EP0026647B1

EP0026647B1 - Mesophase pitch, processes for its production and fibers produced therefrom

Info

Publication number: EP0026647B1
Application number: EP19800303384
Authority: EP
Inventors: Irwin Charles Lewis
Original assignee: Union Carbide Corp
Current assignee: BP Corp North America Inc
Priority date: 1979-09-28
Filing date: 1980-09-26
Publication date: 1985-05-22
Also published as: JPH0157715B2; JPS5657881A; DE3070671D1; EP0026647A1

Description

The invention relates to a process for producing a mesophase pitch and particularly relates to a process for producing a mesophase pitch containing at least about 70% by weight mesophase.
Mesophase pitch has been known to be suitable for producing carbon fibers having excellent properties suitable for commercial exploitation. It is known that mesophase derived carbon fibers are lightweight, strong, stiff, electrically conductive and both chemically and thermally inert. The mesophase derived carbon fibers perform well as reinforcements in composites and have found use in aerospace applications and quality sporting equipment.
Generally, carbon fibers have been primarily made from three types of precursor materials; rayon, polyacrylonitrile (PAN), and pitch. The use of pitch as a precursor is attractive economically.
Low cost carbon fibers produced from ordinary pitch exhibit little preferred molecular orientation and relatively poor mechanical properties.
In contrast, carbon fibers produced from mesophase pitch exhibit high preferred molecular orientation and relatively excellent mechanical properties.
As used herein, the term "pitch" is to be understood as used in the instant art and generally refers to a carbonaceous residue consisting of a complex mixture of primarily aromatic organic compounds which are solid at room temperature and exhibit a relatively broad melting or softening temperature range. When cooled from the melt, the pitches behave as glasses.
As used herein, the term "mesophase" is to be understood as used in the instant art and generally is synonymous with liquid crystal. That is, a state of matter which is intermediate between crystalline solid and a normal liquid. Ordinarily, a material in the mesophase state exhibits both anisotropic and liquid properties.
As used herein, the term "mesophase-containing pitch" is a pitch containing less than about 40% by weight mesophase and the non-mesophase portion or isotropic phase is the continuous phase.
As used herein, the term "mesophase pitch" is a pitch containing more than about 40% by weight mesophase and is capable of forming a continuous anisotropic phase when dispersed by agitation or the like in accordance with the prior art.
The conventional method for preparing mesophase pitch from a precursor pitch includes heat treating at a temperature greater than 350°C to effect thermal polymerization. This process produces large molecular weight molecules capable of forming mesophase.
A typical conventional method is carried out using reactors maintained at about 400°C for about 20 hours. The properties of the final material can be controlled by the reaction temperature, heat-treatment time, and volatilization rate. The presence of the high molecular weight fraction results in a melting point of the mesophase pitch of at least 330°C. An even higher temperature is needed to transform the mesophase pitch into fibers. This is termed "spinning" in the Art.
The following patents are representative of the prior art and are incorporated herein by reference: U.S. Patent No. 4,005,183 to Singer, U.S. Patent No. 3,919,387 to Singer, U.S. Patent No. 4,032,430, U.S. Patent No. 3,976,729 to Lewis et al., U.S. Patent No. 3,995,014 to Lewis, and especially British Patent 2,005,298 to Chwastiak.
In U.K.-A-2,002,024 there is disclosed a process in which a carbonaceous pitch is extracted with benzene, toluene or a toluene-heptane mixture to separate a solvent-insoluble fraction which is capable of being converted to an optically anisotropic, deformable pitch by heating to a temperature of from 230°C to 400°C.
The amount of mesophase in pitch can be evaluated by known methods using polarized light microscopy. The presence of homogeneous bulk mesophase regions can be visually observed by polarized light microscopy and quantitatively determined by the method disclosed in the aforementioned Chwastiak patent. Previously, the criteria of insolubility in certain organic solvents such as quinoline and pyridine was used to estimate mesophase content. There could, however, be present in the precursor pitch certain non-mesophase insolubles and it is a common practice to remove these insolubles before treating the precursor pitch.
In accordance with the prior art, "%Q.I." refers to quinoline insolubles of a pitch quinoline extracted at 75°C. Also, "%P.I." refers to pyridine insolubles of a pitch by Soxhlet extraction in boiling pyridine at about 115°C.
Softening point or softening temperature of a pitch, is related to its molecular weight constitution, the presence of a large amount of high molecular weight components generally tends to raise the softening temperature. It is a common practice in the art to characterize in part a precursor pitch by its softening point. For mesophase pitches, the softening point is used to determine suitable spinning temperature. Generally, the spinning temperature is about 40°C or more higher than the softening temperature.
Generally, there are several methods for determining the softening temperature and the temperatures measured by these different methods vary somewhat from each other.
Generally, the Mettler softening point procedure is widely accepted as the standard for evaluating precursor pitches. This procedure can be adapted for use on mesophase pitches.
The softening temperature of a mesophase pitch can also be determined by hot stage microscopy. In this method, the mesophase pitch is heated on a microscope hot stage in an inert atmosphere under polarized light. The temperature of the mesophase pitch is raised under a controlled rate and the temperature at which the mesophase pitch commences to deform is noted as the softening temperature.
As used herein, softening point or softening temperature will refer to the temperature determined by the Mettler procedure for both precursor and mesophase pitches.
According to the present invention there is provided a process for producing a mesophase pitch containing at least 70% by weight mesophase which comprises solvent extracting a carbonaceous precursor pitch capable of forming a large domained mesophase pitch by a conventional thermal process, characterised by using as a solvent a N,N-dimethylformamide, carbondisulfide, or a mixture of toluene and petroleum ether and by the absence of a heating step subsequent to the solvent extraction.
Preferably, the mesophase pitch produced in accordance with the invention possesses x-ray properties of interlayer spacing Co/2 of less than 3.60 Angstroms and an apparent stack height Lc of greater than 20 Angstroms.
Preferably, the mesophase pitch produced in accordance with the invention contains at least 90% by weight mesophase and more preferably 100% by weight mesophase.
The solvent extraction utilized in the process of the present invention may, for example, include a sequence of solvent extraction steps. Further, when the solvent extraction is carried out utilizing a mixture of toluene and petroleum ether, this may, for example, be effected by carrying out a solvent extraction using toluene and, as a separate step, carrying out a solvent extraction using petroleum ether.
A mesophase pitch containing a predetermined amount of mesophase may, for example, be obtained by blending together soluble and insoluble portions resulting from the solvent-extraction.
The solvent extraction utilized in the process of the present invention may, for example, be preceded by distilling the carbonaceous pitch.
According to another aspect of the present invention a fiber, e.g. a carbonised fiber, is formed from the mesophase pitch produced in accordance with the invention. The fibre may be formed without any chemical operations e.g. by spinning. Preferably, the spinning is at a temperature less than 370°C. Conventional spinning does not subject the pitch to a chemical change for a temperature below 370°C for the period of time the mesophase pitch is at that elevated temperature.
Thus, the instant invention allows the formation of a pitch fiber from a carbonaceous pitch without any chemical operations.
In the conventional thermal mesophase process, the carbonaceous precursor pitch is heated to effect polymerization. The resultant mesophase pitch is characterized by a molecular weight distribution which contains two major peaks. The low molecular weight peak corresponds to components of the precursor pitch and the high molecular weight peak corresponds to the molecules produced by the thermal polymerization.
In contrast, the mesophase pitch produced in accordance with the present invention possesses a molecular weight distribution having a single major peak generally positioned between the two peaks which would have arisen if the thermal polymerization had been carried out.
In carrying the invention into effect, certain embodiments have been selected for illustration in the tables and for description in this Specification.
Illustrative, non-limiting examples of the invention (Examples 2, 4, 6 and 10) and comparative examples (Examples 1, 3, 5, 7, and 9) are set out below. Numerous other examples can readily be evolved in the light of the guiding principles and teaching herein. The examples given herein are intended to illustrate the invention and not in any sense to limit the manner in which the invention can be practiced. The parts and percentages recited herein, unless specifically stated otherwise, refer to parts by weight and percentages by weight.
Generally, the carbonaceous precursor pitch suitable for carrying out the invention should be a precursor pitch capable of forming a large-domained mesophase pitch by conventional thermal processes as set forth in the aforementioned Singer Patent No. 4,005,183. The suitability of any carbonaceous pitch can be determined in a straight forward manner in accordance with the teachings herein.
There is, of course, an interrelationship between the carbonaceous precursor pitch and the physical operations suitable and preferably for producing the mesophase pitch. The teachings herein provide the guidelines for selecting and optimizing the physical operations for a particular carbonaceous pitch.

Example 1 (Comparative)

A commercially available petroleum pitch was used in a solvent extraction operation. The petroleum pitch had a softening temperature of about 130°C and has 0% P.I. and contained no mesophase.
Ten grams of the pitch were stirred with 200 ml of toluene at room temperature for about one hour and then filtered by vacuum filtration. The dried insolubles or yield was about 8% by weight. The yield had a softening point of about 319°C, exhibited 47% P.I., and contained about 40% by weight mesophase.

Example 2

A commercially available pitch different from the Example 1 was selected. The petroleum pitch had a softening temperature of about 123°C and has about 0% P.I. and contained no mesophase.
The operations of the Example 1 were repeated except that the solvent was a 1:2 mixture of petroleum ether and toluene. About 14% by weight yield was obtained and the yield had a softening temperature of about 239°C and had about 3% P.I. and contained about 100% by weight mesophase.

Example 3 (Comparative)

The Example 2 was repeated except that the solvent was a 1:1 mixture of petroleum ether and toluene. The yield was about 32% and contained about 50% by weight mesophase.

Example 4

The mesophase pitch of the Example 2 was spun into fibers on a monofilament spinning machine at about 290°C. The fibers had a diameter of about 20 microns and were thermoset by heating them in air at 2°C per minute to about 375°C. The thermoset fibers were examined by polarized light microscopy and were determined to contain about 100% anisotropic state. The thermoset fibers were carbonized to 1700°C in an inert atmosphere in accordance with conventional methods and tests on the carbonized fibers exhibited a modulus of about 25x10⁶ psi (172 GPa) and 25,000 psi (1.72 GPa) tensile strength.
The methods for making carbonized fibers from mesophase pitch are well known in the art and particular reference is had to the aforementioned Singer Patent No. 4,005,183 and Chwastiak Patent No. 2,005,298.

Example 5 (Comparative)

An air-blown petroleum pitch having a softening temperature of about 152°C was solvent extracted with toluene at room temperature and gave a yield of about 34% by weight. The yield contained no mesophase. A similar result was obtained for a pitch derived from pyrolysis tar.
Both of these pitches do not yield large domain mesophase when heat treated in accordance with conventional methods. This shows that such pitches do not appear suitable for the invention.

Example 6

The petroleum pitch of the Example 2 was solvent extracted with petroleum ether. The yield was about 77% by weight and contained no mesophase. Further solvent extraction with toluene in accordance with the Example 2 produced about 1% by weight yield which contained about 100% by weight mesophase.

Example 7 (Comparative)

Solvent extraction was carried out on a coal tar pitch having a softening temperature of about 125°C and about 0% mesophase. At room temperature, 60 grams of pitch was stirred with 1200 ml of toluene for about two hours. The yield was about 47% by weight. The yield had a softening temperature of about 318°C, exhibited about 53% P.I., and contained about 60% by weight mesophase.

Example 8 (Comparative)

Tests were carried out to determine mesophase content as a function of blending soluble and insoluble portions of the solvent extracted pitch.
For a naphthalene pitch extracted at room temperature with toluene, it was determined that there was a substantially linear relationship between mesophase content by weight and insoluble content by weight for the range of from about 10% to about 100% by weight mesophase which correspond to the range of from about 35% to about 82% by weight of insolubles.
This relationship was substantially the same when blending experiments were performed using the soluble and insoluble portions from the naphthalene pitch with the corresponding fractions from the Example 2. For example replacing a given weight of insolubles for the naphthalene pitch with the same weight of insolubles from the Example 2 produced the same mesophase content. This is an unexpected result in view of the differences in the chemical compositions of the precursor pitches.

Example 9 (Comparative)

Tests were conducted to determine the effect of repeated solvent extraction.
It has been found that the exhaustive solvent extraction with toluene at room temperature results in a diminishing yield and virtually no measurable mesophase.
Thus, this is an additional guideline for the practice of the invention.

Example 10

Tests were carried out on the various yields from the examples to determine the characteristics of both the molecular weight distributions and x-ray diffraction.
Using gel permeation chromatography, the values were determined for number average molecular weight (Mn), weight average molecular weight (Mw), and z-average molecular weight (Ñ_fz). Also, the quantitative distribution of molecular weights was determined.
In addition, x-ray diffraction measurements were made to determine the values of Co/2 and Lc.
Table 1 shows the results of these tests for mesophase pitches of various examples.
The molecular weight distribution data was obtained in accordance with the aforementioned Lewis et al. Patent No. 3,976,729 and Chwastiak British Patent 2,005,298. The solvent employed in the gel permeation chromatographic procedure was quinoline.
For comparison, Table 2 shows mesophase pitches produced by conventional thermal processes for pitches used in various examples.
For completence, Table 3 shows x-ray data for precursor pitches prior to any operations.
A comparison between the values for Co/2 and Lc for the precursor pitches as shown in the Table 3 to the values of the instant mesophase pitch shows the instant mesophase pitches exhibit substantial molecular order due to the instant process. In particular, the instant solvent extraction without any applied heat transforms a relatively disordered precursor pitch into a substantially ordered mesophase pitch.
ft is interesting to compare the solubility parameter a of various solvents to the mesophase content of the pitch after treatment. Table 4 shows that the acceptable and unacceptable solvents appear to fall within the same range.

Example 11

The mesophase pitch of the Example 2 was melted at a temperature of about 300°C and stirred for about 30 minutes in an inert atmosphere in order to remove the solvent. The product was spun into monofilaments at a temperature of about 300°C. The as-spun fibers had a diameter of 15 pm and 25 µm.
The 25 um fibers were crushed to a powder and examined by x-ray. The Co/2 was measured as 3.54 Anstrom and Lc was measured as 34 Angstrom. The x-ray data for the mesophase pitch prior to spinning showed Co/2=3.58 Angstroms and Lc of about 25 Angstroms.
The preferred orientation of the as-spun fibers was measured as being about 30°.
By comparison, the as-spun fibers made by conventional processes as disclosed in the aforementioned Singer Patent No. 4,032,430 had a Co/2 of from 3.45 to 3.55 Angstroms, Lc of from 30 to 50 Angstroms, and a preferred orientation of from 25° to 30°.
Additionally, the 15 µm fibers were thermoset by heating in air at 2° per minute to about 375°C. The thermoset fibers were examined and contained about 100% anisotropic state. The thermoset fibers were carbonized to 1700°C in an inert atmosphere in accordance with conventional methods and the carbonized fibers exhibited a modulus of about 28x106 psi (193 GPa) and a tensile strength of 273,000 psi (1.88 GPa).

Claims

1. A process for producing a mesophase pitch containing at least 70% by weight mesophase which comprises solvent extracting a carbonaceous precursor pitch capable of forming a large domained mesophase pitch by a conventional thermal process, characterised by using as a solvent a N,N-dimethylformamide, carbondisulfide, or a mixture of toluene and petroleum ether and by the absence of a heating step subsequent to the solvent extraction.

2. A process as claimed in Claim 1, characterised in the solvent extraction includes a sequence of solvent extraction steps.

3. A process as claimed in Claim 1 or 2, characterised by the use in the solvent extraction of a mixture of toluene and petroleum ether.

4. A process as claimed in Claim 3, characterised in that the solvent extraction using a mixture of toluene and petroleum ether is effected by carrying out a solvent extraction step using toluene and, as a separate step, carrying out a solvent extraction using petroleum ether.

5. A process as claimed in any one of the preceding claims, characterised in that a mesophase pitch containing a predetermined amount of mesophase is obtained by blending together soluble and insoluble portions resulting from said solvent extraction.

6. A process as claimed in any one of the preceding claims, characterised in that the solvent extraction is preceded by distilling said carbonaceous pitch.

7. A process as claimed in any one of the preceding claims, characterised by the further step of producing a fiber from said mesophase pitch.

8. A process as claimed in any one of the preceding claims, characterised by the further step of producing a carbonized fiber from said mesophase pitch.