JPS62268821A - Production of carbon fiber - Google Patents

Abstract

PURPOSE:To obtain high-strength and high-modulus carbon fibers useful as fiber-reinforced composite materials, etc., with good productivity, by spinning raw material pitch, infusibilizing the resultant fibers and repeatedly heat-treating the infusibilized fibers plural times while successively increasing the heating temperature in a carbonization step. CONSTITUTION:Pitch raw material is spun and then infusibilized. The resultant infusibilized fibers are then repeatedly subjected to heat treatment in which the fibers are heated and cooled plural times (preferably twice) while successively increasing the heating temperature of each heat treatment in a carbonization step to afford the aimed carbon fibers. The heating temperature in the first heat treatment is preferably 300-600 deg.C and that of the final heat treatment is preferably 900-1,500 deg.C. The cooling temperature is preferably 100 deg.C lower than that in the previous heat-treating temperature. The heat treatment in the carbonization step can be carried out while applying tension to fibers to further improve the strength and elastic modulus.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing carbon fibers, particularly pitch-based carbon fibers, used in fiber-reinforced composite materials and the like.

[Prior art] Carbon fiber is lightweight and has excellent properties such as wear resistance, so composite materials of carbon fiber and synthetic resin or metal are attracting attention as structural and functional materials for automobiles, aircraft, etc. .

Currently, polyacrylonitrile (PAN) is used as carbon fiber.
The mainstream is PAN-based products made from
Carbon fibers with an elasticity of 0 Kg/u" or more and an elastic modulus of 20 t/mm" or more have been obtained. However, since PAN, which is necessary for PAN-based carbon fiber materials, is expensive, it is difficult to significantly reduce the price.

On the other hand, pitch-based carbon fibers made from coal or petroleum pitch are advantageous in that the raw materials are inexpensive. However, commercially available fibers have a tensile strength of 60 to 8.
0 Kff/mm" and an elastic modulus of about 2 to 3↑/mm", and it is desired to improve its performance.

Regarding pitch-based carbon fibers, various attempts have been made to obtain high-strength products. For example, a method of subjecting specific condensed polycyclic aromatics to heat treatment etc. to use them as pitch raw materials (Japanese Patent Publication No. 49-
8634), a method for modifying pitch raw materials by treating them with a Lewis acid (Japanese Patent Publication No. 7533-1983), a method for modifying pitch raw materials by extracting them with a solvent (Japanese Patent Publication No. 130809-1987)
, a method of modifying pitch raw materials by hydrogenating them (Japanese Unexamined Patent Publication No. 1983)
-113289) etc. have been proposed. However, the reality is that even with these methods, it is not possible to obtain carbon fibers with characteristics similar to those of PAN-based carbon fibers on an industrial basis.

[Problems to be Solved by the Present Invention] The present invention was made in view of the above-mentioned circumstances, and aims to improve the tensile strength and elastic modulus of pitch-based carbon fibers, and to improve the pitch-based carbon fibers with high strength and high elastic modulus. The purpose of this invention is to provide a method for producing carbon fibers with good productivity, thereby solving the problems of the conventional methods.

[Means for Solving the Problems] The production of pitch-based carbon fibers generally involves a raw material conditioning process in which pitch raw materials are heat-treated at 350°C to 450″C1 for several hours to several tens of hours in a nitrogen atmosphere, and the adjusted raw materials are heated to several tens of hours. The spinning process involves spinning and winding the resulting fibers in the air for 250 min.
An infusibility step of holding at a temperature of ℃ to 350 ℃ for several hours to make it infusible;
The infusible fibers are heated at 900°C to 150°C in an inert gas.
It is manufactured by a carbonization process in which carbonization is performed by heat treatment at 0° C. for several tens of minutes to several hours. Further, after the carbonization step, a graphitization treatment is further performed if necessary.

As a result of various experiments and research, the inventors found that one of the major reasons why high strength products cannot be obtained with conventional pitch-based carbon fibers is that the side chains and their oxides in the structure are removed during the carbonization process. It was found that this was due to withdrawal from the group.

In other words, in the conventional carbonization treatment in which the fiber is immediately heated to about 1000°C after the infusibility treatment, the side chains, co, and carbon in the fiber are
O2 etc. suddenly fly out, causing large defects and distortions in the fibers, which continue to polymerize with these defects remaining, and when cooled to room temperature, it is recognized that the pores and distortions reduce the strength of the product. Ta.

The free energy of formation that determines side chain transitions is generally larger for side chain elimination than for side chain cyclization.

However, in conventional carbonization treatment, the elimination of side chains occurs in 1
It was recognized that excessive energy was given to carry out carbonization by heating to around 000° C. at once, and as a result, side chains were released one after another, exceeding the order of free energy of formation.

The present invention has been made based on the above findings, and is characterized in that in the carbonization step, heat treatment of heating and cooling the fibers is repeated multiple times, and in this case, the heating temperature of each heat treatment is increased sequentially. Two heat treatments are practically sufficient. Although it is recognized that the properties of the carbon fiber obtained by increasing the number of times to three or more are improved, the effect is not significant. The heating temperature in the heat treatment was 300°C for the first treatment.
It is desirable that the final treatment is 900°C to 1500°C. The cooling temperature is set to be 100° C. or more lower than the heating temperature of the preceding heat treatment.

In the present invention, the carbonization treatment can be performed with the fibers under tension. In this case, the strength is improved as the number of heat treatments increases, but in practice it only takes a few times.

[Actions and Effects] In the carbonization process, by heating the first heat treatment to about 300°C to 600°C instead of heating to about 1000°C as in the conventional method, the side chains in the fiber structure are free from rings. oxidation reaction or crosslinking reaction occurs preferentially, and elimination of side chains is prevented. Subsequent cooling causes the molecules of the fibers to be misaligned with each other, creating microscopic paths in the tissue. When the next high-temperature carbonization treatment is performed in this state, residual volatile components such as side chains and CO1CO2 pass through this path and fly out more easily than in conventional carbonization treatment, leaving only minor defects. In addition, polymerization progresses with little residual strain, and high strength is achieved.

In addition, in the present invention, the amount of oxygen taken into the fibers in the infusibility process can be reduced compared to the conventional method.The incorporation of oxygen in the infusibility process is done to prevent the fibers from melting in the subsequent carbonization process. be. Therefore, a sufficient amount of uptake is required to prevent melting. However, on the other hand, since oxygen atoms are released from the fiber as CO2 etc. in the next carbonization step, the smaller the amount of oxygen atoms, the higher the strength of the carbon fiber can be obtained. Under such circumstances, in the carbonization treatment of the present invention, self-melting due to excessive reaction heat caused by conventional unilateral temperature rise is alleviated, so it is possible to reduce the amount of oxygen required to be taken in in the infusibility step. can. Note that the amount of oxygen taken in in the infusibility step can be easily reduced by lowering the treatment temperature or shortening the treatment time.

Furthermore, in the present invention, the carbonization treatment can be performed while the fibers are under tension.

One of the reasons why PAN-based carbon fibers can achieve high strength is
Because the molecular weight of the raw material itself is high, with a molecular weight of tens of thousands or more, the raw fiber after spinning has a certain degree of strength, making it infusible,
One example is that the carbonization process is carried out under stress. The stretching action caused by tension orients the constituent molecules in the longitudinal direction of the fibers, improving tensile strength and elastic modulus. On the other hand, pitch-based raw fibers are extremely fragile and cannot be drawn because they are hydrocarbon molecules with a small molecular weight of several hundred to several thousand.

However, in the present invention, since a certain degree of fiber strength is imparted during the first heat treatment in the carbonization step, the subsequent heat treatment can be performed while applying tension to the fibers and stretching them. The fiber strength is improved by heating the fiber to about 400° C. or higher in the above-mentioned heat treatment. The tension is preferably as high as possible, depending on the fiber strength at that stage, without breaking the fibers. By applying tension to the fibers during the carbonization process, both strength and elastic modulus are further improved.

[Example 1] Petroleum pitch was heat-treated to obtain a pitch raw material with a softening point of 260°C. This was melt-spun using a conventional method to obtain a fibril having a fiber diameter of 10 μm, and was further treated in air at 30 O'C for 13 hours to make it infusible with an oxygen uptake of 10% by weight.

Subsequently, this infusible fiber was subjected to a two-step carbonization treatment. In the pre-treatment, multiple samples were each treated in a nitrogen atmosphere at temperatures ranging from 300'C to 900°C.
The temperature was increased by 00°C for 1 hour, and then cooled to room temperature. The samples pre-treated at each of the above temperatures were heated for 10 minutes at temperatures ranging from 900°C to 1300'C in a nitrogen atmosphere.
After holding the temperature for 1 hour at a temperature that was increased by 0° C., a subsequent treatment of cooling to room temperature was performed. Temperature increase/decrease rate is all 3°C7m1
n was set constant. The tensile strength and elastic modulus of each carbon fiber obtained were measured.

On the other hand, for comparison, the infusible fibers obtained in the same manner as above were subjected to one-step carbonization treatment in the range of 900 °C to 1300 °C by the conventional method, and the tensile strength and elastic modulus were measured in the same manner as above. .

The above results are shown in FIGS. 1 and 2. The figure shows only a typical example in which the pre-treatment was carried out at 300'C, 1400°C and 1600°C. Further, in the figure, O1△ and X marks are the results of the carbonization treatment performed in two stages according to the present invention, and * marks are the results of the conventional method.

As can be seen from the figure, the carbon fiber obtained by the present invention has both tensile strength and elastic modulus superior to those obtained by the conventional method. Those in which the first stage treatment was performed at 400'C and the second stage treatment at 1000°C to 1100°C are particularly excellent, with a tensile strength of 10ONg/mm2 to 11O
Ng/mm", elastic modulus 7.5t/mm2~8t/mm
Indicates the value of 2. Furthermore, the present invention improved the carbonization yield by 3%.

Incidentally, when the first stage treatment was carried out at 700° C. to 900° C., depending on the temperature of the second stage treatment, either a slight improvement in properties was observed compared to the conventional method, or no significant effect appeared.

[Example 2] In the same manner as in Example 1, 1 q of fibrils were heated in air at 250°C.
The mixture was treated for 12 hours, and an infusible treatment was carried out at an oxygen uptake of 8% by weight. Subsequently, the infusible fibers were subjected to a two-step carbonization treatment in a nitrogen atmosphere, including heating at 400°C for 11 hours and cooling to seedling temperature, followed by rapid treatment at 1000°C for 1 hour. The obtained carbon fiber has a tensile strength of 125 Nff/
mm2, and the elastic modulus was 10t/mm''.

On the other hand, the fibers treated to be infusible in the same manner as above were subjected to carbonization treatment at 1000° C. for 1 hour in a nitrogen atmosphere using a conventional method. The obtained material was partially melted or fused and did not take the form of fibers.

From this result, it was confirmed that the present invention can reduce the amount of oxygen taken in during the infusibility treatment.

[Example 3] Infusible fibers obtained in the same manner as in Example 2 were subjected to a first stage treatment of heating at 400° C. for 1 hour in a nitrogen atmosphere and cooling to room temperature. Subsequently, the fiber was subjected to a second step of heating at 600° C. for 1 hour in a nitrogen atmosphere while being stretched under a load of 15 mg/d, and then cooled to room temperature. Next, add 30IIt to the fiber.
A third stage treatment was performed at 1000° C. for 1 hour in a stretched state with a load of g/d applied. The carbon fiber obtained in this way through multi-stage carbonization treatment under tension has a tensile strength of 180 to 9.
/mm2 The elastic modulus was 20t/mm2.

On the other hand, carbon fiber obtained by performing the same multi-stage carbonization treatment as above without applying any load has a tensile strength of 130K.
g/mm2, and the elastic modulus was 12t/1m2.

Even when carrying out the multi-stage carbonization treatment according to the present invention as described above, if this is carried out under tension of the fibers, both the strength and the elastic modulus are further improved.

[Brief explanation of the drawing]

The figures show experimental results regarding the processing conditions of the present invention and the conventional method and the properties of the obtained carbon fibers, with Figure 1 relating to tensile strength and Figure 2 relating to elastic modulus.

Claims (3)

[Claims] (1) Includes a spinning process to spin raw material pitch, an infusible process to make the spun fibers infusible, and a carbonization process to carbonize the infusible fibers, and in the carbonization process, the fibers are heated and cooled. 1. A method for producing carbon fibers, which comprises repeating heat treatment a plurality of times and increasing the heating temperature of each heat treatment in sequence. (2) The method for producing carbon fibers according to claim 1, wherein the heat treatment in the carbonization step is carried out in a state where the fibers are stretched. (3) The carbon fiber according to claim 1, wherein the heating temperature of the first heat treatment in the carbonization step is 300°C to 600°C, and the heating temperature of the final heat treatment is 900°C to 1500°C. Production method.