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Plastics are one of the largest sources of pollution on Earth, lasting for years on land or in water. But a new type of brilliantly colored cellulose-based plastic detailed in ACS Nano could change that. By adding citric acid and squid ink to a cellulose-based polymer, researchers created a variety of structurally colored plastics that were comparable in strength to traditional plastics, but made from natural biodegradable ingredients and easily recycled using water.

Many plastics are dyed using specialized colorants, which can make these materials hard to recycle using typical processes. Over time, dyes can fade or leach into the environment, posing risks to wildlife. One way to make these colorants largely unnecessary could be a phenomenon called structural color. This occurs when tiny structures in a material reflect certain wavelengths of light rather than a dye or pigment molecule. Structural color gives peacock feathers and butterfly wings their vibrant hues and dazzling shine, but certain synthetic polymers display structural color as well.

Hydroxypropyl cellulose (HPC), a derivative of cellulose often used in foods and pharmaceuticals, is one example of a material that can display structural color. In liquid form, it shines in iridescent tones, but its chemical properties have historically made it difficult to form into a solid plastic. Researchers Lei Hou, Peiyi Wu and colleagues wanted to see if they could fine-tune the chemistry of HPC to create vibrant, structurally colored plastics that worked as well as existing petroleum-based plastics and were environmentally friendly.

The researchers added citric acid, squid ink powder and water to the HPC polymer, which formed additional hydrogen bonds within the polymer, creating a firm material as it air-dried at room temperature. The dried material's final hue depended on the amount of citric acid, so the researchers were able to create blue, green, orange and red versions. The final color intensity depended on the amount of squid ink powder present.

Next, they 3D-printed this liquid formulation into a variety of shapes that were molded into small structures, formed into a thin film and gently folded into pinwheels and origami cranes.

Because the plastics dissolved in water, the original HPC-based plastic could be reformed into new shapes after being dried again. The recycled plastic had mechanical properties that were comparable or superior to those of most commercial, newly manufactured plastics. This work provides an efficient strategy to develop the next-generation of sustainable, dye-free plastics, the researchers say.