PAPERmaking! g FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
Introduction The advancing technologies of printing and packaging have placed greater demands on the surface of the paper sheet. To meet the more stringent requirements, many papers are coated with suitable pigment-rich formulation to provide gloss, smoothness, colour, print detail, and brilliance by filling in the void area on the surface of the paper sheet and covering the highest sitting fibres on the base paper surface (Smook 1997). The coating mixtures are highly concentrated water-based suspensions containing, among other additives, inorganic pigments, binder thickeners and other additives. Pigment is the most abundant component in the coating, so pigment is naturally the most important factor affecting the properties of the coating materials (Gullichsen et al. 2000). Pigments are used as a blend of different sizes and shapes of various pigment materials. Various speciality pigments with higher cost are often introduced in small amounts to optimise the coating properties (Ninness et al. 2003). One of the most commonly used paper coating pigments are clays. Clays are hydrous aluminium silicates combined with many other mineral species. Kaolin as one kind of clay minerals have been used as the leading white pigment in paper fillers, paper coating and other different applications such as ceramics, paint, cracking catalyst, cements, waste water treatment, and pharmaceutical industries. Although Kaolin is a commercially available and low-cost paper coating pigment, the need for special paper with special optical characteristics has limited its use in paper coating. Kaolin has comparable whiteness, but lower light scattering index (83.5m 2 /kg) than that of the TiO 2 micropowder pigment (254.7m 2 /kg), nowadays used commonly (Murray and Elzea 2005). Recently, titanium dioxide nanopowder has received much interest. This is due to its use in various applications such as cosmetics, paper and medical devices coating and gas sensors (Deorsola et al. 2008; Kobayashi et al. 2008; Wang et al. 2008). Several papermaking trends necessitate more use of TiO 2 including reduction in basis weight and the increasing use of cheap, discoloured fibres. Also, because TiO 2 is used in a large variety of products, its global demand growth is increasing rapidly along with its exceptionally high price. It has variety of advantages such as high surface tension, specific surface area, magnetic property, lower melting point, good thermal conductivity and environmentally friendly (John 2006; Qiong et al. 2008). TiO 2 is a polymorphic compound that exists mainly in three crystallographic phases: anatase, rutile and brookite. They are different in their synthesis and properties among which rutile and anatase are the most commonly synthesised phases owing to their good thermodynamic characteristics and physical properties. Rutile phase has a more compact tetragonal crystal structure than anatase pigment, which might be the reason for its higher refractive index (2.903). In photocatalysis research, where an external UV source is used, anatase titania is usually considered to be more active than rutile but rutile titania possesses better photoabsorption property in visible light wavelength range. In addition, rutile titania exhibits an excellent refractive index, high dielectric constant, higher hiding power and superior chemical stability (Bok et al. 2007; Yamamoto et al. 2009 and Jiaguo et al. 2003). Titanium dioxide nanopowder was prepared by different methods such as combustion (Deorsola and Vallauri 2008), solvent evaporation (Fumin et al. 2007), hydrothermal (Kobayashi et al. 2008; Yamamoto et al. 2009; Inagaki et al. 2001; Kumar et al. 2009) and hydrolysis (Mahshid et al. 2007; Zhang et al. 2009; Jun et al. 2007; Dahlvik et al. 2000). Its particle size could be precisely controlled in industrial applications to maximise its reflectance. It is used in paper coating to increase both brightness and opacity. Because of
Page 2 of 13
Article 3 – Titanium Coating Pigments
Made with FlippingBook Digital Publishing Software