UV coating applied to sports shoes
In recent years , water-based, high-solids, 3D tactile coatings1 have been a great success on the market, with its main application area being sports shoes. The combination of exciting multicolor effects on multi-layer 3D coatings with tactile effects and high performance properties allows for a whole new freedom of functional design.
New light-sensitive tactile coatings are now being introduced to the market. These coatings are based on fully waterborne polyurethane (PUD) resins with high elasticity, flexibility and excellent bonding properties and are typically applied to textile substrates. Based on graphene oxide nanoparticles – graphene oxide (GO) nanoparticles – highly photosensitive coatings can be developed. The tactile layer of graphene oxide doped shows high transparency and is almost colorless. When the graphene oxide doped coatings are exposed to natural sunlight or artificial UV light, the graphene oxide nanospheres initiate a chemical reduction process that generates black reduced graphene oxide (RGO) nanospheres. 2,3 Relatively large islands of sp2 form, completely altering the absorption properties of these coatings and turning them (over time) into a deep black color. By using a masking device, the coatings on the sneakers could be partially exposed to UV light and the design patterns could be created by selective UV exposure. In addition, GO-doped tactile coatings can be further colored with standard pigments or iridescent effect pigments. Creative color change effects can be achieved during UV exposure and change the surface appearance over time.
Customers are first attracted to the visual appearance of a product. Therefore, the color design of sports shoes plays a crucial role in selling the product. A beautiful color or the right combination of colors will make a product successful. Today’s consumer market requires emotional stimulation of the product, and if the product is not unique, it will quickly become aesthetically fatiguing. Light sensitive tactile coatings add a vivid color effect by changing colors to respond to the environment and consumer usage habits. Whenever a photosensitive tactile coating is exposed to sunlight, a slow process of color change begins and the coating will gradually darken. If used on athletic shoes, the color of the shoes will vary on each sunny day. The color will gradually change over time, and consumers can observe the color change every day, curiously waiting to see the final color.
With the new trend of mass personalization of consumer products, photosensitive tactile coatings open up more opportunities to personalize the point of purchase for products such as athletic shoes. Monotypes can be digitally cut right in the store, while individual designs can be created with a UV-irradiated device that inspires and gives the customer a sense of involvement in the shoemaking process. This can increase the interaction between the customer and his/her shoes, enabling true personalization.
Application of photosensitive tactile coatings
The application of photosensitive tactile coatings is essentially done in the same way as established tactile coatings – using either manual or automated screen printing techniques. 4 The process discussed in this paper, with improvements to the screen printing process, allows for primer coating thicknesses of 0.2-1.2 mm. multiple layers are stacked to form a three-dimensional structural coating – potentially combining multiple colors and multi-layer effects. Typically, tactile coatings are applied to textile materials, such as polyester or nylon fibers, and are often used in the field of sports shoes. In order to obtain the best light-sensitive color effect, a special coating was developed, as shown in Figure 1. Multiple layers of transparent primer are first applied to create the desired 3D coating effect and to ensure maximum bond strength to the fabric. On top of the primer, the first color coat, usually white, is applied first to ensure the consistency of the final color. A GO-doped photosensitive layer is used as the primary effect coating to provide a color change mechanism throughout the 3D coating system. Typically, the GO layer is 0.2 mm thick to produce a strong visible color change, and a thin iridescent coating can be selectively applied as the final topcoat. Iridescent colors are particularly interesting because of their strong visibility against a dark background. Of course, the final topcoat can also be varied to provide other tactile effects such as soft touch, rough touch and matte or glossy effects, among others.
Analysis of UV photosensitive mechanism of GO coatings
In the photosensitive tactile coating system, GO coating is the most important part of the color change process. It is well known that graphene oxide is gradually reduced under UV light, and the color change from light yellow to dark black occurs during the reduction process. However, so far, there are not many ideas about how to utilize this color change in everyday consumer products. To investigate the photosensitive mechanism of graphene oxide coatings, we prepared GO coatings with a film thickness of 0.2 mm by adding 0.02 wt% of graphene oxide to an aqueous polyurethane dispersion (WPU), which was pale yellow and appeared almost colorless in appearance. The part of the graphene oxide coating was covered by a butterfly meter and then irradiated with UV light for 30 min, and the results are shown in Figure 2a. The uncovered butterfly pattern changed to a very dark (almost black) color. Figure 2b shows the results of the Raman study of the bright and dark areas of the sample. The results for the untreated sample are labeled WPU/GO and the dark area is labeled WPU/RGO with corresponding ID/IG values of 1.00 and 0.98, respectively. This clearly indicates that some oxygen-containing functional groups on the GO surface have been removed and the photochemical reduction of graphene oxide under UV irradiation is diminished.
Color Design and Features
By combining photosensitive tactile coatings with other pigments, exciting color designs can be created where the color changes gradually in sunlight and darkens significantly under UV exposure. In order to create great consumer products, color development must be approached in two directions: 1) the initial color before UV exposure needs to be attractive to reach the initial purchase decision, and 2) the final darker color also needs to be very attractive to maintain consumer satisfaction with the product.
Iridescent pigments have a very good color change effect and, interestingly, iridescent pigments have a very different color appearance on light and dark backgrounds. In addition to their iridescent effect, iridescent pigments show a highly transparent light gloss on light backgrounds and a unique dark effect (such as dark purple, dark blue and dark green) on dark backgrounds. Figure 3 shows the effect of the coating on a black and white sample, where the color change due to the background color can be clearly seen. The photosensitive tactile coating can now be used as a background color, and the GO-doped coating is essentially translucent. Thus, in our standard coating, a white layer is placed underneath the graphene oxide doped coating. Before UV radiation is applied, the photosensitive coating system will thus show a light, iridescent appearance, and after UV irradiation, the GO doped layer will darken and eventually become black. As the GO-doped layer gradually darkens, the iridescent color of the top coat becomes more pronounced and eventually reaches the color on a black background.
Color changes with time and UV intensity
For the practical application of photosensitive tactile coatings, it is important to understand the conditions and timing of the color change from initial color to final color. Therefore, we investigated the effects of UV intensity, time and temperature on color change. The results are summarized in Table 1 on the following page.
The conclusions reached are as follows: 1) higher intensity UV light shortens the color change time; 2) discontinuous exposure with frequent applications takes more time to change color than continuous UV light exposure; 3) in the absence of UV light, the coating color does not change even at temperatures up to 80°C.
Considering that the coating was first applied to athletic shoes and taking into account the normal habits of consumers using running shoes, it may take about 100 days for the coating on athletic shoes to fully change color. Obviously, the time to complete color change depends greatly on the climate zone, time of use and local weather conditions, but the study found an interesting range of color change in outdoor athletic shoes.
Personalization of athletic shoes
Not only can photosensitive tactile coatings enable gradient color changes in athletic shoes, but they can also be used for spot customization, and shoes with photosensitive coatings can be efficiently mass-produced. Brands and retailers can install digital cutters and UV light boxes in their stores. The digital cutters can quickly cut any shape of masking paper and personalize customer designs. Under intense artificial UV light, the masked photosensitive coating can be discolored within minutes. In the store, an individual shoe design can be completed in less than 30 minutes. Figure 5 shows some of the individual designs created in this way.
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