Structural, Optical and Thermal Properties of Thermochromic Composites

Dear Colleagues,

Materials that change in accordance with external stimuli are an indispensable basis for smart applications that are undependable on any power supply or IT network. A special group of such materials is thermochromics—they convert the change in temperature into visible color information.

The temperature of objects is often difficult to determine. Therefore, thermochromic materials form the ground for very many applications, including temperature indicators, color-changing textiles, security printing etc. Changing the optical properties of a surface over the entire visible spectral region may cause significant thermal changes upon interaction with radiation. This is why thermochromics are entering different applications also in the surface finish of buildings and for the exterior surfaces of building envelopes.

Organic thermochromic composites are among the most promising types of thermochromic materials. As a rule, they consist of at least three components—a color former (leuco dye), a color developer and a co-solvent—and their color change is reversible. They color in the solid state and discolor when liquefied. The ideal optical properties, i.e., complete discoloration and very deep coloration, are a complex problem with many constraints. As a phase change material, thermochromics also have heat storage capabilities. However, poor lightfastness is an intrinsic problem of organic materials and can significantly affect the functionality of organic thermochromics.

Phase change materials, such as thermochromic composites, must be protected against diffusion in the surrounding area. Microencapsulation is most commonly used for their protection, although other possibilities such as integration inside microfibers are also known. Such procedures make thermochromic composites manageable and allow for the pigmenting of practically any type of binder or media, whether it be polymer, water or even concrete. Thermochromic fibers and microcapsules are widely entering textile applications. However, the confinement may affect the properties of the composite, which should be taken into account.

Many open questions and challenges that need to be solved make this field interesting for researchers from different disciplines. For example, what is the influence of space confinement on the composite’s structural and thermal properties? Toxicity of the components and poor lightfastness may critically limit possible applications. The temperature-dependent optical properties have to be measured over a wide temperature region, which might become problematic for high temperatures. Such questions make the field of thermochromic composites increasingly attractive.

Applicability of thermochromics in numerous smart applications generated immense interest for basic and applied research in this special field. I warmly invite all colleagues to submit manuscripts (full papers, reviews or short notes) to this open access Special Issue. Please feel free to disseminate this invitation to anyone that might be interested in contributing.

Prof. Dr. Marta Klanjšek Gunde
Guest Editor

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• thermochromic composite
• thermochromics
• leuco dyes
• molecular structure
• color complex
• discoloration mechanism
• color transition temperature
• color change
• color measurement
• phase change
• hysteresis
• optical properties
• crystallization
• supercooling
• thermochromic pigments
• microcapsules
• core-shell fibers
• thermal analysis
• heat capacity
• energy storage
• thermochromic printing inks
• thermochromic coatings
• thermochromic textile
• thermal barrier coatings
• thermochromic mortar coating
• wood coatings
• thermal conductivity
• lightfastness
• photofading
• accelerated ageing
• temperature indicator
• temperature sensing
• chameleon facade