DE102009038531B4 - Process for the preparation of phenolic resin foams using a microwave-sensitive material, foam structure produced by this process and their use - Google Patents

Abstract

A process for producing phenolic resin foams comprising the steps of: providing a mixture comprising: a polymerizable system; and micron sensitive particles as nucleating agents to promote homogeneous pore formation, wherein the micron sensitive particles are selected from the group consisting of carbon nanosphere chains, carbon nanotubes, carbon black, graphite, graphene and silicon carbide activating the micron sensitive particles by means of Microwave irradiation wherein the particles sensitive to microwaves are locally heated and serve as nucleating agents for controlled and uniform pore formation; and foaming the polymerizable system by means of hydrolysis product released by the initiated crosslinking of the polymerizable system, water as blowing agent.

Description

Field of the invention

The invention relates to a process for the preparation of homogeneous phenolic resin foams having improved pore structure based on a polymerizable system and a microwave-sensitive material as a nucleating agent. Furthermore, the invention relates to a foam structure produced by the method and the use of the foam structure.

Technological background

Organic foams based on thermoplastics such as polyvinyls, polyolefins, elastomers such as polyurethane and thermosets such as phenolic resins are suitable for thermal and acoustic insulation. While thermoplastics are predominantly represented by polymers, the group of thermosets is characterized in particular by polycondensates, which can cure by heat or suitable catalysts.

The foams mentioned can be prepared by various methods. On the one hand, gas can be introduced into the basic composition or basic mixture by stirring, as in the production of the phenolformaldehyde rigid foam. On the other hand, suitable chemical blowing agents can be added during the preparation of the basic mixture, which become gaseous only during processing. If the mass is injected into a mold, then a structural foam can be produced which has an outer skin with a core structure that saves weight in the foam structure. An essential part of modern lightweight constructions are sandwich structures. For example, phenolic foams are often used for core materials in sandwich structures in commercial aircraft.

Out US 3,238,157 For example, a process for the production of foams based on an elastomer, which consists of mixing rubber-like high-viscosity silicone compositions initially with particulate cavity-forming solids. The mixture cures to a silicone elastomer, from which foam structure the lumpy material is subsequently dissolved out either by washing with water or by heating in such a way that pores are formed which correspond to the size of the solids removed.

It is also known to use microspheres for regulating the cell structure in foams, which are intended, for example, in a resin material to improve the foaming process. In WO 2006/051302 For example, a process for producing a composite foam using phenolic resin having a plurality of expandable thermoplastic microspheres is described. This mixture of phenolic resin and microspheres is exposed to microwave radiation to cure the composite material and cause expansion of the microspheres. Since the not yet expanded particles are much easier to mix to a processing matrix than already expanded microspheres, the processing in the production of the foam can be facilitated. In the described method, however, no uniformly distributed pore formation is achieved by the use of external blowing agents.

The microwave technique used in the latter method aids in the polymerization of resins because, for example, this heat source can cause faster cure by heating at a high water level of the base mixture. In the prior art, the microwave technique is used for example for the polymerization of vinyl monomers. It is also known that the microwave technology is suitable for faster drying of aqueous solutions.

Furthermore, cell structures of foams can be regulated by the addition of nucleating agents by promoting pore formation. As nuclei nuclei, for example, very fine particles so-called microparticles or nanoparticles can be made available, such as inorganic fillers. The use of known nucleating agents, such as. However, for example, clay or fumed silica may prove to be unsuitable for even expansion of the base mixture during the polymerization process, resulting in nonuniform pore structures and higher density foams than desired.

Nucleating agents considered by those skilled in the art include nanoparticles such as nanotubes of layered mineral silicates or other inorganic small particles such as fumed silica structures. These small particles may have a high surface area of between 50-400 m ² / g. In particular, phyllosilicates have been used repeatedly for the production of foams, but have a disadvantageous effect on the viscosity of the base mixture and can be difficult to disperse, which makes it difficult to process the matrix or allows only small addition amounts of the nucleating agent. The nanoparticles mentioned are used in analogy to the widely used microparticle fillers and serve for a finer cell structure of the foam. However, the ability to produce cells evenly is not sufficient with these nanoparticles.

From the EP 0 989 158 A2 For example, a foamed article of agglomerated, foamed particles is known, which particles comprise a thermoplastic starch, a thermoplastic polymer, and water. Microwaves can be used to foam the particles.

A method of frothing silicone compositions using microwaves is described in U.S. Pat US 4,026,844 disclosed.

In the WO 2009/155066 A2 discloses a process for producing foamed polymers, wherein, among other things, activated carbon can be used as a foam aid. The use of microwave radiation is not disclosed.

A water-absorbing, foam-like, crosslinked polymer of monoethylenically unsaturated monomers is described in US Pat DE 195 40 951 A1 described.

Metal powders, metal compounds or metal oxides are used according to the EP 1 900 767 A1 used in the production of a microwave foam.

The DE 602 23 228 T2 discloses a polymeric composite foam of a continuous phase of a furan or a phenolic and furan polymer and a dispersed phase of foamed polystyrene polymer. For example, expandable graphite may be used as the fire retardant.

Finally, the concerns EP 0 010 432 B1 a plastic polymer material which may also be used to make a syntactic foam when, for example, hollow flyash balls are introduced.

As conventional foam composite fabrication processes use complicated, multi-phase, and costly processes, there is a need to find more efficient and simpler ways of manufacturing. Furthermore, when using conventional nucleating agents such as mineral microparticles, only a slight improvement in the pore structure was observed.

Summary of the invention

It is an object of the invention to provide a foamable mixture for the preparation of improved foams having a more homogeneous pore structure as well as a lower density, e.g. B. less than 100 kg / m ³ .

It is another object of the present invention to provide an improved foaming process.

These objects are achieved by a process for producing phenolic foams, a foam structure prepared by the process, and the use of the foam structure according to the independent claims, wherein further developments are embodied in corresponding dependent claims.

According to an exemplary embodiment of the invention, a mixture for producing foams comprising a polymerizable system and micron-sensitive particles as nucleating agent to promote homogeneous pore formation is provided.

In this way, microwave-sensitive particles can be added to a polymerizable system which can be locally heated and serve as suitable nucleating agents for controlled and uniform pore formation. In the context of nucleating agents, microwave technology is a technology option for improving the material properties of the foam products of polymeric materials. The microwaves used in the invention are known to be a form of electromagnetic radiation and are characterized, for example, by wavelengths in the range between 1 mm and 1 m, which corresponds to a frequency range of 300 MHz and 300 GHz.

Furthermore, the use of microwaves as a heat source over conventional heat processes offers the following advantages: Firstly, the process time can be shortened because not the entire microwave oven, but only the sample in the oven is heated. A local heat input, for example, in sandwich structures only heat the foam and not the already cured laminates. On the other hand, the rapid heating by microwave technology promotes nucleation. Shorter heating and cooling times, in turn, result in significant cost savings and lower energy consumption over conventional methods. This is also beneficial for the environmental audit.

Furthermore, a homogeneous heating can be achieved by the microwave technique, which results in homogeneously uniform pore structures and enables the production of strong foams. Care must be taken in the case of microwaves initiated exothermic reactions that the temperature remains under control. This means that with known heat formation by chemical reactions a lower Microwave power can be used to obtain the desired process for optimal process control process temperature.

The microwave active nucleating agent is added to the base mix to aid in the formation of cell structures prior to the foaming process. In this way, the nucleation rate of the mixture in the production of the foam can be improved. These microwave-active nucleating agents promote a finer and more uniform cell structure of the foam. By means of the microwave activity, the nucleating agents, for example in a thermoplastic polymer, cause a local increase in the temperature, which takes place rapidly and forms ideal nuclei for bubble formation. Also, the microwave-active nucleating agents can assist in heating the material to be foamed. In this case, the large surface introduced by the nanoparticles seems to be of central importance for improved nucleation or nucleation. The nucleating agents may also improve mechanical properties such as the compression property.

According to an exemplary embodiment of the mixture, the microwave-sensitive material is selected from the group consisting of carbon nanosphere chains (CNSCs), carbon nanotubes, carbon black, graphite, graphene, and silicon carbide (SiC).

The mentioned materials for nucleating agents are carbonaceous. Carbonaceous nanostructures are made in the prior art from graphite structures. These graphite structures are a well-known carbon material that has important properties such as electrical conductivity and microwave activity. The carbon nanotubes include carbon nanofibers having average diameters of, for example, less than or about 2000 nanometers (nm).

In order to render the carbonaceous and, for example, nanotube fibrous nanomaterial more dispersible, the carbonaceous material may be treated to attach functional groups such as acid groups from the surface. For example, oxidized functional groups can be attached. Other treatment options of this nanomaterial to increase the dispersibility are possible, such as a heat treatment or a cleaning process.

It is also possible to use carbon nanosphere chains that are relatively similar in their properties to the carbon nanotubes. They can also be heated under microwave irradiation and can promote nucleation in the base mixture by the local heating. Furthermore, the use of carbonaceous nanostructures as an additive in polymer foams can improve the mechanical properties such as compressibility of the composite material.

By using the nucleating agent, it is furthermore possible to bring about a positive influence on the burning behavior of the foam produced. Even the addition of small amounts of carbon black, for example, in an amount of 2 percent by weight, increases the flame retardant property of the foam product - especially in highly flammable polystyrenes or polyurethanes. Finally, moisture resistance of the foam products can also be promoted by the addition of hydrophobic nucleating agents such as the carbon nanosphere chains or carbon nanotubes.

According to an exemplary embodiment of the mixture, the polymerizable material may be selected from the group consisting of thermoplastics, elastomers, thermosets and mixtures thereof.

As the polymeric base material, any polymer or polymerizable material compatible with the use of graphite nanostructures or iron oxide nanoparticles can be used. As thermoplastics, for example, the groups of polyolefins and polyvinyls for foam production are known. As exemplary resin systems, phenolic resins and epoxies may be mentioned. In addition to polyurethanes and silicones, it is also possible to use the combination of the stated polymers or derivatives of these or copolymers. The polymerizable material may be a polymer or a polymerizable material such as a monomer or oligomer or other polymerizable systems.

When the foam mixture contains addition amounts of inorganic processing agents or flame retardant additives in addition to the organic foam base, the foam product is less likely to be flammable and therefore particularly suitable for fire protection purposes.

According to an exemplary embodiment of the mixture, the mixture comprises an accelerator or catalyst for curing the resin system.

The catalysts or accelerators are suitable for making the liquid preparation of a polymer or monomer mixture faster to cure or polymerize. For this purpose, both organic acids and inorganic acids can be used as catalysts. Inorganic acids include sulfuric acid, phosphoric acid or hydrochloric acid. The organic acids include, for example, benzenesulfonic acid or phenolsulfonic acid.

The polymerizable system is based on phenolic resin.

By using microwave-active nucleating agents, it is possible to achieve improved mechanical properties of the foam compared with the conventional phenolic foams, which may be partly brittle.

When using phenolic resin systems, external blowing agents can be dispensed with since phenolic systems form water during the polymerization which can be vaporized. In this way, the addition of external blowing agents can be circumvented, whereby a difficult to control gas formation and thus uneven pore formation can be avoided. With the aid of the nucleating agents, foaming can be controlled in phenol resin systems in order to improve the properties of the produced foam in terms of mechanical properties and density as well as uniform pores over conventional foams.

Unlike polystyrene, polyolefins or polyurethanes, which are relatively flammable, phenolic foams are relatively fire resistant. Since phenolic foams have low flammability, low smoke emission rate, and low toxicity of the flue gases produced, they are well suited for use in aircraft. In addition, phenolic foams have good thermal, thermal and sound insulating properties.

According to an exemplary embodiment of the invention, the accelerator or catalyst for curing the polymer or polymerizable system phenolsulfonic acid, which is added, for example, with a weight fraction based on the basic mass between 5 and 15 percent, in particular about 7.5 weight percent.

In this way, it is possible to achieve cell structures which have better mechanical properties than conventional foams, since they can be exposed to higher loads or obtain better pressure resistance values, for example. For example, the cells are smaller with a 7.5 percent addition than with an addition of about 5 percent phenolsulfonic acid.

An exemplary embodiment of the invention comprises a mixture, wherein the weight fraction of the nucleating agent is between 0.5 and 5 percent and in particular makes up about 2 percent.

The proportion of nucleating agents or nanoparticles of the nucleating agent can be adjusted so that the resulting mixture contains between 0.01 and 5 percent by weight. As nanoparticles in the form of carbon nanosphere chains, those with a weight fraction based on the basic mass of 2 percent can be used, wherein the average particle diameter of the nanoparticles of carbon nanosphere chains can be in a range of 10 and 200 nm, for example between 50 to 60 nm With these small amounts of use, based on the weight of the total mixture, significant changes, that is to say a multiple of the cell formation, can be achieved compared to conventional non-microwave-active nucleating agents.

In addition to the carbon nanosphere chains, the microwave-active carbon nanotubes can also be used. The cell structure produced from the mixture is essentially controlled by the amount of nucleating agent and the diameter of the nanoparticles in the nucleating agent.

Optionally, further additives can be added to the starting materials which are soluble in liquid or in one or more of the components used for foam production. Examples of additives are, for example, flameproofing agents, surface-active additives, foam stabilizers and other cell regulators or dyes. In particular, in the use of phenolic resins, the foam product may be brittle, additions of glass or aramid fibers are common. Foams reinforced with aramid fibers have less brittleness and a higher resistance to compressive stress. Glass fiber reinforced foams, on the other hand, have significantly stiffer and stronger properties. These reinforced phenolic foams are often used in sandwich structures because they can absorb high tensile and compressive forces.

According to one aspect of the invention, there is provided a process for the production of phenolic resin foams, comprising the steps of: mixing an aqueous solution based on a polymerizable system with microwaves sensitive particles, activating the microwaves sensitive particles by means of microwave irradiation Microwave-sensitive particles are locally heated and serve as nucleating agents for controlled and uniform pore formation and foaming the polymeric or polymerizable system by means of hydrolysis products released by the initiated crosslinking of the polymeric or polymerizable system.

The matrix used for the mixture is a polymerizable system. For example, liquid or viscous prepolymerized molecular chains called resin are used. For example, if phenolic resin is used as a resin system, which are used in particular in aircraft because of their good fire protection behavior, an aqueous solution is present as a basic mixture. This aqueous resin solution with a water content of, for example, 20 to 30 percent is mixed with the particles sensitive to microwaves in such a way that a stable and uniform distribution of the microparticles or nanoparticles takes place. If nanoparticles are mixed in the form of carbon black, this leads to a discoloration of the foam.

A fine distribution of the particles in the mixture of solvent and resin system can be achieved by the introduction of shear energy. In this way, the nanoparticles can be predominantly deagglomerated distributed in the mixture. Agglomerates can reduce the activity of foaming. The introduction of shear energy can be easily realized by appropriate mixing devices. In the laboratory, a stepwise dispersion with different rotation rates per minute proved to be particularly efficient for a homogeneous distribution. Before the shearing action, other additives can be added in addition to the nucleating agents. If the addition of catalysts is desired, this is done shortly before foaming. Accelerators or catalysts can support the crosslinking, whereby sometimes the intrinsic moisture content can be sufficient.

The resulting mixture is then exposed to a source of microwaves to produce the foam. The micelles sensitive particles contain carbon structures such as graphite or iron nanoparticles and can cause local heating. These local heat sources promote a particularly uniform gas bubble formation and thus a foam with low density. The controlled cell formation rate allows the microwave-active nucleating agents to produce a controlled and uniformly shaped foam. The microwave-sensitive material is so stable in the mixture that there is no premature inhibition of foaming. As materials suitable for microwaves, for example, carbon-containing organic compounds in the size range of nanometers can be used, or even metal oxides such as iron oxide.

If a suitable polymerizable system is used, the microwave irradiation heat action can initiate a reaction process in which water is released. In this way, the hydrolysis product of the resin system can be used as a blowing agent and no additional blowing agents need to be added before foaming. In this way can be dispensed with environmentally harmful chemical blowing agents. For example, the crosslinking reaction in phenolic resins causes the formation of water which evaporates upon heating, whereby the steam can be used as the blowing agent.

According to an exemplary embodiment of the production method according to the invention, the mixing comprises the introduction of an accelerator or catalyst, and further the mixture is introduced into a mold before foaming.

The crosslinking of the polymeric resin can be accelerated with hardeners or catalysts as a further component. In the process of the present invention, the acidic curing agent may be any acidic compound conventionally used in curing foams, such as hydrochloric, sulfuric, nitric, phosphoric, or phenolic sulfonic acids. All of these acids are suitable for use in an aqueous solution. A particularly acidic curing agent is the phenolic sulfonic acid or the phosphoric acid, which are inorganic and are used in weight percentages, in particular 5 to 10 percent and about 15 percent, respectively.

In order to obtain a suitable foamed structure, these hardening agents should, according to an exemplary embodiment, be incorporated shortly before foaming. Depending on the amount of the curing agent added, the reaction rate can be controlled. This means that the more catalyst added, the shorter the reaction time. After addition of the accelerator phenolsulfonic acid at less than 10 percent by weight resulted in a microwave irradiation time of a few minutes in the range between 4 and 10 minutes.

According to an exemplary embodiment of the invention, a foam structure based on a polymer or polymerizable system is provided, wherein the foam structure comprises microwave-sensitive particles as nucleating agent.

Due to the incorporation and use of microwave active nucleating agents, homogeneous foams having a high pore density can be obtained. Suitable nucleants substances may, for example, be carbonaceous organic nucleating agents such as carbon nanosphere chains or carbon nanotubes and carbon black, graphite, graphene, silicon carbide or metal oxides such as iron oxide. Using these nucleating agents, low densities of the foam structure can be achieved.

In an exemplary embodiment of the invention, the foam structure is based on a phenolic resin and a phenol-sulfonic acid based accelerator and includes the microwave-sensitive carbon nanosphere chains.

An advantage of the phenolic foam structures is that they are flame retardant and thermally dimensionally stable. For this reason, they are used in particular for the isolation of rooms in which the fire protection takes a high priority. Furthermore, the phenolic resin can produce foams which have good heat and sound insulation. The material can be made in various forms by placing it in suitable molds.

The use of phenolic foams in microwave assisted manufacture allows the hydrolysis product to be used as a blowing agent while avoiding flammable and environmentally harmful chemicals. Due to their high water content, phenolic foams are particularly suitable for a production process by means of microwave irradiation.

To accelerate the curing, the organic phenolsulfonic acid can be used with. By using the phenolsulfonic acid at about 7.5% by weight, the curing time can be only a few minutes. Experiments showed that a degree of crosslinking of phenolic foams of over 90% can be achieved.

The microwave-sensitive carbon nanosphere chains can locally heat the mixtures and thus promote controlled cell formation. In particular, carbon nanosphere chains yield cell structures with relatively stable cell walls, which can withstand mechanical loads and tensile forces. Furthermore, carbon nanosphere chains have a positive influence on the moisture behavior. Because the carbon nanosphere chains are hydrophobic, these foam structures absorb less moisture than foams that do not contain these particles.

In a further embodiment of the invention, a foam structure of a density of less than 25 kg / m ^{3 is} provided, preferably about 20 kg / m ³ .

In this way, the lightweight potential of the foam structure can be increased. In particular, for use in aircraft weight savings are due to low densities of building materials of importance. Further, by the use of microwave techniques, the manufacturing cycles can be shortened and cost-effective production can be provided.

According to an exemplary embodiment of the invention, a foam structure for use as a material is selected from a group comprising core material for sandwich structures, insulation material and flame-retardant structures.

In this way, the foam structure according to the invention allows an ideal core structure for sandwich structures at a reasonable production cost. Foamed composite bodies or sandwich constructions and composites are used in particular for insulation and thermal insulation. Sandwich structures are shaped bodies which are constructed from an outer cover layer and an inner core material, wherein the core material comprises the foam according to the invention. As core materials, for example, materials with a low density are typically used in the range of less than 150 kg / m ³ .

The use of rigid foams for coating materials such as phenolic foam can provide an isotropic foam that can absorb shear forces in all spatial directions. Furthermore, these foam structures can be easily edited with the usual for example for wood tools and machines.

Phenol foams used in an interior as a core material for sandwich structures have a very good fire behavior. The flame retardant properties of these foam products can be further increased by the addition of a suitable nucleating agent, such as carbon black. Furthermore, in addition to the nucleating agent additional flame-retardant additives can be added to meet the high fire protection requirements, for example in the aviation and aviation sector.

According to an exemplary embodiment of the invention, the foam structure is used for an aircraft.

In particular for isolation purposes or for use in sandwich structures for Interior equipment of aircraft may be suitable for said foam structures. The microwave irradiation offers a possibility to accomplish an in situ production of the composite material. In this way, a simple manufacturing process - the in-situ foaming of composite materials - can be provided. By adding suitable nucleating agents, the brittleness of the material can also be reduced according to the invention. By using low densities, a lightweight construction can be realized, which in aviation in fuel savings.

It should also be noted that the above features and method steps can also be combined. The combination of the above process steps or features may result in interactive effects and effects that go beyond the individual effects of the respective features, although not explicitly described in detail.

Exemplary embodiments of the invention will be described below with reference to the following drawings.

Brief description of the drawings

1 Figure 3 is a schematic view of the process of producing improved foams using micelle sensitive particles as nucleating agents.

2 FIG. 10 is a schematic flow diagram of the process for producing improved foams through the use of microwave active particles as a nucleating agent. FIG.

Detailed description of the drawings

1 shows a schematic view of the manufacturing process for improved foams with evenly distributed pores. The mixture 100 is bounded by a rectangle and contains the nucleating agents 140 , Suitable nucleating agents are nanoparticles of diameter in the nanometer range. Particularly suitable as nucleating carbon nanosphere chains, which can be easily dispersed in an aqueous solution of a resin system.

Carbon nanosphere chains, for example, are commercially available from Clean Technology International Corporation. The carbon nanosphere chains have a spherical shape, which are mostly hollow and connected in chains and due to their shape also called nano onions or onion-shaped nanostructure. For example, these onion-like spheres have diameters in the range of 2 nm to 250 nm, preferably about 60 nm. The carbon nanosphere chains, possibly due to their chained structure, are readily dispersed in hydrophilic systems such as phenolic resin. Experiments showed a simpler distribution of carbon nanosphere chains in an aqueous phenolic resin system compared to carbon nanotubes, which may be fibrous.

Furthermore, experiments have shown that nanoparticles sensitive to microwaves have a significantly higher activity for homogeneous cell formation. In particular, Carbon Nanosphere Chains were tested. In order to bring about the effect of better pore formation, even small proportions by weight of carbon nanoparticles based on the basic composition are sufficient.

As further possible nucleating agents, the carbon nanotubes similar to the carbon nanospheres chains or metal oxides such as iron oxides can be used. Carbon black, graphene and graphite nanoparticles also show strong microwave activity and can be used as nucleating agents for the mixture. At least one of these nucleating agents may be added as an additive to the base mixture in order to favor the nucleation of gas bubbles or foam cells during foaming.

Furthermore, further additives can be introduced into the mixture, in particular additives such as surface-active substances, which can also be used to control the pore size and structure. Frequently, in conventional manufacturing processes, blowing agents are added to the base mixture, whereupon foaming is based. By contrast, propellants can also be produced system-internally, for example by a chemical reaction or due to physical processes, namely by, for example, evaporation at elevated temperatures. All propellants cause gas to be released, allowing a cell structure to be made.

In the mixture can be dispensed with a blowing agent addition, if phenolic resin is used as a base, since the polycondensation in the foaming process water is produced, which can serve as a blowing agent in the evaporation.

Phenolic resins are formed by polycondensation of phenol or similar cyclic compounds with formaldehyde with elimination of water. Phenols can be divided into two different classes, the novolacs and the resoles. The so-called novolacs are thermoplastic resins. The group of resoles is suitable for the production of foams and contain a mixture of mono-, di- or trimethylphenols, condensation products of various types and of different molecular weight, as well as monomeric formaldehydes and phenol.

For foam production, liquid or viscous prepolymerized molecular chains can be used which crosslink in the foaming step. An example of a common phenol is the commercially available Bakelite. The precondensates are solid powdery or viscous resins which are soluble and can cure by heat or with certain catalysts cold. Using a suitable acidic catalyst, the phenolic foams can be easily prepared, and it should be noted that in the curing reaction of the resole product, water is formed. This hydrolysis product can be used as a blowing agent as described above, wherein an external blowing agent can be dispensed with.

The mixed solution, consisting of matrix and nucleating agents and possible other additives, is initiated in a mold, not shown, and placed in an oven. Microwave production can be carried out in modular microwave systems such as HEPHAISTOS (High Electromagnetic Power Heating Automated Injected Structures Oven System) ovens developed at Forschungszentrum Karlsruhe. These hexagonal microwave ovens, whose geometry enables uniform distribution of the microwaves, can generate, for example, microwave radiation of about 2.45 GHz. Furthermore, these ovens can be realized from the laboratory scale to the large-scale facility of the aviation industry. The use of this product line of microwave ovens thus provides the advantage that in situ syntheses of composite foam structures are made possible.

In the said microwave oven, the desired heat can be supplied to the mixture by microwave radiation. After microwave irradiation, the particles are activated and a mixture is formed 110 a, the locally heated nucleating agent 144 contains. By the local heating of the nucleating agent, represented by the circles around the nucleating agent 144 , Homogeneous pore formation can be initiated. In comparison to phyllosilicates as nucleating agents, the use of microwave-active carbon nanosphere chains has experimentally demonstrated a higher reactivity of the resin with respect to cell formation. Laboratory experiments also showed that, for example, carbon nanosphere chains significantly increase the temperature of the resin even at low additions of 5 percent by weight. Lower weight fractions are expected to produce lower temperature increases.

On the one hand, a rise in temperature can reduce the viscosity and surface tension of the mixture and make the walls of the bubble thinner compared to lower temperatures. These thin-walled structures are easier to tear. This leads to larger cells. On the other hand, an increase in temperature can also increase the nucleation rate and nucleation rate, which involves the formation of more cells. Experiments showed, however, that the walls, which contain carbon nanosphere chains as nucleating agents, were able to withstand higher temperatures and did not break down compared to conventional nucleating agents. In this way, a controlled and homogeneous cell growth could be achieved by increasing the temperature.

The microwave irradiation further initiates the crosslinking of the resin used and the curing begins in a period of a few minutes. After curing, the mixture may occupy a larger volume, represented by the right image, which has a larger volume 160 represents the foam structure. The nucleating agents are now as germs 145 the foam bubbles 150 to see. The propellant used may preferably be the hydrolysis product of the resin, in particular of the phenolic resin, water vapor being produced by the heating. The activated and cured mixture 160 represents a higher cell density and more uniform pore structure than foams made with conventional non-microwave active nucleating agents. Further, the hydrophobic property of, for example, the carbon nanosphere chains can be utilized, which reduces water absorption of the foams, which is beneficial because moisture reduces or degrades the mechanical properties of the material.

In order to obtain good foam products, a shape is necessary, however, in 1 not shown. The use of molds allows for a variety of different foam bodies. Furthermore, a mold supports the homogeneity of the foams and at the same time controls the density. For example, the mold may be closed to control the density of the mixture while the microwave radiation is being applied. As gases are generated by heating, the mold should also contain holes so that steam can escape from the mold. It can be prevented with the help of only permeable to gases membranes an exit of the foam mass. To quickly dissipate the resulting vapor, it is also possible to provide an additional venting device.

The material to be used for a mold should be permeable to microwave radiation such as Teflon, polyetheretherketone (PEEK) or silicone. Open forms can also be used. However, experiments showed that closed molds favor more homogeneous foam production. The mold can also consist of several parts such as a base part, a central wall part and a connectable to the other parts cover. Multi-part molds can be closed with suitable devices such as screws or other fixing means.

In the form, temperature sensors are advantageous because temperature control may be necessary if exothermic chemical reactions are involved in the curing process. Through experiments on the temperature curve, the optimal activation irradiation can be found out.

2 shows the time course of the inventive method for producing improved foams. The first step 200 Figure 3 shows the mixing of an aqueous solution based on a micropellin sensitive particle resin system. In this step, other desired additives may also be added, such as spherulites or microspheres capable of expansion, as well as surfactants, dispersants, flame retardants, thermal stabilizers, pigments, dyes, fillers, compression enhancement additives or the like. Just before injection into the mold, that is before the next process step 201 Also, a curing agent or accelerator may be added. In the next step 201 the mixture is introduced or injected into a mold. The mold should be suitable for producing the volumes to be produced later and may have openings which serve to escape the gases produced.

In step 202 The nucleating agents are activated by microwave radiation, so that a local heating is generated around the nucleating agent. The duration of the microwave irradiation and exposure time depends on the amount of microwave-sensitive material added and the intensity of the microwave dose. The generation of microwaves can be carried out in the usual way by suitable devices, preferably by microwave ovens of the HEPHAISTOS series. In these furnaces, the mixture can be uniformly and rapidly heated homogeneously, which is desirable when using foams, in particular with regard to a uniform pore formation. The heat source can be switched on and off immediately, which is associated with energy savings.

By heating the mixture, the crosslinking of the resin system is initiated and, when using phenolic resin, the hydrolysis product water is simultaneously produced. As a result of the heating, the water can evaporate and with the resulting gas bubbles, the resin system can be foamed. The foaming step 202 involves foaming, whereby a blowing agent, which becomes gaseous under the foaming conditions, is used to form the pores. The cells form on the nucleating agents or active nanoparticles.

In the last step 203 the foam structure is cured and a solid foam structure has formed in the mold. After cooling, the foam can be removed from the mold. The foam structure has a homogeneous pore formation and a low density. By the method according to the invention foams of very low densities of about 20 kg / m ³ or less can be produced.

It should be noted that the term "comprising" does not exclude other elements or method steps, just as the term "a" and "an" does not exclude multiple elements and steps. The reference numerals used are for convenience of reference only and are not to be considered as limiting, the scope of the invention being indicated by the claims.

Claims (10)

Process for the preparation of phenolic resin foams, comprising the following steps: Providing a mixture comprising: A polymerizable system; and Micron sensitive particles as nucleating agents to promote homogeneous pore formation, the micelles sensitive particles being selected from the group consisting of carbon nanosphere chains, carbon nanotubes, carbon black, graphite, graphene and silicon carbide Activating the micron-sensitive particles by means of microwave radiation, wherein the micellesensitive particles are locally heated and serve as nucleating agents for controlled and uniform pore formation; and Foaming the polymerizable system by means of hydrolysis product released by the initiated crosslinking of the polymerizable system, water as blowing agent. The method of claim 1, wherein the mixture comprises an accelerator or catalyst for curing the polymers. The method of claim 2, wherein the accelerator or catalyst comprises an acidic curing agent, the proportion of which is between 5 and 15% by weight, in particular about 7.5% by weight. A process according to any one of claims 2 or 3, wherein the accelerator or catalyst comprises a phenolsulfonic acid. Method according to one of the preceding claims 1 to 4, wherein the proportion of the particles of the nucleating agent between 0.1 wt .-% and 5 wt .-%, and in particular about 2 wt .-% is. The method of any one of preceding claims 1 to 5, wherein said providing comprises introducing an accelerator or catalyst; and wherein prior to activating the mixture is poured into a mold. Foam structure produced by a process according to one of claims 1 to 6. A foam structure according to claim 7, wherein the foam has a density of less than 25 kg / m ³ . Foam structure according to one of the preceding claims 7 or 8, wherein the foam structure is used as core material for sandwich structures, as insulation material or as a flame-retardant structure. Use of a foam structure according to any one of claims 7 to 9 for an aircraft.

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