DE10300979B4 - Ultralight composites, process for their preparation and their use - Google Patents

Abstract

Composites made of fiber-reinforced Plastic and / or carbon aerogels, characterized that the surfaces essentially completely with the fibers in the airgel matrix at least one dense layer of the dried material of the Aerogels are wetted and covered

Description

object The invention relates to ultra-lightweight composite materials of high load-bearing capacity with strongly anisotropic properties, in particular thermal conductivity, a process for their preparation and their use as construction materials in aircraft and vehicle construction.

Fiber composites with plastic or carbon matrix and embedded glass or carbon fibers have been described in the art for various applications. CFRP and GRP composites have a specific gravity, which is usually greater than 1.5 g / cm ³ . The elastic properties and the strengths are defined by fiber type and volume content of fibers. The interweaving of the fibers defines the anisotropy of the mechanical properties.

In principle, it is possible to produce a variety of materials from aerogels, for example plastic or carbon aerogels. For example, this describes DE 199 11 847 A1 the use of moldings of highly porous, open-pore plastic and / or carbon aerogels, which are obtained by sol-gel polymerization of organic plastic materials, optionally followed by pyrolysis, for the fine and cast molding of metals or metal alloys. DE 199 39 062 A1 describes the use of plastic / carbon aerogels as a core material for molding. These applications have in common that the aerogels used, which may have a filler content of up to 30 or 60 vol .-%, no special requirements in terms of mechanical strength are provided.

The same applies to other plastic or carbon aerogels already described in the prior art. In the US 6,099,965 A For example, for use as supports for catalysts, solid, porous carbon structures are described which have a high surface area substantially free of micropores. These may be, for example, carbon fiber-reinforced carbon or plastic aerogels, which are distinguished by different chemical "bonding techniques" of the fibers among themselves at the joints. The US 6,099,965 A points out that the nanofiber gel must undergo a supercritical drying step to produce a fiber-reinforced airgel, otherwise a denser xerogel will form.

DE 19721600 A1 describes nanoporous interpenetrating organic / inorganic networks in which the inorganic networks are silicon-containing materials. These networks may be fiber reinforced plastic aerogels, it being understood that a supercritical drying step of the gel is necessary to maintain the airgel structure and result in subcritical drying to xerogels, calcination into compact solids. The networks of DE 197 21 600 A1 are used for the production of moldings or surface coatings with thermal insulation, sound damping and / or adsorption and / or barrier properties against water and / or organic solvents.

DE 195 23 382 A1 describes platelet-shaped, hydrophobic carbon aerogels having a fiber reinforcement of electrically non-conductive, inorganic materials for use as electrodes in polymer electrolyte fuel cells.

Fiber-reinforced carbon or plastic aerogels are used in EP 0 629 810 A1 because of their heat-insulating properties for cryogenic systems proposed as intermediate and filling material between an inner and an outer container layer.

WHERE 97/17308 A1 describes a composite of fibers and airgel and a method for its production.

A key problem of all types of composites is the adhesion between the fibers and matrix into which they are embedded. For example, only when the adhesion is perfect can mechanical loads be adequately transferred from the matrix to the fibers without, for example, fibers breaking, contributing to reinforcement, or being pulled out of the composite. For the applications of fiber-reinforced aerogels described in the prior art, such a strong incorporation of the fibers is not necessary, since the composite material is not used as a construction material of high load-bearing capacity. Therefore, even in the prior art, there is no information on the incorporation of the fibers or how they can be optimized. However, the incorporation of carbon fibers or other fibers such as silicon carbide, alumina or glass fibers is of crucial importance for the production of ultralight composite materials. If you want to produce ultralight composites with densities less than 1 g / cm ³ , a matrix is needed that has virtually no density.

Of the The present invention was therefore based on the object fiber composites which are ultralight and yet high in load capacity and mechanical strength, and thus for use in aircraft and vehicle construction are particularly suitable. Furthermore, the invention was the object of a method for producing such composites provide.

The The object is achieved by Composites made of fiber-reinforced plastic and / or carbon aerogels, characterized in that the surfaces the fibers in the airgel matrix substantially completely with at least a dense layer of the dried material of the airgel wetted and are covered

Essential for the production of solid and stiff composites is the Producing a sufficient wetting of the fibers by an aqueous Sol which is chemically identical to that used for gelation Sol. Fibers of the prior art are either as raw fibers used or are not closer coated materials specified. Uncoated fibers lead by themselves Investigations on composites with poor mechanical properties Properties such as low fracture toughness, low tensile and Compressive strength and "fiber-pull-out". At undefined coated fibers, the bond may be better, but can not sufficiently controlled and adjusted. By the composite materials according to the invention as well as the production method according to the invention this problem is solved and at the same time an excellent bond between fibers and matrix set.

Prefers is the surface the composite materials according to the invention at least partially or completely provided with one or more coatings showing the function take over protective layers against mechanical and / or chemical stress, the coatings) high and / or low molecular weight polymers such as epoxy resin or polyester resin, metal foils, and / or sheets, and in particular a thickness in the range of 20 to 500 μm can have.

reinforcing fibers in the sense of the invention selected be inorganic, organic and / or carbon fibers.

The Matrix of the aerogels of the invention and hence the dense coating of the fibers embedded therein is particularly preferably hydroxymethylated Resorcinalharz. This is for the RF aerogels are given an optimal primer.

The surface coating by polymers such as epoxy resin, polyester or metal foils or bears metal sheets essential for improving the usability of ultralight composites of the invention as construction materials. The surface of aerogels is not only sensitive to mechanical stress in case of scratches, grinding or abrasion. Due to its porosity, it also has a tendency to gases of all kinds or to allow moisture to penetrate. For use with the environment in contact areas, in particular in aircraft and vehicle construction is thus a protective layer that prevents the composites low molecular weight Absorb compounds such as rainwater like a sponge, of eminent advantage. Only a correspondingly dense surface layer made of metal foil, sheet metal, or best with high molecular weight polymers, penetrating to a certain degree in the porous airgel body, can build up a suitable protection. A coating that only rests on the surface, is less suitable because the binding to the airgel is worse. Evaporated layers are therefore not appropriate. Metal foils and sheets are therefore preferred over an adhesive layer is connected to the airgel body. Preferably penetrate the layers in the range of 10 to 80 microns, more preferably 20 to 70 μm, ideally 30 to 60 μm into the airgel. To low-viscosity coating materials such as aqueous solutions should be avoided as they penetrate too deeply into the airgel matrix and thereby increase the weight of the composite materials too much.

The composite materials according to the invention preferably have a density in the range from 0.4 to 1.2 g / cm ³ , wherein a density of 0.4 to at most 1.0 g / cm ^{3 is} particularly preferred. Preferably, the composites of the invention have an elastic modulus in the range of 100 to 200 GPa. Furthermore, the composites of the invention have strengths in the range of 500 to 1000 MPa. The peak values achieved are thus well above the best currently available CFRP materials.

The specific gravity of the composites according to the invention is calculated from that of the matrix (for an RF airgel typically 300 kg / m ³ ) and that of the fibers (for example for carbon: 2250 kg / m ³ ) weighted by the volume fraction. A special feature of the composite materials according to the invention is not only their extremely low weight with excellent mechanical properties such as elastic modulus and / or strength, but also their low thermal conductivity. RF aerogels have a thermal conductivity in the range of 0.3 W / (Km). If the network of fibers is completely embedded in the airgel, the thermal conductivity of the matrix determines that of the overall composite. The ultralight composite materials according to the invention therefore preferably have a thermal conductivity in the range from 0.1 to 0.5 W / Km, in particular from 0.2 to 0.4 W / Km.

• a) Herstellen einer polymerisierbaren Lösung als Vorläufer des Kunststoffaerogels,
• b) gegebenenfalls Zugabe eines Polymerisationskatalysators,
• c) Herstellen eines im wesentlichen blasenfreien Gemisches oder einer Dispersion der polymerisierbaren Lösung mit anorganischen, insbesondere Glas- und/oder Siliziumcarbidfasern, organischen, insbesondere Polymerfasern und/oder Kohlenstofffasern, durch Einlegen der Fasern oder Benetzen der Fasern mit der polymerisierbaren Lösung,
• d) Herausnehmen aus der Lösung und Trocknen der Fasern an der Luft,
• e) gegebenenfalls einfaches oder mehrfaches Wiederholen der Schritte c) und d) bis zur Bildung einer im wesentlichen vollständigen Bedeckung der Oberfläche der Fasern mit einer Schicht der polymerisierbaren Lösung,
• f) Einbringen der bedeckten Fasern in eine polymerisierbare Lösung oder Dispersion als Vorläufer des Kunststoffaerogels,
• g) Gelieren der Lösung bei Temperaturen im Bereich von 30 bis 70 °C, insbesondere 40 bis 60 °C unter Luftausschluss und
• h) Trocknung bei Temperaturen im Bereich von 30 bis 70 °C, insbesondere 40 bis 60 °C unter Bildung des Kunstoffaerogels sowie gegebenenfalls
• i) Pyrolyse bei Temperaturen im Bereich von 1000 bis 1200 °C unter Bildung eines Kohlenstoffaerogels.

The composites of the invention may be prepared by a process comprising the following steps:

• a) preparing a polymerisable solution as precursor of the plastic aerosol,
• b) optionally adding a polymerization catalyst,
• c) producing a substantially bubble-free mixture or a dispersion of the polymerisable solution with inorganic, in particular glass and / or silicon carbide fibers, organic, in particular polymer fibers and / or carbon fibers, by inserting the fibers or wetting the fibers with the polymerisable solution,
• d) removal from the solution and drying of the fibers in the air,
• e) optionally repeated or repeated steps c) and d) until substantially complete coverage of the surface of the fibers with a layer of the polymerizable solution,
• f) introducing the covered fibers into a polymerisable solution or dispersion as a precursor of the plastic aerosol,
• g) gelling the solution at temperatures in the range of 30 to 70 ° C, in particular 40 to 60 ° C with exclusion of air and
• h) drying at temperatures in the range of 30 to 70 ° C, in particular 40 to 60 ° C to form the Kunstoffaerogels and optionally
• i) pyrolysis at temperatures in the range of 1000 to 1200 ° C to form a carbon aerosol.

there you lead the gelation f) and / or the drying g) preferably via a Period of 4 to 36 hours, especially 6 to 24 hours off to ensure optimum solidification of the airgel.

critical In this process is that there is an excellent bond between Provides fibers and matrix. The reinforcing fibers that are preferred in a volume fraction of 20 to 60% based on the total volume and in the form of nonwovens, felt or fabric mats and / or Fiber deposits may be present according to the invention in the polymerizable solution, preferably an RF sol, are dipped and moved in the sol bath, to visually no air bubbles more can be seen on the fibers. The fleeces, felts, fabric mats and / or fiber scrims are then taken out of the bath / solution and preferably dried directly in air. This arises the fibers a thin Layer of, for example, hydroxymethylated resorcinol resin, the is dense and the ideal primer for RF aerogels in this case represents. Depending on the fiber material and chemical nature of the polymerizable solution or sol, this process (dipping / drying) must be done up to three times be repeated.

The impregnated in this way Fibers are then according to the invention as described further treated.

According to the invention preferred is, if in a further step k) the composite materials fiber reinforced Plastic and / or carbon aerogels at least partially with one or more coatings as protective coatings against mechanical and / or chemical stress. This is best done with metal foils or metal sheets or with high molecular weight polymers, in particular epoxy resin or polyester resin. This is the bond between open-pored airgel and polymeric coating, for example, thereby produces that the polymers up to some 10 microns, preferably 10 to 80 microns, especially preferably 20 to 70 μm, Ideally 30 to 60 μm penetrate into the pore space of the airgel before they harden. metal layers are preferably applied by the films or sheets, the as well as polymeric protective layers preferably has a thickness in the range from 20 to 500 μm on a polymer coated composite glued on. This can be done in the form that you have one Coating of metal foil or metal sheet thereby applying that one the composite material and / or the metal foil / sheet metal coated with epoxy resin and / or silicone rubber, the metal foil / Glue metal sheet to the composite and cure.

In a particularly preferred embodiment find the ultra-light of the invention Composite materials Use as functional materials of high load-bearing capacity in aircraft and vehicle construction.

EXAMPLES

• 1. A solution of resorcinol and formaldehyde in a molar ratio of 1: 1.3 was prepared, to which sodium bicarbonate as a catalyst and water (in a variable amount: at least 1 part of water per 1 part of resorcinol plus formaldehyde) was added (polymerizable solution). In this solution, non-woven fabrics, felts and fabric mats made of carbon; Aluminum oxide (Saffillos from DuPont) immersed and moved until no more air bubbles were visible on the fibers. The fabrics, nonwovens and felts were pulled out of the bath and dried directly in air. In the case of carbon fibers and felts, this process was repeated up to three times. Subsequently, the now impregnated fiber webs, felts and fabric mats were placed in a die and covered with a polymerizable solution as above. For example, up to 20 layers of carbon fiber fabric (manufacturer: Cra mer, type T300, binding atlas, style CCC495). The template was filled with R / F aerosol solution and closed with a stainless steel punch. Any excess aerosol solution could escape through overflow channels. Typically, a volume content of fibers between 40 and 60% could be adjusted in this way. The hermetically sealed template was then stored in a dry box at 40 ° C to 60 ° C for 6 to 24 hours until the RF sol gelled to a polymeric airgel. At the same time, the fibers were bound by this gel. Drying of the wet airgel at 40 ° C to 60 ° C in a drying oven with open die generated within 6 to 8 hours the composite material according to the invention.
• 2. The composite material of Example 1 was at the surface coated with epoxy resin. After 30 minutes, the viscous liquid was about 20 μm in penetrated the pore space of the airgel and was now cured at 100 ° C (2h).
• 3. On the composite materials, the bending strength in the three-point bending test and the tensile strength determined according to DIN EN 2561. The tensile strengths at 50 percent by volume fiber web was 796 ± 10 MPa.

Claims (16)

Composites of fiber-reinforced plastic and / or carbon aerogels, characterized in that the surfaces of the fibers in the airgel matrix are substantially completely wetted and covered with at least one dense layer of the dried material of the airgel Composite materials according to claim 1, characterized that the reinforcing fibers are selected of inorganic, organic and / or carbon fibers. Composite materials according to claim 1 or 2, characterized characterized in that the surface is at least partially or Completely provided with one or more coatings. Composite materials according to claim 3, characterized in that that the coatings are high and / or low molecular weight polymers, in particular epoxy resin or polyester resin, metal foil, and / or Include sheet metal. Composite materials according to claim 3 or 4, characterized characterized in that the coatings have a thickness in the range of 20 to 500 μm exhibit. Composite materials according to one of claims 1 to 5, characterized in that they have a density in the range of 0.4 to 1.2 g / cm ³ , in particular from 0.4 to 1.0 g / cm ³ . Composite materials according to one of claims 1 to 6, characterized in that it has an elastic module in the area from 100 to 200 GPa. Composite materials according to one of claims 1 to 7; characterized in that it has strengths in the range of 500 to 1000 MPa. Composite materials according to one of claims 1 to 8, characterized in that it has a thermal conductivity in the range of 0.1 to 0.5 W / Km, in particular 0.2 to 0.4 W / Km have. Process for the production of composite materials according to one the claims 1 to 9, comprising the steps: a) producing a polymerizable solution as a precursor of plastic aerogels, b) optionally adding a polymerization catalyst, c) Preparation of a substantially bubble-free mixture or a Dispersion of the polymerizable solution with inorganic, in particular Glass and / or silicon carbide fibers, organic, in particular polymer fibers and / or carbon fibers, by inserting the fibers or Wetting the fibers with the polymerisable solution, d) take out out of the solution and drying the fibers in the air, e) optionally simple or repeating steps c) and d) until the Forming a substantially complete covering of the surface of the Fibers with a layer of the polymerisable solution, f) introduction of the covered fibers into a polymerizable solution or dispersion as a precursor of the plastic aerosol, G) Gelling the solution at temperatures in the range of 30 to 70 ° C, in particular 40 to 60 ° C with exclusion of air and h) drying at temperatures in the range of 30 to 70 ° C, in particular 40 to 60 ° C forming the Kunstoffaerogel and optionally i) Pyrolysis at temperatures in the range of 1000 to 1200 ° C with formation a carbon aerosol. Method according to claim 10, wherein the gelation and / or drying in the course of 4 to 36 hours, especially 6 to 24 hours. Method according to claim 10 or 11, characterized in that the fibers in a volume fraction from 20% to 60% of the total volume. Method according to one the claims 10 to 12, characterized in that the fibers in the form of Nonwovens, felts, fiber fabrics and / or fiber fabrics. Method according to one the claims 10 to 13, characterized in that in a further step j) the composites partially with one or more coatings provides. Method according to claim 14, characterized in that a coating of metal foil or sheet metal by applying the composite material and / or the metal foil / metal sheet with epoxy resin and / or Silicone rubber spreads on the metal foil / metal sheet stick the composite and harden. Use of the composites of fiber-reinforced plastic and / or carbon aerogels according to one the claims 1 to 9 as a functional material in aircraft and vehicle construction.