DE102014218931B4 - Sandwich structure with a core material containing airgel - Google Patents

Abstract

Sandwich structure (3) comprising a core material (1), the core material (1) being arranged between two cover layers (2a, 2b), the two cover layers (2a, 2b) being the same or different from each other, the core material (1). a carrier material with macroscopic cavities, in which the cavities are filled with an airgel, characterized in that the core material (1) is obtainable with a method for producing a core material (1), in which an airgel solution is introduced into the cavities of the carrier material and The airgel solution is then dried to form an airgel without shrinkage.

Description

The present invention relates to a sandwich structure with a core material made of a carrier material with macroscopic cavities, in which the cavities are filled with an airgel.

Composite materials or fiber composite materials are increasingly being used in vehicle construction, especially in aircraft construction, due to their high stiffness and strength and their comparatively low weight. Sandwich structures are used in these areas in particular due to their high flexural rigidity. A sandwich structure usually consists of a moderately shear-resistant core material, which is connected to two external, expansion-resistant cover layers in a shear and tensile strength. A lightweight material that is suitable for increasing the distance between the two cover layers is usually used as the core material. The cover layers can each be rigid. Due to their construction, sandwich structures are very well suited to withstand bending loads. The tensile and compressive loads caused by the bending are maximum in the cover layers. Ideally they should therefore be very stiff and strong. The core material is subjected to shear and pressure.

Known core materials usually consist mainly of foams or aramid, aluminum, polypropylene, cardboard, polyacryletherketone, thermoplastic polymethacrylamide, polyvinyl chloride, thermoset polyurethane honeycomb or balsa end wood. Compared to core materials made from foam, honeycomb core materials are able to absorb higher mechanical forces without creeping and have better fatigue strength. In addition, honeycombs are particularly light and still have good mechanical properties.

To maintain a rigid and dimensionally stable core material, the honeycomb structure can be filled with spherical or fibrous fillers, the fillers being connected to one another at their points of contact with a binder, as shown, for example, in DE 10 2011 082 087 A1 is described.

However, in the event of a fire, known sandwich structures quickly exhibit poor stability. The matrix material and often also the binder begin to thermally decompose at temperatures of around 200 °C, so that the structure no longer has any stability after a short time.

One way to protect a sandwich structure in the event of a fire is to select a flame-retardant matrix system. Phenol-formaldehyde resins are often used here. Although these have good fire behavior, they are significantly inferior to other systems in terms of their mechanical properties. Another option is to add various flame retardants to the matrix system. For example, materials are used here that release water in the event of a fire, thereby cooling the material and diluting the smoke gases. These additives often have to be added in very large proportions by weight and thus lead to an increase in weight and often to embrittlement of the material. Another option is to apply flame-retardant layers or varnishes. These can expand up to 200 times under the influence of heat and thus form a barrier layer that hinders heat conduction and mass transport.

A composite material with a core material that is light in weight and at the same time suitable for fire protection is in DE 10 2004 046 495 A1 described. Here, a hydrophobic airgel is mixed with an inorganic binder and an inorganic hydrophilic dispersant. The binder is cement, plaster, lime, clay, water glass and/or clay. Ammonium fluoride, for example, can be used as a dispersant. The composite material described there can be used as a load-bearing element in buildings. The airgel particles give the material thermal insulation properties and also allow fire protection.

With fire-protected composite materials known from the prior art, the mechanical properties of the sandwich structure are usually impaired. The weight of the composite often increases, meaning that it can no longer be used, particularly in aircraft construction. Paints that are used as fire retardants can flake off, meaning that they no longer have a flame retardant effect.

WO 03/100364 A2 relates to a structure with a low weight, which reduces the detectability by means of IR or radar of a vehicle which has a corresponding structure.

DE 34 28 285 C2 relates to an insulating body and a method for its production.

US 7,291,373 B2 reveals a composite connection with good thermal insulation. US 2010/0047516 A1 relates to a honeycomb structure that acts as the core of a sandwich structure.

US 5,306,555 A reveals airgel matrix composites. In US 6,068,882 A Flexible aerogels are described which have good thermal insulation properties. US 7,078,359 B2 concerns fiber-reinforced airgel composites.

The object of the present invention is therefore to provide a sandwich structure which eliminates the disadvantages mentioned above.

The object on which the present invention is based is achieved by a sandwich structure according to claim 1. Preferred embodiments are claimed in claims 2 to 10.

The sandwich structure 3 according to the invention comprises a core material 1, which is arranged between two cover layers 2a, 2b. A corresponding structure is shown schematically in 1 shown. The core material 1 is made of a carrier material with macroscopic cavities, the cavities being filled with an airgel.

A core material 1, which is part of the sandwich structure 3 according to the invention, is produced by means of a method which is characterized in that the carrier material has macroscopic cavities into which an airgel solution is introduced as a filler and into which the airgel solution is then shrink-free in the carrier material to form an airgel is dried. This results in the core material 1 made of a carrier material with macroscopic cavities, in which the cavities are filled with an airgel.

Aerogels are highly porous solids whose volume consists largely of pores. They have been known as a material for decades. They are characterized by unique properties such as low density, a very high specific surface area, low thermal conductivity and high acoustic impedance.

Silicate-based aerogels, i.e. inorganic aerogels, are widely used. Organic aerogels are also known. These are produced, for example, based on resorcinol and formaldehyde, melamine and formaldehyde, phenol and furfural, cresol and formaldehyde, polymerizable isocyanates, phenol and melamine or polyvinyl chlorides. A special feature of these purely organic aerogels is that they can be converted into carbon aerogels by pyrolysis, while the structure of the aerogels is retained.

Aerogels are typically synthesized using a sol-gel process. Depending on the manufacturing process and starting materials, the resulting gel is dried supercritically or subcritically.

Before the gel obtained is dried, the airgel solution, i.e. the colloidal sol, is introduced into the macroscopic cavities of the carrier material. The airgel solution is only allowed to dry on site, i.e. in the carrier material. The drying takes place without shrinkage, so that the airgel obtained adheres firmly to the carrier material.

According to the invention, organic or inorganic airgel solutions known from the prior art can be used. The core material 1 thus includes organic or inorganic aerogels in the cavities of the carrier material. The core material 1 preferably consists of an organic or inorganic airgel and the carrier material.

Inorganic airgel solutions must be dried under supercritical conditions to enable shrinkage-free drying. In contrast, organic airgel solutions, especially resorcinol-formaldehyde airgel solutions, can also be dried under subcritical conditions. In particular, the production and drying of organic airgel solutions is described in detail in the prior art, for example in DE 10 2013 216 965 A1 . Inorganic airgel solutions are preferred in view of their properties as fire retardants in a corresponding application. With regard to a simple, cost-effective manufacturing process, organic airgel solutions are preferred.

Both inorganic and organic airgel solutions enable the production of an insulating core material. This enables both acoustic insulation (sound insulation) and thermal insulation (heat insulation, cold insulation).

The core material 1 surprisingly has a high bending strength. The flexural strength of the core material 1 is approximately 1.7 times higher than the flexural strength of the pure airgel. The core material 1 thus not only enables insulation, but also increases the stability in a structure in which it is used.

Known carrier materials such as open-pored/open-cell foams or a honeycomb structure can be used as the carrier material. A honeycomb structure is preferably used. The honeycomb structure can be paper honeycomb, cardboard honeycomb, aluminum honeycomb, aramid honeycomb or polymer honeycomb. The advantage of the honeycomb structure is that they are widespread and therefore inexpensive and can also be made from different materials. The disadvantage is that a thermal bridge is present through the webs of the honeycomb structure. However, this disadvantage is outweighed by the advantages that arise from the airgel as a filler in the cavities of the carrier material, such as low weight and good insulation.

A honeycomb structure, like other carrier materials with macroscopic cavities, usually has two open sides through which a liquid can flow. For example, if you put a liquid on the first open side of the carrier material, it flows out of the second open side. To ensure that this does not happen when the airgel solution is introduced, a first layer of material is preferably applied to the first open side of the carrier material before the airgel solution is introduced. If the support structure is placed on a surface that prevents the airgel solution from flowing through, but from which the airgel solution or the later hardened airgel can be easily removed, it is not necessary to apply a layer of material before introducing the airgel solution. Such a material layer is not necessary even if the support structure is immersed in the airgel solution, whereby the airgel solution is also introduced into the cavities of the support structure. Due to their manufacturing process, support structures, in particular honeycomb structures, often already have a layer of material on one side, whereby an open side is already closed. In this case, there is no need to apply another layer of material.

If the first open side of the carrier material is not covered with a layer of material before introducing the airgel solution, this can also be done after the airgel solution has dried. Furthermore, after the airgel solution has dried, a layer of material can also be applied to the second open side, which lies opposite the first open side of the carrier material. This material layer on the second open side of the carrier material can be the same as the first material layer. However, different material layers can also be applied. A core material 1 can thus have a material layer on the open sides opposite one another.

The materials that are applied as a material layer to the open sides of the support structure can be, for example, filters made of fleece or paper.

The process enables the cavities of the carrier material to be completely filled with an airgel obtained after drying. So that the carrier material is completely covered with the airgel and there are no cavities, it is possible to introduce the airgel solution into the carrier structure under the influence of ultrasound. Any gas bubbles that may be present are removed from the carrier material or the airgel solution using ultrasound.

In order to obtain a stable and compact core material 1, it may also be necessary to match the hydrophobicity or hydrophilicity of the carrier material with that of the airgel or the airgel solution. The material adhesion of the airgel can be tested, for example, on a film that consists of the same material as the carrier material. In particular, by varying the pH value of the airgel solution, the adhesion of the airgel to the carrier material can be adjusted and improved.

It is also possible to provide the support structure with a coating before introducing the airgel solution. This coating is tailored to the hydrophobicity or hydrophilicity of the airgel solution and thereby enables improved adhesion between the airgel and the carrier structure. Here the core material 1 then comprises a coated support structure and an airgel.

Hydrophobic in the sense of the present invention means that the material has a contact angle of more than 90° compared to water at room temperature (25 ° C). Hydrophilic materials show a corresponding contact angle of less than 90° with water.

With the help of the method described, a core material 1 is obtained from a carrier material with macroscopic cavities, in which the cavities are completely filled with an airgel.

The carrier material preferably comprises a honeycomb structure, in particular a paper honeycomb, cardboard honeycomb, aluminum honeycomb, aramid honeycomb or polymer honeycomb structure. The cavities of the carrier material can be filled with an organic or inorganic airgel known from the prior art. Organic aerogels, particularly preferably resorcinol-formaldehyde aerogels, are present as fillers in the cavities since they can also be dried under subcritical conditions, which enables good adhesion to the carrier material. If the core material 1 is to be used in particular in fire protection, it is preferred that the airgel is an inorganic airgel, since this is still stable even at higher temperatures.

The three-dimensional network structure of the dried airgel preferably consists of particles with a size of less than 5 μm, in particular less than 3 μm, particularly preferably the particle size is in the range from 100 nm to 3 μm. Small particles enable the cavities of the support structure to be filled particularly well and as completely as possible. The size of the particles is determined using scanning electron microscope (SEM) images.

The core material 1 preferably has a density in the range of 0.1 to 0.3 g/cm ³ and/or a thermal conductivity in the range of 0.04 to 0.06 W/(m·K). The low density and low thermal conductivity enable it to be used as a fire protection material, which can also be used in areas where low weight is necessary.

A core material 1 as described above is part of a sandwich structure 3 according to the invention. In this case, the core material 1 is arranged between two cover layers 2a, 2b. 1 shows schematically the structure of a sandwich structure 3 according to the invention.

The two cover layers 2a, 2b can be the same or different from each other. If the two cover layers 2a, 2b are different from each other, the sandwich structure 3 can comprise a supporting skin as one of the two cover layers 2a, 2b and an outer skin as the other cover layer 2a, 2b. A flame-retardant core is arranged between the load-bearing skin and the outer skin. This core is the core material 1. The core material 1 serves as a spacer between the two cover layers 2a, 2b. Due to its low weight, the weight of the sandwich structure 3 according to the invention is kept low overall. Due to the low thermal conductivity of the core material 1, when heat is irradiated onto one of the cover layers, the second cover layer is not thermally stressed. In the event of a fire, the cover layer, which is not directly exposed to fire, is protected by the core material 1.

• Bereitstellen des Kernwerkstoffes 1, welcher ein Trägermaterial und ein Aerogel umfasst, und
• Kaschieren des Kernwerkstoffes 1 mit den Deckschichten 2a, 2b derart, dass der Kernwerkstoff 1 zwischen den Deckschichten 2a, 2b angeordnet ist.

A method for producing a sandwich structure 3 according to the invention comprises the following steps:

• Providing the core material 1, which comprises a carrier material and an airgel, and
• Laminating the core material 1 with the cover layers 2a, 2b in such a way that the core material 1 is arranged between the cover layers 2a, 2b.

The lamination of the core material 1 with the cover layers 2a, 2b can be done, for example, by first applying an adhesive layer to the core material 1, onto which the actual material of the cover layers 2a, 2b is then applied. However, it is preferred that a prepreg material, which includes both an adhesive layer and the actual cover layer 2a, 2b, is applied directly to the core material 1.

The cover layers 2a, 2b are preferably selected from fiber-reinforced plastics, such as CFK (carbon fiber-reinforced plastic), GFK (glass fiber-reinforced plastic), AFK (aramid fiber-reinforced plastic) or NFK (natural fiber-reinforced plastic), or metals such as aluminum or Titanium.

The sandwich structure 3 can not only comprise a core material 1, which is located between two cover layers 2a, 2b. It is also preferably possible for one of the two cover layers 2a, 2b to be followed by a further core material 1, which in turn is provided with a further cover layer 2a, 2b. This results in a layer structure in which the core material 1 and a cover layer 2a, 2b alternate with one another. The composite material obtained in this way ends on the outer layers with a cover layer 2a, 2b.

In a preferred embodiment, the sandwich structure 3 according to the invention consists of a core material 1 and two cover layers 2a, 2b, with the core material 1 being arranged between the cover layers 2a, 2b.

If a layer of material is applied to the core material 1, this results in a layer structure of first cover layer 2a, material layer, core material 1, material layer, second cover layer 2b. The two material layers, like the cover layers, can be the same or different from each other.

Due to the airgel as a component of the core material 1 or the sandwich structure 3, the material has thermal insulating properties. In addition, the weight is low. This enables the use of the sandwich structure 3 as a component in land, water and/or aircraft, such as cars, airplanes and/or trains. For example, the sandwich structure 3 can be used as an outer skin in an aircraft. Due to the increased thermal insulation of the outer skin, insulation material could be saved and aircraft could therefore become lighter and therefore more energy-efficient. The same also applies to other vehicles in which the sandwich structure 3 according to the invention is used.

Example 1:

• • Preparation of the solution:
  • First, 127.4 g of deionized water (W) was weighed into a beaker. 50.0 g of resorcinol (R) were then added and the solution was stirred at room temperature until the resorcinol had completely dissolved (5 to 7 minutes). 76.8 g of formaldehyde (F) (24 wt% solution) were then added and the mixture was stirred for a further 5 minutes. Next, 0.032 g of Na ₂ CO ₃ (C) (solid) was added to the solution. After stirring for a further 5 minutes, the pH was adjusted with dilute nitric acid. The pH of the solution was adjusted to 5.7. The solution was stirred for a further 20 minutes at room temperature.

• • Zusammensetzung der Aerogellösung (Verhältnisse der Bestandteile):
  • R/C 1500; R/F 0,74; R/W 0,044
• • Gelation:
  • Die Lösungen mit der Aramid-Wabenstruktur wurden in dicht verschließbare Behälter aus Polypropylen gefüllt und in Ofen bei 80°C zwei Tage lang gelieren gelassen.
• • Trocknen:
  • Das Gel wurde 24 Stunden bei 80°C getrocknet.
  • Das Resultat sind Waben, die vollständig mit RF-Aerogel ausgefüllt sind. Das Wabenmaterial wird nahezu perfekt benetzt, was aus der Rasterelektronenmikroskopaufnahme in 2 zu erkennen ist.

After preparation, a piece of HexWeb ^® from Hexcel, Welkenraedt, Belgium, measuring 3.0x3.0 cm, was placed in approx. 13 mL of resorcinol-formaldehyde solution (RF solution). HexWeb ^® is an aramid honeycomb structure coated with a phenolic resin.

• • Composition of the airgel solution (ratios of the components):
  • R/C 1500; R/F 0.74; R/W 0.044
• • Gelation:
  • The solutions with the aramid honeycomb structure were filled into tightly sealable containers made of polypropylene and allowed to gel in an oven at 80 ° C for two days.
• • Dry:
  • The gel was dried at 80°C for 24 hours.
  • The result is honeycombs that are completely filled with RF airgel. The honeycomb material is wetted almost perfectly, as can be seen from the scanning electron microscope image in 2 can be recognized.

Example 2:

Coating cardboard with RF airgel

• R/F=0,74; R/C=1500; R/W=0,044.

An airgel solution was prepared as in Example 1 with the following composition:

• R/F=0.74; R/C=1500; R/W=0.044.

• > Reihe 1: Pappe wurde direkt nach dem Rühren für 30 bis 300 Sekunden (s) eingetaucht;
• > Reihe 2: RF-Lösung wurde bei 80°C für 30 Min im Ofen gehalten, dann wurde Pappe für 30 bis 300 s eingetaucht;
• > Reihe 3: RF-Lösung wurde bei 80°C für 60 Min im Ofen gehalten, dann wurde Pappe für 30 bis 300 s eingetaucht.

The pH of the solution was adjusted to 5.62. The solution was stirred for 20 minutes. Cardboard was then immersed in this RF solution at different times for different immersion times. The size of the cardboard samples was approximately 4 cm x 4 cm.

• > Row 1: Cardboard was immersed immediately after stirring for 30 to 300 seconds (s);
• > Row 2: RF solution was kept in the oven at 80°C for 30 min, then cardboard was immersed for 30 to 300 s;
• > Row 3: RF solution was kept in the oven at 80°C for 60 min, then cardboard was immersed for 30 to 300 s.

The samples were kept in sealed containers at 80°C for 1 day and then dried at 80°C for 1 day. The finished samples were hard and dark brown.

In the 3 until 8th Scanning electron microscope (SEM) images of the samples are shown. The assignment can be found in the following Table 1: Table 1: Fig. Row Immersion duration 3 1 300s 4 1 30s 5 2 30s 6 2 300s 7 3 30s 8th 3 300s

9 shows an SEM image of the cardboard used before coating.

In the SEM you can see in the figures that the airgel has particle sizes in the range of 2 µm and the airgel, which lies directly on the cardboard, is smaller than 500 nm. The exception is the sample “row 3, 300 s” ( 8th ), in which the particles on the cardboard are also about 2 µm in size. You can see that the airgel particles adhere well to the cardboard surface. The airgel did not penetrate the cardboard even after 300 s of immersion.

In order not to influence the material properties of the honeycomb structure itself, it is important that the airgel solution does not penetrate into the material from which the honeycomb structure is made. The material, i.e. the honeycomb structure, should be coated on the surface. However, this should be done completely and with good adhesion.

As can be seen from the SEM images in exemplary embodiment 2 ( 3 until 8th ), the airgel sticks to the surface of the cardboard. Cardboard is a material from which a honeycomb structure can preferably be made. By dipping the cardboard into the airgel solution, the solution does not penetrate the cardboard. It is simply coated so that the material properties of the cardboard, i.e. the carrier material, are not changed but remain intact.

Example 3:

1. 1. Ansetzen der Aerogellösung (Silica-Lösung): Die Ausgangschemikalien Methyltrimethoxisiloxan (MTMS), Methanol, H₃O⁺ (zum Beispiel verdünnte HCl) und Cetyltrimethylammoniumchlorid (CTAC) wurden in einem Molmassenverhältnis von MTMS: Methanol 35:1, MTMS:H₃O⁺ von 4:1 und MTMS : Cetyltrimethylammoniumchlorid (CTAC) von ebenfalls 4:1 angesetzt und 180 Minuten unter ständigem Rühren bei Raumtemperatur zur Hydrolyse gebracht, anschließend erfolgte die Zugabe von Ammoniumhydroxid, durch welche die Gelation initiiert wurde. Typischerweise wurden 12,2 g MTMS in 100,24 g Methanol in einem Reagenzglas gelöst, danach wurden 2,034 g CTAC und 6,445 g einer 0,0001 M HCl-Lösung zugegeben. Das Reagenzglas wurde mit Aluminiumfolie abgedeckt, die Lösung 3 Stunden gerührt. Die Kondensationsreaktion wurde durch Zugabe von 12,533 g NH₄OH (28-30%) initiiert.
2. 2. Befüllung der Wabenstruktur:
   • Nach dem Ansetzen wurde jeweils ein Stück HexWeb^® (Firma Hexcel, Belgien) (C1-6,4-24; C1-3,2-29) mit einer Größe von 3x3 cm in ca. 13 mL Silica-Lösung gelegt.
3. 3. Gelation des Aerogels in der Wabenstruktur:
   • Die mit Aerogel-Lösungen gefüllten Aramid-Wabenstrukturen (HexWeb^®) wurden in dicht verschließbare Behälter aus Polypropylen (PP) gefüllt und gelierten in einem Ofen bei 50 °C über fünf Tage.
4. 4. Waschen des Verbundmaterials:
   • Nach fünf Tagen wurden die Behälter über weitere 5 Tage hinweg zum Waschen und zum Lösungsmittelaustausch geöffnet. Dazu wurde zweimal täglich die über den Aerogelen stehende Lösung entfernt und durch Ethanol ersetzt. Auf diese Weise wurden Methanol, Wasser und nicht abreagierte Ausgangsstoffe sowie alle Zusatzstoffe aus dem Gelkörper entfernt.
5. 5. Trocknen des Verbundmaterials:
   • Anschließend wurden die Gele in einer Extraktionsanlage im Durchfluss mittels überkritischem CO₂ bei 90-100 bar getrocknet.
6. 6. Resultate Das Wabenmaterial ist nahezu perfekt benetzt. Eine Rasterelektronenmikroskopische Aufnahme der mit super-flexiblem Silica-Aerogel gefüllten Aramid-Wabenstruktur zeigt 12. Die gute Haftung des Aerogels auf dem Trägermaterial ist hier deutlich zu erkennen. Die mittlere Dichte des Verbundmaterials betrug 0,037 g/ccm und die Wärmeleitfähigkeit 0,034 W/(m·K). Die Ergebnisse der Kompressionstests (13) zeigen, dass der Verbundwerkstoff im Vergleich zu reinen Stoffen einen deutlich höheren elastischen Modul aufweist. Der Kompressionstest von super-flexiblen Silica-Aerogelen, Aramid-Wabenstrukturen und Verbundwerkstoffen in der Ebene ( 13a) und senkrecht zur Ebene (13b) ist in 13 gezeigt. In der nachfolgenden Tabelle 2 sind die Ergebnisse zusammengefasst:

Tabelle 2: Material E-Modul, MPa Aramid-Wabenstrukturen 0.03-0.108 5.2-25.5 in der Ebene senkrecht zur Ebene super-flexibles Silica-Aerogel 0.0025 Verbundwerkstoff (Aramid-Waben gefüllt mit super-flexiblem Silica-Aerogel) 0.028 - 0.137 6.5 - 21.3 in der Ebene senkrecht zur Ebene Aramid honeycomb structure with super-flexible silica airgel

1. 1. Preparation of the airgel solution (silica solution): The starting chemicals methyltrimethoxysiloxane (MTMS), methanol, H ₃ O ⁺ (for example diluted HCl) and cetyltrimethylammonium chloride (CTAC) were in a molar mass ratio of MTMS: methanol 35:1, MTMS:H ₃ O ⁺ of 4:1 and MTMS: cetyltrimethylammonium chloride (CTAC) of also 4:1 were prepared and hydrolyzed for 180 minutes with constant stirring at room temperature, followed by the addition of ammonium hydroxide, which initiated the gelation. Typically, 12.2 g of MTMS was dissolved in 100.24 g of methanol in a test tube, followed by 2.034 g of CTAC and 6.445 g of 0.0001 M HCl. Solution added. The test tube was covered with aluminum foil and the solution was stirred for 3 hours. The condensation reaction was initiated by adding 12.533 g of NH ₄ OH (28-30%).
2. 2. Filling the honeycomb structure:
   • After preparation, a piece of HexWeb ^® (Hexcel, Belgium) (C1-6.4-24; C1-3.2-29) with a size of 3x3 cm was placed in approx. 13 mL of silica solution.
3. 3. Gelation of the airgel in the honeycomb structure:
   • The aramid honeycomb structures (HexWeb ^® ) filled with airgel solutions were placed in tightly sealable containers made of polypropylene (PP) and gelled in an oven at 50 °C for five days.
4. 4. Washing the composite material:
   • After five days, the containers were opened for washing and solvent exchange for an additional 5 days. For this purpose, the solution above the aerogels was removed twice daily and replaced with ethanol. In this way, methanol, water and unreacted starting materials as well as all additives were removed from the gel body.
5. 5. Drying the composite material:
   • The gels were then dried in a flow-through extraction system using supercritical CO ₂ at 90-100 bar.
6. 6. Results The honeycomb material is almost perfectly wetted. A scanning electron microscope image of the aramid honeycomb structure filled with super-flexible silica airgel shows 12 . The good adhesion of the airgel to the carrier material can be clearly seen here. The average density of the composite material was 0.037 g/ccm and the thermal conductivity was 0.034 W/(m K). The results of the compression tests ( 13 ) show that the composite material has a significantly higher elastic modulus compared to pure materials. The in-plane compression test of super-flexible silica aerogels, aramid honeycomb structures and composites ( 13a) and perpendicular to the plane ( 13b) is in 13 shown. The results are summarized in Table 2 below:

Table 2: material Modulus of elasticity, MPa Aramid honeycomb structures 0.03-0.108 5.2-25.5 in the plane perpendicular to the plane super-flexible silica airgel 0.0025 Composite material (aramid honeycomb filled with super-flexible silica airgel) 0.028 - 0.137 6.5 - 21.3 in the plane perpendicular to the plane

Example 4:

1. 1. Ansetzen der Aerogellösung:
   • Die Ausgangschemikalien Methyltrimethoxisiloxan (MTMS) und (3-Glycidoxypropyl)-methyldiethoxysilan (GPTMS) wurden im Molmassenverhältnis MTMS:GPTMS = 0,25 vorgelegt. Das Verhältnis von MTMS: Methanol betrug 30:1, das von MTMS: H₃O⁺ 30:1 und das von MTMS:Cetyltrimethylammoniumchlorid (CTAC) = 0,071. Es wurde eine entsprechende Lösung angesetzt und 180 Minuten unter ständigem Rühren bei Raumtemperatur zur Hydrolyse gebracht, anschließend erfolgte die Zugabe von Diethylentriamin (DETA) im Verhältnis MTMS:DETA = 0,125, durch welche die Gelation initiiert wurde. Beispielsweise wurden 14,64 g MTMS und 6,36 g GPTMS in 103,32 g Methanol gelöst. Danach erfolgte die Zugabe von 2,43 g CTAC und 58,11 g HCl. Im verschlossenen Reagenzglas wurde die Lösung 3 Stunden gerührt. Im letzten Schritt wurden 1,38 g DETA zur Lösung zugegeben.
2. 2. Befüllung der Wabenstruktur:
   • Nach dem Ansetzen wurde jeweils ein Stück HexWeb^® (Firma Hexcel, Belgien) (C1-6,4-24; C1-3,2-29) in einer Größe von 3x3 cm in ca. 13 mL Silica-Lösung gelegt.
3. 3. Gelation des Aerogels in der Wabenstruktur:
   • Die mit Lösungen gefüllten Aramid-Wabenstrukturen wurden in dicht verschließbare Behälter aus Polypropylen (PP) gefüllt und gelierten in einem Ofen bei 50°C über drei Tage.
4. 4. Waschen des Verbundmaterials:
   • Nach fünf Tagen wurden die Behälterüber weitere 5 Tage hinweg zum Waschen und zum Lösungsmittelaustausch geöffnet. Dazu wurde zweimal täglich die über den Gelen stehende Lösung entfernt und durch Ethanol ersetzt. Auf diese Weise wurden Methanol, Wasser und nicht abreagierte Ausgangsstoffe sowie alle Zusatzstoffe aus dem Gelkörper entfernt.
5. 5. Trocknen des Verbundmaterials:
   • Anschließend wurden die Gele in einer Extraktionsanlage im Durchfluss mittels überkritischem CO₂ getrocknet.
6. 6. Resultat:
   • Die Wabenstruktur ist vollständig mit Silica-Aerogel ausgefüllt. Das Wabenmaterial wird nahezu perfekt benetzt. 14 zeigt eine Rasterelektronenmikroskopische Aufnahme der mit semi-flexiblem Silica-Aerogel gefüllten Aramid-Wabenstruktur. Die gute Haftung des semi-flexiblen Aerogels auf dem Trägermaterial ist deutlich zu erkennen.

Aramid honeycomb structure with semi-flexible silica airgel

1. 1. Preparation of the airgel solution:
   • The starting chemicals methyltrimethoxysiloxane (MTMS) and (3-glycidoxypropyl)-methyldiethoxysilane (GPTMS) were presented in the molar mass ratio MTMS:GPTMS = 0.25. The ratio of MTMS:methanol was 30:1, that of MTMS:H ₃ O ⁺ 30:1 and that of MTMS:cetyltrimethylammonium chloride (CTAC) = 0.071. An appropriate solution was prepared and hydrolyzed for 180 minutes at room temperature with constant stirring, followed by the addition of diethylenetriamine (DETA) in the ratio MTMS:DETA = 0.125, which initiated the gelation. For example, 14.64 g of MTMS and 6.36 g of GPTMS were dissolved in 103.32 g of methanol. 2.43 g of CTAC and 58.11 g of HCl were then added. The solution was stirred in a sealed test tube for 3 hours. In the last step, 1.38 g of DETA was added to the solution.
2. 2. Filling the honeycomb structure:
   • After preparation, a piece of HexWeb ^® (Hexcel, Belgium) (C1-6.4-24; C1-3.2-29) measuring 3x3 cm was placed in approx. 13 mL of silica solution.
3. 3. Gelation of the airgel in the honeycomb structure:
   • The aramid honeycomb structures filled with solutions were placed in tightly sealable containers made of polypropylene (PP) and gelled in an oven at 50°C for three days.
4. 4. Washing the composite material:
   • After five days, the containers were opened for washing and solvent exchange for an additional 5 days. For this purpose, the solution above the gels was removed twice daily and replaced with ethanol. In this way, methanol, water and unreacted starting materials as well as all additives were removed from the gel body.
5. 5. Drying the composite material:
   • The gels were then dried in a flow-through extraction system using supercritical CO ₂ .
6. 6. Result:
   • The honeycomb structure is completely filled with silica airgel. The honeycomb material is wetted almost perfectly. 14 shows a scanning electron micrograph of the aramid honeycomb structure filled with semi-flexible silica airgel. The good adhesion of the semi-flexible airgel to the carrier material can be clearly seen.

The average density of the composite material was 0.092 g/ccm and the thermal conductivity was 0.038 W/(m K). The results of the compression tests show ( 15 ), that the composite material has a significantly higher elastic modulus compared to pure materials. The in-plane compression test of semi-flexible silica aerogels, aramid honeycomb structures and composites ( 15 a) and perpendicular to the plane ( 15 b) is in 15 shown. The results are summarized in Table 3 below: Table 3: material Modulus of elasticity, MPa Aramid honeycomb structures 0.03-0.11 5.2-25.5 in the plane perpendicular to the plane semi-flexible silica airgel 0.074 level independent of Composite material: (aramid honeycomb filled with semi-flexible silica airgel) 0.06 - 0.16 67 - 20.2 in the plane perpendicular to the plane

Example 5:

Honeycombs produced according to embodiment 1 were carried out using thermogravimetric analyzes coupled with infrared spectroscopy to determine the thermal decomposition behavior.

Thermogravimetric analysis (TGA), also known as thermogravimetry, is an analytical method or method of thermal analysis or thermoanalytics in which the change in mass of a sample is measured as a function of temperature and time. The samples were heated in an oven in a temperature range of 26 °C to 1000 °C. The mass loss was determined. At the same time, the gases produced during heating were analyzed using infrared spectroscopy.

11 shows the TGA curves of an aramid honeycomb without airgel (dashed line) and an aramid honeycomb coated with RF airgel (solid line) (produced according to embodiment 1).

The TGA curve of aramid honeycombs (coated with phenolic resin) without airgel showed a mass loss of approx. 50% in the measured temperature range from 26 to 1000 °C, with 2 steps at 100 °C and 450 °C. Although aramid decomposes at 500-550 °C ( KE Perepelkin, Fiber Chemistry 35 (4), 265-269, 2003 ), you can see from the TGA curve that the decomposition continues up to 1000 °C. This may be due to the resin coating on the honeycomb.
For aramid honeycombs coated with airgel, the mass loss was approximately 57%. You can see a slight decrease in the range 100-400 °C and a step at 450 °C.

IR spectra recorded during pyrolysis in TGA showed the decomposition products that were formed. The mass loss initially corresponds to the amount of escaping CO ₂ and physically absorbed water (peaks at 2400 cm ^-1 , 3500 and 1800 cm ^-1 ). Table 4 shows the absorption bands of the infrared spectrum (IR absorption bands) and the assignment to the chemical groups as they arise during the pyrolysis of the aramid honeycomb. Table 5 shows the additional IR absorption bands that occur during the pyrolysis of aramid honeycombs filled with RF airgel. Table 4: Band [cm ^-1 ] group 3900-3700 Water 3500 -NH valence oscillation 2300, 695 CO ₂ valence oscillation 3000-2600 -CH valence oscillation 1739 -C=O valence vibration 1700-1500 NH ₂ deformation vibration Table 5: Band [cm ^-1 ] group 3015 CH ₂ stretching vibration 1081 CO stretching vibration 3619 OH valence oscillation

Example 6:

Cardboard honeycombs were coated with an RF airgel analogously to embodiment 1. The same thermogravimetric analyzes were carried out as in Example 5. In 10 the TGA curves of paper honeycomb without filling (dashed line) and paper honeycomb with airgel filling (solid line) are shown.

The TGA curve of paper honeycombs without airgel showed a mass loss of 83% in the measured temperature range from 26 °C to 1000 °C. This takes place in 2 stages at 100 °C and 300-400 °C. For paper honeycombs filled with airgel, the mass loss was 39.9%. You can see in the spectrum ( 10 ) a slight decrease in the range 100-300 °C and a further step at 350-400 °C.

IR spectra recorded during the pyrolysis of the TGA measurement show the decomposition products that were formed. The initial mass loss corresponds to CO ₂ and physically absorbed water (peaks in the IR spectrum at 2400 cm ^-1 and at 1800 cm ^-1) ( Wang Shurong et. al, front. Energy Power Eng. China 1 (4), 413-419, 2007 ). Several peaks appear between 300-400 °C. Table 6 shows the signals (bands) in the IR spectrum of the paper honeycomb. Table 7 shows the additional signals that occur with the honeycombs filled with airgel. Table 6: Band [cm ^-1 ] group 3900-3500 Water 2175 CO 2300, 695 -CO ₂ valence oscillation 1739 -C=O valence vibration 1500-900 CC and CO Table 7: Band [cm ^-1 ] group 3000 Aryl-H valence vibration of aromatic compounds 2175 C=C valence vibration of aromatic rings 1300 -OH stretching vibration

The results of embodiments 5 and 6 show that less honeycomb material decomposes when it is coated with airgel.

It should be noted that the samples for the measurements were cut after being filled with airgel, so that honeycomb material was exposed at the interfaces. These areas are therefore unprotected and begin to decompose more quickly or earlier during pyrolysis.

Example 7:

To determine the stability of support structures filled with airgel, an aramid honeycomb, as was also used in exemplary embodiment 1, was examined for its flexural strength in a 3-point bending test. Likewise, an aramid honeycomb filled with RF airgel produced according to exemplary embodiment 1 was examined for its flexural strength in a 3-point bending test.

The bending strength is a value for the bending stress in a component subject to bending stress. If the bending strength is exceeded, the component fails and breaks. It therefore describes the resistance of a workpiece, which is opposed to its deflection or breakage. The bending strength is usually tested in a bending test. The 3-point bending test is often used. In the 3-point bending test, the test sample is positioned on two supports and loaded in the middle with a test stamp.

In the 3-point bending test, the aramid honeycomb showed a bending strength of 1.04 N/mm ² . The aramid honeycomb filled with RF airgel showed a flexural strength of 1.78 N/mm ² . The bending strength of the filled aramid honeycomb is therefore approximately 1.7 times greater than that of the unfilled aramid honeycomb.

Claims (10)

Sandwich structure (3) comprising a core material (1), the core material (1) being arranged between two cover layers (2a, 2b), the two cover layers (2a, 2b) being the same or different from each other, the core material (1). a carrier material with macroscopic cavities, in which the cavities are filled with an airgel, characterized in that the core material (1) is obtainable with a method for producing a core material (1), in which an airgel solution is introduced into the cavities of the carrier material and The airgel solution is then dried to form an airgel without shrinkage. Sandwich structure (3). Claim 1 , characterized in that the core material (1) is obtainable by coating the carrier material before introducing the airgel solution. Sandwich structure (3). Claim 1 or 2 , characterized in that the carrier material comprises an open-cell or open-pored foam or a honeycomb structure. Sandwich structure (3). Claim 3 , characterized in that the honeycomb structure comprises paper honeycomb, cardboard honeycomb, aluminum honeycomb, aramid honeycomb or polymer honeycomb. Sandwich structure (3) according to one of the Claims 1 until 4 , characterized in that the core material (1) further comprises at least one material layer, in particular a filter made of fleece or paper. Sandwich structure (3) according to one of the Claims 1 until 5 , characterized in that the core material (1) has a density in the range of 0.1 to 0.3 g/cm ³ and/or a thermal conductivity in the range of 0.04 to 0.06 W/(m K). Sandwich structure (3) according to one of the Claims 1 until 6 , comprising a load-bearing skin as a first cover layer (2a), an outer skin as a second cover layer (2b) and a flame-retardant core arranged between the load-bearing skin and the outer skin as a core material (1). Sandwich structure (3) according to one of the Claims 1 until 7 , characterized in that the cover layers (2a, 2b) are selected from fiber-containing plastic or metal. Sandwich structure (3). Claim 8 , characterized in that the fiber-containing plastic is selected from CFK (carbon fiber-reinforced plastic), GFK (glass fiber-reinforced plastic), AFK (aramid fiber-reinforced plastic) or NFK (natural fiber-reinforced plastic). Sandwich structure (3). Claim 8 , characterized in that the metal is selected from aluminum or titanium.

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