1.3. TiO2 coatings

One of the most promising areas in photocatalytic building materials is photocatalytically active tiles, which are self-cleaning but also can decompose air pollutants. The standard approach has been to apply TiO2-based materials on ceramic surfaces and to attempt to immobilize the TiO2 at as low a temperature as possible. In the case of spray coating of aqueous dispersions of TiO2 on clay roofing tiles, the heating temperature was 290C [27]. Chemical vapor deposition (CVD) using gaseous precursors, following by heating at 550C, also has been investigated [28] but these films suffered from poor adhesion. By contrast, plasma-enhanced chemical vapor deposition (PECVD), involving reheating at 400C, yielded high-quality materials [29]. Aerosol flame synthesis (temperature probably <700C) has been used to produce ultra-fine TiO2 nanoparticles that could be deposited directly on supports [30]. Sol-gel TiO2 precursors have been coated on cement-based materials by dipping and heating at 500C [31]. TiO2 suspensions in water or alcohol also have been sprayed on limestone with no subsequent heating since the porous microstructure immobilized the powder [32]. All of these approaches used nanosized TiO2.

The TiO2 was deposited on porcelain grès tiles that were manufactured by GranitiFiandre S.p. A. These are dry-pressed tiles fired at 1200–1300C. The phase assemblage as such consists of

Photocatalytic TiO2: From Airless Jet Spray Technology to Digital Inkjet Printing

http://dx.doi.org/10.5772/intechopen.72790

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Conventional, industrial-scale, airless, jet spray deposition was used for the deposition of coatings, as described in brief previously by the authors [37]. However, with the new, innovative, automated decoration of ceramic surfaces by digital instruments, it has become possible to manufacture products of considerably superior quality. Consequently, this work also reports an investigation of the deposition of coatings of photocatalytic TiO2 by digital inkjet printing

A prototype digital printer manufactured by Projecta S.p.A. was used to apply the suspensions. Following deposition, the tiles were calcined at 680C for 80 min using an industrialscale kiln designed to avoid thermal shock. The cooled tiles then were washed with water, brushed vigorously to remove loose particles, and air jet dried. The process is summarized in Figure 1. The water used for degreasing was recovered and purified prior to recycling; any

2.2. Digital inkjet printing and airless jet spraying procedures

using a proprietary ink containing suspended microsized TiO2 particles.

glass, mullite, and quartz.

damaged tiles were recycled.

Figure 1. Summary of digital inkjet printing process.

Although spray coating remains the simplest and least expensive coating technique, recent work [33] reports the functionalization of commercial glazed ceramic tiles with pigment-grade, microsized, anatase (Kronos 1001) using an industrial-type process. This method offers a reduction in the risk of inhalation as it does not involve the use of nanosized TiO2 powders. The TiO2 powder was deposited by airless jet spraying aqueous suspensions containing a silica-based commercial product as binder. The tiles were dried and then fired at 680C in an industrial kiln for 80 min (cold-to-cold cycle).

This work reports a new deposition technique, which is digital inkjet printing. Despite the advantages of conventional spray coating methods, digital inkjet printing offers an advantageous, precise, coating technique that utilizes piezoelectric heads to effect directional deposition of an aqueous/organic ink suspension within an electrostatic field [34]. The suspension contained pigment grade, microsized anatase (Kronos 1077), and this was printed on the surface of porcelain grès (viz., stoneware) tiles using a digital printer. This process has several advantages, including rapid and precise and more uniform deposition and essentially no waste of raw materials, which are favorable for both the economics of the process and the environment. Moreover, with this technique, slabs of large size up to 120 360 cm can also be activated. The data for the thin films deposited by digital inkjet printing are contrasted with those for the more conventional method of jet spraying.
