Foaming of polymers containing blowing agents using microwaves.

Abstract

The present invention relates to the production of (rigid) foams by heating a blowing agent-containing polymer by a combination of thermal energy and irradiation with microwaves.

Description

[Technical Field]

[0001] The present invention relates to the production of (rigid) foams by heating a blowing agent-containing polymer by a combination of thermal energy and irradiation with microwaves.

[0002] Conventional technology The foaming process of polymers to produce (rigid) foams is well known. Known processes include, in particular, continuous extrusion processes, in which the polymer is melted in an extruder, charged with a blowing agent, and foamed by a pressure drop upon discharge from a nozzle. Also known are particle foaming processes, in which blowing agent-containing polymer granules are foamed in a mold by the action of energy (heat, steam), and batch processes, in which the polymer is charged under supercritical conditions, e.g., under CO or N pressure, and foamed upon decompression. Also known is the block foaming process, in which a blowing agent-containing polymer is softened in an oven by heating above its glass transition temperature and then foamed by the blowing agent present therein.

[0003] The foaming agent-containing polymer sheet is g The block foam process, in which the block foam is heated to a temperature exceeding 1000 KPa (2726259), is particularly used to produce PMI- or PMMA-based rigid foams (see German Patent Application Publication Nos. DE 2726259, DE 1817156, and EP 3277748). One of the reasons for using these materials is that the density of the resulting foam can be appropriately adjusted by adjusting the foaming temperature / foaming time. Furthermore, the PMI- or PMMI-based rigid foams mentioned above cannot be produced by extrusion processes due to their high molecular weight, which is important for their mechanical properties, and therefore are not economically available through other production processes.

[0004] Microwave technology has been used only to a very limited extent in the production of foams, for example in the production of elastic melamine foams, where a liquid melamine-formaldehyde precondensate is foamed and crosslinked by heating with microwave radiation (EP 0 037 470 A1).

[0005] EP 3277748 A1 describes a microcellular PMMA foam and its production. In particular, a suitable nucleating agent is provided. The document describes a classical foaming process using thermal energy. It mentions that PMMA cast polymers containing a foaming agent can also be foamed using a combination of thermally supplied energy and microwaves. However, no further details of such a process are disclosed.

[0006] assignment The problem addressed was to develop an economical process for expanded (rigid) foam.

[0007] There was a need to develop an economical foaming process for producing rigid foam blocks (e.g., PMI and PMMA-based foams) that cannot be produced by, for example, an extrusion process due to the high molar mass of the matrix polymer.

[0008] Rigid foam blocks are preferably produced from polymer sheets obtained by a casting polymerization process, in which a blowing agent is added to the monomer solution before polymerization. To foam such a foam-containing polymer sheet, the sheet must be heated above the boiling / decomposition point of the foaming agent present and simultaneously above the glass transition temperature of the polymer. Simple thermal heating in an oven has the disadvantage that energy input to the polymer occurs solely through convection and conduction of thermal energy. This process can take 2-3 hours, as plastic itself is a poor thermal conductor and the core of the polymer sheet slowly reaches foam temperature. Furthermore, the foamed polymer insulates itself against oven temperatures.

[0009] The aim was to shorten this process significantly and make the foaming process more economical.

[0010] solution It has been found that the foaming time can be significantly reduced by providing energy in the form of microwave radiation in addition to the mere thermal energy input during heating.

[0011] The problem is a method for producing a (rigid) foam, in which a blowing agent-containing polymer composition is foamed in an apparatus comprising a thermal heating means and a microwave, and the microwave is used in combination with heat to raise the glass transition temperature T gC and heating the polymer composition to a temperature exceeding 100° C.

[0012] High temperature polymers suitable for the production of rigid foams include those with glass transition temperatures T between 180°C and 235°C. gP Materials with lower glass transition temperatures are often not suitable to meet the desired property profile of high temperature foams. gP Materials with higher values ​​are rarely available. According to the present invention, the definition of the glass transition temperature relates to the most important (energetically largest) thermal transition of the material. This means that the material is very likely to exhibit a second order thermal transition below 210°C. This occurs, for example, in phase-separated systems, especially polymer blends (polymer mixtures).

[0013] For materials for which no explicitly disclosed glass transition temperature is known, this temperature can be determined by DSC (differential scanning calorimetry). In this regard, those skilled in the art will recognize that DSC is sufficiently determinative only if, after an initial heating cycle to a temperature at least 25°C above the highest glass transition or melting temperature but at least 20°C below the material's lowest decomposition temperature, the material sample is held at this temperature for at least 2 minutes. The sample is then cooled to a temperature at least 20°C below the determined lowest glass transition or melting temperature, with the cooling rate not exceeding 20°C / min, preferably not exceeding 10°C / min. The actual measurement is then performed after a further waiting period of several minutes, in which the sample is heated to a temperature at least 20°C above the highest melting or glass transition temperature, generally at a heating rate not exceeding 10°C / min.

[0014] Further work on the DSC, for example with respect to sample preparation, can be carried out by a person skilled in the art in accordance with DIN EN ISO 11357-1 and ISO 11357-2. DSC itself is a very stable method, and only deviations from the temperature program can lead to significant variations in the measurement results.

[0015] According to the present invention, the term (rigid) foam should be understood to mean a low-density, artificially produced substance with a cellular structure. This includes thermoplastic foams (e.g., polystyrene, PP, PVC), elastic foams (e.g., flexible PUR foams), as well as thermosetting foams (e.g., rigid PUR foams). According to the present invention, this term should preferably be understood to mean PMI (polymethacrylimide) and PMMA (polymethyl methacrylate) foams. Polysulfone and poly(ether)imide foams are also suitable.

[0016] The preferred rigid foams according to the present invention have a viscosity of 800 kg / m 3 Less than 25-500 kg / m 3 , particularly preferably 50 to 300 kg / m 3 It has a density of

[0017] Glass transition temperature T g depends on many factors. Every polymer without additives, especially without blowing agents, has a specific glass transition temperature, T gP The addition of additives increases the glass transition temperature T gC changes.

[0018] The blowing agent-containing polymer composition was heated to the glass transition temperature T of the pure polymer by microwave irradiation and thermal energy before foaming. gP It has been found that heating above 0.15 gives particularly good (rigid) foams.

[0019] The temperature of the heating means is set to the glass transition temperature T gP It has also been found that good results are achieved when the temperature is 30°C or less below the glass transition temperature. The energy input required to reach the glass transition temperature can be supplemented by microwave radiation, which significantly improves the heat distribution in the polymer composition and results in a surprisingly homogeneous foam.

[0020] The temperature of the heating means is the glass transition temperature T gP At the same time, the temperature of the polymer composition during the foaming operation is at least the glass transition temperature T gC It is particularly preferable that the

[0021] Blowing agent-containing polymer compositions are usually obtained from monomer mixtures by sheet polymerization. For example, PMMA sheet polymers are produced from monomer mixtures containing mainly MMA (methyl methacrylate), or from polymer syrups consisting mainly or entirely of MMA, and monomer mixtures consisting mainly or entirely of MMA, in the presence of non-gaseous blowing and nucleating agents under polymerization conditions.

[0022] The nucleating agent is silicon oxide particles having a diameter of 4 to 1000 nm. This composition contains 0.01 to 2.0 wt%, preferably 0.2 to 1.5 wt%, of one or more initiators; 2 to 20 wt%, preferably 3 to 15 wt%, of one or more blowing agents; 0.2 to 10 wt%, preferably 0.5 to 8 wt%, of silicon oxide particles having a diameter of 4 to 1000 nm, preferably 5 to 500 nm; and 70 to 97.79 wt%, preferably 75 to 97.8 wt%, of a polymer-forming mixture, wherein at least 75 mol% of the polymer-forming mixture is composed of MMA or MMA repeat units, and 0 to 80 wt%, preferably 0 to 50 wt%, may be present in the form of polymers and / or oligomers. This composition is first polymerized at a temperature of 20 to 100°C, preferably 30 to 70°C.

[0023] In addition to the listed components, the composition may contain up to 27.79% by weight of additional components. Examples of these additional components are, in particular, additional polymer components other than MMA-containing polymers, UV stabilizers, fillers and pigments. For example, additional conventional additives such as crosslinkers, chain transfer agents, release agents, etc. may also be present.

[0024] In addition to MMA, the polymer-forming mixture may contain up to 25% by weight of additional components. These additional components may include monomers copolymerizable with MMA, chain transfer agents, and / or crosslinking agents. MMA and copolymerizable monomers may be used entirely in the form of monomers. The polymer-forming composition may, in particular, contain up to 0.5% by weight of crosslinking agent and / or up to 1.5% by weight of chain transfer agent.

[0025] However, in a more conveniently manageable embodiment of the invention, up to 80% by weight, preferably not more than 50% by weight, of MMA and copolymerizable monomers can be present in polymeric and / or oligomeric form. The advantage of such a syrup consisting of monomers and polymers / oligomers is that it has a higher viscosity than the pure monomer mixture and generates a lower vapor pressure during polymerization.

[0026] Monomers copolymerizable with MMA can be, in particular, acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, or n-butyl acrylate. The copolymerization of acrylates helps to further stabilize the foam, especially at high foaming temperatures, because these foaming temperatures can exceed the ceiling temperature of pure MMA. If no stabilizing comonomer is incorporated, shorter foaming times are preferred.

[0027] Further examples of suitable comonomers include (meth)acrylic acid, methacrylates such as ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, tert-butyl (meth)acrylate, isopropyl (meth)acrylate, isobutyl (meth)acrylate, styrene, (meth)acrylamide, N-alkyl (meth)acrylamides having 1 to 12 carbon atoms in the alkyl group, hydroxyalkyl (meth)acrylates having 1 to 4 carbon atoms in the alkyl group, and polyether (meth)acrylates, where the polyethers may have a molecular weight of 200 to 5,000. These comonomers can also be in the form of a mixture of two or more thereof. When these comonomers are n-butyl (meth)acrylate and / or n-propyl (meth)acrylate, their total proportion in the overall composition should not exceed 3% by weight.

[0028] The term "(meth)acrylate" as used herein refers to both methacrylates, such as methyl methacrylate and ethyl methacrylate, and acrylates, such as methyl acrylate and ethyl acrylate, as well as mixtures of the two.

[0029] If crosslinkers are used, they are preferably di-, tri- or tetra(meth)acrylates, allyl(meth)acrylates, triallyl cyanurate, triallyl isocyanurate, or mixtures comprising at least two of these crosslinkers.

[0030] When chain transfer agents are used, they are preferably compounds having 1 to 5 mercaptan groups, γ-terpinene, or a mixture of at least two of these chain transfer agents. The chain transfer agents are particularly preferably pentaerythritol tetrathioglycolate, 2-mercaptoethanol, alkyl mercaptans having 2 to 12 carbon atoms, thioglycolic acid, thioglycolate, γ-terpinene, or a mixture of at least two of these chain transfer agents.

[0031] Particularly suitable blowing agents are tert-butanol, n-heptane, MTBE, methyl ethyl ketone, alcohols having 1 to 4 carbon atoms, water, methylal, urea, isopropyl (meth)acrylate, and / or tert-butyl (meth)acrylate. When isopropyl (meth)acrylate and / or tert-butyl (meth)acrylate are used, they are also components of the monomer composition described and are first fully or partially polymerized into the polymer formed during polymerization. Subsequently, (meth)acrylic acid repeat units are formed in the polymer during foaming by elimination of propene and isobutene. In certain embodiments, polymers made from a large proportion of these monomers or made entirely from these monomers may be used. The use of polymerizable and / or polymerizable comonomers that release such blowing agents can, for example, result in particularly small and regular cells.

[0032] Particularly suitable blowing agents are tert-butyl (meth)acrylate, isopropyl (meth)acrylate, tert-butanol, isopropanol, tert-butyl methyl ether, and poly(tert-butyl (meth)acrylate).

[0033] A further preferred group of rigid foams is derived from PMI (polymethacrylimide) polymers.

[0034] The preparation of the polymer, for example in the form of a cast polymer, involves first preparing a monomer mixture containing (meth)acrylic acid and (meth)acrylonitrile as the main components, preferably in a molar ratio of 2:3 to 3:2. It is also possible to use additional comonomers, such as esters of acrylic or methacrylic acid, styrene, maleic or itaconic acid or their anhydrides, or vinylpyrrolidone. The proportion of the comonomer should be no more than 30% by weight, preferably no more than 10% by weight, of the two main components. Small amounts of crosslinking monomers, such as allyl acrylate, can also be used. However, the amount should preferably be no more than 0.05 to 2.0% by weight.

[0035] The copolymerization mixture further comprises a blowing agent that decomposes or vaporizes to form a gas phase at a temperature of about 150°C to 300°C.

[0036] The mixture can optionally be mixed with conventional additives. Suitable total amounts of additives are, for example, 0 to 20 wt. %, 0 to 10 wt. %, or 0 to 5 wt. % of the monomer mixture. The conventional additives are different from the monomers, crosslinkers, blowing agents, or initiators described above.

[0037] Common additives include, inter alia, antistatic agents, antioxidants, mold release agents, lubricants, dyes, flow improvers, fillers, light stabilizers, and organic phosphorus compounds, such as phosphites or phosphonates, pigments, release agents, weathering protectants, and plasticizers. A further possible additive is a flame retardant. In addition to halogen-containing flame retardants (some of which contain antimony oxide), phosphorus-containing compounds can also be used. Phosphorus-containing compounds are preferred because they produce less toxic smoke gases upon combustion. Phosphorus compounds include, inter alia, phosphanes, phosphane oxides, phosphonium compounds, phosphonates, phosphites, and / or phosphates. These compounds may be organic and / or inorganic in nature, such as phosphoric acid monoesters, phosphonic acid monoesters, phosphoric acid diesters, phosphonic acid diesters, and phosphoric acid triesters, or polyphosphates.

[0038] Conductive particles that prevent the foam from becoming statically charged are another class of preferred additives. These include, in particular, metal particles and carbon black particles, which may also be in the form of fibers, having a size in the range of 10 nm to 10 mm, as described in EP 0 356 714 A1.

[0039] The polymerization is advantageously carried out by a variant of bulk polymerization, for example but not exclusively by the so-called chamber process described in the prior art.

[0040] Polymerization is advantageously carried out in the form of blocks in the presence of a radical polymerization initiator. For example, to produce flat blocks of layers up to 80 mm thick, the monomer mixture is placed between two glass plates, each sealed at its edge to form a flat chamber. This chamber is surrounded by a water bath set at the desired polymerization temperature.

[0041] The polymerization is preferably carried out in a forming vessel, in particular in the form of a chamber polymerization between two sheets, e.g., glass plates. In the simplest case, a rectangular vessel can be used. Polymerization in such a vessel subsequently produces a sheet whose thickness is determined by the fill level / sheet spacing in the vessel. However, more complex vessel shapes are also possible. The polymerization is preferably carried out at temperatures between 30°C and 70°C. Usable initiators include well-known free-radical initiators, such as peroxides and azo initiators, as well as redox and UV initiators. These redox and UV initiators, in particular, are suitable for polymerization temperatures below 40°C. UV initiators are initiated by irradiation with suitable UV light, while redox initiators include two-component systems initiated by mixing the two components with the monomer.

[0042] The foaming process according to the present invention is characterized in that a polymer composition containing a foaming agent is foamed by a combination of microwave irradiation and thermal energy.

[0043] The polymer composition was measured at a glass transition temperature T gC It has been found that the temperature must be at least 5°C, preferably at least 10°C, above the normal temperature.

[0044] Surprisingly, to obtain fine-celled rigid foams, the blowing agent-containing polymer composition is heated to the glass transition temperature T of the pure polymer by microwave irradiation and thermal energy. gP It has been found that the temperature should be above 100°C.

[0045] The thermal energy can be supplied by various heating devices. It is preferable to use an oven or a heatable microwave device. Depending on the energy source, suitable devices include, for example, industrial ovens, hot air ovens, radiant ovens, and heat treatment ovens.

[0046] Depending on the polymer composition, the temperature during the thermal energy supply should be between 100°C and 250°C, preferably between 160°C and 200°C. In particular, since additional heating is performed in combination with microwave radiation, the ambient temperature in the space of the thermal heating means should be between 100°C and 250°C.

[0047] Both the foaming and the preceding polymerization can be carried out in multiple temperature steps. Later temperature increases in the polymerization can further improve conversion and therefore reduce residual monomer content. Stepwise increases in foaming temperature can be used during foaming to influence cell distribution, cell size, and cell count.

[0048] This process can optionally be such that polymerization proceeds only incompletely, preferably to at least 80% conversion, with final polymerization occurring during foaming. Such a process has the advantage that no plasticizing compounds remain in the final foam, and the relatively short polymer chains and residual monomers have a plasticizing effect at the start of the foaming operation. Thus, in such an embodiment, polymerization and foaming are carried out simultaneously at a single foaming temperature.

[0049] Furthermore, it has been found that the temperature must be at most 30° C., preferably at most 25° C., and particularly preferably at most 20° C., below the temperature required for purely thermal foaming of the same material. If the temperature is below this limit, uniform foaming will not occur.

[0050] Furthermore, it has surprisingly been found that to avoid field non-uniformities, the polymer to be foamed must be moved relative to the microwave field during foaming. In the case of a static microwave field, the polymer to be foamed can be moved, or in the case of a spatially static polymer, the microwave field can be varied. If this is not done, the resulting foam will exhibit non-uniform foaming.

[0051] Relative movement between the polymer sheet and the microwave field can be achieved by exposing a fixed polymer sheet to microwave radiation using a movable hollow conductor. The microwave power, distance, and path length can be individually adapted to the geometry of the polymer sheet, thereby producing a homogeneous foam of the desired density. Movement can be achieved using a linear guideway or a freely programmable handling system.

[0052] A further preferred variant for improving uniformity in terms of density distribution and cell morphology in microwave-based foaming of polymer sheets is to use a rotating mirror to continuously change the vibration mode of the microwave radiation, thereby reducing the formation of so-called hot spots, which can cause local overheating and therefore excessive energy input, which can lead to undesirable high foaming with locally insufficient density or collapse of the foam cells.

[0053] Further optimization is achieved by moving the polymer composition to be foamed relative to the microwave field. Here, the polymer sheet (polymer composition to be foamed) is positioned at the center of the microwave turntable. The rotation of the turntable creates a varying radiation field on the polymer sheet, reducing spot overheating. As the turntable heats up over time, the bottom of the polymer sheet heats by conduction, causing the side edges to expand upward. The high oven temperature and turbulent hot air flow in the microwave oven ensure that all areas of the polymer sheet are foamed. The polymer sheet has a uniform level. Non-uniform foaming in parts of the polymer sheet causes slight differences in the length of the edges of the foam sheet, resulting in warping. Nevertheless, this positioning of the polymer sheet, combined with the high oven temperature and long foaming time, results in a uniformly foamed sheet.

[0054] An additional option involves placing the polymer sheet vertically on the microwave turntable and disabling the rotation function of the turntable. This alternative arrangement results in a horizontal foamed sheet. Slight deformation at the edges and small irregularities at the sides may be observed. The foamed sheet does not exhibit cracks on the lateral edges or surface. The sheet will foam further despite the change in position within the oven and therefore a different radiation field than the microwave.

[0055] A suitable microwave device should provide a power output of 0.1 kW / kg to 10 kW / kg (input power / kg of polymer to be foamed), preferably 0.8 kW / kg to 1.2 kW / kg, and very particularly preferably 1 kW / kg, and should provide the option of moving the polymer to be foamed relative to the microwave field or moving the microwave field relative to the polymer, depending on the application. According to the invention, the microwave device must also be capable of thermal heating.

[0056] By applying the above method, it was possible to reduce the foaming time of PMMA and PMI-based foams from several hours to about 15 minutes.

[0057] The process according to the invention allows for the foaming of polymer sheets of various thicknesses.

[0058] Furthermore, it has been surprisingly found that for the same formulation of blowing agent-containing polymer, the cell size of the resulting foam is finer than in the case of pure hot foaming of the same starting material. The finer cell structure leads to lower resin absorption in the production of sandwich composites, which is advantageous for the weight of the component.

[0059] According to the invention, a fine cellular structure is to be understood as meaning an average pore size of 20 to 300 μm, preferably 50 to 280 μm, very particularly preferably 60 to 250 μm.

[0060] The resulting fine cell structure is very important for various applications. In thermal insulation applications, the insulating effectiveness of a foam generally increases as the cell size decreases, assuming the same density and cell gas. In lightweight construction applications where the foam core faces a resin-impregnated outer layer, resin absorption by the foam core should be minimized to keep weight down. The finer the cells of the closed-cell foam used for this purpose, the less resin is absorbed.

[0061] The PMMA foams produced according to the invention further have a surprisingly high strength and, therefore, a surprisingly low brittleness, and can therefore be used, for example, in lightweight construction. The good material properties also make it possible to avoid the use of plasticizers, such as long-chain alkyl (meth)acrylates or phthalates, which, according to current knowledge, have a positive effect on flow / foamability but at the same time a negative effect on the mechanical properties of PMMA foams, in particular their strength.

[0062] Example Example 1 Moving the polymer composition to be foamed relative to the microwave field ROHACELL® is based on methacrylonitrile (MAN) and methacrylic acid (MAA) monomers, which react to produce the desired product in a multi-stage process with the addition of additives. These monomers are then mixed with a specific amount of blowing agent, crosslinker, and stabilizer, depending on the formulation. Examples of blowing agents used include formamide and various alcohols. In the next step, these two monomers and additives are polymerized to produce a copolymer in a chamber process. The liquid monomer mixture is introduced and fixed between two glass plates sealed with a rubber ring. This is then placed in a water bath at approximately 50°C for a predetermined period of time. The residence time depends on the thickness of the sheet and varies from 3 to 10 days. During this process, an exothermic free-radical reaction begins, where the heat decomposes the initiator. A methacrylonitrile-methacrylic acid copolymer is formed from these two monomers. The copolymer sheet is then heat-treated to allow any residual monomers to react. After this step, large sheets are cut to size. The polymer sheet is cut to dimensions of 70 x 70 x 30 mm.

[0063] During the subsequent preheating, the sheet becomes yellow-orange to transparent. The preheating temperature is approximately 160-180°C, and the preheating time is approximately 120 minutes. The sheet is first preheated above its glass transition temperature, which makes it elastic. Actual foaming follows after the sheet reaches the desired preheat temperature.

[0064] For this purpose, the sheet is introduced into a device consisting of a thermal heating means and a microwave, the temperature of which is equal to or higher than the glass transition temperature T of the polymer before foaming. gP The microwave radiation and thermal energy are used to heat the blowing agent-containing polymer composition to at least the glass transition temperature T gCThe sheet is heated to 1000 K. To measure the temperature profile of the sheet, four holes are drilled in the sheet and temperature sensors are then placed in these holes. One temperature sensor measures the temperature on the surface, one temperature sensor measures the temperature inside the sheet, and two temperature sensors measure the temperature at the edges of the sheet. The sheet is placed on a turntable. Rotating the sheet helps to reduce spot overheating that may occur due to microwave radiation incident on the material. Infrared analysis can be used to demonstrate that rotation results in uniform heating of the sheet.

[0065] The foaming cycle time is significantly reduced and the electromagnetic radiation can heat the polymer sheet deep to its core.

[0066] Comparative example 1C The polymer composition to be foamed does not move relative to the microwave field A polymer sheet was prepared according to Example 1. It was placed in the center of the microwave plate. The rotation function was left disabled. IR images reveal multiple hot spots and uneven temperature distribution. Spot overheating caused premature foaming.

[0067] Comparative Example 2 ROHACELL® TT g Foaming in less than ROHACELL® g It was investigated whether it was possible to foam at temperatures below 205°C.

[0068] Experimental parameters Sample dimensions: 50 x 50 x 23 mm Oven temperature: 160℃ Cooking time: 180 minutes

[0069] Experimental Procedures / Process Steps Precondition the samples for 48 hours at 23°C and 50% atmospheric humidity. · Before starting, preheat the oven to 160°C for at least 60 minutes. The sample is placed in the oven. Record the time when foaming begins.

[0070] Because the chemical reaction is exothermic, the temperature within the core was able to increase faster than the heat could dissipate to the outside. Foaming only began after 85 minutes. The increase in core temperature allowed further foaming in the interior regions. Thermal conductivity, and therefore heat transfer, changed during foaming. After approximately 180 minutes, the entire sample was foamed. However, due to the relatively low temperature, the expansion / expansion of the foam block was relatively limited, thus resulting in a very high final density for the foam block.

[0071] Although the foaming was limited, this experiment revealed that the glass transition temperature (T g It has been found that ROHACELL® can be foamed at temperatures below the glass transition temperature, but the time required to initiate the process is significantly longer than for normal foam temperatures above the glass transition temperature.

[0072] Example 3 Analysis of ROHACELL® cell size (cell morphology) To analyze the cell morphology of ROHACELL®, approximately 71 kg / m 3 Three cube specimens were analyzed, cut from a sheet with a density of 1000 MPa. Each cube had dimensions of 80 x 80 x 45 mm. 5 mm thick strips were then cut from each cube and broken in three places. The broken pieces were observed from above in the direction of the arrows to determine the cell size.

[0073] The sheet has the largest volume in the center, and therefore the lowest density there. Cell counts and cell sizes were determined from individual sections of the strip. The average cell sizes for the individual cube specimens in the x-, y-, and z-axes are shown in the table below.

[0074] [Table 1]

[0075] It is clear that the samples have roughly equal cell sizes in the x and y directions, but larger cell sizes in the z direction. Cube 8 has the lowest density and the largest cell size in the z direction. The cells are long, round, rod-like, and elongated in the z direction. This cell shape is called prolate and cannot be detected by conventional foaming.

[0076] Example 4 Analysis of ROHACRYL® Cell Size (Cell Morphology) To analyze the cell morphology of ROHACRYL®, the analysis was performed as in Example 3, which gave the following cell sizes: [Table 2]

[0077] The cell size is nearly equal in all three directions, thus demonstrating a special fine cellularity.

[0078] Comparative Example 4C Analysis of ROHACRYL® Cell Size (Cell Morphology) Conventional foam To analyze the cell morphology of conventionally foamed ROHACRYL®, an analysis was performed similar to that in Example 3, which resulted in the following cell sizes: [Table 3]

[0079] Conventionally foamed Rohacryl® has uniform cell size in all three directions, but is significantly less finely cellular, in contrast to the cell size obtained in Example 4 of the present invention.

Claims

1. A method for producing a rigid foam, comprising: heating a blowing agent-containing polymer composition by a thermal heating means having a temperature 30°C or less lower than the glass transition temperature T gP of the polymer before foaming; and then foaming the polymer in an apparatus comprising the thermal heating means and microwaves; and ｇＣ heating the polymer composition to above and The polymer composition to be foamed is moved relative to the microwave field, or the microwave field is moved relative to the polymer composition to be foamed. A method characterized by:

2. The temperature of the thermal heating means during the foaming operation is greater than the glass transition temperature T ｇＰ and the temperature of the polymer composition during the foaming operation is not lower than the glass transition temperature T ｇＣ The method for producing the rigid foam of claim 1, wherein the temperature is greater than 100°C.

3. The blowing agent-containing polymer composition is heated to the glass transition temperature T ｇＣ 2. The method for producing a rigid foam according to claim 1, wherein the temperature is at least 5°C above the

4. The blowing agent-containing polymer composition is heated to the glass transition temperature T ｇＣ 2. The method for producing a rigid foam according to claim 1, wherein the temperature is at least 10°C above the

5. 2. The method for producing a rigid foam according to claim 1, wherein the ambient temperature of the space around the heating means is 100°C to 250°C.

Images