Solvent-Extracted Pitch Precursors for Carbon Fiber - American

15 Solvent-Extracted Pitch Precursors for Carbon Fiber D. M. RIGGS

Downloaded by CORNELL UNIV on July 23, 2016 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch015

Exxon Enterprises Materials Division, Fountain Inn, SC 29644 Pitch i s a "pseudo solution" of a wide variety of d i f ferent generic classes of hydrocarbons ranging from paraffins at one extreme to very highly aromatic species at the other. By using the Theory of S o l u b i l i t y for Non-Electrolytes, s p e c i f i c fractions can be isolated from a pitch by properly selecting a solvent system and extraction conditions. "Tailored" precursors for carbon fiber and other carbon products, such as carbon/carbon matrices and bulk graphites, can thus be obtained. The technique of extraction, the charact e r i s t i c s of different precursors, and the structure and properties of carbon fiber and composites made from solvent extracted precursors will be discussed. Pitches are composed of a wide variety of different generic classes of compounds ranging from low molecular weight paraffins at one extreme to very highly condensed aromatic species at the other. The r e l a t i v e proportion of each of the molecular types and the subtlet i e s of their molecular structures are very much a function of the pitch source (e.g. coal, o i l , synthetic, etc.) and i t s previous thermal history. When processing carbon products from pitch precursors, the d i s t r i b u t i o n of molecular types and the inherent charact e r i s t i c s of the the parent pitch acquire varying degrees of importance depending upon the actual carbon product being manufactured. In the case of carbon fiber processing, control over the pitch precursor composition i s very important since this composition u l t i mately controls the pitch thermal s t a b i l i t y , i t s carbon y i e l d , the l i q u i d c r y s t a l (or mesophase) forming a b i l i t y , the pitch rheological properties, and most importantly, the properties (e.g. strength and modulus) of the fibers themselves. Since i t i s very d i f f i c u l t to find a source of pitch with a consistent batch to batch d i s t r i b u t i o n of molecular types and molecular structural c h a r a c t e r i s t i c s , then the i n i t i a l steps of a pitchbased carbon fiber process should " t a i l o r " both the d i s t r i b u t i o n and

0097-6156/ 84/0260-0245S06.00/0 © 1984 A m e r i c a n C h e m i c a l Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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POLYMERS FOR FIBERS AND ELASTOMERS

the c h a r a c t e r i s t i c s to a predetermined point in order to achieve the consistency desired for s p i n n a b i l i t y and fiber properties. There are various ways of accomplishing this " t a i l o r i n g " including extended thermal treatments^), sparging(2), and solvent extract ionC3). Of these methods, solvent extraction may be the most e f f e c t i v e since the pitch i s r e a l l y an " i n t r i n s i c " solution i t s e l f and hence, solub i l i t y relationships can be manipulated to force the extraction of desirable fiber feed components. A "desirable" carbon f i b e r feed from a pitch precursor should s a t i s f y at least four main c r i t e r i a . These c r i t e r i a include the following: a. the precursor f r a c t i o n should be f u l l y l i q u i d c r y s t a l l i n e b. the thermal s t a b i l i t y and carbon y i e l d should be high c. the rheological c h a r a c t e r i s t i c s and s p i n n a b i l i t y should be "manageable" d. the softening point and glass t r a n s i t i o n temperature should be high A f u l l y l i q u i d c r y s t a l l i n e precursor i s desirable for two reasons. F i r s t , i t i s inherently easier to spin a single phase materia l compared to a multiphase material, and second, the orientation that i s imparted to the mesophase during spinning gives rise to the graphitic orientation developed i n the fiber during the carbonizat i o n step. The graphitic orientation i n turn governs the moduli of the carbonized f i b e r . A thermally stable precursor with a high carbon y i e l d i s also desirable for reasons related mainly to v o l a t i l e s and gas evolution. Lower evolved levels of gas and v o l a t i l e s s i g n i f i c a n t l y reduce the chances for the generation of voids or bubbles in the f i b e r . Thus, the chances of obtaining low strength values are reduced. The rheological c h a r a c t e r i s t i c s and s p i n n a b i l i t y considerations are important for more or less self-evident reasons. E s s e n t i a l l y , i f the precursor i s non-spinnable, then good carbon f i ber most assuredly cannot be obtained. High softening points and glass t r a n s i t i o n temperatures are required because the second stage of fiber processing involves an oxidative thermosetting step at temperatures usually above 240°C. The fiber should not relax, soften, or melt during this process step, and hence, the precursor fraction should have a glass t r a n s i t i o n temperature at least in this range. In this paper, a process for producing a carbon fiber from a solvent extracted pitch feed w i l l be discussed. General Characteristics of Pitches A t y p i c a l pitch i s e s s e n t i a l l y a solution of hydrocarbons. As such, the interelationships among the hydrocarbons comprising the pitch are governed by the s o l u b i l i t y relationships for non-electrolytes developed by Hildebrand and Scott(4) many years ago. The formation of a regular solution requires that the free energy of mixing among the components be negative. In order for this to occur, the heat of mixing of the components must be small r e l a t i v e to the product of the temperature and entropy of mixing (Equation 1). AGmix = AHmix - TASmix (1) AGmix = free energy of mixing AHmix = heat of mixing ASmix = entropy of mixing



15.

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Pitch Precursors

247

The entropy of mixing for a hydrocarbon solution i s primarily a function of the temperature and the melting points of the individual components. The heat of mixing on the other hand is primarily dependent upon the difference in s o l u b i l i t y parameters (or cohesive energy densities) of the components and, to a lesser degree, volume fractions of the components and the temperature. A large difference in s o l u b i l i t y parameter between two components w i l l result i n a high heat of mixing, and hence, poor mutual s o l u b i l i t y . I f one examines a t y p i c a l petroleum pitch and plots the r e l a t i v e proportions of the d i f f e r e n t molecular species by their respective s o l u b i l i t y parameters, a d i s t r i b u t i o n similar to that shown in Figure 1 w i l l be obtained. It can be seen from Figure 1 that the s o l u b i l i t y parameters for components i n a t y p i c a l petroleum pitch range from low values of about 7-7.5 (F^-fe for the p a r a f f i n i c material to over 11 £ § c ^ for the more highly condensed aromatics. The mesophase forming components of the pitch, which are of utmost importance to carbon fiber manufacturers, possess s o l u b i l i t y parameters at the upper end of this distribution. The mesophase forming components have these higher s o l u b i l i t y parameters because they are predominantly composed of large aromatic species which possess the necessary molecular s t r u c t u r a l features for l i q u i d c r y s t a l l i n i t y (or mesomorphisra) to occur. (It i s important to r e a l i z e that mesophase is a phase composed of components.) The d i s t r i b u t i o n in s o l u b i l i t y parameters for the pitch w i l l change for different pitch types as well as for d i f ferent thermal treatments, etc. The range of differences depicted in Figure 1 for the component s o l u b i l i t y parameters suggests, from a thermodynamic standpoint, that an " i n t r i n s i c " pitch solution i s actually unfavorable because the heat of mixing between the large aromatics and the more paraff i n i c and naphthenic material i s large. Since many pitches appear to be more or less homogeneous solutions, however, (based on measurements of glass t r a n s i t i o n , temperature, C/H, s o l u b i l i t y , o p t i c a l isotropy, e t c . ) , then in order to reconcile the thermodynamics, a three dimensional localized micellar structure (or "gradient" solution) must exist throughout the pitch. A two-dimensional cut through one of these postulated micelles i s shown i n Figure 2. The heat of mixing for the o v e r a l l pitch would be reduced i f the low s o l u b i l i t y parameter paraffins s o l u b i l i z e d the naphthenic material which i n turn s o l u b i l i z e d the aromatics (and hence, mesophase forming components). In order for large scale coalescence of the highly condensed aromatic mesophase forming components to occur, the nonmesophase forming fractions on the outer surfaces of the micelles must be removed. As discussed previously, there are various techniques for accomplishing this removal including vacuum stripping and heat-treatment, sparging, and solvent extraction. Extensive thermal treatments remove the majority of the p a r a f f i n i c and naphthenic components from the pitch (by d i s t i l l a t i o n and cracking reactions), and as a result disrupt the micelle, s h i f t the average pitch s o l u b i l i t y parameter to higher levels, and allow for the coalescence of the mesophase forming fractions which are so desirable for carbon fiber manufacture. Additions of large amounts of a given solvent, on the other hand, result in an oversaturation of that part of the micelle most similar to the solvent in terms of s o l u b i l i t y parameter. Those components of the micelle which are most d i s s i m i l a r to the solvent

American Chemical Society Library 1155 16th St., N.w. Arthur et al.; Polymers for Fibers and Elastomers Washington, D . C . 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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POLYMERS FOR FIBERS AND ELASTOMERS


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Figure 1 SCHEMATIC REPRESENTATION OF THE DISTRIBUTION OF MOLECULAR TYPES IN A TYPICAL PETROLEUM PITCH (RANKED BY SOLUBILITY PARAMETER)

Figure 2

HYPOTHETICAL "MICELLE" FOUND IN PITCH


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w i l l precipitate from both the " i n t r i n s i c " pitch solution and the " e x t r i n s i c " pitch/solvent solution. By properly selecting the s o l vent, the pitch/solvent r a t i o and the extraction temperature, i t becomes possible to remove the mesophase forming fractions from the pitch, and hence, to t a i l o r the pitch for s p e c i f i c applications.


Characteristics

of Solvent Extracted Petroleum Pitch

An Exxon cat cracker bottom pitch was extracted with solvents of varying s o l u b i l i t y parameter in order to evaluate the relationships which exist between extraction conditions and the c h a r a c t e r i s t i c s of the extracted product. The precursor pitch possessed a carbon/ hydrogen r a t i o of 1.49 and a c a l c u l a t e d (5, 6^) o v e r a l l average solub i l i t y parameter of 10.48 ( . The reflux quinoline insolubles content of the pitch was 2.3% and the refluxing toluene insolubles were at a l e v e l of 20.3%. The pitch was not o p t i c a l l y anisotropic. Upon heating i n N2 to 530°C i n a TGA, a coke y i e l d of 37% was obtained. The precursor pitch was extracted in solvents with s o l u b i l i t y parameters ranging from 7.40 C ^ i ) ^ to 8.93 (-^f P . The results of the characterizations of the extracted fractions are shown in Table I. Samples 2 and 7 were extracted by f i r s t grinding the precursor pitch followed by both a s l u r r y and f i l t r a t i o n step. The s l u r r y of sample 1 was done in heptane at room temperature, while 7 was s l u r r i e d in refluxing toluene. Both of these samples represent the recovered insolubles after f i l t r a t i o n . Samples 3, 4, 5 and 6 were produced by f i r s t grinding the pitch, s l u r r y i n g the pitch in reflux ing toluene, f i l t e r i n g the ash and other insolubles at refluxing temperatures and then adding heptane in amounts ranging from 0 to 30% depending upon the sample. The r a t i o of pitch to solvent was 8:1. The toluene/heptane solution was then cooled to approximately 30°C, and the insolubles were f i l t e r e d . Samples 3-6 represent the f i l t e r e d insolubles. An examination of Table 1 reveals some interesting trends. As the s o l u b i l i t y parameter of the extraction solvent increases, the mesophase content of the extracted f r a c t i o n goes to >99%. Samples 3, 4, 5 and 6 are "neomesophases"(3) i n that upon melting, they i n stantly transform to a l i q u i d c r y s t a l l i n e state. Their quinoline insolubles contents were less than 1%. The extraction process breaks down the micelle and removes the non-mesomorphic species in the pitch which had inhibited the coalescence of the mesophase forming species. Sample 2, the heptane extracted f r a c t i o n , did not exh i b i t neomesophase behavior. Extended heat treatment at 400+°C was required to convert this sample to a mesomorphic state. The heptane did not remove a s u f f i c i e n t amount of the non-mesophase formers. Shown in Figure 3 i s a plot of the coke yields for the d i f f e r ent fractions as a function of the s o l u b i l i t y parameter of the ext r a c t i o n solvent. The coke yields were measured by heating the f r a c t i o n i n a Perkin-Elmer TGS II under N2 to a temperature of 530°C at a rate of 10°C/minute. As can be seen from both Figure 3 and Table I, the coke yields increase more or less l i n e a r l y as the s o l u b i l i t y parameter of the extraction solvent increases. Coke y i e l d s of over 90% are obtainable for samples extraqj^ed with s o l vents having s o l u b i l i t y parameters over about 8.8 ( -QQ-)% . Weight



7.40

8.46

8.62

8.79

8.93

-

3

4

5

6

7

-

SOLVENT

2

PITCH

RAW

SAMPLE

6

95.0

91.2

87.7

86.3

85.5

54

37

YIELD

COKE

1.88

1.79

1.73

1.76

1.74

1.58

1.49

C/H

-

1600

1310

1120

1000

695

-

MW

344

285

266

246

232

-

Tg

100

85.0

81.1

78.5

73.9

20.3

%rTI

-

99+

99+

99+

99+

0

0

%MESO

11.04

10.92

10.84

10.88

10.86

10.62

10.48

6 PROD

Table I: Fraction Characteristics from Extracted Exxon CCB Pitch


Solvent-Extracted Pitch Precursors for Carbon Fiber - American

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