Patent Application
20070259185
The single FIGURE of the drawing is a diagrammatic, cross-sectional view of a cylindrical component containing an inner wound layer and an outer layer made up of individual segments of pressed graphite expandate according to the invention.
In contrast to conventional fillers such as milled coke, the particles added to the carbonizable binder according to the present invention are planar, i.e. their dimension in the flat area (diameter) is significantly greater than their thickness.
One filler which is suitable for the purposes of the invention is, for example, natural graphite whose particles are flake-like. An alternative is particles which are obtained by comminuting (breaking up such as cutting, milling, chopping or shredding) of graphite expandate which has been compacted to form planar structures (e.g. graphite foil). Offcuts which are inevitably obtained in the production of seals or other articles from graphite foil are advantageously utilized for this purpose. The particles obtained in this way are platelet-like.
The mean diameter of the particles added according to the invention to the carbonizable binder is from 1 to 250 μm; preference is given to particles having a mean diameter of from 5 to 55 μm.
In both variants of the particles added according to the invention to the carbonizable binder, i.e. both in the case of natural graphite flakes and in the case of the platelet-like particles obtained by comminuting of graphite foil, the typical layer plane structure of the graphite, which is responsible for the high anisotropy of the thermal conductivity which is typical of graphite, is present. The thermal conductivity along the layer planes, i.e. in the plane of the particles, is at least a factor of 10, preferably at least a factor of 20, higher than perpendicular to the layer planes. In the case of graphite foil, for example, the thermal conductivity perpendicular to the plane of the foil is only about 3-5 W/K*m, while the conductivity parallel to the plane of the foil reaches values in the range from about 100 W/K*m (at a density of 0.6 g/cm3) to about 260 W/K*m (at a density of 1.5 g/cm3).
Since the planar anisotropic particles in the binder layer are aligned parallel to the adjoining layers of material, the thermal conduction across the interface between the various materials is only low. In the case of milled coke as a filler, the thermal conduction between the layers of material is suppressed to a lesser extent because of the more isotropic thermal conductivity of the coke.
A further advantageous effect of the planar graphite particles in the binder layer is that a binder layer containing such particles functions as stress equalization layer, i.e. mechanical stresses between the various materials joined to one another via the binder layer are reduced. It is assumed that this effect is attributable to the known lubricating action of graphite, but the invention is not tied to this explanation.
The mass of the particles is at least 5% of the mass of the binder used. Particle masses of from 10 to 30% of the binder mass have been found to be particularly useful, but the invention is not restricted to this range of values for the mass ratio of particles to binder.
As carbonizable binders, it is possible to use the binders known from the prior art, e.g. phenolic resins, furan resins, epoxy resins, pitch or the like. The curing, carbonization and, if appropriate, graphitization of the binder in the layer composites is carried out in a way known to those skilled in the art. Curing can, for example, be effected using the known vacuum bag process. The carbonization or graphitization occurs in a known manner at temperatures in the range from 800 to 2000° C.
The carbonizable binders containing planar anisotropic graphite particles are suitable for producing layer composites composed of various carbon or graphite materials which can be used for high-temperature applications or precursors thereof, for example layers of compacted graphite expandate of widely varying density (including graphite foil), hard carbon fiber felts, soft carbon fiber felts, carbon fiber reinforced carbon, and fabric prepregs. The composites of the invention contain at least two layers of high-temperature-resistant carbon-based or graphite-based materials.
Depending on the application, a person of average skill in the art will choose suitable materials according to their known specific advantages and join them according to the invention by use of a carbonizable binder containing planar anisotropic graphite particles (graphite flakes or platelets) to form a layer composite having the desired sequence of layers and subsequently carbonize the binder.
For the purposes of the present invention, carbon fibers are all types of carbon fibers regardless of the starting material, with polyacrylonitrile, pitch and cellulose fibers being the most widely used starting materials. In the carbon fiber reinforced carbon materials, the carbon fibers can be present as, for example, individual fibers, short fibers, fiber bundles, fiber mats, felts, woven fabrics or non-crimped fabrics, also combinations of a plurality of the fiber structures mentioned. The woven fabrics can include long fibers or carbon fibers which have been broken by drawing and respun (known as staple fibers).
Materials made up of carbon fibers or reinforced with carbon fibers contribute to the stiffness and strength of the composite. Layers having a low density, e.g. layers composed of carbon fiber felt or graphite expandate which in contrast to graphite foil is compacted only to a density of from 0.02 to 0.3 g/cm3, have a particularly good thermally insulating effect because of their low thermal conductivity. A further advantage is their low weight.
The particular advantages of graphite foil for use in thermal insulations and the like have already been presented.
Suitable materials for the layer which directly adjoins the interior of the furnace, reactor or the like are graphite foil and also materials containing staple fibers. Owing to the drawing/breaking treatment, these fibers produce hardly any dust which could contaminate the interior of the furnace or reactor. The problem of dust formation by fine fibrils from conventional carbon fiber reinforced thermal insulations, which lead to contamination of the interior of the furnace or reactor to be insulated, is referred to in, for example, international patent disclosure WO 2004/063612.
The production of, in particular, curved carbon fiber reinforced layers by the winding technique is known, see European Patent EP 1 157 809 (corresponding to U.S. Pat. No. 6,641,693). Here, elongated (threads, yarns, rovings, ribbons or the like) or/and planar textile structures (woven fabrics, lay-ups, felts) composed of carbon fibers which may, if appropriate, be impregnated with a carbonizable binder are wound up on a shaping mandrel.
Curved layers containing graphite foil can be produced by winding up a long sheet of graphite foil.
Between the various layers of material, a layer of the carbonizable binder containing planar anisotropic graphite particles is applied in each case. Thanks to the high viscosity of the binder containing planar anisotropic graphite particles, which is in the range from 20,000 to 30,000 mPa, this can be applied (e.g. applied by a spatula) without problems even to areas which are not horizontal, for example to curved surfaces as are typical of furnaces, reactors and pipes and are produced, for example, by the winding technique.
In the case of components which have to meet high demands in terms of mechanical stability, the layer composite preferably contains at least one layer which is cross-wound. Therefore, the wound layer contains strata which have been wound up at an opposed angle, e.g. +/−45°. Such layers can be obtained by winding up of elongated fiber structures such as threads, yarns, rovings or ribbons, with the fiber structures being able to be impregnated with a carbonizable binder. In the subsequent carbonization or graphitization, the binder which may be present in the wound fiber structures is carbonized or graphitized so as to produce a carbon fiber reinforced carbon material (CFRC).
It has been found to be advantageous for the surfaces to be joined to one another or at least the surface to which the binder is applied to be roughened.
It is also possible for individual components, e.g. tubes or plates, made of high-temperature-resistant carbon-based or graphite-based materials of construction to be joined to one another to form complex components by use of the carbonizable binder containing planar anisotropic graphite particles, e.g. natural graphite flakes or platelets of compacted graphite expandate.
A specific variant of the invention relates to curved components, for example cylindrical insulations for furnaces or reactors. The winding technique is preferably used for producing these. However, some of the materials coming into question, for example graphite expandate which has been compacted to a density in the range from 0.02 to 0.3 g/cm3, are not flexible enough to produce a layer of the corresponding material by winding a corresponding long sheet of material without the latter breaking. For this reason, it is proposed according to the invention that such curved layers be produced by assembling individual segments of the corresponding material. These segments are produced by customary shaping techniques, for example by pressing of graphite expandate in a mold corresponding to the shape of the segment to be produced or by cutting the segment from a block of pressed graphite expandate.
The cohesion between the segments within a layer is, like the cohesion between the individual layers, produced by use of a carbonizable binder to which planar anisotropic graphite particles have been added and which is subsequently cured and carbonized.
The FIGURE of the drawing schematically shows the cross section of a cylindrical component 1 containing an inner wound layer 2 which has been obtained, for example, by winding up a layer of graphite foil on a mandrel and an outer layer 3 which is made up of individual segments 3a, 3b, 3c . . . composed of pressed graphite expandate. A carbonizable binder to which planar anisotropic graphite particles had been added was applied both to the interface between layer 2 and layer 3 and also to the interfaces between the individual segments 3a, 3b, 3c.
Of course, further layers can be applied to the layer 3 composed of the segments 3a, 3b, 3c . . . , for example a stabilizing outer layer of wound-up carbon fibers.
Some combinations of materials which are joined by a carbonizable binder containing planar anisotropic graphite particles to form layer composites which are suitable for use in thermal insulations, heat shields, etc. are proposed below.
A first example of a composite according to the invention contains a heat-reflecting inside (i.e. facing the interior of the furnace or reactor) layer of graphite foil, a layer of graphite expandate in which the graphite expandate is less highly compacted than in the graphite foil and which suppresses thermal conduction and a stabilizing outer layer of carbon fiber reinforced carbon (CFRC). The fiber reinforcement of the CFRC is formed either by a woven fabric or by layers of carbon fiber threads, yarns or rovings which are wound crosswise, e.g. at an angle of +/−45°.
A second example of a composite according to the invention contains a layer of pressed graphite expandate and a layer of CFRC.
A third example of a composite according to the invention contains a sandwich made up of two layers of graphite foil which enclose a layer of less highly compacted graphite expandate.
Table 1 lists the typical thicknesses and densities of the individual materials.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06 009 214.5 | May 2006 | EP | regional |
