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Curtains down on self-cleaning windows?
22 Jul, 2010 11:29 am
We take the green credentials of nano-TiO2 cleaning agents for a test ride and arrive at some surprising results!
So, there is now a large-scale marketing of 'self-cleaning' glass with a coating of nano-TiO2 - being advertised as 'self-cleaning' windows.
Such windows demonstrate a reduced need of cleaning when regularly exposed to sun and rain. There is also an increased market presence of paints, cement products and paving blocks with TiO2 nanoparticles. They have reportedly revealed an ability to clean the air. The applications of TiO2 nanoparticles for the purpose of cleaning are also being claimed to be 'green'.
However, there has been limited research on the persistence of the 'self cleaning' properties of windows.
A study shows that the reduced need of cleaning persists during at least several years. The environmental benefits should however be weighed against the environmental burden of producing self-cleaning glass.
Energy wise, for instance, the number of cleanings should be reduced by 750-1000 times to break even with the increased energy input in the production of glass. Also, it may be noted that the photocatalytic property of TiO2 nanoparticles present on 'self cleaning' glass leads to the breakdown of hazardous substances but also generates hazardous substances (1-3).
The net effect, therefore, on the presence of, and exposure to, hazardous substances is highly uncertain.
The environmental benefit of TiO2 nanoparticles in paints is even more doubtful. The reason for this is that the photocatalytic effect of the nanoparticles will lead to the degradation of the organic polymers which form the paint film. This will shorten the lifetime of the paint and will lead to the release of TiO2 nanoparticles which may be considered an environmental hazard (4).
Research on the application of TiO2 in concrete and paving blocks has shown rapid inactivation of the ability to photocatalitically convert compounds. Chen and Poon (5) found significant inactivation of photocatalytic activity of paving blocks coated with nano-TiO2 in downtown Hong Kong after four months. And Lackhoff et al. (6) found a 90% reduction of photocatalytic activity of cement modified with nano-TiO2 over a period of four months. This means that the 'cleaning' activity of cement products and paving blocks will be shortlived and that the net effect on the hazard of pollutants is uncertain.
All summed up, it would seem that the green credentials of nano-TiO2 cleaning agents, as currently applied, remain unproven.
(1) A. Mills et al., Research on Chemical Intermediates 31 (2005) 295-308
(2) J. Auvinen and L. Wirtanen, Atmospheric Environment 42 (2008) 4102-4112
(3) J.M. Langridge et al., Atmospheric Environment 43 (2009) 5128-51531
(4) L. Reijnders, Polymer Degradation and Stability 94 (2009) 873-876
(5) J. Chen and C. Poon, Building and Environment 44 (2009) 1899-1906
(6) M. Lackhof et al., Applied Catalysis B: Environmental 43 (2003) 205-216