Preparation and characterization of biodegradable

Preparation and characterization of biodegradable PLA/organosilylated clay nanocomposites R. Olivieri, L. Di Maio, P. Scarfato, and L. Incarnato Citation: AIP Conference Proceedings 1736, 020102 (2016); doi: 10.1063/1.4949677 View online: http://dx.doi.org/10.1063/1.4949677 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1736?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nanostructure and thermal properties of melt compounded PE/clay nanocomposites filled with an organosilylated montmorillonite AIP Conf. Proc. 1695, 020047 (2015); 10.1063/1.4937325 Preparation and characterization of biodegradable active PLA film for food packaging AIP Conf. Proc. 1593, 338 (2014); 10.1063/1.4873795 Fatty hydrazides modified clay for polylactide/polycaprolactone (PLA/PCL) nanocomposite preparation AIP Conf. Proc. 1482, 550 (2012); 10.1063/1.4757532 Alternative synthetic routes for the preparation of PLA/montmorillonite nanocomposites AIP Conf. Proc. 1255, 67 (2010); 10.1063/1.3455667 CLAY FUNCTIONALIZATION WITH DIFFERENT AMINOSILANES FOR NANOCOMPOSITES PREPARATION AIP Conf. Proc. 1042, 181 (2008); 10.1063/1.2988993

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Preparation and Characterization of Biodegradable PLA/Organosilylated Clay Nanocomposites R. Olivieri, L. Di Maio, P. Scarfato, L. Incarnato Department of Industrial Engineering, University of Salerno Via Giovanni Paolo II 132 – 84084 Fisciano (SA), Italy Abstract. In this work a new organosilylated clay was successfully synthesized by functionalization of a natural sodium montmorillonite (MMT) by (3-glycidyloxypropyl)trimethoxysilane (GOPTMS). This organosilylated clay was used as nanofiller for preparation, by solvent casting, of poly(lactic acid) nanocomposite systems. Similar systems, containing as nanofiller the commercial Cloisite 30B (i.e. a natural sodium montmorillonite organically modified with alkylammonium salt), were also prepared for comparison. All the obtained nanocomposite films were characterized using several techniques (XRD, permeability and mechanical tensile tests). Obtained results pointed out that nanocomposite system containing the organosilylated clay showed a better intercalation of the polymer chains into the clay layers and a higher improvement of the oxygen barrier properties, when compared to both the neat PLA film and the PLA film loaded with Cloisite 30B. Keywords: polylactid acid (PLA); montmorillonite; (3-glycidyloxypropyl)trimethoxysilane (GOPTMS); organosilylated clay; nanocomposite; oxygen permeability. PACS: 81.05.Lg, 81.07.Pr, 82.35.Lr.

INTRODUCTION Mineral clays present a set of structural characteristics that make them attractive for the development of fillers for polymer-based nanocomposites. However, to allow the establishment of chemical interactions between the inorganic clay and the polymer matrix, the clay surfaces have to be organically functionalized in order to render them hydrophobic. Main routes employed to modify clays include ion exchange of interlayer cations with organic ammonium or phosphonium compounds and grafting by covalent bonding of silylating agents with the silanol groups on clay surfaces. While the ion exchange is a well-known procedure, which has led to the development of numerous commercial products, the silylation procedure is much less investigated and the benefits of using organosilylated clays on the chemical compatibility with polymeric matrices, the degree of dispersion in the polymer and the properties of the polymer-organoclay nanocomposite systems need to be further explored. In this work the attention was focused on the functionalization of a natural sodium montmorillonite (MMT) with (3-glycidyloxypropyl)trimethoxysilane (GOPTMS) as organic modifier, and on the use of the obtained organoclay as nanofiller for the preparation of biodegradable polylactic acid (PLA) nanocomposites. Hybrid systems added with 5wt% of organosilylated clay were prepared by solvent casting, using a commercial PLA as matrix, and were characterized using several techniques (XRD, permeability and mechanical tensile tests), in order to investigate the effect of the new clay on their morphology, barrier properties and mechanical response of the. Using the same preparation conditions, hybrid systems containing the same amount of the commercial Cloisite 30B nanoclay, derived from the same natural montmorillonite modified with alkylammonium salt, were also produced and characterized, for comparison.

VIII International Conference on “Times of Polymers and Composites” AIP Conf. Proc. 1736, 020102-1–020102-4; doi: 10.1063/1.4949677 Published by AIP Publishing. 978-0-7354-1390-0/$30.00

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EXPERIMENTAL Materials and Processing The clay used for functionalization experiments was a natural sodium montmorillonite (MMT) organomodified with (3-glycidyloxypropyl)trimethoxysilane (GOPTMS). The MMT, having surface area (N 2 , BET) of 760 m2/g and cationic exchange capacity (CEC) of 92 meq/100g was purchased by Southern Clay (Southern Clay Products, Inc., Gonzales, TX, USA). The GOPTMS (MW = 236.34 g/mol) was supplied by Sigma-Aldrich. The obtained organosilylated clay was referred in the following as SC1. Cloisite 30B (supplied by Southern Clay Products, Inc., Gonzales, TX, USA), a layered sodium montmorillonite organically modified by N,methyl-N,tallow-N,N’,2-hydroxyethyl-ammoniumchloride (90 meq/100 g clay), was used to have a comparison with the new organo-modified system. The polymer matrix used was poly(D,L-lactide) with 1.5% of D-lactoyl unity (PLA 4032D, Mw= 155000 g/mol, Mn= 93400 g/mol), obtained from NatureWorks (NatureWorks BV, Naarden, The Netherlands). All the other chemicals were of reagent grade and were used as received. The preparation of the organosilylated clay was performed according to the procedure reported by Chen et al. with a minor modification [1]. The obtained product, referred below as SC1, was characterized by X-ray diffraction and thermogravimetric analyses in order to determine its interlayer spacing and the silane-grafting amount. Unfilled PLA film, obtained by solvent casting, was prepared dissolving 5 g of polymer in 100 mL of chloroform under magnetic stirring at room temperature. The resulting solution was transferred onto a leveled glass plate (23.5x39.5 cm) and left to dry for about 24 hours at room temperature, before peeling, and then at 60°C in a vacuum oven, to remove the remaining solvent that could acted as plasticizer [2]. To prepare hybrid systems with 5wt% of clay added (PLA_5SC1), a calculated amount of clay was dispersed in the solvent and stirred for 1h, followed by sonication for 1.5h at room temperature using an ultrasonic processor (UP200S, HIELSCHER). The clay solutions were added to a PLA solution previously prepared and stirred for 30min, sonicated for 2h and then cast onto a glass plate [3]. For the sake of comparison, PLA nanocomposites at 5wt% of Cloisite 30B (PLA_5C30B) were produced under similar conditions of solvent casting.

Characterization X-ray diffraction (XRD) patterns were obtained by an automatic Bruker D8 Advance powder diffractometer, operated at 35 kV and 40 mA, in reflection mode, using the nickel-ILOWHUHG&X.ĮUDGLDWLRQ Å), at a scanning rate of 0.2 deg/min. The d 001 spacings of clays were calculated using the Bragg’s law. Thermogravimetric analysis (TGA) of silylated clay was performed using a Shimadzu TGA-50 Instrument to quantitatively determine the grafting degree and yield as well as the thermal stability. Samples of about 10 mg were heated in platinum pans from room temperature to 900°C at a heating rate of 20°C/min, under a constant nitrogen flow of 20 ml/min. The amount of grafted organosilane, expressed as the percentage of organosilane moieties with respect the total inorganic mass, was calculated using the following relationship:

Silane grafted amount %

100 W200600 100 W200600

(1)

where W 200-600 , i.e. the mass loss between 200°C and 600°C, corresponds to silane degradation [4]. Mechanical tests were realized according to ASTM D882 method using a SANS dynamometer (MTS Systems, China). Rectangular film specimens of 12.7x110 mm were cut using a manual punch cutter. Tensile tests were performed at room temperature with a speed of 4 mm/min (gauge length 40 mm). Mechanical parameters were measured on 10 identical specimens, and the average values and the corresponding standard deviations are reported. Oxygen permeability measurements were performed by means of a GDP-C permeabilimeter (Brugger Feinmeckanick, Germany) connected to a thermally controlled water bath (Thermo Haake) in order to assess the gas permeability of the films at 23°C with a gas flux of 80 ml/min (ISO 15105-1).


RESULTS AND DISCUSSION With the aim to verify that the layered sodium montmorillonite was successfully modified with (3glycidyloxypropyl)trimethoxysilane, XRD analyses were carried out on the SC1 functionalized clay and on the original MMT, for comparison. The obtained results are reported in Figure 1.

FIGURE 1. X-ray diffraction patterns of the original MMT sodium montmorillonite and of the SC1 organosilylated clay.

The XRD patterns in Figure 1 show that the peak of SC1 was shifted to a smaller angle (2T = 6.35 deg) than that of MMT (2T = 9.15 deg), demonstrating that the adopted organosilylation procedure produces, in our experimental conditions, an enlargement of the d 001 interlayer spacing of the clay, which passes from 0.97 nm to 1.39 nm. TGA measurements allowed determining the initial degradation temperature, T onset , that was 320°C, and the temperature of maximum degradation rate, DT max , that was 357°C; both values are comparable or significantly higher than those of many commercial montmorillonites modified with alkylammonium salts and are an evidence of the good thermal stability of the SC1 organosilylated clay. The grafting amount of the organosilane, calculated from TGA curves according to eq. 1, was 7.82 wt%. In order to verify that the SC1 and C30B clays were dispersed on a nano-scale into the PLA matrix, XRD experiments were performed. Figure 2 shows XRD curves of PLA hybrid systems with 5wt% of SC1 (a) and 5wt% of C30B (b) added, compared with those of the corresponding pristine clays. a)

b)

FIGURE 2. XRD patterns of SC1, PLA_5SC1 (a), C30B and PLA_5C30B (b).


The XRD patterns in Figure 2(a) show that the peak of PLA_5SC1 was shifted to a smaller angle (2T = 5.70 deg) than that of SC1 (2T = 6.35 deg), demonstrating that the intercalation of clay occurred with an enlargement of the d 001 interlayer spacing of the clay, which passes from 1.39 nm to 1.55 nm. From Figure 2(b), instead, it can be observed an increase of 2T value in PLA_5C30B if compared to the same clay from 5 deg to 5.8 deg, with a reduction in interlayer spacing of the clay. Table 1 reports the results of the oxygen permeability tests of the unfilled and nanocomposite PLA films. TABLE 1. Oxygen permeability values at 23°C of the unfilled and the nanocomposite polylactic acid films. P [cm3 mm/m2 d bar] Film Sample Neat PLA 21.5 PLA_5SC1 13.5 PLA_5C30B 19.1

Compared to the unfilled PLA film, the PLA_5C30B shows only a moderate reduction (ca. 11%) of the permeability to oxygen, whereas the PLA_5SC1 sample shows a much stronger decrease (ca. 37%). This behavior is coherent with the better dispersion level of SC1 clay platelets in the polymer matrix inferred by XRD analyses. The clay dispersion on a nano-scale hinders and makes more tortuous the diffusive path for the permeant gas through the polymer [5-7]. Table 2 reports the results of the mechanical tensile tests performed on the investigated systems. Film Sample Neat PLA PLA_5SC1 PLA_5C30B

TABLE 2. Tensile results of unfilled PLA, PLA_5SC1 and PLA_5C30B. E [MPa] ࢽ b [MPa] 2630±200 38.9±5.0 2840±60 39.3±1.1 3070±160 39.9±2.2

İ b [%] 7.1±3.1 7.4±2.2 9.5±2.9

The presence of nanoclay, interacting with polymer chains, determines a slight improvement of the mechanical properties. In particular, for both nanocomposite systems it was observed an increase in the Young’s modulus and in elongation at break. The phenomenon is probably due to the intercalation of clay platelets in a brittle PLA matrix, that absorbing energy by shearing, determine an improvement in tensile ductility [5,8-9].

CONCLUSIONS An organoclay containing epoxy groups (SC1) was successfully produced, with good efficiency of the grafting process, by functionalization of a natural montmorillonite (MMT) by (3-glycidyloxypropyl)trimethoxysilane (GOPTMS). The new clay was dispersed on a nano-scale, by solvent casting method, in a commercial PLA matrix. The obtained biodegradable PLA nanocomposites showed a significant improvement of the gas barrier properties, compared not only to neat PLA film, but also to a similar PLA nanocomposite system containing 5 wt% of a commercial organoclay modified with alkylammonium salt, used as reference materials. Thanks to the nanometric dispersion of the clay, the nanocomposites also showed slight improvements in the mechanical resistance of the material without reduction in terms of ductility.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

G.-X. Chen, H.-S. Kim, J.-H. Shim, and J.-S. Yoon, Macromolecules, 2005, vol. 38, pp. 3738-3744. J. W. Rhim, A. K. Mohanty, S. P. Singh, and P. K. Ng, Journal of Applied Polymer Science, 2006, vol. 101, pp. 3736-3742. J.-W. Rhim, S.-I. Hong, and C.-S. Ha, LWT-Food Science and Technology, 2009, vol. 42, pp. 612-617. N. N. Herrera, J.-M. Letoffe, J.-L. Putaux, L. David, and E. Bourgeat-Lami, Langmuir, 2004, vol. 20, pp. 1564-1571. L. Di Maio, P. Scarfato, M. R. Milana, R. Feliciani, M. Denaro, G. Padula and L. Incarnato, Packaging Technology and Science, 2014, vol. 27, pp. 535-547. R. K. Bharadwaj, Macromolecules, 2001, vol. 34, pp. 9189-9192. G. Russo, G. Simon, and L. Incarnato, Macromolecules, 2006, vol. 39, pp. 3855-3864. S. Tjong, Materials Science and Engineering: R: Reports, 2006, vol. 53, pp. 73-197. C. Zilg, R. Mülhaupt, and J. Finter, Macromolecular Chemistry and Physics, 1999, vol. 200, pp. 661-670.
