Materials for intelligent sun protecting glazing

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Abstract

The aim of this review is to present the last 10 years of development in sun protection by use of electrochromic devices, thermotropic gels and polymer dispersed liquid crystalline systems. Furthermore, the properties of new lyotropic liquid crystalline and thermotropic hydrogels are discussed. Thermochromic hydrogels, which change their colour by changes in the temperature, also are presented. The relationship between gel composition, optical behaviour and thermodynamic properties as investigated by differential scanning calorimetry (DSC) is also demonstrated.

Introduction

Electrochromic and thermotropic organic materials form systems, which react accordingly to different external influences with a reversible colour modification or clouding. Granqvist reviews the development of these materials in the years before 1990 in detail [1]. In the past decade this topic has gained further significance. The motivation for all efforts to develop chromogenic materials is their potential technical application as electrically adjustable or thermally self-adjusting light and heat filters in the external glazing of buildings. These glazing items are frequently called “intelligent” or “smart” windows. Further potential applications are large area displays for information and traffic engineering, as well as temperature sensoring applications in medical technology.

Section snippets

Electrochromic materials and polymer dispersed liquid crystalline systems

The development and the application of electrochromic systems (ECD=ElectroChromic Devices) strongly increased in the last years. Over 2000 patents are already announced in this technology. The advantages of electrochromic materials are:

  • Electric energy consumption only appears during the switching process and with a low switching voltage (1–5 V).

  • ECDs have a grey scale and low polarisation.

  • ECDs are always transparent-typical ECDs has an coverage transmissions of Tcolourless=70–50% and in the fully

Hydrogels and polymer blends

These materials are usually organic compounds and show, in a system depending temperature range, a reversible and more or less sharp transition from transparent to cloudy. Glazing based on these materials will be applicable in areas of buildings, which do not require constant transparency of the glazing, as for instance roof glazing, glazing in shopping centres or industrial buildings.

The use of thermotropic polymerfoils consisting of polymer blends [31] coated directly onto the glazing or the

Thermochromic hydrogels

Gels with reversible colour changes, e.g. thermochromic gels are rarely described [77], [78]. For many applications, e.g. dye sensors [79], large area displays with higher information density or intelligent windows, it is interesting to produce transparent hydrogels which in closed systems exhibit thermochromic behaviour in the practical relevant temperature range of 10–80°C. Recently we have found [80], that pH-sensitive dyes, like the Reichardt-dye ET(30), [81], [82] or cresol red, embedded

Acknowledgements

This work was supported by the Federal Ministry of Education, Science, Research and Technology (BMBF), Germany - Projekt No. 0329820B.

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