Poly(lactic acid) (PLA), a bio-based polymer, has several attractive properties such as excellent stiffness, reasonable strength, excellent flavor and aroma barrier, as well as good grease and oil resistance. Despite these attributes, PLA's applicability as a flexible food packaging material is limited due to several drawbacks such as brittleness, poor water vapor and moderate oxygen barrier properties as well as film processing difficulties due to its insufficient melt strength. This study was aimed at overcoming these drawbacks to widen PLA's applicability in the food packaging industry.Firstly, the effectiveness and efficiency of two newly developed food grade multifunctional epoxies with low and high epoxy equivalent weights in chain extending/branching PLA were studied in a torque rheometer, in order to overcome the issue of PLA's insufficient melt strength. Both chain extender (CE) grades not only chain-extended PLA effectively as indicated by a significant increase in the mixing torque as well as PLA's melt viscosity and molecular weight, but also branched it leading to its reduced crystallinity. Infrared results indicated that chain extension occurred through the ring opening reaction of epoxy groups in the CE with PLA's hydroxyl and/or carboxyl groups. This chain extension/branching was beneficial in overcoming PLA film's brittleness since its impact strength increased almost linearly with the CE content.Secondly, cellulose nanocrystals (CNCs) were added to a PLA matrix to increase its crystallinity and act as impermeable regions in order to improve its barrier properties. However, CNCs were difficult to disperse in non-polar polymers due to their high polarity and strong hydrogen bonding forces. Therefore, two different solvent-free approaches of incorporating and dispersing CNCs into the PLA matrix were examined. The first approach consisted of melt-blending PLA and CNCs in an internal mixer whereas the second method involved direct dry-mixing of PLA and CNCs in a high intensity mixer, before film manufacture through the blown film extrusion process. Good distribution and barrier performance improvement were achieved by both methods. However, the direct dry-blending technique appeared to be the better approach for adding CNCs into the PLA matrix because it exposed the samples to less heat; thus, minimizing thermal degradation as demonstrated by the quantified number of chain scissions, molecular weight and melting temperature results.Thereafter, the influence of CNC addition level and environmental testing conditions on the water vapor (WVP) and oxygen (OP) permeability of direct dry-blended PLA/CNC films were studied. Both WVP and OP of PLA and PLA/CNC nanocomposite films varied exponentially with temperature as expected from the Arrhenius equation, whereas the WVP remained constant with relative humidity (RH) as expected from Fick's law. Additionally, the values of WVP and OP negatively correlated with the degree of crystallinity. Depending on testing temperature or humidity, optimum improvements in WVP (30-40%) and OP (65-75%) of PLA films occurred at 1% CNCs, a CNC content that correlated very well with the maximum increase in crystallinity.Finally, the potential of the developed PLA/1% CNC films with enhanced barrier performance in extending the shelf-life of a moisture-sensitive food product (crackers) was assessed and mathematically modeled. Interestingly, the crackers packaged in the CNC-based films had approximately 40% longer shelf-life compared to the ones packaged in neat PLA, irrespective of the RH. The overall results of this research indicate that the PLA/CNC films developed in this study have tremendous potential for food packaging applications.
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