Effects of modified cellulose nanocrystals on the barrier and migration properties of PLA nano-biocomposites

Abstract

The aim of this paper is to report the impact of the addition of cellulose nanocrystals on the barrier properties and on the migration behaviour of poly(lactic acid), PLA, based nano-biocomposites prepared by the solvent casting method. Their microstructure, crystallinity, barrier and overall migration properties were investigated. Pristine (CNC) and surfactant-modified cellulose nanocrystals (s-CNC) were used, and the effect of the cellulose modification and content in the nano-biocomposites was investigated.

The presence of surfactant on the nanocrystal surface favours the dispersion of CNC in the PLA matrix. Electron microscopy analysis shows the good dispersion of s-CNC in the nanoscale with well-defined single crystals indicating that the surfactant allowed a better interaction between the cellulose structures and the PLA matrix. Reductions of 34% in water permeability were obtained for the cast films containing 1 wt.% of s-CNC while good oxygen barrier properties were detected for nano-biocomposites with both 1 wt.% and 5 wt.% of modified and un-modified cellulose nanocrystals, underlining the improvement provided by cellulose on the PLA films. Moreover, the migration level of the studied nano-biocomposites was below the overall migration limits required by the current normative for food packaging materials in both non-polar and polar simulants.

Introduction

Production of innovative “green materials” derived from natural sources is currently one of the main points of interest in the academic and industrial areas of material research. Studies based on nano-biocomposites using different reinforcements for poly(lactic acid), PLA, have been reported by many research groups in recent years (Fortunati et al., 2012a, Petersson et al., 2007, Sanchez-Garcia et al., 2008). PLA is a biodegradable thermoplastic polyester produced from l- and d-lactic acid, which is obtained from the fermentation of corn starch. PLA is currently commercialized and used in food packaging application for fresh or relatively short shelf-life products as containers, drinking cups, salad cups, overwrap and lamination films, and blister packages (Auras et al., 2006, Schwach et al., 2008). However, some drawbacks such as low thermal stability and poor barrier properties have reduced their application in food packaging. The addition of nanomaterials could be considered an adequate alternative to limit such problems and consequently to improve the possibilities for PLA based packages (Bordes et al., 2009, Martino et al., 2011). The use of cellulose nanowhiskers or nanocrystals has been proposed as the load-bearing constituent in developing new and inexpensive bio-materials due to their high aspect ratio, good mechanical properties and fully degradable and renewable character (Sturcova, Davies, & Eichhorn, 2005). If compared to other inorganic reinforcing fillers for biopolymers, cellulose nanocrystals have some additional advantages, including their wide availability of sources, low-energy consumption, ease of recycling including combustion, high sound attenuation and comparatively easy processability due to their non-abrasive nature, allowing high filling levels and significant cost savings (Samir & Dufresne, 2005). The use of cellulose nanocrystals as nano-reinforcements is an emerging field in nanotechnology, but there are still some obstacles for their use. Firstly, cellulose nanocrystals are not commercially available since their production is still associated with low yields. In addition, they are difficult to use with water insoluble polymers like PLA, because their high ability to produce strong hydrogen bonding. Therefore, cellulose nanocrystals have to be transferred from water to an appropriate solvent in order to produce and process PLA composite systems (Araki et al., 2001, Goussé et al., 2004, Heux et al., 2000).

The control of the degradation rate and the release of potentially migrant compounds, are key issues for the design of biodegradable systems for food packaging. PLA is the most interesting biopolymer for manufacturing these materials, and consequently it should be prepared to be in contact with different types of food (Mutsuga, Kawamura, & Tanamoto, 2008). Lactic acid is the lone monomer in the PLA structure but migrants in PLA based systems could be lactic acid itself joined to dimmers and other oligomers, produced by the PLA hydrolysis (Jamshidian, Tehrany, Imran, Jacquot, & Desobry, 2010). However, different results could be found for PLA blends, composites and copolymers, with more complex migration processes. In general, food packaging materials should be designed to control the gas permeability and to minimize migration of additives and other compounds during storage or processing.

The aim of this research is to study the impact of cellulose nanocrystals derived from highly purified cellulose fibres on the properties of PLA nano-biocomposites, with special focus on the barrier properties and on their interaction with food simulants.