Ceramic Processing Research

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1 Journal of Ceramic Processing Research. Vol. 13, No. 5, pp. 635~640 (2012) J O U R N A L O F Ceramic Processing Research An investigation of the effect of alkaline oxides on porcelain tiles using factorial design Zahide Bayer Ozturk* and Nuran Ay Anadolu University, Faculty of Engineering and Architecture, Department of Materials Science and Engineering, Iki Eylul Campus, 26555, Eskisehir, TURKEY In this study, the effect of alkaline oxides (MgO, CaO, Na 2 O and K 2 O) was investigated in porcelain tile compositions. Talc (MgO) and dolomitic clay (CaO, MgO) were added to porcelain tile compositions instead of feldspar (Na 2 O, K 2 O). In the experiments mixed-level factorial design was used, the individual effects of three main factors and their interactions were determined. The effect of the amount of talc (0, 2, 4 wt. %), dolomitic clay (0, 2, 4 wt. %) and firing conditions (1, 2) on porcelain tile properties were examined. Firing shrinkage, water absorption and the strength of samples were studied by means of factorial design. The porosity and bulk density of samples, color measurement and SEM analyses of these samples were also investigated. It was found that the strength increased and water absorption decreased by means of alkaline oxides variation in the porcelain tile body. According to factorial design, it has been found that the amount of dolomitic clay and talc are the main factors affecting the strength, firing shrinkage and water absorption values. It appeared that the magnitude of these properties were dependent on the amount of alkaline-earth oxides (CaO, MgO). Key words: Experimental design, Porcelain tile, Talc, Dolomitic clay. Introduction Ceramic tile production is a highly energy-intensive process due to drying and firing operations. Considering this fact, to reduce the energy costs, significant among the production costs, studies which increase the production rate and reduce the costs by using new raw materials have become more of an issue. Recent studies have focused on the use of magnesium-containing raw materials in porcelain and glass-ceramic compositions. These materials promote not only sintering kinetics and densification but also early vitrification. MgO is one of the important additives stimulating mullite formation at a low temperature [1, 2]. In another study [3], the effects of MgO addition to Egyptian clays with different Al/Si ratios in the phase formation and morphology of the mullite developed were investigated. Results indicated that MgO additions reduced the temperature of liquid phase formation and the viscosity of the glassy phase. The addition of raw materials containing magnesium led to an increased liquid phase volume and correspondingly decreased viscosity, and also decreased the surface energy caused by the development of the densification behavior and physical properties [4-7]. Moreover, as a result of the improvement in major properties, ceramic bodies could be produced with a lower porosity and *Corresponding author: Tel : ext.6590 Fax: zbayer@anadolu.edu.tr firing temperature [8-10]. According to the literature about alkaline oxides, it is common to employ sintering promoters such as nepheline-syenite, talc and alternative fluxing raw materials rich in K 2 O and Na 2 O (sludge waste, granite rock) to the vitrification of different bodies in order to accelerate the process resulting in the formation of a melt and develop technical properties of bodies [11-13]. There are many factors affecting the quality and technical performances of the processing during production, such as raw material selection, process equipment etc. In order to determine factors influencing the characteristics of the product, an experimental design method is used. This is created by mathematical models and used to survey the qualification and determination of the output and also its effectiveness [14]. One of the experimental design methods is factorial design in which all combinations of the factors examined and each of their repetitions can be investigated [15]. Talc and dolomitic clays, which are alternative fluxing raw materials rich in MgO, promote sintering in porcelain tiles but, also increase shrinkage and thus a determination of the optimum amount of talc and dolomitic clay in a porcelain tile becomes vital to control its physical properties, particularly shrinkage. The aim of this study was to explore the influence of alkaline oxides on porcelain tile bodies using factorial design experiments. Within the framework of this study, the effect of alkaline-earth oxides through the combined use of talc and dolomitic clay in varying amounts into porcelain tile compositions and firing 635

2 636 Zahide Bayer Ozturk and Nuran Ay temperature/time were investigated by employing multi-factor experimental design. For this purpose, mixed level factorial design was used and the individual effects of three main factors and their interactions were determined for firing shrinkage, water absorption and the strength. Moreover, the porositybulk density and color properties were investigated. Experimental Procedure Porcelain tile raw materials (clays, feldspar and pegmatite) were provided by Yurtbay Ceramic Plant (Turkey). Talc and dolomitic clay were added to porcelain tile formulations instead of feldspar in a 2-4 wt. percentage ratio used in the standard recipe. The chemical composition and amounts of raw materials that were modified in recipes are given in Table 1-2. Moreover, porcelain tile bodies provided by all the raw materials (clays, feldspar, pegmatite, talc and dolomitic clay), whose weight percent of alkaline oxides are given in the Table 3. The total of alkaline-earth oxides of bodies with talc and dolomitic clay and the total of (MgO + CaO + Na 2 O+K 2 O) is higher than in the standard porcelain tile body. Porcelain tile mixtures were wet-ground in a ball mill long enough until the residue on a 63 µm sieve was reduced to the required value. The slips obtained were dried in an oven at around 110 ο C, deagglomerated, humidified (7 wt. % moisture content) and finally sieved down to 1 mm before forming. Rectangular samples, mm in size, were prepared by uniaxial pressing at a forming pressure of kg/ Table 1. Chemical Analyses of the modified raw materials (wt. %) Raw Loss on materials ignition SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO Na 2 O K 2 O Feldspar Talc Dolomitic clay cm 2 (Gabbrielli Press). All samples initially had the same dry bulk densities. Subsequently, the samples were single fast fired in a roller furnace under industrial conditions at different peak temperatures. Total firing times (cold-to-cold) of 35 and 38 minute were implemented at 1200 ο C and 1205 ο C, respectively in the Yurtbay Ceramic Plant. The water absorption capacity of the fired samples was measured according to ISO Part 3 and also the strength of the samples was measured according to ISO Part 4 [16, 17]. Moreover, color values of fired samples were measured with the aid of a spectrometer (Minolta CR Colormeter). Bulk densities and open porosity of the sintered samples were calculated by the water immersion Archimedes principle by using the related equations (Eqs. (1) and (2)): w d w w w s Bulk density = w w w w ( w Open porosity(%) d ) = ( ) 100 (1) (2) where w d is the dry weight of the sample, w w is the wet weight of the sample and w s is the weight of solid Table 2. Amount of modified raw materials in recipes Samples Feldspar (wt. %) Talc (wt. %) Dolomitic clay (wt. %) w s Table 3. Amounts of alkaline oxides in chemical analyses of porcelain tile bodies (wt. %) Samples CaO MgO Na 2 O K 2 O (CaO + MgO) (Na 2 O+K 2 O) Σ (Alkaline oxides)

3 An investigation of the effect of alkaline oxides on porcelain tiles using factorial design 637 suspended in water [18]. The sample microstructures were examined by scanning electron microscopy (EVO-50, Carl-Zeiss). The SEM samples were polished using a polishing machine (Metkon- Forcipol-300-1N) and their surface etched for s with 5 weight percentage of HF solution. Results and Discussion The firing strength and shrinkage, water absorption values of 36 samples which were prepared according to and two repeated mixed level factorial designs were measured. The main factors and their levels are given in Table 4 and the main effects of each factor and their interactions are given in Table 5. These interactions were analyzed with an ANOVA table for physical properties of samples. An ANOVA table provides a means to understand whether there is a difference among the factor levels and to be able to determine whether their interactions are meaningful [19]. Table 4. Factors examined and their levels Factors Levels The amount of talc (wt. %) The amount of dolomitic clay (wt. %) Firing temperature/time ( ο C/minute) 1205/ /35 - Table 5. Main factors and their interactions Main factors The amount of talc The amount of dolomitic clay Firing temperature/time Two factor interactions Three factor interaction The amount of talc* The amount of dolomitic clay The amount of talc* The amount of talc* Firing temperature/time The amount of dolomitic clay* Firing temperature/ time The amount of dolomitic clay* Firing temperature/ time In the ANOVA table (Table 6) which includes the effective main factors and their interactions according to firing shrinkage. DF: meaning degrees of freedom, Seq SS: meaning sum of squares, MS meaning square, F distribution is used to determine the differences between factor variances. So the largest F value is the most effective factor in the model. P values indicating the ratio of the unadmitted region (P values which are lower than the admitted α are effective). α = 0.01 value is used (with a 99% confidence interval). The main factors and their interactions whose effectiveness are meaningful are added to the ANOVA table error term [15]. If the value P of the main effect and the interactions of the factor is higher than 0, 01, this interaction and effects are added to the error table. The interaction graphs of effective factors are given in Fig % dolomitic clay, with 8.8% talc, a firing temperature/time with 7.9%, dolomitic clay *firing temperature/time with 3.5% are significant factors for firing shrinkage and these factors constitute 90.9% of the total effect. Firing shrinkage values of porcelain tiles are 6-7 in the standard. The result of shrinkage values of samples meets the requirements of the standard. According to the alkaline oxide content of Fig. 1. Main effects plots for firing shrinkage. Fig. 2. Main effects plots for water absorption. Table 6. ANOVA table of firing shrinkage value for effective factors Factors DF Seq SS MS F 0 P The amount of talc The amount of dolomitic clay Firing temperature/time The amount of talc* The amount of dolomitic clay The amount of talc* Firing temperature/time The amount of dolomitic clay* Firing temperature/time The amount of talc* The amount of dolomitic clay* Firing temperature/time Error Total

4 638 Zahide Bayer Ozturk and Nuran Ay Table 7. Bulk densities and open porosity of the tile bodies versus firing temperature 1205 οc/38 minute Samples 1200 οc/35 minute Bulk Bulk Open Open Samples density density porosity % porosity % (gr/cm3) (gr/cm3) Fig. 4. Secondary electron image of sample with 1 code (M: Mullite, Q: Quartz, G: Glassy Phase). Fig. 3. Main effects plots for firing strength. samples, it has been found that increasing the alkalineearth oxides and a slight decreasing of alkaline oxides in samples promotes the development of densification. Thus alkaline-earth oxides are raised in the amount of shrinkage. Dolomitic clay is the main effective factor on shrinkage due to its large amount of MgO. 43% dolomitic clay, with 21.8% talc, a firing temperature/time with 7.1%, talc *dolomitic clay with 25.1% are significant factors for water absorption and these factors constitute 97.2% of the total effect. As seen Fig. 2, the variation is increasing with the amounts of dolomitic clay and talc. A porcelain tile is characterized by a low water absorption (usually less than 0.5%). The water absorption values are almost zero in the second level of amount of talc and dolomitic clay factors and in the first level of firing temperature/ time factor. The water absorption value is sensitive to talc amount*dolomitic clay amount interaction factors when their amounts are at a high level (4 %). The interaction factors except for of talc amount*dolomitic clay amount do not have a significant effect on water absorption. Considering the water absorption data obtained the in factorial design and the effect of alkaline oxides; bulk densities and percentage open porosity of the sintered samples were also measured (Table 7). As seen from the table, in 1205 οc/38 minute the bulk density and porosity percentage of the non-added sample (1) Fig. 5. SE image of sample with 3 code. reached 2.30 gr/cm3-5.12, while the 4% added talc and dolomitic clay samples reached gr/cm3-0.09% (sample 3) and 2.36 gr/cm3-0.11% (sample 5), respectively. Increasing the alkaline-earth oxides in samples and the firing temperature/time resulted in not only a decrease of the open porosity but also an increase of the bulk density. Thus, water absorption of samples (directly related to the open porosity) is decreased. With 44.8% dolomitic clay, 27% talc, a firing temperature/time with 18.8%, talc *dolomitic clay with 4.4% are significant factors for the firing strength (Fig. 3) and these factors constitute 95% of the total effect. The improved development in the firing strength can be explained by both the increased alkaline-earth content in samples and bulk densities of sintered samples. The effect of the alkaline oxides, namely MgO, was to increase the strength of samples. The results showed that the use of talc and dolomitic clay as a fluxing agent instead of feldspar in a porcelain tile noticeably increased both the strength and densification. In order to evaluate how the strength increases with the amount of alkaline-earth oxides open porosity in the function of temperature was decreased with the

5 An investigation of the effect of alkaline oxides on porcelain tiles using factorial design 639 Fig. 6. SE image of sample with 5 code. Fig. 8. SE image of sample with 7 code. Fig. 7. SE image of sample with 6 code. degree of densification proceeded from the amount of glassy phase. The presence of alkaline and alkalineearth oxides in porcelain tile bodies promotes the development of a less viscous liquid phase, which in turn improves the densification kinetics as long as it is operated within defined limits of composition [20]. Thus, the rate of the elimination of the porosity increased. The liquid phase development in the presence of talc and dolomitic clay (a high amount of MgO) resulted in a lower porosity in comparison with the non-added sample. Depending on the data of the open porosity and bulk density, it was revealed that dolomitic clay and talc provided an increase in strength. In order to explain the effect of the amount of talc and dolomitic clay which is among the most effective factors on firing shrinkage, strength and water absorption, by taking samples from the samples 1, 3 and 5, which were subjected to the same the firing conditions (1205 οc/38 minute), the microstructures of the samples were investigated (Figs. 4-6). Microstructures of samples are composed of mullite (M), quartz (Q) and glassy phase (G) when the firing temperature/time factor is stable. A result of using magnesium oxide with different clays in the structure is known to reduce the formation temperature of the glassy phase and viscosity of glassy phase. These two Fig. 9. SE image of sample with 8 code. effects not only provide chemical transport and a morphological structure of mullite crystals but also play a significant role in the mechanical characteristics of samples and in their densification [3]. Accordingly, these results of talc*dolomitic clay is a very effective factor between interaction factors for the firing strength and water absorption. In order to explain the effect of this factor, taking representative samples from the samples 6, 7, 8 which were fired at 1205 οc/38 minute the microstructure of the samples has been investigated (Figs. 7-9). As a result of using dolomitic clay and talc together, it is observed that mullite crystals, which also increase in relation to the amount of liquid phase in comparison with sample 1, are great in number and that the distribution of mullite crystals was homogenous. According to Rietveld analyses when the alkaline-earth oxide content was increased, the amount of glassy phase was increased (Table 8). A variation of the amount of alkaline oxides in samples effects the L*, a*, b* color parameters. The L* values of fired samples are reduced with an increasing alkaline-earth oxide content in Table 9. The reason for the decrease in L* values is the Fe2O3 content of dolomitic clay (Table 1) and the other one, the matured temperature of samples with talc and dolomitic clay is low due to an increasing amount of glassy phase.

6 640 Zahide Bayer Ozturk and Nuran Ay Table 8. Quantative analyses results of samples fired at 1205 ο C/ 38 minute. Samples Quartz Mullite Glassy phase ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.68 Table 9. Color parameters of fired samples 1205 ο C/38 minute 1200 ο C/35 minute Samples L* a* b* Samples L* a* b* Conclusion The present study demonstrated that the amount of talc and dolomitic clay and firing temperature/time have significant effects on firing strength, shrinkage, and water absorption of samples in a mixed level factorial design. A large amount of alkaline-earth oxides coming from dolomitic clay and talc in the samples caused a lower water absorption capacity and higher strength compared to the samples without dolomitic clay and talc resulting from matured structures and better sintering. Whiteness values of samples decreased gradually with the dolomitic clay content. A large amount of alkaline-earth content of talc and dolomitic clay, reduced the open porosity of industrial porcelain tiles; therefore, giving higher bulk densities of samples than that of the non-added sample. Due to the large amount of glassy phase coming from total alkaline oxides in the samples provided by the degree of the densification, a lower open porosity and higher firing strength were obtained. It is also possible that the addition of talc and (especially) dolomitic clay with 2-3 wt. percentages contribute to an improvement of the physical properties (shrinkage, strength etc.) which facilitates the mechanical manipulation of a porcelain tile. The addition of dolomitic clay to the industrial porcelain tile structure has reduced the whiteness ratio of the structures. An increasing firing temperature and impurities in the clay also caused degradation of the L * value in samples with dolomitic clay. The experimental design and porosity-density measurements demonstrated that a suitable combination alkaline, alkaline-earth oxides provide an increase in the strength and a decrease in the water absorption capacity created advantages in terms of its use in certain ratios instead of feldspar. References 1. L. Montanaro, C. Esnouf, C. Perrot, G. Thollet, G. Fantozzi, A. Negro, J. Am. Ceram. Soc. 83 (2000) L. Montanaro, J.M. Tulliani, C. Perrot, A. Negro, J. Euro. Ceram. Soc.,17 (1997) W.M.N. Nour, H.M. Awad, J. Aust. Ceram. Soc. 44 [2] (2008) V. Biasini, M. Dondi, G. Guarini, M. Raimondo, A. Argnani, S. Di Primio, Silicates Industriels 68 [5-6] (2003) G.N. Maslennikova, T.I. Koneshova, Translated from Steklo i Keramika 4 (1987) A.W.A. El-Shennavi, M.M. Morsi, G.A. Khateer, A.M. Abtel-Hameed, J. Therm. Anal. 51 (1998) V.K. Marghussian, M.H. Dayi Niaki, J. Euro. Ceram. Soc. 15 (1995) V.P. Ilina, G.A. Lebedeva, G.P. Ozerova, I.S. Inina, Glass and Ceramics 62 (2005) E.M. Sallam, H.W. Hennicke, Trans. J.Br. Ceram. Soc. 82 (1983) S. Ferrari, A.F. Gualtieri, T. Bellarni, C. Pasquali, Ceramurgia e Ceramica Acta 34 (2004) I.M. Bakr, E. El-Fadaly, M.R. Abo Breka, Interceram 58 (2009) K.O. Teixeira, S.N. Monteiro, C.M.F. Vieira, Materials Science Forum (2010) M.I. Ryshchenko, E.Y. Federenko, M.A. Chirkina, E.L. Karyakina, S.A. Zozulya, Glass and Ceramics 66 (2009) web site: D.C. Montgomery, in Design and Analysis of Experiments, 5th ed., (John Wiley & Sons, New York (2001) ISO , Ceramic Tile Part 3 Determination of Water Adsorption, Apparent Porosity, Apparent Relative Density and Bulk Density, Switzerland (1997). 17. ISO , Ceramic Tile Part 4, Determination of Modulus of Rupture and Breaking Strength, Switzerland (1997). 18. E. Suvacl, N. Tamsu, J.Euro.Ceram.Soc. 30 (2010) Z. Bayer, N. Ay, N. Erginel, VII.International Ceramic Congress Abstracts Book (2008) G. Cigdemir, A. Kara, F. Kara, Industrial Ceramics 30 (2010)