EFFECT OF ANTIMICROBIAL AGENTS ON CELLULOSE ACETATE NANOCOMPOSITES PROPERTIES Francisco J. Rodríguez*, Julio E. Bruna, María J. Galotto, Abel Guarda and Hugo Sepúlveda Center for the Development of Nanoscience and Nanotechnology (CEDENNA). Universidad de Santiago de Chile. Faculty of Technology. Department of Food Science and Technology. Food Packaging Laboratory. Obispo Manuel Umaña 050. Estación Central. Santiago, Chile. francisco.rodriguez.m@usach.cl. Abstract - Nanocomposites based on cellulose acetate, Cloisite 30B, triethyl citrate and thymol or cinnamaldehyde were prepared using a dissolution casting technique. The effect of thymol and cinnamaldehyde on the cellulose acetate nanocomposite properties was evaluated by XRD and DSC. Important changes on the thermal properties and morphological structure were observed according to thymol and cinnamaldehyde content. Keywords: nanocomposites, organoclays, essential oils, cellulose acetate. Introduction Nowadays, excessive levels of plastic waste in the environment have caused the concern of scientific community to develop eco-friendly materials. Research on these materials has mainly been oriented to biopolymers or its derivates. However, applications of this kind of polymers are limited due to their poor properties (e.g brittleness, high gas permeability, low melt viscosity for further processing) that prevent its use. Nevertheless, the use of nanofillers to produce bionanocomposites has risen as an important alternative to face this problem [1]. Regarding to clays, montmorillonite has been the most commonly employed clay mineral in polymer composites. To ensure good compatibility between polymer and montmorillonite in composites, this layered silicate must be modified with organic molecules such as quaternary ammonium salt with alkyl chains [2]. This way, the organoclays structure favors the nanoparticle polymer interactions necessary to produce nanocomposites. On the other hand, introduction of different additives inside plastic structures has been an important strategy to product functionalized materials. In this way, developing of materials with antimicrobial activity has been considered as important materials with high potentiality to be used as active food packaging [3]. Additives based on derived from essential oils have shown to be an important alternative to produce antimicrobial activity [4]. The aim of this work was evaluate the effect of different antimicrobial agents derived from essential oils on the cellulose acetate nanocomposite properties. 5280
Experimental Commercial organoclay (Cloisite 30B) was provided by Southern Clay Products, Inc. Cellulose acetate (39.8 wt. % acetyl content with Mn ca. 30,000), triethyl citrate (> 99 %), cinnamaldehyde (99 %) and thymol ( 99.5 %) were supplied by Aldrich. The nanocomposites were prepared according to a procedure previously reported [5]. Nanocomposites consisted of 5 wt. % of Cloisite 30B, 5 wt. % of triethyl citrate (TEC) and variable content of thymol and cinnamaldehyde (0, 0.5, 1.0, 1.5 and 2.0 wt. %). Nanocomposites were prepared by means of casting technique using acetone as solvent. 10 g of cellulose acetate, TEC and antimicrobial agent were dissolved in 150 ml of acetone under vigorous stirring for 1 h at ambient temperature. 0.53 g of Cloisite 30B was dispersed in 50 ml of acetone and sonicated for 30 min at room temperature. Then, the cellulose acetate solution was added on organoclay suspension under vigorous stirring and the mixture was stirred during 60 min at room temperature. After that the mixture was added on Petri disc and dried at 40ºC in oven for 4 hrs. Finally, the films were removed from the glass disc and stored in polyethylene bags to avoid contamination. X-Ray Diffraction (XRD) analysis were carried out in a Siemens Diffractometer D5000 (30 ma and 40 kv) using CuKα ( =1.54 A) radiation at room temperature in a 2 range 2 10º at 0.02 /seg. Differential Scanning Calorimetry (DSC) analyses were conducted with a Mettler DSC-822e calorimeter where samples were heated from 25ºC to 300 ºC at a rate of 10 ºC/min under the purge of dry nitrogen. The OTR of films was determined with an Oxygen Permeation Analyzer (MOCON OX-TRAN MS2/20), equipped with a Coulox oxygen sensor with a sensitivity of 0.1 [cc/(m 2 day atm)]. Measurements were carried out at 23 C and 0% RH until a steady-state oxygen transmission rate was achieved. Output values were expressed as the oxygen transmission rate in [cc/(m 2 day)]. The SEM micrographs of nanocomposites films were obtained 205 from an JSM-5410 Jeol Scanning Microscope with accelerating voltage at 10 kv. Samples were coated with gold palladium using a 207 Sputtering System Hummer 6.2. To the cross-section analysis, the 208 samples were previously fractured under liquid nitrogen. Results and Discussion Table 1 resumes the results of diffraction X-Ray (XRD) and differential scanning calorimetry (DSC) analysis. Based on maximum diffraction peak from Cloisite 30B pure (4.78, d=1.85 nm) it 5281
was possible to confirm the introduction of cellulose acetate inside the clay structure producing an intercalated structures to all nanocomposites prepared. Differences in the interlaminar distances to nanocomposites based on cinnamaldehyde and thymol were observed. In this way, nanocomposites based on cinnamaldehyde presented higher interlaminar distances than those elaborated with thymol. At the moment, there is no explanation for this behavior; however, it is clear that these essential oils are not able to modify the interlaminar distance of Cloisite 30B when they are mixed previously [6]. Regarding effect of antimicrobial additives on thermal properties a plasticizer effect was also observed. So, a reduction of Tg and Tm values was observed according to increase of cinnamaldehyde and thymol content in nanocomposites. Fig. 1 shows the SEM images of cross section of selected nanocomposites. Presence of cinnamaldehyde in the nanocomposite produced the formation of cavities in the structure. In previously works, it has been observed that these cavities are favored when the organoclay and essential oils content are increasing [7-8]. These cavities could be formed by the preparation technique used to produce the nanocomposites. In this sense, solvent evaporation could be the key to explain the cavities formation because essential oils or clays would retard the solvent evaporation both for chemical interactions and tortuous path generation [9] which would favor the formation of this kind of morphology. In spite of the clays have been identified as additives that can reduce the gas permeability, these nanocomposites presented oxygen transmission rates above 2,000 (cc/m 2 day) values significantly higher than those reported to cellulose acetate/cloiste30b nanocomposites without other additives [5]. These results could be explained due to presence of the cavities which favors the gas permeability through the nanocomposite films. Further studies will be oriented to determine the antimicrobial activity of this kind of nanocomposites against E. Coli and L. Inocua. 5282
Table 1. XRD and DSC results of different nanocomposites. Sample Antimicrobial agent (AM) [AM], wt. % 2, degree Interlaminar distance, Tg, Tm, c C Cc nmb Controla ------ 0.0 4.12 2.14 162 202 CACi-0.5 0.5 3.84 2.30 152 190 CACi-1.0 1.0 4.05 2.18 151 186 CACi-1.5 1.5 3.81 2.32 152 193 CACi-2.0 2.0 3.96 2.23 140 177 CATh-0.5 0.5 4.25 2.08 155 193 CATh-1.0 1.0 4.07 2.17 151 188 CATh-1.5 1.5 4.16 2.12 149 188 CATh-2.0 2.0 3.99 2.21 147 186 a Cellulose acetate/5 wt % TEC/5 wt % Cloisite30B b Based on Bragg s Law c From the second scan Figure 1 SEM images of cross section of nanocomposites (x5000). a) Control, b) CACi-1.0, c) CACi-2.0, d) CATi-1.0 and e) CATi-2.0. 5283
Conclusion Intercalated nanocomposites based on cellulose acetate/cloisite 30B/Triethyl citrate/cinnamaldehyde or thymol were prepared using a dissolution casting technique. Nanocomposites presented a reduction of glass transition and meting point temperatures, and changes in morphology according to cinnamaldehyde or thymol content. Acknowledgements The authors thank to Comisión Nacional de Investigación Científica y Tecnológica, CONICYT, for the financial support from Programa Bicentenario de Ciencia y Tecnología (Project PDA-22), Programa de Financiamiento Basal para Centros Científicos y Tecnológicos de Excelencia (Project FB0807) and Fondo Nacional de Desarrollo Científico y Tecnológico (Project FONDECYT 11100389). References 1. M. Darder; M. Colilla; E. Ruiz-Hitzky Chem. Mat. 2003, 15, 3774. 2. A. Vázquez, M. López, G. Kortaberria, L. Martin, I. Mondragón App. Clay Sci. 2008, 41, 24. 3. P. Suppakul, J. Miltz, K. Sonneveld, S. W. Bigger J. Food Sci. 2003, 68, 408. 4. P. Suppakul, J. Miltz, K. Sonneveld, S. W. Bigger Pack. Tech. Sci. 2006, 19, 259. 5. F. J. Rodríguez, M. J. Galotto, A. Guarda, J. E. Bruna J. Food Eng. 2011 doi:10.1016/j.jfoodeng.2011.05.004. 6. R. Quintero, M. J. Galotto, A. Guarda, F. Rodríguez. V Coloquio de Macromoléculas. December 1 3, 2010. Parral Chile, 43. 7. S. Bustamante, D. Koscina. Food Enginnering Thesis, Universidad de Santiago de Chile, 2011. 8. F. Rodríguez Mercado, J. Bruna, M. J. Galotto, A. Guarda. 25 th IAPRI Symposium on Packaging. May 16 18, 2011. Berlin Germany, Tu.2.B.1. 9. S. Solovyov, A. Goldmen, Mass Transport & Reactive Barrier in Packaging. Theory, Applications & Design, DEStech Publications, Pennsylvania, 2008. 5284