GB2156378A

GB2156378A - Pitch materials

Info

Publication number: GB2156378A
Application number: GB08507267A
Authority: GB
Inventors: Masaaki Itoi; Masakatsu Ohta; Taizo Sugioka; Kunio Yoshihara; Hiroshi Nishitani
Original assignee: Idemitsu Kosan Co Ltd
Current assignee: Idemitsu Kosan Co Ltd
Priority date: 1984-03-26
Filing date: 1985-03-20
Publication date: 1985-10-09
Also published as: DE3509861C2; DE3509861A1; GB2156378B; GB8507267D0

Abstract

A pitch material useful for the manufacture of carbon fibres has an optically anisotropic phase content of at least 80% by weight, a pyridine-insoluble matter content of from 30 to 70% by weight, a number average molecular weight of from 1000 to 1400 and a softening point of from 330 to 380 DEG C. This pitch material may be obtained by a two step heat treatment performed on a starting pitch comprising a petroleum-based residual oil freed from light oily matter. The first step is performed at 400 to 460 DEG C under a pressure of 5 to 50 mmHg and the second step is performed at 450 to 500 DEG C for 0.2 to 30 minutes under a pressure of 0.1 to 5 mmHg. The heat treatment is preferably performed in a film evaporator.

Description

SPECIFICATION Pitch materials The invention relates to a pitch material useful for the manufacture of carbonaceous bodies, particularly carbon fibres, and to a method for the preparation of such a pitch material.

The invention provides a pitch material having an optically anisotropic phase content of at least 80% by weight, a pyridine-insoluble matter content of from 30 to 70% by weight, a number average molecular weight of from 1000 to 1400 and a softening point of from 330 to 380"C.

The pitch material of the invention must have all the above characteristics (optically anisotropic phase content, pyridine-insoluble matter content, number average molecular weight and softening point). A pitch material having a pyridine-insoluble matter content of above 70% by weight and/or a number average molecular weight above 1400 and/ or a softening point above 380"C may have a disadvantageously poor spinnability. A pitch material having less than 80" by weight of the optically anisotropic (or meso) phase has poor miscibility between the meso and isotropic phases. Difficulties are then encountered in the spinning of the pitch material to form filaments, and any carbon fibres prepared from the filaments may have disadvantageously low mechanical strengths.A pitch material having a pyridine-insoluble matter content of less than 30% by weight and/or a number average molecular weight of less than 1000 may have too low a softening point; with a softening point of less than 330 C, the infusibilization treatment of filaments spun from the pitch material may take an excessively long time.

Preferably a pitch material according to the invention has an optically anisotropic phase content as close as possible to 100% by weight, a pyridineinsoluble matter content of from 40 to 60% by weight, a number average molecular weight of from 1000 to 1300 and a softening point of from 330 to 370 C.

The softening point of the pitch material according to the invention is relatively high as a consequence of the relatively large number average molecular weight of 1000 to 1400, but the pitch material is spinnable at a temperature higher than the softening point by 10 to 600C. Therefore, pitch filaments can be obtained without the danger of coking or thermal decomposition and, in addition, the infusibilization treatment thereof can be complete within a relatively short time.

The spinnability of the pitch material according to the invention into pitch filaments is excellent because the spinning can be performed at a temperature not excessively high in comparison with the softening point of the pitch or, in other words, at a temperature sufficiently lower than the decomposition temperature of the pitch material. In addition, the infusibilization treatment of the pitch filaments is complete within a remarkably shortened time in comparison with those prepared from conventional pitch materials.

Several methods are applicable to the preparation of the pitch material of the invention. It is preferable, however, to prepare the pitch material by heating a starting pitch material, obtained by removing the light oil matter from a petroleum based residual oil, at a temperature of from 400 to 460"C under a pressure of from 5 to 50 mmHg, and heating the resultant pitch material at a temperature of from 450 to 500"C for from 0.2 to 30 minutes under a pressure of from 0.1 to 5 mmHg. This method is within the scope of the invention.

The starting pitch may be obtained from a petroleum-based residual oil, preferably one having a high content of aromatic hydrocarbons, for example the residual oils from the catalytic cracking of petroleum fractions or thermal cracking of naphtha. The petroleum-based residual oil is distilled under reduced pressure to remove the light oily matter having a boiling point of about 400"C or below. It is preferable that the distillation is preceded by the removal of the ash in the petroleum-based residual oil by filtration or other suitable method.

The first heat treatment is preferably performed at a temperature of from 410 to 450"C and at a pressure of from 10 to 50 mmHg. The period for which the heat treatment is performed is usually from 0.1 to 20 hours and preferably from 0.2 to 10 hours. If the temperature in the first heat treatment is less than 400" C, the reaction velocity is unduly low so that the heat treatment must be performed prolongedly. If the temperature exceeds 460"C, an increased amount of volatile material is removed by vaporization with a consequent decrease in the yield of the product. Excess temperature may also cause coking.If the pressure in the first heat treatment is less than 5 mmHg, the yield of the product is decreased as a result of the increase in the amount of the volatile materials lost by vaporization whereas, if the pressure is greater than 50 mmHg, the molecular weight distribution of the product is broadened as a result of the insufficient removal of the light fractions.

The first heat treatment yields an isotropic pitch containing no meso phase or very little thereof.

The second heat treatment is more severe, preferred conditions being a temperature of from 460 to 500"C, a pressure of from 0.5 to 3 mmHg and a period of from 0.2 to 30 minutes. If the temperature in this second heat treatment is less than 450"C, removal of the lighter matter proceeds less efficiently so that the length of time for the heat treatment must be extended. A temperature higher than 500"C is undesirable because of the decrease in the yield, the possibility of coking, and the difficulty in the control of the reaction velocity. If the second heat treatment is performed under a pressure lower than 0.1 mmHg, the yield of the product pitch is decreased and an elaborate or large-scale vacuum equipment is required.A pressure higher than 5 mmHg is undesirable, on the other hand, due to the insufficient removal of the lighter matter with a consequently broadened molecular weight distribution. When increase of the softening point of the pitch product is desired in the second heat treatment, the effective means therefor is to de crease the pressure or to extend the length of time for the treatment.

The use of a conventional reaction vessel equipped with a stirrer has disadvantages in that the staying time of the treated material in the reaction zone has a broad distribution function and an undesirable phenomenon of coking is sometimes induced due to the low volume efficiency. The use of a tubular reactor, on the other hand, is disadvantageous due to the inhomogeneity of the reaction product as a result of the large temperature distribution and concentration distribution in the radial direction of the reactor. The most suitable reactor has been found to be a film evaporator, which can be used for either or both of the heat treatments. The film evaporator may be any of the various types of apparatuses used in conventional vaporization treatments with no particular limitations.For example, the apparatus may be of a vertical or a horizontal type or may be of a type in which scraper blades rotate in contact with the vessel walls. Centrifugal film evaporators are particularly suitable for the purpose. Film evaporators are effective in accelerating the evaporation and dissipation of the volatile matter from the surface of the liquid film which is under continuous refreshment by the rotating scraper blades. The number of the rotating scraper blades is usually from 2 to 16 and the velocity of revolution is usually about 10 to 500 r.p.m. although it should be determined in consideration of various factors.

When a film evaporator is used in either heat treatment, the heat treatment is preferably performed in an atmosphere of an inert gas or a nonoxidizing gas in order to prevent degradation of the pitch product by oxidation. The inert gas here implied includes neon, helium, argon, nitrogen and the like while methane, ethane and the like are named as the non-oxidizing gas.

Although the thickness of the liquid film in the film evaporator is controllable by adjusting the clearance between the vessel wall and the periphery of the rotating blades, it is usually preferable for high efficiency to keep the clearance below 10 mm or, more preferably, from 0.5 to 5 mm. When there is no clearance between the vessel wall and the periphery of the rotating blades, the liquid film on the vessel wall is completely scraped off by each of the rotating blades in contact with the vessel walls under the centrifugal force or by use of springs.The staying time of the pitch material under the treatment in the film evaporator is controllable by suitably selecting the conditions such as the thickness of the liquid film, form of the rotating blades and velocity of revolution of the blades and should be in the range from 0.1 to 60 minutes depending on the nature of the starting pitch material and the conditions of the reaction.

The pitch material according to the invention can be processed into pitch-based carbon fibres according to a conventional process including spinning of the pitch into pitch filaments and infusibilization treatment of the pitch filamaents followed by calcination.

Preferable conditions in each of these steps are subject to a wide variation depending on various factors. For example, the infusibilization treatment of the pitch filaments may be performed by heating the pitch filament, preferably in an oxygen-containing atmosphere, to such an extent that the oxygen content in the infusibilized pitch filaments is in the range from 5 to 12% by weight and this infusibilization treatment may be followed by the calcination treatment in an atmosphere of an inert gas at a temperature of 1000"C or higher.The pitch filaments, which may have a diameter of 5 to 15 Fm, may be obtained by spinning the pitch material according to the invention at a temperature of from 340 to 430"C or, preferably, from 340 to 4000C at a spinning velocity of from 50 to 2000 meters/ minute or, preferably, from 100 to 1000 meters/ minute.

In greater detail, the infusibilization treatment of the pitch filaments may be performed in an oxygen-containing atmosphere or, preferably, in air with a rate of temperature elevation of 5 to 1000C per minute in the temperature range from 150 to 250"C to 350 to 450"C. More preferably, the temperature is increased at a rate of 5 to 40"C per minute in the range from 150 to 250"C to 270 to 3000C and then at a rate of 20 to 1000C per minute up to the maximum temperature of 350 to 450"C. An excessively large rate of temperature elevation of 40"C per minute or larger at a relatively low temperature of 270 to 300"C or below may cause a danger of melt-adhesion of the pitch filaments.The rate of temperature elevation can be increased when the temperature has exceeded 270 to 300"C due to the disappearance of the danger of meltadhesion by virtue of the already infusibilized surface layer of the pitch filaments in order to reduce the overall time for the infusibilization treatment by accelerating the infusibilization with an increased diffusion velocity of oxygen into the filaments. The calcination treatment to follow is performed at a temperature of 1000"C or higher in an atmosphere of an inert gas such as nitrogen or argon. A preferable temperature for the calcination treatment is from 1000 to 15000C when carbonization is desired and from 2000 to 2500"C when graphitization is desired.

Despite the relatively high softening point as a consequence of the narrow distribution of the molecular weight, the pitch material according to the invention has a very good spinnability and, differently from conventional pitch materials for carbon fibres, can be spun into pitch filaments at a temperature higher than the softening point by only 10 to 60"C without the danger of undesirable coking or thermal decomposition. The relatively high softening point of the pitch provides an additional advantage that the infusibilization treatment of the pitch filaments can be complete within a greatly decreased length of time in comparison with conventional pitch materials. Furthermore, the carbon fibre product is of a very excellent quality with high mechanical properties such as tensile strength. The pitch material of the invention is useful as a base material in the manufacture of various kinds of carbonaceous products including not only carbon fibres but also films, filaments and yarns.

The following Examples illustrate the invention.

Example 1 A residual oil obtained from the catalytic cracking of heavy gas oil was freed from ash content by filtration. It was then distilled under reduced pressure to remove the lighter oily matter and to leave a residual oil having a boiling point of 430"C or higher under normal pressure as extrapolated from the actually determined value. This starting pitch material was subjected to heat treatment at a temperature of 420"C for 30 minutes under a pressure of 10 mmHg to give an isotropic pitch containing about 40% by weight of a toluene-insoluble matter (specified in JIS K-2425).Subsequently, this isotropic pitch was subjected to further heat treatment at a temperature of 460"C for 20 minutes under a pressure of 1 mmHg to give a finished pitch material composed substantially of the optically anisotropic phase alone. The thus obtained pitch had a number average molecular weight of 1130, a pyridine-insoluble matter content of 63.3% by weight and a softening point of 345"C.

This pitch material was subjected to spinning at a spinning temperature of 368"C at a spinning velocity of 500 meters/minute to give pitch filaments having a diameter of 7 Fm followed by the infusibilization treatment at a rate of temperature elevation of 20 C/minute from 200 to 400"C taking 10 minutes. Thereafter, the infusibilized pitch filaments were calcined at 1500"C for 10 minutes to give carbon fibres having a tensile strength of 253 kg/mm2.

Example 2 The conditions for the two-step heat treatment of the starting pitch material were substantially the same as in Example 1 excepting the reduction of the time for the second step heat treatment to 15 minutes instead of 20 minutes. The thus obtained pitch material was composed of the optically anisotropic phase alone and had a number of average molecular weight of 10800, a pyridine-insoluble matter content of 42.4% by weight and a softening point of 335 C.

The pitch material was then subjected to the process of spinning, infusibilization and calcination to give carbon fibre having a tensile strength of 250 kg/mm2.

Example 3 A residual oil obtained from the catalytic cracking of heavy gas oil was freed from ash content by filtration. It was then distilled under reduced pressure to remove the lighter oily matter. The residual oil after distillation was introduced into a mixing vessel and heat-treated therein for 1 hour at a temperature of 420"C under a pressure of 10 mmHg to give a treated pitch containing 35% by weight of toluene-insoluble matter. The thus obtained pitch material was then introduced continuously into a centrifugal film evaporator equipped with 4 rotating blades in which the pitch material was subjected to a heat treatment at 470oC under a stream of nitrogen gas at a pressure of 1 mmHg. The staying time of the pitch material was 10 minutes when the blades were rotated at a velocity of 100 r.p.m. with a clearance of 0.5 mm between the vessel wall and the periphery of the blades. The thus obtained pitch material was composed of the meso phase pitch alone and had a pyridine-insoluble matter content of 69% by weight, a number average molecular weight of 1200 and a softening point of 350"C.

The pitch material obtained in the above described manner was spun at 380"C into pitch filaments having a diameter of 7 m and the filaments were infusibilized by heating in air at a temperature of 300"C for 20 minutes followed by calcination at 1500"C for 10 minutes in an atmosphere of argon to give carbon fibres having a tensile strength of 290 kgimm2.

Claims

1. A pitch material having an optically anisotropic phase content of at least 80% by weight, a pyridine-insoluble matter content of from 30 to 70% by weight, a number average molecular weight of from 1000 to 1400 and a softening point of from 330 to 380-C.

2. A pitch material according to claim 1 composed substantially of the optically anisotropic phase alone and having a pyridine-insoluble matter content of from 40 to 60% by weight, a number average molecular weight of from 1000 to 1300 and a softening point of from 330 to 3703C.

3. A method for the preparation of a pitch material according to claim 1, the method comprising heating a starting pitch material, obtained by removing the light oily matter from a petroleumbased residual oil at a temperature of from 400 to 460"C under a pressure of from 5 to 50 mmHg, and heating the resultant pitch material at a temperature of from 450 to 500UC for from 0.2 to 30 minutes under a pressure of from 0.1 to 5 mmHg.

4. A method according to claim 3 wherein the first heat treatment is performed in a film evaporator under an atmosphere of an inert gas or a nonoxidizing gas.

5. A method according to claim 3 or claim 4 wherein the second heat treatment is performed in a film evaporator under an atomsphere of an inert gas or a non-oxidizing gas.

6. A method according to any preceding claim wherein the first heat treatment is carried out at a temperature of from 410 to 4505C under a pressure of from 10 to 50 mmHg for from 0.1 to 20 hours.

7. A method according to any preceding claim wherein the second heat treatment is carried out at a temperature of from 460 to 500 C under a pressure of from 0.5 to 3 mmHg.

8. A method for the preparation of a pitch material according to claim 1, the method being substantially as described herein with reference to any of the Examples.

9. A pitch material according to claim 1, the pitch material being substantially as described herein with reference to any of the Examples.