Fast responsive photochromic materials through nanoemulsion entrapments

Hector Torres Pierna

Claudio Roscini

The aim of this thesis was to explore the potential of nanostructured photochromic materials as photoprotective coatings for eyeglasses to provide user comfort and for smart glasses that save energy. Initially, the photochromic dyes were dissolved inside the liquid-core nanocapsules. The capsules were then dispersed into polymer films that could be integrated into commercial products. However, maintaining the appropriate transparency when integrating the capsules proved to be very difficult, even when the capsules were at the nanoscale, due to aggregation. Fortunately, a new methodology was developed and patented (WO2017105666A1) that did not require the use of core-shell nanocapsules. Instead, the photochromic solutions were nanoemulsified in aqueous solutions of film-forming polymers and then the resulting nanoemulsion was precipitated as films that retained the transparency and rapid color change rate of the liquid solutions. As a result of this thesis, a scientific article has also recently been published explaining the operation and advantages of this technology (Mater. Horiz., 2020,7, 2749). Thanks to the interaction with potentially interested companies, other key objectives were identified to overcome so that the photochromic film could be used in commercial products. These objectives were 1) improve the activated coloration, 2) improve the stability to humidity, 3) reduce the turbidity of the films, 4) improve fatigue to solar exposure and 5) demonstrate the scalability of the manufacturing process. These objectives were achieved between the second and fourth years of the thesis and the materials developed in this way are now suitable for optically clear industrial applications. In addition, the developed technique allows modifying the photochromic and mechanical properties of the films in order to meet the specific requirements of each industrial application. Meanwhile, prototypes of sunglasses, smart windows and a motorcycle helmet visor were produced. Prototypes of construction glass and windshields with mock-ups of a vehicle and a building were also prepared. These prototypes were used to compare them with some photochromic products currently available on the market, showing promising results and competitive advantages for products based on nanoemulsion trapping. These prototypes are currently used as demonstrators by Futurechromes S.L. in order to contact industrial manufacturers and partners to promote the commercialization of innovative products based on photochromic nanoemulsion trapping technology.

The main application of these materials is to incorporate them into products that require high transparency, but also rapid and real-time control of light transmission: The clearest example of this is prescription glasses, which can be clear and colorless inside buildings, but darken to provide comfort to the user once exposed to sunlight, thus avoiding having to carry a spare pair of sunglasses. In fact, photochromic ophthalmic glasses are currently the largest industrial application sector for photochromic dyes, with a global market of 4.8 billion dollars in 2018. In this sector we find products such as Transitions® lenses, with a change time of 3 to 5 minutes. With the nanoemulsion trapping technology developed in this thesis, Futurechromes wants to offer a superior response to eyeglass users and capture a significant part of this market. In the same vein, there are also sports and automotive applications, where the most important thing is to avoid glare caused by the sun and sudden changes in the weather, but it is also necessary to guarantee maximum visibility when entering or exiting tunnels or shaded areas. This market is smaller than that of ophthalmic glasses, as it is much more sensitive to the speed of color change, since the safety of the athlete or driver is at stake. Current sports products, such as Pinlock® ProtectTINT motorcycle helmet lenses, also have a change time of between 3 and 5 minutes. This response is too slow for many users and therefore, it is not a product in high demand. On the other hand, products that incorporate our nanoemulsion trapping technology react much faster, in just a few seconds. It is for this reason that we believe that the technology of this thesis could not only be well received in this sports market, but also boost it, generate more demand for this type of product and generate a positive impact on society, making the practice of sports and driving safer. Therefore, at Futurechromes we have prepared prototypes of these two products, incorporating our nanoemulsion trapping technology (Figure 2). With these prototypes at Futurechromes we have contacted companies from both the world of ophthalmics and sports equipment, as well as producers of raw materials. The reception by the companies contacted is good and we hope to sign specific technology licensing agreements in the coming months. The second area of ​​application for our technology is that of smart glass, which allows saving energy invested in air conditioning by reducing heat input through windows, but without reducing their visibility, and at the same time letting in as much light as possible on foggy and cold days to save on artificial light and heating. In this sector, the speed of change is not as relevant as in the previous ones, but fatigue resistance and price are of great importance. The smart window market is valued at 2.6 billion dollars (2018) and is currently dominated by electrochromic systems, such as Saint-Gobain's SageGlass®. These materials are expensive to produce and require specialized installation and electricity consumption. On the other hand, current photochromic options for smart windows, such as those offered by the company ColorVu, are much cheaper and easier to install, but have much less acceptance in the market due to their short useful life. At Futurechromes, we have discovered that the nanoemulsion trapping technique allows us to extend the life of photochromic dyes, since our matrix makes it difficult for oxygen (the main degrading agent of these dyes) to enter the material. In addition, tests carried out during the PhD showed that prototypes of smart windows prepared with our technology (Figure 3) had an energy saving efficiency similar to those obtained with commercial smart windows. Therefore, we hope to introduce our technology to the smart glass market, to obtain a product at an affordable price, save energy and reduce the environmental impact of buildings, and that also have good fatigue properties and resistance to the elements. We are currently in contact with a smart window production company and hope to carry out pilot production tests before 2021 in order to evaluate the commercial viability of these products.

Nanostructured Photochromic Materials; Photoprotective Coatings; Eyeglasses; User Comfort; Smart Glasses; Energy Saving; Photochromic Dyes; Liquid-Core Nanocapsules; Polymer Films; Commercial Products; Transparency; Aggregation; Novel Methodology; Patented Technology; WO2017105666A1; Core-Shell Nanocapsules; Photochromic Solutions; Nanoemulsification; Aqueous Solutions; Film-Forming Polymers; Nanoemulsion Precipitation; Films; Rapid Color Change Rate; Liquid Solutions; Scientific Article; Mater. Horiz., 2020,7, 2749; Activated Coloration; Humidity Stability; Turbidity Reduction; Solar Exposure Fatigue; Manufacturing Process Scalability; Optically Clear Industrial Applications; Photochromic Properties; Mechanical Properties; Industrial Application Requirements; Sunglasses Prototypes; Smart Windows Prototypes; Motorcycle Helmet Visor Prototypes; Construction Glass Prototypes; Windshields Prototypes; Mock-ups; Commercial Photochromic Products; Promising Results; Competitive Advantages; Nanoemulsion Trapping; Demonstrators; Futurechromes S.L.; Industrial Manufacturers; Partners; Commercialization; Innovative Products.