Silica microsphere encapsulation enables colorant separation from plastics
Last update on Jul 21, 2026
Recycling colored plastics without loss of material quality is one of the more persistent challenges in plastics circularity. When differently colored plastics mix in conventional recycling streams, colorants combine to produce dark, low-value output, a process known as downcycling. Chemical methods that remove colorants more selectively require temperatures of 300-500°C, creating significant energy costs.
A research team from Osaka Metropolitan University's Graduate School of Engineering, in collaboration with Fuji Pigment Co., Ltd., has developed an encapsulation approach that separates colorants from plastics at room temperature, with recovery efficiencies of close to 100%.
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How does encapsulation and separation process work?
The approach uses a spray-drying method to encapsulate colorants within silica microspheres before they are incorporated into the plastic. Silica, a primary component of sand and glass, is chemically resistant and thermally stable, allowing the capsules to maintain their properties through manufacturing and repeated processing cycles.
For recycling, the plastic is dissolved in a solvent such as acetone. The plastic dissolves; the silica capsules do not. Centrifugation then separates the colorless plastic solution and the intact capsules, each recoverable at close to 100% efficiency. To enable color-selective separation, the researchers assigned capsules of different sizes to different colors, which can then be sorted using standard mesh sieves.
"A key advantage of this technology lies in its ease of recycling," said associate professor Kenji Okada. "When the plastic is dissolved in a solvent such as acetone, only the plastic dissolves, while the color capsules remain intact. Using centrifugation, both the colorless plastic solution and the capsules can be recovered separately with nearly 100% efficiency."
What do multiple recycling cycles show?
Testing showed that both color quality and material properties were retained across multiple recycling generations, with no observed degradation. Operating at room temperature also avoids the energy-intensive decomposition required by existing chemical colorant removal methods.
Professor Masahide Takahashi stated, "This approach enables used colored plastics previously destined for disposal or downcycling to be repeatedly reused as high-value resources. 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 contribute to the realization of a sustainable circular recycling society."
The research was partially supported by Grant-in-Aid for Transformative Research Areas (A) "Supra-ceramics" (JSPS KAKENHI Grant Numbers JP22H05142 and JP22H05144) and by the JST FOREST program, grant number JPMJFR235Q.