Improved stain resistance in polished ceramic tiles may now be possible, say researchers at the Instituto de Ceramica y Vidrio in Spain. They have introduced an organic fugitive phase during liquid sintering of green tiles to achieve reduced porosity volume with various pore sizes.

Lead researcher Julian Reniosa at the Instituto de Ceramica explains, ‘Porosity represents a problem in stoneware because it is related to a decrease in stain resistance. The addition of low volume per cent (in vol.) fugitive phase reduces the porosity by up to 50%, whereas the mechanical properties remain constant. So the study is useful for achieving better quality stoneware tiles with higher densification.’

Using almond shell as the fugitive phase, three different average particles have been added to the green porcelain compositions of 30, 200 and 600mm in 1, 3, 5, 10, 15, 20 and 25 vol.%. He adds, ‘This allowed us to correlate the final properties of the stoneware and study the mechanism and porosity of the tiles. The ash residues of almond shell, after heating at 600ºC for one hour, was lower than 0.3 wt%, serving as a porosity former during the sintering process’.

According to Reniosa, the lower the particle size of the fugitive phase, the higher the shrinkage level of the pores. The most significant reduction of porosity was obtained when the added fugitive phase was less than five per cent by volume.

He says, ‘The decrease in porosity could benefit the polishing of stoneware ceramics, as during this step, the closed porosity appears at the surface and improves stain resistance.

‘Up to now, different attempts to reduce porosity in stoneware were reported related to compositional changes, complex processing steps and polishing strategies that failed to be transferred to standard stoneware production.’

He continues, ‘Incorporation of a fugitive phase has demonstrated effective in porosity reduction and is fully compatible with standard stoneware processing. Higher valuated stoneware tiles could be obtained because of the low porosity, if one to three per cent of fugitive phase is added to the green composition’.

Standard stoneware

Sample with one weight per
cent of fugitive phase of 30µm

Reniosa also insists that the processing technique will not raise the cost of final products, as the fugitive phase price is comparable to other rheological additives used in the ceramic industry’, yet comparisons with other additives are yet to be tested.

Furthermore, decreased porosity of up to 50%, is dependent on the volume of initial porosity of the tile and inversely proportional to the fugitive size.

Ceramics expert, Barry Lye says, ‘The research is promising and could clearly lead to far more control of porosity in tiles during the production process, which in turn could lead to improved stain resistance, water absorption and frost resistance in a more controlled and predictable process.

‘However, I would like to see a controlled production run to see if this mirrors the laboratory result’.

Studies on controlled porosity formation have been achieved before, yet a decrease in porosity has not been easily achieved, argues Reniosa. He says, ‘By the addition of fugitive phases in determinate sizes, porosity can be decreased quicker and easier so the properties of the materials are improved’.

 

Author : Ledetta Asfa-WossenClay Technology Magazine, 12 Aug 2010