Alina Elena Coman

Julien Peyrton

Gheorghe Hubca

Andrei Sarbu

Augusta Raluca Gabor

Cristian Andi Nicolae

Tanta Verona Iordache

Luc Avérous

Luc Averous
email: luc.averous@unistra.fr

Synthesis and characterization of renewable polyurethane foams using different biobased polyols from olive oil

Keywords: biobased polyols, hysteresis, polyurethane foams, green chemistry, sustainable

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1.

Introduction

The climate change, the depletion of some fossil resources and environmental degradations push the industry and research to focus their investigation on the production of more eco-friendly products in a more sustainable way. The substitution of fossil materials by renewable feedstock from biomass is driven by the development of new molecular architectures with improved or new properties [START_REF] Mosiewicki | A short review on novel biocomposites based on plant oil precursors[END_REF][START_REF] Miyagawa | Novel biobased nanocomposites from functionalized vegetable oil and organically-modified layered silicate clay[END_REF][START_REF] Lligadas | Synthesis and characterization of polyurethanes from epoxidized methyl oleate based polyether polyols as renewable resources[END_REF].

In this context, different resources such as the vegetable oils are highly investigated. Vegetable oils are mainly composed of triglycerides, a glycerol molecule esterified by fatty acids. The fatty acids can vary in term of length (number of carbons), number of double bonds and with or without the presence of reactive group such as hydroxyl or epoxide groups. Distribution of fatty acid in vegetable oils is highly dependent upon the specie type and growing conditions of the plant. In 2019-2020, the worldwide production of vegetable oils was over 200 million metric tons [START_REF]Global production of vegetable oils[END_REF]. The amount of consumed vegetable oils is increasing every year, because of their renewability, but mainly due to their high degree of unsaturation and several active chemical groups, low costs and high availabilities to bring new molecules or to substitute fossilbased ones.

Although polyurethanes (PUs) can also be obtained without isocyanate (Non isocyanate polyurethane, NIPU) [START_REF] Carré | From the Synthesis of Biobased Cyclic Carbonate to Polyhydroxyurethanes: A Promising Route towards Renewable Non-Isocyanate Polyurethanes[END_REF], PUs are conventionally synthesized by polyaddition between polyisocyanates and polyols. A large variety of high-performance PU-based materials can be developed. PUs are a major polymer family. For instance, 22 million metric tons of PUs were produced in 2016, for a wide range of applications with different architectures. However, up to 65% of the market is focused on foams based on open or closed cells, with rigid or flexible foams (FPUF), respectively [START_REF] Akindoyo | Polyurethane types, synthesis and applications -a review[END_REF]. FPUFs present specific viscoelastic, shocks absorber and resilience properties, which are particularly favorable for applications in several sectors, e.g. cushioning, packaging, bedding, upholstery, automotive seating or footwear.

Without competition with food or feed, vegetable oils can be transformed by oleochemistry into polyols for PU production [START_REF] Maisonneuve | Vegetable oils: a source of polyols for polyurethane materials[END_REF][START_REF] Ghasemlou | Polyurethanes from seed oil-based polyols: A review of synthesis, mechanical and thermal properties[END_REF][START_REF] Singh | Recent Advancement in Plant Oil Derived Polyol-Based Polyurethane Foam for Future Perspective: A Review[END_REF], also leading to new macromolecular architectures [START_REF] Ionescu | Ethoxylated Soybean Polyols for Polyurethanes[END_REF][START_REF] Landers | Additive for adjusting the glass transition temperature of viscoelastic polyurethane soft foams[END_REF]. One of the most investigated pathways for polyols production is the epoxidation of triglycerides double bonds and subsequent ring-opening with a nucleophile. Different vegetable oil-based polyols from: rapeseed oil [START_REF] Kirpluks | Three different approaches for polyol synthesis from rapeseed oil[END_REF][START_REF] Hu | Rigid polyurethane foam prepared from a rape seed oil-based polyol[END_REF], soybean [START_REF] Campanella | Polyurethane foams from soyoil-based polyols[END_REF][START_REF] Aremu | Production of flexible polyurethane foam using soya bean oil and palm kernel oil as surfactant and polyol respectively[END_REF], sunflower oil [START_REF] Peyrton | New Insight on the Study of the Kinetic of Biobased Polyurethanes Synthesis Based on Oleo-Chemistry[END_REF], palm oil [START_REF] Lumcharoen | Preparation and characterization of flexible polyurethane foams from palm oil-Based polyol[END_REF], castor oil [START_REF] Mosiewicki | Polyurethane foams obtained from castor oil-based polyol and filled with wood flour[END_REF], linseed oil [START_REF] Sharmin | Linseed oil polyol/ZnO bionanocomposite towards mechanically robust, thermally stable, hydrophobic coatings: A novel synergistic approach utilising a sustainable resource[END_REF], cotton oil [START_REF] Narute | Study of the performance of polyurethane coatings derived from cottonseed oil polyol[END_REF], safflower oil [START_REF] Meshram | Producing Phosphate-Polyols by Ring-Opening Hydrolysis of Wild Safflower Oil Epoxides[END_REF], algae oil [START_REF] Peyrton | Biobased Polyurethane foams based on new polyols architectures from microalgae oil[END_REF] were investigated for producing biobased PU foams with different methods.

Despite the important use in the industry and as far as we know, olive oil (OO) was never reported as potential raw material for producing polyols for PUF fabrication. OO is one of the most vegetal oil produced in the world. In 2019-2020, the global production volume of OO was around 3.12 million metric tons [START_REF]Production volume of olive oil worldwide[END_REF]. The European Union is the principal producer, consumer and exporter of OO. EU Mediterranean countries, are dedicated to the cultivation of olive trees [START_REF]Olive oil: An overview of the production and marketing of olive oil in the EU[END_REF]. OO contains up to 99% mixed triglyceride esters from which around 55-83% monounsaturated (e.g., oleic acid), 3.5-21% of polyunsaturated (e.g., linoleic acid), 7.5-20% of saturated (e.g., palmitic acid) and other minor compounds (e.g., stearic acid, α-linolenic acid, squalene, phenolics or sterols) [START_REF] Beltra | Influence of Harvest Date and Crop Yield on the Fatty Acid Composition of Virgin Olive Oils from Cv. Picual[END_REF]. Glycerides profile depends on several factors such as the plant variety, altitude, location, extraction process or harvest-time [START_REF] Tripoli | The phenolic compounds of olive oil: structure, biological activity and beneficial effects on human health[END_REF][START_REF] Viola | Virgin olive oil as a fundamental nutritional component and skin protector[END_REF].

Figure 1. FPUF from multistep process

In this context, the aim of this study was to synthesize new OO-based polyols and to analyze the effect of substitution of conventional fossil-based polyol, with the prepared thereof, in the attempt to fabricate "green" FPUFs with open cells, obtained with a chemical blowing agent (water). In this multistep process, each intermediate and the final cellular materials will be largely characterized. Figure 1 covers the summary description of the envision steps of the study.

The study explored the flexible foams elaboration based on polyols obtained from renewable raw materials, respectively from OO. As far as literature shows, such bio-based polyols from OO are being used for the first time for preparing PU flexible foams. The advantages of using OO as raw material refer to availability and the ease of manipulation during synthesis. Furthermore, the adequate mechanical and physical properties of prepared foams are clear proof that "greener" foams can be obtained by a partial replacement of petrochemical polyols with OO based polyols.

Materials and methods

A commercial OO produced in the EU from Carrefour (France) was used. OO is predominantly based on triglyceride structures. Iodine value (IV) was measured by the WIJS method to determine the double bond content expressed in grams of iodine (I2) per hundred grams of OO. The double bond content of OO was 78 g I2/100 g (3 mmol double bond/g). Glacial acetic acid (99.7%), toluene (99.8%), ethyl acetate (99.8%), cyclohexane, sodium thiosulfate, potassium iodide, anhydrous sodium sulfate, tetrahydrofuran (THF) UPLC grade and WIJS solution were obtained from Fisher Scientific (France). Amberlyst® 15H (strong acidic cation exchanger, dry), Amberlite® IR 120 (strong acidic cation exchanger, hydrogen form), deuterated chloroform (CDCl3), hydrogen peroxide solution (H2O2) 30% (w/w) p.a. were provided by Sigma-Aldrich (France). Absolute ethanol was purchased from VWR (France). The reference polyol is mainly fossil-based and named ARCOL (Arcol Polyol 1374, Covestro). This polyether triol is mainly obtained by alkoxylation from glycerol, and then based on secondary hydroxyl groups. This soft polyeter-triol is conventionally used for flexible foams. The diisocyanate was a mixture of diphenylmethane-4,4'-diisocyanate (MDI) with isomers and homologues of higher functionality (Desmodur® 10WB94, Covestro, Germany) with NCO index = 32%. N-{2-[2-(dimethyl amino) ethoxy]ethyl}-N-methyl-1,3-propanediamine (DABCO NE 300, Evonik, Germany) and dibutyltin dilaurate (DBTL -DABCO T-12, Evonik, Germany) were used as blowing and gelling catalysts. A polyether polysiloxane copolymer (Tegostab B8773 LF2, Evonik, Germany) was used as a surfactant. The chemical blowing agent used for foams production was distilled water. All the raw materials were used as received without further purification.

Synthesis of OO-based polyols

Epoxidation of OO

The epoxidation reaction of OO is described in detail in the Supplementary Material.

Ring-opening of EOO with AA

The reaction was performed in a round bottom flask of 250 ml, equipped with a reflux condenser and a mechanical stirrer. According to previously described protocols [START_REF] Sinadinovic-Fiser | Kinetics of in situ Epoxidation of Soybean Oil in Bulk Catalyzed by Ion Exchange Resin[END_REF][START_REF] Dinda | Selective epoxidation of natural triglycerides using acidic ion exchange resin as catalyst[END_REF][START_REF] Abdullah | Epoxidation of vegetable oils and fatty acids: catalysts, methods and advantages[END_REF][START_REF] Dinda | Epoxidation of cottonseed oil by aqueous hydrogen peroxide catalysed by liquid inorganic acids[END_REF][START_REF] Park | Thermal and mechanical properties of tetrafunctional epoxy resin toughened with epoxidized soybean oil[END_REF][START_REF] Petrovic | Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids-kinetics and side reactions[END_REF], the flask was filled with 100 g of EOO (0.3 mol, 1 eq.) and 33.34 ml of acetic acid (0.6 mol, 2 eq.).

The mixture was stirred (600 rpm) at 110 °C for 24 hours, after which 100 ml of ethyl acetate was added to decrease the viscosity of the product. The organic phase was washed with saturated NaHCO3 solution until neutral pH, dried with anhydrous sodium sulfate and then filtered. The solvent was evaporated using a rotary evaporator. Then, the resulting EOO_AA was dried overnight in a vacuum oven at 40 °C.

Ring-opening of EOO with EtOH

The reaction was carried out in a round bottom flask of 1 L, equipped with a reflux condenser and a mechanical stirrer. According to previously described protocols [START_REF] Sinadinovic-Fiser | Kinetics of in situ Epoxidation of Soybean Oil in Bulk Catalyzed by Ion Exchange Resin[END_REF][START_REF] Dinda | Selective epoxidation of natural triglycerides using acidic ion exchange resin as catalyst[END_REF][START_REF] Abdullah | Epoxidation of vegetable oils and fatty acids: catalysts, methods and advantages[END_REF][START_REF] Dinda | Epoxidation of cottonseed oil by aqueous hydrogen peroxide catalysed by liquid inorganic acids[END_REF][START_REF] Park | Thermal and mechanical properties of tetrafunctional epoxy resin toughened with epoxidized soybean oil[END_REF][START_REF] Petrovic | Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids-kinetics and side reactions[END_REF], the flask was filled with 100 g of EOO (1 eq.), 450 mL of ethanol (25 eq.) and 10 g of Amberlyst® 15H. The mixture was stirred (600 rpm) at 90 °C for 4 hours, after which 100 ml of ethyl acetate was added to decrease the viscosity of the sample. The solid catalyst was removed through filtration. The organic phase was washed with saturated NaHCO3 solution until neutral pH and dried with anhydrous sodium sulfate. The solvent was evaporated using a rotary evaporator. Then, the resulting EOO_EtOH was dried overnight in a vacuum oven at 40 °C.

Synthesis of Flexible Polyurethane Foams (FPUFs)

All FPUFs were prepared in a two-step method. After several preliminary foaming trials, a substitution range of 15 to 35% petroleum-based polyol was found to be adequate. Foams with 35% OO based polyol proved to be the threshold for process control, as PU foams with higher substitution degrees could not be analyzed due to physical instability. Foams were prepared from two mixes (A and B) and their mixing in a PP cup, according to formulations given in Table 1. Mix A consisted of the polyol's mixture where the fossil-based polyol was progressively substituted (by 15, 25 and 35 wt.%) by the biobased polyol(s) (alone or 50/50 wt/wt mixed).

Then the catalysts, water, and surfactant were added and the mix was homogenized using a mechanical stirrer at 2000 rpm, for 2 min. Mix B is only based on the diisocyanates, which were rapidly poured into mix A by means of a syringe. The mixture A+B was vigorously stirred at 2000 rpm for 5 s, after which the foam was let to rise freely. The foams were cured at room temperature for at least 24 h before analysis. The RF is based 100% fossil-based polyol and was used as a reference. Finally, 3 different series (Series A, B and C) have been performed with different formulations (Table 1)

The optimal isocyanate/hydroxyl molar ratio (NCO/OH) was set at 0.9 in all formulations. This particular ratio was established after several preliminary trials, starting from data from the literature and from commercial protocols for flexible foams. In this respect, all reactive hydroxyl groups from water (blowing agent,) or polyols, were considered to determine the added isocyanate. Afterwards, different surfactants, catalysts and fossil-based polyols were tested (not presented in the study). The surfactant, blowing and gelling catalysts were set at 0.15, 0.1 and 0.28 parts by weight (pbw), respectively, for all the foams. It is also worth mentioning that the required conditions to obtain opened cells in the FPUFs assumed a value of isocyanate/hydroxyl index of 0.9 and 0.15 pbw surfactant.

2.3.Characterization techniques

1 H NMR was used to study the chemical structures. All the samples were solubilized in 600 µL of CDCl3. Spectra were recorded using Bruker UltraShield 400 MHz at room temperature. The relaxation delay was set to 1 s with 32 scans. Spectra were calibrated using the CDCl3 peak at δ=7.26 ppm. The spectra were analyzed by a dedicated software.

FTIR spectra were recorded on Fourier transformed infrared spectrometer Nicolet 380 from Thermo Scientific (US), in reflection mode, equipped with an attenuated total reflectance (ATR) diamond module (FTIR-ATR). An atmospheric background was collected before each sample analysis (32 scans, resolution 4 cm -1 ).

The viscosity measurements were carried out on TA instrument discovery HR-3 using parallel plate geometry. The viscosity was measured at room temperature, with a shear rate varying from 10 -5 to 100 s -1 in 5 min, and after 10 min of rest, the shear rate was varied from 100 to 10 -5 s -1 in 5 min. The viscosity values were subtracted from the plateau region. SEC was used to determine molar masses. The samples were dissolved in THF at room temperature and filtered on 0.2 µm polypropylene membranes directly in sample vials. An Advanced Polymer Chromatograph (Acquity APC from Waters, Milford, Massachusetts, USA) was used for SEC analysis, with THF mobile phase at 40 °C. Three columns (Acquity APC XT 450 Å 2.5 µm 4.6 × 150 mm, 200 and 45) were connected. 10 µl from each sample was injected and a run of 11 min with a flow rate of 0.6 mL•min -1 was applied. A calibration curve with polystyrene (PS) standards was built for molar mass determination.

The thermal stability was analyzed using thermo-gravimetric analysis (TGA), performed using a TA Instrument Hi-Res TGA Q5000 under N2 (flow rate 40 mL•min -1 ). Samples of 4-5 mg were heated from room temperature to 700 °C, at 10 °C•min -1 . The main characteristic degradation temperatures were determined at the maximum weight loss from the (DTG) curve.

Experiments for DSC thermograms were carried out using DSC Q2000 from TA Instruments, working in temperature-modulated mode (MTDSC) with dynamic purge gas (helium 99.99% with flow rate 25 ml•min -1 , modulated scan). Approximately, 3 -5 mg samples were weighted and heated from -110 to 155 °C with a heating rate of 10 °C•min -1 .

Compression tests were carried out on TA Instrument Discovery HR-3 using parallel plate geometry. The samples were cut to measure on average 15 x 15 x 15 mm (HxWxL). First, the foam was compressed until the upper geometry was in contact with the foam (force of 0.1 N), then compressed at a linear rate of 40 µm•s -1 for 5000 µm (loading cycle). Afterwards, the upper geometry was lifted off at a 40 µm•s -1 rate for 5000 µm (unloading cycle). The stress was calculated using the ratio between the normal force, obtained by the machine, and the measured top area under compression.

The cell sizes of foams were determined according to ASTM D-3574-11 test, using an OPTIKA B-150DBR optical microscope, at 202 µm magnification (scale provided on each figure 4X, 202 µm) and then analyzed using an open-source software. Five measurements were made for each sample and the cell average diameter was the result of 25 measurements for which 4X magnification lens was used.

Apparent foam density was determined on parallelepipedic shaped foam samples with a caliper and weighed. In this regard, 3 to 5 measurements were performed for each studied sample.

Compression tests, density and cell size measurements were performed considering the increasing, longitudinal direction of foams (parallel to the foam growth). The standard error of means (SE) and standard deviation (SD), were calculated for density values and cell size values, using Equations 1 and 2:

= Σ( ) ) (1) = . ( 2
)
Where n is the number of measurements, and is the average measured value.

Results and discussion

3.1.Analysis of the raw materials and intermediates

As, the OO is composed of more than 70% of oleic acid, the structure of OO and EOO were represented by a triglyceride composed of three oleic acids. This idealized structure eased the 1 H NMR. peak identification. The spectra are presented in Supplementary Material Figure S1. The bio-based polyols were obtained by ring-opening of EOO with either AA or EtOH. In The selectivity of the ring-opening reaction for EOO_EtOH and EOO_AA was also evaluated from NMR spectra, as the ratio between twice the integral of the proton located in alpha position of the hydroxyl (9 and 9') and the integral of initial epoxide group from EOO. The results indicated that the selectivity of the ring-opening was higher when using ethanol (98%) than acetic acid (72%). The hydroxyl index (IOH), the molar masses (Mw and Mn) and the viscosity of OO-based polyols (in Table 2) are key parameters that impact the foaming process of and, consequently, the properties of the FPUFs [START_REF] Sahasrabudhe | Density, viscosity, and surface tension of five vegetable oils at elevated temperatures: Measurement and modeling[END_REF][START_REF] Nierat | Temperature-dependence of olive oil viscosity[END_REF]. Table 2 shows that viscosities increased with the molar masses, confirming the success of the epoxidation reaction. The difference in IOH between the theoretical value (calculated for 100% selectivity) and the experimental IOH determined by analysis, confirmed the difference in selectivity as calculated from 1 H NMR analysis. The side reactions that occur especially after the ring-opening refer to oligomerization and, thus, could explain the difference in IOH between the two bio-based polyols. The EOO_AA was synthesized in more acidic conditions, which favored the oligomerization of the epoxide. This hypothesis was confirmed by the molar mass values measured by SEC, presented in Table 2 and Figure S2 in Supplementary Material. The dominant peaks for EOO_AA and EOO_EtOH corresponded to the hydroxyl derivate of the EOO. The broad and minor peaks observed in the chromatograms corresponded to dimers and trimers that resulted during the ring-opening reactions of EOO. Epoxide oligomerization was also reported in previous studies [START_REF] Sahasrabudhe | Density, viscosity, and surface tension of five vegetable oils at elevated temperatures: Measurement and modeling[END_REF][START_REF] Nierat | Temperature-dependence of olive oil viscosity[END_REF]. Although polyols with higher molar masses are usually preferred for the formulation of FPUF [START_REF] Furtwengler | Renewable polyols for advanced polyurethane foams from diverse biomass resources[END_REF], the obtained OObased polyols should deliver promising results.

Conversion of double bonds from OO into epoxide groups led to an increase of viscosity from 70 (OO) to 170 mPa•s (EOO) and the ring-opening reaction of EOO increased even more the viscosity of thereof. However, the EOO_AA was more viscous (2490 mPa.s) than EOO_EtOH (380 mPa.s). In this regard, the formation of hydroxyl groups with the ability to form intramolecular H-bonding could explain the viscosity increase for EOO_AA polyol. On the other hand, the higher content of oligomers may have also contributed to the viscosity increase registered for EOO_AA. FTIR spectra of the OO, EOO, EOO_AA and EOO_EtOH (Figure 3), compared to ARCOL, confirmed the completion of the epoxidation and the ring-opening reactions. OO and thereof presented several typical bands, characteristic to triglycerides. Intense bands were observed in the region of 3000-2800 cm -1 corresponding to C-H stretching vibrations. The stretching vibrations of methylene (-CH2-) and methyl (-CH3) groups from aliphatic fatty acids were recorded at 2924 and 2849 cm -1 , respectively. The large band recorded at 1737 cm -1 due to ester carbonyl stretching was common for both OO and thereof [START_REF] Pretsch | Hydrolytic degradation and functional stability of a segmented shape memory poly(ester urethane)[END_REF]. OO double bonds were identified by the characteristic CH stretching of double bonds at 3007 cm -1 . After epoxidation, the presence of the oxirane ring stretching vibrations were observed at 830 cm -1 [START_REF] Heintz | A spectroscopic analysis of the phase evolution in polyurethane foams[END_REF]. The OH stretching at around 3485 cm -1 , indicated the presence of hydroxyl groups in the polyols compared to OO and EOO. For both OO-based polyols, a symmetric stretching vibration characteristic to ester bonds (-C-O-C-) was revealed at 1084-1023 cm -1 , which was due to epoxide ring-opening with alcohol or acetic acid, respectively. The reference polyol obtained by alkoxylation from glycerol is characterized by a large band at 1084 cm -1 related to the ethylene ether units.

Table 2. Physical properties of OO, EOO and OO-based polyols

Figure 3. FTIR Spectra for OO, EOO and OO-based polyols, compared to the reference polyol

Several studies [START_REF] Roy | Kinetic modeling of the epoxidation of natural rubber with in-situ-formed peracid[END_REF][START_REF] Hwanga | Preparation and Properties of Lubricant Basestocks from Epoxidized Soybean Oil and 2-Ethylhexanol[END_REF][START_REF] Atanu | Synthesis of Diethylamine-Functionalized Soybean Oil[END_REF][START_REF] Chou | Acetic-acid as an Oxygen Carrier, Between Two Phases for Epoxidation of Oleic-acid[END_REF][START_REF] Guo | Structure and Properties of Halogenated and Nonhalogenated Soy-Based Polyols[END_REF][START_REF] Singh | Thermal Stability of Polyester vs. Polyether-Based Urethans[END_REF] have shown that the presence of double bonds in the fatty acid chains of vegetable oil speed up oxidative degradation. Both, the reference polyol and EOO_EtOH contain soft ether bonds, while EOO_AA is based on more rigid ester bonds. The oxidative cleavage of ether bonds occurred at lower temperatures than the ester bonds [START_REF] Gibson | Cellular Solids, Structures and Properties[END_REF], explaining the difference in thermal stability between both polyols. It was also observed that EOO_AA presented lower maximum weight loss, equivalent to the one registered for EOO (88 wt.%). The other polyols (EOO_EtOH and ARCOL) and OO registered around 91 -93 wt.% maximum weight loss. Thermal analysis of OO and derivates can be found in Figure S3 from Supplementary Material.

The increase in the molar masses as together with the occurrence of crystalline phases, shown by DSC analysis (Figure S4 in Supplementary Material) may explain the viscosity increase within OO thereof (Table 2) [START_REF] Singh | Thermal Stability of Polyester vs. Polyether-Based Urethans[END_REF][START_REF] Harwood | The Lipid Handbook with CD-ROM[END_REF]. The DSC thermograms of OO-based polyols significantly differed. We can also show the clear evolution from OO to the corresponding polyols. The primary crystallization and melting temperatures of the analyzed materials are given in Table 3. OO registered the lowest melting temperature, -47 °C and these values increased after each modification in the following order: EOO_AA<EOO_ETOH<EOO< ARCOL. With respect to the crystallization temperature, OO attained the highest value around -5 °C while the reference polyol the lowest (-68 °C) followed by EOO_AA, EOO_ETOH and EOO. Interestingly, EOO, EOO_AA and EOO_EtOH presented two crystallization temperatures (the second one between -18 and -10 °C), ascribed to different crystalline polymorphs, in which case may be due to the presence of oligomers in the composition.

Table 3. Melting and crystallization temperatures of OO, EOO and OO-based polyols compared to the reference polyol

3.2.FPUFs analysis

Foams with 0.9 isocyanate/hydroxyl index were prepared according to the formulations presented in Table 1. This index is commonly used in FPUF formulation as the excess of polyol prevents the remaining of residual toxic isocyanate in the foam. The molded foams were achieved by replacement of 15, 25 and 35 wt.% fossil-based polyol with OO-based polyols. These different contents were defined after some preliminary tests. They had shown that for higher substitution content, foaming and gelling reactions were not well balanced with the catalyst quantities and, the resulting foams were not acceptable for analysis.

Figure 4. FTIR spectra for FPUFs

The FTIR spectra of the FPUFs with the highest content of OO-based polyols were compared with the reference polyol (Figure 4). On FPUF, the stretching vibration of NCO located at 2269-2273 cm -1 almost disappeared. This indicated that isocyanate groups have reacted quantitatively with the hydroxyl groups of polyols to form urethane bonds. The specific urethane, N-H, stretching vibration at 3317-3319 cm -1 was clearly identified for all the FPUFs. Furthermore, the presence of C=O stretching vibration from the urethane bond was also observed around 1730 cm -1 in all FPUF spectra [START_REF] De Haseth | Characterization of Polyurethane Foams by Mid-Infrared Fiber/FT-IR Spectrometry[END_REF]. The transmittance band at 1595 cm -1 corresponded to phenyl stretching, which indicated the presence of ordered rigid domains [START_REF] Tu | Water-blown rigid and flexible polyurethane foams containing epoxidized soybean oil triglycerides[END_REF] responsible for the semi-flexible character of the foams.

Foam density is a key parameter of flexible polyurethanes in terms of durability in connection with the application and usages. Flexible polyurethane foams are available in a broad range of densities, ranging from 14 kg/m 3 to as high as 120 kg/m 3 [START_REF] Gibson | Cellular Solids, Structures and Properties[END_REF]. Herein, density values of FPUFs synthesized with OO-based polyols were in the same range than the RF (Table 4). Density of the RF was 51 kg/m 3 while the FPUFs with OO-based polyols ranged from 41 to 74 kg/m 3 . Furthermore, all the series of foams presented similar trends, meaning the densities increased with the amount of OO-based polyol. Similar observations were made by different authors, for instance in studies regarding soybean-based polyols [START_REF] Liu | CertiPUR-US® Compliant MDI Flexible Polyurethane Foams, Composition-Structure-Property Relationships of Commercial Polyurethane Foams for Mattresses[END_REF]. One explanation of this phenomenon was linked to the IOH value of OO-based polyols. The higher the hydroxyl content the faster the polymerization reaction with the isocyanate. Thus, foams have less time to expand leading to denser structures. The optical micrographs of synthesized FPUF (Figure 5) revealed a clear open cell structure in perfect agreement with the focus of the study and the used blowing agent. The cells were irregular (polyhedral) in all foam series, consisting mainly of large cells with an average diameter of pores ranging from 100 to 124 µm and with a wide distribution of cells size (Table 4). Foams with higher blowing agent content (F_EOO_EtOH and F_EOO_EtOH+AA) revealed larger cell size that widen with the increasing amount of OO-based polyol. However, the series C with mixed OO-based polyols were more homogenous, with narrower distributions of cell size (112-119 µm), similar to the reference foam (around 124 µm). Series C presented more regular cell shapes and the recorded densities were in a close range (51-56 kg/m 3 ).

Figure 5. Optical micrographs of FPUFs

The hysteresis given by compression tests is usually referred as the energy ratio between the dissipated and the stored energy (meaning the area enclosed by the hysteresis loop and the area under the unloading curve). For semi-flexible foams, the value obtained from compression is a qualitative measure for the recovery degree after removing the applied load. In this case, the hysteresis was used as a measure for the lost or absorbed energy when a force was applied on the foam. High values of hysteresis indicated high recovery and elasticity of the foams. Hysteresis loss (the dissipated heat loss) was determined at 25% deformation (Table 4). The energy loss in FPUFs is influenced by several parameters such as the chemical architecture and the formulation, the cellular structure, the morphology of the solid matrix, the temperature and the relative humidity. All the foam series revealed a typical behavior of cellular materials under compression tests (Figure 6). Three different deformation mechanisms were identified, as follows i) a linear elastic response (cellular strut bending) in the first region (low strains), ii) a long plateau caused by cell collapse and cellular wall deformation (cell strut buckling) at medium deformation, and iii) densification (cell strut interaction) at higher strains, when the cells are completely collapsed and further stress compresses the foam, leading to a region of suddenly increasing stress. It can also be mentioned that an increase in hysteresis loss was noticed upon addition of OO-based polyols. This behavior was in agreement with previous studies [START_REF] Prociak | Flexible polyurethane foams synthesized with palm oil-based bio-polyols obtained with the use of different oxirane ring opener[END_REF][START_REF] Prociak | Flexible polyurethane foams modified with natural oilbased polyols[END_REF][START_REF] Scarfato | Structure and physical-mechanical properties related to comfort of flexible polyurethane foams for mattress and effects of artificial weathering[END_REF]. Lower hysteresis loss corresponded to a more rigid (macro)molecular structure, which required lower energy consumption while moving. High hysteresis loss indicated that the foam ability to absorb energy was higher. Compared to RF, foams with OO-based polyols presented higher hysteresis loss. Furthermore, it was observed that the stress increased with the amount of OO-based polyol. This phenomenon is due to the behavior of each polyol. The reference polyol, which is based on soft ether grafted chains, is replaced by stiffer polyols. The stress applied on the series C with equivalent mixture of EOO_EtOH and EOO_AA was higher (19 kPa) compared to F_EOO_AA (9.5 kPa), F_EOO_EtOH (11 kPa) or RF (7 kPa). The highest deformation (31%) was achieved for F_EOO_EtOH+AA 25% but the gap with some other systems is not really significant. Some differences between the series were probably due to cell anisotropy (as observed from optical microscopy) and to some variations on IOH values, which are key factors that affect the mechanical properties. However, it can be noted that the cell homogeneity of series C (F_EOO_EtOH+AA samples) has led to improved mechanical properties, with a hysteresis loss values between 56 and 66 %. These FPUFs exhibited similar physical properties than those of flexible foams found on the market, to fit to mattress or bedding applications [START_REF] Chrissafis | Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)[END_REF].

The thermal stability of FPUF was also evaluated. It is well known that the first stage in PU degradation corresponds to urethane bonds scission leading to the formation of primary or secondary amines and CO2. Ester and ether bonds decomposition through chain scission takes place in the second stage of degradation [START_REF] Xia | Castor-Oil-Based Waterborne Polyurethane Dispersions Cured with an Aziridine-Based Crosslinker[END_REF] while in the third stage occurs at higher temperatures (around 540 °C) when the polymer backbone was broken down almost entirely [START_REF] Lu | Soybean Oil-based, Aqueous Cationic, Polyurethane Dispersions: Synthesis and Properties[END_REF]. The degradation between 250 and 400 °C corresponded to urethane bonds dissociation. Minimal differences were observed in this step between Series A, B and C (Table 5, and Figure S5 in Supplementary Material), with small shifts of the maximum degradation temperatures in the following order: RF > 15% OO-based polyol > 25% OO-based polyol > 35% OO-based polyol. The maximum mass loss was recorded in the second stage of degradation (380-480 °C) for all the foams, when chains scission of the polyols occurred [START_REF] Xia | Castor-Oil-Based Waterborne Polyurethane Dispersions Cured with an Aziridine-Based Crosslinker[END_REF][START_REF] Lu | Soybean Oil-based, Aqueous Cationic, Polyurethane Dispersions: Synthesis and Properties[END_REF]. Increasing amounts of OO-based polyol led to higher thermal stability due to higher ester group content. The last decomposition step of FPUFs with small mass losses (4-10%) was registered above 480 °C and attributed to the thermal oxidation of the remaining materials.

Series B and C with EOO_EtOH+ AA and F_EOO_EtOH presented improved properties compared to the F_EOO_AA-based system. For Series C, the first maximum degradation temperature was close to the one registered for RF and for the F_EOO_EtOH series, T2 was higher. Glass transition temperature (Tg) of FPUF networks was determined by DSC analysis. The thermograms are presented in Figure S6 in Supplementary Material. The results showed that there were no significant changes in Tg for FPUFs compared to RF (Table 5). However, distinct trends were observed in each series of foams. For instance, in Series A with EOO_AA the highest Tg was registered at 15% content of bio-polyol, in Series B with EOO_EtOH at 25% biopolyol content and in Series C at 15% bio-polyol mixture. However, taking into account that SD in DSC is around 1°C, most Tg gaps are too low to be always significant.

Conclusions

Different intermediates and sustainable PPUFs have been successfully synthesized from OO with different biobased macromolecular architectures and formulations. The synthesis and addition of two types of OO-based polyols into the composition of FPUFs has clearly proven to have a major impact on the properties of the final materials. The structural analysis of OO-based polyols, prepared via epoxidation of OO and ring-opening with ethanol or acetic acid, confirmed the presence of significant amounts of hydroxyl groups, perfectly suitable for FPUF formulations. The highest hydroxyl index and of OO-based polyols contents conducted to higher foams densities, with high crosslink densities. Open cells with homogenous shapes and sizes were observed. It can be drawn that FPUFs based on the mixture of OO-based polyols presented overall superior physical, mechanical and thermal properties compared to RF, or compared to the FPUFs obtained with each OO-based polyol. In conclusion, it seems that in the frame of environmentally friendly foams, the systems based on F_EOO_EtOH+AA present the optimal properties to fulfill requirements with various applications where semi-flexible foams can be used such as bedding application.

Perspectives of this valuable work are large. First, they can be targeted to improve the global biobased content of the FPUF, using for instance biobased diisocyanate such as DDI (2heptyl-3,4-bis(9-isocyanatononyl)-1-pentylcyclohexane), obtained by dimerization of fatty acids. However, DDI is a soft building block which could decrease some mechanical properties of the FPUFs. Several other biobased chemicals could also be tested. Some additional tests could be added to perform a complete the technical evaluation of these FPUF with more complex formulations. We could mention the climatic aging, or in complement to thermal degradation test, fire resistance test can be performed in connection with the addition of corresponding additives, in order to fulfill specific requirements in connection with different applications.

FPUF from multistep process

The climate change, the depletion of some fossil resources and environmental degradations force the industry and research to focus their investigation on the production of more eco-friendly products in a more sustainable way. The substitution of fossil materials by renewable feedstock from biomass is driven by the development of new molecular architectures with improved or new properties. In this context, different resources such as the vegetable oils are highly investigated.

The aim of this study was to synthesize new olive oil-based polyols and to analyze the effect of substitution of conventional fossil-based polyol, with the prepared thereof, in the attempt to fabricate "green" FPUFs with open cells, obtained with a chemical blowing agent (water). In this multistep process, each intermediate and the final cellular materials will be largely characterized. Figure above covers the summary description of the envision steps of the study.

Figure 2 -

 2 a and -b,1 H NMR spectra of EOO_AA and EOO_EtOH are presented, respectively.The specific signal of protons in triglycerides was observed in both spectra. Proton signals from the epoxide group at 2.89 (Figure2-a) or 2.87 ppm (Figure 2-b) decreased. New signals appeared due to ring-opening. The proton located in alpha position of the hydroxyl group was observed at 3.56-3.62 ppm in both spectra (9 and 9'). The signal of the proton from CH-OCOCH3 was located at 4.81 ppm (Figure 2-a). The proton from the ethanol molecule attached to the triglyceride -CH-O-CH2-CH3 (10) was spotted at 3.44 ppm and the proton located near ethyl group at 3.05-3.06 ppm (Figure 2-b). The CH3 of EtOH is slightly visible at 1.1. We can spot a triplet next to the 1.23 forest of peaks.

Figure 2 .

 2 Figure 2. 1 H NMR spectra of EOO_AA (a) and EOO_EtOH (b)

Figure 6 .

 6 Figure 6. Mechanical hysteresis behavior measured in a compression mode for all the FPUFs from Series A (a), series B (b) and series C (c)

Table 1 .

 1 FPUFs formulations

		Ref.		Series A			Series B			Series C
	Product code	RF	F EOO AA 15%	F EOO_ AA 25%	F EOO AA 35%	F EOO EtOH 15%	F EOO EtOH 25%	F EOO EtOH 35%	F EOO_EtOH +AA 15%	F EOO_EtOH +AA 25%	F EOO_EtOH +AA 35%
								Mix A
	ARCOL (wt.%)	100	85	75	65	85	75	65	85	75	65
	EOO_EtOH (wt.%)	-	-	-	-	15.0	25.0	35.0	7.5	12.5	17.5
	EOO_AA (wt.%)	-	15	25	35	-	-	-	7.5	12.5	17.5
	Water (wt.%)	3.70	3.78	3.83	3.63	3.82	3.89	3.97	3.80	3.86	3.93
								Mix B
	MDI (wt.%)	54.17	57.50	59.72	51.27	59.10	62.39	65.68	58.30	61.06	63.81

Table 4 .

 4 Density, cell size and hysteresis loss from compression tests on FPUFs

	Series	Notation	ρ, kg/ m 3	SD	SE	Cell size (µm)	SD	SE	Hysteresis loss, %
	R. Foam	RF	51	±8	±4	124	±9	±2	52
		F_EOO_AA 15%	48	±8	±4	109	±14	±3	55
	A	F_EOO_AA 25%	41	±3	±2	115	±20	±4	56
		F_EOO_AA 35%	54	±6	±4	119	±14	±3	60
		F_EOO_ETOH 15%	53	±10	±5	100	±13	±4	55
	B	F_EOO_ETOH 25%	62	±15	±9	130	±20	±5	57
		F_EOO_ETOH 35%	74	±11	±5	140	±16	±3	58
		F_EOO_ETOH+AA 15%	54	±7	±3	112	±11	±2	56
	C	F_EOO_ETOH+AA 25%	51	±2	±1	114	±12	±2	64
		F_EOO_ETOH+AA 35%	56	±2	±1	120	±18	±5	66

Table 5 .

 5 Main thermal properties of FPUF

	Sample	T1, °C	T2, °C	T3, °C	Mass loss (second stage), %	Tg, °C
	Ref. foam	302	389	488	71	58
	F_EOO_AA 15%	298	395	478	65	60
	F_EOO_AA 25%	296	400	475	66	59
	F_EOO_AA 35%	294	404	474	65	60
	F_EOO_EtOH 15%	297	394	485	71	56
	F_EOO_EtOH 25%	298	402	478	71	61
	F_EOO_EtOH 35%	298	405	480	70	56
	F_EOO_EtOH+AA 15%	300	395	482	66	61
	F_EOO_EtOH+AA 25%	300	401	476	61	60
	F_EOO_EtOH+AA 35%	295	404	474	64	55

Acknowledgments

This work was realized through an internship realized in ECPM/BioTeam-ICPEES, University of Strasbourg, under the supervision of Prof. Luc Avérous whom I want to thank for this opportunity. The work has been funded by a grant awarded by UEFISCDI in framework of the PCCDI competition, contract N o 70PCCDI/2018 SECURE-NET. A small part of this work has been also funded with Eurocrust project (Inter Carnot Mica Project-France).


J Croquesel

C Meunier

C Petit

F Valdivieso

S Pillon

A C Robisson

J Martinez

F Lemont

Design of an instrumented microwave multimode cavity for sintering of nuclear ceramics

Keywords: Microwave sintering, Nuclear fuel, Finite Elements

This paper describes the development of an innovative instrumented and automated 2.45GHz multimode microwave cavity for sintering of uranium oxide based nuclear fuels (UO2, (U,Pu)O2, (U,Am)O2 …). The cavity and the sintering cell were designed using Finite Element simulation and the expertise acquired over years on microwave heating. This original setup allows for hybrid and homogeneous heating of nuclear fuels. Reliable in situ monitoring of the samples' shrinkage and temperature is performed with an optical dilatometer and infrared pyrometers calibrated with a specific protocol. A dedicated software including a Proportional-Integral-Derivative (PID) controller was developed to precisely control the sintering cycles like under conventional heating. The setup is also designed for operating in a glovebox under reducing sintering atmosphere.

Introduction

Microwave sintering was deeply studied in the last decades due to its potential advantages compared to conventional heating [START_REF] Rybakov | Microwave Sintering: Fundamentals and Modeling[END_REF][START_REF] Singhal | Microwave Sintering of Advanced Composites Materials: A Review[END_REF]. Studies on various ceramics (e.g alumina, zirconia, hydroxyapatite) have shown densification of fine-grained samples with reduced processing times compared to conventional sintering [START_REF] Gil-Flores | Microstructure and Mechanical Properties of 5.8 GHz Microwave-Sintered ZrO2/Al2O3 Ceramics[END_REF][START_REF] Sikder | Microwave Processing of Calcium Phosphate and magnesium phosphate based orthopedic bioceramics: A state-of-the-art review[END_REF][START_REF] Chen | Microwave Sintering and Fracture Behavior of Zirconia Ceramics[END_REF]. These benefits are related to the heating process: the electromagnetic field directly heats the samples by dipolar polarization, leading to higher heating rates and lower dwell times [START_REF] Bykov | High-temperature microwave processing of materials[END_REF]. Therefore, microwave sintering has a potential economic and environmental interest, especially for ceramics which require a long sintering time like nuclear fuels. Nuclear ceramics are generally prepared by the conventional metallurgical powder process with the following steps: preparation of powder, milling, pressing into green pellets and sintering. This last step is carried out at a temperature range of 1650-1750°C with a minimum dwell time of 4h under reducing atmosphere (generally Ar/H2) [START_REF] Kutty | Densification behaviour of UO2 in six different atmospheres[END_REF][START_REF] Le Guellec | Sintering Investigations of a UO2-PuO2 Powder Synthesized Using the Freeze-Granulation Route[END_REF]. Few studies have reported densification of UO2 by microwave sintering. Ford et al. succeeded in heating in few minutes UO2 pellets until 1100°C [START_REF] Ford | High Temperature Chemical Processing via Microwave Absorption[END_REF]. Subramanian et al. achieved a relative density of 95.5% Theoretical Density (TD) within a sintering time of less than 1h [START_REF] Subramanian | A novel method of sintering UO2 pellets by microwave heating[END_REF]. Yang et al. attempted to sinter the specimens at heating rates of 20-30°C/min but they noted formation of cracks [START_REF] Yang | Microwave process for sintering of uranium dioxide[END_REF]. Despite promising results, these investigations showed the difficulty to obtain defect-free samples and to precisely control heating cycles. This problem has also been encountered for microwave sintering of other ceramics [START_REF] Ramesh | Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia[END_REF][START_REF] Vijayan | Microwave sintering of nanosized hydroxyapatite powder compacts[END_REF] and is mainly due to the use of thermocouples for temperature measurement. The reliability of a thermocouple is low because it generally interacts with the electromagnetic field and gives a wrong measurement of the temperature. Moreover, thermocouples can disturb the field in the vicinity of the samples and induce heterogeneous heating of ceramic parts. It is necessary to use specific temperature measurement methods to precisely control and follow up the microwave sintering. Pert et al. first proposed to use pyrometers as a contactless temperature measurement technique [START_REF] Pert | Temperature Measurements during Microwave Processing: The Significance of Thermocouple Effects[END_REF]. Similarly, contactless dilatometric methods have been set up to measure the samples' shrinkage [START_REF] Żymełka | Contactless Monitoring of Shrinkage and Temperature Distribution during Hybrid Microwave Sintering[END_REF][START_REF] Croquesel | Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder[END_REF].

Another characteristic of microwave heating is the volumetric heating of materials. Unlike conventional solutions, where heat is transferred by convection or conduction from the outside to the product through its surface, microwaves heat the entire product mass directly and homogeneously. However, interaction of ceramics with microwaves depends on the material's dielectric properties. Some ceramics (e.g., alumina) have a low dielectric loss tangent at low temperature and a coupling susceptor (i.e. a material having high dielectric loss at room temperature) can be required to initiate heating. The effect of this susceptor (nature, geometry, relative position to the sample) on the electromagnetic field distribution in the cavity has to be taken into account [START_REF] Heuguet | Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina[END_REF].

Moreover, sintering of nuclear pellets in a microwave cavity requires to take into account different specifications: high sintering temperatures, control of the atmosphere during the thermal treatment and possibility to use the cavity in a glove box (for radiological protection).

For these reasons, a complete study on microwave sintering of nuclear fuels firstly necessitates to design a specific microwave cavity. Such an experimental setup was developed and is described in this paper. A particular attention was paid to the dimensions and geometries of the cavity and the sintering cell (i.e., the insulation box which contains the specimen to heat and the susceptors). Both were designed using Finite Element (FE) simulation to optimize the electromagnetic field distribution in susceptors and samples.

Reliable in situ monitoring of the samples' shrinkage and temperature is performed with an optical dilatometer and calibrated pyrometers. Temperature measurement by pyrometers is also used by a dedicated software, including a PID controller, which automatically modifies the microwave power in the cavity to precisely control sintering cycles. A detailed description of the whole setup is given in this paper. Results of UO2 pellets sintering are also presented to demonstrate the ability of the setup to sinter nuclear fuels but also the accuracy of dilatometric measurements and control of thermal cycles. The microwave furnace designed in this study is a 2.45 GHz multimode parallelepipedic cavity equipped with standard elements of a microwave setup (Figure 1). Microwaves are produced by a magnetron powered with a 3 kW generator (GMP 30KSM 56T208 FST 3 IR, Sairem, France). Power sensors are placed between the magnetron and the cavity to measure the incoming power inside the cavity and the outgoing reflected power from the cavity. These incident and reflected powers are continuously measured during sintering to precisely control heating cycles (see part 2.3.3). A standard WR340 waveguide with a TE10 propagation mode is used to transfer microwaves from the magnetron to the cavity. This waveguide is equipped with an impedance tuner (AI EH WR340, Sairem, France) which aims at minimizing the reflected power outgoing from the cavity, i.e. at maximizing the power transmission from the source (magnetron) to the load in the cavity. A mode stirrer, made of copper blades (for its high electrical conductivity), is used inside the cavity to obtain a homogeneous heating and to limit hot spots. To use this setup in a glovebox, the surface temperature of the cavity must remain under 50°C to avoid melting of the gloves. For this purpose, the walls of the cavity are equipped with a water cooling system designed with copper tubes for efficient transfer of calories (Figure 1). Moreover, the cavity is equipped with a sliding door and specific lockers to facilitate its use inside the glovebox (Figure 1). A gas system is also used to control the sintering atmosphere with Ar/H2 and the cavity is sealed to allow for a slight Ar/H2 overpressure during sintering of samples.

The microwave setup

Main elements of the setup

Internal sizes of the multimode cavity are important parameters to optimize the sample heating. According to previous work [START_REF] Żymełka | Densification and thermal gradient evolution of alumina during microwave sintering at 2.45GHz[END_REF], internal sizes were chosen to be close to nλ/2 (with n being an integer and λ the wavelength corresponding to the frequency of the magnetron) to obtain a homogeneous distribution of the field inside the cavity. In order to get the highest field intensity at the cavity center, n was chosen to be an odd number. Moreover, the cavity sizes must be determined considering also the size and the properties (permittivity and electrical conductivity) of the components constituting the sintering cell (see part 2.2). Furthermore, sizes of the microwave furnace are also limited by the depth and height of the glovebox (around 800 mm). Taking all these conditions into account, FE simulations were carried out with COMSOL Multiphysics software (Electromagnetic Wave module) to calculate the internal sizes of the cavity (close to nλ/2), with n an odd number and nλ/2 < 800 mm.

Chimneys are used to measure temperature (pyrometers) and shrinkage (optical dilatometry) of the UO2 sample during sintering (see part 2.3 and Figure 3). Chimneys dimensions were also determined by FE simulation (not shown here) to prevent from electromagnetic leakage outwards.

The sintering cell

The sintering cell developed for heating of nuclear ceramics is made of thermal insulators and susceptors. Thermal insulators are made of aluminosilicate fibers plates (KVS 184-400, Rath ® , Germany) to minimize heat losses from the sample and the susceptors by conduction and radiation. The Rath ® material is used because it follows the main requirements for microwave applications: transparency to microwaves, stability at high temperature (until 1800°C) and low thermal conductivity (0.33 W.m -1 .K -1 at 1400°C). External dimensions of the cell were chosen in accordance with the internal dimensions of the cavity and close to n'λ/2, with n' an odd number. This design limits the creation of reflected waves on the crucible surface which could be in phase opposition with incident waves (otherwise, the total wave would be cancelled).

UO2 properties change with temperature, including the electrical conductivity which increases above 1300°C [START_REF] Bates | Electrical conductivity of uranium dioxide[END_REF]. Increase of the electrical conductivity has a strong impact on the penetration depth of the field inside UO2 ( [START_REF] Bates | Electrical conductivity of uranium dioxide[END_REF][START_REF] Mishra | Microwave-material interaction phenomena: heating mechanisms, challenges and opportunities in material processing[END_REF] penetration depth decreases with the electrical conductivity). Due to this effect, less energy is dissipated in UO2 at sintering temperatures and it is difficult to keep a high and constant heating rate of the sample (> 10°C/min), unless using high power levels (limited to 3kW with the generator). To get around this problem, susceptors are used in the sintering cell to provide complementary thermal energy (mainly by radiation) to the sample in order to keep high and constant heating rates.

Moreover, the use of susceptors allows for a hybrid and uniform heating through the entire sintering process. FE simulation (COMSOL -Electromagnetic Waves module) was used to determine the appropriate dimensions and positions of these susceptors inside the sintering cell and the furnace. In the simulation, cavity boundaries were considered as perfect electric conductors and the incoming power in the cavity was set to a constant value of 500W. Inside of the cavity was considered as air with a relative permittivity ε = 1. The Rath ® material used as insulator for the sintering cell was considered as perfectly transparent to microwaves and not taken into account in FE simulation. Only the UO2 sample (cylindrical pellet 10 mm height and 8 mm diameter) and the susceptors were simulated in the cavity. Silicon carbide and 3Y-TZP (3 mol% yttria stabilized zirconia) were selected for the nature of the susceptors because they are frequently used for this application due to their interesting dielectric properties (high permittivity and low electrical conductivity) [START_REF] Heuguet | Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina[END_REF]. Table 1 shows the values of real and imaginary parts of permittivity (values at room temperature) used in the FE calculations for UO2, SiC and 3Y-TZP [START_REF] Heuguet | Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina[END_REF][START_REF] Gesi | Dielectric Constant of UO2 at 9.4 GHz[END_REF][START_REF] Manière | Fully coupled electromagnetic-thermalmechanical comparative simulation of direct vs hybrid microwave sintering of 3Y-ZrO2[END_REF]. Dielectric properties of UO2 are poorly known and few values are available in the literature. The real part of the permittivity of UO2 given by Gesi et al. [START_REF] Gesi | Dielectric Constant of UO2 at 9.4 GHz[END_REF] at 9.4GHz was used for the calculations and assumed to be equivalent at 2.45GHz. The imaginary part of the permittivity was set to ε'' = 0.4 because we hypothesize a similar heating behavior of UO2 and SiC under microwave heating. 2), because the same field pattern is calculated. This effect will enable for equivalent heating of both plates and will lead to homogeneous heating of the UO2 sample by susceptors. However, the average electric field is more uniform and of higher intensity in 3Y-TZP susceptors than in SiC (see Figure 2 and Table 2), due to the higher imaginary part of the permittivity of 3Y-TZP and its lower electrical conductivity (see Table 1). Table 2 indicates that the average electric field in the UO2 pellet is lower than in the susceptors. Despite this effect, the higher permittivity of UO2 will allow for direct heating by the field. However, the average electric field in the UO2 pellet is higher using 3Y-TZP susceptors than SiC. This effect, combined with the higher average electric field in 3Y-TZP susceptors compared to SiC, demonstrates that the use of 3Y-TZP should be of better interest to limit the power needed to heat the UO2 sample.

To conclude, FE simulations demonstrate that the arrangement of the susceptors in the cell enables for hybrid heating of the pellet. No "screening effect" of the susceptors or the UO2 sample occurs with this arrangement, whatever the nature of susceptors. SiC and 3Y-TZP are both of interest but the use of 3Y-TZP could limit the power needed to heat and sinter UO2

sample. The less the power inside the cavity, the less the probability to create hot spots, arcing or plasma when sintering at high temperatures under reducing atmosphere.

Control and monitoring of the sintering 2.3.1. Optical dilatometry

Optical dilatometry is used to record the shrinkage of the samples during sintering. This technique was developed in previous studies [START_REF] Żymełka | Contactless Monitoring of Shrinkage and Temperature Distribution during Hybrid Microwave Sintering[END_REF][START_REF] Croquesel | Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder[END_REF]. A CCD camera (RT-mvBC-X104iG, Opto Engineering, Italy) records pictures of the flat circular face of the cylindrical sample through a chimney and a hole in the sintering cell (see Figure 3a and3b). A low-pass filter (filtering wavelengths higher than 700 nm) and a band pass filter (600-650 nm) are used to select only the wavelength range for which the camera lens is optimized. Filters also reduce chromatic aberrations, which can lead to errors in the sample diameter measurement. The camera is placed face to the central chimney with a specific support designed to precisely adjust it (see Figure 3a and3b). Three LEDs (CREE XREWHT-L1-0000-009E7, Conrad, France) are placed on two other chimneys to light the sample at low temperature. For high temperatures (> 700°C), the sample is visible due to its own radiation. Recorded images are processed after sintering with a home-made Labview® software (National Instruments, USA)

which automatically detects pellet's edges, to measure its diameter [START_REF] Żymełka | Contactless Monitoring of Shrinkage and Temperature Distribution during Hybrid Microwave Sintering[END_REF][START_REF] Croquesel | Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder[END_REF]. The program finally output the evolution of the pellet diameter during sintering.

Temperature measurement

Temperature measurement is performed with IR pyrometers. Due to the large temperature range to cover (from room temperature to 1700°C), two pyrometers are used: a monochromatic pyrometer sensitive to wavelengths 2-2.5 µm in the temperature range 75-1300°C (IGA6/23, Lumasense Technologies, Germany) and a bichromatic pyrometer sensitive to wavelengths 0.9-1.05 µm in the temperature range 700-1800°C and equipped with a thermal camera (ISR6-TI, Lumasense Technologies, Germany). Pyrometers are focused on the flat circular face of the UO2 pellet through chimneys and the hole in the sintering cell and precisely arranged using the same support as the camera (see Figure 3). To get an accurate measurement with pyrometers, it is necessary to know the apparent emissivity ε of UO2 in the experimental conditions, which depends on the sample surface (nature, roughness…) and its environment (susceptors and insulator material). A calibration protocol of the apparent emissivity, based on the melting point of selected oxide materials, was used to determine ε in the real sintering conditions. A detailed description of this protocol can be found elsewhere [START_REF] Croquesel | Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder[END_REF][START_REF] Macaigne | Microwave Sintering of Pure and TiO2 doped MgAl2O4 Ceramic Using Calibrated, Contactless Insitu Dilatometry[END_REF]. Briefly, a small amount of oxide powder with a known melting point was inserted in the hole of a UO2 annular pellet (diameter of 9.6 mm, height of around 13 mm and hole diameter of 2.6 mm). The pellet was slowly microwave heated (< 5°C/min) in the sintering cell until the melting of the oxide (detected with the camera). When the oxide started to melt, the emissivity of the pyrometers was modified to match the pellet temperature with that of the oxide melting temperature. Two oxides were used according to their melting temperature and stability at high temperature: GeO2 with a melting temperature of 1115°C, (Puratronic, Alfa Aesar) and Nb2O5 with a melting temperature of 1512°C (HC-Starck). Three calibration tests for each oxide were carried out for reproducibility. GeO2 was used to determine ε for the monochromatic pyrometer and Nb2O5 enabled to calculate the emissivity ratio (k = ε1 / ε2) of the bichromatic pyrometer. Average values of 0.525 and 1 were determined for ε and k respectively.

Control of the thermal cycle

A specific control software was developed using Labview® to carry out sintering experiments with controlled heating cycles. This software uses a PID controller based on the temperature measured by pyrometers. The incident power delivered by the generator is continuously adjusted during the sintering cycle to match the measured temperature with the set temperature. Moreover, the impedance tuner is adjusted manually during heating to minimize the reflected power. The power sensors, the camera and the pyrometers are also connected to this Labview® software in order to record the data (power, temperature, images) used to control the thermal treatment and to obtain sintering curves (as conventional sintering in a dilatometer).

Experimental validation of the setup

Sintering tests were conducted using the developed microwave setup with the following two main targets:

-check the possibility to heat and sinter uranium oxide based samples in the cavity with the designed sintering cell, -characterize the samples densification after sintering to demonstrate that equivalent characteristics of the pellets than under conventional heating can be obtained.

Preparation of samples and sintering parameters

UO2 green pellets were prepared using uranium dioxide powder synthesized by the Ammonium Di-Uranate (ADU) process. The BET specific surface area of the powder is 3.02 m 2 .g -1 . Cylindrical compacts were shaped by double-effect uniaxial cold pressing in a 10 mm diameter die at 350 MPa. To reduce the friction coefficient between the powder and the die and facilitate the compression, stearic acid was sprayed on the die surface. The density of the green bodies was around 5.8 g.cm -3 . The samples were inserted into the sintering cell described in part 2.2. Two types of plates (SiC and 3Y-TZP) were tested as susceptors inside the cell. The following thermal cycle was used: heating at a rate of 4°C/min until 150°C (for degassing and to avoid sample cracking) and then heating at a rate of 20°C/min until 1650°C with a dwell time of 10 min. After the dwell time, the microwave power was turned off for natural cooling. A flow and a slight overpressure of Ar/4%-H2 was imposed in the cavity during the whole thermal cycle.

Figure 4 presents the evolution of the recorded data (temperature, incident, absorbed and reflected power vs time) during heating of UO2 pellets with SiC and 3Y-TZP susceptors (Fig. 4a and4b). Figure 4a shows the data for hybrid heating of a UO2 pellet with SiC susceptors. A constant heating rate of the UO2 pellet of around 15°C/min is measured until 1400°C. From room temperature to 1400°C, the PID controller increased constantly the incident power until 1400W, to keep the heating rate of the pellet. It can be noticed that reflected power (outgoing from the cavity) is very low, all of the incident power is used by the pellet and susceptors.

Above 1400°C, an increase of the incident power is observed to keep a constant heating rate of the pellet. Strong variations of the incident power are also evidenced until 1586°C, where heating was stopped due to the creation of unexpected gases and plasmas in the cavity. A visual control of susceptors and insulators after heating evidenced a change in the color of SiC plates and the presence of a yellow/brown deposit on thermal insulators. Microscopic analysis (not shown here) showed a change in the morphology of SiC plates and XRD analysis demonstrated that the yellow/brown deposit was mainly composed of silicon. These analyses were explained by a reaction between SiC and H2 above 1400°C, leading to the creation of gaseous hydrides. This reaction explains the strong increase and variations of the incident power above 1400°C (a part of the power was used to sustain reaction) and the difficulty to heat the pellet.

Figure 4b shows the data for hybrid heating of a UO2 sample with 3Y-TZP susceptors. A constant heating rate of the UO2 pellet of 18.5°C/min is measured until 1650°C and the dwell time of 10 min was correctly followed. Only two small disturbances in the heating cycle can be observed. The first disturbance is located around 700°C with a slight decrease of the pellet temperature, but quickly adjusted by the PID controller. This effect is due to the change in dielectric properties of 3Y-TZP susceptors around 700°C. Indeed, the imaginary part of the 3Y-TZP permittivity quickly increases around 700°C [START_REF] Song | Grain size control of UO2 pellets by adding heat-treated U3O8 particles to UO2 powder[END_REF], leading to faster heating. The power inside the cavity was lowered to avoid thermal runaway of the susceptors, explaining the small decrease of the incident power and of the UO2 pellet temperature around 700°C, for a few tens of seconds. Afterwards, the PID controller adjusted progressively the incident power to follow the set thermal cycle. Optimization of the PID controller parameters will allow for a better control of this phenomenon. Automation of the impedance tuner is also planned to help minimizing the problem. The second disturbance can be observed between 1500°C and 1600°C. Figure 5 presents a zoom on the temperature and incident power vs time curve for sintering with 3Y-TZP susceptors (Figure 4b). This disturbance went along with important variations in the incident power sent to the cavity by the PID controller. This phenomenon can be attributed to the increase of electrical conductivity of UO2 [START_REF] Bates | Electrical conductivity of uranium dioxide[END_REF] in this temperature range, which lead to a decrease of the electric field depth penetration inside the pellet. But, the increase of electrical conductivity was compensated by the increase of the dielectric loss of 3Y-TZP in this temperature range [START_REF] Arai | Comparison of Techniques for Measuring High-Temperature Microwave Complex Permittivity: Measurements on an Alumina/Zircona System[END_REF]. Therefore, as visible in Figure 5, the disturbance lasted around 3 min and the follow-up of the set temperature took over just after, until 1650°C.

Figure 5: Zoom on the temperature and incident power vs time curve (see Figure 4b) for sintering with the 3Y-TZP susceptors in the range 1400-1650°C.

Thermal images obtained with the bichromatic pyrometer were used to measure the temperature of a UO2 pellet and its environment during microwave heating at 15°C/min with 3Y-TZP susceptors (see Figure 6). Figure 6a shows that the pellet temperature is homogeneous but higher than its environment at low temperatures (under 1000°C), due to the higher dielectric properties of UO2 compared to 3Y-TZP susceptors (see Table 1). This image demonstrates that the designed sintering cell enables for hybrid heating of the UO2 pellet. For higher temperatures, Figure 6b and Figure 6c show that the temperature of the pellet environment (sintering cell) increases because more power is dissipated in 3Y-TZP susceptors (change in their dielectric properties). The temperature of the cell and the pellet is equivalent for sintering temperatures (>1500°C). The equivalent field distribution pattern inside both 3Y-TZP susceptors (see §2.2) induces equivalent heating of the lower and upper plate, which leads to homogeneous temperature inside the cell and the pellet (Figure 6c). Although less energy is dissipated inside UO2 at sintering temperatures due to the increase of its electrical conductivity (see §2. Figure 4b also shows that the total power needed to heat the pellet until 1650°C is 1200W, lower than the power needed with SiC susceptor to heat only until 1400°C (Figure 4a). This is well correlated with FE simulation, which demonstrates a higher field intensity inside 3Y-TZP susceptors (see Table 2).

Carried out sintering experiments evidenced drawbacks for the use of SiC as susceptors. For the same thermal cycle, more power is needed with SiC than 3Y-TZP to heat the UO2 pellet.

However, more the power more the probability to create plasma or electrical arcs in the cavity under reducing atmosphere. Furthermore, reaction of SiC with H2 above 1400°C damages SiC plates, disturbs heating of the UO2 pellet and pollutes the sintering cell and the sample.

These drawbacks prevent the use of SiC susceptors and only 3Y-TZP susceptors are suited for sintering of UO2 pellets in the designed microwave setup.

Densification of the samples

Figure 7 presents pictures of a green UO2 pellet and a UO2 pellet sintered with 3Y-TZP susceptors. The sintered pellet is crack-free and the comparison with the green pellet shows the homogeneity of shrinkage. It demonstrates that heating of the pellet is homogeneous due to hybrid heating: pellets are heated in the bulk by direct interaction with microwaves and on their surface by radiation of the susceptor. Contrary to conventional sintering [START_REF] Żymełka | Densification and thermal gradient evolution of alumina during microwave sintering at 2.45GHz[END_REF], high heating rates (>10°C/min) can be used to sinter UO2 pellets due to bulk heating, without leading to heterogeneous shrinkage. Moreover, no color change can be identified between green and sintered pellets (Figure 7), assuming that no phase transformation from UO2 to U3O8 or U4O9 phases have occurred during sintering: Ar/H2 flow was appropriate to avoid detrimental oxidation [START_REF] Kutty | Densification behaviour of UO2 in six different atmospheres[END_REF]. These findings validate the use of the setup for homogeneous heating of UO2 pellets until high temperatures (1650°C) under reducing atmosphere, with controlled thermal cycles. The final density of the pellet sintered with 3Y-TZP susceptors (Figure 7b) was measured with the Archimedes method using ethanol as liquid medium. The pellet reached a final density of 10.2 g.cm -3 (93 %TD). This high density was obtained in a total sintering time of only 4 h, cooling comprised (see Figure 5), although no particular optimization of the conventional sintering with a total sintering time of 24 h and higher temperatures ( [START_REF] Song | Grain size control of UO2 pellets by adding heat-treated U3O8 particles to UO2 powder[END_REF][START_REF] Radford | Properties, blending and homogenization of (U, Th)O2-UO2 powder[END_REF][START_REF] Harada | UO2 sintering in controlled oxygen atmospheres of three-stage process[END_REF][START_REF] Kumar | Low temperature two-step sintering of nanocrystalline UO2[END_REF]).

The time needed to sinter UO2 pellets with this innovative microwave sintering setup is divided by six compared to conventional sintering. The radial shrinkage of the pellet vs temperature, computed from the optical dilatometer and pyrometers data, is plotted in Figure 8. A final radial shrinkage of -18% was reached. The accuracy of the optical dilatometer and the possibility to follow the shrinkage over time during microwave are demonstrated. The ability to calculate the instantaneous relative density for each temperature using radial shrinkage of the samples will be of high interest for future experiments. It will be possible to directly and reliably compare the sintering behavior of nuclear pellets under microwave heating and conventional heating.

Conclusion

The original instrumented and automated multimode cavity described in this paper allows reliable studies on the microwave sintering of nuclear ceramics under specific atmosphere.

Dimensions of the cavity and the sintering cell were designed using FE simulation in order to perform hybrid microwave heating and to integrate the setup in a glovebox. The other originality of this setup is the use of a PID controller, coupled with an optical dilatometer and calibrated pyrometers, to control the heating rate and to measure the shrinkage of specimens.

Recorded data during heating can be used for reliable comparison studies with conventional sintering behavior. These different elements of the setup are the main innovation of this work compared to previous studies on microwave sintering of nuclear ceramics in the literature.

The first tests evidenced the possibility to control the heating of samples until high temperatures (1650°C) under reducing atmosphere, without sparks or plasma. The homogeneous heating of the pellet is confirmed by thermal IR imaging during sintering. The first pellets of UO2 sintered in the cavity reached 93 %TD without deep optimization of the sintering parameters. This high density was obtained with a higher heating rate (20°C/min) and thus, a shorter thermal cycle compared to conventional sintering. Therefore, this setup provides the opportunity for a more systematic sintering study of uranium oxide based nuclear materials (UO2, (U, Pu)O2, (U, Am)O2 …) with the aim to obtain high densities in a considerably reduced sintering time compared to conventional sintering. Moreover, this setup will be used to study possible effects of microwaves on densification and microstructure of nuclear fuels.

Conflict of interest

None

Data availability

Figure 1 :

 1 Figure 1: a) Picture showing the main standard elements of the setup and b) drawing showing

Figure 2

 2 Figure2shows the results of the electric field distribution inside the cavity with the optimized

Figure 2 :

 2 Figure 2 : Computed electric field pattern in the furnace with a) two SiC plates in horizontal

Figure 3 :

 3 Figure 3: a) Picture and b) drawing of devices (pyrometers, camera, filters and LEDs) and

Figure 4 :

 4 Figure 4: Temperature and incident, absorbed and reflected powers vs time for the pellets

  2)[START_REF] Bates | Electrical conductivity of uranium dioxide[END_REF], thermal energy supplied by susceptors enables to keep a constant heating rate and homogeneous temperature of the pellet.

Figure 6 :

 6 Figure 6: Thermal images of a UO2 pellet (annular shape) during microwave heating in the

Figure 7 :

 7 Figure 7: Picture of a) green UO2 pellet and b) UO2 pellet sintered with 3Y-TZP susceptors

Figure 8 :

 8 Figure 8: Radial shrinkage vs temperature for the pellet sintered with 3Y-TZP susceptors at

Table 1 :

 1 Material data for FE simulation

	UO2	SiC	3Y-TZP

Table 2 :

 2 Average electric field norm inside the UO2 pellet and the SiC and 3Y-TZP susceptors calculated for an input power of 500W in the cavity

	Electric field norm (V/m)
	SiC susceptors	3Y-TZP susceptors

The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

Acknowledgements

This work was supported by the Commissariat à l'Energie Atomique et aux Energies Alternatives.