Meaning
A high pressure separation process utilizes dense gas above its critical temperature and pressure to dissolve and remove low molecular weight impurities, residual monomers, oligomers and additives from solid polymer matrices. Continuous or batch processing uses supercritical fluid extraction, primarily employing carbon dioxide, to purify recycled plastics without degrading the underlying polymer chains. The dense fluid diffuses rapidly through swollen polymer structures, extracting volatile organic compounds, degradation fragments, off-odors and residual catalysts.
Standard solvent washing removes surface contaminants, whereas supercritical fluids penetrate deep within pellets to extract absorbed species.
Process Mechanism
Supercritical carbon dioxide acts as a non-polar solvent with gas-like diffusivity and liquid-like density when pressurized beyond seventy-three bar at thirty-one degrees Celsius. Operating supercritical fluid extraction vessels near these conditions allows precise tuning of solvent density by adjusting pressure and temperature. High fluid density increases solubility of target contaminants, including residual monomers and oxidized fragments.
Reducing vessel pressure precipitates extracted species in a separation chamber, allowing carbon dioxide recovery and recycling.
Decontamination Yield
Extraction efficiency depends on pressure, temperature, contact time and polymer particle size. Implementing supercritical fluid extraction removes post-consumer contaminants, migrating species, odor compounds and residual solvents down to sub-parts-per-million levels. Pellets purified by this method meet strict food contact migration standards that mechanical washing processes cannot achieve.
Deep decontamination restores post-consumer polyolefins to high purity levels suitable for sensitive packaging applications.
Polymer Cleaning
Solvent power decreases when pressure drops, preventing solvent retention within the cleaned resin matrix. Process economics depend on carbon dioxide recycling rates and cycle times required for complete contaminant extraction from thick pellets. Supercritical processing avoids hazardous organic solvent disposal, offering an environmentally benign purification route for recycled polymers.
