COATING COMPOSITION FOR THE FOUNDRY INDUSTRY, CONTAINING PARTICULATE, AMORPHOUS SILICON DIOXIDE AND ACID

Abstract

A coating composition is described, for use in the foundry, in particular comprising particulate, amorphous silicon dioxide (SiO.sub.2) and an aqueous phase having a pH of at most 5, and also coated, waterglass-bound foundry molding elements, especially coated, waterglass-bound foundry molds and foundry cores, which each comprise a coating composition of the invention. Further described is the use of a coating composition of the invention for producing a coating on a waterglass-bound foundry molding element and a method for producing a waterglass-bound foundry molding element (mold or core) coated with a water-containing refractory coating. Likewise specified is a kit whose contents include a coating composition of the invention.

Claims

1. A coated, waterglass-bound mold or coated, waterglass-bound core for use in the foundry, comprising a coating composition comprising (a) an aqueous phase having a pH of at most 5, (b) particulate, amorphous silicon dioxide, and (c) one or more further refractories.

2. The coated mold or coated core as claimed in claim 1, where the waterglass-bound mold or the waterglass-bound core comprises particulate, amorphous silicon dioxide.

3. The coated mold or coated core as claimed in claim 2, where the primary particles of the particulate, amorphous silicon dioxide of constituent (b) (i) are spherical and/or (ii) possess a D90<10 μm, determined by laser diffraction, where the primary particles of the particulate, amorphous silicon dioxide of constituent (b) (i) are spherical and possess a sphericity of 0.9 or more, determined by evaluation of two-dimensional microscope images.

4. The coated mold or coated core as claimed in claim 1, where the constituent (c) comprises 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, and/or where the constituent (a) comprises one or more acids, having a pKa<5, which are selected from the group consisting of inorganic and organic acids, where the organic acids are selected from the group consisting of mono-, di-, and tricarboxylic acids, and/or where the inorganic acids are selected from the group consisting of hydrochloric acid, nitric acid, and phosphoric acid, and/or comprising in or as constituent (b) a particulate, amorphous silicon dioxide which as a secondary constituent comprises (i) zirconium dioxide, (ii) carbon and/or (iii) a Lewis acid and/or where the aqueous phase (a) has a pH of at most 4.

5. The coated mold or coated core as claimed in claim 1, comprising 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.

6. The coated mold or coated core as claimed in claim 1, where the ratio of the total mass of inorganic and organic acids in the aqueous phase (a) in relation to the total mass of the coating composition is in the range from 0.1 to 10%, and/or where the ratio of the water to the total mass of the aqueous phase of constituent (a) is greater than 50%, and/or where the coating composition possesses a solids content of less than 80 wt %, based on the total mass of the coating composition, and/or where the coating composition possesses a fraction of particulate, amorphous silicon dioxide of constituent (b) in the range from 1 to 30 wt %, based on the total mass of the coating composition and/or where the coating composition possesses a total fraction of particulate, amorphous silicon dioxide of constituent (b) and of further refractories of constituent (c) in the range from 25 wt % to 80 wt %, based on the total mass of the coating composition.

7. The coated mold or coated core as claimed in claim 1, comprising one or more binders, in a total amount of not more than 2 wt %, based on the total mass of the coating composition.

8. The coated mold or coated core as claimed in claim 1 for use in the casting of a metal melt having a temperature >900° C.

9. The coated mold or coated core as claimed in claim 4, wherein the organic acids are selected from the group consisting of mono-, di-, and tricarboxylic acids, which are solid at 25° C. and 1013 mbar.

10. The coated mold or coated core as claimed in claim 9, wherein the organic acids are selected from the group consisting of citric acid and oxalic acid.

11. The coated mold or coated core as claimed in claim 5, wherein the one or more binders comprises polyvinyl alcohol.

12. The coated mold or coated core as claimed in claim 7, comprising one or more binders, in a total amount in the range from 0.05 to 0.80 wt %, based on the total mass of the coating composition.

13. The coated mold or coated core as claimed in claim 6, where the ratio of the total mass of inorganic and organic acids in the aqueous phase (a) in relation to the total mass of the coating composition is in the range from 1 to 5%.

14. A kit for producing the coated, waterglass-bound mold or coated, waterglass-bound core of claim 1, including in separate components (U) a coating composition comprising (a) an aqueous phase having a pH of at most 5, (b) particulate, amorphous silicon dioxide, and (c) one or more further refractories, and (V) a binder comprising waterglass.

15. A process for producing a waterglass-bound mold coated with a water-containing refractory coating or a waterglass-bound core coated with a water-containing refractory coating, for use in the foundry, comprising the following steps: (1) providing or producing a coating composition comprising (a) an aqueous phase having a pH of at most 5, (b) particulate, amorphous silicon dioxide, and (c) one or more further refractories, (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.

16. A coated mold or coated core, producible by a process as claimed in claim 15.

17. The process as claimed in claim 15, where the provided or produced uncoated mold or the provided or produced uncoated core comprises particulate, amorphous silicon dioxide, and/or where the uncoated, waterglass-bound mold or the uncoated, waterglass-bound core is produced in step (2) by curing a provided or produced molding material mixture by gassing with carbon dioxide, by admixing esters or phosphates or by gassing with hot air in a heated tool.

18. The process as claimed in claim 15, where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place at a temperature of the provided or produced mold or provided or produced core of >50° C., and/or where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place by an application process selected from the group consisting of spraying, dipping, flow coating, and spreading.

19. The process as claimed in claim 18, where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place at a temperature >70° C. and <100° C.

20. The process as claimed in claim 18, where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place by dipping.

Description

EXAMPLES

[0110] 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

[0111] 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:

[0112] 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 “concentrates”, 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.

[0113] 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 43.3 47.1 46.0 Rheological additive 1.5 1.5 5.0 Phyllosilicate (pyrophyllite DIN ./. 11.0 26.0 140 grind) Phyllosilicate (mica DIN 160 12.0 18.0 ./. grind) Zircon flour (zirconium silicate, 13.5 10.0 9.0 DIN 60 grind) Graphite (DIN 80 grind) 11.0 11.0 8.0 Polyvinyl acetate ./. ./. 0.9 Polyvinyl alcohol 0.4 0.4 ./. Biocide (benzisothiazolinone, 0.3 0.3 0.3 10% w/w aqueous solution) Modified starch ./. ./. 0.3 Yellow iron oxide ./. ./. 1.2 Wetting agent 0.6 0.6 0.3 Defoamer 0.1 0.1 ./. Propylene carbonate ./. ./. 3.0 Particulate, amorphous silicon 14.3 ./. ./. dioxide Citric acid 3.0 ./. ./. TOTAL: 100.0 100.0 100.0 “./.”: containing no fraction;

[0114] 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 100.0 100.0 100.0 Table 1), parts by weight: Water, parts by weight 30.0 40.0 30.0 Properties of the ready-to-use coating compositions resulting from the above dilution: Density [g/ml] 1.32 1.35 1.36 Flow time [s] 13.4 13.3 13.7 pH 2.1 6.7 7.2

[0115] 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).

[0116] The densities of the ready-to-use coating compositions, indicated in Table 1a, were measured according to the standard test method DIN EN ISO 2811-2:2011 (method A).

[0117] 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.

[0118] 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.

[0119] The coating compositions SZ1 and SZ2 each contained attapulgite as rheological additive. Coating composition SZ3 is of the type described in document WO00/05010.

Example 2: Investigation of the Softening of Foundry Cores

[0120] To determine the softening of foundry cores (i.e., the maximum drop in flexural strength), “test cores” (test specimens) were produced conventionally (in accordance with the “core system 1” indicated in Table 4) 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 core production, the test cores were 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 (1 hour at 120° C.), and the change in their flexural strengths under the drying conditions was investigated.

[0121] 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) were 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).

[0122] Table 2, for the coated test cores investigated, reports in each case the values for the maximum dropping of flexural strength within the stated time 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 Maximum drop in refractory flexural strength on Observation coating drying, as % of of core failure on test core the initial value during drying SZ 1 90 No SZ 2 0 Yes SZ 3 25 No

[0123] 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.

[0124] 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) is significantly smaller than with a noninventive, comparative composition (SZ2 and SZ3). It is further apparent from the values in Table 2 that with the noninventive, comparative coating composition SZ2 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

[0125] To determine the storage stability, waterglass-bound test cores (test specimens) were produced in a conventional way (analogous to 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%, temperature in the range from 20 to 25° C.) as indicated above; cf. Table 3 (entry “uncoated after 1 h”).

[0126] 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 SZ2, 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 then 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 seven 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 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 Coated after with type with type Uncoated Core 1 h SZ1 SZ2 on storage system Flexural strength [N/cm.sup.2] 1 300 119 Not core failure determinable after 131 min.

[0127] 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 seven-day storage reliably still had approx. 40% of the initial strength, whereas a test core coated with a noninventive, comparative coating composition (SZ2) 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 Silica sand parts by 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

[0128] Core system 1 consisted only of the molding material, binder, and additive constituents, as indicated in Table 4:

[0129] The binder indicated in Table 4 for core system 1 was a commercial alkali metal waterglass binder “Cordis® 8511” (Hüttenes-Albertus Chemische Werke GmbH).

[0130] The additive indicated in Table 4 for core system 1 was 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 Strengths of Coated Foundry Cores

[0131] Waterglass-bound test cores (test specimens) 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).

[0132] Furthermore, test cores as indicated below in Table 5 were coated at different core temperatures 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.

[0133] The results of the determinations of the flexural strengths are reported below in Table 5. In this case, two different test cores (“core system A” and “core system B”) were used, the production conditions for each of which are reported below in Table 6. The inventive coating composition SZ1 here was also applied to test cores which had different temperatures (25° C., 50° C., and 90° C., respectively).

TABLE-US-00006 TABLE 5 Determination of the flexural strengths of coated foundry cores Un- Coated Coated Coated Coated coated with with with with after 1 h type SZ1 type SZ1 type SZ1 type SZ2 Core 25° C. 50° C. 90° C. 25° C. temperature Core system Flexural strength [N/cm.sup.2] A 350 310 330 350 production of a coated core not possible B  350* 320 not not production deter- deter- of a mined mined coated core not 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.

[0134] From the values reported in Table 5 it can be seen that foundry cores coated with inventive coating compositions at different core temperatures achieve high flexural strengths. In particular the values reported in Table 5 show that with inventive coating compositions foundry cores can be coated successfully even at relatively high temperatures, for instance at temperatures in the range from 50 to 100° C. With a noninventive, comparative coating composition (SZ2), on the other hand, under comparable conditions, it was not possible to produce a usable core, this core instead failing in the course of drying.

TABLE-US-00007 TABLE 6 Production conditions for core systems A and B Parameter Core system A Core system B Molding Silica sand (100.0 parts by Silica sand (100.0 parts by material weight) weight) Binder Alkali metal waterglass Alkali metal waterglass solution, 25-35 wt % solution, 25-35 wt % waterglass content in water waterglass content in water (wt/wt) (wt/wt) (2.2 parts by weight) (2.2 parts by weight) Additive Particulate, amorphous Particulate, amorphous silicon dioxide silicon dioxide (1.0 part by weight) (1.0 part by weight) Core box 120° C. 120° C. temperature Gassing 150° C. 150° C. temperature Cure time  50 s  30 s

[0135] The core systems A and B were produced in an identical manner from the identical constituents, with the exception of the different curing times.

[0136] The binders and additives indicated for the core systems A and B in Table 6 corresponded in each case to the binders (“Cordis® 8511”) and additives (“Anorgit® 8396”) indicated in relation to Table 4.

[0137] 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.