New way to recycle coloured plastics using silica microspheres

When plastics are recycled, the various colours mix to create dark, low-value materials in a process known as downcycling. Conventional recycling methods cannot effectively separate colourants from plastics.

Alternatively, chemical decomposition methods to remove colourants exist, but they require high temperatures of 300–500°C and lead to substantial energy consumption. These limitations thus present a growing need for upcycling technologies that enable efficient separation of colourants under milder conditions and allow plastics to be repeatedly reused as high-quality materials.

In search of a solution, a research team led by Associate Professor Kenji Okada and Professor Masahide Takahashi from Osaka Metropolitan University’s Graduate School of Engineering, in collaboration with Fuji Pigment Co Ltd., developed a novel approach using silica microspheres to encapsulate colourants utilising a spray-drying method.

Silica, a primary component of sand and glass, is highly resistant to heat and chemicals, which allows these colour capsules to withstand harsh manufacturing processes while maintaining vivid colouration in plastic products.

“A key advantage of this technology lies in its ease of recycling,” Okada said. “When the plastic is dissolved in a solvent such as acetone, only the plastic dissolves, while the colour capsules remain intact. Using centrifugation, both the colourless plastic solution and the capsules can be recovered separately with nearly 100% efficiency.”

Furthermore, by using capsules of different sizes for different colours, the researchers demonstrated that the colours can be selectively separated using simple mesh sieves. Even after repeated recycling over multiple generations, the materials retained their original colour and quality without degradation.

“This approach enables used coloured plastics previously destined for disposal or downcycling to be repeatedly reused as high-value resources,” Takahashi said. “Moreover, because the process operates at room temperature and avoids energy-intensive decomposition, it significantly reduces energy requirements and environmental impact. In the future, we hope this technology is widely applied to used plastics, such as PET bottles and polyethylene bags, and potentially contributes to the realisation of a sustainable circular recycling society.”

The research has been published in Green Chemistry.

Image caption: Colour-embedded silica and plastic were combined to form various shapes. Reusability was tested over multiple generations of moulding, decomposition and reuse. Credit: Osaka Metropolitan University