L.M. Matuana, S.S. Karkhanis, N.M. Stark
Michigan State University,
Keywords: cellulose nanocrystals, water and gas barriers, poly(lactic acid), extrusion blown film
Summary:Bio-based plastics from sustainable resources such as biomass are the best alternative to petroleum-based polymers because they provide solutions for reducing the general dependency on oil availability and environmental concerns about the use of common petroleum-based plastics. One bio-based plastic that has become an attractive alternative to petroleum-based polymers because of its physical properties (high stiffness, reasonable strength, excellent flavor and aroma barrier) and favorable degradation characteristics is poly(lactic acid) (PLA), a plastic that comes from fermented plant starch such as corn and sugar beets. Despite these attractive properties, PLA has relatively few commercial applications, concentrating mainly in textile and specialty biomedical niches such as sutures and drug delivery devices. Additionally, it is being used as a commodity polymer for packaging food and consumer goods where the physical properties of PLA make it suitable for manufacture of rigid containers and bottles. However, the widespread applicability of PLA in flexible sheets and films is limited because of its poor water and oxygen barrier properties, brittleness, lower impact resistance at room temperature, and narrow processing window. PLA has a very low melt strength as compared to other plastics currently in the market, making it difficult to melt-process. Additionally, the use of PLA films to package water and/or oxygen sensitive food products is avoided because of its poor barrier properties that can significantly shorten the products shelf life. All these factors greatly limit its applicability. Therefore, it is of paramount importance to overcome these difficulties in order to broaden the range of PLA commercial applications. Cellulose nanocrystals (CNCs) have been shown to improve the barrier properties of polymer films. However, most of the research reported in the literature used solvent casting techniques (a batch process) instead of melt-processing (a continuous process). Melt-processing is industrially viable for processing many types of packaging materials, such as films, sheets, bottles or containers that are often melt-blown or molded. Essentially, no research has been reported on the barrier properties of melt-processed PLA and CNC nanocomposite materials. Therefore, the objective of this work was to evaluate the barrier properties of melt-processed PLA-CNC nanocomposites. The effects of CNC content and types on the water vapor, oxygen, and carbon dioxide transmission rates of extrusion-blown PLA/CNC films recorded at various temperatures and relative humidities were examined. Experimental results indicate that the addition of CNCs into PLA matrix through a melt processing approach improves the barrier properties of PLA due to the tortuosity effect induced by the nanoparticles. This implies that novel bio-based composites with improved barrier properties suitable for packaging applications could be developed by combining CNCs with poly(lactic) acid (PLA).