A new type of tetraphenylborate salts derived from highly basic and nucleophilic amines, namely 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) and triazabicyclodecene (TBD), was applied to the preparation of networked poly(thiourethane)s (PTUs), which showed a vitrimer-like behavior, with higher stress-relaxation rates than PTUs prepared by using dibutyl thin dilaurate (DBTDL) as the catalyst. The use of these salts, which release the amines when heated, instead of the pure amines, allows the formulation to be easily manipulated to prepare any type of samples. The materials prepared from stoichiometric mixtures of hexamethylene diisocyanate (HDI), trithiol (S3) and with a 10% of molar excess of isocyanate or thiol were characterized by FTIR, thermomechanical analysis, thermogravimetry, stress-relaxation tests and tensile tests, thus obtaining a complete thermal and mechanical characterization of the materials. The recycled materials obtained by grinding the original PTUs and hot-pressing the small pieces in the optimized time and temperature conditions were fully characterized by mechanical, thermomechanical and FTIR studies. This allowed us to confirm their recyclability, without appreciable changes in the network structure and performance. From several observations, the dissociative interchange trans-thiocarbamoylation mechanism was evidenced as the main responsible of the topological rearrangements at high temperature, resulting in a vitrimeric-like behavior.
Keywords: vitrimers; poly(thiourethane); thermosets; recyclability; covalent adaptable networks; organocatalyst
Thermosetting polymers, thanks to their permanent covalent bonds in the network structure, show outstanding mechanical and thermal properties. However, their permanent three-dimensional structure hindered this material to be melted or dissolved in any solvents, making it impossible to reprocess and recycle them. In view of the increasing amount of plastic generated, the recycling of this type of polymers is extremely important to prevent them from ending up in landfills and contaminating the environment. The development of covalent adaptable networks (CANs), covalently crosslinked polymers with the ability to be reshaped, to flow and to self-repair, represents a promising approach to improve the lifetime and recyclability of the thermosetting polymers [[
Recently, networked poly(thiourethane)s (PTUs) have been reported as a new class of CANs thanks to the presence of thiourethane groups that can undergo exchange reactions at moderate temperatures [[
Poly(urethane)s (PUs) have already been reported to experiment network reconfiguration by trans-carbamoylation [[
In an excellent paper from Hillmeyer's group, a mechanistic study of the exchange mechanism in poly(urethane) networks in the presence of a Lewis acid, tin (II) octoate, was reported [[
The preparation of PTU networks has been classically performed by the use of dibutyltin dilaurate (DBTDL) as the catalyst, since the use of bases such as amines leads to a too quick reaction [[
As the use of catalysts plays a key role also in the rate of dynamic covalent-bonds exchange, their appropriate selection could affect the mechanism and the kinetics of the exchange process, tuning the desired reconfiguration properties [[
Regarding the trans-thiocarbamoylation process, Torkelson et al. [[
Very recently, Bowman et al. [[
In light of these considerations, the use of thermally generated base catalysts represents an interesting opportunity to reach a temporal control of the curing reaction, offering, at the same time, the possibility to increase the amount of catalyst, which could help to improve the rate of the exchange process. The use of such catalysts does not require any solvents, as it was reported by Torkelson [[
In the present work, we explore and report the ability to undergo stress-relaxation of poly(thiourethane) networks prepared by using different tetraphenylborate salts of the following amines: 1-methylimidazole (1MI), 4-(dimethylamino)pyridine (DMAP), 1,8-diazabicyclo- (5.4.0)undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and triazabicyclodecene (TBD). Moreover, we investigate the relaxation behavior of these materials and their thermal behavior by thermogravimetry, to reach a good understanding of safe recycling conditions.
As in our previous study [[
Trimethylolpropane tris(3-mercaptopropionate) (S3), hexamethylene diisocyanate (HDI), dibutyltin dilaurate (DBTDL), 1-methylimidazole (1MI) (pK
Base generators (BGs) from different amines were synthesized by us, according to a reported methodology [[
To prepare the different formulations, the amount of base generator (BG) was conveniently selected to release 0.05% mol of the selected base. The BG of the selected base was first added to the thiol, and the system was kept under stirring, at 110 °C, for 45 min, until complete solubilization. The mixture was cooled down at room temperature, and then the required amount of diisocyanate was added, manually stirred and immediately poured into a mold or sent to analysis.
As an example, the formulations were composed by 1.90 g (11.29 mmol) of HDI, 3 g (7.53 mmol) of S3 and 0.77 mg (0.0094 mmol) of 1MI or 3.99 mg (0.0094 mmol) of BG1MI.
For the recycling and degradation studies, the thermosetting PTUs were also prepared with different stoichiometric ratios, and 0.1% mole of BGDBU was added as the catalyst. Typical amounts used are reported in Table 1.
For DMTA analysis, films were prepared by pouring the formulations on pre-silanized glasses and using Teflon spacers to ensure a homogeneous thickness of 0.5 mm. The formulations were cured at 60, 100 and 130 °C for 1 h at each temperature. The films were die-cut to obtain a rectangular specimen of 20 × 5 × 0.5 mm
Tensile stress-relaxation tests were conducted in a TA Instruments DMA Q800 analyzer (New Castle, DE, USA), using a film tension clamp on samples with the same dimensions as previously defined. To compare the rate of relaxation of the poly(thiourethane) vitrimers obtained by using different catalysts, a single stress-relaxation test was performed at 180 °C, for 1 h, at a constant strain of 1.5%.
To obtain the activation energy (E
(
where τ is the time needed to attain a given stress-relaxation value (0.37σ
The thermal stability of the cured samples was studied by thermogravimetric analysis (TGA), using a Mettler TGA/SDTA 851e thermobalance (Columbus, OH, USA). All experiments were performed under inert atmosphere (N
Fourier-transform infrared (FTIR) spectra were registered with a Bruker Vertex 70 (Billerica, MA, USA) in absorbance mode at a resolution of 4 cm
The evolution of the FTIR spectra of the poly(thiourethane) materials was followed isothermally at 180 °C in the ATR. The materials before and after being degraded in dynamic TGA, until 260 and 340 °C, were analyzed at room temperature. The spectra of original and recycled PTUs were also recorded at room temperature.
The stoichiometric PTU was degraded by heating 2 g of the material at 200 °C, for 1 h, in a sealed vial. The detection of the derived volatile products was performed in a HP6890 gas chromatograph and 5973 Mass selective detector (Agilent Technologies. Waldbronn, Germany), using an HP-5MS capillary column (30 m × 0.25 mm × 0.25 m) provided by Agilent.
The recycled samples were obtained by cutting the crosslinked polymers and hot-pressing at 15 MPa into an aluminum mold, at 130 °C for 3 h. Recycled samples were die-cut in Type V from the new film obtained and were tested in tensile, under the same conditions.
Original and recycled samples were tested until break in tensile mode, at room temperature, using an electromechanical universal testing machine Shimadzu AGS-X (Shimadzu Co., Kyoto, Japan) with a 1000 N load cell at 5 mm/min and using Type V samples according to ASTM D638-14 standard. Three samples of each material were tested, and the average results are presented.
Dissolution experiments of crosslinked polymers were performed by the following procedure. Pieces of poly(thiourethane) samples of 0.1–0.2 g, which were weighed before the experiment, were placed into a vial. The vial was filled with 1,2-dichlorobenzene (DCB) or dimethyl sulfoxide (DMSO) closed and heated at 150 °C for 24 h, and then the vial was cooled down to room temperature. The polymer sample (if it still exists) was washed by dichloromethane and dried under reduced pressure, at 80 °C, overnight. After cooling down to room temperature, the sample was weighed, and the gel fraction was calculated.
From the dynamic experiments carried out by using a thermobalance, the mass loss of the sample is recorded. The degree of conversion is defined as follows:
(
where m is the mass corresponding to a temperature T, m
In non-isothermal kinetics of heterogeneous condensed phase reactions, it is usually accepted that the reaction rate is given by Equation (
(
where α is the degree of conversion, T is temperature, t time, f (α) is the differential conversion function, R is the gas constant, β is the linear constant heating rate β = dT/dt and A and E are the pre-exponential factor and the activation energy given by the Arrhenius equation.
By integrating Equation (
(
where g (α) is the integral conversion function.
By using the Coats-Redfern approximation to solve Equation (
(
For each conversion degree, the linear plot of ln(β/T
By integrating Equation (
(
This equation relates, for each conversion, the temperature and the time of degradation. The constant ln [g(α)/A]] is directly related to the value ln [AR/g(α)E] by E/R, which can be deduced from the non-isothermal adjustment (Equation (
In this work, we used ln [AR/g(α)E] and E/R obtained by dynamic experiments and Equations (
The viscoelastic properties of poly(thiourethane) thermosets prepared from stoichiometric amounts of HDI and S3, in the presence of a base catalyst (1MI) and without using any solvent, were firstly investigated. In the presence of a base, the thiol-isocyanate reaction is very fast, and the system is difficult to process due to the low pK
Based on the expertise of our research group on temporal and kinetic control of the curing processes, we chose as the catalyst a base generator that releases the base after the application of an external stimulus, as shown in Scheme 3.
The use of such latent bases could also allow us to increase the amount of catalyst in the formulation, which hypothetically could lead to an acceleration of the exchange mechanism.
The effect of using the BG on the thermal and rheological properties of the material was evaluated and compared with a similar system that was prepared by using the corresponding base catalyst. We selected as the base the one with lower pK
The thermal behavior of the materials prepared with the same mol proportion of 1MI and BG1MI was compared by means of TGA and DMTA tests. The plots are shown in Supplementary Materials Figures S1 and S2 in the supporting information. The use of BG1MI instead of 1MI does not affect the thermal stability of the material, and the TGA curves appear almost overlapped, with a 2% of weight loss at 269 °C for the material obtained with 1MI and at 271 °C for the other one. The peak of tan δ obtained by DMTA remains practically unchanged for both materials, with a shift in the temperature peak of only 2 °C higher in the case of the material prepared with 1MI. These results put in evidence that the use of the base generator derivative facilitates the manipulation of the initial formulation without any impact on the thermal characteristics of the material.
The stress-relaxation curves obtained by DMTA of both PTUs are presented in Figure 1.
In the figure, we can see how the materials relax the stress at 180 °C, showing a quite slow relaxation process in both cases. However, it is evident that the use of BG1MI affects positively the relaxation rate if compared with 1MI. This effect is probably due to the presence of tetraphenylboronic acid released during the activation of the 1-methylimidazolium tetraphenylborate that acts as an additional catalyst. From this result, we can deduce that 1MI at this concentration is not an efficient catalyst to enhance the carbamate exchange process. Therefore, the effect on the stress-relaxation process of base generators prepared from DMAP, DBN, DBU and TBD, with higher values of pK
The relaxation rates of PTUs prepared with the different base generators were investigated at elevated temperatures (Figure 2), and the resulting relaxation behavior was compared with a PTU prepared by using DBTDL as the catalyst [[
The relaxation times (τ
To see the effect of the proportion of BGDBU on the relaxation rate, we prepared a sample by doubling the amount of the catalyst (0.1% mol) in the formulation, proving that the rate of the exchange process increased significantly, as it can be seen in Figure 3. The τ
According to these results, we selected as the optimal catalyst molar proportion 0.1% to follow the study. Higher proportions of catalyst were not used, since the BG becomes difficult to dissolve in the thiol during the preparation of the formulation.
The second main objective of this work consisted in the evaluation of the influence of the isocyanate/thiol ratio on the exchange mechanism and the relaxation behavior. The materials tested in the following sections were prepared to build evidence of the associative and/or dissociative trans-thiocarbamoylation exchange mechanism and their effect on the viscoelastic behavior. It was reported by Torkelson [[
A series of stress-relaxation experiments were conducted on these three different systems, calculating their activation energy as it is represented in Figure 4.
As shown in Figure 4, a fast stress-relaxation was observed for all the systems at all temperatures analyzed. The relaxation times are very similar for the three PTUs prepared, and the characteristic time of relaxation decreased from approximately 30 to 1 min as the temperature increased from 130 to 170 °C. The highest relaxation rate was obtained for the system with an excess of isocyanate.
The logarithm of the relaxation times was plotted as a function of the inverse of temperature for all the materials prepared, fitting perfectly with an Arrhenius-like behavior. The activation energies and the topology freezing temperatures were calculated by the Arrhenius equation, using the slope of the straight line. The values obtained are collected in Table 3. As it can be appreciated, the activation energies are similar for the three PTUs studied, around 120–130 kJ/mol. Moreover, from the stress-relaxation experiments, no significant differences in the behavior are appreciable that can lead us to consider a clear change in the trans-thiocarbamoylation pathways by changing the stoichiometry of the formulation in the selected range. It can be observed that the materials prepared with an excess of thiol, which should enhance the associative exchange rate, are the slowest materials in the relaxation process. This behavior seems to suggest that dissociative mechanism is predominant with the catalyst used.
The thermal stability of the PTUs prepared by using BGDBU as the catalyst was evaluated by TGA and compared with the material prepared from DBTDL, since it is important to prove that, at the reshaping temperature, the material does not lose any volatile product. Moreover, the study of the thermal degradation mechanism could help to disclose about the adoption of a dissociative interchange mechanism. The TGA and the derivative of the loss weight curve (DTG) curves are shown in Figure 5.
As we can see from Figure 5, the thermal stability of the material based on BGDBU decreases if compared with the PTU prepared with the acidic catalyst, DBTDL.
The DTG plot shows three clear degradation steps for both materials. The maximum of the first degradation peak appears at 305 °C for the DBTDL sample, whereas, in the case of BGDBU samples, it is shifted to a lower temperature (around 220 °C) and becomes well separated from the second one. Since the first degradation step was attributed, in several studies on PU [[
By analyzing the TGA curves of the PTUs prepared with different thiol-isocyanate stoichiometric ratio (see Figure 6) and in presence of BGDBU, it can be observed that the 2% of loss in weight (given in Table 4) occurs at temperatures around 210 °C for all the samples, without significant differences among them. DTG curves are very similar for all the materials and show the three decomposition processes. The first degradation step, at around 220 °C, relates to a 15% of mass loss; the second peak, at 340 °C, is related to a mass loss of an additional 45%; and the third step is associated with a complete degradation of the network. No differences in the thermal degradation are appreciable on changing the stoichiometry of one of the monomers used in the preparation of the material, and this could be related to the similar behavior observed in the relaxation experiments of the three samples.
The good separation between the three different processes has driven us to perform a kinetic analysis of the degradation process. Thus, we carried out an isoconversional analysis of the three degradation steps, to calculate the activation energy and the kinetic parameters which best describe the degradation processes of PTUs. To this aim, a series of TGA dynamic experiments were performed at different heating rates, as shown in Supplementary Materials Figure S3, for the stoichiometric PTU. As expected, as the heating rate increases, the TGA and DTG curves shift to higher temperatures.
From the dynamic isoconversional kinetic study, we calculated the activation energy for the three different materials, using the Equation (
The kinetic of each degradation step was studied, considering in Equation (
For a better understanding of the processes that occur during the decomposition of the PTU, FTIR analyses were performed on the partially degraded material after step 1 and step 2 of degradation. The material was heated up in the TGA until finishing each degradation step, and then the partially degraded samples were analyzed by FTIR-ATR at room temperature. In Supplementary Materials Figure S4, the FTIR of the initial and the degraded material until 260 °C are presented. From the spectra it is possible to observe how the characteristic carbonyl band of thiourethane at 1670 cm
Additionally, the volatiles produced during the first decomposition step were studied by gas chromatography coupled to a mass spectrometer detector (GC-MS), following the procedure described in the Section 2.8. Supplementary Materials Figure S6 illustrates the chromatographic profile of volatile compounds obtained by GC-MS, where two main peaks were detected and identified by mass spectroscopy (Supplementary Materials Figures S7 and S8). The peak at 2.28 min is due to the formation of benzene in the degradation, and it can be associated to the aromatic ring of the tetraphenylboronic acid. The peak at 1.21 min is due to the formation of carbonyl sulfide, which indicates that, at high temperatures, the thiourethane moiety is decomposed mainly through a CSO elimination, which is the responsible of the initial weight loss. This is in accordance with the results of the degradation study previously reported by Rogulska et al. [[
Although no significant differences in weight loss were determined by TGA on changing the stoichiometry of the material, FTIR-ATR was used to analyze structural effects of a change in stoichiometric when heating the samples at 180 °C during 4 h. We have selected this temperature to detect the occurrence of a possible dissociation process of poly(thiourethane) in isocyanate and thiol groups, which could be evidenced by the appearance of an isocyanate absorption peak. For comparison purposes, we also analyzed a PTU sample obtained with DBTDL. The spectra registered during the thermal treatment are shown in Figure 7.
It is possible to see in Figure 7 that the starting spectra of the samples prepared with BGDBU or DBTDL are identical, and only in the initial spectra of the PTU with an excess of isocyanate can be appreciated the typical absorption peak at 2270 cm
To investigate the recyclability of the three different crosslinked PTUs prepared, the materials were cut into small pieces and hot-pressed at 15 MPa, in an aluminum mold. The temperature of the recycling process was selected according to the isothermal degradation times estimated by using Equation (
Figure 8 shows the pictures taken from the original, grinded and reprocessed samples prepared as described in the experimental part. As we can see, the original and the recycled materials both show good transparency. It can be also appreciated the excellent uniformity of the recycled sample.
To evaluate the recyclability of the PTUs prepared, the original and the recycled samples were subjected to uniaxial tensile test, DMTA and FTIR analysis. Dog-bone-shaped samples of the original and recycled PTUs were tested at room temperature, until break, and their stress-strain behavior was analyzed, obtaining the tensile modulus and the yield stress and strain. These mechanical parameters, as well as thermomechanical data obtained, are collected in Table 5.
From the tensile tests, it can be observed that the mechanical properties of the PTUs after the first recycling process are very similar to those of the original samples for all the systems studied. The stoichiometric material, as expected, is the most rigid at room temperature, showing the highest values of tensile modulus and yield stress, in the original and recycled samples. Moreover, the recycled sample of the stoichiometric material presents excellent mechanical performance in comparison with the original one, i.e., a tensile modulus around 85% of the original modulus and a yield stress of almost 96%. The materials prepared out of stoichiometry also showed a very similar mechanical behavior, with a tensile modulus of 1.7 and 1.8 GPa for the material prepared with an excess of isocyanate and thiol, respectively. The mechanical properties of the recycled samples in these two materials are perfectly recovered, with values that fall within the range of the experimental error.
To compare the thermomechanical behavior of the original and recycled samples, the variation of the storage modulus and the tan δ curves with the temperature extracted from DMTA analysis are presented in Figure 9, and the most representative data are collected in Table 5. In the figure, it can be appreciated that the thermomechanical behavior of the original and recycled materials is very similar, with both the shape and position of tan δ curves remaining unaltered. From these results, we can assert that these thermosets can be recycled without significant difference in the thermomechanical properties, showing the promising use of BGDBU as an efficient catalyst to the trans-thiocarbamoylation exchange of poly(thiourethane)s.
To confirm that no changes in the chemical structure of the materials have occurred after the recycling process, the FTIR spectra of the original and recycled materials were recorded and overlapped (see Figure 10). The FTIR spectra of the PTUs prepared remain almost identical after the recycling process. The only appreciable changes are detected in the material prepared with an excess of isocyanate, where it is possible to see how the absorption band of the isocyanate cannot be detected in the recycled material. Thus, we can confirm the recyclability of this class of thermosets, even in different stoichiometric ratios, in the presence of a strong basic and nucleophilic catalyst, as reported by Bowman [[
Typical thermosets do not solubilize in any type of solvent. However, covalent adaptable networks can be solubilized by a suitable solvent when their chemical-exchange process proceeds through a dissociative mechanism. The possibility to undergo complete dissolution of these reprocessable thermosets could constitute an alternative chemical recycling route to the mechanical recycling previously studied.
Hillmeyer et al. [[
In this work, the catalytic effectiveness of tetraphenylborate derivatives of DBN, DBU and TBD in the trans-thiocarbamoylation process in poly(thiourethane) thermosets was demonstrated. The relaxation rate of the PTUs prepared with the abovementioned catalysts is faster than the one prepared with DBTDL. The great catalytic effect is due not only to the high basicity and nucleophilicity of the amines, but also to the presence of the tetraphenylboric acid, which is formed when the amine is thermally released.
Moreover, the use of the tetraphenylborate salts of these amines is advantageous in comparison to the use of the free amines, concerning the manipulation of the formulation: It helps to delay the pot-life of the mixture and gives the possibility to increase the amount of catalyst in the formulation without using any solvent, increasing thus the exchange rate of the covalent bond.
The poly(thiourethane)s networks show, in the presence of these new catalysts, a vitrimer-like behavior with the possibility to be reshaped and recycled. The materials obtained degrade at temperatures lower than those obtained with DBTDL as the catalyst; therefore, the recycling parameters, temperature and time were properly selected by a thermal degradation study, demonstrating that, under adequate recycling conditions, the materials keep their good thermal and mechanical performance and their chemical structure. Materials with a 10% of molar stoichiometric imbalance, in isocyanate or thiol groups, do not show appreciable differences in relaxation rate and recycling capability.
Moreover, it has been demonstrated that the thermal degradation leads to an initial loss of carbonyl sulfide without dissociation of thiourethane moieties to isocyanate and thiol groups. This observation, together with the proved solubility of the material in DMSO and the unaffected relaxation rate in the presence of an excess of thiol, confirms that the exchange mechanism is mainly dissociative, but with a very fast equilibrium process that leads to a vitrimer-like behavior.
Graph: Scheme 1 Associative-dissociative trans-thiocarbamoylation exchange mechanisms catalyzed by bases and nucleophiles, proposed by Bowman et al. [[
Graph: Scheme 2 Proposed mechanisms of poly(thiouretane) formation in the presence of a basic catalyst.
Graph: Scheme 3 Thermal activation of the base generator BG1MI.
Graph: Figure 1 Normalized stress-relaxation behavior at 180 °C of the samples prepared with the same mol amount of 1MI and BG1MI.
Graph: Figure 2 Normalized stress-relaxation plot at 180 °C of different poly(thiourethane) samples prepared with 0.05% mol of the different catalysts.
Graph: Figure 3 Normalized stress-relaxation plot at 180 °C of PTUs prepared with different mol proportions of BGDBU.
Graph: Figure 4 (A–C) Normalized stress-relaxation plots as a function of time at various temperatures from 130 to 170 °C during 30 min. (D) Arrhenius plot for the different poly(thiourethane) samples with 0.1% of BGDBU.
Graph: Figure 5 (A) Thermogravimetric analysis (TGA) curves and (B) DTG curves of the poly(thiourethane)s prepared with 0.1% of BGDBU and 1% of DBTDL.
Graph: Figure 6 TGA curves (A) and DTG curves (B) of the poly(thiourethane)s prepared from different stoichiometric formulations with 0.1% of BGDBU.
Graph: Figure 7 FTIR of the different poly(thiourethane) samples with 0.1% of BGDBU and 1% of DBTDL registered during 4 h at 180 °C.
Graph: Figure 8 Photographs of the original, grinded and recycled sample prepared from stoichiometric PTU with a 0.1% of BGDBU.
Graph: Figure 9 Dependence of tan δ and storage modulus versus temperature of the different materials prepared before and after recycling.
Graph: Figure 10 FTIR of poly(thiourethane)s before and after recycling registered at room temperature.
Table 1 Formulations used to prepare poly(thiourethane) thermosets.
Formulation Diisocyanate Thiol BGDBU 0.1% 10% exc HDI 2.09 3.00 9.30 Stochiometric 1.90 3.00 9.30 10% exc S3 1.90 3.30 9.30
Table 2 Characteristic relaxation time in stress-relaxation experiments for poly(thiourethane) (PTU) samples with a 0.05% in mol of the different catalysts used.
Samples τ0.37 (min) 1 BG1MI Not reached in 1 h BGDMAP 45 BGTBD 1.7 BGDBN 1.7 BGDBU 1.3 DBTDL Not reached in 1 h
Table 3 Activation energy, topology freezing temperature and time for reach a value of σ/σ
Samples 10% exc HDI 132 94 27.8 11.7 4.7 1.9 0.8 Stoichiometric 117 87 21.5 13.0 4.8 1.9 1.1 10% exc S3 127 96 45.2 16.0 6.9 2.7 1.5
Table 4 Temperature of initial degradation and activation energies of the degradation steps of the PTUs prepared with 0.1% of BGDBU.
Samples T2% Εa I peak Εa II peak Εa III peak 10% exc HDI 209 127 170 257 Stoichiometric 209 130 158 275 10% exc S3 212 139 149 259
Table 5 Mechanical and thermomechanical results of the original and recycled PTUs materials tested. The average value of the results for three different samples tested is presented. Coefficients of variations are less than 7% for stress-and-strain results and less than 5% for the tensile moduli.
Original Samples FWHM 5 10% exc HDI 1.7 30.4 3.0 56.3 9.6 11.4 Stoichiometric 2.5 45.1 2.5 57.4 9.9 12.7 10% exc S3 1.8 31.4 2.7 49.0 10.8 9.6 10% exc HDI 1.6 28.9 3.1 56.3 10.7 11.3 Stoichiometric 2.1 43.3 2.8 57.5 9.2 13.0 10% exc S3 1.8 32.2 2.5 49.2 10.9 9.2
F.G. performed the experimental part, S.M. performed the recycling studies, A.S., S.D.l.F. and X.R. conceived and designed the experiments. All the authors analyzed the data and discuss the results. F.G. wrote the article and A.S., S.D.l.F and X.R. revised it. All authors have read and agreed to the published version of the manuscript.
This research was funded by MCIU (Ministerio de Ciencia, Innovación y Universidades) and FEDER (Fondo Europeo de Desarrollo Regional) (MAT2017-82849-C2-1-R and MAT2017-82849-C2-2-R), Generalitat de Catalunya (2017-SGR-77) and Universitat Rovira i Virgili (2019PFR-URV-81).
The authors declare no conflict of interest.
The authors would like to thank MCIU (Ministerio de Ciencia, Innovación y Universidades) and FEDER (Fondo Europeo de Desarrollo Regional) (MAT2017-82849-C2-1-R and MAT2017-82849-C2-2-R), Generalitat de Catalunya (2017-SGR-77) and Universitat Rovira i Virgili (2019PFR-URV-81 for the financial support.
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By Francesco Gamardella; Sara Muñoz; Silvia De la Flor; Xavier Ramis and Angels Serra
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