Biocomposites based on poly(lactic acid) and hydrotalcite

The benefit of adding a filler may be outweighed by matrix degradation. In recent decades, a desire to reduce negative impacts on the environment has seen interest grow in biodegradable or compostable polymeric materials from renewable sources.1,2 The chemical and…

The benefit of adding a filler may be outweighed by matrix degradation. In recent decades, a desire to reduce negative impacts on the environment has seen interest grow in biodegradable or compostable polymeric materials from renewable sources.1,2 The chemical and physical properties of poly(lactic acid) (PLA) make it a suitable alternative to conventional petroleum-based polymers in several common applications, such as food packaging.3–5 However, wider use of PLA is limited by its relatively poor mechanical and gas/vapor barrier properties. Adding small amounts of lamellar particles or nanoparticles remarkably improves these properties,1,6–8 even if the nature of the components and final morphology also play a vital role.Recently, among inorganic materials, attention has turned to layered double hydroxides, also known as hydrotalcites (HTs).9–11 HTs are compounds of magnesium and aluminium with a layered structure. The HT interlayer anions are highly tuneable and HTs can be organically modified for specific functionalities.12We studied the influence of two different kinds of HTs—organically modified (OM-HT) and unmodified (U-HT)—and their concentrations on the morphology and properties of PLA/HT composites. We prepared the composites by melt compounding the PLA with either OM-HT or U-HT at concentrations of 1, 2, and 5wt%, using a co-rotating modular twin screw extruder. For comparison, neat PLA was processed under the same conditions. The top processing temperature was 200°C.We characterized the materials by scanning electron microscope (SEM), gel permeation chromatography (GPC), and mechanical and rheological measurements, paying particular attention to the effects of PLA degradation. (Thermomechanical stresses during processing induce chain scission of the PLA, reducing its mechanical performance.)SEM analysis indicates that the materials containing OM-HT show a better morphology (better dispersion and smaller HT dimensions) in comparison with those incorporating U-HT (see Figure 1). However, when the concentration of either type of HT increases, the morphology becomes coarser, with worse dispersion and larger dimensions of filler particles.Figure 1.Scanning electron micrographs of poly(lactic acid) (PLA) and unmodified hydrotalcite (U-HT) or organically modified hydrotalcite (OM-HT) with composition: (a) PLA and 1wt% U-HT; (b) PLA and 5wt% U-HT; (c) PLA and 1wt% OM-HT; and (d) PLA and 5wt% OM-HT.Figure 2 shows the complex viscosity as a function of frequency for neat PLA and all the composites. It is evident that the viscosity of the HT-containing materials is lower than that of pure PLA for the same conditions across the whole frequency range. In particular, the viscosity decreases as the HT content increases. This apparently surprising result has already been reported for similar systems13 and can be explained by the effects of degradation outweighing those of reinforcement by HT. We hypothesized that the organic modifier was responsible for the additional reduction in viscosity seen for composites containing OM-HT compared to those containing U-HT, and that degradation products of the modifier accelerate the degradation of the PLA matrix, thus further reducing the melt viscosity.Figure 2.Complex viscosity, η#, as a function of frequency for neat PLA and its composites with HT.To test this hypothesis, we used GPC analysis to measure molecular weights: see Table 1 for number average molecular weight (Mn) and weight average molecular weight (Mw) of neat PLA and all the composites. We found that the molecular weight of the composites was lower than that of the PLA, and this reduction becomes more obvious as the concentration of HT increased. Moreover, the reduction is even more marked when OM-HT was used. These results confirm that HT degrades the PLA matrix, and the organic modifier further depolymerizes the PLA.Table 1.Values of number average molecular weight (Mn) and of the weight average molecular weight (Mw) of neat PLA and its composites with HT.SampleMn(g/mol)Mw(g/mol)PLA 81000168400PLA + U-HT 1%77550161800PLA + U-HT 2%66660147800PLA + U-HT 5%56610109400PLA + OM-HT 1%52340110400PLA + OM-HT 2%4312082440PLA + OM-HT 5%3185080040Adding HT caused only a slight increase of the elastic modulus (E) of filled materials even when 5wt% of filler was incorporated (see Table 2). The tensile strength (TS) of the filled materials decreased compared with neat PLA, particularly for the composites with 5wt% of HT. Elongation at break (EB) slightly decreased as HT content increased. Moreover, the composites incorporating U-HT had better mechanical properties than those incorporating OM-HT.Table 2.Elastic modulus (E), tensile strength (TS) and elongation at break (EB) of neat PLA and its composites with HT.SampleE (MPa)TS (MPa)EB (%)PLA 170550,93,5PLA + U-HT 1%177448,53,4PLA + U-HT 2%186946,493PLA + U-HT 5%187731,31,7PLA + OM-HT 1%174434,92,1PLA + OM-HT 2%178126,61,5PLA + OM-HT 5%179623,91,3To explain these results, we considered that two phenomena are influencing the mechanical behavior of the filled materials: HT dispersion and matrix degradation. The former is improved by the presence of the organic modifier, as shown by the SEM images in Figure 1, but at the same time, OM-HT causes more intense degradation of the PLA matrix as revealed by the molecular weights. Moreover, even when the two hydrotalcites were added at the same concentration, the composites incorporating the OM-HT contained less inorganic fraction than those incorporating the U-HT.In summary, the choice of HT and its amount clearly influence the morphology (and, consequently the properties) of PLA-HT composites. Although materials containing OM-HT showed a finer morphology, they benefit from only a slight increase in the elastic modulus. Improvements from better filler dispersion are counterbalanced by matrix degradation.AuthorsRoberto ScaffaroUniversity of PalermoRoberto Scaffaro is an associate professor of material science and technology. His research focuses on science and technology of polymeric materials with particular reference to polymer blends, composites, nanocomposites, recycling, degradation, stabilization, biopolymers and polymeric materials for special uses (biomedical applications, active packaging, and functional nanohybrids).Luigi BottaUniversity of PalermoLuigi Botta is a postdoctoral researcher. His research focuses on science and technology of polymeric materials with particular reference to polymer blends, composites, nanocomposites, degradation-stabilization, and polymeric materials for special uses (biomedical applications, active packaging, and functional nanohybrids).ReferencesS. S. Ray and M. Bousmina, Biodegradable polymers and their layered silicate nanocomposites: In greening the 21st century materials world, Prog. Mater. Sci. 50 (8), pp. 962-1079, 2005. L. Yu, K. Dean and L. Li, Polymer blends and composites from renewable resources, Prog. Polym. Sci. 31 (6), pp. 576-602, 2006. R. M. Rasal, A. V. Janorkar and D. E. Hirt, Poly(lactic acid) modifications, Prog. Polym. Sci. 35 (3), pp. 338-356, 2010. J. Ahmed and S. K. Varshney, Polylactides-chemistry, properties and green packaging technology: A review, Int. J. Food Prop. 14 (1), pp. 37-58, 2011. R. Scaffaro, M. Morreale, F. Mirabella and F. P. La Mantia, Preparation and recycling of plasticized PLA, Macromol. Mater. Eng. 296 (2), pp. 141-150, 2011. R. Scaffaro, L. Botta and F. P. La Mantia, Preparation and characterization of polyolefin- based nanocomposite blown films for agricultural applications, Macromol. Mater. Eng. 294 (6-7), pp. 445-454, 2009. S. S. Ray, P. Maiti, M. Okamoto, K. Yamada and K. Ueda, New polylactide/layered silicate nanocomposites. 1. Preparation, characterization, and properties, Macromolecules 35 (8), pp. 3104-3110, 2002. P. Bordes, E. Pollet and L. Avérous, Nano-biocomposites: Biodegradable polyester/nanoclay systems, Prog. Polym. Sci. 34 (2), pp. 125-155, 2009. S. Coiai, M. Scatto, L. Conzatti, F. Azzurri, L. Andreotti, E. Salmini, P. Stagnaro, A. Zanolin, F. Cicogna and E. Passaglia, Optimization of organo-layered double hydroxide dispersion in LDPE-based nanocomposites, Polym. Adv. Technol. 22 (12), pp. 2285-2294, 2011. L. A. Utracki, M. Sepehr and E. Boccaleri, Synthetic, layered nanoparticles for polymeric nanocomposites (PNCs), Polym. Adv. Technol. 18 (1), pp. 1-37, 2007. J. U. Ha and M. Xanthos, Novel modifiers for layered double hydroxides and their effects on the properties of polylactic acid composites, Appl. Clay Sci. 47 (3-4), pp. 303-310, 2010. F. R. Costa, A. Leuteritz, U. Wagenknecht, D. Jehnichen, L. Haüßler and G. Heinrich, Intercalation of Mg-Al layered double hydroxide by anionic surfactants: Preparation and characterization, Appl. Clay Sci. 38 (3-4), pp. 153-164, 2008. R. Scaffaro, L. Botta, M. Ceraulo and F.P. La Mantia, Effect of kind and content of organo-modified clay on properties of PET nanocomposites, J. Appl. Polym. Sci. 122 (1), pp. 384-392, 2011. DOI:  10.2417/spepro.005169

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