Solvent-based removal of colorants from post-consumer recycled HDPE plastics

Overview

This research offers a solvent-based purification process for removing colorants from post-consumer recycled high-density polyethylene (PCR-HDPE). Colorants and other contaminants limit the reuse of recycled plastics in high-value applications, reducing their market value. By applying polymer solubility theory and Hansen solubility parameters (HSP), the approach screens and selects solvents and anti-solvents capable of dissolving PCR-HDPE while extracting unwanted colorants. The result is purified recycled HDPE that retains its original polymer properties, enabling manufacturers to incorporate higher percentages of recycled content into packaging and other products without sacrificing appearance or performance.

Technical specifications

Process approach:

• Solvent dissolution-precipitation technique using organic solvents, sub- and super-critical fluids, deep eutectic solvents, or ionic liquids
• Hansen solubility parameters used to screen and match solvents and anti-solvents based on dispersion forces, hydrogen bonding, and polarity
• Process parameters optimized include dissolution temperature (110–200°C), solvent-to-polymer ratio (1:5 to 1:20), and solvent-to-antisolvent ratio (1:1 to 1:3)
• Colorant removal rate evaluated using spectrophotometry and microscopy
• Purified PCR-HDPE characterized by density, melt viscosity, and oxidation extent to confirm processability

Key advantages:

• Targets colorants, a major barrier to PCR-HDPE reuse in visible packaging applications
• Preserves polymer molar mass and mechanical properties comparable to virgin feedstock
• Builds on demonstrated removal rates near 100% for blue and orange colorants using limonene and alcohols
• Applicable to other PCR plastics including polystyrene, polycarbonate, styrene-acrylonitrile, and ABS

Technology readiness level

The underlying solvent dissolution-precipitation method has been validated across multiple plastic waste streams, including e-waste plastics, toy waste (ABS), and packaging waste (HDPE and LLDPE), achieving polymer recovery yields of 80–90% and up to 94% removal of legacy additives. Prior work has demonstrated successful re-processing of reclaimed polymers by injection molding with properties comparable to feedstock. The current project is a 9-month applied study to validate colorant removal from a specific PCR-HDPE sample provided by an industry partner. Task 1 involves sample acquisition and colorant identification, Task 2 covers parameter optimization for colorant removal, and Task 3 focuses on verifying the processability of the purified material. The technology is at the lab-scale validation stage, ready for partner-supplied sample testing and process optimization.