Dissolution Recycling: How Science is Fixing the Grossest Problem With Plastics

Would You Eat From Barf-Scented Plastic?

Didn’t think so. But that’s the trouble with recycling molded plastic from many food containers. They’re often made of polypropylene (PP), a.k.a the #5 plastic, a long hydrocarbon chain that readily traps food residues like diallyl disulfide (garlic smell) and eicosapentaenoic acid (fish odor).

[Captions:]

diallyl disulfide
eicosapentaenoic acid

As time passes, those malodorous molecules can decay into even nastier compounds like butyric acid (one of the compounds in vomit).

Residue molecules are small enough to squeeze between the long hydrocarbon chains of polypropylene, where they stay!

This Kind of Recycling Stinks!

One thing you can do with polypropylene is grind it up, melt it together, and mold it into something “new.” But it still smells! That’s OK for a garbage can, but not so good for a yogurt cup.

Another option is pyrolysis, which splits the polypropylene molecules apart at high temps so they can be purified, then melts them back together. But it gives off almost as much CO2 as simply burning all that plastic for electricity!

[Caption:]

PP
Heat
Impure Plastic Pellets

The Recycling Solution is Dissolution Recycling

Dissolution recycling is a new technique that uses a special alkane solvent to remove those impurities.

Alkanes are single-bonded hydrocarbons that withstand extreme conditions, like really high temperatures and pressures.

Dissolution Recycling Process

Step One:

• Submerge dirty polypropylene solids in the solvent and heat it.
• While high pressure keeps the solvent in liquid state, high temps dissolve the impurities from the polypropylene into the solvent.
• Drain away the dirty solvent and impurities.

Step Two:

• Add a fresh batch of solvent.
• Heat it up enough to dissolve the polypropylene itself.
• Draw off the solvent with the dissolved polypropylene.
• Discard the remaining, undissolved impurities.
• Cool the solution until the polypropylene solidifies, and drain the solvent away.

Sources:

• https://omnexus.specialchem.com/selection-guide/polypropylene-pp-plastic
• https://www.compoundchem.com/2014/05/05/what-compounds-cause-garlic-breath-the-chemistry-of-garlic/
• https://cen.acs.org/articles/91/web/2013/09/Source-Fishy-Odors-Identified.html
• https://www.frontiersin.org/articles/10.3389/fenrg.2019.00027/full
• https://patents.google.com/patent/WO2017003796A1/en
• https://purecycle.com