EP0348599A2 - Process for producing an anisotropic pitch for carbon fibres - Google Patents

Abstract

Anisotropic pitch for generating carbon fibres is produced from blown coal tar pitch by extraction with a pitch solvent and thermal treatment of the solvent-free soluble fraction in vacuo.

Description

The invention relates to the production of a highly anisotropic intermediate for carbon fibers from coal tar pitch.

The production of carbon fibers from coal tar pitch is known per se. Due to the lower raw material costs and the higher yields, it can be expected that the fibers produced therefrom can be produced from polyacrylonitrile at lower costs than those. From isotropic pitches, however, it is not possible to produce high-strength carbon fibers such as polyacrylonitrile, which have a tensile strength of more than 2 GPa at one Elongation at break of more than 1%. Only anisotropic pitches are suitable for this. For this reason, intensive research work on the development of such pitches is carried out worldwide.

Such a pitch has liquid crystalline properties, in particular an orderly arrangement of large planar aromatic molecules while maintaining the fluidity. Such pitches are referred to as 'mesophase pitches', and in addition to the predominant anisotropic phase, an isotropic phase can also be present. The anisotropy is assessed by observing the ground pitch surface with a polarizing microscope.

In addition to anisotropy, a pitch suitable for the production of high-strength carbon fibers should have the following properties:
- The lowest possible content of solid particles to avoid defects in the fiber;
- low flow temperature to avoid polymerization during processing;
- high carbon content, coking residue and low volatile content to achieve a high yield and simplify spinning, stabilization and carbonization;
- low content of components insoluble in quinoline with high content of components insoluble in toluene (high ß-span = TI - QI).

It is obvious that a targeted refining of the multi-substance mixture pitch is necessary in order to achieve such a property profile.

Careful separation of the solid particles present in the pitch is essential and is part of the known state of the art.

It is known from the literature that the direct generation of mesophase pitches from filtered coal tar or petroleum pitches by thermal treatment only leads to moderate results:

If the treatment time is too short, a high isotropic content is present, which reduces the yield and affects the fiber strength. With further treatment, the high-molecular pitch components form infusible particles, which make spinning difficult and deteriorate the fiber quality.

Pitch fractionation with the aim of narrowing the molecular weight distribution has been described in numerous publications.

Starting from a soft pitch, in the process according to EP 0 17 29 55 A1 (1), mesophases are formed in a first thermal treatment and the insoluble pitch constituents formed are separated off by extraction with an aromatic solvent followed by filtration. The filtrate is worked up by distillation to remove the solvent and subjected to a second thermal treatment, producing a highly anisotropic pitch for carbon fiber manufacture, but which has a high pour point and a high quinoline insoluble content. The strength properties of the carbonized fiber are not sufficient.

Another way to produce spinnable mesophase pitch involves pitch hydrogenation. In the process according to DE 3 33 05 75 C2 (2), pitch with a hydrogenating agent, for. B. tetrahydroquinoline, hydrogenated and after filtration and distillative removal of the solvent for mesophase generation thermally treated under vacuum.

This measure lowers the pour point and improves the strength properties of the fiber carbonized at 1500 ° C.

A more complex process for producing spider pitch is described in EP 0 24 75 65 A1 (3):

After extraction with xylene, insolubles are filtered off from coal tar. The soluble fraction is subjected to a heat-pressure treatment, flashed to remove low molecular weight by distillation, and the heavy phase is hydrogenated. After the hydrogenation agent has been separated off by distillation, the mesophase is produced by thermal treatment.

The disadvantage of the last two methods for producing spider pitch is the high technical outlay due to the high pressures and temperatures required for pitch hydrogenation. In particular, the last process with additional heat-pressure treatment, flashing and distillation is complex and can cancel out the cost advantage provided by the cheap raw material.

The properties of the spinches and carbonized pitch fibers produced according to the selected patents are shown in Table 1: Table 1 Procedure according to (1) (2) (3) Spider pitch properties Floating point 350 330 316 optical anisotropy Vol .-% 94 100 90 Toluene insoluble % By weight 93.7 85 95.5 Quinoline insoluble % By weight 45 45 16 Carbon fiber properties tensile strenght GPa 1.3 2.76 2.67 modulus of elasticity GPa 86 196 153 Final carbonization temperature ° C 1000 1500 1000

From the prior art it can be seen that so far it has only been possible with hydrogenated pitches, carbonized pitch fibers with a tensile strength of to produce more than 2 GPa and an elongation at break of more than 1%.

The object was therefore to develop a simpler process for producing an anisotropic pitch without hydrogenation, from which fibers can be spun which, after carbonization, have a tensile strength of more than 2 GPa with an elongation at break of more than 1%.

The object is achieved in that a coal tar pitch in a temperature range of 330 to 400 ° C for 8 to 12 hours with 1 to 10 · 10⁻³ kg / kg pitch · h of an oxygen-containing gas and then extracted with a pitch solvent, and the soluble pitch fraction after the distillative removal of the solvent at a heating rate of 1 to 50 K / min under a pressure of 0.5 to 50 mbar to a temperature between 400 and 480 ° C, the final temperature being maintained for up to 50 min .

The blowing of coal tar pitch with air is known per se. It serves to increase the softening point and the coking residue. The pitch treated in this way is used, for example, as an insert for pitch coking in horizontal chamber furnaces. It has also been proposed to filter coal tar pitch, blow it with air, and then spin it (Fuel, 1981, Vol 60, pp. 848-850). Here, too, the purpose of blowing is to increase the softening point so that the pitch fiber in the others Treatment does not stick. In addition, the pitch obtained is isotropic and therefore not suitable for producing high-strength fibers.

Coal tar pitch is understood to mean the residue from the distillative processing of high-temperature coal tar, preferably a normal pitch coal pitch with a softening point (Kraemer-Sarnow) of approximately 70 ° C. All pitch solvents can be used as solvents which correspond to pyridine, quinoline or anthracene oil in their dissolving behavior.

The invention is explained in more detail using the example below:

example

In a heated retort, 500 kg of coal tar pitch with a softening point (Kraemer-Sarnow) of 70 ° C are heated to 335 ° C with stirring and blown for 10 hours with 6 · 10⁻³ kg air / kg pitch · h until the temperature reaches 393 ° C has increased. The air is forced into the liquid pitch via pipes that end just above the bottom of the test tube.

The bad luck blown off has the following material data: Floating point 160 ° C Toluene insoluble 52% by weight Quinoline insoluble 21% by weight Coking residue (Alcan) 68% by weight Ash (900 ° C) 0.2% by weight

The blown pitch is ground and 1 part by weight of pitch is dissolved in 2 parts by weight of quinoline at 180 ° C. with stirring. After about 2 hours, the undissolved components of the pitch are separated from the soluble components by sedimentation. The liquid phase is suctioned off and filtered through a sintered metal filter (pore size: 1 µm) in order to separate even the finest solid particles. The quinoline is distilled off from the pitch solution under a pressure of 200 mbar up to a bottom temperature of 300 ° C.

The remaining pitch fraction has the following material data: Floating point 170 ° C Toluene insoluble 46% by weight Quinoline insoluble less than 1% by weight Coking residue (Alcan) 65% by weight Ash (900 ° C) Rummage

The pitch fraction is heated from 250 to 440 ° C. under a pressure of 5 mbar in 60 minutes and the final temperature is maintained for 20 minutes.

The mesophase pitch produced in this way has the following properties: Floating point 320 ° C optical anisotropy more than 90 vol .-% Toluene insoluble 85% by weight Quinoline insoluble 18% by weight Coking residue (Alcan) 94% by weight Pressure filter test (1 µm) practically no infusible particles present

The mesophase pitch can be spun at 380 ° C without thread breaks occurring. The pitch fibers are stabilized in air up to a temperature of 350 ° C and then carbonized up to 1200 ° C. The carbon fibers obtained in this way are characterized by the following data: diameter 9-10 µm Tensile strength 2.4 GPa modulus of elasticity 210 GPa

The strength properties exceed those of the fibers obtained from double thermally treated pitch (EP 0 17 29 55 A1) and correspond approximately to those of the fibers produced from hydrogenated and thermally treated pitches.

The following shows the influence of blowing on pitch and fiber properties Comparative example:

Comparative example

The same initial pitch as in the example is filtered after adding filter aids at 270 ° C. The filtrate has the following properties: Softening point (Kraemer-Sarnow) 70 ° C Toluene insoluble 22% by weight Quinoline insoluble less than 0.1% by weight ash traces

The filtered pitch is treated thermally under the same conditions as in the example. In order to obtain an optical anisotropy of at least 90% by volume, the final temperature must be increased to 465 ° C and the holding time to 30 minutes. The mesophase pitch produced in this way has the following properties: Floating point 340 ° C optical anisotropy 90 vol .-% Toluene insoluble 88% by weight Quinoline insoluble 51% by weight Coking residue (Alcan) 95% by weight Filter test (1 µm) 0.5% by weight infusible particles

The mesophase pitch can only be spun at 405 ° C. Thread breaks often occur. The service life of the filter elements arranged in front of the spinnerets is short.

The carbon fibers carbonized up to 1200 ° C can be characterized by the following typical properties: diameter 9-11 µm tensile strenght 1.6 GPa modulus of elasticity 230 GPa

The tensile strength corresponds to that of double thermally treated pitches. However, the elongation at break is less than 1%.

The comparison of the analysis data clearly shows that the mesophase formation is favorably influenced by blowing with air as pretreatment of the pitch. The mesophase pitch according to the invention surprisingly has a lower pour point and a lower quinoline-insoluble content with higher anisotropy. This simplifies spinning and significantly improves the strength properties of the carbonized fiber.

Claims (4)

1. A process for producing an anisotropic pitch for the production of carbon fibers, characterized in that a coal tar pitch in a temperature range of 330 to 400 ° C for 8 to 12 hours with 1 to 10 · 10⁻³ kg / kg pitch · h of an oxygen-containing gas and then extracted with a pitch solvent, and the soluble pitch fraction after the distillative removal of the solvent is heated to a temperature between 400 and 480 ° C. at a heating rate of 1 to 50 K / min under a pressure of 0.5 to 50 mbar , the final temperature being maintained for up to 50 minutes. 2. The method according to claim 1, characterized in that the coal tar pitch is a residue from the distillative workup of a high-temperature tar with a softening point (Kraemer-Sarnow) of about 70 ° C. 3. The method according to claim 1 or 2, characterized in that pyridine, quinoline or anthracene oil is used as pitch solvent. 4. The method according to any one of claims 1 to 3, characterized in that the oxygen-containing gas is air.