The Effects of Weather onto Glazing and Their Influence - Part II: The Influence of the Soiling by Weather on the Outside Surface of Glazing by Coatings

Hans Joachim Gläser

The effects of weather on the outside surface of glazing are as follows: Corrosion, soiling, dew, and frost, respectively.

In the report, on the one hand, the causes and the behaviour of the soling by weather are reviewed as well as its preventing by an outside coating of glazing. The comparative outside exposition of samples with different coatings discussed today has shown that the outside soiling of glazing by weather can be diminished best by a photocatalytic TiO2 coating. The reason for this is the considerably increased self-cleaning effect of this coating during outside exposition, based on a more intense wash-cleaning effect by weather during outside exposition. However, it must be stated that the photocatalytic TiO2 coating is not a panacea against soiling. There are pollutions or kinds of installations of glazing which counteract the photocatalytic and wash-cleaning effects. The special glazing directives of the manufacturer are to be observed.

On the other hand, the causes of dew and frost formation on the outside surface of glazing is treated in the report being an urgent problem for the today's heat-insulating glasses especially with Ug values ≤ 1.0 W/m2K. The reason for this is mainly the emission of heat from the outside surface of glazing, especially to the sky. It can be shown that the outside condensation can be counteracted by outside coatings having thermal emissivity values є ≤ 0.10, suppressing this heat emission to a large extent. Hereby, the formation of frost on the outside of all today's glazing can be prevented completely everywhere and the frequency of dew formation can be reduced considerably in regions with middle and northern European climate. Unfortunately, coatings with these emissivity values being enough weather-resistant cannot be manufactured economically today. The best applicable coating marketed today has є ≤ 0.17 based on a SnO2:F coating being also efficient, but less.

The question arises, whether such low emissivity outside coatings on glazing would also reduce their thermal transmittance, as positioned inside the today's heat-insulating glass units. But it can be shown that nearly no reduction of the thermal transmittance is possible for the today's glazing with Ug values in the range of 0.5 to 0.8 W/m2K with such a coating and, for glazing with Ug values in the range of 1.0 to 1.5 W/m2K, only a reduction of maximum 0.2 W/m2K is possible to the upper end of this value range. The highest Ug reduction shows a single glazing with a low emissivity coating on its outside. Therefore, the author's opinion is that such a coating on vehicle glazing can reduce the heat consumption for the air-conditioning of vehicle cabins as well as prevent the frost formation on their outside most efficiently.

A positive side-effect has the photocatalytic TiO2 as well as the low emissivity SnO2:F coating: They seal the flat glass surface and, in this way, they simultaneously prevent the leaching and consequently the corrosion of the flat glass surface.

The challenge of the glass industry for the future is to develop a coating on flat glass, simultaneously preventing all three effects of weather onto the outside surface of glazing, i.e. corrosion, soiling, dew and frost formation, respectively.

The article is Part II of a series of articles based on the research report "Auswirkungen der Witterung auf Verglasungen und ihre Beeinflussung" sponsored and orginally published by Interpane Company.

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The Authors

Prof. Dr. Hans Joachim Gläser Physicist / Consultant Interpane (retired) Prof. Gläser is one of the pioneers in flat glass coatings with over 30 years of experience in several executive positions in the flat glass and machine manufacturing industries. In the beginning of h... Read more