TITANIC ACID ESTER-CONTAINING BINDER FOR COATING MATERIALS AND REFRACTORY SHAPED BODIES AS WELL AS PROCESS FOR THE PRODUCTION OF THESE BINDERS

I IIB84 i0 The subject of the present invention is binders based on chelated titanic acid esters. These blndersare employed both in the production of ceramic shaped bodies, as well as for example casting moulds or casting cores for metal casting, and also in coating materials for the protection of for example crucibles formed of material which is susceptible to attack by metal melts, in which aggressive metals or alloys are to be melted.

It is known to cast metals in ceramic casting moulds. In the production of such moulds, finely divided refractory material is mixed with a predominantly silicate binder and the mixture is then applied by means of dipping, spraying or casting on a pattern. The curing of such mouldings takes place by means of simple drying or with use of a gas forming liquid or solid hardener, for example ammonia so that there is formed from the fluid binder a gel which in the case of the most employed silicate binders is converted on firing to silicon dioxide. In the frequently employed wax melting or high quality casting process, the mould is built up layer wise by means of a repeatedly carried out sequence of working steps of dipping of the pattern to be melted in ceramic slips, sprinkling coarsely particle refractory sanding material over the still moist layer and in between, drying; after the drying of the last layer, the pattern is removed by warming and the mould is fired.

Casting moulds and cotes produced in this way according toknown procedures will not always yield satisfactory casting results. In particular, when casting high alloy steels or steels with high Contents of carbon or chromium and of titanium, frequently there occur reactions of the metal melt with constituents of the mould so that the cast pieces obtained possess defective surface'layers.

Also on melting of the indicated metals or alloys i0 or during interim storage of such metals or alloys for the purpose of casting in crucibles, which consists of siliconcontaining material, for example made of clay, the indicated defects can be experienced.

One already known possible way of avoiding these defects is to use silicon-free binders for the casting moulds or covering material. The organic derivatives of titanium, aluminium or zirconium analogous to ethyl silicate possess however, in contrast to the organic silicates, a by far greater susceptibility to hydrolysis since they are already decomposed by atmospheric moisture. It is therefore not merely possible to transfer the procedures known from silicate chemistry to these binder starting materials. The procedure described in German Patent Specification 22 04 531 shows a practicable manner of overcoming this difficulty, in which, by addition of specific chelate formers, the susceptibility to hydrolysis of for example aluminium-sec.-butylate, zirconium-n-butylate or -n-propylate and tetra-isopropyltitanate is reduced to such an extent that binders and silicon-free slips in admixture with corresponding refractories can be produced therefrom. The slips produced with these silicon-free binders have however the disadvantage that coatings produced therefrom for crucibles made of clay or mullite yield surfaces on firing at 700°C which are either relatively soft or cracked and accordingly do not optimally fulfil the aforesaid purpose.

There therefore existed the object of finding a binder for slips made of refractory material which, after the hardening of the slip produced therewith yields casting moulds for aggressive metal or metal alloys whose surface possesses a high strangth, is crack free and is not attacked by the indicated metals in molten form. It should.furthermore be possible to produce from the slips coatings for crucibles or other containers made of clay or silicon con- ,i " i0 taining meterial, in which melts o aggressive metals or metal alloys car be kept.

The present invention• provides a binder for a coating material comprising at least one chelated titanic acid ester, characterized in that the titanic acid ester is a polymeric titanic ester and in that the polymeric titanic acid ester is chelated with 0.2 to 1 mole of chelate former per titanium atom.

In another aspect the present invention provides a process for the preparation of a binder as defined, abOve, characterized in that a tetraalkyltitanate is heated with the necessary amount of water for the desired degree of condensation of the polymeric titanic acid ester and with a chelate former andthen the amount of alkanol spi1 t off in the condensation is, if desired, distilled off.

In a further aspect the present invention provides a slip material comprising a carrier liquid, a silicon-free finely divided retractory substance and a binder as defined above.

Slips which are produced with such a binder contrast with those slips considered for the production of high quality casting moulds whose binders are produced from chelated monomeric metal compounds, as corresponds to the above-indicated state of the art. Chelated polymeric titanium compounds are obtained by mixing of polymeric titanic acid esters, for example polymeric butyl titanate, with chelate formers, for example acetylacetone, acetic acid esters or triethanolamine. The polymeric alkyl titanate is previously produced by partial condensation from the corresponding monomeric ikyl titanate or, as is usually the case, from titanium tetrachloride by reaction with aqueous alcohol with simultaneous esterification and partial condensation occurring.

The production of the chelated polymeric titanium compounds can however also take place from a monomeric titanate, for example tetra-n-buty! titanate, if this is reacted simultaneously wìth water and the chelate former, whereupon neither the water affects the chelation nor the chelate former affects the condensation; the alcohol which is split off is moreover distillatively removed. Thus, for example there is obtained from one mole of butyl titanate by addition of a mixture of 0.8 mole of water, 0.8 mole of acetylacetone and 0.6 mole of butano (for solvation), heating to about 120°C and distillation off of 2.2 mole of butanol, a product with a TiO2 content õf about 25% which corresponds to that product which is obtained by mixing of (theoretically) one gram atom of titanium in the form of C 3a 1; 11884 polymeric butyltitanate with 0.8 mole of acetylacetone.

This simple and unexpected procedure for obtaining the new binder has furthermore the additional advantage that predetermined molar ratios or desired properties are to be obtained on starting from monomeric, exactly defined titanates far sooner and much more precisely than if one was proceeding from a polymeric titanate which for its own part already possesses as a technical product a range of variations in degree of condensation, in TiO2 content and in viscosity.

The ester component of the titanate is preferably an alkyl ester group with 1 to 8 C-atoms, preferably 2 to 4 C-atoms.

Suitable for use as chelate formers for the binders according to the invention are the 8-dioxo compounds known as chelate formers, for example acetylacetone, acetoacetíc acid ethyl ester, benzoylacetone and benzoylmethane, or polyvalent alcohols, which are 2-, 3or higher valent and can optionally contain nitrogen, for example ethanediol-l,2, 2-methylpentanediol-(2,4), 2-ethylhexanediol-(l,3), glyderol, dior triethanolamine. In addition compounds from the group of oxy-oxo compounds, such as lactic acid or diacetone alcohol may be employed as chelate formers.

The amount of chelate former added must on the one hand suffice to prevent immediate hydrolysis Of the titanate on preparation of the slip; it may on the other hand however not be too great so that the reactivity of the binder with atmospheric moisture after application of the slip to a substrate is not produced in undesired manner. Additions of about 0.2 to 1 mole of chelate former per titanium atom fulfil the aforesaid purpose most satisfactorily.

If the preparation of the binder according to the invention takes place from a monomeric titanate by addition of chelate former and water, the amount of water which is 1Zll 4 used for the formation of the polymers preferably lies between 0.5 and 1 mole of water per titanium atom. The titanium content of the polymers obtained then lies from to about 35 % TiO2 according to which alkyltitanate and which chelate former is employed and to what extent the alcohol split off from the titanate was distilled off. The titaniun content of the new binder preferably lies from 22 to 33 % by weight Ti02 ; this corresponds to a degree of condensation of 3-5 Ti-atoms per polymerized titanium compound.

The alcohol liberated in the chelate formation does not need to be distilled off since it can then serve at the same time as solvent or transporting liquid during the use of the binder according to the invention in slips for coating materials or foundry cores. A higher TiO2 content of binding material is however sometimes, according to the usage, desired when using certain refractories in the slips; in this case, the alcohol split off is partially or completely distilled off. This can then also be the case if the slip should have an alcohol other than the one that is being split off as solvent or transporting liquid.

Slips in which the present binder materials are used, which are suited to the layering of substrates and also for immersion baths in the high quality casting process, are composed of the chelated polymeric titanate and a silicon-free finely divided refractory substance such as for exanple zirooni moxide, al niniumoxide,(e.g, corundum) or æagnesiumoxide; in addition Ca0 or Y2 03 can be employed as refractory. Here both the particle size distribution of the refractory and also the viscosity of the slip which may be between about to 80 sec run out time (DIN run out beaker, 4 mm nozzle) and optionally can be corrected by addition of suitable solvents such as ethyleneglycol, paly a role. When using zirconium oxide, a mixing ratio of binder: ZrO2 = 1:3.3 and C 1; 11SS4 a partìcle composition 2 parts 0 to 60 m + 1 part 0 to i0 m has proved satisfactory.

The slips obtained with the binders according to the invention are further worked in known manner to coatings or high quality casting moulds. The coatings obtained with these materials already have, after drying for 12 hours in air, a surface hardness of H (pencil hardness). Heating for one hour at 200°C increases the pencil hardness to 2H, and also on heating to 700°C the hardness decreases only to HB.

In each case the coatings are crack free. In contrast, Coatings which are produced according to the technique set out in German Patent Specification 22 04 531 are considerably softer after firing at the indicated temperatures during the same period; such essentially softer layers contrastingly show less resistance to molten metal flowing thereover and are more easily mechanically torn than is the case with the coatings with layers made with the binders according to the invention.

The use of the aforedescribed slip in the production of high quality casting moulds takes place in known manner in which, after dipping of the wax pattern in the dipping bath, the still moist layer is sanded with a suitable coarse material and after drying for many hours and in-depth hardening of the layer, the procedure is repeated. On economical grounds, the further build-up of the mould from a third or fourth layer is undertaken in known manner w±th silicate binder and corresponding refractory material. Also in this way there are obtained after the firing at 1000°C tear resistant inner layers which, in comparison with those which were produced according to the state of the art, possess greater hardness and a finer surface structure.

Example 1 470 g of butyltitanate polymer (54 % TiO2 2 gatom Ti) are provided in a 1 litre four-necked flask with _r i0 stirrer, thermometer, dropping funnel and reflux condenser and 160 g of acetylacetone (pentanedione-2,4 1.6 mole) are added in portions with stirring. As a result of this the temperature of the starting material rose to about 45 to 50°C. Then the mixture is heated for completion of the reaction to boiling of the butanol which has been split off (to about i00 to I05°C) and then is allowed to cool. The binder thus produced is a reddish-brown liquid whích contains 25.4 % TiO2 and 18.8 % butanol.

Example 2 510 g of butyltitanate monomer (23.5 % Ti02 , 1.5 mole) are placed in a 1 litre four-necked flask with stirrer, thermometer, dropping funnel and reflux condenser and, while undergoing stirring, a mixture of 120 g acetylacetone (1.2 mole), 21.6 g water (1.2 mole) and 66.6 g of n-butanol (0.9 mole) is added in portions. As a result of this the starting temperature increases to 55 to 60iC. Then the reflux condenser is replaced by a distillation bridge and the reaction mixture is heated to boiling; n-butanol (3.3 mole = 244 g) is dist£11ed off in this way. The binder produced by thís method is the same material as that obtained in Example i.

Example 3 510 g of butyltitanate monomer (23.5 % TiO2 , 1.5 mole) are placed in a 1 litre four-necked flask with stirrer, thermometer, dropping funnel and reflux condenser and, while undergoing stirring, is supplied with a mixture of 134 g of triethanolamine (0.9 mole), 30.5 g water (0.75 mole) and 22 g of n-butanol (0.3 mole) added in portions. As a result of this the starting temperature rises to 45 to 50°C. Then the reflux condenser is replaced by a distillation bridge and the reaction mixture is heated to boiling; n-butanol (1.8 mole = 133 g) is distilled off in this way. The binder contains about 22 % TiO2.

i0 Example 4 (Comparative Example) 510 g of butyltitanate monomer (23.5, % TiO2 , 1.5 mole) are placed in a 1 litre four-necked flask with stirrer, thermometer, dropping funnel and reflux condenser and are so provided, while undergoing stirring, with 120 g of acetylacetone (1.2 mole) that the starting temperature increases to about 65 to 70°C. To achieve completion of reaction, the starting material is heated up to boiling (starting temperature about I00 to I05°C) and then is allowed to cool while undergoing stirring. The comparative binder thus produced is, like the products of Examples 1 and 2, a reddish-brown liquid, containing however only 19.0 % TiO2.

Example Preparation of coating on clay plates by means of the inventive and comparative binder material in combination with zirconium dioxide.

(a) Production of coating material:

215 g of zirconium dioxide of Particle size 0 to B and 115 g of zirconium dioxide of particle size 0 to are stirred into each 100 g of the binder according to Examples 2 and 4 (for comparison). These mixtures possess a viscosity of 45 to 60 seconds run-out time from the DIN beaker with 4 mm nozzle.

(b) Commercially available unglased clay plates are coated with the materials produced according to (a) by means of a brush applicátor and dried for 12 hours in air or one hour in a muffle oven at 200°C or 700°C. The table which follows shows the results of the pencil hardness test on the coatings obtained.

I,Z11884 i0 15.

Binder Acc. Example 2 Acc. Example 4 (Comparison) Pencil hardness after drying at 200C/12 h 20û°C/I h 700°C/I h H 2H HB H 2B 6B Pencil hardness scale:

Example 6 H > 6H harder HBmedium B > 6B softer Production of high quality casting moulds by means of the binder according to the invention in combination with zirconium dioxide.

Binder-zirconium dioxide mixture for use as immersion baths for high quality casting wax patterns are produced according to the details set out in Example 5(a).

Test-tubes having a size of i0 x i00 mm which are covered with paraffin are immersed as wax model substitute in these immersion baths and the layer remaining thereon is then immediately sanded in a loaded fluidised bed with zirconium dioxide of particle size of 0.12 to 0.25 mm. After overnightdrying in an atmosphere saturated with moisture, the immersion and sanding is repeated as above. After drying once again overnight, four layers of support material formed of commercially available silicic acid ester hydrolysate and synthetic mullite are applied in known manner and hardened by means of ammonia vapour.

The paraffinated test-tube is removed from the thus obtained high quality shell moulds by immersion for a short time in hot water. The remaining ceramic mould is fired in a muffle oven at 1000°C for three hours and after cooling down is checked in order to judge the inner layer.

The results obtained in Example 5 with respect to surface hardness were established with this working procedure.

The embodiments of the invention in which an exclusive