JP2003020516A - Polyacrylonitrile precursor fiber for carbon fiber and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide both a polyacrylonitrile precursor fiber for obtaining a graphitized fiber suppressing crystallinity of a surface layer part, having good adhesion to a matrix resin and having a high flexural strength and a method for producing the precursor fiber. SOLUTION: This polyacrylonitrile precursor fiber for a carbon fiber is characterized by comprising a water-soluble protein in at least the surface layer. The method for producing the polyacrylonitrile precursor fiber for the carbon fiber is characterized by applying the water-soluble protein to a water- swollen fiber prepared by wet or dry jet-wet spinning of a polyacrylonitrile polymer and then drying and densifying the resultant fiber.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a polyacrylonitrile-based precursor fiber for producing a graphitized fiber having excellent adhesiveness with a matrix resin and high bending strength which is a practical property of a composite, and a method for producing the same. Regarding

[0002]

2. Description of the Related Art Carbon fibers have been developed for various uses by taking advantage of their high specific strength and high specific elastic modulus.
In particular, carbon fibers starting from polyacrylonitrile (hereinafter referred to as PAN) -based precursor fibers are widely used because they have high specific strength and excellent workability. Carbon fibers are generally used as a composite material having a thermosetting resin or a thermoplastic resin as a matrix. In order to effectively utilize the strength and elastic modulus of carbon fibers in a composite material, it is necessary that the carbon fibers and the resin that is the matrix be firmly bonded. For this reason, it is common to give a functional group to the surface of the carbon fiber by subjecting it to an oxidation treatment as the finish of the manufacturing process of the carbon fiber. However, when the carbonization temperature is increased and the elastic modulus is increased, it is observed that the adhesion between the carbon fiber and the resin deteriorates. The reason for this is that as the carbonization temperature is raised, the crystals of the graphite structure of the carbon fiber become larger and are less likely to be oxidized. It is presumed that, along with this, it becomes difficult to impart a functional group sufficient for adhesion to the resin. For this reason, the higher the elastic modulus of the carbon fiber, the stronger the oxidation treatment is required, but there is a problem that it is difficult to obtain sufficient adhesive force with the resin only by the oxidation treatment. Further, there is a problem that the bending strength decreases as the carbonization temperature increases and the elastic modulus increases. This problem becomes particularly noticeable in the graphitized fiber obtained by firing the carbon fiber at a temperature of about 2000 ° C. or higher because of its large crystal size.

Bending strength indicates the strength up to the point of destruction when bending is applied in the direction perpendicular to the fiber axis direction. When a flexure is applied, each stress of "pulling, compressing, shearing" is applied to the product, and the fracture starts from the weakest place of each strength.
Therefore, high bending strength means "tensile, compression,
It shows that each strength of “shear” has a good balance, and it is an important strength as a practical property.

The crystallinity of the carbon fibers is not always uniform in the radial direction of the single fiber cross section, and the surface layer portion is often higher than the inner layer portion in many cases. Along with this, the elastic modulus of the surface layer portion of the single fiber becomes high, that is, it becomes difficult to stretch. In this case, when stress is applied to the fiber bundle, a higher stress is distributed in the surface layer portion than in the inside of the single fiber, and there is a problem that the strength reduction due to defects on the fiber surface is further promoted.
This is because oxygen diffuses from the surface of the fiber in the step of making the PAN-based precursor fiber flame resistant in heated air, and thus the surface layer portion has a higher degree of flame resistance progressing and a structure with higher heat resistance. Presumed.

As a means for solving the above problems and lowering the crystallinity of the fiber surface layer portion contributing to the adhesion with the matrix resin, facilitating the oxidation treatment, and at the same time improving the strength of the carbon fiber, the PAN-based precursor fiber It has been proposed that a flame retardant retarding element such as boron be present in the surface layer of the fiber (JP-A-10-88430).

However, when the carbonization temperature of the element such as boron becomes high and the graphitization region is 2000 ° C. or higher, a catalytic action for promoting the graphitization is exerted, so that the crystallinity of the surface layer is rather increased. Was higher, and there was a problem that adhesiveness and bending strength were reduced.

[0007]

SUMMARY OF THE INVENTION In view of the background of the prior art, the present invention provides a graphitized fiber which suppresses the crystallinity of the surface layer portion, has good adhesion to a matrix resin, and has high bending strength. The present invention provides a PAN-based precursor fiber and a method for producing the same.

[0008]

The PAN-based precursor fiber of the present invention has the following constitution in order to solve the above-mentioned problems. That is, the PAN-based precursor fiber for carbon fiber is characterized in that at least the surface layer contains a water-soluble protein.

The method for producing a PAN-based precursor fiber of the present invention has the following constitution in order to solve the above problems. That is, the method for producing a PAN-based precursor fiber for carbon fiber is characterized in that a water-soluble protein is added to water-swelled fiber obtained by wet- and dry-wet spinning of a PAN-based polymer, and then dried and densified. .

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have made extensive studies from the above viewpoints, and when various substances were contained in the surface layer portion of a PAN-based precursor fiber and baked, the water-soluble protein was found. It was found that in a system containing carbon, the amorphous portion of the graphite structure in the surface layer of the carbon fiber can be increased and the average crystallinity of the surface layer can be suppressed. It has been clarified that this can solve the above-mentioned problems all at once by providing a graphitized fiber having good adhesion with a matrix resin and high bending strength.

The carbon fiber in the present invention refers to a precursor fiber fired at a temperature of 1000 ° C. or higher in an inert atmosphere, and a graphitized fiber is 2000 in an inert atmosphere for the purpose of further increasing the elastic modulus. It refers to one baked at a temperature of ℃ or more. The effect of the present invention is obtained by controlling the crystallinity of the surface layer of carbon fiber by using a precursor fiber containing a water-soluble protein, and is particularly remarkable in graphitized fiber having high crystallinity. .

The PAN-based precursor fiber of the present invention contains a water-soluble protein at least in its surface layer.

The protein used in the present invention must be water-soluble. A non-water-soluble material is not preferable because it is difficult to impregnate the surface layer of the fiber. Water-soluble refers to a state in which it is dissolved or dispersed in water in a uniform state, and it is possible to use water-soluble proteins or to force water-insoluble proteins with a surfactant etc. A dispersed product can also be used. Further, the water-soluble protein contained in the precursor fiber, the precursor fiber is fired,
When it is converted into carbon fiber, it decomposes and scatters due to the heat.
If this decomposition is rapid, it may cause defects on the surface of the carbon fiber, leading to a decrease in strength. As an index of heat resistance, the carbonization residual rate is preferably 5% by weight or more.

As the water-soluble protein, for example, gelatin, peptone, peptide and the like are preferably used, but not limited to these.

Since the proper content of the water-soluble protein varies depending on the protein used, it is difficult to say that it is roughly 0.5 to 10% by weight based on the weight of the fiber. If it is less than 0.5% by weight, the effect of suppressing the crystallinity of the surface layer part is insufficient, and if it exceeds 10% by weight, the average crystallinity is largely lowered, and the elastic modulus of the carbon fiber is greatly lowered. is there.

Further, in order to exert the effect of containing the water-soluble protein more effectively, it is more preferable that the monofilament is contained mainly in the surface layer portion in the radial direction of the cross section.

The PAN-based precursor fiber of the present invention can be manufactured by the following method. That is, when the PAN-based polymer is wet- or dry-wet-spun to produce a PAN-based precursor fiber, a water-soluble protein is added to the water-swelled fiber before the dry densification, and then the dry densification is performed. As the spinning method, either a wet method or a dry-wet method is used.

The PAN-based polymer is not particularly limited, but acrylonitrile, preferably 90
It is preferable that the content is not less than wt%, more preferably not less than 95 wt%. If necessary, a second component copolymerizable with acrytonitrile can be added and copolymerized.
When spinning, the polymer can be used as a spinning stock solution dissolved in a solvent.

The solvent for the above-mentioned polymer is not particularly limited, but for example, an inorganic salt system such as zinc chloride and sodium thiocyanate, an organic system such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone and the like. Can be used.

The spinning dope prepared by dissolving the polymer in a solvent is usually spun into a fiber by wet or dry-wet spinning.
Then, the resultant is washed with water and then subjected to bath stretching, or subjected to bath stretching and then washed with water to obtain a water-swelled fiber having a swelling degree of 50 to 300% by weight. Factors that determine the degree of swelling include the hydrophilicity of the polymer, the concentration of the spinning dope, the type of solvent, the concentration of the solvent in the coagulation bath, the coagulation bath temperature, the bath stretching temperature, and the bath stretching ratio, which impairs productivity. Within the range that does not exist, the swelling degree of the fiber is 50-
These factors can be properly set so as to be 300% by weight. In order to contain the water-soluble protein in the surface layer of the fiber, the degree of swelling before drying and densification is 50 to 300.
It is preferred to add water-soluble proteins to the weight percent water swollen fiber. If the degree of swelling is less than 50% by weight, the water-soluble protein may be difficult to penetrate from the surface layer of the fiber and the remarkable effects of the present invention may not be sufficiently exerted, and if it exceeds 300% by weight, conversely to the inner layer of the fiber. The elastic modulus of the entire carbon fiber after infiltration and firing may decrease.

Next, a water-soluble protein is applied to the swollen fiber. The application method is not particularly limited, but a solution obtained by dissolving a water-soluble protein in water is adopted, for example, a dipping method, a spraying method, a touch roll method, a guide feeding method, etc., but a dipping method that is easy to treat is preferably used. It The concentration of the treatment liquid is set so that the final precursor fiber contains 0.5 to 10% by weight of the water-soluble protein. In order to contain the water-soluble protein, there is often no problem if the concentration of the water-soluble protein is adjusted within the range of 2.0 to 10% by weight. Further, an oil agent for preventing adhesion between single fibers can be added at the same time as the water-soluble protein.
Alternatively, after applying the protein, an oil agent for preventing adhesion between the single fibers can be applied. The oil agent for preventing the adhesion between the single fibers is not particularly limited, but silicone compounds modified with various functional groups are preferably used.

The swollen fiber to which the protein has been added is then densified by drying, and the drying temperature is 130 to 200 ° C.
Is preferred. If the drying temperature is lower than 130 ° C., water may be slowly evaporated, and the protein may be diffused to the inner layer portion of the single fiber, leading to a decrease in elastic modulus of the entire fiber. Further, at a high drying temperature of more than 200 ° C., adhesion between single fibers is likely to occur.

After the drying and densification, if necessary, it is stretched in a heating heat medium such as pressure steam to adjust the orientation, and if necessary, further heat-treated at 130 to 200 ° C. and wound up. It is provided as a PAN-based precursor fiber.

When the carbon fiber is produced from the PAN-based precursor fiber of the present invention, the PAN-based fiber obtained by the above-mentioned method is oxidized at 200 to 300 ° C., preferably 230 to 270 ° C., such as air. 0.95 in the atmosphere
~ 1.05 times and then converted into flame resistant fiber,
Maximum temperature is 600-900 ° C, preferably 700-80
Stretched 1.0 to 1.1 times in an inert atmosphere such as nitrogen at 0 ° C., and further stretched 0.95 to 1.0 times in an inert atmosphere such as nitrogen having a maximum temperature of 1000 to 1800 degrees. Can be carbon fiber.

The carbon fiber thus obtained has a maximum temperature of 2000 to 3000 ° C., preferably 20
1.01 in an inert atmosphere such as nitrogen at 00 to 2800 ° C
Stretched to 1.2 times and, if necessary, subjected to surface oxidation treatment, preferably electrolytic oxidation treatment of 10 to 200 coulomb / g in an acid or alkali aqueous solution to form a functional group on the fiber surface that enhances adhesiveness. To form graphitized fiber.

The graphitized fiber thus obtained has excellent properties such as excellent adhesiveness with the matrix resin and high bending strength.

[0027]

EXAMPLES The present invention will be described in more detail below with reference to examples. <Measurement of Strand Strength and Elastic Modulus> After impregnating a carbon fiber bundle with a resin having the following composition and curing at 130 ° C. for 35 minutes,
A tensile test was conducted according to JIS R7601.

(1) Resin composition-Epoxy resin ERL-4221 (manufactured by Union Carbide) 100 parts-Boron trifluoride monoethylamine (BF3 MEA) 3 parts-Acetone 4 parts <interlayer shear strength (hereinafter referred to as ILSS ), Measurement of flexural strength> The carbon fiber wound on a metal frame is put into a concave side mold having a groove width of 6 mm for uneven engagement so that the volume content (Vf) of the carbon fiber is 60%, and the resin is poured. After that, it was heated and degassed in vacuum. After defoaming, set it on a press machine to a thickness of 2.
Engage the uneven mold with a 5 mm spacer, and heat while applying pressure to cure the resin. Width 6 mm,
A test piece having a thickness of 2.5 mm was prepared. The measurement was performed using an Instron tester, and the bending strength was converted to Vf = 60%.

(1) Resin composition: Ep828 (manufactured by Petrochemicals Co., Ltd.) 100 parts: Boron trifluoride monoethylamine 3 parts (2) Molding condition method: 80 under vacuum (10 mmHg or less)
℃ × 4 hours molding; press pressure 4.9MPa, 170 ℃ ×
1 hour after cure; 1 after removing the test piece from the mold
70 ° C. × 2 hours (3) Measurement conditions ILSS; Cut the length of the test piece to 18 mm, use a three-point bending jig, set the supporting span to 4 times the thickness of the test piece, and measure n = 6, and the average value. I asked.

Bending strength: The length of the test piece was cut to 90 mm, a three-point bending jig was used, the supporting span was set to 24 times the thickness of the test piece, and n = 5 measurements were performed to obtain an average value. Example 1 2 of a copolymer having a copolymer composition of 99.0% by weight of acrylonitrile and 1.0% by weight of itaconic acid and an intrinsic viscosity of 1.7.
Ammonia gas was blown into a 0 wt% dimethylsulfoxide (hereinafter referred to as DMSO) solution to adjust the pH to 8.0 to prepare a spinning dope. The spinning solution was introduced into a water-based coagulation bath having a DMSO concentration of 60% by weight and a temperature of 60 ° C. through a 3000-hole spinneret and was taken at a speed of 10 m / min.

Then, after washing with water in a 6-stage warm water bath at 55 to 75 ° C. while maintaining a total draw ratio of 1.15 times,
It was stretched 3.91 times in a hot water bath to obtain a water-swollen fiber having a swelling degree of 200%.

Next, a gelatin having a concentration of 2% by weight as a water-soluble protein and a viscosity of 30 cP at a temperature of 20 ° C. (manufactured by Wako Pure Chemical Industries, Ltd., grade 1, 077-0315).
5%) was used to dip the water-swollen fiber while keeping the 2% by weight aqueous solution at 70 ° C., and the excess surface-adhering liquid was removed by a nip roll.
After passing through an aqueous dispersion treatment liquid of amino-modified silicone of St, it was passed through a nip roll again and dried and densified by a heating roller having a surface temperature of 150 ° C.

Then, in pressurized steam at 145 ° C.
After being stretched 89 times, it was dried and heat-treated with a heating roller having a surface temperature of 170 ° C. to wind up a precursor fiber having a single fiber fineness of 0.8 dtex.

The precursor fiber was heated to 245 ° C., then 255 ° C.
The heated air was passed through at a draw ratio of 1.0 and heated until the fiber specific gravity became 1.34 to obtain flame-resistant fibers.

Next, the obtained flame-resistant fiber was passed through a nitrogen atmosphere having a maximum temperature of 750 ° C. at a draw ratio of 1.05, and then passed in a nitrogen atmosphere having a maximum temperature of 1700 ° C. at a draw ratio of 0.97. Then, the film was passed through a nitrogen atmosphere having a maximum temperature of 2300 ° C. at a draw ratio of 1.03.

Next, a surface electrolysis treatment of 40 coulomb / g was carried out using dilute sulfuric acid as an electrolytic solution, washed with water and dried to obtain a graphitized fiber.

The resin-impregnated strand strength and elastic modulus were evaluated to be 4.65 GPa and 420 GPa, respectively. As a result of measuring ILSS and bending strength, they were as high as 72.1 MPa and 1650 MPa, respectively. (Comparative Example 1) A graphitized fiber was obtained in the same manner as in Example 1 except that no water-soluble protein was added. As a result of evaluating the resin-impregnated strand strength and elastic modulus, each is 4.60 G
Pa was 425 GPa. Moreover, as a result of measuring ILSS and bending strength, 65.1 MPa and 1430 M, respectively.
It was Pa.

[0038]

EFFECTS OF THE INVENTION According to the present invention, it is possible to stably provide a graphitized fiber having excellent adhesiveness with a resin and bending strength of a composite.

Claims (6)

[Claims] 1. A polyacrylonitrile-based precursor fiber for carbon fiber, which contains a water-soluble protein in at least the surface layer. 2. The polyacrylonitrile-based precursor fiber for carbon fiber according to claim 1, wherein the water-soluble protein is gelatin. 3. The polyacrylonitrile precursor fiber for carbon fiber according to claim 1, wherein the water-soluble protein is contained in an amount of 0.5 to 10% by weight based on the polyacrylonitrile precursor fiber. 4. A polyacrylonitrile-based precursor fiber for carbon fiber, which is characterized in that water-swelling fiber obtained by wet or dry-wet spinning of a polyacrylonitrile-based polymer is provided with a water-soluble protein and then dried and densified. Manufacturing method. 5. The water-soluble protein has a concentration of 2.0 to 1.
5. It is provided as a 0% by weight aqueous solution.
A method for producing a polyacrylonitrile-based precursor fiber for a carbon fiber as described above. 6. The water-swellable fiber has a swelling degree of 50 to 3
The method for producing a polyacrylonitrile-based precursor fiber for carbon fiber according to claim 4, wherein the content is 00% by weight.