Chitosan-based Self-Adaptive Crosslinker-Driven Functional Integration of Polyurethane Foam: High Strength, Intrinsic Flame Retardancy, and Upcycling

doi:10.21203/rs.3.rs-8431643/v1

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Chitosan-based Self-Adaptive Crosslinker-Driven Functional Integration of Polyurethane Foam: High Strength, Intrinsic Flame Retardancy, and Upcycling

https://doi.org/10.21203/rs.3.rs-8431643/v1

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Semi-rigid polyurethane foam plays an indispensable role in the field of safety protection. However, its widespread use has brought challenges of unsustainability and potential fire hazards. Bio-based foams offer a new path to solve these problems, but the complexity of traditional manufacturing processes and their insufficient performance hinder their industrialization. This study synthesized a chitosan-based multifunctional crosslinking agent using a scalable one-pot method, which was subsequently utilized to construct a bio-based semi-rigid polyurethane foam (SPUR/CS-D) that integrates flame retardancy, recyclability, and high performance. The foam exhibits exceptional mechanical properties, capable of withstanding loads exceeding 9,000 times its own weight without significant deformation. Furthermore, SPUR/CS-D possesses excellent flame retardancy, maintaining structural stability under sustained flame attack at around 1200°C. Notably, based on the dynamic exchange behavior of urethane bonds and ester bonds catalyzed by tertiary amines, SPUR/CS-D can be rapidly recycled and upgraded into a high-performance transparent film through simple process. Furthermore, the multifunctional composite material formed by stacking the film with graphene exhibits an ideal electromagnetic shielding effect. This study successfully addressed the comprehensive challenges of traditional foams in terms of sustainability, manufacturing complexity, fire risk, and mechanical properties through green design and structural innovation.

Bio-based foam

Intrinsic co-retardant flame retardancy

reprocessing

multi-functional

To meet the core requirements of driving comfort and safety in the transportation sector, internal materials must possess stable mechanical properties and reliable structural support.[1] Semi-rigid polyurethane (SPU) foam plays a crucial role in this regard due to its excellent support stiffness, high compressive strength, and excellent impact energy absorption characteristics.[2, 3] Its superior adaptability, lightweight potential, and long-term durability in high-load and complex shape scenarios (such as engine compartments, chassis, seats, and body modules) have led to its continued popularity in the global automotive manufacturing and aftermarket sectors.[4] However, the environmental and health problems caused by the difficulty in degrading and disposing of waste SPU foam remain a long-standing unresolved challenge.[5] Furthermore, the high flammability of traditional petroleum-based SPU foam results in shortcomings in fire safety.[6, 7] Therefore, there is an urgent need to develop a sustainable/recyclable high-performance flame-retardant semi-rigid polyurethane foam. To improve the life cycle of SPU foam materials, it is necessary to develop natural bio-resources to replace petrochemical-based raw materials. Currently, bio-based polyols derived from wood fiber, polysaccharides, rosin, vegetable oils, or other natural resources have partially replaced traditional petroleum-based polyols, leading to the development of a range of bio-based polyurethane SPU foam materials. Among them, castor oil and rosin are frequently used to achieve a rigid-flexible balance in materials due to their unique structural characteristics; however, it is difficult to achieve functional integration while imparting high performance. Chitosan, sourced from seafood waste, is an abundant and renewable natural resource. Its numerous cyclic structures and amino and hydroxyl active groups on its molecular chain endow it with excellent mechanical reinforcement and functional designability.[8–11] Thus, integrating rosin, castor oil, and chitosan into a polyurethane system through rational structural design holds promise for addressing the multiple challenges of sustainability, safety, and functional integration inherent in traditional SPU foams.

Upgrading and recycling waste foam materials into high-value products represents a sustainable approach to extending the life cycle of SPU foam. Among numerous recycling technologies, the simple hot-pressing method has attracted much attention due to its significant economic and environmental benefits.[12, 13] However, the stable three-dimensional cross-linked network of traditional thermosetting SPU foam severely limits its recyclability through melt reprocessing. In recent years, constructing recyclable thermosetting materials based on covalent adaptive networks (CANs) has become a promising strategy. These materials can achieve reversible network recombination under external stimuli such as heat, pH, or ultraviolet light through dynamic covalent bonds (such as phenolic hydroxyl bonds, ester bonds, disulfide bonds, phosphate ester bonds, and carbamate bonds). Although dynamic chemical systems have been incorporated into foam materials in existing studies, most systems still face challenges such as demanding activation conditions or high raw material costs, hindering their industrialization.[12, 14] In this context, the inherent dynamic ester bonds present in widely available and low-cost bio-based oils are increasingly regarded as an ideal choice for constructing CANs, as they combine green and sustainable industrialization potential with excellent dynamism performance. Furthermore, introducing small molecules containing amine-based catalytic functional groups into the CANs can effectively reduce the energy barrier of dynamic reactions while avoiding the cost and compatibility issues associated with external catalysts.[15] Nevertheless, systematic research on amine-ester synergistic dynamic exchange systems in polyurethane foams remains scarce, and their network formation mechanisms and performance regulation principles require further investigation.

Fire safety is an indispensable part of the entire life cycle management of materials. However, the inherent contradiction between mechanical properties and flame-retardant properties has long constrained the application expansion of SPU foam applications.[16, 17] Currently, halogen-free flame retardant strategies such as phosphorus-based, nitrogen-based, silicon-based, and inorganic hydroxides have become the mainstream research. Among them, 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives exhibit excellent flame-retardant efficiency due to their rigid biphenyl structure and the unique gas-phase and condensed-phase dual flame-retardant mechanism of phosphorus. However, the small-molecule DOPO derivatives are prone to migration, which limits their long-term durability.[18–21] At the same time, benzoxazine rings have attracted much attention due to their thermally induced ring-opening polymerization characteristics. Under heating, they can form a phenolic resin-like polybenzoxazine crosslinked network, which can not only catalyze the dehydration and cyclization of the polymer matrix and promote the formation of a dense and stable nitrogen-rich char layer, but also act as a physical barrier to effectively block heat and oxygen transfer, significantly delaying combustion. Research indicates that combining benzoxazine rings with phosphorus-based flame retardants offers significant advantages.[22–24] For example, Que et al. prepared a polymer using benzoxazine monomers and DOPO that not only possessed excellent thermal stability and mechanical properties but also achieved a high char residue of 53.9% at 800°C.[25] Zhang et al. introduced benzoxazine rings and phosphorus-based flame retardants into polyurethane foam, resulting in a limiting oxygen index exceeding 26%, self-extinguishing capability, and the ability to withstand loads exceeding 10,000 times its own weight without deformation.[3] These results demonstrate that the combination of benzoxazine ring and phosphorus-based flame retardants provides a highly promising solution for reconciling the contradiction between flame retardancy and mechanical properties. However, how to simultaneously embed phosphorus-based flame retardant units and benzoxazine structures into the SPU system through stable chemical bonds to achieve long-lasting and migration-resistant flame retardant effects remains a core challenge in current molecular design and synthesis methods.

To address the aforementioned issues, this study proposes a low-cost, scalable one-pot synthesis strategy, successfully developing a bio-based semi-rigid polyurethane foam (SPUR/CS-D) that integrates inherent flame retardancy, high mechanical properties, and complete recyclability, providing a new solution for the safety protection field. We employed a simple and economical method, replacing traditional petroleum-based polyols with a self-made rosin-castor oil polyol (COGER) and a chitosan-based crosslinking agent containing benzoxazine rings (CS/DOPO-HQ), to prepare a series of SPUR/CS-D materials. This material exhibits superior compressive strength, reaching 2.47 MPa at 10% strain, far exceeding most reported polyurethane foams. Furthermore, based on a multiphase synergistic flame-retardant mechanism, SPUR/CS-D retains its non-flammable properties even when exposed to flames at approximately 1200°C. It is worth emphasizing that, thanks to the dynamic exchange behavior of urethane bonds, ester bonds, and hydrogen bonds under tertiary amine catalysis, SPUR/CS-D can be rapidly recycled through simple hot pressing or solvent treatment, and upgraded into a high-performance transparent film. Furthermore, this film, when combined with graphene, can form a multifunctional material exhibiting excellent electromagnetic shielding properties, potentially expanding its application prospects in low-altitude flight, electronic devices, and the automotive industry. This research, through green design and structural innovation, successfully overcomes the comprehensive challenges of traditional foam materials in terms of sustainability, manufacturing process complexity, fire risk, and mechanical properties.

2.1 Materials

Castor oil (CO), rosin acid, Chitosan (Viscosity 100–200 mPa·s), and 1,8-diazabicycloundec-7-ene (DBU) were purchased from Anaiji Chemical Co., Ltd. Castor oil glycidyl ether (COGE, AR) was provided by Guangzhou Ruishi Biological Company. Surfactant (AK-158), N,N-Dimethylcyclohexylamine, DOPO-HQ (99%), and graphene were purchased from the Alibaba e-commerce platform.

2.2 Synthesis of castor oil-based polyols (COGER)

COGE (100 g), rosin acid (66.54 g), and DBU (1.0 g) were added to a 250 mL conical flask and heated to 130°C. The reaction was carried out at this temperature for 8 h. Finally, a brownish-red oily liquid (COGER) was obtained.[3]

2.3 Synthesis of chitosan crosslinking agent(CS/DOPO-HQ)

Chitosan (10 g) was dissolved in a 7% aqueous acetic acid solution, followed by the addition of paraformaldehyde (3.72 g) and DOPO-HQ (20 g). The mixture was stirred for 30 minutes, then slowly heated to 110°C and reacted for 6 hours. After the reaction was complete, the crude product was filtered and ultrasonically washed several times with aqueous acetic acid and anhydrous ethanol. Finally, it was vacuum dried to obtain a light yellow multifunctional chitosan crosslinking agent (CS/DOPO-HQ).

2.4 Preparation of Semi-rigid polyurethane foam

The obtained COGER and CO were mixed and stirred at 1500 r/min for 1 min. Then, a certain amount of surfactant AK-158, N, N-Dimethylcyclohexylamine, and foaming agent water was added with continuous stirring for 30 s. Then, add an appropriate amount of PM-200, stir for 8 s, and leave it to cure at room temperature for 1 h. The final polyurethane foam SPUR is obtained. Using the same method, polyurethane semi-rigid foams with different properties were prepared by changing the ratio of polyols (CO, COGER, CS/DOPO-HQ) in the system. Specific formulations are detailed in Supporting Information Table S1.[6, 9]

The more detailed experimental section is available in the Supporting Information.

3.1 Materials Design and Structural Characterizations

To prepare multifunctional, high-performance bio-based SPU foams, we used DOPO-HQ, a derivative of 9,10-dihydro-9-oxa-10-phosphenanthrene-10-oxide with high flame-retardant properties, and chitosan, a multi-carbon source, as raw materials. Through an economical and green Mannich reaction, the bio-based crosslinking agent CS/DOPO-HQ containing a benzoxazine ring was successfully prepared (Fig. 1). Simultaneously, we grafted rosin acid, which possesses the phenanthrene ring structure, onto castor oil, which has unique flexible segments, to prepare a bio-based polyol with both rigidity and flexibility (COGER, Scheme S1, Supporting Information). Using COGER and CS/DOPO-HQ as core building blocks, and by adjusting the appropriate ratios, a series of high-performance bio-based SPUs were prepared (Table S1, Supporting Information). The characteristic structure of COGER was characterized by Fourier transform infrared spectroscopy (FT-IR). As shown in Figure S1a (Supporting Information), the epoxy group peak at 910 cm^− 1 in COGER disappeared.[26] In contrast, the hydroxyl -OH peak at 3460 cm^− 1 reappeared, indicating that the epoxy group had undergone a ring-opening reaction and the grafting modification was successful.[27] In addition, in the ¹H NMR spectrum of COGER (Figure S1b, Supporting Information), the epoxy group-related proton peak at 2.7 ppm disappeared;[28] at the same time, the hydroxyl proton peak at 4.3 ppm and the conjugated double bond protons of rosin acid appeared at 5.7 ppm,[29] further confirming the successful synthesis of COGER.

CS/DOPO-HQ, synthesized via a green route, is used as a crosslinking agent for assembling SPU foam (Fig. 1). FT-IR spectroscopy clearly revealed changes in the characteristic functional groups of CS/DOPO-HQ (Fig. 2a): the amine peak at 3388 cm^− 1 was significantly weakened, indicating that the amino group involved in the reaction was modified. Furthermore, new characteristic peaks appeared at 1232 cm^− 1, 1283 cm^− 1, 945 cm^− 1, and 918 cm^− 1, corresponding to characteristic vibrations of P = O, Ar-O-C, the C-N-C, and the P-O-C bond, indicating that the DOPO-HQ structure had been incorporated into the CS matrix and a new oxazine ring structure had been formed.[30–32] To further reveal the chemical state of the elements in CS/DOPO-HQ and verify its structural composition, X-ray photoelectron spectroscopy (XPS) analysis was performed on the sample (Figs. 2b-e), obtaining high-resolution spectra of C 1s, N 1s, O 1s, and P 2p. The high-resolution C 1s spectrum exhibits three characteristic peaks at binding energies of 286.2 eV, 283.8 eV, and 281.8 eV, corresponding to C–O, C–N, and C–C bonds. Furthermore, the P 2p peaks at 134.1 eV, 133.5 eV, and 133.0 eV are attributed to P = O, P–O, and P–C bonds, respectively. The O 1s peaks at 530.0 eV and 529.3 eV correspond to C–O and P–O/P = O bonds, respectively.[33–35] The relative contents of C, O, N, and P in each of these forms are shown in Fig. 2g and Figure S2 (Supporting Information, determined by energy-dispersive X-ray spectroscopy (EDS). The C, N, O, and P contents in CS/DOPO-HQ are 64%, 2.7%, 24.4%, and 8.9%, respectively. SEM images clearly demonstrate the structural characteristics of the CS before and after modification, further confirming the successful formation of the modified system. Thermal analysis (TG) results indicate (Fig. 2f) that the residual char content and initial decomposition temperature of CS/DOPO-HQ have been significantly enhanced, demonstrating improved thermal stability. This phenomenon can be attributed to the fact that DOPO-HQ and its degradation products promote charring of the matrix, while the resulting oxazine ring structure significantly enhances the thermal stability of the material.

3.2 Mechanical Properties and Impact Resistance

The mechanical properties of a material are key indicators that determine its practical application.[36] The study found that under conditions of similar density, the Shore hardness and compressive strength of SPUR increased with increasing content of the bio-based polyol COGER (Figure S4, Supporting Information). The phenomenon is mainly attributed to the enhancement effect brought about by the aggregated phenanthrene ring structure in COGER.[37] Subsequently, by introducing CS/DOPO-HQ into the polymer system, SPUR/CS-D with superior Shore hardness was obtained. When the proportion of CS/DOPO-HQ in the polymer system reached 7.0%, the compressive strength of SPUR/CS-D reached its highest value of 2.47 MPa (with a strain of 10%), which was 54.0% higher than that of pure SPUR (see Fig. 3a and Figure S4, supporting information). This indicates that under the same strain conditions, SPUR/CS-D has a stronger load-bearing capacity. Furthermore, the Young's modulus of SPUR/CS-D reaches 80.34 MPa, surpassing all currently reported SPUs. This significant improvement is primarily due to the abundant cyclic structures in the CS/DOPO-HQ system, as well as the large number of hydroxyl/phenolic hydroxyl and amino groups in the system that can form hydrogen bonds.[38, 39] To more intuitively demonstrate the material's compressive properties, we used only 1.48 g of SPUR/CS-D to support a weight exceeding 9,000 times its own weight and easily lifted a 1.2 kg weight (Fig. 3c).

Notably, compared to SPU, SPUR, and SPUR/CS-D exhibit a richer range of energy dissipation behaviors at different collision rates, enabling them to function in a variety of operating environments.[40] With increasing compressive strain during continuous loading and unloading, both materials exhibit a pronounced hysteresis effect. However, at the same strain, SPUR/CS-D dissipates significantly more energy than SPUR, and the dissipation rates of both materials remain above 80% (Figs. 3d-f and Figure S5, Supporting Information). After the initial loading and unloading, the hysteresis area of SPUR/CS-D decreases slightly during subsequent cycles without relaxation. However, encouragingly, after 3–10 min of rest, the material's dissipation capacity recovers to 95% or more (Figs. 3g, 3h, and Figure S6, Supporting Information). Overall, the glucose units in SPUR/CS-D undergo significant energy consumption during the interconversion between chair and boat conformations;[41] coupled with the ordered hydrogen bond network formed by hydroxyl/phenolic hydroxyl groups and amino groups,[42, 43] the two work synergistically to endow the material with excellent energy dissipation and recovery capabilities. To more intuitively demonstrate the superior mechanical properties of SPUR/CS-D, we compared them with those of currently reported polyurethane foams (Fig. 3i). The results show that SPUR/CS-D significantly outperforms both mechanical properties and sustainability.

3.3 Fire-Retardant Properties

The importance of SPU in flame retardancy lies in the sharp contradiction between its inherent flammability and the high safety requirements of its wide range of applications.[17] Therefore, improving its fire safety through effective flame-retardant treatment is not only a prerequisite for expanding its application range but also a key technical measure for ensuring public safety. To evaluate the synergistic flame retardancy of benzoxazine and phosphorus in the CS/DOPO-HQ, we used the Limiting Oxygen Index (LOI) and UL 94 vertical burn tests to visually demonstrate the material's fire performance (Fig. 4a and Figure S7, Supporting Information). The experimental results show that pure SPU is extremely flammable, with a LOI of only 19.3%. Although the SPUR material incorporating COGER failed the UL 94 burn test, its LOI significantly increased to 22.4%, still not reaching the ideal level. With the addition of CS/DOPO-HQ, SPUR/CS-D exhibits significant flame spread suppression; the introduction of only 7.0wt% can increase the LOI to 32.8%, achieving a UL 94 V-0 rating. Furthermore, a fire shield device (Fig. 4b) was constructed to observe the material's flame retardancy under sustained high-temperature flame attack. After 60 seconds of continuous exposure to a flame torch, SPUR/CS-D remained unignited, demonstrating its safety in fire incidents.

Cone calorimetry (CCT) was used to further evaluate the flame retardancy of the materials. Heat and smoke release are shown in Figs. 4c-f and Table S2 (Supporting Information). Under the same thermal radiation conditions, the maximum heat release rate (PHRR) of pure SPU reached 830.4 KW/m². The PHRRs of SPUR and SPUR/CS-D decreased to varying degrees. SPUR/CS-D, in particular, saw its PHRR drop to 172.2 KW/m², a significant 78.9% reduction. Compared to the total heat release (THR) of pure SPU (338.1 MJ/m²), the THRs of SPUR and SPUR/CS-D decreased to 255.4 MJ/m² and 102.6 MJ/m², respectively, with SPUR/CS-D achieving a remarkable 69.8% reduction. These significant reductions in both parameters enhance the material's fire safety during the initial combustion phase. In terms of smoke release, the total smoke release (TSP) of pure SPU was 21.1 m²/m², and the carbon monoxide release rate (COP) was 0.015 g/m², indicating a high risk of smoke and hazardous gases. In contrast, the TSP and COP of SPUR/CS-D were significantly reduced, by 75.7% and 53.1%, respectively, indicating a significant improvement in smoke release. Furthermore, to verify the flame retardancy advantages of SPUR/CS-D over traditional polyurethane foams, its flame retardant efficiency (THR, PHRR, and TSP reduction rate) was compared with LOI (Figs. 4g-h). The results show that SPUR/CS-D outperforms other flame-retardant polyurethane foams in both flame-retardant efficiency and limiting oxygen index.

To explain the flame retardant mechanism of SPUR/CS-D, we systematically investigated the cross-linking behavior of benzoxazine copolymerization during the combustion of polyurethane foam and its synergistic effect with phosphorus-based flame retardants. The microscopic mechanism was revealed through various characterization methods. Simultaneous thermal analysis (TG-DSC) simulated the temperature changes during actual combustion. As shown in Figs. 5a and b, SPU exhibited lower thermal stability and char residue (Table S4, Supporting Information), and lacked an exothermic peak indicative of cross-linking. In contrast, SPUR and SPUR/CS-D exhibited more ideal thermal stability and char residue, with SPUR/CS-D exhibiting an exceptionally high char residue of 35.4wt% at 600°C. Furthermore, an exothermic peak at 258°C indicated cross-linking within the material structure.[44–46] To eliminate the influence of SPUR itself, SPU/CS-D was further prepared to independently verify the cross-linking behavior of benzoxazine (Figure S8 in the Supporting Information). Compared to pure SPU, SPU/CS-D exhibits a sharper exothermic peak, confirming the cross-linking of benzoxazine. To investigate the cross-linking behavior in the material, the original CS/DOPO-HQ was cured in an oven for 2 h and then compared using FT-IR to verify the chemical structure changes (Figures S9 and 10, Supporting Information). The results show that the characteristic peak of benzoxazine disappears in the cured product, while the absorption peak of the phenolic hydroxyl band is significantly enhanced, indicating that the benzoxazine cross-linking reaction has occurred. In addition, the heated CS/DOPO-HQ exhibits new peaks characteristic of hexasubstituted benzene rings (1639 cm^− 1) and pentasubstituted benzene rings (874 cm^− 1), while the characteristic peak of the original trisubstituted benzene ring (803 cm^− 1) disappears.[44] These results further confirm the high-temperature cross-linking behavior of CS/DOPO-HQ. Raman spectroscopy and char residue morphology analysis (Fig. 5c, d) revealed that SPUR/CS-D exhibited a higher degree of graphitization and a smoother, denser char layer, demonstrating a superior char structure compared to SPU and SPUR. Combined with TGA and FT-IR analysis (Fig. 5e-j and Figure S11, Supporting Information), we systematically investigated the gaseous products of SPUR/CS-D after pyrolysis. Compared with SPUR, the peak intensities of the pyrolysis products of SPUR/CS-D (including hydrocarbons, carbon dioxide, and carbonyl compounds) were significantly reduced, indicating that the introduction of CS/DOPO-HQ effectively mitigated the release of pyrolysis products.[16, 33] Furthermore, the maximum absorption intensity of the pyrolysis products of SPUR/CS-D was lower than that of SPUR, indicating its enhanced gas barrier capability.

Overall, the excellent flame retardancy of SPUR/CS-D stems from the synergistic effect between the benzoxazine units and phosphorus in CS/DOPO-HQ (Fig. 6). Under flame attack, the benzoxazine ring undergoes a ring-opening reaction under the influence of high temperature and the proton-donating environment of the phenolic hydroxyl group, generating zwitterionic intermediates.[3, 44] These dynamic intermediates can further react with substituted sites on the aromatic ring of DOPO-HQ to form a polybenzoxazine copolymer with a cross-linked network structure (Pathways I and II). This crosslinking significantly enhances the char-forming abilities of SPUR/CS-D, thereby enhancing its flame retardancy. Furthermore, the phosphorus in DOPO-HQ, during pyrolysis, generates free radicals such as PO· and HPO·, which can combine with active H· and O· atoms, interrupting the free radical chain reaction, reducing flame intensity, and inhibiting flame spread.[16, 34] Furthermore, the generated phosphorus-containing radicals help promote matrix charring, synergizing with the benzoxazine structure to further enhance the flame retardancy of the material.

3.4 Recycling and Reuse

To achieve efficient, economical, and environmentally friendly treatment of waste polyurethane foam, this study prepared upgradable, recyclable bio-based polyurethane (SPUR) foam containing a large number of dynamic ester bonds by introducing rosin acid. First, the SPUR foam was pulverized and passed through a 120-mesh sieve to obtain powder. Then, it was hot-pressed at 10 MPa and 140°C for 30 min to successfully prepare a recycled RSPUR film with good mechanical properties. RSPU films and RSPUR/CS-D films containing CS/DOPO-HQ were prepared using the same method (Figs. 7a and S11, Supporting Information). Furthermore, RSPUR/CS-D could be completely recycled after 12 h in a hydrothermal reactor at 140°C. To further investigate its recycling mechanism, the dynamic transformation and shift of the chemical bonds in the material were observed using variable-temperature infrared spectroscopy (Figs. 7d and S12, Supporting Information). Initially, NCO bonds were not initially observed at 60°C, but as the temperature increased, NCO bonds gradually appeared, indicating that the urethane bonds began to break.[3, 9] Furthermore, we observed that with increasing temperature, N-H and C = O bonds gradually shifted towards directions with higher wavenumbers, indicating the dissociation of hydrogen bonds and the exchange of ester bonds.[3, 47] This phenomenon is mainly due to the extensive exchange of dynamic ester and hydrogen bonds within the system, which jointly promote the breaking and recombination of the crosslinked network.[3] Activation energy (Ea) is an important kinetic parameter for evaluating the depolymerization reaction. As shown in Figure S13 (Supporting Information), at 120°C, dynamic exchange was not very active, with a stress relaxation time of 50.95 min. With increasing temperature, dynamic exchange gradually became more active, with a stress relaxation time of only 3.46 min at 180°C. These results indicate that high temperatures favor the reversible breaking and recombination of the polyurethane network.[6, 39]

The mechanical properties of the three films were determined by universal testing (Fig. 7b). RSPUR/CS-D exhibited the best mechanical properties (tensile strength of 31.16 MPa) due to its abundant rigid structure and rich hydrogen bonding. Notably, this film maintained high tensile strength even after several repeated heating cycles (Figure S14, Supporting Information). Furthermore, using this film as a substrate, we prepared a high-performance graphene composite material RCF (Fig. 7a) through a "sandwich" stacking method. Compared to the original graphene film (Fig. 7c), the mechanical properties of RCF were significantly improved (tensile strength increased from 0.26 MPa to 29.23 MPa). The high mechanical properties of the graphene film offer broader application possibilities. It is worth noting that due to the recyclability of RSPUR/CS-D in solvents, the graphene material of the composite RCF film can be recycled and reused in a hydrothermal reactor under the same conditions (Figure S15, Supporting Information).

Conductivity significantly affects the EMI shielding performance of materials. Figures 7e and S16 (Supporting Information) show that the composite RCF film exhibits excellent conductivity, which is comparable to that of the original graphene film, enabling the composite film to achieve excellent shielding performance. Figure 7f shows a similar trend in EMI shielding performance and conductivity of the X-band RCF film. Notably, although the EMI SE of the RCF film is slightly lower than that of the original graphene film, its maximum EMI SE still reaches 72.4 dB, which means that it can block the vast majority of incident electromagnetic waves and meet the commercial requirements for electromagnetic shielding materials.[48] Subsequently, we investigated the EMI shielding capability of the RCF film. Absorption efficiency (SE_A), reflection efficiency (SE_R), and total SE (SE_T) represent the shielding material's ability to reflect, absorb, and completely shield electromagnetic waves, respectively.[49] As shown in Fig. 7f and g, the SE_A value is consistently greater than the SE_R value of the composite film, indicating that SE_A is the primary contributor to total EMI SE. However, EMI shielding materials with high SE_A may not absorb most of the electromagnetic wave energy because electromagnetic waves can only penetrate the material after reflection. The reflection coefficient (R), absorption coefficient (A), and transmission coefficient (T) represent the energy coefficients of reflected, absorbed, and transmitted electromagnetic waves, indicating the actual energy loss of the electromagnetic wave.[48] The R, A, and T values of the composite thin film are shown in Figure S17 (Supporting Information). Clearly, the R value is always higher than the A value; therefore, the primary EMI shielding mechanism of the composite thin film is reflection, which is due to its ultra-high conductivity. A Tesla coil was used to demonstrate the electromagnetic shielding performance of the composite film.[48] When the LED is near the Tesla coil system, it lights up; when the composite film is placed in the system, the LED quickly turns off, a stark contrast to the RSPUR/CS-D film, which lacks electromagnetic shielding performance (Fig. 7h). These results highlight the excellent electromagnetic shielding performance of the composite film, laying a solid foundation for its widespread application in electronic devices. These findings demonstrate that RCF film is a reusable organic material. It possesses not only mechanical strength and excellent electromagnetic shielding performance but also recyclability and reusability. This dual-benefit characteristic makes it advantageous both economically and ecologically (Fig. 7i and Table S5, Supporting Information).

In summary, this study successfully introduced CS/DOPO-HQ containing a benzoxazine ring and rosin-castor oil-based polyols into the SPU foam system via a simple, economical, and scalable synthetic strategy, preparing a high-strength, flame-retardant, and recyclable green bio-based foam material (SPUR/CS-D). Benefiting from the aggregation effect of the rigid cyclic structure in CS/DOPO-HQ and rosin acid, this foam exhibits excellent compressive strength, reaching 2.47 MPa at 10% strain, and can withstand external forces exceeding 9000 times its own weight without significant deformation. Notably, based on the biphase synergistic flame-retardant mechanism of CS/DOPO-HQ, SPUR/CS-D maintains structural stability under continuous flame burning at approximately 1200°C and achieves self-extinguishing upon removal of the flame. Furthermore, thanks to the dynamic exchange behavior of urethane bonds, ester bonds, and hydrogen bonds under tertiary amine catalysis, this foam material can be efficiently recycled through hot pressing or solvent treatment and can be further upgraded to create high-performance transparent films. When this film is combined with graphene, it can form a multifunctional material with good electromagnetic shielding properties, showing broad application prospects in low-altitude aircraft, electronic devices, and the automotive industry. Although this study effectively overcomes multiple challenges of traditional foam materials in terms of sustainability, manufacturing process complexity, fire risk, and mechanical properties through green design and structural innovation, the actual performance of this material under harsh working conditions and diverse application scenarios still needs further systematic evaluation and optimization.

Funding

This work acknowledged the financial support of the National Natural Science Foundation of China (Grant Nos. 32371825, 32271819) and the Central Non-profit Research Institution of CAF (Grant No. CAFYBB2023QB009).

Conflict of Interest

The authors declare no conflict of interest.

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Chitosan-based Self-Adaptive Crosslinker-Driven Functional Integration of Polyurethane Foam: High Strength, Intrinsic Flame Retardancy, and Upcycling

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Abstract

Figures

1. Introduction

2 Experimental Section

2.1 Materials

2.2 Synthesis of castor oil-based polyols (COGER)

2.3 Synthesis of chitosan crosslinking agent(CS/DOPO-HQ)

2.4 Preparation of Semi-rigid polyurethane foam

3 Results and Discussion

3.1 Materials Design and Structural Characterizations

3.2 Mechanical Properties and Impact Resistance

3.3 Fire-Retardant Properties

3.4 Recycling and Reuse

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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