Vacuum-supported method for the production of polyurethane foam

Abstract

The present invention is related to a method for the production of polyurethane foam, comprising the steps of: providing an isocyanate-reactive component A comprising a polyol component A1 which further comprises a physical blowing agent T; combining at least the isocyanate-reactive component A and an isocyanate component B, thereby obtaining a polyurethane reaction mixture; providing the polyurethane reaction mixture in a cavity (11); and reducing the pressure within the cavity (11) to a pressure lower than ambient pressure;
characterized in that the cavity (11) is ventilated to ambient pressure before the gel time of the polyurethane reaction mixture is reached.

Claims

1. A method for the production of a polyurethane foam, comprising the steps of: providing an isocyanate-reactive component A comprising a polyol component A1 which further comprises a physical blowing agent T, wherein the physical blowing agent T is present in the isocyanate-reactive component A in the form of an emulsion with the polyol component A1 constituting the continuous phase and droplets of the physical blowing agent T the dispersed phase of the emulsion, wherein the average size of the droplets of the physical blowing agent T is ?0.1 ?m to ?20 ?m, the droplet size being determined by using an optical microscope operating in bright field transmission mode, wherein the polyol component A1 comprises: A1a: a polyether polyol with a hydroxyl number of ?15 mg KOH/g to ?550 mg KOH/g and a functionality of ?1.5 to ?6.0 obtained by the addition of an epoxide to one or more starter compounds selected from the group of carbohydrates and/or at least difunctional alcohols; and A1b: a polyether polyol with a hydroxyl number of ?100 mg KOH/g to ?550 mg KOH/g and a functionality of ?1.5 to ?5.0 obtained by the addition of an epoxide to an aromatic amine; and A1c: a polyester polyether polyol with a hydroxyl number of ?100 mg KOH/g to ?450 mg KOH/g and a functionality of ?0.1 to ?3.5 obtained by the addition of an epoxide to the esterification product of an aromatic dicarboxylic acid derivative and an at least difunctional alcohol; combining at least the isocyanate-reactive component A and an isocyanate component B, thereby obtaining a polyurethane reaction mixture; providing the polyurethane reaction mixture in a cavity; and reducing the pressure within the cavity to a pressure lower than ambient pressure, wherein the pressure is reduced by ?1 mbar up to ?900 mbar; wherein the cavity is ventilated to ambient pressure before the gel time of the polyurethane reaction mixture is reached.

2. The method according to claim 1, wherein the pressure within the cavity is reduced before the polyurethane reaction mixture is provided in the cavity.

3. The method according to claim 1, wherein the pressure within the cavity is reduced after the polyurethane reaction mixture is provided in the cavity.

4. The method according to claim 1, wherein the pressure is reduced by ?50 mbar to ?300 mbar.

5. The method according to claim 1, wherein the cavity is ventilated to ambient pressure when 60 to 99% of the gel time of the polyurethane reaction mixture is reached.

6. The method according to claim 1, wherein the polyurethane reaction mixture has a gel time of ?50 seconds.

7. The method according to claim 1, wherein before ventilating to ambient pressure, the step of reducing the pressure within the cavity to a pressure lower than ambient pressure is conducted in such a way that after an initial reduction of the pressure, the pressure is allowed to rise as a consequence of an expansion of the polyurethane reaction mixture.

8. The method according to claim 1, wherein before ventilating to ambient pressure, the reduced pressure is kept constant.

9. The method according to claim 1, wherein the pressure within the cavity is adjusted to different levels at different cavity areas by using two individually operatable vacuum systems.

10. The method according to claim 7, wherein the pressure level within different cavity areas is adjusted, wherein the pressure level within a cavity area having a first shape is adjusted to a first pressure level, wherein the pressure level within a cavity area having a second shape is adjusted to a second pressure level, wherein the first shape is different than the second shape, and wherein the first pressure level is different than the second pressure level.

11. The method according to claim 9, wherein the physical blowing agent T is present in the isocyanate-reactive component A in the form of an emulsion with the polyol component A1 constituting the continuous phase and droplets of the physical blowing agent T the dispersed phase of the emulsion, wherein the average size of the droplets of the physical blowing agent T is ?0.1 ?m to ?15 ?m, the droplet size being determined by using an optical microscope operating in bright field transmission mode.

12. The method according to claim 9, wherein the polyol component A1 further comprises: A1c: a polyester polyol with a hydroxyl number of ?100 mg KOH/g to ?450 mg KOH/g and a functionality of ?1.5 to ?3.5 obtained by the esterification of a polycarboxylic acid component and a polyalcohol component, wherein the total content of aromatic dicarboxylic acid derivatives employed in the esterification, based on free aromatic dicarboxylic acids, is ?48.5 mass-%, based on the total mass of polyalcohol component and polycarboxylic acid component, and/or A1d: a polyether polyol with a hydroxyl number of ?500 mg KOH/g to ?1000 mg KOH/g and a functionality of ?1.5 to ?5.0 obtained by the addition of an epoxide to an aliphatic amine and/or a polyfunctional alcohol, and/or A1e: a di-, tri- or tetrafunctional aminic or alcoholic chain extender or cross-linker.

13. The method according to claim 1, wherein the physical blowing agent T is selected from the group consisting of hydrocarbons, halogenated ethers, perfluorinated hydrocarbons with 1 to 6 carbon atoms and mixtures thereof.

14. The method according to claim 9, wherein the mass ratio of A1:T is ?5:1 to ?12:1.

15. The method according to claim 9, wherein the polyol component Al has a viscosity according to EN ISO 3219 at 20? C. of ?1000 mPas to ?18000 mPas.

16. The method according to claim 1, wherein the isocyanate-reactive component A further comprises: A2: water; A3: at least one stabilizer selected from the group of polyether polydimethylsiloxane copolymers; and A4: at least one catalyst selected from the group consisting of triethylenediamine, N,N-dimethylcyclohexylamine, tetramethylenediamine, 1-methyl-4-dimethylaminoethylpiperazine, triethylamine, tributylamine, dimethylbenzylamine, N,NN-tris-(dimethylaminopropyl)hexahydrotriazine, dimethylaminopropylformamide, N,N,N,N-tetramethylethylenediamine, N,N,N, N-tetramethylbutanediamine, tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, dimethylpiperazine, 1,2-dimethylimidazole, 1-azabicyclo[3.3.0]octane, bis(dimethylaminopropyl) urea, N-methylmorpholine, N-ethylmorpholine, N-cyclohexylmorpholine, 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, triethanolamine, diethanolamine, triisopropanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, and dimethylethanolamine.

17. The method according to claim 1, wherein the isocyanate component B comprises: B1: at least one isocyanate selected from the group consisting of toluylene diisocyanate, diphenylmethane diisocyanate, polyphenylpolymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicylclohexylmethane, and isophorone diisocyanate; and/or B2: an isocyanate-terminated prepolymer obtained from at least one polyisocyanate B1 and at least one isocyanate reactive compound selected from the group consisting of: A1c: a polyester polyol with a hydroxyl number of ?100 mg KOH/g to ?450 mg KOH/g and a functionality of ?1.5 to ?3.5 obtained by the esterification of a polycarboxylic acid component and a polyalcohol component, wherein the total content of aromatic dicarboxylic acid derivatives employed in the esterification, based on free aromatic dicarboxylic acids, is ?48.5 mass-%, based on the total mass of polyalcohol component and polycarboxylic acid component; A1d: a polyether polyol with a hydroxyl number of ?500 mg KOH/g to ?1000 mg KOH/g and a functionality of ?1.5 to ?5.0 obtained by the addition of an epoxide to an aliphatic amine and/or a polyfunctional alcohol; and A1f: a polyether carbonate polyol with a functionality of ?1.5 to ?8.0 and a number average molecular weight of ?500 g/mol to ?10000 g/mol.

18. The method according to claim 1, wherein the cavity into which the polyurethane reaction mixture is provided is a refrigerator insulation frame.

Description

EXAMPLES

(1) The invention will be further described with reference to the following examples and figures without wishing to be limited by them.

(2) FIG. 1 shows a bright field transmission microscopy image of the soluble polyol formulation according to comparative example 1.

(3) FIG. 2 shows an image of the emulsion system according to example 2.

(4) FIG. 3 shows a scheme of the apparatus for the reduced pressure assisted foaming technology (RAF).

(5) FIG. 4 shows a photograph of a molded polyurethane foam in the freezer department of an appliance cabinet being prepared according the reduced pressure technology of the present invention.

(6) FIG. 5 shows a photograph of a molded polyurethane foam prepared of the identical polyurethane reaction mixture in the freezer department of another appliance cabinet being prepared under standard conditions.

(7) In FIG. 3, an apparatus 1 for the reduced pressure assisted foaming (RAF) is presented. The apparatus 1 comprises two vacuum pumps 2, 3 for reducing the pressure in the system, two vacuum buffer tanks 4, 5 for maintaining the pressure level, which are connected to the vacuum pumps 2, 3, valves 6, pressure gauges 7 for measuring the pressure, an automatic control system 8 for adjusting the pressure, sound silencers 9 for noise reduction and pressure release devices 10 for instant ventilation.

(8) The apparatus 1 further comprises a foaming jig 11 for the fabrication of foamed parts, to which a connect panel 12 can be attached as can be seen in the sectional view at the intersection A-A included in FIG. 3. The foaming jig 11 comprises two cavity areas 11a, 11b, whereas the pressure in each of the cavity areas 11a, 11b can be separately regulated by the automatic control systems 8 which dynamically regulate the pressure between the vacuum buffer tanks 4, 5 and the respective cavity areas 11a, 11b during the foaming cycle. The polyurethane reaction mixture is injected into the cavity 11 by an injection pipe 13.

(9) In the present invention, the reduced pressure foaming jig is connected to at least two separated vacuum systems. However, the inventive method can also be carried out with a single vacuum system. Since no absolute vacuum is necessary to successfully apply this technology minor sealing faults of the jig are acceptable in production which greatly reduces manufacturing costs. The pressure is no longer reduced than the gel time of the polyurethane reaction mixture. The period for the reduced pressure treatment and the level of the pressure reduction are adjusted and optimized individually with respect to the shape and the volume of a cavity and the reaction profile of the polyurethane reaction mixture. Foam leakage can be prevented by immediate venting of the cabinet once it has been fully filled.

(10) The PUR rigid foams can be prepared according to the one-step procedure known in the art where the reaction components are reacted with each other in a continuous or discontinuous fashion and then are applied onto or into suitable forms or substrates. Examples include those published in G. Oertel (editor) Kunststoff-Handbuch, volume VII, Carl Hanser Verlag, 3.sup.rd edition, Munich 1993, pages 267 et seq., and in K. Uhlig (editor) Polyurethan Taschenbuch, Carl Hanser Verlag, 2.sup.nd edition, Vienna 2001, pages 83-102.

(11) In the present case the two-component system with an emulsion (A side) of physical blowing agent in the polyol formulation and an isocyanate (B side) was processed by conventional mixing of these components in a laboratory scale stirring apparatus.

GLOSSARY

(12) Polyol 1: Polyether polyol with a hydroxyl number of 450 mg KOH/g, a theoretical functionality of 4.7 and a viscosity of 15000 mPas at 25? C. (Bayer MaterialScience); Polyol 2: Polyether polyol with a hydroxyl number of 470 mg KOH/g, a theoretical functionality of 4.0 and a viscosity of 8000 mPas at 25? C. (Bayer MaterialScience); Polyol 3: aromatic polyetherester polyol with a hydroxyl number of 300 mg KOH/g, a theoretical functionality of 2.0 and a viscosity of 6500 mPas at 25? C., prepared from the reaction of phthalic acid anhydride with diethylene glycol, followed by ethoxylation (Bayer MaterialScience); Polyol 4: Polyetherpolyol with a hydroxyl number of 380 mg KOH/g, a theoretical functionality of 4.6 and a viscosity of 5350 mPas at 25? C. (Bayer MaterialScience) Polyol 5: Polyether polyol with a hydroxyl number of 400 mg KOH/g, a theoretical functionality of 4.0 and a viscosity of 26500 mPas at 25? C. (Bayer MaterialScience); Tegostab? surfactant: Foam stabilizer (Evonik) amine catalyst tertiary amines which are standard catalysts in rigid foam applications and well known to the skilled person in this art Isocyanate: Polymeric MDI (Desmodur? 44V20L, Bayer MaterialScience)
Preparation of the Emulsions

(13) A reaction vessel was charged with the polyols according to the recipes as given in table 1. The required amounts of additives such as water, catalysts and stabilizers were metered in individually. Cyclopentane as the physical blowing agent was then added and all components were homogenized for 60 seconds at 4200 rpm. The emulsions thus prepared were stored at 20? C. to assess their stability and visually inspected for phase separations daily.

(14) Determination of Droplet Sizes

(15) The quality of an emulsion was evaluated directly after preparation by measuring the droplet size. To this effect, the emulsion was inspected visually in an optical microscope using bright field transmission microscopy in a layer thickness of the specimen of 20 ?m to 40 ?m. The microscope used was an Axioplan 2 microscope from Zeiss. Average droplet sizes of a non-aged emulsion thus determined were below 10 ?m.

(16) PUR Foam Preparation

(17) In general, only freshly prepared emulsions were used in PUR foam preparation. Between the preparation of an emulsion and its processing into PUR foam a time period of at most one hour had lapsed. Emulsions and the isocyanate were mixed in a laboratory using a stirrer at 4200 rpm, brought to reaction with each other and poured into a mould. The starting materials had a temperature of 20? C. and the mould had a temperature of 40? C. The foams thus prepared were analyzed with respect to their core density, cell size and thermal conductivity.

(18) Reactivity and Free-Rise Density Measurement

(19) To determine the reactivity and the free rise density, a total of 250 g material was mixed and poured into a card box. The cream time, gel time and tack free time were measured during foam rise using a wooden stick. The free rise density was determined 24 hours after foaming using foam pieces out of the foam core and following the principle of Archimedes.

(20) Cell Size Determination

(21) PUR rigid foam samples were cut into slices of 90 ?m to 300 ?m thickness using a vibrating microtome (Microm HM 650 V, Microm). Bright field transmission microscopy pictures (Axioplan, Zeiss) were taken. For statistical reasons, per analysis for at least 500 cell windows the area of two orthogonal spatial directions was determined. Using the area of each analyzed cell window, a dodecahedron was calculated whose diameter was equated to the diameter of a PUR rigid foam cell. These diameters were averaged and correspond to the cell diameters as stated below.

(22) PUR Foam Preparation Under Reduced Pressure

(23) For comparative reasons machine trials under reduced pressure and under standard conditions were prepared with the identical equipment (HP machine of Hennecke, MT 18 mixing head) in identical cabinet models (BCD-570WFPM). Unless otherwise stated, the raw material temperature was 20? C. (tank), the pressure was 130 bars (mixing head) and the mold temperature was 40? C.

(24) Thermal conductivities were determined according to DIN 12664 and, unless stated otherwise, were measured at 10? C. central temperature.

(25) Core densities given were determined on the samples for thermal conductivity according to DIN 12664 using the corresponding mass.

(26) Table 1 compares the recipe of an emulsion system (2) with a soluble polyol formulation (1) including the properties of the manually prepared PUR rigid foams obtained in the lab.

(27) Table 2 summarizes representative machine data acquired with an emulsion system (2) and a soluble polyol formulation (1).

(28) Table 3 compares representative machine data acquired with the emulsion system (2) under standard conditions and under reduced pressure conditions.

(29) The comparative example 1 involves a modern PUR recipe for current demands on insulation applications for example in appliances. It is already optimized for low thermal conductivities and is a so-called soluble formulation where the physical blowing agent used is completely dissolved in the polyol mixture. Therefore, the nucleating action of droplets can be excluded. It is still appropriate to compare examples 1 and 2 because their reaction profiles are similar. The gel times of 36 seconds for the soluble system (1) and 27 seconds for the emulsion system (2) are similarly short and the free-rise densities of the PUR foams are nearly identical.

(30) In a representative machine trial pieces of PUR rigid foam were prepared with identical dimensions, thereby ensuring that the properties determined therefrom can be compared with each other. It is noted that the minimum fill density in example 1 is significantly lower than in example 2. This is the density that is created when a mould is filled by the PUR rigid foam without overpacking. The higher minimum fill density can be explained by less favorable flowing characteristics of the emulsion system (which can be, of course, addressed by applying a vacuum during processing). Therefore, more reaction mixture must be injected at normal pressure to fill the foam as in the case of the soluble system.

(31) Still the thermal conductivity value is better by 0,5 mW m.sup.?1K.sup.?1 although the PUR rigid foam displayed a higher core density. Without wishing to be bound by theory, it is believed that this difference can be attributed to the nucleation effect of the droplets in the emulsion. This creates more nucleation sites and therefore lowers the average cell size. A reduction by 40% in cell size is observed from comparative example 1 to example 2.

(32) TABLE-US-00001 TABLE 1 Lab data Example 1 (comparative) 2 Polyol system 1 2 Polyol 1 Weight-parts 35.0 40.0 Polyol 2 Weight-parts 12.0 Polyol 3 Weight-parts 40.0 Polyol 4 Weight-parts 40.0 Polyol 5 Weight-parts 25.0 8.0 Water Weight-parts 2.4 1.3 Tegostab? surfactant Weight-parts 2.0 2.0 amine catalyst Weight-parts 4.18 2.22 Cyclopentane.sup.a Weight-parts 16 14 Isocyanate.sup.a Weight-parts 151 120 Index NCO/OH 118 110 Appearance qualitative clear cloudy Droplet size.sup.b ?m 8 Emulsion storage stability d >4 Cream time s 8 6 Gel time s 36 27 Tack-free time s 70 42 Free-rise density kg/m.sup.3 23.7 25.1 .sup.afor 100 weight-parts of polyol formulation; .sup.bdetermined on a non-aged emulsion.

(33) TABLE-US-00002 TABLE 2 Representative machine data Example 1 (comparative) 2 Polyol system 1 2 Cream time s 2 2 Gel time s 23 17 Free-rise density kg/m.sup.3 23.7 24.0 Minimum fill density kg/m.sup.3 30.7 34.7 Core density kg/m.sup.3 30 33 Thermal conductivity mW m.sup.?1K.sup.?1 18.9 18.4 Cell size.sup.a ?m 150 90 .sup.aaccording to an in-house procedure described previously.

(34) TABLE-US-00003 TABLE 3 Cabinet filling trial results with emulsion system (2) under standard conditions and reduced pressure conditions. Example 3 (comparative) 4 Polyol system 2 2 Foam filling weight kg 11.35 11.35 ? Foam jig pressure mbar 0 ?200 Reduced pressure time s 0 15 Cream time s 4 4 Gel time s 21 21 Free-rise density kg/m.sup.3 23.3 23.3 Demold time Minutes 10 10 Foam core density.sup.a kg/m.sup.3 34.5 36.0 Thermal conductivity.sup.a (10? C.) mW m.sup.?1K.sup.?1 18.3 18.4 Average compression strength.sup.a kPa 233 243 Cell size.sup.b ?m 90-100 90-100 Void formation serious no .sup.afoam samples taken from the divider part of the cabinet, .sup.baccording to an in-house procedure described previously.

(35) As shown in Table 3, the reduced pressure foaming technology does not interfere with the properties of so obtained polyurethane foams. The thermal conductivity values and cell sizes of foams obtained under standard conditions (3) are very well comparable to those obtained under reduced pressure conditions (4). Furthermore, the core density is even higher in case of example (4) although identical filling weights had been used in case of both examples. Finally cabinets having been filled with polyurethane foam under reduced pressure as in case of example (4) do not show any voids on the foam surface which directly illustrates a very even distribution of the polyurethane foam within the cabinet. This even foam distribution is known to induce a significant reduction of the overall energy consumption of appliances which therefore exceeds the performance of foams having been molded under standard conditions.