CA1078167A

CA1078167A - Graphite powder-polyphenylene mixtures and composites

Info

Publication number: CA1078167A
Application number: CA257,247A
Authority: CA
Inventors: Chen-Shen Wang; Eli W. Blaha
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1975-10-07
Filing date: 1976-07-19
Publication date: 1980-05-27
Also published as: IT1066026B; GB1509879A; FR2327275A1; DK336176A; NL7608254A; US3971748A; FR2327275B1; JPS5410580B2; JPS5245656A; LU75585A1; DE2633496A1; BE844564A

Abstract

ABSTRACT OF THE DISCLOSURE
Moldable resin mixtures comprising branched soluble polyphenyl-enes and graphite powder are useful as plates in acid electrolytic fuel cells and in other applications.

Description

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BACKGROUND OF THE INVENT~
This invention relates to moldable resin mixtures comprising polyphenylenes and graphite powder.
There has been a long felt need for polymeric compositions which can be used in extreme environmental conditions such as prolonged exposure to heat and acid. One application in which such a composition is required is in plates used in acid electrolytlc fuel cells. Generally, suitable plate compositions must be electrically conductive and stable in concentrated acid solutions at temperatures about 200~F. for extended periods of time. An example of a plate structure useful fuel cells is described in Dews et al. U.S. Patent 3,801,374, in which a plate was fashioned from a graphite powder - vinylidene fluoride resin.
We have found that a graphite powder-branched polyphenylene composite formed by compression molding without solvent produces a superior plate useful in fuel cells. Such plates can withstand over 3000 hours in 100% phosphoric acid at 400F. and 0.9 volts applied electrical potential with only minimal weight loss.

. . . . .
In general, polyphenylenes are composed essentially of carbon and hydrogen in aromatic ring type s~ructures, with ~he rings chemically linked to each other through the ortho, meta and para positions. Such polymers are to be distinguished clearly from other chemically similar ~ phenylene type structures, such as polyphenylene oxide, polyphenylene ;

- sulfide, polyphenylene sulfone and other polymers contalning the ': ' designation l'phenylene". Polyphenylenes have generally been produced by techniques such as acid catalyzed oxidative coupling of the benzene ring - in various aromatic compounds. The polyphenylenes produced by these ~-, processes possess some degree of high temperature thermal stabili~y, but they are generally linear ~para-polyphenylene) polymers which are relaeively insoluble and infusible. Polyphenylenes have been produced which do possess certain limited solubility, but these have generally been at ' ?~, ' ' ''' number averaÆe molecular wei~hts of only about 1000 to 2000. Generally, these low molecular weight polyphenylenes contain only a low degree of branching, that is, they are still relatively linear polymers which contain long linear segments.
The branched polyphenylenes usefu~ in producing superior graphite powder composites are those novel polyphenylenes dlsclosed by Wennerberg and Wang in U.S. Patent 3,792,099 and produced by the process described in U.S. Patents 3,829,518 and 3,798,281. The carbon hydrogen ratio of our polymer is generally between about 1.50 and 1.60. Ultraviolet spectral analysis shows no band at 3.16 x 104c~n 1, The absence of a band at that point indicates the absence of lengthy linear chains of benzene rings. Nuclear magnetic resonance indicates the presence of meta-linked phenyl units. Pyrolytic gas chroma~ography at 900C and 1,200e.
resulted in the detection of large amounts of m- and p-terphenyls. This indicates that the polymer ls co~posed from simple phenyl~ne units rather ;-than from fused rings. These polyphenylenes possess increased solubility over prior art polyphenylenes and excellent thermal stabllity over a number average molecular weight range from 1000 to over 10,000. Also, small amounts of branched nitropolyphenylene can be incorporated within ` 20 the composites of our invention. Such nitropolyphenylenPs are described ln U.S. Patent No. 3,974,121 filed December 5, 1974.
SUMMARY OF OUR INVENTION
Our invention comprises a moldable resin mixture comprising:
(a) about 25 to 95% graphite powder;
(b) about 5 to 75g branched polyphenylene comprising benzene ring structures bonded onto a polymer chain wherein the linear infrared absorbance spectrum integrated peak area within the frequency range 726-930 cm 1 is distributed as follows: from 10 to 18% ~ the integrated peak area is ~ithin the frequency range 854-903 cm 1, from 15 to 30% is 30 wlthin ~he frequency ra~ge 806-853 cm 1~ from 13 to 20~ ~s wqthi~ the - r . ... .. .. .. . .. . - -. - .. . ~ . - .. .. ; . . ~ -frequency range 778-805 cm l, and the remainder of the integrated peak area within the frequency range 726-930 cm is - within the frequency range 726-777 cm l; and (c) 0 to about 30% branched nitropolyphenylene com-prising benzene ring structures bonded into a polymer chain and from 0.25 to 15 percent by weight of nitrogen wherein infrared absorbance occurs at frequencies of about 1345 cm l and about 1525 cm l and at least 8% of the linear in~rared ;
absorbance spectrum integrated peak area within the frequency range 726-930 cm l is within the frequency range 865-930 cm l - BRIEF DESCRIPTION OF THE INVENTION :
Moldable resin mixtures of our invention, described above, are useful in forming composites. These composites of our invention comprise graphite powder bonded with polyphenyl-enes. More specifically our composites comprise about 25 to 95% graphite powder, about 5 to 75~ branched polyphenylene and `
0 to about 30% nitropolyphenylene. For use under prolonged highly acidic conditions, such as in fuel cell plates, our composites should contain about 85 to 95% graphite powder, about 5 to 15~ branched polyphenylene and less than about 5% nitro-polyphenylene. The preferable fuel cell plate composite contains about 90% graphite powder and about 10~ branched poly-phenylene. All percentages are in weight percent.
Generally, as the amount of graphite powder is de-creased and correspondingly the quantity of polyphenylene is increased, the composite's toughness, impact s-trength, and pliability is enhanced up to about 15% resin, after which point , acid resistance will decrease. Thus, a mechanical seal fitted into a compressor or pump not subject to prolonged acid exposure can be fashioned from a composite preferably having about 40 to 70% g~aphite powder, about 30 to 60% branched -:
;'~' ~7~67 polyphenylene and 0 to about 10~ nitropolyphenylene. Such a mechanical seal element is self-lubricating.
Branched polyphenylene useful in this invention should have at least about 8% by weight of its benzene ring structures bonded to three or more other benæene ring structures, that is, it should be at least 8% branched. Such branched polyphenylene can also be characterized by the relative amounts of the linear infrared absorbance spectrum integrated peak area within the ~requency ran~e 726-930 cm 1, In general, about 7 to 18%, preferably 10 to 18~, of the total integrated peak area within the frequency range 726-930 cm 1 should fall within the frequency range 854-930 cm 1 (I region).
The frequency range 806-853 cm (P region) generally accounts for about 15 to 30%, preferably 18 to 26%, of the total integrated peak area. The frequency range 778-805 cm 1 (M region) accounts for about 13 to 20% of the total integrated peak area.
Branched polyphenylene can alternatively be characterized by the amount of the various types of benzene ring structures present in the polymer chains, which is deter-.~
mined according to the following equation: c = b a*~ In :. , this equation, A is the-planimeter area reading for the - particular absorption frequency range corrected by a constant factor relating to the planimeter used in the measurement and is in units of cm 1. The values of A for the region between 854 and 930 cm 1 are corrected for the presence of meta-disubstituted benzene ring structures by applying a correction factor obtained from the value of A for the region 778 805 cm 1.
The correction factor is one-third of the A value for the 778-805 cm 1 region. The term "b" is the thickness of the KBr pallet in units of cm. The term "a*l' is the integrated :- . :

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absorptivity in units of g 1 1 cm 2. The values for ~* are obtained from the integrated peak areas of the reference compounds determined under essentially the same operating - conditions used for obtaining the spectra for the polyphenylenes.
The term "c" is the concentration, in grams per liter, of any of the characteristic benzene ring structures associated with the regions I, P, M and pH. The amount of the various types of benzene ring structures present in the polymer chain is determined by dividing the measured concentration obtained from a particular frequency range by the sum o~ the concentrations obtained from the four frequency ranges involved. Further details of this procedure are described in U.S. Patent 3,792,099.
- By the above-described analysis, the amount of benzene ring structures in the branched polyphenylene polymer ` chains which are at least trisubstituted, that is, bonded to three or more other benzene ring structures, is at least about 8~ by weight, preferably 10% by weight and is more pre-erably from about 12 to about 25 percent by weight. The amount of benzene ring structures which are disubstituted, bonded to two other benzene ring structures through either the para, meta, or ortho positions, is preferably from about 45 to about 65 percent by weight. The amount of benzene ring structures which are meta-disubstituted, bonded through the meta position to two other benzene ring structures, is pre-ferably from about 15 to about 35 percent by weight. The terminology "double bonding through the meta position" refers to the bonding of a benzene ring structure to two other ben-zene ring structures through the meta positions of the benzene ring structure. The remaining benzene ring structures , _ 5 _ ~
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in the polymer chains are bonded to only one othex benzene ring structure.
The inherent viscosity of the branched polyphenylenes can vary from about 0.025 or less to more than 0.17 when measured in trichlorobenzene at 135C. at a concentration of 0.02 g/ml. This roughly corresponds to a nu~ber average molecular weight range o~ from 1000 or less to greater than 10,000. A number average molecular weight range of about 3,000 to 10,000 is particularly advantageous for the preparation of the nitropolyphenylenes of this invention.
A particularly preferred branched polyphenylene resin is that prepared from biphenyl by the dehydrogenative coupling process described in U.S. Patents 3,829,518 and 3,798,281.
Branched polyphenylene can be converted to branched ; nitropolyphenylene useful in this invention by a nitration reaction. A preferred nitration reaction consists of the addition of a mixed nitric acid and sulfuric acid nitrating agent to a solution consisting of th~ branched polyphenylene, water and sulfuxic acid. This reaction is preferably conducted at from 0C. to 50C. for from 3 hours to 10 hours, more pre-ferably from about 5-15C. for about 2 to 4 hours followed by a few hours at a temperature of about 30~50C.
The branched nitropolyphenylenes of this invention also contain at least 0.25 percent by weight of nitrogen and at least about 0.58 percent by weight of oxygen. The nitropolyphenylenes exhibit infrared absorbance at both 1345 cm 1 and 1525 cm 1 which indicate the presence of nitro groups.
Preferably, the branched nitropolyphenylene will have a softening point between 150C. and 350C., and it will contain ~ ;
at least 0~5 percent nitrogen and at least 1.15 percent oxygen, and no more than about 15 percent nitrogen and about -~ ~8~

35 percent oxyg~n. More pre~erably, the amount of nitrogen contained in the polymer will be rom 0.75 percent to 5 percent and most preferably from 1 to 4 percent.
Using the integrated peak area oktained from linear infrared absorption spectra for the region from about 600 cm 1 to 1000 cm 1, it has been determined that the branched nitro-polyphenylene of this invention must have at least 8 percent of the linear infrared absorbance spectrum integrated peak area in the frequency range 726-930 cm 1 within the I frequency range 865-930 cm 1. It is this type of absorption which is indicative of polymer chain branching.
Preferably, the total linear infrared absorbance spectrum integrated peak area within the frequency range 726-930 cm 1 is distributed as follows: from 8 to 22~, most preferably from 12 to 20~, of the integrated peak area ~alls within the frequency range 865-930 cm 1; from 20 to 45%, most preferably from 25 to 40~, of the integrated peak area falls -within the frequency range 806-864 cm 1; from 7 to 20~, most preferably from 10 to 18%, of the integrated peak area falls - 20 within the frequency range 778 805 cm 1; and the remainder of the integrated peak area within the frequency range 726-930 cm 1 falls within the frequency range 726-777 cm 1 Perferably, the branched nitropolyphenylene has an inherent viscosity of at least 0.04 when measured in trichloro-benzene at 135C. at a concentration of 0.02 g/ml. The number average molecular weights of the nitropolyphenylene --range from as low as 1000 to greater than lO,jOOO, and are ;~
preferably greater than about 4000 .
' Unlike graphite and carbon fiber composites using ~ 30 polyphenylenes, graphite powder-polyphenylene composites can be prepared by dry molding, which is simpler, ~uicker and less . ~ , "'- '., , . . ,, . ~ . . . .. .
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expensive than the solvent casting method used in forming fiber composites and avoids any residual solvent in the final composite. A preferred method of preparation is to blend graphite powder and polyphenylene (either branched polyphenylene alone or a mixture with nitropolyphenylene) in a Waring* blender for at least 5 minutes. This blend is compacted into a pre~s-ing mold which is inserted into a well ventilated press.
Useful molding conditions range ~rom about 600 to 950F. a-t about 1600 to 8000 p.s.i. for about 5 to 30 minutes. Typically, as the amount of polyphenylene in a mixture is increased, the temperature requixed to form a suitable composite must be decreased. The effect of various mixture compositions and molding conditions are shown in Table IV.
Our invention is demonstrated but not limited by the following examples.
EXAMPLE I
Into a stirred autoclave there was charged 20 grams of a MoO3-SiO2. A12O3 catalyst, 1000 grams of biphenyl and 300 psig of hydrogen. Constant heat input conditions were ~ -applied until the temperature reached 900F. at which time ~
the heat input was reduced. The reaction was continued for ~ `
5-1/2 hours during which time the maximum temperature was 1070F. and the maximum pressure was 1785 psig~ ~he inherent viscosity of the worked-up polyphenylene product was 0.14 in `-trichlorobenzene at 135C. at 0.02 g/l using a Cannon-Ubbelohcle* viscometer. The softening point was about 210C.
A linear absorbance infrared spectra was obtained - (Perkin-Elmer* Model 180) and analyzed in accordance with the procedure outlined in U.S~ Patent 3,792,099 using the constant factor to convert the planimeter readings to the * Trade Marks - g - ~, .

~lO7B167 values of A as 1/20.75 cm 1. The integrated absorptivity values (a*, in units of g 1 1 cm ) were as ~ollows: Region I
-13.04, P-17.15, M-12.86, and PH-40.27. The resin concentration in the KBr pellet was 2g.66 g/l which had a thickness of 0.0566 cm. The normalized weight pPrcents attributable to each of the characteristic benzene ring structures is shown in Table I. The percent recovery was 101.6%.

TABLE I
Planimeter Integrated A/b c Normalized Region (1) rea (cm2) Area (%) (cm~2) ~g/l) Weight Percent I 126.0 I (2) 86.2 12.2 73.34 5.62 19.30 P 178 25.3 151.45 8.83 30.32 M 119.5 17.0 101.67 7.90 27.13 PH 320.5 45.5 272.69 6.77 23.25 (1) As described and defined in U.S. 3,792,099.

(2) Corrected as indicated in U.S. 3,792,099.
A portion of the polyphenylene prepared above was ground and powdered to under 200 mesh. In a blender, 197.1 grams of graphite powder (A. Daigger, Chicago, Illinois) was blended for 5 minutes with 27.9 grams of the powdered poly- -phenylene. This mixture was compaated in a 7" x 7" mold for pressing~ The mold was inserted into a press (Pasadena Hydraulics) having a platen temperature of 950F. A pressure of 3,200 psi was applied and then quickly released to remove - air from the prepreg. Pre~sure was again applied and was released three times at 5-minute intervals af~er the tempera- -ture equilibrated to 950F. Total pressing time was 30 minutes at 950F. The 1/8-inch thick composite was cooled to 100F. in the mold while the pressure was maintained. The -; flexural strength was 4,900 psi and the flexural modulus was 2,290,000 psi. The composite had an elec~rical conductance _ ~ _ .
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similar to graphite.
EXAMPLE II
A nitropolyphenylene was prepared byplacing 50 grams of branched polyphenylene prepared as in E~ample I into a stirred reaction container togethar with 700 milliliters of concentrated suluric acid and 140 milliliters of water. I'o this was added over a one-hour period with stirring at 5-10C.
a mixture of 9.8 milliliters of concentrated sulfuric acid and 4.2 milliliters of nitric acid (Sp. gr. 1.42). The reaction mixture was maintained for an additional 3 hours at 5-10C. and for 4 more hours at 40C. The dark solid product was washed to neutrality with distilled water and dri~d at ;
120C. under vacuum. The nitropolyphenylene product had a -; softening point of about 185C. and an inherent viscosity of 0.05 at 135C. in trichlorobenzene at 0.2 g/l ml. The elemental analysis was N, 0.93~; O, 2.33%; C, 91.79~ and H, 4.88%. Infrared absorbance was observed at 1345 cm 1 and 1525 cm A linear absorbance spectrum was obtained from a KBr pellet prepared according to the procedure described in Example I. The KBr pellet concentration was 24.80 g/l and ~ -thickness was 0.0496 cm. The planimeter integrated peak areas and percent of the integrated peak area associated with each region is shown in Table II.
TABLE II
Region Planimeter Area ~ Integrated Area I (865-930 cm 1) 60.9 12.6 P (806-864 cm 1~ 115 23.7 M 74 15.3 P~ 235 48.5 In a blender, 92 grams o~ graphite powder was blended ~' ~
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~78~;7 for 5 minutes with 8 grams o~ the unmodified branched poly-phenylene described in Example I and 4 grams of the ni-tropoly-phenylene described above. Both the unmodified polyphenylene and the nitropolyphenylene were powdered to under 200 mesh and blended toge-ther for 10 minutes before being blended with the graphite powder. This mixture was compacted in a 7" x 7"
mold for pressing. The mold was inserted into a press with a platen temperature of 950F., and a pressure of 3,200 psi was applied and quickly released to remove air ~rom the prepreg.
Pressure was again applied and released three times at 5-minute intervals after the temperature equilibrated to 950F. Total pressing time at 950F. was 30 minutes. The 1/16-inch thick composite was cooled to 100F. in the mold while the pressure was maintained. The composite had a heat distortion tempera-ture of more than 540F. at 264 psi. The flexural strength was 4,600 psi and the flexural modulus was 1,809,000 psi. The composite had an electrical conductance similar to graphite.
EXAMPLE III
In a manner similar to that described in Examples I
and II above, a composite was made rom 67 parts of graphite .
powder, 23 parts of the unmodified branched polyphenylene described in Example I and 10 parts of the nitropolyphenylene of Example II. Essentially the same molding procedure was used as in Examples I and II. This composite also had a heat distortion temperature of more than 540F. at 264 psi and had a flexural strength of 4,900 p9i and a flexural modulus of 1,683,000. The composite had an electrical conductivity similar to graphite.
EXAMPLE IV
~ branched polyphenylene-graphite powder composite was formed in a 2" x 2.5" mold using 2 grams of polyphenylene y (inherent viscosity 0.07) and 18 grams of graphite powder.
The sample was prepared in a manner similar to that described in Example I using a platen temperature of 950F., a pressure of 3,200 psi and a total pressing time of 30 minutes.
This 1/8-inch thick composite was placed in a bath containing 100% phosphoric acid at 400F. An electrical potential of 0.9 volts was applied across the composite. The results of this test are shown in Table III.
TABLE III
Ex~_s re Time thours) Total Weight Loss (%) 240 1.6 500 1.6 1000 1.6 3000 0.4*

* The increase in weight is attributed to failure to wash out all phosphoric acid before weighingO
EXAMPLES V-XXVI
A series of polyphenylene-carbon powder composites :-were formed using the method described in Examples I and II.

20 The results are given in Table IV.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A moldable resin mixture comprising:
(a) about 25 to 95% graphite powder;
(b) about 5 to 75% branched polyphenylene comprising benzene ring structures bonded into a polymer chain wherein the linear infrared absorbance spectrum integrated peak area within the frequency range 726-930 cm-1 is distributed as follows: from 10 to 18% of the integrated peak area is within the frequency range 854-930 cm-1, from 15 to 30% is within the frequency range 805-853 cm-1, from 13 to 20% is within the frequency range 778-805 cm-1, and the remainder of the integrated peak area within the frequency range 726-930 cm-1 is within the frequency range 726-777 cm-1; and (c) 0 to about 30% branched nitropolyphenylene comprising benzene ring structures bonded into a polymer chain and from 0.25 to 15 percent by weight of nitrogen wherein infrared absorbance occurs at frequencies of about 1345 cm-1 and about 1525 cm-1 and at least 8% of the linear infrared absorbance spectrum integrated peak area within the frequency range 726-930 cm-1 is within the frequency range 856-930 cm-1.

2. The moldable resin mixture of Claim 1 comprising about 85 to 95%
graphite powder and about 5 to 15% branched polyphenylene and less than about 5% nitropolyphenylene.

3. A composite formed by bonding with heat and pressure the mixture of Claim 1 or 20

4. An electrically conductive fuel cell plate formed by bonding with heat and pressure the mixture of Claim 2.

5. The moldable resin mixture of Claim 1 comprising about 90%
graphite powder and about 10% branched polyphenylene.

6. An electrically conductive fuel cell plate formed by bonding with heat and pressure the mixture of Claim 5.

7. A composite formed from the mixture of Claim 1 containing about 40 to 70% graphite powder, about 30 to 60% branched polyphenylene and 0 to about 10% nitropolyphenylene.

8. A mechanical seal element formed by bonding with heat and pressure the composite of Claim 7.