U.S. patent application number 10/006419 was filed with the patent office on 2002-09-12 for acrylic resin-impregnated bodies formed of expanded graphite, process for producing such bodies and sealing elements, fuel cell components and heat-conducting elements formed of the bodies.
Invention is credited to Bacher, Jurgen, Ottinger, Oswin.
Application Number | 20020127390 10/006419 |
Document ID | / |
Family ID | 7666154 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020127390 |
Kind Code |
A1 |
Ottinger, Oswin ; et
al. |
September 12, 2002 |
Acrylic resin-impregnated bodies formed of expanded graphite,
process for producing such bodies and sealing elements, fuel cell
components and heat-conducting elements formed of the bodies
Abstract
Bodies made of expanded graphite are impregnated with
low-viscosity, solvent-free, storage-stable, polymerizing acrylic
resins up to resin contents of 50% by weight. A primary product
made of expanded graphite has an open pore system, with a
particularly preferred range of bulk densities of from 0.5 to 1.3
g/cm.sup.3 and with an ash value of not more than 4% by weight.
Such bodies can also contain a proportion of additives. The
impregnated, shaped and rapidly curable graphite bodies are
employed as sealing elements, as components in fuel cells or as
heat-conducting elements. A process for producing the bodies is
also provided.
Inventors: |
Ottinger, Oswin; (Meitingen,
DE) ; Bacher, Jurgen; (Wertingen, DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
POST OFFICE BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Family ID: |
7666154 |
Appl. No.: |
10/006419 |
Filed: |
December 7, 2001 |
Current U.S.
Class: |
428/323 ;
428/292.1 |
Current CPC
Class: |
C04B 41/009 20130101;
F28F 21/02 20130101; C08K 9/08 20130101; C04B 2111/00853 20130101;
Y10T 428/25 20150115; C04B 41/483 20130101; C04B 41/83 20130101;
C04B 35/522 20130101; Y10T 428/249924 20150401; C04B 2111/1012
20130101; C04B 41/009 20130101 |
Class at
Publication: |
428/323 ;
428/292.1 |
International
Class: |
D04H 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 7, 2000 |
DE |
100 60 838.8 |
Claims
We claim:
1. A synthetic resin-impregnated body, comprising: expanded or at
least partially recompressed expanded graphite; said graphite
containing one of: at least one solvent-free, low-viscosity,
storage-stable, polymerizable acrylic resin system; and polymers
obtained by curing said at least one resin system.
2. The synthetic resin-impregnated body according to claim 1,
wherein said at least one acrylic resin system contains
triethyleneglycol dimethacrylate and at least one initiator
system.
3. The synthetic resin-impregnated body according to claim 2,
wherein said at least one acrylic resin system contains azo
initiators as said at least one initiator.
4. The synthetic resin-impregnated body according to claim 3,
wherein said azo initiators contained in said at least one acrylic
resin system are selected from the group consisting of
2,2'-dimethyl-2,2'-azodipropiononit- rile,
1,1'-azobis(1-cyclohexanecarbonitrile) and azoisobutyric acid
dinitrile.
5. The synthetic resin-impregnated body according to claim 1,
wherein said at least one acrylic resin system has a storage
stability at room temperature of more than two days.
6. The synthetic resin-impregnated body according to claim 1,
wherein said at least one acrylic resin system has a storage
stability at room temperature of more than two weeks.
7. The synthetic resin-impregnated body according to claim 1,
including up to 50% by weight of acrylic resin.
8. The synthetic resin-impregnated body according to claim 1,
including 5 to 25% by weight of acrylic resin.
9. The synthetic resin-impregnated body according to claim 1,
including 10 to 20% by weight of acrylic resin.
10. The synthetic resin-impregnated body according to claim 1,
wherein a primary product contains fillers selected from the group
consisting of ceramic fillers, mineral fillers, electrically
non-conductive fillers and electrically conductive fillers.
11. The synthetic resin-impregnated body according to claim 1,
including at least two independently held together networks, one of
said networks being formed of a connected framework made of
expanded or expanded and thereafter at least partially recompressed
graphite with good electrical conductivity as well as good thermal
conductivity, and the other of said networks being a connected
network made of synthetic material having penetrated into said
graphite.
12. The synthetic resin-impregnated body according to claim 1,
including a surface, regions close to said surface, and another
part, said at least one acrylic resin system disposed only in one
of said part and said regions.
13. The synthetic resin-impregnated body according to claim 1,
wherein a continuous resin surface film is not present and the body
is electrically conductively contactable.
14. A process for producing a body containing at least one
synthetic resin, which comprises: providing a primary product made
of expanded or at least partially recompressed expanded graphite
with an open pore system; impregnating the primary product with at
least one solvent-free, low-viscosity, storage-stable,
polymerizable acrylic resin system to form a resin-containing,
uncured intermediate product; and finally subjecting the
resin-containing, uncured intermediate product to a curing
treatment for the at least one resin system.
15. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
processing the resin-containing, uncured intermediate product to
form a shaped body; and carrying out the subjecting step by
subjecting the uncured shaped body produced from the uncured
intermediate product to a curing treatment for the at least one
resin system.
16. The process for producing a body containing at least one
synthetic resin according to claim 15, which further comprises
simultaneously shaping the acrylic resin-containing body and curing
the resin system that is present as a result of temperature
impact.
17. The process for producing a body containing at least one
synthetic resin according to claim 16, which further comprises
providing the primary product made of expanded or at least
partially recompressed expanded graphite having an open pore
system, with an ash value of not more than four per cent.
18. The process for producing a body containing at least one
synthetic resin according to claim 16, which further comprises
providing the primary product made of expanded or at least
partially recompressed expanded graphite having an open pore
system, with an ash value of not more than two per cent.
19. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
providing the primary product made of expanded or at least
partially recompressed expanded graphite having an open pore
system, with a bulk density in a range of from 0.1 to 1.8
g/cm.sup.3.
20. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
providing the primary product made of expanded or at least
partially recompressed expanded graphite having an open pore
system, with a bulk density in a range of from 0.3 to 1.5
g/cm.sup.3.
21. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
providing the primary product made of expanded or at least
partially recompressed expanded graphite having an open pore
system, with a bulk density in a range of from 0.5 to 1.3
g/cm.sup.3.
22. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
carrying out the step of impregnating the primary product made of
expanded or at least partially recompressed expanded graphite
having an open pore system, with acrylic resins having a viscosity
at room temperature of less than 100 mPa.multidot.s.
23. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
carrying out the step of impregnating the primary product made of
expanded or at least partially recompressed expanded graphite
having an open pore system, with acrylic resins having a viscosity
at room temperature of less than 50 mPa.multidot.s.
24. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
carrying out the step of impregnating the primary product made of
expanded or at least partially recompressed expanded graphite
having an open pore system, with acrylic resins having a viscosity
at room temperature of less than 20 mPa.multidot.s.
25. The process for producing a body containing at least one
synthetic resin according to claim 14, wherein the primary product
made of expanded or at least partially recompressed expanded
graphite having an open pore system, takes-up up to 100% by weight
of its own weight of acrylic resins, during the impregnating
step.
26. The process for producing a body containing at least one
synthetic resin according to claim 14, wherein the primary product
made of expanded or at least partially recompressed expanded
graphite having an open pore system, takes-up 5 to 35% by weight of
its own weight of acrylic resins, during the impregnating step.
27. The process for producing a body containing at least one
synthetic resin according to claim 14, wherein the primary product
made of expanded or at least partially recompressed expanded
graphite having an open pore system, takes-up 10 to 25% by weight
of its own weight of acrylic resins, during the impregnating
step.
28. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
carrying out the step of curing the acrylic resins in less than ten
minutes under the effect of temperatures of up to 200.degree.
C.
29. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
carrying out the step of curing the acrylic resins in less than
three minutes under the effect of temperatures of up to 200.degree.
C.
30. The process for producing a body containing at least one
synthetic resin according to claim 14, which further comprises
mixing the expanded graphite with fillers selected from the group
consisting of ceramic fillers, mineral fillers, electrically
non-conductive fillers and electrically conductive fillers;
processing the mixed expanded graphite to form a filler-containing
primary product; and then impregnating the primary product with
resin.
31. A sealing element, comprising: a synthetic resin-impregnated
body having expanded or at least partially recompressed expanded
graphite; said graphite containing one of: at least one
solvent-free, low-viscosity, storage-stable, polymerizable acrylic
resin system; and polymers obtained by curing said at least one
resin system.
32. A fuel cell component, comprising: a synthetic
resin-impregnated body having expanded or at least partially
recompressed expanded graphite; said graphite containing one of: at
least one solvent-free, low-viscosity, storage-stable,
polymerizable acrylic resin system; and polymers obtained by curing
said at least one resin system.
33. A heat-conducting element, comprising: a synthetic
resin-impregnated body having expanded or at least partially
recompressed expanded graphite; said graphite containing one of: at
least one solvent-free, low-viscosity, storage-stable,
polymerizable acrylic resin system; and polymers obtained by curing
said at least one resin system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a synthetic resin-impregnated body
made of expanded or at least partially recompressed expanded
graphite, a process for producing such a body and sealing elements,
fuel cell components and heat-conducting elements formed of the
bodies. In this contest, the phrase "synthetic resin-impregnated
body" is understood to mean a body made of expanded graphite which
is impregnated by synthetic resin.
[0003] Material composites of graphite and plastics are widely used
in many technical applications. For example, particles of
electrographite are processed with fluoroplastics into highly
corrosion-resistant components for the construction of chemical
apparatus, but they are comparatively expensive due to the costs of
the fluoroplastics and the processing technique required. A subject
which in terms of content is even closer to the present application
is set out in U.S. Pat. No. 4,265,952: expanded graphite is mixed
with fine PTFE powder, for example, and subsequently compressed. To
that extent, the production technique differs from the impregnating
technique described in the present application.
[0004] Another example of a material composite of graphite and
plastic is superficially resin-impregnated foils made of natural
graphite, which are predominantly employed in the form of flat
seals against particularly aggressive media. Many references to
that second example are found in the technical literature.
[0005] Today, thousands of tons of foils made of natural graphite
are produced worldwide every year. The process used therefor is
described in European Patent Application 0 087 489 A1, U.S. Pat.
No. 3,404,061 and U.S. Pat. No. 3,494,382. The disclosures of those
references are incorporated by reference in the present
application. To summarize, the following takes place: an
intercalating agent such as, for example, concentrated sulfuric
acid, acts on natural graphite, preferably platelike or flaky
natural graphite, in the presence of an oxidizing agent such as
concentrated nitric acid or hydrogen peroxide, for example. That
results in graphite intercalation compounds in the graphite flakes
or graphite platelets. The flakes are thermally decomposed by brief
heating, for example by introduction into the flame of a gas burner
and, as a result of the gas pressure arising in their interior
during that decomposition process, puff up to form loose graphite
particles with a wormlike shape. That product is also referred to
as "expanded" graphite or as graphite expandate.
[0006] Expanded graphite is extremely plastic and can be readily
shaped without the aid of a special binder while being compressed
to a greater or lesser degree. Economically, the most important
product thus produced is a flexible graphite foil, which can be
produced efficiently on calender belts. Such products have typical
bulk densities of between 0.7 and 1.3 g/cm.sup.3. However, other
parts having different geometry, for instance individual sealing
bodies which, on average, are compressed to a greater degree and
have bulk densities of 1.0 to 1.8 g/cm.sup.3, are also possible.
There are also sponge-like parts on average having low bulk
density, with values of 0.1 to 1.0 g/cm.sup.3. All of those bodies
with different shapes and different bulk densities have an open
pore system. They are referred to hereinbelow as a "primary
product".
[0007] Material composites formed of such a primary product and of
synthetic resins or plastics materials perform a variety of tasks.
Synthetic resins or plastics materials lower the permeability,
improve the surface properties, for example the scratch resistance,
increase the strength to a small extent, lower the thermal
stability of a material composite containing expanded graphite, and
can reduce the electrical conductivity or modify the resistance to
media. An expedient technique for the production of the material
composites is impregnation.
[0008] According to German Patent DE 32 44 595 C2, corresponding to
UK Patent Application GB 2 131 500 A, the sticking action of
graphite foils to metal surfaces can be reduced by impregnating the
primary product with furan resin in regions close to the
surface.
[0009] According to the prior art, the substantial impregnation of
shaped bodies made of expanded and partially recompressed graphite
is difficult. In order to overcome such difficulties, International
Publication No. WO 99/16141 (U.S. Pat. No. 6,037,074) teaches that
such a body can be satisfactorily impregnated when it is
interspersed with mineral fibers, which also pass through the
surface of the particular bodies. In that way, small channels are
formed along those mineral fibers, in which the resin can flow into
the interior of the bodies during the impregnation. In that
reference, a phenolic resin dissolved in acetone, i.e. a
solvent-containing thermosetting resin with condensation reactions
during the curing, is cited as the impregnating agent.
[0010] Another method for achieving good impregnation of bodies
made of expanded graphite resides in converting the desired resins
through the use of solvents into low-viscosity liquids, whereby the
impregnation becomes more complete. In Published Japanese Patent
Application JP 1 100 040 A2, the thermosetting resins cited are
based on phenols, epoxides, polyimides, melamines, polyesters and
furans, which are used in a mixture solution with
polyvinylbutyral.
[0011] Published Japanese Patent Application JP 1 308 872 A2
describes the solution to other problems. A material composite
formed of a glass fiber nonwoven fabric and an expanded graphite
foil is produced in order to thus strengthen the latter and overall
obtain a liquid-tight material. That is achieved by impregnating
with epoxy resin. The resin penetrates the nonwoven fabric and at
the same time also penetrates into the surface, i.e. partially into
the foil. During subsequent curing, the supporting part of the
composite material is formed from the impregnated nonwoven fabric
and is then also sealed at the surface.
[0012] The impregnation of expanded graphite foil with phenolic
resin or epoxy resin, set out in Published Japanese Patent
Application JP 60 24 2041 A2 (German patent DE 35 12 867 C2),
serves similar purposes, namely to improve strength and
gas-tightness. The special feature in that case lies in a degassing
process for the liquid resins and the foil present therein which is
repeated a number of times, presumably with the aim of improving
the quality of the impregnation.
[0013] German Published, Non-Prosecuted Patent Application DE 43 32
346 A1 describes the impregnation of the expanded graphite foils
for the purpose of improving adhesion to elastomer layers lying
thereon. The viscosity of the epoxy resins used in that case is
2100 to 2400 mPa.multidot.s.
[0014] Published Japanese Patent Application JP 11 35 4136 A2
entitled "Fuel Cell, Separator for Fuel Cell, and Manufacture
Therefor" describes the production of expanded graphite in
sheet-like form. That partially recompressed expanded graphite is
subsequently comminuted (pulverized) and then mixed selectively
with resins, solvent-free epoxy resin, solid epoxy resin, melamine
resin, acrylic resin, phenolic resin, polyamide resin, and the
like. The mixture is subsequently shaped. As will be shown below,
that technique differs from the bodies according to the invention
which have an entirely different structure in that the resins are
mixed into an expanded graphite granulate.
[0015] International Publication No. WO 98/09926 describes a
graphite foil which is coated with a plastic on at least one side.
That is accomplished firstly with an aqueous solution of an acrylic
resin which is applied to the surface, remains there, but also
penetrates into regions of the foil close to the surface, and is
then dried in.
[0016] The prior art set out above discloses various synthetic
resin-containing bodies produced by using expanded graphite and
processes for their production. That it is difficult to produce
high-quality, synthetic resin-containing graphite bodies from
recompressed, expanded graphite is easy to see. All of the
described processes have disadvantages, some of which are serious:
if resins that are diluted by solvents and thus have lower
viscosity are used during the impregnation, it is true that the
impregnation is easier. However, the vapors from the, in most
cases, readily volatile solvents cause serious problems during the
impregnation itself, especially during subsequent process steps. In
particular, as a result of the fact that they escape during the
curing of the resins, they leave behind fine channels which raise
the permeability of the bodies being produced. If an increased
permeability can neither be tolerated nor is desired, there is
furthermore a general problem: if the curing is not performed very
slowly, i.e. is time-consuming, blisters and cracks are formed in
the bodies, which lower their quality considerably. The same
applies to resin systems which release gases from condensation
reactions during the curing.
[0017] As a result of the fact that solvents or other gases and
vapors escape, a residual porosity arises in the bodies. Attempts
are now frequently made to eliminate the residual porosity by one
or more additional impregnating operations. The attendant increase
in expenditure is clear and the success is really limited. In
addition, solvent-containing resins always above all require
measures to allow their safe handling and the harmless removal or
recovery of the solvents, which increases the expenditure even
further. However, the solution of the problem through the addition
of fibers penetrating the surfaces of the body may improve the
impregnating properties of the body but does not eliminate the
problems outlined for the use of solvent-containing resins
releasing vapors or gases. In addition, one always has a product
containing certain fibers, which is more expensive to produce.
[0018] The problems with solvents present in the resin systems
which have been discussed also apply to aqueous resins, for example
according to International Publication No. WO 98/09926.
Furthermore, according to that publication, a graphite foil is
provided with a resin system which results in the formation of a
coat of plastics material at the surface for the purpose of
reinforcement. During the application of that resin, it also
penetrates into the surface of the foil. The plastics material
coating on one hand has the effect that a second coating with
improved adhesion can be applied and on the other hand has the
effect of electrical insulation.
[0019] Both aspects, i.e. the resin system dissolved in water and
the electrical insulation at the surface of the body, are regarded
as disadvantageous for the use of the bodies according to the
present application.
SUMMARY OF THE INVENTION
[0020] It is accordingly an object of the invention to provide
acrylic resin-impregnated bodies formed of expanded or at least
partially recompressed expanded graphite having a liquid-accessible
pore system which is completely or partially filled with an uncured
or partially or completely cured synthetic resin, a process for
producing such bodies and sealing elements, fuel cell components
and heat-conducting elements formed of the bodies, which overcome
the hereinafore-mentioned disadvantages of the heretofore-known
products and processes of this general type and in which the body
does not contain any defects such as blisters or cracks that may be
caused by reactions of the synthetic resin during the curing, the
body is producible with comparatively little expenditure and the
body is corrosion-resistant, electrically and thermally conductive
and is from liquid-permeable to gas-tight, depending on the degree
of compression.
[0021] With the foregoing and other objects in view there is
provided, in accordance with the invention, a synthetic
resin-impregnated body, comprising expanded or at least partially
recompressed expanded graphite. The graphite contains either at
least one solvent-free, low-viscosity, storage-stable,
polymerizable acrylic resin system or polymers obtained by curing
the at least one resin system.
[0022] Therefore, the object of the present invention is achieved
with a body of the type mentioned at the outset by subjecting the
primary product or the body obtained from the impregnated primary
product to at least partial compression containing either
solvent-free, low-viscosity, storage-stable acrylic resin systems
or cured acrylic resin systems. The resin systems enter into the
body by impregnating the primary product with solvent-free,
low-viscosity, storage-stable and polymerizable acrylic resin
systems.
[0023] In order to eliminate the aforementioned disadvantages of
solvent-containing resin systems and to achieve the advantages of
resin systems of low viscosity, the following special solvent-free
resin systems were employed according to the invention presented
herein:
[0024] The main component is triethyleneglycol dimethacrylate and
the initiator systems come from the azo initiators group.
[0025] Examples are 2,2'-dimethyl-2,2'-azodipropiononitrile and/or
1,1'-azobis(1-cyclohexanecarbonitrile) and/or azoisobutyric acid
dinitrile. A possible selection of the proportions of the
individual components in the overall mixture is mentioned in the
examples.
[0026] The low viscosities at processing temperature of the resin
systems ensure good and efficient impregnation of the primary
product and the polyadditions which take place during the curing do
not give rise to any low-molecular-weight cleavage products, which
could cause blistering or even cracks in the body. The testing of
the resin systems is described in more detail in the examples.
[0027] At room temperature, the specified mixture has a viscosity
of between 10 and 20 mPa.multidot.s which is markedly below that of
solvent-free, low-viscosity, storage-stable and polymerizable resin
systems from the group of isocyanates and their co-reactants and/or
epoxides. The main component of the acrylic resin system can be
characterized from the development of the viscosities over time in
a unit [mPa.multidot.s] at room temperature, as follows: fresh
mixture.apprxeq.13, after eight days.apprxeq.13 and after 48
days.apprxeq.14.
[0028] The small rate of the changes of the viscosity of the resin
at room temperature and over a period of several weeks is
demonstrated through the use of these viscosity measurements. That
small rate of the changes will be referred to hereinafter by the
term "high storage stability".
[0029] The expanded graphite used to produce the primary product is
formed of fanned-out, wormlike structures, in which very fine
graphite platelets are joined together in the form of a defective
accordion bellows. During the compression of the primary product,
these platelets slide in and over one another. They become
interlocked and thus come into contact again so as to no longer be
able to be released without destruction. This gives rise in the
primary product to a porous graphite framework or network which has
good electrical as well as good thermal conductivity due to the
good contacts between the graphite platelets. Since these
properties are based on the framework function of the graphite in
the primary product, they are not adversely affected by the
impregnation with synthetic resin. They can even be further
improved during a subsequent compression of the primary product
impregnated with resin.
[0030] The primary product is permeated throughout by open pores
which are interconnected in a variety of ways. As a result of this
network of interconnected pores, the synthetic resin penetrates
into the primary-product body during the impregnation and may even
completely fill it under suitable conditions. The network of pores
then becomes a network of synthetic resin. Both networks, the
graphite network and the pore/synthetic resin network, in
combination result in the outstanding properties of the end
products thus produced. By adjusting them in a specific manner, it
is also possible to control the level of properties of the end
products. For example, on one hand, a primary-product body which
has undergone little precompression and is thus highly porous has a
lower electrical and thermal conductivity and a lower degree of
anisotropy than a more highly compressed primary-product body. On
the other hand, it can take-up more synthetic resin and has
modified strength properties. This situation is reversed with
greatly compressed primary-product bodies. After the impregnation
and curing of the synthetic resin, they yield products with
improved electrical and thermal conductivity, as well as good
mechanical strengths. All of the bodies according to the invention
which are described herein are highly impermeable to liquids and
gases when their pore network has been completely filled with
synthetic resin.
[0031] All of the known methods, such as those described in DE 35
12 867, for example, can be used for the impregnation of the
primary-product bodies. It is preferable, however, to use immersion
methods, in particular immersion methods with prior evacuation of
the vessel containing the primary-product body and flooding of the
evacuated vessel with the synthetic resin. Where appropriate, the
vessel is also subjected to a gas pressure after it has been
flooded with the synthetic resin. If the primary-product body is to
be merely impregnated close to the surface or is to be partially
impregnated, the impregnating period is shortened or the surfaces
from which the impregnation is to start are suitably coated or
sprayed with synthetic resin or the body is only partially
immersed. Following this treatment, the excess resin is removed
from the surface.
[0032] An essential aspect of the present invention is efficient
and damage-free impregnation and curing. The rapid blister-free and
crack-free curing which is possible by virtue of the polyaddition
reactions has been discussed above. Efficient impregnation depends
essentially on the viscosity of the resin system. The present
acrylic resin system has a very low viscosity at less than 20
mPa.multidot.s, which is why the impregnating success is very
high.
[0033] The primary product can take-up an amount of up to 100% of
its own weight of resin, depending on the degree of compression of
the primary product and the open pore volume conditional thereon.
If, however, a high electrical conductivity of the end product is
desired, it is expedient to start with a primary-product body which
has undergone greater precompression, has a lower open pore volume
and can then take-up, for example, only 20% by weight of resin
based on its own weight. After the curing of the resin, such a body
can be highly impermeable to liquids and gases, as is seen in Table
2, and has good strength properties.
[0034] The kinetics of the curing reaction are extremely
temperature-dependent with the acrylic resin systems that are used.
Whereas at room temperature virtually no curing reaction takes
place, at higher temperatures and when the azo initiators become
effective, it starts suddenly. Virtually no viscous transition
state of the resin systems can be observed. The resin systems cure
fully all of a sudden. The curing times of the acrylic resin
systems fall in proportion to the rise in the temperature. Examples
are:
1 Temperature 60.degree. C. 80.degree. C. 100.degree. C.
150.degree. C. Curing time days 35 10 1 minute minutes minutes
[0035] If larger series of components or bodies are to be produced
by using the techniques described above, it will be desired to
efficiently combine a number of process steps. This is possible
particularly with the shaping of impregnated primary-product bodies
with simultaneous curing. For this purpose, the impregnated primary
product, which is generally in the form of a semifinished product
or blank, is expediently put into a mold which is already hot and
the mold is closed. The semifinished product thereby takes on the
desired geometry, is simultaneously thoroughly heated and cures
completely.
[0036] A relatively wide variety of graphites based on synthetic
production and natural occurrence exists, both types being
mentioned in U.S. Pat. No. 3,404,061. Only natural graphite will be
discussed hereinbelow, the graphite which is present as raw
material in the bodies described herein.
[0037] Natural graphite is obtained by mining and is separated from
the gangue rock with considerable effort. Nevertheless, very small
amounts of rock also remain, attached to the natural graphite
flakes or having intergrown into the flakes. Those "foreign
constituents" are characteristic of every source of natural
graphite and can also be specified as an ash value. A method for
determining such ash values is described in DIN (German Industrial
Standard) 51 903 under the title "Testing of Carbon
Materials-Determination of the Ash Value".
[0038] In view of the end uses of the synthetic resin-containing
graphite bodies according to the invention, the ash values and ash
composition of the graphite that is present are quite important. If
such bodies are employed, for example, as inherently
corrosion-resistant seals in installations subjected to corrosive
media, certain ash constituents together with the corrosive medium
may result in pitting in the corrosion-resistant seals adjoining
flanges or bushes of stuffing-box packings and eventually lead to
the failure of the sealed joint.
[0039] Another example of a possible adverse effect of too high an
ash value or an unfavorable ash composition of the graphite in a
synthetic resin-containing body according to the invention is found
in fuel cell technology. Thus, for example, bipolar plates of
proton exchange membrane fuel cells can be produced from the
material according to the invention. If such a plate has too high
an ash content, some of the harmful ash constituents may be
released from the plate during the operation of the fuel cell and
poison the sensitive catalysts located close to the surfaces of the
bipolar plate, resulting in a premature loss of power of the
cell.
[0040] Due to the potential adverse effects of an excessively high
ash content, the ash content of the graphite used to produce the
bodies according to the invention is 4 percent by weight and less,
preferably less than 2 percent by weight and in special cases no
more than 0.15 percent by weight.
[0041] It may be convenient to strengthen the body according to the
invention with fillers. The selection of the fillers has to be
matched to the application (e.g. fuel cell). Fillers may be
electrically conductive materials closely related to expanded
natural graphite, such as, for example, materials from the group
consisting of naturally occurring flake graphites, synthetically
produced electrographites, carbon blacks or carbons, and graphite
or carbon fibers. Furthermore, use may be made of silicon carbide
in granular or fibrous form or else electrically non-conductive
ceramic or mineral fillers in granular, platelike or fibrous form,
such as silicates, carbonates, sulfates, oxides, glasses or
selected mixtures thereof.
[0042] The bodies according to the invention can be used wherever
electrically and thermally conductive components having low weight
together with good corrosion resistance are required. Further
properties which are essential for various applications are low ash
values and relatively high impermeability. The bodies according to
the invention are used in particular for components of fuel cells,
for seals and for heat-conducting elements, for example for
conducting away excess heat from integrated circuits.
[0043] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0044] Although the invention is described herein as embodied in
acrylic resin-impregnated bodies formed of expanded graphite, a
process for producing such bodies and sealing elements, fuel cell
components and heat-conducting elements formed of the bodies, it is
nevertheless not intended to be limited to the details given, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0045] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying examples.
In addition, methods for obtaining data on electrical properties
and gas-tightness for the examples are dealt with now.
[0046] In order to determine gas-tightness, a resin-impregnated
graphite body was pressed as a separating plate (test specimen)
between two chambers of a testing apparatus. A constantly
maintained helium gas pressure of 2 bar absolute prevailed in a
first chamber. A metal grid which mechanically supported the test
specimen was disposed in a second chamber. In addition, this
chamber was connected at ambient pressure to a liquid-filled
burette such as is used, for example, in the leakage measurement of
flat seals according to DIN 3535.
[0047] The helium gas emerging from the first chamber and diffusing
through the test specimen was collected in the second chamber and
measured by displacement of the liquid in the burette. It was thus
possible to determine the volume of the helium gas which diffused
through the sample per unit of time. A leakage rate was ascertained
which is specified by the unit mg/(m.sup.2.multidot.s) by taking
the helium density and the testing area into account.
[0048] The material composite of partially recompressed expanded
graphite and synthetic resin has anisotropic properties, i.e. the
individual graphite platelets of the expanded graphite have a
preferred orientation due to the production technique. For example,
the electrical resistance parallel to this preferred orientation is
low and perpendicularly thereto it is higher. In the present case,
the cured shaped bodies according to the invention were
characterized comparatively by measuring the electrical resistance
perpendicularly to the preferred orientation of the graphite
layers. For this purpose, the body was clamped between two
gold-plated electrodes having a diameter of 50 mm, with defined and
in each case identical surface pressure. The electrical resistances
R established with the aid of a device (Resistomat 2318) from the
firm Burster (Gernsbach, Germany) are specified by the magnitude
[m.OMEGA.] hereinbelow.
EXAMPLE 1
[0049] The following primary-product plates were impregnated at
room temperature by immersion:
2 TABLE 1 Type of primary- Bulk Ash value product Thickness density
[% by plate [mm] [g/cm.sup.3] weight] Example 1a F02510C 0.25 1.0
<2.0 Example 1b L10010C 1.0 1.0 <2.0 Example 1c L40005Z 4.0
0.5 <0.15
[0050] Primary-product plates made of partially recompressed
expanded graphite used for impregnation with an acrylic resin
system.
[0051] The resin system which was used had the following
composition:
[0052] 99.2% of triethyleneglycol dimethacrylate (methacrylic acid
ester)
[0053] 0.3% of 2,2'-dimethyl-2,2'-azodipropiononitrile
[0054] 0.5% of 1,1'-azobis(1-cyclohexanecarbonitrile)
[0055] The methacrylic acid ester came from the firm Rohm GmbH
(Darmstadt, Germany) and had the trade name PLEX 6918-O. The two
other components of the resin system had the function of an
initiator. 2,2'-Dimethyl-2,2'-azodipropiononitrile came from the
firm Pergan GmbH (Bocholt, Germany) and had the trade name Peroxan
AZDN. 1,1'-Azobis(1-cyclohexanecarbonitrile) came from the firm
Wako Chemicals GmbH (Neuss, Germany) and bore the designation V40.
The viscosity of the resin system was in the range from 10-15
mPa.multidot.s at room temperature.
[0056] The primary-product plates were completely immersed in the
resin bath and after one, five and nine hours they were removed
from the immersion bath and the resin adhering to the surface was
wiped off. The plates were subsequently put into a circulating-air
oven at 100.degree. C. and cured for 30 min. The impregnated
primary-product plates showed no blisters or cracks at all despite
this shock curing. The values of the resin content, volume
resistance R and helium permeability .lambda. determined on the
plates are summarized in Table 2 and compared with the values for
non-impregnated plates.
3TABLE 2 Comparison of primary-product plates (varying thickness
and bulk density) impregnated with an acrylic resin system with
non-impregnated primary-product plates (of likewise varying
thickness and bulk density). The volume resistances R and the
helium permeabilities .lambda. after various impregnating periods
are compared. Impregnating Impregnating Impregnating period 1 h
period 5 h period 9 h Type of Resin .lambda. Resin .lambda. Resin
.lambda. primary- content R [mg/ content R [mg/ content R [mg/
without impregnation product plate [%] [m.OMEGA.] (m.sup.2
.multidot. s)] [%] [m.OMEGA.] (m.sup.2 .multidot. s)] [%]
[m.OMEGA.] (m.sup.2 .multidot. s)] Resin content [%] R [m.OMEGA.]
.lambda. [mg/(m.sup.2 .multidot. s)] F02510C 23.8 0.10 0.010 33.3
0.19 -- 36.1 0.37 <0.001 0 <0.1 1.5 L10010C 10.2 0.35 0.016
16.3 0.48 0.005 19.9 0.43 <0.001 0 0.28 0.323 L40005C 24.8 0.77
-- 37.4 0.76 -- 39.3 0.77 -- 0 0.43 --
[0057] As is evident from Table 2, the resin content of the
composite materials is greatly dependent on the bulk density of the
primary product, its geometry (plate thickness) and the
impregnating time. The volume resistance of the impregnated plates
rises comparatively little with increasing resin content, since the
electron conduction is borne by the existing graphite network. The
helium permeability of the plates is drastically reduced by the
impregnating treatment. Depending on the resin content of the
plate, the permeability falls by more than 2 powers of ten as
compared with corresponding primary-product plates without
impregnation.
EXAMPLE 2
[0058] The resin system that was used was the same as the resin
system in Example 1. The primary product had a thickness of 2.7 mm
and a density of 0.65 g/cm.sup.3 and the ash value of the graphite
was less than 0.15% by weight. After an impregnating period of one
hour at room temperature, the now impregnated plate was taken out
of the resin bath and weighed after the resin adhering to the
surface had been wiped off. The proportion of resin which was
determined was 20% by weight. The impregnated plate was placed in a
pressing die preheated to 1500.degree. C. The die, which was
furnished with an anti-stick coating, was closed and the
impregnated graphite was pressed into the mold, in the course of
which a further compression of the composite material took place.
After five minutes under the effect of pressing force and
temperature, the die was opened and the cured shaped body was
removed. The shaped body was free from cracks and blisters and the
surface showed no resin film visible to the eye.
[0059] Besides these above-mentioned examples, a multiplicity of
further bodies and procedures can be realized according to the
teaching of the present invention. Accordingly, the invention is
not restricted to the embodiments provided in the examples.
Variants which are not described but which a person skilled in the
art could produce due to the information offered by this disclosure
are therefore also to be included in the present patent
application.
* * * * *