GB2201412A

GB2201412A - Aqueous vitreous enamel slips

Info

Publication number: GB2201412A
Application number: GB08703866A
Authority: GB
Inventors: Roger Frank Price; Dena Strettle
Original assignee: TI
Current assignee: TI
Priority date: 1987-02-19
Filing date: 1987-02-19
Publication date: 1988-09-01
Anticipated expiration: 2007-02-19
Also published as: GB2201412B; GB8703866D0

Abstract

Aqueous vitreous enamel slips having improved drainage characteristics are disclosed containing a combination of locust bean gum and xanthan gum as the suspension agent.

Description

AQUEOUS VITREOUS ENAMEL SLIPS This invention relates to aqueous vitreous enamel slips, and more particularly a method of improving the rheology of such slips.

In the enamelling industry it is conventional practice to apply the vitreous enamel frit to the substrate to be enamelled, prior to fusing, as a slurry or slip comprising a suspension of the vitreous enamel frit in an aqueous suspension medium. It is also known to apply the frit by other techniques, e.g. powder sprinkling or electrostatic powder spraying, and to use organic solvents as the suspension medium, but these are not relevant so far as concerns the present invention.

Conventionally, the aqueous vitreous enamel slip contains finely ground particles of vitreous enamel frit suspended in an aqueous medium using a variety of suspension agents, usually but not necessarily, one or more clay minerals in combination with an electrolyte such as sodium nitrite or potassium carbonate. It is also known to use water-soluble or waterdispersible natural or synthetic resins and gums, such as carboxymethylcellulose, or xanthan gum or gum tragacanth as a suspending agent, see for example, GB-A-2152027.

Whether the aqueous slip is to be applied to the substrate by spraying or dipping, the rheology of the slip is highly important. If the slip is to be applied by dipping, the main requirements are that excess slip drains rapidly from the substrate, thus saving processing time, leaving a uniform film on the substrate which is of uniform thickness, free from pin holes and other imperfections, and free from tears or drainage lines or marks.

When gums are used in place of clays as the suspension agents, problems are encountered in preparing slurries with a rheology suitable for application by dipping. Such slurries tend to have insufficient thixotropy and the slurry drains from the dipped components for an excessively long time. This problem is particularly severe when a thin coating is to be deposited (i.e. between 0.00020 and 0.00050 g of slurry per mm2 of surface to be coated).

In accordance with the present invention, it has been found that the rheology of aqueous vitreous enamel slips containing gums can be improved, particularly with respect to drainage time, i.e. the time taken for excess slip to drain from the substrate after dipping or spraying, if there is used as the suspending agent a combination of xanthan gum and locust bean gum (LBG), and electrolytes such as sodium nitrite and potassium carbonate.

The total amount of suspending agent, xanthan gum and LBG, used in preparing the slip may range from 0.01 to 0.5%, preferably from 0.05 to 0.2%, based on the weight of the frit. The proportion of LBG to xanthan gum is preferably from about 1:1 to 2:1, but proportions can range from 10:90 to 90:10.

The invention also extends to a method of enamelling a substrate in which there is used an aqueous vitreous enamel slip containing electrolytes in combination with xanthan gum and LBG as the suspending agent.

The aqueous vitreous enamel slips according to this invention are prepared in a conventional way, e.g. by ball milling the frit to 75 microns or less, e.g. down to 30 to 40 microns, in the presence of water and incorporating the suspending agent, and electrolyte, into the suspension as mill additions. The invention is equally applicable to vitreous enamels and to cermet vitreous enamels, i.e. containing a powdered metal such as aluminium, titanium or zirconium. Other mill additions may be made, e.g.

surfactants, corrosion inhibitors etc. as is conventional in the art.

A wide variety of electrolytes may be used, such as magnesium carbonate, magnesium sulphate, sodium nitrite and potassium carbonate and mixtures of two or more thereof. Amounts of electrolyte may range from 0.01 to 2%.

The aqueous vitreous enamel slips of this invention may be applied by spraying or dipping.

To illustrate the invention a conventional vitreous enamel frit was prepared having the following composition: Potassium and Sodium Oxides 17.8% by wt.

Boric Oxide 16.0% by wt.

Alumina 7.7% by wt.

Silica 51.1% by wt.

Calcium Fluoride 5P /'n by wt.

Cobalt Oxide 0.5% by wt.

Manganese Oxide 0A'oLi; by Nickel Oxide 0.@% by wt.

Aqueous vitreous enamel slips were prepared in a conventional manner by wet milling the frit in a ball mill until 99% by weight of the frit was of a particle size less than 75 microns, and using the suspending agents and electrolytes hereinafter identified.

To test the rheological properties of the slips thus prepared a standardised steel plate was dipped into each slip, removed, and allowed to drain. The time taken for each plate a) to cease continuous drainage and b) to stop dripping completely was measured. The results are presented below in Table 1. Comparison was also made with a conventional slip using clay as the suspending agent, and having the following formulation: Frit 100 parts by wt.

Clay 4 parts by wt.

Electrolyte (NaNO2) 0.8 parts by wt.

Water 46 parts by wt.

Compared with Kelzan (Xanthan Gum) alone, the combination of Kelzan and LBG shows a substantial reduction in drainage time in terms of the time taken for the continuous flow of slurry from the plate and especially the flow of slurry to cease completely (i.e. all dripping stopped).

Furthermore, the drainage times of the slurry prepared from a combination of Kelzan and LBG were more similar to those of the conventional slip in which clay was used as the suspending agent than when Kelzan was used alone.

TABLE 1

SORRY A B C Suspension Agents 0.15% Keizan 0.05% Kelzan 4% Enamelling Clay and Electrolytes 1.2% NaNO2 0.05% LBG 0.8% NaNO2 (% by weight of 0.4% NaNO2 frit weight) Specific Gravity 1.65 1.66 1.65 Dip Plate Results: i) Time until con- 12 s 8 s 5 s tinuous drainage stopped ii) Time until all More than 42 s 21 s drainage (i.e. 180 s dripping) ceased iii) Weight of 0.00028 g/mm2 0.00026 g/mm2 0.00027 g/mm residue on plate

Claims

CLAIMS 1. An aqueous vitreous enamel slip containing finely divided particles of a vitreous enamel frit suspended in an aqueous medium containing (aDn electrolyte and a suspending agent, wherein the suspending agent comprises a mixture of xanthan gum and locust bean gum.
2. A slip according to claim 1, wherein the suspending agent is present in an amount of from -0.01 to 0.5% by weight based on the weight of the frit.
3. A slip according to claim 2, wherein said amount is from 0.05 to 0.2%.
4. A slip according to any one of claims 1-3, wherein the suspending agent comprises xanthan gum and locust bean gum in a weight ratio in the range 1:1 to 2:1.
5. A slip according to any one of claims 1-4, which also contains in suspension in said medium a finely divided metallic powder.
6. A slip according to claim 5, wherein said finely divided metallic powder is aluminium.
7. A method of enamelling a substrate, which comprises dipping or spraying the substrate with an aqueous vitreous enamel slip, allowing excess slip to drain from the substrate, and stoving the coated substrate to fuse the vitreous enamel coating on the substrate, wherein there is used a-slip as claimed in any one of claims 1-6.