Insight into Chemical Recycling of Flexible Polyurethane Foams by Acidolysis

Acidolysis is emerging as a promising method for recycling polyurethane foam (PUF) waste. Here, we present highly efficient acidolysis of PUFs with adipic acid (AA) by heating the reaction mixtures with microwaves. The influence of experimental conditions, such as reaction temperature, time, and amount of the degradation reagent, on the polyol functionality, molecular weight characteristics, the presence of side products, and the degree of degradation of the remaining PUF hard segments was studied by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS), nuclear magnetic resonance (NMR), size-exclusion chromatography (SEC) coupled to a multidetection system, and Fourier transform infrared (FT-IR) spectroscopy. The purified recycled polyols were used for the synthesis of flexible PUFs. The morphology and mechanical properties of the PUFs show that the degree of functionalization of the polyol by the carboxylic end groups, which is higher for larger amounts of AA used to degrade the PUFs, significantly affects the quality and performance of the flexible PUFs from the recycled polyols.


List of
. Reaction conditions used for microwave-assisted acidolysis of various PUFs with AA. ...... 5 Table S2. Properties of PPO-based VP and its recycled analogs obtained from PUF5611 by acidolysis using 1.  Figure S5. Magnified 1 H NMR spectra of purified RPs obtained by PUF acidolysis at 210, 220, 230 °C for 40 min and AA/urethane group ratio of 1.1. 1 H NMR spectra were normalized to polyol methyl group at δ 1.04 ppm. Traces of EtOAc used as an extraction solvent and partially amidated 2,4-TDA in RPs are marked with asterisk and h', respectively. The peak assignment refers to the structures in Table S4 Figure S8. A) FTIR spectra and B) SEC/UV-MALS-RI chromatograms of purified RPs obtained after acidolysis of PUF with 1.1 (red), 2.0 (blue) and 3.0 (green) equivalents of AA per urethane group at 220 °C for 30 min together with VP of the same type (black  Figure S9. A) 1 H NMR spectra of purified RPs obtained by acidolysis of PUF5611 in bulk (red) and PUF5611 synthesized from 100 % RP (blue) together with VP of the same type (black). B) 1 H NMR spectra of purified RPs obtained by acidolysis of PUF4811 (green) and post-consumer PUF waste (orange) together with VP of the same type (black). .  Figure S13. Stress-strain curves of PUFs synthesized according to formulations in Table S3 using 100 wt% VP (black), 50 wt% (red) and 100 wt% (green) of RP containing 5.6 mol% of carboxyl and 3.2 mol% of amine functional groups, and 50 wt% (

The non-hydroxyl end group content of the RPs
The non-hydroxyl end group content of the RPs was determined from 1 H NMR spectra recorded in DMSO-d 6 with added TFA to shift the signal of the amine groups, which overlaps S6 with that of the polyol methyne groups, to a lower magnetic field. The non-hydroxyl end group content of the RPs was determined according to equation S1 from the integral of the overlapping signals of the polyol methyne protons adjacent to the urethane and ester groups at δ 4.88 ppm (c' and r), and the integral of the methyl signal (-CH 3 ; a) of the polyol at δ 1.04 ppm, assuming 50 PO repeating units in the polyol and polyol functionality of 3.

Non-hydroxyl end group content
The content of polyol carboxyl (-COOH) and amine (-NH 2 ) end groups The content of carboxyl (-COOH) and amine (-NH 2 ) end groups was determined from 1 H NMR spectra of the purified RPs recorded in DMSO-d 6 . The content of carboxyl groups was determined according to equation S2 based on the intensity of the proton signals at δ 2.20 and 2.26 ppm (denoted as p and p′) corresponding to the methylene groups of the AA residue adjacent to the carboxyl and ester groups, respectively, and the intensity of the methyl signal -COOH content (%) =

The content of TDA in recycled polyols
The content of TDA in unpurified RPs (upper polyol phases) was determined from 1 H NMR spectra recorded in DMSO-d 6 according to equation S4  Astra 8 software was used for data acquisition and analysis (Wyatt Technology Corp., USA).
A 0.4 µL of the prepared solution was spotted onto the target plate (dried-droplet method).
The mass spectra of the samples were recorded in reflective positive ion mode. Calibration was performed externally using a mixture of poly(methyl methacrylate) standards dissolved in THF (MALDI validation set PMMA, Fluka Analytical) that covered the measured molecular weight range. Sample preparation for the standard mixture was the same as for the samples. The standard mixture was spotted to the nearest neighbor positions.

Fourier-transform infrared spectroscopy (FTIR)
FTIR transmission spectra were recorded using a Spectrum One FTIR spectrometer (Perkin-Elmer, Waltham, MA, USA) in ATR mode in a spectral range of 400−4000 cm −1 with a spectral resolution of 4 cm −1 .

Acid value (AV)
AV was determined according to the adapted standard ASTM D4662-08 ( where A is the volume of NaOH solution (mL) required to titrate the sample; B is the volume of NaOH solution (mL) required to titrate the blank; N is the normality of the NaOH solution, and w is the weight of the sample (g).

Hydroxyl number (OH number)
The OH number was determined according to ASTM D4274-05 standard, where the esterification process of the polyol with phthalic anhydride was catalyzed by imidazole.
Phenolphthalein dissolved in pyridine (50 µL) was used as the color indicator. An esterification reagent (phthalic anhydride) was used as a blank. Titrations with an aqueous 0.5 N NaOH solution were performed in triplicate to determine the average value. The end point of the titration was determined visually by a change in color of the solution to pink. The OH number was calculated according to equation S6, where A is the volume of NaOH solution (mL) required to titrate the sample; B is the volume of NaOH solution (mL) required to titrate the blank; N is the normality of the NaOH solution and w is the weight of the sample (g).
The OH number was corrected considering AV and calculated according to equation S7.
The water content The water content in the polyols was determined by the adapted standard method ASTM D4672 12, in which chloroform was used as an additional solvent to improve the solubility of the polyols. Karl Fischer titration was performed using a C10S Compact KF coulometer (Mettler Toledo, Columbus, USA).

Scanning electron microscopy (SEM)
The morphology of the PU foams was studied by scanning electron microscopy (SEM) on a high-resolution SEM Zeiss Ultra plus instrument (Carl Zeiss, Germany). The foams were cut with a razor blade perpendicular to the foam rise direction. For charge dissipation during SEM analysis, the obtained cross-sections were coated with a 10 nm thick gold layer using a Gatan PECS 682 (Gatan, USA). Pore size analysis was performed with ImageJ software using at least 100 pores from three different cross-sections.

Compressive properties
The compressive properties of the PU foams were determined using a DMA Q800 dynamic mechanical analyzer (TA Instruments) and 40 mm diameter compression discs. The specimens were cut into a rectangular cuboid shape (10 mm height, 25 mm width, 25 mm length) and compressed at 50 % min -1 up to 70 % of the original specimen height and then decompressed at the same rate to the original specimen height. The procedure was repeated three times and at the fourth compression cycle, the stress-strain curve was recorded from which the Young's modulus was determined from the initial linear range and the stress at 40 % strain. The average values of three specimens with standard errors are given for each sample.
To determine the compression set, the specimens with rectangular cuboid shape (10 mm height, 25 mm width, 25 mm length) were compressed to 50 % of the initial height (d 0 ) and heated to 70 °C. After 22 h, the foams were allowed to recover for 30 min at ambient S11 conditions. The height (d r ) was measured again and the compression set, expressed as a percentage, is calculated according to equation S8: where d 0 and d r are the heights of the original specimen and the specimen after compression testing, respectively.  Figure S1. Magnified 1 H NMR spectra of residues of hard segments obtained by acidolysis of PUF with 1.1 equivalents of AA per urethane group after 40 min at 210 and 230 °C. The peak assignment refers to the structures in Table S4.  The peak assignment refers to the structures in Table S4. S16 Figure S6. Magnified 1 H NMR spectra of residues of hard segments obtained after acidolysis of PUF with 1.1, 2.0 and 3.0 equivalents of AA per urethane group at 220 °C, 30 min. The peak assignment refers to the structures in Table S4.  Figure S9. A) 1 H NMR spectra of purified RPs obtained by acidolysis of PUF5611 in bulk (red) and PUF5611 synthesized from 100 % RP (blue) together with VP of the same type (black). B) 1 H NMR spectra of purified RPs obtained by acidolysis of PUF4811 (green) and post-consumer PUF waste (orange) together with VP of the same type (black). Figure S10. MALDI-TOF mass spectra of RPs obtained by acidolysis of PUF5611 in bulk (top) and PUF5611 synthesized from 100 % RP (bottom). The measured monoisotopic S19 signals are denoted in the magnified regions of the mass spectra and are in good agreement with the calculated exact masses (M) ionized with the sodium ion for the proposed structures. Figure S11. A) FTIR spectra of purified RPs obtained by acidolysis of PUF5611 in bulk (red) and PUF5611 synthesized from 100 % RP (blue) together with VP of the same type (black). B) FTIR spectra of purified RPs obtained by acidolysis of PUF4811 (green) and post-consumer PUF waste (orange) together with VP of the same type (black).  Table S3 using 100 wt% VP (black), 50 wt% (red) and 100 wt% (green) of RP containing 5.6 mol% of carboxyl and 3.2 mol% of amine functional groups, and 50 wt% (blue) of RP containing 14.0 mol% of carboxyl and 1.6 mol% of amine functional groups.