JPS59150116A - Production of high-strength carbon fiber - Google Patents

Abstract

PURPOSE:Preoxidized fibers resulting from treating acrylonitrile synthetic fibers so that they reach a specific density are heat-treated in 2 stages under tension in an inert atmosphere, then the temperature is further raised and heat treatment is effected to produce the titled fibers of remarkably increased tensile strength. CONSTITUTION:Acrylonitrile synthetic fibers are preoxidized in a preoxidizer so that their density becomes 1.25-1.37g/cm<3>, then the resultant preoxidized fibers are heat treated at 300-500 deg.C under tension in an inert atmosphere, subsequently at 500-800 deg.C under increased tension in an inert atmosphere, and at over 800 deg.C in an inert atmosphere to give the objective carbon fibers.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength carbon fibers by heat-treating acrylonitrile-based synthetic fibers.

Carbon fiber has high specific strength and specific modulus and is suitable for aviation.

It is widely used as a reinforcing material in various composite materials for space-related, leisure goods, and industrial materials. However,
It is still difficult to say that its strength is sufficient, and the development of high-strength carbon fiber is desired. As a conventionally developed manufacturing method, for example, Japanese Patent Application Laid-open No. 147222/1984 is known. This method has a fiber density of 1.30 to 1.42P.
/an'' flame-retardant treatment system in an inert atmosphere at 300~8
Carbonization treatment is carried out while elongating up to 25% in a temperature range of 00°C, followed by heat treatment at a temperature of 800°C or higher to complete the carbonization treatment. However, with this method, as seen in the elongation changes in Figure 1,
Since it undergoes significant physical changes at around 500°C, the structural changes during the treatment process are complex, making it difficult to suppress shrinkage relaxation.

The inventors of the present invention have focused on this point of view and have conducted extensive studies, and have divided the flame-retardant treatment system into 72 stages of heat treatment at 800°C or lower before carbonizing it by heat treatment at 800°C or higher. The present invention has been completed to produce high-strength carbon fibers by smoothly changing the structure by elongating the fibers during the process.

That is, the gist of the present invention is to provide fibers obtained by flame-retardantly treating acrylonitrile synthetic fibers to have a density of 1.25 to 1.37 P/:ca'' at a temperature of 200 to 300°C in an oxidizing atmosphere. 300-500℃ in an inert atmosphere
After heat treatment is performed under tension of , heat treatment is performed under tension in an inert atmosphere at 500 to 800°C. Further after that 8
Carbonization is completed in an inert atmosphere at 00°C or higher, and 4
00 kP/mvt” or more preferably 5 Q Oky/
The present invention provides a method for producing high-strength carbon fiber having a strength of urn' or higher.

The acrylonitrile synthetic fiber used in the present invention is polyacrylonitrile or acrylonitrile with a weight ratio of 85:i or more) IJ
and other copolymerizable monomers, such as methyl acrylate/
I/l Acrylic esters such as ethyl acrylate, or unsaturated carboxylic acids such as methacrylic acid, acrylic acid, and itaconoic acid.

Also vinyl chloride, styrene, and acrylamide.

A polymer obtained by appropriately copolymerizing diacetone acrylamide, allyl sulfonic acid, etc. is dissolved in a solvent and then made into fibers by a wet method or a dry method.

In carrying out the present invention, first, acrylonitrile-based synthetic fibers are subjected to flame-retardant treatment in an oxidizing atmosphere such as air at 200 to 300°C to give a density of 1.25 to 1,377/cwL'. At this time, the fiber density of the flame-resistant treated system is 1.2
If the ratio is less than 5 no/ro 3, the flame-retardant treatment system will soften in subsequent steps, causing problems such as thread breakage in subsequent treatment steps. On the other hand, if the density is 1.37cm3 or more, it is not possible to add a large elongation, and the carbon fiber tensile strength cannot be improved. Therefore, the fiber density of the flame-retardant treatment system used in carrying out the present invention is 1.25 to 1,377/cm.
” is necessary.

(As shown in Figure 1, the heat treatment process is divided into a stretching process and a shrinking process, with the temperature around 500°C as the boundary.
a step of heat treatment under tension in a temperature range of 0 to 500°C;
By dividing the process into heat treatment under tension in an inert atmosphere at 500 to 800°C, it is possible to control the tension while controlling the degree of reactivity in each process. Fewer intermediate structures can be formed. 30 when carrying out the present invention
The tension in the 0-500°C step and the 500-800°C step is preferably an elongation rate of 3% or more. At this time, if the elongation rate is less than 3 inches on either side, no improvement in strength can be expected.

Finally, high-strength carbon fibers can be produced by completing carbonization in an inert atmosphere at 800° C. or higher, as is generally practiced.

The present invention will be explained in more detail with reference to Examples below, and the strength was determined by the method shown below.

Epoxy resin (manufactured by Shell Chemical Co., Ltd.: Ebicoat 828)'
& Impregnated and hardened carbon fiber bundle with trial length of 200mm,
It was determined by a tensile rupture test method at a tensile speed of 5 m*/min.

Example 1 An acrylonitrile/methyl acrylate/methacrylic acid (95/4/1) copolymer was made into fibers by a wet method to obtain a multifilament with 6,000 filaments and a single tenier of 165 d. This fiber bundle is 220 to 260
Flameproofing treatment was performed by stretching in air with a temperature gradient of °C. Table 1 shows the following process conditions and the results of performance evaluation of the obtained carbon fibers.

Table 1 Numbers 3.4 and l'th7 in Table 1. 8 indicates the tensile strength of the carbon fiber obtained by the method and implementation conditions of the present invention, %1 and Arashi 6 are for the case where the conventional pre-carbonization step is only one stage, Na3. 5. 9 and 1blo to 12 indicate cases outside the conditions of the present invention.

It can be seen that the carbon fibers obtained by the method and conditions of the present invention have significantly higher tensile strength than those of the comparative examples. It is also understood that the fiber density of the flame-retardant treatment system must be within the range specified by the present invention.

[Brief explanation of drawings]

Figure 1 shows the change in fiber length when the flame-retardant treatment system used in the present invention is heat-treated, with the vertical axis representing the heat treatment temperature and the horizontal axis representing the change in fiber length when the flame-retardant treatment system is continuously heat-treated at elevated temperatures. This shows the change in length.

Claims (1)

[Claims] Acrylonitrile synthetic fiber with a density of 1.25 to 1.3
7 P/crIL" flame-resistant fibers are heat-treated under tension at 300 to 500°C in an inert atmosphere, and then heat-treated under tension at 500 to 800°C in an inert atmosphere. A method for producing high-strength carbon fibers, which further comprises performing a heat treatment in an inert atmosphere at 800° C. or higher.