FOAMING OF BLOWING AGENT CONTAINING POLYMERS THROUGH THE USE OF MICROWAVES

Abstract

(Rigid) foams can be produced by heating a blowing agent containing polymers through the combination of thermal energy with irradiation by microwaves.

Claims

1: A process for producing rigid foams and foams, the process comprising: foaming a polymer composition comprising a blowing agent in an apparatus consisting of a thermal heating means and a microwave, which in combination heat the polymer composition above a glass transition temperature of the polymer composition T.sub.gC.

2: The process for producing rigid foams and foams according to claim 1, wherein the temperature of the thermal heating means is not more than 30° C. below a glass transition temperature of the polymer T.sub.gP before foaming.

3: The process for producing rigid foams and foams according to claim 1, wherein the temperature of the thermal heating means is not less than 30° C. below a glass transition temperature of the polymer T.sub.gP while simultaneously the temperature of the polymer composition during the foaming corresponds at least to the glass transition temperature of the polymer composition T.sub.gC.

4: The process for producing rigid foams and foams according to claim 1, wherein the polymer composition is heated at least 5° C. above the glass transition temperature of the polymer composition T.sub.gC with microwave irradiation and thermal energy.

5: The process for producing rigid foams and foams according to claim 1, wherein the polymer composition is heated at least 10° C. above the glass transition temperature of the polymer composition T.sub.gC with microwave irradiation and thermal energy.

6: The process for producing rigid foams and foams according to claim 1, wherein the polymer composition is moved relative to a microwave field.

7: The process for producing rigid foams and foams according to claim 1, wherein a microwave field is altered relative to the polymer composition.

8: The process for producing rigid foams and foams according to claim 1, wherein an ambient temperature in a thermal heating means space is between 100° C. and 250° C.

Description

EXAMPLES

Example 1

Polymer Composition to Be Foamed Moved Relative to Microwave Field

[0062] ROHACELL® is based on the monomers methacrylonitrile (MAN) and methacrylic acid (MAA) which are reacted to afford the desired product in a multistage process by addition of added substances. Depending on the formulation, a defined amount of blowing agent, crosslinker and stabilizers is added to the monomers. Blowing agents employed are for example formamide and various alcohols. In the next step the two monomers and the added substances are polymerized to afford a copolymer in a chamber process. Liquid monomer mixture is introduced between two glass sheets sealed with a rubber ring and secured. This resides in a water bath at about 50° C. for a certain time. The residence time depends on the sheet thickness and varies between 3-10 days. The exothermic free-radical reaction in which the initiators decompose due to the action of heat commences during this process. A methacrylonitrile-methacrylic acid copolymer is formed from the two monomers. The copolymer sheet is then heat-treated so that the residual monomers can react. After this step the large sheets are cut to size. The polymer sheets are cut to dimensions of 70×70×30 mm.

[0063] During the subsequent preheating the sheets are yellowish-orange to transparent. The preheating temperatures are about 160-180° C. and the preheating time is about 120 minutes. They are initially preheated to above the glass transition temperature in order that they become elastic. Once the sheets have achieved the desired preheating temperature the actual foaming follows.

[0064] To this end a sheet is introduced into an apparatus consisting of a thermal heating means and a microwave. The temperature of the thermal heating means is set to about 20° C. below the glass transition temperature of the polymer before foaming T.sub.gP. By means of microwave radiation and thermal energy the blowing agent containing polymer composition is heated to at least the glass transition temperature of the polymer composition T.sub.gC. To measure the temperature profile in the sheet this is provided with four bores before the temperature sensors are placed in the bores. One temperature sensor measures the temperature at the surface, one temperature sensor measures the temperature in the sheet interior and two further temperature sensors measure the temperature at the edge of the sheet. The sheet is placed on a turntable. The rotation of the sheet serves to reduce spot overheating which can occur due to the microwave radiation incident on the material. Infrared analyses may be used to demonstrate that the sheet is heated uniformly as a result of the rotation.

[0065] The cycle time for foaming was significantly shortened and the electromagnetic radiation allows the polymer sheet to be heated deep in its core.

Comparative Example 1C

Polymer Composition to Be Foamed Without Movement Relative to Microwave Field

[0066] A polymer sheet was produced according to example 1. This was placed in the centre of the microwave plate. The rotation function remained disabled. IR images reveal a plurality of hotspots and a nonuniform temperature distribution. Spot overheating has resulted in premature foaming.

Comparative Example 2

ROHACELL® Foaming Below T.SUB.g

[0067] It was investigated whether it is possible to foam ROHACELL® at a temperature below T.sub.g (205° C.).

[0068] Experimental Parameters [0069] Sample dimensions; 50×50×23 mm [0070] Oven temperature: 160° C. [0071] Duration of heating: 180 min

[0072] Experimental Procedure/Process Steps [0073] The sample is preconditioned at 23° C. and 50% atmospheric humidity for 48 hours. [0074] Before commencement the oven is preheated to 160° C. for at least 60 min. [0075] The sample is placed in the oven. [0076] The time at which foaming commences is noted.

[0077] Since the chemical reaction is exothermic the temperature in the core was able to increase faster than the heat can be dissipated outward. Foaming commences only after 85 min. The increase in the core temperature allows further foaming of the inner regions. Thermal conductivity, and thus heat transfer, changes during the foaming. After about 180 min the entire sample was foamed. However, the expansion/enlargement of the foam block is relatively limited on account of the relatively low temperature, thus giving the foam block a very high end density.

[0078] Despite the limited foaming the experiment showed that the ROHACELL® was also foamable at temperatures below the glass transition temperature (T.sub.g). However, the time required to commence the process is markedly longer than in the case of the customary foam temperatures above the glass transition temperature.

Example 3

Analysis of Cell Size (Cell Morphology) of ROHACELL®

[0079] To analyse the cell morphology of ROHACELL® three cube specimens cut out of a sheet having a density of about 71 kg/m.sup.3 were analysed. The individual cubes have dimensions of 80×80×45 mm. Subsequently a strip of 5 mm in thickness was cut out of the individual cubes and broken in three places. The broken pieces are observed from above in the arrow direction and the cell size determined.

[0080] The sheet has the greatest volume in the middle and thus the density there is lowest. The number of cells and the cell size was determined from the individual parts of the strip. The average values of the cell size for the individual cube specimens in the x-, y- and z-axes are shown in the table below.

TABLE-US-00001 TAB 1 Average cell sizes in three-dimensional coordinate system Cube Cube Cube specimen specimen specimen Cube specimen number 1 8 15 Average cell size in x-direction 116 μm 135 μm 117 μm Average cell size in y-direction 110 μm 127 μm 115 μm Average cell size in z-direction 158 μm 202 μm 168 μm

[0081] It is apparent that the sample has an approximately equal cell size in the x- and y-directions while the cell size in the z-direction is greater. Cube eight has the lowest density and has the highest cell size in the z-direction. The cells are long, round and rod-shaped and are elongated in the z-direction. This cell shape is referred to as prolate and is not detectable in conventional foaming.

Example 4

Analysis of Cell Size (Cell Morphology) of ROHACRYL®

[0082] To analyse the cell morphology of ROHACRYL® analyses were performed analogously to example 3. This resulted in the following cell sizes:

TABLE-US-00002 TAB 2 Average cell sizes in three-dimensional coordinate system Average cell size in x-direction 166 μm Average cell size in y-direction 158 μm Average cell size in z-direction 158 μm

[0083] The cell size is approximately equal in all 3 directions. An exceptional fine cellularity was thus demonstrated.

Comparative Example 4C

Analysis of Cell Size (Cell Morphology) of ROHACRYL® Conventional Foaming

[0084] To analyse the cell morphology of conventionally foamed ROHACRYL® analyses were performed analogously to example 3. This resulted in the following cell sizes:

TABLE-US-00003 TAB 3 Average cell sizes in three-dimensional coordinate system Average cell size in x-direction 451 μm Average cell size in y-direction 450 μm Average cell size in z-direction 377 μm

[0085] The conventionally foamed Rohacryl® has a uniform cell size in all 3 directions but, in contrast to the cell sizes achieved in inventive example 4, has a substantially lower fine cellularity.