Baptiste Quienne

Rinaldo Poli

Julien Pinaud

Sylvain Caillol

Baptiste Quienne

Enhanced aminolysis of cyclic carbonates by β-hydroxylamines for the production of fully biobased polyhydroxyurethanes

Keywords: DFT, reactivity, β-hydroxylamines, cyclic carbonate, polyhydroxyurethanes, biobased polymers

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Polyurethanes (PUs), ranking 6 th among all polymers based on annual worldwide production, are commonplace in everyday life as foams, coatings, adhesives, elastomers, insulation, composites, and so forth. [START_REF] Randall | The Polyurethanes Book[END_REF][START_REF] Tenorio-Alfonso | [END_REF][3] They are present in a wide range of applications due to the numerous industrially available polyols, which allow to cover a large panel of properties. 4,5 However, their synthesis requires the use of isocyanates, known to be harmful for human health and for the environment. 6,7 In order to limit the use of isocyanates, the development of isocyanate-free PUs has emerged over the last decade and has gained increasing attention in both the academic and industrial communities. [8][9][10] Hence, the aminolysis of 5-membered cyclic carbonates, which yields polyhydroxyurethanes (PHUs), is nowadays the most promising and described route to non-isocyanate polyurethanes (NIPUs). [11][12][13][14] This route has the advantage that these carbonates are easily obtained by the carbonation of commercial epoxides.

Moreover, this route valorizes CO 2 as a cheap, renewable and non-toxic resource, which makes this pathway the least toxic and most eco-friendly route to NIPU and PHUs. [15][16][17] Indeed, larger ring size cyclic carbonates require the use of toxic agents such as phosgene or cyclic chloroformates for their synthesis. 18,19 However, five-membered cyclic carbonates suffer from lower reactivity compared to isocyanates or larger cyclic carbonates. 20,21 Therefore, the aminolysis of cyclic carbonates has been thoroughly investigated in order to understand and optimize the ring opening reaction. For this reason, the influence of many parameters such as solvent, temperature, catalyst, cyclic carbonate substituents or amine structure has been studied. [22][23][24] However, depending of the final application, the use of solvent or catalyst is not always possible. Therefore, the choice of both cyclic carbonate and amine structures remains a crucial step. Five-membered cyclic carbonates are more reactive when they are substituted by electron withdrawing groups, because of increased electrophilicity of the carbon from the carbonyl group. 25 On the other hand, primary amines with NH 2 attached to a primary carbon atom with α or β electron withdrawing substituents are the most reactive amines, while primary aromatic amines and aliphatic ones with NH 2 attached to a secondary carbon atom are the less reactive ones. 26 Furthermore, secondary amines can also react with cyclic carbonates but only at higher temperatures, around 90 °C for the most reactive amines and up to 200 °C for sterically hindered ones. 27 Recently, our team demonstrated the higher reactivity of primary β-hydroxylamines toward epoxy monomers compared to alkylamines. 28 Indeed, 1,2-epoxy-3-phenoxypropane was fully converted with 1-amino-2-butanol after only 20 min at 60 °C in bulk, whereas only 31% was consumed with butylamine under the same conditions and full conversion took 2 hours. In addition to high reactivity, the use of β-hydroxylamines also presents strong advantages. Indeed, biobased β-hydroxylamines can be easily synthesized by reacting biobased epoxides with aqueous ammonia. 29,30 This one-step process is inexpensive, does not require complex purifications and can be easily implemented. While the carbonation of a polyfunctional epoxide affords a polyfunctional cyclic carbonate, the ring opening with aqueous ammonia leads to the equivalent polyfunctional β-hydroxylamines. 30 Di-, tri-and tetraβ-hydroxylamines were already reacted with polyfunctional cyclic carbonates by Blattmann et al. for the synthesis of 100% biobased PHU thermosets. 31 However, the reactivity of the β-hydroxylamines were only assessed by comparing the gelation time of different formulations. The authors reported longer gelation times for the formulations containing higher amounts of β-hydroxylamines and concluded that they are less reactive, which is quite surprising in regards to their reactivity with epoxides.

We thus thoroughly investigated, for the first time, the aminolysis of a cyclic carbonate by comparing a β-hydroxylamine to non-functionalized alkylamine. Along with the kinetic study of the ethylene carbonate aminolysis with 1-amino-2-butanol and butylamine by 1 H NMR, quantum chemical calculations of the mechanism were carried out by the DFT method. The higher reactivity of βhydroxylamines toward cyclic carbonates was then confirmed by rheological experiments and 100% biobased thermosets were synthesized and characterized by structural and thermo-mechanical analyses (IR, DSC, TGA, DMA). The higher reactivity of β-hydroxylamines was confirmed by the gelation time gap between the two formulations studied.

Experimental Materials

Ethylene carbonate (purity 99%) was purchased from Alfa Aesar (Kandel, Germany). Butylamine (purity 99.5%), tetrabutylammonium bromide (purity 99%) and trimethylolpropane triglycidyl ether were purchased from Sigma-Aldrich Merck (Darmstadt, Germany). 1-Amino-2-butanol (purity 98.5%) was purchased from TCI EUROPE N.V (Zwijndrecht, Belgium). N,N'-Carbodiimidazole (purity 98%) and glycerol carbonate (purity 90%) were purchased from abcr GmbH (Karlsruhe, Germany). Aqueous ammonia (25% NH 3 ) and ethyl acetate were purchased from VWR International S.A.S (Fontenay-sous-Bois, France). Pripol TM 1012 was purchased from Croda (East Cowick, United Kingdom). All materials were used as received. The NMR solvent CDCl 3 was purchased from Eurisotop (Saint-Aubin, France).

Characterizations Nuclear Magnetic Resonance

Proton Nuclear Magnetic Resonance ( 1 H NMR) analyses were carried out in deuterated chloroform (CDCl 3 , 99.50% isotopic purity) using Bruker Avance III 400 MHz NMR spectrometer at a temperature of 25 °C.

Fourier Transform Infrared Spectroscopy

Infrared (IR) spectra were recorded on a Nicolet 210 Fourier transform infrared (FTIR) spectrometer.

The characteristic IR absorptions mentioned in the text are reported in cm -1 . Materials analyses were recorded using an ATR accessory.

Thermogravimetric Analyses

Thermogravimetric Analyses (TGA) were carried out using TG 209F1 apparatus (Netzch).

Approximately 10 mg of sample were placed in an aluminum crucible and heated from room temperature to 580 °C at a heating rate of 20 °C/min under nitrogen atmosphere (60 mL/min).

Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) analyses were carried out using a NETZSCH DSC200F3 calorimeter, which was calibrated using indium, n-octadecane and n-octane standards. Nitrogen was used as purge gas. Approximately 10 mg of sample were placed in a perforated aluminum pan and the thermal properties were recorded between -150 °C and 190 °C at 20 °C/min to observe the glass transition temperature. The T g values were measured on the second heating ramp to erase the thermal history of the polymer. All the reported temperatures are average values.

Dynamic mechanical analyses

Dynamic mechanical analyses (DMA) were carried out on Metravib DMA 25 with Dynatest 6.8 software. Samples were tested according to uniaxial tension mode while heating at a rate of 3 °C/min from -100 °C to 100 °C at a frequency of 1 Hz with a fixed strain of 10 -5 m. All the thermosets were analyzed at least two times for repeatability.

Rheology experiments

Gelation times were measured using a Thermo Fisher HAAKE MARS rheometer with a plate-plate geometry with a diameter of 25 mm. Gelation times were analyzed by the crossover of the storage modulus (G') and loss modulus (G") during an oscillatory experiment at 1 Hz at 80 °C and 10 Pa of stress, according to the previously determined linear domain.

Titration of the amine equivalent weight of Priamine™1071 by 1 H NMR

The Amine Equivalent Weight (AEW) is the amount of product needed for one equivalent of reactive amine function. It was determined by [START_REF] Randall | The Polyurethanes Book[END_REF] H NMR using an internal standard (benzophenone). A known mass of product and benzophenone were poured into an NMR tube and 500 µL of CDCl 3 were added.

The AEW was determined using Equation 1 by comparing the integration value of the signals assigned to the benzophenone protons (7.5-7.8 ppm) with the integration of the signals arising from amine moiety protons (2.67 ppm). Determination of the secondary amine content of TMPT-β-HA by 13 C NMR TMPT-β-HA was dissolved in CD 3 OD at a concentration of 200 mg/mL and 13 C NMR spectra were carried out on a Bruker spectrometer Avance I at 25 °C using the following parameters: 30 kHz for spectral width, 10 kHz for transmitter frequency offset, 2 s for acquisition time, 2864 scans. Quantitative conditions could be approached (as best without T1 measurement) with a 30° pulse (3.3 µs) and a relaxation delay of 30 s (D1). [START_REF] Randall | The Polyurethanes Book[END_REF] H decoupling was applied during acquisition (waltz16 method with a pulse of 90 µs). Pulse program was zgig30 from the Bruker library. The spectrum was calibrated with the CH 3 peak at 8.00 ppm.

Titration of the amine equivalent weight of TMPT-β-HA by 13

C NMR

The AEW of TMPT-β-Ha could not be measured by 1 H NMR because of the overlap of the primary and secondary amines α proton peaks. Therefore, it was determined by 13 C NMR using the approached quantitative conditions, presented in the previous section, with an additional internal standard (phenol). A known weight of product and phenol were poured into an NMR tube and 500 µL of CDCl 3 were added. The AEW was determined using Equation 2 by comparing the integrals of the phenol carbons (148-160 ppm) and of the primary amine moiety carbons (44.26-45.14 ppm). Equation 2* 𝐴𝐸𝑊 = ∫𝑃ℎ𝑂𝐻 ∫𝑎𝑚𝑖𝑛𝑒 * 𝐻 𝑃ℎ𝑂𝐻 𝑚 𝑎𝑚𝑖𝑛𝑒 𝑚 𝑃ℎ𝑂𝐻 * 𝑀 𝑃ℎ𝑂𝐻 ∫ PhOH : integration value of the phenol carbons peak; ∫ amine : integration value of the peak from carbons in α of the primary amine function; H PhOH : number of phenol carbons; m amine : product mass; m PhOH : benzophenone mass; M PhOH : phenol molecular weight.

Titration of the carbonate equivalent weight by 1 H NMR

The Carbonate Equivalent Weight (CEW) is the amount of product needed for one equivalent of reactive cyclic carbonate function. It was determined by [START_REF] Randall | The Polyurethanes Book[END_REF] H NMR using an internal standard (benzophenone). A known weight of product and benzophenone were poured into an NMR tube and 500 µL of CDCl 3 were added. The CEW was determined using Equation 3 by comparing the integration values of the peak corresponding to benzophenone protons (7.5-7.8 ppm) with those of the cyclic carbonate moiety protons (4.81 ppm for TMPTC and 4.91 ppm for Pripol-C 5 ).

Cross-linking density

From rubber elasticity theory, the uniaxial stretching was studied on the rubbery plateau at T > T α +50, and at very small deformations. Under these hypotheses, the cross-linking density (ν'), can be obtained from Equation 4, where E' is the storage modulus (Pa), R is gas constant (8.314 J.K -1 .mol -1 ) and T α is the temperature, in K, of transition from vitreous to rubber domain, determined at the maximum of the tan δ curve. Calculated values are only qualitative and not absolute. Equation 4𝜈 ′ = 𝐸 ′ 𝑎𝑡 𝑇𝛼 + 50 3𝑅𝑇 𝛼 + 50

Swelling index

Three samples from the same material, of around 20 mg each, were separately immersed in THF for 24 h. The swelling index (SI) was calculated using Equation 5, where m 2 is the mass of the swollen material and m 1 is the initial mass. Reported swelling index are average values of the three sample.

Equation 5

𝑆𝐼 = 𝑚 2 -𝑚 1 𝑚 1 × 100

Gel Content

After the SI measurements, the three samples were dried in a ventilated oven at 70 °C for 24 h. The gel content (GC) was calculated using Equation 6, where m 3 is the mass of the dried material and m 1 is the initial mass. Reported gel content are average values of the three sample.

Equation 6 𝐺𝐶 = 𝑚 3 𝑚 1 × 100
General procedure of 1 H NMR kinetic monitoring Typically, 8.8 mg of ethylene carbonate (0.1 mmol, 1 eq.) were placed in 1.490 g of CDCl 3 (1 mL) in a hemolysis tube, followed by stirring using a vortex. Once the carbonate was fully dissolved, the desired amount of amine was carefully added and the mixture was homogenized with 30 s of vortex stirring.

Then, 450 µL of the solution were transferred to a NMR tube and the reaction was monitored at fixed time intervals by 1 H NMR analyses using a Bruker Avance III 400 MHz NMR spectrometer.

Computational details

All calculations were carried out using the Gaussian09 suite of programs. 32 Gas-phase geometry optimizations were performed without any symmetry constraint using the BP86 functional and the 6-311G (d,p) basis functions for all atoms. A correction for dispersion was also included during the geometry optimizations using Grimme's D3 empirical method (BP86-D3). 33 All final geometries were characterized as local minima (or first-order saddle points for the transitions states) by verifying that all second derivatives of the energy were positive (or that only one was negative). Thermochemical corrections were obtained on the basis of frequency calculations, using the standard approximations (ideal gas, rigid rotor and harmonic oscillator). The final free energy values also included a solvation correction in chloroform, estimated by the polarizable continuum model using the SMD model with standard parameters. 34 A further correction of 1.95 kcal/mol was applied to bring the G values from the gas phase (1 atm) to the solution (1 mol/L) standard state. 35 The Cartesian coordinates and energies of all optimized structures are deposited as Supporting Information.

Synthesis Synthesis of trimethylolpropane tricarbonate (TMPTC)

Trimethylolpropane triglycidyl ether (TMPTGE; 20 g, 0.139 mol) and tetrabutylammonium bromide (TBAB; 0.6 g, 1.86 mmol) were solubilized in 50 mL of AcOEt and added to a 100 mL sealed reactor.

The reaction was carried out at 80 °C under 20 bars of CO 2 for 72 h. The crude mixture was then washed with water and brine to remove TBAB. The organic layer was then dried with magnesium sulfate and under vacuum. The pure product was obtained as a yellowish viscous liquid (yield = 88%).

Synthesis of trimethylolpropane tri-β-hydroxylamine (TMPT-β-HA)

TMPTGE (10 g, 0.099 mol, 1 eq.) was dissolved in 139 mL of a 25% NH 3 aqueous solution (0.99 mol, 10 eq.) in a sealed reactor. The reaction was carried out at 100 °C for 4 h. The ammonia solution was then evaporated with a rotatory evaporator and then freeze-dried to remove water traces. The pure product was obtained as a yellow viscous liquid (yield = 99%).

CEW

The cyclic carbonate and the amine were stirred using a SpeedMixer TM under vacuum for 5 minutes at 2 500 rpm. Then, the resulting homogeneous mixture was poured in a silicon mold and cured at 80 °C for 10 h, then 6 h at 120 °C and finally 1 h at 150 °C.

Results & Discussion

Kinetic and DFT studies

The aminolysis was first studied by 1 H NMR using ethylene carbonate (EC) as a model five-membered cyclic carbonate, comparing the reactivity of butylamine (BuNH 2 ) and 1-amino-2-butanol (BuNH 2 -βOH). The conversion was monitored by measuring the intensity of the EC signal in 1 H NMR (δ = 4.41 ppm) over time (Figure S1 and Figure S2). The resulting conversion plots are presented in Figure 1. The 1-amino-2-butanol is much more reactive compared to butylamine. Indeed, after one hour, the reaction with the β-hydroxylamine reached a 50% conversion against only 7% for the alkylamine.

However, even after 14 hours of reaction with β-hydroxylamine, the conversion was stalling around 85%. This phenomenon is well-known for the aminolysis reaction, indeed, the formation of the new hydroxyl group during the reaction creates a dense network of hydrogen bonds, increasing the viscosity and decreasing the reagents mobility, which prevents quantitative conversion. Even though the 1-amino-2-butanol contains a pendant hydroxyl group, which further increases the density of hydrogen bonds, the EC conversion could still reach 85%, which is comparable to the literature values (around 80%). 25 On the other hand, the reaction with butylamine only reached a 57% conversion after 14 h. Therefore, the 1-amino-2-butanol hydroxyl group allows to strongly increase the amine reactivity.

To understand this effect, experimental (kinetic) and theoretical (DFT calculations) mechanistic investigations were carried out.

Kinetic studies gave us access to the rate law and rate constants for the reaction with each amine. To this end, observed rate constants were first obtained using a significant excess (from 10 to 50 eq.) of 1-amino-2-butanol and butylamine compared to the EC concentration (0.1 mol/L). The EC decay followed a first order rate law in each case, see Figure 2. The pseudo-first order rate constants, k obs , were determined with Equation 9and are given in Table 1. The dependence of k obs on the amine concentration is shown in Figure 3. The trends are clearly non-linear with an upward curvature (positive second derivative), indicating that the reaction order in amine is greater than one. The data were analyzed on the basis of Equation 10, as already described by Zabalov et al. 36 for the reaction of EC with butylamine in n-butanol and dioxane:

Equation 10 k obs = k 1 [Amine] + k 2 [Amine] 2
The meaning of this rate law is that the reaction may occur by two parallel channels with the participation of one (k 1 ) and two (k 2 ) amine molecules. The k 1 and k 2 values were determined by fitting Equation 2 to the data in Table 1, with a correlation coefficient R² > 0.99 in each case, and the results are given in Table 2. These results are interesting for a few reasons. First of all, the rate law for the reaction with the butylamine exhibits a pure second order for the amine reagent, without significant contribution of the pathway involving only one amine molecule. This result is in contrast with those reported by Zabalov et al. for the same reaction in other solvents, where both pathways contributed to the reaction with [k 1 (L mol -1 s -1 ); k 2 (L 2 mol -2 s -1 )] = (0.75•10 -5 ; 1.88•10 -5 ) in n-butanol at 28 °C36 and (0.24•10 -5 ; 2.5•10 -5 ) in DMSO at 55 °C. 37 However, the k 1 pathway was also found insignificant in dioxane (k 1 = 8.4•10 -8 L mol -1 s -1 , vs. k 2 = 2.98•10 -5 L 2 mol -2 s -1 at 28 °C). 36 Secondly, the reaction in chloroform is much faster, since k 2 = (179 ± 2)•10 -5 L 2 mol -2 s -1 at 25 °C. The rate law for the reaction with 1-amino-2-butanol, on the other hand, in addition to having a greater k 2 value, also shows a relevant contribution of the pathway involving a single amine molecule. The involvement of a single amine molecule is not surprising when the reaction is conducted in a solvent that can participate in the proton transfer chain (e.g. n-butanol), but is less obvious in a solvent that cannot ensure this role such as chloroform.

A rationalization of these differences requires a detailed analysis of the reaction mechanism. The EC aminolysis has previously been analyzed by computational methods by Zabalov et al. [36][37][38] Two main pathways, concerted and stepwise, have been highlighted as shown in Scheme 1. In the concerted pathway, the proton from the nucleophile is directly transferred to the leaving group in one step, . 36 A subsequent thorough analysis of the effect of several MeOH molecules (up to six) for both concerted and stepwise mechanisms revealed the presence of several distinct pathways with similar activation barrier and a complex role of the H-bonding solvent, including the possible stabilization of zwitterionic intermediates (e.g. EC-NHR -•••H 2 X + ). 38 Finally, a recent contribution detailed the catalytic action of guanidine as H-X for the EC aminolysis by MeNH 2 in DMSO. 37 The latter study underlines the assistance of the DMSO solvent for the stabilization of cyclic intermediates and transition states, even though it cannot be directly involved in proton transfer chains. This mechanistic scheme allows us to formulate the following hypotheses. The rate law for the reaction with butylamine in chloroform has only a second order term in amine because the chloroform solvent (like the previously investigated 36 dioxane solvent) cannot play any assistance in the proton transfer chain. The k 2 value in chloroform is much higher than in n-butanol, dioxane or DMSO, because unlike all other solvents, chloroform cannot operate as a proton acceptor in H-bonding. Therefore, the only proton acceptor in the reaction mixture is the EC substrate and consequently the amine can display enhanced reactivity as a proton donor. Conversely, chloroform is a weak proton donor in H-bonding because of the inductive effect of the CCl 3 group. 39 Therefore, it can assist the reaction at the transition state level through a screening of the developed negative charge, similarly (but better) to what was shown for the CH 3 protons of DMSO. 37 Concerning the reaction with 1-amino-2-butanol, we must rationalize two different effects: i) the greater k 2 value relative to butylamine; ii) the contribution of a first-order term in amine (k 1 ). The first effect can be easily rationalized, on the basis of the published computational analysis, by a transition state stabilization linked to the H-bonding ability of the OH substituents. The second effect, given that the solvent cannot participate in the proton chain, suggests that the 1-amino-2-butanol must somehow allow a proton chain with the involvement of a single amine molecule.

The last proposition is the most relevant result of the present kinetic study and was therefore further analyzed by the DFT approach. We wish to underline that the purpose of this DFT investigation was not to quantitatively reproduce the relative rates and activation barriers, because the previously published has demonstrated that this requires the explicit inclusion of several solvent molecules and a lengthy investigation of a large number of pathways. [36][37][38] We only wished to highlight the key role of the β-OH substituent in opening a single amine molecule pathway for the reaction. Therefore, no solvent molecules (CHCl 3 ) were explicitly included in the calculations and the effect of the solvent was only accounted for by the inclusion of a polarizable continuum. To capture the β-OH substituent effect, the chosen computational models for the reagents are CH 3 CH 2 NH 2 (for the alkylamine) and HOCH 2 CH 2 NH 2 (for 1_amino-2-butanol). In addition, our study was limited to the analysis of the stepwise process. Other computation details are given in the Experimental section.

We have comparatively analyzed three reaction pathways: i) the two-molecule (k 2 ) pathway for the alkylamine; ii) the two-molecule pathway for the alkoxylamine; iii) the one-molecule pathway (k 1 ) for the alkoxylamine. The first one only serves as a reference since they do not present unprecedented features relatively to the work of Zabalov et al. [36][37][38] The starting reactant complex features two H bonds, one between a first EtNH 2 molecule as a donor and the EC carbonyl O atom as an acceptor, the second one between the second EtNH 2 as a donor and the first EtNH 2 as an acceptor,

EC•••H 2 N(Et)•••H 2 NEt.
In the optimized structure, the NH 2 group of the second EtNH 2 molecule is suitably positioned for the nucleophilic attack at the carbonyl C atom. The transition state of each step, the tetrahedral intermediate and the product were optimized, yielding the reaction profiles shown in the SI (Figure S3). The highest free energy TS is that of the second step, leading from the tetrahedral intermediate to the opened product, at 16.3 kcal/mol from the reactant complex. The additional EtNH 2 molecule remains H-bonded as proton acceptor to the OH group in both intermediate and product, and also acts as proton donor to the carbonyl group in the product.

The alkoxylamine two-molecule pathway is shown in

[(EC)NHCH 2 CH 2 OH] -[NH 3 CH 2 CH 2 OH] + ,
which sits in a shallow minimum at 7.4 kcal mol -1 from the reactant complex between two transition states (TS1 and TS1') at only slightly higher free energy. This intermediate has the formally negative O atom stabilized by two H-bonds, one from the β-OH of the attacking hydroxylamine and the second one from the cationic NH 3 group of the hydroxylamine shuttle. The tetrahedral intermediate obtained from the charge-separated species via TS1' (at 4.4 kcal mol -1 ) must rearrange to prepare the proton transfer chain for the second step, leading to a second tetrahedral intermediate at 6.0 kcal mol -1 , differing from the previous one only in the H-bonding scheme. The second proton transfer occurs in one step via TS2. This corresponds to the highest point along the free energy profile and the overall barrier from the reactant complex (at 11.4 kcal mol -1 ) is significantly reduced relative to that of the EtNH 2 system (16.3 kcal mol -1 ). The fundamental steps of the two pathways are the same, but the two additional O-H•••O interactions that remain present throughout the reaction profile have a greater stabilizing effect on the intermediates, transition states and product than on the reactant complex. The barrier difference between the one-molecule pathway (Figure 5, 24.8 kcal mol -1 ) and the twomolecule pathway (Figure 4, 11.4 kcal mol -1 ) appears too large when considering that both pathways are competitive. In the real system, this difference may be attenuated by specific stabilizing effects by solvent molecules. In conclusion, the DFT analysis of the single-hydroxylamine pathway, though only qualitative and limited to the stepwise mechanism, indicates that the reaction channel with the firstorder dependence on the amine concentration, in a solvent that cannot participate directly as a proton shuttle in the proton transfer chain, is made possible by the ability of the β-OH group to fulfil this role intramolecularly.

Synthesis of monomers

The PHU synthesis suffers from the low reactivity of five membered cyclic carbonates. Therefore, many studies described the use of catalysts or the optimization of the reaction parameters. 23,24 For certain applications, where solvents and catalysts must be avoided, the monomer structure is the only available parameter to tune the aminolysis reactivity. The kinetic study has highlighted the difference of aminolysis reactivity between alkylamines and β-hydroxylamines. Moreover, β-hydroxylamines can easily be synthesized from biobased epoxy compounds. Therefore, we focused on the synthesis of polyfunctional β-hydroxylamine and polyfunctional cyclic carbonate monomers in order to synthesize 100% biobased PHU thermosets. To this end, two polyfunctional cyclic carbonates and a trifunctional β-hydroxylamine monomer were synthesized (Figure ).

Trimethylolpropane tricarbonate (TMPTC) was accessed by condensation of trimethylolpropane triglycidyl ether (TMPTGE) and CO 2 (20 bar) in ethyl acetate in the presence of tetrabutylammonium bromide (TBAB) as catalyst. Full conversion after 72 h of reaction was confirmed by [START_REF] Randall | The Polyurethanes Book[END_REF] H NMR (Figure S4). On the other hand, aminolysis of TMPTGE with 25% aqueous ammonia solution at 100 °C for 4 h yielded the trimethylolpropane tri-β-hydroxylamine (TMPT-β-HA) equivalent. The epoxides were fully converted with a high β-hydroxylamine content (85%) and around 15% of secondary amines according to approached quantitative 13 C NMR (Figure S7). Moreover, Pripol™ 1012 from Croda, a biobased diacid derived from fatty acids, was reacted with glycerol carbonate in the presence of N,N'carbonyldiimidazole (CDI) in ethyl acetate at 25 °C for 3 days, producing Pripol™ 1012 dicarbonate (Pripol-C 5 ). The reaction was monitored by 1 H NMR and stopped once the full conversion of the acid functions was achieved (Figure S5). To compare two similar thermoset networks, Priamine™ 1071 (Figure S8), a biobased diamine derived from fatty acids, was selected for its similar structure with the Pripol™ 1012.

PHU thermosets

Rheological experiments

Since the higher reactivity of β-hydroxylamines compared to alkyl amines for the aminolysis of cyclic carbonates was demonstrated by the model reactions on EC, the next step was to highlight this reactivity difference in the synthesis of 100% biobased PHU thermosets. In order to compare two structurally similar networks, two simple formulations were defined with the four monomers presented in the previous section. The first one consisted of a TMPTC and Priamine™ 1071 formulation (PHU1), while the second one was composed of TMPT-β-HA and Pripol-C 5 (PHU2). In both formulations, the cyclic carbonate/amine ratio was 1:1. Therefore, both thermoset networks will contain similar aliphatic backbone from Priamine™ 1071 and Pripol™ 1012 and trimethylolpropane backbones.

However, TMPT-β-HA/Pripol-C 5 material would present supplementary ester functions and hydroxyl groups coming from the β-hydroxylamines. Those two functions may increase the hydrogen bond density in the material and could improve its mechanical properties.

The pot life and gelation time of the two formulations were measured by oscillatory rheology. Indeed, the pot life corresponds to the time during which the mixture viscosity remains low enough to allow its processing by casting. Typically, the pot life is determined as the time needed for the mixture to reach a viscosity of 10 Pa.s. On the other hand, the gelation time is the time required for the gelation of the polymer system, resulting from the aminolysis crosslinking. Since the gelation time is representative of the network formation rate, it depends on the monomer functionality. Indeed, the more the monomer functionality increases, the more the gelation time decreases, as the polymer network is achieved at a lower function conversion. Therefore, thanks to the similar structure and functionality of our monomers and formulations, the gelation times of both PHU1 and PHU2 are representative of their reactivity. Typically, the gelation time is determined as the time at which the storage modulus (G') and loss modulus (G") crossover takes place. The oscillatory rheological experiments were performed with plate-plate geometry at 80 °C at 1 Hz and an applied stress of 10 Pa (Figure 7), according to the determined linear domain. Pot life and gelation times of both PHU1 and PHU2 are given in Error! Reference source not found.. PHU1 achieved the gelation point after 5 h 22 with a pot life of 3 h 05, whereas PHU2 crosslinking was quicker with a gelation time of 1 h 04 and a pot life of 50 min. Conversely, in 2016 Blattmann et al. 31 reported prolonged gelation times with the increase of β-hydroxylamine ratio in their formulations, probably because they were using very viscous, highly functional β-hydroxylamines, which entailed a lack of reactive function mobility. Therefore, the crosslinking rate was most likely governed by the mixture physical parameters than the chemical kinetic itself. Hence, gelation times measured was not necessarily representative of the function reactivity. In our case, the use of β-hydroxylamine (TMPT-β-HA) strongly increased the crosslinking aminolysis in comparison to alkylamine (Priamine™ 1071) in agreements with our previous kinetic study.

Thermal properties

The thermal stability of the PHUs was explored by TGA from room temperature to 600 °C under a N 2 atmosphere (Figure S12). PHU2 degraded at lower temperature (T d5% = 246 °C) compared to PHU1 (T d5% = 300 °C). Indeed, ester functions are less resistant to temperature than hydroxyurethane linkages.

Hence, the thermal stability of PHU2 was strongly reduced. However, the two thermosets had close char values, 8 % and 10 % for PHU1 and PHU2 respectively.

Conclusion

β-hydroxylamines are readily available from biobased epoxides by aminolysis with ammonia. Indeed, polyfunctional β-hydroxylamines are obtained through a one-step process without involving complex purifications. In addition to their eco-friendly synthesis, we have highlighted their higher reactivity toward cyclic carbonates compared to conventional alkylamines. Kinetic studies have revealed that butylamine exhibits a pure second order rate law for the EC aminolysis in CDCl 3 , whereas both first and second order pathways contribute to the aminolysis with 1-amino-2-butanol. Moreover, 1-amino-2butanol had a greater k 2 value of 5.6•10 -3 L² mol -2 s -1 against 1.79•10 -3 L² mol -2 s -1 for butylamine, which also justifies its higher reactivity. A DFT investigation highlighted the key role of the β-OH substituent in lowering the activation barrier through H-bonding assistance relative to the alkylamine. The β-OH substituent is able to participate in the proton transfer chain both inter-and intramolecularly.

Nevertheless, the single molecule pathway still represents a minor contribution relative to the twohydroxylamine pathway.

Furthermore, the higher β-hydroxylamine reactivity toward cyclic carbonates was confirmed for PHU thermoset synthesis by rheology. Indeed, the gelation time was reduced by a factor of five compared to alkylamine-based PHUs. Moreover, the two fully biobased PHU thermosets synthesized had very similar physico-chemical properties as expected from their complementary formulations. However, PHU2 presented a heterogeneous network with two T α probably due to the ester linkages from Pripol-C 5 . These ester linkages also reduced the thermal stability of PHU2 compared to PHU1. βhydroxylamines are very promising curing agents combining a green synthesis pathway and a higher reactivity toward epoxides and cyclic carbonates.

∫

  PhCOPh : integration of the signal from benzophenone protons; ∫ amine : integration of the signals from protons in α of the amine function; H amine : number of protons in α of the amine function; H PhCOPh : number of benzophenone protons; m amine : product mass; m PhCOPh : benzophenone mass; M PhCOPh : benzophenone molecular weight.

∫

  PhCOPh : integration value of the signal from benzophenone protons; ∫ carbonate : integration value of the signal from protons in α of the carbonate function; H carbonate number of protons in α of the carbonate function; H PhCOPh : number of benzophenone protons; m carbonate : product mass; m PhCOPh : benzophenone mass; M PhCOPh : benzophenone molecular weight.

Figure 1 :

 1 Figure 1 : Comparative kinetic monitoring of the ethylene carbonate (1 mol/L) ring opening with butylamine (1 mol/L) and 1-amino-2-butanol (1 mol/L) by 1 H NMR in CDCl 3 at 25 °C.

Equation 9 :Figure 2 :

 92 Figure 2 : Kinetic of the aminolysis of ethylene carbonate (0.1 mol/L) with (a) butylamine and (b) 1amino-2-butanol at different concentrations by 1 H NMR in CDCl 3 at 25 °C.

Figure 3 :

 3 Figure 3 : Dependence of k obs of ethylene carbonate reaction with butylamine and 1-amino-2-butanol in CDCl 3 on amine concentration at 25 °C.

  mechanism involves proton transit through the carbonyl O atom with generation of a tetrahedral intermediate. However, a simple two-molecule collision (i.e. carbonyl substrate and nucleophile) leads to high barriers in both cases because of the strained 4-membered cycles in the transition state, Scheme 1(a). The reaction has a much lower barrier when assisted by at least one additional proton shuttle molecule (H-X), which must have amphiphilic H-bonding properties, namely both proton and acceptor. Scheme 1(b) shows the example with only one H-X molecule. In a first computational study of the EC aminolysis by MeNH 2 , Zabalov et al. considered only the concerted pathway with one H-X molecule (MeNH 2 or MeOH, models of the n-BuNH 2 reactant and n-BuOH solvent)

Scheme 1 :

 1 Scheme 1: Accepted mechanism for EC aminolysis.

Figure 4 .

 4 The starting reactant complex shows strong O-H•••O interactions between the β-OH groups and the EC O atoms (a ring O atom for one alkoxylamine molecule and the C=O atom for the second one) and an additional N-H•••N bond between the two amine functions. One NH 2 function is once again correctly positioned in front of the EC carbonyl group (N•••C = 2.69 Å), even closer than for the EtNH 2 system (3.06 Å). The NH 2 addition to the carbonyl group is accompanied by transfer of a proton to the second amine NH 2 function. The shuttle for this system occurs in two separate steps, via a charge-separated intermediate,

Figure 4 :

 4 Figure 4 : Free-energy profile for the two-amine-molecule pathway of the ethylene carbonate aminolysis with HOCH 2 CH 2 NH 2 .

Figure 5 :

 5 Figure 5: Free-energy profile for the single-amine-molecule pathway of the ethylene carbonate aminolysis with HOCH 2 CH 2 NH 2 .

Figure 6 :

 6 Figure 6: Monomer structures with their synthetic pathways. Pripol™ 1012 and Priamine™ 1071 representations are ideal structures.

Figure 7 :

 7 Figure 7: Rheological experiments for determination of gelation and pot life time at 80 °C for PHU1 and PHU2 at 1 Hz and 10 Pa of stress.

Figure 8 :

 8 Figure 8: DMA measurements of PHU thermosets (right: storage modulus (E'); left: tan δ).

  2 ), 54.38 (CH 2 -NH-CH 2 dimer), 65.32-66.15 (CH-OH dimer), 70.39 (CH-OH), 73.12 (CH 2 ), 74.89 (CH 2 ), 77.08 (CH 2 ).Materials were synthetized with a cyclic carbonate/amine molar ratio of 1/1. The amine and carbonate masses were calculated with the Equation 7 and Equation8, respectively, where n carbonate and n amine are the number of moles of cyclic carbonate and amine, respectively, and AEW is the amine equivalent weight and CEW is the carbonate equivalent weight.

	1 H NMR (400 MHz, CD 3 OD): δ/ppm = 0.87 (t, CH 3 ), 1.40 (m, CH 2 ), 2.51-3.09 (m, CH 2 -NH 2 ), 2.28-3.77 (m, Equation 7 𝑚 𝑎𝑚𝑖𝑛𝑒 = 𝑛 𝑐𝑎𝑟𝑏𝑜𝑛𝑎𝑡𝑒 AEW CH General procedure of PHU thermoset synthesis Equation 8 𝑚 𝑐𝑎𝑟𝑏𝑜𝑛𝑎𝑡𝑒 = 𝑛 𝑎𝑚𝑖𝑛𝑒

2 ), 3.82 (m, CH-OH).

NH

Table 1 :

 1

Pseudo-first order rate constants for the ethylene carbonate (0.1 mol/L) aminolysis in CDCl 3 at 25 °C. [BuNH 2 -βOH] (mol/L)

k obs x10 3 (s -1 ) [BuNH 2 ] (mol/L) k obs x10 3 (s -1 )

	0.96	20.4 ± 0.2	1.04	1.795 ± 0.006
	2.01	65.6 ± 0.5	2.19	7.54 ± 0.03
	2.96	122 ± 2	2.99	16.8 ± 0.1
	4.02	190 ± 3	4.08	29.5 ± 0.2
	4.93	247 ± 2	5.07	46.0 ± 0.4

Table 2 :

 2 Rate constant for the ethylene carbonate aminolysis in CDCl 3 at 25 °C.

Table 4 :

 4 Physico-chemical properties of PHU thermosets.

	PHU T d5%	Char	T g	T α	E' glassy	E' rubbery	ν'	Swelling index	Gel content
		(°C)	(%)	(°C)	(°C)	(Gpa)	(MPa)	(mol/m 3 )	(%)	(%)
	1	300	8	-5	10	1.28	0.6	76	251	94
	2	246 10	-7	2 / 22	1.44	0.6	72	213	96

Green Chemistry

Table 3: Formulation, pot life and gelation time of PHU1 and PHU2.

Thermoset synthesis

To obtain PHU thermosets, both formulations PHU1 and PHU2 were prepared and cured at 80 °C for 10 h, and 6 h at 120 °C, then post-cured 1 h at 150 °C. The conversion of cyclic carbonates into polyhydroxyurethanes was confirmed by IR spectroscopy with the appearance of the urethane bands at 1787 cm -1 and 1797 cm -1 , for PHU1 and PHU2 respectively (Figure S9). Moreover, PHU2 exhibited the supplementary ester band at 1726 cm -1 from Pripol-C 5 (Figure S10). Furthermore, both thermosets had a high gel content of 94 % and 96 % for PHU1 and PHU2 respectively, which confirmed an efficient crosslinking.

Thermo-mechanical properties

Both PHU1 and PHU2 were first analyzed by DSC to determine their T g (Figure S11), yielding the results summarized in Table 4. As expected, very close T g values were obtained, -5 °C and -7 °C for PHU1 and

Author information

Co-corresponding authors * E-mail address: rinaldo.poli@lcc-toulouse.fr (R. Poli); sylvain.caillol@enscm.fr (S. Caillol)

Notes

The authors declare no competing financial interest


K Abouzaid

David Bassir
email: david.bassir@gmail.com

S Guessasma
email: guessasma@inrae.fr

H Yue

MODELLING THE PROCESS OF FUSED DEPOSITION MODELLING AND THE EFFECT OF TEMPERATURE ON THE MECHANICAL, ROUGHNESS, AND POROSITY PROPERTIES OF RESULTING COMPOSITE PRODUCTS

Keywords: additive manufacturing, fused deposition modelling, composite materials, surface roughness, porosity, parameter identification

HAL is

Introduction

The Additive Manufacturing (AM) is a sophisticated technology used for three-dimensional processing by joining materials, usually layer by layer. This process offers the benefit of creating any complex object starting from a 3D representation of its external surface. It goes with a short realization time, cost efficiency, and minimum need for tooling. A wide variety of materials can be used in this process, such as ceramics, polymers, metals, alloys, and composites [START_REF] Guessasma | Challenges of additive manufacturing technologies from an optimisation perspective[END_REF][START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF], which allow them to invade different industrial sectors including, aerospace [START_REF] Kumar | Current trends of additive manufacturing in the aerospace industry[END_REF], building [START_REF] Nadal | 3D printing for construction: a procedural and material-based approach[END_REF][START_REF] Tay | 3D printing trends in building and construction industry: a review[END_REF][START_REF] Xu | Digital reproduction of historical building ornamental components: From 3D scanning to 3D printing[END_REF], biomedical [START_REF] Mangat | Experimental investigations on natural fiber embedded additive manufacturing-based biodegradable structures for biomedical applications[END_REF][START_REF] Avila | Additive manufacturing of titanium and titanium alloys for biomedical applications[END_REF][START_REF] Liravi | Additive manufacturing of silicone structures: A review and prospective[END_REF][START_REF] Trivedi | Additive manufacturing of pharmaceuticals for precision medicine applications: A review of the promises and perils in implementation[END_REF], education [START_REF] Ford | Invited review article: Where and how 3D printing is used in teaching and education[END_REF], and also to use it for personal needs, such as art and hobbies.

Actually, the AM is the most usable manufacturing process in medical industry to combat the recent pandemic (Covid-19 virus) [START_REF] Srinivasan | Additive Manufacturing and How 3D Printing is Fighting COVID-19[END_REF][START_REF] Zuniga | The role of additive manufacturing and antimicrobial polymers in the COVID-19 pandemic[END_REF][START_REF] Pearce | A review of open source ventilators for COVID-19 and future pandemics[END_REF]. New AM applications have appeared to withstand this virus, such as manufacturing medical face masks.

In a few recent years, the range of AM feedstock has been enlarged significantly by adding new materials to it, such as nanoparticles and smart and biobased materials to expand and to strengthen its application domains by helping in the protection of environment [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF][START_REF] Lee | Fundamentals and applications of 3D printing for novel materials[END_REF][START_REF] Bhushan | An overview of additive manufacturing (3D printing) for microfabrication[END_REF][START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF][START_REF] Cuellar | Additive manufacturing of non-assembly mechanisms[END_REF][START_REF] Rejeski | Research needs and recommendations on environmental implications of additive manufacturing[END_REF].

The AM includes a large number of processes. All they differ in the principle of operation, the layer deposition manner, and the materials used to produce parts. There are three different categories of AM processes -liquid-, solid-, and powder-based ones. Depending on these categories, the material transformation process ranges from polymerization, binding, to melting [START_REF] Teng | A review of defect modeling in laser material processing[END_REF]. In Fig. 1, the AM processes are compared in terms of material and product characteristics [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF][START_REF] Kumar | Comparison of bonding strength of Ti-6Al-4V alloy deposit and substrate processed by laser metal deposition[END_REF][START_REF] Carter | Control of solidification in laser powder bed fusion additive manufacturing using a diode laser fiber array[END_REF][START_REF] Khodabakhshi | Effects of laser additive manufacturing on microstructure and crystallographic texture of austenitic and martensitic stainless steels[END_REF]. The cost and speed are among the primary considerations when choosing an adequate printing machine. The Fused Deposition Modelling (FDM) is the least expensive process, with a low expenditure of energy. For this reason, it is the main technique used nowadays, especially to improve the quality of FDM-printed products, including their mechanical and thermal characteristics.

This article reflects the challenges regarding the parameter identification of printed products and the sensitivity analysis of the parameters affecting their quality the most. It also analyzes their main characteristics, including surface quality, porosity, and mechanical features.

Fused Deposition Modelling (FDM)

The FDM is an additive manufacturing process where thin filaments of a plastic material are used to build objects by adding their layers. It was patented by Scott Crump in 1988 [START_REF] Crump | Apparatus and method for creating three-dimensional objects[END_REF] and commercialized by Stratasys in the USA. The FDM printing process begins with adjustment of the melting temperature. Then, the machine is fueled with a filament feedstock, which is pushed to the heated liquefier via a stepper motor to a pinch roller mechanism to be melted there. The different steps of the feeding process present multiple mechanical and thermal actions (tension, compression, and heating). The melted material obtained is then extruded selectively through a small print nozzle that traces the cross-sectional geometry of parts by roads or by beads given by the STL model (Fig. 2) [START_REF] Goh | Process-structure-properties in polymer additive manufacturing via material extrusion: A review[END_REF][START_REF] Wu | Recent developments in polymers/polymer nanocomposites for additive manufacturing[END_REF][START_REF] Daminabo | Fused deposition modeling-based additive manufacturing (3D printing): techniques for polymer material systems[END_REF].

The nonlinearity of the melt properties generated during printing mainly depends on its temperature and shear rate. The feedstock feed rate is controlled in the liquefier in order to maintain the volumetric flow rate constant. The melt flow in the liquefier is described by the generalized Newtonian power law

τ γ =        Φ 1 m
, where τ is the shear stress,  γ is the shear rate, and m and Φ are material constants. The liquefier geometry is divided into different sections, as shown in Fig. 2 [START_REF] Zhang | Linear-time thermal simulation of as-manufactured fused deposition modeling components[END_REF], and the temperature variation during and after the extrusion process is described by the equation

ρ λ c T t T x T y T z p ∂ ∂ = ∂ ∂ + ∂ ∂ + ∂ ∂         2 2 2 2 2 2 ,
where ρ , c p , and λ are the density, the specific heat capacity (at a constant pressure), and the coefficient of thermal conductivity, respectively. Furthermore, the latent heat in the transition phase of polymers during the extrusion process also cannot be ignored. The enthalpy in this case is

H C T dT = ∫ ρ ( ) ,
where ρ is the density, C is the specific heat, and T is temperature

Parameter Analysis and Discussion

Many researchers have investigated the properties of parts made by the FDM and the process parameters characterizing the deposited layer geometries, which are presented in Figs. 3 and4.

Surface roughness and porosity

Among the critical properties of FDM products treated are their surface quality and dimensional accuracy [START_REF] Qattawi | Design consideration for additive manufacturing: fused deposition modelling[END_REF]. According to the recent reports and results, the surface roughness is affected most by the layer thickness, followed by the raster width and building orientation of a part [START_REF] Vahabli | Improvement of FDM parts' surface quality using optimized neural networks-medical case studies[END_REF]. The latter parameter greatly affects the dimensional accuracy, followed by the layer thickness [START_REF] Mahmood | Taguchi based process optimization for dimension and tolerance control for fused deposition modelling[END_REF]. A small layer thickness, as well as a small raster width of filaments, improves the surface quality.

A void between deposited filaments, called "air gap", which is a crucial parameter of the final part of FDM, has a greater impact on the dimensional accuracy than the surface roughness. There are three types of air gaps. A positive air gap means the presence of a void between roads, a zero gap means that there are no voids between them (they touch each other), and a negative one means that roads overlap. The porosity of a part is indispensable in the case of FDM-mode products, due to the manufacturing method based on road deposition next to each other. The use of negative air gap factors may decrease the porosity, which reduces dimensional errors, but impairs the surface quality of final parts [START_REF] Nouri | Structural imperfections in additive manufacturing perceived from the X-ray micro-tomography perspective[END_REF]. Therefore, to achieve a minimum dimensional error and ensure a good surface quality, it is recommended to use the following combination: a small layer thickness, an average raster width, a zero air gap, and the 0°/90° orientation of rods [START_REF] Garg | Multi-objective optimization of dimensional accuracy, surface roughness and hardness of hybrid investment cast components[END_REF]. Besides, some esthetic defects can be removed by postprocessing techniques, including mechanical ones, such as the milling, the CNC, the abrasive or vibratory bowl machining, or chemical techniques by applying vapor or a slight amount of acetone to dissolve the rough or porous parts in order to achieve the best final quality [START_REF] Chohan | Pre and post processing techniques to improve surface characteristics of FDM parts: a state of art review and future applications[END_REF].

The effect of raster angle on the esthetic properties of final parts have also been investigated. It is found that the effect of this parameter depends on the material printed. Generally, the +45°/-45° angles have found to give the best surface quality and dimensional accuracy [START_REF] Armillotta | Edge quality in fused deposition modeling: I. Definition and analysis[END_REF]. The aesthetic properties of final FDM products depend mainly on the layer thickness and printing orientation.

Mechanical properties

When printing functional parts, their crucial features are Young's modulus, the ultimate tensile strength, yield strength, and ductility. These parameters influence the mechanical properties of FDM-made parts to different degrees.

It is worth mentioning that the main FDM parameter to obtain a functional part is the part fill that specifies the percentage of material [START_REF] Zhou | Modelling and estimation of tensile behaviour of polylactic acid parts manufactured by fused deposition modelling using finite element analysis and knowledge-based library[END_REF][START_REF] Aw | Effect of printing parameters on tensile, dynamic mechanical, and thermoelectric properties of FDM 3D printed CABS/ZnO composites[END_REF][START_REF] Deng | Mechanical properties optimization of poly-ether-ether-ketone via fused deposition modeling[END_REF]. This parameter should be fixed at high values, which are linked to negative values of the air gap parameter.

There exists an extensive range of FDM feedstock materials, of which acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) have been widely studied owing to their good mechanical properties. A literature survey shows that up to 2/3 of the studies reported concern exactly these two materials. It reveals that the building orientation and raster angle are the factors that influence the tensile properties of products, including Young's modulus, the ultimate tensile strength, and ductility most significantly, followed by the layer thickness and other FDM parameters. Building ABS and PLA parts in the plane direction is the optimal direction to ensure the highest Young's modulus [START_REF] Chacón | Additive manufacturing of PLA structures using fused deposition modelling: Effect of process parameters on mechanical properties and their optimal selection[END_REF][START_REF] Liu | Mechanical property parametric appraisal of fused deposition modeling parts based on the gray Taguchi method[END_REF][START_REF] Türk | Mechanical characterization of 3D printed polymers for fiber reinforced polymers processing[END_REF], but the impact of raster angle is substantially different. The highest ultimate strength is obtained by printing at 45° and 0°/90° angles for PLA [START_REF] Liu | Mechanical property parametric appraisal of fused deposition modeling parts based on the gray Taguchi method[END_REF] and ABS [START_REF] Cantrell | Experimental characterization of the mechanical properties of 3D-printed ABS and polycarbonate parts[END_REF][START_REF] Mahmood | Tensile strength of partially filled FFF printed parts: experimental results[END_REF] samples, respectively. The layer thickness is also an important input parameter affecting their tensile properties. It has been shown that small layer thicknesses (less than 1 mm) are the most adequate to realize FDMmade parts with good mechanical characteristics [START_REF] Zhou | Modelling and estimation of tensile behaviour of polylactic acid parts manufactured by fused deposition modelling using finite element analysis and knowledge-based library[END_REF][START_REF] Chacón | Additive manufacturing of PLA structures using fused deposition modelling: Effect of process parameters on mechanical properties and their optimal selection[END_REF][START_REF] Liu | Mechanical property parametric appraisal of fused deposition modeling parts based on the gray Taguchi method[END_REF][START_REF] Song | Measurements of the mechanical response of unidirectional 3D-printed PLA[END_REF]. The interlayer and interraster bondings are among the weakest ones in the FDM process. Therefore, the raster width should be great to cover the full adhesion surface in order to ensure a high degree of cohesion in the part [START_REF] Liu | Mechanical property parametric appraisal of fused deposition modeling parts based on the gray Taguchi method[END_REF][START_REF] Uddin | Evaluating mechanical properties and failure mechanisms of fused deposition modeling acrylonitrile butadiene styrene parts[END_REF].

The FDM feedstock is not restricted to ABS and PLA materials. There are several polymers that are also classified as stiff and strong FDM materials, e.g., polycarbonate (PC), polypropylene (PP), PEEK, UTEM, and polyamide (PA).

For PC, PEEK, and PLA, the same raster angles and building orientation are recommended [START_REF] Deng | Mechanical properties optimization of poly-ether-ether-ketone via fused deposition modeling[END_REF][START_REF] Cantrell | Experimental characterization of the mechanical properties of 3D-printed ABS and polycarbonate parts[END_REF]. PP, PA and UTEM 9085 are similar to ABS, for which the zero raster angle is the best to reach a high Young's modulus and the ultimate tensile strength [START_REF] Milosevic | Characterizing the Mechanical Properties of Fused Deposition Modelling Natural Fiber Recycled Polypropylene Composites[END_REF][START_REF] Wang | Effect of fused layer modeling (FLM) processing parameters on impact strength of cellular polypropylene[END_REF][START_REF] Stoof | Sustainable composite fused deposition modelling filament using recycled pre-consumer polypropylene[END_REF][START_REF] Zaldivar | Influence of processing and orientation print effects on the mechanical and thermal behavior of 3D-Printed ULTEM ® 9085 Material[END_REF].

As for the fracture toughness, it is highest at the zero raster angle, which may increase the fracture toughness K c to about 2 and 5 MPa•m 1 2 / for ABS [START_REF] Mclouth | The impact of print orientation and raster pattern on fracture toughness in additively manufactured ABS[END_REF][START_REF] Aliheidari | Fracture resistance measurement of fused deposition modeling 3D printed polymers[END_REF][START_REF] Hart | Fracture behavior of additively manufactured acrylonitrile butadiene styrene (ABS) materials[END_REF] and PLA [START_REF] Song | Measurements of the mechanical response of unidirectional 3D-printed PLA[END_REF] respectively. The fracture toughness can also be improved by increasing the roughness degree of material mixture, because the grains present in the material tend to resist crack growth [START_REF] Damodaran | Improving the Mode-II interlaminar fracture toughness of polymeric matrix composites through additive manufacturing[END_REF].

Fig. 5. Low adhesion along the junction between layers in a FDM-printed part [START_REF] Abouzaid | Thermal mechanical characterization of copolyester for additive manufacturing using FDM[END_REF].

Impact of printing temperature

Much attention has been focused on finding the optimal tool path parameters and on clarifying their impact on the printing pattern and the 3D-printing object. But the relation between these printing parameters and the printing temperature is often neglected. One of the major concerns related to the fused deposition modelling process is to maintain an adequate printing temperature to ensure a smooth printing operation.

An inadequate extrusion temperature range can damage the product. Low temperatures can lead to a low layer adhesion (Fig. 5), peeling off of layers, and even nozzle clogging. At a high printing temperature, large thermal gradients can arise, and attempts to of maintain a constant layer height can alter the desirable geometry of parts [START_REF] Mahmood | Taguchi based process optimization for dimension and tolerance control for fused deposition modelling[END_REF][START_REF] Abouzaid | Thermal mechanical characterization of copolyester for additive manufacturing using FDM[END_REF].

In all conditions, high or moderate temperatures are recommended. An increased temperature raises the average temperature between layers, and more time above T g is spent, which improves the interlayer welding. Moreover, the raster angle and width become more adjusted and ensure a high adhesion in layers [START_REF] Braconnier | Processing parameter correlations in material extrusion additive manufacturing[END_REF][START_REF] Coogan | Bond and part strength in fused deposition modeling[END_REF][START_REF] Davis | Mechanical strength of welding zones produced by polymer extrusion additive manufacturing[END_REF]. The layers are flattened, and the surface roughness of the whole part decreases.

The anisotropy of printed parts is better controllable at high printing temperatures. The voids in the mesostructure with a cross-linked filament arrangement is the principal factor increasing the porosity f [START_REF] Guessasma | Understanding the microstructural role of bio-sourced 3D printed structures on the tensile performance[END_REF][START_REF] Guessasma | Microstructure, Thermal and mechanical behavior of 3D printed acrylonitrile styrene acrylate[END_REF][START_REF] Lekhnitskii | Elasticity Theory of Anisotropic Bodies[END_REF] A low temperature hinders the adequate flow deposition through the nozzle and may generate discontinuous rasters and decrease the cohesiveness between the deposited filaments. A high printing temperature decreases the porosity of printed parts by minimizing the thermal transfers of deposited rasters and maximizing the cooling time during the printing operation (Fig. 6) [START_REF] Guessasma | Printability and Tensile Performance of 3D Printed Polyethylene Terephthalate Glycol Using Fused Deposition Modelling[END_REF]. Guessasma [START_REF] Guessasma | On the Tensile Behaviour of Bio-sourced 3D-Printed Structures from a Microstructural Perspective[END_REF] confirmed the existence of this phenomenon and proposed an approximate formula for the porosity f and the printing temperature T p based on microtomography measurements, namely, This linear model shows that pores are not distributed arbitrarily in the printed part -they are concentrated at the intersection of printed rasters (Fig. 7).

The printing temperature strongly correlates with mechanical properties, with anisotropy of the printed pattern being the main reason for its mechanical weakness. As already mentioned, a high printing temperature improves the interfilament welding and leads to a high adhesion between layers [START_REF] Guessasma | Printability and Tensile Performance of 3D Printed Polyethylene Terephthalate Glycol Using Fused Deposition Modelling[END_REF][START_REF] D'amico | Effect of layer thickness on irreversible thermal expansion and interlayer strength in fused deposition modeling[END_REF][START_REF] Abouzaid | Printability of co-polyester using fused deposition modelling and related mechanical performance[END_REF], which increases the elastic modulus and tensile strength. A summary of the results of studies is presented in Fig. 8, where it is shown that the mechanical properties of different FDM-made polymer composites depend on printing temperature [31, 38, 39, 42-44, 46, 48, 50-52, 54, 64-73]. Further investigations are needed to determine the optimal values of FDM process parameters by employing various design optimization methods and techniques [START_REF] Bochkareva | A unified approach to determining the effective physicomechanical characteristics of filled polymer composites based on variational principles[END_REF][START_REF] Barkanov | Optimized comparative analysis of an active twist for helicopter rotor blades with C-and D-spar designs[END_REF][START_REF] Galichyan | Parameter optimization of laminated multiferroic composites[END_REF][START_REF] Turusov | Stress state and optimization of the parameters of interrupted and machine surgical sutures of human intestine[END_REF][START_REF] Majak | Orientational design of anisotropic materials using the Hill and Tsai-Wu strength criteria[END_REF].

Conclusion

The first part of this article was dedicated to an overview of the additive manufacturing and fused deposition processes. Various printing parameters and their influence on the quality of printed patterns were considered and recent results of temperature influence on their roughness and porosity during the FDM process were judged.

The crucial factors to be taken into account before building a part were examined, and it was found that the most significant ones were those related to tool path parameters, such as the layer thickness and part building orientation.

The influence of the principal printing parameter was described by a theoretical model characterizing the thermal behavior of parts during the printing process. Finally, the impact of printing temperature on the porosity and the mechanical behavior of printing polymers was summarized.

Nonetheless, there still are a few impediments which hinder one to obtain a product with excellent mechanical properties. The control of porosity of printed parts, which worsens their mechanical properties is still a problem. Voids in deposited trajectories and between layers reduce cohesion and, as result, small cracks may appear and propagate in parts. Therefore, fatigue studies are required to obtain a further insight into this phenomenon.

Fig. 1 .

 1 Fig. 1. AM classification based on the Main machine characteristics (a) and esthetic features (b): 1 -build cost, 2 -build speed, 3 -dimensional accuracy, and 4 -surface quality.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Fused deposition modelling (FDM) (a) and nozzle geometry (b): 1 -build platform, 2 -foam base, 3 -part, 4 -build material filament spool, 5 -support material filament spool, 6 -print nozle, 7 -feed pinch rollers, 8 -heated liquefier, 9 -section 1, 10 -section 2, and 11 -section 3.

Fig. 4 .

 4 Fig. 4. FDM tool path parameters: 1 -raster, 2 -raster angle, 3 -raster width, 4 -perimeter, and 5 -air gap.

Fig. 6 .

 6 Fig.6. Filament cohesiveness at various printing temperatures (a) and the apparent density ρ (b) and porosity f (c) as functions of printing temperature[START_REF] Abouzaid | Printability of co-polyester using fused deposition modelling and related mechanical performance[END_REF].

Fig. 8 .Fig. 7 .

 87 Fig. 8. Uniaxial tensile strength (UTS) (a), Young's modulus E (b), and fracture toughness K c (c) as a functions of printing temperature T p .

Acknowledgement. The authors are grateful to the French national Research Center (CNRS), National Institute of Research for Agronomy (INRA), and Avenir technology Ltd. for their support to this project.