Civil Engineering Reference
In-Depth Information
bleaching of organic dyes and solvents when irradiated with UV light from
a mercury lamp (Dooedeve and Kitchener, 1938). Further experiments
demonstrated that the anatase crystalline structure of TiO 2 possessed a
considerably higher photoactivity than that of rutile and brookite TiO 2
(Hashimoto et al. , 2005).
Different protocols have been described in the literature for the prepara-
tion of TiO 2 photocatalysts involving sol-gel processes, vapour decomposi-
tion of titanium alkyloxides and TiCl 4 and calcination of titanium oxysulphate
at 400-600°C (Kaneko and Okura, 2002; Wang et al. , 2004; Sun et al. , 2008;
Akpan and Hameed, 2010).
Once prepared, the photocatalyst is added to the paint formulation, in a
concentration typically ranging from 5 to 40% which, depending on the type
of application of the paint, is equivalent to a critical pigment volume con-
centration (CPVC) of around 10% w/v (US Patent, 2009). The CPVC
represents the maximum amount of pigment that can be used for a certain
amount of non-volatile solvents in the paint formulation without causing a
deterioration of the paint properties.
Paints are homogeneous mixtures which contain the following:
Binder or resin: The resin imparts adhesion and binds the pigments
together. It has a direct infl uence on properties such as gloss, durability,
fl exibility and toughness. Typical binders include alkyl-, chloride-, vinyl-,
epoxy- and polyurethane resins. The paint is often named after the main
binder used in its formulation. In most cases, the resin is cured after the
application of the paint, which generally involves the polymerization of
its constituents.
Pigments: Pigments are the materials responsible for the colour and
opacity of the paint, and they have a direct infl uence on its mechanical
resistance. Pigments include compounds such as minerals (e.g., mica,
talc), inorganic salts (Fe, Cr, Cd, Mo, Ti or Pb oxides and hydroxides,
calcium carbonate, zinc phosphate, etc.) and organic dyes (toluidines,
phthalocyanines). Some pigments confer antioxidant properties on the
paint, thereby increasing its stability towards the degradation of paint
constituents.
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Solvents and diluents: The appropriate mixture of solvent and diluents
is selected in terms of both their volatility and the time required for the
curing process of the resin. In this sense, an inappropriate match between
these two parameters would result in paints drying rapidly on their
surface, but remaining wet in the interior coating. Additionally, solvents
have a crucial role in controlling the viscosity of the paint and, thus, the
ease of the painting process. The most commonly used solvents are
aliphatic solvents, aromatic solvents, alcohols, ketones and esters, which
in some cases (e.g., low or zero-VOC paints) are mixed with water.
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