Biodegradable poly(lactic acid)-based hybrid coating materials for food packaging films with gas barrier properties

https://doi.org/10.1016/j.jiec.2011.12.004Get rights and content

Abstract

The inclusion of biodegradable poly(lactic acid) (PLA) as an organic component into the inorganic silica networks was attempted to prepare environmentally friendly hybrid coating materials with improved gas barrier properties by using sol–gel method. The PLA film obtained by melt extrusion casting process was used as a substrate for coating with prepared PLA/SiO2 hybrids. Interfacial attraction between the organic and the inorganic phases in the hybrid was promoted by employing 3-isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase interaction, morphologies, crystallization behavior, and optical transparencies for the prepared hybrids were investigated not only to evaluate the phase compatibility, but also to present an evidence for the gas permeation behavior through the hybrid coated PLA film. The incorporation of the silica component at appropriate level of content substantially increased the resistance to gas permeation. The films retained high transparency, with optical transmittance of over 92%, and showed oxygen and water vapor barrier properties improved by 69.7% and 45.7%, respectively, over those of neat PLA film. Aging process improved the barrier properties of the hybrid coatings due to created dense network structures.

Introduction

Packaging materials for food and medicine require high resistances to oxygen and water vapor permeation to ensure products do not deteriorate during storage and handling [1], [2]. Polymeric packaging materials with high barrier properties are usually produced with multilayered structures by coextrusion or lamination process [3], [4]. These multilayer films include various nondegradable polymer resins, such as polyolefin, polyamide, poly(ethylene terephthalate), and ethylene-vinyl alcohol copolymers.

Since nondegradable polymer resins synthesized from petrochemicals can cause environmental problems after their useful life, ecologically safe, biodegradable polymers have been used for the application of short-term storage packaging films [5], [6]. A variety of biodegradable polymer materials have been developed and commercialized to replace conventional nondegradable polymer resins. Biopolymers for use as biodegradable packaging synthesized from petrochemicals or renewable resources include aliphatic thermoplastic polyesters, such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(ɛ-caprolactone) (PCL), and poly(hydroxyl butyrate) [7]. Among these, PLA synthesized from renewable resources has attracted much attention as it can achieve excellent mechanical properties, biodegradability, and biocompatibility, at competitive cost [8], [9], [10].

Despite its advantages, crystalline PLA shows a limitation for the application of gas barrier films to be used for the food or medical packaging materials, as it has relatively low resistance to oxygen and water vapor permeation compared with conventional nondegradable polymer resins [11]. Therefore, much work has been performed to improve the gas permeation resistance of biodegradable PLA resins by combining it with inorganic materials.

The most frequently used approach is the introduction of nanoscale, organically modified, layered silicates into polymer matrices to prepare biodegradable nanocomposites [12], [13], [14], [15]. Gas barrier, mechanical, and thermal properties can be improved upon the addition of small amounts of nanosized clay, due to the homogeneous dispersion of intercalated or exfoliated layered silicate platelets in the continuous matrix. However, clay loadings above a critical content can result in poor dispersion of silicate layers that exhibit intercalated clay tactoids with large domain size, which ultimately deteriorate the physical properties of the nanocomposites. Hence, this nanotechnology approach using nanoclay has a limitation in increasing the barrier properties to the level required as the high barrier packaging materials.

An alternative approach to improve gas barrier properties involves incorporating nanostructured silica components synthesized from inorganic precursors into organic polymer phases using sol–gel technology. Organic–inorganic hybrid coatings of various nondegradable polymer substrate films has been reported to improve gas barrier properties [16], [17], [18], [19]. In these hybrid coating materials, the nondegradable polymer resins such as poly(vinyl alcohol), poly(vinyl acetate), poly(vinylidene chloride), and poly(ethylene-co-vinyl alcohol) were applied as the organic components. However, such nondegradable polymer-based hybrid coatings would severely limit the environmental benefits of using biodegradable polymer substrates. Therefore the preparation of biodegradable polymer-based hybrid coatings that can be combined with biopolymer substrate films has been reported for less environmentally damaging biodegradable food packaging films [11].

This work reports the incorporation of biodegradable PLA resin as an organic polymer into inorganic silica networks to prepare biodegradable organic–inorganic hybrid materials with high gas barrier properties that could be coated onto PLA substrate films. The PLA/SiO2 hybrid materials were synthesized by sol–gel process using tetraethoxysilane (TEOS) as an inorganic precursor and 3-isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase microstructures, thermal properties, and crystallization behaviors of the resulting hybrid materials were investigated by SEM, DSC, and XRD. The gas barrier properties of the biodegradable hybrid-coated PLA films were assessed by measuring the permeabilities of oxygen and water vapor through the coated films.

Section snippets

Materials and preparation

Tetraethoxysilane (TEOS, Acros Organics, 98%) and 3-isocyanatopropyl-triethoxysilane (IPTES, Aldrich, 95%) were used as inorganic silicate precursor and silane coupling agent, respectively. Poly(lactic-acid) (PLA, 2002D, NatureWorks Co. Ltd.) was used as the organic polymer in the PLA/SiO2 hybrid materials. Hydrocholoric acid (HCl, Samchun Chemical, 37 wt%) was used as a catalyst. Tetrahydrofuran (THF, Ducsan, Korea) was used as a co-solvent to facilitate hydrolysis during the sol–gel reactions

FTIR analysis

In the organic–inorganic hybrid materials, the phase attraction between two phases has been considered as a crucial factor to the production of hybrid materials with high performance. Particularly, in nanostructured hybrid materials synthesized via sol–gel method using a polymer resin system as an organic component, the phase morphology in association with the physical properties of the hybrid materials is greatly influenced by phase interaction between the polymer chain segments and the

Conclusions

Environmentally less damaging hybrid coating materials with low gas permeability were prepared by incorporating biodegradable organic PLA resin into inorganic silicate with superior barrier characteristics via sol–gel process. The resulting PLA/SiO2 hybrids showed improved phase compatibility with addition of IPTES silane coupling agent capable of creating covalent bonds between the organic and the inorganic phases. This strong bonding resulted in homogeneous microstructures with silica

Acknowledgment

This work was supported by Kyonggi University Research Grant 2010.

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