USE OF A PARTICULATE MATERIAL COMPRISING A PARTICLE-SHAPED SYNTHETIC AMORPHIC SILICON DIOXIDE AS AN ADDITIVE FOR A MOLDING MATERIAL MIXTURE, CORRESPONDING METHOD, MIXTURES, AND KITS

Abstract

What is described is the use of a particulate material comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, as additive for a molding material mixture at least comprising: a refractory mold base material having an AFS grain fineness number in the range from 30 to 100, particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, and water glass, for increasing the moisture resistance of a molding producible by hot curing of the molding material mixture. Also described are corresponding processes, mixtures and kits.

Claims

1. (canceled)

2. A process for producing a hot-cured molding having increased moisture resistance, having the following steps: (i) producing a molding material mixture by mixing together at least the following constituents: refractory mold base material having an AFS grain fineness number in the range from 30 to 100, particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, and water glass, (ii) forming the molding material mixture, (iii) hot curing the formed molding material mixture, so as to result in the molding, wherein  the constituents of the molding material mixture are additionally mixed with a particulate material as additive comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering.

3. The process as claimed in claim 2, wherein the molding material mixture is produced by creating a solid-state mixture or suspension by mixing at least the following solid constituents: particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, and as additive, a particulate material comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, wherein the solid-state mixture or suspension created is mixed with the further constituents of the molding material mixture.

4. A mixture for use in a process as claimed in claim 2, at least comprising the following solid constituents: particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, and as additive, a particulate material comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, wherein the mixture is a solid-state mixture or a suspension of solid constituents in a liquid carrier medium, preferably a solid-state mixture.

5. The mixture as claimed in claim 4, at least comprising the following constituents: refractory mold base material having an AFS grain fineness number in the range from 30 to 100, particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, water glass, and as additive, a particulate material comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering.

6. The mixture as claimed in claim 5, wherein, in the mixture,  the proportion of particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, is less than 2% by weight and preferably greater than 0.015% by weight, more preferably greater than 0.02% by weight, based on the total mass of the mixture,  and/or  the proportion of particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, is less than 2% by weight and preferably greater than 0.015% by weight, more preferably greater than 0.02% by weight, based on the total mass of the mixture,  and/or  the total proportion of particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, and particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, is less than 2% by weight and preferably greater than 0.3% by weight, based on the total mass of the mixture,  and/or  the total proportion of amorphous silicon dioxide is less than 2% by weight and preferably greater than 0.3% by weight, based on the total mass of the mixture.

7. The mixture as claimed in claim 4, producible by a process comprising the following steps: (i) providing or producing a separate amount of a particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, (ii) providing or producing an amount of a particulate material comprising, as its sole constituent or as one of multiple constituents, a particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, (iii) mixing the amounts provided or produced in steps (i) and (ii).

8. The mixture as claimed in claim 4,  wherein the ratio of the total mass of particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering,  to the total mass of particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  is in the range from 20:1 to 1:20, preferably in the range from 5:1 to 1:20, more preferably in the range from 3:1 to 1:20, especially preferably in the range from 2:1 to 1:20, most preferably in the range from 1.5:1 to 1:20.

9. The process as claimed in claim 2, wherein the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering,  is selected or are independently selected from the group consisting of particulate synthetic amorphous silicon dioxide containing silicon dioxide in a proportion of at least 90% by weight, based on the total mass of the particulate synthetic amorphous silicon dioxide, and at least carbon as secondary constituent, preferably producible by reducing quartz in an arc furnace; particulate synthetic amorphous silicon dioxide comprising oxidic zirconium as secondary constituent and preferably producible by thermal decomposition of ZrSiO.sub.4 particulate synthetic amorphous silicon dioxide producible by oxidizing metallic silicon by means of an oxygenous gas; particulate synthetic amorphous silicon dioxide producible by quenching a silicon dioxide melt  and mixtures thereof.

10. The process as claimed in claim 2, wherein the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, contains silicon dioxide in a proportion of at least 90% by weight based on the total mass of the particulate synthetic amorphous silicon dioxide, and at least carbon as secondary constituent, and is preferably producible by reducing quartz in an arc furnace;  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, is a particulate synthetic amorphous silicon dioxide comprising oxidic zirconium as secondary constituent and preferably producible by thermal decomposition of ZrSiO.sub.4.

11. The process as claimed in claim 2, wherein one or more constituents are added to the molding material mixture or mixture or are selected from the group consisting of: barium sulfate, oxidic boron compounds, graphite, carbohydrates, lithium compounds, phosphorus compounds, hollow microbeads, molybdenum sulfide, lubricants in platelet form, surfactants, organosilicon compounds, alumina and alumina-containing compounds.

12. The process as claimed in claim 2, wherein  the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering,  has pozzolanic activity.

13. The process as claimed in any of claim 2, wherein the activity of Ra226 in the molding material mixture or mixture is not more than 1 Bq/g.

14. A kit for production of a mixture as claimed in claim 4, at least comprising as or in a first constituent of the kit, an amount of particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, as or in a second constituent of the kit, an amount of particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  wherein the first and second constituents of the kit are arranged in spatial separation from one another.

15. The mixture as claimed in claim 4 for use in the production of casting molds or cores for metal processing.

16. The mixture as claimed in claim 4, wherein the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering,  is selected or are independently selected from the group consisting of  particulate synthetic amorphous silicon dioxide containing silicon dioxide in a proportion of at least 90% by weight, based on the total mass of the particulate synthetic amorphous silicon dioxide, and at least carbon as secondary constituent, preferably producible by reducing quartz in an arc furnace;  particulate synthetic amorphous silicon dioxide comprising oxidic zirconium as secondary constituent and preferably producible by thermal decomposition of ZrSiO.sub.4  particulate synthetic amorphous silicon dioxide producible by oxidizing metallic silicon by means of an oxygenous gas;  particulate synthetic amorphous silicon dioxide producible by quenching a silicon dioxide melt  and  mixtures thereof.

17. The mixture as claimed in claim 4, wherein the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, contains silicon dioxide in a proportion of at least 90% by weight based on the total mass of the particulate synthetic amorphous silicon dioxide, and at least carbon as secondary constituent, and is preferably producible by reducing quartz in an arc furnace;  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, is a particulate synthetic amorphous silicon dioxide comprising oxidic zirconium as secondary constituent and preferably producible by thermal decomposition of ZrSiO.sub.4.

18. The mixture as claimed in claim 4, wherein one or more constituents are added to the molding material mixture or mixture or are selected from the group consisting of: barium sulfate, oxidic boron compounds, graphite, carbohydrates, lithium compounds, phosphorus compounds, hollow microbeads, molybdenum sulfide, lubricants in platelet form, surfactants, organosilicon compounds, alumina and alumina-containing compounds.

19. The mixture as claimed in claims 4, wherein  the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering,  and/or the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, has pozzolanic activity.

20. The mixture as claimed in claim 4, wherein the activity of Ra226 in the molding material mixture or mixture is not more than 1 Bq/g.

Description

FIGURES

[0246] FIG. 1 shows results of the determination of the core weight of test bars (cf. example 3) and results of the determination of the moisture resistance of test bars (cf. example 4).

[0247] The X axis indicates the proportion of RW filler sieved in the total amount of RW filler sieved and RW filler Q1 Plus in the molding material mixture in percent. The Y axis indicates the core weight determined according to example 3 in grams. The Z axis indicates the moisture resistance determined according to example 4 in percent.

[0248] The filled circles indicate experimentally ascertained measurements of the core weight of test bars (according to example 3). The dashed-and-dotted line schematically illustrates the progression of the measurement points. The dashed line illustrates the linear correlation expected by the person skilled in the art between the proportion of RW filler sieved in the total amount of RW filler sieved and RW filler Q1 Plus in the molding material mixture and core weight (linear combination based on the values for the pure materials).

[0249] The crosses indicate experimentally ascertained measurements of the moisture resistance of test bars (according to example 4). The solid line schematically illustrates the progression of the measurement points. The dotted line illustrates the linear correlation expected by the person skilled in the art between the proportion of RW filler sieved in the total amount of RW filler sieved and RW filler Q1 Plus in the molding material mixture and the moisture resistance (linear combination based on the values for the pure materials).

[0250] FIG. 2 shows results of the determination of the core weight of test bars (produced from mixtures 1.1, 1.2 and 1.3, cf. table 5, example 6) and results of the determination of residual strength after 3 hours of test bars (produced from mixtures 1.1, 1.2 and 1.3, cf. table 5, example 6).

[0251] The X axis, here and in FIG. 3, FIG. 4 and FIG. 5, indicates the proportion of RW filler Q1 Plus in the total mass of Elkem Microsilica® 971 and RW filler Q1 Plus in the molding material mixture in percent. The Y axis, here and in FIG. 3, FIG. 4 and FIG. 5, indicates core weight in g, determined according to point 6.5 of example 6. The Z axis, here and in FIG. 3, FIG. 4 and FIG. 5, indicates residual strength after 3 hours in percent, determined according to point 6.7 of example 6.

[0252] The filled circles, here and in FIG. 3, FIG. 4 and FIG. 5, indicate experimentally ascertained measurements of the core weight of test bars (according to example 6). The dashed line, here and in FIG. 3, FIG. 4 and FIG. 5, illustrates the linear correlation expected by the person skilled in the art between the proportion of RW filler Q1 Plus in the total mass of Elkem Microsilica® 971 and RW filler Q1 Plus in the molding material mixture and core weight (linear combination based on the values for the pure materials).

[0253] The crosses, here and in FIG. 3, FIG. 4 and FIG. 5, represent experimentally ascertained values of residual strength after 3 hours (according to example 6). The dotted line, here and in FIG. 3, FIG. 4 and FIG. 5, illustrates the linear correlation expected by the person skilled in the art between the proportion of RW filler Q1 Plus in the total mass of Elkem Microsilica® 971 and RW filler Q1 Plus in the molding material mixture and moisture resistance (linear combination based on the values for the pure materials).

[0254] FIG. 3 shows results of the determination of the core weight of test bars (produced from mixtures 2.1, 2.2 and 2.3, cf. table 5, example 6) and results of the determination of residual strength after 3 hours of test bars (produced from mixtures 2.1, 2.2 and 2.3, cf. table 5, example 6).

[0255] FIG. 4 shows results of the determination of the core weight of test bars (produced from mixtures 3.1, 3.2 and 3.3, cf. table 5, example 6) and results of the determination of residual strength after 3 hours of test bars (produced from mixtures 3.1, 3.2 and 3.3, cf. table 5, example 6).

[0256] FIG. 5 shows results of the determination of the core weight of test bars (produced from mixtures 4.1, 4.2 and 4.3, cf. table 5, example 6) and results of the determination of residual strength after 3 hours of test bars (produced from mixtures 4.1, 4.2 and 4.3, cf. table 5, example 6).

[0257] FIG. 6 shows results of the determination of core weight of test bars (produced from mixtures 5.1, 5.2 and 5.3, cf. table 5, example 6)

[0258] The X axis indicates the proportion of RW filler sieved in the total mass of Elkem Microsilica® 971 and RW filler sieved in the molding material mixture in percent. The Y axis indicates core weight determined according to point 6.5 of example 6 in g.

[0259] The filled circles indicate experimentally ascertained measurements of the core weight of test bars (according to example 6). The dashed line illustrates the linear correlation expected by the person skilled in the art between the proportion of RW filler sieved in the total mass of Elkem Microsilica® 971 and RW filler sieved in the molding material mixture and core weight (linear combination based on the values for the pure materials).

EXAMPLE 1

Determination of Particle Size Distribution by Means of Laser Scattering

[0260] The selection of the substances in this example is merely illustrative, and it is also possible to determine particle size distributions or medians of other particulate silicon dioxide species to be used in accordance with the invention by means of laser scattering according to the procedure in this example.

1.1 Sample Preparation

[0261] By way of example, particle size distributions of silica fume particles (CAS number: 65012-64-2) that are commercially available (from RW Silicium GmbH) and in particulate powder form from Si production, RW filler sieved [“RW-Füller gesiebt”], and from ZrO.sub.2 production, RW filler Q1 Plus [“RW-Füller Q1 plus”], were determined experimentally by means of laser scattering.

[0262] In each case, about 1 teaspoon of the particulate silicon dioxide was admixed with about 100 mL of demineralized water, and the resultant mixture was stirred with a magnetic stirrer (IKAMAG RET) at a stirrer speed of 500 revolutions per minute for 30 seconds. Subsequently, an ultrasound probe (from Hielscher; model: UP200HT) preadjusted to 100% amplitude, equipped with the S26d7 sonotrode (from Hielscher), was immersed into the sample, and the sample was sonicated therewith. The sonication was continuous (not pulsed). For the silica fume particles examined from Si production, RW filler sieved, and from ZrO.sub.2 production, RW filler Q1 plus, optimal sonication times of 300 seconds (for RW filler sieved) or 240 seconds (for RW filler Q1 Plus) were chosen, which had been ascertained beforehand as described in point 1.3 of example 1.

1.2 Laser Scattering Measurements

[0263] The measurements were conducted with a Horiba LA-960 instrument (LA-960 hereinafter). For the measurements, circulation speed was set to 6, stirrer speed to 8, data recording for the sample to 30 000, convergence factor to 15, the mode of distribution to volume, and refractive index (R) to 1.50-0.01 i (1.33 for demineralized water dispersion medium) and refractive index (B) to 1.50-0.01i (1.33 for demineralized water dispersion medium). Laser scattering measurements were conducted at room temperature (20° C. to 25° C.).

[0264] The measurement chamber of the LA-960 was filled to an extent of three quarters with demineralized water (maximum fill level). Then the stirrer was started at the set speed, the circulation was switched on and the water was degassed. Subsequently, a zero measurement was conducted with the parameters specified.

[0265] A disposable pipette was then used to take a 0.5-3.0 mL sample centrally from the sample prepared according to point 1.1 of example 1 immediately after the ultrasound treatment. Subsequently, the complete contents of the pipette were introduced into the measurement chamber, such that the transmittance of the red laser was between 80% and 90% and the transmittance of the blue laser was between 70% to 90%. Then the measurement was started. The measurements were evaluated in an automated manner on the basis of the parameters specified.

[0266] For the silica fume particles examined from Si production (RW filler sieved), a particle size distribution was ascertained with a median of 0.23 micrometer, rounded to the second post-decimal place.

[0267] For the silica fume particles examined from ZrO.sub.2 production (RW filler Q1 Plus), a particle size distribution was ascertained with a median of 0.84 micrometer, rounded to the second post-decimal place.

1.3 Determination of Optimal Sonication Time

[0268] The optimal duration of ultrasound sonication, which is dependent on the type of sample, was ascertained by conducting a measurement series with different sonication times for each species of particulate silicon dioxide. This was done by extending the sonication time, starting from 10 seconds, by 10 seconds each time for every further sample, and determining the respective particle size distribution by means of laser scattering (LA-960) immediately after the end of sonication, as described in point 1.2 of example 1. With increasing duration of sonication, the median ascertained in the particle size distribution fell at first, until it ultimately rose again at longer sonication times. For the ultrasound sonications described in point 1.1 of example 1, the sonication time chosen was that at which, in these measurement series, the lowest median of the particle size distribution was determined for the respective particle species; this sonication time is the “optimal” sonication time.

EXAMPLE 2

Production of Test Bars

[0269] This example describes, by way of example, the production of test bars (moldings); the dimensions of the test bars are merely by way of example, and the selection of the substances used is also merely illustrative of further substances to be used in accordance with the invention.

2.1 Production of Molding Material Mixtures

[0270] For the purposes of this example, RW filler (having a particle size distribution with a median of 0.23 micrometer, rounded to the second post-decimal place, determined by means of light scattering; by way of example of a particulate synthetic amorphous silicon dioxide to be used in accordance with the invention having a particle size distribution with a median in the range from 0.1 to 0.4 micrometer, determined by means of laser scattering) and Q1 Plus (having a particle size distribution with a median of 0.84 micrometer, rounded to the second post-decimal place, determined by means of light scattering; by way of example of a particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 micrometers, determined by means of laser scattering) were mixed together in dry form; the amounts added are apparent from table 1. The resulting pulverulent mixture of RW filler sieved and RW filler Q1 Plus was mixed manually with H31 sand (quartz sand; from Quarzwerke GmbH, AFS grain fineness number 46).

[0271] Then a water glass-based liquid binder having a solids content of about 36.2% by weight, a molar modulus of about 2.1 and an Na.sub.2O to K.sub.2O ratio (molar) of about 7.7, and containing 2.0% by weight of HOESCH EHS 40 (from Hoesch; ethylhexyl sulfate, active content about 40.0% to 44.0%; CAS No. 126-92-1) was added, and all components were mixed with one another in a bull mixer (model: RN 10/20, from Morek Multiserw) at 220 revolutions per minute for 120 s.

[0272] By way of example, noninventive and inventive mixtures were produced with the proportions by weight of the components used that are specified in table 1.

TABLE-US-00001 TABLE 1 Proportion of RW filler sieved in the Addition of Addition of total amount of RW Addition of Addition of RW filler RW filler filler sieved and RW sand binder sieved Q1 Plus filler Q1 Plus in the Mixture (parts by (parts by (parts by (parts by molding material mixture no. weight) weight) weight) weight) (percent) 1 100 2.2 0.80 0.00 100 2 100 2.2 0.76 0.04 95 3 100 2.2 0.72 0.08 90 4 100 2.2 0.64 0.16 80 5 100 2.2 0.60 0.20 75 6 100 2.2 0.48 0.32 60 7 100 2.2 0.40 0.40 50 8 100 2.2 0.32 0.48 40 9 100 2.2 0.20 0.60 25 10 100 2.2 0.16 0.64 20 11 100 2.2 0.08 0.72 10 12 100 2.2 0.04 0.76 5 13 100 2.2 0.00 0.80 0

2.2 Production of Test Bars

[0273] Molding material mixtures produced according to point 2.1 of example 2 were formed to test bars having the dimensions of 22.4 mm×22.4 mm×185 mm. For this purpose, the respective molding material mixtures were introduced with compressed air (4 bar) and a shooting time of 3 seconds into a mold for test bars having a temperature of 180° C. Subsequently, the test bars were hot-cured at 180° C. for 30 seconds, while additionally being aerated with heated ambient air at an aeration pressure of 2 bar and an aeration and aeration hose temperature of 180° C. Thereafter, the mold was opened, and the cured test bars were removed and stored for cooling.

EXAMPLE 3

Determination of Core Weight

[0274] This example describes, merely by way of example, the determination of the core weight of test bars (moldings).

[0275] Test bars with mixture numbers 1, 2, 3, 5, 7, 9, 11, 12, 13 that had been produced according to example 2, after a cooling time of about one hour, were weighed on a laboratory balance. Results are shown in table 2, with the respective core weight figure corresponding to an average from 9 individual measurements. The mixture number in table 2 corresponds to the mixture number in table 1, such that an identical mixture number in this respect means an identical composition of the molding material mixture.

TABLE-US-00002 TABLE 2 Core weight Mixture no. (grams) 1 148.3 2 149.2 3 149.8 5 151.8 7 154.0 9 155.9 11 156.6 12 157.0 13 157.3

EXAMPLE 4

Determination of Moisture Resistance

[0276] This example describes, merely by way of example, the determination of the moisture resistance (moisture stability) of test bars (moldings).

4.1 Determination of One-Hour Strength

[0277] Test bars that have been produced according to example 2 (mixture numbers: 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13), after a cooling time of one hour, were introduced into a Georg Fischer strength tester, equipped with a 3-point bending device (from Morek Multiserw), and the force that led to fracture of the test bar was measured. The value read off (in N/cm.sup.2) indicates the one-hour strength.

4.2 Determination of Absolute Residual Strength After 22 h in a Climate-Controlled Cabinet

[0278] Test bars produced according to example 2 (mixture numbers according to example 4.1), after a cooling time of one hour, were stored for 22 hours under controlled conditions of 30° C. and 75% relative humidity in a climate-controlled cabinet (VC 0034, from Vötsch).

[0279] Thereafter, absolute residual strength was determined by introducing the respective test bars into a Georg Fischer strength tester, equipped with a 3-point bending device (from Morek Multiserw), and measuring the force that led to fracture of the test bars. The value read off (in N/cm.sup.2) indicates the absolute residual strength. For cores that had already fractured before the 22 h had elapsed, an absolute residual strength of 0 N/cm.sup.2 was assumed.

4.3 Determination of Moisture Resistance

[0280] For the determination of moisture resistance, for each mixture number, an average of a total of 6 measurements of absolute residual strength (example 4.2) was formed and divided by the average of 3 measurements of one-hour strength (example 4.1). The value thus obtained was multiplied by 100%; the result is the moisture resistance. Values of moisture resistance ascertained in this way are reported in table 3. The mixture number in table 3 corresponds to the mixture number in table 1, such that an identical mixture number means an identical composition of the molding material mixture.

TABLE-US-00003 TABLE 3 Moisture resistance Mixture no. (percent) 1 42 3 41 4 37 5 42 6 40 7 36 8 36 9 29 10 29 11 24 13 4

EXAMPLE 5

Synergistic Effect

[0281] The results from example 3, table 2, and example 4, table 3, are summarized hereinafter in an overview table 4. The overview table 4 is accompanied by a diagram according to FIG. 1 created from the table.

TABLE-US-00004 TABLE 4 Proportion of RW filler sieved in the total amount of RW filler sieved and RW filler Q1 Plus in the Moisture Mixture molding material mixture Core weight resistance no. (percent) (grams) (percent) 1 100 148.3 42 2 95 149.2 — 3 90 149.8 41 4 80 — 37 5 75 151.8 42 6 60 — 40 7 50 154.0 36 8 40 — 36 9 25 155.9 29 10 20 — 29 11 10 156.6 24 12 5 157.0 — 13 0 157.3  4

[0282] It is apparent from overview table 4 and the accompanying FIG. 1 that, generally, for a ratio of the total mass of particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, namely the material RW filler Q1 Plus (particle size distribution with a median of 0.84 μm rounded to the second post-decimal place), to the total mass of particulate synthetic amorphous silicon dioxide in a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, namely RW filler sieved (particle size distribution with a median of 0.23 μm rounded to the second post-decimal place), advantageous values are in the range from 20:1 to 1:20, since there is a significant double synergistic effect within this range, which is manifested in an unexpectedly high (synergistically elevated) moisture resistance and simultaneously an unexpectedly high (synergistically elevated) relative molding weight (core weight here) (the respective measurements are each higher than the values expected). Preferably, the values are in the range from 5:1 to 1:20, more preferably in the range from 3:1 to 1:20, especially preferably in the range from 2:1 to 1:20, most preferably in the range from 1.5:1 to 1:20. Particular preference is therefore given to a proportion of at least 40% by weight of the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering (in the example, this is RW filler sieved having a particle size distribution with a median of 0.23 μm rounded to the second post-decimal place), based on the total mass of the two types used.

[0283] Corresponding products thus firstly ensure high storage stability (especially stability against the action of moisture) and secondly high compaction of the formed molding material mixture, which leads to a high-quality surface containing few defects in the hot-cured molding obtained therefrom, which in turn leads to a high-quality surface containing few defects in metallic cast parts produced in the inventive manner that has come into contact with the hot-cured molding in the casting operation.

EXAMPLE 6

Comparative Studies

6.1 General Pointers for Understanding of the Studies

[0284] This example relates to comparative studies on a total of 15 different molding material mixtures specified in table 5. More particularly, inventive experiments were compared with noninventive experiments that were conducted in accordance with WO2009/056320 A1.

[0285] Studies in accordance with the invention are those with molding material mixtures 1.3, 2.3, 3.3 and 4.3 according to table 5. All other molding material mixtures are not in accordance with the invention.

[0286] In all molding material mixtures examined, the same quartz sand and the same alkali metal water glass were used in equal amounts in each case; cf. table 5 and the details of the composition of the alkali metal water glass specified in the accompanying footnote 1.

[0287] Particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, that was used in the total of 10 molding material mixtures 1.1, 1.3, 2.1, 2.3, 3.1, 3.3, 4.1, 4.3, 5.1 and 5.3 was Elkem Microsilica® 971 U. As stated in footnote 5 relating to table 5, the median of the particle size distribution (rounded to the second post-decimal place) was 0.20 μm, according to the determination method from example 1. The optimal sonication time (cf. point 1.3 in example 1) ascertained was 1020 seconds.

[0288] In the total of eight molding material mixtures 1.2, 1.3, 2.2, 2.3, 3.2, 3.3, 4.2 and 4.3, the particulate amorphous silicon dioxide having a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering, that was used was an RW filler Q1 Plus; according to example 1.2, this material had a particle size distribution with a median of 0.84 micrometer rounded to the second post-decimal place.

[0289] In the two molding material mixtures 5.2 and 5.3 (alongside Elkem Microsilica® 971 U in molding material mixture 5.3), the particulate synthetic amorphous silicon dioxide having a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, that was used was an RW filler sieved; according to example 1.2, this material had a particle size distribution with a median of 0.23 micrometer rounded to the second post-decimal place.

[0290] No surfactant was used in molding material mixtures 1.1 to 1.3; a total of three different surfactants were used in the further molding material mixtures, always in the same amounts. For physical details of the surfactants, reference is made to footnotes 2, 3 and 4 of table 5.

[0291] Studies were conducted on 5 groups of molding material mixtures (1.1 to 1.3, 2.1 to 2.3, 3.1 to 3.3, 4.1 to 4.3 and 5.1 to 5.3): [0292] The first of the studies in each group (molding material mixtures 1.1, 2.1, 3.1, 4.1, 5.1) in each case relates to a molding material mixture comprising solely Elkem Microsilica® 971 U as the only particulate synthetic amorphous silicon dioxide. [0293] The second of the studies in each group relates in each case to a molding material mixture without Elkem Microsilica® 971 U, but with either RW filler Q1 Plus (molding material mixtures 1.2, 2.2, 3.2, 4.2) or RW filler sieved (molding material mixture 5.2) as the only particulate synthetic amorphous silicon dioxide. [0294] The third of the studies in each group relates in each case to a molding material mixture with Elkem Microsilica® 971 U and additionally either RW filler Q1 Plus (molding material mixtures 1.3, 2.3, 3.3, 4.3) or RW filler sieved (molding material mixture 5.3).

[0295] In molding material mixtures 1.3, 2.3, 3.3, 4.3, two species of particulate synthetic amorphous silicon dioxide were used in each case, of which one species (Elkem Microsilica 971 U) had a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering, and the other species (RW filler Q1 Plus) a particle size distribution with a median in the range from 0.7 to 1.5 μm, determined by means of laser scattering.

[0296] In molding material mixture 5.3, two species of particulate synthetic amorphous silicon dioxide were used, each of which has a particle size distribution with a median in the range from 0.1 to 0.4 μm, determined by means of laser scattering.

6.2 Production of the Molding Material Mixtures

[0297] For production of the molding material mixtures defined in table 5, the alkali metal water glass and any surfactant (surface-active substance) were added to the initial charge of H32 quartz sand. The mixture was stirred at 200 revolutions per minute in a bull mixer (model: RN 10/20, from Morek Multiserw) for 1 minute. Thereafter, the particulate amorphous silicon dioxide was added and the resulting mixture was then stirred in the bull mixer for a further minute.

TABLE-US-00005 TABLE 5 Particulate amorphous silicon dioxide Elkem Molding H32 Alkali metal Microsilica ® Further particulate material quartz sand water glass.sup.1 Surfactant 971 U.sup.5 amorphous silicon dioxide Inventive mixture PW.sup.8 PW.sup.8 Name PW.sup.8 PW.sup.8 Name PW.sup.8 yes/no 1.1 100 2.0 — — 0.50 — — no 1.2 100 2.0 — — — Q1 Plus.sup.6 0.50 no 1.3 100 2.0 — — 0.25 Q1 Plus.sup.6 0.25 yes 2.1 100 2.0 EHS.sup.2 0.05 0.50 — — no 2.2 100 2.0 EHS.sup.2 0.05 — Q1 Plus.sup.6 0.50 no 2.3 100 2.0 EHS.sup.2 0.05 0.25 Q1 Plus.sup.6 0.25 yes 3.1 100 2.0 Melpers.sup.3 0.05 0.50 — — no 3.2 100 2.0 Melpers.sup.3 0.05 — Q1 Plus.sup.6 0.50 no 3.3 100 2.0 Melpers.sup.3 0.05 0.25 Q1 Plus.sup.6 0.25 yes 4.1 100 2.0 SOS.sup.4 0.05 0.50 — — no 4.2 100 2.0 SOS.sup.4 0.05 — Q1 Plus.sup.6 0.50 no 4.3 100 2.0 SOS.sup.4 0.05 0.25 Q1 Plus.sup.6 0.25 yes 5.1 100 2.0 SOS.sup.4 0.05 0.50 — — no 5.2 100 2.0 SOS.sup.4 0.05 RW filler 0.50 no sieved.sup.7 5.3 100 2.0 SOS.sup.4 0.05 0.25 RW filler 0.25 no sieved.sup.7 .sup.1Alkali metal water glass with molar modulus (SiO.sub.2:M.sub.2O with M = Na, K) of about 2.2; about 36.2% by weight of solids and a molar ratio of Na.sub.2O to K.sub.2O of about 3.6:1.0. .sup.22-Ethylhexyl sulfate in water (from Hoesch) .sup.3Melpers ® VP 4547/240 L (modified polyacrylate in water, from BASF) .sup.4Texapon ® 842 UP (sodium octylsulfate in water, from BASF) .sup.5Elkem Microsilica ® 971 U (pyrogenic silica; production in an arc furnace; median of particle size distribution determined by means of laser scattering 0.20 micrometer, determination according to example 1). .sup.6RW filler Q1 Plus (from RW Silicium GmbH, silica fume from ZrO.sub.2 production; median of particle size distribution determined by means of laser scattering 0.84 micrometer, determination according to example 1). .sup.7RW filler sieved (from RW Silicium GmbH, silica fume from SiO.sub.2 production; median of particle size distribution determined by means of laser scattering 0.23 micrometer, determination according to example 1). .sup.8PW means part(s) by weight

6.3 Production of Test Bars

[0298] Molding material mixtures of the respective compositions specified in table 5 that had been produced according to point 6.2 were formed to test bars having the dimensions of 22.4 mm×22.4 mm×185 mm. For this purpose, the respective molding material mixtures were introduced with compressed air (2 bar) into a mold for test bars at a temperature of 180° C., and remain in the mold for a further 50 seconds. For acceleration of the curing of the mixtures, hot air (3 bar, 150° C.) was passed through the mold for the last 20 seconds. Thereafter, the mold was opened and the test bar (22.4 mm×22.4 mm×185 mm) was removed.

[0299] The test bars were used in studies according to points 6.4 to 6.7 below; the noninventive test bars based on the group of molding material mixtures 5.1 to 5.3 were used only in the study according to 6.5 (determination of core weight).

6.4 Determination of Heat Resistance

[0300] Immediately after removal from the mold, test bars produced according to point 6.3 were introduced into a Georg Fischer strength tester, equipped with a 3-point bending device (from Morek Multiserw). 10 seconds after the mold had been opened, the force that led to fracture of the test bars was measured. The value read off (in N/cm.sup.2) indicates the hot strength. Table 6 gives the results of the measurements of hot strength; the values reported are medians from 3 measurements in each case.

6.5 Determination of Core Weight

[0301] Test bars produced according to point 6.3, after a cooling time of about one hour, were weighed on a laboratory balance. Results are shown in table 6, with the respective core weight figure corresponding to a median from 9 individual measurements.

6.6. Determination of One-Hour Strength

[0302] Test bars produced according to point 6.3, after removal from the mold, were placed horizontally on a frame such that they rested on the frame only in the region of the two ends of their longest extent, and the test bars spanned a range of about 16 cm without contact between the contact surfaces. After a cooling time of 1 hour after removal from the mold, the test bars were introduced into a Georg Fischer strength tester, equipped with a 3-point bending device (from Morek Multiserw), and the force that led to fracture of the test bars was measured. The value read off (in N/cm.sup.2) indicates the one-hour strength. Results are shown in table 6, with the values reported being medians from 3 individual measurements in each case.

6.7 Determination of Residual Strength After 3 Hours and Relative Residual Strength After 3 Hours

[0303] Test bars produced according to point 6.3, after removal from the mold, were cooled down under ambient conditions in the laboratory for one hour as described in point 6.6, and then, mounted on the same frame, stored under controlled conditions of 30° C. and 75% relative humidity in a climate-controlled cabinet (VC 0034, from \kitsch) for 3 hours (3 h).

[0304] Thereafter, (absolute) residual strength after 3 hours was determined by placing the respective test bars in a Georg Fischer strength tester, equipped with a 3-point bending device (from Morek Multiserw), and measuring the force that led to fracture of the test bars. The value read off (in N/cm.sup.2) indicates the (absolute) residual strength after 3 hours.

[0305] For cores that had already fractured before the 3 hours had elapsed, an absolute residual strength of 0 N/cm.sup.2 was noted. Results are shown in table 6, with the values reported being medians from 3 individual measurements in each case.

[0306] For the determination of relative residual strength after 3 hours, the values of absolute residual strength after 3 hours were each divided by the corresponding values of one-hour strength. The values thus obtained were multiplied by 100%; the respective result is the relative residual strength after 3 hours. The results are reported in table 6.

6.8 RESULTS

[0307] Selected results of the measurements from 6.4 to 6.7 are shown in FIGS. 2 to 6 (see the above elucidations for the figures). The results of all measurements from 6.4 to 6.7 are additionally summarized in table 6; for reasons of clarity, measurements therein are rounded to the first post-decimal place. The numbers of the molding material mixtures in table 6 correspond to those in table 5.

TABLE-US-00006 TABLE 6 Relative Residual residual strength strength after 3 h after 3 h Molding Core Hot One-hour (30° C./ (30° C./ material weight strength strength 75% RH) 75% RH) mixture [g] [N/cm.sup.2] [N/cm.sup.2] [N/cm.sup.2] [N/cm.sup.2] 1.1 136.0 112 257 101 39 1.2 146.7 88 317 0 0 1.3 143.4 112 275 95 35 2.1 140.6 135 341 197 58 2.2 148.7 141 432 167 39 2.3 146.2 140 402 197 49 3.1 137.9 146 378 239 63 3.2 148.2 153 461 185 40 3.3 144.5 158 426 233 55 4.1 140.1 141 317 124 39 4.2 148.1 146 411 59 14 4.3 145.4 123 356 119 33 5.1 140.1 — — — — 5.2 141.4 — — — — 5.3 140.7 — — — —

[0308] It is apparent from table 6 and from FIGS. 2 to 5 corresponding to the groups of 3 molding material mixtures (1.1-1.3 to 4.1-4.3) that, in each case of joint use of Elkem Microsilica® 971 U (particulate amorphous silicon dioxide having a particle size distribution with a median of 0.20 micrometer, i.e. in the range from 0.1 to 0.4 μm, determined by means of laser scattering) and of RW filler Q1 Plus (particulate amorphous silicon dioxide having a particle size distribution with a median of 0.84 micrometer, i.e. in the range from 0.7 to 1.5 μm, determined by means of laser scattering), the core weight of the test bars produced is surprisingly high, namely higher than the linear combination of the values for test bars comprising Elkem Microsilica ® 971 U alone or RW filler Q1 Plus alone (linear combination shown by the dashed line in each case).

[0309] A significant double synergistic effect is found in each case, which is manifested in the unexpectedly high (synergistically increased) relative shaped body weight (core weight) and a simultaneously unexpectedly high (synergistically increased) relative residual strength after 3 hours.

[0310] It is apparent from table 6 and from FIG. 6 corresponding to the group of 3 molding material mixtures 5.1-5.3 that, for molding material mixture 5.3, i.e. in the case of joint use of Elkem Microsilica ® 971 U (particulate amorphous silicon dioxide having a particle size distribution with a median of 0.20 micrometer, i.e. in the range from 0.1 to 0.4 μm, determined by means of laser scattering) and of RW filler sieved (particulate amorphous silicon dioxide having a particle size distribution with a median of 0.23 micrometer, i.e. likewise in the range from 0.1 to 0.4 μm, determined by means of laser scattering), the core weight of the test bars produced is not higher than the linear combination of the values for test bars comprising Elkem Microsilica® 971 U alone (molding material mixture 5.1) or RW filler sieved alone (molding material mixture 5.2) (linear combination shown by the dashed line in each case); no double synergistic effect can be observed.

[0311] The surprising advantages of the invention are especially apparent by comparison with experiments relating to the noninventive molding material mixtures 1.1, 2.1, 3.1, 4.1, 5.1 that were conducted in accordance with WO2009/056320 A1. The core weight of molding material mixtures of the invention is significantly higher in each case; at the same time, relative residual strength after 3 hours is not reduced to a degree of relevance for industrial practice (double synergistic effect).