Field of the Invention:

The present invention pertains to an absolute hardness glaze composition for preparation of ceramic tiles, a method of making said ceramic composition and articles manufactured therein.

Abstract of the Invention:

The present invention is a corundum based coating composition for producing ceramic tiles for achieving Moh's scale on hardness at '8' and the method of applying the corundum based coating composition on the ceramic tiles. The corundum based coating composition of the present invention comprises frit, high hardness materials selected from corundum, quartz, feldspar, zirconium silicate, calcined alumina, calcite and wollostonite, and binding agents such as talc, VST powder, ball clay and china clay.

The present invention further discloses a method of applying said corundum based composition on ceramic tiles comprising a first engobe composition, a second corundum base composition and a third corundum paste base composition.

The method of glazing said ceramic tile further comprises applying plurality of glaze composition from said first engobe composition, said second corundum base composition, and said third corundum paste base composition on the body of said ceramic tiles.

Background of the Invention:

Ceramic articles such as tiles and vessels, or containers are very useful, easy and economical to produce. However, the practical application and use of ceramic articles are limited by its strength, durability and pH resistance.

A ceramic tile is basically a utility product that finds increasing application as floor and wall tiles. Popular housing projects are increasingly switching over to ceramic tiles from the traditional mosaic and even granite or marble, owing to several factors viz ease in laying ability, versatility, low price and most importantly hygiene.

The ceramic tiles are classified as per BIS (Bureau of Indian Standards):

The criterion for the classification is based on the water porosity of the tiles.

1) Bla: Water absorption below 0.08% - ceramic fully vitrified tiles.

2) Bib: Water absorption above 0.08% and below 3% - ceramic porcelain tiles.

3) BIT: Water absorption below 6% - ceramic floor tiles.

4) Bill: High water absorption tiles above 10% - ceramic wall tiles.

The ceramic vitrified tiles and porcelain tiles are produced from raw materials such as ball clays, china clays, quartz and feldspar that are wet milled to very fine particles. The wet milled body dust is produced by spray drying and pressed to form definite size and shape. The tiles are then dried and designed prior to being fired at very high temperature up to 1200°C. The prepared tiles are mirror polished and sold in the market.

In case of BII and Bill, the tiles have 2 components, the body contributing 95% to 97% of
raw materials and the glaze contributing 3% to 5% of the raw materials. The body composition consists of 70% plastic clay and 30% non-plastic minerals. These materials are milled in dry or wet condition to fine particles. The dried powder is pressed to definite size and shape and the glaze is applied after drying. Glaze is composed of processed raw materials like ball clay, china clay, quartz, feldspar, calcified alumina, zirconium silicate, frits, coloring agents, etc. After the glaze application, print decoration is applied and finally the tile is fired at specific temperature and firing cycle.

BI and BII classified tiles are used for floor application and Bill tiles used specifically for wall applications. For use in floor application the surface hardness of BI &. BII grouped tiles plays a significant role. Tiles used in bath rooms, toilets differ from those used in bed rooms, kitchen, halls, dinning halls, porticoes, and commercial complexes. The awareness of different type of tiles and where to apply is of importance while laying the suitable tiles. Especially for high traffic usages, the hardness of tiles surface is of prime importance, which depends on the tile surface glaze recipe. These tiles should be hard enough and also impervious. Therefore it is important to know the hardness and absolute hardness of the natural minerals on moh's scale prior to preparation of tiles incorporating these minerals.
%

Table1: List of minerals as per Moh’s Scale of hardness

One of the complexities in the manufacturing of ceramic tiles is incorporating high hardness minerals such as calcified alumina, quartz, felspar etc., in the composition for glaze preparation. The said hardness materials when incorporated as an ingredient in glaze compositions does not fuse because of high melting temperature of these minerals in the firing cycles.

To overcome the above advantage patent documents US5256611A, US5134096A, GB1341720A and BG51712A1 listed in the prior art disclose the use of pre-melted alumina or corundum replacing calcined alumina in the glaze composition. However, while incorporating the fused alumina, the amount of frit in the glaze composition need to adjusted for allowing appropriate melting and fusing of the said glaze composition onto the ceramic tiles.

Another disadvantage while handling these high hardness materials is the percentage of frit composition used in preparation of glaze. Use of high frit composition in the glaze preparation leads to melting of the glaze while fusing with the body resulting in molten or semi melted tiles. On the other hand decreased use of frit composition reduces the melting of hard minerals such as quartz, felspar and calcined alumina into the tiles resulting in difficulties producing higher absolute hardness.

In view of the limitations now present in the prior art, the present invention provides a new and useful method for an absolute hardness glaze composition for ceramic tiles, which is simpler in construction, more universally usable and more versatile in operation than known method of this kind.

Summary of the Invention

The present invention is a corundum based coating composition for producing ceramic tiles for achieving Moh's scale on hardness at '8'. The present invention further discloses a method of applying said corundum based composition on ceramic tiles comprising a first engobe coating, a second corundum base coating and a third corundum paste base coating.

The corundum based coating composition comprises of Corundum, Quartz, Feldspar Zirconium silicate, frits (9690, 522, 9330, 2772 and 422), VST powder, Calcite Wollostonite, Talc, Ball clay, China clay and Calcined Alumina. The method to apply said corundum based composition on ceramic tiles involves multiple application comprising said first engobe coating, said second corundum base coating and said third corundum paste base coating.

Detailed Description of the Invention

The invention particularly relates to multiple coatings of said glaze composition and more particularly an engobe coating, a corundum base coating and a corundum paste base coating.

The above mentioned 3 coatings contains mainly Corundum 10 to 25 wt.%, China clay 5 to 20 wt.%. Quartz 3 to 10 wt.%. Potash Feldspar 10 to 20wt.%, Wollostanite 3 to 7%wt., Zirconium silicate 5 to 15%wt., Ball clay 10 to 20%wt., Frits 20 to 30%wt., V.S.T powder 5 to 10%wt., Calcined Alumina 3 to 5wt.% and Talc 3 to 5wt.%.

Brief Description of the Drawings

FIG.l. Flow chart for slip house

FIG.2. Flow chart of milling plant

FIG.3. Press and drier flow chart

FIG.4. Glaze preparation flow chart

FIG.5. Glaze line flow chart

FIG.6. Firing Curve

Body Preparation

The body component of the ceramic tiles are prepared by two methods namely wet process and dry process. The dry process method is highly energy efficient. However, the main aim of both the process is to achieve fine ground clay/minerals particles to be used in the body of the ceramic tiles.

The raw materials used for body preparation are 70 to 80wt.% plastic clay raw materials and 20 to 30wt.% minerals. The role of plastic clay is to provide binding strength while the minerals act as a fluxing agent. The composition are designed to achieve the physical, chemical and thermal-properties of the tile. Mechanical or physical means such as the press stage, provide enough strength for the safe transportation of the body of the tile till it reaches the firing zone. Prior to the powder being pressed it is subjected for thorough magnetic separation to avoid the iron particles in the body, otherwise it result in black specks in the finished tile surface.

The chemistry of the tile comprises SiO2- 60 to 65wt.%; Al2O3- 16 to 20wt.%; F32O3- 6 to l0wt.%; CaO- 1 to 2wt.%; MgO- 1 to 2wt.%; Na2O- 0.5 to lwt.%; K2O- 1.5 to 2.5wt.% and LOl- 6 to 8wt.%.

The thermal properties meets the B II classifications as per the Bureau of Indian standards. (Reference FIG. 1. Flow chart for slip house and FIG.2. Flow chart of milling plant)

Table2: Body raw materials and composition

Table 3: Powder granulometry/Milled powder sieve analyze

The dried powder is allowed to age for a minimum of 24hrs before it is conveyed to a press hopper through a vibrosieve. The powder is pressed in the hydraulic press of capacity 1700 to 2080 tons to a definite size at 250 to 300 Kgs/Cm2 specific pressure. To eliminate the moisture content in the green tiles, the tiles are conveyed to dryer with a temperature around 200°C for a cycle of 20 to 25min. The moisture content of the exit tile is maintained below 1% with surface temperature between 90 to 100°C. The body of the tiles is now prepared for glaze application. (Reference FIG.3 Press and drier flow chart)

Glaze preparation:

The ceramic tile is coated with the novel glaze composition of the present invention for achieving the Moh's scale of hardness '8'. The equipments used here comprises of Ball mills, capacity from l00Kgs to 2500Kgs and a microniser. These Ball mills are lined with high alumina lining and used for grinding high alumina pebbles while the microniser is used for achieving the paste base with fine particles of (- 350#). (Reference FIG.4. Glaze preparation flow chart)

Table 4: Ceramic engobe, ceramic base glaze and ceramic paste base coat composition

1st Coat ie Engobe.

As per the composition listed above the raw materials are used to prepare the first engobe coating to the body of the tile. Ball clay and China clay are used as binders to keep the minerals in suspension in liquid state. VST powder, Frits, Feldspar are used as flux and this combination help fuse the other hard materials at low temperature. Quartz, improves SiO2 content and helps in thermal expansion.

2nd coat:- Corundum base:

Corundum based glaze composition as disclosed in this specification is used as the base composition to achieve the required hard and tough tile surface. The corundum base is applied to the engobe composition over the body of the tile. The composition comprises of 10-l4wt.% of corundum with 24- 34wt.% frit to yield the required melting temperature. The base composition comprises in addition to other ingredients

Corundum: - On Moh’s scale it’s hardness is 9. It also helps in reducing the firing temperature as it is in fused status.

Feldspar: - On Moh's scale it's hardness is 6 and it is used as a flux.

Quartz: - On Moh's scale its hardness is 7, improves SiO2 content and helps in thermal expansion.

Zirconium silicate: - It is used as an opacifier and induce the brightness.

Frits/V.S.T powder: - It is used as a main flux and helps in fusing the hard materials. Frit is a pre- melted glaze.

Calcite/ Wollostonite:- It lowers the viscosity of the melted oxides. It acts as an autocatalytic in dissolving the other ingredients.

China clay: - It keeps the glaze in suspension and used as a filler material.

Rustic frit: - It contains hard materials like Quartz, feldspars, Calcined alumina etc, in fused status.

The various colored ceramic coatings are stored in storage tanks for a minimum of 24hrs aging and further processing.

3rd coat Corundum paste base:-

The novelty of the invention in applying the multiple coats of corundum to achieve high absolute hardness for ceramic tiles. The corundum paste base composition comprises hardness yielding materials like corundum, feldspar, quartz, and zirconium silicate which are fused along with the frits. This paste base is used along with pigments to develop different shades and transferred as prints on the 2nd coating. There can be single, double or multiple transfer of prints depending on the design requirements.

Glaze line.

As per the flow chart FIG.5. (Glaze line flow chart ) the glaze line is designed for the application of engobe, ceramic coating and printing coatings. Engobe and ceramic coating are applied either by disc booth ie disc spray or bell/ vela fall method. The designs and shades are decided in glaze line by the way of different inputs and different applications. These applications are scientifically matched to body thermal expansion. As the ceramic body and ceramic coatings are fired together the significance of thermal expansion of both are very important and critical. After different ceramic coat application and transfer of prints as per the required designs, the tile is ready to enter the firing zone. At this stage the ceramic coated green tiles contains 2-3% moisture.

Firing:-

The equipment / device used for firing of green ceramic coated tiles is called kiln. The kiln is divided in to six zones. (Reference FIG.6. Firing curve)

a) Pre drier zone: The temperature is maintained from room temp to 300°C. In this zone the moisture content is totally eliminated.

b) Pre heating zone: Here, the temperature will be from 300 °C to 900 °C. All inorganic incombustible matter will get combusted and allotropic transformation of a to P quartz takes place.

c) Heating zone: Here, the temperature will be from 900 °C to 1150 °C. In this zone, the sintering process starts due to decomposition of kaolinite to Meta kaolinite, formation of Al-Si spinal structure, formation of amorphous silica and formation of mullite phase. Here, the ceramic coating gets fused to the ceramic body making surface impervious.

d) Rapid cooling zone: Here, the temperature drastically reduced from 1150 to 1050 °C, by blowing of cold air which leads to quenching of tile.

e) Slow cooling zone: Here, the temperature is brought from 1050°C to 900°C. The stability of the ceramic coating surface, squareness of the tile and the warpage of the tile will be determined by slow cooling.

f) Final cooling zone: Here, the temperature will be brought down from 900 °C to 150 °C. From the kiln exit the tiles enter the sorting department for classification, packing and palletizing.

Test for hardness

The tiles are subjected for surface hardness on Moh's scale as per IS-13630 part 13 prescribed by Bureau of Indian standards. The standard covers a principle method for determining the surface hardness on Moh's scale by drawing minerals of defined hardness by hand over the surface of the tile

Table 4: Reference Mineral in Hardness Testing Kit

Test specimens: A minimum of three tiles produced as disclosed in the description is tested for surface hardness on Moh's scale.

Procedure:

The tiles are placed under test on a firm support with the glazed or proper surface on the uppermost. A sharp freshly broken edge of the reference mineral is drawn over the surface to be tested by hand, applying a uniform effort such that at the end of the test, its edge or the tile surface is intact. The procedure is repeated for at least four times with a sharp freshly broken edge of the reference mineral for each tile with each mineral that is necessary to obtain a result. The tile surface is examined for any scratches of marks by naked eye/with the aid of spectacles. The mineral of highest Moh's hardness that produce no more than one scratch is noted for each tile. In the case of tiles having variable scratch hardness, the lowest Moh's hardness is recorded.

Observation:

The specimen tile after testing with topaz mineral did not develop any marking or scratch on the tile. The test indicates that the said tile has passed no '8' on Moh's scale of hardness.

CLAIMS

1. A corundum based glaze composition for application over a body of a ceramic tile comprising

1) a first engobe composition, 2) a second glaze base composition and 3) a third glaze paste base composition.

2. The corundum based glaze composition according to claim 1, comprising frit, a plurality of combination of minerals and binding agents.

3. The composition according to claim 2, wherein said frit is selected from a rustic frit 9330, an opaque frit 9690, an opaque frit 522, a transparent frit 2772, a transparent frit 422 and combinations thereof.

4. The composition according to claim 3, comprising said frit upto 34 wt.% of said glaze composition.

5. The composition according to claim 2, wherein said minerals are selected from the group comprising corundum, quartz, feldspar. Zirconium silicate, wollostonite, calcined alumina and combinations thereof.

6. The composition according to claim 5, comprising said corundum up to 27 wt.% of said glaze composition.

7. The corundum based glaze composition according to claim 1, wherein the first engobe composition comprises; CMC - 0.1-0.3 wt.%, STPP - 0.3-0.6 wt.%, quartz powder- 5-7 wt.%, china clay 18-22 wt.%, calcined alumina 1-3, feldspar 15-20 wt.%, VST powder - 7-10 wt.%, ball clay 25-30 wt.%, transparent frit (2772) - 8-11 wt.%, transparent frit(422) - 8-11 wt.% and combinations thereof

8. The corundum based glaze composition according to claim 1, wherein the second glaze base composition comprises; rustic frit 9330 - 8 to 12 wt.%, opaque frit 9690 - 8 to 11 wt.%, opaque frit 522 - 8 to 11 wt.%, corundum - 10 to 14 wt.%, quartz - 4 to 6 wt.%, feldspar - 13 to 17 wt.%, zirconium silicate - 8 to 12 wt.%, wollostonite - 2 to 4 wt.% china clay - 5 to 7 wt.%, calcite - 8 to 12 wt.%. Talc - 2 to 5 wt.%, VST powder - 5 to 8 wt.% and calcined alumina - 0.5 to 1.5 wt.% and combinations thereof

9. The corundum based glaze composition according to claim 1, wherein said third glaze paste base composition comprises; corundum -19 to 23 wt.%, quartz - 2 to 5 wt.%, feldspar - 12 to 16 wt.%, zirconium silicate - 8 to 12 wt.%, transparent Frit 2772 - 10 to 13 wt.%, VST powder - 8 to 11 wt.%, china clay - 4 to 6 wt.%, calcite - 8 to 12 wt.%, wollostanite - 3 to 4 wt.% and combinations thereof

10. The corundum based glaze composition according to claim 1 when applied on body of said ceramic tile produces a scratch resistance of'8' on Moh's scale of hardness.

11. A method of glazing said ceramic tile, said method comprising applying plurality of glaze composition further comprising;

applying said first engobe composition,

applying said second glaze base composition and,

applying said third glaze paste base composition on the body of said ceramic tiles.

12. A method of glazing said ceramic tile composition according to claim 11, wherein said glaze composition is applied as spray coating, print coating, paint coating or combination for uniform application of said glaze.

13. A means for glazing the ceramic tile, said means for applying plurality of glaze composition further comprising;

means for applying said first engobe composition,

means for applying said second glaze base composition and,

means for applying said third glaze paste base composition on the body of the ceramic tiles.

14. An apparatus for facilitating the application of said glaze composition comprising;

a transfer device;

a plurality of spraying devices; to apply said first engobe composition, to apply said second glaze base composition and, to apply said third glaze paste base composition on the body of said ceramic tiles and

a firing device; to fuse said glaze composition onto the body of said ceramic tiles.