Pitch for Direct Spinning Into Carbon Fibers Derived From a Steam Cracker Tar Feedstock

This invention pertains to an aromatic pitch containing a high liquid crystal (optically active) fraction, and more particularly to a pitch which can be directly spun into carbon fibers.

As is well-known, the catalytic conversion of virgin gas oils containing aromatic, naphthenic and paraffinic molecules results in the formation of a variety of distillates that have ever-increasing utility and importance in the petrochemical industry. The economic and utilitarian value, however, of the residual fractions of the cat cracking processes (also known as cat cracker bottoms) has not increased to the same extent as have the light overhead fractions. One potential use for such cat cracker bottoms is in the manufacture of carbon artifacts. As is well-known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest is carbon fiber. Hence, particular reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifacts in a general sense, with emphasis upon the production on shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets, etc.

The use of carbon fibers for reinforcing plastic and metal matrices has gained considerable commercial acceptance. The exceptional properties of these reinforcing composite materials, such as their high strength to weight ratio, clearly offset their high preparation costs. It is generally accepted that large scale use of carbon fibers as reinforcing material would gain even greater acceptance in the marketplace, if the costs of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.

Many materials containing polycondensed aromatics can be converted at early stages of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be fundamental in obtaining a high quality carbon fiber. Thus, one of the first requirements of a feedstock material suitable for carbon fiber production, is its ability to be converted to a highly optically anisotropic material.

In addition, suitable feedstocks for carbon artifact manufacture, and in particular carbon fiber manufacture, should have relatively low softening points and sufficient viscosity suitable for shaping and spinning into desirable articles and fibers.

Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, infusible materials, and/or high softening point components, are detrimental to the fiber-making process. Thus, for example, U.S. Pat. No. 3,919,376 discloses the difficulty in deforming pitches which undergo coking and/or polymerization at the softening temperature of the pitch.

Another important characteristic of the feedstock for carbon artifact manufacture is its rate of conversion to a suitable optically anisotropic material. For example, in the above-mentioned U.S. Patent, it is disclosed that 350.degree. C. is the minimum temperature generally required to produce mesophase from a carbonaceous pitch. More importantly, however, is the fact that at least one week of heating is necessary to produce a mesophase content of about 40%, at that minimum temperature. Mesophase, of course, can be generated in shorter times by heating at higher temperatures. However, as indicated above, incipient coking and other undesirable side reactions take place at temperatures in excess of about 425.degree. C.

In U.S. Pat. No. 4,208,267, it has been disclosed that typical graphitized carbonaceous pitches contain a separable fraction which has important physical and chemical properties. Indeed, this separable fraction exhibits a softening range and viscosity suitable for spinning. It also has the ability to be converted rapidly (at temperatures in the range generally of about 230.degree. C. to about 400.degree. C.) to an optically anisotropic, deformable, liquid crystalline material structure. Unfortunately, the amount of separable fraction present in well-known commercially available petroleum pitches, such as Ashland 240 and Ashland 260, to mention a few, is exceedingly low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to a deformable anisotropic phase.

In U.S. Pat. No. 4,184,942, it has been disclosed that the amount of the aforementioned fraction yielding an optical anisotropic pitch can be increased by heat soaking the feedstock at temperatures in the range of 350.degree. C. to 450.degree. C., until spherules visible under polarized light begin to appear.

In U.S. Pat. No. 4,219,404, it has been disclosed that the polycondensed aromatic oils present in isotropic graphitizable pitches are generally detrimental to the rate of formation of highly anisotropic material in such feedstocks when they are heated at elevated temperatures and that, in preparing a feedstock for carbon artifact manufacture, it is particularly advantageous to remove at least a portion of the polycondensed aromatic oils normally present in the pitch simultaneously with, or prior to, heat soaking of the pitch for converting in into a feedstock suitable in carbon artifact manufacture.

More recently, in U.S. Pat. No. 4,271,006 (June 2, 1981), a process has been disclosed for converting cat cracker bottoms to a feedstock suitable in carbon artifact manufacture. Basically, the process requires stripping cat cracker bottoms of fractions boiling below 400.degree. C. and thereafter heat soaking the residue followed by vacuum stripping to provide a carbonaceous pitch.

Cat cracker bottoms like all other heavy aromatic residues obtained from steam cracking, fluid cracking or coal processing are composed of two components: (1) a low molecular weight oil fraction which can be distilled; and (2) an undistillable fraction of high molecular weight. This high molecular weight fraction is insoluble in paraffinic solvents such as n-heptane, iso-octane, pet ether, etc. This fraction is generally called "asphaltene".

It is preferred to use an asphaltene-free feed for the production of pitches. These asphaltenes have a very high molecular weight (up to 10,000), a very high coking characteristic (coking value as high as 67.5 wt% coke yield at 550.degree. C.), and a very high melting point (200.degree.-250.degree. C.).

It is desired to use an asphaltene-free cat cracker bottom. The asphaltene-free cat cracker bottom is free of ash, coke particles and other impurities. The absence of asphaltene, ash, coke particles and other organic and inorganic impurities make the cat cracker bottom distillate an ideal feed for the production of an aromatic pitch with a very high content of liquid crystals. This asphaltene-free cat cracker bottom can be prepared by two methods: (a) by a distillation proces; e.g., vacuum or steam distillation; and (b) by deasphaltenation of the cat cracker bottom. The deasphaltenation can be made readily by solvent extraction with a paraffinic solvent.

In application U.S. Ser. No. 291,990 (filed Aug. 11, 1981) and assigned to a common assignee a process is described for heat soaking a deasphaltenated cat cracker bottom.

In application U.S. Ser. No. 225,060 (filed Jan. 14, 1981) and assigned to a common assignee a process is described for obtaining a feedstock with a low liquid crystal fraction by heat soaking a distillate derived from a cat cracker bottom. The pitch produced in the above applicaton, Ser. No. 225,060 cannot be used directly for carbon fiber production. The liquid crystal fraction has to be extracted from the pitch and used for fiber production.

Whereas, application U.S. Ser. No. 225,060 teaches that all of the cat cracker bottoms can be used to obtain a pitch having low toluene insolubles (Ti), the present invention teaches the opposite, i.e. obtaining a pitch from fractions of the cat cracker bottoms which has a high Ti content (a high content of liquid crystals).

The present invention uses deasphaltenated feedstock fractions to provide a pitch having a high Ti content, and one which does not require Ti solvent extraction prior to spinning into fibers.

The deasphaltenated fractions of a feedstock in accordance with this invention is generally free of ash and impurities, and has the proper rheological properties to allow direct spinning into carbon fibers. The pitch obtained from this fraction produces fibers which have high strength and performance. For example, a deasphaltenated cat cracker bottom fraction obtained in accordance with the present invention, has virtually no coking value at 550.degree. C. compared with a 56% standard coking value for Ashland 240. The deasphaltenated cat cracker bottom fraction is composed of 4, 5, and 6 polycondensed aromatic rings. This provides a uniform feed material which can be carefully controlled to produce a uniform product with a narrow molecular weight distribution.

The present invention pertains to a high Ti pitch for direct spinning into carbon fibers. An aromatic pitch with a very high liquid crystal fraction (80-100%) can be prepared by thermally reacting a deasphaltenated fraction of either a cat cracker bottom, steam cracker tar or a coal distillate, that are respectively rich in 4, 5 and 6); (2, 3, 4 and 5); and (3, 4, 5 and 6) aromatic rings. The various feedstocks are heat soaked in a temperature range from 420.degree. C. to 450.degree. C. at atmospheric pressure, and then vacuum stripped to remove at least a portion of the unreacted oils at a temperature in the approximate range of from 320.degree. C. to 420.degree. C. at 0.1 to 100 mmHg, and preferably at greater than 400.degree. C. at 5.0 mmHg of pressure.

More specifically, in the case of cat cracker bottoms the fraction is heat soaked at approximately 440.degree. C. for 2-4 hours at atmospheric pressure. In the case of steam cracker tars, the fraction is heat soaked at 430.degree. C. for approximately 40 hours; and in the case of coal distillate, the fraction is heat soaked at approximately 440.degree. C. for 1/4 to 1/2 hour. All the heat soaked materials are then vacuum stripped and spun directly into carbon fibers. The pitch of this invention is definable only in terms of deasphaltenated fractions of a feedstock.

For the purposes of definition the terms "deasphaltenated feedstock" and/or "deasphaltenated middle fraction of a feedstock" shall mean: a deasphaltenated material obtained from a middle cut of a feedstock, and/or one caused to be relatively free of asphaltenes by means of obtaining a distillate portion of said feedstock which when further treated will form a precursor which can be spun into a carbon fiber and which has the following general characteristics:

(1) a relatively low coking value;

(2) a relatively low content of ash and impurities; and

(3) a relatively narrow average molecular weight range.

(4) consisting of 3, 4, 5 and 6 polycondensed aromatics.

A typical weight percentage of asphaltenes in a deasphaltenated stream cracker tar being in a range of approximately 0.5 to 2.0%.

A directly spinnable pitch of this invention has the proper rheological properties characterized by a glass transition temperature (Tg) in the approximate range of 180.degree. C. to 250.degree. C. at atmospheric pressure, and/or a viscosity of less than approximately 2,500 cps in a temperature range of approximately 300.degree. C., to 360.degree. C., at atmospheric pressure.

It is an object of this invention to provide an improved pitch which can be directly spun into carbon fibers.

It is another object of the invention to provide a pitch for manufacturing carbon fibers which is more uniform, and which is relatively free of ash and impurities.

It is a further object of this invention to provide a pitch having high toluene insolubles, and which does not require Ti solvent extraction prior to spinning into fibers.

These and other objects of this invention will be better understood and will become more apparent with reference to the following detailed description considered in conjunction with the accompanying drawings.

FIG. 1 is a graphical representation of deasphaltenated fractions of various feedstocks used to provide the inventive pitches for direct spinning into carbon fibers, including the deasphaltenated steam cracker tar bottom of this invention;

FIG. 2 depicts a graph of a glass transition temperature scan for the pitch of FIG. 1.

Generally speaking, the steam cracker tar which is used as a starting material in the process of the present invention is defined as the bottoms product obtained by cracking gas oils, particularly virgin gas oils, such as naphtha, at temperatures of from about 700.degree. C. to about 1000.degree. C. A typical process steam cracks gas oil and naphtha, at temperatures of 800.degree. C. to 900.degree. C., with 50% to 70% conversion to C.sub.3 olefin and lighter hydrocarbons, by stripping at temperatures of about 200.degree. C. to 250.degree. C. for several seconds. The tar is obtained as a bottoms product. A gas oil is, of course, a liquid petroleum distillate with a viscosity and boiling range between kerosene and lubricating oil, and having a boiling range between about 200.degree. C. and 400.degree. C. Naphtha is a generic term for a refined, partly refined or unrefined liquid petroleum product of natural gas wherein not less than 10% distills below 175.degree. C. and not less than 95% distills below 240.degree. C., as determined by ASTM method D-86. Steam cracker tars typically consist of alkyl substituted polycondensed aromatic compounds.

Obviously, the characteristics of a steam cracker tar vary according to the feed in the steam cracking plant.

Characteristics of typical steam cracker tars obtained from the steam cracking of naphtha, gas oil and desulfurized gas oil are respectively given in Table 1, below:

In the process of the present invention, the steam cracker tars are distilled by heating to elevated temperatures at reduced pressures. For example, the stream cracker tar is heated to temperatures in the range of 130.degree. C. to 320.degree. C. at an approximate pressure of 10 mm of mercury. Basically, the steam cracker tar is separated into a middle distillate fraction having a boiling point at 760 mm mercury in the range of from about 270.degree. C. to about 490.degree. C. In a particularly preferred embodiment of the present invention, the distillate fraction of the steam cracker tar which is employed in forming a suitable carbonaceous pitch for carbon artifact manufacture, is that fraction boiling in the range of about 370.degree. to about 490.degree. C. at 760 mm of mercury.

An ASTM D1160 distillation of a typical steam cracker tar is given in Table 2, blow:

The middle fraction distillate taken at 370.degree.-490.degree. C. @ 760 mmHg has high aromaticity and narrow molecular weight. It contains no ash or solid particulate and does not contain high coking asphaltene. Chemically it is composed of polycondensed 2, 3, 4 and 5 aromatic rings. Table 3 below gives the physical and chemical characteristics of a typical middle distillate fraction of a steam cracker tar:

The molecular structure of a typical steam cracker tar middle distillate fraction as determined by high resolution Mass Spectrometer, is given below in Table 4:

Another method to prepare an asphaltene-free steam cracker tar fraction is by removing the asphaltene from steam cracker tar by a solvent extraction of the asphaltene with a paraffinic solvent such as n-heptane, iso-octane, n-pentene, or pet-ether. Table 5, below gives the characteristics of a deasphaltenated oil obtained from a steam cracker tar using n-heptane as a sovlent (Feed: solvent ratio=1:30):

After separating the steam cracker tar middle fraction distillate, the middle fraction distillate is heat soaked at temperatures of about 430.degree. C. at atmospheric pressure. In general, heat soaking is conducted for about forty (40) hours. In the practice of the present invention, it is particularly preferred that heat soaking be done in an atomsphere such as nitrogen, or alternatively in hydrogen atmosphere.

After heat soaking the distillate, the heat soaked distillate is then heated in a vacuum at temperatures generally about 400.degree. C. and typically in the range of about 370.degree. C. to 420.degree. C., at pressures below atmospheric pressure, generally in the range of about 1.0 to 100 mm mercury. This additional heating removes at least part of the oil present in the heat soaked distillate. Typically, from about 90 to 100% of the oil which is present in the heat soaked distillate is removed.

As can be readily appreciated, the severity of the heat soaking conditions outlined above, will affect the nature of the pitch produced. The higher the temperature chosen for heat soaking, and the longer the duration of the heat soaking process, the greater the amount of toluene insoluble components that will be generated in the pitch.

The inventive process can prepare pitches with a very high toluene insolubles content (80-100% by weight), as well as a high content of quinoline insolubles (greater than 10%, at least 15%), and one which can be spun directly into carbon fibers, as shown in FIG. 1.

For a better understanding of the treatment particular used to convert these distillates into pitch, please refer to U.S. application, Ser. No. 346,624 filed on Feb. 8, 1982, and which is meant to be incorporated herein by way of reference.

The present invention distinguishes over the invention of this referenced application most particularly in the heat soaking step of the process.

The pitches of all these inventions are definable only in terms of deasphaltenated fractions of a feedstock (FIG. 1).

Table 6 below, summarizes the heat soaking conditions for a variety of deasphaltenated feedstocks, and the resultant characteristics of each pitch:

The following Table 7, presents data derived from additional examples of steam cracker tar pitches A, B, C and D in accordance with this invention:

The rehology of pitches used for direct spinning is of great importance to obtain good spinnability. It is desired to have pitches with low viscosity at the spinning temperature which is preferrably below around 400.degree. C., in order to avoid pitch cracking and volatilization which could lead to serious foaming of the fiber and substantial reduction in the fiber strength. The pitch for direct spinning is also desired to be less sensitive to heat, i.e. does not change its viscosity too much when changing temperature. The sensitivity of the pitch to temperature variation can be determined from viscosity-temperature curves.

Differential Scanning Calorimetry (DSC) is used to obtain information on glass transition and softening characteristics of pitches. An OMINITHERM Corp. DSC Model (QC25) is used to obtain the glass transistion (Tg) data. The method comprises heating a small sample of the pitch in the DSC pan, allowed to cool and the DSC trace was then obtained by heating at the rate of 10.degree. C./min under nitrogen (30 cc/min). From the DSC trace three DSC data points are determined; the onset of Tg (Ti), the termination of Tg (Tf), and the Tg point which is at the midway between the Ti and Tf point. It has been reported that there is a relationship between the Tg of the pitch and its softening point as determined by the traditional method such as the ring and ball method. The softening point is higher by around 60.degree. than the Tg.

FIG. 2 depicts a glass transition temperature scan for Example B in Table 7 above.

Table 8, below, illustrates glass transition temperatures for the previous examples A-D (Table 7):

Having thus described this invention, what is desired to be protected by Letters Patent is presented in the following appended claims.