Natural Zeolite as a Cost-effective Opacifier

• Ali Ghafarinazari^{1, 2} , Esfandiar Amiri¹, Mahnaz Karbassi³, Morteza Soroor¹, Talieh Rajabloo³

Abstract

Zircon as an opacifier material is under the very real risk of being replaced. This is mainly because of two reasons: the anticipated shortage of high-quality grade zircon, and high costs associated with the production of zircon as an opacifier material, which is resulting in upward pressure on zircon prices. This study aimed at assessing the influence natural zeolite as opacifier of on both the technological behavior during processing and the technical performances of ceramic glaze for tile manufacturers. Moreover, preliminary investigations show this category of tiles has potential at antibacterial activity in a cost-effective way.

Keywords: Glass-ceramic; Anti-bacterial Tile; Titanates; Zirconia;

1. Introduction

The ceramic tile industry is being progressively moving its worldwide production toward new materials with improved aesthetic and technical properties. The availability of these raw materials in the huge amounts required by the tile industry is a problem in many areas, while in other contexts it is the high price to make the tile manufacture disadvantaged in competition with other producers of building materials, whose manufacturing costs are lower. Thus, the ceramic industry is continuously searching for cheap raw materials able to replace the traditional without altering the process and product characteristics [1].

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In glaze production, amongst commercial frits, zircon (ZrSiO₄) is most conventional opacifier [2]. Nevertheless, these frits are quite expensive and therefore, there have recently been certain attempts to lower the production cost such as optimizing amount of zircon [3], or taking another suitable glass–ceramic glaze systems [4]. Another common opacifier is titania (TiO₂). Although refractive indices of titania (2.52 for anatase and 2.76 for rutile [5]) is higher than zirconium oxide (2.17 [1]), but it has technical problems. First is low chemical stability during melting. In addition, rutile, which is stable structure of titania in standard conditions, is the main problem. Because rutile phase, in opposite of anatase, is yellow and leads to increase roughness of surface.

Zeolites are high porous and crystalline alumino-silicates with a three-dimensional structure based upon repeated units of silica (SiO₄) and alumina (AlO₄) tetrahedral [6]. Based on high-temperature phase transformations of natural zeolites [7], possibility of using Iranian natural zeolite as opacifier of tiles are investigated in this study. Zeolites belong to the tectosilicate mineral group and are building up by a framework of corner-sharing. The framework arranges as such to form a microporous structure with large cages (diameters less than 2 nm) connected into channels. They possess special properties, such as ion exchange, molecular sieves, a large surface area, and catalytic activity, which make them a preferable material for tremendous industrial applications in industries such as domestic and commercial water purification, softening, petrochemical industry, biogas industry, heating and refrigeration, detergents, medical, agriculture [8]. Now a days, they are promising for implementation at ceramic production, such as brick [9], ceramic pigments [10], porcelain and tiles bodies [11] also self-glazing ceramic tiles [12].

The main objective of this study is to use Iranian natural zeolite and develop from this a zeolite opacifier product for glazes, which possesses superior opacifying properties or whiteness values after application on a ceramic body, compared to that of the current zircon, more expensive commercial ceramic or prime grade zircon materials.

2. Experimental Procedure

2.1. Fabrication of Samples

In the first part of the experimental study, one single fast-firing opaque tile glaze selected as standard frit (Table 1). The basic Standard frit composition was selected consisting of 6 wt% suspending agent (kaolin), 0.2 wt% deflocculated (sodium tri-poly-phosphate), and 0.1 wt% ligand (Carboxyl Methyl Cellulose, CMC) in an eccentric mill used at selected suspension compositions. At this research, all of materials were of commercial grades (less than 98 wt% purity) from industrial clays.

2.2. Sintering

The frit suspensions applied on the surface of tile supports using a regulated glaze applicator for the deposition of raw layers with 0.4 mm thickness. The test specimens were then fired in a roller kiln corresponded to the temperature and rate used industrially to manufacture the product with the support and glaze in question. Heating and cooling rates were about 40 ºC/min and soaking time at 1000 ºC was 3 minutes.

2.3. Characterization of samples

The opacity of glaze was evaluated based on a colorimetric analysis using a Minolta CM-2600d spectrophotometer. The results are expressed by the tri-chromatic coordinates: L* means the degree of whiteness, a* indicates the variation between green and red colors, and b* presents the variation between blue and yellow colors, therefore investigation of opacity amount of L* is very important. To better understand the gloss results of the glazes (β₆₀) was determined by Zehntner ZGM1110 glossiness analyzer.

To complement the results, the glazes were characterized micro structurally by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). Crystalline phase identification was performed on glazes prepared from ground samples using a X-ray diffractometer (Philips PW 170) operating with (Cu-K_α = 1.54056 Å, 35 kV, 40 mA) radiation in the range of 10-60° 2θ, using the following settings: 0.1 mm receiving slit, 0.4 s/0.04° 2θ counting time. FESEM images were taken by Hitachi S-4160.

On account of experiments method and results, particularly SEM, we predicted that the new tile has potential for antibacterial activity. The popular method for antibacterial activity is ½ McFarland. We described in detail this method for antibacterial tile [13].

As briefly speaking, E. coli and S. aureus were deposited on tile slide (5 cm × 5 cm). Each slide was placed in a sterile vial. Tryptone soy broth was then added to each vial. An aliquot (10 ml) of S. aureus or E. coli suspension was added to each vial (1.6 × 10³ ml^-1) containing the slides. The vials were incubated with agitation at 35 °C, 220 rpm. The bacteriostatic activity was evaluated after 24 h and the percentage of bacteria reduction was calculated using the following equation (1):

(1)

In which R is the reduction rate, A is the number of bacterial colonies from untreated tiles and B is the number of bacterial colonies from the treated tiles.

3. Results and discussion

In order to investigate effect of zeolite in glaze at different chemical conditions, some experiments carried out in the same firing profile. Output of glossiness and spectrophotometer these experiments showed in Table 2. Before going to the results, it is better to remind that depend on type of body, in general acceptable amount of L* for an opaque glaze is 83.24, it is noticed that this reference opaque glaze has 14wt% Zircon. In addition, acceptable amount for glossiness for shiny tile is at least 88 such as standard frit which is perfectly transparent.

After checking standard, conventional opacifires, zircon and titania, were added. When 10wt% zircon was added to frit as state of compound, degree of whiteness and covering increased sharply, without significant effect on glossy. Uniformity distribution of unfired zircon powders on standard frit exhibited on Figure 1.

Another conventional opacifier, titania, was added in amount of 5 and 10wt% to the standard frit. Outcomes in Table 2 confirm that amounts of whiteness (L*) and covering (a*) is very upper than zircon but yellowish amount (b*) and decreasing of shiny confirm that titania is in rutile crystallographic state. Morphology of titania on glaze is directly depended on crystallographic state. Microstructural studies confirmed that rutile crystals always have had an acicular morphology, however, anatase crystals were observed with cubic and rectangular morphologies [14]. The other stable phase of titanium in glaze is sphene (CaTiSiO₅ [15]), which is completely sphere in glaze [4]. Visual effect of sphene and anatase is white, while anatase has benefits such as semiconductor and antibacterial effects. Bou et al. published that in order to improving smoothness and glossiness adding 1-3wt% P₂O₅, rutile transformed to sphene. The problem of sphene is non-uniform distribution in glass matrix [4], which decrease chemical and mechanical resistant.

Based on Figure 2, existence of rutile confirmed. This big crystal leads to roughness of surface and diminishing glossiness to 25. One of interesting results of FESEM of glaze containing titania is elemental distribution on glass-ceramic. In glassy matrix, atomic percentages of fluxes are 0.93, 4.87, 7.01, and 2.36 % for Na, K, Ca, and Zn respectively, but these amounts are 3.46, 5.25, 12.35, and 3.11 %; i.e. around crystalline part flux materials especially sodium and calcium lead to nucleation. And about 3 at% titanium was solved in glass, due to low chemical stability.

In order to investigation of effect of natural zeolite in glaze and comparing with other opacifiers, 10wt% natural zeolite supplemented to the standard. Result of XRD shows that the zeolite has crystallinity based on having less amount of background with sharp peaks (Figure 3). The zeolite leads to covering and whitening surface without significant effect on glossy. As it can be seen in Figure 4, there are large amounts of unfired or crystalline materials in glass matrix which is due to existence of zeolite. In contrast, for higher temperature (1100 °C) and time duration (2 h) zeolite acts as a flux [1], but at this condition it acts as a refractory. Although these results (Table 2) were comparable with zircon, we could not achieve a sufficient opaque glaze because of refractive index of zeolite which is about 1.48 – 1.60 [16], and is close to silicate-leadless glasses (1.5 – 1.7) [5].

For sufficient covering with whitening in glaze by zeolite, possibility of using zeolite as a nucleus for titanium oxides was inspected. For this purpose, 3wt% TiO₂ and 7wt% zeolite added to the standard. Table 2 confirms that this glaze is an acceptable as an opaque glaze. This glaze has a shiny surface, well covered tile body, high whiteness, and especially low price due to cheap raw materials. To complement the results, FESEM was carried out on these tiles. Figure 5 exhibits rectangular nanoparticles (25-500 nm, 85 nm average) with uniform distribution in glaze. Based on previous lectures, this structure is related to anatase [5]. Owing to EDX analysis, elements distribution on matrix and ceramic generally was the same with previous samples but percentage of Ti in matrix relented to 0.42 (instead of 3 at% in Figure 2). Indeed, natural zeolite is a cheap agent for nucleation of nanocrystalline anatase without side effect on other properties of tiles.

One proposed mechanism for this phenomenon from thermodynamic point of view is alumina and silica which leads to stability of anatase [9]. Based upon zeolite is alumino-silica, this reasoning was checked by kaolin, which is a mineral clay contains mainly alumina and silicates. Results of this experiment are illustrated in Table 2 and Figure 6. Kaolin had no effect on phase and morphology of rutile, and it led to increasing of roughness and firing temperature of glaze. In a conclusion ability of natural zeolite in making glass-ceramic by nanoparticles of anatase is unique according to high porosity and crystallinity (Figure 3) structure.

Due to the fact that there are anatase nanoparticles in the glaze, it is promising having antibacterial activity [17]. This phenomenon investigated by ½ McFarland method as wrote in experimental procedure at end of section 2.3.

In this case it has been shown achieving stable nanostructural anatase is not enough for antibacterial activity [18]. In fact, anatase must be excited by Ultraviolet illumination. This tile has just about 35% bacteriostatic which is not significant comparison by naked eye and it needs to measure by microscope. This result is very low in comparison with other components which had more than 95% [13].

It would be obvious that titania at glaze initially melted and then solidification carried out in the form of anatase. The increase zeolite anti-bacteria agent is prepared by achieving smaller anatase particles and better distribution which can be realized by chemistry of glass network. However, due to the fact that propose of this research was just opacity, this result also is very interesting and beyond our goal.

4. Conclusion

Natural zeolite is a new mineral source for construction and ceramic glass materials. Our research showed that zeolite is economically expedient to use natural zeolite for fabricating opaque glaze. Indeed natural zeolite led to stabilized nano crystalline of anatase. This glaze has potential for antibacterial tiles. Initial investigation showed potential of antibacterial activity if this kind of products in a cost-effective way.

References

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Figure captures

Figure1 FESEM of glaze with 10wt% zircon.

Figure2 FESEM of standard frit with a) 5 and b) 10wt% titania

Figure3. XRD from Iranian natural zeolite

Figure4. FESEM of standard frit containing 10wt% natural zeolite

Figure5. FESEM of standard frit containing 3wt% TiO₂ and 7wt% Zeolite; a) morphology of crystal is in range of 25 – 500 nm, in average of 85 nm; b) fine distribution of crystals.

Figure6. FESEM of standard frit containing 3wt% TiO₂ and 7wt% kaolin

Table captures

Table1. Composition of the standard frit

Table2. Results of spectrophotometer and glossiness

1

 Corresponding author. Tel.: +39 3886598606 E-mail address: [email protected] (A. Ghafarinazari)