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.

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...

AI Technical Summary

Benefits of technology

[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.

[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.

[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.

[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.

Problems solved by technology

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.

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.

According to the prior art, the substantial impregnation of shaped bodies made of expanded and partially recompressed graphite is difficult.

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.

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 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.

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.