New scientific test method for self-cleaning glass

A new method has been developed under a European research project, testing the effectiveness of self-cleaning glass. The results show that industrial glass with hydrophilic titanium dioxide coating stays clean for longer periods of time.

Self-cleaning glass that is coated with a layer of titanium dioxide during the production process has been in use for windows, façades, conservatories and a wide range of glass roofs for over half a decade. The benefit of this industrial glass is its ability to actively break down organic dirt with the help of UV light and photocatalysis. Moreover, the invisible paper-thin coat reduces the surface tension on the glass, thus creating a hydrophilic environment. When water hits the coat, it forms a very thin film that washes away the disintegrated dirt.

Thanks to this self-cleaning effect, glass stays clean for longer periods of time and requires less cleaning. This reduces the maintenance costs of a building.

Project with international participation

The market launch of self-cleaning glass with titanium dioxide coating has led to a discussion in the glass industry how much of a cleaning effect such coats actually have. The central issue is always: how long does self-cleaning glass stay clean compared with either uncoated glass or glass where coating has been applied at a later stage?

Until now there has been no standard in the assessment of this self-cleaning function. Glass has only been tested for durability under the European standard EN 1096. To remove this deficit, a multinational consortium was set up, comprising two industrial partners (incl. Pilkington Group Limited and Saint-Gobain Glass Deutschland GmbH) as well as seven scientific partners and an independent consultancy firm. These partners launched a STREP project entitled “Self-Cleaning Glass”.

The aim of the project was to obtain an objective comparative study of soiling mechanisms at work on glass surfaces, to develop an accelerated and objective test method to determine the self-cleaning effect and to formulate a proposal for a European standard.

Two-year open-air trials

The project started with the simultaneous testing of four different glasses at various European locations: two self-cleaning glasses, one hydrophilic glass product and a standard float glass. The panes were set up vertically in the open air. During the two-year test phase they were given regular visual inspections by independent persons. At the same time, readings were taken to measure the degree of light diffusion on each of the glasses.

Both the subjective inspections and the diffusion readings showed that the self-cleaning glasses with photocatalytic and hydrophilic qualities were cleaner than the uncoated glasses. The latter, in turn, was substantially cleaner than glass with a hydrophobic coat.

The data analysis of this part of the project yielded a good correlation between the subjective assessment of the human eye and objective readings of light diffusion. The test therefore showed that diffusion readings are an appropriate test method to determine the cleanliness of a glass surface.

Direct comparison through diffusion readings

A second test run was subsequently set up with the aim of finding an accelerated test method. To do so, soiling of the glass first had to be standardised and analysed. The resulting findings then led to the definition of a so-called test dirt, comprising organic and inorganic components.

To simulate cleaning processes on the test panes, the test dirt was sprayed on with a specially developed spraying device, then the UV radiation of the sun light was simulated, and finally the panes were sprinkled with water. After each stage, diffusion readings were taken at different points on the panes.

After two cycles – according to one of the participating companies – the self-cleaning glasses were very close to their initial values. The float glass and the hydrophobic product apparently displayed diffusion values that were clearly above one per cent. Even during the first part of the project it was noted that glasses with diffusion values above one per cent were perceived as extremely dirty in the subjective visual evaluation.

According to the project team, the test setup had succeeded in providing an objective assessment of the cleaning properties of glass products. In a press release published in early 2008, Pilkington Deutschland AG said that the specified test method had been submitted to the relevant European committee, CEN TC 129 (Glass in Building).

New information at glasstec 2008

glasstec, the most important trade fair in the international glass industry (held from October 21 to 25, 2008) and the accompanying special exhibition glass technology live will be giving an idea of how far the development of glass with functional coating has progressed and where current research is heading at the moment.

Results of diffusion readings: If glass has a diffusion value above one per cent, it is classified as dirty. The readings taken in the STREP project showed that self-cleaning glass with hydrophilic titanium dioxide coating delivers considerably better results than normal float glass or glass with hydrophobic coating - even after several soilings.
Grafic: Pilkington Deutschland AG