The Meso-Porous Ceramics Research Group (headed by Tohru Nonami) of Ceramics Research Institute (CRI, Director: Tetsuya Kameyama), National Institute of Advanced Industrial Science and Technology (AIST, President: Hiroyuki Yoshikawa), one of independent administrative institutions, succeeded in synthesis and commercialization of titanium oxide photocatalyst coated with apatite fluoride in collaboration with Tomei Co., Ltd. (Tomei, President:. Shoji Yamashita and Executive Director Naoto Tone). The newly developed apatite fluoride-coated TiO₂ photocatalyst has much higher resistance to acid than that coated with hydroxyapatite previously developed by CRI-AIST, and is expected to expand the application area to paint for exterior coating owing to the tolerance to corrosion by acid rain.

The technology for synthesizing TiO₂ photocatalyst coated with hydroxy apatite has been transferred to a private enterprise and commercialized. The composite catalyst is characterized by dual actions: adsorption of odorous and/or noxious substance by apatite with or without light, and decomposition of adsorbed matter under illumination. Besides, apatite coating protects photocatalyst mixed in resin or paper, ensuring broader area of application.

Apatite is a compound of calcium phosphate group, which constitutes major components of tooth and bone. It is less tolerant to acid, and readily affected by acid rain. The existing apatite-coated TiO₂ photocatalyst is not suited for use under harsh conditions of exterior exposed to acid rain. The CRI-AIST in collaboration with Tomei embarked on the research and development of photocatalyst coated with apatite fluoride having much better acid-fast property in comparison to hydroxy apatite. Consequently, the joint efforts succeeded in coating TiO₂ photocatalyst powder with apatite fluoride leading to the synthesis of new apatite-coated titanium dioxide with acid-proof capability drastically improved without degrading existing property.

Tomei is expecting to apply the new catalyst-paint not only to interior coating, but also to exterior use such as painting for road walls, tunnels and buildings. Test executions with paint manufactured on trial basis have already provided a promising result, allowing to aim at commercialization in the near future.

In general, apatite refers to biological materials including hydroxy apatite constituting main ingredients of bone and tooth enamel, and its industrial application involves extraction column for proteins and amino acids. It is a very attractive material, covering a broad spectrum of applications, based on chemical (carrier in fluorescent paint), mechanical (artificial bone) and photonic (photocatalyst) characteristics. Apatite belongs to pseudo-hexagonal system with a generic molecular formula, A₁₀B₆X₂.The A site concerns adsorption, and hydroxy apatite has hydroxy group at the X site.

However, hydroxy apatite has a certain shortcoming: poor resistance to acid, readily dissolving in acidic solution. This prevents hydroxy apatite from being applied to exterior coating directly affected by acid rain, and recalcification of decayed tooth, which is exposed to strong acidic medium. In order to clear this problem, the development of material with hydroxy apatite-like property and upgraded acid resistance, has been urgently sought for.

For the purpose of overcoming this shortcoming, the CRI/AIST-Tomei joint team has addressed to the synthesis of apatite fluoride, replacing hydroxy group at the X-site with fluorine (F). The conventional method of coating TiO₂ with hydroxy apatite consists in simply immersing titanium dioxide powder or film in simulated body fluid with pH and composition adjusted to facilitate apatite formation and temperature close to body temperature. Based on this simple process, the synthesis using newly prepared simulation fluid has led to successful coating with apatite fluoride, opening the way to the mass production. The weight ratio of apatite fluoride to titanium dioxide can be adjusted according to the application purpose, with usual composition: 10-20 % apatite fluoride and 90-80% TiO₂.

When immersed in 1% aqueous solution of hydrochloric acid, hydroxy apatite dissolved out promptly, while apatite fluoride remained intact. In this way, superior anticorrosive property of apatite fluoride in acidic medium to hydroxy apatite has been confirmed.

It has been also ascertained that apatite fluoride has equivalent formaldehyde adsorbing capability to hydroxy apatite, and its application to exterior coating is to be sought for. A trial sample for outer wall coating has been used in an actual scene providing satisfactory results. It is expected that apatite fluoride-coated TiO₂ photocatalyst will play central roles in stain prevention for exterior structures, deodorant measures in the interior, countermeasures against sick house syndrome, and so on.