Synthesis and Properties of Fully Biobased Plastics from Biuret and Diamines

Researchers nowadays pay attention to the environmental and resource issues on polymer materials which are applied broadly in our daily life [1]. These materials rely substantially on petroleum oil resources. To deplete the dependence on the limited petroleum oil and find environmentally friendly materials, biobased polymers are currently in high demand [2]. Biobased polymers are those synthesized from natural resources biologically, chemically, or physically [3]. Polyureas (PUrs) are the polymers bearing urea groups in the backbone [4]. The existence of urea groups contributes to the formation of bidentate hydrogen bonds, which brings the polymers with diverse, remarkable, and excellent properties, such as waterproof, wear-resistant, and anticorrosive [5]. PUrs are therefore widely applied in many fields, such as construction, industrial, and military [6]. Traditional PUrs are synthesized from isocyanates and amines [7]. The reactivity of isocyanates is so high that amines are able to react quickly with isocyanates at room temperature, however, the choice of isocyanates is so limited that retards the expanding use of PUrs, especially fully biobased PUrs [8,9]. The high reactivity of isocyanates increases transportation and storage costs. Another issue for isocyanates is that they are toxic and harmful to physical health [10]. Therefore, alternative synthetic routes, namely non-isocyanate routes, are becoming more attractive, especially in the current underground of green chemistry. There are several environmentally friendly synthetic routes that have been proposed after numerous years of development and research, for instance, the dicarbamate route, the carbon dioxide (CO₂) route, and the urea route [11,12,13].

Urea (NH₂CONH₂) exists naturally or can be manufactured from CO₂, so it is considered a cost-efficient and environmentally friendly chemical [14,15]. The high reactivity of urea with amines makes it more attractive and feasible in the synthesis of PUrs [16]. Lin et al. [11] applied urea and two fully biobased diamines, namely 1,6-hexanediamine (HDA) and 1,10-decanediamine (DDA), to synthesize fully biobased high-performance PUrs without using any catalyst and solvent. Considering that the choice of fully biobased diamines is still limited nowadays, another strategy can be applied: utilizing biuret, an environmentally friendly compound, in the construction of polymers to enhance their properties. The simplest biuret (NH₂CONHCONH₂) is the dimer of urea, and it shows similar reactivity to urea when reacting with amines. The biuret is commercially available and cost-efficient. The polymers synthesized from biuret and diamines are named as polybiurets (PBUs). Polymers with higher hydrogen bonding density brings them enhanced thermal and excellent mechanical properties [17,18,19]. The introduction of the biuret function has been proved by Pan et al. [20] that it is able to considerably improve the mechanical properties of polyureas based on priamine. Compared to other existing solutions, biuret is proposed to be utilized to react with two diamines (HDA or/and DDA), achieving the formation of PBU plastics with enhanced performance in this work. Due to the existence of biuret groups, PBUs, compared with PUrs, are expected to possess higher hydrogen bonding density, investing those polymers more extraordinary and outstanding thermal and mechanical properties.

Biuret was purchased from Senrise Technology Co., Ltd. (Anqing, China). HDA (99.5%) was bought from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). DDA (97%) was bought from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All chemicals were used as received without further treatment.

The synthetic procedures and reaction conditions for the five polymers are the same, with different feedings of the two diamines. Taking the synthesis of PBU-6₅₀10₅₀ as an example, biurets (100 mmol, 10.31 g), HDA (50 mmol, 5.81 g), and DDA (50 mmol, 8.62 g) were added into a 250 mL four-neck spherical flange reactor equipped with a gas inlet, a PTFE mechanical stirrer, and a gas outlet. The reaction was first performed at 180 °C under argon until the mixture became turbid. Then, the temperature was increased to 280 °C, and the reaction was run for another 2 h. Vacuum (0.08 MPa) was applied, and the reaction was kept for another 1 h to allow further chain extension. The products were collected when the reaction finished.

Solid-state ¹³C nuclear magnetic resonance (NMR) spectra were recorded by a JEOL-ECZ400R/M1 (400 MHz) spectrometer (JEOL RESONANCE Inc., Japan) with an HXY 3.2 mm probe. Samples were spun at 15,000 Hz, and the spectra were recorded at 25 °C. Fourier transform infrared (FTIR) was carried out on a SHIMADZU IR Tracer 10 (SHIMADZU, Japan). The measurements were taken out with an accumulation of 32 scans at room temperature. The spectra were obtained in the range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. Thermogravimetric analysis (TGA) was carried out by a NETZSCH TG209 F3 analyzer (NETZSCH, Germany) from 100 to 600 °C with a heating rate of 20 °C/min, under N₂ atmosphere. Differential scanning calorimetry (DSC) measurements were carried out under N₂ atmosphere at a heating rate of 20 °C/min and a cooling rate of 10 °C/min using a NETZSCH DSC 204 F1 calorimeter (NETZSCH, Germany), from 20 to 280 °C. Dynamic mechanical thermal analysis (DMA) was conducted on a DMA 242E dynamic mechanical analyzer (NETZSCH, Germany). For each experiment, a sample cut into a rectangular shape (10 × 5 × 0.5 mm³) was loaded on a tension-type fixture with a frequency of 1 Hz. The temperature was set ranging from 15 °C to 220 °C with a heating rate of 5 °C/min. The samples for both DMA and tensile tests were prepared by compression molding. Compression molding was performed at 195 °C for 10 min, with a clamping pressure of 20 MPa. The obtained film samples were cut into dumbbell shape according to the ISO 37 Type 4 standard (the size of the test section is 10 × 2 × 0.5 mm³). Tensile tests were performed on an INSTRON 5960 (INSTRON, USA) universal tensile tester equipped with a load cell of 10 kN, and a 50 mm/min tensile speed was applied. For each sample, at least five specimens were tested, and the results were given by taking the average value.

PBUs are proposed in this work to be synthesized from biurets and diamines via melting polycondensation, without using any catalyst or solvent (see Scheme 1). The two diamines utilized are fully biobased, namely HDA and DDA. The synthesized PBUs are nominated as PBU-6_x10_100−x, in which x is the molar fraction of HDA (x = 0, 33, 67, 100). The synthetic process is designed to proceed in three steps. In the first step, the polymerization was carried at lower temperature at 180 °C to facilitate the consumption of the volatile diamines. The temperature was increased in the second step to afford a faster polymerization rate and a higher conversion of the functional groups. A vacuum was then applied in the final step to allow further chain extension and to achieve higher molecular weight for the products.

PBUs are resistant to most organic solvents, including acetone, methanol, dichloromethane, tetrahydrofuran, acetic acid, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethylsulfoxide, hexafluoroisopropanol, and even trifluoroacetic acid. Therefore, solid state ¹³C NMR was chosen to verify the chemical structure of PBUs, which are shown in Figure 1. The carbon signals next to the urea (CH₂-NHCONH-) and biuret groups (CH₂-NHCONH-CONH-) can be observed at 41 ppm. The signals of carbonyl carbon in both urea (-NHCONH-) and biuret functions (-NHCONHCONH-) can be observed at 160 and 150 ppm, respectively, which implies the formation of PUrs and PBUs [20]. The formation of urea groups can be explained by the attack of both carbonyl functions by the amino group. The structure of PBUs was detected by FTIR as well, and the results are shown in Figure 2. The peaks at 1720 and 1684 cm⁻¹ are the signals of the stretching vibrations of C=O in biuret groups with disordered and ordered hydrogen bonds, respectively. The peak at about 1627 cm⁻¹ corresponds to the stretching vibrations of C=O in the urea group. The stretching vibrations of N–H in biuret and urea can be observed at about 3320 cm⁻¹. Red shift in the N–H stretching vibrations is present because of the denser hydrogen bonding of PBU-6₁₀₀10₀ [21]. A red shift of N–H stretching vibration appears for PBU-6₁₀₀10₀ and PBU-6₀10₁₀₀, comparing with PBU-6₃₃10₆₇, PBU-6₅₀10₅₀, and PBU-6₆₇10₃₃, due to the less hydrogen bonding in the copolymers [11]. As the proportion of DDA increases to 67%, the stretching vibration of N–H shifts from 3307 cm⁻¹ to 3326 cm⁻¹. The peaks at about 2920 and 2852 cm⁻¹ are the signals of the stretching vibrations of -CH₂ in the main chains.

The thermostability of PBUs was tested by thermogravimetric analysis (TGA), and the curves are shown in Figure 3. For all the PBU samples, the initial decomposition temperature (T_d,5%) is found to be above 310 °C. There is a slight increase in T_d,5% with the increase of DDA content. The degradation behavior of these samples seems similar, which includes two steps: the breakage of biuret or urea functions and the degradation of alkylene functions. Thermal transition behavior of the samples was also probed by differential scanning calorimetry (DSC) (see Figure 4), and the results are listed in Table 1. There is no melting peak observed in the second heating curves for all PBUs samples. In addition, no evident changes are observed in the DSC cooling curves. It is interesting that those PBUs have a higher glass transition temperature (T_g) than PUrs with similar structures [11]. For instance, the T_g of PUr-6 was detected to be 90.4 °C, but that of PBU-6₁₀₀10₀ increases to 131.2 °C. This should be attributed to the strong hydrogen bonding effects [22] provided by the biuret groups, which would restrict the flexibility of hexamethylene segments in PBUs. The T_g of PBU decreases gradually as the increase of DDA content. It is understandable since the introduction of DDA portion would be able to enhance the flexibility of PBU chains.

The curves of dynamic mechanical thermal analysis (DMA) and T_g was measured by the Tan δ curves of DMA (see Figure 5 and Table 1). On the storage modulus curves, a rubbery plateau is observed above 150 °C, indicating that all polymers possess a micro-crosslinked structure. For copolymers, two peaks can be observed in the tan δ curves, which dedicates to the different flexibility of hexamethylene and decamethylene moieties [23]. Tensile tests were conducted to explore the mechanical properties of PBUs (see Figure 6), and the data are listed in Table 2. As we can see from the typical stress-strain curves, PBU-6₁₀₀10₀ seems rigid and brittle, which may be due to the rigid structure with high hydrogen bonding density. When introducing the part of DDA, for instance, PBU-6₆₇10₃₃, the product is also rigid but less brittle and the elongation at break (ε_b) increases from 8% to 44%. The value of strength at break (σ_b) achieves the highest level at 77 MPa. Further increasing the feeding ratio of DDA, the materials become tougher, the toughness increases from 1.3 (PBU-6₁₀₀10₀) to 85.7 J·cm⁻³ (PBU-6₅₀10₅₀). When compared to the corresponding PUr, PBU-6₆₇10₃₃ shows higher σ_b (54.6 vs. 77.4 MPa) [11]. The values of σ_b for other PBUs are quite similar to those of the corresponding PUrs.

In this work, fully biobased PBUs were synthesized from biuret with two fully biobased diamines (HDA and DDA) via solvent-free and catalyst-free melting polycondensation. In the PBUs, not only the biuret group but also the urea group can be observed, and they are lightly cross-linked. Compared with traditional PUrs, the introduction of biuret groups results in a rise of hydrogen bonding density and affords polymers with enhanced thermal and mechanical properties. The glass transition temperature is about 40 °C higher and the strength at break achieves as high as 77.4 MPa. The mechanical properties of PBUs can be feasibly tuned by adjusting the fraction of these two fully biobased diamines. The strength becomes higher when the feeding ratio of HDA increases. The toughness reaches at the highest level when the feeding ratio of HDA and DDA is at 1/1 (PBU-6₅₀10₅₀). This work provides a novel solution in developing fully biobased plastics by simply introducing biuret functions in the backbone to improve the thermal and mechanical properties.

The authors are grateful for the supports of the Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (South China University of Technology, No. 2023B1212060003).

Conceptualization, D.T. and B.P.; Methodology, D.T., B.P. and Y.C.; Validation, D.T., B.P. and Y.C.; Formal Analysis, B.P. and Y.C.; Investigation, D.T.; Resources, D.T.; Data Curation, B.P. and Y.C.; Writing—Original Draft Preparation, Y.C.; Writing—Review & Editing, D.T. and Y.C.; Supervision, D.T.; Project Administration, D.T.; Funding Acquisition, D.T.

Not applicable.

Not applicable.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

This research was funded by the Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (South China University of Technology, No. 2023B1212060003).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.