DE102013223347A1 - Honeycomb filled with poly (meth) acrylimide foam - Google Patents

Abstract

The invention relates to a process for the production of honeycomb structures filled with poly (meth) acrylimide (P (M) I) foams. In this process, the individual honeycombs of a honeycomb structure are filled with P (M) I particles, which may preferably be prefoamed, and then thermally expanded. At the same time, the present invention also encompasses the novel materials produced by means of this method which, at a very low weight, have the advantages of honeycomb structures, such as high vertical load-bearing capacity and impact resistance, with the advantages of P (M) I foams, in particular the high compressive strength in all directions and the high rigidity, interconnecting.

Description

Field of the invention

In the field of lightweight construction, especially in aircraft construction, but increasingly also in the automotive industry, two technologies occupy an outstanding position in terms of weight savings combined with high strength: honeycomb structures and hard foam materials. The invention relates to a process for producing honeycomb structures filled with poly (meth) acrylimide (P (M) I) foams. In this process, the individual honeycombs of a honeycomb structure with P (M) I particles, which may preferably be prefoamed, are filled and then thermally foamed. At the same time, the present invention also encompasses the novel materials produced by this method, which, at a very low weight, have the advantages of honeycomb structures, such as high vertical load-bearing capacity and impact strength, with the advantages of P (M) I foams, in particular the high compressive strength in all directions and the high rigidity, interconnects.

State of the art

Honeycomb structures, in particular of metals such as aluminum, titanium or steel, have long been known in lightweight construction. In addition to the low weight of these structures have the great advantage that they are very good vertical load. These honeycomb structures are sometimes referred to simply as honeycombs. Vertical means in this context in the direction of the open honeycomb. This load capacity is very high, in particular, when the corresponding surface is coated with a material, such as e.g. a metal or wooden plate or a composite, is covered. Disadvantage of these structures, however, is that they are extremely unstable to a lateral pressure load.

In addition, metal honeycombs in the application of a cover layer in the form of a resin-based composite have the disadvantage that they are difficult to process in a resin infusion process, since the individual cells must not run full of resin for cost and weight reasons. In contrast to the rigid foams described below, they must first be laboriously sealed. Further disadvantages of the honeycomb structures are that beveled honeycomb edges must be filled out, since the honeycomb can absorb only poor lateral forces, and that not completely sealed honeycomb can fill with condensed water. The latter is a major problem, especially in aviation, if only because of the associated weight gain.

An alternative to such metal honeycomb are rigid foams, in particular of polymethacrylimide (PMI) or polyacrylimide (PI) - collectively referred to briefly as poly (meth) acrylimide (P (M) I). Rigid foams can be used as sheet material in lightweight construction. For this purpose, the top and optionally the underside are covered with a metal, wood or composite board as a cover layer, analogous to the honeycomb. The connection of the cover to the rigid foam then called foam core or core material is usually done by gluing, sewing or by connecting with pins. Such a sewing method is for example in EP 2 203 298 described. The bonding of the cover layers to the foam core by means of pins results, for example EP 1 907 193 , However, rigid foam materials have the disadvantage that they are not impact resistant. Although this can be partially counteracted by introducing the cover layers, there is usually no protection against lateral action. Furthermore, the stability in the vertical direction on the top layer for both honeycomb cores and hard foam cores is in need of improvement.

In DE 18 17 156 respectively. DE 27 26 260 the production of PMI foams are described which have excellent mechanical properties even at high temperatures. The production of the foams is carried out by casting, ie the monomers and required additives are mixed and polymerized in a chamber. The polymer is foamed by heating in a second step. In addition to these PMI foams, foams based on methacrylic acid and acrylonitrile (PI foams) are also known with similar properties. These are used for example in the CN 100420702 C described. These foams are initially produced as plates.

Apart from these processes, which start from a non-foamed polymer plate, so-called in-mold-foaming processes starting from a granulate are also known. Compared to the described methods, however, these have in principle several disadvantages. Thus, only an uneven pore structure that differs between the interior of the original particles and the interfaces between the original particles is achieved. Furthermore, the density of the foam due to the uneven distribution of the particles during foaming - as described above - additionally inhomogeneous. Furthermore, one can observe this granulated products foamed a poorer cohesion at the interfaces that form between the original particles during foaming, and thus compared to foamed from a semi-finished sheet materials poorer mechanical properties. in WO 2013/05947 becomes an in-mold process described, in which at least the latter problem was solved in that the particles are coated before filling in the forming foaming tool with an adhesion promoter, for example with a polyamide or a polymethacrylate. This achieves a very good grain boundary adhesion. The uneven pore distribution in the final product is not avoided by this method.

task

Against the background of the prior art discussed, it was therefore an object of the present invention to provide new lightweight construction materials, in particular usable as core materials for composite sandwich materials for lightweight construction, which can withstand particularly high mechanical loads. Mechanical resilience includes both a high impact strength and a high compressive strength.

In particular, it was an object of the present invention to provide novel composite materials which, if they are provided on two sides with outer layers and thus as a sandwich material, perpendicular to these outer layers have improved compared to the prior art pressure resistance.

In addition, these composite materials should have an improved impact resistance in the case of a lateral action, that is to say in the case of a direct effect on the core material, compared to hard foam cores, as compared to metal honeycomb cores an improved pressure resistance.

Furthermore, a process for producing these lightweight materials, especially in the form of sandwich materials, should be quick and easy to carry out.

In addition, the core materials should be simple, e.g. by Harzinfusions- or injection processes for the production of outer layers, be further processed.

Other, not explicitly discussed at this point, tasks may arise from the prior art, the description, the claims or the embodiments.

solution

In the following, the term poly (meth) acrylimide (P (M) I) polymethacrylimides (PMI), polyacrylimides (PI) or mixtures thereof understood. The same applies to the corresponding monomers such as (meth) acrylimide or (meth) acrylic acid. Thus, for example, the term (meth) acrylic acid is understood to mean both methacrylic acid and acrylic acid and mixtures of these two.

The objects are achieved by providing a new lightweight material, which has honeycomb filled with a P (M) I foam.

In particular, this lightweight material may be such that the honeycombs filled with the P (M) I foam are in the form of a plate as core material, and that this core material is provided with cover layers on one or two sides.

The cover layers in this case are preferably metal, wood or composite plates which are connected to the core material by sewing, gluing or by means of pins. In particular, aluminum, magnesium, steel and titanium are suitable metals.

Surprisingly, the choice of the cover material is relatively free. In the case of composite materials, these may be, for example, pure thermoplastics, woven or knitted fabrics or composites thereof, such as, for example, so-called organo sheets or plastic-coated textile carrier webs such as e.g. Imitation leather act. The cover material is preferably a fiber-reinforced plastic. The fibers may in turn be, for example, aramid, glass, carbon, polymer or textile fibers. In turn, the thermoplastic material may preferably be PP, polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), an epoxy resin, an isocyanate resin, an acrylate resin, a polyester or a polyamide.

In the case of thermosetting or elastomeric cover materials, the fibers mentioned are processed by impregnation with a reactive resin, such as a polyurethane, epoxy, phenolic, vinyl or (meth) acrylate resin, followed by curing to form a composite material.

The material for the foam core in this case represents P (M) I, preferably PMI. Such P (M) I foams are also referred to as rigid foams and are characterized by a particular strength. The P (M) I foams are normally produced in a two-stage process: a) production of a cast polymer and b) foaming of this cast polymer.

To prepare the cast polymer, monomer mixtures which contain (meth) acrylic acid and (meth) acrylonitrile, preferably in a molar ratio of between 2: 3 and 3: 2, as main constituents, are first prepared. In addition, other comonomers can be used, such as Esters of acrylic or methacrylic acid, styrene, maleic acid or itaconic acid or their anhydrides or vinylpyrrolidone. However, the proportion of the comonomers should not be more than 30% by weight. Small amounts of crosslinking monomers, such as allyl acrylate, may also be used. However, the amounts should preferably be at most 0.05 to 2.0% by weight.

The copolymerization mixture further contains blowing agents which either decompose or vaporize at temperatures of about 150 to 250 ° C to form a gaseous phase. The polymerization takes place below this temperature, so that the cast polymer contains a latent blowing agent. The polymerization suitably takes place in block form between two glass plates. For the production of foamed boards, the foaming of the cast polymer then takes place in accordance with the prior art in a second step, then at a suitable temperature. The production of such PMI foams is basically known to the person skilled in the art and can be described, for example, in US Pat EP 1 444 293 . EP 1 678 244 or WO 2011/138060 be read. As PMI foams in particular ROHACELL ^® types of the company Evonik Industries AG may be mentioned. With regard to production and processing, acrylimide foams are to be regarded as analogues for the PMI foams. For toxicological reasons, however, these are much less preferred than other foam materials.

The density of the rigid foam material is relatively freely selectable. P (M) I foams can be used, for example, in a density range from 20 to 250 kg / m ³ , preferably from 25 to 220 kg / m ³ . Particular preference is given to using a PMI foam having a density of between 30 and 200 kg / m ³ .

The honeycombs of the lightweight material according to the invention may preferably consist of paper, plastic or a metal. Suitable metals are in particular aluminum, magnesium, titanium or steel in question. A particularly suitable embodiment is, for example, the use of an aramid paper, which is impregnated with a phenolic resin and then cured. Likewise particularly preferably, the honeycomb made of aluminum.

The shape, the diameter and the height of the individual honeycombs are generally known to the person skilled in the art from lightweight construction and can easily be transferred to this invention. It is only important that the honeycombs can be partially filled with the non-foamed or pre-foamed particles of the later foam material.

The honeycombs are preferably square from square view, trapezoidal, hexagonal or octagonal. Also combinations of different forms are conceivable. Preferred is an embodiment in which approximately the same size honeycomb cover the area. The diameter of such a honeycomb is preferably at the thickest point of the honeycomb between 0.1 and 20 cm, in particular between 1 and 10 cm. The height of the honeycomb, however, is absolutely freely selectable. It is only to be noted that the volume of the individual honeycomb naturally determines, taking into account the density of the foam to be achieved, the amount of foamable particles with which a single honeycomb is filled before foaming. this amount can be easily determined by those skilled in the honeycomb volume, the type of material to be foamed, the density to be achieved, the concentration of blowing agent in this material and the temperature during foaming and even in a known composition of the material to be foamed even without preliminary tests ,

The density of the honeycomb material used, as well as the honeycomb size and the web thickness determine in part the weight and the mechanical properties of the lightweight material according to the invention. By choosing a suitable P (M) I foam, the choice of its properties, in particular the density, the lightweight materials can be easily optimized in terms of reducing the total weight or on an improvement in mechanical strength.

In addition to the described honeycomb filled with a P (M) I hard foam, a method for the production of foam-filled honeycombs is also part of the present invention. In this case, this method according to the invention is not restricted exclusively to honeycombs filled with P (M) I, but can also be applied to other foamable plastics which are present in particulate form in the non-foamed state.

• a. Füllen eines Wabenmaterials mit schäumbaren Partikeln,
• b. Ausschäumen der Partikel, wobei die schäumbaren Partikel aus PE, PP, PMMA, PS oder P(M)I bestehen.

The inventive method for the production of lightweight materials, comprising honeycomb-filled honeycomb, characterized in particular by the following process steps:

• a. Filling a honeycomb material with foamable particles,
• b. Foaming of the particles, wherein the foamable particles of PE, PP, PMMA, PS or P (M) I exist.

The same applies to the honeycomb material as previously described for honeycombs filled with P (M) I hard foam.

For the P (M) I or the foam produced therefrom, the above characterization of preferred characteristics also applies.

PE or PP foams are known above all as insulation material, in transport containers and as sandwich material. PE or PP foams can contain fillers and are usually commercially available in a density range between 20 and 200 kg / m ³ .

By contrast, PMMA foams are distinguished by particularly good weathering stability and high UV resistance. To date, however, PMMA foams are technically of minor importance.

Polystyrene (PS) as a foam material, however, is well known. When PS is used, most of the previously described advantages of a P (M) I-filled honeycomb structure are eliminated, as PS foams are not nearly as robust mechanically. However, the great advantage over an unfilled honeycomb structure is that it can be processed much better due to the filling. This is especially true when liquid resins are to be used to form topcoats.

Foamable particles of P (M) I, more preferably PMI, are preferably used in the inventive process.

With respect to P (M) I particles, these can preferably be obtained by means of two alternatives. In the first alternative, the P (M) I is prepared as described above as P (M) I semifinished product, in particular as a plate. It can then be obtained by painting the granules in a particle size that is easy to set.

In a second, as it were preferred alternative, the P (M) I particles are suspension polymers. The preparation of such P (M) I suspension polymers can, for example, in the European patent application with the filing 13155413.1 , registered on 15.02.2013, can be read.

Particularly preferably, the honeycombs are filled with prefoamed particles, in particular with a density between 25 and 220 kg / m ³ . It has surprisingly been found that when using pre-expanded particles, the later foam filling of the individual honeycomb is particularly uniform and therefore with particularly good mechanical properties.

Both the optional pre-foaming and the foaming in process step b. can be done thermally and / or by means of microwaves. The respective process parameters to be used depend on the blowing agent used, the foam material, the density to be achieved and the honeycomb size. These parameters are easy to deduce for the skilled person from the known prior art.

A particularly uniform pore distribution and thus uniform density within a honeycomb is obtained if the honeycombs filled with the particles are moved horizontally and / or vertically and / or rotated during foaming. Particularly preferred is a horizontal rotation, which can optionally be combined with other movements. An appropriate device for movement can be easily installed in a furnace. Alternatively, this device may, for example, comprise sources of microwaves directed to the honeycombs.

The lightweight construction material produced according to the invention, in particular the honeycomb filled with the P (M) I foam, preferably has a density between 30 and 350 kg / m ³ , preferably between 40 and 300 kg / m ³ and particularly preferably between 50 and 250 kg / m ³ up. This density includes both the webs of the honeycomb material and the foam filling, as well as any open spaces. In the case of adding cover layers, this density refers to the core material without these cover layers.

Alternatively or additionally, but especially preferably, method step b. done in a press. This press is constructed so that the foam at the edges of the honeycomb after removal from the press closes. This means that the press ideally rests on the open edges of the honeycomb. A great pressure does not have to be exercised. Rather, a pressure sufficient to withstand the foaming pressure.

In a further process step, the foam-filled honeycomb after the completed process step b. and the subsequent cooling by means of sewing, piercing with pins or gluing with one or two cover layers as described above for the honeycombs filled with P (M) I foam.

Alternatively and at the same time preferred is an embodiment in which the cover material is inserted in the press. In such an approach, it is optional to refrain from further sticking, sewing or puncturing with pins. It is also possible to provide the cover material with an adhesive layer.

Due to the direct contact between the tool shells of the press and the cover material can be done very fast curing of the resin. The temperature used to cure the resin will depend on the particular resin used and will be readily apparent to those skilled in the art. As a rule, such temperatures are between 100 and 300 ° C. Thus, in particular, the preferred foaming of the foam core temperatures between 170 and 250 ° C for most resin systems are suitable. For the less preferred case of a higher required temperature, the curing of the resin may be carried out in another heating station.

In addition, adhesion promoters can be used to improve adhesion between the foam core material and cover layers. These adhesion promoters may be contained in the matrix material of the decoating layers. Alternatively, the adhesion promoters can also be applied to the surface of the cover layers or of the foam core prior to the combination. Alternatively, suitable adhesives may be used in this procedure. Particularly suitable adhesion promoters are polyesters, polyamides or poly (meth) acrylates. However, it is also possible to use low molecular weight compounds which are known to the person skilled in the art from the preparation of composite materials, in particular as a function of the matrix material used for the covering layer.

For the entire process according to the invention, the process parameters to be selected depend on the design of the installation used in the individual case and its design, as well as the materials used. They can easily be determined by a few preliminary tests for the expert.

In principle, the lightweight construction materials according to the invention or the lightweight construction materials produced according to the method according to the invention are very widely applicable. Lightweight construction material produced according to the invention can be used in particular in mass production, e.g. For bodywork or interior trim in the automotive industry, find parts for interior design in rail vehicle or shipbuilding, in the aerospace industry, in engineering, for the production of sports equipment, in furniture construction or in the construction of wind turbines.

embodiments

example 1

For foaming the honeycomb a prefoamed PMI granulate was used. This was prepared from a PMI granules with a particle size of <0.1 mm, which was prefoamed at 200 ° C for 15 min in an oven and then had an average particle size of about 1 mm. The PMI granules used were a material sold as PMI foam under the product name ROHACELL RIMA by Evonik Industries. The honeycomb used was an aramid honeycomb with a cell size of 5 mm and a thickness of 13 mm. The density of the honeycomb was 50 kg / m ³ . The honeycomb was cut to size so that it could be inserted into a metal frame (size 16 × 16 cm) with a metal base plate. The metal frame in this case had a height which corresponded to the thickness of the honeycomb, so that she finished honeycomb top flush with the frame. In the thus inserted honeycomb 31 g of the previously prefoamed granules were filled evenly, so that all holes of the honeycomb were filled to about half their height with prefoamed granules. The thus filled honeycomb was covered with a PTFE film and placed in a heated press at 230 ° C. The press was closed (closing force 50bar). After 60 minutes, the press was cooled to room temperature and the metal frame and the PTFE film removed. As the product, a honeycomb structure uniformly filled with PMI foam was obtained.

Example 2

Analogously to Example 1, but the filling of the honeycomb was made so that first the bottom plate of the metal frame was uniformly covered with 31 g prefoamed granules and then the honeycomb was pressed down to the bottom plate of the metal frame in this uniform layer of granules. This resulted in a very uniform filling of the holes of the honeycomb with prefoamed granules. The foaming of the honeycomb proceeded as in Example 1.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2203298 [0004]
• EP 1907193 [0004]
• DE 1817156 [0005]
• DE 2726260 [0005]
• CN 100420702 C [0005]
• WO 2013/05947 [0006]
• EP 1444293 [0021]
• EP 1678244 [0021]
• WO 2011/138060 [0021]
• EP 13155413 [0036]

Claims (18)

Lightweight material, characterized in that the lightweight material has a filled with a poly (meth) acrylimide foam (P (M) I) honeycomb structure. Lightweight material according to claim 1, characterized in that the filled with the P (M) I foam honeycomb in the form of a plate present as a core material and are provided on at least one side with cover layers. Lightweight material according to claim 2, characterized in that it is the cover layers to metal, wood or composite plates, which are connected to the core material by sewing, gluing or by means of pins. Lightweight material according to claim 3, characterized in that it is the composite material is a material consisting of glass, carbon, aramid or textile fibers and a polyurethane, polyester, epoxy, phenolic or (meth) acrylate resin , Lightweight material according to at least one of Claims 1 to 4, characterized in that the P (M) I foam is a PMI foam having a density of between 20 and 250 kg / m ³ . Lightweight construction material according to at least one of Claims 1 to 5, characterized in that the honeycombs are made of paper, plastic or a metal and the honeycombs filled with the P (M) I foam have a density between 30 and 350 kg / m ³ . Lightweight construction material according to claim 6, characterized in that the honeycombs of aluminum, magnesium, titanium or steel. Method for producing lightweight construction materials, characterized in that the method comprises the following method steps: a. Filling a honeycomb structure with foamable particles, b. Foaming of the particles, wherein the foamable particles of PE, PP, PMMA, PS or P (M) I exist. A method according to claim 8, characterized in that the foamable particles of P (M) I, preferably consist of PMI A method according to claim 9, characterized in that the P (M) I particles were previously obtained as granules by painting from a P (M) I semifinished product. Process according to Claim 9, characterized in that the P (M) I particles are suspension polymers. Method according to one of claims 8 to 11, characterized in that it is in the process step a. in the honeycomb-filled particles to prefoamed particles having a density between 25 and 250 kg / m ³ is. Method according to one of claims 8 to 12, characterized in that the foaming in step b. thermally and / or by means of microwaves. Method according to one of claims 8 to 13, characterized in that the honeycombs filled with the particles are moved and / or rotated horizontally and / or vertically during the foaming. Method according to one of claims 8 to 14, characterized in that method step b. is carried out in a press and the foam ends after removal from the press at the edges of the honeycomb. A method according to claim 15, characterized in that in the press filled with the foamable particles honeycomb is covered with at least one cover layer. A method according to claim 16, characterized in that the cover layers are inserted as prepregs and these simultaneously with the foaming in step b. be cured. A method according to claim 15, characterized in that the foam-filled honeycombs are connected by means of sewing, pins or gluing with at least one cover layer according to claim 3 or 4.