JP2002173872A - Chopped carbon fiber having excellent form - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a chopped carbon fiber having excellent collectability and form at a low sizing ratio and useful for a fiber-reinforced composite material. SOLUTION: The chopped carbon fiber has an applied sizing agent content of 0.5-5 wt.%, a collectability of <=1.05 and a flatness of <=12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon fiber chop excellent in form. More specifically, the present invention relates to a carbon fiber chop having excellent bunching properties and morphology with a low size adherence rate for a fiber-reinforced composite material.

[0002]

2. Description of the Related Art In recent years, fiber reinforced composite materials obtained by mixing carbon fibers and various matrices have excellent properties,
That is, due to its high strength, high elastic modulus, high electrical conductivity, high wear resistance, low specific gravity, etc., it is attracting attention as an industrially important material. For example, when trying to obtain a fiber-reinforced composite material using a thermoplastic resin as a matrix, a carbon fiber is cut in advance into one to several tens of mm to form a carbon fiber chop, and a thermoplastic resin pellet or powder is extruded. The mixture is melt-kneaded into a mixed pellet as a fiber-reinforced composite material, and the mixed pellet is applied to a molding machine to obtain a target molded product.

What is important in obtaining the mixed pellets is that the carbon fiber chops are quantitatively fed into the extruder. If not, there will be uneven discharge,
A constant extrusion speed cannot be obtained, strand breaks occur, and productivity is greatly reduced. A factor that has a great effect on the quantitative supply is the biting property of the carbon fiber chop in the hopper of the extruder, which is closely related to the convergence and form of the carbon fiber chop.

When the convergence of the carbon fiber chop is low, the fiber is opened at a portion in contact with the screw at the lower part of the hopper to form a fluff clump, which hinders a quantitative supply. In addition,
When the resin pellets and the carbon fiber chop are pre-mixed before the hopper is charged, if the convergence is low, fluff is generated and the quantitative supply to the extruder is impaired. And it becomes difficult to uniformly mix the same, which also causes a problem in quantitative supply. from these things,
The convergence and form, that is, the flatness, are important indicators when evaluating carbon fiber chops.

[0005] The following method is generally used to obtain a carbon fiber chop to be used for a fiber-reinforced composite material. First, carbon fibers are immersed in a sizing agent to be impregnated, and in a subsequent drying step, a solvent such as water is removed, and the bundled carbon fibers are continuously or discontinuously subjected to a predetermined length using a roving cutter or a guillotine cutter. Cut into a carbon fiber chop.

[0006] After sizing the carbon fiber and then drying it through a dryer, it is cut to a predetermined length by a cutter. The drying step is the most time-consuming of these steps, and the processing speed is several m / min to several tens m / min.
It is. For this reason, in order to increase the processing amount, it is conceivable to increase the drying capacity and increase the processing speed by increasing the drying distance or increasing the drying temperature by increasing the number of carbon fibers passed through the dryer. However, in either case, this method requires extensive equipment or modification.

[0007] Further, since the dried carbon fibers are bundled only with the binder component of the sizing agent, chopping occurs when an impact shearing force is applied during cutting, and this drawback causes fluff without being repaired. As a result, the next step, quantitative supply to the extruder, becomes difficult. As is evident from this, the conventional method of manufacturing carbon fiber chops tends to increase the sizing agent adhesion rate in order to maintain the convergence at a favorable level. However, an increase in the sizing agent adhesion rate is not preferable because there is a concern that the final composite properties may be reduced.

[0008] The shape of the carbon fiber chop greatly affects the dry blending property of the pellets of the extruder and the carbon fiber chop. In other words, as the flatness of the carbon fiber chop decreases, the shape of the pellet approaches the shape of the pellet, so that the dry blending property improves. The roving cutter referred to in the present invention is a cutter roll having a plurality of blades mounted on a roll, a rubber anvil roll pressed against the cutter roll for cutting, and supplying a fiber bundle to a cutting portion. A nip roll pressed against an anvil roll.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a carbon fiber chop having a low sizing ratio, excellent convergence and excellent morphology for use in a fiber reinforced composite material.

[0010]

SUMMARY OF THE INVENTION The present invention provides a carbon fiber chop bundled using a sizing agent, wherein the carbon fiber chop has a sizing agent adhesion rate of 0.5 to 5% by weight and a sizing property. A carbon fiber chop characterized by having an aspect ratio of not more than 1.05 and an aspect ratio of not more than 12.

The sizing agent adhesion rate in the present invention is closely related to the characteristics of the carbon fiber chop. That is, as the sizing agent adhesion rate increases, the bulk density and apparent density of the carbon fiber chop increase, and the convergence improves. However, on the other hand, an increase in the sizing agent adhesion rate is not preferable because there is a fear that the final composite properties may be reduced. The sizing agent adhesion rate in the present invention is 0.5 to 5% by weight. If the amount is less than 0.5% by weight, the carbon fiber chop is opened to form fluff and fluff, and it is difficult to quantitatively supply the extruder into the extruder. I do. On the other hand, if the content exceeds 5% by weight, there is a concern that the composite characteristics may be deteriorated, which is not preferable.

Next, the convergence of carbon fiber chops is 1.0
5 or less, and when this value is exceeded, as in the case of the sizing agent adhesion rate, the fiber is liable to be spread and fluff is liable to be produced. Is reduced. The convergence is determined by dividing the bulk density of the carbon fiber chop after stirring the carbon fiber chop with a mixer by the bulk density before the stirring treatment. 1 is the best value, and the convergence deteriorates as the numerical value increases.
The flatness of the carbon fiber chop according to the present invention is 12 or less. The flattening rate is determined by dividing the width of the carbon fiber chop in the cross-sectional direction by the thickness.

The carbon fiber chop of the present invention is obtained by sizing the twisted carbon fiber and cutting it with a roving cutter before drying. According to this method, carbon fibers are bundled by twisting and then cut to obtain a carbon fiber chop having an excellent form.

[0014] Further, in this method, the carbon fiber is bundled by the surface tension of the sizing agent, and the shearing force at the time of cutting is absorbed in a soft state without drying, thereby preventing chop cracking and repairing the generated defects. High quality carbon fiber chops are obtained by utilizing the effect of

The number of twists applied to the carbon fiber is suitably from 3 to 50 turns / m, and preferably from 3 to 30 turns / m. If the amount is less than 3 turns / m, the effect of twisting cannot be obtained, and if it exceeds 50 turns / m, there is almost no difference in the flatness.

The sizing agent in the present invention may be obtained by emulsifying a binder component in water using an emulsifier,
Alternatively, the binder component is directly dissolved or dispersed in a solvent, and the concentration is 0.8 to 17% by weight, preferably 1.7 to 9% by weight. When the amount is less than 0.8% by weight, the effect as a binder is not exhibited, and when it exceeds 17% by weight, it becomes difficult to obtain a target sizing agent adhesion rate within a range of a pickup amount suitable for cutting.

The binder component of the sizing agent is not particularly limited, and a binder suitable for the present application from various known binders is appropriately used. For example, from the viewpoint of convergence, it is one method to use a mixed system of a high molecular weight component and a low molecular weight component as a binder component, and to find the optimum conditions by changing the ratio. Here, the high molecular weight component gives rigidity, and the low molecular weight component gives plasticity and flexibility to the carbon fiber chop. In addition, when actually selecting, the convergence of carbon fiber chops, the ability to pass through the chopping process, the gum-up properties,
It is necessary to consider the influence on the composite characteristics.

The sizing method in the present invention is as follows.
Any method may be used as long as the pick-up amount of the sizing agent can be arbitrarily controlled. This makes it possible to adjust the liquid holding amount of the carbon fiber at the time of cutting to an appropriate range.

The tension of the carbon fibers during the sizing process is set to 0.
1-2 kg / yarn, preferably 0.3-1 kg / yarn. If the tension is less than 0.1 kg / yarn, the pick-up amount of water or sizing agent is not constant, and if it exceeds 2 kg / yarn, fluff is likely to occur.

In the present invention, the carbon fiber chopping step and the subsequent steps of drying, sorting and metal removal can be operated continuously, which is a highly productive process. Attention should be paid to the bonding of the carbon fiber chops during the drying process. In addition, productivity can be enhanced by continuous drying. It is necessary to determine appropriate values for the drying temperature and the drying speed depending on the chopping speed and the type of the sizing agent. For sorting and metal removal, it is necessary to use appropriate equipment to maintain the required quality.

According to the present invention, the water spray treatment can be performed before the sizing treatment. For example, if water is directly sprayed on the bobbin during unwinding the carbon fiber, the convergence of the carbon fiber is improved, and the process passability and the processability are improved. Workability is improved. At this time, the amount of water spray is 0.1 to 10% by weight, preferably 0.3 to 5% by weight of the carbon fiber in terms of the attached amount. If this amount is less than 0.1% by weight, it is not sufficient, and if it exceeds 10% by weight, the sizing agent is not sufficiently replaced with a sizing agent during the sizing treatment.
The convergence of carbon fiber chops is reduced.

According to the present invention, it is possible to obtain short carbon fibers without attaching the sizing agent by using water instead of the sizing agent in the sizing treatment. In addition, if necessary, various treating agents can be dissolved or dispersed in water for use. The short carbon fiber thus obtained has excellent dispersibility since no sizing agent is attached, and CF
The present invention can be applied to a carbon fiber nonwoven fabric used for a substrate of RC or C / C composite. The carbon fiber in the present invention is not particularly limited, and can be arbitrarily selected from various known carbon fibers.

[0023]

Next, the present invention will be described in more detail by way of examples. Various evaluations in this example were performed as follows. (1) Bulk density of carbon fiber chop 70 g of carbon fiber chop is put into a 500 ml measuring cylinder, and the weight is divided by the volume of the carbon fiber chop after giving up and down vibration to the measuring cylinder 60 times for 1 minute. Ask. (2) Bundleability of carbon fiber chop The bulk density of the carbon fiber chop after stirring the carbon fiber chop with a mixer at 400 rpm for 3 minutes is determined by dividing the bulk density of the carbon fiber chop before the stirring treatment. 1 is the best value, and the convergence deteriorates as the numerical value increases.

(3) Flatness of carbon fiber chop The width (W, mm) of the carbon fiber chop in the cross-sectional direction divided by the thickness (D, mm) is defined as the flatness. Here, the width (W, mm) of the carbon fiber chop is an actually measured value, and the thickness (D, mm) is calculated by the following equation. Thickness (D) = (total number of filaments × single yarn diameter) / (W / single yarn diameter)

Example 1 A carbon fiber bundle composed of 12,000 carbon fibers made of polyacrylonitrile and twisted at a rate of 30 turns / m was subjected to water spray treatment so as to have an adhesion amount of 3 to 5% by weight. While the bobbin was unwound in the axial direction at a speed of 60 m / min, the tension was adjusted to 0.5 to 0.7 kg / yarn via a tension roll and a gotet roll, and sizing was performed by a touch roll method. The pickup amount of the sizing agent was adjusted to be 43 to 47% by weight. The undried carbon fiber bundle thus obtained was continuously cut to a length of 6 mm with a roving cutter to produce a carbon fiber chop.

The sizing agent used here was a mixture of a bisphenol A type epoxy resin having a molecular weight of 470 and 2,900 / aqueous emulsion, and was adjusted to a solid concentration of 8% by weight. As a result, a carbon fiber chop with low flatness and excellent dry blending properties was obtained. Table 1 shows the results. The dry blending property was evaluated by mixing an appropriate amount of a carbon fiber chop and a resin pellet, stirring, and then observing the state of dispersion of the carbon fiber chop.

Comparative Example 1 A carbon fiber was unwound in a direction perpendicular to the bobbin axis, and a carbon fiber chop was prepared in the same manner as in Example 1. As a result, a carbon fiber chop with high flatness and poor dry blendability was obtained. Table 1 shows the results.

Examples 2 and 3 Carbon fiber chops were prepared in the same manner as in Example 1 under the conditions shown in the following table. Table 1 shows the results.

[0029]

[Table 1]

[0030]

Industrial Applicability The carbon fiber chop of the present invention is a high quality carbon fiber chop which has few cracks, has a low size adhesion rate, and is excellent in convergence and form, and has an extremely high industrial value.

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Claims (1)

[Claims] 1. A carbon fiber chop bundled using a sizing agent, wherein the carbon fiber chop has a sizing agent adhesion of 0.5 to 5% by weight, a convergence of 1.05 or less, and a flatness of 12%. An excellent carbon fiber chop characterized by the following: