JP2001271230A - Chopped strand of carbon fiber and method for producing the strand - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-quality chopped strand of carbon fiber having low adhesion rate of a sizing agent, high application uniformity of the sizing agent and excellent collectability and suitable for a fiber-reinforced composite material and a method for surely producing the chopped strand in high efficiency. SOLUTION: The chopped strand of carbon fiber has a flat cross section of the strand having a major diameter/minor diameter ratio of >=3. The strand is collected with 0.1-10.0 wt.% sizing agent and has a collectability of <=1.05 and a density of 0.004-0.400. The method for the production of the chopped strand comprises the cutting of a sized flat carbon fiber bundle before drying.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chopped carbon fiber strand suitable for use in a fiber-reinforced composite material, the carbon fiber chopped strand having a low adhering rate of a sizing agent and having excellent convergence, and its production. About the method.

[0002]

2. Description of the Related Art A fiber reinforced composite material formed by molding a mixture of carbon fiber and various matrix resins has excellent properties such as high strength, high elastic modulus, high electrical conductivity, high wear resistance, and low specific gravity. , Are attracting attention as various industrial materials.

A fiber reinforced composite material using a thermoplastic resin as a matrix resin has a carbon fiber bundle of 1 mm to several tens.
A carbon fiber chopped strand cut to 0 mm and a thermoplastic resin pellet or powder are put into an extruder, melt-kneaded and formed into a mixed pellet of carbon fiber and thermoplastic resin, and then the mixed pellet is formed. It is formed by performing predetermined molding as a material.

[0004] In forming the above mixed pellets, it is important to quantitatively supply the chopped carbon fiber strands to the extruder. If the supply of chopped carbon fiber strands is not carried out quantitatively, discharge unevenness occurs, a constant extrusion speed cannot be obtained, and strand breaks occur, which greatly reduces the productivity of mixed pellets.

A factor that has a great adverse effect on the quantitative supply of chopped carbon fiber strands is the biteability of the chopped carbon fiber strands in the hopper of the extruder, which is closely related to the convergence of the carbon fiber chopped strands used. In a relationship. That is, when the convergence of the carbon fiber chopped strand is low, the carbon fiber chopped strand is spread at the portion in contact with the screw at the lower part of the hopper of the extruder to form a ball of lump. Is inhibited.

[0006] Even when the thermoplastic resin pellets and the carbon fiber chopped strands are mixed before the hopper is charged, if the convergence of the carbon fiber chopped strands is low, the mixing step is similar to that described above. The fluff is generated in the fiber chopped strand, and the quantitative supply to the extruder cannot be performed.

Therefore, the convergence of the chopped carbon fiber strand is an important factor when molding a mixed pellet of carbon fiber and thermoplastic resin, which is a molding material of the fiber reinforced composite material.

[0008] The carbon fiber chopped strand provided for the fiber reinforced composite material is generally impregnated by immersing a carbon fiber bundle in a sizing solution, followed by removing a solvent such as water in a subsequent drying step to be bundled. Further, it is cut continuously or discontinuously to a predetermined length by a roving cutter, guillotine cutter or the like.

By the way, the carbon fiber bundle is immersed in a sizing solution for impregnation, and then dried by passing through a drier.
Among the above steps of cutting into a predetermined length with a cutter, the step that requires the longest time is the drying step, and the processing speed is several meters.
/ Min to several tens m / min.

[0010] Therefore, in order to increase the productivity of chopped carbon fiber strands, the amount of carbon fiber passed through the dryer after the sizing step is increased, or the drying distance of the dryer is increased, or It is conceivable to increase the drying ability by increasing the drying temperature. However, any of these methods requires large-scale equipment or remodeling, and is economically disadvantageous.

The dried carbon fiber bundle is bundled only with the sizing agent (binder component) used in the sizing liquid. For this reason, when the carbon fiber bundle is cut into chopped strands, an impact shearing force is applied to the carbon fiber bundle to cause chop cracking. This makes it difficult to quantitatively supply a carbon fiber to an extruder when forming a mixed pellet of a fiber chopped strand and a thermoplastic resin.

For this reason, in the conventional method for producing chopped carbon fiber strands, there is a tendency to increase the adhering rate of the sizing agent in order to maintain the convergence at a favorable level. However, increasing the adhesion rate of the sizing agent is not preferable because it leads to an increase in winding due to gum up on the roll.

In order to solve the above-mentioned problems, a method of producing a carbon fiber chopped strand has been proposed in which a carbon fiber bundle is subjected to a sizing step and then cut by a roving cutter before drying the carbon fiber bundle (Japanese Patent Application Laid-Open No. H5-105). No. 261729). This method utilizes a convergence effect of carbon fibers due to the surface tension of water or a sizing liquid to be used, and an effect of preventing a chop crack by absorbing a shocking shear force at the time of cutting in a soft state without drying. .

However, according to the above-described method, unevenness in the thickness of the carbon fiber bundle causes unevenness in the attachment of the sizing agent in the sizing step, and inferior cut length (cut mistake) in the cutting step. Furthermore, drying spots are generated in the drying step, and the purpose of obtaining carbon fiber chopped strands of good quality cannot be sufficiently achieved.

[0015]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a fiber-reinforced composite material which has a low adhering rate of a sizing agent, has no spots on the adhering sizing agent, and has excellent convergence. It is to provide a high quality carbon fiber chopped strand suitable for being provided.

Another problem to be solved by the present invention is that
An object of the present invention is to provide a method for efficiently and accurately producing a high-quality chopped carbon fiber strand having the above-mentioned properties suitable for use in a fiber-reinforced composite material.

[0017]

The above object is achieved by a carbon fiber chopped strand of the present invention and a method for producing the same as described below. That is, the carbon fiber chopped strand of the present invention is a flat carbon fiber chopped strand having a cross section of a major axis / minor axis of 3 or more, and the chopped strand is 0.1 to 1 mm.
It is bundled with a sizing agent of 0.0% by weight, has a convergence of 1.05 or less, and has a compactness determined by the following formula (1) in the range of 0.004 to 0.400. It is in. Denseness = Nr ² / R ² ... Formula (1) (where R is (major axis + minor axis) / 2 of the cross section of the carbon fiber chopped strand, and r is the carbon fiber chopped strand) The diameter of the single fiber, N, is the number of the single fibers constituting the carbon fiber chopped strand.) The convergence of the carbon fiber chopped strand is determined by the volume after stirring at 400 rpm for 3 minutes by a mixer. x ₁ / x ₀ when the density was x _1, the bulk density of the carbon fiber chopped strands prior to stirring treatment to x ₀
It is.

Further, the method for producing a chopped carbon fiber strand of the present invention is to obtain a chopped carbon fiber strand having the above-mentioned structure, and before drying the flat carbon fiber bundle after the sizing step. The method consists of cutting.

In the method of the present invention for producing a chopped carbon fiber strand, the sizing step can be performed after flattening the carbon fiber bundle.

Further, in the method for producing a carbon fiber chopped strand of the present invention having the above configuration, flattening can be performed after the sizing step of the carbon fiber bundle.

Further, in the method of manufacturing a chopped carbon fiber strand of the present invention having the above-described structure, the carbon fiber bundle can be subjected to the sizing step while widening.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION A chopped strand of a carbon fiber according to the present invention has a cross section of a major axis / minor axis of 3
The above flat carbon fiber chopped strand,
0.1 to 10.0% by weight of sizing agent,
It has a convergence of not more than 1.05 and a density of 0.004 to 0.4 determined by the following equation (1).
00 is in the range of 00. Denseness = Nr ² / R ² ... Formula (1) (where R is (major axis + minor axis) / 2 of the cross section of the carbon fiber chopped strand, and r is the carbon fiber chopped strand) And N is the number of single fibers constituting the carbon fiber chopped strand.)

If the carbon fiber chopped strand subjected to the sizing treatment has a high adhering ratio of the sizing agent, the bulk density and apparent density of the carbon fiber chopped strand increase, and the convergence is improved. However, an increase in the adhesion rate of the sizing agent not only causes an increase in winding due to gum-up on a roll, but also may cause a decrease in properties of a fiber-reinforced composite material as an end product.

On the other hand, the chopped carbon fiber strands of the present invention are bundled with 0.1 to 10.0% by weight of the sizing agent as described above, that is, the adhesion ratio of the sizing agent is 0.1 to 10%. Since it is 0% by weight, not only does the above-mentioned increase in winding due to gum-up on the rolls disappear, but also there is no cause for deteriorating the properties of the fiber-reinforced composite material as the final target product.

When the adhesion of the sizing agent is less than 0.1% by weight, the chopped carbon fiber strands are spread and fluffs or lumps are liable to be produced, making it difficult to supply the extruder quantitatively. As a result, discharge spots occur, strand breaks occur, and the productivity of mixed pellets of carbon fiber and thermoplastic resin decreases. On the other hand, if the sizing agent adhesion rate exceeds 10.0% by weight, not only is it easy to increase winding due to gum-up on a roll, but also it is obtained by molding a mixed pellet of carbon fiber and thermoplastic resin. This may cause the properties of the resulting fiber reinforced composite material to deteriorate.

[0026] The carbon fiber chopped strands of the present invention, the bulk density after stirring 400 rpm, 3 minutes by carbon fiber chopped strands mixer and x _1,
Converging displayed the bulk density of the carbon fiber chopped strands prior to stirring treatment at x ₁ / x ₀ when the x ₀ is 1.05 or less.

If the convergence of the carbon fiber chopped strands exceeds 1.05, the carbon fiber chopped strands open and fluff even if the adhesion of the sizing agent is 0.1% by weight or more. And fluff clumps are likely to occur, making it difficult to feed quantitatively into the extruder, resulting in uneven discharge and the occurrence of strand breaks, thus lowering the productivity of mixed pellets of carbon fiber and thermoplastic resin. I do.

The chopped carbon fiber strand of the present invention has a flattened cross section of the major axis / minor axis of 3 or more, and the fineness determined by the following formula (1) is 0.004 to 0.400. It is within the range. Denseness = Nr ² / R ² ... Formula (1) (where R is (major axis + minor axis) / 2 of the cross section of the carbon fiber chopped strand, and r is the carbon fiber chopped strand) And N is the number of single fibers constituting the carbon fiber chopped strand.)

If the cross section of the carbon fiber chopped strand is not flat with a major axis / minor axis of 3 or more, the adhesion unevenness of the sizing agent tends to increase. Further, when the above-mentioned denseness of the carbon fiber chopped strand is less than 0.004,
The carbon fiber chopped strands are liable to cause strand cracks, and the adhesion spots of the sizing agent are likely to increase. Furthermore, the fineness of the carbon fiber chopped strand is 0.40
If it is more than 0, the thickness of the carbon fiber chopped strand inevitably increases in the process even if the major axis / minor axis of the chopped strand cross section is 3 or more. If the cut length is increased (miscut), or if the drying rate is increased to increase the processing amount, drying tends to be insufficient, and drying spots are likely to occur.

As a result, the chopped strand of the carbon fiber of the present invention has a flattened cross section of the major axis / minor axis of 3 or more and a compactness of 0.004.
~ 0.400, more preferably 0.002 to
It is in the range of 0.380.

The chopped carbon fiber strand of the present invention having the above structure is obtained by cutting a flat carbon fiber bundle after the sizing step before drying it.
That is, it can be manufactured by cutting a flat carbon fiber bundle in a wet state before drying by a sizing process or by a water treatment after the sizing treatment.

The flattening step for flattening the carbon fiber bundle may be performed before the sizing step, after the sizing step, at the same time as the sizing step, or may be performed at a plurality of points in these steps.

The flattening step of flattening the carbon fiber bundle includes, for example, a method of widening the carbon fiber bundle by passing it through a roll, a method of widening the carbon fiber bundle by applying a carbon fiber bundle to a fixed guide, It can be performed by using a method of widening by applying air from a slit.

The amount of liquid retained at the time of cutting the carbon fiber bundle, that is, the amount of liquid retained at the time of cutting the flat carbon fiber bundle in a wet state into a carbon fiber chopped strand is determined by the amount of water or sizing liquid. The pickup amount is usually 30 to 60% by weight, preferably 40 to 50% by weight. If the pick-up amount of water or sizing liquid is less than 30% by weight, chopping of the carbon fiber chopped strands and generation of fluff will increase during cutting, and the extruder will be extruded when forming a mixed pellet of carbon fiber and thermoplastic resin. Quantitative supply cannot be provided. If the content exceeds 60% by weight, in the case of cutting with a roving cutter, carbon fiber chopped strands adhere to each roll, causing clogging of the blade and miscutting. In the case of cutting with a guillotine cutter, the carbon fiber bundle is slippery,
Miscutting is induced during cutting, and the productivity of carbon fiber chopped strands decreases.

The sizing liquid used in the sizing treatment is, for example, a solution obtained by emulsifying a sizing agent in water with an emulsifier, or a solution obtained by dissolving a sizing agent directly in a solvent and having a sizing agent concentration of 0.8 to 17%. A sizing solution of about 0.0% by weight, preferably about 1.7 to 11.0% by weight is used. The sizing solution has a sizing solution concentration of 0.
If the amount is less than 8% by weight, it is difficult to obtain the sizing effect, and if it exceeds 17.0% by weight, it becomes difficult to obtain a suitable amount of the sizing liquid picked up in the above-mentioned carbon fiber bundle cutting step.

The sizing agent used in the sizing solution is not particularly limited, and a sizing agent that is suitable for the intended use among various known sizing agents is appropriately used. For example, if you want to give strong convergence to carbon fiber chopped strands,
A mixed sizing agent of a high molecular weight component and a low molecular weight component is suitable.

By changing the ratio between the high molecular weight component and the low molecular weight component as a sizing agent, a sizing agent with optimal conditions can be obtained. That is, the high molecular weight component as a sizing agent imparts rigidity to the carbon fiber chopped strand, and the low molecular weight component imparts the carbon fiber chopped strand with adhesiveness and convergence to the carbon fiber chopped strand. In consideration of the convergence of the material, the ability to pass through the chopping process, the gum-up property, the characteristics to be provided for the fiber reinforced composite material to be molded, etc.
The sizing agent used for the sizing liquid can be specified.

The sizing treatment of the carbon fiber bundle may be performed by any method capable of arbitrarily controlling the pickup amount of the sizing liquid. By controlling the pickup amount of the sizing liquid, the sizing treatment of the carbon fiber bundle at the time of cutting is performed. The liquid holding amount can be adjusted within an appropriate range.

Further, if the tension applied to the carbon fiber bundle in the sizing step is less than 0.1 kg / yarn, it becomes difficult to keep the pick-up amount of water or sizing liquid constant, and 2 kg / y.
If it exceeds the yarn, fluff is likely to occur in the carbon fiber bundle during the sizing step. For this reason, in the sizing step, the tension applied to the carbon fiber bundle is preferably about 0.1 to 2 kg / yarn, more preferably 0.3 to 1 kg / yarn.

The step of cutting the carbon fiber bundle and the subsequent steps such as drying, sorting and metal removal can be operated continuously, thereby increasing the productivity of chopped carbon fiber strands.

In the step of drying the carbon fiber chopped strands subsequent to the step of cutting the carbon fiber bundle, the carbon fiber chopped strands are prevented from adhering to each other during the drying process. In addition, productivity can be increased by making drying continuous. Further, proper values of the drying temperature and the drying speed in the drying step can be determined depending on the chopped strand forming speed, the type of the sizing agent, and the like. The step of cutting the carbon fiber bundle and the subsequent steps of sorting and metal removal after the drying step are performed using an appropriate device in order to maintain the quality required for the carbon fiber chopped strand.

In the method of the present invention for producing chopped carbon fiber strands, water spraying can be performed before the sizing step of the carbon fiber bundle. When water is sprayed directly on the carbon fiber, the convergence of the carbon fiber bundle is improved, and the process passability and workability are improved. The amount of water spray at this time is
If the amount of water adhered is less than 0.1% by weight of the carbon fiber, there is no effect of the water spray treatment, and if it exceeds 5.0% by weight, it is difficult to sufficiently replace the sizing agent in the sizing step.
The convergence of the carbon fiber chopped strand is reduced. For this reason, the water spray treatment is carried out when the water adhesion amount is 0.1% of the carbon fiber bundle.
The content is preferably set to about 5.0% by weight, preferably 0.3 to 3.0% by weight.

In the carbon fiber chopped strand and the method for producing the same according to the present invention, there is no limitation on the type of carbon fiber of the carbon fiber bundle to be used, and carbon fiber bundles of various known carbon fibers can be used.

[0044]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The concrete constitution of the carbon fiber chopped strand of the present invention and the method for producing the same will be described below based on production examples.

Various evaluations in this example were performed as follows. (1) Bulk density of carbon fiber chopped strand 50 into which 70 g of carbon fiber chopped strand was charged
The 0ml graduated cylinder, subjected over 60 times / minute vertical vibration per minute to meter the volume v ₀ of the carbon fiber chopped strands (cc), was determined by bulk density = v _0/70.

(2) Bundling property of chopped carbon fiber strands
0 rpm, the bulk density after stirring for 3 minutes and x _1, was determined focusing the bulk density of the carbon fiber chopped strands before performing the stirring process by x ₁ / x ₀ when the x _0. When the numerical value of the convergence is 1, the convergence of the carbon fiber chopped strand is the best, and the convergence deteriorates as the numerical value increases.

Example 1 12,000 carbon fibers made from polyacrylonitrile
The carbon fiber bundle is wound at a speed of 60 m / min while water spraying is performed on the carbon fiber bundle composed of zero fibers so that the water adhesion amount is 2 to 3% by weight, and the tension roll and the godet roll are separated from each other. The tension applied to the carbon fiber bundle is adjusted to 0.7 to 1.0 kg / thread so that the tow width immediately before the sizing tank becomes 2 to 3 mm through a total of 7 rolls,
After flattening the carbon fiber bundle, it was subsequently subjected to a sizing process using a touch roll method.

In the sizing step, a sizing solution composed of a 7.4% by weight aqueous emulsion of a mixed sizing agent of a bisphenol A type epoxy resin having a molecular weight of 470 and a bisphenol A type epoxy resin having a molecular weight of 2900 was used. The number of rotations of the roll was adjusted so that the pickup amount was 33 to 37% by weight of the carbon fiber bundle.

Next, the undried carbon fiber bundle is continuously cut to a length of 6 mm with a roving cutter, and further dried continuously using a vibrating feed hot air drier to obtain a carbon fiber chopped strand. I got

The presence or absence of miscuts in the step of cutting into chopped strands during the production process of the carbon fiber chopped strands, the adhesion rate of the obtained carbon fiber chopped strands to the sizing agent (to the carbon fiber bundle), the chopped strands The major axis / minor axis, compactness, and convergence of the cross section are shown in Table 1 together.

Comparative Example 1 A carbon fiber bundle composed of 12,000 carbon fibers made of the same polyacrylonitrile as used in Example 1 was mixed with a bisphenol A type epoxy resin having a molecular weight of 470 and a bisphenol A type epoxy having a molecular weight of 2900. After immersing in a sizing solution composed of a 7.0% by weight aqueous emulsion of a sizing agent mixed with a resin and impregnating the sizing solution with a pickup amount of the sizing solution of 82 to 86% by weight and sizing, a hot air drier is used. 2.6m / mi processing speed
n and dried.

After that, using a guillotine cutter, 6
It was cut at a cutting speed of 1.6 m / min into a mm length to obtain a carbon fiber chopped strand for comparison.

The presence or absence of miscut in the step of cutting into chopped strands during the production process of the carbon fiber chopped strands, the adhesion rate of the obtained carbon fiber chopped strands to the sizing agent (to the carbon fiber bundle), the chopped strands The major axis / minor axis, compactness, and convergence of the cross section are shown in Table 1 together.

COMPARATIVE EXAMPLE 2 Water spraying was carried out on a carbon fiber bundle composed of 12,000 carbon fibers made of the same polyacrylonitrile as used in Example 1 so that the amount of adhered water was 2 to 3% by weight. While performing the treatment, the carbon fiber bundle was unwound at a speed of 60 m / min, and the tension applied to the carbon fiber bundle through a total of three rolls of a tension roll and a godet roll was reduced to 0.2.
After adjusting to about 0.5 kg / yarn and flattening the carbon fiber bundle, a sizing treatment by a touch roll method was subsequently performed.

In the sizing step, a sizing solution composed of a 7.4% by weight aqueous emulsion of a mixed sizing agent of a bisphenol A type epoxy resin having a molecular weight of 470 and a bisphenol A type epoxy resin having a molecular weight of 2900 was used. The number of rotations of the roll was adjusted so that the pickup amount was 53 to 57% by weight of the carbon fiber bundle.

Next, the carbon fiber bundle in the undried state was continuously cut to a length of 6 mm with a roving cutter to obtain a carbon fiber chopped strand for comparison.
In the cutting process of the carbon fiber bundle, occurrence of miscut was observed.

Example 2 Water spray was applied to a carbon fiber bundle composed of 12,000 carbon fibers made of the same polyacrylonitrile as used in Example 1 so that the amount of water adhered was 2-3% by weight. While performing the treatment, the carbon fiber bundle was unwound at a speed of 60 m / min, and passed through a total of seven rolls, a tension roll and a godet roll, so that the tow width immediately before the sizing tank was 4 mm.
The tension applied to the carbon fiber bundle is adjusted to
After adjusting to kg / yarn and flattening the carbon fiber bundle, it was subsequently subjected to a sizing step by a touch roll method.

In the sizing step, the molecular weight is 470.
A sizing solution consisting of an aqueous emulsion of 8.1% by weight of a sizing agent of a mixed system of a bisphenol A type epoxy resin having a molecular weight of 2900 and a bisphenol A type epoxy resin having a molecular weight of 2900 is used. The rotation number of the roll was adjusted to be 47% by weight.

Next, the undried carbon fiber bundle was continuously cut into a length of 6 mm with a roving cutter, and further subjected to the same drying step as in Example 1 to obtain a carbon fiber chopped strand.

Example 3 Water spraying was performed on a carbon fiber bundle consisting of 12,000 carbon fibers made of the same polyacrylonitrile as used in Example 1 so that the amount of water adhered was 2 to 3% by weight. While performing the treatment, the carbon fiber bundle was unwound at a speed of 60 m / min, and passed through a total of nine rolls, a tension roll and a godet roll, so that the tow width immediately before the sizing tank was 6 mm.
The tension applied to the carbon fiber bundle is set to 3-4
After adjusting to kg / yarn and flattening the carbon fiber bundle, it was subsequently subjected to sizing by a touch roll method.

In the sizing step, a sizing solution composed of a 9.8% by weight aqueous emulsion of a mixed sizing agent of a bisphenol A type epoxy resin having a molecular weight of 470 and a bisphenol A type epoxy resin having a molecular weight of 2900 was used. The number of rotations of the roll was adjusted so that the pickup amount was 43 to 47% by weight of the carbon fiber bundle.

Next, the undried carbon fiber bundle was continuously cut to a length of 6 mm with a roving cutter, and further subjected to the same drying step as in Example 1 to obtain a carbon fiber chopped strand. .

Further, by extruding a mixed pellet of the carbon fiber chopped strand and the (specific name) thermoplastic resin obtained in each of the above Examples and Comparative Examples, the hopper of the extruder during the molding process is extruded. Of the chopped strands of carbon fiber in the neck was observed. The results are also shown in [Table 1].

[0064]

[Table 1]

[0065]

The carbon fiber chopped strand of the present invention has a low adhering rate of the sizing agent, so that the carbon fiber chopped strand does not have an increase in winding due to gum up on the roll in the step of obtaining the carbon fiber chopped strand, and has a fiber reinforced composite. When used in a material, the properties of the composite material are not reduced.

The carbon fiber chopped strand of the present invention has no sticking of the sizing agent and has excellent convergence. Therefore, the carbon fiber chopped strand as the molding material of the fiber reinforced composite material is made of a thermoplastic resin. Quantitative supply to the extruder when molding the mixed pellets can be accurately performed.

Further, the method for producing a chopped carbon fiber strand of the present invention employs a step of cutting a flat carbon fiber bundle in an undried state, so that the occurrence of chop cracks in this cutting step is small. Moreover, there is no generation of spots of the sizing agent. For this reason, it is possible to produce a carbon fiber chopped strand having the above-mentioned various characteristics without causing miscut and at a high speed.

Claims (5)

[Claims] 1. A flat carbon fiber chopped strand having a cross section of a major axis / minor axis of 3 or more, wherein the chopped strand is 0.1 to 10.0% by weight.
Characterized by having a convergence of not more than 1.05 and a compactness determined by the following formula (1) being from 0.004 to 0.400. Fiber chopped strand. Denseness = Nr ² / R ² ... Formula (1) (where R is (major axis + minor axis) / 2 of the cross section of the carbon fiber chopped strand, and r is the carbon fiber chopped strand) And N is the number of single fibers constituting the carbon fiber chopped strand.) 2. A method for producing a chopped carbon fiber strand according to claim 1, wherein the flattened carbon fiber bundle after the sizing step is cut before drying. . 3. The method for producing a carbon fiber chopped strand according to claim 2, wherein a sizing step is performed after flattening the carbon fiber bundle. 4. The method for producing a chopped carbon fiber strand according to claim 2, wherein flattening is performed after the sizing step of the carbon fiber bundle. 5. The method for producing a chopped carbon fiber strand according to claim 2, wherein the sizing step is performed while widening the carbon fiber bundle.