Removal of color pigments from recycling plastics

Abstract

The presence of additives in plastics poses a significant environmental threat and complicates plastic recycling. These additives hinder effective recycling, leading to recycled products being mostly used in lower-quality applications. Colorants in plastics worsen this issue, causing the final recycled product to have a grey color. As a workaround, it has become common to add dark colored additives to recycled plastics to preserve aesthetics. However, this limits the versatility and widespread use of recycled plastics. The complex interaction of colorants in plastics highlights the urgent need for advanced solutions in plastic recycling. In this study, the treatment of plastic waste to extract colorant using organic solvent was investigated. The plastic wastes used were Polystyrene (PS), High Density Polyethylene (HDPE), and Polypropylene (PP). As the first step, acetic anhydride was selected as the colorant extraction solvent among the other organic solvents. It was used to extract colorant from PS plastic with orange, yellow, and pink colors. Three cycles of extraction were carried out for each of them. Then identified the type of colorant (pigment or dye) present in the PS pieces using microscope observations and found that all the three were pigments. Additional examination of these pigments using FTIR revealed their specific functional groups. The application of acetic anhydride was found to be ineffective in the extracting colorants from HDPE and PP. Furthermore, activated carbon was used to successfully remove pigments from extracts to encourage the reuse of solvents. Absorbance was measured at 300-700 nm range using UV-visible spectrophotometer verified that the solvent can be reused up to eight times without losing its effectiveness. The results highlight a notable reduction in pigment levels of PS resulting in 85–90% in yellow and pink pigments and 70–75% in orange pigment demonstrating the effectiveness of the proposed removal technique. Sustainable pretreatment helps reducing plastic waste and has economic benefits. This method can improve the quality of recovered polymers by shifting darker shades to lighter tone, highlighting the potential to create bright and visually appealing recycled items which produce revenue. Therefore, it provides a potential and economical approach to improving sustainable recycling methods for plastics.