Lignosulfonate Blends as Carbon

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Use of microfibrillated cellulose/lignosulfonate blends as carbon precursors: impact of hydrogel rheology on 3D printing. Ying SHAO, Didier Chaussy, Philippe Grosseau, and Davide Beneventi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02763 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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Industrial & Engineering Chemistry Research

Use of microfibrillated cellulose/lignosulfonate blends as carbon precursors: impact of hydrogel rheology on 3D printing. Ying Shao,†, ‡,¥ Didier Chaussy, ,†, ‡,¥ Philippe Grosseau,§ and Davide. Beneventi.*,†, ‡,¥ †

Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France.

‡

CNRS, LGP2, F-38000 Grenoble, France. Email: davide.beneventi@pagora. grenoble-inp.fr

¥

Agefpi, LGP2, F-38000 Grenoble, France.

§

Ecole des Mines de Saint Etienne, Centre Sciences des Processus Industriels et Naturels

(SPIN), 158, Cours Fauriel, CS 62632, 42023 Saint Etienne, France.

ABSTRACT: The aim of the present study was to investigate the rheological properties of microfibrillated cellulose/lignosulfonate hydrogels and to use them for the manufacturing of carbon objects by 3D printing and carbonization. To this purpose, both flow mode and thixotropic mode were used to characterize the hydrogel rheological behaviour which was subsequently used to search for formulation/processability correlations during 3D printing of square cuboids. At a concentration of 2%, microfibrillated cellulose (MFC) displayed excellent printability, i.e. a shear thinning behaviour with high yield stress and a viscoelastic response to a step-down shear rate variation. The addition of lignosulfonate (LS) induced a drop in the yield stress and, above a LS mass fraction of 30%, the MFC/LS hydrogel displayed an inelastic

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thixotropic response with a drop in printability (viz. the printed cuboids underwent a continuous deformation until hydrogel complete spreading). Above 50% of LS, the high viscosity slowed down the flow of MFC/LS hydrogels and printed cuboids had minor deformation. Freeze and air drying of cuboids printed with LS mass fraction lower than 20% and higher than 50%, respectively, allowed keeping the original shape and their carbonization under inert gas led to the production of highly conducting objects. In line with the high density of air dried samples, carbonized samples displayed an irregular structure with pores and crackles generated during drying and carbonization, whereas freeze dried samples had the typical lamellar structure of icetemplated materials. 1. Introduction As a renewable and the most abundant polymer source in nature, wood and its derivatives, in particular cellulose, have been widely used in medical, packaging, construction and energy industries after suitable forming processes. Among all these applications, cellulose use in energy storage devices is raising an ever increasing interest and it is used as binder in flexible electrodes, reinforcing agent in gel-polymer electrolytes,1 separator in Li-ion batteries2 or nanoporous template in supercapacitors.3 Moreover, cellulose nanofibers and lignin have been successfully used as precursors for manufacturing carbon nanofibre aerogels4,5 and carbon fibres6 and mats,7 respectively, displaying interesting application potential as anodic material in Li-ion batteries. Nowadays, nano scaled cellulose can be produced in 3 types of forms: microfibrillated cellulose (MFC), nanocrystalline cellulose (NCC) and bacterial nanocellulose (BNC) by different ways of extraction from different sources. MFC hydrogels are normally obtained by delamination of wood pulp by mechanical pressure before and/or after chemical or enzymatic


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treatment.8 MFC shear-thinning and thixotropic properties have been reported in several articles.9,10 Besides, since MFC is slightly negatively charged, its flocculation pattern in flow shear was investigated with or without polyelectrolyte additives.11,12 Thanks to its large availability as wood delignification by-product,13 high carbon content and carbonization yield, lignin is one of the best carbon/graphite precursors among wood-derived polymers and it has been extensively used for the production of bulk14,15 and carbon nanotube doped16 carbon fibres and powders.17 Thereafter, in this study, lignosulfonate was used as water soluble carbon precursor to replace lignin. Industrial LS is separated from spent sulphite liquor by methods such as precipitation, dialysis and ion exchange, it has a polyelectrolyte structure built of guaiacyl-propyl units with the sulfonate groups attached to the aliphatic chains.18,19 Unlike lignin, LS is water soluble with a maximum solubility near 0.47 (volume phase), making it easier to be processed in form of viscous solutions. Furthermore, LS dissolved in water is negatively charged and the charge level of LS molecules decreases progressively with an increase of the concentration.19 The ability to process MFC/LS colloidal suspensions with high solid loading and viscosity, highlight these systems as promising candidates for 3D shaping by filament-based writing in order to manufacture structured electrodes. However, this additive material shaping technique, extensively used in ceramics20,21 and biomedical applications,22 requires inks with adequate rheological properties. In the deposition of non-reactive/non stimuli responsive suspensions, most of the time ink is stored in a syringe-type reservoir connected to a dispensing nozzle placed on the printing head. The displacement of the syringe piston (and ink filament flow through the nozzle) is computer controlled and synchronized with the movement of the printing head along the x-y-z axis in order to insure a precise filament deposition and object build-up. The printed


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object is finally dried in air or in an oven. In order to achieve good printability, inks must be formulated in order to: i) oppose low resistance to extrusion through circular nozzles, ii) rapidly recovery elasticity to prevent ink viscous flow and to retain the printed shape20, iii) have sufficiently high yield stress and fast elasticity recovery kinetics to prevent the collapse of the wet printed object under its own weight,23 and iv) have limited shrinkage during drying in order to avoid object deformation/cracks formation.21 MFC hydrogels already demonstrated excellent processability in 3D printers equipped with syringe-type gel extruders.24 After printing and freeze drying, MFC forms a tough and highly porous 3D structure with nearly no deformation of the pristine printed object. Whereas, MFC hydrogels undergo a huge volume shrinkage25 (i.e. up to 60-75 times for a 2% MFC gel) and deformation during air drying. Therefore, the aim of this work was to use MFC hydrogels/LS blends in order to increase: the solid phase content without impairing the MFC hydrogel printability and the overall carbon content to limit shrinkage during air drying and improve the carbonization yield, respectively. Even if the rheological behaviour of MFC or LS suspensions has been extensively investigated, to the best of our knowledge, there are no studies on the flow and relaxation behaviour of mixed MFC/LS hydrogels at comparatively different concentrations. The impact of high concentrations of LS on the rheology of MFC hydrogels, their printability and subsequent drying and carbonization was the focus of this study. 2. Experimental 2.1 Materials Sodium lignosulfonate (LS) with purity ≥ 93%, was purchased from Carl Roth GmbH+ Co. KG (France) in form of brown powder. Due to its good water-solubility, a series of LS solutions at


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various concentrations were prepared by adding corresponding quantities of LS into deionized water with the subsequent stirring step. The concentrations (w/w) are 20%, 30%, 40%, 50%, 52.5%, and 55%, corresponding to volume fractions ranging between 0.15 and 0.47. Microfibrillated cellulose (MFC) in the form of a 2% (w/w) hydrogel was provided by FCBA (Saint Martin d’Hères, France). Bleached hardwood (birch) kraft pulp was used for MFC production by following a mechano-enzymatic protocol with the subsequent homogenization at high pressure.26 1% and 0.5% MFC water dispersions were prepared by adding distilled water to the original 2% hydrogel followed by stirring. Whereas, a 11.4% concentration MFC suspension (as determined by sample drying in an oven during 24h at 110°C) was obtained by centrifugation of the pristine 2% suspension during 15 min at 10000 rpm ( 11000 g) and 9°C. MFC/LS mixed suspensions with LS concentration ranging between 20 and 50% (w/w, calculated regarding to water) were prepared by dissolving corresponding quantities of LS in the initial 2% MFC suspension and extensively stirred with a mechanical stirrer. The same protocol was used with 0.5 and 1% MFC suspensions. 2.2 Suspensions rheology All rheological measurements were performed by using a rotational physical MCR 301 rheometer (Anton Paar) in a plate-cone configuration. A cone with 50 mm diameter and 1° angle was used and the gap was 1 mm. A transparent cover was used to prevent water evaporation during measurements. Temperature of the plate was maintained at 23 °C. Viscosity measurements were carried out for all suspension samples by repeating several cycles with shear rate ranging between 10-3 and 103 s-1 with about 10 min relaxing time between each cycle. Four measuring points were set for decay with 10 s between each measuring point.


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Thixotropic measurements were carried out just for samples with 2% MFC concentration. The shear rate was maintained at 1000 s-1 for 20 s then it was decreased to 0.1 s-1 with a step-down profile. Samples’ viscosity and stress responses were recorded as a function of time. In order to get rid of inertial instabilities, data were recorded 1 s after the step-down variation. 2.3 3D printing and pyrolysis A commercial 3D printer (Model 3, Seraph Robotics) was used to test the processability of 2% MFC suspensions with LS content ranging between 0 and 50%. Two series of square cuboids (LxWxH: 2x2x1 cm) were printed using a 0.96 mm syringe needle and a printing speed of 35 mm s-1 (corresponding to a shear rate in the needle tip of ca. 300 s-1). The first series was firstly dried at room temperature during 5 days, then in an oven at 110°C (48h). The second series, was frozen in a refrigerator (-12°C) after 2h relaxation at room temperature. Frozen samples where then freeze dried. Dry samples where carbonized in a tubular oven (RHT 08/17, Nabertherm) under argon flux. The oven temperature was increased by 5°C min-1 up to 800°C with a 30 min intermediate plateau at 150°C in order to limit sample damage due to water removal. The internal structure, before and after pyrolysis, was imaged by X-ray tomography (Nanotom 180, Phoenix X-Ray). Since carbonized cuboids cannot be cut/shaped without sample damage, the electrical resistance along the Z-axis was measured using a commercial Ohmmeter (Fluke) after the deposition of zinc electrodes with ca. 7 mm diameter on the cuboid square faces. The electric conductivity was then calculated with the Ohm law using the sample average thickness, the apparent contact area of the zinc electrodes and the cross sectional area of the whole square face in order to get an estimation of maximum and minimum conductivity values.


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3.

Results and discussion 3.1 Rheology of LS and MFC suspensions Fig. 1 shows that, whatever their concentration, LS solutions displayed a Newtonian behaviour

with a progressive viscosity increase from 0.003 to 100 Pa s when LS concentration rose from 20 to 55%. The rheological behaviour of LS solutions was therefore described by plotting relative viscosity as a function of the volume fraction of dissolved LS (inset in Fig. 1). Below a volume fraction of ca. 0.35, relative viscosities slightly increased with concentration, whereas, above that value an abrupt increase was observed and for LS volume fractions higher than 0.47 it was no longer possible to measure viscosity.

Fig. 1 Rheological behaviours of LS solutions of different concentrations. The inset represents the relationship between the relative viscosity and the volume fraction. The dashed line represents experimental data fitting with the K-D equation, respectively.


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The Krieger-Dougherty (K-D) equation,27,28 was used to fit experimental data and to correlate the relative viscosity of the polymer solution (ηr) to its volume dissolved fraction, ():

= 1 −

Eq. 1

where and [] are the maximum packing fraction (which corresponds to the critical phase volume at which the fluid’s viscosity tends to infinity) and the intrinsic viscosity of LS, respectively. Values obtained for and [] by fitting experimental data with the K-D equation (Eq.1), i.e. 0.49 and 8.14, were in line with those obtained by Vainio et al.28 for similar systems, thus indicating that intermolecular interactions above a volume fraction of 0.49 lead to a fluid to solid transition and LS solutions are no longer processable by extrusion. In order to limit the viscosity of MFC/LS suspensions the maximum LS volume fraction used in this study was 0.42, corresponding to a LS mass fraction of 50% and a solution viscosity of 3.3 Pa s. Fig. 2 shows that MFC hydrogels displayed a typical shear thinning behaviour,9,10 which was associated to the progressive destructuration of the MFC network upon shearing and modelled using the Herschel-Bulkley equations: = + + =

+ +

Eq. 2 Eq. 3

where σ is the shear stress, σy the yield stress, the applied shear rate, ηs the viscosity of the Newtonian suspending medium (i.e. water or LS solutions), η the suspension viscosity, K the concentration factor and n the flow index which represents the fluid flow behaviour (i.e. n = 1 Newtonian, n > 1 shear-thickening, n < 1 shear thinning). A rise in MFC concentration led to a


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general increase of the shear stress and viscosity, which was mainly due to the progressive formation of dense nanofiber networks opposing a growing resistance to shear.29 The plot of the yield stress and of the flow index obtained from Eq. 3 as a function of the MFC concentration (Fig. 3), shows that the dilution to 0.5% of the pristine 2% MFC suspension led to the hydrogel disruption and to a shift from a shear thinning towards a Newtonian behaviour, i.e. the yield stress dropped from 4 to 0.3 Pa and flow index increased from 0.5 to 0.85. Despite a high viscosity at low shear rate, the pronounced shear thinning and high yield stress of 2 and 11.4% MFC suspensions led to hydrogel fluidization in the printing needle (for a shear rate of ca. 300 s-1, 2 and 11.4% hydrogels viscosity drops to ca. 0.1 and 2 Pa s, respectively) and, as highlighted in previous works,20 3D objects can be easily printed. In line with the disruption of the fibre network, 0.5% MFC suspensions displayed a one order of magnitude drop in viscosity and yield stress which led to hydrogel spreading over the printing substrate.

a)

b)

Fig. 2 Viscosity (a) and shear stress (b) responses of MFC suspensions at 4 different concentrations in a shear flow. Dashed lines represent experimental data fitting with Eqs. 2 and 3.


9


25

1.0 Flow index

20

0.8

15

0.6

10

0.4

5

0.2

0

Flow index

Yield stress

Yield stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 0

2

4

6

8

10

12

MFC mass fraction (%)

Fig. 3 Yield stress and flow index of MFC suspensions obtained from experimental data fitting with Eq. 2. 3.2 Rheology of MFC/LS mixtures Flow curves for mixed MFC/LS suspensions show that, at 2% MFC (Fig. 4.a), the increase of the LS concentration results in a general shift of the flow curve towards higher viscosity. This is in line with the viscosity increase in the suspending medium. Whereas, at 1 and 0.5% MFC (Fig. 4.b-c), LS addition induced a drop in viscosity at shear rates below ca. 100 and 10 s-1 for 1 and 0.5% MFC concentration, respectively. Above those shear rate values, the viscosity of MFC/LS systems was higher than that of pure MFC gels indicating that at low shear rate the viscosity of MFC/LS mixtures was dictated by the disruption of weak MFC networks. At high shear rates, rheology was dominated by the viscosity of the suspending medium. Whatever the MFC concentration, in the presence of 50% LS, the viscosity of MFC/LS systems was higher than that of the pristine MFC hydrogel and at high shear converged to ηs. The plot of the yield stress obtained from experimental data fitting with Eq. 2 (Fig. 5), shows that at concentrations below 30%, LS induced a drop in the MFC gel strength. Indeed, a yield stress


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drop was clearly detected with 1 and 0.5% MFC. At a MFC concentration of 2%, only a slight decrease was observed.

a)

b)

c) Fig. 4 Viscosity vs shear stress of MFC/LS. a) 2% MFC series, the inset represents shear stress vs shear rate data; b) 1% MFC series; c) 0.5% MFC series. Dashed lines represent data fitting with Eqs. 2 and 3.


11


10.00

Yield stress (Pa)

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1.00

0.10

0.5%

1%

2%

0.01 0

10

20

30

40

50

LS mass fraction (%)

Fig. 5 Influence of MFC and LS mass fraction on the yield stress obtained from experimental data fitting with Eq. 2. As highlighted for diluted MFC/soluble polysaccharides mixtures,30 the sharp decrease of the yield stress at extremely low LS concentration (i.e. 0.5% LS) was associated to the adsorption on the fibre surface of LS which, acting as lubricant, decreased the friction among nanofibers and the stress necessary to break the entangled nanofiber network. The further increase of the LS concentration from 0.5 % to 30 % poorly affected the yield stress thus indicating that, in this LS concentration range, LS no longer affects interfibre friction. Therefore, we can go on the assumption that, between 0.5% and 30% LS, the fibre surface is saturated by adsorbed LS molecules and their effect on interfibre lubrication attains a steady state. Moreover, dissolved LS molecules have limited/no effect on fibre interactions. The minor effect of LS on the yield stress for a MFC concentration of 2% was ascribed to the formation of dense networks of entangled fibres where interfibres friction was supposed to have a minor contribution to the yield stress compared to the mechanical fibre bending and deformation necessary to break the MFC network.


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Above a LS concentration of ca. 30-40%, corresponding to an abrupt increase in the viscosity of the LS solution (Fig. 1), the yield stress increased for all tested MFC concentrations attaining values close to that of pristine MFC hydrogels. At high LS concentration, interfibre lubrication by LS had a secondary role and the strength of the MFC network was supposed to be mainly affected by the high viscosity of the suspending medium which, by decreasing fibre mobility, increased the stress necessary to break the MFC network (i.e. the yield stress). Overall, at concentration below 30%, the viscosity of LS solutions is below 10 Pa s, LS lubricates nanofiber contact nodes and favours their relative motion and network disruption when shear is applied. At higher concentration, the high viscosity of LS solutions (η > 70 Pa s) screens the lubricating action by inducing a drop in fibre mobility thus restoring the network resistance to shear. 3.3 Thixotropy Under oscillatory deformation, MFC suspensions are characterized by viscoelastic response with high storage modulus and low phase shift.30,31 This quasi-elastic behaviour indicates that structure changes in the MFC network (i.e. break down and rebuild) induced by shear rate variations occurs with no/low time delay. Nevertheless, Sorvari et al.30 showed that carboxymethylcellulose (at a concentration 0.11%) increased the viscosity of the suspending solution up to ~80 Pa s and the viscous behaviour of the MFC hydrogel corresponding to an increase in the time delay of structure change of the MFC network. In order to investigate the role of LS in affecting the MFC network rebuild kinetics, MFC/LS suspensions were subjected to a step-down shear variation simulating shear conditions applied during the suspension extrusion through the 3D printer needle. Network rebuild after shearing was described using a first order kinetic to describe MFC/LS suspensions response32


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$

= ! + " − ! 1 − # % +

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Eq. 4

where σ0 is the shear stress immediately after the step-down variation in the shear rate, σ∞ is the shear stress after infinite time, τ is a time constant and ηs is the viscosity of the Newtonian surrounding medium, i.e. the bare LS solution. Fig. 6 shows that in MFC/LS systems with LS concentration below 30%, the shear stress had a quick monotonical decrease, which reflected the viscoelastic response of the MFC network to the sudden shear variation.33 The time constant obtained from data fitting with Eq. 4, progressively increased from 4 to 7 s when the LS concentration was increased from 0 to 30% thus indicating that LS slowed down the response of the MFC network.

Fig. 6 Shear stress responses to the step-down experiments for 2% MFC series. Shear rate drop from 1000 to 0.1 s-1. Dashed lines represent data fitting with Eq. 4.


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The shear stress drop and its following increase until stabilization observed at a LS concentration of 40% revealed a transition from a viscoelastic to an inelastic thixotropic behaviour. Due to the increased viscosity of the LS solution, MFC network rebuild was delayed and, as observed for papermaking fibers,34 it occurred progressively after the step-down shear variation. The increase of LS concentration up to 45 and 50% further emphasized the thixotropic response with an increase of the time constant from 0.7 s at 40% LS to 14 and 27 s at 45 and 50% LS, respectively. 3.3 Cuboids printing and pyrolysis As illustrated in Fig. 7, square cuboids with good spatial definition, i.e. sharp edges and no spreading of the cuboid base on the substrate, where printed using the bare (2%) MFC hydrogel. LS additions lead to a progressive degradation of cuboid shape: i) Up to 30% LS, the cuboid base was subjected to an ever increasing spread, and the height of the printed object decreased from about 9 to 2 mm when the LS concentration was increased from 0 to 30%. According to experimental data shown in Fig. 6, this behaviour was associated to a decrease in the viscosity (from 130 to 60 Pa s) at low shear rate. ii) With 40% LS, the MFC/LS suspension completely spread over the printing substrate and the thickness of the final liquid film was below 1 mm. This trend was ascribed to the transition from a viscoelastic to an inelastic thixotropic behaviour. Indeed, after shearing in the extruder needle and the destruction of the MFC network, flock rebuild kinetics and the fluid viscosity were too low to stop the MFC/LS suspension which spread over the printing substrate. iii) With 50% LS, square cuboids were easily printed without major deformations. However, edges displayed rounded profiles indicating that the flow of the MFC/LS suspension did not stop


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immediately after extrusion. This behaviour was associated with the high viscosity of the suspending medium (η = 3.3 Pa s) which, despite the pronounced thixotropic behaviour and long MFC network rebuild time (τ = 27 s), slowed down spreading providing a sufficient time for network rebuild and object geometry “freezing” before inducing major deformations. As highlighted in Fig. 7, only samples with a LS concentration ranging from 0 to 10% and of 50% hold the pristine geometry after printing. However, with 0-10% LS, freeze drying was necessary to hold the original shape since air drying induced extensive shrinkage. With 50% LS, air drying led to a limited deformation of the square faces and of a 40% shrinkage along the vertical axis which, to the purpose of this study, was considered acceptable.

Fig. 7 Shape and main characteristics of 3D printed cuboids before and after drying and carbonization.


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X-ray tomography of the cuboid sections (Fig. 8.a-b) shows that before and after carbonization, the freeze dried sample containing 4% of LS (corresponding to a dry sample composition of 33.3 % MFC and 66.7 % LS) displayed a lamellar structure which was associated to solute accumulation at boundaries between ice crystals which acted as template.35,36 According to the slow convective freezing used in this study, Fig. 8 shows fine and continuous lamellar structures originating from the cuboid surface, converging to the cuboid core and forming a continuous network.

Fig. 8 X-ray tomography sections of dry cuboids. a) Top and b) lateral view of freeze dried sample obtained from the 2% MFC/4% LS suspension (i.e. 33.3% MFC and 66.7 LS in the dry sample). c) Top and d) lateral view of air dried sample obtained from the 2% MFC/50% LS suspension (i.e. 3.8 % MFC and 96.2 % LS in the dry sample). After carbonization, this particular structure led to the formation of an extremely light carbon cuboid with apparent density of ca. 63 kg m-3 and electronic conductivity of ca. 5.5-55 S m-1.


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After freeze drying, in the sample containing 50% LS, the lamellar structure was absent and the solid phase displayed the presence of fine cracks presumably due to the formation of small ice crystals. As highlighted for inorganic hydroxyapatite particles,36 this morphology change was interpreted as indicating that LS was not repelled from the ice front during crystal growth. The air dried sample with 50% LS (corresponding to a dry sample composition of 3.8 % MFC and 96.2 % LS) had high internal porosity with macropores and large cracks (Fig. 8.c,d). This irregular structure was associated to the presence of large amounts of residual water in the sample before carbonization. Indeed, as determined by direct weighing, the water content decreased from 64 % w/w to 38 and 30 % after drying in air and at 110°C, respectively. Despite the high density of the continuous phase (highlighted by the pronounced contrast of X-ray tomography images) and the high apparent density of the carbonized cuboid, i.e. 428 kg m-3, the electronic conductivity along the Z-axis was lower than that of the freeze dried sample, i.e. 3-21 S m-1. This difference was associated to discontinuities in the conductive path generated by macropores and cracks. This work shows that microfibrillated cellulose/lignosulfonate appear as promising biosourced precursors for the manufacturing of carbon objects with simple geometry by 3D printing and carbonization. Even if several aspects concerning the control of object shaping by 3D printing and its geometric stability during drying/carbonization remain to be explored, carbon cuboids obtained by freeze drying and from a dry precursor made of 33% MFC and 67% LS, displayed interesting properties (i.e. low density, geometric stability, homogeneous structure). Moreover, when compared to carbon materials obtained from other wood-derived precursors (see Table 1) they display extremely low density, good electric conductivity and come out as promising materials for manufacturing 3D structured porous electrodes for energy storage devices.


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Table 1 Comparison of electrical conductivity of wood-derived carbon materials. Reference work

Conductivity (S m-1)

Density (kg m-3)

Physical form

Precursor

This work

5.5-55

63 ± 20

3D printed cuboid

MFC/Lignosulfonate blend

Teng et al.16

2.3-3

nd

Fibre, 639-816 nm diameter

Kraft Lignin/MW Carbon Nanotube blend

Snowdon et al.17

0.9

nd

Powder compressed at 1.12 MPa

Hydrolysis Lignin

Deraman et al.37

243-500

1000

Bulky pellet

Lignin/precarbonized carbon

Rhim et al.38

10000

2000

Bulky pellet

Microcrystalline cellulose

*Lu et al.39

90-2000

65-600

Monolithic aerogel

Polycondensed resorcinol-formaldehyde gel

*SanchezGonzalez et al.40

44-215

250-700

Compressed carbon black

Carbon black

*Carbon from sources other than wood given for comparison 4.

Conclusions MFC/LS systems displayed a complex rheological behaviour which was affected by LS

concentration. At low concentration, i.e. below 30%, LS acted as lubricant favouring the shearinduced disruption of the MFC network and inducing a progressive decrease of both yield stress and viscosity. Cuboids print quality underwent to a progressive degradation due to the MFC/LS spreading over the printing surface and 10% LS was the maximum LS concentration to print cuboids. At intermediate LS concentration (ca. 40%), MFC/LS suspensions shifted from viscoelastic to inelastic thixotropic fluids with a complete degradation of the print quality, i.e. the MFS/LS suspension completely spread over the printing substrate. At high LS concentration cuboid


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printing quality was restored since, despite a pronounced thixotropic behaviour, the high viscosity of the suspending medium slowed down fluid spreading during printing thus providing sufficient time to rebuild a continuous MFC network and “freeze” the cuboid geometry. Freeze drying and carbonization of cuboids printed using MFC/LS suspensions with a maximum LS concentration of 10%, allowed obtaining carbon cuboids with minimal shape variations which displayed an ice-templated continuous lamellar structure, extremely low apparent density (ρapp = 63 ± 20 kg m-3) and high electronic conductivity (σel = 5.5-55 S m-1). Owing to the high dry material content cuboids printed with 50% LS suspensions underwent acceptable shirinkage upon air drying and carbonization. Nevertheless, despite interesting conducting properties, large defects, i.e. macropores and crackles, generated during drying led to a heterogeneous and brittle object. Overall, this work demonstrates that MFC/LS have the potential to be used as a biosourced precursor for manufacturing 3D printed carbon objects but work remains to be done to demonstrate the level of control that would be necessary for a manufacturing environment. AUTHOR INFORMATION Corresponding Author *Tel: +33 4 76 82 69 54. Fax: +33 4 76 82 69 33. E-mail: davide.beneventi@pagora. grenobleinp.fr. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by a Grenoble-INP and Joseph Fourier University grant (AGIR). LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir – grant agreement n° ANR-11-LABX0030) and of the Energies du Futur and PolyNat Carnot Institutes (Investissements d’Avenir – grant agreements n° ANR-11-CARN-007-01 and ANR-11-CARN-030-01). REFERENCES (1)

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Table of Contents Graphic and Synopsis - Cellulose nanofiber/lignosulfonate hydrogels with shear thinning and thixotropic behavior are formulated and used for the 3D printing of square cuboids. - Highly conductive carbon object are obtained by printed samples freeze/air drying and their carbonization under inert gas.


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Lignosulfonate Blends as Carbon

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