Note: Descriptions are shown in the official language in which they were submitted.
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1
ACRYLIC RESIN-IMPREGNATED BODIES FORMED OF EXPANDED GRAPHITE
Description
The invention relates to a body made of expanded or
at least partially recompressed expanded graphite impregnated
with a synthetic resin and to a process for producing such a
body.
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 these are
comparatively expensive owing to the costs of the
fluoroplastics and the processing technique required. A
subject which in terms of content is even closer to the
application concerned here is set out in US Patent No.
4,265,952 which describes that expanded graphite is mixed
with, for example, fine PTFE powder and subsequently
compressed. To this extent, the production technique differs
from the impregnating technique described in the present
application.
Another example of a material composite of graphite
and plastic is a superficially resin-impregnated foil made of
natural graphite, which is predominantly employed in the form
of flat seals against particularly aggressive media. Many
references to this second example are found in technical
literatures. Today, thousands of tons of foils made of
natural graphite are produced worldwide every year.
Processes used therefor are described in Patent Publications
EP 0 087 489 US Patent No. 3,404,061 and US Patent No.
3,494,382.
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2
The teaching of these references can be summarized
as follows: 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 oxidising agent, such as, for example,
concentrated nitric acid or hydrogen peroxide. This results
in graphite intercalation compounds in the shape of flakes or
platelets. By brief heating, for example by introduction into
the flame of a gas burner, the flakes are thermally
decomposed and, as a result of the gas pressure arising in
their interior during this decomposition process, puff up to
form loose graphite particles of wormlike shape. This product
is also referred to as "expanded" graphite or as graphite
expandate.
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 between 0.7
and 1.3 g/cm3. However, other bodies of 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/cm3, are also possible. There are also sponge-like
bodies of, on average, low bulk density, having values of 0.1
to 1.0 g/cm3. All of these bodies of different shapes and
different bulk densities have an open pore system. They are
referred to hereinbelow as "primary product".
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
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3
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.
According to German Patent Publication DE 32 44
595, 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.
According to the prior art, the substantial
impregnation of shaped bodies made of expanded and partially
recompressed graphite is difficult. To overcome the
difficulties, WO 99/16141 (US Patent No. 6,037,074) teaches
that such a body can be satisfactorily impregnated when it is
interspersed with mineral fibres, which also pass through the
surface of the particular bodies. In this way, small channels
are formed along these mineral fibres, in which the resin can
flow into the interior of the bodies during the impregnation.
In this specification, 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.
Another method for achieving good impregnation of
bodies made of expanded graphite consists in converting the
desired resins by means of solvents into low-viscosity
liquids, whereby the impregnation becomes more complete. In
Japanese Patent Publication JP 1987-257526, the thermosetting
resins cited are based on phenols, epoxides, polyimides,
melamines, polyesters and furans, which are used in a mixture
solution with polyvinylbutyral.
Japanese Patent Publication JP 1-308,872 A2
describes a solution to another problem. A material composite
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formed of a glass fibre nonwoven fabric and an expanded
graphite foil is produced in order to strengthen the latter
and obtain a liquid-tight material. This is achieved by
impregnating the nonwoven fabric with epoxy resin, the resin
penetrating the nonwoven fabric, with the composite material
being formed during the subsequent curing. At the same time
the resin also penetrates into the surface, i.e. partially
into the foil, and seals the surface.
The impregnation of expanded graphite foil with
phenolic resin or epoxy resin, set out in Japanese Patent
Publication JP 60-242,041 A2 (DE 35 12 867 C2), serves
similar purposes, namely to improve strength and gas-
tightness. The special feature here 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.
German Patent Publication 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 this case
is 2100 to 2400 mPa~s.
Japanese Patent Publication JP 11-354,136 A2
entitled "Fuel Cell, Separator for Fuel Cell, and Manufacture
Therefor" describes the production of expanded graphite in
sheet-like form. This partially recompressed expanded
graphite is subsequently comminuted (pulverised) and then
mixed selectively with resins, solvent-free epoxy resin,
solid epoxy resin, melamine resin, acrylic resin, phenolic
resin, polyamide resin, and the like. This mixture is
subsequently shaped. As will be shown later, this technique
differs from the bodies according to the invention which are
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of an entirely different structure in that the resins are
mixed into an expanded graphite granulate.
Patent Publication WO 98/09926 describes a graphite
foil which is coated with a plastic on at least one side.
5 This is done by first applying an aqueous solution of an
acrylic resin to the surface, which resin remains there, but
also penetrates into regions of the foil close to the
surface, and then drying the resin in.
The prior art set out above discloses various
synthetic resin-containing bodies produced using expanded
graphite and processes for their production. It would be
easily understood that it is difficult to produce high-
quality, synthetic resin-containing graphite bodies from
recompressed, expanded graphite. All the processes described
have disadvantages. Some of these disadvantages are serious.
For instance if resins diluted by solvents and thus of lower
viscosity are used during the impregnation, it is true that
the impregnation is easier, although the vapours, in most
cases, from the readily volatile solvents cause serious
problems during the impregnation itself, especially during
subsequent process steps. In particular, as a result of the
fact that the vapours escape during the curing of the resins,
the vapours leave behind fine channels, raising the
permeability of the bodies produced. If an increased
permeability is neither tolerated or desired, a further
general problem exists. Namely if the curing is not
performed very slowly, a time-consuming process, blisters and
cracks are formed in the bodies, which lower their quality
considerably. The same disadvantages apply to resin systems
which release gases from condensation reactions during the
curing.
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As a result of the fact that solvents or other
gases and vapours 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 obvious
and the success is really limited. Also, solvent-containing
resins always require, above all, measures to allow their
safe handling and the harmless removal or recovery of the
solvents, which increases the expenditure even further. The
addition of fibres penetrating the surfaces of the body may
improve the impregnating properties of the body, but this
solution does not eliminate the problems outlined for the use
of solvent-containing resins releasing vapours or gases. In
addition, one always has a product containing certain fibres,
which is more expensive to produce.
The problems with solvents present in the resin
systems which have been discussed also apply to aqueous
resins. For example, according to Patent Publication
WO/09926 a graphite foil is provided with an aqueous resin
system which results in the formation of a coat of plastics
material at the surface of the foil for the purpose of
reinforcement. During the application of this resin, it also
penetrates into regions of the foil close to the surface.
The plastics material coat has on the one hand the effect
that a second coating with improved adhesion can be applied
and on the other hand the effect of electrical insulation.
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.
An object of the invention is to provide a body
made of expanded or at least partially recompressed expanded
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graphite, having a liquid-accessible pore system completely
or partially filled with an uncured or partially or
completely cured synthetic resin. This body should not
contain such defects as blisters or cracks that may be caused
by reactions of the synthetic resin during the curing, that
is, curing the resin system to polymerise the resin does not
generate a low molecular weight cleavage product. The body
should be producible with comparatively little expenditure.
It should be corrosion-resistant, electrically and thermally
conductive and, depending on the degree of compression, be
liquid-permeable or gas-tight.
Hence, the present invention provides bodies made
of expanded or partially recompressed expanded graphite
impregnated with at least one solvent-free, low-viscosity and
storage-stable acrylic resin systems or cured acrylic resin
systems. The resin systems are introduced into the body by
impregnating the primary product with solvent-free, low-
viscosity, storage-stable and polymerisable acrylic resin
systems.
A preferred embodiment of the solvent-free, low-
viscosity, storage-stable acrylic resin systems or cured
acrylic resin systems of the present invention includes the
following acrylic resin systems: acrylic acids, esters
thereof, methacrylic acids, esters thereof, acrylonitriles
and acrylamides.
Preferentially, the acrylic resin system comprises
a methacrylic acid ester as the main component.
Polymerisation of the methacrylic acid ester resin system, is
preferentially achieved by using a thermal initiator such as
an azo initiator. Preferred azo initiators comprise azo
substituted dinitriles.
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The polymer that is the result of the curing
comprises at least two to four monomer units.
The acrylic resin system preferentially comprises
99.0-99.5 by weight of the main component with 0.5-1.0$ by
weight of the initiator.
The acrylic resin system generally has a viscosity
of less than 100 mPa~s at room temperature, preferably of
less than 50 mPa~s and particularly preferably of less than
20 mPa~s.
In order to eliminate the aforementioned
disadvantages of solvent-containing resin systems and to
achieve the advantages of resin systems of low viscosity and
storage stability, the following~special solvent-free resin
system is an example of a particularly preferred acrylic
resin system employed according to the invention presented
herewith:
The main component is triethyleneglycol
dimethacrylate and the initiator systems come from the azo
initiators group. 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.
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 generate 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.
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At room temperature, the specified mixture has a
viscosity of between 10 and 20 mPa~s which is markedly below
that of solvent-free, low-viscosity, storage-stable and
polymerisable resin systems based on isocyanates and their
co-reactants or epoxides. At room temperature the acrylic
resin systems have a storage-stability of more than two days
and preferably of more than two weeks. The main component of
the acrylic resin system can be characterised from the
development of the viscosities over time in the unit [mPa~s]
at room temperature: fresh mixture approx. 13, after eight
days approx. 13, and after 48 days approx. 14.
The small rate of change, which is demonstrated by
means of these viscosity measurements, of the resin at room
temperature and over a period of several weeks will be
referred to hereinafter by the term "high storage stability".
The expanded graphite used to produce the primary
product consists of fanned-out, wormlike structures, in which
very fine graphite platelets are joined together in the form
of a defective concertina 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 no longer to 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 and also good thermal conductivity owing 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.
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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
5 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
10 adjusting them in a specific manner, it is also possible to
control the level of properties of the end products. For
example, 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 give products with improved electrical and thermal
conductivity, as well as good mechanical strengths. All the
bodies according to the invention which are described here
are highly impermeable to liquids and gases when their pore
network has been completely filled with synthetic resin.
Known methods, such as, for example, those
described in Patent Publication DE 35 12 867, 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
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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.
An aspect of this invention is efficient and
damage-free impregnation and curing. Rapid blister- and
crack-free curing possible by virtue of the polyaddition
reactions has been discussed above. Efficient impregnation
depends on the viscosity of the resin system. The present
acrylic resin system has a very low viscosity at less than
mPa~s, which is why the impregnating success is very high.
The primary product can be impregnated with an
15 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 is desired of the end product, it is
expedient to start with a primary-product body which has
20 undergone greater precompression and 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, see Table 2, and has good strength properties.
The kinetics of the curing reaction are extremely
temperature-dependent with the acrylic resin systems used.
For instance, at room temperature virtually no curing
reaction takes place, whereas at higher temperatures, and
when the preferred azo initiators become effective, it starts
suddenly and can be completed in less than one hour at
temperatures in excess of 60°C. In general, virtually no
viscous transition state of the resin systems is observed.
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The curing times of the acrylic resin systems fall in
proportion to the rise in the temperature. Examples are:
Temperature 60C 80C 100C 150C
Curing time days 35 10 1 minute
minutes minutes
If large series of components or bodies are to be
produced using the techniques described above, it is
desirable to efficiently combine a number of process steps.
This is possible particularly with the shaping of impregnated
primary-product bodies with simultaneous curing. In a
preferred embodiment, the impregnated primary product -
generally in the form of a semifinished product or blank - is
put into a mould which is already hot and the mould is
closed. The semifinished product thereby takes on the desired
geometry, is simultaneously thoroughly heated and cures
completely.
A relatively wide variety of graphites based on
synthetic production and natural occurrence exist, both types
being mentioned in the US Patent No. 3,404,061. Only natural
graphite will be discussed hereinbelow, the graphite being
present as raw material in the bodies described herein.
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. These "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 51 903 under the title
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"Testing of carbon materials - determination of the ash
value".
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 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 the flanges or
bushes, of stuffing-box packings and eventually to the
failure of the sealed joint.
Another example of a possible adverse effect of too
high an ash value or an unfavourable 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 now 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.
Owing 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.
It may be convenient to strengthen the body
according to the invention with fillers, the selection of the
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14
fillers having 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 fibres. Furthermore, use may be made of silicon
carbide in granular or fibrous form or else electrically
conductive or electrically non-conductive ceramic or mineral
fillers in granular, platelike or fibrous form, such as
silicates, carbonates, sulfates, oxides, glasses or selected
mixtures thereof.
The present invention also relates to a process for
producing the above-mentioned body. In one embodiment, the
process comprises the following process steps:
(a) providing a primary product that is a body
made of expanded or at least partially recompressed expanded
graphite;
(b) impregnating the primary product with at least
one solvent-free, low-viscosity, storage-stable,
polymerisable acrylic resin systems and
(c) curing the impregnated primary product.
Prior to step (b), the primary product can first be
mixed with ceramic or mineral electrically non-conductive or
electrically conductive fillers and then processed to form a
filler-containing primary product.
Step (b) is preferably an immersion method.
Prior to step (c), the uncured product can be
processed to form a shape. This process may be performed
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using a mould. Further, this processing to form a shape may
be conducted simultaneously with the curing step (c).
The curing step (c) may be conducted at
temperatures between about 60°C to about 200°C, with curing
5 times ranging from days to less than one minute.
The bodies according to the invention can be used
wherever electrically and thermally conductive components of
low weight together with good corrosion resistance are
required. Further properties which are essential for various
10 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 the excess heat from integrated circuits.
15 The invention is explained in more detail
hereinbelow with the aid of examples. For the purpose of the
examples, the following are methods for obtaining the data on
the electrical properties and gas-tightness.
To determine the gas-tightness, the 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 the first chamber. In the second chamber there
was a metal grid which mechanically supported the test
specimen. In addition, this chamber was connected at ambient
pressure to a liquid-filled burette, as used, for example, in
the leakage measurement of flat seals according to DIN 3535.
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. Taking
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16
account of the helium density and the testing area, a leakage
rate was ascertained which is specified by the unit
mg/ (m2 ~ s ) .
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
characterised 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 with a diameter of 50 mm,
with defined and in each case identical surface pressure. The
electrical resistances R established with the aid of a
(Resistomat 2318)* device from Burster (Gernsbach, Germany)
are specified by the magnitude [m~] hereinbelow.
* Trade-mark
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17
Example 1
The following primary-product plates were
impregnated at room temperature by immersion:
Type of Thickness Bulk Ash
primary- [mm] density value[~
product [g/cm3] by
plate weight]
Example F0251OC 0.25 1.0 < 2.0
la
Example L10010C 1.0 1.0 < 2.0
1b
Example L40005Z 4.0 0.5 < 0.15
lc
Table 1:
Primary-product plates made of partially
recompressed expanded graphite used for the impregnation with
an acrylic resin system.
The resin system used had the following
composition:
99.2 of triethyleneglycol dimethacrylate (methacrylic acid
ester)
0.3$ of 2,2'-dimethyl-2,2'-azodipropiononitrile
0.5~ of l,1'-azobis(1-cyclohexanecarbonitrile)
The methacrylic acid ester came from Rohm GmbH
(Darmstadt, Germany) and had the trade name PLEX 6918-0*.
*Trade-mark
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18
The two other components of the resin system had the function
of an initiator. 2,2'-Dimethyl-2,2'-azodipropiononitrile came
from Pergan GmbH (Bocholt, Germany) and had the trade name
Peroxan AZDN*. 1,1'-Azobis(1-cyclohexanecarbonitrile) came
from Wako Chemicals GmbH (Neuss, Germany) and bore the
designation V40*. The viscosity of the resin system was in
the range from 10 - 15 mPa~s at room temperature.
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°C and cured for 30
min. On visual inspection, 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 A determined on the plates were
summarised in Table 2 and compared with the values for non-
impregnated plates.
* Trade-mark
CA 02364748 2001-12-06
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19
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CA 02364748 2001-12-06
25861-32
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
5 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
10 plate, the permeability falls by more than 2 powers of ten
compared with corresponding primary-product plates without
impregnation.
Example 2
The resin system used was the same as the resin
15 system in Example 1. The primary product had a thickness of
2.7 mm and a density of 0.65 g/cm3, the ash value of the
graphite was less than 0.15. 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
20 adhering to the surface had been wiped off. The proportion of
resin determined was 20$ by weight. The impregnated plate was
placed in a pressing die preheated to 150°C. The die, which
was furnished with an anti-stick coating, was closed and the
impregnated graphite pressed into the mould, 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
removed. On visual inspection, the shaped body was free from
cracks and blisters and the surface showed no resin film
visible to the eye.
CA 02364748 2001-12-06
25861-32
21
Besides these above-mentioned examples, a
multiplicity of further bodies and procedures can be realised
according to the teaching of this invention. Accordingly, the
invention is not restricted to the embodiments illustrated in
the examples. Variants which are not shown but which a person
skilled in the art could produce owing to the information
offered by this disclosure are therefore also to be included
in this patent application.
