PRODUCTION OF FINE-PORED PMMA FOAMS USING NUCLEATING AGENTS

Abstract

Fine-cell PMMA foams are produced using a production process including nucleators in addition to suitable blowing agents. It was found that, surprisingly, a simple-to-produce stable PMMA foam having very fine cells and very good properties can be produced.

Claims

1. A process for producing a polymethacrylate foam, polymerizing a composition containing from 0.01 to 2.0 wt % of an initiator, from 2 to 20 wt % of a blowing agent, from 0.2 to 10 wt % of silicon oxide particles having a diameter between 4 and 1000 nm and from 70 to 97.79 wt % of a polymer-forming mixture, wherein said polymer-forming mixture consists of MMA and/or MMA repeat units to an extent of not less than 75 mol % and may be present as polymer and/or oligomer to an extent of from 0 to 80 wt %, wherein said polymerizing proceeds at a temperature between 20 C. and 100 C., to obtain a polymerized composition, and subsequently said polymerized composition at between 130 C. and 250 C.

2. The process according to claim 1, wherein the composition contains from 0.5 to 8 wt % of SiO.sub.2 particles having a diameter between 5 and 500 nm.

3. The process according to claim 1, wherein the composition contains from 0.2 to 1.5 wt % of the initiator, from 3 to 15 wt % of the blowing agent, from 0.5 to 8 wt % of SiO.sub.2 particles and from 75 to 97.8 wt % of the polymer-forming mixture, wherein said polymer-forming mixture consists of MMA to an extent of not less than 75 mol % and is present as polymer and/or oligomer to an extent of from 0 to 50 wt %.

4. The process according to claim 1, wherein the temperature is between 30 C. and 70 C. for the polymerization and between 150 C. and 230 C. for the foaming.

5. The process according to claim 1, wherein the polymer-forming composition contains up to 0.5 wt % of crosslinker and/or up to 1.5 wt % of chain transfer agent.

6. The process according to claim 1, wherein the polymer-forming mixture contains MMA-copolymerizable comonomers and/or -copolymerized comonomers in the polymers and/or oligomers, and wherein the comonomers comprise (meth)acrylic acid, methyl acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, isopropyl (meth)acrylate, isobutyl (meth)acrylate, styrene, (meth)acrylamide, an N-alkyl(meth)acrylamide having one to 12 carbon atoms in the alkyl group, a hydroxyalkyl (meth)acrylate having one to 4 carbon atoms in the alkyl group or mixtures of two or more thereof.

7. The process according to claim 1, wherein the polymerization and/or the foaming are/is effected stagewise at different temperatures.

8. The process according to claim 1, wherein the polymerization and the foaming are effected simultaneously to some extent at least.

9. The process according to claim 1, characterized in that wherein the blowing agent comprises tert-butyl (meth)acrylate, isopropyl (meth)acrylate, tert-butanol, tert-butyl methyl ether, isopropanol and poly(tert-butyl (meth)acrylate), wherein isopropyl (meth)acrylate and/or tert-butyl (meth)acrylate are/is at the same time part of the recited monomer composition and are wholly or partly copolymerized into the polymers formed therefrom.

10. A PMMA foam, comprising: a solid fraction which contains from 0.2 to 12.5 wt % of silicon oxide particles and from 72 to 98 wt % of a polymer incorporating not less than 75 mol % of MMA repeat units, and wherein the foam has a density between 25 and 250 kg/m.sup.3 and an average cell size below 500 m.

11. The PMMA foam according to claim 10, which has an overall cell size below 500 m.

Description

EXAMPLES

Example 1

[0047] In this example, foaming was done with a blowing agent in the form of a constituent part of a polymer.

[0048] A mixture of 266.25 g of a polymethacrylate prepared exclusively from MMA, 1065.00 g of MMA, 0.75 g of 2,2-azobis(2,4-dimethylvaleronitrile), 0.60 g of pentaerythritol tetrathioglycolate as chain transfer agent, 15.00 g of n-butyl acrylate, 105.00 g of tert-butyl methacrylate, 1.50 g of Rewopol SB-DO 75 release agent and 0.90 g of ethylene glycol dimethacrylate as crosslinker was prepared. Into this mixture were then stirred 45.00 g of AEROSIL OX50 nucleating agent, by the complete addition being followed by a further 20 min of stirring and subsequent dispersing using an UltraTurrax (2 min at 3000 rpm, 1 min at 5000 rpm). This mixture was subsequently polymerized at 42 C. for 24 h between two 400 mm*300 mm glass plates separated from each other by 10 mm and laterally sealed off by a rubber strip. This was followed by 4 h conditioning at 115 C. After removal of the glass plates, the PMMA slab obtained was finally foamed in an oven at 215 C. for 1.5 h. The mixture foamed very well and the distribution of the cells was uniform. The foam obtained had a density of about 100 kg/m.sup.3 and very fine cells having an average diameter of 250 m.

[0049] The release agent is known from experience not to affect the polymerization or the foaming operation, and solely serves to facilitate removal of the glass plates from the polymer slab.

Example 2

[0050] In this example, foaming was done with a blowing agent not a constituent part of a polymer and the amount of nucleating agent was doubled.

[0051] A mixture of 257.25 g of a polymethacrylate prepared exclusively from MMA, 1029.00 g of MMA, 0.75 g of 2,2-azobis(2,4-dimethylvaleronitrile), 0.60 g of pentaerythritol tetrathioglycolate as chain transfer agent, 15.00 g of n-butyl acrylate, 105.00 g of tert-butyl methyl ether (MTBE), 1.50 g of Rewopol SB-DO 75 release agent and 0.90 g of ethylene glycol dimethacrylate as crosslinker was prepared. Into this mixture were then stirred 90.00 g of AEROSIL OX50 nucleating agent, by the complete addition being followed by a further 20 min of stirring and subsequent dispersing using an UltraTurrax (2 min at 3000 rpm, 1 min at 5000 rpm). This mixture was subsequently polymerized at 42 C. for 24 h between two 400 mm*300 mm glass plates separated from each other by 10 mm and laterally sealed off by a rubber strip. This was followed by 4 h conditioning at 115 C. After removal of the glass plates, the PMMA slab obtained was finally foamed in an oven at 200 C. for 20 min. The mixture foamed very well and the distribution of the cells was uniform. The foam obtained had a density of about 100 kg/m.sup.3 and very fine cells having an average diameter of 100 m.

Comparative Example 1

[0052] In this comparative example, a nucleating agent was omitted for a direct comparison versus Example 1.

[0053] A mixture of 281.25 g of a polymethacrylate prepared exclusively from MMA, 1125.00 g of MMA, 0.75 g of 2,2-azobis(2,4-dimethylvaleronitrile), 0.60 g of pentaerythritol tetrathioglycolate as chain transfer agent, 15.00 g of n-butyl acrylate, 75.00 g of tert-butyl methacrylate, 1.50 g of Rewopol SB-DO 75 release agent and 0.90 g of ethylene glycol dimethacrylate as crosslinker was prepared. This mixture was subsequently stirred for 20 min. This mixture was subsequently polymerized at 42 C. for 24 h between two 400 mm*300 mm glass plates separated from each other by 10 mm and laterally sealed off by a rubber strip. This was followed by 4 h conditioning at 115 C. After removal of the glass plates, the PMMA slab obtained was finally foamed in an oven at 215 C. for 1 hour. The mixture foamed very well. However, the distribution of the cells was nonuniform and the cells were coarse. Average cell diameter in the foam obtained was about 2000 m.

Comparative Example 2

[0054] In this comparative example, a nucleating agent was omitted for a direct comparison versus Example 2.

[0055] A mixture of 281.25 g of a polymethacrylate prepared exclusively from MMA, 1125.00 g of MMA, 0.75 g of 2,2-azobis(2,4-dimethylvaleronitrile), 0.60 g of pentaerythritol tetrathioglycolate as chain transfer agent, 15.00 g of n-butyl acrylate, 75.00 g of tert-butyl methyl ether (MTBE), 1.50 g of Rewopol SB-DO 75 release agent and 0.90 g of ethylene glycol dimethacrylate as crosslinker was prepared. This mixture was subsequently stirred for 20 min. This mixture was subsequently polymerized at 42 C. for 24 h between two 400 mm*300 mm glass plates separated from each other by 10 mm and laterally sealed off by a rubber strip. This was followed by 4 h conditioning at 115 C. After removal of the glass plates, the PMMA slab obtained was finally foamed in an oven at 215 C. for 1 hour. The mixture foamed very well. However, the distribution of the cells was nonuniform, and the foam was extremely inhomogeneous and coarsely cellular. Average cell diameter in the foam obtained was significantly above 2000 m.

Comparative Example 3

[0056] In this comparative example, the nucleating agent used for a direct comparison versus Example 2 was not in accordance with the present invention.

[0057] A mixture of 272.25 g of a polymethacrylate prepared exclusively from MMA, 1089.00 g of MMA, 0.75 g of 2,2-azobis(2,4-dimethylvaleronitrile), 0.60 g of pentaerythritol tetrathioglycolate as chain transfer agent, 15.00 g of n-butyl acrylate, 75.00 g of tert-butyl methyl ether (MTBE), 1.50 g of Rewopol SB-DO 75 release agent and 0.90 g of ethylene glycol dimethacrylate as crosslinker was prepared. Into this mixture were then stirred 45.00 g of Finntalc M03-AW (a talcum powder) nucleating agent, by the complete addition being followed by a further 20 min of stirring and subsequent dispersing using an UltraTurrax (2 min at 3000 rpm, 1 min at 5000 rpm). This mixture was subsequently polymerized at 42 C. for 24 h between two 400 mm*300 mm glass plates separated from each other by 10 mm and laterally sealed off by a rubber strip. This was followed by 4 h conditioning at 115 C. After removal of the glass plates, the PMMA slab obtained was finally foamed in an oven at 215 C. for 1 hour. The mixture foamed extremely slowly and nonuniformly. Furthermore, the distribution of the cells was nonuniform, and the foam was extremely inhomogeneous and coarsely cellular. Average cell diameter in the foam obtained was about 2000 m.