Rheological behavior of alginate-lignosulfonate blend solutions

Transcription

1 Rheological behavior of alginate-lignosulfonate blend solutions RALUCA PETRONELA DUMITRIU 1,2*, GEORGETA CAZACU 2, DAN SORIN VASILESCU 1 1 Faculty of Applied Chemistry and Materials Science, University POLITEHNICA of Bucharest, 313 Independentei Spl., , Bucharest, Romania 2 Physical Chemistry of Polymers Department, Petru Poni Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley, , Iasi, Romania ROMANIA rdumi@icmpp.ro Abstract: - The rheological properties of polymer solutions can be decisive for the development of new materials with improved properties, for various applications, by bringing essential information about the viscoelastic response and processability conditions. Alginate-lignosulfonate blend solutions prepared in different mixing ratios with the aim to obtain thin films were investigated in terms of rheological behavior. Alginate and lignin derivatives are biodegradable and non-toxic biopolymers based on renewable resources that can be used in food, cosmetics, medical and pharmaceutical applications. Steady shear and dynamic oscillatory measurements were performed at room temperature in order to assess the influence of blend solutions composition on their flow behavior and viscoelastic response. The rheology measurements revealed the viscosity decrease occurred as lignosulfonate was added in blends composition, which exhibited non- Newtonian shear thinning behavior. Key-Words: - Sodium alginate, lignosulfonate, flow behavior, dynamic viscoelasticity, blend solutions, thin films 1 Introduction In the last years there has been a growing interest in materials based on biomass products, which are aimed to reduce the use of the traditional synthetic polymers. Such challenging task became a major concern in terms of both economic and environmental points of view. Polymeric films represent an emerging class of materials that have shown promising qualities for applications in the biomedical field or in food packaging. The use of polymers as food packaging materials received tremendous attention due to their advantages over other traditional materials [1]. Polymer packaging provides many advantageous properties including strength and stiffness, barrier to oxygen and moisture, resistance to food component attack and flexibility [2]. Composite films can be designed to combine the advantages of each component. Nowadays many attempts are made to obtain novel, high-performance polymer materials for food packaging from biopolymers, having also the advantages of being biodegradable and safer for human health and the environment, which are important requirements of actual packaging materials [3]. Proteins and polysaccharides have good film-forming abilities, however their barrier and mechanical properties are naturally limited, being hydrophilic [4]. Therefore, the search for improved polymers with enhanced structures and properties using novel ingredients, production processes and formulations is required, in order to improve the quality of biodegradable packaging forms which are suitable for final product usage. Films based on biopolymers can be used to reduce water vapor, oxygen, lipid and flavor migration between components of multi-component food products, and between food and the surroundings. The film properties (composition, microstructure and physical properties) depend on the type of material used and the process conditions employed which will determine then their applications [5,6]. The combination of varying ratios of polysaccharides and proteins in the form of blends offers the possibility of manufacturing composite films with improved properties to meet consumer expectations [7]. Sodium alginate and lignin derivatives are biodegradable and non-toxic biopolymers based on renewable resources, with wide possibilities to be used in food, cosmetics, medical and pharmaceutical ISBN:

2 applications. Alginate is a linear polysaccharide extracted from brown seaweed, consisting in β-dmannuronate (M) and α-l-guluronate (G) residues linked by 1-4 glycosidic bonds [8]. It is a biocompatible, biodegradable, non-toxic polymer with high viscosity in aqueous solutions, wellknown for its property to form homogeneous films [9]. Films with desirable mechanical and water sorption properties were obtained by incorporation of alginate into pullulan films [10] and edible films with improved mechanical, barrier properties and water solubility were obtained by judicious blending of alginate and carboxymethylcellulose with pullulan as blend films components [11]. Lignin is the second most abundant biopolymer after cellulose and one of the main components of plant cell walls. Ammonium lignosulfonate is one of the typical lignin derivatives, produced through the sulfite pulping process as a by-product in the production of cellulose [12]. Due to their properties and various potential applications, sodium alginate and ammonium lignosulfonate have been chosen as the components of the studied blend solutions prepared with the aim to obtain thin films for food packaging application. In order to obtain polymeric films having the desired functional properties required for certain application, the properties of the film forming solutions should be previously established. Understanding the rheological properties of polymer solutions can be decisive for the development of new materials, with improved properties for various applications (e.g. packaging films) by bringing essential information about the viscoelastic response and processability conditions. The rheological behavior of the alginatelignosulfonate film-forming blend solutions was investigated by steady shear and dynamic oscillatory measurements performed at room temperature. The aim of this study was to evaluate the effects of blend solutions composition on their rheological properties. 2 Experimental 2.1 Materials The biopolymers used for preparation of the film forming solutions were alginic acid sodium salt from brown algae purchased from Sigma-Aldrich (medium viscosity 2000 cp, 2%, 25 o C) and hardwood ammonium lignosulfonate from Zarnesti, Romania. Twice distilled water was used to prepare the solutions. 2.2 Preparation of the blend solutions Alginate (ALG) and lignosulfonate (LS) solutions (2%, w/v) were prepared separately in twice distilled water under continuous stirring at room temperature. After complete dissolution the solutions were mixed and homogenized so as to produce the film forming solutions containing different proportions of alginate and lignosulfonate, namely 0, 10, 20, 30, 40 and 50 g LS/100 g biopolymers. The blend solutions prepared were immediately analyzed to determine their rheological behavior by steady shear and dynamic oscillatory tests performed at room temperature. 2.3 Rheology measurements The rheological behavior of sodium alginate/lignosulfonate blend solutions was studied using an Anton Paar Physica MCR 301 Rheometer (Anton Paar, GmbH, Germany), equipped with a 50 mm diameter cone-plate geometry with a 1 o angle. The rheometer utilized a Peltier heating system for accurate temperature control. Steady shear flow and dynamic oscillatory measurements were performed at 25 o C. 3 Results and discussion 3.1 Obtaining of ALG/LS blend solutions The homogeneous ALG/LS film forming aqueous solutions have been obtained by mixing for about 3 h the required amounts of ALG and LS solutions previously prepared in order to achieve the following weight ratios (wt/wt dry matter content): 100/0; 90/10; 80/20; 70/30; 60/40 and 50/50 ALG/LS. 3.2 Dynamic viscoelastic behavior For dynamic viscoelastic measurements, the linear viscoelastic range of the blend solutions was established at first. Strain sweep tests were performed at 25 o C and 10 rad/s in order to estimate the linear viscoelastic range (LVE); the results showed a LVE regime up to a strain value of 0.5 %. Thus, the dynamic oscillatory viscoelastic measurements were carried out by frequency sweep tests on the angular frequency range between rad/s at the constant strain amplitude of 0.5 %, ISBN:

3 which was established previously within the linear viscoelastic range Oscillatory frequency sweep tests The viscous and elastic responses of viscoelastic systems can be quantified by undertaking dynamic oscillatory measurements. The basis of these measurements is the application of a sinusoidal strain of frequency (ω) to the system and the measurement of its corresponding strain. Complex viscosity (η*) of a polymer is a frequency dependent material property which could be measured during forced harmonic oscillation [13]. The dynamic oscillatory frequency sweep tests were carried out to determine the viscoelastic behavior of the studied ALG/LS samples in the linear viscoelastic range LS (wt%) Complex viscosity (Pa.s) 0,01 rad/s 0,1 rad/s 1 rad/s 10 rad/s 100 rad/s film forming ability of the solutions was affected, the complex viscosity decreasing significantly. At higher frequencies, the structural changes occurred by breaking and reformation of molecular bonds might become irreversible, leading to disentanglement of long chain polymers, and consequently, decrease of complex viscosity [14]. 3.3 Steady shear flow behavior The rheological properties of ALG and ALG/LS blend solutions with different compositions were investigated also by rotational measurements at 25 o C. The shear stress (τ) and apparent viscosity (η) were recorded as a function of shear rate. Fig. 2 shows the flow curves of the aqueous blend solutions plotted as lg τ vs. lg scale. lg 10 2 ALG 90/10 ALG/LS 80/20 ALG/LS /30 ALG/LS 60/40 ALG/LS 50/50 ALG/LS Fig. 1. Complex viscosity dependence on composition at various angular frequencies lg As shown in Fig. 1, within the tested frequency range, the complex viscosity decreases with the angular frequency (ω) increase, indicating a shear thinning behavior corresponding to all tested compositions. The most sudden decrease was recorded for neat ALG sample, while for the other compositions tested the viscosity decreased more smoothly as LS percent increased. This behaviour can be attributed to the entangled polymeric chains of alginate having more time to relax at lower deformation rate, whereas as the sweeping frequency increased the available time for ALG polymeric chains to relax declined, leading to decrease in complex viscosity. By adding LS in higher quantity in composition the rheological properties and the Fig. 2. Shear stress vs. shear rate curves for different ALG/LS blend compositions It can be noticed that the shear stress increases with the increase of shear rate for all compositions and exhibits higher values as ALG content increases in sample solution, probably due to the enhanced intermolecular interactions occurred. Generally, all compositions tested displayed non- Newtonian shear thinning flow behavior, more pronounced as ALG concentration increased. Similar behavior was reported for alginate-pullulan blend solutions [10]. The relation between the zero-shear viscosity (η o ) and film forming solutions composition is depicted in Fig. 3. The neat ALG sample showed the highest ISBN:

4 value for the zero shear viscosity (1.28 Pa s) compared with the other compositions, for which η 0 decreased gradually as LS was added. Such behavior was expected due to the presence of long chains entanglements within alginate structure, undisturbed at low shear rate. Zero-shear viscosity (Pa.s) 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 T=25 o C LS (wt%) Fig. 3. Zero-shear viscosity dependence on composition The shear thinning behavior of the polysaccharide systems is related to the orientation of the macromolecules along the stream line of the flow [15]. The stretching macromolecules of the polysaccharides may form aggregates in the low shear rate range, which exhibit flow resistance and results in the high viscosity. The aggregates will be destroyed as the shear rate increases, dispersing macromolecules along the flow direction, causing the decrease of the apparent viscosity. Thus, increasing the content of sodium alginate in the blend solutions composition led to a more non- Newtonian flow behavior. The rheological properties of biopolymer solutions can affect spreadability, thickness and uniformity of liquid coating layer and film performance. A reduction in the solution s viscosity provides a processing advantage during high shear processing operations, such as pumping, filling and spraying application; whereas high apparent viscosity at low shear rates provides a desirable mouth-feel during mastication [16]. The method of combining various biopolymers having different rheological properties proves to be an effective strategy to control film forming solutions rheology and consequently to obtain new materials with tailored properties for desired applications. 4 Conclusion The rheological properties of ALG/LS blend solutions were discussed, concentrating on the principles of flow behavior and viscoelastic deformation of the systems. The studied film forming solutions showed typical non-newtonian shear thinning behavior at 25 o C. The added LS produced a significant decrease of blend solutions viscosity and of the shear stress, attributed to structural changes induced upon alginate macromolecular entangled chains. Acknowledgements The authors gratefully acknowledge the financial support of the Sectorial Operational Programme Human Resources Development of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/ References: [1] J. Jordan, K.I. Jacob, R. Tanenbaum, M. Sharaf, I. Jasiuk, Experimental trends in polymer nanocomposites-a review. Materials Science and Engineering A, 393(1-2), 2005, [2] C. Silvestre, D. Duraccio, S. Cimmino, Food packaging based on polymer nanomaterials, Progress in Polymer Science, 36(12), 2011, [3] J.W. Rhim, P.K.W. Ng, Natural biopolymerbased nanocomposite films for packaging applications, Critical Reviews in Food Science and Nutrition 47(4), 2007, [4] L. Yang, A.T. Paulson, Mechanical and water vapour barrier properties of edible gellan films, Food Research International, 33(7), 2000, [5] J.M. Krochta, C. De Mulder-Johnston, Edible and biodegradable polymer films: challenges and opportunities, Food Technology, 51(2), 1997, [6] M.S. Rao, S.R. Kanatt, S.P. Chawla, A. Sharma, Chitosan and guar gum composite films: Preparation, physical, mechanical and antimicrobial properties, Carbohydrate Polymers 82(4), 2010, [7] E. Kristo, C.G. Biliaderis, A. Zampraka, Water vapour barrier and tensile properties of composite caseinate-pullulan films: biopolymer composition effects and impact of beeswax lamination, Food Chemistry 101(2), 2007, [8] P. Gacesa, Alginates, Carbohydrate Polymers 8(3), 1988, ISBN:

5 [9] K.I. Draget, S.T. Moe, G. Skjåk-Bræk, O. Smidsrod, Food Polysaccharides and Their Applications, CRC Press Taylor&Francis, Boca Raton FL pp , [10] Q. Xiao, L.-T. Lim, Q. Tong, Properties of pullulan-based blend films as affected by alginate content and relative humidity, Carbohydrate Polymers, 87(1), 2012, [11] Q. Tong, Q. Xiao, L.-T. Lim, Preparation and properties of pullulan alginate carboxymethylcellulose blend films, Food Research International, 41(10), 2008, [12] G. Cazacu, M.C. Pascu, L. Profire, A.I. Kowarski, M. Mihaes, C. Vasile, Lignin role in a complex polyolefin blend, Industrial Crops and Products, 20(2), 2004, [13] J. Ma, Y. Lin, X. Chen, B. Zhao, J. Zhang, Flow behavior, thixotropy and dynamical viscoelasticity of sodium alginate aqueous solutions, Food Hydrocolloids, 38, 2014, [14] M.H. Tunick, Small-strain dynamic rheology of food protein networks. Journal of Agricultural and Food Chemistry, 59(5), 2011, [15] B.S. Chagas, D.L.P. Machado, R.B. Haag, C.R. De Souza, E.F. Lucas, (2004). Evaluation of hydrophobically associated polyacrylamidecontaining aqueous fluids and their potential use in petroleum recovery, Journal of Applied Polymer Science, 91(6), 2004, [16] M.A García,. A. Pinotti, M. N. Martino, N. E. Zaritzky, Edible films and coatings for food applications, New York: Springer, pp.169, ISBN: