EP1250496A1 - Flexible graphite article - Google Patents

Abstract

Graphite article (10) in the form of a flexible graphite sheet having parallel, exposed planar surfaces (30) which has decreased transverse electrical and thermal anisotropy and decreased longitudinal permeability fluids due to transverse mechanical deformation of a sheet surface (50) which is subsequently pressed to restore the deformed surface to a planar condition (30).

Description

FLEXIBLE GRAPHITE ARTICLE

Field of the Invention

This invention relates to an article formed of flexible graphite sheet which has

enhanced isotropy with respect to electrical and thermal conductivity and reduced

longitudinal fluid permeability.

Background of the Invention

Graphites are made up of layer planes of hexagonal arrays or networks of carbon

atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and

are oriented or ordered so as to be substantially parallel and equidistant to one another. The

substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as

basal planes, are linked or bonded together and groups thereof are arranged in crystallites.

Highly ordered graphites consist of crystallites of considerable size: the crystallites being

highly aligned or oriented with respect to each other and having well ordered carbon layers.

In other words, highly ordered graphites have a high degree of preferred crystallite

orientation. It should be noted that graphites possess anisotropic structures and thus exhibit

or possess many properties which are highly directional e.g. thermal and electrical

conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated

structures of carbon, that is, structures consisting of superposed layers or laminae of carbon

atoms joined together by weak van der Waals forces. In considering the graphite structure,

two axes or directions are usually noted, to wit, the "c" axis or direction and the "a" axes or

directions. For simplicity, the "c" axis or direction may be considered as the direction

perpendicular to the carbon layers. The "a" axes or directions may be considered as the directions parallel to the carbon la^ ers or the directions perpendicular to the "c" direction. The natural graphites suitable for Manufacturing flexible graphite possess a very high degree

of orientation.

As noted above, the bonding forces holding the parallel layers of carbon atoms

together are only weak van der Waals forces. Natural graphites can be treated so that the

spacing between the superposed carbon layers or laminae can be appreciably opened up so

as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction and thus form an expanded or intumesced graphite structure in which the

laminar character of the carbon layers is substantially retained.

Natural graphite flake which has been greatly expanded and more particularly

expanded so as to have a final thickness or "c" direction dimension which is at least 80 or

more times the original "c" direction dimension can be formed without the use of a binder

into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers,

strips, tapes, or the like. The formation of graphite particles which have been expanded to

have a final thickness or "c" dimension which is at least 80 times the original "c" direction

dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion

which is achieved between the voluminously expanded graphite particles.

In addition to flexibility, the sheet material, as noted above, has also been found to

possess a high degree of anisotropy with respect to thermal and electrical conductivity and

fluid diffusion, comparable to the natural graphite starting material due to orientation of the

expanded graphite particles substantially parallel to the opposed faces of the sheet resulting

from very high compression, e.g. roll pressing. Sheet material thus produced has excellent

flexibility, good strength and a very high degree of orientation. Briefly, the process of producing flexible, binderless anisotropic graphite sheet

material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or

compacting under a predetermined load and in the absence of a binder, expanded graphite

particles which have a "c" direction dimension which is at least 80 times that of the original

particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded

graphite particles which generally are worm-like or vermiform in appearance, once

compressed, will maintain the compression set and alignment with the opposed major

surfaces of the sheet. The density and thickness of the sheet material can be varied by

controlling the degree of compression. The density of the sheet material can be within the

range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The

flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the

alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet,

with the degree of anisotropy increasing upon roll pressing of the sheet material to increased

density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction

perpendicular to the opposed, parallel sheet surfaces comprises the "c" direction and the

directions ranging along the length and width, i.e. along or parallel to the opposed, major

surfaces comprises the "a" directions and the thermal, electrical and fluid diffusion

properties of the sheet are very different, by orders of magnitude, for the "c" and "a"

directions.

This very considerable difference in pioperties, i.e. anisotropy, which is directionally

dependent, can be disadvantageous in some applications. For example, in gasket

applications where flexible graphite sheet is used as the gasket material and in use is held

tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more

readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for greate. gasket performance, if the resistance to fluid flow

parallel to the major surfaces of tha graphite sheet ("a" direction) were increased, even at the

expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet ("c" direction).

With respect to thermal properties, the thermal conductivity of a flexible graphite

sheet in a direction ("a" direction) parallel to the upper and lower surfaces of the flexible

graphite sheet is relatively high, while it is relatively very low in the "c" direction transverse to the upper and lower surfaces.

The foregoing situations are accommodated by the present invention.

Summary of the Invention

A graphite article which comprises a compressed mass of interlocked, elongated,

expanded graphite particles in the form of a sheet of flexible graphite which has, between its

opposed outer planar surfaces, regions of interlocked particles of expanded graphite which

are substantially unaligned with the outer planar surfaces. The regions are formed by

mechanically impacting a surface of the sheet to transversely deform the surface and

displace graphite within the sheet at a plurality of locations and subsequently pressing the deformed, impacted surface to a planar surface.

Brief Description of the Drawings

Figure 1 is a side elevation view of a sketch of a sheet of prior art flexible graphite;

Figure 2 is an enlarged sketch of the view of Figure 1;

Figure 3 is a side elevation view of the sheet of Figure 1 having a transversely deformed surface; Figure 3(A) is top plan view of the deformed sheet of Figure 3,

Figure 4 shows an apparatus suitable for the surface deformation of a flexible

graphite sheet in accordance with the present invention,

Figure 5 is an enlarged sketch of the view of Figure 3,

Figures 5(A)-(C) are photographs at 500X original magnification corresponding to Figures 2, 6(A), 7(A),

Figure 6 is a side elevation view of the transversely deformed sheet of Figure 3

subsequent to compression of the deformed surface to planar form,

Figure 6(A) is an enlarged sketch of the view of Figure 6,

Figure 7 is an enlarged side elevation view of the sheet of Figure 2 which is

transversely deformed at both opposed surfaces,

Figure 7(A) is a side elevation view of the sheet of Figure 7 subsequent to

compression of the deformed surfaces to planar form,

Figures 8(A)-(C) show alternate embodiments of transverse deformations of a

flexible graphite sheet, and

Figure 9 shows a gasket in accordance with the present invention

Detailed Description of the Invention

Graphite is a crystalline form of carbon comprising atoms covalently bonded in flat

layered planes with weaker bonds between the planes By treating particles of graphite,

such as natural graphite flake, with an intercalant of, e g a solution of sulfuπc and nitric

acid, the crystal structure of the graphite reacts to form a compound of graphite and the

intercalant The treated particles of graphite are hereafter referred to as "particles of

intercalated graphite" Upon exposure to high temperature, the particles of intercalated graphite expand in dimension as much as 80 or more times its original volume in an

accordion-like fashion in the "c" direction, i.e. in the direction perpendicular to the

crystalline planes of the graphite. The exfoliated graphite particles are vermiform in

appearance, and are therefore commonly referred to as worms. The worms may be

compressed together into flexible sheets which, unlike the original graphite flakes, can be

formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact.

A common method for manufacturing graphite sheet, e.g. foil from flexible graphite

is described by Shane et al in U.S. Pat. No. 3,404,061 the disclosure of which is

incorporated herein by reference. In the typical practice of the Shane et al method, natural

graphite flakes are intercalated by dispersing the flakes in a solution containing an oxidizing

agent of, e.g. a mixture of nitric and sulfuric acid. The intercalation solution contains

oxidizing and other intercalating agents known in the art. Examples include those

containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid,

potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium

dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated

nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or

mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent

soluble in the organic acid.

In a preferred embodiment, the intercalating agent is a solution of a mixture of

sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid,

perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or

periodic acids, or the like. Although less preferred, the intercalation solutions may contain

metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic

solvent.

After the flakes are intercalated, any excess solution is drained from the flakes and

the flakes are water- washed. The quantity of intercalation solution retained on the flakes

after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight

of graphite flakes (pph) and more typically about 50 to 120 pph. Alternatively, the quantity

of the intercalation solution may be limited to between 10 to 50 parts of solution per

hundred parts of graphite by weight (pph) which permits the washing step to be eliminated

as taught and described in U.S. Pat. No. 4,895,713 the disclosure of which is also herein incorporated by reference. The thus treated particles of graphite are sometimes referred to

as "particles of intercalated graphite". Upon exposure to high temperature, e.g. 700°C to

1000°C and higher, the particles of intercalated graphite expand as much as 80 to 1000 or

more times its original volume in an accordion-like fashion in the c-direction, i.e. in the

direction perpendicular to the crystalline planes of the constituent graphite particles. The

expanded, i.e. exfoliated graphite particles are vermiform in appearance, and are therefore

commonly referred to as worms. The worms may be compressed together into flexible

sheets which, unlike the original graphite flakes, can be formed and cut into various shapes

and provided with small transverse openings by deforming mechanical impact as hereinafter

described.

Flexible graphite sheet and foil are coherent, with good handling strength, and are

suitably compressed, e.g. by roll-pressing, to a thickness of 0.003 to 0.15 inch and a density

of 0.1 to 1.5 grams per cubic centimeter. From about 1.5-30% by weight of needle-shaped

ceramic additives, can be blended with the intercalated graphite flakes as described in U.S.

Patent 5,902,762 (which is incorporated herein by reference) to provide enhanced resin impregnation in the final flexible t raphite product. The additives include ceramic fiber

particles having a length of 0.15 tc 1.5 millimeters. The width of the particles is suitably

from 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive and non-adhering to

graphite and are stable at temperatures up to 2000°F, preferably 2500°F. Suitable ceramic

fiber particles are formed of macerated quartz glass fibers, carbon and graphite fibers,

zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral

fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide

fibers and the like.

With reference to Figure 1 a compressed mass of expanded graphite particles is

shown in the form of a flexible graphite sheet 10 having opposed parallel planar surfaces 30,

40. Figure 2 is an enlarged sketch of the flexible graphite sheet 10 shown in Figure 1

showing schematically a typical orientation of expanded graphite particles 80 aligned

perpendicular to transverse axis 45 substantially parallel to the parallel, planar opposed

surfaces 30, 40 of flexible graphite sheet 10 and longitudinal axis 55. Figure 5(A) is a side

view photograph (original magnification 500X) corresponding to the material of Figure 2.

In the practice of the present invention, a planar surface 30 of flexible graphite sheet 10 of

Figure 2, Figure 5(A), is transversely deformed, advantageously in a continuous pattern, by

mechanically impacting the planar surface 30 with penetration to a predetermined depth, e.g. 1/8 to 1/2 of the thickness of sheet 10, to displace graphite within the sheet 10, for example,

by means of a device 15 such as shown in Figure 4 which includes a roller 75, having

grooves 50 and ridges 60, co-acting with smooth surfaced roller 80. The resulting article is

illustrated in the side elevation view of Figure 3 and the top plan view of Figure 3(A) and

also in the enlarged sketch of Figure 5 which shows at 800 the deformation of the parallel

orientation of expanded flexible graphite particles 80. The spacing of the penetrations is suitably about 1-4 times the depth of the penetrations. The misalignment of the flexible

graphite particles is due to displacement of graphite entirely within flexible graphite sheet

10 resulting from mechanical impact. The transversely deformed article of Figures 3, 3(A),

5 is compressed, e.g. by roll-pressing, to restore the surface 30 to a planar condition as

illustrated in Figure 6(A). With reference to Figure 6(A), after restoring surface 30 to a

planar condition, sheet 10 has a region 70, adjacent planar surface 30, in which expanded

graphite particles 800 are substantially unaligned with parallel, planar opposed surfaces 30,

40. This is optically observable at magnifications of 500X and higher and is illustrated in the photograph (original magnification 500 X) of Figure 5(B) of a side of a flexible graphite

sheet 10 which was processed in the manner described above, i.e. transversely deformed at

one surface and subsequently roll-pressed to restore the surface to a planar condition. With

reference to Figure 7, a flexible graphite sheet 10 can be transversely deformed at both

opposed surfaces 30, 40 either sequentially or simultaneously, and subsequently compressed

to provide planar, parallel opposed surfaces 30, 40 as shown in Figure 7(A). This is

optically observable in the photograph (original magnification 500X) of Figure 5(C). The article of Figure 7(A) has region 70 of substantially unaligned expanded graphite particles

respectively adjacent both of the parallel, planar surfaces 30, 40.

Articles formed of compressed expanded graphite particles in sheet form, as

described above, exhibit increased thermal conductivity in a transverse ("c") direction

through the thickness of the sheet, and reduced fluid permeability between the planar

surfaces of the sheet ("a" direction) as shown below:

Since most fugitive emissions are the result of low gasket load, a conventional test

was performed to determine the result of the use of the material of this invention in a low

gasket load condition. Annular gaskets, 50mm inside diameter and 90mm, outside diameter were clamped

between flanges under a gasket face pressure of 1000 psi, and a nitrogen gas pressure of 300

psi was introduced into the annulus. A control and samples, in accordance with the present

invention, as respectively represented in Figures 2 and 5(A) -"control"; 6(A) and 5(B) -

"one-side deformation"; and 7(A) and 5(C) - "two-side deformation, were tested identically.

Leakage in the longitudinal direction parallel to the face of the gasket surfaces was

3.9, 2.0 and 0.8 ml/mm respectively for the control, and the "one-side" deformation" and "two-side deformation" samples in accordance with the present invention which

demonstrates the reduced longitudinal permeability obtained by articles in accordance with

the present invention.

In addition to the continuous "saw tooth" ridge and groove transverse deformation of

sheet 10, as described above and shown in Figure 4, smooth channels 180 shown in Figure

8(A), closely spaced indentations 182, shown in Figure 8(A), and serrations 184, shown in

Figure 8(C) which are established in sheet 10 by mechanical impact and displacement of

graphite in sheet 10 can alternatively be incorporated in the device 15 of Figure 4 to develop

the required surface adjacent regions 70 of graphite particle deformation to result in

increased "c" direction thermal conductivity and reduced "a" direction fluid permeability.

In accordance with the present invention, sheet material of the type shown in Figures

6(A), 7(A) is formed, by punching into a gasket of the type shown at 156 in Figure 9,

secured between metal plate 154 and metal block 158 by bolts 152 which pass through

openings 159 in gasket 150. The gasket 150 exhibits increased thermal and electrical

conductivity in the direction transverse to surfaces 30, 40 and decreased longitudinal

permeability to fluids, i.e. in the direction parallel to and between surfaces 30, 40. In the practice of the present invention, the flexible graphite sheet can, at times, be advantageously impregnated with resin prior to and subsequent to mechanical impacting of

the sheet and the absorbed resin, after curing, enhances the moisture resistance and handling

strength, i.e. stiffness of the flexible graphite sheet. Suitable resin content is preferably 20

to 60% by weight.

The above description is intended to enable the person skilled in the art to practice

the invention. It is not intended to detail all of the possible variations and modifications which will become apparent to the skilled worker upon reading the description. It is

intended, however, that all such modifications and variations be included within the scope

of the invention which is defined by the following claims. The claims are intended to cover

the indicated elements and steps in any arrangement or sequence which is effective to meet

the objectives intended for the invention, unless the context specifically indicates the

contrary.

Claims

WHAT IS CLAIMED IS:

1) A graphite article comprising a compressed mass of interlocked, elongated,

expanded graphite particles in the form of a sheet having parallel, opposed first and second

planar surfaces, said sheet having a plurality of regions of interlocked particles of expanded

graphite which are randomly oriented and substantially unaligned with the opposed, parallel,

planar surfaces.

2) Article in accordance with claim 1, said regions being formed by mechanically

impacting a first surface of said sheet to transversely deform said surface and displace

graphite entirely within said sheet at a plurality of locations and subsequently pressing the

deformed, impacted first surface to a planar surface.

3) Article in accordance with claim 2 wherein both the first surface and the second

surface of said sheet are mechanically impacted and deformed and both the first and second surface of said sheet are subsequently pressed to planar surfaces.

4) Article in accordance with claim 1 wherein from about 1.5 to 30% by weight of

ceramic additives are incorporated in said sheet prior to mechanically impacting said sheet.

5) Article in accordance with claim 1 wherein said sheet contains 20 to 60% by weight

resin. 6) Method for making a graphite article which comprises:

(i) providing a compressed mass of expanded graphite particles in the form of a

sheet having parallel, opposed, planar first and second surfaces;

(ii) mechanically impacting the first surface of said sheet at a plurality of

predetermined locations to displace graphite within said sheet to deform said surface and

cause particle of expanded graphite at said locations to be substantially unaligned with said

first and second surfaces; and

(iii) subsequently pressing said deformed first surface to a planar surface.

7) Method in accordance with claim 6 wherein both the first and second surfaces of

said sheet are impacted and deformed and subsequently respectively pressed to a planar

surface.

8) Method in accordance with claim 6 wherein from about 1.5 to 30% by weight of

ceramic additives are incoφorated in said sheet prior to mechanically impacting said sheet.

9) Method in accordance with claim 6 which includes the step of impregnating the

sheet with 20 to 60% by weight of resin prior to the pressing step (iii).

10) A sealing gasket comprising a compressed mass of interlocked, elongated, expanded

graphite particles in the form of a sheet having parallel, opposed first and second planar

surfaces, said sheet having a plurality of regions of interlocked particles of expanded

graphite which are substantially unaligned with the opposed, parallel, planar surfaces. 11) Sealing gasket in accordance with claim 10, said regions being formed by

mechanically impacting a first surface of said sheet to transversely deform said surface and

displace graphite entirely within said sheet at a plurality of locations and subsequently

pressing the deformed, impacted first surface to a planar surface.

12) Sealing gasket in accordance with claim 1 1 wherein both the first surface and the

second surface of said sheet are mechanically impacted and deformed and both the first and

second surface of said sheet are subsequently pressed to planar surfaces.

13) Sealing gasket in accordance with claim 10 wherein from about 1.5 to 30% by

weight of ceramic additives are incoφorated in said sheet prior to mechanically impacting

said sheet.

14) Sealing gasket in accordance with claim 12 wherein said sheet contains 20 to 60%

by weight resin.

15) Sealing gasket in accordance with claim 13 wherein said sheet contains 20 to 60%

by weight resin.