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Fast-responsive photochromic coating

Scientists at the CSIC, at the Institut Català de Nanociencia i Nanotecnologia (ICN-CSIC) and at the Universidad Autónoma de Barcelona, have developed a photochromic coating based on micro and nanocapsules, whose optical properties change very quickly depending on the external light.

 

Photochromic polymer. It takes less than 15 seconds to change from the opaque state (left) to the transparent state (right). Image: ICN2The main advantage of this development compared to others already in the market is the short time it takes to change from one opaque state to one transparent state (less than 15 seconds). It is a substantial improvement compared to existing technologies, which need minutes to complete the opaque-to-transparent process -as it happens, for instance, in some smart windows. That makes feasible applications in which the fast response is an imperative condition, like in the case of cars windows. 

Claudio Roscini and Nuria Vázquez-Mera, two scientists at the Institut Català de Nanociencia i Nanotecnologia (CSIC-ICN) and at the Universidad Autónoma de Barcelona, and two of the creators of the new material, explain: “photochromic materials are made of molecules which change their absorption properties, and therefore their colours, depending on the light passing through”.

In many commercial photochromes, the colour change is due to a reversible change of the molecular structure. The change is then faster when photochromic molecules are within a liquid in which there are no restrictions for the molecular motions involved in the interconversion. On the contrary, if photochrome molecules are right inside a polymer, as usually happens in polymeric coatings, “the polymeric chains tie-up so much the photochrome that at the end photochrome reduces or even loses its interchange capacity”, Roscini explains.

The key of the new development is that the photochrome is maintained in a liquid. Scientists have applied hollow and sealed micro and nanocapsules which contain a solution of the photochromic molecules. Then, the micro and nanocapsules can be integrated into a polymer.

The key of the new development is that the photochrome is maintained in a liquid.

The oil-core capsules not only give the photochrome molecules a faster optical response but also other advantages: they make the photochrome molecules more stable (e.g. longer lifetime) and allow them to be active in a broad range of temperatures, from -5 ºC to 80 ºC.

Moreover, it is easier to embed the encapsulated photochrome in a polymeric matrix than the photochrome itself. The capsules enable the photochrome to keep its interconversion properties also in rigid polymers, such as polyamides, which usually block the photochrome in the transparent state. And the capsules can be applied on a wider range of materials.

The encapsulation process is easily scalable and the cost of the new photochromic material is mainly given by the cost of the photochrome molecules, rather than by the particle synthesis. Another advantage is that the photochromic coating does not need to be activated by an electric field as it is intrinsically activated by the light passing through.

There are different methods to generate the polymeric films with the capsules embedded (spin coating, drop-casting, in-situ polymerization…) “At the moment”, scientists say, “we can produce the films by drop-casting, from a solution made of a polymer and the capsules with photochrome”.

These photochromic nanocapsules have a wide range of applications, such as cosmetics, greenhouses, crash helmets, car windows, sun-glases, mirrors, smart windows, blinds, canopies or textile sector.

Contact:

Isabel Gavilanes-Pérez  This email address is being protected from spambots. You need JavaScript enabled to view it.
Institutional Coordination of CSIC
in Catalonia.
Deputy Vice-Presidency for
Knowledge Transfer, CSIC
Tel.: + 34 93 594 77 00