Universal Color Retrofit to Polymer-Based Radiative Cooling Materials

The global drive towards energy efficiency stems from increasing energy consumption and associated environmental concerns. This pursuit is crucial across various sectors, including building infrastructure, power generation, and transportation. Notably, a significant portion of this energy consumption is attributed to cooling, particularly in regions experiencing high solar radiation. Radiative cooling materials offer a promising solution to the challenges posed by traditional cooling methods in terms of energy consumption and environmental impact. These materials work by exploiting the natural process of thermal radiation to cool objects or structures without the need for external energy sources. Among the various approaches to radiative cooling, polymer-based films have gained significant attention due to their potential for mass production and cost-effectiveness. However, one limitation of traditional polymer-based radiative cooling materials is their colorlessness, which can be visually unappealing and limit their practical applications. In a recent study published in the Journal ACS Applied Materials & Interfaces by Postdoctoral Fellow Dr. Yun Zhang, Wenkai Zhu, and Xiwei Shan led by Professor Tian Li from the School of Mechanical Engineering at Purdue University in collaboration with Dr. Wei-Jie Feng, Wei-Kuan Lin and Professor L. Jay Guo from the Department of Electrical Engineering and Computer Science at University of Michigan, the researchers addressed this limitation by introducing a novel coloration strategy that can be universally applied to polymer-based radiative cooling materials. “Structural coloration, resilient and vibrant, is frequently observed in nature, exemplified by the striking hues on butterfly wings and bird feathers. Unlike the absorption of sunlight, which typically generates heat, the periodic structure by NIL modulate light reflection to generate colors,” explained Professor Tian Li. “This approach can be readily applied to render coloration to numerous radiative cooling materials, provided they can undergo the imprinting process.” Dr. Yun Zhang commented, “This universal approach circumvents the trade-off between aesthetic appeal and radiative cooling efficiency.” By leveraging nanoimprinting technology, their novel approach enables the generation of vibrant structural colors on the material’s surface without compromising its radiative cooling properties. This innovation has the potential to revolutionize the field of radiative cooling and open up new avenues for its widespread implementation.

The researchers employed nanoimprinting lithography (NIL), a versatile technology capable of tuning optical spectra through nanostructure patterning. The nanoimprinting method involves creating nano-scale patterns on the surface of the polymer materials. These patterns cause light interference, which results in the generation of colors visible to the human eye. Significantly, this approach allows for the coloring of the materials without altering their intrinsic properties that are essential for effective radiative cooling. Nanoimprinting as used in this study does not involve adding pigments or dyes, which traditionally increase solar absorption. Instead, the colors are produced through structural manipulation at the nano level. NIL can be categorized into thermal and ultraviolet (UV) imprinting, both of which allow for precise mold replication and integration into roll-to-roll manufacturing systems. This scalability is a crucial feature for the large-scale production of colored radiative cooling materials. The authors achieved structural coloration in their experiments by selectively reflecting light without absorbing solar radiation, a key requirement for effective radiative cooling. They tested the new approach on various polymer substrates that are known for their flexibility, stretchability, and cost-effectiveness, including polyurethane acrylate on polyethylene terephthalate (PET), ethylene tetrafluoroethylene (ETFE), polydimethylsiloxane (PDMS), and polyolefin plastomer (POP). Wei-Kuan also pointed out that “NIL is a fast-growing technology. Recently, this method has started to be used for logic CMOS fabrication. The cost-effective and high-throughput features also  allow the researchers to fabricate various photonics devices, such as structure color and optical sensors without the need of delicate tools.”

The authors’ findings demonstrated the successful retrofitting of structural colors onto polymer-based radiative cooling materials using nanoimprinting technology. The colored polymer films exhibited specular reflection, high IR emissivity, and low solar absorption, making them ideal candidates for both radiative cooling and aesthetic applications. The hemispheric optical responses of the nanoimprinted films were measured to assess their radiative cooling performance. These films displayed minimal solar absorption, with absorption percentages ranging from 1.7% to 3.7%. Additionally, they exhibited strong mid-infrared (MIR) radiation emission within the atmospheric transparent window (8-13 μm), effectively removing heat from the films. When the researchers conducted field tests on the nanoimprinted films, they demonstrated their sub-ambient cooling capabilities. For instance, a PET film achieved a sub-ambient temperature of -4°C during the day and -15°C at night when exposed to clear skies. Importantly, the coloration induced by nanoimprinting had a negligible impact on the radiative cooling performance of the polymer films. The new study also highlighted the flexibility and stretchability of the nanoimprinted films, which could endure mechanical deformation and shocks without compromising their radiative cooling properties. Mechanical tests confirmed the materials’ excellent flexibility and elasticity. Moreover, the scalability of the proposed approach was demonstrated through large-scale roll-to-roll (R2R) fabrication of colored radiative cooling films. This process showed the potential for high-throughput production of these materials, making them suitable for various practical applications.

The authors’ introduction of coloration to polymer-based radiative cooling materials opens up numerous possibilities for their practical applications. For example, in building Infrastructure where colored radiative cooling films can be applied to building roofs, walls, and facades to provide both energy-efficient cooling and aesthetic appeal. The films can replace traditional roofing materials, reducing the heat absorbed by buildings and lowering cooling energy consumption. They also be applied to outdoor pipelines and cable sheaths where these films can prevent heat buildup in exposed infrastructure, enhancing their durability and energy efficiency. Another exciting application is on jet bridges at airports to reduce cooling loads while displaying color codes for different airlines and such application would potentially improve energy efficiency in the aviation industry. Furthermore, the flexible and stretchable nature of colored radiative cooling films makes them suitable for automotive applications, such as vehicle coatings and interior components where they can contribute to energy savings in vehicles operating in hot climates. In conclusion, the new study represents a significant step forward in the field of radiative cooling materials. It offers a solution to the longstanding challenge of adding color to these materials without impairing their cooling properties. This advancement has the potential to broaden the application of radiative cooling materials in various industries, contributing to more sustainable and energy-efficient cooling solutions.