Method for producing foamed molded bodies

Abstract

The invention relates to a method for producing foamed molded bodies comprising the steps of A) providing a mold, and B) introducing a foam-forming reaction mixture into the mold under variable pressure of injection,
wherein the method is characterized in that
the foam-forming reaction mixture has a fiber time of ≧20 s to ≦60 s.

Claims

1. A method for producing foamed molded bodies comprising the steps of A) providing a mold, and B) introducing a foam-forming reaction mixture from a mixing head into the mold under variable pressure of injection, wherein the foam-forming reaction mixture exits the mixing head at a speed which decreases over time from an initial exit speed to a decreased exit speed, wherein the initial exit speed and the decreased exit speed are each in the range of >1 m/s to <5 m/s, and wherein the foam-forming reaction mixture has a fiber time at 20° C. of ≧20 s to ≦60 s wherein the foam-forming reaction mixture comprises an emulsion, and wherein the emulsion contains the following constituents: (I) an isocyanate-reactive composition A containing a polyol mixture A1 of at least three polyols A1a, A1 and A1e as continuous phase and (II) at least one physical blowing agent T as disperse phase, wherein: (i) A1a is a polyether polyol having a hydroxyl number of 15 mg KOH/g to 550 mg KOH/g and having a functionality of 1.5 to 6.0, obtained by addition reaction of an epoxide onto one or more starter compounds selected from the group consisting of carbohydrates and di- or higher-functional alcohols; (ii) A1b is a polyether polyol having a hydroxyl number of 100 mg KOH/g to 550 mg KOH/g and having a functionality of 1.5 to 5.0, obtained by addition reaction of an epoxide onto an aromatic amine; and (iii) A1e is a polyester polyether polyol having a hydroxyl number of 100 mg KOH/g to 450 mg KOH/g and having a functionality of 1.5 to 3.5, obtained by addition reaction of an epoxide onto the esterification product of an aromatic dicarboxylic acid derivative and a di- or more highly functional alcohol.

2. The method as claimed in claim 1, wherein the foam-forming reaction mixture is introduced into the mold at a temporally variable rate.

3. The method as claimed in claim 1, wherein the ratio of the time in which the foam-forming reaction mixture is introduced into the mold at a temporally variable rate to the cream time of the foam-forming reaction mixture is in the range from 0.1 to 10.

4. The method as claimed in claim 1, wherein the temporal variation of the introduction rate and/or injection pressure of the foam-forming reaction mixture is effected by varying the power output of a pump motor acting on the reaction mixture.

5. The method as claimed in claim 1, wherein the foam-forming reaction mixture is obtained from the reaction of a first reaction component comprising said emulsion and a second reaction component and the a first reaction component comprising said emulsion and a second reaction component are each introduced via nozzles into a mixing chamber in said mixing head.

6. The process as claimed in claim 1, wherein the foam-forming reaction mixture has a fiber time of ≧25 s to ≦55 s.

7. The method as claimed in 1, wherein the injection pressure is reduced over time in step B).

8. The method as claimed in claim 1, wherein the injection pressure in step B) is varied by at least 15 bar/s.

9. The method as claimed in claim 1, wherein the mold in cross section comprises a horizontally disposed floor volume and also vertically disposed volumes in communication with the floor volume.

10. The method as claimed in claim 1, wherein the mold comprises a pipework line on the outside and a pipework line disposed therein on the inside and the foam-forming reaction mixture is introduced between the inside and outside pipework lines.

11. The method as claimed in claim 1, wherein the mold comprises two spaced-apart areal elements and the foam-forming reaction mixture is introduced between these areal elements.

12. The method as claimed in claim 1, wherein the foam-forming reaction mixture is introduced by the mixing head having a mixing chamber and wherein furthermore the outflow cross section of the mixing chamber is varied during the introducing.

13. The method as claimed in claim 1, wherein the at least one physical blowing agent T is at least one member selected from the group consisting of hydrocarbons, halogenated ethers and perfluorinated hydrocarbons having from 1 to 8 carbon atoms each.

14. A foamed molded body obtainable by a method as claimed in claim 1.

15. A refrigerator, a freezer or a fridge-freezer combination comprising a foamed molded body as claimed in claim 14.

16. The method as claimed in claim 2, wherein the time during which the foam-forming reaction mixture is introduced into the mold at a temporally variable rate is ≧1 s to ≦20 s.

17. The method according to claim 3, wherein the ratio of the time in which the foam-forming reaction mixture is introduced into the mold at a temporally variable rate to the cream time of the foam-forming reaction mixture is in the range from 0.5 to 5.

18. The method as claimed in claim 4, wherein the power output of the pump motor is effected by varying the rotary speed of the motor using a frequency converter.

19. The method as claimed in claim 5, wherein the foam-forming reaction mixture is selected so as to obtain a rigid polyurethane foam.

20. The process as claimed in claim 6, wherein the foam-forming reaction mixture has a fiber time of ≧30 s to ≦50 s.

21. The method as claimed in 7, wherein the injection pressure is reduced over time in step B) in a very substantially linear manner.

22. The method as claimed in claim 8, wherein the injection pressure in step B) is varied by at least 20 bar/s.

23. The refrigerator, freezer or fridge-freezer combination as claimed in claim 15, wherein the provided mold is a housing component of the refrigerator, of the freezer or of the fridge-freezer combination.

Description

(1) The invention additionally provides a refrigerator, a freezer or a fridge-freezer combination comprising a foamed molded body obtainable according to the present invention, wherein the provided mold is in particular a housing part of the refrigerator, of the freezer or of the fridge-freezer combination. The present invention is further elucidated with reference to the following examples and drawings without, however, being limited thereto. In said drawings:

(2) FIG. 1 shows a method of filling a mold with a foam-forming reaction mixture,

(3) FIG. 2 shows an alternative method of filling a mold with a foam-forming reaction mixture,

(4) FIG. 3 shows a mixing head in a first operative position,

(5) FIG. 4 shows the mixing head of FIG. 3 in a second operative position, and also

(6) FIG. 5 shows a housing mold in unfolded depiction.

(7) FIG. 1 shows in schematic form the state shortly after filling a mold 1 with a foam-forming reaction mixture 6. The mold 1 is executed as a hollow body and depicted in cross-sectional view. An insulating element for a combination of refrigerator and chest freezer may be concerned in the case of mold 1. The horizontally lying mold 1 thus has vertically disposed portions 2, 3 and 4. Portion 2 forms the floor portion, portion 3 separates the cooling compartment and the freezing compartment from each other, and portion 4 forms the head portion. A horizontally disposed floor volume is formed. The cavities of portions 2, 3 and 4 form vertically disposed volumes in communication with the floor volume.

(8) To fill the mold, an output pipe 5 is connected to a corresponding inlet orifice of mold 1. The outlet 5 pipe fills a foam-forming reaction mixture 6, which preferably produces a polyurethane foam, into said mold. In the case depicted in FIG. 1, the reaction mixture 6 was initially introduced into the mold at high injection pressure which was thereafter reduced from 170 bar down to 120 bar in a continuous manner. The initially high injection pressure and the associated high exit speed of the reaction mixture from an undepicted mixing head serves to convey the reaction mixture 6 into the rear region of mold 1. Gradually reducing the injection pressure conveyed the reaction mixture 6 into the front part of mold 1. In this way, reaction mixture 6 is applied uniformly across the full length of mold 1.

(9) Using a reaction mixture having the short fiber time of the present invention provides in this way the wedge-shaped profile depicted in FIG. 1 to reaction mixture 6, since foam formation in the rear part of mold 6 has already ensued during filling. The cavity in portion 4 of mold 1 is the first to be filled out with foam. A further onset of the foaming reaction will force material into the cavities of portions 3 and 2. This combines with an improved full-areal predistribution of the initially still liquid reaction mixture to produce more uniform flow path distances within mold 1. The result is a more homogeneous apparent density distribution and also more isotropic cellular geometries with improved mechanical and insulating properties within the foamed molded body obtained. In addition, the risk of void formation is appreciably reduced in this way.

(10) FIG. 2 shows mold 1 being filled in a manner opposite to FIG. 1. The reaction mixture 6 was initially introduced at comparatively low injection pressure (120 bar) and then the injection pressure was raised to 170 bar. A comparatively large volume close to the inlet opening, as represented by the cavity of portion 2, can be efficiently reached in this way, for example.

(11) FIG. 3 depicts a mixing head for use with the method of the present invention. It is designed as a diverter type mixing head in the present case. The reaction components are each injected through nozzles 1 into a cylindrical mixing chamber 2 under an injection pressure which can be set separately for the two components and is varied during the course of the foaming operation. The reaction components become mixed in the cylindrical mixing chamber 2 through kinetic energy and then flow via a 90° diversion into an outflow pipe 3, the cross-sectional area of which increases significantly and thereby causes a flow quiescence of the mixed stream.

(12) After the discharge of the mixture has ended, the component streams are switched into a recirculation position via grooves in a control piston 4. At the same time, the control piston 4 removes mixture residues from the mixing chamber 2 into the outflow pipe. This is followed by outflow pipe 3 being cleaned via a further ram 5. The switching operations are accomplished via schematically drawn hydraulics “H1” and “H2” at pressures of about 100 to 160 bar in order that fast but also forceful switching movements may be realized.

(13) In addition to a cleaning function, the cleaning ram 5 also forms as throttle means. Usually, a stroke limiter which is manually adjustable via a fine thread will be used to stop limit the displacement path of ram 5 in such a way that the lower ram end in the flow direction creates an superimposition with the transition between mixing chamber 2 and outflow pipe 3. The free outlet cross section 6 varies with the degree of superimposition to influence the quality of mixing as well as the mixing chamber pressure level.

(14) The mixture output is severely restricted in the operation position shown in FIG. 3. In the present mixing head, the manual hand adjuster has been removed and replaced with a gear pair 7. A servomotor “S” 8 is secured to the mixing head housing and forms a positive connection with the stroke limiter via the gear pair 7, and is integrated in the control system for the installation.

(15) To reduce the sticking and rubbing resistances acting on the adjusting thread, the hydraulic switching pressure of the cleaning ram 5 is reduced to <10 bar via a bypass circuit 9 during the mixture exit phase. An axial ball bearing 10 protects the contact face between the travel stop and the hydraulic piston from momentum transfers. Therefore, depending on the direction of movement, the hydraulic pressure only serves to reposition the hydraulic piston against the variable stroke limiter, or to fix its position against the stop face.

(16) On completion of the exit of the mixture, the servomotor 8 drives the stroke limiter into the upper end position, while the axial ball bearing 10 is plunged into the upwardly limiting cylindrical plate 11. In this position, the bypass valve closes and the cleaning ram 5 can be operated at the usual hydraulic pressure. The use of a servomotor achieves a highly precise and reproducible throttle position which can be adjusted according to the mixture output over the mixture output time.

(17) FIG. 4 shows the mixing head as per FIG. 3, except that the free outlet cross section 6 is larger because of the different position of ram 5. Mixture output is unrestricted as a result.

(18) FIG. 5 shows a housing mold 20 for a fridge-freezer combination with a refrigerating region 21 and a freezing region 22 for the foaming tests of the present invention in unfolded depiction. The mold 20 corresponds to the common dimensions of a fridge-freezer combination and has an empty volume of 164 L. The individual walls are identified by the positional references Pos. 1 to Pos. 9. The mold 20 is filled with a polyurethane reaction mixture as described hereinbelow at an inlet position 22 on the compressor stage of the fridge-freezer combination.

EXAMPLES A

(19) The rigid PUR foams of the present invention are formed by the one-shot process known to a person skilled in the art, wherein the reaction components are continuously or discontinuously reacted with one another and then cured in/on suitable molds/substrates. This process has been described, for example in U.S. Pat. No. 2,761,565 A, in G. Oertel (editor) “Kunststoff-Handbuch”, volume VII, Carl Hanser Verlag, 3.sup.rd edition, Munich 1993, pages 267 ft., and also in K. Uhlig (editor) “Polyurethan Taschenbuch”, Carl Hanser Verlag, 2.sup.nd edition, Vienna 2001, pages 83-102.

(20) In the present case, the 2-part recipes consisting of a blowing agent-containing polyol formulation 1 or 2 as per table 1 and an isocyanate were processed using a conventional high pressure machine (HK 650 from Hennecke) and high pressure mixing head (MX 18 from Hennecke). The FIG. 5 housing mold to be foamed out corresponds to the common dimensions of a fridge-freezer combination and has an empty volume of 164 L. Importation took place at the compressor stage. Mold temperature was 38-40° C., the raw materials at a temperature of 20-22° C.

(21) The following materials were used: polyol 1: polyether polyol having an OH number of 450 mg KOH/g, a theoretical functionality of 4.7 and a viscosity of 15 000 mPas at 25° C. (Bayer MaterialScience); polyol 2: polyether polyol having an OH number of 380 mg KOH/g, a theoretical functionality of 4.6 and a viscosity of 5350 mPas at 25° C. (Bayer MaterialScience); polyol 3: polyether polyol having an OH number of 400 mg KOH/g, a theoretical functionality of 4.0 and a viscosity of 26 500 mPas at 25° C. (Bayer MaterialScience); polyol 4: polyether polyol having an OH number of 112 mg KOH/g, a theoretical functionality of 2.0 and a viscosity of 140 mPas at 25° C. (Bayer MaterialScience); polyol 5: aromatic polyether ester polyol having an OH number of 300 mg KOH/g, a theoretical functionality of 2.0 and a viscosity of 6500 mPas at 25° C., obtained from the reaction of phthalic anhydride with diethylene glycol and subsequent ethoxylation (Bayer MaterialScience); stabilizer: Tegostab® (Evonik) amine catalyst: tertiary amines that are standard catalysts in PUR chemistry and very well known to a person averagely skilled in the art isocyanates: polymeric MDI (Desmodur® 44V20L, Bayer MaterialScience)

(22) Polyol formulations 1 and 2 had the following compositions:

(23) TABLE-US-00001 TABLE 1 Processing recipes, particulars in parts by weight. Polyol formulations: Formulation 1 Formulation 2 polyol 1 43.0 35 polyol 2 35 polyol 3 42.0 25 polyol 4 5.0 5 polyol 5 10.0 water 2.0 2.4 stabilizer 2.0 1.7 amine catalyst 3.2 1.7 Recipes: Recipe 1 Recipe 2 formulation 1 100 formulation 2 100 cyclopentane 16.0 13.5 isocyanate 130 143

(24) The tests were carried out such that the output rate of the machine could be varied throughout the entire mold-filling time. For this, the output rate was varied in a linear manner from output rate 1 at the start of the shot to output rate 2 at the end of the shot, if necessary (cf. table 2). Tests were carried out both with increasing output rate and with decreasing output rate. Minimum and maximum output rates were further adjusted such that the delivery times needed for the delivery of a constant mass were <10 s, ≧10 s, ≧15 s and ≧20 s. The processing parameters are summarized in table 2.

(25) TABLE-US-00002 TABLE 2 Processing parameters of machine tests with dynamic input. Example 1 2 3 4 5 6 7 (inv.) (comp.) (comp.) (inv.) (inv.) (inv.) (comp.) recipe 1 X X X X X X recipe 2 X cream time s 5 5 5 5 5 5 5 fiber time s 27 39 28 30 27 27 29 apparent free-rise kg/m.sup.3 22.2 23.1 22.0 21.0 22.3 22.2 21.0 density apparent feed kg/m.sup.3 36.4 36.0 36 35.8 36.5 35.8 35.8 density input time s 7.2 7.6 20.0 15.0 10.0 7.3 6.1 output rate 1 g/s 960 960 580 608 960 608 960 output rate 2 g/s 608 608 0 175 175 960 — input time/cream 1.4 1.5 4 3.8 2 1.5 1.4 time output speed 1 m/s 3.8 3.8 2.3 2.4 3.8 2.4 3.8 output speed 2 m/s 2.4 2.4 0.0 0.8 0.8 3.8 —

(26) Foam properties were measured at the positions identified as Pos. 1 to Pos. 9 in FIG. 5 after foam cure. The results are summarized below in table 3.

(27) TABLE-US-00003 TABLE 3 Results of machine tests. Example 1 (inv.) 2 (comp.) 3 (comp.) 4 (inv.) 5 (inv.) 6 (inv.) 7 (comp.) Thermal conductivity number at 10° C. midpoint temperature [mWm.sup.−1K.sup.−1]: Pos 1 18.53 19.07 — — — 18.27 18.51 Pos 2 18.22 18.78 — 19.24 18.68 18.28 18.51 Pos 3 18.48 19.10 — 20.13 19.18 18.36 18.57 Pos 4 18.20 18.87 — 19.02 18.65 18.46 18.40 Pos 5 18.33 18.88 — 18.22 18.22 18.25 18.19 Pos 6 18.60 19.10 — 18.56 18.66 19.28 18.86 Pos 7 18.28 18.88 — 18.77 18.80 18.24 18.47 Pos 8 18.87 18.73 — 18.65 18.88 19.32 19.05 Pos 9 18.65 18.99 — 18.55 18.59 19.42 19.00 MW 18.46 18.93 — 18.89 18.71 18.65 18.62 STABW 0.22 0.14 — 0.59 0.27 0.52 0.31 Apparent core density [kg/m.sup.3]: Pos 1 32 31 — — — 33 32 Pos 2 32 33 — 31 32 34 33 Pos 3 33 34 — 30 32 32 33 Pos 4 32 32 — 30 31 32 32 Pos 5 33 34 — 31 33 31 32 Pos 6 32 32 — 32 34 30 31 Pos 7 33 33 — 33 34 31 32 Pos 8 32 32 — 32 34 30 31 Pos 9 33 34 — 33 36 30 31 MW 32.4 32.8 — 31.5 33.3 31.4 31.9 STABW 0.53 0.93 — 1.20 1.58 1.42 0.83 Assessment of foam distribution: surface voidless voidless severely voids at voids at voids at voids at disrupted Pos. 1-4 Pos. 1-4 Pos. 6-9 Pos. 6-9 generally — — mold only Pos. 1 not shrinkage — — filled up to filled, too severe Pos. 5, severe shrinkage at Pos. 1 shrinkage at Pos. 1-4

(28) Example 1 was carried out in accordance with the present invention by processing the fast polyol formation 1 with dynamic input and a gradient optimized to the housing geometry used. The distribution of the apparent core densities and of the thermal conductivities found is plainly best in Example 1, as is clear from the low standard deviation of 0.53. By contrast, mean thermal conductivity and thermal conductivity standard deviation are worse in Example 7, emphasizing the positive influence of an optimum predistribution as in Example 1.

(29) Example 7 was carried out with a constant output, as generally customary in the refrigerator industry, without optimizing the distribution. The disadvantage with this procedure, viz., of continued input into the already expanding mixture, is also apparent from the distinctly worse voidage compared with Example 1. Example 6 was carried out using the same gradient for the dynamic injection as in Example 1 with the fast polyol formulation 1 except that this time the gradient was traversed in the opposite direction, from minimum output rate to maximum output rate. The disadvantages show up in the worse voidage at positions 6 to 9 and in the poorer apparent core density distribution associated with an altogether higher average thermal conductivity, i.e., a worse insulating effect.

(30) Examples 3, 4 and 5 were each carried out using dynamic output in that the output rates were in each case adjusted such so as to produce input times of 20 s, 15 s and 10 s. The predistribution in the case of Example 3 was so bad that the housing mold could not be filled out, since the foam only flowed as far as position 5. With shorter input times, as in Examples 4 and 5, the housing mold was almost (Example 4) and completely (Example 5) filled. However, these moldings each exhibited some severe shrinkage at position 1, so no values could be determined for this position. Distinct voids were further apparent at positions 1 to 4 at the far end of the flow path. The poor predistribution of the reaction mixture is evident from the distinctly higher mean thermal conductivities and from the greater scatter in thermal conductivity and apparent density.

(31) Example 2 was carried out by processing the slower polyol formulation 2 in recipe 2 under identical conditions to recipe 1 in example 1. The effectiveness of the injection procedure according to the present invention becomes clear here, since although mean thermal conductivity differs by about +0.5 mWm.sup.−1K.sup.−1 from that of Example 1, the scatter is minimal at a standard deviation of 0.14. Comparing the results of Example 2 with those of Example 4 reveals that dynamic injection with a less reactive recipe gives better results with regard to foam distribution, surface quality and voidage than with a fast one for comparable mean thermal conductivities.

EXAMPLES B

(32) General tests were also carried out on insulating elements for fridge-freezer combinations. The objective of these tests was to investigate which methods can be used to minimize the amount of foam-forming reaction mixture needed for homogeneous foaming out of the mold. The results of using a rigid polyurethane system blown with liquid blowing agent were visually assessed by technically trained personnel. The mold to be foamed out was the same in each case.

(33) Cream time was determined visually. A qualified person observed the reaction mixture after exit from the mixing head. The cream time is the time which elapses from mixing the components to the time where the incipient creaming of the reaction mixture becomes visually discernible in a shift in color to a lighter coloration and some initial foaming.

(34) Fiber time is determined by mixing the reactants of the reaction mixture with one another at 20° C. and dipping a thin spatula of wood into the expanding reaction mixture at short intervals. The time from mixing the components to the time at which it is first possible to draw out strings with the spatula is the fiber time.

EXAMPLE 8 (IN ACCORDANCE WITH THE PRESENT INVENTION)

(35) Example 8 was carried out by filling the mold with reaction mixture using an initial injection pressure for the individual components of 170 bar into mixing chamber 2 of a mixing head depicted in FIG. 3 while continuously reducing the injection pressure down to a final value of 90 bar over a period of 3 s. This corresponds to an injection pressure change rate of about 26.7 bar/s. The reaction mixture had a fiber time of 30 s at 20° C. The injection pressure was decreased in a linear manner as far as practically possible. Altogether 4940 g of reaction mixture were introduced. The mold was continually foamed out. More particularly, even the upper edges were crisply contoured and the foam was fine-cell and voidless.

EXAMPLE 9 (COMPARATOR)

(36) The mold was again filled with 4940 g of reaction mixture using the same mixing head as in Example 8 but in this case at a constant injection pressure of 150 bar for the individual components. The reaction mixture had a fiber time of 50 s. On completion of the foaming operation, the absence of material was noted at the upper end of the insulating element due to incomplete foaming. The foam was also conspicuously more coarse-cell than the foam of Example 8.