Supercritical impregnation of thymol in poly(lactic acid) filled with electrospun poly(vinyl alcohol)-cellulose nanocrystals nanofibers: Development an active food packaging material

Highlights

• Obtaining of electrospun poly(vinyl alcohol) nanofibers containing cellulose nanocrystals.

• Development of a PVA/CNC/PLA nanocomposite with improved mechanical and thermal properties.

• Thymol was incorporated to the PVA/CNC/PLA nanocomposite by supercritical impregnation.

• Thymol release rate is significantly reduced by the presence of the nanofiller in the PLA.

Abstract

This work describes the procedures to obtain poly(lactic acid) (PLA) films containing natural nanofibers as well as to impregnate this material with an antimicrobial agent to be used in active food packaging. Poly(vinyl alcohol) (PV) nanofibers containing cellulose nanocrystals (CNC) were successfully prepared by electrospinning, these electrospun PV/CNC nanofibers were incorporated into PLA films with a concentration equal to 4.7% (w/w) by the casting method. Subsequently, these nanocomposite films were impregnated with thymol, which was previously dissolved in supercritical carbon dioxide (scCO₂).

The active nanocomposite obtained from the whole procedure shows a significant improvement of its mechanical and thermal properties and the incorporation of thymol is not significantly modified when it is compared to the impregnation response of nanofibers-free PLA. However, the release rate of thymol was significantly slower when PV/CNC nanofibers are incorporated in the polymer structure.

Introduction

Nowadays, active food packaging technologies represent a step forward in preservation capacity through different type of applications such as oxygen scavengers, carbon dioxide emitters or absorbers, moisture and ethylene absorbers, ethanol emitters, flavour releasing/absorbing systems among others (Ozdemir and Floros, 2004). Simultaneously, the packaging industry is concerned to use sustainable materials and techniques to develop systems with low or controlled environmental impact.

Poly(lactic acid) (PLA) is one of the most studied materials used in the preparation of biodegradable packaging because it shows competitive structural, mechanical and thermal properties (Garlotta, 2002, Arrieta et al., 2014a, Arrieta et al., 2014b). Furthermore, PLA exhibits high availability and low cost as the main arguments to replace the conventional polymers commonly used in food packaging. Nevertheless, the properties of PLA have to be improved by different methods in order to increase the wide range of applications where the traditional polymers obtained from petrochemical sources are still used. One of the ways to improve the mechanical and chemical properties of biodegradable polymers considers the incorporation of nanostructured fillers (Arrieta et al., 2015a, Arrieta et al., 2015b, Battegazzore et al., 2014, Kowalczyk et al., 2011, Graupner et al., 2009, Oksman et al., 2006), which can act as mechanical reinforcement of the polymer structure. Natural nanofibers can be an option of this functionalization because of their low density, high toughness, low cost and biodegradability (Graupner et al., 2009). Several studies reported properties of natural fibers obtained from jute, wood, apple, kenaf, flax, cellulose and abaca and the modification in the properties of polymer matrices containing them (López de Dicastillo et al., 2017, Oksman et al., 2006, Zini et al., 2004, Bogren et al., 2006, Masirek et al., 2007, Pan et al., 2007, Ogbomo et al., 2009, Meister et al., 2017, Arfat et al., 2017). These studies show improvement in the mechanical and barrier properties increasing the competitiveness of the biodegradable polymers.

On the other hand, antimicrobial capacities of food packages have been largely investigated proposing the use of different antimicrobial agents, which can be slowly released to the contained food (Abarca et al., 2016, Torres et al., 2014, Tawakkal et al., 2016b, Petchwattana and Naknaen, 2015, Torres et al., 2017). In this framework, thymol is a well-known compound with antimicrobial properties that can be obtained from essential oil extracts and it has been widely studied (Torres et al., 2014, Tawakkal et al., 2016b, Petchwattana and Naknaen, 2015, Torres et al., 2017), identifying also a plasticizer effect that improves the thermal and mechanical properties when incorporated in polymers (Torres et al., 2014, Torres et al., 2017).

Notwithstanding the foregoing, sustainable materials should be prepared by greener processes, which show a rational requirement of energy, chemicals and water. In this way, the incorporation of active compounds obtained from natural sources in biopolymers has been proposed by means of supercritical impregnation using dense carbon dioxide (Torres et al., 2014, Torres et al., 2017, Rojas et al., 2015, Ivanovic et al., 2016). The choice of carbon dioxide as impregnation medium can be explained by its high solvent capacity, low cost, non-toxicity and non-flammability (Champeau et al., 2015). Supercritical impregnation can be represented by a reversed extraction process where the mass transfer and the thermodynamic equilibrium determine the performance. The impregnation efficiency depends on different variables such as the properties of the polymer and the chemical interactions with the impregnated compound and the supercritical fluid as well as some operational aspects as the impregnation time and the depressurization rate (Goñi et al., 2016). Currently, the supercritical impregnation of active compounds is being used in food industry (Hassabo et al., 2015, Rojas et al., 2015, Knez et al., 2014, Sovilj et al., 2011).

In this context, this study is focused on the development of an active bio-nanocomposite and the improvement of its properties due to the incorporation of nanofibers, which were completely obtained from natural sources, and how the nanofibers’ presence modifies the release of an antimicrobial agent.