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RESEARCH ARTICLE | Updated:2023-05-26
    • Recyclable Polyurea-Urethane Thermosets with De-Crosslinking Capability in Acetic Acid

    • Tao Xinglei

      ,  

      Yi Wentian

      ,  

      Xu Xiao-Qi

      ,  

      Wang Yapei

      ,  
    • Chinese Journal of Polymer Science   Vol. 41, Issue 6, Pages: 859-865(2023)
    • DOI:10.1007/s10118-022-2872-4    

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  • Cite this article

  • Xinglei Tao, Wentian Yi, Xiao-Qi Xu, et al. Recyclable Polyurea-Urethane Thermosets with De-Crosslinking Capability in Acetic Acid. [J]. Chinese Journal of Polymer Science 41(6):859-865(2023) DOI: 10.1007/s10118-022-2872-4.

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    Abstract

    Covalent crosslinking points within thermosets generally result in excellent mechanical properties and solvent resistance yet lead to limited degradability and recyclability. Those thermosets become degradable or recyclable if crosslinking points are cleavable or reversible. Following this principle, we report a kind of polyurea-urethane thermoset with borate ester as its crosslinking point to enable a controllable de-crosslinking in response to acetate acid. Such a thermoset presents remarkable mechanical properties as well as outstanding solvent resistance capability, due to the high crosslinking density and intermolecular hydrogen bonding. Furthermore, the de-crosslinked product can be reporcessed to generate a brand new thermoplastic material.

    Keywords

    Recyclable thermosets; Borate ester; Polyurea-urethane; Stimuli-cleavage

    INTRODUCTION

    Owing to the excellent mechanical properties, solvent resistance, and chemical stability, thermosets have been widely applied in coatings, composites, electronic packaging, and so on.[

    1] Despite significant successes in broad daily applications, the increasing consumption of thermosets products in recent decades have resulted in uncontrolled plastic wastes problems all around the world.[2−4] Remarkable endeavours have been devoted to the development of recyclable thermosets by introducing dynamic covalent bonds into crosslinking networks, which provides promising strategies for solving thermosets waste issues.[5−17] However, the extraordinary expensive cost makes such strategy merely applicable to high value-added fields. Moreover, the rearrangement of dynamic covalent networks will unfortunately lead to the deterioration of mechanical performance during the reprocessing process of thermosetting polymers. In face of these disadvantages, introducing stimuli-cleavable bonds for controllable degradation of polymer networks offers another adoptable and competitive approach for thermosets recycling.[18−24] Following such recycling strategy, raw materials can be achieved and reused for the reproduction of new thermosets that can be applied to areas where there is high demand for electronics packaging, building materials, wind turbine blades, and so on. Considering that consumers tend to use brand new products, those down-gradable recycling strategies help to maintain the mechanical strength as well as promote the commercial competitiveness of the reproduced materials.

    Amongst all types of promising stimuli-cleavable linkages, borate ester unit possesses unique tripodal molecular structure that can endow the polymer networks with considerable crosslinking density and controllable degradability. Yet the extreme water sensitivity inevitably causes the instability of borate-ester-based thermosets in humid environments.[

    25−28] Although nitrogen-coordinated borates have been studied in some brilliant researches to improve the chemical stability against water, the complicated synthetic process and rather high cost still limit the industrialization of borate-ester-based thermosets.[29]

    Herein, we leveraged the activity of borate ester to design a novel recyclable thermoset which serves as an eco-friendly polymer to cope with the problems of polymer waste. Using polyurea-urethane as the main chain, this thermoset possessed outstanding mechanical strength and solvent resistance capability. Based on the stimuli-cleavable feature of boronic ester linkages, the thermoset could be specifically de-crosslinked under acetic acid environment, and dissociation products could be further reprocessed to form a linear polyurea-urethane copolymer.

    The well-designed one-step synthesis of borate-crosslinked polyurea-urethane (BPU) is depicted in Scheme 1. The condensation between boric acid and hydroxyl group generates borate easter linkages along with water molecules which will further react with diisocyanate to form urea bonds. The residual isocyanate group can react with hydroxyl groups to form carbamate bonds. It is worth noting that the amino group applied in the formation of polyurea is generated from the hydrolysis of isocyanate group. The urea bonds are expected to rapidly arise around the borate ester units due to the high reactivity of water and amino with isocyanate.

    fig

    Fig 1  Synthetic route to BPU.

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    EXPERIMENTAL

    Materials

    Poly(ethylene glycol) (PEG, Mw=400 g/mol), hexamethylene diisocyanate (HDI), acetic acid, dioxane, and dibutyltin dilaurate (DBDTL) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Boric acid was supplied by Beijing Chemical Reagent Company. All reagents were used without further purification.

    Fabrication of BPU

    The borate crosslinked polyurea-urethane was synthesized via one-pot reaction among PEG-400 (Mw=400 g/mol, 4.0 g, 10 mmol), HDI (1.060 mL, 13.75 mmol), and H3BO3 (155.0 mg, 2.5 mmol) under the catalysis by DBDTL (40 μL, 0.5%). PEG-400 and H3BO3 were dissolved in 4.0 mL of dioxane. Then HDI and DBDTL were successively mixed with the above solution and the reaction was carried out on a glass plate at 60 °C for 6 h. The cured film was then transferred to a vacuum oven at 80 °C overnight to evaporate the remaining solvent and cooled to room temperature to obtain the BPU film. Finally, the BPU film (named as PEG-4-400) was processed into predesigned shapes (the 4 referred to the molar ratio of PEG-400:H3BO3=4:1 and the 400 referred to the molecular weight of PEG). Other BPU films with different ratios of raw materials were prepared in the same way.

    Characterizaitons of BPU

    The solid-state 13C-NMR and 11B-NMR spectra of BPU were recorded on Bruker Avance 600 MHz NMR spectrometer. The reflection Fourier infrared spectroscopy (FTIR) spectra were measured on infrared spectrometer (Tensor 27, Bruker). Thermogravimetric analysis (TGA) was performed on a TA INSTRUMENTS Q50. Sample was typically loaded on a platinum pan and the temperature was ramped to 600 °C at a rate of 10 °C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on a DSC8000 (PerkinElmer) apparatus. Two heating cycles between −50 and 200 °C were collected at a heating rate of 10 °C/min. The tensile test of the material was carried out by Instron tensile tester, and the tensile rate was 10 cm/min at room temperature. The Loss factor and storage modulus were collected by a dynamic mechanical analysis (DMA) machine (Q800). Storage modulus and loss factor were specified via a temperature scan method with a temperature ramping rate of 5 °C/min at fixed frequency (1 Hz) and strain (0.05%). According to the DMA results, the crosslinking density of BPU-6-400, BPU-4-400, BPU-2-400 was calculated to be 566.4, 424.0, 59.1 mol/m3, respectively (detailed calculation method see the electronic supplementary information, ESI).

    De-crosslinking Process of BPU

    Solid samples of BPU-400 were placed in 20 mL vials followed by the addition of 10 mL of acetic acid. After 24 h, all the solid samples of BPU-400 were dissolved in the acetic acid and 10 mL of water was added to precipitate the dissociation products from the solution. The precipitated solid material was filtered and washed with H2O to remove acetic acid, and then dried in a vacuum oven for 24 h. The obtained colourless polyurea-urethane (PUU) was hot-pressed into a thin film. For the cyclic reprocessing test, the PUU film was chopped into small pieces and repossessed into another film for 3 times.

    RESULTS AND DISCUSSION

    Synthesis and Structural Characterizations of BPU

    As shown in Fig. 1(a), the BPU polymer films were prepared via solvent evaporation. Typically, the reactant of hexamethylene diisocyanate (HDI) and the catalyst of dibutyltin dilaurate (DBDTL) were successively added into the mixture of PEG and boric acid in a dioxane solution. After vigorous stirring at room temperature the resultant solution was rapidly poured onto a clean glass plate to prepare the BPU films using a blade-casting method. BPU films with remarkable transparency could be peeled off from the glass plate after drying the casted films in vacuum oven at 80 °C overnight. The optical image in Fig. 1(a) shows that the BPU polymer film with a size of 16 cm × 18 cm is uniform and defect-free. According to the solid-state 11B-NMR spectra of BPU (Fig. 1b) and boric acid (Fig. S1 in ESI), the NMR signal at 19.88 ppm is assigned to boronic acid, and the signal at 15.27 ppm is assigned to borate ester linkages generated by the condensation between boronic acid and PEG. The disappearance of the signal at 19.88 ppm suggests the decrease of the boronic acid, while the appearance of signal at 15.22 ppm indicates the formation of borate ester via esterification of PEG and boric acid. In addition, solid-state 1H-NMR spectrum of BPU (Fig. S2 in ESI) shows that the signal of hydroxyl group at 6.50 ppm is no longer distinct after BPU is formed, suggesting a complete condensation process. As shown in X-ray diffraction spectrum in Fig. S3 (in ESI), the appearance of characteristic peak at 28.2° that is assigned to boric acid also proves the high consumption of boric acid in the esterification process. Referring to the solid-state13C-NMR spectrum (Fig. 1c), the doublet peak at 160 ppm illustrates the generation of both urethane bonds (―NH―COO―) and the urea bonds (―NH―CONH―). BPU films with adding different amounts of H3BO3 as reactant were also investigated by FTIR (Fig. 1d). Representatively, the C=O deformation vibration band of urea bonds at 1624 cm−1 increases with the rise of H3BO3 ratio, confirming H3BO3-mediated formation of polyurea, while the characteristic band of boronic ester appears at 1045 cm−1, verifying the generation of BPU network cross-linked by boronic ester units (Fig. S4 in ESI).

    fig

    Fig 1  (a) Fabrication of BPU film via blade-coating method; (b) 11B-NMR and (c) 13C-NMR spectra of BPU samples; (d) FTIR spectra of BPU samples with different ratios of PEG-400: H3BO3, where BPU-4-400 refers to the molar ratio of PEG-400 (400 g/mol):H3BO3=4:1.

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    Mechanical Properties of BPU

    Typically, the mechanical properties of crosslinked polymer could be manipulated by varying the crosslinking degree and intermolecular interaction such as hydrogen bond (Fig. 2a). On the one hand, the growth of the H3BO3 molar ratio will definitely increase the crosslinking density of the polymer network. On the other hand, the generation of more water as by-product led to the increased number of urea bonds. It is predictable that the polymer network can be strengthened through the increase of crosslinking density and hydrogen bonding interaction. Tensile tests were applied to measure the mechanical properties of BPU-2-400, BPU-4-400 and BPU-6-400. As shown in Figs. 2(b) and 2(c), the mechanical properties including break stress, break strain, Young’s modulus and toughness changed with the molar ratio of H3BO3. For BPU-2-400, BPU-4-400 and BPU-6-400, their elongation at break values are 1000%, 1700% and 3000%, respectively. Their Young's modulus are 100, 600 and 900 MPa, respectively, and their toughness values are 120, 180 and 160 MJ/m3, respectively. The Young’s modulus of BPU increases with the growth of the boronic acid ratio, while the elongation at break of BPU presents an opposite tendency. These observations are attributed to covalent crosslinking network that anchors the topological network, and hydrogen bonding network that assists the energy dissipation process. However, high crosslinking degree reduces the length of polymer chain between adjacent crosslinking points, thus decreasing the elongation at break and toughness of the material. To further illustrate the viscoelastic behaviour of BPU, the temperature dependence of the storage modulus, loss modulus and tanδ were recorded by dynamic mechanical analysis. As shown in Fig. 2(d), through the comparison of reported recyclable crosslinked thermosets based on borate ester, BPU-4-400 possesses both outstanding Young’s modulus and remarkable toughness (calculated by the integration of the stress-strain curve).[

    26,29−35] Dynamic mechanical analysis reveals that three crosslinked polymers tend to be softened as represented by the decreased storage modulus, upon increasing the temperature (Fig. 2e). At temperature lower than −30 °C, BPU-6-400 possesses a higher storage modulus than other samples because of the dense crystalline region of PEG. At temperatures above 80 °C, the storage modulus and the loss modulus of BPU-6-400 receive quick decrease due to the breakage of hydrogen bonds. In comparison, the storage moduli of BPU-4-400 and BPU-2-400 are sustained over 150 °C, indicating great thermal stability due to the higher crosslinking degree. Considering the best mechanical property, BPU-4-400 was chosen as the representative material for further studies on de-crosslinking and recycling.

    fig

    Fig 2  (a) Possible intermolecular hydrogen bonding in BPU; (b) Strain-stress curves of BPU-2-400, BPU-4-400 and BPU-6-400; (c) Young’s modulus and toughness of BPU-2-400, BPU-4-400 and BPU-6-400; (d) The comparison of mechanical properties between BPU and other reported recyclable cross-linked thermosets; (e) The storage modulus and loss modulus of BPU-2-400, BPU-4-400 and BPU-6-400 in temperature range from −50 °C to 150 °C. Black, red and blue lines refer to PEG-6-400, PEG-4-400 and PEG-2-400, respectively.

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    Swelling Properties of BPU

    Classical cross-linked polymers are swellable because of the penetration of a solvent into the polymer network. The permanent covalent crosslinkers maintain the topology of polymer network and prevent the BPU from dissolving, which is essential for the material to work in complex conditions. To evaluate the chemical stability of BPU, the swelling tests by recording the mass swelling ratio of BPU in organic solvents and PBS were carried out. According to the swelling kinetics in Fig. 3(a), the swelling of BPU in six types of frequently used solvents reaches equilibrium within 2 h. The excellent organic solvent resistance of BPU is attributed to the network crosslinked by borate ester linkages and urea bond. Besides organic solvents, the swelling test was also performed in PBS with pH values varying from 1.00 to 9.18 to study the stability of BPU with the existence of acid or base. As shown in Fig. 3(b), the mass swelling ratio of BPU was fluctuated between 40% and 50%, suggesting there is no specific correlation between swelling performance and pH value. Typically, borate ester linkage is sensitive to water, yet it is stabilized in BPU via the interpenetration of hydrogen-bonding network.

    fig

    Fig 3  (a) Swelling tests of BPU in various solvents; (b) Swelling tests of BPU in PBS buffer with varied pH values from 1.00 to 9.18.

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    Specific Capability of De-crosslinking

    Based on the stimuli-cleavage behaviour of borate easter linkage, the covalent network can be de-crosslinked into linear polyurea-urethane in acidic condition (Fig. 4a). Intriguingly, there is no obvious sign of de-crosslinking while BPU is emerged in acidic PBS buffer. Only when the glacial acetic acid was applied as solvent could BPU be de-crosslinked and dissolved in the degradation solution. During the decrosslinking process, acetic acid first dissociated the hydrogen-bonding network by means of swelling and protonation followed by the dissociation of borate ester linkage. According to the optical images as shown in Fig. 4(b), after immersed in acetic acid in 24 h, BPU was fully dissociated to be a form of linear polyurea-urethane residual, which could be precipitated by water and extracted from the dilute solution. The dissociated product was characterized by 1H-NMR, 13C-NMR and 11B-NMR spectroscopy (Fig. 4c), confirming the formation of boric acid and linear polyurea-urethane. The 11B-NMR of degradation product shows no signal of borate easter but only the signal at nearly 20 ppm that is assigned to boronic acid. As shown in Fig. 4(d), a characteristic band at 1647 cm−1, which is assigned to the deformation vibration of carbonyl group of acetate, is observed to confirm the existence of acetate units on linear polyurea-urethane. It should be noted that BPU exhibits high stability and no sign of dissociation can be observed when it was emerged in 50% acetic acid aqueous solution for 24 h. Furthermore, using 50% acetic acid in DMSO solution instead of acetic acid aqueous solution, BPU was first swollen and partially dissociated into fragments, and then was completely dissociated at 80 °C (Fig. S5 in ESI), which might be caused by the breakage of hydrogen bonds and the transition of crystalline phase at high temperature.

    fig

    Fig 4  (a) Dissociation process of BPU in acetic acid; (b) Optical images of BPU being dissociated and dissolved in acetate acid; (c) NMR spectra of the dissociated linear product; (d) FTIR spectra of BPU (black line) and the dissociated linear product (red line).

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    Characterizaiton of the Recycling Products

    The dissociation product of linear PUU could be reprocessed by hot-pressing to achieve recycling products. To evaluate the recovered product as a thermoplastic material, the mechanical properties of PUU recycling products were tested, including storage modulus, loss modulus and stress-strain curves. The transition from crosslinked network to linear polymer provides the recycling product with thermoplastic capability. As shown in Fig. 5(a), according to the temperature dependence of storage modulus and loss modulus by DMA, the glass transition temperature of thermoplastic polyurea-urethane is measured as −20 °C. The tanδ keeps rising above 50 °C due to the breakage of hydrogen bonding until 130 °C, which corresponds to the melting transition of plastic PUU. As shown in Fig. 5(b), the mechanical properties of the recovered polymer samples remain stable in three hot-pressing recycling cycles, with elongation at break, breaking strength and Young's modulus to be 700%, 7 MPa and 378 kPa, respectively. It revealed the transition from thermoset to thermoplastic after recycling, while the hot-press process may cause that modulus increase. Fig. 5(c) shows the optical images of PUU reprocessed at temperature of 130 °C, which presents excellent mechanical properties and can be employed for electronic package, adhesives and wearable devices, etc.

    fig

    Fig 5  (a) Temperature scanning spectra of the recycling products characterized by DMA; (b) Strain-stress curves of the recycling products; (c) Optical images of the recycling products.

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    CONCLUSIONS

    In conclusion, a kind of recyclable polyurea-urethane thermosets was synthesized and well characterized. Benefiting from the considerable crosslinking density and intermolecular hydrogen bonding, such thermosets presented excellent mechanical properties as well as outstanding solvent resistance capability. Based on the stimuli-cleavage property of borate ester unit, the polyurea-urethane polymer network could be specifically dissociated in acetate acid to generate a linear polymer product. The de-crosslinking product could be further fabricated through hot-press process to achieve recycling thermoplastic products. It is envisioned that this particular class of recyclable polyurea-urethane thermosets with de-crosslinking capability may be readily extended to wide range of applications in optoelectronics, reinforcement, and encapsulation.

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