There are several methods described in the literature for producing nano-cellulosic material. For example, by using mechanical shearing1, often combined with chemical or enzymatic treatments2,3, disintegration of the cellulose fibres can be achieved yielding cellulose nanofibrils (CNF). By instead using acid hydrolysis of the cellulose, cellulose nanocrystals (CNC) can be formed4. These two materials have very different geometries (and usually also surface characteristics); the nanofibrils are flexible in an aqueous phase, are long in relation to their width i.e. have a high aspect ratio typically in the order of 100-1505, and form, an entangled network at very low concentrations, whereas the CNCs have a significantly lower aspect ratio typically 10 to ca 654, displaying a colloidal behaviour usually. It is thus not unexpected that these two types of nano-cellulosic materials will exhibit quite different rheological properties (viscosity and moduli) in an aqueous medium even though the raw material is the same. This includes also the effect of concentration on their rheology. However, differences in the surface charge density of these nanomaterials will also contribute to their rheological behaviour making it difficult to distinguish geometrical effects (on the rheology) from those associated with the

surface characteristics of the material. In the production of CNF the fibres are usually modified in some way, changing the surface properties of the fibrils, this is done to decrease the energy consumption used in the mechanical shearing step. When making the acid hydrolysis used for creating CNC one can also introduce other chemical groups to the surface of the cellulose fibrils that can influence the rheological behaviour of the materials.

In this study, the rheological properties of aqueous nano-cellulosic dispersions with different particle/fibre geometries have been evaluated. The aim has been to keep the surface charge density of the different materials as close as possible in order to demonstrate and clarify the geometry effect of the rheology (viscosity and moduli) of the dispersions. Three types of nano-cellulosics dispersions were used; one CNF, one CNC obtained from acid hydrolysis of cellulose, and one CNC-similar dispersion obtained from homogenisation of a bioethanol residue. The latter particles had a shape which was more fibrillar-like than the CNC obtained from acid hydrolysis as can be seen in Fig 1. By using AFM and TEM it is possible to image the cellulose materials in order to assess the aspect ratio of the particles. In a

Rheological properties of aqueous nano-cellulosic

dispersions

Tobias Moberg1,5, Karin Sahlin2,5, Kun Yao3,5, Shiyu Geng4,5, Gunnar Westman2,5, Qi Zhou3,5, Kristiina Oksman4,5 and Mikael Rigdahl1,5

1Chalmers University of Technology, Department of Materials and Manufacturing

Technology, SE-412 96 Gothenburg, Sweden 2Chalmers University of Technology, Department of Chemistry and Chemical Engineering,

SE-412 96 Gothenburg, Sweden 3Royal Institute of Technology, School of Biotechnology, SE-100 44 Stockholm, Sweden

4Luleå University of Technology, Division of Materials Science, SE-971 87 Luleå, Sweden 5Wallenberg Wood Science Center, Chalmers University of Technology, SE-412 96

Gothenburg and Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Simultaneous In-situ Analysis of Instabilities and First Normal StressDifference during Polymer Melt Extrusion Flows

Roland Kádár1,2, Ingo F. C. Naue2 and Manfred Wilhelm2

1 Chalmers University of Technology, 41258 Gothenburg, Sweden2 Karlsruhe Institute of Technology - KIT, 76128 Karlsruhe, Germany

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 24, 2016

ABSTRACTA high sensitivity system for capillaryrheometry capable of simultaneously de-tecting the onset and propagation of insta-bilities and the first normal stress differ-ence during polymer melt extrusion flowsis here presented. The main goals of thestudy are to analyse the nonlinear dynam-ics of extrusion instabilities and to deter-mine the first normal stress difference inthe presence of an induced streamline cur-vature via the so-called ’hole effect’. Anoverview of the system, general analysisprinciples, preliminary results and overallframework are herein discussed.

INTRODUCTIONCapillary rheometry is the preferredrheological characterisation method forpressure-driven processing applications,e.g. extrusion, injection moulding. Themain reason is that capillary rheometry isthe only method of probing material rheo-logical properties in processing-like condi-tions, i.e. high shear rate, nonlinear vis-coelastic regime, albeit in a controlledenvironment and using a comparativelysmall amount of material.1 Thus, it isof paramount importance to develop newtechniques to enhance capillary rheome-ters for a more comprehensive probing ofmaterial properties. Extrusion alone ac-counts for the processing of approximately35% of the worldwide production of plas-tics, currently 280⇥ 106 tons (Plastics Eu-rope, 2014). This makes it the most im-portant single polymer processing opera-

tion for the industry and can be found ina variety of forms in many manufacturingoperations. Extrusion throughput is lim-ited by the onset of instabilities, i.e. prod-uct defects. Comprehensive reviews on thesubject of polymer melt extrusion insta-bilities can be found elsewhere.4,6 A re-cent method proposed for the detectionand analysis of these instabilities is that ofa high sensitivity in-situ mechanical pres-sure instability detection system for cap-illary rheometry.8,10 The system consistsof high sensitivity piezoelectric transducersplaced along the extrusion slit die. In thisway all instability types detectable, thusopening new means of scientific inquiry. Asa result, new insights into the nonlinear dy-namics of the flow have been provided.9,14

Moreover, the possibility of investigatingthe reconstructed nonlinear dynamics wasconsidered, whereby a reconstructed phasespace is an embedding of the original phasespace.2,14 It was shown that a positive Lya-punov exponent was detected for the pri-mary and secondary instabilities in lin-ear and linear low density polyethylenes,LDPE and LLDPE,.14 Furthermore, it wasdetermined that Lyapunov exponents aresensitive to the changes in flow regime andbehave qualitatively different for the iden-tified transition sequences.14 It was alsoshown that it is possible to transfer thehigh sensitivity instability detection sys-tem to lab-sized extruders for inline ad-vanced processing control and quality con-trol systems.13

A very recent possibility considered

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series of experiments, some of these materials were also grafted with poly(ethylene glycol). The effect of this grafting on the rheological behaviour will be discussed.

ACKNOWLEDGEMENT

The authors thanks the Wallenberg Wood Science Center and Chalmers University of Technology for the financial support REFERENCES 1. Turbak, A. F., Snyder, F. W. and

Sandberg, K. R. J. (1983), Microfibrillated cellulose, a new cellulose product: Properties, uses and commercial potential. J. Appl. Polym. Sci. 37, 815–827.

2. Henriksson, M., Henriksson, G., Berglund, L. A. and Lindström, T. (2007), An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. Eur. Polym. J. 43, 3434–3441.

3. Isogai, T., Saito, T. and Isogai, A. (2010), Wood cellulose nanofibrils prepared by TEMPO electro-mediated oxidation. Cellulose.

4. Mariano, M., El Kissi, N. and Dufresne, A. (2014), Cellulose nanocrystals and related nanocomposites: Review of some

properties and challenges. J. Polym. Sci. Part B Polym. Phys. 52, 791–806.

5. Siró, I. and Plackett, D. (2010), Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose 17, 459–494.

Figure 1, Three different cellulosic dispersions. The leftmost is a CNC dispersion made by acid hydrolysis of cellulose, the middle one is CNC made from bio-ethanol residue, the rightmost is CNF obtained from TEMPO-mediated oxidation followed by mechanical shearing.

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