Polyurethanes are thermoset materials that are crosslinked and therefore they cannot be as easily recycled as some thermoplastics that can be melted and pelletized after sorting. The inventors in Hildebrand; J., Terheiden; A., Hinrichs-Tontrup; N., Pfefferle; W. (Evonik) US Patent Application Publication US 2023/0357530, 09-Nov-2023 sought a feasible method to avoid disposing of polyurethanes, such as in landfills, by depolymerizing them into polyols and polyamines that can be reused. Glycolysis, acidolysis and hydrolysis without PTC suffer from a variety of disadvantages.
The inventors screened the use of 13 different inorganic weak bases in saturated aqueous solutions, for the depolymerization of polyurethane pieces at 150 deg C in a Parr reactor (with PTFE liner) under pressure and in the presence of quaternary ammonium phase-transfer catalysts, tetrabutylammonium hydrogen sulfate and methyl tributyl ammonium chloride.
They found that when stirring 25 g of compressed polyurethane foam pieces (1 cm x 1 cm) with 75 g of saturated aqueous potassium carbonate as base and 5 wt% tetrabutylammonium hydrogen sulfate, at 150 deg C for 14 hours, they were able to recover 95% yield of polyamine and 81% yield of polyether polyol. The inventors reported the use of the polyether polyols obtained from these depolymerization runs in the formation of new polyurethane foams.
Bases with pKb’s in the range of 1.7 to 4.7 worked best. Bases with higher pKb’s were much less effective. Reducing the phase-transfer catalyst loading from 5 weight% to 2.5 weight% showed a significant reduction in yield for methyl tributyl ammonium chloride.
We speculate, but don’t know, that methyl tributyl ammonium chloride (MW 235 g/mole) was screened since it is less expensive than tetrabutylammonium hydrogen sulfate (MW = 340 g/mole) on a weight basis and even less expensive on a molar basis.
Though not addressed in the patent application publication, the cost of the phase-transfer catalyst should be an important factor for economic feasibility, especially considering that the quat cations may not survive these reaction conditions and therefore the quat salts are likely not recyclable themselves.
In the end, we don’t know if the economics of this process are feasible, but the chemistry and effectiveness of phase-transfer catalysis for this depolymerization are interesting to PTC chemists like us.
