JP7553391B2 - Manufacturing method of carbon fiber reinforced resin composite - Google Patents

Description

The present invention relates to a carbon fiber reinforced resin composite and its manufacturing method.

Carbon fiber reinforced plastic (CFRP), which is made by compounding carbon fiber with a matrix resin, has strength and elastic modulus comparable to metal materials, but has a smaller specific gravity than metal materials, allowing for lighter components. It also has no rust problems and is resistant to acids and alkalis, so it is being used increasingly in electronic equipment materials, electrical equipment materials, civil engineering materials, construction materials, automotive materials, aircraft materials, and in manufacturing parts such as robots and rolls used in various manufacturing industries.

The most common carbon fiber reinforced resin composites are composites made by laminating and integrating prepregs made of long fiber woven fabrics, spread fabrics, or unidirectional (UD) sheets made of continuous carbon fibers with thermosetting resins. However, there are problems with these composites, such as the difficulty of designing the lamination to achieve mechanical properties, the fact that they are not homogeneous materials, the molding process takes a long time, and they are expensive.

As a method for solving these problems, carbon fiber reinforced resin composites consisting of nonwoven prepregs containing discontinuous short carbon fibers and thermoplastic resin fibers have been proposed (see, for example, Patent Documents 1 to 3). The use of carbon fiber nonwoven fabric and thermoplastic resin makes it easier to design, allows for long-term storage, and shortens molding processing time. In addition, leftover materials (remaining yarns, fabric, etc.) from the manufacturing process, prepregs and discontinuous carbon fibers recycled from scraps and retired waste materials from carbon fiber reinforced resin composites can be used as inexpensive materials.

Methods for manufacturing nonwoven prepregs include the wet method, the dry pulp method, and the dry method (card method). In the card method (carding), the short fibers are combed, and the constituent fibers tend to be oriented in the longitudinal direction. There are methods that use the card web with oriented fibers as is, methods that use a cross wrapper to fold the card web alternately in the width direction and then layer the card web and entangle the fibers by needle punching, and methods that use a thermoplastic resin that is melted by heat treatment.

Carbon fiber reinforced resin composites made from these nonwoven fabric prepregs have improved mechanical properties by minimizing voids and gaps within the composite and increasing adhesion between the carbon fiber and resin through increased density, with the aim of replacing metal materials.

However, this method makes it difficult to achieve further weight reduction. In addition, the high-pitched sound generated when a carbon fiber reinforced resin composite is struck is unpleasant in structures that use the carbon fiber reinforced resin composite and in the vicinity of the area where it is used, making it necessary to use a separate soundproofing material, etc. In some cases, the mechanical strength may be excessive when replacing materials other than metal, such as glass fiber reinforced resin composites, and the high strength of carbon fiber reinforced resin composites leaves the problem that they are hard and difficult to process, such as by cutting.

Special Publication No. 2013-519546 Patent No. 5309563 International Publication No. 2014/021366

The objective of the present invention is to provide a carbon fiber reinforced resin composite that is lightweight, suppresses unpleasant noise, and is easy to process.

As a result of intensive research to solve the above problems, we have invented the following carbon fiber reinforced resin composite and manufacturing method.

That is, the present invention comprises the following:
(1) A carbon fiber reinforced resin composite containing carbon fiber and thermoplastic resin fiber, at least a portion of the thermoplastic resin fiber being melt-bonded to the carbon fiber, the carbon fiber mixing ratio being 25 to 75 mass %, the density being 0.35 to 0.8 g/cm ³ , the Frazier air permeability being 1.0 cm ³ /cm ² ·s or more, and the flexural modulus being 6 GPa or more.

(2) The method for producing a carbon fiber reinforced resin composite described in (1) above further comprises a molding step of heating a nonwoven prepreg containing carbon fibers and thermoplastic resin fibers under pressure or non-pressure at a temperature exceeding the melting point of the thermoplastic resin fibers to melt at least a portion of the thermoplastic resin constituting the thermoplastic resin fibers, and then cooling under pressure to solidify the thermoplastic resin, wherein the cooling start temperature in the molding step is equal to or higher than the glass transition temperature of the thermoplastic resin.

The present invention provides a carbon fiber reinforced resin composite that is lightweight, suppresses unpleasant noise, and is easy to process.

Depending on the type of precursor fiber, carbon fibers are industrially produced in a variety of types, including PAN (polyacrylonitrile), pitch (isotropic pitch, anisotropic pitch), phenol, and rayon. The carbon fiber used in the present invention can be selected according to the intended use. To achieve the mechanical properties of a carbon fiber reinforced resin composite, medium-elasticity carbon fiber with a tensile modulus of elasticity of 200 GPa or more is preferred.

It is also possible to use recycled carbon fibers obtained by using process scraps, defective parts, and retired waste materials from prepregs and CFRP generated during the CFRP manufacturing process as raw materials and removing the matrix resin (thermosetting resin or thermoplastic resin) using regeneration processing methods such as atmospheric pressure dissolution, subcritical decomposition, supercritical decomposition, electrolysis, thermal decomposition, and superheated steam method.

CFRP is lightweight and has high specific strength and rigidity, so it is used in golf shafts, tennis rackets, fishing rods, etc. Recently, it has also been used in major structural components of large aircraft such as wings and fuselages, and the market size is expanding. Along with this market size expansion, the amount of CFRP discarded during the manufacturing process is also increasing. For example, in the case of aircraft, safety is very important, so the quality standards are particularly strict, and the yield of CFRP is said to be 50 to 60%. In other words, there are many parts that are discarded due to trimming. In addition, prepregs cut to fit a mold, and prepregs in an uncured state, semi-cured state, or cured state that have expired are also types of discarded CFRP, and are similarly discarded in large quantities. In this situation, it is preferable to actively use recycled carbon fibers from the perspective of reducing the environmental load. In addition, the carbon fibers recycled from the materials provided for recycling are in a discontinuous state, and are also suitable for use as nonwoven fabrics.

In the present invention, it is preferable to use a nonwoven prepreg to manufacture a carbon fiber reinforced resin composite. By using a low-density, high-basis-weight nonwoven fabric, the number of layers of nonwoven prepreg can be reduced, making it easier to adjust the density of the carbon fiber reinforced resin composite. A preferred method for obtaining a low-density, high-basis-weight nonwoven fabric is to prepare a card web by a dry method (card method), laminate the web by cross-wrapping, and then use a very low-density, felt-like nonwoven prepreg in which the fibers are entangled by a needle punch method.

The fiber diameter of the carbon fibers usable in the present invention is preferably 5 to 20 μm, and more preferably 5 to 15 μm. If the fiber diameter of the carbon fibers is smaller than 5 μm, the fiber opening will be extremely insufficient, and the fibers may remain in the carding device, making it difficult to obtain a stable carded web. If the fiber diameter is larger than 20 μm, the carded web may have large unevenness in basis weight. Furthermore, the fiber length of the carbon fibers is preferably 25 to 100 mm, and more preferably 25 to 75 mm. If the fiber length of the carbon fibers is shorter than 25 mm, the fibers may easily fall off the carding device, making it difficult to obtain a stable carded web. If the fiber length is longer than 100 mm, the fibers may easily remain in the carding device, causing frequent neps.

Examples of the thermoplastic resin of the thermoplastic resin fiber include polyolefin resins such as polyethylene and polypropylene; styrene-based resins such as polystyrene, rubber-reinforced polystyrene, acrylonitrile-styrene copolymer, and ABS resin (acrylonitrile-butadiene-styrene copolymer synthetic resin); polyester resins such as polyethylene terephthalate and polybutylene terephthalate; polyamide resins such as nylon 6, nylon 66, and nylon 46; polyether-based resins such as polyphenylene ether and modified polyphenylene ether; engineering resins such as polycarbonate resin, polyarylate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, and polyetherimide; and thermoplastic epoxy resins. These thermoplastic resins may be used alone or in combination of two or more. Examples of the use of two or more types include blended polymers such as polycarbonate/ABS, polyphenylene ether/polyamide, polycarbonate/polybutylene terephthalate, and polyphenylene ether/polybutylene terephthalate. Thermoplastic resins are not limited to these, and all thermoplastic resins that are compatible with the manufacturing method of the present invention can be used. From the viewpoint of mechanical properties, polyamide is preferred, from the viewpoint of heat resistance, polyphenylene sulfide, polyetherimide, and polyether ether ketone are preferred, and from the viewpoint of economy, polypropylene is preferred.

Composite fibers using multiple thermoplastic resins are also preferred materials, and polyester-based sheath-core fibers with low melting points on the surface, polyolefin-based sheath-core fibers, unstretched polyester fibers, etc. can also be used.

The fiber diameter of the thermoplastic resin fiber is preferably 3 to 20 μm, and more preferably 5 to 15 μm. If the fiber diameter of the thermoplastic resin fiber is smaller than 3 μm, the fiber may remain in the carding device, and a stable carded web may not be obtained. If it is larger than 20 μm, the carded web may have large unevenness in basis weight. The fiber length is preferably 25 to 100 mm, and more preferably 25 to 75 mm. If the fiber length of the thermoplastic resin fiber is shorter than 25 mm, the fiber may fall off the carding device, and a stable carded web may not be obtained. If it is longer than 100 mm, the fiber may be easily retained in the carding device, and frequent neps may occur.

In the present invention, the carbon fiber blend ratio is preferably 25 to 75 mass% and more preferably 35 to 60 mass% relative to the nonwoven fabric prepreg. If the carbon fiber blend ratio is less than 25 mass%, the mechanical properties of the carbon fiber reinforced resin composite may be insufficient. If the carbon fiber blend ratio exceeds 75 mass%, the amount of thermoplastic resin relative to the carbon fibers becomes small, resulting in insufficient continuity as a matrix resin between the carbon fibers and insufficient adhesion to the carbon fibers, and the mechanical properties of the carbon fiber reinforced resin composite may be significantly reduced.

In the present invention, it is possible to mix small amounts of carbon fibers and thermoplastic resin fibers that are not within the preferred ranges for fiber length and fiber diameter described above, but the content of these fibers must not be in a range that impairs the performance of the carbon fiber reinforced resin composite.

In addition, in the present invention, a sizing agent that has a surface modification effect on the thermoplastic resin fiber and the carbon fiber can be sprayed, applied, or impregnated. It is also possible to mix other natural fibers, synthetic fibers, inorganic fibers, etc., but the content of these should not be in a range that inhibits the performance of the carbon fiber reinforced resin composite.

The density of the carbon fiber reinforced resin composite of the present invention is 0.35 to 0.8 g/cm ³ , more preferably 0.4 to 0.7 g/cm ^3. If it is less than 0.35 g/cm ³ , the mechanical strength is inferior, which is not preferable. If it is more than 0.8 g/cm ³ , the mechanical strength is high, but the Frazier air permeability is low. As a result, when the carbon fiber reinforced resin composite is struck, an unpleasant high-pitched sound such as "clang" is generated. That is, the Frazier air permeability of the carbon fiber reinforced resin composite of the present invention is 1.0 cm ³ /cm ² ·s or more, more preferably 3.0 cm ³ /cm ² ·s or more. The upper limit of the Frazier air permeability is not particularly limited, but is preferably 30 cm ³ /cm ² ·s or less. If the Frazier air permeability is less than 1.0 cm ³ /cm ² ·s, it is not preferable because it generates unpleasant sounds as described above. In the carbon fiber reinforced resin composite of the present invention, such unpleasant noise can be suppressed by controlling the density and Frazier air permeability to maintain them at constant levels.

The flexural modulus of the carbon fiber reinforced resin composite of the present invention is 6 GPa or more, and more preferably 7 GPa or more. If the flexural modulus is less than 6 GPa, the mechanical strength is insufficient and the composite is not suitable as a reinforcing material.

Next, the manufacturing method will be described. The materials are mixed in a blender, and a card web consisting of carbon fibers and thermoplastic fibers is produced using a carding machine. The basis weight of the card web obtained by this method is preferably 10 to 35 g/m ^2. A cross wrapper is used to stack a plurality of the obtained webs, and needle punching is performed to produce a nonwoven prepreg of 100 to 1000 g/m ^2. The density of the nonwoven prepreg is preferably 0.0045 to 0.01 g/cm ^3. If the density of the nonwoven prepreg exceeds 0.01 g/cm ³ , the carbon fibers may break during needle punching, causing significant dust generation and contaminating the process. In addition, the mechanical strength of the carbon fiber reinforced resin composite may decrease. If the density is lower than 0.0045 g/cm ³ , the mechanical strength of the nonwoven prepreg is weak, breakage may occur easily, and fiber dropout may occur, which is not preferable.

The nonwoven prepreg obtained by the above method can be laminated in a single layer or multiple layers, and molded into a carbon fiber reinforced resin composite by heating, pressurizing, and cooling.

The method for producing the carbon fiber reinforced resin composite of the present invention using the nonwoven fabric prepreg is described in detail below.

The method for producing a carbon fiber reinforced resin composite of the present invention includes a molding step in which a nonwoven prepreg containing carbon fibers and thermoplastic resin fibers is heated under pressure or non-pressurized conditions at a temperature exceeding the melting point of the thermoplastic resin fibers to melt at least a portion of the thermoplastic resin that constitutes the thermoplastic resin fibers, and then cooled under pressure to solidify the thermoplastic resin. The molding step is characterized in that the cooling start temperature is a temperature equal to or higher than the glass transition temperature of the thermoplastic resin.

Examples of heating and cooling methods include a method in which a single layer or multiple layers of nonwoven fabric prepreg are inserted between heated upper and lower molds, heat treated for a certain period of time, and then cooled under the same pressure (heat and cool method), and a method in which after heating and pressurization, the pressure is temporarily reduced to a non-pressurized state, and the prepreg is inserted between upper and lower molds that have been cooled to a temperature lower than the melting point of the thermoplastic resin, and then cooled and pressed (cold press method).

A carbon fiber reinforced resin composite can be produced by placing a single layer or multiple layers of nonwoven fabric prepreg between the dies of a pressure device having a pair of male and female dies with flat or uneven surfaces made of metal or polytetrafluoroethylene, etc., and adjusting the distance between the dies. In order to achieve the density of the carbon fiber reinforced resin composite of the present invention, a spacer having a certain thickness that will give the desired density can be placed between the dies, and pressure can be applied while maintaining the distance so that the nonwoven fabric prepreg is not crushed too much.

The pressurizing device may be a single-stage press, or a multi-stage press equipped with multiple press devices may be used.

It is preferable that the heating temperature be at least 20°C higher than the melting point of the thermoplastic resin that constitutes the thermoplastic resin fiber, but not exceeding the decomposition temperature of the thermoplastic resin.

The method of heating under pressure and then cooling and solidifying while maintaining the pressure is called the heat and cool method, and is preferable because the cooling start temperature in this method is necessarily equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin. For example, when the thermoplastic resin is nylon 6, the Tg is 50°C.

The method of heating in an unpressurized state and then cooling in a pressurized state is called the cold press method, and in this method, the cooling start temperature is a temperature equal to or higher than the Tg of the thermoplastic resin. If cooling is performed at a temperature below the Tg, the cooling and solidification of the molten thermoplastic resin will occur first on the pressurized surface and proceed rapidly, so that the resin will gather on the surface of the obtained carbon fiber reinforced resin composite and form a film, inhibiting breathability. In addition, the resin will be unevenly distributed in different places, and in places with little resin, the fuzz of the carbon fiber will be exposed on the surface, and if it is rubbed or processed by cutting, the carbon fiber will fall off, which is not preferable.

Cooling should be performed until the thermoplastic resin solidifies and the carbon fiber reinforced resin composite does not deform.

The pressure required for pressing should be 0.1 MPa or more, and should be such that the spacer used does not deform.

In the present invention, the melting point and Tg are measured by differential scanning calorimetry (DSC) as described in JIS K 7121:1987 "Method for measuring transition temperatures of plastics."

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. All parts and percentages in the examples and comparative examples are by weight unless otherwise specified. The physical properties of the carbon fiber reinforced resin composites obtained in the examples and comparative examples were measured and evaluated to confirm the effectiveness of the present invention.

(Examples 1 to 7, Comparative Examples 1 to 4)
PAN-based carbon fiber (T-700 manufactured by Toray Industries, fiber diameter 7 μm, fiber length 51 mm) and nylon 6 fiber (manufactured by Taiwan Kinlong Chemical, fineness 1.5 denier, fiber length 51 mm) as thermoplastic resin fiber were mixed in a blender and then carded with a roller carding machine to obtain an oriented carded web of 25 g/ ^m2 . After lamination with a cross wrapper, the fibers were entangled by a needle punch method to obtain a nonwoven fabric-like prepreg with a basis weight of 500 g/ ^m2 .

Two sheets of the nonwoven prepreg were stacked together and heated in an unpressurized environment at 260°C for three minutes. The sheets were then preheated to 60°C, and pressed at a pressure of 0.5 MPa in a press equipped with spacers to achieve the desired density. The sheets were then cooled for three minutes to obtain a carbon fiber reinforced resin composite.

(Comparative Example 5)
A carbon fiber reinforced resin composite was obtained in the same manner as in Example 1, except that no spacer was used and pressing was performed at a pressure of 8 MPa.

(Comparative Example 6)
A glass fiber reinforced resin composite was obtained in the same manner as in Comparative Example 5, except that the PAN-based carbon fiber was replaced with glass fiber (manufactured by Kyoseki Group Co., Ltd., fiber diameter 11 μm, fiber length 50 mm).

(Example 8)
A carbon fiber reinforced resin composite was obtained in the same manner as in Example 1, except that the cooling start temperature was 50° C. (Tg of nylon 6).

(Comparative Example 7)
A carbon fiber reinforced resin composite was obtained in the same manner as in Example 1, except that the cooling start temperature was 40°C.

Example 9
A carbon fiber reinforced resin composite was obtained in the same manner as in Example 1, except that the heating and pressurizing method was to heat the material preheated to 260°C at 0.5 MPa for 3 minutes and then cool it to 50°C while maintaining the pressurized state.

Example 10
A carbon fiber reinforced resin composite was obtained in the same manner as in Example 1, except that the number of the nonwoven fabric prepregs was one.

The fiber composition (mass%), heating temperature, cooling start temperature, and spacer thickness used in the above examples and comparative examples are shown in Table 1.

The physical properties of the carbon fiber reinforced resin composites obtained in the above examples and comparative examples were measured and evaluated by the following methods, and the physical properties and evaluation results are shown in Table 2. In the present invention, unless otherwise specified, the physical properties and evaluation were performed under conditions of 25°C and 50% RH.

<Metaku>
The carbon fiber reinforced resin composite was cut into a size of 20 x 25 cm, and the masses of five pieces were averaged to determine the mass per unit area (weight per unit area) in g/ ^m2 .

<Thickness>
For each of the carbon fiber reinforced resin composites for which the weight per unit area was measured, the thickness was measured at four points per sheet using a micrometer, and the average value of the five sheets was calculated. The unit is mm.

<Density>
The density was calculated by dividing the basis weight by the thickness, and the unit is g/ ^cm3 .

<Flexural modulus>
The evaluation was performed by measuring the flexural modulus of the carbon fiber reinforced composite material by the three-point bending method according to JIS K 7074:1988. The unit is GPa. The flexural modulus of the glass fiber reinforced resin composite of Comparative Example 6 was used as the standard, and the quality was judged according to the following criteria. In addition, ⊚ and ◯ were regarded as "good".

"Flexural modulus of each Example or Comparative Example - Flexural modulus of Comparative Example 6"
1GPa or more: ◎
0 GPa or more, less than 1 GPa: ○
Less than 0 GPa: ×

<Breathability>
The air permeability was evaluated by measuring the Frazier air permeability using an air permeability tester (device name: KES-F8-AP1, manufactured by Kato Tech Co., Ltd.) in accordance with the Air Permeability A Method (Fragile type method) specified in JIS L 1096:2010. The unit is ^cm3 / ^cm2 ·s. The symbols ⊚ and ◯ indicate "good".

Fragile air permeability of 3.0 cm ³ /cm ² ·s or more: ◎
1.0 cm ³ /cm ² ·s or more and less than 3.0 cm ³ /cm ² ·s: ○
Less than 1.0 cm ³ /cm ² ·s: ×

<Reflected sound>
When the surface of the carbon fiber reinforced resin composite was flicked with the nail of the index finger in the same way as flicking something, those that produced a high-pitched "clang" sound were rated as "X", those that produced a relatively low-pitched "bokoboko" sound were rated as "◎", and those that produced sounds in between were rated as "○". ◎ and ○ were rated as "good".

<Surface properties>
The surface of the carbon fiber reinforced resin composite was visually observed, and was evaluated as follows: ⊚: the carbon fiber and the molten thermoplastic resin fiber were uniformly integrated; ◯: the composite was generally uniform with some unevenness; and ×: the composite was mottled in parts. ⊚ and ◯ were rated as good.

<Processability>
The carbon fiber reinforced resin composite was cut into a size of 20 x 25 cm, placed on a ruler, and cut with a cutter knife to evaluate the condition. The evaluation was performed as follows: ◎: easy to cut without applying a large force; ◯: easy to cut but a small amount of fiber was observed to fall off from the cut surface; △: many fibers fell off from the cut surface and many fluffs were observed; and ×: required to cut with the cutter knife multiple times or could not be cut with the cutter knife. ◎ and ◯ were evaluated as good.

<Friction resistance>
The surface of the carbon fiber reinforced resin composite was rubbed with an index finger and evaluated for the presence or absence of fiber shedding and fuzz. The composite was rated as follows: no shedding of carbon fibers, ◯, carbon fibers with a small amount of shedding but no fuzz on the surface, △, carbon fibers with shedding and fuzz on the surface, and ×, carbon fibers with a lot of shedding and fuzz. ◯ and ◯ were rated as good.

The results of the above evaluation are shown in Table 2.

The present invention provides carbon fiber reinforced resin composites that can be used for electronic equipment materials, electrical equipment materials, civil engineering materials, building materials, automotive materials, aircraft materials, and manufacturing parts such as robots and rolls used in various manufacturing industries. In particular, it provides carbon fiber reinforced resin composites that can be used as interior and exterior materials for automobiles, and that are lightweight, have reduced unpleasant noise, and are highly processable.

Claims (1)

A method for producing a carbon fiber reinforced resin composite containing carbon fibers and thermoplastic resin fibers, at least a portion of the thermoplastic resin fibers being melt-bonded to the carbon fibers, the carbon fiber mixing ratio being 25 to 75 mass%, the density being 0.35 to 0.8 g/cm3 ^, the Frazier air permeability being 1.0 ^cm3 /cm2 ^· s or more, and the flexural modulus being 6 GPa or more. The method for producing a carbon fiber reinforced resin composite includes a molding step of heating a nonwoven fabric prepreg containing carbon fibers and thermoplastic resin fibers at a temperature exceeding the melting point of the thermoplastic resin fibers under pressure or non-pressurization to melt at least a portion of the thermoplastic resin constituting the thermoplastic resin fibers, and then cooling under pressure to solidify the thermoplastic resin, wherein the cooling start temperature in the molding step is a temperature equal to or higher than the glass transition temperature of the thermoplastic resin.