Effect of nanoclay-type and PLA optical purity on the characteristics of PLA-based nanocomposite films

https://doi.org/10.1016/j.jfoodeng.2013.01.021Get rights and content

Abstract

Nanocomposite films were produced by blending semicrystalline PLA with different types of OMMTs using extrusion. In order to study their effect on optical, structural, and thermal properties, they were added at a fixed nominal concentration of 5% w/w and compared with pure PLA films. The XRD measurements showed that the incorporation of small amounts of nanoclays resulted in a formation of nanocomposites, with the C30B demonstrating the greatest compatibility with PLA. An increase in UV barrier properties was observed in nanocomposites, with Closite® C20A showing the best performance. The content of L-isomer in PLA significantly affected the thermal properties of the films, with a decrease in Tm of about 14 °C and increase in Tc of about 20 °C when the matrix, possessing a higher D-isomer content, was used. FTIR results showed a strong interaction between the nanoclays and the two different PLA matrices, due to the appearance of new peaks in nanocomposites films at around 520 and 627 cm−1.

Highlights

► Incorporation of nanoclays in PLA formed nanocomposites with intercalated structures. ► Addition of nanoclays in PLA matrix increased barrier properties to UV radiation. ► Thermal properties were slightly affected by the addition of nanoclays. ► Incorporation of nanoclays had weak effects on the decomposition process of PLA. ► Barrier properties were highly improved by addition of nanoclays.

Introduction

Over the last few decades, plastics have been the most used material in the world, playing a central role in modern industrial economies (Carol et al., 2004). It is estimated that the expansion of plastic materials will continue to increase, due in particular to the growing demand for such materials in the developing economies of Asia, South America, and within the states which recently joined the European Union (Plastics Europe, 2008). Nevertheless, the growing reliance on oil-based polymers has raised several environmental and human health issues because of their persistence in the environment. Moreover, food packaging has recently been affected by significant changes in food distribution, due to the globalization of food supply and the increasing consumer demand for better, fresher, and safer quality foods (Lopez-Rubio et al., 2004). This new approach has been a key factor in the development of alternative sustainable packaging, targeted to be beneficial, safe, and healthy for individuals and communities throughout its life cycle, while at the same time, addressing market criteria for performance and cost. Examples of such packaging materials include bio-based polymers, bioplastics, or biopolymer packaging products made from raw materials originating from agricultural or marine sources (Cha and Chinnan, 2004).

A global interest in biopolymer usage in industrial and packaging applications has developed, and the consumption of bioplastics has increased from 15,000 to 225,000 tons during the years between 1996 and 2008 (Gross and Kalra, 2002, Pilla, 2011), with an estimated growth potential of between 20% and 30% anticipated for each year thereafter (Nampoothiri et al., 2010).

Poly lactic acid (PLA) is a compostable polymer derived from renewable sources, in particular from starch and sugar. During the last decade, it has been used primarily for medical applications such as implant devices, tissue scaffolds, and internal sutures, due to its high cost, low availability, and limited molecular weight (Datta and Henry, 2006). Recently, production costs have been lowered through the introduction of new technologies, such as the ring-opening polymerization (ROP) process and large-scale production. Thus, the use of PLA has been extended to other areas, such as packaging, textiles, and composite materials (Drumright et al., 2000, Garlotta, 2001, Groot et al., 2010). PLA is classified as GRAS (Generally Recognized As Safe) in the USA by the FDA (Food and Drug Administration) and approved for use in all the applications related to food and beverage packaging (Conn et al., 1995, FDA, 2002, Datta and Henry, 2006). While the main advantages of PLA consist of being a thermoplastic material with good processability, which can be processed in machines used for conventional plastic, its drawbacks such as brittleness, moderate barrier properties to water vapor and oxygen and low temperature degradation (200 °C) limit its use to films, thermoformed and blow molded containers, food service ware and short shelf-life bottles (Gross and Kalra, 2002). It is therefore suitable for use with fresh products and foodstuffs not requiring a packaging with low oxygen permeability performance (Jamshidian et al., 2010).

Over the years, many studies have been carried out as a means of improving the properties of PLA films (Pluta, 2006). For example, orientation has been used to improve the mechanical properties of PLA (Auras et al., 2005). Similarly, new formulations and plasticizers, blending with other polymers, co-polymerization, co-extrusion, and lamination with more flexible and higher barrier polymers, coating, and the formation of new structures with the addition of fillers ranging from non-biodegradable to biodegradable materials have all been used to improve the properties of PLA-based packaging materials. The progress which has developed recently in the field of nanoscience and nanotechnology has given a unique opportunity to develop revolutionary materials which overcome the typical performance compromises of the conventional materials by benefiting through a synergism occurring between components below certain dimensions (Sinha Ray, 2010). For instance, nanoparticles have been shown to improve the properties of polymeric materials through their low level addition, from 2% to 8% w/w (Lim et al., 2008), thereby allowing them to function as nanofillers.

Among the nanofillers, montmorillonite (MMT) has an interesting structure. It is a layered silicate (2:1 phyllosilicate) characterized by a crystalline structure constituted of stacks of clay platelets. Each platelet has an average thickness of about 1 nm and contains two external layers of silicon oxide tetrahedra with a central sheet of aluminum or magnesium oxide octahedra. A net negative charge is located on both faces of platelets, so that electrically positive cations (Ca2+, Na+, etc.) are attracted in the gallery area between them (Duncan, 2011, Pavlidou and Papaspyrides, 2008).

Due to the hydrophilic nature of MMT, alkali counter-ions are exchanged with cationic–organic surfactants in order to make them more compatible with hydrophobic polymers. This allows polymers, within which MMTs are dispersed, to be improved in terms of their properties, such as barrier to permeants and mechanical strengths (Pilla, 2011). A performant concentration of the MMT in a polymer matrix has been demonstrated to be around 4–5% w/w (Lim et al., 2008, Pluta, 2006). Moreover, some nanoclays have been shown to disperse the UV–visible radiation because of the inherent reduced scattering phenomena and absorbing properties determined by the highly dispersed clay nanolayers (Sanchez-Garcia and Lagaron, 2010). According to these authors, this characteristic allows food packaging to be protected against light radiation, thus preserving the quality of many food products such as fruit and vegetable juices, vitamins and sport drinks, dairy products and edible oils. In particular, materials containing 1–5 wt.% of MMT have been reported to be able to show significant improvements in barrier properties and to disperse to a very little amount the UV–visible radiation (Petersson and Oksman, 2006). According to Rhim et al. (2009), Closite® 20A showed a better performance than Closite® 30B in improving the UV barrier properties, due the reduced transmittance.

In relation to food contact applications, the use of nanomaterials may be considered as a potential risk for the consumer health, due to the migration of nanoparticles from the food packaging into food. Toxicology research and evaluations of the risks derived from the use of nanotechnologies seem to be practically nonexistent, in particular in the food sector (Tiede et al., 2008). According to Munro et al. (2009), a risk assessment must to be carried out on a case by case basis. A potential risk in using the MMT was investigated by Avella et al. (2005), who studied the migration from a nanocomposite biopolymer/nanoclay into vegetables by monitoring the increase in the amount of silicon. They found that the increase in this mineral was not significant, thus complying with the EC normative on food contact materials. According to the FDA (Food and Drug Administration), MMT is considered GRAS (Generally Recognized As Safe, 21CFR184.1155) and thus can be used for food packaging. However, further investigations on potential migration of nanoclay and, in particular, the organic modifiers are still necessary. A number of studies have investigated the effects of developing PLA nanocomposites containing organically modified MMT (OMMT) (Jiang et al., 2007, Fukushima et al., 2009, Pluta, 2006, Thellen et al., 2005), but few (Zhou and Xanthos, 2008, Zhou and Xanthos, 2009) have considered a simultaneous effect of combining nanoclays and PLA possessing different degrees of stereoisomeric purity.

The objectives of this study were to investigate the combined effects of the addition of four types of OMMT possessing different chemical structures in two PLA matrices consisting of distinct contents of L-isomer on optical, morphological, and thermal properties of nanocomposites manufactured through extrusion.

Section snippets

Materials

PLA 4032D and 4042D with 98.5% and 96% L-isomer lactide content, respectively, and average molecular weights of 210 kDa (according to manufacturer’s specification sheet) were used as the polyester matrix (NatureWorks® LLC, Blair, NE, USA). Four types of nano-sized Cloisite®, C10A, C20A, C30B, and C93A (Southern Clay Products Inc., Gonzales, TX, USA), were purchased and used as fillers. These organoclays are obtained by modification of natural montmorillonite with different ammonium salts. In

Tga

The effective organoclay load in the PLA 4032D and 4042D series as assessed by TGA is shown in Table 3. Unlike unfilled control samples, a black residue was observed on the platinum sample holders following the decomposition of nanocomposite materials and may be attributable to the presence of clay in the film. The residue measured was higher than the standard deviations measured for each sample, and this may be due to the presence of inorganic contaminant traces or as a consequence of

Conclusions

The incorporation of nanoclays in two PLA matrices consisting of a different L-isomer content resulted in the formation of nanocomposites characterized by intercalated structures and a partial preservation of OMMTs layered configuration independently from the organic modifier and the PLA matrix used, as shown by the XRD patterns. The thermal analysis indicated that the addition of nanoclays slightly affected the thermal properties of the films. The organoclays had an effect on the transparency

Acknowledgements

Stefano Molinaro would like to thank Arcadia SPA (Italy) for the financial support. Thanks also to Loredana Incarnato and Maria Rosaria Galdi from the Department of Chemical and Food Engineering (DICA), University of Salerno (Italy) for the manufacturing of PLA nanocomposite films and the technical support.

References (66)

  • J.W. Gilman

    Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites

    Applied Clay Science

    (1999)
  • L. Jiang et al.

    Comparison of polylactide/nano-sized calcium carbonate and polylactide/montmorillonite composites: reinforcing effects and toughening mechanisms

    Polymer

    (2007)
  • H.J. Lehermeier et al.

    Gas permeation properties of poly(lactic acid)

    Journal of Membrane Science

    (2001)
  • L.T. Lim et al.

    Processing technologies for poly(lactic acid)

    Progress in Polymer Science

    (2008)
  • E. Pamula et al.

    FTIR study of degradation products of aliphatic polyesters–carbon fibres composites

    Journal of Molecular Structure

    (2001)
  • S. Pavlidou et al.

    A review on polymer-layered silicate nanocomposites

    Progress in Polymer Science

    (2008)
  • L. Petersson et al.

    Biopolymer based nanocomposites: Comparing layered silicates and microcrystalline cellulose as nanoreinforcement

    Composites Science and Technology

    (2006)
  • J.-W. Rhim et al.

    Tensile, water vapor barrier and antimicrobial properties of PLA/nanoclay

    LWT-Food Science and Technology

    (2009)
  • M.G. Rimoli et al.

    Synthesis and characterisation of poly(d,L-lactic acid)-idoxuridine conjugate

    Journal of Controlled Release

    (1999)
  • S. Sinha Ray et al.

    New polylactide-layered silicate nanocomposites. 2. Concurrent improvements of material properties, biodegradability and melt rheology

    Polymer

    (2003)
  • S. Solarski et al.

    Ageing of polylactide nanocomposite filaments

    Polymer Degradation and Stability

    (2008)
  • C. Thellen et al.

    Influence of montmorillonite layered silicate on plasticized poly(L-lactide) blown films

    Polymer

    (2005)
  • C.-C. Tsai et al.

    Crystallinity and dimensional stability of biaxial oriented poly(lactic acid) films

    Polymer Degradation and Stability

    (2010)
  • L. Wang et al.

    Effect of pH and addition of corn oil on the properties of gelatin-based biopolymer films

    Journal of Food Engineering

    (2009)
  • M. Yasuniwa et al.

    Melting behavior of poly(L-lactic acid): X-ray and DSC analyses of the melting process

    Polymer

    (2008)
  • M. Yasuniwa et al.

    Crystallization behavior of poly(L-lactic acid)

    Polymer

    (2006)
  • J.M. Zhang et al.

    Differences in the CH3Odouble bondC interactions among poly(L-lactide), poly(L-lactide)/poly(D-lactide) stereocomplex, and poly(3-hydroxybutyrate) studied by infrared spectroscopy

    Journal of Molecular Structure

    (2005)
  • Q. Zhou et al.

    Nanoclay and crystallinity effects on the hydrolytic degradation of polylactides

    Polymer Degradation and Stability

    (2008)
  • Q. Zhou et al.

    Nanosize and microsize clay effects on the kinetics of thermal degradation of polylactides

    Polymer Degradation and Stability

    (2009)
  • H. Abe et al.

    Crystal growth and solid-state structure of poly(lactide) stereocopolymers

    Biomacromolecules

    (2005)
  • E. Almenar et al.

    Permeation, sorption, and diffusion in poly(lactic acid)

  • R.A. Auras et al.

    Evaluation of oriented poly(lactide) polymers vs. existing PET and oriented PS for fresh food service containers

    Packaging Technology and Science

    (2005)
  • G.W. Beall

    New conceptual model for interpreting nanocomposite behavior

  • Cited by (126)

    View all citing articles on Scopus
    View full text