Polyurethane Foam Chemical Recycling: Fast Acidolysis with Maleic Acid and Full Recovery of Polyol

Chemical recycling of polyurethane (PU) waste is essential to displace the need for virgin polyol production and enable sustainable PU production. Currently, less than 20% of PU waste is downcycled through rebinding to lower value products than the original PU. Chemical recycling of PU waste often requires significant input of materials like solvents and slow reaction rates. Here, we report the fast (<10 min) and solvent-free acidolysis of a model toluene diisocyanate (TDI)-based flexible polyurethane foam (PUF) at <200 °C using maleic acid (MA) with a recovery of recycled polyol (repolyol) in 95% isolated yield. After workup (hydrolysis of repolyl ester and separations), the repolyol exhibits favorable physical properties that are comparable to the virgin polyol; these include 54.1 mg KOH/g OH number and 624 cSt viscosity. Overall, 80% by weight of the input PUF is isolated into two clean-cut fractions containing the repolyol and toluene diamine (TDA). Finally, end-of-life (EOL) mattress PUF waste is recycled successfully with high recovery of repolyol using MA acidolysis. The solvent-free and fast acidolysis with MA demonstrated in this work with both model and EOL PUF provides a potential pathway for sustainable and closed-loop PU production.


Table of contents:
Page S2: Table S1 Properties of common organic dicarboxylic acids for PUF acidolysis Table S2 Model PUF components/preparation Page S3: Table S3 Calculations of molar content for urethane and urea bonds in model PUF Page S4:

Equivalent molar functionality: (Molar input) Functionality ×
Polyol was added as limiting reagent for PUF synthesis, thus the mass% calculated for polyol represents the urethane component in resulting PUF.Similarly, the TDI and water were reacted to form urea in the resulting PUF and thus their mass% was added together.CO 2 was produced and released during the urea formation.Therefore, the mass of CO 2 (negative mass input) was subtracted from the overall mass.
Resulting PUF: sum of Mass input of all components.

𝑅𝑒𝑠𝑢𝑙𝑡𝑖𝑛𝑔 𝑃𝑈𝐹
According to the formulation of model PUF, polyol and water are added as limiting reagent.Therefore, the moles of input polyol represent the moles of resulting urethane while the moles of water equal the moles of urea.

Molar density:
ℎ ( )   () For example, 3 g PUF was used for acidolysis reaction.After the acidolysis reaction, 2 g of repolyol and 0.4 g of TDA were collected after hydrolysis and purification.The yield of repolyol and TDA was calculated as (by mass): Yield of repolyol:    Gas evolution burette setup used to monitor acidolysis.Volume in burette was recorded at beginning of reaction and at each time point selected, and the volume of gas produced was calculated as V g (t) = V l (0) -V l (t), where V g and V l are volumes of gas produced and liquid in the burette, respectively.Time = 0 was chosen to be the time at which the reaction flask reached 175 °C (i.e., the time at which gas evolution from thermal expansion was observed to cease).

Figure S7
. APC GPC THF of a repolyol ester from acidolysis with maleic acid at 175 , ℃ along with two references: VORANOL™ 8136 polyol and its ester with maleic acid, produced from heating VORANOL™ 8136 and maleic acid at 175 for 3 h.℃

DFT Calculations
Geometry optimizations using the cc-pVTZ correlation consistent basis set and the Perdew, 1, 2 Burke, and Ernzhof (PBE) exchange-correlation functional were implemented in Gaussian 16. 3 The reaction enthalpy, entropy, and free energies were defined as the enthalpy, entropy, and free energy differences between the final and initial states.No solvation model was used, so all calculations were carried out in vacuum.

Simplified Reaction Scheme Determination
Figure S-9 shows a monomer of PUF.The complexity of the PUF monomer is computationally prohibitive for a high number of calculations.To lower the computational burden of the calculations, we simplified the reaction system as shown in Figure S10.
We hypothesize that the results of our proposed calculations with this simplified system will scale to understand the chemistry at the polymer scale.To do this, we tested different chain lengths, represented by 'x' in Figure S10, to understand how increasing the size of the carbon chain in PU affects the thermodynamics of acidolysis.We determined that the chemistry of acidolysis is very local, indicated by the non-changing free energy with increasing 'x'.As a result, we used 'x' = 1 for future computations.The resulting simplified PUF acidolysis reaction scheme is shown in Figure S11.  1,4,5 om this, we selected cc-pVTZ as it yielded the same results as cc-pVQZ, a larger and thus more accurate basis set, with smaller computational time.

Free Energy of Acidolysis
With PBE functional, cc-pVTZ basis set, and selected representative reaction (Figure S11), we calculated the free energy of acidolysis at the pertinent reaction temperatures (Figure S14).From this, we determined that the acidolysis of PUF is thermodynamically viable, which is consistent with the observed experimental results.
Figure S1 SEM images of PUF samples Page S5: Figure S2 FT-IR spectra of PUF samples Page S5: Figure S3 TGA analysis of model and EOL PUF samples Page S6: Figure S4 ATR FT-IR spectra of maleic acid, fumaric acid, and acidolysis solid residue Page S7: Figure S5 Reaction setup and mass balance of PUF acidolysis at PUF/MA 1:2 for 3 hr Page S8: Figure S6 Gas evolution burette acidolysis setup Page S9: Figure S7 APC GPC THF spectra of polyol samples Page S10: Figure S8 Mass Spectrometry analysis of TDA products from model PUF Page S11: Figure S9 2D 1 H-15 N HSQC NMR of TDA products from model PUF Page S12 -S14: DFT calculations S12: Figure S10 Example of PUF monomeric unit Figure S11 Simplification of PUF monomeric unit S13: Figure S12 Simplified PUF acidolysis reaction scheme Figure S13 Free energy of PUF acidolysis calculated with B3LYP and PBE functionals S14: Figure S14 Free energy of PUF acidolysis calculated with PBE functional Figure S15 Free energy of PUF acidolysis reaction Page S15: Figure S16 Comparison of repolyol obtained from different purification methods Page S16: References

Figure S2 :
Figure S2: FT-IR spectra of (a) comparison between intact model PUF vs shredded model PUF; (b) comparison between intact EOL PUF vs. shredded EOL PUF.

Figure S3 :Figure S5 .
Figure S3: TGA analysis of model and EOL PUF.The weight loss (%) was determined based on temperature.The TGA analysis was carried out under N2.The weight change between 220 -320 °C was assigned to hard segment decomposition while the weight change between 320 -450 °C was assigned to soft segment decomposition.The assignments of hard and soft segment weight/content were based on the known composition of the model PUF formulation provided by Dow.

Figure S8 .
Figure S8.Mass spectrum (MS) obtained from GC-MS of isolated TDA from model PUF acidolysis with MA after hydrolysis and workup.Inset: NIST MS of authentic TDA standard.97% match to isolated TDA product from PUF acidolysis.

Figure S9 .
Figure S9.2D 1 H-15 N HSQC NMR of isolated TDA from model PUF acidolysis with MA after hydrolysis and workup.The TDA was observed in the Amine region (blue colored); trace amount of amide product as TDI-derivative was observed in Amide region (green colored); no other N-contained product was observed in region with red color indicates the full decomposition of model PUF through acidolysis with MA.

Figure S10 .
Figure S10.Example of a PUF monomeric unit

Figure S15 .
Figure S15.Free energy of PUF acidolysis reaction in Fig. S11, R=R'=methyl, cc-pVTZ basis set, PBE functional, no solvation.Line added to guide the eye.

Figure S16 .
Figure S16.Comparison between original hydrolyzed repolyol vs. the toluene/acid purification treated repolyol from model PUF.(a) shows the original hydrolyzed repolyol obtained from NaOH hydrolysis and isolated by EtOAc; (b) shows the hydrolyzed repolyol obtained from NaOH hydrolysis and isolated and purified by toluene/acid treatment.

Table S1 :
Properties of common organic dicarboxylic acids for PUF acidolysis

Table S3 .
Formulation of model PUF and examples of calculation