MD Nokovitch Lara

Deneuve Sophie
email: sophie.deneuve@lyon.unicancer.fr

Caractéristiques anatomiques du système veineux temporal superficiel et implications en microchirurgie. Anatomical characteristics of the superficial temporal venous system and implications in microsurgery

Introduction :

Les vaisseaux temporaux superficiels sont facilement abordables chirurgicalement en région pré-auriculaire, et se prêtent particulièrement à la reconstruction microchirurgicale du tiers moyen et supérieur de la face du fait de leur localisation [START_REF] Hansen | Superficial temporal artery and vein as recipient vessels for facial and scalp microsurgical reconstruction[END_REF] [START_REF] Herrera | Microvascular approach to scalp replantation and reconstruction: a thirty-six year experience[END_REF]. Ils peuvent également représenter une option intéressante en situation post-radique et en cas de chirurgies itératives au niveau cervical, car ils se situent la plupart du temps en dehors des champs d'irradiation et des zones de fibrose [START_REF] Chia | An algorithm for recipient vessel selection in microsurgical head and neck reconstruction[END_REF] [START_REF] Frohwitter | Microvascular reconstruction in the vessel depleted neck -A systematic review[END_REF]. Ces derniers évoluant au niveau préauriculaire directement sous la peau au sein du fascia temporal superficiel [START_REF] Zwetyenga | The pedicled superficial fascia temporalis flap][END_REF], un contrôle Doppler de l'anastomose peut être facilement réalisé en post-opératoire [START_REF] Li | Superficial Temporal Versus Cervical Recipient Vessels in Maxillary and Midface Free Vascularized Tissue Reconstruction: Our 14-Year Experience[END_REF]. Enfin, en l'absence d'évidement cervical nécessaire, la voie d'abord pré-auriculaire utilisée pour la rechercher des vaisseaux temporaux superficiels laisse une cicatrice beaucoup plus discrète qu'une incision de cervicotomie [START_REF] Revenaugh | Minimizing morbidity in microvascular surgery: small-caliber anastomotic vessels and minimal access approaches[END_REF].

Les travaux de Ricbourg ont aidé à préciser l'anatomie du système vasculaire temporal superficiel [START_REF] Ricbourg | L'artère temporale superficielle: étude anatomique et déductions pratiques[END_REF]. Ainsi, l'artère temporale superficielle (ATS) nait de la branche de bifurcation externe et superficielle de la carotide externe et se divise au-dessus de l'arcade zygomatique en 2 branches pariétale et frontale [START_REF] Ricbourg | L'artère temporale superficielle: étude anatomique et déductions pratiques[END_REF]. En revanche, si l'on sait que la veine temporale superficielle (VTS) naît de la réunion de nombreuses branches antérieures et supérieures en provenance des régions latérales du front et du cuir chevelu [START_REF] Mitz | Several radio-anatomical aspects of superficial veins in the face[END_REF], il est plus difficile de définir précisément l'anatomie du réseau veineux temporal superficiel, d'une part du fait du peu d'études anatomiques réalisées, et d'autre part en raison de la variabilité importante présentée par l'arbre veineux temporal superficiel [10][11][9].

Bien que considérés comme des vaisseaux receveurs fiables par plusieurs auteurs [START_REF] Li | Superficial Temporal Versus Cervical Recipient Vessels in Maxillary and Midface Free Vascularized Tissue Reconstruction: Our 14-Year Experience[END_REF][12] [START_REF] Nahabedian | Recipient vessel analysis for microvascular reconstruction of the head and neck[END_REF], les vaisseaux temporaux superficiels restent sous-exploités en microchirurgie du fait de leur faible calibre, avec une incongruence entre vaisseaux donneurs et receveurs pouvant être majeure pour la veine [START_REF] Hansen | Superficial temporal artery and vein as recipient vessels for facial and scalp microsurgical reconstruction[END_REF] [START_REF] Revenaugh | Minimizing morbidity in microvascular surgery: small-caliber anastomotic vessels and minimal access approaches[END_REF]. La réticence de certains chirurgiens à utiliser les vaisseaux temporaux superficiels comme vaisseaux receveurs, s'explique également par supposée inconstance de la VTS rapportée dans la littérature [START_REF] Hansen | Superficial temporal artery and vein as recipient vessels for facial and scalp microsurgical reconstruction[END_REF]

ii. Rapports vasculaires :

Concernant les rapports artério-veineux temporaux superficiels, au niveau de la région pré-auriculaire, le tronc veineux pariéto-temporal commun évoluait toujours au contact et en arrière du tronc artériel pariéto-temporal superficiel. En amont, les rapports entre branches artérielles et veineuses étaient en revanche variables : en effet, l'affluent veineux pariétal était dans 70% des cas (7/10) accolé à la branche pariétale de l'ATS, tandis que dans 30% des cas (3/10), il évoluait à distance de cette dernière.

L'affluent veineux frontal, quand il existait, était accolé à la branche frontale de l'ATS dans 90% des cas (4/5). Dans 10% des cas (1/5), il évoluait à distance de la branche frontale de l'ATS.

b. Etude écho-Doppler sur sujets sains :

Les veines temporales superficielles de 5 hommes et 5 femmes, d'âge moyen de 34 ans [26 -42] ont été étudiées par écho-Doppler. Trois sujets présentaient un tabagisme actif.

Aucun sujet ne présentait de facteurs de risques ou d'antécédents cardiovasculaires ou thromboemboliques.

La VTS en région pré-auriculaire a pu être repérée par écho-Doppler chez tous les volontaires sains de façon bilatérale (Figure 3). Cette dernière évoluait de façon systématique en arrière de l'ATS.

Le diamètre de la VTS présentait d'importantes variations interindividuelles, avec un diamètre médian de 1,4 mm [0,6-2] pour la VTS droite, et 1,2 mm [0,5-1,9] pour la VTS gauche (Tableau 2).

III. Discussion :

Un tronc veineux commun pariéto-temporal a constamment été retrouvé dans notre étude, que ce soit sur sujet anatomique ou in vivo par écho-Doppler. Cette constance de la VTS dans la région pré-auriculaire a également été constatée par Delgove en 1991, qui rapporte l'existence d'un tronc collecteur pariéto-temporal constant présentant un trajet vertical préauriculaire [START_REF] Delgove | Drainage of the scalp by the superficial temporal vein: surgical implications[END_REF]. L'inconstance de la VTS décrite par certains auteurs [ Notre étude a aussi permis de préciser la fréquence des variations du territoire de drainage du tronc veineux temporo-maxillaire, aucune information n'étant donnée à ce sujet dans les ouvrages d'anatomie [17][18]. La dénomination de tronc veineux temporomaxillaire, tronc veineux formé par la réunion de la VTS et la veine maxillaire interne, se base sur les travaux de Rouvière qui relève le caractère inconstant de la veine rétromandibulaire [START_REF] Rouvière | Anatomie humaine: descriptive, topographique et fonctionnelle[END_REF]. Ainsi, le tronc veineux temporo-maxillaire se draine dans deux tiers des cas conjointement dans la veine jugulaire externe, et la veine jugulaire interne par l'intermédiaire de la veine rétromandibulaire, tandis que dans un tiers des cas ce dernier se draine exclusivement dans la veine jugulaire externe, la veine rétromandibulaire étant alors absente. Des cas de drainage exclusif du tronc veineux temporo-maxillaire dans la veine jugulaire interne ont également été décrits dans la littérature [START_REF] Osborn | Craniofacial venous plexuses: angiographic study[END_REF], néanmoins, nous n'en avons pas retrouvé au cours des dissections réalisées. Cela peut s'expliquer d'une part, par le faible nombre de pièces anatomiques étudiées, et d'autre part, par la faible fréquence de cette voie de drainage. Ces variations du territoire de drainage du tronc veineux temporomaxillaire soulèvent néanmoins la question de la fiabilité d'une anastomose sur la VTS en cas de ligature homolatérale de la veine jugulaire externe ou interne.

Les branches pariétales des vaisseaux temporaux superficiels en région pré-auriculaire ont une longueur exploitable d'environ 6,5 cm [START_REF] Li | Microvascular reconstruction of nasal ala using a reversed superficial temporal artery auricular flap[END_REF]. Le diamètre de la VTS en région préauriculaire est la plupart du temps supra-millimétrique [START_REF] Wong | Microvascular reconstruction in the vesseldepleted neck[END_REF]

  [START_REF] Li | Superficial Temporal Versus Cervical Recipient Vessels in Maxillary and Midface Free Vascularized Tissue Reconstruction: Our 14-Year Experience[END_REF].La principale problématique dans l'usage des vaisseaux temporaux superficiels en microchirurgie est ainsi veineuse. L'objectif de cette étude était donc de préciser les caractéristiques anatomiques du système veineux temporal superficiel à l'aide d'une étude anatomique cadavérique et d'une étude écho-Doppler sur sujets sains. Afin d'étudier les variations anatomiques de la veine temporale superficielle et de ses différentes branches au niveau de la région temporo-pariétale, 10 hémifaces (6 hémifaces droites et 4 hémifaces gauche) de 10 corps donnés à la science (3 sujets frais, 5 sujets frais congelés et 2 sujets formolés) ont été étudiés. Les hémifaces controlatérales des sujets étaient utilisées dans le cadre d'un DU d'implantologie. Le protocole de l'étude a été préalablement approuvé par le laboratoire d'anatomie Rockfeller de la faculté de médecine de Lyon. Tous les donneurs avaient préalablement donné leur consentement éclairé écrit indiquant leur souhait de donner leur corps à la science [14]. La cohorte de sujets étudiée comportait 3 hommes et 7 femmes. Avant toute dissection, les réseaux veineux et artériel étaient préalablement injectés par du latex liquide de couleur bleue pour les veines et rouge pour les artères. Pour ce faire, la carotide commune et la veine jugulaire interne étaient repérées et disséquées de façon bilatérale au niveau de la base du cou, avant d'être canulées à l'aide d'une sonde en silicone. Une ligature péri-canulaire au Vicryl 0 était réalisée afin d'éviter toute fuite lors de l'injection de latex. Les systèmes veineux et artériel étaient ensuite lavés abondamment à l'eau chaude injectée à l'aide d'une seringue de 50 cc, jusqu'à ce que le retour par les veines et artères controlatérales soit clair. Les artères carotides communes et les veines jugulaires internes étaient ensuite injectées de façon bilatérale par une solution de latex rouge pour les artères et bleue pour les veines. Un total de 90 mL de latex était injecté dans chaque vaisseau. Les têtes étaient conservées à 4°C pour une durée minimum de 24h et un maximum de 3 jours. Le travail de dissection débutait dans cet intervalle de temps. Le tracé de l'incision était dessiné au préalable au crayon dermographique. Une incision cutanée hémi-coronale, étendue en région pré-auriculaire et en avant de la ligne des cheveux en région cervicale était ensuite réalisée à la lame froide. La peau de l'hémiface était décollée en restant dans un plan superficiel situé sous les racines des cheveux au niveau du scalp, et au-dessus du fascia temporal superficiel en région temporo-pariétale, laissant apparaître les vaisseaux temporaux superficiels. Sous l'arcade zygomatique et au niveau de la région parotidienne, le plan de dissection s'approfondissait pour passer sous le SMAS, et continuait en région cervicale sous le muscle platysma. Une fois la peau et les tissus sous-cutanés réclinés vers l'avant, la dissection des veines cervicales pouvait débuter. La veine jugulaire externe et ses différentes branches étaient disséquées de bas en haut jusqu'à atteindre le pôle inférieur de la parotide. Une parotidectomie superficielle était ensuite réalisée afin d'exposer le tronc veineux temporo-maxillaire, permettant ainsi d'exposer l'ensemble des veines de drainage du réseau veineux temporal superficiel. Les paramètres étudiés étaient : a) le nombre de branches veineuses temporales superficielles principales en provenance de la région temporo-pariétale, b) leurs rapports anatomiques avec les branches artérielles correspondantes, c) le territoire de drainage du tronc veineux temporo-maxillaire. b. Etude écho-Doppler sur sujets sains : Le diamètre des vaisseaux étant difficilement objectivable sur sujet anatomique, une étude préalable du réseau veineux temporal superficiel a été réalisée par écho-Doppler sur sujets sains. La VTS a ainsi été repérée et étudiée par écho-Doppler au niveau de la région préauriculaire, chez 10 volontaires sains recrutés parmi le personnel médical et paramédical entre novembre 2017 et juin 2018. Les sujets étaient inclus dans l'étude après avoir signé un formulaire de consentement éclairé, et s'ils répondaient aux critères d'inclusion suivants : âge > 18 ans, absence d'antécédent traumatique majeur ou chirurgical au niveau cervicofacial. Un écho-Doppler cervico-facial effectué par un médecin vasculaire expérimenté, a été réalisé en décubitus dorsal chez tous les sujets à l'aide d'une sonde haute fréquence 12-16 MHz, avec mode Doppler pulsé et couleur (Affinity 70 Gy Philips). Par souci de reproductibilité, tous les examens ont été pratiqués par le même praticien. Les paramètres mesurés étaient : le diamètre maximum de la VTS, ainsi que les rapports artério-veineux temporaux superficiels en région pré-auriculaire. anatomique cadavérique : i. Origine, trajet, terminaison : Les caractéristiques des VTS disséquées sont présentées dans le Tableau 1. Un tronc veineux pariéto-temporal commun a été retrouvé chez tous les sujets anatomiques étudiés. Le tronc commun de la VTS recevait en moyenne 1,6 affluents majeurs (1 à 3) en provenance de la région temporo-pariétale. Cinquante pourcent des spécimens anatomiques (5/10) présentaient un affluent pariétal unique, sans affluent frontal associé (Type I) (Figure 1.A). 40% des spécimens anatomiques (4/10) présentaient 1 affluent pariétal et 1 affluent frontal (Type II) (Figure 1.B). Enfin, 10% des spécimens anatomiques (1/10) présentaient 2 affluents pariétaux, et 1 affluent frontal (Type III) (Figure 1.C). L'ATS se divisait quant à elle de façon systématique en 2 branches pariétale et frontale. Quand il existait plusieurs affluents majeurs, la réunion des affluents pariétal et frontal en un tronc veineux pariéto-temporal commun s'effectuait en moyenne 2,18 cm audessus de l'arcade zygomatique. Comparativement, la bifurcation de l'ATS en branche pariétale et frontale s'effectuait en moyenne 0,59 cm au-dessus de l'arcade zygomatique. La terminaison du tronc veineux temporo-maxillaire dans lequel se draine la VTS se faisait dans 60% des cas (6/10) à la fois dans la veine jugulaire interne et externe par l'intermédiaire de la veine rétromandibulaire, et dans 40% des cas (4/10) uniquement dans la veine jugulaire externe (Figure 2.A et 2.B).

Figure 2 :

 2 Figure 2 : Terminaison du tronc veineux temporo-maxillaire. 2.A : Drainage mixte dans la veine jugulaire externe et interne. 2.B : Drainage exclusif dans la veine jugulaire externe.

Figure 3 :Tableau 1 :Tableau 2 :

 312 Figure 3 : Etude par écho-Doppler de la veine temporale superficielle. Le diamètre maximum de la veine temporale superficielle droite est évalué à 1,2 mm. VTS DT: veine temporale superficielle droite.

Figure 1 :Figure 2 :

 12 Figure 1 :

Figure 3 :

 3 Figure 3 :

  En amont de la région pré-auriculaire, les rapports artério-veineux présentaient en revanche une variabilité importante, les principaux affluents veineux pouvant même évoluer à distance des branches artérielles correspondantes[START_REF] Ricbourg | L'artère temporale superficielle: étude anatomique et déductions pratiques[END_REF].

	Si le tronc veineux commun temporo-pariétal présente un caractère constant, les
	principaux affluents de la VTS présentent une certaine variabilité en nombre et dans leur
	trajet. Notre étude cadavérique en retrouve 1 à 3, l'affluent veineux frontal étant souvent
	absent. Ces observations correspondent à celles faites par Imanishi en 2002[15], et plus
	récemment Onishi en 2017 [16], rapportant une absence ou une hypoplasie de l'affluent
	frontal de la VTS dans environ 30% des cas. Cette absence d'affluent frontal légèrement plus
	élevée dans notre étude sur sujets anatomiques (50%), peut s'expliquer par un défaut
	paraître absents.
	L'étude du réseau veineux temporal superficiel au niveau de la région temporo-pariétale
	nous a par ailleurs conduit à préciser les repères anatomiques permettant un repérage facile
	de la VTS. En effet, la branche pariétale de la VTS était aisément repérable au niveau pré-
	auriculaire, et suivait systématiquement la branche artérielle correspondante, se retrouvant
	ainsi dans une « bande de repérage » décrite initialement par Lassau pour la branche

1]

[START_REF] Li | Superficial Temporal Versus Cervical Recipient Vessels in Maxillary and Midface Free Vascularized Tissue Reconstruction: Our 14-Year Experience[END_REF]

pourrait être la conséquence d' un défaut de dissection des vaisseaux temporaux superficiels. En effet, la qualité et la délicatesse de la dissection des vaisseaux temporaux superficiels est primordiale, car ses derniers une fois lésés peuvent se spasmer au risque de pariétale de l'ATS

[START_REF] Ricbourg | L'artère temporale superficielle: étude anatomique et déductions pratiques[END_REF]

. Pour rappel, cette dernière constitue une bande fixe de passage de 2cm de large axée sur le conduit auditif externe et parallèle au plan frontal. Dans cette zone, on retrouvait dans la grande majorité des cas une conformation classique artère-veine d'avant en arrière. d'injection de l'ensemble du réseau veineux temporal superficiel par le latex. Le mode de conservation des pièces anatomiques (frais, frais congelé, formol) n'a pas semblé être un facteur influant sur la qualité de l'injection du système veineux temporal superficiel lors des dissections réalisées. Le faible nombre de sujets anatomiques étudiés, ainsi que la contrainte de ne disséquer qu'une hémiface par sujet ne nous a pas permis de décrire les variations du nombre d'affluent de la VTS en fonction du côté.

Variations anatomiques du réseau veineux temporal superficiel. 1.A : Réseau veineux temporal superficiel de type I.

  [START_REF] Delgove | Drainage of the scalp by the superficial temporal vein: surgical implications[END_REF], suffisant à la réalisation des anastomoses vasculaires en termino-terminal. Dans notre série, les diamètres sont mesurés entre 0.6mm et 2mm. Les mesures du diamètre de la VTS par écho-Doppler étant difficiles à réaliser, le même praticien expérimenté a procédé aux mesures afin de limiter le biais lié à l'opérateur dans notre étude. D'autres paramètres peuvent également biaiser ces mesures, comme la température de la salle ou du patient, ou encore la pression exercée par la sonde d'échographie sur la veine, qui tend à en modifier le diamètre en l'aplatissant. Les problèmes d'incongruence de calibre en cas d'anastomose sur les vaisseaux temporaux superficiels ne sont pas différents de ceux retrouvés en cas de réalisation d'un lambeau perforant[START_REF] Taylor | The angiosomes of the body and their supply to perforator flaps[END_REF][23][START_REF] Sinna | About perforator flaps…20 years later][END_REF]. Le risque d'engorgement et de thrombose veineuse en cas d'anastomose sur une veine de petit calibre reste au demeurant faible[START_REF] Li | Superficial Temporal Versus Cervical Recipient Vessels in Maxillary and Midface Free Vascularized Tissue Reconstruction: Our 14-Year Experience[END_REF], les veines étant dix fois plus distensibles que les artères pouvant ainsi recueillir un volume trois fois plus important[START_REF] Schneider | Clinical Physiology of the Venous System[END_REF]. Le taux de complications vasculaires rapporté dans la littérature en cas Présence d'un affluent pariétal unique, sans affluent

	LÉGENDES :
	IV.	Conclusion :
	Les vaisseaux temporaux superficiels sont des vaisseaux receveurs constants, et qui présentent de nombreux avantages en reconstruction microchirurgicale cervico-faciale. La VTS est aisément repérable en région pré-auriculaire et précède l'ATS à ce niveau, pour se Figure 1 : frontal ; 1.B : Réseau veineux temporal superficiel de type II. Présence de 2 affluents
	trouver dans la « bande de repérage » décrite par Lassau. Avec une longueur exploitable veineux principaux (1 affluent pariétal et 1 affluent frontal) venant se jeter dans le tronc
	moyenne de 6,5 cm et diamètre médian de 1,3 mm, la branche pariétale de la VTS présente commun de la veine temporale superficielle ; 1.C : Réseau veineux temporal superficiel de
	un calibre suffisant à la réalisation des anastomoses vasculaires. type III. Présence de 3 affluents veineux principaux (2 affluents pariétaux et 1 affluent
	frontal) venant se jeter dans le tronc commun de la veine temporal superficielle.
	L'auteur [Les auteurs] déclare[ent] ne pas avoir de liens d'intérêts
	d'anastomoses sur les vaisseaux temporaux superficiels est par ailleurs de seulement
	5,3%[6], et n'est pas supérieur à celui retrouvé en cas d'anastomoses au niveau cervical.
		L'usage des vaisseaux temporaux superficiels comme vaisseaux receveurs est tout
	particulièrement indiqué pour la reconstruction microchirurgicale du scalp et du tiers moyen
	du visage [2][6][26]. En l'absence d'évidement cervical associé, s'anastomoser sur les
	vaisseaux temporaux superficiels permet par ailleurs d'éviter la réalisation d'une
	cervicotomie, au seul prix d'une discrète cicatrice pré-auriculaire [3]. Les vaisseaux
	temporaux superficiels présentent aussi l'avantage d'être la plupart du temps situés en
	dehors des zones de fibrose en cas d'antécédent de chirurgie cervico-faciale et de
	radiothérapie [21], et représentent un réservoir non négligeable quand on ne dispose plus
	des veines communément employées en microchirurgie de la tête et cou (tronc de
	Faraboeuf, veine thyroïdienne supérieure…)[21]. L'usage des vaisseaux temporaux
	superficiels en microchirurgie peut néanmoins être rendu impossible en cas d'antécédent de
	parotidectomie totale, impliquant une ligature de la VTS et l'ATS. Enfin, le réseau veineux

temporal superficiel semble présenter une certaine plasticité, Shih

[START_REF] Shih | An alternative option to overcome difficult venous return in head and neck free flap reconstruction[END_REF]

rapportant s'être anastomosé avec succès de façon rétrograde sur la VTS à de multiples reprises, chez des patients présentant un « désert vasculaire » cervical.

V. BIBLIOGRAPHIE :


Jérémy Marie

Julie Bourret

Pierre-Marie Geffroy

Thierry Chartier

Marguerite Bienia

Vincent Chaleix

Luc Picton

Agnès Smith

Impact of bio-based binders on rheological properties of aqueous alumina slurries for tape casting

This work describes the preparation of concentrated alumina suspensions based on biosourced additives with a low environmental impact and with the rheological properties adapted to the tape casting process. Natural polysaccharides extracted from plants and fruits (pectin, psyllium…) were characterized (molar weight) and identified as promising binders for substituting the classical organic binders from petrochemical industry. The rheological behavior of the additives and of the suspensions containing these bio-sourced additives was studied in flow and in oscillations. The mechanical properties of the green tapes obtained with the different bio-sourced binders were determined from tensile experiment. This paper studies the relations between the nature of the binder, the rheological behavior of the suspension, the ability to tape cast the suspensions and the final properties of the green and sintered tapes.

Introduction

Tape-casting is widely employed in the ceramic industry for the shaping of thin and flat ceramic tapes, for example, for the production of alumina substrates or multi-layer capacitors in the microelectronic industry [START_REF] Mistler | Tape casting: theory and practice[END_REF] or for the manufacturing of solid oxide fuel cell devices (SOFC) [START_REF] Mistler | Tape casting: theory and practice[END_REF] [START_REF] Baquero | Aqueous tape casting of micro and nano YSZ for SOFC electrolytes[END_REF]. The cast suspension generally consists in the ceramic powder dispersed in a solvent thanks to a dispersing agent, and the addition of a binder and a plasticizer. Traditionally, tape casting implies the use of organic solvent and solvent-based binders [3] [4]. For several years, water-based systems have been developed to reduce the health and environmental risks. Other advantages of water-based systems are lower costs linked to the better working conditions for the operators.

Different types of binder have been reported in the literature for aqueous systems. A lot of them come from the petrochemical industry, like latex binders, which are defined as a stable colloidal dispersion of a polymeric substance in an aqueous medium [START_REF] Albano | Aqueous tape casting of yttria stabilized zirconia[END_REF]. Latex binders used for tape casting are generally synthesized from acrylic polymers (such as poly (acrylic acid), poly(vinyl acetate), poly(methylacrylate) …). For example, Baquero et al. [START_REF] Baquero | Aqueous tape casting of micro and nano YSZ for SOFC electrolytes[END_REF] added a styrene acrylic copolymer as a binder to manufacture SOFC electrolytes through water-based tape casting. Polyvinyl alcohol (PVA) has also been used as a binder, for instance, by Albano et al. [START_REF] Albano | Aqueous tape casting of yttria stabilized zirconia[END_REF] for the aqueous tape casting of yttrium stabilized zirconia or by Zhou et al [START_REF] Zhou | Translucent Al2O3 ceramics produced by an aqueous tape casting method[END_REF] to 2 produce translucent Al2O3 ceramics. Other binders, more recent, are water-soluble binders derived from bio-sourced polymer, such as methylcellulose (a cellulosic ether), mentioned by Kristoffersson et al. [START_REF] Kristoffersson | Comparison of different binders for waterbased tape casting of alumina[END_REF] and Chartier and Bruneau [START_REF] Chartier | Aqueous tape casting of alumina substrates[END_REF] , or gelatin based systems as reported by Snijkers et al [START_REF] Snijkers | Aqueous tape casting of yttria stabilised zirconia using natural product binder[END_REF]. However, from the life cycle analysis of organic additives, their environmental impact could still be improved with the substitution by more ecologically friendly biopolymers.

The aim of the present work is then (i) to substitute synthetic additives coming from petrochemical industry by eco-friendly products, in particular bio-sourced polysaccharides, in alumina suspensions and (ii), to determine if such suspensions can be adapted for tape casting. In this respect, citrus pectin is a promising binder candidate [START_REF] Marie | Eco-friendly alumina suspensions for tape-casting process[END_REF]. This recent study shows that the incorporation order of the biosourced additives plays a strong role on the final rheological properties of the alumina slurries. Pectins belongs to a large family of polysaccharides, also named carbohydrates polymers, with molar weights typically between 50 and 150 kg.mol -1 , found as apple pomace, citrus peel and pulp, or sugar beet pulp, among other sources. They are largely used in the food industry as stabilizing agent in jams, jellies, preserves, confections and beverages. Pectin chains consists in a few hundreds to thousands galacturonic acid monomer units linked via α-(1→4)-glycosidic [START_REF] Bayón | Biopolymers from Wastes to High-Value Products in Biomedicine[END_REF]. The main chain can be substituted with α-(1→2) rhamnopyranose units from which side chains of galactose, mannose, glucose, and xylose may occur. Pectins are water-soluble polymers, which become gels by mixing with water thanks to a combination of hydrogen bonds and hydrophobic interactions forming a three-dimensional network.

The study focused on a commercial pectin from citrus. It has been selected as a promising binder for alumina systems because of its highly anionic character and also because it is the most commonly marketed pectin thus ensuring a good reproducibility of the product.

In this work, two other polysaccharides bio-sourced binders have been tested in addition to citrus pectin: microcrystalline cellulose and psyllium. Microcrystalline cellulose (MCC) with size about few micrometers are widely used in cosmetics (as a thickener or binder), food (as a stabilizer, fat substitute or emulsifier), defense and medical industries (as a binder or adsorbent). MCC is currently produced from different cellulosic sources, such as wood and mainly cotton [START_REF] Trache | Microcrystalline cellulose: Isolation, characterization and biocomposites application-A review[END_REF] and is generally obtained after the hydrolysis of pure cellulose. The hydrolysis treatment is usually followed by neutralization, washing and drying processes. Currently, the common manufacturing process is performed by spray-drying a neutralized aqueous slurry obtained from the hydrolysis of cellulose [START_REF] Thoorens | Understanding the impact of microcrystalline cellulose physicochemical properties on tabletability[END_REF]. By using low shear energy (dissolver, kneader) it is possible to convert MCC into a stable gel, which can (i) increase the yield stress and/or the viscosity of water-based systems and (ii) bring cohesion to a green ceramic part after drying. Indeed, the gelation of microcrystalline cellulose is achieved by the association of particles using hydrophobic interactions leading to a colloidal suspension [START_REF] Michon | Gels de biopolymères naturels pour formulation agroalimentaire[END_REF].

Psyllium is, in this work, the most eco-friendly additive because it has not undergone any industrial or chemical transformation, which is not the case for MCC. It is a poorly water soluble polysaccharide extracted from the husks of the seeds of Plantago ovata [START_REF] Kumar | Psyllium Mucilage and Its Use in Pharmaceutical Field: An Overview[END_REF][START_REF] Masood | Psyllium: Current and Future Applications[END_REF]. The polymer backbone consists in a α-(1→3) -α-(1→4) xylan with substitution on C-2 or C-3 by L-arabinose, D-xylose, and α-D-galactouronyl-(1→2)-L-rhamnose. Chemically, the polysaccharide is arabinosyl (galacturonic acid) rhamnosylxylan [START_REF] Madgulkar | Characterization of Psyllium (Plantago ovata) Polysaccharide and Its Uses[END_REF]. Psyllium seed mucilage contains 22.6 % arabinose, 74.7 % xylose and traces of other sugars. This polymer is an excellent water absorber, widely used in pharmaceutics as a therapeutic agent to prevent diabetes, cholesterol, colon cancer, constipation or as drug delivery agent [START_REF] Farahnaky | The impact of concentration, temperature and pH on dynamic rheology of psyllium gels[END_REF], [START_REF] Khaliq | Industrial Application Of Psyllium: An Overview[END_REF][START_REF] Singh | Psyllium as therapeutic and drug delivery agent[END_REF]. Psyllium aqueous solutions are known to show pseudo gel like behavior at low polymer concentration with weakly temperature dependent rheological behavior [START_REF] Farahnaky | The impact of concentration, temperature and pH on dynamic rheology of psyllium gels[END_REF]. It appears as a good potential candidate as binder.

To go more deeply in this approach, ammonium lignosulfonate was chosen as biosourced dispersing agent. The rheological behavior of the suspensions formulated with these bio-additives has to be adapted to the tape casting process. Then this paper focuses to establish the relation between the nature of the binder, the rheological behavior of the suspensions, the ability to tape cast the suspensions and the final properties of the green and sintered tapes. Indeed, the green tapes must be easily handled without cracks or bubbles, and the sintered tapes must be dense.

Materials and methods

Starting materials

Alumina powder

Alumina powder was supplied by Alteo (grade P172SB, France). According to the technical data given by the supplier, the median particle size is 0.4 µm, the specific area is 7.5 m² g -1 and the chemical purity is 99.7%.

Binders

Three natural polysaccharides were identified as potential binders for the tape casting process: (i) citrus pectin (Alfa Aesar), (ii) microcrystalline cellulose (Arbocel P4000, Rettenmaier & Söhne, Germany) and (iii) psyllium extracted from plantago (Psyllium P99, Rettenmaier & Söhne, Germany). These polysaccharides have been selected in relation with their availability to form a gel by mixing them with deionized water, which can act as a binder and ensure the cohesion of the ceramic particles in the green part. Moreover, these three polysaccharides have been chosen according to their different organization in water. Pectin is expected to give rise to classical polyelectrolyte polymer entangled in water. Psyllium is expected to give rise to associated aggregated systems in water. Finally, MCC is known to give structured suspensions in water as MCC is not water soluble.

Dispersant, plasticizer and defoamer

The bio-based ammonium lignosulfonate dispersant (Arbo T11 N5, 6 600 g/mol, Tembec Avebene, France) extracted from maritime pines of the Landes region (France) has been selected to stabilize the alumina suspensions. Marie et al. [START_REF] Marie | Eco-friendly alumina suspensions for tape-casting process[END_REF] have already highlighted the stabilizing properties of the ammonium lignosulfonate for aqueous alumina suspensions. Glycerol (ProLabo, France) was used as a plasticizer and Byk LPC 22787 (Byk Chemie, Germany) was used as a defoamer.

Production and consolidation of the tapes

Previous works [START_REF] Marie | Eco-friendly alumina suspensions for tape-casting process[END_REF] have described the protocol for preparing tape casting suspensions containing these bio-sourced binders (Figure 1). The first step corresponds to the preparation of the binder suspension. The binder powder was mixed by mechanical stirring in deionized water. The amount of the corresponding polymer is adjusted from the rheological properties of the gel with respect to the used shear rate domain (see Table 2).

The alumina powder, the dispersant, the plasticizer and the defoamer were then added to the binder suspension, and mixed by ball milling during 20 h at 120 rpm for homogenization and de-agglomeration. Finally, the resulting suspension was de-aired on rollers during 24 h. Tapes were cast with a blade at a speed of 1.5 m.min -1 . The thickness of the deposited film was 0.8 mm. This corresponds to a shear rate of around 31 s -1 . Tapes were finally dried at room temperature in a saturated humidity atmosphere.

Green tapes were cut in 30 mm diameter disks. Some of them were laminated at 75°C under uniaxial pressure according to the following cycle: 10 MPa during 10 min and 50 MPa during 5 min. Differential thermal (DTA) and thermogravimetric (TGA) analysis (SETERAM, France) showed that all the organic compounds were removed from green tapes below 600°C in air. Thus, the debinding was performed between 100°C and 600°C at a slow heating rate (1•C.min -1 ). Finally, a thermal treatment in two steps was performed: a first treatment at 1200°C for 1 h (5°C.min -1 ) for debinding and a first consolidation necessary to be able to handle the product, followed by a treatment at 1600°C during 1 h (10°C.min -1 ).

Characterizations

In order to better understand the behavior of these biopolymers in solution or in suspension, it is necessary to characterize their properties such as molecular weight and rheological behavior in water.

Molecular weights measurements of binders and dispersant

Average number and weight molar masses (respectively Mn and Mw), distributions, molar masses dispersity, intrinsic viscosity [η] and hydrodynamic radii (Rh) of citrus pectin, psyllium and ammonium lignosulfonate were determined by Size Exclusion Chromatography (SEC) coupled on-line with triple detection: multi-angle light scattering (MALS) (HELEOS II, Wyatt Technology, Ca, USA), a viscometer detector (VD) (Viscostar II, Wyatt Technology, Ca, USA) and a differential refractive index (DRI) (RID 10 A Shimadzu, Japan). This method cannot be applied to microcrystalline cellulose because of its non-solubility in water.

SEC line comprises a degazer, a pump (LC10 Ai Shimadzu, Japan), a guard column and 2 columns OHPAK SB 804 and 806 HQ (Shodex, Japan). The eluent LiNO3 (0.1 mol.l -1 ) is filtered on 0.1 µm and the flow rate is 0.5 ml.min -1 . Polymers samples were solubilized in eluent by stirring at room temperature (excepted for Psyllium at 80°C) during 24 hours. Polymer concentrations were 1.35, 0.5 and 5.2 g.l -1 for citrus pectins, psyllium and ammonium sulfonate respectively. 100 µL were injected with an automatic injector (SIL-20A, Shimadzu, Japan) after filtration on 0.45 μm. dn/dc value of 0.15 ml.g -1 usually used for polysaccharides (citrus pectin [START_REF] Fracasso | Chemical modification of citrus pectin: Structural, physical and rheologial implications[END_REF] and psyllium) and 0.18 ml.g -1 as average value for ammonium lignosulfonate [START_REF] Fredheim | Molecular weight determination of lignosulfonates by size-exclusion chromatography and multi-angle laser light scattering[END_REF] were used. The analysis was performed via data processing with Astra® 6.1.7 soft from Wyatt using Zimm order 1 mode with angles from 44.8° to 140° Equation 1 gives the dispersity of the molar masses (Ð).

Ð = Equation 1

The critical concentration at which chains overlap (C * ) can be calculated from [η], according Equation 2 [START_REF] Gennes | Scalling Concepts in Polymer Physics[END_REF] . Flow measurements have been conducted with up and down stress or shear rate ramps in order to evidence thixotropy.

For the dynamic mode, the linear viscoelastic domain (LVED) for the different systems was checked at 1 Hz leading to the determination of the suitable stress or strain for frequency sweep chosen on the plateau. The LVED and all critical values collected (elastic and viscous moduli ! and " , critical stress and strain # and $ ) were taken as the range where stress and strain display a linear relationship. Then mechanical spectra were obtained thanks to a frequency sweep at fixed stress or a fixed strain in the LVED.

Tensile tests on the green tapes

Tensile tests were performed using an Easy test EZ20 equipment (Lloyd Instruments, UK) with a sensor of 100 N in order to compare the mechanical resistance of green tapes with different formulations. The tensile test specimens (dog-bone shape, 15 mm tensile width and 40 mm gauge length) were cut in the dried green tapes by a laser-cutting machine. The traverse speed was set to 0.5 mm.min -1 . Specimens were cut transversally and parallel to the tape casting direction in order to evaluate its influence on the mechanical properties.

Microstructures of the tapes

Microstructure of the alumina green and sintered tapes were observed by an environmental Scanning Electron Microscope (Quanta 450 FEG, FEI, USA). The porosity and the particle size in the sintered tapes have been analyzed using Image J software.

3 Results

Molecular weight and intrinsic viscosity measurements

Table 1 shows the results of the molecular weight and intrinsic viscosity measurements. Details about these measurements are given in Appendices.

Composition and rheological behavior of the binder systems

Three aqueous solutions or suspensions called "binder systems" were prepared by mixing a binder (powder) with water. According the nature of the added binder, they are respectively named B_Pectin, B_MCC and B_Psyllium. The mass concentration of the binders with respect to water are respectively 2, 3 and 1.2 %wt. for B_Pectin, B_MCC and B_Psyllium. These concentrations of pectin in water is slightly above the C* value. They were adjusted according to the suitable viscosity of the final formulations (between 0.5 and 1 Pa.s at the expected shear rate of 31 s -1 ) (see Figure 5). The compositions (%wt) of these formulations are reported in Table 2.

The flow rheological properties of the binder systems are shown in Figure 2. Pectin aqueous solution (B_Pectin) (2 %wt) shows a Newtonian behavior, which is normal considering the concentration which is just above the C*.

B_Psyllium (1.2 %) and B_MCC (3 %) are largely more viscous, between 2 and 3 orders of magnitude above water viscosity at 31 s -1 . Psyllium and MCC systems are strongly shear thinning with a low anti-thixotropic behavior for B_Psyllium and clear and expected thixotropic one for B_MCC dispersion (respectively -330 and 216 Pa.s -1 as indicated by TRIOS software thixotropy analysis) (Table 3). For the case of psyllium, the anti-thixotropy could be associated with the role of shear rate on aggregated structures. Psyllium aggregates in polysaccharide entanglement due to associated properties (hydrogens bonds, hydrophobic interactions and so on) has been evidenced thanks to light scattering analysis. Such aggregates could be disrupted under high shear leading to more associated structures when back to low shear rates.

The decreasing ramps for B_MCC and B_Psyllium (completely unstructured state) are best fitted by Herschel-Bulkley model # = # %& + $( ) , where * is the consistency (Pa.s n ) and + the rate exponent with a very low yield stress # %& . These behaviors are consistent with psyllium systems corresponding to a large pseudogel lumps stacked together. All fits results are reported in Table 3. Except for pectin, binder suspensions also display yield stresses subsequently called # , for increasing ramps, larger than for the decreasing one for B_MCC and lower for B_Psyllium. Due to thixotropic or antithixotropic effects, no time independent model can be fitted on increasing ramps, so these yield stresses could only be estimated from close scrutiny of the flow curves, and are reported in Table 4.

In order to assess the viscoelastic properties of the binder systems, oscillatory measurements have been conducted on B_Pectin, B_MCC (Figure 3) and B_Psyllium (Figure 4) systems. Dynamic critical stress σc and critical strain γc correspond respectively to the values of stress and strain from which the stress-strain relationship is no longer linear. The linear viscoelasticity domain has been checked at 1 Hz, for all binder suspensions. A frequency sweep was performed for B_MCC and B_ Psyllium systems only (see figures Figure 3b and Figure 4 b). According to the Newtonian behavior of pectin system, the LVED could not be determined ($ and σ ). Only a few correct values at extremely low oscillation stress amplitude (<10 -2 Pa) were obtained on the plateau. The value of Gc' is around 10 -2 Pa and Gc'' around 5. 10 -2 Pa. These values are too low for the working range of the rheometer. The loss modulus is larger than the storage modulus, indicating a liquid-like behavior. Thus, no frequency sweep could be performed on this sample.

Both B_Psyllium and B_MCC systems display a linear domain and a larger storage modulus, indicating a solid-like behavior coherent with the presence of yield stress estimated at the end of the linear domain. Dynamic critical stresses σc give a good estimate of the stress above which the system begins to flow and are reasonably well correlated with yield stresses σ0 obtained from increasing ramp of the flow curves (0.5 and 3.5 Pa for B_Psyllium and B_MCC for flow and 0.9 and 1.3 Pa for oscillation, see Table 4). There can be small variations between the yield stresses measured in flow and in oscillations. MCC polymers can be thought of as an assembly of sticks, stuck together. It is then conceivable that from a certain volume fraction, if a shear stress is applied to the system, the polymers will orient themselves in the same direction and cause the flow.

Concerning the B_Psyllium system, the pseudo-gel consists of polymeric chains and aggregates bonded together, a typical structure of a soft material. As usual, dynamic critical stress values are slightly different, but similar to the ones obtained from flow curves. The critical strain is almost four times larger for B_MCC, which can be interesting for other shaping techniques like micro-extrusion.

Frequency sweeps measured at 10 -2 Pa (i.e. in the linear domain) show a storage modulus always higher than the loss modulus on the whole range of studied frequencies indicating a solid like behavior. B_MCC may reach a modulus crossover at a frequency slightly outside the available measurement range. Such structures with large excess of water (98.5 here), with yield stresses and elastic rheological behaviors (a very slight frequency dependence of the modulus) are fully consistent with a pseudogel behavior. This behavior appears less pronounced for B_MCC compared to B_Psyllium since the ratio between loss and storage moduli (i.e. loss tangent) is noticeably higher.

Both B_Psyllium and B_MCC systems alone can be considered as pseudogels, but as a confirmation of the conclusion of appendices (i.e. very aggregative behavior), B_Psyllium seems more structured than B_MCC. On the other hand, B_Pectin corresponds only to a modest viscosifying agent.

Composition and rheological behavior of the alumina suspensions

Three suspensions were elaborated from the three binder systems previously studied. They are respectively called S_Pectin, S_MCC and S_Psyllium. For tape casting, the viscosities of the suspensions should be ideally between 0.5 and 1.0 Pa.s at the shear rate of 31 s -1 which is applied during casting. Amounts of binder systems in the formulation have been adjusted according to the suitable viscosity at the expected shear rate (Figure 5.a). The compositions (wt %t) of these formulations are reported in Table 2. The rheological results summarized in Table 3 indicate that by the addition of alumina and other additives, the rheological behavior of S_Pectin, S_MCC or S_Psyllium is not the same as for each binder system alone. The three suspensions evidence strong shear thinning (Figure 5 and Table 3). This behavior is beneficial for the tape casting process [START_REF] Mistler | Tape casting: theory and practice[END_REF]. No clear yield stress was observed in the range of the measurements on the decreasing ramps. A modeling of the stress evolutions according to the shear rate was carried out on the decreasing ramp using the Ostwald power law model # = *$( + where * is the consistency (Pa.s n ) and + the rate exponent, which is quite similar for all the suspensions (Table 3). This can be attributed to the interaction of alumina particles through the gel. This idea will also be developed in the following using dynamic rheological measurements.

At the shear rate of 31 s -1 for the tape casting conditions, the three alumina suspensions have a viscosity of respectively 0.5, 0.7 and 0.4 Pa.s for S_Pectin, S_MCC and S_Psyllium. Among the three suspensions, S_Psyllium has the lowest viscosity.

It should be noted that the thixotropy is reversed between the formulation S_Psyllium and S_MCC and the gel alone. A possible explanation for the case of Psyllium could be the following. During the shear involved in the slip preparation process, the Psyllium gel reaches an equilibrium swelled state, which was evidenced by anti-thixotropy on the hysteresis loop of the binder alone. With the addition of alumina, a network is formed, which can be destroyed by shear and take a long time to form again, thus leading to thixotropic behavior of the complete suspension. The fact that this does not occur for pectin-based systems may be related to its low concentration in water. As for MCC systems, the origin of the observed reversal is still unclear. Similarly to binder gels analysis, yield stresses # , observed on the increasing ramps were estimated and reported in Table 4, to be compared with oscillatory results.

Table 3. Fit results for gels alone and for complete formulations on the decreasing ramp or steady state when available. B_Pectin: Newtonian fit, other gels: Herschel-Bulkley, formulations: power law. Thixotropy was calculated using TRIOS software. Figure 6 shows evolutions of the storage (G'), the loss modulus (G") as a function of the stress for the three suspensions, obtained by oscillation measurements. For the suspensions S_MCC and S_Psyllium, G' is larger than G", which is characteristic of a solid like behavior with a structural stability under small deformations. Conversely, the suspension S_Pectin displays a liquid-like behavior with G">G'.

Table 4 shows a summary of all the oscillatory results for both binders and slips. Binders and suspensions have very close LVED, especially $ ,suspensions having slightly larger values for the moduli. This indicates that the binder gel dictates the overall behavior, and that alumina adds some rigidity to the systems. As for the frequency dependence, measurements in the LVED for MCC and psyllium systems show a gel-like behavior with no moduli crossover on the investigated frequency range and almost no dependence on frequency.

Interestingly, the suspension S_Pectin shows a liquid behavior where G" > G'. Therefore, this suspension appears more suitable for tape casting because it has a pseudo-liquid behavior. With a pseudo-solid behavior, suspensions S_MCC and S_Psyllium appear more suitable for extrusion. Indeed, such formulations of alumina pastes containing psyllium have been developed for the micro-extrusion process [START_REF] Bourret | Micro extrusion of innovative alumina pastes based on aqueous solvent and eco-friendly binder[END_REF] Moreover, the frequency dependence is more pronounced.

Mechanical resistances of the green tapes

Tensile tests have been performed on the dried green tapes obtained from the three suspensions S_Pectin, S_MCC and S_Psyllium. A preloading of about 400 kPa or 60 kPa has been applied before carrying out experiments. Figure 8 shows the stress-strain curves for the green tapes in the directions perpendicular and parallel to the casting direction (respectively (Ox) and (Oy), see Figure 7). The mechanical properties of green tapes are anisotropic, and the maximum stress before rupture is higher in the casting direction than perpendicular to the casting direction. The plastic deformation domain of the green tapes is fairly large, (for example, from 0.2 to 0.6% for the tape based on pectin) especially parallel to the casting direction. The main mechanical properties of green tapes obtained with the three suspensions are reported in Table 5. The nature of binder has a large impact on the maximum stress of the green tapes.

The green tape obtained with S_Psyllium shows a lower maximum tensile strength at fracture (σm) in comparison to the values obtained with S_Pectin or S_MCC suspensions (Table 5). Besides, these values are close to those found in the literature [START_REF] Descamps | Tape-casting: Relationship between organic constituents and the physical and mechanical properties of tapes[END_REF] (no data found for systems based on psyllium) which are of the order of a few MPa for tapes obtained from aqueous and non-aqueous suspension and additives from petrochemicals.

The green tapes obtained from citrus pectin binder show the highest maximum stress with a lower relative density of green tape (Table 5). This suggests that the citrus binder leads the formation of cohesive polymeric bridge between alumina particles, which are favorable to higher mechanical properties of the green tapes. Moreover, for each suspension, the samples cut perpendicularly to the direction of casting do not show a plastic domain but instead exhibit a sudden rupture. The specimens cut parallel to the casting direction have a more or less large plastic domain with slightly larger elongations. Besides, the green tapes obtained with psyllium binder have maximum stress values larger in parallel direction than in perpendicular direction. The bio-sourced polymers are likely oriented preferentially by the shear imposed between the substrate and the blade during the casting. This phenomenon has already been put forward by Doreau et al [START_REF] Doreau | Mechanical and lamination properties of alumina green tapes obtained by aqueous tape-casting[END_REF]. The relative densities of the sintered tapes obtained with the three suspensions (S_Pectin, S_MCC and S_Psyllium) are reported in Table 6. The relative densities of sintered tapes obtained from suspension S_Pectin and S_Psyllium show similar values, close to 93-94%, and the sintered density of tapes obtained using MCC is lower, close to 90% (Table 6).

According to the formulations (Table 2), the tape containing MCC has the higher amount of additives after water removal (drying). This could explain that after sintering, the MCC_tape is less dense. Finally, the use of citrus pectin as a binder (suspension S_Pectin) corresponds to the best compromise between the mechanical resistance of the green tapes and the density of the sintered tapes.

As an indication, Figure 9 shows the microstructure of a typical S_Pectin alumina tape, after drying (a) and after sintering (b). No apparent defects are observed. Small alumina particles of average sizes between 0.2 and 0.5 µm, cohesive with each other, are observed for the green tape while the average grain size (5-10 µm) is larger in the sintered tape indicating an important grain growth during sintering.

Conclusion

The aim of this work was to formulate aqueous suspensions of alumina from bio-based additives for the tape casting process. These suspensions required a rheological behavior adapted for tape casting and led to green and sintered tapes, with characteristics similar to those obtained from traditional suspensions. The first step was to identify eco-friendly molecules (binder, dispersant and plasticizer) in order to replace the additives from petrochemicals usually used in such suspensions.

Thus, three aqueous solution, hydrogels and suspension have been chosen as binders in order to maintain the cohesion of the granular compact after drying. Citrus pectin, microcrystalline cellulose and psyllium were selected respectively. Among these three polysaccharides, psyllium has the advantage of being mechanically extracted without any chemical treatment. In addition, the ammonium lignosulphonate extracted from the Landes maritime pine was selected as a dispersant because its dispersing ability has already been demonstrated in the literature. Finally, glycerol, a residue from the production of biodiesel and soap, already widely used in casting suspensions, has been used as a plasticizer. A biosourced anti-bubble agent from the company Byk, LP-C 22787, was also used.

The second step of this work was to characterize the selected bio-sourced additives. Thus, the molar masses of ammonium lignosulfonate and then those of the polysaccharides used as binder were determined. Very diverse molar weight Mw are obtained: approximately 1,000,000 g.mol -1 for psyllium, 165,000 g.mol -1 for pectin, MCC corresponds to nano-object in suspension.

Subsequently, the rheological behavior and the binding ability of citrus pectin, microcrystalline cellulose and psyllium were evaluated. First, the rheological properties of the formed systems from these polysaccharides were studied in flow and in oscillations. Citrus pectin mixed with water presents a slightly shear thinning almost Newtonian and nonthixotropic behavior. This corresponds to slightly entangled polymer solution just above the entangled critical concentration (C*). Psyllium and microcrystalline cellulose systems can be considered as pseudo gels: associated aggregates structure bonded in polymer entanglement and structured suspension, respectively. They appear largely more viscosifying. They are respectively shear thinning with an anti-thixotropy and a thixotropy and display yield stress. This point suggests that they can also been used as a binder in pastes for micro-extrusion [START_REF] Bourret | Micro extrusion of innovative alumina pastes based on aqueous solvent and eco-friendly binder[END_REF].

Finally, using an optimized protocol, suspensions were prepared from citrus pectin, microcrystalline cellulose and psyllium. The composition of these suspensions has been optimized in order to have rheological characteristics adapted for the tape casting process. The rheological behavior of suspension based on citrus pectin seems to be the most suitable for this process. However, all three suspensions lead to easily handled green tapes with no apparent surface defects.

In addition, the green tapes obtained from citrus pectin showed higher tensile strength and the relative densities of the sintered tapes are about 93 %, knowing that the sintering thermal cycle has not be optimized.

Finally, the objective of this study, which was to develop aqueous alumina suspensions composed of eco-friendly additives and adapted for the tape casting process, is achieved.

Figure 1 .

 1 Figure 1. Protocol of the ceramic slurry preparation with the use of bio-sourced binders for tape casting process

2

 2 Rheological measurements of the binder suspensions and the alumina slurriess Viscosity measurements were performed using an ARG2 equipment, a controlled stress rotating rheometer from TA Instrument (USA). A cone/plate geometry ( = 30 mm, tip angle 2°) or a coaxial geometry with double wall concentric cylinders ( = 20 & 22.38 mm, = 20.38 & 21.96 mm, immersed height = 20.5 mm) were used, at room temperature.

Figure 2 .

 2 Figure 2. Rheological behavior of the three binder systems at 22°C. The stress was raised from 0.01 to 20 Pa for 60 s, followed by a peak hold at 20 Pa during 60 s, and finally a decrease from 20 to 0.01. Pa in 60 s. The relevant fits for each decreasing ramp are also shown. a. Stress as a function of shear rate. b. viscosity as a function of shear rate, zoom below 31 s -1 .

Figure 3 :

 3 Figure 3: MCC rheological oscillatory behavior (a: evolution of the storage modulus and of the viscosity as a function of the stress, f = 1 Hz; b: evolution of the storage and the loss modulus and of the viscosity and the phase angle as a function of the frequency as a function of frequency, σ = 10 -2 Pa)

Figure 4 .

 4 Figure 4. B_Psyllium oscillatory rheological behavior (a: evolution of the storage modulus and of the viscosity as a function of the stress, f = 1 Hz; b: evolution of the storage and the loss modulus and of the viscosity and the phase angle as a function of the frequency, σ = 10 -2 Pa)

Figure 5 .

 5 Figure 5. Rheological flow behavior of the S_Pectin, S_MCC and S_Psyllium suspensions. The shear rate was raised from 0.01 to 200 s -1 in 100 s, followed by a dwell at 200 s -1 during 180 s, and finally a decrease from 200 to 0.01 s -1 in 100 s. The grey area corresponds to the optimal conditions for tape casting process [1]. a. Shear stress (Pa) as function of shear rate (s -1 ) for the S_Pectin, S_MCC and S_Psyllium suspensions; b. Viscosity (Pa.s) as function of shear rate (s -1 ) for the S_Pectin, S_MCC and S_Psyllium suspensions

Figure 6 .

 6 Figure 6. Stress sweep and frequency sweep measurements for S_Pectin, S_MCC and S_Psyllium suspension. a. Stress sweep (at 1Hz); b. Frequency sweep for strain= 1 % corresponding to a strain in the LVER

Figure 7 .

 7 Figure 7. Photograph of the casting of a tape. Indications of the considered directions.

Figure 8 .

 8 Figure 8. Stress -strain curves for the green tapes in perpendicular (Ox) and parallel (Oy) to the casting direction. a. Stress -strain curves for green tapes obtained with the suspensions S_Pectin and S_MCC; b. Stress -strain curves for green tapes obtained with the suspension S_Psyllium in perpendicular (Ox) and parallel (Oy) to the casting direction

a)Figure 9 .

 9 Figure 9. Microstructure of green or sintered alumina tape initially based on S_Pectin

Table 1 .

 1 Characterization of the bio-based additives

			Mn (g.mol -1 ) Mw (g.mol -1 )	Ð (± 0.5%)	[η] (mL.g -1 )	Rhw (nm)	% recovery	C* (g/L)
	Binder	Psyllium Citrus Pectin	190 000 (± 6%) 77 000 (± 1%)	995 000 (± 1%) 165 000 (± 0.5%)	5.1 2.1	-330 (± 0.4%)	26 (± 4%) 18 (± 0.1%)	11 100	-3
	Dispersing agent	Ammonium lignosulfonate	38 000 (± 1%)	113 000 (± 0.5%)	3.0	5.2 (± 8%)	4 (± 2%)	75

Table 2 .

 2 % weight composition of the S_Pectin, S_MCC and S_Psyllium respectively containing citrus pectin,

	microcrystalline cellulose and psyllium as binders
	Function		S_Pectin	S_MCC	S_Psyllium
	Mineral powder	Alumina	40	40	40
	Dispersing agent	Ammonium Lignosulfonate	0.4	0.4	0.4
	Plastizicer	Glycerol	3.3	4.8	4.0
	Defoamer agent	Byk LP-C 22787	0.1	0.1	0.1
		Citrus Pectin	1.1	/	/
	Binder	Microcrystalline cellulose (MMC)	/	1.6	/
		Psyllium	/	/	0.7
	Solvent	Water	55.1	53.1	54.8
	Total		100	100	100
	Binder/ (binder system) (% wt)	2	3	1.3

Table 4 .

 4 Summary

		Flow		Oscillations
	System	-1 (Pa) from flow ramps	-. (Pa)	2 . (%)	3′ . (Pa)	3" . (Pa)
	B_Pectin	-	-	-	8.0E-03 ±3.0E-03	4.2E-02 ±0.3E-02
	S_Pectin	-	0.3 ± 0.0	7.5 ± 0.7	1.5 ± 0.1	2.9 ± 00
	B_MCC	3.5	1.3 ± 0.1	19.0 ± 1.6	6.0 ± 0.3	3.9 ± 0.1
	S_MCC	1.7	3.5 ± 0.3	14.8 ± 1.6	22.5 ± 0.7	7.7 ± 0.1
	B_Psyllium	0.5	0.9 ± 0.3	5.5 ± 1.5	15.5 ± 0.8	6.3 ± 0.2
	S_Psyllium	3.8	0.3 ± 0.0	1.3 ± 0.0	23.0 ± 0.1	6.8 ± 0.1

of LVER detection for all systems, with the average results of binder systems alone and of complete formulations associated with their respective standard deviation.

Table 5 .

 5 Mechanical properties of the green tapes obtained with the suspension S_Pectin, S_MCC and

	S_Psyllium
		Relative density of	Parallel to the casting direction	Perpendicular to the casting direction
		green tape (after drying)	Strain at break (%)	Maximum stress σm (MPa)	Strain at break (%)	Maximum stress (MPa)
	S_Pectin	60 ± 1,0 %	0.44 ± 0.02	0.78 ± 0.06	0.32 ± 0.02	0.74 ± 0.06
	S_MCC	64 ± 1,0 %	0.40 ± 0.02	0.63 ± 0.05	0.27 ± 0.02	0.61 ± 0.05

Table 6 .

 6 Relative densities of sintered tapes manufactured with the suspensions S_Pectin, S_MCC and

	S_Psyllium
		Relative density
		of sintered tapes
	Tape obtained from S_Pectin	93 ± 1.0 %
	Tape obtained from S_MCC	90 ± 1.0 %
	Tape obtained from S_Psyllium	94 ± 1.0 %

Acknowledgments

The authors wish to thank Nouvelle Aquitaine Region for its financial support.

Appendices

Molar masses Mn and Mw of the citrus pectin together with dispersity (Ð), intrinsic viscosity and size appear classical of pectins [START_REF] Fishman | Effect of long term cold storage and microwave extraction time on the physical and chemical properties of citrus pectin[END_REF]. About 100% of the pectin is recovered after analysis, that means this polysaccharide is completely water soluble as expected for a low methylated pectin.

Psyllium binder has higher molar masses and much larger dispersity than citrus pectin binder. Nevertheless, the analysis evidences only about 10 % of recovered polymer, determined by the difference between known injected mass and integrated analyzed mass from DRI signal under the peak. This result means firstly that the given results concern only a few part of the sample and is not characteristic of the polymer. Secondly about 90 % of the psyllium is retained on the 0.45 µm filter indicating that large aggregates probably predominate in aqueous solution. Thus, psyllium binder appears likely as large aggregated structure with large sizes even when solution is mixed at 80°C (LiNO3 0.1M, pH ≅ 6). It has been reported that psyllium gives rise to pseudogels in aqueous solvent [START_REF] Farahnaky | The impact of concentration, temperature and pH on dynamic rheology of psyllium gels[END_REF] [START_REF] Rao | Characterization of Psyllium (Plantago ovata) Polysaccharide and its Use as a Binder in Tablets[END_REF], that seems coherent with the aggregation tendency observed.

Finally, the analysis of ammonium lignosulfonate is considered as representative of the sample with a recovering of 75 % and classical values.

These results are consistent with the expected binder role of the used polysaccharide notably for citrus pectin, which evidences large intrinsic viscosity and hydrodynamic radii compared to the molar masses. This shows that the conformation of pectin appears as expanded coils, due to both semi-rigid backbone and polyelectrolyte character. Large aggregates evidenced for psyllium permit also to envisage that it could be used to increase the viscosity with probably gelling properties. MCC dispersions in water at a chosen volume fraction are also expected to give rise to pseudogel like behavior. Ammonium lignosulfonate appears logically more flexible and compact than polysaccharides as evidenced by the very low values of the intrinsic viscosity and hydrodynamic radii compared to the molar masses. These characteristics, added with the anionic character of lignosulfonate, are consistent with the expected role of dispersant in alumina suspension.