Casting process - Patent Review 4915890

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BACKGROUND OF THE INVENTION

The present invention relates to a method of forming a whiteware cast article and to compositions useful therein.

Slip casting is an ancient art. In the practice of this art, a particulate material is suspended in a slip medium to form a slip. The slip is poured into a mold typically made of plaster of paris. The slip medium is drawn out of the casting slip and a cast is left behind in the mold. The mold and cast are separated and the cast article is processed to give the final properties desired in the cast article. Slip casting is employed in the manufacture of whiteware including sanitary ware, such as sinks and toilets.

Pressure casting is a relatively recent modification of slip casting. In pressure casting, pressure is used to drive liquid out of the slip. Polymeric molds are commonly employed rather than the plaster molds useful in ordinary slip casting. For examples, see U.S. Pat. Nos. 4,464,485; 3,929,685; and 3,825,506.

While slip casting and pressure casting are both techniques well-known in the art, there are some problems associated with casting processes and the articles produced therefrom and attempts have been made to use various casting additives to correct some of these problems.

It would be desirable to have an additive for casting which would be of low molecular weight, would have good burnout characteristics, and would give higher casting rates without significantly adversely impacting the other casting properties of the slip. It would be further desirable to have such an additive which would maintain the green strength of the cast article.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an improved process for the preparation of whiteware ceramic articles employing such a casting additive. The process, which comprises preparing a slip from a particulate material and a slip medium, placing the slip into a mold, removing a portion of the slip medium through the mold and recovering a cast article, is improved by employing a slip comprising a polyalkylene polyamine in an amount effective to increase the casting rate.

In another aspect, the invention is a slip composition comprising: a slip medium: a particulate material: an amount of a polyalkylene polyamine, having an average molecular weight of less than about 1,000, which is effective to increase the casting rate of articles cast from the slip composition: and, optionally, a binder in an amount sufficient to maintain the green strength of articles prepared from the slip composition.

In a third aspect, the invention is an additive, useful in the preparation of whiteware ceramic articles, consisting essentially of a polyalkylene polyamine, a binder and water which may be added to a slip composition.

Surprisingly, the use of a polyalkylene polyamine improves the casting rate without significantly adversely impacting the rheology of the slip.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention advantageously employs a slip composition comprising a particulate material, a slip medium, a polyalkylene polyamine and, optionally, a binder. The slip composition will also preferably comprise a dispersant and additional additives. The process can be a slip casting, vacuum casting or pressure casting process, and is applicable to all casting methods.

The particulate material can be any particulate substance which can be suspended in a slip medium and then cast into a cast whiteware article. Whiteware articles include sanitary ware such as sinks and toilets, china, artware and giftware. Preferred particulate materials include clay materials. Examples of preferred materials include ball clays, kaolin clays, feldspar, flint and the like. Mixtures of particulate materials can be employed. Several particulate materials are well-known to those familiar with the art of casting whiteware ceramic articles.

The slip medium serves to suspend the particulate material until it can be cast. Typically, the slip medium is a liquid and can be organic, inorganic, or a mixture thereof. Several slip media are well-known to those skilled in the art of slip casting. Examples of slip media include alcohols, hydrocarbons, chlorinated hydrocarbons and water. The preferred slip medium is water. The particulate material can be employed in any amount which will result in a cast article. Typically, the amount of particulate material employed is from about 5 to about 95 weight percent based on the weight of the slip. Preferably, the amount of particulate material employed is from about 25 to about 90 weight percent. However, the slip composition can vary widely and can contain more or less particulate material. Mixtures of slip media can be employed.

Polyalkylene polyamines are well-known compounds. For the purposes of the present invention, it is preferred to employ a polyalkylene polyamine having an average molecular weight of less than about 1,000, more preferably less than about 800. Examples of polyalkylene polyamines include ethylenediamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, piperazine, aminoethylpiperazine, propylenediamine, butylenediamine, pentamethylenediamine, hexamethylenediamine, diethylenetriamine, dipropylenetriamine, dibutylenetriamine, bis(pentamethylene)triamine, bis(hexamethylene)triamine, tripropylenetetramine, tris(hexamethylene)tetramine, tetrapropylenepentamine, tetrabutylenepentamine, tetrakis(pentamethylene)pentamine, tetrakis(hexamethylene)pentamine, pentapropylenehexamine, pentabutylenehexamine, pentakis(pentamethylene)hexamine, pentakis(hexamethylene)hexamine, bis(aminopropyl)ethylenediamine, bis(aminopropyl)diethylenetriamine, tris(aminopropyl)ethylenediamine, and the like. The preferred polyalkylene polyamines generally have the formula: ##STR1## wherein n can be from zero to about 6, m can be from 1 to about 6, and p can be from 1 to about 15, with the proviso that n and m can vary independently within a molecule from one repeating unit to the next. Preferably, n is zero, p is 1 and m is from about 3 to about 6.

Thus, preferred polyalkylene polyamines for use in the practice of this invention include ethylene polyamines. It is also preferred that the ethylene polyamines used have an average molecular weight greater than about 150 and less than about 800. Examples of preferred polyalkylene polyamines include triethylenetetraamine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine and mixtures thereof.

The polyalkylene polyamine additives of this invention may be used individually or mixtures of polyalkylene polyamines may be used. For example, some polyalkylene polyamines such as triethylenetetraamine and tetraethylenepentamine (hereinafter TEPA) are generally available commercially only as a mixture of isomers having similar boiling points. Also suitable for use in the practice of this invention are mixtures of isomers having similar boiling points and higher molecular weights up to about 800. In addition to the linear isomers of the polyalkylene polyamines, branched and cyclic isomers of these compounds may also be employed.

The polyalkylene polyamine is employed in an amount which is effective to increase the casting rate of the slip relative to a slip which is identical except for the presence of the polyalkylene polyamine. Preferably, the polyalkylene polyamine is employed in an amount which is from about 0.0001 to about 1 percent of the weight of the slip. More preferably, the amount of polyalkylene polyamine is from about 0.05 to about 0.5 percent of the weight of the slip. Most preferably, the amount of polyalkylene polyamine is from about 0.1 to about 0.3 percent of the weight of the slip.

A binder is optionally employed in the process of the present invention. The binder serves to maintain or improve the green strength of the cast article. Examples of preferred binders include latexes such as styrene-butadiene latexes, acrylic latexes, ethyleneacrylic acid latexes, lignosulfonates, polyvinylalcohols, alkanolamines and other binders known in the art. Styrene-butadiene latexes are more preferred. When a binder is employed, it is employed in an amount sufficient to maintain or improve the green strength of the cast article relative to an article cast from a slip which is identical except for the presence of the binder. Preferably, the amount of binder is from about zero to about 30 percent of the weight of the slip. More preferably, the amount of binder employed is from about 0.05 to about 0.5 percent of the weight of the slip. Most preferably, the amount of binder employed is from about 0.1 to about 0.3 percent of the weight of the slip.

The ratio of polyalkylene polyamine to binder, when a binder is used, is any which will result in a slip composition from which a whiteware ceramic article may be produced at a faster cast rate and with a green strength comparable to the casting rate and green strength of an article prepared from a slip without the polyalkylene polyamine and binder. A preferred weight ratio of polyalkylene polyamine to binder is at least about 5:95 and no greater than about 95 5. It is more preferred that the weight ratio is at least about 30:70 and no greater than about 70:30. It is most preferred that the weight ratio of polyalkylene polyamine to binder is about 50:50.

The polyalkylene polyamine and the binder, when used, may be added to the slip composition separately. Alternatively, the polyalkylene polyamine and binder may be mixed together, optionally with water, and added to the slip composition at once. When the polyalkylene polyamine and binder are mixed together before being added to the slip composition, it is preferred to mix the two components by preparing separate aqueous solutions of the binder and the polyalkylene polyamine and adding the amine solution to the binder solution with stirring. When mixed with water, the weight ratio of active ingredients, i.e., polyalkylene polyamine and binder to water is preferably about 1:1. However, other ratios may be used.

Other additives which are well-known in the art of slip casting are also typically employed. Examples of additional additives include soda ash, calcium sulfate, and dispersants. Dispersants useful in the practice of this invention are those known to one skilled in the art of whiteware ceramic casting. A non-limiting example of such a dispersant is sodium silicate.

The casting process of the present invention can be practiced using the conditions commonly employed in the art of slip casting and pressure casting. Typically, slip casting is conducted at ambient temperature, although temperatures of from ab