Introduction

Polymers find extensive applications in household appliances, packaging, construction, medicine, electronics, automotive, or aerospace components. However, the predominant source of polymers lies in petroleum feedstocks. Furthermore, not all polymers are readily recyclable, leading to their disposal in landfills or incineration for energy generation [1, 2]. Consequently, polymer-related pollution has emerged as a critical environmental concern, as the disposal of these materials surpasses the world’s capacity to manage them. This, in turn, has adverse effects on biodiversity, results in oceanic accumulation, and ingestion by wildlife and organisms [3]. In response to these challenges, there is a growing trend towards producing petroleum-based products using eco-friendly feedstocks [4,5,6,7,8,9,10,11]. Simultaneously, significant advancements have been achieved in the development of recycling technologies aimed at transforming polymer waste into valuable raw materials [12, 13].

Yet recycling commences with a collection phase, which currently exhibits a low rate (only about one third of polymers produced is collected [14]). Collecting polymer waste faces a significant bottleneck due to the common occurrence of contamination, either from dirt or mixed with other types of polymers. Fortunately, advancements in technology have addressed this challenge by enabling more effective separation of polymers than previously possible. Sorting and washing processes play a crucial role because, in the recycling of polymers, a mixture can led to a rapid deterioration in mechanical properties. Therefore, extracting various polymers from the waste stream is essential, making sorting techniques pivotal.

PU ranks among the most adaptable polymers due to its versatility in various applications, including elastomers, adhesives, paints, and foams. The synthesis of this polymer occurs through the creation of urethane linkages, which result from the reaction between the OH groups of a polyol and the NCO groups of an isocyanate, as outlined in Scheme 1 [15]:

Scheme 1
scheme 1

[Reprinted Materials, 11, N. Gama, A. Ferreira, A. Barros-Timmons, Polyurethane Foams: Past, Present, and Future, Materials Copyright (2018) with permission from MDPI]

Reaction of urethane production [15].

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Due to its wide range of uses, the production of PU has seen significant growth in recent decades, primarily in the form of foam (PUF). Indeed, in 2023, the global PU market size reached USD 75.19 billion and is projected to experience a compound annual growth rate (CAGR) of 4.4% from 2023 to 2030 [16]. This surge in production has resulted in an increase in waste generation, raising concerns about its proper disposal [17].

The majority of PUs are thermoset polymers, which are not easily biodegradable. Additionally, the presence of various additive compounds complicates their recycling [18]. Consequently, only 29.7% of PU is currently recycled, with 39.5% being used for energy recovery, while the remaining 30.8% is disposed of in landfills [19]. Moreover, low-density polymers, particularly foams, demand significant land resources [20]. Considering that the production of PU was nearly 26 million metric tons in 2022, forecasted to grow to 31 million tons in the year 2030, measures must be carried out to mitigate its effects on the environment [21]. In response to this, the European Commission has been implementing measures to prevent the disposal of urban waste, including items like mattresses [19]. As a result, the European Union has emerged as a leader in the global effort to combat marine litter and polymer pollution. Notably, it enacted Directive 2019/904 on June 5, 2019, to reduce the environmental impact of single-use polymer products [22]. Several European Union countries, including the Netherlands, Denmark, Sweden, Switzerland, Austria, and Germany, have also introduced new directives to prohibit landfill disposal [20]. Embracing a “zero waste to landfill by 2025” approach, the inclusion of polymer waste as a recyclable and reusable material holds significant promise in the development of new materials [19].

PU foam waste presents two potential disposal routes: as a fuel source for cement manufacturers due to its relatively high calorific value or through enzymatic processes for biodegradation [23]. However, neither of these strategies is considered sustainable or economically viable, making PU waste recycling a necessity. Hence, there are two primary approaches for PU recycling which can be sustainable or economically viable: mechanical and chemical. Mechanical recycling involves physical processes that transform PU scraps into flakes or powder, which can then be used in the production of new materials. Various technologies, such as gridding, rebinding, compression molding, or injection molding, are employed in this process [24]. Mechanical recycling is a straightforward and cost-effective method that can enhance the eco-efficiency of PU. While it allows for PU scrap reuse, it has limited market applications, such as agglomerate production [22, 24,25,26]. On the other hand, chemical recycling consists in the depolymerizing PU waste into oligomers and smaller molecules, like polyols, which can be used to manufacture new PU products [24]. Despite the equipment, energy, and reactants required, chemical recycling provides raw materials suitable for a wide range of material production, making it economically advantageous. Several chemical methods are available for chemical PU recycling, including hydrolysis, aminolysis, alcoholysis, and glycolysis. After the reaction, the recovered polyol (RP) typically undergoes separation and purification processes [27]. Nowadays, a novel approach to recycle PU scraps is gaining prominence - acidolysis. This process converts PU waste into a RP by utilizing one or more DA [28,29,30]. Despite the fact that initial research on PU acidolysis dates back to 1982 [31], it is only recently that acidolysis has emerged as the most promising method for the chemical recycling of PU [32]. Examining the scientific papers on SCI Papers published over the years (Fig. 1), one can note that acidolysis has gained significance only in recent years.

Fig. 1
figure 1

SCI Papers published over the years, where “recycling” AND “polyurethanes” and “pyrolysis” OR “acidolysis” OR “Glycolysis” OR “Aminolysis” OR “hydrolysis” OR “alcoholysis” were used as keywords

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As noted by Sołtysiński et al. [33], acidolysis offers the advantage of not containing amines in the RP, including carcinogenic substances like toluene diamine (TDA) and methylene diamine (MDA) [33]. In that sense, the recycling technology of acidolysis is garnering significant attention from manufacturers of both rigid and flexible PU foams, owing to its numerous advantages. Companies like the Rampf Group [34] and H&S Anlagentechnik [35] are actively researching PU acidolysis, underscoring the significance of this field. Notably, H&S Anlagentechnik’s technology is already in industrial use at the Dendro foam processing plant in Poland [36]. Additionally, the PUReSmart consortium represents a comprehensive collaboration involving all stages of the PU reprocessing value chain, bringing together nine partners from six different countries [37]. Its primary objective is to explore innovative methods, technologies, including acidolysis, to transform PU into a truly circular material. Furthermore, ongoing developments include a deeper understanding of the acidolysis degradation mechanism, the impact of reaction conditions on RP properties, and the application of this process to various types of PU materials, as documented in previous studies [29, 30, 38, 39].

Moreover, acidolysis can serve as a complementary recycling process. He et al. [40] introduced a multi-stage degradation approach for recycling PU materials, outlined in Fig. 2. This innovative strategy begins with an initial PUF degradation step through ammonolysis, followed by depolymerization using acidolysis. Subsequently, the resulting RP are directly used, without purification, at concentrations of up to 30 wt% as a partial replacement for conventional polyols in the production of flexible foams.

Fig. 2
figure 2

[Reprinted from Green Chemistry, 25, H.W. He, K.M. Du, H.J. Yu, Y.F. Zhu, H. Su, F. Yang, M. Ma, Y.Q. Shi, X.J. Zhang, S. Chen, X. Wang, A new strategy for efficient chemical degradation and recycling of polyurethane materials: a multi-stage degradation method, Pages No. 6405–6415, Copyright (2023) with permission from RSC]

Schematic diagram of the multi-stage degradation process proposed by He et al. [40].

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In this process, PU flakes were introduced into an acidolysis mixture containing polyethertriol, phthalic acid, maleic acid, acrylic acid, and a radical starter. This procedure aimed to yield a reaction mixture with a PU content of 40% by weight. However, the inefficiency of this method was attributed to the numerous process steps and reactants required.

Moreover, applying pre-treatments to PU waste can expedite the chemical recycling process. He et al. [41] introduced a mechanochemical technique for recycling PU foams. Initially, the foam waste was subjected to milling, followed by chemical recycling. According to the authors, the milling process fractures the C-O bonds within the PU chains, significantly enhancing the reactivity of the resulting powder (120 mesh). A comparable outcome was observed when a ball milling pre-treatment method was employed to pulverize commercial PUF into a powder with a particle size of 74.5 μm and an impressively high specific surface area of 2991.0 m2.kg− 1, which further facilitated chemical reactions [42]. Similarly, an efficient pre-treatment approach for PU was patented [43], involving the pulverization of PU foam to yield a dry powder, which could then be chemically recycled.

In summary, chemical recycling methods like acidolysis present viable approaches for the valorization of PU waste [44]. The mechanism of PU depolymerization via acidolysis was initially proposed by Behrendt et al. [45] in 2009, a proposition later supported by Sołtysiński et al. [33] in 2018. Significant contributions to the chemistry of this recycling method have also been made by other authors such as Fonseca et al. [23], Digvijay Singh Yadav [18], and Gadhave et al. [46]. In the current landscape, Rampf Ecosystems and H&S Anlagentechnik have emerged as the most relevant enterprises operating in this sector [32]. In addition to the mentioned works, Table 1 list relevant Science Citation Index (SCI) papers related with acidolysis of PU, where the reaction conditions are depicted.

Table 1 List of relevant SCI papers related with acidolysis of PU
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It is worth noting that chemical recycling of PU waste remains relatively underdeveloped, despite its substantial potential for economic benefits. Furthermore, this aligns with the objectives outlined in the 2030 Agenda for Sustainable Development, adopted by all United Nations Member States in 2015, with a particular focus on Sustainable Production and Consumption. Consequently, there is an opportunity to achieve prosperous economic, social, and technological progress while maintaining harmony with nature. Additionally, recycling PU materials can help raise public awareness of sustainable practices, encouraging companies to consider circular economy models as genuine alternatives. Ultimately, by utilizing recycled feedstocks, it enhances the eco-efficiency of the resulting PU materials. This review consolidates the most relevant papers on the acidolysis of PU, with the aim of disseminating knowledge about the subject across academia and industry.

Degradation Mechanism and Reaction Products

As previously stated, acidolysis requires the utilization of a cleavage agent known as DA (depolymerization agent). The role of DA is to interact with the carbamate group present in PI. This interaction leads to the depolymerization of these components into polyols, other oligomers, and small molecules. Additionally, this process results in the liberation of water and CO2, as outlined in Scheme 2.

Scheme 2
scheme 2

[Reprinted Materials, 11, N. Gama, A. Ferreira, A. Barros-Timmons, Polyurethane Foams: Past, Present, and Future, Materials Copyright (2018) with permission from MDPI]

Depolymerization of PU using dicarboxylic acids [15].

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where, Rpolyol correspond to the conventional polyol chains, Riso corresponds to the isocyanate chain and RDA corresponds to the DA chain.

In addition to the typical use of DA, as outlined in DE 102,013,106,364 A1 [51] dicarboxylic anhydrides can also serve as viable alternatives. Moreover, patents have been granted for the utilization of various other organic acids, including saturated fatty acids, unsaturated acids, and both monobasic and polybasic carboxylic acids [52].

In addition to depolymerizing PU chains through DA reactions, at temperatures nearly 200 °C, the urethane and urea groups can experience thermal degradation. This results in their dissociation into amines and isocyanates [53, 54]. These molecules can then engage in subsequent reactions with the DA, ultimately leading to the formation of species terminated with acid groups and the release of water, as illustrated in Scheme 3.

Scheme 3
scheme 3

[Reprinted from Chemical Engineering Journal, 395, N. Gama, B. Godinho, G. Marques, R. Silva, A. Barros-Timmons, A. Ferreira, Recycling of polyurethane scraps via acidolysis, Page No. 125102, Copyright (2020) with permission from Elsevier]

Thermal degradation of the urea groups and subsequently reaction with DA [29].

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Indeed, the thermal degradation of PU has been a subject of research since the 1950s [15]. However, the reported results have exhibited unavoidable variation and discrepancies, primarily attributed to the diverse array of PU types and products studied. Crucial factors influencing these variations include the specific isocyanate, polyol, and chain extenders employed in PU production. This is because the various linkages within the polymer chain possess distinct thermal dissociation temperatures, as demonstrated in Table 2. Therefore, depending on the specific PU polymer structure, its thermal degradation has been documented to occur within a temperature range spanning from 110 to 270 °C.

Table 2 Thermal dissociation temperature ranges for linkages in PU [55]
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Consequently, based on the conditions of the recycling process, it becomes possible to recover both the polyol and the isocyanate components. However, it’s worth noting that under certain circumstances, the isocyanate can undergo conversion into diamines, a transformation that may not be desirable, as previously noted.

As showed, the degradation process of PU is intricate and can result in the formation of various chemical species. Godinho et al. [39] conducted a study on the depolymerization of PUF through acidolysis, using succinic and phthalic DA. It was determined the molecular weight distribution of the resulting RP, confirming that the primary molecule found in RP closely resembles the size of the conventional polyol used in the production of the original PUF. This confirmation strongly suggests that the predominant molecule in RP closely matches the size of the conventional polyol used in PUF production. Additionally, smaller molecules were identified within RP, which could be linked to the moieties illustrated in Scheme 4(b) to (d). These smaller molecules may have formed due to hydroxyl or diamine-terminated moieties, which could subsequently react with the DA, resulting in the creation of different by-products, as previously discussed. The presence of moieties with elevated molecular weights can be attributed to the incomplete depolymerization of PU chains.

Furthermore, the characterization of the reaction products involved the application of Fourier-transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance (NMR). Through these techniques, researchers were able to identify the stretching vibration of the C = O ester groups, indicating that the use of the DA indeed yields O-H groups, as previously mentioned. These O-H groups can subsequently undergo polymerization by reacting with acid groups, resulting in the formation of polyester groups, as illustrated in Scheme 4(e).

Scheme 4
scheme 4

[Reprinted from Polymer Science, B. Godinho, N. Gama, A. Barros-Timmons, A. Ferreira, Recycling of polyurethane wastes using different carboxylic acids via acidolysis to produce wood adhesives, Pages No. 1–9, Copyright (2021) with permission from Wiley]

Identification of the most probable chemical environments present in the RP during depolymerization of the PU network [50].

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Lastly, water is generated during the reaction, as depicted in Schemes 2 and 3. In fact, reports indicate that approximately 85.4% of the water is liberated within the initial hours of the reaction [29].

As emphasized, the recycling of PU through acidolysis constitutes a multifaceted system that encompasses both chemical and thermal depolymerization. Therefore, to acidolysis be used as a viable recycling process, it is imperative to manufacturers to meticulously adjust the conditions of the recycling process to achieve a RP with properties suitable for substituting conventional petroleum-based raw materials in the production of new PU materials.

Reaction Process

As documented by H&S Anlagentechnik and visually represented in Fig. 3, the process sequence for recycling PU through acidolysis closely resembles that of other chemical methods.

Fig. 3
figure 3

[Reprinted from Polymers, 13, P. He, H. Ruan, C. Wang, H. Lu, Mechanical Properties and Thermal Conductivity of Thermal Insulation Board Containing Recycled Thermosetting Polyurethane and Thermoplastic, Page No. 4411, Copyright (2021) with permission from MDPI]

Process sequence and duration of acidolysis by H&S [32].

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It starts by filling the reactor with polyol (used as reaction solvent) and other reactants. At this point, heat is started being the PU scraps continuously fed to the reactor. After all scraps being added, and the reactor reached the desirable temperature, the reaction continues by around 2 h. At the end of the reaction, the RP is cooled down and discarded from the reactor. The temperature and duration of each step is dependent from the type of PU and required properties of RP. It has been reported that the recycling of PU via acidolysis requires PU/DA wt.wt− 1 ratios between 4.0 and 20, depending on the type of PU and DA, and that is carried out at 180–220 oC, during 4.5 h up to 12 h [28,29,30, 38, 39, 56].

Typically, acidolysis is conducted under atmospheric pressure in the presence of either an air or nitrogen atmosphere. Regarding to the cleaving agent, aliphatic or aromatic DA can be used, such as oxalic acid, malonic acid, succinic acid, adipic acid, phthalic acid or sebacic acid, among others. Additionally, it can be used DA individually or as a combination of at least two of them. Alternatively, anhydrides can also be used.

Initially, there were reports of the use of unspecified chemicals, including catalysts [35]. The literature mentions the use of inorganic acids as catalysts [46, 57], but employing HCl in acidolysis results in the generation of toxic aromatic amines [44]. Meanwhile, catalysts such as AlCl3, ZrO2, WO3, or ZrO2 have demonstrated the ability to expedite the recycling process [58]. Furthermore, He et al. [47] accomplished the chemical recycling of waste PU foams through acidolysis, employing zinc acetate as a catalyst, which is emerging as a highly effective catalyst, facilitating rapid and efficient recycling, enabling a solvent-free reaction (refer to Scheme 5b). The authors stated that acetate activates the acidolysis agent, leading to faster and more comprehensive PU degradation, achieving a high degradation degree (97%) within a short time (3 h at 200 °C).

Alternative processing conditions, such as the application of microwave radiation, have been documented. Grdadolnik et al. [59] examined the acidolysis of PUF using adipic acid by subjecting the reaction mixtures to microwave radiation. This method was reported to be highly efficient, significantly reducing the reaction time to less than one hour when compared to conventional acidolysis.

Given that the primary role of the cleavage agent is to interact with the carbamate group of the PU, as previously explained, there are no restrictions regarding the type of PU scraps that can be utilized. Consequently, this approach is applicable for recycling PU in various applications, including elastomers, adhesives, paints, foams, and more. Additionally, acidolysis can be employed for the recycling of PU without any limitations concerning the nature of the original PU’s polyols, the geometry (shape or size) of the scraps, or their color.

Conventional petroleum-based polyol can be employed either as a reaction solvent [38, 39] or not [29, 30]. In the former scenario, the amount of conventional petroleum-based polyol utilized is deducted from the subsequent PU production, in contrast to the latter case, as illustrated in Scheme 5(c).

Scheme 5
scheme 5

Acidolysis of PU using conventional polyol as reaction solvent (a), without using conventional polyol as reaction solvent (b) and using inert solvent (c)

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Moreover, alternative chemicals can serve as reaction solvents, including paraffin waxes, which require removal after the reaction. By adopting this method, the inert solvent can be recycled, as outlined in Scheme 5 [60]. Following the reaction, separation techniques like filtration may be employed to refine the RP however, separation or post-treatment procedures are typically unnecessary.

All these reaction conditions can be fine-tuned and optimized according to the specific type of PU, significantly influencing the properties of the resulting RP.

Effect of Reaction Parameters

Acidolysis involves the depolymerization of PU using DA at relatively high temperatures for a specified duration. Consequently, key reaction parameters such as the weight ratio of PU to DA, reaction temperature, and reaction time hold significant importance in the acidolysis process, thereby influencing the properties of the RP. Both the Rampf Group [34] and H&S Anlagentechnik [35] have documented very similar reaction conditions, including a weight ratio of PU to DA at 4, a reaction temperature of approximately 200 °C, and a reaction time ranging from 7 to 12 h. However, there are also reports suggesting that acidolysis can be carried out at lower temperatures (around 60 °C) within shorter timeframes [19, 45].

The impact of reaction conditions on the properties of RP was previously regarded as an industrial secret, but it has since been systematically assessed through statistical analysis, specifically utilizing the design of experiments (DOE) [30].

Reaction Temperature and Reaction Time

In theory, elevating both the temperature and the reaction time enhances the kinetics of depolymerization, facilitating the breakdown of the PU network. Indeed, Grdadolnik et al. [59] observed these trends, with higher temperature and longer reaction times leading to RP with lower molecular weight, reduced viscosity, and an increased abundance of free hydroxyl groups. Essentially, a higher reaction temperature contributes to an elevated hydroxyl number (OHnumber) in the resulting RP and encourages the consumption of the DA, consequently reducing the acid value (AV) of the RP. Moreover, it was documented that the reaction temperature is the input variable exerting the greatest influence (62.4%) on the depolymerization of PU [30].

Concerning the impact of reaction time, a comparable effect was observed. Prolonging the reaction time yielded RP with a higher OHnumber and lower AV. This outcome is attributed to the cleavage of PU chains and the consumption of DA, as previously explained and substantiated by spectroscopic findings [29]. The statistical analyses also demonstrated that an extended reaction time led to an elevation in the OHnumber of RP. In essence, it positively contributes to a greater content of hydroxyl groups.

Both the reaction temperature and reaction time must be carefully controlled within a specific range to ensure that the final product exhibits the desired characteristics, including color, OHnumber, molecular weight, AV, water content, viscosity, and molecular weight, making it suitable for use in PU production.

PU/DA Ratio

DA serves as a cleavage agent as previously mentioned and have been used as 1/4 to 1/6 of the mass of PU. Consequently, the quantity of DA employed can govern the extent of the depolymerization reaction and thus contribute to the generation of free hydroxyl groups. Based on findings reported in the literature [30], it was observed that the PU/DA ratio exerts a significant influence (31.5%) on the OHnumber of the resulting RP, positively contributing to its increase.

Type of DA and Type of PU

The depolymerization of PU was also explored using different types of DA, such succinic acid and phthalic acid, among others [39]. Both were found to facilitate the cleavage of PU chains, but the effect was more pronounced when succinic acid was used, resulting in the resulting RP exhibiting higher AV, a greater OHnumber, and lower viscosity. This was attributed to succinic acid’s enhanced capacity to depolymerize PU chains, leading to a higher yield of depolymerization. The authors proposed that despite phthalic acid’s stronger acidic character (succinic acid - pKa1 = 4.2, pKa2 = 5.6; phthalic acid - pKa1 = 2.89, pKa2 = 5.5), which might suggest a greater potential for depolymerization, the steric hindrance caused by phthalic acid could impede its access to the PU network [61]. On the other hand, the lower acidity of succinic acid may limit its reactions with OH groups in the polymer [62]. Furthermore, succinic acid has a melting point of 184 ºC, while phthalic acid’s boiling point is 210 ºC. Consequently, at the reaction temperature (190–200 ºC), succinic acid is in a liquid state, whereas phthalic acid is solid. This difference in physical state may facilitate the higher diffusion of succinic acid, contributing to its greater depolymerization capability. Additionally, the increased solubility of succinic acid can also play a role in its enhanced depolymerization capability. These factors collectively explain why RP derived from succinic acid exhibits a higher OHnumber.

The recycling of various PU types was also evaluated by depolymerizing different PUF variants, including polyester, polyether, and viscoelastic types [38]. In general, it was observed that all resulting RP exhibited higher viscosity compared to the original polyols. This can be attributed to the presence of higher molecular weight components, i.e., molecular chains that were not fully depolymerized in the RP. Additionally, in line with the proposal by Beneš et al. [63], the elevated content of aromatic carbonate moieties in the RP contributes to increased viscosity.

In addition, other characteristics of foams can also influence the degradation process. Notably, foam hardness has been observed to contribute to its degradation. This can be ascribed to the friction and stress generated within the reactor, which can mimic a mechanical recycling process.

Overall, it has been documented that various types of PU can be effectively depolymerized through acidolysis. Despite the observed disparities between the characteristics of the resulting RP and the original polyols, they have proven to be suitable for the production of new PU.

Applications

All varieties of PU share a common underlying chemistry, suggesting that, in theory, acidolysis could be applied for recycling across all PU types, and the resulting RP could be employed in the production of various PU products. However, it’s worth noting that, to date, acidolysis has primarily been explored for recycling PU foams. Consequently, the resulting RP has been successfully utilized in the production of rigid foams [30], flexible foams [29], adhesives [39], and coatings [38].

Foams

Acidolysis has predominantly found application in the recycling of PU foams, and the resulting RP has primarily been employed in the manufacturing of various foam products. Notably, the RP has been utilized as a partial replacement for petroleum-based components in the production of both flexible [29] and rigid [30] PU foams.

Flexible foams are predominantly characterized by an open-cell structure, whereas rigid foams primarily exhibit a closed-cell configuration. This structural difference has distinct consequences, influencing the compressibility of flexible foams and the thermal insulation properties of rigid foams. When considering this in context, the typical cellular structure was readily evident in all foams derived from RP, with flexible foams displaying an open-cellular structure and rigid foams exhibiting a closed-cellular structure, as depicted in Fig. 4.

Fig. 4
figure 4

[Reprinted from Chemical Engineering Journal, 395, N. Gama, B. Godinho, G. Marques, R. Silva, A. Barros-Timmons, A. Ferreira, Recycling of polyurethane scraps via acidolysis, Page No. 125102, Copyright (2020) and from Polymer (Guildf), 219, N. Gama, B. Godinho, G. Marques, R. Silva, A. Barros-Timmons, A. Ferreira, Recycling of polyurethane by acidolysis: The effect of reaction conditions on the properties of the recovered polyol, Page No. 123561, Copyright (2021) with permission from Elsevier]

SEM images of the cross-section surface of flexible foams produced using 0% wt/wt RP (a) and 30% wt/wt RP (b), as well as rigid foams produced using 0% wt/wt RP (c) and 30% wt/wt RP (d) [29, 30].

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The incorporation of RP did not notably alter the foam morphology, which maintained a highly homogeneous structure [29, 30]. However, upon a closer examination of the cell size distribution, a discernible trend appears: an increased RP content tends to reduce the cell size of the foams. Moreover, when RP is utilized in larger quantities, it can potentially impact both foam expansion and foam morphology. Grdadolnik et al. [59] illustrated this by preparing PUF using conventional polyol and RP, with corresponding images displayed in Fig. 5.

Fig. 5
figure 5

[Reprinted from ACS Sustain Chem Eng, 10, M. Grdadolnik, A. Drinčić, A. Oreški, O.C. Onder, P. Utroša, D. Pahovnik, E. Žagar Pages No. 1323–1332, Copyright (2022) with permission from ACS]

Cross-sectional images of PUF prepared using: 100% conventional polyol (a), 50% RP containing 5.6 mol% carboxyl and 3.2 mol% aromatic amine end groups (b), 100% RP containing 5.6 mol% carboxyl and 3.2 mol% aromatic amine end groups (c), and 50% RP containing 14.0 mol% carboxyl and 1.6 mol% aromatic amine end groups (d). The average pore sizes in µm are given in the lower left corners of the images [59].

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The findings revealed that the foam produced with conventional polyol (A) exhibited an open-cellular structure, whereas the foams created using 50 and 100 wt% RP displayed a reduced average pore size and a progressively closed-cell structure. Beyond affecting the morphology, incorporating RP can also influence the color of the foams. Kiss et al. [64] similarly generated foams using RP obtained from acidolysis. As depicted in Fig. 6, the reference foam appeared white, whereas the foams manufactured using RP exhibited a light brown coloration. Similar results were achieved by He et al. [40]

Fig. 6
figure 6

[Reprinted from Polymers (Basel), 13, G. Kiss, G. Rusu, G. Bandur, I. Hulka, D. Romecki, F. Péter, Pages No. 1–15, Copyright (2021) with permission from MDPI]

Optical photographs of the cross-section surface of the reference foam and of PU foams obtained at different conventional polyol/RP ratios, 90:10 in EXP. 1, 80:20 in EXP. 2 and 70:30 in EXP 3 [64].

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Furthermore, it was reported in prior studies [29, 30] that the partial replacement of conventional polyol with RP did not significantly impact the density or thermal conductivity of the foams.

In terms of mechanical performance, incorporating a higher content of RP obtained from the recycling of flexible foams led to the production of stiffer flexible foams, as noted in a prior study [29]. This increase in stiffness was attributed to the higher concentration of aromatic structures present in the RP. Conversely, when RP from the recycling of flexible foams was employed in the production of rigid foams [60], it resulted in a reduction in foam stiffness.

Typically, conventional polyols used for manufacturing flexible PUF have a high molecular weight (approximately ~ 3500 g.mol− 1) and a low OHnumber (around ~ 50 mgKOH.g− 1), while those used for producing rigid PUF possess a low molecular weight (~ 700 g.mol− 1) and a high OHnumber (~ 250 mgKOH.g− 1) [29]. Consequently, low molecular weight and high OHnumber polyols contribute to a high crosslinking density and, consequently, very rigid structures [65]. In this context, substituting a “rigid” polyol with RP, which can be described as a “flexible” polyol, was linked to a reduction in crosslinking density and, consequently, a decrease in the stiffness of the PU structure.

Other instances of utilizing RP for foam production can be found in the literature. For instance, Sołtysiński et al. [33] demonstrated that RP obtained through acidolysis can be effectively employed as a substitute for petroleum-based materials at levels of up to 40 wt.wt%−1 in the production of rigid foam. The resulting panels exhibited favorable performance throughout the foaming process, displayed a well-structured foam, and maintained dimensional stability, akin to conventional PU rigid foam.

He et al. [40] developed foams using RP through a two-stage recycling process involving ammonolysis and acidolysis. The study revealed that ammonolysis, as the initial degradation step, facilitated the achievement of a uniform-phase reaction for subsequent acidolysis. This allowed for the preliminary breakdown of the three-dimensional cross-linked network structure of PU foams under mild conditions at 160 °C. The subsequent acidolysis step further fragmented the long chains of PUF, resulting in the production of high-quality RP. During this process, any excess ammonolysis agent and aromatic amine compounds were reacted and removed. As a result, the obtained RP, without the need for additional purification, demonstrated suitability as a partial replacement for conventional polyol, comprising up to 30% by weight, in the synthesis of new flexible PUFs. Importantly, these recycled PUFs still met the requirements for commercial applications.

Adhesives

RP was also employed in the synthesis of PU adhesives, which were applied to the surfaces of wooden pieces and cured at room temperature [39]. It was noted that after 7 days of curing, the adhesive strength of the RP-based adhesive exceeded that of a conventional polyol-based adhesive significantly.

Typically, PU adhesives contain urethane, aliphatic and aromatic hydrocarbons, esters, ethers, amides, and urea and allophanate groups, all of which contribute to their adhesive properties. However, RP-based adhesive, as previously discussed, contains a higher proportion of aromatic moieties, which partially accounts for its enhanced adhesive strength. Furthermore, a more highly cross-linked adhesive tends to exhibit greater adhesive strength compared to adhesives derived from linear polyols, owing to the formation of hydrogen bonds with the wooden substrate.

The mode of adhesive failure was also analyzed to assess adhesive performance. The conventional polyol-based adhesive primarily exhibited cohesive failure (failure occurring within the adhesive system [66]). Conversely, in the case of RP-based counterparts, both cohesive failure and substrate failure (indicating that the strength of the adhesive bond exceeded that of the substrate itself [66]) were observed. In other words, the adhesive strength of the RP-based adhesive was so substantial that it led to the failure of the wood specimen during lap shear strength measurements.

Coatings

Similarly, RP was employed in the synthesis of PU coatings, and its surface properties were assessed in terms of Shore D hardness, gloss, hydrophilic/hydrophobic characteristics, and compared to conventional polyol-based PU coatings [38].

The RP-based coating exhibited a higher surface hardness compared to the conventional polyol-based coating, attributed to the increased content of higher crosslinking density and aromatic/hard segments within the RP, as already discussed.

Of particular significance in the coating industry is the reflection of coatings, known as gloss, which depends on various factors including color, particle packing, pore structure, surface roughness, and refractive index. However, based on the reported results [38], the PU material derived from RP displayed lower gloss. This was linked to the degree of crosslinking density in RP.

Similarly, the hydrophilic/hydrophobic properties of the surfaces were also examined, with findings indicating that the RP-derived PU possessed a more hydrophobic surface. This was attributed to the increased presence of non-polar C − C and C − H bonds in the aromatic segments of the RP. Furthermore, the RP-based coating surface exhibited no wettability, which was again attributed to the presence of aromatic moieties, resulting in a hydrophobic coating.

Environmental Impact

Traditional PU materials primarily rely on fossil feedstocks derived from crude oil. However, growing environmental concerns have been driving the adoption of renewable and recycled feedstocks. In the context of a circular economy, recycling PU waste and utilizing it in the production of new PU, rather than disposing of it in landfills, is generally considered a viable and sustainable option.

Some life cycle assessment (LCA) studies are available in the literature. The findings consistently indicate that, in most evaluated cases, mechanical recycling has the least impact on total energy consumption and global warming potential [67]. Lazarevic et al. [68] compared the results and uncertainties in a life cycle perspective of polymer waste management in the context of a European recycling society and stated that the environmental impact of greenhouse gas emissions (GHG) from incineration surpasses that of landfill by + 0.96 kg CO2/kg. Feedstock recycling mitigates GHG emissions by reducing them between − 0.08 to -0.37 kg CO2/kg through feedstock substitution. Both incineration and feedstock recycling exhibit lower acidic emissions compared to landfill. Furthermore, they conducted a comprehensive analysis based on the LCA framework, encompassing global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), abiotic resource depletion potential (ADP), energy use (EN), and solid waste destined for landfill (SW). As previously discussed, mechanical recycling emerges as the preferred scenario for GWP, EP, and AP. Similarly, feedstock recycling is favoured for GWP, EP, and AP, and it generates less solid waste (SW). Energy use (EN) heavily depends on the chosen technology. In the comparison between incineration and feedstock recycling, the latter consumes fewer abiotic resources and produces less SW, especially for single waste streams. However, incineration is preferred for mixed waste streams. When contrasting incineration with landfill, landfill scenarios exhibit lower GWP than incineration scenarios, while other impact categories favour incineration. Overall, the choice between incineration, feedstock recycling, and landfill depends on specific environmental impact categories and waste stream compositions.

In another study [69], when considering global warming potential, landfilling PU waste may be a favourable option than incineration due to the extremely slow biodegradation of PU over a 100-year time frame. While landfilling exhibits favourable performance due to its low energy inputs, incineration with energy recovery, despite having relatively low efficiency, generally demonstrates better environmental performance than landfilling. Among all the scenarios evaluated incineration with energy recovery leads to the lowest solid waste production, although it is associated with the most adverse global warming impact. In various impact categories, pyrolysis generally outperforms both landfilling and incineration.

Regarding the chemical recycling, a recent study compared the environmental impacts of rigid PU foams produced using polyol derived from crude oil with the environmental implications of recycling rigid PUF waste [69]. It was evaluated the impact of life cycle of PU on global warming (GW), fossil resource scarcity (FRS), mineral resource scarcity (MRS), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME) and ozone formation, human health (OF–HH) and it was reported that the recycling of PU waste offers a means to reduce the consumption of petroleum-based feedstocks for manufacturing new materials, initially suggesting a decrease in overall environmental impacts. However, the recycling process itself contributes to environmental impacts, primarily due to the utilization of the cleavage agent. The report highlighted that these environmental impacts may outweigh the benefits when replacing 25% of the virgin polyol, resulting in a slight increase in total impacts. The study also revealed that MDI (methylene diphenyl diisocyanate) constitutes the primary source of impact across all impact categories, ranging from 50 to 98% of the total impacts. Similarly, in another study, it was also reported that isocyanates are the main hotspot for climate change in PUF [70]. This is largely attributable to the use of aniline in MDI production. Nevertheless, RP contains isocyanate-derived components, potentially reducing the need for isocyanate content in the production of new PU materials. This possibility was not explored in the study and should be considered in future research. Consequently, further investigations are warranted to enhance the environmental performance associated with RP. In such studies, other factors, including economic and social aspects, should be incorporated to adopt a more comprehensive perspective on sustainability.

While LCA studies concerning the chemical recycling of PU through acidolysis have only begun to emerge, Deng et al. [71], conducted a review of thermo-chemical recycling methods for waste PU foam. They asserted that, among the available options, LCA assessments rank chemical approaches as more favourable than incineration with energy recovery, with both being preferable to landfilling.

Overall, in the limited number of reports employing product-oriented LCA to compare the environmental impact of recycled polyol-based PU against their crude oil-based counterparts, it was observed that the PU sample containing 100% recycled polyol exhibited higher environmental burdens than its virgin counterpart [72, 73]. This diminished sustainability performance of RP-based PU can be attributed to various factors. Notably, higher quantities of isocyanates and additives are incorporated to compensate for the lower quality of the RP, affecting the physical properties of the material and resulting in increased use of fossil fuels and higher energy consumption [72, 73]. Furthermore, the environmental impact of PU relying solely on RP is influenced by energy consumption and emissions throughout the product lifecycle. This encompasses energy-intensive stages like PU waste collection, transport, treatment of PU scrap for conversion into high-quality recycled feedstock (e.g., sorting, grinding), and the depolymerization process itself [73]. However, LCA results indicate that a more favourable environmental performance is achieved when using 50% and 75% RP content, compared to PU richer in virgin polyols, across various environmental impact categories (Fig. 4b) [73]. Additionally, the quality of the recycled product plays a crucial role, as one-to-one substitution is not always feasible due to lower performance in certain cases.

In addition to the environmental impact associated with the production of RP and PUF, the utilization of flexible PUF can raise concerns regarding consumer safety, particularly concerning airborne pollutants like volatile organic compounds (VOC), which may be emitted from indoor products. In this context, Kiss et al. [64] optimized the incorporation of a high proportion of RP into low-density flexible PUF. Increasing the RP content from 10 to 40% resulted in a nearly 200% reduction in VOCs signifying a substantial improvement in the environmental impact of the PUF.

Another noteworthy characteristic of RP-based PU materials is their thermal stability. To evaluate this, the thermal stability of RP and conventional polyols was assessed through oxidative-induction time (OIT) measurements [39]. The findings indicated that the conventional polyol exhibited significantly lower thermal stability. Moreover, it was reported that only by adding antioxidants to the conventional polyol could similar thermal stability be achieved as with the RP. The authors suggested that the enhanced thermal stability of RP stems from its high content of aromatic moieties. According to ASTM D3895 [74], which is the test method for OIT of polyolefins, polymers with OIT values exceeding 20 min are considered to possess good oxidative stability. The RP exhibited an OIT of 80 min, underscoring its antioxidant effect. The superior thermal stability of RP was also documented by Kiss et al. [64]. In their study, flexible foams were recycled via acidolysis, and the thermal stability of the resulting polyol was analysed using thermogravimetric analysis (TGA). All samples displayed a single degradation step, yet the onset temperature for the degradation process for RP was 374 °C, in contrast to the conventional polyol’s onset temperature of 318 °C.

In their study, Liu et al. [44] discussed the possibilities of closed-loop molecular recycling for end-of-life PU. They presented the metrics of PUF recycling to create polyol and evaluated the environmental impact using a parameter denoted as “E,” representing the ratio of the total input material weight to the weight of output product(s). Sustainability was the key focus, with a higher E value often indicating greater material input or waste generation, such as excess reactants or solvents. It was observed that acidolysis and aminolysis could be executed without the use of solvents and typically in stoichiometric proportions with PU, resulting in lower material requirements. Consequently, acidolysis and aminolysis appeared to be more environmentally friendly options in terms of material input when compared to hydrolysis or glycolysis. However, it’s important to note that hydrolysis produced aromatic amines as byproducts, while acidolysis using organic acids yielded amides, which are easier to handle than aromatic amines. Therefore, recycling PU through aminolysis might necessitate additional safety precautions compared to acidolysis.

Economics

The concept of the circular economy contributes not only to a reduction in greenhouse gas emissions but also to a decrease in the costs associated with PU materials. This is driven by various factors. Firstly, there has been a notable increase in the prices of raw materials. Additionally, the expenses linked to disposing of PU scraps have risen significantly. In Europe, for instance, PU scraps can no longer be simply disposed of in landfills, which can be a costly endeavor. Moreover, previously employed methods for recycling PU waste, such as converting it into rebound foam for carpet underlay and sports mats, are no longer economically viable. For these reasons, PU manufacturers have increasingly shown interest in chemical recycling. This approach is not only a response to pollution prevention requirements but also a necessity for reducing production costs. European consortiums, for instance, have been actively exploring ways to transform PU waste into high-value recycled products [37, 75].

According to H&S Anlagentechnik [76], the cost of producing RP was approximately 1.1 €.kg− 1, which was lower than the cost of conventional polyol at 1.6 €.kg− 1. Despite of the report dates from 2015, the cost of RP was roughly 30% lower than the market price of the original basic polyol used in flexible PUF production. Furthermore, the return on investment for establishing a recycling plant was calculated to be 1.7 years, demonstrating the economic advantages of this recycling method. Similarly, Sołtysiński et al. [33] converted post-consumer PU mattresses into polyols through acidolysis, and they reported that this recycling method is exceptionally cost-effective. In fact, it was claimed that the costs of obtaining RP are approximately 40% lower than the market price of the original polyol for rigid PUF.

Future Perspectives

In the face of present economic and environmental challenges, which are expected to become even more severe in the near future, it is imperative a new approach to the use of polymers. Recycling, particularly of polymer materials, offers a pathway to mitigate pollution and enhance the eco-efficiency of our everyday products. It commences with a collection phase, which currently exhibits a low rate (only one third of polymer produced is collected). However, various strategies, including the implementation of improved collection systems, the promotion of recycling awareness, enhancements to infrastructure, and the exploration of innovative technologies, are being considered to augment and optimize the overall collection process. One of the prominent types of polymers that has not been consistently recycled thus far is PU. This has largely been due to the absence of suitable recycling methods; however, acidolysis is emerging as a promising alternative.

In this comprehensive review, we have examined the existing data regarding the recycling of PU through acidolysis and the subsequent utilization of the RP in the production of new PU materials. It is evident that the recycling of PU via acidolysis constitutes a complex system involving both chemical and thermal depolymerization processes. Consequently, the properties of the resulting RP are sensitive to various reaction parameters such as reaction temperature, reaction time, PU/DA ratio, as well as the type of DA and PU employed. Nevertheless, meticulous optimization of these recycling conditions ensures RP with suitable properties, thereby serving as a replacement for petroleum-based raw materials in the production of new PU materials. Furthermore, we have examined the environmental and economic advantages of this approach and the advantages are clear.

Overall, acidolysis is proving to be a crucial pathway for recycling PU waste, prompting the involvement of numerous consortiums in adopting this methodology. With this perspective in mind, our review contributes to the dissemination of knowledge regarding acidolysis, further promoting its widespread use in the recycling of PU materials.