Try This Approach to Clean Up Your Recycled Material

Recycling is getting lots of attention these days due to public and media outcries about plastic in the environment. About half of all the plastics produced go into single-service food packaging of some type. Food packaging usually contains various oils, printed surfaces, surface treatments and adhesives that are difficult to remove, even by very aggressive washing, and require the use of toxic solvents that often make the reclaim unusable for further food-packaging applications. This makes it hard for processors running reclaim to produce the clean products the application may demand.

However, supercritical CO₂ can be used as a totally “green” solvent to remove many kinds of contamination in recycled plastics without need for toxic solvents. CO₂ is naturally occurring in the atmosphere, and when stripped of residual contamination after processing it can either be reclaimed or simply vented back into the atmosphere. The use of supercritical CO₂ is a well-established technology in food and drug processing. For many years it has been used to decaffeinate coffee and tea, for example.

CO₂ exists naturally as a gas in the atmosphere; at reduced temperatures it is a solid (dry ice). However under certain conditions CO₂ can exist as a liquid if the pressure and temperature are increased above a certain point. The phase diagram shows the pressures and temperatures necessary for CO₂ to exist in the liquid phase.

At the triple point (T_P) all three phases can exist, but as the temperature and pressure are increased the CO₂ becomes a liquid. At a pressure of 1070 psi and 89.8 F the liquid enters a supercritical phase (CP) and remains in that phase with increasing pressure and temperature. That is important because in that phase it has some unusual properties—between a gas and a liquid—that allow it to more easily penetrate through many solids (like polymers) and act as a strong solvent to dissolve many materials. Its solvent properties are so strong, in fact, that when the supercritical phase exists it must be contained in highly corrosion-resistant materials.

CO₂ treatment has very little effect on the polymer morphology, making the recycled polymer useful for the original performance requirements.

Although the technology of decontamination with supercritical CO₂ is old and well understood, this is typically a batch process because of the difficulty in controlling pressure from the treatment phase, where the solvent (CO₂) absorbs the contaminants, to the solvent-cleaning phase, where the pressure is reduced and the contaminants drop out of solution. If the pressure is released during the treatment phase, the residual contamination simply drops out of the supercritical CO₂ and right back into the polymer. The use of an extruder simplifies that by allowing the pressures to be easily segregated by use of viscous seals between the treatment and solvent-cleaning phase.

The use of a viscous seal before and after the area to be cleaned by the supercritical CO₂ isolates the “treatment area” so that the pressure and temperature necessary to maintain the CO₂ in the supercritical condition can easily be maintained without elaborate pressure chambers and transfer valves. The CO₂ can be simply exhausted into a containment container at a lower pressure, allowing the residuals to drop out, and then return to the atmosphere with no net environmental effect. However, in the interest of economics the “clean” CO₂ can be recompressed and reused.  

Fluropolymers are the only common polymers that are dissolved by the supercritical CO₂ so this treatment has broad applicability. Amorphous polymers seem to be more effectively cleaned using this technique, but crystalline polymers apparently can be cleaned effectively with longer treatment times and additional mixing in the treatment section.

The CO₂ treatment has very little effect on the polymer morphology, making the recycled polymer useful for the original performance requirements. Although somewhat of a change from typical decontamination processes used in recycling, it offers the potential to greatly expand the amount of recycle that can be reused for its original purposes.