TITLE: Polymer Adsorption on Nano Fibrillar Cellulose and its Effects on Suspension Rheology

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1 TITLE: Polymer Adsorption on Nano Fibrillar Cellulose and its Effects on Suspension Rheology AUTHORS: Kristýna Hlisnikovská and Lars Järnström, Karlstad University, Dept. of Chemical Engineering, Karlstad, Sweden Presenter: Kristýna Hlisnikovská CONTACT INFO: ABSTRACT: Effects of addition of polymers and simple additives on gel formation and viscosity of suspensions based on Nano Fibrillar Cellulose (NFC) have been investigated. One of the major obstacles for wider use of NFC in industrial scale is the high water content of the suspensions and the formation of gel-like structures already at low solids content (Pääkko et al. 2007). The objectives of this study were to control these parameters and prepare NFC suspensions of variable properties, such as viscosity. The mechanisms behind gel formation in the investigated suspensions were found to be very complex and were affected by several of the experimental parameters. Effects of time and strategy of mixing were found to play significant role during polymers mixing. Rheological measurements were used as an analytical tool to describe the flow and structural behavior of NFC/polymer mixtures, where small-amplitude oscillatory shear have been studied. The adsorption behavior of polymers on cellulose nanofibers was investigated using solution depletion methods. The performed adsorption studies gave an insight in the mechanism and processes behind the viscosity reduction of prepared mixtures. We believe that the results are indicating several different techniques to control the viscosity of NFC suspensions, which could be applied in industrial use of NFC. MATERIALS NFC suspensions were produced by VTT, Finland and CTP, France in cooperation. Random copolymer poly(vinyl alcohol/vinyl acetate) (Alcotex 72,5) supplied by Synthomer, UK (denoted as PVA) were used to investigate the effect of polymer adsorption on viscosity of NFC suspension. Deionized water with conductivity less than 1 µs/cm was used in all experiments. The NFC-suspensions and polymer solutions were prepared without addition of salt, i.e. the ionic strength was given by the purity of the NFC and polymer samples. METHODS 40 g of NFC/PVA suspensions were prepared by addition of deionized water to original NFC suspension followed by drop-wise addition of PVA solution under stirring to obtain final concentration of 1.5 wt.% for NFC and required concentration of PVA. After complete addition of PVA, samples were kept under stirring on magnetic stirrer for whole duration of experiment. The rheological behavior of the suspensions was investigated using a rheometer (Physica MCR 300, Anton Paar GmbH, Austria) with plate/plate geometry (diameter 5 cm, gap 1 mm), at 20 C. Deformations in small-amplitude oscillatory shear (SAOS) have been studied in the frequency range of Hz at strain 0.1 %. All samples were tempered for 300 s before each measurement. The adsorbed amount was measured spectrophotometrically based on a method described by Zwick (1965). Supernatant was separated from suspension by 2 steps centrifugation: first at 4000 rpm for 30 min and second at 6000 rpm for 30 min. RESULTS & DISCUSSION Effect of PVA on NFC viscosity The addition of small amount of PVA to NFC suspension causes the decrease in viscosity. The exact mechanism of this behavior is not known yet and it can take several days before the effect is fully achieved. Flocculation processes that may take longer time have been reported for mechanisms Capillary-Induced Phase Separation (CIPS) by Wennerström et al We believe that the slow flocculation observed in the NFC/PVA case is due to slow diffusion and rearrangement of the NFC. The PVA-treatment resulted in formation of NFC aggregates and also a less extended structure of individual NFC fibers. A decrease in viscosity of the NFC suspension after addition of PVA in order of two magnitudes was observed. The value of suspension viscosity did not level out within an experimental period of time of ca 2 weeks. This general behavior is illustrated in Figure 1 for NFC suspension containing 1.5 wt.% NFC and 0.07 wt.% PVA. 1

2 The method of PVA addition and intensity of mixing played a significant role. It was observed that the decrease in suspension viscosity was faster when PVA was added slowly (drop-wise) to the NFC suspension while stirring. Adsorption isotherms Figure 2 shows the adsorption of PVA on NFC in aqueous suspension as a function of the concentration of PVA in supernatant. A plateau-value of adsorption was observed around 0.04 wt.% PVA in solution. The plateau-value corresponded to 8 mg PVA/g NFC. The concentration of PVA used in the rheological experiments was much higher than the plateau-value shown in Figure 2. Thus the decrease in viscosity with time shown in Figure 1 was most likely due to re-conformation of NFC leading to a less extended configuration, and not directly to the adsorbed amount of PVA. However, additional experiments are needed before a final statement can be made. complex viscosity (Pa s) ,1 1.5 wt.% NFC wt.% PVA time (day) adsorbed amount PVA/NFC (mg/g) ,00 0,02 0,04 0,06 0,08 0,10 equilibrium concentration of PVA (wt.%) Figure 1. Complex viscosity of aqueous NFC/PVA suspensions at 20 C and ph~8 at 1 Hz vs. time after addition of PVA. Figure 2. Adsorption of PVA on NFC in aqueous suspensions containing 1.5 wt.% NFC vs. the equilibrium concentration of PVA. Samples were stirred for 8 days after addition of PVA at 23 C with ph~8 before adsorption experiments. CONCLUSIONS It was discovered that addition of polyvinyl alcohol to NFC suspension changed the gel structure of a NFC suspension resulting in a decrease in viscosity. The effects on suspension viscosity were rather slow and it could take several days or weeks for the full effect to be developed, depending on the strategy of mixing (i.e. geometrical design, energy input, and rate of addition of PVA). A slow addition of the soluble polymer reduced the viscosity more rapidly than did a rapid addition. The decrease in viscosity can most likely be explained in terms of shrinkage of individual NFC fibers taking place in the poly(vinyl alcohol/vinyl acetate)-containing NFC suspension as a consequence of polymer adsorption. ACKNOWLEDGEMENTS The research leading to these results has received funding from the European Community s Seventh Framework Programme under grant agreement n REFERENCES Pääkko, M., Ankerfors, M., Kosonen, H., Nykanen, A., Ahola, S., Osterberg, M., Ruokolainen, J., Laine, J., Larsson, P.T., Ikkala, O. & Lindstrom, T. (2007). Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels. Biomacromolecules, 8 (6), Wennerström, H., Thuresson, K, Linse, P, Freyssingeas, E; Long Range Attractive Surface Forces Due to Capillary-Induced Polymer Incompatibility, Langmuir, 14, (1998) Zwick, M.M. 1965, "Poly (vinyl Alcohol) - Iodine complexes", Journal of Applied Polymer Science, vol. 9, pp

Polymer Adsorption on Nano Fibrillar Cellulose and its Effects on Suspension Rheology Kristýna Hlisnikovská and Lars Järnström, Karlstad University, Sweden

3 Polymer Adsorption on Nano Fibrillar Cellulose and its Effects on Suspension Rheology Kristýna Hlisnikovská and Lars Järnström, Karlstad University, Sweden Presented by: Kristýna Hlisnikovská Karlstad University, Sweden Introduction Experimental part Results adsorption Outline flow properties Concluding remarks Summary 3

fiber interactions : H-bonds strong network transparent t film hydroxyl groups at the surface

4 Research motivation Project for paper industry reduce amount of petroleum-based materials recyclability and biodegrability New materials nanofibrillar cellulose Nanofibrillar cellulose (NFC) fiber interactions : H-bonds strong network transparent t film hydroxyl groups at the surface further modification possible Potential applications coating (graphic and barrier paper) increase of paper strength 4

5 NFC properties high viscosity and gel structure low solids content t affinity to water Research objectives: hydrophobization h of NFC surface by physical adsorption of polymers viscosity reduction solids content increase Materials NFC 1.5 wt.%: shear viscosity (10 2 s -1 )= Pa.s complex viscosity it (1 Hz) = Pa.s PVA: Alcotex 72.5 (6 mpa.s, viscosity at 4%, 20 C) Mowiol 4-98; 6-98; (viscosity at 4%, 20 C degree of hydrolysis) Na 2 SO 4 p-dadmac cationic polyelectrolyte 5

samples were stirred for whole duration of experiment T = 23±1 C (up to 30 C during mixing), ph 7-8 all mentioned concentrations are in wt.%! Table: 1.

6 Sample preparation lab scale magnetic stirrer 1. addition of deionized i d water to 2.28% 28% NFC suspension 2. addition of 1% PVA slow (drop-wise) fast (water layer in between NFC and PVA) slow fast Mixing intensity of mixing slow ( rpm) fast (~600 rpm) samples were stirred for whole duration of experiment T = 23±1 C (up to 30 C during mixing), ph 7-8 all mentioned concentrations are in wt.%! Table: 1.5% NFC with 0.06% PVA (visc 4, DH 98), 20 C complex viscosity it (Pa.s) at drop-wise mixing slow 1 Hz after 1 day of mixing addition fast fast slow addition fast pure NFC

01 10 Hz; strain 10-3 sample Experimental methods Adsorption of PVA on NFC fibrils solution depletion technique NFC/PVA

7 Experimental methods Viscosity rotational viscometer plate-plate geometry, 20 C C, tempering for 300 s flow curve shear rate s -1 oscillations frequency Hz; strain 10-3 sample Experimental methods Adsorption of PVA on NFC fibrils solution depletion technique NFC/PVA solution separation 2 steps centrifugation PVA in supernatant analytical method described by Zwick (1965) formation of colored complex (5 C) analyzed in spectrophotometer (23 1 C) calculation of adsorbed amount 7

8 Viscosity viscosity reduction by PVA adsorption (2 orders of magnitude) time dependence Increase of solids content blends of NFC and PVA mixed for 13 days on magnetic stirrer centrifugation (30 min; 2000 rpm; 20 C) discarding the supernatant NFC residual homogenized by magnetic stirring viscosity measurement comparison with data for pure NFC (different concentrations) Sample: Viscosity 1% NFC with 0.15% PVA (mpa.s) (visc 6/DH 72.5) Comments 4% NFC with ~0.15% PVA 202 viscosity is approx. as for pure pure 1.25% NFC % NFC (ca 3.1x increase pure 1.5% NFC 321 in solid content) 8

9 PVA adsorption isotherm 1.5% NFC with additions of % PVA (visc 6/DH 72.5) Different PVA grades 1.5% NFC with 0.06% PVA after 4 days of mixing; i viscosity it at shear rate 100 s -1 9

10 Effect of sulfate ion sulfate ions supposed to facilitate PVA adsorption (Th. Tadros and B. Vincent, 1979, adsorption on PS particles) 1.5% NFC with 0.06% PVA (visc 4/DH 98) + 0.1M Na 2 SO 4 Effect of cationic polyelectrolyte p-dadmac (Polydiallyl Dimethyl Ammonium Chloride) effect of electrostatic t ti repulsion 10

5) 200 μm 200 μm Summary viscosity decreases by PVA

to further decrease of viscosity in NFC/PVA mixtures

decrease in viscosity than addition of cationic

11 Do NFC properties change? The modification by adsorption of PVA changes the structure! 1.5% NFC 1.5% NFC with 0.06% PVA (visc 6/DH 72.5) 200 μm 200 μm Summary viscosity decreases by PVA adsorption the presence of sulfate ions did not lead to further decrease of viscosity in NFC/PVA mixtures addition of uncharged PVA caused a more significant decrease in viscosity than addition of cationic polyelectrolyte advantage of physical adsorption method relatively simple method and equipment PVA is, like NFC, also biodegradable 11

12 Acknowledgements The research leading to these results has received funding from the European Community s Seventh Framework Programme under grant agreement n NFC suspensions were produced by VTT, Finland and CTP, France in cooperation Thank you PRESENTED BY Kristýna Hlisnikovská kristyna.hlisnikovska@kau.se Please remember to turn in your evaluation sheet... 12

Physical adsorption of PVOH to adjust the rheological profile of NFC

Physical adsorption of PVOH to adjust the rheological profile of NFC Lars Järnström and Kristýna Hlisnikovská, Karlstad University SUNPAP Workshop 5.10.2011 Outline Introduction Experimental part Results