USE OF A COATING COMPOSITION CONTAINING AN ACID IN THE FOUNDRY INDUSTRY

Abstract

The present invention describes the use of a coating composition, comprising an aqueous phase having a pH of at most 5 and one or more refractories, in the foundry industry and also coated, waterglass-bound foundry molding elements, especially foundry molds and/or foundry cores, which each comprise such an aforementioned coating composition. The invention further describes a method for producing a coated, waterglass-bound foundry molding element. The invention likewise describes a kit whose contents include an aforementioned coating composition, comprising aqueous acid and one or more refractories.

Claims

1. The method of a coating composition comprising (a) one or more refractories, and (b) an aqueous phase having a pH of at most 5, for producing a coating on a waterglass-bound mold or on a waterglass-bound core, for use in the foundry, where the waterglass-bound mold or the waterglass-bound core comprises particulate, amorphous silicon dioxide.

2. The method as claimed in claim 1, where the coated, waterglass-bound mold or the coated, waterglass-bound core, in comparison to a coated, waterglass-bound comparative mold or a coated, waterglass-bound comparative core produced, under otherwise identical conditions, using a comparative coating composition obtained from the coating composition by adding sodium hydroxide until a pH of 7 is reached, possesses a flexural strength which decreases less on drying and/or possess an increased storage stability.

3. The method as claimed in claim 1, where the aqueous phase (b) comprises (b1) water, and (b2) one or more acids, preferably having a pKa<5, more preferably having a pKa<4, where preferably the ratio of the mass of constituent (b1) to the mass of constituent (b2) is in the range from 10:1 to 200:1, more preferably in the range from 10:1 to 100:1, and/or where preferably the ratio of the mass of constituent (b1) to the total mass of the aqueous phase (b) is greater than 50%, more preferably greater than 70%, very preferably greater than 90%, and/or where preferably the aqueous phase possesses a pH of at most 4.

4. The method as claimed in claim 3, where the constituent (b2) comprises one or more acids which are selected from the group consisting of inorganic and organic acids, where the organic acids are preferably selected from the group consisting of mono-, di-, and tricarboxylic acids, preferably mono-, di-, and tricarboxylic acids which are solid at 25° C. and 1013 mbar, more preferably citric acid and oxalic acid, and/or where the inorganic acids are preferably selected from the group consisting of hydrochloric acid, nitric acid, phosphoric acid, and acidic phosphates, more preferably from the group consisting of hydrochloric acid, nitric acid, and phosphoric acid, and/or where the ratio of the total mass of inorganic and organic acids of constituent (b2) to the total mass of the coating composition is in the range from 0.1 to 10%, preferably in the range from 0.5 to 5%, more preferably in the range from 1 to 5%, very preferably in the range from in the range from 1 to 3.5%, and especially preferably in the range from 2.5 to 3.5%.

5. The method as claimed in claim 1, where the constituent (a) comprises particulate, amorphous silicon dioxide, preferably particulate, amorphous silicon dioxide whose primary particles (i) are spherical and/or (ii) possess a D90<10 μm, preferably a D90 of <1 determined by laser diffraction, more preferably particulate, amorphous silicon dioxide which comprises as a secondary constituent (i) zirconium dioxide and/or (ii) a Lewis acid, and/or one or more substances selected from the group consisting of quartz, aluminum oxide, zirconium dioxide, aluminum silicates, phyllosilicates, zirconium silicates, olivine, talc, mica, graphite, coke, feldspar, diatomite, kaolins, calcined kaolins, metakaolinite, iron oxide, and bauxite.

6. The method as claimed in claim 1, where the coating composition comprises one or more or all of the following constituents: one or more biocides, one or more wetting agents, one or more rheological additives, and one or more binders, preferably polyvinyl alcohol.

7. The method as claimed in claim 1, where the coating composition possesses a solids content of less than 80 wt %, preferably less than 45 wt %, based on the total mass of the coating composition, and/or where the coating composition comprises one or more binders, preferably comprising polyvinyl alcohol, in a total amount of not more than 2 wt %, preferably in an amount in the range from 0.05 to 0.80 wt %, based on the total mass of the coating composition.

8. The method as claimed in claim 1, where the coating composition is applied to a waterglass-bound mold or a waterglass-bound core for use in the casting of a metal melt with a temperature >900° C., preferably >1250° C., preferably for use in the casting of a metal melt comprising iron and/or steel, and/or where the coating composition is applied to a waterglass-bound mold or a waterglass-bound core for use in the casting of iron or steel and/or where the coating composition is applied to a waterglass-bound mold or a waterglass-bound core at a temperature of the waterglass-bound core or the waterglass-bound mold of >50° C., preferably >70° C., more preferably at a temperature <100° C.

9. The method of acid for setting a pH of at most 5 in the aqueous phase of a coating composition for application to a waterglass-bound mold or a waterglass-bound core.

10. The method as claimed in claim 9, where the acid is selected from the group consisting of inorganic and organic acids, where the organic acids are preferably selected from the group consisting of mono-, di-, and tricarboxylic acids, preferably mono-, di-, and tricarboxylic acids which are solid at 25° C. and 1013 mbar, more preferably citric acid and oxalic acid, and/or where the inorganic acids are preferably selected from the group consisting of hydrochloric acid, nitric acid, phosphoric acid, and acidic phosphates, more preferably from the group consisting of hydrochloric acid, nitric acid, and phosphoric acid, and/or where the waterglass-bound mold or the waterglass-bound core comprises particulate, amorphous silicon dioxide, where the acid is used preferably for setting a pH of at most 4.

11. A process for producing a coated, waterglass-bound mold with high storage stability, or a coated, waterglass-bound core with high storage stability, for use in the foundry, comprising the following steps: (1) providing or producing a coating composition as defined in claim 1, (2) providing or producing an uncoated, waterglass-bound mold or an uncoated, waterglass-bound core, and (3) applying the provided or produced coating composition from step (1) to the provided or produced mold or the provided or produced core.

12. The process as claimed in claim 11, where the provided or produced mold or the provided or produced core comprises particulate, amorphous silicon dioxide.

13. The process as claimed in claim 11, where application to the mold takes place at a temperature of the core or of the mold of >50° C., preferably >70° C., more preferably at a temperature <100° C.

14. A coated mold or coated core for use in the foundry, in each case comprising (X) a waterglass-bound mold or a waterglass-bound core, and (Y) a coating comprising a coating composition as defined in claim 1.

15. A coated mold or coated core, producible by the process as claimed in claim 11.

16. The coated mold or coated core as claimed in claim 14, where the waterglass-bound mold and/or the waterglass-bound core comprises particulate, amorphous silicon dioxide.

17. The coated mold or coated core as claimed in claim 14 for use in the casting of a metal melt having a temperature >900° C., preferably for use in the casting of a metal melt comprising iron and/or steel.

18. A kit including in separate components (U) a coating composition comprising (a) one or more refractories, and (b) an aqueous phase having a pH of at most 5 for producing a coating on a waterglass-bound mold or a waterglass-bound core, for use in the foundry, (V) a binder comprising waterglass, and (W) particulate, amorphous silicon dioxide.

19. A kit, comprising as component (U) a coating composition, where the coating composition is as defined in claim 1, (V) a binder comprising waterglass, and (W) particulate, amorphous silicon dioxide.

Description

EXAMPLES

[0130] The examples given below are intended to describe and explain the invention in more detail without limiting its scope.

Example 1: Production of Coating Compositions

[0131] The inventive coating composition indicated in Table 1 (“SZ1”) and also the noninventive, comparative coating compositions (“SZ2” and “SZ3”) were produced in a conventional way by mixing the respectively indicated ingredients with one another:

[0132] For this purpose in each case the required amount of water was introduced initially in a glass beaker (batch size in each case around 2 kg of coating composition as “concentrate”, cf. Table 1), the rheological additives and the refractories (phyllosilicates, zircon flour, graphite) were added, and these ingredients were then incorporated in a conventional way using a high-shear dissolver stirrer for 3 minutes. The other constituents of the coating compositions (cf. Table 1) were then added in the proportions indicated, followed by stirring for a further 2 minutes with a high-shear dissolver stirrer. This gave the dilutable refractory coating-composition concentrates indicated in Table 1 in each case.

[0133] The references to “DIN grinds” in Table 1 denote that the respectively indicated constituent of the coating composition is present in the ground state and, after the sieving of a sample of this constituent with an analytical sieve having a nominal mesh size in μm corresponding to the stated numerical value (e.g.: “80” denotes “analytical sieve with mesh size 80 μm”) (according to DIN ISO 3310-1:2001-09), a residue remains in each case that is in the range from 1 to 10 wt %, based on the amount of sample used.

TABLE-US-00001 TABLE 1 Inventive and noninventive coating compositions, each obtained as dilutable “concentrates” Coating compositions (“concentrates”): SZ1 SZ2 SZ3 Ingredients: [wt %] [wt %] [wt %] Water 44.1 46.0 47.1 Rheological additive 1.5 5.0 1.5 Phyllosilicate (pyrophyllite, ./. 26.0 11.0 DIN 140 grind) Phyllosilicate (mica, DIN 160 25.0 ./. 18.0 grind) Zircon flour (zirconium silicate, 14.0 9.0 10.0 DIN 60 grind) Graphite (DIN 80 grind) 11.0 8.0 11.0 Polyvinyl acetate ./. 0.9 ./. Biocide (benzisothiazolinone, 0.3 0.3 0.3 10% wt/wt aqueous solution) Modified starch ./. 0.3 ./. Polyvinyl alcohol 0.4 ./. 0.4 Iron oxide (yellow) ./. 1.2 ./. Wetting agent 0.6 0.3 0.6 Defoamer 0.1 ./. 0.1 Propylene carbonate ./. 3.0 ./. Citric acid 3.0 ./. ./. TOTAL: 100.0 100.0 100.0

[0134] The dilutable refractory coating-composition concentrates indicated in Table 1 above were subsequently diluted with water to produce coating compositions ready for use for the purpose intended here (for application to molds and/or cores by means of a dipping operation, preferably in the form of a dipping bath). The respective dilution employed and also other properties of the ready-to-use coating compositions resulting in each case from the dilution employed are indicated below in Table 1a:

TABLE-US-00002 TABLE 1a Production and properties of ready-to-use (for dipping bath or dipping tank) coating compositions Coating compositions (ready to use for dipping tank or dipping bath): SZ1 SZ2 SZ3 Concentrate (as per Table 1), 100.0 100.0 100.0 parts by weight: Water, parts by weight 40.0 30.0 40.0 Properties of the ready-to-use coating compositions resulting from the above dilution: Density [g/ml] 1.32 1.36 1.35 Flow time [s] 13.5 13.7 13.3 pH 2.1 7.2 6.7

[0135] As is apparent from Table 1a, the coating compositions for the purpose intended here, application to test cores by means of a dipping application or a dipping bath, were produced in such a way as to ensure easy comparability (i) of their respective properties on application to the test cores and also (ii) of the respectively resulting properties of the coated test cores (densities and flow times were set to be as similar as possible; but differing pH for inventive coating composition SZ1 relative to noninventive coating compositions SZ2 and SZ3).

[0136] The densities of the ready-to-use coating compositions, indicated in Table 1a, were measured according to the standard test method DIN EN ISO 2811-2 (method A).

[0137] The flow times of the ready-to-use coating compositions, indicated in Table 1a, were measured according to the standard test method DIN 53211 (1974) by determination with the DIN 4 cup.

[0138] The pH values of the ready-to-use coating compositions, indicated in Table 1a, were measured in accordance with the standard test method DIN 19260:2012-10 in each case from the suspension.

[0139] The coating compositions SZ1 and SZ3 each contained attapulgite as rheological additive. Coating composition SZ2 is of the type described in document WO00/05010.

Example 2: Investigation of the Softening of Foundry Cores

[0140] To determine the softening of foundry cores (i.e., the maximum drop in flexural strength), test cores (test specimens) were produced (in accordance with the “core system 1” indicated in Table 4) conventionally in a core shooting machine from Multiserw (model LUT, gassing pressure: 2 bar, shot time: 3.0 s; shooting pressure 4.0 bar). An hour after the core production, the test cores were coated (coated) with the above-stated ready-to-use coating compositions “SZ1”, “SZ2” and “SZ3” (cf. Table 1a) at room temperature (25° C.) by dipping (conditions: 1 s immersion; 3 s hold time in the coating composition, 1 s removal). The wet film thickness of the refractory coatings was adjusted in each case to about 250 μm. Thereafter the coated test cores were dried in a forced-air oven under the conditions indicated below (1 hour at 120° C.), and in each case the change in their flexural strength under the drying conditions was investigated.

[0141] The coated test cores were each dried over a period of an hour, during which their flexural strengths (in N/cm.sup.2, corresponding to the definition as indicated in data sheet R 202 of the Verein Deutscher Gießereifachleute, October 1978 edition) was measured at different times during the drying and then once more one hour after the end of the drying operation using a standard testing instrument of the type “Multiserw-Morek LRu-2e”, in each case with a standard measurement program “Rg1v_B 870.0 N/cm.sup.2” (3-point bending strength).

[0142] Table 2, for the coated test cores investigated, reports in each case the values for the maximum dropping of flexural strength under drying conditions, in %, based in each case on the flexural strength of the respective freshly coated (still wet) test core before the start of drying (initial value).

TABLE-US-00003 TABLE 2 Drop in strength of coated test cores under drying conditions Type of refractory Maximum drop in flexural Observation of coating on strength on drying, core failure test core as % of the initial value during drying SZ1 80 No SZ2 25 No SZ 3 0 Yes

[0143] The expression “core failure” denotes here and below in each case the invalidation of a coated core during the drying procedure, meaning that the coated core was in each case unusable for the measurement of flexural strength and also for a subsequently envisaged casting procedure.

[0144] From the values reported in Table 2 it can be seen, among other things, that the maximum drop in the flexural strength of a test core coated with an inventive coating composition (SZ1) was significantly smaller than with noninventive, comparative coating compositions (SZ2 and SZ3). It is further apparent from the values in Table 2 that with the noninventive, comparative coating composition SZ3 it was not possible under the selected conditions to produce any usable coated cores.

Example 3: Investigation of the Storage Stability of Coated and Uncoated Foundry Cores

[0145] To determine the storage stability, waterglass-bound test cores (test specimens) were produced in a conventional way (in the same way as described in Example 2) and their flexural strengths were determined in each case uncoated shortly after their production (one hour storage time, relative humidity in the range from 30 to 60%, storage temperature in the range from 20 to 25° C.) as indicated above; cf. Table 3 (entry “Uncoated after 1 h”).

[0146] Furthermore, corresponding test cores were coated as indicated below in Table 3 one hour after core production (i.e. after the same respective time interval from their production) at room temperature (25° C.) with the coating compositions SZ1 and SZ3, in each case by dipping (conditions: 1 s immersion; 3 s hold time in the coating composition, 1 s removal) (the coating compositions are designated as in Example 1) and dried in each case for an hour at 120° C. in a forced-air oven. The coated, dried test cores were then subjected to a storage test for a duration of four days (insofar as it was possible to produce the coated core or insofar as core failure was not observed before). The temperature during the storage was in each case 35° C.; the relative humidity was in each case 75%. After the end of the storage test, the flexural strengths of the test cores were determined as indicated above. The results of these storage tests are reported below in Table 3. The test cores (“core system 1”) used respectively for all of the tests in Example 3 are cores whose production conditions are specified below in Table 4.

TABLE-US-00004 TABLE 3 Determination of the storage stability of coated and uncoated foundry cores Uncoated Coated with Coated with Uncoated after type SZ1 after type SZ3 after on/during Core 1 h storage (4d ) storage (4 d) storage system Flexural strength [N/cm.sup.2] 1 300 149 not core failure after determinable 131 min.

[0147] From the values reported in Table 3 it can be seen, among other things, that a waterglass-bound test core coated with an inventive coating composition (SZ1) after four-day storage still had >40% of the initial strength, whereas a test core coated with a noninventive, comparative coating composition (SZ3) was unusable under comparable conditions; its flexural strength was no longer determinable under the conditions defined above, since it fell apart during storage. Under the test conditions, an uncoated comparative core failed after just 131 min., i.e., just the application of an inventive coating composition to a test core resulted in the test core being stabilized under drying conditions.

TABLE-US-00005 TABLE 4 Production conditions for core system 1 Parameter Core system 1 Molding material (100 parts by Silica sand weight) Binder (2.2 parts by weight) Alkali metal waterglass solution, 25-35 wt % waterglass content in water (wt/wt) Additive (1.0 part by weight) Particulate, amorphous silicon dioxide Core box temperature 120° C. Gassing temperature 150° C. Cure time  30 s

[0148] Core system 1 consisted only of the molding material, binder, and additive constituents, as indicated in Table 4:

[0149] The binder indicated in Table 4 for core system 1 was in this case a commercial alkali metal waterglass binder “Cordis® 8511” (Hüttenes-Albertus Chemische Werke GmbH).

[0150] The additive indicated in Table 4 for core system 1 was in this case a commercial binder additive whose main constituent 95 wt %) was particulate, amorphous silicon dioxide, “Anorgit® 8396” (Hüttenes-Albertus Chemische Werke GmbH).

Example 4: Investigation of the Flexural Strength of Coated Foundry Cores

[0151] Waterglass-bound test cores (test specimens) in each case with and without a content of particulate, amorphous silicon dioxide were produced in a conventional way (analogous to that described in Example 2, but after interim maintenance of the core shooting machine used) and their flexural strengths were determined for comparative purposes in each case uncoated shortly after their production (one hour storage time at a temperature in the range from 20 to 25° C., relative humidity 30 to 60%) as indicated above (for the production conditions of the test cores, see Table 6).

[0152] Furthermore, test cores as indicated below in Table 5 were coated by dipping (conditions: 1 s immersion; 3 s hold time in the coating composition, 1 s removal) (designation of the coating compositions as in Example 1) and dried in each case in a forced-air oven at 120° C. for an hour. After cooling to room temperature and a storage time of 24 hours (relative humidity in the range from 30 to 60%, temperature in the range from 20 to 25° C.), the flexural strengths were then determined as indicated above on the coated, dried test cores.

[0153] The results of the determinations of the flexural strengths are reported below in Table 5. In this case, three different test cores (core systems A, B and C) were used, the production conditions for each of which are reported below in Table 6.

TABLE-US-00006 TABLE 5 Determination of the flexural strengths of coated and uncoated foundry cores Uncoated Coated Coated after 1 h with type SZ1 with type SZ3 Core system Flexural strength [N/cm.sup.2] A 401 335 production of a coated core not possible B 413* 317 production of a coated core not possible C 347 269 production of a (without particulate, coated core not amorphous SiO2) possible * Deviation in the measured value from the corresponding value in Table 3 for core system 1 is interpreted essentially as a consequence of the maintenance on the core shooting machine.

[0154] From the values reported in Table 5 it can be seen that foundry cores coated with an inventive coating composition achieve high flexural strengths. Furthermore, the values reported in Table 5 show that with an inventive coating composition (SZ1), foundry cores produced under different conditions can be successfully coated with good success (high flexural strengths). With a noninventive, comparative coating composition (SZ3), on the other hand, under comparable conditions, it was not possible to produce usable coated cores.

TABLE-US-00007 TABLE 6 Production conditions for core systems A, B and C Parameter Core system A Core system B Core system C Molding material Silica sand (100.0 Silica sand (100.0 Silica sand (100.0 parts by weight) parts by weight) parts by weight) Binder Alkali metal Alkali metal Alkali metal waterglass solution, waterglass solution, waterglass solution, 25-35 wt % 25-35 wt % 25-35 wt % waterglass content in waterglass content in waterglass content in water (wt/wt) water (wt/wt) water (wt/wt) (2.2 parts by weight) (2.2 parts by weight) (3.2 parts by weight) Additive Particulate, Particulate, None amorphous silicon amorphous silicon dioxide dioxide (1.0 part by weight) (1.0 part by weight) Core box temperature 120° C. 120° C. 120° C. Gassing temperature 150° C. 150° C. 150° C. Cure time  50 s  30 s  50 s

[0155] The binders and additives indicated for the core systems A, B and C in Table 6 corresponded in each case to the binders (“Cordis® 8511”) and additives (“Anorgit® 8396”) indicated in relation to Table 4.

[0156] The above-stated core systems A, B and C consisted in each case only of the molding material, binder and optionally additive constituents, as indicated in Table 6.