Thermochromic polymers

Materials that change color in reaction to a change of temperature is equipped with thermochromic properties.

Thermochromic polymers

Color change as desired – Thermochromic polymers

Thermochromic effects are caused by either structural changes of dyes or a change in the lattice arrangement of periodic structures. The focus of the work at Fraunhofer IAP lies on dye systems based on leuco or pH indicator dyes. In the case of the leuco dye systems, we distinguish between a color-to-colorless switching with an increase in temperature and an inverse switching from colorless to color with increasing temperatures.

In our material development, the thermochromic effects are converted into polymer matrices - thermoplastics, thermosets, elastomers, paints and coatings - without significantly changing their material-specific properties. The thermochromic additives to be developed for this purpose not only have to be stable in the polymer matrix, but also survive their processing process unscathed. In the case of thermoplastics, this is the processing by means of extrusion processes, in the case of thermosets and elastomers the crosslinking process and in the case of coatings the application and curing process. All of these matrices place different demands on the chemical, mechanical and thermal stability of the thermochromic additives.

Thermochromic foil in the mini plant.
© Fraunhofer IAP

Thermochromic foil in the mini plant.

Color change of thermochromic materials under the influence of heat.
© Fraunhofer IAP

Color change of thermochromic materials under the influence of heat.

Bent glass pane magenta.
© Fraunhofer IAP

Bent glass pane magenta.

Focus topic: Bio-Thermochromism

© Fraunhofer IAP

Principle of the irreversible thermochromic packaging film for monitoring the storage and transport temperature.

Special requirements apply to the use of thermochromic plastics in packaging films for food, pharmaceuticals or toys. In addition to the use of biopolymers from renewable raw materials such as polylactic acid (PLA) or polyhydroxybutyrate (PHB), the effective additives must be replaced by natural dyes as well.

Thermochromic biopolymers can be used for temperature monitoring. As irreversibly switching materials, they can indicate a temperature overshoot, for example the interruption of the cold chain.

A challenge arises in the thermoplastic processing, which should be independent of the actual color effect.