Recovery of Polyol and Aromatic Amines from Rigid Polyurethane Foams via Ammonolysis

Abstract

Polyurethane ranks as one of the most significant plastics globally in terms of production volume and economic value, serving a crucial role in modern society. Due to complexities in the chemical recycling of polyurethane, replacement rates in recycled materials tend to be low, and the recovery of the isocyanate-derived aromatic compounds is often neglected. While many recycling efforts try to address these shortcomings, they primarily focus on TDI-based flexible foams, as recycling rigid PU foams is more challenging. In this work, ammonolysis is reported as an effective method for the recycling of rigid PU foams using ammonia to efficiently produce aromatic amines and polyols, while enabling easy recovery of the excess ammonia. A quantitative ¹H NMR analysis method was developed for rigid PU materials, allowing for the identification and quantification of the polyol and isocyanate-derived products of rigid PU. For ammonolysis, volumetric productivities of up to 300 g·L^–1·h^–1 were obtained, surpassing current rigid PU recycling processes and highlighting its industrial significance. Kinetic studies provided mechanistic insight into PU depolymerization, with urea linkages splitting fastest, followed by carbamates and finally isocyanurates. Furthermore, solid-state NMR analysis demonstrated ammonia’s hydrogen bonds disrupting effect in PU hard segments, presenting ammonolysis as an optimal method for recycling rigid PU foams, where the high content of hard segments makes degradation more challenging. The ammonolysis mixture could be purified into pure polyol and aromatic amine product fractions using two different separation methods. Finally, to highlight its industrial potential, our ammonolysis approach was demonstrated on various PU materials, extending the scope to end-of-life rigid PU waste materials. A comprehensive recycling process was applied, surpassing current rigid PU recycling efforts by reaching high isolated yields for isocyanate derivatives and polyol.

Introduction

The production of plastics surged to an all-time high of 414 million metric tons in 2023 and is anticipated to continue its exponential growth. , Of these plastics, polyurethanes (PU) are the sixth most produced polymer, holding a market share of USD 80 billion, which is predicted to surpass USD 120 billion by 2032. In essence, polyurethanes are formed through the reaction of polyols with di- or polyisocyanates, resulting in the production of foams or CASEs (coatings, adhesives, sealants, elastomers). , Flexible PU foams are mainly applied for mattresses and furniture, while rigid PU foams are used as insulation in, for instance, construction and household appliances. In general, flexible PU foams are synthesized from long-chain polyols with low OH-values and toluene diisocyanate (TDI); they constitute approximately 30 wt % of the total foam volume. In contrast, rigid PU foams generally incorporate a short-chain polyol in combination with polymeric methylene diphenyl diisocyanate (pMDI). ,− The isocyanate content typically ranges from 50 to 70 wt %, with formulations containing high isocyanate content often classified as polyisocyanurates (PIR) due to the increased formation of isocyanurate. , One way to recycle PU materials is by shredding and rebonding, forming low-value products like carpet underlay and fillings for pillows or acoustic products. ,, However, this technique is generally only applied on flexible PU foams and the market is small and already saturated by industrial trim foam waste. Therefore, chemical recycling, which allows for the recovery and valorization of the polyol and isocyanate-derived molecular building blocks, appears more suitable for recycling rigid PU materials. ,,,− In recent years, single-phase glycolysis, often in combination with acidolysis, has emerged as the most promising chemical recycling technology for PU waste from an industrial perspective. ,, In a typical single-phase glycolysis reaction, PU foam is reacted with a low molecular weight diol and optionally a diacid. When combined, both reactants work in synergy, with the glycols cleaving the polymer linkages and the dicarboxylic acids reacting with the locally formed aromatic amines. This ultimately yields a mixture of hydroxyl-terminated components with trace amounts of aromatic amines. Although recycled polyol can substitute virgin polyol in the synthesis of new PU materials, only up to 30 wt % of the virgin polyol can be replaced by recycled polyol without compromising foam quality. ,,− Given this limitation, there has been considerable interest in the development of split-phase glycolysis systems. ,,− In such a process, the apolar polyol phase-separates from the more polar alcoholizing agent and aromatic isocyanate derivatives, resulting in a polyol of superior purity. ,−

However, this technique relies on the phase separation between the polyol and the splitting agent, limiting its application to long-chain polyols, such as those found in flexible PU foams. Moreover, the aromatic compounds, typically anilines, are trapped within the high-boiling glycol, complicating their isolation. Hence, given that rigid PU foams consist for up to 70 wt % of aromatic isocyanates and only for a minor fraction of relatively polar, short-chain polyols, more suitable alternative recycling methods should be explored for rigid PU materials. , In the hydrolysis of PU, water is utilized as a splitting agent, reacting to form a carbamic acid intermediate, which quickly decomposes, releasing CO₂ and producing a final mixture of aromatic amines and polyols. Although frequently discussed in patent literature, hydrolysis has seen limited industrial application due to its energy-intensive requirements, the need for temperatures of at least 200 °C, and the challenging purification process, particularly the difficulty of removing water. ,,− Alternatively, acidolysis utilizes organic acids as a splitting agent. The structural difference between the polyol and the acidolysis intermediate allows for the isolation of both components. However, for rigid PU foams, significantly lower yields were obtained than for flexible PU foams. ,−

In this work, we propose ammonolysis as an effective method for the chemical recycling of rigid PU foams. The use of ammonia as a splitting agent enables the direct and efficient formation of isocyanate-derived aromatic amines and polyols at high volumetric productivities. Furthermore, the excess low-boiling ammonia can readily be recovered, facilitating the fractionation and recovery of the desired product streams in high purities.

Results and Discussion

Analysis of a Rigid PU Ammonolysis Mixture

A benchmark ammonolysis reaction was performed on an industrial, low-additive rigid PU foam (rigid PU-1), based on pMDI and a short-chain propylene oxide (PO)-based polyol. The pMDI had an average functionality of 2.7 (AF = average number of N-containing functional groups per molecule in pMDI = average number of arene rings per molecule in pMDI; see section S2.1.1). The reaction on 2 g of rigid PU foam was performed without solvent at 150 °C with a reaction time of 2 h. Ammonia (1.2 g) was added, corresponding to an 8:1 molar excess relative to the number of isocyanate-derived groups (IC) in the rigid PU sample. It should be noted that in addition to carbamate groups, rigid PU foams also comprise N,N′-diarylurea and isocyanurate linkages, which we expect to be split during ammonolysis into compounds containing urea and/or primary amine functional groups (Figures and S1).

1.

Ammonolysis reaction of rigid PU with the formation of monoarylated urea intermediates.

After ammonolysis of the rigid PU material, the chemolysis mixture was analyzed in detail via ¹H NMR spectroscopy to identify and quantify the obtained splitting products (Figure , see section S4 in the SI). The polyether polyol can be identified by its characteristic signal at 0.99–1.16 ppm, corresponding to the methyl protons of the PO units. Given that rigid PU polyols do not only comprise PO repeating units, the weight fraction of PO was taken into account for quantification of the polyol (see section S4.2 in the SI). Notably, when this methyl group is located next to a carbamate group, indicating that the polyol is still attached to the isocyanate-derived carbamate, the signal shifts to 1.16–1.28 ppm. In the aromatic region, the signals of the isocyanate-derived aromatic splitting products can be identified. The shifts of the aromatic protons vary depending on the functional group on the arene ring, which is here referred to as a nucleus. Notably, the chemical ¹H shifts on a nucleus are independent of the functional groups on an adjacent nucleus that are linked via a methylene bridge. Signals of polymeric methylene diphenyl diamine (pMDA) compounds are observed at 6.42–6.94 ppm, while the signals of polymeric methylene diphenyl diurea (pMDU) and polymeric methylene diphenyl dicarbamate (pMDC) are shifted further downfield (see section S4.1.1 in SI). To accurately quantify the isocyanate-derived aromatic splitting products, the polymeric nature of pMDI was taken into account, since the number of aromatic protons on a nucleus depends on the average functionality of pMDI (see section S4.1.1 in the SI). Aromatic ureas were quantified based on the signal of their ortho protons at 7.27 ppm, while aromatic carbamates were quantified based on the signal at 7.36 ppm, with the slightly higher chemical shift attributed to a stronger electron-withdrawing effect of the alkoxy oxygen compared to the nitrogen.

2.

¹H NMR (600 MHz, DMSO-d ₆) spectrum of an ammonolysis mixture of a low additive, pMDI-based rigid PU foam. Reaction conditions: 2 g of rigid PU, 8:1 (NH₃/IC), 150 °C, 2 h. The characteristic signals of each proton are highlighted in bold and indicated on the spectra.

Although all products could be accurately identified and quantified by using ¹H NMR, GPC can provide additional insights into the composition of the ammonolysis mixtures. Since ¹H NMR is not ideal for the analysis of oligomers, both techniques work in tandem. Upon an increase of the aromatic yield (which is defined as the molar fraction of isocyanate-derived groups that have reacted with NH₃ to form either amine or urea groups and are recovered in the liquid phase), a clear trend becomes apparent in the GPC chromatogram (Figure , see section S2.4). There is a decrease in high molecular weight components and a simultaneous appearance of distinct peaks at lower molecular weights, corresponding to MDA and its higher nuclei homologues. Notably, at an aromatic yield of 80%, only low amounts of pMDA are detected, owing to the multifunctionality of pMDA, which necessitates high aromatic yields to completely remove bonded polyol from the molecule.

3.

GPC chromatogram of rigid PU ammonolysis mixtures with aromatic yields of 50, 80, and 97%. Reaction conditions for an aromatic yield of 50%: 2 g of rigid PU, 4:1 (NH₃/IC), 10 mL of THF, 150 °C, 2 h. For an aromatic yield of 80%, the conditions were identical except for an increased NH₃/IC excess of 10:1. For an aromatic yield of 97%, both the excess (10:1) and temperature (200 °C) were increased.

Optimization of the Ammonolysis Reaction

The ammonolysis of rigid PU toward polyol and aromatic amines was assessed by varying the reaction parameters (Table ). In the reference reaction at 150 °C with 10 mL tetrahydrofuran (THF) and an ammonia excess of 8:1 (1.2 g), a polyol yield of 72% and an aromatic yield of 74% were obtained after 2 h. The selectivity, defined as the molar fraction of amines formed relative to the total amount of isocyanate-derived splitting products, including both amines and ureas, was 74%. Importantly, a pressure of 20 bar was recorded under these conditions. Decreasing the temperature to 140 °C resulted in a lower aromatic yield (61%) and selectivity (71%) (entry 2). Accordingly, an increase in aromatic yield and selectivity was noticed by increasing the reaction temperature to 160 or 170 °C (entries 3 and 4). Besides the reaction temperature, NH₃ excess was also found to significantly influence the yield and selectivity. Ratios of 4:1 and 24:1 were tested, revealing that lower NH₃ excess led to a decreased aromatic yield of 53% and a decreased selectivity of 70%, whereas higher excess resulted in an increased aromatic yield of 91% and an increased selectivity of 87% (entries 5 and 6). Furthermore, increasing both the temperature and NH₃ excess could push the system to an aromatic yield and amine selectivity of >97% (entry 7). Notably, by performing the reaction in the solventless mode (entry 8), the aromatic yield increased from 74 to 90%. Besides THF, protic solvents such as water and methanol (MeOH) were also evaluated (entries 9 and 10). Both reactions resulted in a lower overall yield but showed increased selectivity for the primary amine, likely as a result of accelerated degradation of the urea intermediate by hydrolysis and methanolysis, respectively. Furthermore, the choice of solvent appears to influence the reaction pressure (Table S1); both water and MeOH reduced the pressure at reference conditions to 10 and 16 bar, respectively, while the solventless reaction showed a pressure buildup similar to that observed in THF, reaching 19 bar. In order to increase the volumetric productivity, the rigid PU was compressed by using a mechanical press before loading it into the reactor. In this way, the mass of PU loaded into the 60 mL reactor could be increased from 2 to 8.3 g per reaction (entry 11). As a result, a productivity of up to 300 g·L^–1·h^–1 could be achieved, after 25 min at 180 °C with a 10:1 NH₃ excess (entry 11). In comparison, state-of-the-art acidolysis and glycolysis methods for rigid PU report productivities below 20 g per liter per hour, even if reported methods were not necessarily optimized toward high productivity (see section S2.8.1 in the SI). , In addition to its higher productivity, ammonolysis also provides a significantly improved process mass intensity (PMI) (section S2.8.2 in the SI). Alternative processes rely heavily on the recovery of high-boiling solvents to approach comparable PMI values. While these approaches offer advantages, such as operating under atmospheric pressure in acidolysis, which lowers capital costs, or the use of less toxic gases in alcoholysis, neither achieves comparable volumetric productivities. This highlights the industrial potential of the ammonolysis method. ,,,, Lastly, the ammonolysis of a flexible MDI-based PU foam, originating from a car seat headrest and of an MDI-based PU elastomer, was demonstrated. The same conditions as in entry 8 were applied to the flexible foam and PU elastomer, achieving aromatic yields of 99 and 69% with selectivities of 93 and 85%, respectively (entries 12 and 13). The higher aromatic yield of the flexible PU foam likely stems from its greater content of N,N′-diarylureas, while for the PU elastomer, the absence of a porous structure led to a reduction of aromatic yield. By an increase in the temperature to 170 °C, the PU elastomer could also be fully depolymerized, resulting in an aromatic yield of 95% and a selectivity of 84% (entry 14).

1. Effect of Temperature, NH₃-Excess, and Solvent on Yield, Selectivity, and Productivity in PU Ammonolysis.

entry	deviation from ref conditions	yield Ar (%)	yield polyol (%)	selectivity (%)	productivity (g·L–1·h–1)
1	none	74	72	74	12
2	T = 140 °C	61	60	71	10
3	T = 160 °C	95	95	87	16
4	T = 170 °C	97	97	93	16
5	excess = 4:1	53	56	70	9
6	excess = 24:1	91	92	87	15
7	T = 160 °C, excess = 24:1	98	94	97	16
8	solventless	90	91	89	15
9	10 mL H2O as solvent	61	62	95	10
10	10 mL MeOH as solvent	43	45	90	7
11	mass = 8.3 g, T = 180 °C, excess = 10:1	89	89	96	300
12	solventless, flexible PU foam, m NH3= 1.2 g	99	98	93	16
13	solventless, PU elastomer, m NH3= 1.2 g	69	70	85	12
14	solventless, PU elastomer, m NH3= 1.2 g, T = 170 °C	95	95	84	16

^aThe yield of aromatics is calculated based on the total amount of isocyanate-derived groups that have reacted with NH₃ to either an amine or urea group and are recovered in the liquid phase.

^bThe polyol yield is defined as the ratio of the amount of free polyol to the total polyol content of the original foam.

^cThe selectivity is calculated based on the ratio of the aromatic amine groups relative to the aromatic urea and aromatic amine groups.

^dReaction was performed without solvent with a reaction time of 25 min.

Ammonolysis of Model Substrates

To have a better understanding of the depolymerization kinetics, the ammonolysis reaction was tested on model substrates that represent the typical polymer linkages present in rigid PU materials (Figures and S1). The splitting rate was assessed by conducting kinetic measurements on diphenylurea (DPU), butyl phenylcarbamate (BPC), and triphenylisocyanurate (TPIC) (Figure ), representing N,N′-diarylated urea, carbamate, and isocyanurate linkage, respectively. Furthermore, monophenylurea (MPU) was selected to represent the monoarylated urea intermediate that is formed during ammonolysis (Figure ). For all model compounds, pseudo-first-order kinetics were observed based on the rate of disappearance, independent of the NH₃-excess (see section S2.6 in SI). Of the model compounds, DPU displayed the highest splitting rate with k _DPU = 1.4 × 10^–3 s^–1, which is more than 30 times higher than for the carbamate (k _BPC = 4.1 × 10^–5 s^–1) and more than 200 times higher than for the isocyanurate (k _TPIC = 6.5 × 10^–6 s^–1) (Figure a). Following the initial cleavage of the cyclic TPIC molecule, a rapid degradation toward MPU and aniline was observed. This is in line with previous results, as the linear degradation product of the isocyanurate contains N,N′-diarylated urea groups (Figure b), which were shown to be split with the highest rates (Figure a). When looking at the initial splitting rate of MPU, a similar splitting rate was observed as for carbamate bonds (k _MPU = 4.6 × 10^–5 s^–1). Besides the splitting rate, these reactions give information about the depolymerization mechanism of ammonolysis. It can be concluded that among the various linkages present in rigid PU, diarylated ureas exhibit the fastest splitting, followed by monofunctional ureas and carbamates, with the splitting of isocyanurates being the slowest. Depolymerization can proceed either through nucleophilic substitution by NH₃ or by dissociation into the corresponding isocyanate, which subsequently reacts with NH₃. , If dissociation were the dominant mechanism, the order of bond cleavage would be expected to follow the thermal stability of the bonds. , However, the opposite is observed, with N,N′-diarylureas showing the fastest splitting, despite their higher thermal stability compared to carbamates. , This trend suggests that depolymerization primarily occurs through a nucleophilic substitution reaction.

4.

(a) Conversion over time for the ammonolysis of diphenylurea (DPU), butyl phenylcarbamate (BPC), triphenylisocyanurate (TPIC), and monophenylurea (MPU). (b) Ammonolysis reactions of DPU, BPC, TPIC, and MPU. Reaction conditions: 5 mmol of substrate, 25 mL of THF, 8:1 (NH₃/IC), 160 °C. Conversion determined by HPLC (see section S2.6 in SI).

Ammonia as an H-Bond Disruptor in Rigid PU

After the depolymerization mechanism was elucidated at the molecular level, solid-state NMR (ssNMR) was used to study the impact of NH₃ on rigid PU morphology. While NH₃’s high reactivity as a splitting agent is partly due to its nucleophilicity, other macroscopic factors may also contribute to its enhanced splitting activity for rigid PU foams. It is proposed that NH₃, through its dual role as an H-bond donor and acceptor, destabilizes H-bonding in the hard segments of rigid PU foam, facilitating further breakdown. Hard segments are formed by the H-bonding between urethane, N,N′-diarylated ureas, and isocyanurate linkages. These regions are characterized by their rigidity and a certain degree of crystallinity. In contrast, soft regions within PU materials, typically formed by polyols, are known to be flexible and mobile. By utilizing ¹H ssNMR, the impact of an ammonia treatment on the mobility of the crystalline parts in rigid PU can be studied.

The ¹H ssNMR measurements were performed on a nontreated reference sample and a rigid PU sample treated at 140 °C for 20 min with a 1:1 mass ratio of NH₃ to rigid PU. At this temperature, taking into account an initial heating phase of ±12 min, minimal to no degradation of the material is expected (Table ). After treatment, the log mean T ₁ and T ₂ relaxation times (T _1LM and T _2LM) were measured (Figure , see section S5 in the SI). For the T ₁ relaxation time, which represents the spin–lattice relaxation time, no large changes were observed in the T _1LM. This is expected, as T ₁ is related to the chemical environment of the molecule, and since the treatment does not aim to depolymerize or alter the chemical structure, this value remains largely unchanged, going from 1.68 to 1.88 s. In contrast, T ₂, which represents the spin–spin relaxation time, shows a clear increase in T _2LM after treatment, from 1.2 to 3.5 ms. T ₂ is determined by the ability of nuclei to transfer their spin to surrounding nuclei; hence, an increase suggests enhanced spin transfer and greater mobility. This suggests that the treatment preserved the polymer’s chemical structure while increasing the mobility of its linkages, likely a result of the disruption of internal hydrogen bonds. For comparison, a sample treated at 160 °C for 40 min with a 1:1 mass ratio of NH₃ to rigid PU was also tested. In this case, a certain degree of depolymerization is expected. For T _1LM, a more pronounced increase is observedfrom 1.68 to 2.03 s, likely due to the breaking of polymer linkages. This suggests that the depolymerization of the rigid polymer structure plays a key role. As for T ₂, although it also increases relative to the reference (from 1.2 to 2.4 ms), the value remains lower than that observed for the NH₃-treated sample. This indicates that depolymerization alone cannot account for the observed changes after the NH₃ treatment. The results further suggest that the increase in T ₂ after NH₃ treatment is caused by the disruption of hydrogen bonding rather than solely by depolymerization. Similarly, as observed in biomass pretreatment, ammonia’s ability to break hydrogen bonds in PU hard segments can enhance degradation. This makes ammonolysis particularly well-suited for recycling rigid PU, given its high content of hard segments. ,

5.

T₁ and T₂ relaxation time distributions from solid-state proton NMR for different samples. These distributions were calculated using ILT from the intensity of the signals as a function of the relaxation times. Reaction conditions for NH₃ treatment: 2 g of rigid PU, 2 g of NH₃, 140 °C, 20 min, NH₃ depolymerization: 2 g of rigid PU, 2 g of NH₃, 15 mL of THF, 160 °C, 40 min.

Fractionation and Purification of the Ammonolysis Mixture

To obtain pure product fractions after ammonolysis, a precipitation method was applied. After the dissolution of the ammonolysis mixture in an organic solvent, which allows for the removal of urea, HCl was added. This leads to the formation of the hydrochloride salts of the aromatic amines, which precipitate from the dissolved polyol. Using this method, it was possible to separate the mixture into a pure polyol (98 wt % purity) and pure pMDA (86 wt % purity) fraction (Figure a). Although this method is remarkably simple, HCl is consumed in stoichiometric amounts. Nevertheless, HCl salts of pMDA can be fed directly into a phosgenation plant, which leads to a more controlled phosgenation process and helps to prevent the formation of isocyanate reformation products. Liquid–liquid extraction offers an alternative fractionation technique, avoiding the use of a strong acid. Recovery and purity were tested for different solvent combinations (Table S3). Among various solvents tested, ethyl acetate (EtOAc) performed best in combination with water, showing high recovery and purity of the polyol and pMDA product fractions (Figure b). Notably, with a one-step liquid–liquid extraction, purities could be achieved similar to those with the precipitation method. After evaporation under reduced pressure, the water content in the polyol fraction was determined to be only 0.83 ± 0.01 wt % via Karl Fischer titration.

6.

Purification of a rigid PU ammonolysis mixture into pMDA, polyol, and urea fractions using (a) precipitation and (b) liquid–liquid extraction. The purity of each component, calculated by ¹H NMR, is reported relative to that of the other components.

Recycling of Industrial Rigid PU Foams via Ammonolysis

After the ammonolysis was studied as well as the subsequent fractionation and purification steps, four different industrial rigid PU foams were tested in a comprehensive recycling procedure (Table , see section S6 in SI). The study encompasses two low-additive foams from different industrial manufacturers (Rigid PU-1 and Rigid PU-2), a commercial foam (Rigid PU-3), and commercial end-of-life rigid PU extrudates originating from refrigerator waste containing >20 wt % impurities (Rigid PU-4). For the low-additive foams (Rigid PU-1 and Rigid PU-2), pMDA yields exceeded 90%, while the polyol yields were 81 and 73%, respectively. The two components were recovered in separate fractions, with pMDA purity exceeding 85 wt % and polyol purity surpassing 97 wt %. These results demonstrate the robustness and effectiveness of liquid–liquid extraction for the separation of pMDA and different rigid PU polyols originating from different industrial manufacturers. The solid residues contained mainly urea (>71 wt %) in addition to small amounts of aromatic compounds and polyol. For Rigid PU-1, the hydroxyl value of the recovered polyol was determined; the value of 720 mgKOH/g is only slightly higher than that measured for the virgin polyol (680 mgKOH/g), and is well within the range reported for rigid PU polyols (see section S2.11). The small increase is presumably caused by the higher hydrophilicity of short-chain polyols compared with their longer counterparts. When the commercial rigid PU foam (Rigid PU-3), similar results as for the low additive foams were obtained. While the urea content in the solid residue was lower, it did not affect the overall process. High yields of pMDA and polyol, at 89 and 78% respectively, were achieved, with pMDA purity at 88 wt % and polyol purity at 99 wt %. Finally, the ammonolysis of end-of-life rigid PU extrudates originating from refrigerator waste (Rigid PU-4) was studied. Centrifugation enabled easy separation of the solid residue, mainly comprising metal impurities and urea, resulting in a homogeneous phase containing pMDA and polyol. Using our standard liquid–liquid extraction method, the pMDA and polyol could be further fractionated, achieving yields of 84 and 80%, and purities of 80 and 96 wt %, respectively. The general process, outlined in Figure S10, shows strong industrial potential and is expected to scale up efficiently. However, appropriate safety measures will be necessary to handle pressurized NH₃ safely on an industrial scale.

2. Ammonolysis of Four Different Industrial Rigid PU Foams and Their Purification by Liquid–Liquid Extraction Using EtOAc/Water.

	mass (mg)			purity (wt %)			yield (%)
sample	pMDA	polyol	urea	pMDA	polyol	urea	pMDA	polyol
Rigid PU-1	1032	773	206	86	98	71	93	81
Rigid PU-2	1308	394	145	85	97	80	90	73
Rigid PU-3	923	653	260	88	99	38	89	78
Rigid PU-4	945	519	392	80	96	/	84	80

^aRigid PU-1 and Rigid PU-2 are low additive foams from different commercial suppliers, Rigid PU-3 is a commercial rigid PU foam, and Rigid PU-4 is a commercial end-of-life rigid PU extrudate originating from refrigerator waste containing >20 wt % impurities (See section S6 in SI).

^bAnalysis is not possible due to metal particles.

Conclusions

In this work, ammonolysis has been demonstrated as an efficient method for valorizing pMDA and polyol from rigid PU foams. By using quantitative ¹H NMR analysis, valuable insights were gained into pMDI-derived products and rigid PU polyols. Building on this, the solvent- and catalyst-free degradation of rigid PU was reported, allowing for easy recovery of excess NH₃ through distillation and achieving a volumetric productivity of 300 g·L^–1·h^–1, exceeding the current values reported for rigid PU recycling. To better understand the depolymerization mechanism of ammonia on a molecular level, a kinetic study was carried out using model substrates, which revealed that diarylated urea linkages split the quickest, followed by carbamates, and finally isocyanurates. The observed order of bond splitting strongly suggests that the reaction proceeds through a nucleophilic acyl substitution. Additionally, the effect of ammonia on the macroscopic morphology of rigid PU was examined using solid-state NMR, which uncovered its hydrogen bond-disrupting properties. This disruption is expected to facilitate breakdown of the PU hard segments, making ammonolysis particularly well-suited for rigid PU materials due to their high content of such crystalline hard segments. The ammonolysis mixture could be efficiently purified into its product fractions via a precipitation method using a strong acid or by liquid–liquid extraction with EtOAc/water. A generalized recycling process, combining ammonolysis and EtOAc/water liquid–liquid extraction, was successfully applied to industrial rigid PU samples, including commercial end-of-life rigid PU extrudates originating from refrigerator waste. Solid contaminants and urea were easily separated and did not further affect the recycling process, highlighting the industrial relevance of this method. Both the pMDA and polyol fractions were obtained in high isolated yields, ranging from 73 to 95%, with purities between 80 and 99 wt %. The high purity of the polyol indicates its potential to replace virgin polyol, while the pMDA could be integrated into the phosgenation process, enabling a fully circular rigid PU recycling process. Hence, ammonolysis may present a viable solution to current PU recycling challenges by offering a robust and efficient method for valorizing both building blocks in high yields and purities.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.5c00496.

• A more detailed explanation of the ¹H NMR analysis and quantification, experimental methodology, and additional characterization data and spectra (PDF)

Under the guidance of N.V.V. and D.D.V., L.V.B. was responsible for the conceptualization, execution, and evaluation of the experiments. J.W. executed the Karl Fisher measurements. R.D. helped with the synthesis of model substrates and ¹H NMR measurements. R.d.O.S. and D.S. performed SS NMR measurements and analysis of the data.

The authors declare no competing financial interest.