded as it is to manufacture a molded product, or when a long-fiber reinforced compound or a long-fiber reinforced sheet is formed and then a molded product is manufactured, the density of the molded product can be lowered by the action of the foaming agent, thereby reducing the weight. The molded product obtained by using the prepreg according to the present invention is a lightweight long-fiber composite material molded product compared to existing SMC materials. In addition, when a molded product is manufactured by forming a long-fiber reinforced compound or a long-fiber reinforced sheet using the prepreg according to the present invention, a lightweight part having a complex shape can be manufactured.

A composite material molded product manufactured using the prepreg according to the present invention can be molded into a complex shape while being lightweight, and therefore can be used for various purposes, such as body protection parts such as protectors, bulletproof plates, and outer shells of helmets that are worn on the body to prevent damage to the body from external impact, automobile interior panels, and automobile rib parts.

Hereinafter, the present invention will be described in more detail through specific examples. The following examples are intended to illustrate the present invention, and the present invention is not limited to the following examples.

[Example 1] Manufacturing of unidirectional prepreg sheet

An epoxy resin composition was prepared by mixing 34 wt% of a liquid bisphenol A type (BPA) epoxy resin, 50 wt% of a solid BPA type epoxy resin, 6 wt% of a hardener (dicyandiamide), and 10 wt% of a blowing agent (Expancel 920 DU20 from Akzonobel). The prepared epoxy resin composition was cured by maintaining it in an oven at 150° C. for 30 minutes, thereby obtaining a cured epoxy resin composition (density: 0.43 g/cm3).

A prepreg was manufactured by impregnating carbon fibers into the above epoxy resin composition. In the manufactured prepreg, the input content of the epoxy resin composition (Resin Content) was 50 wt%, and the input amount of carbon fibers per unit area (Fiber Areal Weight) was 150 g/㎡.

The manufactured unidirectional prepreg was cut to a size of 300 x 300 mm in width and length, laminated into 5 sheets (weight of 5 prepreg sheets laminated: 135 g), placed into a mold measuring 300 x 300 mm in width and length preheated to 150°C, pressurized to 5 MPa, and maintained for 30 minutes to manufacture a lightweight prepreg sheet having a thickness of 1.57 mm. The density of the manufactured lightweight prepreg sheet was 0.95 g/cm3.

[Example 2] Manufacturing of long fiber reinforced sheet

In order to manufacture a molded product having a complex shape, the lightweight prepreg sheet was slitting and cutting processed. Specifically, the lightweight prepreg sheet was slit to a width of 8 mm, and then cut to a length of 50 mm. Then, the cut prepreg pieces were randomly distributed and allowed to fall freely to disperse. Thereafter, heat and pressure were applied to process the prepreg pieces into a sheet shape. 135 g of the sheet thus manufactured was put into a mold measuring 300 x 300 mm and preheated to 150° C., pressurized at 5 MPa, and maintained for 30 minutes to manufacture a long-fiber reinforced sheet having a thickness of 1.68 mm. The density of the long-fiber reinforced sheet thus manufactured was 0.89 g/cm3.

[Comparative Example 1] Manufacturing of unidirectional prepreg sheets

An epoxy resin composition was prepared in the same manner as in Example 1 except that no foaming agent was used, and the prepared epoxy resin composition was cured by maintaining it in an oven at 150° C. for 30 minutes, thereby obtaining a cured epoxy resin composition (density: 1.20 g/cm3). Using the cured epoxy resin composition, a prepreg and a lightweight prepreg sheet having a thickness of 1.05 mm were prepared. The density of the prepared lightweight prepreg sheet was 1.44 g/cm3.

[Comparative Example 2] Manufacturing of long fiber reinforced sheet

Using the lightweight prepreg sheet obtained in Comparative Example 1, it was processed into a sheet shape in the same manner as in Example 2, and 135 g of the sheet thus manufactured was placed into a mold preheated to 150°C, pressurized to 5 MPa, and maintained for 30 minutes to manufacture a long-fiber reinforced sheet having a thickness of 1.11 mm. The density of the manufactured long-fiber reinforced sheet was 1.35 g/cm3.

The properties of the prepreg and long-fiber reinforced sheet obtained in the above examples and comparative examples are summarized in Table 1 below.

division Molded product Thickness (mm) Density (g/cm3) Example 1 Lightweight unidirectional prepreg molding
(FAW 150 g/cm2, resin content 50%, 5-sheet laminated molding) 1.57 0.95 Example 2 Lightweight long fiber reinforced sheet molding (135g injection molding) 1.68 0.89 Comparative Example 1 General unidirectional prepreg molding
(FAW 150 g/cm2, resin content 50%, 5-sheet laminated molding) 1.05 1.44 Comparative Example 2 General long fiber reinforced sheet molding (135g injection molding) 1.11 1.35

As shown in Table 1 above, the prepreg according to the present invention and the long fiber reinforced sheet molded using the prepreg can form a relatively lightweight molded product.

Claims (6)

A prepreg sheet in which prepreg chips formed by cutting prepreg in the longitudinal and vertical directions are dispersed and laminated in the form of a sheet,
The above prepreg is,
A matrix resin comprising an epoxy resin composition comprising an epoxy resin, a curing agent for curing the epoxy resin, and a foaming agent; and reinforcing fibers impregnated in the matrix resin.
The above prepreg is formed by heating the epoxy resin composition to a temperature of 60 to 100°C to reduce the viscosity and then coating the epoxy resin film on a release paper, and then laminating or bonding the epoxy resin film with the reinforcing fibers using heat and pressure to impregnate the reinforcing fibers with the epoxy resin composition.
The above blowing agent is a heat-expandable microsphere blowing agent comprising a thermoplastic shell encapsulating the blowing agent,
A prepreg sheet having a reinforcing fiber content per unit area of 150 to 300 g/ ^m2 . delete A prepreg sheet in claim 1, wherein the content of the epoxy resin is 70 to 96 wt%, the content of the curing agent is 1 to 10 wt%, and the content of the foaming agent is 3 to 20 wt%. A prepreg sheet according to claim 1, wherein the content of the matrix resin (RC, Resin Content) in the prepreg is 20 to 70 wt%. A step of impregnating reinforcing fibers into an epoxy resin composition comprising an epoxy resin, a curing agent for curing the epoxy resin, and a foaming agent;
A step of manufacturing a prepreg by curing an epoxy resin composition impregnated with the reinforcing fibers to form a matrix resin;
A step of manufacturing prepreg chips by cutting the prepreg in the longitudinal and vertical directions; and
A step of dispersing the above prepreg chips into a sheet form and laminating and pressing them;
A method for manufacturing a prepreg sheet, comprising:
The above blowing agent is a heat-expandable microsphere blowing agent comprising a thermoplastic shell encapsulating the blowing agent,
The step of impregnating the epoxy resin composition with reinforcing fibers comprises heating the epoxy resin composition to a temperature of 60 to 100°C to reduce the viscosity, then coating it on a release paper to produce an epoxy resin film, and then combining or laminating the produced epoxy resin film with the reinforcing fibers using heat and pressure to impregnate the reinforcing fibers with the epoxy resin composition.
A method for manufacturing a prepreg sheet having a reinforcing fiber content per unit area of 150 to 300 g/ ^m2 . delete