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A touch of gloss

   
 
Picture
There was a need in the ceramic tile industry to develop floor tiles with a very white and glossy surface and with very good resistance to wear by abrasion.
Glazed ceramic floor tiles are becoming fashionable world-wide. But glossy finishes are not very hard-wearing and shiny tiles show wear more easily than rough ones. So a consortium of tile makers and distributors got together to commission research on how to make glossy but hard-wearing glazes for floor tiles.

 

Shops and restaurants like glazed floor tiles because the smooth, shiny surface does not retain dirt and is easier to clean. But until now glossy tiles have not been as hard-wearing as rougher tiles, and they scratch easily. In areas of high traffic the surfaces rapidly deteriorate, losing the gloss and getting dirty.

"There was a need in the ceramic tile industry to develop floor tiles with a very white and glossy surface and with very good resistance to wear by abrasion," says Ana Gozalbo, of the Instituto de Tecnología Cerámica (ITC) in Castellón, a research institute dedicated to ceramics. "The problem is that where you have a smooth and white or very clear surface you can see wear more easily than with very rough and decorated surfaces."

ITC were approached by Keraben SA, a well-known Spanish manufacturer of wall and floor tiles, to see if they could devise a new kind of glaze that would not be so vulnerable to wear. Together with tile manufacturers Gres de Nules and other international partners they put together a CRAFT project that started in 1993. Other partners included distributors of tiles in Germany, Ireland and the United Kingdom, and an Italian company - Esmalglass - who supply the materials for making glazes for tiles.

Melting the frit
Tile glazes are made of a kind of glassy coating. "The tile has a ceramic body with a very thin layer of vitreous material on top and this is made of 'frit', which is a sort of glass," explains Ana Gozalbo. "You have a raw material like quartz, kaolin, or feldspar, then you mix them finely and then they are melted in a furnace at about 1500°C and then quenched in water. So you obtain fine particles of vitreous material, a glass, which is applied to the surface of the ceramic body then fired to obtain the final product. This surface layer is what gives the product its decoration."

When the tile is fired, the glassy layer "devitrifies" forming a tough, crystalline material that gives the surface its hard-wearing properties. "You need those crystalline faces to be as hard as possible," says Mrs Gozalbo. The ITC researchers chose as a starting material a mixture of silica, alumina and zinc oxide which was used to make a range of frits. By using different mixtures of the basic components and adding extra ingredients the team made a wide range of different frits of varying properties. "With these frits you can mix them with other constituents in order to give different characteristics to the surface. And every different mixture is treated in a different heating cycle to give a different glaze. There were hundreds of them because not only were we making products here, but also Esmalglass and Gres de Nules were doing experimental work in pilot and industrial plants."

Picture
Cross-section of one of the glazes developed in the project, viewed with a scanning electronic microscope. The white zones correspond to gahnite crystals that have formed surrounding the surface of the frit particles..

No cracking
But hardness is not the only desirable property of a glaze. "The whole crystalline and glassy mass has to have an expansion coefficient similar to that of the ceramic body. As you fire the product the glass sticks to the ceramic body. When you cool the product down you get problems if they have a difference in their expansion coefficients." The glaze can crack and come away if it does not contract at the same rate as the tile itself.

Another important characteristic is porosity, because porous surfaces will hold dirt. But because the final glaze contains crystalline particles, it is impossible to make a glaze that is not porous to some degree. "The most important thing is that it isn't an open porosity," Mrs Gozalbo points out. "It is just in the inner part of the glaze so it is not open at the surface. But if it is very porous then it will retain dirt and in that case it is not useful even if it is very hard."

Of the hundreds of glazes tested, a few were selected to be developed on a pilot scale at the premises of Keraben and Gres de Nules. CRIBC in Belgium tested the mechanical properties. The new glazes were excellent in most respects, but although they were hard they were not as hard as the researchers had hoped.

Glazes in production
"Some of the goals were too high to attain," admits Mrs Gozalbo. "It's very difficult if you have a smooth glassy product to raise the Mohs' hardness. But the glazes did have characteristics that are needed in a ceramic style floor: they weren't attacked by acids and bases, they didn't stain, they didn't retain dirt, and the most important thing was their resistance to wear by abrasion. Also they were quite glossy for a floor tile."

The project finished early in 1995, and Keraben and Gres de Nules then had exclusive use of the new glazes for two years. Now being commercialised more widely, the glazes can be employed in normal manufacturing processes and are no more costly than conventional glazes. The methods developed to identify the mechanical properties of the glazes in the project can now be used to characterise glaze properties in a systematic way that was not possible before.

 

 

Project Title:  
Obtaining smooth ceramic glazes with improved wear resistance and hardness

Programmes:
CRAFT/BRITE-EURAM

Contract Reference: CR100191/BRE20395

CORDIS databaseFor more information on this project,
go to the Cordis database Record

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