Eco-Friendly Synthesis of Non-Isocyanate Polyurea Using PET and CO2 Upcycling: A Double Green Approach

doi:10.21203/rs.3.rs-4912937/v1

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Eco-Friendly Synthesis of Non-Isocyanate Polyurea Using PET and CO2 Upcycling: A Double Green Approach

https://doi.org/10.21203/rs.3.rs-4912937/v1

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From the perspective of sustainability, this research explored both the recycling of polyethylene terephthalate (PET) and the conversion of carbon dioxide (CO₂) into polymer materials. To this end, a series of copolyurea specimens were synthesized through the polycondensation of carbon dioxide with hexamethylenediamine (HDA) and terephthalic dihydrazide (TDH), where TDH was derived from the chemical recycling of PET via the aminolysis method. FTIR spectroscopy, 1HNMR, DSC, and TGA characterized the products of PET recycling and synthesized copolyureas. Additionally, we sought to identify the optimal parameters for both the recycling and copolymerization processes. The results of identification analyses for both the recycling and copolymerization reactions confirmed the formation of the desired structures. An increase in the hard segment (TDH) led to discrepancies between the intended and actual monomer ratios in the synthesis, possibly due to dimethyl sulfoxide (DMSO) solubility issues or unreacted TDH. Furthermore, the TGA analysis indicated that the initial degradation temperature (T_d,5%) of the copolymer increased with the proportion of aromatic rings in the hard segment, underscoring the complex interactions within the copolymer matrix affecting its thermal and structural properties. The DSC results revealed that for copolyureas with the formula HDA_xTDH_100–x (where x = 50, 60, 70, 80, 90), reducing the x fraction or increasing the TDH content lowered the melting temperature. Furthermore, at a high percentage of TDH, the melting point disappeared entirely due to the destruction of the crystalline structure, indicating a critical threshold where the copolymer transitions from a semi-crystalline to an amorphous state.

Polyurea

Copolyurea

CO2

Upcycling

PET

Sustainability

Aminolysis

In today's era, we witness the growing significance of polymers in various facets of modern life due to their wide-ranging applications across different scientific, technological, and industrial domains, spanning from fundamental uses to biopolymers and stimuli-responsive polymers.

This remarkable proliferation of polymers can be attributed to their outstanding characteristics, including their physicochemical properties, processability, and cost-effectiveness. However, it is indisputable that the disposal of these materials contributes significantly to environmental waste, with polyethylene terephthalate (PET) being a significant contributor. Given the limitations of landfill space, environmental concerns associated with accumulation, government-imposed regulations, dwindling fossil fuel resources, and the poor biodegradability of these materials, the recycling of PET is imperative.[1]

PET recycling typically involves four primary methods: primary (re-extrusion), secondary (mechanical), tertiary (chemical), and quaternary (energy recovery). It is argued that upcycled products can only be achieved through tertiary recycling, a method that aligns with the principles of sustainable development. Through this approach, not only can valuable materials such as PET constituent monomers be obtained, but environmental considerations are also addressed, leading to a reduction in solid waste accumulation.[1]

In recent centuries, the world population has grown alongside the industrialization of civilization, resulting in a substantial increase in energy demand, primarily met through the consumption of fossil fuels and coal. This surge in energy consumption has led to a significant rise in the production and emission of greenhouse gases, notably carbon dioxide, which has detrimental and long-lasting environmental effects. Carbon dioxide production has far surpassed its recycling capacity by the environment, exceeding safe levels.[2]

Polyurea is a versatile polymer known for its exceptional mechanical properties, chemical resistance, and wide-ranging applications in various industries, including automotive, construction, and coatings. The synthesis of polyurea has traditionally relied on the reaction between isocyanates and amines, resulting in the formation of urea linkages. However, the environmental and health concerns associated with isocyanates have prompted the exploration of alternative routes for polyurea synthesis.[3]

In recent years, using carbon dioxide (CO₂) as a feedstock for polymer synthesis has gained significant attention due to its abundant availability, non-toxicity, and potential for reducing greenhouse gas emissions. The incorporation of CO₂ into polymers not only offers a sustainable approach to polymer production but also contributes to the valorization of this greenhouse gas.[2]

One promising approach to harnessing CO₂ in polyurea synthesis involves the reaction between diamines and CO₂, forming polyurea linkages. Diamines serve as the primary building blocks, providing amine functionalities for the reaction with CO₂ while offering structural diversity and tailoring the properties of the resulting polyurea. Incorporating CO₂ into the polyurea backbone enhances the synthesis process's sustainability and imparts unique properties to the polymer, such as improved thermal stability and flame retardancy.[4–17]

Yamazaki et al. were the first to report the direct transformation of CO₂ with diamine to polyurea. They synthesized polyureas through direct polycondensation from aromatic diamines and CO₂, employing a stoichiometric amount of phosphate as a catalyst. Nevertheless, the polyureas produced exhibited relatively low molecular weights, necessitating a significant quantity of catalyst. [18, 19]

Studies indicated that this reaction could occur without a catalyst at temperatures of up to 180°C, and the polycondensation process could be accelerated by employing specific new catalysts. Because gaseous reactants are involved, the reaction requires an autoclave to sustain the high temperature. Nonetheless, the formation of water as a byproduct during polycondensation results in the hydrolysis of the polyureas, impeding the achievement of elevated molecular weights.[10, 11, 14, 20, 21]

Cheng et al. have recently proposed a two-step polycondensation technique to overcome this obstacle. Initially, they synthesized polyurea oligomers through the polycondensation of diamines with CO₂ in an autoclave at 180°C. Subsequently, these polyurea oligomers were subjected to a second-stage polymerization at a higher temperature (such as 250°C), with a continuous flow of CO₂ introduced. This continuous CO₂ flow helps eliminate the produced water, thus aiding in the completion of polycondensation.[14]

This study involves synthesizing and characterizing a copolyurea derived from hexamethylene diamine (HAD) and terephthalic hydrazide (TDH), with the latter obtained from the chemical recycling of PET and CO₂ as a carbonyl resource.

2.1. Materials

All of the chemicals for the chemical recycling of PET and synthesis of polyurea were used as received without further purification.

sodium acetate, hydrazine monohydrate (HMH, purity > 80), dimethyl sulfoxide (DMSO, purity > 99), Hexamethylenediamine (HAD, purity > 98) were obtained from Merck Chemicals Ltd. Cyclohexylamine (CHA, purity > 99) was obtained from Sigma-Aldrich.

N₂ (purity > 99) and Co₂(purity > 99) were purchased from a domestic company.

In order to recycling PET, post-consumer PET bottles underwent a process where the caps and labels were removed. Following this, the bottles were meticulously cleaned using soapy water and then rinsed with distilled water. Subsequently, the PET bottles were dried at 80°C for 10 h. The dried bottles were cut into small flakes measuring approximately 5 mm x 5 mm.

2.2. Aminolytic depolymerization of PET

The aminolysis process of polyethylene terephthalate was carried out in the presence of hydrazine monohydrate (40 g) as an amine reagent, prepared PET flakes (10 g), and DMSO (25 ml) and sodium acetate(1% w/w of PET total weight) in a three-neck round bottom glass flask equipped with a mechanical stirrer and a reflux condenser at a temperature below 65°C, a time of 4 h, and atmospheric pressure. A fiberglass mesh mantle was used to provide the necessary thermal energy for the reaction and achieve the desired temperature. After the completion of the aminolysis reaction in the defined period and the separation of the batch reactor from the heating and reflux system, the unreacted PET flakes, if any, were separated, and the reactor was cooled in an ice bath to a temperature of 10 degrees for several minutes. Then, the resulting sediment (terephthalic dihydrazide) is separated from the reactor and poured into a Buchner funnel on filter paper, washed well with plenty of distilled water and alcohol, and then filtered using a vacuum pump. After filtering, the product is boiled with water and then washed with alcohol to remove the catalyst and other impurities from the surface of terephthalic dihydrazide. Finally, terephthalic dihydrazide was collected as a yellowish-white solid powder from filter paper and placed in a dry oven at 70 degrees for 3 h to dry completely.

2.3. Oligourea synthesis

The synthesis of oligourea involves charging a 100 ml autoclave reactor equipped with a magnetic stirrer with HAD (20 mmol) and TDH(20 mmol) obtained from the PET aminolysis process. The reactor undergoes three purges with nitrogen gas to eliminate air from the reaction environment before sealing. Following this, CO₂ gas is introduced into the reactor under high pressure, typically 11 MPa. The reaction proceeds at 210°C for 10 h. Once this stage is complete, the reactor is cooled and depressurized, and the resulting material is washed with distilled water. Subsequently, it is dried under a vacuum at 60°C.

2.4. Copolyurea synthesis

The final polyurea is synthesized by post-polymerizing the resulting oligourea at this stage. For this purpose, after charging the required amount of synthesized oligourea in a three-neck flask equipped with a mechanical stirrer, The reaction takes place initially at a temperature of 200°C, with the stirrer rotating at 170 rpm and under the flow of carbon dioxide gas for 1 h with flow rate of 50ml/min. Subsequently, the temperature is raised to 250°C, and the reaction continues for approximately another hour until the Weissenberg effect, characterized by an increase in viscosity, is observed. Finally, the reaction product is dried in a vacuum oven at 60°C to eliminate water which inhibits the extension of the carbon chain. The polyureas derived from the reaction of CO₂, HAD, and TDH are named HDA_xTDH_100–x (x = 50,60 ,70,80,90), where x represents the mole fraction of HAD.

2.5. Product characterization

The ¹H NMR spectra were recorded on a Varian Unity INOVA 500 MHz spectrometer. All ¹H NMR chemical shifts are reported as δ in parts per million (ppm).

Fourier-transform infrared (FTIR) spectra of solid samples were collected at room temperature

in the range of 4000–400cm^–1 with a PerkinElmer Spectrum Two FTIR infrared spectrometer.

X-ray diffraction (XRD) analyses were conducted with a Philips PW1730 diffractometer with Cu Kα radiation.

Thermal gravimetric analysis (TGA) experiments were performed using a TA Q600 Thermal Analyzer from 25 to 600°C at a heating rate of 10°C/min in a flowing N₂ gas.

Differential scanning calorimetry (DSC) experiments were conducted on a Netzsch DSC 200 F3 apparatus with heating and cooling rates of 10°C /min from 25 to 300°C in an N₂ flow.

3.1. Preparation and identification of TDH

The aminolysis of PET was conducted using HMH as the amine reagent, catalysts, and DMSO, involving PET flakes sourced from recycled post-consumer bottles, as depicted in Scheme 1.

FTIR spectrometry was used to analyze and confirm the structure of TDH derived from the aminolysis of PET. The spectrum depicted in Fig. 1 displays a carbonyl absorption peak, identified as a primary amide, at 1633 cm^–1. An observed peak at 1545 cm^–1 corresponds to the bending vibrations of NH₂ or NH (secondary amide bond). A broad and sharp peak at 3327 cm^–1 is attributed to the N–H stretching vibration; its absence in the PET spectrum confirms the degradation of the polymer and the formation of amide groups. The peak at 1340 cm^–1 results from the C–N stretching vibration, indicating the breaking of the C–O bond in PET and subsequent bonding of the carbon to the nitrogen atom in HMH. Additionally, aromatic C = C and C–H stretching vibrations are noted at 1336 cm^–1 and 3050 cm^–1, respectively. Sharp peaks at 1490 cm^–1 and 1541 cm^–1 are associated with the presence of the aromatic ring, observable in both the synthesized TDH and PET. These results from FTIR spectroscopy confirm that the degradation of PET and the production of TDH as a depolymerization product have been successfully achieved.

Figure 2 displays the ¹H NMR spectrum of the product obtained from the aminolysis of PET flakes. This spectrum features peaks at 7.89 ppm (singlet, 4H, aromatic protons), 9.9 ppm (singlet, 2H, NH from –CONHNH₂), and a broad peak at 4.58 ppm (singlet, 4H, NH₂ from –CONHNH₂). The spectral data for the aminolyzed product from PET flakes align well with the values reported in the literature for TDH.[22–24] The ¹H NMR spectrum complements the findings from the FTIR analysis, providing a cohesive understanding of the product's structure.

The differential scanning calorimetry (DSC) thermograms, displayed in Fig. 3, compare the thermal behavior of a sample obtained from PET depolymerization and PET flakes across two continuous thermal scans. The thermogram in Fig. 3 reveals the start of an endothermic peak at 328°C with a narrow and elongated distribution, indicative of the melting point of the synthesized product. This peak contrasts with the shorter and broader peak of the PET flakes, aligning with the reported melting point of terephthalic dihydrazide in the literature. The melting point of terephthalic dihydrazide has previously been reported by Shukla and George to fall within this range.[25, 26] The DSC results confirm the formation of terephthalic dihydrazide as the final product of the aminolysis of PET flakes in this study. The presence of an aromatic ring in the structure of terephthalic dihydrazide contributes to its high thermal stability and delays its decomposition.

An exothermic peak at 336°C signifies the total decomposition of the product at that temperature, highlighting a close temperature range between melting and TDH degradation. Meanwhile, the melting peak for the utilized PET sample appears at approximately 245°C, showing a significantly wider temperature gap between the melting and degradation points compared to the small TDH molecule.

According to Fig. 4, thermogravimetric analysis curves for the TDH sample and PET flakes are presented side by side for comparison. The TGA diagram, shows TDH remains stable up to 300°C, and the initial weight loss (d5%) of the synthesized sample occurs at a temperature above 300°C. This loss could be due to residual moisture or small molecules resulting from depolymerization despite the sample being dried in an oven at 60°C for 24 h. In contrast, the initial weight loss for PET flakes starts at around 380°C. Additionally, the TGA, DTA, and DTG curves shown in Fig. 5 highlight three major areas of weight change in the synthesized TDH. The behavior of the TGA curve and the peaks observed in the DTG and DTA curves indicate that the melting temperature of TDH, resulting from depolymerization, is 328 degrees Celsius. This finding is consistent with the results reported by other researchers. Furthermore, this temperature obtained from the thermal gravimetric analysis aligns perfectly with the results from the differential scanning calorimetry tests.[25, 26]

3.2. Influence of reaction parameters on TDH yield

In order to investigate the effect of the catalyst, the concentration of sodium acetate used as a catalyst for the aminolysis of PET flakes was varied from 0.5–2% by weight relative to the weight of the PET flakes, and the results from the depolymerization are reported in Table 1. As the results in Table 1 indicate, the maximum yield (92%) of terephthalic dihydrazide was achieved at a catalyst concentration of 2%. On the other hand, using a catalyst at 50% of this amount, namely 1% by weight, resulted in a yield of 91%. Therefore, considering the economic aspect of the process, the catalyst concentration for advancing subsequent processes was set at 1% by weight.

Table 1

the effect of catalyst on reaction yield
Catalyst Concentration(wt.%)	TDH Yield (%)
0.5	76
1.0	91
1.5	91
2.0	92

Figure 6 illustrates the changes in the aminolysis reaction yield of PET flakes in the presence of HMH as the temperature increases from 40°C to 100°C in increments of 10°C. The graph shows that the overall reaction yield increases with temperature, reaching a peak of 92% at 70°C. Beyond this temperature, the yield declines due to the formation of secondary side products, as noted in the literature on this topic.

To investigate the effect of reaction time on the yield of TDH formation during the depolymerization process, the PET aminolysis reaction time was considered from 1 to 6 h. Complete conversion of PET waste to TDH using HMH as the degrading agent in the presence of sodium acetate as a catalyst (1% by weight) was achieved in 4 h, compared to the reaction without a catalyst, which reportedly took about 24 h for complete degradation.[27] Table 2 shows the effect of reaction time on TDH yield. The results indicate no significant increase in reaction yield when extending the time from 4 to 6 h.

Table 2

The effect of reaction time on the yield of TDH
Reaction Time (h)	TDH Yield (%)
1	33
2	80
3	84
4	91
5	91
6	91

Table 3 shows the effect of the PET to HMH ratio on the yield of TDH, the final product of the reaction. After reaching an optimal ratio, no significant changes were observed in the yield. Therefore, the optimal PET: HMH ratio for the aminolysis reaction of PET was established at 1:4. Previous research in this field typically used approximately 5–10 mL of HMH to aminolyze one gram of PET bottle waste.[27] The required amount of the amine reagent was reduced by using sodium acetate as a catalyst, which significantly decreased the consumption of HMH to 4 grams for the aminolytic decomposition of one gram of PET.

Table 3

Effect of PET to HMH Ratio on TDH Yield
PET/HMH ratio (wt./wt.)	TDH Yield (%)
1:1	31
1:2	69
1:3	86
1:4	91
1:5	91.5
1:6	91.7

3.3. Preparation and identification of oligomer and copolyurea

Polyureas were produced using a two-step polymerization process involving diamines and CO₂, as outlined in Scheme 2. As previously discussed, the synthesis process comprises an initial stage of oligomer preparation, followed by the creation of copolymer samples. To achieve the best conditions for the synthesis reaction, various tests were performed based on the materials’ structure and findings from other research studies. Consequently, to identify the appropriate conditions for synthesizing HDA and TDH-containing copolyurea in a high-pressure autoclave reactor, the optimization of the initial stage of synthesis, known as oligomerization, has been conducted.

To the best of our knowledge, the TDH monomer has not previously been employed in the synthesis of polyurea.

In this research, we synthesized both copolyurea and homopolyurea(scheme 3). The data and results from the homopolymer were used as a basis for developing copolyureas that incorporate TDH.For this purpose, HDA100 samples were prepared within a pressure range of 9 to 12 MPa, a temperature range of 160 to 190 degrees Celsius, and a reaction time of 6 to 12 h. Considering the influence of the three parameters—temperature, pressure, and time—on reaction efficiency, and the need to achieve optimal conditions, each series of samples was tested by keeping two of the parameters constant while varying the third. The optimal value for each parameter was thus determined. To estimate the reaction yield, the weight ratio of the product (Wp) to the initial feed (Wi) was calculated for various HDA₁₀₀ samples.

According to the results of the reaction yield in the first stage of the synthesis, the optimum temperature, time, and pressure were determined to be 180°C, 8 h, and 11 MPa, respectively. These optimal conditions for the HDA₁₀₀ have been used as the basis for the synthesis of copolymers. For the first stage of copolymer synthesis, the optimal conditions were 210°C, 10 h, and 11.5 MPa, respectively. Based on our initial observations during the synthesis of TDH-containing copolymers, samples with high percentages of the hard segment (TDH), specifically 60%, 70%, 80%, 90%, and 100%, were found to be unprocessable. Consequently, we opted against prototyping or synthesizing compounds exceeding a 50% hard segment composition. This decision resulted in setting the maximum allowable TDH content in the copolymers at 50%.

After determining the optimal values for the key factors influencing the reaction during the oligomerization stage, the second synthesis stage was initiated. In this phase, three main parameters must be assessed to establish their optimal values. Additionally, considering the synthesis reaction of the second stage in a three-neck glass balloon and the limitation on applied pressure, all polyurea samples at this stage were synthesized under an applied pressure of 0.1 MPa, as suggested by existing literature and reports. Observations also indicated that a total duration of 2 h for the post-polymerization reaction (t_2nd and t_3rd) was the optimal time for conducting the reaction. In light of the cases mentioned and considering the composition of the desired polymer for synthesis, the focus was solely on the temperature parameter (T_2nd and T_3rd). Generally, during sample preparation, the molar percentage of the hard segment in the formulation was incrementally raised, and based on these adjustments, the suitable temperatures were assessed. Table 4 displays the optimal values for the polyurea synthesis process.

Table 4

the optimal values for the synthesis process of both homopolymer and copolymer polyureas
Entry	Sample	P_1st (MPa)	T_1st (°C)	t_1st (h)	P_2nd (MPa)	T_2nd (°C)	t_1st (h)	T_2nd (°C)	t_1st (h)
1	HDA₁₀₀	11	180	8	0.1	200	1	250	1
2	HDA₉₀TDH₁₀	11.5	210	10	0.1	200	1	250	1
₃	HDA₈₀TDH₂₀	11.5	210	10	0.1	210	1	260	1
4	HDA₇₀TDH₃₀	11.5	210	10	0.1	220	1	270	1
5	HDA₆₀TDH₄₀	11.5	210	10	0.1	230	1	270	1
6	HDA₅₀TDH₅₀	11.5	210	10	0.1	230	1	270	1

The structure of the synthesized polyureas was analyzed using FTIR spectroscopy. The results are shown in Fig. 7 for the polyurea oligomer and copolymer samples. The FTIR spectrum graphs of these samples reveal five distinct peaks within the tested wavelength range. Notably, the peak at 3334 cm^–1 is attributed to the stretching vibration of the N–H group in the oligomer.[9, 11, 12] This peak shows a redshift of about 66 cm^–1 from the free N–H group peak at 3450 cm^–1, indicating the presence of intermolecular hydrogen bonds in the synthesized polyurea. Research by Jiang and colleagues on polyurea synthesis via direct polymerization of 1,6 hexane diamine and carbon dioxide also reported a similar N–H stretching vibration peak at 3329 cm^–1.[12] Moreover, in studies aiming to synthesize thermoplastic polyureas using 1,12-aminododecane (DAD) and/or 4,7,10-trioxa-1-13-tridecane diamine (TTD) with carbon dioxide, they observed this N–H stretch at 3338 cm^–1.[9]

In separate research led by Ruhui Shi and his team, the stretching vibration of the N–H group was detected at a wave number of 3429 cm^–1.[11]

The FTIR spectra also highlight a peak at 1619 cm^–1, associated with the stretching vibration of the –C(= O)– group in the synthesized samples. This peak exhibits a shift of approximately 70 cm^–1 to a lower wavelength compared to its free state at 1690 cm^–1, which suggests the formation of hydrogen bonds. According to literature in the field, the –C(= O)– peak for oligomer samples typically ranges from 1618 to 1614 cm^–1, and for polyurea from 1630 to 1622 cm^–1.[8]

Additionally, the absorption peaks at 2930 cm^–1 and 2857 cm^–1 are identified as related to the stretching vibration of the C–H group along the main aliphatic chain. Jiang and colleagues reported similar absorption peaks at 2863 cm^–1 and 2932 cm^–1, while another study reported them at 2850 cm^–1 and 2922 cm^–1. Some studies also mention a bending vibration of the N–H group in the urea –CO–N–H– bonding group at 1573 cm^–1. In this research, this peak appears independently in some homopolyurea and copolyurea samples, while in others, it overlaps with the C = O stretching vibration peak.

Based on the FTIR spectra of the synthesized samples and a comparison with existing literature, the formation of the urea functional group in the synthesized product can be confirmed.

Additionally, the chemical compositions of the synthesized polyurea were analyzed using 1HNMR, as shown in Fig. 8. 1H NMR results (Fig. 8) show that the actual molar ratio of HDA to TDH in the sample HDA₈₀TDH₂₀ is 4, corresponding to the initial molar ratio used in the feed.

This consistency is also seen in the HDA₁₀₀ and HDA₉₀TDH₁₀ samples, based on their ¹HNMR results. However, for samples with a hard segment content of 30–50%, such as HDA₇₀TDH₃₀, the peak distinction is compromised due to poor solubility in DMSO solvent (Fig. 9), resulting in fewer observable peaks. Further, the molecular weight and ratios of monomers in the final product can be assessed by comparing the integrated areas of characteristic peaks. The discrepancies between the initial feed ratios and the ¹HNMR results, particularly in samples with a higher hard segment content, are attributed to the incomplete dissolution in DMSO and some TDH monomers not reacting under the experimental conditions. According to the NMR data, the number-average molecular weight (Mn) for the HDA₈₀TDH₂₀ sample is approximately 5600 g/mol, while the Mn for the HDA₇₀TDH₃₀ oligomer is around 3600 g/mol.

The thermal properties of the prepared polyureas, HDA_xTDH_100–x, were analyzed using DSC and TGA. The DSC curves from the second heating run can be found in Fig. 6a. The data reveal that the melting temperatures (Tm) for HDA₁₀₀, HDA₉₀TDH₁₀, and HDA₈₀TDH₂₀ are 269°C, 244°C, and 238°C, respectively. Additionally, the melting temperature of the homopolymer HDA₁₀₀ aligns with the findings from previous research conducted by other scholars.[12]

The decrease in melting temperature observed in this series of polyurea samples indicates that as the HDA content decreases, or conversely, as the TDH content increases, the melting temperature drops by approximately 30°C.

However, according to Fig. 10a for the other samples HDA₇₀TDH₃₀, HDA₆₀TDH₄₀, and HDA₅₀TDH₅₀, the melting peak disappears as the TDH content increases in the copolymer structure. It can be concluded that the increase in TDH reduces crystalline structure, thereby promoting the formation of an amorphous structure. Figure 10b illustrates the thermal behavior during the DSC cooling process, showing that the crystallization temperature (Tc) undergoes a similar alteration with the addition of TDH to HDA.

Figure 10b demonstrates a trend in the crystallization temperatures of the samples similar to that observed in their melting temperatures. Specifically, the crystallization temperatures for HDA₁₀₀, HDA₉₀TDH₁₀, and HDA₈₀TDH₂₀ are 200°C, 189°C, and 181°C, respectively. It is inferred that an increase in the TDH fraction within the copolyureas leads to a decrease in crystallinity.

Table 5 provides the thermal characteristics of synthesized polyurea samples, including melting enthalpy (ΔH_m), crystallization enthalpy (ΔH_c), melting temperature (T_m), crystallization temperature (T_c), and the initial decomposition temperature at 5% weight loss (T_{d, 5%}).

Given the influence of molecular weight, degree of polymerization, and molecular structure on polymers’ thermal properties and stability, a TGA (Thermogravimetric Analysis) test was employed to study the structure of homopolyurea copolyureas.

Figure 11 presents the spectra from the thermal gravimetric tests, which compare the thermal decomposition processes of the synthesized polyurea samples.

Figure 11 shows that the homopolymer sample's initial decomposition temperature (T_{d, 5%}) is 217°C. Increasing the content of the TDH monomer in the copolymer structure, which includes an aromatic ring as a hard segment, results in higher thermal stability in the copolymer samples compared to the homopolymer. According to the TGA curve in Fig. 11, the initial decomposition temperatures of the copolyurea samples for HDA₆₀TDH₄₀ and HDA₅₀TDH₅₀ are approximately 336°C and 366°C, respectively. In samples with a lower TDH or higher HDA content, the increase in the initial decomposition temperature is less pronounced compared to the homopolymer. For example, the initial decomposition temperatures for HDA₉₀TDH₁₀, HDA₈₀TDH₂₀, and HDA₇₀TDH₃₀ are 220°C, 235°C, and 285°C, respectively.

Table 5

Thermal properties of polyureas
		DSC				TGA
Entry	Sample	T_m (°C)	T_c (°C)	ΔH_m (J/g)	ΔH_c (J/g)	T_d,_5% (°C)
1	HDA₁₀₀	269	200	90	50	217
2	HDA₉₀TDH₁₀	244	189	57	48	220
3	HDA₈₀TDH₂₀	238	181	53	45	235
4	HDA₇₀TDH₃₀	-	-	-	-	285
5	HDA₆₀TDH₄₀	-	-	-	-	336
6	HDA₅₀TDH₅₀	-	-	-	-	366

Copolyurea based on CO₂, designated as HDA_xHMDA_100–x, is synthesized through a two-step polycondensation process that involves CO₂, HDA, and TDH, the latter being derived from the chemical recycling of PET consumer bottles. Aminolysis of PET to produce TDH as an upcycled monomer is performed under various conditions to determine the optimal process parameters. Ultimately, the conditions are set at 1 wt.% catalyst concentration, 70°C, 4 h, and a PET: HMH molar ratio of 1:4. Additionally, the structure of TDH is confirmed by FTIR and 1H NMR analyses. TGA and DSC results also show that the synthesized TDH, despite having a higher melting temperature, exhibits a narrower temperature range between melting and decomposition compared to PET.

Additionally, the synthesis of the copolyurea was successfully carried out. The results from FTIR and ¹HNMR analyses indicated that the desired structure of the copolymer was accurately achieved. Optimal process parameters, including time, temperature, and applied pressure, were established for different compositions of copolyurea.

The ¹HNMR results showed that as the proportion of the hard segment (TDH) in the copolymer composition increased, discrepancies arose between the monomer ratios fed into the reactor and the calculated values. These differences could be attributed to either the weak solubility of the copolymer in the ¹HNMR solvent (DMSO) or the presence of unreacted TDH. The TGA results indicated that the initial degradation temperature (Td, 5%) increased with a higher proportion of the hard segment in the copolymer composition, likely due to the presence of aromatic rings. Conversely, the DSC results revealed that only copolymers with a lower content of TDH exhibited a crystalline structure, which diminished as the TDH content increased.

Declaration of competing interest

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.

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Schemes 1 to 3 are available in the Supplementary Files section

S1.png
Scheme 1. Aminolysis of PET for TDH Synthesis Using HMH
S2.png
Scheme 2. Two-step polymerization of CO₂with HAD and TDH
S3.png
Scheme 3. Typical structure of :(a) homopolyurea based on HDA and (b) copolyurea containing HDA and TDH

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Eco-Friendly Synthesis of Non-Isocyanate Polyurea Using PET and CO2 Upcycling: A Double Green Approach

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Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Materials

2.2. Aminolytic depolymerization of PET

2.3. Oligourea synthesis

2.4. Copolyurea synthesis

2.5. Product characterization

3. Results and Discussion

3.1. Preparation and identification of TDH

3.2. Influence of reaction parameters on TDH yield

3.3. Preparation and identification of oligomer and copolyurea

4. Conclusions

Declarations

Declaration of competing interest

References

Scheme

Supplementary Files

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Version 1