Thermochromic polymers

Thermochromic effects are based either on structural changes of functional dyes or on changes in the lattice constants of periodic structures. Focus of research at Fraunhofer IAP are dye systems on the basis of pH-indicator- or leuco dyes. In the leuco dye systems, we distinguish between conventional switch (from colored to colorless with increasing temperature) and inverse switch (from colorless to color with temperature increase).

Material innovations at the Fraunhofer IAP

 

In the focus of our material development is the introduction of thermochromism into commercially mass market polymers - thermoplastics, thermosetting plastics, elastomers, lacquers and coatings – by doping them with thermochromic additives. Whereby, all other material properties of the resulting thermochromic polymers are aimed to keep unchanged. This requires thermochromic additives which are stabile in the polymer matrix and which also withstand the polymer manufacturing or production process. In case of thermoplastics it is the manufacturing process by extrusion technology, in case of thermosetting plastics and elastomers the cross-linking process and in case of lacquers and coatings the curing process. Thus for all these polymer matrices a different set of requirements on the chemical, mechanical and thermal stability of thermochromic additives has to be fulfilled.

Thermochromic foils and injection molded parts

 

 

Thermoplastics are non-crosslinked polymers. Their primary feature is their moldability. This class of plastic includes polyolefins, polyesters, polyamides, polyacrylates and polyvinylchloride. Most thermoplastics melt without degrading and therefore can be processed into films and injection molding parts using extruder technology. During processing the raw materials are subjected to not only thermal, but also a high degree of mechanical stress. Our thermochromic pigments can be added to the polymer raw material before extrusion. This enables, for the first time, a technology-compatible production of thermochromic polymer films and injection molding parts by which the thermochromic dye is homogenously distributed throughout the entire volume of the polymer.  Switching temperatures between 0 and 70° C and a broad array of color changes can currently be achieved. The color changing can be both reversible as well as irreversible.  

 

Hybrid active-passive color changing is achieved through vapor coating of transparent electrodes onto the thermochromic polymer material in a high vacuum of 10-7 Torr. The electrodes can be prepared using a thermal process as well as an electrode beam.

© Photo Fraunhofer IAP

Small pilot.

© Photo Fraunhofer IAP

Color change of thermochromic materials under the influence of heat.

Thermochromic hydrogels

 

Hydrogels can achieve reversible color change and high transparency by adding suitable pH sensitive indicator dyes in combination with tensides. This thermochromic behavior is based on the interaction between the dye’s molecules and the hydrogel’s micro-environment. In addition to a change between two colors, multiple color change and colorless-to-color changes can be produced.

 

Color and transparency changes (thermotropism) are combinable in hydrogels with color changing temperatures that are independent of one another. Our many years of experience in this area allow us to produce hydrogels with customized thermochromic and/or thermotropic properties. In terms of their optical properties, these materials are suitable like no other for use in hybrid light- and heat-regulating solar protection glazing. Such demo windows of up to 1 m² are currently being tested under practical conditions.

© Photo Fraunhofer IAP

Thermochromic foil.

Thermochromic thermosets

 

Thermoset plastics are crosslinked polymers. Unlike thermoplastics they are neither meltable nor moldable. Thus the thermosets are molded during production. In order to achieve thermochromic thermosets, the thermochromic pigment must be consistently added to the thermoset while it is being manufactured. This puts a lot of demand on the chemical stability of the pigments being used since the elements used in thermoset production are usually highly reactive components. The thermochromic pigments which were specially developed for this application currently achieve a single to multiple step color change at between 0 and 100° C.

Thermochromic thermoset.

© Photo Fraunhofer IAP

Form glass cyan.

© Photo Fraunhofer IAP

Form glass cyan

© Photo Fraunhofer IAP

Form glass magenta.

Thermochromic pigments

 

We develop thermochromic pigments for use in various applications. By using the primary colors and black and white, the color can be customized to meet customer requirements.

© Photo Fraunhofer IAP

Pigment powder.

© Photo Fraunhofer IAP

Pigment powder.

© Photo Fraunhofer IAP

Pigment powder.

Classification of thermochromic polymers

Types of thermochromic polymers, classified according to the light effect which causes the thermochromic behavior.

 

Light effect

Origin

macroscopic Behavior

Polymer Class

reflection

periodic structures

ΔI, Δλmax

    cholesteric liquid-crystalline polymers

    cristalline colloidal grids
    embedded in a gel network

    gels

absorption

chromophoric groups

ΔI, Δλmax

    conjugated polymers

    hydrogels containing indicator dyes

scattering

areas with different refraction indices

ΔT %

    polymer mixtures with LCST

    hydrogels with LCST

    lyotropic liquid-cristalline hydrogels

LCST: lower critical solution temperature